Jeremy Joseph Jackson

Pulsed Growth of Vertically Aligned Nanotube Arrays with Variable Density

The density of vertically aligned carbon nanotube arrays is shown to vary significantly during normal growth by chemical vapor deposition and respond rapidly to changes in feedstock flux. Pulsing the feedstock gas to repeatedly stop and start nanotube growth is shown to induce density variations up to a factor of 1.6 within ca. 1-2 μm long layers, allowing the synthesis of new array architectures with distinct regions of controllable length and density variation. Z-Contrast scanning transmission electron microscopy of corresponding sections of the arrays is used to provide unambiguous measurements of these density variations. Time-resolved optical reflectivity measurements of the height and optical extinction coefficient of the growing arrays are shown to provide a real-time diagnostic of both array density and growth kinetics.

Flux-Dependent Growth Kinetics and Diameter Selectivity in Single-Wall Carbon Nanotube Arrays

The nucleation and growth kinetics of single-wall carbon nanotubes in aligned arrays have been measured using fast pulses of acetylene and in situ optical diagnostics in conjunction with low pressure chemical vapor deposition (CVD). Increasing the acetylene partial pressure is shown to decrease nucleation times by three orders of magnitude, permitting aligned nanotube arrays to nucleate and grow to micrometers lengths within single gas pulses at high (up to 7 μm/s) peak growth rates and short ∼0.5 s times. Low-frequency Raman scattering (>10 cm(-1)) and transmission electron microscopy measurements show that increasing the feedstock flux in both continuous- and pulsed-CVD shifts the product distribution to large single-wall carbon nanotube diameters >2.5 nm. Sufficiently high acetylene partial pressures in pulsed-CVD appear to temporarily terminate the growth of the fastest-growing, small-diameter nanotubes by overcoating the more catalytically active, smaller catalyst nanoparticles within the ensemble with non-nanotube carbon in agreement with a growth model. The results indicate that subsets of catalyst nanoparticle ensembles nucleate, grow, and terminate growth within different flux ranges according to their catalytic activity.

Incremental growth of short SWNT arrays by pulsed chemical vapor deposition.

Very short arrays of continuous single-wall carbon nanotubes (SWNTs) are grown incrementally in steps as small as 25 nm using pulsed chemical vapor deposition (CVD). In-situ optical extinction measurements indicate that over 98% of the nanotubes reinitiate growth on successive gas pulses, and high-resolution transmission electron microscopy (HR-TEM) images show that the SWNTs do not exhibit segments, caps, or noticeable sidewall defects resulting from repeatedly stopping and restarting growth. Time-resolved laser reflectivity (3-ms temporal resolution) is used to record the nucleation and growth kinetics for each fast (0.2 s) gas pulse and to measure the height increase of the array in situ, providing a method to incrementally grow short nanotube arrays to precise heights. Derivatives of the optical reflectivity signal reveal distinct temporal signatures for both nucleation and growth kinetics, with their amplitude ratio on the first gas pulse serving as a good predictor for the evolution of the growth of the nanotube ensemble into a coordinated array. Incremental growth by pulsed CVD is interpreted in the context of autocatalytic kinetic models as a special processing window in which a sufficiently high flux of feedstock gas drives the nucleation and rapid growth phases of a catalyst nanoparticle ensemble to occur within the temporal period of the gas pulse, but without inducing growth termination.

Laser Interactions for the Synthesis and In Situ Diagnostics of Nanomaterials

Laser interactions have traditionally been at the center of nanomaterials science, providing highly nonequilibrium growth conditions to enable the synthesis of novel new nanoparticles, nanotubes, and nanowires with metastable phases. Simultaneously, lasers provide unique opportunities for the remote characterization of nanomaterial size, structure, and composition through tunable laser spectroscopy, scattering, and imaging. Pulsed lasers offer the opportunity, therefore, to supply the required energy and excitation to both control and understand the growth processes of nanomaterials, providing valuable views of the typically nonequilibrium growth kinetics and intermediates involved. Here we illustrate the key challenges and progress in laser interactions for the synthesis and in situ diagnostics of nanomaterials through recent examples involving primarily carbon nanomaterials, including the pulsed growth of carbon nanotubes and graphene.

NONEQUILIBRIUM LASER SYNTHESIS AND REAL-TIME DIAGNOSTICS OF CARBON NANOMATERIAL GROWTH

Lasers provide unique growth conditions for the synthesis of novel nanomaterials. In addition, they can serve as remote spectroscopic probes of the growth environment. Ultimately, through the process understanding they provide, real-time laser diagnostics that can be used to control the nanomanufacturing of nanomaterials. Here, progress in the laser-based synthesis and investigations of carbon nanomaterial growth kinetics will be reviewed with an emphasis on single-wall carbon nanotubes (SWNTs), single-wall carbon nanohorns (SWNHs), and graphene. Two synthesis methods will be compared. First, the unique high-temperature growth environment of a laser plasma will be examined using time-resolved imaging and laser spectroscopy to understand how pure carbon can self-assemble rapidly into a variety of forms including SWNHs and graphene flakes, and with catalyst-assistance, SWNTs. Atomic resolution images of SWNTs, SWNHs, and graphene reveals that graphene flakes are likely building blocks for the growth of these materials. Second, lower-temperature, chemical vapor deposition (CVD) methods suitable for mass production of nanomaterials will be examined. Pulsed-CVD and pulsed laser deposition (PLD) are described to investigate the catalyst-assisted growth kinetics of graphene and SWNTs. Time-resolved laser reflectivity and Raman spectroscopy studies show that autocatalytic kinetics imply the existence of intermediates crucial to the efficient nanomanufacturing of these materials for energy applications.Lasers provide unique growth conditions for the synthesis of novel nanomaterials. In addition, they can serve as remote spectroscopic probes of the growth environment. Ultimately, through the process understanding they provide, real-time laser diagnostics that can be used to control the nanomanufacturing of nanomaterials. Here, progress in the laser-based synthesis and investigations of carbon nanomaterial growth kinetics will be reviewed with an emphasis on single-wall carbon nanotubes (SWNTs), single-wall carbon nanohorns (SWNHs), and graphene. Two synthesis methods will be compared. First, the unique high-temperature growth environment of a laser plasma will be examined using time-resolved imaging and laser spectroscopy to understand how pure carbon can self-assemble rapidly into a variety of forms including SWNHs and graphene flakes, and with catalyst-assistance, SWNTs. Atomic resolution images of SWNTs, SWNHs, and graphene reveals that graphene flakes are likely building blocks for the growth of thes...