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Biography |
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------Prof. Yongfeng Lu received bachelor, master, and PhD degrees in electrical engineering from Tsinghua University (China) in 1984 and Osaka University (Japan) in 1988 and 1991, respectively. He was a faculty in the Department of Electrical and Computer Engineering at National University of Singapore before we went to USA in 2002. He is currently the Lott University Professor of Electrical Engineering at the University of Nebraska - Lincoln. He has around 20 years experience in laser-based material processing and characterization at micro/nanoscales. His group has research projects funded by NSF, AFOSR, ONR, DoE, NRI, private companies, and other foundations in USA and Japan, with research expenditures more than ten million dollars in past several years. Dr. Lu has authored or co-authored around 210 journal papers and 260 conference papers. He has also edited many conference proceedings and book chapters. Besides the fundamental research works which lead to a large number of publications and a number of national and international awards, he has also developed a number of technologies and commercialized them in industries. He was elected to SPIE and LIA fellows in 2008 and 2009, respectively. He has served as chair and general chair for numerous international conferences, including the general congress chair for International Congress of Applications of Lasers and Electro-Optics (ICALEO, Laser Institute of America) in 2007 and 2008. Prof. Lu has received a number of international awards in this research field (e.g., National Technology Award, Singapore, and International Laser Award, Germany). He has served as the keynote speaker in several key international laser conferences, such as ICALEO 2004 and 2009.
------His research encompasses work on laser-assisted scanning probe microscopy for nanoscale patterning, surface modification, and characterization; laser writing of sub-wavelength structures, laser synthesis of Si nano-crystals; laser annealing for the formation of sub-30 nm p-n junctions; fabrication of 2-D and 3-D photonic bandgap crystals using laser imprinting of self-assembled nanoparticles; deposition of diamond-like carbon thin films on nanoscale metallic tips; and, more recently, laser-assisted CVD of CNTs, CNOs, carbon nano-sheets, and diamond crystals and thin films. His group has also engaged in optical spectroscopy including laser-induced breakdown spectroscopy, laser-induced fluorescence, optical emission spectroscopy, nanoscale Raman spectroscopy, and coherent anti-Stokes Raman spectroscopy. |
Yongfeng Lu.
Department of Electrical Engineering
University of Nebraska-Lincoln
Lincoln, NE, 68588-0511 |
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Abstract |
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Laser-based Synthesis of Micro/nano-structured Carbon Materials |
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------Carbon materials of versatile structures, from diamond, graphite, carbon nanotubes (CNTs) to carbon nano-onions (CNOs), exhibit distinct properties and find applications in different fields. Laser-assisted synthesis methods were developed for growing materials with unparalleled advantages, such as selective interaction, localized heating/interacting area, rapid non-equilibrium heating/cooling rate, and enhanced deposition rate, by making use of both thermal and photolytic effects. Here, we introduce a number of laser-assisted processing methods in producing a vast diversity of good carbons, including diamond, carbon nanotubes, and carbon nano-onions, with high quality and superior controllability. The laser-assisted processing is demonstrated be to a cost effective and highly efficient material preparation technique. In this talk, the speaker will also introduce their research activities in spectrally and specially controlled laser-material interactions for sustainable synthesis of materials with improved efficiency and reduced energy consumptions.
1. Direct synthesis of single-walled carbon nanotubes bridging metal electrodes
------Direct synthesis of single-walled carbon nanotubes (SWNTs) bridging pre-patterned 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. The LCVD method also provides a potential approach to in-situ remove SWNTs with specific chiralities during the growth.
2. Self-aligned growth of single-walled carbon nanotubes using optical near-field effects
------Self-aligned growth of ultra-short SWNTs was realized by utilizing optical near-field effects in an LCVD process. By introducing the optical near-field effects, bridge structures containing single suspended-SWNT channels were successfully fabricated through the LCVD process at a relatively low substrate temperature. Raman spectroscopy and I-V analyses have been carried out to characterize the SWNT-bridge structures. Numerical simulations using High Frequency Structure Simulator (HFSS) revealed that significant enhancement of local heating occurs at metallic electrode tips under laser irradiation, which is about one order of magnitude higher than the rest part of the electrodes. This technique suggests a novel approach for in-situ low-temperature fabrication of SWNT-based devices in a precisely controlled manner due to the nanoscale heating enhancement induced by the optical near field effects.
