1. Comment on "Single-Crystals of Single-Walled Carbon Nanotubes Formed by Self-Assembly"
M. F. Chisholm, Y. Wang, A. R. Lupini, G. Eres, A. A. Puretzky, B. Brinson, A. V. Melechko, D. B. Geohegan, H. Cui, M. P. Johnson, S. J. Pennycook, D. H. Lowndes, S. Arepalli, C. Kittrell, S. Sivaram, M. Kim, G. Lavin, J. Kono, R. Hauge, and R. E. Smalley
Science 300, 1236b (2003)   Download PDF file (308 kB)

Schlittler et al. (1) reported the production of single crystals of single-walled carbon nanotubes (SWCNTs) by the thermolysis of nanopatterned structures of alternating layers of C60 and nickel. Electron diffraction, high-resolution phase contrast imaging, and electron energy loss spectroscopy (EELS) were used to characterize the resulting crystals. In this comment, we report the reproduction of their experimental results; however, we disagree with their interpretation of the data. We suggest that the crystals formed in our experiments consist not of SWCNTs, but rather of calcium molybdenum oxide. ©2003 The American Association for the Advancement of Science.

2. Nucleation of Single-Walled Carbon Nanotubes
X. Fan, R. Buczko, A. A. Puretzky, D. B. Geohegan, J. Y. Howe, S. T. Pantelides, and S. J. Pennycook
Phys. Rev. Lett. 90, 145501 (2003)   Download PDF file (424 kB)

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene. ©2003 The American Physical Society

3. Growth behavior of carbon nanotubes on multilayered metal catalyst film in chemical vapor deposition
H. Cui, G. Eres, J. Y. Howe, A. Puretkzy, M. Varela, D. B. Geohegan and D. H. Lowndes

Chem. Phys. Lett. 374, 222 (2003).   Download PDF file (431 kB)

The temperature and time dependences of carbon nanotube (CNT) growth by chemical vapor deposition are studied using a multilayered Al/Fe/Mo catalyst on silicon substrates. Within the 600–1100 °C temperature range of these studies, narrower temperature ranges were determined for the growth of distinct types of aligned multi-walled CNTs and single-walled CNTs by using high-resolution transmission electron microscopy and Raman spectroscopy. At 900 °C, in contrast to earlier work, double-walled CNTs are found more abundant than single-walled CNTs. Defects also are found to accumulate faster than the ordered graphitic structure if the growth of CNTs is extended to long durations. (C) 2002 Elsevier Science B.V. All rights reserved.

4. Operation of individual integrally gated carbon nanotube field emitter cells
M. A. Guillorn, M. D. Hale, V. I. Merkulov, M. L. Simpson, G. Y. Eres, H. Cui, A. A. Puretzky , and D. B. Geohegan
Appl. Phys. Lett. 81, 2860 (2002)   Download PDF file (200 kB)

In this work, we examine the operation of individual field emitter cells contained in a field emitter array composed of integrally gated multiwalled carbon nanotube (MWNT)-based field emission cathodes. These devices were found to behave in a manner consistent with a multiple emission site model of Fowler-Nordheim field emission. These results show considerable variation in the operational characteristics of cells contained within the same array and indicate that data obtained from arrays of cells are not necessarily indicative of individual cell performance. (C) 2002 American Institute of Physics.

5. The electrodeposition of metal at metal/carbon nanotube junctions
D. W. Austin, A. A. Puretzky, D. B. Geohegan, P. F. Britt, M. A. Guillorn, and M. L. Simpson
Chem. Phys. Lett. 361, 525 (2002)   Download PDF file (424 kB)

We deposited a semiconducting single-walled carbon nanotube on Pd electrodes, and the initial charge transport measurements showed the usual large contact resistance between the electrodes and the nanotube. We electroplated Au over the electrodes with no obvious deposition of An along the sidewalls of the nanotube between the electrodes. Post deposition charge transport measurements indicated more than a factor of six decrease in the electrode/nanotube contact resistance, yet the semiconducting behavior of the nanotube was maintained. A significant difference in the post deposition I-V characteristics may be explained by an electronic or mechanical modification of the nanotube/electrode junction. (C) 2002 Elsevier Science B.V. All rights reserved.

