Chemically assisted directed assembly of carbon nanotubes for the fabrication of large-scale device arrays

Article abstract of DOI:10.1021/ja073647t

We report the directed assembly of single-walled carbon nanotubes (SWCNTs) at lithographically defined positions on gate oxide surfaces, allowing for the high yield (～90%) and parallel fabrication of SWCNT device arrays. SWCNTs were first chemically funct

Full text of DOI:10.1021/ja073647t

Published on Web 09/07/2007
Chemically Assisted Directed Assembly of Carbon Nanotubes
for the Fabrication of Large-Scale Device Arrays
George S. Tulevski,* James Hannon, Ali Afzali, Zhihong Chen,
Phaedon Avouris, and Cherie R. Kagan
Contribution from the IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
Received May 21, 2007; E-mail: gstulevs@us.ibm.com
Abstract: We report the directed assembly of single-walled carbon nanotubes (SWCNTs) at lithographically
defined positions on gate oxide surfaces, allowing for the high yield (∼90%) and parallel fabrication of
SWCNT device arrays. SWCNTs were first chemically functionalized through diazonium chemistry with a
hydroxamic acid end group that both renders the SWCNTs water-soluble and discriminately binds the
SWCNTs to basic metal oxide surfaces (i.e., hafnium oxide (HfO2)). The functionalized SWCNTs are then
assembled from an aqueous solution into narrow trenches etched into SiO2 films with HfO2 at the bottom.
The side walls of the patterned trenches induce alignment of the SWCNTs along the length of the trenches.
Heating the structures to 600 °C removes the organic moieties, leaving pristine SWCNTs as evidenced by
Raman spectroscopy and electrical measurements. Palladium source-drain electrodes deposited perpen-
dicular to the trench length readily contact the ends of the aligned SWCNTs. The resultant devices exhibit
the electrical performance expected for SWCNT devices, with no performance deterioration as a result of
the placement process. This technique allows for the directed assembly and alignment of SWCNTs over
a large area and results in a high yield of working devices, presenting a promising path toward large-scale
SWCNT device integration.
Introduction
functionalize SWCNTs with compounds that selectively bind
to patterned surfaces. The functionalized SWCNTs were
Single-walled carbon nanotubes (SWCNTs) are a promising
emerging material for the continued scaling of logic circuits
due to their excellent properties as the active channel in field-
effect transistors.1-5 To fully exploit their superior transport
properties, fabrication tools need to be developed to address
many substantial processing challenges such as their separation,
chemical doping, and selective placement. This paper details a
method for the selective placement of SWCNTs, via chemically
assisted directed assembly, that allows for large area, high-yield
fabrication of electronic devices. There are numerous reports
on using patterned self-assembled monolayers (SAMs) to
selectively place SWCNTs.6-10 They rely on patterning SAMs,
usually on SiO2, with terminal functional groups that either
promote or inhibit SWCNT adhesion. More recently, reports
from this laboratory11,12 have detailed a method to reversibly
dispersed in alcohol, dried onto the patterned surface, and then
sonicated to remove the loosely bound functionalized SWCNTs.
Although the process results in selective placement, the drying/
sonication process could not produce orientation control or a
high yield over a large area. Here, we were able to overcome
these limitations by using chemistry that renders the SWCNTs
water-soluble, allowing for control over the subtle acid-base
surface reactions via carefully modifying the pH. The function-
alized SWCNTs can simply diffuse to the surface in the aqueous
solution and bind to the surface reaction sites leading to
alignment and large-area assembly.
The important results of this work are that the precise location
and orientation of water-soluble chemically functionalized
SWCNTs can be controlled, allowing for the fabrication of large
device arrays in a high yield. The process is illustrated in Figure
1. The SWCNTs are first modified with organic compounds
that are end-functionalized with organic acids (hydroxamic
acids) that impart water solubility and selectively bind to basic
metal oxides, exemplified in this study by hafnium oxide (HfO2),
a technologically important high-κ gate oxide. Substrates are
fabricated with narrow trenches etched through the top SiO2
layer (in green in Figure 1) of a SiO2 on HfO2 (in red in Figure
1) gate dielectric stack. Immersion of the patterned substrates
in functionalized SWCNT (f-SWCNT) solutions directs the
assembly of the f-SWCNTs onto the HfO2 surfaces, forming
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J. P. Nano Lett. 2005, 5, 451.
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S.; Burghard, M. Surf. Sci. 2000, 462, 195.
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2005, 4, 589.
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C. A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 2026.
