Crystallization Of Thin Films Research Articles

Optimum conditions for deposition of amorphous WS2 thin films and changes in structure and optical properties during solid state crystallization

Transition metal dichalcogenides exhibit a unique properties, which make them interesting for fundamental studies and for applications in many fields of daily life. Here, we report on optimum conditions for deposition of amorphous stoichiometric WS2 thin films by (non-reactive) magnetron sputtering using a stoichiometric WS2 target and subsequent application of solid state crystallization of amorphous WS2 thin films for fabrication of crystalline WS2 over a large area. By ex-situ annealing of samples up to 1000 °C, we found that the optical contrast change occurs in two steps, which are separated by a plateau that exhibits a gradual increase in optical absorbance. We suggest that the dissociation of W–W homopolar bonds, associated with the transformation of 1T’ - like symmetry into 2H-like symmetry units, plays a key role in the first abrupt change in the optical properties of WS2 thin films between 300 and 400 °C while the formation of excitons occurred between 700 and 1000 °C is associated with crystallization, as confirmed by X-ray photoelectron spectroscopy, X-ray diffraction and Scanning electron microscopy studies.

Heteroepitaxial Growth of Colloidal Crystals: Dependence of the Growth Mode on the Interparticle Interactions and Lattice Spacing.

Epitaxial growth is one of the most important techniques for the control of crystal growth, especially for growing thin-film semiconductor crystals. Similarly, colloidal epitaxy, a template-assisted self-assembly method, is a powerful technique for controlling the structure of colloidal crystals. In this study, heteroepitaxial growth, which differs from homoepitaxial growth of conventional colloidal epitaxy, using foreign colloidal crystals as a substrate, was used to grow single-component colloidal crystal films. The Frank-van der Merwe (FM), Stranski-Krastanov (SK), and Volmer-Weber (VW) modes were observed, and the mode varied with the lattice-misfit ratio and interparticle interactions between the substrate and epitaxial phase. The transition of the growth mode (from SK to VW) and the coexistence of different growth modes (FM and VW) were observed, and their processes were revealed by in situ observation. Colloidal heteroepitaxy was confirmed to be useful for controlling structure, which will enable exploration of novel colloidal self-assembly structures.

Effects of substrate pretreatment and annealing processes on AlN thin films prepared by EVPE

Herein, a novel process of substrate pretreatment and high temperature annealing for elementary source vapor phase epitaxy (EVPE) is shown to be capable of optimizing the surface flatness of the AlN single crystal thin film, thus increasing its suitability for application in deep ultraviolet optoelectronic devices. Specifically, the increased surface smoothness/flatness of the so-prepared AlN samples are confirmed by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). In addition, the crystal quality, stress conditions and luminescence performance are significantly improved.

(Invited) Solar CO2 Conversion into Liquid Fuels By Photoelectrochemical Approaches

