Directed assembly of single-walled carbon nanotubes via drop-casting onto a UV-patterned photosensitive monolayer

Article abstract of DOI:10.1021/ja802407f

We report the use of a novel UV-sensitive self-assembled monolayer to selectively deposit single-walled carbon nanotubes from solution using heterogeneous surface wettability. This process combines ubiquitous photopatterning techniques with simple solution processing to yield highly selective and densely packed carbon nanotube patterns. The essential concept behind this process is the change in surface chemistry caused by the UV-induced monolayer reaction. Selective deposition of carbon nanotubes was achieved by drop-casting, and the resulting films show local ordering, indicating that further development of this process will lead to simple technique for large-scale integration. Copyright

Full text of DOI:10.1021/ja802407f

Published on Web 05/16/2008
Directed Assembly of Single-Walled Carbon Nanotubes via Drop-Casting onto
a UV-Patterned Photosensitive Monolayer
Julie A. Bardecker,†,‡ Ali Afzali,*,† George S. Tulevski,† Teresita Graham,† James B. Hannon,† and
Alex K.-Y. Jen‡
IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, and Department of Materials Science and
Engineering, UniVersity of Washington, Seattle, Washington 98195
Received April 2, 2008; E-mail: afzali@us.ibm.com
Scheme 1
Over the past decade, single-walled carbon nanotubes (SWCNTs)
have emerged as a promising material for use in a variety of
electronic applications due to their unique electrical and physical
properties.1 The versatility of SWCNTs yields an impressive range
of potential applications, including sensors, flexible electronics, and
photovoltaics. Semiconducting nanotubes, in particular, have at-
tracted considerable attention because of their demonstrated per-
formance in field-effect transistors.2 However, the fabrication of
such devices depends upon the ability to selectively place oriented
SWCNTs. The concept of using heterogeneous surface wettability
for selective deposition was explored using a multitude of systems,3
including the use of self-assembled monolayers (SAMs) to selec-
tively deposit inorganics from a solution.3a,d,4 The wettability of
SAMs was also utilized to selectively deposit organics, including
polymers,3b polymer microspheres,3c nanoparticles,3d and carbon
nanotubes.3d,e Previous work from this laboratory5 utilized end
groups (i.e., phosphonic acid and hydroxamic acid) that selectively
bind to basic oxide surfaces in order to prevent or enhance the
adhesion of carbon nanotubes to certain regions of a patterned
surface. In these studies, oxide surfaces were patterned lithographi-
cally to produce the regions to which the carbon nanotubes would
bind. Here, we bypass the need for complex lithography by utilizing
a simple and scalable approach to solution-based placement of
SWCNTs based on photopatterning a photosensitive self-assembled
monolayer.
The key results of this study are the design and synthesis of a
photoreactive self-assembling molecule (Scheme 1), the successful
patterning of the monolayer by exposure to UV radiation through
a mask, and the selective placement of SWCNTs on the patterned
monolayer surface. The photosensitive compound (phosphonic acid
derivative 6) was designed to possess the following three properties
that are essential to the formation of a UV-patterned SAM: (i) a
photoreactive moiety that breaks down or rearranges upon exposure
to UV light, (ii) an end group functionality capable of bonding to
the oxide surface, and (iii) the ability to undergo a drastic change
in surface chemistry (i.e., change from hydrophobic to hydrophilic)
upon exposure to UV radiation. The phosphonic acid group in 6
bonds to metal oxides, leading to the formation of uniform SAMs.
The molecule is photoreactive and cleaves upon exposure to UV
radiation. Additionally, the monolayer is highly hydrophobic before
UV exposure due to the presence of a long chain alkyl group, and
after exposure, the exposed carboxylic acid end group creates a
hydrophilic surface. Selective placement of the SWCNTs was
achieved by drop-casting an aqueous dispersion of SWCNTs in
2% sodium cholate surfactant. The aqueous SWCNT solution is
repelled by the hydrophobic regions and wets the hydrophilic
regions of the monolayer. The wettability of the carboxyl-terminated
surface, in combination with the affinity of SWCNTs for the
carboxyl group (which is not expected to affect transport in the
SWCNT),6 enabled a high density of SWCNTs to be deposited on
the UV-exposed surfaces. The use of the photosensitive monolayer
eliminates the need for complex lithography and therefore reduces
the number of processing steps. This method is compatible with
current silicon microprocessing techniques and could be imple-
mented into a large-scale processing line.