3. Controlled growth of carbon nanotubes on electrodes under different bias polarity
------CNTs of different alignments, such as surface-bounded and vertically aligned arrays, enable applications in different fields. Controlled growth of CNTs with different alignments was achieved by electrically biasing catalyzed electrodes with different polarities in an LCVD process. CNT growth was suggested to be guided by the movement of electrically charged catalyst-nanoparticles under the influence of an external electric field. This discovery provides a convenient approach to control the alignment of CNT arrays for different applications.
4. Enhanced chemical vapor deposition of diamond films by resonant vibrational excitations of ethylene molecules using tunable CO2 laser irradiation
------Resonant vibrational excitations of ethylene (C2H4) molecules using a tunable carbon dioxide (CO2) laser was employed to significantly enhance the chemical vapor deposition (CVD) of diamond in open air using a precursor gas mixture of C2H4, acetylene (C2H2), and oxygen (O2). The CH2-wag vibration mode (n7) of the C2H4 molecules was selected to achieve the resonant excitation in the CVD process. Both laser wavelengths of 10.591 and 10.532 mm were applied to the CVD processes to compare the C2H4 excitations and diamond depositions. Compared with 10.591 mm produced by common CO2 lasers, the laser wavelength of 10.532 mm is much more effective to excite the C2H4 molecules through the CH2-wag mode. Under the laser irradiation with a power of 800 W and a wavelength of 10.532 mm, the grain size in the deposited diamond films was increased by 400% and the film thickness was increased by 300%. The quality of the diamond crystals was also significantly
5. Fast growth of diamond crystals in air by resonant laser energy coupling
------Fast growth of diamond crystals in air was achieved by combustion synthesis with resonant laser energy coupling. A pre-mixed C2H4/C2H2/O2 gas mixture was used as precursors for growth of diamond crystals. Through the resonant excitation of the CH2 wagging mode of the ethylene (C2H4) molecules using a CO2 laser tuned at 10.532 µm, high-quality diamond crystals were grown on silicon substrates with a high growth rate at ~139 µm/hr. Diamond crystals with a length up to 5 mm and a diameter of 1 mm were grown in 36 hours. Sharp Raman peaks at 1332 cm-1 with full width at half maximum (FWHM) values around 4.5 cm-1 and distinct X-ray diffraction spectra demonstrated the high quality of the diamond crystals. The effects of the resonant laser energy coupling were investigated using optical emission spectroscopy.
6. Optical emission spectroscopy of the tunable CO2 laser excited C2H4/C2H2/O2 flame for diamond growth
------Optical emission spectroscopy (OES) measurements were carried out to study the C2H4/C2H2/O2 combustion flame for diamond deposition with and without the CO2 laser excitation. According to the OES spectra, strong emission from radicals C2 and CH was observed in the visible range. It was discovered that the flow rate and volume ratio of the gas precursors could significantly influence the emission intensity of the radicals. By adding a continuous-wave CO2 laser to irradiate the flame at a wavelength of 10.591 µm, it was discovered that the emission intensities of the C2 and CH radicals were increased due to the laser beam induced excitation. Another wavelength - 10.532 µm of the tunable CO2 laser was used for more precisely exciting the CH2 – wagging vibration mode. OES measurements of the C2 and CH radicals were performed using different gas combinations and laser powers. The rotational temperatures of the CH radicals in the flame were determined by analyzing the spectra of the R-branch of the A2Δ→X2Π (0, 0) electronic transition near 430 nm. Information obtained from the OES spectra, including the emission intensities of the C2 and CH radicals, the intensity ratios, and the rotational temperatures, was integrated to the study of diamond deposition on tungsten carbide substrates for mechanism analysis on the laser-induced vibrational excitation and laser-assisted diamond deposition. |
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