We explore near-infrared optical transitions and photocurrent dynamics in few-layer PdSe2 phototransistors through spatially-resolved photocurrent measurements.

Gate‐/wavelength‐dependent scanning photocurrent measurements of black phosphorous (BP)–MoS2 heterojunctions have shown that the Schottky barrier at the MoS2–metal interface plays an important role in the photoresponse dynamics of the heterojunction. When the Fermi level is close to the conduction band of MoS2, photoexcited carriers can tunnel through the narrow depletion region at the MoS2–metal interface, leading to a short response time of 13 µs regardless of the incident laser wavelength. This response speed is comparable or better than that of other few‐layer BP–MoS2 heterojunctions. Conversely, when the MoS2 channel is in the off‐state, the resulting sizeable Schottky barrier and depletion width make it difficult for photoexcited carriers to overcome the barrier. This significantly delays the carrier transit time and thus the photoresponse speed, leading to a wavelength‐dependent response time since the photoexcited carriers induced by short wavelength photons have a higher probability to overcome the Schottky barrier at the MoS2–metal interface than long wavelength photons. These studies not only shed light on the fundamental understanding of photoresponse dynamics in BP–MoS2 heterojunctions, but also open new avenues for engineering the interfaces between 2D materials and metal contacts to reduce the response time of 2D optoelectronics.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe 2 -based heterostructures in which heavily p-doped WSe 2 and MoS 2 flakes share an undoped MoSe 2 channel, allowing us to directly compare the optoelectronic properties of MoSe 2 -based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe 2 −WSe 2 and MoSe 2 −MoS 2 heterojunctions with a photoresponse time constant of ∼16 μs, surmounting previous MoSe 2 -based devices by three orders of magnitude.Further studies have shown that the fast response is independent of the integrated 2D materials (WSe 2 or MoS 2 ) but is likely attributed to the high carrier mobility of 260 cm 2 V −1 s −1 in the undoped MoSe 2 channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe 2 −WSe 2 /MoS 2 heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼10 14 Jones has been achieved in MoSe 2 -based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe 2 -based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.