Symposium FF04—Crystal Engineering of Functional Materials—Solution-Based Strategies

This research is devoted for making nanoparticles and fine particles of metallic copper by solution-based strategies. Gelatin and other polymers can work as stabilizing polymers to keep the particle surface in metallic state. These particles can be used as the materials for the inner electrode of MLCC by using a high-temperature annealing process. But in the field of printed electronics as well as organic electronics, conductive materials for low-temperature sintering are highly demanded.
In this study, crystal engineering and surface engineering was applied for copper fine particles prepared by solution-basted strategies. By using polymer-stabilized metallic copper fine particles, and transition metal oxides via solution syntheses that can be scalable for mass production. Our solution-based preparation of copper fine particles was carried out by chemical reduction of CuO micro particles by using hydrazine at alkaline conditions. Because this process is a low temperature process, the particles are consisted of small grains. According to the grain boundary in the copper fine particles, the diffusion of copper atoms becomes higher. Therefore, lower temperature sintering can be carried out with these copper fine particles even the particle sizes bigger than 100 nm. This situation indicates that low temperature sintering cannot be performed only by the melting temperature depression.
More detailed analyses of copper fine particles prepared by solution-based strategies found that the surface of the particles was slightly oxidized. But the structure is not Cu₂O or CuO. These slightly oxidized states can be reduced readily in the presence using some reductive organic small molecules and metallic copper surface generates during sintering. Moreover, when oxygen atoms are eliminated from the surface, re-arrangement of copper atoms carries out. This situation also proceeds necking and sintering of the particles.
We discuss in this paper that the strategy of the crystal engineering of copper fine particles for the application of electroconductive materials and dyebonding materials.

Precise, large-scale assembly of polymer-grafted nanoparticles (PGNs) has shown great promise for the scalable manufacturing of sensors, energy storage devices, and photonic elements. Different than more traditional ligand-coated nanoparticles, PGNs are stabilized by high molecular weight polymers at relatively low grafting densities. In this regard, PGNs are considered soft colloids, possessing the favorable processing properties of polymers while also retaining the ability to pack into ordered nanoparticle assemblies. Using a simple flow-coating technique, we demonstrate that large scale, highly uniform monolayer films of gold-polystyrene nanorods can be fabricated from PGN inks within a few seconds. Applying a combination of atomic force microscopy, scanning electron microscopy, and grazing incidence X-ray scattering, we observe positional and orientational order dependent on coupling between surface energy, process methodology, and the architecture of the grafted polymer canopy. More specifically, we show that globally oriented films possessing two-dimensional orientation parameters greater than 0.7 can be achieved through evaporation induced-assembly when processing at slower deposition velocities on hydrophilic surfaces. Additionally, optical extinction measurements reveal strong plasmonic coupling between the gold nanorods which can systematically be tuned by varying film morphology. With fundamental understanding of structure-processing relationships, we demonstrate polarized optical notch filters using laser-induced reshaping of resonant nanoparticle subpoulations.

The development of new chemical methods for the next generation of nanoparticles with very high magnetic moment, fine tuning Au nanorods and novel hybrid and multifunctional nanostructure is presented.
Detailed mechanistic studies of their formation by sophisticated and advanced analysis of the nanostructure allows tuning of the physical properties at the nanoscale; these can subsequently be exploited for diagnosis and treatment of various diseases. The studies are conducted to provide insight for future material design approaches. It will also help to identify the critical process parameters that can be manipulated in order to obtain the suitable physical properties for the intended applications.
In collaboration with chemical engineers in designing the reactors in microfluidic systems, this allowed for the first time to detailed study the physical properties such as X-ray diffraction (XRD), magnetic moment and morphology to unravel the particle formation mechanism during co-precipitation synthesis of iron oxide magnetic nanoparticles, which is very important for their robust, reproducible formation process for cancer treatment.

References
LaGrow, A.P., Besenhard, M.O., Hodzic, A., Sergides, A., L. K Bogart, Gavriilidis, A., and Thanh, N. T. K* (2019) Unraveling the Growth Mechanism of the Co-Precipitation of Iron Oxide Nanoparticles with the Aid of Synchrotron XRD in Solution. Nanoscale. 11: 6620-6628.

Hobson, N.J., Weng, X., Siow, B., Veiga, C., Ashford, M., Thanh, N.T.K., Schatzlein, A.G., Uchegbu, I.F. (2019) Clustering Superparamagnetic Iron Oxide Nanoparticles Produces Organ Targeted High Contrast Magnetic Resonance Images. Nanomedicine. In Press

Famiani, S., LaGrow A. P., Besenhard, M. O., Maenosono, S., Thanh, N.T.K.* (2018) Synthesis of fine tuning highly magnetic Fe@FexOy nanoparticles through continuous injection and a study of magnetic hyperthermia. Chemistry of Materials. 30: 8897-8904

Besenhard, M.O., Baber, R., LaGrow, A.P., Mazzei, L., Thanh, N. T. K* and Gavriilidis, A. (2018) New insight into the effect of mass transfer on the synthesis of silver and gold nanoparticles. CrystEngComm. 20: 7082-7093. Inside Front Cover.

Pallares, R.M., Sutarlie, L., Thanh, N.T.K.* and Xiaodi Su (2018) Tunable plasmonic colourimetric assay with inverse sensitivity for extracellular DNA detection. Chemical Communications, 54: 11260 - 11263. Gold Open Access. Front Cover.

Predicative synthesis of metal nanocrystals with desired structures relies on precise control over the reduction pathways of metal ion precursors. Using capping ligand is an effective method to affect the reduction kinetic in nanocrystal formation. However, comprehensive understanding and predicative manipulation of reduction pathways is yet to be achieved by conventional capping ligand. DNA as one of the promising biomolecular capping ligand has been used to generate sequence specific morphologies in varies metal nanocrystals. Taking advantage of the well-defined binding mode between DNA and metal ions, we quantitatively analyzed the precise kinetic control of different DNA sequences over Ag+ reduction process and the relationship between reduction pathways and the structure of resulting nanocrystals. The complexation between DNA and Ag+ was shown to significantly affect the reduction potential of the precursor. The difference in binding equilibrium between DNA sequences and Ag+, as well as the interaction between DNA and the surface of the nanocrystals, synergistically determined the distinguishable Ag+ reduction path-ways with different DNA sequences. The DNA sequence specific control in reduction pathways also results in different mode of nanocrystal formation: layer growth, island growth, and the mixed of two. The discovery from this study demonstrates how rational design of metal-ligand interactions can be used to control precisely the growth kinetics of nanocrystals and the real potentials on using functional DNA sequences for the predictable synthesis of metal nanostructures.

The impact of defects on the photophysical and chemical properties of colloidal semiconductor quantum dots (QDs) is dominated by their large surface-to-volume ratios compared to bulk analogs. Understanding the formation mechanism of colloidal QDs is still largely unknown, however, the chemistry of well-defined molecular clusters has revealed their ability to also serve as suitable pre-nucleation clusters in QD synthesis. This talk will highlight our recent studies in using high-resolution mass spectrometry and other spectroscopic techniques to study the solution chemistry and surface dynamics of a class of cadmium thiophenolate clusters with varying size and nuclearity (from 4 Cd ions to 17 Cd ions). The conversion of the tetrameric clusters to the heptadecameric clusters is complex involving numerous metastable intermediates including octameric and decameric clusters. We will also present synthetic design tools where we exploit this complex conversion chemistry to successfully dope Co(II) and Mn(II) ions into specific sites in the the decameric and heptadecameric clusters, including the core sites with metal sulfide bonds.

Transforming nanostructures into hollow ones represents a unique synthetic strategy for achieving new properties that do not exist in their solid counterparts, boosting applications in a broader range. The chemical nature of the reactive nanostructures (or sacrificial template nanostructures) determines the reaction kinetics of hollowing them along with the transformation of chemical compositions, which influences the controllability of synthesis. For example, uniform silver chlorobromide nanocubes can be used a class of sacrificial template nanostructures to regulate and maintain an extremely low concentration of available Ag⁺ that reacts with different thiol molecules to form silver thiolate with high controllability over the reaction kinetics. Such kinetic controllability enables the transformation of the silver chlorobromide nanocubes to hollow nanocages of silver thiolate. In contrast, the direct reaction of Ag⁺ ions with thiol molecules forms silver thiolate particles with irregular sizes and geometries. Moreover, the sacrificial silver chlorobromide nanocubes expose their surfaces to provide the heterogeneous nucleation sites for condensation of silver thiolate, further improving the controllability to benefit their transformation into hollow nanocages. Due to the improved controllability over reaction kinetics, the process of hollowing silver chlorobromide nanocubes can be terminated at any desired stage to form silver thiolate nanocages with the appropriate wall thickness. Time-dependent electron microscopic, X-ray diffraction, and UV-visible spectrometric studies reveal detailed insights into the structural evolution of crystals in terms of amount and dimensions during the hollowing transformation. This method represents scalable and versatile strategy to control the kinetics of fast reactions involved in the synthesis of colloidal nanoparticles, thus enabling the synthesis of uniform nanocages with desirable parameters. The availability of various silver-based hollow nanostructures provides the opportunity to explore potential interesting properties with real-world applications such as in electrocatalysis and photocatalysis.

Plasmonic chiral nanostructure has drawn significant attention due to its potential as a novel route for extraordinary optical properties such as negative refractive index and light polarization control. However, achieving chiral nanostructure remains as a challenge because of the complexity and resulting high cost of asymmetric morphology control at nanoscale. Inspired by natural process of chirality interaction between organic and inorganic materials, we reported a new pathway to control chiral morphology of gold nanoparticles. The resulted Helicoid nanoparticles were a new class of three-dimensional free-standing plasmonic chiral nanostructure, which exhibited strong chiroptical behavior such as anisotropic dissymmetry factor of 0.2. The chiral morphologies of Helicoid nanoparticles were further extended by establishing general model of chirality transfer from amino-acid or peptides to gold nanoparticles based on the concept of surface facet development pathway during nanoparticle growth. Asymmetric growth of chiral element is directed by enantioselective adsorption of amino-acids or peptides on high-Miller-index facet of nanoparticle which is formed in the course of particle growth. As the morphologies of nanoparticles are determined by exposed surface facets, we could establish nanoparticle growth pathway on stereographic projection of crystal plane by analyzing series of different surface facets Helicoid nanoparticles undergo. It was revealed that nanoparticle growth pathways not only depends on the types of seed particles and growth solution conditions, but also can be altered by interaction with amino-acids or peptides. The sequences of amino-acids or peptides determine on which step of growth pathway chiral morphologies are developed and which high-Miller-index facets and final outline of nanoparticle are exposed. Understanding on peptide recognition and crystal growth enables design of diverse chiral morphologies and tailored optical response, and provides insight into further optical modification through coupling with molecular dipoles.

Alloy semiconductor magic-size clusters (MSCs) have been reported rarely, and little is known about their formation pathway. Here, we explored a room-temperature approach to alloy CdTeSe MSC-399, which is labelled according to the peak position of its sharp optical absorption at 399 nm. Also, we investigated its formation pathway. We mixed two binary CdTe and CdSe induction period samples at room temperature; after incubation, the evolution of MSC-399 was detected by optical absorption spectroscopy in tolunen. The induction period is the prenucleation stage which occurs prior to nucleation and growth of quantum dots. We argue that substitution reactions take place after the two binary samples are mixed, which result in the formation of CdTeSe MSC-399 from its precursor compound (CdTeSe PC-399). The former is faster than the latter. Furthermore, we demonstrate that the formation of PC-399 is not via anion exchange nor addition reactions. The present study provides a room-temperature avenue to engineering alloy MSCs and an in-depth understanding of their probable formation pathway, making the synthesis less empirical. Given the complexity of the subject, it is impossible to extract complete mechanistic insight from one study. Even so, we believe that the present study is one solid piece, in a very large puzzle. Also, similar to the advance of organic chemistry, the field of colloidal nanocrystals is pressing forward and transforming from an empirical art to science.

Keywords: magic-size clusters, alloy, colloidal semiconductor nanocrystals, prenucleatioin stage, induction period

We investigate the structure and growth of soft epitaxial nanocrystal (NC) superlattices, formed on the surface of designed templates. At first, we prepare template of ordered monolayer superlattice of PbS NCs, coupled with copper tetraaminophthalocyanines (CuTAPc) molecules [1-3]. Afterwards, we deposit as-prepared PbS NCs of different sizes on top of the templates and finally add short ligands on those adlayer assemblies. Here, we determine the structure of the templates and the adlayer NC assemblies by using grazing-incidence small angle x-ray scattering (GISAXS), grazing incidence x-ray diffraction (GIXD) and microscopy [4]. The deposited NCs self-assemble into ordered superstructures (sc, bcc) and show preferred atomic orientation with respect to the templates. After treating with short ligands, the bcc superlattices undergo a notable orientational change from [110]_SL to [100]_SL. This study provides new insight for designing epitaxial nanostructures of desired crystallographic orientation [5,6].
References:
[1] M. Scheele et al, Phys. Chem. Chem. Phys. 2015, 17, 97.
[2] S. Maiti et al, J. Phys. Chem. Lett. 2018, 9, 739-744.
[3] S. Maiti et al, J. Phys. Chem. C 2019, 123, 1519-1526.
[4] S. Maiti et al, J. Phys.: Condens. Matter 2017, 29, 095101.
[5] S. M. Rupich et al, Nat Commun. 2014, 5, 5045.
[6] M. X. Wang et al, ACS Nano 2017, 11, 180-185.

Oriented attachment of nanoparticles in a colloidal solution is one of the important processes to form larger-scale anisotropic functional nanomaterials. However, the mechanism of oriented attachment is still poorly understood. Our recent study on the growth of colloidal PbSe nanorods proves that the oriented attachment is driven by the dipole-dipole interaction among the seed quantum dots. The dipole-dipole interaction strength depends on their separation distance as well as their orientations. Hence the one-dimensional oriented attachment can be controlled by the quantum-dot surface capping ligands used as the cosolvent in the synthesis. In particular, changing the length of the surface ligands can significantly change the dipole-dipole interaction strength. We demonstrate that the oriented attachment can be reliably turned on or off during the growth of nanocrystals by choosing the proper length of the surface ligand.
This kinetic-controlled growth of the nanorods prefers low reaction temperatures. Colloidal PbSe nanorods with length > 100 nm and diameter < 4 nm can be synthesized at 110 ^oC. The as-synthesized PbSe nanorods exhibit a single exponential decay and an absolute photoluminescence quantum yield more than 30%.

Metal-organic frameworks (MOFs) are attractive crystalline molecular materials for enabling multifunctional properties in optoelectronic devices. Of particular interest are MOFs whose phases and properties can be dynamically and reversibly switched in response to external stimuli. We identify new MOF topologies prepared through tunable assembly of a modular cluster, Mo₂(INA)₄, and explore their electronic structures and redox capabilities. Several synthetic innovations allow us to prepare new MOFs from this Mo₂(INA)₄ cluster, including the use of self-assembled monolayers that template large-area thin film growth, and sacrificial pillaring molecules that control the ordering of two-dimensional (2D) layers within the MOF. This latter 2D phase exhibits anomalous gas adsorption characteristics, redox activity, and a significant photocurrent response. Reversible structural conversion between the multiple topologies is readily accomplished and associated with significant tuning of the MOF electronic properties. These efforts enable the development of sensors and actuators from stimuli-responsive molecular frameworks.

This presentation will describe our research in creating electronic materials from molecular and superatomic building blocks. We are designing crystalline and self-assembled systems from these building blocks. We have created clusters of atoms that behave like the functional equivalent of atoms, but are on the nanoscale. These materials form solid solutions that show drastically enhanced electrical conductivity. Thin films of these materials have low thermal conductivity and point to a new direction to optimize thermoelectric materials. In addition, using this building blocks, we have made new materials with unusual magneto-optical properties.

The large unit cell compounds Co₆E₈(PEt₃)₆ (E = S, Se, Te) and [Co₆E₈(PEt₃)₆][C₆₀]₂, and C₆₀ are modeled using molecular dynamics simulations to understand how intermolecular and intramolecular interactions affect their macroscopic properties. These compounds are examples of superatomic crystals (SACs), a diverse family of compounds that span the periodic table. To enable study across the spectrum of SACs, the Universal Force Field (UFF) and the Extended Charge Equilibration (EQEq) scheme are employed. The resulting zero-pressure lattice constants are within 5% of the experimentally measured results and the predicted elastic moduli encompass the experimental value. Unique trends in thermal conductivity are explained by qualitative assessments and quantitative measures of the rotational diffusion coefficient of the C₆₀ molecules. Large rotational diffusion coefficients, indicative of freely spinning C₆₀ molecules, correspond to temperature-independent thermal conductivities, while librating C₆₀ molecules with proportionally small rotational diffusion coefficients correspond to crystal-like thermal conductivities. Finally, equivalent point mass models of the six compounds are generated and used to assess the impact of the C₆₀ rotation on thermal conductivity.

In a seminal experiment performed in 1848 Louis Pasteur separated the two mirror-images of chiral tartaric acid salt crystals. The chirality of the molecular building blocks manifested itself in the macroscopic shape of the crystals, that exhibited a chiral arrangement of facets. Following this work, many observations of macroscopic shape chirality in crystals followed, and yet relations between molecular scale and overall shape chirality are not well understood. Such understanding is highly desirable for the deterministic synthesis of crystals with chiral shapes. Nanocrystals represent an embryonic stage of crystal growth, and allow direct high spatial and temporal resolution studies of crystal growth mechanisms and the way defects, ligands, impurities and other reaction conditions affect growth. Working with tellurium nanocrystals that exhibit both a chiral crystal structure and a chiral shape, we shed light on a general mechanism that leads materials to exhibit chiral shapes in crystallization. This mechanism has to do with a unique topology of partial dislocations that we believe can be very general in crystal growth reactions. Alongside a fundamental understanding of how chiral shapes might emerge sometimes in crystals we offer design principles to achieve these shapes with control over their handedness, and therefore chiral properties.

Gibbsite (α-Al(OH)₃) is an important archetype hydroxide of aluminum in nature that also plays diverse roles across a plethora of industrial applications. In addition, gibbsite is a prominent component of high-level nuclear waste stored in large quantities at the Hanford Site, Washington and at the Savannah River Site, South Carolina, with future processing plans dependent on developing a predictive understanding of its nucleation, growth and transformation in highly alkaline solution. However, mechanisms of gibbsite nucleation and growth remain poorly understood. In this work, magic angle spinning nuclear magnetic resonance (MAS-NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), high resolution powder X-ray diffraction (XRD), and X-ray Pair Distribution Function (PDF) techniques were used to investigate the crystallization of gibbsite from amorphous aluminum hydroxide gel precursors. By focusing on understanding the role of aluminum coordination change dynamics from tetrahedral in solution to octahedral in solids and vice versa, and by quantifying intermediate polyoxoaluminate cluster formation, some unifying principles governing these transformations emerge. Furthermore, various advanced techniques reveal the transformation and aggregation of aluminum clusters during the hydrothermal process, which indicate cluster assembly pathways to gibbsite nucleation and growth. These findings are important for developing new methods to morphology and size controlled synthesis of gibbsite, and may aid in the design of chemical processes for managing the aluminum inventory in nuclear waste.

Superatomic solids are an emerging class of functional materials assembled from atomically precise clusters. Attributing to the unique properties of constituent clusters and their collective communications, superatomic solids show intriguing electronic, magnetic, and thermal properties. Previously, the assembly of atomically precise metal chalcogenide clusters combined with fullerenes resulted in electroactive superatomic crystals with high electrical conductivities up to 0.6 S/m. Here in this presentation, I would like to introduce two new chemistries for the superatomic solids that bring greatly enhanced or previously unattainable properties to this class of materials. Our first chemistry demonstrates solid solutions at the cluster level, namely superatomic solid solutions. We have mixed two metal chalcogenide clusters with similar sizes, Co₆Se₈(PEt₃)₆ and Cr₆Te₈(PEt₃)₆, with fullerene C₆₀ and demonstrate that the resultant superatomic solids maintain the structure of the original single component but with tunable cluster ratios [Co₆Se₈(PEt₃)₆]x[Cr₆Te₈(PEt₃)₆]_1-x[C₆₀]₂ (x = 0~1). We found that the superatomic solid solutions show up to 100 times enhanced electrical conductivities as well as tunable electrical and magnetic properties. Our second chemistry aims to explore the feasibility of integrating superatomic solids in devices. One key merit is the solution-processability of these materials, unfortunately, superatomic solids tend to be crystals due to the strong electrostatic interactions between the positive and negative charged clusters. Here, we show how to endow solution-processability to superatomic solids by frustrating the strong electrostatic interactions. By grafting flexible side chains to the clusters, we show that their crystallization tendency could be fully inhibited and they can be solution-processed into homogenous and amorphous thin films with arbitrary compositions that are not achievable in crystals ([Co₆Se₈(PPr₃)₆][PCBM]_x (x=2,3,4,5,7,9)). We then show these thin films possess high optical transparency of up to 92%, up to thousands of times higher electrical conductivities of up to 300 S m^-1 and several times lower thermal conductivities as low as 0.05 W m^-1 K^-1 compared to crystalline superatomic materials, as such, they are promising as solution-processable transparent conductors and thermoelectric materials (ZT=0.02 at RT).

