Field Emission Process Research Articles

Role of carbon nanotube interlayer in enhancing the electron field emission behavior of ultrananocrystalline diamond coated Si-tip arrays.

We improved the electron field emission properties of ultrananocrystalline diamond (UNCD) films grown on Si-tip arrays by using the carbon nanotubes (CNTs) as interlayer and post-treating the films in CH4/Ar/H2 plasma. The use of CNTs interlayer effectively suppresses the presence of amorphous carbon in the diamond-to-Si interface that enhances the transport of electrons from Si, across the interface, to diamond. The post-treatment process results in hybrid-granular-structured diamond (HiD) films via the induction of the coalescence of the ultrasmall grains in these films that enhanced the conductivity of the films. All these factors contribute toward the enhancement of the electron field emission (EFE) process for the HiDCNT/Si-tip emitters, with low turn-on field of E0 = 2.98 V/μm and a large current density of 1.68 mA/cm(2) at an applied field of 5.0 V/μm. The EFE lifetime stability under an operation current of 6.5 μA was improved substantially to τHiD/CNT/Si-tip = 365 min. Interestingly, these HiDCNT/Si-tip materials also show enhanced plasma illumination behavior, as well as improved robustness against plasma ion bombardment when they are used as the cathode for microplasma devices. The study concludes that the use of CNT interlayers not only increase the potential of these materials as good EFE emitters, but also prove themselves to be good microplasma devices with improved performance.

Field emission luminescence of nanodiamonds deposited on the aligned carbon nanotube array.

Detonation nanodiamonds (NDs) were deposited on the surface of aligned carbon nanotubes (CNTs) by immersing a CNT array in an aqueous suspension of NDs in dimethylsulfoxide (DMSO). The structure and electronic state of the obtained CNT–ND hybrid material were studied using optical and electron microscopy and Infrared, Raman, X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. A non-covalent interaction between NDs and CNT and preservation of vertical orientation of CNTs in the hybrid were revealed. We showed that current-voltage characteristics of the CNT–ND cathode are changed depending on the applied field; below ~3 V/µm they are similar to those of the initial CNT array and at the higher field they are close to the ND behavior. Involvement of the NDs in field emission process resulted in blue luminescence of the hybrid surface at an electric field higher than 3.5 V/µm. Photoluminescence measurements showed that the NDs emit blue-green light, while blue luminescence prevails in the CNT–ND hybrid. The quenching of green luminescence was attributed to a partial removal of oxygen-containing groups from the ND surface as the result of the hybrid synthesis.

The technique of field emission parameters research for nanostructured materials improvement

A new technique was developed to record and in situ process the data on the evolution of multiwall carbon nanotubes polymer (MWCNT-polymer) nanocomposite field emitters. It includes multichannel acquisition system of current voltage characteristic (IVC), vacuum level, temperature and mass-spectrum data with computer on-line processing of these data based on the LabVIEW program. We obtained the time dependence of the field enhancement factor, the number of emission sites and kinetics of main volatile products during field emission process.

Modeling of Liquid Nuclei Generation for Field-Emission Silicon Nanocathode

This paper presents the results of mathematical modeling of heat transfer in the field emission process in a conic cathode of small dimensions with its possible melting considered. It is shown that the possibility of melting is determined by the cathode vertex angle. The melting is modeled in the framework of the phase field system using the proposed methods for the formation of the liquid phase zone.

Origin of graphitic filaments on improving the electron field emission properties of negative bias-enhanced grown ultrananocrystalline diamond films in CH4/Ar plasma

