(Invited) Improving Photochemical Properties of I-III-VI-Based Quantum Dots through Size and Composition Control
Authors: Torimoto Tsukasa , Mori Tatsuya , Jiang Chang , Akiyoshi Kazutaka , Kameyama Tatsuya , Uematsu Taro , Kuwabata Susumu 
Published: December 2024

Semiconductor nanocrystals smaller than 10 nm, known as quantum dots (QDs), exhibit optoelectronic properties that are dependent on their size due to the quantum size effect. Recently, QDs made of I-III-VI group multinary semiconductors, such as CuInS2 and AgInS2, have attracted significant attention for their potential in highly efficient solar energy conversion systems. This is because these QDs were composed of less toxic elements and their optical properties were broadly tuned in the visible and near-IR wavelength regions through chemical composition. It was reported in our previous papers (1,2) that solid solution QDs composed of Zn-Ag-In-S and Zn-Ag-In-Se exhibited the photocatalytic activity for H2 evolution, which varied according to their size and composition. In this study, we introduce the solution-phase synthesis of less-toxic QDs composed of AgIn(1-x)GaxS2 (AIGS) and CuIn(1-x)GaxS2 (CIGS). The photocatalytic activity for H2 evolution was investigated for AIGS QDs under visible light irradiation. The photoluminescence (PL) properties of CIGS QDs were improved for the application to QD-based light-emitting diodes (QD-LEDs).

AIGS QDs with different In/(In+Ga) ratios were synthesized by thermal decomposition of corresponding metal complexes and S powders in an oleylamine-dodecanethiol solution.(3) The energy gap (E
g) of AIGS QDs was increased from 2.07 to 2.54 eV with a decrease in the In fraction in the QDs from 1.0 to 0.20. After the surface coating with GaSx shell, the resulting AIGS@GaSx QDs exhibited sharp band-edge PL peak, the wavelength of which was tunable from 610 to 500 nm with an increase in the E
g. The photocatalytic activity of AIGS QDs was imoroved by the modification with ZnS shell of 2 nm in thickness. The resulting AIGS@ZnS QDs were dispersed in the aqueous solution containing Na2S as a hole scavenger. Visible light irradiation (λ> 350 nm) to the dispersion caused the H2 evolution, the amount of which linearly increased with the elapse of the irradiation time. The H2 evolution rate was remarkably dependent on the composition of AIGS core. A volcano-type dependence was observed between the photocatalytic activity rate and the In fraction. The highest photocatalytic activity was obtained for AIGS@ZnS QDs with In/(In+Ga)=0.4. This result can be explained by the changes of the conduction band level of AIGS cores and the amount of photon absorbed by the photocatalysts.

CIGS QDs were prepared in a similar way.(4) The E
g of CIGS QDs also increased from 1.74 eV to 2.77 eV with a decrease in In/(In+Ga) ratio from 1.0 to 0. The GaSx coating on CIGS QD surface enlarged the intensity of PL peak without changing its peak width: CIGS@GaSx QDs exhibited a narrow PL peak (FWHM: 0.20 eV) at 671 nm, being comparable to the PL peak of CIGS QDs (the peak wavelength of 675 nm and the FWHM of 0.23 eV). The PL QY was also increased to 27% from 8% by the GaSx coating on CIGS QDs. QD-LEDs fabricated with CIGS@GaSx QDs showed a red color with a narrow emision peak at 688 nm with a fwhm of 0.24 eV.

In cnclusion, the tunabilities of optical properties and energy structure of I-III-VI-based QDs are useful for constructing efficient light energy conversion systems and luminescent devices.

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