Influence of excitation energy on microscopic quantum pathways for ultrafast charge transfer in van der Waals heterostructures

Abstract

Efficient charge separation in van der Waals (vdW) heterostructures is crucial for optimizing light harvesting and detection applications. However, precise control over the microscopic pathways governing ultrafast charge transfer remains an open challenge. These pathways are intrinsically linked to charge transfer states with strongly delocalized wave functions that appear at various momenta in the Brillouin zone. Here, we use time- and angle-resolved photoemission spectroscopy (trARPES) to investigate the possibility of steering carriers through specific charge transfer states in a prototypical WS\textsubscript{2}-graphene heterostructure. By selectively exciting electron-hole pairs at the K-point (A-exciton resonance) and close to the Q-point (C-exciton resonance) of WS\textsubscript{2} with different pump photon energies, we find that charge separation is faster at higher excitation energies. This behavior is attributed to the fact that absorption at the C-exciton resonance generates electron-hole populations at energies well above the direct band gap. The resulting elevated carrier temperatures open an additional, highly efficient charge-transfer channel for holes in the WS\textsubscript{2} valence band, leading to an overall acceleration of interlayer hole transfer for C-exciton excitation. The microscopic insights gained in this work can be leveraged to optimize the performance of vdW heterostructures in optoelectronic devices.

Figures (5)

• Figure 1: trARPES data of WS2-graphene heterostructure.a) ARPES snapshots for negative pump-probe delay taken along the $\Gamma$K direction of graphene. Orange and blue arrows indicate the two different excitation schemes for $\hbar\omega=2.0\,\mathrm{eV}$ and $\hbar\omega=3.1\,\mathrm{eV}$, respectively. Dotted, dashed and continuous lines represent the theoretical band structures for WS2 with twist angles of 0° and 30° Zeng2013 and graphene Wallace1947, respectively. b) Pump-induced changes of the photocurrent in a) 240fs after excitation with $\hbar\omega=2.0\,\mathrm{eV}$. c) Same as b) but $310\,\mathrm{fs}$ after excitation with $\hbar\omega=3.1\,\mathrm{eV}$. Colored boxes in b) and c) mark the regions of integration for the pump-probe traces displayed in Fig. \ref{['figure2']}.
• Figure 2: Population dynamics obtained by integrating the counts over the areas marked by colored boxes in Figs. \ref{['figure1']}b and c. a) Gain (red) and loss (blue) inside the Dirac cone. b) Gain above the equilibrium position of the WS2 VB at K. c) Population of the WS2 CB at K (yellow) and close to Q (orange). Rows 1 and 2 present data for excitation at $\hbar\omega=2.0\,\mathrm{eV}$ and $\hbar\omega=3.1\,\mathrm{eV}$, respectively. Continuous lines are single-exponential fits. Dashed colored lines indicate the pump-probe delay where the pump-probe traces reach their respective maximum or minimum. Grey-shaded areas indicate the temporal profile of the pump-probe cross-correlation.
• Figure 3: Transient band structure.a) Transient band position of WS2 CB (top) and VB (bottom). b) Transient band gap. c) Charging shifts of WS2 obtained from the VB (blue, orange) and CB (dark blue, red). d) Charging shift of Dirac cone. Data points for excitation at $\hbar\omega=2.0\,\mathrm{eV}$ and $\hbar\omega=3.1\,\mathrm{eV}$ are shown in orange and blue, respectively. Orange and blue curves are single-exponential fits. Grey curves in a) are guides to the eye calculated from the VB fit in a) and the gap fit in b) for the CB, and from the gap fit in b) and the charging shift fit in c) for the VB. Vertical colored dashed lines indicate the pump-probe delay where the pump-probe traces reach their respective maximum or minimum. Grey-shaded areas indicate the temporal profiles of the pump-probe cross-correlations.
• Figure 4: Schematic of quantum pathways for ultrafast charge transfer. The band structure sketch is based on density functional theory calculations from Hofmann2023. The color code indicates whether the wave function is localized on the graphene layer (gray curves), the WS2 layer (black curves) or delocalized over both layers (red dots). Blue (orange) arrows indicate direct optical transitions excited at $\hbar\omega=3.1\,\mathrm{eV}$ ($\hbar\omega=2.0\,\mathrm{eV}$) and subsequent carrier relaxation via different charge transfer states.
• Figure 5: Extracting transient peak positions. a) WS2 EDCs for negative pump-probe delay and at $t=0.2ps$ fitted with a constant offset and three Gaussian peaks. The dotted vertical lines mark the peak positions for negative pump-probe delay. b) Pump-induced changes of the EDC through the WS2 conduction band together with a Gaussian fit. c) MDC through the graphene Dirac cone at negative pump-probe delay fitted with the sum of a constant offset and a Lorentzian peak.