Photoelectrochemical (PEC) system for the reduction of CO2 into liquid fuels of formaldehyde/acetaldehyde, methanol and ethanol with trace amount of hydrogen gas bubbling have been described in the aspect of thermodynamics and kinetics of the CO2 reduction reaction to have efficient method by lowering activation energy of CO2 reduction on the electron transfer reaction and to do reduction potential tuning of CO2 reduction reaction for the selective reduction products. Ca/Fe doped TiO2 photoanode oxidizes water and generates the large amount of O2, electrons and protons. On the other side, rGO(reduced graphene oxide)/PVP(poly(4-vinyl)pyridine)/Nafion multi-layers have been coated and fabricated on the surface of Cu foil cathode to reduce CO2 into formaldehyde and acetaldehyde. When solar light was irradiated on the surface of photoanode, electrons get excited to the conduction band of Ca/Fe doped TiO2 and transported to the cathode via external wire with low external bias potential. rGO on Cu foil has been used for the dual functions of reduction potential tuning and multi-electron shuttling process of CO2 reduction reaction. The multi-electron shuttling function was illuminated by larger number and longer life-time of excited electrons and generating electron cloud, which have been confirmed by time-resolved photoluminescence (TR-PL) decay curves and 2D time-resolved photoluminescence (TR-PL) mapping images of Cu/rGO electrode. This allows the sequential multi-electron transport process from Cu/rGOcathode to CO2, which was studied with time-resolved chronoamperometry. N-heterocyclic poly(4-vinyl)pyridine (PVP) helps to capture and do chemical activation of reactant CO2 molecule by complexation as [PVP-CO2*] complex via charge separation and lowering transition state energy level of the electron transfer for the formation of anion radical of carbon dioxide complex with PVP as [e- + PVP-CO2*]≠ complex via electron delocalization, which is basically possible due to high basicity of lone pair electrons of nitrogen atoms of N-heterocyclic PVP compounds. These functions result in the lowered activation energy for the CO2 reduction reaction to have faster kinetics. The decreased amount of activation energy was determined with Tafel plots by measuring the slopes of it and semicircles of the electrochemical impedance spectroscopy (EIS) Nyquist plots at different temperatures. Nafion film coated on the surface of cathode assists the faster transport of H+ ions to CO2 reduction reaction sites for the simultaneous one-pot reaction of proton coupled electron transfer reactions. In the photocataytic system with semi-conducting polymers, CO2 was conversed into liquid solar fuels like formaldehyde and alcohols with solar to fuel efficiency over 2.5% to produce liquid fuel products over mM concentration for 4 hour reaction. Chang Woo Kim, So Jin Yeob, Hui-Ming Cheng, Young Soo Kang*, Selectively Exposed Crystal Facet-Engineered TiO2 Thin Film Photoanode for the Higher Performance of Photoelectrochemical Water Splitting Reaction, Energy and Environmental Sciences. 2015, 8, 3646.Amol U.Pawar, Chang WooKim, Myung Jong Kang, Young Soo Kang*, Crystal facet engineering of ZnO photoanode for the higher water splitting efficiency with proton transferable nafion film, Nano Energy 2015, 20, 156.Chang Woo Kim, Young Seok Son, Myung Jong Kang, Do Yoon Kim, Young Soo Kang*, (040)-Crystal Facet Engineering of BiVO4 Plate Photoanode for Solar Fuel Production, Energy Mater., 2016, 6, 1501754.Jin You Zheng, Amol Uttam Pawar, Chang Woo Kim, Yong Joo Kim, Young Soo Kang*, Highly enhancing photoelectrochemical performance of facilely-fabricated Bi-induced (002)-oriented WO3 film with intermittent short-time negative polarization, Applied Catalysis B 2018, 233, 88.Chang Woo Kim, Myung Jong Kang, Sohyun Ji, and Young Soo Kang*, Artificial Photosynthesis for Formaldehyde Production with 85% of Faradaic Efficiency by Tuning the Reduction Potential, ACS Catalysis 2018, 82, 968-974.Myung Jong Kang, Chang Woo Kim, Amol U. Pawar, Hyun Gil Cha, Sohyun Ji, Wen-Bin Cai, and Young Soo Kang*, Selective Alcohol on Dark Cathode by Photoelectrochemical CO2 Valorization and Their in-situ Characterization, minor revision, ACS Energy Lett. 2019, 4, 1549.Hong Ryeol Park, Amol Uttam Pawar, Umapada Pal, Tierui Zhang, Young Soo Kang, Enhanced solar photoreduction of CO2 to liquid fuel over rGO grafted NiO-CeO2 heterostructure nanocomposite, Nano Energy, 2021, 79, 105483.Amol U. Pawar, Umapada Pal, Jin You Zheng, Chang Woo Kim and Young Soo Kang*, Dynamics controlled CO2 reduction on Cu/rGO/PVP/Nafion multi-layered cathode for selective production of Acetaldehyde and Formaldehyde, Applied Catalysis B: Environmental, 2022, 303, 120921.

