Ambient effects on photogating in MoS2 photodetectors.

Atomically thin transition metal dichalcogenides (TMDs) are ideal candidates for ultrathin optoelectronics that are flexible and semitransparent. Photodetectors based on TMDs show remarkable performance, with responsivity and detectivity higher than 103 AW-1 and 1012 Jones, respectively, but they are plagued by response times as slow as several tens of seconds. Although it is well established that gas adsorbates such as water and oxygen create charge traps and significantly increase both the responsivity and the response time, the underlying mechanism is still unclear. Here we study the influence of adsorbates on MoS2 photodetectors under ambient conditions, vacuum and illumination at different wavelengths. We show that, for wavelengths sufficiently short to excite electron-hole pairs in the MoS2, light illumination causes desorption of water and oxygen molecules. The change in the molecular gating provided by the physisorbed molecules is the dominant contribution to the device photoresponse in ambient conditions.

Highly sensitive MoS2 photodetectors with graphene contacts.

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) are ideal candidates to create ultra-thin electronics suitable for flexible substrates. Although optoelectronic devices based on TMDs have demonstrated remarkable performance, scalability is still a significant issue. Most devices are created using techniques that are not suitable for mass production, such as mechanical exfoliation of monolayer flakes and patterning by electron-beam lithography. Here we show that large-area MoS2 grown by chemical vapor deposition and patterned by photolithography yields highly sensitive photodetectors, with record shot-noise-limited detectivities of 8.7 × 1014 Jones in ambient condition and even higher when sealed with a protective layer. These detectivity values are higher than the highest values reported for photodetectors based on exfoliated MoS2. We study MoS2 devices with gold electrodes and graphene electrodes. The devices with graphene electrodes have a tunable band alignment and are especially attractive for scalable ultra-thin flexible optoelectronics.