Ultrafast band renormalization and defect-driven surface photovoltage in PdSe2
Layered van der Waals semiconductors are being explored as new materials for ultrafast optoelectronics, where device performance depends not only on the equilibrium band structure but also on how photoexcited carriers relax and redistribute on femtosecond-to-picosecond timescales. PdSe2 is a promising example: its puckered lattice yields strongly anisotropic electronic properties, while its environmental stability offers an advantage over black phosphorus. Yet two questions have remained open: the intrinsic indirect bandgap of bulk PdSe2 and the microscopic role of native defects in its nonequilibrium electronic response. These issues are particularly relevant in low-symmetry materials, where in-plane anisotropy and intrinsic defects can reshape carrier pathways and generate transient internal electric fields.
Here we combine time- and angle-resolved photoemission spectroscopy (TR-ARPES) at the T-ReX facility of the FERMI free-electron laser with density functional theory (DFT) to probe ultrafast carrier dynamics in bulk PdSe2. TR-ARPES transiently populates unoccupied conduction-band states with an ultrashort optical pump and maps the evolving electronic structure with a time-delayed probe, providing direct access to the indirect electronic gap and its dynamics.
A key result is the direct determination of the bulk indirect bandgap of 0.55 eV, obtained by tracking the pump-induced population of the conduction-band minimum together with the response of the valence-band maximum (see Fig. 1a). DFT captures the strong out-of-plane (kz) dependence of the band edges and supports extrapolation to the minimum-gap plane, resolving the spread in previously reported values.
Figure 1: TR-ARPES on PdSe2 acquired at the T-ReX facility of the FERMI free electron laser. (a) differential ARPES map at +1 ps, evidencing the photoinduced conduction-band population. (b) the pump–probe dynamics reveals an ultrafast bandgap renormalization (BGR) and a long-lived surface photovoltage (SPV) component. (c) fluence-dependent rigid shift of the valence-band edge, evidencing SPV formation (adapted from Abdul-Aziz et al., npj 2D Materials and Applications 9, 110 (2025), CC BY 4.0).
Beyond band-gap mapping, we observe an ultrafast many-body response of the valence band: within a few ps (Fig. 1b), the band exhibits a transient energy shift and linewidth broadening consistent with about 15 meV bandgap renormalization and enhanced carrier scattering, reflecting rapid carrier redistribution and electron–phonon coupling.
Most strikingly, a pronounced surface photovoltage (SPV) exceeding 67 meV emerges and persists for >50 ps. In TR-ARPES, this appears as a rigid shift of the electronic spectrum, indicating a transient modification of near-surface band bending (Fig. 1c). The SPV amplitude follows a logarithmic saturation versus pump fluence, consistent with depletion-layer screening and filling of a finite density of surface states. Our analysis supports a defect-assisted mechanism: electrons relax toward the conduction-band minimum while holes are trapped in mid-gap states associated with intrinsic Se and Pd vacancies. The resulting charge separation stabilizes a surface dipole layer and sustains the electrostatic potential long after excitation.
Overall, PdSe2 emerges as a model low-symmetry semiconductor in which ultrafast band renormalization and defect-mediated surface fields can be disentangled in a TR-ARPES experiment, providing a microscopic basis for defect-engineered photo-gating and surface-potential control.
This research was conducted by the following research team:
Omar Abdul-Aziz1, Manuel Tuniz2, Wibke Bronsch3, Fulvio Parmigiani3, Federico Cilento3, Daniel Wolverson4, Charles J. Sayers4, Giulio Cerullo5, Claudia Dallera5, Ettore Carpene6, Paul H. M. van Loosdrecht1, Hamoon Hedayat1
1 II. Physikalisches Institut, Universität zu Köln, Cologne, Germany
2 Dipartimento di Fisica, Università di Trieste, Trieste, Italy
3 Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
4 Department of Physics and Centre for Photonics and Photonic Materials, University of Bath, United Kingdom
5 Dipartimento di Fisica, Politecnico di Milano, Italy
6 Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche, Milano, Italy
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Reference
O. Abdul-Aziz et al., “Photoinduced electronic band dynamics and defect-mediated surface potential evolution in PdSe2”, npj 2D Materials and Applications 9, 110 (2025); DOI: 10.1038/s41699-025-00629-2.
