Alex A. Puretzky

Time-resolved imaging of gas phase nanoparticle synthesis by laser ablation

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 SiOx 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.

Photoluminescence from gas-suspended SiOx nanoparticles synthesized by laser ablation

Time-resolved photoluminescence (PL) spectra are reported for gas-suspended 1–10 nm diameter SiOx 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 SiO1.4 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.

Single walled carbon nanohorns as photothermal cancer agents.

Nanoparticles have significant potential as selective photo‐absorbing agents for laser based cancer treatment. This study investigates the use of single walled carbon nanohorns (SWNHs) as thermal enhancers when excited by near infrared (NIR) light for tumor cell destruction.

Separation of junction and bundle resistance in single wall carbon nanotube percolation networks by impedance spectroscopy

Single wall carbon nanotube (SWNT) networks of different loadings were measured by impedance spectroscopy. The resistances of the junctions and bundles have been separated by modeling ac impedance spectroscopy data to an equivalent circuit of two parallel resistance-capacitance elements in series. The junction resistance was found to be 3–3.5 times higher than the bundle resistance. The dc and ac properties of the SWNT networks were found to obey a percolation scaling law, with parameters determined by dispersant type and SWNT purity. The values of the critical exponent in all cases were higher than the expected value of 1.3, which is related to widely distributed bundle and junction conductivities.

In Vitro and in Vivo Studies of Single-Walled Carbon Nanohorns with Encapsulated Metallofullerenes and Exohedrally Functionalized Quantum Dots

Single-walled carbon nanohorns (SWNHs) are new carbonaceous materials. In this paper, we report the first successful preparation of SWNHs encapsulating trimetallic nitride template endohedral metallofullerenes (TNT-EMFs). The resultant materials were functionalized by a high-speed vibration milling method and conjugated with CdSe/ZnS quantum dots (QDs). The successful encapsulation of TNT-EMFs and external functionalization with QDs provide a dual diagnostic platform for in vitro and in vivo biomedical applications of these new carbonaceous materials.

High-density vertically aligned multiwalled carbon nanotubes with tubular structures

Ammonia (NH3) gas was thought to be essential for the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) and led to the formation of bamboo-like structures. Here, we show that VA-MWCNTs with ideal tubular structures can be grown on substrates by various mixed gases with or without NH3 gas. The growth of these VA-MWCNTs is guided by a growth model that combined the dissociative adsorption of acetylene molecules (C2H2) and the successive vapor-liquid-solid growth mechanism. Results indicate that the key factor for growing these VA-MWCNTs is a balance between the decomposition rate of the C2H2 molecules on the iron catalyst and the subsequent diffusion and segregation rates of carbon.

The electrodeposition of metal at metal/carbon nanotube junctions

Abstract 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 Au 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.

Time-resolved diagnostics of single wall carbon nanotube synthesis by laser vaporization

Abstract Three questions important to nanosecond laser ablation synthesis of single wall carbon nanotubes (SWNTs) have been addressed using in situ spectroscopic diagnostics: determining the temperature of the nanoparticles within the propagating plume at different times after ablation, monitoring the aggregation of the nanoparticles in the plume, and measuring the growth rates of the SWNTs. Short SWNTs were synthesized using nanosecond Nd:YAG-laser ablation of a C–Ni–Co target inside a high-temperature laser vaporization reactor by controlling and restricting the growth times. The time spent by the plume inside the oven was varied by positioning the target at various locations and imaging the plume using Rayleigh scattered light induced by a 308xa0nm XeCl laser. Statistical analysis of the short SWNT length distribution was performed using TEM images. The upper and lower limits of the growth rates of SWNTs were estimated as 0.6 and 5.1xa0μm/s. The particle temperature within the propagating plume was measured at different times after ablation through time-resolved measurements of the plume’s blackbody emission. The onset of SWNT growth was estimated based on the time when the particle temperature drops below the eutectic temperature for C/Co, C/Ni. For the first time, absorption spectroscopy was employed to study the aggregation of carbon nanoparticles in the propagating plume. It was shown that the aggregation rate increases rapidly at lower oven temperatures. A general picture of SWNT growth by laser ablation based on imaging, spectroscopy, and pyrometry of ejected material at different times after ablation is discussed.

Structural control of vertically aligned multiwalled carbon nanotubes by radio-frequency plasmas

Plasma-enhanced chemical vapor deposition is the only technique for growing individual vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) at desired locations. Inferior graphitic order has been a long-standing issue that has prevented realistic applications of these VA-MWCNTs. Previously, these VA-MWCNTs were grown by a one-plasma approach. Here, we demonstrate the capability of controlling graphitic order and diameters of VA-MWCNTs by decoupling the functions of the conventional single plasma into a dual-plasma configuration. Our results indicate that the ionic flux and kinetic energy of the growth species are important for improving graphitic order of VA-MWCMTs.

A laser-deposition approach to compositional-spread discovery of materials on conventional sample sizes

Parallel (multi-sample) approaches, such as discrete combinatorial synthesis or continuous compositional-spread (CCS), can significantly increase the rate of materials discovery and process optimization. Here we review our generalized CCS method, based on pulsed-laser deposition, in which the synchronization between laser firing and substrate translation (behind a fixed slit aperture) yields the desired variations of composition and thickness. In situ alloying makes this approach applicable to the non-equilibrium synthesis of metastable phases. Deposition on a heater plate with a controlled spatial temperature variation can additionally be used for growth-temperature-dependence studies. Composition and temperature variations are controlled on length scales large enough to yield sample sizes sufficient for conventional characterization techniques (such as temperature-dependent measurements of resistivity or magnetic properties). This technique has been applied to various experimental studies, and we present here the results for the growth of electro-optic materials (SrxBa1−xNb2O6) and magnetic perovskites (Sr1−xCaxRuO3), and discuss the application to the understanding and optimization of catalysts used in the synthesis of dense forests of carbon nanotubes.