Preparation and optical properties of CoS QDs using liquid phase ultrasonic exfoliation

Cobalt sulfide (CoS) is a zero bandgap transition metal chalcogenides (TMCs). However, the bandgap of CoS can be altered when it exists as low-dimensional material, such as quantum dots (QDs), via different preparation methods. In this work, CoS QDs were successfully prepared by liquid-phase ultrasonic exfoliation method and CoS QDs film was obtained by spin coating CoS QDs solution onto a substrate. The morphology, structural, chemical properties, thickness, vibration peaks and chemical bonds of CoS QDs were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), atomic force microscopy (AFM), Raman spectroscopy (Raman) and X-ray photoelectron spectroscopy (XPS). The absorption and fluorescence characteristics of the CoS QDs were studied using ultraviolet-visible-near infrared (UV-Vis-NIR) and fluorescence spectroscopies. Results show that the average size of CoS QDs was 9.1 nm and average thickness was 8.4 nm and CoS QDs solution and film exhibited absorption in the infrared band. With an increase of excitation and emission wavelength, both the photoluminescence (PL) peak and photoluminescence excitation (PLE) peak of CoS QDs solution and film showed red-shift, which demonstrates Stokes shift effect and dependency on wavelength and have near-infrared luminescence characteristics. The infrared absorption and near-infrared luminescence properties of CoS QDs solution and film imply that they can be found important application in the field of infrared detection. Such novel material is expected to play a crucial role in low-cost, high performance infrared photodetector.