Contact resonance atomic force microscopy using long elastic tips.

In this work, a new theoretical model for contact resonance atomic force microscopy, which incorporates the elastic dynamics of a long sensing tip is presented. The model is based on the Euler-Bernoulli beam theory and includes coupling effects from the two-beam structure, also known as an 'L-shaped' beam in the literature. Here, high-accuracy prediction of the sample stiffness, using several vibration modes with a relative error smaller than 10% for practical working ranges, is demonstrated. A discussion on the model's capability to predict the dynamic phenomena of eigenmode veering and crossing, as the force applied to the sample increases, is presented. The L-shaped beam model presented here is also applicable for structural applications such as: micro-electro-mechanical systems, energy harvesting, and unmanned aerial vehicle landing gear.

Experimental validation of contact resonance AFM using long massive tips.

In this work, we present an experimental validation of a new contact resonance atomic force microscopy model developed for sensors with long, massive tips. A derivation of a new technique and graphical method for the identification of the unknown system parameters is presented. The technique and contact resonance model are experimentally validated. The agreement between our contact resonance experimental measurements and values obtained from nanoindentation show a minimal error of 1.4%-4.5% and demonstrate the validity of the new contact resonance model and system parameter identification technique.

Functional connectivity within an anxiety network and associations with anxiety symptom severity in middle-aged adults with and without autism.

Anxiety is highly prevalent in autism spectrum disorders (ASDs). However, few functional magnetic resonance imaging (fMRI) studies of ASDs have focused on anxiety (and fewer still on anxiety in middle-aged adults). Thus, relationships between atypical connectivity and anxiety in this population are poorly understood. The current study contrasted functional connectivity within anxiety network regions across adults (40-64 years) with and without autism, and tested for group by functional connectivity interactions on anxiety. Twenty-two adults with ASDs (16 males) and 26 typical control (TC) adults (22 males) completed the Beck Anxiety Inventory and a resting-state fMRI scan. An anxiety network consisting of 12 regions of interest was defined, based on a meta-analysis in TC individuals and two studies on anxiety in ASDs. We tested for main effects of group and group by anxiety interactions on connectivity within this anxiety network, controlling for head motion using ANCOVA. Results are reported at an FDR adjusted threshold of q < 0.1 (corrected) and p < 0.05 (uncorrected). Adults with ASDs showed higher anxiety and underconnectivity within the anxiety network, mostly involving bilateral insula. Connectivity within the anxiety network in the ASD group showed distinct relationships with anxiety symptoms that did not relate to ASD symptom severity. Functional connectivity involving the bilateral posterior insula was positively correlated with anxiety in the ASD (but not the TC) group. Increased anxiety in middle-aged adults with ASD is associated with atypical functional connectivity, predominantly involving bilateral insula. Results were not related to ASD symptom severity suggesting independence of anxiety-related effects. LAY SUMMARY: Anxiety is very common in adults with autism but the brain basis of this difference is not well understood. We compared functional connectivity between anxiety-related brain regions in middle-aged adults with and without autism. Adults with autism were more anxious and showed weaker functional connections between these regions. Some relationships between functional connectivity and higher anxiety were specific to the autism group. Results suggest that anxiety functions differently in autism.

Scanning speed phenomenon in contact-resonance atomic force microscopy.

This work presents data confirming the existence of a scan speed related phenomenon in contact-mode atomic force microscopy (AFM). Specifically, contact-resonance spectroscopy is used to interrogate this phenomenon. Above a critical scan speed, a monotonic decrease in the recorded contact-resonance frequency is observed with increasing scan speed. Proper characterization and understanding of this phenomenon is necessary to conduct accurate quantitative imaging using contact-resonance AFM, and other contact-mode AFM techniques, at higher scan speeds. A squeeze film hydrodynamic theory is proposed to explain this phenomenon, and model predictions are compared against the experimental data.

Quantitative Contact Resonance Force Microscopy for Viscoelastic Measurement of Soft Materials at the Solid-Liquid Interface.

Viscoelastic property measurements made at the solid-liquid interface are key to characterizing materials for a variety of biological and industrial applications. Further, nanostructured materials require nanoscale measurements. Here, material loss tangents (tan δ) were extracted from confounding liquid effects in nanoscale contact resonance force microscopy (CR-FM), an atomic force microscope based technique for observing mechanical properties of surfaces. Obtaining reliable CR-FM viscoelastic measurements in liquid is complicated by two effects. First, in liquid, spurious signals arise during cantilever excitation. Second, it is challenging to separate changes to cantilever behavior due to the sample from changes due to environmental damping and added mass effects. We overcame these challenges by applying photothermal cantilever excitation in multiple resonance modes and a predictive model for the hydrodynamic effects. We demonstrated quantitative, nanoscale viscoelastic CR-FM measurements of polymers at the solid-liquid interface. The technique is demonstrated on a point-by-point basis on polymer samples and while imaging in contact mode on a fixed plant cell wall. Values of tan δ for measurements made in water agreed with the values for measurements in air for some experimental conditions on polystyrene and for all examined conditions on polypropylene.

Vibrational shape tracking of atomic force microscopy cantilevers for improved sensitivity and accuracy of nanomechanical measurements.

