1D Sensors

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CHAPTER 30

Gas Sensors Based on One-DimensionalNanostructures

C. H. Xu,1 S. Q. Shi,1 C. Surya2

1Department of Mechanic Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong2Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1. Why are 1D Nanostructure Gas Sensors Needed? . . . . . . . . . . . . . . . . . . . . . 2

1.2. What is a 1D Nanostructure Gas Sensor? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3. Performance Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. 1D Nanostructure Gas Sensors Based on Electrotransducers . . . . . . . . . . . . . . . . . . 3

2.1. Field Effect Transistors (FET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2. Electro Gas Sensors Manufactured by Microelectromechanical System

(MEMS) Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3. Resistive Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. 1D Nanostructure Sensors Based on Optical Transducers . . . . . . . . . . . . . . . . . . . . 9

3.1. Chemiluminescence (CL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2. Photoluminescence (PL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4. 1D Nanostructure Sensors Based on Quartz Crystal Microbalance Sensors . . . . . . . 10

5. Mechanism of 1D Nanostructure Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.1. Reaction Between Crystal Defects and Gases . . . . . . . . . . . . . . . . . . . . . . . . 12

5.2. Mechanism for 1D Nanostructure Gas Sensors . . . . . . . . . . . . . . . . . . . . . . 13

5.3. Electrical Sensitivity of 1D Nanostructure Materials to Manufacture

History and Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.4. Temperature Dependence of Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.5. Sensitivity–Concentration Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6. 1D Nanostructure Sensors Based on Composite Sensing Elements . . . . . . . . . . . . . 16

6.1. Double Metal Oxide Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6.2. 1D Nanostructure Composite Metal Oxide Gas Sensors . . . . . . . . . . . . . . . . 16

7. Summary and Further Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

ISBN: 1-58883-114-0Copyright � 2008 by American Scientific PublishersAll rights of reproduction in any form reserved.

Handbook of Nanoceramics and Their Based NanodevicesEdited by Tseung-Yuen Tseng and Hari Singh Nalwa

Volume XX: Pages (1–22)

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1. INTRODUCTION

1.1. Why are 1D Nanostructure Gas SensorsNeeded?

Modern industrialized society has brought a series of prob-lems to our world. Increasing industrialization makes it nec-essary to constantly monitor and control pollution in theenvironment, chemical factories, food processing plants,laboratories, hospitals, and technical installations in gen-eral. Nitrogen dioxide (NO2) gas, for example, is one of themost dangerous air pollutants; it contributes to the forma-tion of ozone and acid rain. A safe concentration of NO2 inthe air is set as 53 ppb by the U.S. Environmental Protec-tion Agency (EPA). Concentrations of NO2 in the airgreater than this limit may cause incidence of acute respira-tory illness [1]. Solid-state gas sensors play important rolesin monitoring pollution in the areas mentioned previously[2, 3]. Up to now, semiconducting ceramics, especiallymetal oxide sensors, have been widely deployed because oftheir small dimensions, low cost, and high compatibilitywith microelectronic processing. The most important typeof gas sensors work based on the change in the electricalconductivity of metal oxides due to the adsorption of gasmolecules on the surface of the materials. The semiconduc-tor metal oxides contain lattice defects due to the excess ordeficiency of metal or oxygen in the lattice. The associationof electrons with these defects following chemisorptionallows a certain change of the electrical conductivity of theoxide. For polycrystalline ceramic or thin film devices, onlya small fraction of the species adsorbed near the grainboundaries is active in modifying the electrical transportproperties. This results in the low sensitivity of the devicebecause of the limited surface-to-volume ratio [4]. More-over, most thin film devices operate at high temperatures(200–600�C) to achieve enhanced chemical reactivitybetween the sensor materials and the surrounding gases [5].Reported sensing limitations of metal oxide ceramics orthin film sensors are about 1 ppm or even greater [6–8],which cannot satisfy the requirements of detecting low con-centration gas standard, such as 53 ppb for NO2.

In recent years, one-dimensional (1D) nanostructurematerials such as nanotubes, nanowires, or nanobelts [9]have become an important field of research in both scienceand technology. To date, 1D nanostructures have beenproduced using various synthetic techniques such as vapor-phase evaporation [10], chemical vapor deposition [11],sol-gel [12], template-based method [13], arc discharge [14],laser ablation [15], solution [16], and thermal oxidation[17–19]. One-dimensional nanostructures have attractedmuch attention because of their great potential in practicalapplications. In particular, such structures are deemedespecially useful in solid-state gas sensors, with great poten-tial for overcoming the limitations encountered by metaloxide ceramics or thin film devices. Studies of the applica-tions of 1D nanostructure materials in gas sensing haverecently emerged. Gas sensors with a mesh structure ofnanobelts [20], ultraviolet (UV)-activated single nanowiresensors in bare [21] and single nanowire or multinanowiresin field effect configuration [22], have been reported. Com-pared with metal oxide ceramics or film ceramic sensors,

1D nanostructure gas sensors offer high sensitivity, lowdetected limitations, and low operating temperatures. Forexample, it is reported that 5 ppb NO2 can be detectedusing In2O3 nanowire gas sensors [23]. Moreover, the elec-tric properties of materials in nanoscale under differentgases can be detected, which offers information on sensingmechanisms.

1.2. What is a 1D Nanostructure Gas Sensor?

A gas sensor has two main parts: a gas sensing element anda transducer. When a gas molecule is adsorbed on the sur-face of the gas sensing element, it produces a response,which is translated into a measurable signal by a transducer.The change in the signal can be detected through a signalprocessor. Figure 1 shows the schematic diagram of a gassensor.

A 1D nanostructure gas sensor uses 1D nanostructurematerials as sensing elements. One type of the 1D structuresensing element is based on ceramics such as zinc oxide(ZnO), tin oxide (SnO2), indium oxide (In2O3), and tita-nium dioxide, or titania (TiO2). ZnO is an important n-typesemiconductor with a wide band gap energy (3.3 eV) and alarge exciting binding energy (60 meV) at room tempera-ture [24]. SnO2 is an n-type semiconductor with a room-temperature band gap of 3.6 eV and high achievable carrierconcentration (up to 6 3 1020 cm�3) [25]. In2O3 is knownto be an n-type semiconductor in its nonstoichiometric formdue to oxygen vacancy doping [26].

The gas has to be intimately connected to the sensing ele-ments. Gas adsorption on the surface occurs because theatoms or ions at the surface of the solid cannot fully satisfytheir valence or coordination requirements [27]. There aretwo types of adsorption: (a) physisorption, the weak attrac-tion of gas on a solid due to van der Waals forces, and (b)chemisorption, in which the gas becomes adsorbed on thesolid with high surface energy through an exchange of elec-trons with the surface, forming chemical bonds. The heat ofchemisorption is higher that of the physisorption. Generally,the heat of chemisorption is in the range of 15 to 200 kcal/mol, whereas physisorption is only up to 15 kcal/mol [27].

At present, most 1D nanostructure gas sensors studiedare constructed around electrotransducers. Some other 1Dnanostructure gas sensors have been reported whose opera-tion is based on changes in the photoluminescence spec-troscopy and the mass of the sensing element, which isaccurately measured using a quartz crystal microbalance.

Gas Sensor

Gas signal

sensing transducer signal element processor

Figure 1. Schematic diagram of a gas sensor.

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2 Gas Sensors Based on One-Dimensional Nanostructures


1.3. Performance Factors

The most important figures of merit for gas sensors are sensi-tivity, response time, minimum detectable gas concentration,repeatability (recovery method, recovery time), and selectivity.

Sensitivity: The ratio of measured parameters to sensitiv-ity is set to <1. The measured parameters after (orbefore) exposure to detected gas is divided by thatbefore (or after) exposure, depending on the relative val-ues of two parameters.

Response time: The time duration for the parameter tochange by a certain percentage of its original value.

The minimum detectable gas concentration: The lowestgas concentration that can be detected.

Repeatability: The gas sensors can be reused; thisincludes the recovery method and time.

Recovery method: The method used to allow the sensorto return to the original state after sensing gases.

Recovery time: The time duration for the parameter toreturn to its original value.

Selectivity: Whether the sensor can give a significant sig-nal for some specific gases.

2. 1D NANOSTRUCTURE GAS SENSORSBASED ON ELECTROTRANSDUCERS

The most promising area for nanotechnology is metal oxide1D nanostructures as gas sensors. Their large surface-to-volume ratio and their function as quasi-1D conductiveelements simultaneously contribute to high sensitivity. Thechange in the electrical conductivity caused by chemisorp-tion of gas molecules on the surface of 1D nanostructuremetal oxides is measured by electrotransducers. The mainstructures of the electrotransducers are field effect transis-tors, resistive gas microsensors, and resistive gas sensors.

