Topographic Contrast Research Articles

Visual quality with corneo-scleral contact lenses for keratoconus management.

To assess the visual quality achieved by fitting corneo-scleral contact lenses (CScL) for keratoconus management. Thirty patients with keratoconus presented to have CScL fitted because of the unsatisfactory visual quality they experienced with their contact lenses or spectacles. The eye examination included visual acuity assessment, anterior eye biomicroscopy, ocular fundus examination, corneal topographic analysis, endothelial-cell count, contrast sensitivity and aberrometry. The fitting process was performed using a diagnostic trial set. Subjective visual quality and comfort, and contact lens wear time were also reported. Patients were monitored for one year. Three patients discontinued CScL wear before one year. Therefore, 27 eyes of 27 patients (19 male and 8 female) participated in this study. The mean age was 36.1 ± 13.1 (mean ± SD) years. Statistically significant differences were found in logMAR visual acuity between the best spectacle-corrected vision and after CScL fitting (mean ± SD, 0.23 ± 0.30 and 0.00 ± 0.14, respectively; p < 0.001). The total high-order aberrations decreased significantly (55%), and the spatial frequencies of contrast sensitivity all improved to normal range values of the population. Furthermore, high subjective visual quality and comfort ratings, and prolonged usage times (mean ± SD, 13.44 ± 2.38 h a day) were reported. No adverse ocular effects or clinically relevant changes in corneal parameters, visual quality, comfort or usage time were found one year after wearing CScL. This CScL seems to be safe and healthy, providing optimal visual quality, comfort and prolonged usage times in patients with keratoconus.

Design and fabrication of pectin-coated nanoliposomal delivery systems for a bioactive polyphenolic: Phloridzin

Nanostructured colloidal delivery systems comprising of pectin-coated nanoliposomes (pectonanoliposomes) were developed as carriers for a bioactive polyphenolic compound (phloridzin). Phloridzin-loaded nanoliposomes were fabricated using a heating-stirring-sonication method, and coated with low methoxyl pectin using an electrostatic deposition approach. Dynamic light scattering, micro-electrophoresis, atomic force microscopy, and UV–Visible spectroscopy were used to investigate the impact of system composition on the size, charge, morphology and stability as well as immobilization, adsorption and encapsulation efficiencies of the pectonanoliposomes. Response surface methodology was used to optimize the composition of the pectonanoliposomes based on particle size and charge characteristics. Linear, quadratic and interaction effects of 1,2-dioleoyl-3-trimethyl ammonium propane/lecithin, phloridzin/lecithin and pectin/liposome ratios significantly influenced the mean hydrodynamic diameter and/or surface charge of pectonanoliposomes. Second-order polynomial regression models were generated for intensity-weighted particle size and zeta potential of the designed carriers. Topographic and phase contrast images showed that pectonanoliposomes exhibited a range of different morphologies. Coating the nanoliposomes with pectin improved their immobilization and encapsulation efficiencies as well as physical storage stability. Cationic pectonanoliposomes were superior to plain systems regarding long-term stability. Our results suggest that pectonanoliposomes may be suitable delivery systems for polyphenolic nutraceuticals, such as phloridizin, in functional food and pharmaceutical applications.

Vickers Hardness of Diamond and cBN Single Crystals: AFM Approach

Atomic force microscopy in different operation modes (topography, derivative topography, and phase contrast) was used to obtain 3D images of Vickers indents on the surface of diamond and cBN single crystals with high spatial resolution. Confocal Raman spectroscopy and Kelvin probe force microscopy were used to study the structure of the material in the indents. It was found that Vickers indents in diamond has no sharp and clear borders. However, the phase contrast operation mode of the AFM reveals a new viscoelastic phase in the indent in diamond. Raman spectroscopy and Kelvin probe force microscopy revealed that the new phase in the indent is disordered graphite, which was formed due to the pressure-induced phase transformation in the diamond during the hardness test. The projected contact area of the graphite layer in the indent allows us to measure the Vickers hardness of type-Ib synthetic diamond. In contrast to diamond, very high plasticity was observed for 0.5 N load indents on the (001) cBN single crystal face. Radial and ring cracks were absent, the shape of the indents was close to a square, and there were linear details in the indent, which looked like slip lines. The Vickers hardness of the (111) synthetic diamond and (111) and (001) cBN single crystals were determined using the AFM images and with account for the elastic deformation of the diamond Vickers indenter during the tests.

