Giant persistent photoconductivity in monolayer MoS2 field-effect transistors

Abstract

Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD-based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (λ = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large number of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis for the defect-based engineering of the electronic and optical properties of TMDs for device applications.

2D van der Waals Heterojunction of Organic and Inorganic Monolayers for High Responsivity Phototransistors

“…The measured thickness of 0.89 nm confirms the monolayer nature of the MoS 2 crystals according to the previous study. [28] The successful transfer of the MoS 2 monolayer onto BTBT-SAM is confirmed by the height profile (Figure S1b, Supporting Information) where a consistent thickness is observed.…”

Energy-Level Alignment at Interfaces between Transition-Metal Dichalcogenide Monolayers and Metal Electrodes Studied with Kelvin Probe Force Microscopy

“…Defects in monolayers of MoS 2 and WS 2 , produced in the same laboratory and under the same conditions, have been studied previously. 10 The areal density of defects in ML-MoS 2 is about 0.8 nm –2 , and they mostly consist of single S vacancies, while double S 2 vacancies amount to ∼8.5% of the total defect density. Although MoS 2 layers exfoliated from bulk crystals may have lower defect density, CVD growth offers significant advantages regarding scalability and is thus of great interest for future applications.…”

Tuning exciton recombination rates in doped transition metal dichalcogenides

“…In particular, the giant enhancement of the photoluminescence (PL) quantum yield (QY) in the monolayer limit [1] holds great promise for integrated, flexible and high-speed light emitting devices [8]. In this regard, it is paramount to consider that light emission in TMDs strongly depends on intrinsic and extrinsic factors such as doping, environment, defects, strain and the photoexcited carrier density [3,9]. This work focuses on the interplay between doping and photoexcited carrier concentration in monolayer TMDs and explores their impact on the radiative and non-radiative recombination pathways of excitons and trions.…”