Size and Composition Dependent Photoelectrochemical Properties of Cu-Ga-S Quantum Dots
Authors: Komada Goro , Ofuji Shuto , Tozawa Makoto , Akiyoshi Kazutaka , Kameyama Tatsuya , Torimoto Tsukasa 
Published: December 2024

Semiconductor nanoparticles with the size of 10 nm or less, so-called quantum dots (QDs), exhibit distinct optoelectronic properties from those of bulk semiconductors due to the quantum size effect. Binary QDs such as CdSe and PbS have been extensively studied for the applications in photovoltaics, photocatalysts and luminescent materials, because the synthetic methods have been established to obtain high-quality QDs. However, the content of toxic elements such as Cd and Pb severely restricts their practical application. On the other hand, we have recently developed a solution-phase synthesis method for multinary QDs such as Ag-In-Ga-S and Cu-In-Ga-S, which exhibited photoluminescence (PL) with tunable peak wavelength depending on the particle composition.[1,2] However, the PL quantum yield of these QDs decreased significantly with decreasing In content, resulting in the absence of a PL peak for Cu-Ga-S (CGS) QDs, which lack the rate metal, In. In this study, we report the solution synthesis of CGS QDs showing PL. Furthermore, we demonstrate the tunability of PL color by doping Ag+ into CGS QDs.
CGS QDs were synthesized by reacting corresponding metal complexes and sulfur compounds in a solution mixture of oleylamine and 1-dodecanethiol at 350 oC for 10 min. After cooling to room temperature, the target QDs were isolated from the solution by adding of methanol and ethanol, and then dispersed in chloroform. Furthermore, Ag+-doped CGS QDs were prepared in a similar manner by adding a silver complex to metal precursors. The thus-obtained QDs were surface-coated with a GaSx shell, forming core-shell-structured CGS@GaSx QDs.
CGS QDs with different compositions were synthesized by adjusting the Cu/Ga ratio of the precursor from 0.4 to 1.0. TEM measurements revealed the formation of spherical particles with an average size of 4-6 nm, irrespective of the Cu/Ga ratio. XRD patterns confirmed the hexagonal crystal structure of CGS QDs. Absorption spectra of CGS QDs were blue-shifted with a decrease in the Cu/Ga ratio from 1.0 to 0.4, the absorption onset being changed from 600 nm to 540 nm. The energy gap (Eg) of QDs, determined from the absorption onset, was tunable between 2.2 and 2.5 eV by varying the Cu/Ga ratio. Through adjustments in synthesis conditions, the average size of CGS QDs could be controlled in the range of 3-20 nm. Consequently, their Eg increased from 2.0 eV to 2.7 eV with a decrease in particle size, attributed to the quantum size effect. Although prepared CGS QDs did not exhibit photoluminescence (PL), the GaSx shell-coated CGS QDs (CGS@GaSx) showed a broad PL peak at 550 nm. Doping Ag+ ions into CGS cores caused a blue shift of PL peak wavelength. These results suggest that defect sites on the particle surface were removed by the GaSx shell coating and that the Eg of CGS@GaSx QDs increased with Ag+ doping.

References

[1] T. Kameyama, et al., ACS Applied. Mater. Interfaces
2018, 10, 42844-42855.
[2] C. Jiang, et al., J. Chem. Phys.
2023, 158, 164708.