Two-dimensional VS2 nanomaterials have emerged as highly efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), and the further improvement of their HER performance depends on the understanding of the catalytic mechanism and activity in various pristine and defective structures. Here, structural stability, electronic properties, and HER activity of monolayer VS2 nanosheets with various intrinsic point defects are studied by using first-principles calculations. Compared to the most-studied 2H-phase MoS2 basal plane, both 2H- and 1T-phase VS2 basal planes exhibit superior catalytic activity due to their metallic properties. With the introduction of intrinsic point defects onto VS2 basal planes, we find that there are four types of stable defects in the 2H phase (i.e., Sad, Svac, Vad, and VS) and three types of stable defects in the 1T phase (i.e., Sad, Svac, and Vad). Moreover, the formation of Svac, Vad, and VS structures in the 2H phase and Vad in the 1T phase can enhance the HER activity of basal planes, which implies that the synthesis of VS2 nanosheets at the V-rich condition facilitates the achievement of high HER performance. The HER activity of pristine and defective VS2 structures can be well understood by a Fermi-abundance model that is also suitable to describe a broad class of electrocatalytic HER systems. This work provides a deep insight into the HER activity of single-layer VS2 and the guidance for synthesizing highly active electrocatalysts in transition-metal dichalcogenides.

Far-infrared part of electromagnetic spectrum and its technological details have been highly sought after due to its myriad applications including imaging, spectroscopy, industry control, and communication. However, lack of efficient components of electronic and photonic sources/detectors working in this particular spectrum has impeded its widespread application. One of the bottlenecks lies in the compact far-infrared polarization-sensitive resonator/modulator in compatible with pixel-detector for far-infrared spectroscopy. In this work, we demonstrate strong electric resonance response in perforated graphene sheet at this particular electromagnetic region. The results demonstrate inherently different natures for the strong electromagnetic response between graphene-based and metallic metamaterials. Unlike the metallic metamaterials relying on the geometrical inductance for magnetic response, the electric resonance caused by localized dipole/multipolar modes is found to be dominated in graphene and thus enabling sub-wavelength confinement of electromagnetic field. The Babinet’s principle is proposed to be applied for broadband far-infrared modulation and resonant filters design of graphene-based metamaterial. The active tunable electric resonance through electrostatic doping on the graphene-based patterns provides efficient route for compact biosensing, far-infrared imaging, and detection.

An advanced visible/infrared dual-band photodetector with high-resolution imaging at room temperature is proposed and demonstrated for intelligent identification based on the 2D GaSe/GaSb vertical heterostructure. It resolves the challenges of producing large-scale 2D growth, achieving fast response speed, outstanding detectivity, and lower manufacture cost, which are the main obstacles for industrialization of 2D-materials-based photodetection.

We propose a dielectric metasurface of 34 phase level, to reproduce the holographic image in the visible region. With normal incident circular polarized illumination, imaging efficiency as high as 59.72% is achieved at 560 nm wavelength. Consisting of non-resonant meta-cells, the proposed polarization-insensitive dielectric metasurface performs well in a broad band from 480 nm to 660 nm. Numerical results also indicate that the metasurface hologram maintains high fidelity within a wide-angle incidence (0–25°), and show high tolerance to fabrication errors. The high efficiency, broadband, wide-angle, and high fabrication tolerance metasurface based on the hologram, can be used in the fields of display, security, data storage, and information processing.

One-dimensional InAs nanowires (NWs) have been widely researched in recent years. Features of high mobility and narrow bandgap reveal its great potential of optoelectronic applications. However, most reported work about InAs NW-based photodetectors is limited to the visible waveband. Although some work shows certain response for near-infrared light, the problems of large dark current and small light on/off ratio are unsolved, thus significantly restricting the detectivity. Here in this work, a novel “visible light-assisted dark-current suppressing method” is proposed for the first time to reduce the dark current and enhance the infrared photodetection of single InAs NW photodetectors. This method effectively increases the barrier height of the metal–semiconductor contact, thus significantly making the device a metal–semiconductor–metal (MSM) photodiode. These MSM photodiodes demonstrate broadband detection from less than 1 μm to more than 3 μm and a fast response of tens of microseconds. A high detectivity of ∼1012 Jones has been achieved for the wavelength of 2000 nm at a low bias voltage of 0.1 V with corresponding responsivity of as much as 40 A/W. Even for the incident wavelength of 3113 nm, a detectivity of ∼1010 Jones and a responsivity of 0.6 A/W have been obtained. Our work has achieved an extended detection waveband for single InAs NW photodetector from visible and near-infrared to mid-infrared. The excellent performance for infrared detection demonstrated the great potential of narrow bandgap NWs for future infrared optoelectronic applications.Keywords: mid-infrared photodetectors; MSM photodiodes; Single InAs nanowire;

Structured plasmonic metamaterials offer a new way to design functionalized optical and electrical components, since they can be size-scaled for operation across the whole electromagnetic spectrum. Here, we theoretically investigated electrical active split ring resonators based on graphene metamaterials on a SiO2/Si substrate that shows tunable frequency and amplitude modulation. For the symmetrical structure, the modulation depth of the frequency and amplitude can reach 58.58% and 99.35%, and 59.53% and 97.7% respectively in the two crossed-polarization orientations. Once asymmetry is introduced in the structure, the higher order mode which is inaccessible in the symmetrical structure can be excited, and a strong interaction among the modes in the split ring resonator forms a transparency window in the absorption band of the dipole resonance. Such metamaterials could facilitate the design of active modulation, and slow light effect for terahertz waves. Potential outcomes such as higher sensing abilities and higher-Q resonances at terahertz frequencies are demonstrated through numerical simulations with realistic parameters.

