1,722,040 research outputs found
Surface morphology dependent photoluminescence from colloidal silicon nanocrystals
No full textMonodisperse 1-2 nm silicon nanocrystals are synthesized in reverse micelles and have their surfaces capped with either allylamine or 1-heptene to produce either hydrophilic or hydrophobic silicon nanocrystals. Optical characterization (absorption, PL, and time-resolved PL) is performed on colloidal solutions with the two types of surface-capped silicon nanocrystals with identical size distributions. Direct evidence is obtained for the modification of the optical properties of silicon nanocrystals by the surface-capping molecule. The two different surface-capped silicon nanocrystals show remarkably different optical properties
Radio frequency plasma-induced hydrogen bonding on kaolinite
No full textThe radio frequency (RF) plasma-modified surfaces of kaolinite were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and deuteration techniques to determine the nature of RF plasma-induced surface functional groups, the altered sites in the lattice, and interaction mechanism between RF plasma and the surface of the kaolinite. It has been concluded that the RF plasma-induced infrared (IR) vibration absorption bands at 2805, 3010, and 3100 cm(-1) are attributable to the stretching vibration of hydrogen-bonded hydroxyl groups, and the band at 1407 cm(-1) is attributable to the bending vibration of (HO-)Al-O or (HO-)Si-O groupings with hydrogen-bonded hydroxyl groups. Structural alteration occurred on both the surface and subsurface region of the kaolinite during RF plasma treatment. Further structural alteration or adjustment was also observed on well-modified and well-deuterated kaolinite. There are two types of OD bands visible in the DRIFT spectra of this kaolinite, one type which decreased rapidly as a function of time in moist air, and the other which remained unchanged during the measurement. Furthermore, the appearance of broad IR bands at 3500-3100 cm(-1) as a result of deuteration is evidence of structural disturbance by RF plasma treatment lattice deuteration. An RF plasma-induced hydrogen bonding model on the surface of the kaolinite is proposed
Preparation and electrochemical properties of the layered material of LixVyO2 (x=0.86 and y=0.8)
No full textLayered lithium-vanadium oxide with a composition of LixVyO2 (x = 0.86 and y = 0.8) was prepared by the hydrothermal reaction of V2O3 with LiOH center dot H2O at 180 degrees C. This material corresponds to a layered rhombohedral structure related to alpha-NaFeO2 in which the vanadium ions are disordered in alternate layers of octahedral 3a (0, 0, 0) and 3b (0, 0, 1/2) sites. The electrochemical properties of this Li0.86V0.8O2 material were investigated and compared with those of the layered Li0.96VO2 made by the conventional solid-state reaction. It was found that the electrochemical capacity and reversibility of the Li0.86V0.8O2 material are significantly improved compared to those of the Li0.96VO2 material; the reversible specific capacities of the Li/Li0.86V0.8O2 and Li/Li0.96VO2 systems are similar to 100 and similar to 50 mAh g(-1), respectively, under the current densities of 7.14 mA g(-1) over 20 charge-discharge cycles with a potential window of 1.50-4.50 V. Such a reversibility results from the structural stability of Li0.86V0.8O2, whereas the increase in the reversible specific capacity can be qualitatively interpreted in terms of the presence of vanadium vacancies in the structure. (c) 2005 The Electrochemical Society
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Wurtzite P-doped GaN triangular microtubes as field emitters
No full textNovel P-doped GaN triangular microtubes were synthesized by a facile chemical vapor deposition method. This novel structure consists of a single hexagonal wurtzite phase with a triangular cross section. The tube lengths range from tens of to several hundred micrometers, and each side has a width between 0.5 and 1 mu m, with a tube wall thickness of several tens of nanometers. The formation mechanism of this triangular tubular structure is a vapor solid methanism, as determined by electron microscopy. Extraordinary and stable infrared emission (centered at similar to 724 nm) from the P-doped GaN triangular microtubes was observed from their photoluminescence spectroscopy. The low turn-on field (2.9 V mu m(-1)), high field-enhancement factor, large current density (3 mA cm(-2) at a field of similar to 9.5 V mu m(-1)), and high stability indicate the suitability of P-doped GaN microtubes as potential field emitters. This field emission property is attributed to the specific crystallographic feature-the rigid triangular structures with effective P doping and rough surface hillocks
