The ability to achieve AFM imaging at rates on the order of frames per second has been leveraged for multiparametric SPM studies to achieve high resolution maps of mechanical, electronic, piezoelectric, conducting, and photoconducting properties. With BiFeO₃ multiferroics, for example, 180° domain switching is uniquely resolved to occur deterministically via a ferroelectric and ferroelastic multistep process, enabling a new, high efficiency, magnetoresistance device. Repeated photoconduction maps for CdTe solar cells illuminated with 1-5 suns, each performed at distinct biases, provides a similar opportunity to gain fundamental insight into materials properties in order to improve device functionality. In this case, solar cell performance is directly mapped at the nanoscale and related to nano- and micro- structural features in these polycrystalline thin films. Finally, the results are analyzed implementing a new approach, which accounts for the local topographic slope and curvature to mathematically identify any correlations between SPM property and topographic maps, addressing seldom discussed challenges with geometric convolution that often plague SPM interpretation.

Raman microscopy has always been a convenient tool for analyzing and imaging various materials as it provides richer information than other imaging techniques based on topographic information. However, Raman scattering is a weak phenomenon and the spatial resolution in any optical microscopy is usually restricted by the diffraction limit of the probing light. Both these problems can be overcome by utilizing tip-enhanced Raman scattering (TERS) microscopy, which provides local enhancement of light as well as super spatial resolution [1]. The spatial resolution in imaging is limited around 10 nm due to the necessity of a reasonable diameter of metallic tip to excite collective electron oscillation and due to the contribution of imaginary part of dielectric constant of probe metal in visible range [2]. The factor of enhancement is also limited due to the necessary of covering the spectrally broad band for the excitation and Raman scattering shift of sample. The effective spectral range is also limited to near UV to near infrared for silver and gold. In this presentation, I will show our research progress in TERS microscopy beyond the limitations. The spatial resolution has been drastically improved by applying pressure on to the sample with a tip to introduce the localized structural deformation in sample [3]. The broadband enhancement by cascading the probe antennae [4], the deep UV resonant Raman TERS [5], and 3D Raman imaging with a gold nano-particle inside a living cell [6] will be discussed.
[1] S. Kawata, Y. Inouye, P. Verma, Nature Photonics.3, 388 (2009).
[2] S. Kawata, Jpn. J. Appl. Phys.53, 010001 (2013).
[3] T. Yano et al.,Nature Photonics, 3, 473 (2009).
[4] S. Kawata, A. Ono, P. Verma, Nature Photonics, 3, 473, 2009.
[5] A. Taguchi, et. al., J. Raman Spectrosc. 40, 1324, 2009.
[6] J. Ando, et. al, Nano Lett.11, 5344 (2011).

Few imaging methods have the potential to resolve nanoscale structures on living cells. Both atomic force microscopy (AFM) and single molecule localization microscopy (SMLM) have individually been shown very useful to unravel details in cellular structure, function and behavior. To obtain structural information from AFM together with functional information from SMLM we constructed a combined AFM/SMLM microscope for correlated imaging of living cells. This setup allows to exploit the full potential of either microscopic method.
As a proof of concept we characterized the resolution of both systems on actin filaments and assessed the local fluorescent labelling density as a function of AFM topography. We found that fluctuations in fluorescent intensities often observed in SMLM measurements of f-actin are due to the agglomeration of actin filaments, rather than fluctuations in labeling densities as was previously believed.
We also expanded the correlated measurements to fixed bacteria and live mammalian cells. We for the first time performed time resolved imaging of living cells using both AFM and SMLM. By using CHO cells transiently expressing a paxillin-mEos2 construct we resolved cell migration of CHO cells by AFM and the formation and desorption of focal adhesions using time resolved SMLM.
We believe that bridging the gap between AFM and SMLM and visualizing structural dynamics with nanoscale resolution directly correlated with the single molecule resolved localization of biomolecules has the potential to enhance our understanding of biomolecules at work.

