Symposium CM01 : Exploring Nanoscale Physical Properties of Materials via Local Probes

Molecular recognition of adsorbates with very high sensitivity and selectivity is critical in chemical sensing. Nanosystems such as nanowires have been envisioned as a sensor platform for the next generation high performance sensors. In general chemical selectivity in miniature sensors is obtained by using immobilized receptors. However, obtaining chemical selectivity is a challenging task because of the lack of highly selective receptor molecules that can be immobilized on sensor surfaces. In addition immobilizing receptors on nanosensors most often results in irreproducible surface coverage. Therefore, developing receptor-free concepts for obtaining selectivity is very attractive. Molecular adsorption-induced variations in work function and its temperature dependence is an attractive approach for receptor-free chemical sensing. Work function of Pt nanowires investigated with Kelvin Probe Force Microscopy show significant changes due to molecular adsorption. Temperature dependent changes in the differential work function shows peaks corresponding to molecular desorption. Because of their high surface to volume ratio and very low thermal mass of nanowires, this approach can provide high selectivity, sensitivity, and fast operation and offers an elegant route to an extremely sensitive and highly selective sensor platform.

It is well known that carbon nanotube (CNT) optical resonances and electrical characteristics are strongly dependent on the atomic structure of the CNT. However, the relationship between electrical properties and CNT atomic structure has been challenging to fully resolve due to the complicating factor of dielectric environment. Early studies of CNTs on gold surfaces (strong dielectric screening) reveal only one slice of a bigger relationship. Here we describe a combination of nanoscale spectroscopy and transport measurements to explore the full relationship between CNT structure, electrical properties, and dielectric environment. We use spectrally-resolved scanning photocurrent spectroscopy to determine the chiral index of individual-contacted suspended CNTs. Dielectric environment is controlled by using various dielectric liquids. In semiconducting CNTs we observe band gap renormalization of approximately 30%. In metallic CNTs we observe a transport gap that is consistent with the theoretical predictions of a Mott gap. The gap is independent of chiral angle, inversely proportional to CNT diameter, and scales inversely with the dielectric constant of the environment. Our results emphasize the importance of spectrally-resolved scanning probe techniques in nanometrology, and highlight the important effect of dielectric environment on nanoscale physical properties.

Electrical scanning probe microscopes (eSPMs), such as the scanning capacitance microscope (SCM) and the scanning Kelvin force microscope (SKFM), measure the electrical properties of a virtual device formed between the tip and the sample. Like all electrical measurements, the measured electrical parameters depend on the electrode shapes (the tip and the electrically active part of the sample), and the fundamental electrical properties (dielectric constant, conductivity, etc.) of all the material between the electrodes. In general, the details of the tip electrical shape are not known, and the electrical shape can differ substantially from the physical shape, especially for co-axially shielded probe tips.

The eSPM tip shape effects the measured contact potential difference (CPD) profile of measurements of electrical field gradients with SKFM. A different response is measured depending on the angle that the tip crosses the boundary in potential. We have determined a method of extracting the electrical tip shape from measurements on well-known structures which produce a calculatable electric field gradient. The method was derived using two-dimensional COMSOL simulations of CPD profiles across abrupt boundaries in potential. An estimate of the tip width at the cantilever is obtained from the inflection points of the simulated response. Knowing the tip length, the cantilever tilt angle, and the determined tip width, a “blind” reconstruction of the details of the tip shape can then be obtained from the Δφ/Δx using an analytical expression. The quality of the reconstruction is best near the tip apex. The estimate of the tip shape can be improved by inserting the initial determined tip shape into COMSOL and recalculating the CPD profile. An adjusted tip shape can then be derived using the difference between the initial CPD profile and the recalculated profile.

For real three-dimensional tips, two types of test structures were prepared. The first structure consists of alternating Au and Pt regions with abrupt boundaries formed on an extremely flat mica surface. The second structure consists of pairs of independent microfabricated electrodes separated by a small distance and beneath a thin cover layer of dielectric. A 2D cross-section of the 3D tip at the angle of scan across the test structure can be obtained using the above method. By rotating the sample and scan angle, multiple cross-sections across the tip can be obtained and a three-dimensional tip reconstructed.

Knowledge of the actual electrical tip shape can be used to better understand electric field gradients measured with eSPMs and to determine the suitability of various types of conducting SPM tips for electrical measurements. For co-axially shielded tips, the above technique is the only way to get an estimate of the electrical tip shape. Electrical tip shape may also prove useful in inverse modeling to improve the accuracy and spatial resolution of images of electrical properties with eSPMs.

Despite extensive documentation of plastic deformation processes in micro-sized samples, there is up to now no clear understanding of the mechanisms governing plastic flow in nanoscale systems. Here, using a quartz-tuning fork based Atomic Force Microscope, we combine electrical and rheological measurements on nanoscale gold junctions, and study the onset of plastic flow in those model atomic-sized systems. By submitting the junction to increasing sub-nanometric deformations, we uncover a transition from an elastic regime to a plastic regime. Increasing deformations even further leads to the complete shear melting/liquefaction of the junction. This typology is reminiscent of the behavior of macroscopic complex fluids, here uncovered for a “molecular foam”. We rationalize and interpret our results in the framework of a harmonically driven Frenkel-Kontorova model. Our measurements allow us to measure the critical yield force governing the onset of plastic flow in the junction, as a function of size. In those molecular systems, plasticity is limited by the direct sliding of atomic planes under shear, as expected for dislocation-free systems.

Today, a wide range of AFM-based electrical characterization methods is routinely applied in the study of electronics materials and devices. Methods are available to measure conductivity, charge, surface potential, carrier density, piezo-electric and other electrical properties with nm scale resolution. Typically, these modes are operated in ‘imaging’ or ‘spectroscopic’ mode. In imaging mode, a fixed set of operating conditions (DC & AC sample bias, AC frequency, etc.) are used while the tip is raster-scanning the surface, resulting in high resolution images of height & electrical properties. In spectroscopic mode, the user selects a few points where one of the operating conditions is varied, while the tip is kept stationary. In many studies, both ‘imaging’ and ‘spectroscopic’ experiments complement each other. In this work, the ‘imaging’ and ‘spectroscopic’ modes are combined into a single mode providing an electrical spectrum in every pixel of the image. The tip is moved from pixel to pixel in a fast force volume mapping (FFV) manner, providing a force-distance spectrum in each pixel. During each force-distance cycle, the probe is held on the surface for a pre-defined time at a fixed force or Z position. During this ‘hold segment’, an electrical spectrum is collected by varying one of the electrical operating conditions. This results in a multi-dimensional datacube whereby electrical & mechanical spectra are present for each pixel. Using a force-mapping scan method also overcomes the limitations of contact mode scanning inherent to the conventional implementation of many of the electrical modes: Providing longer tip lifetime and capability to measure soft or fragile samples. Optimization of the force mapping movements, and the electrical measurement hardware & method, allowed us to maintain a relatively high imaging speed (typ. 20-100 ms/pixel). Analysis of the datacubes provides correlation of electrical & mechanical data, and images of a wider range of electrical properties. For example, when collecting an I-V spectrum in every pixel, one can extract a high-resolution map of current barrier properties. This approach is illustrated for Tunneling AFM by ramping the DC sample bias resulting in I-V spectra (FFV-TUNA), for Scanning Capacitance Microscopy by ramping the DC sample bias resulting in dC/dV-V spectra (FFV-SCM), for Scanning Microwave Impedance Microscopy by ramping the DC sample bias resulting in C-V and dC/dV-V spectra (FFV-sMIM), and Piezoforce Microscopy by ramping the DC sample bias (switching loops) or AC frequency (contact resonance spectra) (FFV-PFM), but is also applicable to other electrical modes such as SSRM and electrochemical modes such as SECM. Examples from a variety of materials including semiconductors, ferro-electrics and nanotubes illustrate the capability to reveal sample properties which are not accessible or easily missed in conventional methods where maps at only one or a few discrete settings are acquired.

CT-AFM is a significant departure from nearly 30 years of scanning probe microscopy. Instead of interrogating surface properties only, up to hundreds of images are acquired for a single location with intervening material removal. In this manner, resolution equivalent to conventional AFM methods is achieved laterally, and uniquely also into the depth of a specimen. Two examples are discussed, demonstrating nanoscale tomography for polycrystalline CdTe solar cells as well as BiFeO₃ multiferroic thin films.

The novel volumetric maps of photovoltaic performance reveal order-of-magnitude enhancements in the short circuit current for distinct microstructural features such as grains and grain boundaries. These results are effectively independent of grain orientation, and instead mediated by percolation pathways for electrons along grain boundaries and holes along photoactive planar defects. The discovery of such orthogonal channels for carrier transport may provide opportunities to dramatically improve device efficiency and reliability.

With multiferroics, ferroelectric domain contrast is detected down to a critical thickness of less than 10 nm based on the local piezoresponse as a function of milled depth. The 3-dimensionally resolved domain walls also reveal both anticipated as well as unexpected feature geometries and properties. Switching studies as a function of film thickness are especially revealing for spatial and energy scaling investigations with respect to ever-diminishing future device dimensions and power budgets.

CTAFM literally provides a new perspective on nanoscale materials properties throughout the thickness of materials devices. The resulting insight suggests new pathways to improve design, efficiency, and reliability for 3-dimensionally engineered materials systems.

There is a growing need to determine the nanoscale functional and electrical properties of subsurface materials and interfaces for future computing devices and biomedical applications. Characterization and control of these interfaces is the key to improved device performance and reliability. Among a variety of electrical and optical techniques for studying subsurface physical properties, scanning probe microscopy (SPM) based techniques are very promising due to their ability to image conductive, non-conductive, and non-optically-transparent samples. SPMs use an ultra-sharp tip (~10 nm in diameter) that contacts or intermittently contacts the local area to detect electrical and topographical information from the surface as well as subsurface. While resolution decreases with depth into the sample, electric and magnetic fields can penetrate 1000s of nanometers into samples and the resulting interactions can be detected using various electrically sensitive SPM techniques.

We have applied electrostatic force microscopy (EFM) to study voltage-biased subsurface structures. We investigated both traditional EFM, which detects changes in amplitude or phase of the cantilever relative to the mechanical drive frequency of the cantilever and a new approach applying an AC+DC signal to the buried structures instead of the cantilever. An external high frequency lock-in amplifier (LIA) monitors the deflection signal of the cantilever, using the AC signal applied to the buried structure as its reference. Small changes in the phase of the cantilever oscillation can then be detected to map subtle electrostatic force variation between the subsurface metal lines and the EFM tip. To realize these goals, custom-designed 4-metalization microfabricated test structures were used. A printed circuit board (PCB) was utilized to allow external and independent electric access to the various subsurface structures in an SPM environment. The influences of the relative voltage biases on the subsurface metal lines and the substrate, the effect of the ac frequency applied to the subsurface structures, and mechanisms of subsurface contrast and contrast reversal will be presented.

In subsurface imaging applications, contrast in EFM depends on the electrostatic force between the tip and sample. In the related technique, scanning Kelvin force microscopy (SKFM) contrast arises from the force due to capacitance gradient with tip-to-sample distance (dC/dz). These measured quantities arise from variations in sample dielectric constant, any charge accumulated on subsurface structures, and subsurface variations in conductivity. Our technique allows us to independently bias the subsurface structures and thereby, separate the effect of buried charge and capacitance gradient.

Three-dimensional (3D) imaging revealing the structures hidden under the immediate object surface, whether it is an ultrasound baby scan or a 3D image of the living cell is undoubtedly a holy grail for any imaging technique, offering wealth of information on the studied object. While Atomic Force Microscopy (AFM) offers spatial resolution down to individual atoms, the contrast originating from the interaction of the nanoscale probe and the surface, restricts AFM to predominantly surface imaging. In order to overcome this, one can use ultrasonic wave that penetrates both transparent and opaque samples carrying 3D information to the sample surface where it is could be picked up by the scanning probe. Whereas such approach was experimentally realised in Ultrasonic Force Microscopy (UFM) [1], Heterodyne Force Microscopy and Ultrasonic Holography [2], and Atomic Force Acoustic Microscopy [3], the fundamental origin of the contrast is far from clear with some publications referring to the scattering of ultrasonic waves and some attributing it to the interaction of elastic field from AFM tip with the subsurface structures. Furtermore, some of the observed subsurface structures are empty voids, and some are stiff regions hidden under soft surface layers. Untangling this requires analysis of the near-field scattering of the ultrasonic waves, and the propagation of the stresses in a generally anisotropic media.

In this paper we show that given that in all cases of nanoscale ultrasonic imaging both the depth and size of subsurface features are orders of magnitude smaller than the ultrasonic wavelength, the ultrasonic wave interaction with the subsurface inclusion has to take into account the phase and amplitude of both the evanescent and radiated wave. We then show that the near-field corrugation of ultrasonic field on the sample surface is of negligible amplitude for the simple elastic discontinuity, but may be observable for the pure void. This strongly suggests that it is the oscillating stress field generated by the SPM tip-surface contact that provides a main source of the contrast in the subsurface ultrasonic imaging.
We further analyse stress propagation in the highly anisotropic material such as graphite, MoS2 or multilayer graphene - a transversely isotropic materials - and find that the stress field propagate as highly “focused” beam elongated according to the ratio of the out-of-plane Young modulus S33 and the in-plane shear modulus S44, explaining unusual experimental observations of “ultrasonic transparency” in [2] that allows to observe defects and structures deep under immediate surface of such materials.

[1] Yamanaka K, Ogiso H, Kolosov O, 1994;64(2):178-80; Dinelli F at al, Nanotechnology. 2017;28(8):085706.
[2] Cuberes MT et al., J Phys D-Appl Phys, 2000;33(19):2347-55; Tetard L at al, Nat Nanotechn. 2008;3(8):501-5; Verbiest GJ, Rost MJ, Nat Commun. 2015;6:6444.
[3] Hu SQ, Su CM, Arnold W. J. Appl Phys, 2011;109(8).

Notable progresses have recently been made in quantitative scanning thermal microscopy (SThM) based on a sub-micron thermocouple junction fabricated on an atomic force microscopy (AFM) tip. Here, the underlying theoretical basis for such measurements is re-examined, with a focus on the effects of the temperature and spatial dependences of the interface thermal conductance. The nature of the temperaure being measured is discussed in relationship with the phonon transmission coeffiicient across the tip-sample gap. Comparison is made between the results obtained form the SThM and Raman thermal mapping measurements. The SThM method has recently been employed to study heat dissipation in emerging electronic devices made of various two-dimensional (2D) materials. In addition, a different type of scanning thermal probe with a resistive heater and thermometer near the tip has been used to measure thermal transprot across a point contact, and an attempt is made to use a scanning thermoelectric microscopy method to probe the variation of the local thermoelectric property near the edges of 2D materials.

Scanning thermal microscopy (SThM) based on Atomic Force Microscopy (AFM) technique is used in order to investigate thermal properties of materials and mechanism of heat transfer at micro-and /or nanoscale [1-2]. Generally, in all SThM, a temperature sensor (thermocouple, thermoresistive …) is integrated into the probe which can measure simultaneously local temperature or generate local heating. A topographic and thermal image of materials with sub-micrometer spatial resolution can be obtained with this technique. Although various types of SThM probes have been developed so far, the probe selection is of great need. The present study deals with Wollaston resistive probe. The aim is to study the influence of sample structure on the thermal signal of Wollaston probe, to characterize and estimate the effect of probe volume on the thermal conductivity measurements. For that, a buried nanostructure sample is considered. The sample is composed of buried silicon steps under polished CVD SiO2. We develop a well-defined heat transfer numerical model of the probe-sample system for a better interpretation of experimental results obtained by SThM. Numerical simulations were performed using COMSOL Multiphysics which is based on finite element method. A 3D realistic geometry of the Wollaston probe is modelled in contact operation mode in order to obtain probe temperature behavior. The numerical model of probe/sample allows evaluating the flux dissipated by the platinum-rhodium wire and into the sample. The heat flux gives access to the thermal behavior of the probe in contact with the nanostructured sample. This work shows that the thermal signal is sensitive to the internal structures. The thermal signal gives access to a local thermal conductance that corresponds to a probed volume. We present and discuss our latest results.
The research leading to these results has received funding from the European Union Seventh Framework (EU FP7) Program FP7-NMP-2013-LARGE-7 (project QUANTIHEAT) under grant Agreement 604868, and from the Champagne-Ardenne Region A2101-03-Excellence.

REFERENCES
Majumdar, Lai, Chandrachood, Nakabeppu, Wu and Shi, Thermal imaging by atomic force microscopy using thermocouple cantilever probes, 1995
F.A. Guo, N. Trannoy, J. Lu, Analysis of thermal properties by scanning thermal microscopy in nanocrystallized iron surface induced by ultrasonic shot peening, Materials Science and Engineering: A, 2004.

This talk summarizes recent developments in the use of scanning thermal microscopy (SThM) for local thermometry and thermal conductance measurements. The pros and cons of SThM in comparison with other thermometry methods will be discussed. We further describe techniques to quantify temperature avoiding typical artifacts of SThM [1]. Examples of high-resolution measurements of nano-electronic semiconductor devices [1,2], metal interconnects [1], phase change devices [2] and molecular and atomic junctions [3,4]. Finally, we will discuss ongoing strategies to reach resolutions down to and below the 1-nm scale at a sensitivity of Picowatts, and approaches using dynamic operation modes of atomic force microscopy to control SThM scans.
[1] F. Menges et al., Nature Communications 7(10874), 2016
[2] F. Menges et al., 2016 IEEE International Electron Devices Meeting (IEDM), 15.8.1-15.8.4, 2016.
[3] T. Meier et al., Physical Review Letters 113, 060801, 2014.
[4] N. Mosso et al., Nature Nanotechnology 12, 430-433, 2017.

Abstract not available.

Graphene and related two-dimensional (2-D) materials have recently emerged as a topical area in which atomic layer control is possible. However, their successful integration into useful applications is heavily reliant on a complete understanding of their physical properties, together with their response when subject to different environments. For graphene-based devices that are processed and operated in ambient, the environmental humidity has an unpredictable influence on the device performance and reliability. In this talk, I will first review our recent scanning Kelvin probe microscopy studies highlighting the impact of relative humidity variation on the electronic properties of graphene and the relevance to chemical sensors applications.
I will then discuss the role of graphene in tuning the electronic and optical properties of other 2-D materials when integrated in vertical heterostructures, giving rise to novel excitonic effects. In particular, I will show that the thickness of the supporting graphene substrate is crucial in modulating the light emission in WS₂ on graphene heterostructures, as supported by scanning Kelvin probe microscopy, which proved to be extremely valuable in unravelling the underlying nature of excitonic effects associated with such heterostructures.

The optical properties of Van der Waals semiconductors such as transition metal dichalcogenides (TMDC), black phosphorous, SnS etc. are dominated by excitons which are stable at room temperature owing to decreased Coulomb screening which yields large exciton binding energies (> 0.5 eV). Investigation of the photoluminescence (PL) response is one of the most direct methods for probing excitons in these materials.
Nanoscale spectroscopic imaging, such as Tip Enhanced Photoluminescence (TEPL) and Tip enhanced Raman Spectroscopy (TERS), provides new capabilities to probe nanoscale structural and opto-electronic inhomogeneities in novel 2D semiconductors. Owing to the improved spatial resolution of TEPL and TERS compared to conventional confocal microscopy due to the near-field coupling of photons, nanoscale inhomogeneities in charge carrier concentration in CVD grown flakes of MoS2 and WS2 have been directly detected and characterized [1,2]. More recently, brightening of dark excitons has also been observed in monolayer TMDCs when the samples are confined under the gap-mode of TEPL [3].
In this work, we report the importance of sample preparation procedures as well as the nanoscale TEPL and TERS imaging of TMDCs for correct characterization and interpretation of observed TEPL spectra. We report that mono-to-few-layer flakes of WS2 and WSe2 transferred to gold via heat-assisted exfoliation feature a large number of nanoscale bubbles ranging from few tens to about a hundred of nanometers across. Depending on the exact location of the optically active SPM probe relative to the bubble, the spectral response can change dramatically, both in terms of the intensity of the TEPL peak, its spectral position, shape and the ratio between the TERS and TEPL signals. We have observed both red and blue-shifted TEPL peaks in WSe2 ( up to 20nm relative to the position of the far-field PL), which implies broader variety of excitonic behavior in gap-mode near field PL of TMDCs compared to what was reported earlier. The TERS/TEPL peak intensity ratio changes as the probe is positioned away from the center of a nanobubble both for WSe2 and WS2, which may be the consequence of varied distance between the layer of TMDC and underlying gold as well as varied mechanical strain in the vicinity of the bubble. Additional signatures of these effects are also observed in the excitonic emission as the relative intensities of different exciton states varies over the bubble.
The observed nano-PL response demonstrates the importance of nanoscale spectroscopic imaging of 2D semiconductors for comprehensive understanding of possible excitonic resonances, and opens the possibility of rational engineering and manipulation of local nanoscale photon emitters in these materials.

