Low-Temperature Photoluminescence Dynamics Reveal the Mechanism of Light Emission by Colloidal CuInS₂ Quantum Dots

Abstract

Copper indium sulfide colloidal quantum dots are in focus of the nanocrystal community as their optical properties make them exciting candidates for applications in bioimaging and biosensing, solar cells, luminescent solar concentrators, and lighting. However, fulfilling this application potential is hampered by poor understanding of the mechanism of light emission by these dots. In this work, we study temperature and magnetic field dependent photoluminescence (PL) dynamics and spin dynamics at cryogenic temperatures. We cast our experimental observations against two prevailing models of PL in CuInS2 quantum dots: an excitonic mechanism and a free-to-bound mechanism, in which a delocalized electron recombines with a hole localized at a copper ion. We unambiguously show that the PL occurs as the result of the free-to-bound mechanism.

Signatures of Energy Transfer in Solid Films of CuInS2 Colloidal Quantum Dots

“…Another signature of ET is observed in the measurements of the PL dynamics at low temperatures. In general, at low temperatures, the PL lifetimes are about ten times longer owing to a suppressed electron trapping and an increased occupation of the dark state of the luminescent fine structure. ,, In Figure A, we show the PL decays measured on the S2-film at 25 K for selected detection energies. In the same way as at room temperature, the PL decay rate increases with the PL energy as shown in the spectral dependence of k presented in the Supporting Information, Figure S6.…”

“…In CIS QDs, the large Stokes shifts imply that the spectral overlaps are small. Moreover, owing to the free-to-bound nature of the PL transition, ,− the radiative lifetimes are on the order of hundreds of ns, almost 2 orders of magnitude longer than for excitonic recombination in CdSe QDs . Thus, it could be expected that in CIS QD films, the influence of ET on exciton dynamics should be weak.…”

Hole Localization and Ultrafast Electron Transfer Dynamics in CuInS2/Organic Molecule Composite

“…The broad emission was explained by the combination of two distinct transitions such as band edge and surface recombination as explained by Jeong et al Two different transitions were assigned to excitonic and Cu-related sub-band gap state by Jara et al The in situ photoluminescence spectro-electrochemical measurement were used to determine the radiative recombination between delocalized e – and the trapped h + , and simultaneously, both the dark and bright nanocrystals were also detected by the Arjan group . Recently, the temperature and applied magnetic field-dependent photoluminescence studies led to the conclusion that the radiative recombination is due to the recombination of delocalized e – and localized h + at the Cu 1+ center …”

“…The radiative and nonradiative recombination have several manifestations by different spectroscopy techniques such as recombination through band edge (BE) and surface trapped, 9,10 radiative recombination between e − and trapped h + , 11,12 radiative recombination between h + and trapped e − , 6,13 recombination through the dark and bright state, etc. 8,14 The QDs having a very large surface-to-volume ratio are very prone to generate surface traps owing to unsaturated dangling bonds on the surface, and it can be either e − or h + traps, or both depending upon the nature of the surface. 15,16 In addition to this surface state, some intrinsic interband trapped states are generated in the I−III−VI QDs due to the use of different cation and their stoichiometry ratio, which provides a degree of freedom in optimizing the photophysical properties of these green light antenna materials.…”

“…8 Recently, the temperature and applied magnetic field-dependent photoluminescence studies led to the conclusion that the radiative recombination is due to the recombination of delocalized e − and localized h + at the Cu 1+ center. 14 Despite debated unusual photophysical behavior, these ternary green QDs have shown quite good power conversion efficiency in quantum-dot-sensitized solar cells and photocatalysis. These green QDs are extensively used in CO 2 reduction in the QD-metal-porphyrin nanocomposite, where these QDs are used as the light antenna and the e − donor to activate porphyrin catalyst.…”