Yung-Ping Chang

Effect of ZnO catalyst on carbon nanotube growth by thermal chemical vapor deposition

In this study, carbon nanotubes (CNTs) were synthesized on a catalyst of ZnO nanorods using thermal chemical vapor deposition. ZnO nanorods were coated on silicon substrates by a hydrothermal process. The CNT growth temperature was maintained at 900°C at an ambient pressure. Methane and argon gases are used for the CNT synthesis. In this work, scanning electron microscopy (SEM) and transmission electron microscopy images were used to observe the surface morphology and sidewall structure, energy-dispersive spectrometry (EDS) was used to identify and analyze the chemical composition of the surface of the CNTs, and Raman spectroscopy was employed to investigate the CNT structures. The research reveals that the carbon nanotubes grown on ZNO nanorods show a multiwalled structure with defective graphite sheets on the walls. The SEM images show that the surface of CNTs grown on the ZnO nanorod catalyst were covered by the nanonode carbon nanoparticles. Raman spectra show that the appearance of the relatively str...

Enhancement of field emission from carbon nanotubes by post-treatment with a chromium trioxide solution

Abstract A simple method is described to functionalize the surface and to modify the structures of multi-walled carbon nanotubes (MWCNTs) grown on silicon substrates using chromium trioxide (CrO 3 ) solution. Unlike nitric acid (HNO 3 ) used in the conventional post-treatment for MWCNTs, the chemical reaction with CrO 3 involves amorphous carbon and the carbon nanotubes themselves. It is expected that the surface morphology, chemical composition, and field emission of MWCNTs should be significantly changed after CrO 3 solution treatment. The results showed that after 20 min of CrO 3 solution treatment, the emission currents were enhanced compared with the as-grown MWCNTs. However, extended treatment over 20 min was found to degrade the field emission properties of the film. The enhancement in field emission after 20 min of CrO 3 solution treatment can be ascribed to the modification of surface morphology, the increase in surface density of MWCNTs, and the lowering of the work function. Prolonged CrO 3 treatment dissolves MWCNTs and thus results in a decrease in field emission.

Synthesis of carbon nanotubes on silicon nanowires by thermal chemical vapor deposition

Abstract Carbon nanotubes (CNTs) were grown on a silicon nanowire substrate with Ni-catalyst layers of different thicknesses using thermal chemical vapor deposition. Scanning electron microscopy was used to observe the surface morphology, and Raman spectroscopy was used to investigate the structural changes in relation to the thicknesses of the catalyst deposited on the substrate. Field-emission characteristics of CNTs were also correlated with the catalyst thicknesses. The catalyst thickness was found to effectively change the field-emission characteristics of the CNTs. Obvious changes in the diameters, population density, and morphology of CNTs were found with differences in catalyst thickness. The field-emission characteristics of CNTs were dependent on their diameter.

Effects of annealing Ni catalyst in nitrogen-containing gases on the surface morphology and field-emission properties of thermal chemical vapor deposited carbon nanotubes

Abstract Ni catalyst was annealed in N 2 O/N 2 /NH 3 in order to study the effects of the annealing on the surface morphology and field-emission characteristics of synthesized carbon nanotubes (CNTs). The morphology and composition of the CNTs show that the grain size and composition of the Ni catalyst can be modified by N 2 O, N 2 , and NH 3 pretreatments. Scanning electron microscopy images of evaporated Ni after N 2 O pretreatment show that the Ni particles are more uniformly distributed, and their sizes are relatively small. It is found that the CNTs grown on Ni pretreated with N 2 O have the highest surface density and the highest emission current. This improvement is accomplished through the more uniformly distributed and smaller Ni particles obtained by N 2 O pretreatment. The growth of amorphous carbon is also suppressed. The reasons for this improvement are that nitrogen and oxygen atoms dissociated from N 2 O in the pretreatment can oxidize the catalyst particles and inhibit their growth so that the resulting Ni metal nuclei become smaller and more uniformly distributed, which is beneficial to CNT growth.

Field Emission of Carbon Nanotubes Grown on Silicon Nanowires Using Thermal Chemical Vapor Deposition

In this study, carbon nanotubes (CNTs) were synthesized on Si nanowires (SiNWs) using thermal chemical vapor deposition. SiNWs were directly synthesized onto Si substrates through a solid-liquid-solid (SLS) phase growth mechanism at elevated temperatures. The growth temperature for CNT was 900℃ at an ambient pressure. Methane and argon gases were used for CNT synthesis. In this study, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were used to observe the surface morphology and sidewall structure, and Raman spectroscopy was employed to investigate the structure of CNTs. The carbon nanotubes grown on SiNWs demonstrated a multiwalled structure with defective graphite sheets in the wall. Raman spectra determined the SiNW carbon nanotubes had poor crystallinity, or more amorphous carbon, than those grown on plain Si substrate. Compared with CNTs grown on flat silicon substrates, CNTs grown on SiNWs exhibit superior field emission characteristics. The Fowler-Nordheim plot displayed a good linear fit, indicating the emission current of carbon nanotubes follows a Fowler-Nordheim behavior.