6. Synthesis and characterization of single-wall carbon nanotube–amorphous diamond thin-film composites
H. Schittenhelm, D. B. Geohegan, G. E. Jellison, A. A. Puretzky, M. J. Lance, P. F. Britt

Appl. Phys. Lett. 81, 2097, (2002). Download PDF file (256 KB)

Thin-film single-wall carbon nanotube (SWNT) composites synthesized by pulsed laser deposition (PLD) are reported. Ultrahard, transparent, pure-carbon, electrically insulating, amorphous diamond thin films were deposited by PLD as scratch-resistant, encapsulating matrices for disperse, electrically conductive mats of SWNT bundles. In situ resistance measurements of the mats during PLD, as well as ex situ Raman spectroscopy, current–voltage measurements, spectroscopic ellipsometry, and field-emission scanning electron microscopy, are used to understand the interaction between the SWNT and the highly energetic (~100 eV) carbon species responsible for the formation of the amorphous diamond thin film. The results indicate that a large fraction of SWNT within the bundles survive the energetic bombardment from the PLD plume, preserving the metallic behavior of the interconnected nanotube mat, although with higher resistance. Amorphous diamond film thicknesses of only 50 nm protect the SWNT against wear, providing scratch hardness up to 25 GPa in an optically transmissive, all-carbon thin-film composite. ©2002 American Institute of Physics.
The American Physical Society

7. Investigations of single-wall carbon nanotube growth by time-restricted laser vaporization
Alex A. Puretzky, Henrik Schittenhelm, Xudong Fan, Michael J. Lance, Larry F. Allard, Jr., and David B. Geohegan

Phys. Rev. B 65, 245425 (2002) Download PDF file (1000 KB)

The growth times of single-wall carbon nanotubes (SWNT's) within a high-temperature laser-vaporization (LV) reactor were measured and adjusted through in situ imaging of the plume of laser-ablated material using Rayleigh-scattered light induced by time-delayed, 308-nm laser pulses. Short SWNT's were synthesized by restricting the growth time to less than 20 ms for ambient growth temperatures of 760–1100 °C. Statistical analysis of transmission electron microscope photographs indicated most-probable lengths of 35–77 nm for these conditions. Raman spectra (E_ex = 1.96 and 2.41 eV) of the short nanotubes indicate that they are well-formed SWNT's. The temperature of the particles in the vortex-ring-shaped plume during its thermalization to the oven temperature was estimated by collecting its blackbody emission spectra at different spatial positions inside the oven and fitting them to Planck's law. These data, along with detailed oven temperature profiles, were used to deduce a complete picture of the time spent by the plume at high growth temperatures (760–1100 °C). The upper and lower limits of the growth rates of SWNT's were estimated as 0.6 and 5.1 µm/s for the typical nanosecond Nd:YAG laser-vaporization conditions used in this study. These measurements permit the completion of a general picture of SWNT growth by LV based on imaging, spectroscopy, and pyrometry of ejected material at different times after ablation, which confirms our previous measurements that the majority of SWNT growth occurs at times greater than 20 ms after LV by the conversion of condensed phase carbon. The American Physical Society

Single-wall carbon nanotubes (SWNT) were grown to micron lengths by laser-annealing nanoparticulate soot containing short (~50 nm long) nanotube "seeds." The "seeded" nanoparticulate soot was produced by restricting the time spent by an ablation plume inside an 800 °C oven following laser vaporization of a C–Ni–Co target. The soot collected from the laser vaporization apparatus was placed inside graphite crucibles under argon, and heated by a CO₂ laser. In situ pyrometry was used to estimate the sample temperature. Length distributions of SWNT bundles in the unannealed and annealed samples were measured by transmission electron microscopy and field emission scanning electron microscopy. Annealing treatments exceeding 1600 °C produced no increase in nanotube length, while lower temperatures in the 1000–1300 °C range were optimal for growth. These experiments indicate that SWNT grow by the conversion of condensed phase nanomaterial during annealing, a similar mechanism to that proposed for growth during normal laser–vaporization production. ©2001 American Institute of Physics.

