Functionalization of graphene

The functionalization or doping of graphene aims at tuning its electronic properties by opening a energy gap, main challenge for possible electronic applications. The functionalization evidence is highlighted using X-ray Photoemission Spectroscopy (XPS), Scanning Tunneling Microscopy (STM), Scanning X-ray Photoelectron Microscopy (SPEM) or Angle Resolved Photoemission Spectroscopy (ARPES).

Using microscopic and spectroscopic tools, the electronic and structural properties of graphene treated by low pressure plasmas are investigated. Pristine graphene samples were prepared by micromechanical exfoliation of single-crystal graphite flakes. Plasma is a simple, fast and scalable process. It is a useful tool to modify graphene as a large variety of functional groups can be attached to it surface: oxygen, hydrogen, fluorine, nitrogen, etc [1,2]. The characterization of the modified graphene is then performed using tools such as micro-XPS, micro-Raman and AFM. The change of reactivity and surface chemistry is also studied when going from monolayer to multilayer graphene [2]. Furthermore, in order to better understand the functionalization mechanisms, diagnostic of the plasma are performed using techniques such as mass spectrometry and Langmuir probe.


Fig. 1. Raman spectra of oxygen plasma treated monolayer graphene under the following conditions: 10 W, 10 s, 0.1 Torr and inside the discharge (blue) or outside the discharge (red). The black curve shows Raman spectrum of pristine graphene.


Fig. 2. (a) Optical image of t bilayer graphene. The cross indicates the regions where the bilayer C1s spectrum is extracted. (b) XPS C1s spectra of bilayer (red curve) and graphite (blue curve), both treated in oxygen plasma for 100s. the upper and lower dotted black curve corresponds to teh C1s of pristine graphite, rsepectively. (c) Curve fitting of the oxidized bilayer graphene with components attributed as follow: red-sp2 carbons, green-sp3 carbons, blue -C-OH, cyan -C=O or C-O-C, and anvy blue -COOH.


Other technique is to expose the graphene samples prepared on C-terminated n-type 6h-Sic(000-1) wafers on  a nitrogen radical flux produced by an RF plasma source, leading to a nitrogen incorporation in the graphene lattice mainly in the form of simple substitutions of carbon atoms by nitrogen atoms in the graphene lattice [3].


Fig. 3. Surface atomically resolved by STM before (a) and after (b) the exposure to the nitrogen flux (the insets shows the honeycomb structure of pristine graphene (a) and a zoom on a simple substitution (b)).


Nitrogen implantation of suspended graphene flakes

Nitrogen inclusion in both chemical vapour deposition and exfoliated few-graphene flakes was performed by nitrogen ion implantation in ultre-high vacuum. Inclusion of up to ~20 at.% nitrogen can be reached through this clean technique with absence of oxygen species in the final product, while maintaining a largely sp2-carbon network. The nitrogen inclusion was observed by scanning X-ray photoelectron microscopy (SPEM) with energy resolution of 0.2 eV ans spatial resolution of 10 nm. SPEM can be used to follow the evolution of nitrogen species: pyridinic, graphitic, and pyrrolic, at the different doping stages and annealing températures. Variations in the ratio between sp2 nitrogen species was observed, for increasing treatment time; annealing results in quenching of the sp3 component, suggesting the graphitic nitrogen as the most thermal stable species. The occurrence of graphitic species together with the absence of pyrrolic is indicative of N-incorporation into a hexagonal graphene-based lattice. Ion irradiation followed by annealing performed in a controlled way is a promising strategy to fine control the nature of the nitrogen species grafted to the graphene while focusing on selected applications [4].


Fig. 4. (a) N1s core level spectra corresponding to 5 and 15 min nitrogen implantation of CVD few-layers graphene at 250°C and 430°C annealing from top to bottom, respectively (N1 is pyridinic, N2 pyrrolic, N3 graphitic-center, N4 graphitic-valley); experimental data (dotted line), peaks resulting from a least-square fitting procedure (continuous red line). A Shirley-type background was substrated. (b) Nitrogen content in the sample for each component and total amount (black squares) as a function of implantation time (full symbols connected by full line) and annealing temperature (open symbols connected by dotted line). The second and third points are connected even though they are two different studies: nitrogen concentration after 15 min of implantation (second point) and after annealing to 250°C (third point).

Highly nitrogen-doped  graphene

Highly nitrogen-doped graphene on copper has been obtained by post-synthesis low-energy ion implantation. Core level and angle resolved photoemission spectroscopies are correlated to link the actual charge carrier doping to the different nitrogen species implanted in the nanostructure. Indeed,, we exploited the possibility of controlling the graphitic/pyridinic ratio through  thermal heating to tune the charge carrier density; this implicates Dirac cone shifts that are directly correlated to the different doping contribution of the nitrogen species. Supported by density functional theory calculations, we identify graphic nitrogen as being responsible for n-doping when the amount of counterbalancing pyridinic initrogen species is reduced upon thermal heating [5].


Fig. 5. Pristine graphene (left panel), 3 min of plasma exposure (central panel) and after thermal annealing at 500°C (right panel): (a) background subtracted HR-ARPES spectra in p (left) and s (right) polarization, respectively, recorded with photon energy of 31 eV and centred at the Dirac cone region (vertical dashed line at 1.703 Å-1); (b) MDC cuts corresponding to the reported énergies: black dots are the raw data, red lines are the Lorentzian fit curves; (c) maxima of the MDCs (black dots) obtained from the fitting procedure and their linear fit (red lines).


[1] A. Felten, B. Flavel, L. Britnell, A. Eckmann, P. Louette, J.-J. Pireaux, M. Hirtz, R. Krupke, C. Casiraghi, Single and double-sided chemical functionalization of bilayer graphene, 2013, Small, 9, 631-639.

[2] A. Felten, A. Eckmann, R. Krupke, J.-J. Pireaux and C. Casiraghi, Controlled modification of mono- and bilayer graphene in O2, H2 and CF4 plasmas, Nanotechnology, 2013, 24, 355705.

[3] 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.

[4] M. Scardamaglia, B. Aleman, M. Amati, C. Ewels, P. Pochet, N. Reckinger, J.-F. Colomer, T. Skaltsas, N. Tagmatarchis, R. Snyders, L. Gregoratti, C. Bittencourt, 2014, Carbon, 73, 371-381.

[5] M. Scardamaglia, C. Struzzi, S. Osella, N. Reckinger, J.-F. Colomer, L. Petaccia, R. Snyders, D. Beljonne, C. Bittencourt, 2016, 2D Materials, 3, 011001.