T. L. Haslett

FIELD EMITTERS BASED ON POROUS ALUMINUM OXIDE TEMPLATES

The field-emission properties of cold cathodes produced using nano-porous anodic aluminum oxide (AAO) templates are reported. Several types of field emitters were fabricated: aligned copper nanowires grown halfway up the parallel nano-pores of the AAO; aligned multiwalled carbon nanotubes grown to the top of the pores; surfaces overgrown with random tangles of carbon nanotubes; and empty AAO templates. Significant field-emission currents (field enhancement values ∼ 1800) were obtained at threshold voltages as low as 80 V (corresponding to fields of 3–4 V/μm) on samples of nanotube tangles. Perfectly aligned carbon nanotubes were less efficient field emitters and had lower field enhancement values. These observations are explained in terms of the mean separation of active tips in the two sets of samples. Empty templates and metal nanowire arrays show lower field enhancements and higher threshold electric fields (40–70 V/μm). In these samples significant field-emission currents are produced at relatively lo...

Synthesis and resonance Raman spectroscopy of CdS nano-wire arrays

Abstract Highly anisotropic arrays of aligned semiconductor nano-wires with diameters in the range 9–22 nm and lengths up to 1 μm were fabricated by electrodepositing CdS into the pores of anodic aluminum oxide. The value of the exciton energy of the semiconductor nano-wires was determined as a function of wire diameter from the excitation wavelength dependence of their polarized resonance Raman spectra in the vicinity of the CdS absorption edge. The resonance Raman (RR) spectra were dominated by a progression in the longitudinal optical phonon mode. The relative intensities of the members of the progression are both excitation-wavelength and particle-size dependent. The LO phonon frequency was also found to be wire-diameter dependent. The RR spectra were found to be almost independent of the polarization of the light exciting them. The optical exciton energy was determined to vary from 2.36 eV for the largest diameter wires to 2.42 eV for the smallest. The absence of polarization dependence is interpreted to be in part due to the fact that the nano-wires consist of strings of semiconductor crystallites so that the local aspect ratio of the crystallites is both not very great and not uniform in the axial direction. For the smallest nano-wires the radial size uniformity is, however, very high. The dependence of the band-gap energy on the diameter of the nano-wires is in good agreement with theory.

Can surface-enhanced Raman scattering serve as a channel for strong optical pumping?

The surface-enhanced Raman scattering spectra of a number of dye and colorless molecules adsorbed on deposited coloidal silver films were systematically studied as a function of power and position using a Raman microscope. The anti-Stokes portions of the spectra of the dyes reproducibly show line intensities much greater than what is expected on the basis of the equilibrium population of the excited vibrational states, even at the lowest incident light intensities used. This behavior was observed previously and attributed to optical pumping of vibrationally excited states of the molecules by unusually intense surface-enhanced Raman transitions, [Phys. Rev. Lett. 76, 2444 (1996)] suggesting either uncommonly large Raman cross-sections or very intense local field strengths exceeding those encountered in the most powerful currently available lasers. Based on this work, however, we ascribe the apparently large anti-Stokes intensities primarily to a difference in the Stokes and anti-Stokes Raman cross-sections...

The resonance Raman and visible absorbance spectra of matrix isolated Mn2 and Mn3

The resonance Raman spectrum obtained from manganese containing argon matrices previously assigned to the dimer [M.Moskovits, D.P.DiLella, and W.Limm, J. Chem. Phys. 80, 626 (1984)] is shown to belong to a zero order D3h trimer subject to Jahn–Teller distortions. The actual resonance Raman spectra of the dimer in both argon and krypton matrices are reported and discussed along with the visible absorption spectra. The resonance Raman of manganese dimer in krypton gives the 1Σ+g ground state constants of ωe =76.4 cm−1 and ωexe=0.53 cm−1. Trimer constants from the resonance Raman spectra in argon are (ωs−2xs)=196.79 cm−1, ωa=130.15 cm−1, Dωa =2.95 cm−1, xa=0.06 cm−1, and xas=0.79 cm−1. Independent values of ωs and xs could not be determined from our spectral data.

Ag5 is a planar trapezoidal molecule

The Raman spectrum of Ag5, mass-selected from a sputtered jet of silver cluster ions, neutralized and matrix isolated in solid argon, is reported. Analysis of the frequencies and intensities of the 7 vibrational modes recorded suggests unequivocally that the molecule has a planar trapezoidal structure. No other reasonable structure is consistent with the observations. Nor is there evidence for more that one structure coexisting with the planar trapezoid. The observation is in accord with the conclusions of most quantum computations on Ag5. The silver pentamer is found to photofragment into the dimer and trimer when irradiated with 458 or 466 nm laser light.

