Nano Research
DOI 10.1007/s12274-015-0933-5
Increased chemical reactivity of single-walled carbon
nanotubes on oxide substrates: In situ imaging and
effect of electron and laser irradiations
Hasan-al Mehedi1, Johann Ravaux2, Khadija Yazda1, Thierry Michel1, Saïd Tahir1, Michaël Odorico2, Renaud
Podor2, and Vincent Jourdain1 (*)
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0933-5
http://www.thenanoresearch.com on Oct. 30, 2015
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Nano Res.
64
Increased chemical reactivity of single-walled
carbon nanotubes on oxide substrates: In situ
imaging and effect of electron and laser
irradiations
Hasan-al Mehedi1, Johann Ravaux2, Khadija
Yazda1, Thierry Michel1, Saïd Tahir1, Michaël
Odorico2, Renaud Podor2, Vincent Jourdain1*
1
Laboratoire Charles Coulomb (L2C), UMR 5221
CNRS-Université
de
Montpellier,
34095
Montpellier, France.
2
Institut de Chimie Séparative de Marcoule,
UMR 5257 CEA-CNRS-UM2-ENSCM Site de
Marcoule, Bat 426, BP 17171, 30207 Bagnols sur
Cèze cedex, France
The chemical reactivity of single-walled carbon nanotubes on oxide
substrates is strongly influenced by the local doping induced by
substrate-trapped charges. The latter is affected by the electrical connection
of the nanotube and by electron and laser irradiations.
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Nano Research
DOI (automatically inserted by the publisher)
Research Article
Increased Chemical Reactivity of Single-Walled Carbon
Nanotubes on Oxide Substrates: In Situ Imaging and
Effect of Electron and Laser Irradiations
1
2
1
1
1
2
Hasan-al Mehedi , Johann Ravaux , Khadija Yazda , Thierry Michel , Saïd Tahir , Michaël Odorico ,
2
1
Renaud Podor , Vincent Jourdain (*)
1
Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS-Université de Montpellier, 34095 Montpellier, France
2
Institut de Chimie Séparative de Marcoule, UMR 5257 CEA-CNRS-UM2-ENSCM Site de Marcoule, Bat 426, BP 17171, 30207 Bagnols
sur Cèze cedex, France
Received: day month year
ABSTRACT
Revised: day month year
We studied the oxygen etching of individual single-walled carbon nanotubes on
Accepted: day month year
(automatically inserted by
the publisher)
silicon oxide substrates using atomic force microscopy and high-temperature
environmental scanning electron microscopy. Our in situ observations show
that carbon nanotubes are not progressively etched from their ends as
frequently assumed but disappear segment by segment. Atomic force
microscopy, before and after oxidation, reveals that the oxidation of carbon
nanotubes on substrates proceeds by a local cutting which is followed by a
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
rapid
etching
of
the
disconnected
nanotube
segment.
Unexpectedly,
semiconducting nanotubes appear more reactive in these conditions than
metallic ones. We also show that exposure to electron and laser beams locally
increase the chemical reactivity of carbon nanotubes on such substrates. These
results are rationalized by considering the effect of substrate-trapped charges
KEYWORDS
Single-walled
carbon
nanotubes,
Environmental scanning
electron
microscopy,
Oxidative
etching,
Substrate-trapped
charges, Electron and
laser irradiations
on the nanotube density of states close to the Fermi level, which is impacted by
the substrate type and the exposure to electron and laser beams.
Introduction
including air [7, 23], O2 [5, 11, 23-25], SO3 [6] water
Understand the mechanisms underlying the
chemical reactivity of single-walled carbon
nanotubes (SWCNTs) is of great importance for
fundamental and applied points of view. The
reactivity of SWCNT toward oxidation is of
particular importance in many fields ranging
from their synthesis and their post-processing
and
including
the
understanding
of
SWCNT-based devices. For instance, SWCNTs
were found to exhibit lower emission currents
and reduced lifetimes in low pressures of oxygen
and water vapor due to reactive sputter etching [1,
2]. Gas-phase oxidizers such as CO and water
vapor are frequently used during SWCNT
growth to selectively burn off amorphous carbon
by-products in order to enhance and preserve
catalyst activity [3-7]. Thermally oxidizing
SWCNT material in air or oxygen is also
routinely performed to remove unwanted
disordered
carbon
materials
or
create
oxygen-containing moieties on SWCNT walls
[3-12]. Several groups already attempted to
utilize the oxidation process to manipulate
SWCNT structures, for instance by opening up
their terminating cap or by thinning the tubes [13,
14]. When controlled, oxidation therefore plays
an important role in the synthesis, modification
and purification of SWCNTs.
conditions, either metallic (m-CNTs) [12, 27, 28] or
Since SWCNTs can be metallic or semiconducting
depending on their diameter and chiral angle [15,
16],
valuable
applications
are
envisioned
in
transparent conductors [17], solar cells [18],
biosensors [19], and nanoelectronics [20]. However,
most of these applications require SWCNTs of
either metallic or semiconducting type. Despite
recent progress in selective synthesis, current
techniques still produce heterogeneous samples of
SWCNTs of varying structure and electronic
character [21, 22]. Post-growth processing of
SWCNTs is suggested to be an alternative approach
for the type-selective isolation of SWCNTs, and
vapor [26], and fluorine gas [27]. Depending on the
semiconducting nanotubes (s-SWCNTs) [29, 30] are
observed to be etched preferentially. Oxidation
rates were found to increase with smaller SWCNT
diameter which was typically explained in terms of
higher C-C bond strain and higher sp3 character
resulting in higher reactivity towards adsorbates
[23-26,
31-35].
Density
functional
theory
calculations suggest that the local curvature radius
determines the weakest carbon-carbon bond and is
consequently thought to be one of the determining
factors in oxidative etching [23, 32]. Other studies
support that chirality has a direct effect on oxygen
sidewall chemisorption and can affect chemical
reactivity in general [36, 37]. Preferential etching of
m-SWCNT is usually attributed to their finite
density of states (DOS) at the Fermi level which
ease charge transfer between the SWCNT and the
HOMO/LUMO of the reactant molecule. However,
the nanotube environment may also significantly
modify the reactivity. For instance, hole doping was
reported to enhance the chemical reactivity of
semiconducting SWCNTs and to lead to their faster
etching compared to metallic SWCNTs [30]. In the
case of SWCNTs lying on substrates, chemical
doping by the substrate is also to be considered as
recently illustrated by its strong influence on the
oxidation of graphene deposited on substrates [38].
In previous studies, the oxidation of SWCNTs
was
usually
characterized
by
ex
situ
measurements on ensembles of SWCNTs. In situ
approaches at the level of individual SWCNTs are
well suited for investigating SWCNT oxidation,
as one can track the dynamics of the process as it
happens and observe trends that might be
difficult
to
detect
in
ex
situ
ensemble
measurements. For instance, earlier studies have
suggested that SWCNT oxidation initiates at the
Address correspondence to Vincent Jourdain, [email protected]
there is already evidence that separation can be
end of the tube and proceeds along its length [13,
achieved
14]. Recent in situ environmental transmission
using
various
gas-phase
oxidants
Nano Res.
