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Two-dimensional (2D) materials with excellent photocatalytic properties and unique piezoelectric response have attracted great attention. However, these characters are rare for traditional 2D structures. With an intrinsic electric field, the Janus 2D materials show great promise in photocatalytic and out-of-plane piezoelectric applications. Herein, we show that Janus In2X2X' (X and X' = S, Se, and Te) monolayers are desirable in the overall water splitting and piezoelectric devices. Comprehensive investigations reveal that the band gaps of these Janus monolayers are from 0.34 to 2.27 eV. With proper band edge positions, strong solar absorption, fast transfer and efficient separation of carriers, and high solar to hydrogen (STH) efficiencies (reaching 37.70%), eight members of them stand out. Besides, the electrons and holes have sufficient driving forces in the process of redox reaction. The piezoelectric response for in- and out-of-plane is superior for all monolayers. These compelling features make them suitable for photocatalysts, sensors, actuators, and energy conversion devices.

The van der Waals heterostructure (vdWH) has attracted widespread attention as a unique structure for future electronic and optoelectronic devices. In this paper, we constructed the ZnO-SeMoS and ZnO-SMoSe vdWHs and systematically investigated their electronic structures and band alignments considering vertical strain and external electric field effects. It is found that the ZnO-SeMoS and ZnO-SMoSe vdWHs both exhibit type-II band alignment with indirect band gaps of 1.31 and 0.63 eV respectively, depending on the interface characteristics. What's more, the band alignment of these two heterostructures can be tuned to type I or type III, and their band gap can be modified to direct feature by applying vertical strain and an electric field. The results reveal that ZnO/MoSSe vdWHs have promising potential in multi-functional nanodevices, and provide a way to modify the electronic properties of Janus-based heterojunctions using interface characteristics.

Doping is a key technology in modern semiconductor industry. With reduced size of semiconductor devices, quantum effect becomes significant, and the classic theory for device design will no longer be valid. Thus the traditional doping technology is facing a great challenge. On the other hand, the third generation wide-gap semiconductors like GaN, ZnO and TiO2 have great application potential in optoelectronics and microelectronics due to their wide band gap, direct gap character and good stability. However, there is a doping bottleneck on the p-type doping of these materials. In the field of application of semiconductor technology, doping mechanism in photocatalysis, spintronic materials, and novel two-dimensional (2D) semiconductors are also the frontier of research. The research in this project focuses on the above key topics. The primary discoveries include the follows: (i) Based on first-principles band structure calculations and analysis of the band-edge wave function characters, we proposed that passivated (Mo+C)-doped TiO2 is a strong candidate for hydrogen production through water splitting. The results received highly attention from the experts in the field of photocatalysis, and the feasibility was confirmed by recent experimental studies. (ii) We discovered that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d0 semiconductors. The origin and enhancement of such hole-induced ferromagnetism is studied, which suggested new possibility of preparing non-magnetically doped spintronic semiconductor devices. (iii) We systematically studied the doping mechanism and bottleneck of donor and acceptor impurities in quantum dots and quantum wires. Due to the effect of quantum confinement effect, the energy of the conduction band minimum (CBM) increases whereas that of the valence and maximum (VBM) decreases. We found that defect formation energy and transition energy level increase when the size of the quantum dot (QD) decreases, leading to the self-purification effect and degrade the performance of devices. Focused on this problem, we proposed the approach of overcoming the doping bottleneck in nanostructures, and developed a model to describe the quantum Stark effect in hole impurity state of quantum dots. These works promoted the research in the application of nano-devices. (iv) For the first time, the band alignment between two-dimensional transition-metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) is calculated. We predicted the type-II band alignment in MoX2/WX2 heterostructures. In addition, direct-indirect gap transition and semiconductor-metal transition in strained MoS2 is observed. Moreover, we studied the mechanical, optical properties and band gap modulation of graphyne, a new allotrope of carbon, and discovered that Ca-doped graphyne had high capacity of hydrogen storage. The project has attracted great attentions, and the related publications are highly cited by researchers all over the world. These works bring better understanding of semiconductor doping theory, and have important scientific value for the device design and property prediction of new generation semiconductors and nano-devices.

