The electron field-emission properties of hydrogenated amorphous carbon and nitrogenated tetrahedral amorphous carbon thin films are examined by measuring the field-emission current as a function of the applied macroscopic electric field. The experimental results indicate the existence of an optimum film thickness for low-threshold electron field emission. The predictions of various emission models are compared to the experimental results.

The observation of electron emission from amorphous carbon thin films at low applied electric fields is explained in terms of an enhancement of the field brought about by dielectric inhomogeneities within the film. These inhomogeneities originate from the differences between conductive, spatially localized sp 2 C clusters surrounded by a more insulating sp 3 matrix. By a more complete understanding of the concentration and distribution of the clusters, a generic model for field emission from amorphous carbon thin films can be developed. Extensions of this model to explain the emission properties of carbon nanotubes and carbon nanocomposite materials are also presented. The possible use of amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) based materials as cold cathodes has now been well documented.1,2 To date there have been numerous reports of field emission ͑FE͒ at low macroscopic electric fields from a range of a-C and a-C:H based materials. [1][2][3][4] In the case of emission from diamondlike carbon ͑DLC͒ films it has been reported that, the main barrier which controls emission may lie at the front film/ vacuum interface.3 By contrast, for low defect density polymeric-like carbon ͑PAC͒ a-C:H films the presence of a heterojunction at the a-C:H/Si rear contact, resulting in hot electron transport through the film has been proposed. 4 This model was used to explain the observed strong dependence of the threshold field (E th ) with film thickness. 5 Although the different types of film possess different physical properties and emission has been explained by different mechanisms, it has been observed that emission is often highly nonuniform across the film surface.3,4,6 One of the major problems hindering the development of thin film carbon cathodes is the lack of understanding of a definitive mechanism for emission at low applied fields, typically Ͻ20 V/m. [3][4][5][6][7] Emission from flat metal surfaces often requires fields in excess of 500 V/m, 2 whereas previous atomic force microscopy ͑AFM͒ studies of a-C and a-C:H films have indicated that the films possess rms roughnesses of less than 1 nm, [1][2][3][4] along with no evidence of surface protrusions which can act as a source of geometric field enhancement. Two recent studies by Ilie et al. 8 and Carey et al. 9 have proposed that it is the sp 2 C cluster size and concentration that play an important role in the FE process. In the study by Ilie, an optimum cluster size for emission of 1.5-2 nm was proposed. Carey showed that for a-C:H films deposited at different selfbiases, the FE could be explained in terms of the connectivity between sp 2 clusters. In this letter, we examine whether there is any dependence of E th with thickness for DLC films and compare these results to the behavior reported 5 for PAC films. Scanning tunneling microscope ͑STM͒ images of the DLC films, show that while the films may be atomically smooth they do exhibit variations in their conductivity, which are attributed to dielectric inhomogeneities within the film. ...

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.

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 ␤ 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 ␤ 0 = ͑1+ ͱ h / ␣r͒ m where ␣ = 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. 1 have shown to possess a fascinating structure, and their use as electron sources in vacuum microelectronics and nanoelectronics has been widely reported.2 The mechanism of field-induced electron emission from a nanotube is understood to be due to the applied electric field undergoing an increase at the tip of the CNT, often referred to as the field enhancement factor ␤. For a single, isolated CNT, the value of enhancement factor is believed to be dependant on the length, radius, and type of structure, i.e., multiwalled ͑MWNT͒, singlewalled ͑SWNT͒, open or closed cap: This has been subject to several computational and experimental investigations.3-8 Geometric enhancement is not just applicable to CNT but also exists in a number of other tip-based structures including: SiC nanowires, 9 MoO 3 nanobelts, 10 tungsten nanowires, 11 spindt tips, 12 and copper sulphide nanowire arrays. 13 Much of the analysis performed on experimental data has relied upon analysis of the emission current I to field E ͑or voltage V͒ characteristics using the wellknown field emission mechanism of Fowler and Nordheim.14 The standard analysis often involves a plot of the log͑I / E 2 ͒ versus 1 / E ͑or equally log I / V 2 against 1 / V͒ and from the slope of the graph an approximate value for the field enhancement factor ␤ can be extracted. The role of ␤ is the enhancement of the applied macroscopic electric field such that under the action of the local electric field, tunneling of electrons from the Fermi level, into the vacuum, through the potential barrier becomes possible. The interpretation of ␤, which is a dimensionless quantity if electric field rather than voltage is used in the analysis, is therefore of great importance.There have been a number of attempts to model the behaviour of ␤ for a range of nanotube height and radius. Early work by Dyke and Dolan 15 showed that for a planar anode and a sphere-on-cone emitter the local field enhancement ͑neglecting V / d͒ close to the tip of the emitter was given bywhere ␤ L is the local field enhancement due to the emitter shape, d is the emitter-anode gap, and n is related to the cone opening angle ͑for ...

The influence of the concentration and size of sp 2 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 sp 2 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.