Scanning tunneling microscopy (STM) provides atomic resolution but fails to provide chemical sensitivity in complex situations. X-rays, however, provide that chemical sensetivity. In this talk we will discuss the development of a novel high-resolution microscopy technique for imaging of nanoscale materials with chemical, electronic, and magnetic contrast [1,2]. It combines the sub-nanometer spatial resolution of STM with the chemical, electronic, and magnetic sensitivity of synchrotron radiation [3]. Drawing upon experience from a prototype that has been developed at the Advanced Photon Source to demonstrate general feasibility [4], current work has the goal to drastically increase the spatial resolution of existing state-of-the-art x-ray microscopy from only tens of nanometers down to atomic resolution. Key enabler for high resolution is the development of insulator-coated “smart tips” with small conducting apex [5] and advanced measurement electronics [6]. The novel microscopy technique will enable fundamentally new methods of characterization, which will be applied to the study of energy materials and nanoscale magnetic systems. A better understanding of these phenomena at the nanoscale has great potential to improve the conversion efficiency of quantum energy devices and lead to advances in future data storage applications.#65279;
The author acknowledges generous funding by the Office of Science Early Career Research Program through the Division of Scientific User Facilities, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant SC70705. Work at the Advanced Photon Source, the Center for Nanoscale Materials, and the Electron Microscopy Center was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract DEAC02-06CH11357.
[1] V. Rose, J.W. Freeland, S.K. Streiffer, “New Capabilities at the Interface of X-rays and Scanning Tunneling Microscopy”, in Scanning Probe Microscopy of Functional Materials: Nanoscale Imaging and Spectroscopy, S.V. Kalinin, A. Gruverman, (Eds.), Springer, New York (2011), pg 405-432.
[2] M.L. Cummings, T.Y. Chien, C. Preissner, V. Madhavan, D. Diesing, M. Bode, J.W. Freeland, V. Rose, Ultramicroscopy 112, 22 (2012).
[3] V. Rose, K.K. Wang, T.Y. Chien, J. Hiller, D. Rosenmann, J.W. Freeland, C. Preissner, S.-W. Hla, Adv. Funct. Mater. 23, 2646 (2013).
[4] V. Rose, J.W. Freeland, K.E. Gray, S.K. Streiffer, Appl. Phys. Lett. 92 (2008) 193510.
[5] V. Rose, T.Y. Chien, J. Hiller, D. Rosenmann, R.P. Winarski, Appl. Phys. Lett. 99, 173102 (2011).
[6] Kangkang Wang, Daniel Rosenmann, Martin Holt, Robert Winarski, Saw-Wai Hla, and Volker Rose, Rev. Sci. Instrum. 84, 063704 (2013).
Webpage: http://www.aps.anl.gov/Xray_Science_Division/Sxspm/

Thermoelectric effects in tunnel junctions are currently being revisited for their prospects in cooling and energy harvesting applications, as well as sensitive probes of electron transport. Tunneling thermovoltage is also a valuable observable in scanning probe microscopy, with the earliest examples of chemical contrast in the images dating back to the dawn of STM. Quantitative interpretation of these effects calls for advances in both theory and experiment, particularly with respect to particle transmission probability across a tunnel barrier which encodes the energy dependence and magnitude of tunneling thermovoltage. Other unknowns include electronic structure of the tip and the thermal gradient across local junctions.
We will present non-contact thermovoltage measurements, focusing on distance-dependence of tunneling thermovoltage in vacuum tunnel junctions [1, 2] and variability of thermoelectronic contrast on noble metal surfaces. We have shown that it is possible to: (1) decouple vacuum and material-specific thermoelectronic contributions in thermovoltage of a nanojunction; (2) directly estimate thermal gradient in the tunnel junction from the distance-dependence of thermovoltage in tunable junctions; (3) account for tip effects, and thus estimate material-specific parameters; (4) experimentally measure the transmission function and convert it to thermovoltage using the Landauer formalism. With this combined approach, we have been able to infer the predominant origin of the thermoelectric contrast due to single-atomic steps and nanoparticles on silver surfaces, finding a large surface-resonant enhancement of thermovoltage. Experimental measurements and analysis have been complemented and confirmed by state-of-the-art theoretical analysis of local thermopower on silver, gold and copper surfaces within the first-principles-based open-boundary transport formalism. Altogether, we paved the way toward systematic analysis of energy and temperature-dependence of local thermoelectronic effects, with future applications in layered, 2D and nanostructured materials where the surface effects will dominate thermoelectric performance. Moreover, surface state enhancement similar to that observed on silver surfaces can make tunnel junctions very competitive if not superior to other junctions so far considered as candidates for thermoelectronic applications.
Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. J.I.C. acknowledges financial support from the Spanish Ministerio de Economía y Competitividad (grant MAT2010-18432).
[1] P. Maksymovych, J. Vac. Sci. Tech. B, 31 (031804) 2013.
[1] P. Maksymovych, S. J. Kelly, M. A. McGuire, and J. I. Cerda, “Resonant enhancement of tunneling thermovoltage on silver surfaces”, submitted (2014).

Owing to its high conductivity graphene holds promise to be used as electrodes in batteries and photovoltaics. However, to this end the work function and doping levels in graphene need to be precisely tuned. We recently showed that controlled covalent modification via electrochemical grafting of molecules^1,2 can be a robust method that allows for local modification of the graphene&’s electronic properties. By using aryldiazonium salts, in particular, bis(4-nitrophenyl)iodonium tetrafluoroborate, the grafting density can be adjusted from 4 x 10¹³ to 3 x 10¹⁴ molecules/cm². The local electronic structure and nature of chemical bonding is studied via a combination of Low Temperature Scanning Tunneling Microscopy (LT-STM) and Scanning Tunneling Spectroscopy (STS) using an Omicron LT-STM/SPM system. Several typed of grafts have been identified. One of the most commonly observed modifications shows a distinctive three fold symmetry in the STM images, while the density of states (DOS) map contains two separated lobs indicative of the presence of two molecules. By using computational work we show that the second molecule attaches on the same graphene sublattice in the meta position suggesting that the grafting mechanism is controlled by kinetics rather than thermodynamics. In addition to the discussed modifications, extended modified regions (larger than 4nm) have been observed. We show that these regions are responsible for opening a band gap in graphene of approximatelly 150meV as identified by STS, transistor and photoemission measurements as well as IR absorption. Transistor data show in addition that some -NO₂Ph grafts of unidentified structure induce n-type doping in graphene consistent with charge transfer from the grafts to the graphene.
In an effort to further tune the doping level and work function of graphene we are currently in the process of covalently modifying the graphene with either donors, such as tertthiophene, perylene acceptors or a mixture of the two.
1. C. K. Chan, T. E. Beechem, T. Ohta, M. T. Brumbach, D. R. Wheeler, K. J. Stevenson, J. Phys. Chem. C2013, 117, 12038.
2. K. J. Stevenson, P. A. Veneman, R. I. Gearba, K. M. Mueller, B. J. Holliday, T. Ohta, and C. K. Chan, Faraday Discussion 1722014.

When one considers spin-1/2 ferromagnetic (antiferromagnetic) spin chain or spin clusters, several ground states can be found. Depending on choice of adsorbate metals and substrates, the apparent exchange coupling and the consequent ground-state structures can be different. In a recent spin-polarized scanning tunneling microscopy (STM) study, the amplitude and sign of magnetic anisotropy of single Fe atom on a Pt (111) surface were manipulated by varying adsorption sites¹. We extended this study by choosing various adsorbates such as Co, Ce, Nd, Sm, Gd. In order to test one-dimensional chain and two-dimensional island effects, Pt(111), Pt(110), Cu(111), Cu(110) and Bi₂Se_xTe_3-x are used. Substrate-mediated RKKY type interaction was observed in ferromagnetic and antiferromagnetic chains. When we applied a magnetic field, the preferred orientation of the magnetic moment varied at different binding sites on different substrates. Amplitude of magnetic anisotropy was estimated and compared with our density functional theory calculation. Difference and similarity between d- and f- electrons will be presented in the present study. Based on observation we made, we found that ground state structure in two dimensional systems are more complex than those in one-dimensional chains.
[1] A. A. Khajetoorians, T. Schlenk, B. Schweflinghaus, M. dos Santos Dias, M. Steinbrecher, M. Bouhassoune, S. Lounis, J. Wiebe, R. Wiesendanger, Phys. Rev. Lett. 111, 157704 (2013)

We report on state-of-the-art scanning probe microscopy measurements performed in a pulse tube based top-loading closed-cycle cryostat with base temperatures as low as 1.5 K and a 9 T magnet [1]. By fine-tuning our instrument, we were able to significanlty lower the level of mechanical and acoustic noise coupling into a measurement. To demonstrate the performance of the system, we successfully imaged the 0.39 nm lattice steps on single crystalline SrTiO₃, as well as magnetic vortices in a high-T_c superconductor (Bi₂Sr₂CaCu₂O_8+x). Fine control over sample temperature and applied magnetic field further allowed us to probe the helimagnetic and the skyrmion-lattice phases in Fe_0.5Co_0.5Si with unprecedented signal-to-noise ratio of 20:1. Finally, Piezo-response Force Microscopy (PFM) was demonstrated on a thin film of BFO in a read and write experiment at low temperatures, as well as on TmFe₂O₄ at 100 K as a function of magnetic field (+/-9 T).
[1] F.P. Quacquarelli, J. Puebla, T. Scheler, D. Andres, C. Bödefeld, B. Sipos, C. Dal Savio, A. Bauer, C. Pfleiderer, A. Erb, and K. Karrai, arXiv:1404.2046v1 (2014).

