单壁碳纳米管的电子速度及有效质量

利用单壁碳纳米管(SWCNTs)能量色散关系, 计算了最低导带的电子速度和有效质量, 重点讨论了SWCNTs中最低导带电子速度和有效质量与波矢及管径大小的关系. 结果表明, 半导体型锯齿SWCNTs的电子速度和有效质量与其结构参量(管径)有直接的关系. 各种椅型SWCNTs(金属型)和金属型锯齿SWCNTs最低导带电子速度和有效质量随波矢的变化规律分别相同, 各种半导体型锯齿SWCNTs最低导带电子速度和有效质量随波矢的变化规律则有明显差别. 这意味着在低偏压下, 不同管径的椅型SWCNTs和金属型锯齿SWCNTs输运性能相同, 而各种不同管径半导体型锯齿SWCNTs输运性能有明显差别.     

First-principle electronic, elastic, and optical study of cubic gallium nitride

The ab initio pseudopotential (PP) method within the generalized gradient approximation (GGA) has been used to investigate the electronic, elastic constants, and optical properties of zinc-blende GaN. An underestimated band gap along with higher DOS and squeezed energy bands around the fermi level is obtained. The d-band effect is briefly discussed for electronic band structure calculations. With the help of elastic constants, acoustic wave speeds are calculated in [100], [110], and [111] planes. The dielectric constant, refractive index, and its pressure coefficient are well illustrated. The effect of hydrostatic pressure is explicated for all these properties. The results of the present study are evaluated with the existing experimental and first-principle calculations.     

Naphthalenebisimides as photofunctional surfactants for SWCNTs – towards water-soluble electron donor–acceptor hybrids

A water soluble naphthalenebisimide derivative (NBI) was synthesized and probed to individualize, suspend, and stabilize single wall carbon nanotubes (SWCNTs). Besides a comprehensive photophysical and electrochemical characterization of NBI, stable suspensions of SWCNTs were realized in buffered D₂O. Overall, the dispersion efficiency of the NBI surfactant was determined by comparison with naphthalene based references. Successful individualization of SWCNTs was corroborated in several microscopic assays. In addition, emission spectroscopy points to the strong quenching of SWCNT centered band gap emission, when NBIs are immobilized onto SWCNTs. The origin of the quenching was found to be strong electronic communication, which leads to charge separation between NBIs and photoexcited SWCNTs, and, which yields reduced NBIs as well oxidized SWCNTs. Notably, electrochemical considerations revealed that the energy content of these charge separated states is one of the highest reported for SWCNT based electron donor–acceptor hybrids so far.     

Adsorption of atomic hydrogen on single-walled carbon nanotubes

We have investigated atomic and electronic structures of hydrogen-chemisorbed single-walled carbon nanotubes (SWCNTs) by density functional calculations. We have searched for relative stability of various hydrogen adsorption geometries with coverage. The hydrogenated SWCNTs are stable with coverage of H/C, theta >/= 0.3. The circular cross sections of nanotubes are transformed to polygonal shapes with different symmetries upon hydrogen adsorption. We find that the band gap in carbon nanotubes can be engineered by varying hydrogen coverage, independent of the metallicity of carbon nanotubes. This is explained by the degree of sp(3) hybridization.     

Effect of high hydrostatic pressure on structural stability of Ti3GeC2: A first-principles investigation

An investigation into the structural stability, electronic and elastic properties of Ti₃GeC₂ under high hydrostatic pressure was conducted using first-principles calculations based on density functional theory (DFT). From the energy and enthalpy calculations, and the variations of elastic constants with pressure, we conclude that α‐Ti₃GeC₂ is most stable upon compression to 100 GPa, which is not consistent with the nonhydrostatic in situ synchrotron X-ray diffraction studies. The higher structural stability was analyzed in terms of electronic level. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of all polymorphs of Ti₃GeC₂.     

Molecular structure and electronic properties of a series of oligoalkylthiophenes: A theoretical investigation

Molecular geometries and electronic properties of 3-alkylthiophenes (ATs) and their oligomers (OATs) are studied by the density functional theory (DFT). Calculations are performed on the oligomers formed by n repeating units, where n ranges from 1 to 6, using the B3LYP/6-31G** level of theory. The results obtained show that the doped oligomers have more satisfactory structural and electronic characteristics for the conducting polymers. The conjugated system in the doped oligomers has more aromaticity, with expanded and planar chains. The calculated energy gap values between the frontal orbitals and also the ionization potential values for the oligomers indicate that with increase in the oligomer chain length, the conductive band gap decreases. Furthermore, our calculations suggest that an electron-donating alkyl substituent at position 3 of the thiophene ring plays an important role in the structural and electronic properties of the polymers.     

