Colloidal Synthesis and Photoluminescence Property of Near-IR-Responsive Ag8GeS6 Quantum Dots

Abstract

Semiconductor nanocrystals of a few nanometers in size, called quantum dots (QDs), exhibit unique photochemical properties due to quantum size effect. In recent years, high-quality near-infrared responsive QDs such as CdTe and PbS have been used for wide-ranged applications from light-energy conversions to biomedical applications. However, the contents of highly toxic elements, Cd and Pb, restrict their practical use. On the other hand, group IV-based multinary compound semiconductors, such as Ag2ZnSnS4, Ag8SnS6, and Ag8GeS6, have attracted much attention because of their less toxicity and light absorption property in visible and near-IR region. However, there are only a few reports to prepare size-quantized nanocrystals for tuning their photoelectrochemical properties. In this study, we report the preparation of Ag8GeS6 core and Ag8GeS6@ZnS core/shell QDs using a feasible liquid-phase chemical synthesis method.As a moisture and air stable Ge precursor, Ge(gly)2(H2O)2 was prepared by the literature method with slight modification[1]. Precursor powders of Ge(gly)2(H2O)2, silver(I) diethyldithiocarbamate, and thiourea were dispersed in a mixture solvent of 1-dodecanethiol and oleylamine, followed by the heat treatment at 423 K for 20 min. Thus-obtained QDs were isolated from the reaction solution by the addition of methanol and ethanol as a non-solvent. Then, the obtained QDs were finally dispersed in chloroform. The obtained Ag8GeS6 QDs were further surface-coated with thin ZnS layer to form Ag8GeS6@ZnS core/shell QDs.The onset wavelengths in the absorption spectra of Ag8GeS6 and Ag8GeS6@ZnS were observed at 850 nm and 830 nm, respectively. The XRD analysis showed that the obtained nanocrystals had an orthorhombic Ag8GeS6 crystal structure. TEM measurements revealed that the prepared Ag8GeS6 QDs had an average particle size of 4.53 nm while that of Ag8GeS6@ZnS was 5.80 nm. From the difference between the sizes before and after ZnS coating, we estimated the ZnS shell thickness of Ag8GeS6@ZnS to 0.63 nm. The energy gap of Ag8GeS6 QDs was slightly decreased from 1.45 eV to 1.37 eV with an increase in Ge/(Ge+Ag) ratio. Ag8GeS6 QDs with Ge/(Ge+Ag) = 0.8 exhibited a broad photoluminescence (PL) peak at 850 nm with PLQY of about 25%. The surface coating with ZnS remarkably increased the PLQY to 65%, which was considerably high in comparison to those of near-IR-emissive multinary QDs in our previous papers such as Ag–In–Ga–Se and Ag–In–Ga–S–Se QDs [2,3]. Thus, the present Ag8GeS6@ZnS QDs seem to be a promising probe for bio-imaging with near-IR photoluminescence.[1] A.S.R. Chesman, et al., Chem. Mater, 26, 5726 (2014).[2] T. Kameyama, et al., ACS Appl. Nano Mater, 3, 3275-3287 (2020).[3] N. Rismaningsih, et al., J. Mater. Chem. C, 9, 12791-12801 (2021).