Raluca Elena Morjan

Optical recombination of ZnO nanowires grown on sapphire and Si substrates

ZnO nanowires have been grown on sapphire and Si substrates using catalytic growth. A strong near-band-gap ultraviolet emission is observed at room temperature. By carefully studying the temperature dependence of ZnO wire emission, we found that the room-temperature UV emission contains two different transitions; one is related to the ZnO free exciton and the other is related to the free-to-bound transition. The bound state has a binding energy of about 124 meV. The results from optical measurements show that a high quality of ZnO nanowires grown on sapphire and Si substrates has been achieved.

Particle size dependence and model for iron-catalyzed growth of carbon nanotubes by thermal chemical vapor deposition

The catalytic particle size dependence of chemical vapor deposition growth of multiwall carbon nanotubes was systematically investigated using two different molecules, C2H2 and C60, as carbon feedstock gases. In the particle size range between 25 and 500 nm, the use of C2H2 leads exclusively to growth of carbon nanotubes. The nanotube diameters increase with increasing catalytic particle sizes but do not scale 1:1. In contrast, nanotube formation from C60 is observed only if the particle sizes are sufficiently small with an optimum between 20 and 30 nm. For catalyst samples with considerably larger diameters, C60 is transformed into a nontubular deposit. A growth model is given that explains the different behavior.

Plasma-enhanced chemical vapour deposition growth of carbon nanotubes on different metal underlayers

One important requirement for future applications of carbon nanotube electronic devices is the ability to controllably grow carbon nanotubes on metal electrodes. Here we show that it is possible to grow small diameter (<10 nm) vertically aligned carbon nanotubes on different metal underlayers using plasma-enhanced chemical vapour deposition. A crucial component is the insertion of a thin silicon layer between the metal and the catalyst particle. The electrical integrity of the metal electrode layer after plasma treatment and the quality of the metals as interconnects are also investigated.

The temperature dependence of Fe-catalysed growth of carbon nanotubes on silicon substrates

Abstract The catalytic particle size (Fe) and temperature dependence of multi-walled nanotubes growth from C2H2 and C60 precursor molecules is studied. The structure and density of the carbon nanotubes produced is critically dependent on the particle size, the growth temperature and the carbon flux rate. Under certain conditions, bundles of single-walled nanotubes, where the bundles appear to consist of nanotubes with the same diameters, can be produced. The nanotubes are characterised by electron microscopy and Raman spectroscopy. Field emission properties of aligned films are studied and the electron emission is correlated with light emission measurements.

Synthesis of carbon nanotubes by CO2-laser-assisted chemical vapour deposition

Abstract The field of carbon nanotube research is remarkable not only because of the unique properties of this new material but also because of the various possible schemes of their synthesis and their applications. In the present study, we have explored the suitability of laser-assisted chemical vapour deposition for the formation and growth of carbon nanotubes. A medium-power continuous-wave CO2 laser was employed to irradiate a sensitized mixture of Fe(CO)5 vapour and acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown. Scanning and transmission electron microscopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron) that seems to catalyze the nanotube growth. Some regions of the deposit also revealed the presence of nanoparticles. The present experimental conditions should be suitable to produce locally structured deposits of carbon nanotubes for various applications.

Carbon nanotube films grown by laser-assisted chemical vapor deposition

Films of pure high-quality multiwall carbon nanotubes are grown by laser-assisted thermal chemical vapor deposition in a cold wall reactor. A CO2 laser is used to locally heat a substrate on which nanotubes are grown, employing the catalytic activity of iron nanoparticles. Two kinds of experiments are reported: In a two-step mode, catalytic iron particles and solid carbon are deposited separately, leading to the formation of homogeneous films of nonaligned pure multiwall nanotubes (MWNTs). The role of gas phase heating was investigated by the addition of a sensitizing molecule that absorbs the laser radiation. It is found that large-volume gas phase heating is not needed for the synthesis of high-quality nanotube films. Focused laser radiation allows growth of locally defined nanotube films. In a second set of experiments, iron and carbon are deposited simultaneously. Films of vertically aligned MWNTs of extremely high packing density were produced in this mode under very lean hydrocarbon supply conditions.

Integrated nanotube microcooler for microelectronics applications

Providing effective and compact heat removal solutions is an essential element of the electronics packaging approach and its importance increases as the trend in the electronics industry moves towards higher packaging density. Many new thermal removal techniques have been developed. Liquid cooling has been considered as one of the very promising solutions. Its major advantages include the high heat transfer coefficient and possible wafer-level integration with chips. The present work reports the integration of nanotubes which have extremely high thermal conductivity in comparison to conventional microchannel coolers for enhancement in cooling capability. By using lithography techniques, chemical vapor deposition and adhesive bonding; a microcooler with two-dimensional nanotube fins was manufactured. Though it has fairly low cooling capability at the present stage, this new cooler has shown promise in the experimental characterization. Additionally, further modifications have been proposed and some possible reliability concerns were sited.

Highly efficient electron field emission from decorated multiwalled carbon nanotube films

The field emission properties of films of aligned multiwalled nanotubes produced by plasma-enhanced chemical vapor deposition are studied as a function of their length. The measured turn-on and threshold electric fields decrease strongly with increasing nanotube length, reaching values of below 1 V∕μm for the longest nanotubes investigated (170 μm). The field enhancement factors are discussed and the remarkable performance of the longest nanotube films is attributed to secondary thornlike growth at the tips.

Fabrication of individual vertically aligned carbon nanofibres on metal substrates from prefabricated catalyst dots

Carbon nanofibres (CNFs) of controlled diameter and length were grown on different metal substrates using plasma-enhanced chemical vapour deposition (PECVD). The diameter control of catalyst dots (and hence CNF diameter) was obtained by using the shot modulation technique in electron beam lithography. Catalyst dots of different sizes within arrays of different pitch were prepared and the dependence of the growth of vertically aligned CNFs on these parameters was studied for different metal underlayers. Good quality vertically aligned CNFs with a narrow length distribution were grown on Mo and W substrates. The structures grown on Nb substrates were significantly shorter for identical growth conditions and showed a lower nucleation rate. We demonstrate that through the shot modulation technique it is possible to control the diameter variation of CNFs from a single design geometry for the catalyst deposition. Individual VACNFs can be grown down to a pitch within the range 100–500 nm.

Synthesis of carbon nanotube films by thermal CVD in the presence of supported catalyst particles. Part I: The silicon substrate/nanotube film interface

The interface between the silicon substrate and a carbon nanotube film grown by thermal CVD with acetylene (C2H2) and hydrogen at 750 or 900 °C has been characterized by high resolution and analytical transmission electron microscopy, including electron spectroscopic imaging. Silicon (0 0 2) substrates coated with a thin (2.8 nm) iron film were heat treated in the CVD furnace at the deposition temperature in a mixture of flowing argon and hydrogen whereby nanosized particles of (Fe,Si)3O4 formed. These particles were reduced to catalytic iron silicides with the α–(Fe, Si), α2–Fe2Si and α1–Fe2Si structures during CVD at 900 °C, and multi-wall carbon nanotubes grew from supported particles via a base-growth mechanism. A limited number of intermediate iron carbides, hexagonal and orthorhombic Fe7C3, were also present on the substrate surface after CVD at 900 °C. The reduction of the preformed (Fe, Si)3O4 particles during thermal CVD at 750 °C was accompanied by disintegration leading to the formation of a number of smaller (<5 and up to 10 nm) iron and silicon containing particles. It is believed that the formation of these small particles is a prerequisite for the growth of aligned multi-wall carbon nanotube films.