Germanene is a single layer allotrope of Ge, with a honeycomb structure similar to graphene. This report concerns the electrochemical formation of germanene in a pH 4.5 solution. The studies were performed using in situ Electrochemical Scanning Tunneling Microscopy (EC-STM), voltammetry, coulometry, surface X-ray diffraction (SXRD) and Raman spectroscopy to study germanene electrodeposition on Au(111) terraces. The deposition of Ge is kinetically slow and stops after 2–3 monolayers. EC-STM revealed a honeycomb (HC) structure with a rhombic unit cell, 0.44 ± 0.02 nm on a side, very close to that predicted for germanene in the literature. Ideally the HC structure is a continuous sheet, with six Ge atoms around each hole. However, only small domains, surrounded by defects, of this structure were observed in this study. The small coherence length and multiple rotations domains made direct observation with surface X-ray diffraction difficult. Raman spectroscopy was used to investigate the multi-layer Ge deposits. A peak near 290 cm−1, predicted to correspond to germanene, was observed on one particular area of the sample, while the rest resembled amorphous germanium. Electrochemical studies of germanene showed limited stability when exposed to oxygen.

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.

This paper will discuss possible formation of germanene electrochemically. Germanene should be a single layer allotrope of Ge. The techniques of in-situ electrochemical STM (EC-STM), voltammetry, coulometry, and micro-Raman have been used to investigate the electrochemical formation of germanene. Studies on Au(111) show that the initial deposition of Ge is kinetically slow and somewhat unstable, whereas the self-limited layer of Ge is stable and shows atomic distances of about 0.44 nm ± 0.02 nm. Micro-Raman was performed on Ge nanofilms, but only displayed a shift near 290 cm-1 in one area. Given the STM results, it appears that the coherence of the germanene domains will need to be increased in order to more consistently produce the Raman signal. The data presented suggest that germanene has been formed electrochemically, although only as a minority species.

No abstract

Efforts are described to identify an atomic layer (AL) passivating agent for Cu, allowing it to be transferred from solution, between solutions, or from solution into vacuum without oxidation. Atomic layers investigated included tellurium, selenium and iodine. Potentials and pH used to form the AL were investigated. Elemental ratios from auger electron spectroscopy (AES), such as O/Cu, and low energy electron diffraction (LEED) patterns were used to characterize the Te, Se and I AL before and after exposure to oxygen. The AL for all three elements resulted in 1/3 ML coverage ( √ 3x √ 3)R30 0 structures. The ability of an AL to passivate the Cu(111) surface was investigated by exposing it to solution vapor in 1 atm of UHP Ar for 5 minutes within the EC ante-chamber. The resulting surface was then characterized in the analysis chamber. The intent was to mimic transfer between tools in a FAB. In addition, AL coated Cu(111) substrates were exposed to atmospheric oxygen for over 10 minutes, followed by characterization. Exposure to solution vapor and 1 atm UHP Ar resulted in no significant oxygen uptake, though small oxygen signals were detected for the Se AL and the I AL. However, only the Te AL provided passivation to atmospheric oxygen for 10 minutes.

