Titanium dioxide (TiO2) is a versatile material with important applications in energy-related developments, including photo-catalysis and solar energy conversion schemes. TiO2 crystallizes in three different structures: rutile, anatase and brookite. Whereas the phenomenology of rutile surfaces has been extensively studied at atomic scale over several decades with both scanning tunneling microscopy (STM) and atomic force microscopy (AFM), the amount of information regarding the structure [1, 2], intrinsic defects [3], and phenomenology [4,5] of the TiO2 anatase surfaces is still scarce, even when the anatase structure is considered the photo-catalytically most efficient form of TiO2.
Here, we present an atomic scale characterization of the TiO2(101) anatase surface by means of bimodal AFM and simultaneous STM measurements. By using Pt-Ir covered cantilevers, we are able to detect the average tunneling current flowing between the cantilever and a conductive sample while performing atomic resolution dynamic AFM [6]. Simultaneously, it is also possible to operate AFM in bimodal mode [7]. In this work, we use the variation of the resonant frequency of the first eigenmode of a rectangular cantilever to acquire topographic images while simultaneously detecting the variation of the resonant frequency of the second eigenmode, which is driven at ultra-small oscillation amplitudes (typically between 8 to 40 pm), and therefore, it is very sensitive to the tip-surface short-range interaction [8]. In this presentation, we will show how the combination of the different channels of information provided by simultaneous bimodal AFM/STM operation allows us to clarify the contribution of the different atomic species of the TiO2(101) anatase surface surface to the atomic resolution images. We will also show that the average tunneling current enables to pinpoint subsurface defects that are otherwise unrevealed in the topographic AFM images. Preliminary results on the characterization of organic molecules of relevance for optoelectronic devices deposited on the TiO2(101) anatase surface will be also shown.
References:
[1] Hebenstreit et al., Physical Review B 62, R16334 (2000).
[2] Gong et al., Nature Materials 5, 665 (2006)
[3] He et al., Physical Review Letters 102, 106105 (2009)
[4] He et al., Nature Materials 8, 585 (2009)
[5] Grinter et al., J. Phys. Chem. C 116, 11643 (2012)
[6] Sawada et al., Appl. Phys. Lett. 94, 173117 (2009)
[7] Kawai et al., Physical Review Letters 103, 220801 (2009)
[8] Giessibl, Reviews of Modern Physics 75, 949 (2003)

Firstly discovered in meteorites, nanodiamonds (ND) are carbon particles with size from 2 to few tens of nanometers. Nowadays synthesized by detonation technique, ND already have broad applications in polishing materials, lubricants as well as applications in biomedical imaging [1,2]. More recently, the accurate control of the doping of the nanodiamond structure with atomic defects having optical and spin properties gives to this material an exceptional candidate for photonic devices [3], sensor for magnetometry [4] and quantum information processing and computing [5]. Indeed, bulk diamond is one of the widest band gap material (5.5 eV) and is optically transparent from ultraviolet to the infrared. However, single optically-active defects within the lattice structure, such as Nitrogen-Vacancy centers (NV-1), adsorb and reemit lights through the crystal without perturbation.
Here, we combine scanning probe microscopy imaging and spectroscopy [6] to locally investigate a monolayer of nanodiamonds adsorbed on highly oriented pyrolitic graphite (HOPG) [7]. Scanning tunneling microscopy (STM) images of nanodiamonds reveals crystalline facets and graphitic defects at the surface. Scanning tunneling spectra (STS) also confirms the insulating nature of those nanostructures (6 eV-band gap). Under illumination with wavelenghts of 400 nm and 470 nm, a modulation of the electronic band gap is observed compared to the dark condition, whereas no relevant variation is obtained for wavelenghts superior to 590 nm. By Kelvin probe force spectroscopy at low temperature [8], a shift of the contact potential difference (CPD) between tip and sample is also detected by comparing spectra in dark and light condition. Since pure diamonds are insensitive to visible light, this effect is a direct and local evidence of the creation of surface charges under illumination, that we assume coming from the NV centers contained into the nanodiamonds. This result might pave the way for integrating those nanostructures in solar cell devices.
[1] V. N. Mochalin et al., laquo;The properties and applications of nanodiamondsraquo;, Nature Nanotechnol., 7, 11, (2012).
[2] Y. R. Chang et al., “Characterization and Application of Single Fluorescent Nanodiamonds as Cellular Biomarkers”. P NAS, 104, 727, (2004).
[2] I. Aharonovich at al., laquo;Diamond Photonicsraquo;, Nature Photonics, 5, 397, (2011).
[4] G. Balasubramanian et al., laquo;Nanoscale Imaging Magnetometry with Diamond Spins under Ambient Conditionsraquo;, Nature 455, 648, (2008).
[5] J. Warchtrup, “Defect center room-temperature quantum processors”, PNAS, 107, 9479, (2010.)
[7] L. Schmidlin et al., to be published.
[8] L. Gross et al., “Measuring the charge state of an adatoms with noncontact atomic force microscopy”, Science, 324, 1428, (2009).

TiO₂ and its derivatives are very important materials due to the availability, low cost, chemical stability, etc. Their applications in dye-sensitized solar cells, polymer hybrid solar cells, photocatalytic hydrogen production and photocatalytic degradation of organics have been widely studied. Nitrogen doping n-type TiO₂ exhibits decreased band gap due to the mixing of N 2p states with O 2p states, and shows photodegradation capability toward methylene blue and gaseous acetaldehyde by visible light. In addition, a Schottky interface forms between metal nanoparticles with large work function (such as Pt, Pd, Au) and n-type TiO₂, while p-type semiconductors in contact with TiO₂ result in p-n junctions at the interface. Recently, Kelvin probe force microscopy (KPFM) has been used to obtain simultaneous mapping of structural and electronic properties of conjugated polymer based photovoltaic materials. KPFM technique adopts a non-contacting tip with a conductive coating to measure the surface potential difference between the tip and the local surface. This is an in-depth study on the photocatalytic performance characterization for single filament of TiO₂ nanofiber photocatalysts by novel photo Kelvin probe force microscopic technique (photo-KPFM) and first principles calculations. Three kinds of TiO₂ nanofibers: anatase TiO₂ nanofibers (anatase TiO₂ NFs), nitrogen doped TiO₂ nanofibers (N- TiO₂ NFs), nitrogen doped TiO₂ nanofibers decorated with platinum nanoparticles (N-TiO₂-Pt NFs) were investigated. The N- TiO₂-Pt NFs exhibits the largest negative photo surface potential shift (-182 mV) as compared to anatase TiO₂ NFs (-29mV). The first-principles calculations based on density function theory (CASTEP simulation software) indicate the significant photo surface potential shift obtained by adding nitrogen and platinum into TiO₂ NFs is induced by two mechanisms: (1) enhancement in absorbance to increase exciton generation and (2) decreased charge recombination to increase surface charge. These changes in the photo surface potential of various TiO₂ nanofibers are closely correlated to their photocatalytic activity. Thus, this novel photo-KPFM provides a useful technique to easily monitor the photocatalytic capability of material in the development of high performance photocatalysts.

Non-contact force microscopy has demonstrated true atomic resolution on metals, semiconductors and insulators. The application of AFM to single molecules is a challenge because of relatively weak bonding to the substrate, which often leads to high diffusion rates of the molecules. We will present molecules, which were designed to interact with specific sites on insulating surfaces. Molecular wires of porphyrin molecules on ionic crystal surfaces are observed [1,2]. A complete immobilization at kink sites of KBr(001) is observed for single truxene molecules at room temperature [3]. Recently, intramolecular resolution is studied on a variety of molecules. A further challenge is the manipulation of molecules on surfaces, including the controlled rotation, which means that the direction of rotation of the molecule can be chosen by the experimentalist [4].
[1] Th. Glatzel, L. Zimmerli, S. Koch, S. Kawai, E. Meyer, Molecular assemblies grown between metallic contacts on insulating surfaces, Appl. Phys. Lett., 94, (2009), 3

[2] Th. Glatzel, L. Zimmerli, S. Kawai, E. Meyer, L.-A. Fendt and F. Diederich, Oriented growth of porphyrin-based molecular wires on ionic crystals analysed by nc-AFM , Beilstein J. Nanotechnol. 2, 34-39, (2011)., 2, (2011), 34-39
[3] B. Such, T. Trevethan, Th. Glatzel, S. Kawai, L. Zimmerli, E. Meyer, A. L. Shluger, C. H. M. Amijs, P. de Mendoza, and A. M. Echavarren , Functionalized Truxenes: Adsorption and Diffusion of Single Molecules on the KBr(001) Surface ACS Nano, 4, (6), (2010), 3429
[4] R. Pawlak, S. Fremy, S. Kawai, T. Glatzel, H. Fang, L.-A. Fendt, F. Diederich, and E. Meyer , Directed rotations of single porphyrin molecules controlled by localized force spectroscopy, ACS Nano, 6, (2012), 6318-6324

The advent of nanoscale engineered materials has started to revolutionize material science and technology. Further improvement is expected when tailoring the materials down to the atomic scale.
For this and for developing functional systems a detailed understanding not only of the electronic but also of the mechanical properties at nanometer scale is of crucial importance.
Here we study an insulating single atomic layer of hexagonal boron-nitride (h-BN) on Rh(111). The lattice mismatch between the substrate and the h-BN produces a strongly corrugated hexagonal superstructure with 3.2 nm periodicity [1,2]. This superstructure has been shown to be an excellent nanotemplate [3] which allows to decouple electronically molecules and clusters from the underlying substrate [4,5].
To measure its mechanical properties we use a home-built combined scanning tunneling and atomic force microscope operating at low temperatures and with sub-nm oscillation amplitudes. From 3-dimensional frequency shift data we calculate the total energy landscape and lateral and vertical forces acting between the probing tip and the h-BN layer. Slight variations in the forces between the different alignments of the rim sites of the hexagonal corrugation in respect to the Rh surface enable us to derive the elastic properties of the corrugated layer. Our findings are further supported by a statistical evaluation of the lateral displacement of the BN hexagons in atom resolved measurements.
[1] M. Corso et al., Science 202, 217 (2004).
[2] R. Laskowski et al., Phys. Rev. Lett. 98, 106802 (2007).
[3] H. Dil et al., Science Sciene 319, 1824 (2008).
[4] S. Bose et al., Natuer Mat. 9, 550 (2010).
[5] S. Kahle et al., Nano Lett. 12, 518 (2012).

The formation of the smallest permanent indentation in the Au(111) surface is studied by depth-sensing indentation achieved by a combination of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). We use field ion microscopy (FIM) to characterize the 9.5 nm radius spherical apex of the W(111) indenter in UHV prior to the indentation experiments [1,2]. Knowledge of the indenter geometry is necessary to extract quantitative parameters such as contact pressures and stresses within the sample during indentation.
Traditional nanoindentation can measure depth to high precision, but it has been shown that they may not possess the force resolution required to detect initial plastic events occurring in the nano-Newton regime[3]. Indentation with standard AFM allows for excellent force resolution, but large piezo displacements required to load the contact with a soft cantilever hamper the extraction of true indentation depth because of quantitative optical beam deflection calibration issues (beam placement, sensitivity to mode shape, etc.) and piezo creep. These limitations make it difficult assess elastic and plastic behaviour in an indentation curve and extract physically meaningful quantities such as the hysteresis energy (work done) of the indentation. Our AFM-based indentation is not affected by these issues because of its interferometric detection method where displacements of the cantilevered sample are directly calibrated to the wavelength of a laser and small piezo displacements ensure minimal piezo hysteresis.
We report on the transition from elastic to plastic deformation in the indentation of a Au(111) single crystal. This is done by producing arrays of indentations to forces near the plastic yield point and examining the resulting force-displacement curves for both elastic and plastic indentation sites. Plasticity can be identified by features in the force displacement curves, such as the sudden displacement excursions of the tip (pop-ins), the work done by the indenter, and the sink-in depth measured at mild repulsive loads. Additionally, these indicators of plasticity can be correlated with the permanent impressions (or lack thereof) in the surface imaged by STM.
Adhesion of substrate material to the tip and its rearrangement upon loading is found to contribute a large variability to the characteristics of both elastic and plastic force-displacement curves. The forces at the initial yield points correspond to shear stresses lower than those expected for the homogeneous dislocation nucleation. We suggest that heterogeneous nucleation involving surface effects and indenter roughness is likely to play a role in the observed plastic behaviour.
[1] W Paul, Y Miyahara, and P Grütter, Nanotechnology 23, 335702 (2012)
[2] D Oliver, J Maassen, M El Ouali, W Paul, T Hagedorn, Y Miyahara, Y Qi, H Guo, and P Grütter, PNAS (accepted Sept 26 2012) (2012)
[3] Minor et al, Nature Materials 5, 697-702 (2006)

Molecular self-assembly on surfaces is regarded as one of the most powerful technologies for the production of advanced nanostructured materials. However, the development and practical use of this technology is limited by the restricted ability to control molecular adsorption and organisation over long length scales where it can be integrated with top-down nanofabrication techniques. We demonstrated that long-range interactions generated by charge transfer at metal-organic interfaces can be used as a means to steer 2D molecular assembly.
Specifically designed donor molecules were deposited at various coverages on different metal surfaces and characterised by low temperature scanning tunnelling microscopy. Density functional theory calculations shed light on the origin of the observed supramolecular assemblies, indicating an interfacial charge transfer regulated by energy level alignment at the metal-organic interface. Work function shifts, measured on a molecular scale by means of scanning tunnelling spectroscopy further confirmed the formation of induced dipoles. The competition between electrostatic repulsion among these dipoles and Van der Waals attraction leads to the supramolecular assembly, as demonstrated by kinetic Monte Carlo simulations.
A coherent picture emerges where long-range forces between charged molecules drive the spontaneous formation of a novel class of supramolecular structures.

Single organic molecules adsorbed on ultrathin insulating films were investigated using scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM), and Kelvin probe force microscopy (KPFM). With all of these techniques submolecular resolution was obtained due to tip functionalization by atomic manipulation. The techniques yield complementary information regarding the molecular structural and electronic properties.

Using NC-AFM with CO functionalized tips, atomic resolution on molecules has been demonstrated [1]. Moreover, different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished [2]. Two different contrast mechanisms for bond-order discrimination were found, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.
Using KPFM information about the distribution of charges within molecules is gained by measuring the z-component of the electrostatic field above the molecule, as demonstrated on the hydrogen tautomerization switch naphthalocyanine [3].
References :
[1] L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer, Science 325, 1110 (2009)
[2] L. Gross, F. Mohn, N. Moll, B. Schuler, A. Criado, E. Guitian, D. Pena, A. Gourdon, G. Meyer, Science 337, 1326 (2012)
[3] F. Mohn, L. Gross, N. Moll, G. Meyer, Nature Nanotechnol. 7, 227 (2012)

We have measured the angular dependence of chemical bonding forces between a carbon monoxide
molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined
scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.