Contrast of positively charged oxide precipitate in out-lens, in-lens and in-column SE image.

Modern scanning electron microscopes are usually equipped with multiple detectors and enable simultaneous collection of two or even three secondary electron images. The secondary electrons become divided between the detectors in dependence on their initial kinetic energy and emission angle. In this study, sharing of the secondary electrons by out-lens, in-lens and in-column detectors has been systematically investigated. Energy filtering of the signal electrons is demonstrated by separation of the voltage and the topographical contrast in the micrographs obtained by out-lens and in-lens/in-column detectors. The presence of two detectors inside the electron column enables further filtering of the low kinetic energy secondary electrons, which results to unusual contrasts and phenomena. In this paper, inversion of the contrast sign between a positively charged oxide particle and conductive steel matrix (i.e. voltage contrast) in SE images collected under specific imaging conditions is demonstrated.

Increasing compositional backscattered electron contrast in scanning electron microscopy

A method for increasing compositional or material contrast of a standard semiconductor BSE detector in a scanning electron microscope (SEM) by compensation of the topographic contrast component is proposed. Compensation is based on the physical properties of backscattered electron emission and topography information of the specimen's surface.Three analytical and semi-empirical compensation algorithms employing different physical models and approximations are implemented and compared to conventional BSE signals to show the effectivity of the proposed compensation approach.

Comparative study of image contrast in scanning electron microscope and helium ion microscope.

Images of Ga+ -implanted amorphous silicon layers in a 110 n-type silicon substrate have been collected by a range of detectors in a scanning electron microscope and a helium ion microscope. The effects of the implantation dose and imaging parameters (beam energy, dwell time, etc.) on the image contrast were investigated. We demonstrate a similar relationship for both the helium ion microscope Everhart-Thornley and scanning electron microscope Inlens detectors between the contrast of the images and the Ga+ density and imaging parameters. These results also show that dynamic charging effects have a significant impact on the quantification of the helium ion microscope and scanning electron microscope contrast.

Diverse landscapes beneath Pine Island Glacier influence ice flow

The retreating Pine Island Glacier (PIG), West Antarctica, presently contributes ~5–10% of global sea-level rise. PIG’s retreat rate has increased in recent decades with associated thinning migrating upstream into tributaries feeding the main glacier trunk. To project future change requires modelling that includes robust parameterisation of basal traction, the resistance to ice flow at the bed. However, most ice-sheet models estimate basal traction from satellite-derived surface velocity, without a priori knowledge of the key processes from which it is derived, namely friction at the ice-bed interface and form drag, and the resistance to ice flow that arises as ice deforms to negotiate bed topography. Here, we present high-resolution maps, acquired using ice-penetrating radar, of the bed topography across parts of PIG. Contrary to lower-resolution data currently used for ice-sheet models, these data show a contrasting topography across the ice-bed interface. We show that these diverse subglacial landscapes have an impact on ice flow, and present a challenge for modelling ice-sheet evolution and projecting global sea-level rise from ice-sheet loss.

Penetration Depth and Defect Image Contrast Formation in Grazing-Incidence X-ray Topography of 4H-SiC Wafers

Synchrotron x-ray topography in grazing-incidence geometry is useful for discerning defects at different depths below the crystal surface, particularly for 4H-SiC epitaxial wafers. However, the penetration depths measured from x-ray topographs are much larger than theoretical values. To interpret this discrepancy, we have simulated the topographic contrast of dislocations based on two of the most basic contrast formation mechanisms, viz. orientation and kinematical contrast. Orientation contrast considers merely displacement fields associated with dislocations, while kinematical contrast considers also diffraction volume, defined as the effective misorientation around dislocations and the rocking curve width for given diffraction vector. Ray-tracing simulation was carried out to visualize dislocation contrast for both models, taking into account photoelectric absorption of the x-ray beam inside the crystal. The results show that orientation contrast plays the key role in determining both the contrast and x-ray penetration depth for different types of dislocation.