Magnetic tunnel junctions (MTJs) have attracted tremendous interests recently because of their potential application in magnetoresistive random access high-density memory and magnetic sensor. However, the performance of them is far from satisfying due to the various problems exist in tunnel barrier materials. Here, we propose to use two-dimensional (2D) Bi 2 O 2 Se material, which is advantageous in reducing the MTJ size, as the tunnel barrier, and demonstrate that it is able to generate very large tunnel magnetoresistance (TMR) when integrated with CoFe electrodes. The underlying mechanism is elaborated by analyzing the band structures, electron transmission and interface properties. These results provide important guidance for designing high-density and high-performance MTJs.

A method to calculate the band alignment between materials with different structures using passivated quantum dots is proposed. By using this method, the rutile/anatase titanium dioxide band offset is determined. The valence band maxima of anatase and rutile are close to each other, whereas the conduction band minimum of anatase is found to be about 0.2 eV higher than that of rutile, which is in agreement with the experimental fact that anatase has higher photocatalytic activity. The reliability of this method is further tested on several semiconductors, and reasonable results are obtained for most cases.

The magnetic and electronic properties of Mn-doped ZnO are studied by first-principles calculations. It is found that the exchange interaction between Mn ions depends on the Mn–Mn distribution configuration and distance. We also found that the ferromagnetism can be existed when the Mn–Mn distance is large and Mn ions are distributed uniformly, and the long ranged ferromagnetism is explained by the interaction between Mn and O atoms. Thus, it is possible to tune ferromagnetism in Mn-doped ZnO semiconductors by controlling the doping position in ZnO lattice.

Inorganic semiconductors are the essential constituents of information society, having enabled most of the devices for microelectronics, optoelectronics, new energy technology, healthcare devices, artificial intelligence, etc. From the view of condensed matter physics and materials science, inorganic semiconductors are not intricate. However, this special class of materials are keeping inspiring new inventions and consequently new technologies, which in turn promote the design of new semiconductors. The kinds of semiconductors in nature are finite. On one hand, the new techniques need semiconductors with better properties than existing ones, on the other hand, the new computers’ software and hardware develop so rapid that it is possible to design new semiconductors according to people’s desire before manufacturing them. In this paper, we review the rational design of inorganic semiconductors by transformation of their elemental constituents, while maintaining their structure motifs.

The electronic structure and binding energy of a hydrogenic acceptor impurity in 2, 1, and 0-dimensional semiconductor nano-structures (i.e. quantum well (QW), quantum well wire (QWW), and quantum dot (QD)) are studied in the framework of effective-mass envelope-function theory. The results show that (1) the energy levels monotonically decrease as the quantum confinement sizes increase; (2) the impurity energy levels decrease more slowly for QWWs and QDs as their sizes increase than for QWs; (3) the changes of the acceptor binding energies are very complex as the quantum confinement size increases; (4) the binding energies monotonically decrease as the acceptor moves away from the nano-structures’ center; (5) as the symmetry decreases, the degeneracy is lifted, and the first binding energy level in the QD splits into two branches. Our calculated results are useful for the application of semiconductor nano-structures in electronic and photoelectric devices.

We propose a method for uniformly calculating the electronic states of a hydrogenic donor impurity in low-dimensional semiconductor nano-structures in the framework of effective-mass envelope-function theory, and we study the electronic structures of this systems. Compared to previous methods, our method has the following merits: (a) It can be widely applied in the calculation of the electronic states of hydrogenic donor impurities in nano-structures of various shapes; (b) It can easily be extended to study the effects of external fields and other complex cases; (c) The excited states are more easily calculated than with the variational method; (d) It is convenient to calculate the change of the electronic states with the position of a hydrogenic donor impurity in nano-structures; (e) The binding energy can be calculated explicitly. (c) 2007 Elsevier B.V. All rights reserved.

Fabrication of semiconductor nanostructures such as quantum dots (QDs), quantum rings (QRs) has been considered as the important step for realization of solid state quantum information devices, including QDs single photon emission source, QRs single electron memory unit, etc. To fabricate GaAs quantum rings, we use Molecular Beam Epitaxy (MBE) droplet technique in this report. In this droplet technique, Gallium (Ga) molecular beams are supplied initially without Arsenic (As) ambience, forming droplet-like nano-clusters of Ga atoms on the substrate, then the Arsenic beams are supplied to crystallize the Ga droplets into GaAs crystals. Because the morphologies and dimensions of the GaAs crystal are governed by the interplay between the surface migration of Ga and As adatoms and their crystallization, the shape of the GaAs crystals can be modified into rings, and the size and density can be controlled by varying the growth temperatures and As/Ga flux beam equivalent pressures(BEPs). It has been shown by Atomic force microscope (AFM) measurements that GaAs single rings, concentric double rings and coupled double rings are grown successfully at typical growth temperatures of 200°C to 300°C under As flux (BEP) of about 1.0×10-6 Torr. The diameter of GaAs rings is about 30-50 nm and thickness several nm.

