Search PubMed⌕ Search

Biomedical subjects

B A Parkinson

Publications and source records attributed to B A Parkinson.

10 recordsLinked to original sources
PubMed2006

Photochronocoulometric measurement of the coverage of surface-bound dyes on titanium dioxide crystal surfaces.

Atomically flat terraced single-crystal anatase and rutile surfaces can be prepared allowing for the reproducible adsorption of covalently attached sensitizing dyes. Once reproducible surfaces and dye coverages are achieved, a photochronocoulometric technique is developed to measure the surface coverage of the dyes, an important parameter in determining the efficiency of sensitization. The surface-bound dyes are irreversibly oxidized by exposure to a light pulse with the n-type oxide semiconductor electrode held in depletion. A double-exponential decay of the subsequent photocurrent is then measured, where the integration of the faster decay is associated with the adsorbed dye coverage and the second much slower decay is attributed to trace regenerators, including water, in the nonaqueous electrolyte. The ruthenium-based N3 dye shows the expected linear dependence of the rate constant on light intensity whereas a dicarboxylated thiacyanine dye shows a square root dependence of its photooxidation rate on light intensity. The sublinear response of the thiacyanine dye is discussed in terms of the more complex surface chemistry that is known for this family of sensitizing dyes.

Adsorption↗
PubMed2006

Multiple two-dimensional structures formed at monolayer and submonolayer coverages of p-sexiphenyl on the Au(111) surface.

The two-dimensional structures formed by monolayers and submonolayers of p-sexiphenyl (p-6P) molecules evaporated onto the Au(111) surface are investigated using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Five different 2D structures corresponding to different surface coverages are discovered and their 2D structures solved. The trends in the molecular alignment with respect to the underlying gold lattice are discussed. An unusual structure that consists of paired rows of p-6P molecules was discovered. A surface structure with alternating domains of slightly differently packed p-6P molecules was also found. The boundary between these two domains contains systematic molecular vacancies.

Journal Article↗
PubMed2006

Fabrication and investigation of nanostructures on transition metal dichalcogenide surfaces using a scanning tunneling microscope.

Nanometer-scale holes have been fabricated on the surfaces of the semiconducting transition metal dichalcogenides (TMDCs) molybdenum ditelluride (MoTe2) and molybdenum disulfide (MoS2) by applying voltage pulses from the tip of a scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV). It was found that the tip geometry (tip shape and sharpness) influences the formation and structure of the atomic-scale nanostructures. Threshold voltage ranges for the surface modification of MoTe2 (3.0 +/- 0.3 V) and MoS2 (3.4 +/- 0.3 V) were determined. Negative sample voltage pulses applied to a p-type MoTe2 surface produced much larger and deeper nanometer-scale holes when compared with those produced by positive voltage pulses. The existence of threshold voltages and the pulse polarity dependence of nanostructure fabrication suggests that an electric field evaporation mechanism is applicable. Support for this mechanism was obtained by nanostructuring metallic TMDC NbSe2, where both the produced features and the threshold voltages (3.0 +/- 0.3 V) were similar for both positive and negative voltage pulses.

Journal Article↗
PubMed2006

Preparation and characterization of terraced surfaces of low-index faces of anatase, rutile, and brookite.

Simple polishing and relatively low temperature annealing procedures for preparing atomically flat terraced surfaces of various single-crystal TiO2 polymorphs are described. Anatase (101), anatase (001), rutile (100), rutile (110), and brookite (111) surfaces could all be prepared with a terraced surface structure as revealed in AFM images. The rutile (100) and (110) and anatase (101) surfaces were also shown to produce acceptable LEED patterns immediately upon insertion into a UHV system without the usual sputter and anneal cycles.

Journal Article↗
PubMed2005

Dye sensitization of the anatase (101) crystal surface by a series of dicarboxylated thiacyanine dyes.