Microstructural evolution of bias-enhanced grown (BEG) ultrananocrystalline diamond (UNCD) films has been investigated using microwave plasma enhanced chemical vapor deposition in gas mixtures of CH4 and Ar under different negative bias voltages ranging from −50 to −200 V. Scanning electron microscopy and Raman spectroscopy were used to characterize the morphology, growth rate, and chemical bonding of the synthesized films. Transmission electron microscopic investigation reveals that the application of bias voltage induced the formation of the nanographitic filaments in the grain boundaries of the films, in addition to the reduction of the size of diamond grains to ultra-nanosized granular structured grains. For BEG-UNCD films under −200 V, the electron field emission (EFE) process can be turned on at a field as small as 4.08 V/μm, attaining a EFE current density as large as 3.19 mA/cm2 at an applied field of 8.64 V/μm. But the films grown without bias (0 V) have mostly amorphous carbon phases in the grain boundaries, possessing poorer EFE than those of the films grown using bias. Consequently, the induction of nanographitic filaments in grain boundaries of UNCD films grown in CH4/Ar plasma due to large applied bias voltage of −200 V is the prime factor, which possibly forms interconnected paths for facilitating the transport of electrons that markedly enhance the EFE properties.

Towards Highly Transparent and Flexible Field Emission Displays Based on Low-Dimensional Nanocarbon

Transparency and flexibility are key words required for the devices of next generation. This is also the case for displays, for example, highly intelligent information displays for the upcoming ubiquitous world. To realize the transparent and flexible devices, nanocarbon, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene, are promising. In the present talk, nanocarbon-based highly transparent and flexible displays will be dealt with. Field emission displays (FEDs) offer promising advantages such as high brightness, low power consumption, wider viewing angle, etc. Recently some groups have demonstrated the fabrication of nanocarbon based flexible (opaque) FEDs [1, 2]. The next step is the realization of transparency. To achieve this, we are tackling several approaches based on single-walled CNTs, ion-induced conical nanocarbons and their hybrid structures formed on transparent and flexible polymer substrates [3-8]. In addition, in situ transmission electron microscope (TEM) observation of graphene and CNT formation from amorphous carbon-based nanofibers by solid phase reaction during field emission process will be demonstrated [9].1. H. S. Sim, et al., Appl. Phys. Lett., 90 (2007) 143103-143105. 2. B-J. Yoon, et al., J. Am. Chem. Soc., 127 (2005) 8234-8235.3. P. Ghosh, et al., J. Am. Chem. Soc., 132 (2010) 4034-4035.4. D. Ghosh, et al., Chem. Commun., 47 (2011) 4980-4982.5. M. Tanemura and S. P. Lau: "Flexible Field Emitters: Carbon Nanofibers" in "Carbon Nanotube and Related Field Emitters: Fundamentals and Applications" Edited by Y. Saitoh, Wiley-VCH (Weinheim) (2010), Chapt. 15.6. D. Ghosh, et al., Phys. Status Solidi RRL 6 (2012) 303–305.7. D. Ghosh, et al., Phys. Status Solidi RRL 7 (2013) 1080–1083.8. P. Ghosh, et al., J. Nanoscience and Nanotechnology, 13 (2013) 5767-5772.9. M. Zamri, M. Tanemura, et al., ASC Nano, 6 (2012) 9567-9573.

Study on the Field Emission of Titanium-Based Nanodiamond Diode Structure by Electrophoretic Deposition

A cathode process of nanodiamond field emission display based on diode structure by electrophoretic deposition has been explored. In this process, a certain proportion of nanodiamond electrophoresis liquid has been developed, the nanodiamond field emission cathode has been deposited on the polished metal titanium substrate by electrophoresis. The surface appearance and microstructures of the coating has been observed by scanning electron microscopy and Raman spectroscopy and the field emission properties of the prepared cathode coating has been tested. The experimental results show, the diamond-coated surface which has been deposited on titanium substrate is thin and uniform. Compared with the cathode prepared by the spin coating process, it has more field emission current density and better field emission current uniformity and stability. With a simple, low cost and large-area production and other advantages, it can be widely used in flat panel displays.

