Narrow Results

A comprehensive investigation of the unexplored mechanical, electronic, Mulliken bond population, vibrational, optical and thermophysical properties of the synthesized compounds NbIn$X_2$ (X = S, Se) have been made for the first time using the density functional theory. The chemical, mechanical and dynamical stabilities of the compounds are established in our calculations. Both compounds are soft, machinable and brittle. The anisotropic nature of the studied compounds is shown by 3D representations of elastic moduli. The density of states and electronic band structure demonstrate that the compounds are metallic. Fermi surfaces of both compounds are almost similar and contain both hole- and electron-like topologies. The characteristics of chemical bonding among different atoms of the compounds are studied via a charge density distribution map and bond population analysis. Both the compounds possess optical anisotropy. Reflectivity is high (above 44%) in the IR–visible–UV region indicating that the phases may be effective in reducing solar heat. Minimum thermal conductivity, $k_{min}$ (used to select appropriate material for thermal barrier coating) and its anisotropy are calculated for the first time. The results show that both compounds have $k_{min}$ much smaller than the reference value of 1.25Wm${}^{-1}$K${}^{-1}$.

The structural, elastic and electronic properties of A₂C₂ (A = Li, Na, K, Rb and Cs) at zero temperature were investigated by first-principles total energy calculations. The optimized equilibrium structural parameters agree well with available experimental values. Elastic constants, bulk modulus, Young's modulus and Poissons ratio were given. All the structures studied are stable mechanically and all stable A₂C₂ studied has strong compressibility, which originates from weak Coulomb repulsion between metal atoms and carbon atoms. The electronic structure calculations show that binary alkali metal carbides studied here are insulators.

We revisit the problem of the electronic properties of a single strand of DNA, formulating the Hückel approximation for π-electrons in both the sugar-phosphate backbone chain and the π-stacking of nitrogenous bases in a single strand of DNA where the nitrogenous bases are adenine (A), guanine (G), cytosine (C) and thymine (T), respectively. We calculate the electronic band structure of π-electrons: (i) in the single nitrogenous base molecules such as A, G, C and T, (ii) in the single sugar-phosphate molecule, (iii) in the single nucleotide systems such as A, G, C, T with the single sugar-phosphate group, and (iv) in the system of a single strand of DNA with an infinite repetition of a nucleotide such as A, G, C and T, respectively. We find the following: In the case of (i), there is an energy gap between the energy levels for the HOMO and LUMO in the nitrogenous base. This guarantees the semiconducting character of the bases as a mother material. In the case of (ii), there are the HOMO localized at the oxygen site with a double bond and the LUMO localized around the phosphorus atom, which have a quite large energy gap. In the case of (iii), the energy levels for the HOMO and LUMO of the nitrogenous base remain almost the same as those of the nucleotide, while those of the sugar-phosphate group remain the same as well. The HOMO of the sugar-phosphate group exists right below the HOMO of the nitrogenous base. Therefore, comparing the energy levels for the HOMOs of the nitrogenous base group with those of the sugar-phosphate group, the nitrogenous base group behaves as a donor while the sugar-phosphate group behaves as an acceptor. In the case of (iv), there are energy bands and band gaps for the extended states in the nitrogenous base group and the sugar-phosphate group as well as the discrete levels for the localized states at the phosphate site in the spectrum. There is a transition from semiconductor to semimetal as the π-electron hopping between the nitrogenous bases of nucleotide is increased. The details of the above will be discussed in the present paper. Thus, we show the powerfulness of the Hückel theory in the study of DNA as well, although this theory is, at the first glance, oversimplified and purely phenomenological.

The spin–curvature interaction (SCI) and its effects are investigated based on curved Dirac equation. Through the low-energy approximation of curved Dirac equation, the Hamiltonian of SCI is obtained and depends on the geometry and spinor structure of manifold. We find that the curvature can be considered as field strength and couples with spin through Zeeman-like term. Then, we use dimension reduction to derive the local Hamiltonian of SCI for cylinder surface, which implies that the effective Hamiltonian of single-wall carbon nanotubes results from the geometry and spinor structure of lattice and includes two types of interactions: one does not break any symmetries of the lattice and only shifts the Dirac points for all nanotubes, while the other one does and opens the gaps except for armchair nanotubes. At last, analytical expressions of the band gaps and the shifts of their positions induced by curvature are given for metallic nanotubes. These results agree well with experiments and can be verified experimentally.