Dynamic force microscopy and spectroscopy in vacuum - also known as noncontact atomic force microscopy (NC-AFM) - enables true atomic resolution of nonconductive surfaces and molecules. As in all scanning probe techniques, however, it is a weakness that the exact tip shape and the chemical identity of its apex atoms are commonly unknown. Therefore, it is extremely difficult to directly compare experimentally recorded tip-sample interactions with theoretical models. Here, I will review two options out of this dilemma.
Field ion microscopy (FIM) enables the atomic-scale analysis of the tip apex. Applying this technique to tungsten wires glued to a “qPlus” sensor enables the reliable combination of FIM and NC-AFM experiments [1]. Here, we present measurements of tip-sample interactions with a tungsten tip on a Ag(111) surface obtained in a low temperature atomic force. If the tip shape parameters from the field ion microscopy analysis were used the resulting van der Waals and electrostatic forces were found to be in quantitative agreement with analytical models.
In cold atom scanning probe microscopy (CA-SPM) the tip is replaced by a gas of ultracold Rubidium atoms that is confined inside an electromagnetic trap representing the cantilever [2,3]. Such a cold-atom SPM can be operated in a dynamic mode by making the gas oscillate within the trapping potential and measuring how the oscillation frequency changes as the trap is scanned over the surface. Since the interaction potential between the cloud and the sample slightly modifies the potential of the trap, the oscillation frequency of the cloud is changed and the interaction potential can be calculated from the frequency shift. Interestingly, the theory behind the dynamic mode of CA-SPM and dynamic force spectroscopy is essentially the same [4]. Therefore, it is very promising to join both techniques in order to measure the interactions between gas atoms and sample surface.
[1] J. Falter, G. Langewisch, H. Hölscher, H. Fuchs, A. Schirmeisen (submitted)
[2] M. Gierling, P. Schneeweiss, G. Visanescu, P. Federsel, M. Häffner, D. P. Kern, T. E. Judd, A. Günther, J. Fortágh: Cold-atom scanning probe microscopy. Nature Nanotechnology 6, 447 (2011).
[3] P. Schneeweiss, M. Gierling, G. Visanescu, D. P. Kern, T. E. Judd, A. Günther and J. Fortágh: Dispersion forces between ultracold atoms and a carbon nanotube. Nature Nanotechnology 7, 515 (2012)
[4] H. Hölscher: Cold atoms feel the force. Nature Nanotechnology 7, 484 (2012)

The characteristic aspect of strongly correlated oxides systems is the strong coupling between the structural, electronic and magnetic properties. A small change in one property can produce a large change in another. Controllable surface tuning provides the opportunity to study how structural, electronic, and magnetic properties respond to the broken symmetry and opens avenues for exploration of completely new physical properties. The extreme sensitivity of properties to external chemical and physical stimuli makes in situ characterization a requirement for controlled tuning of complex correlated materials. We report some recent progress in observations and tuning of physical and chemical phenomena on the surfaces of in situ grown, single crystalline epitaxial perovskite manganites films with different thicknesses, including atomic-level structural studies, control, and tuning of the physical properties.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Recent reports of oxide hetero-structures, exhibiting exceptionally high oxygen reduction reaction (ORR) activity and ionic conductivity, promise an alternative approach for achieving high-performance Solid oxide Fuel Cell (SOFC) cathodes. A prime example is the (La,Sr)CoO3/(La,Sr)2CoO4 (LSC113/214) hetero-structure which show three to four orders of magnitude faster oxygen exchange kinetics near the interfaces at about 500 °C compared to either material individually. While the interfacial regions are believed to be responsible for such enhanced ORR kinetics, the physical origin of these empirically observed results remains unknown to date. It is well known that the electronic structure of electrocatalysts plays an essential role in determining their oxygen reduction reaction (ORR) activity, and the electronic structure can vary significantly at the hetero-interface compared to that in the bulk. Therefore, obtaining electronic structure information near the interface is essential to understand the role of interfaces.
However, identifying the electronic structure at a buried hetero-interface as in a multilayer superlattices and confined within few nm from the interfaces is not an easy task, especially under operando conditions, and requires special techniques. Scanning probe techniques are ideal in revealing local electronic, magnetic or electrochemical properties on the surface. Nevertheless, challenges arise when attempting to apply them to interrogate interfaces of multilayers buried beneath the surface and under the harsh working conditions of SOFC cathodes (high temperatures and in oxygen environments). Previous successful attempts at exposing the local interface properties of oxides to scanning probe characterization have not provided a generalized capability to expose buried interfaces in a controllable fashion and have been limited to room temperature measurements. Heating and then quenching samples to room temperature for analysis does not ensure that reversible electronic structure changes with temperature on the surface can be captured.
In our work, we implement a novel combination of in situ scanning tunneling microscopy and spectroscopy (STM/STS) and grazing incidence focused ion beam (FIB) milling. This capability, for the first time, enables us not only to probe the electronic structure across hetero-interfaces of LSC113/214 with nano-scale resolution, but also to perform these measurements at high temperature and in an oxygen/gas environment. Our in situ approach enabled us to visualize an electronic structure transition over a 1 nm-thick interfacial zone between LSC113 and LSC214 layers in a nano-scale multilayer (ML) structure at room temperature, and to discover the high-temperature electronic activation of LSC214. This new knowledge is important for advancing our understanding the role of dissimilar oxide interfaces in determining the electro-catalytic activity.

The structures of two stepped Pt crystal surfaces, Pt(332) and Pt(557), were studied at high gas pressures at room temperature by high-pressure STM and ambient-pressure XPS. These crystals posses the same type of terraces but different types of steps. HP-STM shows that the two crystal surfaces respond differently to the exposure of 0.5 Torr of ethylene. Furthermore, after stabilizing the two crystals under 0.5 Torr of CO to form clusters, a subsequent introduction of 0.5 Torr of ethylene can remove the clusters and restore the stepped structure on Pt(332), whereas the CO-induced clusters remain on Pt(557). AP-XPS experiments were performed by following the same procedures to account for the phenomena. The step orientation is crucial in influencing the surface restructuring processes at high gas pressures.

Scanning tunneling microscope (STM) is a primary characterization tool for resolving fundamental aspects of surface activities and kinetics of solid/liquid interfaces and has been extensively used for underpotential deposition (upd) studies of various combinations of electrolytes and electrodes to produce sub-/monolayer films. Upd of Pb on Cu(100) surface represents a model system for studying surface alloying in the otherwise immiscible Pb-Cu system. Previous studies based on electrochemical and vacuum deposition techniques have exclusively revealed clear evidence of surface alloying although limited information is available on the kinetics of the polymorphic evolution that occur in the presence of halides. In this study, the relative coverage and surface structure that arise from the competition between Pb and Cl- is explored using in situ STM and electroanalytical measurements. Experiments were conducted on Cu(100) in 0.1 mol/L HClO4 + 1 mmol/L HCl + 1mmol/L PbClO4. At small overpotentials surface alloy formation initiates via place exchange at the step edges to form c(4x3) Pb structure and the 2-D alloy phase expands laterally across the surface. After a fixed time the alloy phase decomposes into a one dimensional Pb ribbon phase and c(2x2) Cl-. If before the c(4x3) decomposes, the potential is stepped to more negative values a different dealloying process occurs whereby Pb is expelled as a surfactant onto the free surface. The influence of these surface phases on the morphological evolution during multilayer co-deposition of Pb and Cu in the Pb upd potential range was also examined.

Metal phthalocyanines (MPc) have been widely employed as channel materials in organic thin film transistors for chemical vapor sensing because MPc molecules act as electron donors during reaction with oxidative analytes. However, this sensing reaction has not been fully understood on the atomic level. This study presents molecular scale observation of NO adsorption on CuPc monolayers using ultra-high vacuum (UHV) scanning tunneling microscopy. CuPc monolayers were deposited on Au(111) surfaces and HOPG by organic molecular beam epitaxy and subsequently dosed at 150 K with diluted NO via a supersonic molecular beam source.
For CuPc/Au(111) after dosing NO for 10 min, STM images reveal small NO chemisorption sites which have modified central metal ion into bright protrusion on the CuPc metal centers; ~7 % of CuPc molecules are reacted with NO which is the saturation coverage consistent with a delocalized chemisorption induced change in electronic structure. The site specific electronic structure of NO/CuPc/Au(111) sites were recorded with scanning tunneling spectroscopy (STS). Unreacted CuPc/Au(111) have a Fermi level almost in the middle of the band gap. However, after NO chemisorption, this EF shifts to valance band edge. This Fermi level shift by NO is conisstent with published theoretical predictions and with the high sensitivity of CuPc films to strong oxidants in CuPc OTFT chemical sensors. STS measurements of CuPc/Au(111) sites next to NO/CuPc/Au(111) sites show an electronic perturbation compared to sites on adsorbate-free CuPc/Au(111) consistent with a delocalized electronic effect which is very favorable for high sensitivity chemical sensing.
After dosing an equal amount of NO on CuPc/HOPG at 150 K, a lower coverage of NO chemisorbates (0.1%) with less electronic perturbation are observed compared to NO/CuPc/Au(111) but, the topographic appearance of the NO/CuPc/HOPG chemisorbates are similar to NO/CuPc/Au(111). It is known that Au has large densities of states and free electrons; therefore Au can transfer electron to CuPc increasing chemisorption probability of NO. Conversely, HOPG has low density of states and π-electron, so it transfers less electron to CuPc molecules than Au. This low electron density of HOPG results in lower coverage of NO chemisorbates on CuPc/HOPG. STS of NO/CuPc/HOPG shows only a small, electronic perturbation consisting with weaker interaction between CuPc and HOPG compared to Au. These results show that oxidant chemisorption on MPc monolayers can readily induce delocalized changes in electronic structure as shown by the low coverage limit for NO/CuPc/Au and NO/CuPc/HOPG and by the direct perturbation of electronic structure even for non-reacted neighboring sites for NO/CuPc/Au; the delocalized perturbation in electronic structure is critical requirement for high sensitivity sensor with MPc monolayers on graphene and other 2D semiconductors.

Solid-liquid interfaces play crucial, fundamental roles in a wide variety of physical, chemical and biological processes, such as crystal growth, electrochemical reactions and various biofunctions. Investigations of atomic-scale structures and interactions at solid-liquid interfaces are, therefore, essentially important for understanding theses microscopic processes. Force mapping method based on frequency modulation atomic force microscopy (FM-AFM) is a remarkable technique for atomic-scale investigations of interaction forces on a specific site of crystal surfaces. The technique has been mainly used in vacuum environments, where highly sensitive force detection can be performed due to the high Q-factor in the cantilever oscillation. However, since significant progress has been made in FM-AFM in liquids over the past several years [1, 2], the force mapping method can be used for atomic or molecular scale investigations of interaction forces at solid-liquid interfaces, such as solvation forces and electric double layer (EDL) forces.
In this study three-dimensional (3D) force mapping method has been applied to the investigations of molecular-scale hydration structures at solid-liquid interfaces as well as those around biomolecules such as proteins and DNA molecules. The 3D visualization of the hydration structures allows us to make a precise comparison of the experimental data with theoretical calculations of water structures, which can provide a molecular-scale understanding of the hydration structures and the roles of water molecules in the biological functions. Furthermore, the EDLs that formed on molecular assemblies in electrolyte solution were investigated by the 3D force mapping method with a modified DLVO theory. We successfully made a quantitative measurement of the EDL forces on anionic and cationic surfactant micelle surfaces, which allows us to estimate the local charge densities of the molecular assembly surfaces.
[1] T. Fukuma, K. Kobayashi, K. Matsushige and H. Yamada, Appl. Phys Lett. 87, 034101 (2005).
[2] S. Rode, N. Oyabu, K. Kobayashi, H. Yamada and A. Kuhnle, Langmuir 25, 2850 (2009).

We have implemented an imaging technique that allows simultaneous acquisition of topography and chemical contrast with atomic resolution. The technique is based on findings by M.Lantz et al. (2001) and Y.Sugimoto, S.Morita et al.(2006) that the local minimum of force or frequency shift in the force curve differs depending on the atom facing the tip apex. Y.Sugimoto et al(2006) implemented lateral drift compensation control to allow averaging of the force curve on a designated site for as much as ten to a hundred times to improve signal to noise ratio of the curves. The findings are important, but the technique is relatively time consuming. We introduce a new technique to obtain realtime chemical contrast by exploiting the merits of multi-frequency modulation, high frequency operation, and small amplitudes of drive.
An “all-optic” Ultra High Vacuum Atomic Force Microscope incorporates a heterodyne laser doppler interferometer, photothermal excitation and a superheterodyne circuit. Photothermal cantilever excitation offers extremely clean excitation in terms of phase noise of the AFM cantilever even at low amplitude (10 pm~) and high frequency (~10 MHz). Established radio frequency signal processing techniques could be applied to implement real-time characterization of the force curves allowing introduction of various control schemes. The use of higher modes of the cantilever in the MHz regime enabled a feasible scan speed. The system can be used for a conventional cantilever operating in its fundamental and higher modes, as well as for small or stiff cantilevers with high resonance frequency.
Here, we will report on the latest results acquired on multi-atom surfaces, and discuss the characteristics and possibilities of the method.

Metal Phthalocyanines (MPc) are well-known sensor materials for organic thin film transistor (OTFT). Compared to other organic sensor materials, they have advantages in thermal stability and strong interactions with gas molecules. Moreover, metal ion site of MPc, in which charge transportation with oxidizing gas molecules would be favorably occurred, can provide good images as a gas sensor. The instrument of Scanning Tunneling Microscope (STM), which provides surface image at the atomic level through electron tunneling, is attractive for applications where molecular images of the surface are needed; especially on the site gas molecules are adsorbed.
Ambient NO2 adsorption onto copper phthalocyanine (CuPc) monolayers is observed using ultra-high vacuum (UHV) scanning tunneling microscopy (STM) to elucidate the molecular scale sensing mechanism in chemical vapor sensors based on copper phthalocyanine. For low doses of NO2 (1 ppm for 5min), isolated chemisorption sites on the CuPc metal centers are observed in STM images. These weak chemisorbates almost completely desorb from the CuPc monolayer after annealing at 100 C for 30 min. Conversely, for high NO2 doses (10 ppm for 5 min), the NO2 induces a fracture of the CuPc domains. This domain fracture can only be reversed by annealing above 150 C for 6 hr consistent with irreversible chemisorption at near ambient temperatures. The behavior of NO2 dosed CuPc monolayers is consistent with two chemisorption mechanisms for strong oxidants on CuPc molecules inducing two different classes of sensor response: reversible sensing at low exposure and dosimetric (irreversible) sensing at high exposures. The observed domain fracturing is consistent as being the source of the highly sensitive dosimetric sensing of NO2 by CuPc OTFT. Comparison of O3 adsorption onto CuPc on Au and highly oriented pyrolytic graphite (HOPG) will be discussed.

