Contribution (Oral/Poster/Keynote)
Two-dimensional molecular crystals of phosphonic acids on graphene B. R. A. Neves, M. C. Prado, R. Nascimento, L.G. Moura, M. J. S. Matos, M. S. C. Mazzoni, L. G. Cancado, H. Chacham Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, 30123-970, Brazil
[email protected]
Much attention has been drawn to graphene in the past years due to its remarkable properties and it is expected that this material will play a major role in nanotechnology and industrial applications [1]. In order to fulfill such great potential, some basic issues concerning graphene’s electronic properties such as control of bandgap and doping levels are in need. In this work, we report synthesis and characterization of two-dimensional (2D) molecular crystals from long and linear phosphonic acids atop graphene (see Fig. 1). These crystals deposited on graphene provide an easy way to determine the flake orientation and induce a well-defined shift in the Fermi-level.
Scanning Probe Microscopy (SPM), Raman Spectroscopy and ab initio calculations were employed to characterize the self-assembled monolayers of two phosphonic acids OPA (octadecylphosphonic acid) and TPA (tetradecylphosphonic acid) atop graphene [2]. Atomic Force Microscopy (AFM) measurements easily detect the period of the 2D crystal after deposition on graphene flake (see Fig. 1). Our calculations show a significant difference in the formation energy of OPA or TPA monolayers when different orientations of the molecule with respect to substrate are considered, clearly showing that the monolayer most stable configuration is along the graphene zigzag direction. Thus, a simple AFM measurement, with no need of atomic resolution, is all it takes to determine the graphene crystallographic orientation.
In addition, Raman spectroscopy measurements, before and after monolayer deposition, confirm the results of our ab initio calculations regarding the doping effect of the OPA/TPA monolayer, where both 13
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show a hole doping of graphene with carrier concentration of about 10 cm . The 2D crystal formation on graphene flakes on top of Si/SiO2 is easily achieved via spread coating with ethanolic solution [3,4] making this a fast and practical method to discover flake orientation and achieve chemical doping of graphene. References [1] – Schwierz, F. Graphene transistors. Nature Nanotechnol. 5, 487-496 (2010). [2] – Prado, M. C.; Nascimento, R.; Moura, L.G.; Matos, M. J. S.; Mazzoni, M. S. C.; Cancado, L. G.; Chacham, H. and Neves, B. R. A. Two-dimensional molecular crystals of phosphonic acids on graphene. ACS Nano 10.1021/nn102211n. [3] - Neves, B. R. A., Salmon, M. E., Russell, P. E. & Troughton, E. B. Spread coating of OPA on mica: From multilayers to self-assembled monolayers. Langmuir 17, 8193-8198 (2000). [4] - Fontes, G. N. & Neves, B. R. A. Effects of substrate polarity and chain length on conformational and thermal properties of phosphonic acid self-assembled Bilayers. Langmuir 21, 11113-11118 (2005).
Contribution (Oral/Poster/Keynote) Figures
Figure 1. AFM characterization of 2D phosphonic acid crystals atop graphene single and multilayers. (a) Schematic representation of both phosphonic acids employed in this work: the octadecylphosphonic acid (OPA), which is 2.5 nm long, andthe tetradecylphosphonic acid (TPA), which is 2.1 nm long. (b) AFM-topography image of 2D OPA crystals (green-yellow) partially covering a multilayer graphene (blue-purple). (c) Fast Fourier transform of the previous image, showing well-defined periodicity and angle between OPA crystals. (d) AFM-topography and (e) AFM-phase images of 2D TPA crystals partially covering a graphene monolayer. (f) Fast Fourier transform of the image in (e) evidencing the regular periodicity of the TPA crystals. Some periodic features are also seen in certain purple regions of image (b). These are simply color rendering artifacts, caused by the natural corrugation of the sample. Therefore, such periodic (purple) regions are, indeed, also OPA-covered regions and not the bare graphene, which shows no periodicity.
