In-situ Cantilever Shape Profiling – Implications for Quantitative Functional and Nanomechanical Measurements

R. Proksch
Asylum Research, US

Keywords: nanoparticle

Summary:

Quantitative mechanical and functional measurements using the Atomic Force Microscope (AFM) are inextricably linked to quantifying the response of the cantilever. The cantilever is an extended mechanical object with normal modes that depend on the boundary conditions both of the cantilever structure itself and on the boundary conditions imposed by the tip-sample interactions. By using the encoded and motorized optical beam controls on a commercial microscope (schematically indicated in Figure 1), I have been able to directly measure the extended shape of the cantilever as it interacts with various samples and in a variety of measurement modes. Integration of the laser scanning mechanism directly in the AFM has enabled these shape measurements during normal AFM operation in a variety of imaging modes ranging from force curves and contact modes to AC modes. This presentation will focus on contact and force curves measurement modes. Piezoresponse Force Microscopy (PFM) and Electrochemical Strain Microscopy (ESM) have led to significant progress in our understanding of the nano-electromechanical origins of function in a large variety of materials. In many cases of interest, the response of a cantilever does not behave as a simple, single-mode system. To elucidate the effects of local and long-ranged drive mechanisms on the multi-modal cantilever probe, the response of the cantilever as a function of both frequency and optical beam spot position has been measured in-situ while the cantilever is directly probing a functional material, either with PFM or ESM. A typical resulting spectrogram is shown in Figure 2. Two interesting results are (i) the crossover between the dc and first contact mode is measurable at surprisingly low frequencies, and (ii) contrary to initial expectations, operating on resonance actually provides more stable and reliable quantification of the cantilever drive. This is explained in terms of the high Q-factor of the resonance. There has been significant interest in quantifying the nanomechanical response of samples using force curves. As has been pointed out, the response of the cantilever during force curves has significant harmonic terms that contain most of the information on the tip-sample interactions. Previously, the detection sensitivity, stiffness and phase of the cantilever response at these harmonic frequencies have been assumed to be constant. I have examined this assumption while modulating the tip-sample interaction forces as a function of load. Surprisingly, there is often a low frequency crossover similar to that observed for the functional imaging the 0th and first contact resonance. Among other considerations, one significant factor in the crossover frequency is the presence of long-range forces. This may have far-reaching implications in quantifying nanomechanical properties based on conventional force curve measurements – especially force curves performed at higher frequencies.