Showing papers by "Richard E. Russo published in 2002"

Controlled growth of ZnO nanowires and their optical properties

Abstract: This article surveys recent developments in the rational synthesis of single-crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power-dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short-wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 1010 cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single-crystalline, well-facetted nanowires can function as self-contained optical resonance cavities. This argument is further supported by our recent near-field scanning optical microscopy (NSOM) studies on single nanowires.

Methods of fabricating nanostructures and nanowires and devices fabricated therefrom

Abstract: One-dimensional nanostructures having uniform diameters of less than approximately 200 nm. These inventive nanostructures, which we refer to as “nanowires”, include single-crystalline homostructures as well as heterostructures of at least two single-crystalline materials having different chemical compositions. Because single-crystalline materials are used to form the heterostructure, the resultant heterostructure will be single-crystalline as well. The nanowire heterostructures are generally based on a semiconducting wire wherein the doping and composition are controlled in either the longitudinal or radial directions, or in both directions, to yield a wire that comprises different materials. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire (LOHN) and a coaxial heterostructure nanowire (COHN).

Nanowires, nanostructures and devices fabricated therefrom

Abstract: One-dimensional nanostructures having uniform diameters of less than approximately 200 nm. These inventive nanostructures, which we refer to as “nanowires”, include single-crystalline homostructures as well as heterostructures of at least two single-crystalline materials having different chemical compositions. Because single-crystalline materials are used to form the heterostructure, the resultant heterostructure will be single-crystalline as well. The nanowire heterostructures are generally based on a semiconducting wire wherein the doping and composition are controlled in either the longitudinal or radial directions, or in both directions, to yield a wire that comprises different materials. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire (LOHN) and a coaxial heterostructure nanowire (COHN).

Femtosecond laser ablation ICP-MS

Abstract: Femtosecond laser ablation was investigated for direct solid sample chemical analysis. The phonon relaxation time in a solid is of the order of 100 fs, which is the same as the laser pulse duration. For such excitation, there should be little time for the matrix to experience a “temperature” during the laser pulse. If the surface explodes before the photon energy is dissipated as heat in the lattice, the ablation process should produce stoichiometric vapor (elemental fractionation should be negligible). Based on this hypothesis, NIST glasses were ablated using 100 fs laser pulses at 800 nm, with subsequent elemental analysis using the ICP-MS. Pb and U intensities, and Pb/U ratios in the ICP, were measured during repetitively femtosecond-pulsed ablation. These data show that fluence (laser energy/spot area) has a significant influence on the amount of mass ablated and on the degree of fractionation. An optimal fluence was found at which the fractionation index approached unity; negligible fractionation. Infrared femtosecond laser ablation produced similar characteristics to UV nanosecond laser ablation.

Delayed phase explosion during high-power nanosecond laser ablation of silicon

Abstract: An important parameter for high-irradiance laser ablation is the ablation crater depth, resulting from the interaction of individual laser pulses on a targeted surface. The crater depth for laser ablation of single-crystal silicon shows a dramatic increase at a laser intensity threshold of approximately 2×1010 W/cm2, above which, large (micron-sized) particulates were observed to eject from the target. We present an analysis of this threshold phenomenon and demonstrate that thermal diffusion and subsequent explosive boiling after the completion of the laser pulse is a possible mechanism for the observed dramatic increase of the ablation depth. Calculations based on this delayed phase explosion model provide a satisfactory estimate of the measurements. In addition, we find that the shielding of an expanding mass plasma during laser irradiation has a profound effect on this threshold phenomenon.

The physics of laser ablation in microchemical analysis.

Abstract: In laser ablation, there are no sample-size requirements, sample preparation is simple, and solid samples can be spatially characterized.

Comparison of 193, 213 and 266 nm laser ablation ICP-MS

Abstract: There is a widespread discussion concerning the ‘better’ wavelength for laser ablation chemical analysis. Wavelength is believed to be an important parameter based on the sample’s optical penetration depth as well as photon energy for bond breaking. The lasers most widely employed for analytical applications are the excimer, based on an ArF mixture with a wavelength of 193 nm, and the solid state Nd:YAG, with wavelengths of 266 nm and 213 nm. NIST glasses were ablated to test the effects of these wavelengths on fractionation and transport efficiency. Crater geometry and volume were measured by using a white-light interference microscope. For all three wavelengths, linear calibration curves were obtained using NIST glasses as standards. The 208Pb/238U ratio in a tuff rock sample was measured using all three wavelengths; the value obtained using the NIST-glass calibration was compared to that measured using liquid nebulization.

