Atomically Resolved STM Images of CVD Grown Carbon Nanotubes
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Atomically Resolved STM Images of CVD Grown Carbon Nanotubes Daniel (Ching-Shih) Chiang1, Philip Zifeng Lei1, Lifeng Dong2, Jun Jiao2. 1 School of Engineering and Computer Science Washington State University Vancouver, 14204 NE Salmon Creek Avenue, Vancouver, WA 98686-9600 2 Department of Physics Portland State University, Portland, OR 97207-0751 ABSTRACT Atomically resolved images of single-wall carbon nanotubes (CNT) grown in a chemical vapor deposition (CVD) chamber were obtained with the scanning tunneling microscope (STM) under ambient conditions. We found that the average diameters d of the CVD-grown CNTs appear to fall into a bimodal distribution of 1.0 and 0.6 nm, and the chiral angle φ was observed to be close to zero degree. The summation of the lattice indices (n+m) was determined to be 14 and 9 for d=1.0nm and d=0.6nm, respectively. The most possible lattice index pairs (n, m) with a chiral angle close to zero degree are (7, 7) and (5, 4), which indicates that the larger nanotubes are metallic and the smaller nanotubes are semi-conductive.
INTRODUCTION Carbon Nanotubes (CNTs) are a novel class of nano-sized material structures that exhibit good mechanical, and electrical [1,2] properties yielding them extremely attractive for use in future nano-scale devices. CNTs can be perceived as graphite sheets rolled up into a seamless hollow tube or cylinder. It has been demonstrated that CNTs can have various electrical properties depending on their structural configuration [2-4]. CNTs can be conductive or semiconductive depending on their chirality or helicity [3,4]. The chirality of a carbon nanotube can be indicated by the indices (n, m) for which C = na1 + ma2
(1)
where C is the roll-up vector on the graphite sheet’s hexagonal plane, a1 and a2 are lattice vectors, and n and m are integers. From the indices (n, m), it is possible to calculate the corresponding chiral angle φ and diameter d given by the equations ⎡
⎤ ⎥ 2 2 ⎣ 2 n + m + nm ⎦
φ = arccos⎢ d=
a
π
3 ( n + m)
n 2 + m 2 + nm
(2)
(3)
where a is the lattice constant of graphite and is equal to 0.246 nm [5]. When the chiral angle φ is equals to 0° the nanotube is called an armchair CNT, and when the φ =30° it is called a zigzag
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CNT. The indices (n, m) are also useful in determining the electrical properties of the CNT. If the difference (n – m) yields a multiple of 3, the CNT is generally a conductor. On the other hand, the CNT will be a semiconductor for all other cases of (n – m). Carbon nanotubes grown under the chemical vapor deposition (CVD) process can possess advantages over other methods such as arc discharge or laser ablation. The CVD process permits the ability to easily vary the growth parameters used in the manufacture of CNTs and therefore produce CNTs with specific tailored properties. The electrical properties of carbon nanotubes have been intensively studied through the use of a scanning tunneling microscope (STM). The STM can render the atomic resolution of the nanotubes and also provide valuable spectrosco
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