Our development of ultra thin liquid cells for transmission electron microscopy (TEM) allowing imaging through liquids with sub-nanometer resolution has opened many opportunities to directly observe growth, shape evolution and interaction of nanoparticles in solution in real time. By tracking single particle growth trajectories, we observed two types of growth such as the classic growth of monomers attachment and the aggregated growth by nanoparticle attachment. The critical role of surfactant effects on shape control has been identified. Studies reveal novel growth mechanisms, which assist to elucidate many years&’ debates. By tracking nanoparticle movements, we have also quantified nanoparticle interaction and distinguished the force fields exerted by single nanoparticles and nanoparticle chains. Such quantification and understanding of nanoparticle growth and interaction may be proven to be important for the design of hierarchical nanomaterials and controlling nanocrystal self-assembly for functional devices.

This presentation will focus on the shape controlled synthesis and assembly of anisotropic nanoscale building blocks. Rare earth doped lanthanide nanophosphor plates and nanorods make ideal models for studies of shape directed assembly allowing both single component systems and collections of complimentary shapes to be explored. Experimental procedures for the synthesis and structural characterization of nanoplate and nanorod assemblies will be shared. An example of binary self-assembly of two anisotropic nanoplate building blocks mediated by shape complementarity will be presented as a case study along with the first example of an inorganic rod sphere co-assembly. We will focus on liquid interfacial assembly techniques that allow the formation of self-assembled single component as well as the formation of binary superlattices from two anisotropic nanocrystals over a micrometer length scale. In these studies of shape-directed self-assembly the shape (faceting) and ligand interactions guide the position of each anisotropic nanoplates to form superlattices which exhibit long-range orientational and positional order. The design of shape complementary anisotropic building blocks offers the possibility to self-assemble binary superlattices with predictable and desirable structures. The potential for further development of the shape complimentary assembly in 2D and 3D will be discussed.

One of the biggest challenges that the nanotechnology faces is the in-depth analysis of on-particle self-assembled monolayers (SAMs). While the structure of SAMs has been extensively studied for flat surfaces, the extension of these techniques to curvilinear nanoparticle (NP) surfaces has proven exceptionally troublesome and is currently highly debated. At present, the structure of a NPs' ligand shell can only be hypothesized, often with large amounts of guidance needed from theoretical analysis. Nonetheless, a better understanding of the structure of on-particle SAMs through experimental observation has the potential to lead to advances in hybrid organic electronic and single molecule devices, sensors, catalysis, etc.
We have employed the very commonly used proton nuclear magnetic resonance (¹H NMR) spectroscopy to analyze a large set of chemically distinct thiol-stabilized gold nanoparticles in solution with an unprecedented level of detail. Specifically, we prove that the 'broad peaks' often observed in ¹H NMR spectra of NPs are due to distributions of protons which are molecularly identical yet chemically distinct due to their surrounding environments. Having proven this point, we are then able to demonstrate how ¹H NMR can be exploited to gain further insight into to the composition, configuration, and kinetic mobility of on-particle SAMs through an entirely non-destructive technique. By precisely defining the ¹H NMR pulse sequence parameters necessary for these types of analysis, we believe that such a technique has great potential in becoming a prominent tool in nanotechnology research and could be rapidly implemented within the research community.

Self-assembly, as a natural phenomenon, is the autonomous organization of components into patterns or structures without human intervention, which is common in nature with complex, functional, and self-assembled structures such as opals and magnetosomes. Progressive achievement in colloid chemistry makes it feasible to prepare many classes of well-defined nonspherical nanocrystals. Lead sulfide (PbS), is one kind of promising materials in many aspects: photovoltaics, thermoelctrics and infrared sensors. These applications require these PbS nanostructures in the form of controllable assemblies with desired functions.
Here, I will present our recent progress on the self-assembly of PbS nanocubes into unexpected face-centered-cubic (fcc) superstructures.1 The use of synchrotron-based small angel X-ray scattering (SAXS) technique together with electron microscopies provides us a powerful tool to perform this study.

(1) Quan, Z.; Siu Loc, W.; Lin, C.; Luo, Z.; Yang, K.; Wang, Y.; Wang, H.; Wang, Z.; Fang, J. Nano Lett. 2012, 12, 4409.

In this talk I will highlight recent progresses in the characterization of gold nanoparticles coated with a binary mixture of dis-like molecules. These advancements will include true high resolution images as well as diffraction and spectroscopic analysis.
Particles that show stripe-like domains as well as particles with Janus arrangements will be presented. These particles have shown unique properties ranging from non-monotonic dependence on composition of the solubility and the interfacial energy, as well as unique molecular recognition properties. A subclass of these particles has shown the ability to cross cell membranes in an energy independent way. In this talk a mechanism for this process will be proposed together with a set of novel properties.

One-dimensional charged gold nanoparticle chains can be reversibly assembled and disassembled in highly controllable manners by manipulation of nanoparticle colloidal interactions. External stimuli including ionic strength change and heating/cooing which can affect on the colloidal interactions were employed as the driven forces for the assembly-disassembly process. The reversibility of this process was ensured by appropriate surface modification of particles with special charged ligands which can provide strong short-range repulsion when particles are in close contact. The as-assembled nanoparticle chains can also be incorporated into polymer matrices and responsively disassembled by external mechanical stress. As the results, the gold nanoparticle-polymer composite is expected to be useful as a colorimetric indicators for pressure.

The incorporation of ultrasmall nanoparticle (UNP) building blocks (le; 3 nm) into mesoscale architectures during pulsed laser deposition (PLD) is investigated. It is well known that clusters and nanoparticles are formed by the interaction of pulsed laser ablation plumes with background gases, however their role in the synthesis of thin films and nanostructures has not been made clear. In this study, TiO₂ is primarily studied due to its extraordinary optical and photocatalytic properties. However, the application of TiO₂ in different functional devices requires control over their physical architecture and crystalline structures. Therefore, we primarily study the synthesis conditions, size distribution, and structural evolution of TiO₂ UNPs. Various types of TiO₂ nanostructures including fractal, nanorods and thin films with unique crystalline phases are demonstrated. Temporally- and spatially-resolved gated-ICCD imaging is employed as an in situ diagnostic to understand and control the plume expansion conditions for synthesis of TiO₂ UNPs. Atomic-resolution transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and E-beam diffraction patterns are used to characterize the stoichiometry and atomic structure of individual and incorporated UNPs synthesized at different experimental conditions. Raman spectroscopy was used to determine the types and structures of the deposited nanostructures on the substrates. These findings provide an insight into the controlled growth of nanoarchitectures with desired structures and functionalities.

Direct observations of solution-phase nanoparticle growth using in situ liquid transmission electron microscopy (TEM) have demonstrated the importance of mesoscale interactions, such as aggregation and coalescence, on the growth and final morphology of nanocrystals at the atomic and single nanoparticle scales. To date, groups have quantitatively interpreted the mean growth rate of nanoparticles in terms of the LSW model for Ostwald ripening, but less attention has been paid to modeling the corresponding particle size distribution. Here we use in situ fluid stage scanning TEM to demonstrate that silver nanoparticles grow by a length-scale dependent mechanism, where single particles grow by monomer attachment but ensemble-scale growth is dominated by aggregation. Although our observed mean nanoparticle growth rate is consistent with the LSW model, we show that the corresponding particle size distribution is broader and more symmetric than predicted by LSW. Following direct observations of aggregation, we interpret the ensemble-scale growth using Smoluchowski kinetics and demonstrate that the Smoluchowski model quantitatively captures the mean growth rate and particle size distribution. Our observations of length-scale dependent nanoparticle growth emphasize the need to quantify and correlate the contributions of growth mechanisms at different length scales to help tune the overall properties of functional nanostructures, biomineralized nanocrystals, heterogeneous catalysts, and other complex nanoscale ensembles formed by mesoscale interactions.

The degree of chromophore coupling is inevitably a crucial design parameter in all photosystems, whether natural or artificial. In some instances, the best case scenario is band transport, which implies delocalization across the entire ensemble of chromophores. In other cases charge motion should occur only after a specific photophysical event has occurred, such as multiple exciton generation (MEG) or carrier-carrier thermalization (e.g., for creation of a hot-carrier distribution). Rational design of quantum dot solar cells requires that the delocalization of the initially-prepared exciton, and the subsequent effects of this delocalization, be well understood. For example, can band transport occur even if photoabsorption and primary photophysical processes occur in a quantum-confined or highly localized regime? What is the ideal amount of delocalization for these processes to be the most efficient yet provide the opportunity for subsequent fast extraction of charges? In order to answer this question, the size of excitations versus time after photoabsorption must be known. We have recently used ultrafast cross-polarized transient grating spectroscopy to investigate the degree of coherent exciton delocalization in CdSe and PbSe quantum dot (QD) arrays on a femtosecond and picosecond time scale. We have found that the technique provides a quantitative measure of exciton size, which exceeds twice the actual physical diameter of the QD in some cases. In addition, we have discovered how particular chemical treatments of the QD surfaces enhance the delocalization length. The effective delocalization length also affects secondary photophysical processes like Auger recombination, measured by transient absorption spectroscopy. Connections with longer time scale events such as transport resulting in conductivity can be made with available literature reports and theory.

Fundamental studies of magnetic alignment of highly anisotropic mesostructures can enable the clean-room-free fabrication of flexible, array-based solar and electronic devices, in which preferential orientation of nano- or microwire-type objects is desired. In this study, ensembles of 100 µm long Si microwires with ferromagnetic Ni and Co coatings are oriented vertically in the presence of magnetic fields. The degree of vertical alignment and threshold field strength depend on geometric factors, such as microwire length and ferromagnetic coating thickness, as well as interfacial interactions, which are modulated by varying solvent and substrate surface chemistry. Microwire ensembles with vertical alignment over 97% within ±5° of normal, as measured by X-ray diffraction, are achieved over cm2 scale areas and set into flexible polymer films. A force balance model has been developed as a predictive tool for magnetic alignment, incorporating magnetic torque and empirically derived surface adhesion parameters. As supported by these calculations, microwires are shown to detach from the surface and align vertically in the presence of magnetic fields on the order of 100 gauss. Microwires aligned in this manner are set into a polydimethylsiloxane film where they retain their vertical alignment after the field has been removed and can subsequently be used as a flexible solar absorber layer.

Porous materials such as activated carbons, zeolites, silica gels, and inorganic oxides are of great interest because of their potential applications in adsorption, catalysis, separation, purification processes, optics, electronics, etc. Among these materials, ordered mesoporous carbons have attracted considerable attention owing to their high surface area, tunable pore structure and narrow pore size distribution. On the other hand, nitrogen doped carbons have shown enhanced mechanical, thermal and electrical properties; therefore, there are increasing effort in making of N-doped mesoporous carbon, which combine the high porosity of the mesoporous framework and unique chemical functionality. N-doped mesoporous carbons are often prepared by nanocasting method using a sacrificial template, usually porous silica. The template is impregnated with nitrogen containing organic molecules, followed by carbonization and removal of silica template or through post-synthesis treatment of mesoporous carbon using acetonitrile or ammonia CVD. These multiple-steps processes are costly and time consuming.
In this work, we report successful synthesis of nitrogen-doped mesoporous carbon through block polymer soft templating method. By designing modified-pyrrole monomer and with the use of molecular co-assembly and oxidative polymerization during sol-gel preparation, ordered mesoporous polymer at mesoscale was achieved. Synthesis of mesoporous polymer is often challenging due to macrophase separation and polymer demixing. Nitrogen rich mesoporous carbon was resulted from carbonization of the respective polymer without structure collapse. High surface area and nitrogen loading enable its application in carbon capture and electric double-layer capacitors.

