Double Vacancy Research Articles

First-principles study of radiation defects in silicon

The study of the properties of defects in silicon forming under irradiation condition has been carried out for many years, however, many open questions remain. Particularly, there are not comprehensive results for variety of radiation centers calculated at the same level of theory. For example the previously calculated transition levels and formation energies show a large scatter. In this work, we focus on the important radiation-induced defect complexes in Si: double vacancy, vacancy-oxygen, vacancy-phosphorus. Additionally, the phosphorus–vacancy-oxygen complex was studied. Formation energy, charge transition levels and binding energy were calculated from first-principles using the hybrid exchange–correlation functional HSE06. Spin-polarized calculations and large supercells allows us to obtain charge transition levels which agree well with experimental measurements.

Structural and energetic properties of vacancy defects in MXene surfaces

Density functional theory calculations were employed to investigate the structure and energetics of vacancy defects on 84 MXene surfaces, composed of different M (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W), X (carbon or nitrogen), T (oxygen of fluorine), and atomic layer stacking (ABC or ABA) combinations. Structurally, we found that, in most cases, the effect of the presence of the vacancy on the surrounding atoms is to push them away from the vacant site due to the missing screening charge and the reinforcement of the remaining interatomic bonds. The reconstruction energy associated to this surface relaxation was found to range from a few tenths of eV to a few eV. On the bare ABC-stacked carbide MXenes, we found M and X vacancy formation energies between 2 and 3 eV. On nitrides, M vacancy formation energies are decreased, while X ones seem to increase. By terminating the MXenes with an O layer, M formation energies are significantly increased, reaching over 9 eV in some cases, in agreement with theoretical and experimental results of the literature, whereas X vacancy formation energies become smaller. The F termination was found to have the same effect as O on X vacancy formation energies with respect to clean MXenes, but to decrease M vacancy formation energies. The F termination allows more easily created M and N vacancies than the O termination. The creation of vacancies on some ABC-stacked ${\mathrm{M}}_{2}{\mathrm{XF}}_{2}$ MXenes led to extreme lattice deformation which suggests that the experimental synthesis of ${\mathrm{M}}_{2}{\mathrm{NF}}_{2}$ with metals from groups 5 and 6 of the Periodic Table, and ${\mathrm{M}}_{2}{\mathrm{CF}}_{2}$ with metals from group 6, through etching using a hydrogen fluoride aqueous solution [HF (aq)] will lead to ABA-stacked MXenes. The formation energies of the MT double vacancies were found to correlate approximately linearly with the sum of the corresponding M and T single-atom vacancy formation energies. All MXenes studied in this work fit in the same correlation line, implying that one can predict the formation energy of double vacancies on MXenes, made of any M, X, and T elements, and with atomic layers stacked in an ABC or ABA fashion, if the formation energies of the corresponding single vacancies are known. The method used by theoretical works, such as this one, to calculate formation energies has two crucial limitations: the values thus obtained depend on the chosen single-atom energy references, and kinetic effects such as energy barriers are not included. Here, we cautiously discuss these limitations, compare formation energies calculated using different energy references, and propose mechanisms for the formation of vacancies that include atomic migration barriers.

Defects-/doping-driven modulation of the electronic and magnetic properties of 2H- and Td-phase [formula omitted] monolayers: A first-principle study

As one of the transition metal dichalcogenides (TMDs) with layered structure, tungsten telluride (WTe2) has been known for its excellent electric and magnetic properties. Here, we applied density functional theory (DFT) to modify and simulate the structures of 2H-phase and Td-phase WTe2 using the first-principle calculations, and systematically investigated the impact of vacancy defects and substitutional doping on the electronic properties of them. Different types of point defect structures were constructed and their stability was proved by formation energy. Both of single and double vacancy defects are considered, and it shows that 2H-VW translates from semiconducting to semi-metallic, while 2H-VTe and 2H-V2Te remain direct band-gap and indirect band-gap semiconductors, respectively. Td-VTe induces spin polarization and Td-VW changes from semi-metallic to semiconductor. For substitutional dopants, the W or Te atom in 2H-phase and Td-phase WTe2 are substituted by Al, B, C, Co, Fe, N, P, Si atoms. The results show that, except for 2H-C and 2H-Si, all the doping systems of 2H-phase have obvious spin polarization and turn to be magnetic semiconductors. But these doped atoms hardly induce spin polarization in Td-phase WTe2 for the spin-up and spin-down states are mostly symmetrical in the density of state. This work is expected to play a positive role in understanding and regulating the properties of WTe2 and accelerating future applications of nanomaterials.

