Dynamics of repulsive dual-frequency atomic force microscopy
Applied Physics Letters 94(6): 063109
In bimodal atomic force microscopy, two flexural modes are driven at their resonances. The oscillation of the second eigenmode, which is usually an incommensurate multiple of the fundamental frequency, perturbs the dynamic system. Numerical simulations show that the tip motion is almost periodic at typical set points and that harmonics and intermodulation frequencies prevail in the spectrum. The simulations also predict a very small increase in the noise of the first mode amplitude and phase due to the second mode oscillation. At small average tip sample separations, however, phases with repulsive and purely attractive forces can occur intermittently.