Recent progress in the ZnO nanostructure-based sensors

Materials Science and Engineering B 176 (2011) 1409–1421

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Review

Recent progress in the ZnO nanostructure-based sensors Ang Wei, Liuhua Pan, Wei Huang ∗ Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NUPT), Nanjing 210046, China

a r t i c l e

i n f o

Article history: Received 1 February 2011 Received in revised form 17 July 2011 Accepted 4 September 2011 Available online 17 September 2011

a b s t r a c t This review focuses on the sensors based on zinc oxide (ZnO) nanostructures, which have fascinating properties including large specific surface area, good biocompatibility, high electron mobility and piezoelectricity. Due to these versatile characteristics, ZnO nanostructures can be based upon to construct gas sensors, chemical sensors, biosensors, UV sensors, pH sensors and other sensors with different sensing mechanisms. The main structures of the sensors and factors influencing the sensitivity are also discussed.

Keywords: ZnO Nanostructures Sensor

© 2011 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 The parameters of sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 2.1. Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 2.2. Response time and recovery time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 2.3. Detection limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 2.4. Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 2.5. Stability and reproducibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1410 The sensors based on ZnO nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 3.1. Gas sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 3.1.1. Reductive gas sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 3.1.2. Oxidative gas sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 3.2. Chemical sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411 3.3. Biosensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1414 3.4. pH sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415 3.5. UV sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415 3.6. Other sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415 The configuration of the sensing elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416 4.1. ZnO film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416 4.2. ZnO bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416 4.3. Single ZnO nanowire/nanorod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416 The influencing factors of the sensor based on ZnO nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1417 5.1. Specific surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1417 5.2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1417 5.3. The electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1418 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1418 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1420

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (W. Huang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.09.005

1410

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

1. Introduction Nanomaterials have been extensively studied for application in various kinds of nanoscale functional devices used widely in the chemical industry, medical diagnostics, food technology, ultraviolet testing, national defense and our daily life [1–5]. Among these, the semiconductor nanomaterials such as ZnO, SnO2 , TiO2 , and ZnS receive most attention due to intriguing nanosize effects on their physical and chemical properties [6–9]. ZnO nanostructures are good candidates upon which to construct functional devices, because of their low toxicity, good thermal stability, good oxidation resistibility, good biocompatibility, large specific surface area and high electron mobility [10]. ZnO is a transparent semiconductor with a direct band gap (Eg = 3.37 eV) and a large exciton binding energy (60 meV), exhibiting near UV emission, ZnO has good conductivity, also worth noting is its piezoelectricity [11]. The morphology-controlled synthesis of ZnO nanostructures have been extensively studied, the morphology of ZnO can vary from nanorods, nanotubes, nanoneedles, and nanocomb to nanoinjector, nanohelixes and nanodisks simply by adjusting preparation method and preparation conditions [12–17]. The various morphologies and mature growth methods lead to easy preparation of ZnO-based devices. With the help of advanced micro-fabrication techniques, ZnO nanostructures have been used widely in field-effect transistors, light emitters, lasers, solar cells and sensing [18–21]. Much attention has been put on ZnO nanostructures for sensing applications, several properties of ZnO are utilized; gas sensors are based on the fact that conductance changes with the reversible chemisorption process of reactive gases on the surface of ZnO [22]. Pressure sensors are based on the piezoelectric property of ZnO, first observed by Wang et al. [11]. Biosensor can be based on ZnO because of the biocompatibility and nontoxicity of ZnO [23]. Different devices with high sensing performances have been reported, however, high selective response still remains a great challenge. In this review paper, we will comprehensively introduce the recent progress of the ZnO nanostructures-based sensors, such as gas sensors, chemical sensors, biosensors, UV sensors and pH sensors. The parameters, structures and mechanisms of each type of nano-ZnO based sensors along with factors influencing the sensitivity will be discussed. 2. The parameters of sensors 2.1. Sensitivity The sensitivity of a sensor is the ratio of the change amplitude of a sensor signal to the original amplitude, which is defined as: S=

R × 100% R

(1)

where R is the change amplitude of the sensor signal, and R is the amplitude of the original signal. The signal could be the resistance, current, voltage or conductance, etc. Taking the reductive gas ethanol as an example [24], R is the resistance of the ZnO nanostructures and the sensitivity is the ratio of the resistance change before and after being ventilated with ethanol to the original resistance of ZnO nanostructures. We can express it as follows: S=

|Rair − Rgas | Rair

(2)

where Rair and Rgas are the resistance of the sensor before and after being ventilated with ethanol respectively.

Fig. 1. Response and recovery characteristic curves of the sensor based on the ZnO nanofibers doped with 1.2 wt% LiCl for one cycle.

2.2. Response time and recovery time Response time is defined as the time taken by a sensor to achieve 90% of the total signal change and recovery time is defined as the time taken by a sensor to achieve 90% of its original signal state. Fig. 1 shows the response and recovery characteristics curve of a humidity sensor based on 1.2 wt% LiCl-doped ZnO nanofiber for one cycle [25]. It can be observed that the response time and the recovery time are less than 6 s. 2.3. Detection limit A high-performance sensor should be able to detect a tiny amount of certain object, and the lowest amount of the object which the sensor could have a response to is called the detection limit. To the best of our knowledge, the sensor devices based on ZnO nanostructures can detect as low as 0.2 ppm ethanol gas [26] or 4 nN force [27]. 2.4. Selectivity The selectivity of a sensor means how well it responses to a certain object compared to other objects. The pure ZnO has poor selectivity because many kinds of reductive gases and organic vapors can change the ZnO surface state thus inducing similar responses. However, the selectivity of the ZnO sensor can be improved by the introduction of certain functional groups to affect the adsorption processes [28,29]. A gas sensor based on the Fe2 O3 –ZnO nanocomposites with different compositions of Fe:Zn was prepared and it exhibits excellent sensitivity and selectivity to NH3 at room temperature [30]. An Irconia-based amperometric sensor using ZnO sensing-electrode shows a selective detection of 100 ppm propene [31]. Park et al. presented a novel technique for ultrasensitive detection of a protective antigen (PA83 ) of anthrax using an array of ZnO nanorods in conjunction with a FITC-labeled PA affinity peptide [32]. 2.5. Stability and reproducibility The stability of a sensor is evaluated by the change of sensing behavior after numerous times of switching between ‘ON’ state and ‘OFF’ state. The stability is good when the sensing performance shows little change after numerous tests [28,29]. Fig. 2 illustrates the good stability of a ZnO nanobelts-based sensor for 120 ppm NH3 [33] and a flexible piezotronic strain sensor at a frequency of 2 Hz under fixed bias of 2 V [34].

