In spite of the growing importance of AFM in materials research, the link between a dynamic AFM “image” and the underlying material structure and properties remains tenuous. In part, this is because of an incomplete connection between the observables in dynamic AFM (probe tip amplitude, phase) on one hand, and material properties [local mechanical response (elasticity, anelasticity, viscosity and plasticity), charge density, magnetic dipole, and topography] and feedback control on the other hand. Without accurate descriptions of tip-sample interactions coupled with probe dynamics, attempts to quantify precisely nanoscale material properties via AFM mapping will remain prone to large uncertainty.
The current state of AFM imaging for polymer morphology is at a cross-road. There now exists a multiplicity of imaging options that take advantage of resonant or non-resonant properties of vibrating cantilevers in contact with surfaces. With respect to mechanically resolved imaging we now have the ability to probe surfaces over a variety of time, force, and displacement regimes. While this is exciting we also are faced with the challenge of determining the best imaging mode for the problem at hand. For example, in order to resolve the phase morphology in a complex blend we may try Force Modulation AFM, TappingModetrade; AFM, quasistatic AFM indentation, HarmoniXtrade; resonance AFM, and PeakForcetrade; Tapping imaging. The last two are newer dynamic methods and may provide quantitative mechanical mapping.
Much of the trial and error approaches to optimizing contrast in AFM imaging of polymer systems are now just that, left to empirical testing. We know that polymeric materials will have frequency and strain rate dependent properties and so we expect that imaging contrast in blends might be improved by understanding and controlling experimental conditions to take advantage of these dependencies. The need therefore is to be able to simulate the various imaging modes on model heterogeneous surfaces where the material property descriptors can be used in conjunction with appropriate contact mechanical models and cantilever dynamic models.
Specifically we have enhanced the continuum based models available through the VEDA suite of simulation modules(1) (http://www.nanohub.com) to now include viscoelasticity and hysteretic surface adhesion interactions (elastic case). Further we are using molecular dynamics to understand time dependence of tip-polymer contacts from first principles calculations.
(1) Gaining insight into the physics of dynamic atomic force microscopy in complex environments using the VEDA simulator. Kiracofe, Daniel; Melcher, John; Raman, Arvind. Review of Scientific Instruments (2012), 83(1), 013702/1-013702/17.
trade; (Trademark of Bruker-Nano)

Atomic force microscopes (AFM) can map the topography of surfaces with sufficient resolution to observe individual atoms. Mechanical property measurements with AFM have evolved from slow force volume to multiple-frequency based dynamic measurements using TappingMode and contact resonance. Recently, real-time control of the peak force of the tip-sample interaction has led to a fundamental change in AFM imaging, providing quantitative mapping of mechanical properties at unprecedented resolution.
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]. 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] M. E. Dokukin and I. Sokolov, Langmuir 28, 16060-71 (2012).
[2] K. K. M. Sweers, K. O. van der Werf, M. L. Bennink, and V. Subramaniam, Nanoscale 4, 2072 (2012).

In many fields including biology, polymer composites and nanomaterials there is high demand for studying of mechanical properties of soft matter on the micro and nanoscale. Force spectroscopy performed by the means of AFM presents fast reliable way of mapping of mechanical properties of the materials with resolution on the order of tens of nanometers. When mapping mechanical properties of soft materials one should consider the changes in the operating conditions (frequency and temperature) under which material is used, since these conditions change the behavior of the material under study. Therefore, more complex analysis of the mechanical properties should be involved to provide complete information about the mechanical properties of the material. Here we show how such analysis could be performed for several well studied polymeric materials. We have demonstrated the way of collection of the viscoelastic properties of the material with force spectroscopy using a constant loading rate. We have employed viscoelastic three-element model and applied it to analyze force-distance curves to get instantaneous and infinite moduli of the material as well as relaxation times in the single measurement. The measurements have been performed on the polymer materials known to present significant viscoelastic properties such as poly(n-butyl methacrylate).

Mechanics of soft materials at the nanoscale is important when studying nano-heterogeneous materials, such as nanocomposites, multi-phased polymers, biomaterials, biological cells, and tissues. Multifrequency AFM methods are capable of measuring the mechanics of soft materials at the nanoscale.
Here we introduce a new low-frequency (up to 500Hz) multi-frequency FT (Fourier transform) dynamic nanoindentation mode (FT-nanoDMA) method to measure both storage and loss modulus of several polymers. This mode allows analyzing viscoelastic (frequency-dependent) properties of materials with a nanoscale probe of AFM by measuring multiple frequencies at the same time, not sequentially as done in the existing nanoindenters. This brings higher spatial resolution and increase the speed of mapping the viscoelastic properties of soft materials -- more than 100x better (both resolution and speed) than the existing technology. In addition, this technique will allow for testing the linearity of strain-stress relation at the nanoscale while doing the measurements (such linearity information is paramount for proper calculation of the rigidity modulus). The method is verified against measurements with DMA and nanoindentation.

Many new drug compounds being developed today, although highly potent, often suffer from poor aqueous solubility which may in turn limit their bioavailability. To increase the kinetic solubility of a drug molecule, one strategy involves formulating the drug in its amorphous state. The amorphous form of the drug is inherently metastable and is thus generally stabilized by means of solubilizing the molecule in a water soluble polymer to form what is known as an amorphous solid-dispersion. The polymer serves as a crystallization inhibitor in the solid-state, can aid in dissolution and potentially prolong supersaturation in by inhibiting crystallization and precipitation. Methods of preparation for these dispersions include spray drying and hot-melt extrusion. In this presentation, the applications of scanning probe microscopy and instrumented nanoindentation to the development of drug compounds formulated as amorphous solid-dispersions is discussed. Specifically, the effects of drug loading, surfactants, humidity (moisture) and temperature on the phase behavior of these systems is explored using scanning probe microscopy. The technique is used to both screen for optimal formulation compositions as well as characterize dispersions prepared on-scale. Additionally, the mechanical properties of these systems are evaluated with nanoindentation to understand how these materials will perform in downstream processes such as milling and tablet compaction.

Thermal Scanning Probe Lithography (tSPL) is an AFM based patterning technique, which uses heated tips to locally evaporate organic resists such as molecular glasses [1] or thermally sensitive polymers [2]. Organic resists offer the versatility of the lithography process known from the CMOS environment and simultaneously ensure a highly stable and low wear tip-sample contact due to the relatively soft nature of the resists. Patterning quality is excellent up to a resolution of sub 15 nm [1], at linear speeds of up to 20 mm/s and pixel rates of up to 500 kHz.[3] In addition, the patterning depth is proportional to the applied force which allows for the creation of 3-D profiles in a single patterning run.[2]
For reliable patterning at high speed and high resolution an efficient control system is essential. Here I will discuss how we implemented a closed-loop lithography control scheme. We obtain the control signals by reading the topography in the retrace motion of the scan, while writing is performed during the trace motion. We use the acquired data to optimize the position stability in vertical direction, the amplitude and offset of the applied writing force and the applied force during reading. Excellent control of all parameters is important for an accurate reproduction of complex 3D patterns. Here we demonstrate that depth levels are reproduced with an error of less than 1 nm.
These novel patterning capabilities are equally important for a high quality transfer of two-dimensional patterns into the underlying substrate. We utilize an only 3-4 nm thick SiOx hardmask to amplify the 8±0.5 nm deep patterns created by tSPL into a 50 nm thick transfer polymer. The structures in the transfer polymer can be used to create metallic lines by a lift-off process or to further process the pattern into the substrate. Here we demonstrate the fabrication of 27 nm wide lines and trenches 60 nm deep into the Silicon substrate. The line-edge roughness is 2.7 nm (3sigma) at a write-pixel-pitch of 9 nm, which enables high throughput fabrication of high resolution structures. The high quality of the transferred patterns is a direct consequence of the 0.5 nm RMS depth control during writing and the high etch selectivity of the SiOx mask. The demonstrated feature density and line-edge roughness fulfill today&’s requirements for mask-less lithography for example for the fabrication of EUV-masks.
Acknowledgment: This work was supported by the Swiss National Science Foundation (SNSF) and by the European Union&’s Seventh Framework Programme FP7/2007-2013 under grant agreement no 318804 (SNM).
[1] D. Pires, J. L. Hedrick, A. De Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U. Duerig and A. W. Knoll, Science 328, 732 (2010).
[2] A. W. Knoll, D. Pires, O. Coulembier, P. Dubois, J. L. Hedrick, J. Frommer and U. Duerig, Adv. Mater. 22, 3361 (2010).
[3] P. Paul, A. Knoll, F. Holzner, M. Despont and U. Duerig, Nanotechnology 22, 275306 (2011).

Plasmonic nanostructures comprising of split-ring resonator arrays are among the most well studied systems in metamaterial research. The plasmonic response of these well understood systems can be efficiently tuned by the design of the split-ring resonator geometry, the meta-atom dimensions and the choice of suitable materials. However, desired responses in the visible range of the optical spectrum require scaling down the device dimensions dramatically. Conventional lithographic approaches require advanced focused ion beam and highest resolution techniques. Electrochemical oxidation lithography on the other hand can address exactly these device dimensions and can utilize chemical interactions to gradually build up the split-ring resonator structures. We will introduce a comprehensive approach which combines theoretical considerations for the required split-ring design, the practical implementation of these design proposals by utilizing probe based structuring techniques with a resolution down to a few ten nanometers.
The lithography process is a probe based scanning force lithography approach which initiates the local chemical activation of an n-octadecyltrichlorosilane (OTS) monolayer by the local application of negative tip bias voltage pulses. These bias voltage pulses result in the formation of polar acid groups which are used as a chemically active template to create metal nanostructures with small dimensions. The presentation will focus on essential challenges to obtain well defined and reliable structures with high precision. This includes the possibility to inscribe a large number of similar split-rings, the improvement of metallization protocols to obtain metallic ring-structures, etc. and practical considerations for the hierarchical assembly of more complex structures, i.e., nanometric gaps,[1] carbon nanotube assemblies[2] or ring structures.[3] Efficient utilization of the oxidation characteristics of the electrochemical oxidation process are here the key to obtain complex nanometric structures.[4] Even though SPM lithography methods are inherently slow the introduced approach is potentially suitable to provide a cost-efficient screening tool for screening applications in a non-clean room environment.
[1] T.S. Druzhinina, S. Hoeppener, U.S. Schubert, Small 2012, 8, 852-857.
[2] T.S. Druzinina, C. Höppener, S. Hoeppener, U.S. Schubert, Langmuir 2013, doi.org/10.1021/la4000878.
[3] T.S. Druzinina, S. Hoeppener, N. Herzer, U.S. Schubert, J. Mater. Chem. 2011, 21, 8532-8536.
[4] D. Meroni, S. Ardizzone, U.S. Schubert, S. Hoeppener, Adv. Funct. Mater. 2012, 20, 4376-4382.

Carbon nanotubes are deposited with demonstrated orientation even at a single, single walled nanotube level using a MultiProbe SPM system with up to 4 independent probes. The deposited nanotubes are then investigated on-line with intermittent contact mode so that even single walled nanotubes are not moved by the scanning operation to characterize their structure. Raman scattering is present on-line to demonstrate the orientation of the deposited nanotubes. The oriented carbon nanotubes can be either accurately localized or contacted with two probes of the multiprobe scanned probe microscopy system. The accurate positioning has considerable potential for a variety of materials and this is also the case for the on-line contacting of such molecular structures for the investigation of their electrical properties. Raman is also employed to monitor the current propagation in such contacted tubes and these results show defined changes in the G band. The writing process, the chemical characterization process and the electrical characterization will be discussed in this paper in detail including the importance of tapping mode in many of these applications where structures can be easily displaced. The platform developed for this application has potential in numerous areas including plasmonics, nanophotonics, nanobiophysics and other optoelectronic molecular device structures.

Chemical composition of materials in the nanoscale is highly desirable yet elusive in the atomic force microscopy community. Recently, an integrated instrument combining atomic force microscopy and infrared (AFM-IR) spectroscopy has been developed which has the ability to collect IR spectral and imaging information below the diffraction limit with a spatial resolution of ~ 100 nm. However, there are still some limitations that prevent its use on many important nanoscale systems. One of the main limitations is the thickness of the sample required for examination (> 100 nm). Overcoming these limitations has a dramatic impact by enabling widespread use of nanoscale IR spectroscopy for spatially resolved chemical characterization. The use of a quantum cascade laser (QCL) as the IR source significantly increases the sensitivity of AFM-IR. Typically, a QCL has repetition rates up to 1000 times higher than previous lasers used for AFM-IR. This excites the sample at the same rate as the resonant frequency of the AFM cantilever, which gives rise to a high IR sensitivity mode referred to as resonance enhanced infrared nanospectroscopy (REINS). Such enhancement would require less sample heating to generate a spectrum, such that samples that were too thin or easily damaged using the conventional AFM-IR technique can now be examined. Using REINS, nanoscale IR spectra and perform chemical imaging can be obtained from films as thin as 15 nm. To broaden the applicability of the AFM-IR technique, the sample preparation requirement is also relaxed and modified to accommodate a wider range of applications.

Recently, a new technique was introduced that couples the nano-scale spatial resolution of atomic force microscopy (AFM) with the ability to characterize chemical species using infrared (IR) spectroscopy. AFM-IR has been reported to chemically identify domains as small as 50 nm in blended samples and 15 nm-size isolated domains, which is at least 2 orders of magnitude better than conventional FTIR methods. Establishment of this novel technique in an industrial environment will be discussed. In the polymer industry, increasingly complex blends are being used to meet customer&’s performance expectations. Refining our understanding of the interactions between polymers in a blended material can help with the intelligent design of new materials. We are currently tackling these questions with AFM-IR and in the process have identified areas which require further technical development to enhance understanding of our results. The practical spatial resolution is important to define since blends of polymers can yield domains much smaller than 100 nm and interfacial chemistry can take place at the very edges of domains. The spatial resolution is governed by several factors including sample thickness, AFM tip dimensions and thermal conductivity of the sample. Samples are prepared by microtoming sections several hundred nanometers thick, and within the individual sample there can be a great variation in height, and therefore thickness, either due to surface roughness or physical contraction of domains after cutting. Both of these circumstances create non-ideal situations and defining spatial resolution is not straight-forward. We will discuss these challenges, as well as thermal drift, which affects where the IR spectra are collected on the sample with AFM-IR. Variations in the spectra could arise from a reaction at the interface, the presence of different species, or simply due to the movement of the tip off of the point of interest, via thermal drift. Overall, IR spectra should be repeatable and reproducible to verify conclusions. Interpretation of AFM-IR data would be aided with incorporation of standards. One example to address IR spectra was conducted by measuring pure components of the polymer blends and comparing to the blended sample measured by AFM-IR as well as traditional FTIR spectra. Statistical analysis of these spectra will be discussed, along with our current understanding of the practical challenges of AFM-IR and our path forward.
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Frequency modulation atomic force microscopy (FM-AFM) has widely been used for atomic-scale investigations on various materials including insulators as well as conductive materials. Although the method has traditionally been used only in vacuum, its operation in liquid with true atomic resolution has recently become possible.
This breakthrough was made by the following three modifications: the use of stiff cantilevers to suppress thermal fluctuation and other instabilities of the cantilever motion, small cantilever oscillation amplitude to enhance the sensitivity to a short-range interaction force, and a low noise deflection sensor to achieve the thermal-noise-limited performance. Contrary to the pre-existing expectations that the atomic-resolution imaging cannot be performed in liquid due to the low Q factor, the breakthrough was made without changing the fundamental performance but improving the instruments and optimizing the operating conditions.
Since then, there has been rapid development in the application techniques of liquid-environment FM-AFM, which has enabled to visualize subnanometer structures of biomolecules and 3D distributions of water and flexible surface structures. On the contrary, the fundamental performance of the technique, such as speed and sensitivity, has remained unchanged. We have been working on the improvement of the fundamental performance to make another breakthrough in the instrumentation of liquid-environment FM-AFM. Here we present some of achievements in this project.
The minimum detectable force (Fmin) is one of the most important parameters to characterize AFM performance. Fmin of the present liquid-environment FM-AFM has been limited by the noise arising from the thermal vibration of a cantilever. It has been expected that the reduction of the cantilever size can lower Fmin. However, it has been difficult to use it in FM-AFM in practice. We found the major difficulty is in the stable cantilever oscillation in liquid. Here, we present a way to overcome this difficulty by using the photothermal excitation method with a thermally-stabilized laser beam source. With the improvement, we experimentally demonstrate seven-fold improvement in Fmin. The improved force sensitivity allows us to visualize detailed hydration structure formed at a calcite/water interface and compare it with a simulated image.
The seven-fold improvement in Fmin implies that we can improve the speed by 50 times without deteriorating Fmin. However, the high-speed operation of AFM also requires enhancement of the bandwidth or the resonance frequency of individual components constituting the tip-sample distance feedback loop. To achieve this goal, we have developed a separate-type high-speed scanner and a low-latency frequency shift detector. Combining the developed system with the small cantilever, we demonstrate in-situ imaging of crystal growth process of calcite in water at a speed of 1 frame/sec with atomic-scale resolution.

