Langmuir blodgett assembly of densely aligned single walled carbon
  nanotubes from bulk materials
Langmuir-Blodgett Assembly of Densely Aligned Single-Walled Carbon Nanotubes 
from Bulk Materials
Xiaolin Li, Li Zhang, Xinran Wang, Iwao Shimoyama, Xiaoming Sun, Won-Seok Seo, Hongjie Dai*
Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
RECEIVED DATE (automatically inserted by publisher); hdai@stanford.edu
Single-walled carbon nanotubes (SWNTs) exhibit advanced 
properties desirable for high performance nanoelectronics.
Important to future manufacturing of high-current, speed and 
density nanotube circuits is large-scale assembly of SWNTs into 
densely aligned forms.
 Despite progress in oriented synthesis and 
assembly including the Langmuir-Blodgett (LB) method,2-9 no 
method exists for producing assemblies of pristine SWNTs (free 
of extensive covalent modifications) with both high density and 
high degree of alignment of SWNTs. Here, we develop a LB 
method achieving monolayers of aligned non-covalently 
functionalized SWNTs from organic solvent with dense packing. 
The monolayer SWNTs are readily patterned for device 
integration by microfabrication, enabling the high currents 
(~3mA) SWNT devices with narrow channel widths. Our method 
is generic for different bulk materials with various diameters. 
 Suspensions of as-grown laser-ablation and Hipco SWNTs in 
1,2-dichloroethane (DCE) solutions of poly(m-phenylenevinylene 
-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV) were prepared by 
sonication, ultra centrifugation and filtration (see supplementary 
information). The suspension contained mostly individual 
nanotubes (average diameter~1.3nm and ~1.8nm respectively for 
Hipco and laser-ablation materials, mean length ~500nm, Fig.1d
and 1e) well solubilized in DCE without free unbound PmPV. 
PmPV is known to exhibit high binding affinity to SWNT 
sidewall via  stacking of its conjugated backbone (Fig.1a) and 
thus impart solubility of nanotubes in organic solvents.10 Indeed, 
we obtained homogeneous suspensions of nanotubes in PmPV 
solutions. However, we found that DCE was the only solvent in 
which PmPV bound SWNTs remained stably suspended when 
free unbound PmPV molecules were removed (Inset of Fig.1b). 
The PmPV treated SWNTs exhibited no aggregation in DCE over 
several months. DCE without PmPV could suspend low 
concentrations of SWNTs (~50X lower than with PmPV 
functionalization), insufficient for LB formation, especially for 
larger SWNTs in laser materials with lower solubility.
The excitation and emission spectra of PmPV bound SWNTs 
(in PmPV-SWNT solution with excess PmPV removed) exhibited 
~20nm and ~3nm shifts respectively relative to those of pure 
PmPV in DCE (Fig.1b), providing spectroscopic evidence of 
strong interaction between PmPV and SWNTs. No change in the 
spectra was observed with the highly stable PmPV-SWNT/DCE 
suspension for months, indicating strong binding of PmPV on 
SWNT without detachment in DCE. The fact that PmPV-SWNTs 
were not stably suspended in other solvents without excess PmPV 
and that addition of large amounts of these solvents (e.g., 
chloroform) into a PmPV-SWNT/DCE suspension causing 
nanotube precipitation suggested significant detachment of PmPV 
from nanotubes in most organic solvents. The unique stability of 
PmPV coating on SWNT in DCE over other solvents is not fully 
understood currently. Nevertheless, it is highly desirable for 
chemical assembly of high quality nanotubes and integrated 
devices since it enables non-covalently functionalized SWNTs 
(both large diameter laser and small diameter Hipco materials) 
soluble in organics in nearly pristine form, as gleaned from the 
characteristic UV-vis-NIR absorbance (Fig.1c) and Raman 
signatures of non-covalently modified SWNTs (Fig.2c).
PmPV-SWNTs were spread on a water subphase from a DCE 
solution, compressed upon DCE vaporization to form a LB film 
using compression-retraction-compression cycles to reduce 
hysteresis (supplementary Fig.S1, S2&S3) and then vertically 
transferred onto a SiO2 or any other substrate (glass, plastic, etc.).
Organic solutions of stably suspended SWNTs without excess 
free polymer are critical to high density SWNT LB film 
formation. Microscopy (Fig.2a&2b) and spectroscopy 
(Fig.2c&2d) characterization revealed high quality densely 
aligned SWNTs (normal to the compression and substrate pulling 
Hipco
Figure 1. PmPV functionalized SWNTs. (a) Schematic drawings of a 
SWNT and two units of a PmPV chain. (b) Excitation and 
fluorescence spectra of pure PmPV in DCE vs. PmPV bound Hipco 
SWNTs in DCE. Inset: photograph of PmPV coated Hipco SWNTs 
suspended in DCE without excess PmPV in the solution. (c) UV-vis-
NIR spectrum of PmPV suspended Hipco SWNTs with no excess 
PMPV. (d) & (e) Atomic force microscopy (AFM) images of Hipco 
and laser-ablation SWNTs randomly deposited on a substrate from 
solution. Insets: Diameter distributions.
