SYNTHESIS OF A SELF ASSEMBLED HYBRID OF ULTRANANOCRYSTALLINE DIAMOND AND CARBON
NANOTUBES
CONTRACTUAL ORIGIN OF THE INVENTION
The United States Government has rights in this invention pursuant to
Contract No. W-31-109-ENG-38 between the U.S. Department of Energy (DOE) and
The University of Chicago representing Argonne National Laboratory.
FIELD OF THE INVENTION
The present invention relates to various combinations of carbonaceous
materials, particularly those with interesting electrical and hardness properties.
BACKGROUND OF THE INVENTION
Recent strong scientific and technological interest in nanostructured carbon
materials (nanocarbons) has been motivated by the diverse range of physical properties
these systems exhibit. These properties arise from the many different local bonding
structures of carbon, as well as the long range order of the bonding structure. For
example, carbon nanotubes (CNTs) are distinct from graphite although both consist
essentially of sp2-bonded carbon. CNT's are the strongest known material and also
exhibit unique electronic transport properties, making them candidates for a wide range
of applications.
Similarly, nanocrystalline diamond films are distinct from single crystal diamond
although both are mostly sp3-bonded carbon, and exhibit high hardness, exceptional
chemical inertness, biocompatibility and negative electron affinity with properly
treatment. The unique mechanical and electrochemical properties of nanocrystalline
diamond make it a promising candidate as the protective coating for machining tools,
hermetic corrosion resistant coating for biodevices, cold cathode electron source, and
the structural material for micro- and nano- electromechanical systems (MEMS/ NEMS).
It is believed that a combination of carbon nanotubes and nanocrystalline
diamond provides materials with novel properties that are advantageously used in
applications such as electronic devices or MEMS/ NEMS. However, until now no
method of providing the concurrent growth of different allotropes of carbon that are
covalently bonded and organized at the nanoscale has been available.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide a synthesis of
nanocrystalline diamond and carbon nanotubes to form a covalently bonded hybrid
material: a nanocomposite of diamond and CNTs
Another object of the invention is to provide a material comprising carbon
nanotubes and diamond covalently bonded together.
Another object of the invention is to provide a method of producing carbon
nanotubes and diamond covalently bonded together, comprising providing a substrate,
depositing nanoparticles of a suitable catalyst on a surface of the substrate, depositing
diamond seeding material on the surface of the substrate, and exposing the substrate
to a hydrogen poor plasma for a time sufficient to grow carbon nanotubes and diamond
covalently bonded together.
Another object of the invention is to provide a hybrid of carbon nanotubes and
diamond made by the method of providing a substrate, depositing nanoparticles of a
suitable catalyst on a surface of the substrate, depositing diamond seeding material on
the surface of the substrate, and exposing the substrate to a hydrogen poor plasma for
a time sufficient to grow a hybrid of carbon nanotubes and diamond.
The invention consists of certain novel features and a combination of parts
hereinafter fully described, illustrated in the accompanying drawings, and particularly
pointed out in the appended claims, it being understood that various changes in the
details may be made without departing from the spirit, or sacrificing any of the
advantages of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of facilitating an understanding of the invention, there is
illustrated in the accompanying drawings a preferred embodiment thereof, from an
inspection of which, when considered in connection with the following description, the
invention, its construction and operation, and many of its advantages should be readily
understood and appreciated.
