Atomic force microscopy-based local dielectric spectroscopy (LDS) allows to probe the dielectric properties of polymeric materials with high lateral resolution, We employed LDS to analyze the miscible blend composed of poly(vinyl acetate) (PVAc) and poly(ethylene oxide) (PEO). The two homopolymers have very different relaxation times and glass temperatures, which give rise to dynamic heterogeneity in their blends. The aim was to study the dynamic heterogeneity in films as a function of the film thickness. Measurements of the local blend composition at the nanoscale show that LDS is indeed sensitive to the dynamic heterogeneity. In thin films, phase segregation of the homopolymers occurs due to heterogeneous nucleation and crystallization of PEO. We were able to follow the kinetics of phase demixing by detecting the change in local composition of blends including depletion zones of PEO in the PVAc/PEO blend around the PEO crystals via LDS spectra. These results open new possibilities for studying surface segregation in polymer blends, local variation in polymer concentration, and interdiffusion at polymer-polymer interfaces as a function of annealing temperature with LDS.

The surfaces of solids, when immersed into a liquid, tend to develop charges. To maintain the neutrality of the system, the counterions and the ions dissolved in the liquid accumulate close to the surface of the solid, forming the so-called Electrostatic Double Layer (EDL). The distribution of the counterions density perpendicular to the surface is usually described with the continuous Gouy-Chapman-Stern model. This model assumes a dense monolayer (called Stern layer) with a thickness of a hydrated ion, adsorbed onto the charged surface, followed by an exponentially decaying diffuse ionic layer. In this representation, ions are considered as point charges in a continuous dielectric media. Although a good approximation in the diffuse layer region, the model tends to fail in the Stern layer where the complex interactions between ions, liquid molecules and solid surface cannot be fully described by continuum theories.¹ Moreover, the lateral spatial organization of ions within the Stern layer is cannot be conceived continuum models.
Most of the experimental techniques currently used to characterize Stern layers or EDLs, such as X-Ray Reflectivity,² cannot provide local information about the spatial distribution of the ions in proximity of the surface.
Here we use Atomic Force Microscopy (AFM) to investigate the properties of the aqueous interfaces in presence of different ionic concentrations and species with two crystalline solids (mica and calcite) and biologically-relevant systems such as organic self-assemble monolayers and lipid bilayers with different head groups. When operated in liquid, small amplitude AM-AFM is able to investigate solid-liquid interfaces, with atomic- or molecular-level resolution.³ When ions adsorb at the surface of the solid, they induce a substantial perturbation of the local solvation environment that can be detected by operating the AFM in this specific regime.⁴ Our AFM study, combined with Molecular Dynamic Simulation, reveals that, depending on the specific hydration properties of the surface and of the ions, the typical distance between surface and ions is varying. Moreover water-induced ion-ion attractive interactions can be detected in the case of mica-water interface and are typical of chaotropic ions. These results show that water alone can provide a sufficient driving force to induce order within the Stern layer.
1. Ben-Yaakov, D.; Andelman, D.; Podgornik, R.; Harries, D. Curr. Op. Col. Interf. Sci.2011, 16, 542-550.
2. Lee, S. S.; Fenter, P.; Nagy, K. L.; Sturchio, N. C. Langmuir2012, 28, 8637-8650.
3. Voïtchovsky, K.; Kuna, J. J.; Contera, S. A.; Tosatti, E.; Stellacci, F. Nature Nanotechnology2010, 5, 401-405.
4. Ricci, M.; Spijker, P.; Stellacci, F.; Molinari, J.F.; Voïtchovsky, K. Langmuir2013, 29, 2207-2216.

The dynamics of electrons in materials plays a fundamental role for electrical conductance, electronic excitations as well as for adsorbate interactions which are mediated by the electronic states of the substrate. Here, we investigate for the first time the quantum coherence of bulk state electrons with scanning tunneling microscopy. As a model system we use Ag(100) as no surface state is reported at the Γ-point providing direct access to the bulk states. By measuring conductance maps above a threshold voltage, we observe standing wave patterns. These originate from electrons in a bulk band edge at the Γ-point, which are scattered at step edges and defects. From the spatially decaying waves, the wave vector and the quantum coherence parameters - coherence length, lifetime, and line width - are determined as a function of energy. The energy of the band edge is extracted from the dispersion relation and agrees with the peak measured in scanning tunneling spectra at 1.9 eV above the Fermi energy. Theoretical calculations confirm the nature of the state elucidating the experimental findings.

In future silicon nanoelectronics, such as proposed end-of-Moore&’s-law metal-oxide-semiconductor devices and silicon-based quantum bits, the number and placement of single dopant atoms in the device active region is critical in determining device performance. Scanning tunneling microscopy (STM) lithography techniques that allow atomic-precision placement of dopants in Si have developed to the extent that field-effect devices based-upon single dopant atoms have been fabricated in a deterministic process. The fabrication technique consists of atomic-precision depassivation lithography on a hydrogen terminated Si(100) surface with an STM tip, phosphine adsorption and phosphorus dopant incorporation via an anneal, and then subsequent burial of the doped regions in ~25-nm-thick epi-Si. The process yields buried 2-d degenerately doped regions just a few atomic layers thick with atomically sharp edges.
One particularly challenging aspect of integrating the optically invisible buried donor structures into conventional microfab processing is nanoscale-precision registration of their location, with respect to alignment marks, for subsequent electrical contact fabrication (via hole definition, etch, and metallization).
We show that post-STM ex-situ scanning capacitance microscopy (SCM) can be used to locate and image the dopant (carrier) distributions of STM defined donor structures with sub-100-nm (tip limited) resolution. This allows the optically invisible buried dopant structures to be located with 100-nm-scale precision with respect to metal alignment markers added after STM processing.
In this talk, we present the first SCM images of STM-fabricated atomic-precision buried donor structures, then describe the process for SCM registration and electrical contacting of the buried structures. Low-temperature (4K) transport measurements on the devices confirm ohmic electrical contact to the buried structures. We will also discuss applications of SCM for other useful purposes including imaging the shape of the carrier distribution, relative doping level, and post-processing failure analysis.
Acknowledgments:
This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000.

Direct characterization of the local electronic properties of self-assembled quantum dots (QD) via transport measurement is extremely challenging because of the difficulty in attaching electrodes. Alternative spectroscopy technique based on scanning probe microscopy can alleviate this problem by using the probe as an electrode. Single-electron electrostatic force microscopy (e-EFM) is such a technique that builds upon frequency modulation atomic force microscopy (FM-AFM) to perform charge detection. We have shown in previous experiments that e-EFM can be used to measure the charging energy, energy level spacing and electron tunneling rates of various nanostructures deposited on thin insulating films from cryogenic up to room temperature.
In this work, we present theoretical and experimental results of the effect of the density of states of a quantum dot (QD) on e-EFM experiments. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the QD, which is detected through its back-action on the capacitively coupled cantilever, depends on the effective tunneling rate of the QD to a back-electrode. By performing bias spectroscopy, the density of states of the QD can therefore be measured through its effect on this tunneling rate. We present experimental data on individual 5nm gold nanoparticles (GNP) chemisorbed on a self-assembled monolayer of hexadecanedithiol (C16S2) which exhibit a near continuous density of state at 77K.
In contrast, our analysis of already published data on self-assembled InAs QDs at 4K, clearly reveals discrete degenerate energy levels. We discuss the main experimental considerations that went into the preparation of those samples in order to achieve sufficient signal-to-noise ratio for e-EFM.
This technique will open up the possibility to study the local density of states of nanostructures supported by thicker insulating layer than that accessible by scanning tunnelling spectroscopy.

Capacitance force microscopy (CFM) is a variant of atomic force microscopy (AFM) employed to characterize gate oxides in metal-oxide-semiconductor capacitors (MOSCAPs). In CFM, the gate electrode is replaced by a conductive AFM tip significantly simplifying the sample preparation procedure for C-V measurements. In conventional CFM only the differential capacitance is measured. To measure the absolute C-V spectra, a commercial AFM (Veeco Multimode®) was modified to connect the tip and sample to a variable frequency, high sensitivity capacitance bridge (AH 2700A). Due to the very small tip/oxide contact area, the electric field distribution and minority carrier response to the applied bias may be different from conventional C-V measurements. Therefore, an experimental method is required to determine the effect of contact area on the shape and frequency-dependence of C-V curves measured by CFM. Test samples were fabricated with gold dots of diameters ranging from 40 to 300 mu;m on n-type heavily-doped silicon with 100 nm thick SiO2 grown on top. The capacitance of resulting Au/SiO2/Si+ stacks with a 90000x range of area were measured both by conventional probe station (Agilent B-1500) and the CFM system. The CFM and probe station measurements were repeated on 60- to 300-mu;m Pd dots on n-type GaN substrates with 8nm thick Al2O3 on top. C-V curves measured by the CFM on heavily-doped silicon and n-GaN showed a good agreement in shape and absolute values with the ones obtained from the probe station. Gateless C-V measurements, using a blunt AFM tip with 50-nm thick gold coating, was performed by placing the tip directly on Al2O3. Gateless C-V curves were recorded at frequencies from 500 Hz to 10 KHz. Despite the small contact area and as a result, small capacitance signal, each C-V measurement was performed in less than 10 min. Effective tip/sample contact area has been found to be much larger than the actual physical contact area as consequence of fringing electric field, propagating from the tip apex into the semiconductor, and relatively low doing of the n-GaN substrate. The advantage of the enlarged effective tip area is that accurate variable frequency C-V characterization can be performed at high speed independent of the exact shape of the AFM tip.