Two-step implantation of gold into graphene

As a one-atom thick, mechanically strong, and chemically stable material with unique electronic properties, graphene can serve as the basis for a large number of applications. One way to tailor its properties is the controlled introduction of covalently bound heteroatoms into the lattice. In this study, we demonstrate efficient implantation of individual gold atoms into graphene up to a concentration of 1.7 × 1011 atoms cm−2 via a two-step low-energy ion implantation technique that overcomes the limitation posed by momentum conservation on the mass of the implanted species. Atomic resolution scanning transmission electron microscopy imaging and electron energy-loss spectroscopy reveal gold atoms occupying double vacancy sites in the graphene lattice. The covalently bound gold atoms can sustain intense electron irradiation at 60 kV during the microscopy experiments. At best, only limited indication of plasmonic enhancement is observed. The method demonstrated here can be used to introduce a controlled concentration of gold atoms into graphene, and should also work for other heavier elements with similar electronic structure.

Density functional theory investigation of As4 adsorption on Ti, V, Cr, Mn-doped graphene

Coal gasification technology can effectively solve the problem of clean coal utilization, but the problem of volatilization of harmful metals such as arsenic in the coal gasification process cannot be underestimated. In this paper, based on DFT (density functional theory), the adsorption characteristics of As4 on the surface of different carbon-based single-atom Ti/V/Cr/Mn adsorbents (nitrogen atom-doped, single vacancy and double vacancy) were studied. The single-atom adsorbent geometry, adsorption configuration, adsorption energy, electronic structure, electron density and other data were analysed. By comparing and analysing the calculation results of sixteen adsorbents, it is found that the catalyst with the double-deficient nitrogen-doped substrate has higher stability. Through the analysis of charge transfer and density of state, it was found that the adsorption of a single-atom adsorbent catalyst to As4 is related to charge transfer and hybridization. In particular, doped nitrogen atoms can enhance the charge transfer and hybridization of the system. amongst the sixteen adsorbents, the double-vacancy nitrogen-doped Ti catalyst is a promising As4 adsorbent, which provides a reference for further research on the adsorption of toxic gases on the surface of single-atom catalysts and the design of new nonnoble metal adsorbents.

Vacancy-Induced Thermal Transport and Tensile Mechanical Behavior of Monolayer Honeycomb BeO.

Because of the rapid shrinking trend of integrated circuits, the performances of nanodevices and nanomechanical systems are greatly affected by the joule heating and mechanical failure dilemma. In addition, structural defects are inevitable during experimental synthesis of nanomaterials, which may alter their physical properties significantly. Investigation of the thermal transport and mechanical behavior of nanostructured materials with structural defects is thus a crucial requirement. In this study, the thermal conductivity (TC) and tensile mechanical behavior of monolayer honeycomb BeO are systematically explored using molecular dynamics simulations. An infinite length bulk TC of ∼277.77 ± 8.93 W/mK was found for the pristine monolayer BeO. However, the insertion of 1% single vacancy (SV) and double vacancy (DV) defects reduces the TC by ∼36.98 and ∼33.52%, respectively. On the other hand, the uniaxial tensile loading produces asymmetrical fracture stress, elastic modulus, and fracture strain behaviors in the armchair and zigzag directions. The elastic modulus was reduced by ∼4.7 and ∼6.6% for 1% SV defects along the armchair and zigzag directions, respectively, whereas the reduction was ∼2.7 and ∼ 5.1% for 1% DV defects. Moreover, because of the strong symmetry-breaking effect, both the TC and mechanical strength were significantly lower for the SV defects than those for the DV defects. The highly softening and decreasing trends of the phonon modes with increasing vacancy concentration and temperature, respectively, were noticed for both types of defects, resulting in a reduction of the TC of the defected structures. These findings will be helpful for the understanding of the heat transport and mechanical characteristics of monolayer BeO as well as provide guidance for the design and control of BeO-based nanoelectronic and nanoelectromechanical devices.

