Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

A novel fabrication approach for carbon nanotube lateral field emission devices

A novel fabrication approach for growing carbon nanotubes (CNTs) laterally and selectively on the tip region of lateral micro-fingers with built-in metallic anode utilizing a single-mask microfabrication process is presented. The selective growth of the CNTs was achieved with a two-step microwave plasma-enhanced chemical vapor deposition process involving a pre-growth hydrogen plasma treatment. Without plasma pretreatment, CNTs were found to grow along edges of the sandwiched tri-metal layer including the anode. Interestingly, with plasma pretreatment, CNTs grew selectively near the sharp tip region. Moreover, specific CNTs could be selectively synthesized on the tip region by optimizing the plasma pretreatment and the growth time. In essence, a lateral field emission device having CNT emitters with integrated metallic anode can be fabricated in just a single-mask microfabrication process. This approach can enhance the feasibility of integrating CNTs into vacuum integrated circuits.     

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts

The effects of palladium (Pd) catalyst film thickness and ammonia (NH₃) in thermal chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) are systematically compared per the resulting morphologies, Raman spectra and field emission characteristics. The CNT field emitters were tested under identical experimental configurations. Field emission characteristics were described with Fowler-Nordheim field emission theory. Experimental results demonstrate that thermally grown CVD CNTs configured as diode field emitters exhibit low turn-on fields and high emission current density. The work is extended to include the study of gated field emitters or field emission triode, important to achieving high-resolution, full gray-scale imaging for field emission, flat-panel displays. The gated device was fabricated utilizing single-mask, self-aligned gate electrode with conventional integrated-circuit (IC) fabrication process. The CNT-triode showed gate-controlled modulation of emission current where higher gate voltage gives rise to higher anode currents. The triode fabrication process using silicon-on-insulator (SOI) wafers is discussed.     

Quantum-mechanical simulations of field emission from carbon nanotubes

We present simulations of field emission from 2-nm long open (5,5), closed (5,5) and open (10,0) carbon nanotubes. Besides usual effects associated with the field-emission process, the total-energy distributions of the field-emitted electrons present peaks that are shifted by the electric field. Their sharpness and the evolution of their amplitude when changing the electric field depend on the semiconducting or metallic character of the nanotube.     

In-situ studies of electron field emission of single carbon nanotubes inside the TEM

Electron field emission characteristics of individual multi-walled carbon nanotubes (MWCNTs) were investigated in situ inside the transmission electron microscope (TEM). For a single MWCNT it was found that while field-emission can hardly occur from the side of the nanotube, a curved nanotube may result in finite side emission and the best emission geometry is the top emission geometry. Current-voltage (I-V) measurements made at different vacuum conditions and voltage sweeps emphasize the importance of the adsorbates on the electron field emission of MWCNTs. For a contaminated MWCNT, although the field emission current was reduced, the stability of its emission was improved. A current of up to several tens of μA was observed for a single MWCNT, but it was found that long time emission usually results in drastic structure damage that may lead to sudden emission failure.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Flame-synthesis of carbon nanotubes on silicon substrates and their field emission properties

We reported the flame-synthesis of patterned multiwalled carbon nanotubes (CNTs) on silicon substrate by a shadow mask and their field emission properties. It was found that CNTs with tangled and curved morphology were preferentially grown around the cracked edges of Ni dot pattern. A crack-induced catalyst-activation growth mechanism was proposed. The patterned CNTs fabricated by such a simple flame-synthesis method exhibited good field emission characteristics with uniform emission patterns and reproducible and stable emission behaviors, although the CNTs possessed many defective graphite layers and showed relatively higher turn-on and threshold field than other reported CNTs grown by chemical vapor deposition. Our results demonstrated that such a low-cost and scaleable CNT pattern fabrication process can be expected to have favorable applications in field emission devices.     

Interaction between carbon nanotubes and substrate and its implication on field emission mechanism

The interaction between carbon nanotubes (CNTs) and substrate plays an important role in the process of field emission. A double-barrier model is adopted to analyze the difference of field emission for various CNT films. Result shows that the width of interface barrier determines the emission performance. For CNTs on titanium, the best emission performance is attributed to the removal of interlayer barrier by the formation of conductive titanium carbide. These facts might shed new light on the field emission mechanism for CNTs.     

Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method

Thin multiwalled carbon nanotube (t-MWCNTs)-based field emitters are made by use of a spray method. The number of tube walls is between 2 and 6, with the corresponding outer diameters between 3 and 6 nm. They were dispersed in dichloroethane and sprayed onto metal-deposited indium tin oxide glass. After heat treatment, they were found to be tightly adhered to metal electrode. Excellent field emission characteristics were exhibited, with a large field enhancement factor and low turn-on voltage, comparable to those of singlewalled CNTs. However, the t-MWCNTs demonstrated a significantly lower degradation rate than SWCNTs in the emission current. This high emission stability was attributed to their stable edge structures, similar to conventional large-diameter MWCNTs. Therefore, t-MWCNTs could be utilized as an alternative material for field emitters.     

Density control of carbon nanotubes and filaments films by wet etching of catalyst particles and effects on field emission properties

Carbon nanotubes and filaments (CNT&F) films with controlled density were grown by low pressure thermal chemical vapour deposition from acetylene on nickel nanoparticles. Density control was achieved by wet etching of the catalyst particles before carbon growth. Field emission measurements were carried out on several films with different CNT&F densities obtained with this method. Despite strong morphological changes, only slight differences in the field emission characteristics between the highest and the lowest density films were detected, suggesting that almost none of the CNT&F suppressed by the etching step took part to the field emission. However, a maximum field amplification factor was reached for the medium density film. Taking into account the field amplification factor distribution, a model is proposed to link the particles diameter distribution, the CNT&F film morphology and the field emission properties.     

Improved field emission stability from single-walled carbon nanotubes chemically attached to silicon

ABSTRACT: Here we demonstrate the simple fabrication of a single walled carbon nanotube (SWCNT) field emission electrode which shows excellent field emission characteristics and remarkable field emission stability without requiring post treatment. Chemically functionalized SWCNTs were chemically attached to a silicon substrate. The chemical attachment led to vertical-alignment of SWCNTs on the surface. Field emission sweeps and Fowler-Nordheim plots showed that the Si-SWCNT electrodes field emit with a low turn-on electric field of 1.5 V mu m-1 and high electric field enhancement factor of 3965. The Si-SWCNT electrodes were shown to maintain a current density of > 740 mu A cm -1 for 15 hr with negligible change in applied voltage. The results indicate that adhesion strength between SWCNTs and substrate is a much greater factor in field emission stability than previously reported.     

Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers

The influence of applied electric field (E_av) and emission current (I_FE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layers’ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below E_av = 3.2–3.9 V/μm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 μm, with an edge thickness of 200 μm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to E_av = 5.7–8 V/μm and at an emission current I_FE = 2 × 10⁻⁵ A these tubes disappeared (or essentially shortened). At E_av = 8 V/μm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of E_av and I_FE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as E_av, I_FE and exposure time increase.     

A strategy for forming robust adhesion with the substrate in a carbon-nanotube field-emission array

A method of how to form a strong adhesion of carbon nanotubes (CNTs) to substrate by introducing an intermediate metal layer in fabricating spray-based field emitters and its effect to the field-emission properties were addressed. An Indium or tin layer was deposited on an indium-tin-oxide glass substrate and thermally annealed to hold the CNTs tightly. This underlying metal layer forms a strong bond to CNTs by wetting while melting without any detectable evaporation of metal atoms. This gave rise to a stable field-emission current at high voltage, which was confirmed by a mass spectrometer. The tube density and field-emission characteristics strongly depend on the annealing temperature of metal layers.     

