Showing papers in "Chemical Physics Letters in 2007"
TL;DR: In this paper, a combination of the aggregation mechanism with the Maxwell and Bruggeman models gives a good prediction of the effective thermal conductivity of the nanofluids, which can be given as eta/eta(0) - 1 = 10.6 phi + (10.6 �i)(2).
Abstract: Ethylene glycol based nanofluids containing titania nanoparticles up to 8 wt% show the Newtonian behaviour over a shear rate range of 0.5-10(4) s(-1) at 293-333 K. The shear viscosity of the nanofluids (eta) depends strongly on temperature and particle concentration (phi). The normalized shear viscosity with respect to the based liquid viscosity (eta(0)) is found to be independent of temperature and can be given as eta/eta(0) - 1 = 10.6 phi + (10.6 phi)(2). Such theological behaviour is explained by the aggregation mechanism. A combination of the aggregation mechanism with the Maxwell and Bruggeman models gives a good prediction of the effective thermal conductivity of the nanofluids. (C) 2007 Elsevier B.V. All rights reserved.
423 citations
TL;DR: In this paper, a method for improving the nucleation density of nanocrystalline diamond growth is demonstrated, where detonation nanodiamond powder was bead-milled and processed to stable aqueous colloid of core particles.
Abstract: A method for improving the nucleation density of nanocrystalline diamond growth is demonstrated. Detonation nanodiamond powder was bead-milled and processed to stable aqueous colloid of core particles. This colloid was applied to various substrates to yield a very high density of individual spaced diamond nanoparticles. These diamond islands act as nucleation sites for chemical vapour deposition of nanocrystalline diamond. � 2007 Elsevier B.V. All rights reserved.
347 citations
TL;DR: In this paper, the surface-enhanced Raman scattering (SERS) activities of these nanoparticles were investigated using methylene blue (MB) as a probe molecule, and the enhancement factor was estimated to be as large as 4.2 × 10 7.
Abstract: Silver nanocaps were prepared by chemical synthesis combined with physical evaporation technique, and characterized using TEM, SEM and UV–vis-NIR spectroscopy. The surface-enhanced Raman scattering (SERS) activities of these nanoparticles were investigated using methylene blue (MB) as probe molecule. The enhancement factor was estimated to be as large as 4.2 × 10 7 . The results demonstrate that these silver nanocaps are promising for SERS applications in detection and analysis of molecules.
278 citations
TL;DR: In this article, the authors report the absorption spectra over an extended frequency band from 0.5 to 6 THz for four explosives: RDX (1,3,5-trinitroperhydro-1, 3,5,7-tetrazocine), PETN (pentaerythritol tetranitrate), and TNT (2,4,6-tritrotoluene).
Abstract: Time-domain terahertz spectroscopy (TDTS) has been shown to be a promising tool in detection of explosives and explosive related compounds. To date, TDTS for many explosives has been limited to spectral regions <3 THz. With recent improvements in emitters and sensors of THz radiation, we report the THz absorption spectra over an extended frequency band from 0.5 to 6 THz for four explosives: RDX (1,3,5-trinitroperhydro-1,3,5-triazine), HMX (1,3,5,7-tetranitroperhydro-1,3,5,7-tetrazocine), PETN (pentaerythritol tetranitrate), and TNT (2,4,6-trinitrotoluene). New distinctive spectral features are shown in these materials between 3 and 6 THz. These data may aide in the spectral identification of these explosives.
273 citations
TL;DR: In this article, a parallel CCSD(T)/QCISD (T) program was used to optimize the geometries of the main conformations and performed potential scans on the benzene dimer at the QCISD-T)/aug-cc-pVTZ level.
Abstract: Using our new parallel CCSD(T)/QCISD(T) program, we have optimized the geometries of the main conformations and performed potential scans on the benzene dimer at the QCISD(T)/aug-cc-pVTZ level. For each optimized geometry, single point energies were evaluated at the QCISD(T)/aug-cc-pVxZ level, where x = D,T,Q. The largest calculation, QCISD(T)/aug-cc-pVQZ involves 30 correlated orbitals and 1512 basis functions and is thus among the largest coupled cluster type calculations performed to date. The results allow a critical evaluation of MP2-based basis set extrapolation procedures.
