Abstract:

Two-dimensional (2D) layered materials have attracted widespread research interest and have significantly promoted the development of chemistry, material science, and condensed matter physics. Since the emergence of graphene, 2D materials with unique mechanical, thermal, optical, and electrical properties have been developed. In the case of graphene, its extraordinary mechanical strength, carrier mobility, thermal conductivity, and light-absorption over the whole spectral range in UV-Vis and near infrared guarantee a wide range of prospective applications. The electronic structure and properties of graphene flakes are dominated by their thickness, twist angle, and dielectric environment. Tailoring the interlayer interactions of graphene layers can provide additional opportunities for developing optical and electrical nanodevices, resulting in pioneering outcomes, such as the magic-angle graphene. Over the past decade, one of the most active research directions in the field of 2D materials has been the development of novel techniques that can probe the thickness-dependent physical properties of layered materials. In contrast with the intensively studied mechanical, electrical, and optical properties, microscopic investigations of the thermal characteristics of graphene flakes remain to be explored. Photothermal (PT) microscopy is a new all-optical microscopic imaging technique that has gained substantial attention and undergone long-term development in recent years, especially in the fields of nanomaterials and life sciences. The fundamental principle of PT microscopy is to heat the target sample based on the absorption of a heating beam and use a probe beam to indirectly capture information on microscale heat generation and transport. Inspired by several pioneering studies, we conducted a comparative study of the thickness-dependent PT properties of mechanically exfoliated graphene flakes in two different PT media, i.e., air and glycerol. Whereas a nonlinear relationship between the PT intensity and sample thickness was observed in both media, the PT intensities from the two media were distinct. A high-contrast and non-monotonic PT response was observed in glycerol. The PT intensity of monolayer graphene was higher than that of bilayer graphene, and the PT intensities of graphene flakes with 2–4 layers exhibited a good linear relationship with the thickness. We also analyzed the relationship between the PT intensity and heating or probe power, demonstrating that the PT intensity as well as the absorption cross-section of graphene derived from the PT signal vary linearly with the power of both laser beams. Our study provides insights into light absorption and thermal relaxation features of graphene flakes of different thicknesses, which can guide future studies on the thermal properties of layered materials and their heterostructures.

Key words: Graphene, Photothermal microscopy, Thickness-dependence, Optical absorption, Nonradiative relaxation