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.

This talk will give an overview of Intermodulation Atomic Force Microscopy (ImAFM) [1] and is application for measurement of mechanical response [2][3], surface potential [4], and tip-surface forces at the transition from static to dynamic friction. ImAFM uses a unique multifrequency lock-in measurement method that eliminates cross-talk, or Fourier leakage between numerous, closely separated frequency bands [5]. The response amplitude and phase are simultaneously measured in ‘real time&’ at multiple tones in a frequency comb, as many as 42 frequencies (i.e. 84 channel lockin). For a nonlinear system, like an AFM cantilever tapping on a surface, the measured response is the result of very high-order intermodulation, or frequency mixing of the applied drive tones. Analysis of this intermodulation reveals the forces acting on the tip.
ImAFM can be considered as a hybrid between the two main branches of dynamic AFM: frequency modulated AFM (FM-AFM) and amplitude modulated AFM (AM-AFM). The difference being that ImAFM actively modulates the harmonic drive force. The measurement reveals how the tip-surface interaction changes the drive modulation. When ImAFM is preformed with a cantilever having a high Q resonance, high force sensitivity is achieved in the narrow band around resonance and tip motion is restricted to this band. The situation lends itself to analysis of tip-surface interactions in terms of dynamic force quadratures [6]. A direct transformation of the intermodulation spectrum reveals the forces in phase with, and quadrature to the harmonic tip motion. This transformation makes no assumptions as to the nature of the tip-surface force, and therefore allows for detailed examination of the models commonly used to describe tip-surface interaction when making a map of the material properties of a surface.
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[6] D. Platz, et al., Nature Comm., 4, 1360 (2012).

Tapping mode AFM imaging, also known as amplitude-modulated (AM) atomic force microscopy (AFM) or AC mode, is fast, gentle and provides the high spatial resolution necessary for imaging nanoscale features. However, until recently, mechanical characterization with tapping mode was limited to only qualitative results. In AM-FM mode, a bimodal (dual-frequency) technique, the first resonant mode is operated in AM, whereas a higher resonant mode is frequency modulated (FM). As expected from regular tapping mode, AM-FM mode delivers topographical information. Additionally, it provides quantitative data on contact stiffness, from which elastic modulus can be calculated with appropriate models for the tip-sample contact mechanics. Experimental results on different samples such as metals, alloys and polymers will be presented to demonstrate the applicability of AM-FM mode for materials with a wide range of modulus (MPa-GPa). Furthermore, recent advances in AM-FM imaging will be discussed, including a calibration approach using a laser Doppler vibrometer and the use of photothermal excitation to achieve molecular-level resolution on crystalline polymers. Finally, AM-FM results in liquid environment will be shown on biological samples including lipid bilayers and double-helix resolution on DNA. With the growing demand for mechanical characterization of materials at the nanoscale, the AM-FM technique provides quantitative nanomechanical information while simultaneously offering all the familiar advantages of tapping mode.

Simultaneous topography and mechanical property measurements have been a long-standing goal of AFM, especially obtaining complementary mechanical information during gentle tapping mode imaging. Bimodal force microscopy is a dynamic atomic force microscopy (AFM) method that excites two eigenmodes of a cantilever simultaneously [1]. The additional information provided by a second eigenmode allows the separation of topographic from mechanical properties. Combined with the benefits of operating the cantilever on resonance, bimodal force microscopy enables high-speed quantitative nanomechanical mapping across six orders of magnitude in modulus.
We present a new mathematical framework for the extraction of indentation depth and Young&’s modulus from bimodal AFM observables that avoids the use of fractional calculus, Laplace transforms, and Bessel functions, which are used by existing theories [2]. The simplicity of our proposed mathematical framework leads to an intuitive interpretation of bimodal AFM data in the context of Hertzian contact mechanics and is more transparent to the approximations required to reach analytical solutions. The proposed framework can be applied to any combination of amplitude modulation (AM) and frequency modulation (FM) for driving both eigenmodes, as in AM-AM, AM-FM, FM-AM, and FM-FM. The pros and cons of these variations will be discussed with respect to different imaging conditions.
Data acquired with the AM-FM technique, which offers robust and stable imaging while maintaining accurate nanomechanical mapping, will be presented. Furthermore, these AM-FM measurements, ranging from soft polymers to stiff metals, will be compared to bimodal cantilever dynamic simulations that were performed to provide insight into data interpretation and better understanding of the sources of variability. The benefits of using photothermal excitation [3] to drive the cantilever as well as recent efforts in accurately calibrating the stiffness and amplitude of the second eigenmode will be presented, and their impact on AM-FM accuracy will be discussed.
[1] R. Garcia, R. Proksch, European Polymer Journal 49, 1897-1906 (2013)
[2] E.T. Herruzo, R. Garcia, Beilstein J Nanotechnology 3, 198-206 (2012)
[3] A. Labuda et al, Rev. Sci. Instrum. 83, 053702 (2012)

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. CR-AFM exploits the resonant frequencies of the AFM probe both in and out of contact. Because of the interaction between the tip and the sample the resonances shift higher and they show an increase in peak width. Analytical models of the probe vibrations can then be used to deconvolve the beam dynamics from the spectra to recover the local material influence. In this presentation, CR-AFM is used to probe the internal variations of polymer crystals that arise during their formation. A polycaprolactone (PCL) thin film was created by confining the film during crystallization. This confinement resulted in well-defined two-dimensional crystals (spherulites). CR-AFM was used to make measurements of several PCL crystals (diameter ~30 microns) in order to capture the transition from the central amorphous region to the outer regions that have aligned lamellae. Here, we used a U-shaped AFM probe and a Lorenz force excitation to sweep the free and contact frequency spectra. The in-plane and out-of-plane AFM probe motions were exploited to quantify variations in the bulk storage and loss moduli as well as the shear storage and loss moduli. Overall trends in stiffness show a clear increase from the center outward. Moreover, the in-plane motion of particular vibration modes allows the in-plane anisotropy to be evaluated and imaged such that the impact of the lamellar alignment on shear stiffness is quantified. Such measurements provide new insight into the organizational processes of crystalline and semi-crystalline polymers.

Contact resonance atomic force microscopy (CR-AFM) is a promising technique for measuring mechanical properties at the nanometer-scale (1, 2). A key element of CR-AFM is reliable and accurate measurement of contact stiffness between the AFM cantilever tip and the surface. Typically, CR-AFM measures contact resonance frequency for a known calibration material and then uses a dynamic mechanical model of the CR-AFM system to resolve the mechanical stiffness of an unknown sample. A key limitation of this approach is that cantilever dynamics models require assumptions about cantilever geometry, which may introduce uncertainties in the method. Eliminating the need to construct a cantilever dynamics model would improve CR-AFM accuracy and also make CR-AFM broadly applicable to arbitrary cantilever geometries including triangular and U-shaped cantilevers, like those used for thermal measurements or Lorentz AFM (3, 4). This paper describes a method to calibrate the contact stiffness of an AFM cantilever that does not require complex mechanical modeling, but rather provides a large range of known contact stiffness over which the cantilever can be calibrated. The calibration sample consists of 0.5 µm copper disks (2 - 100 µm diameter) patterned on top of a 350 µm thick silicone layer. The contact stiffness is thus governed by the stiffness of the tip-copper system and the stiffness of the copper-silicone system, which can be easily modeled. Finite element simulations of tip-sample behavior validate the assumptions used in the calibration approach. For the measured calibration samples, we are able to achieve contact stiffness over a large range: 1 - 120 N/m. When used to calibrate CR-AFM, this range of stiffness corresponds to sample elastic modulus of approximately 0.01 - 25 GPa. Our measured calibration curves exhibit the expected S-curve shape with resonance frequency being fairly insensitive to changes in contact stiffness for low and high contact stiffness and resonance frequency changing significantly for contact stiffnesses between these plateaus. The presentation will show contact resonance measurements on a variety of samples, including several polymers, metals, and ceramics. The polymers include polystyrene, Teflon, and polyethylene, while the metals include chrome and copper.
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Since Scanning Probe Microscopy (SPM) emerged as a powerful tool for probing materials from meso- to atomic level, a multitude of methods have been developed to study local mechanical, electromechanical, and electrochemical properties. However there remains a challenge in extracting high sensitivity, high veracity, and quantitative information from SPM measurements. In atomic force microscopy (AFM), operating on cantilever resonances maximizes sensitivity, but often at the expense of quantitativeness due to viscoelastic variations across material surfaces and non-linearities inherent to mechanics of the tip-surface junction. Band Excitation has emerged as a robust method enabling reliable on-resonance measurements by continuously measuring the cantilever transfer function. In Band Excitation SPM (BE-SPM) the system is excited through a digitally synthesized signal waveform with a finite spectral density in a band (or bands) centered on a resonance peak(s), as opposed to the single sine wave used in classical SPM. (1-3) The response is detected via a photodetector and is Fourier transformed. The ratio of the Fourier transform of the response to the excitation signals yields the transfer function of the system. The frequency dependence of the response can be analyzed to yield parameters including the amplitude, resonant frequency, and Q-factor thus yielding multiple decoupled channels of information on local linear and non-linear viscoelastic properties. Furthermore, BE combined with complex bias waveforms can be used as a sensitive probe to map properties including local ferroelectric and polarization properties (SS-PFM) (3-4), electrochemical activity (ESM) (5-6), and surface potential (contact-KPFM) with 10&’s nm resolution thus providing a multifaceted view of functionalities critical to understanding energy materials. Discussed in this presentation are the application of BE combined with complex probing and analysis methods to reveal the nanoscale behavior of ionic activity in energy materials.

The photo-carrier lifetime plays a major role in the overall efficiency of a solar cell because it limits the proportion of photo-generated charges collected at the electrodes. This lifetime, which should be ideally as large as possible in organic or inorganic solar cells, is rather difficult to measure in: nanostructured materials or in more complex hybrid systems, indirect band-gap semiconductors, and ultra-thin layers. Identifying the losses mechanisms is one of the main objectives for increasing the performances of solar cells. Most of the experimental approaches developed so far consist in studying recombination by techniques such as: transient photovoltage or charge extraction. All these techniques average sample properties over macroscopic scales, making them unsuitable for directly assessing the impact of local heterogeneity on the recombination process. In this paper, we propose a steady method to measure the photo carrier lifetime by photo-modulated techniques based on Kelvin probe force microscopy (KPFM). [1] This technique provides a spatially resolved measurement, which is applicable on the overall of solar cells.
We will present the principle of this original method based on the nanometrically resolved measurement of the surface potential by KPFM under a modulated illumination. Photo-carrier lifetime down to µs scale is reachable with our experimental setup. The modulation-dependent surface potential is plotted as a function of the frequency. Assuming an immediate generation time under illumination and an exponential decay of the surface potential during the dark condition, the averaged surface potential over a cycle can be fitted as a function of the frequency by simple equation where the fit parameter is the photocarrier-lifetime. [2] Instrumental aspects as well as data treatment will be reviewed. Measurements obtained on third generation solar cells like: silicon nanocrystals embedded in 30 nm film of silicon dioxide [3] or organic donor-acceptor blend (PBTFB and PCBM), [4] will be presented to illustrate the potential of the technique.
This work was supported by the French “Recherche Technologique de Base” Program and performed in the frame of the trSPV Nanoscience project. The measurements were performed on the CEA Minatec Nanocharacterization Platform (PFNC).
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4. N. Delbosc et al. RSC Adv 4, 15236 (2014).

Research on perovskite solar cells consisting of methylammonium lead halides (MAPbX₃) has recently led to efficiencies comparable to commercial silicon solar cells. Nevertheless, there is still a lack of basic understanding for the exact details of the charge generation and extraction mechanisms in perovskites. In particular, ferroelectricity in the perovskite material was suggested as one of the reasons for hysteretic behavior in solar cell devices. However, no clear evidence for ferroelectric domains could be found so far.
In our piezoresponse force microscopy (PFM) study on µm-sized MAPbI_3-xCl_x grains we observed a periodically alternating structure reminiscent of ferroelastic domain patterns. We suggest that the ferroelastic behavior is induced by internal strain during the sample preparation. From vertical and lateral PFM, a domain orientation is proposed. Ferroelasticity is often connected to ferrolectricity. However, we were not able to prove ferroelectricty in PFM switching experiments. This result could be explained by the domain orientation with respect to the sample surface.
This first observation of a ferroelastic domain pattern will provide a better understanding of the ferroic behavior of MAPbX₃.

The structure and properties of ionic liquids (ILs) at the solid-liquid interface govern their performance in numerous applications including lubrication, energy storage and catalysis.The complex interfacial structure of ionic liquids is very different to that of classical dilute electrolytes, and a comprehensive understanding of the ion structure at solid interfaces is still lacking. The layered ion structure at the interface can be probed with Atomic force microscopy (AFM) force-distance measurements, and this method offers advantages over other techniques (e.g. x-ray and neutron reflectivity, surface force apparatus) due to its high spatial resolution allowing for the ion structure to be examined in a spatially resolved manner. Although there are several studies demonstrating the impressive ability of AFM to probe the interfacial ion structure in ionic liquids, several questions remain regarding the measured AFM response. (e.g. How does the nature of the AFM probe affect the measured force profiles? Do we measure the positions of the cations or anions? Can we tune ion selectivity?)
This work examines the AFM force profiles of various ionic liquids at different substrates to examine how the substrate and ionic liquid properties affect the interfacial ion structure. We also use a series of different AFM probes of different geometry and chemistry to determine the effect on the measured force profiles. Our experimental results are compared directly with molecular dynamics simulations, and through a combination of experimental and theoretical approaches we are able to develop a better understanding of the measured force response and factors determining the interfacial structure at the ionic liquid-solid interface.

