Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit remarkable optical and electronic behaviors, including tunable bandgaps and strong light–matter interactions, making them ideal platforms for next-generation devices. Noble-metal TMDs remain a comparatively underexplored subset, even though they exhibit distinctive properties. Notably, PdSE₂and PdS₂ are structurally different from other TMDs. They adopt a highly anisotropic orthorhombic layered structure built from the puckered pentagonal tiling, also known as the 2O phase.
Our colleague Vedran Brusar led a team of scientist from Institute of Physics and Ruđer Bošković Institute, while the PdS₂ were synthesized at the Yunnan University, P.R. China. In a recently published paper the authors described the steady-state and ultrafast optical properties of multilayer PdS2 film based on polarization-dependent Raman and second-harmonic generation microscopy, ellipsometry, and ultrafast differential reflectance. This enabled them to extract the dielectric function, an indirect bandgap of 1.25 eV and confirm the 2O phase. Ultrafast measurements reveal multi-timescale dynamics, including a blueshift during photoinduced carrier relaxation, as well as coherent phonon oscillations of the Ag1 optical mode. Together, these results provide a comprehensive picture of the steady-state and ultrafast properties of PdS₂, linking its equilibrium optical response to its nonequilibrium carrier and phonon dynamics. This work represents the first report of ellipsometry, SHG, and ultrafast transient spectroscopy measurements on multilayer PdS₂.
Ultrafast broadband differential reflectance (DR) measurements were used to probe photoinduced carrier dynamics and coherent phonon response. Since pump photon energy (2.8 eV) significantly exceeds the material’s bandgap, electrons are efficiently excited from the valence band to the conduction band, generating a hot electron-hole plasma. Following pump excitation, a nonthermal carrier distribution relaxes toward a thermalized state through carrier–carrier and carrier–phonon scattering, followed by carrier cooling toward the band-edge extremes. The early-time DR map of PdS₂ is shown in Figure 2, along with selected spectral slices at various probe pulse delays,and time traces at specific probe wavelengths. As seen in similar materials, bandgap renormalization (BGR) takes place upon ultrafast pump absorption due to enhanced screening of the Coulomb potential caused by photoinduced carriers and reduces the effective bandgap. The subsequent reduction of screening within the first few picoseconds leads to recovery of the bandgap and a blueshift of the absorption edge. At the same time, carrier cooling and redistribution toward the band extrema result in state filling near the band edge, further modifying the effective absorption. Owing to the steep absorption profile in this spectral region, these changes produce large variations in the absorption coefficient and, consequently, in both the imaginary and real parts of the refractive index via the Kramers–Kronig relations, giving rise to the observed DR response. The recovery dynamics of the photoinduced response at later times, corresponding to cold carriers, reflects contributions from multiple recombination pathways, such as intrinsic recombination and lattice mediated processes,as well as defect- or surface-assisted recombination.
The importance of this research lies in promising functional behavior as PdS2 films have been used to fabricate field-effect transistors, as well as broadband photodetectors, showing high carrier mobility andstrong photoresponse. Few-layer nanosheets have served as saturable absorbers, enabling doubly Qswitched and mode-locked optical parametric oscillation, and have also been used in ultrafast fiber lasers and passive photonic devices. Detailed knowledge of steady-state and ultrafast transient optical properties of this material may be important in some of these applications.
The full publication can be found on: doi.org/10.1039/d6nr00537c