Contributions from radiation damping and surface scattering to the linewidth of the longitudinal plasmon band of gold nanorods: a single particle study

The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been examined by dark field microscopy. The results show that the longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Analysis of the data using gamma = gamma(bulk) + Anu(F)/L(eff), where L(eff) is the effective path length of the electrons and nu(F) is the Fermi velocity, allows us to determine a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were also examined. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in excellent agreement with recent work on spherical gold nanoparticles by S?nnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.     

Vibrational energy transfer in O2(X 3sigma(g)-, upsilon=2,3) + O2 collisions at 330 K

Vibrational relaxation of O2(X 3sigma(g)-, upsilon=2,3) by O2 molecules is studied via a two-laser approach. Laser radiation at 266 nm photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O2(a 1delta(g)) that is rapidly converted to O2(X 3sigma(g)-, upsilon=2,3) in a near-resonant adiabatic electronic energy-transfer process involving collisions with ground-state O2. The output of a tunable 193-nm ArF laser monitors the temporal evolution of the O2(X 3sigma(g)-, upsilon=2,3) population via laser-induced fluorescence detected near 360 nm. The rate coefficients for the vibrational relaxation of O2(X 3sigma(g)-, upsilon=2,3) in collision with O2 are 2.0(-0.4)(+0.6) x 10(-13) cm3 s(-1) and (2.6+/-0.4) x 10(-13) cm3 s(-1), respectively. These rate coefficients agree well with other experimental work but are significantly larger than those produced by various semiclassical theoretical calculations.     

Labeled gold nanoparticles immobilized at smooth metallic substrates: systematic investigation of surface plasmon resonance and surface-enhanced Raman scattering

This paper experimentally and theoretically investigates the influence of an underlying metallic substrate (i.e., gold and silver) on the surface plasmon resonance (SPR) of labeled gold nanoparticles and the concomitant impact on the surface-enhanced Raman scattering (SERS) signal from the labels. These experiments employ nanoparticles of varied sizes (30-100 nm) that are coated with a bifunctional Raman scatterer composed of (1) a disulfide for chemisorption to the nanoparticle surface, (2) a succinimidyl ester for formation of a covalent linkage to an amine-terminated self-assembled monolayer on the underlying substrate, and (3) an aryl nitro group with an intrinsically strong Raman active vibrational mode. This approach allows facile systematic assessments of how variations in nanoparticle size, substrate composition, and the gap between the nanoparticle and substrate affect the SPR of the bound particles. Both UV-vis transmission and reflection absorption (incident angle of 58 degrees ) spectroscopy are used to characterize the effect of each of these parameters on SPR. These results are then correlated with SERS enhancement factors (EFs) that were determined by accounting for particle surface concentrations, which were measured by atomic force microscopy, and the absolute number of labels, which were calculated on the basis of the surface area of each of the different-sized particles. All SERS spectra were collected at an incident angle of 58 degrees with respect to the surface normal. As expected, the SPR for particles in solution red-shifts with increasing particle size. More importantly, the SPR moves to even longer wavelengths as the size of immobilized particles increases and as the gap between the immobilized particle and substrate decreases. The red shift is also greater for a gold nanoparticle tethered to a gold substrate compared to a silver substrate. A theoretical model for the extinction of a particle above a flat substrate, corrected for surface scattering, radiation damping, and dynamic depolarization, is also briefly detailed. SPR results calculated with the model are consistent with the shifts observed in the SPR position for each of the manipulated experimental variables. The largest EFs are found for samples with an SPR maximum (lambda(max)) between the wavelengths for laser excitation (633 nm) and the Raman band for the symmetric nitro stretch of the particle coating (690 nm). As an example, an order of magnitude in the SERS enhancement factor is gained for a 60-nm particle immobilized 1.2 nm above a gold substrate (SPR lambda(max) = 657 nm) compared to that for a 30-nm particle (SPR lambda(max) = 596 nm).     

