Accelerating Gas Adsorption on 3D Percolating Carbon Nanotubes

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T10074 Carbon Nanotubes in Composites

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Abstract In the field of electronic gas sensing, low-dimensional semiconductors such as single-walled carbon nanotubes (SWCNTs) can offer high detection sensitivity owing to their unprecedentedly large surface-to-volume ratio. The sensitivity and responsivity can further improve by increasing their areal density. Here, an accelerated gas adsorption is demonstrated by exploiting volumetric effects via dispersion of SWCNTs into a percolating three-dimensional (3D) network in a semiconducting polymer. The resultant semiconducting composite film is evaluated as a sensing membrane in field effect transistor (FET) sensors. In order to attain reproducible characteristics of the FET sensors, a pulsed-gate-bias measurement technique is adopted to eliminate current hysteresis and drift of sensing baseline. The rate of gas adsorption follows the Langmuir-type isotherm as a function of gas concentration and scales with film thickness. This rate is up to 5 times higher in the composite than only with an SWCNT network in the transistor channel, which in turn results in a 7-fold shorter time constant of adsorption with the composite. The description of gas adsorption developed in the present work is generic for all semiconductors and the demonstrated composite with 3D percolating SWCNTs dispersed in functional polymer represents a promising new type of material for advanced gas sensors.

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## OPEN

received: 07 December 2015

accepted: 20 January 2016

Published: 18 February 2016

# Accelerating Gas Adsorption on 3D Percolating Carbon Nanotubes

Hui Li 1,* , Chenyu Wen 1,* , Youwei Zhang 1 , Dongping Wu 1 , Shi-Li Zhang 2 & Zhi-Jun Qiu 1

In the field of electronic gas sensing, low-dimensional semiconductors such as single-walled carbon
nanotubes (SWCNTs) can offer high detection sensitivity owing to their unprecedentedly large surfaceto-volume ratio. The sensitivity and responsivity can further improve by increasing their areal density.
Here, an accelerated gas adsorption is demonstrated by exploiting volumetric effects via dispersion
of SWCNTs into a percolating three-dimensional (3D) network in a semiconducting polymer. The
resultant semiconducting composite film is evaluated as a sensing membrane in field effect transistor
(FET) sensors. In order to attain reproducible characteristics of the FET sensors, a pulsed-gate-bias
measurement technique is adopted to eliminate current hysteresis and drift of sensing baseline. The
rate of gas adsorption follows the Langmuir-type isotherm as a function of gas concentration and
scales with film thickness. This rate is up to 5 times higher in the composite than only with an SWCNT
network in the transistor channel, which in turn results in a 7-fold shorter time constant of adsorption
with the composite. The description of gas adsorption developed in the present work is generic for all
semiconductors and the demonstrated composite with 3D percolating SWCNTs dispersed in functional
polymer represents a promising new type of material for advanced gas sensors.

Nano-structured carbon materials, notably one-dimensional (1D) carbon nanotubes and two-dimensional (2D)
graphene, have been explored for their potential in chemical and biological detection applications 1–3 . Although
monolayer graphene, via Hall effects, has been reported to be able to detect single molecules 4, its intrinsic gapless
characteristic, and therefrom small current ratio of “on”-state to “off”-state current, inclines to defeat its detection
sensitivity in the field-effect transistor (FET) configuration 5 . In contrast, semiconducting single-walled carbon
nanotubes (SWCNTs) display orders of magnitude change in conductance upon electrostatic gating or doping 6 .
This characteristic makes them promising as active elements in the FET design for sensing. In early studies, FETs
each consisting of a single SWCNT in the channel were used as the model system for the study of adsorption processes and properties 7,8 . Compared to two-terminal sensing devices, the three-terminal FET sensors are advantageous in yielding multiple electrical responses upon exposure to a target gas. Such responses can be quantified
and their likely correlations found based on the well-established device physics, _e.g._ shift in threshold voltage ( _V_ t),
variations in drain current ( _I_ d), mobility, _etc_ . The advantage with multiple inputs can lend us the opportunity to
unveil the mechanism(s) in gas adsorption and desorption.