•The respective effects of direct and indirect couplings on EIT-like transmission are investigated in our work.•The consistency between theoretical and numerical results indicates that the direct coupling can give rise to EIT-like phenomenon in symmetrical structure.•We find that the direct and indirect couplings can offset each other when the coupling distance L reach to a certain value. Consequently, the PIT transmission dips shift back to the original resonant frequency when the two couplings offset each other.We investigate respectively the effects of direct and indirect couplings on electromagnetically induced transparency (EIT)-like in a Metal–Insulator–Metal (MIM) bus waveguide coupled to two aperture-resonators (ARS). Adjusting the intensity of direct and indirect couplings, we can intentionally realize, modulate and eliminate the EIT-like transmission in the proposed plasmonic structures. The consistency between theoretical results and finite-difference time-domain (FDTD) simulations indicates that the direct coupling can give rise to EIT-like phenomenon in symmetrical structure. Moreover, the EIT-like transmission dips can be shifted back to the original resonant frequency when the two couplings offset each other. These results may provide a helpful guideline for the control of light in highly integrated optical circuits.

The design of a reflection-type subwavelength microstructure has been numerically investigated to concentrate incident light onto pixels for improved photoresponse of InSb infrared focal-plane arrays. Compared with traditional microlenses placed on top of the detector substrate, this reflection-type microstructure is better suited for extremely small pixel pitches. The structure is simulated using a joint numerical method combining the finite-difference time-domain method based on Maxwell’s curl equations and the finite-element method based on the Poisson and continuity equations. The results show that this advanced design could effectively improve device response without sacrificing crosstalk. The optimal structure parameters are obtained theoretically, with response increase of approximately 100%.

Plasma waves in graphene field-effect transistors (FETs) and nano-patterned graphene sheets have emerged as very promising candidates for potential terahertz and infrared applications in myriad areas including remote sensing, biomedical science, military, and many other fields with their electrical tunability and strong interaction with light. In this work, we study the excitations and propagation properties of plasma waves in nanometric graphene FETs down to the scaling limit. Due to the quantum-capacitance effect, the plasma wave exhibits strong correlation with the distribution of density of states (DOS). It is indicated that the electrically tunable plasma resonance has a power-dependent V0.8TG relation on the gate voltage, which originates from the linear dependence of density of states (DOS) on the energy in pristine graphene, in striking difference to those dominated by classical capacitance with only V0.5TG dependence. The results of different transistor sizes indicate the potential application of nanometric graphene FETs in highly-efficient electro-optic modulation or detection of terahertz or infrared radiation. In addition, we highlight the perspectives of plasma resonance excitation in probing the many-body interaction and quantum matter state in strong correlation electron systems. This study reveals the key feature of plasma waves in decorated/nanometric graphene FETs, and paves the way to tailor plasma band-engineering and expand its application in both terahertz and mid-infrared regions.

A series of epitaxial V1−xWxO2 (0 ≤ x ≤ 0.76%) nanocrystalline films on c-plane sapphire substrates have been successfully synthesized. Orbital structures of V1−xWxO2 films with monoclinic and rutile states have been investigated by ultraviolet-infrared spectroscopy combined with first principles calculations. Experimental and calculated results show that the overlap of π* and d∥ orbitals increases with increasing W doping content for the rutile state. Meanwhile, in the monoclinic state, the optical band gap decreases from 0.65 to 0.54 eV with increasing W doping concentration. Clear evidence is found that the V1−xWxO2 thin film phase transition temperature change comes from orbital structure variations. This shows that, with increasing W doping concentration, the decrease of rutile d∥ orbital occupancy can reduce the strength of V–V interactions, which finally results in phase transition temperature decrease. The experimental results reveal that the d∥ orbital is very important for the VO2 phase transition process. Our findings open a possibility to tune VO2 phase transition temperature through orbital engineering.

Recent observations of the negative differential conductance (NDC) phenomenon in graphene field-effect transistors (FET) open up new opportunities for their application in graphene-based fast switches, frequency multipliers and, most importantly, in high frequency oscillators up to the terahertz regime. Unlike conventional two-terminal NDC devices that rely on resonant tunneling and inter-valley transferring, in the present work, it has been shown that the universal NDC phenomenon of graphene-based FETs originates from their intrinsic nonlinear carrier transport under a strong electric field. The operation of graphene-NDC devices depends strongly on the interface between graphene and dielectric materials, the scattering-limited carrier mobility, and on the saturation velocity. To reveal such NDC behavior, the output characteristics of GFET are investigated rigorously, with both an analytical model and self-consistent transport equation, and with a multi-electrical parameter simulation. It is demonstrated that the contact-induced doping effect plays an important role in the operational efficiency of graphene-based NDC devices, rather than the ambipolar behavior associated with the competition between electron and hole conductances. In the absence of a NDC regime or beyond one, ambipolar transport starts at Vds > 2Vgs at the drain end, and as the dielectric layer begins to thin down, the kink-like saturation output characteristic is enhanced by the quantum capacitance contribution. These observations reveal the intrinsic mechanism of the NDC effect and open up new opportunities for the performance improvement of GFETs in future high-frequency applications, beyond the current paradigm based on two-terminal diodes.

Spongelike CuInS2 3D microspheres were synthesized through a solvothermal method employing CuCl, InCl3, and thiourea as Cu, In, and S sources, respectively, and PVP as surfactant. The as-prepared products have regular spherical shapes with diameters of 0.8–3.7 μm, the spheres consisted of small nanosheets, which are composed of small nanoparticles. As an important solar cell material, its photovoltaic property was also tested and the results showed a solar energy conversion efficiency of 3.31%. With the help of reduced graphene, its conversion efficiency could be further increased to 6.18%. Compared with conventional Pt material used in counter electrodes of solar cells, this new material has an advantages of low-cost, facile synthesis and high efficiency with graphene assistance.Keywords: counter electrodes; CuInS2; graphene assisted; microspheres; photovoltaic property; power conversion efficiency;

•We studied all different atomic arrangement (α, β, and γ phases) of PdMnBi.•Detailed explanation of electronic properties for three phases has been given.•We did observed half-metallic properties in alpha and beta phase.•The spin-orbital effect is considered and we made a comparison with NiMnSb.The dependence of the electronic and magnetic properties on the atomic arrangements of three different phases (i.e. α, β, and γ phases), of the half-Heusler alloy PdMnBi, is investigated based on spin-polarized density functional theory. For each phase, the optimized lattice constant is determined and the possibility of finding a half-metal is explored. Throughout this study, the bonding features of each phase are not supported by the large electronegativity of Pd given in the public domain. Both α and β phases PdMnBi show half-metallic (HM) properties for a range of lattice constants, and their magnetic moments are consistent with the values given by the modified Slater-Pauling rule. Additionally, the effects of the spin–orbit (S-O) interaction are examined by comparing the relative shifts of the valence bands and the indirect semiconducting gap, with respect to the spin-polarized results.