Atomic force microscopes (AFM) and scanning tunneling microscopes (STM) image surfaces with atomic resolution, and perform local spectroscopy of current versus voltage, forces and dissipation. The qPlus force sensor [1] is a sensitive eye and hand to the nanoworld that combines STM and AFM capability, enables highly precise imaging and spectroscopy functions and measures the forces that act during atomic manipulation [2]. While STM had better spatial resolution than AFM in the past, the situation is reversed now with modern AFM [3]. Angular dependencies of chemical bonding forces have been observed before for Si tips interacting with Si surfaces [4], W tips interacting with graphite [5] and similarities exist between metal tips interacting with CO molecules on Cu and Si adatoms [6]. In the latter two cases, light atoms such as carbon or oxygen interacted with much heavier and much larger metal atoms. Recently, Gross et al. found that CO is an excellent probe for organic molecules. For example, pentacene can be imaged at excellent resolution with CO terminated tips [7], although the softness of CO on tips can lead to image distortions [8,9]. Tips made of permanent magnets such as CoSm allow to resolve the spin order in the antiferromagnetic insulator nickel oxide [10]. The stiff cantilever/small amplitude technique used here also allows true atomic resolution in ambient conditions [11], and small iron clusters on Cu (111) (a trimer and a dimer on the figure above) are resolved by force microscopy [12].
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Interactions between atomic and molecular objects are to a large extent defined by the nanoscale electrostatic potentials which these objects produce. Consequently, a tool for nanometre scale imaging and quantification of local electrostatic fields could help in many areas of nanoscience research. In this contribution we introduce a scanning probe technique that for the first time enables truly three-dimensional imaging of local electrostatic potential fields with sub-nanometre resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of a tuning fork atomic force microscope operated at 5 K, we image the quadrupole field of a single molecule adsorbed on a metal surface. To demonstrate quantitative measurements, we investigate the Smoluchowski dipole field created by a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

At the interface with immersed solids, liquid molecules tend to behave differently than in the bulk due to their interactions with the solid. The altered molecular organisation and dynamics of this ‘interfacial liquid&’ strongly depends on the local nanoscale chemistry of the solid and can hence be exploited as a reporter for chemical mapping of the surface in liquid.
Atomic force microscopy (AFM) can map sub-nanometre details of the interfacial liquid [1] and gather quantitative information about its local dynamics [1-2] when operated in the appropriate regime [3]. Here I present a few examples of how this approach can be used to derive nanoscale information about the solid's chemical composition, including single adsorbed ions and their type-specific solvation structure [4]. Results show how interfacial water can drive ordering and correlation between single metal ions at various soft and hard interfaces, and dramatically slow down the ions&’ dynamics. I also explore novel strategies based on multifrequency AFM to enhance resolution while retaining chemical information and preserving soft interfaces.
References:
[1] Voitchovsky et al., Nat. Nanotechnol., 5, 401, (2010)
[2] Ortiz-Young et al., Nat. Commun., 4, 2482, (2013)
[3] Voitchovsky, Phys. Rev. E, 88, 22407, (2013)
[4] Ricci et al., Nat. Commun., 5, 4400, (2014)

Single organic molecules were investigated using scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM), and Kelvin probe force microscopy (KPFM). Using NC-AFM and CO functionalized tips, atomic resolution on molecules [1], molecular structure identification [2], bond-order discrimination [3], measurment of the charge distribution within a molecule [4], and adsorption-height determination [5] were demonstrated.
We recently applied this technique to generate and study reaction intermediates as arynes [6] and to investigate complex molecular mixtures as asphaltenes [7].
[1] L. Gross et al. Science325, 1110 (2009)
[2] L. Gross et al. Nature Chem.2, 821 (2010)
[3] L. Gross et al.Science337, 1326 (2012)
[4] F. Mohn et al.Nature Nanotech. 7, 227 (2012)
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Scanning Tunneling Microsocopy (STM) has opened the way to the study of surface reactions a-molecule-at-a-time, giving a substantial impetus to the field of reaction dynamics. Injecting an electron of known energy from the tip of the STM into adsorbates on semiconductor or metal has been found to result in dissociative reaction, involving the selective rupture of either one or two chemical bonds. Here these modes of reaction will be interpreted in terms of a two-electronic-state model which describes the molecular dynamics in terms of forces obtained from DFT. Theory and experiment show the effect on the dynamics of such variables as the reagent and its alignment, and also charge-flow in the molecule . Short range migration of highly reactive product can be understood in terms of the 'walking' of the free radical across the surface. Long-range migration of product, previously baffling, is explainable in terms of the computed mechanics of 'ballistics and bounce', in which the product leaps and bounds across the surface.

The investigation of friction by atomic force microscopy assisted nanoparticle manipulation is a very useful approach to gain insight into tribological processes of extended nano- and mesocontacts. Currently, this approach is utilized to analyze the significance of dynamic processes at the interface formed between particles and substrates. One of the most intriguing concepts in the framework of dry friction is 'superlubricity, also referred to as `structural lubricity', where flat surfaces are thought to slide past each other virtually frictionless if their atomic structures are incommensurate. We analyze the fundamental mechanisms that govern the area-dependence of friction in extended but atomically flat contacts of dissimilar materials. The resulting sublinear power laws, which link mesoscopic friction to atomic principles, are then confirmed by measuring the sliding resistance of gold and antimony particles on graphite [1]. On one hand these findings suggest that engineering surfaces with very low friction can be realized up to mesoscopic contact areas. Furthermore, it is shown that nanoparticles can even co-exist in two frictional states, exhibiting ‘frictional duality&’: Some particles show linear scaling with contact area reminiscent of Amonton&’s friction law while others remain in the superlow friction state of structural lubricity [2]. This duality phenomenon is explained by a model of partial interface contamination. Lastly, we investigated the effect of contact ageing, detrimental in the field of earthquake modelling: The shear strength of tectonic plates is believed to increase logarithmic in time, leading to the infamous strong sudden energy dissipation events, i.e. earthquakes. Interestingly, we find that similar ageing dynamics exist for nanoparticles, as evidenced by stick-slip movements of those objects. A complex interplay of ageing dynamics with thermally activated stick-slip friction explains the commonly observed friction peak at low temperatures [3]. These examples demonstrate how nanoparticle manipulation by atomic force microscopy techniques can contribute to the understanding of fundamental friction processes of atomically defined interfaces [4].
[1] D. Dietzel et al., Physical Review Letters 111, 235502 (2013)
[2] D. Dietzel et al., Physical Review Letters 101, 125505 (2008)
[3] M. Feldmann et al., Physical Review Letters 112, 155503 (2014)
[4] D. Dietzel, U.D. Schwarz and A. Schirmeisen, Friction 2, 114 (2014)

This contribution is devided in two sections. The first section is devoted to examine some relevant issues regarding force microscopy imaging of biomolecules such as spatial resolution, molecule deformation and quantitative mapping of mechanical properties. Specifically, we present a method to obtain the stress-strain curve of a single protein in liquid. The second section is devoted to present an advanced AFM method to genearte three dimensional maps of solid-liquid interfaces. We develop a force microscope to map with true atomic-scale spatial resolution and three-dimensional depth the formation of hydration layers and the adsorption of ions on solid-liquid interfaces. Some applications include to resolve the atomic structure of hydration layers generated from alkali chloride solutions on an atomically flat mica surface.

Force distance curves uniquely provide quantitative mechanical information of surface properties at the nanoscale, and are a sought after technique in a wide range of scientific fields including physics, material science and medicine. Current force curves methods are still limited to the data acquisition rates at the speed of imaging over a large areas, and require a dedicated experiment force distance experiment. Contrarily, amplitude modulated tapping mode AFM is a fast, widely used imaging technique applicable in a variety of environments that lacks a quantitative component. We present a technique that allows direct dynamic force distance curve reconstruction after imaging in G-mode amplitude modulated tapping mode.
In G-mode Atomic Force Microscopy imaging and analysis is based on information theory approach to data handling as its streamed directly from the detector. This allows unprecedented insight into complex tip-surface interactions, spatial mapping of multidimensional variability of material&’s properties, and imaging at the information channel capacity limit. We illustrate two paths for force distance curve reconstruction, first with maintaining the cantilever transfer function, and second via utilization of multiple SHO (Simple Harmonic Oscillator) fits to the G-mode tapping data.
A number of material systems, dynamic acquisition methodology, and benchmarking against well-established force distance methodologies will be presented and discussed.

A method that combines high spatial resolution, quantitative and non-destructive mapping of surfaces and interfaces is a long standing goal in nanoscale microscopy. The method would facilitate the development of hybrid devices and materials made up of nanostructures with different properties in air or liquid.
Bimodal atomic force microscopy is a multifrequency dynamic force method based on the simultaneous excitation of two eigenmodes of the cantilever. Different schemes of bimodal AFM have been developed, FM-FM bimodal AFM[1] and AM-FM bimodal AFM[2][3], which enable the simultaneous mapping of the nanomechanical spectra of soft matter surfaces with nanoscale spatial resolution. The use of several information channels (first and second mode) allows the calculation of Young modulus and viscosity coefficients which do not depend on the applied force. In addition, these methods can provide the peak force and the indentation. The methods have been tested on different polymers and proteins in air and liquid with variations in the elastic modulus of near four orders of the magnitude, from 1 MPa to 3 GPa.
The advantage of the employed methods is that they do not limit the data acquisition speed or the spatial resolution of the force microscope. Also, they are non-invasive and minimize the influence of the tip radius on the measurements.
[1] E. T. Herruzo, A.P. Perrino and R.Garcia. Nat. Comm. 3, 3126 (2014)
[2] R. Garcia and R. Proksch. Eur. Polym. J.49, 1897-1906 (2013)
[3] D. Martinez-Martin, E. T. Herruzo, C. Dietz, J. Gomez-Herrero and R. Garcia, Phys. Rev. Lett. 106, 198101 (2011).

Grafted polymer chains (aka polymers brushes) have been studied with atomic force microscopy for a while. By analyzing the force curves, one can obtain such parameters as grafting density of the brush and size of the grafted polymer chain (including the Flory radius). This was done on solid substrates with negligible deformation occurred during collecting the force curves. However, the substrate is frequently soft. As a result, the collected force curves have information about the substrate deformation. To properly extract brush parameters, one needs to take into account such deformations. We (I.S.,M.D.) recently suggested a method which takes into account such deformations (“the Brush model” of elastic deformations). The method produces not only results for the brush but also the value of the substrate elastic modulus. Nevertheless, a quantitative validation of this method is still absent.
Here we describe and verify this brush model for polyethylene oxide brush grafted to the cross-linked poly(2-vinyl pyridine) substrate. By analyzing the force curves, our method allows to obtain simultaneously the grafting density, size of the grafted polymer chain including the Flory radius) , and the elastic (Young&’s) modulus of this soft substrate. The method is verified by independent measurements of the substrate Young&’s modulus and ellipsometric measurements of the brush thickness. The method demonstrates an excellent agreement between the derived and directly measured elastic modulus and the size of the brush.

Dynamic mechanical spectroscopy (DMS), which allows measuring frequency-dependent viscoelastic properties, is important to study soft materials, tissues, biomaterials, polymers. However, the existing DMS techniques (nanoindentation) have limited resolution when used on soft materials, preventing them from being used to study mechanics at the nanoscale. The nanoindenters are not capable of measuring cells, nanointerfaces of composite materials. Here we present a highly accurate dynamic mechanical spectroscopy mode, which is a combination of three different methods: quantitative DMS (nanoDMA), gentle force and fast response of atomic force microscopy (AFM), and Fourier transform (FT) spectroscopy. This new spectroscopy (which we suggest to call FT-nanoDMA) is fast and sensitive enough to allow DMS imaging of nanointerfaces and single cells, while attaining about 100x improvements on polymers in both spatial (down to 10nm) and temporal resolution (down to 0.7 sec/pixel, multiple frequencies are measured simultaneously) compared to the current art. The uses of 10 simultaneous frequencies are demonstrated in both ambient conditions and liquid environment. The frequency range is up to 300Hz which is a rather relevant range for biological materials and polymers. The method is quantitatively verified on known polymers and demonstrated on cells and polymer blends. Analysis shows that FT-nanoDMA is highly quantitative. The FT-nanoDMA spectroscopy can easily be implemented in the existing AFMs.

