Quantum phenomena in small semiconductor structures and devices

The ability to artificially structure new semiconductor materials on an atomic scale, using advanced crystal growth methods such as molecular beam epitaxy and metal organic chemical vapor deposition, has led recently to the observation of new physical phenomena as well as the creation of entirely new classes of devices based on band gap and wave function engineering. In these lectures an elementary introduction is given to the quantum aspects of these new structures.     

Tailoring Semiconductor Crystals to Atomic Dimensions

This article provides an introduction to the growth by molecular beam epitaxy (MBE) of semiconductor structures which have dimensions of the same order as interatomic distances in solids. The basic process technology is first described, followed by a brief account of surface reaction mechanisms involved in the growth of GaAs from an atomic beam of Ga and molecular beams of As₄ and As₂. From the study of growth dynamics using electron diffraction techniques it is shown how reduced dimensionality structures can be grown and some indication is given of the effect of quantum confinement on material properties. Finally, some recent modifications of MBE based on flux interruption are described.     

High-resolution analytical transmission electron microscopy of semiconductor quantum structures

Analytical transmission electron microscopy was applied to characterize the size, shape, real structure, and, in particular, the composition of different semiconductor quantum structures. Its potential applicability is demonstrated for heterostructures of III-V semiconducting materials and II-VI ones, viz. (In,Ga)As quantum wires on InP and (In,Ga)As quantum dots on GaAs both grown by metal organic chemical vapor deposition, and CdSe quantum dots on ZnSe grown by molecular beam epitaxy. The investigations carried out show that the element distribution even of some atomic layers can be detected by energy-dispersive X-ray spectroscopy, however, exhibiting a smeared profile. Contrary to that, sub-nanometre resolution has been achieved by using energy-filtered transmission electron microscopy to image quantum dot structures.     

Electrical and optical properties of zeolite y supported sno2 nanoparticles

 Meso-and nanoporous solids used as supports for highly dispersed metal or semiconductor nanoparticles represent a promising class of materials for potential nanoscale devices. The electrical and optical properties of zeolite Y supported SnO₂ nanoparticles were studied by use of impedance and UV diffuse reflectance spectroscopy. When subjected to reductive and oxidative atmospheres the samples reveal sensitive changes in their properties which are different to that of bulk SnO₂. Received: 18 June 1996 Accepted: 29 August 1996     

Nanoindentation in Crystal Engineering: Quantifying Mechanical Properties of Molecular Crystals

Nanoindentation is a technique for measuring the elastic modulus and hardness of small amounts of materials. This method, which has been used extensively for characterizing metallic and inorganic solids, is now being applied to organic and metal–organic crystals, and has also become relevant to the subject of crystal engineering, which is concerned with the design of molecular solids with desired properties and functions. Through nanoindentation it is possible to correlate molecular‐level properties such as crystal packing, interaction characteristics, and the inherent anisotropy with micro/macroscopic events such as desolvation, domain coexistence, layer migration, polymorphism, and solid‐state reactivity. Recent developments and exciting opportunities in this area are highlighted in this Minireview.     

1,1-二氨基二硝基乙烯晶体的密度泛函理论研究

The electronic structural properties of 1,1-diamino dinitroethylene crystal have been studied by the DFT method with B3LYP function. The calculated crystal energy is -05.81 kJ/mol,which is comparable to the reference value. The frontier bands are quite flat,indicating that the molecular states are hardly perturbed by the crystalline environment. The distribution of electronic charges determines that the molecules pack through head-end to form wave-shaped layers with extensive intermolecular hydrogen bonding within the layers and ordinary van der Waals interactions between the layers. Judged by the value of band gap of 4.0 eV,it could be predicted that the conductivity of DADNE is between the semiconductor and insulator. The frontier orbital consists of atomic orbitals of C-NO2 group,indicating that there exists a stronger conjugation among the molecules and that C-NO2 is the region with high reactivity. The population of C-NO2 bond is much less than those of all the other bonds and therefore the detonation is initiated by the breakdown of this bond.     