267 citations
TL;DR: In this paper, the luminescence of Ce3+ in garnet phosphors and established a relationship between the excitation/emission wavelengths and the deviation from cubic symmetry around the rare-earth ion.
Abstract: The discovery of inorganic phosphors that can be excited by GaN light emitting diodes in the wavelength range 380–460 nm is essential for improving the efficiency and light quality of solid state lighting devices. Garnets doped with Ce3+ are of particular interest for this application. We have studied the luminescence of Ce3+ in garnet phosphors and established a relationship between the excitation/emission wavelengths and the deviation from cubic symmetry around the rare-earth ion. Data are presented for both known yttrium aluminum and yttrium gallium garnet systems, as well as newly synthesized inverse garnets of the general formula Mg3Y2−yGdyGe3−zSizO12:Ce (0 ⩽ y ⩽ 2, z = 0, 1).
257 citations
TL;DR: In this article, a one-step chemical treatment of graphite fluoride with alkyl lithium reagents was demonstrated, which allowed the manipulation of a soluble form of graphene and the solubility and extinction coefficient were determined by UV-vis-near infrared spectroscopy.
Abstract: Soluble graphene layers were formed by reacting graphite fluoride with alkyl lithium reagents. IR spectral studies confirmed the covalent attachment of alkyl chains to the graphene layers. Raman scattering and XRD studies of the starting materials and products revealed that the chemical process partially restores the sp2 carbon network. The solubility and extinction coefficient were determined by UV–vis-near infrared spectroscopy. Annealing of bulk samples further extends the sp2 lattice due to dealkylation as followed by Raman, XRD and mid-infrared spectroscopy. The present study demonstrates a one-step chemical treatment of graphite fluoride that allows the manipulation of a soluble form of graphene.
234 citations
TL;DR: In this paper, a long path length FTIR-smog chamber technique was used to determine k(Cl+CF3CF CH2) = (7.03−± 0.59)×10−11.
Abstract: Long path length FTIR-smog chamber techniques were used to determine k(Cl + CF3CF CH2) = (7.03 ± 0.59) × 10−11, k(OH + CF3CF CH2) = (1.05 ± 0.17) × 10−12, and k(O3 + CF3CF CH2) = (2.77 ± 0.21) × 10−21 cm3 molecule−1 s−1 in 700 Torr of N2, N2/O2, or air diluent at 296 K. CF3CF CH2 has an atmospheric lifetime of approximately 11 days and a global warming potential (100 yr time horizon) of four. CF3CF CH2 has a negligible global warming potential and will not make any significant contribution to radiative forcing of climate change.
224 citations
TL;DR: In this paper, the authors developed an efficient parameterization of the dielectric properties of Au and Ag in the energy range between 6 eV and 66 eV, which can be used for finite-difference time-domain simulations of the optical properties of nanoparticles.
Abstract: We have developed an efficient parameterization of the dielectric properties of Au and Ag in the energy range between 06 eV and 66 eV The resulting time-domain dielectric function can be used for finite-difference time-domain (FDTD) simulations of the optical properties of nanoparticles We present FDTD calculations of the extinction spectra of various Au and Ag nanoparticles and show that the calculated spectra agree very well with exact calculations using experimentally measured dielectric functions
218 citations
TL;DR: In this article, the effective charge of bovine serum albumin in aqueous solution is determined by dynamic NMR as a function of pH and local counterion condensation.
Abstract: The effective charge of bovine serum albumin in aqueous solution is determined by dynamic NMR as a function of pH. The effective charge results from the degree of dissociation varying with pH and the local counterion condensation. While the hydrodynamic friction is determined from the diffusion coefficient measured by pulsed-field gradient NMR, the electrophoretic mobility is determined by electrophoresis NMR. The effective charge per molecule is calculated from the force balance between hydrodynamic friction and force in the electric field acting on the charges. Since it is influenced by counterion condensation it is smaller than the net charge from titration.
