Dynamic electrostatic force microscopy in liquid media

MRS Proceedings, 2014

In electrostatic force microscopy (EFM), a conductive atomic force microscopy (AFM) tip is electrically biased against a grounded sample and electrostatic forces are investigated. This methodology has been broadly used in the scientific community to characterize dielectric properties of samples at the nanoscale. Two are the main operating conditions associated with this technique. The oscillation amplitude is usually kept to very small values to allow a linearized approach to the force reconstruction and the tip-sample distance is maintained elevated. However, this latter condition negatively affects the lateral resolution of the technique. Thus, electrostatic interaction should be probed in the vicinity of the sample. Theoretically, in this region the force can be linearized using oscillation amplitudes in the order of Å. This might cause the trapping of the tip on the surface (snap-in). Furthermore, at small distances, short-range forces (i.e. Van der Waals’) might reach values co...

Scientific reports, 2016

Atomic force microscopy (AFM) force-distance measurements are used to investigate the layered ion structure of Ionic Liquids (ILs) at the mica surface. The effects of various tip properties on the measured force profiles are examined and reveal that the measured ion position is independent of tip properties, while the tip radius affects the forces required to break through the ion layers as well as the adhesion force. Force data is collected for different ILs and directly compared with interfacial ion density profiles predicted by molecular dynamics. Through this comparison it is concluded that AFM force measurements are sensitive to the position of the ion with the larger volume and mass, suggesting that ion selectivity in force-distance measurements are related to excluded volume effects and not to electrostatic or chemical interactions between ions and AFM tip. The comparison also revealed that at distances greater than 1 nm the system maintains overall electroneutrality between ...

Ultramicroscopy, 2011

In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tip-sample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2-3 Â 10 4 Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tip-sample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tip-sample distances.

Journal of Applied Physics, 2013

…, 2007

Electromechanical coupling is ubiquitous in biological systems with examples ranging from simple piezoelectricity in calcified and connective tissues to voltage-gated ion channels, energy storage in mitochondria, and electromechanical activity in cardiac myocytes and outer hair cell stereocilia. Piezoresponse force microscopy (PFM) has originally emerged as a technique to study electromechanical phenomena in ferroelectric materials, and in recent years, has been employed to study a broad range of non-ferroelectric polar materials, including piezoelectric biomaterials. At the same time, the technique has been extended from ambient to liquid imaging on model ferroelectric systems. Here, we present results on local * Corresponding author, [email protected] electromechanical probing of several model cellular and biomolecular systems, including insulin and lysozyme amyloid fibrils, breast adenocarcinoma cells, and bacteriorhodopsin in a liquid environment. The specific features of SPM operation in liquid are delineated and bottlenecks on the route towards nanometer-resolution electromechanical imaging of biological systems are identified.

Nanomaterials

Mapping the dielectric properties of cells with nanoscale spatial resolution can be an important tool in nanomedicine and nanotoxicity analysis, which can complement structural and mechanical nanoscale measurements. Recently we have shown that dielectric constant maps can be obtained on dried fixed cells in air environment by means of scanning dielectric force volume microscopy. Here, we demonstrate that such measurements can also be performed in the much more challenging case of fixed cells in liquid environment. Performing the measurements in liquid media contributes to preserve better the structure of the fixed cells, while also enabling accessing the local dielectric properties under fully hydrated conditions. The results shown in this work pave the way to address the nanoscale dielectric imaging of living cells, for which still further developments are required, as discussed here.

In atomic force microscopy, the tip experiences electrostatic, van der Waals, and hydration forces when imaging in electrolyte solution above a charged surface. To study the electrostatic interaction force vs distance, curves were recorded at different salt concentrations and pH values. This was done with tips bearing surface charges of different sign and magnitude (silicon nitride, A1203, glass, and diamond) on negatively charged surfaces (mica and glass). In addition to the van der Waals attraction, neutral and negatively charged tips experienced a repulsive force. This repulsive force depended on the salt concentration. It decayed exponentially with distance having a decay length similar to the Debye length. Typical forces were about 0.1 nN strong. With positively charged tips, purely attractive forces were observed. Comparing these results with calculations showed the electrostatic origin of this force.

Nanotechnology, 2004

The phase mode of electrostatic force microscopy (EFM-phase) is a scanning probe microscopy (SPM) technique used to measure electrostatic force gradient. EFM-phase has a higher resolution than scanning Kelvin probe microscopy (SKPM or SKM), but unlike SKPM it does not yield a direct measurement of local potential. Analytical calculations of tip-surface capacitances and their gradients are presented, and the origin of the measurement resolution in EFM-phase and SKPM is explained based on the calculation results. We show that EFM-phase data fit the analytical calculation well, and can be interpreted using a simple analytic model, which allows phase shift to be related to the local surface potential. The analytic formula is easy to calibrate and can be used to convert the EFM-phase data to potential data. This procedure is demonstrated on a poly-(3-hexylthiophene-2,5-diyl) (P3HT) thin film, contacted with Au electrodes, and the potential distributions of the Au/P3HT/Au structure under various biases are presented.