The key spatial and temporal scales for single-wall carbon nanotube (SWNT) synthesis by laser vaporization at high temperatures are investigated with laser-induced luminescence imaging and spectroscopy. Graphite/(Ni, Co) targets are ablated under typical synthesis conditions with a Nd:YAG laser at 1000 °C in a 2-in. quartz tube reactor in flowing 500-Torr Ar. The plume of ejected material is followed for several seconds after ablation using combined imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. The ablation plume expands in stages during the first 200 µs after ablation and displays a self-focusing behavior. Interaction of the plume with the background gas forms a vortex ring which segregates and confines the vaporized material within a \sim1-cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 µs) and cobalt (2 ms) at 1000 °C. This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled (\sim100 nm) length were grown and the first estimate of a growth rate on single laser shots (0.2 µm/s) was obtained. ©2000 Springer-Verlag



Online publication: February 8, 2000 . ©2000 Springer-Verlag

PACS: 42.62.Fi; 52.70.Kz; 81.05.Tp

The synthesis of single-wall carbon nanotubes by Nd:YAG laser vaporization of a graphite/(Ni, Co) target is investigated by laser-induced luminescence imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters at 1000 °C in flowing 500 Torr Ar. These laser-induced emission images under typical synthesis conditions show that the plume of vaporized material is segregated and confined within a vortex ring which maintains a ~1 cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 µs) and cobalt (2 ms). This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled length were grown at an estimated rate of 0.2 µm/s. ©2000 American Institute of Physics.

PII: S0003-6951(00)02102-1
PACS: 61.48.+c, 78.66.Tr, 81.05.Tp, 78.55.Hx, 78.47.+p

PII: S0031-9007(99)09662-3
PACS: 79.20.Ds, 47.70.Nd, 82.80.Ms, 87.80.

The gas-phase growth and transport of nanoparticles are characterized at the low background oxygen pressures used for pulsed laser deposition of high-T_c Y₁Ba₂Cu₃O_{7 – d} superconducting films. Onset times and pressures for gas-phase nanoparticle formation were determined by intensified charge-coupled device imaging and optical spectroscopy of laser-induced fluorescence from diatomic oxides and Rayleigh scattering from gas-suspended nanoparticles. Nanoparticles are detected for oxygen pressures above 175 mTorr at room temperature, with growth continuing during seconds within the cloud of stopped vapor near the heater surface. Elevated heater temperatures create background density gradients which result in reduced resistance to the initial plume expansion. The temperature gradient also moves nanoparticles away from the heater surface as they grow, effectively limiting the time and spatial confinement necessary for continued growth or aggregation, and inhibiting deposition by thermophoresis. © 1999 American Institute of Physics.

The dynamics of nanoparticle formation, transport, and deposition by pulsed laser ablation of c-Si into 1?10 Torr He and Ar gases are revealed by imaging laser-induced photoluminescence and Rayleigh-scattered light from gas-suspended 1?10 nm SiO_x particles. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles, while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. Nanoparticles unambiguously formed in the gas phase were collected on transmission electron microscope grids for Z-contrast imaging and electron energy loss spectroscopy analysis. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described. © 1998 American Institute of Physics.

