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ABSTRACT: Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter.
ACS Nano 03/2013; · 10.77 Impact Factor
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ABSTRACT: We propose a unique experimental technique in which isotopically labeled ethanol, e.g., 12CH3-13CH2-OH, is used to trace the carbon atoms during the formation of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition (CVD). The proportion of 13C is determined from Raman spectra of the obtained SWNTs, yielding the respective contribution of ethanol's two different carbon atoms to SWNT formation. Surprisingly, the carbon away from the hydroxyl group is preferably incorporated into the SWNT structure, and this preference is significantly affected by growth temperature, presence of secondary catalyst metal species such as Mo, and even by the substrate material. These experiments provide solid evidence confirming that the active carbon source is not limited to products of gas-phase decomposition such as ethylene and acetylene, but ethanol itself is arriving at and reacting with the metal catalyst particles. Furthermore, even the substrate or other catalytically inactive species directly influence the formation of SWNTs, possibly by changing the local environment around the catalyst or even the reaction pathway of SWNT formation. These unexpected effects, which are inaccessible by conventional techniques, paint a clearer picture regarding the decomposition and bond breaking process of the ethanol precursor during the entire CVD process and how this might influence the quality of the obtained SWNTs.
ACS Nano 03/2013; · 10.77 Impact Factor
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ABSTRACT: We demonstrate wide-range diameter modulation of vertically aligned single-walled carbon nanotubes (SWNTs) using a wet chemistry prepared catalyst. In order to ensure compatibility to electronic applications, the current minimum mean diameter of 2 nm for vertically aligned SWNTs is challenged. The mean diameter is decreased to about 1.4 nm by reducing Co catalyst concentrations to 1/100 or by increasing Mo catalyst concentrations by five times. We also propose a novel spectral analysis method that allows one to distinguish absorbance contributions from the upper, middle, and lower parts of a nanotube array. We use this method to quantitatively characterize the slight diameter change observed along the array height. On the basis of further investigation of the array and catalyst particles, we conclude that catalyst aggregation-rather than Ostwald ripening-dominates the growth of metal particles.
ACS Nano 07/2012; 6(8):7472-9. · 10.77 Impact Factor
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ABSTRACT: We present a systematic study of the effects of surfactants in the separation of single-walled carbon nanotubes (SWNTs) by
density gradient ultracentrifugation (DGU). Through analysis of the buoyant densities, layer positions, and optical absorbance
spectra of SWNT separation using the bile salt sodium deoxycholate (DOC) and the anionic salt sodium dodecyl sulfate (SDS),
we clarify the roles and interactions of these two surfactants in yielding different DGU outcomes. The separation mechanism
described here can also help in designing new DGU experiments by qualitatively predicting outcomes of different starting recipes,
improving the efficacy of DGU and simplifying post-DGU fractionation.
KeywordsSingle-walled carbon nanotubes–density gradient ultracentrifugation–sodium deoxycholate–sodium dodecyl sulfate
Nano Research 04/2012; 4(7):623-634. · 6.97 Impact Factor
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ABSTRACT: Field-effect transistors (FETs) have been fabricated using as-grown single-walled carbon nanotubes (SWNTs) for the channel
as well as both source and drain electrodes. The underlying Si substrate was employed as the back-gate electrode. Fabrication
consisted of patterned catalyst deposition by surface modification followed by dip-coating and synthesis of SWNTs by alcohol
chemical vapor deposition (CVD). The electrodes and channel were grown simultaneously in one CVD process. The resulting FETs
exhibited excellent performance, with an I
ON/I
OFF ratio of 106 and a maximum ON-state current (I
ON) exceeding 13 μA. The large I
ON is attributed to SWNT bundles connecting the SWNT channel with the SWNT electrodes. Bundling creates a large contact area,
which results in a small contact resistance despite the presence of Schottky barriers at metallic-semiconducting interfaces.
The approach described here demonstrates a significant step toward the realization of metal-free electronics.