2.1. Field Effect Transistors (FET)

2.1.1. Sensor Structure and FabricationTo fabricate 1D nanostructure FETs, 1D nanostructurematerials must be produced by a reliable technique [28].These 1D nanostructure materials in a solution such as iso-propyl alcohol are then sonicated into a suspension. Thesuspension is deposited onto a degenerately doped siliconwafer covered with 100–500 nm SiO2. Photolithography andmetallization of the source and drain electrodes areachieved using Ti/Au (50/200 nm) [29, 30] Ti or Au [31,33]. The back gate consists of a doped Si substrate or adoped Si substrate with a metallic layer [32]. The alterna-tive way of accomplishing electrical contact to the 1D nano-structure materials is achieved by depositing the suspensiondirectly on prefabricated electrodes [33]. The channellength between the two electrodes is 100 nm to 7 lm [33,30]. Figure 2(a) shows a FET fabricated using a single ZnOnanoribbon. Its cross-sectional structure and working prin-ciple are given in Figure 2(b).

2.1.2. Physical Characteristics of FETsTypical drain-source current IDS vs drain-source voltage VDS

characteristics are shown in Figure 3(a). The transfer char-acteristics of IDS versus gate voltage VG curves at VDS ¼ 2V are shown in Figure 3(b). The following physical proper-ties of FETs can be obtained from these curves.

Conductivity of nanowire: Calculation based on theIDS–VDS curve in Figure 3(a) and nanowire size.

Leakage current: The current IDS at VG ¼ �20 to �10Vin Figure 3(b).

N-type semiconductor: As VG increased, IDS in Figure3(a) increased.

Threshold voltage Vth: The VG to extrapolate a linearregion (dotted line) in Figure 3(b) to VG axis.

Current switching ratio: The large and small current onthe curve in Figure 3(b).

The gate capacitance CG: Estimation from Eq. (1) [34]:

CG ¼2pee0L

lnð2h=rÞ ð1Þ

where r and L are the nanowire radius and the length ofnanowire channel, e0 is permittivity in vacuum, and h ande are the thickness and the average dielectric constant ofthe device, respectively.

Electron density ne per unit length of nanowire: FromEq. (2) [35]:

ne ¼CG Vthj j

eLð2Þ

where e is the electronic charge.

Figure 2. (a) E-beam lithography fabricated field-effect transistor using asingle ZnO nanoribbon, and (b) the cross-sectional structure and workingprinciple of the FET device. Reprinted with permission from [31],Z. L. Wang, Adv. Mater. 15, 432 (2003). � 2003, Wiley-VCH.

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Gas Sensors Based on One-Dimensional Nanostructures 3


Transconductance gm: Calculation from the slope of thedashed line in Figure 3(b), according to Eq. (3) [35]:

gm ¼dIDS

dVGð3Þ

The mobility l of the transistor: Calculation from parame-ters in Figure 3(b), using Eq. (4) [34]:

l ¼ dIDS=dVg

ðCG=L2ÞVDSð4Þ

Subthreshold swing S: [35] assuming completely depletedcarriers (VG < Vth) [see Figure 3(b)],

s ¼ dVG

d lgjIDSjð5Þ

2.1.3. Sensor PropertiesThe FET devices are mounted in a sealed chamber withelectrical feedthrough and gas inlet/outlet during a mea-surement. Sensing experiments are carried out by monitor-ing the single nanowire conductance under different gasconcentrations [36, 37]. As an example, the sensor proper-ties of In2O3 nanowire FET to detect NO2 gas can bedrawn from Figure 4 [32]. The system is usually pumped tovacuum first to clean the surface of the nanowires, andthen the conductance of the nanowires is monitored while

Figure 3. (a) Typical characteristics of the IDS–VDS curves at differentgate voltages: from �7 to 3 V in 1 V steps as VDS varies. (b) The trans-fer characteristics with various VG at VDS ¼ 2 V. The transconductancewas inferred from the dashed line. Reprinted with permission from[35], Q. H. Li et al., Appl. Phys. Lett. 85, 6389 (2004). � 2004, Ameri-can Institute of Physics.

Figure 4. (a) IDS–VDS curves measured before and after exposure to100 ppm NO2 plus Ar. Inset: SEM image of a metal/In2O3-nanowire/metal device. (b) The IDS –VG curves before and after exposure to 100ppm NO2 plus Ar with VDS ¼ �0.3 V. (c) Response time of the In2O3

nanowire FET to various NO2 concentrations. Reprinted with permis-sion from [32], C. Li et al., Appl. Phys. Lett. 82, 1613 (2003). � 2003,American Institute of Physics.

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flowing diluted NO2 (0.5–100 ppm) in Ar. Figure 4(a)shows IDS–VDS curves recorded before and after exposingthe nanowire FET to 100 ppm NO2 plus Ar for 5 minutesat the gate voltage of 0 V. The shapes of IDS–VDS curvesrecorded before and after the exposure are significantly dif-ferent. The maximum change in IDS of the device occurs atVDS ¼ 0.3 V, at which the conductance of the In2O3 nano-wire FET reduces by six orders of magnitude after the expo-sure. If the ratio of currents before and after exposure toNO2 gas is defined as sensitivity, the sensitivity of the FETis about 10�6 from Figure 4(a). After each exposure, thesystem was pumped to vacuum, followed by UV illumina-tion to desorb the NO2 molecules. The device was fullyrecovered to its initial status immediately after the UV wasturned on.

The sensing properties of devices can also be studied bymonitoring the current dependence on the gate bias. Figure4(b) shows two IDS–VG curves recorded before and afterexposure to 100 ppm NO2 plus Ar with VDS ¼ �0.3 V. Thethreshold voltage shifted from �48 V before the exposureto 20 V after the exposure. The threshold voltage differenceof 68 V before and after exposure can also be regarded assensitivity to judge the sensor properties. The response toNO2 can be understood by considering the interactionbetween NO2 and the n-type doped In2O3 nanowires.Adsorption of oxidizing gases (such as NO2) reduces thenumber of free electrons in the nanowire and thus reducesthe conductivity.

Besides the sensitivity, another important parameter forgas sensors is the response time for the detected gas. Figure4(c) shows the response of an In2O3 nanowire FET to 100ppm NO2 plus Ar with VG ¼ 0 V and VDS ¼ 0.3 V. If theresponse time is defined as the time duration for the con-ductance to change by one order of magnitude, theresponse time for concentrations of 20, 2, and 0.5 ppm NO2

are 5 s, 5 min, and 10–12 min, respectively. The lowestdetectable concentration for NO2 is 0.5 ppm, as shown inFigure 4(c). A response time of 5 s for the In2O3 nanowireFET is significantly better than the response time of 50 sfor thin-film-based semiconducting oxide sensors operatingat elevated temperatures upon exposure to 100 ppmNO2 [38].

Besides the sensitivity, response time, the lowest detecta-ble concentration of the gas, and recovery time discussedpreviously, the advantages of FET sensors are the changesof electrons in nanowires, based on the change in the physi-cal properties in Section 2.1.2. For example, carrier concen-tration and mobility can change with various gases and atvarious gas concentrations. The estimation of the change ofcarrier contraction (DN) in a FET after the gas exposurecan be calculated as in Eq. (6);

DN ¼ CG DVthj jep r2L2

ð6Þ

where DVth is the shift of the threshold voltage in Figure4(b), CG is the capacitance of the nanowire with respect tothe back gate, e is the electronic charge, and r and L areradius and length of the nanowire channel, respectively.

Another advantage of FET type gas sensors is the possi-bility of the amplification of the signal through the controlof the gate voltage as shown in Figure 3(a). Table 1 lists asummary of the sensing properties of 1D structure FET gassensors. The parameters include the lowest detectable gasconcentrations (LDC), sensitivity, and response time, whichrelate to measured gas concentrations, recovery methods,and measurement temperature (T).

2.1.4. Recovery MethodsRepeatability is another important requirement for a gassensor. Gas is adsorbed on the surface of the nanowiresduring sensing. It is important for the gas sensor to returnto its original state after sensing. Some methods to refreshthe sensors have already been reported. UV illuminationresults in a jump in the conductance [39] by increasing thenet carrier density and decreasing the depletion width.Conductance will decrease when UV illumination is turnedoff. The reducing rate of conductance, however, is muchslower than the increasing rate [33]. Infrared (IR) is alsoused for recovery. The response of conductance to IR, how-ever, is slower than that to UV [39]. Heating sensors at anelevated temperature in inert gases such as N2 and Ar canbe used as a recovery method [40]. Sometimes air can beused as a recovery gas, depending on the sensitivity in the

Table 1. Summary of the sensing properties of 1D nanostructure FET sensors.

Nanowire Gas LDC SensitivityResponsetime (s) T (�C)

Recoverymethod/time Ref.

In2O3 NO2 0.5 ppm 106 5 25 UV/30 s [32]NH3 200 ppm 105 10

In2O3 NO2 0.02 ppm 25 UV [23]multiwires 0.005 ppm 600In2O3 NH3 10000 ppm 25 [86]ZnO NO2 1 ppm 25 �Vg/60 s [41]

NH3 5000 ppmZnO O2 920 Pa 25 �10�4 [35]ZnO O2 5 mPa 25 �10�6 mPa [33]SnO2 Ethanol 250 ppm 200 200 UV [31]

NO2 0.5 ppm 3000CO 5 ppm 70

SnO2 O2 5 3 10�4 torr 250 200 200 �10�5 [36]SnO2 Ethanol 4.7V% 15 >100 400 Ar [37]

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test [51]. The electric field applied over the back gate elec-trode of the ZnO nanowire FET significantly affects thesensitivity as it modulates the carrier concentration. Astrong negative field is used to refresh the sensors by anelectroadsorption mechanism [41]. A high vacuum environ-ment can also be used for deadsorption of gas [35].

