Isolating the optical response of a ${\mathrm{MoS}}_2$ monolayer under extreme screening of a metal substrate

Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices. Interaction with three-dimensional (3D) metals, like Au, profoundly affects their optical properties, posing challenges in characterizing the monolayer's optical responses within the semiconductor-metal junction. In this study, using precise polarization-controlled final-state sum frequency generation (FS-SFG), we successfully isolated the optical responses of a ${\mathrm{MoS}}_2$ monolayer from a ${\mathrm{MoS}}_2/\mathrm{Au}$ junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A or B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape illustrates the expected constant density of states (DOS) at the band edge of the 2D semiconductor, a feature often obscured by excitonic interactions in weak-screening conditions such as in a free-standing monolayer. Extrapolation yields the onset of a direct quasiparticle bandgap of about $1.65\pm 0.20\mathrm{eV}$, indicating a strong bandgap renormalization. This study not only enriches our understanding of the optical responses of a 2D semiconductor in extreme screening conditions but also provides a critical reference for advancing 2D semiconductor-based photocatalytic applications.

Figure 1

Schematic representations of exfoliated ${\mathrm{MoS}}_2$ monolayer on Au is investigated by the FS-SFG. Inset: Schematic energy level diagram of the FS-SFG, where $g$, $v$, and $s$ represent the ground state, virtual state, and excited state, respectively. Only the SFG process will resonate with the excited state in the ${\mathrm{MoS}}_2$ monolayer.

Figure 2

(a) Raman spectra of ${\mathrm{MoS}}_2$ monolayer on Au and Si/${\mathrm{SiO}}_2$ substrate. The inset illustrates two Raman-active modes ($E^{\prime }$ and $A_1^{\prime }$). (b) PL spectra obtained at room temperature for ${\mathrm{MoS}}_2$ monolayer on Au and Si/${\mathrm{SiO}}_2$ substrate with an excitation wavelength of 532 nm. The PL signal of ${\mathrm{MoS}}_2$ on Au is strongly quenched.

Figure 3

(a) The normalized FS-SFG spectra of the ${\mathrm{MoS}}_2$ monolayer on Au under spp (top) and ssp (bottom) polarization combinations. (b) and (c) are the azimuth-dependent SFG intensity of ${\mathrm{MoS}}_2/\mathrm{Au}$ under spp and ssp polarization combination, respectively. There is a ${13}^{\circ }$ offset in the azimuth between spp and ssp polarization combination. The spectra in (a) were collected at specific azimuths, marked in (b) and (c) with corresponding colors.

Figure 4

The onset of bandgap in the ${\mathrm{MoS}}_2$ monolayer on Au under spp polarization combination was determined using the analogous Tauc plot method. A linear fit of the dependence of $|{\chi }_{Mo{S}_2}^{\left(2\right)}|$ on the SFG photon energy was represented by the red solid line. The onset of the bandgap was extracted by extrapolating the linear fit to hit the energy axis ($|{\chi }_{Mo{S}_2}^{\left(2\right)}|=0$) and marked by an arrow.