Exposure to UV light promotes cleavage of the phosphonic acid
molecule at the nitrobenzyl ester junction, yielding a carboxyl-
terminated surface. Patterning of the monolayer using UV light
through a mask will create a surface of alternating alkyl-terminated
(hydrophobic) and carboxyl-terminated (hydrophilic) regions (Figure
1a). Figure 1b illustrates the reaction yielded by exposing the
photosensitive SAM 6 (HD-UV-PA) to UV light.7
The initial surface of bare HfO2 is very hydrophilic with a water
contact angle of less than 10°. After self-assembly of the HD-UV-
PA monolayer, the water contact angle value of the HfO2/HD-UV-
PA surface is 92 ( 5°. Comparatively, the reported water contact
angle of an octadecylphosphonic acid (ODPA) monolayer on the
native oxide of aluminum is 122°.8 The slightly lower water contact
angle measured for the phosphonic acid monolayer from 6 compared
† IBM.
‡ University of Washington.
9
7226 J. AM. CHEM. SOC. 2008, 130, 7226–7227
10.1021/ja802407f CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. AFM image of the UV-patterned HD-UV-PA-modified surface
after drop-casting SWCNTs form a H2O/methanol solution (3:1 volume).
The higher resolution images on the left and right illustrate the high
selectivity of the deposition.
In conclusion, we have patterned the surface of an oxide
substratate to produce both hydrophobic and hydrophilic regions
using self-assembly of a photosensitive compound. The use of UV
light to pattern the surface energy of a monolayer is scalable and
can be readily implemented in current semiconductor processing.
Moreover, the combination of UV patterning with simple solution
processing methods yields patterns of SWCNTs with excellent
selectivity and high density. Further development of this process,
including the use of more advanced exposure techniques, may lead
to a simple and effective method for the selective placement and
alignment of individual SWCNTs.
Figure 1. (a) Illustration of creating the photopatterned monolayer, (b)
illustration of the UV-induced reaction of SAM 6, and (c) AFM image of
the photopatterned monolayer surface.
to that of the ODPA monolayer is probably due to the larger
footprint of the former leading to a lower density of alkyl chains
on the surface after assembly. After blanket UV exposure, the water
contact angle of the monolayer decreased significantly, yielding a
water contact angle of about 15 ( 5° after a 45 min exposure.
Bain et al. reported water contact angles of approximately 5° for
carboxyl-terminated alkane thiols of various chain lengths on gold.9
The higher contact angle measured for the carboxyl-terminated
monolayer is also attributed to the larger molecular footprint of
the HD-UV-PA molecule.
Acknowledgment. This material is based on work partially
supported by DARPA under Contract N66001-06-C-2047. J.A.B.
thanks the University of Washington Center for Nanotechnology
for the Nanotechnology Fellowship. The authors thank Dr. Dario
Goldfarb for use of the photopatterning equipment, and the IBM
MRL fabrication facility for fabrication of wafers and deposition
of dielectrics.
Due to the large size of the cleaved portion of the assembled
molecule (approximately 2 nm fully extended), atomic force
microscope (AFM) images were useful in verifying the monolayer
reaction. Figure 1b shows an AFM image of the patterned HD-
UV-PA surface. The measured height difference between the UV-
exposed and unexposed regions of the monolayer was approxi-
mately 0.7 nm (Supporting Information). The lower value of the
measured height difference compared to the theoretical value is
most likely due to the flexibility of the alkyl chain. The monolayer
reaction was also verified by X-ray photoelectron spectroscopy
through the elimination of the N 1s peak in the spectrum after UV
exposure (Supporting Information).
Supporting Information Available: Detailed synthesis experimental
procedures and SAM characterization. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Avouris, Ph.; Appenzeller, J.; Martel, R.; Wind, S. Proc. IEEE 2003,
91, 1772–1784. (b) Avouris, Ph. Acc. Chem. Res. 2002, 35, 1026–1034.
