On number density of the CNSs and growth mechanism discussion

The vapor-liquid-solid model is a commonly accepted CNT/CNF growth mechanism, which suggests that carbon diffusion in the liquefied metal catalyst is a required step for CNF Fig. 4-8 HRTEM images of CNFs near the tip under different Co electroplating times: (a) 60 and (b) 90 s. (For SP. # A3 and A5)

growth. The cone-shaped catalyst and the segmentation of Co particles shown in the TEM images is an implication of Co liquefaction during the CNF growth in the AAO template. It has been proposed that a temperature gradient can be developed in the catalyst particle as the concentration distribution of the dissolved carbon is nonuniform in the particle [Kanzow 1998-525]. The catalyst has a higher temperature in the region supersaturated with carbon, and graphene sheets develop from the region with a smaller carbon concentration [Harutyunyana 2005-153113]. The particle segmentation probably resulted from the temperature gradient developing in the Co particle. The top portion of the Co particle was liquefied earlier than the bottom portion due to the quicker supersaturation with carbon, which was supplied by the inflowing carbonaceous plasma species under the CNF growth condition. As the liquefied Co particle moved upward with the growing CNF shaft, the Co particle was eventually separated leaving the unliquefied part at the bottom of the AAO pore channel. Besides, because Co has a reasonable wetting strength with aluminum oxide [Wang 2004-081401, Chambers2002-827], it was likely that capillary force might assist lifting the liquefied Co particle and thus allow easy precipitation of graphitic sheets.

As discussed above, CNF growth on the AAO template was accompanied by deposition of a:C, and more a:C was deposited for a smaller Co particle. The a:C accumulation on the pore wall surface could hinder the motion of the liquefied catalyst particle in the pore channel, thereby changing the moving direction and geometric shape of the catalyst during the CNF growth. This can explain why most CNFs grown under a condition of heavy a:C deposition showed a twisted feature inside the pore channels and had a fiber diameter smaller than the AAO pore size. In addition, from Fig. 4-7(b), many nanosized particles, probably Co, adhered to the tube surface of the CNFs. It is possible that the rough wall surface of the AAO pore channel, resulting from the a:C deposition, disturbed the movement of the liquefied Co particle on the tip of the growing CNF, and thereby stripped off surface atoms from the Co catalyst particle, which might later agglomerate and adhere to the growing CNFs.

The growth mechanism of CNFs in AAO pore channels is schematically illustrated in Fig. 4-9. Because the AAO pore channels not only limit the flow direction and the flux of plasma species toward the Co catalyst but also define the exposing area of the Co particle, the impinging rate of carbonaceous and hydrogen radicals and ions onto the catalyst surface should be independent of the height of the Co particle. Once adsorbed and decomposed on the Co catalyst surface, carbon species may diffuse on the surface and dissolve into the particle.

The Co particle becomes supersaturated with carbon on the top region earlier than the lower part, resulting in the development of a temperature gradient in the particle and thus Co liquefaction occurs earlier in the top region. Carbon atoms precipitating at the bottom of the liquefied Co segment then grow into graphitic sheets, forming carbon nanofibers in the AAO pore channel via the tip-growth mode. Because etching the a:C deposit by plasma species is not effective in the pore channel due to the high aspect ratio, removal of carbonaceous deposit in the pore channel is more sluggish than on the AAO template surface. Carbon dissolution into the Co particle becomes the major pathway for reducing surface concentration of the carbon deposit. Because carbon becomes saturated more quickly in the smaller particle than the bigger one, more a:C may remain on the particle surface. If the a:C deposition prevails over CNF growth, extensive a:C accumulation can occur on the top of the catalyst and the pore wall, thus retarding the CNF growth. The a:C deposit can modify the geometric shape of the liquefied catalyst and thus diminish active catalytic area, thereby leading to growth of CNFs with a smaller diameter and a distorted shape. In the extreme situation, the pore opening may be completely blocked by the a:C deposit before the CNF grows out of the pore, thus terminating the inflow of the precursor species and stopping the CNF growth. On the other hand, the bigger Co particle dissolves more carbon atoms and, therefore, accumulation of a:C on the surface can be effectively diminished, making CNF growth in the pore channel more favorable. Once CNFs grow out of the AAO pore surface, plasma etch become effective to remove excessive carbon deposit on the Co catalyst and plasma precursors can reach the

catalyst surface without much obstacle. Thus CNFs can be grown continuously under a growth condition similar to that in an open space.

Fig. 4-9 Schematic illustration of carbon nanostructure growth mechanism in the AAO pore channel with different Co plating time: (a) 90 s and (b) 60 s, respectively.