Synthesis and Study of Stable and Size-Controlled ZnO-SiO2 Quantum Dots: Application as a Humidity Sensor

This study reports a synthesis by the sol-gel method of stable and size-controlled ZnO quantum dots (QDs) embedded into a SiO2 matrix in the diameter range of 2.7-5.5 nm with strong visible emission in the blue-green region. X-ray diffraction and selected area electron diffraction techniques evidence a wurtzite structure of the prepared ZnO QDs. Transmission electron microscopy investigation highlights that the ZnO nanoparticles produced in methanol solution have a nonuniform shape. Addition of tetraethyl ortho-silicate (TEOS) in the solution at different times blocks the nucleation of the QDs and allows the control of the QD size due to a SiO2 capping. Moreover, Fourier transformed infrared analysis and X-ray photoelectron spectroscopy (XPS) were used to show the ZnO QD composition and to confirm their incorporation into the SiO2 matrix by the creation of a Zn-O-Si crossing link. A strong UV band and a weak visible band are revealed at room temperature by photoluminescence (PL) measurement for pure ZnO nanoparticles, whereas, when TEOS is added, the UV band is quenched and a strong visible emission in the range 400-650 nm is enhanced. PL spectra fitting, using Gaussian curves, shows three possible transitions that induce the visible emission. This PL study is correlated to the quantitative XPS study combined with a simple model describing the nanoparticles giving the relative amount of species involved in this emission mechanism. These QDs are then used to manufacture humidity sensors. Indeed, electrical measurements show a high sensitivity to the relative humidity. Moreover, a remarkable enhancement of the sensor performance is observed when the ZnO QD size decreases.