Fast ion conductors are ionic materials with high electrical conductivity comparable with that of liquid electrolytes. They are of great importance in the area of solid-state ionics, and are useful in a variety of applications, including batteries, sensors, and solid oxide fuel cells. Thus, there have been tremendous efforts to understand basic mechanism and improve their properties. However, elucidating underlying mechanisms of fast ionic conduction necessitates probing electrochemical processes in the materials on the nanometer scale level. Recently, newly developed scanning probe microscopy (SPM) techniques, such as electrochemical strain microscopy (ESM) [1] and first-order reversal curve current-voltage (FORC-IV) method [2], open pathways for probing local electrochemical phenomena and ionic dynamics at the nanoscale.
Here we report nanoscale probing of ion dynamics and conduction in (AgI)_0.2(AgPO₃)_0.8 glasses by using a variety of SPM-based spectroscopies. To investigate the ion dynamics and conduction in fast ion conductors, we have chosen silver conducting glasses as a model system. Silver conducting glasses are one of the best known solid electrolytes and some compositions present a conductivity as high as 10^-2 S/cm [3]. In particular, Ag is much more stable than Li in air and does not react with electrolytes. We observed that their electrochemical window is small and nucleation potential is close to the Ag-metal reduction potential. From the band excitation ESM measurements, we observed that the formation of Ag occurs near the sites with high ESM signals. In addition, FORC-IV measurements show intersting two slope behavior of I-V in the positive side. Based on these experiemental results, we discuss the underpinning dynamics of Ag ions and their conduction behavior.
[1] N. Balke et al., Nat. Nanotechnol. 5, 749 (2010)
[2] E. Strelcov et al., Nano Lett. 13, 3455 (2013); Y. Kim et al., Scientific Reports 3, 2924 (2013)
[3] A. C. M. Rodrigues et al., J. Chem. Phys. 135, 234504 (2011)

Electrochemical reactions in solids underpin multiple applications ranging from electroresistive non-volatile memory, to chemical sensors, to energy storage and conversion systems including metal-air batteries and fuel cells. Traditionally, these effects are studied on the macroscopically averaged level, but a more comprehensive understanding of these phenomena is possible if they can be studied on the nanoscale - on the level of individual grain boundaries, catalyst particles, and defects. Furthermore, understanding the functionality in these systems at such length scales requires probing ion mobility and the thermodynamics and kinetics of reversible and irreversible electrochemical processes over a range of temperatures and atmospheric conditions.
Scanning probe microscopy (SPM) has emerged as a powerful tool for probing nano and atomic scale functionality of materials and devices through the detection of electronic currents and tip-surface forces coupled to phenomena of interest. Here I discuss the development of Electrochemical Strain Microscopy (ESM) and Spectroscopy as an AFM based nanoscale probe of ionic functionality in solids. In ESM, the tip is used to concentrate an electric field in a nanometer scale volume of material, inducing local ion transport and local surface strain. The simultaneous measurement of electromechanical response and current provide information on bias-induced changes in a material. Here, I illustrate how these methods can be extended to study and visualize local electrochemical transformations, including oxygen vacancy dynamics in oxides and lithium ion dynamics in a range of materials of interest for Li-ion and Li-air batteries.
Combining time-dependent, multi-frequency electrochemical strain measurements with current and other information channels over many spatial locations and as a function temperature results in highly nontrivial challenges for data acquisition and analysis, necessitating a transition from 1-dimensional (1D) spectroscopic and 2D imaging studies to 6D data acquisition protocols and analysis of resultant 1-100 GB data sets. Methods for processing this data including multivariate and multi-resolution analysis will be presented.
This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Here we introduce a simple and fast method to reliably image ferroelectric and piezoelectric domain walls using charge gradient microscopy (CGM). Various scanning probe microscopy (SPM) modes have been invented to characterize the local piezoelectric properties and polarization charges of ferroelectric thin films and bulk materials. In particular, piezoresponse force microscopy (PFM), operating under the principle of the inverse piezoelectric effect to map nanoscale ferroelectric domains, is the primary technique for recording and investigating ferroelectric domain patterns. Yet the scan speed limitations of PFM by the resonance frequencies of the cantilever and the time constant of the lock-in amplifier restrict its application to investigate dynamic properties of piezoelectric and ferroelectric materials. Additionally, the necessary excitation voltage has the potential of influencing dynamic behavior in ferroelectric films. In this work, we collected the current flow from the grounded CGM tip while scanning a periodically poled lithium niobate (PPLN) single crystal and single crystal LiTaO3 thin film on a Cr electrode. We found a strong current signal at the domain walls originating from the displacement current and the relocation of bound surface charges, which enabled us to visualize the domains at scan frequency above 78 Hz with spatial resolution of about 200 nm. The interplay between polarization charge, screening charges and their removal and replenishment all contribute to the complex CGM signal and will be discussed.
In addition to the fast scanning CGM does not require a lock-in amplifier enabling domain imaging with any standard scanning probe microscope that incorporates a current amplifier. We envision that CGM will be used in high-speed ferroelectric and ionic domain imaging and novel piezoelectric energy harvesting devices.
This work was supported by US Department of Energy, Office of Science, Office of Basic Energy Sciences-Materials Science. The Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357

To meet the strict requirements of miniaturization and power consumption, new semiconductor materials such as high mobility III-Vs are being developed and progressively introduced. They are typically grown in narrow trenches on a Si substrate. In this work we present a study based on scanning probe microscopy (SPM). Our work is composed of two parts: development of a new procedure for sample preparation of III-Vs and a combination of non-contact atomic force microscope (NC-AFM) with scanning tunnelling microscope (STM) by using the same AFM probe.
III-Vs consist of native oxide layer on the surface which needs to be removed in order to analyze their surface crystallographic organisation. In addition, measurements need to be carried out in ultra-high vacuum (UHV) environment to preserve the surface from further oxidation. Removal of the native oxide from InP surface by classical thermal annealing leads to degradation of the sample due to P desorption and In nucleation [1]. Wet cleaning is efficient [2] but cannot be done in UHV environment. We developed a new procedure for oxide removal consisting of a wet cleaning step (H2O2/HCl+HCl) combined with a lower temperature thermal annealing. It allows to remove the native oxide layer avoiding surface degradation. Oxide-free InP surfaces were confirmed by STM images with atomic resolution showing 4x2 surface reconstruction. The developed procedure was also applied to patterned samples but in this case different behaviours have been noticed due to the different properties of the material when grown in trenches on a Si substrate (defect density, process steps like CMP and limited dimensions). Nonetheless, successful oxide removals were confirmed by the first STM images of surface reconstruction of InP when grown in trenches.
STM measurements on trenches were performed using a SPM combined with scanning electron microscope in the same UHV chamber allowing us to land the STM tip on very small areas (trenches width=500nm) in an oxide sea. However, final devices (e.g. III-V nMOS), have much smaller trench widths (20-100nm) which cannot be localized using this procedure. We have therefore implemented a new method consisting of two steps: localization of the trench in the oxide by NC-AFM and then analysis of very localized areas within the trench by switching to the STM mode. This is feasible due to the high stability of the system allowing to keep the position during the switch between the two modes. Since NC-AFM needs a cantilever as a force sensor, STM must be performed using the same conductive AFM tip. First results obtained, push us to explore sets of tips finding a trade-off in tip stiffness to allow STM with a certain stability but at the same time sensitive enough to have topographic information by NC-AFM.
REFERENCES
[1] F. Riesz et al., J. Vac. Sci. Technol. B (16)5, Sep/Oct 1998.
[2] D. H. van Dorp et al., ECS J. Solid State Sci. and Technol., 2 (4), 190-194, 2013.

In this paper we illustrate how high resolution two-dimensional carrier profiles from scanning spreading resistance microscopy (SSRM) can be applied to predict and understand device performance of dynamic random access memory (DRAM) peripheral transistors with high-k metal gate and ultra shallow junctions. In earlier work [1], we demonstrated the proof of concept for using SSRM two-dimensional (2D)-carrier profiles as an input to device simulator for better understanding of device characteristics such as drain-induced barrier lowering (DIBL). As in this case high source/drain and halo concentrations were involved, the 2D-carrier profiles from SSRM could be introduced as 2D-active dopant profiles as mobile carrier diffusion (~2nm) between different impurity regions was very limited.
In comparison to high-speed logic, the devices such as peripheral MOSFETs in DRAM technology make use of low doping concentrations whereby the spatial extent of mobile carrier diffusion becomes much more pronounced (~10nm). Therefore, the difference between dopant and carrier profiles that is the difference in metallurgical versus electrical junction becomes quite significant and can no longer be neglected. Hence in order to provide the correct input for the device simulator it is, in this case, necessary to reconstruct starting from the carrier profile, as measured by SSRM, the accurate active 2D doping profiles. In principle the latter can be done using the backward solution of the 2D-poisson&’s equation but is non-trivial as it is very time consuming and numerically unstable as it is quite sensitive to small variations in the measurement data.
Therefore, in this paper we take an alternative approach whereby we predict, starting from the known process conditions, the 1D and 2D dopant profiles and subsequently the 2D-carrier profiles. The latter are then compared and validated with the high-resolution 2D experimental SSRM carrier profiles and modifications in the process simulations are implemented to accommodate observed deviations. Using this procedure agreement between simulated and experimental device results can be obtained for sensitive parameters such as DIBL and threshold voltage roll-off.
References:
1 A. Nazir, P. Eyben, T. Clarysse, G. Hellings, A. Schulze, J. Mody, K. De Meyer, H. Bender, and W. Vandervorst, Solid State Electronics 74, 38 (2012).