Germanene is the Ge analogue Silicene or Graphene. A number of ab-initio calculations in the literature suggest that Germanene sheets should exist. Those reports suggest that the material would have a buckled structure, where half the atoms are about 0.7 nm higher than the other half. Micro Raman, electrochemistry and in-situ scanning tunneling microscopy (EC-STM) have been used to better understand the electrochemical growth of Ge from aqueous solutions, from the first atomic layers on Au(111). Our initial studies of the electrodeposition of Ge resulted in some interesting, though not clearly understood results, such as why only about 2 ML of Ge could be formed by direct electrodeposition on Au, depending on the pH. Some reports in the literature indicate that thicker deposits of Ge can be formed by electrodeposition on electrode surfaces other than Au. It is not yet clear why Au would be different. In-situ STM studies of the first atomic layers of Ge have shown a number of processes which indicate that nanoscopic atomically flat layers are forming on the surface. The authors believe that Germanene is being formed. In addition, there is clear evidence of the formation of a surface alloy with the Au substrate, though the phase diagram suggests no significant alloying at room temperature, used in these studies. Further, though Au(111) displays the expected reconstructed (√3X23) “herringbone” structure expected, incorporation of Ge into the this structure occurs very slowly by reduction at negative potentials, starting at defects and step edges. Previous work by this group, lead to development of an electrochemical ALD cycle for the growth of thicker Ge films. That cycle is being further investigated, as some spots in early deposits displayed a Raman spectrum indicative of the presence of germanene. Presently it appears that at sufficiently negative potentials, germanene is being formed. That is, a honeycomb structure is observed, but the domain size is on the order of one or two nm, at which point, defects result in a twist in the structure. References Liang, et al., Langmuir, 26(4) (2010) 2877-2884 Xuehai Liang and John L. Stickney, Chemistry of Materials, 23(7)(2011)1742-1752 Xuehai Liang, et al., J. Am. Chem. Soc., 133(2011)8199-8204 M. A. Ledina, et al., ECS Transactions, 66 (6) 129-140 (2015)

The group IV elements including C, Si, Ge and Sn have all shown to form 2D allotropes that have drastically different properties from their bulk component. Following the discovery of graphene and silicene, the study of germanene has garnered increasing interest and has raised many questions pertaining to its exact structure, properties and formation. A number of theoretical calculations have proposed structures for germanene with varying bond distances and degree of buckling. Some experimental works suggest that germanene could be deposited via molecular beam epitaxy (MBE) using germanium wafer as source. This study proposes an alternative route of forming germanene via electrodeposition 1. Electrochemical scanning tunneling microscopy (EC-STM), and in situ surface-enhanced micro Raman (SERS) are used as means to gather information on the deposition process of Ge on Au(111). The optimal condition to grow a single atomic layer of Ge was selected based on atomic resolution images from EC-STM. Various phases of Ge were observed, detailing the transition from a precursor stage to complete honeycomb rings of germanene. The nucleation process started with growth of some small Ge domains, followed by a lateral expansion through the edges. After forming the second layer, the surface became passivated as expected for an insulating gap of a 2D bulk material. Current efforts are being directed toward forming a thicker germanene film using our previously developed methodology called Ge-Te bait-and-switch electrochemical ALD cycle 2-4. The approach involves using Te as a capping layer and as a basal support to allow Ge to form new layer. This work is supported by the National Science Foundation. References 1. Ledina, M. A., Liang, X., G. Kim, Y., Jung, J., Perdue, B., Tsang, C., Soriaga, M.P., Stickney, J. L., Investigations into the Formation of Germanene using Electrochemical Atomic Layer Deposition (E-ALD). ECS Transaction 2015, 66(6), 129-140. 2. Liang, X. H.; Jayaraju, N.; Thambidurai, C.; Zhang, Q. H.; Stickney, J. L., Controlled Electrochemical Formation of GexSbyTez using Atomic Layer Deposition (ALD). Chem Mater 2011, 23(7), 1742-1752. 3. Liang, X. H.; Kim, Y. G.; Gebergziabiher, D. K.; Stickney, J. L., Aqueous Electrodeposition of Ge Monolayers. Langmuir 2010, 26(4), 2877-2884. 4. Liang, X. H.; Zhang, Q. H.; Lay, M. D.; Stickney, J. L., Growth of Ge Nanofilms Using Electrochemical Atomic Layer Deposition, with a "Bait and Switch" Surface-Limited Reaction. J Am Chem Soc 2011, 133 (21), 8199-8204.

