Low-macroscopic-field Emission Research Articles

Low-macroscopic field emission structure: Using the shape of the conductor to identify a local difference in electric potential

The emission of electrons from an insulating crystal deposited on a conductor occurs at a macroscopic electric field of a few volts per micrometer, 3 orders of magnitude below the field emission from a clean metal. This is due to the local field enhancement induced by the presence of the insulating crystal. The emission profiles depend on the shape of the conductive substrate; analyzing these profiles enables the local difference in electric potential and the opening angle to be traced. Given the thickness of the crystal, the local difference in potential indicates the local field enhancement of a few volts per nanometer applied to the conductor.

Advances in Novel Low-Macroscopic Field Emission Electrode Design Based on Fullerene-Doped Porous Silicon

Perspective low-macroscopic field (LMF) emission prototype cathodes based on fullerene C60—doped porous silicon were realized via a two-stage technique comprising the electrochemical etching process of a monocrystalline silicon wafer and functionalization of the acquired porous silicon (PS) matrix with silver-doped fullerene-based carbon structures. The resulting LMF cathode prototypes were studied with SEM and EDS techniques. The formation of an amorphous silver-doped C60-based layer consisting of nanosized aggregates on the matrix surface was established. The emission characteristics of the prototypes were analyzed, crucial parameters including threshold field strength values, emission current density, and effective potential barrier height for electrons were considered. A novel LMF emission model is suggested. It was established that the emitter prototypes realized during this study are on par with or superior to modern and promising field cathodes.

尖鋭構造を持たない炭素膜からの電界放出

Field emission features from Si and W tips are drastically improved upon coating even with simple carbon films. In this paper, in order to obtain clues to clarify the mechanism, we examine atomistic STM observations including local barrier height (LBH) and field emission imagings of the carbon films. And we also examine the field emission features from the shape-controlled W tips before and after the carbon coatings, enabling us to estimate the local electric field at the tip apex. From the STM observations, we find that the carbon film consists of nm-scale grains with various electronic properties and that the FE current varies grain by grain and is higher near the edge of each grain. The effective work functions evaluated from the slopes of the Fowler-Nordheim (FN) plot are constant regardless of the thickness of the carbon coating, although they are largely scattered for the films thinner than 1 nm. On the basis of the observed results, we discuss the mechanism of low macroscopic field emission from carbon films.

Morphology-dependent low macroscopic field emission properties of titania/titanate nanorods synthesized by alkali-controlled hydrothermal treatment of a metallic Ti surface

One-dimensional (1D) and two-dimensional (2D) titania/titanate nanostructures are fabricated directly on a self-source metallic titanium (Ti) surface via in situ surface re-construction of a Ti substrate using potassium hydroxide (KOH) under a hydrothermal (HT) condition. The effect of temperature and the concentration of KOH on the variations in morphology and titania-to-titanate phase changes are studied and explained in detail. A growth model is proposed for the formation process of the platelet-to-nanorod conversion mechanism. The field emission (FE) properties of titania/titanate nanostructures are studied, and the effects of the morphologies (such as 1D nanorods, 2D nanoplatelets, and a mixture of 1D nanorods and 2D platelets) on the FE properties of the samples are investigated. The samples depict a reasonable low turn-on field and emission stability. The FE mechanism is observed to follow standard Fowler–Nordheim (FN) electron tunneling. The geometrical field enhancement factor (β) is measured to be very high, and is compared with theoretical values calculated from various existing models to explore the feasibility of these models. The surface modification of metallic Ti by a simple non-lithographic bottom-up method and the low-macroscopic FE properties can provide a potential alternative to field emission displays for low-power panel technology.

Low-macroscopic field emission properties of wide bandgap copper aluminium oxidenanoparticles for low-power panel applications

Field emission properties of CuAlO2 nanoparticles are reported for the first time, with a low turn-on field of approximately2 V µm − 1 and field enhancement factor around 230. The field emission process follows the standardFowler–Nordheim tunnelling of cold electron emission. The emission mechanism is found tobe a combination of low electron affinity, internal nanostructure and large fieldenhancement at the low-dimensional emitter tips of the nanoparticles. The field emissionproperties are comparable to the conventional carbon-based field emitters, and thuscan become alternative candidate for field emission devices for low-power panelapplications.

Field penetration into amorphous‐carbon films: consequences for field‐induced electron emission

AbstractThe phenomenon of low‐macroscopic‐field (LMF) electron emission is sometimes exhibited by thin films of non‐metallic materials. Both wide‐band‐gap semiconductors and hopping conductors such as amorphous carbon exhibit this phenomenon, and its origin has been under discussion for some years. This paper first uses a simple theory to discuss field penetration into hopping conductors with a high density of localised states near the charge‐neutrality level. This theory is physically similar to widely used existing theories, in that it assumes continuous charge distributions and uses the one‐dimensional Poisson's equation. The theory suggests that little field penetration occurs, and that models of LMF emission based on uniform barrier lowering are not viable. Unexpected self‐consistency difficulties are also created for some types of local‐field‐enhancement model. A re‐assessment of the validity of this simple field‐penetration theory suggests the following. Contrary to a basic assumption of this type of theory, the granularity of the screening charge distribution is a dominant feature of the physics associated with LMF emission from nanoscale‐sized emission sites on thin films of hopping conductors. The authors think that the screening is carried out by discrete charges in localised states, although details are not clear. This conclusion is potentially very far‐reaching. In particular, new forms of field‐penetration theory may be required before definitive decisions can be made about the self‐consistency of some models of local field enhancement. Copyright © 2007 John Wiley & Sons, Ltd.