Li-ion batteries (LIBs) have been used as energy storage devices for various applications, e.g. mobile electronic devices and vehicle. As gradually miniaturizing electronic devices, there has been recently much interest on the miniaturization of Li-ion batteries. However, since electrochemical analysis of the LIBs has been primarily concentrated at the point of micro/macroscopic views, there is no sufficient information on the nanoscale electrochemical properties of the LIBs. In this presentation, we explored electrochemical properties of Li-ion on Li-ion conductive glass ceramics (LICGC) using advanced atomic force microscopy techniques such as electrochemical strain microscopy (ESM) and I-V spectroscopy. We observed correlation between electrochemical reactivity and ESM hysteresis loop opening. We further observed spatial map of electrochemical coefficients which was also correlated with electrochemical reactivity and showed non-linear behavior. These results could provide additional information on the electrochemical properties to understand basic operational mechanisms of LIBs at the nanoscale.

Atomic force microscopy (AFM) has been used commonly in order to measure the surface properties of materials, such as Topography, Electric Force Microscopy and Magnetic Force Microscopy and so on. This powerful equipment can be more powerful tool by replacing cantilever into quartz tuning fork, because the quartz tuning fork has some advantages compared with the cantilever. First, quartz tuning fork has a high quality factor. This means the quartz tuning fork react with very weak force between sample and tip. So, the quartz tuning fork AFM can obtain high resolution image. A representative example is qPlus sensor [1, 2]. Giessbl group achieved a sub-atomic resolution topography using it. Second, quartz tuning fork can be a sensor without both laser diode and optical detector. For this reason, quartz tuning fork AFM can be utilized for a dark room experiment. Because of these advantages of quartz tuning fork, many groups are manufacturing quartz tuning fork AFM. Moreover, quartz tuning fork AFM is successfully deployed on a commercial scale [3].
In order to make the quartz tuning fork vibrate stably, it is attached on an alumina plate. And a tungsten tip is made by electrochemical etching from 500 mu;m tungsten wire, which is used as a tip of the sensor. This tip is attached inside of one prong of the quartz tuning fork. A sinusoidal electrical signal with its resonance frequency is inputted into the quartz tuning fork sensor to make it vibrate. And automatic gain controller is used for keeping the amplitude of vibration to be constant. And then a lock-in amplifier is used for detecting the phase shift between input and output signals of quartz tuning fork sensor. This phase shift is due to the force between the sample and sensor. The phase signal is fed into a PI controller. This circuit acts to maintain the distance of between sample and tungsten tip by moving a z-scanner (piezo tube scanner). From this feedback circuit the topography of sample is obtained on a PC. As a result, we obtained a quartz tuning fork sensor with high quality factor from 300 to 5000. This value is higher than qPlus sensor [1, 2] and Akiyama probe [3]. The resultant topography image shows a high resolution similarly to a commercial AFM.
Reference
1. Giessibl, F.J., High-speed force sensor for force microscopy and profilometry utilizing a quartz tuning fork. Applied Physics Letters, 1998. 73(26): p. 3956-3958.
2. Giessibl, F.J., Atomic resolution on Si(111)-(7 x 7) by noncontact atomic force microscopy with a force sensor based on a quartz tuning fork. Applied Physics Letters, 2000. 76(11): p. 1470-1472.
3. Akiyama, T., et al., Implementation and characterization of a quartz tuning fork based probe consisted of discrete resonators for dynamic mode atomic force microscopy. Review of Scientific Instruments, 2010. 81(6): p. 063706-063706-8.

We determine the density of states (DOS) including gap states in organic semiconductors by means of scanning Kelvin probe microscopy (SKPM). We apply this energy resolving technique with high spatial resolution to bottom gate organic field effect transistors (OFETs) using the small molecule organic semiconductor pentacene on either HMDS modified or unmodified silicon dioxide gate dielectric. Biasing the gate electrode leads to a filling and emptying of electronic states in the semiconductor. Hereby we indirectly gain the density of trap as well as band states. The measurements were performed in both ambient atmosphere and ultra-high vacuum. Additionally we compare the results to transistors made of Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) which is widely used as an air stable solution processable organic p-type semiconductor. The DOS of PTAA exhibits several additional peaks compared to pentacene, probably related to impurity induced energy levels and a higher degree of structural disorder. Finally, we correlate the obtained DOS distributions to the device performances

Functionalized graphene is a versatile material that has well-known physical and chemical properties depending on functional groups and their coverage. However, selective control of functional groups on the nanoscale is hardly achievable by conventional methods utilizing chemical modificaitons. We demonstrate electrical controls of various functionalizations of graphene with the desired chemical coverage on the nanoscale by atomic force microscopy (AFM) lithography and their full recovery through moderate thermal treatments. Surprisingly, our controlled coverage of functional groups can reach 94.9% for oxygen and 49.0% for hydrogen, respectively well beyond those achieved by conventional methods. This coverage is almost at the theoretical maximum, which is verified through scanning photoelectron microscope (SPEM) measurements as well as first-principles calculations. We believe that the present method is now ready to realize ‘chemical pencil drawing&’ of atomically defined circuit devices on top of a monolayer of graphene.

Atomic force microscopy (AFM) is widely used in liquid environments, where true atomic resolution at the solid-liquid interface can now be routinely achieved. It is generally expected that AFM operation in more viscous environments yields poorer performance due to an increased noise contribution from the thermal motion of the cantilever resulting in a reduced signal-to-noise ratio (SNR) [1]. Thus, viscous fluids such as ionic liquids and organics have been generally ruled out for high-resolution AFM studies. This is in stark contrast to the relevance of viscous liquids and their liquid-solid interfaces. Here, AFM studies would be of great interest as they enable in situ investigations of various processes including chemical reactions [2], lubrication [3] and molecular ordering [4]. Despite the scientific need for studies on such systems with high spatial resolution, AFM in highly viscous liquids remains underutilized.
Here, we investigate the thermal noise limitations of dynamic AFM operation in both amplitude and frequency modulation mode. We report that these limitations for high viscosity environments are significantly different from the well-known equations for ambient and vacuum environments. In particular, we found that the assumption of a reduced SNR in viscous environments is not a property inherent to the technique. We demonstrate that SNR values comparable to ultra-high vacuum systems can be obtained in high viscosity environments and show true atomic resolution images of highly ordered pyrolytic graphite and mica surfaces. This new understanding of the noise contributions to the imaging process serves as a basis for widening the scope of high-resolution AFM away from water based applications to a wide variety of energy related materials, ionic liquids and organic solutions.
References
[1] Giessibl F J and Quate C F 2006 Phys. Today. 59 44-50
[2] Domanski A L, Sengupta E, Bley K, Untch M B, Weber S A L, Landfester K, Weiss C K, Butt H-J and Berger R 2012 Langmuir. 28 13892-9
[3] Jones R E and Hart D P 2005 Tribol. Int. 38 355-61
[4] Labuda A and Grütter P 2012 Langmuir. 28 5319-22

Recent research indicates that cilia have functional significance in maintaining health. Ciliary dysfunctions have been implicated in a number of clinical disorders such as polycystic kidney disease and are grouped together as ciliopathies. Understanding ciliary structures on cell membranes is important to addressing ciliary dysfunction and related diseases. In our study, tetrahymena thermophila has been chosen to be an experimental model because it is a highly ciliated single-celled protozoan. We have characterized ciliary structures of tetrahymena thermophila with atomic force microscopy (AFM). In particular, we have observed ciliary structures at nanometer scales that have not been reported before. Results of our measurements of the number density, the size distribution and length distribution of these ciliary structures will be reported. We have also developed algorithms for extracting three-dimensional structural information from AFM images. With these algorithms, we are able to measure the geometrical shape of cilia. These geometrical characterizations results will be presented. Such information may help us understand ciliary beating and ciliary structural evolution.

Understanding cancer cells is necessary for finding approaches to cure cancer. Obtaining more information on the structures of these cells is critical to this effort. The atomic force microscope (AFM) has been a powerful tool in studying surface morphologies of a variety of materials since its invention. In the past decade, atomic force microscopy has found wide and growing applications in biological sciences. In particular, the ability of AFM to examine biological samples in their physiologically native environment makes it an ideal tool to study cell structures. Cell mechanics of cancer cells, including rigidity of cancer cells in terms of Young&’s Modulus, has already been studied with AFM. In this study, we will report our AFM studies of liver cancer cells. In contrast to previous studies by other groups, we will focus on the surface morphology of cancer cells and compare measurements on cancer cells with measurements on normal cells. In particular, we will measure surface roughness as another characteristic of cells and observe how surface roughness changes as cells become cancerous. Our results may provide insight in the understanding of cancer cells and their growth.