REFERENCES
Wei Bao et.al. NATURE COMMUNICATIONS | 6:7993
Christoph Kastl et.al. 2D Mater. 4 (2017) 021024
Kyoung-Duck Park et.al., arXiv:1706.09085v1

Transition metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quantitative nanometer-scale characterization of defects over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL) and electrical conductivity. We report direct observation of defects in monolayer WS₂ with nanometer-scale precision over large length scales (up to 20 µm distances) using conductive atomic force microscopy (CAFM) at room temperature in ambient laboratory conditions. The measurements were made possible by several advances in sample preparation. The observed defects are highly conductive when probed out-of-plane with CAFM, which enables precise identification of defect locations and direct quantification of areal defect density. The defect density ranged from 2.3 x 10¹⁰ cm^-2 to 4.5 x 10¹¹ cm^-2 in our samples. We correlate the measured defect density with spatial variations in PL, and observe a pronounced inverse relationship between PL intensity and defect density. Importantly, we observed the same inverse relationship on multiple different WS₂ grains, which indicates that the relationship is generally true. Finally, we propose a model in which the observed electronically active defects serve as non-radiative recombination centers, and obtain good agreement with the experimental data. Performing PL measurements on both PDMS (before mechanical transfer) and on graphite (after mechanical transfer) allowed us to decouple the effect of defect-related non-radiative recombination and substrate-related non-radiative recombination. This decoupling has not been possible in previous studies because the defect density of monolayer TMDs in other studies was unknown. Our results provide important information for understanding the cause of spatial variations in TMD properties, and are a critical demonstration of a technique for mapping defect density over length scales relevant for observed TMD behaviors.

In order to develop a new generation of electronic and optoelectronic devices based on the novel properties of nanoscale and low-dimensional materials, there remains a strong need for techniques capable of measuring electronic properties non-destructively and with high spatial resolution. In scanning microwave microscopy (SMM) the near-field impedance between a scanning probe tip and a sample of interest is measured at GHz frequencies, enabling local measurement of the free carrier density and the associated nanometer-resolved imaging of conductivity variations.

Here we use SMM to study the electronic properties in field-effect transistors of the 2D transition metal dichalcogenides MoS₂, WSe₂, and MoTe₂. We identify significant differences in carrier density across the individual devices arising from interactions with the oxide substrate, Schottky barriers at the source and drain electrodes, as well as evidence of interlayer interaction in multilayer devices. We develop a new method combining conventional backgate control of carrier density with a high-frequency backgate bias modulation to perform SMM-based differential capacitance measurements that allow us to directly address and image the spatial inhomogeneity in the gating response. We find preferential carrier accumulation in response to gating near the source and drain electrodes while the interior of the device shows an inhomogeneous response dominated by crystal-substrate interactions.

The properties of 2D materials can be affected by doping or defect engineering. However the creation of selected defects and their role in changing surface properties such as hydrophilicity remain unclear, mostly due to the paucity of tolls to probe materials properties at the sub-micrometer scale. This limitation can be surpassed by exploiting the functional modes of atomic force microscopy, including force spectroscopy, electrostatic force microscopy and nanoscale infrared spectroscopy.
In this study, we discuss several means of controllably introducing defects in 2D materials, including using heat treatment and high energy bombardment. We show that the defect created affect the structural, mechanical and electrical properties. In turn, we present some evidence that the defects can become sites for selected chemical reaction. Finally, we monitor the changes in hydrophilic properties and explore the affinity of selected molecules with defect-laden 2D materials using functionalized AFM cantilevers. The study constitutes a new approach to understanding the mechanisms of catalysis at the nanoscale, with implications for large scale catalysts production and reactions.

Recent advances show that localized surface plasmon resonances can be utilized to drive important photochemical reactions at low incident photon flux at room temperature. Central to this application is a localized surface plasmon near-field that drives the surface photochemical reactions and enables us to detect the reactant and product species in situ with high sensitivity using for example surface enhanced Raman spectroscopy. The presentation will start with a brief discussion of plasmon near-field localization based on experimental results recently obtained in our lab using super-resolution near-field scanning optical microscopy. Plasmon-driven photochemical reactions will be discussed based on our recent exciting experimental observations, where the remarkable effect of surface ligands is demonstrated. We show that the same plasmonic optical nanoantennas can lead to different reaction pathways depending on the surface ligands. We propose that the surface ligands are influencing the reaction pathways through their interaction with the charge carriers (electrons and holes), thereby changing the concentration of reactive intermediates. The results provide important insight into the mechanism of plasmon-driven surface photochemistry, and at the same time, indicate the hidden chemistry on metal surfaces, where looking at molecules based on their fluorescence is no longer a possibility.

The photothermal induced resonance (PTIR)[1] is a scanning probe technique that combines AFM with IR (or visible) absorption spectroscopy, enabling material identification, molecular conformational analysis, mapping of composition and electronic bandgap at the nanoscale. In PTIR, the absorption of a laser pulse induces a rapid thermal expansion of the sample. Conventional cantilevers are too slow to track the sample thermal expansion dynamics; however, the fast sample expansion kicks the cantilever in oscillation (like a struck tuning fork), with amplitude proportional to the absorbed energy.
After a brief introduction, I will leverage PTIR to provide direct proof of ion electron migration[2] and ferroelasticity[3] (but not ferroelectricity) in organic-inorganic perovskites solar cells. As second example, I will show that the PTIR near-field mechanical detection enable the observation of dark-polaritonic modes in hexagonal boron-nitride nanostructures (hBN), for the first time.
Later, I will discuss how we revolutionize AFM (and PTIR) signal transduction by integrating cavity-optomechanics for sensing the motion of fast, nanosized/picogram scale AFM probes with unprecedented precision and bandwidth, thereby breaking the trade-off between AFM measurement precision and ability to capture transient events.[4] Applied in PTIR, the probe near-field ultralow detection noise and wide bandwidth improves the time resolution, signal-to-noise ratio and throughput by a few orders of magnitude each. Remarkably, this synergy enables a new PTIR measurement modality: capturing the previously inaccessible fast thermal-expansion response of the sample to nanosecond laser pulses, thus allowing concurrent measurement of the chemical composition and thermal conductivity, at the nanoscale.
We validate these new capabilities using polymer films and measure the intrinsic thermal conductivity (η) of metal-organic framework (MOF) individual microcrystals, a property not measurable by conventional techniques. MOFs are a class of nanoporous materials promising for catalysis, gas storage, sensing and thermoelectric applications where accurate knowledge of η is critically important. Additionally, the improved sensitivity enable measurement of nanoscale IR spectra of monolayer this sample with high signal to noise ratio (≈ 170).
We strongly believe that the radical AFM innovation enabled by nanofabrication and cavity-optomechanics is broadly-applicable and will benefit a wide range of AFM-based dynamic observations in nanoscience and biology and it greatly improve the impact of the PTIR technique in those fields.
1. Centrone, A. Annu. Rev. Anal. Chem. 2015, 8, (1), 101-126.
2. Yuan, Y., et al. J. Adv. Energy Mater. 2015, 5, (15), 1500615.
3. Strelcov, E., at al. Sci Adv 2017, 3, (4), e1602165.
4. Chae, J.; An, S.; Ramer, G.; Stavila, V.; Holland, G.; Yoon, Y.; Talin, A. A.; Allendorf, M.; Aksyuk, V. A.; Centrone, A. Nano Lett. 2017, 17, (9), 5587-5594.

Nanoscale thermometry is vital for the experimental characterization of sub-continuum thermal transport, such as nanoscale solid conduction [Park et al., Journal of Heat Transfer (2008)], near-field thermal radiation [Kim et al., Nature (2015)], and heat transfer in atomic junctions [Mosso et al., Nat. Nanotech. (2016)]. At such small scales, thermal transport greatly deviates from macroscale observations. Therefore, to understand the underlying physics of sub-continuum thermal transport, nanoscale thermometry should be implemented to quantify both the temperature gradient and heat transfer rate for geometric constrictions typically much smaller than 1 μm. While advancements in nanothermometry have enabled groundbreaking experimental research in sub-continuum thermal transport, no work has combined temperature feedback control with nanothermometry to actively control the local temperature of a nano-hotspot.

There are two major challenges when conducting experiments without local temperature feedback control: (1) undesirable variations in the background heat transfer to the surrounding environment during an experiment and (2) temperature varying discrepancies in the nanoscale thermal resistance. To address these challenges, we report the active control of a local nano-hotspot temperature for accurate nanoscale thermal transport measurement. To this end, we have fabricated resistive on-substrate nanoheater/thermometer (NH/T) devices that have a sensing area of ~350 nm × 300 nm, which forms a local nano-hotspot upon Joule heating. The NH/T devices operate with a 4-probe detection scheme: an electric current flows through the outer electrical leads to Joule heat the nano-patterned platinum (Pt) wire, the local electrical resistance is computed by measuring the voltage drop between the inner electrical leads. The key advantage of using resistive nanoscale thermometry is its simultaneous use of the sensing area as both a heater and thermometer.

To examine the controller’s integration with the NH/T device, feedback-controlled temporal heating and cooling experiments of the nano-hotspot reveal that the integral gain plays a dominant role in the device's response time for various temperature setpoints. The NH/T device with temperature feedback control is then applied to a local tip-induced cooling experiment, where a silicon atomic force microscope (AFM) tip is scanned over the NH/T’s nano-hotspot. The tip-induced solid conduction local cooling experiments are performed in both air and vacuum with optimized experimental parameters to separately identify the dominant modes of heat transfer: solid conduction, air conduction, water meniscus, and near-field thermal radiation. The obtained results demonstrate the precision controllability of a local nano-hotspot temperature and its application towards characterization of nanoscale thermal transport, which will provide insight to sub-contiuum heat transfer.

With a STM based near field scanning thermal microscope (NSThM) [1] we investigate the heat transfer at ultimate short distance 0.2 nm - 7 nm. At these separations the observed heat transfer is five orders of magnitudes larger than than black body radiation and three orders of magnitudes larger than predictions by conventional theory of fluctuational electrodynamics [2 - 9]. After an accurate calibration procedure we are able to measure the local heat transfer in a quantitative way[10]. We have investigated the influence of thin films of metals and dielectric material on the near-field mediated heat transfer at the fundamental limit of single monolayer islands on a metallic substrate. Spatially resolved measurements by NSThM are presented which are showing a distinct enhancement in heat transfer above NaCl islands compared to the bare Au(111) film [11]. Experiments at this sub-nanometer scale call for a microscopic theory beyond the macroscopic fluctuational electrodynamics used to describe near-field heat transfer today. The method facilitates the possibility to develop designs of nanostructured surfaces with respect to specific requirements in heat transfer down to a single atomic layer. These findings open up the possibility for a local surface modification by means of local heating, e.g. chemical modification and heat assisted magnetic recording, on a scale of a few nanometers.

References
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[3] S.-A. Biehs, O. Huth, and F. Rüting, Phys. Rev. B 78, 085414 (2008).
[4] A. Kittel, U. F. Wischnath, J. Welker, O. Huth, F. Rüting, and S.-A. Biehs, Appl. Phys. Lett. 93, 193109 (2008).
[5] S.-A.Biehs and J.-J. Greffet, Phys. Rev. B 81, 245414 (2010).
[6] S.-A. Biehs and J.-J. Greffet, Phys. Rev. B 82, 245410 (2010).
[7] A. W. Rodriguez, M. T. Homer Reid, and S. G. Johnson, Phys. Rev. B 86, 220302(R) (2012).
[8] A. W. Rodriguez, M. T. Homer Reid, and S. G. Johnson, Phys. Rev. B 88, 054305 (2013).
[9] K. Kloppstech, N. Könne, S.-A. Biehs, L. Worbes, D. Hellmann und A. Kittel, Nature Communications 8:14475 DOI: 10.1038/ncomms14475 (2017).
[10] K. Kloppstech, N. Könne, L. Worbes, D. Hellmann, and A. Kittel, Rev. Sci. Instrum. 86, 114902 (2015).
[11] L. Worbes, D. Hellmann, and A. Kittel, Phys. Rev. Lett. 110, 134302 (2013).

The interaction of a probe with the thermal near field emitted by a surface is of importance in near-field thermal spectroscopy (De Wilde et al., Nature 444, 740, 2006), tip-based manufacturing (Hawes et al., Opt. Lett. 33, 1383, 2008) and localized radiative cooling (Guha et al., Nano Lett. 12, 4546, 2012). Experimental studies (Jones and Raschke, Nano Lett. 12, 1475, 2012; Babuty et al., Phys. Rev. Lett. 110, 146103, 2013; O’Callahan, et al., Phys. Rev. B 89, 245446, 2014; O’Callahan and Raschke, APL Photonics 2, 021301, 2017) reported that the resonance frequency of the far-field scattered signal is spectrally redshifted compared to the near-field spectrum of the surface as predicted via fluctuational electrodynamics. It is not clear if the resonance redshift is due to near-field coupling between the probe and the surface or if it is an experimental artifact. Simplified models are inadequate for explaining the resonance redshift. In this study, the thermal discrete dipole approximation (T-DDA) with surface interaction (Edalatpour and Francoeur, Phys. Rev. B 94, 045406, 2016) is applied for modeling probe-surface interactions. The heat rate between the probe and the surface as well as the scattered far-field signal is calculated and analyzed. The results reveal that the resonance redshift is a physical phenomenon caused by the multiple reflections of the thermally generated waves between the probe and the surface. The magnitude of the resonance redshift is greatly affected by the shape and material properties of the probe. The possibility of designing a probe which does not introduce a resonance redshift in the scattered signal is discussed. This work will pave the way to the development of spectroscopy techniques enabling measurements of the near-field thermal spectrum.
Acknowledgment: This work is supported by the US Army Research Office under Grant No. W911NF-14-1-0210.

Microorganism pathogenicity strongly relies on the generation of multicellular assemblies, called biofilms. Surface attachment of a planktonic bacteria, mediated by adhesins and hydrated extracellular polymeric substances (EPS), is a crucial step for biofilm formation. Indeed, some pathogens can modulate cell adhesiveness, impacting host colonization and virulence. However, identification of changes in EPS composition during biofilm life cycle, as well as their trigger mechanisms, are still challenging, particularly in early stages. In this work, we analyzed the entire biofilm formation process of the economically important phytopathogen Xylella fastidiosa at single-cell resolution, using several nanometer-resolution spectro-microscopy techniques. Our Scanning Probe Microscopy results reveal electrical and elastic signatures for spatial and temporal EPS distribution at different stages of the bacterial life cycle. Ex-vivo single-cell adhesion forces mediated by EPS were probed using a different approach, based on nanowire arrays. Larger adhesion forces at the cell poles and additional mechanical support from secreted EPS layers were shown for X.fastidiosa. Significant adhesion force enhancements were observed for single cells anchoring a biofilm and particularly on XadA1 adhesin-coated surfaces, evidencing molecular mechanisms developed by bacterial pathogens to create a stronger holdfast to specific host tissues.

Surfactants dissolved in water will self assemble into micellar structures when exposed to a hydrophobic surface. Our work focuses primarily on the study of these films via AFM, where they can be directly probed and their mechanical properties examined. In our earlier studies, we focused on the effect of salt additions upon the breakthrough force of sodium dodecyl sulfate (SDS) and dodecylamine HCl (DAH) films. It was found that the addition of around 1.5 mM of NaCl, MgCl₂, and Na₂SO₄ all produced significant increases (~40-70%) in the average breakthrough force at a surfactant concentration of 10 mM. A model was developed in an effort to explain the cause of this strengthening, which predicted that the breakthrough force was tied to a free energy of micellar formation. Experiments were performed using spectrofluorometry to determine how addition of salt affected the CMC and, consequently, the free energy of formation. Our model showed excellent fits for most added salts, but struggled specifically in the case of MgCl₂ added to 10 mM SDS. We theorized that the mismatch between our theory and data was the result of strong ion binding at the surface, which might have changed the activation volume at breakthrough. More recent work has focused on analyzing the force curves near these micellar surfaces. Force curves contain a wealth of information about potential surface structures, and can be used to determine mechanical moduli for thin films. We found that, although our film was only 1-2 nm thick, we observed strong repulsive forces at tip-film separations of ~15-20 nm. The decay length of these forces, as well as their magnitude, increased with added salts, implying that they were not the result of electrostatic repulsion. At low salt concentrations (<1 mM), observed trends were well explained by theory for steric repulsion. The large decay lengths of 2-3 nm for SDS and DAH at these concentrations implied that the forces were the result of collective movement of micelles, rather than protrusions of individual molecules. At high concentrations of salt (>1 mM), a change in behavior was observed for SDS: A second region with a different force decay length appeared near the surface. To explain this, we applied a model for charged polymer brushes in the osmotic regime to our surface, and found good agreement between the model and our results. DAH films did not display this decay length splitting, which we hypothesized was the result of lower surface charge and ion concentration. Using force curves for SDS, we were able to extract a value for the Young's modulus of 80+/-40 MPa, which is comparable to results from literature for lipid bilayers. The DAH film was too fragile to acquire accurate information about mechanical moduli from our trials. Current work focuses on using micropipette aspiration to compare information about nanomechanical strength to a more practically sized system.

The mechanical properties of living cells provide structural stability and mechanical flexibility, crucial for their function. Thus, molecular understanding of the mechanics of the cell is relevant to understand biological function. High-speed atomic force microscopy (HS-AFM) is a unique technology that combines nanometric-imaging capabilities at video rate (1). We have recently adapted a HS-AFM system to develop high-frequency microrheology to probe the viscoelastic response of living cells from 1 Hz to 100 kHz (2). We report the viscoelasticity of different cell types under cytoskeletal drug treatments. At high frequencies, cells exhibit rich viscoelastic responses that reflect the state of the cytoskeleton filaments. The comparison of benign and malignant cancer cells revealed remarkably different scaling laws at short timescales. Microrheology over a wide dynamic range provides mechanistic understanding of cell mechanics and a univocal fingerprint, applicable for diagnosis or prognosis of disease.

1. T. Ando et al., A high-speed atomic force microscope for studying biological macromolecules. Proceedings of the National Academy of Sciences 98, 12468 (2001).
2. A. Rigato, A. Miyagi, S. Scheuring, F. Rico, High-frequency microrheology reveals cytoskeleton dynamics in living cells. Nat Phys 13, 771 (2017).

In nature, protein molecules can self-assembly into 2D crystalline arrays on templates like, cell wall and membrane. The created protein matrixes, like bacterial S-layer and purple membrane, can have unique structure and biological function. Protein self-assembly structures have also been synthesized artificially on solid-liquid interface to fulfill certain applications, like energy conversion. However the general model to describe the self-assembly process of protein at solid-liquid crystal is still missing. It can be the potential obstacle for designing and applications of protein 2D matrix in future.
In this project, we used the designed helical repeat protein (DHR10-micaX) [1] as the model to elucidate entropy-driven effect to the dynamic and final self-assembly structure of protein at solid-liquid interface of mica via high-speed atomic force microscopy (AFM). After carefully selection of cations and subsequent changes of the amount at interface, DHR10-micaX with size in nanometer can form 2D crystal across area of million times of their length with unique orientation. This class of protein can also form nanowire arrays with uniform orientation, if the protein-protein interaction is tuned. Combining in-situ data of high-speed AFM and simulation, we elucidated the role of entropy in the whole process, and we also discussed the effect of hydration layer to protein dynamics at solid-liquid interface.


1. Brunette, T.J., et al., Exploring the repeat protein universe through computational protein design. Nature, 2015. 528(7583): p. 580-584.

Nano-indentation experiments and their subsequent analysis can provide useful insights into materials' mechanical properties. Moreover, in living cells, knowledge of mechanical properties can lead to improved understanding of function. Despite impressive progress in the field, it is not always possible to acquire adequate information for the characterization of complex biological systems. Real biological samples are often highly heterogeneous and dynamic, and thus nanoindentation experiments must be interpreted carefully under contact mechanics theory¹. Here we use the plant leaf as a model complex biological material structure. Plant cells have a thick wall that maintains shape, prevents cell disruption under the action of high osmotic pressure, and plays a vital role in the varied functions of different plant tissues. We focus on the leaf surface and particularly stomata, specialized cells involved in gas and moisture exchange, which are designed to open or close under varying internal pressure.
Beginning from the observation that not all the force curves of a “force-volume” map can be treated universally under the same theoretical model, an automated approach was generated for the analysis of multiple force curves. In the approach presented here, apart from the calculation of Young’s modulus for the whole map, we present three important features for an improved insight into the complexity of cell mechanics. Firstly, a “contact mechanics model” map suggests the appropriate model to be used for the calculation of modulus of each force curve of the map (depending on adhesion, hysteresis cycle and fitting constants α,β and in P=α*h^β). Secondly, a “yield point” map shows the points of the scan where the elastic limit has been exceeded along the indentation depth. Thirdly, a “modulus tomography” movie is generated, from which a possible change of modulus value along fractions of indentation depth, for a non-yield-point force curve, can reveal bad fitting of the experimental data under the current theoretical models (β=3/2 & 2) for non-adhesive/non-plastic contact. Finally, a map of the fitting residuals is presented to provide an immediate insight into the areas of the data that cannot be treated with confidence.
We have applied this approach to the spatial variation of Young’s modulus over the surface of the plant cell wall, and how this feature couples to the structure and the biological function of the cell. We have observed an unexpected stiffening of the polar regions of the cell², in-line with the mechanical role of this region during the opening and closing of stomata, providing new insights into the mechanics of the cell’s function.