Despite great potentials, the realization of practical sensors based on individual SWCNTs awaits breakthroughs in material synthesis, fabrication process, and device reproducibility. An alternative is to use SWCNT
thin films where the electrical conduction occurs via the networked SWCNTs so as to alleviate fluctuations and
uncertainties caused by the aforementioned technological difficulties 9 . An important added advantage is that sensors made of SWCNT thin films can be processed with conventional microfabrication techniques 10 . Field-effect
gas sensors with randomly networked SWCNTs have shown the sub-ppm detection capability towards small gas
molecules such as NO2 and NH3 at room temperature 11,12 . Hence, this type of sensors remains highly attractive for
low-cost and disposable applications as opposed to using individual SWCNTs or monolayer graphene. Since sensitivity and responsivity can improve by incorporating and exposing more nanotubes to gas molecules, it is desirable to stack two-dimensional (2D) SWCNT networks to a three-dimensional (3D) architecture. This stacking
can increase the SWCNT areal density. However, SWCNT films obtained by means of facile solution-processing
often consist of nanotubes with a sub-monolayer surface coverage and conduct current via a 2D percolation
network 13 . A 3D structure cannot establish due to a so-called self-limiting mechanism where the first-adsorbed
SWCNTs tend to block further nanotube adsorption onto the substrate 14 . A general strategy is, therefore, to form

1State Key Laboratory of ASIC and System, School of Information Science and Technology, Fudan University,
Shanghai 200433, China. 2 Solid-State Electronics, The Ångström Laboratory, Uppsala University, Uppsala Box 534,
SE-751 21, Sweden. * These authors contributed equally to this work. Correspondence and requests for materials
[should be addressed to Z.-J.Q. (email: zjqiu@fudan.edu.cn)](mailto:zjqiu@fudan.edu.cn)

Figure 1. Basic device structure. ( a ) Schematic structure of a back-gate FET with an F8T2/SWCNT composite
film as the semiconductor channel. Random adsorption of gas molecules in the channel film is schematically
illustrated. Inset: the chemical structure of F8T2. ( b ) AFM image in height mode of the composite channel.
Scale bar, 1 μ m.

a 3D high-specific-surface mesoporous composite by dispersing the SWCNTs in a functional polymer matrix
of high gas permeability 15 . This approach is particularly appealing since the composite still retains the attractive
mechanical properties as well as the processing advantages of the polymer. It has, indeed, been found that the
SWCNT/polymer composite displays an improved sensitivity in comparison to using SWCNTs alone 16,17 . There
have been numerous reports on the development of gas sensors based on various SWCNT/polymer composites
processed in solution, with a primary focus on studying the equilibrium adsorption of gas molecules on the composites. Their adsorption isotherms and kinetics are, however, less investigated and remain poorly understood.

Here, we have fabricated high-fidelity FET sensors with a well-characterized composite-semiconductor for a
systematic gas adsorption study 18 . We will establish that assembling nano-sized sensing components into a 3D
sensing layer presents an advantageous and practical approach. The composite comprises the air-stable semiconducting π -conjugated poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) 19 and highly purified semiconducting
SWCNTs. These two materials share a similar aromatic structure and tend to hybridize due to their strong π -π
interaction 18,20 . With the SWCNT density exceeding the percolation threshold, there is a high likelihood that the
nanotubes will intersect with one another and form continuous electrical paths in the polymer, _i.e._ a 3D percolating conductive network 21 .

Results
A schematic representation of our SWCNT/F8T2-composite FET is depicted in Fig. 1a, with the Si substrate and
SiO2 layer as the gate electrode and gate dielectric, respectively. The device fabrication is detailed in Methods. The
semiconductor channel between the two Au electrodes ( _i.e._ the source and drain terminals) is typically 20 μ m
long and 200 μ m wide. A high density of nanotubes is well dispersed in the polymer as shown in the micro-image
of Fig. 1b obtained by means of atomic force microscopy (AFM). For sensor evaluation, NO2, one of the most
concerned air pollutants, is used as a test gas.