The hybrid structure of periodic gold stripes overlaid with a gold film is proposed to enhance the infrared absorption of quantum well infrared photodetector. The reflection spectra and the field distributions are numerically calculated by a finite difference time-domain method. The results show that the electric field component perpendicular to the quantum well is strongly enhanced when the plasmonic resonance of the hybrid structure occurs at the operation wavelength of the quantum well infrared photodetector. The effect of the structural parameters on the plasmonic resonant wavelength is discussed, which indicates that the localized surface plasmon as well as surface plasmon polariton plays a role in the light coupling with the hybrid structure. A high coupling efficiency can be obtained with the optimized structural parameters. In addition, the performance of the hybrid structure at the tilted incidence is investigated.

•The magnetization characteristic of ferromagnetic thin strip is obtained through measuring anisotropic magnetoresistance and ferromagnetic resonance in different configurations.•Indicating how the magnetization vector rotates by changing the intensity of external magnetic field of certain direction.•Demagnetization and magnetic anisotropy are considered.The magnetization characteristic in a Permalloy thin strip is investigated by electrically measuring the anisotropic magnetoresistance and ferromagnetic resonance in in-plane and out-of-plane configurations. Our results indicate that the magnetization vector can rotate in the film plane as well as out of the film plane by changing the intensity of external magnetic field of certain direction. The magnetization characteristic can be explained by considering demagnetization and magnetic anisotropy. Our method can be used to obtain the demagnetization factor, saturated magnetic moment and the magnetic anisotropy.

Although molecular beam epitaxy technology-based arsenic-doped Hg1−xCdxTe has been extensively studied, according to the newly proposed framework of the defect-complex-based p-type doping mechanism, heavier group V elements such as antimony (Sb) should have a different doping behavior because of their larger radius which can cause larger lattice distortion. In this work, we performed first-principles calculations and took As and Sb as examples to study this issue. The substitutional doping, interstitial doping (including split, tetrahedral, and hexagonal interstitial sites), and defect complex doping forms for arsenic and antimony are all investigated. A significant lattice distortion is found in hexagonal and split-site interstitial-Sb-doped Hg0.75Cd0.25Te due to the larger covalent radius of Sb. Compared with As, Sb can lead to a more complicated configuration change in the case of SbHg-VHg-SbHg tridoping, and the interstitial Sb is found to be stable even with the coupling of Hg vacancies through detailed energetic calculations, indicating that the interstitial Sb has greater ability to form stable defect complexes, and thus great potential to be a more appropriate p-type dopant. This study provides more complementary understanding of the behaviors of group V impurities in HgCdTe.

Here we report InAs nanowire (NW) near-infrared photodetectors having a detection wavelength up to ∼1.5 μm. The single InAs NW photodetectors displayed minimum hysteresis with a high Ion/Ioff ratio of 105. At room temperature, the Schottky–Ohmic contacted photodetectors had an external photoresponsivity of ∼5.3 × 103 AW–1, which is ∼300% larger than that of Ohmic–Ohmic contacted detectors (∼1.9 × 103 AW–1). A large enhancement in photoresponsivity (∼300%) had also been achieved in metal Au-cluster-decorated InAs NW photodetectors due to the formation of Schottky junctions at the InAs/Au cluster contacts. The photocurrent decreased when the photodetectors were exposed to ambient atmosphere because of the high surface electron concentration and rich surface defect states in InAs NWs. A theoretical model based on charge transfer and energy band change is proposed to explain this observed performance. To suppress the negative effects of surface defect states and atmospheric molecules, new InAs NW photodetectors with a half-wrapped top-gate had been fabricated by using 10 nm HfO2 as the top-gate dielectric.Keywords: half-wrapped top-gate; InAs nanowire; infrared photodetectors; photoresponsivity; surface defect states

Mg0.05Zn0.95O thin films were prepared on silicon substrates by a sol–gel dip-coating technique. Microstructure, surface topography and optical properties of the thin films were characterized by X-ray diffraction, atom force microscopy, Fourier transform infrared spectrophotometer and fluorescence spectrometer. The results show that the thin film annealed at 700 °C has the largest average grain size and exhibits the best c-axis preferred orientation. As annealing temperature increases to 800 °C, the grain along c-axis has been suppressed. Roughness factor and average particle size increase with the increase of annealing temperature. The IR absorption peak appearing at about 416 cm−1 is assigned to hexagonal wurtzite ZnO. The thin film annealed at 700 °C has the maximum oxygen vacancy, which can be inferred from the green emission intensity. Photocatalytic results show that the thin film annealed at 700 °C exhibits remarkable photocatalytic activity, which may be attributed to the larger grain size, roughness factor and concentration of oxygen vacancy. Enhanced photocatalytic activity of Mg0.05Zn0.95O thin films after a cycle may be attributed to the increase of surface oxygen vacancy and photocorrosion of amorphous MgO on the surface of thin film under UV irradiation.

Longitudinal twinning Zn2GeO4 nanowires, with diameters from several tens of nanometers to 200–300 nm along the growth orientation and lengths of several tens of micrometers, were successfully synthesized by a simple thermal evaporation method. A distinct twin plane of (101) was observed from the twinning Zn2GeO4 nanowires. Zn2GeO4 nanowires in both sides of the twinning nanowires are single crystalline, with a rhombohedral crystal phase, and grow along the [21] direction. The angle between the (300) plane (or (300)t plane) and the twin plane is identified with the results of the SAED pattern and the atomic model. A possible growth mechanism for longitudinal twinning Zn2GeO4 nanowires was proposed. The vibrating and luminescence properties of the twinning Zn2GeO4 nanowire were investigated by Raman and photoluminescence spectroscopy at room temperature.