The electronic structure and electron g factors of HgTe quantum dots are investigated, in the framework of the eight-band effective-mass approximation. It is found that the electron states of quantum spheres have aspheric properties due to the interaction between the conduction band and valence band. The highest hole states are S (l = 0) states, when the radius is smaller than 9.4 nm, the same as the lowest electron states. Thus strong luminescence from HgTe quantum dots with radius smaller than 9.4 nm has been observed (Rogach et al 2001 Phys. Status Solidi b 224 153). The bandgap of HgTe quantum spheres is calculated and compared with earlier experimental results (Harrison et al 2000 Pure Appl. Chem. 72 295). Due to the quantum confinement effect, the bandgap of the small HgTe quantum spheres is positive. The electron g factors of HgTe quantum spheres decrease with increasing radius and are nearly 2 when the radius is very small. The electron g factors of HgTe quantum ellipsoids are also investigated. We found that as some of the three dimensions increase, the electron g factors decrease. The more the dimensions increase, the more the g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension.

The coupling of surface plasmons to the photonic modes in hexagonal textured metallic microcavity was studied. Two types of sample structures (symmetric and asymmetric) were investigated. Both angle-resolved transmission and photoluminescent measurements showed the presence of surface plasmon modes for the textured metallic microcavity samples. For the asymmetric structured sample, the bandgap was observed at the Brillouin zone edge. The simulation of the photonic band structure using decoupled approximation for the standing wave modes agrees with the experimental result. We have derived the surface modes due to various interfaces in the microcavity. It was found that the surface plasmon modes from metal/air interface are most dominant, and was observed to couple strongly with both the transverse electric and transverse magnetic modes. Light extraction enhancement due to surface plasmon coupling was achieved with directional enhancement as much as 12 times compared with planar microcavity for the symmetric structured sample.

A quantum waveguide theory is proposed for hole transport in the mesoscopic structures, including the band mixing effect. We found that due to the interference between the 'light' hole and 'heavy' wave, the transmission and reflection coefficients oscillate more irregularly as a function of incident wave vector geometry parameters. Furthermore conversion between the heavy hole and light hole states occurs at the intersection. (C) 2003 Elsevier Ltd. All rights reserved.

In the framework of effective-mass envelope function theory, the valence energy subbands and optical transitions of the InAs/GaAs quantum ring are calculated by using a four-band valence band model. Our model can be used to calculate the hole states of quantum wells, quantum wires, and quantum dots. The effect of finite offset and valence band mixing are taken into account. The energy levels of the hole are calculated in the different shapes of rings. Our calculations show that the effect of the difference between effective masses of holes in different materials on the valence subband structures is significant. Our theoretical results are consistent with the conclusion of the recent experimental measurements and should be useful for researching and making low-dimensional semiconductor optoelectronic devices. (C) 2002 American Institute of Physics.

Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS 2 Nanoflakes

Layered GaS nanosheets have been attracting increasing research interests due to their highly anisotropic structural, electrical, optical, and mechanical properties, which are useful for many applications. However, single-layer or few-layer GaS-based photodetectors have been rarely reported. Here a few-layer GaS two-terminal photodetector with a fast and stable response has been fabricated. It shows different photo-responses in various gas environments. A higher photo-response (64.43 A W(-1)) and external quantum efficiency (EQE) (12,621%) is obtained in ammonia (NH3) than in air or oxygen (O2). A theoretical investigation shows that the charge transfer between the adsorbed gas molecules and the photodetector leads to the different photo-responses.

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. (C) 2001 American Institute of Physics.

Photonic microcavity has modified photonic modes that have intense localized electric field, which can couple strongly with the embedded emission centers. In this work, we fabricated 2D photonic microcavities with Penrose quasicrystal pattern with 10-fold symmetry. Organic luminescence material tris(8-hydroxyquinoline) aluminum (Alq 3 ) was embedded into the microcavity and the angle resolved transmission (ART) and photoluminescence (ARPL) spectra of the microcavity were measured. The results showed that the normal Gaussian photoluminescence spectrum of Alq 3 has been strongly modified by the microcavity dispersion characteristic. In addition, omni-directional photonic band gap exists in the microcavity. The higher symmetry of Penrose quasicrystal pattern means that there was minimal difference in the directional dispersion characteristics.

