Carbon Nanotube Field Emitter Arrays Research Articles

Vertically aligned carbon nanotube field emitter arrays with Ohmic base contact to silicon by Fe-catalyzed chemical vapor deposition

In this study, dense arrays of aligned carbon nanotubes are obtained by thermal catalytic chemical vapor deposition, using Fe catalyst dispersed on a thin Ta layer. Alignment of the carbon nanotubes depends on the original Fe layer thickness from which the catalyst dispersion is obtained by thermal treatment prior to the synthesis step, as well as on the synthesis temperature, which in this study was varied between 680 and 740°C. For the intended application of the nanotubes as field emitters, it is essential that electrical conductivity between the tubes and the substrate is guaranteed. In a novel approach, a noble metal layer, of 100nm Pt or Au, was applied between the Ta layer and the support, which ensures an Ohmic contact throughout the metal multilayer which connects the silicon substrate with the CNT fibers, because oxidation of Ta at its interface with Si is prevented. For optimized dense arrays of aligned nanotubes with low Ohmic resistance, field emission in nitrogen atmosphere is demonstrated, with field enhancement factors of up to ca. 2000 for nanotubes grown using Fe/Ta catalyst deposited on Au and Pt.

Field emission from optimized structure of carbon nanotube field emitter array

The authors report a detail study on the emission properties of field emitter array (FEA) of micro-circular emitters of multiwall carbon nanotubes (CNTs). The FEAs were fabricated on patterned substrates prepared with an array of circular titanium (Ti) islands on titanium nitride coated tantalum substrates. CNTs were rooted into these Ti islands to prepare an array of circular emitters. The circular emitters were prepared in different diameters and pitches in order to optimize their structure for acquiring a high emission current. The pitch was varied from 0 to 600 μm, while a diameter of circular emitters was kept constant to be 50 μm in order to optimize a pitch. For diameter optimization, a diameter was changed from 50 to 200 μm while keeping a constant edge-to-edge distance of 150 μm between the circular emitters. The FEA with a diameter of 50 μm and a pitch of 120 μm was found to be the best to achieve an emission current of 47 mA corresponding to an effective current density of 30.5 A/cm2 at 7 V/μm. The excellent emission current was attributed to good quality of CNT rooting into the substrate and optimized FEA structure, which provided a high electric field on a whole circular emitter of 50 μm and the best combination of the strong edge effect and CNT coverage. The experimental results were confirmed with computer simulation.

High Field Emission Current Density from Patterned Carbon Nanotube Field Emitter Arrays with Random Growth.

High field emission (FE) current density from carbon nanotube (CNT) arrays grown on lithographically patterned silicon substrates is reported. A typical patterned field emitter array consists of bundles of nanotubes separated by a fixed gap and spread over the entire emission area. Emission performance from such an array having randomly oriented nanotube growth within each bundle is reported for different bundle sizes and separations. One typical sample with aligned CNTs within the bundle is also examined for comparison. It is seen that the current density from an array having random nanotube growth within the bundles is appreciably higher as compared to its aligned counterpart. The influence of structure on FE current densities as revealed by Raman spectroscopy is also seen. It is also observed that current density depends on edge length and increases with the same for all samples under study. Highest current density of -100 mA cm(-2) at an applied field of 5 V/μm is achieved from the random growth patterned sample with a bundle size of 2 μm and spacing of 4 μm between the bundles.

Operation of a Carbon Nanotube Field Emitter Array in a Hall Effect Thruster Plume Environment

The Hall effect thruster is an electric propulsion device for space applications that efficiently reduces the propellant mass requirements on a spacecraft in comparison with chemical rockets. To date, the Hall effect thruster technology relies on the thermionic cathodes that consume up to 10% of the total propellant used in the system to neutralize the ion beam of the thruster; however, such propellant usage does not contribute to thrust generation. An array of thin-film, carbon nanotube field emitters that emit electrons through field emission can potentially neutralize the ion beam without consuming any propellant. This paper examines the effects of 40 min exposure of carbon nanotube field emitter arrays to the plasma environment in the exit plane of a Hall effect thruster. The physical structures that enable field emission appear largely unaffected by placement in the plasma as well as operation in the plasma. This indicates that a refined design of this carbon nanotube field emitter array may potentially provide an alternative to the thermionic cathode used on contemporary Hall effect thrusters and verifies that no fundamental incompatibilities exist between these two technologies.

Carbon Nanotube Electron Emitter for X-ray Imaging

The carbon nanotube field emitter array was grown on silicon substrate through a resist-assisted patterning (RAP) process. The shape of the carbon nanotube array is elliptical with 2.0 × 0.5 mm2 for an isotropic focal spot size at anode target. The field emission properties with triode electrodes show a gate turn-on field of 3 V/µm at an anode emission current of 0.1 mA. The author demonstrated the X-ray source with triode electrode structure utilizing the carbon nanotube emitter, and the transmitted X-ray image was of high resolution.

