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Impact of Defects on Electronic Properties of Heterostructures Constructed From Monolayers of Transition Metal Dichalcogenides
摘要： Electronic properties of heterostructures composed of two single molecular layers (monolayers) of MoS2, WS2, WSe2, and MoSe2 are ab initio simulated with an emphasis to the stacking peculiarities and an influence of point defects in their lattices. MoS2/MoSe2, MoS2/WS2, WS2/WSe2, and MoSe2/WSe2 heterostructures with the monolayers shifted like in the bulk material have been found to behave like semiconductors with the energy gaps of 0.88, 1.25, 1.06, and 1.07 eV, respectively. Such heterostructures possess indirect gaps in contrast to individual monolayers, while direct-gap character is preserved in two layer stacking variants in WS2/WSe2 heterostructures and in MoS2/MoSe2 heterostructure with mirror stacking of the monolayers. Vacancies and Te atoms substituting other chalcogen atoms reduce the band gaps. The calculated orbital compositions of first direct band gap transitions in the defect-free heterostructures and those with the point defects have demonstrated d-electrons of Mo or W atoms to be mainly involved in the transitions.
关键词： defects,two-dimensional crystals,transition metal dichalcogenides,heterostructures,electronic structure
更新于2025-09-23 15:22:29
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Ferromagnetic Order in Substitutional Fe-doped In2O3 Powder
摘要： We report an experimental investigation of the room-temperature ferromagnetism of an Fe-doped In2O3 system. (In1?xFex)2O3(x = 0.00, 0.02, 0.06,0.10,0.14 or 0.18) powders were prepared by a standard solid-state reaction method followed by sintering in air at 1200 °C for 48h. The influence of Fe-doping concentration on the structural and magnetic properties of the In2O3 samples was studied. X-ray diffraction analysis reveals that Fe ions are incorporated into the In3+ sites of the In2O3 lattice without altering the cubic bixbyite structure. Magnetic characterization shows ferromagnetic behavior at room temperature for all the Fe-doped In2O3 samples. The observed ferromagnetism is attributed to the oxygen vacancies induced by substitution of Fe into In3+ sites and vacuum annealing. A model of ferromagnetic exchange interactions was proposed to explain the ferromagnetism in this system.
关键词： DMS,RTFM,Fe doped In2O3,Defects,Ferromagnetism
更新于2025-09-23 15:21:21
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Improved Solar Energy Photoactivity over Defective BiOBr Ultrathin Nanosheets towards Pollutants Removal and Oxygen Evolution
摘要： Defective BiOBr nanosheets with ultrathin thickness and surface confined pits have been synthesized through a dual control of cetyltrimethyl ammonium bromide (CTAB) and polyvinyl pyrrolidone (PVP). The photocatalytic activity of the obtained defective BiOBr nanosheets was evaluated for the removal of rhodamine B (RhB), ciprofloxacin (CIP) and oxygen evolution from water. The as-prepared defective BiOBr nanosheets displayed significantly increased activity for pollutant degradation and oxygen evolution. The enhanced photocatalytic activity was ascribed to the improved light harvesting, suppression of charge recombination, and an increase in the number of active sites for photocatalytic reaction. The electronic structure of the defective BiOBr ultrathin nanosheets with confined pits has been tuned and thus varied the photocatalytic mechanism for pollutant degradation. The active species have changed from hole for BiOBr nanoplates to superoxide radical (O2??) and hole for defective BiOBr nanosheets determined by ESR analysis and trapping experiments.
关键词： Ultrathin nanosheets,Solar energy,Defects,BiOBr,Photocatalytic
更新于2025-09-23 15:21:21
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Buried defects induced by plasma assisted molecular beam epitaxy of AlN and GaN on Silicon
摘要： The present work is dedicated to the study of peculiar defects observed in GaN/AlN structures grown on Silicon substrate using the plasma assisted Molecular Beam Epitaxy under metal-rich conditions. Optical microscopy shows that these defects have unde?ned shape when AlN is grown on Silicon. On the contrary such defects are not observed with AlN ?lms grown under nitrogen-rich plasma assisted or ammonia source Molecular Beam Epitaxy, but triangular shape defects appear in case of GaN regrowth on the latter. As scanning electron microscopy is not able to evidence these defects but only protuberances connected with them, we deduce that they are located in the Silicon substrate. Complementary analysis with atomic force microscopy and energy dispersive X-ray analysis lead us to propose a scenario involving the etching of the Silicon substrate with Ga or Al di?using through weak points in the nucleation layer.
