Revealing Exciton and Metal–Ligand Conduction Band Charge Transfer Absorption Spectra in Cu-Zn-In-S Nanocrystals

Abstract

Heavy metal-free semiconductor nanocrystals such as copper indium sulfide quantum dots (QDs) have attracted substantial attention in recent years due to environmental issues and diverse applications. We report the synthesis and characterization of copper-zinc-indium-sulfide (CZIS) QDs and CZIS treated with excess Zn 2+ at different temperatures, denoted here as CZIS/ZnS 100 and CZIS/ZnS 200. Zn 2+ can diffuse into the lattice by an exchange-cation reaction, replacing Cu + and In 3+. We employed transient absorption (TA) spectroscopy to study the role of Zn 2+ in the lattice. The data were treated by global analysis, which yielded the decay associated spectra (DAS). Through DAS and second-order derivative absorption spectra, we could isolate the excitonic contribution from the metal ligand conduction band charge transfer (MLCBCT) absorption in the TA spectrum, finding the hole localization lifetime by its spectral and dynamical features. The hole localization lifetime increases from 0.3 to 1.7 ps by increasing the Zn 2+ concentration. We also measured the electron trapping constants to be 30 ps and larger than 1 ns. Finally, we concluded that the improvement in the photoluminescence quantum yield originates from accelerated radiative transition relative to the nonradiative component. A detailed mechanism was proposed, and our results suggest that the introduction of Zn 2+ in the lattice of CIS QDs could be beneficial for charge extraction.

Resonant Band-Edge Emissive States in Strongly Confined CsPbBr3 Perovskite Nanoplatelets

“…Therefore, the fast decay at shorter wavelengths and the rise at longer ones support the hole confinement from band-edge states. The same behavior has been observed in the literature and is ascribed to hole trapping. , Here, we can exclude the possibility of charge or energy transfer between different particle sizes because the sample concentration was kept low …”

“…The same behavior has been observed in the literature and is ascribed to hole trapping. 49,50 Here, we can exclude the possibility of charge or energy transfer between different particle sizes because the sample concentration was kept low. 51 Until now, we have discussed the PIA just in terms of time dependence.…”

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots

“…The room-temperature absorption and PL spectra of the bare CIS cores and CIS/ZnS QDs are presented in Figure a. The wz-CIS cores exhibit a broad PL line width and a featureless absorption transition with a low-energy absorption tail that extends far below the bulk CIS bandgap (∼1.3 eV). , The tail is also commonly observed in chalcopyrite CIS QDs − and can be attributed to Cu-related sub-bandgap state absorption. − ,,, After capping the CIS core with ZnS, the overall PL quantum yield significantly improved to 55 ± 5% and a slight blue shift by ∼40 meV was observed in the absorption spectrum, indicating an increase in the band-edge exciton energy. This can be ascribed to partial Zn 2+ -for-In 3+ and Cu + cation exchange followed by interdiffusion, which leads to an increase of the core semiconductor bandgap. , Another possibility is that the core diameter shrinks due to either etching prior to the shell overgrowth , or shell ingrowth by cation exchange, , resulting in a stronger quantum confinement of the charge carriers.…”

“…tens to hundreds of nanoseconds), and large Stokes shifts (∼300–500 meV) with a wide photoluminescence (PL) tunability from the visible to the second near-infrared biological window. − These properties make them particularly attractive for QD-sensitized solar cells, luminescent solar concentrators, , and bioimaging. , Bare CIS QDs usually show low PL quantum yields and a poor photostability, thus restricting their direct use for applications. − These limitations can be circumvented by overcoating them with a shell of wide bandgap semiconductors (e.g., ZnS and CdS). , With the advanced development of colloidal synthesis and postshelling procedures, the state-of-the-art CIS-based core–shell QDs exhibit low size dispersion, excellent photostability, and competing PL quantum yields over 80%. ,, Strikingly, their absorption and emission bandwidths (∼200–400 meV) are much larger than those of the prototypical binary QDs − regardless of their size and shape dispersions or their compositions. After more than a decade of extensive experimental and theoretical work, the emission from these nanostructures is considered to stem from the recombination of a delocalized conduction band (CB) electron with a localized hole. ,,,,− Yet, the nature of the hole localization site has still not been elucidated. The hole may be captured by either a Cu + -related defect ,, or self-trapping, ,, leading to an ongoing debate on the origin of the PL broadening in CIS QDs.…”

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

“…Despite the large progress in synthetic tailoring of the optical properties, the mechanism underlying the PL of CuInS 2 QDs is still a matter of debate. ,, Two pathways are considered. In the free-to-bound mechanism, a delocalized conduction band electron (1 S ) recombines with a hole localized on the copper cation. ,,− In the excitonic mechanism, the hole is delocalized on a P-type symmetry state and the PL is due to a parity-forbidden 1S–1P transition. , The poor understanding of the nature of the luminescent excited state limits further development of these materials toward existing and emerging applications.…”