Dissociation energies of transition metal diatomics

Abstract The dissociation energies of Fe 2 , Sc 2 , Ti 2 , and Mn 2 were recalculated from previously reported mass spectrometric data and the most up-to-date molecular parameters available (MS method). These are compared with dissociation energies obtained from LeRoy-Bernstein (L-B) analyses performed on resonance Raman data for these dimers. For Fe 2 the L-B analysis produces values ranging from 1.24 to 1.75 eV as compared with 1.15 eV obtained from the MS analysis, indicating that the density of states in Fe 2 near the ground states is sparse and hence d -electrons do take part in the bonding of the dimer. We also determined that for the L-B analysis to produce a significant estimate of D e the vibrational data must include sufficient data reflecting anharmonicity at least to the level of ω e y e . The L-B results for Sc 2 and Ti 2 of 0.79 and 1.05 eV are inconclusive due to the lack of sufficient vibrational data. Mn 2 is well characterized as an antiferromagnetic van der Waals molecule. The L-B dissociation energy was found to be 0.15 eV as compared with 0.02 eV calculated from the MS data.

TRICAPPED TETRAHEDRAL AG7: A STRUCTURAL DETERMINATION BY RESONANCE RAMAN SPECTROSCOPY AND DENSITY FUNCTIONAL THEORY

The resonance Raman spectrum of Ag7, mass selected out of a cation beam of sputtered silver, neutralized, and codeposited with solid Ar, is presented. By comparing the observed spectrum with one calculated using density functional theory one concludes the structure of the silver septamer to be a tricapped tetrahedron. A partial resonance Raman spectrum of Ag9 is also included. Both spectra are dominated by totally symmetric “breathing” modes at ∼165 cm−1 (as is also true for Ag5 [T. L. Haslett et al., J. Chem. Phys. 108, 3453 (1998)]). This frequency is close to the Debye frequency (156 cm−1) of solid silver, implying that the nature of the chemical bond in these silver clusters already approximates closely that which exists in bulk silver.

RESONANCE RAMAN SPECTROSCOPY OF MATRIX-ISOLATED MASS-SELECTED FE3 AND AG3

Raman spectroscopy of matrix-isolated, mass-selected Fe3 and Ag3 reveal that both are Jahn–Teller distorted triangular molecules. The observed spectrum of Ag3 can be accounted for adequately using an approximate Jahn–Teller potential truncated at the quadratic term [Wedum et al., J. Chem. Phys. 100, 6312 (1994)] with parameters ωa=119 cm−1, ωe=99 cm−1, k=1.92, and g=0.29. For Fe3, the spectrum is more complex, most likely due to the fact that spin–orbit coupling plays an important role in this high-spin cluster in addition to the quadratic Jahn–Teller terms. The overall pattern of the spectrum suggests that in Fe3 the Jahn–Teller distortion is likely small, and a peak at 249 cm−1 is tentatively assigned to its symmetric stretch.

Self-affine silver films and surface-enhanced Raman scattering: Linking spectroscopy to morphology

The relationship between the self-affine structure of cold-deposited films and the surface-enhanced Raman (SERS) intensity of benzene adsorbed on the films is examined. Based on variable temperature STM studies the structure of cold-deposited silver films is shown to be self-affine with a fractal dimension ∼2.6, more or less independent of temperature for T less than ∼270 K. The fractal structure is shown to collapse to a more or less compact structure when the films are annealed to ∼280 K. SERS activity ceases at a somewhat lower temperature (∼250 K) for all the films examined. SERS enhancements rise by factors as great as 8 as a function of annealing temperature over and above their values at the lowest deposition temperatures used (24 K). The rise reaches a maximum at a temperature that depends both on the deposition temperature of the films and the excitation wavelength. (Such an annealing effect on the SERS intensity has been known for many years.) We suggest that the observations are consistent with...

Resistance and tunneling spectra of aligned multiwalled carbon nanotube arrays

The resistance and tunneling spectra of samples formed by depositing silver electrodes at the two ends of aligned, template-grown, carbon nanotube arrays were measured in the temperature range 0.67–440 K. Two types of samples were fabricated, one with small oxide tunnel junctions separating the carbon nanotubes from the metal electrodes, the other with a significant Al2O3 tunnel barrier. The measurements indicate the presence of three regimes for dI/dV(V). For T>220 K, dI/dV(V) and the zero-bias conductivity show a broad minimum and an activation temperature dependence suggesting semiconductor behavior. In the temperature range 10<T<140 K, the zero-bias conductivity shows a square-root temperature dependence. For T<2 K, a very steep rise in the zero-bias tunneling resistance is observed with a strong simultaneous suppression of the tunneling conductivity near the Fermi energy. Coulomb blockade is suggested as a plausible explanation of the observed behavior.