NANO: Nanotube Molecular Wires as Chemical Sensors
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Nanotube Molecular Wires as Chemical Sensors
Jing Kong, 1* Nathan R. Franklin, 1* Chongwu Zhou, 1 Michael
G. Chapline, 1 Shu Peng, 2 Kyeongjae Cho, 2 Hongjie Dai 1
Chemical sensors based on individual single-walled carbon nanotubes
(SWNTs) are demonstrated. Upon exposure to gaseous molecules such as
NO2 or NH3, the electrical resistance of a semiconducting SWNT is
found to dramatically increase or decrease. This serves as the basis
for nanotube molecular sensors. The nanotube sensors exhibit a fast
response and a substantially higher sensitivity than that of existing
solid-state sensors at room temperature. Sensor reversibility is
achieved by slow recovery under ambient conditions or by heating to
high temperatures. The interactions between molecular species and
SWNTs and the mechanisms of molecular sensing with nanotube molecular
wires are investigated.
1 Department of Chemistry,
2 Department of Mechanical Engineering, Stanford University, Stanford, CA
* These authors contributed equally to this work.
To whom correspondence should be addressed. E-mail: hdai@...
Carbon nanotubes are molecular-scale wires with high mechanical
stiffness and strength. A SWNT can be metallic, semiconducting, or
semimetallic, depending on its chirality (1). Utilization of these
properties has led to applications of individual nanotubes or
ensembles of nanotubes as scanning probes (2, 3), electron field
emission sources (4), actuators (5), and nanoelectronic devices
(6). Here, we report the realization of individual semiconducting-SWNT
(S-SWNT)-based chemical sensors capable of detecting small
concentrations of toxic gas molecules.
Sensing gas molecules is critical to environmental monitoring, control
of chemical processes, space missions, and agricultural and medical
applications (7). The detection of NO2, for instance, is important to
monitoring environmental pollution resulting from combustion or
automotive emissions (8). Detection of NH3 is needed in industrial,
medical, and living environments (9). Existing electrical sensor
materials include semiconducting metal oxides (7-9), silicon devices
(10, 11), organic materials (12, 13), and carbon black-polymer
composites (14). Semiconducting metal oxides have been widely used for
NO2 and NH3 detection (7-9). These sensors operate at high
temperatures (200� to 600�C) in order to achieve enhanced chemical
reactivity between molecules and the sensor materials for substantial
sensitivity (7). Conducting polymers (12) and organic phthalocyanine
semiconductors (12, 13) have also been investigated for NO2 or NH3
sensing. The former exhibit limited sensitivity (12), whereas the
latter tend to have very high resistivity (sample resistance of >10
gigohms) (13). In this report, we show that the electrical resistance
of individual semiconducting SWNTs change by up to three orders of
magnitude within several seconds of exposure to NO2 or NH3 molecules
at room temperature. Miniaturized chemical sensors based on individual
SWNTs are thus demonstrated. Furthermore, we combine theoretical
calculations with experiments to address the underlying fundamental
question regarding how molecular species interact with nanotubes and
affect their electrical properties.
Semiconducting SWNTs are chiral (m, n) tubes with m - n 3 �
integer. The band gap Eg of an S-SWNT scales with its diameter d as Eg
~ 1/d (Eg 0.5 eV for d ~ 1.4 nm) (1). It was previously found that
when two metal contacts were used to connect an S-SWNT, the
metal/S-SWNT/metal system exhibits p-type transistor characteristics
with several orders of magnitude change in conductance under various
gate voltages (6, 15, 16). Our nanotube chemical sensors were based on
these S-SWNT transistors, obtained by controlled chemical vapor
deposition growth of individual SWNTs from patterned catalyst islands
on SiO2/Si substrates (Fig. 1A) (16, 17). Gas-sensing experiments were
carried out by placing an S-SWNT sample in a sealed 500-ml glass flask
with electrical feedthrough and flowing diluted NO2 [2 to 200 parts
per million (ppm)] or NH3 (0.1 to 1%) in Ar or air (flow rate of 700
ml/min) through the flask while monitoring the resistance of the SWNT.
