J. Shi

Direct synthesis of single-walled carbon nanotubes bridging metal electrodes by laser-assisted chemical vapor deposition

Direct synthesis of single-walled carbon nanotubes (SWNTs) bridging prepatterned Mo electrodes has been achieved using laser-assisted chemical vapor deposition (LCVD). The synthesized SWNTs are found predominantly semiconducting. By controlling the spot size of the focused laser beam, synthesis of SWNTs can be achieved in a localized manner, which is governed by the thermal and optical properties of materials as well as the laser parameters. The synthesis process is fast and can be achieved in both far- and near-infrared laser wavelength regions. LCVD method provides a potential approach to in situ remove SWNTs with specific chiralities during the growth.

Structure and magnetic properties of Mn-doped ZnO thin films

We report the structure and magnetic properties of Zn1−xMnxO thin films grown on Si(001) substrates by pulsed laser deposition. Structure and phase evolution with Mn doping has been studied using x-ray diffraction, electron diffraction, and high-resolution electron microscopy. The undoped and 1% Mn-doped ZnO films are completely (001) oriented, and further Mn doping deteriorates the (001) orientation. For Mn concentrations below 3%, only the hexagonal ZnO phase exists in the films without secondary phases. As the Mn concentration reaches 5%, secondary phase Mn2O3 was found aggregating at grain boundaries. All the Mn-doped films show ferromagnetic properties at room temperature, and the magnetic moment decreases as the Mn concentration increases. Our results suggest that the ferromagnetism observed in Zn1−xMnxO thin films is intrinsic rather than associated with secondary phases.

Laser-assisted nanoscale deposition of diamond-like carbon films on tungsten tips

Diamond-like carbon (DLC) films were deposited on tungsten tips under KrF excimer laser irradiation in benzene solution. The deposition process was found to be highly dependent on tip sharpness. Tips with larger curvature radii and smaller aspect ratios could not be coated with DLC films under the same condition as that for sharp tips. Raman spectra showed that more sp3 tetrahedral structures were present in the DLC films on a tip with a smaller curvature radius. Simulation results showed that the tip sharpness dependent local optical enhancement played an important role in the DLC deposition process. An optical field gradient from apex to tip body was also found in the simulation. We suggest that there are two modes in the process of DLC deposition on nanotips under different laser fluences, i.e., local apex DLC deposition under low laser fluences and phase-graded DLC deposition under high laser fluences.

Phase-graded deposition of diamond-like carbon on nanotips by near-field induced chemical vapor deposition

Diamond-like carbon (DLC) films were deposited on tungsten (W) tips under the KrF excimer laser in a laser chemical vapor deposition (LCVD) chamber. Raman spectroscopy showed that the deposited DLC films were phase-graded along the tips from the apexes. The DLC films were more diamondlike at or near the tip apexes. From numerical simulation, there is a strongly confined and enhanced optical field at the tip apexes. The simulation also indicates that there is an optical-field gradient from tip apexes to tip bodies. Therefore, the variations in the phases of deposited DLC films were attributed to the corresponding variations in local optical intensities along the tips. Hence, optical local near field was confirmed to be responsible to the DLC deposition.

Catalytical growth of carbon nanotubes/fibers from nanocatalysts prepared by laser pulverization of nickel sulfate

Dispersed nickel sulfate (NiSO4) microclusters on Si substrates were fragmented by pulsed excimer laser irradiation to serve as catalysts for carbon nanotube/nanofiber (CNT/CNF) growth. At proper fluences, NiSO4 clusters were pulverized into nanoparticles. The sizes of clusters/nanoparticles were found to be dependent on laser fluence and laser pulse number. By increasing the laser fluence from 100to300mJ∕cm2, the size of disintegrated particles decreased drastically from several micrometers to several nanometers. It was found that laser-induced disintegration of as-dispersed NiSO4 clusters was mainly due to physical fragmentation by transient thermal expansion/contraction. Thermal melting of nanoparticles in a multipulse regime was also suggested. Hot-filament chemical vapor deposition (HFCVD) was used for growth of CNTs from the pulsed-laser treated catalysts. For samples irradiated at 100 and 200mJ∕cm2, CNFs were dominant products. These CNFs grew radially out of big NiSO4 clusters, forming dendritic C...