Nanoscale structures and functions of soft materials such as polymer membranes have been studied for their biological and medical applications. At the interface between soft materials and water, flexible parts of the surface structure and mobile water molecules show thermal fluctuations to present non-uniform three-dimensional (3D) spatial distribution. Although such 3D interfacial structures have importance in understanding the functions of soft materials, nanoscale measurement techniques for investigating them have not been established.
We have recently developed a technique referred to as 3D scanning force microscopy (3D-SFM). In the method, an AFM tip is scanned in Z as well as in XY directions to probe the whole 3D interfacial space. Combined with atomic-resolution frequency modulation AFM (FM-AFM), the method allows us to record 3D distribution of frequency shift (Δf) induced by the tip-sample interaction force at solid/liquid interfaces. Previously, we reported that 3D distributions of mobile water molecules are directly visualized at mica/water interface by 3D-SFM.
In this study, we have investigated a hexa(ethylene glycol) (EG6)-terminated self-assembled monolayer (EG6-SAM) by FM-AFM and 3D-SFM in liquid. The EG6-SAM has widely been used as a model surface for studying the inhibitory property of soft materials against non-specific biomolecular adsorption. To reveal the molecular-scale origin of the inhibitory effect, understanding of the conformation of flexible EG6 chains and the distribution of water molecules at the interfaces is of critical importance.
The FM-AFM images of the EG6-SAMs show a closely-packed hexagonal arrangement with a molecular spacing of ~0.5 nm. The observed molecular-scale contrast is similar to the one observed at the surface of OH-SAMs. The results reveal that the molecules show a well-ordered molecular arrangement in spite of the high flexibility of the EG6 chains.
The 3D-SFM images obtained on EG6-SAMs and OH-SAMs show similar hydration structures. Although we cannot find significant difference in the XY cross sections of the 3D-SFM images, the averaged force curves show clear difference. The long-range repulsive force appears only at the EG6-SAM/water interface, which may explain the higher inhibitory activity of EG6-SAMs against molecular adsorption compared with that of OH-SAMs. We also discuss possible molecular-scale origins for the observed long-range repulsive force based on the observed 3D-SFM images.

Surface science techniques, and particularly high-resolution Scanning Probe Microscopy (SPM) approaches, now offer unprecedented levels of understanding and control of solid/vacuum interfaces. By contrast, the physics of liquid/solid interfaces is less developed, although it is often more relevant for real-world applications. It is important in such diverse fields as heterogeneous catalysis, next generation battery technology and corrosion. The solid/liquid interface is also particularly relevant to biological systems, where measurements are made in physiological conditions.
In this work we briefly review our general approach for simulating high resolution scanning probe microscopy in a wide variety of systems [1], but then focus on recent results from studies of solid-liquid interfaces. We apply a combination first principles and atomistic simulation approaches [2] to study how water interacts with a variety of insulating and organic surfaces, providing atomic-scale insight into hydration structures, dissolution and high-resolution imaging.
[1] Phys. Rev. Lett. 109 (2012) 146101; Adv. Mater. 24 (2012) 3228; Phys. Rev. Lett. 107 (2011) 036102; Adv. Mater. 23 (2011) 477
[2] J. Chem. Theo. and Comp. 9 (2013) 600, J. Chem. Phys. 138 154703 (2013)

Recent progress in dynamic force microscopy techniques operating in liquids has opened the possibility for us to explore the liquid-solid interfaces, which play important roles in a wide variety of physical, chemical, and biological processes. We developed a low-noise and low-thermal-drift atomic force microscope (AFM) and demonstrated atomic resolution imaging of various inorganic samples (mica, graphite, etc) and molecular resolution imaging of biological samples such as proteins and DNA in liquids, employing frequency-modulation atomic force microscopy (FM-AFM). Moreover, the state of the art two-dimensional (2D) and three-dimensional (3D) force mapping techniques with FM-AFM now allow us to collect the 2D and 3D frequency shift and force data at the liquid-solid interface. By precisely analyzing the oscillatory hydration force or the electric double layer force at the liquid-solid interface, we can even visualize molecular-scale hydration structures and charge distribution at the interface.
In this presentation, we will review the technical details of FM-AFM and 2D/3D force mapping techniques and present some recent exploration results of the liquid-solid interfaces of the ionic crystal surfaces and their saturated solution (calcite, alkali halides, etc). We will also share the tips for quantitative force measurements and separation of conservative and dissipative forces using FM-AFM.
[1] K. Kobayashi, N. Oyabu, K. Kimura, S. Ido, K. Suzuki, T. Imai, K. Tagami, M. Tsukada, and H. Yamada, J. Chem. Phys. 138, 184704 (2013).
[2] A. Labuda, K. Kobayashi, K. Suzuki, H. Yamada, and P. Grütter, Phys. Rev. Lett. 110, 066102 (2013).

nc-AFM in liquid environment offers the potential of visualization of individual molecules in real space under physiological environments at atomic resolution. Gypsum, a crystal of calcium sulfate dihydrate, has a monoclinic crystal structure and cleaved readily parallel to its (010) lattice plane . This plane is parallel to double sheets of water molecules between which there is only weak H-bonding. Although the growth of gypsum has been intensively studied by AFM in various environments because of its atomic flatness and inertness but true atomic-resolution imaging of gypsum surface at room temperature is still extremely difficult owing to its solubility in water. The sample used was taken from a large single crystal geological specimen of good clarity. Experiments were performed in pure water using a High Resolution Atomic Force Microscopy (AFM) system from Nanomagnetics Instruments Ltd. A gypsum substrate was cleaved by an adhesive tape in air and immersed in pure water. The dissolution of gypsum in water was simultaneously observed where the atomic terraces and atomic lattice were obtained.
*The authors would like to thank TUBITAK 2218 National Post Doctoral Research Scholarship.

Local potential distribution at a solid/liquid interface plays important roles in various chemical and biological processes. To understand the mechanism of these processes, it is important to directly measure local potential distribution in liquid. Although Kelvin probe force microscopy (KFM) has been used for local potential distribution measurements in air and vacuum, KFM cannot be used in liquid due to electrochemical reactions and redistribution of ions and water caused by the application of ac and dc bias voltages between a tip and sample. These phenomena generate uncontrollable spurious forces, which disturb the stable operation of KFM in liquid.
Recently, we have overcome these problems and developed a method to measure local potential distribution in liquid, which is referred to as open-loop electric potential microscopy (OL-EPM). In OL-EPM, sum of two different ac bias voltages with relatively high modulation frequencies (f1 and f2) are applied between a tip and a sample. Owing to the slow time response of electrochemical reactions and redistribution of ions and water, the application of ac bias voltages with high modulation frequencies does not cause the problems. Potential values are calculated from the ac bias voltage at f1 (Vac) and the amplitudes of the cantilever oscillations at f1 and fL (=f1-f2) (A1 and AL, respectively) induced by the ac bias voltages. Thus, combined with atomic force microscopy, surface structure and local potential distribution at a solid/liquid interface can be imaged simultaneously.
In OL-EPM, we apply an ac bias voltage between the tip and the sample surface. Thus, it has been difficult to measure potential distribution on an insulating substrate. If we prepare a bottom electrode underneath the substrate, we can apply an ac bias voltage between the electrode and the tip. However, it has been difficult to estimate what percentage of the voltage applied between the tip and the bottom electrode is actually applied between the tip and the sample surface. This problem may also matter even in the measurements on a conductive substrate if any insulating materials are on the surface.
In this study, we have developed a method for estimating the damping ratio of the applied ac bias voltage and calibrating the measured potential values. In the method, we vary the electrochemical potential of the tip using a potentiostat and measure the induced change in A1 signal. The A1 versus potential curve typically shows a V-shape profile, from which we can estimate the damping ratio and calibrate the potential values. With the developed technique, we demonstrate nanoscale potential measurements of DNAs on mica in buffer solution.

The structure and dynamics of liquid molecules close the surface of solids determines the outcome of countless phenomena, ranging from bio-molecular function to self-assembly processes and electrochemistry.
Recent advances in the field of atomic force microscopy (AFM) [1-2] have shown than when operated dynamically in liquid and with small vibration amplitudes (<2nm), AFM can provide sub-nanometer images of the solvation structures formed by the liquid close to the surface of the solid.
In parallel, developments in multifrequency AFM (MF-AFM) [3-5] have established that in liquid, the non-linear tip-sample interaction can induce momentary excitation of higher harmonics during the tip oscillation cycle. This excitation is related to the solid&’s viscoelastic properties and can be exploited to gain compositional contrast over the solid [5].
The main difference between these two regimes is the amplitude of oscillation of the vibrating tip. Here, the interplay between the solvation forces at the interface, the anharmonicity of the tip motion and the imaging resolution is examined as a function of the tip vibration amplitude A. The results, confirmed by computer simulations, show that the thickness d of the solvation region is a key parameter to determine the imaging regime: For Ad the tip trajectory becomes rapidly anharmonic and the resolution deteriorates due the tip tapping the solid. A non-linear transition between the two regimes occurs for A~d and can be quantified with the second harmonic of the tip oscillation.
References:
1. Fukuma, et al., Phys. Rev. Lett. 104, 016101 (2010)
2. Voïtchovsky, et al., Nat. Nanotechnol. 5, 401 (2010).
3. Garcia &. Herruzo, Nat. Nanotechnol. 7, 217 (2012).
4. Xu et al., Phys. Rev. Lett. 102, 060801 (2009)
5. Payam et al., ACS Nano 6, 4663 (2012).

Local ionic, electronic, and electrochemical phenomena at the solid-liquid interface are central to fields such as biological and energy research. The small length scales at which these phenomena take place require techniques capable of probing the solid-liquid interface at the nanometer scale. Kelvin probe force microscopy (KPFM) is a widely used method for measuring local electrochemical potentials under vacuum and ambient conditions. The successful implementation of Kelvin probe force microscopy (KPFM) in liquid environments offers the potential to finally unravel the mechanisms of important processes at the solid-liquid interface including adsorption, electronic transfer, and catalysis. KPFM operation in polar liquid environments, however, is complicated by the presence of mobile ions, which can lead to the convolution of materials and system responses with the measurement timescale and to irreversible electrochemical processes. Here, we develop and implement multidimensional (M)-KPFM to probe both the time and bias dependence of tip-surface interactions on model systems. We further establish M-KPFM as an imaging mode, allowing visualization of the spatial variability of material-dependent local electrostatic and electrochemical behavior in polar liquids.

Bimodal force microscopy is a dynamic force-based method with the capability of mapping simultaneously the topography and the nanomechanical properties of soft-matter surfaces and interfaces. The operating principle involves the excitation and detection of two cantilever eigenmodes. The method enables the simultaneous measurement of several material properties. A distinctive feature of bimodal force microscopy is the capability to obtain quantitative information with a minimum amount of data points. Furthermore, under some conditions the method facilitates the separation of the topography data from other mechanical and/or electromagnetic interactions carried by the cantilever response. This presentation aims to give an overview of the current developments and applications of bimodal AFM.
Recent references:
E. T. Herruzo, H. Asakawa, T. Fukuma, R. Garcia, Nanoscale 5, 2678 (2013);
H. V. Guzman, A.P. Perrino, R. Garcia, ACS Nano 7, 3198 (2013); R. Garcia and E. T. Herruzo, Nat. Nanotechnol. 7, 217-226 (2012).

Since the advent of atomic force microscopy, cantilevers have predominantly been driven by piezos for AC imaging and data acquisition. The ease of use of the piezo excitation method is responsible for its ubiquity. However, the well-known “forest of peaks”, which is clearly observed while tuning a cantilever in liquids, renders AC imaging in liquids problematic because the peaks move around with time. Effectively, these shifting peaks result in a setpoint that changes with time causing stability problems while AFM imaging. Furthermore, the same “forest of peaks” prevents the quantitative interpretation of forces in liquids[1], air[2], and vacuum environments[3], even if the cantilever tune looks clean. Dissipation studies in all these environments have especially suffered due to piezo excitation of the cantilever.
Photothermal excitation is an alternative method for exciting a cantilever by heating/cooling the base of the cantilever to drive the cantilever. Photothermal excitation results in a repeatable, accurate and time-stable cantilever tunes, as seen in the Figure. Therefore, the setpoint remains truly constant while imaging, preventing tip crashes, or unwanted tip retractions. A true atomic resolution image of calcite in water, shown in the inset of the Figure, were made for hours with no user intervention, testifying to the stability of photothermal excitation. Unlike other specialized drive methods, photothermal excitation is compatible with almost any cantilever and with all AFM techniques. The introduction of a blue laser into the AFM also enables several other functionalities, such as tuning the temperature of the cantilever. Furthermore, because the photothermal tune represents the true cantilever transfer function, existing AFM theories can be applied to accurately recover conservative and dissipative forces between the tip and the sample. This is especially important for force spectroscopy, dissipation studies, as well as the frequency modulation AFM techniques.
Our recent developments in perfecting photothermal excitation and its benefits to the AFM community will be discussed in this talk.
[1] A. Labuda, K. Kobayashi, et al. AIP Advances 1, 022136 (2011)
[2] R. Proksch and S. V Kalinin, Nanotechnology 21, 455705 (2010)
[3] A. Labuda, Y. Miyahara, et al. Phys. Rev. B 84, 125433 (2011)

Since 2004, Multifrequency Atomic Force Microscopy (MF-AFM) has emerged as a powerful surface characterization tool that allows users to obtain compositional contrast simultaneously with topographical imaging. The original MF-AFM method, introduced by Garcia and coworkers [T. Rodriguez and R. Garcia, Appl. Phys. Lett. 84, 449 (2004); N.F. Martinez et al., Appl. Phys. Lett. 89, 153115 (2006)], utilized the fundamental cantilever eigenmode to measure the sample topography through the amplitude-modulation scheme, while the second eigenmode was driven in open-loop mode to map the surface properties. In essence, the method created separate “control knobs” to optimize topographical acquisition and compositional mapping. In 2010 we proposed a trimodal (triple-frequency) extension of this technique by driving one additional eigenmode using either the frequency-modulation or open-loop schemes [S.D. Solares and G. Chawla, J. Appl. Phys. 108, 054901 (2010); Meas. Sci. & Technol. 21, 125502 (2010)]. We described the relationship between the expected contrast of the two higher eigenmodes, depending on the control scheme used for each of them, and provided general imaging guidelines and physical insight, but it was not immediately clear whether there were any advantages in driving three eigenmodes simultaneously. Not only was it not clear what each of them could be used for, but we also identified additional dynamic and controls complexities which could, depending on the conditions, preclude stable imaging. This talk focuses on the use of trimodal AFM in a way that each eigenmode serves as a separate control knob for the optimization of topographical imaging, compositional mapping and sampling depth, respectively, while performing stable and repeatable imaging. The talk includes a discussion of the underlying theory, controls schemes and recommended parameters and setup, as well as computational and experimental results illustrating the application of the method. This approach is ideal for the characterization of samples with sub-surface features and depth-dependent properties, since it allows the user to easily modulate the level of tip penetration into the sample in subsequent scans or successive (duplicate) scan lines, while mapping topography and composition, thus gradually revealing “buried” features.

Atomic and nanoscale interactions give rise to the rich diversity of phenomena that can be found in the macroscale world. The origin of such diversity is intimately connected to even small variations in range and magnitude of conservative and dissipative forces that are ultimately encoded in what could be termed the force profile. Here, we enhance the capabilities of the atomic force microscope to show that the force profile can be reconstructed without restriction by monitoring the wave profile of the cantilever during a single oscillation cycle. Two approaches are provided to reconstruct the force profile in both the steady and transient states in what we call single cycle measurements. The instantaneous force is accurately reconstructed thus capturing the details of conservative and dissipative phenomena. These include a broad range of phenomena from the formation and rupture of bonds and atomic reorientation to the local detection and probing of water molecules. With single cycle measurements, we add high temporal resolution (possibly microsecond range) to the impressive spatial resolution of AFM, to study kinetic processes. The access to simultaneous high throughput and high sensitivity further opens the door to a variety of feedback options for imaging with the potential of adding higher spatial resolution and speed.

Accurate and precise measurement of cantilever spring constant (k) and sensitivity is essential for any quantitative force measurement in Atomic Force Microscopy. This is particularly vital for accurate nanomechanical measurements. Spring constants are intrinsic properties of the cantilever which are not subject to change based on instrumentation, while sensitivity - a measure of the optical lever which relates the physical displacement of the cantilever, in meters, to the measured beam deflection, in volts - is determined primarily by instrumentation and experimental setup. Several calibration methods of varying precision, accuracy, duration and complexity have been developed and described elsewhere. Amongst these, the more widely used are the Cleveland added mass method, the reference spring method, and the thermal noise method. Common to each of these methods is that the cantilever must be physically touched: either by a mass, another cantilever, or a hard surface. Consequently these procedures can render a cantilever unusable after measurement or can damage fragile levers before experimental data are even collected. Further, a desire to avoid damaging the tip may preclude calibration of functionalized or chemically modified levers until after the experiment is complete, which is non-ideal.
A recent method developed by Sader and colleagues allows for calibration of levers without the need for mechanical contact. Briefly, a thermal noise spectrum is taken and the Q and the resonance frequency of the cantilever are measured. For rectangular levers, using the known viscosity of the measurement medium and the known plan (XY) dimensions of the cantilever, the k and the sensitivity of the lever can be determined. For other lever shapes, calibration factors can be pre-determined for a given cantilever type by, for example, measuring the Q and resonance frequency of a representative lever as a function of pressure or viscosity. This expands the method&’s applicability to several popular and commercially available cantilevers. In this work we present our implementation of this contact-free method for sensitivity and k calibration. Data comparing the thermally calculated sensitivities and the measured ones will be presented.

