Symposium NT2 : Oxide and Chalcogenide-Based Thin Films and Nanostructures for Electronics and Energy Applications

The Pb (Zr, Sn, Ti)O₃ family of perovskite antiferroelectrics, modified with La, has attracted much attention in recent years because of its high longitudinal strain and short switching time at the depolarization temperature (T_d) in the vicinity of the morphotropic phase boundary (MPB). However, its electrical properties and structure evolution while depolarized from induced ferroelectric phase (FE_in) to antiferroelectric phase (AFE) still need further investigation. In order to understand the influence of domain structure on the phase transition and depolarization behaviors, (Pb, La)(Zr, Sn, Ti)O₃ single crystals with MPB composition have been grown by flux method. The virgin sample was firstly induced to a metastable ferroelectric state by applying a sufficiently large electric field. Thereafter, the temperature induced phase transition of [100]-orientated PLZST single crystal was explored by electric measurements and in situ polarized light microscope (PLM). T_d is proved to be around 115^oC by exploring temperature-dependent dielectric constants of a poled sample. A temperature induced phase transition from FE_in to AFE state could be proved by its electric measurement: typical ferroelectric behavior below T_d with single polarization versus electric field hysteresis loop (P-E loop) and current versus electric field curve (I-E curve) with only two peaks; when the temperature is above T_d, AFE phase is confirmed by double loops in P-E measurements and I-E curves with four peaks. By observing the domain evolution before and after the phase transition, it was confirmed that the structure changed from rhombohedral to tetragonal state by their extinction angles and the orientation of domain walls. A large strain (0.77%) was obtained at a low electric field of 2.5 kV/mm. The remarkable strain accompany with induced phase transition makes the PLZST single crystal a promising candidate for switch and actuator application.

Potassium niobate (KNbO₃), a lead-free ferroelectric compound with perovskite-type structure, is considered a promising nanomaterial for numerous applications due to their superior optical and ferroelectric properties. Despite progress in 1D perovskite materials, preparing high aspect ratio KNbO₃ nanostructures is still a challenge.

Here, we report a simple and effective method for synthesizing ultra-long KNbO₃ nanofibers with different morphologies (solid and porous) using sol-gel assisted far-field electrospinning process. XRD data and Raman spectra confirmed that KNbO₃ nanofibers with pure perovskite – orthorhombic phase were obtained. SEM images showed the morphological feature and a large quantity of KNbO₃ nanofibers were centimetres long, with the diameter of 100 nm. AFM images reveal that uniform grains of ~40 nm are closely stacked along the direction of nanofiber axis. Due to its bio-eco-compatibility, chemical stability and large surface-to-volume ratio, KNbO₃ nanofibers are utilized as an active material for environmental applications.

The as-synthesized KNbO₃ nanofibers were collected on a SiO₂/Si substrates with interdigitated Ta electrodes to fabricate a fast and highly sensitive humidity sensor. The sensor displays a logarithmic-linear dependence behaviour of the conductance with relative humidity (RH). The resistivity decreases from 7x10⁹ Ω to 5x10⁵ Ω (> 4 orders of magnitude) when relative humidity varies from 11% to 95% RH at room temperature. In addition, the sensor exhibits an ultra-fast response (2 s) and recovery (< 10 s) time, retaining its linearity and reproducibility.

Furthermore, photocatalytic activity of the as-synthesized KNbO₃ nanofibers has been evaluated by photo degradation of Methylene Blue (MB) under direct sunlight irradiation and UV LEDs (370 nm). KNbO₃ nanofibers have large surface area providing enough reactive sites to facilitate redox reactions. The results show MB dye removal percentage of ~80% after 120 min at a degradation rate constant of 0.013 min^-1. KNbO₃, being a ferroelectric material with an indirect bandgap of 3.35 eV, the changes in dipole and internal field should promote photo-generated charge carriers under UV irradiation. Effect of spontaneous polarization of ferroelectric KNbO₃ nanofibers in humidity sensing and photocatalytic efficiency is studied.

Piezoelectric and ferroelectric properties form the basis for many information and energy technologies by converting mechanical to electrical energy or vice versa. Local characterization of electromechanical response is needed to measure nanoscale responses, particularly for structure-function relationships of inhomogeneous materials. Unfortunately, existing approaches fail to provide quantitative values and pose significant ambiguity with respect to the origin of electromechanical response, for example in distinguishing between the effects of charge injection and piezoelectricity.
Piezoelectric Force Microscopy (PFM) has become a very successful methodology to probe piezoelectric and ferroelectric materials, by detecting surface deformation in the electric field of a biased microscope tip. While PFM is an easily accessible characterization technique for thin films, quantification of the piezoelectric properties and the role of measurement artifacts are still poorly understood. Hysteresis of electromechanical response can originate from electrostatic interactions between the tip and sample involving slow surface charging or ionic mechanisms that mimic ferroelectric responses. Similarly, charge writing and strong electrostatic tip-sample interactions can appear as persistent domains in PFM. Distinguishing intrinsic and extrinsic contributions to the electromechanical sample response in oxide and chalcogenide-based films has been widely discussed, however, clear guidelines on how to differentiate charge injection from ferroelectric response is missing.
We present a unified approach to electromechanical measurements that that enables effective differentiation of ionic displacement, piezoelectric deformation, and electrostatic interactions in a nanoscale contact geometry. Furthermore, by modeling cantilever dynamics, the surface displacements can be quantitatively extracted from the measurements, providing a systematic way to quantify piezoelectric response. These studies establish new opportunities for scanning probe microscopies to evaluate electroactive materials outside of the limited family of strongly polarized ferroelectric materials, now including ionic conductors, biological materials, and electroactive polymers.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program (NB) and US DOE MSED (SVK). Experiments were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Solution-processed metal oxide materials have been studied extensively in the past several years for their use in a variety of thin-film electronic applications such as display backplanes, solar cells, smart windows, gas sensors, and resistive RAM. They exhibit a wide range of properties that spans the spectrum of dielectrics, semiconductors and metals, and can be deposited from solution using sol-gel chemistries and techniques such as printing, spray coating, dip coating, or spin coating. The field of resistive switching for non-volatile memory applications has also grown significantly due to its potential for low power consumption and high density of devices. Transition metal oxides such as Ta₂O₅, NiO, HfO₂ and ZrO₂ are commonly employed as the switching material in such systems. The mechanism of operation usually involves the reversible formation of conductive filaments within the oxide that can bridge between metallic contacts. These elements can then be incorporated into arrays with a selector element to allow reliable memory addressing. Recently, complementary resistive switching (CRS) using two back-to-back memory elements has been demonstrated as a route to eliminate the selector [Linn, Nat. Mater. 9, 403-406]. Typically these materials are vapor deposited to achieve high device density and uniformity, however, the demonstration of solution-processed memory arrays is less well studied and of great interest from a low-cost, large-area manufacturing perspective.

In this work we have fabricated arrays of Au-ZrO₂-Ag back-to-back memory elements using inkjet printed metal contacts and sol-gel ZrO₂ deposited by spin coating on glass substrates. The gold acts as the inert contact with silver filament formation occurring in the oxide. Complementary resistive switching is observed at voltages less than 5V with good cycle endurance (10⁴ cycles) and retention times (10⁵ sec). Careful control of printing to obtain reliable contact overlap, thus defining the device area, is found to be critical for high yields. Additionally, the sol-gel ZrO₂ can be tuned to give symmetric SET and RESET voltages, which is needed for optimal CRS behavior. Combining these factors allows us to design simple device arrays that operate with negligible crosstalk during memory addressing and offer the potential for novel memory applications.

Interdigitated electrode (IDE) transducers with lead zirconate titanate (PZT) thin films have interesting properties for MEMS applications. Apart of a higher voltage response for harvesting devices, they also display excellent polarization stability, lower dielectric losses and lower leakage currents. However, the derivation of material properties is quite difficult because the electric field shows a complex line pattern and is not homogeneous. The electric field has to be rescaled to obtain correct polarization (PV) and capacitance (CV) loops, and dielectric constants. Since the direction of the electric field lies mostly in the film plane, it is expected that the grain boundaries of columnar, polycrystalline PZT thin films play a different role in the electric and ferroelectric properties of the film, as compared to the case of parallel plate geometry. For use with a Si substrate, the film has to be grown on an insulating buffer layer for proper device operation. This insulating layer influences as well the electric field distribution.
In this work, we grew (100)-textured PZT thin films by a sol-gel route on an electrically insulating buffer layer of MgO/SiO₂ on silicon substrates. The process was well reproducible. All films obtained in this way show a dense, columnar microstructure with a smooth surface. In PZT thin films much thinner than the IDE gap, the electric field between the fingers is nearly constant and always lower than the simple expression of voltage divided by finger distance. In the case of high permittivity substrates, such as strontium titanate (STO) as used for epitaxial growth of PZT films, the electric field becomes less homogeneous and is even more reduced inside PZT. Both kinds of substrate contribute capacitively to the film properties and this should be taken into account for determining the real PV and CV loops of the ferroelectric film. We can extract the dielectric constant of the film (about 1100 at zero bias) from the CV loop by means of a mathematical model for IDE capacitance computation, and those results are line with finite element simulations. Sputter deposition of the IDE is found to introduce some damage near the electrode/film interface, which causes the PV loops to become slanted. These damages can be partially cured by annealing at high temperatures under oxygen flow. Finally, IDE on PZT thin films show a strong tendency to age, i. e., the PV loops of poled samples display a voltage offset building up with time. Quicker aging is observed for Ti-richer compositions. We believe that grain boundaries act as dielectric layers where interface charges accumulate, driven by the electric field generated by the bound charges of the polarization of the neighbouring grains. The accumulating charges are originating from charge detrapping within the grain boundary. The aging contributes to the stabilization of the poled state, which is beneficial for small field operations, and in particular for energy harvesting.

Lead-free ferroelectric/piezoelectric BiFeO₃ (BFO) thin films can provide superior piezoelectric properties in both epitaxial and polycrystalline thin films for potential applications to multifunctional MEMS/NEMS devices (actuators and sensors), particularly for medical applications, for which BFO may provide biocompatibility as opposed to Pb(Zr_xTi_1-x)O₃, which contains non-biocompatible lead. The remnant polarization P_r and out-of-plane converse piezoelectric coefficient d₃₃ of BFO films are comparable to those of tetragonal Ti-rich PZT films. However, BFO films exhibit large coercive fields and large leakage current, which might limit the applicability of BFO in devices. Prior work focused on reducing the leakage current of BFO films via insertion of dopants in the material, but still the reduction in leakage is limited. This talk will focus on reviewing the R&D performed during the last three years on a novel approach to reduce the leakage current and increase the piezoelectric response of BFO film-based MEMS/NEMS devices by producing structured nanolaminates involving BFO films via insertion of an insulating layer like SrTiO₃ between two BFO layers, all with nanometer-scale thickness. The BFO/STO/BFO nanolaminates provide the means to achieve a strain-engineered structure at the BFO/STO interfaces, which result in very high piezoelectric deflection with low leakage current (~ 10^-7 - 10^-8 A/cm² at 1 V). This review will include a discussion of the mechanism responsible for the observed high piezoelectricity and low leakage current behaviors of the BFO/STO/BFO nanolaminates, as revealed by systematic high-resolution TEM and PFM based studies. The BFO/STO/BFO NLs properties provide the bases for excellent potential application of the BSB-NLs into lead-free piezoelectrically actuated MEMS/NEMS devices, especially for biomedical applications (e.g., biosensors and drug delivery systems) based on the potential demonstrable biocompatibility of BFO components.

Barium titanate (BTO) is one classic perovskite ferroelectric oxide which has attracted lots of research interest due to its high-dielectric constant and large piezoelectric coefficient. In recent years, lots of works have been done for the doped BTO systems to further improve its properties, such as ferroelectricity and piezoelectricity. However, the researchers need to face the problem of sharp decreasing of Curie temperature (C_p) after the doping.

In our work, utilizing pulsed laser deposition (PLD) technique, high quality epitaxial Sn-doped BTO thin films have been successfully grown on various substrates with different lattice coefficient. The observation of much better ferroelectric properties have been found from the Sn-doped BTO thin film compared with the bulk material of the same chemical composition. Moreover, with the demonstration of successfully epitaxial growth, The C_p of those films has been enhanced due to the effect of epitaxial constraint which prevent external dimension changes of thin film. Furthermore, giant piezoelectric coefficient has been obtained from that Sn-doped BTO thin film grown on certain substrate, which would provide particular strain to the film.

The microstructure of those thin films has been examined by the state-of-the-art electron microscopy for the mechanism of the origin of such giant electro-mechanical property of epitaxial thin film under that certain strain compared to other ones. Those improvements of doped BTO thin films would provide lots of potential application possibilities in multi-functional materials areas.

Ferroelectric material integration on semiconductors offers great potential for nanoelectronic and integrated-photonic applications. In particular, introducing a ferroelectric as a gate oxide could decrease the sub-threshold slope below the thermodynamic limit of 60 mV/dec in field-effect transistors (FETs) operating at room temperature. There are still major challenges with growing a ferroelectric oxide on a semiconductor and progress are still needed in obtaining a high quality interface (structurally and electrically), controlling the crystalline structure and orientation, controlling the cationic stoichiometry and oxygen content, and controlling the ferroelectric polarization and domain structure. Beyond film growth, also major progress is needed for the fabrication of integrated devices using the current Si-based nanoelectronic technologies.
In this talk, we will review our work on the molecular beam epitaxy of BaTiO₃ on silicon and silicon germanium surfaces. A detailed study of the impact of oxygen partial pressure during the growth of BaTiO₃ films on SrTiO₃-buffered Si(001) will be presented. The elemental profiles throughout the stacks were determined from electron energy loss spectroscopy (EELS) analyses. A quantitative analysis of atomic structure images (HRTEM or STEM-HAADF) using the geometric phase analysis (GPA) was carried out in order to map the strain in the stacks. We will show that local composition and crystalline parameters are strongly connected.
Si_1-xGe_x is of interest as a channel for future FETs and, via its Ge content, has the potential to be a lever for strain engineering. Passivation of the starting surface of Si_1-xGe_x with Sr or Ba fluxes has been studied by in situ X-ray photoelectron spectroscopy; only partial passivation with Sr is evidenced when using our standard recipe for Si surface. The direct growth of BaTiO₃ on Si_1-xGe_x will be discussed.
We will discuss the ferroelectricity of the films grown on Si and Si_1-xGe_x, as a function of BaTiO₃ – and if relevant - SrTiO₃ buffer thicknesses. Piezoresponse force microscopy imaging and hysteresis loops as well as Kelvin probe microscopy were carried out in various conditions.
Finally, we will describe i) the fabrication of Metal Oxide Semiconductor (MOS) capacitors using pre-patterned Si substrates with oxide isolation and using TiN as a metal gate. Ii) the fabrication of TiN-gated BaTiO₃ gate transistors based on a low temperature route starting from damascene pre-patterned structures. Technical challenges and achievements will be discussed.

Polycrystalline morphotropic-phase-boundary lead (Pb) free 0.5BaZr_0.2Ti_0.8O₃-0.5Ba_0.7Ca_0.3TiO₃ (BZT-BCT) thin films were deposited on Pt(111)/TiO₂/SiO₂/Si substrates by pulsed laser deposition (PLD). X-ray diffraction confirmed a polycrystalline growth of the as-deposited films with single-phase perovskite crystallographic structure with body centered tetragonal symmetry. The optimal ferroelectric response with a high spontaneous polarization Ps~110 μC/cm², remnant polarization of 32.5 μC/cm² with a coercive field of 0.18 MV/cm is observed from P-E hysteresis loops with a frequency of 1 kHz at room temperature. A giant recoverable energy-storage density of 33.39 J/cm³ at 3.58 MV/cm. The optimized BZT–BCT thin film exhibited a high dielectric constant of with a low dielectric loss. Distinct polarization contrast with a complex mosaic-like domain structure was observed in the out-of-plane mode of piezoresponse force microscopy (PFM). Switchable ferroelectric PFM hysteresis loops were obtained. Ferroelectric switching is not detected from PFM phase, amplitude images, after applying ±15V which exceeding the coercive bias voltage (from the obtained hysteresis loops). This may be due to rapid polarization relaxation.

