Robert Vajtai

Reliability and current carrying capacity of carbon nanotubes

The current-carrying capacity and reliability studies of multiwalled carbon nanotubes under high current densities (>109 A/cm2) show that no observable failure in the nanotube structure and no measurable change in the resistance are detected at temperatures up to 250 °C and for time scales up to 2 weeks. Our results suggest that nanotubes are potential candidates as interconnects in future large-scale integrated nanoelectronic devices.

Microfabrication technology: Organized assembly of carbon nanotubes

Nanoscale structures need to be arranged into well-defined configurations in order to build integrated systems. Here we use a chemical-vapour deposition method with gas-phase catalyst delivery to direct the assembly of carbon nanotubes in a variety of predetermined orientations onto silicon/silica substrates, building them into one-, two- and three-dimensional arrangements. The preference of nanotubes to grow selectively on and normal to silica surfaces forces them to inherit the lithographically machined template topography of their substrates, allowing the sites of nucleation and the direction of growth to be controlled.

Direct growth of aligned carbon nanotubes on bulk metals

There are several advantages of growing carbon nanotubes (CNTs) directly on bulk metals, for example in the formation of robust CNT–metal contacts during growth. Usually, aligned CNTs1,2,3,4,5,6,7,8,9 are grown either by using thin catalyst layers predeposited on substrates1,2,3,4,5,6,7 or through vapour-phase catalyst delivery7,8,9. The latter method, although flexible, is unsuitable for growing CNTs directly on metallic substrates. Here we report on the growth of aligned multiwalled CNTs on a metallic alloy, Inconel 600 (Inconel), using vapour-phase catalyst delivery. The CNTs are well anchored to the substrate and show excellent electrical contact with it. These CNT–metal structures were then used to fabricate double-layer capacitors and field-emitter devices, which demonstrated improved performance over previously designed CNT structures. Inconel coatings can also be used to grow CNTs on other metallic substrates. This finding overcomes the substrate limitation for nanotube growth which should assist the development of future CNT-related technologies.

Chip cooling with integrated carbon nanotube microfin architectures

Efficient cooling of silicon chips using microfin structures made of aligned multiwalled carbon nanotube arrays is achieved. The tiny cooling elements mounted on the back side of the chips enable power dissipation from the heated chips on the level of modern electronics demands. The nanotube fins are mechanically superior compared to other materials being ten times lighter, flexible, and stiff at the same time. These properties accompanied with the relative simplicity of the fabrication makes the nanotube structures strong candidates for future on-chip thermal management applications.

Density control of single-walled carbon nanotubes using patterned iron nanoparticle catalysts derived from phase-separated thin films of a polyferrocene block copolymer

Single-walled carbon nanotube (SWNT) density and bundle size has been controlled by a simple one step CVD growth process using a polyferrocenylsilane block copolymer as the pre-organized catalyst source.

Laser-assisted metal deposition from liquid-phase precursors on polymers

Abstract In this work, a short review is presented for results utilizing the technique of laser-assisted metallization of dielectrics. Experimental efforts and results related to the metal (palladium (Pd), copper (Cu) and silver (Ag)) deposition on polymeric materials (polyimide (PI), mylar) are reported. These polymers and metals are chosen due to their growing importance in the rapidly-developing microelectronics packaging industry. The method of laser-induced chemical liquid-phase deposition (LCLD) offers many advantages compared to other techniques such as laser-induced forward transfer (LIFT), pulsed-laser deposition (PLD) and laser-assisted chemical vapor-phase deposition (LCVD). The LCLD is time and cost effective because vacuum tools and special pre-treatments are not required. The consumed chemicals used in precursors are non-harmful and easy to handle due to the liquid phase. For the optimal physical and chemical properties of deposits, the laser and solution parameters are varied. XeCl and KrF excimer and Ar + lasers are employed for executing the palladium, Ag and/or Cu formation on the polymer substrates. Chemical and physical analyses of the formed metal patterns are performed by EDX, XRD, FESEM, SEM, resistance and adhesion measurements.