FIGURE Ia is a SEM showing the evolution of the hybrid UNCD/ CNTs
structures via adjustment of the relative fraction of catalyst and nanodiamond seeds;
FIG. Ib is a SEM showing the hybrid structures of UNCD and CNTs with a
low fraction of CNTs and UNCD;
FIG. Ic is a SEM having a fully dense hybrid structure of UNCD and CNTs
with a high fraction of UNCD;
FIG. Id is a SEM showing pure UNCD;
FIG. 2a is a TEM image of CNTs prepared using PECVD with Ar/ CH4 as
precursor with different diameters of CNTs ranging from 2 to 10 ran;
FIG. 2b is a HRTEM image of CNTs multiwalled with well-ordered graphene
sheets and typical defect densities;
FIG. 3 is a graphical representation of a Raman spectra of CNTs, UNCD and
UNCD/CNTs hybrid structures corresponding to the samples shown in Figs. Ia, b-
d, respectively;
FIG. 4 is a graph of C Is NEXAFS of CNTs, UNCD and UNCD/CNTs hybrid
structures, corresponding to the samples shown in Figs, la-d, respectively,
nanodiamond seeds; and
FIGS. 5-14 are SEM images of covalently bonded diamond and CNTs of the
hybrid materials; and
FIG. 15 is a schematic representation of a combination of carbon nanotubes
and diamond nanoparticles, each in predetermined patterns.
DETAILED DESCRIPTION OF THE INVENTION
One of the most commonly used processes for preparing nanostructured carbon
materials is plasma enhanced chemical vapor deposition (PECVD), in which chemically
activated carbon-based molecules are produced; however, this invention includes any
known method of depositing nonstructural carbon materials. For instance, different
carbon-rich combinations of C2H2/ H2, C2H2/ NH3, and CH4/ Ar have been employed for
growing CNTs. In contrast, hydrogen-rich (~99% H2) CH4/ H2 plasmas are the most
common mixtures used for growing microcrystalline diamond films, wherein large
amounts of atomic hydrogen play a critical role in both the gas-phase and surface
growth chemistries. Importantly, atomic hydrogen is also needed to selectively etch the
non-diamond carbon during growth. Over the past several years Argonne National
Laboratory (ANL) has developed hydrogen-poor Ar/ CH4 (99% Ar, 1 % CH4) chemistries
to grow ultrananocrystalline diamond (UNCD) films, which consist of diamond grains
3-5 nm in size and atomically abrupt high energy grain boundaries, as described by A.
Krauss, O. Auciello, D. Gruen, A. Jayatissa, A. Sumant, J. Tucek, D. Mancini, N.
Moldovan, A. Erdemir, D. Ersoy, M. Gardos, H. Busmann, E. Meyer, M. Ding, Diamond
Relat. Mater. 2001, 10, 1952, incorporated herein by reference.
The special nanostructure of UNCD yields a unique combination of properties,
such as low deposition temperatures, described by X. Xiao, J. Birrell, J. E. Gerbi, O.
Auciello, J. A. Carlisle, J. Appl. Phys. 2004, 96, 2232, incorporated herein by reference,
excellent conformal growth on high-aspect ratio features, described by A. Krauss, O.
Auciello, D. Gruen, A. Jayatissa, A. Sumant, J. Tucek, D. Mancini, N. Moldovan, A.
Erdemir, D. Ersoy, M. Gardos, H. Busmann, E. Meyer, M. Ding, Diamond Relat. Mater.
2001, 10, 1952, incorporated herein by reference and the highest room-temperature
n-type electronic conductivity demonstrated for phase-pure diamond films via nitrogen
doping at the grain boundaries, as described by S. Battacharyya, O. Auciello, J. Birrell,
J. A. Carlisle, L. A. Curtiss, A. N. Goyete, D.M. Gruen, A. R. Krauss, J. Schlueter, A.
Sumant, P. Zapol, Appl. Phys. Lett. 2001, 79,1441. incorporated herein by reference.
It is important to recognize that the composition and morphology of the material
grown is not simply a function of the gas mixture and plasma conditions, but also
depends sensitively on the pretreatment of the substrate prior to growth as well as the
substrate temperature. It is widely known that there is a high nucleation barrier for
growing carbon based materials and that certain pre-treatments are necessary to
provide the initial nucleation sites. For example, nanoparticles of transition metals, such
as Ni, Fe and Co are used as catalysts for growing CNTs, whereas micro or
nano-diamond UNCD powders are typically needed to be present on the substrate
surface prior to the diamond growth. In addition, the temperature window for PECVD
growth of CNTs ranges from 1500C while UNCD films can be prepared at temperature
ranged from 4000C to 8000C.