In characterizing carbon-based (SWCNT and graphene) FET sensors in the ambient environment, caution
must be exercised since a large hysteresis in _I_ d can appear when the gate voltage ( _V_ g) is swept forwardly and then
reversely 22,23 . The hysteresis in _I_ d represents an instability issue of the sensor device and can render the sensing
outcome invalid. This is especially true for detecting gas molecules at low concentrations where weak signals
can be marred by false variations due to device instability. It is well known that the adsorption of H2O/O2 molecules from the ambient environment induces hysteretic behaviors and irreproducible transport phenomena in
the SWCNTs and their FETs 24,25 . A simple N2 purging commonly used in gas sensing experiments is unable to
completely remove the adsorbed H2O/O2 molecules at room temperature. In order to reduce the cumulative hysteretic effects, a treatment in vacuum at high temperature for long duration is required 22 . But such a demanding
treatment can degrade the thermal stability of the polymer, and thereby complicating the sensing pursuit. Besides,
gas sensing is usually practiced in non-controlled environments as opposed to experimenting in the laboratories.

The typical _p_ -type transfer characteristics, _i.e. I_ d versus _V_ g, of an SWCNT/F8T2-composite FET operated in
dry N2 are depicted in Fig. 2a, at a constant drain bias _V_ d = - 1 V. A severe hysteresis in _I_ d is evident with two
distinct _V_ t values for the _I_ d- _V_ g curves when _V_ g was first continuously swept from − 20 to 20 V and then back to

- 20 V as depicted by one of the insets in Fig. 2a, _i.e._ in DC mode. The _V_ t uncertainty has severe consequences;
when the device was biased at _V_ g = 20 V and _V_ d = - 1 V, a substantial increase in the “off”-state current, _I_ d,off, of
the device by 3 orders of magnitude over about 300 s is found in Fig. 2b. This increase in _I_ d,off actually slows down
significantly beyond 200 s and the slowly-varying current level would conventionally be used as the baseline
for sensing. When exposing the FET to NO2, _I_ d,off drastically increases immediately. However, we will show that
such a wished-for response does not result from NO2 alone and it arises from a collaborative effort of both the
adsorbed NO2 molecules and the interfering H2O/O2 molecules.

In order to facilitate a quantitative study of the SWCNT/F8T2-composite FETs as a sensor device, a unique
electrical characterization procedure with _V_ g pulses of alternating polarities, _i.e._ AP mode, is employed to eliminate the _I_ d hysteresis 25–27 . In the AP mode, each positive pulse is followed by a negative pulse of equal amplitude,

Figure 2. Electrical characteristics and gas-sensing behavior. ( a ) Transfer characteristics of the 13-nm-thick
composite-FET with _V_ g sweeping in DC or AP mode, with the arrows indicating the sweep direction. Insets:
schematic of the DC and AP modes of _V_ g sweep. Dashed rectangle: effective detection window, EDW.
( b ) Electrical response of the composite sensor to 50 ppm NO2 in DC or AP mode, with the arrow indicating the
time point for the sensor to be exposed to NO2. In the DC mode _V_ g is fixed at 20 V, while in the AP mode the
train of the _V_ g pulses is + 20 V/− 20 V.

