Substrate-dependent synergistic many-body effects in atomically thin two-dimensional $\mathrm{W}{\mathrm{S}}_2$

Mott transition has been realized in atomically thin monolayers (MLs) of two-dimensional (2D) semiconductors ($\mathrm{W}{\mathrm{S}}_2$) via optically excited carriers above a critical carrier density through many-body interactions. The above nonlinear optical transition occurs when excited electron hole pairs in the ML $\mathrm{W}{\mathrm{S}}_2$ continuum heavily interact with each other followed by transformation into a collective electron-hole-plasma phase, by losing their identity as individual quasiparticles. This is manifested by the alluring redshift-blueshift crossover phenomena of the excitonic peaks in the emission spectra, resulting from the synergistic attraction-repulsion processes at the Mott transition point. A systematic investigation of many-body effects is reported on ML $\mathrm{W}{\mathrm{S}}_2$, while considering the modulated dielectric screening of three different substrates, viz., silicon dioxide, sapphire, and gold. Substrate doping effects on ML $\mathrm{W}{\mathrm{S}}_2$ are discussed using the Raman fingerprints and photoluminescence spectral weight, which are further corroborated using theoretical density functional theory calculations. Further, the substrate-dependent excitonic Bohr radius of ML $\mathrm{W}{\mathrm{S}}_2$ is extracted via modeling the emission energy shift with Lennard-Jones potential. The variation of the Mott point, as well as the excitonic Bohr radius, is explained via the substrate-induced dielectric screening effect for both dielectric substrates, which is, however, absent in ML $\mathrm{W}{\mathrm{S}}_2$ on Au. In this paper, we therefore reveal diverse many-body ramifications in 2D semiconductors and offer decisive outlooks on selecting impeccable substrate materials for innovative device engineering.

Figure 1

(i) Optical microscopic image, (ii) R-channel grayscale image, (iii) atomic force microscopy (AFM) topographic image, and (iv) Raman mapping image ($E_{2g}^1$ peak) of corresponding monolayer (ML) $\mathrm{W}{\mathrm{S}}_2$ flakes on three substrates: (a) ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate, (b) $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate, and (c) $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate. In the inset of the grayscale image (ii), a contrast profile has been shown along the pink dashed line, extracting data using Image-J software and the height profile of the ML region shown in the inset of AFM topographic image (iii) along the yellow dashed line. The green dashed areas in Raman mapping images (iv) represent the ML region on each substrate.

Figure 2

(a) Comparative Raman spectra of monolayer (ML) $\mathrm{W}{\mathrm{S}}_2$ on all three substrates, collected at a very low laser power excitation $\left(0.3\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2\right)$. (b) Comparative normalized photoluminescence (PL) emission spectra at a low laser power density $\left(0.3\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2\right)$ of ML $\mathrm{W}{\mathrm{S}}_2$ flakes on three substrates: $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ (red line), ${\mathrm{Al}}_2{\mathrm{O}}_3$ (green line), and $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ (blue line) showing clear peak shift and variation of full width at half maximum (FWHM). (c) The variation of $A_{1g}$ Raman peak and PL intensity ratio of trion to exciton for different substrates, revealing substrate-induced doping. The green and orange dashed lines are drawn for the guidance of the eye. (d)–(f) Deconvoluted PL spectra of $\mathrm{W}{\mathrm{S}}_2$ on three different substrates, showing two distinct peaks corresponding to excitonic (red line) and trionic (blue line) emissions at a low laser power density $\left(\sim 0.3\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2\right)$. The black line symbolizes the cumulative fit of two distinct peaks. The PL mapping of excitonic peak originating from the monolayer region is illustrated in the top left inset. (g)–(i) Schematic representation of substrate doping-induced exciton-to-trion intensity variation for $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, and $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates, respectively.

Figure 3

Orbital-projected band structures (i) for $\mathrm{W}{\mathrm{S}}_2$ monolayer (ML) with W-up (up-spin channel), W-down (down-spin channel) $5d$ orbitals and S-$3p$ orbitals, (ii) Al-$3s$ and $3p$ and O-$2p$ projected bands for ${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{W}{\mathrm{S}}_2$, (iii) W-up (up-spin channel), W-down (down-spin channel) $5d$ and S-$3p$ projected bands for ${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{W}{\mathrm{S}}_2$, (iv) Si-$3s$ and $3p$ and O-$2p$ projected bands for $\mathrm{Si}{\mathrm{O}}_2/\mathrm{W}{\mathrm{S}}_2$, (v) W-up (up-spin channel), W-down (down-spin channel) $5d$ and S-$3p$ projected bands for $\mathrm{Si}{\mathrm{O}}_2/\mathrm{W}{\mathrm{S}}_2$, (vi) Au-$5d$ and $6s$ projected bands for $\mathrm{Au}\left[111\right]/\mathrm{W}{\mathrm{S}}_2$, (vii) W-up (up-spin channel), W-down (down-spin channel) $5d$ and S-$3p$ projected bands for $\mathrm{Au}\left[111\right]/\mathrm{W}{\mathrm{S}}_2$.

Figure 4

Photoluminescence (PL) spectra of the monolayer (ML) $\mathrm{W}{\mathrm{S}}_2$ for different excitation powers on (a) ${\mathrm{Al}}_2{\mathrm{O}}_3$, (b) $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$, and (c) $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates. The color gradient signifies the intensity variation, i.e., first increases up to a maximum value (from blue to red) and then decreases further (from red to blue). The green (yellow) dotted line is a guide to the eye of exciton (trion) peak energy variation with laser power density. The variation of exciton resonance energy and integrated intensity (plotted for higher excitation power regime) with excitation-induced charge density in ML $\mathrm{W}{\mathrm{S}}_2$ on (d) ${\mathrm{Al}}_2{\mathrm{O}}_3$, (e) $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$, and (f) $\mathrm{Au}/\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates. The redshift-blueshift crossover and integrated intensity fall of the exciton peak at the Mott transition point are indicated for dielectric substrates ${\mathrm{Al}}_2{\mathrm{O}}_3$ and $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ via interface between two regions, Region I (green) and Region II (yellow).

Figure 5

Exciton energy shift ($\mathrm{\Delta }E$) variation with respect to average distance between excitons ($r_s$) for (a) ${\mathrm{Al}}_2{\mathrm{O}}_3$ and (b) $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrates. The curves are fitted with the Lennard-Jones potential, and the excitonic Bohr radii are extracted for each substrate. In the inset of the two figures, the schematic representations of the exciton Bohr radius variation with dielectric environment alteration are illustrated. (c) Schematic of the increasing exciton density-induced many-body effect in transition metal dichalcogenide (TMD) materials leading to Mott transition in $\mathrm{W}{\mathrm{S}}_2$. (d) Schematic illustration of photoluminescence (PL) redshift-blueshift crossover due to interplay between band renormalization and the Pauli blocking effect of carriers.