Most polymeric materials and composites have heterogeneities at the nanometer length scale. Mechanical property measurements with AFM have the sensitivity and resolution necessary to visualize these features and better understand their influence on bulk properties. In the past few years, AFM mechanical property measurements have evolved from slow force volume to faster, but conceptually very similar, PeakForce Tapping. In addition, resonance based techniques which provide less information than force distance based methods have been shown to be useful under some conditions.
During material property mapping, the time scale of tip-sample interaction now spans from microseconds to seconds, tip sample forces can be controlled from piconewtons to micronewtons, and spatial resolution can reach sub-nanometer (where continuum mechanics fails). AFM has become a unique mechanical measurement tool having large dynamic range (1kPa to >100GPa in modulus) with the flexibility to integrate with other physical property characterization techniques in versatile environments.
In particular, researchers have begun to take advantage of the wide range of deformation rates accessible to AFM in order to study time dependent properties of materials such as viscoelasticity [1][2][3]. More traditional measurements with indentation DMA are usually limited in frequency to a few hundred Hz and have limited spatial resolution. In contrast, AFM measurements can extend from less than one Hz to KHz and beyond while retaining the high resolution needed to see the details in distribution of properties near domain boundaries in nanocomposites and other materials.
This presentation will review this recent progress, providing examples that demonstrate the dynamic range of the measurements, and the speed and resolution with which they were obtained. Additionally, the effect of time dependent material properties on the measurements will be explored.
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[1] K. Nakajima, M. Ito, D. Wang, H. Liu, H. K. Nguyen, X. Liang, A. Kumagai, and S. Fujinami, Microscopy dfu009 (2014).
[2] M. Chyasnavichyus, S. L. Young, and V. V Tsukruk, Langmuir 30, 10566 (2014).
[3] M. E. Dokukin and I. Sokolov, Langmuir 28, 16060-71 (2012).

Intermittent-contact resonance atomic force microscopy (ICR-AFM) pairs the contact stiffness measurement capability of the conventional contact resonance AFM (CR-AFM) with the less-invasive surface probing of a force-controlled intermittent AFM mode. Unlike conventional CR-AFM, great stability for the scanning conditions is achieved with ICR-AFM as the tip contacts intermittently the surface during scanning: the tip geometry is better preserved, the feedback control can be adjusted faster, and surface features can be tracked with less damage. At each tap, the tip is pushed in and out of contact with the surface and the tip-sample contact stiffness varies accordingly. This change in the contact stiffness is observed in the change of the eigenmode frequencies of the cantilever and a fast detection is required to measure the frequency changes (e.g. phase-locked-loop) during each tap. By collecting the depth dependence of the contact resonance frequency at each point in the scan, a three-dimensional (3D) data volume is generated. This data can be used to obtain nanoscale tomographic views of the sub-surface elastic properties of a material. In this work, ICR-AFM was performed on high-aspect ratio SiOC:H patterned lines (width from 25 nm to 90 nm) to map the depth and width dependencies of the material stiffness. The spatial resolution and depth sensitivity of the measurements were assessed from tomographic cross-sections over various regions of interest. Furthermore, the depth-dependence of the measured contact stiffness during scanning was used to determine sub-surface variation of the elastic modulus at each point in the scan. This was achieved by fitting the local elastic composition of the sample until the resulting contact stiffness showed the depth dependence measured by ICR-AFM. The collection of all these results was compiled into nanoscale subsurface tomographic images of the elastic modulus of the SiOC:H patterns. The observed variations in the elastic modulus were linked to possible alterations sustained by these structures during processing.

We present an emerging nanospectroscopic technique that measures the local polarizability of a sample by force, referred to as Photo-induced Force Microscopy (PiFM). The technique measures, on one of the mechanical resonances of an Atomic Force Microscope (AFM) cantilever, the attraction between an optically induced molecular dipole and the mirror image dipole in a metal-coated tip. By utilizing multiple mechanical resonances of the cantilever, PiFM can acquire both sample&’s polarizability and topography concurrently. Both linear and nonlinear polarizability can be probed via PiFM with nanoscale spatial resolution.
The dipole-dipole force measured by PiFM is localized to the tip-enhanced electric field generated at the imaging probe tip apex, resulting in an optical resolution that is limited to roughly half of the tip&’s end diameter. The PiFM technique allows for simplified optics since no far-field optical detection is required. Another advantage over the other tip-enhanced optical techniques that rely on detection of near-field photons is that PiFM does not have to contend with the far-field background signal. Combining PiFM with the tunability of visible and mid-IR excitation sources, a robust near-field nano spectral microscopy and nanospectroscopy technique is realized.
Linear PiFM&’s capability to detect electronic modes in chormophores, nanoparticle plasmonics, and layer detection in graphene and molybdenum disulfide will be presented in the talk. In the mid-IR, linear PiFM is able to detect vibrational modes allowing for chemical identification based on known molecular absorption bands. PiFM&’s nanoscale chemical and topology imaging capability will be highlighted by presenting data on several block copolymer (BCP) systems where closely packed BCPs and random defects are clearly resolved chemically and topologically based on their absorption signature.

Mid-infrared (mid-IR) photothermal microspectroscopy is of interest for vibrational spectroscopy and hyperspectral imaging. Potential applications range from chemical analysis and detection of hazardous materials to biophysical studies such as the characterization of secondary protein structure. Many molecules have strong vibrational resonances within this spectral region, often referred to as the “fingerprint region”, allowing for label-free characterization of samples with high chemical specificity and contrast. Here, we investigate photothermal microspectroscopy, a pump-probe technique that relies on the absorption peaks in the mid-IR for sample characterization with high contrast. We report on the signal-to-noise ratio (SNR) of a mid-IR photothermal microspectroscopy system.
In photothermal microspectroscopy, a modulated pump beam is focused onto the sample, resulting in a temperature-induced change in the refractive index. Here, a quantum cascade laser (tunable between 1575 - 1745 cm^-1) operating in pulsed mode serves as the pump. A continuous wave fiber laser operating in the near-infrared serves as the probe beam, allowing use of a sensitive near-infrared detector at room temperature in comparison to cryogenically-cooled mid-IR detectors. The photothermal microspectroscopy system is set up in transmission mode where forward scatter of the probe beam is measured in a heterodyne detection scheme by a lock-in amplifier. Fiber lasers are an attractive probe laser choice due to their robust operation and can be easily power scaled by fiber amplifiers.
We present the SNR of the photothermal microspectroscopy system, targeting the C-H scissoring band at 1607cm^-1 of a liquid crystal sample, 4prime;-octyl-4-biphenylcarbonitrile (8CB). The photothermal signal strength is linear with increasing probe powers. The spectral characteristics, such as the full width at half maximum and central wavenumber, remain constant. The background can similarly be described by a linear function, with the choice of lock-in time constant contributing to this trend. Based on linear fits to the photothermal signal strength and background, the SNR converges to a limit of 1230 ± 30 with asymptotic behavior. We demonstrate a 3-fold increase in the SNR through optimization of the probe power. At a probe power of 1.8mW, a high SNR of 1117 ± 30, close to the asymptotic limit, is measured.

Quantitative evaluation of surface coverage at the nanoscale is a fundamental requirement for many modern surface science applications. The characterization of chemical vapour deposited¹ films has usually been accomplished by scanning electron microscopy. However, this technique suffers from surface charging and it provides at best semi-quantitative information about surface coverage. Recently, optical methods² introduced significant advantages over conventional particle-beam and X-ray diffraction spectroscopy, due to their surface sensitivity, non-destructive interaction and ambient operating conditions.
Here, we present a novel method that can be used to quantify the surface coverage of carbon nanotubes (CNTs) and silver nanowires (Ag NWs), deposited from solution on SiO₂ substrates, by exploiting topographic information acquired via standard atomic force microscopy (AFM). We develop a Gaussian statistical model to describe and quantify the variation of the CNTs height density by extracting the coverage coefficients for all measured CNTs height configurations. In the case of Ag NWs, the Gaussian model is no longer valid, and their height description becomes a more complex statistical function.
The CNTs used have a diameter (d < 1 nm) smaller than the AFM probe (r_p = 8 nm) and comparable to the lateral step size set during the scanning. Because the CNT diameter is deterministically known, their height density has been modelled by a sequence of Delta distributions, where each Delta is centred at a multiple of the CNT diameter. The superposition of CNTs with SiO₂ results in the convolution between the CNTs height density and the Gaussian distribution of the SiO₂ background height, which turns out to be a sequence of Gaussian distributions, where the first is centred at the average SiO₂ background height while the others are spaced by the CNT diameter. The measurement uncertainty is included in the standard deviation of the Gaussian distribution, representing the SiO₂ background roughness and the AFM resolution. Because of the larger diameter of the probe with respect to the CNT diameter, we assumed each height sample to coincide with an integer multiple of the CNT diameter. This hypothesis is no longer valid for tubes that are larger than the probe, like Ag NWs, whose diameter ranges between 60 nm and 120 nm. In this case, Ag NWs are probed several times during the lateral motion, and their height density can no more be approximated by a Delta distribution, leading to a more complex function of the diameter statistics. Once the surface has been described in terms of normalized statistical distributions, the coverage coefficients can simply be obtained from the relative area of the different distributions. Our work highlights the tremendous potential of scanning probe techniques for direct quantitative surface characterization at the nanoscale.
1. R.J. Soave et al., J. Vac. Sci. Technol. A1995, 13, 1027
2. H. Harle et al., Appl. Phys. B 1999, 68, 567

This work describes atomic force microscope infrared spectroscopy (AFM-IR) measurements on single-walled carbon nanotubes (SWCNTs) with diameters near 1 nm. In AFM-IR, an AFM tip measures nanometer-scale thermomechanical expansions induced by an IR laser. This measurement results in IR absorbtion spectra with spatial resolution of about 50 nm (1, 2). Because AFM-IR is governed by nanometer-scale photothermal responses, it can be challenging to measure very small structures and structures that have a low thermal expansion coefficient (3, 4). In this work, we place a thin polymer layer between the SWCNT sample and its substrate, which amplifies the thermal expansion of the sample by up to two orders of magnitude. Placing the polymer layer between the sample and the substrate drastically decreases the heat flow between the SWCNT and the substrate, which leads to a larger temperature rise and thermal expansion for a given absorbed power. Additionally, heat flow from the SWCNT into the polymer causes the polymer temperature to increase, which amplifies the expansion. We report infrared absorption measurements of isolated metallic and semiconducting SWCNTs with diameters between 0.8 and 3.0 nm. To better understand the AFM-IR experiments, we used finite element simulations to predict temperature rise and thermomechanical expansion in the SWCNT-polymer layer system. There is a tradeoff between signal enhancement and spatial resolution: larger polymer layer thickness results in increased signal at the expense of larger apparent feature size. The signal enhancement and full width at half maximum (FWHM) of the thermomechanical expansion increases about linearly with polymer thickness, up to a polymer layer thickness of 150 nm. The simulations predict that for a polymer layer of thickness 20 nm, the full width at half maximum is 27 nm. With 150 nm of polymer, the FWHM is 185 nm. The finite element simulation results agree well with measurements. We use the measurements and the simulations to build a model that can recommend appropriate polymer thickness depending on the desired spatial resolution and sample absorptivity. For the purpose of studying infrared absorption of the sample, it is best to make measurements away from resonances of the polymer layer. However, for the study of antenna-like electric field enhancement effects, the polymer resonances provide a probe of the electric field enhancement (3). The approach to AFM-IR described here enables signal enhancement up to two orders of magnitude compared to previous approaches.
1. Dazzi et al. Opt Lett30, 2388 (2005).
2. Dazzi et al. J Appl Phys107, (2010).
3. Lahiri et al. Nano Lett13, 3218 (2013).
4. Felts et al. Rev Sci Instrum84, (2013).

Tip-enhanced Raman spectroscopy (TERS) is an optical characterization technique that combines the chemical specificity of Raman spectroscopy with the spatial resolution of scanning probe methods. One of the biggest challenges in TERS is the fabrication of reliable and robust probe tips. We have developed a new TERS probe based on the self-assembly of colloidal metal nanoparticles that support strong optical resonances. Tuning the shape, size, composition, and arrangement of these nanoparticles allows us to precisely control TERS performance. We observe exceptionally high Raman enhancement factors for commercially available AFM probes decorated with Ag nanocubes, which we demonstrate for identifying and mapping molecular surface layers patterned with features below the diffraction limit.

With the ongoing research in the field of nanostructures (wires, tubes, films, etc.), there is a need for intelligent and efficient imaging tools. The go-to method is usually Scanning Electron Microscopy which has advantages in its ability to quickly image large areas while allowing the operator to zoom into a specific area for detailed analyses. These can go beyond imaging with methods such as element analysis, crystal structure, etc. Furthermore, adding an ion beam to the setup opens up more possibilities in fabricating test setups (e.g. trenches), mounting samples (e.g. with Ion Beam Induced Deposition, IBID), or modifying structures at will. One feature is distinctly missing from SEM and FIB/SEM imagery: 3D information. In order to address this, we have developed multiple ultra compact AFM designs that can be easily attached to virtually any SEM or FIB/SEM tool on the market. Applications for the introduced systems include static and dynamic mode topography imaging, force spectroscopy, nano-indentation, cryo-afm, etc. A number of these applications will be briefly discussed and results presented.