Sticking of CO to crystalline and amorphous ice surfaces

We present results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of crystalline ice Ih and to the surface of amorphous ice Ia. The calculations were performed for normal incidence at a surface temperature Ts = 90 K for ice Ia, and at Ts = 90 and 150 K for ice Ih. For both surfaces, the sticking probability can be fitted to a simple exponentially decaying function of the incidence energy, Ei: Ps = 1.0e(-Ei(kJ/mol)/90(kJ/mol)) at Ts = 90 K. The energy transfer from the impinging molecule to the crystalline and the amorphous surface is found to be quite efficient, in agreement with the results of molecular beam experiments on the scattering of the similar molecule, N2, from crystalline and amorphous ice. However, the energy transfer is less efficient for amorphous than for crystalline ice. Our calculations predict that the sticking probability decreases with Ts for CO scattering from crystalline ice, as the energy transfer from the impinging molecule to the warmer surfaces becomes less efficient. At high Ei (up to 193 kJ/mol), no surface penetration occurs in the case of crystalline ice. However, for CO colliding with the amorphous surface, a penetrating trajectory was observed to occur into a large water pore. The molecular dynamics calculations predict that the average potential energy of CO adsorbed to ice Ih is -10.1 +/- 0.2 and -8.4 +/- 0.2 kJ/mol for CO adsorbed to ice Ia. These values are in agreement with previous experimental and theoretical data. The distribution of the potential energy of CO adsorbed to ice Ia was found to be wider (with a standard deviation sigma of 2.4 kJ/mol) than that of CO interacting with ice Ih (sigma = 2.0 kJ/mol). In collisions with ice Ia, the CO molecules scatter at larger angles and over a wider distribution of angles than in collisions with ice Ih.     

Syntheses and characterizations of bismuth nanofilms and nanorhombuses by the structure-controlling solventless method

Substrate-free bismuth nanofilms with an average thickness of 0.6 nm (sigma = +/-14.1%) and monodisperse layered Bi nanorhombuses with an average edge length of 21.5 nm (sigma = +/-14.7%) and thickness of 0.9 nm (sigma = +/-25.8%) have been successively synthesized by structure-controlling solventless thermolysis from a new layered bismuth thiolate precursor with a 31.49 A spacing. The morphologies result from self-control at an atomic level by the layered Bi(SC(12)H(25))3 crystal structure. The formation of the Bi nanofilm intermediate provides significant substantiation for this synthesis method, and detailed evidence on the conversion progress has been obtained. Both the films and the rhombuses have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), high-resolution TEM (HRTEM), and atomic force microscopy (AFM) measurements. Special UV-vis electronic absorption spectra of the nanoproducts have been studied.     

Determination of the inelastic mean-free-path and mean inner potential for AlAs and GaAs using off-axis electron holography and convergent beam electron diffraction.

The mean-free-paths for inelastic scattering of high-energy electrons (200 keV) for AlAs and GaAs have been determined based on a comparison of thicknesses as measured by electron holography and convergent-beam electron diffraction. The measured values are 77 +/- 4 nm and 67 +/- 4 nm for AlAs and GaAs, respectively. Using these values, the mean inner potentials of AlAs and GaAs were then determined, from a total of 15 separate experimental measurements, to be 12.1 +/- 0.7 V and 14.0 +/- 0.6 V, respectively. These latter measurements show good agreement with recent theoretical calculations within experimental error.     