Recently, thin gate oxide films become thinner as semiconductor devices have been miniaturized continuously. On these devices, it has become more important to understand physical properties of materials in an atomic-scale. In particular, electric dipoles at material surfaces and interfaces affect characteristics of the highly miniaturized devices such as a threshold voltage. Therefore, to obtain such highly miniaturized and high performance devices, we have to develop a simultaneous observation technique for the electric dipoles and the topography at material surfaces. Non-contact scanning nonlinear dielectric microscopy (NC-SNDM) can image topography and distribution of electric dipoles at material surfaces simultaneously. For example, this microscopy has been applied to acquire the distribution of atomic dipole moments on a clean Si(111)-7×7 surface [1] and a hydrogen-adsorbed Si(111)-7×7 surface [2] so far. Since, mainly on the Si(100) surface, thin gate oxide films of semiconductor devices have been grown, the study of the electric dipoles on this surface is of practical importance for the further miniaturization of semiconductor devices.
Therefore, in this paper, we present the application of NC-SNDM imaging to a Si(100) surface. At first, we show experimental result that negative electric dipole moments are locally formed on individual dimers on the surface from simultaneous images of topography and dipoles. The observed negative dipoles are formed on a dimer by a charge transfer between the two atoms in the dimer. One of these two atoms forming the dimer receives about one single electron from the other atom. Then, forming a sp³ like structure, the atom receiving about one single electron protrudes to the vacuum side. In contrast, the other atom, which loses about one single electron in a dangling-bond, sinks to the bulk side by forming a sp² like structure [3]. Therefore, a formed dipole moment direction is toward the lower atom from the higher atom and then a negative normal component is observed on the dimers. In addition, we obtained the dc-bias voltage dependence of the dipole moment on a specific dimer by using an atom-tracking technique. As a result, we observed that the dipole generated a surface potential of about -250mV on the dimer. These results indicate that we can examine a dipole moment on individual dimers on the cleaned Si(100) surface, and, potentially, initially oxidized one by using NC-SNDM.
[1] Y. Cho and R. Hirose: Phys. Rev. Lett. 99, 186101 (2007).
[2] D. Mizuno, K. Yamasue and Y. Cho: Appl. Phys. Lett. 103, 101601 (2013).
[3] L. Pauling and Z. S. Herman: Phys. Rev. B 28, 6154 (1983).

Noncontact scanning nonlinear dielectric microscopy (NC-SNDM) is a useful tool for simultaneously imaging topography and dipole moment distribution of material surfaces in a nanoscale. This microscopy measures the lowest order nonlinear dielectric constant (ε₃) of a sample surface through the modulation of tip-sample capacitance by an external electric field. Since the polarity of ε₃-signal is sensitive to the direction of a dipole under the tip, we can determine dipole moment distribution on a sample surface. The high sensitivity up to 10^-22 F to the capacitance variation has been achieved by frequency modulation based detection. As a result, NC-SNDM can atomically resolve topography and dipole moment distribution on a cleaned and hydrogen-adsorbed Si(111)-(7×7) surfaces [1, 2].
In this presentation, we propose a novel method for the quantitative measurement of the potentials induced by permanent surface dipoles. In the proposed method, a 2D map of the dipole-induced-potentials is acquired by applying a dc potential nulling local ε₃-signal during the scan of a surface. This can be easily achieved by implementing an additional feedback loop to control the sample potential. Since this additional loop is operated together with the main z-feedback, we can simultaneously acquire topographic and potential images. The fundamental difference from Kelvin Probe Force Microscopy (KPFM) is that the proposed method is sensitive to surface dipoles. This is because ε₃-signal is only governed by the response of dipole moments to external fields. In contrast, KPFM senses a electrostatic force resulting from fixed charges and contact potential difference as well as dipoles. It is therefore difficult for KPFM to distinguish dipole-induced potentials (In particular, the dipole-induced-potentials cannot be measured in air due to charge compensation). The feasibility of the method is demonstrated by the measurement of a Si(111)-(7×7) surface. We successfully acquired an atomic resolution potential image simultaneously with topograhy. As a result, the localized potentials about 0.5 V were observed on individual adatoms. The result is consistent with the previous measurement using tunneling current [3]. In addition, the method was applied to monolayer graphene synthesized on a 4H-SiC(0001) surface to investigate the effects of hydrogen-intercalation [4]. These results indicate that the proposed method is useful for quantitative investigation of surface dipoles on an atomic-scale.
The authors would like to thank Mr. Kazutoshi Funakubo, Prof. Hirokazu Fukidome, and Prof. Maki Suemitsu, Tohoku University, for providing a high quality monolayer graphene sheet on 4H-SiC(0001).
References:
[1] Y. Cho and R. Hirose, Phys. Rev. Lett. 99, 186101(2007).
[2] D. Mizuno, K. Yamasue, and Y. Cho, Appl. Phys. Lett. 103, 101601(2013).
[3] K. Yamasue, M. Abe, Y. Sugimoto, and Y. Cho, (submitted).
[4] K. Yamasue, H. Fukidome, K. Funakubo, M. Suemitsu, and Y. Cho, (submitted).

The development of surface microscopy methods to image buried interfaces has generated a good deal of recent interest. A number of scanning-tunneling microscopy (STM) works of thin insulating oxide films grown on metallic substrates have suggested that for low biases within the band gap of the insulating film, the STM should probe the metal atoms at the interface. A prototypical system is rock-salt MgO films on silver substrates where theory and experiment have shown STM contrast through ultrathin (1-2 monolayers) films of MgO, and the contrast has been assigned to the Ag substrate. However, for commensurate structures, the Mg and O atoms in the film overlap the positions of the Ag atoms complicating any such assignment. We have used and developed first-principles theoretical methods to study this system and are able to show that the STM contrast is in fact dominated by the surface atoms of the thin film. Hence, in such a system, the electronic states at low bias originate from the metal but are transmitted evanescently via the discrete lattice of the oxide film, which can create complex patterns that cannot be simply related to the positions of the underlying metal atoms.

Ferroelectric copolymer P(VDF-TrFE) has been attractive for various applications such as information storage devices and energy harvesting systems due to flexibility and low processing temperature, etc. There have been a couple of reports on the irradiation effect on the ferroelectric and electrical properties in complex oxides. Although the irradiation can be expected to significantly affect material properties of polymer systems, there has not been reported in the polymer systems. Here, we present x-ray irradiated effect on the ferroelectric and electrical properties in the P(VDF-TrFE) co-polymer films using atomic force microscopy. Interestingly, it was found that new phase was formed after the irradiation. Accordingly, ferroelectric and electrical properties have been changed after the irradiation. Using switching spectroscopy piezoresponse force microscopy, we observed that local switching of the irradiated state has been degraded than that of the pristine state. Furthermore, we were able to observe changed internal and surface electrical properties using Kelvin probe force microscopy. These results show that x-ray irradiation directly affects ferroelectric and electrical properties of the P(VDF-TrFE) by phase transition. Furthermore, the obtained results could provide additional information on the ferroelectric and electrical properties of the P(VDF-TrFE) copolymer films at the nanoscale.

Recent developments in combining lightinduced molecular excitations with mechanical force detection are of particular interest, as these approaches seek to add chemical selectivity to force microscopy. There is a novel technique, optical force microscopy, which probes the optically induced changes in the dipolar interactions between a sharp polarizable tip and the sample. We provide a simple description of the operating principle of the optical force microscope in terms of classical fields and forces. By including both the attractive dipole-dipole interactions and the repulsive scattering force, we simulate the characteristics of the force-distance curve and compare the predicted profile with experimental results obtained from gold nanowires and molecular nanoclusters. Our study identifies the distance regimes in which attractive dipole-dipole forces are dominant and points out under which conditions molecular selective signals are optimized. Finally, our experiments show that the technique can be carried out both in cw illumination mode as well as in femtosecond illumination mode, paving the way for more advanced nonlinear optical investigations at the nanoscale level.

Advanced measurement techniques possessing nanoscale and atomic scale resolution have played a pivotal role in the development of photovoltaic, energy storage, and fuel cell technologies over the past decade. Amongst all such characterization tools, scanning tunneling potentiometry (STP) provides indispensable insight into the nature of the potential drop within such devices. Recent STP advances have been driven by a desire to better understand charge transport across active interfaces at ever increasing levels of resolution. The ultimate resolution limit of any scanning probe method lies at the scale of single atoms and even atomic orbitals. Though this is now routinely achievable with scanning tunneling microscopy (STM), the conclusive observation of atomic scale features in STP imaging has remained elusive. This is partly due to the fact, that it is not fully understood what unique potential drop features one might expect to observe at atomic length scales minus; if any at all. Motivated by the desire to better chart this unknown, we present a theoretical first-principles study of atomic scale STP imaging.