Electrical characteristics of nanometer gaps in vacuum under direct voltage

The electrical characteristics of nanometer gaps in vacuum were studied with the tungsten electrodes under dc and pulsed voltage. A novel experimental technique to study the electrical characteristics of nanometer gaps was presented in the paper. In the experimental setup, the tungsten was fabricated and shaped to be a perfect sphere through the electrochemical etch and Joule melting method, and the nanogap was controlled precisely by the scanning electron microscope (SEM) and nanometer manipulator. The effects of electrode geometry, gap separation and injected voltage waveform were investigated. The current-voltage curves and Fowler-Nordheim plots showed the difference of the field emission process before breakdown between the sphere-sphere electrodes and needle-sphere electrodes. The gap separation dependence of dielectric strength demonstrated the similar trend to the previous work but better performance. The breakdown voltage for pulsed voltage was 4-5 times higher than that for the dc voltage. The analysis of the physical damage indicated that the current, duration time and electrode geometry played important roles in electrode modification. In addition, a possible mechanism of nanoscale vacuum breakdown was also proposed in the paper.

Direct observation of structural change in Au-incorporated carbon nanofibers during field emission process

We present a direct observation of structural change in Au-incorporated carbon nanofiber (CNF) during a field emission (FE) process using an in situ transmission electron microscopy (TEM). The TEM images and their electron diffraction pattern together with electron energy loss spectroscopy (EELS) indicated that the amorphous carbon structure of CNF was transformed to hollow graphitic structure catalyzed by dispersed Au particles before FE. Thus, the in situ TEM study will be promising to reveal the formation process of nanocarbon materials by solid phase reaction depending on the catalyst metal element for metal-incorporated CNFs.

Improvement in plasma illumination properties of ultrananocrystalline diamond films by grain boundary engineering

Microstructural evolution of ultrananocrystalline diamond (UNCD) films as a function of substrate temperature (TS) and/or by introducing H2 in Ar/CH4 plasma is investigated. Variation of the sp2 and sp3 carbon content is analyzed using UV-Raman and near-edge X-ray absorption fine structure spectra. Morphological and microstructural studies confirm that films deposited using Ar/CH4 plasma at low TS consist of a random distribution of spherically shaped ultra-nano diamond grains with distinct sp2-bonded grain boundaries, which are attributed to the adherence of CH radicals to the nano-sized diamond clusters. By increasing TS, adhering efficiency of CH radicals to the diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nano-sized diamond grains break, whereas the addition of 1.5% H2 in Ar/CH4 plasma at low TS induces atomic hydrogen that preferentially etches out the t-PA attached to ultra-nano diamond grains. Both cases make the sp3-diamond phase less passivated. This leads to C2 radicals attaching to the diamond lattice promoting elongated clustered grains along with a complicated defect structure. Such a grain growth model is highly correlated to explain the technologically important functional property, namely, plasma illumination (PI) of UNCD films. Superior PI properties, viz. low threshold field of 0.21 V/μm with a high PI current density of 4.10 mA/cm2 (at an applied field of 0.25 V/μm) and high γ-coefficient (0.2604) are observed for the UNCD films possessing ultra-nano grains with a large fraction of grain boundary phases. The grain boundary component consists of a large amount of sp2-carbon phases that possibly form interconnected paths for facilitating the transport of electrons and the electron field emission process that markedly enhance PI properties.

Field Emission from Decorated Carbon Nanotube–QDs Microstructures with a View to the Dominant Electron Paths

We present a study on the field emission properties of a hybrid system comprised of carbon nanotube (CNT) micropillars decorated with quantum dots (QDs). With controlled decoration of QDs on the CNT micropillars through a simple assisted self-assembly process, further enhancement in the field emitting property of the hybrid microstructures was detected. Upon irradiation of the hybrid structure with a broad visible-light laser beam, additional enhanced field emission was observed. Analyses using fluorescence and confocal microscopy, as well as ultraviolet photoelectron spectroscopy, suggested that electron transfer from QDs to the CNT strands and the reduced work function of the hybrid system as the contributing factors behind the enhanced field emissions. In addition, we discovered that the field emission process gave rise to lost of the QDs’ fluorescence luminosity on the microstructures in specific patterns attributable to transfers of charge carrier from QDs to the CNTs. This observation provided a new...