In amplitude modulation atomic force microscopy (AM AFM) the irreversible loss of energy that takes place when a nanoscale tip oscillates over a surface can be quantified reliably [1]. One can identify two distinct dissipative channels related to viscous and hysteretic forces [2][3]. Here, we report the simultaneous experimental mapping of conservative forces and the fine distinction between viscous and the more elusive hysteretic and long-range capillary dissipative interactions [4]. The conservative force versus tip-sample-distance field is reconstructed by means of the formalism proposed by Sader-Jarvis-Katan [5] while the presence of viscous and complex hysteretic dissipative interactions as a function of tip-sample distance is identified with the use of recently proposed methods and variation of these [4]. Furthermore, our capability of controlling and in situ monitoring the effective tip radius allows investigating the variations in nanoscale interactions as a function of tip size in addition to other operational parameters [6]. In the short-range, we provide experimental evidence of a tip size dependent transition from viscous prevalent to hysteretic prevalent dissipation [7]. In the long-range, dissipation related to capillary interactions is discussed in terms of observables and by interpreting long-range dissipation as a function of tipminus;sample distance and the energy dissipation expression [7]. In particular, by numerically integrating the equation of motion, we show that the onset of hysteretic capillary dissipation can be related to a discrete step in both signals. Experiments conducted on freshly cleaved mica samples have provided evidence of such steps. We also show experimentally that energy dissipation increases with tip size and relative humidity in the long-range. These findings should pave the road for future developments in the field in terms of the establishment of nanoscale laws and their size dependencies.
[1] Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B., Appl. Phys. Lett. 1998, 72 (20), 2613minus;2615.
[2] Garcia, R.; Gomez, C. J.; Martinez, N. F.; Patil, S.; Dietz, C.; Magerle, R. Phys. Rev. Lett. 2006, 97, 016103minus;4.
[3] Santos, S.; Gadelrab, K.; Barcons, V.; Stefancich, M.; Chiesa, M., New J. Phys. 2012, 14, 073044.
[4] Gadelrab, K. R.; Santos, S.; Chiesa, M., Langmuir 2013, 29 (7), 2200-2206.
[5] Katan, A. J.; van Es, M. H.; Oosterkamp, T. H., Nanotechnology 2009, 20, 165703minus;165711.
[6] Santos, S.; Guang, L.; Souier, T.; Gadelrab, K. R.; Chiesa, M.; Thomson, N. H., Rev. Sci. Instrum. 2012, 83, 043707minus;043717.
[7] S. Santos, C. A. Amadei, A. Verdaguer and M. Chiesa, The Journal of Physical Chemistry C 117 (20), 10615-10622 (2013).

A ubiquitous problem for Atomic Force Microscopy (AFM) is that optimal accuracy demands smooth surfaces, slow scanning, and expert users, contrary to many AFM applications and practical use patterns. A simple correction to AFM topographic images has thus been developed, incorporating simultaneously acquired deflection and/or amplitude signals that have long been available but were underexploited. This is shown to substantially improve both height and lateral accuracy for expert users, with a decrease in image error of 3-5x. Common image artifacts due to inexperienced AFM use, poorly scanned surfaces, or high speed images acquired in as fast as 7 seconds are also all shown to be substantially corrected, yielding results equivalent to ‘expert-user&’ images. This concept is demonstrated for contact AFM, AC mode, and high speed imaging, as well phase contrast and other property mapping variations of AFM, and is easily employed for future as well as legacy AFM systems and data. Error Corrected AFM therefore provides a widely applicable approach for more accurate, efficient, and user-independent scanning probe micrsocopy.

In our previous works we have shown that the structure of some thin-film surfaces is “frozen” self-organized system. It concerns all other spatial scales: from macro to nano levels. That&’s why traditional approach to the analysis of this system in terms of statistical characteristics such as spatial spectrum, correlation scale doesn&’t give the information about its deterministic origin. Fourier analysis turns out also to be non-effective as Fourier spectra don&’t contain useful resonance frequencies. So it turns out that AFM and STM imaging of the surfaces are often have the value simply as visual images which don&’t have clear mathematical and physical interpretation.
In our work we have presented the results of the analysis of AFM and STM imaging of the surfaces of different materials with the help of the methods of non-linear dynamics. We have developed the algorithm of the calculation of the average mutual information (AMI) for two-dimensional surfaces on the non-directional vector and the circumference. (Actually AMI characterizes the correlations in the complex non-linear systems). We have also used well-known in nonlinear dynamics Takens&’ approach for the analysis of the sequence in the structure. With the help of it we can develop: the type of the surface (regular, chaotic, random), the dimension of phase space, fractal dimension (FD) of attractor, Lyapunov exponents and other invariants.
Silicon-based films, carbon, gallium arsenide, tungsten surface profile obtained by the scanning tunneling microscopy and atomic power microscopy were the objects of the investigation. On the pictures AMI we have found different forms of ordering. We have discussed their physical nature. We can&’t find these forms with the help of any other methods.
From the analysis which was done with the help of Takens&’ approach we have developed FD of the surfaces. We have found that the surfaces structure has complex, determined chaotic character with many levels, and can be described by a limited number of order parameters. The nature of various levels of organization is being discussed. By these results the algorithms for direct modeling of nano- and microstructures and the control of growth processes were processed. The analytical connection between parameters of structure and dynamical characteristics of solidification is established.

In order to fabricate complementary circuits and solar cells from organic semiconductors, both p- and n-channel materials are needed. There are comparatively fewer known n-channel materials than p-channel materials, in large part due to the difficulty of stabilizing the radical anion in the presence of water and/or oxygen. Moreover, very little is known about the microscopic nature of charge trapping, kinetics of trap formation and trap clearing, charge mobility, and charge injection in n-channel organic semiconductors. In this study, we use electric force microscopy to examine the spatially-resolved and wavelength-resolved kinetics of charge trapping and de-trapping [1,2] and measure the local mobility [3] in working perylene diimide transistors. We find that traps in perylene diimides can be cleared with light, revealing the optical absorption of the chemical species assisting in trap neutralization, and that the mobility is charge density dependent.
[1] Jaquith, M.; Muller, E. M. & Marohn, J. A. Time-Resolved Electric Force Microscopy of Charge Trapping in Polycrystalline Pentacene J. Phys. Chem. B, 2007, 111, 7711-7714.
[2] Luria, J. L.; Schwarz, K. A.; Jaquith, M. J.; Hennig, R. G. & Marohn, J. A. Spectroscopic characterization of charged defects in polycrystalline pentacene by time- and wavelength-resolved electric force microscopy Adv. Mater., 2011, 23, 624-628.
[3] Burgi, L.; Sirringhaus, H. & Friend, R. Noncontact potentiometry of polymer field-effect transistors Appl. Phys. Lett., 2002, 80, 2913 - 2915.

Several studies have shown the potential for using scanning probe microscopy (SPM) as a tool to investigate various forensic evidences collected at a crime scene[1]. Bloodstains are one of the most common evidences that can be collected and AFM provides a valuable tool to study the structural integrity of blood in order to estimate the precise point of time when the crime took place distinguishing between the victim and the perpetrator [1-4]. The current methods rely on measuring the change of blood&’s elasticity [4] on random spots on different blood components to derive the aging of the sample and thus recovering the time of the event, but unfortunately the results suffer of large deviation. In this study, we enhanced the accuracy of blood age estimation by monitoring the change in elasticity of one single component of the bloodstain. Furthermore we map simultaneously the conservative forces [5] and we discriminate between viscous and more elusive hysteretic dissipation interaction [6] as a function of aging time. The conservative forces are reconstructed with the use of the Sader-Jarvis-Katan [7] formalism, while dissipative interactions are monitored and identified with the use of recently proposed methods and variations of these [8, 9].
References
1. Chen, Y.-f., Forensic Applications of Nanotechnology. Journal of the Chinese Chemical Society, 2011. 58(6): p. 828-835.
2. Bremmer, R.H., et al., Forensic quest for age determination of bloodstains. Forensic science international, 2012. 216(1-3): p. 1-11.
3. Griffin, T.J., Principles of Bloodstain Pattern Analysis: Theory and Practice. Journal of Forensic Identification, 2006. 56(3): p. 435-437.
4. Strasser, S., et al., Age determination of blood spots in forensic medicine by force spectroscopy. Forensic science international, 2007. 170(1): p. 8-14.
5. Payam, A.F., J.R. Ramos, and R. Garcia, Molecular and Nanoscale Compositional Contrast of Soft Matter in Liquid: Interplay between Elastic and Dissipative Interactions. ACS Nano, 2012. 6(6): p. 4663-4670.
6. Sergio, S., et al., Quantification of dissipation and deformation in ambient atomic force microscopy. New Journal of Physics, 2012. 14(7): p. 073044.
7. Allard, J.K., H.v.E. Maarten, and H.O. Tjerk, Quantitative force versus distance measurements in amplitude modulation AFM: a novel force inversion technique. Nanotechnology, 2009. 20(16): p. 165703.
8. Gadelrab, K.R., S. Santos, and M. Chiesa, Heterogeneous Dissipation and Size Dependencies of Dissipative Processes in Nanoscale Interactions. Langmuir, 2013. 29(7): p. 2200-2206.
9. Karim, R.G., et al., Disentangling viscosity and hysteretic dissipative components in dynamic nanoscale interactions. Journal of Physics D: Applied Physics, 2012. 45(1): p. 012002.

Atomic force microscopy (AFM) has enabled the study of nanoscale friction of a vast array of materials. Nanoscale frictional properties of silicon oxide (silica) surfaces are particularly interesting both in geophysics and industry. From a geophysical point of view, since silica is the main component of rocks, thus, studies of silica friction at the nanoscale could help establish physical basis for the current descriptions of rock friction at larger scales, namely “rate and state friction”, a widely-used law whose basis is mostly empirical.
In this study, we explore the complex time- and rate-dependent friction of silica-silica contact using an oxidized silica AFM probe and a silica substrate under controlled humidity. A novel time-dependent static friction measurement scheme is employed. We use “slide-hold-slide” tests, where friction is measured after holding the silica surfaces in contact for a defined period. We find that the friction drop (the maximum static friction force minus steady-state friction force) increases with the hold time (“ageing”) and the dependence is logarithmic[1]. This is consistent with the idea that interfacial chemistry, namely siloxane bridge formation, is occurring at the interface. The logarithmic dependence on time indicates that the reaction rate is affected by the strain-dependent local structure, consistent with recent atomistic simulations[2] and in contrast to the competing hypothesis that the behavior is a result of plastic deformation.
To further examine the chemical ageing hypothesis, we examine the load dependence and the dynamics of friction. We find that the friction drop increases with applied load for all hold times. These results enable the pressure-dependence of the siloxane bond formation mechanism to be estimated for the first time. Furthermore, we observe nanoscale periodic stick-slip events during sliding of these amorphous surfaces, with the behavior depending strongly on the sliding speed. This is used to further study the dynamics of frictional ageing.
Finally, using lateral force modulation, we explore variation of the lateral contact stiffness as a function of both shear and normal loading. This provides further insights into the mechanisms of frictional ageing, the manner in which chemical bond formation is affected by applied stress, and the way in which shear and normal loadings couple to affect the mechanical behavior of nanoscale contacts.
1. Li, Q., Tullis, T.E., Goldsby, D. and Carpick, R.W. "Frictional Ageing from Interfacial Bonding and the Origins of Rate and State Friction," Nature, 480, 2011, 233-236.
2. Liu, Y. and Szlufarska, I. "Chemical Origins of Frictional Aging," Phys. Rev. Lett., 109, 2012, 186102.

This study forced the relationship between the microscopic tribological properties of pure magnesium and the deformation behaviors in grain interior of the magnesium after scratch process from the viewpoints of micro-tribology and metallurgy. Micro-tribology tests were conducted using a nano-scratch tester with a Berkovich tip under 500 µN and the indentation depth was about 200 nm. The average grain size of the magnesium was less than 1 mm and its surfaces for the tests were previously polished as fine as 5-7 nm Ra.The crystal orientations of the surfaces were checked by means of EBSD before the scratch tests.To investigate the deformation behavior during the scratch tests, the microstructural observations were performed by means of TEM for the cross-section pieces cut by FIB.As for the surface with crystal orientation of {0001}, a horn-shaped shadow was observed by TEM on the side of the scratch track even after the single scratch test, and electron diffraction pattern proved that the shadow was {10-12}-type deformation twins induced during the scratch process.

The AC impedance spectroscopy is a common way to characterized properties of various materials including inorganic, organic, and biomolecular substances. Comparing to the DC resistance measurement, the AC impedance can be used to characterize insulating or capacitive substances. When the AC impedance spectroscopy is combined with a conductive atomic force microscopy (c-AFM), this system can be applied to characterize the nanoscaled materials using c-AFM tip as an electrode. In this study, protein-immobilized nanoparticles were examined using an AC impedance spectroscopy combined with AFM. Specifically, some protein-immobilized polystyrene nanoparticles were prepared and their impedance spectra were obtained. From the Ac impedance spectroscopy, characteristic impedance spectra were obtained from every protein-immobilized nanoparticle such that the impedance spectra can be used as an index identifying the protein immobilized. Further analyses on the impedance spectra using an electrical circuit model revealed that every protein had its own characteristic electrical component such as capacitance and parasitic inductance. Exploiting the characteristic impedance spectrum signal, blind test for protein identification was conducted leading to ~ 80% accuracy. This study demonstrated that the AC impedance spectroscopy combined with c-AFM is a promising tool to sensor diverse non-conductive biomolecules and nanoparticles.

Acid-base sites on solid surfaces at the nanoscale can be determined using Kelvin force microscopy (KFM) under controlled atmospheric humidity, from metals and insulators. Surface potential for these materials changes with relative humidity (even within an electrically shielded and grounded environment) but following different patterns that depend on the acid-base surface properties: acidic surfaces (from magnesium sulphate, iron oxides on iron surface, aluminum oxide on aluminum surface, silica and cellulose) become more negative under higher humidity, due to excess OH- ion adsorption relative to H+ ions. On the other hand, basic surfaces (from calcium oxide, magnesium oxide, nickel oxide on nickel-iron alloy and aluminum phosphate) become more positive, due to preferential capture of H+ ions from vapor. These results verify a recent hypothesis: moist atmosphere is a charge reservoir for solids and liquids, where charge transfer is coupled to water vapor adsorption, including water molecule clusters and related complex species incorporating other atmospheric gases. Therefore, KFM under controlled relative humidity is a new possibility to determine and map acid-base sites with great advantages: it uses only one amphoteric and simple reagent (water vapor) and it is intrinsically endowed with high spatial resolution. These features are currently not found in any other method for catalyst and absorbent acid-base site characterization.

Recent works [1,2] have shown that Kelvin Probe Force Microscopy (KPFM) allows the separation of surface and sub-surface features in different channels of cantilever vibration. These methods have been systematically applied to image sub-surface percolating networks of single walled carbon nanotubes embedded in polymers [2]. A fundamental question in KPFM based sub-surface imaging methods is how deep is it possible to see below the surface? Here we study experimentally the limits on depth sensing for sub-surface imaging of carbon nanotube (CNT) polymer composites.
A polymerization in-situ method is used to fabricate single-walled CNT/Polyimide (SWCNT/PI) composites. After the polymerization is achieved, the material is spin-coated on a conductive substrate and cured with a slow isothermal process to obtain a smooth composite film. A set of ten PI thin film layers, of 100 nm thickness each, is deposited on top of the sample. KPFM sub-surface imaging is performed systematically every time a PI layer is deposited to study the limit of depth sensing. In addition the KPFM measurements are performed as a function of the tip-sample voltage (in the range of 1-8 V) and distance (in the range of 10-30 nm).
The results show a decrease in the contrast between the CNTs and the polymer with the increase on the number of PI layers on top of the composite. Based on the part;C/part;z images, we conclude that the limit of SWCNT detection within the PI is found in the fifth layer of PI (500 nm thickness) where the contrast is no longer detected. The part;C/part;z measurements dependent on the tip-sample voltage indicate the presence of an optimum contrast point in the ranges of 3-4V, a minimum contrast point at 1V and a cross taking effect above 6V. Additionally we report the experimental part;C/part;z sensitivity of the applied KPFM technique. This work aims to help establish a methodology to determine the depth of nano-fillers within polymeric matrixes using electrical techniques based on dynamic AFM.
[1] S. Magonov and J. Alexander. Single-pass kelvin force microscopy and dC/dZ measurements in the intermittent contact: applications to polymer materials. Beilstein Journal of Nanotechnology , 2:15-27, 2011.
[2] M. J. Cadena, R. Misiego, K. C. Smith, A. Avila, B. Pipes, R. Reifenberger and A. Raman. Subsurface-imaging of carbon nanotube-polymer composites using dynamic AFM methods. Nanotechnology, 24: 135706 (13pp), 2013.

