Alvin W. Orbaek

Increasing the Efficiency of Single Walled Carbon Nanotube Amplification by Fe–Co Catalysts Through the Optimization of CH4/H2 Partial Pressures

Single walled carbon nanotubes (SWNTs) seeds are grown using Fe-Co nanoparticles on spin-on-glass. The relative efficiency of nucleation and amplification (versus etching) was investigated as a function of the CH(4)/H(2) feedstock ratio and growth temperature. At 900 °C, maximum amplification is obtained with CH(4)/H(2) ratio of 80:20 but 60:40 for nucleation. Amplification is further enhanced at 800 °C, compared with etching dominating at 1000 °C. Amplification of SWNTs is in equilibrium with etching; higher carbon feedstock pressure and decreased temperature increase the rate of amplification; the converse increases etching.

The development of a ‘process map’ for the growth of carbon nanomaterials from ferrocene by injection CVD

Ferrocene was used as a catalyst in a hydrocarbon solution to carry out injection chemical vapour deposition (CVD) growth of carbon nanomaterials in a horizontal tube furnace. The presence and quality of various carbon materials was determined using Raman spectroscopy, thermogravimetric analysis, and electron microscopy. Products ranged from amorphous carbon (a-C), multi walled carbon nanotubes (MWNTs) and vapour grown fibers (VGFs). Catalyst concentration, growth temperature, injection rate, carrier gas flow rate, and the choice of hydrocarbon were found to influence the product outcome. Slower injection rates gave higher isolated yield. High catalyst concentrations were found to produce less pure carbon materials with iron oxides impurities. Higher temperatures were favourable to nanotube formation. The use of benzene was found to increase yield compared with the other hydrocarbons. In this work we developed a process map to chart the various carbon nanomaterial allotropes that were created according to reaction parameters.

Wet Catalyst-Support Films for Production of Vertically Aligned Carbon Nanotubes

A procedure for vertically aligned carbon nanotube (VA-CNT) production has been developed through liquid-phase deposition of alumoxanes (aluminum oxide hydroxides, boehmite) as a catalyst support. Through a simple spin-coating of alumoxane nanoparticles, uniform centimer-square thin film surfaces were coated and used as supports for subsequent deposition of metal catalyst. Uniform VA-CNTs are observed to grow from this film following deposition of both conventional evaporated Fe catalyst, as well as premade Fe nanoparticles drop-dried from the liquid phase. The quality and uniformity of the VA-CNTs are comparable to growth from conventional evaporated layers of Al(2)O(3). The combined use of alumoxane and Fe nanoparticles to coat surfaces represents an inexpensive and scalable approach to large-scale VA-CNT production that makes chemical vapor deposition significantly more competitive when compared to other CNT production techniques.

Preparation and evaluation of polyethyleneimine-single walled carbon nanotube conjugates as vectors for pancreatic cancer treatment

High quality single-walled carbon nanotubes (SWNTs) were obtained following a new purification procedure, based on using Cl2 gas at high temperature. Cl2-treated SWNTs were fluorinated and modified with branched polyethyleneimine (PEI) to afford covalently functionalised PEI-SWNTs, which were then tested for cytotoxicity both in vitro (HPNE and BxPC3 pancreatic cell lines) and in vivo (BxPC3 xenografts from nude mice) to establish that functionalization with lower molecular weight PEI (600 and 1800 Da) achieved higher cell viability in MTT assay. A shortened version of the nanotubes, PEI(1800)-cut-SWNT (1800 Da branched PEI), was also prepared and tested for cellular internalization in the BxPC3 adenocarcinoma cell line. Laser confocal imaging of the cells after incubation in the presence of RhoB-PEI(1800)-cut-SWNT (covalently labelled with rhodamine B) indicates that the PEI(1800)-cut-SWNTs can reach both the cytoplasm and nucleus of pancreatic cancer cells.

Dendrimer-Assisted Self-Assembled Monolayer of Iron Nanoparticles for Vertical Array Carbon Nanotube Growth

Self-assembled monolayers (SAMs) of iron oxide nanoparticles have been prepared using carboxylic-acid-terminated dendrimers. The iron-containing SAM was used as the catalyst for growth of vertical arrays of carbon nanotubes (CNTs). This approach has the potential for producing diameter controlled CNTs from premade catalyst nanoparticles as well as large scale production of CNTs by chemical vapor deposition.

