Studies of Metal Dichalcogenide Formation Using Electrochemical ALD
Authors: Stickney John Lewellen , Tsang Chu 
Published: February 2020

Group VI transition metal dichalcogenides (TMDCs) are graphene-like 2D layered semiconductors that exhibit anisotropic conductivity, which favors conductivity along the basal planes.1 In particular, MoS2, MoSe2 and WSe2 have received special interests for their potential toward photoelectrochemical and photoelectronic applications. These materials have been shown to be viable hydrogen evolution reaction (HER) catalysts in photoelectrochemical water-splitting devices.1-2 Applications in solar energy conversion are feasible due to the direct band gaps these materials possess, which have energies range from near-IR to visible. They are also highly stable against photocorrosion due to the d-d transition that occur upon photoexcitation.3 Among the TMDCs, MoSe2 synthesis and characterization is currently the main focus in this group. In photovoltaic (PV) devices that commonly use Mo as the back contact, having MoSe2 as the intermediate layer between the PV absorber and the Mo back contact may create a more favorable ohmic-type contact that reduces resistive losses.4 A lower resistivity ohmic contact helps to reduce carrier recombination and improves conversion efficiency of a PV device.
Formation of MoSe2 nanofilms by electrochemical atomic layer deposition (E-ALD) is currently being investigated by this group. E-ALD can be used to grow high quality nanofilms by alternating precursor solutions through a flow cell to deposit the desired elements up to one atomic layer at a time. Such fine control is possible by utilizing the underpotential deposition (UPD) of elements on each other. Pure Mo electrodeposition is hampered by the presence of HER, but “induced co-deposition” is possible by electrodepositing Mo in the presence of other metal ions.5 In E-ALD of MoSe2, UPD Se plays the role of this other metal ion. Electrochemical studies for growing MoSe2 by E-ALD as well as characterization of preliminary MoSe2 films will be presented.
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