A. Hu

Low temperature sintering of Ag nanoparticles for flexible electronics packaging

We achieve robust bonding of Cu wires to Cu pads on polyimide with silver nanopaste cured at 373 K. The paste is prepared by simply condensing Ag nanoparticle (NP) solution via centrifuging. The bonding is formed by solid state sintering of Ag NPs through neck growth and direct metallic bonding between clean Ag–Cu interfaces. Both experiment and Monte Carlo simulation confirm that the melting point of joint clusters increases during sintering. This creates improved bonds for use at an elevated operating temperature using Ag NPs.

Spectroscopic characterization of carbon chains in nanostructured tetrahedral carbon films synthesized by femtosecond pulsed laser deposition

A comparative study of carbon bonding states and Raman spectra is reported for amorphous diamondlike carbon films deposited using 120 fs and 30 ns pulsed laser ablation of graphite. The presence of sp(1) chains in femtosecond carbon films is confirmed by the appearance of a broad excitation band at 2000-2200 cm(-1) in UV-Raman spectra. Analysis of Raman spectra indicates that the concentrations of sp(1)-, sp(2)-, and sp(3)-bonded carbon are approximately 6%, approximately 43%, and approximately 51%, respectively, in carbon films prepared by femtosecond laser ablation. Using surface enhanced Raman spectroscopy, specific vibrational frequencies associated with polycumulene, polyyne, and trans-polyacetylene chains have been identified. The present study provides further insight into the composition and structure of tetrahedral carbon films containing both sp(2) clusters and sp(1) chains.

Femtosecond laser welded nanostructures and plasmonic devices

Nanojoining, a burgeoning research area, becomes a key manufacturing of complicated nanodevices with functional prefabricated components. In this work, various nanojoining methods are first reviewed. For nanojoining of Ag/Au nanoparticles, three methods are investigated comparatively. Thermal annealing shows a two-step solid state diffusion mechanism. Laser annealing by millisecond pulses displays the thermal activated solid state diffusion. Meanwhile, two effects have been identified in femtosecond laser irradiation with different laser intensities: photofragmentation at rather high intensity (∼1014 W/cm2) and nanojoining at low intensity (∼1010 W/cm2). The photofragmentation forms a large number of tiny nanoparticles with an average size of 10 nm. Control over irradiation conditions at intensities near 1010 W/cm2 results in nanojoining of most of the nanoparticles. This nanojoining is obtained through a nonthermal melting and a surface fusion welding. Joined Au nanoparticles are expected to have numerou...

Subwavelength plasmonic waveguides based on ZnO nanowires and nanotubes: A theoretical study of thermo-optical properties

We show theoretically that plasmonic waveguide structures in ZnO nanowires and nanotubes working at optical frequencies can achieve photonic waveguiding in a subdiffraction limit. The output intensity distribution, propagation length, and thermo-optical properties with different waveguide configurations are investigated. Our results show that these waveguides have the potential to develop either high performance thermally controlled nanoscale plasmonic devices or thermally insensitive waveguides by optimizing waveguide configurations.

Controllable plasmonic antennas with ultra narrow bandwidth based on silver nano-flags

We show theoretically plasmonic antennas based on two-step chemical synthesized silver nano-flags constructed by a silver nanowire and a nanoplate. The silver nano-flag antennas exhibit high polarization sensitivity and diversity of spectral signatures dependent on the structural parameters arising from the observed mode competition. With specific configuration engineering, the antennas show controllable electric-field enhancement and ultra narrow bandwidths down to about 2u2009nm. These nano-antennas promise exciting applications in lasing spaser, optical modulation, and enhanced nonlinear processes.