Self-assembly processes have a key role in the bottom-up fabrication of supramolecular structures and advanced functional materials. Traditionally, the control over non-covalent synthesis has relied on tailoring the building blocks for recognition and interaction, while targeting the global minimum of the free energy landscape (thermodynamic control). Recently, however, there is a growing interest in nonequilibrium approaches and methods that can potentially engender an improved spatio-temporal control over molecular self-assembly (kinetic control). Living systems, for example, master reaction-diffusion (RD) processes to finely tune the self-assembly of individual components in space and time leading to materials with structure and functions of remarkable complexity. In this communication, we will show how the unique physical conditions offered by microfluidic environments (e.g. laminar flow regime, mixing only through molecular diffusion, fine-tuning of the reactant gradients) facilitate an improved kinetic control over self-assembly, as well as they help to unveil the pathways underlying the formation of self-assembled materials. In particular, we will demonstrate how by using continuous flow microfluidic devices we have been able to isolate kinetically trapped crystal states of a coordination polymer (CP) (a result that leads to unprecedented materials’ properties), to enhance chiral induction in a supramolecular polymer, to uncover different crystallization pathways undertaken by a metal-organic framework (MOF) towards its thermodynamic product, as well as to shape MOF crystals at the millimeter length scale.

Functional and complex structures can be generated during the self-assembly process via local interactions among a large set of simpler components. The ability to program the self-assembly of components enables novel bottom-up control strategies for multi-particle active systems, fabrication methods for functional materials, and exploration of fundamental physics. However, reprogramming requires multi-stability, and reversible selection of self-assembling particle patterns which remains a challenge. We demonstrate the statistical reprogramming of two-dimensional, porous self-assembled structures by isothermal quenching of millimetric, orbitally-shaken, and magnetically repulsive particles. We select among a finite set of self-assembled patterns repeatably and reversibly by controlling the rate of change of the boundary volume. We report the transition from fluid- to solid-like states of driven granular particles during quenching to realize out-of-equilibrium self-assembly. With this method, we demonstrate the reprogrammable stiffness of granular self-assemblies. This work can extend to the tuning of mechanical, optical and electromagnetic properties of functional materials that depend on the spatial configuration of their components. The particles in the current work are morphologically and functionally symmetric; however the introduction of particle anisotropy may allwo the programming of additional functionalities of the self-assembled structures.

Pourbaix (E-pH) diagrams are often used to guide the hydrothermal synthesis of transition metal oxides. However, non-equilibrium oxide phases often precipitate from aqueous solution, which are difficult to anticipate from equilibrium Pourbaix diagrams. Here, we construct a theoretical framework to reveal the nanoscale and metastable energy landscapes of Pourbaix diagrams, providing quantitative insights into the size-dependent thermodynamics of metastable oxide nucleation and growth in water. We show that a thermodynamic analysis that takes into account these additional thermodynamic regimes can successfully capture which metastable MnO₂ polymorphs appear during hydrothermal synthesis, and the order of their appearance. We further extend our theoretical framework to address the ‘inverse’ stability question for hydrothermal synthesis. That is, rather than plotting Pourbaix diagrams under varying solution conditions to search for a stable phase, can we instead solve for solution conditions where a target phase is stable? We demonstrate this 'inverse' formalism by computing the aqueous stability conditions for the relatively soluble compound LiFePO4, as a function of [Li], [Fe], [P], E, and pH.

[1] B.R. Chen, W. Sun, et al., Nature Communications (2018)
[2] W. Sun et al., Nature Communications (2019)
[3] W. Sun et al., In Submission (2019)

Additive manufacturing has enabled low cost, large area, and solution-based processing. Moreover, it is also possible to assemble full-scale working devices with quantum dots (QDs) as the active layer. Small fundamental building blocks (<100 nm) in the form of nanocrystals offer the unique flexibility to assemble crystals in complex shapes realizing novel functional properties via bottom-up assembly. In this work, we demonstrate the assembly of nanocubes capped with polyvinylpyrrolidone (PVP) into arbitrary patterns including curves of varying radii of curvature. We use polydimethylsiloxane (PDMS) as the flexible template to directly print and pattern the cubes at the same time on the substrate from a colloidal solution. Subsequently, we remove the ligands or use epitaxial overgrowth to convert them into single crystals. The effect of angular misalignment between adjacent cubes and the resulting crystallinity of the welded structure was investigated with high-resolution transmission electron microscopy (HRTEM). We demonstrate that single crystalline interfaces can form which are strain-free and without dislocations. The technique proposed enables room temperature fabrication of patterned single crystalline materials which makes it compatible with low heat resistant flexible polymeric substrates and roll to roll processing to enable next-generation optoelectronic devices.

There have been significant advances in the field of self-assembly of functional, nanoscale materials into periodic 2D and 3D arrays. A number of strategies have been demonstrated for the crystallization of these materials using complementary DNA shells or other linking motifs. However, most notable methods are system-specific, dominated by particle shape, and remain challenging to obtain multiple lattices using a single set of particles. Instead, a more broadly accessible strategy for the assembly of crystalline organizations with programmable nanoscale features can be achieved using well defined DNA constructs integrated with nano-objects. Each of the integrated units can self-assemble in solution into periodic arrays, and can be probed in-situ by Small Angle X-ray Scattering (SAXS). The symmetry of the resulting crystalline assemblies is determined by the unit geometry and the pairwise DNA bonds. Due to the sequence specific nature of DNA, it is possible to encode unique bonds at these specified sites. The addressability of the inter-frame bonds provides the foundation for encoding multiple crystallographic organizations through specific sets of pair-wise bonds. We show that this strategy allows for the design and assembly of complex designed nanoscale architectures, including an analog of a perovskite’s structure, multiscale organizations, and chiral crystals.

Electronic devices in the future sustainable societies require maximum function with minimum amount of constituent materials and energy cost for production. Electronic functions are often originated from two-dimensional material properties so that device components of large area and small thickness saves material consumption. The fundamental electronic functions such as switching “zero” and “one” in the digit of information are governed only by a nanometer-scale thin layer at the very surface of the semiconductor material due to the very short penetration length of electric field. Future ten-meter scale large-area display also needs to be more “two-dimensional” to save materials and save energies to carry and equip on billboards.
This presentation focuses on recently developed ultrathin organic semiconductor single crystalline films that is suitable for large-area production with low energy consumption; the films is easily formed to large area from solution at relatively low temperature at 80 degrees centigrade. Extremely thin crystal films are controllably grown to a-few molecular layers with the thickness of only 10 nm, so that material cost can be only 0.01 USD per cm2. The talk begins with material chemistry of designing the semiconductor molecules that causes very high carrier mobility more than 10 cm2/Vs. Furthermore, Recent development of key technologies for printed integrated circuits which can provide future low-cost platforms for RFID tags and sensing circuitries. Finally, a technology for large-area light-weight display sheets will be demonstrated.
Such prospect bears increasing reality because of recent research innovations in the field of material chemistry, charge transport physics, and solution processes of printable organic semiconductors. With excellent chemical and thermal stability in recently developed new materials, we are developing simple integrated devices based on CMOS using p-type and n-type printed organic FETs. Particularly important are new processing technologies for continuous growth of the organic single-crystalline semiconductor “wafers” from solution and for lithographical patterning of semiconductors and metal electrodes. Successful rectification and identification are demonstrated at 13.56 MHz with printed organic CMOS circuits.

Solution-processed organic thin-film transistors (OTFTs) have received enormous interest due to their great potential for realizing low-cost, lightweight and flexible electronics applications, such as displays, wearable sensors and electronic skins. Although several novel solution-processing techniques for achieving highly-crystalline organic semiconductor (OSC) films have been introduced to OTFTs, there still remains a challenging work to control the thin-film properties, such as crystallinity, morphology and uniformity accompanying the rapid coating speed demanded for industrial-scale production. In this talk, I will discuss the challenges and our long lasting progress in the field of printed electronics. First, I will introduce our previous results on crystal engineering based on inkjet printing. The lessons achieved through a series of the works have advanced our understanding in the nucleation and growth of organic semiconductors in a solution phase. Next, I will present our new solution coating method that is highly compatible with a fast-moving roll-to-roll system by using a gap-controlled wire-bar and a bar-motion programming strategy. The developed method is highly practical and industry-friendly as well as feasible to fabricate high-quality OSC thin films in a large area at a high rate. A crystallization and growth mechanism of the bar-coated films and structure-charge-transport properties will be discussed.

Thin film transistors (TFTs) are essential building blocks in a variety of electronic devices, and play important roles in transmitting electronic information. Generally, the sequential deposition of each component such as a semiconductor layer, an insulator layer, and contact electrodes is required in TFT fabrication processes. In order to achieve high performance devices, it is necessary to control the interface between these layers, which makes the device fabrication process complicated. This difficulty is even more crucial in the case of organic TFTs (OTFTs), which are promising building blocks in printed electronic devices thanks to the soluble nature of organic semiconductors (OSCs). By utilizing the solution process, single-crystal thin films of OSCs composed of only a few molecular layer can be obtained with large areal coverage[1]. However, to obtain perfect single crystal, conditions of underlayers need to be precisely controlled. So far, there have been strict limitation on candidates for underlayers of OSCs. For example, they must satisfy the requirements such as heat tolerance, organic solvent durability and surface energy. In this study, we demonstrate water exfoliation to obtain self-standing thin films of solution-processed single-crystal OSC thin film. Furthermore, the developed transfer method allows the facile integration of OSC layer of OTFTs.
A single-crystal thin film of the analogs of our benchmarked material[2], 3,11-dinonyldinaphtho[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (C₉–DNBDT–NW), was grown via continuous edge-casting method[3] on a freshly cleaved piece of natural mica. Then, the C₉–DNBDT–NW/mica was carefully immersed in water. In only a few seconds, the C₉–DNBDT–NW thin film was peeled off from mica substrate. The preservation of original single crystallinity during this step was confirmed by transmission electron microscope measurements. The successful exfoliation of an OSC thin film from a mica substrate can be attributed to the difference in the surface energy. C₉–DNBDT–NW has a highly hydrophobic alkyl chain that stands perpendicular to the substrate, which gives a high contact angle of water of 108°, while a freshly cleaved mica has a super hydrophilic surface as indicated by a contact angle of 3°. The large difference in the surface energy allows water to infiltrate between the interface of OSC and mica.
In order to employ the exfoliated single-crystal OSC thin film as an active semiconductor layer in OTFTs, the transfer technique was developed. First, the super hydrophilic substrate on which the OSC thin film was fabricated was mounted on the destination substrate. Then, a few droplets of water were applied near the edge of the template substrate, which promotes an exfoliation of OSC thin film from the template substrate and results in perfect transfer to the destination substrate. The present transfer technique allows a wide usage for the underlayers of OSC layer. As representative examples, we demonstrated fabrication of OTFTs on (i) a highly hydrophobic surface (fluorinated polymer coated substrate), and (ii) a substrate with low thermal stability and solvent resistance (commercially available food wrap). The fabricated OTFT showed remarkably high electronic performance with a mobility up to 12 cm² V⁻¹ s⁻¹. We also demonstrated the deposition of large-area OSC thin films of 3 cm × 3 cm, which shows the scalability of the present technique. All of 100 OTFTs fabricated with photolithography process showed transistor operation with a high average mobility of 10 cm² V⁻¹ s⁻¹. These results represent that the water exfoliation and the transfer method developed in this study enable an ideal production of OTFTs in printed electronic devices.
[1] A. Yamamura et al., Sci. Adv. 4, eaao5758 (2018).
[2] C. Mitsui et al., Adv. Mater. 26, 4546 (2014).
[3] J. Soeda et al., Appl. Phys. Express 6, 076503 (2013).

Spray pyrolysis is a cheap, scalable process for deposition of thin functional layers that can be used in electronic or optic applications. Temperature is one of the crucial parameters that influence the properties of the thin layer. Deposition properties change around the Leidenfrost temperature where smoother films are obtained at higher temperatures. Leidenfrost temperature depends mainly on the solvent and the substrate. Other deposition parameters like the droplet lifetime, droplet movement and evaporation rate are usually neglected.

In this work we follow a new real time approach to determine the microscopic properties and the interaction of small droplets (smaller than 80 µm) on hot surfaces on a timescale below 1 ms. A uniform distribution of small droplets are produced by an ultrasonic atomizer and transported by an inert carrier gas onto the temperature controlled substrate. The behavior of the droplets at the surface is monitored by a high speed camera with up to 10.000 frames per second. Droplet properties are automatically extracted by an computer algorithm and statistically analyzed. For time-dependent properties the droplets are being automatically tracked on the surface. The findings enable to obtain physical parameters of the droplets during their lifetime. Furthermore, they help to optimize parameters of the spray pyrolysis deposition process, e.g. for thin film zinc oxide deposition used in thin film transistors.

The pen-writer solution deposition method is used to deposit 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) organic semiconductor thin films and also polymer dielectric layers. The performance of top-contact bottom-gate C₈-BTBT transistor is found to improve with time in ambient air, which is consistent with passivation of shallow traps during exposure to air. In addition, severe non-ohmic injection and large contact resistance resulted from gold penetration during the evaporation process are observed with these top-contact transistors. This aging effect can be eliminated in the bottom-contact geometry when gold was deposited first without the penetration issue and contact resistance was reduced an order of magnitude by treating the gold electrode with pentafluorobenzenethiol (PFBT). An optimized geometry is obtained by using pen-written Cytop^TM dielectric in a top-gate bottom-contact structure, which exhibits a near-intrinsic average saturation mobility of 4.73 cm²/V-s with C₈-BTBT thin films deposited at high writing speed (25mm/s) and contact resistance down to 5.52*10² W^.cm. We will report temperature-dependent carrier transport results for the high mobility C₈-BTBT top gate transistors.

Metal oxide thin-film transistors (MOTFTs) are expected to play a vital role in enabling printed transparent and flexible electronics, given their excellent charge transport, wide bandgaps and solution processability. Compatibility with plastic electronics, however, strictly requires MOTFTs to be processed at low temperatures in a scalable fashion, a necessity that has so far been a challenge. Herein, we explore blade-coating of indium oxide (In₂O₃) TFTs via sol-gel and combustion chemistries using 2-methoxyethanol (MeOEt) as the processing solvent. Blade coating is an established prototyping technique for slot-die coating. We find that the sol-gel chemistry enables amorphous In₂O₃ TFTs annealed at 200°C with moderate electronic mobility (ca. 1 cm²V^-1s^-1). Subtle increase in annealing temperature to 212°C preserves the amorphous state whilst increasing the mobility (ca. 5 cm²V^-1s^-1). Films coated using combustion synthesis are found to bypass the electronically-active amorphous state leading to an early onset of crystallization. This behavior is expected given the lowered nucleation barrier thanks to the added fuel in the combustion ink. Paradoxically, blade-coated combustion TFTs are found to possess poor charge transport at low temperatures of 200-250°C. Small-angle X-ray scattering evidence suggests that early nucleation during combustion leads to formation of nanocrystalline domains that are deleterious to charge transport. Our results highlight that surprisingly it is not crystallization, rather the absence of it that is required to fabricate high-mobility, low-temperature In₂O₃ FETs using blade coating.

High-temperature operation (i.e., beyond 150°C) is of great interest for many electronics applications. To achievie charge transport characteristics for organic semiconductors at elevated temperatures is fundamentally challenging. We report a general strategy to make thermally stable high-temperature semiconducting polymer blends, composed of interpenetrating semicrystalline conjugated polymers and high glass-transition temperature insulating matrices. When optimized, such polymer blends can display a temperature-insensitive charge transport behaviors. Further, We employ polyimide substrates, polyimide dielectrics, and a polyimide-based semiconducting blend to achieve flexible and thermally-robust transistors. An in situ temperature-dependent charge transport study was used to demonstrate that these devices can operate in extremely high temperatures and can sustain prolonged baking. Our devices maintained stable charge-transport characteristics with charge mobilities of 0.20 cm²/V s, ION/IOFF of 10⁴, threshold voltage of 3 V, and operational voltage of 10 V when baked at 195 °C for 2 h.

3D printing, one of the most disruptive technologies to emerge in recent years, has attracted increasing attention due to its potential impact on science, industry, and society. Winning high-quality materials is one of the most important challenges in modern 3D printing technology. To this end, beyond exterior shaping, 3D printing techniques that can also engineer crystallinity or molecular orientation are in a great demand.
Here, we have developed a new method to incorporate solution-mediated crystallization into nanoscale 3D printing. The key idea is to exploit a femtoliter solution meniscus formed on a nanopipette to guide evaporation-induced crystallization in mid-air. Evaporative loss of solvent rapidly creates a supersaturated concentration field of solutes inside the femtoliter meniscus, leading to the nucleation and growth of nanocrystals. Furthermore, omnidirectional guiding of the meniscus with moving the nanopipette realizes freeform 3D nanostructures with excellent crystallinity. By employing this method, we have successfully demonstrated nanoscale 3D printing of two promising material systems: (1) organic-inorganic metal halide perovskites and (2) self-assembled peptides. In this talk, we will present the theoretical and experimental results including examples of 3D printed crystalline structures and discuss prospects of our work for potential applications in electronics and optoelectronics.

Microreactor-Assisted Nanomaterial Deposition (MAND) process combines the merits of microreaction technology and solution phase synthesis of nanomaterials. This technique uses continuous flow microreactors for the synthesis, assembly and deposition of nanomaterials. MAND processes control the heat transfer, mass transfer and reaction kinetics using well-defined microstructures of the active unit reactor cell that can be replicated to produce higher chemical production volumes. This important feature opens a promising avenue in developing scalable nanomanufacturing. Furthermore, the continuous flow microreactor opens up the opportunity to conveniently assemble unique nanostructures and nanostructured thin films. In this presentation, we will report our recent progress in using continuous microreactors to achieve controlled synthesis and transport of reactive building blocks for the fabrication of micro- and nanostructured thin films of inorganic compound semiconductors such as metal oxides and metal chalcogenides and organic-inorganic hybrid materials like metal organic frameworks along with the applications of these functional materials.

Process scaling-up is an essential step to apply laboratory innovations into mass-manufacturing. Unfortunately, the small scale of laboratory synthetic routes makes them difficult to be scaled-up due to the significant difference of transport phenomena between the two scales and the resultant kinetics dilemma during scaling up. Development of the scalable process is a promising strategy to address that issue. Among those scalable processes, the ones based on aerosol science and technology are intrinsically easier to be scaled-up by simply increase the feed solution concentration or reduce the residence time. The product morphology, composition, and crystallinity can be designed a priori. Currently, three aerosol processes, spray pyrolysis, spray drying, and flame spray pyrolysis, are widely used to fabricate metals, composites, and oxides.
To meet the demands of the US Department of Energy (DOE) on the high energy density battery materials and large scale synthesizing, Argonne National Laboratory has built a state-of-art facility based on flame spray pyrolysis. Feed solutions containing salts are continuously atomized and fed into self-supported flame. Particle formation by gas-to-particle route is completed within milliseconds. A scalable process has been developed to manufacture nickel-rich cathode materials at a scale of 20 g/h. The synthesized materials displayed excellent battery performance in terms of cycling stability and rate performance. This solution-based synthetic strategy is a promising method to tackle the bottleneck of increasing production rate.

Large-area fabrication of vanadium dioxide (VO₂) on polymeric substrates has been a great challenge to manufacture the flexible thermochromic smart films which control the influx of radiant heat transfer by attaching to pre-installed windows. These smart window films are promising for applications in building windows and automotive glasses to improve the energy efficiency. However, the formation of VO₂ layer on flexible substrates has been rarely reported due to the high temperatures of VO₂ growth process. Herein, we demonstrate the fabrication of VO₂ layers from hydrothermal synthesized VO₂ nanoparticle ink using multi-step intense pulsed light (IPL) sintering. IPL irradiation rapidly triggers the evaporation of organic vehicles and the formation larger VO₂ clusters, and improves significantly the crystallinity and thermochromic performance of VO₂ granular films. These films exhibit visible light transmittance of 65.7%, excellent infrared modulation at 2000 nm of 43.8%, and salient solar modulation efficiency of over 12%. IPL sintering of VO₂ nanoparticles effectively enhances the crystallinity and stoichiometry of VO₂ layers on a polyethylen terephthalate substrate. It is believed that the proposed strategy provides new insight into the manufacturing of flexible theromochromic smart films for energy saving.