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

1411

The sensor signal could be the change of resistance, current, conductivity or voltage. Wang et al. have fabricated a large-area ZnO single crystal thin film sensor with mesoporous nanostructure, which has a response to CH4 down to 100 ppm, as shown in Fig. 4(a) [42]. Through the nanoscale spacer lithography, the ZnO nanowire CO sensor has been fabricated by Ra et al. [43]. Fig. 4(b) compares the sensitivity of the CO sensor based on ZnO nanowire and film. The voltage change of the sensor device has been used as the sensor signal to test NH3 by Li et al. [44] and a nano-ZnO based H2 S sensor has been fabricated via vapor-phase transport method by Zhang et al. [45], which are demonstrated in Fig. 4(c) and (d), respectively. 3.1.2. Oxidative gas sensor The oxidative gas such as NO2 [46], NO [47], O3 [48] and O2 [49] can also be detected using ZnO nanostructures-based sensors, but the mechanism is opposite to that of the reduced gases. After being absorbed on the surface of ZnO nanostructures, the O atom of oxidative gas molecular extracts the electrons from the ZnO nanostructures instead of releasing electrons. As shown in Fig. 5, the depletion layer becomes thicker due to the decreasing of the carrier concentration. The thicker depletion layer means the increasing of the resistance, or the decreasing of the current, which can be used as the sensor signal. The sensor curves of ZnO nanostructures to O2 [50], NO2 [51] are shown in Fig. 6. 3.2. Chemical sensor

Fig. 2. (a) Current responses of the ZnO nanobelts sensors when the surrounding gas is switched between air and 500 ppm NH3 and (b) current response of a sensor device that was repeatedly stretched at a frequency of 2 Hz under fixed bias of 2 V.

3. The sensors based on ZnO nanostructures 3.1. Gas sensor 3.1.1. Reductive gas sensor ZnO nanostructures are considered as one of the most potential candidates for testing reductive gases due to their high sensitivity. Once adsorbed on the surface of ZnO nanostructures, the reductive gas molecules such as H2 [35,36], CO [37,38], CO2 [39], H2 S [40], NH3 [33] and CH4 [41] will react with the adsorbed O2 − , O− or O2− ions and release the electrons back to the ZnO nanostructures. Because of the large specific surface area and high electron mobility, the ZnO nanostructures will adsorb a large quantity of gas molecules after exposing to the gas, resulting in a large change of conductivity. Fig. 3 schematically shows how the depletion layer forms in the ZnO nanostructures. The depletion layer is formed after extracting electrons and the resistance of the ZnO nanostructures increases accordingly. The reductive gas reacts with the O− ions, which can be expressed as follows: R + O− = RO + e−

(3)

where R is the reductive gas molecule. The released electrons in this process will increase the carrier concentration, making the depletion layer thinner. The resistance of the ZnO nanostructures will decrease as well. The sudden increasing of the current or the decreasing of the resistance can be monitored by the electrical instruments.

The detection of chemical gases is very important in chemical laboratories or medical facilities because of their volatility, toxicity and combustibility. With the help of various kinds of the sensors based on ZnO nanostructures, the chemical gases, such as methanol [52], ethanol [53], acetone [54], butane [55,56], dimethylamine [57], triethylamine [58], chlorobenzene [7], and liquefied petroleum gas [59,60] can be detected. The chemical gas is also a kind of reductive gas which would react with the adsorbed O ion, releasing the electrons. The sensor signal could be the variation of resistance or current. Fig. 7 is an example of the porous ZnO nanoplates for chlorobenzene and ethanol sensing [7]. Besides testing the conductance, other methods have also been developed to develop ZnO nanostructures-based sensors. Yang et al. successfully grown ZnO nanowires on the surfaces of infrared (IR) internal reflection elements for detecting volatile organic compounds [61]. By interaction of the evanescent field with the surrounding volatile organic compounds, an IR spectrum can be obtained. Because of the high surface-to-volume ratios of the ZnO nanowires, large amounts of gas molecules will be adsorbed on the surface of the ZnO nanowires which improves the sensitivity of the IR internal reflection elements about 10 times relative to that of untreated sensing elements. An m-lines technique at 633 nm is developed for chemical gas detection [55,56]. The ZnO is used as the planar waveguide in the totally reflecting prism coupler, as shown in Fig. 8(a). A laser beam enters the prism with the incident angle  s , then a dark line can be observed by the resonant coupling of the laser beam into the waveguide. The dark line is known as the mode line or m-lines that appear in the reflected beam. From the propagation constants of the guided modes determined from angles corresponding to the m-lines, we can get the refractive index n of the waveguide. After exposing to carbohydrate, the variation of the refractive index of ZnO can be used as the sensing signal. Effective index variations down to 0.005 can be detected in the case of 500 ppm of butane diluted in N2 . Another ZnO waveguide used as a gas sensing element on the side-polished fiber has been fabricated by Dikovska et al. [62]. The cross section of a side-polished fiber sensor is shown in

1412

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

Fig. 3. The schematic image of the reductive gas sensor.

Fig. 4. Signals of the sensor based on ZnO nanostructures to (a) CH4 , (b) CO, (c) NH3 , and (d) H2 S.

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

1413

Fig. 5. The schematic image of the oxidative gas sensor.

Fig. 6. Signals of the sensor based on ZnO nanostructures to (a) O2 and (b) NO2 .

Fig. 8(b). Based on a distributed coupling between the fiber mode and the corresponding mode of the metal oxide planar waveguide, a spectral behavior of the channel-dropping filter can be got after exposing to gases. The spectral behavior of the waveguide struc-

ture will be changed with the variation of the refractive index of the metal oxide film. A change of refractive index in the ZnO film by 3 × 10−5 for 1.5% butane diluted in N2 has been obtained with the response time and the recovery time in the range of 3–5 s.

Fig. 7. Signals of the sensor based on ZnO nanostructures to (a) chlorobenzene and (b) ethanol.

1414

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

Fig. 8. (a) Totally reflecting prism coupler and (b) a cross section of the side-polished fiber sensor element.