Resonant Raman scattering from polyacetylene and poly(p-phenylene vinylene) chains included into hydrogenated amorphous carbon

The resonant Raman scattering in N-IR–UV range from amorphous hydrogenated carbon (a-C:H) reveals inclusions of trans-polyacetylene [trans-(CH)x] chains with approximate length of up to 120 Cue5fbC units and inclusions of poly(p-phenylene vinylene) (PPV) polymer chains. The PPV is evidenced by a strong dispersive mode at ∼1175cm−1. It was found that the Raman response from core Ag trans-(CH)x modes incorporated into a−C:H to changing excitation energy is identical to that of freestanding chains, thus facilitating identification of trans-(CH)x in complex carbonaceous materials spectra.

Cryogenic graphitization of submicrometer grains embedded in nanostructured tetrahedral amorphous carbon films

A growth mechanism for submicrometer grains in diamondlike carbon films has been investigated. Films were formed by laser deposition on quartz substrates at 20–573K. Atomic force microscopy shows that films deposited at 20–100K consist of a matrix assembled from 40nm nanoparticles that further aggregate into 400–500nm grains which become embedded in the matrix. Nanohardness measurements show that these embedded grains possess a lower hardness than that of the matrix. Electronic structure obtained from resonant Raman spectra shows a remarkable clustering of sp2 bonds in the sp3 bonded matrix. It is believed that the sp2 bonded phase is concentrated in these submicrometer grains. X-ray photoelectron spectra show that this segregated structure has an enhanced affinity for oxygen.

Nanostructure and sp1/sp2 clustering in tetrahedral amorphous carbon thin films grown by femtosecond laser deposition

Diamondlike amorphous carbon films have been deposited on silicon and quartz substrates by laser ablation of graphite using 120u2002fs pulses from an amplified Ti:sapphire laser operating at 800u2002nm. Ultraviolet/visible (UV/VIS) and micro-Raman spectra of these materials have shown that the sp3-bonded carbon fraction in these films is ≈27%, 55%, and 20% when deposition occurs at substrate temperatures of 77, 300, and 573u2002K, respectively. The presence of sp1 chains in these films is indicated by the appearance of an excitation band at 2000–2100u2002cm−1 in UV-Raman spectra. We also find a remarkable increase in the Tauc energy gap calculated from in situ UV/VIS optical spectra immediately after exposure to air together with a 1u2002eV redshift of the Cu20021s core-level energy in x-ray photoelectron spectra of these samples. The properties of sp1-, sp2- and sp3-bonded components of these materials have also been studied using UV/VIS Raman spectroscopy. The enhanced stability of sp1 chains in tetrahedral carbon matrix is di...

Microstructures and photocatalytic properties of nitrogen-implanted titania nanostructured films

We report the influence of nitrogen implantation and annealing on the microstructures and photocatalytic properties of a nanostructured titania (TiO2) film. Titania samples were implanted at 40 keV and ion dose range of 1016/cm2 to 4×1016/cm2. From X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses it was found that the anatase phase of titania predominated with small amount of brookite, and the structure was stable at annealing temperatures up to 973 K. The samples showed narrower XRD peaks corresponding to larger mean-grain sizes comparing to the un-implanted titania samples. The SIMS (secondary ion mass spectroscopy) nitrogen depth profile showed a maximum nitrogen concentration at about 70 nm beneath the film surface. The absorption edge of the titania samples as measured using spectrophotometer was found to shift toward longer wavelengths with the increase of ion dose. The experiments of photodegradation of phenol were performed under a UV illumination for the N-implanted titania film which exhibited improved photocatalytic properties with the increase of annealing temperature.

Nanostripes in superconducting REBa 2 Cu 3 O 7 -δ superconductors: robust vortex pinning sites

We report a naturally grown stripe structure with a nanometer scale wavelength in REBa2Cu3O7-δ (RE = Sm and Eu) superconductors investigated with scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Such a periodic array was unveiled owning to the 3 dimensionally spatial oscillation of RE and Ba around the stoichiometric ratio. The study displayed that novel nanostripes function as robust pinning sites and effectively enhance the peak effect and the irreversibility line at 77K. This illustrates an approach to fabricate high performance REBa2Cu3O7-δ superconductors for application in liquid nitrogen temperature.