Experimental
Iron films with different thickness (~5 ~ 40 ran) were deposited on silicon
substrates using an ion beam sputtering deposition system with a Kr ion gun. The
coated samples were then immersed into a suspension of ~5 nm diamond particles in
methanol and ultrasonically vibrated for different periods of time in order to control the
nucleation density for the growth of UNCD. Next, the seeded films were inserted into
a microwave plasma deposition system (IPLAS) and heated at 800 0C in flowing
hydrogen (90 seem, 20 mbar) for 30 minutes to coalesce the iron films into nano-sized
iron particles to catalyze CNTs formation. The iron film thickness determines the size
of the catalyst particles, which subsequently determines the diameter of CNTs.
Following the pretreatment described above, the substrate was cooled down to 7000C
and a plasma consisting of 99% Ar with 1% CH4 was initiated to grow the carbon
nanocomposite .
A number of specific experiments used the following protocol:
Experimental details:
1. Clean the substrate (Silicon, Silcion oxide, W and other carbide formed
metal) using acetone and methanol for 5 minutes separately.
2. Sputter the transition metals (Fe, Ni, Co) to the cleaned substrate with
different thickness (0, 5, 10, 20 and 40nm).
3. Ultrasonically seed the substrate in nanodiamond suspension (3mg nano
diamond powder in 100ml methanol) with different time (0, 5, 15, 30 minutes), then
rinse with methanol.
4. Heat the sample up to 8000C and input H2 flow (90 seem, 20 mbar) for 20
minutes to reduce the possibly oxidized metal and break the continuous film into nano
particles. The size and density of nano particles are dependent of thickness of metal
films and in turn influence the diameters and density of carbon nanotubes accordingly.
5. Decrease the substrate temperature down to 600-7000C and switch off the
hydrogen flow, wait for 5 minute pumping down.
6. Expose the treated substrate to hydrogen poor Ar/CH4 plasma (49 seem
Ar and 1 seem CH4 the typical flow rate for growing ultrananocrystalline diamond) for
different time (10, 20, 30 minutes).
We determined the following from the experimental data:
1. The relative fraction of ultrananocrystalline diamond and carbon
nanotubes is controlled by the combination of seeding time, thickness of catalyst thin
films and growth time.
2. Thickness of the catalyst thin films not only control the catalyst particle
size but also control the catalyst density, which in turn control the diameter and density
of catalyst.
• Pure ultrananocrystalline diamond is obtained without catalyst
deposition on substrate, as shown in Fig. Ia;
• Nerve structures are obtained with process of 5 minute seeding, 10
ran catalyst and 10 minute growth; as shown in Fig. Ib;
a
• Structure with the protrusion of carbon nanotubes through
supergrain boundaries are obtained with the process parameters
of 30 minute seeding, 10 nm catalyst, 30 minute growth, as shown
in Fig. Ic;
• Pure UNCD are obtained without transition metal sputtering as
shown in Fig. Id;
3. Setting the process parameters in the overlapped process windows
resulted in carbon nanotubes and ultrananocrystalline diamond.
4. Patterned templates for seeds and catalyst were utilized to simultaneously
and selectively grow carbon nanotubes and ultrananocrystalline diamond to fabricate
the prototype of electronic devices.
5. Uniform distribution of carbon nanotubes in diamond matrix enhances
the fracture roughness of diamond thin films and overcomes the shortcomings of
brittleness.
