MoS 2 Heterojunctions by Thickness

In this work, we report lateral heterojunction formation in as-exfoliated MoS2 flakes by thickness modulation. Kelvin probe force microscopy is used to map the surface potential at the monolayermultilayer heterojunction, and consequently the conduction band offset is extracted. Scanning photocurrent microscopy is performed to investigate the spatial photocurrent response along the length of the device including the source and the drain contacts as well as the monolayermultilayer junction. The peak photocurrent is measured at the monolayer-multilayer interface, which is attributed to the formation of a type-I heterojunction. The work presents experimental and theoretical understanding of the band alignment and photoresponse of thickness modulated MoS2 junctions with important implications for exploring novel optoelectronic devices. Semiconducting transition metal dichalcogenides (TMDCs) with a layered crystal structure exhibit unique electrical 1,2 and optical properties 3–5 . TMDCs provide opportunities in exploring new device concepts given their atomic level flatness, and ability to form van der Waals (vdW) heterostructures with strong interlayer coupling 6–8 . For instance, vdW heterobilayers of MoS2/WSe2 have been recently reported to exhibit spatially direct light absorption but spatially indirect light emission, representing a highly intriguing material property 9,10 . Here, we explore the optoelectronic properties of lateral “hetero”-junctions formed on a single crystal of MoS2 of varying thickness (i.e., number of layers). As a result of the quantum confinement effect 11 , when the thickness of a MoS2 crystal is scaled down to a monolayer the optical band gap increases from 1.29 eV (indirect) to 1.85 eV (direct) 12,13 . The change in the band structure and the electron affinity of MoS 2 with layer number opens up the path to the formation of atomically sharp heterostructures, not by changing composition but rather by changing layer thickness 14 . We experimentally examine the surface potential of this thickness modulated heterojunction by using Kelvin probe force microscopy (KPFM). We further use scanning photocurrent microscopy (SPCM) to probe the photoresponse of the junction. A large photocurrent response is observed at the monolayer/ multilayer junction interface which confirms the presence of a strong built-in electric field at the inter face. Device modeling is used in parallel to experiments to understand the underlying mechanism of the observed photocurrents and the band-alignments at the junction interface, suggesting the formation of a type-I heterojunction. SPCM has been previously used to study the photoresponse of metal contacted MoS2 transistors, where the channel thickness for MoS2 was uniform throughout the device 15,16 . The results have shown that the photoresponse is primarily driven by the metal/MoS2 Schottky contacts and photothermoelectric effect 16 . In distinct contrast to previous studies, we observe that the peak photoresponse is spatially located at the MoS2 monolayer/multilayer junction for our lateral heterojunctions and not at the metal contacts.