Improvement of transparent conducting nanotube films by addition of small quantities of graphene.

We demonstrate a water-based method to prepare transparent, conducting graphene/single-walled nanotube hybrid films. While the transmittance decreases slightly with increasing graphene content, the DC conductivity, sigma(DC), and sheet resistance scale non-monotonically with film composition. We observe an optimum composition of approximately 3 wt % graphene, which results in a peak in the DC conductivity. We have calculated the figure of merit, the DC to optical conductivity ratio, sigmaDC/sigmaOp, which also shows a peak at this composition. We find that this effect is only present for small graphene flakes. In addition, acid treatment increases both the sigmaDC and sigmaDC/sigmaOp by x2.5. Interestingly, acid treatment is more effective for films close to the optimum composition. This has the effect of sharpening the peaks in both sigmaDC and sigmaDC/sigmaOp. For acid-treated films, addition of 3 wt % graphene results in a 40% increase in sigmaDC/sigmaOp compared to the nanotube-only film, from 12.5 to 18. Optimized, acid-treated films display transmittance of 80% coupled with a sheet resistance of 100 Omega/[square].