Analytical Chemistry, 2000

Acombinedscanningelectrochemicalmicroscope(SECM)atomic force microscope (AFM) is described. The instrument permits the first simultaneous topographical and electrochemical measurements at surfaces, under fluid, with high spatial resolution. Simple probe tips suitable for SECM-AFM, have been fabricated by coating flattened and etched Pt microwires with insulating, electrophoretically deposited paint. The flattened portion of the probe provides a flexible cantilever (force sensor), while the coating insulates the probe such that only the tip end (electrode) is exposed to the solution. The SECM-AFM technique is illustrated with simultaneous electrochemical-probe deflection approach curves, simultaneous topographical and electrochemical imaging studies of tracketched polycarbonate ultrafiltration membranes, and etching studies of crystal surfaces.

Physical review letters, 2006

High-resolution imaging of ferroelectric materials using piezoresponse force microscopy (PFM) is demonstrated in an aqueous environment. The elimination of both long-range electrostatic forces and capillary interactions results in a localization of the ac field to the tip-surface ...

Biophysical Journal, 1997

The interaction of DMPC (L-a-dimyristoyl-1,2-diterradecanoyl-sn-glycero-3-phosphocholine, C36H72NO8P) lipid-coated Si3N4 surfaces immersed in an electrolyte was investigated with an atomic force microscope. A long-range interaction was observed, even when the Si3N4 surfaces were covered with nominally neutral lipid layers. The interaction was attributed to Coulomb interactions of charges located at the lipid surface. The experimental force curves were compared with solutions for the linearized as well as with exact solutions of the Poisson-Boltzmann equation. The comparison suggested that in 0.5 mM KCI electrolyte the DMPC lipids carried about one unit of charge per 100 lipid molecules. The presence of this surface charge made it impossible to observe an effective charge density recently predicted for dipole layers near a dielectric when immersed in an electrolyte. A discrepancy between the theoretical results and the data at short separations was interpreted in terms of a decrease in the surface charge with separation distance.

Quantitative measurement of the low-frequency dielectric constants of thick insulators at the nanoscale is demonstrated utilizing ac electrostatic force microscopy combined with finite-element calculations based on a truncated cone with hemispherical apex probe geometry. The method is validated on muscovite mica, borosilicate glass, poly͑ethylene naphthalate͒, and poly͑methyl methacrylate͒. The dielectric constants obtained are essentially given by a nanometric volume located at the dielectric-air interface below the tip, independently of the substrate thickness, provided this is on the hundred micrometer-length scale, or larger.

Nano Letters, 2009

We present the experimental demonstration of low-frequency dielectric constant imaging of single-layer supported biomembranes at the nanoscale. The dielectric constant image has been quantitatively reconstructed by combining the thickness and local capacitance obtained using a scanning force microscope equipped with a sub-attofarad low-frequency capacitance detector. This work opens new possibilities for studying bioelectric phenomena and the dielectric properties of biological membranes at the nanoscale.

Applied Physics Letters, 2010

Electron paramagnetic resonance investigation of polar nanoregions mobility in the relaxor PbMg1/3Nb2/3O3 and solid solutions PbMg1/3Nb2/3O3 -PbTiO3 J. Appl. Phys. 111, 014104 The dielectric relaxation behavior of (Na0.82K0.18)0.5Bi0.5TiO3 ferroelectric thin film J. Appl. Phys. 110, 124109 Dielectric and spin relaxation behaviour in DyFeO3 nanocrystals J. Appl. Phys. 110, 124301 Dielectric relaxation and alternating current conductivity of polyvinylidene fluoride doped with lanthanum chloride J. Appl. Phys. 110, 114119 Defects control for improved electrical properties in (Ba0.8Sr0.2)(Zr0.2Ti0.8)O3 films by Co acceptor doping Appl. Phys. Lett. 99, 232910 (2011) Additional information on Appl. Phys. Lett.

Nanotechnology, 2013

The need to resolve the electrical properties of confined structures (CNTs, quantum dots, nanorods, etc) is becoming increasingly important in the field of electronic and optoelectronic devices. Here we propose an approach based on amplitude modulated electrostatic force microscopy to obtain measurements at small tip-sample distances, where highly nonlinear forces are present. We discuss how this improves the lateral resolution of the technique and allows probing of the electrical and surface properties. The complete force field at different tip biases is employed to derive the local work function difference. Then, by appropriately biasing the tip-sample system, short-range forces are reconstructed. The short-range component is then separated from the generic tip-sample force in order to recover the pure electrostatic contribution. This data can be employed to derive the tip-sample capacitance curve and the sample dielectric constant. After presenting a theoretical model that justifies the need for probing the electrical properties of the sample in the vicinity of the surface, the methodology is presented in detail and verified experimentally.