Ferroelectric and piezoelectric materials have been considered as an important functional material for various applications such as NVRAM&’s, energy density capacitors, actuators, MEMs, sonar sensors, microphones to the high technology scanning electron microscopes (SEM). Recently, lead-free barium zirconium titante- barium calcium titante (BZT-BCT) ceramics and thin films have been gaining attention owing to its high ferroelectric and piezoelectric properties. Low leakage current in BZT-BCT leads to saturated ferroelectric hysteresis loops. By suitable site engineering both at Ba and Ti-sites in BaTiO3, one can observe an improvement in the ferroelectric and piezoelectric properties. In this study, lead-free BZT-BCT thin films, with LSCO as bottom electrode were grown on MgO substrates by pulsed laser deposition technique. We have observed high remanent polarization and saturated polarization behaviour in BZT-BCT thin films. Structure, ferroelectric, dielectric, energy density properties, AFM, and piezoresponse (PFM) results will be presented.

Dielectric spectroscopy is a cornerstone technique to characterize the frequency dependence of the complex dielectric constant of composites or heterogeneous materials [1, 2]. AFM based methods allow the mapping of local material properties with nanometer scale resolution. However, the extension of dielectric spectroscopy using the AFM is challenging due to the broadband frequency in which the cantilever is used as the sensing element and the quantification of the acquired data. Thus far frequency dependent measurements have only been made at a single point [3, 4] or in contact with the sample [5]. Moreover, AFM based methods are challenged by spatial resolution due to stray capacitance from the cantilever body.
In this work, we propose the mapping of the local dielectric properties in polymer composites in a frequency range from 10kHz to 1MHz using ultra small, ultra high frequency cantilevers. For this purpose, we use Kelvin Probe Force Microscopy technique to acquire the amplitude (A2omega;) and phase (phi2omega;) response of the second harmonic due to the electrostatic field between the tip and the sample. This is performed during a second pass, while the tip is not mechanically driven and follows the topography taken during the first pass. Ultra high frequency cantilevers are used to have the capability of imaging over the proposed broadband frequency range and also their small size (asymp;10 microns) reduces significantly the stray capacitance and thus improving the spatial resolution of the method. From a theoretical model, A2omega; is used to obtain quantitative data of the capacitance gradient (δC/δz), which in turn gives us the real part of the complex dielectric. Meanwhile, phi2omega; is useful to obtain the dielectric losses in the frequency range.
We apply this technique to polymer composites in which the complex dielectric is frequency dependent, such as carbon nanotube (CNT) polymer composites, where δC/δz increases with local effective dielectric constant, and has also been used in the context of sub-surface imaging of CNT networks embedded in the polymer matrix [6].
References
[1] M.E. Achour, et al. Journal of Applied Physics 103, 094103 (2008).
[2] D. Nuzhnyy, et al. Nanotechnology 24, 055707 (2013).
[3] P.S. Crider, et al. Applied Physics Letter 91, 013102 (2007).
[4] C. Riedel, et al. Applied Physics Letters 96, 213110 (2010).
[5] R. Shao, et al. Applied Physics Letters 82, 1869 (2003).
[6] M. J. Cadena, et al. Nanotechnology 24, 135706 (2013).

The ability to rapidly and accurately map mechanical properties on the nanoscale is of fundamental importance for fields as diverse as material science, biology and nanotechnology. The high resolution and force sensitivity of the atomic force microscope (AFM) makes it an attractive approach for characterization of nanomechanical properties. Several dynamic AFM techniques, such as atomic force acoustic microscopy (AFAM) have been established, in which the tip-sample contact force is modulated, usually under purely sinusoidal excitation, at or near its resonant frequency. However, these single frequency techniques lack the ability to separate contributions from viscous and elastic sample properties, which become important when investigating compliant materials. In order to determine both conservative (elastic) and dissipative (viscous) material properties we have applied the BE method to AFAM. BE-AFAM is an alternative detection scheme to traditional LIA techniques, which instead of using a single drive frequency, uses a predefined amplitude and phase content across a continuous band of frequencies allowing tracking of not only changes in resonant frequency (elastic) but also the quality factor (viscous) of the AFM cantilever. The increased amount of information obtained in such a multidimensional approach provides insight into the tip-sample forces that is unavailable in single frequency AFAM measurements. We apply this approach for imaging multicomponent organic systems, including mono- and multilayer graphene, fossil fuels and biomaterials.

Atomic force microscopy (AFM) provides a means to fully investigate biological material systems in a precise and non-destructive manner. One such system of interest is the lyriform organs in the exoskeleton of a Central American wandering spider (Cupiennius salei). These organs comprise up to almost 30 strain-sensitive slits. One of the organs, HS10, is primarily used to detect substrate vibrations of the plant on which the spider sits. HS10 works in conjunction with a cuticular pad, which picks up the stimulus from the tarsus (last segment of the leg) and transmits it to the sense organ proper. This pad has high-pass characteristics mechanically increasing the strain-sensing system's signal to noise ratio by keeping out irrelevant background vibrations. AFM was employed to study the morphology and nanomechanical response to loads applied at different loading frequencies and temperatures. Considering the spiders' very humid natural habitat our measurements were carried out in fluid to avoid dehydration of the exoskeleton and a corresponding change of its natural mechanical properties. The exoskeleton of C. salei is a layered structure asking for measurements on the outer surface as well as on important areas of the pad's cross section in order to map.its viscoelastic mechanical properties in detail.

Zinc dialkyl dithiophosphate (ZDDP) is a lubricant additive used nearly universally in engine oils to reduce wear and enhance engine life and performance. Despite the generation of phosphorous- and sulphur-containing compounds that reduce the working life of the catalytic converter and adversely affect emissions, the unrivaled wear protection of ZDDP makes it essential to lubricant performance since its introduction in the 1930s. ZDDP works by decomposing under tribological sliding in the engine to form nanoscale anti-wear films whose formation mechanisms are poorly understood despite several decades of research. One reason for this is that a multitude of phenomena occur in macroscopic sliding interfaces and they are extremely challenging to directly analyze and visualize. However, greater understanding of the formation of anti-wear films is required to enable rational design of more environmentally-friendly and energy-efficient engine oil formulations. Here we report novel experiments visualizing and quantifying the formation of ZDDP anti-wear boundary films in-situ in a single asperity contact with nanometer spatial resolution using atomic force microscopy (AFM). Experiments performed on iron-coated silicon surfaces at 100-110 °C in the presence of lubricant show that thermal films grow on the substrate in the absence of tribological contact. These films are easily removed by sliding the tip at applied normal forces of only a few nanonewtons. However, continued sliding at sufficiently high normal loads reveals the nucleation and growth of much more robust films with a pad-like lateral structure, similar to the morphology of anti-wear boundary films formed by ZDDP in macroscopic contacts. This provides the first direct confirmation of asperity-level formation of such films. We find the growth rate is nonlinear with time. Furthermore, the growth rate increases with applied pressure, enabling us to directly compare with atomistic predictions of pressure-induced crosslinking of zinc polyphosphates. We will discuss how these parameters, as well as temperature, scanning time, and tip and substrate materials affect the film growth. These findings provide new insights into the mechanisms of formation of anti-wear films.

In electrostatic force microscopy (EFM), a conductive atomic force microscopy (AFM) tip is electrically biased against a grounded sample and electrostatic forces are investigated[1]. This methodology has been broadly used in the scientific community to characterize dielectric properties of samples at the nanoscale. Two are the main operating conditions associated with this technique. The oscillation amplitude is usually kept to very small values to allow a linearized approach to the force reconstruction and the tip-sample distance is maintained elevated. However, this latter condition negatively affects the lateral resolution of the technique[2]. Thus, electrostatic interaction should be probed in the vicinity of the sample. Theoretically, in this region the force can be linearized using oscillation amplitudes in the order of Å[3]. This might cause the trapping of the tip on the surface (snap-in)[4]. Furthermore, at small distances, short-range forces (i.e. Van der Waals&’) might reach values comparable to electrostatic forces.
Here we present a framework that combines EFM and dynamic amplitude modulation AFM to achieve decoupled reconstruction of forces. It permits reconstructing the real shape of the electrostatic force and the capacitance of the tip-sample system even in the vicinity of the surface. This is done using a technique proposed in literature by Sader[5] and Katan[6] to reconstruct the force without the linearization approximation. The steps needed to decouple short-range and electrostatic forces are explained in detail. This data can be employed to derive the electrical properties of thin films with enhanced lateral resolution with respect to the commonly used EFM techniques.
[1] P. Girard, "Electrostatic force microscopy: principles and some applications to semiconductors," Nanotechnology, vol. 12, p. 485, 2001.
[2] S. Gomez-Moñivas, L. S. Froufe, R. Carminati, J. J. Greffet, and J. J. Sáenz, "Tip-shape effects on electrostatic force microscopy resolution," Nanotechnology, vol. 12, p. 496, 2001.
[3] H. Hölscher, U. D. Schwarz, and R. Wiesendanger, "Calculation of the frequency shift in dynamic force microscopy," Applied Surface Science, vol. 140, pp. 344-351, 1999.
[4] R. Garcia and R. Pérez, "Dynamic atomic force microscopy methods," Surface Science Reports, vol. 47, pp. 197-301, 2002.
[5] J. E. Sader, T. Uchihashi, M. J. Higgins, A. Farrell, Y. Nakayama, and S. P. Jarvis, "Quantitative force measurements using frequency modulation atomic force microscopy—theoretical foundations," Nanotechnology, vol. 16, p. S94, 2005.
[6] A. J. Katan, M. H. Van Es, and T. H. Oosterkamp, "Quantitative force versus distance measurements in amplitude modulation AFM: a novel force inversion technique," Nanotechnology, vol. 20, p. 165703, 2009.

Atomic force microscopy (AFM) suffers from an important limitation: it does not provide quantitative information about the scanned sample. This is because too many unknowns come into play in AFM measurements. The shape of the tip is probably the most important[1, 2]. A technique able to characterize in situ the shape of the tip apex would represent an important step ahead to turn the AFM into a quantitative tool.
Standard methods[3, 4] can be destructive to the tip and are time consuming. Two main methods are currently used to characterize the tip radius in situ without affecting its shape. The first consists of characterizing the tip radius by monitoring the dynamics of the cantilever[5]. The value of free amplitude, for which transitions from the attractive to repulsive force regimes are observed, strongly depends on the curvature of the tip. The second method to characterize the tip radius consists instead on fitting the capacitance curve of the tip-sample system with an analytical function[6].
In this work we compare the two methods to characterize in situ the tip radius and results are verified with SEM images. The value of the free amplitude is correlated with the value of R while the capacitance curve is derived with a method we proposed[7]. Tips with different tip radii are used. The investigation is conducted with the aim of determining the most reliable technique for characterizing the tip radius for both sharp and blunt tips.
[1] B. V. Derjaguin, V. M. Muller, and Y. P. Toporov, "Effect of contact deformations on the adhesion of particles," Journal of Colloid and Interface Science, vol. 53, pp. 314-326, 1975.
[2] S. Santos, C. A. Amadei, A. Verdaguer, and M. Chiesa, "Size Dependent Transitions in Nanoscale Dissipation," The Journal of Physical Chemistry C, vol. 117, pp. 10615-10622, 2013/05/23 2013.
[3] M. Bloo, H. Haitjema, and W. Pril, "Deformation and wear of pyramidal, silicon-nitride AFM tips scanning micrometre-size features in contact mode," Measurement, vol. 25, pp. 203-211, 1999.
[4] R. Geiss, M. Kopycinska-Müller, and D. Hurley, "Wear of Si Cantilever Tips used in Atomic Force Acoustic Microscopy," Microscopy and Microanalysis, vol. 11, pp. 364-365, 2005.
[5] S. Santos, L. Guang, T. Souier, K. Gadelrab, M. Chiesa, and N. H. Thomson, "A method to provide rapid in situ determination of tip radius in dynamic atomic force microscopy," Review of Scientific Instruments, vol. 83, pp. 043707-043707-11, 2012.
[6] S. Hudlet, M. Saint Jean, C. Guthmann, and J. Berger, "Evaluation of the capacitive force between an atomic force microscopy tip and a metallic surface," The European Physical Journal B - Condensed Matter and Complex Systems, vol. 2, pp. 5-10, 1998/03/01 1998.
[7] C. Maragliano, D. Heskes, M. Stefancich, M. Chiesa, and T. Souier, "Dynamic electrostatic force microscopy technique for the study of electrical properties with improved spatial resolution," Nanotechnology, vol. 24, p. 225703, 2013.

The nanometre thick water layers present on surfaces in ambient conditions have been partially responsible for the development of some of the most common atomic force microscopy (AFM) modes of operation1, 2. Here, we describe the phenomena involved in the tip-sample interactions for dynamic AFM in the presence of adsorbed water layers including forces, energies and the minimum distance of approach3-6. An attractive plateau in force that extends up to several nm above the surface is found to be responsible for smoothly driving the tip to the proximity of the surface. Through the force plateau phenomenon, the tip can be made to oscillate inside the water layer without the requirement of large driving amplitudes (and associated cantilever stored energies) that would be detrimental to image soft matter7. Several outcomes are particularly interesting in such a regime of operation which we term small amplitude small set-point (SASS). On one hand, only the more localized short range forces responsible for high resolution imaging control the cantilever dynamics via the interaction with the front atoms of the tip. On the other hand, the damping typical of ambient conditions still controls the dynamics of the micro-cantilever implying high sensitivity to amplitude and phase relative to liquid environments. High resolution imaging of dsDNA molecules is demonstrated with the use of SASS in standard conditions and AFM equipment.
1. A. l. Weisenhorn, P. K. Hansma, T. R. Albrecht and C. F. Quate, Applied Physics Letters 54, 2651-2653 (1989).
2. Q. Zhong, D. Inniss, K. Kjoller and V. B. Elings, Surface Science Letters 290, L688-L692 (1993).
3. S. Santos, V. Barcons, H. K. Christenson, D. J. Billingsley, W. A. Bonass, J. Font and N. H. Thomson, Applied Physics Letters Submitted (2013).
4. V. Barcons, A. Verdaguer, J. Font, M. Chiesa and S. Santos, Journal of Physical Chemistry C 116 (14), 7757-7766 (2012).
5. S. Santos, C. A. Amadei, A. Verdaguer and M. Chiesa, The Journal of Physical Chemistry C 117 (20), 10615-10622 (2013).
6. D. S. Wastl, A. J. Weymouth and F. J. Giessibl, http://arxiv.org/abs/1303.5204 (2013).
7. S. Santos and N. H. Thomson, Applied Physics Letters 98, 013101-013103 (2011).

Boron nitride nanotubes (BNNTs) are a type of one-dimensional tubular nanostructure with many extraordinary mechanical, electrical and thermal properties, and are promising building blocks for a number of applications. The tube length is a critical structural parameter in design, manufacturing and functioning of BNNT-based devices and material systems. In this talk, we present a study of controlling the tube length of individual BNNTs by means of nanomechanical cutting inside an atomic force microscope (AFM). By laterally impacting a tube staying on a substrate with an AFM probe, the nanotube lattices are broken by the AFM tip, resulting in a reduction of the tube length or formation of fracture slits. Our experimental studies revealed that the cutting of nanotubes using this nanomechanical approach depends on several factors, including the tube diameter and the number of tube walls, the impact velocity, the applied compressive load, and the dynamic frictional interaction on the tip-tube collision contact. The mechanical response of nanotubes during the tip-tube collision process and the roles of the impact velocity and the frictional interaction on the tip-tube collision contact in cutting nanotubes were quantitatively investigated through cutting double-walled BNNTs. Our measurements quantified the required normal cutting load resulting in tube fracture and the corresponding peak lateral collision force. The fracture strength of BNNTs was also quantified based on the measured collision forces and their structural configurations using contact mechanics theories. Our data of the fracture strength of BNNTs obtained from the AFM cutting measurements are consistent with the reported values in the literature that were obtained by using tensile testing techniques. The nanomechanical study presented in this talk demonstrates that the AFM-based nanomechanical cutting technique not only enables effectively controlling the length of nanotubes with high precisions, but also is promising as a new nanomechanical testing technique for characterizing the mechanical properties of tubular nanostructures.

Dynamic atomic force microscopy (dAFM) is a powerful tool for high resolution imaging of biomolecules such as DNA and proteins. Also, it has been widely recognized that the condition of the substrate and the sample preparation process affect the resolution and quality of imaging greatly. In this study, we investigate this effect systematically by exploring the conservative and dissipative forces at nanoscale. The force between the tip and the sample is reconstructed by implementing the standard Sader-Jarvis-Katan formalism [1]. Force profiles of mica surface under various nickel ion concentrations along with different incubation times are recovered and analyzed in the senses of distance dependency and localized chemical interactions. Details in the spatial and temporal changes in surface properties will benefit our understanding in the behavior of ion distribution as they play crucial roles in biomolecule imaging using AFM. For DNA imaging, ions act as bridges connecting our target molecules to the mica surface through the interplay between positive and negative charges. Under varied absorption conditions and by using a sharp tip, four imaging modes including attractive, repulsive, non-contact, and small amplitude small set-point (SASS) are demonstrated on linear DNA molecules [2]. The resolutions and sensitivities from different channels are compared and will be used in building the relationship between ions and surface properties. This work has implications in finding the optimal condition for surface functionalization, which is an integrated outcome of buffer composition, incubation time, and the aging process after treatment.
1. S. Santos, C. A. Amadei, A. Verdaguer and M. Chiesa, The Journal of Physical Chemistry C 117 (20), 10615-10622 (2013).
2. S. Santos, V. Barcons, H. K. Christenson, D. J. Billingsley, W. A. Bonass, J. Font and N. H. Thomson, Applied Physics Letters Submitted (2013).