We are currently developing an “all-optic” Ultra High Vacuum Atomic Force Microscope for improvement in resolution of force gradient and real time chemical identification. It incorporates a heterodyne laser doppler interferometer, photothermal excitation and a superheterodyne circuit with an intermediate frequency of 10.7 MHz. Photothermal cantilever excitation offers extremely clean excitation of the AFM cantilever at low amplitude (10 pm) and high frequency (~10 MHz), making the system a good approach for high frequency, small amplitude multimodal AFM with various control schemes. The system can be used for a conventional cantilever operating in its fundamental and higher modes, as well as for small or stiff cantilevers with high resonance frequency. Fundamental, second and third mode of flexural, or torsional, can be confirmed by scanning a small laser spot with a diameter of 1.2 µm on the backside of the cantilever.
By using multi-frequency modulation, small amplitude of drive and high frequency operation mode, the gradient of frequency shift profiles could be efficiently obtained. Flexural resonance mode of a Si cantilever was used to detect the interaction between tip and Si(111) 7x7 surface and the structures could be correctly identified. Recently, with the lateral mode, gradient of frequency shift profiles and topography could also be obtained on the same sample. In this mode, the magnitude of the deflection is determined by the topography of the sample surface, the frictional coefficient, the cantilever&’s lateral spring constant and the direction of the cantilever movement. This method can be useful for studying inhomogeneous alloys since it enhances contrast at the boundary between different compounds. We will report on these recent results obtained with this new all optic AFM and discuss the characteristics of this technique.

Infrared spectroscopy is a valuable tool for materials characterization (e.g. identification of chemical composition, orientation, impurity content, etc.). Unfortunately, the long wavelengths of light characteristic of the technique (2-16 µm) result in a diffraction-limited spatial resolution that is typically several micrometers at best; precluding characterization at the nanoscale which is so relevant to many areas of current research and development. Photothermal Induced Resonance (PTIR) is a technique that circumvents the diffraction limit by employing an atomic force microscope to locally monitor sample expansion resulting from pulsed laser light absorption/heating at chemical resonance to obtain nanoscale spectra and maps.
In this work we have expanded the spectral range and spectral resolution of a PTIR instrument by integrating a broadly tunable (1.55 -16 µm), narrow line-width (0.5 cm^-1) laser based on a difference frequency generation scheme. The result is a materials characterization platform capable of spectroscopy and chemical imaging, in registry with atomic force images, with a spatial resolution (100 nm) that notably surpasses the light diffraction limit throughout the entire mid-IR spectral range. We characterize organic, inorganic, and composite samples suggesting the potential for wide-spread applicability. Additionally, we compare PTIR spectra obtained with two broadly tunable lasers with differing pulse lengths (10 ns and 100 ps) to experimentally address controversy in the theory of PTIR signal generation. Results show that the difference in signal produced by the two lasers, predicted to vary by two orders of magnitude, is negligible for the sample investigated. This can be understood in terms of the forcing function imparted on the cantilever, which acts as a harmonic oscillator, by a particular sample according to the illumination conditions. Such analysis also offers guidelines for engineering PTIR experiments for optimal signal generation.

We report rapid chemical imaging of a polymer blend with 100 nm spatial resolution using atomic force microscope infrared spectroscopy (AFM-IR). AFM-IR enables chemical characterization and identification in an AFM, but requires a relatively long acquisition time for high resolution mapping due to its low signal to noise ratio. In AFM-IR, tunable IR laser light incident upon a sample drives local photothermal expansion. An AFM tip in contact with the sample senses this nanometer-scale expansion, which is captured by resulted cantilever vibration. The cantilever response is measured either in terms of the peak-to-peak amplitude of time-domain data or the integrated magnitude of frequency-domain data. Here, we apply a continuous Morlet wavelet transform to show how the cantilever dynamics during AFM-IR vary as a function of both time and frequency. This time-frequency domain analysis reveals the region of energy localization in both time and frequency domain. Based on this observation, a time and frequency window is applied to the wavelet transformed response in the range where the energy is confined. With this approach, we can increase the signal to noise such that the throughput is increased by 32X over state of the art. We demonstrate how technique can improve the chemical characterization with a film of polystyrene in the spectral range 2800-3100 cm^-1. Finally, we can reduce the acquisition time accordingly to map chemically distinct domains of a polymer blend (polystyrene-polymethyl methacrylate) with 100 nm resolution.

The need for better analytical tools that can provide high sensitivity, detailed molecular information with high spatial resolution is well recognized and is evidenced by the fact that it is a goal sought by many researchers and the impetus behind the development of several different chemical imaging techniques. Mass spectrometric imaging (MSI) is a powerful technique that uses molecular surface sampling to generate images that correlate the physical features on a surface with the presence of chemical species. The key to visualizing the spatial distribution of molecular species present in a sample is effectively sampling small volumes of material from a surface from specific locations with high spatial resolution. With this in mind, we have developed an AP sampling/ionization technique that focused on achieving nanometer scale MSI resolutions using an AFM in combination with nano-thermal probes for multimodal imaging of material surfaces with sub-micrometer chemical imaging spatial resolution. The combination of AFM and mass spectrometry on one platform allows for multimodal imaging of various physical and chemical characterizations of relevant materials systems. We will present our results of multimodal chemical imaging using this technique on test substrates and show application of this approach for the multimodal analysis of polymeric systems. This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, United States Department of Energy. ORNL is managed by UT-Battelle, LLC for the U.S. Department of Energy under contract DE-AC05-00OR22725.

The holy grail of surface science is to “image” the chemical makeup of a surface at high spatial resolutions, i.e., to identify, interrogate, and image chemical functionality (bonding) of materials at the nm-scale to connect physicochemical properties with function. To fill this need, we are developing hybrid scanning probe microscopy (SPM) instruments that manipulate light below the diffraction limit to “chemically” interrogate surfaces using localized vibrational (Raman) spectroscopy. Laser light is coupled to the plasmon modes of a nanoscale optical antenna tip to create an enhanced optical field at the tip apex; the sub-wavelength (evanescent) optical field near the tip is subsequently used as a nanoscale light source to conduct tip-enhanced Raman spectroscopy (TERS) of a surface.
This talk will highlight the design and validation of our confocal, side-on TERS system for chemical imaging of surfaces with emphasis on (1) confocal operation of the Raman microscope (e.g., multi-wavelength capability, point spread function, and optical sectioning of samples), (2) plasmonic properties of the tip-substrate system (e.g., theoretical vs. experimental), (3) point spectroscopy mode on various samples (e.g., thin polymer films and semiconductors), and (4) spatial resolution of TERS via Raman imaging of phonons in SiGe nanowires and catalytic nanoparticles.
Key results of the work have been (1) demonstration of all-optical chemical imaging of surfaces with spatial resolutions < 6 nm and (2) rigorous measurement of the distance scaling of plasmonic enhancements in Raman scattering, which shows that tip-enhanced optical experiments are often plagued by “artifacts,” rather than true tip-surface plasmonic coupling [1,2]. For example, it is shown that spatial variation in Raman scattering at distances less than the diffraction limit is not a sufficient indicator of TERS; in fact, the true TERS signal decays on the same length scale as the tip size, in accord with plasmonics theories. Such artifacts have been attributed to multiple scattering events involving the tip shaft; experimental considerations to alleviate these near-field artifacts and anomalous TERS signals will also be discussed.
[1] R. Ramos and M.J. Gordon, “Near-field artifacts in tip-enhanced Raman spectroscopy”, Appl. Phys. Lett. 100, 213111 (2012).
[2] R. Ramos and M.J. Gordon, “Reflection-mode, confocal, tip-enhanced Raman spectroscopy system for scanning chemical microscopy of surfaces”, Rev. Sci. Inst. 83, 093706 (2012).

The characterization of nano-materials continues to grow in importance and to impact key applications in the fields of materials science. The development of advanced materials for various applications requires detailed information about both physical and chemical properties of these materials at nanometer scale. However, conventional chemical characterization techniques are limited in an amount of situations due to the inability of these methods to chemically differentiate materials with good spatial resolution and without damage, staining or preferential solvent washing; and more difficultly, associating observed physical properties with chemical compositions correspondingly at the nanometer spatial resolution. True Surface Microscopy can follow the surface topography with high precision, so that even rough or inclined samples always stay in focus. The topographic coordinates from the profilometer measurement are used to perfectly follow the sample surface in confocal Raman imaging mode. The result is an image revealing chemical properties at the surface of the sample, even if this surface is rough or inclined.

In the ongoing effort to miniaturize the functional elements in electronic devices, molecular dimensions are currently approached. Scanning probe microscopy has demonstrated fascinating capabilities for bottom-up fabrication of atomically defined prototype structures. However, little is known about the underlying interactions during the manipulation of large functional molecules with a scanning probe tip. Here, we demonstrate the use of non-contact atomic force microscopy at cryogenic temperatures for the controlled lateral displacement of the organic prototype molecule PTCDA on the Ag(111) surface. During repeated manipulation cycles, we measure the precise lateral and vertical tip-molecule force profiles as well as the energy dissipation before and during the manipulation process. The jump of the molecule to an adjacent equivalent substrate lattice site occurs in the regime of repulsive lateral forces, thus constituting a &’pushing&’ mechanism. Our results allow to quantify the tip-molecule force interactions at the manipulation threshold for a pure displacement and report the success factors for an accurate measurement of manipulation forces.

In this study, we employed scanning electron beam induced deposition (SEBID) to form a high-aspect-ratio carbon nano-rod at the tip apex of an atomic force microscope (AFM). The SEBID-modified nano-rod shows diamond-like carbon according to the analysis of carbon k-edge by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy. Using SEBID, one can fabricate or modify AFM tip apexes for high-resolution and high-aspect-ratio applications. The modified tips have an apex diameter of about 10 nm and an aspect ratio of about 60, which can provide topographic images with high resolution and explore deep trench structures. We note that tip apex modification allows us not only to improve the tip apex but also to recycle the used probes and reduce the consumption of AFM probes.

Understanding of the interfacial properties in organic field effect transistor (OFET), i.e., molecular structure of the interfacial transition layer, energy level alignment, traps, etc. is of vital importance to improve the device performance. In this study, we have fabricate a high mobility (0.15 cm2/Vs) CuPc thin film transistor operating at Vg=-2V by using a high-k AlOx/TiOy (ATO) thin film as gate dielectric. The crystallinity of the CuPc on the dielectric, the contact effect and the density of states at the interface are investigated in-situ by exploiting Kelvin force microscopy on a working device. We find that the superior performance of the CuPc/ATO FET comes from the low trap density of states, which is at the order of 7×1017cm-3. Low surface energy and good crystallinity of CuPc interfacial layer lead to the reduction of trap density of states. In addition, we find a doping level above the HOMO edge, which is considered to be the doping level of O2.

The electric characterization of Au islands on TiO2 at nanometer scale (as a Schottky nanodiode) has been studied with conductive atomic force microscopy in ultra-high vacuum. The diverse sizes of the Au islands were formed by using self-assembled patterns on n-type TiO2 semiconductor film using the Langmuir-Blodgett process. Local conductance images showing the current flowing through the TiN coated AFM probe to the surface of the Au islands on TiO2 was simultaneously obtained with topography, while a positive sample bias is applied. The boundary of the Au islands revealed a higher current flow than that of the inner Au islands in current AFM images, with the forward bias presumably due to the surface plasmon resonance. The nanoscale Schottky barrier height of the Au/TiO2 Schottky nanodiode was obtained by fitting the I-V curve to the thermionic emission equation. The local resistance of the Au/TiO2 nanodiode appeared to be higher at the larger Au islands than at the smaller islands. The results suggest that conductive atomic force microscopy can be used to reveal the I-V characterization of metal size dependence and the electrical effects of surface plasmon on a metal-semiconductor Schottky diode at nanometer scale.

Gold nanoparticles can be coated with a monolayer of thiolate molecules to be colloidal stabilized or to obtain desired functionalities. In some cases, this monolayer can be composed of different types of molecules. Usually, this is done to take advantage of some specific property of each molecule. When the molecules are not similar, it is possible to have a patchy distribution on the particle&’s monolayer. Therefore, it is of fundamental interest to understand how these thiolate molecules are spatially distributed. Scanning Tunneling Microscopy (STM) has allowed a direct observation of ligand coatings on nanoparticles. Mixed self-assembled monolayers on the surface of gold nanoparticles have been shown to form defined domain structures such as alternating stripe-like domains. Here, we present our observation of Janus-type structures for mixtures of ligand molecules, where the longer tends to crystalize. The visualization of Janus topography by STM is confirmed by various imaging procedures and image analysis.

A molecular wire containing polypyrrolyl conjugate bonds was prepared by chemical adsorption using 6-pyrrolylhexy 1-12,12,12-trichloro-12-siladodecanonate (PEN) and an electrooxidative polymerization, and the conductivity of the molecular wire without any dopant was measured by a conductive AFM at room temperature. When the measured sample dimensions were about 0.3 nm (thickness of conductive portion in PEN monomolecular layer) : 100 mu;m (average width of electric path) : 2 mm (distance between Pt electrode and the AFM tip covered with Au), the conductivity of the polymerized PEN molecular wire at room temperature exceeded 2.5E5 S/cm both at atmospheric pressure and in a vacuum chamber of 1E-5 torr. The activation energy obtained from Arrhenius plots was almost zero between 320 and 450 K. We confirmed electric paths on different samples fabricated by the same technique using conductive AFM tips, and obtained similar data. The conductivity of molecular wires containing polypyrrolyl conjugate bonds should be exceed 2.5E5 S/cm may have even exceeded 1E7 S/cm. This value is much higher than the conductivity of Au. On the other hand, along an electric path (molecular wire), it is unlikely that all the PEN molecules were polymerized linearly to connect the Pt+ electrode and the tip covered with Au. The polypyrrolyl conjugate bonds have no dopant. Thus, it is reasonable to conclude that only a few molecular wires made of continuous polypyrrolyl conjugate bonds were formed in the electric path made of the polymerized PEN monomolecular layer to connect the Pt+ electrode with the AFM tip covered with Au, and these wires mainly contributed to the electric conductance. The conduction mechanism of this molecular wire might be also different from that of semiconductors and metals. The conduction mechanism of PEN may be different from that of semiconductors and metals. The electric conductance of the electric path is extremely high, as mentioned above. It is unlikely that all the PEN molecules at the electric path were polymerized to form a connection between the Pt electrode and the tip covered with Au. Although, using our technique it was difficult to confirm that the conjugated bond chains (molecular wires) connecting the Pt electrode and the tip covered with Au were formed in the PEN monomolecular layer, it is reasonable to conclude that some of the molecular wires in the polymerized PEN monomolecular layer connected the Pt electrode and the tip covered with Au, and these wires mainly contributed to the electric conductance. Thus, a plausible model of the electric conduction path formed during current measurement is schematically shown.

The detection and use of the tip-sample force interactions for exploring surface topography and material properties is the essence of Atomic Force Microscopy (AFM). The AFM probe serves as a force sensor and its deflection or changes of oscillatory parameters reflect the forces exercised primarily by the tip apex. In AFM contact mode and in the non-resonant oscillation mode, the probe deflection, which is related through the cantilever spring constant to the overall tip-sample force, is employed for topography tracking. Although such imaging can be connoted as constant force operation, we could not exclude the possible variations of the net force constituents such as van der Waals, electrostatic, adhesive and repulsive interactions. In case of resonant oscillatory modes, in which the probe oscillates near its 1st flexural resonance, the changes of the amplitude (amplitude modulation - AM) or frequency (frequency modulation - FM) are applied for surface tracking. The relations between these parameters and the tip-force are rather complicated but can be theoretically described using the asymptotic Krylov-Bogoliubov-Mitropolsky method of solving the Euler-Bernoulli equation for the probe oscillation [1]. Furthermore, this theoretical approach also allows extracting the maximal tip-force in AM modulation mode using one of the solid deformation models. For practical purposes, it is quite desirable to perform a rational comparison of the tip-forces in different AFM modes. We have performed such comparative imaging of different samples using the non-resonant and resonant oscillatory modes. The results obtained on soft polymers and rigid materials are analyzed in framework of the described theoretical approach.

