(Invited) Understanding Electronic and Optoelectronic Properties of MoS2 and Its Junctions with Graphene
Authors: Strano Michael S 
Published: February 2020

Atomically thin MoS2 is of great interest for electronic and optoelectronic applications because of its unique two-dimensional (2D) quantum confinement, however, the scaling of optoelectric properties of MoS2 and its metallic junctions with layer number remains unaddressed. In this work, we utilize photocurrent spectral atomic force microscopy (PCS-AFM) to image the current and photocurrent generated between a biased PtIr tip and MoS2 between n = 1 to 10 layers. Dark current measurements in both forward and reverse bias reveal characteristic diode behavior well described by Fowler-Nordheim tunneling with a monolayer barrier energy of 0.605 eV and an effective barrier scaling linearly with layer number. Under illumination at 600 nm, the photocurrent response shows a marked decrease up to n = 4 but increasing thereafter, which we describe using a model that accounts for the linear barrier increase at low n, but increased light absorption at larger layer number creating a  minimum at n = 4. Comparative 2D Fourier analysis of physical height and photocurrent images shows high frequency spatial variations in substrate/MoS2 contact that exceed the frequencies imposed by the ITO substrates. These results should aid in the design and understanding of optoelectronic devices based on quantum confined MoS2.  We also studied MoS2-graphene heterostructures as a means of combining the advantages of high carrier mobility in graphene with the permanent band gap of MoS2. We report the electron transfer, photoluminescence, and gate-controlled carrier transport in such heterostructures, showing that that the junction behaves as a Schottky barrier, whose height can be artificially controlled by gating or doping graphene. When the applied gate voltage (or the doping level) is zero, the photoexcited electron hole pairs in monolayer MoS2 can be split by the heterojunction, significantly reducing the photoluminescence. By applying negative gate voltage (or p-doping) in graphene, the interlayer impedance formed between MoS2 and graphene exhibits a 100-fold increase. For the first time, we show that the gate-controlled interlayer Schottky impedance can be utilized to modulate carrier transport in graphene, significantly depleting the hole transport, but preserving the electron transport. Accordingly, we demonstrate a new type of FET device, which enables a controllable transition from NMOS digital to bipolar characteristics. In the NMOS digital regime, we report a very high room temperature on/off current ratio (ION/IOFF ∼ 36) in comparison to graphene-based FET devices without sacrificing the field-effect electron mobilities in graphene. By engineering the source/drain contact area, we further estimate that a higher value of ION/IOFF up to 100 can be obtained in the device architecture considered. The device architecture presented here may enable semiconducting behavior in graphene for new electronic applications.  Lastly, we discuss graphene as an interfacial optical biosensor, showing that it possesses two pKa values in alkaline and basic ranges.  We use this response to measure dopamine in real time, spatially resolved at the interface with living PC12 cells which efflux dopamine, indicating graphene’s promise as an interfacial sensor in biology.