不同层数石墨烯的光热显微成像

Photothermal Microscopy of Graphene Flakes with Different Thicknesses

二维层状材料(石墨烯、二维过渡金属硫族化合物等)因具有独特的物理性质,引起了研究学者们的广泛关注,极大促进了化学、材料科学和凝聚态物理学的发展。开发能够探究层状材料中层数依赖的光学、电学、力学和热学特性的新技术一直是二维材料领域最活跃的研究方向之一。光热显微镜利用光激发后非辐射跃迁产生的热效应,可实现在单个颗粒或单分子水平上成像与检测,并实时捕捉微观尺度热弛豫和热传输过程。本文对比研究了石墨烯薄片在不同光热介质(空气、甘油)中随厚度变化的光热特性,发现了在两种介质中光热信号强度与样品厚度之间均存在非线性依赖关系。相比于空气介质,甘油介质中光热信号强度具有更高的对比度,且随着厚度增加表现出非单调变化。该研究提供了不同介质环境中不同层数石墨烯光吸收和热弛豫特征的细节信息,相关研究结论将为层状材料及其异质结的热学性质研究提供依据。

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 highcontrast 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.