as illustrated by the other inset of Fig. 2a. The immediate application of a negative _V_ g pulse aims at depriving
away the cumulated charges during the positive _V_ g pulse; the reverse by exchanging the polarity or order of _V_ g
pulses is also operational. This practice with the AP mode has indeed resulted in stable electrical characteristics
with hysteresis-free transfer curves in Fig. 2a (i.e. the forward and reverse curves completely overlap) and a
horizontal baseline in Fig. 2b. The pulsed measurement allows, as expected, the current to be monitored without
interference from the ambient environment. Upon exposure to NO2, a much weaker response than that in the
DC mode is obtained in Fig. 2b. This comparison indicates that the environmental interference can complicate
the sensing process and make the sensing results unpredictable. Apparently, the use of the AP mode effectively
suppresses the charge transfer between the adsorbed H2O/O2 molecules and the SWCNTs without affecting that
between the adsorbed NO2 molecules and the SWCNTs. However, the NO2 adsorption on nanotubes represents
a complex process, which involves possible charge-transfer-mediated chemical reactions 28,29 . These reactions are
hardly affected by pulsing _V_ g as used in this study. Therefore, unless otherwise stated, all our gas-sensing measurements in the remainder of this work are performed in the AP mode. It is worth reminding that the competition
for adsorption sites by H2O, O2, and NO2 prevails even if the hysteresis originating from the adsorbed H2O/O2
molecules is suppressed by operating the sensor device in the AP mode. Since the density of the active adsorption
sites in the composite is invariant, no further adsorption can take place once a specific site is occupied. A lower
response in air than in N2 at the same NO2 concentration observed in Supplementary Fig. S1 indeed confirms that
the adsorption sites already occupied by O2/H2O molecules are not accessible to NO2. This observation motivates
the adsorption kinetics study (below) of NO2 in a 3D SWCNT percolating network dispersed in the F8T2 matrix,
all in N2 atmosphere.

The majority of recent reports 30–33, including theoretical studies, conclude that the charge transfer between the
adsorbed gas molecules and SWCNTs is the primary sensing mechanism. The charge transfer can lead to the
so-called molecular gating effect, _i.e._ the accumulated charge causes _V_ t to shift and the conductance to change,
and can eventually turn the transistor state from “off” to “on” or the other way around. However, the role of gas
adsorption in modulating the Schottky barrier at the SWCNT/metal contact as well as the consequences in gas
sensing remains to be clarified 34–36 . In an SWCNT network, a further sensing mechanism at work is the modulation of the energy barrier at the tube-tube junctions by adsorbed molecules 37–39 . Upon NO2 exposure, _I_ d,off initially
follows an exponential increase with time, _t_, and then tends to saturate for longer time, see Fig. 3a. This _I_ d,off- _t_
behavior is induced by the molecular gating effect, and it resembles very well the transfer _I_ d- _V_ g characteristics.
These two curves are plotted together in Fig. 3a with their subthreshold regimes overlapping. This practice leads
to a correlation between the electrostatic and molecular gating that can be exploited to quantify the adsorption
rate of NO2, _dN_ _dt_ , with _N_ as the areal density of occupied adsorption sites or adsorbed molecules. From the slope of

rate of NOthe time-response curve and that of the transfer characteristic curve in the semi-logarithmic scale, 2, _dN_ _dt_ , with _N_ as the areal density of occupied adsorption sites or adsorbed molecules. From the slope of _i.e._ _dt_

_d_ ( _V_ _g_ - _V_ _t_

and _d_ ( _V_ _g_ - _V_ _t_ )

_dt_
_d_ (log _Id_ )

and _d_ ( _V_ _g_ - _V_ _t_ ), respectively, the rate of _V_ t change during adsorption, _dV_ _t_, can be obtained for a constant _V_ g. Of the

first-order approximation, which assumes that the gas molecules are uniformly adsorbed in the composite chan- _d_ (log _Id_ ) _dt_
nel, the following is readily obtained,

_g_ - _V_ _t_ )

, respectively, the rate of _V_ t change during adsorption, _dV_

_dt_

_d_ (log _Id_

(log _Id_ )

_dt_

_d_

_Cox_ _dV_ _t_

_δdNdt_ = _Cox_ _dVdt_ _t_ (1)

_t_

_ox_ _dt_ (1

where, _C_ ox is the capacitance of the gate dielectric per unit area and _δ_ the amount of transferred charge per molecule. Both experimental work and theoretical calculations have shown that the electron transfer from SWCNTs to
NO2 is approximately 0.1 electrons per NO2 molecule 30,40,41 . With 100-nm-thick SiO2, _dN_ _dt_ for the SWCNT/

F8T2-composite sensors and the SWCNT-network-only reference is plotted as a function of NO2 concentration