Scanning Force Microscopy (SFM) is based on the interaction between the tip and the surface or nanoscale object to be investigated. Tip-sample interaction can be understood as the interaction between two surfaces, the very curved surface of the tip, and the usually less curved surface of the sample. The Derjaguin Approximation (F(d)=2p Rtip w(d) with w(d) surface energy between two infinite planes) shows that the tip-radius is a fundamental parameter in Scanning Force Microscopy, and that the force generally scales linearly with the tip-radius. Unfortunately, tip-radius is a parameter that is not easy to determine by “intrinsic” Scanning Probe Techniques. Tip-radius can be determined using Transmission Electron Microscopy (TEM). However, since tips are changed quite often, and, even more importantly, since the tip may easily change during the evolution of a typical SFM experiment, TEM cannot be considered a standard and fast method for tip characterization. Moreover, most SFM users do not have easy and direct access to an appropriate TEM system.
As described in more detail elsewhere [1] the electrostatic and the Van der Waals contribution to tip-sample interaction can be separated and precisely measured using non-contact Scanning Force Microscopy. The spectroscopic technique discussed in that work is based on the analysis of “Interaction Images” where tip-sample interaction I is measured as a function of tip-sample voltage and tip-sample distance: I=I(U,d). Such an “Interaction Image” can be processed to obtain very precise curves for the Van der Waals interaction on the one hand and for the electrostatic interaction on the other. By fitting these curves to the theoretical relations for Van der Waals and Electrostatic interaction, we will show that a Van der Waals tip radius RvdW and an Electrostatic tip radius Restat can be determined which describe on the one hand the response of conducting as well as non-conducting parts of the tip (everything that interacts by Van de Waals forces) and on the other hand the electrostatic response of the tip. We will present our experimental values for the radii of different tips and compare these values to those obtained from TEM images. We believe that the “intrinsic” SFM-technique proposed will provide the fast, easy and reproducible method for tip-radius characterization that the SFM community has been waiting for.
Reference
[1] E. Palacios and J. Colchero, Nanotechnology 17 (21), 5491 (2006).
http://dx.doi.org/10.1088/0957-4484/17/21/033.

Artificial molecular rotors, which have been a focus of attention in the last decade of nanoscience, are fascinating subjects for scientists not only in terms of fundamental understanding of molecular motion but also due to their potential utility in the context of molecular scale machinery. The construction of regularly oriented two-dimensional arrays of molecular rotors, rather than isolated single rotors, is a key requirement to demonstrate and miniaturize functional systems for signal processing, sensing and controlling dielectrics. Such a molecular system of regular arrays of molecular rotors has been achieved by utilizing a single layer of Bisphenol A (BPA) molecules on the very weakly corrugated Ag(111) surface. We employ a combinatorial approach of surface science methods consisting of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure to investigate the self-assembled networks built with BPA molecules on the Ag(111) surface under ultra-high vacuum conditions. We found that the molecules show temperature-dependent polymorphism, including an assembly where mobile molecules showing a rotational movement are trapped in the cavities of hexagonally arranged trimeric structures. STM and X-ray spectroscopy studies allow us to propose molecular models for the observed molecular structures, which are stabilized by intermolecular hydrogen bonding and explain the rotational motion of the trapped molecules by their specific adsorption sites.

Understanding of the local electrostatic, electrochemical, and double layer ion distribution at the solid-liquid interface is crucial to the study of corrosion, sensing, energy storage, and biological processes. These phenomena are governed by charge transport, diffusion, and electrostatic screening by mobile charge species from a bulk electrolyte, as well as a diverse set of electrochemical reactions that may take place at the solid-liquid interface. Understanding such complex processes requires techniques capable not only of probing electrostatic and electrochemical phenomena in liquid, but ultimately mapping such phenomena at the intrinsic length scales of electrochemical and transport phenomena and any surface heterogeneity. In this work, we demonstrate electrochemical force microscopy (EcFM), a multidimensional technique capable of bias- and time-resolved mapping of ion dynamics and tip-sample electrochemical processes. We show that EcFM can be used to determine surface potential in low molarity (< 10 mM) solutions by probing the fast dynamics regime (< ~ 2 ms), and also to measure relaxation due to slower (>> ~ 3 ms) screening mechanisms governed by mobile ion dynamics. We demonstrate that EcFM can also detect slower electrochemical processes (>> 50 ms) using the ferri-ferrocyanide redox couple as a model highly reversible electrochemical reaction. In this way, we show that EcFM can be used to probe charge dynamics, ion diffusion and electrochemical processes in the tip-sample junction depending on what regime (e.g. bias, time, and concentration) is probed. Finally, we establish EcFM as an imaging mode, allowing visualization of the spatial variability of local electrochemical behavior.

Materials interfaces have emerged as central functional elements in both applications and fundamental physics, enabling multiple phenomena absent in the bulk and arising from the symmetry breaking at the media boundary. The vivid examples of bipolar junction transistors, 2D electron gas in SrTiO3-LaAlO3, heteroegenous doping of alumina to AgI, etc. show that both electronic and ionic transport through a device can be controlled via a rational engineering of the interface. Of special interest are heterostructure oxide interfaces, where the interplay between the mechanical strain energy of the lattice and electrostatic energy of charge distribution can give rise to interwoven cross-coupled phenomena and lead to new fundamental discoveries. The nanoscale nature of interfaces calls for techniques capable of probing electronic and ionic transport at the sub-micron level. Recently, we have developed novel first-order reversal curve current-voltage (FORC-IV) SPM method for measuring electrochemical activity locally. Here we employ it to study a nanocomposite of BiFeO3 (BFO) matrix with embedded LiMn2O4 (LMO) nanopillars. FORC-IV-z spectroscopy was used to collect simultaneously IV curves and monitor surface expansion across a spatial grid of points, providing information on the local difference of the bias-induced current and mechanical responses. A complex ferroelectric/ionic interface manifests complicated behavior that is sensitive to partial pressure of water vapor and oxygen. The observed behavior is explained via modeling of charge species transport for ionically-blocking electrodes conditions.
Research was supported (E.S., S.V.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted at the Center for Nanophase Materials Sciences (E.S., S.V.K.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

The energy conversion in electrochemical energy conversion systems based on gas-solid interactions such as solid oxide fuel cells (SOFC) is underpinned by a series of complex mechanisms like ion and vacancy diffusion, electronic transport and solid-gas electrochemical reactions at surfaces and triple phase junctions. One of the critical steps in the SOFC and Li-air battery operation leading to large overpotentials and charge-discharge hysteresis is the kinetics of the oxygen oxidation reaction (ORR). It is important thus to explore the mechanisms behind these reactions which remain elusive, largely due to the lack of experimental techniques capable of probing local ionic currents and ORR on the nanoscale. Oxygen vacancies also play a significant role in determining the functionality of electro-resistive devices and non-volatile memories based on resistive switching. Traditionally, the study of the role of oxygen vacancies in these processes is limited by high activation temperature and macroscopic measurement techniques. Here, we demonstrate spatially resolved local probing of the thermodynamics and kinetics involving the generation and diffusion of oxygen vacancies by utilizing chemical expansivity of these oxides upon application of concentrated electric fields. The principles and applications of electrochemical strain microscopy, a technique based on probing minute local deformations induced by applied electric bias applied to the tip will be discussed. Systematic mapping of ORR/OER activity on bare and Pt-functionalized YSZ surfaces is demonstrated with direct visualization of ORR\OER activation process at the triple-phase boundary. Spatial localization of the oxygen reduction/evolution reactions on lanthanum strontium cobaltite (LSCO) surfaces with perovskite and layered perovskite structures is studied on the sub-10 nanometer level. The electrical field-dependence of ionic mobility is explored to determine the critical bias required for the onset of electrochemical transformation, potentially allowing to deconvolute reaction and diffusion processes in the fuel cell system on a local scale. Comparison between Electrochemical Strain Microscopy (ESM) and structural imaging by scanning transmission electron microscopy (STEM) suggest that small-angle grain boundaries act as diffusion pathways for oxygen vacancies which may contribute to enhanced electrochemical activity. The local electrochemical activity is compared across a family of LSCO samples, demonstrating excellent agreement with macroscopic behaviors. The challenges and recent progress associated with measuring and controlling ionic transport and electrochemical phenomena on the nanoscale will be discussed. Probing irreversible bias induced transformation involving electrochemical nucleation processes on solid Li-ion electrolytes require somewhat different paradigms for SPM detection which will also be discussed.

Understanding friction or mechanism of energy dissipation is nowadays among few priorities in nanoscience. The concepts of friction control, wearless sliding or superlubricity are now successfully examined down to the atomic level by means of Atomic Force Microscope (AFM). Bodies in relative motion, separated by few nanometers gap experiences a tiny friction force, whose nature is not yet fully understood. This non contact form of friction can be successfully measured by highly sensitive cantilever oscillating like a tiny pendulum over the surface. The force sensitivity in this configuration is typically few orders of magnitude better as compared to the standard AFM geometry.
We have measured the friction forces acting on a sharp probe tip across the critical temperature of Nb. The tip was oscillating below 3nm distances from 140nm thick Nb surface. Measurements reveal a reduction of dissipation in the superconducting state compared to the normal state by a factor 3. Therefore, electronic friction is found to be the dominant dissipation mechanism with power losses of 80ueV/cycle at separations of 0-3nm. Measurement across the critical temperature of Nb film shows that the character of transition is smooth reflecting the increasing normal electron population which are giving rise to the electronic induced friction. A good agreement with the BCS theory has been found in the drop of friction coefficient, as predicted by the theory. We have also measured distance d and voltage V dependence of the friction coefficient Γ. The Γ is found to be proportional to ~d^minus;1 and ~V^2 in the metallic state, whereas in the superconducting state Γ ~ d^minus;4 and Γ ~ V^4 . That suggests that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state.
A significant scientific interest, triggered by recent discovery of graphene, is focused on 2 dimensional (2D) materials. Owing to breaking of translational symmetry quasi-2D materials exhibit a variety of electronic and structural peculiarities such as Charge Density Waves (CDW) and accompanied periodic lattice distortion (PLD). We have measured non-contact friction between AFM tip and NbSe2 sample - an intercalated CDW compound. We report the existence of several dissipation maxima extending up to a few nm above the surface. Each peak appears at a well defined tip surface interaction force of the order of a nN, and persists until T = 70 K where CDW short-range order is known to disappear. A theoretical model shows that giant enhancement from a few up to hundreds meV/cycle is due to local 2π CDW phase slips when oscillating tip perturbs the CDW underneath. As the tip oscillates to and fro, each slip gives rise to a hysteresis cycle, appearing at a selected distance, the dissipation corresponding to “pumping” in and out a local slip in the surface CDW phase of NbSe2.

