Synthesis of carbon nanoparticles by microwave plasma chemical vapor deposition and their field emission properties

Abstract

The search for excellent electron field emission (FE) materials has long been an important subject for related researchers. The desired objective for field emission materials is to obtain the highest electron emission current density at the lowest applied electrical field. Much attention has been focused on carbon based materials because of their superior field emission characteristics. Diamond [1, 2], diamond-like carbon [3, 4], and tetrahedral amorphous carbon [5] have all been demonstrated to emit electrons at reasonably low fields. Recently, much attention is attracted to the field emission from carbon nanotubes (CNTs). [6] Indeed, the CNTs exhibit excellent field emission properties including low threshold field, high emission current density, and high emission site density due to their unique structure. [7–9] However, one important and widely neglected fact is that the carbon nanoparticles (CNPs) may also exhibit good field emission characteristics comparable to that of the CNTs because the CNPs fall in the same dimension range and possess the same graphitic structure as the CNTs. To date very few works were reported on the field emission from the CNPs. [10] Because of the ball shape and simplicity in synthesis the CNPs have their own advantages different from the CNTs as field emission materials. In this letter, we report on the synthesis and electron FE measurements of the CNPs produced by microwave plasma assisted chemical vapor deposition (MWCVD). As a comparison, the results of the field emission from the CNTs are also reported. The samples were prepared by an AsTex MWCVD system. Fe films deposited on Si substrates by ion beam sputtering and about 200 nm thick were used as catalysts for growing the CNPs and CNTs. The source gas for growing the CNPs is the mixture of H2 and CH4 while that for growing the CNTs is the mixture of N2 and CH4. The gas flow rates of H2 and CH4 for growing the CNPs are 6.68 × 10−6 and 3.34 × 10−7 m3/s and that of N2 and CH4 for growing the CNTs are 8.35 × 10−7 and 3.34 × 10−7 m3/s, respectively. The growth time, pressure, and growth temperature are 15 min, 4000 Pa, and 670 ◦C, respectively, for all the samples. The structure of the samples was analyzed by scanning electron microscopy (SEM) and Raman spectroscopy. The field emission characteristics were measured by a transparent anode imaging system. The transparent anode was made by coating phosphor on