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www.acsmaterialsletters.org Efficient Lone-Pair-Driven Luminescence: Structure−Property Relationships in Emissive 5s2 Metal Halides Kyle M. McCall, Viktoriia Morad, Bogdan M. Benin, and Maksym V. Kovalenko*

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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 5s2 cations Sn2+ and Sb3+ 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 5s2 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 5s2 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 5s2 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 fi 2 Downloaded via LIB4RI on October 15, 2020 at 14:20:54 (UTC). directions in the eld of low-dimensional 5s metal halides.

+ 2+ 3+ 3+ 12 he rise of lead halide perovskites APbX3 (with A being primarily Sn ,Sb , and Bi . Spurred by solar cell research, a large cation and X− a halide) as exceptional which requires higher dimensionalities and the heavier halogens ffi See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. optoelectronic compounds has kindled immense to maintain the low band gaps needed to provide e cient solar T ff 13 interest in related main-group metal halides over the past absorption, initial e orts began with CsSnI3, Cs3M2I9 (M = Sb, − 14,15 decade.1 4 These are unique semiconductors because of the mix Bi), and the quasi-two-dimensional (2D) Ruddlesden− 16 of covalent bonding in the inorganic [PbX ]− framework and Popper perovskites. The shift to lower dimensionality 6/2 fl ionic bonding to the A cation, yielding a dynamic structure that permitted a great deal of structural exibility, and a plethora of allows for local polar fluctuations,5 electron−phonon coupling,6 metal halides soon followed, from 3D double perovskites + 3+ + 3+ 17 and the formation of large polarons to screen charge carriers.7 A2M M X6 (M = Ag, In, Tl; M = Sb, In, Bi), to fully zero- dimensional (0D) hybrid systems with large cations completely The remarkable optoelectronic features of these three-dimen- 18 19 sional (3D) materials include long charge carrier diffusion isolating square-pyramidal or octahedral metal halide units. lengths and lifetimes8,9 as well as bright, narrow emission in Even considering only octahedral metal halides, the myriad nanocrystalline form,10 properties that are enhanced drastically by their defect-tolerant nature. This defect tolerance is based on Received: May 22, 2020 the antibonding properties of the 6s2 lone pair of the heavy metal Accepted: August 4, 2020 Pb2+ coupled to halide p orbitals, which forces defects to be very Published: August 4, 2020 shallow or even lie outside the band gap.3,11 To replicate or improve upon the lead halide perovskites, the community turned to main-group metals with ns2 lone pairs,

© 2020 American Chemical Society https://dx.doi.org/10.1021/acsmaterialslett.0c00211 1218 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective

Figure 1. Structural versatility of octahedral metal halides: dimensional reduction of the aristotypical AMX3 3D perovskite to low-dimensional metal halides. choices of compatible cations (organic and inorganic) generate begin with a structural overview. In particular, we highlight the an impressive array of structure types (Figure 1), described stereoactive effects of the 5s2 lone pair, which generates the further below. appropriate atomic environments for such emission, and unify The varied dimensionalities of metal halide perovskites give the language used to describe this emission by considering the rise to a plethora of different properties deriving from the terminologies of ion emission, the atomic orbital approach, and the self-trapped exciton model. We believe that this burgeoning The varied dimensionalities of metal field will benefit from a common mechanistic understanding and halide perovskites give rise to a a summary of the available structure types that exhibit broadband emission. plethora of different properties deriv- ing from the disruption of the 3D ■ DIMENSIONALITY AND EMISSION IN perovskite framework. OCTAHEDRAL METAL HALIDES Here a brief introduction to dimensional reduction of metal disruption of the 3D perovskite framework. For example, in 2D halides is given through the series of perovskite-derivative hybrid perovskites, the alternating organic−inorganic layers lead structures based largely on corner-connected octahedral metal to anisotropic charge transport20 and dramatic enhancements in cations (Figure 1). We must mention that a variety of other non- the exciton binding energy due to dielectric confinement.21 perovskite polytypes exist, such as the face-shared octahedra of 41 Compositional tuning is similarly critical. For example, the hexagonal perovskites or entirely non-octahedral structures, 36,42−45 importance of electronic dimensionality has been demonstrated with similar diversity of dimensionalities and properties. in the double perovskites, which often behave similarly to 0D The aristotypical 3D perovskite exhibits a cubic structure, with 46 systems despite their 3D crystal structure.22,23 lower-symmetry structures derived from octahedral tilts in 1 The structural confinement found in low-dimensional metal cases with a non-ideal Goldschmidt tolerance factor. Halide halides generates vastly different properties compared with their perovskites exhibit band-edge emission at RT with high internal 47,48 3D counterparts, in particular with regard to the mechanism for PLQYs and efficient photon recycling, but self-absorption light emission. Low dimensionality tends to favor the formation leads to weak PLQYs in bulk, so isolation as nanocrystals is 4,10 of self-trapped excitons (STEs),24,25 the recombination of which needed to achieve emission with high external PLQYs. yields broadband photoluminescence (PL) with generally The shift to 2D structures already leads to a variety of different − longer lifetimes.25 27 The emission of related compounds has structure types, the properties of which are dictated by the also been interpreted using the frameworks of ion emission and direction along which the perovskite framework is cut.49,50 − atomic orbital theory.28 31 Recent work has begun to realize the ⟨100⟩ 2D perovskites consist of flat slabs of octahedral layers potential of these low-dimensional metal halides for a variety of interspersed with layers of large organic cations, yielding the applications complementary to the solar cells and narrow Ruddlesden−Popper or Dion−Jacobsen perovskites (Figure emitters of the 3D halide perovskites, namely, in solid-state 1).2,16,51 The shape of the perovskite framework is maintained lighting,18,32 X-ray scintillation,33,34 and remote thermometry.35 with such flat sheets, so the ⟨100⟩ 2D materials somewhat Of these, the highest photoluminescence quantum yields resemble their 3D counterparts with high absorption (PLQYs) at room temperature (RT) have been based on Sn2+ coefficients, dispersive band structures within the inorganic − or Sb3+ cations containing the active 5s2 lone pair.19,34,36 40 layer, and good solar cell efficiencies.20,52,53 However, the In this Perspective, we seek to provide insight into the introduction of large interlayer organic cations interrupts the structure−property relationships that dictate the performance of covalent perovskite framework, generating a quantum well low-dimensional, emissive 5s2 materials. While the focus here is structure with anisotropic optical and electronic properties and on 0D compounds, there is a rich chemistry of these materials extremely high exciton binding energies due to dielectric that has not been fully explored for light emission, and thus, we confinement.21 These large binding energies stabilize free