Time-resolved photoluminescence (PL) spectra are reported for gas-suspended 1-10 nm diameter SiO_x particles formed by laser ablation of Si into 1-10 Torr He and Ar. Three spectral bands (1.8, 2.5 and 3.2 eV) similar to PL from oxidized porous silicon were measured, but with a pronounced vibronic structure. Particle size and composition were determined with Z-contrast transmission electron microscopy imaging and high resolution electron energy loss spectroscopy linescans of individual nanoparticles. Maximized violet (3.2 eV) PL from the gas-suspended nanoparticles was correlated with an ex situ SiO_1.4 overall particle stoichiometry. Cryogenically-collected gas-suspended nanoparticles produced web-like-aggregate films exhibiting very weak PL. Standard anneals restored strong PL bands without vibronic structure, but otherwise in agreement with the PL measured from the gas-suspended nanoparticles. © 1998 American Institute of Physics.
 

The dynamics of laser-ablated yttrium plume propagation through background argon have been investigated with fast time- and spatially-resolved plasma diagnostics in order to characterize a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). During expansion into low-pressure background gases, the ion flux in the laser ablation plasma plume is observed to split into fast and slow components over a limited range of distances including those typically utilized for PLD. Optical absorption and emission spectroscopy are employed to simultaneously identify populations of both excited and ground states of Y and Y ⁺ . These are correlated with intensified-CCD (ICCD) photographs of visible plume luminescence and ion fluxes recorded with fast ion probes. These measurements indicate that plume-splitting in background gases is consistent with scattering of target constituents by ambient gas atoms. The momentum transfer from these collisions produces a transition from the initial, ``vacuum'' velocity distribution into a velocity distribution which is significantly slowed in accordance with shock or drag propagation models. © 1995 American Institute of Physics.
 

A comparative study of ArF- and KrF-laser generated carbon plasmas has been performed under PLD conditions of amorphous diamond-like carbon (DLC) films. Gated-ICCD species-resolved imaging, luminescence spectroscopy and ion probe diagnostics have revealed distinct differences between the carbon plumes generated by ArF- and KrF-lasers. KrF-laser (6.7 J/cm) irradiation produces a less energetic carbon plasma containing larger amounts of luminescent C₂ compared with ArF-laser ablation at the same energy fluence. Spectroscopic ellipsometry and EELS analysis of the DLC films deposited on Si 100 and NaCl substrates were utilized to characterize the high quality ArF- and KrF-laser deposited films (up to 84% of sp bonded carbon for 7 J/cm ArF-laser DLC film). The more energetic and highly-atomized ArF-laser carbon plasma appears to be responsible for the better diamond-like properties.
 

Combined diagnostic measurements are employed to characterize the penetration of energetic ablation plumes through background gases during a key transitional regime in which the ion flux is observed (with fast ion probes) to split into distinct fast and slowed components. This apparently general phenomenon occurs over a limited range of distances at ambient pressures typically used for PLD (as reported for YBCO ablation into O₂) and may be important to film growth by PLD because a 'fast' component of ions can arrive at the probe (or substrate) with little or no delay compared to propagation in vacuum (i.e., high 10-100 eV kinetic energies). At longer distances, this 'fast' component is completely attenuated, and only slowed distributions of ions are observed. Interestingly, this 'fast' component is easily overlooked in imaging studies because the bright plume luminescence occurs in the slowed distribution. Time- and spatially-resolved optical absorption and emission spectroscopy are applied to experimentally determine the composition of the 'fast' and 'slow' propagating plume components for a single-component target ablation (yttrium) into an inert gas (argon) for correlation with quantitative imaging and ion probe measurements. The yttrium/argon system was chosen because optical absorption spectroscopy of both Y and Y was simultaneously possible and the inert nature of argon. Experimental results for several other systems, including Si/Ar, Si/He, YBCO/O₂ are presented to illustrate variations in scattering mechanisms. Species-resolved imaging of YO and Ba is presented for the YBCO/O₂ system to illustrate the similarities and differences in the spatial regions of observed luminescence. These measurements confirm that, in addition to the bright significantly-slowed front which has been described by shock or drag propagation models, a fast-component of target material is transmitted to extended distances for some ambient pressures with near-initial velocities.