Other works on Graphene

Different recent works realized on graphene are summarized in this last part.



Counting the Number of Graphene Layer


The determination of the number of layer is an important issue for the future development of the graphene science and technology.

Few-layer graphene (FLG) samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO2/Si substrate and mechanical exfoliation) are characterized by combined optical contrast and micro-Raman mapping experiments. We examine the behavior of the integrated intensity ratio of the 2D and G bands (A2D/AG) and of the 2D band width (G2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A2D/AG decreases and G2D increases with N as expected for commensurately stacked FLG. For CVD FLG, both similar and opposite behaviors are observed and are ascribed to different stacking orders. For small (respectively, large) relative rotation angle between consecutive layers (u), the values of the A2D/AG ratio is smaller (respectively, larger) and the 2D band is broader (respectively, narrower) than for single-layer graphene. Moreover, the A2D/ AG ratio decreases (respectively, increases) and, conversely, G2D increases (respectively, decreases) as a function of N for small (respectively, large) u. An intermediate behavior has also been found and is interpreted as the presence of both small and large u within the studied area. These results confirm that neither A2D/AG nor G2D are definitive criteria to identify singlelayer graphene, or to count N in FLG.

 graphene layer

Fig. 1: (a) Optical image of a single-layer graphene flake (1LG) including multilayer patches (darker regions) synthesized by CVD on copper and transferred on a 90 nm SiO2/Si substrate. Inset: zoom in the region delimited by the red square in the main image. (bd) Different maps of the region highlighted by the red square in (a): (b) laser optical contrast, (c) width of the 2D band, and (d) integrated intensity ratio of the 2D and G bands. The right floweris labeled as region 1 and the left one as region 2.


M. Bayle, N. Reckinger, J.-R. Huntzinger, A. Felten, A. Bakaraki, P. Landois, J.-F. Colomer, L. Henrard, A.-A. Zahab, J.-L. Sauvajol, M. Paillet, Physica Status Solidi b 252 (2015) 2375-2379.



Biological intercations with graphene


Understanding how graphene interact with biological materials (cells, tissues, etc.) is critical for biomedical applications.

Accurate assessment of the antibacterial activity of graphene requires consideration of both the graphene fabrication method and, for supported films, the properties of the substrate. Large-area graphene films produced by chemical vapor deposition were grown directly on copper substrates or transferred on a gold substrate and their effect on the viability and proliferation of the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli were assessed. The viability and the proliferation of both bacterial species were not affected when they were grown on a graphene film entirely covering the gold substrate, indicating that conductivity plays no role on bacterial viability and graphene has no antibacterial activity against S. aureus and E. coli. On the other hand, antibacterial activity was observed when graphene coated the copper substrates, resulting from the release of bactericidal cupric ions in inverse proportion to the graphene surface coverage.

Bacterial

Fig.1. Bacterial viability was determined using the LIVE/DEAD assay after a 24 h incubation of S. aureus (a) and E. coli (b) on bare Cu, bare Au, graphene+/-@Cu, graphene@Cu and graphene@Au. Cu and Au concentrations were measured by AAS after incubating S. aureus (c) and E. coli (d) for 24 h under the same experimental conditions. Stars denote the statistical significance (pvalues) in increasing order: p<0.1 (*), p<0.05(**), p<0.01(***). Error bars indicate standard deviation values.

 

L. Dellieu, E. Lawarée, N. Reckinger, C. Didembourg, J.-J. Letesson, M. Sarrazin, O. Deparis, J.-Y. Matroule , and J.-F. Colomer, Carbon 84 (2015) 310-316 .



Damage evaluation in graphene underlying atomic layer deposition dielectrics


One of the most explored applications domains for graphene is nanoelectronics because of its high carrier mobility and atomic thickness. However, gate dielectric deposition is an important challenge for transferring graphene transistors from laboratory level to industrial production. Dielectric or metal deposition induces defects in monolayer graphene and at the interface between dielectric and few-layer graphene. The carrier mobility is very sensitive to the graphene lattice defects and interface quality.

Mild plasma conditions are used to directly grow Hafnium oxide (HfO2) and Al2O3 dielectric on graphene by a remote plasma-enhanced atomic layer deposition (PE-ALD). The structural damage induced in monolayer graphene underlying HfO2 and Al2O3 upon different oxygen plasma power levels has been studied. The damage levels in AB-stacked bilayer, twisted bilayer and trilayer graphene underlying HfO2 and Al2O3 for a fixed oxygen plasma power have been also evaluated.


ALDgraphene.jpgFig. 1. Optical and transmission HAADF-STEM images:(a) as-transferred graphene on SiO2/Si substrate, (b) HfO2/graphene/SiO2/Si stack and (c) cross-section of HfO2/graphene/SiO2/Si stack.


X. Tang, N. Reckinger, O. Poncelet, P. Louette, F. Urena, H. Idrissi, S. Turner, D. Cabosart, J.-F. Colomer, J.-P. Raskin, B. Hackens, A. Francis, Scientific Reports 5 (2015) 13523.



Probing graphene χ(2) using a gold photon sieve

 

Nonlinear second harmonic optical activity of graphene covering a gold photon sieve was determined for different polarizations. The photon sieve consists of a subwavelength gold nanohole array placed on glass. It combines the benefits of efficient light trapping and surface plasmon propagation in order to unravel different elements of graphene second-order susceptibility χ(2). Those elements efficiently contribute to second harmonic generation. Actually, a graphene-coated photon sieve produces a second harmonic intensity at least two orders of magnitude higher compared to a bare flat gold layer, much of this signal being attributed to the graphene layer itself. The measured second harmonic gen-eration yield supplemented by semi-analytical computations provides an original method to constrain the graphene χ(2) elements. The obtained values are |𝑑31+𝑑33|≤814 pm²/V and |𝑑15|≤1.45×105 pm²/V for a second harmonic signal at 780 nm. This original method can be applied to any kind of 2D materials covering such a plasmonic structure. SHG.jpg


Fig. 1. (a) Schematic representation of the device and of second harmonic generation (SHG) measurements in transmission mode. (b) Polarization maps of the SHG intensity as a function of the azimuthal angle φ for bare flat gold and graphene-coated flat gold (green triangles), bare gold photon sieve (orange squares) and graphene-coated gold photon sieve (blue circles) for SS polarization in logarithmic scale.

 

M. Lobet, M. Sarrazin, F. Cecchet, N. Reckinger, A. Vlad, J.-F. Colomer, D. Lis, Nano Lett. 16 (2016) 48-54.