Separation of atomic-scale spin contrast on NiO(0 0 1) by magnetic resonance force microscopy

In magnetic exchange force microscopy, images contain the topographic contrast mixed with the spin contrast on the sample surface. In this study, we propose a new method of magnetic resonance force microscopy using ferromagnetic resonance to extract only the spin contrast. In this method, the magnetization of a magnetic cantilever is modulated by ferromagnetic resonance to separate the spin contrast and topographic contrast. We succeeded in obtaining a spin image of Ni atoms on a NiO (0 0 1) surface. Furthermore, we successfully detected the superexchange interaction between the tip apex atom and the second layer of Ni atoms.

Impact of inter-seasonal solar variability on the association of lower troposphere and cold point tropopause in the tropics: Observations using RO data from COSMIC

Association of lower tropospheric variations with the cold point tropopause (CPT) is examined on inter-seasonal basis over the tropical region (30°N-30°S) during 2007–2010 using COSMIC/FORMOST-3 Radio Occultation (RO) data. Temperature analyses for this association are shown over different regions of the globe having contrast topography namely over Western Pacific sector, Indian sector, and African sector. Correlation coefficient (r), taken as a measurement of association, show specific longitudinal differences between the lower troposphere (from 1km to 5km height) and the CPT. The northern and southern hemispheres show contrast coupling of temperature variation between lower tropospheric region and the CPT. Land and ocean effects are found to contribute in a different way to the correlation coefficient. Analyses show symmetrical structure of ‘r’ on both sides of the equator over the African region, as data include mostly land region on both side of equator. Data represent positive correlation (r~0.5) over 15°–20° latitudes on either side of the equator over the African region, suggesting strong hold of the inter-seasonal variation of solar diabatic heating influence over the tropic of Cancer and tropic of Capricorn. On the other hand, there is a contrast behaviour over the Indian region, ‘r’ is nearly negative (~−1.0) each year in the southern hemisphere (SH) and positive (~0.4) in the northern hemisphere (NH) with a maxima near tropic of Cancer. Western Pacific region is found to display a linear increase in ‘r’ from negative (~−1.0) in SH to positive (~0.8) in NH. In general, ‘r’ (positive) maximizes over the land region around 15°–20° latitudes, suggesting a control of in phase inter-seasonal solar heating on the coupling of boundary layer/lower troposphere and CPT region, whereas it turns negative over water body. Analyses suggest that variabilities in CPT over different regions of globe show significant inter-seasonal association with the lower troposphere. Thus CPT variabilities are not only governed by QBO, ENSO, gravity waves and Kelvin wave system as reported in earlier studies but also considerably affected by inter-seasonal changes taking place in the lower troposphere.

“Secondary electron spectra of semi-crystalline polymers – A novel polymer characterisation tool?”

The nano-scale dispersion of ordered/disordered phases in semi-crystalline polymers can strongly influence their performance e.g. in terms of mechanical properties and/or electronic properties. However, to reveal the latter in scanning electron microscopy (SEM) often requires invasive sample preparation (etching of amorphous phase), because SEM usually exploits topographical contrast or yield differences between different materials. However, for pure carbon materials the secondary spectra were shown to differ substantially with increased order/disorder. The aims here is to gain an understanding of the shape of secondary electron spectrum (SES) of a widely used semi-crystalline polymer regioregular poly(3-hexylthiophene-2,5-diyl), commonly known as P3HT, and its links to the underlying secondary electron emission mechanisms so SES can be exploited for the mapping the nano-morphology. The comparison of simulated and experimental SES shows an excellent agreement, revealing a peak (at about 0.8eV) followed by a broad shoulder (between 2eV and 4.5eV) with respective relative intensities reflecting order/disorder.

Resolving the Discrepancy between Observed and Calculated Penetration Depths in Grazing Incidence X-Ray Topography of 4H-SiC Wafers

Synchrotron X-ray Topography with grazing incidence geometry is useful for discerning defects at different depths below the crystal surface, particularly for 4H-SiC epitaxial wafers. However, the penetration depths measured from X-ray topographs are much larger than the theoretical values. In order to interpret this discrepancy, we simulate topographic contrast of dislocations based on two of the most basic contrast formation mechanisms – orientation contrast and kinematical contrast. Orientation contrast considers merely the displacement fields associated with dislocations while kinematical contrast also takes the diffraction volume into account. The diffraction volume is defined by the effective misorientation around dislocations and the rocking curve width for particular diffraction vector. Ray Tracing Simulation has been carried out to visualize dislocation contrast for both models, taking into account the photoelectric absorption of X-ray beams inside the crystal. Results show that orientation contrast plays the key role in determining both the contrast and X-ray penetration depths for different types of dislocations.

