Multilayer and functional coatings on carbon nanotubes using atomic layer deposition

Article abstract of DOI:10.1063/1.2053358

Atomic layer deposition (ALD) can be used to deposit ultra-thin and conformal films on flat substrates, high aspect ratios structures and particles. In this paper, we demonstrate that insulating, multilayered and functionalized ALD coatings can also be deposited conformally on carbon nanotubes. Multilayered coatings consisting of alternating layers of dielectric and conductive materials, such as Al2 O3 and W, respectively, are deposited on conductive multi-walled carbon nanotubes. This coated carbon nanotube can function as a nanoscale coaxial cable. Thin layers of Al2 O3 ALD are also used as a seed layer to functionalize nanotubes. A carbon nanotube was made highly hydrophobic using an Al2 O3 ALD seed layer followed by the attachment of perfluorinated molecules.

Full text of DOI:10.1063/1.2053358

Multilayer and functional coatings on carbon nanotubes using atomic layer deposition
C. F. Herrmann, F. H. Fabreguette, D. S. Finch, R. Geiss, and S. M. George
Citation: Applied Physics Letters 87, 123110 (2005); doi: 10.1063/1.2053358
View online: http://dx.doi.org/10.1063/1.2053358
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/87/12?ver=pdfcov
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APPLIED PHYSICS LETTERS 87, 123110 ͑2005͒
Multilayer and functional coatings on carbon nanotubes using atomic
layer deposition
C. F. Herrmann
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
and Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427
F. H. Fabreguette
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215
D. S. Finch
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427
and Materials Reliability Division (853), National Institute of Standards and Technology, Boulder,
Colorado 80305-3328
R. Geiss
National Institute of Standards and Technology, Boulder, Colorado 80305-3328
S. M. Georgea͒
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215
and Department of Chemical and Biological Engineering, University of Colorado, Boulder,
Colorado 80309-0424
͑Received 21 March 2005; accepted 29 July 2005; published online 14 September 2005͒
Atomic layer deposition ͑ALD͒ can be used to deposit ultra-thin and conformal films on flat
substrates, high aspect ratios structures and particles. In this paper, we demonstrate that insulating,
multilayered and functionalized ALD coatings can also be deposited conformally on carbon
nanotubes. Multilayered coatings consisting of alternating layers of dielectric and conductive
materials, such as Al2O3 and W, respectively, are deposited on conductive multi-walled carbon
nanotubes. This coated carbon nanotube can function as a nanoscale coaxial cable. Thin layers of
Al2O3 ALD are also used as a seed layer to functionalize nanotubes. A carbon nanotube was made
highly hydrophobic using an Al2O3 ALD seed layer followed by the attachment of perfluorinated
molecules. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2053358͔
Widespread interest exists for using carbon nanotubes to
fabricate devices. Nanotubes possess many extraordinary
properties such as high strength and large electrical
conductivity.1–5 Carbon nanotubes are desirable for future
generations of mechanical and electrical devices with appli-
cations ranging from backend interconnects to biological
probes. The success of many of these applications depends
on the deposition of insulating, passivating, or functional
films on the carbon nanotubes.6,7 In this paper, we demon-
strate the deposition of insulating, multilayer, and functional
films on carbon nanotubes using atomic layer deposition
͑ALD͒ techniques.
ALD is a vapor phase thin film growth technique allow-
ing atomic-scale thickness control.8,9 The technique is based
on a sequence of two self-limiting reactions between gas
phase precursor molecules and a solid surface. During the
reaction sequence, only one reactant is present in the reaction
zone at a time. This procedure prevents unwanted gas phase
reactions in contrast to chemical vapor deposition. Because
only a finite number of reactive sites exist on the surface,
reactions with these surface species are inherently self-
limiting. Dense and pinhole-free films are produced when
these two reactions are allowed to go to completion. Further-
more, since gas phase reactants are utilized, ALD does not
require line-of-sight. Conformal coatings can be applied to
very high aspect ratio geometries and porous structures.
Conformal Al2O3 ALD coatings have been previously
reported on multi-walled carbon nanotubes.10 This paper ex-
pands on this work and presents three applications of ALD
on carbon nanotubes. First, a dielectric layer of Al2O3 ALD
is coated on carbon nanotubes to isolate and insulate the
nanotubes from their surroundings. Second, multiple insulat-
ing and conductive ALD films are deposited on a conductive
carbon nanotube core to fabricate a nanotube multilayer co-
axial cable. This novel coaxial cable could be used as a neu-
ronal probe similar in principal to a patch clamp.11 Third, a
seed layer of Al2O3 ALD is grown on carbon nanotubes to
provide a high concentration of surface hydroxyl ͑-OH͒
groups for the attachment of functional molecules. High-
resolution transmission electron microscopy ͑TEM͒ images
of the ALD coatings are obtained to demonstrate the pres-
ence and conformality of these films on the carbon
nanotubes.
Multi-walled carbon nanotubes were obtained from com-
mercial sources. After dispersion using toluene and sonica-
tion, the nanotubes were placed on holey carbon films on
copper grids for subsequent TEM analysis. After initial char-
acterization with TEM, the grids were placed in the ALD
reactor and coated with the desired film.12 No special prepa-
ration steps were necessary before coating the nanotubes.
Al2O3 ALD coatings were deposited by the following two
sequential reactions where the asterisks designate the surface
species:13,14
a͒
Electronic mail: Steven.George@Colorado.Edu
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0003-6951/2005/87͑12͒/123110/3/$22.50 87, 123110-1 © 2005 American Institute of Physics
141.217.58.222 On: Thu, 27 Nov 2014 10:55:26