References
[1] Kuznetsova et al. Micron, Volume 38, Issue 8, 2007, 824-833
[2] Carter, Ross et al. Current Biology, Volume 27, Issue 19, 2017, 2974 - 2983.e2

Since its development in the eighties, atomic force microscopy (AFM) has proved itself to be the perfect tool to image a wide range of samples and characterize their mechanical properties with an unprecedented accuracy. Force spectroscopy emerged in the nineties as an ideal technique to extract the sample’s stiffness or elasticity, and, to date, is the most commonly used AFM technique in biology. But over the last decade, other quantitative modes have offered benefits in terms of resolution and information. Recently an oscillating technique called PeakForce Tapping (PFT) has been released. Whereas the cantilever is oscillated far below its resonance frequency (typically 1 kHz), a force/distance curve is recorded each time the tip contacts with the surface. From each of those force curves, mechanical information can be extracted in real time. If the tip is calibrated prior to the experiment, all information is displayed in a quantitative manner. This technique has proven suitable for a wide variety of samples ranging from stiff biopolymers to live cells.
In this presentation, we will be focusing on a few examples where PFT has been successfully used on biological samples in near-physiological conditions to address issues of medical or pathological order. More in particular, via the use of PFT, we’ll be reviewing how AFM can be considered as a nanotool to distinguish cancer cells from their normal counterparts by extracting the cell’s mechanical properties or measuring their migration speed; or to investigate the effects of diabetes or atherosclerosis on tissue sections with a submicrometer resolution. The interest of the technique also lies in the possibility to combine it in real time to other more traditional microscopy techniques, thus providing complementary information.

Polymers and metals have been fashioned into functional composites utilized in electronics, product packaging and decorative films despite their disparate mechanical, thermal, and bonding properties. Metallic nanoparticles on polymeric architectures hold potential for catalysis, antimicrobial filtration membranes and solar cells. However, adhesion between polymers and metallic particles is notoriously weak due to the differences in atomic and molecular configuration of these materials; as a result, their functional application can be undermined. Furthermore, the process of delamination of metallic particles on polymeric substrate is exacerbated by the differences in compliance; delamination and has been shown to be dependent on adhered metal particle size. This work explores the adhesion characteristics of electrolessly deposited copper particles on two distinct polymer architectures: thin films and fibers. Polyacrylonitrile was spun-coat into a thin film of a few microns as well as electrospun into a non-woven fiber mat, with fiber diameters on the order of 1000 nm. Catalytic palladium nanocrystals on these structures was achieved via a classical pretreatment processes. Electron microscopy was used to characterize the time evolution of copper particle size as well as distribution. The chemistry as well as crystallography of the fabricated structures were assessed using FT-Infrared Spectroscopy and X-ray Diffraction. Adhesion was measured using a scanning probe nanoindentation tool to perform scratch testing on the metal particle on thin film structure in a constant load scratch test. The adhesion performance as a function of particle size and specific pretreatment is then compared to the delamination of metal particles deposited on the non-woven fiber mats using tensile stretching (by flat punch nanoindentation and mechanical tensile straining). The similarity in the local probe versus the macroscopic behavior allows future adhesion improvements to be quantified using the local probe prior to the more complex non-woven fiber geometry.

A development of fast imaging for most atomic force microscopy (AFM) applications is a current trend of this technique. We illustrate the potential of combining fast imaging and variable temperature studies, which were performed on samples of mesomorphic polymer – poly(diethylsiloxane) – PDES, of semicrystalline polyethylenes of different type and bitumen. Quick Scan in scanning probe microscopes of Keysight Technologies is based on use of the fast-response piezo nose-cone embedded into a regular scanner and high-frequency probes with 1^st flexural resonance around 1.5 MHz. In variable temperature experiments the imaging of surfaces at scales from hundreds of nanometers to a hundred of mm with scanning rates up to 50 Hz allows fast recording of morphological transformations and nanoscale structures in a time of milliseconds. Quick Scan provides not only the substantial time savings in experiments, but also enables recording of structural changes at heating rates of 5-10 degrees/min, which are common for other characterization techniques such as differential scanning calorimetry (DSC), dynamic mechanical analysis, etc. Structural changes can be visualized practically at every temperature degree, and this capability empowers comprehensive material characterization. The data, which were recorded on PDES samples prepared in different ways, have confirmed a coexistence of several crystalline, mesomorphic and isotropic states of this polymer. Crystallization from different mesomorphic states proceeds in different path for samples with various thermal histories. Transition from crystalline to mesomorphic order took place in a broad temperature range compared to crystallization. In rubbed sample mesomorphic order has originated from isotropic state at oriented strips of material serving as the nuclei. Newly grown lamellar aggregates are more extended compared to those formed by rubbing. In variable temperature experiments on polyethylene samples we have obtained novel results regarding crystallization, melting and related structural transformations in samples with different crystallinity and chain order. Particularly, we have monitored crystallization process and recorded kinetic data in study of polyethylene based on ethylene/octene copolymer. These results can be obtained with high time efficiency that is invaluable in industrial research environment. Other technologically important material – bitumen has been examined in the temperature range from +90C to -20C with Quick Scan. These studies have revealed specifics of wax crystallization and a formation of surface wrinkling in different shapes depending on thermal history and cooling rate from melt. This technique has a strong potential for a more thorough examination of composition and thermal behavior of bitumen. The demonstrated successful use of Quick Scan at different temperatures is only one of many areas of AFM applications that strongly benefit from this advanced technique.

Electrospun fibers have the potential to play an important role in fields including filtration, tissue engineering and drug delivery. Applications often require precise fiber mechanical properties and orientations. For example, in tissue engineering cells sense and respond to the stiffness, orientation and size of elements in their environment. Researchers can control the orientation of electrospun fibers by altering the electric field in the region of the collector. By placing two collector parallel to one another, aligned fibers are produced in the region between the collectors. Here we hypothesize that electric field alignment of electrospun nanofibers will cause changes in mechanical properties of the fibers.

Fibers were electrospun from 8% polyethylene oxide (PEO) solution under electric field conditions for alignment and nonalignment. Individual fibers were manipulated by AFM in combination with an inverted microscope to determine mechanical properties such as Young’s modulus and extensibility. The force required to stretch the fiber was found using the Sader method to calculate the torsional constant of the cantilever. The diameters of fibers were determined through AFM and SEM imaging.

We found aligned fibers formed under the same working distance, voltage and polymer solution to be significantly smaller in diameters than nonaligned fibers. Fibers aligned using a parallel plate collector had a diameter of 182 +/- 10 nm (average ± standard deviation of the mean) while nonaligned fibers had a diameter of 262 +/- 14 nm. Consistent with previously published data, the smaller aligned fibers had a higher initial modulus on average than nonaligned fibers, 130 ± 10 MPa and 40 ± 10 MPa, respectively. Previous data showed aligned fibers to be less extensible, with an average strain of 1.4 ± 0.2, than nonaligned fibers, with an average strain of 3.2 ± 0.3.

These results indicate a significant effect on electrospun fiber properties due to changes to the electric field and lead to further questions about the effect of the collector and therefore the electric field on electrospun fiber mechanical and physical properties.

Layered metal thiophosphates have recently been considered in the context of quasi-2D non-linear dielectric and ferroelectric materials, with properties distinct from a more common dichalcogenide family. A specific representative, CuInP₂S₆ (CIPS), exhibits order-disorder ferroelectric transition with T_c around room temperature. Although mobility of Cu⁺¹ ions in CIPS has been inferred from previous measurements, the mechanism of the ionic transport and the accompanying change in material properties are presently unknown.
Here we employed scanning probe microscopy (SPM) to investigate ionic motion in paraelectric CIPS [1]. Above critical electric field, we observe surface expansion well beyond what is expected of dielectric tunability. However, the expansion is almost completely reversible, indicating lack of permanent lattice damage. At 473K, the surface expansion can reach a giant 100 nm, nominally corresponding to over 30% strain, while still remaining reversible. All the measurements were carried out under strictly controlled environmental conditions, ruling out trivial artefacts.

We propose a hierarchical model where surface deformation in applied field proceeds through a sequence of piezoelectric deformation where Cu⁺¹ maintains its intralayer position, Cu⁺¹ hopping into the van-der-Waals gap and finally nucleation and growth of Cu on the surface. The reversibility of this process is tied to the resilience of the CuInP₂S₆ to cationic vacancies, which has been remotely hinted on by the prior studies of intentionally Cu-deficient phases, as well as the existence of a stable Cu-free In_4/3P₂S₆ phase [2].

Within the model, we further developed the technique of electrochemical strain microscopy, an almost decade-old proposal to study ionic motion in solids via the associated strain deformation. Specifically, we directly measured the frequency and temperature dependence of surface expansion, and extracted both activation barrier and diffusivity coefficient for ionic motion.

Altogether, ionic mobility in CuInP₂S₆, when probed by local electric field, translates into giant shape-memory type of response, involving almost completely reversible extraction and reinsertion of Cu atoms in and out of the parent van-der-Waals crystal. CIPS has therefore provided the best yet model system for electrochemical strain microscopy, enabling further insight into how to utilize this technique for nanoscale studies of energy materials.

Research conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. Research partly supported by a research grant from Science Foundation (SFI) under the US-Ireland R&D Partnership Programme Grant Number SFI/14/US/I3113.
References:
[1] Balke et al., submitted (2017)
[2] Susner et al., ACS Nano, 11, (2017), 7060

Despite being one of the oldest phenomena known to mankind and its vast use, there still are open questions about the frictional process between two surfaces. At the nanometer scale for instance, the energy dissipations mechanisms, the influence of the crystalline orientation and the correlation between macroscopic and microscopic scales are still under debate.

Recently, development of 2D materials such as graphene has been widely studied due to its prominent properties and potential applications. Therefore, it is necessary to understand how such materials behave when in contact. Their tribological behavior at the nanoscale may present novel and unexpected features as compared to their bulk counterparts.

In this work we study the interaction between silicon tips and graphene by friction force microscopy. A giant anisotropy according to the crystallographic direction is observed, which is counterinuitive due to the isotropic behavior of its elastic properties. This behavior is associated with a nonlinear mechanism for energy disspation during scanning.

The velocity dependence of friction was also measured. The effective interaction potential between the tip and the surface was observed to vary linearly with the normal force, ranging from ~ 0.5 to ~0.6 eV. A critical velocity related to thermally activated dynamics of the contact atoms have also been determined to vary linearly with the applied load in the range of 1.5 to 2.0 µm/s.

Characterization of lead-free ceramics on the base of sodium-bismuth titanate and sodium-potassium niobate via PFM local probes
E.D. Politova¹, G.M. Kaleva¹, N.V. Golubko¹, A.V. Mosunov¹, N.V. Sadovskaya¹, D.A. Kiselev², A.M. Kislyuk², S. Yu. Stefanovich^1,3, P.K. Panda<sup>4

1</sup>L.Ya.Karpov Institute of Physical Chemistry, Vorontsovo pole str. 10, Moscow 105064 Russia,
²National University of Science and Technology “MISiS”, Leninskii pr. 4, Moscow 119991 Russia,
³Lomonosov Moscow State University, Leninskie gory 1, Moscow 119992 Russia,
⁴National Aerospace Laboratories, Kodihalli, Bangalore-560017 India
E-mail: [email protected]

Lead-free piezoelectric oxide materials are being intensively studied in order to replace widely used Pb-based ones during last ten years. We studied influence of cation substitutions on stoichiometry, structure parameters, dielectric, ferroelectric, and piezoelectric properties of ceramics based on (Na_0..5Bi_0.5)TiO₃ (NBT) and (K_0.5Na_0.5)NbO₃ (KNN) perovskites.
The samples were characterized using the X-ray Diffraction, Scanning Electron Microscopy (SEM), Second Harmonic Generation (SHG), Dielectric Spectroscopy, and Atomic Force Microscopy in Piezorespone Force Microscopy mode (PFM) methods.
Ceramic samples in systems (Na_0.5Bi_0.5)TiO₃ - BaTiO₃ (NBT-BT) and (K_0.5Na_0.5)NbO₃ – BaTiO₃ (KNN-BT) with compositions close to Morphotropic Phase Boundaries (MPB) were prepared by the two-step solid-state reaction method at temperatures of 900 – 1500 K. The samples were additionally modified by small amounts of Li₂O, MnO₂, Ni₂O₃, and Fe₂O₃ (1-5 mol.%) oxides.
Changes in the unit cell volume of the KNN- and NBT-based ceramics observed correlate with cation substitutions. Ferroelectric phase transitions near ~ 400 K and ~ 550 K (NBT) and at ~700 K (KNN) were revealed in the dielectric permittivity versus temperature curves of ceramics studied. Ferroelectric phase transitions near ~ 400 K (NBT) revealed typical relaxor-type behavior confirming presence of polar nanoregions in a nonpolar matrix.
Remnant hysteresis loops were received for separate grains in the samples using switching spectroscopy PFM method. In PFM nucleation of a single domain occurs under a sharp tip, and the PFM signal follows the development of domains at a single location. Local PFM hysteresis loops for the samples studied were observed indicating ferroelectric polarization switching at nanoscale. Increase in the spontaneous polarization value was proved for modified ceramics using the SHG method.
Finally, non monotonous changes of the dielectric parameters ε_rt, tanδ_rt and maximum effective local d₃₃ values were observed in modified BNT- and KNN-based compositions, thus confirming their prospects for new lead-free materials development.

Acknowledgment
The work was supported by the Russian Foundation for Basic Research (Project 16-53-48009).

Electrohydrodynamic jet printing (EHD) has emerged as a competitive technique in the field of additive nanomanufacturing ¹. With the wide range of materials it can accommodate, it has been shown to produce printed features of up to 50 nm in resolution ². Whilst this is remarkable, the printing speed used to achieve this is not. One way to improve this important parameter (for upscaling purposes) is to better control the wetting properties of the substrate being printed on. Their modification is a key step in the quest for high resolution printing. A reliable technique to achieve this is by using self-assembled monolayers (SAMs) to modify the surface energy.
It has been shown that the presence of a SAM can increase the resolution obtained during EHD printing ³. However, there has been insufficient analysis into whether this monolayer survives the printing process. With electric fields of the order of MV/m passing through several nanometres of soft matter, does the dielectric SAM experience breakdown? Previous studies suggest that the monolayer does suffer breakdown ⁴, but this has not been demonstrated with a scanning probe microscopy technique.
In this study, we present our findings to these questions. We use conductive atomic force microscopy (C-AFM) to probe the electrical characteristics of four different SAM layers grown on an indium tin oxide (ITO) surface to determine properties such as conductivity and breakdown threshold. We do this by studying I-V plots over a certain potential window and assessing any deviations from normal Ohmic behaviour of the ITO. We relate the observed results to the physical properties of the SAMs such as chain length and terminal groups. We also use Kelvin Probe Force Microscopy (KPFM) to study the surface potential of SAM coated substrates which have been exposed to high electric fields using EHD. We find that there is a difference in contrast between E-field exposed regions and those not exposed, suggesting that field effects do influence the monolayer. Finally, we suggest ways to preserve the monolayer integrity during the printing process.

References
1. Porter, B. F., Mkhize, N. & Bhaskaran, H. Nanoparticle assembly enabled by EHD-printed monolayers. Microsystems Nanoeng. 3, 17054 (2017).
2. Galliker, P. et al. Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets. Nat. Commun. 3, 890 (2012).
3. Jeong, Y. J. et al. Directly drawn poly(3-hexylthiophene) field-effect transistors by electrohydrodynamic jet printing: Improving performance with surface modification. ACS Appl. Mater. Interfaces 6, 10736–10743 (2014).
4. Haag, R., Rampi, M. A., Holmlin, R. E. & Whitesides, G. M. Electrical Breakdown of Aliphatic and Aromatic Self-Assembled Monolayers Used as Nanometer-Thick Organic Dielectrics Electrical Breakdown of Aliphatic and Aromatic Self-Assembled Monolayers Used as Nanometer-Thick Organic Dielectrics. 7895–7906 (1999).

Exploring both topographical and chemical information down to nanoscale has intrigued broad research attention. Tip-enhanced Raman spectroscopy (TERS) imaging, which combines both the high spatial resolution of Scanning Probe Microscopy (SPM) and the enhanced signal sensitivity of surface-enhanced Raman Spectroscopy, has been demonstrated as a promising technique for the near-field Raman imaging research. The quality of TERS heavily relies on the Raman signals Enhancement Factor (EF) using the metallic tip. Here, we have developed a reliable and low-cost fabrication method to prepare STM-TERS tip. Using a Sharp-Tip Silver Nanowire (ST-AgNW) introduced in our previous work, both high spatial resolution and high EF can be achieved. By putting the ST-AgNW on the platinum-iridium wire with a micromanipulator, the protruding length of ST-AgNW can be precisely controlled and reduced to less than 5 µm, which results in the minimized thermal vibration. Utilizing conical shape at the ST-AgNW apex, atomic scale resolution (~0.1 nm) STM image of Highly Ordered Pyrolytic Graphite (HOPG) has been achieved and multi-channel tunneling current can be avoided. Moreover, TERS performances of the ST-AgNW have been evaluated by approaching the ST-AgNW to monolayer graphene on the 30nm gold substrate. Owing to the small tip radius of our ST-AgNW (~5 nm), a gap mode with small mode volume between the ST-AgNW and the gold substrate can be supported when the incident laser was tightly focused into the tip-substrate gap. Comparing with the regular AgNW tips in other TERS tip preparation methods, a much higher enhancement factor (EF) from our STAgNW tip has been demonstrated by both Raman spectroscopy measurement and finite element analysis simulation.

In bias induced strain measurements such as piezoresponse force microscopy and electrochemical strain microscopy, the contact resonance (CR) of the surface-coupled cantilever-sample system can serve as an amplifier of cantilever motion, providing orders of magnitude improvement in strain sensitivity compared to off-resonance measurements. However, use of the CR can significantly complicate relative and absolute quantification of AC strain because the optical lever sensitivity of the CR differs dramatically from the quasistatic bending shape typically used for optical calibration. Furthermore, the CR optical lever sensitivity varies with the CR frequency, which itself is a function of sample modulus, sample curvature, and contact area, all of which can vary spatially over a scan area. The net result is that a response that appears as contrast in AC strain amplitude can be predominantly associated with a change in the mechanical boundary conditions of the probe, rather than actual electromechanical function. Recently, the concept of a shape factor to relate the CR amplitude to the actual tip motion has been developed by Balke et al. through use of an Euler-Bernoulli beam model. However, epistemic uncertainties between the beam model and the experiment may still result in unacceptable errors. Here, we introduce an experimental method with a calibrated reference artifact to calculate the shape factor across the range of relevant boundary conditions that the unknown sample may exhibit. The reference artifact is a high-frequency (fundamental resonance >2 MHz) ultrafast cantilever affixed to an ultrasound transducer. The base motion of the cantilever is quantitatively calibrated using a Michelson interferometer while the transducer is driven at frequencies encompassing the CR frequencies of the unknown cantilever-sample system. When an unknown cantilever with precisely placed detection-laser is brought into contact with the reference cantilever, the resultant contact resonance is monotonically related to the position of the unknown cantilever’s tip on the reference cantilever beam (i.e. positions closer to the free-end result in lower stiffness and lower CR frequency, closer to the base result in higher stiffness and higher frequency). Here, we show that the relation between a known sample-motion drive-force and the CR-amplitude (i.e. the CR shape factor) can be continuously measured. We then apply the calibrated cantilever to measure quantitative surface strain on a piezoelectric material.

Atomic Force Microscopy (AFM) is widely used for nanoscale characterization of materials by scientists worldwide. The long held believe of AFM, is that the tip is generally chemically inert but can be functionalized in respect to the studied sample. This implies that basic imaging and scanning procedures do not affect surface and bulk chemistry of the studied sample. However, an in-depth study of the confined chemical processes taking place at the tip surface junction, and the associated chemical changes to the material surface have as of now been missing.
Here, we used a hybrid system that combines Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) with an AFM to investigate the chemical interactions that take place at the tip-surface junction. Investigations showed that even basic contact mode AFM scanning is able to modify the surface of the studied sample. In particular, we found that AFM tips are heavily contaminated by the silicone oils, which were determined to come from standard gel-boxes used for the storage and handling of the tips. These oils were deposited from the AFM tip into the scanned regions during the scanning process, which modifies surface state of the studied sample and can significantly affect results of the surface investigations performed by the AFM. Furthermore, we found that silicone oils can be spread to distances exceeding 15 microns from the static tip.
These results demonstrate that chemical phenomena have to be taken into the consideration for interpreting and understanding results of AFM mechanical and electrical studies relying on state of the tip-surface junction.
This work was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility.