Hybrid organic metal halides, such as CH₃NH₃PbI₃, have garnered significant attention because they are earth-abundant, solution-processable materials that can be used to form solar cells with high power conversion efficiency (>20%). Despite their high performance in lab-based devices, there are significant questions about the phase behavior of these materials and their resulting properties. For example, an interesting feature of these compounds is the ability to form layered Ruddlesden-Popper phases with quantum confinement by introduction of mixed organic cations. We will present our work on understanding the electronic properties of 3D and 2D Pb- and Bi-based systems. The microstructure of thin films of layered Ruddlesden-Popper compounds, (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 (n = 1, 2, 3, 4) suggests that they are not phase pure with varying composition depending on the growth conditions used. While these compounds are structurally disordered, apparent from quantitative analysis of grazing incidence X-ray scattering and electron microscopy, they surprisingly still have sharp band edges. Recent results on the optoelectronic properties of these materials will be discussed.

Lead halide-based perovskite nanocrystals are promising as optoelectronic materials for their easy tunable optical range, high absorption coefficinet and photoluminescence quantum yield and solution processible preparation. However, ionic properties of the halide-based peorvskite nanocrystals have a stability problem against humidity, heat, and long-term light irradiation. Especially the perovskite nanocrystals consisting of iodide as anion, which have larger ionic radius and lower electronegativity than chloride and bromide, show low crystalline phase stability and photoluminescence quantum yield without any surface post treatments. In order to enhance the stability of the materials and also tune the optical range up to near infrared region, it is necessary to intorduce elements with low ionic radius and high electronegativity
Herein we report new perovskite nanocrystals substituted with the lead cation with iso-valent tin cations which have low ionic radius and high electronegativity for high stability issue. The tin cation can control the the distortion of the metal halide octahedron and make the strong bond with iodide in nanocrystals. To control the activity of two different metal cations and enhance the stability of the nanocrystals, we use phosphine as a functional group of base ligands rather than alkyl amine ligands. Considering the pKa of the conjugate acid of the phosphine and amine as brønsted bases, the phosphine is weaker brønsted base than amine. And phosphine ligands are expected to have lower reaction constant for the acid-base reaction with oleic acid which is used for acid ligands for synthesis of perovskite nanocrystals. This results in the tuning the reaction between the oleate and metal cation to change the activity of the metal cation.Therefore, base ligands having phosphine as a head group are used to finely control the activity of the metal cation via acid-base reaction with acid ligands.
This results demonstrate that the acid-ase reaction between ligands should be considered to control the acitivity of the precursor of the nanocrystals. Furthermore, the phosphine ligands as base ligands can control the acitivity of two different metal cations to mediate the growth of the alloyed perovskite nanocrystals via acid-base reaction. It is highly expected that the stable Pb-Sn alloyed iodide perovskite nanocrystals can be useful for various optoelectronic applications in the near future.

Solution-processed perovskite solar cells (PSCs) have attracted tremendous attention due to their rapidly rising efficiency over 24% in several years, mainly owing to the huge freedom in compositional engineering of mixed perovskites. However, complex composition of perovskite challenges the fundamental understanding of the crystallization mechanism, which are closely coupled with the device performance, stability and reproducibility. Here, we are going to present two of our recent works on mixed perovskite systems. In the first work, we employ a small molecule (ITIC-Th) to stabilize the perovskite precursor solution by effectively suppressing the formation of yellow δ-phase in the films made from aged precursor solutions. Consequently, the PSCs fabricated from the aged precursor solution with ITIC-Th experience much less efficiency drop with the increase of the precursor aging time, compared with the devices made from conventional precursor solutions. In the second work, the crystallization pathways of mixed perovskites under spin-coating have been investigated via in situ grazing-incidence wide-angle X-ray scattering, revealing the existence of “annealing window” for the first time. The as-cast film should be timely annealed within the annealing window to achieve a high-quality perovskite film. Remarkably, the incorporating of Cs⁺ could extend the annealing window, thereby improving the device performance and reproducibility. These studies unveil underlying reasons of the degradation of precursor solution and time sensitivity in fabricating mixed-perovskite devices, providing insightful guidelines for manipulating the perovskite crystallization pathways towards higher performance.


Reference:
1) M. Qin, J. Cao, T. Zhang, J. Mai, TK. Lau, S. Zhou, Y. Zhou, J. Wang, Y-J. Hsu, N. Zhao, J. Xu, X. Zhan, X. Lu*, Fused-ring electron acceptor ITIC-Th: a novel stabilizer for halide perovskite precursor solution, Adv. Energy Mater. 2018, 1703399.
2) M. Qin, K. Tse, T.-K Lau, Y. Li, C.-J. Su, G. Yang, J. Chen, J. Zhu, U-S. Jeng, G. Li, H. Chen, and X. Lu*, Manipulating the Mixed-Perovskite Crystallization Pathway Unveiled by In Situ GIWAXS, Adv. Mater. 2019, 1901284.

Hybrid organic-inorganic perovskites have been of paramount interest for solution-processible solar cells, reaching photon conversion efficiencies larger than 23%. We will study in detail the effect of different processing routes, including use of different coating and annealing methods, antisolvent drip, as well as mixtures of organic cations, halogen anions, and metal cation additives. We have performed in-situ time-resolved x-ray scattering experiments during processing, that provide detailed information about the emergence and disappearance of different crystalline phases, and how these affect the final film morphology. Our method allows us to assess, how different material composition and processing conditions influence the crystallization process and we can correlate these findings with the resulting photon conversion efficiency of the films.

In order to realize scalable fabrication of perovskite optoelectronic devices, large area deposition techniques such as solution shearing must be realized and perfected. However, the mechanisms of fabrication processes are still not well studied in different solution shearing regimes. By monitoring the precursor species during crystallization process using In-situ Grazing Incidence Wide-Angle X-ray Scattering, the crystallization process in low shearing speed meniscus regime is proven to happen under lower supersaturation. The low supersaturation contributes to the bigger crystal size of thin film while high speed shearing results in thin films with small size crystals. Photodetectors with perovskite thin films coated at low shearing speed meniscus regime show much higher internal quantum efficiency because of the larger crystal domain size and uniform orientation inside of domains. This shows continuing study and rational design of the crystallization process is needed to push perovskite technology toward useful applications.

Cesium lead halide perovskites are promising candidates for photovoltaic and light-emitting diode applications. Bulk CsPbX₃ however, are structurally unstable, transforming into an orthorhombic phase at room temperature. Improved thermal stability can be achieved in nanocrystal form with stability and optoelectronic properties dependent on the size of the nanocrystal.

Using first-principles calculations, we investigate phase stability and optoelectronic changes as a function of varying nanocrystal size.
The stabilization of the CsPbI₃ cubic phase against the orthorhombic phase at nanocrystal dimensions of a few nanometers is explained by the competition between short-range bond formation and long-range electrostatic forces. We demonstrate lattice constants and band gap scaling behavior with nanocrystal size, explaining discrepancies between experiments and simple quantum confinement models previously proposed.

We also show that superlattices of CsPbBr₃ nanocrystals preserve the photoluminescence signature of the constituent nanocrystals, while undergoing structural changes that give rise to new photoluminescence peaks. With multi-scale modeling, we predict the rate of these changes and simulate their optical properties.

Lead chalcogenides are one of the important class of semiconducting materials. PbS and PbSe have shown promising performance in the field of photovoltaic devices when synthesized in the form of quantum dots with power conversion efficiencies exceeding 11% while PbSe and PbTe in the pure and alloyed form are one of the best performing thermoelectric materials with the figure of merit (zT) greater than 2. These materials promise further scope in performance improvement by introducing controlled nano-structuring of material for device fabrication which is realized via solution chemistry.
In this work, we have synthesized lead chalcogenide materials via solution processing route at ambient reaction conditions using lead halide and pure chalcogen precursors. Unlike traditional syntheses routes, we have demonstrated phase pure, carbonaceous impurity free particle synthesis using a highly volatile organic mixture of amine and thiol. For these syntheses, we have observed instantaneous reactions with simultaneous self-assembly yielding nano to micron scale structures from PbS nanoparticles. These assembly sizes were further controlled such that the desired quantum confinement yielding specific optical bandgap of PbS nanoparticles is achieved in sub-micron structures. Similar controlled nano to micron scale structures for PbSe and PbTe materials were also synthesized and then studied using cross-sectional FIB milling and TEM/STEM analysis to understand the different interiors in the structures. Along with the size of the structures, the morphology and shape of the structures were also manipulated giving highly faceted cubic structures in addition to original spherical assemblies.
Similar to morphological evolution, compositional variation was also achieved at room temperature using this new solution route. Homogeneous alloys of PbSSe in the form of nanoparticles and self-assemblies were obtained via reaction and room temperature anion exchange strategy respectively. This alloy formation approach was further extended in the case of PbSeTe material where a homogeneity in alloy formation was realized by gradual diffusion of chalcogens in the structure as a function of heat treatment.

Hybrid organic-inorganic metal-halide perovskites have excited the photovoltaic community due to their high light to power conversion efficiencies. One surprising aspect of HOIPs is that they are synthesized with solution processing and under mild conditions (e.g., 100C annealing). Understanding the role of processing on crystallization pathways of HOIPs can provide insight into developing new processing strategies. In this talk I will show our recent results on the HOIP formation mechanisms during solution processing. We have investigated the film formation of Cs-FA-containing perovskites during in situ spin coating, blade coating, and the subsequent thermal annealing using X-ray scattering. Different processing approaches such as anti-solvent [chlorobenzene (CB)] and compressed-gas N2 treatments were applied during film casting. I will show how these different processing routes affect the competition between the formation of the non-perovskite δ-phase and perovskite α-phase during film formation and subsequent annealing. I will also discuss the formation of MAPbI3 films from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3(CH3NH3I)). We use X-ray scattering and absorption to determine the film formation mechanisms, revealing the importance of a self-regulating mechanism that slows the film growth. This work provides insight into optimizing processing conditions and realizing enhanced optoelectronic properties in HOIPs.

Multinary metal oxides (MO) are the key active materials in a range of energy applications, from batteries to fuel cells, from water oxidation catalysts to light absorbing photoanodes for artificial photosynthesis. The synthesis of compositionally homogenous and morphologically-controlled films is crucial to understand how composition and structural parameters impact performance.
Here, we discuss our work on using colloidal nanocrystals (NCs) as nanoscale precursors to access tunable multinary MOs of interest as light absorbers for water splitting. We have already demonstrated that copper vanadates with variable grain size can be obtained when reacting copper NCs of different sizes with a molecular vanadium precursor.^[1] By reacting two NC seeds (Cu +WO₃) with a moecular vanadium precursor, Cu₂V₂O₇/WO₃ heterostructures were also synthesized, thus increasing the level of complexity achievable by our NC-seeded growth.^[2] To further advance the understanding of the solid state reactions occurring between NCs, we are now taking advantage of the formation of ordered binary NC superlattices.^[3] The idea is to use in-situ electron microscopy to monitor the transformation of CuO NCs and Fe₃O₄ NCs towards copper ferrites.^[4] We show that Fe₃O₄-Cu NC in AlB₂ and NaCl superlattices can be obtained by properly tuning the NC size, ligand shell and solvents. We use a combination of electron microscopy techniques together with elemental compositional mapping and X-ray techniques, like GIWAXS and GISAXS, to study the growth of compositionally homogenous copper ferrite from the reaction of the two components in the superlattice. We believe that this knowledge can guide the development of a general synthesis procedure for achieving ternary MO films with tunable composition and morphology.
[1] Gadiyar, C.; Strach, M.; Schouwink, P.; Loiudice, A.; Buonsanti, R. Chemical Transformations at the Nanoscale: Nanocrystal-Seeded Synthesis of β-Cu2V2O7 with Enhanced Photoconversion Efficiencies. Chem. Sci. 2018, 9.
[2] Scarongella, M.; Gadiyar, C.; Strach, M.; Rimoldi, L.; Loiudice, A.; Buonsanti, R. Assembly of β-Cu₂V₂O₇/WO₃ Heterostructured Nanocomposites and the Impact of Their Composition on Structure and Photoelectrochemical Properties. J. Mater. Chem. C 2018, 6,12062-12069.
[3] Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Binary Nanocrystal Superlattice Membranes Self-Assembled at the Liquid–Air Interface. Nature 2010, 466, 474–477.
[4] Gadiyar, C., D’Ambra, F., Stoian, D. ,Oveisi,E., Mantella,V., R Buonsanti, in preparation

In this work, we report the synthesis of Pt–Cu nanoparticles with hollow core and porous shell based on self-assembly of ultrasmall particles. At the initial stage, Pt–Cu cage frames with porous outermost shells were formed. These pores on the shells served as tunnels to afford getting across by residual Pt and Cu ions in the solution. These Pt and Cu ions then mostly deposited on the inside shells to thicken the width of the shells, leading to the smaller dimension of the hollow core. By carefully governing the reaction conditions, the thickness of the shell and the size of the hollow core could be precisely tuned. Furthermore, we compared the catalytic performance of Pt–Cu nanoparticles with hollow-core structures and Pt–Cu nanoparticles without a hollow-core structure for the selective hydrogenation of furfural to furfuryl alcohol. It is found that the Pt–Cu nanoparticles with a hollow-core structure exhibited an over 4-fold increase in catalytic activity, compared to Pt–Cu nanoparticles without a hollow-core structure. In particular, these Pt–Cu with hollow-core nanoparticles achieved alomost 100% selectivity toward furfuryl alcohol,which cannot be obtained using bimetallic Pt–Cu nanoparticles without hollow-core structure.

Nanocrystals (NCs) present unique physicochemical properties arising from their size and the presence of ligands. Comprehending and controlling the ligand−crystal interactions as well as the ligand exchange process is one of the central themes in NC science nowadays. However, the relationship between NC structural disorder and the ligand exchange effect in the NC atomic structure is not yet sufficiently understood. Here we combine pair distribution function analysis (PDF) from electron diffraction data (ED), extended X-ray absorption fine structure (EXAFS), and high-resolution transmission electron microscopy (HRTEM) as experimental techniques and first-principles density functional theory (DFT) calculations to elucidate the ligand exchange effects in the ZrO₂ NC structure. In our work, we have demonstrated that the combination of advanced characterization techniques (EXAFS and PDF from ED) with theoretical DFT calculations is a powerful tool to reveal the nature of NC structures. We presented the unprecedented result that the ligand exchange process causes a decrease in the static structural disorder of the ZrO₂ NCs, mainly due to strain rearrangement caused by solvothermal treatment at elevated temperatures and not due to the type of functional organic group (R) present in the X₂- type ligand. These results have a direct impact on NCs manipulation processes involving ligands, properties associated with their structural disorder degree, and the development of functional nanomaterials, especially for functionalities wherethe structural disorder located on the surface plays an important role in the physicochemical properties, such as in mechanical, photoluminescence, electronic transport, and catalytic properties.

III-V semiconductors have promising composition and size dependent optoelectronic properties for a variety of applications in nanophotonics and high speed electronic devices. Due to their increasing demand, large-scale production of the III-V semiconductors is required. New growth techniques have been proposed to tackle the resource- and energy-intensive techniques which are currently used. One of the proposed techniques is electrochemical mediated crystal growth, which has already been demonstrated for the III-V semiconductor growth at ambient conditions. One advantage of using electrochemical mediated growth is the possibility of controlling the crystal growth with the applied potential.
In this work, we assess via Raman spectroscopy the voltage effects on crystal quality in InAs grown at ambient conditions directly onto Indium foils. We find that the average crystal correlation size is not affected by using more cathodic potentials up to a critical potential. Beyond that critical value, the average correlation length decreases, following the phonon confinement model. This decrease can be explained by an increase in the nanostructured roughness and or porosity of the grown InAs crystals. Gas evolution of hydrogen and or arsine and the overgrowth of As can induce this nanostructured roughness and or porosity. Therefore, we find that the crystal structure of the electrochemically grown InAs can be modified with applied potential, giving rise to the possibility to fine-tune optoelectronic properties at ambient conditions.

Over the last decade, the transition from conventional vacuum based thin film deposition techniques to solution processes has remarkably empowered the evolution of a novel manufacturing scheme that involves state-of-the-art applications, such as flexible and high-performance electronics. Besides time and cost-efficiency included in their major advantages, solution processes additionally present an outstanding compatibility with a plethora of materials; small molecules, polymers and colloids, to name a few. Amongst them, metal oxides (MOs) constitute an attractive research field, as their significantly enhanced optoelectronic properties, concomitantly with high optical transparency and mechanical stability render them as highly promising, especially in delivering trend-of-the-future applications (e.g. IoT) into high throughput manufacturing.
“Sol-gel” is established as one of the currently favorable solution processes, as it uniquely links metal oxides to solution processing. This deposition of organometallic precursors via wet techniques, followed by a subsequent thermal treatment, prevails the formation of high quality, uniform and homogenous metal oxide thin films. Nevertheless, considering its demanding high processing temperatures (>250 ), this post-deposition treatment remains challenging, as it constitutes a major obstacle that hinders the integration of temperature-sensitive materials (plastic) and thus large-scale manufacturing. Therefore, a breakthrough method of converting as-deposited organometallic precursors into uniform MO thin films with tunable morphological and chemical structure is essential.
A radically new idea for the processing of metal oxides is the implementation of a combination of solution-processing and laser annealing techniques. These methods marry two important attributes that are currently absent from the existing state-of the-art commercial technologies, namely scalable manufacturing and high temperature annealing via ultra-fast laser post-processing. Because of the temporal nature of the laser-annealing step (typically nanoseconds), efficient conversion of the printed metal oxide precursor can be achieved even when the layer is deposited atop temperature-sensitive substrates such as plastic. There is currently no other method that combines these two highly attractive attributes while being compatible with existing manufacturing processes.
The applicability of this approach has already been demonstrated [S. Dellis et al. J. Mater. Chem. C] by the successful In(NO₃)₃ conversion into high quality In₂O₃ thin films, leading to the realization of high performance (μ>13 cm²/Vs) Thin Film Transistors (TFTs). We now expand the method to a pallet of metal-oxides of different opto-electronic nature (conductors, semiconductors and dielectrics) in an effort to develop a platform for a new era of metal-oxide based printable and flexible devices, on the route towards mass production of modern electronics. In particular, an extensive study regarding the optoelectronic response of three highly promising metal oxides such as ITO, In₂O₃ and Al₂O₃ after laser annealing is presented, shedding light onto the precise mechanism under which the photochemical conversion of their organometallic precursors occurs. Optical, structural and electrical behavior of the above-mentioned thin films will be reported, including their implementation in TFT manufacturing.

During the last two decades, silicon carbide (SiC) has been widely studied for power electronics application because of its superior physical and chemical properties especially under high power and high frequency. For the reliable performance of SiC devices, control of the crystal quality related to polytype, defect density and residual stress, are very important. Thus, both thermodynamic and kinetic factors must be considered during SiC growth process.
Despite the big advancement in SiC technologies, there still remain some open questions. For instance, the fabrication of high quality SiC wafers is still topical, because the commercialized crystal growing technique, physical vapor transport (PVT) method, could not easily grow high quality SiC crystal. As an alternative method growing SiC crystal, top seeded solution growth (TSSG) method has recently studied by many researchers. This method has a big advantage growing SiC crystal at low temperature and a big potential getting a high quality SiC crystal with low dislocation density. However, the thermal stress, which is the most important issue in TSSG, still exists without a key solution. During TSSG process over 1900°C, a seed crystal always experiences an abrupt temperature change when it reached the melt surface. Consequently, residual stress could remain after crystal growth, which brings about the deformation or the crack of the crystals in the subsequent processes. The melt droplet, which is often observed on the as-grown surface after process due to the surface tension, was benefited to reduce the residual stress locally but resulted in the generation of micropipes. Therefore, it is even more problematic in epitaxy growth and device performance.
Note that such stress is mostly caused by the differences in the thermal expansion coefficients between graphite and SiC material. Thus we consider that some kind of buffer layer would be helpful to reduce the thermoelastic stress. In the present study, we will discuss the recent progress on TSSG method for growing SiC crystal from the basic to the recent progress focused on the thermoelastic stress relaxation on the entire grown surface.

Solution-processed metal oxide films have shown stunning properties of high charge carrier mobility, high electrical conductivity, large dielectric constants, or optical transparency in the application of flexible solar cells, thin film transistors, light emitting devices and various sensors. However, their high annealing temperatures are still incompatible with flexible polymeric substrates even though various chemical approaches have been studied to reduce the annealing temperature. In this study, we adopted intense pulse light sintering for solution-processed indium tin oxide films instead of thermal annealing. Multi-pulses of intense light consisting of 5 on/off pulses for 2 ms ware repeatedly irradiated on the films. The electrical sheet resistance of indium tin oxide film decreased effectively as the pulse number increased. However, the decreased sheet resistance tended to return back before the microstructure of film was stabilized. After the full densification, the sheet resistance did not change again, and the solution-processed indium tin oxide film showed a sheet resistance of 95 Ω/sq and a high optical transparency of 98 %. The intense pulse changed the chemical organization of oxide bonding from M-OH to oxygen deficient M-O-M structure, which was a critical factor to complete the photonic sintering of metal oxide films.

