M. Mitchell

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

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Publications (12)30.6 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.
    Crystal Growth & Design 06/2013; 13(7):3171–3176. · 4.69 Impact Factor
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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.
    Nanotechnology 05/2012; 23(20):205702. · 3.84 Impact Factor
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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.
    Applied Physics Letters 03/2011; 98(12):123109-123109-3. · 3.52 Impact Factor
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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.
    Proc SPIE 02/2011;
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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.
    Applied Physics Letters 01/2011; 99. · 3.52 Impact Factor
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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.
    MRS Proceedings. 12/2010; 1365.
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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.
    Nanotechnology 10/2010; 21(39):395601. · 3.84 Impact Factor
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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.
    Nanotechnology 08/2010; 21(31):315601. · 3.84 Impact Factor
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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.
    Proc SPIE 02/2010;
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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.
    Nanotechnology 12/2009; 20(49):495202. · 3.84 Impact Factor
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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.
    Femtosecond Laser Microfabrication; 10/2009
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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.
    Applied Physics Letters 01/2009; 95. · 3.52 Impact Factor