Scanning probe microscopy, having the capability of nano-positioning and nano-manipulation, enables the characterization of material properties at a very small scale. In our previous work, the investigation of localized electrochemical reactions in Si3N4-TiC ceramic nanocomposites had been demonstrated using a single conductive scanning probe in a scanning impedance microscope (SIM). The results have provided experimental evidence that links the relations among microstructural heterogeneity, electrochemical property, and sintering behavior of spark plasma sintered ceramics. This single-probe SIM measurement gave through-body electrochemical information of specific surface feature of interest; however, the characterization of across-surface material properties in nanoscale is still much desired and unavailable.
To further investigate the heterogeneity of materials, we have designed and developed a dual-electrode scanning probe (DESP), which is capable of localized electrochemical characterization across the surface of a material. These probes were designed based on computer simulation and iterations, and fabricated using common semiconductor processing techniques. The span of two probes (electrodes) in our first prototypes was 10~15 microns, which can be further reduced with optimized parameters. The DESP probes have been evaluated on Si3N4-TiC nanocomposites to demonstrate their functionality in topography scanning and in-situ impedance measurement. The impedance spectroscopy revealed two distinct impedance patterns for measurements across TiC-rich and Si3N4-rich surface regions. The design, fabrication, and evaluation of DESP were discussed in addition to the analysis of Si3N4-TiC nanocomposites.

Recently it was demonstrated that the conductivity of the LaAlO3/SrTiO3 (LAO-STO) interface could be reversibly tuned through a switchable electromechanical response. In this study, we have investigated dielectric and electronic properties of the LAO ultrathin (several unit cells) films. Piezoresponse force microscopy (PFM) studies show a genuine switchable hysteretic electromechanical behavior resembling that one observed in ferroelectric films associated with induced polarization in the LAO layer. An insight into the underlying mechanism of such behavior has been gained by using temperature-dependent PFM spectroscopic, dielectric and structural measurements. The effect of inhomogeneous strain gradient induced by the PFM probe on polarization in LAO has been also studied. The proposed mechanism of switchable electromechanical response should be active in many other oxide heterosystems, but its detailed manifestation likely depends on a number of subtleties, such as electrical boundary conditions, strain, and stoichiometry.

We present a new technique to measure the piezoelectric response quantitatively using an atomic force microscope (AFM). During last two decades, piezoresponse force microscopy (PFM) has been widely used to detect the local piezoelectric deformation of various ferroelectric materials with high lateral resolution. However, its capability of quantitative measurement is limited by several factors, including the unwanted electrostatic (background) force between metal-coated-cantilever and sample; difficulty in analyzing the electric field distribution confined locally near the sample surface in contact with an AFM tip. These complications result from using an AFM cantilever to apply E-field and to detect the deformation simultaneously. In our new technique, we apply the electric field to the top electrode of a sample using an additional probe and measure the deformation using an uncoated AFM tip. By doing so, the electric field is distributed across the ferroelectric film between the top and bottom electrodes, making it easy to estimate the actual electric field. Besides the even electric field distribution across the film also enables our technique to investigate the piezoresponse of compositionally inhomogeneous ferroelectric films. We also measure the current from this ferroelectric capacitor to characterize the ferroelectric response at the same time. We demonstrated our new technique by measuring the piezoelectric and ferroelectric hysteresis loops quantitatively on a 100 nm thick ferroelectric film of PbZr_0.2Ti_0.8O₃, of which results were in good agreement with the respective data. The mapping of the measured piezoresponse identifies typical domain wall structures of PbZr_0.2Ti_0.8O₃ with the same lateral resolution shown in PFM. We also measured a ferroelectric thin film having a bilayer heterostructure, a 50 nm PbZr_0.8Ti_0.2O₃ layer grown on a 50 nm layer of PbZr_0.2Ti_0.8O₃. The conventional PFM image gives the similar mapping with the result of a PbZr_0.8Ti_0.2O₃ thin film due to its confined electric field in the top layer. On the other hand, our technique was able to measure the combined piezoelectric deformation of the heterostructure and recognize the domain walls developed in the bottom layer of PbZr_0.2Ti_0.8O₃.

Resistive random access memories (RRAMs) has emerged in recent years as it can be applied for data storage with high speed and data density, as well as lower the power and can overcome the scaling problem simultaneously. The ZnO thin film has also attracted a great numbers of attentions as the resistive switching (RS) behavior has been also reported. There are many possible mechanisms underlying the RS behaviors. However the real mechanism is still unclear as many factors may contribute to the phenomena. Especially, the ferroelectric-like behavior in undoped ZnO may complicate the RS mechanisms. To study the RS phenomena, Scanning Probe Microscopy (SPM) techniques are widely used. In this study, conductive Atomic Force Microscopy (c-AFM) is used to characterize the morphological and resistive switching behavior, Piezoresponse Force Microscopy (PFM) is used to study the electromechanical coupling properties, and Kelvin Probe Force Microscopy (KPFM) is used to measure of the surface potential distributions and the dynamic charge distribution at micro- to nano-scales. Therefore, the characteristics and mechanisms of resistive switching behavior and the polarization switching behavior as well as the charge distribution after a poling process in ZnO thin films have been simultaneously studied by these techniques. In addition, the effects of oxygen partial pressure during film deposition and the uncompensated polarization orientations are also studied. It reveals the resistance states are accompanied by the polarization orientations and charges storage. Furthermore, by comparing the KPFM, PFM and c-AFM results at the same location, it is found that the oxygen vacancy is a dominant factor over the holes or electrons injection in the region with high resistance state (HRS). However, the low resistance state (LRS) may be dominated by the electrons injection. This study gives the dominated factors that affect the characteristics of the resistive switching behavior and proposed a more persuasive RS mechanism in ZnO films.

The spring constant of an AFM cantilever is often needed for quantitative measurements. The calibration method of Sader et al. [Rev. Sci. Instrum. 70, 3967 (1999)] for a rectangular cantilever requires measurement of the resonant frequency and quality factor in fluid (typically air), and knowledge of its plan view dimensions. In this presentation, I will present the generalization of this method to cantilevers of arbitrary shape. Results for commercial cantilever geometries of arrow shape, small aspect ratio rectangular, quasi-rectangular, irregular rectangular, non-ideal trapezoidal cross sections, and V-shape will be presented. This enables the spring constants of all these cantilevers to be accurately and routinely determined from measurement of their resonant frequency and quality factor in fluid (such as air).
Two practical issues dealing with the robust and accurate measurement of the resonant frequency and quality factor will also be discussed. These parameters are often determined from the thermal noise spectrum of an AFM cantilever, and its subsequent fit to a Lorentzian distribution. It will be shown that if no spectral window is used, the thermal noise spectrum retains its original Lorentzian distribution but with a (artificially) reduced quality factor. A simple correction formula is derived enabling extraction of the true device quality factor from measured data. Common windows used to reduce spectral leakage such as the Hanning window are found to distort the (true) Lorentzian shape, potentially making fitting problematic. I will also discuss the effect of sampling noise in the measured thermal noise spectrum on the uncertainty in the fitted resonant frequency and quality factor. This will include explicit formulas for the uncertainties in the final fit parameters. Comparison and validation of these effects using measurements on AFM cantilevers will be given.

Spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 is explored on the atomic level using continuous imaging tunneling spectroscopy (CITS). Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Statistical analysis of calculated density of states of chemically inhomogeneous structures further confirms that the two types of chalcogens - (Te,Se), can be identified by their electronic signature and further differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the STM data including separation of atomic identities, proximity and local configuration effects, and can be universally applicable to chemically- and electronically inhomogeneous surfaces.

Heat, a measure of entropy, is largely perceived to be diffusive and transported incoherently by charge carriers and lattice vibrations in a material, which is hard to be spatially localized. Heat transport is therefore considered a challenging means of the local imaging of a material and its electronic states. However, Cho et al. [1] reported a series of striking wavefunction images of epitaxial graphene by measuring thermoelectric voltages with a heat-based scanning probe microscopy. Here we present how the thermoelectric signal is related to the atomic-scale wavefunctions and what the role of the temperature is at such a length scale. An expression of local thermoelectric voltage is deduced, and a computer-based thermoelectric imaging simulation method with first-principles wavefunction calculations is developed and performed on pristine and defective graphene. From this analysis, we find that coherent electron and heat transport through a point-like contact produces an atomic Seebeck effect. We also discuss the connection between Seebeck coefficient and thermal properties of a material, such as electronic heat capacity and quantum of thermal conductance, by introducing the statistically defined Fermi temperature [2].
[1] S. Cho, S. D. Kang, W. Kim, E.-S. Lee, S.-J. Woo, K.-J. Kong, I. Kim, H.-D. Kim, T. Zhang, J. A. Stroscio, Y.-H. Kim and H.-K. Lyeo, arXiv:1305.2845, Nature Mater. 12, 913 (2013).
[2] E.-S. Lee, S. Cho, H.-K. Lyeo and Y.-H. Kim, arXiv:1307.3742, submitted (2013).

