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NANO: Nanotube Molecular Wires as Chemical Sensors

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  • Eugene Leitl
    (((ripped from ScienceMag))) Nanotube Molecular Wires as Chemical Sensors Jing Kong, 1* Nathan R. Franklin, 1* Chongwu Zhou, 1 Michael G. Chapline, 1 Shu Peng,
    Message 1 of 1 , Feb 1, 2000
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      (((ripped from ScienceMag)))

      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
      94305, USA.
      * 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
      file)]

      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
      investigations.

      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
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