2.1.5. Position of 1D NanostructureMaterials on the Electrodes

1D nanostructure FET gas sensors have some advantages,such as good sensitivity, as described in Sections 2.1.2 and2.1.3. The change in physical properties of the FETs underdifferent gases and concentrations can be used to investi-gate the sensing mechanism of gas sensors [35]. The fabri-cation of reliable contacts to the devices is a crucialtechnical issue in the manufacturing process of the sensors.

To increase placement yield, several groups investigatedthe placement of nanowires on selected positions from asuspended solution [42]. Nanowires can be assembled intoparallel arrays with control of the average separation bycombining fluidic alignment with surface-patterning techni-ques [43]. A direct placement method has been reported byapplying a DC or AC voltage bias [44, 45]. A solution con-taining nanowires is dropped on the top of the electrodearray. The nanowires are aligned along the direction ofelectric field. Different species such as a nanowire or con-taminant species will respond to different frequencies ofAC bias. Therefore, applying an AC bias with a correctivefrequency offers the advantage of selectively choosing nano-wires from other contaminant species in the solution [46].Solutions such as vertical field effect transistors [47], fieldemission growth [48], and self-assembled nanowires [49, 50]have been proposed.

2.2. Electro Gas Sensors Manufactured byMicroelectromechanical System (MEMS)Technology

Placing nanowires on electrodes is a time-consuming proc-ess in the manufacture of FET sensors. Other types of elec-trosensors based on MEMS techniques have been reportedrecently. A voltage is applied on the two electrodes duringmeasurement and the current or resistance is used as themeasurement signal.

2.2.1. E-beam Lithographic Nanowire-PatternsA SnO2 nanowire-pattern on a SiO2/Si substrate is fabri-cated by electron beam lithography and lift-off technique[51]. Figure 5(a) shows two Pt electrodes (right side) formeasuring the conductance of SnO2 nanowire-pattern anda heater element Pt to operate from room temperatures upto 500�C.

The measurement is performed by monitoring thechanges in the current or the resistance of the device. Fig-ure 5(b) shows the change in current for a thin film sensor,as indicated by dotted lines, and a nanowire-pattern sensor,as indicated by solid lines. The operating temperature waskept at 300�C and a relative humidity of 40%. Conductivityof both sensors increases when exposed to reducing gases

such as CO, ethanol, and acetone and decreases whenexposed to oxidizing gases such as NO2. The responses ofthe nanowire-pattern sensor are higher for all tested gases.For example, for acetone, an 80 nm wire-pattern sensorpresents a relative response (DG/G0) (where DG is the con-ductance variation and G0 the original value), increasingfrom 1.95 to 7.20 when acetone concentration ranges from10 up to 100 ppm, whereas the continuous film devicereports a maximum response of 1.62 [51].

2.2.2. Chemical Oxidation of Metal Ti toNanostructured Titania

Nanostructured titania (NST) arrays are also used for gassensors [52]. Sensing devices are fabricated as follows. A500 nm thick Ti film is first evaporated on a SiO2/Si wafer.

Figure 5. (a) Nanowire-pattern sensor device completed with circuitryelements; the nanowire pattern is in the middle of the sample (highmagnification in right bottom), whereas all around are the visibleheater element and parallel electrode contacts for SnO2 nanowires.(b) Functional characterization of 80 nm wire-pattern sensor (solidline) and continuous film sensor (dotted line). Reprinted with permis-sion from [51], P. Candeloro, et al., J. Vac. Sci. Technol. B 23, 2784(2005). � 2005, American Institute of Physics.

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Another SiO2 layer is then deposited on the Ti film. TheSiO2 layer on Ti film is subsequently etched, exposing Tipatterns, which are then oxidized in aqueous 10% hydrogenperoxide at 80�C to form NST arrays. After annealing at300�C, Ti/Pt electrodes are evaporated. The top view of theelectrodes on the NST arrays is shown in Figure 6(a). Thestructure of the sensor can be seen clearly on the cross sec-tion of the devices, as shown in Figure 6(b).

Electrical characterization indicates that contacts areohmic and nanostructured titania is highly sensitive to O2.Variations of hundreds of oxygen molecules over a padsensing element are detected at 250�C.

2.2.3. Drop of Nanowire PasteThis type of gas sensor is manufactured by combiningMEMS technology with dropping nanowires on the device[53, 54]. For example, a silicon-based membrane is preparedas follows: (1) Pt heater and the temperature sensors aresputtered on Si3N4/SiO2/Si substrates and patterned byliftoff methods; (2) an insulating SiON layer 1 mm thick is

deposited using plasma enhanced chemical vapor deposition(PECVD); (3) a Pt interdigitating electrode is prepared onthe SiON layer by sputtering; and (4) a window of 1.4 3

1.4 mm2 is formed through the backside etching of siliconwith KOH solution. The structure of the sensors is shown inFigure 7(a). The synthesized ZnO nanowires are ultrasoni-cally dispersed in ethanol for 30 min. The resulting ZnOnanowire paste is then deposited onto the silicon-based mem-brane by spin coating [see Figure 7(b)]. Finally, the sensorsare dried at 400�C for 1 h. Ethanol sensing characteristics ofthe sensor have been studied, demonstrating that ZnO nano-wire sensors exhibit a high sensitivity to ethanol gas and fastresponse time at operating temperatures of 300�C.

The sensing properties of electro gas sensors manufac-tured by MEMS technology are summarized [51–55] inTable 2.

2.3. Resistive Gas Sensors

2.3.1. Structure of Resistive SensorsThe channel length between the two electrodes for resistivesensors is usually in the millimeter scale. The typical struc-ture of a resistive sensor is illustrated in Figure 8(a) [56].A mixture of 1D nanostructure materials and ethanol isdeposited as a thin film by span coating across the two Auelectrodes on the outer surface of the alumina tube. Afterheating at 380�C in air for 4 h, a small NiACr alloy coil is

Figure 7. (a) Top-view SEM image of the fabricated substrate embed-ded with Pt interdigitating electrodes and Pt heater. (b) Schematicshowing the cross-section of the fabricated sensor. Reprinted with per-mission from [54], Q. Wan et al., Appl. Phys. Lett. 84, 3654 (2004).� 2004, American Institute of Physics.

Figure 6. (a) Optical micrograph of sensor with nanostructured titania(NST) arrays as sensing elements. (Inset) SEM image of a singlemetallized nanostructured pad. (b) Cross-sectional TEM of a nano-structured titania pad. Reprinted with permission from [52], A. S. Zur-uzi et al., Appl. Phys. Lett. 88, 102904 (2006). � 2006, AmericanInstitute of Physics.

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crossed through the alumina tube as a heater to producethe operating temperature by adjusting the heater voltageVh. The fabricated sensors are aged at 300�C for 96 h. Thetesting principle is schematically shown in Figure 8(b) [56].The heating voltage (Vh) is supplied to the coils for heatingthe sensors. The circuit voltage (Vc) is supplied across thesensors. The load resistor (RL) is connected to the sensorin series. The signal voltage (Vout) across the load is used asthe measurement signal, which will change depending onthe ambient gas and its concentration. Measurement canalso be set as in Figure 8(c). In this case, current (I) is usedas the detected signal. A given amount of gas such as SO2

is injected into the chamber by a microinjector. The sensi-tivity of the sensor can be measured when the detecting gasis mixed with air homogeneously.

A similar structure with a plane shape ceramic as sub-strate has been reported [57, 58]. For the fabrication ofsensors, the 1D nanostructure materials are dispersed interpineol, which is used as a binder to form pastes. Plati-num interdigitated electrodes are deposited with shadowmasking on an alumina substrate. The alumina plane with plat-inum electrodes is then dipped into the nanowire–terpineol

paste several times to obtain a gas sensing film. Then thedevice is annealed at 350�C for 1 h in an ambient atmosphereto evaporate the terpineol. Finally, the device and an Ni–Crheater is welded onto a BakeliteTM substrate.

2.3.2. Properties of Resistive SensorsVout or Rs can be used as detecting signals. Vout can bemeasured directly, as shown in Figure 8(b). Sensor resistor(Rs) can obtained from Eq. (7):

RS ¼Vc � Vout

VoutRL ð7Þ

Current through the sensor can also be detected directlywhen the measurement is set up as in Figure 8(c). In thiscase, current (I) can be used as the detecting signal.

The signal can be Vout, Rs, or I in Figure 8(b, c). Forexample, CO and H2 are measured using a polycrystallineSnO2 nanowire resistive sensor. Air is often used as recoverygas for the resistive sensors. The sensitivity of the sensor S ¼Rair/Rgas, where Rair is the resistance in atmospheric air andRgas is the resistance of the SnO2 nanowires in a CO (orH2)–air mixture. Figure 9 shows the changes in resistance ofthe sensor at room temperature under the exposure of 20ppm CO (dotted curve) and 500 ppm H2 (solid curve).Resistive sensors usually show fast recovery, which is nearlyequal to the response time. Compared with an FET sensoror a MEMS electrosensor, a resistive sensor contains morenanowires, which results in larger signals and faster recovery.

