1. Introduction
Nowadays, there has been a rapid advancement in the development of the sensors for application in monitoring various parameters such as humidity, temperature and chemical gases. A number of sensors have been fabricated on the basis of vanadium oxides[1, 2, 3, 4, 5, 6]. Vanadium oxide, VO2, shows a large reversible change of electric, magnetic and optical properties at temperatures around 68-70 C[7, 8, 9]. Transition of VO2 from semiconductor to metal is observed. At transition temperature the optical properties of vanadium dioxide are quickly changed: the optical transmission is decreased and reflectivity is increased. Due to this anomalous behavior, vanadium dioxide is an attractive material for smart windows for solar energy control, electrical and optical switches. Microstructure and crystallinity of the films affect the hysteresis of the transition. By the addition of transition metals such as niobium, molybdenum or tungsten, the transition temperature of vanadium dioxide of 68 may be decreased.
It was observed that[10] VO2 films demonstrate holographic storage and bit recording properties by using a near-infrared laser[11]. Switching time of about 30 ns and writing energy of the order of a few mJ/cm2 were reported[12]. The vanadium dioxide is an interesting candidate for modern applications of active thin films in optical or electric[13] switches as well.
Vanadium oxide is considered as an n-type semiconducting material[7]. Due to the semiconducting properties, vanadium oxides and their complexes have been reported to be used in different kinds of sensors. The lamellar nature of vanadium oxide makes it possible to modulate the adsorption and conduction properties. The electrical conductivity of the vanadium oxides can be enhanced by the formation of oxygen vacancies. Therefore, it is important to introduce different metal based materials into existing technology, which may bring a considerable improvements in functionality and/or cost of organic electronics[14].
Although the existence of CNTs and cementite nanowires in ancient engineering products was proved a few years ago[15] but since their discovery in 1991, CNTs have been intensively investigated[16]. Due to their unique electronic and mechanical properties, CNTs have proved very interesting for researchers. Depending on the orientation of graphene lattice with respect to the axis of the tube, CNTs are electronically classified as metallic, semiconducting, or small-gap semiconducting (SGS)[17]. Various types of sensors have been fabricated and investigated on the basis of single-walled nanotubes (SWNTs)[18, 19, 20, 21], double-walled carbon nanotubes (DWNTs)[22], and multi-walled carbon nanotubes (MWNTs)[23].
The temperature dependent electrical transport in highly disordered multiwalled carbon nanotubes of large outer diameter (60 nm) was investigated by Friedman et al.[24]. These nanotubes were fabricated by means of the chemical vapor deposition process. Kumar et al. also investigated the temperature and chemical sensitivity of carbon nanofilms made on diamond surface by high temperature surface modification followed by plasma treatment[25]. The conductance of these amorphous nanofilms was found to be sensitive for temperature and exposure to vapors of different organic compounds.
Recently the flexible electronic sensors[26, 27, 28, 29, 30] have become more attractive for researchers. Among these sensors, the flexible temperature sensors are very interesting due to their ability to improve the functionality of integrated bio-parameter monitoring systems such as the body temperature controlling systems. Sibinski et al.[31] fabricated a miniaturized flexible temperature sensor based on polymer composite filled with multiwalled carbon nanotubes. This sensor did not show the tensometric effect that was very common for other carbon-polymer sensors. The temperature sensitivity of the sensor was around of 0.13%, K−1. Zaitsev et al.[32] developed temperature sensor based on an array of carbon nanowires, and it was fabricated by a 30 keV Ga+ focused ion beam on diamond substrate. The sensor shows increase of current with increase in temperature in the range from 40 to 140 . Saraiya et al.[33] fabricated a low-temperature (10-300 K) resistance sensor based on CNTs, which were grown on nickel film by using the ion beam deposition technique. The results revealed that CNTs behave as semiconductors.
The surface-type CNTs film temperature sensors were designed, fabricated and investigated[34]. Resistance of the sensors decreased as the temperature increased, i.e. resistance-temperature relationship had semiconductive behavior. The average temperature sensitivity of the samples was −0.24% C-1. It was fabricated and investigated the resistive-type temperature sensor based on vanadium complex (VO2(3-fl))[35]. It was found that resistive-temperature relationships of the sensors showed exponential behavior. The resistance temperature coefficients of the sensors were in the range of -(3.2-3.6)%. It seems reasonable to investigate the temperature sensor based on vanadium complex VO2(3-fl) and CNT, having potential advantages of high sensitivity of the complex and lower resistance of the CNT temperature sensors. This paper reports the fabrication of surface-type resistive temperature sensor, employing the composite of vanadium complex VO2(3-fl) and CNT as an active sensing element.