La_0.7Sr_0.3MnO₃ (LSMO) and SrRuO₃ (SRO) are two ferromagnetic materials with Curie temperatures of respectively 350K and 150K. A positive exchange bias has been observed in bilayers and superlattices of LSMO/SRO [1,2,3] which originates from an antiferromagnetic coupling at the interface between these two materials. This phenomenon is of particular interest to obtain a hard layer for magnetic oxide junctions where the stabilization of a reference layer is a crucial point.
LSMO/SRO bilayers have been grown epitaxially by pulsed laser deposition on SrTiO₃ (STO) substrate. By controlling the substrate properties (orientation and surface properties) and the layer growth mode, we are able to control the SRO crystallographic variants present in LSMO/SRO bilayers. X-Ray diffraction and AFM characterizations show the good crystalline quality of the deposited layers but also reveal the SRO crystallographic variants.
In this paper, we will show that the SRO crystallographic variants seem to strongly affect the magnetic behavior of the bilayer measured by magnetometry. If several crystallographic variants are present in the SRO, an asymmetry in the LSMO hysteresis loop is observed [4]. We attributed it to the presence of two different exchange coupling strengths at the interface between LSMO and SRO by combining magnetometry, Polarized Neutrons Reflectivity (PNR) measurements and simulations based on an extended Stoner-Wohlfarth model. When the SRO has one crystallographic variant, the hysteresis cycles of the LSMO, coupled to SRO, are also asymmetric but in an unexpected way. PNR measurements show that the LSMO magnetization reversal is uniform and that only one exchange coupling strength is, this time, present at the SRO/LSMO interface. Simulations and complementary PNR experiments are currently performed to understand the origin of the hysteresis cycle asymmetry. One possibility is the SRO uniaxial anisotropy which possesses an out-of-plane component.

[1] X. Ke, M. S. Rzchowski, L. J. Belenky and C. B. Eom, Appl. Phys. Lett. 84, 5458 (2004)
[2] P. Padhan, W. Prellier and R. C. Budhani, Appl. Phys. Lett. 88, 192509 (2006)
[3]M. Ziese, I. Vrejoiu, E. Pippel, P. Esquinazi, D. Hesse, C. Etz, J. Henk, A. Ernst, I.V. Maznichenko, W. Hergert and I. Mertig , Phys. Rev. Lett. 104, 167203 (2010)
[4] A. Solignac, R. Guerrero, P. Gogol, T. Maroutian, F. Ott, L. Largeau, Ph. Lecoeur, M. Pannetier-Lecoeur, Dual antiferromagnetic coupling at La_0.67Sr_0.33MnO3/SrRuO₃ interfaces, Phys. Rev. Lett. 109, 027201 (2012).

Multiferroic magnetoelectric materials – those which exhibit simultaneous ferroelectric and ferromagnetic behaviors and permit control and switching of the magnetic order parameter, magnetization M via electric field E and, vice versa, polarization P with magnetic field H, have drawn significant interest in recent years because of their intriguing physical origin and great potential for multifunctional applications. Understanding the coupling of the ferroic (electric and magnetic) order parameters in multiferroics is a long-standing scientific challenge. We report studies of the ferroelectric and magnetic phase transition of Pb(Fe_0.5Nb_0.5)O₃-Co_0.65Zn_0.35Fe₂O₄ (x = 0.2) composite with emphasis upon the nature of magnetoelectric coupling at room temperature. The presence of all cationic elements with their required stoichiometry has been confirmed by SEM and XPS studies. The composite shows well-saturated ferroelectric and ferromagnetic (multiferroic) behavior at room temperature. A ferroelectric-paraelectric phase transition around 428 K has been confirmed from the temperature dependent dielectric spectra along with DSC and Raman spectroscopic studies. Existences of different magnetic orderings have been observed in this composite. The nature of magnetoelectric coupling is found to be biquadratic (P²M²) from magnetic and magnetocapacitance measurements. The mechanism responsible for the magnetoelectric effect in this composite takes place indirectly via ferroelastic strain rather than a direct coupling between the magnetic and electric order parameters. This composite shows strong bulk (not interfacial) biquadratic magnetoelectric coupling at room temperature, which can be useful for potential multifunctional device applications.

Topological insulators (TIs) are materials in which spin-orbit interaction and time-reversal symmetry cause exciting quantum properties such as electronic spin-momentum interlocking and dissipationless current flow. These phenomena emerge at the interface of the TI with an insulator, e.g. vacuum. Bismuth chalcogenide TIs have a high n-type bulk conductivity mostly due to vacancies and defects which masks electronic transport through surface electrons. Hence, TI nanowires are a natural choice to enhance the surface effects and suppress the relative contribution of bulk carriers.
Typically, TI nanowires, such as Bi₂Te₃, are grown using wet-chemical synthesis and chemical vapor deposition (CVD). CVD is more suited to produce clean samples with pristine surfaces. Catalyzed nanowire growth usually employs gold nanoparticles as seeds. We found three shortcomings following this approach: (1) The nanowires are contaminated with Au. (2) The crystal quality is low. (3) Undesired, differently shaped nanostructures are grown alongside the nanowires.
Therefore, we explored more exotic catalysts such as SiC and TiO₂, as well as self-catalyzed nanowire growth for comparison [1]. A special mixture of TiO₂ nanoparticles, P-25, delivered excellent results and performed much better than the conventional Au catalyst. P-25 catalyzed Bi₂Te₃ nanowires are of high crystalline quality, free of catalyst contaminations, and they grow almost purely as nanowires without the undesired formation of ribbons or clusters.
P-25 also supports the integration of Sn into the crystal structure of the TI nanowires. Sn atoms act as p-type dopants to counter the intrinsic, defect-related n-type doping of the material so that the bulk becomes truly insulating as desired for applications.

[1] Schönherr, P. et al. Comparison of Au and TiO₂ based catalysts for the synthesis of chalcogenide nanowires. Applied Physics Letters 104, 253103 (2014).

During the last few years, two-dimensional (2D) inorganic materials have emerged as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered Transition Metal Chalcogenides and Oxides is particularly high. These materials are being heavily researched due to their novel functionalities and suitability for a wide range of electronic and optoelectronic device applications. In this talk, I will present the latest work going on in this area in my lab at the university of Utah. Major focus will be on the synthesis and the layer dependent evolution of crystal, phonon, and electronic structure of these materials. A pulsed laser assisted deposition method capable of producing layer-by-layer growth will be introduced. Detailed characterization results using atomic force microscopy (AFM), Raman spectroscopy, UV-Vis spectroscopy, and photoluminescence measurements will be presented.

Recently the layered transition metal dichalcogenide WTe2 was reported to have an extremely large, non-saturating magnetoresistance (XMR), reaching values of 13 million % at 0.53 kelvin (K) and 60 tesla (T). Larger than the giant MR (GMR) or colossal MR (CMR) effects previously seen, this makes WTe2 of interest due to both its fundamental materials physics and its potential for low-temperature applications in magnetic sensors or computing-related devices. Following this work, additional studies on WTe2 have been conducted with findings including pressure-induced superconductivity, orbital spin texturing, a large and linear Nernst effect, predictions of a quantum spin Hall insulating state under strain, and the realization of one of the first Weyl Semimetals and, in particular, the first example of a Type II Weyl Semimetal. Several other materials such as Cd3As2, NbSb2, TaAs, NbAs, and NbP, ZrSiS, have also recently been reported to show a similarly large MR effect.
Here we present a correlation of RRR, MR ratio and average carrier mobility in the crystal growth of WTe2. A preferred method of single crystal synthesis is presented, consisting of a Te flux followed by a cleaning step involving self-vapor transport. The method is reproducible and yields consistently higherquality single crystals than are typically obtained via halide-assisted vapor transport methods. Magnetoresistance (MR) values at 9 tesla and 2 kelvin as high as 1.75 million %, nearly an order of magnitude higher than previously reported for this material, were obtained on crystals with residual resistivity ratio (RRR) of approximately 1250. The MR follows a near B2 law (B = 1.95(1)) and, assuming a semiclassical model, the average carrier mobility for the highest-quality crystal was found to be 167, 000 cm2/Vs at 2 K. Also, we will present the first successful creation of crystalline thin films of WTe2 using a simple and scalable technique (8” wafer processing is demonstrated). These are the first thin films of any Weyl Semimetal; film characterization and transport properties will be discussed.

The use of various nanostructures such as nanotubes and 2D sheets in electrical and electromechanical devices is the subject of intensive research in recent years. In particular, the electronic properties of inorganic compounds such as the dichlcogenides sparked the research of their incorporation into nano-electro-mechanical systems (NEMS). WS₂ nanotubes (INT-WS₂) have been shown to exhibit superior mechanical properties and interesting stick-slip mechanical phenomena¹ and thus are a natural candidate for electro-mechanical devices.
We show here that INT-WS₂ possess significant field-effect mobility and surprisingly high current carrying capacity². We further present the first demonstration of a significant electro-mechanical response in pure inorganic nanotubes³. The INT-WS₂ exhibited a highly repeatable increase of the conductivity in response to strain and/or torsion. These results are in qualitative agreement with the theoretical calculations presented here for torsion and previous theoretical predictions for strain. The large sensitivity to torsion and tension suggests INT-WS₂ as promising in NEMS such as nano-gyroscopes and accelerometers.
References
(1) Nagapriya, K. S.; Goldbart, O.; Kaplan-Ashiri, I.; Seifert, G.; Tenne, R.; Joselevich, E. Physical Review Letters 2008, 101, 195501.
(2) Levi, R.; Bitton, O.; Leitus, G.; Tenne, R.; Joselevich, E. Nano Letters 2013, 13, 3736-3741.
(3) Levi, R.; Garel, J.;Teich, D.; Seifert, G.; Tenne, R.; Joselevich, E. ACS Nano 2015, Article ASAP.

Two dimension materials, such as graphene and transition metal dichalcogenides (TMDs), could enable atomically thin devices and circuits with novel electrical, optical, and mechanical properties, including the valley Hall Effect observed in MoS2. However, the integration of these materials into functional devices is challenging due to the large contact resistance between them and the conventional metal electrodes (e.g. Ti, Au) with the typical values greater than 50 kΩ.µm. The large Schottky barrier height at the metal/TMD junction limits the injection of electrons to the TMDs and thus degrade the performance of the device, an effect even more significant when the dimensions are scaled down to sub-micron level where the contact resistance becomes dominant.
Here we demonstrate the use of CVD graphene as a universal contact electrode to various TMDs in which the junction is atomically sharp and shows low resistance ohmic behavior. To form the lateral contact, CVD graphene is transferred and patterned on a SiO2/Si substrate, after which TMDs are grown using MOCVD. The resulting graphene and TMD lateral heterostructures are mechanically strong and form atomically sharp junction in which the properties of each individual materials are well preserved. The low temperature measurements, down to 77K, shows the independence between contact resistance and temperature which implies a very small Shottky barrier height. At the same time, we have shown that graphene can be used as a universal contact electrode to TMDs by demonstrating its similarly low contact resistance (~20 kΩ.µm) to MoS2 and WS2 with excellent spatial controllability and scalability.

This lecture covers novel two-dimensional materials, including graphene and graphene oxide, topological insulators, and discovery of new phase of carbon (Q-carbon). Independent control of crystal structure and chemistry can lead to transformative new functional materials and devices. These materials are grown by laser annealing or by templating the structure of the underlying growth substrate. By laser annealing, near surface regions are modified by introducing defects and /or dopants which lead to desirable 2D electrical and magnetic properties. Templating leads to 2D metamaterials that assume the structure of the growth substrate and exhibit novel properties not otherwise achievable. We have created novel two-dimensional metamaterials such as bcc Ni, NiO, ZnO (wurtzite and zinc blende) and novel SrSnO3 and BiTeSe2 TIs. High-power pulsed laser annealing will be used to introduce defects in the top few layers of ZnO and NiO and modify the electrical, optical and magnetic properties in a controlled way.
The second part focuses on the discovery of new phase (distinct structure, bonding and entropy) of carbon, named as Q-carbon. The properties of Q-carbon are as follows: (1) The Q-carbon is amorphous and it can be semiconductor or metallic, and it has robust ferromagnetism with Curie temperature over 500K; (2) The Q-carbon has very high electron emission (negative electron affinity), needed for a variety of display devices; (3) Q-carbon can be harder than diamond as average bond length in Q-carbon is shorter C-C length in diamond; (4) Q-carbon can be converted into diamond rather inexpensively in the form of nanodots, microdiamonds, nanoneedles and microneedles, and large-area single-crystal films; and (5) diamond can be doped both n-type and p-type, creating a new frontier in high-power devices and high-temperature transistors. We have obtained recently parallel results from hBN, Q-BN and cBN.

Using a novel high vacuum CVD process we synthesize large area monolayer MoS₂ films. Our process involves exposure of a hot Mo filament to organic chalcogen which then precipitate on a thermally-controlled substrate. The resultant films are photoluminescent at the 1.87 eV as expected for monolayer material; their Raman modes are indistinguishable from exfoliated material. Metal contact formation to these films was investigated under UHV conditions utilizing X-Ray Photoelectron Spectroscopy. These measurements permit us to follow the formation of a Schottky Barrier with increasing metal film thickness on the Angstrom scale. We utilize core level spectroscopy to indicate the evolution of the MoS₂ valence band under metal deposition. Our measurements provide direct indication on the charge transfer direction at metal contacts and the ensuing band-bending in two-terminal devices.

Molybdenum disulfide (MoS₂) is a transition metal chalcogenide that has unique properties on a two-dimensional level. MoS₂ has potential to be used in different electronic applications, but more needs to be known about doping and controlling the properties of few layer films. This study presents a way to chemically modify the surface of MoS₂, which leads to doping the film without disruption of the crystal structure. There are examples of doping MoS₂ with an adsorbed molecule, but few where the dopant is covalently bound to the surface. By taking advantage of the sulfur defects on the surface, this process is able to create a covalent bond between MoS₂ and an organic molecule.
First, MoS₂ was grown through a chemical vapor deposition (CVD) process to produce large area films with uniform composition. An organic molecule, 3-aminopropyltrimethoxysilane (APTMS) was used to functionalize the surface through a CVD process. The presence of APTMS on the surface was confirmed using X-ray photoelectron spectroscopy (XPS) by the appearance of a Si peak. The O 1s XPS peaks show the appearance of an Mo-O peak, which suggests that the APTMS bonds to the MoS₂ through an Mo-O bond. The Mo 3d and S 2p XPS peaks show no change, which supports the fact that APTMS functionalization offers a stable, yet non-destructive, method to dope MoS₂.
The APTMS has a p-doping effect on the MoS₂, which is seen through XPS and photoluminescence (PL) spectroscopy. First, the APTMS functionalization causes a downshift in binding energy of both the Mo and S XPS peaks. This indicates a lowering of the Fermi level, making the MoS₂ more p-type. A Fermi level change is also suggested through valence band XPS, which shows a change in the valence band energy of MoS₂ after functionalization. Second, the PL peak shows a large increase in intensity after functionalization. It has been previously reported that p-doping of MoS₂ will cause an increase in PL intensity. This result agrees with the XPS that the APTMS does p-dope the MoS₂.
Lastly, this experiment was run using a different organic molecule, octyltrichlorosilane (OTS). This molecule is believed to bond to the MoS₂ surface in the same way, through an Mo-O bond. The OTS molecule causes very similar changes in the MoS₂ properties and also points toward p-type doping of the material. Even though the two molecules have different end groups, they give a similar effect. This is evidence that the main influence on the doping of the MoS₂ is the silane at the interface and the covalent bond and not the rest of the molecule. This gives insight into how changes at the interface of MoS₂ are important to the underlying properties.
This procedure provides a way to tune the optical properties and the doping of MoS₂, which in turn may affect the electronic properties. This technique can be expanded to include more organic molecules and has potential to be used for different chalcogenide semiconductors, leading to new ways to dope different materials.