[1] S. Belikov, and S. Magonov, “Classification of Dynamic Atomic Force Microscopy Control Modes Based on Asymptotic Nonlinear Mechanics” Amer. Control Confer. 979-985 (2009).

The hybrids consisting of the n-type CdSe/ZnS quantum dots (QDs) as a core and the p-type π-conjugated molecules as a shell were synthesized. The nanoscale photoluminescence (PL) and photoresponsive molecular electronic characteristics of the hybrid nanoparticle (NP) were investigated. For π-conjugated molecules, the macromolecular dioctyloxybenzodithiophene-based polythiophene (P3000) or the single molecular carbazole (CB) were attatched to the QDs with or without the insulating molecular blocks, respectively. The formations of the core-shell hybrid NPs of the QD-P3000 and QD-CB were confirmed by the high resolution transmission electron microscope (HR-TEM) and electron energy loss spectroscopy (EELS). The nanoscale PL characteristic of the single NP was measured using a high spatial resolution laser confocal microscope (LCM). For the hybrid QD-P3000 single NP, the nanoscale PL characteristics were changed because of both energy and charge transfer effects between QD and P3000. However, for the hybrid QD-CB NP, the LCM PL of the QD was dominant because of weak energy transfer due to the relatively longer insulating molecular block, which confirmed by the exciton lifetime obtained from the time-resolved PL measurements. From conducting atomic force microscope, the photocurrents of the QD-P3000 were higher and actively responsed both forward and reverse biases due to the energy and charge transfer effects, while those of the QD-CB exhibited the diode characteristics.

The effect of the Hf doping on hard magnetism of nanocrystalline Zr18-xHfxCo82 ribbons (x=0, 2, 4, and 6) are investigated by magnetization curves and magnetic force microscopy (MFM). Emphasis are on the local magnetic domain structures in the polycrystalline rapidly solidified Zr18-xHfxCo82 ribbons for four different samples with small fractions of Hf dopants (x = 0, 2, 4, 6). The typical size of nanocrystalline ribbons is about 2 mm wide and 50 mu;m thick prepared by arc-melting followed by vacuum melt spinning using a rotating copper wheel at a speed of 40 m/s. The investigation of the magnetic properties of the Zr18-xHfxCo82 ribbons revealed all the samples under investigation are ferromagnetic in nature at room temperature, and the corresponding MFM images have bright and dark contrast patterns with up-down magnetic domain structures. The average magnetic domain sizes of samples are about 38 nm, 48 nm, 15 nm, and 76 nm for x = 0, 2, 4, and 6 respectively. It is found that the saturation magnetizations and the coercivities depend on substituting Zirconium by limited amounts of Hafnium. For a sample with Hf concentration x=4, the maximum energy product (BH)max value is 3.7 MGOe. The magnetic correlation length of this sample is about 15 nm as calculated from the MFM images as an average over the lateral dimension of the domains sizes. The relatively short magnetic correlation length indicates weak inter-granular exchange coupling. The improvement of the energy product of the rare-earth-free high temperature permanent-magnet ribbons upon the Hf addition can be attributed to the refinement of the magnetic microstructure.

We analyze the heat transfer between a self-heated scanning thermal microscopy probe and a cold sample in ambient conditions versus the probe temperature. Measurements are performed while the probe and the sample are in contact. A predictive modeling of the measurement is used to identify from these measurements the global thermal conductance of the thermal exchange between the probe and the sample. A variation of the identified thermal conductance is observed versus the probe temperature. A new description of this thermal conductance, based on the several possible physical mechanisms of probe-sample energy transfer, namely solid-solid conduction, conduction by the surrounding gas, near field radiation and conduction through the liquid nanofilm formed by capillary condensation on the probe and sample surfaces, versus the probe/sample contact temperature is proposed as well as a new scanning thermal microscopy probe calibration protocol for thermal conductivity measurement. This work underlines and discusses a strong change of the predominant physical mechanism of heat transfer between the probe and a sample around a critical probe/sample contact temperature.

Atomic force microscopy (AFM) and Raman spectroscopy were used to characterize the morphology and physical properties of polypropylene-based graphene nanocomposites. An Asylum Research MFP-3D-Bio AFM was used to perform phase angle measurements to estimate the loss tangent along with the local modulus of composite&’s surface as a function of graphene content. We have observed an increasing trend in phase angle as the graphene content increased. This trend correlates with other physical properties such as thermal conductivity within the composite. In addition to the composite trends observed, we also identified wrinkled graphene flakes embedded in the polymer matrix. The graphene corrugation and the mismatched strain between polymer and graphene sheets show a variation in the phase angle that is corroborated with Raman measurements. Mechanically exfoliated graphene on SiO2 was also characterized as a baseline to understand the effect of graphene wrinkles compared to graphene surfaces on phase angle. The Raman results further revealed that there are changes in the crystalline morphology of the polymer with the addition of graphene as well as a correlation between composite thermal stability and polymer structure.

An all-optic liquid atomic force microscope incorporating a heterodyne laser doppler interferometer and photothermal excitation was implemented. The instrument enabled acquisition of atomic resolution images for both flexural and torsional modes of the AFM cantilever. However, the operating points of the images acquired by the two modes have so far remained unknown. We have implemented a dual mode excitation loop to access the onset of the two modes with respect to the distance of the tip apex to the sample surface. The amplitude of drive of the two modes are changed in the range of a few pm to a few 100 pm to verify the effect of the amplitude of drive on the structuring of the liquid molecules, the force curves and the acquired images.

Breakthroughs and further improvements of atomic force microscopy (AFM) methods critically depend on the availability of high-performance tips. Although silicon-based tips are very versatile and are therefore most widely used today, they fail in applications where ultra-high contact pressures (typically in GPa range) are needed such as in scanning spreading resistance microsocpy (SSRM). Doped diamond has become the material of choice for such measurements. Diamond coated Si tips do break close to the apex during scanning at the high lateral forces and show low spatial resolution. Full diamond tips (FDT) with a pyramidal shape show a superior spatial resolution of le;1 nm in routine SSRM measurements. In this work, we demonstrate the enabling nature of various FDT probes by expanding the AFM application domain from two-dimensional (2D) measurements towards three-dimensional (3D) applications.
This is achieved by FDT probes operated in a slice-and-view approach: first a 'slice' scan at high force removes a certain amount of material; then the 'view' scan performs an actual measurement at lower force. In this way, FDT probes allow the collection of a series of 2D images which can then be assembled to an electrical tomogram. We show that the developed FDT allow for adjustable scan removal rates of about 1-10 nm/scan on hard materials like silicon, germanium and silicon oxide. Operating such FDT probes in slicing motion only, can be exploited for nano-machining purposes. We demonstrate this for making reference marks and removing passivation layers. Besides pyramidal FDT, we introduce knife-shaped (45° rotated on cantilever) and in-plane (triangular shape with tapered end) FDT. The latter configuration strongly facilitates locating the region of interest as the tip region is clearly visible in the optical microscope of the AFM. The knife-shape FDT configuration is advantageous when material removal is the main purpose. The presented FDT are integrated into metal cantilevers and are fabricated in a state-of-the-art 200-mm wafer manufacturing line. Our work illustrates that solid diamond tips have the potential to lead AFM towards 3D centered profiling and machining applications not possible with Si-based probes.

Atomic Force Microscopy (AFM) force distance curves were obtained in the surface of different asphalt binders in order to relate the features observed on the surface, their phase contrast images to their local stiffness and elastic recovery. Indentations were performed in different points of the surface and a significant variation of elasticity was observed between the points on the so-called bee structure and the matrix. Also, a surface modification that could be induced by the tip pressure was observed and related to the local surface structure. Indentations, varying the maximum force, were performed on similar white spots of the bee structure and the elastic recovery was followed up to 1 hour after indentation. The final surface state of the binder, close to the bee for usual bees is not the same as the initial one indicating severe plastic deformation and the elastic recovery is very much dependent on the colloidal structure of the bee. Also, permanent phase change and surface hardening could be observed depending on the force and structure of the bee arrangements.

Paper is a material that is known to mankind for a long time and used commonly in everyday life. The applications range from an information carrier to packaging, with each having different requirements. A crucial knowledge to design paper would be the exact mechanisms, which are holding the individual fibers together, allowing to form a sheet.
Here, methods based on atomic force microscopy (AFM) are presented, which allow to gain insights into bonding mechanisms on the nanometer scale. The first method is designed to directly test the strength of single fiber-fiber bonds. Stiff AFM cantilevers are used to separate the bond by pressing on a fiber and recording force vs. distance curves. Scanning the formerly bonded area in tapping mode, allows to address morphological features such as microfibrils and fibril bundles to features observed in force-distance curves [1].
A different method deals with single unbonded pulp fibers. By employing a setup which allows to change the relative humidity in a certain range, swelling of pulp fibers can be studied. Besides a topographical investigation, AFM based nanoindentation was used to estimate mechanical properties of pulp fiber surfaces as a function of the relative humidity [2]. Additionally, pulp fibers were characterized in their fully swollen state in water.
Supported by Mondi, Kelheim Fibres and the Christian Doppler Research Society, Vienna, Austria.
[1] F.J. Schmied, C.Teichert, L. Kappel, U. Hirn and R. Schennach . Joint strength measurements of individual fiber-fiber bonds: An atomic force microscopy based method, Rev. Sci. Instrum. 83, 073902 (2012)
[2] B. N. J. Persson, C. Ganser, F. Schmied, C. Teichert, R. Schennach, E. Gilli, and U. Hirn. Adhesion of cellulose fibers in paper. submitted to J. Phys.: Condens. Matter.

We study photochemistry in model polymer blend solar cells by combining surface IR with electrical scanning probe microscopy to study the kinetics and spatial distribution of photochemical defect formation. In addition to showing that photochemical defect formation is non-linear in light intensity, we also correlate quantitatively the degree of photo-oxidation with the loss of photocurrent in nanostructured photovoltaics. Finally, we show that spatial composition variation can play unexpected roles. Photocurrent generation from acceptor-rich domains appears to decline much more dramatically although the donor material is the preferentially being degraded. We attribute this effect to faster degradation of hole transport paths, resulting in a loss of percolation pathways for hole transport in the acceptor-rich phases. We believe our results will help the organic photovoltaic community better understand the critical mechanisms behind photo-oxidation in these important devices.

Recently, BiFeO3 (BFO) thin films with periodically ordered ferroelectric domains have shown to generate open circuit photovoltages as large as 15 V by illumination with light having the photon energy above the band gap. The origin of this abnormal photovoltaic effect was supposed to be the ferroelectric domain walls. It was assumed that the strong local electrical field existing at the domain walls will separate the electron-hole pairs photo-generated within the domain wall whereas the carriers will strongly recombine within the bulk of the domains. In order to closely investigate this nanoscale photovoltaic mechanism, we have developed AFM-based local measurements of photoelectric and photovoltaic effects.
The present talk will address general aspects of local probing of photovoltaic and photoconductive effects. We will discuss photoelectric scanning probe microscopy and tip-enhancement of photocurrents in BFO single crystals.[1] Furthermore, we will show a novel time-resolved spectroscopic scanning probe method, called photo-induced transient spectroscopy (PITS) scanning probe microscopy (SP-PITS), which allows rapid evaluation and local mapping of the generation and recombination rates. [2]
Specific on BFO we will show that the photovoltaic currents are rather uniformly distributed at the crystals surface, suggesting that there is no strong electron-hole recombination within the ferroelectric domains.
[1] M. Alexe and D. Hesse , Nature Communications 2:256, DOI: 10.1038/ncomms1261.
[2] M. Alexe, Nanoletters 12, 2193, 2012, DOI: 10.1021/nl300618e

Advanced near field microscopy techniques are essential tools for fundamental and technological research in the field of organic electronics and photovoltaics. The Kelvin probe force microscope (KPFM) has become very popular among the different atomic force microscopy (AFM) techniques because it allows to map the electric surface potential of organic thin films and devices. Under ultra-high vacuum (UHV) conditions and in non-contact mode (nc-mode) it is possible to record simultaneously the topography and the contact potential difference (CPD) with an improved level of resolution.
In the case of donor-acceptor bulk-heterojunction blends it has been recently demonstrated that KPFM enables the investigation of the charge carrier generation on the sub-10nm scale [1]. However, to further enhance the understanding of these processes it is advantageous to study model molecular systems that possess better defined electronic and structural properties than the one found in bulk heterojunctions.
In this communication, nc-AFM/KPFM and scanning tunneling microscopy (STM) investigations of model self-assembled polymers and oligomers on highly oriented pyrolytic graphite (HOPG) will be presented. The nature of the local CPD contrasts [2] and the tip surface interactions will be discussed. First of all, the influence of long-range and short-range forces on the molecular stacking height measured by nc-AFM/KPFM [3] is clarified in the case of sub-monolayer poly(3-dodecylthiophene) (P3DDT) samples. In a second step, a new generation of donor-acceptor dyads will be presented that has been synthesised following the concept developed by W. Li et al. [4]. Low-current STM and nc-AFM measurements proof the auto-organization of this model system in the form of periodic lamella on the HOPG substrate. Additionally the results clearly reveal the existence of an, as well auto-organized, "face-on" layer which serves as a buffer-layer for the formation of the overlying lamella. The scanning probe microscopy data will be compared to the results of molecular mechanics (MM) and molecular dynamics (MD) simulations. Finally, the surface potential contrasts will be examined and a comparison of CPD measurements in dark and under illumination will be discussed. The results affirm the value of this new generation of donor-acceptor dyads as a promising system for organic photovoltaics.
[1] Evan J. Spadafora, Renaud Demadrille, Bernard Ratier, Benjamin Grevin, Nano Letters 10, 3337-3342 (2010)
[2] Evan J. Spadafora, Mathieu Linares, Wan Zaireen Nisa Yahya, Frederic Lincker, Renaud Demadrille, Benjamin Grevin, Applied Physics Letters 99, 233102 (2011)
[3] Sascha Sadewasser, Martha Ch. Lux-Steiner, Physical Review Letters 91, 1-4 (2003)
[4] W. Li, A. Saeki, Y. Yamamoto, T. Fukushima, S. Seki, N. Ishii, K. Kato, M. Takata, T. Aida, Chem. Asian J. 5, 1566-1572 (2010)