Conventional closed loop-Kelvin probe force microscopy (KPFM) has emerged as a powerful technique for probing electric and transport phenomena at the solid-gas interface. The extension of KPFM capabilities to probe electrostatic and electrochemical phenomena at the solid-liquid interface is of interest for a broad range of applications including corrosion, sensing, energy storage, and biological processes. However, the operation of KPFM implicitly relies on the presence of a linear lossless dielectric in the probe-sample gap, a condition which is violated for ionically-active liquids (e.g., when diffuse charge dynamics are present). In this work, we demonstrate electrochemical force microscopy (EcFM), a multidimensional technique capable of bias- and time-resolved mapping of ion dynamics and tip-sample electrochemical processes. Electrostatic and electrochemical measurements are demonstrated in ionically-active (polar isopropanol, milli-Q water and aqueous NaCl) and ionically-inactive (non-polar decane) liquids. In the absence of mobile charges (ambient and non-polar liquids), KPFM and EcFM are both feasible, yielding comparable contact potential difference (CPD) values. In ionically-active liquids, KPFM is not possible and EcFM can be used to measure the dynamic CPD and a rich spectrum of information pertaining to charge screening, ion diffusion, and electrochemical processes (e.g., Faradaic reactions). In this way, we show that EcFM can be used to probe charge dynamics, ion diffusion and electrochemical processes in the tip-sample junction depending on what regime (bias or time) is probed. Finally, we establish EcFM as an imaging mode, allowing visualization of the spatial variability of local electrochemical behavior

Atomic force microscope (AFM) has become an indispensable tool to study surfaces at atomic resolution; investigate local oxidation, thermal conductivity and electrochemical processes; move, stretch and compress single-molecules and cells; and to deliver and collect material with sub-nanometer precision. Over the past three decades, while the performance of AFMs has steadily improved and the variety of scanning probe techniques have exploded, the basic design of the AFM cantilever has remained largely unchanged. Despite the fine level of control afforded to an investigator by modern AFMs to study material properties, the process of manipulation and precise delivery of materials has remained difficult. Here, we present, Fluid force microscopy (FluidFM®), a technique that combines the positional accuracy and force sensitivity of an AFM with the picoliter precision of nanofluidics to provide a unique platform for materials and biological research. The FluidFM integrates an AFM with hollow cantilevers and a pressure controller to enable applications such as single-cell manipulation, colloidal probe spectroscopy and lithography. Data will be presented that demonstrates manipulation of cells and other micron sized particles using FluidFM. By applying a negative pressure, micron sized particles can be reversibly and serially attached to the hollow cantilevers. Once the particle has been attached to the cantilever, they can be used to quantify the forces of interaction. Different cell types were picked up by the cantilevers and adhesion to glass and fibronectin was quantified. The data showed a 10 fold difference in interaction forces between cell types. In another experiment, the relationship between surface textures and focal adhesion was studied using endothelial cells. Data will also be presented quantifying the bacterial detachment force from poly-L-lysine coated substrates.

Combining Atomic Force Microscopy (AFM) with Mass Spectrometry (MS) enables the correlation of physical and chemical properties of samples. Traditional MS techniques such as Electrospray Ionization or Matrix-Assisted Laser Desorption/Ionization typically result in partial or complete damage to the sample during sample preparation or molecular desorption / ionization. AFM probes, with integrated heaters near the tip, serve as inexpensive and precise alternatives for thermal desorption (TD) and pyrolysis of molecules. Current heating techniques for such heated probes have resulted in minimum spot sizes of 1-2 µm for certain non-volatile samples. Spot sizes are even larger for materials where the melting and vaporization temperatures are further apart since the majority of the heat from the probe only melts or damages the sample. Here we present novel heating functions for the heated AFM probe that can shrink the spot size to 0.325 µm while maintaining comparable MS signal levels. A numerical model predicts ideal heating functions that ensure optimal localization of thermal energy in the material to maximize the TD efficiency. We study the effects of parameters defining the heating function on the spot size and MS signal experimentally.
This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy
Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. We thank Anasys Instruments for loaning the AFM+ instrument.

In this paper, we describe a novel method of nanoscale printing which involves chemo-mechanical material addition within a nanoscale tribological contact constituted by a conventional AFM probe and a substrate. The AFM probe is operated within a liquid carrier environment, which contains a dispersion of the ink material in either molecular or nanoparticle form. Patterning occurs at the sliding interface due to the in-situ confinement and stress-induced coagulation of the feedstock ink material with itself and to the substrate due to the large and highly localized tribological stresses within the contact. We demonstrate the flexibility of this method to print patterns with varying lateral and vertical dimensions (linewidths at least as small as 100 nm, and thicknesses ranging from a few nm to 100 nm) through straightforward variations in tribological contact parameters including probe size (0.01-100 µm) and normal load (0.5-500 µN). We report metrics for print speed and quality, as well as the mechanical properties of the tribologically-printed materials. Using a combination of these parameters, complex custom patterns as large as 20 µm by 20 µm were printed using programmed tip motion. We further demonstrate the ability of this technique to create nanoscale prints with different ink materials, including zinc dithiophosphates and zirconium oxide nanoparticles, as well as multi-material nanoprinted structures. We demonstrate printing on different substrate materials and across a range of temperatures. Finally, we show that, in some cases, the mechanical properties of the formed structures can approach bulk values, which is often a key drawback of several traditional additive manufacturing methods.

