Graphene is well-known not only for the singular behavior of its massless electrons, but also for its remarkable mechanical properties. Although graphene is only one atom thick, it forms an extremely rigid (in plane), resistant and flexible membrane. The ultra-strong adhesion of graphene to typical substrates, on which it is deposited, and its impermeability to standard gases allow the formation of graphene “blisters”, whose shape and motion can be controlled with accuracy (a). Such graphene drums are very promising as electro- or opto-mechanical nano-systems. Up to now, their static mechanical properties have been studied by local probe techniques like atomic force microscopy or nano-indentation.
Researchers of the Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS, UMR 7504) have performed the first all-optical study of the mechanical properties of a graphene blister under pressure. Using micro-Raman spectroscopy, the physicists were not only able to map the distribution of the mechanical stress on the blister surface, but also the topography of the blister. This work, which has been published in Physical Review Applied, can be adapted to other two dimensional membranes, such as transition metal dichalcogenides. Moreover, this optical (hence contactless) technique offers promising perspectives for precise pressure sensing and for real-time detection of mechanical motion in nano-resonators.
Figure 1: (a) Schematic and optical image of a graphene blister. (b) Raman spectra measured at the center of the blister for difference pressure differences. (c) Topography reconstruction of the blister at Dp=74 kPa from the Raman G mode intensity. (d) Cube of the height at the blister center as a function of the pressure difference. The Young’s modulus is deduced with the slope of the linear fit.
In order to prepare graphene blisters, the researchers have first etched cylindrical pits of a 8 µm in diameter, with a depth of several hundreds of nanometers in a silicon substrate covered with a thin silicon oxide layer. These pits have then been hermetically sealed with graphene monolayers to form optical micro-cavities. By placing such samples under a total or partial vacuum, a pressure difference builds up between the inside of the micro-cavity, which contains a fixed number trapped air molecules, and the outside of the pit. Due to this pressure load, graphene bulges up and forms a blister (a). In these conditions, the spectrum of the light scattered by the elementary vibrations (phonons) of the graphene lattice, contains a wealth of information (b). On the one hand, we observe that the frequency of the Raman scattering peaks decreases due to the mechanical stress that is induced by the pressure difference. On the other hand, the intensity of the Raman peaks varies considerably as a function of the pressure difference, i.e., the height of the graphene blister. The latter effect arises from optical interferences, and its analysis allows an accurate reconstruction of the blister topography (c). As a result, it becomes possible to: i) determine the average strain in the blister and to correlate it to the Raman peak shifts. This analysis provides the Grüneisen parameters. ii) correlate the blister height to the pressure difference. This height is proportional to the cubic root of the pressure difference, as it has been theoretically predicted by Hencky for a membrane without flexural rigidity (like graphene), one hundred years ago. The proportionality coefficient provides a measurement of the Young’s modulus of graphene of 1 TPa (d). This measurement is in excellent agreement with nano-indentation measurements, which have been a landmark in the history of graphene research.
ii) correlate the blister height to the pressure difference. This height is proportional to the cubic root of the pressure difference, as it has been theoretically predicted by Hencky for a membrane without flexural rigidity (like graphene), one hundred years ago. The proportionality coefficient provides a measurement of the Young’s modulus of graphene of 1 TPa (d). This measurement is in excellent agreement with nano-indentation measurements, which have been a landmark in the history of graphene research.
“All-optical blister test of suspended graphene using micro-Raman spectroscopy » , Dominik Metten, François Federspiel, Michelangelo Romeo, & Stéphane Berciaud, Physical Review Applied, 2, 054008
Contact chercheur : Stéphane Berciaud, maître de conférences
Institut de physique et chimie de matériaux de Strasbourg (IPCMS, UMR 7504)