Image contrast of dielectric specimens in transmission mode near-field scanning optical microscopy: imaging properties and tip artefacts

Near-field scanning optical microscopy (NSOM) is a scanned probe technique utilizing a subwavelength-sized light source for high-resolution imaging of surfaces. Although NSOM has the potential to exploit and extend the experimental utility of the modern light microscope, the interpretation of image contrast is not straightforward. In near-field microscopy the illumination intensity of the source (probe) is not a constant value, rather it is a function of the probe–sample electronic environment. A number of dielectric specimens have been studied by NSOM to elucidate the contrast role of specimen type, topography and crystallinity; a summary of metallic specimen observations is presented for comparative purposes. Near-field image contrast is found to be a result of lateral changes in optical density and edge scattering for specimens with little sample topography. For surfaces with considerable topography the contributions of topographic (Z) axis contrast to lateral (X,Y) changes in optical density have been characterized. Selected near-field probes have also been shown to exhibit a variety of unusual contrast artefacts. Thorough study of polarization contrast, optical edge (scattering) contrast, as well as molecular orientation in crystalline specimens, can be used to distinguish lateral contrast from topographic components. In a few cases Fourier filtering can be successfully applied to separate the topographic and lateral contrast components.     

Adhesion artefacts in atomic force microscopy imaging

Artefacts that affect contrast and arise from adhesion forces in atomic force microscopy images of aramid fibres (both fresh and plasma-treated) are investigated. It is demonstrated that these stem not only from variations in the chemical composition of the surface but also from certain topographical features (which may appear hidden or enhanced in the images), resulting in changes in the lateral forces that are detected by the cantilever and are comparable to the vertical forces. It is also shown that both types of contribution to the forces can be uncoupled to yield images free from these artefacts, thus allowing more accurate quantitative measurements. These artefactual effects are also generally applicable to many other materials.     

Calibration of the scanning (atomic) force microscope with gold particles

Scanning force microscopy (SFM) holds great promise for biological research. Two major problems that have confronted imaging with the scanning force microscope have been the distortion of the image and overestimation in measurements of lateral size due to the varying geometry and characteristics of the scanning tip. In this study, spherical colloidal gold particles (10, 20 and 40 nm in diameter) were used to determine (1) tip parameters (size, shape and semivertical angle); (2) the distortion of the image caused by the tip; and (3) the overestimation or broadening of lateral dimensions. These gold particles deviate little in size, are rigid and have a size similar to biological macromolecules. Images of the colloidal gold particles by SFM were compared with those obtained by electron microscopy (EM). The height of the gold particles as measured by SFM and EM was comparable and was little affected by the tip geometry. The measurements of the lateral dimensions of colloidal gold, however, showed substantial differences between SFM and EM in that SFM resulted in an overestimate of the lateral dimensions. Moreover, the distortion of images and broadening of lateral dimensions were specific to the SFM tip used. The calibration of the SFM tip with mica provided little clue as to the type of distortion and the amount of lateral broadening observed when the larger gold particles were scanned. The SFM image also depended on the orientation of the tip with respect to the specimen. Our results suggest that quantitative SFM imaging requires calibration to identify and account for both the distortions and the magnitude of lateral broadening caused by the cantilever tip. Calibration with gold particles is fast and nondestructive to the tip. The raw imaging data of the specimen can be corrected for the tip effect and true structural information can be derived. In summary, we present a simple and practical method for the calibration of the SFM tip using gold particles with a size in the range of biomacromolecules that allows: (1) selection of a cantilever tip that produces an image with minimal distortion; (2) quantitative determination of tip parameters; (3) reconstruction of the shape of the tip at different heights from the tip apex; (4) appreciation of the type of distortion that may be introduced by a specific tip and quantification of the overestimation of the lateral dimensions; and (5) calculation of the true structure of the specimen from the image data. The significance is that such calibration will permit quantitative and accurate imaging with SFM.     

Morphological investigation of various orthodontic lingual bracket slots using scanning electron microscopy and atomic force microscopy

Various labial and lingual orthodontic appliances with aesthetic materials have been developed due to an increased demand in aesthetic orthodontic treatment. However, there are few reports regarding the morphology of lingual orthodontic appliances. Therefore, this study evaluates the roughness of slot surfaces of various orthodontic lingual brackets using field emission scanning electron microscopy (FE‐SEM) and atomic force microscopy (AFM). Three types of stainless steel lingual brackets (Stealth^®, 7th Generation^®, and Clippy L^®) and one gold lingual bracket (Incognito?) with a slot size of 0.018 inches × 0.025 inches (0.457 × 0.635 mm²) were selected as representative lingual materials. Both FE‐SEM and AFM examinations showed that the Stealth^® and Clippy L^® brackets had the lowest surface roughness, while the 7th Generation^® bracket had the highest surface roughness. There was a significant difference in surface morphology between the types of lingual brackets, even when composed of the same material. The surface roughness of the bracket slot was dependent on the manufacturing process or surface polishing process rather than the fundamental properties of the bracket materials. There was no significant difference in the mean surface roughness of the slot floor between gold and stainless steel lingual brackets. These findings suggest that, although the gold lingual bracket is very expensive, it has great potential for use in patients with nickel allergy.     

