Davide SCIACCA's thesis
Monday 25 January 2021 at 10.30am.
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Jury :
Henri HAPPY, University Professor, IEMN, Chairman of the jury
Laurence MASSON, University Professor, CINAM, Rapporteur
Andrew MAYNE, Research Director, ICMO, Rapporteur
Geoffroy PREVOST, Research Director, INSP, Examiner
Bruno GRANDIDIER, Research Director, IEMN, Thesis Director
Pascale DIENER, Associate Professor, IEMN, Thesis co-supervisor
Summary:
In this thesis, we studied the properties of two 2D germanium materials: germanene, the equivalent of graphene, and a multilayer stack of germanene terminated by methyl groups. Because of their flamed atomic structure and strong spin-orbit coupling, these materials differ from graphene and graphite. Although they have been extensively studied in theory, their physical properties remain poorly characterised.
In the case of germanene, the study was carried out by depositing germanium on an aluminium (111) surface under ultra-high vacuum. For relatively low temperatures, around 100°C, the growth of germanene is epitaxial with two structures: reconstruction (3×3) and reconstruction (√7x√7). Scanning tunneling microscopy was used to further our understanding of these phases. Firstly, we looked at the electronic properties. Spectroscopic measurements were carried out at temperatures of 77K and 5K. Unfortunately, they did not reveal the true nature of germanene due to the strong electronic coupling of this material with the aluminium surface. Through an unexpected diversity of spectra, this analysis showed the weak adhesion of germanene to the Al (111) surface, which contaminated the tip of the microscope.
In addition to spectroscopic measurements, the growth of small leaves enabled us to study their edges. Observations using scanning tunneling microscopy have shown that these leaves grow in the plane of the aluminium atomic terraces. Their edges generally show a sharper contrast than the rest of the leaf. To better understand this change in contrast, ab-initio calculations based on density functional theory (DFT) were carried out. They showed the key role of aluminium atoms in the formation of edges, with zigzag or armchair structures.
Unlike single-leaf germanene, which requires epitaxial growth, germanan crystals can be synthesised chemically, ensuring electronic decoupling of the material from a host substrate. We carried out a multi-physics analysis of crystals passivated by methyl groups. Two types of behaviour were discovered. The largest crystals, around 10 micrometres in lateral dimension, are polycrystalline, contain water molecules intercalated between the layers or have oxidised surfaces and charge under electron irradiation due to the presence of isopropanol at the interface with the host substrate. In contrast, the smallest crystals, identified as the purest, were the most likely to be characterised by four-probe ultra-high vacuum transport measurements. These measurements revealed hole transport, which occurs within the volume of the microstructure. Compared with hydrogen-terminated germanan, the crystals exhibit higher conductivity and stronger chemical inertness, making them a promising candidate for integration into devices.
Abstract:
In this PhD thesis, we have studied the properties of two 2D materials made of germanium: germanene, the equivalent of graphene, and a multilayer stack of germanene terminated with methyl groups. Due to a buckled atomic structure and a strong spin orbit coupling, these materials stand out from graphene and graphite. Although much studied in theory, their physical properties remain little characterized.
In the case of germanene, the study was carried out by depositing germanium on an aluminium (111) surface in ultra-high vacuum. For relatively low temperatures, around 100° C, the growth of germanene is epitaxial with two structures: the (3×3) reconstruction and the (√7x√7) reconstruction. Scanning tunneling microscopy has been used to deepen our knowledge of these phases. First of all, we were interested in the electronic properties. Spectroscopic measurements were carried out at temperatures of 77K and 5K. Unfortunately, they did not reveal the true nature of germanene due to the strong electronic coupling of this material with the aluminum surface. Throughout an unexpected diversity of spectra, this analysis showed the weak adhesion of germanene to the Al(111) surface, which was found to contaminate the tip of the microscope.
In addition to spectroscopic measurements, the growth of small-sized sheets enabled the study of their edge. Observations by tunneling microscopy showed that these sheets grow in the plane of the aluminium atomic terraces. Their edges generally present a clearer contrast than the rest of the sheet. To better understand this change of contrast, ab-initio calculations based on density functional theory (DFT) have been performed. They showed the key role of aluminium atoms in the formation of edges, with both zigzag or armchair structures.
Unlike single-sheet germanene which requires an epitaxial growth, germanane crystals can be chemically synthesized, which ensures the electronic decoupling of the material from a host substrate. We carried out a multi-physics analysis of crystals passivated by methyl groups. Two types of behaviour were discovered. The largest crystals, around 10 micrometres in lateral dimension, are polycrystalline, contain water molecules intercalated between the layers or have oxidized surfaces and become charged under electron irradiation due to the presence of isopropanol at the interface with the host substrate. In contrast, the smallest crystals, identified as the purest, were most prone to be characterized by ultra-high vacuum four-probe transport measurements. These measurements revealed a transport of holes, which occurs in the volume of the microstructure. Compared with hydrogen-terminated germanane, the crystals show a higher conductivity and stronger chemical inertness, making it promising for their integration into devices.
Publications
1. Sciacca, D., Peric, N., Berthe, M., Biadala, L., Pirri, C., Derivaz, M., ... & Grandidier, B. (2019). Account of the diversity of tunneling spectra at the germanene/Al (1 1 1) interface. Journal of Physics: Condensed Matter, 32(5), 055002.
2. Franchina Vergel, N. A., Post, L. C., Sciacca, D., Berthe, M., Vaurette, F., Lambert, Y., ... & Grandidier, B. (2020). Engineering a Robust Flat Band in III-V Semiconductor Heterostructures. Nano Letters.
