Research on the Relationship between Density of States and Conducting Properties of Single-walled Carbon Nanotubes

The analytical expression of the electronic density of states (DOS) for single-walled carbon nanotubes (SWNTs) has been derived on the basis of graphene approximation of the energy E(k) near the Fermi level EF. The distinctive properties of the DOS, the normalized differential conductivity and the current us bias for SWNTs are deduced and analyzed theoretically. The singularities in the DOS (or in the normalized differential conductivity) predict that the jump structure of current (or conductance)-bias of SWNTs exists. All conclusions from the theoretical analysis are in well agreement with the experimental results of SWNT's electronic structure and electronic transport. In other words, the simple theoretical model in this paper can be applied to understand a range of spectroscopic and other measurement data related to the DOS of SWNTs.

GGA+U study of the electronic energy bands and state density of the wurtzite In_(1-x)Ga_xN

The electronic band structures, densities of states （DOSs）, and projected densities of states （PDOSs） of the wurtzite In1-xGaxN with x=0, 0.0625, 0.125 are studied using the generalized-gradient approximation （GGA） and GGA＋U in density functional theory. Our calculations suggest that in the case of wurtzite InN it is important to apply an on-site Hubbard correction to both the d states of indium and the p states of nitrogen in order to recover the correct energy level symmetry and obtain a reliable description of the InN band structure. The method is used to study the electronic properties of the wurtzite In1-xGaxN. The conduction band minimum （CBM） energy increases, while the valence band maximum （VBM） energy decreases with the increase of the gallium concentration. The effect leads to broadening the band gap （BG） and the valence band width （VBW）. Furthermore, the compressive strain in the crystal can cause the BG and the VBW to increase with the increase of gallium concentrations.

First-principles calculations of structural and electronic properties of Tl_xGa_(1-x)As alloys

The zincblende ternary alloys Tl_xGa_（1-x） As（0 〈 x 〈 1） are studied by numerical analysis based on the plane wave pseudopotential method within the density functional theory and the local density approximation. To model the alloys,16-atom supercells with the 2 × 2 × 2 dimensions are used and the dependency of the lattice parameter, bulk modulus,electronic structure, energy band gap, and optical bowing on the concentration x are analyzed. The results indicate that the ternary Tl_xGa_（1-x） As alloys have an average band gap bowing parameter of 4.48 eV for semiconductor alloys and 2.412 eV for semimetals. It is found that the band gap bowing strongly depends on composition and alloying a small Tl content with GaAs produces important modifications in the band structures of the alloys.

Structural,electronic and elastic properties of YCu from first principles

The structural, electronic and elastic properties of YCu compound in the B2 （CsCl） phase were investigated using the density functional theory （DFT） within the generalized gradient approximation （GGA）. The electronic density of states （DOS） obtained in this way accorded weU with the results of a recent study utilizing the full-potential linearized augmented plane wave （FLAPW） method. We also found that the density of d-states at the Fermi energy was low. The calculated equilibrium properties such as lattice constant, bulk modulus and its first derivative, and the elastic constants were in good agreement with experimental and theoretical results.

Orbital electronic heat capacity of hydrogenated monolayer and bilayer graphene

The tight-binding Harrison model and Green＇s function approach have been utilized in order to investigate the contribution of hybridized orbitals in the electronic density of states（DOS） and electronic heat capacity（EHC） for four hydrogenated structures, including monolayer chair-like, table-like, bilayer AA- and finally AB-stacked graphene. After hydrogenation, monolayer graphene and bilayer graphene are behave as semiconducting systems owning a wide direct band gap and this means that all orbitals have several states around the Fermi level. The energy gap in DOS and Schottky anomaly in EHC curves of these structures are compared together illustrating the maximum and minimum band gaps are appear for monolayer chair-like and bilayer AA-stacked graphane, respectively. In spite of these, our findings show that the maximum and minimum values of Schottky anomaly appear for hydrogenated bilayer AA-stacked and monolayer table-like configurations, respectively.

Nature of the band gap of halide perovskites ABX_3(A=CH_3NH_3,Cs;B=Sn,Pb;X=Cl,Br,I):First-principles calculations

The electronic structures of cubic structure of ABX3（A=CH3NH3, Cs; B=Sn, Pb; X=Cl, Br, I） are analyzed by den- sity functional theory using the Perdew-Burke-Ernzerhof exchange-correlation functional and using the Heyd-Scuseria- Ernzerhof hybrid functional. The valence band maximum （VBM） is found to be made up by an antibonding hybridization of B s and X p states, whereas bands made up by the π antibonding of B p and X p states dominates the conduction band minimum （CBM）. The changes of VBM, CBM, and band gap with ion B and X are then systematically summarized. The natural band offsets of ABX3 are partly given. We also found for all the ABX3 perovskite materials in this study, the bandgap increases with an increasing lattice parameter. This phenomenon has good consistency with the experimental results.

First-principles study of orbital ordering in cubic fluoride KCrF_3

Comprehensive first-principles calculations are performed to provide insight into the intriguing physical properties of the ternary cubic fluoride KCrF3. The electronic structures exhibit a prominent dependence on the effective local Coulomb interaction parameter Ueff. The ground state of the cubic phase is a ferromagnetic （FM） half-metal with Ueff equal to 0, 2, and 4 eV, whereas the insulating A-type antiferromagnetic （A-AFM） state with concomitant homogeneous orbital ordering is more robust than the FM state for Ueff exceeding 4 eV. We propose that the origin of the orbital ordering is purely electronic when the cooperative Jahn-Teller distortions are absent in cubic KCrF3.