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.     

Atomic and magnetic force microscopy imaging of thin-film recording heads

Investigations on the use of the scanning probe microscope (SPM) in the atomic force microscopy (AFM) mode for topography imaging and the magnetic force microscopy (MFM) mode for magnetic imaging are presented for a thin-film recording head. Results showed that the SPM is suitable for imaging the surface profile of the recording head, determining the width of the pole gap region, and mapping the magnetic field patterns of the recording head excited under current bias conditions of different polarity. For the cobalt sputter-coated tips used in MFM imaging, it was found that the magnetic field patterns obtained under different polarities of the current bias to the recording head were similar. This can be explained by the nature of the thin-film MFM tip, in which the direction of the tip magnetic moment can follow the stray magnetic field of the sample as the current bias to the recording head reverses in direction.     

Ribosome substructure investigated by scanning force microscopy and image processing

Scanning force microscopy was used to study the ultrastructure of eukaryotic ribosomes from Chironomus pallidivittatus in the polysomal complex. Positively stained polysomes were imaged, and the resulting three-dimensional ribosomal structures were further processed by statistical analyses of virtual cross-sections parallel to the substrate plane. Structural investigations were based on parameters which are minimally influenced by the tip geometry, like section plane centre or axis ratio. In the lower part of the structure a shift of the section centres was observed, suggesting an attached structure. The geometry of the sections revealed an elliptical shape in the upper part (axis ratio 1.52 ± 0.22), with a less elongated shape in the lower region (axis ratio 1.41 ± 0.18). A model for the surface topography of a positively stained ribosome exhibiting a small subunit attached along the long side of an elliptical large structure is proposed.     

Influence of atomic force microscopy acquisition parameters on thin film roughness analysis

A reliable procedure for measuring parameters connected to surface roughness is needed to compare the gas sensing properties of various thin films or the effect of different fabrication procedures on the surface roughness and the sensing properties. In this article, we propose to investigate how the acquisition parameters specific to atomic force microscopy investigations such as pixel size, scan area and scan speed influence the roughness parameters, namely root mean square and surface area ratio, commonly used for characterizing the gas sensing properties of porphyrins and other materials.     

Fractal compression and adaptive sampling: reducing the image acquisition time in scanning probe microscopy

In typical scanning probe microscope experiment a three‐dimensional image of a substrate is obtained. For a given scanning mechanism, the time needed to image an area depends mainly on the number of samples and the size of the image. The imaging speed is further compromised by drifts associated with the substrate and the piezoscanner. It is therefore desirable to improve the imaging speed with limited impact to the effective resolution of the resulting image. By utilizing an adaptive sampling scheme with fractal compression technique, we have demonstrated that the number of the required samples can be significantly reduced with minimal impact to the image quality. SCANNING 30: 463–473, 2008. © 2008 Wiley Periodicals, Inc.     

Atomic force and scanning electron microscopy of atmospheric particles

Atomic force microscopy (AFM) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) have been used for both morphological and elemental mass analysis study of atmospheric particles. As part of the geometrical particle analysis, and in addition to the traditional height profile measurement of individual particles, AFM was used to measure the volume relative to the projection area for each particle separately, providing a particle shape model. The element identification was done by the EDS analysis, and the element mass content was calculated based on laboratory calibration with particles of known composition. The SEM-EDS mass measurements from two samples collected at 150 and 500 m above the surface of the Mediterranean Sea were found to be similar to mass calculations derived from the AFM volume measurements. The AFM results show that the volume of most of the aerosols that were identified as soluble marine sulfate and nitrate aerosol particles can be better estimated using cylindrical shapes than spherical or conical geometry.     

Optoelectronic detector probes for scanning near-field optical microscopy

A brief explanation of the optoelectronic probe concept and a comparison between the implementation of passive waveguide probes and optoelectronic probes in scanning near-field optical microscopy (SNOM) is presented. The first probe realizations using cleaved semiconductor crystals and the work at present in progress using microfabricated Si pyramids are described. These crystals with evaporated metal electrodes forming a slit aperture with subwave-length dimensions work as metal–semiconductor–metal photodetectors. Their optical detection behaviour is investigated by measuring the intensity distribution of a laser focal point. Measurements where the external bias voltage is changed show that it is possible to modify the detection behaviour of the device because of the varying depletion widths. The last part of the article describes a concept where pyramidal probes should function simultaneously as sensors for scanning force microscopy (SFM) to measure topography and as optoelectronic probes for scanning near-field optoelectronic microscopy (SNOEM).     

A method for measuring the size distribution of latex particles by scanning force microscopy

A methodology has been developed to accurately determine the size distribution of latex particles using the scanning force microscope (SFM). Unlike other workers, who have generally measured the lateral dimensions of monolayers of latex particles using a global quantification method, we have measured the heights of individual latex particles located at the edges of latex monolayers that were immobilised onto mica substrates. In agreement with other work, we noted that the edges of monolayers of latex particles provided stable and reproducible scanning force imaging.

