Synthesis and Optical Properties of Ag–Ga–S Quantum Dots

Abstract

Ag–Ga–S quantum dots (QDs) are obtained by colloidal synthesis in aqueous solutions with glutathione ligands at mild conditions. The QDs can be assigned to metastable orthorhombic, rhombohedral, or rocksalt‐type phases, assuming a significant internal pressure within crystallites, while the conventional tetragonal AgGaS2 phase is less likely formed. Similarly to these metastable phases, the QDs reveal a relatively narrow indirect bandgap contrary to the wide and direct bandgap of tetragonal AgGaS2. Size‐selected fractions of the colloidal solutions are separated by repeated centrifuging with addition of 2‐propanol. The QD size is evaluated from atomic force microscopy, while the crystallite size determined using the Scherrer formula is underestimated. X‐ray photoemission spectroscopy reveals that the QD composition changes from nearly stoichiometric for the fraction with the largest QDs toward Ag deficient and S rich for smaller QDs. The synthesized QDs are characterized by the absorption edge blueshift with decreasing QD size. A blueshift of the photoluminescence (PL) peak with decreasing QD size is observed as well as a nonmonotonous size dependence of the PL intensity. Raman spectra of the Ag–Ga–S QDs are discussed compared to those of bulk crystals and a strong contribution of surface phonons is revealed.

Structural and Optical Characterisation of Size-Selected Glutathione-Capped Colloidal Cu–In–S Quantum Dots

“…3). Note that we earlier observed a similar broad feature for the smallest-size fractions of Ag-Ga-S QDs synthesized by the same technique [65]. This maximum can be related to by-products or unreacted complexes with GSH ligands.…”

Glutathione-Capped Quaternary Ag–(In,Ga)–S Quantum Dots Obtained by Colloidal Synthesis in Aqueous Solutions

“…Such value is obtained for all QD compositions except the end-point Ag-Ga-S QDs, for which the diffraction peak is noticeably broader. In our recent study [37], it was concluded from the position and width of the XRD peak near 30 ∘ that Ag-Ga-S QDs synthesized by this method possess a metastable (presumably, orthorhombic, rhombohedral, or rocksalt-type) structure rather than the conventional tetragonal chalcopyrite-type structure. The major (112) XRD peak of the latter is reported around 28 ∘ for the AgGaS 2 QDs prepared by the thermal decomposition [23], solvothermal process [25], or one-pot hot injection synthesis [31].…”

“…The colloidal synthesis of Ag-(In,Ga)-S QDs, including the end-point Ag-In-S and Ag-Ga-S ternary compounds, was based on an exchange reaction between Na 2 S and a mixture of Ag(I), In(III), and Ga(III) thiolate complexes in the desired proportions with GSH in an aqueous solution (with addition of NH 4 OH) at mild heating (93-98 ∘ C) similarly to the earlier reports on obtaining brightly luminescent Ag-In-S [10,11,33,34], Cu-In-S [35,36], and Ag-Ga-S [37] QDs. We gave preference to GSH over MAA for the synthesis, since formerly [38] we found that GSHcapped Ag-In-S QDs exhibit a better stability upon long-term room-temperature storage than those stabilised by MAA.…”

“…Then we added 1.0 ml of 0.2 M NH 4 OH solution to the reaction mixture, followed by the addition of 2.0 ml of aqueous 0.1 M AgNO 3 solution and the rapid injection of 1.0 ml of 1.0 M aqueous Na 2 S solution and adding 0.25 ml of 2.0 M aqueous solution of citric acid. Thus, all samples were prepared with a molar ratio of [Ag] : [In+Ga] : [S] = 1 : 4 : 5 in the reaction mixture, which earlier showed a considerable long-term stability for GSH-capped Ag-In-S QDs [11,38] and Ag-Ga-S QDs [37] obtained by a similar route. Depending on the [In]/[Ga] precursor ratio in the reaction mixture, the resulting solution acquired an intense red or dark red color.…”

Narrow-Bandwidth Blue-Emitting Ag–Ga–Zn–S Semiconductor Nanocrystals for Quantum-Dot Light-Emitting Diodes

“…And the size of AGZS NCs gradually decreases with the decline of Ag/Ga feeding ratio, reaching the minimum at a feeding ratio of 1:12, as shown in Figure S5. , The AGZS NCs exhibit a uniform size distribution (Figure S6), suggesting a high monodisperse property . However, the optical energy bandgap reaches the maximum at the ratio of 1:8 (Figure S7), which may be attributed to the appearance of more Ag + vacancies. ,− In addition, optical characterizations were utilized to investigate the luminescent properties of AGZS NCs obtained at different Ag/Ga feeding ratios. As shown in Figure d, the emission peaks of the samples present an extended blueshift from 512 to 470 nm with a decreased Ag/Ga feeding ratio.…”