3.3. Biosensor Due to the biocompatibility, stability, and high isoelectric point, ZnO nanostructures have been used widely in the biological detection, such as in the detection of urea [63], DNA sequence [64], cholesterol [65], carbohydrate antigen [66] and glucose [67,68]. The enzyme or DNA segments used as the detecting elements are immobilized on the surfaces of ZnO which serve as the nanoscale platforms and then the testing signal can be transformed into surface plasmon resonance [66], fluorescence [69,70], electrochemical [71,72] or the field emitter signal [73]. Our research group reported a glucose biosensor based on glucose oxidase (Gox) immobilized on ZnO nanorod array prepared via hydrothermal method [67]. All of the sensing signals were tested with a three electrode electrochemical systems. In a phosphate buffer solution with a pH value of 7.4, negatively charged GOx was immobilized on the positively charged ZnO nanorods through electrostatic interaction. ZnO nanorods-based biosensor presented a high and reproducible sensitivity of 23.1 ␮A cm−2 mM−1 with a response time less than 5 s. The biosensor shows a linear range from 0.01 to 3.45 mM with a detection limit of 0.01 mM. A high affinity has been shown with Michaelis–Menten constant of 2.9 mM between glucose and GOx immobilized on ZnO nanorods, as show in Fig. 9.

Fluorescence-based biosensor is another kind of sensor with high sensitivity for detecting analyte but requires multi-step functionalization and skillful adjustment of the affinity of the interacting biomolecules due to the use of labels. Dorfman et al. demonstrated that the engineered nanoscale ZnO can significantly enhance the detection capability of biomolecular fluorescence [70]. The biosensors based on these nanoscale platforms can identify the Bacillus anthracis by successfully discriminating its DNA sequence. Their detection limit can be as low as a few femtomolar of the target concentration. Fig. 10(a) shows a fluorescence emission image based on the open square ZnO arrays [64]. The electrochemical methods have been used widely for detecting biomolecules in the solution [74]. The enzyme is immobilized on the surface of ZnO to serve as the sensing element. The electron which is generated by the reaction between sensing element and the analyte will shuttle between the work electrode and auxiliary electrode. The generated current can be monitored by the computer-controlled electrochemical station. Yang et al. [72] immobilized the glucose oxidase on the surface of ZnO nanotube by a two-step electrochemical/chemical process. The high sensitivity of 30.85 ␮A cm−2 mM−1 at an applied potential of +0.8 V vs. SCE (saturated calomel electrode) with a detection limit of 10 ␮M has been demonstrated. Cyclic voltammograms of Nafion/GOx/ZnO nanotube arrays/ITO electrode in the PBS solution (pH 7.4) in the absence (dashed) and the presence (solid) of 1 mM glucose are shown in Fig. 10(b). A method based on the field emitter signal of ZnO nanorod arrays has been developed by Liu et al. to detect glucose [73]. The field emitter signal is measured using a two-parallel-plate configuration in a homemade vacuum chamber at a base pressure of about 1 × 10−6 Pa at room temperature and varies significantly according to the concentration of the glucose in the PBS solution. The higher concentration of the glucose, the more H2 O2 will be produced, and the more electrons will be trapped by O ions, then the smaller amplitude of the field emitter signal is generated. The detection limit of glucose sensors based on the filed emitter of ZnO nanorod arrays can be down to 1 nM. However, sensors based on field emitting are not suitable for large glucose concentration. Fig. 10(c) shows the field emission properties of the ZnO nanorod arrays immersed in PBS solution with different glucose concentrations. The surface plasmon resonance spectroscopy is based on the surface plasma wave (SPW) which is an electromagnetic wave and propagates along the boundary between a dielectric and a metal [75]. Once the biomolecular recognition elements on the surface of metal recognize and capture analyte in the solution producing a

Fig. 9. (a) Amperometric responses of GOx/ZnO nanorods/Au electrodes with successive addition of glucose to the 0.01 M pH 7.4 PBS buffer under stirring. (b) Calibration curves for glucose using GOx/ZnO nanorods/Au electrode (solid squares) and the Lineweaver–Burk plot (open circles). The straight line is a linear fit to the plot.

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

1415

Fig. 10. (a) The fluorescence emission image based on an open square ZnO arrays; (b) a cyclic voltammograms of Nafion/GOx/ZnO nanotube arrays/ITO electrode in the PBS solution with 1 mM glucose; (c) field emission J–E curves of the ZnO nanorod arrays immersing in the glucose concentration from 1 to 500 nM in 6 mM PBS buffer solution and 10 U ␮l−1 glucose oxidase with a pH value of 5.8; and (d) real-time intensity of CA15-3 antigen as detected by the Au/ZnO thin film biosensor.

local change in the refractive index at the metal surface, a change of the propagation constant of SPW propagating alone the metal surface can be accurately measured by optical means. Chang et al. fabricated an Au/ZnO thin film SPR biosensor for detecting carbohydrate antigen 15-3(CA15-3) with a detection limit reaching 0.025 U/mL at a signal-to-noise ratio of 3:1. A real-time intensity of CA15-3 antigen detected by the Au/ZnO thin film biosensor is shown in Fig. 10(d). 3.4. pH sensor A change in surface potential at the ZnO/liquid interface upon exposing to polar liquids has been used to design the pH sensor. The pH responses of ZnO polar and nonpolar surfaces have been modeled [76–78]. After the ZnO is immerged into the polar liquid or electrolyte solution, a surface charge will be developed, which can be expressed as: ZnO(s) + H+ = ZnOH+ , for acid,

(4)

ZnO(s) + 2H2 O = Zn(OH)3 − + H+ , for base,

(5)

The ion form is ZnOH+ in the acid environments (Eq. (4)) while Zn(OH)3 − under neutral or moderately basic conditions (Eq. (5)) [79]. Fig. 11(a) shows the change of conductance of ZnO with and without ultraviolet light in the pH range of 2–12 [80]. The ZnO nanorods used as intracellular pH sensors have been fabricated by Willander et al. [79,81,82]. A working electrode covered with bare ZnO nanorods was used to test the pH value in a human fat cell

with an Ag/AgCl reference microelectrode. The functionalized ZnO nanostructures have also been reported [19]. 3.5. UV sensor The ZnO nanostructure can be used as an ultraviolet light (UV) sensor due to its wide band gap of 3.37 eV [83,84]. Being exposed to air, electrons in ZnO nanostructure will transfer to the O molecule absorbed on it, resulting in the conductance change of the ZnO. Upon absorbing a photon, the hole and electron carriers will be generated. The positive-charged holes will neutralize the chemisorbed O ions, which release the electrons back to ZnO at the same time. Then an increment of conductivity of ZnO nanostructures will be obtained, in another word, the sensitivity of the ZnO gas sensors can be improved by the radiation of UV [85]. A ZnO film based acoustic-wave resonator (FBAR) can also be used to detect ultraviolet light [86]. The resonant frequency will be up-shifted after UV illumination, as shown in Fig. 11(b). The mechanism may be related to the decreased density of ZnO under UV illumination which causes the desorption of oxygen ions from the ZnO surface. 3.6. Other sensors ZnO nanostructures can be used in some other sensors, such as humidity sensor, mass (or pressure) sensor, nanoforce sensor. The ZnO nanostructure can sense humidity but some opposite results were obtained in different laboratories. With the increase of