Recent advances in the field of two-dimensional (2D) dichalcogenide materials have indicated that the use of vapor deposition techniques such as Chemical Vapor Deposition (CVD)¹ or Atomic Layer Deposition (ALD)² could be instrumental to the production of large area two-dimensional materials and simultaneously offer superior alternatives to exfoliating methods. In this work we consider the transition metal dichalcogenide MoS₂ – specifically its growth starting from an ALD route comprised of (^tBuN)₂(NMe₂)₂Mo and O₃ to produce amorphous MoO₃,³ followed by subsequent sulfurization steps to produce MoS₂. The chemical conversion to MoO_2-xS_x and finally 2H MoS₂ through the multistep sulfurization process is studied. Data from a variety of direct measurement techniques for determining the existence and quality of monolayer MoS₂, including atomic force microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy will be presented. A method of thinning oxide films from the post nucleation growth regime by plasma etching to produce the desired thickness of MoS₂ is demonstrated. Finally, the effects of temperature, incorporation of plasma, precursor exposure, and surface cleaning – both wet and plasma – on ALD nucleation kinetics are explored and shown to be critical to the production of high quality films in the monolayer and few layer regime without an intermediate etching step.

References
1. Shi et. al. Chem. Soc. Rev. 2014; DOI: 10.1039/c4cs00256c
2. Song et. al. ACS Nano, 2014, 7 (12), pp 11333-11340; DOI: 10.1021/nn405194e
3. Bertuch. et.al. J. Vac Sci Tech. A 32, 01A119, 2014; DOI: 10.1116/1.4843595

Topological insulators (TIs) showcase fascinating quantum effects that have attracted more and more interest during the last decade, as well as offering a promising platform for applications in spintronics. The signature of a TI is its topologically protected surface state. The 3D TI Bi₂Se₃ was among the first that were experimentally realized. It is part of the chalcogenide family of TIs which includes the layered, binary compounds formed by Bi, Sb, Se, and Te among which only Sb₂Se₃ is not layered and a trivial insulator. The Dirac point of Bi₂Se₃ and Bi₂Te₃ is located under the valence band maximum due to n-type doping caused by vacancies and defects. Counterdoping can reduce the intrinsic carriers and align the Fermi level with the Dirac point. Another approach is simply the reduction of the surface-to-volume ratio by using nanomaterials, where the contribution of surface electrons to the electronic transport is increased.
We explored the growth of ternary chalcogenide nanowires using chemical vapor deposition (CVD) with a special catalyst, TiO₂, that increased the yield of nanowires. We found a new TI with a non-layered structure [1]. The material, (Bi_0.6Sb_0.4)₂Se₃, has an orthorhombic crystal structure characterized by conducting quasi one-dimensional atomic chains. Its electronic band structure displays a single Dirac cone and the Fermi energy is aligned with the bandgap as measured by nano angle-resolved photoemission spectroscopy (nanoARPES). The new material resembles Sb₂Se₃, which has been shown to be a TI at high pressures as well.

[1] Schönherr, P. et al. A new topological insulator built from quasi one-dimensional atomic ribbons. Phys. Status Solidi RRL 9, 130–135 (2015).

Chalcopyrite semiconductors such as Cu(In_1-xGa_x)Se₂ (CIGS) are promising materials for high-efficiency thin-film photovoltaics; however, due to significant lateral spatial inhomogeneity in composition (and consequently structure), the recombination mechanisms that limit power conversion efficiency (PCE) can vary at a single-grain level. Although the PCE of CIGS solar cells is believed to be largely limited by the radiative and non-radiative recombination processes in the bulk (space-charge region and the quasi-neutral region), these measurements have been performed on large area devices, thereby averaging out the effects of spatial inhomogeneities. To understand the effects of these inhomogeneties on the recombination mechanism, we carry out electrical characterization on specific grains as a function of temperature to investigate the nanoscale recombination dynamics of CIGS photovoltaic devices with an optimum gallium composition of ~0.3 and overall copper composition of ~0.9. We correlate these nanoscale electrical measurements with energy dispersive X-ray spectroscopy to determine composition, confocal Raman spectroscopy for phase, and aberration corrected transmission electron microscopy with selected area electron diffraction to ascertain structure at the specific CIGS region to identify the source of the recombination. We find that regions with a higher fraction of copper-rich grains in the absorber material show interfacial (as opposed to bulk) recombination, as revealed using temperature-dependent current-voltage measurements, even though the overall copper composition of the CIGS is sub-stoichiometric. These results provide additional insight into the complexity of recombination dynamics in chalcopyrite materials, and can prove beneficial in identifying strategies for lowering recombination rates in an effort to develop high-efficiency thin-film solar cells.

Nanoporous and nanoflake tungsten oxide (WO₃) films were produced on metallic W foil via one-step potentiostatic anodization method. The effect of annealing temperature on the morphological, structural and optical properties of the material was studied. The nanoflake films conserved their topology under different annealing temperatures (up to 500 ^oC) with particle sizes in the range of 37 to 46 nm. However, the nanoporous films showed a gradual deterioration in the porous structure upon annealing up to 500 ^oC. X-ray diffraction and Raman spectroscopy measurements were done to investigate the structural properties of the materials. The photoelectrochemical activity tests reveal that nanoflakes annealed at 500^oC exhibited a superior photocatalytic performance among all other samples, achieving a photocurrent density of 0.98 mA cm^-2 at 1.0 V_SCE under AM 1.5 illumination. The UV-Vis absorption and incident photon-to-current efficiency measurements are used to explain the activity-morphology relationship in such nanostructured films. Also, the stability of the films was investigated under light on/off conditions.

Cu₂ZnSnS₄ (CZTS) films were prepared by atmospheric sulfurization of chemical bath deposited (CBD) CuS-ZnS-SnS precursors using ditert-butylsulfide [(C₄H₉)₂S: DTBS] to serve as an alternative sulfur source for CZTS sulfurization owing to the merits of its improved environmental friendliness and reduced bond dissociation energy of DTBS comparing to that of H₂S. In this study, CuS-ZnS-SnS precursors were prepared by immersing glass substrates in diluted solutions containing copper, zinc, tin and sulfur species. The CBD-deposited precursors were, then, sulfurized by using DTBS in an atmospheric pressure quartz reactor at a constant temperature ranging from 400 to 600°C for 10 (or 20) min, under a constant DTBS flow rate of 2 (or 4) μmol/min carried by purified nitrogen gas. The characteristics of DTBS-sulfurized CZTS films were investigated by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), absorption spectroscopy, and x-ray photoelectron spectroscopy (XPS). It was found that increments of sulfurization time interval and DTBS admittance at high sulfurization temperatures tended to enable the formation of CZTS structure with improved optical quality and stoichiometry. Sufficient DTBS replenishment and thermal energy are essential to complete the reactions of sulfides through sequential pathways so that high quality DTBS-sulfurized CZTS films can be achieved.

In the field of photovoltaics, especially the third generation, light trapping structure has been widely developed and utilized to improve device performances. A common approach is to texture the surface of the front electrode usually made of transparent conductive oxides (TCOs). One of the important TCOs is polycrystalline fluorine-doped SnO₂ (FTO) thin film. Differently textured FTO thin films have been studied and were shown to improve the quantum efficiency of microcrystalline Si (µc-Si) thin film solar cells ^[1].

However, the texturing of FTO surface is usually realized by specific surface morphology development during film growth, which involves complicated studies thus is time-consuming and not at all straightforward. In this study, we report a cost-effective and easy-to-reproduce routine to fabricate high haze FTO electrodes by introducing the use of nanoparticles. In the first studies, ZnO and sulfur doped TiO₂ (S:TiO₂) nanoparticles have been used to make ZnO-FTO and S:TiO₂-FTO nanocomposites. The structural, electrical and optical properties of such nanocomposites are presented and discussed. By varying the nanoparticle surface density, the haze factors of the nanocomposites can be varied from 0.4% up to 64% in the visible range without degrading total optical transmittance (~90%), while maintaining good electrical properties (i.e. sheet resistance 10-15 ohm/square) ^[2][3]. With such high haze factors, the nano-composite FTOs are very valuable in integration in solar cells.

^[1] T. Oyama et al. MRS online Proc. Libr. (2008) 1101
^[2] G. Giusti et al. ACS Appl. Mater. and Interf. 6 (2014) 14096
^[3] S.T. Zhang et al. Phys. Status Solidi A (2015), submitted.

Silver antimony telluride (AgSbTe₂) thin films for the thermoelectric (TE) applications near room temperature were electrochemically synthesized for the first time. This study allows to investigate the key factor that optimizes the TE power of the ternary system as well as to correlate the crystallinity and transport properties with respect to composition. To investigate the effects of crystallinity and composition on electrical and TE properties, amorphous Ag_xSb_yTe_z solid solutions with various compositions were first electrodeposited by tailoring the Ag⁺ concentration in the electrolytes, followed by a thermally driven solid-state amorphous-to-nanocrystalline phase transition. Due to the relatively lowered carrier concentration in the nanocrystalline phases, almost two-times higher Seebeck coefficients compared to the values observed in the bulks and higher Hall mobility led to the enhanced power factor of 90 ~ 553 μW/mK². The optimum TE property of 553 μW/mK² was obtained from the nanocrystalline AgSbTe₂ thin films at room temperature, which is highly comparable to the literature data of the electrodeposited p-type V-VI compounds. This improvement of the TE properties of electrodeposited AgSbTe₂ thin films shows the possibility to be utilized as p-type legs for the room temperature operation using the simple and cost-effective electrodeposition method.

The recent increase of interest in indium-free transparent conducting oxides (TCOs) warrants the development of alternatives to replace indium tin oxide (ITO). A valid candidate is the transparent composite electrode (TCE) that has a layered structure of transparent metal oxide (TMO)/metal/TMO. As potential TCE candidate, structures of TiO₂/Au/TiO₂ (i.e., with various Au thicknesses) are fabricated on flexible substrate by physical vapor deposition at room temperature. The influence of Au thicknesses on optical, electrical properties is also addressed by UV-VIS, four-point probe and Hall measurement. Our results show that the optimal transmittance and sheet resistance are achieved when the Au layer reaches a critical thickness. For this thickness, the optical transmittance at 550 nm is as high as 86% and a figure of merit corresponds to a value of ~ 18 Χ10^-3 Ω^-1 (which is one of the highest values reported to date).

Nowadays, over-heat-induced performance degradation of application processor (AP) is a hot topic in the mobile device industry. It is expected that application of Peltier devices, cooling system based on thermoelectric (TE) materials such as chalcogenides, will be the best solution for that problem. Peltier devices have been developed based on bulk TE materials. Because heat problems occur at focused area and space for cooling system is limited on AP, planar (thin-film) and radial type will be the best solution for mobile device rather than bulk type. However, as dimension of TE materials shrinks from bulk- to thin-films, unprecedented problems raised, which are degradation of TE properties, poor adhesion with substrate, and self-heating due to contact resistance. In this study, we design the planar type radial peltier devices for focusing at the heating area, with considering the problems of TE thin-films. We improved TE material properties with controlling defects in TE thin-film, adhesion property, and selected electrode metal satisfying ohmic contact between metal and TE materials for carrier transport with low resistance.
TE films were deposited on SiO₂/Si wafer by RF magnetron sputtering. TE films were co-deposited with Bi₂Te₃ and Te for N-type film and Bi₂Te₃ and Sb₂Te₃ for P-type film. Annealing, N doping, and Ar implantation was proceeded for improvement of TE properties. Ge₂Sb₂Te₅ (GST) film was selected for adhesion layer. Electrode was deposited with DC magnetron sputtering.
For improving TE properties, Seebeck coefficient (α) and electrical conductivity (σ) of the films were increased with annealing temperature below 300 °C. N doping, Ar implantation changed native defects of TE thin film. Carrier concentration and mobility were increased simultaneously at Ar implanted thin film. After all, TE thin films were optimized similarly with other process for deposition TE thin film, which has about 1300 S/cm and -110 μW/K²cm for N-type and 1800 S/cm 130 μW/K²cm for P-type, α and σ respectively. For enhancement of adhesion, using GST under layer improved adhesion property. Both GST and TE materials have the similar hexagonal unit cell structure which can reduce the lattice mismatch. Furthermore, the low thermal conductivity of GST prevent heat spreading into substrate which causes power wasting of device. For lowering contact resistance, the work function of metal has to lower than N-type and higher than P-type for ohmic contact. Electrode is deposited with various metal, Ti, Cu, Au. Among them, Au satisfied above conditions and showed lowest contact resistance. Based on the studies of each of TE type film deposition above, planar type radial peltier device can be fabricated with overall sputtering process, which cool down the small and focused heat area. N-type and P-type TE materials are connected in series with low contact resistance metal. The performance of planar radial peltier devices will be also discussed using thermos-graphic camera.

This work demonstrates the fabrication, characterization and integration of thin-film high density capacitors (HDCAP) based on tantalum in a power module for low power applications. Existing discrete tantalum capacitor technology meets the HDCAP requirements. However, they have incompatibility issues with organic/silicon packages bringing in several packaging challenges. Similarly, silicon trench capacitors employ ALD process rendering it expensive and non-scalable.

Demand for higher volumetric efficiency capacitors with electronic package compatibility requires high surface area architectures, low temperature, low cost and scalable processes and a more elegant approach to fabricate HDCAPs. In this work, thin-film capacitors were fabricated by printing and subsequent sintering of Ta nanoparticles to form high surface area anode followed by conformal dielectric oxide growth through the process of anodization and dispensing of cathode fluid inside the porous electrode structure. The pre-fabricated capacitor foils were laminated to the silicon substrate with a thin, ultra-low loss adhesive using a vacuum laminator. A planarization layer was laminated onto the tantalum films. Multiple blind vias were drilled through the Tantalum and polymer layers. Some of the vias were used to access the copper pads on the Si and the others to access the capacitor cathodes and anodes. As a final step, semi-additive copper process was used to form via connections and also the top assembly and test-pads.

The anodization process leads to growth of thin films with higher capacitance densities and higher yield at low temperatures. The self-limiting nature leads to uniform oxide coating on porous foils. The low temperature dielectric synthesis could enable their integration in ICs and packages. This will assist system miniaturization marred by a host of process incompatibility /manufacturability issues. Such integrated HDCAPs could be used in (a) high speed digital applications for noise suppression in power supply, (b) bio-electronic systems (c) miniaturized RF components, (d) storage cells and gate dielectrics for semiconductor devices. The capacitors demonstrated stable volumetric capacitance densities of more than 4 µF/mm3 upto 10 MHz with leakage of less than 0.1 µA/µF at 3 V. Compositional (EDS), Structural (SEM/XRD) and electrical studies were done to elucidate tantalum-pentoxide films.