2.Experimental procedure
The molecular structure of the vanadium complex is shown in Figure 1. The VO2(3-fl) was obtained from Aldrich and used as received. Commercially available glass slides were used as substrates which were primarily cleaned. In the first stage, the silver electrodes were deposited in a co-planar structure on the glass substrates using the Edwards AUTO 306 vacuum evaporation technique. The pressure inside the chamber was maintained at 10−5 mbar. The thickness of the electrodes was 200 nm and the gap was 40 μm. Commercially produced (Sun Nanotech Co Ltd., China) CNTs powder was used to make composite with VO2(3-fl). The diameter of multiwalled nanotubes (MWNTs) varied between 10-30 nm. 2.5 wt.% of CNT and 2.5 wt.% of VO2(3-fl) were mixed in benzol and deposited by drop casting on the pre-patterned Ag surface-type electrodes. The device was kept at room temperature for 10 h to let the moisture evaporate from the films. The composites' films thickness was in the range of 20-30 μm. The width of the surface-type sample's films was in the range of 6-8 mm. The cross sectional view of the fabricated resistive type temperature sensor is shown in Figure 2. For SEM measurements, the composite films were deposited on glass substrate. SEM micrographs of thin film of the composite were obtained from a Hitachi SU-1500 scanning electron microscope. SEM micrographs have been obtained at 4 K magnification (Figure 3). As requisite for morphological study through SEM, initially thin film of the composite was made conductive by depositing a thin film of platinum-palladium (Pt-Pd) (thickness ∼ 20 nm) on it. A Hitachi E-1010 ion sputter was used for Pt-Pd thin film deposition. Morphological study showed that sizes of CNT particles were ∼ 100 nm.
The temperature measurements were carried out in a self made chamber which has been designed and developed in our laboratory. Resistance measurements were carried out using a Digital HiTester 3256 and an ESCORT ELC-132A meter at DC and AC at frequency of 100 Hz, respectively. The temperature was measured by a multimeter FLUKE 87. The experimental error for the measurement of the temperature was equal to ±1 and the accuracy of the electric resistance measurement was equal to ±2% (it was estimated as described by Dally et al[36]).
3.Results and discussion
Figure 4 shows DC resistance versus temperature relationships for Ag/VO2(3-fl)-CNT/Ag surface-type resistive temperature sensor, and Figure 5 exhibits AC resistance versus temperature relationships for the same sensor. The AC measurements were made at 100 Hz frequency. The resistance of the sensor measured at DC and AC decreased quasi-linear with increasing the temperature in the range of 25-80 . The overall resistance of the sensor decreases in the range of 1.8-2.1 and 1.9-2.0 times for DC and AC voltages, respectively. The resistance temperature coefficient (S) of the samples can be calculated by[36]:
S=1RoΔRΔT×100%, | (1) |
Using a linear function[37]:
Y=kx+b. | (2) |
The relative resistance can be represented as:
RRo=kT+b, | (3) |
The mechanism of conductivity in the VO2(3-fl)-CNT composite samples can be considered as transitions between spatially separated sites that can be attributed to the Percolation theory[38, 39]. According to Percolation theory, the effective conductivity (σ) of the samples can be calculated as:
σ=1LZ, | (4) |
As experimental resistance-temperature (Figures 4 and 5) relationships for the Ag/VO2(3-fl)-CNT/Ag sensors are quasi-linear, they can be linearized easier by nonlinear op-amps[40].
A comparison of the properties of VO2(3-fl)-CNT with VO2(3-fl)[35] and CNT[34] based sensors shows that the composite sensors have lower values of resistances and these can be changed by changing the ratio of VO2(3-fl) and CNT in the composite. On the other hand, the resistance temperature coefficients of the composite temperature sensors are lower than the sensors based on VO2(3-ft), but much larger than those of the sensors fabricated by use of CNT. The non-linearity of the resistance-temperature relationships of the composite temperature sensors is lower than that of the sensors based on VO2(3-fl). These properties of the VO2(3-fl)-CNT composite based temperature sensors are very important for practical applications.
4.Conclusions
The resistive-type temperature sensor based on a composite of vanadium complex (VO2(3-fl)) and CNT was fabricated by drop-casting. It was found that resistive-temperature relationships of the sensors showed quasi-linear behavior. The resistance temperature coefficients of the sensors were in the range of -(0.9-1.3)% and -(1.1-1.3)% at DC and AC voltages, respectively. The mechanism of change of conductivity with change of the temperature in the VO2(3-fl)-CNT composite was considered as transitions between spatially separated sites that can be attributed to the Percolation theory: with an increase in temperature, an increase of the concentration and mobility of charge carriers takes place. Fabrication of the VO2(3-fl)-CNT composites allows one, firstly, to decrease the value of the sensor's resistance due to the higher conductivity of the CNT and, secondly, to decrease the non-linearity of the resistance-temperature relationships. These characters are important for practical applications of temperature sensors.
Acknowledgements
The authors are thankful to the GIK Institute of Engineering Sciences and Technology for its support of this work.