The self-assembly of nanocrystals into ordered superlattices is a powerful strategy for the production of functional nanomaterials. While much progress has been made assembling nanocrystals with homogeneous composition into superlattices with novel structures and properties, high-quality multicomponent nanocrystals and their ordered assemblies are rarely reported. Besides, controlling the orientations of nanocrystals in superlattices also remains a challenge. Here, we combine experiments with computer simulations to study the self-assembly of two unique quantum dot−gold (QD-Au) heterostructural nanocrystals (HNCs), namely heterodimers and satellites. Assembly of the heterodimers results in a superlattice with long-range orientational alignment of dimers while the assembly of satellites results in a close-packed superlattice with pronounced orientational orders. Molecular dynamics simulations of a coarse-grained model suggest that anisotropic interactions between the QD and Au is responsible for superlattice formation. We also demonstrate experimentally that the degree and type of orientational alignment can be controlled by changing ligand populations. This study provides guidance for the design and fabrication of nanocrystal superlattices with enhanced structural control and potential optoelectronic properties.

Lanthanide ion-based coordination solids are majorly sought after for their distinctive optical properties’ applications in OLEDs, lasers and sensors. Recent trends include incorporating suitable ligands to increase the quantum efficiency of parity forbidden f-f transitions through synergetic effects while simultaneously preventing the coordination of solvent molecules that can quench emissive processes. In our work, we employed the well-known Anderson-Evans cluster, {CrMo₆O₂₄}^n–, a molecular cluster, that exhibits ruby–like red emission and its optically inactive analogue,{AlMo₆O₂₄}^n–, as inorganic ligands to coordinate with optically active lanthanide ions (Ln=Sm, Eu, Tb, Dy). We have successfully synthesized the whole series of closely related structures with the general composition, [Ln(OH₂)₇{X(OH)₆Mo₆O₂₄}].xH₂O with varying dimensionality; the difference in supramolecular aggregation of the same set of molecular species appears to depend on the size of the lanthanide ions. Photophysical properties were dominated by Ruby-like emission in chromium analogues while characteristic emissions of lanthanide ions were observed in the aluminium counterparts. Our results strongly suggest a possible energy transfer between lanthanide ions to chromium levels resulting in the quenching of lanthanide emission in lanthanide coordinated chromium molybdates. We have also succeeded in preparing new analogues wherein more than one lanthanide ions are present in the same crystal as well as hybrid inorganic-organic solids based on these molecular building blocks. The structural features of the system change when a competing organic ligand is added to it leading to another series of photoluminescent materials. In this talk, we discuss the structural landscape, stability and photoluminescent behaviour of all the materials with an emphasis on structure-property correlation.

The evolution of nanomaterials over the last few decades has prompted widespread research on their integration into a range of applications resulting from their outstanding physical, optical and electric properties. Nanowires (NWs) have been at forefront of these developments due to the variety of semiconductor materials, highly controllable growth coupled with the large surface to volume ratio and the high aspect ratio. ZnO in particular is highly researched due to its wide band gap (3.37 eV) and high exciton binding energy (60 meV) that confers interesting piezoelectric and optoelectronic properties. In recent years, research interest in metal-organic frameworks (MOFs) has developed rapidly. MOFs are a zeolite-like nanomaterial that have been extensively studied due to their remarkably high surface area, enabling catalytic and gas storage properties. MOFs are reticular nanomaterials formed by transition metal cations coordinated by multidentate organic linkers. This combination results in a crystalline material with a flexible structure. Among the many existent MOF families, the zeolitic imidazolate frameworks (ZIF) are one of the most researched because of their exceptional thermal and chemical stability. ZIF-8 is a standout MOF comprised of zinc as transition metal nuclei and imidazolate linkers as the coordinating agent.

Here, these different forms of nanoscale materials prompted the idea for synergetic combination to create novel nanocomposite materials that exploit the individual outstanding properties, in this case for ultimate application in hydrogen production. In this work, the idea is the realisation of the integration of ZnO NWs and ZIF-8 to form a core-shell structure in which ZnO is the core and ZIF-8 is the shell around it (ZnO@ZIF-8) via a low-cost and fast production processes. A two-step wet-chemistry process is used to grow the ZnO NWs: first, a thin ZnO seed layer is deposited using atomic layer deposition. Then, the sample is introduced in a solution containing the Zn precursor and a hydroxide source. The shell is grown by means of surface conversion of the ZnO NWs. For this, we introduce a novel method based on spin coating. A methanol solution containing the imidazole linker is spin coated on top of the NWs and, afterwards, the sample is heated above the melting point of the imidazole so the surface reaction with the ZnO NWs can take placing, thus using the ZnO NWs as sacrificial pattern to grow the ZIF-8 shell. The cheap and easy route to grow the core-shell structure through this process is remarkable, especially considering that the most commonly used alternative is a hydrothermal growth that requires the use of a pressure vessel, water/dimethylformamide mixtures and long reaction times. In our case, only few droplets of a methanol solution are required to obtain comparable results.

This work presents the structural analysis and the optoelectronic characterisation of these nanocomposite materials and aims to show the first results using ZnO@ZIF-8 structures as a photoanode for the photoelectrocatalytic (PEC) water splitting of water. Electron microscopy and x-ray analysis are used to study the structure of the NWs and the core-shell structure, while voltammetry and optical methods are sued to study the optoelectronic properties. Different NW and shell growth conditions are compared to demonstrate the control over the growth of the NWs and the MOF shell in relation the long-range homogeneity and shell thickness. It is shown that the shell thickness is a critical factor that can directly influence the performance of the photoanode by controlling the electrolyte diffusion time to the ZnO NW core. The effectiveness of the core-shell structure as photoanode is compared to as-grown ZnO NWs. The PEC tests also show that the presence of the shell in the core-shell structure passivates the surface states of the ZnO core, improving its stability over time, and protecting it from photocorrosion.

In this work, a series of novel Au-Pt-Pd ternary self-assembled porous nanospheres with featured building segments and radially distributed composition have been successfully synthesized via a facial solution approach with tri-block copolymer F127 as soft templates and applied as surface enhanced Raman scattering (SERS) chemical sensors for identifying trace amount of pesticides. Based on our previously developed techniques used for synthesizing Pt-Pd binary self-assembled nanoparticles arrays[1], in this work, to further enhance the SERS performance, Au was mainly selected and has been successfully introduced as the third active component to form the Au-Pt-Pd ternary self-assembled porous nanospheres. By adjusting the amounts of the Au precursors and F127, two unique presentative Au-Pt-Pd nanospheres respectively with spherical-like and petal-like building segments were achieved. The decoration of tiny Au nanoparticles on the surface layer of the preformed Pt-Pd nanospheres could effectively enhance the SERS hot spots via increasing coupling modes for generating large local electromagnetic fields without crushing the specialized, transparent porous architectures. The liquid penetrable copolymer networks in the synthesized Au-Pt-Pd nanospheres could effectively enhance the ability to absorb target molecules and the consequent SERS performance. Also, the extremely uniform dimension and morphology of the synthesized Au-Pt-Pd nanospheres are essential for having steady SRES signals. It was found that the petal-like Au-Pt-Pd nanospheres exhibit a lower limitation approaching 50 ppb in detecting malachite green oxalate (MGO) and show promising potential in sensing pesticides such as thiabendazole directly on apple skins. The unique architectures as well as the active chemical composition of the present ternary Au-Pt-Pd nanospheres improve the molecular detection limitation and also show high potential for practical applications in the fields of biochemistry, chemical analysis, food safety and environmental monitoring.

1. Chang, F.-c., et al., Pt-Pd Floating Nano-Arrays Templated on Pluronic F127 Micelles as Effective Surface-Enhanced Raman Scattering Sensors. ACS Applied Nano Materials, 2019.

Many efforts have been made to produce low cost, flexible and high performance transparent conductive materials, replacing high cost, brittle and high performance Indium tin oxide (ITO) films. Cu nanowires (NWs) with high aspect ratios are promising materials as an alternative to ITO because of their high performance, low cost and flexibility. To take the cost advantage, the process of Cu NWs synthesis is required to be low-cost and environmentally-friendly. Cu NWs are usually synthesized via chemical reduction as this method often requires high temperature, high pressure, organic solvent and toxic reagent^[1], resulting in high cost. Recently, we have developed a low-cost and environmentally-friendly synthesis method in which aqueous copper complexes are reduced to Cu NWs by using ascorbic acid in acidic region near room temperature under atmosphere. Cu atoms were produced by reduction of the Cu complexes using ascorbic acid, forming decahedral Cu seeds. Subsequently, PVP bound to the newly formed (100) planes of the seeds rather than the (111) planes, and Cu atoms were continuously deposited on the (111) facets along the [110] direction, leading to the Cu NW synthesis^[2]. It is noted that the particle size depends strongly on the driving force for the reduction. As ascorbic acid is a mild reducing agent, the driving force is smaller than that of commonly employed reducing agents. As a result, Cu seeds become larger, leading to low aspect ratios. Besides, the small driving force caused long reaction time (48 hours or more), limiting practical use of Cu NWs. The rapid synthesis of Cu NWs with high aspect ratios requires large driving force. In this study, we reported the rapid synthesis of Cu NWs with high aspect ratios based on the environmentally-friendly method. The driving force is the difference between the half-cell reactions potential of ascorbic acid and Cu complex. The redox potential of ascorbic acid changed depending on pH, enhancing the driving force in alkali region. In addition, easily reducible Cu complexes also increase the driving force. Thus, we precisely controlled pH values in the synthesis and changed Cu complexes to increase the reduction potential, leading to Cu NWs with high aspect rations in short reaction time (1h). Detailed results and discussion will be introduced in our presentation. Part of this work has been supported by the Grant-in-Aid for Scientific Research (B) (18H03416)

Reference
[1] Ducamp-Sanguesa et al. ,J. Solid State Chem. ,100 ,272(1992)
[2] Yokoyama et al. ,Journak of Colloid and Interface Science ,531 109-118(2018)

Transparent conductive films are currently used in various optical devices such as solar cells and smartphones. Indium tin oxide (ITO) is widely used as a high performance transparent conductive film, but it is toxic and expensive rare metal. Fabrications of ITO layer on substrates usually require a sputtering system with high material loss and high vacuum environment. Metal doped ZnO nanocrystals are promising materials as an alternative to ITO because of its low cost, high transparency and low resistivity. Besides, metal doped ZnO film can be fabricated on substrates by sintering metal doped ZnO nanocrystals without material loss and high vacuum environment. In order to make high-performance ZnO transparent conductive film, ligand on crystal surfaces and crystal size controls are needed to facilitate the sintering to become bulk ZnO at low temperatures. In addition, precise metal doping is required to optimize carrier concentration and mobility in ZnO for high conductivity. These control methods about ZnO nanocrystal have been studied using thermal decomposition method as a typical synthesis method[1]. As the synthesis method often requires high reaction temperatures and toxic reagents, we developed the synthesis of ZnO nanocrystals via esterification reaction between zinc carboxylates and alcohols at low temperature.
In this study, we changed surface ligands and crystal size of ZnO nanocrystal synthesized from the esterification method[2]. We precisely controlled the amount of Ga doping into ZnO. Finally, we fabricated Ga-doped ZnO films and discuss the effect of ligand, crystal size and Ga doping on performances of the film. In the esterification between zinc oleate and oleyl alcohol, oleic acid remained on ZnO nanocrystal. Low molecular zinc carboxylates and alcohols were used to change surface ligand from oleic acid to low molecular carboxylic acids. Although low molecular carboxylic acids also work as a surface ligand, the steric hindrance of the carboxylic acid modified ZnO nanocrystals decreased, leading to bigger nanocrystals. The size was decreased by decreasing the amount of zinc carboxylates to prevent nucleus growth. The sintering of low molecular carboxylic acid modified ZnO nanocrystals with small size were facilitated at low temperatures. Metal carboxylates react with alcohols to form M-OH species with the ester, and then H₂O molecule is dehydrated by condensation to form a metal oxide. When a different element is doped, two different M-OH species formed with the ester react to form M-O-M₁-OH species, and then H₂O molecule is dehydrated, finally produces the doped metal oxide. The amount of Ga doping into ZnO were precisely controlled by changing the amount of Ga carboxylate.We will present details of these controls and performance of ZnO conductive films.
Part of this work has been supported by the Grant-in-Aid for Scientific Research (B) (18H03416).

Reference
[1] Jongnam Park, et al., Nature Materials ,2004, 3, 891–895
[2] D.Ito, et al., ACS Nano, 2014, 8, 1, 64-75

P-channel thin film transistors (TFTs) are essential components of complementary circuits, displays and flexible electronics. However, development of high performance p-channel TFTs has been delayed due to the charge carrier accumulation, large effective mass/low conductivity of semiconductor, and device optimization issues. Although tremendous efforts have been devoted to improve the device performance, the field effect mobility of p-channel TFTs still remained 0.01 ~ 5 cm²/Vs.
Recently, Electrolyte gated transistors (EGTs) have attracted attention for switching devices due to their low operating voltage and great device performance. We have employed EGT structure into p-channel TFTs to improve the electrical performance of devices. During the operation, EGT form high capacitance (> 1 uF/cm²) electrical double layer (EDL) and 3-dimensional hole transport channel. Thus, the huge amounts of charge carriers can be accumulated in the channel region and we can expect realization of the high performance p-channel TFTs. Using this EGT structure, we successfully demonstrated p-channel TFTs with ultra-high mobility exceeding 100 cm²/Vs and low operating voltage of ~ 0.5 V. As a semiconductor, copper iodide (CuI) was employed due to its small effective hole mass and high Hall mobility. For electrolyte gating, polymer electrolyte was used as a gate insulator. All the fabrication processes were conducted by low temperature (< 100 ^oC), solution process. We also examined the electrical and mechanical stability of CuI-TFTs. Finally, we demonstrated complementary inverters by the combination of CuI- and ZnO-TFTs and the circuits exhibited low operation voltage (0.7 V) and high voltage gain value of 18.

Copper(I) thiocyanate (CuSCN) has wide optical band gap (E_g = 3.5 ~ 3.9 eV), making it transparent in the optical spectrum, high charge carrier mobility, and solution processability. Due to these advantages, it has been used as a p-type semiconductor in thin film transistors (TFTs) and hole transport materials in organic/inorganic solar cells. Especially, high Hall mobility (10 ~ 70 cm²/Vs) of CuSCN allows for the possibility of realization of high performance p-channel TFTs. However, previously reported CuSCN-TFTs showed low field effect mobility in the range of 0.01 ~ 0.5 /Vs. This low mobility might be originated from the hole accumulation issue, non-stoichiometry of CuSCN and charge carrier scattering effect in the channel regions.
In this work, to realize high mobility CuSCN-TFTs, we introduced vertical structure and chlorine doping method. For vertical structured TFTs, the channel length (L) is defined as the thickness of CuSCN film. Comparing to the conventional planar structure, the L can be reduced from the several tens of micrometers to nanometer scale. Thus, employing vertical structure can minimize the charge carrier scattering effect in the channel region and our device exhibited high mobility about 3 /Vs with low operation voltage of 1V. This mobility value is 10 times higher than those of TFTs with conventional planar structure. To more improve the device performance, we doped SnCl into CuSCN. Our XPS and UPS analysis revealed that by the doping of SnCl, the Cl induces additional p-type doping effect with formation of acceptor states. The work function of material increased by 0.20 eV and the mobility of device also increased up to 4.13 cm²/Vs. Using this high performance CuSCN-TFTs, we could successfully demonstrate resistor-loaded inverters. The inverter was completely switched with in 1V of V_in variation. The dynamic response of the inverter was measured and it worked well at 500 Hz without a large rise-fall delay.

Sensors and actuators in various industrial, lab and environmental monitoring applications often require deposition of piezoelectric thin films like lead zirconium titanate (PZT). Such films require deposition temperatures (~600^○C), are incompatible with traditional silicon-based foundries, and the high temperatures of deposition present unique process challenges. In recent years, several new classes of piezoelectric films such as sputtered lead-free potassium sodium niobate (KNN) and AlN have been developed. In the former case, owing to the differential response of potassium and sodium to needed annealing steps, and the result stoichiometric shifts near the (K:Na)=50:50 morphotropic phase boundary and the high temperature of annealing (~ 600-800^○C) has created challenges to widespread adoption of KNN. In contrast, sol-gel based methods coupled with simple deposition methods such as spin-coating, dip-coating and printing offer process simplification at lower temperatures. However, the resulting amorphous films exhibit low values of d₃₃, resulting in inferior overall performance. In this work, we have examined the possibility of using high in-plane electric fields to modify the packing density of the material. A concentrated KNN sol (50:50) was synthesized using the sol gel method. Powder X-ray diffraction (XRD) measurements on the dried sol films reveal an amorphous product. Oxide substrates (University Wafers) were cleaned (RCA) and metal bottom contacts were deposited using successive DC sputtering (Cr/100Å) and RF sputtering (Au/500Å) at a base pressure of 2 x 10^-6 Torr. The metal was patterned using maskless lithography (Intelligent SF100 Express) and etched (Sigma Aldrich CAS: 7681-11-0) with two electrode spacings of ~ 10μm. Films of KNN were deposited via drop casting of concentrated sol (~10 μL) onto the two electrode lateral structures and dried at 90^○C prior to measurements. Energy dispersive X-ray fluorescence (EDX) reveals (K:Na)=50:50, which was confirmed using X-ray Photon Spectroscopy (XPS) surface survey measurements. The lateral devices were biased on a probe station (Cascade Microtech) with a Keithley 2450 source meter at 10V. Under the microscope, we observed sporadic possible reflow of the dried film on the positive pad, along with a 300% increase in the spot temperature estimated using a non-contact IR thermometer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements were used to gather information about the morphology of the film in the vicinity of the device, which shows cracks in shapes that closely resemble calculated electric field patterns (COMSOL), which predict a maximal electric field of ~330 kV/cm with the averaged field far away from the device subsiding to ~100 kV/cm. The resistance between the electrodes increased by ~44% as a result of the annealing, attesting to changes in local thickness due to crack formation. Post-annealing EDX measurements showed a ~16% reduction in the volatile potassium content suggesting a local migration of ions, resulting in a stoichiometric shift to (K:Na)=44:56. SEM and AFM measurements of the annealed film were combined to estimate a volume reduction of ~4.8% over an area of 100 μm². We are currently developing the above process in combination with successive spin-coating of sol on annealed films and reannealing to develop densified films of KNN suitable for measurements of relevant electromechanical coefficients.

We demonstrate sol-gel processed Mg-doped SnO₂ thin film transistors (TFTs) with high performance and improved stability. To improve the performance of tin oxide, which is unstable at a high negative threshold voltage, magnesium is used to suppress oxygen vacancy formation. As the Mg doping concentration increases, the oxygen deficiencies decrease, resulting in a negative shift in the threshold voltage and improved stability in negative bias stress environments. In this experiment, Mg-doped (0 to 1 wt%) tin oxide TFTs are fabricated. The tin oxide prepared from a synthesized solution of tin (II) chloride dehydrate precursor and ethanol has a mobility of 13.84 cm²/Vs and -32.13 V threshold voltage shift under negative bias stress. On the other hands, the SnO₂ TFTs doped with 0.5 wt% Mg also show a mobility of 4.23 cm²/Vs and a noticeable -6.98 V threshold voltage shift under negative bias stress. Our results suggest that sol-gel processed Mg-doped SnO₂ TFTs are a promising candidate for use in high stability and high-performance applications in transparent devices.

Colloidal synthesis and surface stabilization of nanocrystals (NCs) have been well-developed and established experimentally over the past 30 years. The role of surface increases as the size of materials shrinks, and this fact shines a spotlight on the surface ligands attached on the colloidal NCs. Ligands form a ‘capping’ layer on the surface which saturate dangling bonds and stabilize surface energy, and affect to the reaction mechanism in synthesis by kinetic and thermodynamic controls. However, detailed understandings on the ligand chemistry of NCs at the atomic level were largely focused on the ionic II-VI (CdSe, CdS) and IV-VI (PbS, PbSe) semiconducting NCs. Surface ligands are classified as L-, X- or Z-type depending on the number of electrons involved in ligand-NC bonds and the identity as electron donor or acceptor. Recent studies have shown that different types of ligands stabilize each facet of ionic NCs, resulting in mixed surface ligands during the synthesis (intentionally or not).
In this study, we investigate the influence of molar ratio between X- and L-type ligands on the synthesis of colloidal transition metal dichalcogenides (TMDCs). Two-dimensional (2D) TMDCs have been considered to be attractive for numerous applications because of their broad range of properties from metals to insulators. However, wet chemical synthesis of 2D TMDCs is challenging because of their additional covalency requiring a high growth temperature, and even complicated by large anisotropy in layered structure. Ideally, layered TMDCs have no dangling bonds on the basal plane (001) and therefore their ligand chemistry is largely expected to rely on only edge sites (110), (100). Reflecting the ligand chemistry of ionic NCs, we control the molar ratio between different types of ligands (X-type and L-type; halides and amines), and monitor how the surface composition changes accordingly. Stoichiometric differences and oxidation states shift in overall TMD NCs show a correlated tendency, resulting in different surface states of colloidal TMDC NCs. Based on the observations, we conclusively demonstrate that the appropriate ligand ratio is required to synthesize the desired colloidal TMDC NCs, introducing an effectiveness of ligand control in colloidal synthesis of 2D TMDCs.