The controlled adsorption and manipulation of transition metal oxide nanoclusters on substrates with known template properties is a promising approach for the creation of new and unconventional memory devices on the nanoscale.
In this work the adsorption of tungsten oxide clusters deposited on an ultrathin titanium oxide film grown on the Pt₃Ti(111) surface has been investigated using high resolution scanning tunneling microscopy (STM). Former studies have shown that the exposition of oxygen under UHV conditions on the bimetallic Pt₃Ti alloy surface at elevated temperatures results in the formation of an atomically thin and highly ordered TiO film, which covers the entire crystal surface [1,2]. This so called w&’-TiO_x phase has a hexagonal symmetry and exhibits a strong template effect for the adsorption of small nanoparticles. By thermal evaporation of WO₃ it is possible to form W₃O₉ clusters in the gas phase with a typical ring structure, which adsorb on the TiO film in a very controlled way. DFT calculations predict the existence of d-aromaticity indicating different stable molecular redox states [3]. In our study we were able to depict these ordered clusters with submolecular resolution and also to specifically manipulate their oxidation state.
[1] S. Le Moal, M. Moors, J. M. Essen, C. Breinlich, C. Becker, K. Wandelt, J. Phys. Condens. Matter 25 (2013) 045013.
[2] C. Breinlich, M. Buchholz, M. Moors, S. Le Moal, C. Becker, K. Wandelt, J. Phys. Chem. C118 (2014) 6186.
[3] X. Huang, H.-J. Zhai, B. Kiran, L.-S. Wang, Angew. Chem. Int. Ed.44 (2005) 7251.

Multiwall Carbon Nanotubes (MWCNTs) were embedded in a polymer matrix to form useful polymer composites. Tapping mode imaging was used to evaluate the dispersion of the carbon nanotubes in the matrix. Running in repulsive mode, tapping mode imaging can track the surface morphology but failed to generate strong contrast in the phase image. However, by applying an AC voltage to the conductive tip and running the AFM at an unusual “Scanning Polarization Force Microscopy” mode, we can generate new phase contrasts based on the electric polarizability differences between the MWCNT and the polymer matrix. In this way, we obtain valuable information regarding the distribution of the MWCNTs within the polymer. Finally, by applying a DC voltage to the conductive tip and running the AFM in a classic “Electrostatic Force Microscopy” mode, we can generate new phase contrasts based on the conductivity differences between the MWCNT and the polymer matrix and establish the structure-property relationship between the overall conductivity and filler morphology of the composites.

Sub-gap absorption to form interfacial charge transfer states occurs in organic photovoltaics (OPVs), but with extremely low quantum efficiency due to the fact that the charge transfer transition is only weakly optically allowed. As a result, measuring charge transfer absorptions typically requires special experimental considerations (e.g. lock-in detection of current), even when one is characterizing macroscopic devices. Here, we show that time-resolved electrostatic force microscopy (trEFM) can provide an exquisitely sensitive probe of charge-transfer state excitations with nanoscale spatial resolution. We demonstrate quantitative agreement between the wavelength-dependent quantum yield of charge-transfer state excitations as measured in bulk devices with that measured via local detection using trEFM. By comparing trEFM images of sub-gap and above-gap excitation, we show how the spatial distribution of charge-transfer excitations differs from that of direct excitonic excitation in several model conjugated polymer bulk heterojunction (BHJ) solar cells.

A new approach is presented for investigating the performance and nanoscale heterogeneities of photovoltaic materials with SPM. The approach provides two important advances over traditional SPM measures of photovoltaics: efficient data acquisition with enhanced spatial resolution, and quantitative mapping of common performance metrics (e.g. Voc, Ish, fill factor, etc.). This approach is based on consecutive photo-conductive atomic force microscopy (pcAFM) scans on a single area in contact mode; each with incremented DC applied biases during simultaneous illumination either obliquely or through an inverted optical microscope. The photoresponse (pcAFM current contrast) is analyzed as a function of the distinct voltage (image) for any given location (image pixel). The local I-V response is easily determined for all pixel positions which is then used to construct drift-free property maps. This technique is applicable with standard or high speed imaging, provides ~5nm spatial resolution over typically 65,536 pixels, and does so in ~10% of the time as compared to more common point-by-point measurements that are particularly susceptible to spatial drift. This scheme is applied to continuous as well as micro/nanopatterned CdTe-CdS thin film solar cells. In addition, CdTe thin films are studied to investigate electroptic property changes near and at grain boundaries. Enhanced performance is clearly observed for certain grains and grain boundaries, even revealing twin boundaries. Finally, this approach is employed to investigate nanoscale heterogeneities in thermochromic materials. These results confirm the importance of nanoscale resolved functional measurements as this widely applicable multi-parametric approach proves to be very beneficial in directly correlating microstructure to performance for optimizing photovoltaic properties.

Solar cells based on perovskite light absorbing materials reached power conversion efficiencies >15%. Today, the knowledge about the local charge generation processes inside these solar cells is limited. The aim of our study is to apply scanning probe microscopy (SPM) for measuring the electrical potentials under working conditions inside the device [1]. In order to study light induced electrical effects on the level of single nanostructures and charge extraction effects into adjacent materials, we added a defined sample illumination to a SPM setup [2]. The entire setup was placed in an inert atmosphere to avoid photo-oxidation of the solar cell device. In particular, we prepared smooth cross sections by means of focused ion beam milling such that the full structure and functionality of the devices were preserved. This way, the internal interfaces between the different materials in the cell are accessible for frequency modulation Kelvin Probe Force Microscopy (FM-KPFM). FM-KPFM revealed local light induced changes in the contact potential difference without cross talk from the electrode potentials. Our measurements indicated that mesoscopic lead methylammonium tri-iodide solar cells exhibit a homogeneous electric field throughout the device representing a p-i-n type junction. Upon illumination under short-circuit conditions, holes accumulate in front of the hole transport layer, which is proof of an unbalanced charge transport. In conclusion, the FM-KPFM method allows us not only to map the local contact potential variation but also to correlate it with the local structure of the functional layers. The insight obtained by FM-KPFM is a step forward in understanding of energy relevant materials such as perovskite solar cells.
References
[1] Q. Peng et al. Nanoscale 6, 1508 (2014).
[2] R. Berger et al. Eur. Polym. J. 49 1907-15 (2014)

A factor that determines the electric performance of a device, inclusive of solar cells, is the electric contacts between the device and its electrodes. Si heterojunction (SHJ) solar cell, which has a higher efficiency than the conventional Si solar cells [1], is composed of a-Si and indium tin oxide (ITO) layers on c-Si substrate. While the high efficiency is usually attributed to the unique interface structure, the detailed mechanisms remain unrevealed due to difficulties in measuring the local electronic properties of the interface between c-Si, a-Si, ITO and surface metal electrode. In this paper, to clarify the nature of the electric contacts at the heterojunction interface, we performed workfunction (WF) measurements of the interface of a cleaved SHJ solar cell using Kelvin probe force microscopy (KFM).
The SHJ solar cell used in the present study consists of 5 nm thick intrinsic a-Si layers deposited and 20 nm thick p-type amorphous Si on an n-type c-Si substrate, with a 70 nm thick ITO layer deposited on the top. Ag is further sputtered on to the surface as metal electrode. For cross-sectional KFM measurements, the sample was cleaved in ambient at room temperature. The measurements were performed at room temperature in vacuum of 10^-4 Pa. KFM measures the potential difference between the AFM tip and the sample surface, which can be converted into the WF map on nm-scale.
The topograph and WF difference between the layers are obtained simultaneously. In the topograph, we can the layers can be distinguished by the surface morphology - c-Si is readily cleaved while the others show distorted structures. WF differences obtained are 500 meV between c-Si and a-Si, -600 meV between a-Si and ITO, and about -100 meV between c-Si and ITO. The WF difference between the c-Si and ITO obtained by the present experiment agrees well with that in the literature [2]. The energy diagram of the interface between the layers was drawn using the measurement result with literature value.
Although ITO is generally taken to act metallically, this result shows that it actually is a semiconductor, and band bending is observed at the interface with p-type a-Si. This may be due to impurities of ITO diffusing into a-Si, and control of the ITO growth condition may be crucial for obtaining an abrupt interface that would allow us to improve the efficiency of the solar cell.
WF mapping of cleaved interfaces on nm-scale should provide us with information that would allow us to better understand and improve the quality of device performances.
This work was supported by High Performance PV Generation System for the Future, R & D on Ultimate Wafer-based Si Solar Cells (Installation of Experimental Process line), NEDO, and Strategic Research Infrastructure Project, MEXT, Japan.
References
[1] M. Tanaka, et al., Jpn. J. Appl. Phys. 31 (1992) 3518.
[2] R. Schlafa,et al., J. Electr. Spectr. Rel. Phen. 120 (2001) 149.

