Size-selected series of water-soluble luminescent non-stoichiometric mercaptoacetate-stabilized silver indium sulfide and core/shell AIS/ZnS QDs were produced by the controlled precipitation techniques. Up to 10–11 fractions of size-selected AIS (AIS/ZnS) QDs were isolated differing distinctly in their optical properties and emitting in a broad color range from deep-red (fractions with larger NCs) through yellow-orange (fractions with intermediate sizes) to bluish-green (fractions with smaller sizes). The PL hue can be additionally varied by adjusting the composition of the starting crude AIS (AIS/ZnS) colloids. The PL quantum yield (QY) of size-selected core/shell AIS/ZnS and copper-doped AIS/ZnS quantum dots (Figure 2) varied among the fractions reaching up to 47% for the intermediate (yellow-orange emitting) fractions. The size-selective precipitation/redissolution can be successfully applied to glutathione (GSH) capped aqueous AIS and AIS/ZnS QDs producing broad (8–16 fractions) series of size-selected QDs with different emission colors and a maximum PL QY of 60% for the most populated fraction of the core/shell AIS/ZnS QDs which is among the highest reported for the direct aqueous synthesis of colloidal AIS QDs. We evaluated the efficiency of photoinduced electron transfer in systems comprising mesoporous titania and AIS/ZnS NCs basing on the results of photoelectrochemical and time-resolved PL measurements. We found that the rate of the photoinduced electron transfer as well as the efficiency of the photocurrent generation in the systems based on TiO2/AIS composites increase with a decrease of the AIS NC size (in the studied range of 2-4 nm) simultaneously with an expansion of the bandgap of the AIS NCs. The copper doping was found to result in an enhancement of the photoelectrochemical activity of CAIS/ZnS QDs introduced as spectral sensitizers of mesoporous titania photoanodes of a liquid-junction solar cells. The doping-induced photoactivity increment increases from 24% for the largest QDs to ~60% for the smallest QDs, the photocurrent density correlating closely with the PL QY of original colloidal CAIS/ZnS QDs.
We reported on the aqueous synthesis of ultra-small (~2 nm) copper(I)- and silver(I)-doped CdS and core/shell CdSe/CdS QDs stabilized by Cd(II) complexes with mercaptoacetate anions and ammonia. The doped QDs reveal high stability toward aggregation and oxidation, retain individual character after complete solvent removal and can be easily redispersed in dilute aqueous ammonia solutions producing concentrated luminescent “inks”.
We found that QDs with a similar size reveal the same normalized rate of vibrational relaxation for different broadband-emitting QDs, including CdS, CdSe/CdS, doped CdSe/CdS, Ag-In-S, and Cu-In-S QDs. The reported results show that the broadband PL of different metal-chalcogenide QDs can be described by a general model which does not require the assumption of charge-trapping defects participating in the radiative recombination.
We characterized the structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 NCs produced by a “green” method directly in aqueous solutions in the form of stable and concentrated “inks” with a CZTS content of up to 33 g/L. A size-selective precipitation using 2-propanol as a non-solvent was found to yield a series of around ten fractions of CZTS NCs with an average size d varying from 3 nm to 2 nm and maintaining roughly the same composition. The size-selected CZTS NCs thus obtained showed a phonon confinement effect, with the frequency position of the main phonon shifted by about 4 cm–1 between 3 nm and 2 nm NC diameters, accompanied by a narrowing of the main phonon feature. The conditions of the preparation of samples for optical studies, the nature of the support of the dried CZTS NC film, as well as the conditions of the registration of Raman spectra were found to affect crucially the spectral properties of the studied samples.
