(424e) Charge Transfer in Junctions of Single Layer Graphene and Metallic Single Walled Carbon Nanotubes

Authors: 
Paulus, G. L., Massachusetts Institute of Technology
Wang, Q. H., Massachusetts Institute of Technology
Ulissi, Z., Massachusetts Institute of Technology
McNicholas, T., Massachusetts Institute of Technology
Strano, M. S., Massachusetts Institute of Technology


We fabricate and study junctions between single walled carbon nanotubes (SWNTs) and monolayer graphene for the first time.  A single layer graphene (SLG) sheet grown by chemical vapor deposition (CVD) was transferred onto a Si/SiO2 wafer with aligned CVD-grown SWNTs.  Raman spectroscopic mapping is used to identify metallic-SWNT/SLG junctions, and we report a method for spectroscopic deconvolution of the overlapping G peaks of SWNT and SLG by making use of the polarization dependence of the SWNT. A comparison of the Raman peak positions and intensities of the individual SWNT and graphene to the SWNT-graphene junction indicates an electron transfer of 1.12 x 1013 cm-2 to the SLG at the junction. We claim part of these electrons come directly from the SWNT where the direction of transfer is in agreement with the work functions of each. It is likely another part of the additional electrons observed in the graphene at the junction results from the fact that the latter is no longer in direct contact with the p-doping SiO2 substrate. We find that the compression of the SWNT by the graphene increases the broadening of the radial breathing mode (RBM) peak from 3.6+/-0.3 to 4.6+/-0.5 cm-1 and the G peak from 13+/- 1 to 18 +/- 1 cm-1, in reasonable agreement with a molecular dynamics simulation we performed of the system.  However we conclude that RBM and G peak position shifts are due primarily to charge transfer with minimal contributions from strain. With this method, we are able to dope the graphene on a nanometer lengthscale.