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.     

Photodetector arrays directly assembled onto polymer substrates from aqueous solution

We report a simple and inexpensive approach to directly assemble arrays of cadmium sulfide (CdS) semiconductors onto transparent flexible poly(ethylene terephthalate) (PET) sheets via a polymer-mediated selective nucleation and growth process from an aqueous solution. This strategy of assembling functional materials onto plastics utilizes the surface functional molecules of the UV photooxidation patterned polymer to direct the nucleation and growth of CdS. We demonstrated that such assembled structures are viable for flexible macroelectronics, as manifested by the fabrication of CdS photodetector arrays on PET that can withstand bending. The best devices exhibited a specific detectivity of 3 x 10(11) cm Hz(1/2) W(-1) at 514-nm excitation wavelength at a modulation frequency of 90 Hz at room temperature. This direct assembly strategy eliminates additional lithography and etching steps during the deposition of the active inorganic semiconductor layer, is general to other inorganic materials and plastic substrates, and can enable low-cost, wearable, and/or disposable flexible electronics.     

有机薄膜晶体管半导体材料的研究进展

有机薄膜晶体管(organic thin-film transistors,OTFTs)具有工艺简单、成本低及柔性良好等优点,在有源显示、传感及逻辑电路等领域有着十分重要的应用前景.实用性有机薄膜晶体管应具备高迁移率、高开关比、低阈值电压及良好的稳定性等性能.有机半导体材料是有机薄膜晶体管的主要组成部分,对器件的性能有着重要影响.介绍了有机薄膜晶体管的基本结构和运行模式,按照p型、n型及双极性分类总结了有机薄膜晶体管半导体材料的研究进展情况,最后对有机半导体材料的前景进行了展望.     

Flexible Metal Oxide Semiconductor Devices Made by Solution Methods

For the fabrication of next-generation flexible metal oxide devices, solution-based methods are considered as a promising approach because of their potential advantages, such as high-throughput, large-area scalability, low-cost processing, and easy control over the chemical composition. However, to obtain certain levels of electrical performance, a high process temperature is essential, which can significantly limit its application in flexible electronics. Therefore, this article discusses recent research conducted on developing low-temperature, solution-processed, flexible, metal oxide semiconductor devices, from a single thin-film transistor device to fully integrated circuits and systems. The main challenges of solution-processed metal oxide semiconductors are introduced. Recent advances in materials, processes, and semiconductor structures are then presented, followed by recent advances in electronic circuits and systems based on these semiconductors, including emerging flexible energy-harvesting devices for self-powered systems that integrate displays, sensors, data-storage units, and information processing functions.     

High-performance organic field-effect transistors: molecular design, device fabrication, and physical properties

In the past decade, tremendous progress has been made in organic field-effect transistors (OFETs). Their real applications require further development of device performance. OFETs consist of organic semiconductors, dielectric layers, and electrodes. Organic semiconductors play a key role in determining the device characteristics. The properties of the organic semiconductors, such as molecular structure and packing, as well as molecular energy levels, can be properly controlled by molecular design. Therefore, we designed and synthesized a series of organic molecules. The synthesized organic semiconductors exhibit excellent field-effect properties due to strong intermolecular interactions and proper molecular energy levels. Meanwhile, the influence of the device fabrication process, organic semiconductor/dielectric layer interface, and organic layer/electrode contact on the device performance was investigated. A deep understanding of these factors is helpful to improve field-effect properties. Furthermore, single-crystal field-effect transistors are highlighted because the single-crystal-based FETs can provide an accurate conducting mechanism of organic semiconductors and higher device performance as compared with thin film FETs.     

Hierarchical self-assembly of semiconductor functionalized peptide α-helices and optoelectronic properties

To determine the ability of semiconductors templated by α-helical polypeptides to form higher order structures and the charge carrier properties of the supramolecular assemblies, L-lysine was functionalized with a sexithiophene organic semiconductor unit via iterative Suzuki coupling and the click reaction. The resultant amino acid was incorporated into a homopolypeptide by ring-opening polymerization of an amino acid N-carboxyanhydride. Spectroscopic investigation of the polypeptide revealed that it adopted an α-helical secondary structure in organic solvents that underwent hierarchical self-assembly to form higher order structures. In cyclohexane, the polymer formed organogels at 2% (w/v). Organic photovoltaic and organic field effect transistor devices were fabricated by deposition of the PCBM blended active layer from chlorobenzene at concentrations shown to induce self-assembly of the polymer. Compared with control compounds, these devices showed significantly greater hole mobility, short circuit current, and efficiency. This work establishes the potential of this previously unreported bioinspired motif to increase device performance.     

