Photoinduced Electronic Band Dynamics and Defect-mediated Surface Potential Evolution in PdSe$_2$

TL;DR

This work resolves ultrafast carrier dynamics in PdSe$_2$ by combining time- and angle-resolved photoemission spectroscopy with density functional theory. It identifies an indirect band gap of $0.55$ eV and observes rapid valence-band renormalization and broadening, followed by hot-electron population of the conduction band and defect-assisted hole trapping. A pronounced surface photovoltage up to $67$ meV emerges and persists beyond $50$ ps, driven by mid-gap defect states related to native vacancies that enable long-lived charge separation. Collectively, the results position PdSe$_2$ as a prototypical low-symmetry layered material with exotic, defect-influenced ultrafast photoresponses and potential for rapid surface-potential engineering in optoelectronic devices.

Abstract

We use time- and angle-resolved photoemission spectroscopy (TR-ARPES) combined with density functional theory to investigate ultrafast carrier dynamics in low-symmetry layered semiconducting PdSe$_2$. The indirect bandgap is determined to be 0.55~eV. Following photoexcitation above this gap, we resolve a valence band shift and broadening, both lasting less than a picosecond, consistent with bandgap renormalization and carrier scattering, indicative of strong many-body interactions. Subsequently, hot carriers populate the conduction band minimum and are captured by defect states. A surface photovoltage (SPV) of $\sim$ 67~meV emerges, persisting for over 50~ps, driven by defect-assisted charge separation. The formation of native vacancies, promoted by the low-symmetry lattice, likely gives rise to the mid-gap states responsible for this long-lived SPV response. Detailed analysis of TR-ARPES spectra disentangles the contributions of bandgap renormalization, carrier scattering, defect states, and SPV. These findings establish PdSe$_2$ as a prototypical layered quantum material exhibiting exotic photoresponses on ultrafast timescales.

Figures (5)

• Figure 1: Electronic structure of PdSe$_2$. (a) Top and side views of the PdSe$_2$ crystal structure (upper part), along with a 3D representation of the Brillouin zone showing high-symmetry points. The lower-right part illustrates the corrugated layer structure of PdSe$_2$. (b) $k$-resolved density of states (DOS) calculated using density functional theory (DFT) along high-symmetry paths in reciprocal space. Red squares and dots highlight the upper valence band and lower conduction band positions. (c) Raman spectrum of bulk PdSe$_2$ measured at room temperature using 532 nm excitation, with the polarization of the incident light parallel to the a-axis (lower panel) and to the b-axis (upper panel).
• Figure 2: Determination of the indirect bandgap. (a) Differential ARPES map of PdSe$_{2}$ along the $\Gamma^\prime$-S$^\prime$ direction at a pump-probe delay of 1 ps, showing conduction band (CB) population. Dark brown indicates enhancement, while dark turquoise represents depletion. The CBM and VBM are observed simultaneously, and the black dashed lines serve as guides to the eye for the band dispersions. Vertical red and blue dashed lines mark the energy cuts at the VB and CB, respectively. (b) DFT calculations for the highest VB and lowest CB along the $\Gamma$-S axis at $k_z = 0$ (brown bands) and along $\Gamma^\prime$-S$^\prime$ at $k_z = 0.165$ plane (green bands) across the BZ (for details see the text). This figure demonstrates the strong $k_z$ dependence as the band gap changes from 0.45 eV up to 0.66 eV for $k_z = 0$ and $k_z = 0.165$, respectively. The grey shaded region indicates the energy range of defect-induced states obtained from DFT calculations (see Supplementary Information). (c) EDCs of the VB (red) and CB (blue), extracted along the corresponding dashed lines in panel (a) with an integration width of 0.04 Å$^{-1}$ from the absolute intensity of the TR-ARPES signal. The EDC intensity corresponding to the CB has been multiplied by a factor of 15. The bandgap at this $k_z$ point is approximately 0.80 eV (from the VB peak to the CB peak), whereas the indirect band gap of PdSe$_2$ is calculated to be 0.55 eV; for details, see the main text.
• Figure 3: Photoinduced bandgap renormalization, depletion, and surface photovoltage. (a) Differential ARPES map at the $\Gamma^\prime{}$ point shows transient VB evolution after photoexcitation. (b) Pump-probe traces or the difference in photoemission (PE) intensity at selected energy regions (1–3), each integrated over a 30 meV window, highlight distinct dynamical behaviors. The error bars scale with the fitting uncertainty. (c) Multi-exponential fit reveals components attributed to band gap renormalization, depletion (Dep), and SPV, with shared time constants. (d) Differential ARPES at $\sim$0.65 Å$^{-1}$ shows CB electron population and below-E$\mathrm{_F}$ depletion. (e) The dynamics of the conduction band minimum (region 4) and the depletion region (region 5), integrated over energy widths of 300 and 130 meV respectively, exhibit asymmetric behavior and delayed hole trapping. (f) Comparative rise times indicate ultrafast VB broadening, CB rise $\sim$ 0.3 ps, and delayed hole depletion ($\sim$ 1 ps), represented by the black and brown arrows, respectively.
• Figure 4: Analysis of the surface photovoltage effect. (a) ARPES maps at three different pump fluences and a negative pump-probe delay $\sim -1 \, \text{ps}$, along with a differential ARPES map at high pump fluence highlighting the negative shift of the top of the VB. (b) EDCs (bullets) and their fits (solid lines) near the top of the valence band, integrated around $\sim$ 0.1 Å$^{-1}$, for five different excitation powers, showing a clear band shift with increasing intensity. The shift of the spectra is the result of SPV. Interpretation of the positive- and negative-delay dynamics in VB. (c) The upper inset illustrates the valence and Fermi bands of the bulk and at the surface. The dashed red line represents the band bending potential, $\Phi_{\text{SPV}}$ (surface photo-voltage). The lower inset in this panel sketches the expected ARPES spectrum. (d) A comparison of the event sequences. The black lines perpendicular to the sample surface represent the SPV-induced electric field. (e) SPV versus pump fluence. Experimental data (black marks) were fitted by the test function Eq.(2) (solid red line).
• Figure 5: Valence band dynamics and carrier scattering pathways. (a) VB energy distribution curves at -1 ps and 0.5 ps delays (black and red dots). The solid lines show background-subtracted spectra fitted with Gaussian profiles, revealing a valence band shift and broadening indication of band gap renormalization. (b) Schematic of carrier redistribution after photoexcitation with a 1.2 eV pump: the top VB is directly excited, and electrons reach the CBM in about 300 fs. Holes are transferred to localized surface defect states, triggering the SPV process.