Efficient and affordable energy conversion and energy storage technologies are required to meet society’s increasing demands. Semiconductor nanocrystals are particularly attractive materials for solar energy conversion applications, as their tunable optoelectronic properties can be manipulated to both optimize the absorbance of solar photons and to afford desirable electronic properties for various solar electricity, photocatalytic, or thermoelectric applications. Further tunability of binary systems can be realized through substitutional doping, where a small fraction of the cations or anions in the nanocrystal lattice are replaced by ions of other elements, thereby introducing new electronic states into the nanocrystal. However, doping can be difficult, as the dopants can cause significant lattice strain in the host crystals. Thus, increased synthetic understanding is necessary to afford new functional material systems. Lead-doped ZnS nanocrystals are one promising material for the conversion of solar photons into storable fuels such as hydrogen gas. The ZnS conduction band is sufficiently high in energy to reduce protons, and the lead dopants are hypothesized to add filled states in the ZnS band gap, thereby extending the absorbance of the crystals into the visible region. This work details progress towards controllable doping of ZnS nanocrystals with lead cations using modified hot injection procedures. Preliminary results suggest that the temperature of the ZnS reaction matrix upon lead precursor injection can be used to afford various doped products. Spectroscopic data will be presented to demonstrate doping-dependent optical and electronic properties in combination with high resolution transmission electron micrographs for structural information. The early results of this work suggest that functional materials for hydrogen gas generation may be readily synthesized from earth-abundant precursors, representing a viable option for future solar energy conversion platforms.

Nowadays, many applications of optoelectronics require materials with tailored properties, specifically, those referred to get a desired electrical conductivity and optical transparency. Most of the investigations in the literature correspond to the development of n-type transparent metallic oxide semiconductors, which are characterized by having a wide bandgap energy value. However, optoelectronics applications also require p-type semiconductors. Recently, besides of metallic oxides, chalcogenide metallic materials have become an important research field in order to get p-type semiconductors. Specifically, CuS is a promising material for device applications such as transistor devices. Here, we show preliminary results on CuS films obtained by a chemical bath deposition method (CBD). For the reaction solutions we used cupric Nitrate (Cu (NO₃)₂) as source of Copper (Cu) ions, Thiourea as source of Sulfur (S) ions and triethanolamine (TEA) as complexing agent. Potassium hydroxide (KOH) was used to adjust the solution to pH 9. Clean glass substrates were vertically immersed into the solution, contained in a beaker at a temperature of 60^oC, and were taken out after 15 minutes. For all the cases, the results from X-ray diffraction shows that CuS thin films are amorphous. However, the Raman spectroscopy showed the presence of CuS. The morphology of the CuS thin films was a homogeneous deposition of rice shaped particles. The size of the particles was approximately 50 nm. The XEDS[L1] analyses showed a stoichiometric deposition of Cu and S. The optical transmittance obtained for the CuS thin films was approximately 75% and the results of the energy band gap was 2.38 eV. The obtained thin films were used to fabricate thin film transistors (TFTs) which have shown good output curves. A study on the electrical response of the TFTs to luminous sources is also shown.

[L1] X-ray energy dispersive spectroscopy.

The lead sulphide (PbS) is one of the semiconductor materials with low band gap suitable to absorb the near-infrarred region of solar spectrum, in which lies nearly half of the energy from the sun. The PbS is abundant and inexpensive with a band gap E_g that can be tuned from 0.4 to 2.3 eV by changing its crystallite grain size. Furthermore, the PbS is soluble in water which allows its deposition by aqueous chemical methods. In the present work is presented the synthesis and characterization of PbS thin films obtained by chemical bath deposition for their application in thin film solar cells. The chemical solution was composed of lead acetate [Pb(C₂H₃O₂)₂ 3H₂O] and thiourea [CH₄N₂S] as the precursors for the metal and sulfur ion sources. Sodium citrate [Na₃C₃H₅O(COO)₃ 2H₂O] was used as complexing agent for metal ions. The PbS thin films were deposited at bath temperature of 40°C during 30 and 45 min. The pH of the solution was kept at 12.5 by adding potassium hydroxide [KOH]. The preliminary results show that the surface morphology of the as-deposited PbS thin films are constituted of particulates of 250-300 nm in size, distributed homogeneously over the surface without the presence of voids or cracks. The XRD patterns of the films, obtained at glazing incidence, resemble the cubic structure Galena, indicating that they are crystalline with a crystallite size of 13 nm. The estimated E_g from the Tauc plots of the as-deposited PbS films is of 1.55 and 1.23 eV for the deposition times of 30 and 45 min. On the other hand, the electrical characterization indicates that the PbS films are photo-sensitive to the light; the photo-response of the film is better for that obtained at 30 min while the conductivity is higher for that obtained at 45 min. The hot-probe measurement of the PbS films gives p-type conductivity. An improvement of the optical, electrical and structural properties of the PbS films will be done by heating them at different temperatures under argon/sulfur atmospheres. The resulting PbS films will be tested in a solar cell using a CdS thin film partner as window into the cell structure.

This study focuses on understanding the influence of incorporating Ammonium Acetate into the chemical bath used for the deposition of CdS thin films, on the final films’ optical, morphology, and microstructural properties. Thus, CdS thin films were deposited on 1” × 2” microscopic glass substrates using chemical bath deposition (CBD) method. The deposition process was carried out in a double jacket beaker with fixed chemical bath temperature of 90°C for a deposition time of 40min. The chemical bath solution consisted of fixed concentrations of Cadmium Acetate, Thiourea, and Ammonium Hydroxide; as such the concentrations values are of 4.8×10^-4M; 0.97×10^-4M; and 0.2M, respectively. However, in order to study the impact of the ammonium acetate incorporation into the chemical bath and to induce variations in the morphology, structural, and optical properties of the final deposited CdS films. Accordingly, Ammonium Acetate was incorporated into the chemical bath with concentrations that were varied from 0.003M to 0.0122M. Meanwhile, for comparison purposes and to indicate the initial conditions and properties of the CdS films, additional CdS films were deposited in the same chemical bath, but without adding Ammonium Acetate. The pH of the chemical bath was measured during the deposition process. The films’ morphology and the chemical composition was examined by Field Emission Scanning Electron Microscopy (FE-SEM), and the Energy Dispersive spectrometer (EDS), respectively. The X-Ray Diffraction (XRD) was applied to study the structure of the films, including the lattice parameters. Atomic Force Microscopy (AFM) was used to measure the Root-Mean-Square (RMS) surface roughness of the films. Dektak Profilometer was used to determine the CdS films’ thickness, where the films’ optical properties were measured using UV-Vis-NIR spectrometer. Optical energy band gap (Eg), and absorption coefficient (α) were calculated from the transmission spectral data. Preliminary results showed a decrease in the PH of the solution with increasing the concentration of the Ammonium Acetate; leading to increase in the growth rate and consequently increase of the final CdS film thickness. The PH value of the bath was decreasing (from 9.2 down to 8.2) with increasing the concentration of the Ammonium Acetate incorporated into the bath. The chemical composition of the films revealed slight increase in the Cd:S ratio beyond 1 with increasing the Ammonium Acetate concentration in the deposition bath. Whereas, the CdS films exhibited hexagonal crystal structure with (002) preferred orientation. The calculated lattice parameters using Cohen’s method, showed reduction in the crystal size (c:a ratio of the lattice parameters) with increasing Ammonium Acetate concentration in the deposition bath. However, further data and interpretation of the results of this research work will be presented in the detailed paper.

MoS₂ is a key industrial catalyst for hydrodesulfurization (HDS) of crude oil. Atomistic investigation of the HDS reaction is hampered by the elevated pressures and temperatures required for HDS. With regards to the chemical/geometric setup of the catalyst particle, though, there is close correlation between the actual catalyst material and model single-layer MoS₂ flakes that can be prepared in a lab. We developed a new experimental approach that takes relies on the strong photoluminescence (PL) characteristics of monolayer MoS₂ (peak @ 670-690 nm). The PL intensity of MoS₂ inversely correlates with its defect density and also with the adsorption of reactive species on its surface. We present data that correlated PL mapping under and immediately following reactive conditions to the reactions product distribution as measured by mass spectrometry. The use of wide field imaging drastically reduces the data acquisition time. Our work is based on recent findings, that sputtering of MoS2 allows removal of sulfur atoms from its basal plane resulting in dimming of its photoresponse. Exposure to an organic sulfur source such as benzenethiol then permits recovery of photoluminescence as a direct indicator of ongoing hydrodesulfurization at basal plane defects. In addition to presenting data on the sputter-induced formation of basal plane defects, we also show how MoS₂ photoluminescence responds to hydrogen exposure at elevated temperatures resembling the surface conditions during hydrodesulfurization.

Cu(In,Ga)Se₂ (CIGS) thin film solar cells have reached record efficiencies greater than 21% on a laboratory scale. A key stepping stone to achieving these efficiencies was the discovery of the beneficial effects of alkali elements such as Na and K. It is known that Na increases the majority carrier concentration, when added through a post deposition treatment (PDT) or by diffusion from the soda lime glass (SLG) substrate during CIGS growth. The distribution of the alkali elements in the CIGS film and the position and role in the crystal lattice are not well understood; however, it is argued that Na could be located at the grain boundaries, aiding in chemical passivation^[1].The KF PDT has been correlated to an increase in V_oc and decrease in J₀ ^[2]. It is argued that KF treatment improves the p-n junction quality by allowing Cd to diffuse into the CIGS layer creating a buried junction ^[3]. While it is understood that alkali elements are beneficial to device performance, the mechanisms that provides the enhancement is still under discussion.
Synchrotron based x-ray fluorescence (XRF) and x-ray beam induced current (XBIC) are excellent techniques to investigate the correlation between composition and charge carrier collection with nanometer scale spatial resolution. These techniques are non-destructive and are capable of quickly sampling areas > 100 μm² to achieve statistically meaningful results over many grains and grain boundaries^[4]. We have employed XRF and XBIC to investigate the impact of alkali PDTs and diffusion from the SLG on grain boundaries in CIGS solar cells. Two sets of samples with KF and NaF PDT are compared with identically grown films that did not receive a PDT, as well as samples with varied Na addition from the substrate. Results from this study will determine the impact of alkali elements and PDTs on the composition distribution of grain boundaries and if a corresponding increase in the distribution of the charge carrier collection is observed. We believe that studying the impact of alkali elements and PDTs in a statistically meaningful way will help shed light on the position of alkali elements in the film and their impact on recombination centers. This will provide a path forward to further increasing the efficiency of these devices.
[1] F. Pianezzi et al., Phys. Chem. Chem. Phys. 2014, 16, 8843.
[2] P. Jackson et al., Phys. status solidi - Rapid Res. Lett. 2015, 9, 28.
[3] A. Chirilă et al., Nat. Mater. 2013, 12, 1107.
[4] B. West et al., Proceedings of the 42nd IEEE Photovoltaic Specialist Conference, 2015, New Orleans, LA.

Copper indium gallium diselenide (CIGSe) thin films are regarded as a promising chalcogenide material to be used as an absorber layer in solar cells. The photovoltaic conversion efficiency of CIGSe solar cells reached up to 21.7% using vacuum method (co-evaporation). Since vacuum based processes have higher production cost and possess difficulty in scaling up, non-vacuum based methods are more encouraging due to efficient use of materials, high throughput and low cost. In the present work, CIGSe thin films were deposited using non-vacuum mechanically alloyed nanoparticles and the impact of annealing and selenization treatments on thin film properties is reported. Nanoparticles were synthesised by mechanical alloying of elemental Cu, In, Ga and Se using ball to powder ratio of 25:1 at milling speed of 400 rpm for 2 h. The structural and compositional properties of synthesised nanoparticles were studied by X-ray diffraction (XRD) and Energy Dispersive Analysis of X-rays (EDAX), which confirmed the formation of single phase chalcopyrite CIGSe nanoparticles with desired composition (Cu/In+Ga=1, Ga/In+Ga=0.3). The ink was formulated by dispersing the nanoparticles in absolute ethanol with the aid of organic binders, ethyl cellulose and terpineol. CIGSe thin films were deposited on glass substrate by paste coating of the ink. The films were air annealed at 300 and 400 °C for 1 h in order to remove the carbon contents. The film annealed at 300 °C is referred as film A and film annealed at 400 °C is referred as film B. The air annealed films were selenised in two steps, first at 400 °C for 30 min and then at 550 °C for 15 min. The structural properties of as-prepared, air annealed and selenised films were characterised using XRD analysis. Compared to the XRD patterns of the as-prepared and air annealed films, the selenised films showed intense and sharp peaks. Film A retained the single phase of CIGSe while film B decomposed into binary phases. After selenization, XRD pattern of film B showed peaks related to CuSe phase along with chalcopyrite phase of CIGSe. In addition, film B showed peak splitting along with a small shift in peak position towards lower 2 theta values whilst film A exhibited well defined peaks corresponding to chalcopyrite CIGSe phase. EDAX data showed increase in Se content from 46.55 to 49.43 at% and from 28.46 to 40.51 at% in the film A and film B respectively, subsequent to selenization. Field Emission Scanning Electron Microscopic images of both the selenized films showed dense surface morphology. As selenised film A showed superior structural and compositional properties than film B, film A was chosen for further studies. The current- voltage (I-V) curve of CIGSe film showed ohmic nature of contact resistance. The current of CIGSe film at 10 V showed 1.2 fold increases in the presence of light, which proves its applicability as absorber layer in solar cells.

Enhanced grain growth and re-crystallization for CuIn_0.7Ga_0.3Se₂ (CIGSe) thin films deposited using particle based approach can be achieved by selenization of off-stoichiometric precursor films. Stoichiometric (CuIn_0.7Ga_0.3Se₂) and off-stoichiometric (CuIn_0.7Ga_0.3Se_0.5 and Cu_0.85In_0.7Ga_0.3Se_0.5) nanopowders were synthesised by mechanical alloying of elemental Cu, In, Ga and Se. X-ray diffraction analysis (XRD) of synthesised nanopowders showed existence of secondary phases such as Cu₁₁In₉, In₂Se₃, Ga₂Se₃, Cu_2-xSe in both the off-stoichiometric nanopowders while stoichiometric nanopowders showed only single phase of chalcopyrite CIGSe. Field emission scanning electron microscopy (FESEM) results revealed that both stoichiometric and off-stoichiometric nanopowders consist of highly agglomerated nanoparticles. Energy dispersive analysis of X-rays (EDAX) exhibited homogeneous distribution of the elements Cu, In, Ga and Se in the stoichiometric nanopowders whereas inhomogeneous distribution of elements was observed in the off-stoichiometric nanopowders. The nanopowders ink was formulated with the inclusion of organic additives in order to deposit thin films by knife-coating. Afterwards, the films were annealed at 300 °C for 1 h in order to eliminate the organic additives and selenized at 550 °C for 15 minutes to obtain dense and continuous films. The structural, compositional and morphological properties of films before and after selenization were studied using XRD, EDAX and FESEM analyses. The XRD pattern revealed that no phase evolution was occurred in the stoichiometric films after selenization whilst the binary phases were vanished and single phase CIGSe was formed during selenization of off-stoichiometric films. The EDAX results showed that both the stoichiometric and off-stoichiometric films had CIGSe phase composition subsequent to selenization indicating the reaction of Cu-In-Ga-Se phases in the off- stoichiometric films with the Se which leads to the formation of single phase CIGSe. The selenized CuIn_0.7Ga_0.3Se_0.5 films were slightly Cu-rich (Cu/Se= 0.6) compared to selenized CuIn_0.7Ga_0.3Se₂ (Cu/Se=0.49) and Cu_0.85In_0.7Ga_0.3Se_0.5 (Cu/Se=0.45) films. Planar and Cross-sectional FESEM images proclaimed improved grain growth and recrystallization in the selenized off-stoichiometric films than selenized stoichiometric films. Hence, the strategy of CIGSe film preparation using off-stoichiometric nanopowders could be a possible way to improve the performance of particle based thin film solar cells.