1219 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective excitons at room temperature, causing bulk ⟨100⟩ 2D perov- isolated 0D structures built of mononuclear emissive metal skites to emit more efficiently at room temperature.54 Broad centers. The emission of fully 0D metal halides is based wholly emission from STEs is common at low temperatures but on STEs and is most efficient for the fully isolated octahedra, requires structural distortion of the Pb−X−Pb angle to manifest especially with large organic cations.19,37 at room temperature,26 so free exciton emission is most 2 common in the ⟨100⟩ 2D perovskites. ■ 0D UNITS AND CHEMISTRY OF THE 5s LONE PAIR The ⟨110⟩-type 2D perovskites possess further anisotropy, as In addition to octahedral units, ns2 metal halides can form corner-sharing occurs in a straight line along one direction while structures based on pyramidal MX3 units, disphenoidal MX4 31,43 the other direction has zig-zag chains that disrupt charge units, and square-pyramidal MX5 coordination (Figure 2), transport within the inorganic layer, providing further confine- ment.55,56 In some sense, the ⟨110⟩ 2D perovskites share more similarities to one-dimensional (1D) perovskites than to their ⟨100⟩ or 3D counterparts, and accordingly, the emission properties of these materials are drastically different, with the manifestation of RT STE emission in ⟨110⟩ perovskites.26,57,58 This class represents an edge case in this regard, as it includes several ⟨110⟩ compositions exhibiting both free exciton and STE emission.26,56 2D defect perovskites formed from trivalent cations (e.g., Sb3+ or Bi3+) must balance the excess charge on the M site with one- □ third occupancy of vacancies; this yields the formula A3M2 X9 (where □ represents a vacancy) and forms bilayers cut along the ⟨111⟩ direction of the 3D perovskite (Figure 1).59,60 It should be noted that the ⟨111⟩ direction of the 3D perovskite corresponds to the R symmetry point of the Brillioun zone, where the band edge states lie, and thus, the 2D defect perovskite most effectively breaks up the 3D perovskite. Furthermore, while this Figure 2. Atomic environments of 0D ns2 metal halides, with the structure maintains 2D hexagonal symmetry within the lone pair visualized in orange. octahedral layer, it is further removed from the 3D perovskite than the ⟨100⟩ or ⟨110⟩ 2D perovskites because no straight lines 61 which can be built into polymeric frameworks and chains just as connect more than two octahedra. Accordingly, these in the case of the octahedral metal halides.43,69 The increasing compounds exhibit STEs and relatively poor charge transport ⟨ ⟩ structural diversity with reduced dimensionality is illustrated in properties compared with either 3D or 100 2D perov- Table 1 for 5s2-based metal halides (Sn2+,Sb3+), with archetypes skites.14,61,62 For further information on 2D perovskites, we refer the reader to recent reviews that describe these compounds in Table 1. Anionic 5s2 Metal Halide Structural Units greater detail.50,51 Further reducing the dimensionality to 1D compounds begets wholly different properties compared with the 3D perovskites because of the limitation of orbital overlap to a single crystallographic axis. While there are a variety of corner- connected octahedral structures ranging from straight trans- connected chains to zigzag cis-connected chains (Figure 1)63 and corrugated double chains,64 their optical properties are relatively underexplored. The corrugated compounds Cs3M2Cl9 (M = Sb, Bi) are known to be emissive at low temperatures only,65 whereas efficient STE emission from 1D metal halides has been obtained from edge-shared double chains.66 0D metal halides possess fully isolated octahedral units, which can be bridged by inorganic cations as in Cs4SnBr6 or large 19,37 organic cations as in (C4N2H14Br)4SnBr6. While corner- connected octahedra form many higher-dimensional frame- works, corner-connected 0D clusters are quite rare: no divalent cations exhibit such anions, while only seven compounds are 5− 67 known to exhibit the dimeric [M2X11] anion. Instead, 0D octahedral clusters tend toward edge-sharing or face-sharing octahedra to help reduce the total charge of the anion. For example, in Sb-based dimers the total charge on the − − anion decreases from 5 in corner-shared Sb2Br11 to 4 in edge- known to exhibit emission at room temperature marked in − fl shared Sb2X10 and 3 in face-shared Sb2X9 (Figure 1). Such green. The coordinative exibility of 0D units permits a wide polynuclear clusters can contain as many as eight or 18 range of structures, many of which possess emission at room octahedra for trivalent and bivalent cations, respectively.44,68 temperature. It should be noted that the table is limited to However, such clusters have not been extensively explored for dimeric units for clarity and that a variety of polynuclear 0D their optical properties, and hence, the focus here is on the fully compounds exist beyond those listed.43,44,87

