In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition

Nano-structured CuS thin films were deposited on the functionalized -NH(2)-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the -NH(2)-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV-vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3Ω/□). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On -NH(2)-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On -OH- and -CH(3)-terminated surfaces, the dominant growth mechanism on the surface is cluster-by-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on -NH(2)-terminated SAMs regions of -NH(2)/-CH(3) patterned SAMs surface.     

Characterization of metal oxide surfaces and thin semiconductor films by inelastic electron tunneling spectroscopy.

Inelastic electron tunneling spectroscopy (IETS) is a unique surface and interface analytical technique using electron tunneling through a metal/insulator/metal tunneling junction at cryogenic temperatures. It gives the vibrational spectrum of a very thin (nm) insulator film and the adsorbed species on it. The high sensitivity, good resolution, and wide spectral range inherent in IETS enable us to analyze the surface and interface of the insulator in detail. The tunneling junction is a good model system for oxide catalysts, electronic devises, and solid state sensors. Information about the surfaces of alumina and magnesia, the adsorption states and chemical reactions of adsorbed species occurring on these oxides can be obtained through an analysis of the tunneling spectra. The structures and properties of evaporated thin semiconductor films can also be studied. In this review, the surface characterization of alumina and magnesia, the adsorption and surface reactions of organic acids, esters, amides, and nitryls on these oxides, and the characterization of thin evaporated films of Si, Ge, and the oxides are summarized.     

High-throughput screening using porous photoelectrode for the development of visible-light-responsive semiconductors

A high-throughput screening system for new visible-light-responsive semiconductors for photoelectrodes and photocatalysts was developed in this study. Photoelectrochemical measurement was selected to evaluate visible-light responsiveness, and an automated semiconductor synthesis system that can be used to prepare porous thin-film photoelectrodes of various materials was also developed. As an example application of our system, iron-based binary oxides were selected as target materials for n-type semiconductors. Fe-Ti, Fe-Nb, and Fe-V with various composition ratios were synthesized. Fe-Ti and Fe-Nb binary oxide systems have been studied previously, and our results showed good consistency with previous reports, demonstrating the capability of our system. In the Fe-V system, the highest photocurrent was observed with 50% vanadium. This ratio corresponds to FeVO4, which is expected to be a new visible-light-responsive material. As another example, screening targets of bismuth-based binary oxides were investigated for p-type semiconductor photoelectrodes, and CuBi2O4 was found as a new visible-light-responsive p-type semiconductor.     

Biomimicry designs for photoelectrochemical systems: Strategies to improve light delivery efficiency

In the past years, photoelectrocatalysis has been developed to offer green and sustainable application to wastewater remediation, water disinfection, H₂ production and CO₂ reduction. The advances of these systems with new semiconductor photoelectrocatalysts that are photoexcited with visible light giving photogenerated charge carriers efficiently, separated in a photoelectrochemical cell, are explained. These studies do not consider the light transport as fundamental aspect of light–matter interaction, although the harvesting of photons at the semiconductor surface limits the quantum yield by the existing semiconductor architectures. This opinion review envisages biomimicry as an alternative natural guide for synthesizing more efficient photoelectrodes. Micro- and nanostructure shapes from nature are identified to prepare new bio-inspired photoelectrodes for photoelectrochemical reactors with the light transport as indispensable element. The implementation of strategies of phototroph organisms to maximize light adsorption and the enhancement of photoelectrocatalytic surface area are analyzed as key factors for such bio-based photoelectrodes.     

Organometallic chemistry related to applications for microelectronics in Japan

This is meant to be a brief overview of the developments of research activities in Japan on organometallic compounds related to their use in electronic and optoelectronic devices. The importance of organometallic compounds in the deposition of metal and semiconductor films for the fabrication of many electronic and opto-electronic devices cannot be exaggerated. Their scope has now extended to thin-film electronic ceramics and high-temperature oxide superconductors. A variety of organometallic compounds have been used as source materials in many types of processing procedures, such as metal–organic chemical vapor deposition (MOCVD), metalorganic vapor-phase epitaxy (MOVPE), metal–organic molecular-beam epitaxy (MOMBE), etc. Deposited materials include silicon, Group III–V and II–VI compound semiconductors, metals, superconducting oxides and other inorganic materials. Organometallic compounds are utilized as such in many electronic and optoelectronic devices; examples are conducting and semiconducting materials, photovoltaic, photochromic, electrochromic and nonlinear optical materials. This review consists of two parts: (I) research related to the fabrication of semiconductor, metal and inorganic materials; and (II) research related to the direct use of organometallic materials and basic fundamental research.     