We report recent advances in AFM-based nanolithography using ultra sharp single crystal diamond AFM probes. Pulsed force operation when the sample is indented with the diamond AFM probe at a set of points located along the pattern lines with a user-defined pitch allows creating high quality nanoscale patterns in metals, silicon and other semiconductors and novel 2D materials with spatial resolution and the pattern quality unachievable with standard “plow-type” force lithography. In addition, elimination of the lateral drag during the nanoindentation dramatically improves the life time of the diamond probes.
We&’ll demonstrate the application of this novel nanolithography in manufacturing of nanowires and nanocontacts as well as in modification of the magnetic, electric and optical properties of metal films through nanopatterning, which can be probed in-situ with the same AFM or combined AFM-Raman system. In addition, we&’ll demonstrate how nanopatterning of 2D carbon-based materials can result in improved sensitivity of nanoscale Raman characterization of such materials by means of tip enhanced Raman scattering (TERS).

Tip-enhanced Raman scattering (TERS), which is a powerful option for scanning probe microscopy (SPM), allows Raman measurement of a solid surface and a single nanostructure, including a polymeric molecule, with nanometer-scale spatial resolution. In TERS, extremely enhanced electric fields of incident and scattered light at the surface of the metal apex of a probe tip enable us to detect ultraweak Raman signals from nanoscale volumes. The electric field enhancement, resulting from localized surface plasmon resonance (LSPR) at metal nanostructures, occurs when the size of the metal nanostructure is well suited to the wavelength of incident light. Typical TERS probes are currently metal probes for scanning tunneling microscopy (STM) or metal-coated silicon probes for atomic force microscopy (AFM). Such probes are, however, supposed to enhance the electric fields less effectively than a probe with a metal nanoparticle on its tip apex; their metal structure along the tip axis is too large to work for the enhancement of visible light, which is commonly used as an excitation light for Raman measurement.
Here, we report a method to prepare a probe with a silver nanostructure on its tip apex: the apex of a silicon AFM tip is irradiated using a focused ion beam (FIB), and then the FIB-irradiated AFM tip is exposed to a pure AgNO₃ aqueous solution. With this method, a silver nanostructure selectively grows on the tip apex where the native oxide layer has been removed in response to FIB irradiation. Our previous works on this FIB-triggered electroless deposition of gold and silver nanostructures on a silicon substrate, not on AFM tips, reveals that electrons flow from silicon into the solution through the silicon-solution and silicon-metal-solution boundaries, owing to the difference in Fermi energy between silicon and the solution, and thus reduce the metal ions on the boundaries [1-3]. Our method provides direct growth of a metal nanostructure on the AFM tip apex, and thus enables us to control the size and shape of the metal nanostructure using experimental conditions, including the exposure time and the concentration of AgNO₃(aq). Such flexibility in size and shape of metal nanostructures can allow metal growth tailored to excitation sources with different wavelengths.
[1] H. Itasaka, M. Nishi et al., MRS proceedings 1748 (2015) mrsf14-1748-ii11-02.
[2] H. Itasaka, M. Nishi et al., J. Ceram. Soc. Jpn. 122 (2014) 543-546.
[3] H. Itasaka, M. Nishi, and K. Hirao, Jpn. J. Appl. Phys. 53 (2014) 06JF06.

Atomic force microscopy (AFM) is moving toward an era of high speed scanning for higher time resolution. Commercial high speed AFM systems commonly adopt metallic mechanical structure which is driven by high capacitance piezo stacks, thus a costly high voltage/current source is needed. We propose an AFM design that implements a 3D framework assembled by printed circuit boards (PCBs) for low cost and high speed scanning. The PCB has similar stiffness but much lower mass (1.7 g/cc) compare with Aluminum (2.7g/cc), thus the PCB based AFM scanner has smaller inertia during high speed scanning. The PCB based scanner is actuated by 4 pieces of piezo buzzers which can be driven with a low voltage (±15 Volt) and low current (<20 mA) source which can be easily found. The system performance is evaluated by measuring a piece of DVD (data tracks with pitch: 740nm, depth: 150nm), the X-Y scanning speed and area of the PCB based AFM scanner are 1,129 mm/s and 26 mm, respectively. The scanning speed can be further improved by shorten the length of the PCB based scanner.

