Bi-layer Functionally Gradiend thick film semiconducting butane sensors

National seminar on Advanced nanomaterials and its applications, Jadavpur University, 8-9 August, 08<

Introduction

Bi-layer functionally gradient thick film semiconducting butane sensors

S. Shrivastava1, R. Biswal2, S. Chakraborty3, I. Roy3, A. K. Halder3, A. Sen3,*

Abstract

Gas sensors based on metal oxide semiconductors like tin dioxide are widely used for the detection of toxic and combustible gases like carbon monoxide, methane and butane. One of the problems of such sensors is their lack of sensitivity, which to some extent, can be circumvented by using different catalysts. However, highly reactive volatile organic compounds (VOC) coming from different industrial and domestic products (e.g. paints, lacquers, varnishes etc) can play havoc on such sensors and can give rise to false alarms. Any attempt to adsorb such VOCs results in sorption of the detecting gases (e.g. butane) too. To get round the problem, bi-layer sensors have been developed. Such tin oxide based functionally gradient bi-layer sensors have different compositions at the top (high resistance in the order of M? range) and bottom layers (low resistance in the order of K?). Here, instead of adsorbing the VOCs, they are allowed to interact and are consumed on the top layer of the sensors and a combustible gas like butane being less reactive, penetrates the top layer and interacts with the bottom layer and the electrical signal generated at the bottom layer from the combustible gas is collected. Such functionally gradient sensors, being very reliable, can find applications in domestic, industrial and strategic sectors. The processing steps for the fabrication of such sensors are also simple and cost-effective.

1. Introduction:

With increasing world-wide distribution of gas sensors for different applications, the demand of sensors fulfilling specific standards is growing in leaps and bounds1-2. Incidentally, volatile organic compounds are posing as a menace for satisfactory sensor performance because they come out from many industrial and domestic products like paints, lacquers, varnishes, cosmetics and automobile exhausts and being highly reactive, they tend to interfere with the sensor operation. In this regard, a particular concern is to avoid VOC cross-sensitivity with the detecting gases as VOCs being highly reactive, can give rise to false alarm. So far, primarily two techniques have been tried to get around the problem. One of the techniques to avoid interference from unwanted vapours is to use charcoal filters3. Such sensors are satisfactory for detection of CO in presence of VOCs. The disadvantage of this method lies in the fact that charcoal filters also adsorb most of the combustible gases and hence cannot be used satisfactorily to detect combustible gases like methane, propane, CNG and LPG in presence of VOCs. The other technique is to use uncoated/coated (with Pt, Pd etc.) filters of Al2O3, SiO2, WO3 etc on SnO2 coatings4-5. Such protecting filters can, to some extent, check specifically alcohol cross-sensitivity. The second method also has drawbacks because VOCs, in general, are either not affected by such filters or, the overall sensitivity of the sensors towards the detecting gases dramatically goes down. In this study we prepare a functionally graded tin dioxide based composition for gas sensors capable of detecting combustible gases in presence of volatile organic compounds.

2. Experimental

A precursor powder (SnO2+0.25 Sb2O3+ 5Pd+0.1B) for the bottom layer was prepared by the following steps6-7. Firstly, reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of hot distilled water containing 20 drops of HCl with continuous stirring. Secondly, reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80oC) containing 5 drops of HCl. In the next step, reagent grade PdCl2 was taken in 100 mL distilled water containing 10 drops of HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80oC under constant stirring for 1 h. The three solutions were mixed and added to ammonia solution under sonication (ultrasonic processor, vibronics, 25 kHz, 250 W) and the pH of the solution has maintained at 9. The precipitate was centrifuged and dried at 100oC for 10 h. Then calculated amount boric acid solution was added into the precursor powder and again dried into a mortar pestle. Finally, the dries powder was calcined at 900oC for 2 h.
To prepare the precursor powder (SnO2+ 10 Pd+ Al2O3 (varying amount 10 to 30 wt% with respect to tin dioxide based composition)) for the top layer, again stannous chloride and PdCl2 were dissolved in distilled water following the above procedure. Reagent grade alumina powder was then mixed with the calcined powder in the different ratio by weight using an agate mortar and a pestle.
A thick paste of the powder formulation for the bottom coating was made by mixing the prepared powder for the bottom layer with dilute alumina gel and cured at 600oC for 1 h. A thick paste of the powder formulation for the top coating was made by mixing the prepared powder for the top layer with dilute alumina gel and cured at 500oC for 30 min. Gold electrodes and platinum lead wires were attached at the ends of the tubes (by curing at a higher temperature) before applying the paste. Kanthal heating coils were placed inside the tubes and the leads were bonded to nickel pins. The electrical resistance and butane (500 ppm) sensitivity of the coatings were measured at 350°C by using a digital multimeter (Solartron), a constant voltage/current source (Keithley 228A) and X–Y recorder (Yokogawa). All the fired samples were initially preheated at 350°C for 72 h to achieve the desired stability before the measurements.

