Fig. 3: Charge separation, interfacial E-field, and transient changes in the energy level alignment.
a Sketch of the real space electron densities (red shaded areas) of the wave functions at the K- and the Σ-valley of the WSe2 conduction band (adapted from Bertoni et al. 20). b Illustration of the charge separation process at the C60/WSe2 interface. After the ultrafast electron transfer from the C60 CT2 state into the WSe2 K-valley, the electrons are confined to the first WSe2 layer. Only the intervalley scattering form the K- into the Σ-valley leads to a delocalization of the electrons in WSe2. c Temporal evolution of the valence band shifts of the WSe2 (VB) and the C60 (HOMO) valence states. The error bars estimate the uncertainty of the fitting procedure to the experimental data. The dynamics of the energy shifts were analyzed with exponential functions. d Cartoon of electrostatic model estimating the transient valence band and HOMO shifts. Interfacial charge transfer creates a hole located on C60 and an electron located initially on the first layer of WSe2, which we assume to form physical dipoles across the interface (solid black equipotential lines). Neighboring dipoles interact by depolarization (dashed black arrows), reducing each other’s magnitude in close proximity (near field region, NF), while creating an increased electrostatic potential far above the dipole layer (far field, FF). Since the origin of the dipole is located between C60 and WSe2, the Stark shifts experienced in the near field region have opposite signs for the two sides of the interface.