Nanolithography is of particular interest in the development of semiconductor industry because of shrinking feature size. Nanolithography methods based on scanning probe microscopes (SPM) are alternatives to the conventional methods which can overcome the Rayleigh limit thanks to their operation in near-field. Among SMP methods, scanning near-field optical microscope (SNOM) has attracted attention because of its great applications in imaging and spectroscopy. Scanning near-field optical lithography (SNOL) is a simple method to use because nanopatterns can be created in a single step without any mask. Unlike the optical lithography that spatial resolution is limited by light wavelength, spatial resolution of SNOL is defined by both the probe size and probe-sample distance. In this study, line nanopatterns are produced on the positive photoresist by SNOM. A laser diode with a wavelength of 450 nm and a power of 250 mW as the light source, and an Al coated nanoprobe with a 70 nm aperture at the tip apex are employed. A neutral density filter is used to control the exposure power of the photoresist. It is shown that changes induced by light in the photoresist can be detected by in-situ shear force microscopy, before the development of the photoresist. SEM images of the developed photoresist are used to optimize the scanning speed, and the power required for exposure, in order to minimize the final line-width. It is shown that nanometric lines with minimum width of 33 nm can be achieved by a scanning speed of 75 µm/s, and a laser power of 113 mW. In addition, the effects of multiple-exposure of nanopatterns on their width and depth are investigated. Although this method is not appropriate for mass production of nanodevices owing to low scanning speed, it is of particular interest for device fabrication in small quantities, prototype nanodevices, and fabrication of nanoscale master patterns.

Multivariate data analytics techniques, such as principle component analysis (PCA) have been successfully applied to multidimensional functional spectroscopies such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). However, introduction of new methods for acquisition and processing, such as general mode (G-mode) scanning probe microscopy (SPM) has resulted in generation of data sets with very high spectral dimensionality, which can contain varying amount of channels containing relevant information about the material based on its properties. In order to speed up the analysis of such data we propose utilizing methods for automatic dimensionality determination, which leads to ability to use much faster randomized algorithm for PCA with computing limited number of components. We demonstrate that results generated by using randomized algorithm with properly chosen number of components perfectly correspond to results of standard full PCA. We further explore speed up and efficiency of the algorithm implementation in C compared to Intel Math Kernel Library function and its parallelization potential.
This research was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SVK). This research was conducted at and partially supported by (AB,SJ,SVK) the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. AM acknowledges fellowship support from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education.

In the last two decades voltage modulated - scanning probe microscopy (SPM) has emerged as primary tool for characterizing electrostatic, electrochemical, electronic properties on the nanoscale. In particular Kelvin probe force microscopy (KPFM) is a technique which can be used to provide a quantitative measure of the contact potential difference (CPD). KPFM is based on a combination of heterodyne detection and closed loop bias feedback. This technique reduces responses at the probe sample interaction into a single parameter map corresponding to the CPD. Notably, details of dynamic cantilever response at sub-microsecond time scales, higher-order eigenmodes and harmonics are lost pertaining information on electrostatic interactions, dielectric sample properties and mechanical properties depending on the mode of operation (i.e. lift mode/tapping mode). Here we present for the first time a purely digital multifrequency KPFM approach based on information-theory analysis of the data stream from the detector. Gmode-KPFM allows full exploration of complex tip-surface interactions, spatial mapping of multidimensional variability of material&’s properties and their mutual interactions, and imaging at the information channel capacity limit. Gmode-KPFM will be useful for measuring fast charging dynamics in photovoltaics, liquids and ionic conductors.

The quality of data recorded and ease of conducting experiments in scanning probe microscopy (SPM) are, among other factors, directly related to the quality and reproducibility of sensor assemblies used in the experiments. This is particularly true for non-contact atomic force microscopy (NC-AFM) when quartz tuning forks in the so-called qPlus configuration are used, as the sensor setup is commonly hand-made and may therefore suffer from large variations in quality. Despite these variations, quartz tuning forks in qPlus configuration have nevertheless gained wide spread use in NC-AFM experiments due to advantages such as their self-sensing feature, high spring constant, flexibility in choice of tip material, and low costs if compared to conventional silicon-based cantilevers. From a sensor quality and reproducibility point of view, the problem is that multiple steps in the assembly of tuning fork-based qPlus sensors have to be performed such as the stabilization of one of the prones to the sensor assembly base, attaching a probe tip to the free prone, and establishing electric connection to the probe tip to allow combined scanning tunneling microscopy and atomic force microscopy experiments. To optimize sensor properties, understanding how the sensor&’s key parameters spring constant, Q-factor, and eigenfrequency are affected by the specifics of the assembly would be required. Unfortunately, such understanding is far from intuitive, and analytical approaches such as solving the Euler-Bernoulli beam equation are problematic due to complex architecture of the sensor assemblies.
To gain the necessary insight, we therefore performed our analysis based on finite-element method simulations. The work presented in this poster will describe the exact contributions of thickness, geometric distribution, and hardness of the glue used to attach the one of the prones to the rest of sensor assembly on the spring constant, Q-factor, and eigenfrequency. We find that as the amount of epoxy used to attach tuning fork to the rest of sensor assembly increases, all the important parameters spring constant, Q-factor and eigenfrequency attenuates. We reached the conclusion that one should use as little epoxy as possible to stabilize one of the prones rigidly.

Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM) are powerful methods that can not only visualize a surface&’s atomic structure, but also probe its electronic (STM) and chemical (NC-AFM) nature with picoampere, piconewton, and picometer resolution. When a conducting probe is attached to a suitable oscillator, it is even possible to conduct simultaneous STM and NC-AFM experiments that deliver complementary information. Towards this goal, quartz tuning forks in qPlus configuration that have a metallic probe tip attached to the end of the free prong have gained considerable popularity in recent years. Sensor assembly is a multi-step process that includes stabilizing the tuning fork rigidly to the sensor&’s base plate, followed by attaching the tip to the tuning fork and establishing a separate electrical connection to the tip to allow collection of the tunneling current. Due to the small size of the sensor assembly and the complexity of its architecture, it is, however, not intuitive to judge how variations in the execution of the individual assembly steps ultimately affect the completed sensor&’s performance. As a result, personal skills have currently a major impact on the quality of the sensor assemblies.
To provide guidance on how to optimize the assembly process, we studied the influence of different tip mounting options on the spring constant, Q-factor, resonance frequency, and perturbation from an ideal vertical oscillation behavior. Towards this end, the entire sensor assembly is modeled using the finite element method (FEM). This approach allows to conveniently simulate the evolution of the sensor performance as a function of the various choices that have to be made during assembly such as glue thickness and choosing the location where to attach the tip in the first place. We find that the more asymmetry is introduced to the sensor setup when attaching the probe tip, the more the spring constant, Q-factor, and eigenfrequency are attenuated. This effect is, however, modest if compared to the effect that an asymmetric tunneling current connection may have. Our calculations show that a poorly implemented connection could induce unwanted horizontal oscillations that render meaningful signal recovery from the sensor impossible. As a consequence, we suggest that establishing a highly symmetric tunneling connection should receive highest priority during sensor assembly.

Scanning tunneling microscopy (STM) studies have shown for more than two decades rectangular formations when sulfur atoms are deposited on gold surfaces. The precursors have ranged from simple molecules or ions, such as SO₂ gas or sulfide anions, to more complex organosulfur compounds. In this work, we investigated the formation of the sulfur phases on Au(111) and Au(100) substrates using an alkaline solution of piperazine bis(dithiocarbamate) sodium and potassium salts as precursors. Characterization of the sulfur modified gold surface was performed by means of photoelectron spectroscopy, Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) calculations.
Photoelectron spectroscopy signals indicated the presence of S-Au bonds, monomeric and polymeric sulfur, and absence of nitrogen and sodium. STM Images showed the sulfur adlayer formation on Au(111) and sulfur multilayers on Au(100).
In the sulfur adlayer are observed square patterns formed by eight sulfur atoms (octomer) adsorbed directly on substrate. In the sulfur multilayers are observed phases of the six sulfur atoms (hexamer) and four sulfur atoms (tetramer with radic;2×radic;2 structure), and these two phases were observed in coexistence with the well-known octomers. A model that explain the sulfur multilayers was proposed, where sulfur multilayers were formed by a radic;2×radic;2 phase adsorbed directly on the gold surface while one of the other structures: hexamers or octomers were deposited on top. The lifting reconstructed surface is accompanied by the formation of sulfur multilayers. Sequential high-resolution STM images allowed the direct observation of the dynamic of the octomers on Au(100) surface, while the radic;2×radic;2 structure remained static. Images also showed the reversible association/dissociation of the octomers.
The DFT model, using the arrangement of sulfur dimers for the octomer formation reproduced the experimental STM images on Au (111) surfaces. Other DFT model, applying the dimers model for the octomer phase, explains successfully multilayer formations and the adsorption of the radic;2 ×radic;2 phase on the gold surface and the deposition of the octomers on top.

Semiconducting single-walled carbon nanotubes (sc-SWCNT) are a promising channel material for solution processable thin film transistors (TFTs). The performance of these TFTs is strongly dependent on the network morphology. In this context, there is a need for methods to evaluate and quantify the morphology of SWCNT networks. Two measurands particularly relevant to sc-SWCNT-based TFTs&’ performance are the network density and the degree of bundling [1].
We use atomic force microscopy (AFM) and scanning tunneling microscopy (STM) to visualize sc-SWCNT networks and assess their morphology. Networks are formed using sc-SWCNT dispersions in toluene where polyfluorene was used to extract the semiconducting fraction [2], and dropcasting them on SiO₂ and HOPG substrates. Small changes in the network preparation, such as rinsing the network with different common solvents to remove excess polymer and contaminants, lead to very different morphologies, where both nanotube density and bundled fraction can vary more than 4-fold. We propose a method based on AFM images to quantify the network density and the degree of nanotube bundling, which performs well even in the case of dense and heavily bundled networks. Procedures to use AFM for reliable height, diameter, and length measurements are also discussed. Finally, STM data is presented that sheds more light on bundling and polymer-nanotube interactions. The AFM-based method we propose enables routine quantification of relevant parameters to predict and optimize the electronic performance of sc-SWCNT networks, and to move beyond just pretty images and qualitative morphology descriptions.
[1] Chem. Soc. Rev., 2013, 42, 2824—2860
[2] Nanoscale, 2014, 6, 2328-2339

Polarization switching in ferroelectric and multiferroic materials forms the basis for the next generation of electronic devices such as race-track memories, field effect transistors, and tunneling devices. In these materials, the switching mechanisms are highly sensitive to the local defects and structural imperfections at the micro and nanometer scale which have undesirable effects on ferroelectric domains. These considerations necessitated the development of Piezoresponse Force Microscopy (PFM) techniques to measure and manipulate local polarization states. However, the current state-of-art PFM spectroscopy techniques suffer from serious compromises in the measurement rate, voltage and spatial resolutions since they require the combination of a slow (~ 1 sec) switching signal and a fast (~ 1 - 10 msec) measurement signal. Furthermore, transients in the cantilever response at higher vibrational modes and harmonics are lost since signal only from a single frequency, or a narrow band of frequencies is typically acquired. We report on a fundamentally new approach that combines the complete acquisition of the cantilever response signal with intelligent signal filtering techniques to directly measure material strain in response to the probing bias. Our technique enables precise spectroscopic imaging of the polarization switching phenomena 3,500 times faster than currently reported methods. The improved measurement speed enables dense 2D maps of material response with minimal drift in the tip position.
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.

Charge carriers in organic semiconductors are typically treated using free diffusion, which neglects interactions between charge carriers; however, organic semiconductors&’ ineffectively screen coulombic forces due to their low dielectric constants, which can cause charge carriers interaction energies to be several times thermal energy at device-relevant charge densities. Recently there has been experimental evidence of the dramatic effects of charge carrier interactions in organic semiconductors. Organic ratchets produced orders of magnitude more power than previously observed [1], and measured atomic force microscope (AFM) cantilever low-frequency noise over organic semiconductors was orders of magnitude lower than the noise predicted by free diffusion [2].
To relate cantilever low-frequency noise to organic semiconductor charge motion, a linear response theory was previously developed that quantitatively explains charge fluctuations by calculating the electric field and electric field gradient fluctuations arising from thermally driven charge density and dielectric fluctuations [3, 4]. Recently, the linear response theory has been extended to better match transistor device geometry; the updated theory predicts that a positive signature of charge carrier interactions can be observed at higher frequencies [5].
High frequency (short time scale) cantilever frequency noise cannot be measured directly because of photodetector noise. These fluctuations can still be quantified by measuring their effect as sample-induced cantilever friction. We will describe measurements underway to detect effects of charge carriers on sample-induced friction over an organic semiconductor. Theory predicts increasing friction at low charge densities as the charge carriers diffuse freely and contribute to the fluctuations linked to cantilever dissipation. At higher charge densities, theory predicts fluctuations should be suppressed, resulting in reduced cantilever dissipation at high charge densities; this has already been observed in silicon by Stowe, et al. [6]. The linear response theory we have in place links these density-dependent changes in friction to the semiconductor mobility; this would allow us to map local mobility in a semiconductor, without interference from contact effects or charge injection.
References
[1] Roeling, E. M. et al., Nat. Mater.10, 51 (2001). doi: 10.1038/nmat2922
[2] Lekkala, S. et al, J. Chem. Phys.137, 124701 (2012). doi: 10.1063/1.4754602
[3] Yazdanian, S. M. et al., J. Chem. Phys.128, 224706 (2008). doi: 10.1063/1.2932254
[4] Yazdanian, S. M. et al., Nano Lett.9, 2273 (2009). doi: 10.1021/nl9004332
[5] Lekkala, S. et al., J. Chem. Phys. 139, 184702 (2013). doi: 10.1063/1.4828862
[6] Stowe, et al. Appl. Phys. Lett. 75, 2785 (1999). doi: 10.1063/1.119522

It is known that top- and bottom-contact organic field-effect transistors (OFETs) exhibit vastly different source-drain currents, but the underlying mechanisms behind their disparate performances remain debated. In our work, 2-dimensional potential plots derived from photocurrent microscopy measurements are presented for pentacene OFETs. Potential maps are presented for top- and bottom-contact devices as well as bottom-contact devices treated with pentafluorobenzenethiol to demonstrate that the origin of inferior performance in bottom-contact devices is most likely not morphological in nature. In addition, our results demonstrate that pentacene charge-transfer excitons are capable of contributing to the photocurrent in bottom-contact devices at the source-channel barrier, despite their increased binding energy as compared to pentacene Frenkel excitons.

