Study of optical fibre structures by atomic force microscopy

A novel method using atomic force microscopy (AFM) to study optical fibre structures at the fibre end-face has been successfully developed. The doping concentration profiles of fibres revealed by differential etching speeds in a saturated solution of ammonium bifluoride at room temperature (25°C) were obtained from AFM topographic images. The superior spatial resolution of AFM made it possible to resolve concentric structures a hundred times smaller than the feature, due to the difference in the known refractive index (n) of 1×10^-3. Fibres with small core diameters and anisotropic structures, such as polarization-maintaining fibres, were studied with ease.     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

Physical bounds of metallic nanofingers obtained by mechano-chemical atomic force microscope nanolithography

To obtain metallic nanofingers applicable in surface acoustic wave (SAW) sensors, a mechano-chemical atomic force microscope (AFM) nanolithography on a metallic thin film (50 nm in thickness)/piezoelectric substrate covered by a spin-coated polymeric mask layer (50-60 nm in thickness) was implemented. The effective shape of cross-section of the before and after etching grooves have been determined by using the AFM tip deconvolution surface analysis, structure factor, and power spectral density analyses. The wet-etching process improved the shape and aspect ratio (height/width) of the grooves and also smoothed the surface within them. We have shown that the relaxed surface tension of the polymeric mask layer resulted in a down limitation in width and length of the lithographed nanofingers. The surface tension of the mask layer can be changed by altering the initial concentration of the polymer in the deposition process. As the surface tension reduced, the down limitation decreased. In fact, an extrapolation of the analyzed statistical data has indicated that by decreasing the surface tension from 39 to 10 nN/nm, the minimum obtainable width and length of the metallic nanofingers was changed from about 55 nm and 2 μm to 15 nm and 0.44 μm, respectively. Using the extrapolation’s results, we have shown that the future SAW sensors buildable by this nanolithography method possess a practical bound in their synchronous frequency (∼58 GHz), mass sensitivity (∼6125 MHz-mm²/ng), and the limit of mass resolution (∼4.88 × 10⁻¹⁰ ng/mm²).     

25 nm resolution single molecular fluorescence imaging by scanning near-field optical/atomic force microscopy

High-resolution single molecular near-field fluorescence images were observed by scanning near-field optical/atomic force microscopy (SNOM/AFM). We modified the SNOM/AFM for both high-resolution fluorescence imaging and high-resolution topographic imaging. The imaged fluorophore, Alexa 532, is prepared with a poly-methyl-methacrylate (PMMA) film coating. A fluorescence resolution of 25 nm was obtained with a simultaneous topographic image of a flat surface. A sample prepared with a lower PMMA concentration exhibited a rough surface in the micro area. The results for the flat surface indicated that the fluorescence resolution is worst in the rough surface sample, that the maximum fluorescence intensities for the individual fluorophore are similar, and that the decay rate is faster. Thus, we concluded that the morphological effect is an important factor in fluorescence image resolution and the apparent lifetimes of the fluorescence molecules.     

Chemical surface composition of the polyethylene implanted by Ag ions studied by phase imaging atomic force microscopy

High density polyethylene (HDPE) has been modified by Ag⁺ ion implantation with the energy of 60 keV. The total amount of implanted silver ions was 1, 5 and 12 × 10¹⁵ ions/cm². The surface topography was observed by atomic force microscopy (AFM), while the surface composition changes were detected using phase imaging AFM. Surface topography changes were studied in detail using 3D surface parameters analyses. The average roughness decreased for the implanted HDPE indicating the flattening of the surface. Phase AFM images indicated the homogenization of the polyethylene during ion implantation, while histogram analyses confirmed the change in surface composition.     

Bias-induced spatially resolved growth and removal of Si-oxide by atomic force microscopy

Three mechanisms for spatially resolved growth and removal of oxide on silicon substrates have been investigated. Thermally grown oxide layers with thicknesses in the range 2–6 nm were the distinctive feature of the system. The layers were characterized and manipulated by methodologies based on atomic force microscopy (AFM) with conducting probes in a vacuum environment of 10^-2–10^-3 Pa. The probe is then effectively a travelling electrode that generates an electrostatic field between the tip and the substrate. Oxide growth was induced for a positive sample bias greater than 5 V, but below the level corresponding to dielectric breakdown. Application of a short pulse of amplitude marginally above that corresponding to dielectric breakdown, on the other hand, had the effect of producing pits of inner diameter of about 10 nm in the pre-existing oxide layer at the point of tip-to-oxide contact. Application of a low positive sample bias (less than that required for measurable oxide growth) in combination with high linear scan speed had the effect of removing a pre-existing oxide layer from the scanned field of view. The most plausible mechanisms are based on transverse ionic diffusion (for oxide growth), controlled dielectric breakdown (for formation of pits) and lateral transport of silicaceous species (for oxide removal). Received: 24 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +617-3875-7656, E-mail: s.myhra@sct.gu.edu.au     

Quantitative atomic force microscopy with carbon monoxide terminated tips

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes.     