1416

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

current response to a repeated compression is shown in Fig. 12(d) [34,95]. 4. The configuration of the sensing elements The configuration of sensing element of ZnO nanostructures can be different according to various applications. The ZnO film, ZnO nanobridge, and the single nanowire/nanorod are the three most common configurations. They are chosen for the sensing study accordingly. 4.1. ZnO film ZnO film can be grown between the electric substrate and the electrode [96] or two electrode plates are deposited on the surface of ZnO nanostructures after the preparation of ZnO film [97,98], as shown in Fig. 13(a) and (b). The ZnO film can also be used as the waveguide for the chemical gas detection, the fluorescence-based biosensors, and the mass or pressure sensors. These all have been discussed in Section 3. 4.2. ZnO bridge

Fig. 11. (a) Change in conductance with pH from 2 to at V = 0.5 V and (b) UV light sensing result with frequency upshift.

the relative humidity, the resistance of some sensors based on ZnO nanostructures decreased [87–89]. Qiu et al. explained that protons could be released from the water molecules to promote the conductance of the ZnO nanostructures where the water molecule has been adsorbed [87]. But Wang et al. argued that in the water molecule, the O atom will easily attract electron from the H atom which has been transformed to H+ [90]. When the water molecule adsorbs on the surface of ZnO nanostructures, the H+ will also easily attract electron from the ZnO nanostructures and then a depletion layer is formed. The resistance of ZnO nanostructures will increase accordingly. Both of the two mechanisms can explain the corresponding sensing phenomenon. The opposite sensing characters are shown in Fig. 12(a) and (b). So far, no appropriate theory can explain the humidity sensing based on ZnO nanostructures. The surface acoustic wave (SAW) pressure sensor [91], the film bulk acoustic resonator (FBAR) [92] and the lateral extensional mode (LEM) piezoelectric resonator [93] based on ZnO nanostructures have been fabricated for mass or pressure detection because ZnO nanostructures have relatively low temperature coefficient of frequency (TCF) and large electromechanical coupling factor. In the thickness-mode FBAR, the acoustic wave propagates along the electrical field direction and the sensitivity of the mass sensor based on FBAR reaches 5 × 103 Hz cm2 /ng. While the LEM piezoelectric resonator has a sensitivity down to 7.7 × 10−15 g/Hz. Fig. 12(c) shows the shift of the resonance frequency before and after mass loading on the FBAR. The piezoelectricity of ZnO nanostructures has been demonstrated by Wang et al. [94]. The combination of piezoelectric and semiconducting properties of ZnO as well as the gating effect of the Schottky barrier transforms the mechanical displacement among ZnO nanowires to an electrical signal. The nanoforce sensor based on a single ZnO nanowire has a detecting limit down to nanonewton [27]. With the help of organic substrate, the flexible piezoelectric strain sensors have also been fabricated. At the fixed bias of 2 V, the

Some smart sensors have been fabricated with the counter electrodes connected by ZnO nanostructures as a bridge. This device configuration can be obtained by preparing the special ZnO nanostructure with the confined condition to connect the near counter electrodes [51,19,99], as shown in Fig. 14(a). A NO2 sensor has been constructed by Ahn et al. through growing ZnO nanowires to bridge the two prepatterned Au catalysts. Its side-view SEM image is shown in Fig. 14(b). Another method to build the ZnO nanobridge sensor is to drop the solution containing ZnO nanorods/nanowires onto the substrate with the patterned electrodes [85,100,101]. After evaporation of the solution, the ZnO nanorods/nanowires will connect the electrodes with random orientation, as shown in Fig. 14(c). Fig. 14(d) is the image of an ethanol sensor made via this method with a slight modification of electrospinning to dry the solution [102]. 4.3. Single ZnO nanowire/nanorod In order to maximize the sensing ability of ZnO nanostructures, it is worthy to construct sensors based on single ZnO nanowire or nanorod. During the process of fabrication, the separation of single ZnO nanowire or nanorod and the connection between it and the electrodes have baffled lots of the researchers. The focus ion beam (FIB) system, which is schematically shown in Fig. 15(a), is the most useful instrument to fabricate the micro-scale sensing device. After the single ZnO nanowire or nanorod is separated on the substrate after ultrasonication, the metal will be deposited on the junction between the two ends of ZnO nanowire or nanorod and the electrodes, thus obtained the sensing unit. A H2 S sensor was fabricated by directly writing the Pt electrodes from the end of the ZnO nanorod to the Au pad by FIB [103], as shown in Fig. 15(b). The dielectrophoresis is utilized to build the single ZnO nanorod or nanowire between two electrodes. The electric field will induce dipoles in the nanostructures and the dielectrophoretic force will cause nanostructures to build up between the electrodes if the suspension has been correctly chosen. Fig. 15(c) is the schematic image of the NiSi nanowires built up between the Pt electrode and the ground via dielectrophoresis [104]. Our group dispersed ZnO nanorods into the ethanol solution by ultrasonication and built up a single ZnO nanorod between the Al

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

1417

Fig. 12. (a) Sensitivity of the ZnO tetrapod sensor as a function of relative humidity at 23 ◦ C; (b) dynamic resistance response to the change of RH; (c) shift of the resonance frequency before and after mass-loading; (d) current response of a sensor device that was repeatedly compressed a frequency of 2 Hz under fixed bias of 2 V.