KeywordsSingle-walled carbon nanotube–field-effect transistor–patterned synthesis–self-assembled monolayer–Schottky barrier–interfacial dipole
Nano Research 04/2012; 4(6):580-588. · 6.97 Impact Factor
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ABSTRACT: In this study, we systematically investigated the influence of catalyst preparation procedures on the mean diameter of single-walled carbon nanotubes (SWNTs) synthesized by the alcohol catalytic chemical vapor deposition (ACCVD) process. It was found that the SWNT diameter is dependent upon both reduction temperature and time, with lower reduction temperature and/or shorter reduction time resulting in smaller diameter SWNTs. The morphology of the SWNTs also changed from vertically aligned to randomly oriented when the reduction temperature was below 500 degrees C. We also found that introducing a small amount of water during the catalyst reduction stage significantly decreased the mean diameter of the SWNTs. Lastly, we report on the use of a new binary catalyst system in which rhodium was combined with cobalt. This new Co/Rh combination produced SWNTs of smaller diameter than the conventional Co/Mo catalyst.
Journal of Nanoscience and Nanotechnology 01/2012; 12(1):370-6. · 1.56 Impact Factor
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Applied Physics Letters. 01/2011; 99:093104.
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ABSTRACT: Adsorption and desorption of atomic hydrogen on single-walled carbon nanotubes were observed by photoluminescence spectroscopy. A satellite peak appeared at the lower energy side of the E11 photoluminescence emission peak after exposure to atomic hydrogen and then disappeared after annealing at 300 °C in vacuum. The energy difference between the satellite peak and E11 peak was 40-80 meV, depending on the tube diameter. The satellite peak was attributed to the triplet dark exciton that became optically active because of the effectively enhanced spin-orbit interaction induced by adsorbed hydrogen atoms.
Physical Review Letters 10/2010; 105(15):157403. · 7.37 Impact Factor
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ABSTRACT: The order of magnitude of Raman differential cross sections of radial breathing modes (RBMs) of individual carbon nanotubes is measured for 633 and 785 nm laser excitations. This is shown by both a calibration applied to previously published data from other authors at 785 nm and our own measurements of individual nanotubes at 633 nm excitation. We find typical values of differential cross sections of RBMs to be on the order of approximately 10(-22) cm(2)/sr for resonant nanotubes on a silicon substrate. This study therefore provides a rigorous quantification of the accepted view that Raman cross sections of carbon nanotubes are "huge".
ACS Nano 06/2010; 4(6):3466-70. · 10.77 Impact Factor
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ABSTRACT: We systematically investigate the catalytic properties of Al2O3 particles in the growth of carbon nanotubes (CNTs) in chemical vapor deposition (CVD). Compared with the conventional metal catalysts, the particle size of Al2O3 catalyst has no influence on the number of nanotube walls because only single-walled carbon nanotubes (SWCNTs) are produced. Interestingly, one large Al2O3 particle can usually nucleate several SWCNTs. However, statistical analysis reveals that the average growth rate of SWCNTs from Al2O3 catalyst is about 200 nm/min, which is much lower than that of CNTs grown from Fe catalyst under the same conditions. Additionally, a low gas pressure is found to be important for the growth of SWCNTs from Al2O3 catalyst. These results reflect that Al2O3 catalyst has different catalytic properties from the conventional metal catalysts, which will be helpful for the elucidation of the nanotube growth mechanism.
Journal of Nanoscience and Nanotechnology 06/2010; 10(6):4068-73. · 1.56 Impact Factor
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ABSTRACT: We have investigated the effects of atomic-scale surface morphology on the single-walled carbon nanotube (SWNT) growth by using atomic step-structure-controlled SiO2/Si substrates. SWNTs initially extended randomly on the surface, and when they became longer than 3 μm, they followed the atomic step morphology with a height of 0.6 nm. This indicates that SWNT extension along the surface is greatly affected by the surface morphology of the substrate. The present results also explain why the SWNT orientation on a normal SiO2/Si surface is random. SWNTs extend along the surface morphology based on the roughness of the SiO2 surface.
