In this talk I will report on our recent experimental and computational study of the mechanical responses to nanoindentation of graphene films on SiC(0001) [1]. The focus of the talk will be on the mechanical properties of two-layer graphene films, which (unexpectedly) show a transverse stiffness comparable to that of diamond. After introducing the main experimental results, I will present results and insights obtained from density functional theory calculations and indentation simulations. In particular, in agreement with the experiments our computational study shows that the stiffening effect exhibited by two-layer graphene may arise from a pressure-induced sp²-to-sp³ phase transformation, from a soft layered graphene film to a hard diamond-like carbon film [1]. This phase transformation occurs favorably in case of a two-layer graphene film in contact with the Si-face of SiC(0001), it involves (most likely) a negative compressibility transition, and it is hindered by unfavorable stacking geometries in case of three-layer (or thicker) graphene films. I will conclude the talk by presenting an outlook on current and future work.
[1] Yang Gao, Tengfei Cao, Filippo Cellini, Claire Berger, Walter A. de Heer, Erio Tosatti, Elisa Riedo, and Angelo Bongiorno, “Ultra-hard carbon film from two-layer epitaxial graphene”, Nature Nanotechnology 13, 133-138 (2018).

Angelo Bongiorno earned the B.S. degree in Physics from the University of Milan (Italy) with the highest honors (Summa Cum Laude). In 2003 under the supervision of Profs. Alfredo Pasquarello and Alfonso Baldereschi, he earned the Ph.D. in Physics from the École Polytechnique Fédérale de Lausanne, EPFL (Switzerland). From 2003 to 2015, he worked at Georgia Institute of Technology (USA); as a postdoc in the group of Prof. Uzi Landman from 2003 to 2007, as an independent research scientist from 2008 to 2009, as untenured Assistant Professor with a joint appointment in the Schools of Physics and Chemistry from 2009 to 2014, and as tenured Associate Professor in 2015. In 2015, he moved to the Department of Chemistry at the College of Staten Island (CUNY) in New York (USA). His research exploits classical and quantum mechanical computational methods to study complex materials at the atomic scale, and so far he has published about 70 refereed research articles. His publications have been cited ~3,500 times, he has an h-index equal to 26, and his research accomplishments have been recognized with the Blanchard Professorship Award.