Virtual state scattering with cold electrons: para-xylene and para-difluorobenzene

The scattering of electrons with kinetic energies down to a few meV by para-xylene and para-difluorobenzene has been observed experimentally with an electron beam energy resolution of 0.95 to 1.5 meV (full width half maximum). At low electron energies the collisions can be considered as cold scattering events because the de Broglie wavelength of the electron is considerably larger than the target dimensions. The scattering cross sections measured rise rapidly at low energy due to virtual state scattering. The nature of this scattering process is discussed using s- and p-wave phase shifts derived from the experimental data. Scattering lengths are derived of, respectively, -9.5+/-0.5 and -8.0+/-0.5 a.u. for para-xylene and para-difluorobenzene. The virtual state effect is interpreted in terms of nuclear diabatic and partially adiabatic models, involving the electronic and vibronic symmetries of the unoccupied orbitals in the target species. The concept of direct and indirect virtual state scattering is introduced, through which the present species, in common with carbon dioxide and benzene, scatter through an indirect virtual state process, whereas other species, such as perfluorobenzene, scatter through a direct process.     

Dynamic scaling of the critical binary mixture methanol-hexane

Acoustical attenuation spectrometry, dynamic light scattering, shear viscosity, density, and heat capacity measurements of the methanol/n-hexane mixture of critical composition have been performed. The critical part in the sonic attenuation coefficients nicely fits to the empirical scaling function of the Bhattacharjee-Ferrell [Phys. Rev. A 24, 1643 (1981)] dynamic scaling model if the theoretically predicted scaled half-attenuation frequency Omega(12) (BF)=2.1 is used. The relaxation rates of order parameter fluctuations, as resulting from the acoustical spectra, within the limits of experimental error agree with those from a combined evaluation of the light scattering and shear viscosity measurements. Both series of data display power law with amplitude Gamma(0)=44x10(9) s(-1). The amplitude of the fluctuation correlation length follows as xi(0)=0.33 nm from the light scattering data and as xi(0)=0.32 nm from the amplitude of the singular part of the heat capacity if the two-scale factor universality relation is used. The adiabatic coupling constant g=0.11 results from the amplitude of the critical contribution to the acoustical spectrum near the critical point, in conformity with g=0.12 as following from the variation of the critical temperature with pressure along the critical line and the thermal expansion coefficient.     

Bound states and scattering resonances of OH(A)-He

The OH-He complex has been observed using laser excitation of the A 2sigma+-X 2pi transition. The bands of the complex were close to the monomer rotational lines that terminate on the n = 0, 1, and 2 levels of OH(A). The unresolved band associated with He.OH (A,n=0) was redshifted from the OH parent line by 1.6 cm(-1), providing a direct measurement of D0'-D0". The complex features associated with n = 1 and 2 were identified as scattering resonances. They have been assigned by comparison with resonance structures derived from close-coupling calculations. The ab initio potential energy surface of H.-S. Lee, A.B. McCoy, R.R. Toczylowski, and S.M. Cybulski, [J. Chem. Phys. 113, 5736 (2000)] was used in these calculations. The level of agreement between the observed and predicted resonances indicated that the ab initio surface is reasonably accurate.     

Electron-impact excitation of D1delta <-- X1sigma+ in carbon monoxide

The dipole-forbidden transition of D1delta <-- X1sigma+ of CO has been observed by a fast electron-energy-loss spectrometer at a large scattering angle of 7 degrees and with an energy resolution of 60 meV. The energy levels and the relative intensity distribution of D1delta, v'=9-25 have been determined, most of the results are reported for the first time.     

Interacting resonances in the F+H2 reaction revisited: complex terms, Riemann surfaces, and angular distributions

We study the effect of overlapping resonances on the angular distributions of the reaction F+H2(v=0,j=0)-->HF(v=2,j=0)+H in the collision energy range from 5 to 65 meV, i.e., under the reaction barrier. Reactive scattering calculations were performed using the hyperquantization algorithm on the potential energy surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)]. The positions of the Regge and complex energy poles are obtained by Pade reconstruction of the scattering matrix element. The Sturmian theory is invoked to relate the Regge and complex energy terms. For two interacting resonances, a two-sheet Riemann surface is contracted and inverted. The semiclassical complex angular momentum analysis is used to decompose the scattering amplitude into the direct and resonance contributions.     