Fig. 1. Changes of electrical characteristics of a semiconducting SWNT
in chemical environments. (A) Atomic force microscopy image of a
metal/S-SWNT/metal sample used for the experiments. Nanotube diameter
is ~1.8 nm. The metal electrodes consist of 20-nm-thick Ni, with
60-nm-thick Au on top. (B) Current versus voltage curves recorded
before and after exposure to NH3. (C) Current versus voltage curves
recorded under Vg = +4 V, before and after NO2 exposure. [View Larger
Version of this Image (39K GIF file)]
We observed that the conductance of S-SWNT samples can be
substantially increased or decreased by exposure to NO2 or NH3. A
current versus voltage (I-V) curve recorded with an S-SWNT sample
after a 10-min exposure to NH3 showed an ~100-fold conductance
depletion (Fig. 1B). Exposure to NO2 molecules increased the
conductance of the SWNT sample by about three orders of magnitude
(Fig. 1C) when the SWNT sample was initially depleted by a back-gate
voltage (Vg) of +4 V (6, 15, 16). The SWNT is a hole-doped
semiconductor, as can be gleaned from the current versus gate voltage
(I-Vg) curve shown in Fig. 2 (middle curve), where the conductance of
the SWNT is observed to decrease by three orders of magnitude under
positive gate voltages (6, 15, 16). The I-Vg curve recorded after the
S-SWNT sample was exposed to NH3 exhibits a shift of 4 V (Fig. 2, left
curve). In contrast, the I-Vg curve was shifted by +4 V after NO2
exposure (Fig. 2, right curve). The low resistance (~360 kilohms) of
the SWNT under zero gate voltage suggests substantial hole carriers
existing in the p-type nanotube at room temperature. Exposure to NH3
effectively shifts the valence band of the nanotube away from the
Fermi level, resulting in hole depletion and reduced conductance. For
the NO2 case, exposure of the initially depleted sample to NO2
resulted in the nanotube Fermi level shifting closer to the valence
band. This caused enriched hole carriers in the nanotube and enhanced
sample conductance. These results show that molecular gating effects
are capable of shifting the Fermi level of S-SWNTs and modulating the
resistance of the sample by orders of magnitude.
Fig. 2. Chemical gating effects to the semiconducting SWNT. Current
versus gate voltage curves before NO2 (circles), after NO2
(triangles), and after NH3 (squares) exposures. The measurements with
NH3 and NO2 were carried out successively after sample recovery.
[View Larger Version of this Image (14K GIF file)]
The conductance of the SWNT sample increased sharply by about three
orders of magnitude after 200 ppm of NO2 was introduced (Fig. 3A). We
investigated five S-SWNT samples and found that the response times
(defined as time duration for resistance change by one order of
magnitude) of the samples to 200 ppm of NO2 were in the range of 2 to
10 s. The sensitivity [defined as the ratio between resistance after
(Rafter) and before (Rbefore) gas exposure] is ~100 to 1000. After the
NO2 flow was replaced by pure Ar, the conductance of the SWNT samples
was observed to slowly recover, and the typical recovery time was ~12
hours. This suggests slow molecular desorption from the nanotube
sample and that the SWNT chemical sensors can be reversibly
used. Heating the exposed sample in air at 200�C led to recovery in ~1
hour. For comparison, a high-performance metal oxide sensor (Cd-doped
SnO2) operates at 250�C for detecting 100 ppm of NO2 with a response
time of ~50 s, a recovery time of ~8 min, and a sensitivity of ~300
(8, 18). A polypyrole-conducting polymer sensor can detect 0.1% NO2 by
an ~10% resistance change in ~5 to 10 min at room temperature
(12). Thus, the S-SWNT sensors have the advantage of room temperature
operation with sensitivity up to 103 over these materials.
Fig. 3. Electrical response of a semiconducting SWNT to gas
molecules. (A) Conductance (under Vg = +4 V, in an initial insulating
state) versus time in a 200-ppm NO2 flow. (B) Data for a different
S-SWNT sample in 20- and 2-ppm NO2 flows. The two curves are shifted
along the time axis for clarity. (C) Conductance (Vg = 0, in an
initial conducting state) versus time recorded with the same S-SWNT
sample as in (A) in a flow of Ar containing 1% NH3. (D) Data recorded
with a different S-SWNT sample in a 0.1% NH3 flow. Read 3.5e-7, for
example, as 3.5 � 107. [View Larger Version of this Image (28K GIF
NH3-sensing results were obtained with the same SWNT sample after
recovery from NO2 detection (Fig. 3C). The conductance of the SWNT
sample was observed to decrease ~100-fold after exposure to a 1% NH3
flow. The response times to 1% NH3 for five S-SWNT samples were ~1 to
2 min, and the sensitivity was ~10 to 100. For comparison, metal oxide
NH3 sensors typically operate at 300� to 500�C, with a response time
of ~1 min and a sensitivity of ~1 to 100 toward 200 ppm to 1% NH3 (8).
Conducting polymer sensors can detect 1% NH3 with a response time of
~5 to 10 min by an ~30% resistance change at room temperature (12).
For the S-SWNT samples, lowering the NO2 concentration to 20 and 2 ppm
led to response times of ~0.5 to 1 min and ~5 min, respectively
(Fig. 3B). Lowering the concentration of NH3 to 0.1% led to a response
time of ~10 min (Fig. 3D). Thus, for detecting an ~10-fold resistance
change of individual S-SWNT samples within minutes of gas exposure,
the lower concentration limit is ~2 ppm for NO2 and ~0.1% for
NH3. Similar sensing results were obtained when Ar or air was used as
the carrier gas. This suggests that NH3 or NO2 dominates the response
of the SWNT samples over molecules in the ambient environment. Over
time, repeated sensing and recovery experiments with the S-SWNT
samples obtained reproducible results.