Isotope-selected measurements of the velocity-controlled Yb atomic beam

1 S₀→¹P₁ and ¹P₁→¹D₂ transitions were employed in this scheme. This method enabled precise measurement of the velocity distribution with the isotope selection and a higher resolution than our previous measurements using only the ¹S₀→¹P₁ transition. This technique was applied to the measurement of the Yb atomic beam whose velocity distribution was optically perturbed and compressed by radiation pressure. Received: 10 June 1996/Revised version: 19 July 1996     

Lipid membrane: inelastic deformation of surface structure by an atomic force microscope

The stability of the 1,2-Dioleoyl-sn-Glycero-3-[phospho-rac-1-Glycerol-Na] liposome in the liquid crystalline state have been investigated using an atomic force microscope (AFM). We have observed the inelastic deformation of the sample surface. The AFM tip causes persistent deformation of the surface of the lipid membrane, in which some of the lipid molecules are eventually pushed or dragged by the AFM tip. The experiment shows how the surface structure of the lipid membrane can be created by the interaction between the AFM tip and lipid membrane. When the operating force exceeds 10^-8 N, it leads to large deformations of the surface. A square region of about 1×1μm² is created by the scanning probe on the surface. When the operating force is between 10^-11N and 10^-8N, it can image the topography of the surface of the lipid membrane. The stability of the sample is related to the concentration of the medium in which the sample is prepared.     

Water-solid interfaces probed by high-resolution atomic force microscopy

Water-solid interfaces play important roles across a broad range of scientific and application fields. In the past decades, atomic force microscopy (AFM) has significantly deepened our understanding of water-solid interfaces at molecular scale. In this review, we describe the recent progresses on probing water-solid interfaces by noncontact AFM, highlighting the imaging of interfacial water with ultrahigh spatial resolution. In particular, the recent development of qPlus-based AFM with functionalized tips has made it possible to directly image the H-bonding skeleton of interfacial water under UHV environment. Based on high-order electrostatic forces, such a technique even enables submolecular-level imaging of weakly bonded water structures with negligible disturbance. In addition, the three-dimensional (3D) AFM using low-noise cantilever deflection sensors can achieve atomic resolution imaging at liquid/solid interfaces, which opens up the possibility of probing the hydration layer structures under realistic conditions. We then discuss the application of those AFM techniques to various interfacial water systems, including water clusters, ion hydrates, water chains, water monolayers/multilayers and bulk water/ice on different surfaces under UHV or ambient environments. Some important issues will be addressed, including the H-bonding topology, ice nucleation and growth, ion hydration and transport, dielectric properties of water, etc. In the end, we present an outlook on the directions of future AFM studies of water at interfaces and the challenges faced by this field, as well as the development of new AFM techniques.     

Maintaining the genuine structure of 2D materials and catalytic nanoparticles at atomic resolution

The recent development of atomic resolution, low dose-rate electron microscopy allows investigating 2D materials as well as catalytic nano particles without compromising their structural integrity. For graphene and a variety of nanoparticle compositions, it is shown that a critical dose rate exists of <100 e⁻/Ų s at 80 keV of electron acceleration that allows maintaining the genuine object structures including their surfaces and edges even if particles are only 3 nm large or smaller. Moreover, it is demonstrated that electron beam-induced phonon excitation from outside the field of view contributes to a contrast degradation in recorded images. These degradation effects can be eliminated by delivering electrons onto the imaged area, only, by using a Nilsonian illumination scheme in combination with a suitable aperture at the electron gun/monochromator assembly.     

Tarantula hemocyanins imaged by atomic force microscopy

Individual 4 x 6-meric tarantula hemocyanins and dissociation products were imaged by AFM in the non-contact mode. Although the resolution was low, the hexamers and topological arrangement within the oligomers can be seen. However, the relative humidity seems to affect the height profiles.     

Correlation of atomic force microscopy detecting local conductivity and micro‐Raman spectroscopy on polymer–fullerene composite films

Thin hetero‐junction composite films of polymer (electron donor) and fullerene (electron acceptor) are prepared on indium‐tin‐oxide coated glass by spin‐coating from solution in dichlorobenzene. Optimized atomic force microscopy (AFM) parameters allowed us to scan these soft composite films in contact mode and to measure their local conductivity with high lateral resolution by current‐sensing AFM. The morphology and local conductivity data are correlated with Kelvin force microscopy and micro‐Raman mapping and discussed with view to their photovoltaic properties. Regions with both compounds present are compared to areas where the components segregated, acting as shunts of the junction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanotubes and force interactions in an atomic force microscope

The conditions for nanotubes to be used as atomic force microscope (AFM) probes are analyzed. It is shown theoretically for the first time that single-and multilayer tubes with diameters ranging from 0.5 to 5 nm give atomic-level resolution of the surface. The presence of cylindrical symmetry makes each surface atom of a nanotube “imaging.” For a definite ratio of the diameter of a single nanotube and the period of the surface structure, the atomic resolution vanishes. Such nanotubes are of special interest for probing the details of the large-scale relief and for investigations in nanotribology. In contrast to silicon and other (solid) probes, nanotubes are not blunted on contact with the surface, but rather they bend and their initial shape is restored when the stress is removed. The critical loads for an AFM to function in the repulsive regime are determined. Zh. Tekh. Fiz. 69, 124–127 (August 1999)     