The sensing properties of resistive sensors [58–68, 82, 87,95] are summarized in Table 3.

3. 1D NANOSTRUCTURE SENSORSBASED ON OPTICAL TRANSDUCERS

Optical based sensors have been developed to overcomethe difficulty in forming a high quality contact to the nano-wire [69]. Optical gas sensors form another important andextensively studied class. Using chemiluminescence (CL)and photoluminescence (PL) for gas detection offers cer-tain advantages: remote sensing, easy reversion, and goodselectivity.

3.1. Chemiluminescence (CL)

An experiment setup for CL is illustrated in Figure 10(a),where a-Fe2O3 nanowire materials were put over the

Table 2. Summary of the sensing properties of 1D nanostructure MEMS electrosensors.

Nanowire Gas Concentration SensitivityResponsetime T (�C)

Recoverymethod/time Ref.

V2O5 amine 30 ppb 42 seconds 25 seconds [55]SnO2 CO 30 ppm 1.95–7.2 300 Air [51]

C2H5OH 10 ppmacetone 10 ppmNO2 0.4 ppm

WO2.72 NH3 100 ppm seconds 20–250 Air/minute [53]TiO2 O2 0.8mtorr 2–60 seconds 200–600 N2/seconds [52]ZnO C2H5OH 1–200 ppm 2–45 seconds 300 Air/seconds [54]

Figure 8. Schematic diagrams of (a) the structure of a resistive gassensor and (b) the working principle, Vh, Vc, Vout, and RL, which repre-sent the heating voltage, circuit voltage, signal voltage, and load resis-tor, respectively. Reprinted with permission from [56], G. Y. Zhanget al., Sens. Actuators B 114, 402 (2006). � 2006, Elsevier. (c) Theworking principle, A, is a current meter.

Page Number: 8



bottom of a quartz tube. A mixture of air and H2S waspassed through the quartz tube. At a certain temperature,H2S was oxidized at the surface of the a-Fe2O3 sample bythe oxygen in the flowing air. The change in CL intensitywas measured with a BPCL chemiluminescence analyzer.The gas flow rate and detection wavelength were set at200 mL/min and 400 nm, respectively. A typical responsecurve is shown in Figure 10(b). The sensor exhibited a verystrong CL intensity of about 2500 absorption units inresponse to 22 ppm H2S in the tube at 134�C [70]. This CLintensity is almost comparable to that of the a-Fe2O3 filmsensor in response to 100 ppm H2S at 360�C [71]. In addi-tion, the nanowire sensor has a short response time of 15 sand a recovery time of less than 100 s, from Figure 10(b).

3.2. Photoluminescence (PL)

The ZnSe nanowires were passivated in (NH4)2S solutionfor 60 min. For PL measurement, the nanowire sampleswere mounted in a small vacuum chamber with ports toallow gases in and out and optical access. The chamber wascyclically filled to 760 torr using gases of H2, N2, Ar and airor was pumped to below 10 m torr.

Figure 11(a) shows the PL spectra of the nanowires invacuum and in gases H2, air, Ar, or N2 at atmosphericpressure [72]. Under normal circumstances, the intensityof the near band edge emissions (NBE) peaks at 2.67 eV.The broad deep level emissions centered at 2.1 eVremain relatively constant. Compared with the spectral

Table 3. Summary of the sensing properties of 1D nanostructure resistive sensors.

Nanowire GasConcentration

(ppm) Sensitivity Response time T (�C)Recovery

method/time Ref.

ZnO CO 200 1.7 225 N2 [58]NO2 200 2 225H2S 100 1.4 100

ZnO C2H5OH 100 45 270 Air [87]ZnO C2H5OH 1000 40 Air [82]ZnO H2S 0.05 20, 350 3 min [95]

C2H5OH 1.7 3–20 mina-Fe2O3 H2 1.7 20 Air [59]

C2H5OH 7SnO2 C2H5OH 10 4.2 1 s 300 Air/1 [60]SnO2 CO 10 2 s 150 Air/10 min [61]

NO2 10 min 100 Air/10 minSnO2 polycrystals CO 20 4 minutes 25 Air [62]

H2 500 3 [63]C2H5OH 60000 >10000

SnO2 C2H5OH 20 22 50 s 300 [64]WO3 hollow spheres C2H5OH 50 2 �10–20 s 400 [65]

acetone, 50 3.5CS2, 50 1.56NH3, 50 1.5H2S, 10 21.8benzene, 500 2.56ether, 500 2.72CH3CN 500 3.18

In2O3 C2H5OH 100 27 10 s 370 Air/20 s [66]V2O5 C2H5OH 10 seconds 200 Air/seconds [67]Co3O4 H2 10 1.9 25 Air [68]

C2H5OH 10 1.7

Figure 9. Change in resistance for a resistive sensor based on polycrys-talline SnO2 nanowires: 20 ppm CO (dotted curve) and 500 ppm H2

(solid curve) under room temperature. Reprinted with permissionfrom [62], X. H. Jiang et al., J. Mater. Chem. 14, 695 (2004). � 2004,The Royal Society of Chemistry.

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peaks obtained in vacuum, exposure to H2 increases theNBE intensity, but exposure to N2, O2, and even Ardecreases it.

The time dependence of the changes in the intensity ofNBE peak in vacuum or various gases is shown in Figure11(b) [72]. The nanowires indicate fast response and recov-ery in the pressure of H2. The response to the increase inthe pressure of N2 and Ar is as fast as it is to H2, but theresponse to the decrease it is considerably slower. This sug-gests a fast response and slower recovery for N2 and Ar.After air is passed through the sample, the intensity ofNBE peak cannot return to the value in vacuum when thesystem was repumped to below 10 mtorr, suggesting theresponses to O2 are not reversible. Photodesorption andphotoadsorption of gases are found to affect the intensityof only the NBE peak, indicating the participation of freecarriers.

The sensing properties of 1D nanowire optical sensorsare listed in Table 4.

4. 1D NANOSTRUCTURE SENSORSBASED ON QUARTZ CRYSTALMICROBALANCE SENSORS

Quartz crystal microbalance (QCM) is an extremely sensi-tive measurement device. The principle of the measurementis based on mass change as the gas adsorbs on the surfaceof the sensing material. It can be measured quantitatively,according Eq. (9):

Df ¼ af 2Dm=A ð8Þ

Figure 11. (a) Comparisons of the PL spectra of ZnSe nanowires invacuum and in different gases at 760 torr and (b) response of theintensity of the NBE emissions to the cyclical changes of pressurebetween 10 mtorr and 760 torr in different gases. Reprinted with per-mission from [72], K. M. Ip et al., Nanotechnology 16, 1144 (2005). �2005, IOP Publishing.

Figure 10. (a) Schematic diagram of the CL sensor measurement sys-tem and (b) the CL response curve. Reprinted with permission from[71], Z. Y. Zhang et al., Sens. Actuators B 102, 155 (2004). � 2004,Elsevier.

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where a is a constant, ƒ is the fundamental frequency of theunloaded piezoelectric crystal, Dm is the mass loading onthe surface of the crystal, and A is the surface area of theelectrode.

Circular-shaped 25 MHz quartz crystals (consisting oftwo silver electrodes with a diameter of 4.5 mm and a thick-ness of �0.2 mm) were used to make QCM sensors [73].The materials for the sensing element were 1D nanostruc-ture ZnO with two different shapes. One was a multilegshape with the average length of 1 lm and a mean diameterof about 300 nm, used as sample I. The other was a nano-wire with a mean diameter of 30–40 nm and a length ofabout 2–5 lm, used as sample II. A 1D nanostructure ZnOin ethanol was deposited on the surface of the QCM elec-trodes using the spin-coating technique. The samples weredried at temperatures of less than 500�C. Humidity sensingexperiments were performed in the system shown inFigure 12(a). The various humidity levels were achievedusing anhydrous P2O5 and saturated aqueous solutions ofCaCl2, LiCl, MgCl2, NaBr, NaCl, KCl, and K2SO4 in a closedglass vessel at room temperature, which yielded approxi-mately 5.0%, 12.0%, 33.2%, 57.6%, 75.8%, 84.3%, and96.7% relative humidity (RH), respectively. The oscillationgeneration and frequency test of QCM were carried out usinga crystal impedance meter and a digital frequency counter.

Figure 12(b) shows the frequency shift (Df) curves of twoQCM humidity sensors coated with the different shapes ofZnO films as a function of the RH within the time intervalof 180 s. The results showed that the QCM sensor II coatedwith ZnO nanowires has a larger frequency response thanthat of the QCM sensor I coated with multileg ZnO. Theinset in Figure 12(b) shows curves of the sensitivity (Sƒ) vsRH for two QCM sensors with different morphology ZnOnanostructure coatings. Here, the frequency changing rateSƒ is defined as frequency sensitivity:

Sf ¼Df

Dt

��

��

ð9Þ

where Dƒ is the response frequency shift within the timeinterval of Dt. The sensitivity of sensor II is higher than thatof sensor I, concluding that thinner and longer 1D nano-structure ZnO can produce a larger response. The responseand recovery times can be found from the frequency shift,about 1.5 min and 2 min, respectively.

