Description
We present the design of a polymeric device for the experimental measurement of out-of-plane surface optical conductivity in bilayer graphene. Out-of-plane optical constants of 2D materials are experimentally elusive because the substrate on which 2D materials are deposited hides the contribution of the out-of-plane constants to their optical response. Until now only the observation of the out-of-plane surface susceptibility of monolayer graphene was reported.
We adopt an experimental approach that consists in two steps. The first is a standard ellipsometric measurement on a bilayer graphene sample, deposited on a transparent polydimethylsiloxane (PDMS) substrate. In a second step, we remove the substrate contribution. We place the same sample in a prism-shaped mold, we pour non-polymerized PDMS on it and wait for complete polymerization. Owing to the replicant properties of PDMS we obtain a bilayer graphene totally immersed in the PDMS prism without spurious interfaces. In this second step, the light reflected from the sample is much less than in the previous experiment. This forced us to develop a homemade ellipsometric setup working at a 633 nm. Our experiment produces 4 experimental data, Δs and Ψs from the sample deposited on the PDMS substrate and Δi and Ψi from the immersed sample (Figure 1). From these it is possible to obtain the in-plane and the out-of-plane surface susceptibilities (χ∥,χ⊥) and surface conductivities (σ∥,σ⊥). Results are reported in Table 1, that compares them with those obtained for the monolayer and for bulk graphite. We note that χ∥ and σ∥ for the bilayer are practically equal to the values measured for monolayer graphene and for bulk graphite. This result is confirmed by a lot of other experimental studies. Out-of-plane optical constants behave differently, as they are shown to increase with increasing number of layers. This is the main result presented here.