Force microscopy is considered the second most relevant advance in Materials Science since 1960. Despite the success of AFM, the technique currently faces limitations in terms of three-dimensional imaging, spatial resolution, quantitative measurements and data acquisition times. Atomic and molecular resolution imaging in air, liquid or ultrahigh vacuum is arguably the most striking feature of the instrument. However, high resolution imaging is a property that depends on both the sensitivity and resolution of the microscope and on the mechanical properties of the material under study. Molecular resolution images of soft matter are hard to achieve. In fact, no comparable high resolution images have been reported for very soft materials such as those with an effective elastic modulus below 10 MPa (isolated proteins, cells, some polymers). Similarly, it is hard to combine the exquisite force sensitivity of force spectroscopy with molecular resolution imaging. Simultaneous high spatial resolution and material properties mapping is still challenging.
This presentation reviews some of the above limitations and some recent developments based on the bimodal operation of the AFM to address and overcome them.
Recent references:
R. Garcia and E. T. Herruzo, Nat. Nanotechnol. 7, 217-226 (2012).
E. T. Herruzo, H. Asakawa, T. Fukuma, R. Garcia, Nanoscale 5, 2678 (2013)
H. V. Guzman, A.P. Perrino, R. Garcia, ACS Nano 4, 3198 (2013)

The development of error-mapping routines to analyse and characterise the motion of video-rate AFM scan stages is extremely beneficial and provides a low-cost method for reducing the measurement uncertainties associated with such stages. This is particularly useful since video-rate scan stages often operate at high-frequencies (greater than 1 kHz), meaning that many conventional closed-loop sensors lack the bandwidth to track their motion.
We demonstrate the successful application of error-mapping techniques to video-rate AFM data collected with an open-loop scan stage. A simple method for calibrating the scan stage at a range of scan amplitudes and frequencies is presented and its application to mapping millimetre and centimetre square regions of a sample surface is demonstrated.

Recent advances in atomic force microscopy (AFM) have enabled subnanometer-scale measurements of three-dimensional (3D) force distribution at solid/liquid interfaces. Although the measured 3D force distribution has been attributed to the water density distribution, i.e. the hydration structure, this interpretation has not generally been accepted. This is because of the lack of a clear explanation for the central issues on the imaging mechanism such as influence of the tip and its hydration, and the quantitative relationship between the measured force and the water density. Here we address these issues by comparing the 3D force distributions at the Calcite/water interface obtained by experiments and AFM simulation. So far, such comparison has been hindered by the difficulties in both experiments and simulation.
In this study, we used ultra-short cantilevers with 3.5 MHz resonance frequency to achieve unprecedented resolution in the 3D hydration force measurements [1]. We performed atomistic simulations of a system consisting of a Calcite surface in water and a nanocluster to model the AFM tip apex. We calculated the free energy of the system as a function of the tip-surface distance, for 32 lateral tip positions. The force on the AFM tip obtained from deriving the free energy profiles also contains entropic contributions to the force on the tip, originating from perturbations of the hydration layer structure by the presence of the tip [2]. This 3D force field was then used to model the experiment in a virtual AFM, matching the experimental parameters for the cantilever oscillation, and obtain simulated AFM images.
With all these efforts, we have achieved a quantitative comparison between theory and experiment. Based on the results, we present convincing evidence that the 3D force distribution measured by AFM indeed reflects the 3D hydration structure at the solid/liquid interface [3].
[1] T. Fukuma, K. Onishi, N. Kobayashi et al., Nanotechnology, 23:135706, 2012.
[2] B. Reischl, M. Watkins and A. S. Foster, J. Chem. Theory Comput., 9:600, 2013.
[3] T. Fukuma, N. Kobayashi, B. Reischl et al., in preparation (2013).

Advanced polymer material systems are often heterogeneous both at the surface and in the sub-surface. Surface heterogeneities on polymers can often be visualized using energy dissipation maps in tapping mode AFM, however linking the observed contrast to material properties has remained challenging. In the first part of this talk, we will discuss advances in computational models of tip-polymer interactions that provide unprecedented insight into mechanisms for tapping mode dissipation contrast on polymer blends. Sub-surface heterogeneities are especially relevant for polymer nanocomposites where sub-surface percolating networks of nano-fillers such as carbon nanotubes or graphene are essential for material performance. Recent advances using Kelvin probe force microscopy have been able to image sub-surface percolating networks, however the depth and spatial resolution of such techniques remain unclear. In this work we discuss the fundamental mechanisms for sub-surface imaging in KPFM and the ultimate limits for depth and spatial resolution. Put together these approaches in dynamic AFM are providing better insight into both surface and sub-surface contrast mechanisms on polymer based systems.

Crystallographic image processing (CIP) is an established technique in the electron microscopy community where it is used for the analysis and enhancement of high-resolution transmission electron microscopy images of crystals and two-dimensional (2D) arrays of membrane proteins. The technique has recently been adapted to the processing of 2D periodic images from scanning probe microscopes (SPMs) [1-4]. A procedure for the unambiguous identification of the underlying Bravais lattice of an experimental or theoretical image of a (2D) periodic array of objects (e.g. molecules or atoms and their respective electron density distribution functions, hellip;) has also been developed [4] within this context.
Our (2D) Bravais lattice identification procedure is independent of which type of microscope has been utilized for the recording of the images. It is particularly useful for the correction of SPM images that suffer from a blunt scanning probe tip artifact. With the crystallographic processing of two molecular resolution Scanning Tunneling Microscope images of periodic arrays of tetraphenoxyphthalocyanine on graphite, it is demonstrated how the classical CIP procedure is augmented by our unambiguous translation symmetry identification method. Finally, we apply CIP to an artificial SPM image that features a blunt scanning probe tip artifact and recover the underlying plane symmetry of the hypothetical sample array even though the periodic image motif had completely lost its striking four-fold point symmetry completely.
[1] http://www.formatex.info/microscopy4/1951-1962.pdf. [2] http://nanocrystallo
graphy.research.pdx.edu/media/thesis14acorr.pdf. [3] http://www.microscopy.org/
MandM/2010/plachinda.pdf. [4] http://nanocrystallography.research.pdx.edu/
media/cms_page_media/6/Taylor_thesis_final.pdf

Solid-liquid interfaces are ubiquitous and have an important, if not fundamental, role in many phenomena from different fields such as surface science, materials science and chemistry. But despite the importance of solid-liquid interfaces, deep understanding of the physical properties of these interfaces remains scarce, and experimental data (especially at the microscopic scale) is still limited, mainly caused by the molecular complexity of these interfaces as well as the mobile nature of liquids. Atomic force microscopy (AFM) has the great advantage of probing the system locally. When using AFM in a liquid environment it allows sub-nanometer studying of the solid-liquid interface, and it is perhaps one of the few tools capable of resolving the nature of the hydration layers at the solid-liquid interface.
Though, from a molecular perspective even AFM experiments with the tip submerged in the liquid span large time scales and individual atomic-level processes cannot be measured. This is exactly where large-scale molecular dynamics (MD) simulations prove to be extremely useful in elucidating the physical origins of the solid-liquid interfaces. In the current work we show how MD simulations can be used to understand the complex structure of the hydration layers for different systems, covering well-known examples of ionic and organic surfaces.
For instance, the hydration structures observed in MD simulations for calcite and mica surfaces solvated in water are in very good agreement with AFM experiments. We also show that despite the charged surface of calcite, ions do not get adsorbed at the interface, but tend to be blocked by the strong ordered water layer [1]. In the case of ions at the muscovite mica surface, we are able to show why these ions cluster at the surface and that these local correlations are driven by the interfacial water hydrating the ions and the mica surface [2].
Recent AFM experiments on the solid-liquid interfaces of another organic system, p-nitroaniline, showed strong ordering of water molecules [3]. Our MD simulations confirm this strong ordering of water at the crystal's surface, but the MD simulations also point to a different and novel surface reconstruction, which is in excellent agreement with the majority of the experimental results and which stands as a challenge for future diffraction techniques [4].
Our joined theoretical and experimental study emphasizes the power of experimental techniques, while simultaneously demonstrating the importance of computer simulations to confirm the details of these complex heterogeneous molecular solid-liquid interfaces.
[1] M. Ricci, P. Spijker, J.F. Molinari, F. Stellacci, K. Voitchovsky, Langmuir, 29:2207 (2013).
[2] M. Ricci, P. Spijker and K. Voitchovsky, submitted (2013).
[3] R. Nishioka, T. Hiasa, K. Kimura and H. Onishi, J. Phys. Chem. C, 117: 2939 (2013).
[4] P. Spijker, T. Hiasa, T. Musso. R. Nishioka, H. Onishi and Adam S. Foster, submitted (2013).