3
electron microscopy (ETEM) investigation of
defects. During our in situ SEM experiments, the
multi-walled CNT oxidation, on the other hand,
nanotubes are first imaged at room temperature
reveals that despite the higher curvature of the
in vacuum (<10-2 Pa) prior to oxidation. The
carbon cap, it is the outer wall which is oxidized
ESEM image of the as-synthesized SWCNTs on a
and etched first [36]. Here, we report on the
selected part of the quartz substrate at higher
oxidation of individual SWCNTs deposited on
magnification (Fig. 1(a)) clearly shows that some
substrates
by
tubes appear very bright compared to the
high-temperature environmental SEM. Using this
substrate while other tubes are less contrasted.
in situ approach, we could continuously identify,
The SEM contrast of individual SWCNTs on
locate and quantify the modifications induced by
insulating substrates originates from the SWCNT
oxidation along the same individual SWCNTs. By
electrical conductivity which reduces the density
coupling these in situ observations with AFM
of surface charges of the substrate in the vicinity
characterization before and after oxidation, we
of the tube [40]. The contrast is strongly enhanced
showed that SWCNT oxidation is associated with
if the tube is electrically connected to a larger
both cutting and etching of the tubes. We
mass of conducting material so that the charges
investigated how the chemical reactivity of
induced by the electron beam can be efficiently
SWCNTs is influenced by the temperature, the
evacuated away from the imaged area. In our
type
or
case, the catalyst line which is several mm long
amorphous SiO2) and the exposure to electron or
and consists of a network of entangled SWCNTs
laser
on
plays such a role. It has also been shown that the
substrates were found to be slightly more reactive
SEM contrast of individual SWCNTs is related to
than m-SWCNTs. A mechanism is proposed to
their electrical properties, the brighter and darker
rationalize these observations and describe the
SWCNTs corresponding respectively to metallic
influence of the substrate and of electron and
and
laser exposures on the chemical reactivity of
metallic/semiconducting
substrate-supported SWCNTs.
clearest if all the tubes are connected to the same
of
by
oxygen
substrate
beams.
monitored
(crystalline
Surprisingly,
quartz
s-SWCNTs
semiconducting
SWCNTs
(M/SC)
[41].
contrast
This
is
electrode (i.e. the catalyst line in our case). For
Results and discussion
instance, in Fig. 1(a), one can assign the SWCNTs
The SWCNTs investigated in this work were
marked as 2, 3, 4 and 5 to semiconducting tubes
grown on ST-cut quartz wafers by catalytic
whereas SWCNTs marked as 1, 8, 14 and 18 are
chemical
described
metallic. After O2 injection at high temperature,
elsewhere [39]. The individual nanotubes are
several tens of seconds were usually required to
straight, long (lengths typically range from 30 to
adjust the imaging parameters and obtain the
170 µm) and horizontally-aligned along the
first image in reactive conditions. The SEM image
X-direction of the quartz substrate (Fig. S1(a) in
in Fig. 1(b) corresponds to the same region as Fig.
the Electronic Supplementary Material (ESM)).
1(a), recorded at 480 °C, 190 s after the
Each tube has one end connected to the catalyst
introduction of 11 Pa of O2. As apparent, most
line (which consists of entangled SWCNTs) while
tubes now appear equally bright as expected
its other end is free from any connection. Figures
from
S1(c) and S1(d) (in the ESM) show typical Raman
semiconducting and metallic SWCNTs becoming
spectra of the studied SWCNTs revealing that
closer with increasing temperature [42, 43]. The
they are mostly single-walled with diameters
few tubes which still appear dark at high
ranging from 1.3 to 1.7 nm. The average ID/IG
temperature (e.g. tube N in Fig. 1(b)) probably
ratio is ~0.03 indicating a very low density of
correspond to small-diameter semiconducting
vapor
deposition,
as
the
electrical
conductivities
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4Nano Res.
SWCNTs remaining poorly conducting due to
the evolution of the SWCNTs shown in Figure 1
their larger band gap. The influence of the
with time. Figure 2 shows a few snapshots of the
temperature therefore agrees with the M/SC
movie at different times in a specific zone (left
assignment of the SEM contrast. In this first
part of Fig. 1).
image, several tubes are already observed to be
shorter than at room temperature (e.g. tubes 2, 3,
5, 8 and 18) due to the O2 exposure at high
temperature.
Movie S1 (in the
ESM)
illustrates
Figure 1. SEM images of as-synthesized SWCNTs on a quartz substrate (a) at room temperature in 10-2 Pa before oxidation and (b)
at 480 °C after 190 s of exposure to 11 Pa of pure O2. The images were recorded at an accelerating voltage of 1.8 kV.
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Nano Res.
5
Figure 2. In situ SEM images of SWCNTs on a quartz substrate during oxidation in 11 Pa O2 at 480 °C showing the
segment-by-segment etching of the SWCNTs. The scale bar is 50 µm.
As shown in Movie S1 and in Fig. 2, the
monitoring its oxidation by in situ SEM.
apparent length of each nanotube decreases
Figures 3(a) and 3(b) show the SEM images of
with time. The most striking feature is that the
the zone before and after 17 min of oxidation,
nanotubes do not disappear progressively from
respectively. Some tubes of interest in this zone
one end or from the two ends simultaneously,
are marked with letters ranging from A to L.
as would be expected from a continuous
Although G and J appear as single lines in the
nanotube
disappear
SEM picture, AFM reveals that each of them
sequentially, that is segment by segment.
actually consists of two close tubes of different
Another feature of our observations is that the
lengths that we labeled as G1, G2 and J1, J2,
segments are always removed from the free
respectively. Note that, although the oxidation
end of the tube. Three hypotheses can be
behavior of each of these four tubes cannot be
proposed to explain these in situ SEM
assessed by the in situ SEM data, it can be
observations:
characterized by ex situ AFM. The features
(I) Local cutting. In this hypothesis, the
(length, M/SC type) and the oxidation behavior
oxidation would induce a local cutting of the
of all these tubes are detailed in Table ST1 (in
tube (at a random position or at a pre-existing
the ESM).
reactive site) resulting in two disconnected
The observed behaviors can be categorized as
nanotube
follows:
etching
but
segments:
instead
the
segment
still
connected to the catalyst line would remain
a) No length change in both SEM and AFM.
visible while the other one would become
Several tubes (tubes A, C, J(1,2), K and L)
invisible in SEM due to the lack of electrical
displayed no apparent reduction of their
connection with the catalyst line.
lengths during in situ observations and still
(II) Fast etching and abrupt stop. Here, the
appeared
oxidation
and
unchanged lengths. AFM pictures of tube
continuous etching of the tube starting from
A before and after oxidation are shown in
the nanotube end until a certain position along
Fig. 3(c) to illustrate this type of SWCNTs
the tube (e.g. a defect) where the etching would
having low reactivity.
would
induce
a
rapid
in
the
AFM
images
with
abruptly stop; here it is hypothesized that the
b) Length change in SEM but not in AFM. We
etching is too fast to be resolved at the
observed one tube (tube F) displaying the
acquisition rate of the SEM pictures (i.e. about 1
features of hypothesis I, that is the segment
s).
which
(III) Bunch of nanotubes. In this scenario, the
observations
observed line would actually consist of two or
unchanged length in the AFM picture after
more tubes of different lengths which are too
oxidation (Fig. (3d)). This observation
close to be resolved by SEM (e.g. a nanotube
agrees with a local cutting of the tube
bundle); the abrupt disappearance of one
causing
segment would be caused by the oxidation of
disconnected segment during the ESEM
the longest tube of the bunch (by cutting or
observations.
etching).
mechanism is additionally supported by in
To test these hypotheses, we performed AFM
situ observations of segments blinking for a
on a nanotube sample before and after
few
disappeared
a
still
during
the
appeared
disappearance
The
seconds
existence
before
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SEM
with
of
of
the
this
complete
Research
6Nano Res.
disappearance (see Movie S2 in the ESM).
features of hypothesis II (tubes B, D, E and
Note that despite our attempts, we were
G(1,2)),
not able to identify by AFM a cut along the
disappeared during the in situ SEM
nanotube at the position expected from the
observations are found to have been
SEM picture. We presume that this is
physically removed (etched) in the AFM
because the width of the cut is under the
images (Fig. 3(e)).
that
is
the
segments
which
lateral resolution of our AFM setup (lateral
resolution: ~ 8 nm).
c)
Same length change in both SEM and AFM.