Nowadays, it is crucial to discover highly active electrocatalysts composed of earth-abundant materials for the oxygen evolution reaction (OER). Combinatorial methods has been reported to provide an efficient way to screen and discover mixed metal oxides as promising OER electrocatalysts. Here, we have tried to explore Ni-based selenides doped with other transition metals as potential OER electrocatalysts using the Ni-Co-Fe ternary phase diagram in a combinatorial approach. In our work, we synthesized transition metal selenide films containing a mixed metal composition of Ni-Fe-Co by electrodeposition on different substrates including Au-coated glass and glassy carbon (GC). We systematically investigated how the doping with Fe and Co affects the electrocatalytic activities of Ni-selenide for water oxidation. The structure and morphology of these electrodeposited films were characterized with power X-ray diffraction, X-ray photoelectron spectroscopy, Scanning and Transmission electron microscopy and Raman spectroscopy. The catalytic activities were studied through electrochemical studies in alkaline conditions using linear sweep voltammetry, while stability of the catalyst was investigated by chromoamperometric studies at constant potential. In the presentation we will show some novel catalyst compositions containing Ni_xFe_yCo_zSe_n, Ni_xCo_ySe₄, and Ni_xFe_ySe₄ which show low onset potentials for O₂ evolution along with extended stability. Some of these catalysts were found to be active for H₂ evolution as well, essentially functioning as full water splitting catalyst. A systematic exploration of binary and ternary Ni:Co:Fe selenides along with electrochemical studies for catalytic efficiency will be presented in the poster. Such studies provide the foundation to investigate other metal selenide combinations.

Cadmium sulfide (CdS) is a wide band gap semiconductor that has emerged as an important material due to its applications in photovoltaic cells, optical filters, multilayer light emitting diodes, photodetectors, thin film field effect transistors, and gas sensors. CdS is naturally an n-type material of a band gap between 2.1 - 2.4 eV. The deposition of CdS film has been explored by various techniques, such as thermal evaporation, sputtering, molecular beam epitaxy, spray pyrolysis, and chemical bath deposition. Among of these techniques, Chemical bath deposition (CBD) is known to be a simple, easy, and inexpensive large-area deposition technique to grow CdS thin films under low temperature with high quality.

We use the CBD technique to synthesize CdS thin films. Unlike most reported work, the reagents we use are cadmium sulfate (CdSO₄), ammonia water (NH₄OH) and thiourea (CS(NH₂)₂). From our results, comparing with the method of using cadmium acetate, cadmium sulfate is easier to synthesize high quality CdS thin films under lower temperatures to prevent the quick evaporation of solution at high temperatures. All of reagents are of analytical grade and used without further purification. The glass substrates were well cleaned with 5% HF solution, left for 20 minutes under ultrasonic duty in isopropyl alcohol, washed with distilled water and finally dried in air. The typical procedures for the film deposition is described as follows. Drop by 25% NH₄OH into a 100 ml beaker containing 25 ml of 1M CdSO₄ solution until the initially formed white precipitate is completely dissolved. The clean substrates are mounted vertically in the bath beaker. 25 ml of 1M CS(NH₂)₂ is then poured into the mixtures. The distilled water is gradually added to make the volume up to 100 ml. The deposition is made at 50 – 80^oC under magnetic stirring for all samples. In order to vary the composition of the films, different concentrations of CdSO₄ and thiourea are used.

The CdS thin films deposited on the substrates using this technique are optically transparent, adherent, homogeneous and yellowish in color without powdered precipitation. After deposition, the deposited CdS films are cleaned thoroughly in distilled water and dried in the air at room temperature. In order to better understand the effect of growth conditions on film quality, the films were characterized by X-ray diffraction, scanning electron microscope, and UV-Vis absorption spectra. The measurements show that as deposited CdS films are polycrystalline and the crystallization of CdS particles is improved with increasing deposition time.

Solution-phase processing of semiconductor thin films is an attractive approach for fabricating low cost devices for optoelectronics and thermoelectrics. While most metal chalcogenide semiconductors are generally insoluble in regular solvents, a number of them can be prepared using hydrazine.[1] Unfortunately the highly hazardous nature of hydrazine poses a problem for industrial application. More recently, the use of less hazardous diamine-dithiol solvent mixtures have demonstrated that these too can enable solution-phase processing of many metal chalcogenides.[2] This diamine-dithiol solvent approach has been successfully used to deposit SnSe and SnS.[3, 4] However, the corresponding thermoelectric properties have not been reported.

In this poster presentation, we present our measurements on the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of SnS, SnSe, and SnS_xSe_1-x films made by solution-processing with a diamine-dithiol solvent mixture. We also explore the incorporation of dopants into the SnS_xSe_1-x films and report the corresponding effects on the thermoelectric properties. The incorporation of these dopants will be achieved by mixing in appropriate amounts of Ag₂S, CdS, CdSe, ZnS, and ZnSe, all of which can also be dissolved in diamine-dithiol solvents.

[1] Mitzi et al., Nature, 428, 299 (2004)
[2] Webber and Brutchey, JACS, 135, 15722 (2013)
[3] Antunez et al., Chem. Mat, 26, 5444 (2014)
[4] Lin et al., ACS Nano, 9, 4398 (2015)

Solution processed Cu₂ZnSnS_xSe_4–x (CZTSSe) has attracted considerable interest as a material capable of driving economical and terawatt capacity photovoltaic module production. It has already shown promising results with record solar cells efficiency 12.6%. The interest to use CZTSSe as an absorber layer for thin film solar cells is due to its large absorption coefficient of over 10⁴ cm^-1 in the visible range, its tunable optical band gap of 1~1.5 eV, the low cost and earth abundance of its constituents copper (Cu), zinc (Zn) and tin (Sn) and sulfur(S). In this paper, non-vacuum solution based method to deposit Cu₂ZnSnS_xSe_4–x (CZTSSe) was investigated. In this approach, a precursor solution of CZTSSe is formed by reacting elemental Cu, Zn, Sn S, and Se in a mix solution of thioglycolic acid, ethanolamine and 2-methoxyethanol. The precursor solution was then spray coated onto a heated solid lime glass substrate, followed by annealing at different temperatures. This process allowed for the growth of homogenous and large grain crystalline layers of CZTSSe. Optical, structural, and chemical composition characterizations of thin films were performed using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), spectrometer and UV-Vis spectrophotometer. Electrical properties of the surface of CZTSSe thin films were investigated with conductive atomic force microscopy (AFM) and Scanning Kelvin probe force microscopy (SKPM). Film thickness and surface roughness were measured using KLA Tencor D-120 surface profilometer. SEM images show the formation of a continuous and homogenous film of large grains with size ranging from 0.5µm to 3µm. The Se/(S+Se) ratio ranges from 30 to 70% as measured by EDS performed in top view. The formation of the desirable Cu-poor and Zn-rich composition of the synthetized CZTSSe was confirmed by EDS also. X-Ray diffractograms show different shifts of the kesterite/stannite (112) peak, which indicate the presence of CZTSSe. Three major peaks corresponding to (112), (220), and (312) planes were respectively located near 28°, 47.5° and 56°. The shift of the peaks depends on the ratios of S/Se in the synthetized material. The lattice constants decrease linearly with increasing S contents in the precursor solution. Raman spectroscopy confirmed the formation of CZTSSe with traces of quaternary CZTS, CZTSe. A higher positive surface potential bending and a higher current flow are observed in the vicinity of the grain boundaries.

Antimony sulfide (Sb2S3) is a semiconductor material of group V-VI with band gap of 1.78 to 2.50 eV, covering the visible and near infrared spectrum. It has a high photosensitivity and good photoconductivity and can be applied as absorbing material in solar cells.There are several methods of synthesis for Sb2S3, such as thermal decomposition, vacuum evaporation, hydrothermal reaction, solvothermal reaction, microwaves, among others. Evaporation methods require high temperature and are difficult to obtain exact stoichiometric compositions. The solvothermal and hydrothermal methods usually employ ligands during the synthesis which are sources of impurities in the final Sb2S3 products. The formation of Sb2S3 under microwave heating is fast and the obtained products show nanostructures and high purity. In this work, Sb2S3 nanorods were synthesized by microwave method without any ligands. Sodium thiosulfate (ST), thiourea (TU) and thioacetamide(TA) were used as sources of sulfur, and antimony chloride as source of antimony. Ethylene glycol (EG) and dimethylformamide (DMF) were used as solvent. The temperature of synthesis was varied from 140 °C to 170 °C, and the reaction time, for 30 min. It is found that EG was a very reactive solvent during the synthesis of Sb2S3 by microwave; with TA no such product was formed and with TU or ST, a high percentage of oxygen in the final products was obtained. With DMF as solvent, nanorods of crystalline (stibnite) was formed in all the Sb2S3 products with a band gap of around 1.7eV. Among the three sulfur sources, ST led to a S:Sb ratio of 7.77:1 in the final product, out of stoichiometry of Sb2S3, and the final product contained a high percentage of oxygen. In the case of Sb2S3 synthesized from TU and TA, the atomic ratio of S:Sb was close to 1.5:1 by EDS analysis. The SEM images of those products show the formation of microrods with average diameter and length of 0.95 and 13.42 um, respectively. XPS spectra of those samples show the characteristic signals of the Sb2S3 (Sb 3d5 / 2 = 528.5 eV, Sb 3d3 / 2 = 538 eV in the Sb-S bond and S^-2 = 161.1-162 eV). A very weak signal around 131 eV was observed that could indicate the presence of traces of oxygen less than 3% in the final products in both cases. It is concluded that Sb2S3 microrods of high purity can be obtained by microwave heating using DMF as solvent and TU or TA as sulfur source. The very long nanorod microstructure of the semiconductor products is quite attractive for hybrid solar cell applications.

Cadmium Sulfide (CdS)--a chalcogenide semiconductor--is of major interest for electronic and optoelectronic applications. Electron Beam Lithography (EBL) is a common method for defining electrode patterns, however, low-angle backscattered electrons (BSE) from the high-atomic number Cd-rich substrate can significantly degrade lithographic resolution using EBL.
In this study, we demonstrate the nature of this back scattered electron problem using CASINO 2.42 Monte Carlo electron trajectory simulations. Using these simulations as a guide, we show how lithographic resolution can be optimized via judicious selection of electron beam energy and dose, and through use of a bi-layer resist comprised of both 495 PMMA and 950 PMMA resists.

Resistive random access memory (RRAM) has attracted as one of the promising future non-volatile memory devices because of its low power consumption, high operation speed, and probability of high density integration. Resistive switching occurs in insulating films that are sandwiched between two metal electrodes as follows metal-insulator-metal (MIM) structure. Resistive switching has been studied in various oxides, such as TiO₂, ZrO₂, Ta₂O₅, and HfO₂. Recently, there is a great interest of HfO₂ for resistive switching insulator films, since HfO₂ is already used as a high-k dielectric in complementary-metal-oxide-silicon (CMOS) technology. Many mechanisms postulated to explain the resistive switching phenomenon in RRAM, the prospective one is oxygen vacancy-oxygen ion based mechanism.[1]
In this study, the resistive switching characteristics of Pt/HfO₂/Pt resistors have been investigated. To generate more oxygen vacancies (V_o) from the HfO₂ insulator, forming gas anneling (FGA) was performed at temperature ranges of 300-500 °C for 60 min. in 5%H₂/N₂ ambient. FGA is a conventional process to remove oxygen in the high-k dielectric by reduction annealing processes.[2] Typical unipolar resistive switching characteristics is observed in Pt/HfO₂/Pt resistors, which displayed similar resistive switching feature for positive bias polarity. Nevertheless, the endurance is prolonged twice, and the reset current is lowered about 10⁴ times for resistors with FGA at 300 and 400 °C. In order to clarify the change of oxygen bonding states of annealed HfO₂ films, the X-ray photoelectron spectroscopy (XPS) measurements were carried out. As a result, it has great potential for low power consumption and good endurance for non-volatile memory applications.

[1] H.-S.P. Wong, H.-Y. Lee, S. Yu, Y.-S. Chen, Y. Wu, P.-S. Chen, B. Lee, F.T. Chen and M.-J. Tsai, Proc. IEEE. 100(6), 1951 (2012).
[2] J.K. Schaeffer, L.R.C. Fonseca, S.B. Samavedam, Y. Liang, Appl. Phys. Lett. 85, 1826 (2004).

Many transition metal thio- and selenophosphates (TMTP) form layered materials, within individual layers comprising a honeycomb sub-lattice of metal cations immersed into a cage of thiophosphate (P₂S₆)^4- or selenophosphate (P₂Se₆)^4- anions. Owing to ionic bonding in this class of materials, the electronic bandgaps range from 1.6-3.5 eV. Furthermore, in small subset of the known TMTP compositions, mostly involving Cu⁺¹ and Ag⁺¹, the ions spontaneously order to develop ferroelectric, antiferroelectric and mutliferroic ground states. Therefore, TMTP possess distinct and unique properties among layered materials, that can greatly benefit the development of future electronic devices derived from layered and 2D conductors and semiconductors, in the form of encapsulated and/or hybrid heterostructures.
Motivated by finite ionic conductivity in these materials, we have investigated them experimentally and theoretically from the view point of solid solutions. Specifically, we have investigated deviations from a perfect stoichiometry in a ferroelectric CuInP₂S₆ compound, using a combination of growth, imaging techniques and first principles theoretical modeling. Our key finding is that depleting CuInP₂S₆ of electroactive Cu⁺¹ ions creates a unique layered structure – which has all the signatures of coherent spinodal decomposition into stoichiometric CuInP₂S₆ and In₂P₃S₉ phases. When probed on the nanoscale, the materials reveal a rich and self-organized texture of ferroelectric and non-ferroelectric domains with nearly atomically-sharp chemical boundaries. Spinodal decomposition occurs without any significant perturbation of the basic layered structure – producing cleavable and air-stable surfaces and quasi-2D sheets. Perhaps most strikingly, we found that chemical phase-separation increases the Curie temperature for ferroelectric ordering in CuInP₂S₆ phase by up to 40K – which we assign to the chemical pressure effect. In combination with hydrostatic pressure dependent data, we have inferred the entropy of ferroelectric ordering of ~2.5k_b which is at least 10 times larger than in conventional ferroelectrics, such as PbTiO₃ and BaTiO₃. This enables large modulation of T_c even by moderate pressures on ~100 MPa and suggests further design criteria for controlling T_c. We are currently exploring the stability of 2D and quasi 2D sheets of these materials as a stepping stone to novel van-der-Waals heterostructures with electronic and semiconducting 2D materials.
This research was sponsored by the Laboratory Directed Research and Development fund at the Oak Ridge National Laboratory.
M. A. Susner et al, “High T_c layered ferrielectric crystals by coherent spinodal decomposition”, ACS Nano in review (2015).

The steady miniaturization of electronic devices and the emerging need for self-powered miniaturized systems (stand-alone sensors, medical implants, MEMS,…) boost the development of miniaturized energy storage solutions such as all-solid-state thin film batteries (i.e. microbatteries). Up to now, most studies on cathodes focused on intercalation compounds (LiCoO₂, LiMn₂O₄, V₂O₅,…), despite their limited capacity (650–1150 mAh.cm^-3) [1,2]. Nevertheless, other materials reacting with lithium according to a conversion reaction described by the general formula M_aX_b + (b.n) Li↔aM + bLi_nX (M = metal; X = O, N, F, S, P, H) are promising [3]. Among them, pyrite (FeS₂ or Fe²⁺S₂^2-) which may insert up to 4 Li⁺ per atom of iron enabling one of the highest theoretical capacity among transition metal sulfides is an ideal candidate: FeS₂ + 4Li⁺ + 4e^- → Fe + 2Li₂S leading to theoretical capacities of 894 mAh.g^-1 and 435 µAh.cm^-2.µm^-1. Moreover, this reaction that occurs at around 1.5 V vs Li⁺/Li meets the requirement for lower supply voltages in various microelectronics systems. Nevertheless, the use of pyrite in commercial systems is still confined to lithium primary cells and thermal batteries. Indeed, similarly to lithium–sulfur cells, the main phenomenon hindering the cycle life is the formation of polysulfide species which are soluble in the liquid electrolyte [4], whereas the practical delivered capacity is limited by the size of the pyrite particles.
Here, striking results obtained in electrochemical cells combining stacked thin film geometry and the use of a solid state electrolyte will be presented. Lithium microbatteries which comprise a FeS₂ positive electrode and a vitreous electrolyte (LiPON) deposited by Radio-Frequency magnetron sputtering, and a lithium negative electrode, allowed demonstrating the efficiency of such a cell design. Physico-chemical characterizations carried out on the deposited pristine materials (EPMA, SEM, Raman and Mössbauer spectroscopies, GIXRD) confirmed the pyrite structure. Besides, ex situ transmission microscopy (HRTEM) analyses at different states of charge couple with EELS, revealed the lithiated pyrite compounds obtained during first lithium insertion/de-insertion. Full Li/LiPON/FeS₂ microbatteries delivered excellent capacities of 300 µAh.cm^-2.µm^-1 for more than 800 cycles. Thorough electrochemical characterization suggested that the reversibility of the conversion reaction was affected by a continuous cycling in the low voltage region, indicating a progressive evolution of the phase distribution inside the electrode material [5].