Synthesis of carbon nanotubes by laser-assisted chemical vapor deposition

With recent advances in the aligned growth of carbon nanotubes (CNTs), there are great interests in CNT-based field-emission and electronic applications. In conventional thermal chemical vapor deposition, substrates as well as chambers need to be globally heated to a sufficiently-high reaction temperature. In this paper, we report a method for direct synthesis of CNTs on pre-defined electrodes using laser-assisted chemical vapor deposition. A CW CO2 laser (wavelength 10.6 μm, beam diameter 2 mm) was used to irradiate the pre-defined structures for CNT growth. The temperature of the substrate was measured by a pyrometer, ranging from 850-1000 °C. By varying catalysts and laser parameters, carbon nanostructures including carbon nanofiber, multi-walled and single-walled CNTs can be controllably synthesized.

Near-field enhanced laser-assisted deposition

Diamond-like carbon (DLC) coated tips have been successfully applied in field emitter arrays, and scanning probe microscope (SPM) based nanofabrications. DLC deposition on tips is conventionally realized by thermal and plasma-enhanced chemical vapor deposition processes. In this study, we use laser-assisted method employing strongly enhanced near field around the tip apex for DLC deposition. DLC films were deposited on tungsten (W) tips under KrF excimer laser irradiation in a benzene solution and in a laser chemical vapor deposition (LCVD) chamber. Simulation results showed a highly localized optical field enhancement at the tip apex. There was also an optical-field gradient from apex to tip body. Experiment results showed that a locally confined DLC film was deposited based on energy dispersive X-ray (EDX) analysis. Raman spectra showed that at positions close to apexes, films tend to be more diamond-like. This implies that quality of DLC film varies according to local optical intensity along the tip. Hence, the deposition process was confirmed to be induced by the local near field generated by laser and nanotip interaction.

Fabrication of multi-layered inverse opals using laser-assisted imprinting

Multi-layered inverse opals were fabricated by laser-assisted imprinting of self-assembled silica particles into silicon substrates. A single pulse (pulse duration 23 ns) of a KrF excimer laser instantaneously melts the silicon substrate, which infiltrates and solidifies over the assembled silica particles on the substrate. By removing silica particles embedded in the silicon surface using hydrofluoric acid, inverse-opal photonic crystals were fabricated. This technique is potentially capable of controlling the photonic crystal properties by flexibly varying the silica particle size and the substrate material.

Laser applications in integrated circuits and photonics packaging

Laser processing has large potential in the packaging of integrated circuits (IC). It can be used in many applications such as laser cleaning of IC mold tools, laser deflash to remove mold flash from heat sinks and lead wires of IC packages, laser singulation of BGA (ball grid array) and CSP (chip scale packages), laser reflow of solder ball on GBA, laser peeling for CSP, laser marking on packages and on Si wafers. Laser nanoimprinting of self-assembled nanoparticles has been recently developed to fabricate hemispherical cavity arrays on semiconductor surfaces. This process has the potential applications in fabrication and packaging of photonic devices such as waveguides and optical interconnections. During the implementation of all these applications, laser parameters, material issues, throughput, yield, reliability and monitoring techniques have to be taken into account. Monitoring of laser-induced plasma and laser induced acoustic wave has been used to understand and to control the processes involved in these applications. Numerical simulations can provide useful information on process analysis and optimization.

Fabrication of back-gated SWNT field-effect transistor by laser chemical vapor deposition

Recent prominent progresses in synthesizing and manipulating single-walled carbon nanotubes (SWNTs) stimulated extensive interests in developing SWNT-based devices for nanoelectronics and nanoelectromechanical systems (NEMS). Thermal chemical vapor deposition (CVD) is one of the most widely accepted technique for growing SWNTs by heating the whole chamber and substrate to required reaction temperatures. In this study, we demonstrated a process for position-controllable synthesis of SWNT-FET by bridging the SWNT across pre-defined electrodes using the laser chemical vapor deposition (LCVD) technique. The SWNT-FET was back-gate modulated, showing p-type semiconducting characteristics. The process is very fast and can be conducted using both far-infrared CO2 laser (10.6 &mgr;m) and near-infrared Nd:YAG laser (1064 nm). We have also demonstrated localized synthesis of SWNTs by a focused laser beam. Due to the unique advantages of LCVD process, such as fast and local heating, as well as its potential to select chiralities during the growing process, it may provide new features and versatilities in the device fabrication.