3. Results and discussion

The X-ray diffractogram of calcined tin dioxide powder (SnO2+0.25Sb2O3+ 5Pd+ 0.1B) is depicted in figure 1, which indicates complete SnO2 phase formation after firing at 900°C. The crystallite size of the SnO2 based powder is calculated by scherrer formula and the value is around 27 nm.

From this spectrum a strong band associated with the anti-symmetric Sn–O–Sn stretching mode of the surface binding oxide can be observed apparently at 600 cm-1.
The percent response (S) of SnO2 based sensors in different gases at 350°C has been calculated by

S = (RA – RG)/RA × 100%

RA and RG being the sensor resistance in air and gas at the same temperature. The measurement temperature, 350°C, was selected for our studies because C4H10 sensitivity is maximum around this temperature.

It is shown that the double coated sensors (30 wt% Al2O3 doped) prepared in this way showed an average sensitivity of around 90% in 500 ppm butane at 350oC. Incidentally, the sensors show a low sensitivity of around 30% or even less when kept inside a container containing standard paint thinner (a source of conc. VOCs) or acetone or alcohol etc. whereas, under the same condition, the sensitivity in such conc. VOCs can be as high as 75%, when the sensor is singly coated, i.e., the top coating of the sensor is absent. Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs. The basic mechanism besides that the top layer of the functionally gradient bi-layer sensor contains tin dioxide, palladium and alumina. Alumina raises the resistance of the top coating to the order of 10–100 MW at the operating temperature. Such increase in resistance can be explained by considering the electronic interaction between semiconducting tin dioxide grains in close contact with the Lewis acid sites (electron acceptor) of alumina grains. Whereas, the bottom coating is devoid of alumina and contains antimony resulting in the resistance value of the order of 10–100 kW at the operating temperature. The VOCs being highly reactive, interact with adsorbed oxygen on the top layer releasing free electrons. However, due to three orders of higher resistance of the top coating with respect to that of the bottom coating, the top coating always remains shunted to the bottom coating. Less reactive gases like methane penetrate the top layer and interacts with the bottom layer of adsorbed oxygen and the change in resistance is picked up by the electrical leads at the bottom.

4. Conclusions

By modifying the chemical compositions of the top and bottom layers, novel functionally gradient bi-layer tin dioxide based sensors have been developed, which show excellent sensitivity towards methane with negligible cross-sensitivity towards volatile organic compounds. The processing steps for the fabrication of such sensors are also simple and cost-effective.

References

1. W. Gopel and K.D. Schierbaum, SnO2 sensors Current Status and Future Prospects, Sens Actuators B, 26-27 (1995) 1-12.
2. G. Sberveglieri (ed.), Gas Sensors – Principle, Operation and Development, kluwer, Dordrecht (1992).
3. M. Schweizer-Berberich, S. Strathmann, U. Weimar, R. Sharma, A. Senbe, A Peyre-Lav and W. Gopel, Sens Actuators B 58 (1999) 318-324
4. K. Komatsu and S. Sakai, U.S. patents 4,592,967 (1986).
5. C.A. Papadopoulos, D.S. Valchos and J.N. Avaritsotis, Sens Actuators B, 32(1996) 61-69.
6. A. Banerjee, A. K. Haldar, J. Mondal, A. Sen, H. S. Maiti, Bull. Mater. Sci., 25 (6) (2002) 497.
7. M. Saha, A. Banerjee, A. K. Halder, J. Mondal, A. Sen, H. S. Maiti, Sens Actuators B79 (2001) 187.