三(五氟苯基)硼烷掺杂有机/高分子半导体的研究进展

有机/高分子共轭聚合物的结构设计是制备高性能有机半导体的有效策略,但该过程存在着设计合成周期长、制备步骤复杂和产率偏低等问题。为了克服这些问题,近年来人们越来越关注对有机/高分子半导体的掺杂。然而,传统电荷转移掺杂剂(如卤族单质I₂、金属氧化物Fe₃O₄、小分子F4TCNQ等)存在掺杂效率低、溶解度差和掺杂条件苛刻等问题。相比之下,三(五氟苯基)硼烷具有溶解度高、掺杂效率高、广泛适应性等优点。本文结合相关文献综述了三(五氟苯基)硼烷掺杂有机半导体的物理机制,并探讨了掺杂有机半导体的性质;此外,还总结了三(五氟苯基)硼烷掺杂在不同光电功能器件中的应用并明确了今后的研究方向。     

Modeling electrochemical deposition inside nanotubes to obtain metal-semiconductor multiscale nanocables or conical nanopores

Nanocables with a radial metal-semiconductor heterostructure have recently been prepared by electrochemical deposition inside metal nanotubes. First, a bare nanoporous polycarbonate track-etched membrane is coated uniformly with a metal film by electroless deposition. The film forms a working electrode for further deposition of a semiconductor layer that grows radially inside the nanopore when the deposition rate is slow. We propose a new physical model for the nanocable synthesis and study the effects of the deposited species concentration, potential-dependent reaction rate, and nanopore dimensions on the electrochemical deposition. The problem involves both axial diffusion through the nanopore and radial transport to the nanopore surface, with a surface reaction rate that depends on the axial position and the time. This is so because the radial potential drop across the deposited semiconductor layer changes with the layer thickness through the nanopore. Since axially uniform nanocables are needed for most applications, we consider the relative role of reaction and axial diffusion rates on the deposition process. However, in those cases where partial, empty-core deposition should be desirable (e.g., for producing conical nanopores to be used in single nanoparticle detection), we give conditions where asymmetric geometries can be experimentally realized.     

聚合物表面自组装电极的制备

采用在聚合物表面制备选择性化学镀电极的方法, 将具有电极结构的模板置于聚萘二甲酸乙二醇酯(PEN)基片表面上, 用紫外光进行照射, 使辐照区域表面形成羧基, 然后通过配位作用使银离子附着到表面上, 再经过紫外光照射还原出金属银颗粒, 最后以表面的金属银颗粒为催化剂进行特定区域化学镀铜形成电极. 利用四探针法测得电极的电阻率为5.063×10-2 Ω·mm2/m, 与纯金属铜的电阻率的数量级相同.     

Transforming Ionene Polymers into Efficient Cathode Interlayers with Pendent Fullerenes

A new and highly efficient cathode interlayer material for organic photovoltaics (OPVs) was produced by integrating C₆₀ fullerene monomers into ionene polymers. The power of these novel “C₆₀‐ionenes” for interface modification enables the use of numerous high work‐function metals (e.g., silver, copper, and gold) as the cathode in efficient OPV devices. C₆₀‐ionene boosted power conversion efficiencies (PCEs) of solar cells, fabricated with silver cathodes, from 2.79 % to 10.51 % for devices with a fullerene acceptor in the active layer, and from 3.89 % to 11.04 % for devices with a non‐fullerene acceptor in the active layer, demonstrating the versatility of this interfacial layer. The introduction of fullerene moieties dramatically improved the conductivity of ionene polymers, affording devices with high efficiency by reducing charge accumulation at the cathode/active layer interface. The power of C₆₀‐ionene to improve electron injection and extraction between metal electrodes and organic semiconductors highlights its promise to overcome energy barriers at the hard‐soft materials interface to the benefit of organic electronics.     