The presence of adsorbed water layers resulting from the exposure of a graphite surface to environmental moisture for prolonged periods of time is investigated by a framework based on dynamic atomic force microscopy AFM. Variations in conservative and dissipative forces are experimentally monitored and quantified as a function of time. In particular, exposure of the surface to environmental moisture generates a (net) distance independent force that spans more than 1 nm above the surface. We further propose and report the appearance of a footprint of capillary bridge formation and rupture with energies ranging from 5 to 20 eV for tips ranging from 5 to 20 nm in radius respectively. The initial conditions are recovered after baking the samples above 100 degress for hours. This phenomenon suggests that the origin of the observed variations in force profiles are related to the presence of thin water layers on the surface. This statement is corroborated by attenuated total reflectance infrared spectroscopy experiments. Future work involving the present methodology should allow studying highly heterogeneous surfaces, which exhibit nano-hydrophilic/hydrophobic regions and used for their anti-biofouling properties, where other techniques might fail.

Tip enhanced Raman spectroscopy (TERS) is evolving in two parallel paths that need to be unified for the further development of the technique both in terms of a fundamental understanding of its origins and its limitations and potential. In one mode of investigation tunneling feedback is used between an etched metallic wire (generally gold) and a metallic surface also generally gold. In such a tunneling mode plasmons are highly
confined and broadband in nature creating a nanocavity plasmon field
in the tunneling gap. This has generally been defined as a gap-mode. Such tunneling based TERS has recently been able in ultrahigh vacuum to chemically map a molecule [1]. The limitations of course are that the technique is applicable only to very flat samples and on conducting substrates. Alternately, probes with gold nanoparticles in glass have been used in atomic force feedback even on non-conducting surfaces. Such probes have been shown to even emulate a gap mode on a non-conducting surface and this application of TERS has broad potential [2]. We have now been able in the same probe to switch between tunneling and atomic force feedback and the results of such investigations will be the subject of this paper within the context of previous results that have been achieved in both AFM and Tunneling feedback.
1. Z. Zhang et al, Nature 2013, 498 doi:10.1038/nature12151
2. H. Wang and Z Schultz, Analyst, 2013, 138, 3150

In spite of the long history of atomic force microscopy (AFM) imaging of soft materials little is known about the detailed effect of a finite tip size and applied force on the imaging performance. Here we exploit the defined scaling of roughness amplitudes on amorphous polymer films to determine the transfer function imposed by the imaging tip. The finite indentation of the nanometer scale tip into the comparatively soft polymer surface leads to a finite contact area, which in turn effectively acts as a moving average filter for the surface roughness. In the power spectral density (PSD) this leads to an attenuation of the roughness amplitudes related to the Airy-pattern known from light diffraction from a circular aperture.
This transfer function is affected by the roughness-induced local modulation of the tip height and contact area, which is studied by performing simulations of the polymer roughness and the imaging process. We find that the contact radius of the tip-sample contact can be recovered from the roughness spectrum for typical polymer material parameters and sharp imaging tips. We experimentally verify and demonstrate the method by measuring the contact radius as a function of applied load and travel distance on a highly cross-linked model polymer. The results for the contact radius are consistent with the Johnson-Kendall-Roberts (JKR) contact model, which is the appropriate model for the system.
Being able to determine the contact radius in-situ allows one not only to quantitatively monitor the state of the tip and test contact and tip wear models on the nanometer scale. The method also provides a calibration scheme enabling a variety of quantitative AFM measurements. Testing mechanical material properties and mechanical contact models is demonstrated here. Other measurements on polymers would include heat or current transport into (conductive) polymeric samples, friction force microscopy, quantitative tip wear, etc.

Theoretical studies of non-contact atomic force microscopy (AFM) play an important role in analyzing the measured images. In general, such studies are computationally intensive as the force scanning process involves a raster scan with a vertical vibration motion. Modeling the AFM tip is another difficult aspect since the exact morphology of the tip is in general unknown. We introduce an efficient simulation method based on a real-space implementation of pseudopotentials, which were constructed within density functional theory. We perform non-contact AFM simulations for the GaAs(110) surface, and analyze the tip-surface forces by comparing with the previous simulations and experiment. We show that our method works especially well for tips made of inert materials, as is frequently the case.

Most solid surfaces acquire a surface charge upon exposure to aqueous environments due to adsorption and desorption of protons and other ionic species. The local distribution of surface charges and adsorbed ions plays a curcial role for many subsequent physico-chemical processes in environmental, biological, and technological processes. We use high resolution Atomic Force Microscopy spectroscopy in frequency as well as imaging in amplitude modulation mode to investigate the adsorption of mono- and divalent cations typically contained in sea water (Na, K, Ca, Mg) to mineral surfaces such as mica, gibbsite, kaolinite, and silica. We consistently find a strong adsorption of the divalent species, as revealed by well-defined monolayers of adsorbed ions with a characteristic symmetry differing from the substrate lattice. The interaction is driven by non-electrostatic interfactions. Substantial variations between the surface charge determined by Poisson-Boltzmann fitting of the spectroscopy curve and the density of imaged adsorbed cations reveals details about the boundary between the Stern layer and the diffuse part of the electric double layer.

We use scanned probe microscopy to investigate the morphology and conductivity of individual ion transport domains in polymer fuel cell membranes. In particular, using atomic force microscopy phase and current imaging, we have resolved ion conducting domains at the surface of Nafion proton exchange membranes. Such measurements were performed at dehydrated, ambient, and hydrated conditions, where we observed a unique morphology at each membrane water content. At ambient conditions, Nafion&’s surface morphology resembles that proposed in the parallel-pore and bicontinuous network models, with the exception that hydrophilic domains are larger at the surface of Nafion compared to the bulk. At hydrated conditions, a network of wormlike, insulating domains extends several micrometers over Nafion&’s surface with more conductive, water-rich regions found between these fibrillar features. Neither the surface morphology observed at ambient conditions nor at hydrated conditions persists in dehydrated membranes, which instead exhibit a low coverage of isolated hydrophilic surface domains that, remarkably, are similar in size to such domains at ambient conditions. These observations affirm properties distinct to Nafion&’s surface and provide morphological evidence for the low conductivity observed in Nafion at dehydrated conditions and the high conductivity observed at hydrated conditions. AFM phase imaging was used to gain quantitative information regarding the size and distribution of proton conducting domains at the surface of Nafion and it was found that the nature of tip-sample interaction forces strongly affects the resolution and subsequent interpretation of such domains. Finally, comparison of current and phase images of a Nafion membrane at ambient conditions revealed that not all aqueous surface domains are electrochemically active to the same extent, an important implication in rationalizing Nafion&’s ion transport properties.

The structure and properties of solid-liquid interfaces directly underpin the multitude of phenomena in virtually all areas of scientific inquiry ranging from energy storage and conversion systems, biology, to geophysics and geochemistry. These interfaces will affect the lifetimes and stability of batteries, erosion and corrosion, and many other phenomena. In many cases, the associated mechanisms include the static properties of interfaces, as well as dynamic transport through solid and liquid phases, and electrochemical properties and reactivity, resulting in systems of extreme complexity. Here, we explore the possibility to study a variety of phenomena linked to ionic transport at the solid-liquid interface on the nanoscale using scanning probe microscopy. This will include measuring strain generated through electrochemical processes for Li-ion batteries and electrochemical supercapacitors, the measurement of surface potentials in liquid, and the probing of electrical double layers. We will discuss these phenomena for different material systems important to energy storage and will comment on their feasibility to become standard characterization tools for future studies of energy storage systems.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program, by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, and by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Biological molecules fulfill a wide variety of unique functions. Their functions are essentially elicited from conformational change and/or interactions with other molecules which are often triggered by binding of ligand/substrate and changes in the external environment. Therefore, studying dynamic processes of individual molecules, for example, how molecules undergo conformational change and how molecules interact, is indispensable to understand the dynamic relationship between structure and function in biological molecules. Nevertheless, a tool with an ability to directly detect and track conformational change in real time has not been available.
Atomic force microscopy (AFM) is a vital technique to study nanoscale structures of materials under various environments. One of the most coveted new functions of AFM is “fast recording” because it allows the observation of dynamic processes occurring at the nanoscale. The visualization of dynamic processes affords deep insights into the target objects and phenomena under the microscope. This new capability of observation should have a great impact particularly on life science[1-4]. In this talk, we demonstrate representative examples of high-speed AFM imaging of molecular behaviors difficult to study with other approaches. We further introduce recent progress on high-speed AFM as a manipulation tool of single proteins beyond imaging.
[1] N. Kodera, D. Yamamoto, R. Ishikawa, and T. Ando, , Nature, 468, 72 (2010).
[2] M. Shibata et al., Nat. Nanotech., 5, 208 (2010).
[3] T. Uchihashi, R. Iino, T. Ando, and H. Noji, Science, 333, 755 (2011).
[4] K. Igarashi et al., Science 333, 1279 (2011).

Recent developments in atomic force microscopy live cell imaging have significantly advanced our understanding of fundamental cell biology. Specifically, Multi-harmonic AFM methods (A. Raman et al., Nature Nanotech., 2011, 6, 809-814; A. Cartagena et al., Nanoscale, 2013, 5, 4729-4736) now allow for the quantitative mapping of local nanomechanical properties of live cells in physiological solutions with the speed and spatial resolution of conventional Tapping mode AFM. There is however a need to further improve material property mapping speed to track time-evolving/dynamic cellular processes. Here we demonstrate that using Lorentz excited cantilevers with Z feedback on the 0th harmonic amplitude (A0) signal instead of the oscillation amplitude, we can boost by ~3-5 times quantitative material property mapping throughput (pixel/min) of live mammalian cells in solution compared to Tapping mode. After the material contrast channels has been recorded during a scan, the data has been post-processed using the Bottom Effect Cone Correction (BECC) tip-sample interaction contact mechanics model (N. Gavara, and R.S. Chadwick, Nature Nanotech., 2012, 7, 733-736) modified with a linear viscoelastic model to quantitatively extract maps of the local unknown material properties.
Here we introduce the method and its measurement application that allows higher speed quantitative mapping of local nanomechanical properties of live eukaryotic cells. The total acquisition time per image for live fibroblast cells and human breast carcinoma cells was below 5mins. This signifies a ~3 orders of magnitude faster material property mapping throughput (pixel/min) than standard Force-Volume method and ~3-4 times faster than Multi-harmonic AFM and Peak Force Tapping, which are well known based quantitative material property mapping methods. We anticipate that this with yet further optimization has the potential to track the time-varying changes in cytoskeleton structure dynamics and its material properties in a time window closer to real time processes which would not be possible before with current methods.

Stiffness mapping of various biological and cellular surfaces has been a topic of great interest in the field of material and surface sciences. Surface stiffness measurements are currently being used to define metastatic properties of various cancerous cell lines and other related biological tissues. Here we present a unique methodology to investigate depth dependent nanomechanical variations in biopolymeric materials and live cells. Specifically we have used 0.5% & 1 % Agarose as well as NIH3T3 cell lines for investigating the role of porosity and inter-molecular associations for Depth-Dependent-Nanomechanical-Analysis (DDNA). This approach can circumvent the issue associated with the contribution of substrates on apparent cell stiffness. DDNA provides crucial information regarding changes in cell stiffness as a function of depth, corresponding to changes in sub-membrane structures such as the cytoskeleton. Sub-membrane indentation mapping can therefore aid in understanding any variations in the growth and distribution of cytoskeletal structures, which can profoundly influence cell function, metastatic behavior, response to therapeutic agents, etc.

Living cells are complex micromechanical systems that are able to govern their mechanical properties within an extremely wide range. The underlying processes allowing the cytoskeleton to dynamically alter its mechanics to such an extent are still not completely understood. Here, we present a new technique for measuring the viscoelastic material properties of living cells based on the scanning ion conductance microscope (SICM). Due to its noncontact imaging character and its high temporal and spatial resolution this technique is particularly well suited for investigating dynamic processes in living cells. We show that the local viscoelastic creep response of living cells exhibits large spatial variations in stiffness and viscosity. By recording quantitative maps of stiffness and viscosity with sub-µm spatial resolution we show that viscous regions are located on the cell body and on the cell extensions, while the lamellum is relatively elastic. The local stiffness and viscosity are furthermore highly correlated and collapse onto a master curve. This is in accordance with the hypothesis that the cytoskeleton can be modeled as a soft glassy material, where the viscosity is related to the motor protein activity of the cytoskeleton. We show that biochemical treatment results in the cell “wandering” on the master curve, which indicates that the cell can alter its mechanical stiffness by one order of magnitude just by changing its cytoskeletal activity.

Scanning electrochemical microscopy (SECM) is increasingly applied to study and image live cells. To reduce the overall analysis time during live cell SECM measurements and maintain cell viability, the microelectrode scan rate can be increased. The use of increasing microelectrode scan rates is challenging because our understanding of the downstream convection effects is tied to the ill-defined topography of the imaged live cells. This presentation illustrates the effect of forced convection on the microelectrode current during SECM imaging of live cells, and model planar and non-planar substrates. Experiments demonstrate that during constant height imaging, the normalized peak current observed during line scans on all three substrates scales linearly with the microelectrode velocity. This linear relationship is corroborated by finite element simulations of non-planar substrates, which further reveal that the slope is closely related to the electrochemical activity of the substrate. Startegies to extract the cell's kinetics are proposed.

Bacteriophytochromes (BphPs) are red-light photoreceptors found in photosynthetic and nonphotosynthetic bacteria that have been recently engineered as infrared fluorescent tissue markers. Light-induced, global structural changes are proposed to originate within their covalently linked biliverdin chromophore and propagate through the protein. Classical BphPs undergo reversible photoconversion between spectrally distinct light absorbing states, Pr and Pfr respectively. RpBph3 (P3) from Rhodopseudomonas palustris is unique because photoconversion occurs between Pr and unique near-red (Pnr) light-absorbing states. Due to size and photosensitivity of BphPs, structures of the intact proteins have not been resolved by nuclear magnetic resonance and/or X-ray crystallography. Therefore, structural details about the light and dark-adapted structures of the intact BphPs are not well understood at the molecular level. We have utilized liquid cell atomic force microscopy (AFM) to investigate the domain structure of intact P3 in its light-adapted state (Pnr). By varying the concentration of the protein solution, deposition time, and the ionic strength of the buffer solution, the aggregation of P3 on a mica surface can be controlled and single dimers may be observed in a biologically relevant media. Domain resolution has been achieved for several orientations of the dimer on the surface. The structural dimensions of the dimer have been compared to a model, intact BphP generated using Pymol software. AFM experiments are currently underway to analyze the dark-adapted state (Pr) of P3 in order to observe the anticipated structural changes. Ultimately, the goal is to use AFM and other surface analytical methods such as scanning tunneling microscopy and scanning electron microscopy to gain new insight into the unique photochemistry of P3.

Scanning probe microscopy offers the unique capability of correlating local functionality with local morphology in nanostructured materials for energy applications. We have applied scanning probe microscopy methods, ranging from photoconductive atomic force microscopy (pcAFM) to time-resolved electrostatic force microscopy (trEFM) for understanding the role of structural heterogeneity in photocurrent generation and loss in thin film solar cells, including organic (polymer bulk heterojunction), nanocrystal-based (PbS), and inorganic (thin film kesterite) solar cells. In each of these materials, understanding how local variations in carrier lifetime arise from compositional heterogeneity in the bulk and at the contacts is critical to rationally improving device performance. In this talk, we provide examples of how combinations of functional imaging can be used to link variations in surface potentials with variations in charge carrier lifetimes at critical interfaces in solution processed thin film solar cells.

