Scanning tunneling microscopy and spectroscopy : I. Semimetals and semiconductors. II. Atom-resolved imaging of DNA

The topographic and electronic structure of semiconductor and semimetal surfaces were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), respectively. The longrange morphology and atomic-scale characteristics of cleaved materials such as highly oriented pyrolitic graphite (HOPG), boronated pyrolitic graphite (BPG), titanium disulfide, and gallium arsenide (GaAs) were revealed by STM performed under ultrahigh vacuum (UHV) conditions. Atomic resolution constant-current and current-imaging data, as well as barrier height information obtained from tunneling gap modulation, are presented. Both point and line defects were observed on these surfaces; the origin and role of native and adsorbed surface defects are discussed. Visual evidence of coulombic screening caused by adsorption of charged species on n-type GaAs(110) is provided. The atomic corrugation of the GaAs surface was measured to be as little as 0.03 Å peak-to-valley, attesting to the stability of the microscope design. The BPG sample used in these studies consisted of up to 0.5% boron; boron is the only known substitutional impurity of graphite. Boron substituent atoms appeared as small protrusions approximately 3 Å in diameter, with an atom density consistent with the assumed concentration.The BPG surface exhibited frequent line defects, including large-angle grain boundaries, and monolayer-depth etch pits. Images of BPG in air using graphite tips showed similar results; the validity of the popular "sliding-planes" mechanism for graphite imaging is evaluated. The effects of anisotropic stress on the morphology and reconstruction of a thermally annealed Si(111) wafer were explored. The height and orientation of step bunches, as well as terrace widths, on the (7x7) surface were determined. Electromigration effects were also observed; although the overall surface slope was conserved, the step bunches were "smeared out" by reversal of the current direction during heating. Line fault defects at step kinks were observed; a theory for the origin and structure of these features based on stress relief is proposed. Current imaging tunneling spectroscopy (CITS) and localized STS revealed differences between the adatom sites of the (7x7) surface. Atom-resolved barrier height images were also obtained;comparison to constant-current images may in fact provide a means of differentiating between defects and adsorbed species on the surface. The local effective barrier height was seen to depend strongly on the "cleanliness" of the STM tip. The barrier height increased dramatically following voltage pulsing on the order of ten volts. The large height of the step bunches also provided a good test to evaluate the sharpness of the STM tip; examples of "tip changes" affecting image resolution and "multiple-tipping" are provided. Silicon samples annealed at temperatures below 1000°C revealed substantial silicon carbide (SiC) contamination which effected step pinning. No SiC islands were observed on samples annealed above 1250°C. In addition, atom-resolved STM images of duplex DNA supported on a HOPG surface were obtained in UHV. These images revealed double-helical structure, major and minor groove alternation, base pairs, and atomic-scale substructure. Comparison of the DNA dimensions derived from the STM data were in agreement with those from x-ray crystallography for "random-sequence" A-form DNA. Cross sectional profiles of the experimental STM data showed excellent correlation with the atomic contours of the van der Waals surface of A-DNA. Barrier height cross-sections showed general correlation with the atomic-scale topography over the phosphate-sugar backbone but distinct anticorrelation (complementarity) over the base pair region. The problems of aggregation and deposition coverage are discussed in the context of possible future applications of STM to DNA sequencing. The use of alternate techniques of DNA deposition, including electrospray ionization, for increased experimental reproducibility are described. The limitations of HOPG as a substrate for biomolecular adsorbates in STM experiments are evaluated.</p

Journal of Microelectronic Research 2010

https://scholarworks.rit.edu/meec_archive/1018/thumbnail.jp

Assessment of elliptic flame front propagation characteristics of iso-octane, gasoline, M85 and E85 in an optical engine

Premixed fuel–air flame propagation is investigated in a single-cylinder, spark-ignited, four-stroke optical test engine using high-speed imaging. Circles and ellipses are fitted onto image projections of visible light emitted by the flames. The images are subsequently analysed to statistically evaluate: flame area; flame speed; centroid; perimeter; and various flame-shape descriptors. Results are presented for gasoline, isooctane, E85 and M85. The experiments were conducted at stoichiometric conditions for each fuel, at two engine speeds of 1200 rpm (rpm) and 1500 rpm, which are at 40% and 50% of rated engine speed. Furthermore, different fuel and speed sets were investigated under two compression ratios (CR: 5.00 and 8.14). Statistical tools were used to analyse the large number of data obtained, and it was found that flame speed distribution showed agreement with the normal distribution. Comparison of results assuming spherical and non-isotropic propagation of flames indicate non-isotropic flame propagation should be considered for the description of in-cylinder processes with higher accuracy. The high temporal resolution of the sequence of images allowed observation of the spark-ignition delay process. The results indicate that gasoline and isooctane have somewhat similar flame propagation behaviour. Additional differences between these fuels and E85 and M85 were also recorded and identified

Burden of injury along the development spectrum: associations between the Socio-demographic Index and disability-adjusted life year estimates from the Global Burden of Disease Study 2017

Background The epidemiological transition of non-communicable diseases replacing infectious diseases as the main contributors to disease burden has been well documented in global health literature. Less focus, however, has been given to the relationship between sociodemographic changes and injury. The aim of this study was to examine the association between disability-adjusted life years (DALYs) from injury for 195 countries and territories at different levels along the development spectrum between 1990 and 2017 based on the Global Burden of Disease (GBD) 2017 estimates. Methods Injury mortality was estimated using the GBD mortality database, corrections for garbage coding and CODEm-the cause of death ensemble modelling tool. Morbidity estimation was based on surveys and inpatient and outpatient data sets for 30 cause-of-injury with 47 nature-of-injury categories each. The Socio-demographic Index (SDI) is a composite indicator that includes lagged income per capita, average educational attainment over age 15 years and total fertility rate. results For many causes of injury, age-standardised DALY rates declined with increasing SDI, although road injury, interpersonal violence and self-harm did not follow this pattern. Particularly for self-harm opposing patterns were observed in regions with similar SDI levels. For road injuries, this effect was less pronounced. Conclusions The overall global pattern is that of declining injury burden with increasing SDI. However, not all injuries follow this pattern, which suggests multiple underlying mechanisms influencing injury DALYs. There is a need for a detailed understanding of these patterns to help to inform national and global efforts to address injury-related health outcomes across the development spectrum