•Cu concentrations enhance preferential c-axis orientation CuxZn1−xO thin films.•The optical band gap and PL spectra of CuxZn1−xO thin films were studied.•Effect of Cu concentrations on optical band gap has been reported.•We report the photoluminescence mechanism of CuxZn1−xO thin films.CuxZn1−xO thin films were prepared by the sol–gel process. Microstructure, surface topography and optical properties were studied by X-ray diffractometry, atom force microscopy, UV–Vis spectrophotometer and fluorescence spectrometer. The monoclinic CuO phase has been observed in CuxZn1−xO (x = 0.05, 0.10, and 0.15) thin films annealed at 800 °C. Preferential c-axis orientation of the CuxZn1−xO thin films annealed at 800 °C increases with the increase of x value. The annealing temperature and CuO component have great effect on the absorption coefficient of CuxZn1−xO thin films in the visible region. The decrease of optical band gap may be attributed to the reduction of the fraction of the amorphous phase and stoichiometry deficiency of ZnO due to CuO doping. The deep level emission in ZnO thin film originates from oxygen vacancy and oxygen interstitial defects. The violet emission peaked at about 400 nm has been assigned to the electron transition from zinc interstitial level and the valence band.

The transmissions through a two-dimensional compound metal periodic hole arrays comprised of rectangle and cross-shaped holes are calculated by the finite-difference-time-domain (FDTD) method. The results show that the transmissions strongly depend on the collective effect of the compound hole cell. Moreover, the comparisons between transmission characteristics corresponding to the asymmetry of the cross-shaped hole in two different directions (vertical and parallel to the light polarization) are also studied. It is found that the 1168 nm peak is split into two peaks when the symmetry of the cross-shaped hole in y-axis direction is broken. However, it is shown that there is no obvious change of transmissions with changing the asymmetry in x-axis direction. Thus, it is concluded that the transmissions are more sensitive to the vertical direction asymmetry. The results may be utilized to tune the electromagnetic wave in subwavelength optics.

•Uniaxial strain impact on the electronic properties of HgTe is studied by DFT.•Uniaxial strain manipulate the local lattice structure along the same direction•Uniaxial strain changes HgTe to an “indirect” zero band gap semimetal.•A positive band gap is opened by uniaxial strain in HgTe.The impact of uniaxial strain along the [111] direction on the structural and electronic properties of bulk HgTe in the zinc blende is studied by DFT. Uniaxial strain can effectively manipulate the local lattice structure along the same direction. The transformation may be caused to form the graphite-like structure in large compression and the layered one under stretching. Meanwhile, the conductive band minimum (CBM) and valence band maximum (VBM) are gradually shifted to form the indirect band structures. Further, the band gap is opened in HgTe for the significant stretching, The uniaxial compression only changes the coordination of HgTe by maintaining the semi-metallic properties.

ZnO thin films were grown on silicon substrates using a hydrothermal method. The XRD patterns show that all of the peaks can be attributed to the wurtzite structures of ZnO. The TC value of (002) plane and average crystal size increase first and then decrease with the increase of solution concentration. SEM and AFM results show that many dense hexagonal cylinder particles have been observed on the surface of the thin films, which grown at 0.08 and 0.10 mol/L. The surface roughness of the thin films deposited at 0.06, 0.08, 0.10, and 0.12 mol/L are 24.5, 38.3, 32.0, and 39.4 nm, respectively. Surface wettability results show that the preferential orientation along c-axis and surface roughness contribute significantly to the hydrophobicity. The reversible switching between hydrophobicity and hydrophilicity is related to the synergy of the transition of wetting model, surface crystal structure, and surface roughness.

Arsenic-related defect complexes have been proven responsible for the charge-compensation effects in arsenic-doped Hg1−xCdxTe, but the underlying mechanism is still unclear. In this study, we systematically investigated the interaction between arsenic donor (AsHg) and mercury vacancy (VHg) versus the AsHg–VHg separation in arsenic-doped Hg1−xCdxTe using first-principles calculations. A new long-range interaction between AsHg and VHg is found, and the related binding energies and electronic structures are calculated to reveal its coupling mechanism. Our results show that VHg can increase the distortion of the lattice collaboratively with AsHg due to the different characteristics of AsHg and VHg in distorting the lattice. The relaxational enhancement as well as the electrical compensation of the AsHg donor is weakened as VHg moves away from AsHg, and the underlying mechanism is revealed. In addition, a set of defect levels in the band gap generated from the donor–acceptor interaction are also shown, and the origin of these levels is explored. The results of this work are important for theoretically explaining the characteristics of complicated defect levels found in experiments.

A general transformation is proposed to design the compact waveguide coupler with homogeneous media. By dividing the coupling region into several triangle blocks and engaging the transformation, material inhomogeneity of the coupler can be eliminated, and thus the device only requires homogeneous and anisotropic media. Also, it is found that the electromagnetic field in the coupling region can be controlled artificially and the field in the two waveguides is little influenced. Thus, much freedom and flexibility is provided in the design of waveguide coupler. Besides, the general transformation can also be extended to design waveguide bender with homogeneous media. Furthermore, Full wave simulation based on finite element method is performed to verify the performance of the waveguide coupler.Highlights► In this paper, a general transformation is proposed to design the compact waveguide coupler with homogeneous media. ► As long as the coupling region is divided into several triangle blocks (two, three or four blocks) and general transformation is employed, material inhomogeneity of the device can be eliminated, and thus the device only requires homogeneous and anisotropic media. ► Numerical results indicate that the electromagnetic field in the coupling region can be controlled artificially and the field in the two waveguides is little influenced. ► As a consequence, much freedom and flexibility is provided in the design of waveguide coupler. ► Besides, our general transformation can also be extended to design waveguide bender with homogeneous media. ► Full wave simulation based on finite element method is performed to verify the performance of the waveguide coupler.

