Michael J. Bronikowski

Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide

Abstract Single-walled carbon nanotubes (SWNTs) have been produced in a gas-phase catalytic process. Catalysts for SWNT growth form in situ by thermal decomposition of iron pentacarbonyl in a heated flow of carbon monoxide at pressures of 1–10 atm and temperatures of 800–1200°C. The SWNT yield and diameter distribution can be varied by controlling the process parameters, and SWNTs as small as 0.7 nm in diameter, the same as that of a C60 molecule, have been generated. This process shows great promise for bulk production of carbon nanotubes.

Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study

We have demonstrated large-scale production (10 g/day) of high-purity carbon single-walled nanotubes (SWNTs) using a gas-phase chemical-vapor-deposition process we call the HiPco process. SWNTs grow in high-pressure (30–50 atm), high-temperature (900–1100 °C) flowing CO on catalytic clusters of iron. The clusters are formed in situ: Fe is added to the gas flow in the form of Fe(CO)5. Upon heating, the Fe(CO)5 decomposes and the iron atoms condense into clusters. These clusters serve as catalytic particles upon which SWNT nucleate and grow (in the gas phase) via CO disproportionation: CO+CO⇒CO2+C(SWNT). SWNT material of up to 97 mol % purity has been produced at rates of up to 450 mg/h. The HiPco process has been studied and optimized with respect to a number of process parameters including temperature, pressure, and catalyst concentration. The behavior of the SWNT yield with respect to various parameters sheds light on the processes that currently limit SWNT production, and suggests ways that the producti...

Dissolution of small diameter single-wall carbon nanotubes in organic solvents?

The solubility of small diameter single-wall carbon nanotubes in several organic solvents is described, and characterization in 1,2-dichlorobenzene is reported.

Structural characterization and diameter-dependent oxidative stability of single wall carbon nanotubes synthesized by the catalytic decomposition of CO

Low T oxidation followed by brief sonication in hot HCl has been optimized to remove Fe catalyst residues from HiPco material with minimal loss of tubes. One pass reduces Fe from 6–10 to 0.6–1.6 at.% with 60% yield, most of the loss being Fe. Raman scattering reveals a broad diameter distribution with a mean of 1.0–1.1 nm. After purification we find excellent correlation between oxidation temperature and Raman spectra whereby the oxidation rate varies inversely with tube diameter. This can be explained by the larger strain associated with greater curvature of small tubes.

An atomically resolved scanning tunneling microscopy study of the thermal decomposition of disilane on Si(001)

Abstract Scanning tunneling microscopy (STM) has been used to study the adsorption and thermal dissociation of disilane (Si 2 H 6 ) on the Si(001) surface. At low coverage, disilane adsorbs dissociatively to produce adsorbed SiH 3 groups which are randomly distributed on the surface. STM images show that these SiH 3 groups spontaneously dissociate into SiH 2 and H within several minutes of adsorption at room temperature, and images depicting individual dissociation events have been obtained. Thermal annealing leads to diffusion and dissociation of the SiH 2 groups through an intermediate which consists of a hydrogenated dimer which has its dimer axis parallel to those of the underlying substrate, instead of rotated. The structure and chemical identity of the intermediates are discussed in terms of the overall dissociation mechanism. Diffusion of hydrogen atoms leads to the formation of segregated surface phase of clean Si and the Si(001)-(2 × 1) “monohydride” structure, and bias-dependent STM imaging permits these species to be identified on a dimer-by-dimer basis. At low temperatures the monohydride distribution is non-statistical, showing a preference for flat terraces over small epitaxial islands. The diffusion, desorption, segregation of hydrogen and its role in blocking diffusion of silicon atoms is also discussed.

Adsorption and dissociation of disilane on Si(001) studied by STM

Abstract The surface-adsorbed fragments resulting from the room-temperature adsorption and dissociation of disilane (Si 2 H 6 ) on Si(001) are observed and identified using scanning tunneling microscopy (STM). The predominant fragments are H and SiH 2 , which are identified by the symmetries of their binding sites on the surface. H atoms often bind near single or double dimer vacancy defects. We find that adsorbed H atoms induce buckling of the dimer rows on the Si(001) surface, while SiH 2 groups do not. This difference is ascribed to differences in the electronic structure of the two surface-bound species. No systematic correlation between the positions of the SiH 2 groups is evident, which indicates that the fragments of a single disilane molecule are not localized in a small region. This fact suggests that at least some of the molecular fragments from disilane dissociation are mobile on the Si(001) surface at room temperature. Further decomposition of the SiH 2 fragments can be induced by annealing, which produces surface structures similar to those seen in molecular-beam epitaxial growth of silicon: small asymmetric islands form with low disilane coverages, whereas higher coverages give multilayer island growth.

