Interlayer charge transfer in ReS2/WS2 van der Waals heterostructures

We observed ultrafast charge transfer between distorted $1T\text{\ensuremath{-}}{\mathrm{ReS}}_{2}$ with anisotropic in-plane electronic and optical properties and $2H\text{\ensuremath{-}}{\mathrm{WS}}_{2}$ that is in-plane isotropic. Heterostructures of monolayer ${\mathrm{ReS}}_{2}$/monolayer ${\mathrm{WS}}_{2}$ and bilayer ${\mathrm{ReS}}_{2}$/monolayer ${\mathrm{WS}}_{2}$ were fabricated by mechanical exfoliation and dry transfer techniques. Significant photoluminescence quenching of ${\mathrm{WS}}_{2}$ in the heterostructures indicates efficient charge transfer. In femtosecond transient absorption measurements, it was found that holes injected in monolayer or bilayer ${\mathrm{ReS}}_{2}$ transfer to ${\mathrm{WS}}_{2}$ on a timescale that is shorter than the time resolution of the measurement. This observation provides evidence that the holes are delocalized in bilayer ${\mathrm{ReS}}_{2}$, revealing strong van der Waals interlayer couplings. These results also show that ${\mathrm{ReS}}_{2}$ and ${\mathrm{WS}}_{2}$ form type-II heterostructures with excellent charge transfer properties.