The goal of this research is to address the critical issues of throughput, repeatability, scalability, and limited functionality of probe-based nanofabrication by designing, fabricating, and testing a novel active cantilever probe with an automated ability to interchange probe tips (tools). Probe-based fabrication enables unmatched spatial/feature resolution and the ability to assemble and pattern hybrid (inorganic and organic) device architectures. However, practical nanofabrication with probe tips is limited by the issues of throughput, tip wear, tip chemical cross contamination, and scalability - all of which act to decrease the quality, reliability, and efficiency of probe-based fabrication.
We address these issues by enabling automated interchanging of probe tips. This is accomplished through the design, analysis, and characterization of a SPM cantilever with an integrated electrothermally actuated microgripper. The microgripper allows for automated probe-tip, here after referred to as tool-tip, exchange. Located at the distal end of a cantilever is an electrically activated microgripper, which is designed to automatically load/unload tips from an array of modular tool-tips. The microgripper has been designed to provide adequate range of actuation, gripping force, stiffness, and dynamic response required for securely holding the tool-tip, and for functioning within existing SPM-based systems. Multi-physics based finite element analysis was utilized to model the frequency response and the coupled electro-thermo-mechanical behavior of the microgripper. Design refinement and optimization was carried out to obtain the ‘best&’ performance of the microgripper. The design refinement involved development of an electro-thermo-mechanical analytical model. Cantilever prototypes were fabricated using established MEMS microfabrication processes. Electrical and mechanical characterization has been completed to establish the operating limits of the microgripper, and assess successful functionality. Modular tool-tips have been fabricated via rapid-prototyping by focused ion beam milling. Demonstration of the cantilever function was shown by gripping of a 37 mu;m wire similar to the size of a modular tool-tip.

Atomic force microscopy (AFM) has become the popular device for material characterization due to its capability to measure various kinds of characteristics such as mechanical and electromagnetical properties even in liquid. However the scanning direction of AFM is limited to Cartesian coordinate system especially for contact mode because its probe has different properties depending on the orientation with respect to scanning direction. And imaging property is dependent on the scanning direction as well. For example, scanning in lateral direction is for friction and axial direction for topography. Because the conventional AFM scans into 2 orthogonal directions for contact mode, rotation of the sample plate is necessary in order to scan sample in arbitrary direction for characterization of properties of anisotropy material such as friction and stiffness. However the rotation of specimen causes the discontinuity of the imaging and loss of the previous information of the same specimen, so the material characterization becomes very time consuming work. Therefore we present the AFM which is able to rotate its probe different from the conventional AFM. The design for a light and compact probe head, which includes microcantilever, microcantilever holder, deflection sensor, and 3-axis positioner, is applied. In particular, we used the optical pickup unit of the optical disk driver to make the deflection sensor compact and light. We also presented the new scanning methodology which combines the raster scan and vector scan. With the developed AFM we could image the topography of the standard sample in arbitrary direction for contact mode. The presented AFM is able to scan in an arbitrary direction for all kinds of imaging mode keeping the functions of the conventional AFM. Therefore it is expected that the presented AFM will be an upgraded tool for various fields of nanotechnology.

Nanoscale in-situ characterization of dynamic molecular and interfacial processes is important for applications as diverse as biology to advanced patterning for semiconductor materials and energy storage materials. Atomic Force Microscopy is an excellent tool for in-situ imaging due to its high spatial resolution and environmental versatility. However, working in fluids compromises the spatial resolution of AFM. This is due to the fluid damping of the cantilever increasing the force noise. Deformation and loss of resolution result from the higher minimum force required to perform an AFM measurement. At the same time, scanning a massive piezo with significant inertia makes scanning with high temporal resolution challenging. We improved spatial resolution by developing scuba probes and temporal resolution by developing spiral scanning with a very fast Z feedback stage.
Scuba Probes reduce the force noise of the cantilever for higher spatial resolution. We build a protective encasement around the cantilever that traps an air bubble and keeps the cantilever dry reducing damping. The tip protrudes from the encasement to the sample in the solution. Encased cantilevers have exceptionally high Q factor and detection sensitivity. The slower time constant relative to high damping cantilevers in liquid can be improved using Q-control. More importantly, Scuba Probes have low force noise and high resonance frequency which together signifi-cantly reduce the impulse to the sample with each tap enabling gentle high speed imaging.
Present raster scan techniques are poorly matched to mechanical limitations of the piezoelectric nanopositioners used for Atomic Force Microscopy. This makes data collection relatively slow and half of the data is thrown away because trace and retrace do not overlap due to piezo nonlinearity. We developed Sensor Inpainting [1, 2] to accurately create images from times series of X, Y, Z triplets from the scanner sensors. Sensor Inpainting allows the use spiral scans with lower acceleration for high speed imaging and display 100% of the data. We are able to collect frame rates 20 times faster than otherwise possible with the same scanner. We used fast spiral scanning to investigate dissolution of crystals and nucleation and growth of the ZIF-8 metal organic framework. Further, we developed a Z piezo with 600 kHz bandwidth and a nested dual Z feedback loop to compliment the high scan velocities.
[1] D. Ziegler, T. Meyer, R. Farnham, C. Brune, A. Bertozzi, P. Ashby, Nanotechnology, 24, 335703.
[2] T. Meyer, D. Ziegler, C. Brune, A. Chen, R. Farnham, N. Huynh, J. Chang, A. Bertozzi, P, Ashby, Ultramicroscopy, in press.

Coaxial and triaxial scanning probe microscope (SPM) tips provide methods to image, assemble, and characterize material structures using dielectrophoresis. A coaxial tip consists of a conducting core, surrounded by a grounded conical conducting shell, separated by an insulator. The end of the tip is cut-off by a focused ion beam, to allow electric field lines to escape. An ac voltage applied between the core and the shell produces a sharply confined dipolar electric field profile for pick and place assembly of microparticles using dielectrophoresis [1]. The polarization force can also be used to image the topography of dielectric surfaces [1]. In addition, coaxial probes can perform high spatial resolution Kelvin probe force microscopy to image the work function of composite materials [2]. A potential difficulty for coaxial-tip assembly is sticking between attracted particles and the tip. Non-contact trapping and manipulation of nanoparticles can be achieved using a triaxial SPM tip, which consists of a conducting core, surrounded by inner and outer conducting shells, separated by insulating layers; the end is cut off to allow electric field lines to escape. A triaxial tip can approximate quadripolar field profiles by grounding the outer shell and applying opposite polarity ac voltages to the core and the inner shell. An electric field zero that traps nanoparticles via negative dielectrophoresis can be created [3]. By varying the ratio of the core and inner shell ac voltages, the trap can be opened and closed to perform pick and place assembly at the nanoscale [3].
PhD research of Keith Brown, with KJ Satzinger, J Berezovsky and JA Aguilar
1. KA Brown, JA Aguilar, RM Westervelt, "Coaxial Atomic Force Microscope Tweezers," Appl Phys Lett 96, 123109 (2010); KA Brown, J Berezovsky, RM Westervelt, "Coaxial atomic force microscope probes for imaging with dielectrophoresis," Appl Phys Lett 98, 123109 (2011).
2. KA Brown, KJ Satzinger, RM Westervelt, "High spatial resolution Kelvin probe force microscopy with coaxial probes," Nanotechnology 23, 115703 (2012); KJ Satzinger, KA Brown, RM Westervelt, “The importance of cantilever dynamics in the interpretation of Kelvin probe force microscopy,” J. Appl. Phys. 112, 64510 (2012).
3. KA Brown and RM Westervelt, "Triaxial Atomic Force Microscope Contact Free Tweezers for Nanoassembly," Nanotechnology 20, 385302 (2009); "Triaxial AFM probes for non-contact trapping and manipulation," Nano Lett 11, 3197 (2011).

Atomic force microscopy (AFM) in liquid environments is a routine technique for attaining atomic resolution. Despite an array of problems with piezo actuation in liquid environments it remains widely used due to its availability and ease of use. The most significant effect of piezo activation is the modification of the driven cantilever transfer function due to the “forest-of-peaks” effect, where the cantilever resonance couples to other resonance modes of the liquid and/or the liquid cell. This coupled motion can result in order of magnitude changes in the driven quality factor (Q) and a shift in the driven resonance frequency away from the natural resonance frequency of the cantilever.
Here, we investigate the thermal noise limitations of dynamic AFM operation in both amplitude and frequency modulation modes for both ideal (electrostatic) and non-ideal (piezo) forms of actuation in liquid environments. Furthermore, we demonstrate a capacity to tune both the intrinsic cantilever transfer function (via solution viscosity) and the driven transfer function (via Q control) to achieve a desired level of performance using conventional cantilevers. We show that the imaging performance (signal-to-noise ratio (SNR)) is directly determined by a combination of both the thermal noise limitations and the influence of the driven transfer function. We demonstrate that, for a given measurement bandwidth, lowering the Q of the system results in an increased SNR. An added benefit is an increase in the mechanical bandwidth of the cantilever, resulting in a pathway to high speed imaging. We observe that SNR values comparable to ultra-high vacuum systems can be obtained in high viscosity environments and/or lowered driven Q systems. Finally, we show that true atomic resolution can be achieved by system optimization through either modification of the cantilever&’s environment (passive) or modification of the driven transfer function (active). Such modifications allow the extension of high performance imaging to a wide variety of fluids and interfacial materials.