Owing to its excellent sensitivity, its high dynamic range and its good depth resolution, Secondary Ion Mass Spectrometry (SIMS) constitutes an extremely powerful technique for analyzing surfaces and thin films. In recent years, considerable efforts have been spent to further improve the spatial resolution of SIMS instruments. As a consequence, new fields of application for SIMS, e.g. nanotechnologies, biology and medicine in particular, are emerging.
State-of-the-art SIMS instruments allow producing 3D chemical mappings with excellent sensitivity and spatial resolution. However, several important artifacts arise from the fact that the 3D mappings do not take into account the sample&’s surface topography. The traditional 3D reconstruction assumes that the initial sample surface is flat and the analyzed volume is cuboid. The produced 3D images are thus affected by a more or less important uncertainty on the depth scale and can be distorted. Moreover, significant field inhomogeneities arise from the surface topography as a result of the distortion of the local electric field. These perturb both the primary beam and the trajectories of secondary ions, resulting in a number of possible artifacts, including shifts in apparent pixel position and changes in intensity.
In order to obtain high-resolution SIMS 3D analyses without being prone to the aforementioned artifacts and limitations, we developed an integrated SIMS-SPM instrument, which is based on the Cameca NanoSIMS 50. This instrument, an in-situ combination of sequential high resolution Scanning Probe Microscopy (SPM) and high sensitivity SIMS, allows topographical images of the sample surface to be recorded in-situ before, in between and after SIMS analysis. Hence, high-sensitivity high-resolution chemical 3D reconstructions of samples are possible with this extremely powerful analytical tool [1-4].
In addition, this integrated instrument allows a combination of SIMS images with valuable AFM (Atomic Force Microscopy) and KPFM (Kelvin Probe Force Microscopy) data recorded in-situ in order to provide an extended picture of the sample under study. The known information channels of SIMS and AFM/KPFM are thus combined in one analytical and structural tool, enabling new multi-channel nanoanalytical experiments. This opens the pathway to new types of information about the investigated nanomaterials.
This paper will present the prototype instrument with dedicated software, its performances and some typical examples of application.
References:
[1] Y. Fleming et al., Appl. Surf. Sci. 258 (2011) 1322-1327
[2] T. Wirtz et al., Surf Interface Anal. 45 (2013) 513-516
[3] T.Wirtz et al., Rev. Sci. Instrum. 83 (2012) 063702
[4] C. L. Nguyen et al., Appl. Surf. Sci. 265 (2013) 489-49

Scanning Kelvin probe microscope images have been used to optimize the performance of both organic and inorganic photovoltaic materials and understand their operation at the 10&’s of nm scale [1]. While most studies have examined the spatial distribution of the potential, photopotential, capacitance, photocapacitance, current, and photocurrent, there is much to be learned by measuring and mapping the time-dependence of these quantities [2]. In organic bulk heterojunction solar cells, Ginger et al. have established that the microscopic photocapacitance charging rate in a film, measured by time-resolved electrostatic force microscopy (tr-EFM), is directly proportional to the external quantum efficiency (EQE) measured in a completed device. Single point measurements with sub-microsecond time resolution have been collected, but acquiring these transients required frustratingly long signal-averaging times [2]. Building on the insight of Moore, Marohn, and co-workers [3], we are developing a method to rapidly acquire transients of photocapacitance indirectly, in a stepped-time experiment, by encoding and measuring the photocapacitance as a change in the phase of a vibrating cantilever. Our new approach may allow us to measure and image photocapacitance on the nanosecond time scale. This capability would represent an exciting new route to understanding geminate recombination in heterogeneous hybrid organic/inorganic photovoltaic materials.
[1] O&’Dea, J. R.; Brown, L. M.; Hoepker, N.; Marohn, J. A. & Sadewasser, S. Scanned Probe Microscopy of Solar Cells: From Inorganic Thin Films to Organic Photovoltaics. Mater. Res. Soc. Bulletin, 2012, 37: 642 - 650 [http://dx.doi.org/10.1557/mrs.2012.143].
[2] Giridharagopal, R.; Rayermann, G. E.; Shao, G.; Moore, D. T.; Reid, O. G.; Tillack, A. F.; Masiello, D. J. & Ginger, D. S. Submicrosecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics. Nano Lett., 2012, 12: 893 - 898 [http://dx.doi.org/10.1021/nl203956q].
[3] Moore, E. W.; Lee, S.-G.; Hickman, S. A.; Wright, S. J.; Harrell, L. E.; Borbat, P. P.; Freed, J. H. & Marohn, J. A. Scanned-Probe Detection of Electron Spin Resonance from a Nitroxide Spin Probe. Proc. Natl. Acad. Sci. U.S.A., 2009, 106: 22251 - 22256 [http://dx.doi.org/10.1073/pnas.0908120106].

The Kelvin force microscopy (KFM) provides a spatially resolved measurement of the surface potential, which is related to the energetic band structure of a material. However, it depends strongly on the physical properties of the tip, e.g. width of the apex, the geometric shape and the stiffness of the cantilever as well as the surface sample state. The goal of this work is to investigate the surface potential measured by KFM on AlGaAs/GaAs heterostructures. For this study, we used a certified reference sample (BAM-L200), which is a cross section of GaAs and Al_0.7Ga_0.3As epitaxially grown layers with a decreasing thickness (600 to 2 nm) and a uniform silicon doping (5x10¹⁷ cm^-3). The resulting stripe patterns are commonly used for length calibration and testing of spatial resolution in imaging characterization tools (ToF-SIMS, SEM, XPEEM). The surface potential measurement is performed under ultra-high vacuum with an Omicron system by using two acquisition modes: the amplitude modulation (AM-KFM), sensitive to the electrostatic force and the frequency modulation (FM-KFM), sensitive to its gradient. Three kinds of tips have been used for this study: platinum or gold nanoparticles coated silicon tips and super sharp silicon tips.
We will present the measurements obtained with these different tips for the narrowest layers (typ. < 40 nm). The surface potential mapping reveals a contrast around 300 meV between Al_0.7Ga_0.3As and GaAs layers. However, we observed that this contrast vanishes when layer thickness becomes thinner. This loss of contrast cannot be only explained by the resolution limit of the KFM technique. Indeed, we will discuss the effect of the band bending length scale at the AlGaAs/GaAs interface related to the dopant concentration. The contribution of band bending between the layers is evaluated by a self-consistent simulation of the electrostatic potential, accounting for the free carriers distribution inside the sample and for the surface and interface dipoles. We will show that the electric fields of the narrowest layers recover each other, resulting in the partial or total loss of the contrast between Al_0.7Ga_0.3As and GaAs layers. The simulation results will be compared to the experimental results in order to emphasize that the surface potential contrast is not only influenced by the resolution limit.
The measurements were performed on the CEA Minatec Nanocharacterization Platform (PFNC).

The local electric field in a semiconductor device is an important quantity, governing many fundamental charge transport processes. Measuring the lateral electric field in operating organic semiconductor devices has yielded much new information about both charge transport and injection. From simultaneous measurements of device current, local potential, and local electric field in a field-effect transistor, one can infer the local mobility and study its dependence on temperature and electric field [1, 2]. In two-terminal devices, the bulk current and local electric field may be analyzed as a function of bias voltage and temperature to rigorously test microscopic theories of metal-to-organic charge injection [3, 4, 5].
To date, scanning-probe techniques such as electric force microscopy (EFM) and Kelvin probe force microscopy (KPFM) have been used to infer the lateral electric field by simply numerically differentiating the measured local electrostatic potential [2, 3, 4, 5]. However, numeric differentiation is effectively a high-pass filter, magnifying high-frequency noise in the data. The signal-to-noise of the inferred electric field is thus far lower than the signal-to-noise of the measured electrostatic potential.
Here, we describe an adaptation of FM-KPFM that employs an additional position modulation to obtain a direct measurement of the local electric field simultaneous with measurement of the local electrostatic potential. Rather than simply scanning the tip continuously across the sample, a small oscillation is introduced to the voltage used to scan the sample position. This position modulation yields an oscillating surface potential signal whose time-dependent amplitude, measured via lock-in detection, is proportional to the local electric field. The electric field profile measured in this way has a signal-to-noise which is greatly improved over the electric field inferred by the direct-differentation approach.
[1] X. Li, A. Kadashchuk, I. I. Fishchuk, W. T. T. Smaal, G. Gelinck, D. J. Broer, J. Genoe, P. Heremans, and H. Bassler, Phys. Rev. Lett., 2012, 108, 066601.
[2] L. Burgi, H. Sirringhaus, and R. Friend, Appl. Phys. Lett., 2002, 80, 2913 - 2915.
[3] L Burgi., T. Richards, R. Friend, and H. Sirringhaus, J. Appl. Phys., 2003, 94, 6129 - 6137.
[4] T. N. Ng, W. R. Silveira, and J. A. Marohn, Phys. Rev. Lett., 2007, 98, 066101.
[5] W. R. Silveira and J. A. Marohn, Phys. Rev. Lett., 2004, 93, 116104.