The robust and facile fabrication strategies for surface-enhanced Raman scattering (SERS) active substrates are important for developing practical SERS sensors. As comparing to traditional SERS substrates such as roughened metallic surface or randomly distributed metal nanoparticles, the highly ordered SERS substrates with controllable geometries are essential to obtain large and stable Raman enhancement with reproducibility. Several approaches to creating dielectro-metallic nanostructures have been developed for the SERS application, including the fabrication of metal-shell nanoparticles and bimetallic nanowires. For the practical SERS application, the individual plasmonic nanostructures are required to integrate as one- or two dimensional arrays with highly periodicity and uniformity. However, most attempts recently made have focused on fabricating dielectric-core metal-shell particles, which resulted in somewhat irregular structures due to the difficulties in precisely controlling the geometry and in positioning the individual units at a desired location.
Here, we report a novel SERS platform composed of highly periodic Au or Au-Ag /chitosan hybrid nanopatterns. The chitosan can not only play a role as the matrix for Au nanoparticles adsorption but also can be readily manipulated into well-ordered patterns via soft nanolithography. The fabrication strategy employing chitosan nanopatterns will provide reliable reproducibility and good sensitivity for SERS based detection of DNA hybridization.

Layer-by-layer (LBL) assembly has come to the forefront in recent years as a reliable method of producing very thin films. LBL can be used for deposition of a variety of organic and inorganic materials; it can produce layers with thicknesses in the nano to micro meter range; it is cheaper and more efficient than comparable methods of producing very thin films. LBL is especially useful for the production of composite materials with a well-ordered laminate structure.
Nacre, also known as mother of pearl, is the material produced by some mollusks for the interior layer of their shells. This material has attracted much attention as a model for engineered materials of superior strength and toughness. The outstanding properties of nacre stem from its unique organic-inorganic layered morphology. The structural component of nacre is made of aragonite tablets, which are connected by thin layers of protein. This layered structure easily dissipates strain energy, and shows a gradual mode of failure, which produces the superior mechanical properties of the material.
In order for the superior properties of nacre to be realized in a synthetic system, the highly ordered character of the natural system must be maintained. Many different methods of producing synthetic nacre-like structures with high strength and toughness have been attempted in the literature, with varying success. Some of the systems which have produced the largest improvements in toughness are based on an LBL technique. However, LBL deposition of inorganic nanoplatets has not yet been explored.
Titania nanoplatelets are of interest for use in this type of layered system because they can be synthesized with aspect ratios similar to those of the aragonite tablets in nacre. Also, it has been shown that titania anatase nanoplatelets display a preferentially oriented attachment mechanism which may be useful in the deposition of highly oriented films.
In this research, we present nanocomposite thin films produced via alternating LBL deposition of titania nanoplatelets and polyelectrolyte. An outline of LBL composite film production procedure will be provided. Characterization of the resulting thin films via techniques like SEM, UV/vis, AFM, and nanoindentation will be discussed. The effects of polyelectrolytes with differing zeta-potentials will be investigated and the effect of variation in film thickness and structure will be analyzed.

The controlled arrangement of functionalized high-permittivity TiO2 nanoparticles in polymer matrix is vital to achieving optimum optical and dielectric properties. An approach to selectively sequester and organize nanoparticles in polymer matrices is the use of nanostructured block copolymers (BCP: PS-b-PMMA) as templates with nanoscale domain spacings of the order of asymp;30 nm. To achieve the desired dielectric properties afforded by the TiO2 nanoparticles, it is necessary to have effective filler-matrix interactions and dispersion. To facilitate the interaction of nanoparticles with the domains, we have synthesized using “grafting to” and “grafting from” PS-g-TiO2 and PS-b-PMMA-g-TiO2 nanoparticles, respectively. Characterization results by FTIR, 1H NMR, GPC and TGA showed that the copolymer chains are covalently bonded to the surface of TiO2 particles. Time dependent dispersibility study of the synthesized PS-b-PMMA-g-TiO2 nanoparticle demonstrated remarkably good dispersability of functionalized nanoparticles in organic solvents resulting from surface grafting of polymer. The TGA analysis indicated higher thermal stabilities for functionalized nanoparticles in comparison with pristine polymers. Thin films of BCP with and without functionalized nanoparticles were cast and processed using dynamic thermal annealing and solvent vapor processing conditions. We examine highly oriented block copolymer cylinders fabricated at industrially relevant dynamic thermal annealing speeds in films over a wide film thickness range, from 100nm up to 1mu;m. Results on the impact of grafted-NP fillers on morphology of BCP and ability to tune the morphology with the composition and film thickness so as to formulate functionalized nanoparticles sequestered confined 3-D polymer geometries for potential dielectric applications will be reported.
Acknowledgements: U.S. Air Force of Scientific Research under contract FA9550-12-1-0306

Surface-enhanced Raman scattering (SERS) has attracted extensive research interest due to its high sensitivity and applications for the detection of chemical and biological analytes down to single molecule levels. SERS offers finger print signatures of molecules, allowing positive identification of analytes, and has been regarded as one of the most promising sensing platforms for assessing food and environmental safety. Herein, I will present results on our recent advances in the fabrication of highly sensitive and reproducible SERS substrates via conducting polymer assisted spontaneous deposition of metal nanoparticles without the use of strong reducing agents or organic solvents. With this newly developed synthetic platform, nanostructured metals with a wide range of sizes, structures, and morphologies have been fabricated. After careful examination of their SERS responses, we have been able to establish structure-property relationships between the SERS response and the metal particle morphology. Our results warrant further development of highly sensitive SERS substrates and a better understanding of the underlying SERS response associated “hot Spots” that were previously not accessible through conventional fabrication techniques.

Arrays of molecularly capped nanoparticles are a unique material system with versatile chemical, optical, and electrical properties. Interest stems from both their potential as controllable artificial solids, allowing research into interfacial self-assembly and nanoscale charge transport, and their potential applications including gas-phase molecule sensing, nanoscale filtration, photo-active conductance switching, and others. The versatility of nanoparticle arrays derives from the tunability of particle size, capping molecules, and assembly processes. This versatility allows control of the electrical characteristics of the arrays. Charge transport through molecule-nanoparticle hybrid monolayers is expected to occur via thermally activated hopping, where energy barriers between individual nanoparticles are determined by the molecules interlinking the particles. The length and height of these barriers can be systematically controlled by changing the length and energy gap of the molecular ligands, thereby controlling the transport efficiency. In this work we investigate particle synthesis, interfacial assembly, array structure, and the electrical properties of these complex materials.
Oleylamine-capped Au nanoparticles are synthesized in toluene and deposited on a water surface for self-assembly. Particle synthesis, stability in the solvent, and deposition variables such as water surface curvature and evaporation rate all influence assembly and array formation. Control of these variables has enabled the reproducible formation of monolayers of molecule-capped gold nanoparticles, enabling characterization of charge transport through these two-dimensional structures.
After molecule-nanoparticle arrays form on the water surface, microcontact printing with patterned PDMS stamps is used to transfer arrays to substrates for SEM imaging or electrical characterization. Four-point probe measurements are used to determine the conductance and contact resistance of the hybrid arrays as a function of array area. Post assembly molecular exchange in solution allows further control of interparticle tunneling barriers, and we have demonstrated increasing array conductivity with the decreasing length of introduced molecules, and reversible exchange between two array conductivities determined by interparticle molecular length.

We demonstrate a fast and scalable method of particle assembly to produce organized micron and nanoparticle structures. By utilizing a patterned electrode and electrophoretic deposition, we show assembly of particles in limitless configurations with control down to the single particle level. Multi-material deposition on the same 2-dimensional platform was made possible by electric field manipulation, enabling this technique to produce mesoscale materials of compositions that are normally not possible by techniques that rely on entropic and electrostatic forces to dictate the final structure. Precise positioning of particles has wide implications for a variety of applications such as in device fabrication, controlled crystallization, and tuning of materials properties.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Self-assembly (SA) is a spontaneous collective process among component building blocks via weak interparticle interactions, e.g. Van der Waals, capillary, hydrogen bonding, etc. SA can be observed in a variety of size regimes from molecular to macroscopic size. Molecular self-assembly, nanoparticle self-assembly, and colloidal self-assembly are the typical examples of the self-organization of molecules, nanoparticles, and colloids, respectively. SA is prevalent phenomena, therefore, it exists already in numerous natural objects and creatures.
Recently, there are increasing numbers of applications using semiconductor quantum dots (QDs). Typically, assembled or aggregated form of QDs has been studied for the fundamental and applied researches, which is practically cost-effective way for a number of applications if conserved the unique quantum properties. Generally, it is expecting that the collective energetic properties can be affected by the ordering manner and/or size of component QDs. However, a limited study has been accomplished on this topic for the assembled QDs (QD superlattices).
Therefore, in this study, we firstly present unique emission properties on semiconductor QD superlattices (SLs) using time-resolved photoluminescence (TRPL) microscopy at a single particle level. Briefly, PL lifetime imaging technique clearly reveals that different shaped QD SL microcrystals have different PL lifetimes. Compared to single QD particles, the QD SL crystals show obviously shorter PL lifetimes. Furthermore, the faceted SL crystals consisted of well-organized QDs showed shorter emission lifetimes than those of the spherical microparticles with randomly-organized QDs, which can be explained by different degree of energetic coupling among component QDs within a single SL. Moreover, we have also tried to conserve the shape and collective properties of the QD SLs by simply atomic layer ZnO coating thereon. We expect that the fundamentals of the observed unique emission properties would provide useful information to manipulate and enhance the performance of devices, using SA QDs, of solar cells, optoelectronics, light emitting devices, etc.

Inorganic nanomaterials have attracted broad attention due to their electrooptical properties. Among other inorganic materials, ZnO stands out due to its band gap in the range of semi-conductors. This property has led to diverse applications in electronic devices, such as light emitting diodes (LED), thin film transistors (TTFT), liquid crystal displays (LCD) and as contact material in solar cells.
In this work, ZnO nanoparticles, which were synthesized with an average diameter of 5 nm and ZnO nanorods were functionalized with cationic porphyrins equipped with a carboxylate and catechol anchor group. Our functionalization protocol gave rise to positively charged surfaces, which come with the advantage that agglomeration is prevented due to electrostatic repulsion. In this way, we were able to obtain nanomaterial solutions that were stable over 4 weeks. By using transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential and UV/Vis measurements, we were able to determine the size and shape of the resulting particles, and gain insight into the functionalization degree.
Finally, we used our positively charged ZnO-porphyrin nanoparticles to prepare a two-component multi-layer material by the so called “layer-by-layer” technique. To achieve this, we immersed etched quartz slides alternately into a cationic ZnO-porphyrin solution and into an anionic gold nanoparticle solution. These multi-layers were characterized via TEM and UV/Vis measurement.

