Dielectric constants by multifrequency non-contact atomic force microscopy

We present a method to obtain capacitive forces and dielectric constants of ultra-thin films on metallic substrates using multifrequency non-contact atomic force microscopy with amplitude feedback in air. Capacitive forces are measured via cantilever oscillations induced at the second bending mode and dielectric constants are calculated by fitting an analytic expression for the capacitance (Casuso et al 2007 Appl. Phys. Lett. 91 063111) to the experimental data. Dielectric constants for self-assembled monolayers of thiol molecules on gold (2.0±0.1) and sputtered SiO2 (3.6±0.07) were obtained under dry conditions, in good agreement with previous measurements. The high Q-factor of the second bending mode of the cantilever increases the accuracy of the capacitive measurements while the low applied potentials minimize the likelihood of variation of the dielectric constants at high field strength and of damage from dielectric breakdown of air.     

Tour de force microscopy

Scanning probe microscopy (SPM) is capable of imaging synthetic polymers and biomolecular systems at sub-molecular resolution, without the need for staining or coating, in a range of environments including gas and liquid, so offering major advantages over other forms of microscopy. However, there are some limitations, which could be alleviated by (i) reducing the force interaction between the probe and specimen and (ii) increasing the rate of imaging. New developments in instrumentation from the SPM group at the University of Bristol to overcome these limitations are discussed and illustrated here.The invention of scanning tunneling microscopy (STM) in 1981 began a revolution in microscopy¹, which has led to a whole new family of about a dozen microscopies known collectively as scanning probe microscopy (SPM). The importance of this development is comparable to that of the invention of electron microscopy in the 1930s and arguably as fundamental as the development of the first optical microscopes, since SPM uses an entirely different principle from optical and electron microscopy to achieve imaging at high resolution.     

Nanofabrication with atomic force microscopy

Atomic force microscopy (AFM) was developed in 1986. It is an important and versatile surface technique, and is used in many research fields. In this review, we have summarized the methods and applications of AFM, with emphasis on nanofabrication. AFM is capable of visualizing surface properties at high spatial resolution and determining biomolecular interaction as well as fabricating nanostructures. Recently, AFM-based nanotechnologies such as nanomanipulation, force lithography, nanografting, nanooxidation and dip-pen nanolithography were developed rapidly. AFM tip (typical radius ranged from several nanometers to tens of nanometers) is used to modify the sample surface, either physically or chemically, at nanometer scale. Nanopatterns composed of semiconductors, metal, biomolecules, polymers, etc., were constructed with various AFM-based nanotechnologies, thus making AFM a promising technique for nanofabrication. AFM-based nanotechnologies have potential applications in nanoelectronics, bioanalysis, biosensors, actuators and high-density data storage devices.     

Combined scanning electrochemical-atomic force microscopy

A combined scanning electrochemical microscope (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 track-etched polycarbonate ultrafiltration membranes, and etching studies of crystal surfaces.     

Gold-coated conducting-atomic force microscopy probes

Some aspects of the performance of gold-coated conductive probes used in conducting atomic force microscopy (C-AFM) technique are discussed. The resistance of the nanocontact between the gold-coated AFM tip and the graphite substrate has been monitored at various applied forces. For small forces (<50 nN), resistance on the order of a few kiloohms was observed. Minimal contact resistance was observed for forces in the range 100-150 nN, beyond which the tip seems to undergo plastic deformation. The resistance of the nanocontact increased when current on the order of 100 microA was allowed to pass through, finally resulting in melting of the gold coating.     

Magnetic force microscopy of superparamagnetic nanoparticles

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.     

Atomic force microscopy of electrochemical nanoelectrodes

Nanometer-sized electrodes have recently been used to investigate important chemical and biological systems on the nanoscale. Although nanoelectrodes offer a number of advantages over macroscopic electrochemical probes, visualization of their surfaces remains challenging. Thus, the interpretation of the electrochemical response relies on assumptions about the electrode shape and size prior to the experiment and the changes induced by surface reactions (e.g., electrodeposition). In this paper, we present first AFM images of nanoelectrodes, which provide detailed and unambiguous information about the electrode geometry. The effects of polishing and cleaning nanoelectrodes are investigated, and AFM results are compared to those obtained by voltammetry and SEM. In situ AFM is potentially useful for monitoring surface reactions at nanoelectrodes.     

The role of nonlinear dynamics in quantitative atomic force microscopy

Various methods of force measurement with the atomic force microscope are compared for their ability to accurately determine the tip-surface force from analysis of the nonlinear cantilever motion. It is explained how intermodulation, or the frequency mixing of multiple drive tones by the nonlinear tip-surface force, can be used to concentrate the nonlinear motion in a narrow band of frequency near the cantilever's fundamental resonance, where accuracy and sensitivity of force measurement are greatest. Two different methods for reconstructing tip-surface forces from intermodulation spectra are explained. The reconstruction of both conservative and dissipative tip-surface interactions from intermodulation spectra are demonstrated on simulated data.     

