Herein, a peripherally clamped stretched square monolayer graphene sheet with a side length of 10 nm was demonstrated as a resonator for atomic-scale mass sensing via molecular dynamics (MD) simulation. Then, a novel method of mass determination using the first three resonant modes (mode11, mode21 and mode22) was developed to avoid the disturbance of stress fluctuation in graphene. MD simulation results indicate that improving the prestress in stretched graphene increases the sensitivity significantly. Unfortunately, it is difficult to determine the mass accurately by the stress-reliant fundamental frequency shift. However, the absorbed mass in the middle of graphene sheets decreases the resonant frequency of mode11 dramatically while having negligible effect on that of mode21 and mode22, which implies that the latter two frequency modes are appropriate for compensating the stress-induced frequency shift of mode11. Hence, the absorbed mass, with a resolution of 3.310−22 g, is found using the frequency ratio of mode11 to mode21 or mode22, despite the unstable prestress ranging from 32 GPa to 47 GPa. This stress insensitivity contributes to the applicability of the graphene-based resonant mass sensor in real applications.

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In order to make the operation of stereo microscope more convenient and acquisition high quality images, we develop a novel motorized stereo microscpe and propose new auto-focusing (AF) algorithm. X, Y and Z (focus) axis positions of the microscopic stage are visual feedback controlled to focus. The focus evaluation function based on the morphological wavelet decomposition pyramid is introduced. By calculating the ratio of energy of high frequency wavelet coefficients and low frequency wavelet coefficients in the second Median-Erosion morphological wavelet domain, the proposed method performs well in the noise environment.