Optimising the visibility of graphene and graphene oxide on gold with multilayer heterostructures

Abstract

Metals have been increasingly used as substrates in devices based on two-dimensional (2D) materials. However, the high reflectivity of bulk metals results in low optical contrast (<3%) and therefore poor visibility of transparent mono- and few-layer 2D materials on these surfaces. Here we demonstrate that by engineering the complex reflectivity of a purpose-designed multilayer heterostructure composed of thin Au films (2-8 nm) on SiO/Si substrate, the optical contrast of graphene and graphene oxide (GO) can be significantly enhanced in comparison to bulk Au, up to about 3 and 5 times, respectively. In particular, we achieved ∼17% optical contrast for monolayer GO, which is even 2 times higher than that on bare SiO/Si substrate. The experimental results are in good agreement with theoretical simulations. This concept is demonstrated for Au, but the methodology is applicable to other metals and can be adopted to design a variety of high-contrast metallic substrates. This will facilitate research and applications of 2D materials in areas such as plasmonics, photonics, catalysis and sensors.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer‐Scale Growth and Beyond

“…c,d) Reproduced with permission. [ 56 ] Copyright 2019, AIP Publishing. e) Reproduced under the terms and conditions of the CC‐BY license.…”

“…Following the criteria mentioned above, substrates that provide high optical contrast for graphene oxide (GO) [ 56 ] and MoS 2 [ 57 ] were developed by introducing a thin gold (Au) film (<13 nm) between the 2D material and the SiO 2 /Si substrate (Figure 2c ). The introduction of the Au film adjusted the reflectivity phase of the substrate to ensure the optimal phase for these 2D materials.…”

“…The introduction of the Au film adjusted the reflectivity phase of the substrate to ensure the optimal phase for these 2D materials. By optimizing the thickness of the Au film and considering the different NAs of the objective lens, the optical contrast for monolayer GO was enhanced to approximately 17% when using a 50× objective (Figure 2d ); [ 56 ] furthermore, the optical contrast for monolayer MoS 2 was enhanced significantly to an extremely high value of 430% when using a 10 × objective with a 2.5 mm aperture stop (AS) (Figure 2e ). [ 57 ] The enhanced optical contrast on the optimal substrates suggests that even very small flakes of a 2D material can be readily located.…”

Current Trends in the Optical Characterization of Two-Dimensional Carbon Nanomaterials

“…For example, the optical contrast between the graphene and the substrate must be sufficiently high to resolve few-layer graphene by optical or electron microscopy. The number of graphene layers on the substrate can be estimated by correlation between the contrast and the specific thickness: e.g., the number of graphene flakes has been determined by using 7.5 nm Au, 1 nm Ti, and 93 nm SiO 2 stacked on a silicon wafer, and by analyzing the reflected light collecting the image after a 520-nm band pass filter by its optical contrast (Velický et al, 2018). A disadvantage of this method is that the graphene layer thickness can be estimated only by knowing the thickness of one single layer of the 2D nanomaterial.…”

“…Also, flake overlapping needs to be avoided. The differences in thickness of <1 nm have been resolved (Khan et al, 2017;Halbig et al, 2018), and also the number of the graphene layers (from one layer up to 10 layers) was determined (Pu et al, 2009;Tan et al, 2017;Velický et al, 2018). To characterize the thicknesses of the deposited graphene layers, focused ion beam (FIB) cuts can be fabricated and analyzed by electron microscopy.…”