The crystal structure and morphology of ZnO, grown on silicon substrate by two-step method, were measured by X-ray diffraction and field emission scanning electron microscopy. The results reveal that the sample is mainly composed of ZnO nanorods and preferentially oriented in the c-axis direction. The photoluminescence properties of the ZnO nanorods were investigated over the temperatures from 10 K to 297 K. There exist three emission bands in near band-edge, green–yellow–orange–red and near-infrared, respectively. Donor bound exciton (D0X) and its phonon replicas emission peaks were observed in low temperature photoluminescence (PL). The D0X and its phonon replicas peak intensity decreased with the increase of temperature and disappeared when the temperature increased up to 87 K. The decay in the D0X and its phonon replicas emission peak intensity stemmed from the thermal dissociation of D0X to free exciton. Temperature-dependent second-order diffraction of the near band-edge emissions were investigated in detail.Highlights► D0X and its phonon replicas emission peaks were observed in low temperature PL. ► The NIR emissions correspond to the second-order diffraction of the NBE emission. ► NBE and NIR emission mechanism were investigated.

Zinc oxide thin films were deposited on silicon substrates via hydrothermal method. Microstructures, surface topographies and optical properties of ZnO thin films were systematically investigated by X-ray diffraction, atomic force microscopy and fluorescence spectrophotometer. The mean grain size and surface roughness of the thin films decrease first and then increase with increasing the concentration of zinc nitrate hexahydrate. The photoluminescence spectra of ZnO thin films, excited by the 240, 320, 360, 380 and 400 nm excitation wavelength, were investigated in detail. Based on our analysis, it can be noted that mechanisms of the ultraviolet, violet and blue emissions are attributed to the transitions from the localized levels below the conduction band, zinc vacancy, interstitial zinc and extended interstitial zinc levels to the valance band, respectively. Blue–violet emissions of ZnO have great potential in light emitting and biological fluorescence labeling applications.Highlights► A novel UV-violet-blue emission band was discovered in the ZnO thin films. ► The blue emission peak blue shifted with the excitation wavelength increase from 380 to 400 nm. ► The blue emission band was assigned to the electrons transition from extended zinc levels to the valance band.

Zn1−xCuxO nanorods with different Cu concentrations are prepared by a hydrothermal method. Bent and aggregated nanorods are obtained, which is attributed to centripetal surface tension of the evaporation and coagulation processes of the water film on the ZnO nanorods. The broad visible band consists of one violet, three blue, and one green emission. The violet emission is due to the transition of electrons from zinc interstitial (Zni) levels to the valance band. The three blue emissions may be attributed to the transition from extended Zni levels, which are slightly below the simple Zni level, to the valance band. The change of the green emission may be the result of competition between oxygen vacancies (VO) and zinc vacancies (VZn).

By use of first-principles calculations, we study the effects of the external electric field F on the structural configurations and electronic properties of gold dimers on graphene. The direction of the applied F is perpendicular to the graphene plane, and the downward (upward) direction is defined as a positive (negative) one. It is shown that the adsorption energy increases with increasing F regardless of its direction owing to an enhancement of the charge transfer between the gold dimers and the graphene. The crossover of adsorbed configurations induced by F is predicted. For F ≥ −0.1 V/Å, an optimal vertical gold dimer on graphene is determined, whereas for F < −0.1 V/Å, the preferred adsorbed configuration of the gold dimer is parallel to the graphene plane. The physical mechanism behind F control of the adsorption of gold dimers deposited on graphene is also discussed in detail. The present results indicate the key role of F as an effective means in tuning the interactions between gold atoms and graphene.

The molecular passivation effect on the doping of InAs nanowires is explored by the first-principles calculation within the density-functional theory. We demonstrate the passivation of two different molecules that is implemented by the adsorption of NH3 and NO2 on the sidewall of InAs nanowire, respectively, but the two molecular passivations indicate different roles in the doping of InAs nanowires. The NH3 adsorption is a physisorption process which makes InAs nanowire with surface dangling bonds (SDBs) stay at n-type, while the adsorption of NO2 molecule is chemisorption that can reactivate the Zn dopant in InAs nanowires and yields a p-type doping characteristic. The difference of molecular passivation effect is attributed to the charge-compensation ability of passivating molecules to the SDBs of nanowires. This mechanism can be applied to explain the experimental observations on the doping of InAs nanowires through the surface passivation.

The energetics and growth kinetics of graphene edges during CVD growth on Cu(111) and other catalyst surfaces are explored by density functional theory (DFT) calculations. Different from graphene edges in vacuum, the reconstructions of both armchair (AC) and zigzag (ZZ) edges are energetically less stable because of the passivation of the edges by the catalytic surface. Furthermore, we predicated that, on the most used Cu(111) catalytic surface, each AC-like site on the edge is intended to be passivated by a Cu atom. Such an unexpected passivation significantly lowers the barrier of incorporating carbon atoms onto the graphene edge from 2.5 to 0.8 eV and therefore results in a very fast growth of the AC edge. These theoretical results are successfully applied to explain the broad experimental observations that the ZZ egde is the dominating edge type of growing graphene islands on a Cu surface.Keywords: chemical vapor deposition; density functional theory; edge reconstruction; graphene

Ground-state structures of supported C clusters, CN (N = 16, ..., 26), on four selected transition metal surfaces [Rh(111), Ru(0001), Ni(111), and Cu(111)] are systematically explored by ab initio calculations. It is found that the core–shell structured C21, which is a fraction of C60 possessing three isolated pentagons and C3v symmetry, is a very stable magic cluster on all these metal surfaces. Comparison with experimental scanning tunneling microscopy images, dI/dV curves, and cluster heights proves that C21 is the experimentally observed dominating C precursor in graphene chemical vapor deposition (CVD) growth. The exceptional stability of the C21 cluster is attributed to its high symmetry, core–shell geometry, and strong binding between edge C atoms and the metal surfaces. Besides, the high barrier of two C21 clusters’ dimerization explains its temperature-dependent behavior in graphene CVD growth.