The Kelvin probe force microscope (KPFM) constitutes an extremely powerful tool of characterization in the field of organic photovoltaics. In ultra-high vacuum (UHV) and in non-contact mode (nc-mode) it is possible to record simultaneously the topography and the contact potential difference (CPD) with an exceptional level of resolution. For donor-acceptor bulk-heterojunction blends it has been demonstrated that KPFM enables the investigation of the charge carrier generation on the sub-10nm scale [1]. However, to further enhance the understanding of these processes it is advantageous to study model molecular systems that possess better defined electronic and structural properties than the one found in bulk heterojunctions.
In this communication, KPFM and scanning tunneling microscopy (STM) investigations of model donor-acceptor self-assemblies on highly oriented pyrolytic graphite (HOPG) will be presented. First, the characteristics of a mixed monolayer of evaporated C60 (acceptor) and the π-conjugated polymer P3DDT (donor) will be discussed. In a second step, a new generation of donor-acceptor dyads will be introduced that has been synthesised following the concept developed by W. Li et al. [2]. Scanning probe microscopy measurements proof the auto-organization of this model system in the form of periodic lamella on HOPG. The scanning probe microscopy data will be compared to the results of molecular mechanics (MM) and molecular dynamics (MD) simulations. Finally, the nature of the CPD contrasts [3] will be examined and a comparison of CPD measurements in dark and under illumination will be discussed. The results affirm the value of these new donor-acceptor model-systems for organic photovoltaics.
[1] Evan J. Spadafora, Renaud Demadrille, Bernard Ratier, Benjamin Grevin, Nano Letters 10, 3337-3342 (2010)
[2] W. Li, A. Saeki, Y. Yamamoto, T. Fukushima, S. Seki, N. Ishii, K. Kato, M. Takata, T. Aida, Chem. Asian J. 5, 1566-1572 (2010)
[3] Evan J. Spadafora, Mathieu Linares, Wan Zaireen Nisa Yahya, Frederic Lincker, Renaud Demadrille, Benjamin Grevin, Applied Physics Letters 99, 233102 (2011)

Kelvin Probe Force Microscopy (KPFM) has proven to be a quantitative method for the observation of local surface potentials, doping concentration of bulk silicon surfaces and surface charges trapped into insulating layers. Even locally generated charges generated by transfer mechanisms in single molecule interactions or photo-induced surface potentials can be studied with KPFM methods, which makes it one of the most promising SPM technique.
High-speed imaging of the surface topographies, on the other hand, became a major topic when advances in SPM were presented in the past years. The combination of KPFM measurements with high-speed imaging, however, is not yet in the focus of current SPM developments. For AM-KPFM, this is because the electrical KPFM signals usually are too noisy to be detected with short time constants. For FM-KPFM, the available PLL feedback limits the imaging rate.
This contributions presents several approaches for closing the gap between the state-of-the-art scanning speed and the limited signal quality for fast KPFM measurements. Results are shown on biological samples, metal surfaces and semiconductive quantum dots.
Special conductive cantilevers with a low force constant and a high resonance frequency, for instance, are employed to significantly increase the detection sensitivity. A dedicated high-speed data acquisition system is extended so that also the electrical channels can be acquired with a bandwidth that is related to the image acquisition speed, the cantilever oscillation and the frequency of the electrical signal. Finally, the feasibility and the limits of each approach has been analyzed and a current limit of high-speed KPFM is derived.

The versatility of SPM has led to the development of a plethora of techniques capable of probing local functionality, such as electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). In these techniques, long range electrostatic interactions are recorded usually at fixed tip-sample distances. The resulting images, however, contain non-local contributions from both microscopic (tip apex) and macroscopic (tip and cantilever) tip to sample interactions. Therefore, an EFM or KPFM image provides only a snapshot of the complex tip-sample interactions, the resolution of which is strongly influenced by the chosen operating distance. Here we demonstrate a multidimensional scanning probe microscopy approach for quantitative, cross-talk free mapping of surface electrostatic properties. Force volume band excitation (FVBE)-KPFM, provides the full cantilever resonance response (amplitude, quality factor and phase vs. frequency) as a function of both voltage and tip-sample distance for each image pixel. The subsequent analysis reconstructs this multidimensional dataset into maps of work function, tip-surface capacitance gradient and resonant frequency maps. The distance dependence of the tip amplitude, frequency shift, and quality factor can be used to; decouple conservative and dissipative interactions, as a measure of the capacitance gradient, and ultimately towards achieving quanititaive high resolution KPFM imaging through deconvolution of non-local tip effects.

I will discuss recent atomic force microscopy studies of nanoscale single asperity contacts that reveal surprising new behavior and insights. First, the behavior of nanoscale contacts with truly 2-dimensional materials will be discussed. For nanoscale contacts to graphene, we find that the friction force exhibits a significant dependence on the number of 2-D layers[1]. Surprisingly, adhesion (the pull-off force) does not. However, studies as a function of scanning history reveal further complexities that arise from the combined effects of high flexibility and variable substrate interactions that occur at the limit of atomically-thin sheets. An even stronger effect occurs when graphene is fluorinated, where experiments and simulations both show that friction between nanoscale tips and fluorinated graphene (FGr) monolayers exceeds that for pristine graphene by an order of magnitude. The results can be interpreted in the context of the Prandtl-Tomlinson model of stick-slip friction.
I will then discuss new insights into the physics of nanoscale wear. A better understanding of wear would allow the development of rational strategies for controlling it at all length scales, and would help enable applications for which wear is a primary limitation such as micro-/nano-electromechanical systems (MEMS/NEMS). We have demonstrated the ability to characterize single-asperity wear with a high degree of precision by performing in-situ wear tests inside of a transmission electron microscope. For silicon probes slid against a flat diamond substrate, the shape evolution and volume loss due to wear are well described by kinetic model based on stress-assisted bond breaking mechanisms[2]. This allows new insights to be gained about the kinetics of atomic-scale wear[3].
[1] Lee, C., Li, Q., Kalb, W., Liu, X.-Z., Berger, H., Carpick, R.W. and Hone, J. "Frictional Characteristics of Atomically-Thin Sheets," Science, 328, 2010, 76-80.
[2] Jacobs, T.D. and Carpick, R.W. "Nanoscale Wear as a Stress-Assisted Chemical Reaction," Nature Nanotech., 8, 2013, 108-112.
[3] Jacobs, T.D., Gotsmann, B., Lantz, M.A. and Carpick, R.W. "On the Application of Transition State Theory to Atomic-Scale Wear," Tribol. Lett., 39, 2010, 257-271

A fundamental understanding of adhesion is important for contacts at all length scales, but is particularly critical in nanoscale devices and applications due to their high surface-to-volume ratio. The classic continuum mechanics models of adhesion between spheres (JKR, DMT, Maugis-Dugdale) are commonly applied to study adhesion between nanoscale bodies. They have been generalized to different geometries, enabling broader sets of contacting situations to be understood. However, whether these models apply at the nanoscale, and what to do when the models break down, is not yet established. These models require knowledge of the interaction potential acting between the two materials; at a minimum, one needs not just the strength of the interaction (characterized by the work of adhesion), but also the length scale of the interaction (commonly called the range of adhesion). Conventionally, the range of adhesion is estimated using order-of-magnitude arguments, and the work of adhesion subsequently calculated. We will show that this can introduce large errors. We will then present a novel experimental technique to simultaneously determine both the work of adhesion and range of adhesion.
In particular, nanoscale adhesion measurements were conducted inside of a transmission electron microscope (TEM), using a modified in situ nanoindentation apparatus. Nanoscale silicon asperities were brought into contact and separated from a flat diamond substrate while being directly imaged in real time. The snap-in distance and the pull-off force were measured with sub-nanometer and sub-nanonewton resolution, respectively. This in situ technique allowed the shape of the asperity to be determined with sub-nanometer resolution immediately before and after contact, to verify that elastic conditions were maintained.
A simple analytical model was used to integrate a Lennard-Jones-type potential over the real shape of the asperity. By fitting the results to experimental pull-off forces and snap-in distances for multiple asperity shapes, the work of adhesion and range of adhesion were determined. We find that the range of adhesion is longer than commonly estimated and that the true work of adhesion is more than an order of magnitude larger than the result calculated using the aforementioned continuum mechanics models. These fundamental parameters describe the adhesive interaction between silicon and diamond, and allow the prediction of adhesive forces between arbitrary geometries.

A plethora of applications in pharmacy, cosmetics, food industry and other areas are directly linked to the research fields of particle technology and contact mechanics. Here, a typical particle ensemble features particle sizes ranging from the nanometer up to the micrometer regime.Up to date, the direct access to particle motion and particle/surface interaction either requires dedicated homebuilt set-ups or is limited with respect to the weight of the particle and/or the accessible load regime.
In the work presented here, a novel approach is introduced to overcome such limitations by using a nanoindentation setup. In addition to that, we demonstrate a relatively simple experimental path capable of probing sliding, rolling and torsional friction. Basically, the concept of the colloid probe technique, which is well established in the AFM community, is transferred to a nanoindenter setup. The potential of such strategy is shown by studying the sliding and rolling friction of silica microspheres featuring radii of about 2.5µm, 10µm and 50µm on various substrates such as Si substrates featuring various roughness as well as flat and rough gold films (300nm film thickness). Key aspects of this work include the influence of surface roughness, adhesion, humidity and the elastic plastic transition on the rolling of particle over Si and gold surfaces. Additionally, experimental results for controlled particle/surface motion in a Si based rail system will be presented. Differences in sliding, rolling and torsional friction of particles as a function of the opening angle of the corresponding rail are evaluated and compared to Discrete Element Method (DEM) simulations.

Taber abrasion (ASTM D-4060) is widely used as a performance wear test for the polymeric silicone hard coatings that protect polycarbonate in automotive applications. However, numerous drawbacks of the Taber test make an alternative measurement method desirable. The use of hardness and modulus values, as obtained from nanoindentation, is documented in the literature as a means to predict wear properties of ceramic and metallic nanocomposite coatings, when expressed as the ratios of hardness to reduced modulus (H/E_r or H³/E_r²). A series of silicone hard coat formulations with a wide range in wear performance (Taber abrasion ΔH @ 500 cycles of 2.7% to 90.1%) were prepared by adjusting formulation and process variables such as: resin type, resin molecular weight (pre-cure), catalyst level, and cure time. These samples were tested using dynamic nanoindentation to obtain hardness and reduced modulus, and the ratio values were compared with the Taber testing results. A strong correlation was obtained (R² = 0.8) which indicates that this alternate method would be an effective screening tool for evaluating the wear performance of silicone hard coats. Additional benefits of the nanoindentation technique include lower test variability, smaller sample size, and faster sample throughput. This study demonstrates that nanoindentation is a valuable alternative to Taber testing for measuring the wear of performance polymeric coatings.

Scanning-probe microscopy (SPM) has been suggested as a means for mapping the mechanical properties of surfaces with nanometer-scale resolution. Under ideal conditions, SPM can be used only to assess elastic properties, not plastic strength. In this presentation, a new measurement technique is presented which overcomes these problems and allows the rapid generation of quantitative and highly resolved mechanical-properties maps. Instead of maintaining continuous contact with the surface, the scanning probe hovers just over the surface and performs an array of discrete indentations. Each indentation cycle requires less than one second, including surface approach, contact detection, force application, withdrawal, and movement to the next indentation site. Traditional nano-indentation analyses are applied to the force-displacement measurements, but information storage and presentation owe much to SPM technology. This new hybrid of scanning-probe microscopy and nano-indentation is used to understand the effect of copper on the growth and location of gold intermetallic compound in lead-free solder.

Characterization methods to identify and discriminate polymer materials based on their mechanical properties with atomic force microscopy is a focus of much research activity. The ability to quantitatively measure on the nanoscale viscoelastic properties of increasingly complex polymer systems such as composites and blends remains a significant challenge. Recent progress is discussed in the ability to quantitatively measure the storage modulus, loss modulus, and loss tangent of polyolefin containing systems with both contact resonance and amplitude modulation based methods, successfully applying these methods to a blend of PP-PE-PS, and PP and elastomers. The loss tangent has also been measured as a function of temperature, revealing important polymer transitions. Finally, the use of multifrequency AFM methods, where the AFM cantilever is excited at multiple eigenmodes, is shown to successfully discriminate materials multi-component blends and materials.

We describe multimodal imaging and nanomechanical investigations of the structure and properties of silicone elastomers consisting of 5-25 wt% of pyrogenic silicone dioxide (“fumed silica”) nanoparticles dispersed in silicone elastomer matrices. The silicone matrices (MPa-regime modulus) are found to include strongly modified interphase regions within nanometers to tens of nanometers of the silica nanoparticles. AFM phase imaging shows that these regions exhibit distinctly elevated dissipative response compared to either the nanoparticles themselves or domains of pure polymer located farther from the silica nanoparticles. So-called "height" images are actually complex convolutions of topography and nanomechanical response. By exploring imaging parameters and modalities we found that both "true" and incremental degrees of "false" topography can be controllably generated as images, revealing a more 3D distribution of nanoparticles (i.e., depth dependence). Thus AFM results are further compared to TEM imaging (i.e., 2D projections of 3D nanoparticle distributions) to enhance understandings of each imaging modality. We also find dramatically different apparent topography in amplitude-modulation dynamic (AC/”tapping”) mode compared to fast force curve mapping (“peak force QNM”). AC-mode phase imaging is further complemented by adhesion and stiffness imaging in fast force curve mapping. [1]
The multiphase morphology was found to be strongly dependent on compositional and processing parameters. AFM results enabled an assessment of the influences of particle size and surface chemical treatment, as well as particle concentration and dispersion, on the multiphase morphology, including the degree of percolation of dissipative domains. We further correlate these observations to the macroscale mechanical properties of the elastomers engineered under variable composition and processing.
[1] G. Haugstad, Atomic Force Microscopy: Understanding Basic Modes and Advanced Applications (Wiley, 2012)

In the past 20 years, the atomic force microscope (AFM) has evolved into a powerful tool for the nanoscale investigation of polymeric materials down to the single macromolecule level. Although AFM can provide images with nanometer spatial resolution on topography and mechanical properties of heterogeneous materials, there is a fundamental need to obtain further material specific information, for instance optical properties with nanoscale resolution. In this regard the integration of an AFM with optical instrumentation into hybrid devices has opened novel avenues for nanoscale material characterization.
We recently introduced a hybrid setup called scanning near field ellipsometric microscope (SNEM) composed of an AFM and an ellipsometer [1,2]. This approach relies on recording changes in the state of polarization of the electromagnetic field by a vibrating AFM probe in the proximity of the specimen surface. For excitation of gold-coated probes, field enhancement at the tip apex would be expected to occur both as a result of the increasing confinement of the surface charge density at the sharp tip apex, the so-called “lightning-rod effect” and due to the resonant excitation of localized surface plasmon modes of the metalized probe tip. SNEM was used to simultaneously obtain optical images and topography images of the microphase separated morphology of block copolymer films. Optical images of the block copolymer films revealed a spatial resolution well below the diffraction limit. Here we report on the investigation of the effect of the tip coating, tip-sample separation distance and the dielectric constant difference in the sample on the optical contrast. These results have improved the understanding of the optical contrast mechanism of SNEM. Calculations of the point dipole model supported the experimental results. Additionally, short-chain thiol monolayers were used to increase the lifetime of the probes.
[1] Cumurcu, A.; Duvigneau, J.; Lindsay, I. D.; Schön, P.; Vancso, G. J. Eur. Polym. J. 2013,doi: 10.1016/j.eurpolymj.2013.03.004.
[2] Tranchida, D.; Diaz, J.; Schön, P.; Schönherr, H.; Vancso, G. J. Nanoscale 2011, 3, 233.

The stability of polymer/metal oxide interfaces plays a crucial role in the design and performance of biomaterials, protective coatings and dye-sensitized solar cells. Chemical Force Microscopy (CFM) is an excellent method to obtain insight into the mechanisms of molecular adhesion, especially for systems which are designed to operate in liquid media.
This paper presents the application of CFM for the investigation of the stability at polymer/metal oxide interfaces. The cantilevers for the CFM experiments were coated with thin films of ZnO, ZrO₂ and TiO₂. Self-assembled monolayers (SAMs) of bi-functional organothiol molecules terminated with phosphonic and carboxylic acid functionalities were deposited on gold substrates to represent the functionalities of a model polymer. Additionally, patterned monolayers were prepared by means of micro-contact printing to enable a direct comparison of the measured adhesion forces on both functionalities.
CFM experiments were performed as a function of the electrolyte pH, in a pH window of 5 - 9 to minimize the dissolution of the metal oxide film on the cantilever. Complementary Fourier Transform - Infrared Reflection Absorption Spectroscopy (FT-IRRAS) and X-Ray Photoelectron Spectroscopy (XPS) studies were performed to investigate the stability of long alkyl chain phosphonic and carboxylic acid monolayers on respective metal oxide surfaces as a function of immersion time in electrolytes with different pH values.
The CFM results illustrated the effect of the electrolyte pH on the molecular adhesion as well as on the interplay of the electrical double layers on the SAM surface and the metal oxide on the cantilever. Phosphonic acid monolayers have shown high adhesion forces in the CFM experiments over a wider pH range in comparison to carboxylic acids due to the presence of double deprotonation states. Lowest adhesion forces with phosphonic acid monolayers have been observed at alkaline pH values. The results of ex situ spectroscopic analysis were in good agreement with the CFM data. Our results demonstrate that CFM can provide valuable information regarding interfacial stability and has a great potential to be used as a predictive method.

Scanning probe microscopy offers a unique tool for probing nanoscale dynamic phenomena in solids ranging from phase transitions to electrochemical reactions, the target of crucial importance for material science to link defect structure to its functionality. In these experiments, the SPM tip focuses an electric or thermal field in a small (5 - 30 nm) region of material, inducing local transformations. In parallel, measured dynamic strain, resonance frequency shift, or quality factor of the cantilever (piezoresponse force microscopy, electrochemical strain microscopy) or tip-surface current (conductive AFM) provides information on processes in the material (polarization, domain size, ionic motion, second phase formation, melting) induced by local stimulus. The uniqueness of this approach is that transformation can be probed in material volumes containing no or single individual extended defects, paving a pathway for studying phase transformations and electrochemical reactions on a single defect level. However, these studies require drastic improvement in capability to collect and analyze multidimensional data sets necessary to capture time- and field dependent behavior at each spatial location. In this presentation, I will illustrate several SPM based approaches for probing dynamic bias-induced phenomena at solid-gas and solid-liquid interfaces, including reversible and irreversible electrochemical reactions, intercalation, and ionic migration. I will further illustrate the use of data mining methods based on supervised and non-supervised learning for analysis of the multidimensional data and some strategies for matching with theoretical models.
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.