electrodes under the AC electric field, which can be observed in Fig. 15(d). 5. The influencing factors of the sensor based on ZnO nanostructures There are two kinds of factors influencing the sensor based on ZnO nanostructures, namely the external and the interior influencing factors. The external ones mainly refer to the specific surface area, the temperature and the connecting electrodes, all of which influence the surface adsorption process which plays an important role in the gas sensor, the chemical sensor, the UV sensor and the humidity sensor. The interior one refers to the piezoelectric property of the ZnO nanostructures, that is, the energy involved in mechanical bending can be transformed to the electric energy, which reveals the fundamental mechanism for the pressure or the mass sensor. 5.1. Specific surface area The specific surface area of ZnO nanostructures plays an important role in sensing. Sensors with different surface morphologies have different surface to volume ratio, thus different sensor performance because the more O ions or biomolecules are adsorbed on the surface of ZnO nanostructures the higher the sensitivity of the sensor will be. Qiu et al. fabricated a humidity sensor based on ZnO tetrepods and investigated the influence of the specific

surface area to the sensitivity [87], as shown in Fig. 16(a). They found ZnO tetrapod film displays much higher sensitivity to humidity than ZnO nanoparticle film, and the sensitivity increases with decreasing tetrapod size, all of which is ascribed to the high specific surface area and high surface activity of the small tetrapod. Liao et al. also reported that the thinner ZnO nanorod-based sensors have significantly better sensing performance due to a larger effective surface area, resulting in a larger quantity of adsorbed oxygen than the thicker nanorod sensors [105]. The sensing performances are clearly illustrated in Fig. 16(b), in which the ZnO nanorods in sample A are the thinnest, and the ZnO nanorods in sample D are the thickest. 5.2. Temperature The temperature has a critical impact on the sensitivity of the sensor. There are three processes proceeding on the surface of ZnO, the adsorption, the desorption, and the activity of the O ions. As the temperature increases, the sensitivity will be prompted because of the increasing activity of the O ions. But the sensitivity will decrease once the temperature increases over the optimum temperature. The main reason is that the process of desorption is the main process compared to the one of adsorption, though the activity of O ions is still increasing. Fig. 16(c) shows a ZnO film sensor of LPG with an optimum temperature of 673 K [59]. Chu et al. studied the responses of ZnO tetrapods-based sensor to volatile gases as a function of operating temperature [106]. They found that the sensor

1418

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

shows the highest response at 270 ◦ C to 1000 ppm CH4 gas and 300 ◦ C to 100 ppm CH3 CH2 OH gas, both are shown in Fig. 16(d).

5.3. The electrodes Due to the different work functions of the electrodes, the Schottky contact or Ohmic contact will be formed accordingly. When one is the Schottky contact and the other one is the Ohmic contact, the sensitivity of the sensor can be improved a lot, as it was demonstrated recently [107]. The improvement results from the current jump due to the reduction of the Schottky barrier height, which is formed by adsorbing the O− . Also, the reverse bias will promote sensitivity in opposite to the forward bias.

6. Summary

Fig. 13. The image of (a) the electrodes plating on the surface of ZnO and (b) the ZnO growing on the electric substrate with a layer of electrode plating on it.

The ZnO nanostructures have been widely used in different sensing applications because of their versatile properties such as large specific surface area, nontoxicity, biocompatibility, good conductivity and piezoelectricity. Objects including reductive and oxidative gases, chemical solvents, biomolecules, mass and pressure can be detected, which are summarized in Table 1. The high sensitivity and the low detection limit have promised a widespread application of the sensors based on ZnO nanostructures in the sensor field. An overview of various kinds of sensors based on ZnO nanostructures is presented, proposed mechanism. The main structures and factors influencing sensor performance of these sensors are discussed.

Fig. 14. (a) The schematic image of the ZnO nanostructure growing to connect the near electrodes; (b) SEM image of ZnO nanowires bridging the gap between two prepatterned Au electrodes; (c) schematic image of the pattern electrodes; and (d) SEM image of the ZnO prepared by electrospuning connecting the electrodes.

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

1419

Fig. 15. (a) A schematic image of the FIB system; (b) the SEM image of connecting Pt electrodes to both ends of ZnO; (c) the schematic image of the dielectrophoresis of NiSi; and (d) SEM image of ZnO nanorod connecting the Al electrodes by dielectrophoresis.

Fig. 16. (a) Sensitivity of different ZnO sensor under 100% relative humidity at 23 ◦ C, (A) diameter of the ZnO tetrapods is 17 nm, (B) diameter of the ZnO tetrapods is 100 nm, (C) diameter of the ZnO nanoparticles is 100 nm. (b) Typical response curves of the four kinds of ZnO nanorod sensors to CH3 CH2 OH; (c) the dynamic response of ZnO film to 0.2 vol% LPG in different temperatures: (A) 623 K, (B) 648 K, (C) 673 K, (D) 698 K; (d) the responses to volatile gases of ZnO sensor based on tetrapods prepared in humidified Ar.

1420

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421

Table 1 Summary of the sensor based on ZnO nanostructures. Type

Detecting object

Structure

Sensitivity

Detection limit

Ref.

Gas sensor

CO2 H2 S CH4 NO O3 O2 NO2 H2 S CO H2 CO NH3 NO2

Film Film Film Film Film Film Bridge Single nanowire Single nanowire Film Film Film Film

22.5 78.7% – – – 0.15 10 – 32,000% 2.6% – – 100

1000 ppm 0.05 ppm 100 ppm 100 ppm 49.8 Pa 1.4 ppm 0.5 ppm 100 ppm – 10 ppm 2 ppm 60 ppb 0.2 ppm

[39] [40] [42] [47] [48] [50] [51] [103] [107] [108] [109] [110] [111]

Chemical sensor

Chlorobenzene Ethanol Methanol Butane DMA Triethylamine Urea DNA Cholesterol CA15-3 Glucose Glucose Glucose Ethanol Acetone LPG Trimethylamine Phenol

Film Bridge Film Film Film Film Film Film – Film Film Film Film Film Film Film Film Film

1.6 10 – – 26% 6 – – 0.059 A/mg dl−1 cm−2 – 23.1 ␮A cm−2 mM−1 30.85 ␮A cm−2 mM−1 – 11 3.8 17% 7 –

100 ppm 0.2 ppm 10 ppm 100 ppm 0.25% 1 ppb 3 mg/dl 2 fM 0.98 mg/dl 0.025 U/mL 0.01 mM 10 ␮M 1 nM 1 ppm 1 ppm 0.1 vol% 1 ppm 0.623 ␮M

[7] [26] [52] [56] [57] [58] [63] [64] [65] [66] [64] [72] [73] [112] [113] [114] [115] [116]

UV and pH sensor

pH pH UV

Single nanorod Single nanorod Film

20 nS/pH 59 mV/pH –

0.1 pH 1 pH 6.5 nW

[80] [81] [86]

Others

Nanoforce Humidity Pressure Mass Mass

Single nanowire Film Film Film Film

– – 35 ppm/mbar 0.05 × 105 Hz cm2 /ng 7.7 × 10−16 g/Hz

4 nN 11% 0.3 bar – 73 fg

[27] [88] [91] [92] [93]

