1. Art and Diseño

Nano Research
Nano Res
DOI 10.1007/s12274-015-0725-y
1
Raman microscopy mapping for the purity assessment
of chirality enriched carbon nanotube networks in thin
film transistors
Zhao Li ( ), Jianfu Ding, Paul Finnie, Jacques Lefebvre, Fuyong Cheng, Christopher T. Kingston, Patrick
R. L. Malenfant ( )
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0725-y
http://www.thenanorese arch.com on January 28, 2015
© Tsinghua University Press 2015
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TABLE OF CONTENTS (TOC)
Raman
microscopy
mapping
for
the
purity
assessment of chirality enriched carbon nanotube
networks in thin film transistors
SWCNT network
Purity and density
Via Raman Mapping
-2
-3
Zhao Li,* Jianfu Ding, Paul Finnie, Jacques Lefebvre,
Fuyong Cheng, Christopher T. Kingston, Patrick R. L.
Log (ISD)
-4
-5
-6
2.5 um
-7
5 um
-8
Malenfant*
Security
and
-10
-5
0
5
10
VG (V)
Disruptive
Technologies
Portfolio,
National Research Council Canada, 1200 Montreal
Road, Ottawa, Ontario, K1A 0R6, Canada
Raman microscopy mapping can be a powerful characterization tool to
quantify residual metallic carbon nanotubes in high density networks of
enriched semiconducting single walled carbon nanotubes.
www.nrc.ca
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Raman microscopy mapping for the purity assessment
of chirality enriched carbon nanotube networks in thin
film transistors
Zhao Li ( ), Jianfu Ding, Paul Finnie, Jacques Lefebvre, Fuyong Cheng, Christopher T. Kingston, Patrick
R. L. Malenfant ( )
Received: day month year
ABSTRACT
Revised: day month year
With recent improvements in carbon nanotube separation methods, the
accurate determination of residual metallic carbon nanotubes in a purified
nanotube sample is important, particularly for those interested in using
semi-conducting single walled carbon nanotubes (SWCNT) in electronic device
applications such as thin film transistors (TFT). This work demonstrates that
Raman microscopy mapping is a powerful characterization tool to quantify
residual metallic carbon nanotubes present in highly enriched semiconducting
nanotube networks. Raman mapping correlates well with absorption
spectroscopy, yet provides greater differentiation in purity. Electrical data from
TFTs with channel lengths of 2.5 and 5 microns demonstrate the utility of the
method in a device context. By comparing samples with nominal purities of
99.0% and 99.8%, a clear differentiation can be made when evaluating the
current on/off ratio as a function of channel length and as such, the Raman
mapping method provides a means to guide device fabrication by correlating
SWCNT network density and purity with TFT channel scaling.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Raman
spectroscopy,
Carbon nanotubes, Thin
film
transistors,
Microscopy
mapping,
purity assessment.
Address correspondence to Zhao Li, [email protected]; Patrick R. L. Malenfant, [email protected]
2
Nano Res.
As prepared SWCNT raw materials contain bundles
of nanotubes with a 2:1 semiconducting (sc-) to
metallic (m-) ratio and other impurities such as
catalyst and amorphous carbon [1-3]. For electrical
devices, these raw materials must be debundled,
purified and enriched [4-11]. It has been
demonstrated that for network thin film transistors
(TFT), the m-tube content should be less than 2% [3].
For more demanding applications, such as high
frequency logic circuits and display backplanes, the
m-tube content should be less than a few ppm given
the need for high mobility and current on/off ratios
[12]. In recent years, multiple efforts have led to
significant progress in solution based enrichment
techniques, with sc-SWCNT purity in excess of 99%
routinely achieved. This leap, however, has now
started to reveal the limits of common
characterization methods and other tools are
required to provide accurate purity assessment
[13,14].
