Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots

Abstract

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium-and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using lowtemperature single quantum dot spectroscopy on core−shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zerophonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.

Semiconductor Nanocrystals Emitting in the Second Near‐Infrared Window: Optical Properties and Application in Biomedical Imaging

“…[ 6 ] Recently, it was reported by Lounis and co‐workers that one subpopulation of the core/shell CuInS 2 /ZnS SNCs exhibited extremely sharp PL linewidths arising from the radiative recombination of band‐edge excitons, while another population in the same ensemble exhibited a broad PL ascribed to the radiative recombination of a band‐edge electron with a defect‐assisted self‐trapped hole. [ 138 ] Another strategy to narrow the PL FWHM is to develop shelling strategies aimed on avoiding intrabandgap defects. Torimoto and co‐workers found out that amorphous InS x and GaS x shells effectively narrowed the PL linewidths of AgInS 2 SNCs down to ≈24 nm, while blue‐shifting the emission peaks due to the occurrence of band‐edge transition rather than original defect emission.…”

“…[ 136,137 ] On the other hand, in a recent study by the Lounis group, the defect‐assisted self‐trapping of the holes was revealed to be the origin of broad PL from one subpopulation of CuInS 2 /ZnS SNCs. [ 138 ]…”

“…[6,[134][135][136][137][138] It has been debated whether the hole is captured by Cu-related defects or through a self-trapping mechanism. [136][137][138] Studies by Klimov and co-workers provided evidence that Cu-related defects are responsible for the intragap emission of CuInS 2 SNCs, where the emission pathways were distinct for different oxidation states of Cu defects (1+ for stoichiometric and 2+ for Cu-deficient ones). [136,137] On the other hand, in a recent study by the Lounis group, the defectassisted self-trapping of the holes was revealed to be the origin of broad PL from one subpopulation of CuInS 2 /ZnS SNCs.…”

“…There is a growing consensus in literature that the emission in I-III-VI SNCs involves the recombination of a delocalized conduction band electron with a localized hole, while the origin of the hole localization remains elusive. [6,[134][135][136][137][138] It has been debated whether the hole is captured by Cu-related defects or through a self-trapping mechanism. [136][137][138] Studies by Klimov and co-workers provided evidence that Cu-related defects are responsible for the intragap emission of CuInS 2 SNCs, where the emission pathways were distinct for different oxidation states of Cu defects (1+ for stoichiometric and 2+ for Cu-deficient ones).…”

“…[136,137] On the other hand, in a recent study by the Lounis group, the defectassisted self-trapping of the holes was revealed to be the origin of broad PL from one subpopulation of CuInS 2 /ZnS SNCs. [138] Using the same Se-precursor to grow CuInSe 2 cores, Rogach and co-workers successfully controlled the crystal structure of CuInS 2 shells between cubic and hexagonal in CuInSe 2 /CuInS 2 core/shell nanostructures by using long-chain thiol (tert-dodecanethiol) as a sulfur precursor and varying its amount. [51] Cubic phase shells started to grow epitaxially on CuInSe 2 cores followed by an island-like growth, finally producing a branched CuInS 2 shell.…”