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ABSTRACT: The ultraviolet optical properties of semiconducting-enriched and metallic-enriched single-walled carbon nanotube (semi-enriched and m-enriched SWCNT) networks were studied using spectroscopic ellipsometry. According to calculated energy loss function, the energy loss peak assigned to the maximum intensity of π-plasmon energy was found to increase from 4.5 eV to 5.0 eV as SWCNT network composition was changed from m-SWCNT enriched to semi-SWCNT enriched. These results clearly demonstrate that the dielectric response in the 4-6 eV range is sensitive to changes in the surrounding dielectric environment depending on the semi-/m-SWCNT content. Therefore, the spectral shift of this energy loss is attributed to the enhanced electron confinement by the presence of the surface plasmon due to a small amount of m-SWCNT, which is an important phenomenon at the SWCNT network.
Nanoscale 09/2012; 4(20):6532-6. · 5.91 Impact Factor
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ABSTRACT: We present an integration process to fabricate single-walled carbon nanotube (SWCNT) three-electrode systems on glass substrate for electrochemical biosensors. Key issues involve optimization of the SWCNT working electrode to achieve high sensitivity, developing an optimal Ag/AgCl reference electrode with good stability, and process development to integrate these electrodes. Multiple spray coatings of the SWCNT film on glass substrate enabled easier integration of the SWCNT film into an electrochemical three-electrode system. O₂ plasma etching and subsequent activation of spray-coated SWCNT films were needed to pattern and functionalize the SWCNT working electrode films without serious damage to the SWCNTs, and to remove organic residues. The microfabricated three-electrode systems were characterized by microscopic and spectroscopic techniques, and the electrochemical properties were investigated using cyclic voltammetry and chrono-amperometry. The fully-integrated CNT three-electrode system showed an effective working electrode area about three times larger than its geometric surface area and an improved electrochemical activity for hydrogen peroxide decomposition. Finally, the effectiveness of miniaturized pf-SWCNT electrodes as biointerfaces was examined by applying them to immunosensors to detect Legionella(L) pneumophila, based on a direct sandwich enzyme-linked immunosorbent assay (ELISA) format with 3,3',5,5'-tetramethylbenzidine dihydrochloride/hydrogen peroxide(TMB/H₂O₂) as the substrate/mediator system. The lower detection limit of the pf-SWCNT-based immunosensors to L. pneumophila is about 1500 times lower than that of the standard ELISA assay.
Biomedical Microdevices 03/2012; 14(3):613-24. · 3.03 Impact Factor
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ABSTRACT: Transferred multi-walled carbon nanotube (MWCNT)-modified platinum thin-film immunosensing electrode material was engineered on a glass substrate and fabricated a fully-integrated electrochemical three-electrode system for monitoring Legionella pneumophila. The transferred MWCNT film was treated with oxygen plasma to improve its electrochemical response and electrical conductivity. We voltammetrically characterized and optimized the electrochemical performance of the fabricated electrode for direct detection of Legionella pneumophila-specific peptidoglycan-associated lipoprotein (PAL) and maltose binding protein (MBP) peptidoglycan-associated lipoprotein (MBP-PAL) fusion. The latter, as an intermediate product to yield the former, has important roles in the growth and purification of PAL, which commercial enzyme-linked immunosorbent assay (ELISA) kits require as a target substrate. Consequently, direct electrochemical detection of MBP-PAL compared to PAL by square-wave voltammetry showed a greater than 50% increase in sensitivity with a lower detection limit of 5 pg mL(-1). We also investigated the affinity properties by determining kinetic parameters of the PAL and the MBP-PAL in relation to polyclonal antibodies immobilized on transferred MWCNT substrates using Michaelis-Menten assumptions and a Hanes-Woolf plot. This new method presented herein could save the time and effort for the separation and purification of PAL form MBP-PAL fusions that are required for performing ELISA-based immunoassay.