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J. AM. CHEM. SOC. 2007, 129, 11964-11968
10.1021/ja073647t CCC: $37.00 © 2007 American Chemical Society

Chemically Assisted Directed Assembly of C Nanotubes
A R T I C L E S
Scheme 1
hydroxamic acid as white crystals (7.9 g, 73%), mp 169-170 °C. NMR
(CDCl3); 5.03 , s 2H; 7.4-7.55, m 5H, 7.93 d, J ) 7 Hz, 2H, 8.31 d,
J ) 7 Hz, 2H.
4-Aminophenylhydroxamic Acid (3). Palladium on activated carbon
(10%, 500 mg) was added to a mixture of O-benzyl 4-nitrophenylhy-
droxamic acid (2.71 g, 0.01 mol) and ammonium formate (3.65 g, 0.05
mol) in 100 mL of anhydrous methanol, and the solution was refluxed
under nitrogen for 4 h. The hot mixture was filtered, and the solvent
was removed under reduced pressure. The residual solid was crystallized
from water to give 4-aminophenyl hydroxamic acid (1.18 g, 78%) as
white needles, mp 144-145 °C. NMR (CD3OD); 6.62 d, J ) 6 Hz,
2H and 7.65 d, J ) 6 Hz, 2H.
In Situ Functionalization of SWCNTs. Nitrosonium tetrafluorobo-
rate (118 mg, 1 mmol) was added to a suspension of 4-aminophenyl-
hydroxamic acid (152 mg, 1 mmol) in 10 mL of anhydrous acetonitrile,
and the solution was stirred under nitrogen for 30 min. The yellow
diazonium (1) solution was then added to a dispersion of laser ablated
carbon nanotubes (5 mg) in 100 mL of 1% aqueous sodium dodecyl-
sulfonate (SDS), and the mixture was stirred for 2 h. The mixture was
diluted with acetone (5×) and centrifuged at 3600 rpm for 25 min.
The supernatant liquid was discarded, and the precipitated SWCNTs
were dissolved in water by sonication to form a stable aqueous solution
of functionalized SWCNTs.
Characterization. Raman spectroscopy was performed on pristine
laser ablated SWCNTs, f-SWCNTs, and annealed (defunctionalized)
SWCNTs. The samples were prepared by drop casting an acetone
dispersion (for pristine SWCNT) or a water dispersion (for f-SWCNT)
of the corresponding nanotubes onto a 1 in. diameter, optically flat
sapphire window followed by drying the solution over a stream of
nitrogen. The 488 nm line from a Coherent Ar-ion laser was used as
the excitation beam, and the back-scattered radiation was analyzed with
a Jobin-Yvon monochromator, equipped with a 1200 groves/mm grating
blazed at 500 nm, in combination with a liquid nitrogen cooled CCD
camera. The Raman data were calibrated to the sapphire peaks from
the substrate.
The device fabrication began with atomic layer deposition (ALD)
of 2.5 nm hafnium oxide (HfO2) onto a heavily doped silicon wafer.
Silicon oxide (20 nm) was then thermally grown on top of the HfO2
layer at low temperatures to prevent diffusion. Electron beam lithog-
raphy was used to define an integrated array of trenches in a PMMA
resist layer. SiO2 was etched away by 30 s immersion in a commercially
available 9:1 Buffered Oxide Etch (Fisher Scientific) composed of an
aqueous NH4-HF solution, exposing the underlying HfO2 at the bottom
of the trenches. The substrates were rinsed in deionized water, followed
by an oxygen plasma treatment. The sample was then placed in an
aqueous dispersion of SWCNTs modified with 1 to assemble the tubes
in the trenches. The devices were annealed at 600 °C for 120 s in argon
to remove the surface functionality and completed by e-beam evapora-
tion of source-drain electrodes oriented perpendicular to the trench
length. The electrical characteristics where measured with a probe
station and parameter analyzer (Hewlett-Packard 4156).
Figure 1. Overview of the directed assembly method used to create large
arrays of SWCNT devices. The process includes chemical functionalization,
assembly, and alignment of functionalized nanotubes into arrayed trench
structures followed by parallel device fabrication. HfO2 trenches are in red,
SiO2 layer is in green, and top metal source-drain electrodes are in yellow.
the bottom of the trenches. The trench sidewalls induce
alignment of the f-SWCNTs along the length of the trenches.
The patterned substrate is then gently rinsed with water to
remove any functionalized tubes from the SiO2 areas. The
SWCNT modification is then reversed via annealing at 600 °C
in argon, to leave pristine nanotubes behind. A Ti/Pd source-
drain electrodes are deposited orthoganol to the trench length
to fabricate large arrays of many SWCNT devices in parallel
with high yield. The resulting devices showed typical SWCNT
device characteristics, with no deterioration of the device
properties as a result of the processing. The method provides a
promising path for the integration of SWCNTs into logic
devices.