The hybrid nanostructures were studied using a Hitachi S-4700 field emission
Scanning Electron Microscope (SEM) at 10 kV accelerating voltage and a TECNAI 20
Transmission Electron Microscope (TEM) with Electron Energy Loss Spectroscopy
(EELS) at 10OkV accelerating voltage. The hybrid films were also analyzed with visible
Raman spectroscopy using a Renishaw Raman microscope in the backscattering
geometry with a HeNe laser at 633 nm and an output power of 25 mW focused to a spot
size of ~ 2 μm. Near Edge X-ray Absorption Fine Structure (NEXAFS) analysis was
performed at the Advanced Light Source of Lawrence Berkeley National Laboratory.
The diamond reference sample was a standard Type Ha diamond. The graphite
reference sample was a highly oriented pyrolitic graphite (HOPG).
By selectively placing the catalyst and nanodiamond powders on the same
substrate, carbon nanotubes and UNCD can be grown. The relative fraction of UNCD
and CNTs can be varied by controlling the relative amounts of transitional metal
catalysts and nanodiamond seeds. The first successful preparation of the hybrid
CNT/ UNCD nanostructures using this approach is set forth hereafter.
Fig. 1 shows SEM images revealing the structural evolution from pure CNTs to
pure UNCD films as the relative fraction of Fe and diamond nanoparticles was varied.
Pure CNTs (Fig. Ia) were observed when only Fe catalyst particles were present on the
substrate, whereas "normal" UNCD resulted when only nanodiamond particles were
present (Fig.1 d) . Seeding with both types of catalyst particles leads to the simultaneous
growth of both UNCD and CNT in all cases, but controlling the relative amounts of
these two allotropes further requires careful control of temperature and deposition time,
since CNTs normally grow much faster than UNCD. This is shown in the SEM data
presented in Fig. Ib and Ic. For sufficiently short deposition times (~30 min.), the
formation of isolated "supergrains" consisting of many nanosized crystalline diamond
grains on the substrate is observed. Since the catalyst and nanodiamond powder were
present at the same time in the plasma, UNCD and CNTs were simultaneously grown
on those seeds and catalyst. The supergrains shown in Fig. Ib appear, in fact to be
interconnected by CNTs, with both ends of some individual nanotubes terminating on
different supergrains. It is possible that the plasma environment causes local charging
effects that lead to attractive forces to arise between the UNCD supergrains and CNTs,
but it is also possible that UNCD and CNT can grow into each other.
It may be that the CNTs and UNCD are covalently bonded together or it may be
that the combination is a hybrid, but whichever form it may be, the composition is new.
To realize useful materials such as for MEMS and wear-resistant coatings, it will be
necessary to produce fully-dense that is substantially free of voids, covalently-bonded
(or hybrid) structures. Fig. Ic shows a SEM image of a material that very nearly realizes
this goal. Further increase of the diamond nucleation density relative to the Fe catalyst
enhanced the growth of UNCD relative to CNTs, and the CNTs are clearly present at
the boundaries between the supergrains (Fig. Ic). Energy-dispersive x-ray (EDX) data
(not shown) revealed the presence of Fe at the tips of the structures between the
supergrains.
The carbon nanotubes shown in Fig. Ia were further investigated by TEM (Fig.
2), which showed a typical bundled multiwall (MWCNT) morphology. The catalytic
particles were also observed, as shown in the top left area of Fig. 2a. HRTEM images
revealed that the nanotubes had diameters in the range of about 2 to 10 nm and the
nanotube walls were comprised of reasonably well-ordered graphene sheets. The
carbon nanotubes are defective, as is typical for CNTs prepared by PECVD under these
conditions. Furthermore, the HRTEM and EELS results on the sample shown in Fig. Ib
confirmed the coexistence of CNTs and UNCD (not shown here).