We demonstrate a novel technique for measuring contact potential which overcomes the limitations of standard Kelvin Probe Force Microscopy (KPFM). The technique is particularly well suited to investigate bias sensitive environments (electro-chemistry cells) and materials with very high work function (for energy storage applications). We apply the technique to investigate photo-generated charge distribution in photovoltaic thin films.
Intermodulation Electrostatic Force Microscopy^[1] (ImEFM) is a multifrequency Atomic Force Microscopy (AFM) technique where the cantilever is driven acoustically at its resonance frequency and electrically at a much lower frequency. The contact potential difference (CPD) between the AFM tip and the sample is obtained from a calibrated measurement of force at multiple mixing frequencies around the cantilever resonance. This resonant measurement scheme allows us to obtain very high signal to noise ratio (SNR) and therefore rapidly resolve small variations in CPD. Moreover, the measurement is performed in a single pass with the cantilever oscillating very close to the sample surface, resulting in fast imaging with high lateral resolution.
The combination of resonant detection and single pass overcomes a common trade-off, where fast imaging and high lateral resolution result in low SNR (Frequency Modulated KPFM), or high SNR requires slow imaging and low lateral resolution (Amplitude Modulated KPFM).
ImEFM exploits the nonlinear nature of the electrostatic force to measure in open loop, i.e. without feedback to control the tip-sample bias. Eliminating feedback avoids crosstalk between the height and CPD images. Removing the requirement of applying a DC potential to the tip makes the technique interesting for electro-chemistry and energy storage applications.
We report the successful application of ImEFM on different photovoltaic materials, both organic and hybrid. We show how accurate and high resolution measurement of CPD highlights variations of charge density at the nanometer scale providing meaningful insights to better design of photovoltaic thin films. Finally, we show how this high speed CPD imaging method reveals dynamic behavior of charging and discharging in these samples.
^[1]R. Borgani, D. Forchheimer, J. Bergqvist, P.-A. Thorén, O. Inganäs, and D. B. Haviland. Intermodulation Electrostatic Force Microscopy for imaging Surface Photo-Voltage. Applied Physics Letters, 105(14), 143113 (2014).

There has recently been great interest in nanomechanical mapping by atomic force microscopy. Additionally, high speed, ease of use and long term stability have been dominant directions for AFM instrumentation.
Peak force tapping (PFT) is one of the most promising modes for atomic force microscopy developed in recent years, offering nanomechanical characterization, ease of use and robust long-term operation. However, classic peak force tapping is relatively slow, as tip-sample distance modulation is provided by the piezo scanner, limiting the rate of force curves to well below the primary scanner resonance.
By using the cantilever as transducer for tip-sample distance modulation we achieve acuation bandwidth of up to 100kHz. We present fast imaging at 31.5kHz PFT rate which is limited by the processing capability of currently available controllers. We implement the direct actuation by heating the base of the cantilever with an auxiliary laser, which bends the cantilever with a bi-metal effect, allowing for large amplitudes beyond 100nm.
The system is implemented in a home-built AFM head compatible with Bruker Multimode systems.

Scanning ion conductance microscopy (SICM) is an emerging tool for the analysis of live cells. It is based on an electrolyte-filled nanopipette and allows imaging of topography without a direct mechanical contact between the nanopipette and the cell. To assess the imaging qualities of SICM, we have performed a direct comparison with atomic force microscopy (AFM) for imaging microvilli, which are small features on the surface of live cells, and for imaging the shape of whole cells. While the imaging quality on microvilli significantly increases after cell fixation for AFM, it remains constant for SICM. Also, the apparent shape of whole cells depends on the imaging setpoint for AFM, while it is independent of the setpoint for SICM. Both observations demonstrate the benefit of the contact-free imaging mechanism of SICM. The lateral resolution on a living cell is limited by the cell&’s elastic modulus for AFM, while it is not for SICM. We determined the lateral resolution of SICM theoretically and experimentally as three times the inner opening radius of the nanopipette.
The nanopipette in SICM can also be used to “focus” an externally applied pressure onto the cell surface, thereby providing a mechanism for studying cell mechanics in a contact-free fashion. We have developed a method for fast and quantitative elasticity mapping on live cells. We have used this method to visualize mechanical properties of live platelets and their dynamics ex vivo. We found that platelets significantly stiffen during spreading and soften during thrombin-induced activation. While the degree of softening was similar compared to cytochalasin D-induced cytoskeleton depolymerization, temporal and spatial modulations of the elastic modulus were substantially different. These results shed new light on the mechanics of hemostasis, with possible application in the investigation of platelet-associated diseases.

