Structural Approach to Improve the Response Characteristics of Copper Phthalocyanine Thin Film-Based NO 2 Gas Sensor
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improvement of the gas sensitivity has been attempted by an insertion of higher-sensitive layer between the a-CuPc thin film and the glass substrate. Secondly, we have attempted to improve the gas response characteristics of the a-CuPc thin films by means of a deposition on a hydrofluoric acid (HF)-treated glass substrate. Measured gas-sensing characteristics are discussed in relation to the detailed film microstructure analyzed by an atomic force microscopy (AFM). EXPERIMENT Since the preliminary studies sugst that vanadyl phthalocyanine (VOPc) shows a relatively higher sensitivity to the NO2 gas than and a different crystal structure from the c-CuPc, the VOPc film was selected as an inserted layer between the ce-CuPc film and the glass substrate. The a-CuPc and VOPc thin films were deposited by the vacuum sublimation method on the glass substrates at room temperature in a vacuum of 10a Pa. They were sublimed at between 450°C and 500°C. The film thickness was monitored by quartz oscillator to be 200 nm with a deposition rate of 0.1 nm/s. The HF treatment of the glass substrate was done in a 55% HF solution for one minute. The 20 finger interdigitated gold electrodes were vacuum evaporated on the thin films to measure an electric conductivity in NO 2 gas diluted in N2 gas, and in N2 gas. The conducting current was monitored by using a Keithley 610C electrometer under the dc bias of 10 V. Details of the measurement are illustrated in Fig. 1. DC Power l .. . Tflienoometer •" Test Chamber T "- I SSensor
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FIGURE 1 Schematic diagram of the measurement system for the ca-CuPc thin film-based NO 2 gas sensors The film crystallographic relation and microstructure were characterized by using the Xray diffractometer (XRD, RIGAKU RAD-2A) with a Ni-filtered Cu-Ka radiation, and the AFM (JEOL JSTM-4200D) with a non-contact mode in air, respectively. RESULTS
Structural Approach to Imorove the Sensitivity
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Figure 2 shows structures of the CuPc and VOPc molecules. It is expected from the difference in the structure (plane and pyramid) that microstructures of the deposited films must be quite different between these two sources. XRD profiles shown in Fig 3 reveal that the aCuPc thin film shows only one peak at 20=6.8' corresponding to an interplane distance of 13 A [10], while no peak is detected from the VOPc films. Therefore, The a-CuPc film tends to orient uniaxially and the VOPc film seems to be amorphous like. 011 V
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Molecule structure of (a) CuPc and (b) VOPc. 20 = 6.8°
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FIGURE 3 X-ray diffraction profiles of a-CuPc, VOPc and a-CuPcNOPc thin films deposited on glass substrate at room temperature. Gas sensitivity of the VOPc thin film is higher than that of the ca-CuPc thin film in the gas concentration between 0 ppm and 100 ppm at a working tempera
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