Finally,
several
tubes
displayed
the
Figure 3. SEM images of individual SWCNTs on quartz substrate (a) before oxidation and (b) after 17 min of oxidation at 540 °C.
(c-e) AFM images before and after oxidation illustrating the three types of behaviors: (c) no change in both SEM and AFM
pictures, (d) the disappeared segment in the SEM picture still appears in the AFM picture and (e) the disappeared segment in the
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Nano Res.
7
SEM picture is physically removed. The scale bar for AFM images is 500 nm.
Note that we observed one tube (tube I) which
and found that this did not lead to the
displayed the consecutive disappearance of
nanotube being preferentially cut at this
two segments during the in situ observation
position (see ESM files “AFM before/after
and exhibited the two types of behavior, i.e. the
oxidation”). This led us to conclude that iron
AFM characterization revealed that the first
catalyst nanoparticles were probably oxidized
segment was physically removed while the
due to air and oxygen exposures and had
second segment was still present on the
therefore no or little catalytic activity to
substrate. This case supports that local cutting
interfere
(mechanism I) may systematically precede the
Alternatively, the observed nanoparticles may
rapid etching of the disconnected segment
be
(mechanism II). This would notably explain
resulting from exposure of the samples to
why progressive etching of nanotubes was
ambient air.
never
observed
but
contaminant
the
nanotube
inorganic
oxidation.
nanoparticles
abrupt
To allow a quantitative analysis, the in situ
disappearances of tube segments. In this
images were analyzed in order to extract the
hypothesis, this raises the question of why
features of the segment disappearance events
disconnected
rapidly
(or cutting events) for each tube: cutting time,
etched than segments that are still connected to
tube length before and after the cutting. The
the catalyst line.
corresponding SEM images and extracted data
To test hypothesis III (i.e. bunch of nanotubes),
are summarized in Figs. S2-5 and Tables ST2-5
we searched by AFM for evidence of nanotube
in the ESM. First, the analysis showed that
bundling or of changes of geometrical features
there is no preferential position of cutting
(width or height) along the nanotubes at the
along the CNTs (i.e. closer to the free end or
positions where the segments were observed to
closer to the catalyst line): when averaged over
be removed during the in situ observations. All
a large number of cuttings, the average cutting
our observations support that the studied tubes
position is close to the middle of the tube
on quartz are individual and have a constant
segment (see Table ST2-5 in the ESM). One can
diameter all along their length. So, even
conclude that the cuts appear at random
though hypothesis III cannot be completely
positions along the nanotube. Second, the
excluded, it finds no experimental support in
average time between two consecutive cutting
the AFM data.
events tends to increase when the total length
In addition, to determine whether residual
of remaining tubes decreases (Fig. S6 in the
catalyst nanoparticles may be involved in
ESM). Third, if normalized by the sum of the
nanotube cutting, AFM characterization was
remaining nanotube lengths, the number of
also used to image the locations where the
cutting events at a given time tends to increase
tubes were cut during ESEM experiments.
with time (Fig. S7 in the ESM). These
However, no correlation was found between
observations support that the probability of a
tube cutting and the presence of a nanoparticle
cutting event is proportional to the tube length
in the close vicinity. At the opposite, in AFM
and to the time of exposure to oxygen. To
observations
after
account for these trends at the individual
oxidation in the same area, we observed
nanotube level, we defined the index of
several instances of nanoparticles located
reactivity (R) of an individual nanotube as:
segments
performed
only
with
are
more
before
and
above or beside a nanotube before oxidation
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8Nano Res.
é i =n
1 ù
êå j j ú
Dt L
lim ê i=1 i i-1 ú = R j
n j ®+¥ ê
ú
nj
ê
ú
ë
û
that the index converges towards a constant
where Lij is the length of the individual tube j
displayed more than 4 cutting events. Figure 4a
after the ith cutting event, Dtij is the interval of
displays the reactivity of individual nanotubes
time between the (i-1)th cut and the ith cutting
as a function of the oxidation temperature. As
event of tube j, and nj(t) is the total number of
may be expected, the individual reactivities
cutting events experienced by tube j after time t.
tend to increase with increasing temperature.
Experimentally, we do not have access to an
However,
infinite number of cutting events but we found
individual reactivities are largely
j
value for a number of cutting events typically
greater than 4 (Fig. S8 in the ESM). In the
following,
the
individual
reactivity
will
therefore only be used for tubes having
at
a
given
temperature,
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the
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Nano Res.
9
Figure 4. Variation of reactivity of (a) individual SWCNTs and (b) of the ensemble of SWCNTs with the oxidation temperature.
Figure 5a corresponds to SWCNTs on quartz only. The first number before the brackets correspond to the initial length of the
nanotubes in microns; the number within brackets indicate the total number of cutting events experienced by each nanotube.
scattered without any apparent correlation
graphite in O2 [44].
with the initial length of the nanotube.To
As previously mentioned, some tubes display
obtain a more statistical view of the effect of
many segment removals while other ones
ensemble
the temperature, the average reactivity R
remain nearly unmodified within the timespan
of the ensemble of nanotubes during a given
of the experiment. Such large differences of
experiment was defined as:
reactivity suggest that the reactivity is strongly
é
1 ù
ê å Dt j L j ú
all tubes
i i -1 ú
lim ê
= R ensemble
t ®+¥ ê n ensemble (t ) ú
ê
ú
ë
û
affected by the nanotube features such as its
metallic/semiconducting (M/SC) type, diameter
and/or density of defects.
In a first approach, the individual reactivities
of fifteen tubes whose M/SC type could be
where nensemble(t) is the total number of cutting
ascribed
events experienced by the nanotube collection
compared. Please note that the least reactive
after time t. For statistical validity, we also
tubes (with less than 4 cutting events) are
checked that R
converges towards a
neglected in this analysis. As shown in Table
constant value within the duration of the
ST6 (in the ESM), the individual reactivities are
experiments (Fig. S9 in the ESM). In this case,
quite scattered varying by a factor 10 for both
all the tubes contribute to the calculated
metallic and semiconducting tubes. On average,
reactivity whatever the number of cutting
the reactivity appears substantially higher for
events they experienced. Figure 4b shows the
semiconducting tubes (~2.6×10-5 µm-1s-1) than
evolution of the ensemble reactivity with the
for metallic tubes (~1.4×10-5 µm-1s-1), supporting
oxidation temperature. The ensemble reactivity
a slightly higher reactivity of semiconducting
tends to follow an Arrhenius law with an
tubes. This trend is also apparent in the
activation energy of 0.9 eV for SWCNTs on
coupled SEM-AFM experimental data (see
crystalline quartz, which is slightly lower than
Table ST1 in the ESM) where most metallic
the activation energy (~1.2 eV) reported for
tubes display no cuts within the timespan of
monolayer etching of the basal plane of
the experiment while most semiconducting
ensemble
from
their
SEM
contrast
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Nano Res.