The two-dimensional (2D) materials have multiple applications in optoelectronics, energy storage, gas- and bio-sensors, and photocatalysis and solar energy conversion, to mention a few. In particular, the atomically layered transition metal dichalcogenides (TMDCs), such as WS₂ and WSe₂ are promising candidates due to their unique electronic and optical properties, ease of manufacturing,
mechanical robustness, low toxicity, as well as composed of relatively abundant elements on Earth . For this research, we study WSe₂ and WS₂ samples growth individually or simultaneously by the Chemical Vapour Deposition (CVD) on the Si or SiO_x surfaces. These materials form complex trigonal and hexagonal faceted structures formed by the individual layers of material, typically with a screw dislocation in the centre of the crystallites defining the growth process .
To identify dislocations and faults between several stacked WSe₂ and WS₂ layers we have used nanomechanical mapping in SPM by combining the Atomic Force Microscopy (AFM) with the ultrasonic vibration – namely, the Ultrasonic Force (UFM) and the Heterodyne Force (HFM) Microscopies. In UFM/HFM amplitude modulated ultrasonic vibrations at frequency up to 10 MHz are applied to the sample resulting in its displacement of few nm normal to its surface. The AFM tip contacting the sample then produces dynamic force at the oscillation frequency that propagates to the inner layers of the 2D materials. The hidden subsurface features such as dislocations and stacking faults have a compressibility that differs from one of the perfect sample that, in turn, modifies the dynamic mechanical impedance sensed by the AFM tip. This is detected as cantilever deflection at the modulation frequency, thanks to the nonlinearity of the tip-surface interaction reflecting the hidden structure of the 2D material [3]. If both tip and sample are vibrated at the adjacent frequencies without amplitude modulation (as in the HFM setup [4]), the amplitude of the nonlinear response reflects the subsurface nanomechanical elastic moduli modulation, whereas the phase detects the dynamic relaxation processes in nanometre volumes with a time-sensitivity of few nanoseconds.
The UFM and HFM study of WS₂ and WSe₂ materials revealed a clear contrast in areas, some of these linked with the topographical features, and some likely to reflect subsurface dislocations and stacking faults. Alternative reasons for the nanomechanical contrast of hidden features such as the misorientation of the crystallographic axis of layers and crystal-surface interaction and the demonstrated link between the screw and misfit dislocations are discussed.
[1] Eftekhari A, J. Mater. Chem. A, 5 (2017) 18299-18325.
[2] Shearer MJ et al., Journal of the American Chemical Society, 139 (2017) 3496-3504.
[3] Dinelli F et al., Nanotechnology, 2017, 28(8):085706.
[4] Cuberes MT et al., Journal of Physics D-Applied Physics, 33 (2000) 2347-2355.

Development of novel high frequency Si, Si₃N₄, and graphene based micro-and nano-electromechanical systems (MEMS and NEMS) requires suitable characterization methods with nanoscale spatial resolution, high frequency (HF) response and high sensitivity. As spatial resolution of existing methods such as Laser Doppler Vibrometry (LDV) is limited by the light wavelength to the micrometre scale [1], it is tempting to use atomic force microscope (AFM) techniques offering nanoscale resolution. Here we use AFM to analyse the vibrations of nanoscale thin membranes over the frequency range from kHz to several MHz using both linear and nonlinear mechanisms for their excitation and detection.
Our model system is a Si₃N₄ membrane (200 nm thickness, 500x500 um², Agar Scientific) on a Si substrate. The AFM (Multimode, Nanoscope 8, Bruker) was modified with a piezoceramic transducer driven by the function generator to excite sample vibrations from kHz to about 10 MHz, with the resulting cantilever deflection detected by a standard lock-in-amplifier. The reference optical vibrometry (OFV-2670 and UHF-120, Polytec) found the membrane fundamental vibrational mode at ~250 KHz suggesting it to be under high tensile stress.
The core idea of our study was to explore the possibility of detection HF membrane vibrations via AFM and effect of the probing tip contact on the resonance frequency. We used three AFM modes: 1) Force Modulation Microscopy (FMM) with tip vibrations detected at the excitation frequency, 2) nonlinear off-resonance regime where HF sample vibration is modulated at low frequency, and cantilever response measured at the modulation frequency (Ultrasonic Force Microscopy, UFM [2]), 3) UFM resonance regime, where the modulation frequency was around the membrane resonance (M-UFM).
While the edge of a membrane was not detectable via topography, it was clearly visible in all ultrasonic modes. FMM mapping at swept excitation frequency showed that the cantilever-tip loading of the membrane increasingly shifted the resonance frequency down as the tip moved towards the centre, with the maximum response reached at a certain distance from the edge, suggesting an optimum position for the detection of vibrations. In M-UFM mode we found that the membrane resonance was also detectable, even though there was no resonance frequency component in the driving oscillation spectrum. We attributed this to the nonlinear nature of the tip-membrane interaction that produced the localised force at the resonance modulation frequency. This study shows that ultrasonic AFM modes will allow the exploration of the vibration of MEMS/NEMS structures of sub-um dimensions including 2D materials based NEMS.
[1] Gates RS, Pratt JR, Nanotechnology, 2012, 23(37).
[2] Bosse JL et al., Journal of Applied Physics, 2014, 115(14):144304.

We report on the systematic investigation by Atomic Force Microscopy of the role of surface nanoscale roughness and morphology on the charging behaviour of nanostructured zirconium dioxide (ZrOx, x≤2) surfaces in aqueous solutions, and in particular on the influence of nanoscale morphology on the isoelectric point of these surfaces. By using supersonic cluster beam deposition to fabricate nanostructured zirconia films, we achieved a quantitative control over the surface morphological properties of ns-ZrOx films, and in particular we were able to control the root-mean square roughness across a wide range of values matching those of other characteristic lengths of electrostatic double-layer interactions in electrolytic solutions, such as the Debye length, and the typical size of nano-colloids (proteins, enzymes, nano-catalysts). We have characterized by direct AFM measurements the interaction forces between rough zirconia surfaces and a micrometer-sized spherical silica probe in NaCl aqueous electrolyte. A method of incorporating surface roughness into theoretical calculations of hydrodynamic and DLVO forces is presented, where the surface roughness is modeled by a Gaussian distribution of surface heights. We performed a systematic exploration of the electrical double layer properties in different interaction regimes characterized by different ratios of characteristic nanoscale lengths of the system: the surface rms roughness R_q, the correlation length ξ and the Debye length λ_D. We observed a remarkable reduction by more than one pH unit of the Isoelectric Point (IEP) on very rough nanostructured surfaces (Rq∼26nm), with respect to the flat amorphous ZrOx. A possible explanation for the behavior of IEP, previously characterized also for titanium dioxide nanostructured thin films¹, is the roughness-induced self-overlap of the electrical double layers, as a potential source of deviation from the trend expected for flat surfaces.

¹ F. Borghi, V. Vyas, A. Podestà, and P. Milani, PLoS ONE 8, e68655 (2013).

Studying light-matter interactions at the molecular level is critical to accelerate our understanding of life sciences. Chemical speciation has successfully been impeded to Atomic Force Microscopy (AFM) measurements by exciting the material with infrared light and using the AFM cantilever to monitor the photothermal expansion resulting from the vibrational modes excited in the sample. With subwavelength spatial resolution, it is expected that single molecule fingerprints are attainable. Previous work shows that it is possible to design plasmonic substrate to locally enhance the electromagnetic field used to excite the molecules for higher sensitivity.
Here we use plasmonic substrate to polarize the field used to excite the molecular vibration. Our study focuses in circular dichroism at nanoscale for chiral biomolecules. Using cavity-coupled achiral and planer plasmonic structures, we show that it is possible to generate a stronger confined electromagnetic field for our nanoscale infrared spectroscopy platform for wavelengths of 1500-1800cm^-1. Chiral biomolecules with absorption and vibrational circular dichroism signature overlapping with the absorption of the cavity-coupled plasmonic structure are identified for the measurements. The sample is excited by both the linearly and polarized tunable IR pulsed laser. For circular polarization we acquire the successive for both the left-handed and right- handed circular polarizations. Upon absorption, energy is transferred to the lattice and heat is generated, leading to thermal expansion. Molecules laying in the confined field at the plasmonic structures show stronger circular dichroism signal. By using this approach, we are able to detect chirality of biomolecules at nanoscale. The results suggest that using the “hot spots” of the cavity coupled plasmonic structures are significant and offer great potential for characterizing the chirality of single molecule.

The organization of eukaryotic DNA into nucleosomes and chromatin involves dynamic structural changes. These dynamics are also relevant to regulate genome processing, including transcription, replication, and DNA repair. However, there is a lack of methodologies that probe structure and structural changes over mesoscopic (10-100nm) length scales within chromatin. We have designed, constructed, and implemented a DNA-based nanocaliper that probes this mesoscopic length scale to address these challenges. Our nanocalipers consist of 2 70nm rigid arms each made up of 18 helices of double stranded DNA (dsDNA), connected by 6 sets of single stranded DNA (ssDNA) at one end. The nanocaliper has the appearance of a hinge joint. The nanocaliper design and its ability to fluctuate between 0-120^o allows for end-to-end distance measurements on the tens of nanometers length scale.

We developed an approach for integrating nucleosomes into our nanocaliper at two attachment points with over 50% efficiency. We focused on attaching a strand of DNA containing a nucleosome consensus sequence to the ends of the two nanocaliper arms, so the hinge angle acts as a readout of nucleosome end-to-end distance. We found that the nanocaliper angle is a sensitive measure of nucleosome disassembly and can read out transcription factor binding to its target site within the nucleosome. Interestingly, the nanocaliper not only detects transcription activator Gal4-VP16 binding but also significantly increases the probability of Gal4-VP16 occupancy at its respective site by contributing to nucleosome unwrapping. This suggests that our DNA nanocalipers can serve as a tool to readout biologically relevant conformational changes and to manipulate nucleosomes to test their function under different physical constraints.

We have also developed a model using this data to characterize how individual nucleosomes adopt unwrapped conformations. We find that this model demonstrates concomitant nucleosome unwrapping and is further validated by hexasome unwrapping experiments. We can use this model to study trends in the dynamics of nucleosomes, chromatin, and regulatory processes involving nucleosome unwrapping.

This project provides a foundation for future mesoscale studies of nucleosome and chromatin structural dynamics, an area that has been largely unexplored. More broadly, mesoscale studies of nucleosomes provide insight into molecular events that lead to misregulated oncogenes and tumorigenesis. The nanocaliper construct can be further scaled up to probe larger biomolecular complexes such as nucleosome arrays. Current work focuses on developing a caliper variation that is more compatible for nucleosome arrays in physiological conditions. Ultimately, these tools could be implemented inside cells to directly monitor or manipulate local chromatin structure and dynamics in single living cells.

Techniques to probe strength of materials at small scales have been established for over a decade but fracture toughness experiments are exclusively standardized for the macroscale. Notched cantilever and nanoindentation based methods have recently been proposed for performing fracture experiments on microscale samples. Asymmetric loading conditions around the crack tip and assumptions about the sample microstructure in these experiments limit their viability, particularly for multi-scale heterogeneous materials such as bone, a composite primarily comprised of collagen fibrils and bioaptite at the nanoscale.
We developed a methodology that enables conducting 3-point bending beam fracture experiments on micron-sized samples with free boundary conditions and notches or fatigued pre-cracks on a variety of different materials that were fabricated using traditional nanofabrication methods and an in situ SEM/nanoindenter. We first extracted beams of roughly 50x10x5 µm from single crystalline silicon using a focused ion beam (FIB) and measure a microscale K_1c of 0.98 MPa m^1/2, in agreement with the accepted value of 1 MPa m^1/2 . We performed these experiments on fused silica and an acrylate reporting K_1c values of 1.33 and 0.82 MPa m^1/2 , respectively. We employ the technique to perform site-specific fracture experiments on similarly-sized dry bone beams, fabricated with collagen fibrils oriented along the length of the beam and orthogonally to the loading direction. We calculate the J integral and report a toughness of 45 J/m^2 over ~2.5 µm of stable crack growth. We discuss these results in the context of the rising R curve of bone and compare observed toughening mechanisms to those reported in similar macroscale experiments in literature.

Inspired by nature one of the most promising and challenging approaches is to synthesize nanocomposites, which consist of a combination of soft organic and hard ceramic materials.^1-3 Iron oxide such as magnetite nanoparticles proved to be very suitable for the synthesis of nanocomposite, because nature makes use of iron oxide in chiton radular teeth. Indeed, chiton teeth are one of the hardest and toughest materials known in nature.⁴
Furthermore, iron oxide nanoparticles are being extensively studied with regard to their size and shape-controlled synthesis. Preparation methods for nanocomposites demands for a superior quality of the NPs. In particular, properties like size, monodispersity, crystal structure and shape are of paramount importance. As the most prominent examples we synthesize monodisperse magnetite nanoparticles as spheres, rods, cubes and octopods.
We present the successful manufacturing of a nanocomposite consisting of oleic acid coated spherical iron oxide nanoparticle with exceptional isotropic mechanical properties.⁵ We developed an easy concept to link iron oxide nanoparticles in a well-ordered superstructure by oleic acid molecules during a thermal process. The synthesis process is divided into four stages: sedimentation, drying, pressing and heat treatment.
Nanoindentation in the nanocomposite shows an increase of the elastic modulus and hardness with increasing annealing temperature. Despite this compaction and heat treatment, HRTEM and SAXS investigations of milled material clearly show that each particle is still isolated from adjacent particles by an organic layer. XPS measurements of the material provide further evidence for thermally induced chemical changes in the iron oxide/oleic acid system and in addition we exclude a significant fraction of oleic acid graphitisation up to 350 °C.
The exceptional mechanical properties - bending modulus of 114GPa, hardness of up to 4GPa and strength of up to 630MPa - are dominated by the covalent backbone of the linked oleic acid molecules. To our knowledge these are the highest combined values of elastic modulus, strength and nanohardness ever reported for a synthetic bioinspired organic/inorganic nanocomposite.
1. Fratzl, P. & Weinkamer, R. Nature’s hierarchical materials. Prog. Mater. Sci. 52, 1263–1334 (2007).
2. Sellinger, A. et al. Continuous self-assembly of organic-inorganic nanocomposite coatings that mimic nacre. Nature 394, 256–260 (1998).
3. Tang, Z., Kotov, N. a, Magonov, S. & Ozturk, B. Nanostructured artificial nacre. Nat. Mater. 2, 413–418 (2003).
4. Weaver, J. C. et al. Analysis of an ultra hard magnetic biomineral in chiton radular teeth. Mater. Today 13, 42–52 (2010).
5. Dreyer, A. et al. Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties. Nat. Mater. 15, 522–528 (2016).

A central idea in electron transfer theories is the coupling of the electronic state of a molecule to its structure. Previous studies used mechanical forces to induce structural and chemical changes in large biological molecules, but mechanically control of the electron transfer reaction of a small molecule without bond rupture, and determining the relationship between force and distribution of redox equilibria have not been reported.
Here we show experimentally that fine changes to molecular structures by mechanically stretching a single metal complex molecule via changing the metal-ligand bond length can shift its electronic energy levels and predictably guide electron transfer reactions, leading to the changes in redox state. We monitor the redox state of the molecule by tracking its characteristic conductance, determine the shift in the redox potential due to mechanical stretching of the metal-ligand bond, and perform model calculations to provide insights into the observations. The work demonstrates that a mechanical force can prompt the molecule into a geometry that favors the electron transfer reaction, shift its redox potential, change its redox state, and thus allow the manipulation of single molecule conductance. This observation reveals experimentally the role of electronic-nuclear coupling in the electron transfer reactions at the single molecule level, and its potential application in mechanically controllable molecular switches.

The presentation describes development of a model to predict the macroscopic thermal resistance of mechanically contacting surfaces — a capability that is important for modeling thermal management in many industrial processes. Contacting interfaces are fractally rough, with small islands of locally intimate contact separated by regions with a wider gas filled boundary gap. Heat flow across the interface is therefore heterogeneous and thus the contact model is based on a network of thermal resistors representing boundary resistance at local contacts and the Sharvin resistance for transport laterally to contacts. In this work, we present the results of molecular dynamics simulations to characterize boundary resistance of silicon alumina interfaces, testing the sensitivity of thermal resistance to contact opening, and the influence of adsorbent layers. In addition, we report on Boltzmann transport simulations of Sharvin resistance in Si in the ballistic transport regime.

Ferroelectricity in ultrathin crystal has been long believed to exhibit rich phase competition physics with quantum confinement and enhanced quasiparticle interactions¹. Its realization is also critical to scale down memory and develop versatile ferroelectric devices with large electrical and mechanical tunablity². However, its own depolarizing electrostatic field prevents the existence of out-of-plane ferroelectricity at two-dimensional (2D) limit. Here we report the discovery of out-of-plane 2D ferroelectricity in atomically thin In₂Se₃ crystal, which was theoretically predicted recently³. We experimentally found that in-plane lattice asymmetry and out-of-plane polarization is strictly locked, a new mechanism to stabilize the polar order. Such unique locking enables robust 2D ferroelectricity at ambient conditions and results in a very high transition temperature (~700 K). In addition, it also enables electrical manipulation of atomic lattice anisotropy, which is the key to 2D spintronics and valleytronics. This discovery is potentially important to the atomically thin sensors and actuators, and ultrahigh density nonvolatile memory devices.


1. Dawber, M., Rabe, K. M. & Scott, J. F. Physics of thin-film ferroelectric oxides. Rev. Mod. Phys. 77, 1083–1130 (2005).
2. Martin, L. W. & Rappe, A. M. Thin-film ferroelectric materials and their applications. Nat. Rev. Mater. 2, 16087 (2016).
3. Ding, W. et al. Prediction of intrinsic two-dimensional ferroelectrics in In2Se3 and other III2-VI3 van der Waals materials. Nat. Commun. 8, 14956 (2017).

Multifunctional oxides like BiFeO₃ are extensively studied for their properties and functionalities. In this study, the growth of (20nm) fully strained tetragonal (T), rhombohedral (R) and a mixed phase of T and R of Bismuth ferrite (BiFeO₃) was achieved by varying the thickness of the template layer. The X-ray diffraction and ferroelectric/piezoelectric characterization revealed improved piezoelectric property in case of the sample with the mixed phase of BFO. An improved piezoelectric constant (d₃₃) of ~45 pm/V is observed in the mixed phase sample compared with the fully strained tetragonal (~12 pm/V) and completely relaxed rhombohedral phase (~16 pm/V) grown on LAO substrate for the given thickness. Further, a tip-induced domain switching was performed along with the ferroelectric domain nucleation and growth. The nucleation of the domains begins from 4V DC bias applied through the tip. The ferroelectric domains study shows the 90^o domains and 180^o domains are present in the fully strained tetragonal sample with a dominant Out-of-Plane response. In case of the mixed phase, the in-plane response is also present with 180^o, 90^o, 71^o, and 109^o domain walls. The fully relaxed film has dominant 71^o and 109^o domain walls. This study is relevant in analyzing the domain dynamics when these number of domain walls are present together. Further, the conductive AFM studies are performed to reveal the role of domain boundaries in conductivity of the film.

The study of mechanical properties in porous materials is challenging. The inhomogeneous distribution of porous with sizes ranging from micro to nanoscale strongly influences the measured mechanical properties. For porous materials, the mechanical properties are then scale dependent and difficult to correlate with macroscale measurements. The atomic force microscope can be used to measure materials surfaces with high spatial resolution allowing the identification of pores and other surface features. The microscope can also be used to measure local mechanical properties on materials surfaces.
In this work, we present the use of the AFM (Nx-10, Park Systems) in the force modulation and acoustic modes to characterize the surface of samples with a wide distribution of pores, with diameters ranging from ~ 5 nm to tens of micrometers. Mapping of the elastic modulus were performed with the use of an external acoustic piezoelectric ceramic between the microscope XY scanner and the sample. The acoustic piezo was dithered by an external function generator with frequencies ranging from 300 kHz to 5MHz while the microscope tip movement was measured with an external lock-in amplifier. Rectangular silicon cantilevers with resonances of ~ 145 kHz and tips with radius ranging from ~ 10 nm to ~ 200 nm were used. The elastic modulus images were then correlated with the materials topography and pore distribution, confirming the high heterogeneity of the samples and its influences on the mechanical properties. The quantitative determination of the elastic modulus, at specific surface sites, was also obtained by measuring the first and second resonance frequencies of the AFM cantilever tip while in contact with the surface. The mechanical properties of the samples were additionally measured by nanoindentation (triboscope, Hysiotron). The elastic modulus measured by AFM was then compared with the values obtained using nanoindenters.

Electrostatic force microscopy EFM, that employs a biased conducting AFM tip, has been used to quantify dielectric constant, surface potential and surface charges of various nanomaterials including Graphene and Carbon nanotubes. It has also been employed for sub-surface imaging of carbon nanotubes embedded and dispersed inside a polymeric thin film.
There are several experimental approaches used in EFM. The most widely used one requires using a “small” oscillation amplitude so that the tip-sample electrostatic force can be linearized. In this approach, the gradient of the electrical force is measured directly from the phase-shift. This approaches, while simple, require a numerical and an experimental validation. Moreover, the use of the small amplitude relatively far from the sample surface (> 20 nm) deteriorates the resolution of the technique.
In this paper, we introduce an algorithm for the electrostatic force reconstruction in amplitude modulation “tapping mode” spectroscopy. The method is based on the recoding amplitude and phase-shift versus tip-sample distance while applying a bias. These two observables (amplitude and phase) are used to calculate the virial of the tip-sample interaction which is used to reconstruct the electric force. The proposed algorithm is verified by numerical simulation and works for an arbitrary amplitude of oscillation. The model is used to extract the long-range electrostatic force versus distance curves on carbon nanotubes. The model is compared to the force-gradient model (linearization) and thus for various “practical” amplitude of oscillation. Finally, the experimental force curves are used to calculate the surface potential as well as a dielectric response of the carbon nanotubes.