关键词： A1. Defects,B1. Nitrides on Silicon,A3. Molecular beam epitaxy,B2. Semiconducting materials
更新于2025-09-23 15:21:21
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Extrinsic Defects in Crystalline MoO <sub/>3</sub> : Solubility and Effect on the Electronic Structure
摘要： The effect of six potential contaminants (Cu, In, Ga, Se, Sn and Zn) and five potential dopants (Ti, Mn, Sc, V and Y) on the electronic and optical properties of molybdenum oxide (MoO3) contact layers for solar cells was investigated using point defect analysis informed by density functional theory simulations. Of the contaminants investigated, Sn, In and Ga were found to be highly insoluble at all relevant temperatures and pressures, and therefore not a concern for solar cell manufacturing. Zn, Cu and Se exhibit some solubility, with the latter two appearing to introduce detrimental defect states near the valence band. This contamination can be avoided by increasing the O2 partial pressure during MoO3 deposition. Out of five potential aliovalent dopants, Sc, Ti and Y were disregarded due to their limited solubility in MoO3, while V was found to be highly soluble and Mn somewhat soluble. The effect of Mn and V doping was shown to be strongly dependent on the O2 partial pressure during deposition with a high pO2 favoring the formation of substitutional defects (potentially beneficial in the case of Mn doping due to the addition of defects states near the conduction band) while low pO2 favors interstitial defects.
关键词： electronic structure,solubility,solar cells,molybdenum oxide,extrinsic defects,point defect analysis,density functional theory,MoO3
更新于2025-09-23 15:21:21
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Effects of vacancy defects location on thermal conductivity of silicon nanowire: a molecular dynamics study
摘要： The improvement of thermoelectric ?gure of merit of silicon nanowire (SiNW) can be achieved by lowering its thermal conductivity. In this work, non-equilibrium molecular dynamics method was used to demonstrate that the thermal conductivity of bulk silicon crystal is drastically reduced when it is crafted as SiNW and that it can be reduced remarkably by including vacancy defects. It has been found that ‘centre vacancy defect’ contributes much more in reducing the thermal conductance than ‘surface vacancy defect’. The lowest thermal conductivity that occurs is about 52.1% of that of pristine SiNW, when 2% vacancy defect is introduced in the nanowire. The vibrational density of states analysis was performed to understand the nature of this reduction and it has been found that the various boundary scatterings of phonon signi?cantly reduce the thermal conductivity. Also, larger mass difference due to voids induces smaller thermal conductivity values. These results indicate that the inclusion of vacancy defects can enhance the thermoelectric performance of SiNWs.
关键词： molecular dynamics,silicon nanowire,thermoelectric performance,thermal conductivity,vacancy defects
更新于2025-09-23 15:21:21
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The Infrared spectrum of very large (periodic) systems: global versus fragment strategies—the case of three defects in diamond
摘要： The calculation of the full vibrational spectrum (Infrared or Raman) of very large systems (say larger than one thousand atoms) is not only very expensive, but also of relatively low interest, as in many (most of the) cases only a subset of modes, well separated from the large, diffuse bands resulting from the superposition of thousands of peaks, is used for the spectroscopic characterization of the specific system under study. Here, a fragment strategy, consisting in computing and diagonalizing a reduced (in size) Hessian matrix centered around the zone of interest, is illustrated, and its accuracy and efficiency documented, by comparison with the full Hessian diagonalization (FHD) scheme. Three test cases are considered, showing different vibrational features. They are defects in diamond: the VN3 H defect (V stands for the vacancy), where the interesting point is the characterization of the bending and stretching modes of H, well separated from the large band resulting from the perturbation of the diamond manifold; the VH4 defect (four H atoms in the vacancy, with vibrational modes related to H appearing both at high and low wave numbers); and the I2N interstitial defect, with modes in which the N atoms are involved, appearing at wave numbers not far from the manifold of the perfect diamond modes. So the three cases, apparently similar, explore three different situations of interest for the fragment strategy: (1) localized modes very well separated from the large diamond continuous band ( VN3H); (2) modes at upper border of the large diamond continuous band ( I2N ): a case in which the modes of interest appear both as separated from and merged with the large continuous band ( VH4 ). It turns out that in all cases relatively small fragments, containing from 2 to 40 atoms, permit to reproduce with high accuracy (the difference with respect to the FHD being always smaller than 5 cm?1 for the wave numbers, and a few percent for the IR intensity) the spectral feature(s) of interest, at a computational cost that is only a small fraction of the one required by the FHD.