Structural Inhomogeneity and Crystallization of Amorphous Thin Films of Group IV Semiconductors

Structural Inhomogeneity and Crystallization of Amorphous Thin Films of Group IV Semiconductors

Biomineral-Inspired Colloidal Liquid Crystals: From Assembly of Hybrids Comprising Inorganic Nanocrystals and Organic Polymer Components to Their Functionalization

ConspectusBioinspired organic/inorganic synthetic composites have been studied as high-performance and functional materials. In nature, biominerals such as pearls, teeth, and bones are self-organized organic/inorganic composites. The inorganic components are composed of calcium carbonate (CaCO3) and hydroxyapatite (HAp), while the organic components consist of peptides and polysaccharides. These composites are used as structural materials in hard biological tissues. Biominerals do not show significantly higher performances than synthetic composites such as glass-fiber- or carbon-fiber-reinforced plastics. However, biominerals consist of environmentally friendly and biocompatible components that are prepared under mild conditions. Moreover, they form elaborate nanostructures and self-organized hierarchical structures. Much can be learned about material design from these biomineral-based hierarchical and nanostructured composites to assist in the preparation of functional materials.Inspired by these biological hard tissues, we developed nanostructured thin films and bulk hybrid crystals through the self-organization of organic polymers and inorganic crystals of CaCO3 or HAp. In biomineralization, the combination of insoluble components and soluble acidic macromolecules controls the crystallization process. We have shown that poly(acrylic acid) (PAA) or acidic peptides called polymer additives induce the formation of thin film crystals of CaCO3 or HAp by cooperation with insoluble organic templates such as chitin and synthetic polymers bearing the OH group. Moreover, we recently developed CaCO3- and HAp-based nanostructured particles with rod and disk shapes. These were obtained in aqueous media using a macromolecular acidic additive, PAA, without using insoluble polymer templates. At appropriate concentrations, the anisotropic particles self-assembled and formed colloidal liquid-crystalline (LC) phases.LC materials are generally composed of organic molecules. They show ordered and mobile states. The addition of stimuli-responsive properties to organic rod-like LC molecules led to the successful development of informational displays, which are now widely used. On the other hand, colloidal liquid crystals are colloidal self-assembled dispersions of anisotropic organic and inorganic nano- and micro-objects. For example, polysaccharide whiskers, clay nanosheets, gibbsite plate-shaped particles, and silica rod-shaped particles exhibit colloidal LC states.In this Account, we focused on the material design and hierarchical aspects of biomineral-based colloidal LC polymer/inorganic composites. We describe the design and preparation, nanostructures, and self-assembled behavior of these new bioinspired and biocompatible self-organized materials. The characterization results for these self-assembled nanostructured colloidal liquid crystals found using high-resolution transmission electron microscopy, small-angle X-ray scattering, and neutron scattering and rheological measurements are also reported. The functions of these biomineral-inspired liquid crystals are presented. Because these biomineral-based LC colloidal liquid crystals can be prepared under mild and aqueous conditions and they consist of environmentally friendly and biocompatible components, new functions are expected for these materials.

Bioinspired macromolecular templates for crystallographic orientation control of ZnO thin films through zinc hydroxide carbonate

The biomineralization-inspired preparation of inorganic hybrid materials has attracted attention. Here, we report a new approach to the orientation control of zinc oxide (ZnO) thin-film crystals through the preparation of zinc hydroxide carbonate (ZHC) by the macromolecular templates of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(vinyl alcohol) (PVA). Using 100-nm-thick PHEMA templates, ZHC thin films with the c-axis oriented parallel to the substrate were obtained, while ZHC thin films prepared by 100-nm-thick PVA templates showed perpendicular orientation. After the thermal treatment of ZHC, the crystal orientations of the ZnO thin films were maintained. The effects of the thickness and annealing time for the polymer templates on the morphologies of the ZnO thin films were examined.

Effects of substrate temperature and sputtering gas composition on physical properties and photocatalytic activities of WOx thin films deposited via radiofrequency sputtering

Tungsten oxide (WOx) is expected to act as a photocatalytic material under visible light. We have deposited WOx thin films using radiofrequency sputtering and evaluated the photocatalytic activities of the films via degradation of methylene blue solution. The optical absorbance, crystallization, and surface morphology of the WOx thin films were also investigated. The absorption edge of the WOx thin films was shifted to the long-wavelength region when the substrate temperature was high and the O2 gas flow rate was low. Crystallization proceeded when the substrate temperature was high and additional WOx diffraction peaks were appearing with low O2 gas flow during growth. Furthermore, the grain size of the WOx thin films was smaller when the O2 gas flow was high. The photocatalytic activity was higher when the substrate temperature and O2 gas flow rate were low.

