Ab initio density functional theory investigation of Li-intercalated silicon carbide nanotube bundles

We present the results of ab initio density functional theory calculations on the energetic, and geometric and electronic structure of Li-intercalated (6,6) silicon carbide nanotube (SiCNT) bundles. Our results show that intercalation of lithium leads to the significant changes in the geometrical structure. The most prominent effect of Li intercalation on the electronic band structure is a shift of the Fermi energy which occurs as a result of charge transfer from lithium to the SiCNTs. All the Li-intercalated (6,6) SiCNT bundles are predicted to be metallic representing a substantial change in electronic properties relative to the undoped bundle, which is a wide band gap semiconductor. Both inside of the nanotube and the interstitial space are susceptible for intercalation. The present calculations suggest that the SiCNT bundle is a promising candidate for the anode material in battery applications.     

The stability and electronic properties of wurtzite and zinc-blende ZnS nanowires

The stability and electronic properties of the pristine wurtzite (WZ) and zinc-blende (ZB) structural ZnS nanowires are investigated and compared by using first-principles approaches. It shows that the WZ-ZnS nanowire is more stable energetically than the ZB-ZnS nanowire. The two kinds of ZnS nanowires have different electronic properties due to both the quantum confinement effect and the surface effect. The band gaps of pristine WZ nanowires become larger than that of the corresponding bulk ZnS, while those of ZB nanowires are smaller. The electronic properties of the hydrogen-passivated WZ-ZnS and ZB-ZnS nanowires are further calculated. The underlying physical reason for their energetic and electronic structures is elucidated.     

Infrared absorption spectra of few-layer graphenes studied by first principles calculations

The infrared (IR) absorption spectra of the undoped, the hole- and electron-doped few-layer graphene (FLG) with layer number of N=1,2,3 have been calculated using the density functional theory in the local density approximation. It is found that in contrast with the featureless optical spectrum of the undoped monolayer graphene, the undoped AB-stacking bilayer and trilayer graphenes exhibit interesting rich IR spectra, e.g., the peaks and jumps in their IR spectra, which are caused by the coupling between different layers. And clear characteristic peaks, lying at different energies, exist in the IR spectra of the hole- or electron-doped bilayer and trilayer graphenes due to the asymmetrical band structures. Beside, based upon their different IR spectra, a powerful experimental tool has been proposed to identify accurately the layer number and doping type of the FLGs.     

Strain Effects on Electronic Properties of Boron Nitride Nanoribbons

We investigate the strain effects on the electronic properties of boron nitride nanoribbons （BNNFts） by using firstprinciples calculations. The results show that the energy gap of BNNRs with both armchair edges （A-BNNRs） and zigzag edges （Z-BNNFts） decreases as the strain increases. As strain increases, the energy gaps of Z-BNNRs decrease rapidly as the width increases and reduce significantly to small values, which makes Z-BNNRs change from wide-gap to narrow-gap semiconductors.     

New template for metal decoration and hydrogen adsorption on graphene-like C3N4

From density functional theory calculations we identify a graphene-like C₃N₄ (g-C₃N₄) as an excellent template for stable and well dispersed decoration of alkali (Li) and 3d transition metal (TM) atoms. The porous sites of g-C₃N₄ accommodate excessive N lone-pair electrons and promote hybridization between the orbitals of N and the metal atoms. The most stable TM decorations (Ti and Sc) on g-C₃N₄ exhibit high capacities of hydrogen adsorption with binding energies suitable for mobile applications. These metal decorated g-C₃N₄ may also prove useful in catalytic and sensing applications for their unique nanoscale structural features unavailable in conventional nano-clusters.     

Effect of S Substitution for P Point Defects in KDP Crystals： First-Principles Study

The electronic structure and geometric distribution of phosphor replaced by sulfur in potassium dihydrogen phosphate （KDP） are investigated by first-principles calculations. The point defect narrows down the energy gap to about 4.9eV, corresponding to a two-photon absorption of 355nm after correction. This can explain the decrease of the laser damage resistance in KDP crystals. Moreover, the defects twist the crystal structure and weaken bonds, especially the O-H bonds, so these bonds may be the first sites to crack under laser irradiation.     

First-principles study of arsenic impurity clusters in molecular beam epitaxy (MBE) grown HgCdTe

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

Absorption spectra of germanium nanocrystals

Absorption spectra of hydrogenated germanium nanocrystals are calculated in real space using ab initio pseudopotentials constructed within the local density approximation. Nanocrystals with over 800 atoms are considered. We find the calculated spectra are essentially bulk-like for systems with ∼250 atoms or more. The energy positions for the E₁ and E₂ reflectivity peaks are blue-shifted with respect to their bulk values owing to quantum confinement. Comparisons with available experimental data reveal good agreement for the energies of the E₁ peak, whereas the energy position of the E₂ peak is underestimated by a constant shift, which is independent of the nanocrystal size. This shift is consistent with local density calculations for bulk germanium.     