Electron-conducting quantum dot solids: novel materials based on colloidal semiconductor nanocrystals

We review the optical and electrical properties of solids that are composed of semiconductor nanocrystals. Crystals, with dimensions in the nanometre range, of II-VI, IV-VI and III-V compound semiconductors, can be prepared by wet-chemical methods with a remarkable control of their size and shape, and surface chemistry. In the uncharged ground state, such nanocrystals are insulators. Electrons can be added, one by one, to the conduction orbitals, forming artificial atoms strongly confined in the nanocrystal. Semiconductor nanocrystals form the building blocks for larger architectures, which self-assemble due to van der Waals interactions. The electronic structure of the quantum dot solids prepared in such a way is determined by the orbital set of the nanocrystal building blocks and the electronic coupling between them. The opto-electronic properties are dramatically altered by electron injection into the orbitals. We discuss the optical and electrical properties of quantum dot solids in which the electron occupation of the orbitals is controlled by the electrochemical potential.     

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.     

Orientation of crystalline polymers by non-classical means

Crystalline polymers can be oriented by epitaxy on various substrates, including low molecular weight, usually aromatic compounds and polymers oriented by epitaxy and by mechanical means (strain induced polyolefin films and friction-deposited PTFE layers). The resulting oriented films are thin, but display unusual morphologies with lamellae standing edge-on and, often, single crystal structure. They are adequate investigation materials for structure determination, show enhanced or maximized orientation-dependent properties, and may contribute to mechanical strengthening in composite polymer films.     

IV-VI semiconductor growth on silicon substrates and new mid-infrared laser fabrication methods

This paper reviews results from research conducted at the University of Oklahoma on the development of new IV-VI semiconductor (lead salt) epitaxial growth and laser fabrication procedures that can ultimately lead to dramatic increases in mid-IR laser operating temperatures. Work has focused on growth of IV-VI semiconductor laser structures on silicon substrates using buffer layers that contain BaF2. Recent experiments show that it is possible to obtain high crystalline quality IV-VI semiconductor layer structures on (111)-oriented silicon substrates using molecular beam epitaxy (MBE) or on (100)-oriented silicon using a combination of MBE and liquid phase epitaxy (LPE). Experimental data for IV-VI semiconductor layer structures grown on silicon substrates including crystalline quality information as determined by high resolution X-ray diffraction (HRXRD) measurements and absorption edge information as determined by Fourier transform infrared (FTIR) transmission measurements are presented. Results show that these materials can be used to fabricate lasers that cover the 3 microns (3333 cm-1) to 16 microns (625 cm-1) spectral range. Removal of IV-VI semiconductor laser structures from the silicon growth substrate by dissolving BaF2 buffer layers with water is also demonstrated. This allows epitaxially-grown laser structures to be sandwiched between two heat sinks with a minimum of thermally resistive IV-VI semiconductor material. Theoretical modeling predicts that IV-VI lasers fabricated this way will have maximum continuous wave (cw) operating temperatures at least 60 degrees higher than those of IV-VI lasers fabricated on PbSe or PbTe substrates.     

Nanostructured metal oxides/hydroxides-based electrochemical sensor for monitoring environmental micropollutants

Nanostructured metal oxides/hydroxides (NMOs/HOs) with unique optical, electrical and molecular properties, chemical and photochemical stability, electrochemical activity, large surface area along with desired functionalities have recently become important as materials to construct electrochemical sensor for monitoring environmental micropollutants. In this review, we present and discuss the NMOs/HOs-based electrochemical sensor for detection of micropollutants including toxic organic micropollutants, heavy metal ions (HMIs), and anions in water. The analytical performance of a NMOs/HOs-based electrochemical sensor can be improved by tailoring the properties of the NMOs/HOs through engineering of morphology, particle size, exposed crystal facets, effective surface area, functionality, adsorption capability and electron-transfer properties. These interesting NMOs/HOs are expected to find potential applications in a new generation of miniaturized, smart electrochemical environmental monitoring devices.     