The electronic properties of carbon nanotubes (CNTs) play an important role in their electrochemical performance for energy storage devices such as supercapacitors and batteries. Information such as charge transport, charge density and band energy diagram are required to fully understand electronic behavior of CNTs in electrochemical devices. This study investigated the charge transport in carbon nanotube thin films using conductive atomic force microscopy (C-AFM) and scanning Kelvin probe microscopy (SKPM). Freestanding single-wall carbon nanotube (SWCNT) films of about 1 um in thickness were prepared using a simple casting method. Transmission electron microscope images and Raman analysis indicate very low amount of amorphous carbon (<1%) and majority of the tubes are metallic type. Specific capacitance of SWCNT supercapacitors is ~ 8mF/cm² or 276 F/g, which is comparable with prior reports. C-AFM data reveal that resistance of bundles of SWCNT is in the range of several MW. The CNT films switch from resistance behavior to capacitance behavior when about 35% volume of it becomes conductive. SKPM images show that work function of CNTs depends on diameter of CNT bundles, meaning the size of CNT bundles regulates the charge and discharge performance of supercapacitors.

Heat management has always been a key factor in the development of novel technologies – from cooling rocket engine nozzles to heat dissipation in today’s computer chips. To help these developments, it is tempting to use the highly heat conductive one- and two-dimensional (1D and 2D) materials, such as carbon nanotubes (CNT), and, more recently, graphene and hexagonal boron nitride material composed of atomically thin Van-der-Waals bonded layers. However, the integration of these materials in the real-life applications is rather challenging and requires novel approaches, in particular, to merge a nanoscale nature of these materials with their application in a real-world macroscale device – in, so-called, thermal interface materials (TIM). Novel measurements approaches are needed to explore the TIM physical properties such as heat conductivity of their components and thermal resistance of the structural interfaces in structures that are often buried within the sample volume.

In our work, we aimed to resolve these by using Scanning Thermal Microscopy (SThM) enhanced with dedicated sample preparation techniques, namely a unique cross-sectional tool (Beam-Exit Cross-section Polishing, BEXP), used to prepare a nanoscale flat tapered section of a material with a shallow angle (<10°). For TIM consisting of CNT brush attached to a Si substrate, BEXP creates a “nanoforest” of vertically aligned carbon nanotubes whose lengths increase from less than 10 nm to more than 1 mm. SThM then measures the thermal resistance of this resulting shaved “nanoforest” by scanning a nanoscale sharp tip integrated with a heater and a temperature sensor as a function of the CNT length. These measurements complemented with an analytical modelling allowed to extract thermal conductivities of in-plane and out-of-plane of CNT “nanoforest”, as a result gaining significant insights into the nature of the TIM sample thermal anisotropy. By using this approach to fully structured CNT TIM’s in combination with focused-ion beam, we were able to map the thermal conductance of the individual materials of the structure. Finally, by applying a temperature gradient across the TIM we measured the temperature drop from the hot to cold side.

In summary, this work demonstrates the usefulness of nanoscale probe microscopy for TIM engineering, allowing to estimate anisotropy of thermal resistance of CNT bundles and interfacial thermal conductance, providing working solutions for the research and development of new generation of heat management components.

As the downscaling of electronic devices pushes dimensions of its components towards the atomic limits, new measurement tools need to be developed to address new challenges. In particular, nanoscale heat transfer is a key mechanism which is known to limit the performance of nanoscale sized transistors in the processor chips and light emitting diodes and to define the performance of thermoelectric materials. In most cases the thermal properties of materials and devices defining their performance are to be investigated through all their three-dimensional (3D) structure (e.g. interconnects and transistor layers in the processor chips) and grain interfaces (e.g. in thermoelectric compounds) with the interfaces and layer thickness’ down to few nanometres. Furthermore, thermal properties at nanoscale lengths present many challenges to traditional techniques, e.g. thermal conductivity of films thinner than 100 nm is affected by surface scattering and mean-free path of the heat carriers (phonons and electrons) and often cannot be directly decoupled from interfacial resistance between the film and the substrate. In this report, we develop a novel approach addressing these challenges.

Here we report combination of an Ar-ion cross-sectional tool ideally suitable for the scanning microscopy (SPM) measurements - beam-exit cross-sectional polishing (BEXP) and a heated probe of scanning thermal microscopy (SThM) to measure thermal transport of buried layers of a wide range of materials. BEXP uses Ar-ion beams impinging on a sample at shallow angle (<10^o). The cross-sectioned surface obtained has low-angle geometry and sub-nm surface roughness making it easily suitable for studies via standard SPM methods. We then use SThM to raster a thermosensitive probe on a surface quantifying probe-sample thermal resistance. By analysing the SThM signal of the wedge-shaped section of the probed material, we are able to extract the intrinsic thermal conductivity of the nanoscale thin layer material separating it from the probe-sample and sample-substrate thermal resistances by combining analytical and finite element modelling of the heat transport in the sample.

To validate our approach, we apply this method on different commonly used materials from semiconductors to insulators such as silicon dioxide, spin-on-glass and spin-on-polymers. Then we investigate multi-layered samples for optoelectronic applications such as MBE grown SiGe alloys and map the thermal conductance across the sample volume. Our results demonstrate its applicability for direct measurements of otherwise hard to obtain quantities for previously unknown materials. The ease of use of our method renders it suitable for a broad range of samples and opens new paths for fundamental and applied research.

Electric Double Layer Capacitors (EDLCs) have received increasing attention during the past two decades since they fill the gap between batteries and conventional capacitors in the Ragone plot. During the charging process of EDLCs, owning to the electrostatic attraction, the electrolyte ions and the charges at the surface of electrode materials will form the electric double layer (EDL). The main challenge today for EDLCs is to increase their energy density without sacrificing their high power nature. Huge amount of effort has been devoted into developing room temperature ionic liquids (RTILs) as EDLC electrolyte materials since they provide a wide electrochemical window (> 5V) thus an efficient energy density enhancement. Understanding the EDL structure at the electrode/electrolyte interface and its dynamics holds the key to improve EDLCs. Of special interest is information on nanometer length scales which would allow us to find correlations between EDL structures, electrode microstructure and the overall electrochemical performance. Scanning Probe Microscopy (SPM) techniques are well suited to characterize electrode/electrolyte interfacial properties due to their high lateral and vertical resolution.

In this work, out-of-plane and in-plane double layer structure of ionic liquid electrolytes on the highly oriented pyrolytic graphite (HOPG) electrode can be directly visualized by performing force-distance curves and topographic imaging. The effect of electrolyte ion size/shape and electrode bias on the ionic liquid EDL structure can be observed. The possibility of applying this methodology to aqueous electrolytes for other applications or moving further to more advanced techniques (ex. band excitation) to study electrode/electrolyte interface will be discussed.

The work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed 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.

Continuing miniaturization of portable electronic and mechanical devices requires development of the novel nanoscale elements. This can be realized using functional materials, such as ferroelectrics. Ferroelectrics exhibit a unique set of the properties, including piezoelectricity, high non-linear optical activity, pyroelectricity and others, and can be used in a wide range of practical applications from non-linear optical converters to data storage and processing devices. However, functionality of ferroelectric materials is defined on the nanometer level, which requires using the nanoscale investigative techniques. Atomic Force Microscopy (AFM) is one of such paradigmatic tools allowing comprehensive investigations of the functional ferroelectric properties. However, the capabilities of AFM are limited in terms of characterization of chemical properties. The introduction of other, concomitant technique into the measurement setup can provide a solution to this shortcoming and enable correlated investigations of the functional properties along with the chemical information.
Here we used multimodal functional and chemical imaging based on a combination of AFM with time-of-flight secondary ion mass spectrometry (ToF-SIMS) for comprehensive characterization of the ferroelectric crystals and thin films. These studies demonstrated significant role of the surface and bulk chemistry on the polarization reversal, which is important for the understanding of fundamental principles of ferroelectrics and their practical applications.
This work was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility.

Organic and organic-inorganic hybrid piezo/ferro-electric materials have been burgeoning recently for their low densities, high flexibilities, and solution processibilities. Despite the successive observations of the physical piezo/ferro-electric phenomena in some organic and organic-inorganic materials, this prospect is still not good-fetched yet. The goal of this work is to unravel the structural functionalities within natural biomacromolecules and artificially formed metal-organic frameworks (MOFs) and to offer a strategy for constructing promising supramolecular piezo/ferro-electric materials. On one hand, our nanoscale probing as well as theoretical calculations indicate that the piezoresponses of collagen fibrils in wild mice bone is larger than those in genetic modified osteogenesis imperfecta murine (OIM) mice bone via delicate dual AC resonance tracking piezoresponse force microscopy (DART-PFM) characterizations and molecular dynamics simulations respectively. We thereby describe the better piezoelectricity by forming heterotrimeric peptide chains comprising polar residues within the oriented structure of collagen biomacromolecules. On the other hand, we have experimentally found that both the NUS-6 and UiO-66 MOFs nanocrystals demonstrate certain piezoelectricity. Besides, the Hafnium (Hf)-based MOFs show better ferroelectricity than their counterparts zirconium (Zr)-based MOFs nanocrystals. These findings evident the low symmetric lattice structure in real NUS-6 and UiO-66 MOFs. To unravel the underlying mechanism, first principles density functional theory (DFT) calculations have been performed on the UiO-66 MOFs with FCC crystal structures (F-43m, 216). It is found that the Born effective charges as well as the polarization changes of atoms in UiO-66 (Hf) are larger than those in the UiO-66 (Zr). Herein, we propose the ferroelectricity of MOFs could be tunable by judicious selection of metal ions to form more polar coordination bond and capable of forming electronic structure in asymmetric lattice. Through both experimental and theoretical simulation, we demonstrate the piezo-/ferro-electricity phenomena in supramolecular materials.

Due to its centrosymmetric crystal structure, graphene is intrinsically non-piezoelectric. However, piezoelectricity can be engineered into graphene through symmetry-breaking strategies. Here, we will report on the piezoelectric contrast, measured with piezoresponse force microscopy (PFM), between regions with different graphene layer numbers on 6H-SiC (0001). Additional experiments on oxygen-intercalated, freestanding graphene and on tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) doped graphene on SiC, indicate that the observed piezoelectric contrast arises from the interfacial dipole moment across the van der Waals (vdW)-bonded graphene and the partially -bonded zerolayer graphene. The interfacial dipole moment is due to the spontaneous polarization of the bulk SiC substrate, which breaks the crystal symmetry in the out-of-plane direction. Our findings demonstrate a new way to turn graphene into a piezoelectric material by manipulating the graphene-substrate interface, which opens new potentials for graphene in piezoelectric material-related applications.

As the power density increases in ever shrinking electronics, high research efforts are focusing on transforming this heat into electricity with the use of high figure of merit thermoelectric materials and exploiting thermoelectric phenomena is data storage such as in phase change memories. While two-dimensional and Van-der-Waals bound materials are progressively being considered for their ground-breaking physical properties, nanoscale measurement methods of thermoelectric systems are lagging behind, hindering the understanding of local thermoelectric phenomena on nanoscale. In this report, we present a novel microscopy technique, Scanning Thermal Gate Microscopy (SThGM) which enables mapping Seebeck coefficient with nanoscale (20-50 nm) spatial resolution.

Our method is based on Scanning thermal microscopy (SThM) which is an AFM technique using a heated sensor tip to locally probe thermal conductance of a sample. SThGM uses the nanoscale heating capabilities of the SThM tip in order to create a local temperature rise with sharp gradient in a probed device. This gradient creates a thermovoltage which can be directly measured at the device terminals and that is proportional to the Seebeck coefficient of the material under the tip and the temperature gradient created. As the nanoscale temperature distribution created by the SThM is well characterised, the SThGM allows to provide both the nanoscale map of the Seebeck coefficient distribution and quantitatively evaluate its magnitude.

The results of nanoscale thermoelectric maps of three different samples: standard thermoelectric thin films, 2D materials heterostructures and graphene nanoconstrictions are discussed. SThGM allows to explore novel thermoelectric phenomena and to open possibilities for investigations and development of more efficient electronic and thermoelectric devices.

The advent of nanoelectromechanical systems is leading to the requirement to quantitatively measure the piezeoelectric coefficients of thin films on a small spatial scale. This paper contrasts the use of three different approaches for local measurements: spatially mapped interferometry measurements, nanobeam synchrotron diffraction measurements, and piezoresponse force microscopy. A single beam laser interferometer based on a modified Mirau detection scheme with a vertical resolution of about 5 pm was developed for localized d_33,f measurements on patterned piezoelectric films. The tool provides high spatial resolution (~ 2 microns), essential for understanding scaling and processing effects in piezoelectric materials. This method can capture both intrinsic and extrinsic contributions to the displacement, and has excellent accuracy providing flexure-induced artifacts associated with contact structures are avoided. Nanobeam synchrotron diffraction measurements have been used to map the local piezoelectric response of patterned arm structures in lead magnesium niobate - lead titanate thin films. In this case, the lateral resolution is constrained by the X-ray optics: in this case a full width at half maximum of 250 nm; typically however, only the intrinsic response is probed, unless stroboscopic techniques are employed. These results are compared quantitiavely with measurements from piezoresponse force microscopy, in which better spatial resolution is achieved, but at the expense of absolute d_33,f values.

We present our recent development of a new group of atomic force microscopy (AFM) modes for non-destructive piezoelectric and thermoelectric studies simultaneously with topography and quantitative nanomechanical measurements. These modes – HybriD Piezoresponse Force Microscopy (HD PFM) and HybriD Scanning Thermoelectric Microscopy (HD SThEM) are extensions of recently introduced HybriD mode (HD mode) – scanning technique based on fast force-distance curves measurements with real-time processing of tip response [1]. In HD AFM mode, the sample or the tip is driven into a vertical oscillation and in every cycle the tip goes from non-contact to contact regime so wealth of useful information can be detected, also mapped during lateral scanning: topography, tip-sample adhesion force and quantitative value of E modulus. Additional signals can be also applied and recorded in operator-defined “time window”. For example current through conductive tip and sample can be measured during tip-surface contact for conductivity mapping of soft and loose samples together with above mentioned properties [2].
HD PFM and HD SThEM modes are also based on “time window” approach. In HD PFM mode an AC voltage is applied on the conductive coating of the tip when tip comes in contact with the sample while fast force spectroscopy. AC voltage causes mechanical oscillations of the piezoelectric (ferroelectric) sample and corresponding vertical and lateral motion of the AFM tip is recorded and processed to get amplitude and phase characterizing local piezoelectric coefficient and local polarization direction respectively. Since AFM tip retracts from the surface in each scanning point, the lateral tip-sample interaction force is noticeably reduced in comparison to conventional contact PFM technique. This opens-up the ability of piezoresponse studies of soft, loose and fragile objects like biological samples, nanoparticles etc. Additionally a thermal drift of the cantilever (for example caused by the samples temperature exchange) can be automatically compensated allowing in situ PFM measurements under variable temperature.
HD SThEM mode working principle is based on direct measurement of generated voltage when conductive tip and sample under different temperatures contact each other (Seebeck’s effect) while fast force spectroscopy measurement. This allows non-destructive mapping of Seebeck coefficient with tip radius-limited spatial resolution.
We demonstrate HD PFM and HD SThEM modes in its application to different types of nanostructures: diphenylalanine peptide nanotubes, collagen type-I matrix, tin-bismuth alloy and triglycine sulfate crystal while second-order phase translation.
[1] S. Magonov et al, Scanning probe based apparatus and methods for low-force profiling of sample surfaces in non-resonant oscillatory mode, US9110092B1, 2015.
[2] J. Montenegro et al, Coupling of carbon and peptide nanotubes, J. Am. Chem. Soc. 136 (2014) 2484–2491.

The atomic force microscope (AFM) is used widely to image surfaces and measure interfacial forces with atomic/molecular scale resolution. In this talk, I will discuss our recent work that explores the foundations of quantitative AFM force measurements.

Measurement of the force between two atoms is now performed routinely by dynamically exciting the cantilever, in a frequency modulation (FM) mode. This has led to remarkable discoveries including the ability to chemically identify individual atoms based on the force that they exert on other atoms. I will show that the shape of this interatomic force law directly controls this capacity. Rapidly varying interatomic force laws, which are common in nature, can corrupt their own measurement leading to spurious and unphysical results. Conditions under which reliable interatomic force measurements can be achieved are given.

Essential to all quantitative force measurements is calibration and standardisation of the measured force. Users of the AFM often calibrate the spring constants of its cantilevers using functionality built into individual instruments. This calibration is performed without reference to a global standard, hindering the robust comparison of force measurements reported by different laboratories. I will describe a virtual instrument (sadermethod.org) whereby users from all laboratories can instantly and quantitatively compare their calibration measurements to those of others – standardizing AFM force measurements – and simultaneously enabling non-invasive calibration of AFM cantilevers of any geometry.

For its capability to remove material while scanning, contact-mode atomic force microscopy has been often used to induce surface modification and small-scale nanofabrication.[1] However, a clear comprehension of the material removal process and its precise control are still missing. Understanding the mechanism of tip-induced wear and formulating a model for its control at the nanoscale, is of great scientific and technological interest as it can enable tomographic sensing using AFM in a slice-and-view methodology.[2] Here, we study the fundamentals of high-pressure tip-induced material removal for various materials using nanosized sliding contacts (made of diamond) in the regime whereby thousands of nm³ are removed. First, we report on the usage of diamond tips to induce the removal of materials with different hardness. Second, the removal is studied as a function of the applied loads with as additional parameter the conditions the AFM scanning conditions. Finally, the impact of the tip-sample interaction inside the worn regions is investigated in order to generate the fundamental understanding on the physical wear mechanisms. We observe a nonlinear variation of the removal rate with the load force which is interpreted as a combination of two contact regimes each dominating in a particular force range. After probing with AFM and SEM the impact of load force and tip scanning conditions, we model the experimental rate of material eroded on each tip passage. A clear challenge is hereby the prediction of the parameters to control the tip-induced removal rate for various materials in order to satisfy a broad range of applications. The gradual transition between the two regimes is used to obtain a controllable removal rate below 5 nm/scan for different materials including semiconductors, metals and oxides.
References
[1] A.A. Tseng, Removing material using atomic force microscopy with single- and multiple-tip sources, Small. 7 (2011) 3409–3427.
[2] U. Celano, L. Goux, R. Degraeve, A. Fantini, O. Richard, H. Bender, et al., Imaging the Three-Dimensional Conductive Channel in Filamentary-Based Oxide Resistive Switching Memory., Nano Lett. 15 (2015) 7970–5.

Nano Fracture Mechanics involves the study of interface adhesion and fracture by stuying well-defined nano-pillars with the scanning force microscope. In this presentation the focus is given to the methods specifically revealing the mechanical properties and fracture thresholds of oxide with respect to semiconductor interfaces like SiO₂/Si and SiO₂/SiC as they are relevant for electronic and MEMS device manufacturing. Through load /unload cycles also called force versus distance curves elastic and plastic properties can be assessed at different time scales. In the different imaging modes, frictional forces can be measured as they depend on the applied force ‘loading’ and as they induce elastic deformation or delamination of nanopillars containing a material interface. In adition to the Nano-fracture mechanics investigations, an in-depth investigation of defects at these interfaces by Microscopic and Spectroscopic Nanoanalysis using STEM and HR-TEM will also be presented.

A specific focus will be given to the application of SFM fracture mechanics towards the investigation of time dependent changes in mechanical properties like in the threshold for fracture due to load/unload cycles (fatigue), thermal cycles (diffusion) and the modifications of elasticity and plasticity due to the presence and creation of defects, due to ‘slow’ processes like the interaction with fluids via the solubilisation and the modification of the surface and interface layers.