Enhancement of Field Emission Characteristics for Multi-Walled Carbon Nanotubes Treated with a Mixed Solution of Chromic Trioxide and Nitric Acid

Abstract A simple acid treatment method was applied to functionalize the surface and to modify the structures of multi-walled carbon nanotubes (CNTs) grown on silicon substrates using a mixed solution of chromic trioxide (CrO 3 ) and nitric acid (HNO 3 ). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and energy dispersive spectrometer (EDS) were employed to investigate the mechanism causing the modified field emission (FE) properties of the CNT films. After 20 min of CrO 3 +HNO 3 treatment, the emitted currents were enhanced by more than one order of magnitude compared with those of the untreated CNTs. This large increase in emitted current can be attributed to the favorable surface morphologies, open-ended structures, and highly curved CNT surfaces in the CNT films. These factors altogether caused an increase in the field enhancement factors of CNTs. We also demonstrated that using a mixed solution of CrO 3 +HNO 3 post-treatment exhibited a higher emission current and a lower turn-on electric field than in the CNTs treated with HNO 3 . The method provides a simple, economical, and effective way to enhance the CNT field emission properties.

The Effect of Hydrogen Plasma Treatment on the Field-Emission Characteristics of Silicon Nanowires

Silicon nanowires (SiNWs) were formed on nickel-coated silicon substrates by a solid–liquid-solid (SLS) growth mechanism. Effect of hydrogen plasma treatment on the field-emission characteristics of the silicon nanowires was investigated. The results show that hydrogen plasma treatment can enhance the field-emission characteristics of the SiNWs showing a decrease in turn-on field as well as an increase in field enhancement factor. It is believed that hydrogen plasma treatment plays an important role in the improvement of field-emission characteristics of silicon emitters.

Enhancement of Field Emission from Silicon Nanowires Treated with Carbon Tetrafluoride Plasma

In this study, a simple method using carbon tetrafluoride (CF4) plasma to functionalize the surface and to modify the structures of silicon nanowires (SiNWs) grown on silicon substrates is presented. A layer of nickel was evaporated onto a (100) silicon substrate to induce the growth of SiNWs at 1000 °C for 2 h. Oxygen derived from native oxide lying on the silicon substrate was incorporated into SiNWs resulting in silica nanowires. Due to its high resistivity, the emitted currents from SiNWs are much lower than those emitted from carbon nanotubes. After 4 min of CF4 plasma treatment, a large increase (more than two orders of magnitude) in the emitted current is observed. It is found that conglomeration of SiNWs is found after CF4 plasma treatment which increases the surface density of emission sites. Furthermore, the SiO2 layer covering the surface of SiNWs has been removed by CF4 plasma etching. These two factors combined to obtain the enhanced field-emission characteristics of SiNWs. However, nonvolatile fluorocarbon polymers may form on the sidewall of SiNW after prolonged CF4 plasma treatment and thus field-emission current is lowered. These results clearly manifest the potential of using SiNWs in field emitter applications.

The synthesis of carbon nanotubes on silicon nanowires by thermal chemical vapor deposition

In this work, carbon nanotubes (CNTs) were grown on the silicon nanowires substrate with different thickness of Ni catalyst layer at temperatures 900°C using thermal chemical vapor deposition (CVD) to study their effects on surface morphology and field emission characteristics. Scanning electron microscopy (SEM) image was used to observe the surface morphology and structural properties, and Raman spectroscopy was employed to investigate the structural changes caused by different catalyst thickness. Our experimental results clearly demonstrate that catalyst thickness can effectively vary the field emission current of CNTs. Obvious changes in the surface density and morphology of CNTs caused by the variation of catalyst thickness can be clearly seen, which attributed to the grain size effect and thus an increase in emission current. The emission characteristics of CNTs were found to be dependent on the diameter of carbon nanotubes. Compared with CNTs grown on the different thickness of catalyst, CNTs grown on silicon nano wires with Ni catalyst of 7.5nm exhibit the largest field emission current.

Controlled growth of carbon nanotubes using zinc oxide nanotubes as the catalyst

In this study, carbon nanotubes (CNTs) were synthesized on a catalyst of zinc oxide (ZnO) nanorods prepared with a hydrothermal process. The surface morphology of the CNTs was studied with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the structural properties and chemical compositions of the CNTs were studied with Raman spectroscopy and energy dispersive spectrometer (EDS), respectively. Our research results reveal that the CNTs grown on ZnO nanorods exhibit a multiwalled structure with defective graphite sheets on the wall. The surfaces of the CNTs grown on ZnO catalyst were covered by the nanonodes carbon nanoparticles. Raman spectra show that the appearance of the relatively strong D peaks in CNT films grown on ZnO catalyst can possibly result from the existence of nanonodes and the structural defects on the nanotubes graphite sheets. EDS results reveal that the CNTs grown on ZnO nanorods do not need any purification process to optimize the field-emission characteristics of the CNTs. The CNTs synthesized on ZnO catalyst exhibit a lower turn-on electric field and a higher emission current density than that of the CNTs synthesized on Ni catalyst.