Microscopic Understanding of Ultrafast Charge Transfer in van der Waals Heterostructures

Van der Waals heterostructures show many intriguing phenomena including ultrafast charge separation following strong excitonic absorption in the visible spectral range. However, despite the enormous potential for future applications in the field of optoelectronics, the underlying microscopic mechanism remains controversial. Here we use time- and angle-resolved photoemission spectroscopy combined with microscopic many-particle theory to reveal the relevant microscopic charge transfer channels in epitaxial ${\mathrm{WS}}_2/\mathrm{graphene}$ heterostructures. We find that the timescale for efficient ultrafast charge separation in the material is determined by direct tunneling at those points in the Brillouin zone where ${\mathrm{WS}}_2$ and graphene bands cross, while the lifetime of the charge separated transient state is set by defect-assisted tunneling through localized sulphur vacancies. The subtle interplay of intrinsic and defect-related charge transfer channels revealed in the present work can be exploited for the design of highly efficient light harvesting and detecting devices.

Figure 1

Tr-ARPES snapshots of ${\mathrm{WS}}_2/\mathrm{graphene}$ heterostructure before and after optical excitation. (a) Photocurrent measured along the $\mathrm{\Gamma }K$ direction at negative pump-probe delay. Colored dashed lines indicate the position of the line profiles used to determine the transient band positions in Figs. 2 and 2. The yellow box indicates the region where the Fermi-Dirac fits for Fig. 4 were performed. (b) Pump-induced changes of the photocurrent at the peak of the pump-probe signal. Red and blue indicate gain and loss of photoelectrons, respectively, with respect to the photocurrent measured at negative pump-probe delay. Colored boxes indicate the areas of integration for the pump-probe traces shown in Figs. 2 and 2. Thin dashed lines represent the calculated band structures of graphene [32] and ${\mathrm{WS}}_2$ [33]. Rigid band shifts of $-0.81\mathrm{eV}$ and $-1.19\mathrm{eV}$ were applied to ${\mathrm{WS}}_2$ CB and VB, respectively. The graphene Dirac cone was shifted by $+0.3\mathrm{eV}$ to account for the observed hole doping [25].

Figure 2

Evidence for ultrafast charge separation: (a) Gain in the ${\mathrm{WS}}_2$ CB (orange) and loss in the ${\mathrm{WS}}_2$ VB (green). (b) Gain above the Fermi level (red) and loss below the Fermi level (blue) inside the graphene Dirac cone. (c) Position of the ${\mathrm{WS}}_2$ CB (orange) and the upper ${\mathrm{WS}}_2$ VB (green) and transient band gap of ${\mathrm{WS}}_2$ (turquois). (d) Charging-induced shift of ${\mathrm{WS}}_2$ valence and conduction band. (e) Transient shift of the graphene Dirac cone. All quantities are plotted as a function of pump-probe delay. Black lines are single exponential fits to the data. Gray lines in panel (c) were calculated from the exponential fit for $E_{\mathrm{gap}}$ in panel (c) and the exponential fit in panel (d).

Figure 3

Fluence dependence of ultrafast charge separation and recombination: (a) Lifetime of graphene gain from Fig. 2. The black dashed line represents our temporal resolution of 200 fs. (b) Lifetime of photoexcited electrons in the ${\mathrm{WS}}_2$ CB from Fig. 2. (c) Lifetime of gain above the equilibrium position of the upper ${\mathrm{WS}}_2$ VB. Thick lines in (a) and (b) were obtained by inverting and rescaling the guides to the eye from Fig. 4 to match the data points. The thick line in (c) is a guide to the eye.

Figure 4

Direct tunneling model. (a) Temporal evolution of the two barrier heights deduced from Fig. 2 together with single-exponential fit. (b) Temporal evolution of the electronic temperature inside the Dirac cone together with double-exponential fit. (c) Temporal evolution of the electron and hole transfer rates deduced from panels (b) and (c) together with double-exponential fit. (d) Pump fluence dependence of the peak transfer rates for electrons and holes determined from exponential fits to the data in Fig. 4 [31]. Thick gray lines are guides to the eye. (e) Sketch of the band structure with tunneling barrier for holes ($\mathrm{\Delta }{E}_h$, red) and electrons ($\mathrm{\Delta }{E}_e$, orange). Our final microscopic model includes direct (orange and red arrows) as well as defect-assisted tunneling (green arrows).

Figure 5

Microscopic theory. Momentum dependence of the tunneling matrix element for the CB (a) and VB (b). $k_x=k_y=0$ corresponds to the $K$ point of graphene. Black lines indicate the location of the intersection between ${\mathrm{WS}}_2$ and graphene bands as illustrated in the inset. Calculated transfer rate for electrons (c) and holes (d) as a function of carrier temperature for different barrier heights.