1220 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective fl This highlights that lone pair expression is also heavily The coordinative exibility of 0D units influenced by the halogen size. For example, the binary halide ff permits a wide range of structures, SnBr2 exhibits a di erent structure than SnCl2, SnI2, PbCl2, and fl many of which possess emission at PbBr2 because of the combination of stereochemical in uence of room temperature. its lone pair electrons with the appropriate size disparity between the metal and halogen, despite the fact that SnCl2 exhibits the 100 strongest repulsion of the lone pair. Similarly, while BiI3 3− adopts a layered structure with perfectly symmetric [BiI6] octahedra, BiCl3 exhibits trigonal distortions that lead to Excited states play a critical role in the optoelectronic 101 properties of metal halides, and in these 0D complexes the pyramidal coordination, requiring additional crystal forces 31 64 geometry is dictated by the outer-shell ns2 lone pair electrons. to return to the high-symmetry octahedra of Cs3BiCl6. The Hence, it is helpful to visualize the stereochemistry of the lone smaller size of Sb causes all three binary halides SbX3 (X = Cl, Br, ff 101 fl pair in each coordination environment, shown in orange in I) to exhibit such o -centering, but the templating in uence of other cations can enforce high-symmetry crystallographic Figure 2. The lone pair induces a statically expressed distortion 3− 79 in these units, from the expected trigonal-planar to trigonal- sites such as the [SbCl6] octahedra in Rb7Sb3Cl16 and Cs NaSbCl .102 The case of Cs NaSbCl is especially interest- pyramidal in MX3 anions, tetragonal to disphenoidal in MX4 2 6 2 6 31 ing, as the enforced octahedral symmetry on a normally anions, and trigonal-bipyramidal to square-pyramidal in MX5. expressed SbCl lone pair leads to a large lattice constant of In contrast, the octahedral MX6 anion can be found in either 3 inert crystallographic environments, where the site symmetry is 10.7780 Å for Cs2NaSbCl6 while that of the isostructural 37,88 102 Cs2NaInCl6 is 10.5313 Å, despite the similar ionic radii of essentially cubic, or in environments where the lone pair is 3+ 3+ 103 expressed through a trigonal or tetragonal deformation.19,85,89 Sb and In (0.76 vs 0.80 Å). This points to a dynamic distortion of the [SbCl ]3− octahedron in which the stereo- Intriguingly, even the inert MX6 octahedron exhibits some 6 ff chemistry of the lone pair is expressed dynamically while the tendency toward o -centering, with signs of dynamic excited- 29 state structural fluctuations observed in tellurium halides88,90,91 cubic long-range symmetry is maintained. However, this does 37,92 not occur in the isostructural Cs2NaSbBr6, in which the ionic and Cs4MX6. The importance of such features in the optical 29 performance will be discussed for the case of 5s2 metal halides radius is consistent with the expected value, highlighting the − based on Sb3+ and Sn2+. importance of metal halogen overlap in determining the The 5s2 lone pair has an exceptionally rich chemistry because dynamics of the lone pair. fl The contrast of lone pair stability and expression makes the of the combination of stereoactivity and structural exibility 2 compared with neighboring metals. Following the inert pair 5s lone pair unique, as it lies on the cusp between the effect, the oxidative stability of ns2 cations increases moving down the periodic table, with 6s2 cations such as Pb2+ and Bi3+ The contrast of lone pair stability and having the most chemically inert lone pairs.93,94 Lone pair expression makes the 5s2 lone pair electrons have strong stereochemical effects on the metal− ligand coordination environment; however, this lone pair unique, as it lies on the cusp between expression is distinct from the chemical stability of the “inert” the dynamically expressed lone pairs of lone pair. The same larger 6s2 cations offer more space for the 6s2 cations and the statically expressed lone pair and reduced surface charge density on the cation than lone pairs of the 4s2 cations. their lighter neighbors, increasing the polarizability and coordination number. These features permit structurally inert lone pairs such as the highly symmetric structures of PbI2 and BiI3, which have centered metal cations with no obvious lone dynamically expressed lone pairs of 6s2 cations and the statically pair expression.69 expressed lone pairs of the 4s2 cations. This interplay of dynamic The 3D halide perovskites AMX based on Ge2+,Sn2+, and 3 and static off-centering is at its peak in the 0D 5s2 metal halides, Pb2+ are an instructive example to explore the differences 2 2 2 permitting structural distortions that can yield efficient between the 4s ,5s, and 6s lone pairs. The small size of the 2 2 Ge2+ cation does not leave sufficient space for the lone pair luminescence in materials where similar 4s or 6s structures fi ff have nonexistent or weak emission. For example, while electrons, and thus, AGeX3 structures possess signi cant o - 104,105 centering of the Ge atom toward a face of the would-be Cs4PbBr6 does not emit at room temperature, Cs4SnBr6 95 and (C4N2H14Br)4SnBr6 have bright emission with PLQYs of octahedron, leading to a trigonal-pyramidal coordination. 37 19 2+ 2+ 15% and 95%, respectively. This can be understood as a Sn - and Pb -based AMX3 compounds exhibit perovskite- 2 related structures with crystallographically inert lone pairs (with function of the enhanced hybridization between the metal ns the exception of the highly distorted low-temperature phases of lone pair and the halide np orbitals, with recent work showing 96 that the higher energy of the Sn 5s level leads to the trend MASnBr3), but this belies a dynamic nature that leads to polar 5 ffi fluctuations at room temperature and exceptional thermal Cs4SnBr6 >Cs4SnI6 >Cs4PbBr6 favoring e cient STE emission 92 expansion cofficients.97 Stereochemical off-centering at high in the former while the latter only emits at low temperatures. fi temperatures was rst observed in CsSnBr3, with the Sn atom Similarly, in the disphenoidal metal bromides (bmpip)2MBr4, exhibiting a trigonal shift toward the face of the octahedron with (bmpip)2SnBr4 has a PLQY of 75%, while that of 98 space group R3̅m, exactly the same space group and off- (bmpip)2PbBr4 is only 24% and that of (bmpip)2GeBr4 is 95 34 centering found in the crystal structure of AGeI3. Subsequent below 1%. To better understand the broad, bright studies have shown that this dynamic nature is more luminescence driven by the 0D 5s2 lone pair, we bridge together pronounced in the Sn2+ and Br− analogues than in the respective the various models that have been utilized to characterize this Pb2+ or I− compounds.99 emission.

