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Graphene curvature, flexibility and reactivity control by means of external fields: theory and computer simulations

Periodic Reporting for period 1 - GRAFLEX (Graphene curvature, flexibility and reactivity control by means of external fields: theory and computer simulations)

Reporting period: 2015-09-08 to 2017-09-07

Graphene, a two-dimensional, all-carbon material, has been referred to as the plastic of the XXI century. It has very high thermal and electrical conductivity, large surface per unit weight, and exceptional mechanical resistance despite its extreme thinness. These properties – mainly related to the combination of its honeycomb structure and the electronic structure of carbon – immediately suggest application in the fields of high-tech (e.g. flexible electronic devices), clean energy (e.g. hydrogen technology), environment (e.g. water and hair purification), and medicine (e.g. nano-prosthetics).
However, bare graphene is not always optimal for applications. It is a metal with high carrier mobility, but needs electronic doping to have sufficient carriers density or manipulation to transform into a semiconductor. As it is light and with a large surface to mass ratio, graphene has great potential for gas storage and catalysis. However, it has low chemical and physical reactivity. As a consequence, for most applications, graphene needs some sort of manipulation, which must be conducted at the nano-scale level to optimally exploit graphene properties. Manipulation implies the disruption of its perfect symmetry through controlled creation of different defects. One can then say that the next challenge in the graphene era is its nano-scale controlled morphing.
Because most morphing actions consist of local structural or chemical transformations (e.g. substitutional doping, defect or added atoms), control of morphing can be translated into control of local reactivity. The main aim of GRAFLEX was to achieve this task by controlling graphene local curvature. This idea stems from the following observations: (i) graphene is extremely flexible as a consequence of its symmetry and 2D nature; (ii) there are indications that reactivity depends directly on local curvature: as graphene is deformed, its electronic structure is locally disturbed and becomes more prone to interactions with other substances; and (iii) graphene curvature is sensitive to external electrostatic fields a phenomenon related to flexo-electricity. The overall objectives of GRAFLEX were therefore quantifying the dependence of reactivity on the local curvature and studying the possibility of using external electric fields to manipulate curvature and reactivity.
The project GRAFLEX was funded by Marie Skłodowska-Curie (CAR) action of EU-Horizon 2020 and hosted at the Istituto Nanoscienze (NANO-Cnr, Pisa). It supported the Georgian researcher Khatuna Kakhiani with a 2-year fellowship (2015-2017) to collaborate with NANO senior scientist Valentina Tozzini. The GRAFLEX project concluded in September 7th, 2017, but it was the beginning of a fruitful collaboration between the two scientists and their respective institutions.
Since GRAFLEX is a theoretical-computational project, the initial months were devoted to the choice of the most appropriate theoretical approach. The chosen framework was the Density Functional Theory, used to address the electronic structure and geometry of the system at the quantum level. Within this scheme, several choices have to be made: the electron density energy functional used to represent electron-electron interactions, the scheme for representing the elusive van der Waals interactions, the choice of the software – which settled on the open source “Quantum Expresso” code, developed under a trans-national foundation involving EU countries and US, and the choice of numerical parameters of calculations. A series of comparative calculations were performed on test model systems. Although a required step, this preliminary study has resulted in outcomes on the relative performance of different setups on low dimensional graphene-like systems. A subset of these results was already published in a review paper, and the rest is the subject of a manuscript currently in preparation. The use of the vdW correction scheme was crucial in the description of the interaction of graphene with molecular hydrogen, but less important in the description of the chemisorption process. In this phase, models for corrugated graphene with ripples of different amplitude were also optimized.
Subsequently, the project entered the hot phase, and simulations of chemi(de)sorption of molecular hydrogen were systematically performed over sites at different corrugations. The researcher underwent training to exploit the extreme parallelism of the High Performances Calculations resources at the CINECA supercomputing center. The main outcome of the second year is the quantification of the dependence of the chemi(de)sorption energy barrier on the local curvature, showing that a catalytic effect is present over most corrugated sites. A manuscript with these results is now in preparation. The last months were devoted to calculations in the presence of an electric field. These were not finished, but the researcher will have access to HPC resources and will continue working on the project in her own institution, consolidating the collaboration started with this MCSA.
"The quantification of the relationship between the reactivity and flexoelectricity of graphene is a step beyond the current state-of-the-art. While most current applications exploit the electronic properties of graphene (e.g. mobile and communication technologies), applications in clean energy and pollution prevention are mainly concerned with its chemical structural and mechanical properties, which are those investigated by GRAFLEX. The obtained results can be exploited in particular in the field of hydrogen storage and, more generally, in clean energy technologies. Public interest in this work was demonstrated by the article “Morphing graphene’’ published in Nov 2016 in the magazine ""Platinum"", distributed in association with the Italian newspaper IlSole24Ore and with other economy-oriented newspapers in the EU.
In the last two years, the results of GRAFLEX were presented to conferences and seminars (3 talks – one invited – and 4 posters) and were preliminary reported in a volume currently in press for the international Publisher Springer. Two technical manuscripts for are currently in preparation and will soon be submitted to international journals for peer-reviewing. The scientists involved in GRAFLEX strongly believe in the social power of the “sense about science”. Therefore, an effort was devoted to popular dissemination. A movie was published and shared with the community via Youtube and Facebook. The MCSA fellow KK also delivered a seminar “Graphene, curved surface and molecules: what do they have in common?” to high school students organized by Junior TSU, Ivane Javakhishvili Tbilisi State University in Tbilisi (Georgia, June 13th, 2017).
Lastly, this MCSA project had a great impact on the groups involved. The KK had the opportunity of resuming her career, after a forced break, which was one of the declared aims of this MCSA call. KK was in fact invited for a public talk: “Career Restart with Marie Skłodowska-Curie Actions” (Tbilisi, Georgia January 17, 2017) in the campaign for the promotion of MSC actions in the associated countries. This project has also started and consolidated a collaboration between the group of hosting and hosted institutions, establishing a bridge between central EU and eastern emerging countries.
Corrugated graphene and chemisorption barrier: effect of an orthogonal electric field