Spatially separated oxygen vacancies and nickel sites for ensemble promotion of selective CO2 photoreduction to CO

The CO2 reduction reaction (CRR), which is key to addressing global environmental and energy issues, must overcome the strong C = O bond and is hindered by low CRR selectivity and activity. Here, we unravel the ensemble effect of catalytically active sites for promotion of CRR while suppressing hydrogen evolution reactions (HERs). We design a 2D/2D hybrid consisting of an ordered assembly of nickel-rich Ni-Mn layered double hydroxide (LDH) nanosheets (NSs) and oxygen vacancies (OVs)-rich Mn3O4 (OVs-Mn3O4) NSs. On its own, Ni-Mn LDH demonstrates high CRR selectivity but mediocre CRR efficiency, whereas OVs-Mn3O4 gives a higher efficiency but poorer selectivity. The 2D/2D hybrid inherits advantages from both components, attaining high CRR selectivity and markedly higher efficiency than Ni-Mn LDH and OVs-Mn3O4. Calculations and in situ Fourier transform infrared spectroscopy (FT-IR) measurements reveal that nickel sites in the 2D/2D hybrid activate CO2 to generate CO2•– via a one-electron injection, whereas OVs in OVs-Mn3O4 promote light harvesting and charge transport without activating unwanted HERs.

Effect of defects on heat transfer at the graphene/epoxy interface

The effects of defects on the interface thermal conductivity (ITC) for graphene/epoxy (G/EP) are investigated by using molecular dynamics simulations (MD). The results indicate that defects in graphene are important for improving the ITC of G/EP. This enhancement depends on defect type and concentration. Among the various defect types, Stone-Wales(SW) defects are detected to be the most effective defect type for improving the ITC. Other defect types such as multi-vacancy (MV), double-vacancy (DV), and single-vacancy (SV) are detected to have negligible enhancement. The influence of graphene layer number on ITC is further investigated for both pristine and SW-defective graphene. As layer number grows, the ITC decreases greatly but becomes steady when layer number exceeds four for both systems. The calculated ITCs are then comprehensively analyzed by using Phonon Density Of States (PDOS). Our study offers meaningful directions on the use of defective graphene in practical thermal management applications.

Strain induced magnetic hysteresis in MoS2 and WS2 monolayers with symmetric double sulfur vacancy defects.

It has been found that magnetism in two-dimensional (2D) transition metal dichalcogenides can be realized by properly introducing vacancies and applying strain. However, no work has clearly clarified the modulation of such 2D magnetism under a sweeping strain. Thus we were motivated in this work to investigate the mechanical and electronic properties of the monolayer MS2 (M = Mo, W) with symmetric S vacancy defects under sweeping strain. The results show that the local structure of the M atoms in MS2 around the defect undergoes a reversible phase transition from a triangular shape (Tri-3M) with short M-M bonds, to a circular one (Cir-6M-12S) with larger M-M bonds as the planar strain increases. The critical tensile strain for the transition from Tri-3M to Cir-6M-12S are 12.53% for MoS2 and 11.46% for WS2, while the critical compressive strain for the reversal from Cir-6M-12S to Tri-3M are -3.60% and -2.16%, respectively. In particular, we find that the magnetism can be continuously modulated and undergoes a hysteresis loop behavior under the sweeping strains, with the residual magnetism being 2 μB. Our work theoretically predicts the promising prospect for exploring low-dimensional semiconductor spintronic devices working without applying a magnetic field.

Selective decomposition of hydrazine over metal free carbonaceous materials.