Contrast of Backscattered Electron SEM Images of Nanoparticles on Substrates with Complex Structure.

This study is concerned with backscattered electron scanning electron microscopy (BSE SEM) contrast of complex nanoscaled samples which consist of SiO2 nanoparticles (NPs) deposited on indium-tin-oxide covered bulk SiO2 and glassy carbon substrates. BSE SEM contrast of NPs is studied as function of the primary electron energy and working distance. Contrast inversions are observed which prevent intuitive interpretation of NP contrast in terms of material contrast. Experimental data is quantitatively compared with Monte-Carlo- (MC-) simulations. Quantitative agreement between experimental data and MC-simulations is obtained if the transmission characteristics of the annular semiconductor detector are taken into account. MC-simulations facilitate the understanding of NP contrast inversions and are helpful to derive conditions for optimum material and topography contrast.

Nanopatterning of GeTe phase change films via heated-probe lithography.

The crystallization of amorphous germanium telluride (GeTe) thin films is controlled with nanoscale resolution using the heat from a thermal AFM probe. The dramatic differences between the amorphous and crystalline GeTe phases yield embedded nanoscale features with strong topographic, electronic, and optical contrast. The flexibility of scanning probe lithography enables the width and depth of the features, as well as the extent of their crystallization, to be controlled by varying probe temperature and write speed. Together, these technologies suggest a new approach to nanoelectronic and opto-electronic device fabrication.

True non-contact atomic force microscopy imaging of heterogeneous biological samples in liquids: topography and material contrast.

The present work analyses how the tip-sample interaction signals critically determine the operation of an Atomic Force Microscope (AFM) set-up immersed in liquid. On heterogeneous samples, the conservative tip-sample interaction may vary significantly from point to point - in particular from attractive to repulsive - rendering correct feedback very challenging. Lipid membranes prepared on a mica substrate are analyzed as reference samples which are locally heterogeneous (material contrast). The AFM set-up is operated dynamically at low oscillation amplitude and all available experimental data signals - the normal force, as well as the amplitude and frequency - are recorded simultaneously. From the analysis of how the dissipation (oscillation amplitude) and the conservative interaction (normal force and resonance frequency) vary with the tip-sample distance we conclude that dissipation is the only appropriate feedback source for stable and correct topographic imaging. The normal force and phase then carry information about the sample composition ("chemical contrast"). Dynamic AFM allows imaging in a non-contact regime where essentially no forces are applied, rendering dynamic AFM a truly non-invasive technique.

Microstructure morphologies of asphalt binders using atomic force microscopy

We investigated microstructure morphologies of three asphalts (SK, Karamay, and Esso) used in China using atomic force microscopy (AFM). The topography and phase contrast images were obtained. Topographic profile and three dimensional images were described. Roughnesses of microstructure were calculated. And the chemical compositions of asphalt were tested to explain the microstructural mechanism of the asphalt. The results show that the topography and phase image in atomic force microscopy are appropriate to evaluate the microstructure of the asphalt binder. There are significant differences in microstructural morphologies including bee-like structure, topographic profile, 3D image, and roughness for three asphalts in this study. There are three different phases in microstructure of asphalt binder. The oil source and chemical composition of asphalt, especially asphaltenes content have a great influence on the microstructure.

Topographic contrast of ultrathin cryo-sections for correlative super-resolution light and electron microscopy.

Fluorescence microscopy reveals molecular expression at nanometer resolution but lacks ultrastructural context information. This deficit often hinders a clear interpretation of results. Electron microscopy provides this contextual subcellular detail, but protein identification can often be problematic. Correlative light and electron microscopy produces complimentary information that expands our knowledge of protein expression in cells and tissue. Inherent methodological difficulties are however encountered when combining these two very different microscopy technologies. We present a quick, simple and reproducible method for protein localization by conventional and super-resolution light microscopy combined with platinum shadowing and scanning electron microscopy to obtain topographic contrast from the surface of ultrathin cryo-sections. We demonstrate protein distribution at nuclear pores and at mitochondrial and plasma membranes in the extended topographical landscape of tissue.