2,6-Bis[2-(4-pentylphenyl)vinyl]anthracene: a stable and high charge mobility organic semiconductor with densely packed crystal structure

The development of new organic semiconductors with improved electrical performance and enhanced environmental stability is the focus of considerable research activity. This paper presents the design, synthesis, optical and electrochemical characterization, crystal packing, modeling and thin film morphology, and organic thin film field effect transistor (OTFT) device data analysis for a novel 2,6-bis[2-(4-pentylphenyl)vinyl]anthracene (DPPVAnt) organic semiconductor. We observed a hole mobility of up to 1.28 cm2/V.s and on/off current ratios greater than 107 for OTFTs fabricated using DPPVAnt as an active semiconductor layer. The mobility value is comparable to that of the current best p-type semiconductor pentacene-based device performance. In addition, we found a very interesting relationship between the charge mobility and molecule crystal packing in addition to the thin film orientation and morphology of the semiconductor as determined from single-crystal molecule packing study, thin film X-ray diffraction, and AFM measurements. The high performance of the semiconductor ranks among the best performing p-type organic semiconductors reported so far and will be a very good candidate for applications in organic electronic devices.     

High-mobility field-effect transistors from large-area solution-grown aligned C60 single crystals

Field-effect transistors based on single crystals of organic semiconductors have the highest reported charge carrier mobility among organic materials, demonstrating great potential of organic semiconductors for electronic applications. However, single-crystal devices are difficult to fabricate. One of the biggest challenges is to prepare dense arrays of single crystals over large-area substrates with controlled alignment. Here, we describe a solution processing method to grow large arrays of aligned C(60) single crystals. Our well-aligned C(60) single-crystal needles and ribbons show electron mobility as high as 11 cm(2)V(-1)s(-1) (average mobility: 5.2 ± 2.1 cm(2)V(-1)s(-1) from needles; 3.0 ± 0.87 cm(2)V(-1)s(-1) from ribbons). This observed mobility is ~8-fold higher than the maximum reported mobility for solution-grown n-channel organic materials (1.5 cm(2)V(-1)s(-1)) and is ~2-fold higher than the highest mobility of any n-channel organic material (~6 cm(2)V(-1)s(-1)). Furthermore, our deposition method is scalable to a 100 mm wafer substrate, with around 50% of the wafer surface covered by aligned crystals. Hence, our method facilitates the fabrication of large amounts of high-quality semiconductor crystals for fundamental studies, and with substantial improvement on the surface coverage of crystals, this method might be suitable for large-area applications based on single crystals of organic semiconductors.     

金属离子的光催化去除研究进展

利用光催化剂表面的光生电子或空穴等活性物种，通过还原或氧化反应去除水相中的金属离子，是与环境保护、贵金属回收或金属担载催化剂制备相关的重要过程。笔者结合半导体的结构特征，综述了利用光催化还原反应或氧化反应，对铬、汞、铜、镍、银及铂、钯等贵金属和锰、铀等的光催化去除效果，介绍了该技术在处理金属离子混合体系实现金属分离过程的应用。结合有关实验数据，对一些可能反应机制进行了探讨。对与环境保护及其它相关工艺过程的应用进行了介绍。     

Pressure-sensitive Transistor Fabricated from an OrganicSemiconductor 1,1'-Dibutyl-4,4'-bipyridinium Diiodide

Although organic semiconductors have attracted extensive interest and been utilized to fabricate a variety of optoelectronic devices, their electrical transportation characteristics under high pressure have rarely been investigated. However, the weak intermolecular interaction of organic semiconductors endows them with a pre- ssure-sensitive crystal structure and electrical transportation performance, especially the latter. Herein, a new pre- ssure-sensitive transistor was fabricated from an organic semiconductor 1,1'-dibutyl-4,4'-bipyridinium diiodide. It was found that this transistor exhibited increasing resistance as the pressure gradually increased and that it eventually shut off under a pressure of 288 MPa. Such a characteristic makes this organic semiconductor a potential candidate for the use in the fabrication of pressure-sensitive switches and regulators. In addition, these results shed light on the electrical performance of flexible organic optoelectronic devices working under high pressure levels resulted from the bending force.     