The development of nanoparticle fabrication utilizing laser ablation, metal vapor synthesis and chemical reduction methods has gained lots of interest. Especially for silver nanoparticles (Ag-NPs), they exhibit special and tunable optical properties through controlling its shape and size.
This work demonstrates the feasibility of fabricating size-controlled Ag-NPs by photocatalytic reduction within confined space. Photocatalytic materials were deposited on glass substrates and then the large-scale uniform patterns were fabricated by polystyrene (PS) nanosphere lithography. Silver nitrade solution was applied on the sample under UV light irradiation and Ag-NPs formed in the repeated patterns. In order to control the shape and size of reduced Ag-NPs, PS beads were tailored by O2 plasma treatment resulting in various size of holes so that Ag-NPs could form in the holes with different size and exhibit various visible colors. The characteristics of Ag-NPs were analyzed by ultraviolet-visible spectroscopy (UV-Vis), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Faceted metal nanoparticles (NPs) are central to numerous research efforts in the fields of catalysis, plasmonics, and nanomaterials growth. Due to the strong dependence of chemical and physical properties on NP shape, much effort has been devoted to producing homogeneous NP shape populations using both synthetic and post-synthetic methods. While progress has been made towards controlling the shape of metal NPs during synthesis, the residual presence of spurious shapes still remains a challenge. Centrifugal sorting schemes to further refine as-produced NP shape distributions have shown great promise among NPs with starkly different aspect ratios or masses, particularly for separating rod-shaped NPs from spherical NPs or sorting NPs of different aggregation states. However, the goal of sorting polyhedral NPs possessing similar sizes and aspect ratios has remained elusive due to their comparable sedimentation coefficients. Here, we demonstrate the ability to centrifugally sort AuNPs by shape using the surfactant cetyltrimethylammonium bromide (CTAB) that preferentially binds to Au(100) facets, thereby providing differences in the sedimentation coefficient as a function of AuNP shape. The resulting tailored sedimentation coefficients enable AuNP shape sorting via density gradient centrifugation (DGC). DGC-refined populations of {100}-faceted AuNPs are shown to significantly enhance the growth rate of InAs nanowires when used as seed particles, emphasizing the importance of shape control for nanomaterials growth applications. Similarly, DGC-based separation of sub-100 nm, {110}-faceted gold nanostructures (i.e., rhombic dodecahedra (RD) and triangular BPs) enables the isolation of highly enriched solutions of BPs and RD as verified by scanning electron microscopy and optical spectroscopy. Due to the strong local field enhancement from the sharp tips of the BPs, the sorted BP sample provides 2.5 times higher refractive index sensitivity than the as-synthesized, unsorted mixture. These ensemble measurements are further corroborated by single-particle localized surface plasmon resonance (LSPR) spectra with the distribution of the plasmon resonance energies of the sorted particles quantified via dark-field optical microscopy.

The propensity of nanoparticles to self-assemble is well recognized although the mechanisms of these processes may not be completely understood. The understanding is important because these processes (1) allow us to produce nano-, micro-, and macroscale structures with fascinating optical and other properties; (2) highlight similarities between nanoparticles and similar biological species; and (3) represent the pathway for scalability. Self-organization also represents an untapped resource for energy efficient green manufacturing of electronic, photonic, and sensing devices. The formation of a large variety of 1D, 2D, and 3D superstructures from nanoparticles can be understood as a balance of attractive and repulsive interactions. The large variety of forces between and geometries of NPs makes theoretically possible engineering sophisticated multi-NP complexes, which will be supported by latest results exemplified by artificial viruses and superchiral plasmonic systems. I will conclude by the discussion of future challenges in nanoparticle self-organization and the pathway to applications.

Nanocomposite thin films containing hierarchically ordered 3D nanoparticle (NP) arrays exhibit new properties to meet the growing material demands for advanced technology. However, translating NP assemblies in bulk nanocomposites to thin films is not trivial because NPs tend to segregate on the film surface to minimize entropic penalty from polymer chain deformation and the surface tension. We recently overcame the difficulties of NP surface segregation in thin films using a block copolymer (BCP)-based supramolecular approach. The non-covalently bonded small molecules on the polymer chains effectively modulate NP/polymer enthalpic interactions and the chain architecture of BCPs. In thin films where cylindrical supramolecules are oriented parallel to the surface, alkyl-ligand-capped NPs selectively localize in the interstitial regions, resulting in hexagonally packed 3D arrays in the interior of the films. Different from that observed previously in thin films of coil-coil BCPs, the coil-comb chain conformation of the supramolecule magnifies the entropic contributions, offsetting the other energetic contributions in the nanocomposite. This allows for hierarchical 3D NP assemblies with sub-10 nm inter-particle distances in thin films. Varying NP size relative to the BCP periodicity further enables us to tune the entropic contributions in the co-assemblies, leading to 1D chain, 2D lattices and 3D arrays/networks of not only a single kind but also NP mixtures in supramolecular thin films. Moreover, the spatial resolution of the hierarchically structured 3D NP assemblies is tunable at the molecular level over macroscopic distances, which remains difficult to be achieved by lithographic techniques and DNA linkers. The supramolecular approach provides not only a powerful platform to test theoretical predictions of nanocomposite properties but also a versatile method to precisely tailor the coupling between the 3D NP arrays containing a wide range of elements on the periodic table for next-generation devices.

Pressure sensors have been developed by using direction related change of surface plasmon resonance of gold nanorod assemblies. Gold nanorods are randomly distributed in polyvinyl alcohol, which can experience the pressure of sapphire anvil cell used for transferring external force. When the force is applied to the polymer, gold nanorods are more inclined in the direction perpendicular to the force. If the incident light is parallel to the force, the ratio between transverse mode and longitudinal mode in surface plasmon resonance absorption of gold nanorod assemblies will decrease. The relationship between plasmon resonance ratio and pressure has been established as the basis of pressure sensors. Considering the deformation of polymer is related to time, plasmon resonance ratio variation with time is also investigated.

Nanoparticles of different functional materials can self-assemble from colloidal solutions into long range ordered periodic structures (superlattices). Through a series of systematic studies of self-assembly phenomena in single- and multicomponent nanoparticle assemblies we demonstrate that self-assembly of nanoscale objects is guided by an intricate interplay of entropy-driven crystallization an interparticle interactions, such as van der Waals and dipolar forces. The surface ligands also play an important and sometimes counterintuitive role. For example, organic ligands with long hydrocarbon chains introduce many-body interactions involving multiple nanoparticles and stabilizing low density structures. Finally, on the example of tetrahedral CdSe nanocrystals, we will show that non spherical shape introduces rotational entropy as an important additional term to the free energy balance.
Epitaxial heterostructures play a key role in electronics and optoelectronics. In a close analogy, performance of nanocrystal based devices depends on the perfection of interfaces formed between nanocrystal layers. We systematically studied the epitaxial growth of nanocrystal superlattices and revealed an exceptional strain tolerance of nanocrystal epitaxy reveals. It follows a universal island size scaling behavior and shows a strain-driven transition from layer-by-layer to Stranski-Krastanov growth regime with non-trivial island height statistics. Kinetic bottlenecks play an important role in nanocrystal epitaxy, especially in the transition from sub-monolayer to multilayer coverage and the epitaxy of nanocrystals with anisotropic shape.

Self-assembly of nanoparticle can greatly improve the charge transport properties of solution-processes materials for electronics. These processes are often investigated in water but not in polar ionic liquids that can offer technological advantages for solution processed electronics. Self-assembly of thioglycolic acid (TGA) stabilized cadmium telluride (CdTe) nanoparticles in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) ionic liquid demonstrated spontaneous formation of hierarchical supraparticles with a diameter reaching several hundreds of nanometers self-limited in dimensions. High-resolution TEM images revealed that individual CdTe nanoparticles assembling with BMIMPF6 and form continuous crystal lattice reminiscent of plumber&’s nightmare structure. Intermediate stages involve initial formation of smaller uniform agglomerates that later transform into chains and then collapse into larger supraparticles. Strong interaction between negative thiol group of CdTe nanoparticles and positive BMIM+ group is believed to be the driving force of supraparticles formation. The produced CdTe-BMIMPF6 possess strong fluorescence indicative of retained quantum confinement.

The self-assembly of nanoparticles can lead to clusters with complex but predictable shapes and properties that can be exploited for new materials. Here we show how defined numbers of particles interact in a confined volume until a minimum-energy situation is reached. This yields regular, anisotropic supraparticles [1]. We use oil-in-water emulsions where the oil phase contains unpolar nanoparticles. This is a variation of a well-known route to microparticle superstructures originally introduced by Pine and Manoharan, who created Pickering emulsions using polymer microspheres [2].
In a typical experiment, monodispersed, sterically stabilized metal nanocrystals with diameters below 10 nm were introduced into the disperse phase of an oil-in-water emulsion. Upon removal of the dispersed solvent from the emulsion droplets, the particles formed cluster-like “supraparticles”. The superstructures formed from the nanoparticles were characterized using dynamic light scattering (DLS), surface plasmon resonance (SPR), and transmission electron microscopy (TEM). Interestingly, the particles did not arrange into small pieces of dense packings, but resembled clusters predicted for particles interaction with Lennard-Jones potentials.
The size distribution of the supraparticles depends on the concentration of the nanoparticles in the dispersed phase and on the droplet size distribution of the emulsion. A narrow distribution of supraparticles requires emulsions with uniform droplet sizes. Conventional shear emulsification does not yield sufficiently narrow droplet distributions. Microfluidic devices offer an alternative route with better definition by producing one drop at a time.
We show first results using a droplet generator based on the work of Weitz and others [3] that can produce emulsions with a diameter below ten microns and a polydispersity of approximately three to five percent. The dispersed phase was injected into the continuous through a fine capillary positioned directly in the axis of the tube carrying the continuous phase. At the capillary tip, single drops of oil detach and form droplets in the continuous phase [3]. Size and size distribution of the resulting emulsion were characterized by optical microscopy and DLS.
1. Lacava, J., Born, P., and Kraus, T., Nanoparticle Clusters with Lennard-Jones Geometries. Nano Letters, 2012. 12(6): p. 3279-3282
2. Manoharan, V.N., Elsesser, M.T., and Pine, D.J., Dense Packing and Symmetry in Small Clusters of Microspheres. Science, 2003. p. 483-487
3. Umbanhowar,P.B., Prasad, V., and Weitz, D.A., Monodisperse Emulsion Generation via Drop Break Off in a Coflowing Stream. Langmuir, 2000. 16: p. 347-351

Heterostructuring of semiconducting quantum dots (QDs) with zero-band gap materials such as graphene is of huge importance for photovoltaic devices and chemical sensors. However, the major challenge remains to be integration of metal, graphene, and quantum dots in a spatially-resolved spherical architecture. Here, we demonstrated a large area patterning of inorganic hybrids comprised of graphene shell encapsulated gold nanoparticles (GNPs) tagged with CdSexS1-x/ZnS core/shell quantum dots. This was achieved by combining CVD growth of graphitic shells around uniformly patterned gold nanoparticles on a Si substrate. Furthermore, the covalent chemistry of the quantum dots and graphitic shells (< 3 nm in thickness) was utilized to tag former on the surface of GNPs. The patterned 3-D architectures were studied using electron microscopy, Raman spectroscopy, and FTIR at every step of fabrication to analysis crystal structures, surface chemistries, and morphologies. A complete band diagram and band edge location of the inorganic hybrid was derived, which showed that graphitic shell play a critical role as electron sink. Such novel inorganic hybrid assemblies of nanoparticles on a substrate are promising for sensing and bioanalysis applications.

Silicon (Si) is a promising candidate for lithium ion battery anodes because of its high theoretical capacity. However, the large volume changes during lithiation/delithiation cycles results in pulverization of Si and loss of contact of conductive (typically carbon) material with the electrodes, leading to rapid fading of capacity and loss of Coulombic efficiency. Here, we report a simple anode fabrication technique that is designed to overcome many of the limitations that deter more widespread adoption of Si based anodes. We confine Si nanoparticles in the oil phase of an oil-in-water emulsion stabilized by carbon black (CB). These CB nanoparticles are both oil- and water-wettable. The hydrophilic/hydrophobic balance for the CB nanoparticles also causes them to form a network in the continuous aqueous phase. Upon drying this emulsion on a current collector, the CB particles located at the surfaces of the emulsion droplets form cages that loosely encapsulate the Si particles that were in the oil. The CB particles that were in the aqueous phase form a conducting network connected to the CB cages. Half-cell experiments using this Si/CB anode architecture show a specific capacity of ~1300 mAh/g Si + C, and a Coulombic efficiency of 97.4% after 50 cycles. Emulsion-templating is a simple, inexpensive processing strategy that directs Si and conducting CB particles to desired spatial locations for superior performance of anodes in lithium ion batteries.