Dye sensitization of the single crystal anatase (101) surface was studied using a structurally similar series of dicarboxylated thiacyanine dyes that bind to the oxide surface through their carboxylate groups. An ultraviolet (UV) light treatment of the anatase (101) surfaces, immediately prior to dye adsorption, improved both the reproducibility of dye coverage and the incident photon-to-current efficiencies (IPCE) for sensitization. The UV treatment does not pit or roughen the anatase surface and results in high IPCEs of more than 1% in some cases and absorbed photon current efficiencies (APCE) from 5 to 100%. The photocurrent spectra showed features associated with surface-bound dye monomers and H-dimers that could be followed as a function of the dye surface coverage. Models for the surface structures of the adsorbed dye layers that are consistent with the measurements are presented, along with a discussion of adsorption isotherms.

Journal Article↗
PubMed2004

Chiral morphologies and interfacial electronic structure of naphtho[2,3-a]pyrene on Au(111).

The adsorption of the two-dimensionally chiral naphtho[2,3-a]pyrene molecule has been studied on Au(111). Both structural and electronic properties of the naphtho[2,3-a]pyrene (NP)/Au(111) interface have been measured. Ultraviolet and X-ray photoelectron spectroscopy have been employed to measure the energies of the molecular orbitals of the NP film with respect to the gold Fermi level. A Schottky junction with a large interface dipole (0.99 eV) is formed between Au(111) and NP. Temperature-programmed desorption was used to determine that adsorbed NP has a binding energy of 102.2 kJ/mol. Chiral domains have been observed with scanning tunneling microscopy due to the spontaneous phase separation of the 2-D enantiomers. Two distinct structural polymorphs have been observed, one of which has homochiral paired molecular rows. Models of the 2D structure are proposed that are in excellent agreement with experimental measurements.

Electrons↗
PubMed2003

Naphtho[2,3-a]pyrene forms chiral domains on Au111.

Chiral domains have been prepared by evaporation of a two-dimensionally chiral molecule, naphtho[2,3-a]pyrene (NP), onto the hexagonal Au(111) surface in an ultrahigh vacuum environment. High-resolution UHV scanning tunneling microscopy (STM) showed that NP formed chiral domains consisting of only one 2D enantiomer rather than racemic two-dimensional unit cells. A structural model is proposed that agrees with the STM observations. Chiral pockets between adsorbed molecules may be useful for binding a specific enantiomer of a 3D chiral molecule.

Aluminum Silicates↗
PubMed2001

Tervalent conducting polymers with tailor-made work functions: preparation, characterization, and applications as cathodes in electroluminescent devices.

A series of conducting polymers have been prepared through thermal polymerization of transition-metal diimine complexes. The as-polymerized material is electrochemically converted into its formally zerovalent form. Due to the proximity of the half-wave potentials of the formal 1+/0 and 0/1- couples, there is substantial disproportionation of the redox sites at room temperature, resulting in a conductive tervalent mixed-valent material. The redox processes that give rise to this mixed-valent material are predominantly ligand-based, and therefore are highly sensitive to substitution on the ligand periphery. Solution redox chemistry of the monomer can be used to accurately predict the work function of the corresponding zerovalent conducting polymer, which has been verified by ultraviolet photoelectron spectroscopy. Many of these materials have especially low work functions (<3.6 eV) making them appropriate materials to use as cathode materials in organic light-emitting devices (OLEDs). Working examples of tris(8-hydroxyquinoline)aluminum(III)-based OLEDs have been fabricated using one of these polymers as a cathode.

Journal Article↗
PubMedDate not supplied

Solid-state 29Si, 113Cd, 119Sn, and 31P NMR studies of II-IV-P2 semiconductors.

Solid-state 29Si, 113Cd, 119Sn, and 31P MAS NMR spectra are reported on a series of II-IV-P2 compounds. In favorable cases (e.g., high degree of crystallinity, low concentration of unpaired electrons), well-defined spectra, with sharp lines for each specific nearest-neighbor configuration, are observed; in such cases, expected J coupling patterns are also seen. High-resolution solid-state NMR studies of this type provide useful information on structure (disorder), doping, and electron-mediated coupling in semiconductor systems.

Cadmium↗