Surface-plasmon-coupled emission of quantum dots

We studied surface plasmon-coupled emission (SPCE) of semiconductor quantum dots (QDs). These QDs are water-soluble ZnS-capped CdSe nanoparticles stabilized using lysine cross-linked mercaptoundecanoic acid. The QDs were spin-coated from 0.75% PVA solution on a glass slide covered with 50 nm of silver and a 5-nm protective SiO(2) layer. Excited QDs induced surface plasmons in a thin silver layer. Surface plasmons emitted a hollow cone of radiation into an attached hemispherical glass prism at a narrow angle of 48.5 degrees. This directional radiation (SPCE) preserves the spectral properties of QD emission and is highly p-polarized irrespective of the excitation polarization. The SPCE spectrum depends on the observation angle because of the intrinsic dispersive properties of SPCE phenomenon. The remarkable photostability can make QDs superior to organic fluorophores when long exposure to the intense excitation is needed. The nanosize QDs also introduce a roughness near the metal layer, which results in a many-fold increase of the coupling of the incident light to the surface plasmons. This scattered incident illumination transformed into directional, polarized radiation can be used simultaneously with SPCE to develop devices based on both quantum dot emission and light scattered from surface plasmons on a rough surface.     

Flexible electronics based on one-dimensional inorganic semiconductor nanowires and two-dimensional transition metal dichalcogenides

Flexible electronics technology is considered as a revolutionary technology to unlock the bottleneck of traditional rigid electronics that prevalent for decades, thereby fueling the next-generation electronics. In the past few decades, the research on flexible electronic devices based on organic materials has witnessed rapid development and substantial achievements, and inorganic semiconductors are also now beginning to shine in the field of flexible electronics. As validated by the latest research, some of the inorganic semiconductors, particularly those at low dimension, unexpectedly exhibited excellent mechanical flexibility on top of superior electrical properties. Herein, we bring together a comprehensive analysis on the recently burgeoning low-dimension inorganic semiconductor materials in flexible electronics, including one-dimensional (1D) inorganic semiconductor nanowires (NWs) and two-dimensional (2D) transition metal dichalcogenides (TMDs). The fundamental electrical properties, optical properties, mechanical properties and strain engineering of materials, and their performance in flexible device applications are discussed in detail. We also propose current challenges and predict future development directions including material synthesis and device fabrication and integration.     

Electroreduction of molecular oxygen on carbon electrode modified by dispersed silver

Silver particles are formed by electrochemical deposition on the carbon electrode surface. It is found that the deposition process occurs according to the progressive nucleation mechanism, which results in formation of silver particles with the size of 95 to 190 nm as dependent on the electrodeposition time. The values of silver particle size and support surface coverage by metal obtained on the basis of microphotographs indicate that cathodic polarization in the presence of dissolved oxygen results in particle size redistribution due to the reaction of silver particle dissolution with further deposition simultaneously with oxygen electroreduction. The reaction of molecular oxygen electroreduction on a carbon electrode with deposited dispersed silver occurs via a mixed two- and four-electron mechanism. The observed limiting reaction current is of diffusion nature.     

Electron donor and acceptor spatial distribution in structured bulk heterojunction photovoltaic devices induced by periodic photopolymerization

Donor and acceptor spatial distributions were directly formed in a surface relief grating of structured bulk heterojunction (BHJ) photovoltaic devices by simple periodic photopolymerization. Enhanced photocurrents were observed in the structured BHJ photovoltaic devices and formation of the D/A spatial distribution was confirmed by Kelvin probe force microscopy. This technique enables the fabrication of structured BHJ photovoltaic devices with solution-processable organic semiconductors, and has tremendous potential for controlling D/A spatial distribution in organic optoelectronics devices.     

Effect of morphology on organic thin film transistor sensors

This review provides a general introduction to organic field-effect transistors and their application as chemical sensors. Thin film transistor device performance is greatly affected by the molecular structure and morphology of the organic semiconductor layer. Various methods for organic semiconductor deposition are surveyed. Recent progress in the fabrication of organic thin film transistor sensors as well as the correlation between morphology and analyte response is discussed.     

Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor

Plasmonic metal nanostructures have been incorporated into semiconductors to enhance the solar-light harvesting and the energy-conversion efficiency. So far the mechanism of energy transfer from the plasmonic metal to semiconductors remains unclear. Herein the underlying plasmonic energy-transfer mechanism is unambiguously determined in Au@SiO(2)@Cu(2)O sandwich nanostructures by transient-absorption and photocatalysis action spectrum measurement. The gold core converts the energy of incident photons into localized surface plasmon resonance oscillations and transfers the plasmonic energy to the Cu(2)O semiconductor shell via resonant energy transfer (RET). RET generates electron-hole pairs in the semiconductor by the dipole-dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor), which greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone. RET from a plasmonic metal to a semiconductor is a viable and efficient mechanism that can be used to guide the design of photocatalysts, photovoltaics, and other optoelectronic devices.