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Characterization and functionalization of germanene films

par Elodie PICO - publié le

Thesis advisor : Pr Thierry ANGOT
Email and address : thierry.angot, Aix Marseille Université, CNRS, PIIM UMR
7345, 13397 Marseille, France
Co-advisor : Dr Eric SALOMON, eric.salomon

Subject description :

The discovery of graphene, a material consisting in a single atomic layer of carbon,
can be considered as a turning point in the development of two-dimensional materials. This breakthrough has opened up the possibility to isolate and explore the fascinating properties of several other materials that offer, to the status of a few atomic layers, new features as well as new applications [1]. Among them, attention has recently been brought to the silicene, the silicon-based analog of graphene. A judicious choice since in 2015 the first silicene-based field-effect transistor was elaborated [2]. In line with this work, we propose to explore the germanene, predicted to be the germanium-based analog of graphene. Ge is in the same column with C and Si in the periodic table, therefore, germanene might share similar novel properties with graphene and silicene. Ge is compatible with Si-technology, which makes germanene as a potential candidate for the next generation of semiconductor materials. In addition, Ge as a larger spin-orbit coupling than C or Si, which involve the germanene to be potentially very attractive for tremendous applications such as topological insulator [3-5]. Like silicene, germanene does not exist as a natural state and have to be synthesized.
To do this, it is possible to mimic the techniques that have made the success of the silicene, i.e. evaporation and adsorption of atoms of germanium on surfaces [6-9]. By the way, it has recently been shown that it is possible to grow the germanene by adsorption of atoms of germanium on Au(111) or Al (111) surfaces [8, 9]. However, as for silicene, there are several surface reconstructions of the germanium atoms, which depend essentially on the coverage and the substrate temperature during the evaporation and detailed studies of these parameters on the evolution of the atomic arrangement of the germanium atoms are necessary, particularly in order to avoid the formation of surface alloys.
The topic of the proposed thesis is therefore to thoroughly identify the growth
parameters of the germanene, namely : the substrate temperature, the evaporation rate and the coverage. To do this we will use low-energy electrons diffraction (LEED), scanning tunneling microscopy (STM) and Auger spectroscopy (AES), to determine the influence of these parameters on the structural properties and growth of germanene films. We will also determine the electronic properties of the germanene by photoelectrons spectroscopy.
Subsequently we will consider the possibility to functionalize the germanene to alter
its electronic properties [10]. With that aim, high-resolution electrons energy loss
spectroscopy (HREELS) will be used to study the adsorption and reactivity of the germanene with respect to various chemical species such as hydrogen, oxygen, halogens or organic compounds.
The thesis will be carried out under ultra-high vacuum conditions, on an experimental
station comprising :
- a sample preparation room equipped with a heating system, three calibrated
evaporation sources, an atomic hydrogen source and an ion gun.
- two analysis chambers housing : a low-energy electron diffraction (LEED)
apparatus, a scanning tunneling microscope (STM), an Auger spectrometer
(AES) and a high-resolution electrons energy loss spectrometer (HREELS).
Finally, to complete the measurements made in the laboratory and particularly for
the purpose of conducting the experiments by photoelectron spectroscopy, the student will also have to work on various synchrotron radiation facilities.

The candidate must have good knowledge in solid state physics and material
sciences. In addition he or she will have to be highly motivated by the experimental aspect of this thesis.

Bibliography :
[1] : M. Xu, et al., Chem. Rev., 113 (2013) 3766
[2] : L. Tao, et al. ; Nat. Nanotech. 10 (2015) 227
[3] : M. Ezawa, Phys. Rev. Lett., 109 (2012) 055502
[4] : P. Liang et al., Solid Sta. Comm. 226 (2016) 19
[5] : G. R. Bhimanapati et al., ACS Nano 9 (2016) 11509
[6] : P. Vogt, et al., Phys. Rev. Lett. 108 (2012) 155501
[7] : E. Salomon, et al., J. Phys. : Condens. Matter 26 (2014) 185003
[8] : M. E. Dávila, et al., New Journal of Physics 16 (2014) 095002
[9] : M. Derivaz, et al., Nano Lett. 15 (2015) 2510
[10] : M. Ye et al., Physica E 59 (2014) 60