References:
1. Influence of Alkali Ions on Tribological Properties of Silicon Surface, Dariusz M. Jarzabek, Dorota Siewert, Wojciech Fabianowski, Helmut Schift, Zygmunt Rymuza, Thomas Jung, Tribol Lett (2015) 60, 28 DOI 10.1007/s11249-015-0603-5

2. Elastic modulus and fracture strength evaluation on the nanoscale by scanning force microscope experiments D M Jarzabek, A N Kaufmann, H Schift, Z Rymuza and T A Jung, doi:10.1088/0957-4484/25/21/215701

3. D. Jarzabek et al manuscript in preparation.

Comparison between experiments, analytical models, and atomistic simulations of thermally-activated single asperity friction have shown good overlap in their ability to describe the energetics of atomic stick-slip and minor differences in the force required to initiate slip. However, a poorer overlap is seen in the magnitude and frequency of thermal vibrations responsible for reducing friction with increasing temperature. This issue indicates that the number of atoms contributing to friction measured in experiments may not be accurately reproduced in simulations. Using conductive atomic force microscopy (AFM), we have performed friction measurements while simultaneously measuring the current through the sliding contact to observe the variation in contact area as the tip-sample junction is moved across the sample surface. More specifically, we have performed atomic stick-slip measurements of platinum and doped diamond AFM tips across highly oriented pyrolytic graphite in an ultra-high vacuum environment. Variations in the current measured through the contact show the same periodicity and structure as the underlying graphite lattice for each of the tips used. Through examination of current, a change in contact structure during the sticking and slipping segments, as characterized in the lateral force, were observed in the simultaneously recorded current signal. Furthermore, local anomalies in the lattice structure were observed in the measured current signal that were absent in the measured lateral force signal. These point towards small area defects that are typically hidden in friction experiments due to the averaging effect from a multi-atom contact.

Nanomechanical property measurement is one of the most popular functions of atomic force microscopy (AFM). Conventional nanomechanical measurement techniques are mainly based on AFM force-volume spectroscopy, which collects force-distance (F/D) curves at each pixel to calculate material elastic properties. However, these techniques have been recognized as being extremely slow, and it usually takes hours to acquire an elasticity map. Driven by the demand for a faster and more efficient technique, Park Systems developed PinPoint^TM Nanomechanical Mode to provide a solution that is at least 100 times faster than traditional techniques. This mode enables acquisition of an elasticity map with a correlated topography image within minutes, and it represents a new application tool for collecting real-time topography and quantitative mechanical property maps of various materials, ranging from hard disks to soft tissues. To better access the capability of the PinPoint^TM Nanomechanical Mode, we select three cantilevers with stiffness ranging from 0.2 N/m to 25 N/m and investigated their influence on the measured modulus values. In addition, for all three cantilevers, we also examine the influence of applied force on the resulted modulus. Findings show that cantilevers with smaller force constants will lead to smaller measured modulus values, and vice versa. With the same cantilever, a larger applied force results in a larger measured modulus compared to that obtained with smaller force values. The most important finding is that, with PinPoint^TM, the relative modulus difference/ratio within a sample can be accurately acquired regardless of the force constant of the cantilever or the applied setpoint force values.

This contribution aims to provide an overview of some recent advances in force microscopy and its applications in nanoscience and biomedicine. The presentation covers instrumentation^1-2, imaging³, nanomechanics^4-6 and applications to study a wide range systems, from metal-organic-frameworks to proteins and cells in air and liquid environments. The first section is devoted to present the physics underlying the operation of an advanced force microscope. The second section provides an introduction to bimodal force microscopy. An advanced microscopy method that enables simultaneous, fast and high resolution mapping of different interfacial properties (mechanical, electrical, magnetic, optical, topography). This section describes some applications to generate atomic and molecular resolution maps of soft matter interfaces. Those maps combine topography and nanomechanical properties. The third section introduces a method to generate three dimensional and atomically-resolved maps of solid-liquid interfaces.

Recent References
1. Y. Dufrene, T. Ando, R. Garcia, D.J. Muller, Nature Nanotechnol. 12, 295 (2017).
2. R. Garcia, E.T. Herruzo, Nature Nanotechnol. 7, 217 (2012)
3. D. Martin-Jimenez, E. Chacon, P. Tarazona and R. Garcia, Nature Commun. 7, 12164 (2016).
4. E.T. Herruzo, A.P. Perrino and R. Garcia, Nature Commun. 5, 3126 (2014)
5. C.A. Amo, A. P. Perrino, A.F. Payam and R. Garcia, ACS Nano 11, 8650 (2017)
6. C. A. Amo and R. Garcia, ACS Nano 10, 717-7124 (2016)

Nanomechanical measurements in Atomic Force Microscopy (AFM) are based on detection of quasi-static deflection (D) versus probe-sample vertical displacement (Z). These experimental D-vs-Z curves are converted into force (F) versus deformation (h) dependencies: F-vs-h. Elastic modulus and work of adhesion are extracted from analysis of F-vs-h curves in framework of solid state deformation models. Studies of D-vs-Z curves are associated with contact and non-resonant oscillatory modes of AFM. Dynamic probe variables, such as amplitude (A), phase (θ) and frequency shift (Δf), change as the oscillatory probe interacts with sample in the resonant oscillatory modes. The A-vs-Z, θ-vs-Z, Δf-vs-Z curves can be collected simultaneously in pairs if either frequency or phase is kept constant as it happens in amplitude modulation and frequency modulation imaging modes. A pair of (A-vs-Z, θ-vs-Z) curves is collected when the probe frequency is fixed at or near its resonance in free oscillatory state. A fixing of the probe phase at 90 degrees with a phase lock loop, which keeps the probe at effective resonance, allows recording a pair of (Δf-vs-Z, A-vs-Z curves). These curves are collected at single surface locations and their maps over the scanning area are often constructing to reflect spatial changes of materials’ properties. Both pairs of curves contain at least the same information as quasi-static D vs Z curve, and the latest can be potentially restored from one of the pairs. We use steady state equations of asymptotic amplitude-phase AFM dynamics derived using Krylov-Bogoliubov-Mitropolsky( KBM) averaging technique. This technique allows a formulation of the force restoration problem as a pair of integral equations. However, solution to these equations is an ill-posed inverse problem that magnifies noise and requires regularization. One form of regularization can use a priori parametric force model, e.g. JKR, but even in this case non-parametric restoration is required to justify the model.
We demonstrate a successful use of Tikhonov regularization for the force restoration from both pairs (A-vs-Z, θ-vs-Z) and (Δf-vs-Z, A-vs-Z) of curves by simulation with different kind of forces, errors and noise levels. The presented theoretical analysis was verified in interplay with experimental data, which were obtained on samples with macroscopic elastic modulus varied in the range form few MPa to tens of GPa. They included neat polymers, polymer blends, more complex organic compositions and metals. The AFM probes with spring constants from few to hundreds of N/m were applied in these studies.

In conventional contact resonance (CR) AFM methods, the AFM tip is brought into contact with the sample, and a mechanical, thermal, or electrical excitation is applied to the tip or sample at a drive frequency equal to a resonance frequency of the coupled tip-sample system. Mechanical or thermal excitation is used to probe nanomechanical properties, while electrical excitation is used to probe piezoelectric properties (piezo response force microscopy, PFM) or ionic conduction (electrochemical strain microscopy, ESM). The motivation for driving the system at a resonance frequency is that the cantilever motion is amplified by up to several orders of magnitude (relative to off-resonance), thus enhancing the small-signal sensitivity. Conventional CR-AFM has the disadvantage that the resonance frequency and hence the vibrational shape of the cantilever varies during a scan, due to spatial variation of the elastic modulus or surface morphology. Hence, for a fixed position of the detection laser on the cantilever, the deflection amplitude may vary all the way from maximum response (at a node) to zero response (at an antinode). This shape-related amplitude variation is a roadblock to quantification of the material properties of interest.

Here, we propose and demonstrate a new type of CR-AFM measurement, isomorphic CR, in which the resonance frequency and vibrational shape is equal for all pixels in a scan, thus eliminating the unwanted shape-related amplitude variation. As a result, relative amplitude contrast is inherently reliable, quality factor contrast is directly related to material damping, and calculated elastic stiffness is free from modeling uncertainties. Furthermore, the detection laser position can be specifically optimized for the selected vibrational shape. Isomorphic CR-AFM is accomplished as follows. First, the AFM scan is conducted in force-mapping mode: the tip is brought into contact with the sample; the contact force is increased and then decreased; and the tip is retracted and moved to the next sample location (this sequence is repeated for each pixel in the scan). Simultaneously, the tip or sample is excited at a sum of two fixed drive frequencies, and the amplitude and phase response at each frequency is measured as a function of time and hence contact force. The resonance frequency is deduced to be equal to f_mean , the mean of the two fixed drive frequencies, at the point in the force curve where the vibration amplitudes are equal. Finally, the amplitude, phase, and Q factor at f_mean are calculated from the measured amplitudes and phases based on a harmonic oscillator model. We will present experimental results for a variety of samples, including mapping elastic and viscous properties of a polymer blend, bias induced strain of a piezoelectric material (PFM mode), and bias induced strain of a lithium battery cathode material (ESM mode), to illustrate the utility and scope of the isomorphic contact resonance method.

Little is still known about the interfacial properties of very thin films of room-temperature ionic liquids (ILs) supported by flat solid surfaces; experimental investigations have been focused so far primarily to the air-(bulk)liquid, and (bulk)liquid-solid interfaces. Understanding the combined effects of surface interactions, presence of water in solution, long range electrostatic forces, and confinement on the structural rearrangement and on the interfacial properties of supported ILs films is a challenging task, which requires theoretical, computational, and experimental efforts. Advancements in this field would be beneficial for several applications where the liquid-solid interface plays a crucial role, such as in photo-electrochemical devices and in macro- and micro-mechanical systems, where ILs are used as lubricants. We present the results of experimental studies of the morphological and structural properties of nanometer-thick thin films of 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) deposited from highly diluted methanol solutions by drop-casting on flat and nanostructured silicon and carbonaceous surfaces. Atomic Force Microscopy (AFM) studies, including high-resolution imaging and nanomechanical tests, have been carried out on the thin IL layers. On smooth silicon surfaces, ordered lamellar nanostructures of mesoscopic area (1–100μm²) with a vertical structural periodicity have been observed at room temperature, while they are not observed on HOPG surfaces^1,2. Nanomechanical investigations reveal that besides being vertically ordered, these structures also resist to normal compressive loads up to few hundreds of MPa. Beyond that limit, destructive penetration occurs in discrete steps. This observation suggest a solid-like character of the islands. The presence of nanostructured features on the flat solid surfaces, with the same chemical composition of the substrates, could in principle promote the solid-like terraces because of the increased area of interaction, or destroy them because of the introduction of a morphological disorder at the interface. We show how the presence of clusters of 6-20nm in diameters on the surface doesn’t prevent the formation of the IL solid-like layers, while they work as pinning points for the liquid droplets. Our findings highlight the potentialities of scanning probe techniques for the quantitative investigation of the interfacial properties of thin IL films. Moreover, the results of this study suggest that surface interactions and nanoscale confinement play an important role in the interfacial restructuring of the ILs, leading to modifications of the physico-chemical properties of the liquid that may have important consequences for the performance of those electrochemical and tribological devices employing these ionic compounds as electrolytes or lubricants.

¹S. Bovio, et al. J. Phys. Chem. B 113, 6600 (2009).
²S. Bovio, et al. J. Phys. Condens. Matter 21, 424118 (2009).

One of the key challenges in the field of bottom-up fabrication is the rational use of existing self-assembling systems, while maintaining the level of control required for actual applications. Here we present a hierarchical bottom-up approach to controlled self-assembly. Self-assembled PS-b-PEO block copolymer (BCP) thin films express nanopatterned structures over macroscopic areas. The Non-specific interactions between chemically heterogeneous nanodomains and individual proteins is used to direct the self-assembly of bacterial surface layers (S-layers) in situ at the nanoscale. A comparison between physically and chemically patterned BCP substrates shows that chemical heterogeneity is required to confine the adhesion and eventual self-assembly of proteins to specific domains.
Collagen is the main structural protein in vertebrates and collagen films have been widely used as substrate to facilitate cell adhesion or as adhesion matrix for specific collagen-binding proteins. For this reason, a high level of local control over the position and orientation of collagen fibers is desired. The principle of hierarchical bottom-up self-assembly has been used to direct the formation of collagen fibers. It has been demonstrated that nanopatterned PS-b-PEO striped patterns can be used to direct the self-assembly of collagen molecules in situ along the local direction of the underlying substrate.
The dynamics of protein self-assembly at the solid-liquid interface is followed using high-resolution AFM measurements in an in situ continuous flow environment. This allows for the extraction of self-assembly rate constants. It has been shown that a pattern of alternating, chemically distinct nanoscale domains drastically increases both the rate of self-assembly as well as the total protein load compared to non-patterned chemically homogeneous substrates.

Photo-polymerizable and photo-degradable materials are the focus of extensive research across a diverse variety of fields ranging from additive manufacturing to regenerative medicine. However, a lack of thorough understanding of material mechanical and rheological properties during polymerization/degradation at the relevant exposure intensities and length-scales limits advancements in part performance, voxel-resolution, and throughput. Voxel-relevant in-situ characterization of these photopolymerizable materials is not currently possible due to the volume and sampling-rate constraints of traditional, bulk techniques (e.g., oscillatory rheometry, Fourier Transform Infrared Spectroscopy (FTIR)). Atomic Force Microscopy (AFM) affords the requisite spatiotemporal resolution to observe the dynamic kinetics inherent to these materials, but has yet to be adapted to suit the specific needs of in-situ photo-processing, such as using an independent light source to initiate reactions. We demonstrate the use of novel atomic force microscopy techniques to locally characterize the mechanical and rheological properties of photo-processable materials on the relevant reaction kinetic time-scales for materials ranging from hydrated, soft (10s kPa) hydrogels to stiff (10s GPa) additive manufacturing materials. By modifying a photothermal AFM excitation source (405 nm) to operate independently from the AFM scanning controls, we locally polymerized/degraded polymer at different transverse locations from the cantilever tip (0, 1, 5, 10, 50 um from tip) and with varied illumination power (0.1, 1, and 3 mW). Concurrently, the AFM was operated in various continuous stiffness sensing modalities (i.e., fast force spectroscopy and contact resonance force microscopy) to detect the voxel-scale mechanical modification of the underlying material. With this technique, materials with fast kinetics (<1 ms) can be observed with sufficient spatial and temporal resolution to identify how the mechanorehological properties develop in real-time.

Triboelectric charging is one of the oldest manifestations of electricity known to mankind, however, its relevance for fundamental studies and applied research has rapidly increased in the recent years. Novel methods of energy harvesting such as triboelectric nanogenerators require strong ability to develop electrical charge while materials on the industrial production line should show the opposite trend to avoid handling issues and eliminate associated costs. Thus, practical applications prompt the development of techniques that can reliably identify the best candidates by their ability to develop tribological charge customized for a specific application.
Most prior works have explored the charging of the material with the localized probe. This approach has a general limitation that contact charging is convolved with the simultaneous discharging into the probe itself, as well as diffusion of static charge across the surface. Moreover, the rate of probe-surface motion is orders of magnitude smaller than the characteristic parameters of macroscopic triboelectric processes.
We developed a different approach that quantifies localized discharge of a previously charged surface. Force-distance curves acquired using scanning probe microscopy, were used to quantify the static charge density and observe its dynamics in real time. We demonstrate the capabilities of this methodology directly on industrial polymer samples – representing surfaces of commercial plastic bottles. Specifically, we investigate the efficiency of macroscopic charging, nanoscale discharging, and nanoscale recharging following a partial discharge. We directly observe the dynamics of the triboelectric charge and quantify its surface mobility, within the approximations of a continuum electrostatic model. At the same time, we show that it is possible to reliably differentiate polymer compositions based on their propensity to accumulate triboelectric charges. This is relevant to the handling of industrial bottles on production lines, where static charge can present significant challenges to handling and scale-up.

Acknowledgements: authors would like to thank Procter & Gamble Company for the funding of the project. The experiments have been carried at the Center for Nanophase Materials Sciences, Division of User Facilities, US Department of Energy.
Borodinov et al., submitted to Nano Lett. (2017)

Atomic Force Microscopy (AFM) is a powerful technique in biophysics, nanomedicine since it allows imaging and manipulating nanostructures in physiological conditions on a single molecule level [1].
Magnetic Force Microscopy (MFM) is an Atomic Force-based technique where a nanometric magnetic probe is raster –scanned in close proximity to a surface detecting the local magnetic fields near the surface. MFM has been applied to the study of a variety of magnetic systems, including magnetic nanoparticles (MNPs) [2] but always in vacuum or atmospheric conditions.
Nowadays, magnetic nanostructures play important roles in different fields such as medicine, biology or catalysis. For example, the use of MNPs is growing a lot of attention for its potential applications that include therapeutic drug, contrast enhancement agents or methods for the catabolism of tumours via hyperthermia. Many studies as well are focused on the encapsulation of magnetic particles in different biological entities as bacteria or virus like particles [3].
Despite the importance of measuring magnetic nanostructures with biological applications in physiological conditions, the applicability of MFM to these systems was limited up to now because of the difficulty in developing MFM for detecting magnetic interactions in liquids. This is a consequence of the high damping forces of the cantilever, which are several times greater than in air. These damping forces are the origin of the low-quality factor (Q) of the cantilever resonance characteristic of liquid measurements, which results in a significant loss of sensitivity in the MFM signal. The work presented here introduces the development of MFM imaging in liquid media and discusses its potential for detecting and imaging nanoscale magnetic domains in biological samples [4].
MFM studies in liquids have been carried out using commercial probes, being able to detect magnetic signals even from a single 30 nm Fe3O4 nanoparticle. To optimize the magnetic contrast in liquid, 3D modes analysis was done for different dynamic modes. By working in Drive Amplitude Modulation DAM-AFM [5] the tip is able to detect the magnetic interaction closer to the sample, which translates into a higher magnetic signal.
A study of fundamental MFM noise shows that further improvement on the performance could be gained using specially designed cantilevers for liquid media. Here we present the development of new magnetic probes by following different routes. These special magnetic cantilevers allow imaging magnetic signals in liquid media with similar quality of images as in air ambient conditions.
[1] Y. F. Dufrêne et al. Nature Nanotechnology 12, 295–307 (2017)
[2] S.Schreiber et al. Small, 4 (2), 270−278 (2008)
[3] M. Jaafar et al. ACS Appl. Mater. Interfaces , 6, 20936−20942 (2014)
[4] P. Ares et al. Small 11, 36, 4731-4736 (2015)
[5] M. Jaafar et al. Beilstein Journal of Nanotechnology 3, 336 (2012)

Clathrin-mediated endocytosis is one of the central pathways for cargo transport into cells, and plays a major role in the maintenance of cellular functions, such as intercellular signaling, nutrient intake, and turnover of plasma membrane in cells. The clathrin-mediated endocytosis process involves invagination and formation of clathrin-coated vesicles. However, the biophysical mechanisms of vesicle formation are still debated. Currently, there are two competing models describing the membrane bending during the formation of clathrin cages: the first involves the deposition of all clathrin molecules to the plasma membrane, forming a flat lattice prior to membrane bending to form clathrin vesicles, whereas in the second model, membrane bending happens simultaneously as the clathrin arrives to the site to form a clathrin-coated cage. We investigate clathrin vesicle formation mechanisms through the utilization of tapping-mode atomic force microscopy for high resolution topographical imaging in neutral buffer solution of unroofed cells exposing the inner membrane, combined with fluorescence imaging to definitively label intracellular constituents with specific fluorescent fusion proteins (actin filaments labeled with green phalloidin and clathrin coated vesicles with the fusion protein (Clathrin ligh chain - Tq2) in SKMEL (Human Melanoma) cells. Results from our work are compared against dynamical polarized total internal fluorescence (TIRF) and transmission electron microscopy (TEM) to draw conclusions regarding the prominent model of vesicle formation in clathrin-mediated endocytosis.

Van der Waals nanoscale structures are of great interest for both fundamental research and future technological applications, including heat dissipation in nano-electronic devices and thermoelectric applications. Therefore, characterization of the thermal properties of such structures are of great importance. The thermal transport in 2D materials depends strongly on the coupling to the underlying material and the temperature. Scanning Thermal Microscopy (SThM) is a leading technique for mapping thermal properties with nanoscale resolution [1], consisting of a self-heated probe which acts as a thermosensor during sample scanning. In this work, we report nanoscale thermal transport measurements at van der walls heterostructures formed by Molybdenum Disulphide (MoS2) and Graphene, as well as Franckeite which is natural heterostructure [2], by SThM in high vacuum. We demonstrate, for first time, that by cooling down the sample to temperatures as low as 150K and using doped Si probes, we are able to map the nanoscale heat transport in the heterostructures. We study the effect of the sample temperature on effective thermal conductivity of the heterostructures, by increasing the temperature up to 300K. In addition, for the franckeite we measure samples of different thicknesses and extract, for first time, the thermal conductivity of a single layer at various temperatures.

Super-resolution fluorescent imaging based on photoswitchable fluorophores provide promising perspective of optical microscopy with nanoscale resolution (10-100 nm). We report a series of fluorescence molecular photoswitches including spiropyrans (SP)^1,2, dithienylethene (DTE)³ and hexaarylbiimidazole (HABI), which have been screened for super-resolution fluorescent imaging (SURF). The molecular photoswitches undergo photoinduced isomerization and photoswitchable fluorescence, which is essential for super-resolution fluorescence imaging. The super-resolution imaging applications of these photoswitchable fluorophores have been demonstrated that fine nanostructures have been characterized with 40 nm resolution in the self-assembly of amphiphiles, block copolymers and polymer nanofibers as well as inorganic nanowires.