The sensing properties of 1D nanostructure OCM sen-sors [73, 74] are listed in Table 5.

Table 4. Summary of the sensing properties of 1D nanostructure optical sensors.

Nanowire Gas Concentration SensitivityResponsetime (s)

1.8 Recoverymethods/time T (�C) PL/CL Ref.

Fe2O3 H2S 22 ppm 2500 unit 15 Air/100 s 134 CL [70]Fe2O3 H2S 3 ppm 15 Air/120 s 360 CL [71]WO3

ZnSe H2 760 torr 5 10 mtorr 25 PL [72]N2 15 5 sAr 15 3 minAir 15 3 min

SnO2 NO2 1–10 ppm 1 25–120 PL [61]

Figure 12. (a) Schematic diagram of a humidity testing system and (b)frequency shift curves of two ZnO nanostructure coated QCM humid-ity sensors as a function of the relative humidity: (I) multileg ZnO; (II)ZnO nanowires. The corresponding insets are curves of the sensitivityvs relative humidity. Reprinted with permission from [73], Y. Zhanget al., Physica B 368, 94 (2005). � 2005, Elsevier.

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5. MECHANISM OF 1D NANOSTRUCTUREGAS SENSORS

Most of the 1D nanostructure gas sensors operate on thebasis of a change in electrical properties of the active ele-ment, which adsorbs gas molecules on the surface of thesensor. The discussions on the mechanism of gas sensors inthis section concentrate on electrotransducer sensors.

5.1. Reaction Between Crystal Defects andGases

An n-type semiconductor can be obtained either through anexcess of metal (for example, ZnO) or a deficit of nonmetal(for example, TiO2 and SnO2), as shown in Figure 13. Here

M and O represent metal and oxygen, respectively. To allowextra metal in the oxide, it is necessary to postulate the exis-tence of interstitial cations with an equivalent number ofelectrons in the conduction band. The structure of ZnO canbe represented as that shown in Figure 13(a) [89]. Here,M2þ is represented as possible occupiers of interstitial sites.Alternatively, n-type behavior of oxides can be caused bynonmetal deficit. This can be visualized as the discharge andsubsequent evaporation of an oxygen ion. The electronsenter the conduction band and a vacancy is created on theanion lattice in Figure 13(b).

For n-type oxide single crystals, the intrinsic carrier (oroxygen ion vacancy) concentration is determined primarilyby the deviation from stoichiometry in the form of equilib-rium interstitial metal ions (or oxygen ion vacancy), whichare predominantly atomic defects. The conduction electronsresulting from the point defects play a major role in the gassensing of the materials. Interstitial metal ion and oxygenvacancies on many oxide surfaces are electrically andchemically active. When interstices and vacancies are cre-ated, the electrons left behind are localized in states whoseenergies lie close to the conduction band and function asn-type donors. As a result, the interstices and vacancy crea-tion increase the conductivity of the oxide.

Oxygen adsorbs on the surface of oxides and ionizes toO�2 ads or O�ads in the formation of a depletion layer on theoxide surface under an oxygen environment first, accordingto Eq. (10) [75]:

O2 þ e� ¼ O�2 ads ð10Þ

Oxygen vacancies can be the best positions for oxygenadsorption. As a result, the adsorption of an electronacceptor lowers the conductivity of the n-type oxide.

The resistance of a metal oxide increases or decreasesdepending on the type of gas it is exposed to. If theadsorbed molecules are charged acceptors, they will act asreducing agents (such as CO or H2S). In the absence of anO2 environment, reducing gas molecules such as CO adsorbon the surface of an n-type oxide as COads. Gas moleculesinteracting with the surface of an oxide tend to bind at theoxygen vacancy sites [76]. Adsorbed COads reacts with theoxide, according to Eq. (11) [77]:

COads , COþads þ e� ð11Þ

In the presence of an oxygen environment, O�2 ads or O�adsare the main adsorbed species, and these form a depletionlayer on the oxide surface [77]. A reducing gas such asCO reacts with the adsorbed O�2 ads or O�ads species as inEq. (12) to change the thickness of the depletion layer [58]:

2COþO�2 ads ! 2CO2 þ e� ð12ÞFigure 13. The structure of an n-type oxide: (a) interstitial cations andexcess electrons and (b) oxygen ion vacancies and excess electrons.

Table 5. The sensing properties of 1D nanostructure OCM sensors.

Nanowire Gas Concentration Sensitivity Response time T (�C)Recoverytime Ref.

ZnO H2O 5%–97% RH 5–80 1.5 m room 2 min [73]ZnO ammonia 40–1000 (ppm) — 5 s room seconds [74]

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In both cases, the reducing action of the CO injects elec-trons into the conduction band, according to Eqs. (11) and(12), which results in the increase in conductivity.

Oxidizing gases such as NO2 react with a negativelycharged surface state and are thereby neutralizing. Thisreduces the depleting region surrounding the surface,resulting in an overall increase in the conductivity of thenanowire [58].

5.2. Mechanism for 1D NanostructureGas Sensors

Various gas sensing mechanisms in semiconductor oxidesensors have been suggested, including the desorption ofthe oxygen atoms that are adsorbed on the surface andgrain boundaries in polyoxides [78]. The exchange ofcharges between adsorbed gas species and the surface ofoxides lead to changes in depletion depth [79] and changesin the surface or grain boundary conduction by gas adsorp-tion/desorption [80]. In all of these cases, the mechanism issurface related. In addition to the previous mechanisms,some mechanisms are used to explain the sensing proper-ties of 1D nanostructure sensing elements.

5.2.1. Nanoparticles and NanowiresResistive sensors are fabricated using NiCo2O4, ZnCo2O4,and CuCo2O4 nanoparticles (20 nm) and nanotubes (poly-crystals with an outer diameter of approximately 200 nmand a wall thickness of about 20 nm). The sensor responseRair/Rgas, the ratio of the resistance in air and the resistancein detected gas, is measured in different gases at a concen-tration of 400 ppm and temperature of 300�C, as listed inTable 6. Although the surface area of nanoparticles is largerthan that of the nanotubes, the response of nanotube sen-sors is higher than that of nanoparticle sensors.

The Co3O4 nanotubes are synthesized via chemicaldecomposition of Co(NO3)2Æ6H2O within the aluminamembranes, whereas Co3O4 nanoparticles are prepared byball-milling the decomposition product of Co(NO3)2Æ6H2Oin Ar ambient at 500 rpm for 1 h. The average outer diame-ter of the nanotubes is about 200 nm and the wall thicknessis about 20�30 nm. The diameter of nanoparticles is about100 nm. The Co3O4 nanotubes and nanoparticles are thenused as sensing elements to make resistive sensors. Figure14 shows the sensitivities of two types of sensors usingnanotubes and nanoparticles as sensing elements to H2 andC2H5OH gases at room temperature. The Co3O4 nanotubes

exhibit superior gas sensing capabilities toward H2 andC2H5OH gases than the Co3O4 nanoparticles. The sensitiv-ity of nanowire and nanotube sensors is much higher thanthat of nanoparticle sensors. This has also been reported inother systems, La0.59Ca0.41CoO3 [81] and a-Fe2O3 [59] forH2 and ethanol.

Table 6. Responses of nanoparticle/nanotube sensors to various gases of 400 ppm at 300�C.

CuCo2O4 NiCo2O4 ZuCo2O4

Detected gas Nanoparticle Nanotube Nanoparticle Nanotube Nanoparticle Nanctube

CH3COOH 1.27 1.40 1.12 35.28 0.944 1.455SO2 1.61 41.65 — 3.78 1.26 2.83C2H5OH 1.38 1.11 1.12 7.63 1.00 6.17CO 1.08 8.37 1.00 1.61 1.35 1.69Cl2 1.39 2.72 1.49 1.50 1.27 2.48NO2 1.07 1.37 1.01 1.51 1.50 1.72

Source: Reprinted with permission from [56], G. Y. Zhang et al., Sens. Actuators B 114, 402 (2006). � 2006, Elsevier.

Figure 14. Sensitivity to (a) H2 and (b) C2H5OH of the sensors madeby CO3O4 nanotubes (triangles) and nanoparticles (squares) at roomtemperature. Reprinted with permission from [68], W. Y. Li et al., Adv.

Funct. Mater. 15, 851 (2005). � 2005, Wiley-VCH.

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5.2.2. Shape of NanowiresThe morphologies of nanowires can also lead to changes inthe sensitivity of nanosensors. T-ZnO and M-ZnO nano-wires are shown in Figures 15(a) and (b), respectively.These two types of nanowires are used as sensing elementsto make resistive sensors. Figure 15(c) shows the typicalresponse curves of the two kinds of ZnO nanowire sensorsto ethanol (1000 ppm) in air. For all the ZnO nanowiresensors, the gas sensitivity increases with increasing concen-tration of ethanol in air [82]. The M-ZnO nanowire sensorshave higher ethanol sensitivity than the T-ZnO nanowiredevices. The M-ZnO nanowire sensors have a fast responsetime, approximately 10 s, wherease the response time ofthe T-ZnO nanowire devices is approximately 16 s. Thisenhanced gas sensitivity of the M-ZnO nanowire sensorshas also been found in their responses to other gases. Forthe T-ZnO nanowires, the intensity of the UV emission ishigher than that of the green emissions, whereas for theM-ZnO nanowires, the intensity of the UV emission islower than that of the green emissions. The higher greenemission for M-ZnO is believed to be due to more oxygenion vacancies in the nanowires [83].

