Eric J. Snyder

Correlation from randomness : quantitative analysis of ion-etched graphite surfaces using the scanning tunneling microscope

The scanning tunneling microscope (STM) was used to quantitatively examine the surface morphology of highly oriented pyrolytic graphite (HOPG) surfaces bombarded with 5 keV Ar+ ions. Constant current topographs clearly showed that the morphology of the resulting nonequilibrium surfaces depended sensitively on the ion flux, the ion fluence and the sample temperature. For low and intermediate ion fluence, an increase in surface roughness with increasing ion flux was observed. For constant ion flux, the surfaces developed structures with heights proportional to the ion fluence and separations characterized by a correlation length that diverged with increasing fluence. Increasing the sample temperature during bombardment produced smoother surfaces as a result of enhanced surface diffusion. The autocovariance function G(L) and the height correlation function 〈 |h(q)|2 〉 in reciprocal space were calculated directly from the STM topographs. The latter was then compared with a linear response theory for the formation of rough surfaces under nonequilibrium conditions and a scaling analysis. The fluence dependence of the surface roughening of graphite could not be explained by shot noise alone in a linear theory, whereas surface diffusion and redeposition of sputtered material satisfactorily accounted for the correlation observed in the bombardment-induced features.

Atomic Force Microscope Studies of Fullerene Films: Highly Stable C60 fcc (311) Free Surfaces

Atomic force microscopy and x-ray diffractometry were used to study 1500 �-thick films of pure C60 grown by sublimation in ultrahigh vacuum onto a CaF2 (111) substrate. Topographs of the films did not reveal the expected close-packed structures, but they showed instead large regions that correspond to a face-centered cubic (311) surface and distortions of this surface. The open (311) structure may have a relatively low free energy because the low packing density contributes to a high entropy of the exposed surface.

How (un)useful is the pelvic ring stability examination in diagnosing mechanically unstable pelvic fractures in blunt trauma patients

OBJECTIVES Our goal was to evaluate the utility of the pelvic ring stability examination for detection of mechanically unstable pelvic fractures in blunt trauma patients. METHODS Retrospective chart review. RESULTS We enrolled 1,502 consecutive blunt trauma patients and found 115 patients with pelvic fractures including 34 patients with unstable pelvic fractures (Tile classification B and C). Unstable pelvic ring on physical examination had a sensitivity and specificity of 8% (95% CI 4-14) and 99% (95% CI 99-100), respectively, for detection of any pelvic fracture and 26% (95% CI 15-43) and 99.9% (95% 99-100), respectively, for detection of mechanically unstable pelvic fractures. The sensitivity and specificity of pelvic pain or tenderness in patients with Glasgow Coma Scale >13 were 74% (95% CI 64-82) and 97% (95% CI 96-98), respectively for diagnosing any pelvic fractures, and 100% (95% CI 85-100) and 93% (95% CI 92-95), respectively for diagnosing of mechanically unstable pelvic fractures. The sensitivity and specificity of the presence of pelvic deformity were 30% (95% CI 22-39) and 98% (95% CI 98-99), respectively for detection of any pelvic fracture and 55% (95% CI 38-70) and 97% (95% CI 96-98), respectively for detection of mechanically unstable pelvic fractures. CONCLUSIONS The presence of either pelvic deformity or unstable pelvic ring on physical examination has poor sensitivity for detection of mechanically unstable pelvic fractures in blunt trauma patients. Our study suggests that blunt trauma patients with Glasgow Coma Scale >13 and without pelvic pain or tenderness are unlikely to suffer an unstable pelvic fracture. A prospective study is needed to determine whether a set of clinical criteria can safely detect or exclude the presence of an unstable pelvic fracture.