Experimental Procedures
O-Benzyl 4-Nitrophenylhydroxamic Acid (2). Oxalyl chloride
(10.16 g, 0.08 mol) was added to a solution of 4-nitrobenzoic acid
(6.68 g, 0.04 mol) in 100 mL of anhydrous dichloromethane. A drop
of N,N-dimethylformamide was added, and the mixture was stirred
under nitrogen for 3 h. The solvent and excess oxalyl chloride were
evaporated under reduced pressure. The residual oily compound was
redissolved in 20 mL of anhydrous dichloromethane and added to a
solution of O-benzylhydroxylamine hydrochloride (0.04 mol) and
triethylamine (0.08 mol) in 50 mL of anhydrous dichloromethane. The
mixture was stirred at room temperature for 4 h and then washed with
dilute hydrochloric acid and brine, dried over anhydrous magnesium
sulfate, and filtered. Evaporation of the solvent gave a light brown solid,
which was crystallized from ethanol to give O-benzyl-4-nitrophenyl-
Results and Discussion
The chemistry employed to carry out the functionalization
and directed assembly is highlighted in Scheme 1. The diazo-
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A R T I C L E S
Tulevski et al.
and 1570 cm-1 are assigned to the G+ and G- bands,
respectively, and are associated with the C-C stretch modes
along the nanotube axis (G+) and the circumferential direction
(G-). The D-band, 1350 cm-1, is commonly referred to as the
defect band and is a phonon mode activated by the presence of
defects in the structure of the SWCNT. The peaks at 378, 417,
and 645 cm-1 are assigned to the Eg and A1g peaks of the
sapphire substrate and were used to calibrate the spectra. Since
the functionalization results in the hybridization of the bonded
carbon atoms (from sp2 to sp3) of the SWCNTs, defects are
introduced into the crystal structure upon functionalization,
increasing the intensity of the D-band mode. Monitoring the
intensity ratio between the D-band and the G-band, ID/IG, will
indicate whether the reaction took place, to what degree, and if
it was reversed. The ID/IG ratio is significantly higher in the
spectrum taken of the functionalized SWCNTs (Figure 2, red
trace). The G-band intensity is attenuated and broadened as well.
This ratio reverts to the initial value upon annealing the
functionalized SWCNTs, as is seen in the blue trace in Figure
2, suggesting that the SWCNTs are defunctionalized and that
the sp3 carbon atoms have rehybridized back to the sp2 state.
The RBM peak at 220 cm-1 also decays upon functionalization
as the symmetry of the radial mode is broken due to covalent
attachment of the organic moiety.24 This mode is also fully
recovered upon annealing. The data confirm that the SWCNTs
are chemically functionalized with 1 and that the functional-
ization is reversible.
Figure 2. Raman spectra of pristine SWCNT (black trace), after function-
alization with 1 (red trace), and after annealing to remove 1 (blue trace).
The data were acquired by depositing the samples onto sapphire windows
and using a 488 nm excitation.
nium salt (1) reacted readily with SWCNTs to yield the
functionalized species (f-SWCNT). The diazonium salt formed
a charge-transfer complex at the surface of the SWCNT with
electron transfer from the SWCNT to the organic compound
leading to the formation of a SWCNT-aryl carbon-carbon bond,
converting the sp2 hybridized SWCNT carbon atom to a sp3
hybridized carbon atom.13-17,22 Annealing the reacted material
to 600 °C, under argon, cleaved the SWCNT-aryl bond, restoring
the original sp2 hybridization of the SWCNT. Compound 1 was
end-functionalized with hydroxamic acid. Hydroxamic acids are
known to bind strongly and selectively to basic metal oxides
(i.e., Al2O3 and HfO2) as opposed to silicon oxide.12,18 We use
a short-chain (x ) 0) hydroxamic acid terminated compound
that renders the functionalized SWCNTs water-soluble. An
advantage of using aqueous solutions, as opposed to common
organic solvents as used in the previous reports,10,11 is that the
solution pH can be precisely controlled to mediate the subtle
acid-base interaction between the terminal hydroxamic acid
and the desired metal-oxide surface allowing for increased
SWCNT density and, subsequently, increased device yield. The
SWCNT functionalization is completely reversible upon an-
nealing to 600 °C. This reversibility is critical since the
attachment chemistry rehybridizes the carbon atoms at the point
of attachment, introducing scattering sites and severely degrad-
ing the SWCNTs device performance.