5.2.3. MechanismThe total energy of adsorption and reaction of molecularoxygen onAQ1 the ZnO (10�10) surface has been calculated byfirst principles [84]. The adsorption is fully molecular with asmall adsorption energy, and the dissociation is not ener-getically favored on stoichiometric ZnO (10�10) surfaces.On a partially reduced ZnO (10�10) surface, the dissociativeadsorption is energetically preferred. The dissociativeadsorption will influence the electronic properties of theZnO (10�10) surface.

The electronic structure of three-dimensional SnO2

nanostructures (aerogels) is studied by soft X-ray absorp-tion near-edge structure (XANES) spectroscopy. High-resolution O K-edge and Sn M3-edge and M4,5-edgeXANES spectra of nanocrystalline rutile SnO2 aerogelswith surface areas from 320 to 55 (m2/g) are compared withthe spectra of bulk rutile SnO2. The result shows that thepresence of under-coordinated surface atoms affects theposition of Fermi level and the structure of the conductionband by introducing additional Sn-related electronic statesclose to the conduction band minimum. These additionalstates arise from oxygen deficiency and are attributed tothe surface reconstruction of SnO2 nanoparticles formingthe aerogel skeleton [85].

Theoretical analysis suggests that that defects on the sur-face of an oxide are important for gas sensing. 1D nano-structure metal oxides, nanowires, and nanotubes are ableto improve the performance of chemical sensors signifi-cantly. The following explains this phenomenon: first, thelarge surface-to-volume ratio of the 1D nanostructure offerslarge areas for gas adsorption and reaction. Second, the 1Dnanostructure favors the carrier motion in two directions.Third, the hollow structure of the nanotubes is extremelyconvenient for gases to drift in and out. Finally, the defectsin 1D nanostructure facilitate the change of the electricalconductivity of the oxide. For n-type semiconducting metaloxides, the sensing mechanism is usually related to the

Figure 15. (a) SEM image of the T-ZnO nanowires. (b) SEM image ofthe M-ZnO nanowires. (c) Typical response curves of the two kinds ofZnO nanowire sensors to ethanol (1000 ppm) in air. Reprinted withpermission from [82], T. Gao and T. H. Wang, Appl. Phys. A 80, 1451(2005). � 2005, Springer.

Page Number: 14



depletion layer (its thickness, Ld) formed on their surfaceas electrons are trapped by adsorbed gas species such asO2. The electrical conductivity of nanocrystalline oxidesdepends strongly on the surface states produced by molecu-lar adsorption that result in space charge layer changes andband modulation. In addition to the surface-related mecha-nisms, bulk properties of the nanowire can contribute toconductance changes in the sensing of gases [93].

5.3. Electrical Sensitivity of 1D NanostructureMaterials to Manufacture History andEnvironments

Electrical sensitivity of nanowire gas sensors dependsstrongly on the history of the fabrication of nanowires. Forexample, In2O3 nanowires with different doping concentra-tions were investigated by examining the conductance of1D In2O3 FET gas sensors in NH3 gas environment. Theconductance of the FETs changes in an opposite directionwhen introducing NH3, because of the effect that doping inthe nanowire has on the density of oxygen vacancies alongthe nanowire sensors [86].

ZnO tetrapod samples prepared by evaporating highpurity Zn in various ambient gases (humidified Ar, dry Ar,humidified N2, and dry N2) are used as sensing elements tomake resistive sensors. The results of experimentation dem-onstrated that the sensor response was strongly dependenton the ambient of the deposition process. The sensorsbased on ZnO tetrapods prepared in a humidified Aratmosphere showed a high response, good selectivity, andshort response time to dilute C2H5OH [87].

The history of the fabrication of nanowires can influencethe defect type and concentration with a consequent effecton the sensing properties. ZnO with a wurtzite structure isnaturally an n-type semiconductor because of the deviationfrom stoichiometry. The deviation is considered as the pres-ence of intrinsic defects such as O vacancies (V 00O or V 0O)and Zn interstitials (Zn��i or Zn�i ). The equilibrium concen-tration of intrinsic defects on the surface and bulk oxidewill be a function of the environmental oxygen partial pres-sure and the temperature [88, 89]. Undoped ZnO showsintrinsic n-type conductivity with very high electron den-sities of about 1021 cm�3 [90]. In addition, defect concen-tration is strongly dependent on the impurity type andconcentration. Obtaining n-type doping of ZnO is relativelyeasy compared to p-type doping. Group III elements Al,Ga, and In as substitutional elements for Zn and group VIIelements Cl and I as substitutional elements for O can beused as n-type dopants [91]. P-type doping in ZnO may bepossible by substituting either group I elements (Li, Na,and K) for Zn sites or group V elements (N, P, and As) forO sites [24].

The conductivities of nanowire devices are also very sen-sitive to the annealing process, including temperature, time,and gases. The conductivity of SnO2 nanowire FETincreases significantly after the device has undergoneannealing in oxygen at 800�C for 2 h [28].

When a SnO2 nanowire FET is annealed at 200–250�C inair or vacuum, generally the conductivity of the devicedecreases after annealing in air and increases after annealing

in vacuum. The changes in conductivity after annealing arecaused by the variations in the number of oxygen speciesadsorbed on surfaces and by the change in the number ofoxygen vacancies on the material bulk due to different oxy-gen partial pressures in the environment. Annealing in vac-uum should decrease the number of adsorbed oxygen speciesand increase the number bulk oxygen vacancies whereasannealing in oxygen or in air should do the opposite. Theequilibrium concentration of intrinsic defects for n-typeoxides increases with the oxygen partial pressure [89]. Whena 1D nanostructure ZnO FET is annealed in hydrogen gas at400�C, the hydrogen-annealed devices are more conductingbecause the hydrogen increases the n-type doping in theZnO [92]. The hydrogen-annealed nanowire sensors wereinsensitive, however, to the measurement gases, such as N2,O2, H2, N2O, and C2H2. Compared with hydrogen-annealednanowires, unannealed samples show a strong sensitivity tomeasurement gases, even hydrogen gas [93]. One mightexpect the current to be dependent on this ambient gas if theconduction were truly dominated by the surface. Therefore,bulk properties of the nanowire may contribute to conduct-ance change in the sensing of gases [93].

5.4. Temperature Dependence ofGas Sensors

The temperature dependence of gas sensors has been stud-ied in detail [58, 66, 87]. The response of the sensor to thepresence of the reducing gases depends mainly on two fac-tors. The first is the density of active sites for oxygen andthe reducing gases on the surface of the sensor materials.The second is the reactivity of the reducing gases. Opera-tion temperature of a sensor is strongly dependent on thebond energy of the gas. The bond energy of HASH is 381kJ/mol [94], so it is easy to break the HASH bond at lowtemperatures. On the other hand, the bond energies ofHACH2, HAOC2H5, and HACH in C2H5OH are 473, 436,and 452 kJ/mol [94], respectively, so it is difficult to breakthe bonds in C2H5OH at low temperatures. Therefore, theZnO nanowire sensors show high responses to H2S at lowtemperatures and to C2H5OH at high temperatures [95].

The resistance variation with temperature (T) reflects anactivated process with the activation energy. The current-voltage IDS – VDS curves of In2O3 FET sensors are meas-ured in air as a function of temperature at 290, 180, 120,70, 30, and 10 K with 0 V applied to the gate electrode. Theconductance vs 1/T shows a linear relationship, which indi-cates that the transport through the device was dominatedby thermal activation of electrons across the metal-semicon-ductor. Thermal activation energy Ea was calculated as6.90 meV, according to the formula

G ¼ exp2Ea

kT

� �

ð13Þ

where G is conductance, k is Boltzmann’s constant, and Tis absolute temperature [30]. The I–V characteristics of sin-gle ZnO nanowires have also been measured as a functionof both temperature and ambient gas. The conductivity ofthe nanowires can be increased by a postgrowth anneal inhydrogen (400�C), and these nanowires show a thermally

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activated current. The apparent activation energy derivedfrom Eq. (13) is 89 6 0.02 meV. This does not correspondto any of the known donor dopant or native defect ioniza-tion energies in ZnO [96].

5.5. Sensitivity—Concentration Curves

The relationship between sensitivity (S) and concentration(C) of detected gases may obey a linear law, a power law,or saturation, as illustrated in Figures 16 (a, b, c),respectively.