Figure 3. Extraction of adsorption rate. ( a ) Overlap in the subthreshold regime of the transfer characteristics
_I_ d- _V_ g and the time-response characteristics _I_ d,off- _t_ of the 13-nm-thick composite-FET to 50 ppm NO2 exposure.
The train of the _V_ g pulses for the _I_ d,off- _t_ curve is + 8 V/− 8 V. ( b ) Dependence of _dN_ _dt_ on NO2 concentration for a

series of composite films of different thickness, _d_, along with an SWCNT-network film. Solid lines: fitting results
according to equation (2). The data points and error bars represent the average value and the standard deviation,
respectively, from six separate sensors of the same material. Inset: dependence of _β_ on _d_, solid line is the linear
curve fitting with a slope of 3.3× 10 11 cm −2 s −1 per nm.

in Fig. 3b. In both cases with composite and SWCNT-network, _dN_ _dt_

in Fig. 3b. In both cases with composite and SWCNT-network, _dN_ _dt_ increases with increasing NO2 concentration

and the rate is higher for the composite than for the SWCNT-network. Moreover, the gas adsorption in the composite sensors is dependent on the thickness of the composite films. In our work, the thickness of the composite
films is controlled by varying the dip-coating time. Longer dip-coating times yield thicker composite films and
the resulting sensors with larger _dN_ _dt_ , simply due to more SWCNTs in the films.

The dependence of _dN_ _dt_ on NO2 concentration is found to follow the Langmuir adsorption isotherm

equation 42 :

the resulting sensors with larger _dN_ _dt_ , simply due to more SWCNTs in the films.

The dependence of _dN_ _dt_ on NO2

_dN_ _βkC_

_dt_ 1 + _kC_ (2)

_kC_

_dt_

_β_
=

1 + _kC_ (2

by assuming a fixed density of independent absorption sites for the gas molecules to bind and no interaction
between the molecules. Here, _C_ is the NO2 concentration and _k_ the Langmuir constant, while _β_ represents _dN_ _dt_ at

between the molecules. Here, _C_ is the NO2 concentration and _k_ the Langmuir constant, while _β_ represents _dN_ _dt_ at

saturation. The excellent fit of the experimental data by equation (2) in Fig. 3b yields _β_ = 0.8, 1.3, 2.0, 2.7, and
3.6× 10 12 cm −2 s −1 for the SWCNT-network and composite films of 6, 8, 10, and 13 nm thickness, respectively. It is
remarkable that _dN_ _dt_ of the 13-nm thick composite film is almost 5 times that of the SWCNT-network. It is impor-

remarkable that _dN_ _dt_ of the 13-nm thick composite film is almost 5 times that of the SWCNT-network. It is impor-

tant to observe a linear relationship between _β_ and the thickness of the composite films, _d_ (inset in Fig. 3b), with
a slope of 3.3× 10 11 cm −2 s −1 per 1 nm. Higher SWCNT areal densities in thicker composite films are favorable in
increasing the adsorption rate and thereby the gas sensitivity.

Operating a sensor device in its subthreshold regime is generally considered advantageous due to an exponential dependence of _I_ d on _V_ t that can shift by gas adsorption 43 . Hence, this regime presents an effective detection
window, EDW, with high sensitivity, marked by the rectangle in Fig. 2a. When the device is biased in its “off” state
on the far right of EDW, _i.e._ at large positive _V_ g, _I_ d,off stays low close to the noise level even if gas molecules have
been adsorbed. This “off” state can change when the adsorbed charge is accumulated to a level sufficient to pull
the device into EDW. In practice, there will be a time delay from gas feeding to sensor response. In the AP mode,
a train of _V_ g pulses of alternating polarity with identical amplitude, _e.g._ + 20 V/− 20 V, is used to characterize gas
sensing. When a negative _V_ g pulse is applied, the device is in its “on” state while a positive _V_ g pulse sets the device
to its “off” state. Hence, the device is consecutively switched between the “on” and “off” states during its exposure
to the test gas NO2. The total gas exposure time used in this study is 200 s. As illustrated in Supplementary Fig. S2,
the device in its “on” state immediately responds to the gas feeding, while it in its “off” state exhibits a certain time
delay for both SWCNT-network and composite. This delay depends on the value of the positive _V_ g pulse relative
to EDW and is confirmed in Fig. 4a,b: the onset of the substantial increase in _I_ d,off is pushed to longer times for
larger positive _V_ g pulses. Concurrently, _I_ d,off tends to saturate at lower current levels because the adsorption sites
occupied by NO2 during the delay period do not contribute to the sensor response. At the extreme, the sensor
would not respond at all if _V_ g could assume an extremely large positive value without device breakdown. Hence,
a delay time for gas response, _t_ d, exists between the “on” and “off” states. During this time, _I_ d,off is unaffected by the
adsorbed gas molecules, but beyond _t_ d, it starts to respond. This feature is important as it can help understand the
adsorption kinetics. Furthermore, _t_ d can be accurately controlled by varying the _V_ g pulses. The dependence of _dN_ _dt_

adsorption kinetics. Furthermore, _t_ d can be accurately controlled by varying the _V_ g pulses. The dependence of _dN_ _dt_

on _t_ d for the SWCNT-network and composite is summarized in Fig. 4c,d, respectively. Increasing _t_ d leads to an
exponential decay of _dN_ _dt_ due to the reduction of the available adsorption sites, in accordance to the Langmuir

exponential decay of _dN_ _dt_ due to the reduction of the available adsorption sites, in accordance to the Langmuir

theory in which _N_ at any time can be described as 44 :

Figure 4. Adsorption kinetics in SWCNT-network and SWCNT/F8T2-composite. ( a,b ) Gas response of the
SWCNT-network and the 13-nm-thick SWCNT/F8T2-composite, respectively, with different _V_ g pulses. Only
positive _V_ g pulses are shown here. Black arrows: time point for the sensor to be exposed to NO2. Colored
arrows: the onset time points at which _I_ d,off starts to increase. Delay in time from gas feeding to sensor response
strongly dependent on _V_ g. ( c,d ) Dependence of _dN_ _dt_ on _t_ d for the SWCNT-network and SWCNT/F8T2-

composite, respectively. Solid lines: fits results according to the first derivative of equation (3).

_N_ = _A_ (1 - _e_ −/ _t_ _τ_
_N_ 0

_t_

= _A_ (1 - _e_ −/ _t_ _τ_ )

(3)

_A_ (1 - _e_ −/ _t_ _τ_ )

Here, _N_ 0 is the total areal density of the available adsorption sites, _A_ the amplitude of the response, and _τ_ the
time constant. The solid lines in Fig. 4c,d represent the fitting results of the experimental data points by the first
derivative of equation (3). The derived _τ_ is found independent of gas concentration, as expected, and it is about
4 s for the composite and 28 s for the SWCNT-network ( _i.e._ 7 times larger). It is, therefore, understood that the
composite is characterized by a more rapid response and higher sensitivity than the SWCNT-network does in a
field-effect sensing device. The demonstrated high gas-sensing performance with the SWCNT/F8T2-composite
FET sensor is primarily due to the formation of a 3D SWCNT percolating network in the F8T2 matrix. However,
other important roles of the polymer should not be overlooked since it can assist absorbing, concentrating, and
retaining the gas molecules around the dispersed SWCNTs 45,46 . As a result, the interaction between the adsorbed
gas molecules and nanotubes can be enhanced, apart from the polymer acting as the matrix for a uniform distribution of SWCNTs.

Discussion
Field-effect gas sensors with their sensing membrane placed at the surface are expected to generate the most
sensitive outcome. Operating such sensors in the ambient environment is, however, susceptible to interferences
from the inevitable adsorption of H2O/O2 molecules on the sensing membrane. The interaction in the form of
charge transfer between the sensing membrane and the H2O/O2 molecules has been found to cause a severe current hysteresis; hysteresis generally represents a specific type of device instability. The instability can be the cause
responsible for a false interpretation of sensing experiments. In this work, we have shown that this hysteresis, and
thereof drift of the sensing baseline, in the SWCNT/F8T2-composite based field-effect sensor can be effectively
suppressed by operating the device in the AP mode. This method allows for the establishment of an analytical
procedure for quantification of the gas adsorption via the unique subthreshold-regime detection window of the
FET sensors. Without the influence of hysteresis, we have further demonstrated that the adsorption rate, and
thereby the detection sensitivity, can be significantly enhanced by exploiting volumetric effects via the formation
of a 3D percolating SWCNT network in the polymer. The function of the gas-molecule-permeable polymer is
multiple and should not be overlooked. This system with the SWCNT/F8T2-composite has allowed us to explore
the adsorption isotherms and the kinetics of gas adsorption in detail, which is of great significance because, in
addition to gas-detection applications, the adsorption process plays an important role in many fields of surface
science ranging from material sciences to biochemistry and nanotechnology.