Fig. 3 compares the Raman spectra of UNCD, CNT, and the UNCD/CNT
nanocomposite in the range 100~300 cm-1. Radial breathing mode (RBM) peaks are
clearly observed in the Raman spectra of CNTs and the nanocomposite, which indicates
the presence of small diameter single- or double-wall CNTs, in addition to the
somewhat larger diamond MWCNT that were observed via TEM. Interestingly, the
peak positions in the pure CNT sample compared to the hybrid UNCD/ CNTs materials
are consistently different, which may be indicative of slightly different growth regimes
for the two materials (e.g. the presence of only Fe particles versus Fe+ nanodiamond
particles). The estimated inner-diameters are on the order of one nm, which may
correspond to the some of the smaller CNTs shown in HRTEM pictures. No RBM is
detected in pure UNCD, even for the graphitic phase along the grain boundaries.
Further research is undergoing in our lab to explore the relationships between the RBM
peaks and process parameters.
Near-edge x-ray absorption fine structure (NEXAFS) is a useful tool to
unambiguously distinguish the Sp2 bonding and sp3 bonding in carbon materials. C (Is)
NEXAFS data obtained from pure CNTs, pure UNCD, and the UNCD/ CNT shown in
Fig. Ic are shown in Fig.4. UNCD films consist of about 95% sp3-bonded carbon, with
5% sp2 bonded carbon within the grain boundaries which occupy 10% of the UNCD
volume. Thus the C Is NEXAFS from UNCD looks similar to data obtained from
high-quality microcrystalline diamond or single crystal diamond except for the presence
of an sp2 π* peak at 285.5 eV. In contrast, the spectrum obtained from the pure CNTs
sample looks very similar to those obtained from a typical graphite reference (highly
oriented pyrolytic graphite), with both the π* at 285.5 eV and the sp2 σ* core exciton at
~291.5 eV clearly visible. This is consistent with the observation of good local order in
the CNTs shown in Figure 2.
The NEXAFS spectrum of a CNT/ UNCD hybrid structure shows the combined
signals from both CNTs and diamond. The peak intensity around 285 eV in the
nanocomposite is higher and the dip around 302 eV is shallower than the corresponding
ones in UNCD, implying a slightly higher fraction of the graphite phase resulting from
CNTs and the grain boundaries of UNCD. These data provide direct evidence that the
growth of UNCD (and probably CNTs) proceed independently in the hybrid as they do
during the growth of the composite.
It is the overlap of the process parameters for growing UNCD and CNTs, in
particular the reduced amount of atomic hydrogen, that makes it possible to
simultaneously grow the UNCD/ CNT hybrid. CNTs grow readily in Ar-rich Ar/CH4
discharges due to the abundance of C2H2 in these plasmas via the thermal
decomposition of CH4 at 1600K plasma temperatures. It is believed that C2H2
decomposed on the Fe nanoparticles, leading to the formation and diffusion of carbon
atoms in the catalyst and the growth process for CNTs. However, several other carbon
species have also been considered as growth species for CNTs, including CH3 which is
widely regarded as the principal growth species for most PECVD deposited diamond
thin films. Our data indicate that the relative proportion of the two species is governed
by kinetics and not the competing energetics of CNTs and UNCD growth. In previous
work it was demonstrated that the same hydrogen-poor plasmas can still selectively
etch the side walls of the horizontally oriented MWCNTs under an Ar-rich Ar/ CH4
discharge, leading to the growth of graphitic structures on the sidewalls, as described
by S. Trasobares, C. P. Ewels J. Birrell, O. Stephen, B. Q. Wei, J. A. Carlisle, D. Miller, P.
Keblinski, P. M. Ajayan, Adv. Mat. 2004, 16, 610, incorporated herein by reference.
Since the process parameters for growing both nanocarbon materials are the
same in Ar/ CH4 plasma, the key factor determining the subsequent nonstructural
development is the initial nucleation sites. Fabricating periodic arrays of UNCD and
CNTs by patterning nanodiamond and catalyst particles with the aid of lithographic
techniques such as electron-beam lithography, n-type conductive various geometries
such as films of heterojunctions between conductive UNCD and CNTs are capable of
being produced, such as but not limited to semiconductors, MEMS devices and the like
and Figures 5-15 are SEM images of the hybrid materials produced by the methods
disclosed herein.
