[Show abstract][Hide abstract] ABSTRACT: Single 4,7,12,15-tetrakis(4'-dimethylaminostyryl)[2.2]paracyclophane molecules adsorb on NiAl(110) in different configurations. When the symmetry axes of the molecules are properly oriented with respect to the surface lattice, three adsorbate states of different conductance can be reversibly induced and directly imaged with a scanning tunneling microscope. Couplings between tunneling electrons and adsorbate vibrational and electronic states are primarily responsible for the transformation. However, change from low to high conductance configuration can also be triggered by electric field in the junction.
[Show abstract][Hide abstract] ABSTRACT: Single 4,7,12,15-tetrakis[2.2]paracyclophane were deposited on NiAl(110) surface at 11 K. Two adsorbed species with large and small conductivities were detected by the scanning tunneling microscope (STM). Their vibrational properties were investigated by inelastic electron tunneling spectroscopy (IETS) with the STM. Five vibrational modes were observed for the species with the larger conductivity. The spatially resolved vibrational images for the modes show striking differences, depending on the coupling of the vibrations localized on different functional groups within the molecule to the electronic states of the molecule. The vibrational modes are assigned on the basis of ab initio calculations. No IETS signal is resolved from the species with the small conductivity.
The Journal of Chemical Physics 01/2008; 127(24):244711. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Purpose: To investigate the characteristics of scattered radiation and its effects on image quality in digital breast tomosynthesis. Method and Materials: A GEANT 4 based Monte Carlo package was used to simulate a rotating target/detector tomosynthesis system. The compressed breast was modeled as a cubic block imbedded with 24 small cylinders of different radii and heights in the central layer. An 11cm air gap between the breast and detector was modeled. The incident photon energy was set to 20 keV to avoid the complexity of beam hardening effects. A primary image and a scatter image were generated for each projection. The gantry was rotated around the breast from −25 degree to 25 degree with a 5 degree increment. Reconstructions of the 3D breast were computed from the 11 projection images using primary x‐rays only, and primary plus scattered x‐rays. Results: The magnitude of scatter radiation does not change very much from one projection to another because the scatter volume does not change. However, the primary radiation detected can be significantly different in different projections due to different path length. As a result, scatter to primary ratio is very different for different projection. Even with the 11 cm air gap, scatter to primary ratio was observed to be as high as 0.4 for a 5 cm thick breast. 3D breast images reconstructed from projections with only primary x‐rays showed higher contrast than those reconstructed from projections with both primary and scatter. Further evaluation is needed to determine if this reduced contrast can affect tumor detectability. Conclusion: Scatter to primary ratio changed significantly from one projection to another and was observed to be as high as 0.4 for a 5 cm thick breast. Tomosynthesis slices showed a moderate decrease in contrast due to scatter.
Medical Physics 05/2005; 32(6):2105-2105. · 2.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An improved synthetic approach was developed for the synthesis of 1,4-bis[9′,9′-bis(6″-(N,N,N-trimethylammonium)-hexyl)-fluoren-2′-yl]benzene tetrabromide (1a), 1,4-bis[9′,9′;9″,9″-tetra(6″′-(N,N,N-trimethylammonium)-hexyl)-7′,2″-bisfluoren-2′-yl] benzene octabromide (1b) and 1,4-bis[9′,9′;9″,9″;9″′,9″′-hexakis(6″″-(N,N,N-trimethylammonium)-hexyl)-7′,2″,7″,2″′-trifluoren-2′-yl] benzene dodecabromide (1c). These molecules provide a size-specific series of water-soluble oligofluorene molecules with increasing numbers of repeat units to model the interactions between cationic conjugated polymers and DNA. Fluorescence quenching and energy-transfer measurements were performed with 1a–c and single-stranded (ss) DNA and double-stranded (ds) DNA, with and without fluorescein (Fl). These studies show that, on a per-negative-charge basis, ssDNA quenches the emission of 1a–c more effectively than dsDNA. Furthermore, we show that the energy-transfer ratios dsDNA–Fl/ssDNA–Fl are dependent on the number of repeat units in 1a–c.