Cesium Manganese Bromide Nanocrystal Sensitizers for Broadband Vis-to-NIR Downshifting

Abstract

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible-to-NIR spectral conversion. However, lead-based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr 3 and Cs 3 MnBr 5 nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr 3 . The correlation of the lanthanide emission to the CsMnBr 3 visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr 3 matrix to the lanthanides was understood by density functional theory calculations. This study shows that lead-free metal halides with an appropriate phase are effective sensitizers for lanthanides and offer a route to efficient downshifting applications.

Highly Reversible Cesium Manganese Iodine for Sensitive Water Detection and X-ray Imaging

“…No difference was found in the PLE and PL spectra at various emission and excitation wavelengths, which suggests that the green emission originates from the relaxation of the same excited state. Actually, previous studies have shown that the tetrahedrally coordinated Mn 2+ ions could emit green fluorescence due to the radiative d–d transitions (i.e., from 4 T 1 to the ground state 6 A 1 ). − Time-resolved PL (TRPL) signal of the Cs 3 MnI 5 NCs can be well fitted by a single-exponential function at both low and high temperatures (Figure S9), similar to a prior report on Cs 3 MnBr 5 NCs . This result suggests that the radiative transition process from 4 T 1 to the ground state 6 A 1 is not influenced by trap states.…”

“…We have also recorded TRPL curves of the Cs 3 MnI 5 NCs under different excitation wavelengths (Figure S10). Unlike PL decay kinetics of CsMnBr 3 NCs, no excitation-dependent PL decay lifetime can be observed for our Cs 3 MnI 5 NCs, similar to that of Cs 3 MnBr 5 , indicating that there is no significant delay in populating the low-energy excited state of Mn 2+ before it returns to the ground state.…”

“…Inspired by previous works by Almutlaq et al and Jalali et al, which demonstrated phase-controlled synthesis of CsMnBr 3 NCs, , we have also tried to synthesize the pure CsMnI 3 by optimizing synthesis parameters, such as precursors’ ratio and reaction temperature. However, instead of the CsMnI 3 NCs, a CsI phase was often produced (Figure S6).…”

“…Figure a shows UV–vis absorption and PL excitation (PLE) spectra of the Cs 3 MnI 5 NCs, from which an obvious band tail absorption up to 600 nm was observed, a strong sign that the crystals contain a certain number of sub-band gap states which lie below its conduction band edge. Figure S7 shows the Tauc plot of the Cs 3 MnI 5 NCs, from which a band gap of 2.4 eV was extracted assuming their direct band gap transitions . Similar to Cs 3 MnBr 5 NCs, the absorption spectrum of Cs 3 MnI 5 NCs exhibits multiple peaks that arise from the d–d transitions present in the tetrahedrally coordinated Mn 2+ ions. , Cs 3 MnI 5 NCs show a PL emission peak at 540 nm with a large Stokes shift of ∼280 nm and a narrow fwhm of 48 nm (Table S1 summarizes and compares fwhm values of Cs 3 MnI 5 NCs, Cs 3 MnBr 5 NCs, and some recently reported Pb-based perovskites).…”

“…Figure S7 shows the Tauc plot of the Cs 3 MnI 5 NCs, from which a band gap of 2.4 eV was extracted assuming their direct band gap transitions . Similar to Cs 3 MnBr 5 NCs, the absorption spectrum of Cs 3 MnI 5 NCs exhibits multiple peaks that arise from the d–d transitions present in the tetrahedrally coordinated Mn 2+ ions. , Cs 3 MnI 5 NCs show a PL emission peak at 540 nm with a large Stokes shift of ∼280 nm and a narrow fwhm of 48 nm (Table S1 summarizes and compares fwhm values of Cs 3 MnI 5 NCs, Cs 3 MnBr 5 NCs, and some recently reported Pb-based perovskites). Under 254 nm UV light excitation, Cs 3 MnI 5 NCs emit bright green fluorescence with a moderate PLQY of ∼30% (see inset of Figure a).…”