References:
[1] J. B. Bates et al., J. Power Sources, 54 (1995) 58–62,
[2] B.Wang et al., J. Electrochem. Soc., 143 (1996) 3203–3213,
[3] J. Cabana et al., Adv. Mater., 22 (2010) E170–E192,
[4] T. A. Yersak et al, Adv. Energy Mater., 31 (2012) 120–127,
[5] V. Pelé, F. Flamary, L. Bourgeois, B. Pecquenard, F. Le Cras, Electrochem. Comm., 51 (2015) 81-84

Introduction The current quest for new cathode materials for rechargeable Mg batteries is focused on intercalation compounds exhibiting high working potential and capacity¹. Vanadium oxides are an attractive alternative, as vanadium is known to exist in a wide range of oxidation states and our recent results² show that it is possible to extend the reversible ion-insertion chemistry of V₂O₅ to Mg²⁺.
Results and Discussion V₂O₅ was used as a model system to develop an understanding of the critical parameters that govern intercalation and de-intercalation of Mg²⁺ ions in layered oxide materials, focusing in particular on the role of water and defects. The strong polarity and active electrochemistry of water make it a likely player in many battery phenomena including cation solvation, SEI formation and redox behavior. Molecular dynamics simulations show that a significant amount of water remains in the structure solvating inserted Mg ions and diminishing the strength of the interaction with apical oxygens. Thermogravimetric analysis of as-prepared bilayered V₂O₅ shows that near 120 ^oC, free (adsorbed) and structural (hydrogen-bonded) water are removed from the structure, while strongly bonded hydroxyl groups persist until temperatures of ~ 300 ^oC. Removal of these structural OH groups results in poor electrochemical performance similar to highly crystalline orthorhombic V₂O_5. We confirm the presence of these unique structural hydroxyl groups with FTIR, focusing on the stretching vibrations of OH groups in the region 3000-4000 cm^-1. These results, together with preliminary NMR, XPS and atom probe tomography data confirming the presence of hydroxyl groups in our structure, will be discussed. We believe that that these strongly bonded hydroxyl groups play two important roles in reversible Mg cycling: 1) they maintain a sufficient interlayer spacing to allow for the diffusion of Mg cations, and 2) they reduce the symmetry of the V₂O₅, which contributes to electron transfer and Mg intercalation.
Conclusions We successfully used V₂O₅ as a model system to develop understanding of the role of the water in the intercalation and de-intercalation of Mg²⁺ ions into layered oxide materials. We have shown that the presence of water and the hydration of Mg ions are important for the reversible cycling of Mg. In the future, we plan to investigate intercaltion into amorphous V₂O₅ thin films.
Acknowledgement. This work was supported by the U. S. Department of Energy, US DOE-BES, under Contract No. DE-AC02-06CH11357.

References
(1). On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials E. Levi, Y. Gofer, and D. Aurbach, Chem. Mater. 2010, 22, 860–868.
(2). Nanoarchitectured Layered Cathode for Rechargeable Magnesium Batteries, Sanja Tepavcevic, Yuzi Liu, Dehua Zhou, Barry Lai, Jorg Maser, Xiaobing Zuo, Henry Chan, Petr Král, Christopher S. Johnson, Vojislav Stamenkovic , Nenad M. Markovic and Tijana Rajh, ACS Nano, 2015, 9 (8), pp 8194–8205.

Presently, safe energy storage is one of the most demanding technologies by the developing society. In this regard, lithium ion batteries (LIBs) have got tremendous attention due to their high energy and power densities; have been considered as promising power source for future electric vehicles (EVs). Here, we have developed different hybrid structures of metal oxides, nitrides, sulfides, hydroxides and metal alloys with doped graphene to control above mentioned problems and to achieve the goals set by USABC. All these composites possess extraordinary performances as electrodes of LIBs with long cyclic stability and excellent rate capability. The high performance of the composites based on the synergistic effect of several components in the nanodesign. These strategies to combine the different property enhancing factors in one composite with engineered structures will bring the realization of these devices in road market.

References
Li, Q.; Mahmood, N.; Zhu, J.; Hou, Y.; Sun, S. Nano Today 2014, 9, 668–683.
Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y., Adv Mater 2013, 25, 4932–4937.
Mahmood, N.; Zhang, C.; Hou, Y., Small 2013, 9, 1321–1328.
Mahmood, N.; Zhang, C.; Liu, F.; Jinghan, Z.; Hou, Y., ACS Nano 2013, 11, 10307–10318.
Mahmood, N.; Zhang, C.; Yin, H.; Hou, Y., J. Mater. Chem. A 2014, 2, 15–32.
Mahmood, N.; Tahirb, M.; Mahmood, A.; Zhu, J.; Hou, Y., Nano Energy, 2015, 11, 267–276.
Yin, H.;Zhang, C.;Liu, F. ; Hou, Y., Adv. Funct. Mater. 2014, 24, 2930–2937.
Mahmood, Hou, Y., Adv. Sci., 2014, 1, 1400012.
.

The significant growth of both the number and the kinds of portable electronic systems, generally battery-powered has triggered the race for the development of high-performance microprocessors and systems on chips using low power consumption integrated circuits. As a consequence, the energy supply of such optimized components can also be ensured today by miniaturized power sources such as Li microbatteries. The latter have the advantage of being manufactured by vacuum deposition process also widely used in the microelectronics industry [1]. Nevertheless to be considered as an electronic components, the microbattery need to sustain temperature higher than 230°C for the reflow soldering to attach the component to the PCB . As Li metal anode melts at 181°C, Li microbatteries are not tolerant to such high temperature. For this reason we have manufactured Li-ion all-solid-state thin films batteries based on Si at the negative electrode and a new lithiated titanium oxysulfide as a cathode [2]. This new thin film materials was prepared by sputtering from a home-made LiTiS₂ target. The resulting films are quite dense, amorphous with a typical composition close to Li_1.2Ti^IVO_0.5S^II_2.1. A full electrochemical delithiation of this electrode corresponding to a capacity of 64 µAh.cm^-2.µm-¹ was achieved in all-solid-state cells. As Ti⁴⁺ cannot be involved in the oxidation process, only sulfide species are oxidized during charge up to 2.9 V/Li⁺/Li forming S₂^2- disulfide anions. The subsequent lithiation and delithiation curves showing no capacity loss, demonstrate the perfect reversibility of the electrochemical processes. Actually, when cycled in the 3.2-1.5 V voltage window versus a lithium anode, extra lithium can be inserted in the material below 2V and titanium is partly reduced. The increase of the current density from 2 to 130 µA.cm^-2 (i.e from C/50 to 2C rates) induces only a slight capacity decrease, mainly due to an increase of the polarization near the end of charge. Besides, the capacity retention is found to be excellent during extended cycling with a capacity fading lower than -0.01 % per cycle and a mean coulombic efficiency close to 100%. Excellent electrochemical performances in terms of capacity value and cycling stability were also obtained in all-solid state Li-ion cells with Si as a negative electrode. After three successive solder-reflow processes (thermal treatment at 260°C), the capacity, the cycle life as well as the voltage profile remained unchanged. Finally, Li_1.2TiO_0.5S_2.1 is confirmed as a new interesting lithiated positive material for thin film lithium and lithium-ion microbatteries. Moreover, as its synthesis does not require any thermal treatment (contrary to the well-known LiCoO₂), is well-adapted to thermally sensitive substrates such as flexible polymers foils

References:
[1] B. Pecquenard, F. Le Cras et al., J. Power Sources, 196 (2011) 10289
[2] F. Le Cras, B. Pecquenard et al., Adv. Energy Mater., 5 (2015) 1501061

Ionic transport dynamics underpin the functionality of most of the energy storage and conversion technologies. Barium zirconate (BZO) has been widely studied for use in proton-conducting solid oxide fuel cells (PT-SOFCs), due to its high proton conductivity and excellent chemical stability at intermediate temperatures (400-700°C). Literature reports focusing on approaches to enhance conductivity in BZO are mostly based on developing new processing methods as well as new dopant and sintering additive systems, with macroscopic characterization such as X-ray diffraction and impedance spectroscopy. In order to identify proton transport mechanisms at the nanoscale, it is important to investigate the influence of atomic scale defects on ion transport mechanisms and kinetics as a function of dopant concentration.
Here we explore the ionic transport mechanisms in pure and Y doped BZO (Y-BZO), using energy discovery platforms, a synergy of nanofabricated device and in-situ characterization methods under controlled external stimuli with time resolved Kelvin probe force microscopy (tr-KPFM). A series of Y-BZO films (Y = 0 to 20 %) were prepared by pulsed laser deposition (PLD). Aberration corrected STEM was used to directly image defects and dopant clusters, which provide localized lattice strain and help facilitate proton conduction.
Through tr-KPFM, the surface potential mapping in both the space and time domains are obtained, enabling analysis of local proton transport dynamics on the 10^-2 to 10² s time scale as a function of temperature and dopant concentration. Experimentally measured activation energy increases from 0.49 eV to 0.61 eV with increasing dopant concentration, which is in good agreement with existing ac conductivity measurements [1]. The enhanced conductivity is explained by local lattice distortions from the dopant clustering and confirmed via density functional theory. The electrochemical and transport processes are also simulated through finite element method, resulting in creation of a physical model consistent with observed phenomena, and deconvolution of physical parameters such as proton injection rate and diffusivity.
The combination of results from theory and modeling suggests that in proximity of an optimal doping of ~20%, dopants begin to cluster and essentially trap protons. This is the reason for reduced conductivity at higher dopant concentrations in BZO and possibly all known oxide materials that require acceptor dopants. The latter is supported by computations. We will also present ways to mitigate such clustering effects using a combined modeling/experimental approach.
The research was sponsored by the Laboratory Directed Research and Development fund at the Oak Ridge National Laboratory. Computations were performed at NERSC.
[1] D.Pergolesi et.al, High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition, Nature Materials 9, 846–852 (2010).

Cobaltate perovskites are promising materials for implementation as cathodes in solid oxide fuel cells (SOFCs) as well as for application as catalysts in the oxygen evolution reaction (OER) taking place during electrochemical water splitting. Although great efforts have been made to understand both oxygen incorporation in SOFC cathodes as well as the OER in electrolysers, details of these processes are still under debate. One particular issue is the lack of model systems to systematically study the effects of various material parameters involved.

One way to generate model systems is to grow epitaxial thin films. In terms of surface area these systems cannot compete with poly-crystalline thin films or ceramics. However, the variation of material parameters (composition, oxygen deficiency, etc.) and thin film parameters (strain, thickness, etc.) can be tuned in a systematic manner.

Recently, the compounds (Pr_x,Ba_1-x)CoO_3-δ (PBCO) has attracted a signifficant amount of attention [1,2]. However, due to its complex composition, multiple material parameters such as oxygen deficiency (δ), valence state of Co (2+/3+) and Pr (3+/4+) may be involved in its catalytic activity. Moreover, additional effects such as phase separation and formation of Co-oxides or even metallic Co may also affect the catalyst's properties.

In this study, we discuss the synthesis of epitaxial PBCO thin films using RHEED-controlled pulsed laser deposition. In terms of epitaxial growth, this compound is particularly challenging due to the large difference in the ionic radii on the A-site compounds Pr and Ba, that causes cation ordering for x=0.5 [3] and a significant change of the lattice constant with Pr/Ba ratio. Moreover, the lattice constant is strongly affected by the oxygen load as revealed by X-ray diffraction measurements. As a result, epitaxial strain and thus the optimum choice of the substrate material varies with both composition and oxygen pressure during growth.

We deposited PBCO thin films on various perovskite substrates including LaAlO₃, SrTiO₃ and DyScO₃. In oxidizing conditions, we find single phase PBCO films that can be manufactured either in a strained or relaxed perovskite structure depending on lattice mismatch. Independent of epitaxial strain, we observe the formation of CoO secondary phases at low growth pressures.

While low-pressure samples show electrically insulating behavior, well-oxidized single phase PBCO thin films show sufficient electrical conduction at room temperature being essential for catalytic performance. We present first results on electrocatalytic properties of these epitaxial samples and discuss their dependence on strain and growth conditions.

[1] D. Bick et al., submitted (2015)
[2] Choi et al., Sci. Rep. 3, 2426 (2013)
[3] Grimaud et al., Nat. Comm. 4, 2439 (2013)

The relentless demand in energy generation using non-fossil fuels inspires the scientific community to develop stable and better performing materials that are composed of abundant, non-toxic, and cost-effective elements. As compared to conventional silicon-based solar cells, thin film solar cell panels are lightweight and flexible and are thus preferable for a number of applications. In objective of identifying materials composed of sustainable and non-toxic elements for thin film solar cells, we have been investigating number of chalcogenide containing compounds. Here we report a detailed solution-based synthesis, optical studies and electronic structure calculations of nanocrystalline Cu-Sb-S and Cu-A-B-S (A = In, Ga, Al; B=Zn, Sn) systems. The study suggests that all these materials are p-type semiconductors with an optical band gap between 0.8 eV and 1.6 eV and large absorption coefficient values over 10⁴ cm^-1. Band structure calculations on these materials predict most of them are direct band gap materials and the energy gap values are in consistent with experimental results. The details of the syntheses methods, structural and optical characterizations and band structure calculations will be presented and discussed

With a band gap of 0.98 eV, CuInSe₂ (CIS) is an important candidate for the bottom cell of tandem devices, as it exhibits excellent current matching to some of the best performing hybrid perovskite solar cells [1,2]. In this work we show a new solution processing route to deposit CuInSe₂-based absorber layers that yield 13.0% efficiency devices [3]. To our knowledge, this is the highest reported conversion efficiency for a CIS (or 1.00 eV bandgap) solar cell from any non-vacuum method, exceeding routes with hydrazine solvent [4] or nanocrystals [5,6]. Devices with gallium-containing absorbers from the same process (CIGS) and bandgap of 1.15 eV show up to 14.7% conversion efficiency. The excellent material properties of absorber layers is further elucidated by time-resolved photoluminescence measurements, which confirm minority carrier lifetimes exceeding 20 ns.

To minimize the processing cost of these layers, this novel molecular-ink route omits sophisticated nanoparticle fabrication and stabilization as well as toxic and explosive solvents and gases, such as hydrazine, H₂, H₂S, or H₂Se. Single crystal and powder XRD, as well as Raman spectroscopy confirm the chemical coordination of metal cations in both solid and liquid state. The formation of metal-organic complexes was found critical to control oxidation states and loss of metals during processing and tailor the final composition of the absorber. These findings may pave the way for all-printed tandem solar modules and dramatically reduced cost of solar electricity.

References

[1] Zhou et al., Science 2014, 345, 542.

[2] Meillaud et al., Sol. Energ. Mat. Sol. C. 2006, 90, 2952.

