Controlling Electronic Energy Structure of Ag–Ιn–Ga–S–Se Quantum Dots Showing Band-Edge Emission
Authors: Rismaningsih Nurmanita , Yamauchi Hiroki , Kameyama Tatsuya , Uematsu Taro , Kuwabata Susumu , Torimoto Tsukasa 
Published: December 2020

Semiconductor quantum dots (QDs) exhibited unique optical properties owing to the confinement of charge carriers or excitons in all three dimensions, that is quantum confinement, and then have engaged considerable attention for possible wide-ranged applications, such as solar cells, photocatalysts, light-emitting devices, and biological imaging. The majority of published studies of QDs have focused on binary QDs containing highly toxic heavy metals such as CdS, CdSe, and PbS, due to their well-developed synthesis methods, though the presence of highly toxic elements (Pb or Cd) limits their practical utility in prospective real-life applications. Recently, group I-III-VI-based multinary QDs, such as ZnAgInS[1], AgInGaS[2], and AgInGaSe[3], have been an environmentally benign alternative to conventional binary QDs, with comparable physicochemical properties. We previously reported that a sharp PL peak assignable to band-edge emission was observed for AgInGaS QDs surface-coated with GaSx shell, the peak wavelength being blue-shifted from 610 to 500 nm by the increase in the content of Ga in QDs[2]. Since the bulk Eg of AgInGaSe is smaller than that of corresponding AgInGaS, the doping of Se into AgInGaS QDs can be another strategy to tune their Eg. In this study, we investigate the photochemical properties of Se-doped AgInGaS (AIGSSe) QDs, being dependent on the ratio of Se/S in the particles.
AIGSSe QDs were synthesized by a solution-phase synthesis method. A mixture of AgOAc, In(acac)3, and Ga(acac)3 was used as a metal ion precursor, and that of thiourea and selenourea was used as a chalcogen precursor. These were added to a test tube with a mixture solvent of oleylamine and dodecanethiol, in which the ratio of Se/(S+Se) varied from 0 to 1.0. The solution was heated at 100~250 oC and then formed QDs were isolated by adding methanol. The resulting wet precipitated were washed several times with methanol, followed by dissolving in chloroform.
Absorption spectra of thus-obtained AIGSSe QDs were red-shifted with an increase in the Se/(S+Se) ratio in preparation from 0 to 1.0, the onset wavelength being changed from 580 nm to 806 nm, respectively. The Eg value of the QDs could be controlled between 2.0 eV and 1.4 eV. Although the as-prepared AIGSSe QDs showed a weak broad PL peak assigned to defect-site emission, the surface coating with GaSx removed the broad PL peak and then enhanced the sharp band-edge emission peak. The PL peak wavelength was red-shifted with a decrease in the Eg of QDs. TEM analysis revealed that core-shell-structured AIGSSe@GaSx QDs had a spherical shape with an average size of 4 nm, which were slightly larger than that of AIGSSe QDs used as a core. Thus, we concluded that AIGSSe and AIGSSe@GaSx QDs were successfully prepared via solution-phase method and their Egs were controlled by tuning the ratio of Se/(S+Se) in the composition.
References:
[1] T. Kameyama, et al., J. Phys. Chem. C
2018, 2, 13705-13715.
[2] T. Kameyama, et al., ACS Appl. Mater. Interfaces, 2018, 10, 42844-42855.
[3] T. Kameyama, et al., ACS Appl. Nano Mater.
2020, 3, 3275-3287.