To understand the chemical gating effects and the nanotube gas-sensing
mechanism, we first considered the fact that S-SWNT samples appear to
be hole doped (p-type) before the molecular sensing experiments. Hole
doping in S-SWNTs has been observed by several groups (6, 15,
16). Possible hole-doping mechanisms include metal electrode-tube work
function mismatch (6) and electrostatic effects due to charged species
existing on the SiO2 surface or bulk (19). Because our nanotubes are
long (>3 �m), we suggested a hole-doping mechanism (for example,
charged chemical groups on SiO2) operating throughout the nanotube
length. As a result of the hole doping, the Fermi level of an S-SWNT
is typically located at ~25 meV above the valence band (19), which is
responsible for the observed conductance of S-SWNT samples at room
temperature (typical resistance of 300 kilohms to 5 megohms). Next, we
considered the chemical nature of the molecules. NO2 has an unpaired
electron and is known as a strong oxidizer. Upon NO2 adsorption,
charge transfer is likely to occur from an SWNT to NO2 because of the
electron-withdrawing power of the NO2 molecules. NH3, on the other
hand, is a Lewis base with a lone electron pair that can be donated to
other species. However, it is necessary to investigate whether these
qualitative pictures represent the correct mechanisms of molecular
sensing with SWNTs.
We carried out first-principles calculations on molecule-SWNT
complexes using density functional theory (20). NO2 is found to bind
with a semiconducting (10, 0) tube with an adsorption energy Ea ~ 0.9
eV (18.6 kcal/mol) and 0.1 electron charge transfer from the tube to a
NO2 molecule. Charge transfer from the nanotube to NO2 should be the
mechanism for increased hole carriers in an S-SWNT and enhanced
conductance. For the NH3-SWNT system, calculations found no binding
affinity between NH3 molecules and the (10, 0) tube. We suggest two
possible indirect routes through which NH3 molecules may affect
S-SWNTs. The first is that NH3 binds to hydroxyl groups on the SiO2
substrate (21), which could partially neutralize the negatively
charged groups on the SiO2 surface and lead to positive electrostatic
gating to the S-SWNT. Second, interactions may exist between NH3
molecules and an SWNT through other species. It was previously found
that NH3 can interact strongly with adsorbed oxygen species on
graphite (22). Preadsorbed oxygen species on a nanotube could interact
with NH3 and affect its electrical properties. These possible
mechanisms require further experimental and theoretical
We also investigated the electrical properties of metallic SWNTs in
various chemical environments. A metallic tube was identified by small
changes in the conductance with gate voltage (a typical resistance of
~20 to 200 kilohms) (16). We found that, for a typical metallic SWNT,
exposure to NO2 or NH3 increased or decreased, respectively, the
conductance of the sample by 30%. The explanation for these small
changes is that, for a metallic SWNT, small shifts of the Fermi level
do not result in a substantial change in the density of states at the
Fermi level and, thus, in the charge carriers in the nanotube.
The interactions between NO2 and NH3 with graphite have been
previously investigated (22-25). For SWNTs, molecular interaction
effects have been studied in the case of Br and I intercalation with
bulk samples of SWNT ropes (26, 27). The intercalation leads to
substantially enhanced sample conductance (26, 27). Our report is
concerned with molecular interactions with individual semiconducting
and metallic SWNTs. We have also investigated the effects of NO2 and
NH3 on the electrical properties of mats of SWNT ropes made from
as-grown laser ablation materials. In a 200-ppm NO2 flow, the
resistance of an SWNT mat is found to decrease from R = 150 to 80 ohms
(Rbefore/Rafter ~ 2) in ~10 min (Fig. 4A). In a 1% NH3 flow, the
resistance of a second SWNT mat increases from 120 to 170 ohms
(Rafter/Rbefore ~ 1.5) in ~10 min (Fig. 4B). In these bulk SWNT
samples, the molecular interaction effects are averaged over metallic
and semiconducting tubes. Also, the inner tubes in SWNT ropes are
blocked from interacting with NO2 and NH3 because the molecules are
not expected to intercalate into SWNT ropes. This explains the small
resistance change of bulk SWNT mats by gas exposure compared to that
of an individual S-SWNT.
Fig. 4. Electrical response of bulk SWNT mats to NO2 and NH3
molecules. (A) Conductance versus time data recorded with an SWNT mat
in 200 ppm of NO2. (B) Conductance versus time recorded with an SWNT
mat in 1% NH3. [View Larger Version of this Image (18K GIF file)]
The main feature of individual S-SWNT sensors, besides their small
sizes, is that they operate at room temperature with sensitivity as
high as 103. An individual nanotube sensor can be used to detect
different types of molecules. The selectivity is achieved by adjusting
the electrical gate to set the S-SWNT sample in an initial conducting
or insulating state. The fast response of a nanotube sensor can be
attributed to the full exposure of the nanotube surface area to
chemical environments. Thus, nanotube molecular wires should be
promising for advanced miniaturized chemical sensors.
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1 October 1999; accepted 24 November 1999