Objective  Compared with other quantum dots (QDs), infrared QDs have narrower band gaps, wider absorption ranges, and longer fluorescence wavelengths. Therefore, they show greater potential in areas such as bioimaging, tumor treatment, photodetector and solar concentrators. As transition metal chalcogenides (TMCs), FeS QDs are promising infrared detection materials due to their narrow band gap, low toxicity, and strong near-infrared absorption. Forming therm into thin films is an effective approach to enhance the stability and processability of QDs. At present, the research about FeS mainly focuses on nanofilms and nanoparticles, and there are few reports on FeS QDs and their composite films. In this paper, we studied the preparation of FeS QDs by liquid-phase ultrasonic exfoliation, and prepared FeS/PVA composite films by mixing FeS QDs with polyvinyl alcohol (PVA). We tested and analyzed the infrared characteristics of FeS QDs in order to explore their potential applications in the field of infrared, and its application in the field of infrared optics was prospected.

  Methods  FeS QDs solution was prepared by liquid phase ultrasonic exfoliation method. The preparation steps were as follows: 0.15 g of FeS powder (purity ≥99.9%) was weighed and placed in a mortar, followed grinding for 2 h. The ground FeS powder was then mixed with 50 mL of isopropyl alcohol (IPA, purity ≥99.7%) dispersant, and placed in the ultrasonic instrument at 120 W power for 2 h. After ultrasonic, the solution was centrifuged at 500 r/min for 5 minutes, taking out the supernatant, FeS QDs solution was obtained. Collect in a reagent bottle for further use.　　FeS QDs/PVA nanocomposite films were prepared using a blending method, following the steps below: 0.4 g of PVA powder was weighed and added to a beaker containing 20 mL of deionized water. The mixture was placed on a magnetic heating stirrer and continuously stirred at elevated temperature for 45 min until the powder was completely dissolved. Then, 4 mL of the FeS QDs solution was added to the mixture, and the heating and stirring kept on an additional 15 min. Subsequently, 4 mL of the mixed solution was drop-cast onto a metal sample holder, and the film was formed by heating the sample holder on a heating plate at 40 ℃ for 4 h. 　　FeS QDs were characterized and analyze for size, morphology, structure, and elemental composition using transmission electron microscopy (TEM), atomic force microscopy (AFM), and energy spectroscopy (EDS). The phase composition and bonding properties of FeS QDs were analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The optical properties of FeS QDs and FeS QDs/PVA nanocomposite films were studied using UV-Vis spectrophotometer and fluorescence spectrometer.

  Results and Discussions   Both FeS QDs and FeS QDs/PVA nanocomposite films exhibit significant absorption and luminescence characteristics in the infrared band (Fig.4(a), (b), (c)). As the excitation wavelength increase, the PL peak of the FeS QDs/PVA nanocomposite film shows a clear redshift, which shows obvious Stokes shift and excitation wavelength dependence (Fig.5(e)).

  Conclusions  FeS QDs with an average particle size of 8.1 nm were successfully prepared by liquid phase ultrasonic exfoliation method. FeS/PVA nanocomposite films were prepared by blending FeS QDs with PVA. UV-Vis tests show that FeS QDs and FeS/PVA nanocomposite films exhibit absorption from ultraviolet to infrared band (200-2500 nm). PL test shows that they have photoluminescence in infrared band. PL peaks show significant redshift and Stokes shift, indicating that both are wavelength dependence. In addition, FeS/PVA nanocomposite film shows excellent infrared optical properties, especially the absorption and luminescence characteristics in the infrared band. These results show that FeS QDs and its nanocomposite films have important application potential in the field of infrared optics, and provide a new idea for the development of infrared optical devices.