Ion-beam nanotexturing of buffer layers for near-single-crystal thin-film deposition: Application to YBa2Cu3O7-δ superconducting films

Abstract: A method of producing biaxially textured template layers for near-single-crystal-quality film growth on substrates that do not provide a template for oriented crystalline growth is described and compared to existing methods. This technique, ion-beam nanotexturing (ITEX), produces a biaxially textured layer by oblique ion irradiation of an amorphous film surface. Using in situ reflection high-energy electron diffraction and ex situ x-ray diffraction, an yttria-stabilized zirconia (YSZ) template layer fabricated by ITEX is shown to have the appropriate surface texture for YBa2Cu3O7-δ coated conductor fabrication. A YBa2Cu3O7-δ thin film deposited on an ITEX YSZ layer has a critical current of 2.5×105 A/cm2 (77 K, 1 μV/cm). ITEX produces texture rapidly and should be ideally suited for future low-cost manufacturing.

Thermal model of phase explosion for high-power laser ablation

Abstract: Although laser ablation of solid materials is finding applications in a growing number of fields, the basic mechanisms underlying laser ablation processes have not been fully understood. One fundamental parameter for high-power laser ablation applications is the ablation depth resulting from the interaction of individual laser pulses. The ablation depth for laser ablation of single-crystal silicon shows a dramatic increase at a laser intensity threshold of approximately 20 GW/cm 2 . Above this threshold, micron-sized particulates have been observed to eject from the target surface. We present an analysis of this threshold phenomenon and demonstrate that thermal diffusion and subsequent explosive boiling after the completion of laser irradiation is a possible mechanism to describe the observed dramatic increase of the ablation depth. Calculations based on this delayed phase explosion model provide a satisfactory estimate of the measurements. In addition, we find that the shielding of an expanding mass plasma during laser irradiation plays an important role on this threshold phenomenon.

Laser ultrasonic in‐process inspection of paper for elastic properties

Abstract: A laser‐ultrasonic (LUS) sensor has been developed that allows measurement of the bending stiffness (BS) and shear rigidity (SR) of paper and paperboard as it is being made on the papermaking machine. A prototype system was recently tested in a paper mill at web speeds up to 5000 ft/min with excellent precision and accuracy. The LUS technique performs well on paper and board with basis weights up to 130 g/m2. Several laboratory methods exist for measuring the bending stiffness in small samples of paper and board. Currently, no commercial method exists for nondestructively measuring this property on the papermaking machine at production speeds. Commercial instruments using contact transducers measure ‘‘tensile strength orientation’’ (TSO) on heavier boards, where marking of the sheet by the contact transducers is not of concern. Unlike contact ultrasonic techniques, LUS does not visibly mark even the lightest grade papers. Contact ultrasonic measurements correlate approximately to the tensile strength of the sheet and can be used to calculate an approximate value for BS. LUS measurements are directly related to BS and should yield more accurate determinations. Optimum use of feed stock, reduced waste, and decreased energy consumption are potential benefits of the LUS technology.

Optical energy conversion in crystalline nanowires

Abstract: One-dimensional nanostructures hold the promise of becoming critical elements for next generation nanoscale electronic and photonic devices. While significant efforts have been devoted to the development of nanotube or nanowire based transistors, little has been done on their photonic counterpart. Here we summarize our recent efforts on one-dimensional crystalline nanostructures, in particular, the zinc oxide (ZnO) nanowires grown on a sapphire substrate. ZnO is a wide bandgap (3.37 eV) compound semiconductor that is especially suitable for blue and ultraviolet (UV) optoelectronic applications. Room-temperature optical energy conversion and stimulated UV light emission from ZnO nanowires are emphasized, along with a discussion of potential applications of nanoscale lasers.

Femtosecond time-resolved studies of laser ablation

Abstract: Laser ablation has proven to be an important technology in an increasing number of applications. The fundamental mechanisms underlying laser ablation processes are quite complicated, and include laser interactions with the target as well as plasma development off the target. While substantial progress has been achieved in understanding laser ablation on the nanosecond and picosecond time scales, it remains a considerable challenge to elucidate the underlying mechanisms during femtosecond laser ablation. We present experimental observations of plasma development inside silica glass during single femtosecond laser pulse (100 fs, 800 nm) irradiation. Using a femtosecond time-resolved imaging technique, we measured the evolution of a laser-induced plasma inside the glass that has an electron number density on the order of 1019 cm-3. Additionally, we observed an air plasma outside the target which forms long before the explosion of a material vapor plume.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.