J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 0, 2 0 0 1, 1009 – 1011
Praseodymium bis[phthalocyaninato] complex based gas sensor using a charge-flow transistor D A N X I E ∗, Y A D O N G J I A N G Department of Materials Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China E-mail: [email protected] JIANZHUANG JIANG Department of Chemistry, Shandong University, Jinan 250100, People’s Republic of China ZHIMING WU, YANRONG LI Department of Materials Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China
Charge-flow transistor (CFT) is a special metal-oxidesemiconductor field effect transistor (MOSFET) in which a portion of the gate electrode is replaced by an organic film (Fig. 1). The charge-flow transistor was first reported by Senturia et al. . The major advantage of using CFT in sensor applications, instead of using a conventional interdigitated electrode pair structure, is the level of current measured. Because the organic films used are typically highly resistive, the current measured using the electrode pair device is in the nanoampere to picoampere range, requiring complex detection and good shielding to avoid excessive problems with noise. While the current measured using CFT device is in the microampere to millampere range. Another advantage includes the reduction in the device size, the possibility of integrating the detection circuitry with sensors, and so on. On applying gate-source voltage Vgs , greater than the FET threshold voltage, the metallic sections of the gate electrode charge rapidly to the applied voltage. Due to the high resistance of the organic film, there is a delay before the film is uniformly charged to the applied voltage. Therefore, a complete conducting source-drain channel is not formed until the threshold voltage is exceeded along the whole film, resulting in a delay between applying Vgs and the drain current reaching its saturation value. The time required for this charging process depends on the sheet resistance of the film, the width of the gap in the gate electrode and the dielectric constant and thickness of the SiO2 insulator. When exposing to different gas ambients, the film resistance will vary, hence, the time taken for the drain current to saturate will change following, which suggests that the CFT device can be used as an effective gas sensor [1–3]. The Langmuir-Blodgett (LB) technique is a promising means to develop highly-ordered conducting organic thin films. Because such ultrathin films have high ratios of surface area to bulk volume, the use of organic gas-sensitive materials and LB deposition technique will show a great potential for improving the per-
formance of gas sensors. It can be expected to obtain an efficient and quick response gas sensor by using LB technique with good molecular packing and with its gas-sensitive molecular groups aligned near the surface of LB film [4–6]. In order to fabricate stable gas sensors, careful selection of the film-forming materials is required. It is known that metal-substituted phthalocyanines are extremely sensitive to oxidizing gas. There are many reports about mono-phthalocyanine gas sensors. However, there has been no systematical research on the gas-sensitivity of the novel phthalocyanine material–substituted bis(phthalocyaninato) rare earth double-deckers, which show great potential on molecular electronics, gas sensor, electrochromic and molecular magnetic, etc. [7, 8]. There are fewer reports on substituted bis(phthalocyaninato) rare earth complex based CFT gas sensor by LB technique. In this paper, we report the characterization of substituted bis(phthalocyaninato) praseodymium complex Pr[Pc∗ ]2 (Pc∗ = Pc(OC8 H17 )8 ) and octadecanol(OA) on different subphases, as well as the gas-sensing property to NO2 gas of CFT based gas sensor. The new substituted bis(phthalocyaninato) praseodymium complex Pr[Pc∗ ]2 was synthesized by the method described in reference . The molecular structure of Pr[Pc∗ ]2 is shown in Fig. 2. Pr[Pc∗ ]2 LB films
Figure 1 A schematic cross-section of Pr[Pc∗ ]2 /OA mixed LB film based CFT sensor.
∗ Author to whom all correspondence should be addressed. C 2001 Kluwer Academic Publishers 0261–8028
Figure 4 The UV-vis spectra of Pr[Pc∗ ]2 /OA mixed LB films with various number of layers (inset the plot of the absorbance at 641.5 nm and 358.5 nm versus the number of LB film layers). Figure 2 Molecular structure of Pr[Pc∗ ]2 (Pc∗ = Pc(OC8 H17 )8 ; R = OC8 H17 )).
