Origin of p-type conductivity in a WSe2 monolayer.

Transition-metal dichalcogenides have promising potentials for high-performance electronic and optoelectronic applications, which could be deeply influenced by defects, including native defects and dopants. Experiments to date have frequently reported p-type conductivity in the WSe2 monolayer, but the origin remains elusive. Here, using the first-principles calculations, we systematically investigate the point defects in the WSe2 monolayer and show that: (1) no intrinsic point defect is responsible for the p-type doping; (2) hydrogen interstitials (Hi) are possible sources for n-type conductivity; (3) oxygen substitution of Se (OSe) can greatly promote the formation of adjacent W vacancy (VW), and finally make VW relatively shallow acceptors by forming the defect complex nOSe + VW (n = 1 to 6). Our work reveals that nOSe + VW is the origin of the p-type conductivity in the unintentionally doped WSe2 monolayer, given that O is present throughout the synthesis conditions of WSe2.

Tuning magnetocrystalline anisotropy by controlling the orbital electronic configuration of two-dimensional magnetic materials.

A suitable magnetic anisotropy energy (MAE) is a key factor for magnetic materials. However, an effective MAE control method has not yet been achieved. In this study, we propose a novel strategy to manipulate MAE by rearranging the d-orbitals of metal atoms with oxygen functionalized metallophthalocyanine (MPc) by first-principles calculations. By the dual regulation of electric field and atomic adsorption, we have achieved a substantial amplification of the single regulation method. The use of O atoms to modify the metallophthalocyanine (MPc) sheets effectively adjusts the orbital arrangement of the electronic configuration in the d-orbitals of the transition metal near the Fermi level, thereby modulating the MAE of the structure. More importantly, the electric field amplifies the effect of electric-field regulation by adjusting the distance between the O atom and metal atom. Our results demonstrate a new approach to modulating the MAE of two-dimensional magnetic films for practical application in information storage.

Enhancing Hole Density and Suppressing Recombination Centers through Illumination in Kesterite Thin Film Solar Cells.

Enhancing carrier density and increasing carrier lifetime are critical for the good performance of thin film solar cells. We apply illumination during the growth of kesterite Cu2ZnSnS4 (CZTS) to enhance hole density and suppress defects of nonradiative electron-hole recombination centers simultaneously. To examine the effect of the injected carriers generated by illumination, we first extend the scheme of detailed balance equations relating free carriers and defects beyond thermal equilibrium conditions by developing an extended Fermi level (EF') to characterize a homogeneous semiconductor with non-equilibrium carriers. On the basis of this scheme, we find that illumination can promote the formation of carrier-providing defects and suppress the formation of carrier-compensating defects. Then, we demonstrate that applying proper illumination during the growth of CZTS will help achieve a higher hole density and simultaneously suppress the formation of the SnZn antisite significantly, which are beneficial for the performance of CZTS solar cells.

Dimensionality-Inhibited Chemical Doping in Two-Dimensional Semiconductors: The Phosphorene and MoS2 from Charge-Correction Method.

Despite the great appeal of two-dimensional semiconductors for electronics and optoelectronics, to achieve the required charge carrier concentrations by means of chemical doping remains a challenge due to large defect ionization energies (IEs). Here, by decomposing the defect IEs into three parts based on ionization process, we propose a conceptual picture that the large defect IEs are caused by two effects of reduced dimensionality. While the quantum confinement effect makes the neutral single-electron point defect levels deep, the reduced screening effect leads to high energy cost for the electronic relaxation. The first-principles calculations for black phosphorus and MoS2 do demonstrate the general trend. Using BP monolayer either embedded into dielectric continuum or encapsulated between two hBN layers, we demonstrate the feasibility of increasing the screening to reduce the defect IEs. Our analysis is expected to help achieve effective carrier doping and open ways toward more extensive applications of 2D semiconductors.