The hydroxoxamic acid functionality was chosen due to its
ability to discriminate between basic metal oxides and silicon
oxide surfaces.12,18 Metal oxides with large isoelectric points
(IEP) are able to deprotonate the weakly acidic hydroxamic acids
(pKa ) ∼9.3), resulting in a hydroxamate ion that can more
readily chelate to the oxide surface. Using the patterned
substrates, the weakly bonding hydroxamic acid species exists
at the acidic SiO2 surface (IEP ) ∼2), while the mostly
deprotonated, strongly bonding hydroxamate species exists on
the more basic HfO2 surface (IEP ) ∼8), leading to selective
binding to the basic metal oxide surface. The pH can be carefully
tuned to mediate this acid-base interaction to maximize the
binding density, while keeping the functionalized tubes stably
suspended. This, coupled with the high oxidation state of the
hafnium ion (4+), leads to strong and selective binding of the
f-SWCNTs to the HfO2 surface. It is also important to note that
using dielectrics deposited via atomic layer deposition (ALD)
yielded a higher density than those deposited via electron beam
deposition. ALD yields atomically smooth surfaces and therefore
a more homogeneous surface structure. It is presumably this
homogeneity that results in these superior results. To take
advantage of the binding selectivity, substrates were lithographi-
cally patterned to expose HfO2 channels for the placement of
the modified SWCNTs, as illustrated in Figure 3a. Trenches of
various line widths were fabricated by chemically etching the
SiO2 overlayer through a photoresist mask. The substrates were
then exposed to a solution of the f-SWCNTs for 3 h, removed
from the solution, and rinsed thoroughly with deionized water
before being imaged with scanning electron and atomic force
microscopes. The images (Figure 3b-d) reveal the functional-
ized carbon nanotubes assembled strictly in the region with
exposed HfO2. The rest of the surface, composed of SiO2, was
completely free of any f-SWCNTs.
Raman spectroscopy is a powerful tool for probing the
dynamics of SWCNT functionalization.19,20 Figure 2 shows the
Raman spectra, at 488 nm excitation, of pristine SWCNTs prior
to any chemical treatment (black), of SWCNTs functionalized
with 1 (red), and of f-SWCNTs annealed to 600 °C (blue). The
pristine SWCNTs (Figure 2, black trace) show all the requisite
peaks associated with typical Raman spectra of SWCNTs.19,20
The radial-breathing mode (RBM), associated with the ring
vibration perpendicular to the long axis of the nanotube, is
assigned to the peak at ∼220 cm-1, while the peaks at 1590
(13) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R.
E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536.
(14) Kooi, S. E.; Schlecht, U.; Burghard, M.; Kern, K. Angew. Chem., Int. Ed.
2002, 41, 1353.
(15) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis,
M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105.
(16) Heald, C. G. R.; Wildgoose, G. G.; Jiang, L.; Jones, T. G. J.; Compton, R.
G. ChemPhysChem 2004, 5, 1794.
(17) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17.
(18) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides,
G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813.
(19) Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A.; Souza Filho, A. G.; Pimenta,
M. A.; Saito, R. Acc. Chem. Res. 2002, 35, 1070.
(20) Jorio, A.; Saito, R.; Hertel, T.; Weisman, R. B.; Dresselhaus, G.;
Dresselhaus, M. S. MRS Bull. 2004, 29, 276.
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Chemically Assisted Directed Assembly of C Nanotubes
A R T I C L E S
Figure 3. (a) Schematic of the side view of the patterned substrates. (b)
SEM image of functionalized SWCNTs (in blue) assembled in a 2 µm HfO2
bottomed trench. (c and d) SEM and AFM image, respectively, of 250 nm
trenches with aligned functionalized SWCNTs inside the HfO2 bottomed
trenches.
Figure 4. (a) SEM images of one section of the array structure and of a
single device (inset) in the array with a single SWCNT spanning the source-
drain electrodes. (b) Schematic of the top view of a single device. (d) ID vs
VG curves for a typical device at VD ) 0.1 V (red trace), 0.3 V (blue trace),
and 0.5 V (green trace).
As the width of the trench decreases (from Figure 3b), to
below the average length of the nanotubes (Figure 3c,d), the
tubes align along the length of the trench. Figure 3b is an SEM
image of a 2 µm wide trench that was exposed to a solution of
f-SWCNTs. The tubes assemble into the trench with a relatively
high density (∼2 SWCNT per µm2); however, the tubes are
oriented in random directions inside the trench. As the width
of the trench becomes smaller than the average length of the
f-SWCNTs, the tubes tend to align along the length of the trench.
