Synthesis and Near-IR Photoluminescence Property of Ag-Au-S Quantum Dots
Authors: Akiyoshi Kazutaka , Hasegawa Mariko , Miyamae Chie , Kameyama Tatsuya , Sato Hiroki , Ohshima Yusuke , Torimoto Tsukasa 
Published: December 2024

Quantum dots (QDs), semiconductor nanoparticles, exhibit size-dependent tunable optical properties, a phenomenon known as the quantum size effect [1]. They have attracted much attention for the application to biomedical imaging, solar cells, photocatalysts, and light-emitting diodes (LEDs) [2]. Over the past few decades, extensive research has been conducted on binary QDs composed of group II–VI and IV–VI elements, such as CdTe, HgTe, and PbS. However, the toxicity associated with the content of heavy metal elements, such as Cd, Hg, and Pb, has severely restricted their practical use [3]. Recently, there has been intensive investigation into QDs with narrower energy bandgaps (Egs) for developing novel materials that exhibit near-IR photoluminescence (PL), with potential applications as in vivo biomarkers and near-IR sensors. In our previous papers, we successfully prepared I-III-VI-based QDs of less toxicity, such as Ag(In,Ga)(S,Se)2, which exhibited a sharp bandedge PL, the peak wavelength being controllable with their size and composition [4]-[6]. In this study, we report the solution-phase synthesis of Ag-Au-S QDs and investigate their near-IR photoluminescence property.

Colloidal Ag-Au-S QDs were synthesized by reacting corresponding metal complexes and sulfur compounds as precursors in a hot organic solution. The resulting Ag-Au-S QDs were isolated by the addition of methanol or ethanol as a non-solvent followed by dispersing in chloroform. Ag-Au-S QDs of several nanometers were formed and their Eg controlled between 2.3 and 1.3 eV by tuning the Ag fraction in the precursors. Since the estimated Eg values were larger than that of the bulk Au2S (1.8 eV) and Ag2S (1.1 eV) [7], it was found that the obtained Ag-Au-S QDs showed a quantum size effect. The electronic energy structure of these QDs was assessed by measuring their ionization potential using photoelectron yield spectroscopy in air (PYSA). The valence band maximum level remained relatively constant, regardless of the Ag fraction, while the conduction band minimum shifted to lower energy as the Ag fraction in the QDs increased. The PL peak originating from defect sites was controlled within the range of 735 nm to 835 nm in the near-IR region, by adjusting the particle composition. Furthermore, the PL quantum yield of Ag-Au-S QD shows remarkable enhancement with InSx shell coating, owing to the removal of non-radiative recombination sites on the surface of Ag-Au-S QD. In conclusion, the prepared Ag-Au-S QDs are composed of low-toxicity elements and exhibit near-IR PL suitable for the application to in vivo imaging.

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