204 citations
TL;DR: In this article, the influence of organics on the formation of H 2 SO 4 -H 2 O clusters and subsequent nucleation in the atmosphere is poorly understood, while atmospheric aerosols are known to contain a significant fraction of organic substances.
Abstract: While atmospheric aerosols are known to contain a significant fraction of organic substances, the influence of organics on the formation of H 2 SO 4 –H 2 O clusters and subsequent nucleation in the atmosphere is poorly understood. In the present Letter, bonding among formic and acetic acids, sulfuric acid, ammonia and water is studied using density functional theory at PW91PW91/6-311++G(3df,3pd) level. The stabilizing effect of formic and acetic acids is found to be close that of ammonia that indicates that the common organic substances may efficiently stabilize small H 2 SO 4 –H 2 O clusters and their involvement, alongside with or without ammonia, in the atmospheric nucleation should be studied further.
TL;DR: In this article, surface-enhanced Raman scattering (SERS) spectra of Rhodamine 6G on as-prepared silver nanostructures of various shapes were measured and their shape-dependent properties were evaluated.
Abstract: Shape-controlled synthesis of metal nanostructures has opened many new possibilities to design ideal building blocks for future nanodevices. By solution-based method, three kinds of silver colloids, nanospheres, triangular nanoprisms and nanorods have been synthesized. Non-resonant surface-enhanced Raman scattering (SERS) spectra of Rhodamine 6G on as-prepared silver nanostructures of various shapes were measured and their shape-dependent properties were evaluated. The intensity of the SERS signal was the smallest for the silver nanorod and nanoprism exhibits the strongest SERS signals. The great differences of the SERS spectra for different shape Ag colloids confirmed that like electromagnetic enhancement, chemical enhancement also plays a very important role on SERS signal enhancement.
TL;DR: The subgraph centrality is interpreted as the partition function of a network and several models of network growing/evolution as well as real-world networks, such as those representing metabolic and protein–protein interaction networks aswell as the interaction between secondary structure elements in proteins are explored.
Abstract: We interpret the subgraph centrality as the partition function of a network. The entropy, the internal energy and the Helmholtz free energy are defined for networks and molecular graphs on the basis of graph spectral theory. Various relations of these quantities to the structure and the dynamics of the complex networks are discussed. They include the cohesiveness of the network and the critical coupling of coupled phase oscillators. We explore several models of network growing/evolution as well as real-world networks, such as those representing metabolic and protein–protein interaction networks as well as the interaction between secondary structure elements in proteins.
TL;DR: In this paper, annealing of TiO 2 nanotubes to anatase structure Cr doping was carried out by ion implantation at an energy of 60 keV and at two nominal fluences, 5.5 × 10 15 cm −2 and 5.1 × 10 16 cm − 2.
Abstract: Layers of TiO 2 nanotubes have been fabricated by electrochemical anodization of Ti. After annealing the tubes to an anatase structure Cr doping was carried out by ion implantation at an energy of 60 keV and at two nominal fluences, 5 × 10 15 cm −2 and 5 × 10 16 cm −2 . XRD measurements reveal that the anatase structure shows a certain degree of ‘amorphization’ after ion implantation. However, this effect can be reversed by subsequent thermal annealing. Photoelectrochemical measurements show a visible photoresponse for the Cr + implanted tubes. Reannealing the tubes, leads to an increase in the photoresponce magnitude in the visible range of the solar spectrum.
TL;DR: In this paper, structural and electronic properties of Boron Nitride nanoribbons with both zigzag and armchair shaped edge (Z-BNNR and A-BNR) were studied by first-principle spin-polarized total energy calculations.
