Property Modulation Within Low Dimensional Semiconductors By Local Strain

Local strain is ubiquitous in nanomaterials due to heterointerface or local deformations while the details remain unclear. In this work, first-principles calculations are performed to investigate the structural and electronic properties of Si/Ge nanowires and layered (monolayer and bilayer) T-phase TaS2 under various strains. With our "cyclic replacement" method we applied surface strains into the (111) facet of Si/Ge<112> nanowires (1D) and applied various isotropic strains on the layered TaS2 nanosheet (2D) by changing the lattice constant, respectively. For the Si nanowires we found only strong surface compression results in band gap decline, while tensile strain always leads to decrease of band gap and impressively indirect-to-direct band gap transition. The local surface strain can result in spatial separation of valence band minimum maximum to the compressed surface and conduction band minimum to the tensed surface. And surface strain applied on the Ge nanowires shows a quadratic elastic relation to the deformation energy. It is found that the compressive strains hardly change the electronic band gap which implies electronic properties of Ge in the Ge/Si core-shell structures are almost preserved. On the other hand, the tensile strains reduce the gap efficiently and even result in spatial separation of valence band maximum and conduction band minimum. On the other hand, when monolayer and bilayer T-phase TaS2 are introduced various isotropic strains, the energetically preferred charge-density wave states are weakened and enhanced by compressive and tensile strains, respectively. Monolayer TaS2 is spin polarized, however, the band gap in one spin channel monotonically decreases to 0 eV from compression to tension, while the other channel remains semiconducting with a maximum band gap at around 3%, showing half-metallic features. When two monolayers form bilayer, the system becomes nonmagnetic semiconducting with two localized electronic states above and below the Fermi level. The band gaps decrease more rapidly under tensile strains than that under compressive strains, leading to a semiconductor-metal transition at tensile strain. These findings offer a feasible route to stabilize or tune related properties of the Si/Ge nanowires and the T-phase in TaS2 in CDW (charge-density wave states) and also offer a possible an effective way to look for and engineer stable and efficient nanoelectronics devices.