Raman Spectroscopy of Graphene Molecules: Size Dependent and Reaction Time Studies
Authors: Dervishi Enkeleda , Ji Zhiqiang , Hartmann Nicolai F. , Sykora Milan , Doorn Stephen K. 
Published: February 2020

Graphene molecules, nanostructures with fascinating optical and electronic properties, also referred to as graphene quantum dots, have attracted significant interest for a number of applications, including photovoltaics, electrochromic, and bio-sensing.[1],[2],[3],[4] Raman spectroscopy is a non-destructive, powerful technique, commonly used to characterize graphitic materials and investigate the doping level, the number of layers, edge-type, and the presence/type of defects. Even though, Raman studies of large-area graphene, as well as, nano-graphene sheets, with lateral dimensions larger than ∼3 nm, have been previously reported, an investigation of structures with well-controlled dimensions below 2 nm has been lacking. Raman spectroscopy can be employed to gain important insights on the structure of well-defined graphene, by studying the dependence of the Raman spectral features on the edge-type and the level of defects present on graphene molecules.
In this study, we present a detailed Raman analysis of graphene molecules with synthetically controlled lateral dimensions between 0.76-1.39 nm, which correspond to structures with 16-79 ordered carbon rings. Graphene molecules with armchair edges and controlled lateral dimensions smaller than 2 nm were synthesized utilizing a new synthetic approach. [3.4] The samples were excited using laser light at discrete wavelengths (405 nm and 532 nm), and their Raman spectral features, including integrated band area ratios and peak positions, were systematically investigated. Our studies indicated that the integrated area ratios of D and G bands (AD/AG) increase with the size of the molecules. These results are in good agreement with the findings of the Raman studies of structures larger than 2 nm, generated via ion-bombardment.[5]The D and G band positions were also found to depend on the size of the graphene molecules. Additionally, we observed that the intensities of the higher-order bands (e.g., 2D, D+G, and 2G), increase for both wavelengths, as the size of the graphene molecules increases. To elucidate the effect of synthesis conditions on the structure of graphene molecule, we varied the reaction time and probed the crystallinity of the resulting graphene molecules. The integrated area band ratios and the band positions of the graphene molecules (with 79 and 48 carbon rings) were strongly dependent on the reaction time. These results demonstrate the suitability of Raman spectroscopy as a useful diagnostic tool for structural characterization and reaction dynamics of graphene molecules smaller than 2 nm, which is currently missing from the studies of larger graphene structures generated via ion-bombardment or other techniques.

[1] S. H. Jin et al., ACSNano, 7 (2), 1239-1245, 2013.
[2] H. Razmin et al., Biosensors and Bioelectronics, 41, 498-504, 2013.
[3] Z. Ji et al., ACS Appl. Mater. Interfaces, 6, 20473-20478, 2014.
[4] Z. Ji et al., ACSNano, 9 (4), 4043-4049, 2015.
[5] L. G. Cancado et al., Nano Lett. 11, 3190-3196, 2011.