A powerful method to detect the thermal fluctuation of graphene

Graphene has been widely studied by scientists since it was found to rely on its unique mechanics, electrical properties, and thermal properties. Graphene bubbles, as an extended structure of graphene, also attracted a great deal of interest. Various methods have been used to prepare graphene bubbles of various sizes and shapes. But so far, the size of graphene bubbles is a few microns. Therefore, people's research on their related properties is also limited by their size.

Dr. Yuan Huang (currently back to the Institute of Physics of the Chinese Academy of Sciences) of the Low-dimensional Carbon Materials Center of the Korean Institute of Basic Sciences and Dr. Wang Xiao and their collaborators have produced super-large graphene bubbles for the first time by using a new mechanical cleavage method. This bubble can reach up to 20 μm without losing stability. More importantly, through the preparation of such large graphene bubbles, many unique physical phenomena hidden behind the surface gradually emerged.

Under an optical microscope, this extra-large graphene bubble can display rainbow-colored colors. This Newton ring-like result was also reported for the first time in the structure of graphene bubbles. This is due to the optical path difference generated by the two reflections produced on the graphene bubble and on the substrate. And only if the bubble is large enough, the optical path difference can satisfy Δd = nλ, so that the interference enhances the generation of Newton rings.

Figure 1. Schematic and optical micrograph of graphene bubbles

However, these graphene bubble structures exhibit much more than just such odd properties. Using Raman's two-dimensional imaging method, they found that on the graphene bubbles, both the intensity and the wave number of the G-peak or 2D-peak showed significant oscillating behavior. This is because when the laser light passes through the graphene bubbles, the reflected light and the incident light on the substrate form a wave superposition, generating a standing wave, which results in the existence of maximum and minimum amplitude points in the direction of the vertical substrate. . This turbulence caused by light interference will be reflected on graphene bubbles by thermal effect, and Raman spectrum is very sensitive to the thermal effect of graphene, so it can be used as a powerful means to detect the thermal fluctuation of graphene.

Figure 2. Two-dimensional Raman imaging of graphene bubbles and G-peak peak profile of the center section.

The graphene bubble structure can provide a simple and important model for studying various properties of graphene. By measuring some direct parameters in graphene bubbles, various properties of graphene, such as thermal conductivity, can be accurately analyzed. The interaction energy between the substrate and the substrate, Young's modulus and other important information. This work proposes a method for preparing bubbles of graphene and other layered materials, and for the first time observed Raman oscillations on graphene bubbles, and also accurately calculates the thermal conductivity of graphene by the movement of Raman spectra. It provides new ideas for studying many properties of graphene and other two-dimensional materials. In the future, more two-dimensional material properties can be observed through the bubble structure.

This work was published in "Raman Spectral Band Oscillations in Large Graphene Bubbles", Physical Review Letters, May 3, 2018, DOI: 10.1103/PhysRevLett.120.186104. The first author, Dr. Yuan Huang, is currently an associate researcher at the Physics Institute of the Chinese Academy of Sciences, and the author of the communication is Professor Rodney S. Ruoff, Director of the Low-dimensional Carbon Materials Center of the Korean Institute of Basic Sciences.