Exciton States and Linear Optical Spectra of Semiconducting Carbon Nanotubes under Uniaxial Strain

Considering the exciton effect, the linear optical spectra of semiconducting single-walled carbon nanotubes （SWNTs） under uniaxial strain are theoretically studied by using the standard formulae of Orr and Ward [Mol. Phys. 20（1971）513]. It is found that due to the wrapping effect existing in the semiconducting zigzag tubes, the excitation energies of the linear optical spectra show two different kinds of variations with increasing uniaxial strain, among which one decreases such as tube （11,0）, and the other increases firstly and then decreases such as tube （10,0）. These variations of the linear optical spectra are consistent with the changes of the exciton binding energies or the （quasi）continuum edge of these SWNTs calculated in our previous work, which can be used as a supplemented tool to detect the deformation degree of an SWNT under uniaxial strain.     

Broadening of Photoluminescence by Nonhomogeneous Size Distribution of Self-Assembled InAs Quantum Dots

The photoluminescence spectrum （PL） of InAs quantum dots （QDs） at 80 K is studied by comparison between the theoretical calculation and experimental measurement. The Gaussian line shape is used to approximate the size distribution of QDs. Its mean volume and the standard full width at half maximum （FWHM） of the PL spectrum. size deviation are well correlated with the peak and The experimental PL spectrum is well reproduced by the theoretical model based on the effect mass approximation including the size distribution without any adjustable parameters. Compared with the standard size deviation value σ = 9 × 10^-2 determined by atomic force microscopic method a small value σ = 7 × 10^-2 is obtained by the best fitting process from the measured and calculated PL spectra.     

First principles study of Eu doped carbon nanotubes

The geometric and electronic structures of Eu doped single-walled carbon nanotubes (SWCNTs) have been studied using density functional theory. Three different doping configurations are considered. All of these configurations are stable upon relaxation, and Eu atom on the top of the inside hole site is the most favorable configuration for most nanotubes, except (3,3) CNT. The formation energies vary regularly with the same trend as in the Co and Fe doped cases. The electronic structures studies indicate that the charge transfer basically occurs between 5d6s of Eu and the antibonding orbital of the C6 ring of the SWCNT. Eu atom is monovalent for the exohedral and substitutional doping, and for the endohedral doping of large radius nanotubes; it is bivalent for endohedral doping of (3,3) tube. As the radius increases, the net charges on Eu atom steadily decrease for exohedral and endohedral doping. The magnetic moments of Eu atoms are preserved in all of the configurations, but they vary with the radius of nanotube and adsorbing sites.     

Photoluminescence of crystalline and disordered BTO:Mn powder: Experimental and theoretical modeling

Disordered and crystalline Mn-doped BaTiO₃ (BTO:Mn) powders were synthesized by the polymeric precursor method. After heat treatment, the nature of visible photoluminescence (PL) at room temperature in amorphous BTO:Mn was discussed, considering results of experimental and theoretical studies. X-ray diffraction (XRD), PL, and UV-vis were used to characterize this material. Rietveld refinement of the BTO:Mn from XRD data was used to built two models, which represent the crystalline BTO:Mn (BTO:Mn_c) and disordered BTO:Mn (BTO:Mn_d) structures. Theses models were analyzed by the periodic ab initio quantum mechanical calculations using the CRYSTAL98 package within the framework of density functional theory at the B3LYP level. The experimental and theoretical results indicated that PL is related with the degree of disorder in the BTO:Mn powders and also suggests the presence of localized states in the disordered structure.     

Effective field theory for dilute fermions with pairing

Effective field theory (EFT) methods for a uniform system of fermions with short-range, natural interactions are extended to include pairing correlations, as part of a program to develop a systematic Kohn-Sham density functional theory (DFT) for medium and heavy nuclei. An effective action formalism for local composite operators leads to a free-energy functional that includes pairing by applying an inversion method order by order in the EFT expansion. A consistent renormalization scheme is demonstrated for the uniform system through next-to-leading order, which includes induced-interaction corrections to pairing.     

First-principles supercell calculations for simulating a shallow donor state in Si

We present first-principles simulations of As-doped Si carried out using several cubic supercells of up to 10 648 atoms. The 1s As donor level in each supercell splits into three states, which have A₁, T₂, and E symmetries, respectively. The 1s(A₁) wavefunction is well converged in the largest cell, and its spread is close to those of the effective-mass theories. However, the calculated binding energies are smaller than experimental values. This discrepancy would be due to the self-interaction error within the approximated exchange-correlation density functional used in this calculation. Therefore, we also show perturbative calculations based on an impurity potential without the self-interaction error to estimate the binding energies of the 1s(A₁) donor state. The estimated binding energy in the largest supercell agrees well with the experimental value.     