Atomic force microscopy and atomic force acoustic microscopy characterization of photo-induced changes in some Ge-As-S amorphous films

Amorphous Ge₂₇As₁₃S₆₀, Ge₁₄As₂₇S₅₉ and Ge₁₆As₂₆S₅₈ thin films were prepared by thermal evaporation. Well annealed films were photodarkened by the photons with energy little exceeding the band gap energy. Using Atomic Force Microscopy we observed significant photoexpansion of studied films. Atomic Force Acoustic Microscopy revealed domains like structure of the surface and near surface parts of the samples which one was found to be more disintegrated after illumination.     

Chaos in dynamic atomic force microscopy

In tapping mode atomic force microscopy (AFM) the highly nonlinear tip-sample interaction gives rise to a complicated dynamics of the microcantilever. Apart from the well-known bistability under typical imaging conditions the system exhibits a complex dynamics at small average tip-sample distances, which are typical operation conditions for mechanical dynamic nanomanipulation. In order to investigate the dynamics at small average tip sample gaps experimental time series data are analysed employing nonlinear analysis tools and spectral analysis. The correlation dimension is computed together with a bifurcation diagram. By using statistical correlation measures such as the Kullback-Leibler distance, cross-correlation and mutual information the dataset can be segmented into different regimes. The analysis reveals period-3, period-2 and period-4 behaviour, as well as a weakly chaotic regime.     

Atomic force microscopy using plastic replicas

Scanning probe microscopy is now an accepted tool in both industrial and research efforts. Its development parallels the advances in technology and imaging applications found in the history of progress of both transmission electron microscopy and scanning electron microscopy. All three forms of microscopy ultimately suffer a fundamental application problem—situations arise where it is either unreasonable or impossible to observe a particular sample within the sample stage of the microscope. For the transmission and electron and scanning electron microscopies, this problem has been resolved by resorting to making a replica of the area of interest on the actual sample and preparing the replica so that it may be imaged directly by the desired microscopy technique. This work attempts to ascertain the suitability of observing replicas using a scanned probe microscope; specifically, employing the techniques of atomic force microscopy to image plastic surface replicas.     

The use of force modulation microscopy to investigate block copolymer morphology

Force modulation microscopy (FMM) is used to characterize the external surface and internal fracture surface morphologies of three different block copolymer samples. A roll-cast poly(styrene-butadiene-styrene) triblock copolymer film, spin-coated poly(styrene-b-methyl methacrylate) thin films, and an ultrathin poly(styrene-b-hexyl isocyanate) rod-coil block copolymer film were investigated. For each sample, height and elasticity images were obtained for the same areas allowing direct comparison. The elasticity images obtained using force modulation microscopy were independent of surface roughness and found to exhibit better contrast and spatial resolution of the respective block copolymer domains than the height images. The lateral resolution of the elasticity images was sufficient to show microphase separated domains having length scales as small as about 10 nm. The poly(styrene-b-methyl methacrylate) samples demonstrate that FMM can even be successfully used to study block copolymers in which both blocks are glassy under the conditions of measurement.     

Chemical force microscopy of single live cells

Traditionally, cell surface properties have been difficult to study at the subcellular level, especially on hydrated, live cells. Here, we demonstrate the ability of chemical force microscopy to map the hydrophobicity of single live cells with nanoscale resolution. After validating the technique on reference surfaces with known chemistry, we probe the local hydrophobic character of two medically important microorganisms, Aspergillus fumigatus and Mycobacterium bovis, in relation with function. Applicable to a wide variety of cells, the chemically sensitive imaging method presented here provides new opportunities for studying the nanoscale surface properties of live cells and for understanding their roles in mediating cellular events.     

Friction force microscopy of mixed Langmuir-Blodgett films

Thin films of mixtures of fluorocarbons and hydrocarbons are investigated by friction force microscopy. For a mixture of equal molar parts of arachidic acid and partially fluorinated carboxylic acid, phase separation manifests itself as circular domains. By shortening the length of the hydrocarbon chain, the disorder of the system is increased and phase separation takes on a fractal geometry. The frictional forces are found to be material specific and distinguish between domains of fluorocarbons and hydrocarbons. With higher normal forces hydrocarbon particles are easily sheared away while the fluorocarbon region remains unbroken. The results are discussed in relation to the phenomenon of boundary lubrication.     

High-speed atomic force microscopy coming of age

High-speed atomic force microscopy (HS-AFM) is now materialized. It allows direct visualization of dynamic structural changes and dynamic processes of functioning biological molecules in physiological solutions, at high spatiotemporal resolution. Dynamic molecular events unselectively appear in detail in an AFM movie, facilitating our understanding of how biological molecules operate to function. This review describes a historical overview of technical development towards HS-AFM, summarizes elementary devices and techniques used in the current HS-AFM, and then highlights recent imaging studies. Finally, future challenges of HS-AFM studies are briefly discussed.