The electronic states and optical transition properties of silicon quantum-well layers embedded by SiO2 layers are studied by the empirical pseudopotential homojunction model. The energy bands, wave functions, and the optical transition matrix elements are obtained for layers of thickness from 1 to 6 nm, and three oriented directions (001), (110), and (111). It is found that for Si layers in the (001) direction the energy gap is pseudodirect, for these in the (111) direction the energy gap is indirect, while for those in the (110) direction the energy gap is pseudodirect or indirect for a thickness smaller or larger than 3 nm, respectively. The optical transition matrix elements are smaller than that of diner transition, and increase with decreasing layer thickness. When the thickness of a layer is smaller than 2 nm, the Si QW layers have larger transition matrix elements. It is caused by mixing of bulk X states with the Gamma(1) state. The calculated results are compared with experimental results.

The electronic states and optical transition properties of three semiconductor nanocrystallites, Si, GaAs, and ZnSe, are studied using the empirical pseudopotential homojunction model. The energy levels, wave functions, optical transition matrix elements, and lifetimes are obtained for quadratic prisms with widths from 11 to 27 A. It is found that the three kinds of prism have different quantum confinement properties. For Si prisms, the energy gaps vary with the equivalent diameter d as , in agreement with previous theoretical calculations. For the same d the energy gaps are slightly different for different shapes: large for the prism with large aspect ratio; small for the prism with small aspect ratio. The exponent of d depends on the boundary barrier height, i.e. the extent of penetration of the wave function into the vacuum. The wave function of the LUMO states consists mainly of bulk X states. The optical transition matrix elements are much smaller than those of direct transition, and increase with decreasing width. The corresponding lifetimes decrease from the millisecond range to the microsecond range, and the change is abrupt depending on the symmetry and composition of the wave function of the LUMO and HOMO states. For GaAs prisms, the energy gap is also pseudo-direct, but the optical transition matrix elements are larger than those of Si prisms by two orders of magnitude for the same width. For ZnSe prisms, the energy gap is always direct, and the optical transition matrix elements are comparable with those of direct energy gap bulk semiconductors. In some cases the symmetry of the HOMO state changes, resulting in an abrupt decrease of the transition matrix element. The calculated lifetimes of the Si prism and the positions of PL peaks are in agreement with experimental results for porous Si.

In the framework of effective mass envelope function theory, absorption coefficients are calculated for intraband (intersubband in the conduction band) optical transition in InAs/GaAs coupled quantum dots. In our calculation the microscpic distributon of the strain is taken into account. The absorption in coupled quantum dots is quite different from that of superlattices. In superlattices, the absorption does not exist when the electric vector of light is parallel to the superlattice plane (perpendicular incident). This introduces somewhat of a difficulty in fabricating the infrared detector. In quantum dots, the absorption exists when light incident along any direction, which may be good for fabricating infrared detectors.

A numerical analysis of a quantum directional coupler based on Π-shaped electron waveguides is presented with use of the scattering-matrix method. After the optimization of the device parameters, uniform output for the two output ports and high directivity are obtained within a wide range of the electron momenta. The electron transfer in the device is found more efficient than that in the previously proposed structures. The study of the shape-dependence of transmission for the device shows that the device structure with smooth boundaries exhibits a much better performance.

A transfer matrix approach is presented for the study of electron conduction in an arbitrarily shaped cavity structure embedded in a quantum wire. Using the boundary conditions for wave functions, the transfer matrix at an interface with a discontinuous potential boundary is obtained for the first time. The total transfer matrix is calculated by multiplication of the transfer matrix for each segment of the structure as well as numerical integration of coupled second-order differential equations. The proposed method is applied to the evaluation of the conductance and the electron probability density in several typical cavity structures. The effect of the geometrical features on the electron transmission is discussed in detail. In the numerical calculations, the method is found to be more efficient than most of the other methods in the literature and the results are found to be in excellent agreement with those obtained by the recursive Green's function method.