1221 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective

Figure 3. (A) Schematic representation of the relationship between the atomic orbital model, the molecular orbital model, and the semiconductor model of an extended solid. (B) Energy band diagram associated with the free ns2 ion. (C) Energy band diagram of the metal− halide molecular orbitals. (D) Configurational coordinate diagram of the simplified STE model in 0D 5s2 metal halides. (E) Unified model with the configurational coordinate diagram, in which the ground and excited states are described using their atomic character as derived from the active ns2 metal ion. ■ UNIFYING THEORY OF 5s2 EMISSION Unification of the two models provides Emission from the isolated center can manifest itself in many a framework that can explain and different forms, e.g., a dopant in a host matrix or pure material, as well as under different conditions, e.g., cryogenic or room predict many properties of emissive 0D temperature. Researchers from different fields of spectroscopy metal halides. and materials science have contributed to isolated center emission studies. Because of this, different theoretical models are The first theoretical description is derived from the simple circulating in the literature. While these models share many fi two-electron Seitz model (Figure 3B). In this model, a free ion features, the absence of a uni ed model (or the discussion of with the ns2 configuration has the singlet electronic ground state, such a model) leads to contradicting and confusing terminology. 1 denoted with atomic term symbol S0. The splitting of the nsnp Here we attempt to bring together the important theoretical excited states into 1P (singlet) and 3P (triplet) states occurs as a fi concepts into a uni ed framework. result of the Coulomb (F) and exchange (G) interactions. Two main models are presently utilized, arising from either 3 3 Further, the P states are split into non-degenerate states P2, (1) treating 0D centers as isolated ions dressed in the halide 3P , and 3P by the spin−orbit coupling interaction. Additionally, fi 1 0 ligand eld or (2) presenting these as the limiting case of the overall energy of all of the described states is non-uniformly complete electronic and structural isolation in the framework of altered by the ligand field (in this case by the coordinating an extended solid. Overall, both approaches lead to the same halides). This interaction with the ligand field for ns2 ions theoretical picture of the isolated MXz centers, as illustrated in includes a large degree of hybridization between the atomic Figure 3A. Unification of the two models provides a framework orbitals of the central metal and ligands, which is described by that can explain and predict many properties of emissive 0D molecular orbital (MO) theory (Figure 3C). In this case, the metal halides. ground and excited states are localized on both central ions and

1222 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective ligands. Atomic term symbols in this description are substituted structural change that the metal halide center undergoes upon 1 ff with molecular term symbols (i.e., S0 becomes a1g). The relative excitation. The di erence between the excited- and ground-state energy of the latter depends directly on the molecular geometry geometries is reflected in the large Stokes shift of 0D 5s2 metal (i.e., octahedral, square-pyramidal, disphenoidal) and has been halide emission. Both absorption and emission transitions can summarized by Vogler and Nikol for different possible occur from different vibronic states (denoted as flat lines in the molecular geometries.31 However, it remains very common in parabola), thus yielding an amalgam of individual transitions the literature to use atomic term symbols even for systems with a with emission probabilities following a Poisson distribution. The high degree of hybridization and denote the resulting molecular convolution of these transitions results in a broadened emission 1 3 38,106,107 59 states as P1-derived, P0-derived, etc. In this way, the peak, often with a tail toward lower energy. The magnitude of model still serves its main purpose: to explain the transitions that the electron−phonon coupling is described by the Huang−Rhys occur between the states in terms of their allowed or forbidden parameter, which is larger for stronger coupling and defines the nature, which can be experimentally reflected in the measured degree of broadening associated with STE emission.59,110 emission decay time (i.e., longer radiative lifetimes for partially The STE model’s configurational coordinate diagram is a forbidden transitions and shorter for allowed ones) or the simplified projection that represents a complex energy surface as intensity of the corresponding excitation band.30,108 a two-dimensional plot and therefore has limitations, especially For example, the dramatic reduction of the radiative lifetime in complex materials with multiple emissive centers. However, it μ of Cs2NaSbCl6 from 1.4 ms at 5 K to 0.15 s at higher has proven quite useful in characterizing STE emitters in recent temperatures with no change in the emission intensity until years,59,111,112 and in conjunction with the Toyozawa model,24 ∼200 K is derived entirely from changes in the character of the important features such as the Huang−Rhys parameter and 3 → 1 ff radiative excited state, from the forbidden P0 S0 transition at e ective phonon energy can be derived. Importantly, this model 3 → 1 ∼ 29 low temperatures to the P1 S0 transition by 25 K. On the also rationalizes a key property of STE emitters: temperature- other hand, the MO approach becomes useful for comparing dependent emission quenching.24,108 The crossing of the optical properties for different geometries as well as correctly ground- and excited-state parabolas provides a nonradiative predicting the ground- and excited-state geometries. This is relaxation pathway from the excited-state minimum to the especially crucial for these 0D systems in which the halide bonds crossing point purely through phonon excitation, known as hot are not shared, and thus, the metal halide anion and its lone pair nonradiative recombination.24 The energy difference (height) are free to distort in the excited state, and the excited-state between the intercept and the excited-state minimum defines geometry is dramatically different than the ground-state the energy barrier (and thereby the temperature, as described geometry. further below) required to quench the radiative process (Figure Historically, the Seitz model was widely used to interpret the 3D). optical properties of ns2 ion impurities in binary alkali metal Thus, the Seitz model mainly focuses on the nature of the halides109 or in various other oxide and halide hosts.29,30 The electronic states and, with the MO extension, on their STE model for interpretation of the optical properties of 0D degeneracy in the electric field of the ligand, while the SCC metal halides became widespread only recently because of the model describes phonon−exciton interactions. The successful research focus on ns2 metal halide semiconductors. While free interpretation of observed phenomena in the optical properties excitons are dominant in the prototypical 3D AMX3 semi- of 0D materials requires the aid of all three theoretical concepts. conductors, the structural versatility of metal halides yields many The unified model is depicted in Figure 3E. It is based on the compounds where STEs dominate.50 SCC diagram, where the excited state can be split into several The typical theoretical description of phonon-assisted STE nondegenerate states, each reflecting the nature of the recombination involves a single configurational coordinate contributing atomic state. As we will show, the splitting and (SCC) diagram, in which the abscissa represents a change in degeneracy of these states vary with the coordination geometry, molecular geometry upon excitation (Q) and the ordinate but this model is flexible enough to describe the observed optical represents a change in energy (Figure 3D).110 Ground and features. excited states in this model are described as parabolic potential 2 energy functions, which are harmonic in the first approximation. ■ PROPERTIES OF EMISSIVE 5s 0D MATERIALS It should be noted that in this SCC diagram, the free exciton Among the various emissive 5s2 0D materials that have been excited state would be represented as a parabola centered at the reported, the simplest case with which to study such structure− ground state with higher energy;57 however, free excitons have property relationships comes from Te4+. The high charge of the not been observed in 0D 5s2 metal halides, and hence, our model metal center necessitates a greater number of halide ligands, considers the localized self-trapped exciton as the only stable restricting the known Te4+ halides to materials containing excited state. This is consistent with the description of Ueta et al. octahedrally coordinated Te4+ regardless of the nature of the demonstrating that 1D or lower-dimensional systems have countercation.87,113 The most commonly adopted structure is unstable free exciton states that localize upon any perturba- that of the vacancy-ordered double perovskite A2TeX6 with tion,24 which means that the self-trapped exciton is the only regular octahedra.87 The isostructural nature of these com- relevant state in 0D 5s2 metal halides. This simplifies the STE pounds has encouraged excellent fundamental studies; for model to an SCC diagram essentially identical to the generalized example, the role of the halide in luminescence quenching has − 114 113 vibronic transitions described by the Franck Condon principle. been studied in the series Cs2TeX6 and A2TeX6, with Here we maintain the use of the term “self-trapped exciton” to be chlorides showing the brightest emission at RT. The dynamic consistent with the literature on higher-dimensional metal nature of these octahedra has also been extensively studied, with halides. many reports on the dynamic Jahn−Teller effect-driven The minimum of each parabola represents the equilibrium tetragonal distortions in the excited state.86,88,90,113,115 The geometry of the ground or excited state, while the horizontal apparent simplicity of these materials, however, has not been difference between these geometries, ΔQ, is interpreted as the manifested in numerous luminescent examples, as the

