Low-field electron emission of self-organized laser-produced micro-tip arrays with incorporated carbon nanotubes

Low-field electron emission is reported from quasi-periodical arrays of micro-tips produced by laser-assisted evaporation of single crystal Si wafers in vacuum and Ni wafers in liquids. To improve the emission from the micro-tips, single-wall carbon nanotubes were incorporated into surface layers of the tips during or after micro relief formation. The best samples showed the emission at threshold fields of as low as 0.3–1 V/μm for Si substrates and of 1.5–2 V/μm for Ni substrates as measured with the flat screen technique. Improvement of the emission surface uniformity and lower threshold fields were achieved by incorporating of carbon nanotubes when compared with ‘undoped’ Si and Ni micro-tips. The experimental data and microstructural analysis enables us to explain the emission from the tip arrays by a complex geometric field enhancement including nanotubes and narrow conducting channels in the tip body. The arrays of micro-tips are promising for a wide range of cold cathode applications.     

Prevention of Si-contaminated nanocone formation during plasma enhanced CVD growth of carbon nanotubes

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of tapered-diameter nanocones containing Si. Either low bias voltage (∼500 V) or deposition of a protective layer (tungsten or titanium film with 10-200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylindrical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the nanocone formation.     

CVD growth of carbon nanotube bundle arrays

Recent discovery of enhanced field emission current intensity from arrays of bundles of carbon nanotubes (CNT) has prompted this investigation of the growth of CNT bundle arrays by metal-catalyzed chemical vapor deposition (CVD), in order to understand and control the growth of these arrays. CNT bundle array growth has been characterized as a function of array geometric parameters: the CNT bundle diameter and inter-bundle spacing. We find that CNT bundle array growth varies significantly with bundle size and spacing, which we suggest is due to the formation of a volatile molecular byproduct of ethylene decomposition that enhances CNT growth in areas with high concentrations of metal catalyst. We have also studied and optimized CNT growth with respect to a variety of CVD process parameters, in order to control the length of the resultant CNT bundles. We find that the length of the CNT can be reliably controlled by varying either the reaction time or the gas pressure. Such control over CNT bundle length will be crucial in the incorporation of these bundle arrays into high-intensity electron field emission devices.     

One-step growth process of carbon nanofiber bundle-ended nanocone structure by microwave plasma chemical vapor deposition

In this work, we report a simple one-step growth process to synthesize a novel and distinct carbon nanostructure, called a carbon nanofiber bundle-ended nanocone (CNFNC) structure, by using microwave plasma chemical vapor deposition (MPCVD) method with CH₄ and H₂ as source gases and Fe catalyst. The nanostructures and their properties after each processing step were characterized by FESEM, HRTEM, ED, AES, and Raman spectroscopy. The preliminary results have demonstrated that the CNFNC structures exhibit excellent field emission properties. The results also show that the favored conditions to form the CNFNC structures include a combination of lower CH₄/H₂ flow ratio, higher substrate negative bias, and proper working pressure and deposition time. The possible growth mechanism of the CNFNC structures is proposed.     

Highly uniform hole spacing micro brushes based on aligned carbon nanotube arrays

Highly uniform hole spacing micro brushes were fabricated based on aligned carbon nanotube (CNT) arrays synthesized by chemical vapor deposition method with the assistance of anodic aluminum oxide (AAO) template. Different micro brushes from CNT arrays were constructed on silicon, glass, and polyimide substrates, respectively. The micro brushes had highly uniform hole spacing originating from the regularly periodic pore structure of AAO template. The CNT arrays, serving as bristles, were firmly grafted on the substrates. The brushes can easily clean particles with scale of micrometer on the surface of silicon wafer and from the narrow spaces between the electrodes in a series of cleaning experiments. The results show the potential application of the CNT micro brushes as a cleaning tool in microelectronics manufacture field.     

Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

A growth mark method for studying growth mechanism of carbon nanotube arrays

A novel and simple growth mark method was developed to make marks during the growth process of carbon nanotube arrays. These marks can be read out under scanning electron microscope or optical microscope. Based on this method, the growth rates at different temperatures and under different acetylene partial pressures were measured, from which the activation energy and the order of reaction were determined. Based on our experimental results, the growth of carbon nanotube arrays in our experimental condition could not be diffusion-limited. The measured activation energy could possibly be attributed to the heterogeneous decomposition of acetylene over the catalyst particle. Furthermore the marked array with special segmental structure may be found some applications in the future.     