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

To improve the photoelectrochemical activity of TiO2 for hydrogen production through water splitting, the band edges of TiO2 should be tailored to match with visible light absorption and the hydrogen or oxygen production levels. By analyzing the band structure of TiO2 and the chemical potentials of the dopants, we propose that the band edges of TiO2 can be modified by passivated codopants such as (Mo+C) to shift the valence band edge up significantly, while leaving the conduction band edge almost unchanged, thus satisfying the stringent requirements. The design principle for the band-edge modification should be applicable to other wide-band-gap semiconductors.

The origin of ferromagnetism in d;{0} semiconductors is studied using first-principles methods with ZnO as a prototype material. We show that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d;{0} semiconductors and can be attributed to the localized nature of the 2p states of O and N. We find that acceptor doping, especially doping at the anion site, can enhance the ferromagnetism with much smaller threshold hole concentrations. The quantum confinement effect also reduces the critical hole concentration to induce ferromagnetism in ZnO nanowires. The characteristic nonmonotonic spin couplings in these systems are explained in terms of the band coupling model.

The Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys are calculated using the first-principles band-structure method. We show that due to wave function localization, the MgGa acceptor energy level in the alloy is significantly lower than that of GaN, although the two materials have nearly identical band gaps. Our study demonstrates that forming AlxInyGa1-x-yN quaternary alloys can be a useful approach to lower acceptor ionization energy in the nitrides and thus provides an approach to overcome the p-type doping difficulty in the nitride system.

Etude des proprietes optiques et electroniques des super-reseaux a courte periode, dans un modele empirique de la liaison forte qui inclue les interactions de seconds voisins. Les niveaux energetiques electroniques de types-Γ et X sont obtenus en fonction du nombre de monocouches de GaAs, du champ electrique applique, et du vecteur d'onde parallele. La transition continue Γ→X calculee est en bon accord avec l'observation experimentale. On examine egalement les super-reseaux a periode courte dont la croissance est obtenue dans la direction [111]. Calcul des permitivites dielectriques sur toute la gamme d'energie, en utilisant une methode empirique developpee recemment pour obtenir les elements de matrice optiques

An improved plane wave expansion method for the numerical calculation of photonic bands of metal/dielectric photonic crystal (PC) are presented. This method is applied to two-dimensional PCs with frequency-dependent dielectric constants. We obtained the photonic band structure of three kinds of structures: sawtooth, cylinder and hole PCs. The results show that the lowest band-1 is relatively flat, and does not approach zero. Also, there is no complete band-gap that extends throughout the first Brillouin zone for these three structures. However, there are partial band-gaps in different directions in the first Brillouin zone. For the complementary cylinder and hole PCs, their photonic bands are similar except for the lowest three bands; the hole PC?s lowest frequency of band-1 is larger than that of cylinder PC for the configuration R/d??=??0.2.

p-type doping is a great challenge for the full utilization of ZnO as short-wavelength optoelectronic material. Due to a large electronegative characteristic of oxygen, the ionization energy of acceptors in ZnO is usually too high. By analyzing the defect wave-function character, we propose several approaches to lower the acceptor ionization energy by codoping acceptors with donor or isovalent atoms. Using the first-principles band-structure method, we show that the acceptor transition energies of V-Zn-O-O can be reduced by introducing F-O next to V-Zn to reduce electronic potential, whereas the acceptor transition energy of N-O-nZn(Zn) (n=1-4) can be reduced if we replace Zn by isovalent Mg or Be to reduce the anion and cation kinetic p-d repulsion, as well as the electronic potential.

The ballistic transport in the semiconductor, planar, circular quantum dot structures is studied theoretically. The transmission probabilities show apparent resonant tunneling peaks, which correspond to energies of bound states in the dot. By use of structures with different angles between the inject and exit channels, the resonant peaks can be identified very effectively. The perpendicular magnetic field has obvious effect on the energies of bound states in the quantum dot, and thus the resonant peaks. The treatment of the boundary conditions simplifies the problem to the solution of a set of linear algebraic equations. The theoretical results in this paper can be used to design planar resonant tunneling devices, whose resonant peaks are adjustable by the angle between the inject and exit channels and the applied magnetic field. The resonant tunneling in the circular dot structures can also be used to study the bound states in the absence and presence of magnetic field.