[3] Uhl et al., Energy Environ. Sci., 2016, 9, 130.

[4] Liu et al., Chem. Mater. 2010, 22 (3), 1010-1014.

[5] Norsworthy et al., Sol. Energ. Mat. Sol. C. 2000, 60, 127-134.

[6] Uhl et al., Prog. Photovolt: Res. Appl. 2015, 23, 1110.

Earth abundant compound semiconductors are promising candidates for the fabrication of high-quality thin film solar cell absorber. In particular, these compound (I2-II-IV-VI4) semiconductor material possess high optical absorption coefficients (10⁵ cm^-1), high power conversion efficiencies, offer good photostability against long-term radiation, and cause less environmental problems due to their relatively low toxicity. However, device efficiencies for these materials are significantly lower than the best efficiencies achieved for chalcopyrite based solar cells. Low device efficiency for earth abundant semconductors have been attributed to difficulties in achieving large grained, single phase thin films with controlled stoichiometry using vacuum-based deposition or precursor annealing techniques. These techniques can also result in the unintentional formation of a large number of secondary phases in the Cu-Zn-Sn-S system that significantly reduces the device performance. Nanocrystal based methods for the fabrication of thin film photovoltaics are an attractive alternative to vacuum deposition processes due to their simplicity and cost effectiveness. Here, we develop a simple and reliable solution approach to process large grained, single phased kesterite Cu2ZnSnS4 films via thermal treatment of assembled nanocrystal-precursor films consisting of wurtzite Cu2ZnSnS4 nanorods. The crystal structure, composition and electrical properties of the final film were investigated with SEM, XRD, Ramna, PL and Hall effect measurements.

1) M. F. A. M. van Hest, D. S. Ginley, Solution Processing of Inorganic Materials, Wiley-VCH, Hoboken, NJ, 2008.
2) H. Katagiri, Thin Solid Films 2005, 480, 426.
3) Q. Guo, G. M. Ford, W. C. Yang, C. J. Hages, H. W. Hillhouse, R. Agrawal, Sol. Energy Mater. Sol. Cells, 2012, 105, 132.
4) T. K .Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D. B. Mitzi, Adv. Energy Mater., 2013, 3, 34.

Synchrotron-based characterization techniques—first of all x-ray fluorescence (XRF) and x-ray beam induced current (XBIC)—have been shown to be very powerful tools for the in-situ investigation of diffusion and growth processes of solar cells [1]. Thanks to the excellent spatial resolution (< 100 nm) and sensitivity of state-of-the-art synchrotron beamlines, we are able to map multi-grain areas of completed CuIn_xGa_1-xSe₂ (CIGS) solar cells, correlating point-by-point the charge collection efficiency from XBIC with the elemental distribution from XRF measurements. This points out the beneficial and detrimental effects of the distribution of Se, Cu, In, and Ga on device performance, which is an important step towards active engineering of high-efficiency devices rather than improvements by experimental optimization [2-4].

The holy grail of engineering high-efficiency solar cells goes even a step further, investigating the distribution of the elements in-situ during the growth of CIGS solar cells. For that purpose, we have built a heating stage for the specific requirements of XRF and XBIC measurements under conditions that are close to those of solar cell manufacturing or processing, covering a temperature range up to 600°C and working under corrosive atmospheres such as H₂Se and H₂S [5]. Furthermore, the mechanical concept of the stage provides excellent stability, allowing to compensate thermal drift and finally tracking sample features in nanoscale over the full temperature range.

Using that heating stage and the expertise in XRF analysis of multi-layered multi-compositional thin films, we investigate in the present work CIGS solar cells during the selenization process. We have deposited the layer stack of CIGS solar cells including the Mo back contact on glass and a layer of metallic Cu/In/Ga precursors with a Se capping layer, as described in [6]. We follow the selenization procedure described therein using a heating phase in an inert atmosphere that is followed by a sulfurization step in helium-diluted H₂S. During these steps, when the Se diffuses into the Cu/In/Ga precursor and forms CIGS crystallites, we measure the distribution of the elements Cu, Ga, In, and Ga in situ. This shows how CIGS grains nucleate and grow to the final state in solar cells, including segregation of elements throughout the grain cores or at grain boundaries. Finally, the correlation of these growth observations with XBIC measured for cells grown under varied conditions gives direct access to the significance of growth parameters for further performance improvements of CIGS solar cells.

[1] M. I. Bertoni, et al, Energy Environ. Sci. (2011)
[2] B. West, et al., IEEE PVSC proc., New Orleans (2015)
[3] B. West, et al., submitted for publication (2015)
[4] M. Stuckelberger, et al., IEEE PVSC proc., New Orleans (2015)
[5] R. Chakraborty, et al., submitted for publication (2015)
[6] D. M. Berg, et al., IEEE PVSC proc., Denver (2014)

In the last years, synthetic methods for magnetic core-shell nanoparticles (NPs) have been developed. Such NPs may show exchange bias (EB) when they comprise an exchange-coupled ferromagnetic-antiferromagnetic (FM-AFM) interface that induces a unidirectional magnetic anisotropy. The interest in NPs displaying EB thus renewed and research focused on the goal of “beating the superparamagnetic limit” for successful application of NPs in high-density magnetic recording.
We prepared thin-film assemblies of Ni@NiO and Ni@CoO core-shell NPs by sequential layer deposition of (i) a thin MO layer (1 nm), (ii) Ni nanoparticles (12 nm), and (iii) a variable-thickness MO layer (0-6 nm for M = Ni; 1-3.5 nm for M = Co). The thickness of the bottom MO layer and the Ni NP size and areal density were kept fixed to study the effect of the thickness of the top MO layer on the magnetic behavior. The NPs were characterized by XPS, SEM, HRTEM, and STEM. The magnetic properties of the NP assemblies were investigated by analyzing the field-cooled (FC) hysteresis loops, the thermal magnetization in FC and ZFC mode, and the thermoremanence.
The Ni@NiO NPs displayed negligible EB when t_NiO ≤ 1.6 nm, but for thicker shells, the EB field H_b steeply increased without sign of saturation up to H_b = 0.57 kOe. Coercivity H_c showed a similar trend and reached H_c = 0.55 kOe for the thickest NiO shell. The microcoercivity spectrum displayed a thickness-related shift towards larger values in the direction opposite to the cooling field. This behavior is related to the morphology of the top NiO layer. At low deposited dose, NiO islands form and enlarge on the Ni cores, achieving a complete shell at t = 1.6 nm, which then grows and finally forms a discontinuous NiO matrix embedding the Ni cores.
Ni@CoO and Ni@NiO@CoO NPs were also investigated. Comparing shells with similar t, one finds that CoO is more effective than NiO in creating EB and increasing coercivity. The maximum H_b = 2.15 kOe and H_c = 2.76 kOe at nominal t_CoO = 3.5 nm. Since the Ni/CoO interface is expected to be (at best) of similar quality as the Ni/NiO interface, the larger EB can be attributed to the larger anisotropy of CoO (K_CoO/K_NiO ≈ 10⁵) or to a larger AFM/FM exchange coupling. The CoO blocking temperature T_B is about 240 K, ca. 50 K lower than T_N of bulk CoO, and does not depend on the CoO layer thickness. Ni@NiO@CoO NPs have larger H_b and H_c than both Ni@NiO and Ni@CoO NPs with similar t. This could be attributed to the synergy between the better quality of the Ni/NiO interface and the larger anisotropy of the CoO coating.
We thus demonstrated the viability of the sequential layer deposition method to both systematically investigate the effect of morphology on exchange-coupled core-shell NPs and to tune the EB by the fine regulation of a single morphological parameter. Our results are a first step in the direction of the rational design and synthesis of NPs able to beat the superparamagnetic limit.

The transition metal oxides (TMOs), including NiO and FeO, are currently being considered to be used in Resistive Random Access Memory devices (ReRAM). ReRAM is based on the reversible switching of a thin TMO layer between a low and high resistance state using the mechanism of soft breakdown. ReRAM’s high integration density, its high endurance and good retention, its low energy use, and its high speed make it an interesting technology to possibly replace Flash in non-volatile memory applications in the future. NiO has been well-studied in this regard, but PyO has never been studied by first principles, and experimental publications on the material are scarce. The addition of other elements in NiO is known to influence the morphology and crystal structure of polycrystalline thin films in particularly important for ReRAM as the switching between the high and low resistance state is inhomogeneous and nano-filaments are formed at the grain boundaries in the thin oxide film. In this paper we consider the effect iron has on the electronic structure of NiO.

The VASP code based on the density functional theory and the spin-local density approximation was used to study Fe-doped NiO in the rock salt structural phase. For the exchange-correlation potential, we adopted the Generalized Gradient approximation (GGA) and the GGA+U methods, with U set to 3.0 eV for both Fe and Ni, and 0.0 eV for O. Calculations were done on a 32-atom super cell consisting of 16 nickel and 16 oxygen atoms. Preliminary calculations show that NiO is an anti-ferromagnetic semiconductor which is in agreement with experimental results. The calculation with the GGA method yielded a direct bandgap of 0.6 eV, while the calculation with GGA+U method yielded a direct bandgap of 2.6 eV, in better agreement with experimental findings (~ 3.7 eV). In order to further improve the band gap description, the Hartree Fock theory using a hybrid calculation (the HSE06 approach) was incorporated in the VASP calculation.

Iron-doped NiO was obtained by replacing one Ni atom with one Fe atom and maintaining the AFM order, i.e. a rhombohedral symmetry in the [111] direction. This corresponds to a Fe concentration of approximately 6.3 % in our supercell calculations. The results obtained with the GGA+U method showed that Fe-doped NiO has a net magnetic moment as the moment on the iron atom differs from the moment on the nickel atoms so they no longer cancel out in the AFM state. We found that Fe introduces a localized state in the band gap of NiO, and the effects of Iron in the optical properties of NiO are analyzed.
We acknowledge financial support from Texas State University (Research Enhancement grant) and from DOD (HBCU/MI grant W911NF-15-1-0394).

We have grown CoTe nanostructures using a wet-chemical synthesis. The CoTe nanostructures exhibit NiAs hexagonal crystal structure and show semiconducting behavior. We have fabricated nanoscale devices of CoTe nanostructures using electron beam lithography. The gate-dependent I-V characteristics show that the CoTe nanostructures are p-type. They are ferrimagnetic with a saturation magnetization of 0.25 µ_B/Co and having a large Curie temperature. They display magnetoresistance effect, which is observed only below 50 K. The maximum magnetoresistance is observed at 10 K, which is around 54 %.
This work was supported by the National Science Foundation under ECCS-0845501 and NSF-MRI, DMR-0922997

Topotactic phase transitions of functional oxides induced by changes in oxygen non-stoichiometry can largely alter multiple important physical and chemical properties, including electrical conductivity, magnetic state, oxygen diffusivity and oxygen reduction/evolution reactivity. This enables conveniently tuning functional oxide properties and switching between distinct states, which is of pivotal importance for applications such as memristors or electro-catalysis (e.g. solid oxide fuel cell cathodes). Here we demonstrate a novel and feasible means to trigger topotactic phase transitions by precisely controlling oxygen non-stoichiometry in a functional oxide. The method involves electrochemical control of oxygen non-stoichiometry as previously demonstrated by Chen et al¹. We use electrochemical potential to change the non-stoichiometry and thereby induce phase transition in strontium cobaltite (SrCoO_x, denoted as SCO) between the Brownmillerite phase SrCoO_2.5 (BM-SCO) and the perovskite phase SrCoO_{3- δ} (P-SCO). In situ high-temperature X-ray diffraction (HTXRD) measurements revealed that the BM-SCO to P-SCO phase transition can be readily induced by applying electrochemical bias at a temperature of 300°C. Moreover, the transition was shown to be fully reversible. We used in situ X-ray spectroscopy methods to probe the evolution of electronic structure during the phase transition. In situ ambient pressure X-ray photoelectron spectroscopy revealed a binding energy shift towards lower energy side upon an anodic bias, consistent with increasing hole concentration and downwards shift of Fermi level in SCO. In situ X-ray absorption spectra (XAS) showed that the unoccupied states had evident O2p characteristics, indicating strong hybridization between O2p and Co3d states. Resonant photoelectron spectroscopy performed on Co-resonance photon energy clearly showed that the Co 3d states are populated near the top of valence band during the BM-SCO to P-SCO phase transition. More importantly, in situ XAS was utilized to assess the kinetics of phase transition and identify the rate-limiting steps. A revised nucleation-growth model, which includes an additional oxygen incorporation term, was used to describe the phase transition process, which matched very well with the experimental data. This study provides a complete picture on the electronic structure evolution of SCO that is important for assessing the mechanisms involved in a red-ox based resistive switch and electro-catalyst applications of this material.
¹ Chen, D. & Tuller, H. L. Voltage-Controlled Nonstoichiometry in Oxide Thin Films: Pr_0.1Ce_0.9O_2−δ Case Study. Adv. Funct. Mater. 24, 7638–7644 (2014).

NiO (E_g = 3.6 eV) and MgO (E_g = 7.8 eV) are both direct band gap materials which crystallize in the cubic rock-salt structure. As the maximum lattice parameter mismatch between the two materials is less than 0.8 % [1], Ni_xMg_1-xO thin films on MgO single crystals can be grown over the entire composition range with high crystalline quality. This epitaxial growth of Ni_xMg_1-xO thus offers enormous flexibility for tailoring the Ni_xMg_1-xO (0 ≤ x ≤ 1) band gap in the deep UV region within energies of 3.7 eV to 7.8 eV.
In contrast with theoretical measurements which predict that the band gap of a binary alloy semiconductor should vary linearly according to Vegard’s law, our optical measurements observe that the band gap increases only slightly from 3.7 eV for NiO to about 4.8 eV for Ni_0.1Mg_0.9O despite the large band gap of MgO (7.8 eV). This asymmetry or non-parabolic bowing has previously been attributed to the presence of Mg rich clusters [2] and oxygen vacancies [3] in the films. This minor band gap increase is surprising and so far very little attention has been given to the exceptional Ni_xMg_1-xO band gap trend as a function of composition. Moreover no nano-scale structural investigations have been carried out to study the reason for this band gap bowing. Therefore our work investigates the nano-scale structural and compositional homogeneity of epitaxially grown Ni_xMg_1-xO films grown on MgO (100) substrates by pulsed laser deposition over the entire composition range of the NiO-MgO solid solution system and its effect on the band gap of the alloy using high-resolution X-ray diffraction analysis employing synchrotron radiation, high-resolution transmission electron microscopy (HRTEM) and aberration corrected scanning transmission electron microscopy (HRSTEM) using the high angle annular dark field detector (HAADF), energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS).
Our HRSTEM results and HRSTEM-EDXS results on Ni_xMg_1-xO (x = 0.1 and 0.2) films show homogenous films with the absence of both Mg and Ni clustering. Moreover both HRTEM and aberration-corrected HRSTEM-HAADF studies showed epitaxially grown low defect density containing films. Oxygen vacancy studies on the Ni_xMg_1-xO films using STEM-EELS and resistivity data also showed the absence of high concentration of point defects around the low Ni content region where a high degree of band gap bowing is observed. These results confirm that Mg clustering and oxygen vacancies do not cause band gap asymmetry and bowing in Ni_xMg_1-xO films as predicted previously, and the band gap asymmetry could be related to other effects such as the presence of localized deep states in the band gap.

[1] A. Kuzmin and N. Mironova, Journal of Physics: Condensed Matter 10, 7937 (1998).
[2] Z.-G. Yang, L.-P. Zhu, Y.-M. Guo, Z.-Z. Ye and B.-H. Zhao, Thin Solid Films 519, 5174 (2011).
[3] R. Boutwell, M. Wei, A. Scheurer, J. Mares and W. Schoenfeld, Thin Solid Films 520, 4302 (2012).