One-step nanomorphology control of self-organized projection coronas in uniform polymeric nanoparticles

Uniform polymeric nanoparticles with various morphologies of projection coronas like the viruses in the coronavirus group have been formed by the self-organization of macromolecular chains polymerizing in a dispersion system of styrene (St), acrylonitrile (AN) and poly(ethylene glycol) monomethoxymonomethacrylate (PEGm) in a polar solvent (water/ethanol). An increase in the water composition reduced the crystallization degree of AN units, resulting in a variety of the nanoparticle morphology such as the increased particle size, the reduced projection size, the increased projection number, and the decreased inter-projection distance. The difference in the projection morphology strongly affected a dispersibility in water.     

Electrical and optical properties of binary CNx nanocone arrays synthesized by plasma-assisted reaction deposition

Light-absorbing and electrically conductive binary CN_x nanocone (CNNC) arrays have been fabricated using a glow discharge plasma-assisted reaction deposition method. The intact CNNCs with amorphous structure and central nickel-filled pipelines could be vertically and neatly grown on nickel-covered substrates according to the catalyst-leading mode. The morphologies and composition of the as-grown CNNC arrays can be well controlled by regulating the methane/nitrogen mixture inlet ratio, and their optical absorption and resistivity strongly depend on their morphologies and composition. Beside large specific surface area, the as-grown CNNC arrays demonstrate high wideband absorption, good conduction, and nice wettability to polymer absorbers.     

Plasma-assisted supercritical carbon dioxide removal process for photoresist stripping applications

Because of increasing interest in environmentally benign fabrication process, the use of dry cleaning processes to remove residues of metal, photoresists, organic moieties and particles from surfaces has grown as a research area. In this study, we designed series of systematic approaches to understand the properties of the hard-baked photoresist on glass substrates. We focused on the effects of de-bonding photoresists on the surface by using a plasma-assisted supercritical carbon dioxide removal process with various control parameters. Changes in the surface morphology of plasma de-bonded photoresists were observed and analyzed. A weight-loss method was used to evaluate quantitatively the efficiency of photoresist stripping by SCCO₂. Compared to the blank experiment (SCCO₂ process only), the photoresist residue reduces to 14.7% from 76.3%. This study demonstrates the possibility of incorporating plasma pretreatment into the supercritical carbon dioxide removal process for photoresist stripping applications.     

Multi-electrolyte-step anodic aluminum oxide method for the fabrication of self-organized nanochannel arrays

Nanochannel arrays were fabricated by the self-organized multi-electrolyte-step anodic aluminum oxide [AAO] method in this study. The anodization conditions used in the multi-electrolyte-step AAO method included a phosphoric acid solution as the electrolyte and an applied high voltage. There was a change in the phosphoric acid by the oxalic acid solution as the electrolyte and the applied low voltage. This method was used to produce self-organized nanochannel arrays with good regularity and circularity, meaning less power loss and processing time than with the multi-step AAO method.     

Effect of carbon deposits on the reactor wall during the growth of multi-walled carbon nanotube arrays

The effect of carbon deposits on the reactor wall during the growth of multi-walled carbon nanotube (MWCNT) arrays was studied. It was found that the carbon deposits were helpful in converting the continuous catalyst film into carbon-containing nanoparticles before growth started, which was beneficial for the nucleation of MWCNT arrays. As a result, MWCNT arrays synthesized in a reactor with carbon deposits were well aligned with high nucleation density. Further study revealed that this effect could be probably ascribed to some activated species adsorbed in the carbon deposits. These results will be helpful in the repeatable synthesis of high quality MWCNT arrays and in understanding the nucleation process of MWCNT arrays.     

Tuning substrate roughness to improve uniform growth and photocurrent response in anodic TiO2 nanotube arrays

Here we report on the effect of Ti substrate surface roughness on the morphology of anodized TiO₂ nanotube arrays (TNAs), as well as their crystal structure and photocurrent response measured in the backside illuminated dye-sensitized solar cells (DSSCs). TNAs grown on Ti substrates with higher roughness exhibited a non-uniform morphology, which encompassed both thick and thin-walled nanotubes. Reduction of the substrate roughness by different polishing methods causes the morphology of all of nanotubes to become uniform with thin wall thickness. Moreover, the compressive strain was reduced by decreasing the roughness according to the X-ray diffraction patterns. DSSCs fabricated using TNAs grown on the smoothest substrate showed a significantly higher conversion efficiency than that of the TNAs grown on the roughest substrate by a factor of 100%. Furthermore, TNAs grown on the smoothest substrate showed higher electron lifetime and lower recombination. Therefore, it has an enhancing effect on the photocurrent response of the anodized TNAs in backside illuminated DSSCs.     

Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres

Vertically aligned carbon nanotube (VACNT) arrays grown on ceramic spheres are obtained from ethylene using a floating catalysis process. The exhaust gas mainly contains light gaseous hydrocarbons, which decreases the contamination at the outlet of the reactor. Linear synchronous growth of the VACNT arrays is demonstrated and the morphology evolution of VACNT array grown on spheres is shown. The VACNT arrays on the spheres crack radially into a flower-like structure when the length of CNT is above 400 μm. The VACNT arrays grown on spheres still possess good flowability even when the length of the array reaches 1100 μm after a 2-h growth at 800 °C. The arrays on the spheres show good alignment, high purity and good graphitization. Meanwhile, with a decrease in temperature, the diameter of CNTs in the array correspondingly decreases, the distribution becomes narrower, and the growth rate decreases. The apparent activation energy is 180 ± 8 kJ/mol, indicating that ethylene is a good carbon source for fast and continuous radial growth of millimeter VACNT arrays on ceramic spheres.     

Effects of carbon film roughness on growth of carbon nanotip arrays by plasma-enhanced hot filament chemical vapor deposition

Carbon nanotip arrays were grown from carbon films deposited on silicon substrates by plasma-enhanced hot filament chemical vapor deposition. The carbon films were characterized by the atomic force microscopy and micro-Raman spectrometry and the carbon nanotip arrays were investigated by micro-Raman spectrometry, scanning electron microscopy and X-ray photon spectroscopy. The results indicate that both carbon films and carbon nanotips are made of polyaromatic carbon with turbostratic structure. Carbon films possess various roughnesses, which greatly influence the nanotip growth rate. The theory related to plasma and sputtering effect is used to discuss the results.     

Superior integrated field emission cathode with ultralow turn-on field and high stability based on SiC nanocone arrays

Low turn-on field (E_to) and stable electron emission are two of key parameters for reliable application of field emission (FE) cathodes. In the present work, we developed a novel high-performance integrated field emission cathode based on well-aligned SiC nanocone arrays via an electrochemical etching approach. The etched SiC nanocone emitters and the underlying remaining SiC wafer are designed into a single-crystalline integrated architecture without interfaces, which favors cathodes with a sturdy configuration to resist Joule heat during long period electron emission process and structural failure caused by the existed strong electrostatic forces. Accordingly, the E_to of the integrated SiC cathode is reduced to 0.32 V/μm, which is the lowest value among all the previously reported SiC nanostructured emitters. In addition, the integrated cathode presented superior stability with an electron emission fluctuation of 3.3% over 10 h. This work provides a new perspective for designing and fabricating advanced FE cathodes for further promising applications in harsh working conditions with high performance.     

Effect of catalyst pattern geometry on the growth of vertically aligned carbon nanotube arrays

We report the effect of catalyst pattern geometry on the growth behaviour of carbon nanotube (CNT) vertical arrays. Larger patterns are seen to produce longer CNT arrays. We show that this is predominantly related to the pattern size dependence of the number of walls and relate this to the local availability of carbon feedstock species. In addition, the vertical alignment of CNT pillar arrays is seen to depend on the pattern design, in particular the relationship between the pillar dimension and the inter-pillar spacing.     

Thickness-, alignment- and defect-tunable growth of carbon nanotube arrays using designed mechanical loads

We demonstrate the thickness-, morphology-, and defect-tunable growth and simultaneous integration of aligned carbon nanotube (CNT) arrays using a novel microscale platform. This platform consists of a micromechanical spring of desired stiffness, which applies a precise vertical load to a vertically aligned CNT array during its growth by chemical vapor deposition (CVD). The micromechanical spring is strained by the extrusive growth force output from the aligned CNT array during its growth and, at the same time, exerts a mechanical restoring force against the buckling resistance of the CNTs. This application of a designed vertical load on the CNTs allows modulation of the thickness and degree of alignment of the CNT array, as well as the structural quality of the individual CNTs. Consequently, the electrical resistance between two opposing CNT arrays can be tuned by adjusting the vertical load. In addition, their sensing responsiveness toward chemical species can also be enhanced by applying larger vertical load on the CNTs. In contrast to conventional growth methods for producing aligned CNT arrays, our approach offers an efficient way for the growth engineering and on-chip integration of aligned CNT arrays in a single step of the CVD.     

One second growth of carbon nanotube arrays on a glass substrate by pulsed-current heating

We report the very rapid growth of carbon nanotubes (CNTs) at high temperatures that can be tolerated by glass substrates. Glass substrates with metal microelectrodes and sputtered catalysts are heated by a pulsed current in a chemical vapour deposition gas environment for 0.5–1 s to synthesize CNTs of several micrometres in height without damaging the glass substrate. CNTs with structures from single-walled to multi-walled and morphologies from entangled networks to vertically aligned forests are grown simply by changing the nominal thickness of the catalyst, and such CNTs grown selectively on the microelectrodes worked as field emitters for cathodoluminescence. Rapid, easy growth of patterned CNT arrays on glass substrates without using furnaces/heaters or vacuum pumps will be useful for various applications of CNTs.