Efficiently converting photonic to nano-plasmonic modes for localizing and enhancing optical near fields is of high interest for applications ranging from nano-optical imaging and sensing to computing. Based on extensive simulations of various “optical transformer” geometries, we propose a novel photonic-plasmonic hybrid Scanning Near-field Optical Microscopy (SNOM) probe called the “campanile” tip. These campanile tips couple the photonic to the plasmonic mode, then adiabatically compress the plasmon mode, over a broad bandwidth, which is crucial for many optical spectroscopy techniques. The confinement of the optical near field is determined by the gap size between the two antenna arms, which can be well below 10nm given the appropriate resolution of the dielectric deposition method. Based on excitation through the back of the tip similar to traditional aperture-based SNOM tips, these campanile tips are an excellent candidate for background-free nanoscale imaging and spectroscopy applications on dielectric, non-transparent substrates. We used FEM to simulate conventional aperture-based probes, the coaxial plasmonic probes, traditional apertureless SNOM tips and the state-of-the-art adiabatic-compression-type probes, and compared them all with the campanile tip geometry. The understanding of relative strengths and weaknesses of each SNOM probe geometry served as the guideline for the design of the campanile tips, resulting in their superior field coupling, enhancement and resolution capabilities. Experimentally, as a proof-of-concept, ~40nm resolution hyperspectral nanoimaging of InP nanowires is performed via excitation and collection through the campanile probes. We map the influence of local trap states on exciton energies and recombination rates, revealing optoelectronic structure along individual nanowires that is not possible to observe with any existing methods. The theory and experiments demonstrates their unique access to physiochemical properties at the length scales relevant to critical processes in nanomaterials and the campanile geometry SNOM probe thus represents a step forward in plasmonics, non-linear optics, and especially scanning near-field investigations, allowing the study of the full scope of nanostructured materials including biological samples and optoelectronic nanomaterials.
Reference: These results were just accepted by Science for publication

Thin-film silicon solar cells require advanced light-trapping to absorb incident light in the optically thin absorber layers. Conventional light-trapping schemes make use of textured interfaces with subwavelength structures which scatter incident light into large angles, resulting in total internal reflection. This way, the layer stack of the solar cell serves as a waveguide, guiding the light within the absorber layer. To study the influence of local interface structures on the light-trapping properties is a matter of ongoing research. The investigation of correlations between interface structures and their optical properties requires experimental methods with subwavelength resolution beyond the Abbe limit. Scanning Near-field Optical Microscopy (SNOM) provides the required resolution. In addition, SNOM measurements in the near-field offer access to evanescent fields which are correlated to total internal reflection.
In this contribution, we will present a two-probe Scanning Near-field Optical Microscope, which is designed to investigate the local light propagation in thin layers. Conventional single-probe SNOMs are limited in the ability to gain information about the light propagation induced by local interface structures. This is mainly due to the fact that in the standard modes of operation either the source of illumination (collection mode) or the location of detection (transmission mode) is placed in the far-field. The usage of two probes allows to overcome the fundamental limitations of the collection and transmission mode. A two-probe SNOM uses one probe for light in-coupling and another probe for light out-coupling, both in the near-field. By placing the illumination probe at a point of interest and scanning the surrounding area with the detection probe, a map is created visualizing the propagation of light in the thin layer for the selected position of the illumination probe.
In particular, we present two-probe SNOM measurements of thin film silicon solar cells. The solar cells are made of hydrogenated microcrystalline silicon (µc-Si:H) and have a thickness of around 1 µm. For wavelengths longer than 500 nm the absorption in µc-Si:H decreases and the propagation length of the trapped light increases. To study this relation SNOM measurements with five wavelengths are performed simultaneously which cover the region of high and low absorption of µc-Si:H. The light propagation in solar cells with the state-of-the-art random texture, as well as flat solar cells as reference, is studied.

In scattering-type scanning near-field optical microscopy (s-SNOM) the evanescent electric fields at the apex of a sharp illuminated tip can be used to probe the dielectric properties of a sample with a sub-wavelength resolution. The interpretation of the near-field coupling to inhomogeneous samples, however, can be highly involved, e.g. due to multiple reflections. Additionally a distinction between geometric contrasts (i.e., the depth of a buried object) and dielectric contrasts (i.e., the dielectric properties of a buried object) is not directly evident. Experimentally, a full three-dimensional reconstruction of the local dielectric properties has not yet been demonstrated. As a step towards quantitative s-SNOM measurements on vertically structured systems we address layered samples and covered nanoparticles.
For the interpretation of s-SNOM measurements on layered samples we present a quantitatively precise model for the prediction and analysis of optical near-field signals on multilayer systems [1]. The model is based on the fully analytic finite dipole model (FDM) by Cvitkovicacute; et al. [2]. We extended this model to layered samples and demonstrate its validity by a comparison to experimental data. Our extension comprises an effective medium approximation and is not restricted in the number of layers or the involved dielectric functions. Therefore our multilayer FDM is a computationally efficient means to interpret s-SNOM signals from layered systems in order to extract a local layer thickness and variations in the dielectric functions of thin films or the underlying material.
We demonstrate that the electrostatic approximation on which the FDM is based is valid in the mid-infrared spectral range. Additionally, we investigated the influence of the tip vibration amplitude on the contrast and the signal strength from regions below the surface of the sample both theoretically [1] and experimentally [3]. Our findings help optimizing this parameter in order to enhance the performance of s-SNOM for subsurface imaging.
The resolution of buried objects in near-field microscopy is addressed experimentally. We demonstrate that spherical particles with a diameter of only 30 nm can be resolved through dielectric membranes with a thickness of up to 50 nm. Together our investigations on s-SNOM signals from layered samples and on the subsurface resolution pave a way towards a more quantitative understanding of near-field optical measurements on inhomogeneous samples.
[1] B. Hauer, A. P. Engelhardt, and T. Taubner, Optics Express 20, 13173 (2012).
[2] A. Cvitkovicacute;, N. Ocelicacute;, and R. Hillenbrand, Optics Express 16, 8550 (2007).
[3] A. P. Engelhardt, B. Hauer, and T. Taubner, submitted (2012).

Among some of the most exciting discoveries in the past few years in condensed matter research is the theoretical foundations of topological insulating electronic phases based on strong spin-orbit coupling and observations of the these states in model materials. On the surface of topological insulators such as Bi₂Se₃ the electronic spectrum is characterized by a single helical Dirac dispersion such that counter-propagating electrons carry opposite spin allowing propagation of pure spin currents. In addition to their fundamental scientific novelty, these states are predicted to have special properties arising from charge dynamics that could be driven optically, making them potentially useful for applications ranging from opto-spintronic devices, quantum computation, nanoscale electronics and nanophotonics. Here we investigate near-field optical properties of Bi2Se3 and Bi₂Se₃ thin films using spectroscopic scattering type near-field optical microscopy (s-SNOM) using PtIr and Co coated probes.
Bulk Bi₂Se₃ and Bi₂Se₃ samples were prepared by stoichiometric mixing in an Ar-filled glove box at high temperature. Formation of single phases Bi₂Se₃ and Bi₂Se₃ were confirmed by powder X-ray diffraction technique. Thin film Bi₂Se₃/Bi₂Te₃ samples were prepared by mechanical exfoliation on silicon wafers. Extensive AFM scans revealed that exfoliation produces a size distribution including nanofilm structures as thin as ~15 nm.
Diffraction-limited optical images correlated with AFM reveal thickness-dependent color and contrast. By imaging several Bi₂Se₃ thin films with s-SNOM, we discovered size-dependent near-field contrast in both amplitude and phase. S-SNOM can directly visualize the Re(ε) of the sample#700;s dielectric function ε in amplitude, while phase images map Im(ε). Retraction curves taken at several locations confirm that the contrasts observed were from evanescent near-field interactions.

High brilliance infrared (IR) sources using bending magnets [1] have provided synchrotron radiation facilities with analytical tool for spectroscopist in this frequency domain. IR synchrotron sources does not have a higher flux than thermal sources, but the small source size provides an increased brightness, roughly between two to three orders of magnitude. Therefore, they are highly suited for microscopy application. Recently, Fourier transform (FT) spectroscopy has been demonstrated with a lateral resolution of 100 nm by a commercial aperture-less scanning nearfield optical microscope (a-SNOM) [2] using a black body radiator as IR source.
In this contribution, we report on a new IR beamline of the Brazilian Synchrotron Light Laboratory (LNLS), which will couple the IR beam to a-SNOM suited for FT-IR spectroscopy. Hereby, the black body IR source will be replaced by the IR light obtained from a 1.67 T bending magnet of the 1.37 GeV storage ring. The source is characterized and calculations carried out to design an appropriate optical layout consisting of 5 mirrors. After the final mirror, we expect a collimated beam of 76.1×8.1 mm with a divergence of 1.13×0.62 mrad.
The a-SNOM (NeaSpec) is currently commissioned and first images from the instrument as well as its integration into the IR beamline will be discussed. The beamline should be opened for users in Mid 2013.
[1] R. A. Bosch, T. E. May, R. Reininiger, and M. A. Green, Rev. Sci. Instrum. 67, 3346 (1996)
[2] F. Hurth, M. Schnell, J. Wittborn, N. Ocelic, and R. Hillenbrand, Nature Mat. 10, 352 (2011)

Similarly to the SERS effect, Surface Enhanced Infrared Absorption Spectroscopy (SEIRA) exploits the large electromagnetic field enhancements of plasmonic nanostructures for sensitive detection. SEIRA enhancement is proportional to the second power of the incident field and enhancements factors of 3 to 5 orders of magnitude have been theoretically calculated for small hot spots. However, hotspots engineering had to rely only on theoretical modeling because the diffraction of the long IR wavelengths (2-15 µm) has prevented SEIRA experimental investigations at the nanoscale. Gold asymmetric split ring resonators (A-SRRs) are plasmonic nanostructures, composed by two metallic arcs with different length sharing a common center of curvature. By changing the resonators diameter it is possible to tune their plasmonic resonances across the whole IR spectral range and match the vibrational absorption resonances of target analytes.
In this work, SEIRA spectra and images of a polymer film on top of gold A-SSRs are measured with 100 nm lateral resolution using Photothermal Induced Resonance (PTIR) technique. PTIR, is a new technique that combines the chemical sensitivity of IR spectroscopy to the lateral resolution of Atomic Force Microscopy (AFM). PTIR uses a broadly tunable pulsed laser for sample illumination in ATR configuration and an AFM tip in contact mode to measure the sample instantaneous thermal expansion induced by light absorption. PTIR spectra are obtained by plotting the amplitude of the tip deflection (proportional to the absorbed energy) with respect to the laser wavelength. In addition to visualizing SEIRA hot spots as a function of wavelength in the near field, PTIR images allow visualizing bright and dark plasmonic modes with high resolution. Since A-SRRs optical response is polarization dependent we introduced a broadband polarization controller in our set-up which allow for the first time to extract SEIRA enhancement factors at the nanoscale, quantitatively. We named this new technique Surface Enhanced Photothermal Induced Resonance (SE-PTIR).
We believe that our work will foster the optimization and engineering of plasmonic materials towards their technological applications.

Thermal scanning probe lithography (tSPL) has demonstrated its potential of patterning high resolution structures at feature sizes down to 15 nm [1] at high throughput [3]. In addition, the method is capable of creating three dimensional topographic patterns with single nanometer depth resolution limited only by the profile of the tip [2].
The method is based on the local desorption of polymeric resist material. Studying the thermo-mechanical and desorption properties of the resists as a function of applied force and temperature yields insight into important materials properties for high quality nano-patterning. In the thermo-mechanical response we can clearly identify the softening temperature of the resist at the micro-second time scales of the tip-sample interaction. We find that the decomposition is already triggered below - and is fully active at - the softening temperature. Therefore, in spite of the inevitable temperature profile caused by the hot tip, the resist stays in the glassy regime in the non-patterned areas. Heat induced swelling is minimized which enables high quality patterning. In addition, at the softening temperature there is no mechanical resistance of the resist against the penetrating tip. As a result the tip penetrates to a depth defined by the applied force working against the cantilever restoring force and not by the volume of material to be removed. Consequently a programmed three dimensional pattern is reliably reproduced even in rough samples.
Additional progress was made on several technical aspects of the technology: For high resolution patterning the resulting depth profiles in the resist layer are only 6-8 nm deep. We optimized a three layer transfer process to amplify the patterns into an underlying silicon substrate. We demonstrate 15 nm wide L-lines at a period of 57 nm transferred 60 nm deep into silicon. Line edge roughness is below 1 nm (1 sigma).
The maximum pattern size of our instruments is determined by the maximum scan area of the piezo translation stages, on the order of 50 x 50 µm. Stitching of smaller fields is required to achieve large area patterning. We demonstrate that we can use the uniqueness of the polymer roughness to obtain a stitching precision of about 10 nm [4]. A careful analysis shows that theoretically sub nanometer precision can be achieved and the experimental result is limited by inaccuracies in the scanning system.

[1] D. Pires, J. L. Hedrick, A. De Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U. Duerig, and A. W. Knoll, Science328, 732 (2010).
[2] A. W. Knoll, D. Pires, O. Coulembier, P. Dubois, J. L. Hedrick, J. Frommer, U. Duerig, Adv. Mat.22, 3361 (2010).
[3] P. C. Paul, A.W. Knoll, F. Holzner, M. Despont and U. Duerig, Nanotechnology22, 275306 (2011).
[4] P. Paul, A. W. Knoll, F. Holzner, U. Duerig, Nanotechnology23, 385307 (2012).

Atomic Force Microscopy is a widely utilized tool for imaging the surface morphology of a wide variety of samples, including thin films, polymer blends and biological materials. However conventional AFM offers little chemical or mechanical information about the material under the AFM tip. Lorentz Contact Resonance (LCR) allows for the clean excitation of the resonance modes of a ThermaLevertrade; AFM cantilever. The resonant frequency and amplitude of these resonances are dependent on the stiffness of the material in contact with the probe. When tuned to a particular resonant frequency and scanned across a sample, the probe can obtain a qualitative map of the varying stiffness of each component on the surface of a sample. Each individual component can then be highlighted by tuning to its resonant frequency and scanning the surface. Another advantage of using a Thermalevertrade; probe is that the temperature of the probe can be ramped and corresponding changes in stiffness can be seen. This allows for the measuring of thermal transitions on materials that have traditionally been difficult to measure, such as, thin films and highly filled epoxys. The contact frequency between tip and sample will change as a sample undergoes a thermal transition, with the resonance shifting to a lower frequency as softening occurs. When LCR is combined with AFM-IR and nanoTA, samples can be characterized chemically, mechanically and thermally with nanoscale resolution.

We investigate the measurement of temperature-dependent viscoelastic properties by contact and intermittent contact dynamic atomic force microscopy (AFM) methods. As polymers are heated through their glass transition, modulus can drop many orders of magnitude; in contrast, loss tangent exhibits a peak near the glass transition and generally increases with temperature outside the peak. Such characteristic response can serve as a “litmus test” for an AFM method&’s ability to capture the thermomechanical information obtained in standard bulk experiments. Recent developments in analysis techniques for both intermittent contact AFM (e.g., tapping mode) and dynamic contact AFM (e.g., contact resonance force microscopy) data enable more accurate determination of viscoelastic properties that depend strongly on temperature. These refined analysis techniques introduce a quantitative scale to the mechanical property response, enabling transitions to be identified by the same criteria as in bulk thermomechanical methods, rather than by mere phenomenological indications. Here, the temperature of the tip-sample contact was controlled globally using a heater stage and locally using a microfabricated heated-tip AFM probe. We discuss how temperature-dependent variations in the frequency and quality factor of the cantilever&’s free-space resonance influence the accuracy of our nanomechanical analysis and how they can be accounted for. Then, using combinations of local/global heating and contact/intermittent contact techniques, we demonstrate how the cantilever&’s dynamic response can be used to determine the glass transition temperature of a polystyrene sample. The AFM techniques enable direct mechanical probing of viscoelastic properties at much higher frequencies than typically possible with bulk methods (kHz to MHz versus 100s of Hz), lending new insight into the high-frequency response of polymers. Temperature-dependent measurements not only aid in the fundamental understanding of polymer properties but also challenge the accuracy and applicability of dynamic AFM methods by increasing the range of property values accessed in a single sample. In this way, experimental and analysis techniques can be improved even further for enhanced measurement capabilities.

