Jason P. Killgore

Viscoelastic Property Mapping with Contact Resonance Force Microscopy

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene-polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time-temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

Engineering plant cell walls: tuning lignin monomer composition for deconstructable biofuel feedstocks or resilient biomaterials

Advances in genetic manipulation of the biopolymers that compose plant cell walls will facilitate more efficient production of biofuels and chemicals from biomass and lead to specialized biomaterials with tailored properties. Here we investigate several genetic variants of Arabidopsis: the wild type, which makes a lignin polymer of primarily guaiacyl (G) and syringyl (S) monomeric units, the fah1 mutant, which makes lignin from almost exclusively G subunits, and a ferulate 5-hydroxylase (F5H) overexpressing line (C4H:F5H) that makes lignin from S subunits. We employ multiscale, multimodal imaging techniques that reveal the biomass of the C4H:F5H transgenic to be more susceptible to deconstruction by maleic acid treatment than the other variants. Enzymatic saccharification assays of the treated materials show that C4H:F5H transgenic tissue is significantly more digestible than the wild type, while the fah1 mutant is clearly the least digestible of these materials. Finally, we show by contact resonance force microscopy, an atomic force microscopy technique, that F5H overexpression in C4H:F5H transgenic plants significantly reduces the stiffness of the cell walls in the region of the compound middle lamella relative to wild type and fah1.

Continuous Measurement of Atomic Force Microscope Tip Wear by Contact Resonance Force Microscopy

devices, [ 3 ] and manufacturing. [ 4 ] In these applications, tip size and shape critically affect the accuracy, resolution, and reliability of measurements and processes. [ 5 ] However, during tip–sample contact the tip can wear and break, undermining the utility of the instrument. [ 6 ] Thus, the development of wear-resistant probes, protocols for their testing and a fundamental understanding of their wear process is of vital importance. Although wear-resistant probes continue to advance, [ 7 , 8 ] tribological test methods and the collection of data for theoretical models still require measurements ex situ to the scanning process. Scanning electron microscopy (SEM) [ 8–12 ] and blind reconstruction while scanning on highaspect-ratio reference samples [ 8 , 12 , 13 ] have both been used to correlate scanning history with tip geometry changes. Such ex-situ approaches are slow and can cause additional wear, fracture, and contamination. More recently, periodic force– displacement adhesion measurements have provided a less disruptive means of monitoring changes in contact area after scanning a fi nite distance. [ 8 , 11 , 12 ] Still, adhesion measurements require interruption of the scan, and quantitative determination of a contact radius can be strongly affected by geometry, contamination, and environmental conditions. Here, we demonstrate how contact resonance force microscopy (CR-FM) methods enable quantitative in-situ evaluation of tip wear by measurement of instantaneous changes in contact radius while scanning Si cantilevers on a Si substrate. It is found that CR-FM measurements do not adversely affect the wear process, and the results compare favorably with ex-situ techniques. Overall, CR-FM is shown to be an effective tool for detecting subnanometer changes in the contact radius while also revealing novel information about tip symmetry and wear rate. Contact resonance force microscopy experiments and analysis are well described in the literature. [ 14 ] Briefl y, an AFM tip is brought into contact with a sample, and the sample or cantilever is vibrated out of plane over a frequency range that excites a fl exural resonance. Due to tip–sample interactions, the contact resonance of a given eigenmode

Anomalous Friction in Suspended Graphene

Since the discovery of the Amontons law and with support of modern tribological models, friction between surfaces of three-dimensional materials is known to generally increase when the surfaces are in closer contact. Here, using molecular dynamics simulations of friction force microscopy on suspended graphene, we demonstrate an increase of friction when the scanning tip is retracted away from the sample. We explain the observed behavior and address why this phenomenon has not been observed for isotropic 3D materials.

Morphing Metal–Polymer Janus Particles

The direct deformation and shape recovery of micron-sized polystyrene particles via nanoimprint lithography is reported. The recovery of the programmed PS particles can be utilized to create a range of smart Janus particles with contrasting properties in conductivity and topography, by use of metal-layer constrained recovery.

Methylammonium lead iodide grain boundaries exhibit depth-dependent electrical properties

In this communication, the nanoscale through-film and lateral photo-response and conductivity of large-grained methylammonium lead iodide (MAPbI3) thin films are studied. In perovskite solar cells (PSC), these films result in efficiencies >17%. The grain boundaries (GBs) show high resistance at the top surface of the film, and act as an impediment to photocurrent collection. However, lower resistance pathways between grains exist below the top surface of the film, indicating that there exists a depth-dependent resistance of GBs (RGB(z)). Furthermore, lateral conductivity measurements indicate that RGB(z) exhibits GB-to-GB heterogeneity. These results indicate that increased photocurrent collection along GBs is not a prerequisite for high-efficiency PSCs. Rather, better control of depth-dependent GB electrical properties, and an improvement in the homogeneity of the GB-to-GB electrical properties, must be managed to enable further improvements in PSC efficiency. Finally, these results refute the implicit assumption seen in the literature that the electrical properties of GBs, as measured at the top surface of the perovskite film, necessarily reflect the electrical properties of GBs within the thickness of the film.

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.

Liquid contact resonance atomic force microscopy via experimental reconstruction of the hydrodynamic function

We present a method to correct for surface-coupled inertial and viscous fluid loading forces in contact resonance (CR) atomic force microscopy (AFM) experiments performed in liquid. Based on analytical hydrodynamic theory, the method relies on experimental measurements of the AFM cantilevers free resonance peaks near the sample surface. The free resonance frequencies and quality factors in both air and liquid allow reconstruction of a continuous hydrodynamic function that can be used to adjust the CR data in liquid. Validation experiments utilizing thermally excited free and in-contact spectra were performed to assess the accuracy of our approach. Results show that the method recovers the air frequency values within approximately 6%. Knowledge of fluid loading forces allows current CR analysis techniques formulated for use in air and vacuum environments to be applied to liquid environments. Our technique greatly extends the range of measurement environments available to CR-AFM.

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.

Hydrodynamic corrections to contact resonance atomic force microscopy measurements of viscoelastic loss tangenta)

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.