10
tubes display 1 or 2 cuts. In a second approach,
Laser exposure in air can induce several
we considered the average number of cutting
modifications
events experienced by metallic tubes and
functionalization
semiconducting tubes during the total duration
including structural damages to SWCNTs and
of a given experiment. For such a measurement,
graphene, which would explain why the
eight semiconducting and twelve metallic
exposed areas became more reactive. However,
tubes of a given experiment (T=480 °C) were
the e-beam effect cannot be explained in terms
selected and their average numbers of cutting
of structural damages caused by the incident
events were normalized by the summed initial
electrons since the accelerating voltage is well
lengths of the tubes (639 µm and 790 µm for
below the knock-on damage threshold for sp2
semiconducting
tubes,
carbons in a hexagonal network [45, 46]. The
respectively) and by the duration of the
most important effect of the electron beam is to
experiment (45 min). This analysis again
cause surface charging.
confirms
tubes
The charging of an insulator surface under
experienced
that
and
metallic
semiconducting
slightly
optical
chemical
doping
and
events
electron beam irradiation depends on the
accelerating voltage which controls the ratio of
µm-1s-1) supporting again a slightly higher
secondary electrons emitted by the surface per
reactivity of semiconducting tubes.
incident electron [47]. At high accelerating
In a separate experiment, we performed the
voltages, the SiO2 surface is negatively charged
Raman characterization of individual tubes of
and appears brighter than neutral SiO2; at
a given zone before monitoring their oxidation
accelerating voltages below a threshold value
by in situ SEM. Unexpectedly, we observed that
(typically 3 kV for a thin SiO2 layer on Si and
all tubes were oxidized extremely rapidly
10 kV for bulk quartz), the surface becomes
within the first tens of seconds of exposure to
positively charged and appears darker than an
oxygen, whatever their type (M/SC) and
uncharged SiO2 surface because its positive
diameter (Fig. S10 in the ESM). This precluded
potential hinders the emission of secondary
the use of Raman spectroscopy to determine
electrons
the
and
voltages (typically less than a few hundred
correlate them with their reactivities. However,
mV), the surface becomes negatively charged
the experiment showed that the first cuttings
again. In our experimental conditions (Vacc=1.8
preferentially appeared along the line where
kV), the SiO2 surface is therefore positively
the laser beam was scanned during the Raman
charged and has a surface potential of up to
characterization. This prompted us to check the
several volts. If SWCNTs are connected to an
influence of the electron beam exposure on the
outer electrode as in our experiments, electrons
SWCNT reactivity. To do so, we performed a
are
long monitoring of SWCNT oxidation by
compensate for the positive charge of the
ESEM in a given zone before zooming out in
substrate thus acting as a positive gate. As
order to compare the reactivities in the zones
shown by the group of Kaili Jiang [42], a part
exposed and not exposed to the electron beam.
of the electrons will leak into the nearby
As shown in Fig. S11 (in the ESM), electron
substrate making it less charged and appear
beam exposure strongly increases the reactivity
brighter
of SWCNTs. Exposure to laser or electron
temperature, electrons are more efficiently
beams can therefore significantly increase the
transported by metallic tubes as evidenced by
reactivity of SWCNTs deposited on substrates.
their higher contrast in SEM. However, at the
features
of
cutting
to
from
(2.3×10-5 µm-1s-1) than metallic tubes (1.9×10-5
structural
more
ranging
SWCNTs
[48].
carried
than
At
very
through
the
rest
low
the
of
accelerating
SWCNTs
it.
At
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11
high temperatures used for SWCNT oxidation,
electron/hole puddles along the tubes) which
the conductivities of m- and s-SWCNTs
would increase the pre-exponential term of the
become closer and so do the surface charge
Arrhenius law.
density of the SiO2 surrounding them, as
Based on these arguments, we propose the
evidenced by the close SEM contrasts of m-
following mechanism for the oxidation of
and s-SWCNTS.
SWCNTs on substrates and for the influence of
In the case of graphite or of several-layer
the laser and electron beam exposure (Fig. 5).
graphene, oxidation is initiated at pre-existing
SWCNTs deposited on SiO2 surfaces and
point defects, which manifests by a rather low
exposed to air are known to be slightly
reactivity and a homogeneous size distribution
p-doped.
of etch pits. However, in the case of monolayer
impurities or substrate-trapped charges can
graphene supported on a substrate, oxidation
locally
is strongly enhanced [49] which was explained
electron/hole puddles to form along the
by the effect of substrate-trapped charges [39]
SWCNTs (Figs. 5(a) and (b)). Such fluctuations
which cause spatial fluctuations of the Fermi
of the electrostatic potential are common
energy of graphene, that is electron and hole
features on SWCNTs deposited on oxide
puddles that alternate in space [50, 51].
substrates [54]. If the doping level reaches the
Evidence of fluctuations of electrical potential
first van Hove singularity, the DOS close to the
along quartz-supported SWCNTs can be found
Fermi level is strongly increased and so is the
in many SEM pictures at room temperature
reactivity.
displaying marked variations of contrast along
constitute preferential sites along the SWCNT
individual semiconducting SWCNTs (see Fig.
for initiating redox reactions, such as oxygen
S12 in the ESM). These contrast fluctuations are
etching. The density of electron/hole puddles
not observed at high temperature probably
along the tubes is influenced by the density of
because of the lower resolution and signal to
charged impurities and of charge-trapping
noise ratio in ESEM conditions.
sites of the substrate but also by the history of
We propose that the reactivity of SWCNTs on
the sample. As demonstrated for graphene on
substrates is ruled by similar processes and
SiO2 by Young Duck Kim et al. [55], laser
that the reactivity can be further enhanced by
exposure can be used to optically eject
charging the surface of the substrate with
SiO2-trapped charges and induce a localized
electron or laser beams. To test the hypothesis
n-type doping. We propose that a similar
that SWCNT oxidation is primarily controlled
process is at the origin of the higher reactivity
by the substrate rather than by intrinsic
of supported SWCNTs exposed to a laser beam
features of SWCNTs, we transferred SWCNTs
(Fig.
from quartz to an amorphous layer of thermal
accelerating voltages of a few kV, the SiO2
silicon oxide (on Si) which is known to contain
surface is positively charged which can cause
a high density of charge trapping sites [52, 53].
n- or p-type doping depending on whether the
As shown in Fig. 4(b), SWCNTs show on
tubes are connected to a reservoir of electrons
average a higher reactivity on amorphous SiO2
outside the irradiated area (Figs. 5(c) and (d)).
than on crystalline quartz and this despite a
If the tube is not connected to a reservoir of
slightly higher activation energy (1.1 eV
electrons, it will become positively charged
compared to 0.9 eV on quartz). This agrees well
(hole-doped) due to the electrons leaking to the
with a higher rate of oxidation caused by the
underlying and positively-charged SiO2. At the
higher density of surface charges (and of
opposite, if the tube is connected to a reservoir
Similarly
shift
the
to
Fermi
Electron/hole
5(a)).
Under
graphene,
level
puddles
e-beam
charged
causing
therefore
exposure
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Nano Res.