非传统晶体生长的早期观察

晶体早期生长的研究揭示,在某些体系中,晶体生长可能并不遵循传统路径.借由某些聚合物或生物分子的帮助,无机晶体的前驱体或纳米晶体在生长初期有可能聚集为无序的大块颗粒.这些聚集体表面晶化形成高结晶度高密度的外壳,随后完成从表面到核心的晶化过程.此逆向晶体生长机理在一些诸如沸石、钙钛矿、金属和金属氧化物等无机化合物体系中均已被发现,在其他材料体系里也将得到验证.认识这一新的晶体生长路径将给予我们更多的自由度来实现晶体形态控制,也有助于我们对于许多天然矿物形成机制的理解.本文简要回顾了最近本领域研究中一些典型逆向晶体生长的例子.     

Advances in Photoelectrocatalysis with Nanotopographical Photoelectrodes

The design of photoelectrodes for high efficiency solar fuel energy conversion devices is based on the search for adequate surface conditioning to achieve efficient light harvesting, stability, minimized surface recombination losses and high electron‐transfer rates at the electrolyte interface. An overview on established and novel approaches is given. A recent viable solution is provided by electroplating of nanoscale catalytic metals on passivated semiconductor surfaces, thereby forming reactive centers and avoiding contact between the semiconductor surface and the electrolyte. At these nano‐dimensioned Schottky‐type junctions, light‐induced excess minority carriers are scavenged and transferred to the electrolyte. Various possible device configurations are outlined and envisaged systems for hydrogen or oxygen evolution and carbon dioxide reduction are presented. The role of ultrathin passivating films is emphasized and methods to fabricate open as well as compact conformal films are described.     

Photovoltaic performance of injection solar cells and other applications of nanocrystalline oxide layers

The direct conversion of sunlight to electricity via photoelectrochemical solar cells is an attractive option that has been pursued for nearly two decades in several laboratories. In this paper, we review the principles and performance features of very efficient solar cells that are being developed in our laboratories. These are based on the concept of dye-sensitization of wide bandgap semiconductors used in the form of mesoporous nanocrystalline membrane-type films. The key feature is charge injection from the excited state of an anchored dye to the conduction band of an oxide semiconductor such as TiO₂. In the use of the semiconductor in the form of high surface area, highly porous film offers several unique advantages: monomeric distribution of a large quantity of the dye in a compact (few micron thick) film, efficient charge collection and drastic inhibition of charge recombination (‘capture of charge carriers by oxidized dye’). Near quantitative efficiency for charge collection for monochromatic light excitation gives rise to sunlight conversion efficiency in the range of 8–10% This has led to fruitful collaboration with several industrial partners. Possible applications and commercialization of these solar cells and also other practical applications of nanosized films are briefly outlined.     

电沉积二氧化钛纳米微粒膜的光电化学性能和表面形貌研究

采用光电流谱、透射光谱和扫描微探针显微镜技术对电沉积法制备的二氧化钛纳米微粒膜的光电化学性能和表面形貌进行了研究.结果表明,不同制备条件下的二氧化钛纳米微粒膜具有与紧密的半导体电极不同的光电化学性质,并探讨了其光电化学性能与表面形貌的关系.     

Imaging of crystal morphology and molecular simulations of surface energies in pentacene thin films

We have investigated the crystal growth of the organic semiconductor pentacene by complementing molecular simulations of surface energies with experimental images of pentacene films. Pentacene thin films having variations in thickness and grain size were produced by vacuum sublimation. Large (approximately 20 microm) faceted crystals grew on top of the underlying polycrystalline thin film. The films were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Single crystals most commonly grew in a truncated diamond shape with the largest crystal face, (001), growing parallel to the substrate. Crystal morphologies and surface energies were calculated using force field-based molecular simulations. The (001) surface was found to have the lowest energy, at 76 mJ/m(2), which was consistent with experimental observations of crystal face size. It was demonstrated that the morphology of the large faceted crystals approached the equilibrium growth shape of pentacene. From contact angle measurements, the critical surface tension of textured pentacene thin films in air was determined to be 34 mJ/m(2).     