First-principles study of the co-effect of carbon doping and oxygen vacancies in ZnO photocatalyst

Although tuning band structure of optoelectronic semiconductor-based materials by means of doping single defect is an important approach for potential photocatalysis application,C-doping or oxygen vacancy(Vo)as a single defect in ZnO still has limitations for photocatalytic activity.Meanwhile,the influence of co-existence of various defects in ZnO still lacks sufficient studies.Therefore,we investigate the photocatalytic properties of ZnOx C0.0625(x=0.9375,0.875,0.8125),confirming that the co-effect of various defects has a greater enhancement for photocatalytic activity driven by visible-light than the single defect in ZnO.To clarify the underlying mechanism of co-existence of various defects in ZnO,we perform systematically the electronic properties calculations using density functional theory.It is found that the coeffect of C-doping and Vo in ZnO can achieve a more controllable band gap than doping solely in ZnO.Moreover,the impact of the effective masses of ZnO_(x)C_(0.0625)(x=0.9375,0.875,0.8125)is also taken into account.In comparison with heavy Vo concentrations,the light Vo concentration(x=0.875)as the optimal component together with C-doping in ZnO,can significantly improve the visible-light absorption and benefit photocatalytic activity.

Study of magnetic and optical properties of Zn1-xTMxTe(TM = Mn,Fe,Co,Ni) diluted magnetic semiconductors：First principle approach

We present structural,magnetic and optical characteristics of Zn_（1-x）TM_xTe（TM = Mn,Fe,Co,Ni and x = 6.25%）,calculated through Wien2 k code,by using full potential linearized augmented plane wave（FP-LAPW） technique.The optimization of the crystal structures have been done to compare the ferromagnetic（FM） and antiferromagnetic（AFM） ground state energies,to elucidate the ferromagnetic phase stability,which further has been verified through the formation and cohesive energies.Moreover,the estimated Curie temperatures T_c have demonstrated above room temperature ferromagnetism（RTFM） in Zn_（1-x）TM_xTe（TM =Mn,Fe,Co,Ni and x= 6.25%）.The calculated electronic properties have depicted that Mn- and Co-doped ZnTe behave as ferromagnetic semiconductors,while half-metallic ferromagnetic behaviors are observed in Fe- and Ni-doped ZnTe.The presence of ferromagnetism is also demonstrated to be due to both the p-d and s-d hybridizations between the host lattice cations and TM impurities.The calculated band gaps and static real dielectric constants have been observed to vary according to Penn＇s model.The evaluated band gaps lie in near visible and ultraviolet regions,which make these materials suitable for various important device applications in optoelectronic and spintronic.

Prediction of chemical short-range order in high-/medium-entropy alloys

Chemical short-range orders(CSROs),as the built-in sub-nanoscale entities in a high-/medium-entropy alloy(H/MEA),have aroused an ever-increasing interest.With multi-principal elements in an H/MEA to form a complex concentrated solution,a variety of sub-systems of species exist to induce the metastable ordered compounds as candidates for ultimate CSROs.The issues remain pending on the origin of CSROs as to how to judge if CSRO will form in an H/MEA and particularly,what kind of CSROs would be stably produced if there were multiple possibilities.Here,the first-principles method,along with the proposed local formation energy calculation in allusion to the atomic-scale chemical heterogeneities,is used to predict the CSRO formation based on the mechanical stability,thermodynamic formation energy,and electronic characteristics.The simulations are detailed in an equiatomic ternary VCoNi MEA with three kinds of potential compounds,i.e.,L1_(1),L1_(2),and B2,in the face-centered cubic matrix.It turns out that L1_(1)is stable but hard to grow up so as to become the final CSRO.L1_(1)is further predicted as CSROs in CrCoNi,but unable to form in FeCoNi and CrMnFeCoNi alloys.These predictions are consistent with the experimental observations.Our findings shed light on understanding the formation of CSROs.This method is applicable to other H/MEAs to design and tailor CSROs by tuning chemical species/contents and thermal processing for high performance.

Photoreflectance system based on vacuum ultraviolet laser at 177.3 nm

Photoreflectance(PR)spectroscopy is a powerful and non-destructive experimental technique to explore interband transitions of semiconductors.In most PR systems,the photon energy of the pumping beam is usually chosen to be higher than the bandgap energy of the sample.To the best of our knowledge,the highest energy of pumping laser in reported PR systems is 5.08 eV(244 nm),not yet in the vacuum ultraviolet(VUV)region.In this work,we report the design and construction of a PR system pumped by VUV laser of 7.0 eV(177.3 nm).At the same time,dual-modulated technique is applied and a dual channel lock-in-amplifier is integrated into the system for efficient PR measurement.The system’s performance is verified by the PR spectroscopy measurement of well-studied semiconductors,which testifies its ability to probe critical-point energies of the electronic band in semiconductors from ultraviolet to near-infrared spectral region.

Anion-site-modulated thermoelectric properties in Ge2Sb2Te5-based compounds

The amalgamation of multi-subjects often elicits novel materials,new concepts and unexpected applications.Recently,Ge2 Sb2 Te5,as the most established phasechange material,has been found to exhibit decent thermoelectric performance in its stable,hexagonal phase.The challenge for higher figure of merit(zT) values lies in reducing the hole carrier concentration and enhancing the Seebeck coefficient,which,however,can be hardly realized by conventional doping.Here in this work,we report that the electrical properties of Ge2 Sb2 Te5 can be readily optimized by anion-site modulation.Specifically,Se/S substitution for Te induces stronger and more ionic bonding,lowering the hole density.Furthermore,an increase in electronic density of state is introduced by Se substitution,contributing to a large increase in Seebeck coefficient.Combined with the reduced thermal conductivity,maximum zT values above 0.7 at 800 K have been achieved in Se/S-alloyed materials,which is ~30% higher than that in the pristine Ge2Sb2 Te5.