Piezoresponse force microscopy (PFM) has been established as a powerful tool for the characterization and manipulation of ferroelectric and piezoelectric materials on the nanoscale. By using a nanoscale biased conductive AFM tip in contact with the surface of piezoelectric samples, one can achieve a highly-localized electric field distribution in the material volume right underneath the tip. Due to the converse piezoelectric effect, the electric field generated this way further induces a three-dimensional (3D) mechanical deformation of the sample surface, which is then reflected in the vertical and lateral PFM signals. By appropriate VPFM and LPFM calibrations, one can even achieve quantitative piezoresponse measurements (units: pm/V).
However, it is worth noting that the quantitative piezoresponse measured this way should not be considered as the piezoelectric coefficients of the studied materials, because the electric field induced by the conductive AFM tip in the PFM measurements is highly inhomogeneous. This is one of key reasons why there is no reports with full materials&’ piezoelectric tensors determined by PFM so far, to the best of our knowledge.
PbTiO₃ is one of the prototype ferroelectric materials, yet with large discrepancies in the piezoelectric strain coefficients reported in the literatures (see reference 1 and 2). One of the reasons is that the c/a ratio (1.069) is so large, that a sizable single crystal with single domain state is not readily obtained, thus shaping a formidable challenge in accurate determination of the piezoelectric coefficients. In this work, we demonstrate the whole procedure in determining PbTiO₃&’s piezoelectric coefficients by PFM. Finite element modeling is found to play a key role in bridging the materials&’ piezoelectric coefficients with the effective piezoresponse measured by PFM. From our quantitative PFM experiments and FEM simulations, we not only show the possibility to determine the full piezoelectric tensors of PbTiO₃, but also are able to distinguish the domain wall orientation inside the material. This technique can be highly efficient in the situations where the geometry size of the materials is small (as small as hundreds nanometer scale) or a single domain state cannot be easily obtained.
Reference
1.Z. Li a , M. Grimsditch a , X. Xu a & S. -K. Chan,
2. A. G. Kalinicheva and J. D. Bass, J. Mater. Res. 12, 10 (1997)

Ferroelectric single crystals with tailored domain structure nowadays are widely used in acoustic, nonlinear optical and data storage devices. Scanning probe microscopy (SPM) provides perfect set of the tools for nanometer-scale investigations of the ferroelectrics. Tip-induced polarization reversal is one of the most prominent of them.
Here we report a number of unexpected phenomena in the thin lithium niobate single crystal caused by the action of electric field produced by biased SPM tip. The first one is the long-range domain-domain interaction which can essentially change group switching dynamics. Formation of the domain chains in this case demonstrates wide range of the domain dynamics, including intermittency, quasiperiodicity and chaos. The second phenomenon can be observed after switching by sequences of the bipolar triangular pulses and gave rise to a surprisingly broad range of symmetric and asymmetric domain morphologies. Detailed studies show that domain growth is multi-stage process with “abnormal” polarization reversal against applied electric field on the one of the stages.
Measurement at different values of relative humidity showed that all diversity of observed phenomena is dependent on the sample surface state and can be controlled by the value of relative humidity in the SPM chamber. This fact allowed us to conclude that polarization reversal process in ferroelectrics is controlled by surface screening charge dynamics.
From the point of view of potential applications, observed set of phenomena is very interesting in connection to the emergent computing paradigm known as memcomputing. It potentially give a rise to new generation of computational and data storage devices.
The 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.

This talk describes recent advances in scanning thermal microscopy, with application to temperature and thermoelectric property measurements wof working electronic devices. When driven with a periodic heating signal, an electronic device heats and cools at a frequency equal to twice the driving frequency. This periodic temperature change generates periodic thermomechanical strain. The AFM tip can measure this thermomechanical strain down to the range of 1 pm, which corresponds to a temperature detection limit of about 10 mK. The spatial resolution of the temperature measurement is typically between 10-100 nm, and depends upon thermomechanical expansion in the substrate as well as the tip shape. We demonstrate this temperature measurement technique on working two-terminal and three-terminal devices of graphene, working phase change memory devices, and working GaAs high voltage power transistors. For the graphene and the phase change memory devices, we demonstrate how it is possible to measure the Seebeck coefficient using this technique.

The emergence of nanoscale devices constructed from stiff (>10 GPa range) materials requires novel approaches to high resolution quantification of nano-mechanical properties. Tapping mode, also referred to as amplitude modulated-atomic force microscopy (AM-AFM) is the most successful, high resolution and gentle of imaging modes. Despite its clear success, quantitative measurements in tapping mode have been somewhat limited. In what follows, we will describe an extension of tapping mode, a bimodal technique we have coined “AM-FM” that is capable of high speed imaging, very high topographic resolution while providing quantitative stiffness and with appropriate models, modulus measurements. The name originates from operating the first resonant mode in AM and the second resonance in FM. We will discuss our approach to quantifying tapping mode images that builds on work in frequency-modulated bimodal imaging.[1] Hand in hand with high resolution measurements of stiff materials, is a very small indentation depth, a fraction of a nm. This small indentation depth requires special care with sample preparation where even very small amounts of contamination can result in errors of the measured modulus. These measurements represent a roughly two order of magnitude improvement over past reported modulus ranges and will now allow us to address a wide range of modern engineering materials and devices.
[1] Herruzo ET, Garcia R. Theoretical study of the frequency shift in bimodal FM-AFM by
fractional calculus. Beilstein J. Nanotechnol. 2012;3:198-206.

Nanoscale mapping material properties based on mechanical resonances of AFM cantilevers in contact with a sample surface became a common method to sensitivity enhancement in many scanning probe imaging modes such as piezoresponse-force microscopy (PFM), electrochemical strain microscopy (ESM), acoustic force atomic microscopy (AFAM), and others. While there exist general considerations for the choice of cantilevers for different imaging modes, the exact choice of a cantilever for a particular experiment is frequently a result of a trial-and-error approach. In this work, we have systematically explored sensitivity of a variety of cantilever probes, from soft to stiff, to surface displacement and electrostatic interaction between the tip and the sample in PFM experiments. The experiments revealed an unexpectedly large range of response intensity and loop shapes for nominally identical experimental conditions in calibrated single-point measurements of piezoresponse hysteresis loops with different cantilevers. This becomes a problem when experiments with different tips are compared, or when values of quantities, such as coercive voltages, are extracted. Therefore, it becomes crucial to understand the cantilever dynamics in contact with the sample. Experimentally, we were able to separate contributions from the piezo-effect-induced surface displacement and a local electrostatic force acting on the tip pyramid apex. The data were analyzed using analytical and numerical modeling of the cantilever vibrations. It was found that the sensitivity of a cantilever to surface displacements is a strong function of probe geometry and contact stiffness and determined to a large degree by competing effects of normal and lateral contact stiffness. Overall, the modeling revealed that cantilever dimensions and stiffness, as well as stiffness of the tip-sample contact are major factors determining the measured value of the cantilever response. In particular, the results show that softer cantilevers are prone to (undesirable) response to electrostatic interactions, while certain cantilever design rules can be proposed for optimization of imaging based on surface displacement measurement. The research was performed at CNMS, which is sponsored at ORNL by the SUFD, BES, US DOE.

Catalytic nanoburning (CNB) of methanol using platinum nanoparticles on a thermoelectric (TE) module is a simple method for converting chemical energy into electrical power. In the CNB process, the platinum particles first catalytically convert methanol into carbon dioxide and water, releasing heat and raising the particle temperature. The hot nanoparticles then initiate the auto-combustion of methanol in the region near the heated particles. Temperatures in the ranges of hundreds of degrees can be achieved using this approach. Understanding the various nanoscale heat transfer routes from the nanoparticles to the TE module is important in designing interfaces for maintaining a high surface temperature. Scanning probe microscopy and its variations offer an ideal technique for investigating thermal maps as well as capillary condensation of byproduct water around the nanoparticles. We will discuss AFM-based thermal imaging techniques that can be used for manipulating and controlling the heat transfer mechanisms to maintain a high uniform surface temperature to eliminate water condensation while sustaining a constant thermal gradient across the TE module for power generation.

Piezoresponse Force Microscopy (PFM) and Electrochemical Strain Microscopy (ESM) measurements depend intimately on the behavior of the mechanical sensor, typically a cantilever beam. Ultimately, changes in the cantilever shape in response to different boundary conditions results in the experimental observables - amplitudes, phases, resonance frequencies - for example, that are in turn used to estimate sample properties. Jungkt et al.1 listed the “ideal” behavior of PFM measurements of ferroelectric materials: (i) frequency-independent response, at least below the first contact resonance frequency. (ii) the amplitude should be independent of the ferroelectric polarization direction and A = deffV , where A is the PFM amplitude, deff is the inverse piezoelectric coefficient and V is the amplitude of the drive voltage. (iii) the phase shift across oppositely polarized domains should be 180°. This is in stark contrast to numerous experimental measurements appearing in the literature and in practical, every day PFM measurements. In this talk I will first show typical PFM measurements on a LiNbO3 sample that violate all three of the above points. This behavior is experimentally explored by measuring “Spectrograms” of the cantilever response. The spectrograms are in-situ measurements of the amplitude and phase response of the cantilever measured versus frequency and optical detector spot location while it is interacting with the sample. The experimental spectrograms are explained by treating the cantilever as a “diving board” Euler-Bernoulli beam with the appropriate boundary conditions. The experimental and theoretical spectrograms show excellent agreement over oppositely polarized domains in the LiNbO3 and under the influence of applied DC biases. This fitting required only four parameters: deff , Cprime;body , the capacitance gradient of the cantilever body, Cprime;tip capacitance gradient of the cantilever tip and kts , the tip-sample stiffness. These results suggest some interesting directions for improving quantitative PFM and ESM measurements that will be outlined at the end of the presentation.