The relationship between the concentration of target gasand the response of a sensor depends on several factors,such as gas type, the concentration range of target gas, testtemperature, and the defect density on n-type semiconduc-tors. If the concentration of active sites on the semiconduc-tor surface is much higher than the adsorbed gas, a linearrelation will be expected. If the active sites on the oxide sur-face are fully covered by the target gas, a saturation situa-tion will occur. Otherwise, the sensitivity vs concentrationof gas may show a power law relationship. Some mathemat-ical models have been used to describe these curves. It hasbeen demonstrated that the sensitivity of oxide semiconduc-tors is usually depicted as [54, 97]

S ¼ a C½ �N ð14Þ

where a denotes a constant and C is the concentration ofthe target gas. N is usually 1, indicating a linear law, or 1/2,indicating a power law, dependent on the charge of the sur-face species and the stoichiometry of the elementary reac-tions on the surface [98]. For thin films composed of SnO2

particles with a diameter of 6–20 nm, N is about 1. N isabout 0.5, however, when the diameter of SnO2 increases tolonger than 20 nm [99]. For example, ethanol (gas concen-tration set to less than 300 ppm) is measured at 300�Cusing a SnO2 resistive sensor with nanowires of diameter of6–20 nm as sensing elements. The sensitivity of the sensoris directly proportional to the concentration of ethanol gas(linear law) [60]. In this case, N is about 1. In a similarexperiment, the In2O3 nanowires had a diameter rangingfrom 60 to 160 nm. The ethanol (gas concentration rangeof 100–1000 ppm) was measured at 370�C. The relationshipbetween the response of the sensor and gas concentrationobeyed the power law [66]. In that case, N was about 1/2.

If the concentration range of the target gas is large, theresponse–concentration curve will be linear at low concen-trations and will tend to saturate with an increase in the gasconcentration, which can be described by the followingequation:

s ¼ a

1þ bC

ð15Þ

where a and b are experimental constants and C is the con-centration of the gas. The explanation is that the surfacecoverage of adsorbed molecules follows Langmuir isotherm[100].

6. 1D NANOSTRUCTURE SENSORSBASED ON COMPOSITE SENSINGELEMENTS

1D nanostructures of semiconducting metal oxides such asZnO, SnO2, TiO2, and WO3 have attracted great attentionbecause of the potential applications in gas sensors, humid-ity sensors, and nanoelectronic circuits. Nowadays, a greatdeal of effort is put into the study of double metal oxidessuch as ZnSnO3, 1D nanostructure composites such as 1Dnanostructure ZnOASnO2, and nanoparticle coated nano-wires such as Pd particles on SnO2 nanowires because oftheir novel properties in nanodevices.

6.1. Double Metal Oxide Gas Sensors

Bulk ZnSnO3 sensing elements have been suggested as hav-ing a higher sensitivity to ethanol gas than bulk ZnO orSnO2 sensors [101, 102]. The mixed valences of the cationsin these composite oxides may help the reversible adsorp-tion of gases by providing donor-acceptor sites for chemi-sorption [103]. The properties of double metal oxide1D nanostructure sensors [56, 81, 104] are summarized inTable 7.

6.2. 1D Nanostructure Composite MetalOxide Gas Sensors

Metal oxide sensing films have been doped with noble met-als (or metal oxides) to increase sensitivity, reduce response

(a)

Concentration (C)

(b)

Concentration (C)

(c)

Concentration (C)

Sens

itivi

ty (

S)

Sens

itivi

ty (

S)

Sens

itivi

ty (

S)

Figure 16. Illustration of the relations between gas concentration and responses of a sensor: (a) linear law, (b) power law and (c) saturation.

Page Number: 16



time, andAQ2 selectivity. There are two mechanisms of sensiti-zation by metal or metal oxide additives: chemical andelectronic sensitizations. The chemical sensitization is per-formed by a spillover effect [105], wherease the electronicsensitization is accomplished by the direct exchange of elec-trons between the semiconductor and the metal additives[106]. Dopants such as Pt or Pd are catalysts that promotechemical reactions between the film and the test gas byreducing the activation energy without being consumedthemselves [106]. This allows the reaction to occur at afaster rate, at lower temperature, and at lower gasconcentrations.

The 1D nanostructure composite materials can be one ofthe following. The doped metal (or oxide) can be nanopar-ticles on the surface of oxide nanowires; doped elements canbe impurities modifying the 1D nanostructure materials; ortwo different types of 1D nanostructure materials can bemixed to form a composite. 1D nanostructure compositematerials can be used as sensor elements to make sensors.The sensing properties of 1D nanostructure composite mate-rial gas sensors [57, 107–115] are listed in Table 8.

6.2.1. Chemical SensitizationThe selectivity of gas sensors and catalysts is usuallyachieved by functionalizing oxides with catalytically activemetals. Enhanced gas sensing by SnO2 nanowires function-alized with Pd catalyst particles has been reported [109].

Either Pd or Au is deposited on a SnO2 nanowire FET, asshown in Figure 17(a). The sensing properties of the FETwere measured in situ in at variable temperatures duringthe deposition.

The deposition of Pd or Au on the SnO2 nanowire wasmonitored during the deposition process by measuringdrain-source current, IDS. The change in IDS with increasingAu deposition time is shown in Figure 17(b). In the initialcluster nucleation stage, the drop in conductance impliesthe formation of depletion regions at the metal–semicon-ductor interface. Current is small in the stage of clustergrowth. The conductance dramatically increases in the stageof cluster percolation, because of shorting out the SnO2

nanowire.The sensing properties of a SnO2 nanowire were meas-

ured in oxygen and hydrogen pulses at temperatures of 443and 473 K before (dashed curves) and after (solid curves)Pd deposition, as shown in Figure 17(c). The SnO2 nano-wire surface with Pd shows larger IDS response for bothgases than the SnO2 nanowire.

The mechanism of the Pt function as a catalyst is illus-trated in Figure 17 (d)-(I). In the case of the absence of Ptparticles on the SnO2 nanowire, ionosorption of oxygenoccurs at defect sites of the nanowire surface, as illustratedin process (1); chemical catalysis of Pt occurs at process (2)and process (3). Oxygen dissociates on Pd nanoparticles fol-lowed by spillover onto the oxide surface (spillover effects)in process (2). In process (3), a Pd nanoparticle captures

Table 7. The sensing properties of 1D nanostructure double oxide sensors.

Nanowire GasConcentration(ppm) Sensitivity

Responsetime (s) T (�C)

Recoverymethod/time (s) Sensor Ref.

CuCO2O4 SO2 400 41 300 Air Resistive [56]NiCO2O4 CH3COOH 32ZnCO2O4 C2H5OH 6ZnSnO3 C2H5OH 1–500 2.7–42 1 300 Air/1 Resistive [104]La0.59Ca0.41 C2H5OH 10 2.7 350 Air Resistive [81]–CoO3 H2 10 1.7

Table 8. SensingAQ8 properties of 1D nanostructure composite gas sensors.

Materials GasConcentration

(ppm) SensitivityResponse

time T (�C)Recoverymethod/time (s) Sensor Ref.

SnO2 on MWNT LPG 100 4-7 1.8 s 20, 335 Air/100 Resistive [57]SnO2 on MWNT NO 2 1.9 300 Air Resistive [107]

NO2 2 1.77C2H5OH 10 1.83C2H2 10 1.5

SnO2 on NaY zeolite H2 500–4000 280 Resistive [108]CdS on SnO2 C2H5OH 100 90 400 Air Resistive [115]Pd on SO2 H2

O2

10�3 torr seconds 200 Vacuum FET [109]

PdO on ZnO LPG 500 19 10 s 332 Air/30 Resistive [110]3%Sb doped-SnO2 C2H5OH 10 25 5 s 300 Air/5 Resistive [111]0.9TiO2–0.1SnO2 H2 5000–20000 0.87 2 min 400 Air/420 Resistive [112]

0.95Zn2SnO4–SnO2 C2H5OH 200 8 seconds 250 Air/s Resistive [113]CuO–SnO2 H2S 3 5 seconds 25 Air/30 Resistive [114]

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weakly adsorbed O2, which has diffused along the SnO2

surface to the Pd nanoparticle’s vicinity (followed by proc-ess [2]). RS is the effective radius of the spillover zone, andRC is the radius of the collection zone. Figure 17(d)-(II)illustrates the band diagram of the pristine SnO2 nanostruc-ture and SnO2 in the vicinity of (and beneath) a Pdnanoparticle.

6.2.2. Electronic SensitizationThe structure of a SnO2 nanobelt is shown in Figure 18(a).The sonochemical synthesis of CdS nanoparticles has beendone in a neutral aqueous solution with the SnO2 nano-belts to fabricate a structure of SnO2 nanobelt/CdS

nanoparticles, as shown in Figure 18(b). The SnO2 nano-wires and the SnO2ACdS core/shell are used as sensing ele-ments to make resistive sensors. Figure 18(c) shows theresponse curves of the SnO2 nanobelt sensors and theSnO2ACdS core/shell heterostructured sensors to 100 ppmethanol vapor in air at an operating temperature of 400�C.The ethanol-sensing performance of the SnO2ACdS core/shell heterostructured sensors shows significant improve-ment compared to the SnO2 nanobelt sensor. Consideringthe efficient charge separations in SnO2ACdS core/shellheterostructured sensors, the CdS nanoparticles wouldserve as additional electron sources by the ‘‘electronic sensi-tization,’’ and can greatly improve electron conduction inSnO2 nanobelts [115].

