Topographic Contrast Research Articles

Flat surface versus present-day topography for cylindrical and spherical sources in temperature field simulation models: The Cerritos Colorados geothermal field, Jalisco, Mexico

Most thermal simulation studies assume a flat upper surface, which is not strictly valid for geothermal areas and volcanic centers. Further, even those thermal simulations that have taken into account the actual topography did not objectively evaluate the simulated temperature differences between the flat surface and topography models. We provide the first 3-D simulations of the Cerritos Colorados geothermal field of Mexico for the flat surface and actual topography models as well as for cylindrical and spherical primary heat sources whose upper surface was assumed to be located at the same subsurface depths. The simulated subsurface temperatures were reported at 9 different subsurface depths from +1275m a.s.l. to −1725m a.s.l. The flat surface models always overestimated the subsurface temperatures as compared to the respective topography models by about 27% near the surface (at +1275m a.s.l.) to only about 1% near the heat source (at −1725m a.s.l.) in the central part of the modelled region, and about 18–70% near the surface to 2–16% near the heat source at eight other selected locations within the area. The percent temperature differences between the flat surface and topography models showed a statistically significant linear dependence for both cylindrical and spherical heat sources on the topographic contrast.

Simultaneous Scanning Electron Microscope Imaging of Topographical and Chemical Contrast Using In-Lens, In-Column, and Everhart-Thornley Detector Systems.

The scanning electron microscope provides a platform for subnanometer resolution characterization of material morphology with excellent topographic and chemical contrast dependent on the used detectors. For imaging applications, the predominantly utilized signals are secondary electrons (SEs) and backscattered electrons (BSEs) that are emitted from the sample surface. Recent advances in detector technology beyond the traditional Everhart-Thornley geometry have enabled the simultaneous acquisition and discrimination of SE and BSE signals. This study demonstrates the imaging capabilities of a recently introduced new detector system that consists of the combination of two in-lens (I-L) detectors and one in-column (I-C) detector. Coupled with biasing the sample stage to reduce electron-specimen interaction volumes, this trinity of detector geometry allows simultaneous acquisition of signals to distinguish chemical contrast from topographical changes of the sample, including the identification of surface contamination. The I-C detector provides 4× improved topography, whereas the I-L detector closest to the sample offers excellent simultaneous chemical contrast imaging while not limiting the minimization of working distance to obtain optimal lateral resolution. Imaging capabilities and contrast mechanisms for all three detectors are discussed quantitatively in direct comparison to each other and the conventional Everhart-Thornley detector.

Topographical and Physicochemical Contrast in Photopatterned SU-8 Films for Microfabrication of Multilayer Structures

During SU-8 standard photolithography process, a patterned topography is formed with a characteristic height profile produced by the different shrinkage of the UV exposed and masked regions. We study the change of wettability, film solubility, and topographic modifications on SU-8 films of different thicknesses and show its relevance in the formation of spinning-flow arrays on top layers made from positive photoresists. Also, considerable contrast in film solubility and surface energy as observed from contact angle measurements is produced. Interface diffusion of the photoresists was also observed and followed by Rutherford Back Scattering. We discuss the derivations of the mentioned effects concerning the limitations to multilayered microfabrication processes and possibilities to take advantage of the surface profiles obtained.

Methods of Angular Scanning in X-ray Imaging and Topography

In an imaging experiment with the use of X-ray topography or phase contrast techniques, generally, the result is a single image, which represents the internal structure of the object. However, identification of the features of the observed structure is a challenge. In these methods, an important parameter is the scattering angle, and methods based on its variation are being actively developed, making possible to obtain a scattering characteristics for each local region of a sample. The present paper discusses the results of applying the methods of the angular scanning to some experiments on synchrotron topography and analyser based imaging.

Fabrication of tungsten probe for hard tapping operation in atomic force microscopy

We propose a method of producing a tungsten probe with high stiffness for atomic force microscopy (AFM) in order to acquire enhanced phase contrast images and efficiently perform lithography. A tungsten probe with a tip radius between 20nm and 50nm was fabricated using electrochemical etching optimized by applying pulse waves at different voltages. The spring constant of the tungsten probe was determined by finite element analysis (FEA), and its applicability as an AFM probe was evaluated by obtaining topography and phase contrast images of a Si wafer sample partly coated with Au. Enhanced hard tapping performance of the tungsten probe compared with a commercial Si probe was confirmed by conducting hard tapping tests at five different oscillation amplitudes on single layer graphene grown by chemical vapor deposition (CVD). To analyze the damaged graphene sample, the test areas were investigated using tip-enhanced Raman spectroscopy (TERS). The test results demonstrate that the tungsten probe with high stiffness was capable of inducing sufficient elastic and plastic deformation to enable obtaining enhanced phase contrast images and performing lithography, respectively.