Scattering of neutral NH3 clusters off LiF(100): angular distributions of NH3 and small clusters

The scattering behavior of neutral ammonia clusters off a LiF(100) surface is studied. Ammonia clusters are produced by a coexpansion of NH₃ and Kr with an average kinetic energy of 48 meV per monomer molecule. Using single photon VUV laser ionization at λ = 118 nm (hv = 10.49 eV) the mass distribution of scattered particles is obtained in a reflecting time-of-flight mass spectrometer. Compared with the incoming cluster beam the average cluster size of the scattered particles is drastically decreased. The angular distribution of NH ₃ ⁺ and NH ₄ ⁺ after scattering reveals a strong inelastic interaction between the clusters and the LiF(100) surface which is described in the context of a thermokinetic model and a phonon excitation along the (001) azimuth of the LiF(100) surface.     

Formation of the background component of the analytical signal in the long-wavelength region of X-ray fluorescence spectrum

Components of X-ray background in the long-wavelength spectral region of a crystal diffraction X-ray fluorescence spectrometer were calculated. The calculations took into account the bremsstrahlung radiation of free electrons, diffuse scattering and fluorescence of the crystal analyzer, and high-order reflections of the scattered radiation of the fluorescent sample by the crystal analyzer. The results of calculations were compared with the intensities of background samples measured in the region of the NaK _α fluorescence line on an SRM-25 wave X-ray spectrometer. The experimental background intensities (response function) well correlate with those found by the regression equation with calculated factors. The importance of particular processes in the formation of X-ray background was assessed.     

Intermolecular pi-pi interactions in solids

A parameter-free DFT/CCSD(T) correction scheme is proposed for precise calculations (close to CCSD(T) accuracy) of weakly bound molecular solids. The correction scheme has been tested for solid benzene and graphite. The CCSD(T)/CBS correction to planewave DFT calculations reproduces the experimentally determined lattice constants for solid benzene. The calculated cohesive energy of benzene (457 meV per molecule) compares well with the experimental values of the heat of sublimation (460-560 meV per molecule). For graphite, the correction yields structural parameters (a = 2.46 A, c = 6.60 A) in good agreement with experiment (a = 2.46 A, c = 6.67 A). The calculated exfoliation energy of 54 meV per atom agrees fairly well with the most recent experimental value of 52 +/- 5 meV per atom.     

Induction of strand breaks by low-energy electrons (8-68 eV) in a self-assembled monolayer of oligonucleotides: effective cross sections and attenuation lengths

Self-assembled monolayers of 5'-32P-labeled 3'-thiolated oligonucleotides chemisorbed on gold were bombarded by low-energy electrons (LEE) of 8-68 eV. Shorter 5'-32P-oligonucleotides produced by LEE-induced strand breaks were separated with denaturing polyacrylamide gel electrophoresis and quantified by phosphor imaging. The yields of short oligonucleotides (y) decrease exponentially with their length (n), following the equation y=ae-bn, where a and b are constants, which are related to the average effective cross section per nucleotide for DNA strand break (sigmaeff) and the attenuation length (AL=1b) of LEE, respectively. The AL decreases with LEE energies from 2.5+/-0.6 nm at 8 eV to 0.8+/-0.1 nm at 68 eV, whereas sigmaeff increases from (3+/-1)x10(-18) to (5.1+/-1.6)x10(-17) cm2 within the same energy range. The energy dependence of sigmaeff shows a resonance peak of (2.8+/-0.9)x10(-17) cm2 at 18 eV superimposed on a monotonically rising curve. Transient electron attachment to a sigma* anion state of the deoxyribose group, followed by dipolar dissociation into H- and the corresponding positive-ion radical, leading to C-O bond cleavage, is proposed to account for this maximum.     

