The design and application of an in-laboratory diffraction-enhanced x-ray imaging instrument.

We describe the design and application of a new in-laboratory diffraction-enhanced x-ray imaging (DEXI) instrument that uses a nonsynchrotron, conventional x-ray source to image the internal structure of an object. In the work presented here, a human cadaveric thumb is used as a test-sample to demonstrate the imaging capability of our instrument. A 22 keV monochromatic x-ray beam is prepared using a mismatched, two-crystal monochromator; a silicon analyzer crystal is placed in a parallel crystal geometry with the monochromator allowing both diffraction-enhanced imaging and multiple-imaging radiography to be performed. The DEXI instrument was found to have an experimentally determined spatial resolution of 160+/-7 mum in the horizontal direction and 153+/-7 mum in the vertical direction. As applied to biomedical imaging, the DEXI instrument can detect soft tissues, such as tendons and other connective tissues, that are normally difficult or impossible to image via conventional x-ray techniques.

Collision-induced annealing of octanethiol self-assembled monolayers by high-kinetic-energy xenon atoms.

Collisions with high-energy xenon atoms (1.3 eV) induce structural changes in octanethiol self-assembled monolayers on Au(111). These changes are characterized at the molecular scale using an in situ scanning tunneling microscope. Gas-surface collisions induce three types of structural transformations: domain boundary annealing, vacancy island diffusion, and phase changes. Collision-induced changes that occur tend to increase order and create more stable structures on the surface. We propose a mechanism where monolayer transformations are driven by large amounts of vibrational energy localized in the alkanethiol molecules. Because we monitor incremental changes over small regions of the surface, we can obtain structural information about octanethiol monolayers that cannot be observed directly in scanning tunneling microscopy images.

Structural changes of an octanethiol monolayer via hyperthermal rare-gas collisions.

In situ scanning tunneling microscopy is used to measure the effect of hyperthermal rare-gas bombardment on octanethiol self-assembled monolayers. Close-packed monolayers remain largely unchanged, even after repeated collisions with 0.4 eV argon and 1.3 eV xenon atoms. In contrast, gas-surface collisions do induce structural changes in the octanethiol film near defects, domain boundaries, and disordered regions, with relatively larger changes observed for xenon-atom bombardment.