Sb-Doped Metal Halide Nanocrystals: A 0D versus 3D Comparison

Abstract

We synthesize colloidal nanocrystals (NCs) of Rb 3 InCl 6 , composed of isolated metal halide octahedra (“0D”), and of Cs 2 NaInCl 6 and Cs 2 KInCl 6 double perovskites, where all octahedra share corners and are interconnected (“3D”), with the aim to elucidate and compare their optical features once doped with Sb 3+ ions. Our optical and computational analyses evidence that the photoluminescence quantum yield (PLQY) of all these systems is consistently lower than that of the corresponding bulk materials due to the presence of deep surface traps from under-coordinated halide ions. Also, Sb-doped “0D” Rb 3 InCl 6 NCs exhibit a higher PLQY than Sb-doped “3D” Cs 2 NaInCl 6 and Cs 2 KInCl 6 NCs, most likely because excitons responsible for the PL emission migrate to the surface faster in 3D NCs than in 0D NCs. We also observe that all these systems feature a large Stokes shift (varying from system to system), a feature that should be of interest for applications in photon management and scintillation technologies. Scintillation properties are evaluated via radioluminescence experiments, and re-absorption-free waveguiding performance in large-area plastic scintillators is assessed using Monte Carlo ray-tracing simulations.

Isolated [SbCl6]3– Octahedra Are the Only Active Emitters in Rb7Sb3Cl16 Nanocrystals

“…The optical absorption spectrum of the Rb 7 Sb 3 Cl 16 NCs was characterized by a narrow peak centered at 3.85 eV (∼325 nm), whereas the PL spectrum consisted of a Stokes-shifted and broad emission peak (full width at half-maximum of ∼0.4 eV) centered at 2.2 eV (∼556 nm) (Figure c). The PL excitation (PLE) spectrum measured at the PL maximum displayed three distinguished excitation peaks at 3.5, 3.8, and 4.4 eV, in agreement with the well-known fine structure of the singlet–triplet transitions occurring in hexacoordinated Sb 3+ ions . In this case, the shape of these peaks was clearly broadened, presumably indicating that both isolated [SbCl 6 ] 3– octahedra and [Sb 2 Cl 10 ] 4– dimers are absorbing light.…”

“…With the aim to better elucidate the optical features of this relatively new material at the nanoscale, we synthesized Rb 7 Sb 3 Cl 16 NCs and compared their optical properties to those of a system featuring only isolated [SbCl 6 ] 3– octahedra and no [Sb 2 Cl 10 ] 4– dimers. This system is represented in this work by Sb-doped Rb 3 InCl 6 NCs (Scheme , right panel), which were recently developed by our group . The outcome of our spectroscopic investigation combined with DFT calculations indicated the following: In Rb 7 Sb 3 Cl 16 NCs the [Sb 2 Cl 10 ] 4– dimers are inactive in PL; i.e., they only promote nonradiative emission channels.…”

“…Lead halide perovskites nanocrystals (NCs) have emerged as promising materials for various optoelectronic applications because of their bright and narrow photoluminescence (PL) emission. − However, their poor stability, mainly against humidity and more in general against polar solvents, and the intrinsic toxicity of Pb 2+ limit their commercial application. , For these reasons, extensive efforts are being devoted to seek alternative nontoxic metal halide NC systems with analogous optical features and possibly higher stability. − Among the systems discovered and investigated so far, Sb-doped metal halides are particularly promising, as some of them have been demonstrated to feature high PL quantum yield (PLQY) in both the bulk and nanoscale. − For example, Sb-doped bulk Cs 2 NaInCl 6 and Cs 2 KInCl 6 double perovskite systems exhibit a blue-green emission with PLQY values of ∼80% and 90%, respectively. ,,, Similarly, Sb-doped bulk Rb 3 InCl 6 and Cs 3 InCl 6 and their hydrated counterparts (namely, Rb 2 InCl 5 (H 2 O) and Cs 2 InCl 5 (H 2 O)) feature a bright PL emission in the green or yellow with PLQY as high as 95%. ,− In addition, Cs 2 SnCl 6 powders, when doped with Sb 3+ ions, exhibit a broadband orange-red emission and a PLQY of 37%, and Sb 3+ doped Cs 2 ZnCl 4 crystals show a near-infrared emission with a 70% PLQY . The Sb-doped NC counterparts, in turn, have been reported to have lower PLQY values (∼20% in the case of Cs 2 NaInCl 6 and Cs 2 KInCl 6 , ∼40% for Rb 3 InCl 6 , and 8% for Cs 2 SnCl 6 ) caused by the undercoordinated surface Cl – ions generating nonradiative defect states. , Such Sb-doped systems are characterized by the presence of [SbCl 6 ] 3– octahedra governing their optical properties since they behave as both sensitizers and recombination centers. More specifically, Sb-doped metal halides reported so far are characterized by (i) analogous near-UV absorption features originating from the parity allowed, although spin forbidden, 1 S 0 → 3 P 0 and 1 S 0 → 3 P 1 transitions of the [SbCl 6 ] 3– octahedra and (ii) PL emission stemming from self-trapped excitons recombining in the Sb centers, with high PLQY values, large Stokes shifts, and long emission lifetimes (microsecond time scale). −<...…”

“…The outcome of our spectroscopic investigation combined with DFT calculations indicated the following: In Rb 7 Sb 3 Cl 16 NCs the [Sb 2 Cl 10 ] 4– dimers are inactive in PL; i.e., they only promote nonradiative emission channels. Only isolated [SbCl 6 ] 3– octahedra are responsible for the emission, and this explains the low PLQY of such NCs (≤1%) compared to that of Sb-doped Rb 3 InCl 6 NCs (PLQY of 40%) Sb-doped Rb 3 InCl 6 NCs and Rb 7 Sb 3 Cl 16 NCs have very similar temperature dependent characteristics, namely, two different emission peaks that emerge when exciting at different energies (namely, 3.5 and 4.66 eV) at cryogenic temperatures.…”

“…In Rb 7 Sb 3 Cl 16 NCs the [Sb 2 Cl 10 ] 4– dimers are inactive in PL; i.e., they only promote nonradiative emission channels. Only isolated [SbCl 6 ] 3– octahedra are responsible for the emission, and this explains the low PLQY of such NCs (≤1%) compared to that of Sb-doped Rb 3 InCl 6 NCs (PLQY of 40%) …”