Synthesis and characterization of ZnO and ZnO:Ga films and their application in dye-sensitized solar cells

Highly crystalline ZnO and Ga-modified zinc oxide (ZnO:Ga) nanoparticles containing 1, 3 and 5 atom% of Ga3+ were prepared by precipitation method at low temperature. The films were characterized by XRD, BET, XPS and SEM. No evidence of zinc gallate formation (ZnGa2O4), even in the samples containing 5 atom% of gallium, was detected by XRD. XPS data revealed that Ga is present into the ZnO matrix as Ga3+, according to the characteristic binding energies. The particle size decreased as the gallium level was increased as observed by SEM, which might be related to a faster hydrolysis reaction rate. The smaller particle size provided films with higher porosity and surface area, enabling a higher dye loading. When these films were applied to dye-sensitized solar cells (DSSCs) as photoelectrodes, the device based on ZnO:Ga 5 atom% presented an overall conversion efficiency of 6% (at 10 mW cm(-2)), a three-fold increase compared to the ZnO-based DSSCs under the same conditions. To our knowledge, this is one of the highest efficiencies reported so far for ZnO-based DSSCs. Transient absorption (TAS) study of the photoinduced dynamics of dye-sensitized ZnO:Ga films showed that the higher the gallium content, the higher the amount of dye cation formed, while no significant change on the recombination dynamics was observed. The study indicates that Ga-modification of nanocrystalline ZnO leads to an improvement of photocurrent and overall efficiency in the corresponding device.     

Synthesis of highly pure nanocrystalline and mesoporous CaTiO3 by a particulate sol–gel route at the low temperature

The low temperature perovskite-type calcium titanate (CaTiO₃) thin films and powders with nanocrystalline and mesoporous structure were prepared by a straightforward particulate sol–gel route. The prepared sol had a narrow particle size distribution about 17 nm. X-ray diffraction and Fourier transform infrared spectroscopy revealed that, the synthesized powders had highly pure and crystallized CaTiO₃ structure with preferable orientation growth along (1 2 1) direction at 400–800 °C. The activation energy of crystal growth was calculated 5.73 kJ/mol. Furthermore, transmission electron microscope images showed that the average crystallite size of the powders annealed at 400 °C was around 3.5 nm. Field emission scanning electron microscope analysis and atomic force microscope images revealed that, the deposited thin films had uniform, mesoporous and nanocrystalline structure with the average grain size in the range 33–39 nm depending on annealing temperature. Based on Brunauer–Emmett–Teller (BET) analysis, the synthesized powders showed mesoporous structure with BET surface area in the range 51–21 m²/g at 400–800 °C. One of the smallest crystallite size and one of the highest surface areas reported in the literature is obtained which can be used in many applications, such as photocatalysts.     

Selective modified SnO2-based materials for gas sensors arrays

An enhancement of selectivity of semiconductor gas sensors, based on nanocrystalline SnO2 is reported. It is shown that modification of the surface of crystallites, forming thick films of conductive sensor materials, with catalytic clusters of gold or oxides of ruthenium, nickel, copper and iron allows selective response of sensors to different gases, such as carbon monoxide, ammonia, hydrogen sulfide, nitrogen dioxide and acetone vapor. These selective sensor responses can be obtained in the ranges of gas concentrations close to or below threshold limit values while the working temperature of sensors can be kept below 300 °C. The described approach for modification of selectivity of sensor materials could be used as perspective route in developingselective gas sensors. These results allow us to propose application of obtained materials in electronic nose sensor systems.     

Study on mechanism of photocatalytic performance of La-doped TiO2/Ti photoelectrodes by theoretical and experimental methods

TiO₂ photoelectrodes with various nanostructures have been successfully prepared by the anodization method. The morphology, microstructure and optical properties of as-prepared photoelectrodes were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet/visible light diffuse reflectance spectra (UV/vis/DRS), surface photovoltage spectroscopy (SPS) and photocurrent. The electronic structure and optical properties of La doped/undoped TiO₂ photoelectrodes with different crystal structures were calculated by the density function theory. The photocatalytic and photoelectrocatalytic activities of as-prepared photoelectrodes were evaluated. The results showed that the anodization potentials played a crucial role in the surface morphology and microstructure. Both results of theoretical calculations and experimental tests demonstrated that La-doped photoelectrodes were more sensitive to light than undoped one. The difference of photoelectrodes performance was ascribed to the crystal configuration, impurity energy levels and long-range orientation moving of photogenerated carriers.     

Nature of Gas Sensitivity in Nanocrystalline Metal Oxides

Problems associated with developing gas-sensitive inorganic materials are discussed. The principle of operation of a semiconductor gas sensor of resistive type is considered, and main band structure parameters sensitive to the gas phase composition are determined. Ways to solve the problem of selectivity of semiconducting oxides are discussed. The influence of microstructure, dopants, and analysis temperature is looked into on the basis of experimental results obtained in studying nanocrystalline tin dioxide and zinc oxide. Prospects for use of systems based on two or more nanocrystalline oxides (nanocomposites) are considered.     

