EventsProf. Ruben Perez

Tailoring Graphene Electronic, Chemical and Mechanical Properties:
Interaction with Metals, Moire Patterns and the Role of Defects

The talk will start with a brief summary of the research activity of the SPMTH group, in particular, our recent developments to describe tip-sample transport and interactions for the simulation of STM and AFM [1,2], reducible oxides for catalysys and optoelectronics [3], graphene edge states and vacancy-induced magnetism [4], and the challenge involved in the imaging of surfaces and large biomolecules in their native liquid environment [5,6].

After that, I’ll focus in the ability of graphene to corrugate and how the interplay of this structural modulation with the interaction with metal substrates can be exploited to tailor its electronic and chemical properties. We have combined high-resolution STM experiments by different groups and our DFT/STM calculations to challenge some of established ideas for G-metal systems. Experiments in strongly interacting G/Rh(111) conclusively prove the formation of different moiré structures with a wide distribution of surface periodicities. A proper simulation of the current beyond the standard Tersoff-Hamman approach is needed to reproduce quantitatively the trends observed in the STM apparent corrugation. Based on this agreement, we discuss the relative contribution of strain, corrugation and G-metal binding to stabilize the observed moires [7]. We prove the enhanced reactivity of certain Moire areas and show how they contribute to oxygen intercalation, a process that converts a strongly coupled system into a freestanding like p-doped graphene layer [8].

In G/Cu, we have demonstrated that STM can selectively visualize either the graphene layer, the substrate underneath or even both at the same time and exploited this tunable transparency to provide a comprehensive picture of the G-metal coupling with atomic precision and high energy resolution [9], with important implications for the accurate description of van der Waals interactions. Our calculations explain the tunable transparency in terms of the short out-of-plane extension of the graphene electronic states, suggesting that it should apply to a good number of graphene/substrate systems.

Finally, we investigate the influence of defects on the thermal expansion coefficient (TEC) of suspended graphene membranes. Experiments show that low densities of monovacancies reduce the graphene TEC up to one order of magnitude. Our molecular dynamics simulations reproduce the observed trend and show that TEC reduction is due to the suppression of out–of–plane fluctuations caused by the strain fields created by monovacancies in their surrounding areas [10]. These results highlight the key role of defects in the properties of “real-life” graphene and pave the way for future electronic and mechanical defect engineering.

[1] M. Ellner et al, Nano Letters 16, 1974 (2016)
[2] H. Monig et al, ACS Nano 10 1201 (2016)
[3] O. Stetsovych et al, Nature Communications 6, 7265 (2015)
[4] L. Rodrigo et al., Carbon 103, 200 (2016)
[5] J. G. Vilhena et al, ACS Nano 10, 4288 (2016)
[6] J. G. Vilhena et al, Nanoscale 8, 13436 (2016)
[7] A. Martin-Recio et al., Nanoscale 7, 11300 (2015)
[8] C. Romero-Muñiz et al., Carbon 101, 129 (2016); and ACS Nano (submitted)
[9] H. Gonzalez-Herrero et al., ACS Nano 10, 5131 (2016) [10] G. Lopez-Polin et al., Carbon 116, 670 (2017).

About The Speaker

Prof. Ruben Perez graduated in Theoretical Physics in the Universidad Complutense de Madrid in 1987 and got his Ph.D. from Universidad Autonoma de Madrid (UAM) in 1992. After a three-year postdoctoral stay at the Cavendish Laboratory, University of Cambridge, as a Marie Curie Fellow and Research Associate, he returned to the UAM, where he is now a Full Professor of Condensed Matter Physics and leader of the Scanning Probe Microscopy Theory and Nanomechanics Group.

His research has focused on the quantum mechanical modeling of different problems in Materials Science and Nanotechnology that involve forces and currents at the atomic scale, working in close collaboration with experimental groups worldwide. His group (www.uam.es/spmth) has pioneered the theoretical analysis and contrast interpretation of probe-based experimental techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). His activity has recently expanded into two new topics: the study of oxide materials for catalysis and energy applications, and the AFM characterization of large biomolecules and the structure of self-assembled monolayers in their native liquid environment.

Ruben has co-authored more than 137 publications in international peer-review journals, including Nature and Science, and given more than 56 invited talks at international conferences. He has been the chairman or member of the Program and Advisory Committees in more than 20 international conferences and workshops, and belongs to the Steering Committee of the Non-contact AFM Conference since 2005. He has performed stays as invited researcher at different international institutions including the Joint Research Center for Atom Technology (Tsukuba, Japan), the Max Planck Institut fur Metallforschung (Stuttgart, Germany), the Fritz-Haber-Institute der MPG (Berlin, Germany) and the National Institute for Materials Science (Tsukuba, Japan); and he has been Visiting Professor in the Osaka University (Osaka, Japan, 2001) and the Lawrence Berkeley National Laboratory (Berkeley, USA, 2012).