Pulsed laser deposition-assisted patterning of aligned carbon nanotubes modified by focused laser beam for efficient field emission

Patterned carbon nanotube (CNT) arrays have been synthesized on patterned substrates created via pulsed laser deposition (PLD) of the precursor catalyst films with a mask. Arrays of CNTs in square and hexagonal patterns with tube lengths of 8 μm and 16 μm were created on silicon or quartz substrates, respectively. Using the method of laser cutting, as-grown CNT patterns were pruned by focused He-Ne laser beam. It is found that after pruning, CNTs tend to cluster together and form welded junctions. The comparison of field emission properties of CNTs before and after pruning shows that laser modification of CNT morphologies effectively enhanced the emission currents.     

Fabrication of self-defined gated field emission devices on silicon substrates using PECVD-grown carbon nano-tubes

Fabrication of novel self-defined gated field-emission devices on silicon substrates using vertically grown carbon nano-tubes (CNT) is reported. Carbon nano-tubes are grown in a PECVD reactor from the Ni catalyst islands at a pressure of 1.6 Torr with a mixture of C₂H₂ and H₂ gases with 5 and 30 sccm flows, respectively. The growth occurs at temperatures ranging between 550 and 650 °C and CNT’s are electrically isolated by a TiO₂ film. Silver is used as the metal gate and complete fabrication of transistors requires removing the insulating layer from top of the tip followed by one step of plasma ashing. With a voltage applied between gate and the cathode electrode, the emission current from cathode to anode shows a significant drop, indicating proper control of gate on the anode current.     

Observation of electron field emission patterns from B-doped diamond films

To develop electron beam sources of carbon materials, field emission patterns were observed in three different setups. The first was a diode-type, in which a carbon specimen was facing to a positively biased fluorescence plate. The second was a triode-type, in which a positively biased grid was placed between them. In the third setup, a commercial electron gun was modified so that it could accommodate a carbon specimen and a grid. A fluorescence plate was placed in a vacuum chamber outside the gun. As the carbon specimen for electron emission source, B-doped diamond films, a single crystal diamond with a B-doped layer, an undoped diamond film and a glass-like carbon both with a fibrous structure at the surface, and a sponge carbon were used. It was found that electron emission from edges was dominant for 1×1 cm diamond films and carbon specimens in the diode-type setup. In the triode-type setup, the edges of the specimens were masked with a Kapton® tape. The electron emission occurred only from some spots on the specimen. In the electron gun setup, it was confirmed that an electron beam was generated, and a fairly uniform circle was seen on the fluorescence plate under defocused situation, while the circle became smaller by adjusting the current of the focusing lens. Although more uniform emission from the electron source materials seemed to be necessary for practical applications, it was demonstrated that an electron beam could be generated even in such a simple setup.     

Carbon nanotube films as electron field emitters

Carbon nanotubes have been recognized as one of the most promising electron field emitters currently available. We review the state of the art of current research on the electron field emission properties of carbon nanotube films and present recent results outlining their potential as field emitters as well as illustrating some current concerns in the research field.     

Focused-ion-beam assisted fabrication of individual multiwall carbon nanotube field emitter

We report the fabrication of an individual carbon nanotube (CNT) electron field emitter using a focused-ion-beam (FIB) technique. The monolithic multiwall CNT with a graphitic shield is synthesized using chemical vapor deposition technique. The FIB technique is applied to attach the monolithic multiwall CNT on an etched tungsten tip. Field emission measurements are carried out in a vacuum of 10⁻⁷ Torr. Threshold voltage as low as 120 V has been obtained.     

Enhancement of the field emission properties of low-temperature-growth multi-wall carbon nanotubes by KrF excimer laser irradiation post-treatment

Multi-wall carbon nanotube (CNT) films were fabricated by microwave plasma chemical vapor deposition at low temperatures ( 500 °C). The films when properly post-treated by laser irradiation exhibited a factor of 2–3 enhancement in the emission current, while the turn-on field (E_on) was reduced from 4.89–5.22 to 2.88–3.15 V/μm. The introduction of excessive oxygen during laser irradiation, however, degrades the performance of field emission properties drastically. Raman spectroscopy measurements revealed the intimate correlation between the parameter I_D/I_G (intensity ratio between the two representative Raman peaks seen in carbon nanotubes) and the field emission performance. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that the irradiation-induced modification of the tube morphology and crystallinity might be responsible for the observations.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.