Transition between direct gap and indirect gap in two dimensional hydrogenated honeycomb Si x Ge1−x alloys

Using first-principles calculations, we have explored the structural and electronic properties of fully hydrogenated honeycomb Si _x Ge_1?x H alloys. Finite band gaps are opened by hydrogenation for x in the whole range from 0 to 1, while their nature and values can be tuned by x. When x is <0.7, the band gap is direct (from Γ to Γ). And when x is ≥0.7, the gap turns into indirect (from Γ to M). For all the computed compositions, the two kinds of energy differences between valence band and conduction band, Γ–Γ and Γ–M, are described well by two polynomial functions of x. The smaller of the two functions gives a good prediction for the overall band gap at any x. The two curves cross at x = 0.7, leading to the change of band gap type. At PBE level, the values of band gap for different x spread from 1.09 to 2.29 eV. These findings give a new route to tune the electronic properties of these materials and may have potential applications in nanoscale optoelectronics.     

Structural,elastic, electronic and optical properties of the cubic perovskite BiAlO3

We report first-principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BiAlO₃ using the pseudopotential plane waves method within the local density approximation. The calculated structural parameters are in good agreement with previous calculations. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffness is found. Band structures show that BiAlO₃ has an indirect band gap between the occupied O 2p and unoccupied Bi 6p states. The density of states and Mulliken charge populations analysis shows that Al–O and Bi–O bonds are covalent with a strong hybridization. The variation of the gap versus pressure is well fitted to a quadratic function and an indirect to direct band gap transition occurs at 15.5 GPa. Furthermore, in order to understand the optical properties of BiAlO₃, the dielectric function, absorption coefficient, refractive index, extinction coefficient, optical reflectivity and electron energy loss are calculated for radiation up to 30 eV.     

Cr、Sn、Co掺杂对层状二维材料MoSi2N4的电子和光学性能的影响(英文)

基于新合成的二维材料MoSi₂N₄(MSN),我们建立了一系列MSN的掺杂模型进行了第一原理计算。首先,我们计算了本征MSN的电子特性,包括其能带结构和态密度。然后我们研究了Cr、Sn和Co掺杂对MSN的电子和光学性质的影响。结果表明,在3种掺杂体系中,Co掺杂体系表现出最低的形成能,这表明Co掺杂体系是最稳定的。通过带隙计算表明,尽管3种掺杂模型都降低了MSN的固有带隙,但却表现出3种不同的电子特性。态密度图也显示,Cr和Co掺杂体系都在导带底(CBM)和价带顶(VBM)附近产生局部尖峰。此外,光学性质的计算中表明,掺杂后体系的光学性质也得到了改善。     

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not ensure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer toward the aromatic ? quinoidal level crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.     

Electronic, linear, and nonlinear optical properties of III-V indium compound semiconductors

We have made an extensive theoretical study of the electronic, linear, and nonlinear optical properties of the III-V indium compound semiconductors InX (X=P, As, and Sb) with the use of full potential linear augmented plane wave method. The results for the band structure, density of states, and the frequency-dependent linear and nonlinear optical responses are presented here and compared with available experimental data. Good agreement is found. Our calculations show that these compounds have similar electronic structures. The valence band maximum and the conduction band minimum are located at Gamma resulting in a direct energy gap. The energy band gap of these compounds decreases when P is replaced by As and As by Sb. This can be attributed to the increase in bandwidth of the conduction bands. The linear and nonlinear optical spectra are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. The calculated linear optical properties show very good agreement with the available experimental data. We find that the intra-and interband contributions of the second-harmonic generation increase when moving from P to As to Sb. The smaller energy band gap compounds have larger values of chi(123) ((2))(0) in agreement with the experimental measurements and other theoretical calculations.     

Density functional theory study of the brookite surfaces and phase transitions between natural titania polymorphs

The present study is concerned with the structural and electronic properties of the low-index surfaces of the brookite form of titanium dioxide. A theoretical investigation is carried out using the density functional formalism under the nonlocal B3LYP approximation to calculate surface energies, band energy values, and to interpret the response to hydrostatic pressure of the bulk and surfaces of the brookite structure. In addition, phase transformations with pressure are predicted from the anatase and the rutile to the brookite polymorph at about 3.8 and 6.2 GPa, respectively. The orthorhombic structure and the fractional coordinates of brookite vary isotropically with the rise in pressure. The calculated band gap energy is 3.78 eV for the bulk brookite. The brookite surface stabilities follow the sequence (010) < (110) < (100), and the minimum gap energy value is found for the (110) surface.     