Whilst SFM imaging noise, image processing artifacts, tip/sample forces and variations in the mica substrate are sources of measurement error that should not be overlooked, our experience has been that the variation over time of the sensitivity of the Z actuator is the greatest potential uncertainty in determining the heights of latex particles. The methodology that we used requires frequent calibration of the Z actuator of the SFM, typically before and after two or three images, in order to ensure that the uncertainties in the Z sensitivity are known and minimised. This methodology was developed for an SFM instrument that was equipped with open loop piezoelectric actuators following a careful study of the behaviour of those actuators.

Using this methodology, we have measured the size distributions of populations of 300–400 latex particles from each of several different latex samples, with the maximum variation in the Z-actuator calibration experienced during the measurement of a sample being less than 2%, often about 1% and occasionally better still. In so doing, we have demonstrated that SFMs equipped with open loop actuators can be used for high confidence quantitative measurements of step heights.     

Research on double-probe, double- and triple-tip effects during atomic force microscopy scanning

Information obtained by atomic force microscopy (AFM) depends strongly on the kind of probe or tip used; therefore, probe and tip effects have to be taken into account when verifying or interpreting the data acquired. In many papers, double-tip effects have been mentioned while other research was done; however, there are only a few special reports on double- or triple-tip effects, especially double-probe effects. In our paper, metaphase chromosomes of Chinese hamster ovary (CHO) cells, aggregates of pectin molecules, membrane surface of mouse embryonic stem cells, and R-phycoerythrin-conjugated immunoglobulin G complexes were imaged by AFM with high-quality probes, double-probe cantilever, and double-tip and triple-tip probes, respectively, in order to determine double-probe, double-tip, and triple-tip effects during AFM scanning. We found that the double-probe, double-tip, and triple-tip effects share the same principle, and that these effects correlate with distance and height differences between probes of double-probe cantilever or tips of double-tip or multiple-tip probes. Since many other factors influence double-probe or double-tip effects, more in-depth studies must be undertaken. However, this initial research will make all users of AFM techniques aware of double-probe and double-tip or triple-tip effects during AFM scanning and aid in verifying or interpreting the data acquired.     

Combined atomic force and scanning reflection interference contrast microscopy

A sphere attached to a cantilever is used simultaneously as an atomic force microscope (AFM) tip and as a curved reflective surface for producing scanning reflection interference contrast microscope (RICM) images of fluorescent beads dried onto a glass slide. The AFM and RICM images are acquired in direct registration which enables the identification of individually excited beads in the AFM images. The addition of a sharp, electron beam-deposited tip to the sphere gives nanometer resolution AFM images without loss of optical contrast.     

A compact sensor head for simultaneous scanning force and near-field optical microscopy

A compact sensor head based on scanning force microscopy (SFM) using cantilever probes has been developed. The idea is to replace the microscope objective of a conventional optical microscope by this compact module and turn the optical microscope into a scanning force and near-field optical microscope with subwavelength resolution. We describe our concept and present initial results showing images of the object’s optical properties and surface topography recorded simultaneously.     

Characterization of biomolecule immobilization by scanning force microscopy using a wet-masking technique

The assemblage of molecular layers was investigated using scanning force microscopy (SFM). A wet-masking technique was used for the preparation of monolayer steps by partial masking of the substrate with an elastomeric mask during incubation in the modification buffers. The subsequent adsorption of biotin, streptavidin, and biotiny lated beads onto a gold substrate was investigated by SFM visualization of the created steps. The molecular layers were characterized based on measurements of step height and surface roughness. The simultaneous visualization of the surface before and after modification minimizes artifacts introduced by changes in tip shape or imaging parameters.     

Real-time atomic force microscopy in lubrication condition

We have studied frictional force and wear problem in real-time atomic force microscopy in contact-mode using a resonator type mechanical scanner allegedly reported. The fast scanning may cause wear in the sample surface or the tip, and may deteriorate the image quality. Mineral oil was used to make a lubricious surface on a polycarbonate sample, and it was found that the interfacial frictional force was decreased. A Si tip which was coated with a hydrophobic film by means of chemical modification was confirmed to diminish the frictional force in the fast scanning process. The resultant image quality was improved due to reduced friction and wear.     

Coaxial probes for scanning near-field microscopy

This paper deals with the development of coaxial aperture tips integrated in a cantilever probe for combined scanning near-field infrared microscopy and scanning force microscopy. A fabrication process is introduced that allows the batch fabrication of hollow metal aperture tips integrated on a silicon cantilever. To achieve the coaxial tip arrangement a metal rod is deposited inside the hollow tip using the focused ion beam technique. Theoretical calculations with a finite integration code were performed to study the transmission characteristics of coaxial tips in comparison with conventional aperture probes. In addition, the influence of the geometrical design parameters of the coaxial probe on its optical behaviour is investigated.