Molecular crystallization of rubrene thin films assisted by gold nanoparticles

We demonstrate that crystalline organic rubrene thin films can be obtained by a facile spin-coating method using gold (Au) nanoparticles (NPs). Dodecanethiol-functionalized Au NPs were dissolved with rubrene molecules in solvent and a thin film of Au/rubrene was prepared by a simple spin coating process. The results of confocal photoluminescence (PL) and absorption spectral mapping confirmed the local formation of orthorhombic crystalline structures of the Au/rubrene hybrid film, in contrast to the monoclinic structure of plain rubrene films. Further, the results of transmission electron microscopy (TEM) and X-ray diffraction analysis, as well as Raman spectroscopy measurements of the rubrene and Au/rubrene films suggested the formation of high crystalline Au/rubrene film. The molecular crystallization of the Au/rubrene hybrid film is attributed to the nucleation effect of the Au NPs.

Morphological Variation of the Poly(ε-caprolactone) Crystals in Bulk, Thin and Ultrathin Films of Poly(ε-caprolactone)/Poly(vinyl methyl ether) Blends

Morphological Variation of the Poly(ε-caprolactone) Crystals in Bulk, Thin and Ultrathin Films of Poly(ε-caprolactone)/Poly(vinyl methyl ether) Blends

Synthesis and characterization of SnO2 thin films using metalorganic precursors

Tetrakis (dimethylamino) tin(IV) (TDMASn) is a metal organic precursor used for fabricating transparent conducting tin oxide (SnO2) thin films via atomic layer deposition process (ALD). This study reported for the first time, a spin coating fabrication process of transparent conducting SnO2 thin films using TDMASn as the precursor for electrode materials of solar cells substrate. The samples were prepared using sol–gel spin-coating process and annealed at different temperatures to form metastable crystal structured SnO2 thin films. The manufactured thin films consequently underwent X-ray diffraction (XRD), scanning electron microscope (SEM), UV–vis spectroscopy, and resistivity analysis. The XRD analysis certified the films to be of polycrystalline structure, with the SEM results revealing variations in film thicknesses. The UV–vis analysis demonstrated a peak absorption band at 250 nm, while the highest transmittance of 83.2% in the visible range was noticed at the 550 °C sample.

Detection of extremely low- and high-concentration organic acids by a highly-aligned lamellar block copolymer photonic crystal film

Detection of organic acids was carried out using lamella-forming polystyrene-block-poly (4-vinylpyridine) (PS–P4VP) block copolymer (BCP) photonic crystals. PS-P4VP photonic crystal thin films featuring highly-aligned lamellar microstructures parallel to the substrate were prepared through solvent annealing method by chloroform. When the lamella-forming PS-P4VP films were immersed in low-concentration organic acids, including formic acid (FA), acetic acid (AA), and propionic acid (PA), red-shifting reflectance wavelengths could be observed with the increase of the acid concentration. In contrast to the AA and PA systems, the lamella-forming PS-P4VP thin film was able to detect extremely low-concentration FA (5 × 10−5 vol%) due to its strongly protonated ability. When the PS-P4VP thin film was immersed in high-concentration FA and AA solutions, the longer and red-shifting reflectance wavelengths were observed with the increase of the acid concentration. In addition, the unexpected blue-shifting reflectance wavelength was observed when the film was immersed in a high-concentration PA solution. This is attributed to the alternation of segregation strength resulted from the compatibility effect. As a result, well-aligned lamellar PS-P4VP photonic crystal thin films are highly sensitive to the stimuli of organic acids for detecting extremely low- and high-concentration organic acids due to the change of protonated ability and compatibility, respectively.