Many efforts have been made to produce low cost, flexible and high performance transparent conductive materials, replacing high cost, brittle and high performance Indium tin oxide films. Random networks of metal nanowires (NWs) possess optically transparent holes between nanowires and good electrical conductivity arose from metallic contacts between nanowires. Among metal nanowires, Cu has a disadvantage of low stability but is a top candidate due to its low cost. To take the cost advantage, synthesis and fabrication processes of Cu NW networks must be low cost and environmentally friendly. In addition, the surface must be modified with stable layers to avoid surface oxidation. Recently, we synthesized Cu NWs with different aspect ratios using an environmentally friendly synthetic method¹. However the surface easily oxidized, hindering NW performance.
In this study, we reported the surface coating of Cu NWs with ZnO layer based on aqueous solution processes for stable and high-perfomance transparent conductive films. To make transparent conductive films made of Cu NWs, a vacuum filtration and transfer method was used. The synthesized Cu NWs were deposited on a membrane filter and filtered under vacuum. Subsequently, the Cu NWs were transferred onto a glass substrate by pressing the filter against the glass substrate. After deposition of Cu NWs on the filter, aqueous ammine-hydroxo zinc complexes were employed to be absorbed on Cu NW surfaces. Although the complex easily converted to ZnO with heat treatment at low temperature, the complexes did not remain on Cu NW surfaces. As the complex condition and reactions between Cu NW surfaces and the complexes dramatically changed depending on pH and zinc concentrations, we identified complexes using ESI-TOFMS and evaluated the surface reactions using surface analyses at different conditions. As a result, the ammine-hydroxo zinc complexes sufficiently remained on the Cu NW surfaces with appropriate conditions and then changed to thin ZnO layers. The thin ZnO layers on the Cu NWs improved the stability of Cu NWs under air without significant decreases in Cu NW performances such as transmittance and sheet resistance. In our presentation, detailed results and discussion will be introduced.
This work is partially supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (A) 17H04721 and Scientific Research (B) 19H04324, and Mayekawa Foundation 2018 and The Iwatani Naoji Foundation.

Reference
1. S. Yokoyama, K. Motomiya, B. Jeyadevan and K. Tohji, Journal of Colloid and Interface Science 531, 109-118 (2018).

Two-dimensional (2-D) nanomaterials have recently received a considerable attention because of their unique characteristics, which were not seen in their bulk or 1-D nanomaterials. Zinc oxide (ZnO) is one of the most industrially-promising materials because it is chemically stable, non-toxic and available at low cost. To synthesize the 2-D ZnO platelets, ZnO must grow in the a-axis direction, while suppressing its growth in the c-axis direction, which is often preferred in the ZnO growth. To control the growth direction, a seed layer plays an important role. In this study, we prepared a seed layer by spin-coating and high-temperature post annealing of the r-plane (1-102)-oriented aluminum oxide (Al₂O₃) thin films on Si (111) substrate. Then, ZnO nanoplates were hydrothermally grown on the seed layer in the a-axis direction with various growth parameters. Since a lattice mismatch between a-plane (11-20) of ZnO and r-plane of Al₂O₃ is only 1.53%, ZnO grows epitaxially along the a-axis. Such nanoplates contained a high concentration of the surface defects and were off-stoichiometric, thereby making them attractive as very effective photocatalytic surfaces for degradation of organic contaminants. In addition, these vertically-stacked ZnO nanoplate surfaces showed the electrical properties that strongly depend on the presence of oxygen and temperature. As a result, the templated growth of the 2-D ZnO nanoplates shows the great potential to form functional surfaces that can be very sensitive to external stimuli such as light and environment by controlling the concentration and type of defects. This presentation will highlight a current research progress made to the control of these defects to tailor the photocatalytic properties of the nanoplate-stacked surfaces.

For decades two-dimensional (2D) van der waals(vDw) materials, which have been isolated as one atomic sheet from their layered-bulk materials, are intensively studied due to their excellent and unique properties such as high mobility, flexibility, and two-dimensional quantum effects. In recent years, interest in 2D vDw materials has expanded to include a new type of nanomaterials, one-dimensional (1D) materials which are isolated as one molecular chain from their chained-bulk inorganic materials. The 1D bulk materials consist of chain-shaped inorganic molecule units which are assembled by weak physical interactions such as van der Waals or ionic bonding in their bulk materials. The inorganic molecular chain itself have a dimension of smaller than 1 nm in a diameter thus produce flexible characteristic even though they have inorganic compositions.
Here in, we synthesized new 1D materials [1,2](LiMo₃Se₃, V₂Se₉, Nb₂Se₉) and developed liquid-phase based exfoliation techniques for them which makes 1D bulk materials as an inorganic molecular chain. These include survey of solvents[3,4], design of dispersant[5] and ion exchange chromatography[6], and provides essential information on the material surface for additional stripping strategy design. The result of liquid exfoliation, colloidal nanomaterial, can be applied to various fabrication techniques for future applications.[7]

[1] S. Oh et al., Phys. Status Solidi RRL, 2018, 1800451
[2] S. Oh et al., RSC Adv., 2018, 8, 33980-33984
[3] S. Chae et al., RSC Adv., 2018, 8, 35348-35352
[4] S. Chae et al., Nanomaterials, 2018, 8, 794
[5] S. Chae et al., Chem. Commun., 2018, 54, 12190-12193
[6] S. Chae et al., Chem. Commun., 2018, 54, 12503-12506
[7] S. Chae et al., Nano Lett., 2018, 18, 7619-7627

Recently, metal oxide nanoparticles have gained much attention owing to their anticipated properties and application in different areas, such as photoelectron devices, sensors, catalysts, and photovoltaic devices. Among these metal oxides, zinc oxide is one of the most important natural n-type semiconductors with a direct wide band gap in the near-UV spectral region (3.36 eV at room temperature). The optical and electrical properties of ZnO are affected by many factors, such as the structure, size, shape, and defect concentration, which makes ZnO a very important and interesting subject to examine the controlled growth of novel materials.
Recent work showed that metal oxide nanoparticles could be synthesized by a new pathway: submerged photosynthesis of crystallites (SPSC). In the SPSC method, the initial metal is surface treated by a submerged liquid plasma process, which creates a metal nano oxide semiconducting layer with surface protrusions. After that, the growth of metal oxide NCs is assisted by a ‘photosynthesis’ reaction, where the metal surface is irradiated with ultraviolet light in water. Thus, the SPSC process requires only light and water and does not require the incorporation of impurity precursors. Moreover, this method is applicable at low temperature and at atmospheric pressure, producing only hydrogen gas as the by-product. These characteristics give rise to the potential application of SPSC as a green technology for metal oxide nanocrystallites synthesis. Therefore, SPSC method is applied for ZnO nanorods synthesis in this work, and the photochemistry reactions and the role of light in the SPSC process are studied.
We carried out surface microstructural analysis and monitored the pH and temperature change of the water during the process. Based on this analysis, ZnO nanocrystallites formation mechanism was clarified. Furthermore, the effect of light irradiation during the process was elucidated by contrast experiments under dark conditions. The resutls show that both photoinduced reactions and hydrothermal reactions contribute to the SPSC process. The former generates OH radicals, which are one of the main sources of OH^- at the crystal tips, whereas the latter involve the ZnO growth reactions. Furthermore, ZnO nanocrystallites can be obtained under both light irradiation and heating in dark conditions, in which ZnO growth and corrosion reactions occur simultaneously. Light irradiation makes ZnO growth dominant and the water pH close to neutral, whereas thermal energy makes ZnO corrosion dominant and the water pH increases. The role of light in SPSC process is to enhance ZnO apical growth at relatively lower temperature by preventing the pH of water from increasing, revealing the environmentally benign characteristics of the present process.

Two-dimensional materials, especially transition metal chalcogenides MX₂, remain a dominant topic in novel material chemistry and physics. As we explore their layered ternary analogs with the formula Cu₂MX₄ (M = Mo/W, X = S/Se), we found few reports on the fabrication and characterization of those ternary selenides. Here we describe a solution processing method towards phase-pure nanoparticles and associated synthetic controls available in our method.

Predictable and consistent synthetic control of nanoparticle sizes, shapes, and properties through synthesis can be guided by comprehensive thermodynamic and kinetic models for nanoparticle growth. Establishing these models requires not only understanding how different growth mechanisms affect the size and morphology of nanoparticles, but also what factors determine the kinetic rates of these growth mechanisms and the resulting growth regimes. Here, electron microscopy characterization of hydrothermally grown solid solution KTa_1-xNb_xO₃ nanoparticles and density functional theory calculations revealed an intricate story, where complex relationships between several properties—including surface segregation, strain, and adsorption chemistry—affect and explain the surface behavior and growth regimes of the nanoparticles. Comprehensive study of these relationships informs mathematical models for predicting nanoparticle growth. In this case, these properties manifested in two surface composition dependent growth modes, where nanoparticles with tantalum-rich surfaces grew via step-flow growth to form smooth facets, while those with niobium-rich surfaces grew in a different growth mode to form rougher surfaces. Understanding how such properties affect growth mechanisms allows for further manipulation of nanoparticle growth, as demonstrated by the effects of adding potassium halide on KTa_1-xNb_xO₃ nanoparticle phase, composition, and morphology.

Prior attempts at reducing the crystallization temperature of metal oxide films have succeeded via chemical solution deposition and photochemical solution deposition [1]. These methods include seeding solutions with crystalline nanoparticles in order to reduce the activation barrier for nucleation and crystal growth, or incorporating photosensitive compounds into the solution and thereby reducing the crystallization temperature during exposure to continuous UV irradiation. In one study, a xenon flash lamp was utilized to obtain nanocrystalline thin films by the use of a metal β-diketonate solution [2]. Thus, the aim of this study is to determine whether such solutions can be used to obtain tantalum oxide films with a xenon flash lamp, and how future studies could allow for crystallization from the amorphous state via pulsed laser irradiation, or from the solution state via seeding or the incorporation of photosensitive tantalum complexes [1].

In this investigation, the characteristics of a photosensitive, tantalum β-diketonate sol-gel ink were studied as a function of flashes from a xenon flash lamp at room temperature. The ink was printed on quartz substrates for UV-Vis spectrophotometry in order to follow the absorption versus flash lamp exposure, and on interdigitated electrodes in order to follow the capacitance versus flash lamp exposure. The results indicate that the UV-Vis absorption spectrum is similar to those found for tantalum oxide films obtained by photo-irradiation [3], [4], and the interdigitated electrode capacitance increases with the number of xenon flash lamp flashes. An increase in capacitance hints at oxide densification and formation [5], thus allowing for a photopatternable material. The amorphous film could be subsequently heat treated to achieve polycrystalline films [6], or exposed to pulsed laser irradiation to similarly achieve high temperatures [1]. The photosensitivity of the ink also allows for future investigations involving low temperature crystallization via photochemical solution deposition, as has been performed for lead-titanate based precursors, among others [1]. Moreover, the air stable solution would further increase the ease of manufacturing and allow for it to be implemented into a roll-to-roll manufacturing line [7].

[1] I. Bretos, R. Jiménez, J. Ricote, and M. L. Calzada, “Low-temperature crystallization of solution-derived metal oxide thin films assisted by chemical processes,” Chem. Soc. Rev., vol. 47, no. 2, pp. 291–308, 2018.
[2] S. Luo et al., “Instantaneous photoinitiated synthesis and rapid pulsed photothermal treatment of three-dimensional nanostructured TiO₂ thin films through pulsed light irradiation,” J. Mater. Res., vol. 32, no. 9, pp. 1701–1709, 2017.
[3] T. Ohishi, S. Maekawa, and A. Katoh, “Synthesis and properties of tantalum oxide films prepared by the sol-gel method using photo-irradiation,” J. Non. Cryst. Solids, vol. 147–148, pp. 493–498, Jan. 1992.
[4] I. W. Boyd and J.-Y. Zhang, “Photo-induced growth of dielectrics with excimer lamps,” Solid. State. Electron., vol. 45, no. 8, pp. 1413–1431, Aug. 2001.
[5] L. A. Silverman, G. Teowee, and D. R. Uhlmann, “Characterization of sol-gel derived tantalum oxide films,” MRS Proc., vol. 72, no. 1966, p. 331, 1986.
[6] M. L. Calzada and A. González, “Tantalum penta-glycolate sol as a precursor of strontium bismuth tantalate ferroelectric thin films,” J. Am. Ceram. Soc., vol. 88, no. 10, pp. 2702–2708, 2005.
[7] D. Levy and M. Zayat, Eds., The Sol-Gel Handbook, 1st ed. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2015.

Vanadium sesquioxide , a transition metal oxide, is an important metal transition insulator (MTI) material for its potential applications in novel electronic and memory devices. thin films of ~230 nm thick were grown on Si/Pt substrate using a D.C magnetron sputtering system deposited at substrate temperature 450 C in a controlled atmosphere of 35:2.5 sccm of Ar:O₂. X-ray diffraction studies confirm the formation of corundum rhombohedral phase. EDAX analysis and X-ray photoelectron spectroscopy were carried out to identify the elemental compositions and their chemical states. Magnetization studies down to 5 K were measured to predicts the paramagnetic to ferromagnetic transition around 53 K. Out of 7 expected Raman active modes, Raman active modes located at 242 and 500 cm^-1 were observed at ambient phase. Temperature dependent Raman spectroscopic studies carried out in the temperature range of 80-300 K to identify the low temperature metal-insulator transition at 153 K. A possible coexistence and hysteresis of metal and insulator phases are evident from the intensity analysis. These results will be presented in detail at the meeting.

Magnesium borohydride (Mg(BH₄)₂, abbreviated MBH) have received tremendous attention as a promising onboard hydrogen storage media due to its excellent hydrogen storage capacity and high volumetric density. While each polymorph of MBH— alpha (α), beta (β), and gamma (γ) — is interesting, their synthetic homogeneity is not well developed owing to complex structures and similar thermodynamic properties of the polymorphs. Here, we demonstrate effective synthetic strategies to obtain pure polymorphic phases of MBH nanomaterials supported by reduced graphene oxide (MBHg) under mild conditions (60 - 190 °C under low vacuum, 2 Torr). Different polymorphs of MBH are obtained from the different starting materials dried in two different ways (under Ar or vacuum) via the phase evolution, which mechanism elucidated by computational analysis. Specifically, we selectively synthesize thermodynamically stable phase (α-MBH) and metastable phase (β-MBH) from γ-MBH within the explored temperature ranges (150 °C to 180 °C). The relevant underlying phase evolution mechanism is analyzed by theoretical thermodynamics and kinetic nucleation modeling. Furthermore, the resulting MBHg composites were found to have an oxidational resistivity, structure stability, and expression of partially reversible [BH₄]⁻ species during de(re)hydrogenation processes, offering a potential for realizing hydrogen storage cyclability.

The ZnO films were deposited on n+-type antimony doped silicon (111) substrate (0.01 Ω×cm) using an electrochemical deposition from aqueous solution containing zinc nitride, nickel nitride and isopropyl (IP). Deposition was carried out in galvanostatic mode at current densities of 15 – 30 mA/cm² and at temperature of 80 °C. The structure and optical properties was correspondingly studied by Raman and photoluminescence spectroscopy.
According to Raman spectra the obtained films consist of ZnO doped with Ni atoms. Existence of ZnO was verified by a typical number of Raman modes for ZnO, such as E₁, A₁ and et. al. Nickel doping was indicated by an increase in the intensity of the 1050 cm^-1 Raman spectra band and smoothing of the band at 1150 cm^-1.
The observed intense photoluminescence band in visible range on all spectra corresponds to radiative transitions through three main group of deep levels in the band gap of semiconductor ZnO: a oxygen atoms in zinc interstitial (2,28 eV or 523 nm); a complex of an oxygen vacancy and zinc interstitial (2,16 eV or 576 nm) and oxygen vacancy (2,00-1,62 eV or 640-765 nm).
ZnO films were created by standard solution (without IP). Films with arrays of crystals on them were obtained for the solution with IP. The surface density of ZnO crystals on the ZnO film surface depends on the current density and increase with increasing current density. The size of crystal also increases by increasing deposition time. For example, at current density of 15 mA/cm² and deposition time 30 min continuous ZnO film with thickness of 7 um with an array of crystals of approximately 30 x 30 um in size and thickness of 7 um on it was obtained.
Based on the data extracted from the photoluminescence spectra of annealed samples, the number of such defects as oxygen atoms in zinc vacancies are significantly reduced and the emission peak redshifts. At the same time, annealing of samples prepared with the addition of 5 vol.% IP leads to a blueshift due to the annihilation of oxygen vacancies.
After annealing, the mode at 280 cm^-1 appeared on all Raman spectra. This mode stand for the mute B₁low mode, the display of which in an optical spectra is related to the disorder of the crystal lattice. But, this Raman mode doesn’t presented for samples that was preparing from solutions with isopropyl.
This work was supported by the Belarus Government Research Programs “Photonics, opto- and microelectronics”, Grant 2.1.02, “Physical materials science, novel materials and technologies”, Grant 1.15.

Some calcitic biominerals have been shown to be formed through the crystallization of a Mg-rich amorphous calcium carbonate (Mg-ACC) precursor. That non-classical crystallization pathway allows the formation of out-of-equilibrium supersaturated solid solutions (such as high-Mg calcite) as well as the formation of intricate rounded crystal shapes. Formation mechanisms of multiscale biogenic single crystal structures at ambient temperatures, in conditions of limited solid phase diffusion, remain largely incomprehensible. Based on experimental results, we develop a model describing the formation of the brittle star Mg-calcite nanostructure from an amorphous Mg-ACC precursor to a Mg-calcite nanostructure containing periodic layers with varying concentrations of coherent Mg-rich nano-inclusions. The amorphous ACC precursor can exist as a gel or as a liquid depending on amount of Mg and acidic proteins present in the system. The formation route is rationalized in a two-step model: the first step involves spinodal decomposition of a liquid or gel-like Mg-ACC precursor into Mg-rich nanoparticles and a Mg-depleted amorphous matrix. The second step is the crystallization of the decomposed Mg-ACC precursor. The crystallization probably starts in the Mg-depleted matrix. We consider two possible crystallization routes leading to the layered structure formation. In the first route, crystallization front faces Mg-rich nano-domains and alternatively repels them to amorphous phase or embeds epitaxially to the crystalline matrix, depending on the local effective viscosity varying with the density of nano-domains. In the second route, the crystallization of Mg-depleted ACC matrix is accompanied by the exclusion of Mg ions into a diffusion zone adjacent to the crystallization front; after crystallization of a certain layer, the Mg concentration ahead of the layer exceeds the critical value above which the Mg-ACC matrix becomes unstable against spinodal decomposition. A secondary spinodal decomposition will then start and result in the formation of additional Mg-rich nano-domains. As a result, the density of Mg-rich nano-domains changes periodically and forms periodic layered structure inside magnesium calcite single crystals. The model was supported by our experimental results in synthetic Mg-calcite, which suggest a spinodal decomposition in the amorphous precursor. These new insights have significant implications for fundamental understanding of the role of Mg-ACC material transformation during crystallization and its subsequent stability.

The Cu₂ZnSnS₄ (CZTS) powder was systematically investigated by energy dispersive X-Ray spectroscopy (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Ramam spectrometer, and further studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA). The Ramam and XRD results showed that the sample is a structure of Kesterite type under a condition of Cu-rich and Zn-poor with few impurity phases. The DSC results show that incubation time, peak time and end time of crystallization are all reduced with the increasing temperature; and the temperature of 550 °C will be more suitable for annealing treatment of CZTS. A CZTS film is also fabricated and annealed at 550 °C to confirm the result of the research. The systematic investigation of CZTS by using differential scanning calorimetry provided a fundamentally dynamic reference for the temperature selection of anneal during the fabrication of CZTS.

The formation of twisted crystals of a family of low molecular weight organic dye molecules from solutions of the compounds has been discovered. This phenomenon is fascinating because the chirality arises over the lengthscale of microns, which is not accessible by simply relying on chiral materials (where helical pitch is short) or by twisting fragile macroscopic organic crystals. The properties of this new kind of material are potentially unique for a range of optoelectronic applications.
The twisting occurs across a broad range of compounds based on the diketopyrrolopyrrole family, substituted at the core of the molecule or at the terminus. Chiral side chains induce single handed crystals with very regular pitch related with the thickness of the objects. Achiral sidechains result in the formation of twisted crystals of more varied mrophology, and a variety of twisted crystal types depending on the constitution of the side chain.
Electron and atomic force microscopy have been used to understand the morphology of the materials, and single crystal diffraction and scanning tunnelling micrioscopy reveal the nature of interactions between the molecules and their long range packing. Circualr dichroism has been used to demonstrate the huge optical activity of the twisted crystals compared with the solution phase systems, thereby demonstrating the interest for chiroptical phenomena.

