Synthesis of graphene

Our group focuses on the graphene synthesis by différent techniques, including micromechanical exfoliation of graphite, epitaxial growth from C-terminated n-type 6H-SiC(000-1) wafers and Chemical Vapor Deposition (CVD) growth, and its characterization.

Graphene synthesis by Chemical Vapor Deposition.

on copper foils

The growth of graphene is investigated on copper by atmospheric pressure chemical vapor deposition in a system free of pumping equipment. In such a system, it is evidenced that it is mandatory to flow hydrogen all along the growth process (especially the growth and cooling steps) to prevent oxidation and etching of graphene by residual oxidizing impurities inevitably present in the atmosphere of the reactor. In that case, micrometer-sized hexagon-shaped graphene domains of high structural quality are achieved  [1].

  optical-images

Fig. 1. Optical microscopy pictures of the samples cooled with or without hydrogen: (a) and (c) just after graphene growth; (b) and (d) after baking on a heating plate. Graphene flakes are evidenced by the white spots and the copper is orange.

characterization

Fig. 2. (a) Scanning electron microscopy image of a monolayer graphene hexagonal flake on copper foil. The scale bar is 1 µm. (b) Raman spectrum of a typical monolayer graphen hexagon. Inset: optical microscopy image of a graphene hexagon transfered onto SiO2.

on oriented Cu(111)

Graphene on oriented (111) copper films was synthesized and characterizedusing scanning tunneling microscopy (STM). Moiré patterns, due to the mismatch between the periodic lattices of graphene and copper, have been observed.

Usually, hexagonal moiré superstructures are observed. However, under certain growth conditions, anomalous moirés patterns can be observed, with well-defined linear periodic modulations. The reason is that graphene has grown on an oxygen-induced reconstructed copper surface, and not directly on the oriented (111) copper film, as expected. These moiré patterns were simulated thanks to a hard-sphere atomic model  [2].

Sample 1

sample 2

  Growth conditions

 

-  0.2 sccm of CH4 (99.5 % purity) for 15 min.

-  Rapid cooling under a Ar/H2 mixture

 

-   2 sccm of C2H4 (99.5 % purity) for 5 min.

- Rapid cooling under Ar

 LEED patterns

 

normalLEED

Image recorded at E = 75 eV

 

oxydLEED

(a) Calculated LEED pattern for a “44-structure” (an oxygen-induced reconstruction of Cu(111)). (b) and (c) modified calculated LEED pattern to evidence the experimental pattern shown in (d) (E=51.1 eV).

 STM images

 

normalSTM

(a) Experimental STM image with a hexagonal moiré superstructure. (b) Hard-sphere atomic model: superposition of a graphene lattice on a Cu(111) surface.

 

oxydSTM

(a) Experimental STM image showing an anomalous moiré pattern, with well defined linear periodic modulations. (b) Hard-sphere atomic model: superposition of a graphene lattice on the surface of a “44-structure” (an oxygen-induced reconstruction of Cu(111)).

 

Structural and electronic characterization of CVD graphene

A combination of magnetotransport and local probe measurements on graphene grown by CVD on copper foil and subsequently transferred onto a sapphire substrate is presented. A rather strong p-doping is observed together with quite low carrier mobility. Atomic force and tunneling Imaging performed on the transport devices reveals the presence of contaminants between sapphire and graphene, explaining the limited performance of our devices [3].

magnetotransport.jpg

Fig. 3.  (a) A picture of a graphene/sapphire sample together with the schematic wiring. (b) Raman spectra of graphene/sapphire. (c) typical magnetotransport results together with a linear fit.

 

Graphene synthesis using SiC.

Epitaxial multi and rotationally disorded graphene layers are grown on C-terminated n-type 6H-SiC(000-1) wafers. The high crystalline quality of this graphene is suitable for STM experiments.

 grapheneSiC

 

Fig. 4. A typical large scale STM image of graphen grown on the carbon face of SiC together with a LEED pattern illustrating the rotational disorder of graphene layers.

 

Fourier transform analysis of STM images of mutilayer graphene moiré patterns

Scanning Tunneling Microscopy images of simple and double moiré patterns resulting from misoriented bi- and tri- layers graphene stacks are analyzed with the help of a simple model. Multilayer graphene samples were obtained on SiC(000-1) by annealing the substrates in UHV at ~1320°C for 12 min. Under a silicon flux of 1ML/min. It is found that the model reproduces surprisingly the non-trivial features oberved in the Fast Fourier Transform of the images [4].

Fourriertransform.jpgFig. 5. (a) Experimental STM image (Vsample = -0.1V; I = 10 nA) of a double moiré pattern (between three graphene layers) D12 = 2.6 nm and D23 = 7.3 nm i.e. Theta12 = 5.5° and Theta12 = 1.9°. Inset: experimental STM image  (Vsample = +0.1V; I = 5 nA) of a double moiré pattern with inverted misorientation angles (Theta12 = 1.8° and Theta12 = 5.3°). (b) Zoom in the FFT of (a) on which replicas of the inner hexagon are seen around each vertex of the outer hexagon (one of them is pointed at by magenta arrows). Inset: zoom in the FFT of the image in the inset of (a). (c) Simulated image of the trilayer stacks with corre^sponding misorientation angle (Theta12 = 5.5° and Theta12 = 1.9°). (d) Zoom of the FFT of (c).

 

 

Direct growth of graphene on Si(111)

Due to the need of integrated circuit in the current silicon technology, the formation of graphene on Si is highly desirable, but is still a challenge for the scientific community. In this context, the direct growth of graphene is reported on Si(111) wafer Under appropriate conditions using an electron beam evaporator. The structural quality of the graphene is investigated in detail by reflection high electron diffraction, Auger electron microscopy, X-ray photoemission spectroscopy and scanning tunneling microscopy [5].

grapheneSi(111).jpgFig. 6. STM images of graphene grown on Si(111).

[1] N. Reckinger, A. Felten, C. N. Santos, B. Hackens, J.-F. Colomer, 2013, Carbon, 63, 84-91.

[2] N. Reckinger, E. Van Hooijdonk, F. Joucken, A. V. Tyurnina, S. Lucas, J.-F. Colomer, 2014, Nano Research, 7, 154-162.

[3] F. Joucken, J.-F. Colomer, R. Sporken, N. Reckinger, 2016,  Appl. Surf. Sci., 378, 397-401.

[4] F. Joucken, F. Frising, R. Sporken, 2015, Carbon, 83, 48-52.

[5] Pham Thanh T., J. Campos-Delgado, F. Joucken, J.-F. Colomer, B. Hackens,  J.-P. Raskin, C. Santos, 2014, J. Appl. Phys., 115, 223704.