Narrow Band-Edge Photoluminescence of Ga3+-Doped AgInS2 Quantum Dots

Abstract

Quantum dots (QDs) composed of multinary I-III-VI semiconductors and related materials exhibit unique composition- and size-dependent physicochemical properties and have attracted much attention for the application to luminescent devices, solar cells, and photocatalyts because of their strong absorption coefficient and low toxicity. It has been reported that the QDs composed of CuInS2, AgInS2 and their solid solution with ZnS exhibited strong photoluminescence (PL) in the visible light wavelength region, PL peak wavelength being tunable by controlling the particle size and composition. Most of these multinary QDs exhibited broad defect-site PL peaks with a large Stokes shift, which may limit large-scale commercial applications. In contrast, recently, we have reported that rod-shaped (AgIn)xZn2(1-x)Te2 QDs with almost stoichiometric composition exhibited a clear exciton peak in the absorption spectra and a narrow band-edge emission peak in the near-IR wavelength region, the peak wavelengths of which were blue-shifted with a decrease in the x value.(1) In this study, we synthesize non-stoichiometric Ag-In-Ga-S (AIGS) QDs and then investigate their PL properties in relation to their chemical compositions.(2,3) AIGS QDs were synthesized in a mixture solvent of oleylamine and dodecanethiol by the thermal decomposition of the precursor mixture of elemental sulfur, Ag(OAc), Ga(acac)3, and In(acac)3 at 300 oC for 10 min with vigorous stirring under an N2 atmosphere, in which the ratios of Ag/(Ag+In+Ga) and In/(In+Ga) were varied. By adding methanol to the resulting solution, the target AIGS nanoparticles were isolated as wet precipitates. GaSx shell coating was carried out by the heat treatment of AIGS QDs in oleylamine in the presence of Ga(acac)3 and thiourea. Stoichiometric AIGS QDs, which had Ag/(Ag+In+Ga)= 0.5, exhibited a broad defect-site PL peak with a small narrow band-edge PL peak. With a decrease in the Ag/(Ag+In+Ga) ratio, the intensity of band-edge peak was enlarged, being optimal at non-stoichiometric composition of Ag/(Ag+In+Ga)= ca. 0.4. The wavelength of band-edge PL peak was blue-shifted from 610 to 500 nm by decreasing the content of In in AIGS QDs from In/(In+Ga)= 1.0 to 0.2. The GaSx coating of AIGS QDs produced amorphous shell layer of ca. 1 nm thickness on AIGS core surface. Thus-obtained AIGS@GaSx core-shell particles predominantly exhibited a band-edge PL peak of 41 nm in FWHM, and then the intensity of broad defect-site peak was significantly reduced to less than 15 %. The optimal PL quantum yield of AIGS@GaSx core-shell QDs was 28% with green band-edge emission at 530 nm. The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic element-free I-III-VI-based QDs in a wide area of applications. Reference (1) T. Kameyama et al., J. Mater. Chem. C 2018, 6, 2034. (2) T. Uematsu et al., NPG Asia Materials 2018, 10, 713 (3) T. Kameyama et al., ACS Appl. Mater. Interfaces 2018 , 10, 42844.