The quasiparticle (QP) band structures of both strainless and strained monolayer MoS${}_2$ are investigated using more accurate many-body perturbation $\mathit{GW}$ theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent $\mathit{GW}$ ${}_0$ (sc$\mathit{GW}$ ${}_0$) calculation, the predicted optical gap magnitude is in good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. The sc$\mathit{GW}$ ${}_0$ and BSE calculations are also performed on monolayer WS${}_2$, similar characteristics are predicted and WS${}_2$ possesses the lightest effective mass at the same strain among monolayers Mo(S,Se) and W(S,Se). Our results also show that the electron effective mass decreases as the tensile strain increases, resulting in an enhanced carrier mobility. The present calculation results suggest a viable route to tune the electronic properties of monolayer transition-metal dichalcogenides (TMDs) using strain engineering for potential applications in high performance electronic devices.

As an electrical energy storage device, supercapacitor finds attractive applications in consumer electronic products and alternative power source due to its higher energy density, fast discharge/charge time, low level of heating, safety, long-term operation stability, and no disposable parts. This work reviews the recent development of supercapacitor based on carbon nanotubes (CNTs) and their composites. The purpose is to give a comprehensive understanding of the advantages and disadvantages of carbon nanotubes-related supercapacitor materials and to find ways for the improvement in the performance of supercapacitor. We first discussed the effects of physical and chemical properties of pure carbon nanotubes, including size, purity, defect, shape, functionalization, and annealing, on the supercapacitance. The composites, including CNTs/oxide and CNTs/polymer, were further discussed to enhance the supercapacitance and keep the stability of the supercapacitor by optimally engineering the composition, particle size, and coverage.

We report ferromagnetism in carbon-doped ZnO. Our first-principles calculations based on density functional theory predicted a magnetic moment of $2.02{\mu }_B$ per carbon when carbon substitutes oxygen in ZnO, and an ferromagnetic coupling among magnetic moments of the carbon dopants. The theoretical prediction was confirmed experimentally. C-doped ZnO films deposited by pulsed-laser deposition showed ferromagnetism with Curie temperatures higher than 400 K. The measured magnetic moment based on the content of carbide in the films [$(1.5–3.0){\mu }_B$ per carbon] was in agreement with the theoretical prediction. The magnetism is due to the Zn-C system in the ZnO environment.

“Noncompensated $n\mathrm{\text{−}}p$ codoping” is established as an enabling concept for enhancing the visible-light photoactivity of ${\mathrm{TiO}}_2$ by narrowing its band gap. The concept embodies two crucial ingredients: The electrostatic attraction within the $n\mathrm{\text{−}}p$ dopant pair enhances both the thermodynamic and kinetic solubilities, and the noncompensated nature ensures the creation of tunable intermediate bands that effectively narrow the band gap. The concept is demonstrated using first-principles calculations, and is validated by direct measurements of band gap narrowing using scanning tunneling spectroscopy, dramatically redshifted optical absorbance, and enhanced photoactivity manifested by efficient electron-hole separation in the visible-light region. This concept is broadly applicable to the synthesis of other advanced functional materials that demand optimal dopant control.

Two-dimensional materials have various applications in next-generation nanodevices because of their easy fabrication and particular properties. In this work, we studied the effects of edge structures on the edge stability, and electronic and magnetic properties of MoS₂ nanoribbons by first-principles calculations. We predicted that S-terminated zigzag nanoribbons are the most stable even without hydrogen saturation because of their low and negative edge energies, although hydrogen saturation of the edge states can stabilize other nanoribbons with different edge structures. MoS₂ zigzag nanoribbons are metallic and ferromagnetic. Importantly, their conductivity may be semiconducting (n- or p-type) or half metallic by controlling the edge structures saturated with H atoms. The magnetic states of the MoS₂ zigzag nanoribbons are enhanced by H-saturation and are much stronger than those of graphene zigzag nanoribbons. The armchair nanoribbons are semiconducting, with bandgaps increased by the hydrogen saturation of their edge states, and are nonmagnetic. These MoS₂ nanoribbons with versatile functions may have applications in spintronics, nanodevices, and energy harvesting.