Photonic microcavity promises to be one of the photonic devices that can have immediate applications such as super bright LED and low threshold laser. Most photonic crystal structures currently used on microcavity are cubic or hexagonal, whose folding symmetry is no greater than 6. In this work, we fabricated 2-D photonic microcavity with Penrose quasicrystal pattern and measured the angular resolved transmission and photoluminescence spectra of the microcacity. From the experimental result it is found that isotropic photonic band gap exists in the microcavity with the Penrose quasicrystal pattern.

A hexagonal lattice photonic crystal was fabricated inside the metallic microcavity. And a thin film of Alq3 was incorporated inside the textured cavity as an active medium. The microcavity is designed such that the modified photonic modes due to the textured structure can couple to the excited electronic states of Alq3. This leads to changes in the emission characteristics of Alq3. From the angle-resolved transmission (ARTR) results, the photonic bandgap was observed at all angles from normal incident to 60°. The presence of surface plasmon (SP) was observed in both TM and TE modes of the transmission. Compare to the bulk Alq3 photoluminescence spectrum, significant modification of the photoluminescence (PL) spectrum was observed in the angle-resolved photoluminescence (ARPL). The photoluminescence spectra showed clear suppression in luminescence intensity for the range inside the photonic bandgap. We use decouple approximation for the standing wave modes and derive the photonic waveguide characteristics for two-dimensional textured metallic microcavities. The theoretical result is in good agreement to the experimental result.

The ability to control the appropriate layer thickness of transition metal dichalcogenides (TMDs) affords the opportunity to engineer many properties for a variety of applications in possible technological fields. Here we demonstrate that band-gap and mobility of ReSe2 nanosheet, a new member of the TMDs, increase when the layer number decreases, thus influencing the performances of ReSe2 transistors with different layers. A single-layer ReSe2 transistor shows much higher device mobility of 9.78 cm(2) V(-1) s(-1) than few-layer transistors (0.10 cm(2) V(-1) s(-1)). Moreover, a single-layer device shows high sensitivity to red light (633 nm) and has a light-improved mobility of 14.1 cm(2) V(-1) s(-1). Molecular physisorption is used as "gating" to modulate the carrier density of our single-layer transistors, resulting in a high photoresponsivity (Rλ) of 95 A W(-1) and external quantum efficiency (EQE) of 18 645% in O2 environment. This work highlights the fact that the properties of ReSe2 can be tuned in terms of the number of layers and gas molecule gating, and single-layer ReSe2 with appropriate band-gap is a promising material for future functional device applications.

Here, we demonstrate self-powered ultraviolet photodetectors that are capable of generating opposite current flow when illuminated at different wavelengths. The photodetectors are composed of n-ZnO/Polyaniline (PANI) p-n and PANI/ZnGa2O4 type-II heterojunctions and operate without any need for external power source. Devices display superior stability in ambient conditions within months. Results provide opportunities for developing devices for optical recognition.

The electronic structure and optical properties of InAs/GaSb/AlSb/GaSb superlattice (M-structure) are investigated in the framework of eight-band effective-mass theory, with the interface potential considered. The calculated energy gaps agree excellently with the experimental results. Via calculations, we find that the electronic structure of M-structure strongly depends on the geometrical structure of superlattice. The electron effective mass increases notably with the thickness of GaSb and AlSb layers, and AlSb layer is more favorable to obtain large electron effective mass than GaSb layer. Increased thickness of AlSb layer also leads to larger variation range of valence band maximum (VBM) and so M-structure has more tunable VBM than InAs/GaSb superlattice. Also the VBM of M-structure rises considerably with the increment of GaSb layer thickness and is almost independent of InAs layer thickness. We further find that M-structure has no remarkable superior optical absorption coefficient over InAs/GaSb superlattice. However, with larger electron effective mass and more tunable valence band maximum compared with InAs/GaSb superlattice, M-structure can be used as barrier in InAs/GaSb superlattice infrared detector to reduce the dark current. And the quantum efficiency of infrared photodiodes will not depend on the bias voltage when the M-structure is appropriately doped and carefully designed based on the dependence of its electronic structure on the superlattice geometry.

The transport properties through a quantum dot are calculated using the recursion method. The results show that the electric fields can move the conductive peaks along the high- and low-energies. The electric field changes the intensity of conductance slightly. Our theoretical results should be useful for researching and making low-dimensional semiconductor optoelectronic devices. (C) 2002 Elsevier Science B.V. All rights reserved.