Contact resonance atomic force microscopy (CR-AFM) methods currently utilize the eigenvalues, or resonant frequencies, of an AFM cantilever in contact with a surface to quantify local mechanical properties. However, the cantilever eigenmodes, or vibrational shapes, also depend strongly on tip-sample contact stiffness. In this paper, we evaluate the potential of eigenmode measurements for improved accuracy and sensitivity of CR-AFM. We apply a recently developed, in situ laser scanning method to experimentally measure changes in cantilever eigenmodes as a function of tip-sample stiffness. Regions of maximum sensitivity for eigenvalues and eigenmodes are compared and found to occur at different values of contact stiffness. The results allow the development of practical guidelines for CR-AFM experiments, such as optimum laser spot positioning for different experimental conditions. These experiments provide insight into the complex system dynamics that can affect CR-AFM and lay a foundation for enhanced nanomechanical measurements with CR-AFM.

Characterizing the free and surface-coupled vibrations of heated-tip atomic force microscope cantilevers.

Combining heated-tip atomic force microscopy (HT-AFM) with quantitative methods for determining surface mechanical properties, such as contact resonance force microscopy, creates an avenue for nanoscale thermomechanical property characterization. For nanomechanical methods that employ an atomic force microscope cantilever's vibrational modes, it is essential to understand how the vibrations of the U-shaped HT-AFM cantilever differ from those of a more traditional rectangular lever, for which analytical techniques are better developed. Here we show, with a combination of finite element analysis (FEA) and experiments, that the HT-AFM cantilever exhibits many more readily-excited vibrational modes over typical AFM frequencies compared to a rectangular cantilever. The arms of U-shaped HT-AFM cantilevers exhibit two distinct forms of flexural vibrations that differ depending on whether the two arms are vibrating in-phase or out-of-phase with one another. The in-phase vibrations are qualitatively similar to flexural vibrations in rectangular cantilevers and generally show larger sensitivity to surface stiffness changes than the out-of-phase vibrations. Vibration types can be identified from their frequency and by considering vibration amplitudes in the horizontal and vertical channels of the AFM at different laser spot positions on the cantilever. For identifying contact resonance vibrational modes, we also consider the sensitivity of the resonant frequencies to a change in applied force and hence to tip-sample contact stiffness. Finally, we assess how existing analytical models can be used to accurately predict contact stiffness from contact-resonance HT-AFM results. A simple two-parameter Euler-Bernoulli beam model provided good agreement with FEA for in-phase modes up to a contact stiffness 500 times the cantilever spring constant. By providing insight into cantilever vibrations and exploring the potential of current analysis techniques, our results lay the groundwork for future use of HT-AFM cantilevers for accurate nanomechanical property measurements.

Hydrodynamic corrections to contact resonance atomic force microscopy measurements of viscoelastic loss tangent.

We present a method to improve accuracy in measurements of nanoscale viscoelastic material properties with contact resonance atomic force microscope methods. Through the use of the two-dimensional hydrodynamic function, we obtain a more precise estimate of the fluid damping experienced by the cantilever-sample system in contact resonance experiments, leading to more accurate values for the tip-sample damping and related material properties. Specifically, we consider the damping and added mass effects generated by both the proximity of the cantilever to the sample surface and the frequency dependence on the hydrodynamic loading of the system. The theoretical correction method is implemented on experimental contact resonance measurements. The measurements are taken on a thin polystyrene film and are used to determine the viscoelastic loss tangent, tan δ, of the material. The magnitude of the corrections become significant on materials with low tan δ (<0.1) and are especially important for measurements made with the first flexural mode of vibration.

Characterization of silver-gallium nanowires for force and mass sensing applications.

We investigate the mechanical properties of cantilevered silver-gallium (Ag(2)Ga) nanowires using laser Doppler vibrometry. From measurements of the resonant frequencies and associated operating deflection shapes, we demonstrate that these Ag(2)Ga nanowires behave as ideal Euler-Bernoulli beams. Furthermore, radial asymmetries in these nanowires are detected through high resolution measurements of the vibration spectra. These crystalline nanowires possess many ideal characteristics for nanoscale force and mass sensing, including small spring constants (as low as 10(-4) N m(-1)), high frequency bandwidth with resonance frequencies in the 0.02-10 MHz range, small suspended mass (picograms), and relatively high Q-factors (approximately 2-50) under ambient conditions. We evaluate the utility of Ag(2)Ga nanowires for nanocantilever applications, including ultrasmall mass and high frequency bandwidth piconewton force detection.

Flexural vibration spectra of carbon nanotubes measured using laser Doppler vibrometry.

Laser Doppler vibrometry is used to measure the thermal vibration spectra of individual multiwalled carbon nanotubes (MWNTs) under ambient conditions. Since the entire vibration spectrum is measured with high frequency resolution, the resonant frequencies and quality factors of the MWNTs are accurately determined, allowing for estimates of their elastic moduli. Because the diameters of the MWNTs studied are smaller than the wavelength of the vibrometer's laser, Mie scattering is used to estimate values for the smallest diameter nanotube or nanowire whose vibration can be measured in this way.