Figure 17. (a) Schematic view of device used for the current measurements under gas exposure and metal deposition, (b) source-drain current,IDS, through a SnO2 nanowire (VDS ¼ 2 V) measured during Au deposition, (c) response of SnO2 nanowire (dashed line) and Pd-functionalized(solid line) nanostructure to sequential O2 and H2 pulses at 473 K (top panel) and 543 K (bottom), and (d) (I) schematic depiction of the threemajor processes taking place at a SnO2 nanowire/nanobelt surface: (1) ionosorption of oxygen at defect sites of the pristine surface; (2) mechanismof spillover effects; (3) mechanism of back-spillover effects, and (II) band diagram of the pristine SnO2 nanostructure and SnO2 in the vicinity of(and beneath) a Pd nanoparticle. Reprinted with permission from [109], A. Kolmakov et al., Nano Lett. 5, 667 (2005). � 2005, American ChemicalSociety.

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PdO doped ZnO nanowires are used as sensing materialsin the manufacture of a single nanowire sensor. The PdOadditive to ZnO nanowires acts as a strong acceptor of elec-trons from the oxide, and induces an enlarged surface spacecharge layer, which results in depletion of electrons nearthe interface. When the PdO additive is reduced on contactwith the target gas, it relaxes the space charge layer by giv-ing back electrons to the oxide. Such a change in the oxida-tion state of the additive is responsible for the promotionof the gas response. Such results agree with the electronicsensitization model [110].

The use of SnO2 nanoparticles on carbon nanotubes hasbeen studied as gas sensors. The resistances of this kind ofsensor are much lower than those of the SnO2 nanowiresensors. The sensor resistance is dominated by the barriersbetween the SnO2 grains on the multiwall carbon nano-tubes (MWNTs), and the barrier height is controlled by theadsorptive gas molecules, which extract or release electronsto produce the sensing response [107].

Both Sb-doped SnO2 nanowires and SnO2 nanowires areused as sensing elements to fabricate resistive sensors. Theresistance change of the Sb-doped SnO2 nanowires is signif-icant when the ambient gas is changed. The response and

recovery times to 10 ppm ethanol are approximately only1 and 5 s, respectively, for the Sb-doped SnO2 nanowire gassensors [111], whereas the recovery time of pure SnO2

nanowire sensors to ethanol was longer than 10 min. Thus,Sb doping can greatly reduce the recovery time of SnO2

nanostructure gas sensors to ethanol [20]. The possiblemechanism is that Sb doping favors or accelerates theadsorption of oxygen molecules and the formation of O2�

ions on the surface of SnO2 nanowires, which is of signifi-cance in the reduction of the recovery times.

6.2.3. p—n Junction Between 1DNanostructure Materials

The SnO2 (nanowires) and 4mol% CuO (nanoparticles)can be mixed mechanically to perform as sensing materialsto make resistive sensors. The sensitivity of the sensor to3 ppm H2S is as high as 18,000 with a response time of 15 sand recovery time of more than 450 s at room temperature.The mechanism of the reaction of a SnO2ACuO sensor toH2S gas is explained by the formation or distortion of p-njunctions between the SnO2 and CuO interface before (air)and after (H2S) exposure, as shown in Figure 19. CuO andSnO2 are p- and n-type semiconductors, respectively. It iseasy to form p–n junctions at CuOASnO2 interfaces.Numerous p–n junctions at CuOASnO2 interfaces causehigh resistance in air, because of the one direction conduc-tion of p-n junctions. On exposure to H2S gas, CuO par-ticles are rapidly converted to CuS by the followingchemical reaction:

CuOþH2S! CuSþH2O ð16Þ

CuS is metallic in nature and its formation shorts out thep–n junctions existing on the intersurface, causing a largedecrease in electrical resistance. The formation of CuS hasbeen confirmed by X-ray photoelectron, X-ray diffraction,and Raman spectroscopic studies [116]. On the other hand,CuS will be oxidized in air and will change back to CuOreversibly through the following reaction:

CuSþ 3

2O2 ! CuOþ SO2 ð17Þ

These two reactive processes dominate the response andrecovery of the CuOASnO2 sensors. Reaction rates forEq. (16) are much higher than those for Eq. (17) at low

Figure 19. Schematic showing the CuOASnO2 nanoparticle/nanorib-bon sensor. Reprinted with permission from [114], X. H. Kong et al.,Sens. Actuators B 105, 449 (2005). � 2005, Elsevier Science SA.

Figure 18. (a) TEM image of the uncoated SnO2 nanobelts. Insetshows the corresponding SAED patterns. (b) General morphology ofthe SnO2 nanobelt/CdS nanoparticle core/shell heterostructures. Insetshows the corresponding SAED patterns. (c) Response curves of theSnO2 nanobelt sensors and the SnO2–CdS core/shell heterostructuredsensors to 100 ppm ethanol vapor in air at a working temperature of400�C. Reprinted with permission from [115], T. Gao and T. H. Wang,Chem. Commun. 22, 2558 (2004). � 2004, The Royal Society of Chemistry.

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temperatures, so the CuOASnO2 sensors have a largeresponse at low temperatures. It must be pointed out thatthe crystal structure of CuS is changeable at 103�C andbecomes Cu2S, an ionic conductor with higher resistivity,when the temperature is >220�C [117]. Finally, 1D nano-structure CuOASnO2 composite gas sensors have goodselectivity to H2S gas.

7. SUMMARY AND FURTHER STUDYCompared with bulk or film ceramic sensors, 1D nanostruc-ture gas sensors offer high sensitivity, low detection limita-tions, and low operating temperatures. In addition, 1Dnanostructure gas sensors also provide a platform to investi-gate the relationship between electrical transport propertieswith dimensionality size in various gas environments. Atpresent, 1D nanostructure gas sensors can be constructedaround electrotransducers, optical transducers, and quartzcrystal microbalance transducers.

1D nanostructure gas sensors using electrotransducerswork based on the change in the electrical conductivity ofmetal oxides due to chemisorption of gas molecules on thesurface of the materials. Among these sensors, 1D nano-structure FET gas sensors have advantages such as goodsensitivity and the possibility of the amplification of the sig-nal through the control of the gate voltage. The electricaltransport properties of 1D nanostructure materials in vari-ous gas environments can be identified because of theknown size of the nanowires, so 1D nanostructure FET gassensors often are used to investigate the sensing mecha-nism. The fabrication of reliable contacts to the devices is acrucial technical issue in the manufacturing process of FETsensors. For resistive sensors, the size of the sensing ele-ment is usually in the millimeter scale. Compared with anFET sensor, a resistive sensor contains more nanowires,which results in large signals and fast recovery. Therefore,air is often used as a recovery gas. Moreover, the resistivesensors are easy to fabricate and cheap because of largerdetected signals. Therefore, the resistive sensors are suita-ble for practical applications. The span coating technique isusually used to deposit the mixture of 1D nanostructurematerials and ethanol on electrodes to fabricate a resistivesensor. As mentioned, a good connection between electro-des and 1D nanostructure materials is a serious issue forresistive sensors. Some electrosensors based on MEMStechniques, such as e-beam lithographic nanowire-patternsand chemical oxidation of metal Ti to nanostructured tita-nia, have been reported recently; both of these can providegood connections between electrodes and 1D nanostructuresensing materials.

Optical based sensors have been developed to overcomethe difficulty in forming high quality contacts to the nano-wires [118]. Optical gas sensors offer advantages such asremote sensing, easily reversible changes, and high selectivityfor some gas species. 1D nanostructure optical sensors aresuitable for gas measurement in dangerous environments.

The quartz crystal microbalance (QCM) is an extremelysensitive measurement device for mass. The sensing signalis directly proportional to the mass change as the gasadsorbs on the surface of the sensing material. Compared

with the sensors based on the electrotransducers, bothchemisorption and physisorption of gas molecules can occurduring the measurement process of 1D nanostructure sens-ing QCM sensors. Therefore, 1D nanostructure sensingQCM sensors are also suitable for detecting the physisorp-tion of gases.

Although promising gas sensing results from the per-formance of 1D nanostructure metal oxides have beenreported, studies of the applications of nanowires in gassensors are still in the preliminary stages. Techniques tomaintain good connections between electrodes and 1Dnanostructure materials and to obtain stable 1D nanostruc-ture sensing materials need to be improved. The mecha-nism of improving the gas sensitivity of 1D nanostructureshas been unclear up to now. Developing highly selectiveand controllably sensitized devices remains a future chal-lenge for 1D nanostructure oxide gas sensors.

ACKNOWLEDGMENTThis work was supported by a research grant from theResearch Grants Council of Hong Kong (Project no. PolyU5236/03E).

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Author Queries

AQ1: Is this correct, or should it be (100)?AQ2: Is the sensitivity changed in my way? e.g., increase sensitivity?AQ3: Please provide publisher and publisher location, and page range.AQ4: Please provide publisher name and location, and page range.AQ5: Who is the publisher?AQ6: What is the location of the publisher?AQ7: Should there be a gas for this LDC in the second column?AQ8: Please provide the missing datas for column head "Response time".

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