Hexagonal silicon nanotube confined inside a carbon nanotube: A first-principles study

We studied the stability, geometrical structures and electronic energy band of hexagonal silicon nanotube (SiNT) confined inside carbon nanotubes based on first-principle calculations. The results show that the encapsulating process of SiNT is exothermic in (9,9) carbon nanotube while endothermic in (8,8) and (7,7) carbon nanotubes. When the SiNT is inserted into (9,9) carbon nanotube, the insertion energy is about 0.09 eV. Energy band of SiNT@(9,9) nanotube is not distorted greatly compared with the superposition of bands of isolated SiNT and (9,9) carbon nanotube. Especially, a parabolic band occurs near the Fermi level of energy band in SiNT@(7,7) nanotube. Such a band could be a nearly free electronic state originating from carbon nanotube. Moreover, we discuss the variation of total energy as the SiNT rotates around its axis inside carbon nanotubes.     

Structural, electronic and optical properties of AgI under pressure

We report results of first-principles total-energy calculations for structural properties of the group I-VII silver iodide (AgI) semiconductor compound under pressure for B1 (rocksalt), B2 (cesium chloride), B3 (zinc-blende) and B4 (wurtzite) structures. Calculations have been performed using all-electron full-potential linearized augmented plane wave plus local orbitals FP-LAPW + lo method based on density-functional theory (DFT) and using generalised gradient approximation (GGA) for the purpose of exchange correlation energy functional. In agreement with experimental and earlier ab initio calculations, we find that the B3 phase is slightly lower in energy than the B4 phase, and it transforms to B1 structure at 4.19 GPa. Moreover, we found AgI has direct gap in B3 structure with a band gap of 1.378 eV and indirect band gap in B1 phase with a bandgap around 0.710 eV. We also present results of the effective masses for the electrons in the conduction band (CB) and the holes in the valence band (VB). To complete the fundamental characteristics of this compound we have analyzed their linear optical properties such as the dynamic dielectric function and energy loss function for a wide range of 0-25 eV.     

Coulomb interactions of massless Dirac fermions in graphene; pair-distribution functions and exchange-driven spin-polarized phases

The quasi-2D electrons in graphene behave as massless fermions obeying a Dirac-Weyl equation in the low-energy regime near the two Fermi points. The stability of spin-polarized phases (SPP) in graphene is considered. The exchange energy is evaluated from the analytic pair-distribution functions, and the correlation energies are estimated via a closely similar four-component 2D electron fluid which has been investigated previously. SPPs appear for sufficiently high doping, when the exchange energy alone is considered. However, the inclusion of correlations is found to suppress the spin-phase transition in ideal graphene.     

Codoping of Potassium and Bromine in Carbon Nanotubes： A Density Functional Theory Study

We investigate the co-doping of potassium and bromine in single-walled carbon nanotubes （SWCNTs） and doublewalled carbon nanotubes （DWCNTs） based on density functional theory. In the co-doped （6,0） SWCNTs, the 4s electron of potassium is transferred to nanotube and Br, leading to the n-type feature of SWCNTs. When potassium is intercalated into inner tube and bromine is put on outer tube, the positive and negative charges reside on the outer and inner tubes of the （7,0）@（16,0） DWCNT, respectively. It is expected that DWCNTs would be an ideal candidate for p-n junction and diode applications.     

First-Principles Studies on Properties of Boron-Related Impurities in c-BN

We investigate, by first-principles calculations, the pressure dependence of formation enthalpies and defective geometry and bulk modulus of boron-related impurities （VB, Cs, NB, and OB） with different charged states in cubic boron nitride （c-BN） using a supercell approach. It is found that the nitrogen atoms surrounding the defect relax inward in the case of CB, while the nitrogen atoms relax outward in the other cases. These boron-related impurities become much more stable and have larger concentration with increasing pressure. The impurity CB^＋1 is found to have the lowest formation enthalpy, make the material exhibit semiconductor characters and have the bulk modulus higher than ideal c-BN and than those in the cases of other impurities. Our results suggest that the hardness of c-BN may be strengthened when a carbon atom substitutes at a B site.     

Artificial Rydberg atom

We analyze bound states of an electron in the field of a positively charged nanoshell. We find that the binding and excitation energies of the system decrease when the radius of the nanoshell increases. We also show that the ground and the first excited states of this system have remarkably the same properties of the highly excited Rydberg states of a hydrogen-like atom, i.e., a high sensitivity to the external perturbations and long radiative lifetimes.