1223 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective

Table 2. Summary of the Optical Properties of Highly Emissive 0D 5s2 Metal Halide Compounds μ compound coordination unit Eexc/abs (nm) Eem (nm) Stokes shift (eV) FWHM (eV) PLQY (%) lifetime ( s) ref

Cs4SnBr6 MX6 340 540 1.28 0.51 15 0.54 37

HDM3SnBr8 MX6 350 600 1.60 0.44 86 3.12 120

(C4N2H14Br)4SnBr6 MX6 355 570 1.32 0.40 95 2.2 19

(PEA)6SnBr8[CCl2H2]2 MX6 340 566 1.46 0.44 89.5 2.7 121

(C4N2H14I)4SnI6 MX6 410 620 1.02 0.38 75 1.1 19

(bmpip)2SnBr4 MX4 350 658 1.66 0.37 75 4 34

(C6H22N4Cl3)SnCl3 MX3 280 638 2.48 0.42 7.6 71

Bzmim3SbCl6 MX6 342 525 1.26 87.5 2.4 85

Bzmim2SbCl5 MX5 375 600 1.24 22.3 2.6 85

Bmim2SbCl5 MX5 370 583 1.22 86.3 4.26 18

(C9NH20)2SbCl5 MX5 380 590 1.16 0.43 98 4.2 19

TTA2SbCl5 MX5 370 625 1.37 0.45 86 7.5 38

TEBA2SbCl5 MX5 360 590 1.34 0.48 98 7.7 38

(Ph4P)2SbCl5 MX5 375 648 1.39 0.41 87 4.6 122

(PPN)2SbCl5 MX5 410 635 1.07 0.44 98.1 4.1 119

Rb7Sb3Cl16 MX6,M2X10 365 560 1.16 0.53 3.8 0.2 79

Figure 4. (A) 0D structures with statically expressed lone pairs (SnX4, SbX5) and the corresponding energy band diagram. (B) Dynamically distorted octahedral MX6 and the corresponding band diagram. (C) Excitation-dependent PL of (bmpip)2SnBr4 showing singlet and triplet 34 emission. (D) PL and PL excitation (PLE) spectra of (TTA)2SbCl5 showing the separate excitation of the triplet and singlet states (reproduced using the previously reported synthesis38 and characterized using an optical setup described elsewhere34). (E) PL and PLE spectra 37 19 118 of Cs4SnBr6, (C4N2H14Br)4SnBr6, and [18-crown-6]2Cs3SbBr6. quenching temperatures are relatively low in these com- examples of luminescent materials containing octahedra, 86 pounds, and those that do emit at RT are plagued by low disphenoids, and trigonal pyramids (Table 2 and Figure 4A). PLQYs. This appears to be due to an apparent concentration Additionally, a 0D structure containing square pyramids quenching effect86 that is less pronounced in other 5s2 0D metal (Rb3SnCl3I2) was also recently discovered; however, its halides, but the root cause deserves further investigation given 116 2+ 3+ potential for luminescence was left unexplored. Though the exceptional performance of Sn and Sb emissive materials 2+ and Te4+’s distinct advantages of stability and its tendency to Sn is a versatile ion for materials discovery, its tendency to 4+ ffi adopt a single coordination that encourages rational design. oxidize to Sn increases the synthetic di culty of discovering 117 At the other end of the spectrum, Sn2+ with its low charge new materials. This may be one of the reasons why so far only exhibits the greatest structural diversity of 0D compounds, with seven room-temperature luminescent 0D structures have been