Superlattices of gold and silicon nanocrystals were studied by small angle X-ray scattering as they were heated. Grazing incidence SAXS data have revealed that superlattices of organic ligand-stabilized gold nanocrystals can undergo a surprising series of ordered structure transitions at elevated temperature. For example, a body-centered cubic (bcc) superlattice can evolve into a hexagonal close-packed (hcp) structure, followed by the formation of binary simple cubic (sc) AB13 and hexagonal (hex) AB5 superlattices. These transitions are driven by controlled nanocrystal growth during heating and the structures that form exhibit qualitative similarity with microphase-segregating diblock copolymers. Binary superlattices of larger silicon nanocrystals combined with smaller gold nanocrystals were also formed. When heated, the gold nanocrystals coalesce and segregate out of the the simple hexagonal (sh) AB2 binary superlattice to leave a simple hexagonal superlattice of silicon nanocrystals. This structure results from the high thermal stability of the silicon nanocrystals compared to the gold nanocrystals.

Self-Assembly has been employed as a very useful method for the fabrication of functional materials from nanoscale building blocks. New properties unavailable from individual nanoparticles are often created through the collective effects such as interparticle coupling and structural ordering. An important consequence of the formation of such secondary structures is that the properties of nanoparticle ensembles can be dynamically controlled by manipulating the assembly and disassembly behaviors, making them excellent candidates for constructing stimuli-responsive or smart materials. In this presentation, I will use a number of examples recently developed in my group to demonstrate that dynamic assembly and disassembly processes can be utilized as powerful tools to construct functional optical materials that can effectively respond to stimuli such as solvent, temperature, and electric and magnetic fields.

Creating tailored responsive polymer structures constitutes one of the major challenges of modern material science. Stimuli responsive materials, based on homo- and block copolymers, which react upon changes in pH, temperature or light are well studied and so far exhibit only slow response to the various stimuli. Considering the high demand on fast acting systems for sensor applications and bionic materials there is a need for systems that exhibit a quick and reversible response. These requirements might be met by introducing microgels as building blocks for fast responsive polymer nanostructures. Microgels show quick responses to temperature changes and can be tailored chemically in a straightforward way. By integrating metallic nanoparticles into the polymeric network additional physical properties can be achieved. The swelling and deswelling of microgels in response to external stimuli reveals itself in more or less isotropic size. Motion along specific directions, as in muscle-like contraction, would necessitate anisotropic response of the microgel. one-dimensional microgel nanostructures, e.g., single and double chains, rods and meshes are attainable by crosslinking well-defined particle arrays. Therefore, highly ordered self-similar wrinkles are used as a template to guide self-assembly of microgel particles. These nanoscaled substrates are prepared by oxidization of stretched poly(dimethylsiloxane) (PDMS). Subsequently, microgels are spin-coated onto these surfaces, forming self-assembled structures within the wrinkle grooves. Transfer of these pre-aligned particles onto silicon wafers yields microgel arrays with well-defined spacings covering large surface areas up to 2 x 2 square cm, as confirmed by GISAXS measurements. UV- crosslinking and redispersion of the arrays yields microgel chains with variable lengths up to 100 µm and widths of 500-2000 nm which are tunable by altering the spacing of the template. By integrating gold nanoparticles into the polymeric network heat can generated within the microgels due to plasmon resonance. This photo-thermal trigger avoids heat convection which inevitably would slow down the response time and enables triggering the anisotropic contraction by laser light.

We demonstrate a semi-automatic electrophoretic deposition (EPD) process to fabricate a colloidal crystal in large area (10×10 cm2). In our work, the polystyrene (PS) microspheres (150-1000 nm) could be assembled on an ITO substrate in highly-ordered arrangement with tunable layer/thickness. To render a desirable inverse opaline structure, the colloidal crystals are served as a template to allow for the filling of selective materials (Au, Ni, Bi2Te3, ZnO, Cu2O, C) in the interstitial voids among the template, followed by the removal of the PS template. Since the template reveals excellent structural integrity, the inverse opal film could be removed from the ITO substrate, resulting in a free-standing porous film. Materials characterizations including XRD, nano-indentation, porosity measurement, SEM, electrical conductivity, and α-step have been performed.

Colloidal nanocrystals exhibit a wide range of size and shape dependent properties and have found application in a myriad of fields such as optics, electronics, mechanics, drug delivery and catalysis to name but a few. In this talk, I will elaborate a general strategy for synthesizing a large variety of functional nanocrystals (nanoparticles and nanorods) with precisely controlled dimensions, compositions and architectures by using star-like and bottlebrush-like block copolymers as templates. This new class of copolymers forms unimolecular micelles that are structurally stable under various experimental conditions and therefore overcomes the intrinsic instability of linear block copolymer micelles. Our approach enables the facile synthesis of organic solvent- and water-soluble nearly monodisperse nanocrystals with desired composition and architecture, including core/shell and hollow nanostructures. Mesoscale self-assembly of nanocrystals driven by irreversible solvent evaporation and directed-assembly of nanocrystals in the linear diblock copolymer matrix rendered by flow-enabled self-assembly over large areas are also explored.

We will describe our recent efforts in constructing solid-state electrochromic devices with near-infrared (NIR) switching capabilities. The working electrode in these devices consists of Sn-doped In2O3 (ITO) nanocrystals arranged in precise, 3-D architectures to facilitate infiltration by polymer electrolyte. By electrochemically tuning the nanocrystals&’ state of charge, their plasmon absorption band in the NIR can be switched from a NIR-blocking state to a NIR-transmissive state. Tunable periodic architectures, with spatially-segregated conductive frameworks for ion and electron transport, allow for the enhancement of coloration efficiency and transport capabilities in comparison to unarchitectured nanocrystal films. This optimized approach to new SMART window technology is envisioned to allow for more effective management of solar heat gain in commercial and residential spaces, thereby realizing greater future energy savings.

Ferrimagnetic inorganic nanorods have been used as building blocks to construct liquid crystals awith optical properties that can be instantly and reversibly controlled by manipulating the nanorod orientation using considerably weak external magnetic fields. Under an alternating magnetic field (5 mT), they exhibit an optical switching frequency above 100 Hz, which is comparable to the performance of commercial liquid crystals based on electrical switching. By combining magnetic alignment and lithography processes, it is also possible to create patterns of different polarizations in a thin composite film and control over the transmittance of light in particular areas. Developing such magnetically responsive liquid crystals opens the door towards various applications, which may benefit from the instantaneous and contactless nature of magnetic manipulation.

Showing the high potential of applications in electronic, biotechnology, energy, etc., nanotechnology has been developed as an important technique for recent researches. Silver nanoparticles are extraordinary efficient at absorbing and scattering light, revealing various colors depending on the size and shape of the nanoparticles. Nevertheless, to apply silver nanoparticles to extensive applications, lots of efforts should be made to control the shape or size of silver nanoparticles and to functionalize their surface.
In this study, to manipulate size and functionalize the surface of silver nanoparticles, polymeric materials were selected as reducing agent and template to form silver nanoparticles. With abundant amino-, hydroxyl group and porous surface, polymeric materials showed the advantages for this research.
The silver nanoparticles, snowflake-shaped, were self-assembled on polymeric material, functionalized with hydroxyl- or amino group and showed yellow color. The optical properties of silver nanoparticles were analyzed by ultraviolet-visible spectroscopy (UV-Vis), exhibiting strong light absorbance at 410-430 nm. Furthermore, the morphology and crystal structure were analyzed via bright-field images and diffraction patterns of transmission electron microscopy (TEM). This work shows the feasibility to manipulate the shape of silver nanoparticles and functionalize their surface for bio-related application.

We used a green and versatile discontinuous Vertical Evaporation-driven Colloidal Deposition (dVECD) method for formatting nanoparticle wires by the direct deposition of nanoparticles from colloid suspensions onto hydrophilic substrates, without any lithographic procedures. Gold nanoparticles in different sizes are deposited into wire arrays for electronic detections of biological molecules. Because of the high surface-to-volume ratio of the porous structures, a sensitive detection of DNA molecules is achieved. Moreover, we can also detect the interaction between DNAs and proteins. Gold nanoparticle wires prepared by the nontoxic and simple dVECD method are promising for detecting virus involved in diseases in the future.

Coupling of optically active materials such as fluorescent organic dyes with plasmonic metal nanoparticles can provide advantageous photonic characteristics including enhanced emission of fluorophores which can be applied to photonic devices and biosensors. One of the key issues in photonic interactions is the controlled spatial organization of photonic nanomaterials for the precise adjustment of their distances in the nanometer scale, which mainly affect near-field plasmonic coupling. Diblock copolymer micelle, which consists of a soluble corona and an insoluble core in a selective solvent, is one of potential candidates to allow the nanoscale control over the distance between fluorophores and plasmonic particles. Since organic fluorophores can be incorporated into the core of micelles, nanometer-sized micelles can be utilized to separate fluorophores from plasmonic metal nanoparticles in a precise way by controlling the length of coronas, which determines the separation between fluorophores and nanoparticles. In this work, we combined diblock copolymer micelles and metal nanoparticles to control the plasmonic coupling of fluorescence. Silver nanoparticles were first grown from gold nanoparticle arrays templated by a single layer of diblock copolymer micelles. A thin film of copolymer micelles containing fluorophores in the cores was then spin-coated on this nanoparticle-decorated substrate. Using this combination as a platform, plasmon-enhanced emission and plasmon-coupled fluorescence resonance energy transfer were investigated along with nanostructural characterizations by FE-SEM and AFM.

Defect-free, long-range assembly of one-dimensional nanorods (NRs) promises the use of quantum rods in display and light-guiding applications yet remains underachieved. The NR assembly via solution-based processes typically results in disclination or vortices, which surprisingly provides the key to a rather perfect and long-range self-assembly.In this presentation, we examined self-assembly of CdS NRs suspended in solution with designed polarity and vapor pressure. It turned out that the nanoscale interaction of CdS NRs could be tailored by hydrophobic attraction using the solvent with rather high polarity index. From the results, we observed that the change of hydrophobic attraction affected the inter-NR distance in hexagonally-packed NR clusters, which was experimentally verified in the small-angle X-ray scattering analysis on solution samples and theoretical calculation of colloid-colloid interaction energy. During the evaporation process for sampling, the CdS NR clusters resulted in vortex formation or disclination, so as to minimize the interface energy due to capillary force between neighboring CdS NRs. Based on the understanding, we successfully demonstrate the assembly of CdS NRs in micron range without vortexes or defects.

Noncovalent electrostatic interactions play an important role in nature, e.g. in aggregation processes in biological systems.[1] The buildup of supramolecular networks by using noncovalent electrostatic interactions is of great interest, as it offers an access to new functional materials. One approach starts with the synthesis of charged organic molecules, equipped with catechol anchor groups for attaching them covalently to inorganic nanoparticles. This leads to nanoparticles that are stabilized in solution due to electrostatic repulsion.[2] In the current work, we present the functionalization of TiO₂ nanoparticles with organic molecules which contain catechol anchor groups as well as either positively or negatively charged units, e.g. quaternized pyridines or carboxylic acids. The successful functionalization of the nanoparticles was confirmed by dynamic light scattering, thermo gravimetric analysis and zeta potential measurements.
These charge-functionalized nanoparticles feature an excellent stability in solution that allows us, among other applications, to build up multi-layer architectures of oppositely charged nanoparticles by using the so-called Layer-by-Layer (LbL) technique.[3, 4] The assembly is formed due to electrostatic attractions. Thereby mono-molecularly thin polyelectrolyte layers, covalently attached to inorganic nanoparticles and mesoporous surfaces, are assembled to build up new functional materials with unique mechanical, optical, electrical, and biological properties.
In a first attempt we built up LbL assemblies by alternated deposition of functionalized, positively charged TiO₂ nanoparticles and negatively charged porphyrin derivatives on different substrates via dip-coating. The formation of the LbL assemblies was followed by UV/Vis spectroscopy, scanning electron microscopy and profilometry techniques. Since both, TiO₂ and porphyrins, due to their unique optical and electronical properties, are ideal systems for fabricating solar energy conversion schemes, we also studied the photogenerated features of the assembled layers.
[1] P. Politzer, J. S. Murray, Z. Peralta-Inga, Int. J. Quant. Chem., 2001, 85, 676. [2] J.-F. Gnichwitz, R. Marczak, F. Werner, N. Lang, N. Jux, D. M. Guldi, W. Peukert, A. Hirsch, J. Am. Chem. Soc., 2010, 132, 17910. [3] G. Decher, Science, 1997, 277, 1232. [4] D. M. Kaschak, J. T. Lean, C. C. Waraska, G. B. Saupe, H. Usami, T. E. Mallouk, J. Am. Chem. Soc., 1999, 121, 3435.