Abstract: Structural and electronic properties have been studied for Boron Nitride nanoribbons (BNNR) with both zigzag and armchair shaped edge (Z-BNNR and A-BNNR) by first-principle spin-polarized total energy calculations. We found that the energy band gap of Z-BNNR is indirect and decreases monotonically with the increasing ribbon width, whereas direct energy band gap oscillation was observed for A-BNNRs. Additionally, C-substitution at either single boron or nitrogen atom site in BNNRs could induce spontaneous magnetization. Our results could be potentially useful to design magnetic nano-devices based on BNNRs.
TL;DR: In this article, the authors generalize Bruggeman effective medium theory to investigate the effective electrical and thermal conductivity of carbon nanotube composites, and they show that the non-spherical shape of the carbon nano-tubes helps to achieve a large enhancement of the effective conductivity.
Abstract: We generalize Bruggeman effective medium theory to investigate the effective electrical and thermal conductivity of carbon nanotube composites. Both the nonlinear dependence of effective thermal conductivity on the nanotube volume fraction in nanofluids and very low percolation threshold for carbon nanotube/polyimide composites are well predicted. Numerical results show that although single-walled carbon nanotubes possess a higher thermal conductivity than multi-walled nanotubes, they induce less effective conductivity due to interfacial resistance. Moreover, the non-spherical shape of carbon nanotubes helps to achieve a large enhancement of the effective conductivity, and the use of disc-shaped particles gives a larger conductivity than the use of needle-shaped particles.
TL;DR: The photophysical properties of [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) are found to differ depending on its concentration in thin films as discussed by the authors.
Abstract: The photophysical properties of [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) are found to differ depending on its concentration in thin films. A film of dispersed PCBM molecules in polystyrene (PS) showed only weak absorption in the visible region (400–700 nm) with a single S 1 → S 0 emission band at around 700 nm. By comparison, a film of pristine PCBM showed heightened absorption in the visible region and the presence of an additional higher energy emission at ∼500 nm assigned to the presence of charge transfer (CT) transitions in crystalline PCBM. The energy of the CT exciton is estimated to be ∼2.8 eV.
TL;DR: In this paper, nonlinear optical properties of 2(3), 9(10), 16(17), 23(24) tetra tert-butyl phthalocyanine were studied using Z-scan technique with 800 nm femtosecond and 532 nm nanosecond pulses.
Abstract: We report our results on nonlinear optical properties of 2(3), 9(10), 16(17), 23(24) tetra tert-butyl phthalocyanine and 2(3), 9(10), 16(17), 23(24) tetra tert-butyl Zinc phthalocyanine studied using Z-scan technique with 800 nm femtosecond and 532 nm nanosecond pulses. Nonlinear absorption behavior in both femtosecond and nanosecond domains was studied in detail. We observed three-photon absorption with femtosecond laser excitation and strong reverse saturable absorption with nanosecond pulse excitation. We have also evaluated the sign and magnitude of the third-order nonlinearity.
TL;DR: In this paper, an improved and simplified version of the string method in collective variables for computing minimum free energy paths is proposed, where the string is discretized into a finite number of images and in the new method these images are evolved concurrently with replicas of the original system while keeping their inter-distances equal via reparametrization.
Abstract: An improved and simplified version of the string method in collective variables for computing minimum free energy paths is proposed. The string is discretized into a finite number of images and in the new method these images are evolved concurrently with replicas of the original system while keeping their inter-distances equal via reparametrization. There is no need to compute the mean force by time-averaging, nor to project the force perpendicularly to the string. In this Letter, the algorithmic aspects of the on-the-fly string method are presented in detail and the method is tested on the solvated alanine dipeptide molecule.
TL;DR: Using statistically uncorrelated solute-solvent configurations generated by Monte Carlo simulation, an average configuration alone is used such that one single quantum mechanical calculation reproduces the converged statistical average obtained from the entire simulation as mentioned in this paper.
Abstract: Using statistically uncorrelated solute–solvent configurations generated by Monte Carlo simulation a simpler and efficient implementation of the averaged solvent electrostatic potential is made. An average configuration alone is used such that one single quantum mechanical calculation reproduces the converged statistical average obtained from the entire simulation. Applications are presented for solvent effects in a variety of properties of acetone and aminopurine in water. In all cases, excellent agreement is obtained using the average configuration and the average from the full statistical distribution.