Hydrogen and humidity sensing properties of C60 thin films

Hydrogen and humidity sensing properties of C60 thin films
G. Sberveglieri a,b, G. Faglia a, C. Perego a, P. Nelli al*, R.N. Marks ‘, T. Virgili ‘, C. Taliani ‘,
R. Zamboni ’
a Department of Chemistry and Physics for Materials, INFM, via Valotti 9, I-25133 Brescia, Italy
b Department of Physics, INFM, via Paradiso 12, I-44100 Ferrara, Italy
’ CNR-Institute of Molecular Spectroscopy, via P. Gobetti 101, I-40129 Bologna, Italy



Abstract

We discuss the sensing properties of C,a thin films towards reducing gases and humidity. C,a thin films were sublimed onto alumina substrates with platinum interdigitated contacts, and a platinum heating element was deposited on the substrate back face. The electrical responses of the C6a films towards 1000 ppm of hydrogen, ethyl alcohol and carbon monoxide were measured both in dry and humid air, in the temperature range between 50 and 300 “C. At 300 “C we observe a reversible increase of the current in the presence of hydrogen, while the water vapour causes a reversible decrease in the current. We suggest that the response to hydrogen may be due to a chemical reaction between the oxygen molecules trapped in the film and the hydrogen, reducing the concentration of oxygen-based electron traps in the CGO,
hence increasing the conductivity. The reduction in conductivity in the presence of humidity is probably due to the formation of extra trap states by water molecules in the film.

1. Introduction
In the past few years, fullerenes have attracted a great deal of attention due to their novel electronic properties. They are semiconducting molecular solids with an optical bandgap of
1.7 eV [ 11; when suitably doped (e.g. with alkali metals) they show metallic or super conducting properties. Cc0 can be sublimed in vacuum in the form of thin polycrystalline films, which have been used in devices such as field-effect transistors [ 2,3] and photodiodes [ 4-61.
CeO crystals are face-centred cubic ( f.c.c.) with large interstitial voids which allow the efficient diffusion of gases through them, and make this material an interesting candidate
for gas sensing applications. In this paper we describe the detection properties of C6,, thin films towards some reducing gases.

Synthesis and characterization of Copper Oxide based nanostructure by wet chemical route

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Synthesis and characterization of Copper Oxide based nanostructure by wet chemical route
S.Das and K.K.Chattopadhyay
Department Of Nano Science And Technology Jadavpur University
Different CuO nanostructues on copper substrate were synthesized successfully using a simple wet chemical method at room temperature by varying pH and reaction time of the solution. The composition, morphology, and growth conditions of the nanostructures were characterized by XRD, AFM, SEM, and EDAX. UV-Visiable spectroscopy measurement indicates that the CuO nanostructures shows reflectivity about 25 -12 % in the wavelength of 400-1500 nm with a direct band gap lying in the range of 1.37-1.47 eV. FT-IR analyses of nanostructured CuO thin films have been performed for the films deposited on Cu substrates. FT-IR spectra of the CuO thin films shows all the bands are due to the absorption peaks of Cu-O, at 600, 421cm-1.Cu-OH shows at 3647, 3428 and at 789 cm-1 and one bond of carbon dioxide at 2334 cm-1.field emission shows that electrons are emitted by cold field emission process and turn on current is 12.2 V/ μm for the self assembled nanorods of diameter lying in the range 180–200 nm with electrode distance 130 μm.
Corresponding author: kalyan_chattopadhyay@yahoo.com