Geometric and electronic structures of W${}_m$Mo${}_n$ (m + n$\le$ 7) clusters have been systematically calculated by density functional theory (DFT) at the generalized gradient approximation (GGA) level for ground-state structures. Geometry optimization shows that clusters are almost bipyramid structures with m + n$>$ 4. E${}_b$ of clusters is mainly dominated by W atoms. And the substitution of atoms between W and Mo in Mo${}_n$ or W${}_n$ (n$\le$ 7) clusters enhances the stability of the original clusters. The calculated IE shows that W${}_{1,3,5}$Mo, W${}_{1,3,5}$Mo₂, W${}_{1,2,3}$Mo₃ and WMo${}_{4,5}$ are relatively more stable in the chemical reaction. In addition, the magnetism of clusters mainly comes from valance d orbitals.

The geometries, stabilities and electronic properties of Al${}_n$Pd${}_m$ (n = 1–10, m = 1, 2) have been systematically investigated by using the DFT method at B3PW91/GENECP level. The optimized results indicate that the lowest-energy structures of Al${}_n$Pd clusters prefer to form three-dimensional (3D) structures and the Pd atom occupies a peripheral position of Al${}_n$Pd clusters. The most stable Al${}_n$Pd₂ clusters can be obtained by adding one Al atom to the most stable structure of Al${}_{n-1}$Pd₂ clusters except for n = 7 and 10. The two Pd atoms are found to occupy the exclusive surface sites. The analysis of stabilities reveals that Al₃Pd${}_m$ and Al₇Pd${}_m$ clusters are more stable than their neighbors. The doping of Pd atoms enhances the stabilities of aluminum clusters. The charges always transfer from Al atoms to Pd atoms in Al${}_n$Pd${}_m$ clusters. There exists strong spd orbital hybridization between Pd and Al. The results of polarizability imply that the nucleus and electron cloud of these clusters are easily affected by the external field and the nonlinear optical effect of Al${}_n$Pd and Al${}_n$Pd₂ clusters is enhanced with the increase of cluster size.

Geometric and electronic structures of W${}_m$Cu${}_n$H₂ (m + n$\le$ 7) clusters have been systematically calculated by density functional theory (DFT) at the generalized gradient approximation (GGA) level for ground-state structures. For all W–Cu clusters, H atoms prefer to attach to W atoms in this system during adsorption. And more electrons transfer from H atom to W atom with the growth of the size of the cluster which benefits the hydrogen storage. Analysis of stability properties and electronic properties shows that hydrogen adsorption and dissociation process take place more efficiently at the W₂Cu₃H₂ cluster than the others. Due to high thermodynamic stability and adsorption energy of W₅CuH₂ cluster among W${}_m$Cu${}_n$H₂ (m + n$\le$ 7) clusters, W₅Cu is more suitable for hydrogen storage.

The elastic constants and phonon dispersion of metallic C${}_{14}$ are calculated by first-principles calculations. The results show that the metallic C${}_{14}$ is mechanically and dynamically stable under high pressure. The variations of G/B ratio, Poisson’s ratio, elastic anisotropy, acoustic velocity and Debye temperature at the pressure range from 0 GPa to 100 GPa are analyzed. The results reveal that by adjusting the pressures the elastic anisotropy and thermodynamic properties could be improved for better applicability.

The effect of strain on the electronic properties of BC₃ sheet was studied by using first-principles density functional theory. It is found that the band gap of BC₃ sheet increases gradually when the applied tensile strain ranges from 0% to 12.5%. While the band gap decreases as the compressive strain is applied, especially resulting in the semiconductor-metal transition at some strain. Further analysis shows that the change of band gap mainly results from the variation of the energy of valence band maximum (VBM), which is related to the strength of the bonding state. The proposed mechanical control of the electronic properties will widen the application of BC₃ sheet in future nanotechnology.