Select Pathways to Carbon Nanotube Film Growth

When such superstructures begin to take the role of basic building blocks, the symmetry of these structures assumes importance in deciding the final film structure and morphology. These superstructures can be found during the growth of organic and inorganic systems, playing an important role in structural evolution during growth, e.g., spherulites during the growth of semi-crystalline polymers. [2] When the fundamental building blocks involved in growth are complex macromolecules or rigid nanostructure units, rather than simple atomic species, the structural evolution of the films will depend on how the units are able to self-organize, attach, and space-fill the substrate surface. The build up of carbon nanotube films on substrates by chemical vapor deposition (CVD) is a striking example of this phenomenon, and has received much attention due to its practical applications. We show in this letter that the film growth of nanotubes occurs via select pathways that involve the nucleation and growth of self-assembled nanotube domains having different shapes and symmetries. Moreover, this process can be controlled by changing the rates of nucleation and growth of the nanotubes on the substrates, leading to domains that are distinctly different in morphology, and leading to nanotube films with a characteristic alignment of individual nanotubes. Individual carbon nanotubes are macromolecular structures with nanosize diameters and micrometer lengths. [3] It has been shown that packed films of nanotubes can be grown on substrates by catalytic CVD of hydrocarbon gases. [4‐10] Although

Anisotropic thermal diffusivity of aligned multiwall carbon nanotube arrays

A photothermoelectric technique was employed to determine the anisotropic thermal diffusivity of thick arrays of multiwalled carbon nanotubes grown by chemical-vapor deposition. The thermal diffusivity along the alignment direction was also determined using a self-heating 3ω method. The agreement between the measured thermal diffusivities with the two techniques is between 2% and 13% in the tested temperature range. The thermal diffusivity along the alignment direction decreases slightly with temperature in the 80–300‐K temperature range and is ∼ two orders of magnitude smaller than the thermal diffusivity along the planes of graphite. The thermal diffusivity across the alignment direction is ∼25 times smaller than along the alignment direction and is between 50% and five times smaller than the thermal diffusivity across the planes of graphite in the measured temperature range.

Building carbon nanotubes and their smart architectures

Carbon nanotubes have fascinating physical properties. In order to use these novel one-dimensional structures for applications (such as in electronic devices, mechanical reinforcements and nano-electromechanical systems) the structure of nanotubes needs to be tailored and various architectures have to be configured using nanotube building blocks. Firstly, in this paper we focus on the directed and self-assembly of nanotubes on planar substrates into hierarchical structures that include oriented arrays, and ordered bundles. These structures are achieved by patterning substrates with or without metal catalysts. Growth of nanotubes is typically achieved by chemical vapor deposition. Various strategies to build two- and three-dimensional architectures of nanotubes are described by this method. In addition to creating pristine nanotube arrays on planar substrates, the paper also covers some of our recent efforts in fabricating nanotube polymer hybrids. Recent efforts and challenges in manipulating nanotubes on surfaces and measuring transport properties are discussed. Results of noise measurements carried out on individual nanotubes; surface potential mapping and very high, long-term current carrying capacity (109–1010 A cm−2) are reported. In conclusion, a perspective is given on our recent efforts in creating controlled structures with nanotubes and measuring some of their properties.

Quantitative analysis of hysteresis in carbon nanotube field-effect devices

The authors present a model to analyze hysteresis in transfer characteristics (TCs) of single-wall carbon nanotube field-effect transistors, based on capacitive charging of the surrounding dielectric by charges injected out of the nanotube. The model identifies the extent and time scale of the hysteresis and correctly describes the dependence of the magnitude of hysteresis on common experimental parameters. The authors propose and experimentally establish a “time-decay” experiment for obtaining accurate device properties in hysteresis-affected devices without actually measuring TCs. The authors further show that values obtained by this method can be used to precisely predict TCs for the same device under different experimental parameters.