The bare M5+13 and ligand-protected nanoparticles M25(SR)−18 and M13(PR)10Cl3+2 (M = Au, Ag, Cu) are investigated using the density functional theory. There are strong interactions between the metal core atoms and the ligands. It is found that the electronic structures agree well with the Jellium model for gold and copper nanoparticles. The superatoms's S and P orbitals are shown. However for silver ones, as the adding of the ligands, the S orbital of the nanoparticle disappears. The binding energy of these nanoparticles are also obtained by our calculation. The Au nanoparticles are most stable, the Cu ones take second place, and the Ag ones are the third stable. Our results could be essential for further understanding of the properties of ligand-protected isolated superatoms.

We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. Odd modes can be selectively excited by the input field of odd symmetry. The input field with different positions can excite different modes due to the field intensity distribution of modes. When the input field locates at the position of the zero field, intensity of waveguide modes is zero and the modes are not excited. The finite-difference time-domain method is used to obtain the excited field distributions.Research highlights► We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. ► Odd modes can be selectively excited by the input field of odd symmetry. ► The input field with different positions can excite different modes due to the field intensity distribution of modes.

A kind of transformation functions is proposed to realize the nonmagnetic invisibility cloak with minimized scattering on the basis of generalized transformation. By matching the impedance at the outer surface of the cloak, the transformations with two parameters are determined. The good performance of the cloak is indicated by the full wave simulation based on the finite element method. Furthermore, based on the calculation of total scattering cross section, it is shown that the scattering cross section is very sensitive to the different parameters even though the impedance at the exterior boundary matches perfectly with the free space. In addition, from the effective media theory, an alternating layered system composed of two isotropic materials is proposed to realize experimentally the cloak.

In this paper, we propose a plasmonic coupler which is composed of a nanoslit with a bump. The slit is used to generate surface plasmon polariton (SPP), and the bump is employed as a SPP reflector. It is found that the phase difference between the SPP propagating the opposite direction to the bump and the one reflected by the bump can be periodically adjusted by the distance between the center of slit and the bump. When the constructive interference between the two SPPs occurs, the proposed structure can be regarded as a undirectional plasmonic coupler. Moreover, we also find that the propagation of the interfering SPPs is influenced by the width and length of bump. It is expected that our results may be utilized to control the electromagnetic wave in subwavelength optics.

The effect of surface dangling bonds (SDBs) on the doping of InAs nanowires is investigated by first-principles calculations within density functional theory. The result of the formation energies shows that the dangling bonds of In atom on the surface of nanowires are a kind of stable defect. Moreover, the surface dangling bonds prefer to be charged and form trap centers of carriers. For the ultrathin nanowires, both the positively and negatively charged SDBs can be produced. With the increase of size, the stable energy region of the negatively charged SDBs has diminished gradually and disappeared, but the positively charged SDBs keep a high stability. The result originates from the quantum confinement effect that makes stronger influence on the conduction band than the valence band of InAs nanowires. The higher stability of the positively charged SDBs means that the SDBs have an ability to capture the holes from the p-type dopants, resulting in the deactivation of dopants. Thus, the SDBs could be fundamental obstacles for the p-type doping of InAs nanowires. On the basis of our results, the surface passivation can be considered as an effective way to suppress the effect of SDBs on the doping of InAs nanowires.

We perform systematic density-functional calculations on the self-catalytic effect and the nucleation mechanism of In adatoms on InP(111)B substrate during the initial growth of self-catalyzed InP nanowires. We first determine the equilibrium structure and stability of pure InP(111)B surface. The calculated surface formation energies show that the (2 × 2) surface with P trimer is a stable phase for a wide range of P chemical potential. Based on the P trimer structure, the adsorption of In atoms can induce the desorption of P trimer on InP(111)B surface. The adsorption−desorption phase diagrams as functions of temperature and P2 pressure indicate that the desorption of P trimer on the (2 × 2) surface without In adatom occurs beyond 970−1190 K, while the desorption with the In adatom does beyond 570−700 K. The P trimer desorption promoted by In adatom suggests the self-catalytic role of In adatom. It can be interpreted by the electron-counting rule. The structural relaxations and the calculated Gibbs free energies of In adatoms for a wide range of coverage indicate that the In adatoms can be stabilized on InP(111)B surface only at a low coverage (ΘIn ≤ 1 ML). In contrast, the In adatoms prefer to form the In droplets on the surface when the In coverage is continuously increased. The In adatoms of InP(111)B surface can be nucleated because the bonding interaction between the In adatoms and surface P atoms becomes weak with the increase of In coverage. Once the substrate is heated under In-rich condition, the surface In atoms easily form the In droplets. Our results offer a valuable information to insight into the initial growth of self-catalyzed InP nanowires by the vapor−liquid−solid mechanism on InP(111)B substrate.

The surface and size effects on the structural stability and electronic properties of InAs one-dimensional nanostructures are investigated by first-principles calculations within density functional theory. No matter what the diameters, the nanowires are more energetically favorable than the nanotubes due to the preferable sp3 hybridization for In and As atoms in the InAs nanostructures. The formation energies of these nanostructures satisfy a linear dependence relationship with the surface atom ratio. The calculated band structures reveal that the band gaps of InAs nanostructures are determined by two competition mechanisms. One is the quantum confinement effect, which favors the increase of the band gaps with the decreasing diameter or wall thickness. Another is the effect of surface dangling bonds, which induces the decrease of the band gaps with the decreasing diameter or wall thickness. With the same diameter, the band gaps of InAs nanowires are still less than those of the nanotubes. The result indicates that the quantum confinement effect in one-dimensional structures can be enhanced by the formation of tubes instead of wires.