By employing first-principle total-energy calculations, a systematic study of the dopability of ZnS to be both n- and p-types compared with that of ZnO is carried out. We find that all the attempted acceptor dopants, group V substituting on the S lattice site and group I and IB on the Zn sites in ZnS, have lower ionization energies than the corresponding ones in ZnO. This can be accounted for by the fact that ZnS has relative higher valence band maximum than ZnO. Native ZnS is weak p-type under S-rich condition, as the abundant acceptor V-Zn has rather large ionization energy. Self-compensations by the formation of interstitial donors in group I and IB-doped p-type ZnS can be avoided when sample is prepared under S-rich condition. In terms of ionization energies, Li-Zn and N-S are the preferred acceptors in ZnS. Native n- type doping of ZnS is limited by the spontaneous formation of intrinsic V-Zn(2-); high efficient n-type doping with dopants is harder to achieve than in ZnO because of the readiness of forming native compensating centers and higher ionization energy of donors in ZnS. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3103585]

Using first-principles methods, we systematically study the mechanism of defect formation and electronic structures for 3d transition-metal impurities (V, Cr, Mn, Fe, and Co) doped in silicon nanowires. We find that the formation energies of 3d transition-metal impurities with electrons or holes at the defect levels always increase as the diameters of silicon nanowires decrease, which suggests that self-purification, i.e., the difficulty of doping in silicon nanowires, should be an intrinsic effect. The calculated results show that the defect formation energies of Mn and Fe impurities are lower than those of V, Cr, and Co impurities in silicon nanowires. It indicates that Mn and Fe can easily occupy substitutional site in the interior of silicon nanowires. Moreover, they have larger localized moments, which means that they are good candidates for Si-based dilute magnetic semiconductor nanowires. The doping of Mn and Fe atom in silicon nanowires introduces a pair of energy levels with t(2) symmetry. One of which is dominated by 3d electrons of Mn or Fe, and the other by neighboring dangling bonds of Si vacancies. In addition, a set of nonbonding states localized on the transition-metal atom with e symmetry is also introduced. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000445]

The electronic structure and optical gain of wurtzite ZnO nanowires are investigated in the framework of effective-mass envelope-function theory. We found that as the elliptical aspect ratio e increases to be larger than a critical value, the hole ground states may change from optically dark to optically bright. The optical gain of ZnO nanowires increases as the hole density increases. For elliptical wire with large e, the y-polarized mode gain can be several thousand cm(-1), while the x-poiarized mode gain may be 26 times smaller than the former, so they can be used as ultraviolet linearly polarized lasers. (C) 2008 American Institute of Physics.

Our Raman measurement indicates that the intensity of the peaks (510 and 645 cm(-1)) related to nitrogen concentration is enhanced in MgZnO compared with that in ZnO. Using first-principles band structure methods, we calculated the formation energy and transition energy level for nitrogen acceptor in ZnO and random MgxZn1-xO (with x=0.25) alloy. Our calculations show that the incorporation of nitrogen can be enhanced as Mg is alloyed into ZnO, which agrees with our experiments. The acceptor energy level deeper in the alloy ascribes to the downward shift of the valence-band maximum edge in the presence of magnesium. (c) 2008 American Institute of Physics.

The electronic structures of N quantum dot molecules (QDMs) are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels are calculated. In the calculations, the effects of finite offset and valence-band mixing are taken into account. The theoretical method can be used to calculate the electronic structures of any QDM. The results show that (1) electronic energy levels decrease monotonically and the energy difference between the N QDMs decreases as the quantum dot (QD) radius increases; (2) the electron energy level is lower and quantum confinement is smaller for the larger N QDM; (3) the hole ground state energy level is lower for the one dot QDM than N (greater 1) QDMs if the QD radius is larger than about 5nm due to the valence-band mixing. The results are useful for the application of the N QDM to photoelectric devices.

Silicon tips used as field emitters have dimensions that are within the quantum confinement regime. Therefore they can be considered as freestanding silicon tips. In this letter, a photoluminescence spectrum of a 100×100 array of silicon tips was taken at 10 K. Narrow ultraviolet luminescence peaks were observed. Using the empirical pseudopotential homojunction model, it is demonstrated that these luminescence peaks come from energy levels arising from quantum confinement. By fitting the theoretical result to the experimental result, we conclude that the luminescence peaks come from Si quantum tips of about 20 A in width and that they are covered by silicon dioxide.