Chemicophysical surface treatment and the experimental demonstration of Schottky-Mott rules for metal/semiconductor heterostructure interfaces

Metal/semiconductor (MS) heterostructure is of wide interest in a number of areas including physics, chemistry, materials science, materials engineering, chemical engineering, and electrical engineering. It is an important element of modern technology. The present investigation describes a novel experimental technique to address the influence of interfacial chemical passivation on the Schottky-Mott [Naturwiss. 26, 843 (1938); Cambridge Philos. Soc. 34, 568 (1938)] rules for MS heterostructure, and to try to establish these rules. The success of the experiment derives from three remarkable findings: First, a semiconductor (Al(x)Ga(1-x)N), which is robust and relatively less susceptible to an easy reaction with foreign chemicals, is needed for the demonstration. Second, reactive ion etching together with wet chemical etching by certain selected chemical (such as KOH), but not by others (for example, H(3)PO(4) or aqua regia), can clean the semiconductor surface well, and remove/passivate the dangling chemical bonds from this surface. Third, a judicious selection of deposition parameters for the deposition of metal(s) preferably on a certain selected semiconductor can lead to metal deposition on the semiconductor surface by van der Waals type of epitaxy. Transmission electron microscopy and x-ray diffraction indicate that MS heterostructures, thus prepared, are very different from others; they appear to provide convincing experimental verification of the Schottky-Mott rules, and to establish these rules without any ambiguity. Others fail to do it.     

纳米材料的概述、制备及其结构表征

纳米材料在电子、光学、化工、陶瓷、生物和医药等诸多方面的重要应用而引起人们的高度重视。本文从以下3个方面加以论述。 一、纳米材料的概述:从分子识别、分子自组装、吸附分子与基底的相互关系、分子操作与分子器件的构筑,并通过具体的例证加以阐述,包括在STM操作下单分子反应;有机小分子在半导体表面的自指导生长;多肽-半导体表面特异性选择结合;生物分子/无机纳米组装体;光驱动多组分三维结构组装体;DNA分子机器。 二、纳米材料的若干制备方法和结构表征方法:制备方法包括:物理的蒸发冷凝法,分子束外延法(MBE),机械球磨法,扫描探针显微镜法(SPM)。化学的气相沉淀法(VCD),液相沉淀法,溶胶-凝胶法(Sol-gel),L-B膜法,自组装单分子层和表面图案化法,水热/溶剂热法,喷雾热解法,样板合成法或化学环境限制法及自组装法。 三、若干结构表征方法包括:X-射线法(XRD),扩展X射线精细结构吸收谱(EXAFS),X-射线光电子能谱(XPS),光谱法,扫描隧道显微镜/原子力显微镜(STM/AFM)和有机质谱法(OMS)。     

Two‐Dimensional Noble‐Metal Chalcogenides and Phosphochalcogenides

Noble‐metal chalcogenides, dichalcogenides, and phosphochalcogenides are an emerging class of two‐dimensional materials. Quantum confinement (number of layers) and defect engineering enables their properties to be tuned over a broad range, including metal‐to‐semiconductor transitions, magnetic ordering, and topological surface states. They possess various polytypes, often of similar formation energy, which can be accessed by selective synthesis approaches. They excel in mechanical, optical, and chemical sensing applications, and feature long‐term air and moisture stability. In this Minireview, we summarize the recent progress in the field of noble‐metal chalcogenides and phosphochalcogenides and highlight the structural complexity and its impact on applications.     

Electrochemical preparation of ZnS/CdS superlattice and its photoelectrochemical properties

Superlattice electrodes composed of three molecular layers of CdS and one to three molecular layers of ZnS were prepared on gold (111) film by using electrochemical atomic layer epitaxy. The prepared electrodes exhibited similar behavior to those of n-type semiconductor electrodes. The energy gap was not changed at all by changing the number of molecular layers of ZnS and by increasing the number of periods in the superlattice structure up to four. The magnitude of the anodic photocurrent was, however, influenced by these variables. It increased with an increase in the number of periods in the superlattice structure and decreased with increasing number of molecular layers of ZnS.     