The onset of damage and its evolution in polymers at the nanoscale (<100 nm), as well as the mechanics that govern it, have not been well-studied. Recently, atomistic simulations have revealed that there is a stark difference between the mechanical behavior of materials at the nanoscale and macroscale. In particular, how local chemical heterogeneities: influence damage nucleation and growth at the nanoscale, alter the makeup and mechanical behavior of the fiber/matrix interface and interphase regions, and manifest themselves at the macroscale. Given that interphase volume fractions can exceed 50 vol. % in a nanocomposite, it has been hypothesized that small changes at the interface and within the interphase may yield drastic changes in bulk behavior. Recent attempts at answering these questions have primarily involved electron microscopy, but unfortunately, polymers are inherently non-conductive and commonly experience charging and electron-induced damage when imaged. AFM can be used as an alternative to electron microscopy. Specimens can be readily molded and tested at ambient conditions, without the need to put under vacuum or sputter coat. Nevertheless, AFM has its own challenges, which include the time required for sample imaging, tip selection and calibration. In this study, carbon nanofibers (CNFs) were decorated with silver (Ag) nanoparticles via a solventless process and subsequently introduced into a commercial polyurethane (PU) to promote the formation of a conductive network. Four sample types were fabricated: neat PU, PU+CNF, PU+CNF+Ag Acetate, and PU+Ag-CNF. Nanofiller concentrations were held at 3.47-6.78 % vol. for CNF, 0.06-1.31 % vol. for Ag Acetate, and 0.01-0.27 % vol. for Ag. High-resolution SEM, and AFM were used to observe the morphology of freeze fractured, and uniaxially strained specimens, respectively, while TEM and nano X-ray computed tomography provided two-dimensional and three-dimensional views of nanofiller dispersion in each system, respectively. DSC and TGA provided T_g and nanofiller concentrations in the composites, respectively. Chemical maps of sample surfaces were acquired via AFM-IR, while the onset and evolution of damage (prior to plastic deformation) was monitored via in situ tensile testing (quasi-static) in AFM coupled with DMT modulus mapping. By using DMT modulus, rather than height or phase signals, the components and damage states within the composite can be clearly differentiated in situ. The ability to conduct these experiments in situ and at ambient conditions provides an unprecedented amount of insight into the influence that chemical/morphological heterogeneities have on the nanomechanical behavior of polymer nanocomposites, which is essential for guiding and refining current computational model frameworks.

The Helium Ion Microscope (HIM), a new form of sub-nanometer microscopy, has several advantages over
traditional electron based forms of imaging, such as the ability to image insulating samples without a
conducting overlayer. HIM images are generated by rastering a helium ion beam across a sample and collecting
the emitted secondary electrons. The number of secondary electrons corresponds to different grayscale values,
creating a black and white image.
Rutherford Backscattering physics presents a method for determining the elemental composition of the atoms
in samples: The energy of a backscattered helium ion depends on the mass of the sample atom it has scattered
from. By measuring the flight time of the backscattered ion, we determine this energy. Therefore, by detecting
the backscattered helium ions, we can create images of samples that convey this elemental information and
how those elements are distributed.
In order to create accurate images, it is important to understand how the sample is damaged by the incident
ions. We therefore exposed a 1.3 nm thin platinum film on silicon to varying ion beam doses and observed
how the composition of the surface changed with dose. From this we calculated the sputtering coefficient for
platinum. In a separate experiment, using X-ray photoelectron spectroscopy, we verified the thickness of the
platinum film. Our results show that elemental identification is possible on the 500 nanometer scale using this
“time of flight” configuration but that there is a limit to our ability to sensitively identify elements as a result
of beam deterioration. We found a sputtering coefficient that is smaller than the accepted value in the literature
of ion beam sputtering; possible systematic errors to explain this difficulty will be discussed.
This project has been supported by funding from National Science Foundation grant PHY-1560077.

Nanoscale probing techniques are becoming a common method for solving problems in complex multiphase materials. Diverse topics, from fundamental metallurgical studies and mineralized biocomposites in academia to industrial researchers involved in dissimilar welding both find themselves trying to find failure and strength controlling mechanisms at very fine scales. One technique for this type of mapping is high speed nanoindentation. While “standard” nanoindentation is truly a high throughput technique in comparison to most other mechanical testing mechanisms, high speed nanoindentation increases this throughput by a factor of 1000. Thus, large arrays of indents numbering in thousands can be used to generate high resolution property maps. This can also produce useful statistics which can be selected and compared from different regions of the map with traceability back to the original location via SPM scanning. This technique can also be coupled to the environment, such as cooling and heating. Here, a fundamental study in a low carbon, 1018, steel is presented. This material is non-exotic, but plays a large role in the nuts and bolts of everyday life. 1018 steel is a two phase steel, containing both ferrite and pearlite phases that are easy to distinguish both via in contact SPM and high speed mapping of the steel, with the high carbon pearlite being much harder than the ferrite. This material also exhibits a ductile to brittle temperature transition at -5C via Charpy impact testing. However, the ability to explore this phase by phase and at a local scale reveals changes in the load-displacement curve from smooth to heavily serrated flow dominated by pop-in behavior.

The decay of surface plasmons into electron/hole pairs via Landau damping can be exploited in a context of surface chemistry and catalysis if these "hot" carriers can be harvested before thermalization with the lattice occurs. We utilize the ultrafast dynamics of this process to control the surface chemistry of silver and gold plasmonic nanoantennas with nanoscale resolution. Specifically, we demonstrate mapping of reactivity hot spots, decoupled from the well-known electromagnetic hot spots of plasmonic nanosystems. We then show how the hot electron dynamics can be utilized for control over nanoscale self-assembly of molecular overlayers, allowing functionalization of specific parts of plasmonic nanoantennas with desired molecules. Coupled with interrogation via super-resolution microscopy, this allows for the local probing of plasmon decay and hence absorption with nanoscale resolution. Applications in controlled self-assembly of molecules and colloids as well as for the purification of collodial mixtures will be discussed.

The silver nanowire (AgNW) represents a novel class of probes for scanning probe microscopes (SPM), including atomic force microscopy (AFM), near-field scanning optical microscopy (NSOM) and tip-enhanced Raman spectroscopy (TERS), due to their unique mechanical and optical characteristics. Here we report our recent advances on the integration of commercial AFM cantilevers with a single sharp-tip AgNWs (5 nm tip radius) through a reliable, cost-efficient bench-top process for high-fidelity and high-resolution (HR) topographic imaging of nanoscale 3D surfaces and nano-antenna-assisted remote-excitation TERS. We have demonstrated that the novel high-aspect-ratio AFM probe is mechanically robust against buckling and bending up to 150 nN, and is capable of high fidelity imaging of deep trenches (10:1) imaging with a lateral resolution of <8 nm, on par with or exceeding the performance of commercial HAR probes fabricated by expensive clean-room techniques. Recently, we have further demonstrated a remote-excitation TERS probe by attaching a silver nanoparticle (AgNP) on AgNW. The AgNP served as a nano-antenna, which remotely couples excitation laser into SPP modes propagating along the AgNW to its tip apex. The remote excitation shrinks the laser illumination area on the sample by up to 10,000 times, thus effectively suppresses the TERS background and avoids sample photo degradation. We have achieved high signal-noise ratio TERS of 4-Aminothiolphenol self-assembled monolayer (SAM) and a sub-50nm spatial resolution chemical mapping of monolayer molybdenum disulfide MoS₂.

Polymer photonic waveguides on flexible substrates have been proposed as a solution for meeting the increased demand for rack-to-rack communication within data centers. These devices have clear advantages over copper electronics in terms of both power consumption and system density. To meet this need, we have been exploring the potential of photopatterned silicone-based optical interconnects consisting of clad and core layers on flexible substrates. When designing materials for polymeric waveguides, one critical metric is the optical crossing loss, which is related in part to the refractive index gradient. From a materials perspective, the refractive index gradient is caused by chemical gradients present across the core layer or at the core/clad interface. AFM-IR analysis has proven extremely fruitful for understanding how migration of small molecules from the clad layer into the 50 x 50-micron core impacts the crossing loss. In cross-sections of full waveguide builds, quantitative structure/function relationships have been demonstrated between crossing loss and the areas of peaks associated with O-H, aliphatic C-H, and aromatic C-H contributions in the core and clad layers. Additionally, AFM-IR spectra of model bilayer systems strongly demonstrate the impact of process variables such as temperature, order of layer deposition, and order of heating vs. UV cure on the extent of small molecule migration.

Spectroscopy methods working in the mid-infrared wavelength range allow for addressing several kinds of fundamental material properties as molecular vibrations and the excitation of phonons and free charge carriers. However, the downscaling of electronic and photonic devices raises the need for characterization techniques with nanoscale resolution. Conventional spectroscopy methods lack this high resolution as they are limited by the diffraction limit to about half the wavelength. In scattering-type scanning near-field optical microscopy (s-SNOM), the high resolution of atomic force microscopy (AFM) is combined with the capability of infrared spectroscopy methods to address fundamental material excitations. A metallized AFM tip is illuminated by laser light. Near-fields are excited at the tip apex leading to a confinement of the light to a region in the order of the tip radius (~25 nm), which is the only limitation for the lateral resolution. Parallel to the acquisition of the samples topography, local changes in the dielectric response of the sample are mapped. In the mid-infrared wavelength range this response is connected with the carrier properties of doped semiconductors as described by the Drude model. Due to this, s-SNOM has proven great potential for the quantification of the amount of activated free carriers in doped semiconductor nanowires. [1]
By performing sequential spectroscopy in the range of the plasma edge we present on the ability to determine the charge carrier concentration profile along silicon nanowires with phosphorous-doped segments in between intrinsic silicon that have been grown with the vapor-liquid-solid (VLS) technique. We show that the combination of the shape of line profiles along the nanowire axis with the spectroscopic evaluation at one point gives us precise information about the level of free charge carriers in the doped segment as well as the sharpness of the boundaries between intrinsic and doped regions. We show that this determination is even possible if the plasmon resonance of the free carriers is overlaid by a phonon resonance of the native SiO2-layer covering the nanowire, by using a multilayer model [2] for the tip-sample interaction.

[1] J. Stiegler et al., “Nanoscale free-carrier profiling of individual semiconductor nanowires by infrared near-field microscopy”, Nano Letters 10, 1387 (2010).
[2] B. Hauer et al., “Quasi-analytical model for scattering infrared near-field microscopy on layered systems”, Optics Express 20, 13173 (2012).

Infrared microspectroscopy is a powerful technique for obtaining unique chemical information from a variety of biological systems. In general, spatial resolutions are limited by the Abbe diffraction laws to ~ λ/2 (3-10 µm). In recent years, the development of the AFM-IR technique has allowed for overcoming these diffraction limitations by using an AFM probe as the detector for IR absorption. Using a quantum cascade IR laser (QCL), the molecular vibrations in a sample are excited, leading to thermal expansion of the surface, which is monitored using an AFM cantilever. This technique reduced the spatial resolution of < 10 nm, however, in certain cases this high resolution is not required and the size of an image in AFM-IR is limited by the piezo stage to sub 100 µm. A new technique has been developed that fills the gap between IR microspectroscopy and AFM-IR. This is an optical based technique that still beaks the diffraction limits associated with conventional IR microspectroscopy, and provides sub-micron spatial resolution. Taking advantage of the same fundamental principles that govern AFM-IR, the thermal expansion of a sample surface induced by a QCL IR laser is monitored using a visible probe laser, instead of an AFM probe. This unique technique provides transmission like IR spectra while operating in a reflective regime. The spatial resolution is limited by the diffraction limit of the visible probe laser, while also being independent of the IR wavelength used. In this talk we will focus on the combination of this new optical probe based IR technique with AFM-IR to fully characterize a variety of materials, from the micro scale to the nanoscale using the same QCL laser source.

Combining tip-enhanced Raman, photoluminescence, and nonlinear spectroscopy with active atomic force tip interaction, I will discuss novel approaches for the study of photo-physical and photo-chemical processes in molecules and nano-solids [1,2,3]. In recent examples, we nano-image the exciton behavior and its correlation with defects, grain boundaries, and local strain in different transition metal di-chalcogenides (TMDs). Based on exciton-plasmon coupling we achieve a 10⁵-fold enhancement of the photo-luminescence yield [4].

Further, we are able to achieve TEPL spectroscopy of the otherwise forbidden radiative emission from excitonic dark states with the optical antenna tip coupling to its out-of-plane transition dipole moment [5]. We achieve room temperature contrast, not possible in conventional approaches, due to the ultrafast radiative dark exciton to the tip-antenna mode with few-nm tip-sample gap localized mode volume induced Purcell factor of > 2x10³. With the atomic-force microscope controlled antenna tip we demonstrate correlative nano- opto-mechanical swit- ching and programmable modulation of the dark exciton emission. This hybrid tip- enhanced nano- spectroscopy and -imaging method allows to probe and control neutral-, multi-, localized-, and dark-excitons and their correlation with lattice and electronic structural heterogeneities in 2D materials and molecular systems.

[1] Park, K. D., Muller, E. A., Kravtsov, V., Sass, P. M., Dreyer, J., Atkin, J. M., and Raschke, M. B. Variable-Temperature Tip-Enhanced Raman Spectroscopy of Single-Molecule Fluctuations and Dynamics. Nano Lett.16, 479-487 (2016),
[2] Kravtsov, V., Ulbricht, R., Atkin, J. M., and Raschke, M. B. Plasmonic nanofocused four-wave mixing for femtosecond near-field imaging. Nat. Nanotechnol. 11, 459-464 (2016).
[3] Kravtsov, V., Berweger, S., Atkin, J. M., and Raschke, M. B. Control of Plasmon Emission and Dynamics at the Transition from Classical to Quantum Coupling. Nano Lett. 14, 5270-5275 (2014).
[4] Park, K.-D. et al., “Hybrid tip-enhanced nanospectroscopy and nanoimaging of monolayer WSe₂ with local strain control,” Nano Lett., Vol. 16, No. 4, 2621–2627, 2016.
[5] Park, K. D., T. Jiang, G. Clark, X. Xu, and Raschke, M. B. “Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect”, Nature Nanotechnol. (in press), 2017.

In the quest for high performance materials for energy production, transmission, and storage, a comprehensive toolbox of characterization techniques – both experimental and theoretical – is an absolute necessity. In recent years, many techniques have been increasingly focussed on combining observations of material structure, composition, and dynamics to develop a complete picture on how various material properties affect overall performance. This is generally achieved either by custom built hardware that incorporates several characterization techniques that can be performed simultaneously, or by successive ‘ex situ’ measurements that each capture a different aspect. There do exist, however, some emerging techniques that can capture both dynamic and structural information simultaneously, one of which is atomic force microscopy (AFM) and its variants. AFM represents a very good candidate for measuring local dynamic processes due to the extremely sensitive and highly localized force sensing nature of the AFM probes. Here we will present some of the leading work on various implementations of AFM and how they are applied to energy materials, including time-resolved electrostatic force microscopy (EFM), which enable the direct probing of charge transport processes with high spatial resolution.

In EFM-based techniques, the probe is used to detect changes in the local electric field, which can arise from a variety of processes including the movement of ions. By driving ionic transport with an applied potential and acquiring the response signal over time, it can be used to directly measure the bulk ionic conductivity of a substrate and – if done at varying temperatures – the local activation energy and hopping barriers. Using this time-resolved technique on a heterogeneous sample, we have observed spatial variations in the response signal on the order of 50nm, demonstrating the true power of this technique [Nano Lett. 17, 4489 (2017)]. The topographic images obtained also allow for further characterization on the exact same physical regions of the sample, which eliminates the effects of averaging over large macroscopic areas.

These measurements allow us to directly peer into the relationship between local structure and dynamics with the spatial and temporal resolution not previously available with many standard approaches. This has lead to the maturation of AFM into a staple in the characterization toolbox for energy materials.

Triboelectricity harvesting techniques are capable of converting mechanical energy into electricity, which show tremendous potential for powering miniaturized sensors, portable electronics, etc. Specifically, converting mechanical energy into direct-current (DC) power with sufficient current density is one of the key challenges. In conventional triboelectricity nanogenerators (TENGs), polymer-based materials are widely used. Due to the large impedance however, they can only generate electrostatic charges via contact electrification and obtain dielectric displacement alternating current (AC) with low current density. Moreover, the AC power output has to be rectified for practical use. Therefore, exploring new physical mechanisms and new material systems for harvesting triboelectric power is highly desirable. By using C-AFM techniques, we found that a high DC output can be generated from two-dimensional semiconducting materials without applying any external bias. The nanoscale electrical measurement results suggest that the triboelectric DC generation has different physical mechanism compared with the dielectric displacement current generation in conventional TENGs. From the analysis of electronic properties at the probe-sample interface, we found that the phenomenon is related to the surface potential difference between the metal-semiconductor materials under a thermodynamic non-equilibrium condition. By a precise control of the interface structure, we found strong evidences showing the tunneling behavior of the triboelectric charges. Accordingly, a new physical model has been proposed to describe the phenomenon. And also, we have successfully demonstrated the concept at a macro-scale level. This new concept shows a promising future as a green energy harvesting technique with a broad range of material candidates and device configurations.

Due to earth’s finite natural resources and greenhouse effect, developing high efficient photovoltaics is one of the primary interests in energy research society. Recently, dielectric nano-resonators-based coating atop solar cells has been considered as a solution to make high absorptive, efficient photovoltaics as they are relatively cheap and can be deposited atop solar cells without surface damage [1]. However, the evaluation and optimization of the nano-resonator coatings have been made only with macro-/mesoscale experiments and calculations. Therefore, characterization in nanoscale where photo-excited carrier generations and collections actually take place is necessary to fully establish the physical origin of the efficiency enhancements. In this presentation, for the first time, we show how excited optical resonances within a monolayer of nano-resonators improve photocurrent of solar cells in nanoscale via a novel near-field scanning photocurrent microscopy technique [2]. We employ a tuning fork-based non-contact mode atomic force microscopy probe attached with a metal-coated multimode optical fiber to inject light in nanoscale. At a wavelength of whispering gallery mode excitation, we image strong resonance-induced local photocurrent enhancements over each nano-resonator. Based on nanoscale photocurrent imaging as well as macroscale measurements and calculations, we conclude that collective optical resonances between whispering gallery modes and thin-film interference improve the photocurrent of solar cells by more than 20 %. We also examine the properties of multiple layers of nano-resonators by the suggested near-field scanning photocurrent microscopy to evaluate the role of possible manufacturing imperfections [3]. We show that the optical resonances and their resultant photocurrent enhancements are dependent on the number of nano-resonator layers and the size of nano-resonators. The in-depth understanding of physical origins of enhancements from macro-/nanoscale experiments and calculations as well as easy, inexpensive, and scalable fabrication process would propel the application of this novel coating on high efficient commercial photovoltaics.

References
[1] Dongheon Ha, Chen Gong, Marina S. Leite, and Jeremy N. Munday, “Demonstration of resonance coupling in scalable dielectric microresonator coatings for photovoltaics, ACS Applied Materials and Interfaces, 8, 24536-24542, 2016 (Cover)
[2] Dongheon Ha, Yohan Yoon, and Nikolai B. Zhitenev, “Nanoscale imaging of photocurrent enhancement by resonator array photovoltaic coatings,” Advanced Optical Materials (under review)
[3] Dongheon Ha and Nikolai B. Zhitenev, “Nano- and macroscale characterization of antireflection coatings made of nanosphere arrays,” Journal of Applied Physics (under review)

Semiconductor nanostructures have attracted a great deal of research interest because of their nanoscale properties that offer a great potential for improving performances in existing devices. Nanowire arrays with radial junctions are one of the most promising nanostructures for photovoltaic (PV) applications due to their light trapping and carrier collection properties that are purposely combined for enhancing solar efficiency with respect to conventional planar structures. The efficiency of nanowire solar cells may be limited by damaged nanowire junctions in the array, nevertheless efficiencies up to 9,6 % have been already reached for silicon nanowire (SiNW) radial junctions based on Si thin-film technology [1]. The characterization of such structures remains a critical issue and notably the possibility to characterize the photoelectrical performances of individual nanowires.
In this study, Kelvin Probe Force Microscopy (KPFM) technique has been used to evaluate the possibility to measure the local open-circuit voltage (VOC) of individual SiNW radial junctions based on thin-film technology [2]. All the photoelectrical measurements were performed in the KPFM platform (AIST-NT “Trios” SPM setup) that allows illumination of the sample from different directions (top, side and bottom) and with different laser wavelengths. In addition, a micropositioner has been coupled to this platform in order to electrically probe the “macroscopic” current-voltage (I-V) properties of completed SiNW devices under the same illumination conditions that will be used for KPFM analysis.
KPFM measurements were first performed on completed SiNW devices at the dark and under illumination conditions. The extracted surface photovoltage (SPV) that corresponds to the local VOC exhibits values over 650 mV, and shows for different illumination conditions a good agreement with the macroscopic values with a relative difference below 5%. Next KPFM measurements performed on individual SiNWs (uncompleted devices without ITO top contact) reveal a strong shading effect coming from the AFM tip when the tip is illuminated from the top. Side illumination greatly minimizes the previous shading effect with a local VOC that varies logarithmically with the incident power, however, the VOC remains 25 % below the values acquired on completed devices. Many issues can explain the following discrepancy notably, an angular dependence of light trapping, a change of the illumination area, a residual AFM tip shading, …. Despite these different issues we show the great interest of the KPFM technique for VOC imaging at the nanoscale on completed SiNW PV devices with good perspectives to characterize individual SiNW.