Atomically resolved scanning tunneling microscopy study of the adsorption and dissociation of methylchloride on Si(001)

The room‐temperature adsorption and dissociation of methylchloride (CH3Cl) on Si(001) using scanning tunneling microscopy has been studied. Fragments are identified by the symmetry of their binding on the underlying Si(001) lattice. Methylchloride adsorbs dissociatively at room temperature, yielding a methyl group and a chlorine atom bound to the surface. Chlorine atoms bind in a variety of geometries, the most common being two chlorine atoms bonded to a single silicon dimer (silicon monochloride dimers). When the surface is heated to 150 °C, all Cl rearranges to this monochloride‐type bonding configuration, and images reveal the beginnings of monochloride island formation. The Cl:CH3 ratio on the surface is found to be approximately 2:1, implying that not all methyl groups are bound to the surface in the reaction.

Growth of carbon nanotube bundle arrays on silicon surfaces

The growth on silicon substrates of arrayed bundles of multiwalled carbon nanotubes (CNTs) by metal catalyzed chemical vapor deposition of carbon from ethylene has been characterized and optimized. We find that, while CNTs can grow on bare Si substrates, the growth is substantially more reproducible if a thin (∼3nm) barrier layer of aluminum oxide is used between the Si surface and iron catalyst. Optimum Fe thickness and growth temperature are 3.0 nm and 650 °C, respectively. We find that the CNT length increases linearly with time at a rate of 3–4μm∕min for up to 2 h of CNT growth, after which the growth ceases. The length of the resulting CNT can thus be controlled up to a maximum length of ∼500μm. Such control over CNT bundle length will be crucial in the incorporation of these bundle arrays into high-intensity electron field emission devices.

Direct dimer‐by‐dimer identification of clean and monohydride dimers on the Si(001) surface by scanning tunneling microscopy

Atomic resolution images of clean Si(001)‐(2×1) and the monohydride phase, Si(001)‐(2×1)H were investigated using scanning tunneling microscopy at various sample‐tip bias voltages. At a sample‐tip bias of −1.9 V, each dimer of the monohydride phase shows two protrusions 3.3 A apart separated by a minimum 0.12 A deep, while clean dimers show a single protrusion per unit cell. Monohydride dimers appear lower than ‘‘clean’’ dimers, with apparent height differences ranging from 1.9 A at −1.6 V to 0.65 A at −3.0 V sample bias. An analysis of the apparent height and spatial distribution of tunneling current within each dimer can be used to unambiguously discriminate between clean dimers, monohydride dimers, and vacancy defects. This methodology is applied to study the distribution of hydrogen on Si(001) surfaces during chemical vapor deposition using disilane, revealing segregation of the monohydride into nearly isotropic islands.

The chemistry of gallium deposition on Si(001) from trimethylgallium : an atomically resolved STM study

Abstract The chemistry of trimethylgallium (TMG, Ga(CH 3 ) 3 ) and its dissociation fragments on the Si(001) surface has been studied using scanning tunneling microscopy (STM). The products of TMG dissociation are identified by their bonding location with respect to the underlying Si(001) lattice, by bias-dependent imaging and from detailed counting statistics. TMG dissociates at room temperature, yielding a methyl group and a dimethylgallium fragment which bind to the surface. The Ga(CH 3 ) 2 groups produced by TMG dissociation are somewhat mobile at room temperature but are bonded more strongly near surface defects. Further dissociation yields gallium atoms on the surface, but no additional methyl groups. It is proposed that this second stage of reaction involves an intramolecular reaction to produce ethane, which desorbs into the gas phase, and gallium atoms. The gallium atoms are observed to arrange into single rows of gallium dimers which bind epitaxially on the Si surface. Heating the surface to 150°C completely decomposes the DMG fragments, yielding Ga atoms and CH 3 groups.