12
of electrons, the latter will flow into the tube
rapidly etched away (Fig. 5(d)). Finally, the
causing it to be less p-doped or even slightly
effect of a moderate doping on the chemical
n-doped. The SiO2 region surrounding the tube
reactivity is expected to be stronger for
will be less positively charged, due to the
semiconducting SWCNTS because the first van
electrons leaking from the nanotube, and
Hove singularities of semiconducting SWCNTs
therefore appear brighter in SEM. As before,
lie
un-neutralized substrate charges near the tube
s-SWCNTs can therefore be greatly enhanced if
would locally induce highly-doped sites along
the doping level reaches their first van Hove
the SWCNT displaying higher reactivity. This
singularity, thereby causing a strong increase
would account for the two experimental
of the DOS close to the Fermi level. This would
observations that i) the reactivity of SWCNTs
explain
increases under e-beam exposure, ii) SWCNT
semiconducting SWCNTs display a slightly
segments
higher reactivity than m-SWCNTs contrary to
which
become
electrically
disconnected from the others after a local
at lower energies. The
why,
in
the
reactivity of
studied
conditions,
common expectations.
cutting become entirely p-doped and are
Figure 5. Proposed mechanism of oxidation of SWCNTs on oxide substrates. a,b) Influence of laser irradiation and of charged
impurities on the local doping of SWCNTs. c,d) Influence of electron beam irradiation on SWCNTs connected and disconnected
to/from an outer reservoir of electrons, respectively. The blue and red colors along the SWCNTs indicate the local degree of n- and
p-type doping, respectively (see DOS in insets). The bright area delimited by a dashed line represents the SiO2 region depleted in
positive charges due to the electrons leaking from the tube and appearing brighter in SEM imaging.
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
Research
Nano Res
1
Conclusion
In summary, we showed how the chemical reactivity of SWCNTs is impacted by the underlying substrate
and how electron and laser beam exposures can locally increase the SWCNT reactivity. By analogy with
the reactivity of monolayer graphene on oxide substrates, we propose that the chemical reactivity of
SWCNTs on substrates is controlled by substrate-trapped charges and that laser and electron beams can
increase the reactivity by increasing the surface density of substrate charges. Controlling the SWCNT
electrical potential with an outer electrode can also be used to alter the density of substrate charges close to
the SWCNT and hence its Fermi level and chemical reactivity towards oxidative etching. It follows that in
conditions favouring high levels of doping, semiconducting SWCNTs can be as or even more reactive than
metallic SWCNTs. An interesting question is whether the observed effect is limited to reactions involving
charge transfer (e.g. oxidation, reduction or reaction involving a charge-transfer complex intermediate). We
believe that these results may have important consequences for the functionalization and the purification
of SWCNTs but also for their growth selectivity. They first show that semiconducting SWCNTs can be
made more reactive than metallic ones by tuning their doping level via the density of substrate-trapped
charges and that a spatial control is possible using electron or laser beams. Second, these results provide a
novel interpretation to the still-unexplained preferential growth of metallic SWCNTs on silicon oxide
substrates [56] and to why it is so dependent on the conditioning history of the catalyst substrate.
Experimental
The SWCNTs investigated in this work were grown on stable temperature-cut (ST-cut) quartz wafers by
catalytic chemical vapor deposition using ethanol as carbon source and iron nanoparticles as catalyst [39].
Briefly, a positive photoresist (Shipley S1818) was loaded with a solution of FeCl3 in methanol and
spin-coated on the quartz wafers. Photolithography was then used to pattern lines of catalyst nanoparticles
with a line width of 30-50 µm and an interline separation of 400 µm. The samples were then calcined in air
at 700°C to remove the organic part and transform the FeCl3 salt into iron oxide nanoparticles.
For some experiments, nanotubes were transferred on Si substrates (p-type doping, resistivity 10-20 Ω.cm,
with a 500 nm layer of thermal SiO2) firstly to investigate any influence of the substrate on the oxidation
kinetics and secondly to facilitate their characterization by Raman spectroscopy (wavelength 532 nm, laser
power 0.5 mW, objective 50x) before oxidation. Briefly, transfer of SWCNTs from quartz substrates to the
patterned SiO2/Si substrates was carried out by a polymeric water-based method using cellulose acetate
butyrate (CAB) [57]. The quartz substrates with SWCNTs were spin-coated by a thin layer of cellulose
acetate butyrate (CAB). After baking (50°C, 1 h), the CAB layer with embedded nanotubes was peeled
from the quartz by soaking in ultrapure (Milli-Q) water overnight. The peeling occurs due to the
intercalation of a layer of water molecules between the hydrophilic surface and the hydrophobic
nanotubes embedded in the hydrophobic polymer (CAB). The peeled layer floating at the surface of water
was then transferred onto silicon substrates (SiO2/Si) patterned with alignment markers (created and
etched on the substrate by lithography using S1818 resin, a positive photoresist, and reactive-ion etching
(RIE)) and baked (70°C, 1 h). The CAB layer was then removed by soaking in ethyl acetate solution (20
min), in acetone (5 min) and isopropanol (5 min).
The in situ oxidation experiments were performed using a Field Emission Gun Environmental Scanning
Electron Microscope (ESEM, model FEI QUANTA 200 ESEM FEG) equipped with a heating cell (5 mm
inner diameter MgO ceramic crucible, 25-1500 °C) under 11 Pa of O2 at temperatures in the range of
480-580 °C. The water cooling system of this device was associated to an expansion tank that limited the
stage vibration during the experiment. A specific detector was used for in situ gaseous secondary electron
2Nano Res
imaging at high temperature. The nanotubes were first imaged at room temperature under high vacuum
(~10-4 Pa), and then the sample was heated at 50 °C/min under vacuum (< 5×10-3 Pa). When the temperature
reached 300 °C, a heat shield with electrostatic bias was introduced between the sample and the electron
column in order to protect the column. O2 was introduced into the chamber when the sample temperature
was stabilized to the desired oxidizing temperature. During oxidation, images were recorded sequentially
(4-10 sec per frame) under 11 Pa of O2 using the optimized beam energy of 1.8 keV, spot size of 5, emission
current of 150 µA and working distance of 20 mm. Note that the beam energy used in our experiments is
well below the knock-on displacement energy of carbon atoms in SWCNTs [45, 46]. Sample temperature
was measured by a home-made thermocouple placed just below the sample [58]. The temperature
accuracy for each heating stage was verified by checking the fusion temperature of a small gold wire.
The samples were characterized, before and after oxidation, by a MULTIMODE 8 AFM (lateral resolution:
~ 8 nm) with a Nanoscope 5 controller from Bruker. Imaging was performed in peak force mode in dry
condition using SNL tips (silicon tip on silicon nitride cantilever) from Bruker with a spring constant of
0.24 N/m.
Acknowledgements
This work was supported by EOARD (European Office of Aerospace Research and Development) and
USAF (United States Air Force) under grant number FA8655-12-1-2059.
Electronic Supplementary Material
Supplementary material (coupled SEM-AFM experiments; SEM images of nanotubes before, after and
during oxidation experiments; raw data of the in situ oxidation experiments; relation between number of
cuttings, time of exposure to oxygen and reactivity of individual SWCNTs; comparison of the reactivity of
metallic and semiconducting SWCNTs; oxidation behavior of a laser-exposed area; SEM contrast
fluctuation along the length of individual nanotubes; effect of prolonged electron beam exposure on
nanotube oxidation; ESEM movies of nanotube oxidation, AFM pictures before and after oxidation on the
same area) 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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Development of an integrated thermocouple for the accurate sample
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Microsc. Microanal. 2015, 21, 307-312.