Scanning spreading resistance microscopy (SSRM) has evolved as a complimentary method of choice together with Scanning Capacitance Microscopy (SCM) for two-dimensional carrier mapping due to its unique spatial resolution and high sensitivity. SSRM relies on the phase transformation of the Si substrate by a diamond tip, which changes the underlying Si phase from cubic semiconducting to five- and six-fold coordinated metallic Si phases. While, the electrical nano-contact between a SSRM probe and Si is well documented, the insight is insufficient to understand or make predictions about the properties of the SSRM contact.
In the present work we compute from first principles the evolution of the TVB and BCB, Fermi level of the cubic (Si-I), β-Sn (Si-II), BCT-5 (Si-III) and BC8 (Si-IV) bulk phases of Si that can form under the diamond tip and map those to the each of the formed phases under the tip, according to the formed phase and degree of stress, present in each of those. The 2D cross-section of the Si/tip model is used to perform TCAD simulations of the current distribution involved in the structure that explain the success of the SSRM technique.

We use scanning gate microscopy to determine the electrostatic limit of detection (LOD) of a nanowire (NW) based chemical sensor with a precision of sub-elementary charge. The presented method is validated with an electrostatically-formed NW whose active area and shape is tunable by biasing a multiple gate field-effect transistor (FET)^[1].
By using the tip of an atomic force microscope (AFM) as a local top gate, we emulate the field-effect of analytes that are adsorbed on the sensing surface acting as a molecular gate. We quantify the tip induced charge with an analytical electrostatic model and show that the NW sensor is sensitive to about an elementary charge and that the measurements with the AFM tip are in agreement with sensing of ethanol vapor. This method is applicable to any FET-based chemical and biological sensor, provides a means to predict the absolute sensor performance limit, and suggests a standardized way to compare LODs and sensitivities of various sensors.
We show by scanning capacitance force microscopy that the electrostatic nanowire can be "moved" in real-time across the active sensing area by changing the depletion width with appropriate biasing. This novel feature is used to further increase the sensitivity of the sensor.
References
[1] A. Henning, N. Swaminathan, A. Godkin, G. Shalev, I. Amit, and Yossi Rosenwaks, Nano Research, in press (2015).

ZnO is considered as a good candidate material for diverse applications such as optoelectronics and sensor devices. For potential application of ZnO nanowires (NWs) in electronic and optoelectronic area, doping characterization of them is carried out by scanning capacitance microscopy (SCM)^1,2 at nano-scale as well as conventional capacitance-voltage (C-V) profiling at macro-scale.
Non-intentionally doped NWs were grown by chemical bath deposition (CBD) on Si substrate and metal-organic chemical vapor deposition (MOCVD) on sapphire. Fisrt, a planarization process was conducted based on dip-coating of SiO₂ matrix to NWs field to consolidate them and chemical-mechanical polishing to obtain a proper surface for following measurements.
In SCM, an alternating voltage (Vac) of 1000 mV (peak to peak) is employed to produce capacitance variation of the tip-sample system. As a result, the NWs embedded in SiO₂ matrix can be well detected in hexagonal form. Determination of carrier concentration is done through calibration method. For this purpose, two ZnO: Ga samples with staircase structures were grown by MBE³ containing multiple layers of Ga⁴ concentrations ranging from 2E17 to 3E20 cm^-3, as measured by secondary ion mass spectrometry (SIMS). SCM on the cross-sections of them reveals that SCM is able to reliably distinguish the doped layers within the doping range, with SCM profile consistent with doping profile from SIMS, indicating the potential use of this technique for ZnO. Then SCM is performed on the staircase structure and the planarized NWs structure in the same session under identical conditions. Calibration dataset is obtained from the SCM signal for layers with different Ga densities. Through comparison of SCM data of NWs with the calibration dataset, the carrier concentration in the NWs is estimated to be around 3E18 cm^-3.
For C-V profiling, Pt is deposited for schottky contact with the collective ZnO NWs through electron beam physical vapor deposition (EBPVD) process. The rectifying characteristics is verified by current-voltage (I-V) measurement and the ratio of area of NWs to the total contact area is estimated under SEM observation. Carrier concentration for the NWs is determined from C-V curve to be higher than 1E17 cm^-3. Comparison of results from the two techniques is done and their difference discussed.
[1] C.C. Williams, Annu. Rev. Mater. Sci. 29, 471 (1999).
[2] R. A Oliver, Reports Prog. Phys. 71, 076501 (2008).
[3] D. Taïnoff et al. Appl. Phys. Lett. 98, 131915 (2011).
[4] S. Sadofev et al. Appl. Phys. Lett. 102, 181905 (2013).

Compositional imaging of individual constituents in polymer materials is one of the important applications of Atomic Force Microscopy (AFM), which is performed in different modes. Recent developments have broadened such AFM studies by combining this technique with spectroscopic methods such as Raman spectroscopy. The AFM/Raman studies are in full development and several aspects of these applications will be demonstrated on various polymer blends and composite materials. The related experiments were conducted on the Raman Confocal microscope DXR (Thermo Fisher Scientific) and NTEGRA Spectra Unit (NT-MDT). In regards to the compositional imaging of heterogeneous materials, AFM-based studies rely on the specific shape and dimensions of individual constituents and differences in their local mechanical, electric and thermal properties. The compositional AFM maps of block copolymers, polymer blends, incomplete metal alloys and semiconductor structures are well documented. However, the identification of the individual components based purely on AFM studies in these complex materials is primarily indirect and has definite limitations. The analysis is further enhanced by combining an AFM microscope and a Raman spectrometer that enables surface profiling and measurements of local materials properties with the collection of Raman maps component-specific molecular information. We will describe the AFM/Raman confocal microscopy studies of a number of heterogeneous materials combined with mapping of their mechanical and electric properties. Several immiscible polymers blends such as polystyrene (PS) with polyvinyl acetate; PS with low-density polyethylene and PS with polybutadiene were explored and the benefits of spectral characterization of the blends&’ constituents are evident. In addition to reporting on the characterization of the polymer blends, various practical issues related with choice of substrates, film thickness and spatial resolution of spectroscopic mapping will be discussed in detail. The analysis of the blend morphology was based on mapping of elastic modulus and polymer-specific Raman bands. The polymers filled with nanoparticles and hybrid materials represent another group of samples that was explored. In the case of blend of polyethylene oxide with polyvinylpyrrolidone, which is filled with Si nanoparticles, the combination of results obtained with AFM-based electric modes and Raman mapping was emphasized.

In this presentation, the utility of atomic force microscopy (AFM) for assessing the quality of mixing of a hot melt extrusion process used for the preparation and stabilization of drugs in their amorphous state is demonstrated. The impact of process parameters such as extruder screw speed and thermal quenching method as well as formulation composition on the resulting extrudate phase morphology are explored with both AFM imaging and force measurements. The AFM results are interpreted in the context of mechanical energy input calculations and modulated differential scanning calorimetry data for each processing condition. In addition, further applications for the practical use of AFM for drug development within the pharmaceutical industry will be discussed.

Gas cluster ion beams (GCIB) coupled with XPS and ToF-SIMS instruments is a relatively new technology that is used for cleaning and depth profiling of organic surfaces. In the past, monatomic beams were used for sputtering and exposing buried interfaces of a surface; however, these beams are quite damaging to the organic material. In contrast, using a large cluster of Ar atoms (2500), damage to the surface is minimized because the energy is spread laterally, maintaining the integrity of the surface chemistry. This is especially useful for soft materials, such as polymers. While this technique is currently used to understand the surface chemistry of buried interfaces and layers, we now consider it as a sample preparation method for polymer microscopy. Using a GCIB to sputter away amorphous material on polymer surfaces may give access to underlying crystalline morphology for more accurate visualization of sub-surface structures without artifacts introduced by cryo-microtomy . To this end, we are developing a novel sample preparation method for AFM using our GCIB sputtering capabilities on polyolefin materials. This preparation method is compared to traditional permanganic acid etching methods.

We report a new method to estimate the elastic adhesion properties of a top-layer graphene on a highly ordered pyrolytic graphite (HOPG) bulk at the atomic scale. Low energy neon ion sputtering is used to intercalate neon atoms to form top-layer graphene blisters on HOPG surface in an ultrahigh vacuum chamber. Detailed characterization of the blister shape and atomic perturbation in the surrounding lattice is then carried out using scanning tunneling microscopy (STM) and image analysis algorithms, coupled to elastic continuum modeling. We estimate the adhesion energy between the top-layer graphene and graphite bulk to be about 0.240 J/m², in close agreement with reported experimental and theoretical values of the graphite adhesion energy. Our method directly demonstrates scaling of the elastic properties of 2D materials to atomic scale (at least for graphene) and it provides a quick and effective route to obtain important elastic mechanical properties of layered graphite. We propose this technology can be further applied to various two-dimensional layered materials.
Reference: Jun Wang, et al., “Estimation of adhesion properties of graphene on HOPG at an atomic scale by intercalation of neon atoms”, submitted 2015.
Acknowledgements
This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

Recent reports showed how a graphene monolayer on metal substrate is either transparent, partially transparent or opaque to water.¹ Macroscopic measurements, fail at providing a complete understanding of the underlying physical phenomenon. For this reason, we investigate the surface wettability of graphitic surfaces with a recently developed Amplitude Modulation force spectroscopy technique,^2,3 which represents a novel nanoscopic approach to the problem. The technique is capable to recover conservative and dissipative interactions, which are characterized by metrics to monitor their behavior. We further relate nanoscopic observables of the tip-sample interaction, such as force of adhesion, to macroscopic contact angle measurements. This represents an attempt to bridge nanoscopic and macroscopic views of wettability, which could value the AFM as a versatile tool to interpret surface wettability.
Reference
1 Mugele, F. Unobtrusive graphene coatings by Mugele. Nature11, 182-183, (2012).
2 Katan, A. J., Van Es, M. H. & Oosterkamp, T. H. Quantitative force versus distance measurements in amplitude modulation AFM: a novel force inversion technique. Nanotechnology20, 165703, (2009).
3 Santos, S., Amadei, C. A., Verdaguer, A. & Chiesa, M. Size Dependent Transitions in Nanoscale Dissipation. The Journal of Physical Chemistry C117, 10615-10622, (2013).

Atomic force microscopy (AFM) is a technique capable of imaging surfaces with atomic resolution, but it is also prone to a wild range of artifacts. With the recent advances in 2D materials, like graphene, h-BN and others, it is of utmost importance to have an artifact-free image for a quantitative analysis. In this work, we investigated an artifact that occurs when operating in the contact mode AFM and we found that, depending of the choice of scan direction, an artifact can yield up to 500% difference in topographic data. This artifact is associated to local changes in the surface friction coefficient, which leads to different bends of the cantilever in the forward and backward scan directions. An analytical theory using Euler-Bernoulli beam equation was used in conjunction with finite-elements analysis using the COMSOL Multiphysics package to explain quantitatively the artifact. It is also shown that no artifact occurs when scanning perpendicularly to the cantilever axis. Finally, we show that this artifact, which might appear as a nuisance, may be effectively used to acquire frictional information from the sample without the need to calibrate the spring constant of the cantilever (normal or torsional). In other words, it enables the calculation of frictional coefficients across the sample without the need to know the forces involved in the process.

One of the ongoing challenges in the field of piezoresponse force microscopy (PFM) is the accurate quantification of the piezoelectric coefficients. Conventional PFM systems almost exclusively use an optical beam deflection (OBD) system where a laser is focused on the back of the cantilever and the angle of the reflected light is used to deduce the cantilever normal and lateral tip motion. However, non-desirable buckling and torsion of the cantilever may be misinterpreted as cantilever tip motion. This is a shortcoming of the OBD method which measures the angle of the cantilever, rather than the displacement of the tip.
Here, we describe results on highly sensitive PFM imaging and spectroscopic studies of ferroelectrics (LiNbO₃ crystals and Pb(Zr,Ti)O₃ and BaTiO₃ thin films) performed with an interferometric AFM. This AFM is based on a commercial Cypher S AFM and combines the existing OBD system with a separate quantitative interferometric Laser Doppler Vibrometer (LDV) system to enable accurate measurements of the displacement and velocity of the cantilever tip [1]. This combined instrument allows a host of quantitative measurements to be performed including measuring a variety of in-situ PFM cantilever oscillation modes, as well as accurately measuring the cantilever spring constant prior to making contact with the surface. Importantly, the piezoelectric coefficients extracted from several LDV measurements showed an order of magnitude less variability compared to the error-prone OBD measurements acquired simultaneously. By performing simultaneous LDV and OBD measurements, we were able to conclude that most of the measurement error and variability in PFM measurements thus far can be attributed to the shortcoming of the OBD method.
We present a systematic methodology for accurate PFM measurement of d₃₃ and d₁₅ coefficients. In this context, the notable differences between the OBD and LDV results are demonstrated and discussed. Even though the interferometer provides an intrinsically quantitative measurement of the cantilever motion, there are additional requirements for quantification of the tip-sample electromechanical response that prevent cantilever dynamics and stray electrostatic interactions from overwhelming the PFM signal. Further considerations about the effects of finite loading forces that may reduce the apparent piezoelectric sensitivity are also discussed. In addition, quantitative lateral PFM results, determined from sequential LDV measurements at various LDV spot positions, are also presented.
[1] A. Labuda and R. Proksch, Appl. Phys. Lett., in press (2015).