Typically, the purity of enriched sc-SWCNT is
estimated from the UV absorption spectrum by
comparing the peak areas associated with the m- or
sc- species [12, 15-17]. This method works well for
samples when these peaks are well defined and
background absorption is not dominant. However, as
the sc-purity increases, the features associated with
m-tube absorption will gradually disappear and the
precise subtraction of the background absorption will
dramatically influence the calculated results and
introduce uncertainty [18]. As an alternative, we have
previously used a different purity metric, denoted φ,
which is based on the ratio of the sc-peak area over
the total absorption background from the metallic
(M11) and semiconducting (S 22) absorption bands [16].
While we presume φ correlates well with purity for
highly pure samples, it does not provide a
quantitative assessment. Furthermore, a solution
sample is not necessarily representative of the
deposited networks on a solid substrate, such as a
transistor channel, where selective adsorption may
occur [19]. There is currently a need for a quick and
reliable method to quantitatively characterize a
SWCNT network for its residual m-SWCNT content
in fabricated TFTs in order to better understand
device performance as a function of SWCNT network
density and purity.
Methods for the measurement of m-SWCNT content
can be divided into bulk sample techniques and
counting-based
techniques.
Counting-based
techniques more accurately enumerate m- and
sc-nanotubes and typically rely on their electrical
and/or optical performance [20-29]. Although these
methods give relatively accurate results, they involve
tedious fabrication processes and costly instruments.
Furthermore, most of these methods are only
applicable to sparse SWCNT networks (<1 tube/µm2),
which is significantly below the typical tube network
density used in TFTs (10-40 tubes/µ m2).
Raman spectroscopy is commonly used to
characterize SWCNT ensembles [30-36] and can also
be used for individual nanotubes due to its high
sensitivity and chirality selective resonance with
laser irradiation wavelength [37-39]. Raman
spectroscopy not only discriminates m- and sc- tubes,
but also enables the assignment of specific chiralities
to individual tubes [40-42]. Raman spectroscopy,
especially in the G band region, can be very sensitive
to metallic SWCNT contamination, and it is useful
for the purity assessment of highly pure SWCNT
samples [36]. Herein, we will demonstrate that
Raman microscopy mapping is an efficient and
effective characterization method for large area, high
density, high sc purity SWCNT networks in TFTs
[43-45].
SWCNT networks were formed on Si/SiO2 substrates
by soaking the later in toluene dispersions [16].
Networks formed by soaking are random in terms of
position and orientation of nanotubes. This type of
network produces TFTs with good performance (high
current density and current on/off ratio) and are
accessible using scalable fabrication processes.
Although the Raman mapping method presented
here will be proven with random nanotube networks,
it should be equally applicable to aligned nanotubes.
Raman microscopy was performed by raster
scanning a defined area. While scanning we can
monitor each spectrum and record the position of
interest to determine tube type and the validity of the
signal to eliminate false positives. Finally, a map or
image can be built by selecting specific Raman
peaks/bands. Figure 1 shows the Raman spectra and
mapping images of the network under 514 nm laser
excitation. According to the Kataura plot, a 514 nm
laser is resonant mainly with sc-nanotubes for the
range of tube diameters used here (~1.3 nm) [46-47].
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Nano Res.
The dark area at the top of the images is the
evaporated TFT electrode. Figure 1(b) and 1(d) show
the intensity mapping of the G + band from 1581 to
1602 cm-1 and RBM band from 140 to 208 cm -1,
respectively (all Raman mapping images have been
processed and raw data can be found in the ESM).
These two maps show similar intensity patterns,
confirming these two bands come from the resonance
of tubes at the same position. Several representative
spectra covering D and G bands extracted at the
marked spots in Figure 1(b) are shown in Fig. 1(a),
which are typical sc-tube Raman spectra [30-36].
Figure 1(c) shows the RBM bands of these spots and
the peaks centered at 190 cm-1 correspond to 1.30 nm
diameter (10, 9) sc-tubes, agreeing well with the
result from PLE mapping [16].