Biotechnology and Bioengineering 12/2011; 109(6):1471-8. · 3.95 Impact Factor
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ABSTRACT: We investigated the chemical doping of the single-walled carbon nanotubes (SWCNTs) networks by a treatment with aromatic amines. Adsorption and intercalation of amine molecules in bundled SWCNTs leads to typical n-type doping observed already for alkali metals. The electron donation to SWCNTs is demonstrated by the X-ray-induced photoelectron spectra (XPS), where the carbon C 1s peak observed at 284.4 eV for the sp2 carbon in pristine samples is shifted by up to 0.3 eV to higher binding energy upon chemical treatment. The development of a Breit-Wigner-Fano component on the lower energy side of the G− mode in the Raman spectrum as well as a shift of the G+ to lower frequency provide evidence for charge accumulation in the nanotube π system, and indication for the n-type doping. The spectroscopic changes are accompanied by the modification of the electrical properties of the SWCNTs. A reduction of conductivity depends on the doping level and implies the decreasing concentration of the charge carriers in the naturally p-doped tubes. Comparing the two selected n-type dopants, the tetramethyl-p-phenylenediamine, shows more pronounced changes in the XPS and the Raman spectra than tetramethylpyrazine, indicating that the sp3 hybridization of nitrogen in the amine groups attached to phenyl ring is much more effective in interaction with the tube π system than the sp2 hybridization of nitrogen in the aromatic pyrazine ring.
physica status solidi (b) 09/2010; 247(11‐12):2793 - 2796. · 1.32 Impact Factor
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ABSTRACT: A novel multiwall carbon nanotube (MWCNT) electrode functionalized with oxygen plasma treatment was prepared and characterized, and its DNA sensing ability for Legionella pneumophila (L. pneumophila) detection was examined using electrochemical measurement. A well-patterned MWCNT working electrode (WE) on a Pt track was fabricated using photolithography, transfer methods and an etching technique. The MWCNT WE was functionalized by oxygen plasma treatment prior to applying for DNA sensor. The surface morphology of the plasma-functionalized MWCNT (pf-MWCNT) WEs were observed by scanning electron microscope (SEM) and the change of chemical composition was characterized by X-ray photoelectron spectroscopy (XPS), and electrochemical measurements were performed using CV with ferricyanide/ferrocyanide redox couple. Effective areas of working electrodes were calculated to be 0.00453 cm(2) for pristine MWCNT electrode and 0.00747-0.00874 cm(2) for pf-MWCNT electrodes with different plasma treatment times. Differential pulse voltammetry (DPV) was carried out in methylene blue solution for DNA sensing. The pf-MWCNT based DNA sensor was successfully operated in a target concentration range of 10 pM to 100 nM and had a lower detection limit than a pristine MWCNT based DNA sensor.
Talanta 08/2010; 82(3):904-11. · 3.79 Impact Factor
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ABSTRACT: We fabricate hybrid coaxial nanotubes (NTs) of multiwalled carbon nanotubes (MWCNTs) coated with light-emitting poly(3-hexylthiophene) (P3HT). The p-type P3HT material with a thickness of approximately 20 nm is electrochemically deposited onto the surface of the MWCNT. The formation of hybrid coaxial NTs of the P3HT/MWCNT is confirmed by a transmission electron microscope, FT-IR, and Raman spectra. The optical and structural properties of the hybrid NTs are characterized using ultraviolet and visible absorption, Raman, and photoluminescence (PL) spectra where, it is shown that the PL intensity of the P3HT materials decreases after the hybridization with the MWCNTs. The current-voltage (I-V) characteristics of the outer P3HT single NT show the semiconducting behavior, while ohmic behavior is observed for the inner single MWCNT. The I-V characteristics of the hybrid junction between the outer P3HT NT and the inner MWCNT, for the hybrid single NT, exhibit the characteristics of a diode (i.e., rectification), whose efficiency is clearly enhanced with light irradiation. The rectification effect of the hybrid single NT has been analyzed in terms of charge tunneling models. The quasi-photovoltaic effect is also observed at low bias for the P3HT/MWCNT hybrid single NT.
