Owing to their perfect 1-D nanostructure, low density, and
high conductivity of both heat and electricity, carbon nanotubes
(CNTs) show great promises in nano-electronics and photonics,
vacuum electronics, sensors and actuators, high strength composite
materials, and aerospace engineering etc. For such an important
material, there must be some large-scale applications in industry.
For many years, we have aimed at achieving controlled synthesis of
CNTs, i.e., synthesizing CNTs with desired diameter, chirality,
wall thickness, and length to meet a variety of industrial demands.
To achieve this aim, one has to studying the growth mechanisms of
CNTs, finding out how CNT nucleates, why growth teminates, how to
control the diameter and chirality etc. Thus the goal of this study
is to clarify the growth mechanisms and achieve controlled
synthesis of CNTs. The approach of this thesis is a combination of
experimental investigation and theoretical modeling which are
stimulative to each other. Finding out basic laws from experimental
exploration will facilitate theoretical modeling. On the other
hand, one can design new growth method according to the growth
mechanism. After several years of efforts, we have made some
progress in the controlled synthesis of super-aligned CNT arrays,
the growth kinetics, and the nucleation mechanisms of
CNTs.
1．Controlled
synthesis of super-aligned CNT arrays
CNT array is a self-organized ordered
structure, in which parallel CNTs with narrow diameter distribution
and nearly identical length are regularly aligned perpendicular to
the substrate. Thus CNT arrays are aggregations of high quality
CNTs. By tuning the growth rate of CNT arrays, super-aligned CNT
arrays are successfully synthesized. The super-aligned CNT arrays
are distinguished from normal arrays by their higher nucleation
density and growth rate, narrower diameter distribution, better
alignment, as well as cleaner surfaces and stronger van der Waals
interaction between CNTs. When trying to pick up a bundle of CNTs
from the super-aligned array, a continuous yarn is obtained. Here
the suer-aligned array has the similar function as a silk cocoon.
Upon drawing, CNTs in it are end to end joined together by van der
Waals force forming continuous yarns of pure CNTs. Optical
polarizers are constructed by parallel aligning the yarns, which
can work even in the UV region. These yarns can also be used as
filament of light bulb, from which incandescence can be emitted at
small power input. Recently a novel method was invented to process
fresh yarn by passing through volatile solvent, which greatly
enhanced the mechanical strength and improved the manipulability.
The processed yarn is both elastic and pliable and can be freely
manipulated and mold to any desired shape to construct a variety of
macroscopic objects for various applications. In 2005, the
synthesis was expanded to 4-inch wafer scale. A super-aligned array
on 4-inch wafer can be converted to a continuous thin film of 10
centimeters wide and 60 meters long, which can be directly employed
as transparent conducting film and thin film transistors. Recently
we have achieved batch growth of 4-inch super-aligned arrays in a
6-inch tube furnace.
2．Growth kinetics of
CNTs
Today, CNTs can be synthesized with a
variety of methods by using a variety of precursors and catalysts.
However, there is still no clear physical picture of the growth
process. To study the growth kinetics of CNTs, we developed a
simple but effective growth mark method to measure the growth rate
during growth at various temperatures, from which the activation
energy of the overall growth reaction can be obtained. It is found
that the surface reaction is the rate-limited step in our
synthesis. This surface reaction limited growth favors the growth
of millimeter high CNT arrays. According the these experimental
results, A model based on VLS (vapor-liquid-solid) mechanisms was
proposed for CVD growth of CNTs, which involves a liquid state
catalyst with homogeneous temperature and carbon concentration
distribution. The growth kinetics was fully controlled by the
super-saturation level which can be expressed as a function of
temperature and carbon concentration in the catalyst droplet. Based
on this model, the steady-state axial growth rate equation was
derived, which fits very well with our experimental results of
kinetic study.
3．Nucleation
mechanisms of CNTs
To achieve large-scale industrial
applications, it’s very important to studying the growth
mechanisms of CNTs, finding out how the CNTs nucleate, why growth
terminate, how to control the diameter and chirality
etc.
We believe that there must be a
unified mechanism for catalytic growth of CNTs, no matter what kind
of catalysts and methods were used. The reason is that, the growth
of CNTs, whether single-walled or mult-walled, whether arc, laser,
or CVD method, all starts from the formation of a super-saturated
carbon-metal solution. Here the start point of the proposed
nucleation mechanism is the carbon-metal solution which will give
birth to a graphene nuclei upon super-saturation or super-cooling.
Then growth takes place at the edge of graphene nuclei. It’s the
super-saturation level that determines the nucleation behavior and
the formation of various structures such as single-walled,
double-walled, multi-walled and bamboo-shaped CNTs.
Then the VLS model was further
generalized to XLS (X-liquid-solid) model, in which the precursor
can be in any X phase (X=gas, liquid or solid). Based on this
model, we applied classical nucleation theory to the nucleation
mechanisms of graphene layer over the surface of carbon-catalyst
solution, in which super-saturation level and microscopic bond
energies were adopted as the basic parameters. Three kinds of
nucleation modes were distinguished. The critical sizes and
activation energy barriers of these nucleation modes were derived,
as well as the influences of super-saturation on them. The
nucleation and growth processes of all kinds of CNTs and carbon
nanofibers were analyzed by using this nucleation mechanism. And
such questions as, what is the role of metal catalyst in nucleation
and growth, is there any chirality correlation between adjacent
layers of multi-walled CNTs, why growth terminates, how to control
the wall thickness and diameter, etc. were answered. This
nucleation mechanism can be generally applied to CNT growths via
arc, laser or CVD and fit well with existing experimental
results.
The work presented here enables us to understand the
nucleation and growth process of CNTs in a general framework
qualitatively and semi-quantitatively. In the future, quantitative
descriptions are expected. Furthermore, based on the understanding
of the growth mechanisms, many kinds of growth methods will be
developed to meet a variety of industrial demands.