Herein we report a combined experimental and computational investigation unravelling the hydrazine hydrate decomposition reaction on metal-free catalysts. The study focuses on commercial graphite and two different carbon nanofibers, pyrolytically stripped (CNF-PS) and high heat-treated (CNF-HHT), respectively, treated at 700 and 3000 °C to increase their intrinsic defects. Raman spectroscopy demonstrated a correlation between the initial catalytic activity and the intrinsic defectiveness of carbonaceous materials. CNF-PS with higher defectivity (ID/IG = 1.54) was found to be the best performing metal-free catalyst, showing a hydrazine conversion of 94% after 6 hours of reaction and a selectivity to H2 of 89%. In addition, to unveil the role of NaOH, CNF-PS was also tested in the absence of alkaline solution, showing a decrease in the reaction rate and selectivity to H2. Density functional theory (DFT) demonstrated that the single vacancies (SV) present on the graphitic layer are the only active sites promoting hydrazine decomposition, whereas other defects such as double vacancy (DV) and Stone-Wales (SW) defects are unable to adsorb hydrazine fragments. Two symmetrical and one asymmetrical dehydrogenation pathways were found, in addition to an incomplete decomposition pathway forming N2 and NH3. On the most stable hydrogen production pathway, the effect of the alkaline medium was elucidated through calculations concerning the diffusion and recombination of atomic hydrogen. Indeed, the presence of NaOH helps the extraction of H species without additional energetic barriers, as opposed to the calculations performed in a polarizable continuum medium. Considering the initial hydrazine dissociative adsorption, the first step of the dehydrogenation pathway is more favourable than the scission of the N-N bond, which leads to NH3 as the product. This first reaction step is crucial to define the reaction mechanisms and the computational results are in agreement with the experimental ones. Moreover, comparing two different hydrogen production pathways (with and without diffusion and recombination), we confirmed that the presence of sodium hydroxide in the experimental reaction environment can modify the energy gap between the two pathways, leading to an increased reaction rate and selectivity to H2.

STRUCTURAL ENVIRONMENT OF FLUORIDE IONS IN APATITES OF CHEMICAL COMPOSITION AND ORIGIN: 19F MAS NMR STUDY

Apatites of markedly different chemical composition and origin were studied using 19F magic-angle spinning nuclear magnetic resonance. Synthetic carbonate fluorapatites (CFAp) containing 2.6 to 4.7 wt% F, 0 to 4 wt% CO2 and 0 to 1 wt% Na2О, hydroxylfluorapatite (OH:F ≈ 1:1) containing about 3 wt% Y (Y-HFAp), natural REE-apatites, and CFAp and Y-HFAp heated at temperature from 700 to 1000 oC were researched. The spectra of apatites with isomorphic substitutions show the signals (chemical shift δ) caused by fluoride ions in fluorapatite structure and, possibly, near defects in Ca sites (from −102.5 to −100 ppm), near water molecules (H2Os) incorporated in the channels (about −96.5 ppm), and one or two signals with δх from −91 to −86 ppm. The spectra of synthetic CFAp and Y-НFAр heated up to 900 oС and original natural REE-apatites show two components, δх1 and δх2 shifted with 2 − 3 ppm, in this range. It is shown that the component δx2 is new, it hasn’t been observed in the spectra previously. Signals in the δх range are caused by Fх ions (up to 12% F) whose structural environment is different substantially from the "ideal" fluorapatite structure. It is found that the contents of Fх ions and CO2 in synthetic CFAp correlate linearly. It is shown that Fx (Fх1) ions can occupied sites in the channels near single vacancies Ca, and Fх2 ions — near double vacancies, Ca and anionic those in the channels, in CFAp with the F content not higher than stoichiometric and Y-НFАр. These vacancies can form through different heterovalent isomorphism mechanisms such as РО43− → СО32− and/or Са2+ → M3+ (М = REE, Al, Fe), vacancies in the channels of heated apatites with partial substitutions F → H2Os, OH — through dehydration and/or dehydroxylation.

Catalytic activity of 1D chains of gold oxide on a stepped gold surface from density functional theory.

The rich surface chemistry of gold at the nanoscale has made it an important catalyst for low-temperature applications. Recent studies point to the possible role of self-organized structures formed by chemisorbed O atoms on the surface of gold catalysts for their catalytic activity and/or deactivation. In this study, we investigate the reactivity of a double O chain running along a step on a Au(221) surface with oxygen vacancies as a prototypical model of a 1D surface gold oxide. We compare CO and O2 adsorption on such a chain with the oxygen-free Au(221) surface model. A systematic study of the reactivity of the double chain with O vacancies was done with respect to the regular Au(221) surface using CO as a probe. The CO oxidation was investigated assuming dissociative and associative mechanisms. Remarkably, O2 adsorbs stronger on the double oxygen vacancy than on the regular Au(221) surface, and its dissociation barrier reduces significantly from 1.84 eV to 0.87 eV, whereas the CO adsorption energy is similar on these surfaces. Calculations suggest that CO oxidation should occur more efficiently on the double O vacancy than on the regular Au(221) surface due to stronger adsorption of O2 and a low activation barrier for O2 + CO surface reaction.

