Ultrafast exciton decay in PbS quantum dots through simultaneous electron and hole recombination with a surface-localized ion pair

Abstract

This paper describes the ultrafast decay of the band-edge exciton in PbS quantum dots (QDs) through simultaneous recombination of the excitonic hole and electron with the surface localized ion pair formed upon adsorption of tetracyanoquinodimethane (TCNQ). Each PbS QD (R= 1.8 nm) spontaneously reduces up to 17 TCNQ molecules upon adsorption of the TCNQ molecule to a sulfur on the QD surface. The photoluminescence of the PbS QDs is quenched in the presence of the reduced TCNQ species through ultrafast (≤15 ps) non-radiative decay of the exciton; the rate constant for the decay process increases approximately linearly with the number of adsorbed, reduced TCNQ molecules. Near-infrared and mid-infrared transient absorption show that this decay occurs through simultaneous transfer of the excitonic electron and hole, and is assigned to a four-carrier, concerted charge recombination mechanism based on the observations that (i) the PL of the QDs recovers when spontaneously reduced TCNQ 1-desorbs from the QD surface upon addition of salt, and (ii) the PL of the QDs is preserved when another spontaneous oxidant, ferrocinium, which cannot participate in charge transfer in its reduced state, is substituted for TCNQ.2

Electronic Processes within Quantum Dot-Molecule Complexes

“…Weiss and co-workers “p-doped” the surfaces of PbS and CdSe QDs by treating them with tetracyanoquinodimethane (TCNQ), which spontaneously oxidizes surface chalcogenides but does not oxidize the core (Figure ). They observed a similar surface doping upon addition of ferrocenium to oleate-capped PbS QDs …”

“…They observed a similar surface doping upon addition of ferrocenium to oleate-capped PbS QDs. 160 Gamelin, Mayer, and co-workers used a dimethylcobaltocene/ dimethylcobaltocenium (CoCp* + /CoCp*) redox indicator to demonstrate the effect of pH on the band-edge positions of colloidal ZnO QDs. 100,161,162 They noted that in more acidic conditions, the ZnO was reduced at less negative potentials and oxidized at more positive potentials.…”

Reversible Electrochemical Control over Photoexcited Luminescence of Core/Shell CdSe/ZnS Quantum Dot Film

“…As the applied potential reaches − 0.9 V, the core emission shows a faster decay and a redshifted spectrum relative to that at 0 V. This is attributed to the adsorption of the cations from the electrolyte onto the surfaces of the QDs. As mentioned above, the electron injection into the surface trap states occurs at − 0.9 V. The adsorbed cations serve as counterions to the injected electrons [ 25 , 37 , 38 ] and charge acceptors [ 39 ], giving rise to the dissociation of the exciton by charge transfer and quenching of the PL. As the applied potential is decreased to more negative values, a greater quenching occurs due to the potential difference-induced penetration of cations.…”

Dedoping of Intraband Silver Selenide Colloidal Quantum Dots through Strong Electronic Coupling at Organic/Inorganic Hybrid Interfaces

“…PL quenching was also observed through TCNQ doping of PbS CQDs. 55 This could be a showstopper for F4-TCNQ doped Ag 2 Se CQDs as well, as photoexcited carriers would be absorbed by excess F4-TCNQ molecules on the surface of the CQDs instead of being transported to the contacts. Keeping the excess F4-TCNQ to a minimum would therefore be of critical importance; hence, we are devising ways to quench the MWIR peak and enable the SWIR absorbance with controlled amounts of F4-TCNQ.…”

“…Additionally, improved performance of CdSe/CdSe nanorod-based LEDs has been observed through F4-TCNQ doping, 56 and electron transfer from CQDs to a molecular dopant has also been shown for the related, but less polar molecule tetracyanoquinodimethane (TCNQ). 54,55,57 A challenge with molecular doping, particularly in organic semiconductors such as poly(3-hexylthiophene) (P3HT), is the low doping efficiency. 58 In some cases, a dopant amount of up to 35−50% F4-TCNQ is necessary to achieve sufficient change in electrical conductivity in organic material.…”