The precise nature of intermolecular interactions, both in solution aggregates and in the solid state, plays a key role in the electronic and photonic properties of any material. We developed a reliable functionalization strategy that allowed the tuning of the solid-state order of a variety of aromatic chromophores, and established a rough set of guidelines to predict how functional group size impacts crystal packing. For this talk, I will discuss several refinements of our models, including (1) the use of computational screening to reduce the time and effort required to engineer the crystal packing of new semiconductors for electronic and photonic applications, (2) the impact of disorder on electronic properties, and how functionalization can diminish or enhance several types of disorder, and (3) efforts to minimize dynamic disorder, and minimize phonon modes that are disruptive to charge transport.

Despite the importance of conjugated materials in organic optoelectronic devices, the rational design of their properties as solid state ensembles is currently not possible. The current design paradigm of conjugated materials uses long, often branched alkyl side chains to impart solubility to otherwise insoluble conjugated backbones that are responsible for the key optoelectronic properties of these materials. This talk will summarize work in which discrete non-covalent interactions between pendants and conjugated backbones can dictate the conformations, and resulting spectroscopic characteristics, of conjugated molecules in the solid state. In particular, intra- and intermolecular arene-fluoroarene stacking interactions twist phenylene-ethynylene linkages as revealed by X-ray crystallography. The weak nature of this interaction results in surprising sensitivity to seemingly insignificant changes in chemical structure that impact interaction strengths and the resultant conformations of these solids. This non-covalent control over solid state conformation and spectroscopy extends to a wide variety of conjugated materials, including organometallic phosphors. Mechanofluorochromism—the reversible change in photoluminescence upon application of force—is a shared feature of many of these solids. The temperatures at which the force-induced phases revert to the thermodynamically favored crystalline states is tunable over a wide range by changing the lengths of alkyl substituents. Overall, this collection of materials opens a new path forward to controlling the properties of solid conjugated materials, for which rational design is most difficult, yet critically important for future technological development.

Control over the out-of-plane molecular orientation of solution-processed organic semiconductors is a long-standing challenge in the organic electronics field. We present a generalizable strategy to orient the nucleation of small-molecule organic semiconductors during solution-phase deposition using nanoconfining scaffolds. Employing a simple dip coating process, triisopropylsilylethynyl-derivatized acene molecules were deposited onto nanoporous anodized aluminum oxide (AAO) and Si scaffolds exhibiting cylindrical pores with average diameters ranging from 60 – 500 nm. Preferentially oriented nuclei were found to form within the pores such that the fast growth direction (i.e. the π-stack direction) aligned with the long axes of the pores. Well oriented vertical crystal arrays with the π-stack direction of organic semiconductor crystals perpendicular to the substrate surface were found to form on both scaffolds. By varying the dip coating speed, the diameters and heights of these crystals on scaffolds were tunable over ranges of 100 – 600 nm and 0.8 to 6.7 m[SL1] m, respectively. Photoluminescence (PL) experiments further revealed an eight-fold enhancement of the PL signal from vertical crystal arrays compared to horizontal crystals deposited on flat SiO₂ substrates due to waveguiding along the crystal length. This strategy is suitable for continuous deposition methods that will enable the large-area manufacturing of flexible and inexpensive optoelectronic devices.

In the organic field-effect transistors (OFETs), the charge accumulation region is usually within several nanometers from the dielectric surface. For small-molecule organic semiconductor material with monolayer thickness, all the molecules in the monolayer organic crystals are in principle contributed in forming the conductive channel. A meniscus-guided solution-processing method is employed to grow monolayer crystals of C₁₀-DNTT with single-crystalline domain size as large as millimeters. The intrinsic field-effect mobility of >12 cm²V^-1s^-1is achieved. More importantly, the ultrathin thickness minimizes the charge injection barrier for the top-contact electrodes, allowing an ohmic hole injection and a low contact resistance (R_c-W) of 75 ohm-cm. Such low contact resistance not only allows short channel devices but also ensures low voltage drops at the metal/semiconductor interface, allowing the device to operate even at V_DS = -1 mV. It opens a new direction of low-power OFET that operates at pico-watts, which can only be achieved by high-mobility short-channel OFETs with very low contact resistance. The physics origin of forming such low contact resistance OFETs will also be discussed.

The weak intermolecular forces inherent to organic semiconductors give rise to polymorphism, or the ability for these materials to adopt multiple solid-state packing arrangements. And since the details of intermolecular packing can substantively influence charge transport, the ability to predict which of the available polymorphs is preferentially accessible and stable should shed light on charge transport characteristics. In this talk, I will discuss a simple framework that directly correlates the presence of short intermolecular contacts with polymorphic stability. Quantification starts with assessing whether intermolecular contacts within a crystal structure are smaller than the sum of the van der Waals radii of the same atoms. Polymorphs with such short intermolecular contacts are substantially more resistant to polymorphic transformation while those without exhibit an added degree of freedom for molecular rearrangement; the said polymorph can thus be easily destabilized with post-deposition processing. Starting with a series of core-chlorinated naphthalene diimide derivatives, we show this framework to be widely applicable to a wide range of molecular semiconductors, and even extending to pharmaceutics and biological building blocks. In this era of machine learning and materials by design, this framework can help refine computational predictions by identifying the polymorphs that are practically stable [1].

[1]. Geoffrey E. Purdum, Nicholas G. Telesz, Karol Jarolimek, Sean M. Ryno, Thomas Gessner, Nicholas D. Cavy, Antohny J. Petty, II, Yonggang Zhen, Ying Shu, Antonio Facchetti, Gavin E. Collis, Wenping Hu, Chao Wu, John E. Anthony, R. Thomas Weitz, Chad Risko, Yueh-Lin Loo, J. Am. Chem. Soc. 140, 7519. (2018).

We present an extensive study of the optical and electronic properties and the phase transition in 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) thin films utilizing in situ x-ray diffraction, polarized optical spectroscopy, terahertz spectroscopy and ab initio density functional theory. The influence of molecular packing on the properties are reported for thin films deposited in the temperature range from 25°C to 140°C, and for films that are strain-stabilized at their as-deposited lattice spacings after cooling to room temperature. Anisotropic thermal expansion causes relative displacement of neighboring molecules while maintaining a nearly constant stacking distance. This leads to a large blueshift in the absorption spectrum as the temperature increases. The blueshift is a signature of charge transfer excitations, which depend sensitively on the alignment of the nodes of the frontier molecular orbitals with those on neighboring molecules. This effect is correlated with an enhancement of the field-effect transistor mobility in strain-stabilized thin films. These results suggest approaches to improve carrier mobility in strained thin films and for optical monitoring of electronic changes.

High performance organic thin-film transistors (OTFTs) are attainable with molecular materials that show high degree of layered crystallinity, because the feature is best suitable to obtain two-dimensional channel layers interfaced with gate-dielectric layers. Here we present and discuss that some asymmetric rod-like organic molecules, composed of p-conjugated backbones linked with relatively long alkyl chains, provide molecular-bilayer-type organic semiconductors that involve intralayer herringbone-type molecular packing [1-5]. The materials exhibit considerably high layered crystallinity, allowing to realize innovative solution processing for obtaining extremely uniform channel semiconductor layers [6,7]. Based on ab initio quantum chemical calculations, we discuss that the high layered crystallinity are strongly enhanced by the intermolecular attractive forces between alkyl chains [3]. Based on molecular dynamics study, we show that the molecules are self-organized at around air-liquid interface via intermediate liquid-crystal phases, which eventually leads to the large-area single-crystal film growth [8]. We also demonstrate that the formation of single molecular bilayers becomes possible by using frustration effects for the alkyl chains [7].
[1] H. Minemawari et al. Appl. Phys. Exp. 7, 091601(2014).
[2] S. Inoue et al. Chem. Mater. 27, 3809 (2015).
[3] H. Minemawari et al. Chem. Mater. 29, 1245 (2017).
[4] S. Inoue et al. Chem. Mater. 30, 5050 (2018).
[5] T. Higashino et al. Chem. Lett. 48, 453 (2019).
[6] T. Hamai et al. Phys. Rev. Appl. 8, 054011 (2017).
[7] S. Arai et al. Adv. Mater. 30, 1707256 (2018).
[6] M. Yoneya et al. J. Phys. Chem. C 121, 8796 (2017).

Organic semiconductors (OSCs) have been the cornerstone of organic electronics in a variety of applications, including photovoltaics, thin-film transistors (OTFTs) and electrochromics. Although OSCs promise to be lower in cost, more flexible and compatible with roll-to-roll processing compared to inorganic semiconductors, they suffer from a lack of long-range order, especially when deposited from solution over large areas. Defects, such as grain boundaries and dislocations, in as-cast OSC films, have been widely recognized to impede charge transport. While the introduction of an external field during solution processing, such as that during blade coating, has resulted in high quality millimetric single-crystalline domains and ribbons, these techniques still face formidable drawbacks because they employ chlorinated organic solvents that pose significant health and environmental hazards, and are often trapped in the resulting OSC films.
Herein, we introduce a scalable, solvent-free technique to prepare single crystalline OSC films with low defect densities over several centimeters. This approach leverages the liquid crystallinity of OSCs; depositing the OSCs at temperatures within their smectic window over a metal frame forms a freely suspended membrane. Given the elasticity of the smectic phase and that the OSCs are individually mobile at the surface, the resulting membrane – comprising of OSC molecules preferentially aligned with their long axis normal to the surface – is defect-free and uniform across its entire area. Transferring this membrane onto desired substrates and cooling it slowly from its smectic phase into its crystal phase preserves this high quality assembly. In the particular example of a equimolar blend of dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and didodecyl[1]-benzothieno[3,2-b][1]benzothiophene (C12-BTBT), we have produced BTBT cocrystalline films with single-crystalline domains that span ca. 30 mm laterally. Varying the deposition temperature and coating speed produces membranes that are 20 nm to a few hundreds of nanometers thick.
To assess the electrical properties of these single crystalline films, we fabricated OTFTs by transferring the OSC membranes to heavily n-doped silicon wafers with a 100 nm thick SiO₂ as the gate dielectric and cooling them slowly into their crystalline phase. Evaporation of gold contacts defined individual transistors with a channel width of 200 mm and a channel length of 500 mm. We chose a particularly large channel length of 500 mm to demonstrate the uniformity and quality of our films over large areas. Such devices exhibit an average hole mobility of 8.5 ± 2.3 cm²V^-1s^-1 and an average on/off ratio that exceeds 10⁶, independent of device location across the entire film. This solvent-free scalable approach of producing single crystalline OSC films at wafer dimensions (ca. 50 mm) should be amenable for broad applications in organic electronics.

A large diversity of π-systems exist, nearly all transport electrical charges but only a few molecular structures qualify as best-performing organic semiconductors with μ ≥ 10 cm²/V.s. But charge carrier mobility is more a material than a molecular property. One has, thus, to consider supramolecular order at all lengthscales. The best organic semiconductors self-organize into large plate-like monocrystals as evidenced in recent papers. Due to their electronic properties but also to their favorable crystalline morphology, thienoacenes derivatives exhibit record charge carrier mobility above 10 cm²/V.s. We will report our latest results on the molecular and supramolecular engineering of BTBT semiconductors, including: design by theory, synthesis, crystal engineering, calculation and observation of crystal morphology, and processing into single crystal thin films for transistor fabrication.

Nanowire morphology combined with one-dimensional (1D) diffusion pathways is advantageous for the enhanced electrochemical performance of materials in a wide range of applications. In this work, we for the first time report the performance of highly ordered Na-2x3 and Na-2x4 tunnel manganese oxide (TuMO) nanowires with 1D crystallographic channels for the hybrid capacitive deionization (HCDI) of water. TuMOs are a family of relatively low cost, sustainable, and highly electrochemically active materials. In addition, the size of the tunnel opening can be modified by altering the nature of the stabilizing ion and hydrothermal treatment parameters of the precursor materials. The Na-2x3 and Na-2x4 tunnel dimensions refer to the number of MnO₆ octahedra on perpendicular sides of the tunnel walls, and the ordered nature of these structures is revealed via scanning transmission electron microscopy (STEM). Thus, TuMOs provide a unique platform to investigate the relationship between the size of the 1D diffusion channels and their electrochemical performance, including potential selectivity for removing various ions from aqueous solutions.

The homogeneity of tunnel sizes was achieved through improving crystallinity of the layered Na-birnessite precursor material via extended aging and control over the pH of the solution during hydrothermal treatment. The Na⁺ ions that stabilize the tunnels have increased radii in basic solutions that result in increased tunnel dimensions [1]. In addition, a study that investigated ion transport properties demonstrated that sodium preferentially diffuses into the larger tunnel sizes [2]. Therefore, the ion removal performance of Na-2x3 and Na-2x4 TuMO nanowires is investigated in a variety of solutions to gain insight into relationships between cation hydrated radii, charge, and electrode ion removal capacities.

NaCl, KCl, MgCl₂ solutions, and mixtures of all three were analyzed. In sodium chloride solutions, the Na-2x3 and Na-2x4 electrodes demonstrated ion removal capacities as high as 20 mg g^-1. Previous research conducted on TuMOs with an irregular distribution of tunnel sizes and incomplete formation of tunnel walls demonstrated higher ion removal capacities up to 45 mg g^-1. This data indicates that structural defects can be a vital factor in determining performance [3]. In addition, the highly ordered nanowires showed capacity retention as high as 90% after 20 cycles compared to 100% in the disordered material. To further understand the relationship between tunnel size and HCDI performance, both Na-2x3 and Na-2x4 tunnel structures were tested with a mixed ion solution containing all three chloride salts: NaCl, KCl and MgCl₂. Our results indicate that Na-stablized TuMO electrodes may preferentially remove ions with larger hydrated radii from solution as demonstrated by a 40% drop in concentration of potassium ions compared with a 50% drop in concentration of magnesium ions. Interestingly, we observed that the concentration of sodium ions in mixed solution increased after cycling, indicating that stabilizing Na⁺ ions can be released from the structural tunnels of TuMO electrodes. This result suggests that there is interesting ion dynamics between stabilizing sodium ions and ions electrochemically removed from the mixed solution. In summary, this work contributes to the overall scientific understanding between ion transport and tunnel structure within manganese oxide materials used as electrodes in HCDI for water desalination.

References:
[1] Shen, et al. Advanced Materials 17 (2005): 805.
[2] Yuan, et al. Joule 3 (2019): 471.
[3] Byles, et al. Nano energy 44 (2018): 476.

A dense array of vertically aligned indium antimonide (InSb) nanowires with high aspect ratio (diameter ~150-200 nm, length 20 µm) were grown in the pores of a track-etched polycarbonate membrane via a one-step electrochemical method. There are several reports on InSb nanowire growth in the pores of mechanically rigid, nano-channel alumina template (NCA), where nanowire growth occurs in the pores of the NCA. However, non-uniform pore size across the thickness of the template and the roughness within the pore tubules translate to the nanowires, resulting in them having non-uniform diameters and rough surfaces. In contrast, InSb nanowires grown in polycarbonate (PC) membranes appear to have uniform diameters and smooth surfaces. However, transmission electron microscopy images show that despite their smooth surfaces, InSb nanowires grown in PC membranes are polycrystalline and in some cases even amorphous. A possible nanowire growth mechanism explaining the variation in crystallinity based on the template type will be presented in this work. The electronic quality of the nanowires was assessed by fabricating a back-gated InSb nanowire field effect transistor (NWFET), with a channel length of 1 µm. Analysis of the transport measurements show the nanowires have high resistivity (4 Ω cm), and a majority hole carrier concentration of the order of 10¹⁶ cm⁻³.

Bottom-up fabrication holds great commercial potential due to the cost-effectiveness of saving material. Solution-based fabrication methods are particularly attractive due to their scalability, low cost, and mild operating conditions. In this respect, electrodeposition using scanning probe microscopy provides a unique platform. Scanning probes allow for additive nanoscale manufacturing under ambient conditions, and additionally provide 3-dimensional spatial freedom which enables the fabrication of structures that are hard to realize in conventional cleanroom environments.

We report on the direct writing of Copper and Cobalt nanostructures using pulsed mechanical and electrical techniques in both an Atomic Force Microscope (AFM) and a Scanning Tunneling Microscope (STM). We demonstrate the writing of arbitrary structures beyond the sizes characteristic of single clusters and with lateral dimensions down to tens of nanometers, which can be read in-situ using the scanning probe. The extent of the deposition is interpreted based on the charging dynamics of ions in the liquid, determined by fundamental constants of the electrolyte such as the concentration, dielectric constant, and diffusion coefficient.

Due to the wide variety of electrochemical deposition and conversion processes, the technique of controlled electrochemical deposition with a scanning probe can be extended to different materials and morphologies. As such, we envision this technique to enable the manufacturing and rapid prototyping of novel devices at the nanoscale such as optical resonators, magnetic elements, and semiconductor devices.

Organic heterojunctions are widely used in electronics and well ordering in the molecular packing inside the heterojunctions is highly desired. However, it is still challenging to interface organic single crystals to form single-crystalline heterojunctions. In this work, two novel methods will be introduced to describe how to prepare, from solutions, single-crystalline heterojunctions. On one hand, organic single-crystalline planar heterojunctions are grown in a single step from a mixed solution of p-type and n-type molecules. Based on crystallization of six organic semiconductors from a droplet on a substrate, two distinct crystallization mechanisms have been recognized in the sense that crystals form at either the top interface between the air and solution or the bottom interface between the substrate and solution. The preference for one interface rather than the other depends on the semiconductor-substrate pair and, for a given semiconductor, it can be switched by changing the substrate, suggesting that the preference is associated with the semiconductor-substrate molecular interaction. Furthermore, simultaneous crystallization of two semiconductors at two different interfaces to reduce their mutual disturbance results in the formation of bilayer single crystals interfaced together for organic heterojunctions. On the other hand, single crystals are prepared in organogels of a typical conjugated polymer. Instead of pure crystals, crystals containing the gel nanofibers are obtained. Essentially, nanofiber networks are incorporated into growing crystals and penetrate through the crystals, resulting in a crystal/gel-network interpenetrating composites. Based the two types of single-crystalline heterojunctions, charge transport and charge transfer process are studied. Hence, by demonstrating these two solution methods for the preparation of single-crystalline heterojunctions, this work should greatly harvest the highly ordered organic crystalline materials for high performance electronics.

The application of nanoscale oxide semiconductors for optical applications is growing, which drives research that seeks to modify and tune the electronic behaviour for technological applications. ZnO is a wide bandgap (3.37 eV) semiconductor with a large exciton binding energy (60 meV) at room temperature, making it a good candidate as a transparent conductive oxide and as part of optoelectronic-based or piezoelectric devices. For ZnO nanowires (NWs), the options for growth are vast, including pulsed laser deposition (PLD), chemical vapour deposition (CVD), molecular beam epitaxy (MBE) or chemical synthesis. Among them, the latter represents a cheap and simple route towards the production-scale synthesis of high quality single crystal ZnO NWs. Typically, the chemical route is a two-step process: first, a seed layer is deposited followed by the NW growth using chemical bath deposition (CBD). The CBD process allows tuning of the electronic properties via the introduction of doping agents to the growth solution that can be incorporated within the ZnO NW lattice during growth.

In this work, the optical properties of Co and Cu-doped ZnO NWs were studied by photoluminescence (PL) and fast-transient absorption spectroscopy (FTAS) with temporal resolution of 50 fs using a pump-probe method. The doping levels used were 1%, 5%, 10% and 20% in order to study how the doping levels affected the optoelectronic properties of ZnO NWs. The NWs were around 4 µm long and 120 nm in diameter.

For undoped NWs, the FTAS measurements show a sharp absorption bleaching at 3.3 eV with bandgap value comparable to those largely reported in the literature. As the cobalt doping was increased, a red-shift in the bandgap bleaching appeared (resulting in bandgap reductions from 3.3 eV to 2.95 eV), we also see that the bandgap bleaching has become broader. Co-doping modifies both spectral and dynamic properties of the ZnO NWs. The defect band became more prominent with increasing doping levels. This may be attributed to the modification of surface states in the presence of Co. Thy dynamics of bandgap bleaching also becomes faster with increasing Co doping. In the case of Cu-doped samples, a similar red-shift was obtained for the bandgap bleaching. The crystallinity and crystalline structure of the NWs was also demonstrated by x-ray diffraction and transmission electron microscopy to show that the original wurtzite phase of the ZnO NWs was maintained after doping. These experiments prove that by including a certain amount of dopant species, the optoelectronic properties of the ZnO NWs can be tuned, opening up a host of new and improved applications in which bandgap engineering and defect states control are key, such as selective gas sensing or photoelectrochemical water splitting.

Wet chemical synthesis provides a simple yet scalable route to synthesize nanomaterials with controlled size and shape. Tuning size and shape of nano crystal often leads to enhancement of various properties which could be very different from their bulk counterpart. Many exciting properties of the nanocrystals arise due to their higher surface area, hence shows excellent catalytic property. Noble metal nanowires are extremely important from fundamental and application point of view. However, anisotropic growth (1D here) in case of high symmetry material (specifically FCC crystal like Au, Ag, Pt) is not trivial to achieve. But, in these systems the inherent symmetry can be broken by using suitable capping agent or template . Though ultrathin Au nanowire of 2nm diameter has been synthesized and studied extensively, achieving Pt nanowire of such dimension remains challenging due to its intrinsic isotropic nature. Herein, we report synthesis and mechanism of foration of ultrathin Pt nanowire which has been achieved via a simple solvothermal method without making use of any special atmosphere.