Mixed magnetic behavior in gadolinium and ruthenium co-doped nickel oxide nanoparticles

Pure and different concentrations from (Gd, Ru) co-doped NiO nanoparticles, capped with Polyvinylpyrrolidone (PVP), were fabricated by the co-precipitation method. The nanoparticles were characterized by different techniques. The Rietveld refinements of x-ray Diffraction (XRD) patterns confirmed the formation of the pure face-centered-cubic NiO phase. The x-ray Photo-induced Spectroscopy (XPS) assured the trivalent oxidation state of the doped ions Gd3+ and Ru3+ and unveiled the multiple oxidation states of nickel ions (Ni2+ and Ni3+), emerging from the vacancies in the samples. The Transmission Electron Microscope (TEM) images showed the pseudospherical morphology of the samples and the Energy Dispersive x-ray permitted the quantitative analysis of the presented elements and their homogeneous distribution. The Raman and Fourier Transform Infra-Red (FTIR) spectra depicted the fundamental vibrational bands of NiO nanoparticles, confirming their purity. The UV-visible spectroscopy enabled the absorption measurements and the energy gap calculations. The co-dopants increased the energy bandgap of NiO nanoparticles from 3.15 eV for pure NiO to 3.62 eV with the highest concentration of the co-dopants (x = 0.02) The photoluminescence (PL) spectra gave insights into the possible defects present in the samples, such as nickel vacancies, single and double oxygen vacancies, and oxygen interstitials. The Vibrating Sample Magnetometer (VSM) studied the room temperature M-H loops of the co-doped samples. A combination of ferromagnetic, antiferromagnetic, and paramagnetic contributions was noticed and treated according to the law of approach to saturation and bound magnetic polaron (BMP) model. The magnetic parameters, such as the saturation magnetization, exchange and anisotropy field, and the BMP concentration were extracted from the fitted models and discussed in terms of the co-dopants’ concentration. The co-doped samples showed a softer magnetic behavior, which is recommended for data storage applications.

Z-Scheme Heterojunctions with Double Vacancies Semiconductors Moo3-X and Fe-Doped W18o49 for Photocatalytic Nitrogen Fixation

Z-Scheme Heterojunctions with Double Vacancies Semiconductors Moo3-X and Fe-Doped W18o49 for Photocatalytic Nitrogen Fixation

A DFT study of the effect of stacking on the quantum capacitance of bilayer graphene materials

Graphene is acknowledged as one of the ideal active electrode materials for electric double-layer capacitors because of its extremely high specific surface area and outstanding electronic conductivity. By introducing defects or heteroatoms into the graphene sheet, the electronic structure around the defects can be altered, which could lead to an increased quantum capacitance (CQ) and therefore te capacitive performance. One of the unavoidable problems for manufacturing and using graphene materials is that the stacking of the layers affects their electronic structure, and eventually their capacitance. DFT calculations were used to investigate the effect of layer stacking in bilayer graphene materials on CQ and the surface charge density. A two layer, AB-stacked graphene model, in which the top layer is defective and the bottom one is perfect was assumed for the calculations. The defective graphenes investigated are those containing Stone-Thrower-Wales defects, single vacancies (SV), three with double vacancies (5-8-5, 555-777 and 5555-6-7777), pyrrole-N graphene and the pyridine-N graphene. Results indicate that both the values and waveform of CQ of the materials are changed by stacking. The CQ values of most of these graphenes are significantly increased after stacking. The CQ waveforms of the SV and N-doped graphene are relatively insensitive to stacking. The basal layer contributes a considerable amount of charge, which is most obvious for the pyrrolic-N double-layer graphene and 5-8-5 double-vacancy graphene. The surface charge density provided by the defective top layer is increased by interlayer interaction, especially for the N-doped graphene. The uniform distribution of charge on the bottom layer partially alleviates fluctuations in the CQ waveform. These findings provide theoretical guidance for the micro-structural design of graphene materials to optimize their performance as electrode active materials.