Here we describe development of a novel non-resonant oscillatory AFM mode, which we suggest to call “Ringing” mode. This mode is based upon utilizing a part of the AFM signal that is currently not used. This part of the signal has been considered parasitic (or noise) and, consequently, filtered out. The Ringing mode is a combination of the oscillatory non-resonant mode and a resonant one. It operates with the non-resonant feedback, but utilizes the signal information from the ringing of the AFM cantilever, free resonance oscillations of the cantilever which occur after detaching the AFM probe from a sample surface. Analyzing the amplitude of the ringing provides less noisy information. The absence of signal filtering produces fewer artifacts.
Compared to its rival, non-resonant oscillatory AFM mode (for example, PeakForce QNM), it provides faster imaging (~10x for material property channels); it produces fewer artifacts; it generates less noise in some channels due to the signal averaging (for example, multiple adhesion amplitudes). In addition, it delivers novel imaging information: Restored adhesion; Dynamic creep phase shift; Viscoelastic adhesive loss; Adhesion height; Zero force height (the latter has been recently introduced by several companies). It works with the standard AFM probes used in non-resonant modes. We demonstrate the application of this mode to various polymers and bio-materials.

Advances in micro-, nano-, and biotechnology put increasing demands on nano-scale microscopy and characterization. In particular, high-resolution microscopes such as scanning- or transmission-electron-microscopes (SEM, TEM) as well as atomic-force-microscopy (AFM) play a central role for nano-scale exploration as it provides material information at and beyond the atomic scale. During the last decade the combination of different microscopic and spectroscopic methods into one instrument gained increasing importance due to the simultaneous acquisition of complementary information. Especially highly-localized probing of mechanical and electric properties on the lower nano-scale represents an indispensable task for a fundamental understanding of material properties and provides the basis for further technological developments (e.g. new electronic- or energy-materials).
Traditional probes for AFM use optical detection of the cantilever sensor, therefore limiting the ability to integrate it with other analysis techniques (e.g. SEM or TEM). In this work, we remove these limitations by implementing a novel self-sensing nano-probe. These cantilevers are equipped with a deflection sensor that directly measures the cantilever signal electrically, which removes the space-consuming requirement for optical readout and allows seamless integration into SEMs. We use these novel nano-probes in a combined AFM and SEM development (AFSEMtrade;) that enables true correlated microscopy and analysis on the nano-scale. We use a 3D nano-printing technology (focused electron beam induced deposition, FEBID) for a variable tip modification to achieve electrically conductive nano-probes with final tip-radii of 10 nm and below. We present a variety of case studies for our correlated AFM & SEM microscopy of nanostructured materials, e.g. micro-mechanical testing and fast material analysis. In addition, we present first results of experiments for in-situ electrical characterization of nanostructures, e.g. in-situ FEBID shape optimization towards plasmonic Au structures, as well as behavior of industrial flexible PCBs, and discuss future developments.

The motion of the tip at the end of the conventional AFM cantilever is essentially constrained during scanning to one direction, that is, approximately perpendicular to the sample surface. Samples that are typically imaged, therefore, are necessarily planar in nature. The tip is unable to image samples that are more three-dimensional in character, as it is unable to image the sample from, for example, the side.
Recently, er have developed an AFM in which the tip is the end of a nanorod attached to a custom-designed microstructure that is steered in three dimensions with up to six degrees of freedom using holographically-generated optical traps. The microstructural probe was fabricated using two-photon polymerisation and contains components for steering and applying a constant force as well as components for determining the position and orientation of the probe at high resolution. As the probe is scanned, image recognition algorithms detect small deviations from the expected probe orientation immediately the tip touches the specimen surface. This results in extremely low forces on the specimen and defines the specimen surface in the scanned image. The nanorod probe is scanned over the sample and detects the sample surface from any direction such that it is capable of scanning and imaging around 3D structures [1-4].
The curvature of the trapping regions of the microstructures determines the spring constant. It is, therefore, possible to design structures with varying (and even b=negative) spring constants as a function of displacement, unrealizable with a material mechanical spring.
[1] Phillips DB, Padgett MJ, Hanna S, Ho Y-L D, Carberry MD, Miles MJ, Simpson HS, NaturePhotonics 8 (2014) 400-405.
[2] Phillips DB, Grieve JA, Olof SN, Kocher SJ, Bowman R, Padgett MJ, Miles MJ, Carberry DM, Nanotechnology22 (2011) Art No.285503.
[3] Phillips DB, Simpson, Grieve JA, Bowman R, Gibson GM, Padgett MJ, Rarity JG, Hanna S, Miles MJ, Carberry DM, EPL99 (2012) Art No. 58004.
[4] Olof SN, Grieve JA, Phillips DB, Rosenkranz H, Yallop ML, Miles MJ, Patil AJ, Mann S, Carberry DM, Nano Letters12 (2012) 6019-6023.

