J. D. Carey

Influence of sp2 clusters on the field emission properties of amorphous carbon thin films

The influence of the concentration and size of sp2 carbon clusters on the field emission properties of hydrogenated amorphous carbon thin films is investigated. In combination with electron paramagnetic resonance and optical measurements, it is shown that the trend in the threshold field for emission for films deposited under certain conditions can be explained in terms of improvements in the connectivity between sp2 clusters. These clusters are believed to be located near the Fermi level, and the connectivity is primarily determined by the cluster size and concentration, which in turn is determined by the choice of deposition conditions. Details of the appropriate emission mechanisms for different types of deposited carbon films are discussed.

Charge transport effects in field emission from carbon nanotube-polymer composites

Electron field emission measurements have been made on multiwall arc discharge carbon nanotubes embedded in a conjugated polymer host. Electron emission at low nanotube content is observed and attributed to an enhancement of the applied electric field at the polymer/nanotube/vacuum interface where the electron supply through the film is attributed to fluctuation induced tunneling in a disordered percolation network. A high network resistance is attributed to a polymer coating surrounding each nanotube, resulting in high resistance nanotube-polymer-nanotube tunnel junctions. The potential use of carbon nanotube-polymer composites for field emission based displays is also discussed.

Room Temperature Photoluminescence From Nanostructured Amorphous Carbon

Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (PAr). By varying PAr from 5to340mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp2 bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp2 rich phase.

Effect of aspect ratio and anode location on the field emission properties of a single tip based emitter

The effect on the field emission characteristics of the aspect ratio of an isolated emitter, together with the position of the anode electrode are reported. We show by computational simulation that the field enhancement factor β is only dependant on the emitter height h, radius r, when the anode to cathode separation D is greater than three times the height of the emitter away from the tip. In this regime the enhancement factor is independent of the anode location and approaches a value depicted by h and r alone and is described by the expression β0=(1+h∕αr)m where α=2 and m=1. As the anode is brought close to the tip of the emitter, the emitter tip and anode approximate a parallel plate configuration and the enhancement factor tends to unity. Extracted enhancement factor and threshold fields are described by a modified applied electric field taking D−h as the separation. Comparison with previously reported experimental results is also given.

Formation of low-temperature self-organized nanoscale nickel metal islands

The growth and evolution of nanometre-sized Ni metal islands deposited under low-temperature non-ultra high vacuum conditions as a function of metal layer thickness, growth temperature and time is reported. The temperature of formation of the islands has been intentionally kept low for possible applications in flat panel display technology and also to act as a catalyst for carbon nanotube growth. It is shown that the size and distribution of the islands depends critically on the annealing temperature and the initial thickness of the metal layer. The mechanism of formation of the islands is described in terms of an Ostwald ripening mechanism of mass transport of either weakly bound individual Ni atoms or smaller clusters into larger more dispersed clusters.

Interpretation of enhancement factor in nonplanar field emitters

A comparison of the field emission properties of exposed nanotubes lying on a tipped carbon nanorope, with the emission properties from a sharpened iron tip of similar dimensions is performed. By varying the electrode separation it is observed that the threshold field for emission for both structures decreases as the electrode separation initially increases; however, for sufficiently large electrode separations, the threshold field is observed to reach an asymptotic value. Our results show that the field enhancement factor is fundamentally associated with the electrode separation, and depending on the experimental conditions in order to obtain a true value for electric field a set of alternative definitions for enhancement factors is required. We further confirm our experimental synopsis by simulation of the local electrostatic field which gives results similar to those obtained experimentally.

Excimer laser nanostructuring of nickel thin films for the catalytic growth of carbon nanotubes

Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst.

Enhancing the electrical conduction in amorphous carbon and prospects for device applications

Abstract The problems and possible solutions associated with producing practical electronic devices based upon amorphous carbon (a-C) thin films are discussed. The clustering of the carbon sp2 phase is shown to be a critical aspect in understanding the current device limitations. In order to exploit the sp2 clustering we show that the use of ion beams to deposit energy into the microstructure in a controlled manner, as opposed to conventional thermal anneal treatments, results in a delocalised electron wavefunction and an enhancement of conductivity. A barrier controlled device is demonstrated by carefully choosing the ion energy and dose. One of the consequences of ion implantation is that film can now be considered as consisting of conductive sp2 C clusters within an insulating sp3 C matrix. We show that the presence of this dielectric inhomogeneity between the conductive sp2 regions and the sp3 matrix plays an important role in understanding the field emission behaviour from a-C based materials.

Laser-nanostructured Ag films as substrates for surface-enhanced Raman spectroscopy

Pulsed-laser (248nm) irradiation of Ag thin films was employed to produce nanostructured Ag∕SiO2 substrates. By tailoring the laser fluence, it was possible to controllably adjust the mean diameter of the resultant near-spherical Ag droplets. Thin films of tetrahedral amorphous carbon (ta-C) were subsequently deposited onto the nanostructured substrates. Visible Raman measurements were performed on the ta-C films, where it was observed that the intensity of the Raman signal was increased by nearly two orders of magnitude, when compared with ta-C films grown on nonstructured substrates. The use of laser annealing as a method of preparing substrates, at low macroscopic temperatures, for surface-enhanced Raman spectroscopy on subnanometer-thick films is discussed.

Electron field emission from room temperature grown carbon nanofibers

The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/μm) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 μm in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/μm) for emission.