Microelectronic Engineering 88 (2011) 1037–1041
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V2O4–PEPC composite based pressure sensor Kh.S. Karimov a,b, M. Abid a, M. Mahroof-Tahir c, M. Saleem a,d,⇑, Adam Khan a, Z.M. Karieva e, M. Farooq a a
GIK Institute of Engineering Sciences and Technology, Topi 23640, District Swabi, Pakistan Physical Technical Institute of Academy of Sciences, Tajikistan c Saint Cloud State University, 720 Fourth Avenue South, Saint Cloud, USA d Government College Township, Lahore 54770, Pakistan e Tajik Technical University, Rajabov St. 10, Dushanbe 734000, Tajikistan b
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 4 January 2011 Accepted 24 January 2011 Available online 1 February 2011 Keywords: Vanadium oxide Poly-N-epoxypropylcarbazole Composite Micropowder Pressure sensor Resistance
a b s t r a c t In this study, V2O4–PEPC based pressure sensor was designed and fabricated by drop-casting the blend of V2O4–PEPC microcomposite thin films of vanadium oxide (V2O4) micropowder (10 wt.%) and poly-Nepoxypropylcarbazole, PEPC (2 wt.%) in benzol (1 ml) on steel substrates. The thickness of the V2O4–PEPC films was in the range of 20–40 lm. The DC resistance of the sensor was decreased in average by 24 times as the pressure was increased up to 11.7 kNm2. The resistance–pressure relationships were simulated. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Pressure sensors are used for control and monitoring of pressure in thousands of everyday applications. Pressure sensors are mostly fabricated on the basis of piezo-resistive, capacitive, inductive and piezoelectric elements [1,2]. Someya et al. [3] fabricated a network of pressure sensors with organic transistors based on pentacene. The drain–source current was increased from 15 nA to 6.7 lA, under an applied pressure of 30 kPa. Piezoelectricity and electrostriction were observed in organic semiconductor Schottky junctions due to the presence of non-uniform spatial electric field distribution in the junction and softness of organic semiconductors. This effect can be potentially used for the fabrication of electromechanical sensors [4]. The strong piezo-resistive effect was observed on the indium-tin oxide (ITO)/tris-(8-hydroxyquinoline) aluminum (Alq, 80 nm) (or bathocuproine (BCP, 70 nm))/Al devices [5]. The current showed a high sensitivity to the pressure. It was found that the pentacene (C22H14) transistors with solution processed polyvinylphenol gate dielectric on glass substrate were sensitive to uniaxial mechanical pressure applied with a needle [6]. A flexible organic field effect transistor (OFET), based on penta⇑ Corresponding author at: Faculty of Engineering Sciences (FES), GIK Institute of Engineering Sciences and Technology, Topi 23640, District Swabi, Pakistan. Tel.: +92 938 271858x2306. E-mail addresses:
[email protected],
[email protected] (M. Saleem). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.074
cene, was fabricated and investigated [7] and it was found that the output current was dependent on the pressure that was applied by a mechanical stimulus by means of a pressurized air flow. Complexes of poly-N-epoxypropylcarbazole (PEPC) are photosensitive organic semiconductors, having good adhesive properties and are used for the fabrication of solar cells and photocapacitors [8]. A number of resistance strain gauges have been fabricated and investigated on the basis of low molecular organic semiconductors tetracyanoquinodimethane (TCNQ) ion-radical salts crystals [9]. High sensitive resistance strain gauges based on PEPC were fabricated and investigated as well [10]. Vanadium oxide (V2) shows the large reversible change of electric, magnetic and optical properties at temperatures around 68–70 °C [11,12]. In the infrared spectrum of this material, transmission of semiconductor to metal is observed. At transition temperature, optical properties of vanadium dioxide are quickly changed: the optical transmission is decreased and reflectivity is increased. Due to this behavior vanadium dioxide is an attractive material for smart windows for solar energy control, electrical and optical switches. Microstructure and crystallinity of the films effect hysteresis of the transition. By the addition of transition metals such as niobium, molybdenum or tungsten, the transition temperature of vanadium dioxide may be decreased. Fabrication of the pressure sensors and investigations of their properties on the basis of vanadium oxide and PEPC composites would be useful from practical point of view and for deepening of the knowledge about the physical properties of the composites. It would be reasonable
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to investigate the resistance–pressure relationships. In this paper we have designed, fabricated and investigated the sandwich-type resistance pressure sensors based on V2O4–PEPC composite.
