Abstract
Vertically aligned single-walled carbon nanotubes (VA-SWCNTs) can be fabricated on the Co/Si(100) substrate in a gravity-assisted chemical vapor deposition process (CVD). The Co/Si(100) substrate is tilted such that its surface normal points downwards in the CVD process. All the SWCNTs are lined up with the direction of gravity. The length of SWCNTs increases with growth time. A modified vapor-liquid-solid mechanism is proposed to explain the role of gravity in the precipitation of SWCNTs and in the line-up behavior of SWCNTs.Temperature and CH4/H2 ratio of gas flow rates are the two factors that strongly affect the qualities of VA-SWCNTs in gravity-assisted CVD. The qualities of SWCNTs and other carbon products grown by gravity-assisted CVD were characterized by scanning electron microscopy and Raman spectroscopy. At temperatures between 850 and 900 oC, SWCNTs of very good quality stand alone on the substrate. At other temperatures, nanofibers or irregular islands of carbon are present on the substrate. The CH4/H2 ratio influences the quality of SWCNTs more abruptly than temperature. At low ratio, no carbon nanotube (CNT) is formed. The window of CH4/H2 ratio for the growth of VA-SWCNTs ranges from 160/80 to 160/40. At a ratio of higher than 160/40, MWCNTs replace SWCNTs and become the dominant product.The effect of growth time on the shapes of SWCNTs grown on Co/Si(100) using the gravity-assisted CVD method has been investigated. A transition of SWCNTs from vertical (1–3 min) to bent (3–10 min) and to vertical (20 min) shape has been observed at different growth time. High quality vertical SWCNTs appear at a growth time less than 3 min, verified by the clear radial breathing mode (RBM) in the Raman spectra and by the high-resolution images taken by transmission electron microscope (TEM). At a growth time of 5 min, vertical and bent SWCNTs co-appear on the substrate. When a SWCNT is longer than ~1 μm, it is easier to bend by its own weight due to the low value of the effective bending stiffness. At a longer growth time such as 20 min, the coating of amorphous carbon (a-C) on a SWCNT becomes dominant which holds the SWCNT straight and keeps the a-C/SWCNT composite structure in a nearly vertical shape. A mechanism has been proposed to explain the coating of a-C on a SWCNT.VA-SWCNTs have been fabricated on carbon nanocones (CNCs) in a gravity-assisted CVD process. The CNCs with nano-scale Co particles at the top were first grown on the Co/Si(100) substrate biased at 350 V in a plasma enhanced chemical vapor deposition process. The CNCs typically are ~200 nm in height, and their diameters are ~100 nm near the bottom and ~10 nm at the top. The nanoscale Co particles of ~10 nm in diameter act as catalysts which favor for growing VA-SWCNTs out of CNCs at 850 oC in the gravity-assisted CVD process. The average length and the growth time of VA-SWCNTs are ~150 nm and 1.5 min, equivalent to a growth rate of ~6 μm/h. The diameters of VA-SWCNTs are estimated to be 1.2–2.1 nm. When VA-SWCNTs are fabricated on CNCs, the turn-on voltage is reduced from 3.9 to 0.7 V/μm and the emission current density at the electric field of 5 V/μm is enhanced by a factor more than 200. The composite VA-SWCNT/CNC structure is potentially an excellent field emitter. The emission stability of the VA-SWCNTs/CNCs field emitter is discussed.