Low-macroscopic field emission from silicon-incorporated diamond-like carbon film synthesized by dc PECVD

Silicon-incorporated diamond-like carbon (Si–DLC) films were deposited via dc plasma-enhanced chemical vapor deposition (PECVD), on glass and alumina substrates at a substrate temperature 300 °C. The precursor gas used was acetylene and for Si incorporation, tetraethyl orthosilicate dissolved in methanol was used. Si atomic percentage in the films was varied from 0% to 19.3% as measured from energy-dispersive X-ray analysis (EDX). The binding energies of C 1s, Si 2s and Si 2p were determined from X-ray photoelectron spectroscopic studies. We have observed low-macroscopic field electron emission from Si–DLC thin films deposited on glass substrates. The emission properties have been studied for a fixed anode–sample separation of 80 μm for different Si atomic percentages in the films. The turn-on field was also found to vary from 16.19 to 3.61 V/μm for a fixed anode–sample separation of 80 μm with a variation of silicon atomic percentage in the films 0% to 19.3%. The turn-on field and approximate work function are calculated and we have tried to explain the emission mechanism there from. It was found that the turn-on field and effective emission barrier were reduced by Si incorporation than undoped DLC.

Low-macroscopic field emission from nanocrystalline Al doped SnO2 thin films synthesized by sol–gel technique

We have observed low-macroscopic field electron emission from wide bandgap nanocrystalline Al doped SnO2 thin films deposited on glass substrates. The emission properties have been studied for different anode-sample spacings and for different Al concentrations in the films. The turn-on field and approximate work function were calculated and we have tried to explain the emission mechanism from this. The turn-on field was found to vary in the range 5.6–7.5 V/μm for a variation of anode sample spacing from 80–120 μm. The turn-on field was also found to vary from 4.6–5.68 V/μm for a fixed anode-sample separation of 80 μm with a variation of Al concentration in the films 8.16–2.31%. The Al concentrations in the films have been measured by energy dispersive X-ray analysis. Optical transmittance measurement of the films showed a high transparency with a direct bandgap ∼3.98 eV. Due to the wide bandgap, the electron affinity of the film decreased. This, along with the nanocrystalline nature of the films, enhanced the field emission properties.

Electro-optical characteristics and field-emission properties of reactive DC-sputtered p-CuAlO 2+x thin films

P-type transparent-conducting CuAlO 2+ x thin films were deposited on silicon and glass substrates by reactive direct current sputtering of a prefabricated metal powder target having 1:1 atomic ratio of Cu and Al in oxygen-diluted argon atmosphere. XRD spectrum confirmed the proper phase formation of the material. UV-Vis-NIR spectrophotometric measurements showed high transparency of the films in the visible region with direct and indirect band gap values around 3.90 and 1.89 eV, respectively. The room temperature conductivity of the film was of the order of 0.22 S cm −1 and the activation energy was ∼0.25 eV. Seebeck coefficient at room temperature showed a value of +115 μV/K confirming the p-type nature of the film. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a carrier concentration ∼4.4×10 17 cm −3. We have also observed the low macroscopic field emission, from the wide band gap p-CuAlO 2+ x thin film deposited on glass substrate. The emission properties have been studied for different anode-sample spacing. The threshold field was found to be as low as around 0.5–1.1 V/μm. This low threshold is attributed primarily to the internal nanostructure of the thin film, which causes considerable geometrical field enhancement inside the film as well as at the film/vacuum interface.

Low-macroscopic field emission from fibrous ZnO thin film prepared by catalyst-free solution route

Fiber-structured zinc oxide thin films were synthesized by catalyst-free chemical process from zinc acetate dihydrate at an elevated temperature of 1073K on Si substrate. X-ray diffraction spectra confirmed the proper phase formation of the films. Scanning electron micrograph depicted the formation of ZnO fibers with an average diameter of 500nm with an average aspect ratio ∼150. Field emission properties of the film showed considerably low turn-on field around 1.4V/μm. This low-threshold field-emission of the films are attributed to the conducting nanostructure inside the film as well as on the tip of the fibers at the film–vacuum interface, which causes the geometrical field enhancement at the emitter-tip and provides the required barrier field for field-emission tunneling. The emission current was as high as ∼70μA at the field of 3.6V/μm. The high emission current, high stability and low turn-on field make the ZnO fibers a strong candidate for field-emission displays.

Field-induced electron emission from electrically nanostructured heterogeneous (ENH) materials

This paper discusses the origins of field-induced electron emission from thin films of electrically nanostructured heterogeneous (ENH) materials. Such materials exhibit low macroscopic field (LMF) electron emission: as thin films on a conducting substrate, they emit electrons into vacuum when LMFs, typically about 1–10 V/μm, are applied. An ENH material comprises: a dielectric matrix, which may contain nanoscale inclusions of higher electrical conductivity; conducting channels that open in the dielectric between the inclusions (if present) and between them and the substrate; and an electron emitting channel that opens into vacuum. Electrically nanostructured heterogeneous materials can have a variety of different detailed structures and theories, but all can exhibit LMF emission under suitable circumstances. This paper provides an updated summary of an integrating overview recently presented to explain LMF emission. A central feature is that electrical nanostructure within the film can create internal field enhancement, thereby producing a high local field at the vacuum interface: this enables thermalised electrons to escape rapidly into vacuum by tunnelling. The question of what aspect of the system controls the emission current is a separate issue. Various features and implications of the theory are set out.

Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism

Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1. This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.