In this work, we construct an ultrathin graphene/GaS heterostructure and investigate its electronic properties as well as the effect of vertical strain using density functional theory. The calculated results of the equilibrium interlayer spacing (3.356 Å) and the binding energy show that the intrinsic properties of isolated graphene and GaS monolayers can be preserved and the weak van der Waals interactions are dominated in the heterostructures. The van der Waals heterostructure (vdWH) forms an n-type Schottky contact with a small Schottky barrier height of 0.51 eV. This small Schottky barrier height can also be tuned by applying vertical strain. Furthermore, we find that the n-type Schottky contact of the vdWH can be changed to p-type when the interlayer spacing is decreased and exceeded to 2.60 Å. These findings show the great potential application of the graphene/GaS vdWH for designing next generation devices.

The effects of the compressive stress on the binding energy and the density of shallow-donor impurity states in symmetrical ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ double quantum wells are calculated using a variational procedure within the effective-mass approximation. Results are for different well and barrier widths, shallow-donor impurity position, and compressive stress along the growth direction of the structure. We have found that independently of the well and barrier widths, for stress values up to 13.5 kbar (in the direct-gap regime) the binding energy increases linearly with the stress. For stress values greater than 13.5 kbar (indirect gap regime) and for impurities at the center of the wells, the binding energy increases up to a maximum and then decreases. For all impurity positions the binding energy shows a nonlinear behavior in the indirect gap regime due to the $\Gamma -X$ crossing effect. The density of impurity states is calculated for a homogeneous distribution of donor impurities within the barriers and the wells of the low-dimensional heterostructures. We have found that there are three special structures in the density of impurity states: one associated with on-center-barrier-, the second one associated with on-center-well-, and the third one corresponding to on-external-edge-well-impurity positions. The three structures in the density of impurity states must be observed in valence–to–donor-related absorption and conduction–to–donor-related photoluminescence spectra, and consequently these peaks can be tuned at specific energies and convert the system in a stress detector.

We study the optical transport properties of the monolayer transition metal dichalcogenides (TMDCs) such as ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$ in the presence of a magnetic field. The TMDCs band structures are obtained and discussed by using the effective massive Dirac model, in which the spin and valley Zeeman effects as well as an external electric field are included. The magneto-optical absorption coefficient (MOAC) is derived as a function of absorbed photon energy when the carriers are scattered by random impurities combined with the intrinsic acoustic and optical phonons in TMDCs and the surface optical (SO) phonons of substrates. Our result shows that the spin-splitting feature appeared in all four TMDC materials. The combination of strong spin-orbit coupling (SOC) and Zeeman fields has doubled the Landau levels but has not changed the energy gap of the TMDCs monolayer, which can be controlled by the electric field. Because of their strong SOC effect, the absorption spectrum in monolayer TMDCs is separated into two separate peaks caused by spin up and down. At the low temperature, the MOAC intensity via impurity scattering is the biggest followed by that of the SO phonons while the intrinsic acoustic and optical phonon scatterings display the smallest. For the monolayer TMDCs on substrates, ${\mathrm{SiO}}_2$ always shows its superiority in comparison with the others. Among the four TMDC materials, ${\mathrm{MoSe}}_2$ shows the biggest MOAC intensity, while ${\mathrm{WS}}_2$ has the biggest value of the absorbed photon energy. The full-width at half-maximum (FWHM) via impurity scattering achieves its highest value in ${\mathrm{WS}}_2$, while this occurs in ${\mathrm{MoSe}}_2$ and ${\mathrm{MoS}}_2$ via intrinsic acoustic and optical phonon scatterings, respectively. Our estimation of mobility from FWHM gives good agreement with the experimental results in ${\mathrm{WS}}_2$.