Highly Luminescent and Temperature Stable Quantum Dot Thin Films Based on a ZnS Composite

A solution processable Zn(EtXn) 2 (octylamine) precursor has been used to deposit nanocrystalline ZnS thin films which can effectively host CdSe–CdS–ZnS quantum dots (QDs) with their native surface chemistry intact. The formation of such hybrid QD:ZnS composites proceeds through the initial decomposition of the octylamine stabilized zinc xanthate precursor to form nanocrystalline ZnS. To gain insight into this decomposition process we have utilized headspace gas chromatography–mass spectrometry (HS GC-MS), thermogravimetric analysis coupled with mass spectrometry (TGA-MS), grazing angle attenuated total reflectance Fourier transform infrared spectroscopy (GAATR FTIR), and grazing angle X-ray diffraction (GAXRD). Through these characterizations we identify that the major decomposition route of Zn(EtXn) 2 (octylamine) to form ZnS begins at 100 °C, generating predominantly CO 2, COS, CS 2, and ethanol as gaseous products. The octylamine used to solubilize the metal complex is found to remain adsorbed within the ZnS matrix up to temperatures of above 200 °C; however, due to its ability to favorably passivate the QDs, its presence only aids to increase the fluorescence thermal stability of the composite. Through the study of various alkyl amines passivating the QD surface within the ZnS composite, we identify that the role of the ZnS is to permit good chemical passivation of the QD surface and to create an electronically passivating host. Each of these factors and the additional presence of residual alkyl amines enables such composites to exhibit significantly higher fluorescence stability factors compared to neat QD films. To exemplify the highly lucrative fluorescence stability offered by these QD:ZnS composites, we study the amplified-spontaneous emission (ASE) properties of the resulting thin films and find unprecedented stability of the optical gain states.