Solution-Phase Synthesis and Composition-Dependent Optical Properties of Less-Toxic Multinary Ag–Mn–Sn–S Quantum Dots

Abstract

Introduction Multinary quantum dots (QDs) have been promising materials as an efficient light absorber in solar light energy conversion systems. These QDs including Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4 QDs, consisting of less-toxic elements, are known for their high absorption coefficient and excellent processability. However, the cation disorder in Cu2ZnSn(S,Se)4 has been observed to cause a bandgap fluctuation and an emergence of additional antisite defects1. An effective approach to mitigating such disorder involves the substitution of Cu or Zn with larger cations. Here we focus on Ag2MnSnS4 (AMTS) due to similar optical properties with Cu2ZnSn(S,Se)4 QDs. The energy gap of bulk AMTS was reported to ca. 2.0 eV2, making it suitable for efficient solar light energy conversion systems. However, creating AMTS structures can be challenging, often requiring high temperatures or pressures. Thus, a simpler synthesis route for preparing QDs will offer a new perspective in addressing this issue, and the size reduction will enable to tune their optical bandgap. In this study, we report for the first time the solution-phase preparation of AMTS QDs. The resulting QDs exhibited composition-dependent optical properties depending on the Ag/metal ratio. Experimental The synthesis of AMTS QDs was conducted by a facile colloidal heating-up method. Precursors of corresponding metal complexes and sulfur powder were dispersed in a mixture solvent of 1-dodecanethiol (DDT) and oleylamine (OLAm), followed by the heat treatment at more than 423 K. Thus-obtained solution was subjected to centrifugation to remove large precipitates, and target QDs were isolated from the supernatant by adding methanol and ethanol as a non-solvent, followed by centrifugation. Results and Discussion AMTS QDs with diameters less than 6 nm were obtained. The absorption spectra of AMTS QDs varied with the Ag/metal ratio. The onset wavelength was blue-shifted from ca. 670 nm to ca. 500 nm with a decrease in the Ag fraction. In the case of Ag/metal= 0.50, that is, the stoichiometric Ag2MnSnS4, the bandgap was determined to 2.0 eV, being similar to the bulk bandgap of monoclinic Ag2MnSnS4 2. The optical bandgap was monotonously increased from 2.0 eV to 2.4 eV as the Ag/metal ratio decreased from 0.50 to 0.05.The electronic energy structure of AMTS QDs was determined from the ionization energy of the QDs, which was evaluated from the onset energy of the photoelectron yield spectra in air. The conduction band minimum level shifted from –3.5 eV to –3.1 eV with a decrease in the Ag fraction, while the valence band maximum was constant at ca. –5.5 eV. The crystal structure and particle morphology were almost unchanged even when the particle composition was varied, indicating that the change in bandgap resulted from the composition. The stability of QDs was improved by surface coating with a ZnS shell. Subsequent ligand exchange with a hydrophilic molecule enabled the uniform dispersion of QDs in an aqueous solution without changing their optical properties and chemical composition. These findings open up possibilities for utilizing ATMS QDs in various applications such as photocatalysis and bioimaging in aqueous media. Reference 1 J. Kumar, and S. Ingole, J. Alloys Compd., 865, 158113 (2021). 2 Frank N. Keutsch, Dan Topa, Rie Takagi Fredrickson, E. Makovicky, and W.H. Paar, Mineral. Mag. 83(2), 233–238 (2019).