| Organic-inorganic hybrid metal halides(OIMHs)are a type of optoelectronic materials composed of organic cations and metal halide anions,which can combine the excellent properties from both the organic and inorganic components,and have potential application value in the fields of photodetectors,lasers,light-emitting diodes,etc.Originally,the research on these materials has mainly focused on three-dimensional(3D)metal halide perovskites.However,owing to the relatively poor stability of 3D perovskite materials largely limits their practical applications,thus prompting attention to low-dimensional OIMHs.By reducing the dimension of OIMHs,not only the stability of the material can be effectively improved,but also the crystal structure and electronic structure can be regulated.Among them,the zero-dimensional(0D)OIMHs exhibit the inherent photophysical properties of individual metal halides due to their unique"host-guest"structure.0D OIMHs embody high exciton binding energies and stable excitonic emission owing to remarkable quantum confinement effects and highly localized charges.In recent years,studies have found that the photoluminescence quantum yields(PLQYs)of(C10H16N)2Mn Br4 and(C24H20P)2Mn Br4 can be significantly improved by introducing large organic molecules,but the PLQYs of some metal halide materials are still not significantly improved.Pressure,as a"clean"effective means,can modulate the optical properties of luminescent materials by regulating the interatomic distance and intermolecular interactions without changing the chemical composition.In-depth study of the intrinsic link between the crystal structure and luminescence behavior not only deepens the understanding of the luminescence mechanism of metal halides,but also provides new ideas for pressure-induced emission enhancement and piezochromic behavior based on0D OIMHs.At present,Mn-based metal halides are favored by researchers due to their high luminescence purity,outstanding optical stability,and environmental protection.In this paper,[PP14]2[Mn Br4]([PP14]+=N-butyl-N-methylpiperidinium)is selected as the research object,in which the[Mn Br4]2-anion and the organic cation are connected by hydrogen bonds,and the hydrogen bonds will constrain the freedom of[Mn Br4]2-.The regulation of intermolecular interactions by pressure should be able to realize the regulation of the emission properties of Mn-based metal halide[PP14]2[Mn Br4]material.Based on this,we conducted the following research:1.We systematically study the optical behavior of 0D OIMH[PP14]2[Mn Br4]by in situ high-pressure photoluminescence experiments.We successfully harvested green emission enhancement in[PP14]2[Mn Br4]OIMH via pressure-treated engineering.When the pressure is completely released from 15.2 GPa to ambient conditions,the[PP14]2[Mn Br4]exhibits an excellent PLQY of up to 90.8%and a nearly 3-fold increase in luminescence intensity.Meanwhile,an ultrabroad emission tunability of 185 nm in[PP14]2[Mn Br4]has been observed from 527 nm(green)to 712 nm(red)below 12.5GPa,exhibiting obvious piezochromic behavior.2.Combined with in situ high-pressure X-ray diffraction,in situ high-pressure infrared and Raman spectra and Hirshfeld surface theory calculations,the relationship between crystal structure and optical properties was analyzed,and then the luminescence mechanism was deeply discussed.The asymmetric structure unit of[PP14]2[Mn Br4]contains two tetrahedral[Mn Br4]2-units and four organic cations,and the anions and the organic cations are connected by hydrogen bonds.In such a configuration,the intermolecular interactions could be regulated via pressure-treated engineering.Hirshfeld surface theory calculations confirmed that the Br···H intermolecular interactions became stronger after pressure treatment.The enhanced intermolecular interactions inhibited the motion freedom of the[Mn Br4]2-and reduced the thermal vibration of[Mn Br4]2-,thereby resulting in improved emission.Moreover,by digging deeply into the factors affecting the luminescent color,the mechanism of its piezochromism was obtained:The emission of Mn(II)complexes is closely related to the d-d spin-forbidden transition(~4T1?~6A1).Upon compression,the length of the Mn-Br bonds continuously decreased,contributing to the enhancement of the crystal field splitting energy.This led to the lowest energy excited state ~4T1 of Mn2+approach the ground state ~6A1,and correspondingly reduced the energy difference of ~4T1?~6A1,thereby exhibiting an obvious piezochromic behavior from green to red. |
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