Applied Physics Letters 03/2010; 96(10):103102-103102-3. · 3.84 Impact Factor
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ABSTRACT: We present a simple method to transfer large-area random single-walled carbon nanotube (SWCNT) films grown on SiO(2) substrates onto another surface through a simple contact printing process. The transferred random SWCNT films can be assembled into highly ordered, dense regular arrays with high uniformity and reproducibility by sliding the growth substrate during the transfer process. The position of the transferred SWCNT film can be controlled by predefined patterns on the receiver substrates. The process is compatible with a variety of substrates, and even metal meshes for transmission electron microscopy (TEM) can be used as receiver substrates. Thus, suspended web-like SWCNT networks and aligned SWCNT arrays can be formed over the grids of TEM meshes, so that the structures of the transferred SWCNTs can be directly observed by TEM. This simple technique can be used to controllably transfer SWCNTs for property studies, for the fabrication of devices, or even as support films for TEM meshes.
ACS Nano 02/2010; 4(2):933-8. · 10.77 Impact Factor
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ABSTRACT: High-density horizontally aligned single-walled carbon nanotubes (SWCNTs) have been grown by annealing Fe catalyst in air and optimizing catalyst thickness in the gas flow process. The aligned SWCNT density reaches >60 nanotubes per 100 microm and the length can be a few millimeters. Interestingly, the growth of aligned SWCNT arrays can be extended from the catalyst substrate to a downstream substrate across the gap between them by gas flow when the two substrates are put close to each other, and thus an aligned SWCNT array has been achieved on an isolated clean substrate.
Nanotechnology 09/2009; 20(34):345604. · 3.98 Impact Factor
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ABSTRACT: A single-wall carbon nanotube (SWNT) is a rolled-up graphene sheet, and all the carbon atoms are in the surface layer. Thus, when lying on the substrate surface, or forming a bundle, SWNTs suffer from strong interaction with the substrate atoms or other nanotubes. However, when a SWNT is suspended between mesa structures, the interactions with the substrate and other nanotubes are minimized, which is important for extracting the intrinsic properties of nanotubes. In this paper, we review the synthesis of suspended SWNTs between mesa structures and their optical properties. The first part focuses on the growth and characterization of suspended SWNTs, the mechanisms of suspended structure formation and control of the structures (individual or bundled). The second part describes photoluminescence and Raman spectroscopy of individual and ensemble SWNTs. Highly enhanced photoluminescence and Raman signals enable us to examine the structure of individual SWNTs. The third part describes device applications of suspended SWNTs.
Reports on Progress in Physics 05/2009; 72(6):066502. · 14.72 Impact Factor
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ABSTRACT: We demonstrate that a tweezers tip can be directly used to manipulate single-walled carbon nanotube (SWCNT) films and individual SWCNTs (or bundles). Specifically, we can align, bend, and even cut SWCNTs on the substrates where they are grown. With this manipulation technique, we are able to control the position, direction, and length of SWCNTs and fabricate complex patterns. Because of the large size of the tweezers tip, long SWCNTs (hundreds of micrometers in length) are found to be critical for the realization of such manipulations. This finding would contribute to the property studies on nanotubes and the fabrication of nanodevices.
Nanotechnology 11/2008; 19(44):445716. · 3.98 Impact Factor
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ABSTRACT: The photoluminescence (PL) spectra of suspended single-walled carbon nanotubes (SWNTs) were measured in an ethanol gas atmosphere. When the gas pressure was decreased, the PL peaks were initially blue-shifted to a small extent before a rapid blue-shift took place at a transition pressure that depended on the temperature and diameter of the SWNT being measured. This pressure dependence is due to the adsorption of ethanol molecules on the SWNT surface. The optical transition energies measured below the transition pressure are intrinsic to the SWNT.