Combining advanced transmission electron microscopy with high-precision first-principles calculations, the properties of Si(111)//6H-SiC(0001) (Si-terminated and C-terminated) heterojunction interface, such as work of adhesion, geometry property, electronic structure and bonding nature, are studied. The experiments have demonstrated that interfacial orientation relationships of Si(111)//6H-SiC(0001) heterojunction are $\mathrm{\text{Si[2-1-1]}}/\mathrm{\text{6H}}$-$\mathrm{\text{SiC}}[10\bar{1}0]$ and Si(111)/6H-SiC(0001). Compared with C-terminated interface, Si-terminated interface has higher adhesion and less relaxation extent.

Exploring the next generation of invincible energy materials with fascinating properties is vital in the challenge of energy crisis. In this paper, we extract T-carbon as a potential candidate and have an insight into the electronic and optical properties by means of first principles. It is found that S1 position doping system is relatively more stable with the formation energy of 0.323 eV, and has the smallest bandgap value of 1.228 eV. Charge density difference maps show that the electron loss is obvious near the S atom and the covalent bond is weakened. The population analysis shows that the S atom will obtain electrons through competition, while the Se atom will lose electrons. Additionally, the peak values of $𝜀$2 in the doped systems decrease significantly, especially for S2 doping system, indicating that S2 doping can effectively improve the service life in related devices. Compared with the instinct system, the absorption coefficient is lower in the UV region and greater in the visible region. The peak of energy loss spectrum reduces after doping, especially for Se1 doping. The results provide a theoretical basis for the industrial application of T-carbon in the energy microdevice fields.

In this work, we have studied the electronic properties of SnO₂ by employing the density functional theory. The aim of the work is to study the comparative effect of Te doping in SnO₂ in Sn and O sites. The CASTEP module is used for the simulation. $2\times 2\times 2$ lattice of SnO₂ was used for the study of the band structure and density of state. The electronic properties change significantly on doping the sample with Te. Also, when Te is doped in different quantities and at different sites in SnO₂, the bandgap is overlapped in 0.75% Te doping at O site and the maximum is found to be 0.587 eV in 0.75% Te doping at Sn site. For pure SnO₂, the bandgap is 1.064. Hence SnO₂ when doped with Te influences the conductivity.

The electronic and optical properties of Mn-doped 3C-SiC films are investigated by the first-principles calculation. The structure of Mn-doped 3C-SiC is modeled by substituting Mn atom for C or Si atom in 3C-SiC lattice. The results suggest that Mn-C and Mn-Si bonds can exist in the Mn-doped 3C-SiC. Mn location in 3C-SiC lattice significantly affects the crystal structure of Mn-doped 3C-SiC, and the Mn atom substitution for C or Si sites of 3C-SiC lattice can induce to the difference of indirect or direct band structure. The calculated results also show that some new impurity energy levels occur in the band gap of Mn-doped 3C-SiC, and the imaginary part of dielectric function of Mn-doped 3C-SiC shifts toward the infrared region in comparison with the primitive 3C-SiC. The adsorption spectrum of Mn-doping 3C-SiC, due to the transition of electrons between Mn 3d states, presents some new prominent peaks at low frequency. These results can further confirm Mn-doped 3C-SiC to act as a potential material for optical applications.

The structure stability and electronic properties of Cu_mCo_nCO ($m+n=2$–7) clusters have been systematically investigated using density functional theory (DFT) within the generalized gradient approximation (GGA). The results indicate that the ground state structures of Cu_mCo_nCO clusters obtained by adsorbing CO molecules on the top sites of stable Cu_mCo_n clusters with C atoms and CO molecules have been activated during adsorption process. Cu₂CO, CuCoCO, Cu₃CoCO, Co₄CO, Cu₄CoCO and Cu₃Co₃CO clusters are stronger than other ground state clusters in thermodynamic stability. Cu₂CO, Cu₄CO and Cu₆CO clusters show stronger chemical stability; Co₂CO, Co₄CO, Cu₅CoCO, Cu₃Co₃CO, Cu₂Co₅CO and Co₇CO clusters show better propensity to adsorb CO for these clusters have larger adsorption energies; Electronic states of Cu₂Co₃CO, CuCo₄CO, Co₅CO, Cu₄Co₃CO, Cu₃Co₄CO, CuCo₆CO and Co₇CO clusters are mainly influenced by those of 3d orbitals in Co and Cu atoms, the contribution to total magnetic moments of these clusters comes mainly from Co atoms and these clusters have high magnetism.