As synthesized Pt nanowires have a diameter of 2-3 nm and lengths in microns. HRTEM images of these wires confirm that these are single crystalline in nature and interestingly the growth direction is <110> which is different from that of Au nanowire, where the growth direction is <111>. Theoretical calculation suggests the nature of binding of capping agent could be responsible for such type of growth direction.To understand the formation and growth mechanism of these nanowires, several control experiments have been performed. TEM analysis of the quenched samples at an earlier stage reveals that initially bigger particles (7-8 nm) form which eventually break down to smaller monodispersed particles of size 2-3 nm. The breaking of larger particles to smaller ones (in presence of excessive capping agent) can possibly be attributed to digestive ripening. Further these monodisperse particles could undergo oriented attachment to form the 1D structure. It has been observed that the presence of capping agent plays a critical role in the kinetics of the reaction.

Since these nanowires are only few atoms thick, they show very interesting electrical transport property. Thermal stability of these wires has been studied by in situ TEM heating as well as ex situ heating. Moreover, these wires show superior thermal stability as compared to Au nanowire of similar dimension and hence can be used as a promising candidate for interconnects. These wires have been further converted to bimetallic nanowires like PtPd and PtCu by using Pt nanowire as template. Electrocatalytic studies on these single crystalline bimetallic nanowire shows better performance.

Platinum (Pt) has been proven to be the most efficient candidate for the cathode catalyst of proton exchange membrane fuel cells (PEMFCs). However, the intrinsically sluggish kinetics and ultrahigh price of Pt severely limit the widespread application of PEMFC technology. Incorporating Pt with less expensive transition metals to form stable bimetallic PtM alloys, where M = Fe, Co, Ni, Cu, etc., has been considered as an effective approach to address these issues. This not only largely reduces Pt consumption but also enhances the activity of Pt-based electrocatalysts due to possible synergistic effects and electronic and lattice shrinking/strain effects. In addition, a nanodendrite (ND) structure with open skeletons can endow Pt-based electrocatalysts with higher atomic utilization and higher active surface area. Therefore, the design and synthesis of PtM alloys with highly open structure is crucial for fuel cells reaction applications.
Here, we report a highly universal approach for preparing PtM bimetallic NDs with M = Fe, Co and Ni by solution phase synthesis. By reacting Pt(acac)₂ and M(acac)_x at elevated temperature, we achieve NDs with a fcc crystal lattice. The as-prepared PtM NDs have a polycrystalline structure and are rich in steps and kinks on the surface, which ensure the PtM alloy has excellent bifunctional catalytic properties in acidic electrolytes for both the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR). The ORR and MOR activities of the PtM NDs both show a sequence of PtCo/C > PtFe/C > PtNi/C > Pt/C. We conclude that the development of a novel strategy to prepare various types of bimetallic or ternary and Pt- or Pd-based alloy NDs can advance fuel cell applications by improving electrocatalytic performance.

Hierarchically porous structured materials with multifunctional properties and higher order complexities are highly desirable for many applications such as separation and energy storage. Here we describe the generation of hierarchically porous organic and inorganic structures coupling block copolymer self-assembly with spatially- and temporally-controlled laser irradiation. A simple and rapid laser irradiation of block copolymer-directed hybrid films with a continuous wave laser in the sub-millisecond timescales enabled synthesis of 3D mesoporous polymer structures and shapes. Backfilling the polymer template with amorphous silicon followed by pulsed laser annealing enabled transient melt transformation of amorphous precursors into 3D mesoporous crystalline silicon nanostructures. Mechanistic studies on laser-induced crystalline silicon nanostructure formation during the nanosecond silicon melt-crystallization process and polymer template stability at temperatures above 1200 degrees Celsius are highlighted.

In this paper, we report the successful fabrication of a superlattice film composed of PbS colloidal quantum dots (QDs) with three-dimensionally-coincident crystal orientations. Semiconductor solar cells using QD superlattice has been expected to realize energy conversion efficiency of 70% or more owing to the intermediate energy bands. The chemically synthesized colloidal QDs self-form densely-packed structures when they are deposited on a substrate in solvent. However, since the QDs can move freely on the substrate, grain size of the packed structures is on the order of micrometers. Long carrier transportation is effective to improve energy conversion efficiency of solar cells. We have proposed a method to realize a large area superlattice structure by restricting the free motion of QD using pyramidal microhole array as a template produced by anisotropic wet etching. In this study, faceted QDs of truncated octahedral shape were prepared and deposited on the template of Si(100) substrate. During the deposition, facets of neighboring QDs came into contact with each other. The three-dimensional crystal orientation alignment was confirmed by X-ray pole figure mapping. QD superlattice film with aligned crystal orientation are expected to further improve carrier transportation. We found that emission lifetime of QDs became longer after the formation of superlattice film, and that the usage of template extended the lifetime of the film especially at the excited states. The emission decay curves were double exponential at the excited state, one of which was a few nanoseconds and the other was several tens of nanoseconds or more. The results suggested that two type regions, where the intermediate bands were formed or not, were mixed in the film.

Control over the conformation and packing of conjugated materials is an unsolved problem that prevents the rational design of organic optoelectronics, such as preventing self-quenching of luminescent molecules. Exacerbating this challenge is a general lack of widely applicable strategies for controlling packing with discrete, directional non-covalent interactions. Here, we present a series of conjugated molecules with diverse backbones of three or four arenes that feature pentafluorobenzyl ester substituents. Nearly all the compounds reveal intramolecular stacking interactions between the fluoroarene (ArF) side-chains and nonfluorinated arenes (ArH) in the middle of the chromophores; a twisted PE linkage accompanies each example of this intramolecular ArF–ArH stacking. Furthermore, these molecules can resist dramatic changes to emission upon transition from organic solution to thin film when ArF rings prevent interchromophore interactions. Not only does this design paradigm apply to fluorophores, but we also demonstrate its utility in inducing solid state phosphoresence in platinum acetylide small molecules. By broadening the structural space of conjugated backbones over which ArF–ArH stacking can twist PE linkages reliably and prevent self-quenching of solids with simple synthetic approaches, this work suggests fluorinated benzyl ester substituents adjacent to phenylene ethynylene linkages as supramolecular synthons for the crystal engineering of organic optoelectronic materials

Hydrogen (H₂) production using the suitable semiconductors through photocatalytic water splitting process is a much sought-after technology to reduce the greenhouse gases and to further contribute to clean solar hydrogen energy production [1][2]. To date, it is still a big challenge to search for a stable semiconductor photocatalyst responding to wide-ranged light illumination. Recently, methylammonium lead halide perovskites have emerged as promise candidates for varieties optoelectronic devices, because of their low-cost, large carrier mobilities, long carrier life time and tuneable bandgap [3]. Moreover, recent advancements have demonstrated that their extraordinary optical and electrical properties make this class of perovskites a promising mediums for the high-performance photocatalysts beyond the optoelectronic device technologies. However, Most of the present works regarding the perovskite photocatalysts usually employed the hybrid perovskites in the form of thin polycrystalline films [4][5], and little effort is paid to apply the perovskite single crystals on photocatalysis, which is aiming at splitting water and generating H₂. Here we synthesized MAPbBr₃ single crystals by Inversed Temperature Crystalization (ITC) method and used them to fabricate the photocatalyst for water splitting with simple device structure, which exhibits high photocurrent, high H₂ evolution activity, and good stability.

Reference
[1] Chi-Hung Liao, Chao-Wei Huang, and J. C. S. Wu, “Hydrogen Production from Semiconductor-based Photocatalysis via Water Splitting,” Catalysts (2073-4344), vol. 2, no. 4, pp. 490–516, Dec. 2012.
[2] S. Chen, S. S. Thind, and A. Chen, “Nanostructured materials for water splitting - state of the art and future needs: A mini-review,” Electrochemistry Communications, vol. 63, pp. 10–17, Feb. 2016.
[3] Y. Liu, Z. Yang, and S. F. Liu, “Recent Progress in Single-Crystalline Perovskite Research Including Crystal Preparation, Property Evaluation, and Applications,” Advanced Science, vol. 5, no. 1, p. 1700471, Jan. 2018.
[4] Gurudayal et al., “Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting,” Nano Lett., vol. 15, no. 6, pp. 3833–3839, Jun. 2015.
[5] J. Luo et al., “Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts,” Science, vol. 345, no. 6204, pp. 1593–1596, Sep. 2014.

Contamination of the drinking water by heavy metal ions is a frightful problem for present and future generations. These contaminants are difficult and expensive to detect and remove and their lethal effect on the human health is of a long term and it’s impossible to trace it immediately. The permissive concentrations for the water in the environment are specified in 10^-6 mol/L to 10^-9 mol/L by WHO, whereas the detection limits of general chemosensors were around 10^-7 mol/L. Thus, the detection properties leave something to be desired. To aim at development of molecular crystal-based chemosensor for highly ion-sensitive materials for optode, curcumin was chosen as molecular core structure because of its high absorbance and emission properties and fine coordination properties with heavy metal ions. We synthesized curcumin-based mesogenic molecules with hydrophobic tethers (1) and hydrophilic tethers (1’). Upon addition of heavy metal ions to the oligomeric assembly of 1 in the solution, UV/vis absorption spectra were more obviously changed than those of monodispersed state. The fluorescent quenching of the assembly was also detected efficiently upon metal ion addition, suggesting that the oligomeric 1D molecular crystals contribute to high detection properties for optode. For metal ion sensing properties of oligomeric 1’ will also be discussed.

The study of cocrystal formation has seen a great deal of attention in the past decade. Cocrystals have important applications as pharmaceutical compounds and in the development of organic electronics. However, for a given set of coformers, different cocrystal stoichiometries may exist, and multiple polymorphs are also more likely than in single component crystals. We have recently developed a model to describe the equilibrium formation of multiple stoichiometries under various crystallization conditions such as evaporation. By tracing the path through the phase diagram, we can identify the conditions under which individual stoichiometries will form. We present work on applying this model to explain the spatial distribution of different cocrystal stoichiometries in a drying film due to the changes in concentration along the contact line.

The thin-film morphology of conjugated polymers regulated by coating conditions can significant impact on electrical properties in organic field-effect transistors. Here, we analyze thin-film morphology of blade-coated poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) thin-films varying with the solvent and coating temperature. In order to achieve P(NDI2OD-T2) thin-films revealing large polymer domain, elongated packing structure and predominant face-on orientations, the consideration of pre-aggregation of P(NDI2OD-T2) in solution and the amount of thermal energy from coating temperature is crucial during film formation. The direct investigation of evolution of film formation is highly desirable with in situ measurements. By combining in situ UV-Vis spectroscopy and in situ grazing incidence wide angle X-ray scattering, we observed five stages which appear during blade coating. The insight will offer fundamental understanding of the relationship between film morphologies and charge transport properties as well as a potentially powerful method to control film microstructure of various organic electronics.

As a new class of crystalline porous materials, metal-organic frameworks (MOFs) have attracted broad attention for their fascinating chemistry as well as potential industrial applications as sorbent materials. However, the stability of MOFs towards water vapor and acid containing environments has been continuously criticized over the past decade and may well be one of the major bottlenecks for its many industrial deployments. Understanding the mechanism of the MOF degradation process may help us find the key towards increasing MOF stability in aggressive environments. In this talk, I will present our recent finding on the dynamic crystallization behavior of MOF particles confined in a polymeric capsule. By compartmentalizing MOF components within a polymeric boundary, the dissolution and crystallization processes are occurring simultaneously which led to a significantly extended MOF’s longevity under aggressive conditions. Such strategy may be extended as a universal method to improve the stability of MOF materials for industrial applications.

Two-dimensional (2D) organic crystals are receiving tremendous attention in next generation phototransistors for high-performing image sensor due to their potential color selective absorption and favorable electrical properties. However, organic crystals showing selective absorption in visible region have not been reported yet. Recently, we developed green color absorbing 2D single crystalline n-type material, 2F-4-TFPTA, which exhibited field effect electron mobility of up to 0.9 cm² V^-1 s^-1. 2F-4-TFPTA could form bilayer thick self-assembled 2D morphology due to the brickwork molecular packing structure with balanced intermolecular interactions along the two principal in-plane directions in the crystal. Based on the excellent optical and electrical properties, 2F-4-TFPTA single crystal exhibited decent photoresponsivity of 3.6 × 10³ A W^-1 under green LED irradiation and fast response time of less than 85 ms.

Lead halide semiconductors with perovskite crystal structure and APbX₃ stoichiometry [A = MA⁺, FA⁺, Cs⁺; X = I−, Br−, Cl− or mixtures there of] have recently become perhaps the most intensely studied class of inorganic optoelectronic materials. These materials are now widely used in light-emitting diodes, lasers, photo-detectors as well as in X-ray and gamma-ray (γ-ray) detectors. In these applications large single crystals (SCs) grown by precursors solutions of perovskites are mostly utilized because of low trap density and less grain boundary. Therefore, the rapid emergence of halide perovskites based single crystalline materials for wide variety of as mentioned applications has drawn much research attention in recent years. Though lead-based perovskites single crystals have made a significant contribution in the various applications in the photodetector and imaging application, many challenges are yet to be overcome. Limitations of lead perovskites such as toxicity of lead, easy decomposition in the presence of moisture, heat, and photo instability make them non eco-friendly and are eventually restricting their further development and commercialization potential. Moreover, Pb can accumulative in the human body causing several physiological disorders such as abdominal pain, constipation, and headaches. It is more important now to develop lead free and stable perovskite materials.

In our present study, we have reported the lead free perovskite single crystals grown by the slow cooling method. The synthesis, crystal growth, and detailed structural and optoelectronic characterization have been carried out by Optical microscopy, EDXA, single crystal XRD, UV-Vis, PL, Raman and TCSPC. The synthesized single crystals have orthorhombic structure and Pnma space group and the corresponding lattice parameters and reciprocal lattice parameters are calculated. The I-V characteristics and impedance spectroscopy of this lead free single crystal are also discussed. With all the measured characteristics, we can conclude that our synthesized lead free inorganic halide perovskite based single crystalline material is very interesting and potential candidate for the optoelectronics and photodetector applications.

Electrochromism (EC) in which the color tone changes reversibly by voltage application is well studied in various fields ranging from inorganic to organic, and polymer chemistries. Among them, organic EC materials possess a lot of advantages such as low environmental impact, easy to design, and rich properties in multicolor. In this research, we have focused on EC supramolecular gel materials which can gelate solvents and prevent from leakage of electrolytes solution. An amphiphilic naphthalene diimide derivative (1) having long alkyl chains and a pyridinium ion was synthesized for one-dimensional (1D) crystal. 1 has fine self-assembling and gelation properties through noncovalent interactions such as π–π interaction and van der Waals interaction. When a voltage was applied to 1 gel on an ITO electrode, the gel showed EC properties in which the color tone reversibly changed from pale yellow to deep brown for at least 3 times. EC properties of polymer blend film prepare from 1D molecular crystals of 1 and conventional polymers will also be discussed.

There are many efforts to achieve flexible or stretchable electronics based on organic semiconductors. A certain decrease in the π-conjugated structure of semiconducting polymer chains can drastically degrade the π-overlap driven chain rigidity and conductivity, which are important factors to enhance the intra- and inter-molecular charge-carrier transport in organic electronics. Diketopyrrolopyrrole (DPP)-based semiconducting copolymers can yield excellent charge-carrier mobilities in solution-based organic thin film transistors (OTFTs) ranged from 0.01 to 6.2 cm²V^-1s^-1, due to well-defined conjugated structures. However, the poor ductile conjugated structures of these DPP-based copolymers are hard to be used as semiconducting materials for flexible or stretchable devices. Here, we simply prepare solutions including a minor portion of high electric performance P29-diketopyrrolopyrrole-selenophene-vinylene-selenophene (P29-DPP-SVS) copolymers and a major portion of flexible regiorandom poly(3-hexyl thiophene)s (rr PHT) dissolved in chloroform respectively, and develop self-assembled structures of the P29-DPP-SVS in the semiconductor/semiconductor polymer blends to improve the ductility of film. On polymer-treated gate dielectrics, these blend films are spun-cast from the solutions optionally pretreated to induce better self-assembled structures of the semiconducting polymer in spun-cast films. The semiconducting polymer blend films show greatly improvement in stretchability of the resulting OTFTs, and they can show enhanced charge-carrier mobility and on/off current ratio values in OTFTs including a hole transport layer between Au electrodes and the blend films. These phenomena indicate that the simple blending of the semiconductor polymers can achieve unexpected high π-π stacking and stretchable structures in the spun-cast films, which improving the electrical properties and flexibility in the OTFTs.

Despite the rapid advancement of perovskite solar cells, the fundamental drawback of toxicity in lead based materials has largely limited their large-scale development and application. Hence, we developed a local temperature reduction induced crystallization (LTRIC) method to directly obtain a lead-free perovskite wafer (CH3NH3SnI3), which is 110 mm-thick, in an oil bath. The XRD diffraction peaks of the MASnI3 wafer are indexed to (001) and its parallel crystal planes, demonstrating its high orientation. The UV-vis-NIR discloses that the absorption onset of the MASnI3 wafer is red-shifted to 1015 nm, corresponding to a narrow bandgap of 1.21 eV.

Coordinated polymers are an important class of designer materials for broad set of applications including energy research and nano-patterning, where single atom substitution can have a dramatic impact on both structure and function. For our research, we have synthesized a novel series of coordinated polymers, which consisted of alkaline earth metals and nitrilotriacetic acid, in particular Ba-NTA, Sr-NTA, Ca-NTA, and Mg-NTA, and we used Raman spectroscopy, in particular low frequency (LF) Raman spectroscopy, as a sensitive and a nondestructive method for characterizing the materials based on their vibrational resonances. Since vibrational modes can originate both from the atomic vibrations of molecular bonds and from more distributed phonon modes of nanoscale structures and crystals, atomic mass changes can have a strong influence the observed vibrational spectra, as described by the harmonic oscillator model. Our goal was to study the effect of one atom replacement on the shift of the related Raman band. According to single crystal x-ray diffraction (XRD) measurements, Ba-NTA and Sr-NTA have the same crystalline structure despite their different central metals, while Mg-NTA and Ca-NTA have different crystalline structures. Raman measurements of the chemical fingerprints region (i.e. above 200 cm^-1) exhibited high similarity, however for the Raman band associated with vibration of the Metal-Oxygen bond, the lightest metals exhibited the highest frequency Raman shifts. This is expected since the lighter metals form the shortest bonds and consequently their vibrational energies are higher according to the harmonic oscillator model. We found the LF-Raman spectral range, which captures the inter-molecular interactions, to be more sensitive to differences between the compounds. Despite the Ba-NTA and Sr-NTA being highly similar and presenting almost identical regular chemical fingerprint Raman spectra, expanding the measurements to the LF-Raman region enabled us to distinguish between them. In addition, we conducted polarized LF-Raman measurements of Ba-NTA single crystals, which provided additional information on the assignment of specific LF-Raman bands to specific vibrational modes and crystallographic planes as determined by XRD.

My research group has focused recent efforts into using molecular simulations and liquid state theory in an integrated fashion to better understand and predict effects of polymer design on the resulting macromolecular material structure and thermodynamics. In this talk I will present these efforts in the context of polymer solutions. I will describe how we have used coarse-grained models, Polymer Reference Interaction Site Model (PRISM) theory and molecular dynamics (MD) simulations to study structure and effective interactions in polymer solutions with linear, cyclic and bottle-brush polymer architectures.

Poly(3,4-ethylenedioxythiophene) (best known as PEDOT) is the one of the most important and most studied conducting polymers due to its superior features such as high electronic and ionic conductivities, versatile optical properties, as well as stability against oxidants and air-humidity. Owing to these properties, PEDOT has been utilized for various applications such as photovoltaic cells, supercapacitors, organic electronic ion pumps, light-emitting diodes and electrochromic displays, field effect transistor and many others. While a number of papers that address various aspects of structural, optical, electronic and transport properties of PEDOT have been published, a few studies have so far focused on its polymerization mechanisms. Moreover, a theoretical understanding of thermodynamics and kinetics of the polymerization of PEDOT is completely missing. For example, it is generally accepted that PEDOT polymerizes into relatively short oligomers consisting of 10-20 monomer units. Factors that limit the chain length are not understood, and, to our knowledge, have never been discussed in the literature. Another important property of PEDOT is its oxidation level. In contrast to most other conjugated polymers, a pristine (i.e. as polymerized) PEDOT is heavily oxidized (p-doped). In most studies the oxidation level of pristine PEDOT is typically found to be 33%, i.e. one charge per three monomer unit. Currently, the understanding of this “magic number” of 33% is not available. Apparently, understanding of the factors that determine and limit the oxidation level during polymerization is instrumental for design and improvement of charge storage material and devices such as supercapacitors and batteries with the enhanced performance.

