Uniform Narrow Excitonic Spectrum in Large-Area Suspended WSe2 Monolayers

Abstract

Uniformity in the excitonic spectrum is a key requirement for accessing intrinsic excitonic physics in two-dimensional semiconductors; however, in supported transition-metal dichalcogenide (TMD) monolayers, exciton energies and linewidths can vary spatially due to inhomogeneities created by contact with other materials or contamination left by fabrication procedures. Suspended TMD monolayers provide an effective route to minimizing substrate-induced disorder. Here we demonstrate the spatially uniform excitonic spectrum from high-quality WSe2 suspended monolayers fabricated by gold-assisted exfoliation directly onto an Au contact electrode of a gate-tunable device. The resulting membranes span narrow suspended regions up to ~80 um and show spatially uniform photoluminescence at cryogenic temperatures with neutral-exciton linewidths as low as ~4.5 meV, comparable to the narrowest values reported for high-quality monolayers. Spectral reproducibility across the suspended regions supports an intrinsic optical response, while gate-dependent measurements resolve multiple excitonic species. This approach provides a practical route to electrically tunable potential landscapes in suspended TMD monolayers with a highly uniform excitonic response.

Figures (4)

• Figure 1: (a) Schematic illustration of the fabrication steps for gate-tunable suspended WSe$_2$ MLs. (b) Optical microscope images of suspended WSe$_2$ ML regions spanning large holes and trenches. (c) Optical microscope image showing the termination of the ML at the edge of the top contact electrode (left), and a contrast-enhanced image highlighting the lateral extent of the large-area WSe$_2$ monolayer, spanning several hundred micrometers (right).
• Figure 2: Low-temperature PL of suspended WSe$_2$ MLs. (a) PL spectra at 7 K under 532 nm continuous-wave (CW) excitation (1 $\mu$W) acquired on a 5 $\mu$m-wide trench (inset) at zero bias (0 V), near charge neutrality ($-3$ V), under $p$-doping ($-30$ V), and under $n$-doping ($+30$ V) conditions. (b) Lorentzian fit of the normalized neutral-exciton peak at zero bias from (a). (c) Neutral exciton linewidth and energy as a function of excitation power, measured at the center of a 5 $\mu$m-wide trench and an 8 $\mu$m-diameter hole. (d) Gate-voltage-dependent PL spectra under 532 nm laser excitation at 1 $\mu$W for a ML suspended over an 8 $\mu$m-diameter hole (inset). Scale bars: 10 $\mu$m. The red dots at the centers of the optical microscope images in (a) and (d) indicate the measurement positions.
• Figure 3: Spatial PL mapping in suspended WSe$_2$ MLs. (a) PL spectra acquired along the center line of a ML suspended over a long trench at $V_{\mathrm{g}} = -3$ V. (b) Spatial PL maps of the $X^{0}$ energy and full width at half maximum (FWHM), extracted from Lorentzian fits of PL spectra for the trench in (a). (c) Histograms of the $X^{0}$ energy and FWHM extracted from the homogeneous region indicated by the dashed area in (b). (d) Spatial PL maps of the $X^{0}$ and $X^{+}$ energies and linewidths for a ML suspended over an 8 $\mu$m-diameter circular hole at $V_{\mathrm{g}} = -3$ V and $V_{\mathrm{g}} = -65$ V. (e) Spatial PL maps of the $X^{+}$ energy and linewidth for a ML suspended over a 4 $\mu$m-wide trench at $V_{\mathrm{g}} = -80$ V.
• Figure 4: Spatial reproducibility of the PL across multiple suspended WSe$_2$ MLs. (a) Schematic map of the device showing the spatial positions of the analyzed suspended MLs spanning trenches with widths of 3, 4, and 5 $\mu$m; the number in parentheses indicates the trench width. (b) Normalized PL spectra acquired at the positions indicated in (a), plotted relative to the $X^{0}$ energy. Spectra are vertically offset for clarity. (c) Gate-dependent PL maps for four representative suspended MLs located near the corners of the device. (d) Relative energy shifts of the main excitonic resonances, plotted with respect to the $X^{0}$ energy, as a function of $V_{\mathrm{g}}$ for the four positions shown in (c).