title: Layer number dependence of MoS2 photoconductivity using photocurrent spectral atomic force microscopic imaging.
abstract: 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 optoelectronic properties of MoS2 and its junctions with metals as a function of layer number as well the spatial variation of these properties remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy (PCS-AFM) to image the current (in the dark) and photocurrent (under illumination) generated between a biased PtIr tip and MoS2 nanosheets with thickness ranging between n = 1 to 20 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 0.61 eV and an effective barrier scaling linearly with layer number. Under illumination at 600 nm, the photocurrent response shows a marked decrease for layers 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 n creating a minimum at n = 4. Comparative 2D Fourier analysis of physical height and photocurrent images shows high spatial frequency spatial variations in substrate/MoS2 contact that exceed the frequencies imposed by the underlying substrates. These results should aid in the design and understanding of optoelectronic devices based on quantum confined atomically thin MoS2.
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Youngwoo
authors.initials: Y
authors.lastname: Son
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Qing Hua
authors.initials: QH
authors.lastname: Wang
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Joel A
authors.initials: JA
authors.lastname: Paulson
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Chih-Jen
authors.initials: CJ
authors.lastname: Shih
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Ananth G
authors.initials: AG
authors.lastname: Rajan
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918, United States.
authors.forename: Kevin
authors.initials: K
authors.lastname: Tvrdy
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Sojin
authors.initials: S
authors.lastname: Kim
authors.affiliation: §KUWAIT-MIT Center for Natural Resources and Environment, Cambridge, Massachusetts 02139, United States.
authors.forename: Bassam
authors.initials: B
authors.lastname: Alfeeli
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Braatz
authors.affiliation: †Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Strano
keywords: MoS2; conductive AFM; layered dichalcogenide; metal-MoS2 junction; photoconductive AFM; photoconductivity