Quantifying temperature and mechanical strain in electronic devices is critical for understanding device degradation. We report atomic force microscope (AFM) based measurements of thermomechanical and piezoelectric strain in AlGaN/GaN high electron mobility transistors (HEMTs) [1]. The lateral spatial resolution of our measurement technique is near 10 nm, and the strain resolution is near 2 pm. This spatial resolution is significantly higher than Raman spectroscopy (~1 mu;m), which is currently used to map temperature and strains in AlGaN/GaN HEMTs [2]. Our measurements are based on scanning Joule expansion microscopy (SJEM), where an AFM cantilever measures periodic surface thermal expansion in response to periodic joule heating [3]. We report SJEM measurements of a two finger AlGaN/GaN HEMT made on SiC with a gate-to-drain spacing of 1 mu;m. One important consideration when using SJEM on a transistor is that the heat generation region changes shape with a change to either the gate-source voltage, V_GS, or the drain-source voltage, V_DS. To investigate the heating at a given combination of V_DS and V_GS, we held either V_DS or V_GS constant and applied a relatively small amplitude sinusoidal signal (25-800 kHz) with a DC offset to the other. Adjusting frequency provided control of thermal penetration depth into the SiC substrate. One novel feature of this work is that we measured the device surface directly, without a polymer layer to enhance expansion as is typical for SJEM. Even at 800 kHz, we measured surface expansion amplitudes as large as 100 pm. The measurements show significant changes in the surface expansion profile for different bias conditions. For low V_DS, the surface expansion profile is nearly symmetric about the gate centerline, indicating nearly uniform heating across the length of the channel. For higher V_DS, the surface expansion is asymmetric, with more expansion on the drain side, indicating localized heating on the drain side, particularly near the drain edge of the gate. In addition to our measurements, we report finite element modeling of the HEMTs to verify the device expansion measurements, provide device temperatures, and to elucidate the impact of device parameters on the sensitivity of the measurement. Finally, aspects of using this technique for nondestructive testing of degradation in AlGaN/GaN HEMTs will be presented.
[1] J. A. del Alamo and J. Joh, "GaN HEMT reliability," Microelectronics Reliability, vol. 49, pp. 1200-1206, Sep-Nov 2009.
[2] S. Choi, E. R. Heller, D. Dorsey, R. Vetury, and S. Graham, "The Impact of Bias Conditions on Self-Heating in AlGaN/GaN HEMTs," Ieee Transactions on Electron Devices, vol. 60, pp. 159-162, Jan 2013.
[3] J. Varesi and A. Majumdar, "Scanning Joule expansion microscopy at nanometer scales," Applied Physics Letters, vol. 72, pp. 37-39, Jan 5 1998.

The link between ionic and electronic transport in semiconductor devices enables numerous applications such as resistive switchers, MIT, neuromorphic electronics, etc. Strong history dependence of the electronic conductivity in these systems is due to the bias-induced electrochemical transformation of the micro- and nano- structures of the device platform (e.g. formation of conductive filaments). Consequently, SPM techniques are a natural candidate for local studies of these phenomena. A wealth of data that can be obtained from SPM, calls for interpretation and extraction of the physical meaning beyond qualitative observations of resulting image maps. Here we demonstrate the power of combining statistical methods with physical models to interpret variations of the local conductance mechanisms in a metal oxide sample. Our model system is BiFeO3 (BFO) matrix with embedded CoFe2O4 (CFO) tubular nanoislands. Accumulation of oxygen vacancies at the BFO/CFO interface compensates for the change in iron&’s oxidation state and leads to formation of three conductive regions in the sample: two of the pure oxide phase and one interfacial. First order reversal curve current-voltage (FORC-IV) spectroscopy was used to collect IV curves across a spatial grid of points, providing raw information on the local difference of the bias-induced current response. We demonstrate that the separation of different conduction mechanisms from FORC-IV data is possible using principal component analysis, independent component analysis and Bayesian unmixing, with each of the methods illustrating their own merits. The separated components are fitted and interpreted using different physical models, revealing reversible vs. irreversible, as well as kinetically vs. thermodynamically driven contributions to the local response.
Research was supported (E.S., A. B., S.J., S.V.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted at the Center for Nanophase Materials Sciences (E.S., A.B., S.J., S.V.K.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Scanning Microwave Impedance Microscopy is a novel mode for Atomic Force Microscopy (AFM) enabling imaging of unique contrast mechanisms and measurement of local permittivity and conductivity at the 10&’s of nm length scale. We will review the state of the art, including imaging studies revealing electrical characteristics of novel materials and nanostructures, such as graphene and patterned optical crystals and ferro-electrics. In addition to imaging, the technique is suited for a variety of metrology applications where specific physical properties are determined quantitatively. Examples include the measurement of dielectric constant (permittivity) and conductivity (e.g. dopant concentration). These capabilities will be presented, including an analysis of sensitivity and resolution for dielectric constant, doping levels and capacitance. For samples where properties such as dielectric constant are known the technique can be used to measure film thickness. We will present results exploring the sensitivity to thin ITO, nitride, and oxide films.

Understanding the crystal orientation at the domain level using a non destructive technique is absolutely crucial for the design and characterisation of nano-scale ferroelectric devices. Here, we have used piezo-force spectroscopy (PFS) for the first time to identify domain orientation, and to experimentally demonstrate the impact of crystalline orientation on the switching field of ferroelectric BaTiO3 at the level of individual domains. Using X-ray diffraction, we determined that the pulsed laser deposited BaTiO3 thin films used in this study have [001], [101], and [111] preferential orientations, which we have mapped onto the PFS spectra showing three corresponding switching fields of 460, 330 and 230 kV/cm. In addition, the spectra also varied in the field-position at which the enhanced piezoresponse occurs due to a phase change. Such variations were observed in two of the three types of spectra, and, consistent with previous first principles modelling, it is therefore possible to identify these domains as [101] and [111] with the polarisation reversal occurring via a 2-step (rotation and switching), leaving the [001] (pure switching) domains as being identified where the piezoresponse enhancement is absent. Significantly, the results demonstrate that electric field induced phase change causes the [101] and [111] domains to reverse polarisation at a low field. We conclude that the results of this study with a novel application of the PFS technique, provides an understanding of the crystallographic orientation and switching response in ferroelectric materials by the virtue of field induced phase transition shown by these materials. The current research presents PFS as a valuable technique in the characterisation of future ferroelectric devices, with a key outcome of the analysis being that switching fields can be reduced by careful design of the film orientation, a critical factor in low power ferroelectric memory devices.

In ferroelectric ultrathin films, the depolarization field arising from bound charges on the surface of the film and at the interface with the substrate can be partially screened by free charges from metallic elec-trodes, ions from the atmosphere, or mobile charges from within the semiconducting ferroelectric itself. Even in structurally perfect metallic electrodes, the screening charges will spread over a small but finite length, giving rise to a nonzero effective screening length that will dramatically alter the properties of an ultrathin film. In the absence of sufficient free charges, a ferroelectric has several other ways of minimi-zing its energy while preserving its polar state, e.g., by forming domains of opposite polarization, or ro-tating the polarization into the plane of the thin ferroelectric slab [1].

Using piezo-force microscopy (PFM), we investigate the intrinsic nanodomain structure of PbTiO3 ultra-thin films at room temperature. By changing the degree of screening, the domain structure can be modi-fied or even suppressed in favour of a uniform monodomain state. On the other hand, reducing the PbTiO3 thickness leads to a change in the size and shape of the domains, which transform from large nanodots to small stripes.
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[1] Ferroelectricity in ultrathin-film capacitors, C. Lichtensteiger et al, Ch. 12 in Oxide Ultrathin Films, Science and Technology, Wiley (2011) (arXiv:1208.5309v1 [cond-mat.mtrl-sci])

In standard scanning probe methods the tip shape and chemical identity of the apex atoms are often unknown preventing the accurate comparison of experimentally recorded tip-sample interactions with theoretical models.
Cold atom scanning probe microscopy (CA-SPM) solves this dilemma in a very elegant way: the tip is replaced by a gas of ultracold Rubidium atoms that is confined inside a electro-magnetic trap representing the cantilever [1,2]. Such a cold-atom SPM can be operated in a dynamic mode by making the gas oscillate within the trapping potential and measuring how the oscillation frequency changes as the trap is scanned over the surface. Since the interaction potential between the cloud and the sample slightly modifies the potential of the trap, the oscillation frequency of the cloud is changed and the interaction potential can be calculated from the frequency shift. Interestingly, the theory behind the dynamic mode of CA-SPM and dynamic force spectroscopy is essentially the same [3]. Therefore, it is very promising to merge both techniques in order to measure the interactions between gas atoms and sample surface.
[1] M. Gierling, P. Schneeweiss, G. Visanescu, P. Federsel, M. Häffner, D. P. Kern, T. E. Judd, A. Günther, J. Fortágh: Cold-atom scanning probe microscopy. Nature Nanotechnology 6, 447 (2011).
[2] P. Schneeweiss, M. Gierling, G. Visanescu, D. P. Kern, T. E. Judd, A. Günther, J. Fortágh: Dispersion forces between ultracold atoms and a carbon nanotube. Nature Nanotechnology 7, 515 (2012)
[3] H. Hölscher: Cold atoms feel the force. Nature Nanotechnology 7, 484 (2012)

Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a longstanding challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. Here we demonstrate the use of noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on Ag(100) as they undergo a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Additional evidence for the proposed reaction pathways is obtained using ab initio density functional theory.

Semiconductor junctions are the basis of electronic and photovoltaic devices. Here, we investigate junctions formed from highly doped (ND~10^20-10^21 cm-3) hydrogen-passivated silicon nanocrystals (NCs) in the 2-50nm size range, using non-contact atomic force microscopy coupled to Kelvin probe force microscopy experiments with single charge sensitivity, in ultra-high vacuum. We first describe the energy equilibrium associated with the charge transfer, as measured from Kelvin probe microscopy measurement. It reveals the NC quantum confinement (blue-shift of the silicon NC conduction band edges as a function of the nanocrystal size, typically up to the eV range), in good quantitative agreement with tight-binding calculations for hydrogen-passivated silicon nanocrystals [1]. We also show that the charge transfer from doped NCs towards a two-dimensional layer experimentally follows a simple phenomenological law, corresponding to formation of an interface dipole linearly increasing with the NC diameter. These feature lead to analytically predictable junction properties down to quantum size regimes: NC depletion width independent of the NC size and varying as ND-1/3, and depleted charge linearly increasing with the NC diameter and varying as ND1/3. This analytical model establishes a “nanocrystal counterpart” of conventional semiconductor planar junctions, valid in regimes of strong electrostatic and quantum confinements [2]. References [1] L. Borowik, K. Kusiaku, D. Deresmes, D. Théron, H. Diesinger, and T. Mélin, T. Nguyen-Tran and P. Roca i Cabarrocas, Phys. Rev. B 82 073302 (2010) ; [2] L. Borowik, T. Mélin , T. Nguyen-Tran and P. Roca i Cabarrocas submitted (2013).

Atomic dipole moments on a Si(111)-(7×7) surface have been imaged by noncontact scanning nonlinear dielectric microscopy (NC-SNDM). Lowest order nonlinear dielectric constant ε₃ images, acquired from the first order response of tip-sample capacitance to electric field modulation (V(t)=V₀ +V_pcos omega;_pt where V₀ and V_pcos omega;_pt are applied dc and ac bias voltage between the tip and the sample surface, respectively), have resolved upward dipole moments on the adatoms. In addition, it has been recently found that ε₃ images resemble simultaneously measured 2nd harmonic tunnelling current I_2omega;p images [1]. These quantities are connected by the electric fields from the atomic dipoles. As the fields cause shift of surface potential (ΔV_d) (built-in potential), they locally enhance asymmetry of current-voltage characteristics. The relationship of I_2omega;p=(part;³I/part;V³)ΔV_dV_p²/4 (at V₀=0) has been then derived [1]. In order to validate this relationship, we experimentally evaluated ΔV_don the adatoms and that on the corner holes from tunnelling current. We measured topography, ε₃ and ΔV_d images simultaneously which were acquired by NC-SNDM. The ΔV_d image resembled ε₃ one. ΔV_d can be determined using I_2omega;p and the 3rd harmonic tunnelling current I_3omega;p,because I_3omega;p is given by I_3omega;p = (part;³I/part;V³) V_p³/24. ΔV_d is also estimated (by using different method) as ΔV_d^ε₃=-V₀, where ε₃ signal crosses zero at V₀ giving an external field cancelling a field from atomic dipoles. We measured the dependence of ε₃, I_2omega;p, and I_3omega;p on dc bias voltage V₀ acquired on an adatom and corner hole. Here, V_p =1.5V and omega;_p= 2π×60 kHz. The tip was fixed on the target positions using atom tracking technique during successive ten times measurement. Although ΔV_d^ε₃ and ΔV_d are obtained from the different physical quantities, both were 0.3V on adatoms and 0 V on the corner hole. These results show that the shift of local surface potential is attributed to the atomic dipoles, which can be directly imaged by NC-SNDM.
[1] Yamasue, K. and Cho, Y.; J. Appl. Phys. 2013, 113, 014307.

This talk is about the use of Kelvin Probe Force Microscopy (KPFM) to map the surface potential of Poly(L-lactic) acid films and shed light on the role of polarization and charge effects on selective protein adsorption and cell proliferation.
It has been shown that piezoelectricity is present in some living tissues such as bone with piezoelectric collagen, dentin, and tendon among others. The piezoelectricity in bone is due to the piezoelectric character of collagen, and the alignment of collagen fibers in response to an applied electric field was observed, although the exact mechanisms behind bone mineralization and bone growth are still unknown. Since then the use of piezoelectric materials for bone growth has been exploited and in vivo tests have shown bone regeneration acceleration.
Poly(L-lactic) acid (PLLA) is one of the few polymers approved by the FDA for clinical human applications presenting a unique set of properties as biodegradability, biocompatibility and piezoelectricity. Within this context we have been studying the electrically induced polarization of a PLLA thin film and our results showed the selective protein adsorption in electrically poled areas of the PLLA films (Barroca 2011, 2010). To clarify the role of charges at the surface of the film, in the present work we use Kelvin Probe Force Microscopy (KPFM) to map the surface potential of PLLA films. PLLA thin films were obtained by spin coating of a 5% poly L-lactic acid solution on Pt/ TiO2 / SiO2 / Si substrates. This solution was prepared by dissolving PLLA pellets (Purac) in 1,4 dioxane (Panreac) at 80 °C, which was then stabilized at room temperature. The films were prepared by spin coating on the substrates. The thickness of the resulting films was approximately 300 nm. Spin-coated PLLA thin films were submitted to a thermal annealing treatment for crystallization. Films were heated at 190 °C for 30 min, then maintained at the crystallization temperature 80 °C for 15 min, and finally frozen at minus;16 °C. Charge writing of the films was carried out through the atomic force microscope tip and surface potential distribution was characterized by KPFM in air. The obtained results are discussed in terms mechanical and electrical charge writing and the dependence on films morphology and crystallography established. The obtained results are also discussed in the light of biomedical applications.

Thin Pb films epitaxially grown on 7x7 reconstructed Si(111) represent an ideal model system for study of electron-phonon interaction at metal-insulator interface. For this system, using combination of scanning tunneling microscopy and inelastic electron tunneling spectroscopy we performed direct real-space imaging of electron-phonon coupling parameter. We found that lambda; increases when electron scattering at Pb/Si(111) interface is diffuse and decreases when electron scattering is specular. We show that the effect is driven by transverse redistribution of electron density inside a quantum well.
Reference:
Igor Altfeder, K. A. Matveev, A. A. Voevodin, ``Imaging the Electron-Phonon Interaction on the Atomic Scale'', Physical Review Letters 109, 166402 (2012)

We report on the first antenna-enhanced optoelectronic microscopy studies on nanoscale devices. Our approach is based on tip-enhanced near-field optical microscopy (TENOM) that has been mainly used for purely optical applications offering sub-diffraction spatial resolution [1]. Here we show that TENOM can be employed to probe optoelectronic signals as present in photovoltaic or light-emitting devices. By coupling either the emission or excitation to a scanning optical antenna, we are able to locally enhance the electroluminescence or photocurrent along a carbon nanotube device. We show that the emission source of the electroluminescence can be point-like with a spatial extension below10 nm. Topographic and antenna-enhanced photocurrent measurements reveal that the emission takes place at the location of highest local electric field indicating that the mechanism behind the emission is the radiative decay of excitons created via impact excitation. Our findings underline the importance of the optical antenna concept within the field of optoelectronic characterization of nanoscale devices.
[1] Hartschuh, A. Angew. Chemie (Int. Edition) 2008, 47, 8178

The scanning tunneling microscope (STM) used in spectroscopic mode (STS) allows electronic measurements at the surface of a conductive material with high lateral spatial resolution, without the need to form ohmic contacts. However, analysis of STS data on semiconductors is complicated by the effect of tip-induced band bending (TIBB) whereby a portion of the tip potential drops within the material itself. Conversely, the potential can be screened by the possible existence of surface states that counteract the TIBB effect. These phenomena can be overlooked when attempts are made to measure electronic band gaps from STS, often leading to quantitative misinterpretation. In this work, we demonstrate a rigorous analysis of STS data obtained on sulfide semiconductors including FeS2. We show how such experiments can provide more accurate, quantitative insight into surface electronic properties by coupling with first principles modeling of potential distributions and tunneling currents, as informed by density functional theory (DFT) calculations. This STM-based approach is applicable to any semiconducting material and can be used to quantify surface electronic band gaps more accurately and without contacts, which is particularly important when contrasting surface states with those of the bulk.