Adsorption of 3d transition-metal atoms on two-dimensional penta-graphene: A first-principles study

The adsorption of single 3d transition metal (TM) atoms (from Sc to Zn) on a (3 × 3) penta-graphene (PG) sheet has been systematically studied by means of density-functional theory calculations. We particularly study the effect of the TM adatom on the structural, electronic, and magnetic properties when adsorbed on the PG sheet. Our calculations have shown that most of the TM adatoms are preferably adsorbed on the T₂ site (i.e., the top of the C₂ atom located at the bottom layer), while Cr, Zn, and Ni are preferably adsorbed on different B sites. The calculated band structures demonstrate that all TM-PG systems remain semiconductors such that the gap between the valence and conduction bands moves to a lower energy state relative to the Fermi level. For this reason, an apparent narrowing in the band gap values for the TM-PG systems has been predicted (0.11 – 0.97 eV) compared to the band gap of the isolated PG sheet (2.23 eV). Additionally, our results indicate that most of the TM-PG systems are magnetic, with the exception of Ni-PG and Zn-PG systems. Consequently, the engineering of the electronic properties of the TM-PG systems implies that such systems might be promising candidates for different applications.     

First principles calculations of electronic and optical properties of Ag2S

A theoretical study of structural, electronic and optical properties of Ag₂S is presented using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, the modified Becke Johnson (MBJ) potential coupled with Local Density Approximation (LDA) was used for the exchange-correlation potential calculation. Ground state properties are determined for the bulk material in monoclinic phase. Band structure reveals that this compound is a direct energy band gap semiconductor. MBJLDA results for the band gap of this compound are much better than those obtained using LDA, Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and Engel–Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gap and corresponding experimental values as compared to other calculations. Optical constants including the dielectric function, refractive index, extinction coefficient, electron energy loss function, reflectivity and absorption coefficient are obtained and discussed.     

A DFT study on the electronic structure for iridium nitride under high pressure

We present density functionary theory (DFT) calculations on the structural parameters and electronic structure for iridium nitride by using the generalized gradient approximation (GGA) and the Perdew–Burke–Ernserhof (PBE) exchange-correlation functional. The lattice parameters and bulk modulus (B ₀) for the ground state are obtained, and the energy band structure and electron densities of states (DOS) of IrN₂ are presented. It is found that IrN₂ has a very close indirect energy gap. There is a strong covalent bond between the two nearest N atoms. This gives rise to a very high elastic modulus of IrN₂ and reveals the quasimolecular nature of the N₂ in IrN₂ crystal. Lattice parameters, bulk modulus, and the electronic structure of IrN₂ under high pressure have also been investigated based on DFT. The compressibility along three cell vectors is very close to each other. The band gap increases a little with the pressure even when the pressure is up to 100 Gpa.     

Two Dimensional Materials Beyond MoS2: Noble‐Transition‐Metal Dichalcogenides

The structure and electronic structure of layered noble‐transition‐metal dichalcogenides MX₂ (M=Pt and Pd, and chalcogenides X=S, Se, and Te) have been investigated by periodic density functional theory (DFT) calculations. The MS₂ monolayers are indirect band‐gap semiconductors whereas the MSe₂ and MTe₂ analogues show significantly smaller band gap and can even become semimetallic or metallic materials. Under mechanical strain these MX₂ materials become quasi‐direct band‐gap semiconductors. The mechanical‐deformation and electron‐transport properties of these materials indicate their potential application in flexible nanoelectronics.     

Electronic and structural properties of the (1010) and (1120) ZnO surfaces

The structural and electronic properties of ZnO (1010) and (1120) surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The stability and relaxation effects for both surfaces were analyzed. The electronic and energy band properties were discussed on the basis of band structure as well as density of states. There is a significant relaxation in the (1010) as compared to the (1120) terminated surfaces. The calculated direct gap is 3.09, 2.85, and 3.09 eV for bulk, (1010), and (1120) surfaces, respectively. The band structures for both surfaces are very similar.     

Effect of Pressure on Electronic Structures and Optical Properties of Rocksalt InN

The electronic structures and optical properties of rocksalt indium nitride (InN) under pres-sure were studied using the first-principles calculation by considering the exchange and cor-relation potentials with the generalized gradient approximation. The calculated lattice con-stant shows good agreement with the experimental value. It is interestingly found that the band gap energy E_g at the Γ or X point remarkably increases with increasing pressure, but E_g at the L point does not increase obviously. The pressure coefficient of E_g is calculated to be 44 meV/GPa at the Γ point. Moreover, the optical properties of rocksalt InN were calculated and discussed based on the calculated band structures and electronic density of states.