were deposited with WM-1 LB instrument. Spreading solution was prepared by dissolving Pr[Pc∗ ]2 in chloroform. The concentration of this solution was 0.42 mg/ml. In order to produce stable Langmuir films, Pr[Pc∗ ]2 was mixed with octadecanol(OA) in different molar ratios of 1 : 1, 1 : 3 and 1 : 6. Two types of subphase were employed: a pure water subphase (pH 6.9) and 10−4 M Cd2+ subphase (pH 5.7). The monolayer was then compressed at a speed of 3 mm/min and the surface pressure was monitored by Wilhelmy balance. Based on standard MOSFET design, an array of four CFTs has been fabricated as a set of devices, the width of the gap in the gate electrode is 0 µm, 15 µm, 25 µm and 35 µm respectively. The basic structure is shown in Fig. 1. UV/vis absorption spectra were measured with UV1100 spectrophotometer. After placing the samples in the gas-testing system, the gas-sensing property of CFT sensor was measured. Surface pressure vs. area isotherms recorded for monolayers of pure Pr[Pc∗ ]2 and Pr[Pc∗ ]2 /OA mixture on both pure water and 10−4 M Cd2+ subphases are shown in Fig. 3. It is found that Pr[Pc∗ ]2 /OA(1 : 3) mixed monolayer in Cd2+ subphase shows a better film-
forming stability. The mixture was transferred to substrates at 36.5 mN/m by the vertical dipping method. Fig. 4 gives the UV-Vis. absorption spectra of Pr[Pc∗ ]2 /OA mixed LB films with various layers. From the inset of Fig. 4, we can see that the plot of the absorbance at 641.5 nm and 358.5 nm of the deposited LB films versus the number of LB film layers results in a straight line respectively, which indicates a constant transfer ratio during sequential dipping of the slide through the film with uniform deposition. To investigate the gas-sensing property, CFT sensor were exposed to NO2 repeatedly at room temperature. Fig. 5 shows the “turn-on” response for 35 µm gate-hold CFT sensor with 60-layer Pr[Pc∗ ]2 /OA(1 : 3) mixed LB films on exposure to different NO2 concentration. With the increase of NO2 concentration, the rate of turn-on response of source-drain current (Ids ) increases. It can be seen that there is a detectable response for concentration of NO2 as low as 5 ppm. Because phthalocyanine is a kind of p-type organic semiconductor, the gas sensitivity is realized through the charge transfer interaction in which the gas molecule to be sensed acts as a planar π -electron acceptor forming a redox couple, and the positive charge produced is delocalized over the two phthalocyanine macrocycles causing the decrease of the resistance, which will lead to the shortening of
Figure 3 Surface pressure vs. area isotherms of Pr[Pc∗ ]2 /OA mixture in the molar ratios (1 : 0; 1 : 1; 1 : 3) on pure water and 10−4 M Cd2+ subphases at temperature of 25 ◦ C.
Figure 5 Source-drain current Ids vs. time for 60-layer Pr[Pc∗ ]2 /OA mixed LB film based CFT sensor with 35 µm gate-hold in various NO2 concentrations.
Figure 6 Sensitivity plot of CFT sensor (35 µm gate-hold) for turn-on response on exposure to different concentration of NO2 gas.
the time taken for LB film to charge [7, 10]. When NO2 gas concentration and the width of the gate-hold vary, the film resistance of the gate area changes too, resulting in the variation of the turn-on response. It takes 40 min for the CFT sensor to recover to 80% of initialization, but complete recovery needs another 40 min again. This may be due to the rapid desorption of the NO2 molecules adsorbing on the LB film surface at initial recovery stage. However, in the period of latter longer recovery stage, desorption of the NO2 molecules adsorbing on the LB film surface and diffusing into the film is a complicated process . Of course, the interaction process between LB film and the adsorption gas is a dynamical process, adsorption of NO2 gas molecules occurs during the process of desorption. Through calculating the gradient of the steepest region of the straight-line section of the turn-on response curve, the sensitivity of the CFT sensor was obtained which can be seen from Fig. 6. There is approximately a linear relationship between the increasing rate of drain current and the concentration of NO2 gas. Mixing Pr[Pc∗ ]2 with octadecanol in different molar ratios of 1 : 1, 1 : 3 and 1 : 6 greatly improves the film-forming characteristic. It is especially obvious for 1 : 3 Pr[Pc∗ ]2 : octadecanol on 10−4 M Cd2+ subphase. UV-vis spectra show the transferred materials in
the spreading solution in each deposition are approximately equivalent. A new gas sensor has been fabricated by incorporating the multilayer Pr[Pc∗ ]2 /OA mixed LB film into the gate electrode of a MOSFET, forming an array of CFT device. It is found that 35 µm gate-hold CFT sensor with 60-layer Pr[Pc∗ ]2 /OA mixed LB film can detect NO2 gas as low as 5 ppm. Therefore, detection to different gas with lower concentration can be realized using the turn-on effect of such CFT device. At the same time, it is feasible to achieve the miniaturization and integration of all kinds of sensors integrating with microelectronic fabrication process. Because Pr[Pc∗ ]2 has two planar phthalocyanine macrocycles, it is very favorable for charge transfer interaction between certain electrophilic gas molecule and π -electron of the phthalocyanine macrocycle, which shows more remarkable sensitivity than monophthalocyanine. Therefore, substituted bis(phthalocyaninato) rare earth complexes are promising organic materials to develop gas sensors. References 1. S . D . S E N T U R I A ,
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Received 23 August 2000 and accepted 6 February 2001