Angle selective backscattered electron contrast in the low-voltage scanning electron microscope: Simulation and experiment for polymers

Recently developed detectors can deliver high resolution and high contrast images of nanostructured carbon based materials in low voltage scanning electron microscopes (LVSEM) with beam deceleration. Monte Carlo Simulations are also used to predict under which exact imaging conditions purely compositional contrast can be obtained and optimised. This allows the prediction of the electron signal intensity in angle selective conditions for back-scattered electron (BSE) imaging in LVSEM and compares it to experimental signals. Angle selective detection with a concentric back scattered (CBS) detector is considered in the model in the absence and presence of a deceleration field, respectively. The validity of the model prediction for both cases was tested experimentally for amorphous C and Cu and applied to complex nanostructured carbon based materials, namely a Poly(N-isopropylacrylamide)/Poly(ethylene glycol) Diacrylate (PNIPAM/PEGDA) semi-interpenetration network (IPN) and a Poly(3-hexylthiophene-2,5-diyl) (P3HT) film, to map nano-scale composition and crystallinity distribution by avoiding experimental imaging conditions that lead to a mixed topographical and compositional contrast

Removing Beam Current Artifacts in Helium Ion Microscopy: A Comparison of Image Processing Techniques.

The development of the helium ion microscope (HIM) enables the imaging of both hard, inorganic materials and soft, organic or biological materials. Advantages include outstanding topographical contrast, superior resolution down to <0.5 nm at high magnification, high depth of field, and no need for conductive coatings. The instrument relies on helium atom adsorption and ionization at a cryogenically cooled tip that is atomically sharp. Under ideal conditions this arrangement provides a beam of ions that is stable for days to weeks, with beam currents in the order of picoamperes. Over time, however, this stability is lost as gaseous contamination builds up in the source region, leading to adsorbed atoms of species other than helium, which ultimately results in beam current fluctuations. This manifests itself as horizontal stripe artifacts in HIM images. We investigate post-processing methods to remove these artifacts from HIM images, such as median filtering, Gaussian blurring, fast Fourier transforms, and principal component analysis. We arrive at a simple method for completely removing beam current fluctuation effects from HIM images while maintaining the full integrity of the information within the image.

Assessment of multi-scenario rockfall hazard based on mechanical parameters using high-resolution airborne laser scanning data and GIS in a tropical area

Rockfall hazard is a main threat for mountainous and hilly areas that can cause loss of life, damage to infrastructures, and traffic interruption. Rockfall frequency and magnitude vary both spatially and temporally; therefore, multi-scenarios related to rockfall characteristics (trajectories, frequency and kinetic energy) can provide early warnings by identifying the areas at risk for mitigation purposes. The aim of this study is to predict the areas at risk from future rockfall incidents and suggest suitable mitigation measures to prevent them. The most significant elements in rockfall analysis are slope topography interpretation or the digital elevation model (DEM) and the rockfall modeling approach. Light detection and ranging (LiDAR) techniques have been widely used in rockfall studies because of their capability to provide high-resolution information regarding slope surfaces. In the current study, airborne laser scanning (ALS) is used to obtain a high-density point cloud (4 pts./m2) of the study area for the construction of an accurate DEM via a geographic information system. Rockfall source areas were identified based on multi-criteria method including DEM derivatives (e.g., slope, aspect, curvature and topographic contrast) in addition to terrain type and aerial photos. A 3D rockfall model has been established to determine rockfall multi-scenarios based on their characteristics according to a range of restitution coefficient (normal and tangential) and friction angle values; these parameters are particularly crucial in rockfall simulation to delineate the spatial prediction of rockfall hazard areas along the Jelapang corridor of the North–South Expressway in Malaysia. In addition, a barrier location was suggested based on limited rockfall height and kinetic energy to mitigate rockfall hazards. Results show that rockfall trajectories (stopping distance) and, subsequently, their frequency and energy are increased; moreover, barrier efficiency is reduced when the values of the mechanical parameters (Rn, Rt, and friction angle) are increased. Nonetheless, the suggested barrier location is an efficient and mitigative measure to eliminate the rockfall effect.