The potential application of ultra-nanocrystalline diamond films for heavy ion irradiation detection

The potential of utilizing the ultra-nanocrystalline (UNCD) films for detecting the Au-ion irradiation was investigated. When the fluence for Au-ion irradiation is lower than the critical value (fc = 5.0 × 1012 ions/cm2) the turn-on field for electron field emission (EFE) process of the UNCD films decreased systematically with the increase in fluence that is correlated with the increase in sp2-bonded phase (π*-band in EELS) due to the Au-ion irradiation. The EFE properties changed irregularly, when the fluence for Au-ion irradiation exceeds this critical value. The transmission electron microscopic microstructural examinations, in conjunction with EELS spectroscopic studies, reveal that the structural change preferentially occurred in the diamond-to-Si interface for the samples experienced over critical fluence of Au-ion irradiation, viz. the crystalline SiC phase was induced in the interfacial region and the thickness of the interface decreased. These observations implied that the UNCD films could be used as irradiation detectors when the fluence for Au-ion irradiation does not exceed such a critical value.

Constructing Ag nanoparticles–single wall carbon hybrid nanostructure to improve field emission properties

The overall process of field emission can be dominated by the contact resistance between the single wall carbon nanotubes (SWCNTs) and the substrate. In order to reduce the contact resistance, we present a wet chemical process of constructing Ag–SWCNTs hybrid nanostructure. Constructing Ag–SWCNTs emitters could improve the electrical contact by increasing the contact area between SWCNTs and substrate. Contact resistance is greatly reduced compared to that of pristine SWCNTs. Field emission properties of Ag–SWCNTs hybrid emitters including current density and emitting image are remarkably improved. Field emission properties based on the modified Fowler–Nordheim (F–N) equation are discussed. Our studies show that this method can enhance the field emission properties of the SWCNTs by improving the contact resistance and is a promising way for mass production of SWCNTs for field emission display.

In Situ TEM Observation of Fe-Included Carbon Nanofiber: Evolution of Structural and Electrical Properties in Field Emission Process

In situ transmission electron microscopy (TEM) of single Fe-included carbon nanofibers (CNFs) revealed that the fine polycrystalline structure in the shank region of CNFs transformed to graphitic, hollow structures during a field emission (FE) process. The iron metal platelets agglomerated during the FE process and perceptibly were emitted from the shank, which featured bamboo-like carbon nanotube (CNT) structures. The structural evolution also improved the electrical properties, and the FE current was remarkably increased, that is, 1000 times higher than the initial value (from 10(-9) to 10(-6) A). The structural transformations were effectuated by Joule heating that generated simultaneously during the FE process. The in situ TEM study of room-temperature-synthesized CNFs could provide essential information regarding CNFs' structural transformation for their possible application in future electron emitter sources.

Electric‐field‐assisted crystallisation in phase‐change materials

AbstractPhase‐change materials are of intense research interest due mainly to their use in phase‐change memory (PCM) devices that are emerging as a promising technology for future non‐volatile, solid‐state, electrical storage. Electrically driven transitions from the amorphous to the crystalline phase in such devices exhibit characteristic threshold switching. Several alternative electronic explanations for the origins of this characteristic behaviour have been put forward, for example Poole–Frenkel effects, delocalisation of tail states, field emission processes and space charge limited currents [for a full discussion, see Radielli et al., J. Appl. Phys. 103, 111101 (2008) and Simon et al., MRS Proc. 1251, H01–H011 (2010)]. However, an alternative to these conventional electronic models of threshold switching is based on electric field induced lowering of the system free energy, leading to the field induced nucleation of conducting crystal filaments. In this paper we investigate this alternative view. We present a detailed kinetics study of crystallisation in the presence of an electric field for the phase‐change material Ge2Sb2Te5. We derive quantitative crystallisation maps to show the effects of both temperature and electric field on crystallisation and we identify field ranges and parameter values where the electric field might play a significant role. Then we carry out physically realistic simulations of the threshold switching process in typical phase‐change device structures, both with and without electric field dependent energy contributions to the system free energy. Our results show that threshold switching can be obtained by a mechanism driven purely by electric field induced nucleation, but the fields so required are large, of the order of 300 MV m−1, and significantly larger than the experimentally measured threshold fields.