Figure 1 Raman spectra and maps for excitation at 514 nm. (a) G
band spectra; (b) G+ map obtained from integrated intensities in
the 1581-1602 cm-1 range; (c) RBM band spectra and (d) RBM
map obtained from integrated intensities in the 140-208 cm-1
range. Circles in (b) and (d) highlight positions from which
spectra were taken in (a) and (c). Scanned area was 16×16 μm2
and scale bar is 2 µm. The concentration of the SWCNT solution
was 7.6 mg/L and the networks were prepared by submerging the
substrate for 10 min in the SWCNT solution.
The purpose of this work is to quantitatively assess
the minority m-SWCNT species buried in the
sc-SWCNT network. Given the diameter distribution
of the sample used here, 633 nm laser excitation
matches well with the M11 absorption resonance.
Figure 2(a) compares the Raman spectra in the 1200
to 1700 cm-1 region under 514 and 633 nm laser
excitation spatially averaged over the whole scanned
area. The spectrum from 633 nm excitation has a
clear metallic signature with a broad G - band
(Breit-Wigner-Fano lineshape), in contrast with the
sharper G- seen at 514 nm [30-36, 49-50]. Since the
sample is highly sc-enriched, the off resonance
contribution from sc-tubes at 633 nm is
non-negligible, especially for the spatially averaged
spectra as in Figure 2(a). However, individual spectra
generated from 1 µ m2 defined pixels will be
dominated by m-SWCNTs resonant under 633 nm
excitation considering their much higher scattering
cross-section.
Figure 2 633 nm Raman spectra and maps taken from the same
area as in Fig. 1. (a) Spatially averaged Raman spectra covering
the whole area (normalized to G+ band); (b) G+ and (d) G- map
obtained from integrated intensities in the 1581-1602 and
1507-1547 cm-1 range; (c) G band spectra; (e) RBM band spectra
and (f) RBM map obtained from integrated intensities in the
149-210 cm-1 range. Circles in (b), (d) and (f) highlight positions
from which spectra were taken in (c) and (e). Scanned area was
16×16 μm2 and scale bar is 2 µm.
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Nano Res.
The Raman intensity mapping of the G + band under
633 nm laser excitation in Fig. 2(b) shows different
patterns compared to 514 nm laser excitation (Fig.
1(b)), meaning Raman signals originated from
different tube species. Figure 2(d) shows the G- band
intensity mapping covering 1507 to 1547 cm -1, where
bright spots correspond only to m-SWCNTs in this
area. By analyzing the raw spectra for each pixel, we
found 21 locations in the scanned area having typical
m-SWCNT G- band signals with the Raman spectra
shown on Fig. 2(c).
We count one metallic tube if the spectrum has a
strong (signal visually well above noise floor; for a
signal:noise ratio of approximately 3:1) and broad Gpeak at 1530-1560 cm-1. Variation in intensity from
pixel to pixel can be related to tube chirality, length
and its overlap with the laser spot [30-37]. For strong
G- signals that extend over several contiguous pixels,
the spectra were analyzed to determine if each spot
contained one or more m-tubes. Given an average
tube length of 1.2 μm and given that a 0.5 m raster
increment was used to assess each pixel, if identical
spectra were collected over multiple adjacent pixels,
only a single m-tube was counted (it is statistically
improbable that two metallic tubes are situated in
adjacent pixels).
Most of the RBM bands of these pixels (Fig. 2(e))
show only one peak, meaning no more than one
diameter of m-SWCNT per pixel were found. In such
cases, we assumed that only one nanotube was
present at that location. Only one RBM band (green
trace in Fig. 2(e)) displays two peaks at 170.8 and
185.0 cm-1, indicating two m-SWCNTs within that
pixel. The diameter of these two m-SWCNTs are
calculated to be 1.45 and 1.34 nm and they are
assigned to (15, 6) or (16, 4) and (14, 5) respectively,
considering their resonance with 633 nm irradiation
[30-36].
