Intrinsic and Extrinsic Exciton Recombination Pathways in AgInS <sub>2</sub> Colloidal Nanocrystals
Matteo L. Zaffalon, Valerio Pinchetti, Andrea Camellini, Sergey Vikulov, Chiara Capitani, Bing Bai, Meng Xu, Francesco Meinardi, Jiatao Zhang, Liberato Manna, M. Zavelani–Rossi, S. A. Crooker, Sergio Brovelli
Abstract
Ternary I-III-VI 2 nanocrystals (NCs), such as AgInS 2 and CuInS 2 , are garnering interest as heavy-metal-free materials for photovoltaics, luminescent solar concentrators, LEDs, and bioimaging. The origin of the emission and absorption properties in this class of NCs is still a subject of debate. Recent theoretical and experimental studies revealed that the characteristic Stokes-shifted and long-lived luminescence of stoichiometric CuInS 2 NCs arises from the detailed structure of the valence band featuring two sublevels with different parity. The same valence band substructure is predicted to occur in AgInS 2 NCs, yet no experimental confirmation is available to date. Here, we use complementary spectroscopic, spectro-electrochemical, and magneto-optical investigations as a function of temperature to investigate the band structure and the excitonic recombination mechanisms in stoichiometric AgInS 2 NCs. Transient transmission measurements reveal the signatures of two subbands with opposite parity, and photoluminescence studies at cryogenic temperatures evidence a dark state emission due to enhanced exchange interaction, consistent with the behavior of stoichiometric CuInS 2 NCs. Lowering the temperature as well as applying reducing electrochemical potentials further suppress electron trapping, which represents the main nonradiative channel for exciton decay, leading to nearly 100% emission efficiency.