[Show abstract][Hide abstract] ABSTRACT: Direct integration of vertically aligned carbon nanotube (VACNTs) patterns on a graphene layer combined with a liquid crystal cell can be utilized as an optical array. The hybrid structures have high transparency and conductivity and, as reported by Sun IL Kim, Jong Min Kim, Young Hee Lee, and co-workers, clear diffraction patterns are obtained by modulating an external electric field.
[Show abstract][Hide abstract] ABSTRACT: Graphene/carbon nanotube (CNT) hybrid structures are fabricated for use as optical arrays. Vertically aligned CNTs are directly synthesized on a graphene/quartz substrate using plasma-enhanced chemical vapor deposition (PECVD). Graphene preserves the transparency and resistance during CNT growth. Highly aligned single-walled CNTs show a better performance for the diffraction intensity.
[Show abstract][Hide abstract] ABSTRACT: The wetting of a metal on carbon nanotubes is fundamentally difficult due to the unusually large difference between their surface tensions and is a bottleneck for making metal–carbon nanotube (CNT) composites. Here, we report a simple method to enhance the wettability of metal particles on the CNT surface by applying aluminum, which is the material with the largest surface tension. This method involves two steps: (i) Al nanoparticles are decorated on multiwalled carbon nanotubes by electroplating and (ii) Al powder is further spread on Al-electroplated CNTs, followed by high-temperature annealing to accommodate complete wetting of the aluminum. The large surface tension difference is overcome by forming strong AlC covalent bonds initiated by defects of the CNTs. The decrease in the D-band intensity, the G-band shift in the Raman spectroscopy and the formation of AlC covalent bonds, as confirmed by X-ray photoelectron spectroscopy, were in agreement with our structural model of CNTvacancyOAl determined by density functional calculations.
[Show abstract][Hide abstract] ABSTRACT: Atomic layer deposition (ALD) is a type of chemical vapor deposition method specially modified to grow thin films via a so called self-limiting growth mechanism. In spite of its excellent conformality due to inherent layer-by-layer growth behavior, the utilization of ALD is restricted mainly to the growth of thin films. In this article we demonstrate that ZnOnanoparticles can be grown with a botryoidal appearance by ALD using a forest of single-walled carbon nanotubes as the substrate, which only has a sparse amount of reactive sites for precursor chemisorption. The nanoparticles are fairly spherical and single crystalline with a wurtzite crystalline structure. The growth rate of the nanoparticles is roughly estimated to be 2.8 Å cycle−1 which is around twice the growth rate of a conformal film on a flat substrate. The size of the nanoparticles is considerably uniform with a standard deviation of 18%.
[Show abstract][Hide abstract] ABSTRACT: By using carbon-free inorganic atomic layer involving heat treatment from 150 to 300 °C, environmentally stable and permanent modulation of the electronic and electrical properties of single-walled carbon nanotubes (SWCNTs) from p-type to ambi-polar and possibly to n-type has been demonstrated. At low heat treatment temperature, a strong p-doping effect from Au(3+) ions to CNTs due to a large difference in reduction potential between them is dominant. However at higher temperature, the gold species are thermally reduced, and thermally induced CNT-Cl finally occurs by the decomposition reaction of AuCl(3). Thus, in the AuCl(3)-doped SWCNTs treated at higher temperature, the p-type doping effect is suppressed and an n-type property from CNT-Cl is thermally induced. Thermal conversion of the majority carrier type of AuCl(3)-doped SWNTs is systematically investigated by combining various optical and electrical tools.
[Show abstract][Hide abstract] ABSTRACT: Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnel's equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.
[Show abstract][Hide abstract] ABSTRACT: The mechanism of doping carbon nanotubes (CNTs) with a salt solution was investigated using the density functional theory. We propose that the anion-CNT complex is a key component in doping CNTs. Although the cations play an important role in ionizing CNTs as an intermediate precursor, the ionized CNTs are neutralized further by forming a stable anion-CNT complex as a final reactant. The anion-CNT bond has a strong ionic bonding character and clearly shows p-type behavior by shifting the Fermi level toward the valence band. The midgap state is introduced by the strong binding of carbon and anion atoms. These localized charged anion sites are highly hygroscopic and induce the adsorption of water molecules. This behavior provides a new possibility for using anion-functionalized CNTs as humidity sensors.
[Show abstract][Hide abstract] ABSTRACT: Hygroscopic effects on AuCl3-doped single-walled carbon nanotubes (SWCNTs) were investigated. Various concentrations of AuCl3 solution were dropped on the random network SWCNT transistors, where the doping effect was confirmed by I−Vg characteristics and a blue shift in the Raman G band. The AuCl3-doped SWCNT transistor was highly sensitive to water vapor in the I−Vg curve, especially in the positive gate bias region, compared with the undoped SWCNT transistors. We also found the selective response of water from ambient gases, such as O2 and N2, in AuCl3-doped SWCNTs. This investigation could be beneficial for high-performance water sensor applications.