Bimodal atomic force microscopy is a multifrequency dynamic force method based on the simultaneous excitation of two eigenmodes of the cantilever [1,2]. It enables to increase the number of channels that information can be extracted from. Over the last years, it has proofed to render compositional contrast on heterogeneous samples both in air and liquid environments.
Recently, simultaneous acquisition of quantitative information such as the effective Young modulus and topographic information has been pursued by several authors. By using bimodal FM-AFM with self-excitation of the first mode and the second mode excited at a fixed amplitude and frequency, we have measured variations of the elastic modulus on a single pentameric IgM antibody when the probe is moved from the end of the protein arm to the central protrusion [3]. However, this included a hypothesis on the indentation.
Recently, by using bimodal FM-AFM with self-excitation of both modes and a model based on fractional calculus, quantitative mapping of the effective Young modulus and the indentation simultaneous with a topographic image can be achieved [4,5]. The two equations which appear in the model allow using the monitored Δf₁ and Δf₂ images to find the effective Young modulus and indentation. Moreover, adding a third equation based on the ΔA_1,drive image allows to extract the viscosity of the sample.
We have checked our method with samples such as Polydimethylsiloxane (PDMS), poly(lactic-co-glycolic acid) (PLGA), and proteins. The results for the effective Young modulus and viscosity do not show any dependence on the indentation and coincide with the results obtained by other well-established methods (static AFM, force inversion methods).
References
[1] R. Garcia, E. T. Herruzo. The emergence of multifrequency force microscopy
Nat. Nanotechnol., 7, 217-226 (2012).
[2] N F Martinez, J R Lozano, E T Herruzo, F Garcia, C Richter, T Sulzbach and R Garcia, Nanotechnology 19 384011 (2008)
[3] D. Martinez-Martin, E. T. Herruzo, C. Dietz, J. Gomez-Herrero and R. Garcia, R. Phys. Rev. Lett.106, 198101 (2011)
[4] E. T. Herruzo, A.P.Perrino, R.Garcia, submitted.
[5] E. T. Herruzo and R. Garcia, Beilstein J. Nanotechnol. 3, 198-206 (2012)

Intermittent contact or tapping mode AFM is a proven technique for nondestructive characterization of materials. A recently developed extension of this mode called Bimodal AFM has become a valuable additional tool and reveals new contrast and information on a wide variety of materials, especially polymer blends. Bimodal imaging employs excitation of a higher eigenmode with no feedback along with the conventional fundamental vibration mode of the AFM micro-cantilever that is used for feedback. However, the nature of the contrast obtained in the higher order mode phase and amplitude are still poorly understood and are complicated by a challenging tip-sample interaction present in bimodal imaging methods.
Blends (e.g., combinations of different rubbers and thermoplastics) are studied and reveal significant differences as a function of the various operating conditions. The contrasts observed in the response amplitude and phase provides important information regarding the material properties of the different components in the blend. The phase and amplitude contrasts depend on different operating conditions of the AFM, such as the combinations of the eigenmodes, amplitudes of the first and the higher eigenmode, set point amplitude, and the properties of different constitutive materials in a multi-component sample. We present both experimental data accompanied by simulations on the effects of these operating conditions to better understand the various operating regimes present in bimodal, with the ultimate goal of improved understanding of the ongoing cantilever dynamics and a more rational approach to bimodal AFM operation.

Quantitative characterization of the material at nanometer-scale is essential for substantially furthering design of material with hitherto unparalleled atomic scale specificity. We present a method for determining stiffness and dissipation properties of material that is particularly well suited for soft-matter such as polymers and bio-matter. The method simultaneously provides estimates of topography, dissipation and stiffness of material in a dynamic (AC) mode of Atomic Force Microscope (AFM) operation. Here, the dissipative and stiffness properties of the sample are mapped to parameters of an equivalent linear time invariant system. Averaging and perturbation theory form an important basis and provide the theoretical foundations for the method. The method significantly expand the previous work by the authors [1] where an adaptive recursive least square based estimation scheme was developed to estimate the equivalent parameters. Theoretical analysis and experimental studies indicate the need for multi-tone excitation of the cantilever for robustly estimating the parameters of the equivalent system. Perturbation analysis based tools are suitably modified to account for the multi-tone excitation. Furthermore, we also derive the equivalent linear time invariant model when multiple modes of the cantilever participate in the nonlinear interaction with the sample forces.
A hardware module using FPGA based architecture is also developed, which implements the method in a commercially available AFM. The hardware module is used to study the properties of different polymer systems in real-time and results presented demonstrate the efficacy of the method. Here real-time images of dissipation and stiffness properties of the material accompany the traditional AC mode images of amplitude, phase and height of the material being imaged. These results also demonstrate the temporal and spatial resolution that our method enables for material discovery and design. Furthermore, the use of the module on a polymer blend indicates the new diagnostics that are made possible by the method.
[1] P. Agarwal and M. Salapaka, "Real time estimation of equivalent cantilever parameters in tapping mode atomic force microscopy," Applied Physics Letters, vol. 95, no. 8, p. 083113, 2009.

Plants and many other biological organisms have developed structures to harness spatial and temporal changes in relative humidity for mechanical actuation. The ability of water confined to nanoscale pores of biomaterials to withstand large negative pressures without cavitation leads to large actuation pressures that, if harnesses in engineering systems, may rival state of the art technologies. We have used atomic force microscopy based experiments to probe energy conversion processes in Bacillus spores. These dormant organisms are dynamic structures. They respond to changes in the relative humidity of their environment by swelling and shrinking dynamically, which provides a model system to test the behavior of water under biological confinement. Atomic force microscopy experiments allowed us to characterize the effective actuation pressure, the amount of water absorbed and released in the process, and the speed of actuation. We will present our findings and discuss possible applications of this phenomenon in energy conversion and storage.

Ferroelectric polarization switching underpins basic operational mechanisms and functionalities of multiple devices and applications. As decreasing device size for the high density memory and miniaturization, knowledge of nanoscale polarization dynamics as controlled by fundamental material properties and defects becomes key step towards knowledge-driven materials and device development. For these fundamental studies and applications, piezoresponse force microscopy (PFM) was suggested to explore complex polarization switching at the nanoscale. More recently, we suggested 5D PFM which allows exploring switching properties on the space, time, voltage, and frequency domains. Since dimensionality of the obtained data is significantly increased from 2D data set of the classical PFM, it necessitates applying proper methods to explore ferroelectric polarization switching. In this presentation, we illustrate that the nucleation kinetics and orientation dependence of domain wall velocities can be explored by application of level set method (LSM) on the 5D-PFM data set, providing a comprehensive picture of domain dynamics as a function of pulse parameters. The results show how polarization switching happens spatially in a capacitor structure at the nanoscale and will open new perspectives for studies on the multidimensional data processing.
Research was supported (S.V.K., Y.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences (S.V.K., S.J.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

The response to a local, tip-induced electric field of Ferroelastic domains in thin
polycrystalline Lead Zirconate Titanate (PZT) films with predominantly (110) orientation has been studied using Enhanced Piezoresponse Force Microscopy (E-PFM). Two types of reversible polytwin switching between well-defined orientations have been observed. When the polarization vector switches by 180° (ferroelectric reversal), the domain walls rotate in-plane by 109.5° whereas when the polarization vector switches by around 90° (ferroelastic reversal), the domain walls rotate by half this value, or 54.75°. We show how the local geometry influences the switching pathways, and how after multiple switching cycles, grains release stress by buckling. We also show how the rotation of polytwins can be used to determine the crystalline orientation of individual grains.

Electrical atomic force microscopy (AFM)-based techniques such as conductive-AFM and scanning spreading resistance microscopy (SSRM) have evolved as mature techniques for the two-dimensional (2D) electrical characterization of nanoscale devices during the last decade. SSRM is considered as the method of choice for 2D carrier mapping of future semiconductor devices by the international technology roadmap for semiconductors due to its sub-nm spatial resolution, 2..3 nm/decade dopant gradient resolution and high sensitivity. However, the introduction of three-dimensional (3D) device architectures such as TriGate- and nanowire-based transistors inevitably entails the need for metrology techniques capable of measuring the distribution of carriers in confined volumes in 3D. Therefore, our work extends the SSRM technique towards 3D for the application as electrical tomography tool. We demonstrate two different approaches: 1) Generating a set of cross-sections with incremental off-set in the third dimension through a confined volume. 2D carrier profiles obtained on the individual device cross-sections are then reconstructed towards a 3D carrier distribution map. 2) A slice-and-view method whereby material removal by successive tip scanning is applied to 3D electrical profiling of carbon nanotube (CNT)-based interconnects. We compare both approaches and highlight their potential future applications.
The first approach is demonstrated on the example of a nanowire-based transistor and a FinFET device, and is facilitated by arranging several devices in a staggered array, allowing to produce a series of cross-sections with incremental setup by a single cleave. A 3D carrier distribution map is then obtained by combining the individual 2D maps. Based on this concept, we obtained 3D-carrier profiles in FinFETs and nanowire-based tunnel-FETs.

The second, slice-and-view approach has been applied to electrically characterize CNT-based interconnects for future integrated circuits (IC). Here, important characteristics such as CNT resistivity and contact resistance between CNT and bottom electrode cannot be derived from 2D resistance measurements across the contact hole. For this reason, we developed a procedure whereby dedicated in-house made full-diamond (boron doped) probes are used for the electrical measurements as well as continuous material removal by successive scanning. This way we obtain 2D resistance maps at different depth respectively CNT length values. By stacking and interpolating between the individual 2D resistance maps we can then construct an electrical tomogram, which enables us to derive the evolution of the CNT resistance along the CNT length for individual CNTs in a CNT-based contact hole. From such a resistance profile, quantitative numbers for the resistivity of the CNT and the contact resistance between CNT and bottom electrode are extracted.

Scanning spreading resistance microscopy (SSRM) has evolved as the method of choice for quantitative two- and three-dimensional carrier mapping of semiconductor device structures [1] due to its excellent characteristics such as sub-nm spatial resolution, (2-3) nm/decade dopant gradient resolution and high sensitivity. Therefore a conductive probe (typically boron doped diamond) is scanned in contact mode over the cross-section of the device under test. At the same time, a DC bias is applied between scanning probe and sample back-contact. Thereby the current spreads out when flowing from the constriction given by the small size of the electrical contact into the sample [2]. The associated spreading resistance is directly linked to the local resistivity and can be converted into carrier concentration values by means of dedicated calibration procedures.
However, the actual spreading resistance can be screened under certain circumstances by series resistances such as the intrinsic resistance of the probe, the resistance linked to the sample back-contact as well as the bulk resistance the current experiences on its trajectory towards the back-contact. The latter hampers in particular in case of confined volumes such as nanowires an accurate quantification.
To overcome this limitation, we developed and implemented a variant of SSRM, so called
Fast Fourier Transform-SSRM (FFT-SSRM). Thereby the spreading resistance is decoupled from parasitic series resistances by a sinusoidal modulation of the force applied to the cantilever. This causes a modulation of the contact size and in turn a modulation of the spreading resistance, whereas the series resistance components remain unaffected. Using FFT, we then extract the resistance amplitude at the applied modulation frequency.

We will present our experimental setup and demonstrate by measuring across dopant staircase samples that the resistance amplitude extracted by FFT-SSRM constitutes a measure of the local carrier concentration underneath the tip, even when an additional series resistance, exceeding the actual spreading resistance, is added into the circuit. We will furthermore discuss the possibility of decoupling the tip resistance caused by the limited conductivity of the boron doped diamond probes typically used for SSRM and compare the sensitivity of FFT-SSRM with the one of conventional SSRM.
References
[1] International technology roadmap for semiconductors (ITRS), Metrology, 2011.
[2] P. De Wolf et al. Applied Physics Letters 66 (12), 1995.

In the growing field of molecular electronics, controlling and accurately measuring the electronic transport properties through molecular junctions, constitutes a crucial issue for the future development of devices. This is also a longstanding and tricky problem because of the complexity and interplay of several mechanisms such as atomic contact geometry, molecular conformation, and molecule/molecule interactions occurring at the nanoscale. To tackle this important problem, statistical methods, using the repetition of hundreds or thousands electrical measurements, are definitely required.
Here we report on a new approach that allows us to measure the conductance of up to a million of junctions in a single Conducting-Atomic Force Microscopy (C-AFM) image [1]. By using e-beam lithography, we fabricated a large, perfectly aligned array of sub-10 nm single crystal gold nanodot electrodes, each molecular junction being made of less than one hundred molecules. We focus on alkylthiol junctions (with increasing carbon chain lengths) as model molecules. We show that the number of the conductance peaks may vary, depending on the atomic structure of the electrodes (i.e., single crystal, polycrystal, amorphous). Such precise formation of small dots enables us to identify the critical size that determines whether a dot is composed of single or multiple crystal domains. Moreover, we show that a post-annealing process at moderate temperature can convert Au dots from amorphous to single-crystalline, and then they are covered with a thin silicon oxide layer. After easy removal of the silicon oxide by HF etching, these nanodots can be used as electrodes for the electrical characterization of multi-functional organic self-assembled monolayers.
In this work, we show that electronic properties of such alkyl-thiolated nanocrystals strongly depend on the strain applied on molecules. Local mechanical properties of the nanocrystals and the molecules were statistically evaluated by using Peak-Force Tapping AFM (PFT) images. Statistics on electrical properties of such molecular junctions were obtained with controlled strain down to 3 nN (by C-AFM [1]) and down to 100 pN (by PFT coupled to C-AFM, i.e. Peak Force Tunneling Atomic Force Microscopy - PF-TUNA). Below strain of 1 nN, electronic current is only observed on side (111) facets. Above 1 nN, current arises mainly at top (100) facets with statistically two peaks of conductance that we can attribute to two phases of molecular organization. Considering the molecular junction as a tunnel barrier for electrons, both the decay tunneling constant and the barrier height of small molecules strongly depend on the load that can be easily controlled by PFT technique.
[1] N. Clément, G. Patriarche, K. Smaali, F. Vaurette, K. Nishiguchi, D. Troadec, A. Fujiwara and D. Vuillaume, Small 7, 2607 (2011).
[2] K. Smaali, N. Clément, G. Patriarche and D. Vuillaume, ACS Nano 6, 4639 (2012).