For accurate and reproducible measurements, a new Atomic Force Microscope (AFM) platform was developed to eliminate the cross-talk between the XY and the Z scan [1]. The XY flexure scanner, driving a sample, is decoupled from the Z scanner to which a probe is attached. The new AFM platform provided not only a highly orthogonal and flat scan, but also fast Z-servo speed, which enabled non-contact mode AFM. AFM is evolving into an ideal methodology for non-destructive sample scan with longer tip life, in various hard disk and semiconductor industry applications: pole-tip recession, surface roughness, automatic defect review, etc.
The automatic defect review AFM locates and images defects during media and substrate and wafer manufacturing. The key technological challenge here is the accurate transfer and remapping of defect map from optical inspection tool to AFM stage, if at all possible, without any reference marking. Using the defect location map transferred, the AFM automatically goes to each of the defect locations and images the defects in two steps: image a larger, survey scan to refine the defect location, and then image a smaller zoom-in scan to obtain the details of the defect. Here, a much extended tip life by the non-contact AFM proved to be crucial in effective defect finding and cost-saving therein.
Recently, a new 3D AFM was introduced where the Z scanner with the AFM tip attached is tilted to one side with respect to the XY scanner where the sample is placed [2]. The new 3D AFM can characterize the sidewall roughness of lithographically produced multilayer resists [3]. High-resolution sidewall images and line profiles obtained by the new 3D AFM technique demonstrate its advantages to characterize the critical device patterns [4]. Taken together with confirmation of CD-SEM imaging, the new 3D AFM imaging can identify a trend in Sidewall Roughness (SWR) on photoresist sample and establish the LER trend.
[1] Atomic force microscope with improved scan accuracy, scan speed, and optical vision, J.Kwon et al., Rev. Sci. Instrum. 74 (2003) 4378
[2] Three-Dimensional Imaging of Undercut and Sidewall Structures by Atomic Force Microscopy, Sang-Joon Cho et al., Rev. Sci. Instrum. 82 (2011) 023707.
[3] Introduction of Next-Generation 3D AFM for Advanced Process Control, J. Foucher et al., Proc. SPIE 8681, Metrology, Inspection, and Process Control for Microlithography XXVII, (April 18, 2013) 868106
[4] 3D AFM Method for Characterization of Resist Effect of Aerial Image Contrast on Side Wall Roughness, Yong-ha Lee et al., Proc. SPIE 8681, Metrology, Inspection, and Process Control for Microlithography XXVII, (April 10, 2013) 868121

Dynamic Scanning Force Microscopy (DSFM) is a very powerful tool for non-invasive imaging of surfaces since extremely low forces are applied on the sample. Most systems are operated in ambient conditions where non-contact DSFM is performed with tip-sample distances of the order of 3-10 nanometers. We will show that the dissipation occurring in ambient conditions within the tip-sample system during NC-DSFM operation leads to significant noise in the tip-sample interaction. To measure noise, the SFM-system is operated in Amplitude Modulation mode with relatively small oscillation amplitude (about 5-10nm) and a small amplitude reduction (asetasymp;0.95afree). Noise images are acquired at constant amplitude (amplitude is signal for topography feedback) with a Phase Locked Loop in order to track the resonance frequency and Kelvin Force Microscopy was performed in order to minimize the local electrostatic field between tip and sample. From the noise image we clearly deduce that the noise in the tip-sample system is inhomogeneous and depends on the physical and/or chemical properties of the sample. In addition to the images shown, high resolution local spectroscopy data of the normal force, the noise signal, the amplitude and the frequency shift will be discussed. From the analysis of these curves we find that the noise increases significantly as soon as a reduction of oscillation amplitude is detected. We will discuss whether this significant increase of noise affects resolution of DSFM-related techniques such as Electrostatic and Magnetic Force Microscopy. In addition, we will compare Amplitude Modulation with Frequency Modulation DSFM when operated in ambient conditions.

Tunable Infrared (IR) laser light can be used for spectroscopy. To achieve submicron spatial resolution, the AFM-based thermomechanical detection is used. If a pulse of a given wavelength is absorbed by a sample, local temperature rise leads to local thermal expansion. This expansion excites mechanical resonance of the AFM cantilever in contact with the sample. We compared LHCII-membrane complexes isolated from spinach leaves. The difference lay in the amount of light the complexes had received: One group came from leaves adapted to the dark and the other from leaves previously exposed to high-intensity light. Using X-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy, it was found that the dark-adapted LHCII-membranes complexes assembled into rivet-like stacks of bilayers (like a typical chloroplast membranes), while the pre-illuminated complexes formed 3-D forms that were considerably less structured. Unprecedented spatial resolution of 20 nm was reached. [1]
[1] E. Janik et al., The Plant Cell June 2013 vol. 25 no. 6 2155-2170

We apply nano-FTIR, a fascinating new imaging technique combining Fourier Transform Infrared Spectroscopy (FTIR) and scanning type optical microscopy (sSNOM), to study bio-medical hard matter at impressive lateral resolution of lambda;/1000.
Spectroscopic methods with high spatial resolution are crucial to understand the physical and chemical properties of nanoscale heterogeneous materials. A widely common used tool is FTIR spectroscopy, which use fingerprint absorbance spectra in the 5-20 µm mid-infrared region to identify and quantify material compositions. The strength of this method relays on the already established spectral databases which allow a rapid molecular identification, however a huge drawback is the spatial resolution of FTIR microscopes which are limited by diffraction to about one wavelength on the order of micrometers. Nano-FTIR is a fascinating new imaging and spectroscopy method [1,2], allowing nanoscale chemical identification and mapping of heterogeneous material. Selecting the high-resolution monochromatic mode we were enabled to map individual lacunae (bone cells) and canaliculus of standard polished human bone sections [3].
Furthermore, we apply this powerful new method to map the local distribution of bone-matrix and PMMA embedded material of human tooth section. In accordance with far-field FTIR measurements we were able to highlight either mineral vibration using 1020 cm-1 radiation or PMMA by tuning the illumination light-source to 1150 cm-1.
Our studies show clear label-free chemical recognition at impressive lateral resolution. In contrast to common used electron microscopy techniques we use low energy photons offering a non-destructive method, which works under ambient conditions and requires low level of sample preparation. Giving that a huge application potential is foreseen in biology and medicine.
[1] S. Amarie, P. Zaslansky, Y. Kajihara, E. Griesshaber, W. W. Schmahl and F. Keilmann, Nano-FTIR chemical mapping of minerals
in biological materials. Beilstein J. Nanotechnol. 3, 312 (2012).
[2] F. Huth, A. Govyadionov, S. Amarie, W. Nuansing, F. Keilmann and R. Hillenbrand, Nano-FTIR absorption spectroscopy of molecular fingerprints at 20 nm spatial resolution, Nano Lett. 12, 3973 (2012).
[3] T. Geith, S. Amarie, S. Milz, F. Bamberg and F. Keilmann, Visualisation of methacrylate-embedded human bone section by infrared nanoscopy, J. of Biophotonics. doi: 10.1002/jbio.201200172 (2012).

In scattering-type scanning near-field optical microscopy (s-SNOM), the evanescent electric fields at the apex of a sharp illuminated tip are used to probe the dielectric properties of a sample with a sub-wavelength resolution [1]. Since the near-fields at the tip apex penetrate into a dielectric sample, nondestructive imaging of subsurface structures and covered samples is also possible. In order to interpret s-SNOM measurements on layered samples we present a quantitative model for the prediction and analysis of optical near-field signals on multilayer systems [2]. The resolution of buried objects in near-field microscopy is also addressed experimentally. We demonstrate that spherical particles with a diameter of only 30 nm can be resolved through dielectric membranes with a thickness of up to 50 nm. The obtained resolution is quantified and compared to theoretical expectations as well as transmission electron microscopy (TEM) images.
For structures further than 100 nm below the surface, we employ the superlensing effect for the detection of nanoscale subsurface features: Near-field superlenses [3] have the ability to break the diffraction limit due to the resonance of surface plasmon (or phonon) polaritons (SPPs) and enable applications like optical lithography and near-field imaging. Combining the superlens with s-SNOM enabled subsurface imaging of metallic nanostructures, which promises the characterization of buried nanoscale features [4]. However, the practical application of the superlens is limited by its narrow bandwidth since a superlensing condition should be met. We propose two new superlenses that can overcome this limitation: The first concept is a multilayer-superlens [5], which - by suitably choosing polar dielectric materials - can operate at distinct wavelengths widely apart. The other new concept makes use of graphene, a one-atom-thick planar sheet supporting surface plasmons at frequencies in the infrared (IR) and terahertz (THz) spectral range [6,7]. Compared to conventional superlenses, the nonresonant operation of our so-called “Graphene-Lens” (GL) [8] provides the advantages of a broad intrinsic bandwidth and a low sensitivity to losses, while maintaining a good subwavelength resolution of better than lambda;/10. Most importantly, thanks to the large tunability of the graphene, we show that our GL is a continuously frequency-tunable subwavelength-imaging device in the IR and THz regions.
1. F. Keilmann, R. Hillenbrand, Nano-Optics and Near-Field Optical Microscopy (Artech House), 235 (2009).
2. B. Hauer, A.P. Engelhardt and T. Taubner, Optics Express 20, 13173 (2012).
3. J.B. Pendry, Physical Review Letters 85, 3966 (2000).
4. T. Taubner, et al. Science 313, 1595 (2006).
5. P. Li and T. Taubner, Optics Express 20, A11787 (2012).
6. J. Chen et al., Nature, 487, 77 (2012).
7. Z. Fei et al., Nature 487, 82 (2012).
8. P. Li and T. Taubner, ACS Nano 6, 10107 (2012).

Electron-hole pair formation and subsequent charge separation by external light illumination are the most basic and core processes for the applications of the photoactive devices. Charge separation and injection in semiconductor-metal nanoparticle junction such as TiO₂/Au nanoparticle, in particular, are controversial. In this study, spatial charge separation in Au nanoparticle(NP)/TiO₂ nanotube(NT) asymmetric nanostructures is investigated by Kelvin probe force microscopy (KPFM). Under continuous photoexcitation (UV, E = 3.4 eV), generation and recombination processes of electron-hole pairs are in equilibrium state and excited electrons in TiO₂ are injected into conducting substrates or Au NPs. Electron injection into Au NPs or substrate change the effective Fermi level of TiO₂ NTs and electrons injected into Au NPs on the surfaces of TiO₂ NTs screen the electric field generated by holes in TiO₂ NTs. Under UV illumi