Germanene, Ge analogue of graphene, is a near-planar 2D Dirac material stable in a low-buckled graphene-like honeycomb geometry and electronic properties.1 Graphene’s interesting features lead to booming interest in graphene, and now we are exploring other elemental group IV 2D materials that have graphene-like geometry and electronic properties such as siliecene2 ,3, stanene4 ,5, germanane (hydrogenated germanene)6, and germanene.7 ,8,9  Theoretical studies of the 2D honeycomb structured germanene (Ge) have been reported earlier10, but experimental evidence of germanene synthesis have been reported only in recent years from various groups utilizing ultra-high vacuum (UHV) system.11 A new photon-assisted electrochemical deposition of germanene has been studied, and evidences for germanene formation are found using in situ scanning tunneling microscope (STM) and in situ Raman spectroelectrochemistry. The STM images show that germanium deposited on a single crystalline gold (111) surface exhibit a honeycomb germanene structure. Using i n situ surface-enhanced Raman spectroelectrochemistry the germanene peak was reproducibly observed when the germanium deposit was exposed to laser; this peak agrees with the theoretical prediction for the “G-like” mode in germanene and previous experimental result.12 ,13  ADDIN EN.REFLIST 1. Wang, J.; Deng, S.; Liu, Z.; Liu, Z., The rare two-dimensional materials with Dirac cones. National Science Review 2015, 2 (1), 22-39. 2. Grazianetti, C.; Cinquanta, E.; Molle, A., Two-dimensional silicon: the advent of silicene. 2d Materials 2016, 3 (1). 3. Houssa, M.; Dimoulas, A.; Molle, A., Silicene: a review of recent experimental and theoretical investigations. Journal of Physics: Condensed Matter 2015, 27 (25), 253002. 4. Zhu, F.-f.; Chen, W.-j.; Xu, Y.; Gao, C.-l.; Guan, D.-d.; Liu, C.-h.; Qian, D.; Zhang, S.-C.; Jia, J.-f., Epitaxial growth of two-dimensional stanene. Nat Mater 2015, 14 (10), 1020-1025. 5. van den Broek, B.; Houssa, M.; Iordanidou, K.; Pourtois, G.; Afanas'ev, V. V.; Stesmans, A., Functional silicene and stanene nanoribbons compared to graphene: electronic structure and transport. 2d Materials 2016, 3 (1). 6. Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E., Stability and Exfoliation of Germanane: A Germanium Graphane Analogue. ACS Nano 2013, 7 (5), 4414-4421. 7. Acun, A.; Zhang, L.; Bampoulis, P.; Farmanbar, M.; Houselt, A. v.; Rudenko, A. N.; Lingenfelder, M.; Brocks, G.; Poelsema, B.; Katsnelson, M. I.; Zandvliet, H. J. W., Germanene: the germanium analogue of graphene. Journal of Physics: Condensed Matter 2015, 27 (44), 443002. 8. Kaloni, T. P.; Schreckenbach, G.; Freund, M. S.; Schwingenschlogl, U., Current developments in silicene and germanene. Physica Status Solidi-Rapid Research Letters 2016, 10 (2), 133-142. 9. Roome, N. J.; Carey, J. D., Beyond Graphene: Stable Elemental Monolayers of Silicene and Germanene. ACS Applied Materials & Interfaces 2014, 6 (10), 7743-7750. 10. Cahangirov, S.; Topsakal, M.; Aktürk, E.; Şahin, H.; Ciraci, S., Two- and One-Dimensional Honeycomb Structures of Silicon and Germanium. Physical Review Letters 2009, 102 (23), 236804. 11. Davila, M. E.; Le Lay, G., Few layer epitaxial germanene: a novel two-dimensional Dirac material. Scientific Reports 2016, 6. 12. Scalise, E.; Houssa, M.; Pourtois, G.; van den Broek, B.; Afanas’ev, V.; Stesmans, A., Vibrational properties of silicene and germanene. Nano Research 2013, 6 (1), 19-28. 13. Tsai, H.-S.; Chen, Y.-Z.; Medina, H.; Su, T.-Y.; Chou, T.-S.; Chen, Y.-H.; Chueh, Y.-L.; Liang, J.-H., Direct formation of large-scale multi-layered germanene on Si substrate. Physical Chemistry Chemical Physics 2015, 17 (33), 21389-21393.