High resolution imaging of the dolomite (104) cleavage surface by atomic force microscopy

In this paper we present high resolution atomic force microscopy (AFM) images of dolomite (104) cleavage surfaces immersed in pure water. These images show a rectangular lattice with surface unit cell dimensions in general agreement with those derived from the dolomite bulk structure. Furthermore, the two-dimensional fast Fourier transform (2D-FFT) plots of the high resolution images exhibit a pattern of periodicities consistent with both the alternate orientation of the carbonate groups and the positions for calcium and magnesium atoms on the dolomite (104) surface. However, the Mg²⁺ and Ca²⁺ sublattices could not be resolved. Finally, the images in both the real and the Fourier space do not reveal any clear evidence of reconstruction of the dolomite (104) surfaces.     

Imaging and focusing of an atomic beam with a large period standing light wave

A novel atomic lens scheme is reported. A cylindrical lens potential was created by a large period ( 45 m) standing light wave perpendicular to a beam of metastable He atoms. The lens aperture (25 m) was centered in one antinode of the standing wave; the laser frequency was nearly resonant with the atomic transition 2³ S ₁–2³ P ₂ (=1.083 m) and the interaction time was significantly shorter than the spontaneous lifetime (100 ns) of the excited state. The thickness of the lens was given by the laser beam waist (40 m) in the direction of the atomic beam. Preliminary results are presented, where an atomic beam is focused down to a spot size of 4 m. Also, a microfabricated grating with a period of 8 m was imaged. We discuss the principle limitations of the spatial resolution of the lens given by spherical and chromatic aberrations as well as by diffraction. The fact that this lens is very thin offers new perspectives for deep focusing into the nm range.     

Resonance lonization mass spectroscopy with a pulsed thermal atomic beam

Resonance ionization mass spectroscopy (RIMS) and pulsed-laser induced desorption (PLID) have been combined for ultrasensitive detection and spectroscopy of very small samples of refractive elements. The method has been tested and applied to laser spectroscopy of 5×10⁹ atoms (1.5 pg) of¹⁹⁵Au (T _1/2= 183d) implanted at the ISOLDE online mass separator with 60 keV into graphite. A pulsed thermal atomic beam was formed by laser desorption with a 10 ns NdYag laser pulse. Subsequently the atoms were photoionized in a three-colour, three-step resonant excitation to an autoionizing state. The selectivity was enhanced by a time-of-flight measurement of the photo ions. In resonance, one ion was detected per 10⁵ atoms implanted resulting in a gain in detection efficiency by three orders of magnitude in comparison to the use of a continuous atomic beam. In the course of the experiments several unknown autoionizing states were found, and the lifetime of the 6d ² D ^3/2 state of gold was determined to be=10.7(6) ns.     

Temperature dynamics of triglycine sulfate domain structure according to atomic force microscopy and dielectric spectroscopy data

A hydrogen-containing ferroelectric triglycine sulfate (TGS) was comprehensively studied with an atomic force microscopy (AFM) and dielectric spectroscopy. The domain structure dynamics was in situ investigated with piezoresponse force microscopy (PFM) during heating and cooling the TGS crystal near phase transition. Relaxation dependencies of domain boundaries general perimeter and domain dimensions were obtained. TGS dielectric spectra measured at the frequency range from 10 to 10¹¹ Hz were analyzed on basis of significant contribution of conductivity into the dielectric response of ferroelectrics and a good agreement with the experimental data was received. It allows us to obtain more information about temperature dynamics of the domain structure.     

Evaluation of nanoscale roughness measurements on a plasma treated SU-8 polymer surface by atomic force microscopy

A comparison between roughness data obtained with an atomic force microscope (AFM) on different surfaces requires reliable roughness parameters. In order to specify the appropriate parameters for nanoscale roughness measurements, we compared the root mean square (rms) roughness and the relative surface area (sdr) as function of varying scan size, speed and pixel size. By using oxygen plasma (24 kJ) treated SU-8 with an average rms roughness of 2.6 ± 0.5 nm as reference surface, the repeatability of the method was evaluated for dynamic (tapping) and contact mode. The evaluation of AFM images indicated a decrease of the effective tip radius after a few measurements. This degradation of the tip lowers the resolution of the image and can affect roughness measurements.     

Detection of atomic oxygen by laser-induced fluorescence spectroscopy at 130 nm

VUV radiation generated by stimulated Raman scattering from H₂ and tunable around 130 nm is applied to the detection of atomic oxygen produced in a flow-tube. Concentrations in the range to the detection of atomic oxygen produced in a flow-tube. Concentrations in the range of 10¹⁰ to 10¹¹ O-atoms per cm³ lead to fluorescence signals that can easily be detected on a nanosecond timescale. We deduce that oxygen impurities generated by plasma-wall interaction in present-day tokamak experiments should be measurable with spatial and temporal resolution applying this vuv source.