M. Mitchell

University of Nebraska at Lincoln, Lincoln, Nebraska, United States

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Publications (18)30.48 Total impact

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    ABSTRACT: The m-plane-oriented gallium nitride (GaN) nanoplates were successfully grown on silicon (Si) substrates at 450 °C, using laser-assisted metal organic chemical vapor deposition (L-MOCVD). The morphology and <101̅0> orientation of the nanoplates were confirmed using high-resolution electron microscopes. GaN nanoplates served as seed crystals for the subsequent growth of m-plane-oriented interlinked GaN nanoplates at a longer growth time. The strong A1 (TO) mode in Raman spectra and the (101̅0) peak in X-ray diffraction confirmed the m plane orientation of the nanoplates. The interlinked GaN nanoplates showed a high-growth rate of 38 μm/h. The results suggest that L-MOCVD is a promising technique for the rapid growth of m-plane-oriented GaN nanoplates on the Si substrates at low-growth temperatures.
    Full-text · Article · Jun 2013 · Crystal Growth & Design
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    X N He · Y Gao · M Mahjouri-Samani · P N Black · J Allen · M Mitchell · W Xiong · Y S Zhou · L Jiang · Y F Lu
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    ABSTRACT: Gold-coated horizontally aligned carbon nanotube (Au-HA-CNT) substrates were fabricated for surface-enhanced Raman spectroscopy (SERS). The Au-HA-CNT substrates, which are granular in nature, are easy-to-prepare with large SERS-active area. Enhancement factors (EFs) of ∼10(7) were achieved using the Au-HA-CNTs as substrates for rhodamine 6G (R6G) molecules. Maximum enhancement was found when the polarization direction (E-field) of the incident laser beam was parallel to the aligned direction of the HA-CNTs. Simulations using the finite-difference time-domain (FDTD) method were carried out for the granular Au-HA-CNT samples. Enhancement mechanisms and determination of EFs were analyzed. Biological samples, including (13)C- and deuterium (D)-labeled fatty acids and Coccomyxa sp. c-169 microalgae cells, were also measured using this SERS substrate. The limits of detection (LODs) of D- and (13)C-labeled fatty acids on the SERS substrate were measured to be around 10 nM and 20 nM, respectively. Significantly enhanced Raman signals from the microalgae cells were acquired using the SERS substrate.
    Full-text · Article · May 2012 · Nanotechnology
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    ABSTRACT: We developed a process to form transparent interconnections using graphene patterns that were synthesized by laser chemical vapor deposition. The number of graphene layers was tightly controlled by laser scan speed. Graphene patterns were fabricated at a high scan speed of up to 200 mum/s with a single-step process. The process time is about a million times faster than the conventional chemical vapor deposition method. The fabricated graphene patterns on nickel foils were directly transferred to desired positions on patterned electrodes. The position-controlled transfer with rapid single-step fabrication of graphene patterns provides an innovative pathway for graphene-based interconnections.
    Full-text · Article · Aug 2011 · Applied Physics Letters
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    ABSTRACT: Rapid single-step fabrication of graphene patterns was developed using laser-induced chemical vapor deposition (LCVD). A laser beam irradiates a thin nickel foil in a CH4 and H2 environment to induce a local temperature rise, thereby allowing the direct writing of graphene patterns in precisely controlled positions at room temperature. Line patterns can be achieved with a single scan without pre- or postprocesses. Surprisingly, the growth rate is several thousand times faster than that of general CVD methods. The discovery and development of the LCVD growth process provide a route for the rapid fabrication of graphene patterns for various applications.
    Full-text · Article · Mar 2011 · Applied Physics Letters
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    ABSTRACT: Periodic diameter modulation of carbon nanotubes (CNTs) by quick temperature variation was successfully achieved in laser-assisted chemical vapor deposition process. Tapered and diameter-alternating CNTs were grown by periodic modulation of the temperature due to inverse relationship between the temperature and the diameter of the CNTs. The diameter-modulated single-walled carbon nanotubes (SWNTs) were integrated into field-effect transistors (FETs) structure to investigate their electronic transport properties. The tapered SWNTs showed electronic properties similar to Schottky diodes indicating clear evidence of different bandgaps at two ends of the tubes. However, the electronic transport of the diameter-modulated SWNTs showed a very small current magnitude which is attributed to the large number of defects and the electron confinement in the periodic quantum well arrays. Transmission electron microscopy and Raman spectroscopy were also studied to investigate the structural and electronic properties of the structures.