123110-2
Herrmann et al.
Appl. Phys. Lett. 87, 123110 ͑2005͒
FIG. 1. TEM image of a multi-walled carbon nanotube coated with 25 nm
of Al2O3 ALD.
FIG. 2. TEM image of a multi-walled carbon nanotube coated with a mul-
tilayered ALD film consisting of an inner Al2O3 ALD layer, a middle W
ALD layer, and an outer Al2O3 ALD layer.
͑A͒ AlOH* + Al͑CH3͒3͑g͒ → AlOAl͑CH3͒2* + CH4͑g͒,
͑1͒
͑B2͒ WSiH2F* + ͑1/2͒Si2H6 → WSiHFSiH3* + ͑1/2͒H2.
͑5͒
*
͑B͒ AlCH3 + H2O͑g͒ → AlOH* + CH4͑g͒.
͑2͒
The carbon nanotubes were first exposed to trimethylalumi-
num ͓Al͑CH3͒3͔ and then to the subsequent water exposure.
This pattern was repeated to obtain the desired film thick-
ness. At 177 °C, the growth rate for Al2O3 ALD was 0.12 nm
per AB cycle. Including purging following each reactant ex-
posure, each AB cycle required 12 sec.
A TEM image of a carbon nanotube coated with Al2O3
ALD at a deposition temperature of 177 °C is shown in Fig.
1. The Al2O3 ALD film thickness is 25 nm. The graphitic
structure of the carbon nanotube is clearly visible in this
TEM image. A fast Fourier transform ͑FFT͒ micrograph of
the graphitic structure indicates that the lattice spacing of the
graphitic structure is 0.34 nm.
Al2O3 ALD films have been found to be excellent
insulators.15 These films display extremely low leakage and
have a dielectric constant of approximately 7.5.15 This Al2O3
ALD film on the carbon nanotube should be useful as a
dielectric layer to isolate the nanotube from its surroundings.
Multilayered ALD coatings were deposited on carbon
nanotubes using successive Al2O3 ALD, W ALD, and Al2O3
ALD. The Al2O3 ALD and W ALD were deposited to obtain
individual film thicknesses of ϳ20 nm. Al2O3 was deposited
first using the reactions described above and a deposition
temperature of 125 °C to form the inner insulating layer. At
125 °C, the growth rate for Al2O3 ALD was about 0.13 nm
per AB cycle.16 A typical AB cycle required 44 sec including
purging following each reactant exposure. The middle con-
ductive layer was deposited using W ALD.17 The W ALD
surface chemistry required alternating exposures of WF6 and
Si2H6. The W ALD film growth was achieved according to
the following surface reactions where silicon serves as a sac-
rificial species:17,18
The growth rate at 125 °C was 0.35 nm per AB cycle and
one AB cycle required 61 sec. Following this W ALD layer,
a final Al2O3 ALD layer was added to insulate the multilayer
structure.
The Al2O3/W/Al2O3 coated nanotubes were examined
using TEM. The TEM image displayed in Fig. 2 clearly re-
veals the presence of discrete layers surrounding the carbon
nanotube. The high atomic number W layer is highly attenu-
ating and produces a dark region compared with the low
atomic number Al2O3 layer. The W layer yields the highest
contrast and obscures the inner Al2O3 ALD layer. This
multilayer coated carbon nanotube could be used as a nano-
tube coaxial cable.
ALD techniques have been previously demonstrated to
functionalize various surfaces and render them highly
hydrophobic.19 This technique utilizes an Al2O3 ALD seed
layer and the subsequent adsorption of a functional hydro-
phobic monolayer. In this paper, an Al2O3 ALD seed layer
with a thickness of 25 nm was deposited at 140 °C using the
chemistry described above. This Al2O3 layer provides a
dense and uniform concentration of surface -OH groups on
the nanotube.14 A hydrophobic precursor, tridecafluoro-
1,1,2,2-tetrahydro-octyl-methyl-bis͑dimethylamino͒silane
͓C8F13H4͑CH3͒Si͑N͑CH3͒2͒2͔, was then chemically bonded
to the surface hydroxyl groups on the ALD seed layer. On
flat substrates, this procedure produces a film that is highly
hydrophobic with a water contact angle of 108°.19
Due to poor contrast, the hydrophobic layer was difficult
to observe on the Al2O3 ALD seed layer with TEM. How-
ever, the TEM images suggested that the hydrophobic layer
was present with a thickness of approximately 1 nm. This
hydrophobic layer thickness agrees well with the expected
thickness of ϳ0.9 nm for the fully extended alkyl chain as-
suming an all trans configuration with tetrahedral angles.
Different molecules could also be reacted with the -OH
groups on the Al2O3 seed layer to produce a variety of func-
͑A͒ WSiHFSiH*3 + 2WF6͑g͒
→WWWF*4 + 2SiF4 + ͑3/2͒H2͑g͒ + HF͑g͒,
͑3͒
*
*
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͑B1͒ WF4 + Si2H6 → WSiH2F + SiHF3͑g͒ + ͑3/2͒H2, ͑4͒
tionalities such as more hydrophilic or biocompatible surface
141.217.58.222 On: Thu, 27 Nov 2014 10:55:26

123110-3
Herrmann et al.
Appl. Phys. Lett. 87, 123110 ͑2005͒
coatings should play a key role in the development of nano-
technology based on carbon nanotubes.
This research was supported by grants from the Air
Force Office of Scientific Research and DARPA ͑Grant#
F33615-99-C-1518͒.
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