Geometric and electronic properties of nitric oxide adsorption on W_mMo_n ($m+n\le$ 6) clusters have been systematically calculated by density functional theory (DFT) at the generalized gradient approximation (GGA) level for ground-state structures. NO molecule prefers top site with nitrogen-end bridging a tungsten atom for W${}_{1,2}$Mo${}_{1,2,3}$ and W₃Mo₂ clusters. While NO tends to locate on the hollow site for WMo₅, W₂Mo₄ and W₃Mo₃ clusters, and dissociation of NO molecule happens on W₃Mo, N–O bond lengths expand in accordance with the variation of adsorption energy with the increasing number of tungsten atoms, originating from metal $\to {\pi }^{\ast }$ back-donation. Electron transfer occurs among 4d state of Mo, 5d state of W, 2p state of N and 2p state of O.

The geometrical structures, stability and electronic properties of Pt_mIr_n and Pt_mIr_nNO ($m+n=8-10$) clusters have been studied by using density functional theory (DFT). Simple cube evolution pattern is revealed for Ir${}_{8-10}$ and Ir-rich clusters. The curve of the second-order energy difference of Pt_nIr${}_{(9-n)}$ clusters shows the evident even–odd oscillations, indicating that PtIr₈, Pt₃Ir₆, Pt₅Ir₄ and Pt₇Ir₂ clusters are more stable than their neighbors. The minimum excess energies values are seen for Pt₇Ir, PtIr₈ and Pt₂Ir₈, which means that these clusters show the stronger mixing tendency. Analysis of magnetic properties shows that the total magnetic moment contributed from Ir and Pt atoms, and the magnetic moment mainly comes from localization of the d orbit electron. All ground-state structures show an adsorption of NO at the top site of the bare cluster via the $N$ atom. In all of the alloy clusters, NO molecule prefers to be adsorbed near the Ir atom site. Fukui function is an accurate predictor of the reactivity on a specific adsorption site in Pt–Ir alloy clusters.

An impact of positions of Te atoms substituting W atoms in two-dimensional WS₂/WSe₂ heterostructures on their electronic properties is investigated by theoretical simulation. The substitution of W by Te tends to reduce the energy band gap and can lead to metallic properties depending on the impurity position and concentration.

Structural and electronic properties of Sr(N₃)₂ under pressure up to 120 GPa are studied by means of SIESTA calculation. The pressure–angle as well as the cell parameters relation respect to pressure is employed to study the structural changes under pressure. The obtained N–N bond length at zero pressure is in agreement with the other works. The energy band gap takes on the trend of decreasing below 20 GPa and this trend could result in the reduction of the stability for Sr(N₃)₂ crystal, but at 30 GPa it increases suddenly. And polymorphic transformation is observed. The ionic configuration for Sr(N₃)₂ in the fundamental state is estimated to be Sr^+1.200N^-0.200. The charge density of N atom is more sensitive to pressure variation than that of Sr atom.

The dye molecules behaving as photosensitizers in dye-sensitized solar cells are the most critical factors to determine the power conversion efficiency. Therefore, ways to design dye molecules with excellent photoelectric properties has been the focus of dye-sensitized solar cells research. Here, we selected four representative different metal-corrole monomers to characterize their structures and photoelectronic properties. Then based on these metal-corrole monomers, six different architectures of metal-dicorroles were designed by varying the linking forms. The most stable architecture $a$ was screened out by binding energy calculations. A further two types of metal-dicorrole-based dyes were constructed by incorporating different bridge groups with the cyanoacry acceptor into the stable metal-dicorroles. A large number of density functional theory calculations and photoelectric properties analysis indicate that among these different metal-dicorrole-based dyes, Ga-dicorrole dyes have two strong and wide absorption bands in the visible region corresponding to Soret and Q bands, respectively, and have high charge separation efficiency under optical excitation. Especially for Ga-SN dye, by incorporating a $\pi$-bridge-conjugated group, its Soret absorption band is greatly enhanced, broadened and red-shifted, resulting in its merging with the Q band into one absorption band. Moreover, its charge transfer efficiency is up to 76.86%, which will facilitate its coupling with semiconductor materials and transfer its electrons to the semiconductor materials.