ACS Nano 07/2010; 4(7):4197-205. · 10.77 Impact Factor
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ABSTRACT: We synthesized thin-multiwalled carbon nanotubes (t-MWCNTs) using Fe-Mo/MgO catalyst by citrate precursor method based on a simple sol-gel technique. Synthesis of high-quality t-MWCNTs with uniform diameters was achieved in large-scale by catalytic decomposition of methane over Fe-Mo/MgO catalyst prepared under Ar-atmosphere. The produced t-MWCNTs had the outer diameters in the range of 4-8 nm, with a Gaussian average diameter of 6.6 +/- 0.1 nm and wall numbers in the range of of 4-7 graphenes. The product yield of the as-synthesized t-MWCNTs was over 450% relative to the used Fe-Mo/MgO catalyst, and TGA showed purity about 85%. The t-MWCNTs showed the low turn-on field about 1.24 V/microm, corresponding to the current density of 0.1 microA/cm2, and the high emission Current density of 10 mA/cm2 at the applied electric field of 2.6 V/microm. The t-MWCNTs indicate good emission stability without any fluctuation of emission current through a lifetime measurement for 20 h. It was confirmed that the field emission performance of t-MWCNTs was similar to DWCNTs and the field emission stability of t-MWCNTs was similar to MWCNTs.
Journal of Nanoscience and Nanotechnology 06/2010; 10(6):3998-4006. · 1.56 Impact Factor
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ABSTRACT: The improved field emission stability of thin multiwalled carbon nanotube (thin-MWCNT) emitters using a tip sonication process has been investigated. The thin-MWCNTs showed short lengths and many open tips after the tip sonication treatment. The field emission properties of the thin-MWCNT emitters were investigated. Field emission stability dramatically increased as the tip sonication time increased. In particular, field emission current at an acceleration condition was quite stable and showed no degradation for over 19 h after tip sonication treatment of 30 min. Tip sonication could effectively cut CNTs short and regulate the length of CNTs. Therefore, field emission stability was significantly improved during a long period of operation because many shortened thin-MWCNTs could participate in field emission after the treatment.
Nanotechnology 01/2010; 21(1):015704. · 3.98 Impact Factor
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ABSTRACT: We highlight the significance of multiwall carbon nanotube (MWCNT) shape on the electrical percolation. For rodlike MWCNTs, percolation threshold depends on aspect ratio (L/d). For random coil-like MWCNTs, the percolation threshold does not depend on L/d but depends on the shape factor of MWCNTs represented by the ratio between static bending persistence length and diameter. Surface resistivities of various MWCNTs converge into single curve when we plot their dimensionless surface concentration versus surface resistivity. The surface resistivity of MWCNT films decrease with increasing temperature, particularly at low concentrations, indicating the films can be used as a negative temperature coefficient thermistor.
Applied Physics Letters 09/2009; 95(13):134104-134104-3. · 3.84 Impact Factor
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ABSTRACT: We fabricated point emitters using a multiwalled carbon nanotube (MWCNT) yarn which was treated by ethylene glycol. The point emitter showed a very high emission current of 3.01 mA (current density of 1.1 x 10(8) A cm(-2)) and good emission stability of over 20 h. We attributed the excellent field emission properties to a large field enhancement factor caused by the large aspect ratio of the sharp tip of the point emitter and the tight bonding of neighboring MWCNTs due to the ethylene glycol treatment. We investigated the field enhancement factor according to the gap between the anode and the emitter tip at a macroscopic gap regime. The measured field enhancement factor of the MWCNT point emitter was in good agreement with theoretical models.
Nanotechnology 09/2009; 20(31):315201. · 3.98 Impact Factor
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ABSTRACT: Thin multi-walled carbon nanotubes (MWCNTs) were successfully synthesized by a rapid thermal chemical vapor deposition (RTCVD) method using a liquid catalyst. The growth of the thin MWCNTs was achieved by decomposition of C2H2 over Fe-Mo/MgO/citric acid directly at 700 degrees C for 30 min. Most thin MWCNTs, which had about 6 approximately 8 graphene layers, showed high purity (approximately 90%) and good crystallinity. Moreover, they showed homogenous morphology and uniform diameters. The average outer diameter of the thin MWCNTs was about 8 nm. The uniform diameter and good homogeneity of thin MWCNTs were mainly attributed to prevention of catalyst agglomeration at high temperature due to a short reaction time, and the high purity of thin MWCNTs was caused by suppression of the residual CNT growth after finishing a reaction in RTCVD process. Field emission properties of the thin MWCNTs were measured in a vacuum chamber at a pressure of less than 2 x 10(-7) Torr. The turn-on field was about 3.35 V/microm at the emission current density of 0.1 microA/cm2, and the emission current density was 2.5 mA/cm2 at an applied field of 6.7 V/microm. Particularly, the thin MWCNTs showed strong emission stability at emission current density of 0.8 mA/cm2 for 20 h.