    No preview · Article · Feb 2011 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The diameter of individual single-walled carbon nanotubes (SWNTs) was successfully modulated along their axes by instant temperature control in a laser-assisted chemical vapor deposition (LCVD) process. SWNTs were grown using different temperature profiles to investigate the effects of temperature variation on their growth. Due to the inverse relationship between SWNT diameter and growth temperature, SWNTs with ascending diameters were obtained by reducing the LCVD temperature from high to low. The diameter-modulated SWNTs were grown across a pair of Mo electrodes to form field-effect transistors (FETs) for investigation of their electronic transport properties. Fabricated devices demonstrated properties similar to Schottky diodes, implying different bandgap structures at the ends of the SWNTs. Raman spectroscopy, transmission electron microscopy, and electronic transport characteristics were studied to investigate the influence of temperature variation on the structural and electronic characteristics of SWNTs.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    J.B. Park · W. Xiong · Z.Q. Xie · M. Mitchell · Y. Gao · M. Qian · Y.F. Lu
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    ABSTRACT: Rapid growth of single-layer graphene using laser-induced chemical vapor deposition (LCVD) with a visible CW laser (λ = 532 nm) irradiation at room temperature was investigated. In this study, an optically-pumped solid-state laser with a wavelength of 532 nm irradiates a thin nickel foil to induce a local temperature rise, thereby allowing the direct writing of graphene patterns about ~10 μm in width with high growth rate on precisely controlled positions. It is demonstrated that the fabrication of graphene patterns can be achieved with a single scan for each graphene pattern using LCVD with no annealing or preprocessing of the substrate. The scan speed reaches to about ~50 um/s, which indicates that the graphene pattern with 1:1 aspect ratio (x:y) can be grown in 0.2 sec. The patterned graphene on nickel was transferred to SiO2/Si substrate for fabrication of electrical circuits and sensor devices.
    Full-text · Article · Jan 2011 · MRS Online Proceeding Library
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    M. Mahjouri-Samani · Y.S. Zhou · W. Xiong · M. Mitchell · Y.F. Lu
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    ABSTRACT: Periodic diameter modulation of individual carbon nanotubes (CNTs) by rapid temperature variation in laser-assisted chemical vapor deposition (LCVD) process was successfully achieved. A CO2 laser was used to quickly and precisely modulate the growth temperature. Due to inverse relationship between the diameter of SWNTs and the growth temperature, tapered and diameter-alternating tubes were grown by modulating the CO2 laser power during the growth. The diameter-modulated SWNTs were grown across a pair of Mo electrodes to form field-effect transistors (FETs) for investigation of their electrical and optical properties. Variations in the bandgap of the individual semiconducting SWNTs were detected meaning the formation of multi bandgaps within a single tube. Transmission electron microscopy and Raman spectroscopy were also studied to investigate the structural and electronic properties of the structures.
    Full-text · Article · Jan 2011
  • W. Xiong · Y.S. Zhou · M. Mitchell · J.B. Park · M. M. Samani · Y. Gao · Y.F. Lu
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    ABSTRACT: Fabrication of nanoscale devices by assembling individual carbon nanotubes (CNTs) remains challenging despite enormous effort made in this field. Fulfilling the promise of CNTs requires more efficient assembly techniques. In this study, we have developed an in-situ assembly method for precise and cost-effective integration of CNTs using a laser-assisted chemical vapor deposition (LCVD) process. Results show that CNTs can be trapped between sharp tip-shaped electrodes due to the optical gradient forces around the tip apexes generated by a CO2 laser irradiation. This method enables the precise assembly of CNT-based field-effect transistors (FETs) and paves the way for the successful implementation of the CNT-based nanoelectronics.
    No preview · Article · Dec 2010 · MRS Online Proceeding Library
  • M Mahjouri-Samani · Y S Zhou · W Xiong · Y Gao · M Mitchell · L Jiang · Y F Lu
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    ABSTRACT: Diameter modulation by fast temperature control in laser-assisted chemical vapor deposition (LCVD) was successfully achieved to tune the diameters of single-walled carbon nanotubes (SWNTs) in different segments. Due to the inverse relationship between the SWNT diameter and the growth temperature, SWNTs with ascending diameters were obtained by reducing the LCVD temperature from high to low. The diameter-modulated SWNTs were integrated in electrodes to form field-effect transistors (FETs) and to investigate their electronic transport properties. The SWNTs in the FET structures have electronic properties similar to Schottky diodes, indicating clear evidence of different bandgap structures at the two ends of the SWNTs. Raman spectroscopy, transmission electron microscopy, and electronic transport characteristics were studied to investigate the influence of temperature variation on the structural and electronic characteristics of the SWNTs.
    No preview · Article · Oct 2010 · Nanotechnology
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    Y S Zhou · W Xiong · Y Gao · M Mahjouri-Samani · M Mitchell · L Jiang · Y F Lu
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    ABSTRACT: Where it starts and where it goes? Controlled integration of single-walled carbon nanotubes (SWNTs) into pre-designed nano-architectures is one of the major challenges to be overcome for extensive scientific research and technological applications. Various serial assembly techniques have been proposed and developed. However, they are still a long way from practical applications due to the drawbacks on reliability, yield and cost. Here we demonstrate a laser-based strategy to achieve parallel integration of SWNTs into pre-designed nano-architectures through an optically controlled in situ growth process. Optical driving forces originated from tip-induced optical near-field enhancement and laser beam polarization were applied in this study to realize the controlled placement of SWNTs at designated sites following wanted orientations on the nanometer scale. Parallel integration of SWNT arrays was achieved by adjusting laser beam diameter to cover interested nano-architectures. The laser-based process suggests an efficient and cost-effective approach for fabricating and integrating SWNT-based devices and circuits.