A novel H₂S sensor which functions at room temperature was assembled from viral-templated nanocrystalline gold nanowires. Hydrogen sulfide (H₂S) is a toxic gas released in petroleum, waste water, and mining industries. Low level exposure of 5 ppm or lower is innocuous, but high level exposure can cause irritation to the eyes and respiratory track, and in the extreme, result in paralysis and death. Hence, compact H₂S sensors with high sensitivity, low detection limit, and low power consumption are desired for mobile and continuous personal monitoring. Nanostructured gold with a high surface-to-volume ratio and a high affinity for H₂S is a promising material candidate for miniature chemiresistive H₂S sensors. Large surface area provides increased adsorption sites and the high gold-sulfur affinity allows specific interaction with H₂S gas analytes. Biological templates have been reported as effective scaffolds for the hierarchical assembly of complex nanostructures. Specifically, the high aspect ratio of the M13 bacteriophage is well-suited for templated inorganic nanowire synthesis. In this study, a previously reported genetically-modified, gold-binding M13 phage was used to form nanocrystalline gold nanowires for H₂S sensing. Scanning electron microscopy was used to determine the morphology and distribution of the nanowires between the device electrodes. The nanocrystal size and nanowire density increased with electroless deposition time and phage concentration, respectively, resulting in decreased device resistance. Sensor behavior and performance was measured through exposure to H₂S gas concentrations from 0.025 ppm to 40 ppm. The viral-templated H₂S sensors showed superior performance with a sensitivity of 654%/ppm, theoretical lowest detection limit of 2 ppb, and 70% recovery time of 9 min for 0.025 ppm. The presence of the M13 template and the gold-binding peptides within the device greatly affected electrical behavior and sensing performance. Upon removal of these organic materials with an oxygen plasma treatment, the electrical resistance of the sensors dropped several orders of magnitude and the sensitivity decreased by a factor 100 to 6%/ppm. Furthermore, no recovery was observed. The M13 bacteriophage was not only template, but also played a functional role, enhancing the performance of the nanostructured H₂S gas sensor.

Assembly of nanoscale materials from nanoparticle (NP) building blocks relies on our understanding of multiple nanoscale forces acting between NPs. These forces may compete with each other and yield distinct stimuli-responsive self-assembled nanostructures. Here, we report structural transitions between linear chains and globular assemblies of charged, polymer-stabilized gold NPs, which are governed by the competition of repulsive electrostatic forces and attractive poor solvency/hydrophobic forces. We propose a simple quantitative model and show that these transitions can be controlled by the quality of solvent, addition of a salt, and variation of the molecular weight of the polymer ligands.

Molecular concepts of synthetic chemistry are becoming a powerful strategy in the bottom-up fabrication of nanomaterials. The analogy drawn between atoms or molecules involved in a chemical reaction and nanoparticles undergoing self-assembly in predictable, well-defined manner enables control over the size and shape of the resulting nanoparticle clusters. Self-assembly of inorganic nanoparticles in linear chains (colloidal polymers or nanopolymers) bears a strong similarity with supramolecular step-growth polymerization, in which individual nanoparticle monomeric units are held together by directional and reversible noncovalent interactions. Drawing on the similarities between conventional step-growth, supramolecular and colloidal polymerizations, control of the degree of polymerization of nanopolymers can be achieved by using monofunctional chain stoppers, which "zip" chain ends, thereby making them incapable of further polymerization reaction. A colloidal analog of the monofunctional molecular chain stopper is a Janus nanoparticle containing a single reactive patch with well-defined dimensions and surface chemistry. In the present work, we explore the concept of colloidal chain stoppers for the chains of gold nanorods. Chains of metal nanoparticles (plasmonic polymers) show tunable optical properties due to the near-field surface plasmon coupling along the chain. Optical properties of plasmonic polymers strongly depend on the aggregation number, which is analogous to the degree of polymerization of the conventional molecular polymers. Here we report the rational design and synthesis of colloidal chain stoppers that allow for the quantitative control of the degree of polymerization of nanopolymers formed by gold nanorods. Analysis of the nanostructures provided information about polymerization kinetics, side reactions, and the distribution of all the species in the reaction system. In addition, this work provided the ability to test theoretical models developed for molecular polymerization.

Pressure-driven membranes with high porosity can potentially be fabricated by removing template, such as low water stability metal-organic frameworks (MOFs) or other nanoparticles, in polymeric matrix. We report on the use of benign MOFs as green template to enhance porosity and interconnectivity of the water treatment membranes. Significantly enhanced separation performance was observed which might be attributed to the mass transfer coefficient of the substrate layer increased in ultrafiltration (UF) application.

Thermodynamic models of colloidal self-assembly with anisotropic basis particles combined with physical confinement of the colloid suspensions (Escobedo 2013) have identified a rich diversity of low-symmetry phases across systematically varied fill fractions and confinement heights which show promise for enhanced photonic bandgap properties. Experimental verification consistent with phase predictions have been realized using wedge confinement cells with synthesized colloidal particles of comparable shape (Riley 2010). Multi-dimensional photonic crystals have been investigated for their properties allowing electromagnetic field localization, switching, spontaneous emission suppression and negative refraction which are desired for applications including integrated optical circuitry, sub-wavelength focusing and planar lenses. For 2D slab structures of finite height, light is confined in the third dimension through index guiding. Restrictions in gap polarization and reductions in bandgap width have been reported as compared to infinite 2D structures, though improvements are seen in structures with reduced symmetry. Here, the self-assembly and photonic properties of the square bilayer phase of cut spheres with cut fraction chi;=0.75 under confinement will be reported. The impact of dielectric contrast, dielectric fill fraction and variation in confinement height will be discussed for the direct and inverted square bilayer structures. Simulations indicate a large stable bandgap between the tenth and eleventh bands in the direct structure which is maximized at the dielectric contrast ε=12 (Silicon). In the inverted structure, two significant bandgaps between the fourth and fifth and the eighth and ninth bands were found.
References
1. C. Avendano, C. M. Liddell Watson, F. A. Escobedo. “Directed Self-Assembly of Spherical Caps via Confinement.” Soft Matter, 9 (2013): 9153-9166.
2. E.K. Riley, C.M. Liddell. “Confinement-controlled self assembly of colloids with simultaneous isotropic and anisotropic cross-section.” Langmuir, 26 (2010): 11648-11656.

Recently our group has been focused on medical applications of various uniform-sized nanoparticles. For example, using 3 nm-sized iron oxide nanoparticles, new non-toxic MRI contrast agent was realized for high resolution MRI of blood vessels down to 0.2 mm, which can be potentially applied to early diagnosis of cancers, stroke, and cardiovascular diseases. We demonstrated that ceria nanoparticles could protect against ischemic stroke in an in vivo animal model. In animal trials, intravenously administered ceria nanoparticles considerably reduced the stroke volume and nerve damage. We reported the first successful demonstration of high-resolution in vivo three-photon imaging using biocompatible and bright Mn2+ doped ZnS nanocrystals. Tumor vascular lining could be visualized with a resolution as small as 2 micrometer, which can be eventually applied to real-time monitoring of tumors during cancer surgery.
We reported the large-scale synthesis of magnetite nanocrystals imbedded in a carbon matrix and hollow iron oxide nanoparticles. These iron oxide-based nanomaterials exhibited very high specific capacity and good cyclability. We demonstrate galvanic replacement reactions in metal oxide nanocrystals. When Mn3O4 nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn3O4/γ-Fe2O3 (“nanoboxes”) were produced. Because of their non-equilibrium compositions and hollow structures, these nanoboxes exhibited good performance as anode materials for lithium ion batteries.

Seeded growth of noble-metal nanocrystals is a good example of directed self-assembly, in which atomic species can be manipulated with various parameters to grow into nanocrystals with complex structures. In this talk, I will discuss how surface capping, reduction kinetics, temperature, and surface diffusion can be used to experimentally control the assembly of atoms on the surface of a nanoscale seed to develop new morphology, including those with unsymmetrical structures. I will also discuss some of the unique optical and catalytic properties associated with these novel nanocrystals.

DNA hybridization has proven promising to meditate versatile crystallization product of nanoparticles (NPs). However, only polycrystalline was realized until recently, we showed that through very slow cooling, DNA functionalized nanoparticles can assemble into superlattices with a specific crystal habit, providing a nanoscale analogue to the crystallization behavior exhibited by conventional atomic crystals. Here we apply multi-scale models with molecular dynamics simulations to study and predict the shapes for such systems. We firstly use a scale-accurate coarse-grained model with explicit DNA chains to estimate surface energy ratios for different surface orientations, e.g. (100), (110), (111). As the orientational dependence of the surface energy determines the equilibrium shape, the corresponding Wulff polyhedra can be calculated accordingly. For the symmetry of body-centered-cubic (BCC) and face-centered-cubic (FCC) of DNA-NP supperlattices, the Wulff polyhedra is predicted to be a rhombic dodecahedron and a truncated octahedron respectively. Secondly, we use a colloidal model in which each DNA-NP building block is represented by a single bead with effective pair-wise potential to simulate the dynamics of crystal shape formation. The repulsive potential comes from the electrostatic repulsion and the attractive potential due to complementary DNA strands is validated by the first set of model. By this method, we reproduce the polyhedra growth in silico, and confirm the shape for the BCC system to be a (110)-enclosed rhombic dodecahedron. But due to defects including twinning and stacking faults in the lattice, the FCC system does not show any uniform shape except triangular features with (111) and (100) facets. The simulated crystal habits of both BCC and FCC system are consistent with experiments.

Rational integration of metal nanostructures into arbitrary patterns is a key foundation in nanotechnology and promises diverse applications in photonics, photovoltaics, and electronics. In particular, aligning plasmonic metal nano-particles, spheres or cuboids, into chains has been used as waveguides to direct light propagation at sub-wavelength scale. From bottom-up approaches, tuning the weak binding interactions among particles, such as DNA hybridization and electrostatic interaction, has produced diverse 1D/2D/3D structures from gold nanoparticles. However, it remains challenging to direct light propagation through arbitrary prescribed directions. Top-down approaches, such as e-beam and dip-pen lithography, have enabled the production of arbitrary complex metal arrays for light propagation. But fast fabrication of large-scaled structures with sub-5 nm resolution is yet to be achieved. Additionally, finding a generalized approach for constructing plasmonic nanostructures with waveguiding capability at visible light region remains an open challenge.
To step-forward to this challenge, we here describe here a versatile strategy for aligning gold nanoparticles into micron-scale ordered low-dimensional waveguide arrays with tunable patterns. We first assemble micron-scale 3D canvas with 2.5 nm spatial positioning resolution through the self-assembly of DNA crystals, and then “print&’&’ gold nanoparticles onto the prescribed positions of DNA canvas through DNA hybridization to form the designed 2D/3D array patterns. By varying the distribution of binding sites on DNA canvas, we have integrated gold nano-particle, size ranging from 5 nm to 20 nm, into arrays with distinct symmetry and intra-/inter-particle spacing, from around 2 nm to 20 nm. Theoretical computation and optical measurements suggest complicate plasmonic coupling existing within the self-assembled nanoparticle arrays.
Printing gold nano-particles onto DNA crystal canvas provides a simple, high-precision, and versatile strategy for design and construction of low-dimensional waveguides to direct light propagation. Simplicity. DNA canvas reduces the challenging task of exploring self-assembly conditions for distinct sized inorganic particles to a much simpler task of designing the surface binding patterns of a DNA nanostructure, which can be readily achieved by computer aided design software tools. High precision. Using DNA canvas, nanoparticles can be aligned with position accuracy around 1 nm, and inter-particle spacing around 2 nm over 5 um scale, which favors strong inter-particle plasmonic coupling. Versatility. Not only 1D chain-like waveguides, but also 2D/3D periodic arrays and chiral structures, assembled from homogeneous or heterogeneous nanoparticle components, can be fabricated from current method.

Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. Assembling nanoscopic objects into meso/macroscopic complex architectures allows bottom-up fabrication of functional materials, which is essential for many nanomaterial-based technological applications. In this seminar, we will discuss the formation of superparticles with supercrystalline structures made from the self-assembly of nanoparticles with a verity of chemical compositions and with well-defined size and shapes. We will show that the self-assembly of CdSe/CdS core/shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Further, functionality-based anisotropic interactions between these CdSe/CdS nanorods can be kinetically introduced during the self-assembly and in turn yield single-domain, needle-like superparticles having parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe/CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as down-conversion phosphors to create polarized light-emitting diodes.

Recently it was shown that using a template-assisted self-assembly process millimeter long linear chains of nanoscale spheres can be arranged in a trench on a silicon wafer [1]. Here we demonstrate that the spheres can be used as a shadow mask for the deposition of two rows of metallic nanotips opposing each other and that the distance between opposing tips can be precisely tuned.
To this end linear trenches were formed on the surface of silicon wafers by photolithography and dry etching. After depositing a self-assembled molecular monolayer (SAM) to functionalize the wafer surface a spreading knife technique was used to arrange 2.1 µm diameter polystyrene (PS) spheres from a colloidal suspension in a close packed line exclusively in the trench. The spheres act as a shadow mask in the subsequent thermal or electron beam deposition of a metallic thin film. After removal of the PS mask two lines of opposing sharply pointed metallic nanotips with tip radii in the nm range are obtained beneath the contact point between each pair of neighboring PS mask spheres. The tips on each side of the trench are interconnected by a large continuous metallic area, which may serve to apply a voltage between the opposing tips. This arrangement allows to address nanoscale objects between the tips and connect them to the macro-world.
Scanning electron microscopy shows that the distance between opposing nanotips is in the range of 300 nm due to mask clogging effects during metal deposition at normal incidence. This large gap between the tips limits the inter-tip electric field enhancement as has been shown by finite element simulations. To solve this problem ray tracing simulations predict that the inter-tip gap can be reduced by tilting the sample during evaporation. Therefore a home built physical vapour deposition system was used to perform the evaporation of metal films at non-normal angles and to tune the distance between opposing nanotips. The experimental results are shown and compared to theoretical predictions of the tip distance.
[1] K. Brassat et al., Phys. Stat. Sol. A 210 (2013) 1485 (cover)

In this talk, I will introduce a new class of Ag-Au hollow nanostructures with great SERS performance and excellent chemical stability. Specifically, with the involvement of a strong reducing agent such as ascorbic acid, the galvanic replacement reaction between Ag nanocubes and HAuCl4 led to the formation of Ag-Au hollow nanostructures with morphologies, compositions, and optical properties different from those previously reported for Ag-Au nanoboxes and nanocages. With the use of 785 nm laser excitation to mitigate plasmon damping, I could optimize the SERS performance by maneuvering morphology and composition to achieve SERS activity 15 times stronger than that of Ag nanocubes. Remarkably, these nanostructures with exceptional SERS activity also embrace significantly improved stability in an oxidizing environment due to the formation of a Ag-Au thin shell on the surface. I will further demonstrate template-assisted and solvent-mediated self-assembly of these novel nanostructures for the formation of SERS hot spots on a solid substrate or a colloidal suspension, respectively. The feasibility of these new SERS substrates for an in-situ SERS detection of reaction kinetics will be discussed.

The design and fabrication of materials that simultaneously contain more than one type of functional components, so-called multifunctional materials, is an active research area with the potential to impact a wide range of technological applications. In particular, co-assembling two types of nanocrystals with distinct properties into larger colloidal particles, especially at the mesoscopic scale, offers the possibility of producing new classes of nanoparticles (i.e., supernanoparticles, SPs) with a set of combined properties, all the while maintaining the colloidal nature of their building blocks. Here, we report a simple and effective method for assembling CdSe-CdS QDs with Fe3O4 MNPs into colloidal SPs with a “core-shell” (CS) superstructure. Additional thermal annealing drives the crystallization of the “core” MNPs into a highly ordered supercrystal with an ideal fcc superlattice. The CS-SPs can be easily coated with a thin silica layer, providing surface functionality and colloidal stability as well as biocompatibility. Importantly, these silica-coated CS-SPs for the first time simultaneously satisfy all the design criteria needed for various applications: uniform and tunable sizes, high magnetic content loading, maximized fluorophore loading on the surface, substantial colloidal stability, and versatile surface functionality. These features allow the utilization of these magneto-fluorescent SPs for in vitro intracellular manipulation and in vivo MR/MP dual-modal imaging.

ZnO is probably the most studied material among various metal oxides due to its wide, direct bandgap, wide variety of nanostructure shapes, and earth abundance. In this study, we prepared ZnO nanofilms with various morphologies using assembly of ZnO nanocrystals. A continuous flow microreactor-assited deposition system was used to yield ZnO assembly composed of colloidal ZnO nanocrystals and ZnO nanofilms with various morphologies. Morphology of ZnO assembly was primarily determined by flow rates of solution, resulting in dispersed nanocrystals, rectangular, or spherical assembly at a solution flow rate of 6.8 mL min-1, 14.7 mL min-1, and 28.1 mL min-1 respectively. These ZnO assemblies, which were made in-situ in the microreactor, were directly delivered onto substrates to serve as seeds. The nanofilms were fabricated by the subsequent reaction of molecular precursors onto the seed layers, which include smooth amorphous layer, flower-like structure, vertical nanowire arrays, and dense crystalline thin film. This study demonstrates the versatility of the continuous flow microreactor-assited deposition system for the production of nanocrystals, nanocrystal assembly, and nanostructured films with various morphologies by tuning the physical parameters while using the same chemical precursors, precursor concentration, and reaction temperature without using surfactants for the synthesis.

Titania&’s photoelectrochemical activity has attracted attention for promising new applications in photocatalysis, photovoltaics, and electrochemical energy storage. In the case of batteries and supercapacitors, TiO₂ is being explored as an alternative to graphitic electrodes. Its higher operating voltages, reduced solid-electrolyte interactions, lower volume change, and higher capacity at higher charge/discharge rates could lead to improvements in device safety, lifetime, and performance. Nanostructured TiO₂(B) [B = Bronze] is a particularly interesting polymorph because its larger lattice and higher surface area facilitate ion diffusion and coordination. Electrochemical studies and density functional theory calculations showed that different TiO₂(B) nanostructures, i.e., nanoparticles versus nanosheets (NS), possess similar Li⁺ capacities, but exhibit dramatically different structure-dependent lithiation mechanisms. It was proposed that two-dimensional TiO₂(B)-NS offer additional degrees of structural freedom that broaden Li⁺/Li redox peaks and stabilize surface-electrolyte interactions. To experimentally study these structure-property-interface relationships at a fundamental level, Langmuir-Blodgett (LB) assembly was used to fabricate conformal and flat lying TiO₂(B)-NS monolayers with controllable density. This technique allowed us to overcome the problems of nanosheet clustering, hydrocarbon contamination, morphological changes, and phase transitions that have hindered access to native TiO₂(B)-NS surfaces. Detailed chemical and structural characterization of the LB-assembled films showed that the dominant phase was stoichiometric TiO₂(B)-NS. A highly preferred (020) surface orientation was observed, indicating a predominantly flat lying morphology. Small concentrations of TiO₂(B)-NP and anatase were also observed in the films, and their concentrations increased with increasingly aggressive post-processing treatments involving O₂-plasma cleaning or annealing. The assembly of well-packed TiO₂(B)-NS films with well-defined (020) surfaces enabled unambiguous studies of lithium intercalation and surface-electrolyte interactions. This work further highlights the ability of Langmuir-Blodgett assembly of nanomaterials to create well-defined substrates for fundamental studies and novel applications.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Stretchable conductors are essential components of next generation electronic devices that can facilitate integration of human-machine interfaces. However, molecular mechanisms in material deformation and stiffening make the combination of high stretchability and conductivity fundamentally difficult. Best known stretchable conductors partially overcome the dilemma using percolated networks from high aspect ratio nanotubes/nanowires. Various strategies, pre-strained substrates, buckled microwires, three-dimensional microfluidic polymer networks, were also tried to improve parameters. Further improvements can be achieved by using fillers with increased aspect ratio, of all-metallic, or with clever alignments. However, one might expect difficulties with synthesis/separation, or increased anisotropy/stiffness. Here we demonstrate that excellent stretchable conductors can be made from spherical nanoparticles (NPs) by two methods, layer-by-layer (LBL) assembly and vacuum-assisted flocculation (VAF), despite their minimal aspect ratio. High conductivity and stretchability were observed from both composites and the properties originated from dynamic self-organization of NPs under stress. Modified percolation theory to incorporate the self-assembly behavior of spherical fillers gave excellent match with experimental data. The self-assembly processes and highly efficient charge transport make NP composites similar to liquid metals. They represent a new approach to nanoscale engineering of stretchable conductors and enable electronic tunability of mechanical properties in solid state. Scanning electron microscope, transmittance electron microscope, atomic force microscopy, electrical properties measurement system, dynamic mechanical analysis, small-angle X-ray scattering were performed to analyze properties of nanocomposites.
This work was funded, in part, by STX foundation, Seoul, Korea, and US Air Force Office of Scientific Research.

Mesoporous graphene-based bulk materials with tunable three-dimensional morphologies can be fabricated by sol-gel techniques, either by self-assembly of individual graphene building blocks or by converting sol-gel derived polymer foams into networks of sp2-hybridized carbon atoms. These materials have a unique combination of properties including high surface area, electrical conductivity, chemical inertness, and environmental compatibility, and thus have received considerable attention for energy related applications such as supercapacitors, rechargeable batteries, capacitive deionization, and catalysis. Polymer derived graphitic carbon aerogels were first developed at Lawrence Livermore National Laboratory (LLNL) in the late 1980s. Since then we have developed synthetic methods to control material morphology and pioneered efforts to functionalize the materials by incorporating inorganic components, with an eye towards real-world applications. More recently we focused on interfacial phenomena such as a charge induced macroscopic strain effect that may enable carbon based actuation. This contribution will highlight recent advances in graphene aerogel research at LLNL ranging from the development of graphene oxide-based aerogels and graphene/metal oxide hybrids to electronic structure engineering by doping, and will discuss the related improvements in electrical, electrochemical, and mechanical properties.
Work at LLNL was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. Project 12-ERD-035 was funded by the LDRD Program at LLNL.

