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We investigate electronic and optoelectronic properties of few-layer palladium diselenide (PdSe 2 ) phototransistors through spatially-resolved photocurrent measurements. A strong photocurrent resonance peak is observed at 1060 nm (1.17 eV), likely attributed to indirect optical transitions in few-layer PdSe 2 . More interestingly, when the thickness of PdSe 2 flakes increases, more and more photocurrent resonance peaks appear in the near-infrared region, suggesting strong interlayer interactions in few-layer PdSe 2 help open up more optical transitions between the conduction and valence bands of PdSe 2 . Moreover, gate-dependent measurements indicate that remarkable photocurrent responses at the junctions between PdSe 2 and metal electrodes primarily result from the photovoltaic effect when a PdSe 2 phototransistor is in the off-state and are partially attributed to the photothermoelectric effect when the device turns on. We also demonstrate PdSe 2 devices with a Seebeck coefficient as high as 74 μV K −1 at room temperature, which is comparable with recent theoretical predications. Additionally, we find that the rise and decay time constants of PdSe 2 phototransistors are ∼156 μs and ∼163 μs, respectively, which are more than three orders of magnitude faster than previous PdSe 2 work and two orders of magnitude over other noble metal dichalcogenide phototransistors, offering new avenues for engineering future optoelectronics. 

Abstract We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS₄in bulk, few-, and single-layer form. Analysis of the mode pattern reveals aC2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS₃where phonons shift as 1/size²and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap.