Origin of the Broadband Photoluminescence of Pristine and Cu⁺/Ag⁺-Doped Ultrasmall CdS and CdSe/CdS Quantum Dots

Abstract

Ultrasmall (∼2 nm) copper­(I)- and silver­(I)-doped CdS and core/shell CdSe/CdS quantum dots (QDs) stabilized by Cd­(II) complexes with mercaptoacetate anions and ammonia were produced in aqueous solutions. The doped QDs emit broadband visible photoluminescence (PL) with a quantum yield reaching 10–12% for Cu+-doped QDs and 5–9% for Ag+-doped QDs. The broadband PL was described by a self-trapped exciton model as a sequence of phonon replicas of a zero-phonon emission line. The shape of the PL bands of CdS, Cu+-doped CdS QDs, and Ag+-doped CdS QDs was modeled by using the energies of optical phonons of CdS, CuS, and Ag2S, respectively. The dependence of the average PL lifetime of both pristine and doped CdS and CdSe/CdS QDs on PL registration wavelength was interpreted in terms of the vibrational relaxation of the self-trapped exciton. The analysis of PL properties of different ultrasmall metal chalcogenide QDs showed that the broadband PL can be described by a general model which does not require the assumption of participation of charge-trapping lattice defects.

Temperature‐Dependent Photoluminescence of Silver‐Indium‐Sulfide Nanocrystals in Aqueous Colloidal Solutions

“…Earlier we reported on the synthesis and PL properties of ultra‐small (the size d <2 nm) CdS NCs stabilized by Cd(II) complexes with PEI or by a combination of MA and ammonia . Both types of CdS NCs revealed very similar reversible variations of the PL intensity when heated up and cooled down in aqueous colloidal solutions (see examples in SI, Figure S3).…”

“…The stabilization of previously studied PEI‐capped CdS NCs originates from the formation of complexes of undercoordinated or adsorbed Cd(II) cations on the NC surface with amino‐groups of PEI . The macromolecular PEI can be substituted by a combination of Cd(II) complexes with ammonia and MA forming in situ during the synthesis of CdS NCs from the mixture of Cd(II)‐NH 3 and Cd(II)‐MA complexes . Both types of CdS NCs capped either by Cd(II)‐PEI or by Cd(II)‐NH 3 ‐MA complexes were found to have almost identical absorption and PL properties indicating a very similar stabilization mechanism.…”

“…Earlier we reported on the synthesis and PL properties of ultra-small (the size d < 2 nm) CdS NCs stabilized by Cd(II) complexes with PEI [20,21] or by a combination of MA and ammonia. [22,23] Both types of CdS NCs revealed very similar reversible variations of the PL intensity when heated up and cooled down in aqueous colloidal solutions (see examples in SI, Figure S3). Similar to the present AIS NCs, the heating-/coolinginduced absorption and PL changes of ultra-small CdS NCs were reversible and could be repeated many times without any deterioration of the integrity/stability of the CdS NCs.…”

“…[23,24] In these studies, we found that luminescent properties of aqueous ultra-small CdX NCs and ternary AIS (CIS) NCs are very similar, both types of NCs emitting relatively intense visible PL characterized by large spectral widths and Stokes shifts. [16,[18][19][20][21][22][23] We found also that the ultra-small CdS NCs stabilized by Cd(II) complexes with PEI or NH 3 /MA reveal unusually strong temperature dependences of both absorption edge and PL intensity when probed in aqueous colloidal solutions, [21,22] showing a peculiar PL quenching mechanism differing from that typical for "regular" CdX NCs capped by covalently bound ligands. [25][26][27][28][29][30][31] In the present work we explore the variations of PL parameters of GSH-capped AIS (AIS/ZnS) NCs produced directly in aqueous solutions in the temperature window of 10-80°C in comparison with previously studied ultra-small CdX NCs capped with Cd(II) complexes.…”

“…In particular, we reported recently on the aqueous syntheses of ternary AIS and core/shell AIS/ZnS NCs stabilized by NC surface complexes with polyethyleneimine (PEI), thioglycolic (mercaptoacetic) acid, and glutathione (GSH) . Our approaches to aqueous ternary NCs stemmed from our previous works on the syntheses of ultra‐small (the size d <2nm) CdS NCs capped with Cd(II) complexes with polyethyleneimine (PEI) or mixed complexes of Cd(II) with ammonia and mercaptoacetate (MA), as well as ultra‐small core/shell CdSe/CdS NCs . In these studies, we found that luminescent properties of aqueous ultra‐small CdX NCs and ternary AIS (CIS) NCs are very similar, both types of NCs emitting relatively intense visible PL characterized by large spectral widths and Stokes shifts .…”