Understanding and optimizing advanced materials frequently requires detailed knowledge of nanoscale electrical properties. Scanning probe techniques such as scanning tunneling microscopy (STM), conductive AFM (cAFM), scanning capacitance microscopy (SCM), and Kelvin probe force microscopy (KPFM) provide such nano-electrical measurements, but are generally limited in the classes of materials they can characterize or the properties they can measure. Scanning microwave impedance microscopy (sMIM) uses GHz frequency microwaves and shielded AFM probes to directly measure the impedance (capacitance and conductance) of the tip sample interface. As such sMIM is sensitive to the permittivity and conductivity of a wide variety of samples including dielectrics, conductors, and semiconductors.
When sMIM is applied to non-linear materials, changing the tip sample bias changes the local electric field thereby changing the local electrical properties of the sample just under the AFM tip. The electric field induced changes in the sample create changes in the tip-sample impedance that can be measured by sMIM. For example, when imaging doped semiconductor samples, the tip sample interface forms either a metal-semiconductor junction or a metal-insulator-semiconductor junction. Plotting the sMIM measured capacitance as a function of the tip sample bias voltage produces the equivalent of a typical capacitance-voltage curve, but from nanoscale regions selected from an AFM image. C vs V results from doped silicon samples that closely match theoretical calculations will be discussed.
The talk will also present results from advanced and novel materials and devices, such as III-V semiconductors, 2D materials and 1D structures where sMIM data has been used to assess linear and non-linear behavior and characterize dopant type and distribution.

Electromechanical effects are defined as a change in material volume as response to an applied electrical field and are widely distributed. Some commonly studied electromechanical effects include piezo-/ferroelectricity, electrostriction, and ionic transport. Scanning probe microscopy (SPM) has been extensively used to study electromechanical effects on the nanoscale to reveal structure-function relationship.
While the development of different SPM techniques are evolving fast, the quantitative analysis of the measured signal and the extraction of quantitative material and tip-sample contact properties are lacking behind. This is especially true when the changes in dynamic surface displacements are measured and when the contact resonance enhancement is utilized to increase the vertical displacement sensitivity.
Here, we present how to move towards more quantitative and reproducible experimental data based on electromechanical effects by considering the shape of the cantilever in contact mode during resonance-enhanced SPM techniques. Examples will include the quantification of surface displacements during piezoresponse force microscopy (PFM) measurements performed on ferroelectric thin films and the quantification of dielectric tunability in ferroelectrics in paraelectric state, such as SrTiO₃ and Ba_xSr_1-xTiO₃ measured through electrostriction. In addition, we will explore how electrostatic tip-sample interactions are present in many SPM techniques and show how to quantitatively measure the electrostatic force between tip and sample when electric field are applied. This allows to estimate the tip displacement caused by electrostatics which can be compared with the total strength of the measured electromechanical effect.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, through the Office of Science Early Career Research Program, and the Materials Sciences and Engineering Division. The experiments were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility which also provided additional support.

Strontium titanate (SrTiO₃) is among the most popular substrates for complex oxide epitaxy, with film quality dependent on the structure of the substrate at the beginning of the growth process. Here, we examined the surface structure of SrTiO₃ (100) single crystals as a function of annealing time and temperature in either oxygen atmosphere or ultra-high vacuum (UHV) for a variety of different preparation schemes using scanning probe microscopy, auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). We find that the SrTiO₃ surface evolves depending on the preparation scheme with respect to surface roughness, surface terminations, and surface reconstruction. Non-contact atomic force microscopy (NC-AFM) images, e.g., reveal a non-monotonic trend of surface roughness with respect to UHV annealing temperature. Interestingly, the surface roughness changes also as a function of the bias voltage applied to the surface. This can be explained by the effect of the electrostatic field induced by both the Nb-doping in the bulk and oxygen deficiencies in the bulk or on the surface, with the latter being a function of the preparation history. As for surface termination, we observe for initially TiO₂-terminated crystals the formation of terraces with half unit cell step heights between them with increasing UHV annealing temperatures, implying that multiple terminations are forming. This conclusion is corroborated by AES data, which expose an increase in Sr amount relative to Ti and O. Complementary LEED data reveals a structural phase transition from (1×1) termination to an intermediate c(4x2) surface reconstruction to ultimately a sqrt(13) × sqrt(13)-R33.7° surface phase by annealing the sample with oxygen flux, while the inverse structural phase transition from sqrt(13) × sqrt(13)-R33.7° to c(4×2) is observed when annealing in UHV. As a result, we suggest that careful selection of preparation procedure combined with applying an appropriate bias voltage during growth may be used to control outcomes of thin film growth.

High mobility semiconductor materials, such as III-V compounds, are at the forefront among non-silicon based high performance devices [1]. In general, these materials are epitaxially grown on Si. However due to the lattice mismatch with Si, crystallographic defects are induced by the stress relaxation during growth. Transmission electron microscopy inspection identifies stacking faults (SFs) and twins defects along the {111} plane as the main relaxing mechanism for (001) InP grown in v-shaped trenches on Si [2]. By a chemical anisotropic etching, defects are decorated and easily identified on the top surface. However, the evaluation of the electrical properties of SFs and twins is still a major challenge. In this work we exploit the properties of conductive atomic force microscopy (c-AFM) technique to study the electrical impact of single SF defects present in InP grown in narrow trenches. Our study reports an enhanced conductivity at the SF location, demonstrating the ability of probing atomic planes by c-AFM. The high lateral resolution in the electrical characterization of atomic planes is achieved by using in-house fabricated conductive diamond tips [3]. In addition, the high wear resistance of our tips combined to the precise load-force application, enables controlled tip-induced material removal often termed scalpel c-AFM [4]. In essence we collect multiple conductivity profiles at different heights within our sample. Finally, all the 2D profiles can be interpolated into a three-dimensional (3D) object. Our results prove that the SFs lie in the {111} plane whereby, as a function of depth and depending on their relative crystal orientation, they remain parallel or approach each other. Of important relevance for device operation, is that we are able to demonstrate that the SF represents a conductive plane running throughout the trench with different electrical properties than the surrounding material. This can impact on local mobility, carrier concentration as well as create junction leakage paths. Moreover the enhanced conductivity can also be affected by the segregation of doping atoms and a higher density of point defects along the SF. To support our findings, first-principles simulations of the electronic and transport properties are carried out for InP structures with and without SFs. The presence of a SF locally influences the electronic properties and the transmission spectrum of InP, leading to a change in the drive current. To conclude, the electrical properties of SFs is investigated in narrow InP trenches. Our observation highlights a possible lower resistance path between channel and bulk in the presence of SFs, leading to an increase in the leakage current component of high performance devices.
1. J. A. del Alamo, Nature 479, 17 Nov. 2011, 317-323.
2. M. Paladugu et al., Cryst. Growth Des. 12, 2012, 4696-4702.
3. T. Hantschel et al., Phys. Status Solidi A 206, 9, 2009, 2077-2081.
4. U. Celano et al., Nano Lett. 14, 5, 2014.

Silicon carbide (SiC) is an attractive material for power semiconductor devices especially in high voltage application area [1]. Although SiC power devices are now practically in use, further research and development (R&D) for enhancing their electrical performance, reliability, and cost reduction are required. Techniques for evaluating device structure including activated carrier distribution are required for effective R&D.
Previously, we have reported that scanning nonlinear dielectric microscopy (SNDM) [2] and super-higher-order SNDM (SHO-SNDM) [3], were powerful tools for visualizing carrier distribution in cross-section of SiC devices. However, previous measurements were performed to static devices. Techniques for carrier profiling of operated SiC device will make the R&D more effective by enabling us to “see” physical origin of actual device&’s electrical characteristics. Although carrier profiling of operated silicon (Si) devices using scanning capacitance microscopy (SCM) have been reported [4][5], in our best knowledge, no such analyses on operated SiC devices has been reported. This is considered to be due to the difficulty of carrier profiling on SiC devices. SiC materials exhibit less capacitance response than Si materials as reported by other groups [6][7].
In this study, techniques for cross-sectional carrier profiling of operated SiC device using SHO-SNDM were proposed. Unlike previously reported methods for carrier profiling of operated Si devices using SCM, mechanisms for nullifying tip-sample voltage were implemented. As a demonstration of our methods, carrier distribution of cross-sectioned operated SiC power double-diffused metal-oxide-semiconductor field-effect transistor (DMOSFET) [8], which is common structure for vertical power MOSFET, was measured.
At first, “on” and “off” state were measured in detail using SHO-SNDM and variation of depletion layer distribution was visualized (experiment 1). Second, carrier distribution as a function of gate-source voltage (V_GS) was measured in detail (experiment 2). As a result of experiment 1, visualization of electron channel formation at “on” state was demonstrated. Moreover, depletion layer redistribution was visualized. In experiment 2, profiling of detailed carrier redistribution depending on the V_GS was demonstrated. These results show the capability of SHO-SNDM for evaluating operated SiC devices.
[1]H. Matsunami, Microelectron. Eng. 83, 2 (2006).
[2] Y. Cho et al., Rev. Sci. Instrum. 67, 2297 (1996).
[3] N. Chinone et al., J. Appl. Phys. 116, 084509 (2014).
[4] V. V. Zavyalov et al., J. Vac. Sci. Technol. B 18, 549 (2000).
[5] C. Y. Nakakura, P. Tangyunyong, D. L. Hetherington, and M. R. Shaneyfelt, Rev. Sci. Instrum. 74, 127 (2003).
[6] O. Ishiyama and S. Inazato, J. Surf. Anal. 14, 441 (2008).
[7] T. Fujita, K. Matsumura, H. Itoh, and D. Fujita, Meas. Sci. Technol. 25, 044021 (2014).
[8] S. M. Sze and Kwok K. Ng, Physics of Semiconductor Devices, 3rd ed. (John Wiley & Sons, Inc., 2007).

The oxidation of Si surfaces has been studied because of its industrial significance in the production of electronic devices. In particular, the oxidation of Si(100) surfaces is important, because commercial devices are often fabricated on Si(100) surfaces. Continuous miniaturization of MOSFETs in the devices now results in the fabrication of atomically thin and nanometer-scale gate oxide insulators. In these miniaturized devices, atomic-scale variation in electronic properties at surfaces and interfaces cause significant variation in device characteristics. Scanning probe microscopy methods are useful for the atomic-scale investigation on the Si surfaces. Among them, noncontact scanning nonlinear dielectric microscopy (NC-SNDM) method can simultaneously image topography and electric dipole moment distribution of material surfaces by measuring tip-sample capacitance. Recently, NC-SNDM has resolved the distribution of atomic electric dipole moments on a Si(100)-(2x1) surface [1]. NC-SNDM has demonstrated that individual dimers on the surface have negative dipole moments, which are oriented from surface to bulk.
Here, we investigate an initial stage of oxygen-adsorption on a Si(100)-(2x1) surface using NC-SNDM. We prepared a cleaned Si(100)-(2x1) surface in ultrahigh vacuum using a flashing technique and then exposed it to oxygen gas at room temperature. The total amount of the exposure was 0.65 L. We applied an ac voltage of 2.0 Vpp at 25kHz to the sample in order to induce the variation in tip-sample capacitance for NC-SNDM measurement. Then, we simultaneously imaged topography and dipole moment distribution of an oxygen-adsorbed Si(100)-(2x1) surface. We observed that buckled dimers make zig-zag patterns along the dimer rows in the topography. This non-local transition from flip-flop to buckled dimers may have been caused by partly adsorbed oxygen. In addition, each buckled dimer has a negative dipole moment. In the cleaned (non-oxygen adsorbed) dimers, this is explained by electron transfer from a lower Si atom to an upper one in one dimer [2]. NC-SNDM detects the normal component of this dipole moment. The oxygen-adsorbed surface had several differences from the bare surface. Among the dimer atoms, a dimer with the oxygen adsorbed atom protruded about 0.2 Å from the neighboring clean atom, while the dipole moment remained negative. According to Ref. [3], this phenomenon is caused through the insertion of an oxygen atom into a back-bond of the lower dimer atom. The inserted oxygen atom protrudes relative to the lower dimer atom and the oxygen atom attracts electrons from the lower atom because of the difference in electronegativity. Thus, the normal component of the total dipole moment remains negative, which explains our experimental results.
[1] M. Suzuki et al., Appl. Phys. Lett. 105(10), 101603 (2014).
[2] D. J. Chadi, Phys. Rev. Lett. 43, 43 (1979).
[3] T. Uchiyama et al., Phys. Rev. B 55, 9356 (1997).