We have investigated the self-imaging phenomena based on the field distribution in two-dimensional multimode photonic crystal waveguides (PCWs) by the finite-difference time-domain (FDTD) method. The new self-imaging conditions are given for the mirrored images and the direct images. The self-imaging distributions in multimode PCWs can be explained by the new self-imaging conditions and the superposition of “sub-images”. The results of numerical simulation show a good agreement with the theoretical predictions.

First-principles calculations have been performed on the C-doped anatase TiO2. The doped TiO2 shows half-metallic properties and a 2.0 μB magnetic moment per cell. The magnetic coupling is closely related to the C–C distance. When the distance is shorter than a typical CC double bond (1.34Ǻ), the system is nonmagnetic and the dopants tend to form a cluster through a direct CC bonding interaction [Appl. Phys. Lett. 93 (2008) 132507]. When the distance is between 3.8 and 5.5Ǻ, there exists strong ferromagnetic or antiferromagnetic coupling between two C atoms. The ferromagnetic coupling is induced by the bent C–Ti–C unit. When the distance is further increased, the system becomes paramagnetic.

The optical properties of amorphous group III–V compound semiconductors were investigated through the first principles calculations. The imaginary parts (ε2ε2) of dielectric function for amorphous GaAs, InAs, and InSb are given, respectively. There is a single broad peak found in the ε2ε2 spectrum. By comparing with the available experimental data of a-GaAs, it is found that the maximum of the ε2ε2 spectrum is sensitive to the topological local structures of amorphous materials. By comparison of the ε2ε2 spectrum for amorphous sample to that of the crystal, the dependence of the E1 and E2 peaks of the crystal on the local structures of amorphous sample becomes evident. The calculated results are in agreement with the available experimental data. The corresponding results should be generalized to cover the amorphous group III–V semiconductors.

The structural and optical properties of amorphous semiconductor mercury cadmium telluride (a-MCT) are obtained by the first principles calculations. The total pair distribution functions and the density of states show that the a-MCT has the semiconductor characteristic. The calculated results of dielectric function show that E2 peak of the imaginary of dielectric function for the crystal mercury cadmium telluride abruptly disappears in the amorphous case due to the long-range disorders. And the imaginary of dielectric function of a-MCT shows a large broad peak, which is in agreement with the available results of other amorphous semiconductors. From the linear extrapolation of the curve ħωɛ2(ω)1/2 versus ħω, it can be obtained that the optical energy gap of amorphous semiconductor Hg0.5Cd0.5Te is 0.51±0.05 eV.

A metallodielectric photonic crystal with photonic band gaps in near infrared regime has been constructed using layer-by-layer stacking of two-dimensional micro-size metal-coated dielectric spheres array. In transmission spectra two photonic band gaps are observed at 1.38 μm and 2.46 μm, which are in agreement with theoretical calculations. Experimental results show that the photonic band gaps can be realized with about ten layers. The structure with metallic microspheres provides us a novel way for fabrication of near infrared metallic photonic crystals.

The lowest-energy structures of AlnPn clusters up to n = 9 have been explored using all-electron density functional calculations with a gradient correction. For smaller AlnPn clusters with n = 1–4, we successfully reproduced the previously reported lowest-energy structures. Novel cage structures with Al–P alternating arrangement were observed for n ⩾ 5. The comparison of the lowest-energy structures of AlnPn clusters with those of Si2n, BnNn, and GanAsn clusters has been made. Size-dependent cluster properties such as binding energy, HOMO–LUMO gaps, electron affinities, and photoelectron spectra have been computed and compared with experiments.The lowest-energy structures of AlnPn clusters up to n = 9 are obtained from density functional theory calculations.

The understanding of the microstructures of the arsenic tetramer (Ast), dimer (Asd), and singlet (Ass) of HgCdTe is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to pp-type behavior. The stable configurations were obtained from the first-principles calculations for the arsenic cluster defects [Asn (n=1n=1, 2, and 4)] in as-grown HgCdTe. According to the defect formation energies calculated under Te-rich conditions, the most probable configurations of Ast, Asd, and Ass have been established. For the optimized Ast and Asd the energy is favorable to combine in a nearest neighboring mercury vacancy (VHg), and the corresponding configurations can be used to explain the self-compensated nn-type characteristics in as-grown materials. Ast is likely to be more abundant than Asd in as-grown materials, but arsenic atoms are more strongly bounded in Ast than in Asd, thus more substantial activation energy is needed for Ast than that for Asd. The atomic relaxations as well as the structural stability of the arsenic defects have also been investigated.

The understanding of the configurations of the arsenic tetramer (Ast), dimer (Asd), and singlet (Ass) is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to p-type behavior. In this work, the possible configurations were optimized from density functional calculations for arsenic defects Asn (n = 1, 2, and 4) in as-grown HgTe. According to the dominant electronic contribution to the defect formation energies, which was calculated under Te-rich conditions, the most probable configurations for Ast, Asd, and Ass have been established. The above discussion applies to the neutral arsenic defects. A further study is necessary to consider the entropy contribution to the defect formation energy.

This paper simulates a kind of new sub-50 nm n-type double gate MOS nanotransistors by solving coupled Poisson–Schrödinger equations in a self-consistent manner with a finite element method, and presents a systematic simulation-based study on quantum-mechanical effects, gate leakage current of FinFETs. The simulation results indicate that the deviation from the classical model becomes more important as the gate oxide, gate length and Fin channel width becomes thinner and the Fin channel doping increases. Gate tunneling current density reduces with the body thickness decreasing. Excessive scaling increases the gate current below Fin thickness of 5 nm. The gate current can be dramatically reduced beyond 1017 cm−3 with the Fin body doping increasing. In order to understand the influence of electron confinement, quantum mechanical simulation results are also compared with the results from the classical approach. Our simulation results indicate that quantum mechanical simulation is essential for the realistic optimization of the FinFET structure.