[1] S. Misra, L. Yu, M. Foldyna, P. Roca I Cabarrocas, IEEE vol.5 p.40-45 (2014) 14820286
[2] S. Misra, L. Yu, W. Chen, M. Foldyna and P. Roca i Cabarrocas, J. Phys. D: Appl. Phys. 47 (2014) 393001

The ongoing research in the field of non-volatile memories aims to overcome the limitation of flash memories in terms of scalability. Several emerging memory technologies, especially phase-change random access memories (PCRAM) and oxide based RAMs (Ox-RAMs) are already in production or explored as potential successors. A major interest of OxRAM is its ability to change the Set and Reset resistance values according to the applied electrical constraints, which is an attractive feature for neuromorphic applications.

In order to improve the understanding of OxRAM resistive switching and area scaling, electrical characterization at nanometric scale by Conductive-AFM (C-AFM) has been performed generally in atmospheric environment. In order to avoid the experimental artefacts from electro-chemical reactions due to humidity, we explore the resistive switching of OxRAMs at nanometric scale by C-AFM under vacuum. Moreover, we compare C-AFM measurements under vacuum to the ones made on standard devices in order to provide information on the switching mechanisms and scalability.

We report the observation of resistive switching cycles performed by Atomic Force Microscopy under 10^-7 Torr vacuum environment by using two samples with different bottom electrodes, TiN/HfO₂ and TiN/Ti/HfO₂. A detailed experimental study concerning the effect of: the polarity, the nature of the C-AFM tip, the bottom electrode, the serial resistance and compliance current on the resistive switching current- voltage cycling characteristics is reported. In particular, the influence of the tip work function and wear out on current-voltage characteristics is analyzed. Those complete switching current-voltage cycling characteristics are compared to those measured on TiN/Ti/HfO₂/TiN devices. The main Set and Reset characteristics are discussed. The large set voltage observed at nanometric scale with respect to the standard devices is analyzed for example. SEM EDX measurements have been carried out to evaluate the chemical composition of the C-AFM tip after I-V measurements on TiN/HfO₂ and TiN/Ti/HfO₂ samples. The experimental data are then compared to model based simulations.

It appears that the difference between C-AFM measurements and those obtained on devices cannot be explained only by top electrode area scaling. It is found that the nature of the tip and the quality of its contact with the substrate surface determine the differences of switching characteristics between all the above mentioned experiments. The model results indicate that those differences might be due to differences in filament morphology according to the nature of the top electrode or C-AFM tip.

Charge-state investigations of single adsorbates on ultrathin insulating films have revealed fundamental charging properties. For example, charge-state determination and control of adatoms (1) and molecules (2,3) on bilayer NaCl as well as the charge-distribution mapping within a molecule (4) was demonstrated. However, multilayered insulating films are necessary to efficiently confine charges at the surface – requiring better understanding of adsorbate charging and their quantification on thicker films.
Here, we exhibit the essential steps towards exploring stable charge states of single molecules. First, we demonstrate reversible charge-state manipulation, leading to a tip-induced lateral single-electron transfer between molecules (5). Second, we present a novel approach based on single-electron detection using an atomic force microscope that allows tunneling spectroscopy measurements of a single molecule on an insulator. This culminates in the characterization of a fundamental property of the molecule/surrounding system, the reorganization energy.

(1) J. Repp et al, Science 305, 493 (2004)
(2) T. Leoni et al, Physical Review Letters 106, 216103 (2011)
(3) I. Swart et al, Nano Letters 11, 1580 (2011)
(4) F. Mohn et al, Nature Nanotechnology 7, 227 (2012)
(5) W. Steurer et al, Nature Communications 6, 8353 (2015)

Frequency modulation atomic force microscopy (FM-AFM) is found to be promising tool for atomic resolution imaging and solid/liquid interface study. Also it could be used in different environmental conditions such as vacuum, atmospheric air, and liquids. Hence this technique has great impact in the various research fields like surface science and nanotechnology. Ionic liquids (ILs), composed of organic cations and weakly coordinating organic/inorganic anions, exhibit remarkable and tunable physicochemical characteristics such as negligible vapor pressure, non flammability, wide electrochemical window, high thermal stability, broad liquid range, and excellent conductivity, which promises their potential for large range of applications like batteries, chemical reactions and processes, solar cells, catalysis, lubricants, etc. When IL is in contact with solid substrate, the properties of IL at the interface are different as compare to bulk IL hence to determine their interfacial behavior is so important for scientific and technological processes. Also to enhance the efficiency and selectivity of particular process the detailed knowledge of IL / solid interface is needed. Therefore, the investigation of IL/solid interfaces is very important study for not only in fundamental understanding point of view but also the industrial applications purpose.
Herein, we demonstrate the atomic resolution imaging of KBr (100) and KBr (111) crystal surface in 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF₆) IL by using FM-AFM with force sensor based on quartz tuning fork [1]. The (100) and (111) surfaces of alkali halides including KBr possess different surface properties because the (100) surface is electrically neutral while the (111) consists only of either cations or anions. Thus it is an interesting task to understand effect of surface nature on IL/KBr interface. The (100) sample was obtained just by cleavage and the (111) sample was fabricated by IL-assisted vacuum deposition [2]. Atomic resolution images of KBr (100) and (111) surfaces which were obtained in the IL reveals that, atomic arrangement of (111) surface was slightly distorted compared to the (100) surface, which would be because of its high surface energy and surface rearrangement. Furthermore, we demonstrate how IL molecules exist on two dimensional graphene nanosheets and at their interface. The detailed study will be discussed during presentation.

References
[1] T. Ichii, M. Negami, and H. Sugimura, J. Phys. Chem. C. 118, 26803 (2014).
[2] M. Yamauchi, S. Maruyama, N. Ohashi, K. Toyabe, and Y. Matsumoto, CrystEngComm, 18 3399 (2016).

Molecular photoswitches have been the focus of intense research for decades as these systems are potentially useful as optoelectronic switches for controllable optical filters, data recording, signal transduction in sensing systems etc. Despite the variety of uses of these systems, many photochromic molecules have proven to be difficult to switch when placed on a metal surface, limiting their utility for integration with electronic systems. This work demonstrates that isomerization from the norbornadiene (NB)-state to the quadricyclane (QC)-state can be electrically induced on metal surfaces. We utilize the STM-Break Junction (STM-BJ) technique to make contact to and investigate the switching properties of these photoswitching molecules synthesized by Moth-Poulsen et al. Specifically, at room temperature under high vacuum, at bias=0.05V, the high conductance NB-state is detected, meanwhile, at bias=0.2V, the low conductance QC-state is observed. Alternatively, when the STM-BJ experiments are performed in a mesitylene solvent environment, the switching does not occur until reaching a bias of ~1.2V. This change in the switching voltage suggests that the solvent serves as a cold bath and that a local heating effect may be responsible for the observed switching behavior. A series of STM-BJ tests under different ambient temperatures ranging from 78K to 300K shows the correlation between the switching bias and the ambient temperatures. The bias-temperature dependence fits well with a model of local heating effect due to electron-phonon interaction by Di Ventra et al. which yields a local effective temperature of 325K at the molecular junction when switching occurs. By modeling the behavior with an Arrhenius-type relation we are able to examine the effects of surface binding on the switching energy barrier. Compared with our previous work of switching molecules from NB-state to QC-state through radiating laser light of 405nm, we demonstrate that electrically controlled excitation from NB to QC could also be achieved. This work brings deeper insight of chemical reaction at a single molecule level on the metal surface as well as provide a new approach to electrically control and engineer photoswitching molecule behavior from a surface science perspective.

The III-V nanowire (NW) technology platform has reached a level of advancement that allows atomic scale control of crystal structure and surface morphology as well as flexible device integration. Combining this with Scanning Tunneling Microscopy/Spectroscopy (STM/S) opens up for studies of the role of nanostructure crystal variations down to the atomic level as well as direct in operando imaging of semiconductor surfaces response to electrical operation. Importantly, novel use of device processing allows all of this to be accomplished with a standard STM.

We have previously demonstrated atomically resolved STM/S on a wide variety of III-V NWs and standard devices [1-5]. We now use these methods for studying atomic scale crystal phase changes and the impact on local electronic properties on both GaAs and InAs NWs as well as demonstrating full atomic resolution STM during device operation [6-8].

Firstly, we explore the surface diffusion and alloying of Sb into GaAs NWs with controlled axial stacking of Wurtzite (Wz) and Zincblende (Zb) crystal phases[6]. We find that Sb preferentially incorporates into the surface layer of the Zb segments rather than the Wz segments. Density functional theory calculations find that this is related to differences in the energy barrier of the Sb-for-As exchange reaction. This demonstrate a simple processing-free route to compositional control at the monolayer level using crystal phase engineering.

Secondly, using 5K STM/S we measure local density of states of Zb crystal segments in Wz InAs NWs down to the smallest possible atomic scale crystal lattice change[7]. We find that the Zb phase signatures are seen in both the conduction and valence bands as well as in the band positions down to the smallest Zb segment. We also find indications of confined state effects due to the difference in bandgap between Wz and Zb.

Thirdly, we demonstrate a novel device platform that allows STM/S with atomic scale resolution across a III-V NW device simultaneously with full electrical operation [8]. The platform presents a significant step forward as it allows STM everywhere on the device surface and high temperature processing in reactive gases of the device. We demonstrate the new method by measurements on both InAs and GaAs NW devices with variable biases across the nanowire of up to 4 V. On InAs NWs we observe a surprising removal of atomic defects and smoothing of the surface morphology. As we use standard fabrication and scanning instrumentation our concept is widely applicable.

[1] E. Hilner et al., Nano Lett., 8 (2008) 3978; M. Hjort et al., ACS Nano 6, 9679 (2012)
[2] M. Hjort et al., Nano Lett., 13, 4492 (2013)
[3] M. Hjort et al., ACS Nano, 8 (2014) 12346
[4] J. L. Webb, et al Nano Lett. 15 (2015) 4865
[5] O. Persson et al., Nano Lett. 15 (2015) 3684
[7] M. Hjort et al Nano Lett., 17 (2017) 3634
[8] J. V. Knutsson et al ACS Nano, 11 (2017) 10519
[9] J. L. Webb et al, Sci. Rep. 7 (2017) 12790

Atomically precise graphene nanoribbons synthesized via wet chemical method opens opportunities for tailoring their structures as well as electronic properties and therefore have ignited enormous interest. Among different techniques for GNR characterization, scanning tunneling microscopy and spectroscopy (STM/STS) provide both morphological details and local electronic structure with atomic resolution. Here we applied a dry contact transfer technique [1] to deposit solution-synthesized doublewide GNRs (dGNRs) and extended-chevron GNRs (eGNRs) [2] onto clean InAs (110) and hydrogen-passivated Si(100) semiconducting surfaces under ultrahigh vacuum conditions. High-resolution STM images were collected to confirm their structures. By performing scanning tunneling spectroscopy, the band gap of the wGNRs and the eGNRs were determined to be 2eV and 2.6eV, respectively. For the wGNRs on InAs, we also carried out detailed analysis on mapping of the electronic density of states both spatially and energetically via STS and current imaging tunneling spectroscopy. We found that the electron orbital shapes at the GNR edges are different from those at the centers. Correspondent DFT simulations of isolated GNRs were also carried out and showed great consistency with our experimental results, indicating a very weak coupling between the GNR and the InAs at an equilibrium position. In addition, we demonstrated the tunable transparency effect of the GNRs with respect to the underlying substrate, creating tunable GNR-substrate interaction by pushing GNRs towards the surface.

References:
1. Ritter, K. A.; Lyding, J. W. The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons. Nat. Mater. 2009, 8 (3), 235−42.
2. Mohammad Mehdi Pour, Andrey Lashkov, Adrian Radocea, Ximeng Liu, Tao Sun, Alexey Lipatov, Rafal A. Korlacki, Mikhail Shekhirev, Narayana R. Aluru, Joseph W. Lyding, Victor Sysoev & Alexander Sinitskii, Nature Communications 8, article 820, October 2017.

Using real-time in-situ scanning tunneling microscopy, Auger electron spectroscopy and density functional theory simulations, we have studied the growth of Si films on Ag(111) beyond the silicene monolayer, evidencing the existence of metastable phases and an original growth mechanism. Above monolayer Si coverage, a first (√3×√3)R30° structure forms, which we identify as a Ag-free Si bilayer with additional Si adatoms. With further deposition, this structure is replaced by a distinct bilayer structure covered by Si trimers and Ag atoms. The formation of these bilayers follows a counterintuitive dynamics: they are partially inserted within the Ag substrate and form by expelling, from the underlying substrate, atoms that reinsert below the adjacent silicene layer. The growth is therefore characterized by an unexpected "surfactant competition" between Ag and silicene: while silicene is a metastable surfactant for the Ag(111) surface, Ag plays the role of a surfactant for thicker diamond-like Si islands. This mechanism also reproduces the evolution of the Si and Ag Auger intensities during growth. Our conclusion is that, due to the high kinetic-barrier for the growth of thicker layers, the silicene monolayer is a remarkably stable structure, in spite of being thermodynamically unfavoured.

We present the properties of molecular junctions fabricated on a large array of sub-10 nm single crystal Au nanodot electrodes (nanodot-molecule junctions : NMJ), contacted by conducting AFM (C-AFM) [1-4] or interferometric scanning probe microscope (iSMM) [5]. Each individual NMJ consists of less than one hundred molecules. Typically, 2000-4000 NMJs are measured in a few μm² C-AFM image. Conductance histograms and 2D histograms of the current-voltage (I-V) curves are extracted from these measurements [2, 3]. We report a C-AFM study of π-π interactions from electrochemical and conductance measurements on a large array of ferrocene-thiolated gold nanocrystals [4]. Despite substantial theoretical progress, a direct experimental measurement of the π-π electronic coupling energy parameter t has remained challenging due to molecular structural variability and the large number of parameters that affect the charge transport. We confirm the theoretical prediction [6] that t can be assessed from a statistical analysis of current histograms for thousands of nanocrystals. The extracted value of t ≈ 35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the sub-angström level. Molecular electronics originally promised that molecule(s) bridging two or more electrodes would generate electronic functions and overcome scaling limits of conventional technology. However, so far, these molecular devices have been only demonstrated at low frequency. Here, we demonstrate molecular diodes operating up to 18 GHz with an estimated cut-off frequency of 520 GHz [5]. With the iSMM, we demonstrate microwave molecular diodes. The DC and RF properties were simultaneously measured on a large array of NMJs with ferrocenyl undecanethiol molecules, contacted by the tip an interferometric scanning microwave microscope (iSMM). We show that the measured S₁₁ parameters exhibit a diode rectification ratio up to 12 dB and up to 18 GHz. This feature can be directly related to the DC conductance and aF range fringing capacitance of the molecular junctions. The RF and DC data are combined using simple model from which we extract a high conductances up to 0.36 mS and aF range of fringe capacitances. From these results, we extrapolate a cut-off frequency of 520 GHz for the molecular diode [5].
[1] N. Clement, et al., Small, 7, 2607-2613, 2011.
[2] K. Smaali, et al., ACS Nano, 6, 4639-4647, 2012.
[3] K. Smaali, et al. Nanoscale, 7, 1809-1819, 2015.
[4] J. Trasobares, et al., Nano Letters, 17,3215–3224, 2017.
[5] J. Trasobares, et al. Nature Communications, 7, 12850, 2016.
[6] M. G. Reuter, et al., Nano Letters, 12, 2243-2248, 2013.

The conformation of a molecule determines its chemical and physical characteristics. Consequently, changes in molecular conformation modify important properties such as molecular conductance, chemical activity and assembly. Clear identification of the conformational structure is therefore crucial for understanding these properties. Whilst submolecular resolution imaging with non-contact atomic force microscopy (nc-AFM) has provided unparalleled insight into many interesting molecular systems [1,2], non-planar molecular structures have proved far more challenging. Here I will discuss studies on tetra(4-bromophenyl)porphyrin (Br₄TPP) molecules deposited onto a Cu(111) surface, which are known to adopt two distinct non-planar conformations. Using nc-AFM we are able to unambiguously determine the adsorption configuration of porphyrin conformers on the Cu(111) surface with submolecular detail. We show through direct measurement that the lateral forces required to manipulate each conformer significantly differ [3], suggesting unusually strong binding arising from vdW interactions that consequently dramatically affect the molecular density of states [4]. Following this, I will discuss the perceived limits of imaging molecules with non-planar structure and the potential to achieve single bond resolution outside of UHV conditions as a general analytical tool. [1] Jarvis, S.P. Jarvis. Int. J. Mol. Sci., 2015, 16, 19936. [2] N Pavlicek and L. Gross. Nature Reviews Chemistry, 2017, 1, 0005. [3] Jarvis, S. P. et al, Nature Commun. 2015, 6, 8338. [4] Jarvis, S. P. et al, J. Phys. Chem. C 2015, 119, 27982.

Two-dimensional (2D) layered hexagonal boron nitride (hBN) is an insulator, isostructural to graphene, and is considered an ideal dielectric for a variety of 2D layer based nanoelectronic devices. In contrast to free-standing hBN layers, substrate-supported hBN can be metallic, semiconducting, or insulating, depending on the chemistry and electronic structure of the hBN/substrate interface. Control over these properties hinges on the ability to grow high-quality, wafer-scale hBN crystals. hBN layers are commonly grown on transition-metal substrates and the hBN layer structures, as well as their interactions with the transition-metals have been fairly well characterized. However, relatively few studies have investigated (i) the mechanisms underlying the growth of the hBN layers and (ii) influence of hBN domain orientation (with respect to the substrate) on its electronic properties.
Using in situ ultra-high vacuum scanning tunneling microscopy (UHV STM), we investigated the chemical vapor deposition (CVD) kinetics of hBN on Pd(111). STM images are acquired during CVD of borazine as a function of substrate temperature, borazine flux, and deposition time. We observe the nucleation and growth of chemisorbed borazine islands on the Pd surfaces. Furthermore, we investigated the surface structure of hBN domains on Pd(111) using STM, scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. STM images acquired from the hBN/Pd(111) sample reveal moiré patterns with at least six different periodicities (λ) corresponding to six rotational domains of hBN. From the STM images, we measured the surface corrugations in each of the moiré patterns and found that the corrugation amplitude △z depends on the tunneling bias and increases with increasing λ. We suggest that △z is a measure of hBN-Pd(111) interaction strength and attribute the higher corrugation amplitudes to stronger interactions between the hBN domains and the Pd(111). We expect that similar approach could be used to investigate growth kinetics and orientation-dependent interactions in other substrate-supported 2D layers as well as in 2D layered heterostructures.

Following theoretical predictions [1], two-dimensional (2D) boron (i.e., borophene) was experimentally realized less than two years ago via molecular beam epitaxy on noble metal substrates [2,3], showing intriguing properties including high in-plane anisotropy and unique surface chemistry [4]. As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, implying that the intrinsic defects of borophene remain unexplored. To fill this knowledge gap, we perform here an atomically-resolved study of borophene structures with ultra-high vacuum scanning tunneling microscopy and density functional theory. Borophene phases corresponding to the v_1/6 and v_1/5 models are found to coexist and intermix under suitable growth conditions. In each phase, we identify parallel line defects with atomic structures that correspond to the constituent units of the other phase. Furthermore, these line defects are observed to preferentially self-assemble into periodic arrays, giving rise to new borophene phases. Therefore, the distinction between borophene crystals and defects is blurred by the fact that line defects become building blocks of new phases with a single tuning parameter being the mixing ratio of the v_1/6 and v_1/5 rows. This unique phenomenon in borophene is enabled by its high in-plane anisotropy, similarities of the v_1/6 and v_1/5 lattice constants, and atomic registry with the underlying Ag(111) substrate. Overall, this work reveals the unique behavior of borophene line defects, which will likely inform future fundamental studies and emerging efforts to realize borophene-based applications.

[1] Y. Liu, E. S. Penev, B. I. Yakobson, Angew. Chem. Int. Ed. 52, 3156–3159 (2013).
[2] A. J. Mannix, X.-F. Zhou, B. Kiraly, J. D. Wood, D. Alducin, B. D. Myers, X. Liu, B. L. Fisher, U. Santiago, J. R. Guest, M. J. Yacaman, A. Ponce, A. R. Oganov, M. C. Hersam, N. P. Guisinger, Science 350, 1513–1516 (2015).
[3] B. Feng, J. Zhang, Q. Zhong, W. Li, S. Li, H. Li, P. Cheng, S. Meng, L. Chen, K. Wu, Nat. Chem. 8, 563–568 (2016).
[4] X. Liu, Z. Wei, I. Balla, A. J. Mannix, N. P. Guisinger, E. Luijten, and M. C. Hersam, Sci. Adv. 3, e1602356 (2017).