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.     

Design magnetischer Materialien: Chemie der Magnete

The field of molecular based magnetism is an active area of research directed toward the design of new magnetic materials. The idea is to introduce molecular strategies in magneto-chemistry. This can open completely new synthetic routes to materials with previously unknown physical properties. Spin carriers used within this approach range from purely organic radicals to metal complexes and organometallic compounds. The design of new magnetic materials with tailor-made properties requires a detailed knowledge about the interactions between possible spin carriers and the strategies necessary to achieve interactions in all three dimensions. The latter is closely related to the field of crystal engineering. Starting from introductory remarks to magnetochemistry the underlaying concepts for the design of magnetic materials on the basis of molecular compounds as well as new developments and possible applications are described.     

Directional Charge Transport in Layered Two‐Dimensional Triazine‐Based Graphitic Carbon Nitride

Triazine‐based graphitic carbon nitride (TGCN) is the most recent addition to the family of graphene‐type, two‐dimensional, and metal‐free materials. Although hailed as a promising low‐band‐gap semiconductor for electronic applications, so far, only its structure and optical properties have been known. Here, we combine direction‐dependent electrical measurements and time‐resolved optical spectroscopy to determine the macroscopic conductivity and microscopic charge‐carrier mobilities in this layered material “beyond graphene”. Electrical conductivity along the basal plane of TGCN is 65 times lower than through the stacked layers, as opposed to graphite. Furthermore, we develop a model for this charge‐transport behavior based on observed carrier dynamics and random‐walk simulations. Our combined methods provide a path towards intrinsic charge transport in a direction‐dependent layered semiconductor for applications in field‐effect transistors (FETs) and sensors.     

Upconversion and Phase Transition Characteristics in Erbium(III)- and Ytterbium(III)-Codoped Vanadium(IV) Oxide

Vanadium(IV) dioxide has emerged as a promising thermochromic material for smart window application through metal–insulator transition, which simultaneously involves an abrupt change in optical, electrical, and magnetic properties. Here, Er³⁺ or Yb³⁺-codoped vanadium(IV) dioxide has been prepared by a hydrothermal and annealing process. The structure, metal–insulator transition, and upconversion luminescence characterizations have been evaluated using X-ray diffraction, differential thermal analysis, and fluorescence spectral analysis. The samples exhibit unique properties, including enhancing the intensity of upconversion emission, decreasing the metal–insulator transition temperature to 41.4°C, and emitting bright green upconversion emission along with extremely weak emission in the red region under 980?nm excitation. Moreover, green upconversion luminescence intensity increased by an order of magnitude from the low-temperature monoclinic structure of vanadium(IV) dioxide to the high-temperature rutile structure of vanadium(IV) dioxide for the first time, which will pave a new pathway for researching the application of photoluminescence in smart materials.     

Nanoparticles as structural and functional units in surface-confined architectures

The nanoscale engineering of functional chemical assemblies has attracted recent research effort to provide dense information storage, miniaturized sensors, efficient energy conversion, light-harvesting, and mechanical motion. Functional nanoparticles exhibiting unique photonic, electronic and catalytic properties provide invaluable building blocks for such nanoengineered architectures. Metal nanoparticle arrays crosslinked by molecular receptor units on electrodes act as selective sensing interfaces with controlled porosity and tunable sensitivity. Photosensitizer/electron-acceptor bridged arrays of Au-nanoparticles on conductive supports act as photoelectrochemically active electrodes. Semiconductor nanoparticle composites on surfaces act as efficient light collecting systems, and nanoengineered semiconductor 'core-shell' nanocrystal assemblies reveal enhanced photoelectrochemical performance due to effective charge separation. Layered metal and semiconductor nanoparticle arrays crosslinked by nucleic acids find applications in the optical, electronic and photoelectrochemical detection of DNA. Metal and semiconductor nanoparticles assembled on DNA templates may be used to generate complex electronic circuitry. Nanoparticles incorporated in hydrogel matrices yield new composite materials with novel magnetic, optical and electronic properties.