Simulations on graphene structures (defects, doping, etc.)

Chemical doping and structural modification are key methods to tailor the electronic and optical properties of graphene. We performed tight-binding and ab-initio simulations to predict the electronic properties of modified graphene and confront our results with experimental results (STM, Raman, XPS) inside ou outside the CARBONNAGe research group.

Grain boundaries in graphene

The structural models of that sort can be used to calculate STM images or STS spectra of the grain boundary for bringing a support to the interpretation of experimental data [1].

boundaries

Figure 1. Computer model of a junction between two graphene crystals misoriented by 21.8°. In addition to several pentagons-heptagon pairs present in the interfacial region, there is a nine-edge polygon, adjacent to a pentagon, that contains a two-fold coordinated atom.

 

The structural model of that sort can be used to calculate STM images or STS spectra of the grain boundary for bringing a support to the interpretation of experimental data [1].

 

Chemical doping of graphene

Chemical doping efficiency by plasma methods depends on the plasma conditions and on the number of layer (see Graphene functionalization by plasma). We have shown, based on abinitio studied, that the fluorine-carbon binding energy depends on the dopant density, the adsorption site (A or B graphene sublattice) and on the number of layer. The diffusion barrier of a fluorine for a carbon site to next site is also shown to depend on the presence of another fluorine in the neighbouring [2]

 

 

Figure 2. Schematic view of graphene-fluorine system.

 

Chemical doping of graphene by nitrogen can be obtained by plasma functionalization. We have simulated the STM images and STS spectra of N and B doped mono- and bi layer graphene [3-5]. Comparison with experimental data demonstrates the importance of the tip-sample distance in the interpretation of the experimental images. Moreover, not only density of nitrogen influence the energy of the Dirac point and of the states localized near the dopant but also the periodicity and the disorder [6]

 

Z = 3.0Å

Z = 7.0Å

 

 

 

 

Simulations

image1 image2

 

 

 

 

 

Expériences

image2 image4

 Figure 3. Comparison between STM simulations and experiences of nitrogen doped graphene.

 

Many potential applications of graphene require either the possibility of tuning its electronic structure or the addition of reactive sites on its chemically inert basal plane. In this context, the interactions between chemical dopants in graphene have important consequences on the electronic properties of the systems and cannot be neglected when interpreting spectroscopic data or setting up devices. In this report [7], the structural and electronic properties of complex doping sites in nitrogen doped graphene have been investigated by means of scanning tunneling microscopy and spectroscopy, supported by density functional theory and tight-binding calculations. In particular, based on combined experimental and simulation works, we have systematically studied the electronic fingerprints of complex doping configurations made of pairs of substitutional nitrogen atoms. Localized bonding states are observed between the Dirac point and the Fermi level in contrast with the unoccupied state associated with single substitutional N atoms. For pyridinic nitrogen sites (i.e., the combination of N atoms with vacancies), a resonant state is observed close to the Dirac energy. This insight into the modifications of electronic structure induced by nitrogen doping in graphene provides us with a fair understanding of complex doping configurations in graphene, as it appears in real samples.

 STM2

Figure 4. (a) STM image (0.5 V, 400 pA, 10x10 nm2) of a nitrogen-doped graphene showing various nitrogen configurations; zoomed STM images of (b) a 1-2 nitrogen pair, (c) a 1-3 nitrogen pair, (d) a 1-4 nitrogen pair, (e) a 1-8 nitrogen pair, (f) a triangular shape labeled P; the color scale used for the 10x10 nm2 image and for the zooms is shown on the right; (g) labels used for the nitrogen pairs. A single substitutional nitrogen atom is labeled N in panel (a).

 STM3

Figure 5. DFT-based computed STM images of a nitrogen doped graphene showing nitrogen pairs at 5 Å above the atomic plane: (a) 1-2 pair; (b) 1-8 pair.

 

 

[1] P. Vancso, G. I. Mark, P. Lambin, A. Mayer, Y.-S. Kim, C. Hwang, L. P. Biro, 2013, Carbon, 64, 101-110.
[2] H. Santos-Esposito, L. Henrard, Journal of Physical Chemistry C (2014) 118, 46, p. 27074.
[3] B. Zheng, P. Hermet, L. Henrard, 2010, ACS Nano, 4, 4165-4173.
[4] S.-O. Guillaune, B. Zheng, J.-C. Charlier, L. Henrard, 2012, Physical Review B, 85, 035444.
[5] F. Joucken, Y. Tison, J. Lagoute, J. Dumont, D. Cabosart, B. Zheng, V. Repain, C. Chacon, Y. Girard, A. R. Botello-Méndez, S. Rousset, R. Sporken, J.-C. Charlier, L. Henrard, 2012, Physical Review B, 85, 161408.
[6] P. Lambin , H. Amara, F. Ducastelle, L. Henrard.  2012 Phys. Rev. B . 86 ,  045448.

[7] Y. Tison, J. Lagoute , V. Repain , C. Chacon , Y. Girard  , F. Joucken, D. Sharma, S. Rousset , L. Henrard, H. Amara, A. Ghedjatti , F. Ducastelle, 2015, ACS Nano, 9, 1, p.670.