In order to calculate the purity in the SWCNT
networks, we use SEM charge contrast imaging to
obtain an accurate measure of the nanotube network
density of the same area (a 4 µ m 2 area was used to
count tubes at high magnification). For the sample in
Fig. 1-2, a density of 40 nanotubes/μm2 was
measured, which amounts to 104 nanotubes for the
16×16 µ m2 area. (Fig. S1). As a result, the residual
m-SWCNT content is estimated to be 0.2%. Note that
this method may underestimate m-SWCNT content
due to off resonance or no G - contribution from
armchair SWCNT [51]. Armchair SWCNTs have
unique Raman spectra and we did not observe any in
our networks [51]. Statistically speaking, only 3
armchair tubes species would be expected given the
diameter range of the enriched SC-SWCNTs we have
isolated [(9,9); (10,10); (11,11)]. Assuming an even
distribution of chiralities in the unsorted starting
material, armchair tubes in our diameter regime
would represent less than 10% of all metallic tubes, a
negligible contribution to the underestimation.
Figure 3 Calibration of G+ band intensity versus carbon nanotube
coverage determined by SEM. The scanned area was 16×16 µm2.
The intensity for both wavelengths was normalized for easy
comparison.
To expedite the method, a Raman mapping
calibration curve that obviates the need for repeated
SEM was established. Networks with tube densities
ranging from 2 to 100 tubes/µm 2 were made using
different SWCNT solution concentrations and
analyzed by SEM (Fig. S2). Figure 3 shows the G+
peak intensity at 1591 cm-1 averaged over a 16×16
µ m2 area under 514 nm or 633 nm laser excitation vs
SEM tube network density. Good linearity is found
for network densities up to 40 tubes/µ m 2. We should
emphasize that at higher tube coverage it becomes
increasingly difficult to accurately count tubes using
charge contrast SEM imaging as evidenced by the
outlying data point observed in Figure 3 (~80
nanotubes/m2). As a result, it is possible to estimate
both the tube network density and sc purity using
exclusively Raman microscopy mapping (vide infra).
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Nano Res.
1000
1
(a)
800
SC1
Intensity
Absorption (a.u.)
0.8
SC2
0.6
0.4
(b)
SC1
600
SC2
400
200
0.2
0
400
800
1200
1600
2000
0
1200 1300 1400 1500 1600 1700 1800
Raman Shift (cm-1)
Wavelength (nm)
SC1, 633 nm (G-)
(c)
25
1E+8
20
1E+6
On/off ratio
Mobility
SC2, 633 nm (G-)
(f) 1E+7
(e)
(cm2/Vs)
(d)
1E+5
15
1E+4
10
1E+3
1E+2
5
SC2
SC1
SC1
SC1
0
2.5
5.0
2.5 5.0
Channel Length (µm)
1E+1
1E+0
SC1
SC2
2.5 5.0
2.5 5.0
Channel Length (µm)
Figure 4 (a) UV absorption spectra of enriched sc-tube samples
SC1 and SC2 in solution. (b) Raman spectra averaged from
20×20 µm2 area for networks prepared from SC1 and SC2 for
excitation at 633 nm. (c) and (d) G- maps obtained from
integrated intensities in the 1507-1547 cm-1 range for networks
prepared from SC1 and SC2 respectively. (e) Mobility and (f)
current on/off ratio of thin film transistors prepared from SC1
and SC2. The channel width is 2000 µm. Transfer curves can be
found in Figure S3 in the ESM.
The validity of the Raman mapping method for the
sc-purity evaluation was further confirmed using two
conjugated polymer enriched sc-SWCNT samples
(SC1 and SC2). As shown in Fig. 4(a), the UV-vis
absorption curve of SC2 (ɸ=0.40) has a much deeper
valley centered at 630 nm than that from SC1 (ɸ=0.33),
and the Raman spectrum of the SC2 network has a
lower intensity of G- band at 1540 cm-1 (Fig. 4(b)).