TL;DR: In this article, the authors developed a new method for describing charge flow in molecules based on charge transfer variables (as opposed to atomic charges) and correctly treated asymptotic behavior near dissociation.
Abstract: Polarization and charge transfer are important effects which are difficult to describe using conventional force fields. Charge equilibration models can include both of these effects in large-scale molecular simulations. However, these models behave incorrectly when bonds are broken, making it difficult to use them in the context of reactive force fields. We develop a new method for describing charge flow in molecules – QTPIE. The QTPIE method is based on charge transfer variables (as opposed to atomic charges) and correctly treats asymptotic behavior near dissociation. It is also able to provide a realistic description of in-plane polarizabilities.
TL;DR: In this article, a combination of experimental THz time-domain spectroscopy and ab-initio density functional perturbative theory is used to accurately predict the terahertz vibrational spectrum of molecules in the crystalline phase.
Abstract: We use a combination of experimental THz time-domain spectroscopy and ab-initio density functional perturbative theory to accurately predict the terahertz vibrational spectrum of molecules in the crystalline phase. Our calculations show that distinct vibrational modes found in solid-state materials are best described as phonon modes with strong coupling to the intramolecular degrees of freedom. Hence a computational method taking the periodicity of the crystal lattice as well as intramolecular motion into account is a prerequisite for the correct prediction of vibrational modes in such materials.
TL;DR: Theoretical studies of the betalains (Betanidin, betanin and geomphrenin) have been performed at the B3LYP/6-31G(d) level as mentioned in this paper.
Abstract: Theoretical studies of the betalains (betanidin, betanin and geomphrenin) have been performed at the B3LYP/6-31G(d) level. As dye-sensitizers in DSSCs, only little is known about their molecular properties relevant to the electron injection process. Here, ground state geometries, electronic transition energies, oxidation potentials, and proton affinities are reported. The HOMO → LUMO transition describes all lowest singlet excited states. The ground and excited state oxidation potentials are calculated to be 1.0 V and −1.3 V (vs. NHE), respectively. Deprotonation order is determined by calculating proton affinities at different sites. The vibrational–electronic coupling and its role in the electron transfer process are analyzed.
TL;DR: In this article, the melting and nucleation of the iron nanoparticles were investigated by molecular dynamics simulation, and the melting point of the bcc single-crystal was uniformly melted from the surface at a melting point during heating and a nucleus was generated near one side of an undercooled liquid droplet and the solidification spread toward another side at a lower temperature during cooling.
Abstract: The melting and nucleation of the iron nanoparticles were investigated by molecular dynamics simulation. The nanoparticle of the bcc single-crystal was uniformly melted from the surface at a melting point during heating, whereas a nucleus was generated near one side of an undercooled liquid droplet and the solidification spread toward another side at a lower temperature during cooling. The melting point and nucleation temperature decreased with particle radius. Moreover, the solid–liquid interfacial energy was estimated to be 0.101 J/m 2 using a Gibbs–Thomson equation, which is of the same order as the experimental value based on Turnbull–Fisher’s classical nucleation theory.
TL;DR: In this paper, the temperature dependence of the fluorescence intensity ratio (FIR) of the green up-conversion emissions about 526 and 549 nm in the range of 296 −723 K was investigated.
Abstract: The Er 3+ –Yb 3+ co-doped silicate glasses have been prepared and the temperature dependence of the fluorescence intensity ratio (FIR) of the green up-conversion emissions about 526 nm and 549 nm in the range of 296–723 K is investigated. The maximum sensitivity and the temperature resolution derived from the FIR technique of the green up-conversion emissions are approximately 0.0033 K −1 and 0.2 K, respectively. The results imply that Er 3+ –Yb 3+ co-doped silicate glass can play an important role in high temperature measurement.