High Rectification Ratio in Nanostructured Organic-Inorganic Photodiodes

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

High Rectification Ratio in Nanostructured Organic-Inorganic Photodiodes
Santanu Karan and Biswanath Mallik*
Department of Spectroscopy, Indian Association for the Cultivation of Science,
2A & 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata-700 032, India.
High quality organic-Inorganic heterojunction photodiodes based on nanostructured copper (II) phthalocyanine (CuPc) and intrinsic zinc oxide (i-ZnO) have been fabricated. The i-ZnO thin films/ layers were grown by RF magnetron sputtering on clean ITO coated glass substrate. These films have been characterized by optical absorption and Field Emission Scanning Electron Microscopy (FESEM). CuPc thin films deposited at room temperature on i-ZnO have exhibited change in their surface morphology with the post-deposition annealing temperature under normal atmosphere. The electrical dark and photoconductivity of ITO/i-ZnO/CuPc/Au sandwich structure have been measured under various photoexcitation intensities using Xenon light source. The devices have shown excellent reproducibility of their electrical characteristics and high rectification ratio. The rectification ratio is nearly 831 calculated above the threshold voltage at room temperature. The effects of annealing temperature on the surface morphology and rectification ratio have been discussed. -1.5-1.0-0.50.00.51.01.50246810111315171678 0.00.51.01.50.00.51.01.52.0876541Total Current (
mA/cm2 )
Biased Voltage (Volt)Total Current (mA/cm2 ) Biased Voltage (Volt)
(a)
(b)
(c)
(d)
Fig. 1 (a) FESEM images of the CuPc thin filmdeposited at room temperature on i-ZnOsurface. Inset shows the morphology of i-ZnOsurface deposited on ITO. FESEM images of theCuPc thin film annealed at 100 (b), 200 (c), and250 oC (d), respectively. Insets show the surfaceat higher magnification.
Fig.2 The I-V characteristics for the devices under dark and different intensity of photoexcitation: The curves represent for (1) Dark, (2) 5, (3) 10, (4) 15, (5) 30, (6) 45, (7) 70 and (8) 100 mW/cm2, respectively. The inset refers to the magnified view in forward
After deposition of CuPc thin films on i-ZnO layer at room temperature, the deposited thin films were annealed at various temperatures. The FESEM images of the CuPc thin films annealed at different temperatures are shown in Figure 1. Figure 1a shows the FESEM image of as deposited CuPc film at room temperature. Most of the particles are almost spherical in shape and the average size is nearly 30-40 nm. The inset of Figure 1a shows the morphology of i-ZnO deposited on ITO. Figure 1b shows the surface morphology of the CuPc film annealed at 100 oC. Clearly, the aggregation of the CuPc nanoparticles is observed. Figure 1c shows the surface morphology of the CuPc film annealed at 200 oC. Here the film has been found to be formed uniformly with aggregated nanoparticles and some nanorod like structure. Figure 1d represents comparatively smoother surface for a CuPc film annealed at 250 oC. The surface shows the compactness of the nanorod like structures. Inset of Figure 1d shows the nanorod composed of small nanoparticles of size below 10 nm. The high tendency of the self-ordering of phthalocyanine molecules could be one of the main reasons for the change in film morphology depending on the annealing condition. The I-V characteristics for Pc 250 devices under different intensity of photoexcitation are shown in Figure 2. From this figure it has been found that all the devices responded to photoexcitation and gave rise to photocurrent. The RR under different intensity of photoexcitation has been calculated from Figure 2 and is listed in Table 1.
RR for the Devices
Intensity (mW/cm2 )
Pc 30
Pc 100
Pc 200
Pc 250
0
5
6.16
7.51
14.37
44.03
1.28
2.90
831.54
5.27
10
5.01
36.10
2.63
9.42
15
5.99
29.48
2.55
12.90
30
7.17
27.28
3.11
30.37
45
7.78
25.28
3.41
54.01
70
8.35
23.34
3.50
126.37
100
11.73
21.24
3.12
356.23 -1.6-1.2-0.8-0.40.0-32-24-16-8087654321Total Current (
μA/cm2 )
Biased Voltage (Volt)
TABLE 1: Rectification Ratio (RR)Calculated above the Threshold Voltage ofthe Devices under Different Intensity ofPhotoexcitation.
Fig. 3 Reverse biased I-V characteristics in dark and under photoexcitation of different intensities for the device Pc 250. The curves represent for (1) Dark, (2) 5, (3) 10, (4) 15, (5) 30, (6) 45, (7) 70 and (8) 100 mW/cm2, respectively. Inset shows the photovoltaic performance of the devices.
Typical I-V characteristics of the photodetector at reverse bias from the devices, under 5-100 mW/cm2 photoexcitation range are shown in Figure 3. I–V response under dark condition is also shown in the figure. The plot shows that upon increasing intensity of photoexcitation, the amplitude of device current at any voltage increases. At higher photoexcitation intensities, in addition to more photo generated excitons and hence more dissociated carriers, higher carrier mobility due to increase in the density of carriers may be the cause of photocurrent. At any particular photoexcitation intensity, the magnitude
of increase is also more at higher bias amplitudes. Field dependent dissociation of excitons at the interfaces can result in higher photocurrent at higher bias amplitudes. Higher bias may also pull out more carriers out of the device and leaves space for more exciton dissociation. At lower bias, charge confinement may hinder exciton dissociation.
In conclusion, for the growth of CuPc films on i-ZnO by vacuum evaporation, surface morphology of the thin films are influenced very strongly on the post-deposition annealing temperature. Good rectification and photosensitivity was observed for the devices. Higher rectification ratio was observed for the device having CuPc layer annealed at 250 oC. For the device having CuPc layer annealed at 200 oC the value of ISC is higher and RR is very small. Above the annealing temperature of 200 oC, i.e. when β-phase occurs, the value of ISC again becomes less and RR becomes higher. The results regarding effects of annealing temperature on CuPc deposited on i-ZnO may be extended to structures involving the growth of multilayer photodiodes.
Corresponing author: spbm@mahendra.iacs.res.in;

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