Journal of Nanoscience and Nanotechnology 04/2009; 9(3):2148-54. · 1.56 Impact Factor
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ABSTRACT: Vertically aligned double-walled carbon nanotube (VA-DWCNT) arrays were synthesized by point-arc microwave plasma chemical vapor deposition on Cr/n-Si and SiO(2)/n-Si substrates. The outer tube diameters of VA-DWCNTs are in the range of 2.5-3.8 nm, and the average interlayer spacing is approximately 0.42 nm. The field emission properties of these VA-DWCNTs were studied. It was found that a VA-DWCNT array grown on a Cr/n-Si substrate had better field emission properties as compared with a VA-DWCNT array grown on a SiO(2)/n-Si substrate and randomly oriented DWCNTs, showing a turn-on field of about 0.85 V µm(-1) at the emission current density of 0.1 µA cm(-2) and a threshold field of 1.67 V µm(-1) at the emission current density of 1.0 mA cm(-2). The better field emission performance of the VA-DWCNT array was mainly attributed to the vertical alignment of DWCNTs on the Cr/n-Si substrate and the low contact resistance between CNTs and the Cr/n-Si substrate.
Nanotechnology 10/2008; 19(41):415703. · 3.98 Impact Factor
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ABSTRACT: We have investigated the electrical transports of double-walled carbon nanotube field effect transistors (DWNT-FETs) with modified contacts. The CNT/Au metal contacts of DWNT-FETs were modified with a self-assembled monolayer of 2-aminoethanethiol molecules. In ambient air, the contact-modified DWNT-FETs showed a decreased conductance in the p-channel (negative gate voltages) and an increased conductance in the n-channel (positive gate voltages), while the original device showed p-type transport. In a vacuum, the n-channel current in the contact-modified DWNT-FET started to rise. We observed a clear n-type transport in the high vacuum. Almost no changes in the gate threshold voltages were observed by means of the contact-modification with a self-assembled monolayer. While the semiconducting DWNT-FET showed a clear transition from a p-type to n-type transistor with contact modification, no apparent changes were observed in semi-metallic DWNT devices.
Journal of Nanoscience and Nanotechnology 10/2008; 8(9):4349-52. · 1.56 Impact Factor
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ABSTRACT: We have synthesized high-purity double-walled carbon nanotubes (DWCNTs) by an arc-discharge method in hydrogen ambient. The DWCNTs were synthesized using a mixture of Fe catalyst and FeS promoter. Without FeS promoter, we only obtained single-walled carbon nanotubes (SWCNTs). The synthesized DWCNTs had outer diameters in the range of 3.0−3.4 nm and an average interlayer distance of 0.38 nm between graphene layers. The FeS promoter played a key role for the DWCNT growth. The DWCNTs indicated high electronic density of states in the binding energy region between 3.88 and 13.23 eV below Fermi energy, indicating that the DWCNTs had alot of delocalized graphite σ and σ/π electrons. For field emission properties, the typical turn-on field of DWCNTs was about 3.0 V/μm at the emission current density of 0.1 μA/cm2, and the emission current density of DWCNTs was about 10 mA/cm2 at the applied field of 6.5 V/μm. It is considered that the higher current densities of DWCNTs were mainly attributed to the emitted delocalized graphite σ and σ/π state electrons of DWCNTs. Moreover, DWCNTs have higher emission stability than SWCNTs due to two neighboring graphene layers.