Magnetic materials are broadly explored for multiple applications including inductors, transformers, electric motors and generators. Many of these applications are underpinned by the existence of eddy current which can be a source of energy loss in these magnetic applications. However, local probing of magnetic response beyond magnetic domains has been relatively less explored compared to its significance on the multiple applications. Here, we develop a new approach for probing multiple types of magnetic properties associated with eddy current and magnetic domains in magnetic inductors (MIs) using so called multimodal magnetic force microscopy (MMFM). The obtained MMFM images show spatially varied local information of static magnetic domains. In addition, the bias dependent MMFM images reveal locally different eddy current behavior of which values depends on the type of materials that comprise the MI. We further analyzed multi-dimensional components of magnetic properties based on the vertical and lateral cantilever deflections. The MMFM approach allows exploring spatially varied magnetic properties at the nanoscale and can be further extended to the analysis of local physical features related to the eddy current and magnetic domains

The real-space observation of magnetic structure using scanning probe microscopy (SPM) methods or synchrotron-based microscopy continues to have a tremendous impact on our understanding of nanomagnetism. However, although SPM methods provide high spatial resolution, they lack direct chemical contrast. X-ray microscopy, on the other hand, can provide chemical as well as magnetic sensitivity. However, the spatial resolution is limited. In order to overcome these limitations, we have developed a new technique that combine synchrotron radiation with the high spatial resolution of scanning tunneling microscopy (STM). The goal is to combine the spin sensitivity of x-ray magnetic circular dichroism with the locality of STM. Recently, synchrotron x-ray scanning tunneling microscopy (SX-STM) showed the capability to obtain elemental contrast with a lateral spatial resolution of only 2 nm and sensitivity at the limit of single-atomic height.[i]
The polarization of synchrotron x-rays provides the opportunity to obtain magnetic properties in addition to the aforementioned chemical information. Hence, the use of polarized x-rays in SX-STM has been proposed before and feasibility experiments were carried out. However, so far the tip could only be located in the far field, several hundred nm outside the quantum mechanical tunneling regime. In this talk, we present the first measurements of the local x-ray magnetic circular dichroism (XMCD) signal of magnetic domains in a Fe film using a non-magnetic-tip that is actually tunneling over the surface. The achievment of localized spectroscopy with simultaneous topographic, elemental, and magnetic information has the potential to significantly impact characterization of complex materials at the nanoscale.
This work was funded by the Office of Science Early Career Research Program through the Division of Scientific User Facilities, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant SC70705.
[i] Nozomi Shirato, Marvin Cummings, Heath Kersell, Yang Li, Benjamin Stripe, Daniel Rosenmann, Saw-Wai Hla, and Volker Rose, Nano Lett. 14 6499 (2014).

Over the last 30 years, scanning probe microscopy has emerged as a powerful tool for imaging structure and functionality of matter on nano- and atomic scales. In most general terms, the SPM images represent the convolution of the dynamic interactions in tip-surface junction as interrogated through chosen measurement protocol. Correspondingly, much of the attention of SPM practitioners was aimed at the development of progressively more complex protocols for excitation and detection, starting from original static modes in contact AFM, to lock-in and phase-locked loops in single frequency SPMs, to multifrequency methods of the last decade. However, these measurements invariably rely on the predefined model for processing of the information flow, as limited by data analysis electronics. In this presentation, I will summarize existing approaches for information processing in SPM and introduce the approach for full information capture in SPM based on recording and complete analysis of data stream from photodetector. This general-mode (G-Mode) SPM is illustrated for classical SPM modes such as intermittent contact mode SPM, as well as piezoresponse force microscopy and spectroscopy (PFM) and Kelvin probe microscopy. The analysis of the information contact allows deducing in which cases classical signal processing allows unbiased representation of the tip-surface interactions and which it incurs significant information loss. The approaches for full mapping on frequency responses providing complete view of tip-surface interactions are discussed. Finally, perspective and critical needs for the full information analysis imaging in SPM are discussed.
This research is supported by and performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.
A. Belianinov, S.V. Kalinin, and S. Jesse, Complete information acquisition in scanning probe microscopy, Nat. Comm. 6, 6550 (2015).

