Biomedical Engineering Reference
In-Depth Information
a function of the relative humidity, but is equally applicable to any
gaseous environments given suitable input parameters.
7.1
Introduction
Carbon nanotubes (CNTs) are often portrayed as the principal
material of the nanotechnology age, due in part to their remarkable
electronic, thermal, and mechanical characteristics [1-3]. As the
nanoscience of CNTs matures into nanotechnology we are beginning
to see development of a variety of real CNT-based devices such as
sensors [4] (including biosensors [5]), tips for scanning probe
microscopy [6], electrochemical actuators [7], and batteries [8].
However, like many other emerging technologies scaling up CNT-
based prototypes to industrial levels requires more than just
advanced fabrication techniques and investment. The successful
commercialization of (these and other) applications using CNTs
also requires control over post-fabrication transformations, and
consideration of the degree to which the devices can tolerate
dispersion and uncertainty in the fundamental properties of
individual components. Repeated delivery of
nanotubes on
the industrial scale is unrealistic, particularly when we need the
devices to be cost effective.
The issue of structural imperfection is not unique to CNT-based
technologies, but is particularly poignant, since the existence (and
co-existence [9, 10]) of multiple allotropes makes the manipulation
of carbon nanostructures challenging [11]. Carbon nanostructures
can undergo changes in their structure when exposed to different
external stimuli, including a very fundamental transformation
involving the re-hybridization of the carbon-carbon (C-C) bonds in
the material. We can even see variations in the distribution of the sp
perfect
3
2
and sp
phases within a single nanostructure [11].
Among the first to characterize the relationship between sp
3
2
and sp
allotropes, and the effect they have upon the physical
properties of the system, were those working on quasi-zero
dimensional nanocarbons such as fullerenes, carbon onions and
nanodiamonds [12]. For example, the transformation of dispersed
nanodiamonds (typically ~4.8 nm in size) into carbon-onions upon
annealing has been observed [13], as has the reverse transition
from carbon-onions into nanocrystalline diamond under electron
irradiation [14], both in ultra-high vacuum. In most cases, however,