= OC8H17
(b) (c)
300 400 500
5.0x10
1.0x10
1.5x10
Wavelength (nm)
 PmPV
 Hipco-PmPV
600 800 1000
Wavelength (nm)
 Hipco-PmPV
1.0 1.5 2.03.00.5
d (nm)
1.0 1.5 2.03.00.5
d (nm)
1.0 1.5 2.0 2.50.5
d (nm)
1.0 1.5 2.0 2.50.5
d (nm)
(d) (e)
direction) formed uniformly over large substrates for both Hipco 
and laser ablation materials. Height of the film relative to tube-
free regions of the substrate was <2nm under AFM, suggesting 
monolayer of packed SWNTs. Micro-Raman spectra of the 
SWNTs showed ~ cos2polarization dependence of the G band 
(~1590cm-1) intensity (Fig. 2d), where  is the angle between the 
laser polarization and the SWNT alignment direction. The peak to 
valley ratio of the Raman intensities was ~8 with little variation 
over the substrate, indicating alignment of SWNTs over large 
areas. Nevertheless, imperfections existed in the quasi-aligned 
dense SWNT assembly including voids, bending and looping of 
nanotubes formed during the compression process for LB film 
formation due to the high aspect ratio (diameter <~2nm, length 
~200nm-1m) and mechanical flexibility of SWNTs.
Our aligned SWNT monolayers on oxide substrates can be 
treated as carbon-nanotube on insulator (CNT_OI) materials for 
patterning and integration into potential devices, much like how
Si on insulator (SOI) has been used for electronics. We used 
lithographic patterning techniques and oxygen plasma etching to 
remove unwanted nanotubes and form patterned arrays of squares 
or rectangles comprised of aligned SWNTs (Fig.3a and 3b). We 
then fabricated arrays of two-terminal devices with Ti/Au metal 
source (S) and drain (D) contacting massively parallel SWNTs in 
~10 m wide S-D regions with channel length ~250nm (Fig.3c 
and 3d). Current vs. bias voltage (I-V) measurements showed that 
such devices made from Hipco SWNTs were more than 25 times 
more resistive than similar devices made from laser-ablation 
SWNTs, with currents reaching ~0.13mA and ~3.5mA 
respectively at a bias of 3 V through collective current carrying of 
SWNTs in parallel (Fig.3e and 3f). Further, Hipco SWNT devices 
exhibited higher non-linearity in the I-V characteristics than laser 
ablation nanotubes (Fig.3e). These results were attributed to the 
diameter difference between Hipco and laser-ablation materials. 
Hipco SWNTs were small in diameter with many tubes ≤1.2nm, 
giving rise to high (non-ohmic) contact resistance for both 
semiconducting and metallic SWNTs.
 Smaller SWNTs could 
also be more susceptible to defects and disorder, contributing to 
degraded current carrying ability.
The LB assembly of densely aligned SWNTs can be combined 
with chemical separation and selective chemical reaction 
methods12 to afford purely metallic or semiconducting SWNTs in 
massive parallel configuration useful for interconnection or high 
speed transistor applications at large scale. The method is generic 
in terms of the type of nanotube materials and substrates. 
Acknowledgment. We thank Dr. Pasha Nikolaev for providing 
laser-ablation SWNTs and MARCO-MSD and Intel for support.
Supporting Information Available: Experimental details are 
available free of charge via the internet at http://pubs.acs.org. 
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Figure 2. LB monolayers of aligned SWNTs. (a) AFM image of a LB 
film of Hipco SWNTs on a SiO2 substrate. (b) AFM image of a LB 
film of laser-ablation SWNTs. (c) Raman spectra of the G line of a 
Hipco SWNT LB film recorded at various angles () between the 
polarization of laser excitation and SWNT alignment direction. Inset:
Raman spectrum showing the radial breathing mode (RBM) region of
the Hipco LB film at ~0. (d) G line (1590cm-1) intensity vs. angle 
for the Hipco SWNT LB film in (c). The red curve is a cos2 fit.
(a) (b)
0 50 100 150
5.0x10
1.0x10
1.5x10
2.0x10
Angle (deg.)
ngle (deg.) Raman s
hift (cm
ngle (deg.) Raman s
hift (cm
150 200 250
Raman shift (cm
(c) (d)
Figure 3. Microfabrication patterning and device integration of SWNT 
LB films. (a) Optical image of a patterned SWNT LB film. The squares 
and rectangles are regions containing densely aligned SWNTs. Other 
areas are SiO2 substrate regions. (b) SEM image of a region 
highlighted in (a) with packed SWNTs aligned vertically. (c) SEM 
image showing a 10-micron-wide SWNT LB film between source and 
drain electrodes formed in a region marked in (b). (d) AFM image of a 
region in (c) showing aligned SWNTs and the edges of the S and D 
electrodes. (e) Current vs. bias (Ids-Vds) curve of a device made of 
Hipco SWNTs (10m channel width and 250nm channel length). (f)
Ids-Vds of a device made of laser-ablation SWNTs (10m channel 
width and 250nm channel length).
-3 -2 -1 0
-3 -2 -1 0
-0.15
-0.10
-0.05
(e) (f)
400m
400nm
80m(b)
Angle (deg.) Raman 
shift (cm
Angle (deg.) Raman 
shift (cm
ABSTRACT FOR WEB PUBLICATION.
Single-walled carbon nanotubes (SWNTs) exhibit advanced electrical and surface properties useful for high performance 
nanoelectronics. Important to future manufacturing of nanotube circuits is large-scale assembly of SWNTs into aligned forms. 
Despite progress in assembly and oriented synthesis, pristine SWNTs in aligned and close-packed form remain elusive and 
needed for high-current, -speed and -density devices through collective operations of parallel SWNTs. Here, we develop a 
Langmuir-Blodgett (LB) method achieving monolayers of aligned SWNTs with dense packing, central to which is a non-
covalent polymer functionalization by poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV) imparting high 
solubility and stability of SWNTs in an organic solvent 1,2-dichloroethane (DCE). Pressure cycling or ‘annealing’ during LB 
film compression reduces hysteresis and facilitates high-degree alignment and packing of SWNTs characterized by 
microscopy and polarized Raman spectroscopy. The monolayer SWNTs are readily patterned for device integration by 
microfabrication, enabling the highest currents (~3mA) through the narrowest regions packed with aligned SWNTs thus far.