It has also been pointed out that some crucial problems for sensors based on ZnO nanostructures still remain unsolved, the first problem is relatively low selectivity, reproducibility and stability of the sensors. For instance, the surface state and conductivity of ZnO nanostructures are easily affected by the testing object and change of surrounding environment. The introduction of multiple functional groups on the surface of ZnO nanostructures may help to solve this problem. Another problem is the lack of standardisation of testing systems. Nearly all the sensor testing systems are designed by researchers themselves and no standard has been agreed upon. This lack of standardisation makes it complicate to accurately reproduce the published results and hinder further investigation. Thus to develop a standard testing system is the key to simplifying research and manufacturing procedure and expanding the application of sensors based on ZnO nanostructures. Needless to say, this paper only reviews some of the research papers in this field and many exciting works have not been cited. The above discussion shows a promising future of ZnO nanostructures-based sensors due to their intriguing properties and excellent performances. With the development of new device concepts and micro-fabrication techniques, sensors based on nanoscale ZnO will surely come for practical applications. Acknowledgements The authors wish to thank Prof. Wenyong LAI, Qunliang SONG and Prof. Wei WU for kindly advice. The financial supports by

the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University (BJ209007), and Natural Science Research Project of Jiangsu Ordinary University (09KJB430008) are kindly acknowledged. References [1] X. Fang, L. Zhang, J. Mater. Sci. Technol. 22 (2006) 18. [2] E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Prog. Mater. Sci. 54 (2009) 1. [3] S. Barth, F. Hernandez-Ramirez, J.D. Holmes, A. Romano-Rodriguez, Prog. Mater. Sci. 55 (2010) 563. [4] X. Fang, T. Zhai, U. Gautam, Prog. Mater. Sci. 56 (2011) 175. [5] X. Fang, L. Wu, L. Hu, Adv. Mater. 23 (2011) 585. [6] X. Fang, Y. Bando, M. Liao, U.K. Gautam, C. Zhi, B. Dierre, B. Liu, T. Zhai, T. Sekiguchi, Y. Koide, Adv. Mater. 21 (2009) 2034. [7] Z.H. Jing, J.H. Zhan, Adv. Mater. 20 (2008) 4547. [8] J.F. Liu, C. Roussel, G. Lagger, P. Tacchini, H.H. Girault, Anal. Chem. 77 (2005) 7687. [9] Y.L. Wang, X.C. Jiang, Y.N. Xia, J. Am. Chem. Soc. 125 (2003) 16176. [10] A. Wei, C.X. Xu, X.W. Sun, W. Huang, G.Q. Lo, J. Display Technol. 4 (2008) 9. [11] Z.L. Wang, Adv. Funct. Mater. 18 (2008) 3553. [12] Y. Chang, W. Yang, C. Chang, P. Hsu, L. Chen, Cryst. Growth Des. 9 (2009) 3161. [13] S. Cho, S. Jung, J. Jang, E. Oh, K. Lee, Cryst. Growth Des. 8 (2008) 4553. [14] X. Kong, Z. Wang, Nano Lett. 3 (2003) 1625. [15] L. Mazeina, Y.N. Picard, S.M. Prokes, Cryst. Growth Des. 9 (2009) 1164. [16] A. Wei, X. Sun, C. Xu, Z. Dong, Y. Yang, S. Tan, W. Huang, Nanotechnology 17 (2006) 1740. [17] H. Zhou, P. Shao, S. Chua, J. van Kan, A. Bettiol, T. Osipowicz, K. Ooi, G. Goh, F. Watt, Cryst. Growth Des. 8 (2008) 4445. [18] H. Gullapalli, V.S.M. Vemuru, A. Kumar, A. Botello-Mendez, R. Vajtai, M. Terrones, S. Nagarajaiah, P.M. Ajayan, Small 6 (2010) 1641. [19] V. Pachauri, A. Vlandas, K. Kern, K. Balasubramanian, Small 6 (2010) 589.