Directed Self-Assembly of Block Copolymer Micelles onto Topographically Patterned Surface.

We report a facile method to control directed self-assembly (DSA) of spherical micelles of block copolymers (BCPs) by topographically patterned surface. A cylinder-forming polystyrene-block-poly(2-vinylpyridine) copolymer [Mn,PS = 175 kg/mol, Mn,P2VP = 70 kg/mol, and polydipersity index (PDI) = 1.08] was phase-separated on a thin film of poly(vinyl alcohol) (PVA) by solvent annealing. By additional treatment with ethanol as a preferential solvent for P2VP block, the surface of BCP thin film was reconstructed to produce nanopores. Nanoporous structures in BCP thin films were transferred to the underlying hydrophilic PVA film by reactive ion etching (RIE). Then spherical BCP micelles were quickly self-assembled within the nanopores in the PVA layer due to topographical contrast and surface energy difference during spin-coating. Consequently, the site-selective array of BCP micelles was utilized as templates to achieve heterogeneous organization of nanoparticles and organic fluorescent dyes over a large area. In addition, it was observed that those heterogeneous assemblies showed a remarkable decrease in fluorescence intensity of organic dyes.

<b>Dissipated energy in tapping mode by the atomic force microscope

Dissipated energy measurements between a tip of an AFM and a sample have been used to analyze variations in mechanical and tribological properties of materials, using tapping mode AFM to generate topography and phase contrast images. Furthermore it's possible to perform indentations in the material with this operation mode and to create micro-controlled defects, in diameter and depth, to growing of quantum dots in semiconductor materials. In this paper, we discuss the fundamentals of this technique.

Micropatterned Surfaces for Atmospheric Water Condensation via Controlled Radical Polymerization and Thin Film Dewetting.

Inspired by an example found in nature, the design of patterned surfaces with chemical and topographical contrast for the collection of water from the atmosphere has been of intense interest in recent years. Herein we report the synthesis of such materials via a combination of macromolecular design and polymer thin film dewetting to yield surfaces consisting of raised hydrophilic bumps on a hydrophobic background. RAFT polymerization was used to synthesize poly(2-hydroxypropyl methacrylate) (PHPMA) of targeted molecular weight and low dispersity; spin-coating of PHPMA onto polystyrene films produced stable polymer bilayers under appropriate conditions. Thermal annealing of these bilayers above the glass transition temperature of the PHPMA layer led to complete dewetting of the top layer and the formation of isolated PHPMA domains atop the PS film. Due to the vastly different rates of water nucleation on the two phases, preferential dropwise nucleation of water occurred on the PHPMA domains, as demonstrated by optical microscopy. The simplicity of the preparation method and ability to target polymers of specific molecular weight demonstrate the value of these materials with respect to large-scale water collection devices or other materials science applications where patterning is required.

X-ray topography contrast from edge dislocations in ZnGeP2 single crystals

X-ray topography images in the form of contrast rosettes from edge dislocations of the \{ {\bar 110} \}\langle {110} \rangle slip system in ZnGeP2 single crystals, obtained under different diffraction conditions, are identified and interpreted. To analyze and model the dislocation images, a semi-phenomenological theory of contrast is applied. The simulation of images of edge dislocations under Borrmann effect conditions was carried out in cases when the diffraction vector was parallel or perpendicular to the slip plane of the dislocation. In both cases, the main part of the experimental image is formed because of waveguiding of Bloch waves in the regions of tension and compression of the lattice on either side of the slip plane. Additional image details arise through relaxation of stresses at the exit surface. Since the color of all lobes of the contrast rosette is related to the sign of the deformation, the location of the dislocation half-plane is unambiguously determined. Thus, from the form of the rosette image, it is possible to obtain the slip plane, the magnitude and sign of the Burger's vector, and the direction of the dislocation line.

