A rigorous and accurate contrast spectroscopy for ultimate thickness determination of micrometre-sized graphene on gold and molecular sensing

The thickness of graphene films can be accurately determined by optical contrast spectroscopy. However, this becomes challenging and complicated when the flake size reduces to the micrometre scale, where the contrast spectrum is sensitively dependent on the polarization and incident angle of light. Here we report accurate measurement of the optical contrast spectra of micrometre-sized few-layer graphene flakes on Au substrate. Using a high-resolution optical microscopy with a 100x magnification objective, we accurately determined the layer numbers of flakes as small as one micrometre in lateral size. We developed a theoretical model to accurately take into account the appropriate contribution of light incident at various angles and polarizations, which matched the experimental results extremely well. Furthermore, we demonstrate that the optical contrast spectroscopy is highly sensitive to detect the adsorption of a submonolayer airborne hydrocarbon molecules, which can reveal whether graphene is contaminated. Though the technique was demonstrated on graphene, it can be readily generalized to many other two-dimensional (2D) materials, which opens new avenues for developing miniaturized and ultrasensitive label-free molecular sensors. INTRODUCTION Gold is widely used as the substrate material in many graphene devices, due to its superior optoelectronic properties and chemical stability. However, there has been little experimental investigation on the optical contrast of graphene films on Au substrates. Here we report accurate measurement of the optical contrast spectra of few-layer graphene flakes on bulk Au. We used a high-resolution optical microscopy with a 100 ́ magnification objective, accurately determining the thickness of flakes as small as one micrometre in lateral size, which are highly desired in many applications. The results are in excellent agreement with theoretical calculations and confirmed by Raman and AFM measurements. Furthermore, we demonstrate that the optical contrast spectroscopy is sensitive enough to detect the adsorption of a sub-monolayer airborne hydrocarbon molecules, which can reveal whether graphene is contaminated and opens the opportunity to develop miniaturized and ultrasensitive molecular sensors. Graphene has attracted a lot of interest since its discovery in 20041, owing to a plethora of extraordinary performance in electronic, mechanical, thermal and optical properties. It is a zero-gap semiconductor due to the fact that the conduction and valence bands meet at the Dirac point2, and has electron mobilities of more than 15000 cm2.V-1.s-1 at room temperature3. As a result, it has the lowest resistivity (~10-6 Ω.cm) of any known conductive material at room temperature4. It also has exceptional mechanical strength and thermal conductivity5. These remarkable properties make graphene one of the most exciting materials widely investigated across multiple areas, including energy, electronics, photonics, and sensors6,7. For graphene devices, the choice of underlying substrate is critical and impacts the functionality of the device in many aspects, e.g., modulating the electrical and thermal conductivities, doping and affecting the Raman and fluorescence spectroscopy, and modifying the electrochemical properties8. Many applications require graphene to be deposited on metallic substrates, such as copper9, nickel10, silver11 and gold12-14, to achieve optimal electronic, plasmonic, catalytic and sensing functionalities15-17. Gold in particular is a popular choice of metallic material, due to its excellent optoelectronic properties and chemical stability. On the other hand, much of the research and applications require high quality pristine graphene films, which are often produced by mechanical exfoliation, resulting in small flakes of a few microns or even less. Such small films hold realistic hopes for graphene commercialization in the near future. Despite important advancement has been made in the CVD synthesis of graphene films, it is still extremely challenging to fabricate uniform and defect-free high-quality large-area graphene films, which poses as a significant barrier for the commercialization of graphene devices. By contrast, micrometre-sized high quality graphene films are readily to make and can be fabricated in large scales, which have been exploited to develop a wide range of miniaturized nanodevices, such as high frequency transistors, ultra-broadband photon detectors, low-threshold mode-locked lasers, ultrasensitive molecular sensors, and even in spintronics18-20. Hence, accurate determination of the thickness of micrometre-sized graphene films on gold substrates is essential in graphene research and technology development. Several methods are commonly used to identify the number of atomic layers in a graphene flake, including atomic force microscopy (AFM), Raman spectroscopy and optical contrast spectroscopy21. AFM is a low-throughout technique, taking up to several hours to produce a detailed enough image from which the important data can be extracted. It is also subject to the influence of a number of factors, such as humidity, surface roughness and contamination. Raman spectroscopy is much faster and can probe the flake’s thickness non-destructively, based on the ratio of the Gand 2D-peaks, as well as the position and shape of the 2D-peak22. However, these spectral features can be influenced by many factors, including temperature23, strain24, and doping25, therefore sometimes giving ambiguous results for the number of layers present. Compared to AFM and Raman spectroscopy, optical contrast spectroscopy has a number of distinct advantages: it is easy to implement, fast, noninvasive, and less affected by the aforementioned factors. As such, it has become an attractive technique for identifying the layer thickness of graphene and a wide variety of other 2D materials26-28. Figure 1. Schematic showing the experimental configuration (a) and the diagram of the incidence and focusing of light (b). The circle in the top panel of b refers to the incident aperture of the objective (top view). The triangle in the bottom panel indicates the focusing cone of light through the objective. The correspondence between the incident annular ring and the focusing cone is indicated by shaded areas. Optical contrast of graphene films on oxidized silicon substrates has been studied extensively29. In contrast, very limited investigation has been carried out on metallic surfaces. Previously there was a theoretical investigation of the contrast of graphene on thin Au films, based on simple Fresnel reflectivity formulae at normal incidence30. However, there have been no experimental investigations of the optical contrast of graphene films on gold surfaces. Here we provide detailed experimental measurements, for the first time, of the optical contrast of few-layer (1-5 layer) graphene flakes on a bulk Au surface, using high-resolution optical microscopy with a 100 ́ magnification objective (Figure 1), which enables us to accurately identify the thickness of flakes as small as half micrometre (the diffraction limit of the 100 ́ magnification objective) in lateral size. The experimental results obtained are in excellent agreement with theoretical calculations from a modified model, taking into account the effects of different incident angles and polarizations, which become significant with the use of a high numerical aperture objective. The accurate determination of the layer numbers was validated by independent AFM and Raman measurements. In addition, we found that the 100x objective NA=0.9 graphene Au, 100nm Ti, 10nm