Interplay of Multiexciton Relaxation and Carrier Trapping in Photoluminescent CdS Quantum Dots Prepared in Aqueous Medium

Abstract

A comprehensive understanding of exciton dynamics of water-soluble quantum dots (QDs), especially those synthesized directly in aqueous media, is critical for devising their applications. Trapping processes are expected to play a pivotal role here, as surface trap states are abundant in such QDs. The present study, based on ultrafast transient absorption, provides insight into exciton dynamics in PEI-capped CdS QDs synthesized by a one-pot reaction in water. They are robust and strongly photoluminescent (PL), unlike many other aqueous QDs. Excitation energy dependence of PL and the underlying mechanisms have been investigated. Analysis of fluence-dependent transient absorption bleach and photoinduced absorption is performed to deconvolute contributions from multiexciton recombination and carrier trapping. A simple kinetic model is used to quantify the rate constants associated with intraband relaxation of hot excitons and hot carrier trapping. Similar magnitudes observed for the two pathways highlight the competitive kinetics between them, which is responsible for a drastic decrease in PL quantum yields for excitation above the band gap. Such efficient hot carrier trapping in these QDs may possibly foreshadow ultrafast trap-mediated electron transfer and thus render them suitable for redox-sensing and photocatalysis.

Mechanistic Insights into Selective Sensing of Pb2+ in Water by Photoluminescent CdS Quantum Dots

“…Broad PL is observed for these aqueous media processed CdS QDs; however, the FWHM of PL bands is unaffected upon excitation at different energies, thus indicating the broad PL originates from trap states rather than polydispersity in the system. 11 Upon addition of metal ions, absorption spectra remain unaffected for all metal ions (Figure 1A,B); however, PL is quenched selectively by Pb 2+ ions (Figure 1C−E, S2) with a concurrent decrease in PL T1), whereas lifetime is unaffected for other metal ions (Figure S3). About 90% quenching is achieved at [Pb 2+ ] = 10 μM (Figures S2 and S5A), with an apparent slight blueshift and decrease in FWHM.…”

“…Synthesis, TEM (Note SN1, Figure S1) and optical absorption spectra of PEIcapped CdS QDs have been elaborated elsewhere. 11 The first excitonic absorption maxima is at approximately 432 nm (∼2.87 eV, Figure 1A). Broad PL is observed for these aqueous media processed CdS QDs; however, the FWHM of PL bands is unaffected upon excitation at different energies, thus indicating the broad PL originates from trap states rather than polydispersity in the system.…”

“…Synthesis, TEM (Note SN1, Figure S1) and optical absorption spectra of PEI-capped CdS QDs have been elaborated elsewhere . The first excitonic absorption maxima is at approximately 432 nm (∼2.87 eV, Figure A).…”

“…The first excitonic absorption maxima is at approximately 432 nm (∼2.87 eV, Figure A). Broad PL is observed for these aqueous media processed CdS QDs; however, the FWHM of PL bands is unaffected upon excitation at different energies, thus indicating the broad PL originates from trap states rather than polydispersity in the system . Upon addition of metal ions, absorption spectra remain unaffected for all metal ions (Figure A,B); however, PL is quenched selectively by Pb 2+ ions (Figure C–E, S2) with a concurrent decrease in PL lifetime (λ em = 510 nm, Figure A, Table T1), whereas lifetime is unaffected for other metal ions (Figure S3).…”

“…Very recently, we have studied excitonic relaxation processes in CdS QDs, synthesized at room temperature in aqueous phase with PEI as a capping agent. 11 PL of these QDs is Stokes-shifted (0.38 eV) with a PLQY of 31%. Upon testing for response to metal ions, we observe PL quenching upon lead-ion (Pb 2+ ) addition.…”