Using first-principles calculations, we investigate the strain effects on the charge density wave states of monolayer and bilayer 1T-TaS2. The modified stability of the charge density wave in the monolayer is understood in terms of the strain dependent electron localization, which determines the distortion amplitude. On the other hand, in the bilayer, the effect of strain on the interlayer interaction is also crucial. The rich phase diagram under strain opens new venues for applications of 1T-TaS2. We interpret the experimentally observed insulating state of bulk 1T-TaS2 as inherited from the monolayer by effective interlayer decoupling.

We explore the electronic properties of the MnO2/graphene interface by first-principles calculations, showing that MnO2 becomes half-metallic. MnO2 in the MnO2/graphene/MnO2 system provides time-reversal and inversion symmetry breaking. Spin splitting by proximity occurs at the Dirac points and a topologically nontrivial band gap is opened, enabling a quantum anomalous Hall state. The half-metallicity, spin splitting, and size of the band gap depend on the interfacial interaction, which can be tuned by strain engineering.

•We introduce surface deformationinto a surface.•We examine the electronic band gap modulated by surface deformations.•Indirect-to-direct band gap transition occurs due to the surface tensile strain.•Spatial separation of valence band minimum and conduction band minimum of the Si nanowires occurs due to surface strain.Based on the models built with our “cyclic replacement” method we introduced local strain into the (111) facet of the Si 〈112〉 nanowires. With ab initio approach, it is found that the electronic band structures of the nanowires are modulated efficiently by the surface strains: the indirect band gap declines by strong surface compression, while it always decreases and impressively changes to a direct band gap with surface tension. Moreover, the local deformations result in spatial separation of the valence band minimum to the compressed surface and the conduction band minimum to the tensed surface.

The origin of the contrast in noncontact atomic force microscopy (NC-AFM) images, which is interpreted as intramolecular and intermolecular bonds, is still under debate. On the basis of the ab initio approach and explicitly including the tilt effect of the flexible CO tip, we reveal that the outermost electron density of the sample dominates the AFM contrast by corrugating of the repulsive force that determines the frequency shift and the lateral behavior of the flexible tip. Consequently, we find that various aspects of bond images in AFM are governed by features of the electron density residing between nuclei; for example, in a π-conjugated system, the brightness of bonds is similar to that of atoms in AFM images due to the gently undulating π electron density; bright lines can arise between two bonded atoms (e.g., in a hydrogen bond) and also between “nonbonded” atoms (e.g., between two Xe atoms) due to the spatial overlapping of the outermost electrons.

In experiments the reconstructed Si(100) surface shows silicon dimers pointing along the [011] direction. However, the origin of the dimer formation is still unclear. Our theoretical studies on dynamics and energetics show that the reconstruction process depends crucially on the initial local surface morphology: starting from different local tilting scenarios various reconstruction domains can appear as a result of thermal excitation. Molecular dynamics simulations show that c(4 × 2) and asymmetric p(2 × 1) reconstructions can appear quite easily while p(2 × 2) domains are less likely to be found even though they are energetically favorable. The latter is consistent with experimental findings of p(2 × 2) domains being observed quite rarely. The simulations show further that spontaneous dimer-flipping in asymmetric p(2 × 1) domains is possible at about 100 K and driven by the stress within the aligned atoms below the tilted dimers. This can result in a transition to c(4 × 2) reconstruction which is consistent with this reconstruction dominating in experiments above 80 K.

Using first-principles calculations, we investigate the strain effects on the charge density wave states of monolayer and bilayer 1T-TaS2. The modified stability of the charge density wave in the monolayer is understood in terms of the strain dependent electron localization, which determines the distortion amplitude. On the other hand, in the bilayer, the effect of strain on the interlayer interaction is also crucial. The rich phase diagram under strain opens new venues for applications of 1T-TaS2. We interpret the experimentally observed insulating state of bulk 1T-TaS2 as inherited from the monolayer by effective interlayer decoupling.