Picosecond energy transfer in a transition metal dichalcogenide-graphene heterostructure revealed by transient Raman spectroscopy

Abstract

Intense light–matter interactions and unique structural and electrical properties make van der Waals heterostructures composed by graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunneling transistors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics, and quantum light emitting devices (QLEDs), bright and narrow-line emitters using minimal amounts of active absorber material. The performance of such devices is critically ruled by interlayer interactions which are still poorly understood in many respects. Specifically, two classes of coupling mechanisms have been proposed, charge transfer (CT) and energy transfer (ET), but their relative efficiency and the underlying physics are open questions. Here, building on a time-resolved Raman scattering experiment, we determine the electronic temperature profile of Gr in response to TMD photoexcitation, tracking the picosecond dynamics of the G and 2D Raman bands. Compelling evidence for a dominant role of the ET process accomplished within a characteristic time of ∼4 ps is provided. Our results suggest the existence of an intermediate process between the observed picosecond ET and the generation of a net charge underlying the slower electric signals detected in optoelectronic applications.

Keywords: Raman scattering; energy transfer; graphene; ultrafast spectroscopy; van der Waals heterostructures.

Fig. 1.

Optical characterization of a WS ₂–Gr heterostructure. (A) Time-delayed pump and probe beams focused onto a diffraction-limited WS₂–Gr spot. (B) PL map generated by probe only (1.58 eV) after integrating the emission spectra in the ranges 1.88 to 2.05 eV (WS₂, two-photon PL) and 1.65 to 1.85 eV (Gr, hot luminescence) and the corresponding optical image (Right). Contour lines indicate bare Gr (blue), few-layer Gr (yellow), WS₂ (red), coupled (green), and uncoupled (black) WS₂–Gr regions. (C) PL spectra for the spots indicated in the map with corresponding colors. (D) Time-resolved, probe-generated Raman spectra of Gr modes in the WS₂–Gr heterostructure for two time delays, corrected from hot PL background (Materials and Methods).

Fig. 2.

Time-resolved Raman spectra. (A and B) Pump-off Raman spectra of the G and 2D modes for WS₂–Gr and bare Gr. Transient differential Raman spectra $\mathrm{\Delta }I(\mathrm{\Delta }t,E)=I(\mathrm{\Delta }t,E)-I(-30ps,E)$: (C and D) color maps and (E and F) vertically offset slices for selected time delays in ps. The 2D mode intensity I_2D, as opposed to $I_{\text{G}}$, decreases around zero delay. This effect is observed to a lesser extent in bare Gr. (G) While I_2D drop—due to electronic heating—recovers in bare Gr within a timescale comparable with the pump–probe temporal overlap (black symbols and guideline), it takes longer in WS₂–Gr (blue symbols and guideline). The dashed blue (black) lines show I_2D without the pump beam in WS₂–Gr (bare Gr), indicating the presence of a small photo-induced phonon heating. The bare Gr profile has been modeled (red thick line) with a picosecond drop (fast electronic term broadened by instrumental resolution) and its convolution $(\mathrm{⊛})$ with an exponential term (transient phononic contribution; red thin line): $f(\mathrm{\Delta }t)=C+A\exp \left[-\mathrm{\Delta }{t}^2/\left(2{\sigma }^2\right)\right]+\mathrm{\Delta }{I}_{\text{2D}}^{\text{ph}}(\mathrm{\Delta }t)$, where $\mathrm{\Delta }{I}_{\text{2D}}^{\text{ph}}(\mathrm{\Delta }t)=B\exp \left(-\mathrm{\Delta }{t}^2/2{\sigma }^2\right)\mathrm{⊛}\left[\theta (\mathrm{\Delta }t)\left(1-e^{-\left(\mathrm{\Delta }t\right)/\tau }\right)\right]$. A, B, and C are fitting parameters. $\sigma =0.66$ ps corresponds to the autocorrelation of the 1-ps FWHM pump and probe pulses, and τ = 5 ps from ref. . $\mathrm{\Delta }{I}_{\text{2D}}^{\text{ph}}$ is also reported with a vertical offset to emphasize its role in the WS₂–Gr case (dotted red line). The accuracy in the I_2D measurement is $\sim 1\%$.

Fig. 3.

Modeling energy transfer in a WS₂–Gr heterostructure. (A) T_e and e–h pair density at different time delays extracted from the dynamics of I_2D for WS₂–Gr (open symbols) and bare Gr (full symbols) are compared with the simulated profiles (solid lines). The uncertainty on the T_e is 20 K. (B) Sketch of the kinetic model used in the simulation. The pump pulse can 1) generate an exciton population in WS₂ (thick orange arrow) and 2) populate the electronic states of Gr with e–h pairs (thin orange arrow). The e–h pairs (represented as blue and white circles, respectively) in Gr decay with a timescale ${\tau }_{\text{G}}$. In contrast, the excitons in bare WS₂ have a long lifetime τ₀. Exciton decay is strongly accelerated in WS₂–Gr due to energy transfer to Gr with a characteristic time ${\tau }_{\text{T}}$.