A. Wei et al. / Materials Science and Engineering B 176 (2011) 1409–1421 [20] X. Fang, Y. Bando, U.K. Gautam, T. Zhai, H. Zeng, X. Xu, M. Liao, D. Golberg, Crit. Rev. Solid State Mater. Sci. 34 (2009) 190. [21] Q.F. Zhang, C.S. Dandeneau, X.Y. Zhou, G.Z. Cao, Adv. Mater. 21 (2009) 4087. [22] J. Kim, K. Yong, J. Phys. Chem. C 115 (2011) 7218. [23] M. Ahmad, C. Pan, Z. Luo, J. Zhu, J. Phys. Chem. C 114 (2010) 9308. [24] T.J. Hsueh, S.J. Chang, C.L. Hsu, Y.R. Lin, I.C. Chen, Appl. Phys. Lett. 91 (2007) 053111. [25] W. Wang, Z. Li, L. Liu, H. Zhang, W. Zheng, Y. Wang, H. Huang, Z. Wang, C. Wang, Sens. Actuators B: Chem. 141 (2009) 404. [26] J.Y. Son, S.J. Lim, J.H. Cho, W.K. Seong, H. Kim, Appl. Phys. Lett. 93 (2008) 053109. [27] X. Wang, J. Zhou, J. Song, J. Liu, N. Xu, Z. Wang, Nano Lett. 6 (2006) 2768. [28] X.J. Huang, Y.K. Choi, Sens. Actuators B: Chem. 122 (2007) 659. [29] X. Fang, L. Hu, C. Ye, L. Zhang, Pure Appl. Chem. 82 (2010) 2185. [30] H.X. Tang, M. Yan, H. Zhang, S.H. Li, X.F. Ma, M. Wang, D.R. Yang, Sens. Actuators B: Chem. 114 (2006) 910. [31] T. Ueda, V.V. Plashnitsa, M. Nakatou, N. Miura, Electrochem. Commun. 9 (2007) 197. [32] H.Y. Park, H.Y. Go, S. Kalme, R.S. Mane, S.H. Han, M.Y. Yoon, Anal. Chem. 81 (2009) 4280. [33] X.G. Wen, Y.P. Fang, Q. Pang, C.L. Yang, J.N. Wang, W.K. Ge, K.S. Wong, S.H. Yang, J. Phys. Chem. B 109 (2005) 15303. [34] J. Zhou, Y.D. Gu, P. Fei, W.J. Mai, Y.F. Gao, R.S. Yang, G. Bao, Z.L. Wang, Nano Lett. 8 (2008) 3035. [35] O. Lupan, G. Chai, L. Chow, Microelectron. J. 38 (2007) 1211. [36] A. Wei, Z. Gao, J. Nanjing Univ. Posts Telecommun. (Nat. Sci.) 28 (2008) 91. [37] J.X. Wang, X.W. Sun, H. Huang, Y.C. Lee, O.K. Tan, M.B. Yu, G.Q. Lo, D.L. Kwong, Appl. Phys. A 88 (2007) 611. [38] A. Wei, W. Huang, C.X. Xu, X.W. Sun, Chin. J. Lumin. 28 (2007) 880. [39] O. Lupan, L. Chow, S. Shishiyanu, E. Monaico, T. Shishiyanu, V. Sontea, B. Roldan Cuenya, A. Naitabdi, S. Park, A. Schulte, Mater. Res. Bull. 44 (2009) 63. [40] C. Wang, X. Chu, M. Wu, Sens. Actuators B: Chem. 113 (2006) 320. [41] D. Gruber, F. Kraus, J. Müller, Sens. Actuators B: Chem. 92 (2003) 81. [42] X.D. Wang, Y. Ding, Z. Li, J.H. Song, Z.L. Wang, J. Phys. Chem. C 113 (2009) 1791. [43] Y.W. Ra, K.S. Choi, J.H. Kim, Y.B. Hahn, Y.H. Im, Small 4 (2008) 1105. [44] L. Li, H.Q. Yang, H. Zhao, J. Yu, J.H. Ma, L.J. An, X.W. Wang, Appl. Phys. A 98 (2010) 635. [45] N. Zhang, K. Yu, Q. Li, Z.Q. Zhu, Q. Wan, J. Appl. Phys. 103 (2008) 104305. [46] Z.Y. Fan, J.G. Lu, Appl. Phys. Lett. 86 (2005) 123510. [47] F.V. Farmakis, T. Speliotis, K.P. Alexandrou, C. Tsamis, M. Kompitsas, I. Fasaki, P. Jedrasik, G. Petersson, B. Nilsson, Microelectron. Eng. 85 (2008) 1035. [48] R. Martins, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, M. Bender, N. Katsarakis, V. Cimalla, G. Kiriakidis, J. Appl. Phys. 96 (2004) 1398. [49] Q.H. Li, Y.X. Liang, Q. Wan, T.H. Wang, Appl. Phys. Lett. 85 (2004) 6389. [50] J.Y. Park, S.W. Choi, S.S. Kim, Nanoscale Res. Lett. 5 (2010) 353. [51] M.W. Ahn, K.S. Park, J.H. Heo, D.W. Kim, K.J. Choi, J.G. Park, Sens. Actuators B: Chem. 138 (2009) 168. [52] J. Zhang, S.R. Wang, M.J. Xu, Y. Wang, B.L. Zhu, S.M. Zhang, W.P. Huang, S.H. Wu, Cryst. Growth Des. 9 (2009) 3532. [53] C.C. Li, Z.F. Du, L.M. Li, H.C. Yu, Q. Wan, T.H. Wang, Appl. Phys. Lett. 91 (2007) 032101. [54] P. Sahay, J. Mater. Sci. 40 (2005) 4383. [55] G. Socol, E. Axente, C. Ristoscu, F. Sima, A. Popescu, N. Stefan, I.N. Mihailescu, L. Escoubas, J. Ferreira, S. Bakalova, A. Szekeres, J. Appl. Phys. 102 (2007) 083103. [56] T. Mazingue, L. Escoubas, L. Spalluto, F. Flory, G. Socol, C. Ristoscu, E. Axente, S. Grigorescu, I.N. Mihailescu, N.A. Vainos, J. Appl. Phys. 98 (2005) 074312. [57] S. Roy, S. Basu, Bull. Mater. Sci. 25 (2002) 513. [58] Y.Z. Lv, C.R. Li, L. Guo, F.C. Wang, Y. Xu, X.F. Chu, Sens. Actuators B: Chem. 141 (2009) 85. [59] V. Shinde, T. Gujar, C. Lokhande, R. Mane, S. Han, Mater. Sci. Eng. B 137 (2007) 119. [60] D. Mishra, A. Srivastava, R. Shukla, Appl. Surf. Sci. 255 (2008) 2947. [61] J. Yang, Y. Shih, I. Chen, C. Kuo, Y. Huang, Appl. Spectrosc. 59 (2005) 1002. [62] A. Dikovska, P. Atanasov, T. Stoyanchov, A. Andreev, E. Karakoleva, B. Zafirova, Appl. Opt. 46 (2007) 2481. [63] P.R. Solanki, A. Kaushik, A.A. Ansari, G. Sumana, B.D. Malhotra, Appl. Phys. Lett. 93 (2008) 163903. [64] N. Kumar, A. Dorfman, J. Hahm, Nanotechnology 17 (2006) 2875. [65] P.R. Solanki, A. Kaushik, A.A. Ansari, B.D. Malhotra, Appl. Phys. Lett. 94 (2009) 143901. [66] C.C. Chang, N.F. Chiu, D.S. Lin, Y. Chu-Su, Y.H. Liang, C.W. Lin, Anal. Chem. 82 (2010) 1207. [67] A. Wei, X.W. Sun, J.X. Wang, Y. Lei, X.P. Cai, C.M. Li, Z.L. Dong, W. Huang, Appl. Phys. Lett. 89 (2006) 123902.