关键词： fragment strategy,vibrational modes,large systems,Infrared spectrum,diamond defects
更新于2025-09-23 15:21:21
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Electronic properties of graphene with point defects
摘要： This article attempts a point-by-point review of the electron spectrum of graphene containing defects (adsorbed atoms, substitutional atoms, vacancies) that can be adequately described within the Lifshitz model. In this regard, the known Hamiltonian for this model is calculated for the case of two-dimensional relativistic electrons, and the criteria for occurrence of an impurity resonance near a Dirac point are given. The theory of concentration band structure transformation of graphene is presented, from which it follows that a transport gap is opened upon reaching a strictly defined value of impurity concentration in the neighborhood of the energy of impurity resonance. At the same time, the question of the possibility (or impossibility) of localization in such a {spatially disordered system} of Dirac quasiparticles is analyzed. Based on this, it is possible to provide an explanation and present a description of a phenomenon recently observed in {impure graphene}, the metal-dielectric transition that appears as a direct consequence of the decrease of the Fermi energy of the system in the region of a transport gap. The concept of local spectrum rearrangement of graphene, which also occurs during the process of increased concentration of defects in it, is introduced and substantiated. Physical reasons are formulated, by which the position of the minimum of the low-temperature conductivity of graphene as a function of the Fermi energy of electrons corresponds to the impurity resonance energy rather than to the Dirac point, as has been validated in a number of theoretical and test studies. Here, it appears that the minimum value is not a universal magnitude, but depends on the concentration of defects. Analytical examination of impurity effects is accompanied by numerical modeling of the system under study, as a result of which complete correspondence has been established between these two approaches. In particular, the overall picture of spectrum rearrangement, localization of electron states, and also the effects having local nature are confirmed.
关键词： electron spectrum,transport gap,Lifshitz model,impurity resonance,metal-dielectric transition,graphene,local spectrum rearrangement,conductivity,point defects
更新于2025-09-23 15:21:21
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Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods
摘要： Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical arrays of nanorods. The ability to define the morphology and structure of the nanorods is essential to maximising the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show the O-polar nanorods (~0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (~0.37 eV). Oxygen plasma treatment is shown by Cathodoluminescence (CL) spectroscopy to reduce mid-gap defects associated with radiative emissions that improves the Schottky contacts from weakly-rectifying to strongly-rectifying. Interestingly, the plasma treatment was shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc vacancy related hydrogen defect complexes (VZn, Hn) in Zn-polar nanorods that evolve to lower coordinated complexes. The effect on HO in the O-polar nanorods coincided with a large reduction in the visible range defects producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled enhancing devices such as dynamic photodetectors, strain sensors, and LEDs while showing the O-polar nanorods can outperform Zn-polar nanorods in such applications.
关键词： ZnO,Cathodoluminescence,Electrical Transport,Polarity,Schottky Contacts,Nanorods,Defects
更新于2025-09-23 15:21:01
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[IEEE 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Chicago, IL, USA (2019.6.16-2019.6.21)] 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) - Control of Vacancy Defects in Reactively Sputtered (Ag,Cu)(In,Ga)Se <sub/>2</sub> Solar Cells
摘要： We report positron annihilation spectroscopy measurements revealing the prevalence of VSe-VCu divacancy defects in reactively sputtered ACIGS solar cells. Together with compositional and structural analysis as well as capacitance-voltage measurements, an intricate interplay is observed between divacancies, (Cu+Ag)/III ratio, grain size, and carrier concentration. These properties can be tuned during absorber growth by varying growth temperature and alkali content. Exploiting this interplay may be the key to achieving high efficiency in ACIGS solar cells.
关键词： ACIGS,positron annihilation,defects,potassium,vacancies,reactive sputtering,photovoltaic cells
更新于2025-09-23 15:21:01