Moiré-Trapped Interlayer Trions in a Charge-Tunable ${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$ Heterobilayer

Transition-metal dichalcogenide heterobilayers offer attractive opportunities to realize lattices of interacting bosons with several degrees of freedom. Such heterobilayers can feature moiré patterns that modulate their electronic band structure, leading to spatial confinement of single interlayer excitons (IXs) that act as quantum emitters with $C_3$ symmetry. However, the narrow emission linewidths of the quantum emitters contrast with a broad ensemble IX emission observed in nominally identical heterobilayers, opening a debate regarding the origin of IX emission. Here we report the continuous evolution from a few trapped IXs to an ensemble of IXs with both triplet- and singlet-spin configurations in a gate-tunable $2H\text{−}{\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayer. We observe signatures of dipolar interactions in the IX ensemble regime which, when combined with magneto-optical spectroscopy, reveal that the narrow quantum-dot-like and broad ensemble emission originate from IXs trapped in moiré potentials with the same atomic registry. Finally, electron doping leads to the formation of three different species of localized negative trions with contrasting spin-valley configurations, among which we observe both intervalley and intravalley IX trions with spin-triplet optical transitions. Our results identify the origin of IX emission in ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayers and highlight the important role of exciton-exciton interactions and Fermi-level control in these highly tunable quantum materials.

Popular Summary

Stacks of two different transition-metal dichalcogenides—atomically thin semiconductors—offer attractive opportunities to realize lattices of interacting particles. Such heterobilayers can feature moiré patterns that confine single interlayer excitons—bonded electron-hole pairs—which behave as quantum-light sources. However, the narrow spectral emission linewidths of these quantum emitters contrast with the spectrally broad emission of interlayer excitons observed in nominally identical heterobilayers, opening a debate about the nature of their emission. Here, we provide a unified picture that resolves this apparent controversy: We bridge the two reported regimes for interlayer exciton emission in a gate-tunable ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayer.

Increasing the power of the excitation laser increases the number of optically generated excitons, driving an evolution from a few quantum-emitter-like interlayer excitons to an ensemble with electron-hole pairs depicting both parallel and opposite spins. We also observe signatures of dipolar interactions in the ensemble regime. When combined with magneto-optical spectroscopy, these reveal that the narrow quantum emitters and broad ensemble emission originate from interlayer excitons trapped in moiré potentials with the same relative lateral atomic stacking of the constituent layers.

Moreover, electron doping leads to the formation of negative interlayer trions—two electrons bound to a hole—in both the quantum-emitter and ensemble regimes, enabling the control of the exciton charge state. Finally, in the ensemble regime, we observe three different species of negative trions with contrasting spin and momentum configurations.

Our results identify the origin of interlayer exciton emission in ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayers and provide further valuable evidence about the nature of quantum emission in moiré-based quantum materials.

Figure 1

Power dependence of moiré-trapped IXs. (a) Sketch of the dual-gated ${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$ heterobilayer. Graphite layers are used as contacts for top, bottom, and heterobilayer, while $h$-BN layers ($D=18\mathrm{nm}$) are used as dielectric spacers. (b) Color plot with the full evolution of the low-temperature ($T=4\mathrm{K}$) confocal PL spectrum of a representative spot in the undoped heterobilayer for $0.4\mathrm{nW}\le P_{\mathrm{exc}}\le 80\mu \mathrm{W}$ in logarithmic scale. (c) Line cuts extracted from the color plot in (b) for $P_{\mathrm{exc}}$ with different orders of magnitude. The spectra are shifted vertically and normalized by the values indicated in the figure for visualization purposes. (d) Schematics of the spin-valley configuration of the proposed optical transitions with the corresponding selection rules for $H_h^h$ atomic registry [24]. (e) Magnetic field dependence of IX PL in the low- and high-excitation regimes (bottom and top panels, respectively) for ${\sigma }^{+}$- and ${\sigma }^{-}$-resolved confocal collection (left and right panels, respectively). (f) Magnetic field dependence of the Zeeman splitting measured for ${\mathrm{IX}}_T^0$ and ${\mathrm{IX}}_S^0$ at high $P_{\mathrm{exc}}$ (top panel) and for a representative moiré-trapped IX at low $P_{\mathrm{exc}}$.

