T. D. Hu

Computed tomography algorithm based on diffraction-enhanced imaging setup

Recently taking advantage of the novel diffraction-enhanced imaging (DEI) method, one very effective and practical phase contrast imaging technique—a new x-ray computed tomography scheme based on DEI (DEI-CT) showed promising results, really superior to those of conventional CT imaging. In this letter, we show that the application of the conventional CT reconstruction algorithm is not always satisfactory for the new DEI-CT imaging and a new mathematical framework for imaging reconstruction is presented. Experimental data collected at the Beijing synchrotron radiation facility are also discussed using the new algorithm.

Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of few nanometers. Here, x-ray absorption spectroscopy has been used to identify the local Ti environment and characterize the related electronic structure. We combine experimental results at the Ti K edge in both bulk and nanocrystal samples to determine the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the x-ray absorption near-edge structure spectra. The correlation between peak intensities and surface-to-volume ratio of nanoparticles is also discussed.

Diamond‐coated tips and their applications

Scanning tunneling microscope (STM) has recently demonstrated its potential application as a powerful tool for nanometer‐scale surface modification. We have successfully fabricated new types of diamond‐coated STM tips by using microwave plasma chemical vapor deposition (MPCVD). The reactant is a gaseous mixture of methane and hydrogen, sometimes adding oxygen. While natural diamond is a good insulator, the electrical conductivity of diamond‐coated tips made by MPCVD technique is always high enough for STM experiments. These tips are quite stable, hard enough to make machining on metal or other solid surfaces in nanometer scale. We have applied the diamond‐coated STM tip to so‐called controlled machining on the metallic surfaces, such as gold, silver, and platinum films, and even polycrystal platinum, single crystal nickel, and palladium. Further investigations, including the results caused by different tips, applications for nanometer‐scale processing and engineering, etc., have been discussed. This techn...

Hardness demonstration of diamond tips by nanometer‐scale controlled scratching on metallic surfaces

A new kind of diamond tip for the scanning tunneling microscope has been fabricated by a microwave plasma chemical vapor deposition method. The extreme hardness of the tips is demonstrated by doing nanometer‐scale controlled direct scratching on various metallic surfaces using these tips. The results indicate the potential of such diamond tips for nanometer‐scale mechanical polishing.

INVESTIGATION OF C-PHYCOCYANIN FROM BLUE-GREEN-ALGA SPIRULINA-PLATENSIS WITH SCANNING TUNNELING MICROSCOPE

C‐phycocyanin (C‐PC) was isolated from blue‐green alga spirulina platensis. A scanning tunneling microscope (STM) has been used to investigate its three‐dimensional structure. The samples were dialyzed before the STM experiment, and then deposited on highly oriented pyrolytic graphite (HOPG). The measurement was carried out in ambient condition at room temperature. STM images showed that C‐phycocyanin was uniformly distributed on solid‐state substrate HOPG. The shape of C‐phycocyanin is disklike with a channel in the center. It is concluded that STM has great potential to observe the structure of biliproteins and phycobilisomes.

Determination of chemical valence states in dilute samples using a combination of fluorescence ion chamber and PIN photon diode

Determination of chemical valence state is an important aspect for x-ray absorption spectroscopy (XAS) application. In this work a simple detecting system was developed for measuring the chemical states and recording x-ray absorption spectra of dilute samples. The new device consists of two parts: a Lytle-type ion chamber and a PIN photon diode detector, which are set side by side. The sample and the standard were placed in front of the two detectors respectively. The unfocused incident x-ray beam with certain size, hits on both the sample and the standard. The Lytle-type ion chamber detects the fluorescence signal from the sample while the PIN photon diode records the transmitted signal from the standard separately and simultaneously. Comparing the two spectra, the energy shift of the sample can be calibrated relative to a selected standard. This method is suitable for measuring high quality XANES spectra of low concentration samples at the 2nd generation synchrotron radiation sources exploiting either bending magnets or wigglers with large horizontal divergence.