Silver Nanowires with Semiconducting Ligands for
Low Temperature Transparent Conductors
Brion Bob,1 Ariella Machness,1 Tze-Bin Song,1 Huanping Zhou,1 Choong-Heui Chung,2 and Yang
Yang1,*
1
Department of Materials Science and Engineering and California NanoSystems Institute,
University of California Los Angeles, Los Angeles, CA 90025 (USA)
2
Department of Materials Science and Engineering, Hanbat National University, Daejeon
305-719, Korea
Abstract
Metal nanowire networks represent a promising candidate for the rapid fabrication of transparent electrodes with high transmission and low sheet
resistance values at very low deposition temperatures. A commonly encountered obstacle in the formation of conductive nanowire electrodes is
establishing high quality electronic contact between nanowires in order to facilitate long range current transport through the network. A new system of
nanowire ligand removal and replacement with a semiconducting sol-gel tin oxide matrix has enabled the fabrication of high performance transparent
electrodes at dramatically reduced temperatures with minimal need for post-deposition treatments of any kind.
Keywords: Silver Nanowires, Sol-Gel, Transparent Electrodes, Nanocomposites
6
1. Introduction.
Silver nanowires (AgNWs) are long, thin, and possess conductivity values on the same order of magnitude as bulk silver
(Ag) [1]. Networks of overlapping nanowires allow light to easily pass through the many gaps and spaces between
nanowires, while transporting current through the metallic conduction pathways offered by the wires themselves. The high
aspect ratios achievable for solution-grown AgNWs has allowed for the fabrication of transparent conductors with very
promising sheet resistance and transmission values, often approaching or even surpassing the performance of
vacuum-processed materials such as indium tin oxide (ITO) [2-6].
Significant electrical resistance within the metallic nanowire network is encountered only when current is required to pass
between nanowires, often forcing it to pass through layers of stabilizing ligands and insulating materials that are typically
used to assist with the synthesis and suspension of the nanowires [7, 8]. The resistance introduced by the insulating junctions
between nanowires can be reduced through various physical and chemical means, including burning off ligands and partially
melting the wires via thermal annealing [9, 10], depositing additional materials on top of the nanowire network [11-14],
applying mechanical forces to enhance network morphology [15-17], or using various other post-treatments to improve the
contact between adjacent wires [18-21]. Any attempt to remove insulating materials the network must be weighed against
the risk of damaging the wires or blocking transmitted light, and so many such treatments must be reined in from their full
effectiveness to avoid endangering the performance of the completed electrode.
We report here a process for forming inks with dramatically enhanced electrical contact between AgNWs through the use
of a semiconducting ligand system consisting of tin oxide (SnO2) nanoparticles. The polyvinylpyrrolidone (PVP) ligands
introduced during AgNW synthesis in order to encourage one-dimensional growth are stripped from the wire surface using
ammonium ions, and are replaced with substantially more conductive SnO2, which then fills the space between wires and
enhances the contact geometry in the vicinity of wire/wire junctions. The resulting transparent electrodes are highly
conductive immediately upon drying, and can be effectively processed in air at virtually any temperature below 300 °C. The
capacity for producing high performance transparent electrodes at room temperature may be useful in the fabrication of
devices that are damaged upon significant heating or upon the application of harsh chemical or mechanical post-treatments.
2. Results and Discussion
2.1. Ink Formulation and Characterization
Dispersed AgNWs synthesized using copper chloride seeds represent a particularly challenging material system for
promoting wire/wire junction formation, and often require thermal annealing at temperatures near or above 200 °C to induce
long range electrical conductivity within the deposited network [22, 23]. The difficulties that these wires present regarding
junction formation is potentially due to their relatively large diameters compared to nanowires synthesized using other
seeding materials, which has the capacity to enhance the thermal stability of individual wires according to the
Gibbs-Thomson effect. We have chosen these wires as a demonstration of pre-deposition semiconducting ligand substitution
in order to best illustrate the contrast between treated and untreated wires.
Completed nanocomposite inks are formed by mixing AgNWs with SnO2 nanoparticles in the presence of a compound
capable of stripping the ligands from the AgNW surface. In this work, we have found that ammonia or ammonium salts act
as effective stripping agents that are able to remove the PVP layer from the AgNW surface and allow for a new stabilizing
matrix to take its place. Figure 1 shows a schematic of the process, starting from the precursors used in nanowire and
nanoparticle synthesis and ending with the deposition of a completed film. The SnO2 nanoparticle solution naturally contains
enough ammonium ions from its own synthesis to effectively peel the insulating ligands from the AgNWs and allow the
nanoparticles to replace them as a stabilizing agent. If not enough SnO2 nanoparticles are used in the mixture, then the wires
will rapidly agglomerate and settle to the bottom as large clusters. Large amounts of SnO2 in the mixture gradually begin to
increase the sheet resistance of the nanowire network upon deposition, but greatly enhance the uniformity, durability, and
wetting properties of the resulting films. We have found that AgNW:SnO2 weight ratios ranging between 2:1 and 1:1
produce well dispersed inks that are still highly conductive when deposited as films.
The nanowires were synthesized using a polyol method that has been adapted from the recipe described by Lee et al. [22,
23] Silver nitrate dissolved in ethylene glycol via ultrasonication was used as a precursor in the presence of copper chloride
and PVP to provide seeds and produce anisotropic morphologies in the reaction products. Synthetic details can be found in
the experimental section. Distinct from previous recipes, we have found that repeating the synthesis two times without
cooling down the reaction mixture generally produces significantly longer nanowires than a single reaction step. The lengths
of nanowires produced using this method fall over a wide range from 15 to 65 microns, with diameters between 125 and 250
nm. This range of diameters is common for wires grown using copper chloride seeds, although the double reaction produces
a number of wires with roughly twice their usual diameter. The morphology of the as-deposited AgNWs as determined via
SEM is shown in Figure 2(a), higher magnification images are also provided in Figures 2(c) and 2(d).
7
The SnO2 nanoparticles were synthesized using a sol-gel method typical for multivalent metal oxide gelation reactions. A
large excess of deionized water was added to SnCl4·5H2O dissolved in ethylene glycol along with tetramethylammonium
chloride and ammonium acetate to act as surfactants. The reaction was then allowed to progress for at least one hour at near
reflux conditions, after which the resulting nanoparticle dispersion can be collected, washed, and dispersed in a polar solvent
of choice. The material properties of SnO2 nanoparticles formed using a similar synthesis method have been reported
previously [24], although the present recipe uses excess water to ensure that the hydrolysis reaction proceeds nearly to
completion.
After mixing with SnO2 nanoparticles, films deposited from AgNW/SnO2 composite inks show a largely continuous
nanoparticle layer on the substrate surface with some nanowires partially buried and some sitting more or less on top of the
film. Representative scanning electron microscopy (SEM) images of nanocomposite films are shown in Figure 2(b).
Regardless of their position relative to the SnO2 film, all nanowires show a distinct shell on their outer surface that gives
them a soft and slightly rough appearance, as is visible in the higher magnification images shown in Figure 2(e) and 2(f).
The SnO2 nanoparticles do a particularly good job coating the regions near and around junctions between wires, and
frequently appear in the SEM images as bulges wrapped around the wire/wire contact points.
