Synthesis and Surface Characterization by Raman Spectroscopy of Water-Dispersible ZnS:Mn Nanocrystals Capped with Mercaptoacetic Acid

Abstract

Semiconductor nanocrystals have been extensively studied due to their size-dependent optical properties and wide applications. Many efforts have been made to develop nanoparticles with superior optical characteristics, e.g., higher quantum yields, more accessible absorption wavelengths, and wider absorption and narrower emission bands. The incorporation of transition metal ions into semiconductor nanocrystals induces radiative recombination of excited electron-hole pair at doping ion sites rather than at surfaces, which shifts photoluminescence wavelength and increases quantum efficiency. Therefore, transition metal-doped semiconductor nanocrystals have great potential applications as a novel material for display and light-emitting devices. The functionalization of surfaces is another important aspect of nanoparticle fabrication. In particular, it is important to make nanoparticles water-dispersible for many biological applications. Surfactants play important roles in the control of dispersed particle sizes, optical properties, and solubility by passivating nanocrystal surfaces. Here, we report on the synthesis and properties of water-dispersible Mn-doped ZnS nanoparticles stabilized by mercaptoacetic acid (ZnS:Mn-MAA). MAA is widely used as a capping agent in the synthesis of nanoparticles. MAA is also the simplest molecule that has two terminal functional groups, a thiol and a carboxylic acid, which can be deprotonated, allowing it to bind to the surfaces of nanoparticles. In this respect, it would be interesting to determine the nature of surface-bound MAA and, in particular, to determine which functional group is involved in this binding. Here, we present the Raman spectra of MAA adsorbed on the surfaces of ZnS:Mn nanocrystals and propose a structure for surfacebound MAA based on spectral analyses and density functional theory (DFT) calculations.