Figure 2

Dipolar interactions of an ensemble of trapped IXs. (a) Ratio of integrated PL intensities between the ${\mathrm{IX}}_T^0$ and ${\mathrm{IX}}_S^0$ peaks (${\mathrm{IX}}_T^0$/${\mathrm{IX}}_S^0$) as a function of $P_{\mathrm{exc}}$ extracted from Fig. 1 (black dots). The inset shows the evolution of the integrated PL intensities for each individual peak in the same $P_{\mathrm{exc}}$ range. The red, blue, and green solid lines represent fits of the experimental data to the rate equation model described in the Supplemental Material [38]. (b) Energy splitting between ${\mathrm{IX}}_S^0$ and ${\mathrm{IX}}_T^0$ ($\mathrm{\Delta }{E}_{S-T}$) as a function of the excitation power (black dots). The inset shows the energy shifts of the individual peaks. The red solid line represents a fit of the experimental $\mathrm{\Delta }{E}_{S-T}$ to Eq. (2), while the green and blue solid lines are fits of the measured $\mathrm{\Delta }{E}_{S,T}$ to Eq. (1), and the shaded areas represent the uncertainty interval corresponding to $d$ values ranging from 0.4 [8] to 1 nm [55]. (c) Estimated power-dependent densities of IXs with spin-singlet (green) and spin-triplet (blue) configurations. The thickness of the lines indicates the confidence interval estimated from the fits to the experimental data. The black dashed line indicates the density of moiré traps estimated for our heterostructure, while the orange shaded area represents the density of sites for ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayers with stacking angles between 54.4° and 58.4°. (d) Sketch depicting the transition between the low- and high-excitation power regimes for IXs (red spheres) trapped in the moiré sites of a ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayer. At low $P_{\mathrm{exc}}$, only a few IXs are a localized in the moiré sites lying inside the circular confocal collection spot (not to scale). As $P_{\mathrm{exc}}$ increases, more and more optically generated IXs are trapped in the moiré sites, giving rise to a broad PL emission band originating from an ensemble of moiré-trapped IXs. Even at the highest $P_{\mathrm{exc}}$ used in this work, less than approximately 2% of moiré sites contain trapped IXs.

Figure 3

Interlayer exciton trions in ${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$. (a),(b) Gate-voltage-controlled PL of IXs in the neutral ($0<V_g\le 0.1\mathrm{V}$) and electron-doping regime ($V_g\ge 0.1\mathrm{V}$) at $B_z=0\mathrm{T}$, for $P_{\mathrm{exc}}=20\mathrm{nW}$ (a) and $P_{\mathrm{exc}}=40\mu \mathrm{W}$ (b) at 4 K. In (b), the PL intensity is multiplied by a factor 20 in the spectral range delimited by the vertical dashed area for visualization purposes. (c) Line cuts extracted from the color plots in (a) and (b) (black and red solid lines, respectively) in the neutral ($V_g=0\mathrm{V}$) and electron-doping regimes ($V_g=0.6\mathrm{V}$). The spectra are normalized by the values indicated in the figure, and the line cuts at $V_g=0.6\mathrm{V}$ are shifted vertically for visualization purposes. The dashed vertical lines indicate the central energies of the different ensemble peaks, illustrating the power-induced blueshift [$\mathrm{\Delta }E(P_{\mathrm{exc}})$] and the energy splittings arising from the binding energies of the charged IXs with spin-triplet ($\mathrm{\Delta }{E}_{{\mathrm{IX}}_T}$) and spin-singlet ($\mathrm{\Delta }{E}_{{\mathrm{IX}}_S}$) configuration.

Figure 4

Interlayer exciton trions in ${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$. (a) Color plot with the full evolution of the PL spectrum of negative IX trions for $0.2\mathrm{nW}\le P_{\mathrm{exc}}\le 40\mu \mathrm{W}$ at $V_g=2\mathrm{V}$ in logarithmic scale, in which four different peaks can be resolved (as indicated by the arrows). (b) PL spectrum acquired for the intermediate $P_{\mathrm{exc}}$ value indicated by the white dashed line in (a) ($2\mu \mathrm{W}$) at $V_g=2\mathrm{V}$, fitted to four Lorentzian peaks corresponding to various exciton species: ${\mathrm{IX}}_{T,\text{inter}}^{-}$ (blue), ${\mathrm{IX}}_{T,\text{intra}}^{-}$ (gray), ${\mathrm{IX}}_{S,\text{inter}}^{-}$ (green), and ${\mathrm{IX}}_T^{{-}^{\prime }}$ (red). (c) Schematic representation of the charge configurations for ${\mathrm{IX}}_{T,\text{inter}}^{-}$ (left), ${\mathrm{IX}}_{T,\text{intra}}^{-}$ (middle), and ${\mathrm{IX}}_{S,\text{inter}}^{-}$ (right) showing the optical transitions that involve a hole in the topmost valence band of ${\mathrm{WSe}}_2$ at $K$. (d) Magnetic field dependence of the IX trions PL for ${\sigma }^{+}$- (purple) and ${\sigma }^{-}$-polarized (orange) collection in the range $0\le B_z\le 5\mathrm{T}$ under $V_g=1\mathrm{V}$ and $P_{\mathrm{exc}}=40\mu \mathrm{W}$. (e) Zeeman splitting of each IX trion species (dots) as extracted from fits of the experimental data shown in (d). The solid lines represent linear fits of the experimental data.