In situ adsorption method for synthesis of binary semiconductor CdS nanocrystals inside mesoporous SBA-15

Binary semiconductor CdS nanocrystals have been prepared inside the channels of mesoporous SBA-15 using an in situ adsorption method combining a surface modification scheme and a wet impregnation technique by functionalizing the SBA-15 surface with thiol groups, adsorbing cadmium cations, and calcining in N₂ atmosphere at 300 °C. The combined results of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) firstly reveal that CdS nanocrystals with uniform size of about 6 nm are formed and mono-dispersed inside the channels of mesoporous SBA-15. And a blue shift is observed in UV–Vis absorption spectrum and photoluminescence (PL) spectrum, indicating the quantum size effect of nanocrystalline CdS.     

Mesoporous oxide junctions and nanostructured solar cells

Mesoporous films of wide-band gap semiconductor oxides are an important new class of electronic materials. They are constituted by a network of nanocrystalline particles of oxides, such as titania, niobia or zinc oxide, sintered together to allow for charge carrier transport to take place. The pores between the nanoparticles are filled with an electrolyte or a solid state organic hole conductor forming an interpenetrating heterojunction of very large contact area. These junctions exhibit extraordinary opto-electronic properties due to their large surface area to volume ratio leading to applications in different domains, such as photovoltaics, intercalation batteries, electrochromic and electroluminescent displays, photocatalysis and chemical sensors. Of particular interest are dye-sensitized heterojunctions, where photo-induced charge separation occurs at the interface between the mesoporous oxide and the hole conductor or the electrolyte. Photovoltaic cells based on this concept form a viable alternative to conventional silicon cells. Solar to electric power conversion efficiencies exceeding 10% have been reached with mesoporous titania films derivatized with molecular charge transfer sensitizers and used in conjunction with organic iodide/triiodide-based redox electrolytes. Long-term accelerated light-soaking tests have shown the system to be intrinsically stable. This article summarized recent developments in this field including a discussion of solid state dye-sensitized heterojunctions employing spirobifluorene-connected arylamines as hole transport materials.     

Colloidal Semiconductor Q-Particles: Chemistry in the Transition Region Between Solid State and Molecules

In semiconductor particles of nanometer size, a gradual transition from solid-state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q-particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q-particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance-specific quantity in macrocrystalline materials, increases by several electron volts in Q-particles with decreasing particle size. In Q-particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico-chemical properties of the particle. Q-particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q-particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q-particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q-particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q-particles, as well as with the spectroscopic, electrochemical, and electron-microscopic investigation of these particles.     

Nanocrystalline structure and soft magnetic properties of nickel–molybdenum alloy thin films electrodeposited from acidic and alkaline aqueous solutions

Nanocrystalline nickel–molybdenum (Ni–Mo) alloy thin films were electrochemically synthesized in acidic and alkaline aqueous solutions. Transmission electron microscope bright-field images and electron diffraction patterns of the electrodeposits made it obvious that pure Ni consists of a submicron crystalline phase with the grain diameter of several hundred nanometers, while Ni–20 %Mo alloy was composed of a nanocrystalline phase with the grain diameter of a few nanometers. It was estimated that the nanocrystalline phase of electrodeposited Ni–Mo alloy thin films was introduced by the formation of supersaturated Ni–Mo solid solution phase with Mo content in the deposit more than 20 %. Submicron crystalline pure Ni thin films were hardly magnetized in perpendicular direction to the film plane while the nanocrystalline Ni–20 %Mo alloy thin films were isotropically magnetized. It was suggested that the isotropical magnetization behavior was caused by decreasing the demagnetizing field and the magneto crystalline anisotropy with a decrease in the magnetic moment and the average crystal grain size. Coercive force of a submicron crystalline pure Ni thin film electrodeposited from an acidic aqueous solution was ca. 100 Oe while that of a nanocrystalline Ni–20 %Mo alloy thin film electrodeposited from an alkaline aqueous solution was only 1～2 Oe. Soft magnetic properties of Ni–Mo alloy thin films electrodeposited from an alkaline aqueous solution were better than that from an acidic aqueous solution and it was improved with an increase in Mo content in the deposit. It was estimated that the electrodeposited Ni–Mo alloy catalysts could be easily recovered with magnetic field less than 1 kOe.     

Nanocrystalline TiO2/ZnO thin films: fabrication and application to dye-sensitized solar cells

Nanocrystalline TiO2 thin films composed of densely packed grains were deposited onto indium-doped tin oxide (ITO)-coated glass substrates at room temperature using a chemical bath deposition technique. A layer-by-layer (LbL) process was utilized to obtain a 1.418-microm-thick TiO2/ZnO structure. The TiO2 surface was super-hydrophilic, but its hydrophilicity decreased considerably after ZnO deposition. Other TiO2/ZnO films were studied to assess their suitability as photoelectrodes in dye-sensitized solar cells (DSSCs).