A study of field emission process in electrostatically actuated MEMS switches

A study of field emission process in MEMS-based capacitor/switch-like geometries is presented. High resolution current–voltage characteristics up to breakdown have been obtained across micro-gaps in fixed–fixed Metal–Air–Metal and Metal–Air–Insulator–Metal structures. In metallic devices the I–V dependence reveals Fowler–Nordheim theory effects. In the presence of insulator the process is found to be limited by the film conductivity following Poole–Frenkel dependence. The data analysis reveals the major importance of surface asperities on the onset of the field emission process while it is also presented that charge transfer may occur between metal and insulator surfaces even in the presence of micrometer scale gaps.

The induction of a graphite-like phase by Fe-coating/post-annealing process to improve the electron field emission properties of ultrananocrystalline diamond films

The electron field emission (EFE) process for ultrananocrystalline diamond (UNCD) films was tremendously enhanced by Fe-coating and post-annealing processes. The extent of enhancement changes with the granular structure of the UNCD films and the post-annealing conditions (temperature and atmosphere). The best EFE properties are obtained by post-annealing the films at 900°C in an H2 environment for 5min. The EFE behavior of the films can be turned on at E0=1.28V/μm, attaining a large EFE current density of 772μA/cm2 at an applied field of 8.8V/μm. Microstructural analysis indicates that the mechanism for the improvement in the EFE process is the formation of graphene-like phase (a-few-layer graphite) with good crystallinity, surrounding the Fe (or Fe3C) nanoclusters. Presumably, the nanographites were formed via the reaction of Fe-clusters with diamond films, viz. the Fe-clusters dissolved the carbons in the diamond grains and the re-precipitated them on the surface of the other side of clusters, a process similar to the growth of carbon nanotubes via Fe clusters as catalyst.

Ultraviolet light-enhanced field electron emission of zinc oxide nanowires

One-dimensional nanomaterial possesses an electric field reinforcing effect, and its field emission properties have aroused much interest. In this paper, ZnO nanowire (NW) arrays are prepared by the thermal chemical vapor deposition (CVD) method, and the characteristic of field electron emission of ZnO NW arrays under the illumination of ultraviolet light is investigated. It is found that, upon ultraviolet light illumination, the turn-on voltage drops off and emission current increases. A process of field emission coupled with semiconducting properties of ZnO NWs is proposed. Ultraviolet photon-excited electron transition from valence band to conductance band and defect energy levels of ZnO NWs can lead the number of emitting electrons to increase, and the photoemission reduces the effective work function of zinc oxide emitters, which largely enhances the field emission performance. The characteristic of field emission of ZnO NWs under ultraviolet light illumination suggests an approach to tuning field emission of semiconductor emitters, which is promising for the applications in optical sensor, cold-cathode flat panel display and field electron source.

Atomic layer coating of hafnium oxide on carbon nanotubes for high-performance field emitters

Carbon nanotubes coated with hafnium oxide exhibit excellent electron emission characteristics, including a low turn-on voltage, a high field enhancement factor, and exceptional current stability. Their enhanced emission performance was attributed to a decrease in the work function and an increase in the electron density of states at the carbon nanotube Fermi level closest to the conduction band minimum of hafnium oxide. In addition, the enhanced current stability was attributed to the ability of hafnium oxide to protect the carbon nanotubes against ions and free radicals created in the electron field emission process.

Photo-enhanced field electron emission of cadmium sulfide nanowires

The response of field electron emission of cadmium sulfide (CdS) nanowires (NWs) to visible light has been investigated. It is found that, upon light illumination, the turn-on voltage drops, emission current increases obviously, and the Fowler-Nordheim behavior deviates from a straight line. A process of field emission coupled with semiconducting properties of CdS NWs is proposed. Photon-excited electron transition from the valence band to the conductance band of CdS nanowires increases the quantity of emitting electrons, and the photoemission decreases the effective work function of CdS emitters, which largely enhances the field emission performance. The response of field emission of CdS NWs to light illumination suggests an approach for tuning field emission of semiconductor emitters.