Overview of the Raman signatures of graphene and carbon nanotubes

Raman spectroscopy is a major characterization technique of carbon nanotubes and graphene. It is widely used to identify their structure, i.e. the diameter of single-walled carbon nanotubes (SWNT), or the number and orientation of layers of graphene, as well as the nature and density of defects. On the other hand, the Raman signatures of carbon nanomaterials are sensitive to electronic and mechanical perturbations, eg doping and strain, and to their dielectric/polar environment. Their orientation in anisotropic materials can be described from polarization studies. Raman studies can also be used to probe the properties of a substrate eg through interferences on thin films or surface-enhanced Raman spectroscopy (SERS) on plasmonic antennas.We will recall the main Raman signatures of SWNT and graphene, discuss the resonance phenomena, stress the limits and traps of Raman characterization, and review some recent and original Raman results from the literature.

Strategic approaches for dispersing and/or orienting SWNT in films and composites are essential for controling and optimizing their properties [1,2]. Dispersion in a liquid phase is the most efficient way to exfoliate the bundles into individual SWNT. Additives, most often surfactants, have to be used because of the poor solubility of SWNT in all solvents. However, the presence of surfactants in the final films/composites can drastically change their properties. In this talk, we show that a simple dialysis process opens a new route for stabilizing aqueous inks with high concentrations of individual SWNT without surfactant, and for preparing surfactant-free polymer-based composites [3]. Coupled Raman/ photoluminescence spectroscopy studies allow to probe in situ both the dispersion of the nanotubes and the nature of the molecules adsorbed on their surface. We demonstrate a spontaneous exchange between surfactant molecules (bile salts) and polyvinyl alcohol (PVA) chains at the surface of the nanotubes, and a forced exchange by dialysis for both PVA and polyvinyl pyrrolidone (PVP) [3].
On the other hand, we study the processing of SWNT aqueous inks into patterned deposits by inkjet printing. We show that a good control of both the morphology of inkjet printing lines and of the alignment of SWNT in these lines can be achieved by tuning the SWNT concentration, inter-drop distance and substrate temperature. Using polarized micro-Raman spectroscopy, we present an original method to determine the spatial distribution of nanotubes in the deposits as well as their orientation. We show that two types of line morphologies can be obtained: homogeneous deposits or pairs of ‘‘railway tracks’’. Highly oriented SWNT patterns can be observed in the “railways tracks” and at the edge of the homogeneous lines.
References:
[1] Ph. Avouris. Acc. Chem. Res. (2002) 35, 1026
[2] P. M Ajayan et al, Nature. (2007) 447, 1066
[3] F. Torres-Canas et al, J. Phys. Chem. C 119, 703 (2015)
[4] F. Torres-Canas et al, in preparation

We propose a simple and original way to prepare surfactant-free SWNT/hydrosoluble polymer composites with high concentrations of individual SWNT. We first disperse single-walled carbon nanotubes (SWNT) in aqueous suspensions using bile salts (BS), and we mix them with a solution of hydrosoluble polymer, poly(vinylpyrrolidone) (PVP) or poly(vinyl alcohol) (PVA). We measure the yield using visible–NIR absorption spectroscopy, and we probe both the chemical environment of the nanotubes and the effectiveness of individualization from coupled Raman/photoluminescence studies. We evidence a direct exchange of BS molecules and PVA chains at the surface of SWNT. By contrast, no direct exchange is observed with PVP. On the other hand, we show that a simple dialysis process leads to the preparation of aqueous suspensions of SWNT covered by PVP or PVA with high yields and an effective individualization of the nanotubes.

Nanotube-based patterns and films have exciting potential applications in electronics and optoelectronics.One of the key issues to optimize the electrical and optical properties of nanotubearrays is the control of their orientation. So far, orientation of nanotubes in thin films wasachieved i) directly during CVD growth [1], ii) using liquid dispersions: by dielectrophoresis [2],in an hydrodynamic flow [3,4], in the field of an host liquid crystal [5], by formation of a liquidcrystal in concentrated suspensions [6], or iii) by stretching composites [7]. However, versatileand industry-compatible methods are still needed, and inkjet printing appears to be a goodcandidate. Recently, Denneulin et al. reported an heterogeneous orientation of SWNT in inkjetprinted lines, with a preferential orientation parallel to the lines at the edges and perpendicularin the sub-surface [8]. On the other hand, Beyer et al. reported an homogeneous alignmentparallel to the lines for inkjet printed SWNT, and assigned it to the formation of a nematic phasefor special printing rates [9].Here, we present a coupled Raman/SEM study of the alignment of SWNT during inkjet printingof aqueous suspensions as a function of temperature, nanotube concentration and printingconditions. We report a very good alignment of the nanotubes, especially at the edges of theprinted patterns, and we discuss the contributions of hydrodynamics and thermodynamics to theorientation.References[1] K. Hata et al, Science (2004), 306, 132002 ; [2] S. Shekhar et al. ACS Nano (2011), 5, 1739; [3] C. Zamora-Ledezma et al, Nano Lett., (2008), 8 (12), 4103 ; [4] Q. Li et al, J. Phys. Chem.B (2006), 110, 13926 ; [5] N. Ould-Moussa et al, Liq. Cryst. (2013), 40, 12 ; [6] C. Zamora-Ledezma et al, Phys. Rev. E. (2011), 84, 062701 ; [7] C. Zamora-Ledezma et al, Phys. Rev. B.(2009), 80, 113407 ; [8] A. Denneulin et al, Carbon (2011), 49, 2603 ; [9] S.T. Beyer et al.Langmuir (2012), 28, 8753.