Near-infrared optical transitions in PdSe2 phototransistors

Abstract

We investigate electronic and optoelectronic properties of few-layer palladium diselenide (PdSe₂) 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 PdSe₂. More interestingly, when the thickness of PdSe₂ flakes increases, more and more photocurrent resonance peaks appear in the near-infrared region, suggesting strong interlayer interactions in few-layer PdSe₂ help open up more optical transitions between the conduction and valence bands of PdSe₂. Moreover, gate-dependent measurements indicate that remarkable photocurrent responses at the junctions between PdSe₂ and metal electrodes primarily result from the photovoltaic effect when a PdSe₂ phototransistor is in the off-state and are partially attributed to the photothermoelectric effect when the device turns on. We also demonstrate PdSe₂ 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 PdSe₂ phototransistors are ∼156 μs and ∼163 μs, respectively, which are more than three orders of magnitude faster than previous PdSe₂ work and two orders of magnitude over other noble metal dichalcogenide phototransistors, offering new avenues for engineering future optoelectronics.