2. Experimental Molecular structure of the PEPC is presented in Fig. 1. The PEPC was synthesized in the laboratory [10] and V2O4 micropowder was commercially purchased from ALDRICH and was used without further purification. The steel substrates of thickness of 3 mm were cleaned by acetone. The blend of vanadium oxide micropowder V2O4, (10 wt.%) and poly-N-epoxypropylcarbazole, PEPC (2 wt.%) in benzol (1 ml) was drop-casted on the steel substrates to fabricate V2O4–PEPC microcomposite thin films. The thicknesses of the V2O4–PEPC films were in the range of 20–40 lm. Fig. 2 shows optical image of V2O4–PEPC microcomposite film obtained by optical microscope, Leica DM 6000M. It is seen that the structure of the film is not uniform and it contains clusters of the particles. The largest sizes of V2O4 microparticles were in the range of few microns to 50–80 lm. Fig. 3 shows SEM micrograph of the V2O4–PEPC film obtained by JEOL JSM-6460. As a top electrode, thin aluminum foil of thickness 40 lm and size of 5 mm 5 mm was used to make the sandwich-type resistance pressure sensor based on V2O4–PEPC (Fig. 4). Fig. 5 shows experimental setup for the investigation of pressure sensor’s properties. The setup consists of the following elements: support (1), weight holder (2), weights (3), metallic squeezing disk (4) of diameter 8 mm and elastic rubber film (5) of 0.5 mm thickness. The pressure sensor ((6) is aluminum foil, (7) is V2O4–PEPC composite film, (8) is steel substrate, (9) is support, (10) and (11) are terminals) is placed between the support and the rubber film. The value of the pressure was changed by changing the values of the weights. The main parts of the experimental setup, i.e. weight holder and weights were used from the conventional laboratory setup ‘‘Flexor: Cantilever Flexure Frame’’.
Fig. 3. SEM image of the V2O4–PEPC composite film.
Pressure
Terminals
Al foil Film Steel Substrate
Fig. 4. Schematic diagram of the steel/V2O4–PEPC/Al resistance pressure sensor.
n N CH=CH-CH2OFig. 1. Molecular structure of poly-N-epoxypropylcarbazole (PEPC).
Fig. 5. Experimental setup for the investigation of pressure sensor’s properties with installed pressure sensor: support (1), weight holder (2) with weights (3), metallic squeezing disk (4) and elastic rubber film (5); pressure sensor: aluminum foil (6), V2O4–PEPC composite film (7), steel substrate (8), support (9) and terminals (10) and (11).
Fig. 2. Optical microscope image of the V2O4–PEPC composite film.
The DC resistance was measured by FLUKE 87 true rms multimeter at room temperature.
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3. Results and discussion Fig. 6 shows resistance–pressure relationships for one of the V2O4–PEPC sensors during increase and decrease in pressure. In the range of experimental errors, no hysteresis is observed during experiments. The DC resistance of the sensor decreased almost 24 times with an increase in pressure up to 11.7 kNm2. Fig. 7 shows relative resistance–pressure relationship for the three V2O4–PEPC sensors. The effect of pressure on the sensor’s resistance was large at thicker V2O4–PEPC films of 40 lm and less at the average films thickness of 30 lm. The sensor’s resistance (R) can be represented by the following expression [13]:
R¼
dq d ¼ A rA
ð1Þ
where d is the length or inter-electrode distance and A is the crosssection of the sample (in this case, area of the aluminum foil electrode), q is resistivity (q ¼ r1 , where r is conductivity). As V2O4– PEPC is a microcomposite, the resistance–pressure relationship shown in Fig. 6 may be due to the change of geometrical parameters of the sample and intrinsic properties. This is due to the decrease in
2.5
Resistance (kΩ )
2.0
1.5
1.0
0.5
0.0 0
2
4
6
8
10
12
-2
Pressure (kNm ) Fig. 6. Resistance–pressure relationships of the V2O4–PEPC based sensor at increasing and decreasing of the pressure.
the thickness (d) of the film under squeezing by the disk and increase in the conductivity (r) of the composite due to the decrease in the distances between the V2O4 particles and PEPC molecules in the PEPC matrix (Eq. (1)) and accordingly increases in the conductivity, concentration of their charge carriers and mobility. It can be assumed that the second process is dominating. For the simulation of the experimental results given in Fig. 6, the exponential function [14] can be used:
f ðxÞ ¼ ex
In this case, Eq. (1) can be represented by the following expression:
R ¼ epK Ro
ð3Þ
where p is pressure, K is resistance pressure factor and Ro is the resistance of the sensor at 0 kNm2. From the experimental data shown in Fig. 6, the pressure factor K was determined (K = 0.21 kN1m2) for p = 8.5 kNm2. Fig. 8 shows good agreement between experimental and simulated results. The decrease in the sensor’s resistance with an increase in pressure can be explained by the following way. The composite consists of two components: V2O4 and PEPC. The conductivity of the PEPC is lower (4 109 cm1 X1 [10]) with respect to the conductivity of V2O4 (6 104 cm1 X1 [6]), and, actually, conductivity of the composite (3 1011 cm1 X1) is controlled by PEPC. Taking into account that the conductivity of the composite is lower than the conductivity of the PEPC and V2O4, it can be assumed that at the interface of the V2O4–PEPC junctions, there are depletion regions. Therefore, V2O4–PEPC system can be considered as a bulk heterojunction system. This may be one of the reasons that the composite shows high sensitivity to uniaxial compression, as observed from the resistance–pressure relationships (Fig. 6). In multicrystalline organic semiconductor thin films as in disordered system, mostly hopping mechanism of conduction was observed [15–17], due to phonon assisted hopping of carriers from one localized state to another. As a rule, the mobility increases with temperature, but it depends on the actual contribution from scattering phenomenon. It was found that the mobilities may increase, decrease or remain constant with change in temperature for different organic semiconductors [16,17]. Usually a value of mobility around 1 cm2 V1 s1 is a boundary between band transport and hopping mechanisms [15–17].
Sample 1 (20 μm V 2O4)
1.0
ð2Þ
Experimental Simulated
1.0
Sample 2 (30 μm V 2O4) Sample 3 (40 μm V 2O4)
0.8
0.8
R/R 0
R/R 0
0.6
0.6
0.4
0.4 0.2
0.2 0.0
0.0
0
2
4
6
8
10
12
-2
Pressure (kNm ) Fig. 7. Relative resistance–pressure relationships for the three V2O4–PEPC based sensors.
0
2
4
6
8
10
12
-2
Pressure (kNm ) Fig. 8. Experimental (solid line) and simulated (dashed line) relative resistance– pressure relationships of the V2O4–PEPC based pressure sensor.
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Ioffe and Regel [18] have shown that at mobility lower than 1 cm2 V1 s1, the mean free path of carriers calculated from band transport approach is less than the inter atomic distances in semiconductors that would be incorrect from physics point of view. Tunneling effect is the most universal phenomenon, but it may contribute mostly at very low temperatures where band and hopping mechanisms have less contribution. It is believed that in the tunneling mechanism mobility shows temperature independent behavior and values of mobility are very low (l 1 cm2 V1 s1). The mechanism of conductivity in PEPC can be considered as thermally assisted hopping transitions between spatially separated sites, molecules or particles that can be attributed to the Percolation Theory [19,20]. The average conductivity (r) of the one component (in this case PEPC), according to Percolation Theory, can be calculated by the following expression:
r¼
1 LZ
ð4Þ
12.0
2.5
10.5
V2O4
1.5
9.0
7.5
Composite
1.0
6.0
0.5
4.5
3.0
0.0 0
2
4
6
8
10
12
-2
where L is a characteristic length depending on the concentration of the sites, Z is the resistance of the path with the lowest average resistance. With an increase in pressure, the composite film will be squeezed between steel substrate and aluminum foil that may cause, firstly, the decrease of L and secondly, lower Z. As a result, conductivity increases and the resistance of the sample decreases accordingly, as observed experimentally (Fig. 6). In V2O4–PEPC system, it may be considered that the conductivity of PEPC is much less than that of V2O4. Therefore, at low concentration of the V2O4 particles, where they have no direct electric contact with each other, the conductivity of PEPC can be controlled by the conductivity of the composite. At larger or critical concentration of V2O4 in the V2O4–PEPC composite, the V2O4 particles can form conductive channels that can increase the composite’s conductivity crucially. Fig. 9 shows log relative resistance–log pressure relationship for the V2O4–PEPC sensor. It is seen that the graph is quasi-linear. It means that the original graphs can be linearized by the nonlinear op-amps [21] that are important for practical application of the sensors. The V2O4–PEPC composite’s pressure sensing properties were compared with press-tablets of size of 5 mm in diameter and 0.8 mm thickness, prepared from pure V2O4 micropowder at pressure of 1500 bar. It was found that the effect of pressure on the resistance of the pure V2O4 sample (Fig. 10) is almost 10 times less in comparison with the composite (Fig. 6).
