RESUMO
In this work, we present a theoretical study of the electronic band structure and the Young's modulus of hydrogen-passivated silicon nanowires (H-SiNWs), grown along the [110] crystallographic direction, as a function of the concentration of interstitial sodium (Na) and lithium (Li) atoms. The study is performed using the supercell scheme and the density functional theory (DFT), within the local density approximation (LDA). The results show that the presence of Na or Li atoms closes the former semiconducting band gap of the H-SiNWs and shifts the Fermi energy into the conduction band. The transition from semiconductor to metal occurs as soon as a single Na or Li atom is added to the nanowire and the number of occupied states near the Fermi level is larger for the H-SiNWs with Li atoms in comparison with those nanowires with the same concentration of Na atoms. The calculated formation energies reveal that the system becomes less stable when the concentration of Na and Li atoms augments. Moreover, the obtained binding energies indicate that Si-Li and Si-Na bonds are formed. It is worth mentioning that the binding energies of H-SiNWs with interstitial Li atoms are larger than those corresponding to the H-SiNWs with interstitial Na atoms. On the other hand, the Young's moduli of H-SiNWs with Na atoms are lower than those of pure H-SiNWs and their values diminish when the concentration of Na atoms increases. In contrast, Young's moduli of H-SiNWs present a non-monotonic behavior as a function of the concentration of interstitial Li atoms and for the largest studied concentration the nanowire fractures. These results give insight into the changes that electronic and mechanical properties of H-SiNWs suffer during the charge-discharge process, which should be taken into account in the design of electrodes of Na or Li-ion batteries.
RESUMO
This work studied the phonon confinement effects at the low temperature specific heat of Si nanowires from first principles using density functional perturbation theory. The nanowires were modeled in the [0 0 1] direction for three different diameters, with the largest cross section being approximately 10 Å. The results indicate the specific heat can be described at low temperatures using a third-grade polynomial of the form c v = λT + ßT 2 + γT3, where the coefficients of quadratic and cubic terms are almost nonexistent for small diameters. These terms begin to have relevance at larger diameters. Further analysis shows λ > ß > γ, which shows the phonon confinement (λ) and surface atoms (ß) become more important than the volumetric contribution (γ) for ultrathin nanowires at low temperatures.
RESUMO
Theoretical studies on the effect of Li on the electronic properties of porous silicon are still scarce; these studies could help us in the development of Li-ion batteries of this material which overcomes some limitations that bulk silicon has. In this work, the effect of interstitial and surface Li on the electronic properties of porous Si is studied using the first-principles density functional theory approach and the generalised gradient approximation. The pores are modeled by removing columns of atoms of an otherwise perfect Si crystal, dangling bonds of all surfaces are passivated with H atoms, and then Li is inserted on interstitial positions on the pore wall and compared with the replacement of H atoms with Li. The results show that the interstitial Li creates effects similar to n-type doping where the Fermi level is shifted towards the conduction band with band crossings of the said level thus acquiring metallic characteristics. The surface Li introduces trap-like states in the electronic band structures which increase as the number of Li atom increases with a tendency to become metallic. These results could be important for the application of porous Si nanostructures in Li-ion batteries technology.