Differences in Late Quaternary primary productivity between the western tropical Pacific and the South China Sea: Evidence from coccoliths

Changes in Late Quaternary oceanic primary productivity in the western tropical Pacific were reconstructed using coccolith counts from the improved SYRACO system in piston core MD01-2386 retrieved from the Halmahera Sea near northwest New Guinea. The calculated primary productivity exhibits cycles on obliquity and precession timescales over the last 192ka. There are marked differences between primary productivity records from the western tropical Pacific and the South China Sea (SCS), with the former being dominated by precession, and the latter showing all three Milankovitch cycles (eccentricity, obliquity and precession). Empirical Orthogonal Function (EOF) analyses reveal two significant EOF modes in the western tropical Pacific and SCS records. EOF-1 accounts for 38% of the total variance and exhibits obvious precessional cycles corresponding to Northern Hemisphere summer insolation, while EOF-2 accounts for 22% of the total variance and exhibits strong 41-kyr periodicity, suggesting different biological responses to hydroclimate changes in the two regions. A more complex hydroclimate regime influenced by the East Asian monsoon and the large contrast in regional topography and circulation during glacial cycles are considered to be the primary drivers of the stronger temporal variability in productivity in the SCS compared to the relatively stable western tropical Pacific.

Synchrotron X-Ray Study on Crack Prevention in AlN Crystals Grown on Gradually Decomposing SiC Substrates

We report on the growth method and the structural characterization of freestanding AlN crystals. An AlN layer is grown on a gradually decomposing SiC substrate yielding a freestanding crack free 2H single crystal with dislocation density 5×104 cm–2 and without grain boundaries as confirmed by synchrotron radiation phase contrast imaging and topography data. Wafers of 600–1000 μm thick and up to 15 mm in diameter are obtained. The thermal stress distribution in a conventional AlN/SiC structure is discussed. Theoretical estimates show that cracking of AlN layers is a natural result of their growth on undecomposed SiC substrates.

Near-field microwave imaging of inhomogeneous KxFeySe2: Separation of topographic and electric features

It is important for modern scanning microwave microscopes (SMMs) to overcome the effect of the surface roughness. Here, we report microwave conductivity imaging of the phase-separated iron chalcogenide KxFeySe2 (x = 0.8, y = 1.6–2), in which electric conductivity-induced contrast is distinguished from topography-induced contrast using a combination of a scanning tunneling microscope and a SMM. We observed the characteristic modulation of the local electric property that originates from the mesoscopic phase separation of the metallic and semiconducting phases in two different scanning modes: constant current mode and constant Q (CQ) mode. In particular, CQ scanning is useful because we obtain a qualitative image in which the topographic contrast is largely eliminated without degradation of the spatial resolution.

RESOLVING THREE-DIMENSIONAL AND SUBSURFICIAL FEATURES OF CARBONACEOUS COMPRESSIONS AND SHELLY FOSSILS USING BACKSCATTERED ELECTRON SCANNING ELECTRON MICROSCOPY (BSE-SEM)

Paleontologists routinely study fossils using high-magnification and high-resolution backscattered electron (BSE) images acquired via scanning electron microscopy (SEM). In BSE imaging, contrast corresponds to differences in backscattering of primary electrons and BSE detection among points in the electron beam raster scan. In general, BSE images are known for compositional contrast corresponding to backscattering monotonically related to average atomic number. However, two other types of contrast are relevant to BSE-SEM of fossils: (1) topographic contrast corresponding to backscattering and BSE detection varying with specimen shape and (2) mass-thickness contrast corresponding to backscattering varying with the relative masses and thicknesses of materials in the uppermost few microns of a sample. Here, we demonstrate the significance of these contrast mechanisms for resolving three-dimensional and subsurficial features of fossils. First, we show—through study of mass-thickness contrast in BSE images of carbonaceous compressions from the Triassic Solite Quarry Lagerstatte (Virginia)—that some tissues (e.g., leaf and insect wing veins) are preserved as thicker carbonaceous films than others (e.g., leaf laminae and insect wing membranes), possibly reflecting taphonomic differences among anatomical tissues. Second, we show that the problematic phosphatic shelly fossil Sphenothallus (lower Cambrian, China) is covered by low-relief transverse ribs and made up of exteriorly sculptured and interiorly unsculptured carbon- and phosphorus-rich layers with microstructures. Taking advantage of both topographic and mass-thickness contrast mechanisms, these case studies demonstrate that BSE imaging elucidates morphological details that are not obvious in surficial light microscopy or secondary-electron SEM and are otherwise only evident via tomography.

Near-Field IR Nanoscale Imaging of the Solid Electrolyte Interphase on a HOPG Electrode

The SEI layer on graphitic carbon electrodes is well known to protect effectively the electrode from further electrolyte reduction during long-term charge-discharge cycling process. Many different techniques have been applied to characterize the chemical and structural composition of this complex surface film. The standard vibrational optical spectroscopies, which offer molecular-level information are subject to the diffraction limit, which restricts their ability to probe at the nanoscale level of the SEI building blocks. This work exploits infrared apertureless near-field microscopy that operates below the diffraction limit to characterize the SEI layer on a model HOPG electrode. Variations in surface topography and chemical contrast are discussed in the context of SEI composition and function. The promise of near-field techniques for characterization of electrochemical interfaces is briefly evaluated.

