Electrodeposition of nanomaterials: examination of growth at the atomic interfaces
Bui, Nhi N.
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The electrochemical formation of ZnS nanofilms has been demonstrated using a derivative of the Electrochemical Atomic Layer Deposition (E-ALD), called Potential Pulse Atomic Layer Deposition (PP-ALD). Similar to E-ALD, PP-ALD focuses on the use of underpotential deposition – an electrochemical surface-limited reaction, to form materials one atomic layer at a time. By changing the potentials instead of exchanging solutions between cycles, PP-ALD can reduce the deposition time. The effects of pHs, potentials, pulse durations, intermediate pulses, and annealing temperatures were investigated to optimize the deposition conditions. Surface-limited reaction was achieved for Zn2+ and thiosulfate precursors. The linear correlation between the number of deposition cycles and the thickness of the deposits suggested that the growth rate can be controlled by PP-ALD. EPMA, SEM, ellipsometry, Photoelectrochemistry (PEC), and XRD were used to characterize the thin film deposits. The chemistry for growing ZnS has opened routes to forming similar compounds such as SnS and CuS, and eventually can be combined with Cu2Se and SnSe to form a superlattice structure of Cu2ZnSn(S,Se)4– a PV absorber materials. The atomic-level processes occurring during the electrodeposition of germanene were investigated using the Electrochemical Scanning Tunneling Microscopy and in situ Surface-enhanced Raman Spectroscopy. In potential range between -0.7 V and -0.85 V (vs Ag/AgCl), Ge is first deposited in the face-center-cubic region of the Au reconstruction. At -0.9 V, Ge domains merge together to form a sheet, expelling some remaining Herringbone lines and form new Au islands that are also covered with Ge layer. Germanene domains are found to be 2-3 nm in size. Within a domain wall, honeycomb structure characteristic of germanene was observed. Formation of a second layer occurs at -1.0 V. An 8 mW 780 nm laser with 3.1 micrometer spot size can induce surface crystallization. The first layer of germanene forms a Raman peak at 200 cm-1, whereas the second layer is at 295 cm-1. It is proposed here that interaction with the Au substrate had induced some strain to the vibration mode of the first layer, shifting it to a lower energy compared to the more liberated second layer. A possible exfoliation of the deposit due to hydrogen evolution at potential more negative than -1.0 V was noticed. More basic pH has resulted in higher Raman intensity. During a cycle of Bait and Switch E-ALD, the first Te deposit is found to go underneath the Ge layers and bonds with Au substrate.