Interlayer charge transfer in ${\mathrm{ReS}}_2/{\mathrm{WS}}_2$ van der Waals heterostructures

We observed ultrafast charge transfer between distorted $1T\text{−}{\mathrm{ReS}}_2$ with anisotropic in-plane electronic and optical properties and $2H\text{−}{\mathrm{WS}}_2$ 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.

Figure 1

(a) An optical microscopy image of the sample. (b) Band alignment of 1L-${\mathrm{ReS}}_2$/1L-${\mathrm{WS}}_2$ according to theory [50]. (c) PL of 1L ${\mathrm{WS}}_2$ (blue), 1L-${\mathrm{ReS}}_2$/1L-${\mathrm{WS}}_2$ (red), and 2L-${\mathrm{ReS}}_2$/1L-${\mathrm{WS}}_2$ (purple).

Figure 2

Differential reflection of individual materials of 1L ${\mathrm{WS}}_2$ (blue squares), 1L ${\mathrm{ReS}}_2$ (magenta circles), and 2L ${\mathrm{ReS}}_2$ (black triangles) over (a) short- and (b) long-time ranges. Photocarriers are injected by a 3.18 eV pump and detected by probes of 2.01 eV for ${\mathrm{WS}}_2$ and 1.61 eV for both ${\mathrm{ReS}}_2$ regions, respectively. The curves are fits (see text).

Figure 3

(a) Schematics of the pump-probe configuration to study hole transfer from ${\mathrm{ReS}}_2$ to ${\mathrm{WS}}_2$. (b) Differential reflection signal obtained from the heterostructures of 1L-${\mathrm{ReS}}_2$/1L-${\mathrm{WS}}_2$ (red diamonds) and 2L-${\mathrm{ReS}}_2$/1L-${\mathrm{WS}}_2$ (purple hexagons) using the configuration shown in (a). Results of the same measurements on regions of 1L ${\mathrm{WS}}_2$ (blue squares), 1L ${\mathrm{ReS}}_2$ (magenta circles), and 2L ${\mathrm{ReS}}_2$ (black triangles) are also shown for comparison. (c) Same as (b), but with a longer-time range and with linear (left) and logarithmic scales. (d) Peak differential reflection signal as a function of the probe photon energy for the two heterostructures and the 1L ${\mathrm{WS}}_2$.