Spatial spectrograms of vibrating atomic force microscopy cantilevers coupled to sample surfaces

Many advanced dynamic Atomic Force Microscopy (AFM) techniques such as contact resonance, force modulation, piezoresponse force microscopy, electrochemical strain microscopy, and AFM infrared spectroscopy exploit the dynamic response of a cantilever in contact with a sample to extract local material properties. Achieving quantitative results in these techniques usually requires the assumption of a certain shape of cantilever vibration. We present a technique that allows in-situ measurements of the vibrational shape of AFM cantilevers coupled to surfaces. This technique opens up unique approaches to nanoscale material property mapping, which are not possible with single point measurements alone. (C) 2013 AIP Publishing LLC

Accurate Vertical Nanoelectromechanical Measurements

Accurate measurements of the nanoscale electromechanical coupling in materials, including piezo and ferroelectrics, twisted 2D layers, and biological systems is of both fundamental scientific and applied importance. Piezoresponse Force Microscopy (PFM) is capable of detecting strains in these materials, down to the picometer range. Following the emergence of weaker materials, the smaller signals associated with them have revealed various crosstalk challenges that have limited the accuracy of measurements. Previous work demonstrated that the use of an interferometric displacement sensor (IDS) positioned appropriately above the tip of the cantilever (x/L~1), where x is the spot position and L is the cantilever length, has enabled sensitive and artifact-free electromechanical measurements. A similar approach has been employed in removing unwanted electrostatic and in-plane response contributions for the optical beam deflection (OBD) measurement technique commonly used in most atomic force microscopes. In the present study, extensive automated sub-resonance spot position dependent PFM measurements were conducted on periodically poled lithium niobate (PPLN). In this work, both IDS and OBD responses were measured simultaneously, allowing direct comparisons of the two approaches. The IDS showed a blind spot at x/L~1, as expected. However, for OBD measurements, the blind spot's location exhibited wider variation, ranging from 0.15<x/L<0.61. Furthermore, the magnitudes of the amplitudes measured with IDS and OBD were typically different, sometimes approaching disagreement by a factor of two. These measurements have important implications, not only for the PPLN measured here, but for more complex and unknown samples that have a heterogeneous polarization and electrical characteristic.Comment: 40 pages, 16 Figures, 24 Supplemental Figure

The Band Excitation Method in Scanning Probe Microscopy for Rapid Mapping of Energy Dissipation on the Nanoscale

Mapping energy transformation pathways and dissipation on the nanoscale and understanding the role of local structure on dissipative behavior is a challenge for imaging in areas ranging from electronics and information technologies to efficient energy production. Here we develop a novel Scanning Probe Microscopy (SPM) technique in which the cantilever is excited and the response is recorded over a band of frequencies simultaneously rather than at a single frequency as in conventional SPMs. This band excitation (BE) SPM allows very rapid acquisition of the full frequency response at each point (i.e. transfer function) in an image and in particular enables the direct measurement of energy dissipation through the determination of the Q-factor of the cantilever-sample system. The BE method is demonstrated for force-distance and voltage spectroscopies and for magnetic dissipation imaging with sensitivity close to the thermomechanical limit. The applicability of BE for various SPMs is analyzed, and the method is expected to be universally applicable to all ambient and liquid SPMs.Comment: 32 pages, 9 figures, accepted for publication in Nanotechnolog

Nanoscale rheology: dynamic mechanical analysis over a broad and continuous frequency range using photothermal actuation atomic force microscopy

Polymeric materials are widely used in industries ranging from automotive to biomedical. Their mechanical properties play a crucial role in their application and function and arise from the nanoscale structures and interactions of their constitutive polymer molecules. Polymeric materials behave viscoelastically, i.e., their mechanical responses depend on the time scale of the measurements; quantifying these time-dependent rheological properties at the nanoscale is relevant to develop, for example, accurate models and simulations of those materials, which are needed for advanced industrial applications. In this paper, an atomic force microscopy (AFM) method based on the photothermal actuation of an AFM cantilever is developed to quantify the nanoscale loss tangent, storage modulus, and loss modulus of polymeric materials. The method is then validated on styrene–butadiene rubber (SBR), demonstrating the method’s ability to quantify nanoscale viscoelasticity over a continuous frequency range up to 5 orders of magnitude (0.2–20,200 Hz). Furthermore, this method is combined with AFM viscoelastic mapping obtained with amplitude modulation–frequency modulation (AM–FM) AFM, enabling the extension of viscoelastic quantification over an even broader frequency range and demonstrating that the novel technique synergizes with preexisting AFM techniques for quantitative measurement of viscoelastic properties. The method presented here introduces a way to characterize the viscoelasticity of polymeric materials and soft and biological matter in general at the nanoscale for any application