Zinc oxide family semiconductors for ultraviolet radiation emission – A cathodoluminescence study

• Study of ZnO and MgZnO grown using a variety of techniques. • UV cathodoluminescence seen in bulk ZnO and in epitaxial MgZnO. • Sputtered MgZnO shows no UV cathodoluminescence. • Atomic layer deposition appears to be suitable for growing UV-luminescent MgZnO films . Zinc oxide (ZnO) family semiconductors that include ZnO and various ternary and even quaternary semiconductors formed by the inclusion of suitable other elements to zinc and oxygen are potential candidates for making UV-emitting solid-state devices. This work is a study of the UV-emitting potential of ZnO and MgZnO through electron beam irradiation of various samples. In this work, we studied materials grown or deposited through different techniques in order to assess their suitability for use in UV-emitting devices. Our work throws light on which growth or deposition methods are more suitable for obtaining radiation emission-capable materials from this family of II-VI oxide semiconductors. We find that single crystal thin film material, produced through either metal organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) appear to be the best, followed by hydrothermally-grown single crystals. In contrast, sputtered material appears unsuitable for making UV-emitting devices.

Crystallization of GeTe phase change thin films grown by pulsed electron-beam deposition

In this work, a technique for crystallization of Germanium Telluride (GeTe) thin films was experimentally studied. The GeTe phase change material was grown by a novel high vacuum physical deposition method called pulse electron-beam deposition (PED). To evaluate the quality of the PED growth method, crystallized GeTe thin films were investigated by contemplating the crystallite quality and the electrical conductivity performance. X-ray diffractometer (XRD) spectrum and resistivity were used to evaluate the crystal quality. Alternative material characterization methods, like field emission scanning electron microscope (FESEM) and energy-dispersive X-ray spectroscopy (EDX), were used to explore the crystallization parameters further. Our previous study observed that the as-grown GeTe thin film was amorphous. In order to convert the GeTe thin film into a crystalline formation, post treatment annealing was performed. To optimize the post annealing procedure, several annealing parameters were experimentally investigated. The thin film resistivity dropped by six orders of magnitude upon the crystallization process.

Layer polymorphous crystallizations of amorphous films: structural and kinetic aspects

Structural and geometric aspects of layer polymorphic crystallization of thin amorphous films were considered. The review of electron microscopic studies of crystallization “in situ” of amorphous films under the influence of the electron beam was provided. Digital video recording in real time t of structural and phase changes occurring in the film was applied. Kinetic crystallization curves were constructed and the parameters reflecting the growth of crystals of various morphologies in amorphous film were defined. Electron-beam crystallization of amorphous films is accompanied by the formation of Cr2O3 and V2O3 crystals of three basic morphological forms: disk-shaped, sickle-shaped and needle-shaped, the kinetics of growth of which is different. The growth of disk-shaped crystals occurs at a constant rate; the degree of crystallinity x ∼ t 2, and the rate of change in the fraction of the crystalline phase x't ∼ t. Sickle-shaped (at the initial stage) crystal, as it grows, changes its morphology in the sequence: sickle-shaped → ring-shaped → disk-shaped crystal.

Kinetic Monte Carlo simulations of Ge–Sb–Te thin film crystallization

Simulation of atomic redistribution in Ge–Sb–Te (GST)-based memory cells during SET/RESET cycling is needed in order to understand GST memory cell failure and to design improved non-volatile memories. However, this type of atomic scale simulations is extremely challenging. In this work, we propose to use a simplified GST system in order to catch the basics of atomic redistribution in Ge-rich GST (GrGST) films using atomistic kinetic Monte Carlo simulations. Comparison between experiments and simulations shows good agreements regarding the influence of Ge excess on GrGST crystallization, as well as concerning the GST growth kinetic in GrGST films, suggesting the crystallized GST ternary compound to be off-stoichiometric. According to the simulation of atomic redistribution in GrGST films during SET/RESET cycling, the film microstructure stabilized during cycling is significantly dependent of the GST ternary phase stoichiometry. The use of amorphous layers exhibiting the GST ternary phase stoichiometry placed at the bottom or at the top of the GrGST layer is shown to be a way of controlling the microstructure evolution of the film during cycling. The significant evolution of the local composition in the amorphous solution during cycling suggests a non-negligible variation of the crystallization temperature with operation time.

