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-dependenc