Correlated Rayleigh Scattering Spectroscopy and Scanning Electron Microscopy Studies of Au-Ag Bimetallic Nanoboxes and Nanocages

The optical properties of hollow nanoparticles (Au-Ag nanoboxes and nanocages) were investigated by recording Rayleigh scattering spectra of single particles, whose morphology and composition had been analyzed by scanning electron microscopy (SEM). This was achieved by depositing the particles on optically transparent substrates with registration marks, which are compatible with SEM imaging. Fitting the experimental spectra to a Lorentzian function yields the frequencies and homogeneous line widths of the plasmon resonance for the particles. The resonances are extremely broad, with dephasing times of 2-5 fs. Analysis of the line width data using the dimensions determined by SEM shows that the broadening is due to a combination of electron-surface scattering and radiation damping. The sensitivity of the plasmon resonance to the dielectric constant of the environment was also investigated by adding a drop of water to the substrate. The nanoboxes show similar dielectric sensitivities compared to other metal nanoparticle systems. A significant increase in the line width was also observed for the nanoboxes in water compared with air. This was attributed to increased radiation damping in the environment with a higher dielectric constant. Both the red shift and the increase in line width are reversible.     

Reverse micelle synthesis of rhodium nanoparticles

Water-in-oil reverse micelles of butyl ammonium laurate in hexanes that contain sodium hexachlororhodate were reduced with sodium borohydride to yield rhodium nanoparticles. The size of the micelle, determined by dynamic light scattering, was from 3 to 20 nm and varied as the water to surfactant ratio (W) was changed. The rhodium nanoparticles exhibited a Gaussian size distribution (sigma=0.35 nm), with average diameters of 1.5, 2.2, and 2.9 nm. The products were characterized with TEM, HRTEM, and X-ray photoemission spectroscopy.     

Surface-enhanced Raman scattering of 4-mercaptopyridine on thin films of nanoscale Pd cubes,boxes, and cages

We have synthesized a variety of Pd nanoparticles of 8–50 nm in size including solid cubes, hollow boxes, and porous cages. Using 4-mercaptopyridine (4MP) as a probe molecule, we have characterized thin films for surface-enhanced Raman scattering (SERS) activity, and have found a significant level of enhancement (with factors ranging from 170 for 8-nm cubes to 1.3 × 10⁴ for boxes). For the cubes and boxes, we observed a trend of stronger enhancement with more red-shifted SPR bands. We evaluated the sensitivity of this approach, and also used SERS to monitor monolayer formation on these particles.     

The (1 X 1) --> hexagonal structural transition on Pt(100) studied by high-energy resolution core level photoemission

The (1 X 1) --> quasihexagonal (HEX) phase transition on a clean Pt(100) surface was investigated by monitoring the time evolution of the Pt4f(72) core level photoemission spectra. The spectral component originating from the atoms forming the (1x1) metastable unreconstructed surface was found at -570+/-20 meV with respect to the bulk peak. Ab initio calculations based on density functional theory confirmed the experimental assignment. At temperatures above 370 K, the (1 X 1) phase irreversibly reverts to the more stable HEX phase, characterized by a surface core level shifted component at -185 +/ -40 meV. By analyzing the intensity evolution of the core level components, measured at different temperatures in the range of 393 - 475 K, we determined the activation energy of the phase transformation, E = 0.76 +/- 0.04 eV. This value is considerably lower than the one previously determined by means of low energy electron diffraction. Possible reasons for this discrepancy are discussed.     

Structure-controlled solventless thermolytic synthesis of uniform silver nanodisks

Monodisperse silver nanodisks are synthesized on the gram scale from a well-characterized layered silver thiolate precursor via thermolysis at 180-225 degrees C under a N(2) atmosphere. XRD, TEM, HRTEM, and AFM analyses indicate that the nanodisks generated at 180 degrees C over 2 h have an average diameter of about 16.1 nm (sigma = +/-12%) and a thickness of 2.3 nm (sigma = +/-14%), and they lie on their (111) faces. The disk shape is considered to be predestined by the crystal structure of the precursor. Important aspects regarding the stability of the precursor, the thermolysis temperature, and the annealing time, as well as a possible conversion mechanism, are discussed.