The performance of organic field-effect transistors (OFETs) is often limited by the electrical properties of the interface between metal electrodes and organic film, and the short-channel effect when the OFETs are miniaturized. For improvement of the performance of the miniaturized OFETs, elucidation of the electrical properties of individual organic semiconductor grains, grain boundaries between them, and the interface with the metal electrodes is indispensable. Point-contact current imaging atomic force microscopy (PCI-AFM) provides simultaneous mapping of topographic information and current-to-voltage (I-V) characteristics by a combination of dynamic-mode AFM and current measurement with the tip in contact [1]. Since the tip does not drag the sample surface during scanning, it is a powerful technique for electrical property measurements of soft materials such as organic semiconducting films with a spatial resolution on the order of nanometers [2].
We utilized PCI-AFM for OFET measurements of single-grain pentacene OFETs. We fabricated a 6-nm-thick Pt electrode on a heavily doped n-type Si substrate with a 100-nm-thick thermally grown SiO₂ layer. Pentacene molecules were deposited on it to obtain pentacene thin film grains connected to the Pt electrode. A Pt-coated cantilever (spring constant: 2 N/m) was oscillated at a frequency closet to its resonance frequency (~70 kHz) during the first period of time for tip height control. In the second period of time, the oscillation was stopped and the tip was brought in contact to measure OFET characteristics of the grain. The sequence was repeated while the tip was scanned to simultaneously obtain topographic and current images. The measurements were performed in a vacuum environment.
We obtained a current profile as a function of distance from the electrode, which showed abrupt reduction of the current with increasing distance. The profile was converted to the resistance profile, which was consistent with the finite element simulation of the resistance profile. We applied the transfer line method to extract the field-effect mobility of the single grain, which found a power-law dependence on the applied gate bias voltage.
[1] Y. Otsuka, et al., Jpn. J. Appl. Phys. 41 (2002) L742.
[2] T. Kimura, et al., Jpn. J. Appl. Phys. 51 (2012) 08KB05.

Single molecule junctions represent an attractive platform to understand and control functionality of materials and devices at the nanoscale. While their electronic transport properties have received tremendous attention thus far, a more complete understanding of the structure-function relationship of these atomic scale devices is obtained through their mechanics [1]. Specifically, quantitative measurement of Van der Waals (vdW) interactions at the single molecule level remains challenging in experiments and its accurate inclusion in first principles theory is also complex. Here we report simultaneous measurement of force and electrical conductance across Au-molecule-Au junctions using a conducting atomic force microscope (AFM) for 4,4&’-bipyridine (BP) and 1,2-bis(4-pyridyl)ethylene (BPE) molecules. For each of these molecules two distinct molecular junction structures are observed with characteristic conductances, consistent with previous studies utilizing scanning tunneling microscopy (STM). Using statistically relevant analysis, these two structures are found to have very different mechanical properties. Specifically, we find that the higher conductance junctions have a significantly larger rupture force and stiffness than those that show the lower conductance. They also have a larger rupture force than Au point contacts, suggesting multiple points of contact. Using density functional theory simulations we show that vdW interactions between the pyridine ring and Au electrodes can play a key role in the junction mechanics. These measurements thus provide a quantitative characterization of vdW interactions at metal-organic interfaces at the single-molecule level [2].
1. Frei, M., Aradhya, S. V., Koentopp, M., Hybertsen, M. S. & Venkataraman, L., Mechanics and chemistry: single molecule bond rupture forces correlate with molecular backbone structure. Nano Letters, 11, 1518-1523 (2011).
2. Aradhya, S. V., Frei, M., Hybertsen, M. S. & Venkataraman, L., Van der Waals interactions at metal/organic interfaces at the single-molecule level. Nature Materials, 11, 872-876 (2012).

In the last years nanoparticles have been harnessed in nanoscience, to improve the performance of electronic devices. For this reason studying the electrical behavior of nanoparticles has become highly desirable. In this work Peak Force Atomic Force Microscopy (AFM) is used to study the morphology and the conductivity of nanoparticles.
Peak Force AFM is a new technique to characterize soft materials such as nanoparticles with high accuracy with only one measurement. Peak Force AFM does not rely on tapping and contact AFM but operates at a frequency below the resonant frequency of the cantilever. This allows for a direct control of the forces and to avoid lateral forces that may damage the sample. Furthermore, the performance characteristics of Peak Force AFM are suitable to work also in Tunneling AFM (TUNA) mode, allowing the study of the electrical properties of materials.
In this work SnS nanoparticles capped with tri-n-octylphosphine oxide (TOPO) have been characterized. By means of Peak Force AFM it has been possible to measure simultaneously topography and current maps of nanoparticles, yielding information about the shape, size and the conductivity of even a single nanoparticle. The topography map clearly showed single nanoparticles with a size less than 5 nm and spherical shape. In the conductivity map it is possible to discern the same nanoparticles, the correlation with the topography map is evident. This confirms the conduction (though not calibrated) of SnS nanoparticles. This type of measurements has been repeated many times in order to check the reproducibility of this technique. Moreover, the same nanoparticles have been measured also by Torsional Resonant TUNA AFM in order to compare it with Peak Force AFM. By means of TR-TUNA it was possible to measure the topography of SnS nanoparticles capped with TOPO but not the current. Besides, the resolution of the topography map acquired by TR-TUNA AFM is much less than by Peak Force AFM. From this comparison it has been found that the conductivity of nanoparticles, even if they are capped with TOPO, can be measured by Peak Force AFM, a result that has thus far been difficult to achieve by other types of AFM.

Magnetic force microscopy (MFM) is the state-of-the-art tool used to study submicron magnetic structures. It is a popular technique due to its high spatial resolution (~30 nm), high sensitivity in large variety of environmental conditions, and simple sample surface preparation. It is based on interaction of a sharp magnetic tip with the sample surface containing magnetic domains. Tip-sample interaction involves both Van der Waals and magnetic forces, separated by means of two-pass scanning method [1]. In the first line scan the topography is determined in contact or tapping mode. During the second pass the tip is scanned at a constant height above the surface (~ 30-50 nm), thus eliminating short range Van der Waals&’ forces. The magnetic interaction depends only on the magnetization of the tip and the spatial change of magnetization on the sample&’s surface.
This approach is no more applicable on novel complex magnetic materials for the future bit-pattern media [2], in which the spatial resolution of the MFM needs to be extended down to 10 nm and beyond. The improved resolution cannot be achieved simply by reducing the lift distance - at that point MFM mixes magnetic, Van der Waals, and electrostatic forces [3-6].
In this work we realize a new scanning technique based on low momentum probes. The switching magnetization MFM allows separation of magnetic forces from Van der Waals and electrostatic forces [7] when these forces are of similar strength. The switching magnetization MFM method is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local Van der Waals forces, while their difference maps the local magnetic forces. The idea is implemented by fabricating low-momentum magnetic probes that exhibit magnetic single domain state, which can be easily reversed in low external field during the scanning. We have applied the technique on high-density magnetic recording media at a lift distance of 5 nm, and have demonstrated clear separation of magnetic and Van der Waals forces. Resulting spatial resolution of the magnetization distribution is on the level of 5 nm at ambient conditions. The method would be very effective in the case of non-contact MFM in ultrahigh vacuum environment, where separation of forces and high spatial resolution are difficult to achieve at the same time with conventional MFM lift mode technique.
[1] http://www.ntmdt.com/spm-principles/view/ac-mfm.
[2] O. Mosendz et al, J. Appl. Phys. 111, 07B729 (2012).
[3] R. Wiesendanger et al, Phys. Rev. Lett. 65 (1990) 247.
[4] S.N. Piramanayagam et al, Phys. Status Solidi RRL 6, 141 (2012).
[5] H. Li, D. Wei, and S.N. Piramanayagam, J. Appl. Phys. 111, 07E309 (2012).
[6] M. Ohtake, K. Soneta, and M. Futamoto, J. Appl. Phys. 111, 07E339 (2012).
[7] V. Cambel et al, Appl. Phys. Lett., submitted.

Local ionic, electronic, and electrochemical phenomena at the solid-liquid interface are ubiquitous in physical chemistry and biological processes. 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 surface potentials under vacuum and ambient conditions. Conventional KPFM is a technique whereby the electrostatic force between an atomic force microscope probe and a sample is minimized by the application of a DC bias using closed-loop feedback control. Applying the same principles of operation in liquid is complicated by the presence of multiple relaxation times and the bias and frequency dependent onset of irreversible electrochemical reactions. Novel, open loop, multidimensional KPFM methods for the characterization of electrostatic and voltage-controlled electrochemical dynamics in liquid have recently been developed. The key to such measurements is the ability to address the local voltage and time dependent responses of the bias induced forces experienced between tip and sample. First-order reversal curve based KPFM spectroscopy measurements combining low bias sweeps with multi-harmonic detection, allows electrochemical dynamics to be explored and the hysteretic response of the electrochemical potentials to be investigated.

Recording the variations of local electric properties in Atomic Force Microscopy enriches the characterization capabilities of this technique. Tip-sample electrostatic forces are the basis of electric force microscopy (EFM), Kelvin force microscopy (KFM) and the detection of capacitance gradients (dC/dZ, dC/dV). Recent progress in applications of these methods is related to the implementation of multi-frequency techniques and, particularly, to the practical realization of the single-pass measurements under ambient conditions. Single-pass KFM and dC/dZ studies of various materials, especially with electrostatic force gradient detection, proved high sensitivity and nanometer-scale resolution in mapping surface potential of organic compounds, polymers, semiconductors and metals [1-2]. Despite the advances, the extraction of quantitative electric properties of the sample from EFM, dC/dZ [3] and dC/dV measurements is still an opened question. A rational analysis of tip-sample forces in these modes can be performed by the asymptotic Krylov-Bogoliubov-Mitropolsky method applied to the Euler-Bernoulli equations that describe the oscillation of a probe interacting with a sample. The electric interaction of a conducting probe with a sample is added as an external force to the Euler-Bernoulli equation. The derived equations relate the changes of probe oscillatory parameters to the tip-sample electrostatic force, and this helps extracting the electrostatic force from the AFM data. Furthermore, the material properties can be obtained once the relationship between the electrostatic force and a particular electric property is established. Finding the force-property relationship, however, requires the development of models of the probe-sample interactions with the help of finite element analysis. We have applied this approach to dielectric layers on a conducting substrate, organic layers with molecular dipoles, metal alloys, semiconductors with various doped regions, etc. A comparison of the experimental data and theoretical calculations of surface potential and dielectric permittivity will be discussed.
[1] S. Magonov, J. Alexander, and S. Wu “Advancing Characterization of Materials with Atomic Force Microscopy - Based Electric Techniques”, pp. 233-300, in “Scanning Probe Microscopy of Functional Materials: Nanoscale Imaging and Spectroscopy" Eds. S. Kalinin and A. Gruverman, Springer, New York, 2011.
[2] S. Magonov, and J. Alexander “Single-Pass Kelvin Force Microscopy and dC/dZ Measurements in the Intermittent Contact: Applications to Polymer Materials” Beilstein Journal of Nanotechnology, 2011, 2, 15-27.
[3] S. Belikov, J. Alexander, S. Magonov, and I. Yermolenko “Towards quantitative local dielectric analysis of polymers” Amer. Control Conference 2012, 3228-3233.

The use of scanning probe microscopy (SPM) to characterize and manipulate surfaces at the nanoscale usually faces the problem of dealing with systems where several parameters are not known. Artificial neural networks (ANNs) have demonstrated to be a very useful tool to tackle this type of problems. Here, we show that the use of ANNs allows us to quantitatively estimate magnitudes such as the dielectric constant of thin films. To improve thin film dielectric constant estimations in EFM, we first increase the accuracy of numerical simulations by replacing the standard minimization technique by a method based on ANN learning algorithms. Second, we use the improved numerical results to build a complete training set for a new ANN. The results obtained by the ANN suggest that accurate values for the thin film dielectric constant can only be estimated if the thin film thickness and sample dielectric constant are known.
To get a deeper knowledge of the physical processes involved in the imaging of thin films by Scanning Probe Microscopy we use artificial neural networks to replace the full structure of a thin film over a dielectric substrate by an equivalent semiinfinite sample described only by an effective dielectric constant. For thin film thicknesses around 1 nm, we demonstrate that thin film dielectric constants between 1000 and 10 000 give very different electric responses. This effect is of great interest in the study of thin materials with a high polarizability such as graphene layers, where we find that for electrostatic purposes, a graphene layer is equivalent to an extremely thin dielectric layer with an effective permittivity that depends on the conductivity of the layer and spans from 5 for the insulating layers, to 2000 for the more conductive ones.

Surface potentials play a key role for both structure formation and functionality in a wide range of molecular systems, e.g., in biological [1] or organic and molecular electronic systems [2]. The process of molecular self-assembly, for example, is often determined by the electrical interaction of different moieties and side groups. For Kelvin probe force microscopy (KPFM) the mapping of surface potentials has been demonstrated at nanometer resolution in air and even at atomic resolution in ultra-high vacuum (UHV) [3]. The operation of conventional KPFM requires the application of a DC voltage between tip and sample. In voltage sensitive materials such as certain biomolecules or ferroelectrics, KPFM can cause reversible and irreversible changes in conformation or the electronic state (e.g. by band bending) of the sample. In aqueous environments, the DC voltage can lead to spurious forces and electrochemical reactions that hamper the operation of KPFM.
Thus, a surface potential measurement technique that can be operated in any environment from UHV to ambient to liquid environments would be highly desirable. Recently, a multi-frequency approach for quantitative surface potential mapping without the necessity to apply a DC voltage was reported [4]. The technique operates by analyzing the response of the cantilever to an AC voltage of frequency f. By recording the response at both the fundamental frequency f and the second harmonic frequency of 2f, quantitative surface potential values can be obtained. We will present a detailed comparison of conventional KPFM techniques with this so-called dual harmonic KPFM (DH-KPFM) in both ambient and liquid environment. We will demonstrate that this multi-frequency approach overcomes many problems of conventional KPFM. In particular, artifacts originating from the feedback mechanism and the distance dependence of the measured surface potential can be significantly reduced.
References
[1] C. Leung et al., Appl. Phys. Lett. 97 (20), 203703 (2010).
[2] S.A.L. Weber et al., Electrical Characterization of Solar Cell Materials Using Scanning Probe Microscopy, in Scanning Probe Microscopy in Nanoscience and Nanotechnology III, edited by Bharat Bhushan (Springer, Berlin, 2013).
[3] S. Kitamura et al., Appl. Phys. Lett. 72 (24), 3154 (1998).
[4] O. Takeuchi et al., Jpn. J. Appl. Phys. 46 (8B), 5626 (2007); N. Kobayashi et al., Rev. Sci. Instrum. 81 (12), 123705 (2010).