This is clear in both the SEM and the AFM images of Figure
3c,d, respectively. The side walls of the trench constrain the
nanotubes to assemble in the orientation of the trench itself.
This is crucial in the fabrication of electronic devices as either
end of the assembled carbon nanotube can be easily contacted
with source-drain electrodes deposited in a direction perpen-
dicular to the trench length. The opposite geometry was also
fabricated, where HfO2 ridges are made on top of a SiO2 sub-
strate. In that geometry, the functionalized carbon nanotubes
have a tendency to hang over the edge and onto the SiO2 sub-
strate, lacking any orientation control.21
behind. Source-drain electrodes (Ti/Pd) were then lithographi-
cally defined perpendicular to the trenches in a highly parallel
fashion to achieve the structure shown in the SEM image. The
inset of Figure 4a shows one device in the larger array. The
source-drain electrodes cross the trench and are spaced by 300
nm, significantly less than the SWCNT length to ensure that
the SWCNT spans the length of the source-drain pair. Since
the source-drain electrodes run perpendicular to the trench, the
alignment along the length of the trench was crucial to achieve
a high yield. Images and transport measurements were taken
of dozens of these device arrays, and approximately 90% of
the source-drain electrode pairs had a single SWCNT spanning
the contacts (>35/40 devices per array out of 8 arrays). Since
the metallic SWCNTs react at a faster rate with the diazononium
salts than the semiconducting SWCNTs, the resulting devices
yielded an enrichment of metallic SWCNTs where 70% of the
devices was metallic. By using a reported presorting process
that separates semiconducting from metallic SWCNTs and
coupling it to this placement process, viable logic circuits could
be fabricated of strictly semiconducting SWCNTs.22,23
The SWCNT density can be tuned by carefully adjusting the
pH and varying the deposition time and the width of the trench,
such that at any given point in the trench there is a high
probability of finding one, and only one, SWCNT to electrically
contact. Once these conditions were determined (pH ) 6.5-
6.9, 3 h deposition, width ) 800 nm), large device arrays were
fabricated using the trench geometry. Figure 4a illustrates a
portion of a typical grid structure that consists of 40 source-
drain pairs. The structures were fabricated by first making the
large grid of trenches, followed by the directed assembly of
f-SWCNTs into the trenches. The samples were then annealed
to defunctionalize the SWCNTs, leaving the pristine SWCNTs
A representative ID versus VG curve of a contacted semicon-
ducting SWCNT is shown in Figure 4c. The SWCNT is
expectedly p-type with the drain current increasing as the gate
voltage is swept in the negative direction with a constant
source-drain voltage. The on-current is high, 1 µA, which is
expected for SWCNT devices, and the on-off ratio was
measured to be 107. These values are consistent with measure-
ments for pristine carbon nanotubes with similar dielectric
(22) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.;
Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science
2003, 301, 1519.
(23) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C.
Nat. Nanotechnol. 2006, 1, 60.
(24) Strano, M. S. J. Am. Chem. Soc. 2003, 125, 2003.
(21) See Supporting Information for images of substrates with inverted geometry
and larger area images.
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Tulevski et al.
thicknesses, diameters, and source-drain lengths, providing
further evidence that the chemical functionalization is fully
reversible. This method produced dozens of single SWCNT
devices with no apparent performance deterioration as a result
of the processing.
superior electronic properties expected of carbon nanotubes, with
no deleterious effects on the electrical performance. The
precision, ease, and resultant high yield of this method provide
a promising route to the parallel fabrication of large-scale carbon
nanotube electronics.
Acknowledgment. This material is based on work supported
by DARPA under SPAWARSYSCEN San Diego Contract
N66001-06-C-2047. We thank Bruce Ek and Teresita Graham
for help with electrode deposition. We also thank the IBM MRL
fabrication facility for fabrication of wafers and deposition of
dielectrics.
Conclusion
Chemical functionalization and directed assembly of SWCNTs
is a powerful route for the fabrication of large-scale arrays of
SWCNT devices. Organic acids impart water solubility to
SWCNTs that allows the pH to be tuned to achieve selective
and more precise placement of SWCNTs on patterned oxide
surfaces having different isoelectric points. This method also
allows for directed assembly into predefined positions of
oriented SWCNTs by confining the SWCNTs to the trenches
of patterned dielectric stacks. Large arrays of functioning
SWCNT devices were fabricated in high yield and with the
Supporting Information Available: Images of assembled
SWCNTs on different geometries and of larger areas. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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