Resistive RAM is a next-generation technology offering efficiency and storage density improvements over memories such as flash, utilising thin dielectric layers sandwiched between conductive electrodes. While insulating in their pristine state, sub-breakdown electrical stress can be used to reversibly change the resistance of the dielectric. In this way devices may be switched between ‘on&’ and ‘off&’ states, a phenomenon known as resistance switching. Several models have been proposed to explain the switching mechanism in different materials, yet a full, experimentally backed, description of switching has not yet been achieved. Many models are based around the formation of conductive filaments through the dielectric. In particular, for oxide dielectrics this filament is believed to be composed of a chain of intrinsic oxygen vacancy defects that bridges the dielectric, shorting the electrodes. Often it is believed that such filaments are heavily branching as many pathways take part in forming the final chain between the electrodes during the electrical stressing. However, these filaments are sub-micron, meaning that they are difficult both to locate and to probe. In order to aid in developing filament-based switching models, we used a conductive atomic force microscopy technique to study the structure of these filaments in silicon-rich silica. By suitably adjusting the cantilever setpoint, we have been able to perform depth-profiling studies in which we have observed the size and shape of these conductive pathways through the dielectric, building up a 3D tomographic image of individual conductive filaments. Our results show that filamentary behaviour is indeed observable and intrinsic within our devices, yet we are able to note for the first time that the form of the filament is very different to the commonly-pictured branching, tree-like structure. Instead, our filaments are very straight, direct pathways, with occasional competing growths observed at one of the electrodes.

Monolayer MoS₂ (ML-MoS₂) has emerged as a prototypical semiconductor for high performance, flexible 2D atomically thin optoelectronics devices including field effect transistors, ultrafast photodetectors, sensors, light emitting devices and even photovoltaics. At only 0.7 nm thick, dielectric screening is reduced, enhancing the coulombic interaction between electrons and holes and yielding tightly bound exciton states with binding energies that exceed room temperature. This complex manifold of excitons in conjunction with local structure and electronic properties govern the performance and capabilities of ML-MoS₂ and other transition metal dichalcogenides, and because these materials form direct bandgap semiconductors, these exciton states show remarkably strong light-matter interactions which are readily probed with optical measurement techniques such as photoluminescence and absorption spectroscopy. In particular, spatially mapping the photoluminescence intensity and energy that arises from radiative relaxation of the exciton states provides direct access to variations in crucial optoelectronic properties such as exciton diffusion length, local carrier concentration, defect density, strain and exciton energy. However, the spatial resolution of traditional optical techniques is constrained by the diffraction limit to hundreds of nanometers, which is well above the characteristic nanoscale dimensions of these optoelectronic properties in ML-MoS₂ and related 2D materials.
Although scanning nearfield optical microscopy (SNOM) is able to image with sub-diffraction resolution, applying the technique to inelastic light-matter interactions such as photoluminescence in 2D systems constitutes a formidable challenge. Using the recently developed Campanile probe [1], nearfield optical microscopy is performed for the first time to map the photoluminescence of synthetically grown ML-MoS₂ with sub-diffraction spatial resolution that approaches critical optoelectronic length scales. The unique structure of the Campanile probe enables background-free, full spectroscopic imaging of the photoluminescence and resolves nanoscale variations in emission intensity and energy. The enhanced resolution combined with detailed spectral analysis reveals a unique disordered peripheral edge state that surrounds a pristine, crystalline interior. Furthermore, the spatial extent of excited state quenching by grain boundaries in polycrystalline ML-MoS₂ is quantified, providing an upper bound on quantities such as the exciton diffusion length. In addition to providing fundamental insight into the optoelectronic properties of ML-MoS₂ and other transition metal dichalcogenides, the results elucidate crucial considerations for the development of reliable, large-scale synthesis procedures for these 2D semiconductors.
[1] Bao et al., Science338, 1317 (2012).

Dynamic Atomic Force Microscopes (AFM) are often used to probe nanomechanical properties of soft materials, in particular polymers. The last two decades have seen numerous efforts to interpret height and phase images in terms of mechanical properties and dissipating processes. Beyond the technical difficulties, mostly related to a proper knowledge of the size and shape of the tip, extracting quantitative information requires a model (ideally providing analytical expressions), in turn allowing the recorded data to be analyzed. Any attempt to model oscillation behavior when the tip touches the sample faces several difficulties, making models of the interaction truly cumbersome.
With the recent development of multifrequency AFM, analysis of the cantilever motion in dynamic AFM can describe the force acting on the tip, from which we can hopefully intuit the viscoelastic properties of the studied surface. Progress in the field of instrument/cantilever calibration, and methods of force measurements set the stage for a critical examination of the different physical models commonly used in this growing field of research. These models are of paramount importance for our understanding and interpretation of the data in order to provide the (ideally quantitative) mapping of the material mechanical properties. In this work, we will review most of them by illustrating their capabilities but also their limitations on a series of samples based on polymer blends and block copolymers.
Material property mapping based on this approach faces many problems, mainly due to the fact that the measurements are performed too rapidly, provoking the appearance of viscous forces. More importantly, when the sample is very soft, the tip penetrates the surface and its interaction with the surface may include entropic forces, capillary forces arising from surface curvature that is not usually taken into account. Actually, many of the methods developed so far employ a linear approximation, treating the mechanical response as changing the parameters of an effective cantilever resonance assumed in rigid contact with the surface.
By using Intermodulation AFM data, we clearly show that we can go beyond the linear response and the rigid interaction models to explain the AFM data. Our analysis based on the dynamic force quadratures, very similar to the macroscopic Dynamic Mechanical Analysis, clearly show that significant viscous response may be explained by a soft material flow, giving the surface its own dynamics. From the comparison with simulations of the dynamics of the system including the surface deformation in a simple model describing the cantilever eigenmode coupled to a linear viscoelastic surface, we are able to find remarkably good agreement between experiment and simulation, providing quantitative mapping of the mechanical properties of very soft materials such poly(dimethylsiloxane) based materials essentially at any point of the (viscous) surface.

Atomic force microscopy provides a variety of approaches to measuring material properties of a substrate. Single point measurements in the form of force spectroscopy, where force vs. tip-sample separation data is collected and then modeled with any number of contact mechanics models, can be especially useful for measuring properties such as material stiffness and adhesion. Current force spectroscopy approaches can be slow, accompanied by a tedious analysis that has to be applied to a very large data set that can make meaningful interpretation of the data challenging.
We present force spectroscopy measurements that incorporate real-time quantitative measurements and analysis of adhesion and stiffness providing a rapid assessment of material properties. Measurements on biological systems and polymer blends reveal a quick, quantitative differentiation in stiffness and adhesion within the different components of the polymer blend and between the different biological samples. In addition to real-time analysis with adhesion-based contact mechanics models, analysis of the properties of interest can be further refined by assessing different parameters in the force curves such as such as bimodality, baseline correction, and splitting for improved assessment.

Studies of soft materials in Atomic Force Microscopy (AFM) are typically performed in the amplitude modulation mode with phase imaging (AM-PI). This is one of three available resonance oscillatory modes, in which the characteristics of probe (amplitude, phase and frequency) are related to the sample topography and tip-sample force interactions. The other two modes: amplitude modulation with frequency imaging (AM-FI) and frequency modulation (FM) are rare applied for the examination of polymer materials yet they can provide the undiscovered values for comprehensive analysis of the materials. In each mode, two of three characteristics are fixed during AFM operation. The third one and the surface profile are the measured signals that form the images or maps. Several issues of the combined applications of the modes will be considered in the analysis of the experimental amplitude, phase and frequency curves and the images that include the calculated dissipation variations. The simulation of the curves, which were performed for different theoretical models of tip-sample interactions as well as for experimental force versus deformation dependencies, will be analyzed together with the results recorded on different materials. A special attention will be paid to operations in the attractive and repulsive force regimes and bifurcation phenomena. It will be demonstrated that the bifurcation is theoretically predicted and experimentally observed in AM-PI and AM-FI modes. A correlation between the phase and frequency contrast changes and local mechanical properties of polymers will be considered in imaging of polymer blends and block copolymers. An extraction of quantitative values of elastic modulus and work of adhesion from the results obtained in AM-PI, AM-FI and FM modes will be discussed in applications to neat polymers and heterogeneous polymer systems. The increasing interest to the analysis of interfaces of nanoparticles embedded into polymers will be addressed in high-resolution mapping of elastic modulus and adhesion in close interplay between the data obtained with AM-PI, AM-FI and non-resonant Hybrid modes.

High-resolution Atomic Force Microscopy (AFM) in liquid is gaining more and more attention from the scientific community. In the past few years it has shown to be able to characterize a huge variety of different solid-liquid interfaces at the atomic/molecular level [1] and provide valuable information on the structure of the liquid molecules [2] and dissolved ions in proximity of complex charged surfaces [3-5]. Yet the interpretation of the images remains difficult, partially because the categorization of forces involved in the enhanced resolution at the surface is still under debate.
Here AFM is used to create images of single metal ions close to the surface of minerals and lipid bilayers immersed in aqueous solutions [3-4]. We are able to ascribe specific features in the AFM images to the presence of single metal ions. We found that their dynamics are significantly slow compared to the bulk and affected both by the structure of the interfacial water at the surface and the hydration properties of the ions. Moreover we show how the use of higher eigenmodes in the case of soft cantilevers can significantly enhance the resolution in the case of soft membrane proteins.
References:
[1] Voitchovsky et al., Nat. Nanotechnol.,5, 401, (2010)
[2] Herruzo et al., Nanoscale5, 2678 (2013).
[3] Ricci et al., Langmuir, 29, 2207, (2013)
[4] Ricci et al., Nat. Commun., 5, 4400, (2014)
[5] Siretanu et al.,Sci. Rep.4, (2014).

The metastable morphology of polymer-surfactant ultrathin films for lubricity applications (model hair conditioner) is explored as a function of relative humidity and temperature using environmental atomic force microscopy. Film ingredients include various combinations of the surfactant behenyl alcohol (BOH, 22-carbon chains), the polysaccharide guar, and polydimethyl siloxane, deposited onto cleaved mica substrates. The minimally perturbative character of attractive-regime dynamic AFM enables imaging at very high humidity (up to 95% RH) of highly fluidized films. In particular, we track reversible swelling (during humidity cycling) of hygroscopic moieties, thereby identified as guar, which further grow or shrink irreversibly via Ostwald ripening due to surface diffusion. Capillary water is also imaged, localized at the edges of surfactant bilayer domains atop open patches of bare mica. At the highest humidity explored, the surfactant bilayers laterally grow at the expense of an incomplete first monolayer, producing larger domains of bare substrate. In further work such activated transformations are investigated as a function of temperature.
The thickness of the first BOH monolayer is approximately 1 nm, half the molecular length, indicating a strong chain ti< whereas the bilayers are 4 nm thick, indicating nearly vertical stacking. Corroborating these inferred structural differences are nanoscale friction force images on the monolayer versus bilayer domains, as sensed in contact mode on dry films. Friction, being sensitive to the dissipative responses of the respective film domains, likely reflects differences in molecular packing and possibly polarizability (the subject of ongoing investigations). Intriguingly, the bilayer domains that grow adjacent to as-deposited bilayer domains during high humidity exposure - comprised of molecules that leave the first monolayer and diffuse to form bilayer structures - exhibit definitively reduced friction and increased thickness on the order of Angstroms, implying slight differences in molecular tilt and perhaps reduced disorder.

On the nanoscale many material systems are heterogeneous. Characterization requires high spatial resolution and sensitivity to chemical composition. Scattering scanning near-field optical microscopy (s-SNOM) provides both, chemical sensitivity and nanoscale resolution down to 10 nm. Our s-SNOM instrument is based on an atomic force microscope where the tip is illuminated with a tunable, infrared laser. The tip-scattered light is detected field-resolved in an asymmetric Michelson interferometer allowing for direct detection of the material's absorption¹. We have combined the instrument with peak-force tapping, a technique that allows pN-level force control between tip and sample, e.g. for imaging fragile material systems. Moreover, one can extract valuable nano-mechanical information such as modulus with molecular resolution².
We use our instrument to study amelogenin protein, the primary matrix component in developing dental enamel³. However, amelogenin self-assembly and its role in enamel organization is not well understood. We investigate this self-assembly into ordered, self-aligned, one-dimensional nanoribbons, which occurs in the presence of calcium and phosphate ions. The ordering is similar to the one observed in phosphate-based apatite crystals that provide the extremely hard material in dental enamel. It is hence likely that the bundles form a template for these apatite crystals. To help clarify that open question, mapping the distributions of phosphate and hydroxyapatite nanocrystals within the bundles is necessary. S-SNOM is ideal to simultaneously obtain topography and chemical content of the <30 nm narrow and only a few nm high nanoribbons.
We present nano-absorption data that clearly track the phosphate distribution of the nanoribbons for different growth conditions involving different calcium/phosphate ion concentrations. For instance, we study a phosphorylated peptide derived from amelogenin amino acid sequence that assembles into ribbons without phosphate ions in solution. Here, s-SNOM is used to locate the organic phosphate within the assembly, providing insight on the precise organization of the peptide molecules. While s-SNOM probes the nano-chemistry, peak-force tapping can access nano-mechanical data and is able to distinguish hard crystalline from soft non-crystalline material. We show that for certain growth conditions apatite nanocrystals with higher modulus than the ribbons have not formed yet, highlighting how the combination of s-SNOM and nano-mechanical mapping can help to address important questions in protein self-assembly.
¹ X. Xu, A. Tanur, G. Walker, J. Phys. Chem. A, 117, 3348 (2013).
² B. Pittenger, N. Erina, C. Su, Bruker Application Note, 128 (2011).
³ O. Martinez-Avila et al. Biomacromolecules 13, 3494 (2012).