O atom etching of graphite in low earth orbit

Abstract Samples of highly ordered pyrolytic graphite were placed aboard the space shuttle Atlantis (mission STS46) and exposed to the atmosphere present in low earth orbit to observe the effects of chemical etching by atomic oxygen on carbon-based materials. During the 43 h exposure, approximately 2.25 μm of graphite were removed from the sample, which corresponds to an etch yield of one C atom for every eight incident O atoms. The topography of the etched samples was analyzed quantitatively within the framework of scaling theory, and the experimental data agree with the static scaling behavior predicted for a model in which surface diffusion and reaction at step edges dominates the etching mechanism.

Lack of evidence to support routine digital rectal examination in pediatric trauma patients.

Background: Current advanced trauma life support guidelines recommend that a digital rectal examination (DRE) should be performed as part of the initial evaluation of all trauma patients. Our primary goal was to estimate the test characteristics of the DRE in pediatric patients for the following injuries: (1) spinal cord injuries, (2) bowel injuries, (3) rectal injuries, (4) pelvic fractures, and (5) urethral disruptions. Methods: We conducted a nonconcurrent, observational, chart review study of a consecutive series of pediatric trauma patients. We enrolled all patients younger than 18 years seen in our ED from January 2003 to February 2005, for whom the trauma team was activated and who had a documented DRE. For each patient, we reviewed all available clinical documents in a computerized medical record system to identify the DRE findings followed by review of radiological reports, operative reports, and discharge summaries to identify specific injuries. Results: Two hundred thirteen patients met our selection criteria and were included in the analysis. We identified 3 patients with spinal cord injury (1% prevalence), 13 patients with bowel injury (6%), 5 patients with rectal injury (2%), 12 patients with a pelvic fracture (6%), and 1 patient with urethral disruption (0.5%). The DRE failed to diagnose (false-negative rate) 66% of spinal cord injuries, 100% of bowel injuries, 100% of rectal wall injuries, 100% of pelvic fractures, and 100% of urethral disruption injuries. Conclusions: The DRE has poor sensitivity for the diagnosis of spinal cord, bowel, rectal, bony pelvis, and urethral injuries. Our findings suggest that the DRE should not be routinely used in pediatric trauma patients.

Effects of tip size and asymmetry on scanning tunneling microscope topographs

Abstract We investigated broadening and skewing of scanning tunneling microscope topographs caused by tip size and symmetry. Model images were generated numerically by convoluting a gaussian response function for the tip with an idealized graphite image. Atomic-scale features were retained even for a 9.4 A FWHM tip function, which demonstrated that single-atom tips are not required to obtain atomic resolution topographs. Model tips with an elliptical radial cross section produced many of the familiar skewed images of graphite. Such response functions could also be used to remove the skew from an experimental topograph, and thus provide an approximation for the shape of the physical tip.

Atomic force microscope studies of CuCl island formation on CaF2(111) substrates

Abstract We have grown by molecular beam epitaxy (MBE) CuCl thin films at various thicknesses and substrate temperatures on CaF2(111) substrates. Atomic force microscope (AFM) topographs reveal that islanding is the dominant growth mechanism. Quantitative analysis of the AFM data enabled us to determine the amount of the substrate remaining exposed after the deposition as well as the total amount of CuCl deposited. We calculated the reciprocal-space height correlation function, 〈|h(q, t)|2〉, for each of our films and compared them to the predictions of the shadoqing growth theory, which enabled us to extract the important kinetic parameter of surface diffusion length for the growth condition of each of the four films.

AFM Study of Film Growth Kinetics in Heteroepitaxy

Abstract : The growth of CuCl/CaF2 heterostructures has been studied with an atomic force microscope (AFM). We have grown by molecular beam epitaxy (MBE) CuCl thin films at various substrate temperatures and thicknesses on CaF2(111) substrates. AFM studies reveal that islanding is the dominant growth mechanism. We calculated the height-height correlation function,/Ih(q,t)12>, for each of our films and compared them to the predictions made by the Shadowing Growth Theory, a preexisting growth model that enabled us to extract the important kinetic parameter of surface diffusion length for the growth condition of each of the four films.