Page 121 - Carbon Nanotubes

PROPERTIES OF BUCKYTUBES AND DERIVATIVES

X. K. WANG, X. W. LIN, S. N. SONG, V. P. DRAVID, J. B. KETTERSON,
and R. P. H. CHANG*
Materials Research Center, Northwestern University, Evanston, IL 60208, U.S.A.

(Received 25 July 1994; accepted 10 February 1995)
Abstract-The structural, magnetic, and transport properties of bundles of buckytubes (buckybundles)
have been studied. High-resolution electron microscopy (HREM) images have revealed the detailed struc-
tural properties and the growth pattern of buckytubes and their derivatives. The magnetic susceptibility
of a bulk sample of buckybundles is -10.75 x lop6 emu/g for the magnetic field parallel to the bundle
axes, which is approximately 1. I times the perpendicular value and 30 times larger than that of C,. The
magnetoresistance (MR) and Hall coefficient measurements on the buckybundles show a negative MR at
low temperature and a positive MR at a temperature above 60 K and a nearly linear increase in conduc-
tivity with temperature. The results show that a buckybundle may best be described as a semimetal. Using
a stable glow discharge, buckybundles with remarkably large diameters (up to 200 pm) have been synthe-
sized. These bundles are evenly spaced, parallel, and occupy the entire central region of a deposited rod.
HREM images revealed higher yield and improved quality buckytubes produced by this technique com-
pared to those produced by a conventional arc discharge.
Key Words-Buckytubes, buckybundle, glow discharge, magnetic properties, transport properties.
1. INTRODUCTION the range of 3000 K to 4000 K. [ll] After arcing for an
hour, a deposited carbon rod 165 mm in length and
Since the initial discovery[l,2] and subsequent devel- 16 mm in diameter was built up on the end of the cath-
opment of large-scale synthesis of buckytubes[3], var-
ious methods for their synthesis, characterization, and ode. The deposition rate was 46 pm/sec.
potential applications have been pursued[4-121. Par- 2.2 Structural measurements by
allel to these experimental efforts, theoreticians have electro It rn icroscopy
predicted that buckytubes may exhibit a variation in
their electronic structure ranging from metallic to Most of the HREM observations were made by
semiconducting, depending on the diameter of the scraping the transition region between the “black ring”
tubes and the degree of helical arrangement[l3-161. material and the outer shell, and then dispersing the
Thus, careful characterization of buckytubes and their powder onto a holey carbon TEM grid. Additional ex-
derivatives is essential for understanding the electronic periments were conducted by preparing cross-sections
properties of buckytubes. of the rod, such that the rod was electron-transparent
In this article, we describe and summarize our stud- and roughly parallel to the electron beam. HREM ob-
servations were performed using an HF-2000 TEM,
ies on the structural, magnetic,and transport proper- equipped with a cold field emission gun (c FEG) op-
ties of buckytubes. In addition, we describe how a
conventional arc discharge can be modified into a sta- erated at 200 keV, an Oxford Pentafet X-ray detector,
and a Gatan 666 parallel EELS spectrometer.
ble glow discharge for the efficient synthesis of well-
aligned buckytubes.
2.3 Magnetic susceptibility measurements
Magnetic susceptibility measurements were per-
2. EXPERIMENTAL formed using a magnetic property measurement sys-
tem (Quantum Designs Model MPMS). This system
2.1 Synthesis of buckytubes has a differential sensitivity of emu in magnetic
Bundles of buckytubes were grown, based on an fields ranging from -5.5 T to +5.5 T over a tem-
arc method similar to that of Ebbesen and Ajayan[3]. perature range of 1.9 K-400 K. The materiz$s studied
The arc was generated by a direct current (50-300 A, included: three buckybundle samples of 0.07 12 g,
10-30 V) in a He atmosphere at a pressure of 500 Torr. 0.0437 g, and 0.0346 g; 0.0490 g of C60 powder,
Two graphite electrode rods with different diameters 0.1100 g of gray-shell material, 0.1413 g of polycrys-
were employed. The feed rod (anode) was nominally talline graphite anode, and a 0.0416-g graphite single
12.7 mm in diameter and 305 mm long; the cathode crystal. Measurements were performed at temperatures
rod was 25.4 mm in diameter and 100 mm long (it re- from 2 K to 300 K and.in magnetic fields ranging from
mained largely uneroded as the feed rod was con- 0.005 T to 4 T. The susceptibility of buckytubes was
sumed). Typical rod temperature near the arc was in measured with the magnetic field (H) either parallel
to (x!) or perpendicular to (xk) the buckybundle
axis. All samples used in this work were enclosed in
*Author to whom correspondence should be addressed. gelatin capsules, and the background of the container
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