12/2007;
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ABSTRACT: Persistence length of multiwalled carbon nanotubes (MWCNTs) is studied using three-dimensional analysis of MWCNT individual particles, employing scanning electron microscopy with various view angles and a capillary viscometer. The root-mean-squared end-to-end distance of MWCNTs follows random-coil scaling in spite of their static bending points. The static bending persistence length (lsp) of the MWCNTs is estimated to be 271 nm. The intrinsic viscosity of the MWCNTs follows the wormlike coil model when the dynamic bending persistence length (lp) is replaced by the static bending persistence length (lsp).
11/2007;
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ABSTRACT: In summary, the SWCNT and DWCNT point emitters were fabricated by attaching CNT bundles onto the AFM tip using a dielectrophoresis method. We could find that the SWCNT point emitter indicated the better field emission performance compared with the DWCNT point emitter. We expect that the probe type CNT based point emitter can be used in the various applications such as x-ray sources and microwave amplifier tubes which require large emission current density.
Vacuum Nanoelectronics Conference, 2007. IVNC. IEEE 20th International; 08/2007
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ABSTRACT: Coaxial Zn/ZnS core/shell fibers were synthesized via a simple thermal evaporation process using ZnO and SnS as the source materials. The coaxial fibers have lengths of several tens to hundreds of micrometers and are composed of inner Zn fibers with diameters of 50−500 nm and outer ZnS shells with wall thicknesses of 50−400 nm. The product was carefully studied using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive spectroscopy. The growth of coaxial Zn/ZnS fibers involves the Sn-catalyzed growth of Zn fibers and then the formation of ZnS shells on them. The photoluminescence spectrum of the product studied at room temperature shows a strong orange emission at about 605 nm.
03/2007;
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ABSTRACT: High-purity double-walled carbon nanotubes (DWCNTs) were synthesized by catalytic decomposition of CH4 over Fe−Mo/MgO catalyst at 900 °C. After purification, the DWCNTs were annealed at 1300 °C in vacuum condition of 10-5 Torr. The DWCNTs before and after the high-temperature thermal annealing were characterized using scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and thermogravimetric analysis. It was observed that the annealed DWCNTs had enhanced crystallinity and reduced defects. In addition, the DWCNTs showed better field emission performance after high-temperature thermal annealing. The annealed DWCNTs indicated the turn-on field of about 1.16 V/μm at 0.1 μA/cm2 and the threshold field of about 2.12 V/μm at 1.0 mA/cm2. Moreover, the annealed DWCNTs exhibited better emission stability compared with the nonannealed DWCNTs. Such a remarkable improvement of field emission performance was attributed to the enhanced crystallinity and reduced defects of the annealed DWCNTs.
02/2007;
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ABSTRACT: The authors investigated the field emission from vertically well-aligned zinc oxide (ZnO) nanoneedles grown on the Au/Ti/n-Si (100) substrate using metal organic chemical vapor deposition. The turn-on field of ZnO nanoneedles was about 0.85 V/μm at the current density of 0.1 μA/cm2, and the emission current density of 1 mA/cm2 was achieved at the applied electric field of 5.0 V/μm. The low turn-on field of the ZnO nanoneedles was attributed to very sharp tip morphology, and the high emission current density was mainly caused by the formation of the stable Ohmic contact between the ZnO nanoneedles and Au film.
Applied Physics Letters 02/2007; 90(8):083107-083107-3. · 3.84 Impact Factor
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ABSTRACT: In situ potassium (K)-doped single-walled carbon nanotube (SWCNT) was synthesized using a hydrogen arc-discharge method. X-ray photoelectron spectroscopy analysis showed that the K-doped SWCNTs consisted of 0.12% K mass composition. The K-doped SWCNTs showed the lower turn-on electric field of 2.0 V /μ m at a current density of 10<sup>-9</sup> A / cm <sup>2</sup> and the higher emission current density of 3.0 mA / cm <sup>2</sup> at an applied field of 4.6 V /μ m compared with the undoped SWCNTs. The improved field emission performance of K-doped SWCNTs was mainly attributed to the decreased work function and the increased density of state near the Fermi energy.
Applied Physics Letters 02/2007; · 3.84 Impact Factor