Well-defined, organically-modified ZnO nanoparticles were prepared via an efficient hydrolysis route, without the need for excess surfactant co-ligands, washing or size-selection steps. The products have a narrow size distribution and are soluble in organic solvents. The synthesis involves reacting a mixture of alkylzinc carboxylate complex and excess diethylzinc with water to yield carboxylate-capped ZnO nanoparticles. Varying the ratio of the different organometallic species enables control of either size or degree of surface modification (1). The method is conveniently compatible with cross-linking resin chemistries, and is applied for the in situ preparation of organically-modified ZnO polymer nanocomposites.
Stabilised nanoparticles with partial-coverage are particularly interesting for catalytic reactions where surface access is required. Combination with copper nanoparticles provides an active catalyst for the reduction of carbon dioxide with hydrogen to methanol. The use introduction of phosphinate ligands for the synthesis of the intial ZnO nanoparticles from organometallic precursors improves the reductive stability and the catalytic activity of the system. The most active catalyst appears to be a self-assembled ‘bow-tie&’ construct of two ZnO tetraheda with a copper core (2).
(1) Orchard et al, Organometallic Route to Surface-Modified ZnO Nanoparticles Suitable for In Situ Nanocomposite Synthesis: Bound Carboxylate Stoichiometry Controls Particle Size or Surface Coverage, Chemistry of Materials, 24:2443-2448, 2012
(2) Brown et al, Phosphinate stabilised ZnO and Cu colloidal nanocatalysts for CO ₂ hydrogenation to methanol, Chem. Commun., 2013, 49, 11074-11076

Materials capable of rapidly storing and delivering hydrogen are currently in demand for numerous energy applications. The high surface area exhibited by nanoporous palladium has the potential to greatly improve its kinetics of hydrogen charge and discharge versus nonporous metal while retaining favorable operating temperatures, pressures, and volumetric capacity. In addition, the void space imparted by porosity should accommodate the volumetric expansion of palladium upon hydriding, mitigating plastic deformation and improving cycle life.
By chemically reducing palladium salts in the presence of various surfactants, we are able to synthesize palladium powders having size-tunable mesopores from a few to tens of nm, in a scalable fashion. Our results suggest that Pd nanoparticles form and sinter around micelles present in the aqueous media. Changing the size of the surfactant molecules affects the size of these micelles and ultimately determines the pore dimensions. We demonstrate that pore size affects not only surface area, but also pore thermal stability under vacuum, and in the presence of hydrogen gas.
In addition to pore geometry, particle size and shape are important factors in determining gas flow characteristics. Using nonporous copper particles as sacrificial reductants for palladium salts, in the presence of surfactants, we are able to synthesize micron-sized particles of uniform size and shape. This is in contrast to continuous aggregates and films that are obtained using soluble chemical reductants or planar electrodes.
We will present details of the synthesis of nanoporous palladium, demonstrating control of both pore and particle size. Bulk measurements of hydrogen storage properties and pore characteristics, as well as microscopic measurements, including in situ heated-stage TEM under vacuum and in the presence of hydrogen, will be included, as well as details on the kinetics of hydriding/dehydriding.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND 2013-3929A

Nanoparticle-biomolecule hybrid systems, such as DNA-gold nanoparticle (AuNP) assemblies, have gained an immense interest because of their unique combination of optical properties and biomolecular recognition abilities.[1,2] In order to precisely tune the functionalities of these systems, it is crucial to generate networks with well-defined sizes and geometries. By mixing different ratios of complementarily functionalized DNA-AuNPs, DNA-AuNP networks with different sizes can be tailored. The size and internal structure of these networks determine their optical properties due to electromagnetic interparticle coupling. The networks are (photo)thermally addressable and thus a reversible network dissociation can be triggered.[3] Upon pulsed laser irradiation, the DNA-AuNP networks are known to disintegrate on a millisecond time scale.[4] Therefore, these systems may be interesting for possible future applications in the fields of drug release or gene regulation.
In this work, we analyze the size and internal structure of DNA-AuNP networks in aqueous environment by combining optical extinction spectroscopy, dynamic light scattering, small angle X-ray scattering, in situ-TEM measurements and generalized Mie calculations. Using these techniques, we determine the different interparticle distances in the networks to establish accurate network models. Small networks are shown to exhibit a center-shell geometry with a total number of ca. 10 AuNPs. Upon cw laser heating, the networks show a photothermal response that correlates with their size and the laser intensity. Since the DNA dehybridizes at a specific temperature, the photothermally generated temperature increase inside the networks can be determined and is found to scale with both quantities mentioned above. By monitoring the scattered laser light on a millisecond time scale, we measured the transient network dissociation process in dependence of the laser intensity and compare it with the calculated heating dynamics inside the laser irradiated water volume. For high laser intensities, the temporal dissociation curve additionally exhibits fast and low-amplitude oscillations whose origin is unclear. The curves also indicate the onset of convective flows inside the sample solution. Furthermore, by depositing a single DNA-AuNP network on a substrate, the photothermally induced disassembly of the network can be visualized by dark-field microscopy. We observe that the laser irradiation leads to a disassembly of the network into small fragments rather than a direct dissociation into single particles.
The comprehensive information obtained here on the internal structure and photothermal properties of DNA-AuNP hybrid systems is expected to be useful for potential biomedical applications based on photothermal actuation.
[1]Mirkin et al., Nature 1996, 382, 607; [2]Alivisatos, A. P.; et al. Nature 1996, 382, 609; [3]M. Reismann et al., Small 2008, 4, 607-610; [4]J. Stehr et al., Nano Letters 2008, 8, 619-623;

We are interested in the interaction between multidentate macromolecules and noble metal particles. In particular we are seeking for macromolecules able to coat inorganic particles and thereby removing them from the reaction mixture during their growth. Using this approach coated particles with narrow size distribution and an organic coat controlling their exposed functional groups (nature and number) were obtained in good yields.
Oligomeric benzylic thioethers as multi-dentate ligands displayed favorable properties for the stabilization of gold nanoparticles. A heptameric structure stabilized particles with diameters around 1 nm and the dimensional comparison pointed at a ratio of only two ligands per particle [1]. By further functionlizing the coating ligand, the integer relation of ligands per particle allowed to control the number of peripheral functional groups. For example, particles exposing two protected ethynyl-functions at opposed sites were synthesized and subsequently interlinked by oxidative acetylene coupling protocols [2]. A perpendicular arrangement at the particles surface of the ethynyl-function bearing molecular rod was obtained by introducing a coordinating pyridine subunit [3]. In order to expand the surface coating of the macromolecular structure, a dendritic design was investigated. A single icosadentate second generation dendrimer turned out to efficiently coat and stabilize a gold nanoparticle with a diameter of about 1.2 nm [4]. The 1/1 ratio of coating ligand per gold nanoparticle made mononot;functionalized particles available which provided exclusively dumbbell type dimers as organic/inorganic hybrid architectures upon oxidative acetylene coupling [5]. In order to expand the scope of wet chemical coupling reactions, these monoethynyl-functionalized particles were interlinked by click-chemistry with small molecules exposing azide-groups.
Our current investigations are geared towards expanding the scope of applicable chemistry and varying the nanoparticles (dimensions and materials).
[1] T. Peterle et al., Chem. Comm., 2008, 3438.
[2] T. Peterle et al., Adv. Funct. Mater., 2009, 19, 3497.
[3] J. Hermes et al., small, 2011, 7, 920.
[4] J. Hermes et al., Chem. Eur. J., 2011, 17, 13473.
[5] J. Hermes et al., J. Am. Chem. Soc., 2012, 134, 14674.
[6] F. Sander et al., small, 2013, DOI: 10.1002/smll.201300839

Nanoparticle (NP) based aerogels represent a sophisticated class of materials with low density, high surface area, open porosity, and excellent noise and heat insulating capabilities.
A synthesis route via the self-assembly of preformed NPs offers beneficial effects in comparison to traditional sol-gel approaches. Defined crystallinity and composition of the NP building blocks and a defined mixture of different building blocks enables the preparation of complex aerogel structures that are not accessible by molecular sol-gel processes. This results in improved properties and functionalities. In particular, examples in the area of photocatalysis[1] and luminescence[2] have been reported.
In this work we present titania NPs as building blocks for photocatalytically active aerogels. These titania structures show good specific surface area, high porosity and well defined crystallinity. By a careful control of the gelation process of well dispersed nanoparticle building blocks, anatase aerogels are produced with outstanding translucency.
These anatase aerogel structures serve as matrix for the incorporation of several other types of nanoparticles and nanowires, offering unique possibilities for the improvement of the photocatalytic properties and for the implementation of further functionalities. Exemplarily the improvement of the photocatalytic properties can be shown by the incorporation of gold NPs.[1] An excellent example for the implementation of further functionalities is the addition of magnetic Fe₃O₄ NPs. This adds magnetic properties to the photocatalyst and enables the controlled release of the catalyst to a reaction solution and the controlled removal therefrom.
The use of preformed building blocks and the easiness to control the gelation process allows the production of aerogels with locally well-defined content of different nanoparticles of specific shape and crystallinity, resulting in the production of complex layered aerogel structures.
[1] F. J. Heiligtag, M. D. Rossell, M. J. Süess, M. Niederberger, J. Mater. Chem. 2011, 21, 16839
[2] J. L. Mohanan, I. U. Arachchige and S. L. Brock, Science, 2005, 307, 397

During the past decades, silver ions/nanoparticles have been largely used in many commercial bioactive devices or coatings. [1] This bactericidal property is also exhibited by Cu (II) ions when leached from Cu nanoparticles (NPs) and could be an alternative when the local environment hinders the efficiency of silver ions. Indeed, bioactive thin films containing Cu nanoparticles would be of great interest for surface treatments and anti-biofilm purposes with low copper loading in the 1% range. [2] However, the control of the stability of the coating as well as the release of the metal ions have to be considered in order to design a smart bactericidal surface.
In this work, we have developed a multilayer deposition process combining copper NPs and PEG-phosphonate dendrimers. The use of copper nanoparticles entrapped in a stabilizing dendrimeric matrix and bearing a PEG repellent top surface is highly relevant to favor a bactericidal/anti-adhesion synergetic effect. On one hand, the Cu NPs included in the matrix are considered as a Cu (II) reservoir of biocide. On the other hand, the functionalized dendrimers play in the same time the role of stabilizing agent and of glue, allowing the strong adhesion of the material onto the substrate thanks to multivalent interactions. Our preliminary results have shown that these dendrimers strongly adhere to several surfaces and that they are also very efficient to stabilize thin films containing Cu NPs. [3-4] The hybrid interface between the support and the coating has been here conceived to maximize the stability of the multilayers by using a technique which has already proved its efficiency for the industrial production of dendrimer-based biochips.[5] Finally, we developed methods to build up stable multilayer materials depending on the choice of the substrate (silicon, glass, titanium oxide...) and the affinity of the NPs for the dendrimers. Different strategies such as in-situ growth of NPs or dip-coating deposition will be compared and their antibacterial activity towards P. aeruginosa, E. coli or S. aureus bacteria will be commented.
[1] F. Cheng; W. J. Betts, S.M. Kelly, J. Schaller, T. Heinze Green Chemistry, 2013, 15, 989-998
[2] K. C. Anyaogu, A. V. Fedorov, D. C. Neckers Langmuir, 2008, 24, 4340-4346.
[3] C. Barrière, K. Piettre, V. Latour, O. Margeat, C.-O. Turrin, B. Chaudret, P. Fau J. Mater. Chem., 2012, 22, 2279
[4] W. B. Zhao, J. Park, A.-M. Caminade, S.-J. Jeong, Y. H. Jang, S. O. Kim, J.-P. Majoral, J. Cho, D. H. Kim J. Mater. Chem., 2009, 19, 2006-2012
[5] A. Hameau, V. Collière, J. Grimoud, P. Fau, C. Roques, A.M. Caminade, C.O. Turrin RSC Adv., 2013, 3, 19015-19026.

Anode materials are key factor for achieving high specific energy and stability of Li-ion battery. Transition metal oxide nanoparticles have a large storage capability, but they suffer from the excessive generation of solid-electrolyte interphase (SEI) on the surface, low electrical conductivity, and mechanical degradation and pulverization. We prepared mesoporous iron oxide nanoparticle clusters (MIONCs) by a bottom-up self-assembly approach. This MIONCs showed with enhanced capacity retention, rate capability, and Coulombic efficiency when they applied to anodes in Li-ion battery. More importantly, this unique structure demonstrated that changing the geometric configuration can lead to the confinement of SEI layer formation, resulting in very stable battery performance. We expect that the bottom-up self-assembly strategy can be considered as new approach to control SEI formation and achieve stable anode materials in Li-ion battery.

