Organic Semiconductor Chemiresistors

The adhesion of noble metals to the insulating substrates is generally poor. In order to overcome this problem, a thin layer (10-50nm) of adhesive metal (Cr, Ti, W) is deposited under the noble metal. Often this adhesive metal migrates through the noble metal and forms an oxide layer that significantly alters its surface 

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The emphasis on the thickness of the selective layer is linked to the mode of interaction between the analyte and the selective layer. If this interaction takes place exclusively at the surface of the selective layer, then the bulk conductivity does not contribute and represents only a shunt which decreases the signal-to-noise ratio. This is a typical case of chemiresistors based on inorganic materials (Fig. 8.6). On the other hand in chemiresistors based on organic semiconductors, the signal usually originates in the bulk of the selective layer. In that case, the response time of the sensor is affected by its thickness. 

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