Adsorption properties of CoPc molecule on epitaxial graphene/Ru(0 0 0 1)

Combining the scanning tunneling microscopy (STM) and density functional theory (DFT), the adsorption properties of cobalt phthalocyanine (CoPc) on monolayer graphene/Ru(0001) [MG/Ru(0001)] have been investigated. At monolayer coverage, CoPc forms an ordered Kagome lattice, and a slight deformation for one lobe of CoPc and charge transfer from CoPc to Ru(0001) substrate take place. The existence of the defect (vacancy) in graphene on Ru(0001) increases the coupling between the Ru substrate and the epitaxial graphene. Such an increase of the coupling brings about an overall CoPc molecular energy level shift toward the low binding energy, which subsequently results in a central topographical contrast between the CoPc molecules on the intact and defective MG/Ru(0001).

Direct observation and analysis of yolk-shell materials using low-voltage high-resolution scanning electron microscopy: Nanometal-particles encapsulated in metal-oxide, carbon, and polymer

Nanometal particles show characteristic features in chemical and physical properties depending on their sizes and shapes. For keeping and further enhancing their features, the particles should be protected from coalescence or degradation. One approach is to encapsulate the nanometal particles inside pores with chemically inert or functional materials, such as carbon, polymer, and metal oxides, which contain mesopores to allow permeation of only chemicals not the nanometal particles. Recently developed low-voltage high-resolution scanning electron microscopy was applied to the study of structural, chemical, and electron state of both nanometal particles and encapsulating materials in yolk-shell materials of Au@C, Ru/Pt@C, Au@TiO2, and Pt@Polymer. Progresses in the following categories were shown for the yolk-shell materials: (i) resolution of topographic image contrast by secondary electrons, of atomic-number contrast by back-scattered electrons, and of elemental mapping by X-ray energy dispersive spectroscopy; (ii) sample preparation for observing internal structures; and (iii) X-ray spectroscopy such as soft X-ray emission spectroscopy. Transmission electron microscopy was also used for characterization of Au@C.

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10−8 yr−1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.

Electron imaging with an EBSD detector.

Electron Backscatter Diffraction (EBSD) has proven to be a useful tool for characterizing the crystallographic orientation aspects of microstructures at length scales ranging from tens of nanometers to millimeters in the scanning electron microscope (SEM). With the advent of high-speed digital cameras for EBSD use, it has become practical to use the EBSD detector as an imaging device similar to a backscatter (or forward-scatter) detector. Using the EBSD detector in this manner enables images exhibiting topographic, atomic density and orientation contrast to be obtained at rates similar to slow scanning in the conventional SEM manner. The high-speed acquisition is achieved through extreme binning of the camera-enough to result in a 5 × 5 pixel pattern. At such high binning, the captured patterns are not suitable for indexing. However, no indexing is required for using the detector as an imaging device. Rather, a 5 × 5 array of images is formed by essentially using each pixel in the 5 × 5 pixel pattern as an individual scattered electron detector. The images can also be formed at traditional EBSD scanning rates by recording the image data during a scan or can also be formed through post-processing of patterns recorded at each point in the scan. Such images lend themselves to correlative analysis of image data with the usual orientation data provided by and with chemical data obtained simultaneously via X-Ray Energy Dispersive Spectroscopy (XEDS).

The Usage of Backscattered Electrons in Scanning Electron Microscopy

Secondary and backscaterred electrons are the most common signals used for imaging in the scanning electron microscopy. Generally, SE are used to obtain topographical contrast while BSE show differences in chemical composition (so called Z-contrast).The aim of the present work is to show possibilities and techniques to obtain not-so-common information using BSE, as e.g. orientation contrast, residual stress, different allotropic modifications, etc.

A method to determine fault vectors in 4H-SiC from stacking sequences observed on high resolution transmission electron microscopy images

A new method has been developed to determine the fault vectors associated with stacking faults in 4H-SiC from their stacking sequences observed on high resolution TEM images. This method, analogous to the Burgers circuit technique for determination of dislocation Burgers vector, involves determination of the vectors required in the projection of the perfect lattice to correct the deviated path constructed in the faulted material. Results for several different stacking faults were compared with fault vectors determined from X-ray topographic contrast analysis and were found to be consistent. This technique is expected to applicable to all structures comprising corner shared tetrahedra.