Planktonic bacteria can be found in nature; however, most types of bacteria live in clusters known as biofilms, which is also formed by a matrix of extracellular polymeric substances (EPS). The most accepted model for this process assumes morphological and phenotypic changes. In order to complement this model, physical and chemical characterization of bacteria and the different types of EPS should be studied. Among the techniques widely used in the study of biological samples, scanning probe microscopy offers high-resolution visualization of microscopic systems and the analysis of some physical properties. However, electrical modes have not been fully explored for this task yet. In this work, we used Kelvin probe force microscopy to study the variations of surface potential (SP) during biofilm formation of the phytopathogen Xylella fastidiosa. We have measured dry samples, with different growth times on gold-coated silicon substrates. The results show heterogeneities on bacterial membrane mainly located at the bacterial pole, which are related to the initial process of polar adhesion. We have also observed, from the electrical point of view, three different types of EPS present at different stages of the bacteria life cycle. The first kind, soluble EPS, gradually covers the substrate surface, increasing SP and facilitating bacterial adhesion. The second to appear is the tightly bound EPS (TB-EPS) and it covers gradually the bacterial body, which then presents a low SP around it, possibly due to chemical bonds formed with soluble-EPS. Finally, we observed a third type of loosely bound EPS (LB-EPS) with high SP that appears primarily on the central region of mature biofilms. We also show differences between these biofilms and those at the dispersion stage; here we observed a large difference between the SP rms variation in the LB-EPS of dispersed biofilms in relation to mature biofilms.

The endothelial cell (EC) lining of the pulmonary vascular system forms a semipermeable barrier between blood and the interstitium and regulates various critical biochemical functions. Collectively, it represents a prototypical biomechanical system, where the complex hierarchical architecture, from the molecular scale to the cellular and tissue level, has an intimate and intricate relationship with its biological functions. We investigated the mechanical properties of human pulmonary artery endothelial cells (ECs) using atomic force microscopy (AFM). Concurrently, the wider distribution and finer details of the cytoskeletal nano-structure were examined using fluorescence microscopy (FM) and scanning transmission electron microscopy (STEM), respectively. These correlative measurements were conducted in response to the EC barrier-disrupting agent, thrombin, and barrier-enhancing agent, sphingosine 1-phosphate (S1P). Our new findings and analysis directly link the spatio-temporal complexities of cell re-modeling and cytoskeletal mechanical properties alteration. This work provides novel insights into the biomechanical function of the endothelial barrier and suggests similar opportunities for understanding the form-function relationship in other biomechanical subsystems.

Since our first paper on nanomechanics of cancer cells from human patients in 2007, the relationship between cell mechanics and physiological function has developed into a field of great interest to the medical and AFM community. In this talk I will present some examples on the use of AFM to study cell stiffness from metastatic cells gathered from Pleural effusions in cancer patients where we compare our data with normal human mesothelial cells. A comparison of cells cultured for a short time was also shown to compare well with cyto-spinning indicating that morphology is not a important factor in determining cell stiffness. We also investigated the effects of green tea extract in the form of catechins, a type of natural phenol and antioxidant and found remarkably that GTE exerts an effect on cytoskeletal actin remodeling and selectively increases cell stiffness in metastatic cells providing further support for the use of GTE as a chemopreventive agent. This also demonstrates the use of cell stiffness measurements to investigate the effects of a variety of drugs.
Along these lines, cisplatin resistant and sensitive ovarian cancer cell lines were investigated as well as the effects of cisplatin treatment on both cell lines. Using cell stiffness and high resolution Stimulated emission depletion (STED) microscopy we were able to correlate our data with the nature of the actin cytoskeleton. Our AFM studies have also revealed the mechanobiology of actin with respect to a variety of binding proteins. Recent biophysical and morphological effects of nanodiamond/nanoplatinum solution (DPV576) on metastatic murine breast cancer cells tracking in vitro changes in cell stiffness and early structural alterations of metastatic breast cancer cells post treatment with DPV576, which may have important implications in the role of nanodiamond/nanoplatinum based cancer cell therapy and sensitization to chemotherapy drugs.
In addition to cell properties, recently exosomes released by cells forming a FedEX system for extracellular communication have become the subject of intense interest as a biomarker for disease. We have pioneered a number of AFM and SMFS studies backed up by FESEM studies to elucidate the role of these sub 100 nm particles.
“Nanomechanical analysis of cells from cancer patients,” Sarah E. Cross, Yu-Sheng Jin, Jianyu Rao and James K. Gimzewski Nature Nanotechnology 2, 780-783 (2007).
"Atomic Force Microscopy for Medicine" in Life at the Nanoscale: Atomic Force Microscopy of Live Cells (ed. Yves Dufrene) S. Sharma and J. K. Gimzewski (Pan Stanford Publishing Pte. Ltd., Singapore, 2011)
“Green tea extract selectively targets nanomechanics of live metastatic cancer cells,” S. Cross, Y. Jin, Q. Lu, J. Rao, J.K. Gimzewski Nanotechnology 22, 215101 (2011)
Correlative nanomechanical profiling with super-resolution F-actin imaging reveals novel insights into mechanisms of cisplatin resistance in ovarian cancer cells S Sharma, C Santiskulvong, LA Bentolila, JY Rao, O Dorigo and J.K. Gimzewski Nanomedicine: Nanotechnology, Biology and Medicine 8 (5), 757-766 (2012)
“Biophysical and morphological effects of nanodiamond/nanoplatinum solution (DPV576) on metastatic murine breast cancer cells in vitro” Ghoneum, A., Huanqi, Z., Woo, J.R., Zabinyakov, N., Sharma, S., and Gimzewski, J.K. Nanotechnology 25, 465101, October 31, (2014)
Analysis of ex vivo cells for disease state detection and therapeutic agent selection and monitoring
JK Gimzewski, SE Cross, Y Jin, J Rao - US Patent 8,652,798, 2014
Nanomechanical biomarkers for disease therapy O Dorig, J Gimzewski, J Rao, S Sharmahellip; - US Patent App. 13/802,080, 2013

Antibody-antigen (Ab-Ag) recognition is the primary event at the basis of many biosensing platforms. In label-free biosensors these events occurring at solid-liquid interfaces are complex and often difficult to control technologically across the smallest length scales down to the molecular scale. Here a molecular-scale technique, such as single molecule force spectroscopy, is performed across areas of a real electrode functionalized for the immuno-detection of an inflammatory cytokine, interleukin-4 (IL-4)¹. The statistical analysis of force-distance curves allows us to quantify the probability, the characteristic length-scales, the adhesion energy and the timescales of specific recognition. These results enable us to rationalize the response of an electrolyte-gated organic field-effect transistor (EGOFET) operated as an IL-4 immuno-sensor. Two different strategies for the immobilization of IL-4 antibodies on the Au gate electrode have been compared: antibodies are bound to i) a smooth film of His-tagged Protein G (PG)/Au; ii) a 6-aminohexanethiol (HSC6NH2) self-assembled monolayer on Au through glutaraldehyde. The most sensitive EGOFET (concentration minimum detection level down to 5nM of IL-4) is obtained with the first functionalization strategy. This result is correlated to the highest probability (30%) of specific binding events detected by force spectroscopy on Ab/PG/Au electrodes, compared to 10% probability on electrodes with the second functionalization. Specifically, this demonstrates that Ab/PG/Au yields the largest areal density of oriented antibodies available for recognition. More in general, this work shows that specific recognition events in multi-scale biosensors can be assessed, quantified, and optimized by means of a nanoscale technique.
[1] Casalini, S.; Dumitru, A. C.; Leonardi, F.; Bortolotti, C. A.; Herruzo, E. T.; Campana, A.; de Oliveira, R. F.; Cramer, T.; Garcia, R.; Biscarini, F. Multiscale Sensing of Antibody-Antigen Interactions by Organic Transistors and Single-Molecule Force Spectroscopy. ACS Nano2015, 9, 5051-5062.

Single cell force spectroscopy (SCFS) has been demonstrated to be a powerful tool for the quantification of cellular adhesion. Typically, SCFS experiments are used to measure the maximum adhesion force between a cell and a surface or another cell. Here, we present a method for quantifying specific integrin-ligand interactions on living cells using atomic force microscopy (AFM) based SCFS experiments. SCFS data from HEK 293 cells expressing α_Mβ₂ leukocyte integrin (HEK Mac-1) and wild-type HEK 293 (HEK WT) cells on surfaces coated with fibrinogen were analyzed to identify specific “rupture events.” The rupture events were classified into 2 types based on the force-load leading up to the rupture. Ruptures with high force load (> 0.2 pN/nm) imply a connection of the integrin with the underlying actin cortex of the cell, while low force load (< 0.2 pN/nm) ruptures result by the formation of a membrane tether. For highly adhesive fibrinogen surfaces, prepared with a coating concentration of 0.6 µg/ml fibrinogen on mica, we found 41% of all rupture events to have a high force load for HEK Mac-1 cells compared to only 9% of rupture events having a high force load for HEK WT data of the same surface. The high force load events in the HEK Mac-1 data showed a median rupture force of 55 pN, whereas the median rupture force of the HEK WT high force load events was 29 pN. After adding monoclonal antibody directed against the α_M subunit of the integrin, HEK Mac-1 cells showed similar rupture force values to that of the HEK WT. Our results indicate that the specific interactions between α_Mβ₂ integrin and fibrinogen are characterized by binding forces with a median of 55pN for loading rates larger 0.2 pN/nm. This analysis demonstrates the ability to quantify specific integrin-ligand interactions within SCFS data.

Alzheimer&’s disease (AD) is a neurodegenerative disease characterized by dementia and memory loss for which no cure or prevention is available. Amyloid toxicity is a result of the non-specific interaction of toxic amyloid oligomers with the plasma membrane. We used Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) to study amyloid fibril formation and interaction of amyloid-beta peptide (1-42) with lipid membrane. Effect of the membrane itself is discussed: we showed that membrane cholesterol creates nanoscale electrostatic domains which induce preferential binding of amyloid peptide to the membrane, while melatonin reduces amyloid-membrane interactions. Using atomic force spectroscopy (AFS) we showed that novel amyloid inhibitors prevent amyloid-amyloid binding, the first step which leads to toxic amyloid aggregates. This demonstrates that AFS can be a useful tool in AD drug screening and development.
References:
E.Drolle, R.M.Gaikwad, Z.Leonenko, Nanoscale electrostatic domains in cholesterol-laden lipid membranes create a target for amyloid binding. Biophys.J. 2012, 103, L27-L29.
B.Moores, F.Hane, L.M.Eng, Z.Leonenko, Kelvin probe force microscopy in application to biomolecular films: Frequency modulation, amplitude modulation, and lift mode, Ultramicroscopy, 2010, 110, 708-711.
E.Drolle, F.Hane, B.Lee, Z.Leonenko, Atomic force microscopy to study molecular mechanisms of amyloid fibril formation and toxicity in Alzheimer&’s disease. J. Drug Metabolism Rev., 2014, 46, 207-223.
F.Hane, S.J.Attwood, Z.Leonenko. Comparison of three competing dynamic force spectroscopy models to study binding forces of Amyloid-β 1-42. Soft Matter, 2014, 10(1): 206-213.
F.Hane, B.Y. Lee, A.Petoyan, A.Rauk, Z.Leonenko, Testing Synthetic Amyloid-β Aggregation Inhibitor Using Single Molecule Atomic Force Spectroscopy. J. Biosensors and Bioelectronics, 2014, 54, 492-498.
F.Hane, E.Drolle, R.Gaikwad, E.Faught, Z.Leonenko. 2011. Amyloid-β aggregation on model lipid membranes: an atomic force microscopy study. J. Alzh. Dis. 26: 485-494.

Single-molecule sequencing methods have been developed to analyze DNA directly without the need for amplification. Here, we present a new approach to sequencing single DNA molecules using atomic force microscopy (AFM). In our approach, four surface-conjugated nucleotides were examined sequentially with a DNA polymerase-immobilized AFM tip. By observing the specific rupture events upon examination of a matching nucleotide, we could determine the template base bound in the polymerase&’s active site. The subsequent incorporation of the complementary base in solution enabled the next base to be read. Additionally, we observed that the DNA polymerase could incorporate the surface-conjugated dGTP when the applied force was controlled by employing the force-clamp mode.
MicroRNAs have attracted great attention in life sciences, because they play important roles in regulating life, and are useful biomarkers for various diseases including cancers. Sensitive detection of a specific microRNA has been challenging, because the conventional tools such as PCR are not applicable. Various new approaches have been developed recently. For example, nanoparticles tethering DNA probes were isolated after capturing a specific microRNA, and the DNA probes were analyzed with or without amplification. We observed that the unbinding force between a DNA/RNA duplex and a hybrid binding domain is about 20 pN, and specific to the duplex. Therefore, scanning a defined area of a capture spot with AFM reveals copy number of the captured microRNA on surface, and provides the concentration of a specific microRNA in a sample solution. In this talk, details of our observation will be presented.