Efficient Lone-Pair-Driven Luminescence: Structure–Property Relationships in Emissive 5s² Metal Halides

Abstract

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of low-dimensional halides based on the 5s 2 cations Sn 2+ and Sb 3+ has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s 2 lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s 2 lone pair to identify the structural features that enable strong emission. We unite the different theoretical models that have been able to explain the success of these bright 5s 2 emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s 2 metal halides.

Expanding the 0D Rb7M3X16 (M=Sb, Bi; X=Br, I) Family: Dual‐Band Luminescence in Rb7Sb3Br16

“…Rb 7 Sb 3 Br 16 is isostructural to Rb 7 Sb 3 Cl 16 , [18] with a quasi‐0D structure consisting of [SbBr 6 ] 3− octahedra and edge‐shared [Sb 2 Br 10 ] 4− dimers that stack in alternating layers along the c ‐axis ( Figure 1). The bioctahedral [Sb 2 Br 10 ] 4− dimers are bent towards one side, likely a result of the repulsion of the stereoactive 5s 2 lone pair of Sb 3+ , [14] as first suggested by Ruck et al . for Tl 7 Bi 3 I 16 [20] .…”

“…The PL of Rb 7 Bi 3 Br 16 at 12 K has a peak maximum at 1.97 eV and excitation maximum at 2.96 eV and quenches rapidly, becoming weak enough to match the background level by 50 K ( Figure S12 ), which is significantly lower than the 200 K quenching temperature of Rb 7 Bi 3 Cl 16 [18] . The weaker luminescence of the Rb 7 Bi 3 X 16 (X=Cl, Br) series compared to the antimony analogues Rb 7 Sb 3 X 16 further highlights the greater optical activity of the stereoactive 5s 2 lone pair of Sb 3+ over the 6s 2 lone pair of Bi 3+ [14,31] …”

“…This absence of optical activity is consistent with the trend of decreasing self‐trapped exciton luminescence quenching temperature with heavier halides (and the correspondingly lower phonon energies), as determined for (TDMP)PbX 4 (X=I, Br, Cl) [33] and Cs 2 TeX 6 (X=I, Br, Cl) [34] and as found here with the reduction of luminescence quenching temperature in the series Rb 7 Bi 3 Cl 16 >Rb 7 Bi 3 Br 16 >Rb 7 Bi 3 I 16 . Similarly, heavier cations with the 6s 2 lone pair such as Bi 3+ or Pb 2+ are often less emissive than their 5s 2 counterparts, [14] indicating that Rb 7 Bi 3 I 16 should have the lowest quenching temperature (and thereby the weakest luminescence) of the Rb 7 M 3 X 16 family. Intriguingly, Rb 3 Bi 2 I 9 is also the only composition of the A 3 M 2 I 9 (A=Cs, Rb; M=Bi, Sb) family which did not luminesce at any temperature (note that Cs 3 Bi 2 I 9 exhibits a different structure than the other three compositions of this family, [35,36] likely explaining why it maintains luminescence despite its compositional similarity with Rb 3 Bi 2 I 9 ) [25] …”

“…Note that this broader class of materials may often lack the corner‐shared octahedra which define the perovskite archetype, and thus the proper term for this family is metal halides (or equivalently halometalates) [8] . Low‐dimensional metal halides in particular are a rising class of semiconductors with complementary properties relative to the 3D perovskites, [9–13] with zero‐dimensional (0D) metal halides containing 5s 2 lone‐pair ions such as Sn 2+ and Sb 3+ demonstrating efficient, broadband emission driven by the structural dynamics associated with stereoactivity of the lone pair [14–16] . Among these luminescent materials, very few all‐inorganic 0D‐ compositions exhibit strong PL at room temperature (RT), namely Cs 4 SnBr 6 [17] and Rb 7 Sb 3 Cl 16 [18] .…”

“…[8] Lowdimensional metal halides in particular are a rising class of semiconductors with complementary properties relative to the 3D perovskites, [9 -13] with zerodimensional (0D) metal halides containing 5s 2 lonepair ions such as Sn 2 + and Sb 3 + demonstrating efficient, broadband emission driven by the structural dynamics associated with stereoactivity of the lone pair. [14][15][16] Among these luminescent materials, very few all-inorganic 0D-compositions exhibit strong PL at room temperature (RT), namely Cs 4 SnBr 6 [17] and Rb 7 Sb 3 Cl 16 . [18] The latter composition is especially interesting due to the greater oxidative stability of Sb 3 + relative to the air-sensitive Sn 2 + cation.…”