Low Driving Voltage and Low Optical Loss Electro-Optic Modulators Based on Lead Zirconate Titanate Thin Film on Silicon Substrate

Heterogeneous integration of electro-optic (EO) crystal thin film on insulator has become an attractive platform to achieve high-performance and compact modulators. While there has been some impressive research on the development of such modulators, a critical consideration is the relatively complex heterogeneous-integrating process. In this work, we demonstrate EO modulators based on lead zirconate titanate (PZT) crystal thin film on SiO₂&#x002F;Si substrate, where the PZT is epitaxially-grown via the straightforward chemical solution deposition method. The presented device exhibits a voltage&#x2013;length product <i>V_&#x03C0;L</i> of 1.4 V&#x00B7;cm and a propagation loss of 1.8 dB&#x002F;cm at the wavelength of 1550 nm. The measured 3-dB bandwidth of a 5 mm-long device is 12 GHz, which can be improved by further optimizing the travelling-wave electrodes. The proposed technique should enable low-cost and mass-production of power-efficient modulators and to promote the development of photonic integration.

Structural evolution and thermal stability of functionally graded NiTi nano-glass thin films alloys during crystallization

Functionally graded and composite materials are known as new materials, which exhibit higher performances and provide a comprehensive application. This paper introduces a bi-layer Ni-rich/NiTiCu alloy with a composite structure comprising nano-crystal, glassy and amorphous microstructure. The successive crystallization of amorphous Ni-rich/NiTiCu bi-layer thin films deposited at room temperature was studied by annealing at 773 K for 15, 30, and 60 min. Different experiments, including scanning electron microscopy (SEM), grazing incidence X-ray diffraction (GIXRD), high-resolution transmission electron microscopy (HRTEM), and thermodynamics calculations (CALPHAD) were conducted to analyze the microstructure. The crystal nucleation, glassy/amorphous thermal stability against alloying elements, and related interfaces were studied. The as-deposited thin films were fully amorphous and had a rounded nano-glassy structure. HRTEM images indicated no long-range periodic lattice and the interface between layers was flat and sharp. Short-time annealing depicts non-spherical crystal nuclei in the amorphous NiTiCu layer with a diffuse interface. In contrast, after 60 min annealing, the NiTiCu and NiTi layers were fully and mostly crystallized with different structures, respectively. The results confirm the higher thermal stability of the amorphous NiTi binary alloy. Thermodynamic and diffusion studies indicate that the higher driving force of amorphous to B2 ordered structure against Cu addition along with the higher Cu diffusivity compared with Ni and Ti are responsible for lower thermal stability of NiTiCu. • The NiTi/NiTiCu bi-layer thin films deposited at RT on a Si substrate using DC magnetron. • The crystallization of amorphous bi-layer thin films was studied at different annealing at conditions. • The structure of the thin film and interface of the layers strongly depended on the composition andthe annealing conditions. • The amorphous NiTi thin film was more stable than the NiTiCu thin film as supported by thermodynamic calculation.

Hysteresis Phenomena and the Effect of Reorientation in a Polymer-Liquid Crystal System Under the Influence of Laser Radiation and Uniaxial Deformation

The orientational structure in a polymer-liquid crystal thin film induced by uniaxial mechanical deformation has been studied. It is shown that the dependence of the deflection angle of the director of a nematic liquid crystal on the direction of the tensile force has a hysteresis region in which the orientation structure spontaneously changes from a state with positive torsion of the director to a state with negative torsion. It is established that the effect of modification of the hysteresis loop leads to an expansion of the hysteresis region and a decrease in the field at which orientation bistability appears.

Room temperature crystallization and stability of halide perovskite thin films

<p indent="0mm">The preparation of metal halide perovskite films with high crystallinity and high stability without heating in air is difficult to be realized, which is important for future large-scale applications of flexible perovskite solar cells. In this paper, we propose a method to realize the crystallization of perovskite thin films at room temperature, by using a mixed solution of three low-boiling polar molecules, 1,2-dimethoxyethane (DME), methylamine (MA), and ethanol (EA). The mixed solution effectively dissolves the perovskite precursor, and high crystallinity perovskite thin films can be prepared without annealing in air. The as-prepared perovskite film still maintains the lattice structure after 120 h of continuous illumination in air, showing high stability under water, oxygen, and illumination conditions. This method is suitable for the preparation of perovskite thin films with a band gap of <sc>1.60–1.78 eV.</sc> Finally, the flexible perovskite solar cells fabricated by this method achieved the highest photoelectric conversion efficiency of 19.63%. This provides a feasible direction for the large-scale commercial production of flexible perovskite solar cells.