Nano Letters 09/2008; 8(10):3097-101. · 13.20 Impact Factor
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ABSTRACT: Fe, Co, and Ni are catalytically effective for growing single-walled carbon nanotubes (SWNTs). On substrates, however, Ni tends to yield only multi-walled carbon nanotubes. Because enhanced surface diffusion at the elevated growth temperature required for deposition might cause coarsening of Ni catalyst nanoparticles, adjusting the nominal Ni thickness should be crucial for controlling the particle size. Using our previously developed combinatorial method, we prepared a thickness profile of Ni on a quartz glass (SiO 2) substrate and found that Ni nanoparticles catalyzed the growth of SWNTs by chemical vapor deposition only when nominal Ni thickness was in the monolayer range.
Chemical Physics Letters 08/2006; 428(4-6):381-385. · 2.34 Impact Factor
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ABSTRACT: Recent advances in high-purity and high-yield catalytic chemical vapor deposition (CVD) generation of single-walled carbon nanotubes (SWNTs) from alcohol are comprehensively presented and discussed on the basis of results obtained from both experimental and numerical investigations. We have uniquely adopted alcohol as a carbon feedstock, and this has resulted in high-quality, low-temperature synthesis of SWNTs. This technique can produce SWNTs even at a very low temperature of 550 degrees C, which is about 300 degrees C lower than the conventional CVD methods in which methane or acetylene is typically used. We demonstrate the excellence of the proposed alcohol catalytic CVD method for high-yield production of SWNTs when Fe-Co on USY-zeolite powder was used as a catalyst. At optimum CVD conditions, a SWNT yield of more than 40 wt % was achieved over the weight of the catalytic powder within the reaction time of 120 min. In addition to the advantages for mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with the newly developed liquid-based "dip-coat" technique to mount catalytic metals on the surface of substrates. This method allows easy and costless loading of catalytic metals without the need for any support or underlayer materials that were usually required in previous studies for the generation of a sufficient quantity of SWNTs on an Si surface. Finally, the result of molecular dynamics simulation for the SWNT growth process is presented to obtain a fundamental insight into the initial growth mechanism on the catalytic particles.
Journal of Nanoscience and Nanotechnology 05/2004; 4(4):360-7. · 1.56 Impact Factor
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ABSTRACT: Single-walled carbon nanotubes (SWNTs), synthesized by a catalytic decomposition of alcohol (alcohol CVD method, ACCVD), are compared with high-pressure CO (HiPco) SWNT samples through optical spectroscopic measurements such as resonant Raman scattering, optical absorption and near infrared fluorescence. By the ACCVD method, SWNTs were synthesized either on zeolite catalyst-support particles or directly on the surface of a quartz substrate; in the latter case, a simple dip-coat technique was employed for mounting the metal catalyst. Specific morphological characteristics of as-grown SWNTs generated on zeolite support are presented using SEM and TEM, revealing that the SWNTs produced by the proposed method possess the significant quality of being almost free from amorphous carbons or metal particle impurities. The quality and diameter distribution of SWNTs were investigated and discussed through the results of Raman scattering and optical absorption. The average diameter was slightly smaller for SWNTs grown on zeolite particles than for HiPco SWNTs. Finally, fluorescent emission spectra from isolated SWNTs in an aqueous surfactant suspension were measured for various excitation wavelengths to determine the structural (n,m) distribution of the SWNTs. The narrower chirality distribution for ACCVD SWNTs grown on zeolite compared with HiPco SWNTs was demonstrated.
New Journal of Physics 10/2003; 5(1):149. · 4.18 Impact Factor
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ABSTRACT: Single-walled carbon nanotubes with narrow diameter-distributions were synthesized by the catalytic CVD technique usingfngv84/Fv C 60 or C 70 , as the carbon source. Fe/Co bimetal particles supported with zeolite powder were exposed to fv8xFxvD vapor in a heated quartz tubefbevFx7 With a precise controlof ftrolv/ vapor pressure, macroscopic amountsof SWNTs with relatively high quality were generated. The diameter distributionsof SWNTs, estimated by an analysisof resonance Raman scattering using excitation wavelengthsof 633, 514.5 and 488 nm, ranged between 0.8 and 1.1 nm. The narrownessof this distribution is ascribed to thefev/Wx.W of nanotube caps due to fovxBN44 collisions.
09/2003;
