Effects of Metal Underlayer Grain Size on Carbon Nanotube Growth

Abstract

In this paper we demonstrate that the nucleation density of single-walled carbon nanotubes (SWNTs), formed by thermal catalytic chemical vapor deposition, strongly depends on the grain size of Al underlayers covered with a native oxide (Al/Al2O3). By varying the substrate temperature during Al sputter deposition it was possible to investigate the effect of Al grain size on growth without inducing changes in the underlayer thickness, surface chemistry, or any other growth parameter. The resulting SWNT growth structures ranged from low-density 2D nanotube networks that lay across the surface of the substrate to high density 3D nucleation which gave rise to vertical “forest” growth. The height of the SWNT “forest” was observed to increase with increasing Al deposition temperature as follows, 200 > 100 > 60 > 20 °C on Si/Al but in the order 100 > 200 > 60 > 20 °C on SiO2/Al substrates for fixed growth conditions. The differences in the SWNT growth trends on Si and SiO2 substrates are believed to be due to the existence of an optimal Al/Al2O3 underlayer grain size for the formation of active catalytic nanoparticles, with larger Al/Al2O3 grains forming on SiO2 than Si at a fixed substrate temperature. Numerous surface analysis techniques including AFM, XPS, FESEM, TEM, and Raman spectroscopy have been employed to ascertain that the observed changes in nanotube growth for this system are related primarily to changes in underlayer morphology.

The Critical Role of the Underlayer Material and Thickness in Growing Vertically Aligned Carbon Nanotubes and Nanofibers on Metallic Substrates by Chemical Vapor Deposition

“…[17][18][19][20] For instance, Lee et al [18] investigated the effects of Cr, Ti, Ta, and Was the metal underlayers for CNTgrowth in a hot filament system with a sample temperature of 700 8C and using a Ni/Fe alloy as catalyst. The metal underlayers were nominally 100-nm thick with a grain size of roughly 20-30 nm in diameter and were sputter-deposited on silicon wafers.…”

Confocal Microscopy for Process Monitoring and Wide-Area Height Determination of Vertically-Aligned Carbon Nanotube Forests

“…By increasing the thickness of Al2O3 buffer layer from 20 nm to 54 nm, the resulting VACNT height increases from 70 μm to 130 μm ( Figure 5). This is probably due to an effect described by Burt et al [24]. They found an influence of the buffer layer's morphology on the density of catalyst nanoparticles and thereby the growth rate of CNT.…”

X-ray photoelectron spectroscopy study on Fe and Co catalysts during the first stages of ethanol chemical vapor deposition for single-walled carbon nanotube growth

“…One possibility is that the oxidation process changes the morphology of the catalyst, which could change the growth characteristics. 9 , we observed that Fe gives higher yields than Co [forests are a factor 2 taller for the Fe than for the Co, while keeping the growth time (5 min) the same]. Therefore we concluded that, although harder to reduce, Fe is more active in carbon nanotube growth than metallic Co.…”

“…[5][6][7][8][9][10] It has been claimed that Fe/Al 2 O 3 interfacial bonding restricts Fe surface mobility and hence allows for high density SWCNT forest growth. 5,6 In addition, it has been suggested 7,8 that Al 2 O 3 , which is a well known catalyst for hydrocarbon formation, 11 aids in decomposing the carbon precursor and therefore enhances SWCNT forest growth from the catalyst.…”