1224 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective reported. Of these, the most prominent examples consist geometries such as disphenoids (SnX4) and square pyramids primarily of disphenoids and octahedra (Figure 4). (SbX5)(Figure 4A). In both cases, the reduced symmetry Intermediate in 0D structural diversity is trivalent Sb3+. Most imposed by these motifs results in nondegenerate excited states, of the luminescent examples in the literature are hybrid which may allow for additional transitions if the energy levels are − 2− ffi organic inorganic materials with square-pyramidal [SbCl5] su ciently separated, although the separation may be quite units isolated by large organic cations (Figure 4A); the small (Figure 4A).31 This has been demonstrated by Morad et 3 → 1 1 → 1 remainder are octahedral or polynuclear octahedral units al., with the observation of both P1 S0 and P1 S0 34 obtained in cation/alkali halide-rich compositions, e.g., transitions at RT in disphenoidal (bmpip)2SnBr4 (Figure 4C). (bzmim)3SbCl6 and Rb7Sb3Cl16 (Figure 4 and Table 2). This also highlights the fact that the triplet STE state tends to be the lowest-energy excited state for such 5s2 metals. This same Some empirical observations lead us to model extends to square-pyramidal Sb (i.e., MX5 in Table 2), for which Wang et al. first observed two distinct emission bands conclude that coordinatively unsatu- 18 from (bmim)2SbCl5, and subsequently several other singlet- rated metal centers such as SnX4 or emitting hybrid antimony chlorides have been found.19,119 One MX5 (M = Sn, Sb) can be obtained only such compound is TTA2SbCl5, which further demonstrates the through the use of organic and excitation-wavelength-dependent nature of the emission (Figure 4D).38 High-energy excitation succeeds in populating the high- sufficiently bulky countercations or a 1 energy singlet P1 state, resulting in emission directly from this combination of halides with very state as well as emission from the thermodynamically favored 3 different sizes. lower-energy P1 triplet state. This situation is simplified in octahedrally coordinated systems (Figure 4B). The larger coordination number reduces From a purely structural perspective, too few structures are ff known at this point to predict the structure-directing effect that a the static expression of the lone pair and therefore o -centering specific cation may have. However, some empirical observations and distortion, often leading to high-symmetry sites and similar lead us to conclude that coordinatively unsaturated metal bond lengths. The regularity (or near-regularity) of such sites obscures the optical properties present in the disphenoidal and centers such as SnX4 or MX5 (M = Sn, Sb) can be obtained only through the use of (a) organic and sufficiently bulky counter- cations (e.g., bmpip, PPh4, bzmim, etc.; Table 2) or (b) a Given the enormous diversity of struc- combination of halides with very different sizes (e.g., mixed Cl−, ff 2 − 116 tures o ered by these 5s systems and I ). Smaller inorganic countercations can enable 0D their recent (re)discovery as optical structures in alkali halide-rich environments that sufficiently isolate the metal halide complex with high coordination materials, it is apparent that the numbers (e.g., Cs4SnBr6), but less alkali halide favors the community has only scratched the formation of higher-dimensional frameworks (e.g., CsSnBr3), as surface concerning their properties and the small cations cannot fully isolate the 0D units (Table 1). On the basis of these various Sn2+- and Sb3+-based materials, how to utilize them. several examples can be compared to see how the geometry and the metal center affect the resulting optical properties. To square-pyramidal coordinations, as the dynamic distortions of examine these differences, the SCC diagrams for 0D Sn and Sb these octahedra lead to stronger mixing of the excited states. units can be redrawn on the basis of their geometry and the This is best illustrated by comparing three 5s2 bromides: “ ” stereoactivity of the lone pair (Figure 4A,B). The static lone Cs4SnBr6,(C4N2H14Br)4SnBr6, and [18-crown-6]2Cs3SbBr6 pair is expressed and found in coordinatively unsaturated (Figure 4E).19,37,118 Although these three materials have

Figure 5. (A) Radioluminescence spectra of (bmpip)2SnBr4 and NaI:Tl under 50 kV Ag tube X-ray irradiation, with images of the NaI:Tl commercial scintillator and a (bmpip)2SnBr4 pellet under ambient light and X-ray irradiation. Adapted from ref 34. Copyright 2019 American Chemical Society. (B) Configurational coordinate diagram showing the change in energy barrier with different shifts of the excited state, with ΔE 79 red and blue representing high and low (and thus quenching temperatures), respectively. (C) PL lifetime vs temperature for Rb7Sb3Cl16, 35 35 Cs4SnBr6, and (C4N2H14I)4SnI6.

1225 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective different metal centers and different degrees of distortion or lone with high Stokes shifts and momentum offsets that inhibit 3 pair expression, the PL spectra appear to be triplet ( P0,1,2)- nonradiative recombination from defect states and favor hot dominated with one broad, featureless emission peak. This can recombination. The only other known source of nonradiative be understood in terms of the higher degeneracy of the regular quenching comes from parasitic impurity phases such as 37 octahedral environment: while the trigonally distorted octahe- CsSnBr3, which absorbs in the emissive region of Cs4SnBr6, dron would have fully distinct singlet and triplet states, the but such quenching is only known to affect the PLQY. regular octahedron does not fully separate these levels, and Thus, hot nonradiative recombination dominates in 0D 5s2 therefore, the singlet remains unobserved.31 This also correlates metal halides, dictated by the energy barrier ΔE that governs the Δ with the reduced Stokes shift and emission line width in quenching temperature of the luminescence, Tquench. E, along ff octahedral compounds relative to their more distorted counter- with the average or e ective phonon energy, Eph, directly parts; e.g., Vogler and Nikol compared the absorption and influences the emission lifetime and PLQY through the − 3− emission of disphenoidal SbCl4 and octahedral SbCl6 in temperature-dependent competition between the radiative and 31 solution. nonradiative relaxation rates (kr vs knr, respectively). The onset Given the enormous diversity of structures offered by these of quenching, with corresponding changes in lifetime, defines 5s2 systems and their recent (re)discovery as optical materials, it the temperature-sensitive regime. For example, in Figure 5C this is apparent that the community has only scratched the surface regime is found at the higher temperatures, where kr and knr ff concerning their properties and how to utilize them. We compete e ectively against one another and kr remains non- highlight two rising applications that take advantage of the negligible. In contrast, the flat region at low temperatures (<325 unique features of these 0D materials: X-ray scintillation and K for (C4N2H14I)4SnI6; <250 K for Cs4SnBr6 and Rb7Sb3Cl16) remote thermometry and thermography. involves a purely radiative process, where knr is negligible, and X-ray scintillation utilizes two features shared by these 0D 5s2 the PL lifetime remains rather unaltered and the PLQY is metal halide materials: (1) efficient relaxation processes for saturated. This trend appears to be general not only for 0D Sn high-energy absorption to the STE triplet state and (2) highly materials but also for 0D Sb materials (Figure 5C).79 This robust Stokes-shifted emission such that the compounds are trans- thermal sensitivity, which can be tuned by structural engineer- parent to their emission. The first such study demonstrated that ing, and the wide temperature and lifetime ranges that it covers 2 (bmpip)2SnBr4 could function as an excellent X-ray scintillator make 5s metal halides a compelling class of novel (Figure 5A).34 A very recent work found that square-pyramidal thermoluminophores. ffi 119 (PPN)2SbCl5 also exhibits e cient X-ray scintillation, and it Cryogenic temperatures can present exceptions to this is likely that many of these materials are radioluminescent. An behavior that may also prove suitable for thermometric open question is whether the long emission lifetimes (Table 2) applications, as below 77 K in Rb7Sb3Cll6 a second temper- are problematic for X-ray detection. Although these materials ature-sensitive regime can be observed without any change in 79 have low overall atomic numbers, they still perform comparably the PLQY. This purely radiative yet thermally sensitive regime to commercially available NaI:Tl or CsI:Tl scintillators and is likely driven by changes in the character of the excited-state 29,108 seem to outperform organic scintillators. This class of materials levels, as described above for Cs2NaSbCl6. The unchanging deserves much more investigation in this context, and we hope intensity with concurrent temperature sensitivity ensures that to see collaborations between traditional scintillator groups and the signal from the thermometric probe will remain strong materials chemists to find effective scintillators among the 5s2 throughout the entire sensitivity regime. This unique behavior metal halides. may enable high thermometric sensitivities at extremely low The second application, thermometry and thermography, is temperatures (below 10−15 K), where accurate temperature based on the steep temperature dependence of the PL lifetimes values can be difficult to obtain through other means without of these materials, as first demonstrated by our group for altering the temperature of the system and hence a simple optical 35 Cs4SnBr6,(C4N2H14I)4SnI6, and [C(NH2)3]2SnBr4. The PL probe may be quite valuable. lifetimes of these compounds vary by up to 2 orders of magnitude over a temperature range of 100 °C, giving rise to ■ POLYNUCLEAR 0D 5s2 COMPOUNDS fi some of the highest speci c sensitivities among known Beyond these isolated 0D units, a plethora of polynuclear 0D thermoluminophores. Temperature sensitivity is also observed cluster compounds based on 5s2 metal cations exist.43,78,87,123 in the PLQY of many materials, including these, but the dramatic 2 Their optical potential remains to be thoroughly explored. RT changes in lifetime of 0D 5s metal halides impart a much 3− PL has been reported for [Sb2I9] dimers in Cs3Sb2I9, but with stronger sensitivity than simply utilizing the emission intensity 59 35 fi rather weak emission intensity. This may be due to the could provide. This property is rationalized by the uni ed asymmetry in these polynuclear systems, which provides stable model presented earlier, which is simplified in Figure 5Bto ff ff lone pair o -centering in coupled units that may enhance highlight the e ect of shifting the excited-state parabola. The nonradiative recombination (Figure 6). However, the discovery intersection of two parabolas (colored and black curves) of Rb Sb Cl and its emissive distorted [Sb Cl ]4− dimer with represents a nonradiative pathway back to the ground state. As 7 3 16 2 10 ’ a PLQY of 3.8% demonstrates that there are more optically the Stokes shift increases and the material s PL red-shifts, the active centers to be discovered.79 Recent reports on a bright upper excited-state parabola shifts further from the ground-state 5− [Pb3Cl11] cluster with near-unity PLQY provide further parabola, lowering the energy barrier ΔE for the phonon- 124 “ ” encouragement, and we expect emissive clusters to be a assisted hot nonradiative recombination. rich avenue of exploration moving forward. Unlike conventional semiconductors, the PL lifetime in 0D metal halides is intrinsically robust, i.e., without noticeable effects of the structural defects and surfaces. This robust nature ■ OTHER EMISSIVE METAL CENTERS is a consequence of the strongly localized excited states of these We also wish to highlight a recent series of metal halides based 0D 5s2 metal halides, which lead to excited-state energy bands on ns0 (d10) cations that have exhibited remarkably efficient and