The precise morphology of the SnO2 shell that effectively surrounded each AgNW was analyzed in more detail using
transmission electron microscopy (TEM) imaging. Figures 3(a) to 3(c) show individual nanowires in the presence of
different ligand systems: as-synthesized PVP in Figure 3(a), inactive SnO2 in Figure 3(b), and SnO2 activated with trace
amounts of ammonium ions in Figure 3(c). The as-synthesized nanowires show sharp edges, and few surface features. In the
presence of inactive SnO2, which is formed by repeatedly washing the SnO2 nanoparticles in ethanol until all traces of
ammonium ions are removed, the nanowires coexist with somewhat randomly distributed nanoparticles that deposit all over
the surface of the TEM grid. When AgNWs are mixed with activated SnO2, a thick and continuous SnO2 shell is formed
along the nanowire surface. In when sufficiently dilute SnO2 solutions are used to form the nanocomposite ink, nearly all of
the nanoparticles are consumed during shell formation and effectively no nanoparticles are left to randomly populate the rest
of the image.
As the AgNWs acquire their metal oxide coatings in solution, the properties of the mixture change dramatically. Freshly
synthesized AgNWs coated with residual PVP ligands slowly settle to the bottom of their vial or flask over a time period of
several hours to one day, forming a dense layer at the bottom. The AgNWs with SnO2 shells do not settle to the bottom, but
remain partially suspended even after many weeks at concentrations that are dependent on the amount of SnO2 present in the
solution.
A comparison of the settling behavior of various AgNW and SnO2 mixtures after 24 hours is shown in Figures 3(d) and
3(e). The ratios 8:4, 8:16, and 8:8 indicate the concentrations of AgNWs and SnO2 (in mg/mL) present in each solution. The
8:8 uncoupled solution, in which the PVP is not removed from the AgNW surface with ammonia, produces a situation in
which the nanowires and nanoparticles do not interact with one another, and instead the nanowires settle as in the isolated
nanowire solution while the nanoparticles remain well-dispersed as in the solution of pure SnO2. The mixtures of nanowires
and nanoparticles in which trace amounts of ammonia are present do not settle to the bottom, but instead concentrate
themselves until repulsion between the semiconducting SnO2 clusters is able to prevent further settling.
Our current explanation for the settling behavior of the wire/particle mixtures is that the PVP coating on the surface of the
as-synthesized wires is sufficient to prevent interaction with the nanoparticle solution. The addition of ammonia into the
solution quickly strips off the PVP surface coating and allowing the nanoparticles to coordinate directly with the nanowire
surface. This explanation is in agreement with the effects of ammonia has on a solution of pure AgNWs, which rapidly begin
to agglomerate into clusters and sink to the bottom as soon as any significant quantity of ammonia is added to the ink.
We attribute the stripping ability of ammonia in these mixtures to the strong dative interactions that
occur via the lone pair on the nitrogen atom interacting with the partially filled d-orbitals of the Ag atoms
on the nanowire surface. These interactions are evidently strong enough to displace the existing
coordination of the five-membered rings and carbonyl groups contained in the original PVP ligands and
allow the ammonia to attach directly to the nanowire surface. Since ammonia is one of the original
surfactants used to stabilize the surface of the SnO2 nanoparticles, we consider it reasonable that ammonia
coordination on the nanowire surface would provide an appropriate environment for the nanoparticles to
adhere to the AgNWs.
8
Scanning Energy Dispersive X-ray (EDX) Spectroscopy was also conducted on nanoparticle-coated AgNWs in order to
image the presence of Sn and Ag in the nanowire and shell layer. The line scan results are shown in Figure 3(f), having been
normalized to better compare the widths of the two signals. The visible broadening of the Sn lineshape compared to that of
Ag is indicative of a Sn layer along the outside of the wire. The increasing strength of the Sn signal toward the center of the
AgNW is likely due to the enhanced interaction between the TEM’s electron beam and the dense AgNW, which then
improves the signal originating from the SnO2 shell as well. It is also possible that there is some intermixing between the Ag
and Sn x-ray signals, but we consider this to be less likely as the distance between their characteristic peaks should be larger
than the detection system’s energy resolution.
2.2. Network Deposition and Device Applications
For the deposition of transparent conducting films, a weight ratio of 2:1 of AgNWs to SnO2 nanoparticles was chosen in
order to obtain a balance between the dispersibility of the nanowires, the uniformity of coated films, and the sheet resistance
of the resulting conductive networks. Nanocomposite films were deposited on glass by blade coating from an ethanolic
solution using a scotch tape spacer, with deposited networks then being allowed to dry naturally in air over several minutes.
The as-dried nanocomposite films are highly conductive, and require only minimal thermal treatment to dry and harden
the film. Without the use of activated SnO2 ligands, deposited nanowire networks are highly insulating, and become
conductive only after annealing at above 200 °C. The sheet resistance values of representative films are shown in Figure
4(a). The capability to form transparent conductive networks in a single deposition step that remain useful over a wide range
of processing temperatures provides a high degree of versatility for designing thin film device fabrication procedures.
Figure 5(a) shows the sheet resistance and transmission of a number of nanocomposite films deposited from inks
containing different nanowire concentrations. The deposited films show excellent conductivity at transmission values up to
85%, and then rapidly increase in sheet resistance as the network begins to reach its connectivity limit. The optimum
performance of these networks at low to moderate transmission values is a consequence of the relatively large nanowire
diameters, which scatter a noticeable amount of light even when the conditions required for current percolation are just
barely met. Nonetheless, the sheet resistance and transmission of the completed nanocomposite networks place them within
an acceptable range for applications in a variety of optoelectronic devices. Figure 5(b) shows the wavelength dependent
transmission spectra of several nanowire networks, which transmit light well out into the infrared region. The presence of
high transmission values out to wavelengths well above 1300 nm, where ITO or other conductive oxide layers would
typically begin to show parasitic absorption, is due to the use of semiconducting SnO2 ligands, which is complimentary to
the broad spectrum transmission of the silver nanowire network itself.
Avoiding the use of highly doped nanoparticles has the potential to provide optical advantages, but can create difficulties
when attempting to make electrical contact to neighboring device layers. In order to investigate their functionality in thin
film devices, we have incorporated AgNW/SnO2 nanocomposite films as electrodes in amorphous silicon (a-Si) solar cells.
Two contact structures were used during fabrication: one with the nanocomposite film directly in contact with the p-i-n
absorber structure and one with a 10 nm Al:ZnO (AZO) layer present to assist in forming Ohmic contact with the device.
The I-V characteristics of the resulting devices are shown in Figure 6(a).
The thin AZO contact layers typically show sheet resistance values greater than 2.5 kΩ/⧠, and so cannot be responsible
for long range lateral current transport within the electrode structure. However, their presence is clearly beneficial in
improving contact between the nanocomposite electrode and the absorber material, as the SnO2 matrix material is evidently
not conductive enough to form a high quality contact with the p-type side of the a-Si stack. We hope that future
modifications to the AgNW/SnO2 composite, or perhaps the use of islands of high conductivity material such as a
discontinuous layer of doped nanoparticles will allow for the deposition of completed electrode stacks that provide both
rapid fabrication and good performance.