Structure and properties of topological defects in soft matter such as liquid crystals and ordered ionic liquids directly control their energetics and functionality. Multiple studies of defect properties, transport, and generation/recombination process have been reported, however, these are generally based on the observation of long-range strain fields by optical methods, and the structure of the defect cores generally remained unknown. Here we report direct observation of the structure and properties of topological defects formed in ordered ionic liquid layers at a carbon interface using atomic force microscopy (AFM) force volume mapping. Measurements were performed on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface in the ionic liquid 1-ethyl-3methyl-imidazolium bis(trifluoromethanesulfonyl)imide (Emim⁺Tf₂N^-). The ion layer structure at structural defects such as a carbon step-edge is investigated and contrasted with molecular dynamics (MD) simulations, defining the spatial resolution of the method. Furthermore, serendipitous dislocation type topological defects in the ordered ion layers are observed on the HOPG basal planes, occurring over length scales of 50-80nm. Within the defects a decrease in ordering of the ion structure is observed, consistent with behavior expected for classical liquid crystals. Universal behavior of the force curves is observed across multiple defects when the peak parameters are plotted as a function of separation distance from the carbon surface. These studies offer a pathway for probing the internal structure of topological defects in soft condensed matter on the nanometer level.
Acknowledgements
The experimental and modeling efforts of JB, GF, and PTC were supported by the Fluid Interface Reactions, Structures and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Additional personal 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 (NB) and 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 (MBO and SVK). We thank the computational resource from the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. G.F. also acknowledges the funding from the Hubei Provincial 100-Talent Program.

Atomic force microscopy (AFM) is often used in non-aqueous liquid environments for in situ investigations of various processes including chemical reactions [1], lubrication [2] and molecular ordering [3]. In contrast to water, these environments often exhibit a much higher damping of the cantilever motion, lowering the quality factor (Q), due to an increased fluid viscosity. It is generally expected that AFM operation in such environments will not yield atomic scale resolution due to increased noise from the thermal motion of the cantilever resulting in a reduced signal-to-noise ratio (SNR) [4].
Recently, we have demonstrated that true atomic resolution can be obtained in a highly viscous environment. In particular, we imaged the atomic structure of highly ordered pyrolytic graphite (HOPG) and mica surfaces with SNR values comparable to ultra-high vacuum systems [5]. To understand these findings, we investigated the influence of the Q-factor of a cantilever on the thermal noise of the relevant AFM signals, namely amplitude, phase and frequency shift. In particular, we found that a reduction of the Q-factor does not necessarily reduce the SNR for dynamic AFM in viscous environments. This new understanding of the noise contributions to the imaging process opens up a new route to high resolution AFM studies in a wide range of viscous fluids. This includes highly relevant energy related materials such as ionic liquids and organics, where interface effects at boundaries (such as step edges) and defects play an important role for the function of the material.
References
1. Domanski, A.L., et al., Kelvin Probe Force Microscopy in Nonpolar Liquids. Langmuir, 2012. 28(39): p. 13892-13899.
2. Jones, R.E. and D.P. Hart, Force interactions between substrates and SPM cantilevers immersed in fluids. Tribology International, 2005. 38(3): p. 355-361.
3. Labuda, A. and P. Grütter, Atomic Force Microscopy in Viscous Ionic Liquids. Langmuir, 2012. 28(12): p. 5319-5322.
4. Giessibl, F.J. and C.F. Quate, Exploring the Nanoworld with Atomic Force Microscopy. Physics Today, 2006. 59(12): p. 44-50.
5. Weber, S.A.L., et al., High viscosity environments: an unexpected route to obtain true atomic resolution with atomic force microscopy. Nanotechnology, 2014. 25(17): p. 175701.

In recent years, atomic force microscopy (AFM) has become a well-established technique for single molecule studies and even sub-molecular scale research. Several new developments in terms of faster AFM imaging and imaging modes, based on the phase or frequency, have been established in order to decrease the cantilever response time and increase the AFM&’s scan speed, e.g., for studying molecular dynamics.
The novel NanoWizard® ULTRA Speed A AFM combines the latest scanner technologies and compact design allowing a full integration of AFM into advanced commercially available optical microscopy. Thus, fast AFM imaging of approximately 1 frame per second can be seamlessly combined with methods such as, fluorescence, confocal, TIRF, STED microscopy and many more. Individual molecule dynamics can now be studied with AFM and simultaneously with optical microscopy by applying JPK&’s tip scanner technology.
With JPK&’s HyperDrivetrade; sub-molecular resolution is achieved even on soft samples imaged in liquid environments. It allows for imaging with smallest amplitudes of often approximately 0.2 nm for lowest tip-sample interaction. Topographical images of membrane proteins and DNA-origami are presented. It has been shown that the phase response in phase modulation AFM (PMAFM) is faster allowing higher imaging speeds for the study of molecule kinetics. In conjunction with JPK&’s NanoWizard® ULTRA Speed A AFM, a dynamic biomechanical study of Bacteriorhodopsin (bR) when interacting with photons will be discussed.
More than half a century after the first high-resolution electron microscopy images of collagen type I banding of 67 nm have been reported, now with the NanoWizard® ULTRA Speed A AFM we could gain a high-resolution temporal insight into the dynamics of collagen I fibril formation and its characteristic 67 nm banding hallmark. The literature still abounds with conflicting data regarding the models of its fibril formation, structural intermediates, and kinetics. AFM is the only currently available high-resolution imaging technique amongst many to offer insight into the collagen I fibrillogenesis by operating in situ. The described technique could be instrumental for future studies of the structural dynamics of protein systems, etc.
The systems newly gained flexibility will also be demonstrated on a study of living fibroblast cells directly imaged in their culture petri dish at 37 degrees C. Here, the dynamics of individual membrane structures is investigated with AFM while simultaneously observing the individual living cell with optical phase contrast. The unambiguous correlation between AFM and optical microscopy is achieved by the DirectOverlaytrade; technique.

In recent years, high-speed AFM (HS-AFM) has become a powerful tool in industry for direct (real-time) visualization of dynamic processes on surfaces with nanometer resolution and high throughput applications where data turn-around is critical. The fields of biological imaging and material defect analysis are already leveraging HS-AFM capabilities. In this presentation, highlighted examples will include: 1). High throughput applications will be exemplified in a chemical mechanical planarization (CMP) optimization study where HS-AFM is applied to correlate tungsten surface roughness and slurry development. In developing new CMP slurry, there is need to systematically decipher effects of each slurry parameters (pH, oxidizer, accelerants or passivators) and how they correlate to resultant substrate roughness and defectivity. 2). A hyphenated approach for applications in dynamic process imaging where HS-AFM is applied to monitor tungsten surface morphology during electrochemical deposition of copper. There is continued need to understand metal nucleation mechanism (related to surfaces roughness and defects) in the manufacture of next generation electronics.

Nowadays ferroelectric materials attract much attention due to possibility of development of new generation of optoelectronic, data storage and processing devices. Investigations of the ferroelectric polarization reversal processes on the level of a single domain are of immense scientific importance. Scanning Probe Microscopy (SPM) provides wider range of tools for ferroelectric investigations. Tip-induced polarization reversal is one of the most prominent and popular of them. Switching under the action of highly inhomogeneous electric field produced by the SPM tip allows both create isolated domains with nanometer size and study switching kinetics on the nanoscale.
Mostly investigators attention is focused on study of the switching of polarization component perpendicular to the sample surface; however tip-induced field configuration allows switching of polarization directed along the surface too. This opens great opportunities for nanometer investigations of the forward domain growth along polar direction on nonpolar cut of uniaxial ferroelectrics.
Here we report result of investigation of polarization reversal under the action of electric field produced by conductive SPM tip on non-polar cuts of uniaxial ferroelectric lithium niobate.
We explored that switching in most cases led to formation of complex domain structures consisted of few isolated domains: small elliptic domain under the tip; long prolate wage-like domain in the direction against spontaneous polarization and short wage-like domain in opposite direction (in the case of negative applied bias).
Complex domain shape has been attributed to switching in two separate stages: 1). formation of prolate wage-like domain under the action of tip-induced electric field; 2). backswitching under the action of injected charges after bias switching off. Offered explanation has been justified by computer simulation of electric fields distribution and additional experiments with biased tip withdraw.
The 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.

