High luminescence efficiency of multi-valley excitonic complexes in heavily doped WSe2 monolayer

TL;DR

This study demonstrates that heavily $n$-doped WSe$_2$ monolayers host multi-particle excitonic complexes (hexciton $H$, oxciton $O$, and a multi-valley complex $M$) whose photoluminescence intensity far exceeds that of neutral states. By combining differential reflectivity and time-resolved photoluminescence, the authors extract lifetimes and show a rising quantum yield with electron density, surpassing $50\%$ at high doping. The enhanced luminescence is attributed to suppression of non-radiative and dark channels and efficient formation of cold complexes, revealing TMD monolayers as a platform for excitons in high-density electron gases and suggesting potential for efficient atomically thin light emitters. A transfer-matrix model ties the reflectivity data to lifetimes and oscillator strengths, providing a quantitative framework for analyzing radiative and non-radiative channels in these systems.

Abstract

Monolayers of group-VI transition-metal dichalcogenides (TMDs) are two-dimensional semiconductors that exhibit exceptionally strong light-matter coupling yet typically suffer from low emission quantum yields. In this letter, we investigate the heavily n-doped regime of a WSe$_2$ monolayer and show that multi-particle excitonic complexes produce photoluminescence signals up to two orders of magnitude stronger than in the neutral state. Time-resolved photoluminescence and differential reflectivity measurements reveal that the quantum yield rises with carrier density and exceeds 50% for electron concentrations above 10$^{13}$ cm$^{-2}$. These findings establish TMD monolayers as a platform for exploring excitonic complexes in high-density electron gases and point toward new opportunities for efficient, atomically thin light emitters.

Figures (8)

• Figure 1: (a) Sketch of the sample. (b) Reflectivity contrast as a function of the gate voltage. (c) 1st Brillouin zone of WSe$_2$. Sketches of (d) hexciton $H$, (e) oxciton $O$, and (f) multi-valley complex $M$. (g) Spectrally integrated intensity of the reflectivity contrast as a function of the gate voltage.
• Figure 2: (a) cw PL intensity as a function of the gate voltage. (b) Spectrally integrated PL intensity of $H$, $O$ and $M$ peaks as a function of gate voltage.
• Figure 3: (a) Normalized TRPL as a function of the gate voltage. The curves are shifted vertically for clarity. The rise times (decay times) are extracted from the data on the left (right) panel with two different time resolutions. Fits are shown by the thin black solid lines. Between 5V and 7V, the decay time is not monoexponential. The red lines represent the short decay time contribution. See the Supplemental Material for more details. (b) Rise time $\tau_\mathrm{rise}$ and decay time $\tau_\mathrm{decay}$ as a function of gate voltage. Coloured shaded area represent the error bars. (c) Reflectivity contrast at 9 V (black solid line) and result of the transfer matrix model for $\tau_\mathrm{life}\equiv\tau_\mathrm{rise}$ (green line) and $\tau_\mathrm{life}\equiv\tau_\mathrm{decay}$ (yellow and purple lines).
• Figure 4: (a) Sketch of the three-level model. (e) Total quantum yield $\eta$ and its components $\eta_{0}$ (b) and $\eta_\mathrm{res}$ (d) as a function of the gate voltage. (c) $I_{PL}/\eta_{0}$ as a function of $\tau_\mathrm{decay}$. (f) PL intensity as a function of the gate voltage for the three regimes $H$ (5V), $O$ (9V) and $M$ (13V) in cw excitation.
• Figure S1: TRPL measurements for different excitation powers measured at 9 V after pulse excitation at 635 nm at $t = 0$.