1226 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective

to achieve high performance; and large Stokes shifts that minimize self-absorption. Applications of such compounds include X-ray scintillation, phosphors for white-light emission, and high-resolution thermography, and we expect great strides in each of these fields in the coming years. That being said, challenges persist that will need to be overcome to enable the development of successful devices that fulfill the promise of these applications. Many open questions deserve further exploration to under- stand the structure−property relationships and unveil the practical potential of these compounds. At present, it is difficult to predict the coordination environment generated by different organic cations, and finding design principles will require many more examples to tease out the effects of cation size, polarity, bonding characteristics, etc. Even in the absence of such design principles, several important trends can be observed on the basis of the available literature. For example, nearly all examples of highly emissive compounds in Table 2 can be grouped into two kinds, namely, Sn(II) bromides and Sb(III) chlorides, indicating that the orbital overlap and/or phonon energies in these combinations are particularly suited for highly efficient emission. Figure 6. Atomic environments of polynuclear 0D 5s2 metal halides derived from square pyramids and octahedra, with the lone pairs In the Cs2TeX6 system, the RT emission intensity increases for visualized in orange. lighter halides, similar to Sb(III) halides but contrary to Sn(II) bromides. Further studies, such as the computational compar- fi ison of Cs4SnBr6,Cs4SnI6, and Cs4PbBr6 that identi ed the broadband luminescence similar to that observed in the 5s2 importance of the Sn 5s2−Br 4p overlap,92 are needed to metal halides. Examples of nd10 emitters have been known for determine whether the energy levels of Sn(II) and Br are ideal − many years among the Cu+ and Ag+ halides,125 127 but this field for efficient emission or whether the lower stability of Sn(II) has been reinvigorated with a flurry of investigations in the last 5 chlorides is the key to the observed differences. The structural years showcasing the utility of these materials in scintillators and and photophysical aspects of the excited state are rather poorly − light-emitting diodes (LEDs).128 131 This group has been understood. While we have elaborated on some intriguing trends further bolstered by several new broadband-emissive com- in the structure−property relationships of these compounds and − pounds based on isoelectronic Zn2+ and Cd2+.132 135 Mixing of the connection to the 5s2 lone pair, a true picture of the nd10ns2 systems has also led in some cases to enhanced dynamics of these systems is currently lacking. Besides emission,136 and we expect that there is much more synergy to commonplace transient absorption spectroscopy, excited states be found among such mixed-metal systems. In particular, of these materials can be probed by ultrafast measurements that combining the efficient excitation of low-band-gap ns2-based combine optical excitation and structure probing (ultrafast − materials with highly emissive nd10 centers could make for electron or X-ray scattering).50,57,99,142 144 attractive white-light phosphors. We also observe that the hybrid organic−inorganic materials In the case of In3+, the reported emission characteristics are routinely exhibit higher PLQYs than their fully inorganic extraordinarily similar to those of Sb3+-doped indium halide counterparts, which may indicate more effective 0D isolation − materials.39,137 139 A new report found that undoped and Sb3+- in the former materials with large and soft organic cations (Table · doped Cs2InCl5 H2O had essentially identical features and 2). This likely derives from the lower-energy phonons and their temperature-dependent intensities,39 while recent work in our enhanced coupling in inorganic materials, whereas the high- · group showed that pure Cs2NaInCl6 and Cs2InCl5 H2O do not energy phonons of the organics improve the structural isolation emit.140 In our view, trace Sb3+ dopants in the precursors may be of the lone-pair-driven vibrational modes in these 0D materials responsible for these emissions, similar to the miniscule levels of and push the temperature region for thermal quenching to Sn impurities that were found by Mitzi et al. to generate STE higher temperatures (Figure 5C). However, the inorganic 141 emission in PEA2PbBr4. Careful experimental work would be compositions benefit from enhanced stability and higher melting required to demonstrate this, perhaps utilizing InX monohalides points, as manifested in the ability to prepare nanocrystals 145,146 as indium precursors that would be chemically less likely to (NCs) of Cs4SnBr6, which has not yet been demonstrated contain trace Sb3+ impurities. A very recent report further for any hybrid organic−inorganic 0D compositions. · validates this view by demonstrating that pure Cs2InCl5 H2O The properties that allow them to excel as thermosensitive does not luminesce without Sb3+ doping.107 phosphors may hinder the utility of metal halides as solid-state lighting phosphors. During LED operation, the excitation source ■ SUMMARY AND OUTLOOK typically heats the phosphor, with local surface temperatures The future is bright for emissive lone pair materials, and the potentially exceeding 70 °C.147,148 While hybrid organic− structural versatility of 5s2 metal halides yields a wide inorganic metal halides generally exhibit higher PLQYs at compositional space for the design of emitters with unique ambient and elevated temperatures, their chemical stability properties. Low-dimensional emissive 5s2 metal halides offer becomes a limiting factor. Furthermore, the majority of these several advantages: quantum efficiencies near unity; earth- materials exhibit excitation peaks below 400 nm, which are abundant, nontoxic metal cations; intrinsic emission that does unsuitable for use even with purple LEDs (405 nm) and require not require doping, electronic passivation or particle size control them to be paired with UV LEDs (for which an inexpensive and

