Simple Use of SiO 2 Film Thickness for the Control of Carbon Nano-Tube Diameter During Ferrocene Catalyzed CVD Growth
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Simple Use of SiO2 Film Thickness for the Control of Carbon Nano-Tube Diameter During Ferrocene Catalyzed CVD Growth N. Chopra, P.D Kichambare, R. Andrews, B.J. Hinds Dept. of Chemical and Materials Engineering, University of Kentucky Center for Applied Energy Research, University of Kentucky Lexington, KY 40506-0046 USA ABSTRACT Selective growth of carbon nano-tubes (CNT) on micron scale patterned substrates has been accomplished by taking advantage of the non-reactivity of ferrocene catalyst on H-terminated Si surfaces in a CVD process. Demonstrated here is that this phenomenon can be used to control the diameter of CNTs when sufficiently narrow lines of SiO2 surrounded by H-terminated Si are used. Narrow lines of SiO2 (12-60nm) are formed at the etched face of a Si/SiO2/Si multilayer structure. This allows the precisely controllable thickness of an SiO2 film to determine an exposed SiO2 line width. There is no need for e-beam lithography since film thickness determines nm-scale line dimensions. CNTs are then formed by CVD with a ferrocene/H2/Ar mixture at 700oC. CNTs are observed to grow only on the exposed SiO2 surface at the edge of the ‘mesa’ structure. CNT diameters of 13.2, 20.5, 34.2, 64.3nm are observed for SiO2 film thickness of 12, 19, 35, and 65 nm. The larger distribution of CNT diameter with increased line width is consistent with wider SiO2 linewidths not being able to affect smaller nucleation centers. These results are consistent with the use of self-assembly chemistry of iron catalyst onto nanoparticles of catalyst support.
INTRODUCTION Carbon nanotubes (CNT) have many potential applications in electronics [1], sensors[2], and nano electromechanical systems (NEMS).[3] Particularly interesting is that due to the long length of tubes, it is possible to use commercially prevalent micron scale lithography to result in nm-scale line features if one can selectively incorporate CNTs. Thus the development of a method to control CNT diameter and its placement becomes critical for applications. A particularly promising study has shown that it is possible to bridge the tops of photolithographically defined ‘posts’ with CNTs grown from nano-scale catalyst support particles.[4,5] Further refinements of the concept of growing CNTs between photolithographically defined pillars could become the basis of nm-scale wide wiring. There are a variety of growth techniques for CNT synthesis, but the CVD method offers the most commercially viable technique due to a large uniform reaction area and high mass flux flows.[6] CNT diameter is determined largely by the size of nm-scale catalyst particle.[7] These catalyst can be metal particles[8] or a coating of transition metal on catalyst support particles.[9,10] Catalytic nano-particles can be supplied during growth in a xylene and ferrocene based CVD process. This process has been used to grow CNTs at high densities with noteworthy vertical alignment.[7] The diameter of resultant CNTs is a complex system that involves the surface free energy of catalyst and substra
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