Why H Atom Prefers the On-Top Site and Alkali Metals Favor the Middle Hollow Site on the Basal Plane of Graphite
No full textIn this work, the different adsorption properties of H and alkali metal atoms on the basal plane of graphite are studied and compared using a density functional method on the same model chemistry level. The results show that H prefers the on-top site while alkali metals favor the middle hollow site of graphite basal plane due to the unique electronic structures of H, alkali metals, and graphite. H has a higher electronegativity than carbon, preferring to form a covalent bond with C atoms, whereas alkaline metals have lower electronegativity, tending to adsorb on the highest electrostatic potential sites. During adsorption, there are more charges transferred from alkali metal to graphite than from H to graphite
Energy transfer dynamics of nanocrystal-polymer composites
No full textSteady-state and time-resolved photoluminescence spectroscopy are used to examine the photoluminescent properties of nanocrystal-polymer composites consisting of colloidal PbS nanocrystals blended with poly(2-methoxy-5(2-ethylhexyloxy)-p-phenylene vinylene). Quenching of the emission from the conjugated polymer due to the PbS nanocrystals is observed along with band edge emission from the ligand capped PbS nanocrystals. A decrease in the photoluminescence lifetime of MEH-PPV is also observed in the thin film nanocrystal-polymer composite materials. Photoluminescence excitation spectroscopy of the PbS nanocrystal emission from the composite shows features attributed to MEH-PPV providing evidence of a Forster transfer process
Transport diffusion of gases is rapid in flexible carbon nanotu
No full textMolecular dynamics simulations of rigid, defect-free single-walled carbon nanotubes have previously suggested that the transport diffusivity of gases adsorbed in these materials can be orders of magnitude higher than any other nanoporous material (A. I. Skoulidas et al., Phys. Rev. Lett. 2002, 89, 185901). These simulations must overestimate the molecular diffusion coefficients because they neglect energy exhange between the diffusing molecules and the nanotube. Recently, Jakobtorweihen et al. have reported careful simulations of molecular self-diffusion that allow nanotube flexibility (Phys. Rev. Lett. 2005, 95, 044501). We have used the efficient thermostat developed by Jakobtorweihen et al. to examine the influence of nanotube flexibility on the transport diffusion of CH4 in (20,0) and (15,0) nanotubes. The inclusion of nanotube flexibility reduces the transport diffusion relative to the rigid nanotube by roughly an order of magnitude close to zero pressure, but at pressures above about I bar the transport diffusivities for flexible and rigid nanotubes are very similar, differing by less than a factor or two on average. Hence, the transport diffusivities are still extremely large compared to other known materials when flexibility is taken into account
Efficient phosphorescence by reducing intrachain chromophore interactions in dendrimer-containing polymers
No full textPoly(dendrimers) comprised of a poly(styrene) backbone with dendrimer side chains containing an iridium(III) complex core, first-generation biphenyl dendrons, and (2-ethylhexyl)oxy surface groups show increased viscosity compared to their individual dendrimer components, which is important for inkjet printing processes. However, intrachain interchromophore interactions lead to lower photoluminescence quantum yields even in solution relative to the simple isolated dendrimers. We demonstrate that the phosphorescence efficiency of a polymer can be enhanced by incorporating the dendrirner monomer unit(s) into a copolymer with poly(styrene) spacer units. The poly(styrene) spacer units remove the intrachain interchromophore interactions between the chromophores in solution. The copolymer gives a >50% increase in solution photoluminescence quantum yields (to 94%) and an improved organic light-emitting diode performance with an external quantum efficiency of 6.7% at 100 cd/m(2) at 11 V when compared to the homopolymer with the same dendrimer side chain
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