1227 https://dx.doi.org/10.1021/acsmaterialslett.0c00211 ACS Materials Lett. 2020, 2, 1218−1232 ACS Materials Letters www.acsmaterialsletters.org Perspective efficient UV light source is unavailable at present) for use as Bogdan M. Benin − Laboratory of Inorganic Chemistry, downconversion phosphors in white-light LEDs.149 The Department of Chemistry and Applied Biosciences, ETH Zürich, combination of these two limitations necessitates further CH-8093 Zürich, Switzerland; Laboratory for Thin Films and structural exploration of 5s2-based materials suitable for white- Photovoltaics, EmpaSwiss Federal Laboratories for Materials light LEDs. Science and Technology, CH-8600 Dübendorf, Switzerland With regard to application in scintillators, a pertinent Complete contact information is available at: challenge is their intrinsically slow radiative recombination, in https://pubs.acs.org/10.1021/acsmaterialslett.0c00211 the low microsecond range (Table 2). Time-resolved radio- luminescence studies are needed to determine the decay time Notes under X-ray excitation and the possibility of afterglow. These fi compounds may therefore not be suitable for applications that The authors declare no competing nancial interest. require photon counting and/or spectroscopic X-ray energy Biographies resolution capabilities, such as positron emission tomography Kyle M. McCall received his Ph.D. in Applied Physics from and nuclear security. However, static X-ray imaging such as used Northwestern University in 2019 under the joint supervision of in dental and physician practices would benefit from the low- Professors Mercouri Kanatzidis and Bruce Wessels. He is now working cost nature of these materials and the high reported as a postdoctoral researcher under Professor Maksym V. Kovalenko in 119 sensitivity, which enables a low dose rate to the patient. the Functional Inorganic Materials laboratory at ETH Zürich. His So far, we have found that emission characteristics and research interests focus on halide materials for radiation detection and 2 durability of known 5s metal halides neatly match the light emission. theoretical and practical requirements of thermometric Viktoriia Morad received her M.S. in Chemistry from ETH Zürich in applications. The intrinsic nature of the STE emission makes 2018 and since then has been pursuing a Ph.D. under the supervision of Prof. Maksym V. Kovalenko at ETH Zürich. Her research is focused on The intrinsic nature of the STE emission synthesis and applications of metal halides for light emission. makes the emission characteristics Bogdan M. Benin received his M.S. in Chemistry from Kent State rather insensitive to the particle size, University in 2016 and is now pursuing his Ph.D. under the supervision surface, and common structural defects of Prof. Dr. Maksym V. Kovalenko at ETH Zürich. His research and hence the synthesis method. currently focuses on the discovery and application of luminescent zero- dimensional lead-free metal halides. the emission characteristics rather insensitive to the particle size, Maksym V. Kovalenko received his Ph.D. from Johannes Kepler surface, and common structural defects and hence the synthesis University Linz in 2007 and was a postdoctoral researcher at the − method. The temperature range of high sensitivity is exceptional University of Chicago in 2008 2011. Since 2011 he has led the ̈ and broadly tunable by chemical design, while the respective PL Functional Inorganic Materials group at ETH Zurich and Empa. He has lifetimes lie in convenient ranges for various detection schemes been a full professor since 2020. The research activities of his group (ns to μs). Future work should also focus on the design of focus on discovery and engineering of novel inorganic materials and nanoscopic thermoluminescent probes. This requires the their applications for light emission and detection as well as in synthesis of size-tunable and environmentally stable nano- rechargeable batteries. particles of 0D metal halides. For high-resolution thermal imaging of biological systems, these NCs must be water- ■ ACKNOWLEDGMENTS compatible and water-dispersible. This work was financially supported by ETH Zürich through the ■ AUTHOR INFORMATION ETH+ Project SynMatLab and by the European Union through Corresponding Author the Horizon 2020 Research and Innovation Programme (Grant Agreement 819740, Project SCALE-HALO). The authors thank Maksym V. Kovalenko − Laboratory of Inorganic Chemistry, Rebecca McClain for constructive input in figure design. Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland; Laboratory for Thin Films and Photovoltaics, EmpaSwiss Federal Laboratories for Materials ■ REFERENCES Science and Technology, CH-8600 Dübendorf, Switzerland; (1) Stoumpos, C. 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