(c) Wildo¨er, J.; Venema, L.; Rinzler, A.; Smalley, R.; Dekker, C. Nature
1998, 391, 59–62. (d) Odom, T.; Huang, J.-L.; Kim, P.; Lieber, C. Nature
1998, 391, 62–64.
(2) (a) Avouris, Ph. MRS Bull. 2004, 29, 403–410. (b) Javey, A.; Guo, J.;
Farmer, D. B.; Wang, Q.; Wang, D.; Gordon, R. G.; Lundstrom, M.; Dai,
H. Nano Lett. 2004, 4, 447–450. (c) Dai, H. Surf. Sci. 2002, 500, 218–
241. (d) Lin, Y. M.; Appenzeller, J.; Knoch, J.; Avouris, Ph. IEEE Trans.
Nanotechnol. 2005, 4, 481–489. (e) Bachtold, A.; Hadley, P.; Nakanishi,
T.; Dekker, C. Science 2001, 294, 1317–1320.
Deposition of the aqueous solution of SWCNTs by drop-casting
on the patterned monolayer surface yielded the desired combination
of excellent selectivity and densely packed SWCNTs. Since the
unexposed areas of the monolayer are very hydrophobic, the
aqueous SWCNT solution prefers to wet the areas that have been
exposed to UV light, forming microdroplets on the exposed areas.
The solvent then evaporates leaving the SWCNTs on the carboxyl-
terminated surfaces. Figure 2 shows a 20 µm AFM image of drop-
cast SWCNTs on a patterned HD-UV-PA surface. Higher resolution
images of the hydrophobic and hydrophilic regions of the monolayer
after SWCNT deposition are shown adjacent to the 20 µm image
in Figure 2.
(3) (a) Nakanishi, T.; Masuda, Y.; Koumoto, K. Chem. Mater. 2004, 16, 3484–
3488. (b) Xiang, J.; Masuda, Y.; Koumoto, K. AdV. Mater. 2004, 16, 1461–
1464. (c) Benor, A.; Hoppe, A.; Wagner, V.; Knipp, D. Thin Solid Films
2007, 515, 7679–7682. (d) Yerushalmi, R.; Ho, J. C.; Jacobson, Z. A.;
Javey, A. Nano Lett. 2007, 7, 2764. (e) Fan, F.; Stebe, K. J. Langmuir
2004, 20, 3062–3067.
(4) Ko, H.; Peleshanko, S.; Tsukruk, V. J. Phys. Chem. B 2004, 108, 4385–
4393.
(5) (a) Hannon, J. B; Afzali, A.; Klinke, C.; Avouris, Ph. Langmuir 2005, 21,
8569–8571. (b) Klinke, C.; Hannon, J. B.; Afzali, A.; Avouris, Ph. Nano
Lett. 2006, 6, 906–910. (c) Tulevski, G.; Hannon, J. B.; Afzali, A.; Chen,
Z.; Avouris, Ph.; Kagan, C. R. J. Am. Chem. Soc. 2007, 129, 11964–11968.
(6) Klinke, C.; Afzali, A.; Avouris, Ph. Chem. Phys. Lett. 2006, 430, 75.
(7) Bochet, C. J. Chem. Soc., Perkin Trans. 1 2002, 125–142.
(8) Hoque, E.; DeRose, J. A.; Hoffmann, P.; Mathieu, H. J.; Bhushan, B.;
Cichomski, M. J. Chem. Phys. 2006, 124, 174710–174716.
It should be noted that some local ordering of bundled SWCNTs
is visible in the AFM image shown in Figure 2. This type of
“woven” structure has been noted previously10 and was attributed
in part to the motion of the liquid-solid-air contact line.
(9) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Eval, J.; Whitesides, G. M.;
Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335.
(10) Xin, H.; Wooley, A. T. Nano Lett. 2004, 4, 1481–1484.
JA802407F
9
J. AM. CHEM. SOC. VOL. 130, NO. 23, 2008 7227