RESEARCH ARTICLE
Copyright © 2008 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Computational and Theoretical Nanoscience
Vol. 5, 1–5, 2008
InfluenceofWaterontheFrequencyof
Carbon Nanotube Oscillators
Zhao Qin, Jian Zou, and Xi-Qiao Feng
∗
Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
Water molecules, which may either be introduced during fabrication of devices or absorbed from
servicing environment, affect the performance and reliability of nanodevices and nanosystems. We
investigate via theoretical analysis and molecular dynamics simulation the effect of water molecules
on the vibration behavior of a double-walled carbon nanotube-based oscillator. It is found that water
confined between the two nanotubes may significantly affect the oscillation frequency of the oscil-
lator as a result of the enhancement of van der Waals interaction. An analytical expression is
derived for the oscillation frequency in terms of the physical and geometrical parameters of car-
bon nanotubes and water. The theoretical results have a good agreement with molecular dynamics
simulations.
Keywords: Carbon Nanotube, Nano-Oscillator, Frequency, Molecular Dynamics Simulation.
1. INTRODUCTION
Owing to their unique and perfect one-dimensional atomic
structure, carbon nanotubes (CNTs) possess outstanding
physical, chemical, and mechanical properties and hold a
promise for a wide variety of applications.
1 2
CNTs have
been suggested, for example, as basic building blocks of
nanodevices and nano-electromechanical systems. Many
CNT-based nanodevices have been reported recently, e.g.,
bearings,
3
springs,
4
motors,
5
nozzles,
6 7
oscillators,
8–19
and
rotors.
20
Cumings and Zettl
8
opened one end of a multi-
walled carbon nanotube (MWNT) and extruded the inner
tube out of the outer ones. They estimated the interlayer
static and dynamic friction force to be about 23 × 10
−14
N
and 15 × 10
−14
N per atom, respectively. As the inner tube
was extruded out and set free, it spontaneously retracted
back into the outer shell in less than a video frame time
(33 ms). Zheng et al.
9 10
proposed that if both the ends of
the outer tubes were opened, the retracted core would not
stop at once but oscillate rapidly with respect to its original
position. This means that MWNTs might be used as nano-
oscillators with a frequency up to gigahertz. CNT-based
oscillators have attracted considerable attention of scien-
tists and engineers due to their characteristics of small size,
high frequency, high strength and stiffness. Using molec-
ular dynamics (MD) simulation, Legoas et al.
11 12
veri-
fied the reliability of CNT oscillators and proposed some
potential methods to initialize such devices. Guo et al.
13
and Zhao et al.
14
investigated the energy dissipation of
∗
Author to whom correspondence should be addressed.
CNT oscillators. Ma et al.
15
and Xiao et al.
19
studied the
tribological property of the oscillator by MD simulation
and concluded that the frictional forces exhibit no pro-
portional dependence upon the normal force but a strong
dependence upon temperature. Wong et al.
16
found a way
to assuage the energy dissipation by placing a single defect
on the outer tube in order to impede intertube rotational
modes. Kang et al. investigated CNT oscillators operated
by applying external electric fields or thermal expansion of
encapsulated gases,
17 18
and used the oscillator to control
the speed of a nano-engine.
5
Most of the above researches
were based on MD simulations since the manipulation of
nanometer devices and systems remains a great challenge.
2
Recently, Zou et al.
21 22
found via MD simulation
that single-walled carbon nanotubes (SWNTs) of differ-
ent lengths, diameters and chiralities can coaxially self-
assemble into multi-walled CNTs in water. The water
molecules confined in the annular gap between the two
CNTs may form a quasi-two-dimensional stable mono-
layer or bilayer of water. This study offers a promising
approach to produce CNT oscillators with desired sizes
and properties through self-assemble in water. In fact,
water molecules may not only be introduced into CNT
oscillators or other nanodevices during such fabrication
processes but also be adsorbed from environment dur-
ing service or other manipulations. The nanotube shells
are already atomically smooth
8
and thus water between
the stator and actor will not lubricate intertube sliding.
22
Therefore, an attractive question arises as to whether water
absorbed in nanodevices can still be impeditive and make
the CNT oscillators lose their reliability in frequency. In
J. Comput. Theor. Nanosci. 2008, Vol. 5, No. 7 1546-1955/2008/5/001/005 doi:10.1166/jctn.2008.027 1