The Journal of Physical Chemistry C 07/2010; 114(26):11618-11622. DOI:10.1021/jp1036662 · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Vertically aligned multiwalled carbon nanotubes (MWCNTs) as an interdigital electrode were fabricated to investigate AC response to NO2 gas. The response mechanism was explained in terms of the polarization effect of gases, charge transfer by gas adsorption on CNT electrodes, and field emission current. The response was affected more by gas adsorption than by the polarization effect and field emission effect. We also found that capacitive response and the yield of device production can be significantly improved by a post-heat-treatment, which can be explained by the removal of adsorbents such as moisture and/or other functional groups on the CNT surface.
The Journal of Physical Chemistry C 02/2010; 114(8). DOI:10.1021/jp910678m · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Critical yarning conditions from vertically aligned carbon nanotubes (VACNTs) using a chemical vapor deposition have been investigated. VACNTs with a diameter of around 15 nm have been synthesized with a length up to 3.7 mm. The yarning was realized exclusively in a limited range of the CNT lengths of about 170-1500 mu m. Although CNTs became long for longer growth times, some of the CNTs were plucked out from the bottom substrate during growth so that the CNT density decreased at later stages of growth, prohibiting continuous yarning by the suppression of interconnection between CNTs at the bottom part of VACNTs.
[Show abstract][Hide abstract] ABSTRACT: A charge trapping layer can serve not only for designing multilevel nonvolatile memory but also for type conversion from p- to n-type and vice versa of carbon nanotube (CNT) channels. Type conversion from p- to n-type and vice versa for CNT field effect transistors can be realized by changing the polarity of trapped charges (see figure). Figure Presented
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotube (CNT)-based point electron emitter was fabricated using a cavity-confined dielectrophoresis. The emission current of an individual multi-walled CNT (MWCNT) was stable up to 10 μA and reached ~ 2 mA (1.7 × 108 A/cm2), about three orders of magnitude higher than the threshold current of the existing single MWCNT tip. At low electric field, the current fluctuated in a stepwise manner. On the other hand, above critical field, CNT point emitter started to disintegrate so that the current fluctuated rapidly and gradually diminished. These anomalous behaviors were explained from the cap opening and field-induced unraveling of tip edges.
Diamond and Related Materials 12/2009; 18(12-18):1435-1439. DOI:10.1016/j.diamond.2009.09.010 · 1.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate doping-free and adaptive inverter to verify that the single ambipolar SWCNT transistors can be utilized both p- and n-type. Furthermore, we fabricate an adaptive logic circuit that can reveal multifunctions such as NOR and NAND gate using four ambipolar transistors. This new approach is innovative in several aspects, for instance, in improving integration density, simplicity without intentional doping, and its multifunctionality and ensures multidisciplinary interests in materials, physics, mechanics, and electronics areas.
Proceedings of SPIE - The International Society for Optical Engineering 08/2009; DOI:10.1117/12.828118 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A CMOS-like inverter was integrated by using ambipolar carbon nanotube (CNT) transistors without doping. The ambipolar CNT transistors automatically configure themselves to play a role as an n-type or p-type transistor in a logic circuit depending on the supply voltage (V(DD)) and ground. A NOR (NAND) gate is adaptively converted to a NAND (NOR) gate. This adaptiveness of logic gates exhibiting two logic gate functions in a single logic circuit offers a new opportunity for designing logic circuits with high integration density for next generation applications.
[Show abstract][Hide abstract] ABSTRACT: Various viologens have been used to control the doping of single-walled carbon nanotubes (SWCNTs) via direct redox reactions. A new method of extracting neutral viologen (V(0)) was introduced using a biphase of toluene and viologen-dissolved water. A reductant of sodium borohydride transferred positively charged viologen (V(2+)) into V(0), where the reduced V(0) was separated into toluene with high separation yield. This separated V(0) solution was dropped on carbon nanotube transistors to investigate the doping effect of CNTs. With a viologen concentration of 3 mM, all the p-type CNT transistors were converted to n-type with improved on/off ratios. This was achieved by donating electrons spontaneously to CNTs from neutral V(0), leaving energetically stable V(2+) on the nanotube surface again. The doped CNTs were stable in water due to the presence of hydrophobic V(0) at the outermost CNT transistors, which may act as a protecting layer to prevent further oxidation from water.
Journal of the American Chemical Society 04/2009; 131(1):327-31. DOI:10.1021/ja807480g · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotube(CNT)-anchoredpolymer nanofiber mats were designed by in situ spraying of carbon nanotubes while simultaneously electrospinningpolymer nanofibers. The improved conductivity of the composite mat, high catalytic current (3400 mA/cm2/mg Pt) and long term stability of the composite mat was attributed to the efficient formation of CNT bridges between nanofibers.