Modern heterogeneous catalysts are relatively simple and are constructed of metallic nanoparticles dispersed on an oxide support. This simple structure extremely limits the catalytic scope and excludes the activation of complex catalytic reactions. The construction of mesoscale catalysts with few incorporated active sites, which are assembled together in specific architecture, will enable the preparation of complex organic products. The preparation and utilization of these multi-functional catalysts will expand the focus of catalysis from the nanoscale into the mesoscale. In this work, mesoscale catalysts with unique reactivity and selectivity were prepared by employing the environment which surrounds the metallic nanoclusters as a co-catalyst. These rationally-designed mesoscale catalysts showed unique reactivity and selectivity. Using a polymer based environment that surrounds 2 nm sized Au nanoparticles we were able to modify the oxidation state of the metallic clusters and by that activate the nanoclusters toward a variety of organic reactions which were previously solely catalyzed by homogeneous, organometallic catalysts. Moreover, by tuning the steric and symmetric properties of the surrounding polymer molecules, high products stereoselectivity and enantioselectivity were obtained. These results demonstrate that by expanding the catalytic system beyond the metallic site and moving to the mesoscale regime, novel catalytic reactivity and selectivity can be gained.

Based on block copolymer micelle nanolithography (BCML) we synthesized and self-assembled monodisperse, stable mono and bi-metal nanoparticles on differently shaped surfaces such as flat interfaces or microparticles. We have advanced two different approaches to use BCML for synthesizing monodisperse, thermally stable mono- and bi-metallic nanoparticles consisting of the noble metals, e.g., gold, palladium and platinum. Depending on the preparation method the used metals form different alloys. With this method the particle size, composition, morphology and density are precisely adjusted. Visiting individual nanoparticles confirm the chemical composition as set by the original macroscopic weight relation between metals. These well assembled alloy nanoparticles on geometrically different substrates proved to be good materials for adjusting optical, and catalytical properties as well as intersting biointerfaces.

Synthesis of interfaced nanoparticle dimers made of asymmetric compositions (i.e., interfaced heterodimers) is challenging because of the difficulty in manipulating the nanoparticles&’ surface chemistries in order to control the assembly and/or growth of different nanoparticles. In this presentation, a seed-mediated, surface-confined epitaxial overgrowth strategy is introduced to demonstrate the synthesis of high-quality interfaced Au-Ag heterodimers with varying sizes. Au and Ag share a common face-centered cubic lattice and have nearly identical lattice constants, which facilitates epitaxial overgrowth and allows direct contact between the Au and Ag domains. The interfaced Ag nanodomains can be chemical transformed to hollow nanoshells of other materials through a galvanic replacement reaction, resulting in the formation of interfaced dimers made of solid Au nanoparticles and hollow nanoshells. Due to the direct contact between the two components in each interfaced dimer, strong coupling between them and the corresponding novel properties are expected. For example, in the interfaced Au-Ag heterodimers with quantum sizes (diameters <10 nm) was observed an unusual enhancement of the characteristic Au surface plasmon resonance and the emergence of a charge transfer plasmon across the Au/Ag domains, which together lead to broad-band absorption spanning visible to near-infrared wavelengths.
This work has been performed at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357.

Owing to the unique properties of titanium dioxide (TiO2) nanomaterials such as stability, a large electronic band gap and nontoxicity, a diverse range of energy and environmental applications have been demonstrated.1 Here we describe an easy and low cost synergistic approach using a polystyrene-block-poly4vinylpyridine (PS-b-P4VP) block copolymer (BCP) and sol-gel titania precursor for the engineering/development of titania nanodots and nanowires. Block copolymers composed of two distinct polymer segments covalently bonded together, microphase separate under appropriate conditions to form a wide range of functional morphologies such as lamella, cylinders and spheres with feature sizes between 3-100 nm. The ability of asymmetric BCPs to pack hexagonally as well as to orient parallel to the substrate through solvent annealing is demonstrated. Nanoporous templates are generated after selective solvent swelling which enables the incorporation of titania from a sol-gel precursor solution following spin coating. The polymer matrix is selectively removed by UV/ozonation and the created titania patterns have one to one registration to the initial PS-b-P4VP template. The titania structures were calcined to different temperatures. The samples were characterized using FTIR, XPS, XRD, SEM, AFM and UV/Vis to investigate their chemical, structural and morphological features and their evolution with respect to heat treatment. A systematic photocatalytic study was carried out examining the degradation of methylene blue to demonstrate their interesting structure-morphology-photoactivity relationship.
References
1. Chen, X.; Mao, S. S.,Chemical Reviews 2007.

Periodically ordered nanohetero inorganic structures offer great promise due to their unique electric, ionic, magnetic, and photonic properties. Many studies have focused on the formation of periodically ordered nano-hetero inorganic structures through layer-by-layer adsorption, sputtering, and self-assembly methods. However, the construction of three-dimensional periodically ordered nanohetero inorganic structures with desired sizes and morphologies remains a great challenge. We present a simple method for producing three-dimensional periodically ordered inorganic nanoheterostructures with controlled shape and size by replicating self-assembled block copolymers (BCPs) containing precursors of metals and metal oxides. Precursors were dissolved with BCPs in a solvent. Upon evaporation of the solvent, each precursor was selectively introduced into a separate polymer block. Subsequent pyrolytic removal of the BCPs produced periodically ordered nanoheterostructures that were structural replicates of the precursor-BCP composites. Self-assembled nanoheterostructures of nanoparticles, nanorods and layers in a matrix were produced. The morphology and domain size can be tailored by controlling the molecular weight and relative block length of block copolymers. The magnetic and electrochemical properties of produced three-dimensional periodically ordered inorganic nanoheterostructures will be presented. The controlled size and morphology of the inorganic nanoheterostructures demonstrate the method&’s utility for producing highly functional materials.

Colloidal inorganic nanocrystals (NCs) are among the most exploited nanomaterials to date due to their extreme versatility. Research on NCs went through much advancement in the last fifteen years, for example in the synthesis, which opened up the possibility to control their size and shape: this includes the ability to combine several materials in the same nano-object, with topological control, which has contributed to expand the functionality of NCs and the range of their applications considerably. An additional step forward was the creation of a wide range of superstructures from the assembly of such NCs, which can be clearly thought of as new types of artificial solids. This, coupled with the possibility to replace the native ligands on the surface of the NCs with shorter molecules, down to single atom ligands, has conferred unique electrical features to films of NCs that make them attractive for low cost alternatives to many technologies in photovoltaics, optics and electronics. Progress also came from the study of chemical transformations in nanostructures, most notably via cation exchange, which involves replacement of the sublattice of cations in a crystal with a new sublattice of different cations, while the sublattice of anions remains in place. Also, a new field of study has emerged recently, aiming at investigating the transformations in colloidal synthesized nanomaterials under conditions like thermal annealing and/or irradiation. In part this research is boosted by the recent availability of microscopy tools by which one can follow the transformations on individual NCs in-situ, i.e. when such perturbations are actually applied to the sample. The present talk will highlight the recent progress by our group in all the above areas (advanced synthesis, assembly, surface functionalization and the study of chemical and structural transformations in NCs).

Plasmonic nanoparticles can be assembled into a wide variety of
crystalline arrays using DNA functionalization and hybridization. Here we
demonstrate that three-dimensional DNA-NP superlattices can be grown
epitaxially, eliminating grain boundaries and enabling fine control over
orientation and size of the assemblies up to hundreds of square microns.
We use for our experiments a DNA-NP superlattice with a BCC crystal
structure, consisting of a 30 nm diameter Au nanoparticle and a 62 nm
lattice parameter. This superlattice is composed of interpenetrating
lattices of nanoparticles with complementary DNA functionalization (A and
A&’), enabling it to be grown from a surface layer-by-layer, by exposing
the surface alternately to nanoparticles with A and then A&’ type DNA. We
use electron-beam lithography followed by Cr/Au deposition and liftoff to
engineer a silicon surface to resemble lattice-matched and strained
BCC(100), BCC(110), and BCC(111) faces of this superlattice. We then
functionalize this surface with DNA and expose it overnight to particles
of complementary type. By continuing in this manner, multilayer epitaxial
DNA-NP superlattice thin films may be built up. We characterize the
resulting crystal growth by embedding the grown crystal in a silica
sol-gel, drying and imaging via scanning electron microscopy.
With crystallization at 23 degrees C, we observe defect densities as low
as 2% in the lattice-matched BCC(100) orientation, clearly demonstrating
the viability of large-scale crystal growth using this technique. At
higher temperatures, however, as the nanoparticles become more mobile on
the surface, it becomes clear that they prefer binding to one another
relative to binding on a template, and island growth becomes dominant. We
are able to understand this process thermodynamically on the basis of our
template geometry and crystal growth parameters, and we are able to alter
it by adjusting the attachment strengths of the template DNA strands.
Ultimately, control over the three-dimensional directed self-assembly of
these nanoparticles leads to the possibility of integrating self-assembled
plasmonic materials into on-chip optical or optoelectronic platforms.

Colloidal silica particles are being intensively studied due to their potential applications in catalysis, intelligent materials, optoelectronic devices, photonic bandgap crystals, masks for lithographic nanopatterning, etc. On the other hand, in nanoscale electronic, photonic and plasmonic devices, feature dimensions shrink towards a critical limit, and new experimental approaches have to be explored in lithographic patterning in order to create ordered arrays of metallic nanostructures with useful optical properties. For this work, spherical submicrometer-sized silica particles were prepared by the sol-gel technique and deposited as a self-assembled monolayer onto silica glass plates using a spin coater system. This silica monolayer is then used as a mask to create regular arrays of nanoscale features in the sample by 1 MeV Ag ion implantation. By this way, after removal of the silica particles and an adequate thermal annealing of the as implanted samples, the formation of Ag nano-objects was confirmed by the presence of the surface plasmon resonance (SPR) in the optical absorption spectra. Some of the samples were irradiated at room temperature with 8 MeV Si ions in order to modify the shape of the Ag nanostructures. Thus, the formation of elongated Ag nanoparticles allows the modification of the optical properties of the Ag arrays, i.e., achieving then tuning of the SPR. The size and shape of the arrays of silica particles and metallic deposits were studied by scanning and transmission electron microscopy, respectively. The total amount of implanted Ag was measured by Rutherford Backscattering Spectrometry (RBS). Finally, the long range order of the Ag nanoparticle assembly and its plasmonics properties were characterized by means of a Fast Fourier Transform study and optical absorption measurements, respectively.

An understanding of the statistical and time-dependent features of cluster formation is essential for the application and control of the dispersion quality of colloidal nanoparticles (CNPs). In particular, the functional nanoparticles decorated with surface modifying molecules involving Janus nanoparticles are of important when they are succesfully dispersed as colloidal phase. We performed computational and theoretical studies on the formation of clusters in Janus CNPs, focusing on the scaling behavior of the growth of the cluster size and size distribution, with analysis of the fractal dimension. For the study, we employed a kinetic Monte Carlo (KMC) algorithm in which NPs are moved by self-avoiding diffusive jumping with a random walk. To describe diffusion of the NPs in a colloidal environment, the diffusivity was modeled as a configuration-dependent function of the interacting potential of the clusters. In order to describe the anisotropic potential by which Janus NPs, we tested conventional Janus particles model and found that the conventional model is not relevant to elucidate the clustering of the Janus NPs. Instead, we introduced the advanced potential model by which we observed the clustering phenomena as well as fractal nature of the clusters. We observed that the scaling exponents for the average cluster size and weight are smaller than the conventional isotropic cluster formation. Strikingly, the fractal dimension of the cluster itself was also considerably smaller than the one observed in the typical reaction-limited aggregation of particles. We provided a semi-empirical explanation of how the scaling exponent of the cluster size and weight should be reduced depending on the scaling behavior of the monomer concentration. The results of this study can be used to design and control the colloidal quality of Janus NP dispersions by understanding the cluster growth behavior and its dynamics.