Although both data indicate higher m-SWCNT
content in SC1, we only have qualitative information
[5]. We then scanned an area 20×20 µ m2 using 633 nm
laser excitation for each network and intensity
mapping was done using a G- window spanning 1507
to 1547 cm-1 (Fig. 4(c) and 4(d)). We counted 135 and
20 m-SWCNT signals for SC1 and SC2 networks
respectively. Using the calibration curve in Figure 3,
the tube density of these two networks was
calculated to be ~ 30 tubes/µ m2. As a result, the
m-SWCNT content was estimated to be 1.0% and
0.2%, thus the sc-nanotube purity was estimated at
99.0% and 99.8% for SC1 and SC2, respectively.
In order to assess the correlation between SC purity
and channel length, samples SC1 and SC2 were
further compared as the semiconducting channel
material in thin film transistors. A SWCNT network
was combined with a bottom gate, bottom contact
device configuration with interdigitated S/D
electrodes having a 2000 µ m channel width [16].
Devices were prepared under identical conditions
with regards to solution concentration and TFT
performance data are summarized in Fig. 4(e) and
4(d). At 5 µ m channel lengths the TFTs have very
similar mobility values of 20±2 cm 2/Vs and excellent
current on/off ratios, reaching as high as 1x10 7.
However, devices with 2.5 µ m channel lengths
clearly reveal the purity difference between SC1 and
SC2 even though similar mobility values of 16±1
cm2/Vs were obtained. In a representative sampling
of four devices, all four 2.5 µ m channel length TFTs
from SC1 had on/off ratios < 1x10 4, with three devices
< 1x10 3 while three TFTs from SC2 had on/off ratios >
1x10 6 and only one device had an on/off ratio
between 10 3 and 10 4. The TFT data at 2.5 µm channel
lengths agrees with the purity differentiation
obtained by Raman mapping, further substantiating
the ability of the method to discriminate between
highly enriched sc-SWCNT samples. A statistical
model was used to determine the probability of a
percolation path to form (see ESM for calculations).
For TFTs with a channel length of 2.5 m, we expect 1
metallic path every 290 μm for a network with 0.2%
metallic tube content (SC2) or every 5 μm for 1%
metallic tube content (SC1). Given that our channel
width is 2000 m, a few short circuits, if any, are
expected for the higher purity sample, while short
circuits should be fairly abundant in the lower purity
sample. Given that the on currents are of the order of
several mA, and with several short circuits
contributing current of the order of 1 μA in the off
state, on-off ratios of the order of 10 3 are expected for
the lower purity sample (SC1), consistent with our
purity assessment.
In summary, Raman mapping has been shown to be
an efficient and effective method to quantitatively
evaluate sc-purity and SWCNT network density
directly in fabricated TFT devices. The accuracy of
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Nano Res.
the method was further validated by sc-SWCNT
network TFT device performance at short channel
lengths. The advantage of using networks of
SC-SWCNT over aligned tubes [52-55] for the active
channel in TFTs is the favorable trade-off between
targeted performance and ease of fabrication [56-58].
Such TFTs will find utility in low cost display driver
and logic circuit elements fabricated using scalable
deposition techniques such as dip-coating, ink-jet or
gravure printing [59-66]. This work bridges the gap
between separation methods that yield high purity
sc-SWCNTs and available characterization tools for
purity assessment thus providing a framework for
device design that considers TFT channel scaling
with SWCNT density and purity.
Acknowledgements
The authors would like to thank Jeff Fraser for SEM
imaging.
Electronic Supplementary Material: Supplementary
material (Materials, SWCNT film preparation, TFT
device fabrication and testing, resonance Raman
spectra mapping method and representative SEM
images) is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
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