Effect of micro and nanoparticle inorganic fillers on the field emission characteristics of photosensitive carbon nanotube pastes

We successfully fabricated field emitter arrays of carbon nanotube (CNT) dots of 10 μm diameter with excellent field emission properties by using photosensitive CNT paste. The CNT paste was investigated in terms of morphologies, current –voltage properties, and luminous uniformities by varying the mixing ratios of micro and nanoparticle inorganic fillers and the amount of CNTs added into the paste. The 3:1 mixing of micro and nanoparticle fillers and the addition of 5% CNTs in the paste brought about the best field emission characteristics of dot-patterned CNT field emitter arrays.

Carbon nanotube pillar arrays for achieving high emission current densities

We introduce an innovative geometry carbon nanotube (CNT) field emitter array capable of achieving stable and high current densities. Arrays of toroid CNT pillars were grown directly on bulk metal alloy substrates and on patterned metal catalyst on silicon substrates. Compared to a solid CNT pillar array (CPA), this toroid CPA (tCPA) provides a larger edge area for achieving a higher stable current density of 50 mA/cm2 at an applied dc field of less than 8 V/μm. Electrostatic simulation data confirming the field enhancement at the inner and outer edges of the tCPA are also presented.

Fabrication and Field Emission Properties of Triode-Type Carbon Nanotube Emitter Arrays

We report here an effective method for the fabrication of a large number of triode-type microgated carbon nanotube field emitter arrays. Our technique combines dual-beam focused ion beam technology and plasma-enhanced chemical vapor deposition, avoiding the tedious lithography and wet chemistry procedures conventionally used to fabricate such structures. Field emission testing revealed that increasing gate voltage by as little as 0.3 V had significant impact on the local electric fields, lowering the turn on and threshold fields by 3.6 and 3.0 V/microm, respectively. The field enhancement factor of the emitter arrays was also increased from 149 to 222. A quantum mechanical model for such triode-type field emission indicates that the local electric field generated by a negatively or positively biased gate directly impacts the tunneling barrier thickness and thus the achievable emitter current.

Efficient Carbon Nanotube Field Emitter using Electrospun Carbon Nanofibers as a Flexible Electrode

AbstractThermal-stable, conductive, and flexible carbon fabric (CF), which is composed of thin carbon fibers prepared by electrospinning, was used for the substrate of carbon nanotube (CNT) field emitter arrays. The field emitter arrays were prepared by chemical vapor deposition (CVD). The current density-electric field characteristics revealed that the CNT field emitter arrays on CF produced a higher current density at a lower turn-on voltage compared to ones on a Si substrate. This emitter integrated with a gate electrode based on hierarchy-structured carbon materials, CNTs on CF, can be used for light sources, displays, and other electronic devices.

Carbon nanotubes on carbon fabrics for flexible field emitter arrays

Flexible carbon fabrics (CFs) prepared by electrospray deposition were used for the substrate of carbon nanotube (CNT) field emitter arrays. CNTs were grown on the CFs by chemical vapor deposition. The CNT field emitter arrays on CFs produced a higher current density at a lower turn-on voltage compared to ones on a Si substrate.

P‐155: Enhanced Electron Emission with Robust CNTs Grown by Resist‐Assisted Patterning Process

AbstractWe developed a novel carbon nanotube field emitter arrays (CNT‐FEAs) with resist‐assisted patterning process by dc‐plasma enhanced chemical vapor deposition (PECVD). Through this method, we obtained a very strong bonded CNT with substrate. The CNTs were grown with −600 V bias to substrate electrode and +300 V bias to mesh grid being placed 10mm above the substrate holder electrode. The structure and electrical properties of the CNTs were strongly related to the growth time. The length was decreased after 80 minute and the diameter increases with growth time. An electron emission current increases with growth time. The growth mechanism, electron emission characteristics, and mechanical robustness were discussed.

열 및 불산 처리를 통한 탄소나노튜브의 전자 방출 특성의 향상 연구

We studied the effect of thermal annealing and hydrofluoric (HF) acid treatment on the field emission properties of carbon nanotube field emitter arrays (CNT-FEAs) grown with the resist-assisted patterning (RAP) process. After thermal and HF treatment, it was observed that the electron emission properties were remarkably improved. The enhanced electron emission was also found to depend strongly on the sequence of the treatments; the electronemission current density is 656 with the process of thermal treatment prior to HF treatment while the current density is reduced by 426 with the reversal processes. This is due to the increased crystalline structure by thermal annealing and then strong fluorine bond was formed by HF treatment.

Effect of electrical aging on field emission from carbon nanotube field emitter arrays

We studied the effect of bias aging on the field emission properties of selectively patterned carbon nanotube field emitter arrays (CNT) grown using the resist-assisted patterning process. After electrical aging using an electric field of 6.87V∕μm for 40h, it was observed that the electron emission properties and uniformity were remarkably improved. X-ray photoelectron spectroscopy spectra show a shift of 0.2eV in the O1s peak and the concentration of oxygen is reduced. Also, transmission electron microscopy measurements showed that Ni catalyst was removed from CNT tips after aging. Therefore, after electrical aging, we obtained enhanced and stable electron emission over a wide current range.

Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm2 and 7.0 mA/cm2, respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.

Fabrication of carbon nanotube-based nanodevices using a combination technique of focused ion beam and plasma-enhanced chemical vapor deposition

This study focuses on the fabrication of two nanodevice prototypes which utilized vertical and horizontal carbon nanotubes used the focused ion beam to localize the catalysts, followed by plasma-enhanced chemical vapor deposition. First, metal-gated carbon nanotube field emitter arrays were fabricated on multilayer substrates containing an imbedded catalyst layer. Second, horizontally aligned single-walled carbon nanotubes were grown on a transmission electron microscopy grid. This allows the carbon nanotubes to be directly analyzed in a transmission electron microscope. It is expected that the methodology introduced here will open up opportunities for the direct fabrication of carbon nanotube based nanodevices.

Electrostatic Shielding in Patterned Carbon Nanotube Field Emission Arrays

As the morphologies of patterned carbon nanotube (CNT) field emission arrays become more complex, it is critical to examine the interactions of individual emitters in close proximity to one another. The number of active emission sites in dense arrays can be severely inhibited by electrostatic shielding. To explore this effect, we have fabricated an array of microscale CNT bundles with varying diameters. Field enhancement was found to increase with decreasing bundle diameter. High-resolution scanning electron microscopy revealed a corresponding increase in the number of stray CNT tips as the bundle diameter decreased. These CNT tips were not as heavily shielded, resulting in improved emission current density. Our results confirm that electrostatic shielding significantly affects the field enhancement of dense arrays, particularly in terms of the number of emitters actively participating in the field emission process.

Diamond/Carbon Field Emission Based Structures for Sensors and MEMS

Chemical vapor deposited diamond/carbon micro- and nanostructures are excellent field emission cathodes for novel applications in sensors, MEMS, and NEMS because of their low electron affinity and excellent mechanical and chemical properties. However, practical micropatterning and fabrication of emitter structures are needed to fully exploit their potential use. This paper describes the use of microfabrication methods to achieve various forms of morphologically-engineered cathodes such as micro pyramidal tips with self-aligned gated membrane, nano-diamond lateral field emitters with built-in anode, and carbon-nanotube field emission arrays with self-aligned gate. We report the achievement of these micropatterned diamond/carbon field emission-based sensing platforms with excellent field emission characteristics. These field emitter-based microstructures are efficient sensing platforms from which novel sensors and MEMS can be derived.

FABRICATION AND ELECTRON MICROSCOPY CHARACTERIZATION OF METAL-GATED CARBON NANOTUBE EMITTER ARRAYS

A technique was investigated for the fabrication of triode-type carbon nanotube (CNT) field emitter arrays, where an integrated extraction gate was built between the nanotube cathode and the anode. The gate improves the control capability of emission currents. To fabricate the metal-gated CNT field emitter arrays, well ordered cells were generated by focused ion beam (FIB) milling of platinum ( Pt ) coated silicon ( Si ) substrate and then modified by chemical etching. Two types of catalyst elements iron ( Fe ) and nickel ( Ni ), were used for growing the CNTs inside the cells. The methods for depositing catalysts into the cells include spin coating sol–gel Fe , FIB induced decomposition of ferrocene and sputter coating pure Ni . CNT growth was carried out by a chemical vapor deposition (CVD) process. The results suggest that the CNTs grew from inside the cells where the catalysts were located. In comparison, the CNTs synthesized from the sol–gel Fe catalyst were straighter than those from ferrocene Fe and pure Ni . The density and orientation of the CNTs in each cell are directly related to the type and quantity of the catalysts and are also affected by the size of the cells.

Anodic aluminum oxide membrane bonded on a silicon wafer for carbon nanotube field emitter arrays

We have developed a method to bond a very thin anodic aluminum oxide membrane (400nm thick) on a Si wafer. Furthermore, we were able to fabricate well-ordered carbon nanotube (CNT) arrays on the membrane at a very high temperature—above 1000°C—without deformation. The CNT arrays fabricated at 800°C exhibited long-term stability and uniform emission. Their current density was higher than 1mA∕cm2; such a density might be required for flat panel displays. When the tip of the CNTs was modified from an open shape to a closed shape by exposure to acetylene gas, the turn-on voltage decreased significantly and the enhancement factor increased significantly.

1 A ∕ cm 2 current density from microgated carbon nanotube field-emitter arrays grown by dc plasma chemical-vapor deposition

We report improved emission performance of microgated carbon nanotube field-emitter arrays. The arrays with 0.1mm2 areas produced anode currents in excess of 1mA (1A∕cm2) and transconductance of up to 174μS (174mS∕cm2) at gate voltages of 75V or less. The carbon nanotubes were grown by dc plasma chemical-vapor deposition, but were not oriented vertically during inspection in the scanning electron microscope. Most of the nanotubes had diameters near 10nm. We suggest that the small tube diameter caused the relatively high emission current and also produced limber tubes, unable to support themselves vertically.