Nanomechanical property measurements with the atomic force microscope (AFM) provide crucial information in the study of polymers, composites, cells, viruses, nanotubes, thin films and other systems containing nanoscale structure. Many of these important systems have material properties that vary as a function of measurement frequency. Understanding this frequency dependence is important when the frequency of the forces the system experiences under operation vary, for example, as in ballistic materials. Furthermore, this frequency dependent variation complicates the comparison between mechanical properties measured with the AFM to macro-scale, lower-frequency measurement techniques. Common dynamic AFM modes for measuring mechanical properties such as tapping mode and contact resonance AFM only measure nanomechanical properties at discrete resonance frequencies of the AFM cantilever, typically hundreds of kilohertz. The most common quasi-static AFM mode for measuring mechanical properties, force spectroscopy AFM, is often only done at a single low frequency, typically a few hertz. To fully characterize frequency dependent samples and to compare AFM measurements with lower frequency macroscale techniques, there is a need for AFM techniques that can determine material properties across a large frequency bandwidth.
We have developed a photothermally actuated force modulation microscopy (FMM) AFM method for mechanical property characterization across a broad frequency range. Photothermal cantilever actuation allows for a driving force that varies smoothly as a function of frequency, avoiding the problem of spurious resonant vibrations that hinder piezoelectric actuation schemes. FMM is a sub-resonant dynamic AFM technique that is capable of measuring mechanical properties over a frequency range varying from a lower limit determined by the low frequency noise floor of the system to an upper limit of about ten percent of the first free resonance frequency of the cantilever. A complication of applying phototheramally actuated FMM is that the quasi-static cantilever vibration shape is fundamentally different compared to piezoelectric actuation. To account for this difference we have rederived the FMM equations using Euler-Bernoulli beam theory in place of the classical series spring model. The photothermal FMM model relates the cantilever vibrational shape to the stiffness of the tip sample contact. To validate this approach photothermal FMM measurements were made on a suspended bridge structure. We measured the contact stiffness of the suspended bridge structure over a frequency bandwidth varying from 3 kHz to 15 kHz and observed good agreement with the stiffness predicted by contact resonance AFM. Ultimately, photothermal FMM enables the expansion of frequency-dependent properties characterization into very large frequency bandwidths by avoiding the problem of spurious resonances associated with piezoelectric cantilever excitation.

Nanomechanical properties are closely related to the states of matter, including chemical composition, crystal structure and mesoscopic domain configuration, etc. Understanding these properties relies on high-veracity spatial imaging of them and revealing their dependence on various external stimuli in spectroscopies. Contact Resonance atomic force microscopy (CR-AFM) works on the principles of cantilever dynamics and contact mechanics, and show merits of e.g., high spatial resolution, large dynamics range and simple implementation. Conventionally employed in CR-AFM, piezoacoustic excitation is known to associate with various drawbacks.
Here, we explored the use of photothermal excitation, which recently became commercially available, in CR-AFM in conjunction with the multi-frequency band-excitation technique. Our measurements show that photothermal excitation is highly efficient and clean and provides a unique capability of cantilever mode selection. Furthermore, we implemented voltage spectroscopy (VS) of CR-AFM in which the sample mechanical properties can be tracked during application of tip biases, similar to piezoresponse spectroscopy. Due to the advantages offered by photothermal excitation, minute changes can be detected in the CR-VS of ferroelectric materials providing reliable criteria of nanoscale polarization switching behavior. A few more examples of quantitative imaging and spectroscopic studies, including those on battery materials, will also be presented.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division of U.S. Department of Energy. Support was provided by U.S. DOE, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program (Q.L., N.B.).