Methods
Solution processing and sensor fabrication. The F8T2 (Sigma-Aldrich) used as purchased was dissolved in toluene (Sigma-Aldrich) in a sonic bath. Highly purified 98% semiconducting SWCNTs (NanoIntegris)
were then ultrasonically added into the F8T2/toluene solution (0.1 mg mL −1 ) followed by centrifugation at
13300 g for 60 min. The supernatant (15 mL) was carefully decanted for the sensor fabrication. Highly doped
_p_ -type Si wafers with a 100-nm thermally grown SiO2 layer were used as the substrate. The SiO2/Si substrate
was pre-cleaned by, in this order, sonication in acetone, isopropyl alcohol, and deionized water. Conventional
electron-beam evaporation in combination with lift-off process was used to make an electrode pattern with a
Ti/Au bilayer of 5 nm/50 nm thickness. The channel area was defined using a second photoresist layer. For each
experiment, one chip was sliced out of the wafer and a thin film of the F8T2/SWCNT composite was deposited
by dip-coating the chip in the polymer solution. The thickness of the composite films is controlled by varying the
dip-coating time. The chip was then carefully pulled out and baked at 90 °C to remove the residual solvent. The
film formed outside the channel region was stripped off in acetone along with the photoresist. After blown-dry in
N2, a 10-min hot-plate baking at 90 °C completed the device fabrication. The film thickness was measured using
AFM (Dimension 3100 with Nanoscope IIIa controller, Veeco) operated in tapping mode under ambient conditions. For 2, 12, 24, and 48 h of dip-coating, composite films of 6, 8, 10, and 13 nm average thickness, respectively,
were obtained. All the processes were performed under ambient conditions. For reference SWCNT-network
FETs, the chip fabrication was identical to the process described above and the SWCNT conduction channel was
formed by dip-coating the chip in the SWCNT/surfactant solution.

Gas sensing measurements. The gas sensing experiment was performed in a homemade vacuum-sealed
stainless steel chamber equipped with a Keithley 4200 semiconductor parameter analyzer in a controlled atmosphere. The desired NO2 concentrations were attained by intermixing calibrated commercial NO2 with purified N2
at specific ratios using respective mass flow controllers (SLA5850E, Brooks). When exposed to different concentrations of NO2, the response was measured by monitoring the change in the current-voltage ( _I_ - _V_ ) characteristics
of the sensors. In the AP mode, the _V_ g pulse width is set to be 0.5 s. All sensing experiment was carried out at
room temperature.

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Acknowledgements
This work was supported partially by the National Natural Science Foundation of China (Nos. 61204090 and
61171010), Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education
of China (No. 708033), Fundamental Research Project of young teachers to enhance research capacity, Fudan
University (No. 20520133248), State Key Laboratory of ASIC & System, Fudan University (No. 2015MS005), and
the Swedish Research Council (No. 2014-5591).

Author Contributions
Z.-J.Q. conceived the research and analyzed the results. H.L. and Y.Z. carried out solution processing and device
fabrication. H.L., C.W. and D.W. performed gas sensing measurements. Z.-J.Q. and S.-L.Z. wrote the manuscript.
All authors discussed the results and revised the manuscript.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Li, H. _et al._ Accelerating Gas Adsorption on 3D Percolating Carbon Nanotubes.
_Sci. Rep._ 6, 21313; doi: 10.1038/srep21313 (2016).

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