Electrochemical Scanning Tunneling Microscopic Study of the Potential Dependence of Germanene Growth on Au(111) at pH 9.0

Abstract

Germanene is a 2D material whose structure and properties are of great interest for integration with Si technology. Preparation of germanene experimentally remains a challenge because, unlike graphene, bulk germanene does not exist. Thus, germanene cannot be directly exfoliated and is mostly grown in ultrahigh vacuum. The present report uses electrodeposition in an aqueous HGeO solution at pH 9. Germanene deposition has been limited to 2-3 monolayers, thus greatly restricting many applicable characterization methods. The in situ technique of electrochemical scanning tunneling microscopy was used to follow Ge deposition on Au(111) as a function of potential. Previous work by this group at pH 4.5 suggested germanene growth, but no buffer was used, resulting in change in surface pH. The addition of borate buffer to create pH 9.0 solution has reduced hydrogen formation and stabilized the surface pH, allowing systematic characterization of germanene growth versus potential. Initial germanene nucleated at defects in the Au(111) herringbone (HB) reconstruction. Subsequent growth proceeded down the face-centered cubic troughs, slowly relaxing the HB. The resulting honeycomb (HC) structure displayed an average lattice constant of 0.41 ± 0.06 nm. Continued growth resulted in the addition of a second layer on top, formed initially by nucleating around small islands and subsequent lateral 2D growth. Near atomic resolution of the germanene layers displayed small coherent domains, 2-3 nm, of the HC structure composed of six-membered rings. Domain walls were based on defective, five- and seven-membered rings, which resulted in small rotations between adjacent HC domains.

Evidence for Facilitated Surface Transport during Ge Crystal Growth by Indium in Liquid Hg–In Alloys at Room Temperature

“…Here, in this work, we in fact did detect some accumulation of crystalline Ge with solid In foil electrodes. This observation stands in stark contrast to our and other groups’ past attempts to perform crystalline Ge electrodeposition with solid metal electrodes. ,, However, the size of these Ge crystallites were smaller than for the liquid metal electrodes, their arrangement was a thin, compact film, and the total quantity was small. Hence, although fast mobility of electrodeposited Ge atoms on an In surface may still occur, this feature alone is not sufficient to mimic the crystal growth process in ec-LLS.…”

Nanoscale Electrochemical Mapping

“…This structure was then relaxed by further growth in the troughs of the face-centered cubic surface. Finally, any further growth created a second layer of small, weakly bound islands before further lateral growth …”

“…Finally, any further growth created a second layer of small, weakly bound islands before further lateral growth. 140 In situ EC-STM has considerable potential for examining energy materials. The oxygen reduction reaction (ORR) catalyzed by iron-pthalocyanine (FePc) layers adsorbed on Au (111) was recently studied using EC-STM, 141 with images suggesting the formation of the FePc-O 2 complex.…”

Edge-Hydrogenated Germanene by Electrochemical Decalcification-Exfoliation of CaGe2: Germanene-Enabled Vapor Sensor

“…UHV germanene has been grown on various substrates such as Au, 24 Pt, 25 Ag, 26 Al, 27 Si, 28 graphite, 29 , and MoS2. 30 Germanene formation is not limited to bottom-up synthesis as shown by the electrochemical deposition of germanene in aqueous, 1,[31][32][33][34] and nonaqueous medium. [35][36][37] However, electrochemical deposition can be self-limited to approximately several germanene layers filled with the defect domains.…”