Multi-eigenmode control for high material contrast in bimodal and higher harmonic atomic force microscopy

Review: Advanced Atomic Force Microscopy Modes for Biomedical Research

Visualization of biomedical samples in their native environments at the microscopic scale is crucial for studying fundamental principles and discovering biomedical systems with complex interaction. The study of dynamic biological processes requires a microscope system with multiple modalities, high spatial/temporal resolution, large imaging ranges, versatile imaging environments and ideally in-situ manipulation capabilities. Recent development of new Atomic Force Microscopy (AFM) capabilities has made it such a powerful tool for biological and biomedical research. This review introduces novel AFM functionalities including high-speed imaging for dynamic process visualization, mechanobiology with force spectroscopy, molecular species characterization, and AFM nano-manipulation. These capabilities enable many new possibilities for novel scientific research and allow scientists to observe and explore processes at the nanoscale like never before. Selected application examples from recent studies are provided to demonstrate the effectiveness of these AFM techniques.

Enhancing higher-order eigenmodes of AFM using bridge/cantilever coupled system

The wide application of multi-frequency atomic force microscopy (AFM) places higher demands on the higher-order modes response of the cantilever. The response of the higher modes however is generally weaker than that of the fundamental mode in air. Researchers have proposed many methods, most of which involve cantilever modification, to enhance higher-order eigenmodes response. These previous results are proved to be effective, but the microfabrication is expensive. In this article, we propose a novel model based on bridge/cantilever coupled system to enhance the higher-order modes response of AFM cantilever. The segmented beam model provides a new thinking to explain the appearance of undesired peaks in mode analysis of cantilever. Through theoretical analysis and simulation, we find that higher resonance modes are enhanced by tuning the bridge to match the high resonances of the single clamped cantilever. The length, thickness of the coupled system and the location of excitation can affect the enhancement. In summary, this model provides a new way to improve higher mode response for multi-frequency and other high bandwidth applications of AFM.

Quantitative Visualization of the Nanomechanical Young's Modulus of Soft Materials by Atomic Force Microscopy

The accurate measurement of nanoscale mechanical characteristics is crucial in the emerging field of soft condensed matter for industrial applications. An atomic force microscope (AFM) can be used to conduct nanoscale evaluation of the Young’s modulus on the target surface based on site-specific force spectroscopy. However, there is still a lack of well-organized study about the nanomechanical interpretation model dependence along with cantilever stiffness and radius of the tip apex for the Young’s modulus measurement on the soft materials. Here, we present the fast and accurate measurement of the Young’s modulus of a sample’s entire scan surface using the AFM in a newly developed PinPoint^TM nanomechanical mode. This approach enables simultaneous measurements of topographical data and force–distance data at each pixel within the scan area, from which quantitative visualization of the pixel-by-pixel topographical height and Young’s modulus of the entire scan surface was realized. We examined several models of contact mechanics and showed that cantilevers with proper mechanical characteristics such as stiffness and tip radius can be used with the PinPoint^TM mode to accurately evaluate the Young’s modulus depending on the sample type.

Multifunctional cantilevers for simultaneous enhancement of contact resonance and harmonic atomic force microscopy

To enhance contact resonance atomic force microscopy (CR-AFM) and harmonic AFM imaging simultaneously, we design a multifunctional cantilever. Precise tailoring of the cantilever's dynamic properties is realized by either mass-removing or mass-adding. As prototypes, focused ion beam drilling or depositing is used to fabricate the optimized structures. CR-AFM subsurface imaging on circular cavities covered by a piece of highly oriented pyrolytic graphite validates the improved CR frequency to contact stiffness sensitivity. The detectable subsurface depth and cavity radius increase accordingly by using the multifunctional cantilever. At the same time, the free resonance frequency of the second mode is tuned to an integer multiple of the fundamental one. Harmonic AFM imaging on polystyrene and low-density polystyrene mixture shows the improved harmonic amplitude contrast and signal strength on the two material phases. The multifunctional cantilever can be extended to enhance other similar AFM operation modes and it has potential applications in relevant fields such as mechanical characterization and subsurface imaging.

Thermal nanometrology using piezoresistive SThM probes with metallic tips

In this paper we present design and application of novel piezoresistive scanning thermal microscopy (SThM) probes. The proposed probe integrates a piezoresistive deflection sensor and thermally active, resistive nanosize tip. Manufacturing technology includes standard silicon MEMS/CMOS processing and sophisticated postprocessing using Focus Ion Beam milling. Authors also describe dedicated measurement technique in order to perform quantitative nanoscale thermal probing. Performance of the developed thermal probes is validated by test scans (topography and temperature distribution) of silicon nanoresistors supplied with current.

Material discrimination and mixture ratio estimation in nanocomposites via harmonic atomic force microscopy

Harmonic atomic force microscopy (AFM) was employed to discriminate between different materials and to estimate the mixture ratio of the constituent components in nanocomposites. The major influencing factors, namely amplitude feedback set-point, drive frequency and laser spot position along the cantilever beam, were systematically investigated. Employing different set-points induces alternation of tip-sample interaction forces and thus different harmonic responses. The numerical simulations of the cantilever dynamics were well-correlated with the experimental observations. Owing to the deviation of the drive frequency from the fundamental resonance, harmonic amplitude contrast reversal may occur. It was also found that the laser spot position affects the harmonic signal strengths as expected. Based on these investigations, harmonic AFM was employed to identify material components and estimate the mixture ratio in multicomponent materials. The composite samples are composed of different kinds of nanoparticles with almost the same shape and size. Higher harmonic imaging offers better information on the distribution and mixture of different nanoparticles as compared to other techniques, including topography and conventional tapping phase. Therefore, harmonic AFM has potential applications in various fields of nanoscience and nanotechnology.

Note: Double-hole cantilevers for harmonic atomic force microscopy

To enhance the harmonic signals in intermittent contact atomic force microscopy, we proposed the double-hole structural modification. Finite element analyses and experiments demonstrated the capability and advantages of the developed method. An infinite set of harmonic cantilevers can be optimized by proper selections of hole size, position, and inter-distance. The second and third resonance frequencies are simultaneously regulated to be integer multiples of the fundamental frequency. In the meanwhile, the alteration of cantilever stiffness is kept minimum. The double-hole modifications have prominent advantages of regular geometry, flexible selection of cutting positions/dimensions, and easy-to-meet fabrication tolerances.

Calibration of higher eigenmodes of cantilevers

A method is presented for calibrating the higher eigenmodes (resonant modes) of atomic force microscopy cantilevers that can be performed prior to any tip-sample interaction. The method leverages recent efforts in accurately calibrating the first eigenmode by providing the higher-mode stiffness as a ratio to the first mode stiffness. A one-time calibration routine must be performed for every cantilever type to determine a power-law relationship between stiffness and frequency, which is then stored for future use on similar cantilevers. Then, future calibrations only require a measurement of the ratio of resonant frequencies and the stiffness of the first mode. This method is verified through stiffness measurements using three independent approaches: interferometric measurement, AC approach-curve calibration, and finite element analysis simulation. Power-law values for calibrating higher-mode stiffnesses are reported for several cantilever models. Once the higher-mode stiffnesses are known, the amplitude of each mode can also be calibrated from the thermal spectrum by application of the equipartition theorem.