Mica is an abundant crystal mineral that has important and interesting bulk and surface properties for a variety of applications. These properties arise from its anisotropic structure, in which layers of aluminum silicate, 1 nm thick, are ionically bonded together, typically with K⁺ ions. The surface properties of mica can be varied through ion exchange with the exposed lattice sites. In this study, the effect of kinetics on ion exchange with nickel ions (Ni²⁺) and cobalt ions (Co²⁺) and its influence on surface water accumulation as a function of time has been investigated. Mica was ion-exchanged for 30 s or 5 min with ion concentrations of 5mM, and its surface properties were measured for up to 96 h after incubation in a controlled environment. Nanoscale variations in the waterlayer accumulation can be detected by reconstructing the conservative force profile between an atomic force microscopy (AFM) tip and the surface. Surface water contact angle of the ion-exchanged mica was also measured after aging in the same controlled environment, which macroscopically verified the validity of AFM force reconstruction method.

Force microscopy is considered the second most relevant advance in Materials Science since 1960. Despite the success of AFM, the technique currently faces limitations in terms of three-dimensional imaging, spatial resolution, quantitative measurements and data acquisition times. Atomic and molecular resolution imaging in air, liquid or ultrahigh vacuum is arguably the most striking feature of the instrument. However, high resolution imaging is a property that depends on both the sensitivity and resolution of the microscope and on the mechanical properties of the material under study. Molecular resolution images of soft matter are hard to achieve. In fact, no comparable high resolution images have been reported for very soft materials such as those with an effective elastic modulus below 10 MPa (isolated proteins, cells, some polymers). Similarly, it is hard to combine the exquisite force sensitivity of force spectroscopy with molecular resolution imaging. Simultaneous high spatial resolution and material properties mapping is still challenging. Furthermore, there not an accepted solution to the problem of force reconstruction in the most popular AFM method: tapping mode.
This presentation reviews some of the above limitations and some recent developments based on dynamic AFM (single and bimodal) to address and overcome them.
Recent references:
E.T. Herruzo, A.P. Perrino, R. Garcia, Nat. Comm. 5, 3126 (2014)
E. T. Herruzo, H. Asakawa, T. Fukuma, R. Garcia, Nanoscale5, 2678 (2013)
H. V. Guzman, A.P. Perrino, R. Garcia, ACS Nano 4, 3198 (2013) R. Garcia and E. T. Herruzo, Nature Nanotechnol. 7, 217-226 (2012).

Contact resonance is a dynamic contact AFM based method that has effectively measured the storage modulus, loss modulus, and loss tangent of materials. One of the challenges of contact resonance is the ability to reliably find and track the contact resonance peak, which can be easily confused with the parasitic cantilever resonances associated with piezo actuation. Magnetic actuation of the tip-sample contact offers distinct advantages where now the signal to noise ratio on the contact resonance spectrum, showing multiple modes, is significantly improved and does not suffer from parasitic resonances.
Magnetic actuation in the experiments described here is accomplished through Lorentz contact resonance, which is based on the Lorentz force where an oscillating current is passed through a specialized probe that interacts with a magnetic field, resulting in an oscillating tip sample force. Additionally, Lorentz contact resonance is conducted with a specialized Thermalevertrade; cantilever, that has two arms that come together at the end where the tip is located. Magnetic actuation of such probes enables the quick, clean measurement of resonance frequencies of multiple cantilever motions (e.g. flexural and torsional modes) at both fundamental and higher order eigenmodes in a single sweep.
Lorentz contact resonance was thus used to probe the viscoelastic properties of different polymer blends. The frequency peak positions and quality factors of a variety of different modes are analyzed and correlated with bulk storage and loss moduli. The Lorentz contact resonance results show that the first and second flexural modes provide excellent sensitivity for the material storage modulus and loss modulus, while the asymmetric flexural modes and torsional modes demonstrate poor sensitivity. These results show Lorentz contact resonance as a powerful new SPM method to probe nanomechanical properties of materials.

Mineralized biological exoskeletons, such as fish scales and teeth, often adapt a multi-layered material design to optimize their mechanical function, including shock absorption, flexibility and weight reduction. As a result of variations in degree of mineralization, crystallinity and collagen texture, these layers exhibit material and mechanical mismatch at their interfaces. Despite this mismatch, the interfaces are not so susceptible to crack initiation or propagation as normally expected for non-biogenic material boundaries. To understand the design principles that improve the interfacial resistance to failure, we studied the ganoine-bone and ganoine-dentin interfaces from scales of two fish species, Atractosteus spatula and Polypterus senegalus, respectively. Moduli at these interfaces are known to undergo two-fold (ganoine-dentin) and three-fold (ganoine-bone) reduction. We cut and polished the cross-section of these scales using a series of aluminum oxide sheets with reducing roughness (1µm to 300nm, TedPella, Inc.), followed by 60 nm silica colloids (TedPella, Inc.). We quantified the distribution of modulus at a ~ 10 nm spatial resolution using a diamond indenter tip (PDNISP-HS, k ~ 385 N/m, R ~ 27 nm) and a Dimension Icon AFM (BrukerNano) in the new PeakForce nanomechanical mapping mode. Images were taken at 2 kHz indentation rate up to ~ 2 mu;N maximum force. The maximum indentation depth was controlled at le; 5 nm to minimize irreversible plastic deformation. Imaging was repeated for at least 5 locations for each type of scale. Using this method, we found two critical features that have not been reported before. Firstly, there exists abrupt material transition at the interface at the nanometer scale, as there is no gradient observed at the nanometer scale. Secondly, nm-scale suture-like zig-zag boundaries are present at the interface. These features highlighted the specialized design principles at these interfaces which accommodate the abrupt mechanical changes while maintaining the tissue integrity. It is likely that the suture-like boundaries are beneficial for energy absorption and crack retardation to reduce the susceptibility to catastrophic failure. Ongoing studies are constructing a nanostructure-specific model to quantitatively explain the experimental observation and reveal the design mechanisms.

Intermittent contact resonance atomic force microscopy (ICR-AFM) is a new dynamic scanning probe microscopy with high-spatial and temporal resolution for quantitative nanoscale mechanical property measurements. It is based on the continuous track of the changes experienced by the resonance frequency of an eigenmode of a driven AFM cantilever during a force-controlled intermittent-contact AFM mode. With nanometer and microsecond scale detection, ICR-AFM provides momentarily measurement of the tip-sample contact mechanics during each oscillation. Force versus resonance frequency curves are obtained at each position in the scan from the measured dynamics of the cantilever in response to two simultaneous excitations, an amplitude modulation (at a non-eigenmode frequency of the cantilever) and a frequency modulation (at one of the cantilever&’s eigenmodes). In comparison with CR-AFM, ICR-AFM benefits by a reduced tip wear due to the reduction of the contact time while retaining the great measurement sensitivity of CR-AFM. Two cases, on compliant and stiff materials, will be discussed in this presentation. In the first example, the ICR-AFM was demonstrated on a two-phase polystyrene/poly(methyl methacrylate) film with emphasize on measuring the elastic and adhesive responses of the two materials. Detailed contact stiffness measurements over the entire contact depth provided in this case robust verification of the contact model used with consideration of both long and short range adhesive forces. In the second example, ICR-AFM was used as a depth sensing technique for 3D characterization of near-surface mechanical response of a Si/SiO₂ patterned sample to obtain high-resolution 3D contact stiffness maps.
G. Stan and R. S. Gates, Nanotechnology 25, 245702 (2014).

One of the key challenges in producing macromolecules composites of high durability is to investigate whether the individual polymers have blended together. It will be of great importance to locate the spatial location and morphology of the individual polymers that constitute the composite polymer material. A special class of polymer called dendrons which are repeatedly branched polymers linked together by a network of cascade branched monomers. A composite of these dendritic polymers with linear polymers may have unique physical and chemical properties. Using contact resonance mode of atomic force microscopy we are able to spatially locate the dendritic formation of the polyethylene oxide (PEO) mixed with polyvinylpyrrolidone (PVP). The aqueous solution of the mixed polymers is spin coated on a cleaved silicon wafer. PEO is known to form nanometric crystallites during this process. However, the dendritic formation in the mixture has not been reported before. Moreover, the Dendron formation is observed only at the contact resonance but not in the topography images. The amplitude and phase of the contact resonance shows a clear dendritic growth of PEO in the composite material. The extend of the polymer crystallization can be several nanometers thick within the composite material. By employing the contact resonance method, we are able to characterize the elastic properties of such polymers with nanometer spatial resolution. Additionally, the intrinsic properties of such polymers to form dentrimers can be explored for fabricating polymer composites having numerous potential applications in chemical sensing, drug-delivery, energy applications and many more.

A longstanding goal in cell mechanobiology has been to link the dynamic processes underpinning cell migration, morphogenesis, and drug response to spatio-temporal changes in local mechanical properties such as viscoelasticity, surface tension and/or adhesion. This requires the development of quantitative mechanical microscopy methods with high spatio-temporal resolution within a single cell. We present in this talk recent advances in the use of dynamic, multi-frequency methods to map the spatio-temporal changes in the nanomechanical properties of live cells over large areas. By combining fast feedback channels in dynamic AFM, multi-frequency Lorentz force excitation of the AFM cantilever, and accurate continuum models of cell mechanics it is possible to achieve 40x40 µm nanomechanical property images of live rat fibroblasts at 256x256 pixel resolution in 50 seconds. The method is used to study the spatio-temporal mechanical response of MDA-MB-231 breast carcinoma cells to the inhibition of Syk protein tyrosine kinase.

