Cadmium selenide quantum dots: Synthesis, characterization and their humidity and temperature sensing properties with poly-(dioctylfluorene)

Abstract

In this paper, we report synthesis, thin film characteristics and sensing properties of cadmium selenide (CdSe) quantum dots (QDs) for enhancing the humidity and temperature sensing of a polymeric semiconductor poly-(9,9-di-n-octylfluorenyl-2,7-diyl) (F8). Transmission electron microscopy (TEM) has been performed to study the texture, distribution over surface and size of the synthesized CdSe QDs. TEM image revealed slightly non-uniform distribution of the QDs with an average size of 4 nm having three-dimensional quantum confinement features. Atomic force microscopy (AFM) is carried out to investigate the average grain size and surface roughness of the F8-CdSe nanocomposite film. To understand the role of CdSe QDs on the sensing properties of F8, a blend F8-CdSe QDs has been prepared at 10:1 wt-percent (wt%) of F8 and CdSe QDs, respectively, in chloroform. An Ag/F8-CdSe QDs/Ag surface-type sensor has been fabricated via spin coating F8-CdSe QDs nanocomposite on a 45 μm gap between the pre-patterned silver (Ag) electrodes on glass substrate. The fabricated Ag/F8-CdSe QDs/Ag sensor has been studied for humidity and temperature sensing by measuring capacitance (C) of the sensor as a function of humidity and temperature, respectively. The Ag/F8-CdSe QDs/Ag sensor exhibits enhancement in the sensing parameters such as response time and recovery time i.e., 9 s and 7 s, respectively, as compared to our previously studied Ag/F8/Ag sensor whose response and recovery time were 15 s and 7 s, respectively. This improvement is due to the addition of CdSe QDs in F8 polymer matrix which changes some of the important properties of the F8-CdSe QDs nanocomposite such as electron polarizability, polarity, hydrophobicity and size dependent properties. The role of porosity (P) in the sensing properties of the active film of Ag/F8-CdSe QDs/Ag device has also been explored. The Ag/F8-CdSe QDs/Ag based sensor shows approximately linear change in C as a function of humidity and temperature with wide range of sensitivity from 25 percent relative humidity (%RH) to 90%RH and from 16 to 186 °C, respectively.