Solid atomic hydrogen: Point defect formation and elastic stability

Within the framework of the density functional theory, the resistance of solid atomic hydrogen with an I41/amd crystal structure to the formation of point defects, such as a vacancy, an interstitial site, and a double vacancy, was studied. At the pressures below 265 GPa, the equilibrium vacancy concentration per atom reaches a critical value, even near the temperature of liquid helium. Thus, the destruction of the metastable atomic state occurs at higher pressures than predicted by the calculations based on the phonon spectra. For atomic hydrogen at room temperature, pressures exceeding 280 GPa are required. The elastic stability of the I41/amd atomic phase in the pressure range from 250–600 GPa was investigated. It is shown that there are regions of instability of the I41/amd atomic hydrogen phase, where a transition to the structure with FDDD symmetry occurs.

Effect of double vacancy near the heating wall on the heating characteristics of particle pile

Reducing carbon emissions is the focal issues of common concern all over the world. It is of great significance for reducing carbon emissions to explore the effect of double vacancy adjacent to the heating wall on heating characteristics of particle pile. A heat exchange model with double vacancy was constructed, which was verified by a particle reactor experimental system. Effect of the change of the double-vacancy's position on the heating characteristics was studied. The results show that with the increase of the distance between the double vacancy and the heating wall, the thermal resistance of the selected area increases first and then decreases. The minimum heat flux and heating efficiency of the selected area, the maximum thermal resistance of the selected area, and the maximum area ratio of the vacancy affected area all appear when the distance from the vacancy to the heating wall is 1 time the particle size. With the increasing of the relative distance between the double vacancy, the total heat flux on the heating wall increases, the thermal resistance of the particle reactor decreases. When the relative distance of double-vacancy increases to 8 times the particle size, the mutual influence between the double-vacancy disappears.

Gas Sensing by Metal and Nonmetal Co-Doped Graphene on a Ni Substrate

Gas sensors based on graphene are gaining more and more attention. The unique properties of the single-atom-thick, flexible, robust, and quanta-sensitive graphene make it an excellent candidate for sensors. However, its characteristics can be modified to suit specific applications by means of metal and nonmetal doping. In this systematic and detailed study, with the aid of density functional theory (DFT)-based simulations, we investigate the gas sensing capabilities of trapped transition-metal (TM) atoms at the graphene double vacancy (2VG). Eight different gas molecules, ranging from electron acceptors (Lewis acids) to donor species (Lewis bases), are investigated, namely, O2, NO2, NO, CO2, SO2, H2O, NH3, and CO. Four TM atoms Fe, Co, Ni, and Cu are considered as active sites for gas sensing. The effects of N-doping and the Ni(111) substrate are also examined. The performance of these systems in terms of stability, sensitivity, selectivity, and reusability is discussed for practical applications.

A First-Principles Study on Titanium-Decorated Adsorbent for Hydrogen Storage

Based on density functional theory calculation, we screened suitable Ti-decorated carbon-based hydrogen adsorbent structures. The adsorption characteristics and adsorption mechanism of hydrogen molecules on the adsorbent were also discussed. The results indicated that Ti-decorated double vacancy (2 × 2) graphene cells seem to be an efficient material for hydrogen storage. Ti atoms are stably embedded on the double vacancy sites above and below the graphene plane, with binding energy higher than the cohesive energy of Ti. For both sides of Ti-decorated double vacancy graphene, up to six H2 molecules can be adsorbed around each Ti atom when the adsorption energy per molecule is −0.25 eV/H2, and the gravimetric hydrogen storage capacity is 6.67 wt.%. Partial density of states (PDOS) analysis showed that orbital hybridization occurs between the d orbital of the adsorbed Ti atom and p orbital of C atom in the graphene layer, while the bonding process is not obvious during hydrogen adsorption. We expect that Ti-decorated double vacancy graphene can be considered as a potential hydrogen storage medium under ambient conditions.

Density functional theory based HSE06 calculations to probe the effects of defect on electronic properties of monolayer TMDCs

Defects usually play an important role in modifying the properties of the materials, therefore in this investigation an attempt has been made to understand the effect of chalcogen vacancy defect in various monolayer transition metal dichalcogenides (TMDCs). En route to probe the effect of defect, the density functional theory (DFT) calculations have been carried out. The results obtained from DFT calculations reveal that: (i) the nature of both chalcogen and metal significantly influences the dissociation energy, (ii) the work function of defective TMDC alters only marginally for both single or double vacancies, (iii) chalcogen vacancy converts direct gap (KV → KC) semiconductors into indirect gap semiconductors (ΓV → KC), and (iv) electronic bands clearly indicate that the d-orbital of the transition metal plays a vital role in the formation of impurity band.