Figure 6(b) contains the top view image of a completed device. The enhanced viscosity of the nanowire/sol-gel composite
inks allows for films to be blade coated onto substrates with a variety of surface properties without reductions in network
uniformity. In contrast with traditional back electrodes deposited in vacuum environments, the nanocomposite can be blade
coated into place in a single pass under atmospheric conditions and dried within moments. We anticipate that the use of
sol-gel mixtures to enhance wetting and dispersibility may prove useful in the formulation of other varieties of
semiconducting and metallic inks for deposition onto a variety of substrate structures.
3. Conclusions
In summary, we have successfully exchanged the insulating ligands that normally surround as-synthesized AgNWs with
shells of substantially more conductive SnO2 nanoparticles. The exchange of one set of ligands for the other is mediated by
9
the presence of ammonia during the mixing process, which appears to be necessary for the effective removal of the PVP
ligands that initially cover the nanowire surface. The resulting nanowire/nanoparticle mixtures allow for the deposition of
nanocomposite films that require no annealing or other post-treatments to function as high quality transparent conductors
with transmission and sheet resistance values of 85% and 10 Ω/⧠, respectively. Networks formed in this manner can be
deposited quickly and easily in open air, and have been demonstrated as an effective n-type electrode in a-Si solar cells when
a thin interfacial layer is deposited first to ensure good electronic contact with the rest of the device. The ligand management
strategy described here could potentially be useful in any number of material systems that presently suffer from highly
insulating materials that reside on the surface of otherwise high performance nano and microstructures.
4. Experimental Details
Tin oxide nanoparticle synthesis. Tin chloride pentahydrate was dissolved in ethylene glycol by
stirring for several hours at a concentration of 10 grams per 80 mL to serve as a stock solution. In a typical
synthesis reaction, 10 mL of the SnCl4·5H2O stock solution is added to a 100 mL flask and stirred at room
temperature. Still at room temperature, 250 mg ammonium acetate and 500 mg ammonium acetate were
added in powder form to regulate the solution pH and to serve as coordinating agents for the growing
oxide nanoparticles. 30 ml of water was then added, and the flask was heated to 90 °C for 1 to 2 hours in
an oil bath, during which the solution took on a cloudy white color. The gelled nanoparticles were then
washed twice in ethanol in order to keep trace amounts of ammonia present in the solution. Additional
washing cycles would deactivate the SnO2, and then require the addition of ammonia to coordinate with
as-synthesized AgNWs.
Silver nanowire synthesis. Copper(ii) chloride dihydrate was first dissolved in ethylene glycol at
1 mg/ml to serve as a stock solution for nanowire seed formation. 20 ml of ethylene glycol was then added
into a 100 ml flask, along with 200 µL of copper chloride solution. the mixture was then heated to 150 °C
while stirring at 325 rpm, and .35g of PVP (MW 55,000) was added. In a small separate flask, .25 grams of
silver nitrate was dissolved in 10 ml ethylene glycol by sonicating for approximately 2 minutes, similar to
the method described here.22 The silver nitrate solution was then injected into the larger flask over
approximately 15 minutes, and the reaction was allowed to progress for 2 hours. After the reaction had
reached completion, the various steps were repeated without cooling down. 200 µL of copper chloride
solution and .35g PVP were added in a similar manner to the first reaction cycle, and another .25g silver
nitrate were dissolved via ultrasonics and injected over 15 minutes. The second reaction cycle was allowed
to progress for another 2 hours, before the flask was cooled and the reaction products were collected and
washed three times in ethanol.
Nanocomposite ink formation. After the synthesis of the two types of nanostructures is complete,
10
the double washed SnO2 nanoparticles and triple-washed nanowires can be combined at a variety of weight
ratios to form the completed nanocomposite ink. The dispersibility of the mixture is improved when more
SnO2 is used, although the sheet resistance of the final networks will begin to increase if they contain
excessive SnO2. AgNW agglomeration during mixing is most easily avoided if the SnO2 and AgNW
solutions are first diluted to the range of 10 to 20 mg/ml in ethanol, with the SnO2 solution being added
first to an empty vial and the AgNW solution added afterwards. The dilute mixture was then be allowed to
settle overnight, and the excess solvent removed to concentrate the wires to a concentration that is
appropriate for blade coating.
Film and electrode deposition. The completed nanocomposite ink was deposited onto any desired
substrates using a razor blade and scotch tape spacer. The majority of the substrates used in this study were
Corning soda lime glass, but the combined inks also deposited well on silicon, SiO2, and any other
substrates tested. Electrode deposition onto a-Si substrates was accomplished by masking off the desired
cell area with tape, and then depositing over the entire region. The p-i-n a-Si stacks and 10 nm AZO
contact layers were deposited using PECVD and sputtering, respectively.
ACKNOWLEDGMENTS
The authors would like to acknowledge the use of the Electron Imaging Center for Nanomachines
(EICN) located in the California NanoSystems Institute at UCLA.
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Figure 1. Process flow diagram showing the synthesis of AgNWs and SnO2 nanoparticles followed
by stirring in the presence of ammonium salts to create the final nanocomposite ink. Transparent
conducting films were produced by blade coating the completed inks onto the desired substrate.
15
Figure 2. (a,c,d) SEM images of as-synthesized AgNWs at various magnifications. (b,e,f) SEM
images of nanocomposite films, showing the tendency of the SnO2 nanoparticles to coat the entire
outer surface of the AgNWs, increasing their apparent diameter and giving them a soft appearance.
16
Figure 3. Schematic diagrams and TEM images of (a) a single untreated AgNW, (b) an AgNW in the
presence of uncoupled SnO2 (all ammonium ions removed), and (c) an AgNW with a coordinating
SnO2 shell. Scale bars in images (a), (b), and (c) are 300 nm, 400 nm, and 600 nm, respectively. (d,e)
Optical images of AgNW and SnO2 nanoparticle dispersions mixed in varying amounts (d) before and
(e) after settling for 24 hours. The numbers associated with each solution represent the AgNW:SnO2
concentrations in mg/ml. The uncoupled solution contains AgNWs and non-coordinating SnO2
nanoparticles, and shows settling behavior similar to the pure AgNW and pure SnO2 solutions. (f)
Normalized Ag and Sn EDX signal mapped across the diameter of a single nanowire, with the inset
showing the scanning path across an isolated wire.
17
Figure 4. Sheet resistance versus temperature for films deposited using (red) AgNWs that have been
washed three times in ethanol and (blue) mixtures of AgNW and SnO2 with weight ratio of 2:1. The
annealing time at each temperature value was approximately 10 minutes. The large sheet resistance
values of the bare AgNWs when annealed below 200 °C is typical for nanowires fabricated using
copper chloride seeds, which clearly illustrate the impact of SnO2 coordination at low treatment
temperatures.
18
Figure 5. (a) Sheet resistance and transmission data for samples deposited from solutions of varying
nanostructure concentration. Each of these samples were fabricated starting from the same
nanocomposite ink, which was then diluted to a range of concentrations while maintaining the same
AgNW to SnO2 weight ratio. (b) Transmission spectra of several transparent conducting networks
chosen from the plot in plot (a).
19
Figure 6. (a) I-V characteristics of devices made with AgNW/SnO2 rear electrodes with (blue) and
without (red) a 10 nm AZO contact layer. The dramatic double diode effect is likely a result of a
significant barrier to charge injection at the electrode/a-Si interface. (b) Top view SEM image of the
AgNW/SnO2 composite films on top of the textured a-Si absorber. (c) Schematic cross section of the
a-Si device architecture used in solar cell fabrication. The thickness of the thin AZO contact layer is
exaggerated for clarity.
20