Speaker
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2012 brought the first reports of a new member of the 2D material family: a hexagonal honeycomb of Si atoms deposited on the Ag(111) surface called “silicene”[1]. The characteristics and stability of freestanding silicene had previously been theoretically explored [2], and there was a strong push to determine if the epitaxial sheets possessed the promising qualities of their hypothetical freestanding counterparts. Initially, ARPES experiments were thought to indicate that epitaxial silicene had a gapped Dirac cone in its electronic structure [1], as would be expected of freestanding silicene with a broken inversion symmetry. This enticing result, however, would be later overturned through a combination of experimental and theoretical techniques [3-5], and it would eventually be concluded that the epitaxial silicene sheet was in fact metallic with a strong cohesion to the underlying Ag(111) face. However, this conclusion would prove controversial [6,7], as the ambiguity of the ARPES data left some room for interpretation as to whether specific electronic features belonged to the epitaxial Si, the Ag substrate, or represented a hybridization between the two.
Soft X-ray emission and absorption spectroscopy (XES and XAS) are synchrotron-based experimental techniques for directly probing the (element-specific) partial density of electronic states (PDOS) in the valence and conduction bands of a material. When performed at the Si L2,3 emission and Si 2p absorption edges, XES and XAS allowed us to unambiguously show that the Si valence and conduction states were continuous across the Fermi energy; i.e. that the silicene overlayer was indeed metallic [3]. However, for the material to be of use, it must be isolated from the substrate. One suggested way of achieving these characteristics is to produce a multilayer of silicene on the Ag(111) surface. However, other reports insist that bilayers and multilayers are inherently unstable, collapsing into bulk Si nanocrystals shortly after the monolayer deposition is complete [8].
Our DFT calculations [9] predict a stable, AA-stacked silicene bilayer on Ag(111) that corresponds nicely to the scanning tunnelling microscopy (STM) bilayer observations. Unfortunately, these same DFT calculations predict a similar electronic structure as that of the monolayers, namely metallic and bound to the Ag(111). However, our XES and XAS measurements indicate a transition to bulk, sp3-hybridized Si beginning shortly after the completion of a monolayer, supporting the low-energy electron microscopy study that first suggested the nucleation of the silicene sheets to bulk crystals [9]. Finally, we will discuss our recent study in which we explore how Silicene oxidizes [10].
References:
[1] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M.C. Asesnsio, A. Resta, B. Ealet and G. Le Lay. Phys. Rev. Lett. 108, 155501 (2012).
[2] S. Cahangirov, M. Topsakal, E. Aktürk, H. Şahin and S. Ciraci. Phys. Rev. Lett. 102, 236804 (2009).
[3] N.W. Johnson, P. Vogt, A. Resta, P. De Padova, I. Perez, D. Muir, E. Z. Kurmaev, G. Le Lay and A. Moewes. Adv. Funct. Mater. 24, 5253 (2014)
[4] S. Cahangirov, M. Audiffred, P. Tang, A. Iacomino, W. Duan, G. Merino and A. Rubio. Phys. Rev. B 88, 035432 (2013).
[5] D. Tsoutsou, E. Xenogiannopoulou, E. Golias, P. Tsipas and A. Dimoulas. Appl. Phys. Lett. 103, 231604 (2013).
[6] S. Huang, W. Kang and L. Yang. Appl. Phys. Lett. 102, 133106 (2013).
[7] J. Avila, P. De Padova, S. Cho, I. Colambo, S. Lorcy, C. Quaresima, P. Vogt, A. Resta, G. Le Lay, M. C. Asensio. J. Phys.: Condens. Matter 25, 262001 (2013).
[8] P. De Padova, P. Vogt, A. Resta, J. Avila, I. Razado-Colambo, C. Quaresima, C. Ottaviani, B. Olivieri, T. Bruhn, T. Hirahara, T. Shirai, S. Hasegawa, M. C. Asensio and G. Le Lay. Appl. Phys. Lett. 102, 163106 (2013).
[9] N.W. Johnson, D. Muir, E.Z. Kurmaev, and A. Moewes, Adv. Funct. Mat. 25, 4083(2015).
[10] N.W. Johnson, D. Muir, A. Moewes, Sci. Rep. 6, 22510 (2016).