Measurement of electrical transport properties such as charge carrier type, mobility and concentration of nanomaterials has been an outstanding challenge. Typically, electrical properties of nanomaterials are investigated via fabricating field effect transistors (FETs) and deriving materials properties from device transport measurements. While this approach has been successful in fundamental research, it requires nanofabrication facility, and is also limited by the low throughput. Furthermore, transport characteristics of many devices are dominated by metal contacts and may also be heavily influenced by testing environments, making it difficult to unveil the intrinsic properties of nanomaterials.
Towards addressing this challenge, we propose new characterization method for electronic properties of nanomaterials based on the probing of low-frequency dielectric responses using scanning probe microscopy. Dielectric response of semiconducting and metallic materials are dominated by their charge carrier drift under external fields, thus dielectric response of materials reflects their electrical transport properties. Scanning dielectric force microscopy (DFM) technique, which we recently developed, is capable of imaging dielectric response, thus characterizing the electronic properties of nanomaterials without the need of metal contacts.
First, we show quantitative measurement of the transverse dielectric constant of individual SWNTs, then we show an assay of SWNT metallicity based on qualitative DFM measurement of longitudinal dielectric properties. Via incorporation of a local gate bias voltage on DFM probe, charge carrier type can be identified in nanomaterials such as ZnO nanowires and SWNTs. This technique avoids the need of electrical contacts and inherits the spatial mapping capability of scanning probe microscopy, as manifested in the imaging of intra-tube metallic/semiconducting junctions in SWNTs. We expect the DFM technique to find broad applications in the characterization of various nanoelectonic materials and nanodevices.
Reference:
1. Wei Lu, Jie Zhang, Yize Stephanie Li, Qi Chen, Xiaoping Wang, Abdou Hassanien and Liwei Chen “Contactless Characterization of Electronic Properties of Nanomaterials Using Dielectric Force Microscopy” J. Phys. Chem. C, 116, 7158-7163 (2012)
2. Wei Lu, Yao Xiong and Liwei Chen “Length-Dependent Dielectric Polarization in Metallic Single-Walled Carbon Nanotubes” J. Phys. Chem. C.113, 10337-10340 (2009)
3. Wei Lu, Yao Xiong, Abdou Hassanien, Wei Zhao, Ming Zheng, and Liwei Chen “A Scanning Probe Microscopy Based Assay for Single-Walled Carbon Nanotube Metallicity” Nano Lett. 9, 1668-1672 (2009)
4. Wei Lu, Dan Wang and Liwei Chen “Near-Static Dielectric Polarization of Individual Carbon Nanotubes” Nano Letters, 7, 2729-2733 (2007)

Switching of ferroelectric polarization by application of mechanical force has recently attracted interest for possible applications in memory devices. The flexoelectric contributions generated by strain gradients under an applied bias and the facile motion of internal ferroeleastic interfaces are two of the possible mechanisms resulting in mechanically-induced polarization switching of ferroelectrics. In this work, we demonstrate a technique to spatially map the hysteretic behavior during mechanical switching of ferroelectric domains in substrate-thinned PZT films. The mechanically-unclamped PZT films exhibit giant electromechanical coupling and local domain mapping suggests the creation of stress/strain gradients at hundreds of nanometer length scales that can contribute to the flexoelectric response. The results are compared to clamped films and other ferroelectrics. The study allows us to gain insight into the possible dominating mechanism of writing in these structures as well as demarcate the switching behavior across individual domains. The platform developed here can also be used to study the physics and potentials for device applications in materials that undergo metal insulator transitions coupled to ionic dynamics which in turn controls their mechanical stimuli.

Defining new standards concerning superior physical properties, smart materials have attracted increased interest during the last two decades and constitute one important focus of modern solid state physics. We report about nano mechanical surface characterization of Ni-Mn-Ga, a member of ferromagnetic shape memory (FSM) alloys. Well known for its high magneto-elastic coupling - this Heusler alloy yields a reversible strain up to 10% at moderate external magnetic fields below 1 T [1].
Since first studies in 1989 [2] the investigative scope shifted from bulk material to thin films due to promising prospects regarding miniaturized sensor- and actuator applications in industry and medicine.
We investigated local indentation module M of epitaxial grown Ni-Mn-Ga thin films by means of the innovative CR-AFM technique [3,4] and compared results to theoretical data based on density functional theory (DFT) [5]. Nano-scale resolved mechanical surface imaging underlines the significance of twin boundaries concerning the FSM effect.
This project is funded by the German BMBF, PTJ-BIO, Grant Number: 0313909
[1] Sozinov A., Likhachev A.A., Lanska N., and Ullakko K., Appl. Phys. Lett. Vol. 80, no. 10 (2002)
[2] Kokorin V.V., Chernenko V.A., Fiz. Met. Metalloved. Vol. 68, no. 6, 1157-1161 (1989)
[3] Rabe U., Arnold W., Appl. Phys. Lett. Vol.64, P1493-1495 (1994)
[4] Yamanaka K., Ogiso H., Kolosov O., Appl. Phys. Lett., Vol.64, P178-180 (1994)
[5] A.M. Jakob, M. Müller, B. Rauschenbach and S.G. Mayr, New J. Phys. 14 (2012)

We studied the surface properties of elastomeric polypropylene, a semicrystalline polymer, by atomic force microscopy. A thin cover layer of amorphous polypropylene was detected using PeakForce-QNM. This technique of fast force-distance data acquisition enables the determination of the elastic modulus modeled by the Derjaguin-Muller-Toporov (DMT) approximation. The DMT modulus of the top layer showed no significant variation and the values correspond to those of the amorphous phase found deeper in the material of the heterogeneous structure. The exposed crystalline regions after wet-chemical etching revealed a remarkably higher DMT modulus in the GPa-range than the amorphous regions. The results are in good agreement with earlier studies [1,2]. Moreover, interaction potential curves measured by bimodal frequency-modulated atomic force microscopy using short cantilevers gave further insight into the interplay between amorphous and crystalline polypropylene.
[1] C. Dietz, M. Zerson, C. Riesch, A. M. Gigler, R. W. Stark, N. Rehse, and R. Magerle, Appl. Phys. Lett. 92, 143107 (2008).
[2] C. Dietz, M. Zerson, C. Riesch, M. Franke, and R. Magerle, Macromolecules 41, 9259 (2008).

As components in devices and microscopy applications shrink to nanometer length scales, adhesion forces play an increasingly dominant role in the physics of contact. In particular, tip-based approaches for data storage, nanomanufacturing, and nanoelectromechanical systems (NEMS) rely on accurate knowledge and control of adhesion between a sharp asperity and a surface. It is well known that surface roughness affects adhesion at macro- and microscopic scales. However, the atomic-scale roughness of sharp tips used for advanced scanning probe microscopy (SPM) is rarely measured or accounted for. Here, we characterized the atomic-scale roughness of carbon-based SPM probes, and measured the corresponding effect on adhesion using simulations and experimental techniques.
In the present study, adhesion tests were conducted inside of a transmission electron microscope (TEM), using a modified in situ nanoindentation apparatus. Nanoscale asperities composed of either diamond-like carbon (DLC) or ultrananocrystalline diamond (UNCD) were brought into contact and separated from a flat diamond substrate. The in situ nature of the testing allowed characterization of the surface roughness with sub-nanometer resolution immediately before and after contact. Additionally, complementary adhesion simulations were conducted using molecular dynamics (MD) with conditions matched as closely as possible with the experiments (e.g., materials, asperity shape, environment). The root-mean-square roughness for the experimental tips spanned 0.18 nm to 1.58 nm; for the simulated tips, the range was 0.03 nm (atomic corrugation) to 0.12 nm. Over the tested range of roughness, the measured work of adhesion was found to decrease by more than an order of magnitude as the roughness increased, with a consistent trend observed between experimental and simulation results. The dependence of adhesion upon roughness was accurately described by a simple analytical model.
This combination of simulation and novel in situ experimental methodologies allowed for an exploration of an unprecedented range of tip sizes and length scales for roughness, while also intrinsically verifying consistent behavior between the two approaches. These results demonstrate a high sensitivity of adhesion to interfacial roughness down to the atomic limit. Furthermore, they indicate that present approaches for extracting work of adhesion values from experimental measurements of adhesion forces contain significant uncertainty due to an unmeasured variable - atomic-scale roughness.

While Atomic Force Microscopy is one of few high resolution characterization techniques that perform well in liquids, cantilever damping by the fluid substantially lowers resolution by increasing the minimum detectable imaging force. We reduced the damping and force noise of the probe by building a protective encasement around the cantilever which keeps the cantilever dry but allows the tip to probe the sample in solution. The substantially lower viscosity of the air leads to orders of magnitude improvement in force noise for higher resolution imaging. Furthermore, encased cantilevers retain high resonance frequencies and Q factors when immersed in liquid such that they are thermally limited even at high stiffness. This enables high resolution imaging and force spectroscopy on all commercial AFM systems in liquid.
Raster scan paradigm is not ideal for high speed AFM. Forcing the piezo to be at specific positions at specific times is frustrated by piezo nonlinearities causing hysteresis and the inertia of the scanner causing ringing without substantial overscanning. Furthermore, control loop delays do not allow trace to be displayed with retrace leading to loss of greater than 50% of scan time for capturing sample dynamics. We have developed a new scan mode that creates images directly from the scanner position sensor data using image processing algorithms leading to more accuracy and the use of all scan data. More importantly, after breaking from the raster scan paradigm, scan algorithms such as spiral scanning can be used which are better suited for piezo scanner limitations leading to 10 times higher frame rates. Spiral scanning together with encased cantilevers will lead to significant advances in temporal and spatial resolution for AFM in solution.

Scanning-probe microscopy (SPM) has been suggested as a means for mapping the mechanical properties of surfaces with nanometer-scale resolution. Inevitably, such maps are qualitative, because surface roughness and incipient plasticity compromise the determination of contact area. Even under the best conditions, SPM can be used only to assess elastic properties, not plastic strength. In this presentation, a new measurement technique is presented which overcomes these problems and allows the rapid generation of quantitative and highly resolved mechanical-properties maps. Instead of maintaining continuous contact with the surface, the scanning probe hovers just over the surface and performs an array of discrete indentations. Each indentation cycle requires less than one second, including approach, contact detection, force application, withdrawal, and movement to the next indentation site. Traditional nano-indentation analyses are applied to the force-displacement measurements, but information storage and presentation owe much to SPM technology. A directionally solidified eutectic alloy of chrome and chrome silicide (Cr-Cr3Si) is used to demonstrate this new technology as an improvement over other SPM techniques for mechanical-properties mapping.

Self-sensing cantilevers have several advantages compared to the bulky optical beam-deflection readout commonly used in atomic force microscopy (AFM) [1]. The application of magnetostrictive tunnel magnetoresistance (TMR) sensors for self-sensing cantilevers should be superior to already employed piezoresistive and piezoelectric sensors since they offer higher strain sensitivity and remarkable miniaturizing opportunities [2]. These advantages can turn into a substantial progress for many applications.
In our study, we used TMR sensors consisting of a magnetically stable layer and a sensing magnetostrictive CoFeB layer separated by a MgO tunneling barrier. Due to the inverse magnetostrictive effect, induced strain rotates the magnetization of the sensing layer. Accordingly, it alters the magnetoresistance of the TMR sensor which can be monitored as a signal for the cantilever deflection. Using microsystem technology techniques, we integrated TMR sensors into AFM cantilevers with standard dimensions. The sizes of the integrated TMR sensors range from 10 × 10 µm2 to 37 × 37 µm2. Their TMR values and resistance-area products are about 150 % and 75 kOmega;µm2, respectively.
With these cantilevers we measured force vs. distance curves as well as resonance curves in a home-built AFM. Furthermore, we successfully scanned sample surface in contact and tapping mode. All AFM data was obtained with the TMR sensors and had the same quality as those simultaneously recorded with an optical beam deflection.
[1] J. C. Doll and B. L. Pruitt, “Design of piezoresistive versus piezoelectric contact mode scanning probes”, J. Micromech. Microeng. 20, 095023 (2010)
[2] D. Meyners, T. von Hofe, M. Vieth, M. Rührig, S. Schmitt, and E. Quandt, “Pressure sensor based on magnetic tunnel junctions” J. Appl. Phys. 105, 07C914 (2009)

With average size of magnetoelectronic devices and components constantly decreasing, there is an increasing need of a technique capable of detection and analysis of a single such entity or component with adequate spatial resolution. Although Magnetic force microscopy (MFM) is a mature technique, it has so far been largely limited by ability to fabricate small enough magnetic probes capable of high-resolution imaging. For several past decades researchers were struggling to extend the resolution limits of this technique via different approaches to probe fabrication, e. g. directional deposition of magnetic thin films on electron beam deposition manufactured tips, focused ion beam (FIB) machining of commercial tips, attachments of functionalized nanotubes, etc. In our work we utilize electron-beam-induced deposition (EBID) process for cobalt patterning to produce sharp high-resolution MFM tips [1].
We have improved spatial resolution of Co EBID spike tips to the order of 10 nm by decreasing the tip diameter to the order of 15-20 nm and by increasing purity of the Co EBID [2]. The tips are grown directly via electron beam from the vapor phase of the organo-metallic precursor dicobaltoctacarbonyl Co₂(CO)₈. The deposition process itself depending on the aspect ratio was taking 1 - 5 s per tip and was accomplished on the recycled cantilevers. The issue of a lower signal-to-noise ratio due to small amount of magnetic material constituting the tip was resolved by additional design of a planar base of Co for each tip.
References:
[1] Belova LM, Hellwig O, Dobisz E, Dahlberg ED. Rapid preparation of electron beam induced deposition Co magnetic force microscopy tips with 10 nm spatial resolution. Review of scientific instruments 83, 093711, 2012.
[2] Belova LM, Dahlberg ED, Riazanova A, Mulders JJL, Christophersen C, Eckert J. Rapid electron beam assisted patterning of pure cobalt at elevated temperatures via seeded growth. Nanotechnology 22, 145305, 2011.

The KolibriSensortrade; from SPECS represents a new quartz sensor on the market that excels in its performance and its reliability. It is based on a symmetrical length extension resonator. The high resonance frequency of 1 MHz and the good signal-to-noise ratio allows for faster data acquisition in scanning microscopy and force spectroscopy. Oscillation amplitudes may be set below 20 pm. High stiffness prevents snap-in and the low noise floor continues to give a good frequency shift signal. The tip of the KolibriSensortrade; has a separate contact, guaranteeing clean separation of the signals from the tunneling tip and from the quartz force sensor.
The new Tyto scan head from SPECS is a milestone in the technology of Scanning Probe Microscopy. The modular design allows for various experimental configurations and for the usage of different sensors. A kinematic mount is used for both the sample and sensor and this feature is combined with accurate position sensors. For the first time, this enables different sensors to access identical locations on a sample and to repeat the procedure after successive sample preparation steps. This opens up opportunities for new experiments and will advance the research of surfaces at the nanometer scale. Additional features of the Tyto scan head are: Four openings for in-situ evaporation, two specular ports for simultaneous optical experiments, and large front openings and windows situated on each side of the body and at the back for broad visual inspection of the sample and sensor. Various sample receptors can be installed in the Tyto scan head, with four or twelve electronic contacts to the sample. Optional extra features include a calibrated Cernox temperature sensor located directly under the sample plate, and a small heater to control the sample temperature to within 1 mK.
For controlling state-of-the-art SPM experiments a new generation of the leading Nanonis Control System is presented.

We have implemented an all-optic Atomic Force Microscope operating inside an UHV TEM. The AFM incorporates heterodyne laser doppler interferometry and photo thermal excitation for sprius free excitation of the AFM cantilever. The AFM head is anchored onto the pole pieces for improved mechanical stability. Force curves could readily be measured with the instrument. High resolution imaging with the AFM, as well as a new control method to enable real time chemical contrast acquisition with the AFM will be presented. The results will be compared in part with real time TEM imaging. The TEMAFM could also be used to monitor decrease in friction and wear by applying vertical vibrations to the tip as previously reported by E.Meyer et al, and M. Lantz et al. Real time observation of the rubbing process will be presented and discussed for a nanometric point asperity.