Contact resonance atomic force microscope (CR-AFM) methods are relatively new measurement techniques used to quantify the elastic and viscoelastic properties of numerous materials such as polymers, elastomers, metallic glasses, asphalt, and biological materials. More recently, AFM thermalevers have been developed to allow local heating of samples and the resonances of these probes are much more complex. These probes have one distinct advantage over rectangular AFM probes in that specific modes allow in-plane and out-of-plane tip-sample motion to be excited independently at the same location using a Lorenz force excitation. Here, a commercial thermalever, a U-shaped probe, is used to determine the in-plane and out-of-plane viscoelastic properties at the same location. The CR-AFM approach involves measurement of the resonant frequencies of the AFM probe both for the free case and the case for which the tip is in contact with the sample. Vibration models of the probe and tip-sample contact models are then used to determine the sample properties from the frequency behavior. A simplified analytical model of these U-shaped probes is described here that is based on a three beam model (TBM) which includes two beams clamped at one end and connected with a perpendicular cross beam at the other end. We also demonstrate the use of this approach for measurement of the material loss tangent at local positions on several polymers including high-density polyethylene and polystyrene. Finally, the measurements for single positions are applied to image the properties of polymer blends over a region of several microns.

Loss tangent imaging is a recently introduced technique to quantify image contrast in amplitude modulation (AM) AFM, also known as tapping or AC mode. It provides a single term that is the ratio of the dissipated to stored energy of the tip-sample interaction and therefore does not require calibration of the cantilever&’s spring constant or deflection amplitude. Loss tangent imaging shows promise as a fast, versatile method for mapping near-surface viscoelastic properties. However, experiments to date have generally obtained values larger than expected for the material loss tangent tan δ. Here, we examine several issues for improved measurements with loss tangent imaging. Uncertainty analysis and signal-to-noise considerations indicate that fundamental thermal (Brownian) noise restricts accurate measurements to materials with approximately 0.01 < tan δ < 5. We also discuss the potential impact of surface effects such as squeeze film damping, adhesion, and plastic deformation and choice of experimental parameters such as setpoint and amplitude. For squeeze film damping, we demonstrate a calibration technique that removes this effect at every pixel. Finally, we present loss tangent imaging experiments on a two-component polymer film using a heated sample stage. The results prove that this technique can identify temperature-dependent polymeric phase transitions, even in the presence of non-ideal interactions. Our results give insight into the opportunities and limits of both loss tangent imaging specifically and AM-AFM phase imaging in general for accurate nanomechanical characterization.

Contact resonance force microscopy (CR-FM) is an atomic force microscope (AFM) based technique for quantitatively determining the nanoscale elastic and viscoelastic properties of materials, but its application in liquid has been limited. A number of biological and industrial applications depend on accurate nanoscale viscoelastic measurements in liquid. We used CR-FM to extract material loss tangent (tan δ) from confounding liquid effects in measurements made in water, arriving at values that agreed with those made in air under certain conditions on polystyrene (PS) and for all cases examined on polypropylene (PP). In CR-FM, the resonant properties of an AFM cantilever in contact with the surface are measured, and interpreted as viscoelastic properties of the sample. However, there are new complications to CR-FM in liquid environments. First, spurious resonances arise during operation in fluid. Direct cantilever excitation enables measurement of idealized, near-Lorentizan contact resonance peaks in liquid. With this technology, the next step becomes interpretation of the liquid CR peaks. Due to surface-coupled fluid effects, the material loss tangent in water is greatly overestimated. Recently we developed a model to compensate for the hydrodynamic loading in the contact resonance data, thus allowing further analysis with existing models developed for use in air. The analysis method relies on reconstruction of the hydrodynamic function based on measurements of the free resonance peaks near the surface. By use of photothermal excitation and hydrodynamic function reconstruction we were able for the first time to accurately measure the loss tangents of polymers in liquid. Without applying the hydrodynamic correction, the measured tan(δ) values for both PS and PP in water were indeed much higher than those measured in air. These materials are not expected to change upon hydration, so this discrepancy stems from confounding liquid effects. After applying the hydrodynamic correction, the values measured in the two environments agreed to within experimental error at some frequencies on PS, and in all conditions on PP. Many important materials, however, are much softer than PS and PP, and their properties are intricately linked to their hydration. The technique was also successfully applied to measure loss tangents of different hydrogel tissue scaffolds. The current developments in quantitative measurements in liquid will expand CR-FM for more accurate characterization of a broader range of materials. This will ultimately allow comparisons with bulk techniques such as dynamic mechanical analysis, advancing development of soft and biological materials.