Radiofrequency electric-field heating behaviors of highly enriched semiconducting and metallic single-walled carbon nanotubes

It is theorized that enhanced thermal heating may result from exposing single-walled carbon nanotubes (SWNTs) embedded in a conductive host to radiofrequency (RF) electric fields. We examine the RF-induced (13.56 MHz) heating behaviors of 95% metallic- and semiconducting-enriched SWNTs (m-/s-SWNTs) suspended in aqueous solutions with varying NaCl molarity (0.001 mM–1 M). The heating effects were only evident for host molarities below 1 mM (equivalent to 0.1 S/m) at which the s-SWNT heating rates dominated those of the m-SWNTs. The heating effects were localized to aligned and aggregated “SWNT ropes” ~1 cm in length that formed in suspension, parallel to the electric-field vector, during the RF exposure. For molarities above 1 mM, no enhancements were evident, owing to the large heating effects of the bulk ionic NaCl suspensions, which were observed in previous studies. Although larger enhancement effects proportional to the host conductivity have been theoretically predicted for m-/s-SWNT suspensions, this was not observed most likely because of the aggregation and screening effects, which diminished the scattered electric field near the m-/s-SWNTs. Our research may further the development of better nanoparticle heating agents for applications such as non-invasive RF-induced cancer hyperthermia.

Reagent control over the composition of mixed metal oxide nanoparticles

Binary (M1 − M2 − O) and ternary (M1 − M2 − M3 − O) metal-oxide nanoparticles (NPs) have been prepared by thermal decomposition in benzyl ether of the appropriate M(acac)n (M = Fe, Mn, Pd, Cu, Al, Gd) compounds in the presence of a mixture of oleic acid and oleylamine templating (surface capping) ligands, and 1,2-hexadecanediol as an accelerating agent. The metal percentage and the particle size were investigated as a function of the starting composition. The NP composition is controlled by the relative reaction rates of the particular precursors, such that prediction of NP composition from reagent ratios is not straightforward. However, understanding reaction rate limitations allows for alternative synthesis to be developed. In some cases, ligand exchange reaction and subsequent decomposition are possibly more important than thermal decomposition.

Towards a ‘catalyst activity map’ regarding the nucleation and growth of single walled carbon nanotubes

Quantification using scanning electron microscopy (SEM) of single walled carbon nanotubes (SWNTs) grown per unit area using a Co-Fe (50:50) catalyst system, prepared by the incorporation of the appropriate metal salts into a Spin-On Glass substrate, at 900°C. The effects of substrate, as well as catalyst precursor concentration, were investigated. SWNT growth density is maximised with a catalyst precursor concentration of ≥2.5 mM, associated with the formation of catalyst nanoparticles of a critical size for SWNT nucleation. Samples were subjected to secondary growth, using a range of H2:CH4 ratios to determine the optimum precursor composition. It was found that nucleation and growth stages are optimal under different conditions. Optimum conditions for nucleation resulted in >10× increase in SWNT density. Optimisation is dependent on temperature and the partial pressure of reagent gas species.

Complications pertaining to the detection and characterization of individual and embedded single walled carbon nanotubes by scanning electron microscopy

Comparison of AFM and SEM images of single walled carbon nanotubes (SWNTs) grown within a dielectric matrix reveal subterranean nanotubes that are present within the matrix, and as such can be charge screened by the dielectric. Under adequate imaging conditions for the SWNT/silica sample the intensity of isolated nanotubes is found to be inversely proportional to the instrument dwell time (i.e., shorter dwell times were found to make SWNT intensities brighter). The threshold dwell time required to enable isolated tubes to be visible was found to be 10 μs; moreover, the degree change in intensity was found to be nanotube specific, i.e., different SWNTs respond in a different manner at different dwell times. The results indicate that care should be taken when attempting to quantify number density and length distributions of SWNTs on or within a dielectric matrix.