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Paul Mulvaney

Publications and source records attributed to Paul Mulvaney.

15 recordsLinked to original sources
PubMed2006

Superhydrophobic effects of self-assembled monolayers on micropatterned surfaces: 3-D arrays mimicking the lotus leaf.

The contact angle of water has been measured on a series of self-assembled monolayers (SAM) on thermally evaporated and sputter coated silver surfaces. It is found that micropatterning the surface using nanosphere lithography leads to large increases in the contact angle and generates superhydrophobic surfaces with contact angles >150 degrees. The type of functional groups on the SAMs, the metal island size, and the metal island thickness all contribute to the measured contact angle. The maximum contact angle found was 161 degrees for a fluorinated alkanethiol on 80 nm thick silver islands.

Alkanes↗
PubMed2006

Optical properties of single semiconductor nanocrystals.

We present an overview of the current progress in the understanding of the (steady state) optical properties of individual II-VI semiconductor nanocrystals. We begin with a presentation of the conceptual development of the theory required to model the electronic structure of these systems. This is followed by an overview of the current experimental results obtained from the spectroscopy of individual semiconductor nanocrystals, and in particular, we focus on the study of photoluminescence intermittency (blinking) and spectral diffusion. Where possible, we link the experimental observations to the predictions of current theories. We conclude that the surface of small semiconductor crystals plays an important role in determining their optical properties.

Anisotropy↗
PubMed2006

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 Sönnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.

Gold↗
PubMed2005

On the temperature stability of gold nanorods: comparison between thermal and ultrafast laser-induced heating.

The response of gold nanorods to both thermal and ultrafast laser-induced heating has been examined. The thermal heating experiments show structural changes that occur on timescales ranging from hours to days. At the highest temperature examined (250 degrees C) the nanorods are transformed into spheres within an hour. On the other hand, no structural changes are observed in the laser-induced heating experiments up to temperatures of 700 +/- 50 degrees C. This is attributed to thermal diffusion in the laser experiments. Measurements of the period of the extensional mode of the nanorods using time-resolved spectroscopy show a significant softening at high pump laser powers. However, the decrease in the period is less than expected from bulk Young's modulus vs. temperature data.

Journal Article↗
PubMed2005

Phosphine-free synthesis of CdSe nanocrystals.

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).

Letter↗
PubMed2005

Nucleation and growth of CdSe nanocrystals in a binary ligand system.

The competing effects of two ligands, oleic acid (OA) and bis-(2,2,4-trimethylpentyl) phosphinic acid (TMPPA), on the nucleation rate and growth of CdSe nanocrystals in octadecene are reported. It is found that TMPPA acts as a high boiling point "nonsolvent" or "nucleating agent". Addition of TMPPA leads to higher initial particle yields and smaller particle diameters. Conversely, oleic acid inhibits nucleation and results in a drastic increase in "early time ripening" (ETR), a phenomenon that causes a rapid reduction in the number of particles within the first minutes of reaction. By controlling the number of nuclei formed with TMPPA and tuning the rate of ETR with oleic acid, high yields of particles can be obtained with sizes between 3 and 7 nm. Furthermore, in the absence of OA, the preparation of very small nanocrystals with diameters approximately 2 nm is facilitated.

Journal Article↗
PubMed2005

Spatially-directed oxidation of gold nanoparticles by Au(III)-CTAB complexes.

Gold nanoparticles are readily oxidized by Au(III) in the presence of cetyl-trimethylammonium bromide (CTAB). Oxidation occurs preferentially at surface sites with higher curvature. Conversely, oxidation with cyanide ions in the absence of CTAB leads to uniform oxidation over the whole surface. Examples of the spatially directed oxidation are provided using large, irregular spheres, nanocubes, and nanorods. We conclude that the mechanism of oxidation depends on whether the oxidant is attached to CTAB micelles. It is postulated that the CTAB micelles approach the nanoparticles preferentially at the tips, leading to spatially directed oxidation.