1421

[68] J.X. Wang, X.W. Sun, A. Wei, Y. Lei, X.P. Cai, C.M. Li, Z.L. Dong, Appl. Phys. Lett. 88 (2006) 233106. [69] E. Taratula, E. Galoppini, R. Mendelsohn, Langmuir 25 (2009) 2107. [70] A. Dorfman, N. Kumar, J.I. Hahm, Langmuir 22 (2006) 4890. [71] J.F. Zang, C.M. Li, X.Q. Cui, J.X. Wang, X.W. Sun, H. Dong, C.Q. Sun, Electroanalysis 19 (2007) 1008. [72] K. Yang, G.W. She, H. Wang, X.M. Ou, X.H. Zhang, C.S. Lee, S.T. Lee, J. Phys. Chem. C 113 (2009) 20169. [73] H.B. Liu, X.M. Qian, S. Wang, Y.L. Li, Y.L. Song, D.B. Zhu, Nanoscale Res. Lett. 4 (2009) 1141. [74] X. Sun, J. Wang, A. Wei, J. Mater. Sci. Technol. 24 (2008) 649. [75] J. Homola, Anal. Bioanal. Chem. 377 (2003) 528. [76] O. Dulub, B. Meyer, U. Diebold, Phys. Rev. Lett. 95 (2005) 136101. [77] A. Wander, N.M. Harrison, J. Chem. Phys. 115 (2001) 2312. [78] Y. Yan, M.M. Al-Jassim, Phys. Rev. B 72 (2005) 235406. [79] S.M. Al-Hilli, M. Willander, A. Öst, P. Strålfors, J. Appl. Phys. 102 (2007) 084304. [80] B.S. Kang, F. Ren, Y.W. Heo, L.C. Tien, D.P. Norton, S.J. Pearton, Appl. Phys. Lett. 86 (2005) 112205. [81] S.M. Al-Hilli, R.T. Al-Mofarji, M. Willander, Appl. Phys. Lett. 89 (2006) 173119. [82] M. Willander, P. Klason, L.L. Yang, S.M. Al-Hilli, Q.X. Zhao, O. Nur, Phys. Status Solidi C 5 (2008) 3076. [83] S. Hullavarad, N. Hullavarad, D. Look, B. Claflin, Nanoscale Res. Lett. 4 (2009) 1421. [84] V.A. Krivchenko, D.V. Lopaev, P.V. Pashchenko, V.G. Pirogov, A.T. Rakhimov, N.V. Suetin, A.S. Trifonov, Tech. Phys. 53 (2008) 1065. [85] J. Gong, Y.H. Li, X.S. Chai, Z.S. Hu, Y.L. Deng, J. Phys. Chem. C 114 (2010) 1293. [86] X. Qiu, J. Zhu, J. Oiler, C. Yu, Z. Wang, H. Yu, Appl. Phys. Lett. 94 (2009) 151917. [87] Y.F. Qiu, S.H. Yang, Adv. Funct. Mater. 17 (2007) 1345. [88] Q. Qi, T. Zhang, S. Wang, X. Zheng, Sens. Actuators B: Chem. 137 (2009) 649. [89] K. Zheng, Y. Zhao, K. Deng, Z. Liu, L. Sun, Z. Zhang, L. Song, H. Yang, C. Gu, S. Xie, Appl. Phys. Lett. 92 (2008) 213116. [90] Y. Wang, J. Yeow, L. Chen, Recent Adv. Sens. Technol. (2009) 257. [91] A. Talbi, F. Sarry, M. Elhakiki, L. Le Brizoual, O. Elmazria, P. Nicolay, P. Alnot, Sens. Actuators A 128 (2006) 78. [92] F. Juriollo, J. Rosolem, J. Said, M. Horiuchi, Microw. Opt. Technol. Lett. 42 (2004) 505. [93] W. Pang, L. Yan, H. Zhang, H.Y. Yu, E.S. Kim, W.C. Tang, Appl. Phys. Lett. 88 (2006) 243503. [94] Z.L. Wang, Nano Res. 1 (2008) 1. [95] K. Sun, H. Zhang, L. Hu, D. Yu, S. Qiao, J. Sun, J. Bian, X. Chen, L. Zhang, Q. Fu, Phys. Status Solidi A 207 (2009) 488. [96] J. Yang, G. Liu, J. Lu, Y. Qiu, S. Yang, Appl. Phys. Lett. 90 (2007) 103109. [97] C. Dandeneau, Y. Jeon, C. Shelton, T. Plant, D. Cann, B. Gibbons, Thin Solid Films 517 (2009) 4448. [98] J. Chang, H. Kuo, I. Leu, M. Hon, Sens. Actuators B: Chem. 84 (2002) 258. [99] M.W. Ahn, K.S. Park, J.H. Heo, J.G. Park, D.W. Kim, K.J. Choi, J.H. Lee, S.H. Hong, Appl. Phys. Lett. 93 (2008) 263103. [100] A. Qurashi, N. Tabet, M. Faiz, T. Yamzaki, Nanoscale Res. Lett. 4 (2009) 948. [101] N. Van Hicu, N.D. Chien, Physica B 403 (2008) 50. [102] W.Y. Wu, J.M. Ting, P.J. Huang, Nanoscale Res. Lett. 4 (2009) 513. [103] L. Liao, H.B. Lu, J.C. Li, C. Liu, D.J. Fu, Y.L. Liu, Appl. Phys. Lett. 91 (2007) 173110. [104] L. Dong, J. Bush, V. Chirayos, R. Solanki, J. Jiao, Y. Ono, J. Conley Jr., B. Ulrich, Nano Lett. 5 (2005) 2112. [105] L. Liao, H.B. Lu, J.C. Li, H. He, D.F. Wang, D.J. Fu, C. Liu, W.F. Zhang, J. Phys. Chem. C 111 (2007) 1900. [106] X.F. Chu, D.L. Jiang, A. Djurisic, Y. Leung, Chem. Phys. Lett. 401 (2005) 426. [107] T.Y. Wei, P.H. Yeh, S.Y. Lu, Z.L. Wang, J. Am. Chem. Soc. 131 (2009) 17690. [108] H. Wang, B. Kang, F. Ren, L. Tien, P. Sadik, D. Norton, S. Pearton, J. Lin, Appl. Phys. Lett. 86 (2005) 243503. [109] T. Wagner, T. Waitz, J. Roggenbuck, M. Froba, C.D. Kohl, M. Tiemann, Thin Solid Films 515 (2007) 8360. [110] Y. Wang, W. Jia, T. Strout, A. Schempf, H. Zhang, B. Li, J. Cui, Y. Lei, Electroanalysis 21 (2009) 1432. [111] J.H. Jun, J. Yun, K. Cho, I.S. Hwang, J.H. Lee, S. Kim, Sens. Actuators B: Chem. 140 (2009) 412. [112] C.C. Li, Z.F. Du, H.C. Yu, T.H. Wang, Thin Solid Films 517 (2009) 5931. [113] Q. Qi, T. Zhang, L. Liu, X.J. Zheng, Q.J. Yu, Y. Zeng, H.B. Yang, Sens. Actuators B: Chem. 134 (2008) 166. [114] C.D. Lokhande, P.M. Gondkar, R.S. Mane, V.R. Shinde, S.H. Han, J. Alloys Compd. 475 (2009) 304. [115] H. Nanto, H. Sokooshi, T. Kawai, T. Usuda, J. Mater. Sci. Lett. 11 (1992) 235. [116] B.X. Gu, C.X. Xu, G.P. Zhu, S.Q. Liu, L.Y. Chen, X.S. Li, J. Phys. Chem. B 113 (2009) 377.