TL;DR: In this article, a robust sensitivity-enhanced 1H/14N MAS HMQC experiment was described for proton-detected 14N NMR of solids, which was achieved by using dipolar recoupling for coherence transfer with a so-called n = 2 rotary resonance.
Abstract: A robust sensitivity-enhanced 1H/14N MAS HMQC experiment is described for proton-detected 14N NMR of solids. The sensitivity enhancement is achieved by using dipolar recoupling for coherence transfer with a so-called n = 2 rotary resonance. Rotary resonance occurs when a cw rf field matches certain ratios with the sample spinning frequency, n = ω1/ωr. The theory of rotary resonance for chemical shift anisotropy, heteronuclear and homonuclear dipolar interactions is presented in the irreducible representation. It is shown that the n = 2 rotary resonance decouples the homonuclear dipolar interactions while recoupling the heteronuclear dipolar interaction for proton-detected 14N NMR. The dipolar recoupling, T2′ lengthening, and 1H/14N HMQC experiment under the n = 2 rotary resonance are demonstrated.
TL;DR: The iterative Hirshfeld-I procedure as mentioned in this paper is a method for removing the dependence of Hirshfield charges on the promolecule chosen, and it has been shown that it is only slightly basis set dependent.
Abstract: The iterative Hirshfeld-I procedure is a method for removing the dependence of Hirshfeld charges on the promolecule chosen. Computational and theoretical evidence for why these self-consistent Hirshfeld-I charges do not depend on the promolecule chosen is presented. Furthermore, it is shown that Hirshfeld-I charges are only slightly basis set dependent.
TL;DR: In this paper, a method of sintering Ag nanoparticles protected by dispersant in an air atmosphere without heating was developed, and the sintered Ag wire had excellent low resistivity of 7.3 · 10 7 Xm.
Abstract: We successfully developed a new method of sintering Ag nanoparticles protected by dispersant in an air atmosphere without heating. Ag nanoparticles on glass substrates were dipped in methanol for 10–7200 s to remove the dodecylamine dispersant. After 7200 s dipping, the removal of the dispersant became clear by surface analysis and Ag nanoparticles were densely sintered. The sintered Ag wire had excellent low resistivity of 7.3 · 10 7 Xm. Microstructural observations revealed that the Ag nanoparticles had agglomerated and coarsened with increased dipping time. Clear connections were found among the grown particles. � 2007 Elsevier B.V. All rights reserved.
TL;DR: In this article, a branched structure for single crystalline ZnO nanowires in dye-sensitized solar cells (DSSCs) was adopted to maximize dye absorption and improve energy conversion efficiency.
Abstract: To maximize dye absorption and improve energy conversion efficiency, we adopted a branched structure for single crystalline ZnO nanowires in dye-sensitized solar cells (DSSCs). The high-density branched structures of these ZnO nanowires were incorporated by a two-step seeding with zinc acetate dihydrate seeds. The ZnO nanowire DSSCs exhibited an overall efficiency of ∼0.46%. This represented a 35% improvement compared to the ZnO nanowire DSSCs with pillar-shaped ZnO nanowires. Our current density–voltage (J–V) characterizations suggest that the current densities and power conversion efficiencies were improved by increasing the surface area with these branched structures of ZnO nanowires in DSSCs.
TL;DR: In this article, carbon-doped TiO2 nanoparticles were successfully prepared using a convenient controlled nonhydrolytic sol-gel method followed by annealing at 400°C.
Abstract: Carbon-doped TiO2 nanoparticles in a pure anatase phase colored dark brown were successfully prepared using a convenient controlled nonhydrolytic sol–gel method followed by annealing at 400 °C. The UV–Vis diffuse reflectance spectra and X-ray photoelectron spectroscopy (XPS) indicate that substitutional and interstitial carbon atoms coexist in the lattice of TiO2. The shift of photo response of TiO2 from UV to near infrared region is responsible for the multi-type carbon doping by the formation of complex midgap states. The photo-initiated graft polymerization of methyl methacrylate (MMA) is also induced by the carbon impurities in the TiO2 structure.