Ji Hoon Yang

Field emitter density control effect on emission current density by Ag–Cu alloy coating on carbon nanotubes

We have investigated the morphological evolution and improvement in field emission properties of carbon nanotube (CNT) emitters coated with an Ag–Cu alloy (ACa). Vertically aligned multiwalled CNTs (MWCNTs) were synthesized by direct current-plasma enhanced chemical vapor deposition. The MWCNTs were then coated with ACa by dc-magnetron sputtering and then annealed. Scanning electron microscopy revealed an increase in the size of the ACa droplets on the CNTs after thermal annealing, and a decrease in the emitter density with increasing deposition time. The emitter density was controlled by the amount of ACa with high surface tension and annealing. A lower turn-on voltage (1.18 V/μm) and higher emission current density of 588.9 μA/cm2 at 5.0 V/μm were achieved from the sample containing ACa droplets with an average radius of 500 nm.

Effects of N2 on the Growth of Multiwalled Carbon Nanotubes Synthesized by Plasma-Enhanced Chemical Vapor Deposition

We have investigated the role of N2 in the synthesis of carbon nanotubes (CNTs) by dc hot-filament plasma-enhanced chemical vapor deposition. An NH3 and C2H2 gas mixture with a ratio of 4:1 was used as a precursor for the synthesis of CNTs on nickel-coated TiN/Si (100) substrates. N2 gas was introduced at a flow rate of 30–120 sccm with the precursor. The structure and composition of CNTs synthesized with/without N2 were investigated by scanning electron microscopy, transmission electron microscopy, X-ray photoemission spectroscopy (XPS), and Raman spectroscopy. The multiwalled carbon nanotubes (MWCNTs) synthesized with N2 were 2–5 times longer than those synthesized without N2, and the morphology of the MWCNTs was improved. An XPS core level spectrum shows that nitrogen is in the CNX and C–NH2 forms.

Field emission properties of ZnO nanorods coated with NiO film

The field emission (FE) properties of ZnO and NiO-coated ZnO (NiO∕ZnO) nanorods are investigated under vacuum of 7×10−7Torr and oxygen rich vacuum of 1×10−5Torr. The ZnO nanorods were synthesized on a Si(100) substrate by metal-organic chemical vapor deposition, and the NiO film with the thickness of ∼15nm was coated by using atomic layer deposition. The turn-on voltages of the NiO∕ZnO nanorod and the ZnO nanorod were ∼5.2 and ∼3.0V∕μm at 1μA∕cm2, respectively. The electron FE stability of the NiO∕ZnO nanorods to the ZnO nanorod was significantly improved in oxygen rich vacuum even.

Synthesis of Crystalline Carbon Nanotube Arrays on Anodic Aluminum Oxide Using Catalyst Reduction with Low Pressure Thermal Chemical Vapor Deposition

Well-aligned crystalline carbon nanotubes (CNTs) were synthesized on anodic aluminum oxide (AAO) template using low-pressure thermal chemical vapor deposition (TCVD). The AAO templates were fabricated using an anodization process, and Fe as a catalyst was electrochemically deposited inside the bottom of the pores on the AAO template. For promotion of catalytic reaction of Fe, pore widening and catalyst reduction were performed. The enhancement of the catalytic reaction between C2H2 and the Fe catalyst particles results in CNTs with good crystallinity and a high packing density without the deposition of amorphous carbon on the pore surface of the AAO templates. The diameter and density of the CNTs were approximately 50 nm and 7×109 CNTs/cm2 (70% of pore density), respectively. When the threshold voltage of the electron emission was measured with a gap of 300 µm, it showed a very low turn-on field of 0.87 V/µm and the field enhancement factor was greater than 5600.

Nitrogen-incorporated multiwalled carbon nanotubes grown by direct current plasma-enhanced chemical vapor deposition

The nitrogen-incorporated multiwalled carbon nanotubes (N-MWCNTs) were synthesized by dc plasma-enhanced chemical vapor deposition with a gas mixture of C2H2, NH3, and N2. Nitrogens in the N-MWCNTs were pyridinic nitrogen and graphitic nitrogen. With increase in the flow rate of N2 gas during the synthesis of MWCNTs, the pyridinic nitrogen increased much more than graphitic nitrogen. The near-edge x-ray absorption fine structure spectra revealed that the density of states such as π*, σ*, and π*+σ* bands of the N-MWCNTs decreased with increase of concentration of pyridinic nitrogen incorporated in the MWCNTs. The intensity ratio of the D band to the G band of Raman spectrum increased with the incorporation of nitrogen into MWCNTs.

Synthesis of Carbon Nanotubes on Metal Substrates by Plasma-Enhanced Chemical Vapor Deposition

Vertically aligned carbon nanotubes (CNTs) on the Ni coated TiN/metal substrates were synthesized with a plasma-enhanced chemical vapor deposition at 650 using a gas mixture of NH3 and C2H2. The diameters of the CNTs were about 80 nm for the stainless steel (SUS304) and 160 nm for the Kovar substrate. The density of the CNTs in the SUS304 substrate is larger than that of the Kovar substrate. In Raman spectra, the relative peak ratio of the D to the G band in SUS304 substrate is about 1.1; higher than that of the CNTs for the Kovar substrate. This indicates that there are lesser defects here than that of the Kovar substrate, which was consistent with the SEM images and the TEM analyses. The emission current density of the CNTs in the SUS304 substrate was about 57 μAcm at 13 Vμm, which is lager than that of the CNTs in the Kovar substrate. Introduction Carbon nanotubes (CNTs) [1] are attractive materials with interesting physical, chemical, and structural properties. Research on the various properties of the CNTs has actively proceeded because of their prominent applications in various fields such as nano-devices [2], fuel cells [3], and field emitters [4]. In applications using the electronic properties of single-walled CNTs (SWCNTs), its structure has to be controlled because the electronic properties of CNT depend on their diameter and chirality [5]. The structure influences the electronic properties of multi-walled CNTs (MWCNTs) and SWCNTs. In order to control the structure, an understanding of the growth of the mechanism of CNTs is required, but this has not yet been proven clearly. There are many reports of structural influences of various parameters such as catalysts, pressure, temperature, the kinds of gas used during film growth, and the on FE (Field emission) properties of the field enhancement factor [6,7,8]. There are also reports on the synthesis of CNTs aligned with Si or SiO2 substrates [9-12]. There are some problems in these cases such as thermal conductivity, CNT-substrate electrical contact, and adhesion between CNT and the substrates. The metal substrate, therefore, is attractive due to the substrate electro-conductivity requirement. In our study, the SUS304 and the Kover are used as metal substrates in order to improve on these problems [13]. The use of a conductive substrate may facilitate the direct manufacture of devices such as field emitters and scanning probes. This resolves the problem of adhesion of nanotube layers on metallic substrate. In this study, the vertically aligned carbon nanotubes on the metal (SUS304 and Kovar) substrates were synthesized through a dc hot-filament plasma-enhanced chemical vapor deposition (dc HF-PECVD) at 650 using a gas mixture of NH3 and C2H2. We compare the emissive properties of the MWCNTs grown on the SUS304 and Kovar substrates. We investigate the structure of the MWCNTs through scanning electron microscopy (SEM), a transmission electron microscope (TEM), and Raman spectroscopy. Field emissions from the as-grown MWCNTs on the metal substrates have been measured. Experimental details Key Engineering Materials Online: 2005-01-15 ISSN: 1662-9795, Vols. 277-279, pp 950-955 d i:10.4028/www.scientific.net/KEM.277-279.950