We present the development of an indium-free anode from a combinatorial material study up to its application in large-area flexible white organic light-emitting diodes (OLEDs). The material developed is a void-free transparent and conductive zinc tin oxide (ZTO) amorphous compound. Following a combinatorial screening of a range of aluminum doped zinc oxide (ZnO:Al) and tin oxide (SnO₂) mixtures by co-sputtering deposition, we determined the optimized ratio of Sn/Zn to achieve the highest conductivity. Extensive chemical, electrical and microstructural characterization of the distinct material phases formed in the combinatorial development was performed to determine the influence of the deposition parameters on the films properties. The optoelectronic properties of the material were subsequently optimized by adjusting the parameters of the sputtering process from a single target manufactured with the optimized Sn/Zn ratio. Transmission electron microscopy (TEM) analysis indicates that this material as-deposited offers a defect-free microstructure which allows achieving high electron mobilities of up to 21 cm²/Vs. The ZTO anodes also present an absorptance of less than 5% in the visible range of the spectra and surface roughness of less than 0.2nm, following the requirements for application as anode for small molecule (sm)-OLEDs. The ZTO films are also extremely stable under thermal and acid treatments. In addition, the optimized void-free amorphous ZTO anodes are fabricated by sputtering deposition, which is a common industrial process that will facilitate upscaling and ITO replacement at the industrial level. Large-area (41 cm²) white sm-OLEDs were fabricated on these ZTO electrodes in combination with a metal grid (ZTO/grid), and compared to devices fabricated with ITO/grid anodes. We show that large-area white sm-OLEDs fabricated with the ZTO/grid anode show better performance than those with ITO/grid anodes, confirming that ZTO can already substitute ITO for large-area OLED lighting applications.

Transparent oxide La:BaSnO₃ demonstrates great potential as a high-mobility electron transport layer for applications in flat panel displays, solar cells, light-emitting diodes, and multi-functional perovskite-based electronic devices. While an extraordinary high carrier mobility of 320 cm²/Vs has been reported for La:BaSnO₃ single crystals^[1], epitaxial La:BaSnO₃ thin films grown on SrTiO₃ substrates suffer from the presence of dislocations and domain boundaries that cause electron scattering. Up to present, optimum electron transport properties have been presented for vapour phase grown epitaxial thin films and by La substitutional doping well beyond the equilibrium solubility limit in BaSnO₃.
This work presents the solid-phase epitaxial growth of high mobility La:BaSnO₃ thin films by crystallization through thermal annealing of pulsed laser-deposited amorphous/nanocrystalline films at room temperature. The solid phase epitaxy excels in its remarkably facile preparation and single crystal-like quality thin film growth as confirmed by 200 out-of-plane rocking curves as narrow as 0.10^o FHWM. Hall mobilities up to 30 cm²/Vs are obtained, which is the highest among those reported with carrier concentrations as low as 4x10¹⁹ cm^-3. The La concentrations in the thin films are accurately determined by SIMS depth profiling to assess the actual solubility after the solid-phase epitaxial growth and to evaluate the donor activation efficiency in La:BaSnO₃.
This work further presents the first experimental report of H doping to BaSnO₃ thin films, as implemented by H plasma treatment and high pressure H₂ annealing. Carrier concentrations as high as 1x10²⁰ cm^-3 are obtained, and the noticeably improved electronic properties support the prediction of first-principles calculations suggesting that interstitial H acts as a shallow donor in high electron affinity oxides^[2] and BaSnO₃ in particular^[3]. The (La,H):BaSnO₃ thin films show enhanced free carrier absorption, and the optical mobility and the electron effective mass are analysed from FT-IR spectra. Carrier scattering mechanism will be discussed based on these results and temperature-dependent electronic transport properties.

[1] H. J. Kim et. al. Appl. Phys. Express 5, 061102 (2012).
[2] Ç. Kiliç and A. Zunger. Appl. Phys. Lett. 81, 73 (2002).
[3] D. O. Scanlon. Phys. Rev. B 87, 161201 (2013).

As the size of semiconductor materials continues to shrink, the surface-to-volume ratio increases and surface defects often dominate the performance of the materials, particularly in solar-energy applications. Surface defect sites can alter the optical and electronic characteristics of materials by changing the Fermi level, charge-carrier mobility, optical absorption, and surface reactivity. Here, we present a new vapor-phase deposition method to selectively and completely passivate defect sites in semiconductor nanomaterials termed targeted atomic deposition (TAD). We demonstrated the capabilities of this selective passivation method by applying a TAD of alumina to the widely used cathode material, Nickel Oxide. By exploiting a temperature-dependent deposition process, we were able to selectively passivate the most highly reactive sites in NiO: the oxygen dangling bonds and Ni vacancies. The new TAD treatment resulted in the complete passivation of all measurable surface defects, optical bleaching of the material, and significant improvements in all photovoltaic performance metrics in dye-sensitized solar cells. The technique was proven to be generic to numerous forms of NiO, and these results suggest that TAD could be generic for many 0D, 1D, and 2D nanomaterials with a variety of highly-reactive vapor-phase precursors.

Recently, metal chalcogenides materials have been intensively researched due to their diverse optical and electronic properties, such as large band gap tunability, high infrared transparency, high conductivity, and high carrier mobility. The metal chalcogenides have been successfully implemented for various applications, such as photovoltaics, photodetector, non-linear optics, telecommunication, transistor, and thermoelectric generator. All of these successful industrial demonstrations relied on the thin-film or bulk materials. Especially the thin film metal chalcogenide expand their usage from the conventional electronic and optoelectronic applications to next generation energy and sensor applications. The cost effective fabrication of metal chalcogenide thin-film become increasingly important especially for large area applications, such as thin-film solar cell, large area display, and sensor arrays. Although the solution processing of metal chalcogenide enable the cheap deposition of large area thin-film, the lack of soluble metal chalcogenide precursor hinder the facile low cost deposition of thin-film.

In this reports, we investigated novel sol-gel type chalcogenide precursor systems on CdS. In order to achieve soluble precursor, cadmium salt and thiourea are used. We can observe cadmium sulfide precursors are dissolved in organic solvent. Then, the precursor solution is spin-coated onto thermally oxidized silicon substrate, with the subsequent thermal annealing at 400'C. After the heat treatment, the carbon impurities in active layer are removed. The continuous cadmium sulfide films with thickness ranging 20-100Å have been successfully achieved. X-ray diffraction confirms that the films are CdS, with the cubic structure. The source and drain electrodes are thermally evaporated onto the CdS film. The n-channel cadmium sulfide shows a saturation field-effect mobility and on/off ratio of 11.8 cm²/V-s and >10⁶, respectively.

In this work, we demonstrate the fabrication of thin and densified sol–gel metal oxide films for gate dielectric layers by a simple bar-printing method in large-area printed metal oxide thin-film transistor arrays. The thickness of single-coated aluminum oxide and hafnium oxide dielectric films can be accurately controlled with a nanometer resolution (10 – 40 nm) by adjustable rheological factors (various bar-shearing speeds and the precursor concentrations), whereas thick metal oxide dielectric films (~100 nm) are accomplished by repeated sol–gel oxide coatings. The fabricated high-quality oxide dielectric layers show remarkable results of very smooth surface morphology with excellent insulating properties, performing very low leakage current density in the range of 10^–8 A cm^-2 at 2 MV cm^-1 and very high areal capacitance of over 400 nF cm^-2, in company with very uniform large-area coverage up to 4-inch silicon wafer. Lastly, in combination with direct-oxide patterning process via selective surface wettability, solution-bar-coated metal oxide transistor arrays were achieved using a self-patterned sol-gel indium–gallium–zinc oxide semiconductor on the aluminum oxide-coated dielectric layer, which obtain sufficient transistor performance along with electron mobilities of >5 cm² V^-1 s^-1, high current on–off ratios of 10⁵, and minimal gate leakage current in the stable range of 10^–9 A cm^-2 at a very low operation voltage below 2 V.

A Cu(In, Ga)Se₂ (CIGS) solar cell is the most promising material owing to its excellent light trapping ability, broadband light absorption, and environment-friendly manufacturing processes. The CIGS solar cell has a crucial advantage of using various flexible substrates such as stainless steel (STS), polyimide. The preparation of CIGS solar cells on STS substrate, which shows higher temperature stability than polyimide during the selenization process, requires Na incorporation into CIGS absorber. Na incorporation is well kwon to improve the efficiency of CIGS solar cells. However, Ga grading in the CIGS absorber occurs when sodium diffuses into the CIGS absorber. It is also reported that sodium influences In and Ga inter-diffusion. In this study, Mo-Na layer and Se interlayer are used for incorporating Na and uniform Ga distribution in the CIGS absorber along the depth.
Mo-Na layer is deposited by sputtering using Mo-Na alloy (90:10) target on STS substrate. A CIGS absorber is grown by co-sputtering system and selenization process followed by deposition of Mo back contact on the Mo-Na layer. The content of Na diffused into the CIGS absorber is controlled by variation of growth pressure for a Mo-Na layer, as is confirmed by SIMS depth profile. Also, in order to confirm Ga inter-diffusion, two CIGS absorbers are prepared by converting CuInGa (CIG) precursors without and with Se interlayer. These CIG precusors consisted of CuGa/Se/CuIn/In/Mo/STS and CuGa/CuIn/In/Mo/STS stacks are selenized at 500 °C for an hour. Through the SIMS depth profile and XRD, there is a change between prepared CIGS absorbers that the amount of Ga elements decreases from the top to bottom layer in the CIGS absorber gradually. We will discuss the effect of Se interlayer in the CIGS absorber and its effect on solar cells.

Conductive-bridge random access memory (CBRAM) is a promising alternative to flash for non-volatile data storage. CBRAM is composed of an electrochemically active anode (copper or silver) and an electrochemically inert cathode, a layer of ion conductor (chalcogenide glass or oxide) is sandwiched between the two electrodes. Resistance change between a high-resistance state (HRS) and a range of low-resistance states (LRS) originates from creation or dissolution of conducting metallic filament in ion conducting layer. Similar to Ag-chalcogenide based CBRAM, CBRAM consisting of a Cu anode and a SiO₂ ion conductor possesses merits such as low power, scalability, and fast speed, and it has much better compatibility with BEOL CMOS processing. Considering these benefits, this CBRAM variant is gaining significant attention from industry.
Doping the ion conductor with anodic metal atoms is a critical step to achieve stable resistive switching and better LRS retention. Unlike chalcogenide glass, SiO₂ can be hardly doped with Cu through photo-doping process. Doping Cu into SiO₂ is achievable through thermal diffusion which requires high temperature process. Therefore, the stability of cathode at high temperature is crucial to achieve reliable resistive switching performance. Generally, it’s ideal to have an absolute inert cathode that does not react and generate unwanted residues, such as metal oxides and/or metal ions, during thermal annealing. Platinum (Pt) are frequently used as CBRAM cathode in researches because of its excellent high temperature stability, however, the high cost and CMOS-incompatibility of Pt hinders its application in industry. Cathode using CMOS compatible materials are needed to replace Pt for foundry adaptability.
In this study, we studied the thermal annealing effects on Cu-SiO₂ CBRAM with tungsten (W) and nickel (Ni) cathode. The devices are fabricated in a 32x32 crosspoint array. The arrays are sealed with SiO₂ and are annealed at 450°C to 550°C with continuous nitrogen gas flow to prevent anodic and cathodic oxidization. The resistive switching performance is characterized before and after thermal annealing. It is found that electroforming voltage of the thermally annealed devices with Ni cathode is much larger than devices with W cathode. The influence of high temperature on W and Ni cathode and cathode/SiO₂/Cu interface are investigated through materials analysis and electrical characterization. The full results of this study will be presented in the paper.

We present a novel strategy for increasing the efficiency of photovoltaic (PV) devices, including dye-sensitized solar cell (DSC) and perovskite solar cell (PSC), through modifying the surface of FTO glass by atmospheric pressure plasma treatment. This strategy allows the suppression of charge recombination at the interface between FTO substrate and electrolyte as well as hole transfer material. HRSEM, water contact angle, and XPS measurements confirmed the change of the FTO surface properties. The plasma treatment (150 W power, 5 lpm Ar gas flow rate, and 1 min treatment time) decreased the water contact angle of FTO surface down to zero, which refers to super-hydrophilicity. This property allows the formation of uniform and thin TiO₂ BL on FTO surface without any aggregation. First, we applied the developed electrode in working electrode of DSCs. DSC employing plasma-treated FTO substrate showed higher efficiency of 8.61% than 8.10% of the device fabricated without plasma treatment. The IVMS data suggest that the improvement of device PV parameters induced by plasma treatment is because of high charge transfer resistance at the interface between treated FTO and BL, resulting in low interface recombination rate. The increase of BL specific surface area led to the improvement of dye loading on BL. Furthermore, the transmittance of the plasma-treated FTO/BL interface is higher than that of plasma-untreated interface. This is because the plasma-treated interface reduces the reflection of light. These effects lead to the decrease in the recombination loss through the junction and the enhancement of light harvesting, in turn to an increase in short-circuit current density. We believe that this work provides the possibility of improving the efficiency of perovskite solar cells through plasma treatment of charge collection layer. Indeed, PSCs based on the newly developed electrode had 39% higher efficiency than reference devices. The obtained results provide direct evidence in favor of the developed strategy.

Thin film having high absorption coefficient is considered as an alternative absorber since it allows the low cost solar cells with a high efficiency. Representatively, Cu(In, Ga)Se₂ showed the impressive solar energy conversion efficiency of 20.4 %. However, scarcity of the In, and Ga restricted the terawatt scale deployment. Thus, Cu₂(Zn, Sn)S₄ only containing the earth-abundant materials has been focused and achieved the 11.1 % energy conversion efficiency. Nevertheless, formation of the secondary phases, such as Cu_xSn_yS_z and SnS is competitive and limits the improving efficiency.
Sb₂Se₃, simple binary compound having high absorption coefficient(>10⁵ cm^-1), 1.0-1.2 eV of band-gap and high mobility (> 42 cm²/Vs) is appropriate for the thin film solar cells. Unlike Cu₂(Zn, Sn)S₄, it is easy to prepare the uniform composition and large grains since it only has fixed composition and low-melting temperature (~885 K). Recently, J. Tang group employed the thermal evaporation method for Sb₂Se₃ layer and more than 5 % of energy conversion efficiency. In our study, Sb-Se precursors were prepared by electrodeposition and conveniently annealed at 573 K to form the Sb₂Se₃ compounds. After that, to manufacture the photovoltaic device, CdS, ZnO and ITO layers were sequently deposited on the Sb₂Se₃ by chemical bath deposition and sputtering method. As a result, we simply achieved thin film solar cells having the energy conversion of > 1 %.

The artificial lighting system has passed through three phases; incandescent lamp, neon light, and Compact Fluorescent Lamp (CFL), and has been advancing towards the fourth phase-semiconductor lighting, especially white Light emitting diode (w-LEDs). White LEDs are believed to be attractive green lighting source of the 21^stcentury and are gaining importance due to their superior features and wide applications. Other light sources are considered very inefficient, owing to inevitable large losses of energy during thermal excitation. Present commercial LEDs exhibit efficiencies less than 50%, are limited by materials quality and device-structure imperfections. Therefore, the LED is considered to have potential to become the next generation lighting. One of the famous and widely used approaches for creating white light by means of LEDs is, using a blue LED with a phosphor having broad yellow-green emission. Indeed, Y₃Al₅O₁₂:Ce (YAG:Ce)/blue LED system has a poor color rendering index (CRI) because of the red spectral deficiency of YAG:Ce phosphor, which hinders the development of w-LEDs with high CRI and low correlated color temperature (CCT). Y₂O₃:Eu and Y₂O₂S:Eu, the traditionally used phosphors to resolve the above-mentioned issue either have negligible (