Motivated by the need for theoretical understanding of the thermodynamics and kinetics of the oxidative polymerization, we performed step-by-step density functional theory calculations of the oxidative polymerization of PEDOT:TOS. Using the density functional theory, we calculated the Gibbs free energy for the conventional mechanism that consists of polymerization of a neutral PEDOT oligomers first, followed by their oxidation (doping). We also corroborated the feasibility of alternative mechanism of polymerization in which already oxidized oligomers are used as reactants, leading to doped(oxidized) oligomers as products during polymerization. Our study, for the first time, provides a theoretical insight into PEDOT polymerization, which has been completely missing so far. Our focus of our study is understanding of the maximum oxidation level that can be achieved during polymerization. Our calculations provide a theoretical explanation of “the magic number” of 33% for the oxidation level typically reported for the pristine (i.e. as polymerized). Another result of our study is that the proposed alternative mechanism is likely to be more feasible for longer PEDOT oligomers (chain length N>6).

Molecular additives are often used to enhance dynamic motion and tune microstructures of polymeric chains, which subsequently alter the functional and physical properties of polymers. However, controlling chain dynamics and microstructures of semiconducting polymer thin films, and understanding the fundamental mechanisms of such changes is a new area of research. Here, cycloparaphenylenes (CPPs) are used as conjugated molecular additives to tune the dynamic behaviors of diketopyrrolopyrrole-based (DPP-based) semiconducting polymers. It is observed that the addition of CPPs results in significant improvement in the stretchability of the DPP-based polymers without adversely affecting their mobility. Specifically, the DPP-based polymer thin films originally form cracks at 15% strain, while the addition of 5 wt% CPP improves the stretchability to 105% strain, which arises from enhanced polymer dynamic motion and reduced long-range crystalline order. The polymer films retained their fiber-like morphology and short-range ordered aggregates, which lead to high mobility. Fully stretchable transistors are subsequently fabricated using CPP/semiconductor composites as active layers. These composites are observed to maintain high mobilities when strained and after repeated applied strains. Interestingly, CPPs are also observed to improve contact resistance and charge transport of the fully stretchable transistors. ln summary, the above results collectively indicated that controlling the dynamic motion and crystalline structures of polymer semiconductors proved to be an effective way to improve their stretchability.

Solution-processable polymeric semiconductors have received an increasing attention in the past two decades due to their wide range of potential applications in large-area, low-cost, flexible devices. In recent years, studies pertaining to 2D films of these semiconductors are gradually emerging owing to the development and wide recognition of 2D inorganic materials which exhibit incredible properties compared to their bulk counterparts. Analogous to the phenomenon observed in inorganic materials, reducing the dimension of polymer semiconductors films to 2D could potentially give rise to unique optoelectronic and mechanical properties. Moreover, the molecular level ultrathin body of the 2D conjugated polymer films can be incorporated in field effect transistors (FETs) to develop highly sensitive sensors for biologically relevant molecules. Despite their growing interest and utilization of a wide variety of solution processable methods, crystallization of these polymers into 2D films over large area still remain challenging. This is partially due to one dimensional nature of their intermolecular interactions as well as high conformational degrees of freedom of the polymer chains. Because 2D charge transport demands highly ordered 2D films, novel methods that enhance 2D polymer crystallization are needed. So far, fundamental questions of how electronic structure and charge transport properties depend on the number of molecular layers are still unclear. Methods for fabricating 2D layers with controlled morphology will lay the foundation for understanding these questions.

In this talk, we present dynamic-template integrated meniscus-guided coating as a new technique to fabricate 2D films of conjugated polymers over centimeter length scale using a wide processing window. Using two different polymeric systems, DPP2T-TT and PII2T, we demonstrate enhanced 2D crystallization on dynamic template leading to highly ordered and aligned 2D layer with edge-on pi-pi stacking. This is in contrast to 1D fiber network morphology obtained on static substrates. Our results show that while static substrates favor ordering of alkyl side chains in 2D films, dynamic template enhances ordering of the conjugated backbone with favorable interaction between the polymer backbone. As a result, charge transport was observed only in 2D films printed on dynamic template but not in monolayer films coated on solid substrates. In addition, we also observed a change in molecular packing from edge-on to bimodal when the film reaches a critical thickness of ~25 nm as well as an abrupt increase in J-aggregation, absorption coefficient and decrease in band gap and HOMO level until the critical thickness. This phenomenon was independent of the substrate and possibly arisen from straightened polymer backbone. Furthermore, we also observed an abrupt increase in hole mobility with film thickness reaching a maximum of 0.7 cm²V^-1s^-1 near the critical thickness which further corroborates with the drastic increase backbone planarity as film thickness increases. Finally we also fabricated chemical sensors using bilayer polymer films which demonstrated ultrahigh sensitivity < 1ppb to breath biomarkers.

In the past two decades, significant progress has been made in the fabrication of polymer-based devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) or organic photovoltaics (OPVs), predominantly due to important improvements of existing materials and the creation of a wealth of novel compounds. Many challenges, however, still exist: from achieving reliable device fabrication, increasing the device stability and, more fundamentally, gaining a complete understanding how structural features over all length scales affect important optoelectronic and photophysical processes in such polymers, including charge transport, charge generation, and general photovoltaic processes. Here we demonstrate how classical polymer science tools can be used to elucidate the structure development of semiconducting polymers from the liquid phase, how such knowledge can be exploited to manipulate their phase transformations and solid-state order and, in turn, their electronic features and device performances. To exemplify our approach, we discuss bulk heterojunction solar cells, which are comprised of active layers that are generally comprised of a complex phase morphology where intermixed and neat phases of the donor and acceptor material co-exist. We show that the fate of photogenerated electron/hole pairs –whether they will dissociate to free charges or geminately recombine– is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower. The local arrangement of the donor and acceptor plays a critical role in this phenomena as we demonstrate on a series of donor polymer:fullerenes binaries by combining 2D-NMR data with time-resolved ultra-fast spectroscopy results as well as detailed structural data. We discuss that similar considerations apply also to other functional blends, such as semiconducting polymer:dopant systems that can lead to highly conductive systems. The spatial arrangement of the polymer with the dopant, which is dictated by the blends phase behavior, strongly affects charge transport in analogy to the photovoltaic donor:acceptor systems. We will conclude with providing a tentative picture of the complex correlation of structure and electronic landscape for the understanding of organic photovoltaic cells as well as doped, conducting “plastics” of metallic-type transport – and beyond.

Plastic transistors do not only create an interesting field of science but also belong to a highly promising technology for future electronic devices, especially for field-effect transistors. Thereby, conjugated small molecules or polymers are used as organic semiconductors for solution deposition onto flexible substrates. The deposition methods are based on roll-to-roll printing such as inkjet or gravure, while the substrate consists of thermoplastic polymers such as PEN, PET or PMMA. The benefits of such printed organic transistors in comparison to their traditional silicon counterparts are mechanical flexibility and low cost fabrication. To reduce the production costs, new processing concepts are required. One approach is focused on the deposition of the substrate polymer together with the organic semiconductor from the same solution or melt. During solvent evaporation or after post-treatment a vertical phase separation between the isolating polymer and the semiconductor occurs^1,2. In the final bilayer structure, the semiconductor crystallizes on top of the polymer layer ensuring a charge carrier transport in the transistor. As further key factors, the molecular order, organization and film microstructure of the organic semiconductor are crucial for the charge carrier transport³.

This work acknowledges the Foundation for Polish Science financed by the European Union under the European Regional Development Fund (POIR.04.04.00-00-3ED8/17-00)

1. E. Dobruchowska, T. Marszalek, J. Ulanski, Thin Solid Films, 564, (2014) 361
2. T. Marszalek, M. Wiatrowski, E. Dobruchowska, J. Jung, J. Ulanski, J. Mater. Chem. C, 1, (2013) 3190
3. K. Zhang, T. Marszalek, P. Wucher, Z. Wang, L. Veith, H. Lu, H.J. Räder, P. M. Beaujuge, P. W. M. Blom, W. Pisula, Adv. Funct. Mater., 28, (2018) 1805594

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are at the forefront in enabling organic thermoelectric (OTE) devices for thermal energy management. The challenge lies in the judicious control of thermoelectric transport properties: electronic conductivity (σ), Seebeck coefficient (α), and thermal conductivity (κ). One approach considers functionally graded materials (FGMs) where one locally tunes the material properties across an operational temperature gradient. While FGMs has shown to improve the performance of inorganic thermoelectric materials, similar concepts have not been considered in the field of molecularly doped organic semiconductors. Here, we present the fabrication, structure and transport characteristics of poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno [3,2-b]thiophene] (PBTTT) thin film (ca. 30nm) with compositional functional grading control of the molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ). By using an in-house designed vapor doping process, we specifically fabricate a double – segmented PBTTT thin film consisting of two segments with different doping levels of F₄TCNQ. Electrical conductivity and structural characterizations (X-ray scattering and Raman spectroscopy) indicate the existence of an interface region of ca. 1000 mm between the two segments within which the F₄TCNQ concentration, polymer morphology, and conductivity gradually vary. The measured effective Seebeck coefficients at two sides of the two segments and across the interface match with known models describing transport in graded thermoelectric systems. Our results demonstrate the first successful fabrication and characterization of functionally graded organic thermoelectric materials, and thus providing guidelines to further develop more complex FGMs leveraging high mobility organic semiconductors and molecular doping.

It has been known that poly(3-hexylthiophene) (PHT), constituted of asymmetric 3-hexyl thiophene repeat units, is a benchmark semiconducting polymer for solution-processed organic electronics. High regioregular (RR) PHTs with a backbone rigidity due to the strict head-to-tail manners approach tends to grow into an ordered domain of π-π overlapping chains in spun-cast films, facilitating the charge-carries to move along the intra- and inter-chains to yield high hole mobilities. Generally, conjugated semiconductors can't have both high electrical performance and flexibility. In order to obtain a flexible device with high optoelectrical properties, researchers have proposed many approaches. For example, semiconducting RR PHTs were chemically modified with thiophene segments generated better π-conjugated and deformable film structures and some semiconducting polymers were physically blended with elastomers or amorphous polymers to achieve stretchable semiconducting films, etc. However, most of these strategies to realize stretchable organic semiconducting films suffer from limitations of available materials and tedious fabrication processes.
Here, via simple blending a high-RR PHT (with a regioregularity of 97%) with the chemically identical but regiorandom (rr) PHT (55%) without any assistance, the RR PHT chains from the rr PHT media and develop highly conjugated nanodomains in both solution and film. In the spun-cast blend film, the rr PHT matrix imparts enough deformability of the channel layer required for stretchable organic thin film transistors (OTFTs), compared to neat RR PHTs and blends with a deformable polymer. OTFTs including RR PHT/rr PHT blend film showed excellent hole mobility (μ) values up to 0.13 cm2 V-1 s-1, surpassing that of the best RR PHT films (0.026 cm2 V-1 s-1) fabricated by ultrasound solution pretreatment. Furthermore, a 50% stretched RR PHT/rr PHT film maintains ~55% of its μ value at no strain, while RR PHT film shows a sudden decrease in μ even at 10% stretch. The simple blending approach imparts deformability to π-conjugated polymer films for application in stretchable OTFTs.

The fabrication of polymer solar cells involves coating processes from a solution of electrically conducting donor and acceptor molecules, resulting in the photoactive layer. During the solvent evaporation, structures with a variety of length scales can be formed, the control of which is key to the high performance recently achieved for polymer solar cells. These structures are initiated through molecular self-assembly in which molecular interactions between donor, acceptor, and solvent have an important role. The choice of coating technique and post-fabrication annealing steps also have a significant effect on the final film morphology.
We will present one route towards estimating the molecular interactions between donor and acceptor molecules. With the help of a large set of solvents, the Hansen Solubility Parameters (HSP) of donor and acceptor are determined separately. Their distance in Hansen space presents an estimate for the mutual miscibility of donor and acceptor. From the HSPs, the Flory-Huggins interaction parameters can be determined, leading to a general strategy to control the donor/acceptor system’s tendency to phase separate.
Soft X-ray based spectro-microscopy techniques provide excellent nanoscale resolution and chemical contrast to resolve the nanostructure of the donor/acceptor bulk heterojunction, as well as the composition of the domains, for systems where the acceptor is a fullerene derivative. However, the transition to non-fullerene acceptors brings along new challenges for morphology studies, because orthogonal photon energies that correspond to X-ray absorption resonances for donor and non-fullerene acceptor molecules are more difficult to find. Taking advantage of distinct functional groups on the non-fullerene acceptor molecules, new strategies for the imaging of the nanoscale morphology are being developed, such as AFM-IR spectro-microscopy.
We will present results from our multi-disciplinary effort to generate fundamental understanding about how molecular interactions drive the structure formation at different length scales in solution-borne polymer blend systems and how these processes can be manipulated by external stimuli. We aim to use this knowledge to formulate general design principles for polymer-polymer structures, as well as to apply these to guide the development of novel materials with structures that are tailor-made to improve properties and functionality of optoelectronic devices.

Organic semiconductors typically exhibit a rich variety of ordered phases. Orientation occurs during processing, often by passing through lyotropic liquid crystalline phases, and dried films exhibit orientation at many length scales. Liquid crystallinity appears almost ubiquitous in organic semiconductors we have studied, including behaviors similar to nematics, smectics, or sanidics. Uniaxial orientation where the conjugated plane exhibits a preferential “face on” or “edge on” orientation is the most commonly characterized aspect of orientational order, and mature techniques such as near edge X-ray absorption fine structure (NEXAFS) spectroscopy and grazing incidence X-ray diffraction (GIXD) are frequently employed to measure it. Significant challenges remain, even for this seemingly simple aspect of orientational order. For example, these approaches are often found to be insufficient when films exhibit a complex depth profile of orientation, composition, or both.

As the materials diversity of organic semiconductors increases thanks to achievements in synthetic chemistry, and as the complexity of formulation, blending, and coating increases as process design knowledge matures, there is a commensurate need for more advanced approaches to characterizing orientational order. Biaxial orientation is increasingly observed, where “face on” or “edge on” preferences are combined with an in-plane preference imparted by a substrate or processing vector. Domain-relative orientations are also been observed, with correlations in domain-to-domain orientation thought to influence charge transport.

I will describe some emerging approaches to measure orientational order in organic semiconductors. Resonant soft X-ray reflectivity (RSoXR), a method capable of measuring thin film depth profiles using the spectroscopic principles of NEXAFS, will be discussed. I will also summarize progress on in-situ spectroscopic and scattering-based structure measurement techniques that are combined with ex-situ probes including imaging and image analysis to determine how orientation originates during the processing of organic semiconductors. These new measurements will support science-based approaches for orientation control and optimization, helping to maintain the rapid pace of organic semiconductor technology development in an era where increasing complexity renders matrix-style empirical approaches too inefficient to contemplate.

This presentation focuses on recent advances in growth control and oriented crystallization of (semi)-conducting polymers for organic electronic applications. The performance and lifetime of organic electronic devices are critically dependent on the morphology of the active layers and structural order of the materials. For instance both molecular and crystalline orientations of the polymers determine optical and charge transport properties in thin films since these properties are by essence highly anisotropic.

Particular emphasis will be given to the progress made in high-temperature rubbing of conjugated polymers films. This effective large scale alignment method can orient a large palette of semiconductors with n- or p-type character without the use of alignment substrate.¹ High degrees of crystallinity and in-plane alignment can be obtained which provide well-defined electron diffraction patterns essential for structure refinement. The concurrent roles of polymer molecular weight distribution and rubbing temperature (T_R) on the in-plane orientation are rationalized for P3HT and PBTTT.^2a Correlations are drawn between nanomorphology/crystallinity on one side and charge transport and optical properties on the other side. It is shown that the exciton bandwidth in P3HT crystals is determined by the length of the average planarized chain segments in the crystals. The high alignment and crystallinity observed for T_R > 200 °C cannot translate to high hole mobilities parallel to the rubbing because of the adverse effect of amorphous interlamellar zones interrupting charge transport between crystalline lamellae of semi-crystalline P3HT.^2b As opposite, hole mobilities along the polymer chains of rubbed PTB7 films are observed to be 6 times higher than the non-rubbed films.³ This is due to the smectic- like character of this alternated donor-acceptor copolymer. Interestingly high-Tr rubbing and post annealing process of PTB7 provide well-defined electron diffraction pattern. Combined with DFT calculation, this helps refining the structure and the chain conformation of this benchmark electron donor polymer for OPV application.
In a second part of this presentation, we show that soft doping of aligned P3HT yields highly oriented conducting polymer films with anisotropic charges and thermal conductivities. The thermoelectric properties are enhanced along the rubbing direction. The unique in-plane orientation in such conducting polymer films helps rationalizing the mechanism of redox doping.⁴



References.
1. M. Brinkmann et al. Macromolecular Rapid. Comm. 2014, 35, 9.
2. a) L. Biniek et al. Macromolecules 2014, 47, 3871. b) A. Hamidi-Sakr et al. Adv. Funct. Mater. 2016, 26, 408.
3. L. Biniek et al. Adv. Electron. Mater. 2018, 1700480
4. A. Hamidi-Sakr et al. Adv. Funct. Mater. 2017, 1700173.

The electrical properties of organic thin film transistors (OTFTs) are strongly influenced by the structural characteristics and thin film morphology. Understanding the growth dynamics that lead to preferential molecular orientation and changes in crystal structure remain an important factor in the design of new high-performance OTFTs as well as in their fundamental studies.
In this contribution, we have investigated the thin film growth behavior of 5,5′-Bis(naphth-2-yl)-2,2′-bithiophene (NaT2) and 5,5′′-bis(naphth-2-yl)-2,2′:5′,2′′-terthiophene (NaT3) during the deposition process using real-time grazing incidence X-ray diffraction (GIXRD) in situ. The thin films were prepared by vacuum sublimation atop various substrates, including monolayer graphene on 90 nm SiO₂, in a custom-built ultra-high vacuum (UHV) chamber with a 360° cylindrical beryllium window that allows for in situ X-ray measurements within realistic deposition time scales. The crystal structure analysis revealed that the preferred orientation of the molecules were dictated by the substrate and that the unit cell underwent significant changes when transitioning from monolayer to multilayer structure. The changes of the unit cell were not readily observed when measuring thin films of corresponding thicknesses ex situ, suggesting that the molecules undergo further re-organization/relaxation upon terminating the deposition. From the evolution of the crystal structure, a connection to the film growth mode and kinetics is made. These findings are rationalized based on the surface energies of the studied substrates and supplemented with AFM and helium ion microscopy. Finally, the crystal structure and morphology are correlated with charge transport properties of the final thin films.

The evolution of organic electronics has now reached the commercial phase, with the recent market introduction of the first prototypes based on organic transistors and organic solar cell modules fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfil their potential. Synthesis of conjugated aromatic polymers typically involves carbon coupling polymerisations utilising transition metal catalysts and metal containing monomers. This polymerisation chemistry creates polymers where the aromatic repeat units are linked by single carbon-carbon bonds along the backbone. In order to reduce potential conformational, and subsequently energetic, disorder due to rotation around these single bonds, an aldol condensation reaction was explored, in which a bisisatin monomer reacts with a bisoxindole monomer to create an isoindigo repeat unit that is fully fused along the polymer backbone. This aldol polymerization requires neither metal containing monomers or transition-metal catalysts, opening up new synthetic possibilities for conjugated aromatic polymer design, particularly where both monomers are electron deficient. Polymers with very large electron affinities can be synthesised by this method, resulting in air stable electron transport, demonstrated in solution processed organic thin film transistors. We present an electrical, optical and morphology characterisation of polymer thin films, illustrating structure-property relationships for this new class of polymers.

Carrier mobility in conjugated polymers continues to increase with recent reports of field-effect mobilities exceeding 10 cm²/V.s. Charge transport is intrinsically dependent on processes occurring across multiple lengthscales. In order to access order parameters at the molecular scale we use charge modulation spectroscopy combined with theory. Furthermore, we study the mesoscale organization of polymers using a new technique in the transmission electron microscope. These techniques are used on homopolymers and donor-acceptor copolymers. Such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport in conjugated polymers.

Doping is a fundamental strategy to increase the carrier density and conductivity of organic electronic layers for opto-electronic devices. The model system for p-doping is poly(3-hexyl)thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ), whereby increases in conductivity occur through integer charge transfer (ICT) to form mobile carriers in the P3HT. Extensive work in the organic electronics field has revealed the interplay between film microstructure, the opto-electronic properties, and conductivity.

This talk asks the simple question: How stable are ICT states in doped films? An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, trap-like state for the hole on P3HT. Starting first with our recent observation that both ICT and CPX states exist simultaneously in a single film, this talk will focus on thermodynamic and kinetic pathways that lead to conductivity loss in F₄TCNQ-doped P3HT films. Both reaction chemistries and microstructural effects will be discussed.