Letter↗
PubMed2005

Tunable whispering gallery mode emission from quantum-dot-doped microspheres.

We describe the synthesis of quantum-dot (QD) doped silica microspheres and a method by which it is possible to fine-tune the position of the whispering gallery modes (WGMs) produced by the fluorescence emission of CdSe@ZnS doped silica microspheres (5.06 mum in diameter). The method is based on small perturbations to the sphere's refractive index caused by adsorption of thin layers of polyelectrolytes and modification of the radial position of the nanocrystals within the microsphere by means of deposited silica shells.

Adsorption↗
PubMed2003

Vibrational response of nanorods to ultrafast laser induced heating: theoretical and experimental analysis.

In this paper, we elucidate the vibrational response of cylindrical nanorods to ultrafast laser-induced heating. A theoretical analysis of the expected behavior is first presented. This analysis predicts that both extensional and breathing vibrational modes of the rods should be excited by laser-induced heating. Analytical formulas are derived assuming that the heating/expansion process is instantaneous, and that the lengths of the rods are much greater than their radii. These results show that the breathing mode dominates the mechanical deformation of the rod. However, because the frequency of the extensional mode is much lower than that of the breathing mode, the extensional mode will dominate the response for a real experiment (a finite-time heating/expansion process). The results of this model are compared to data from transient absorption experiments performed on gold nanorods with average lengths between 30 and 110 nm. The transient absorption traces show pronounced modulations with periods between 40 and 120 ps, which are only observed when the probe laser is tuned to the longitudinal plasmon band. The measured periods are in good agreement with the expected values for the extensional modes of the rods. For rods wider than 20 nm, the breathing mode can also be observed and, again, the measured periods are in good agreement with the theoretical calculations. The breathing mode is not observed for thinner rods (<20 nm width) because, in this case, the period is comparable to the time scale for lattice heating.

Journal Article↗
PubMed2003

Two-photon fluorescence scanning near-field microscopy based on a focused evanescent field under total internal reflection.

We present two-photon fluorescence near-field microscopy based on an evanescent field focus produced by a ring beam under total internal reflection. The evanescent field produced by this method is focused by a high-numerical-aperture objective, producing a tightly confined volume that can effectively induce two-photon excitation. The imaging system is characterized by the two-photon-excited images of the nanocrystals, which show that the focused evanescent field is split into two lobes because of the enhancement of the longitudinal polarization component at the focus. This feature is confirmed by the theoretical prediction. Unlike other two-photon near-field probes, this method does not have the heating effect and requires no control mechanism of the distance between a sample and the probe.

Journal Article↗
PubMed2002

Preparation of CdSe nanocrystals in a micro-flow-reactor.

A micro-reactor was utilized for continuous and controlled CdSe nanocrystal preparation. Effects of reaction conditions on optical properties of the nanocrystals were investigated; in this current system, rapid and exact temperature control of the micro-reactor was beneficial for controlling particle diameter and reproducible preparation of particles; additional effort was made towards narrower particle-size distributions.

Journal Article↗
PubMed2002

Characterization of the adhesive mucilages secreted by live diatom cells using atomic force microscopy.

Atomic Force Microscopy (AFM) resolved the topography and mechanical properties of two distinct adhesive mucilages secreted by the marine, fouling diatom Craspedostauros australis. Tapping mode images of live cells revealed a soft and cohesive outer mucilage layer that encased most of the diatom's siliceous wall, and force curves revealed an adhesive force of 3.58 nN. High loading force, contact mode imaging resulted in cantilever 'cleaned' cell walls, which enabled the first direct observation of the active secretion of soft mucilage via pore openings. A second adhesive mucilage consisted of strands secreted at the raphe, a distinct slit in the silica wall involved in cell-substratum attachment and motility. Force measurements revealed a raphe adhesive strand(s) resistant to breaking forces up to 60 nN, and these strands could only be detached from the AFM cantilever probe using the manual stepper motor.

Adhesiveness↗