-0.75
-0.80
-0.85
Log [R/R0]
3.0
2.0
R (kΩ)
1040
-0.90
-0.95
-1.00
-1.05 0.6
0.7
0.8
0.9
1.0
1.1
-2
Log[Pressure (kNm )] Fig. 9. Log Relative resistance–log pressure relationship for the V2O4–PEPC based sensor.
Pressure (kNm ) Fig. 10. Resistance–pressure relationship for the pure V2O4 sample.
4. Conclusion The sandwich-type steel/V2O4–PEPC/Al pressure sensor was designed, fabricated and investigated. The resistance–pressure relationships were simulated. The resistance of the sensor was observed to decrease as the pressure increased. For the explanation of the conduction mechanism, the Percolation Theory is used. The V2O4–PEPC system is assumed as a bulk heterojunction system that results in high sensitivity of the composite due to the squeezing effect. The DC resistance of the sensor decreased almost 24 times with an increase in pressure up to 11.7 kNm2. Acknowledgements The authors acknowledge support of Higher Education Commission (HEC) Pakistan and GIK Institute of Engineering Sciences and Technology in all respects in completing this research study. References [1] C.D. Simpson, Industrial Electronics, Prentice Hall, Inc., New Jersey, USA, 1996. [2] J.W. Dally, W.F. Riley, K.G. McConnell, Instrumentation for Engineering Measurements, second ed., John Willey & Sons, Inc., New York, USA, 1993. [3] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y. Murase, H. Kawaguchi, T. Sakurai, PNAS 102 (2005) 12321–12325. [4] G. Dennler, C. Lungenschmied, N.S. Sariciftci, R. Schwodiauer, S. Bauer, H. Reiss, Appl. Phys. Lett. 87 (2005) 163501–163503. [5] G.Y. Zhong, Y. Liu, J. Song, Q. Zhao, Y.S. Li, F.Y. Li, J. Phys. D: Appl. Phys. 41 (2008) 205106. [6] G. Darlinski, U. Bottger, R. Waser, J. Appl. Phys. 97 (2005) 093708. [7] I. Manunza, A. Sulis, A. Bonfiglio, Appl. Phys. Lett. 89 (2006) 143502. [8] Kh.S. Karimov, K. Akhmedov, I. Qazi, T.A. Khan, JOAM 9 (2007) 2867–2872. [9] Kh.S. Karimov, Electrophysical properties of low-dimensional organic materials at deformation, Thesis of D.Sc., Academy of Sciences, Tashkent, Uzbekistan, 1993. [10] Kh. Akhmedov, Synthesis, properties and application of carbazolile containing polymers, Thesis of D.Sc., Academy of Sciences, Dushanbe, Tajikistan, 1998. [11] G. Guzman, Vanadium dioxide as infrared active coating, , (accessed 14.10.10). [12] E.E. Chain, Appl. Opt. 30 (1991) 2782–2787. [13] J.D. Irwin, Basic Engineering Circuit Analysis, sixth ed., John Wiley & Sons, New York, 1999. [14] A. Croft, R. Davison, M. Hargreaves, Engineering Mathematics, A Modern Foundation for Electronic, Electrical and Control Engineers, Addison-Wesley Publishing Company, Great Britain, 1993. [15] C.D. Dimitrakopoulos, D.J. Mascaro, IBM J. Res., & Dev. 45 (2001) 11. [16] F. Gutman, L.E. Lyons, Organic Semiconductor, Part A, Robert E. Krieger Publishing Company, Malabar, Florida, 1980. p. 251. [17] F. Gutman, H. Keyzer, L.E. Lyons, R.B. Somoano, Organic Semiconductors, Part B, Robert E. Krieger Publishing Company, Malabar, Florida, 1983. p. 122.
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