Richard A. Farrer

Field-emission studies on thin films of zinc oxide nanowires

Studies on field emission (FE) from thin films of zinc oxide (ZnO) nanowires found that both the turn-on voltage and emission current density depend on the areal density of nanowires. The density of ZnO nanowires is controlled by the gold (Au) nanoparticle density deposited on the silicon substrates. The growth of ZnO nanowires was achieved by the thermal evaporation/condensation method. It is shown that the same screening effect observed on carbon nanotube field emitters also affects the FE from thin films of ZnO nanowires. Thin films with the lowest areal density of ZnO nanowires showed much better FE characteristics, comparable to that of carbon nanotubes. More importantly, the FE characteristics of ZnO nanowire thin film were further improved with annealing in hydrogen.

Acrylic-based resin with favorable properties for three-dimensional two-photon polymerization

We describe an acrylic-based prepolymer resin that is ideally suited for the fabrication of three-dimensional structures with two-photon polymerization. We characterize the photochemical and photophysical properties of the photoinitiator and present representative structures that demonstrate the favorable mechanical and optical properties of the polymer.

Exponential intermolecular dynamics in optical Kerr effect spectroscopy of small-molecule liquids

Optical Kerr effect spectroscopy has been employed to study the behavior of six symmetric-top liquids (acetonitrile, acetonitrile-d3, benzene, carbon disulfide, chloroform, and methyl iodide) over a broad range of temperatures. In all of the liquids, an exponential intermolecular response is observed on a time scale of a few hundreds of femtoseconds. Comparison of the temperature dependence of the time scale of this relaxation with the viscosity and single-molecule and collective orientational times in the liquids suggests that the exponential relaxation arises from motional narrowing.

Dynamics of a wetting liquid in nanopores: An optical Kerr effect study of the dynamics of acetonitrile confined in sol-gel glasses

The orientational dynamics of acetonitrile and acetonitrile-d3 confined in nanoporous glasses have been studied using optical Kerr effect spectroscopy. The decays can be fit to the sum of three exponentials, the fastest of which corresponds to relaxation of bulk-like liquid. We present evidence that the intermediate exponential arises from the exchange of molecules bound to the pore surfaces into the bulk liquid, whereas the slowest exponential corresponds to surface relaxation. A comparison to nuclear magnetic resonance data demonstrates that the liquid at the pore surfaces is more highly ordered than that in the bulk. Surface-modification studies demonstrate that hydrogen bonding is responsible for the extreme inhibition of dynamics at the pore surfaces.

LEVEL-DEPENDENT DAMPING IN INTERMOLECULAR VIBRATIONS : LINEAR SPECTROSCOPY

A treatment of stimulated Raman intermolecular spectroscopy is presented that employs a Landau–Teller model of damping. This model incorporates a quantum-number dependence to population relaxation and pure dephasing, thereby introducing a specific temperature and frequency dependence into the damping in the intermolecular spectrum. Optical-heterodyne detected Raman-induced Kerr effect data obtained in CS2 and acetonitrile over a broad temperature range are shown to agree with the basic predictions of the model.

Single-molecule detection with a two-photon fluorescence microscope with fast-scanning capabilities and polarization sensitivity

We describe a laser-scanning two-photon fluorescence microscope that is capable of observing single molecules with excellent temporal resolution and three-dimensional spatial resolution. To demonstrate the capabilities of the instrument we present single-molecule fluorescence data obtained in several different scanning modes. In addition, a polarization-sensitive detection scheme can provide detailed three-dimensional information about the orientations of molecules in any of these scanning modes.

Orientational diffusion of n-alkyl cyanides

Ultrafast optical Kerr effect spectroscopy has been used to study the temperature-dependent orientational dynamics of a series of nitriles with n-alkyl chains ranging from one to 11 carbons in length. In all cases the orientational diffusion is found to be described by a single-exponential decay. Analysis of the orientational correlation times using the Debye–Stokes–Einstein equation suggests that the molecules adopt extended configurations and reorient as rigid rods. The liquids with shorter alkyl chains undergo an apparent ordering transition as they are cooled.

Efficient multiphoton polymerization for the fabrication of 3-dimensional microstructures

A commercially-available photoinitiator has been employed to perform efficient multiphoton polymerization of acrylate resins. We demonstrate 2- and 3-dimensional structures with micron feature sizes that have been created with this resin system at low laser powers using objectives with modest numerical apertures.

Novel ZnO nanostructures

A variety of novel ZnO nanostructures such as nanowires, nanowalls, hierarchical nanostructures with 6-, 4-, and 2-fold symmetries, nanobridges, nanonails have been successfully grown by a vapor transport and condensation technique. Doping both In and Sn into ZnO hierarchical nanostructures can be created. The 2-fold eutectic ZnO structures can also be created without any doping in the source. It was found that the hierarchical nanostructures can be divided into two categories: homoepitaxial and heteroepitaxial where heteroepitaxy creates the multifold nanostructures. The novel ZnO nanowalls and aligned nanowires on a-plane of sapphire substrate have also been synthesized and the photoluminescence is studied. The ZnO nanowires also demonstrated very good field emission properties, comparable to carbon nanotubes. These nanostructures may find applications in a variety of fields such as field emission, photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength and multifunctional nanocomposites, etc. that require not only high surface area but also structural integrity.

Multiphoton photopolymerization with a Ti:sapphire oscillator

We demonstrate that three-dimensional multiphonon microfabrication using photopolymerization can be accomplished using a small fraction of the output of a mode- locked Ti:sapphire laser by employing an appropriate commercially-available photoinitiator. Using this photoinitiator we have been able to develop resins with a broad range of physical and chemical characteristics for use in microfabrication. The combination of optical and chemical nonlinearity in the photopolymerization reaction allows us to make structures readily with submicron features. The use of a variable beam expander allows us to create features with a broad range of sizes using a single objective while avoiding out-of-focus polymerization.