To summarize, a new synthesis pathway has been developed to combine
different allotropes of carbon at the nanoscale in covalently bonded structures. The
synthesis of a hybrid nanocarbon material consisting of ultrananocrystalline diamond
and carbon nanotubes has been successfully demonstrated for the first time, via the
exposure of a surface consisting of nano-sized diamond powders and iron nanoparticles
to a hydrogen-poor carbon-containing plasma. This method offers a novel approach to
modulate the relative ratio of sp2- and sp3-bonded carbon to form self-assembled carbon
nanostructures that is amendable to modern patterning techniques to further organize
these structures for useful purposes. Potential applications of these new hybrid
structures ranging from nano-electronics to bio-MEMS.
In the manufacture of a variety of devices, such as semiconductors, a substrate
such as but not limited to W, Ta, Ti, Mo, Cu, Si, SiO2, mixtures and alloys thereof may
be used. The diamond may be nanocrystalline or UNCD and may be electrically
conducting or not. Nitrogen doping of UNCD provides an n-type electrical conductor.
The growth plasma used to grow the composite materials can be further tailored
to change the electrical transport properties of the UNCD. In addition to the "normal"
plasmas used to grow undoped UNCD (about 99% Ar and 1% CH4), plasmas with
either extra nitrogen (N2) or boron (either diborane or trimethylboron) added to the gas
mixture leads to the growth of n- type or p-type UNCD, respectively. Co-integration of
highly room-temperature conductive n-type UNCD with CNTs will further increase the
electrical conductivity of the hybrid material and also improve its use in cold-cathode
electron sources or as electrodes in supercapacitors. P-type UNCD, when integrated
with carbon nanotubes which are normally n-type conductive, will give rise to
interesting photonic properties suitable to the conversion of solar energy into electrical
energy.
The use of patterning techniques (such as photolithography, e-beam lithography,
and others approaches) are used to control the location of the nanotubes and UNCD
relative to each other. As seen in Fig. 15, a nanotube catalyst can be patterned on a
substrate as an arrays of dots on a substrate surface with arbitrary diameter and pitch
(1), after which diamond nanoparticles (which lead to the growth of UNCD) are
ultrasonicated onto the patterned surface (2), and the growth then leads to the UNCD
and CNTS having distinct locations within the hybrid material. The pattern and
location of the diamond nanoparticles themselves are controlled (instead of or in
addition to) the CNT catalyst particles to grain further control over the growth and
microstructure.
The alignment of the carbon nanotubes within the hybrid thin film materials can
be controlled via the application of electric fields (commonly accomplished by applying
a bias voltage during growth). Vertically aligned carbon nanotubes have been grown
by many groups using microwave plasma chemical vapor deposition. Such an
integration of vertically aligned carbon nanotubes within a UNCD matrix will have
anisotropic transport and mechanical properties. For instance, the electrical
conductivity of the hybrid thin film as a whole is higher in the direction parallel with
the alignment of the CNTs and much lower perpendicular to the alignment. Mechanical
properties such as the fracture toughness & strength, hardness, and Young's modulus
will also exhibit similar behavior.
The use of more exotic catalyst materials (such as CoMo alloys) produces
predominantly single wall (instead of multiwalled) carbon nanotubes and the growth
of single-walled nanotubes that are semiconducting versus metallic. Integration of
single walled material (which is distinct from multiwalled and exhibit ballistic (zero
resistance) electron transport, ultra-high thermal conductivities, and the highest
material strength of any known material is a very valuable aspect of the invention.
While the invention has been particularly shown and described with reference
to a preferred embodiment hereof, it will be understood by those skilled in the art that
several changes in form and detail may be made without departing from the spirit and
scope of the invention.