Optoelectronic properties in 2D junctions

CIC nanoGUNE Seminars

Nieves Morquillas, Nanoimaging Group
nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
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Optoelectronic properties in 2D junctions Transition metal dichalcogenides (TMD) MX2 (M=Mo, W, Nb; X= S, Se, Te) are promising 2D materials for optoelectronic applications due to their electrical and structural characteristics. Since 2D materials are characterized by a weak interlayer Van der Waals interaction, it is possible to isolate a single layer and create heterostructures by mechanical exfoliation of TMDs crystals. Monolayers (ML) of semiconducting TMDs present different optical and electrical properties than bulk. Here, we investigate and characterize the optoelectronic properties of TMD devices by combined photoluminescence and electric transport measurements, using a scanning confocal microscope (Scanning Photocurrent Microscopy-SPCM). First we study devices made of a MoS2 single layer flake with Ti/Au contacts. The contact between a metal and the TMD semiconductor shows a Schottky barrier with a characteristic symmetric diode behavior. We show that the Schottky barriers actively participate in the generation of photocurrent by separating the excitons created there by a focused green laser. Interestingly, MoS2 devices including an interface between a single and a double layer also show a potential barrier at the boundary created by the different bandgap of the ML (Eg = 1.9 eV) and BL (Eg = 1.2 eV). The presence of this new barrier induces an asymmetric diode-like behavior in the device’s performance. We also investigated the barriers at TMD heterostructure devices formed by a MoS2 monolayer on top of a thick NbSe2 flake. NbSe2 is a strong electron acceptor metal (work function 5.8 eV). We found very effective photoactive areas on the MoS2 monolayer at the interface with NbSe2 flake. We interpret this in terms of a strong change of the MoS2 doping due to the strong electron acceptor character of the NbSe2.