A new measurement technique using a cantilever probe with an integrated thermal sensor is investigated for measuring thermal conductivity at the nanometer scale. The probe is used in a configuration wherein the laser from an atomic force microscope (AFM) heats the tip of the probe above ambient temperature. The heating is enhanced by the shape of the tip which is shown to have a focusing effect on the laser. Heat is transferred from the probe to a sample based on the thermal conductivity of the sample. The heat flow creates a temperature change, as small as 0.01°C, which is detected by the thermal sensor.
The measurement technique presented offers a simple and effective method for mapping the thermal conductivity of a number of materials. We explore the ability of the technique to map carbon fibers, gold nanoparticles, and multi-walled carbon nanotubes. Analysis shows that the technique can be used to produce images with a thermal resolution surpassing 25nm.

Measurement of properties beyond simple topography has been an ongoing and active area of Atomic Force Microscopy (AFM) research. The sample stiffness and the elastic modulus are quantities of considerable interest, since they can greatly vary over very short length scales. Contact Resonance Force Microscopy (CRFM) is a leading AFM technique for measuring the elastic properties, especially on stiffer materials [1-4]. In CRFM, vibrational resonances of the AFM cantilever are excited while the tip is in contact with the sample. The CRFM frequency is measured during scanning, and the measured frequencies are used to determine a map of elastic modulus. Recently, CR force microscopy has been extended to measure viscoelastic properties of materials where the frequency and quality factor Q of the contact resonance spectrum are used to derive storage and loss moduli of the material under investigation [5-7]. Previously, unlike many other AFM techniques, CRFM measurements have been limited to imaging in air, since the "forest of peaks" associated with acoustic excitation methods effectively masks the true cantilever resonance. Using a new blueDrive^TM photothermal excitation module, we have been able to produce clean contact resonance spectra that closely match the Brownian spectra, allowing unambiguous and simple resonant frequency and quality factor measurements. Both stiff metals and softer more compliant polymer materials were successfully imaged and analyzed.
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[2] Yamanaka, K.; Ogiso, H.; Kolosv, O. V. Applied Physics Letters (1994), 64, 178.
[3] Hurley, D. C.; Shen, K.; Jennett, N. M.; Turner, J. A. Journal of Applied Physics (2003), 94, 2347.
[4] Hurley, D. C. Applied Scanning Probe Methods Vol. XI; Bhushan, B., Fuchs, H., Eds.; Springer-Verlag, 2009; pp 97-138.
[5] Yuya, P. A.; D.C.Hurley; Turner, J. A. Journal of Applied Physics (2008), 104, 074916
[6] Killgore, J. P.; Yablon, D. G.; Tsou, A. H.; Gannepalli, A.; Yuya, P. A.; Turner, J. A.; Proksch, R.; Hurley, D. C. Langmuir (2011), 27, 13983.
[7] Gannepalli, A.; Yablon, D. G.; Tsou, A. H.; Proksch, R. Nanotechnology (2011), 22, 355705.

Mapping of physical properties of materials with Peak-Force QNM gives possibilities to study various surface properties down to the nanoscale, such as stiffness, deformation, adhesion, viscous energy loss, and topography. Similar parameters can also be obtained with NT-MDT HybriD mode.
Here we describe novel real-time data processing of raw signals collected in Scan-Assist mode. We call this “extension” of Peak-Force QNM mode. It allows obtaining stiffness, deformation, adhesion, viscous energy loss, and topography 5-10 times faster and less noisy compared to the Peak-Force QNM mode. In addition, we record and visualize a few additional parameters related to viscoelastic properties of the sample material. We demonstrate the work of this new mode on various polymers and fixed biological cells.

The arrangement of molecules at interfaces between solids and liquids is of interest due to the role it plays in determining behavior of phenomena such as colloidal flocculation, crystal growth and dissolution, catalysis and biological activity. Atomic Force Microscopy is the only tool available to directly study the details of these solid-liquid interfaces at the relevant length scales of a few nanometers or less. PeakForce Tapping, Tapping Mode and FM-AFM have all succeeded in visualizing ordered molecules in liquids near surfaces including adsorbed ions, hydration layers, and self-assembled monolayers. Recent simulations have aided interpretation of the details of these systems.
To gain more information about the extent of the structures at varying distance above the surfaces, Tapping Mode and FM-AFM are often combined with Force Volume, ramping the Z position above the surface while measuring the varying amplitude and/or frequency of the cantilever vibration. Since the ramp rates are limited to a few ramps per second, this mapping is fairly time consuming. PeakForce Tapping in contrast, uses sub-resonant Z modulation to provide tip-sample force data throughout the interface volume at piconewton force levels without additional ramping. PeakForce Tapping ramping occurs at around one thousand ramps per second allowing much faster mapping of tip-sample interaction as a function of distance above the surface. These maps are collected simultaneously with the topographic imaging, so it is possible to identify the precise location on an image where an interaction occurred -- even down to the atomic level.
In this talk we will discuss the latest high resolution results using PeakForce Tapping in liquids and the implications of this with regard to studies of dissolution, adsorption, and structure of surfaces at the molecular level.

Streptomyces, filamentous soil bacteria, are well known for their ability to produce antibiotics and other molecules useful in medicine or agriculture. Under specific growth conditions some strains can store an excess of carbon into TriAcylGlycerols (TAGs), a direct precursor for Bio-diesel ^{1, 2}. Streptomyces is thus an interesting canditate to generate bio-oils by fermentation ³.
Our goal is to evaluate, at the subcellular scale, the size/shape and localization of storage lipid inclusions in different Streptomyces strains by using a combination of atomic force microscopy and infrared spectroscopy (AFM-IR).
AFM-IR technique is a user-friendly benchtop technique that enables infrared spectroscopy with a spatial resolution well below conventional optical diffraction limits. It acquires IR absorption imaging spectrally resolved with lateral resolution down to 100 nm ^{4, 5}.
For the study of the local repartition of TAGs inside the cells, AFM-IR was employed to create sub-cellular chemical maps that allows label-free identification of TAGs inclusions in Streptomyces cytoplasm. This was possible since TAGs molecules show a specific response in the mid-infrared region, quite distinct from that of the other cellular constituants (C=O stretching of the esters at 1741 cm^-1).
Hence, the AFM-IR technique is likely to provide new insights into the constitution of the fatty inclusions and the role of TAGs in the morphological and metabolic differentiation processes that characterize Streptomyces developmental cell cycle.
1. Holmbäck, M.; Lehestö, M.; Koskinen, P.; Selin, J. Process and Microorganisms for Production of Lipids. WO2011/148056A1, 2011.
2. Packter, N. M.; Olukoshi, E. R.; Tag, A. Ultrastructural Studies of Neutral Lipid Localisation in Streptomyces. Arch. Microbiol. 1995, 164, 420-427.
3. Deniset-Besseau, A.; Prater, C.; Virolle, M-J. and Dazzi, A. Monitoring TriAcylGlycerols Accumulation by Atomic Force Microscopy Based Infrared Spectroscopy in Streptomyces Species for Biodiesel Applications. J. Phys. Chem. Lett., 2014, 5 (4), pp 654-658
4. Dazzi, A.; Glotin, F.; Carminati, R. Theory of Infrared Nanospectroscopy by Photothermal Induced Resonance. J. Appl. Phys. 2010, 107, 1-7.
5. Lahiri, B.; Holland, G.; Centrone, A. Chemical Imaging beyond the Diffraction Limit: Experimental Validation of the PTIR Technique. Small 2013, 9, 439-45.

Neaspec&’s near-field optical microscopy systems (NeaSNOM) allow to overcome the diffraction limit of classical optical microscopy and spectroscopy enabling optical measurements at a spatial resolution of 10nm not only at visible frequencies but also in the infrared or terahertz spectral range.
Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) [1] employs an externally-illuminated sharp metallic AFM tip to create a nanoscale hot-spot at its apex. The optical tip-sample near-field interaction is determined by the local dielectric properties (refractive index) of the sample material and detection of the elastically tip-scattered light yields nanoscale resolved near-field images simultaneous to topography.
Development of a dedicated Fourier-transform detection module for analyzing light scattered from the tip which is illuminated by a broadband laser source enabled IR spectroscopy of complex nanostructures (nano-FTIR) [2]. Identification of polymers [2], analysis of embedded structural phases in biominerals [3], or investigation of the secondary structure of individual protein complexes [4] demonstrated the successful application of nano-FTIR as an material characterization technology.
Equipping NeaSNOM systems with cw-light sources near-field imaging can be performed at time scales of 30-300s per image. Patented signal detection and analysis technology allows i.e. investigation of phase change materials, analysis of Graphene nanostructures [5], or to study energy storage materials in near-field amplitude and phase at unprecedented scanning speed and signal quality.
The patented modular system design enables tailored system configurations where the ultimate spectral coverage can be achieved by using synchrotron-based broadband IR light sources [6]. Based on reflective optics design of the system novel time-resolved near-field measurements [7] or the integration of THz-TDS systems have been realized.
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