    Full-text · Article · Aug 2010 · Nanotechnology
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    ABSTRACT: Growing carbon nanotubes (CNTs) of different alignments, including surface-bounded and vertically aligned arrays, on metallic electrodes was achieved by applying electric voltages of different polarities on metallic electrodes during the laser-assisted chemical vapor deposition process. Surface-bounded CNTs were found to crawl out from the positively charged electrodes. In contrary, vertically aligned CNTs dominated the negatively charged electrodes. The alignment control was ascribed to the movement of catalyst-nanoparticles (NPs) under the influence of external electric field. The surface-bounded CNTs were ascribed to the repulsive forces between the catalyst NPs and the anodes. The vertically aligned CNTs were ascribed to the joint interactions of catalyst-cathode interactions and tube-tube interactions. This investigation suggests a convenient approach to control the alignment of CNT arrays for applications in different fields.
    No preview · Article · Feb 2010 · Proceedings of SPIE - The International Society for Optical Engineering
  • W. Xiong · Y.S. Zhou · M. Mahjouri-Samani · M. Mitchell · Y.F. Lu
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    ABSTRACT: Despite significant progress made in controllable synthesis of single-walled carbon nanotubes (SWNTs), fabrication of SWNT-based devices and systems is still lagged behind due to the absence of efficient integration technique to precisely place SWNTs into nano/micro-architectures. Here we demonstrate a novel in-situ synthetic method for precise and direct integration of SWNTs into predesigned nano/micro-architectures. Pre-patterned substrates containing sharp Ru tip arrays were used in a laser-assisted chemical vapour deposition for growing SWNTs. The sharp metallic tips were fabricated to enhance optical near-field effects which in turn stimulate and confine the growth of SWNTs through localized heating and optical trapping. We show that the integration of SWNTs has nanoscale precision defined by the localized heating and optical trapping effects. Moreover, the laser based synthetic approach also demonstrated laser polarization dependence and low-substrate-temperature growth, which made this approach a potentially scalable technique for fabricating SWNT-based devices and systems.
    No preview · Article · Jan 2010
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    M Mahjouri-Samani · Y S Zhou · W Xiong · Y Gao · M Mitchell · Y F Lu
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    ABSTRACT: Distinguishing between carbon nanotubes (CNTs) according to their individual electronic properties is of significant importance for developing CNT-based electronics and devices. In this study, selective removal of metallic CNTs from CNT mixtures on silicon substrates was investigated using controlled laser irradiation. Free electron movement and eddy currents are induced within the metallic CNTs by the strong electric field and optical near-field effects caused by the laser irradiation. Selective heating of metallic CNTs in air results in selective removal of metallic CNTs when the laser fluence and wavelength are properly selected. Through this process, metallic nanotubes are successfully removed from the CNT mixtures. This technique provides an efficient single-step approach for selective removal of metallic CNTs from CNT mixtures.
    Full-text · Article · Dec 2009 · Nanotechnology
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    ABSTRACT: Controllable growth and integration of single-walled carbon nanotubes (SWNTs) were achieved using an optically controlled approach. By applying optical near-field effects in a laser-assisted chemical vapor deposition process, controllable growth of SWNTs was realized.
    No preview · Conference Paper · Oct 2009
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    ABSTRACT: Carbon nanotubes (CNTs) of different alignments, such as surface-bounded and vertically aligned arrays, enable applications in different fields. In this study, controlled growth of CNTs with different alignments was achieved by electrically biasing catalyzed electrodes with different polarities in a laser-assisted chemical vapor deposition 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.
    Full-text · Article · Oct 2009 · Applied Physics Letters
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    ABSTRACT: Carbon materials of versatile structures, from diamond, graphite, carbon nanotube to graphene, exhibit distinct properties and find applications in different fields. Laser-assisted deposition methods were developed for growing materials with unparallel 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 briefly introduce applications of laser-assisted deposition method in depositing a vast diversity of good carbons, including diamond, diamond-like carbon and carbon nanotubes, with high quality and superior controllability. The laser-assisted deposition was demonstrated to be a cost effective and highly efficient material preparation technique.
    No preview · Article · Jan 2009
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    ABSTRACT: Single-walled carbon nanotubes (SWNT) are regarded as one of the most promising materials for next-generation nano-electronics. However, there are still several challenges limiting its wide applications, including the inability in controlled growth of SWNT connections. In this study, we developed a laser-based in-situ growth approach to simultaneously fabricate SWNT-bridge arrays on a single silicon substrate with precise control. Localized thermal enhancement induced by optical near-field effects and an external electric field enabled the SWNT growth with precise control of growth sites and directions. Furthermore, laser polarization also shows significant influence on the control of growth site for SWNTs. Simultaneous growth of SWNT-bridge arrays in various patterns was achieved. Raman spectroscopy and I-V analysis demonstrated the successful growth of SWNT bridge structures. The laser-based growth method suggests a promising solution for the fabrication of SWNT-based systems in nano-electronics.
    No preview · Article · Jan 2009