It has been difficult to compute the band structures and transmission spectra for photonic crystals (PCs) with dispersive components included in the periodic units. Here we show that by using an extended plane-wave-based transfer-matrix method, we are able to formulate the problem for computing optical properties of dispersive PCs, including magnetic and left-handed PCs. This approach is very general, since it can treat PCs with arbitrary Bravais lattice composed of materials with arbitrary dielectric permittivities and magnetic permeabilities. Combined with the supercell method, this method can further simulate defective PCs such as PC-based waveguides and microcavities.

We use a transfer-matrix method (TMM) to investigate light coupling into and out of single-end single-mode photonic crystal waveguides. Without multi-reflection complexity, we give clearly the unambiguous quantitative determination of the coupling efficiency of external light into guided mode and the transition among guided modes. It is shown that the waveguide with a line defect along ΓJΓJ direction exhibits a much better coupling efficiency than that with a line-defect orientation along ΓMΓM direction.

By means of a plane-wave-based transfer-matrix method, we present an extensive study of propagation loss in two-dimensional (2D) photonic crystal waveguides with limited cladding wall thickness. We examine the dependence of propagation loss on both the propagation direction and the waveguide wall thickness. It is shown that the propagation loss is a function of excitation frequency, wall thickness and waveguide outermost surface morphology. The conclusion that the propagation loss essentially decays exponentially with respect to the cladding wall thickness of a waveguide is valid only to a certain degree. In addition, we find that the propagation loss exhibit very complex behavior with respect to the excitation frequency.

The photoconductivity of BaTiO2.5 with oxygen vacancy has been studied by the linear muffin-tin orbital method in the atomic sphere approximation (LMTO-ASA). The ground-state structure of BaTiO2.5 is obtained by minimization of the total energy. The partial densities of states show that the occupied states at the bottom of the conduction band have primarily Ti d orbital character. The photoconductivity shows that two novel features, in the low energy side, can be attributed to the intraband transition of free electronic carriers in the vicinity of the Fermi level and the interband transition of the Ti 3d(yz) related band states, to the Ti 3d(xy,xz) related band states, respectively. In addition, it is also found that the anisotropy of photoconductivity is enhanced because of the introduction of oxygen vacancy. The system can show the conductive behavior of electronic carriers, which is qualitatively in agreement with a recent experimental finding.

The electronic and optical properties of Nb doped SrTiO3 are studied by ab initio linear muffin-tin orbital method in the atomic sphere approximation. The equilibrium lattice constants of SrTi1−xNbxO3 with x=0.0, 0.25 and 0.5 are found by minimization of the total energy curves. The computated lattice constants are in good agreement with experimental data. Our electronic band calculation shows that the Fermi level of SrTi1−xNbxO3 with x≥0.125 moves into the conduction bands and the system shows metallic behavior. The numerical results indicate that the Nb impurity atoms would lead to the distortion of the band edges. The complex dielectric function of SrTiO3 and Nb doped SrTiO3 are calculated using the random-phase approximation. The doping effect on the optical properties of SrTi1−xNbxO3 is discussed.

We study the effect of the electron–electron interactions in a one-dimensional disordered Anderson model with specific long-range correlations in the disorder, characterized by a correlation exponent α. The topology of the system is that of a ring with a magnetic flux φ threading through it, and we study the so-called charge stiffness Dc, which is essentially the second derivative of the total energy with respect to Φ. In the absence of interaction, for α>2, we find a metallic region around zero Fermi energy. In this metallic region, Dc is positive, whereas in the insulating region outside, Dc vanishes. In contrast, for sufficiently strong interaction, the long-range correlations leading to the metallic behavior are destroyed. At the same time it is also shown that the competition between the interactions and correlation determines the electronic transport behavior of the system.

The dark current of separate absorption grading charge multiplication (SAGCM) InGaAs/InP avalanche photodiodes has been numerical analyzed. SRH current, TAT current, BBT current and avalanche amplification combined together as the dark current have been extracted by simulation separately. The trend of punch-through voltage and breakdown voltage have been discussed, meanwhile the influence of structure parameters also has been investigated.

A metallodielectric photonic crystal with photonic band gaps in near infrared regime has been constructed using layer-by-layer stacking of two-dimensional micro-size metal-coated dielectric spheres array. In transmission spectra two photonic band gaps are observed at 1.38 μm and 2.46 μm, which are in agreement with theoretical calculations. Experimental results show that the photonic band gaps can be realized with about ten layers. The structure with metallic microspheres provides us a novel way for fabrication of near infrared metallic photonic crystals.

The bare M5+13 and ligand-protected nanoparticles M25(SR)−18 and M13(PR)10Cl3+2 (M = Au, Ag, Cu) are investigated using the density functional theory. There are strong interactions between the metal core atoms and the ligands. It is found that the electronic structures agree well with the Jellium model for gold and copper nanoparticles. The superatoms's S and P orbitals are shown. However for silver ones, as the adding of the ligands, the S orbital of the nanoparticle disappears. The binding energy of these nanoparticles are also obtained by our calculation. The Au nanoparticles are most stable, the Cu ones take second place, and the Ag ones are the third stable. Our results could be essential for further understanding of the properties of ligand-protected isolated superatoms.

A series of epitaxial V1−xWxO2 (0 ≤ x ≤ 0.76%) nanocrystalline films on c-plane sapphire substrates have been successfully synthesized. Orbital structures of V1−xWxO2 films with monoclinic and rutile states have been investigated by ultraviolet-infrared spectroscopy combined with first principles calculations. Experimental and calculated results show that the overlap of π* and d∥ orbitals increases with increasing W doping content for the rutile state. Meanwhile, in the monoclinic state, the optical band gap decreases from 0.65 to 0.54 eV with increasing W doping concentration. Clear evidence is found that the V1−xWxO2 thin film phase transition temperature change comes from orbital structure variations. This shows that, with increasing W doping concentration, the decrease of rutile d∥ orbital occupancy can reduce the strength of V–V interactions, which finally results in phase transition temperature decrease. The experimental results reveal that the d∥ orbital is very important for the VO2 phase transition process. Our findings open a possibility to tune VO2 phase transition temperature through orbital engineering.