Photothermal microscopy has rapidly emerged as the most sensitive label-free optical microscopy method rivaling even fluorescence microscopy. Photothermal microscopy uses the change in temperature from the absorption of the pump laser beam, which causes change in optical index of refraction. Because the thermal diffusion properties are different at different time-scales, the modulation frequency affects the spatial resolution. Here, we use a photothermal heterodyne imaging system with Ti:sapph 100-fs pulsed laser to image dyed polystyrene beads and red blood cells. We present the differences in spatial resolution between images (both the amplitude and phase images) obtained simultaneously with a broad modulation frequency range, from 3 kHz to 20 MHz. The time dependence is also tested by varying the pump-probe delay time from few ps to ~12.5 ns.
We thank the National Institutes of Health (NIH) the National Science Foundation (NSF) for support.

Mid-infrared light confined to nanoscale volumes is a powerful and versatile probe of Graphene physics. For instance, recent IR-sSNOM experiments show that one can excite, image, and spectrally characterize Dirac plasmons in Graphene with nanoscale sensitivity. Due to the uncertainty principle: Δx Δk ge; 0.5, conventional IR microscopy lacks both the spatial momentum and spatial resolution necessary to launch and simultaneously image such surface waves. In the IR-sSNOM technique, nanoconfinement of IR light between a sharp metallic tip (<10nm radius) and the sample surface can reduce Δx to <10nm and consequently increase Δk by as much as 3 orders of magnitude over the diffraction limited value. The IR frequency of the excitation, however, remains unchanged. Beyond the recently demonstrated plasmonic applications, we believe the potential for using IR light with such a large spatial momentum is not yet fully recognized by the Graphene or SPM communities. In this work, we demonstrate yet another consequence of the extremely large Δk of naconfined IR light: ultrasensitivity to Graphene thickness. We show this phenomenon experimentally with high resolution (<20nm) IR sSNOM images which clearly show monotonically increasing contrast for 1,2,3, 4 Graphene layers. We confirmed the layer number by colocalized confocal Raman and high resolution AFM measurements. Our proof of principle experiment confirms a 3D sensitivity for nanoconfined IR light to be better than 20 x 20 x 0.35nm for single to multilayer Graphene samples.

Electrical impedance characterization at the nanoscale is a challenge for beyond CMOS investigations and for understanding the electronic properties of nanomaterials. Among the various scanning probe microscopes, Scanning Microwave Microscope (SMM) is of particular interest because it combines nanometric lateral resolution of atomic force microscopes and sub-fF capacitance sensitivity.[1,2] In particular, using an interferometer, such sensitivity may be further increased, but for reaching aF scale capacitance sensitivity, the probe parasitic capacitance starts to be the limitation.[2] For example, sub-100 nm diameter metallic pads evaporated on 100 nm-thick thermal SiO2 are not distinguished on S11 amplitude signal.
In order to overcome this limitation, but also to address clearly the lateral electrical resolution of SMMs, we have fabricated two types of devices: 1- An “on chip calibration kit” composed of patterned metallic pads evaporated on 100 nm-thick SiO2 that provides reference capacitors. 2-“gold nanodots” are nanoscale bottom gold electrodes (from 5 nm to 200 nm) with an ohmic contact to a highly doped silicon subtrate[3], covered either by self-formed SiO2 during annealing process or by atomic layer deposition of Al2O3 (5 Å and 5 nm). The top electrode is the SMM Pt tip.
Nanodot-based Capacitance estimation by finite element analysis with a given load applied on the tip and measured capacitance by SMM (with consideration of the calibration kit) are corroborated. Capacitances as small as 10 aF are measured with a lateral resolution of 30 nm. Finally, we address the role of tip shape, atmosphere (presence of N2), impact of hydrophobic molecules and discuss models when tunnel current is large.
We thank D. Ducatteau for helpful comments and University Lille1, Young researcher grant Singlemol from Nord pas de Calais and ANR Excelsior project for fundings.
1 H. P. Huber, M. Moertelmaier, T. M. Wallis, C. J. Chiang, M. Hochleitner, A. Imtiaz, Y. J. Oh, K.Schilcher, M. Dieudonne, J. Smoliner, P. Hinterdorfer, S. J. Rosner, H. Tanbakuchi, P. Kabos, and F.Kienberger, Rev.Sci.Instr. 81, 113701 (2010)
2 T. Dargent, K. Haddadi, N. Clément, D. Ducatteau, B. Legrand, H. Tanbakuchi and D. Théron, submitted to Rev.Sci.Instr.
3 N. Clément, G. Patriarche, K. Smaali, F. Vaurette, K. Nishiguchi, D. Troadec, A. Fujiwara and D.Vuillaume, Small 2011 doi: 10.1002/smll.201100915

Scanning probe microscopy is an essential research tool for both fundamental and technology-driven studies, mapping diverse functional properties in a wide range of materials with a nanoscale resolution, defined by the specific tip geometry and properties. Its imaging capacity can be significantly enhanced by carbon-nanotube-based (CNT) tips [1], whose small size and outstanding mechanical and electrical properties improve resolution in topographical, electrostatic and surface potential microscopy modes. Further modification of the CNT tips [2-3] extends their application to magnetic force microscopy, when ferromagnetically coated, and to contact-based techniques, when rigidified with an insulating silicon dioxide layer. In the rapidly developing field of domain wall nanoelectronics, such ultra-high resolution tips could provide unprecedented access to the novel and highly localized functional properties of domain walls in ferroelectric and magnetic materials.
Here, we report on extended studies of functionalized CNT tips, benchmarking their properties against commercially available tips in different applications. Introducing molecular beam epitaxy deposition of cobalt on single-walled CNT, we demonstrate sub-10 nm resolution in hard magnetic samples, and non-perturbative magnetic force microscopy imaging of complex domain and domain wall (DW) structures in micropatterned permalloy, both at ambient conditions [4]. Integrating insulator-encapsulated tips in local conduction measurements, we demonstrate ultra-high current carrying capacity, with the dielectric providing a Joule heat sink, as well as a chemical barrier against oxidation [5]. In both cases, we also develop theoretical models allowing us to extract the key imaging parameters and their dependence on the tip properties, and tailor the tip design to optimize resolution and reliability under given measurement conditions.
References:
[1] J.H. Hafner. et al. J. Am. Chem. Soc. 121, 9750 (1999)
[2] H. Kuramochi. et al. Nanotechnology. 16, 24 (2005)
[3] N. Tayebi. et al. Appl. Phys. Lett. 93, 103112 (2008)
[4] Y. Lisunova et al. Nanotechnology 24 105705 (2013)
[5] Y. Lisunova, I. Levkivskyi, and P. Paruch. Ultra-high currents in dielectric-coated carbon nanotube probes. In submission.

Contact resonance (CR) is an emerging mode of atomic force microscopy (AFM) that operates in dynamic contact at or near the cantilever resonance. The value of CR to obtain nanomechanical data (stiffness, damping, etc.) has been demonstrated on materials ranging from stiff polymers to metals and ceramics. Here, we describe recent advances to achieve viscoelastic measurements for a wider range of materials and operating conditions. We first show how quantitative values of the viscoelastic loss tangent tan δ can be directly determined from the CR peak frequency f and quality factor Q. CR results for tan δ acquired in air and at room temperature compare favorably with those from macroscale dynamic mechanical analysis and microscale dynamic nanoindentation. Because viscoelastic properties depend strongly on temperature, extension of quantitative CR methods towards material heating is a logical and necessary step. The dramatic changes in tan δ near a polymer&’s glass transition temperature also provide a further test of our methods. Temperature-dependent CR data with local (tip) or global (sample stage) heating are presented, and challenges to quantitative analysis such as temperature-dependent f and Q, viscoelastic creep, and the nonideal geometry of heated-tip cantilevers are explained. Finally, we describe progress on CR operation in liquids, the natural or operating environment for many industrial and biological materials. Measurement and analysis issues include resonance excitation that minimizes spurious vibrations and modeling the changes to f and Q from air to liquid. All of these results move us closer to the goal of a versatile AFM tool for accurate mapping of viscoelastic properties.

Many advanced dynamic Atomic Force Microscopy (AFM) techniques require a vibrating cantilever tip to be continuous contact with the sample. Contact resonance, force modulation, piezoresponse force microscopy, electrochemical strain microscopy and AFM infrared spectroscopy are several examples. These techniques usually assume a certain shape of cantilever vibration in order to achieve quantitative results; however, these vibration shapes are not directly measured. Here we present a technique that allows in-situ measurements of the vibrational shape of AFM cantilevers coupled to surfaces, thus opening unique approaches to nanoscale material property mapping that are not currently possible.

Our experimental procedure is applied to contact resonance AFM experiments and described as follows. First, the cantilever is brought into contact with the sample surface and held at a constant normal force. Second, the excitation frequency is swept over the range of experimental interest. Third, the laser position is moved to a new spot and the excitation frequency swept again. This procedure is repeated until the laser spot position is moved along the entire length of the cantilever. This measurement is facilitated on the test AFM system by use of controlled electrical actuators to adjust the laser spot position. The result of this experiment is a “spectrogram” in which amplitude and phase of the cantilever response is plotted as a function of laser spot position and excitation frequency. This procedure has the advantage over similar competing techniques in that it is implemented “in situ” in a standard commercial AFM and does not require interfacing additional equipment, such as an interferometer.
Experimentally measured spectrograms are compared to spectrogram predicted with contact resonance AFM theory. These comparisons show that most aspects of the cantilever response matches between theory and experiment, providing important experimental validation of the applied contact resonance AFM model. However, some interesting differences between theory and experiment are observed suggesting that some aspects of the applied model, namely force transfer between the tip and the sample and damping, could be improved. More work is needed to study the effects of such improvements on contact resonance AFM material property predictions; however, it is hoped that this study provides new insights into the prediction of material properties with contact resonance AFM. It should be noted that the employed experimental technique can be extended to study any dynamic AFM method allowing for a wide potential applications.

Equipped with subnanoscale spatial resolution, atomic force microscopy (AFM) has been proven to be the most versatile tool in probing mechanical properties of nanoscale volumes. In the last few years, various AFM capabilities have been developed to characterize elastic, viscoelastic, and adhesive properties of materials at the nanoscale. In this work, we performed depth-dependent contact resonance AFM measurements over polystyrene-polypropylene polymer blends to detail their surface and near sub-surface mechanical response in terms of elastic modulus and dissipated power. The depth-dependences of the measured parameters were analyzed in great detail from cross-sectional images of three-dimensional tomographic reconstructions. Through a suitable normalization of the measured contact stiffness and indentation depth, the depth-dependence of the contact stiffness was analyzed by linear fits to obtain the elastic moduli of the materials probed. The analysis allowed us also to differentiate the contribution of adhesive forces (short-range versus long-range) to contact on each material without a priori assumptions. The adhesion analysis was complemented by an unambiguous identification of distinct viscous responses during adhesion and in-contact deformation from the dissipated power during indentation. With these developments we added three-dimensional capabilities to contact resonance atomic force microscopy with dynamic characterization of the probe-sample mechanics.

Contact resonance (CR) methods are dynamic contact AFM techniques that provide accurate, sensitive nanomechanical spectroscopy and mapping. To date, CR measurements have been performed in air. Here we present work to achieve CR spectroscopy in a liquid, an environment necessary for correct evaluation of many biological and industrial materials. We show how the liquid environment introduces significant challenges to both data acquisition (resonance excitation) and data analysis (model interpretation). For resonance excitation we utilize thermal, or Brownian, motion of the cantilever to mitigate the “forest of peaks” phenomenon, which is exacerbated in contact compared to free space. Passive thermal excitation couples significantly less spurious vibration through the liquid than conventional acoustic (piezoelectric) excitation. For data analysis we must account for changes in the CR frequency and quality factor from air to liquid. In our experiments, the contact frequencies were observed to decrease by up to 60 % from air to liquid. To accurately model the changes, we include surface-coupled hydrodynamic forces. A new technique is developed to experimentally reconstruct the theoretical hydrodynamic function of the system, which governs the frequency-dependent fluid loading present in the CR system. Results are shown for thermal CR spectra obtained in water and corrected for hydrodynamic effects. The CR frequencies in liquid after correction compare favorably to those obtained from thermal spectra acquired in air, and are within 10 % of the original air values. CR operation in liquids will enable nanoscale characterization in a wide range of new applications including energy production and storage, civil infrastructure, and health care technology.

Contact resonance (CR) AFM is a technique that can provide quantitative and sensitive stiffness mapping of surfaces with nanoscale lateral resolution. In typical CR-AFM, a piezoelectric transducer under the sample acoustically induces the resonances of an AFM cantilever in contact, and these contact resonances are detected to determine the viscoelastic property distribution on the surface. However the sample variety for CR-AFM imaging has been limited, since it can only be used in air. To date, CR-AFM in liquid has been considered challenging due to the complicated contact dynamics in liquid and due to additional experimental problems, such as very low quality factors and vibrational couplings.
We developed an analytical model for liquid CR-AFM by accounting for the hydrodynamic damping of cantilevers, and combining this model with cantilever contact dynamics models. With this model we can calculate the stiffness of surfaces in liquid by using the contact resonance spectra. In our liquid CR-AFM model, the fluid loading and damping on the cantilever are frequency dependent, which directly affects the contact resonance frequencies and quality factors. We verified the validity of the CR-AFM model by measuring the contact resonances experimentally for different cantilevers. Furthermore, our experimental and analytical liquid CR-AFM studies show that the quality factors of the contact resonances are reduced significantly due to the high fluid damping, which decrease the sensitivity of the method. In addition, the acoustic actuation by a transducer underneath the sample support, as it is common in dry CR-AFM, causes fluid motion and undesired vibrational couplings to the cantilever. Combined with the low quality factors, these vibrational couplings affect and deteriorate the contact resonances and make reliable contact resonance detection difficult.
To address these issues, we employed three methods in liquid CR-AFM: Q-control, magnetic actuation, and cantilever modifications. We performed liquid CR-AFM experiments with Q-control and observed that the quality factors can be increased electronically, but the vibrational couplings still affect the contact resonances. Our experiments show that an effective way of reducing the vibrational coupling is to actuate the cantilever directly, as it can be done in magnetic actuation. However, even with a direct actuation the low quality factor reduces the stiffness detection sensitivity. Therefore, we modified the cantilever with a concentrated magnetic mass to enhance the quality factor and to provide direct magnetic actuation at the same time. We modified the liquid CR-AFM model to analyze cantilevers with magnetic actuation and concentrated mass. Then, we verified the model experimentally and mapped the stiffness of organic and inorganic samples by liquid CR-AFM.

Atomic force acoustic microscopy (AFAM) constitutes a highly
promising upcoming scanning probe microscopy technique, that allows to quantify
mechanical properties of samples in surface proximity with nanometer
resolution. Basically, a cantilever is brought into direct contact with the
sample surface with a predefined static load, while broad band longitudinal
acoustic waves are fed into the sample by an ultrasound transducer and excite
the cantilever-sample system to vibrations. From the cantilever eigenfrequencies
during material contact information on the mechanical surface properties, viz.
the indendation modulus, is accessible with nanometer resolution.
Within the present contribution we first review the fundamentals of this technique, report
about our implementation into a standard atomic force microscope (AFM) and
address model assumptions that are employed within finite element calculations
to extract indentation moduli from experimental data. We then report about application
to a particularly intersting example, viz. nanomechanical characterization of single-crystalline
14M modulated martensitic Ni-Mn-Ga films, which were fabricated by sputter deposition
on magnesium oxide substrates at elevated temperatures. One central focus is the relation between
mechanical response and nanostructure. Comparing experimental indentation
moduli obtained with CR-AFM with our theoretical predictions based on
density functional theory (DFT) indicates a central role of pseudoplasticity
and intermartensitic phase transitions for mechanical response. Spatially
highly resolved mechanical measurements allow for quantification of mechanical
properties around twin boundaries, while imaging of mechanical contrast
identifies them as mechanical heterogeneities of low indentation modulius
on the surface. As the AFAM technique constitutes a rather new approach,
advantages, drawbacks and possible imaging artifacts are also carefully addressed.
[1] A.M. Jakob, M. Müller, B. Rauschenbach and S.G. Mayr, New Joural of Physics,
14, 033029 (2012)