Directed assembly of layered perovskite heterostructures as single crystals.

The precise stacking of different two-dimensional (2D) structures such as graphene and MoS2 has reinvigorated the field of 2D materials, revealing exotic phenomena at their interfaces1,2. These unique interfaces are typically constructed using mechanical or deposition-based methods to build a heterostructure one monolayer at a time2,3. By contrast, self-assembly is a scalable technique, where complex materials can selectively form in solution4-6. Here we show a synthetic strategy for the self-assembly of layered perovskite-non-perovskite heterostructures into large single crystals in aqueous solution. Using bifunctional organic molecules as directing groups, we have isolated six layered heterostructures that form as an interleaving of perovskite slabs with a different inorganic lattice, previously unknown to crystallize with perovskites. In many cases, these intergrown lattices are 2D congeners of canonical inorganic structure types. To our knowledge, these compounds are the first layered perovskite heterostructures formed using organic templates and characterized by single-crystal X-ray diffraction. Notably, this interleaving of inorganic structures can markedly transform the band structure. Optical data and first principles calculations show that substantive coupling between perovskite and intergrowth layers leads to new electronic transitions distributed across both sublattices. Given the technological promise of halide perovskites4, this intuitive synthetic route sets a foundation for the directed synthesis of richly structured complex semiconductors that self-assemble in water.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-).

The Cs8AuIII4MIIIX23 (M = In3+, Sb3+, Bi3+; X = Cl-, Br-, I-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au3+ to Au+ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs8Au4InCl23 from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

Single Ensemble Non-exponential Photoluminescent Population Decays from a Broadband White-Light-Emitting Perovskite.

The mechanism of white-light emission from layered Pb-X (X = Cl or Br) perovskites following UV excitation has generated considerable interest. Prior time-dependent studies indicated that the broadband photoluminescence (PL) from (110) perovskites arises from a distribution of self-trapped excitonic sites emitting in different regions of the visible spectrum with different decay dynamics. Here, using time-correlated single photon counting to study single crystals, we show that the white-light emission decay from the (110) perovskite (EDBE)PbBr4 (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) behaves as a single ensemble. Following the rapid decay (0.6 ns) of a small spectral side band, the broad emission line shape is constant to 100 ns. We propose that rapid local structural fluctuations cause the self-trapped excitons (STEs) to experience a wide range of energies, resulting in the very broad PL. The STEs sample fluctuating local environments on time scales fast compared to the PL, which averages the PL decay at all emission wavelengths, yielding single ensemble PL dynamics. Although emission occurs from a very wide, inhomogeneously broadened spectral line with time-averaged single ensemble luminescence dynamics, the decay is tri-exponential. Two heuristic models for the tri-exponential decay involving defects are discussed. Spin-coated films show faster non-exponential decays with the slowest component of the crystal PL absent. Like the crystals, the film PL decays as a single ensemble. These results demonstrate that the broadband emission decay of (EDBE)PbBr4 arises from a time-averaged single ensemble and not from a set of excited states emitting with distinct luminescence decays at different wavelengths.

Dynamically Disordered Lattice in a Layered Pb-I-SCN Perovskite Thin Film Probed by Two-Dimensional Infrared Spectroscopy.

The dynamically flexible lattices in lead halide perovskites may play important roles in extending carrier recombination lifetime in 3D perovskite solar-cell absorbers and in exciton self-trapping in 2D perovskite white-light phosphors. Two-dimensional infrared (2D IR) spectroscopy was applied to study a recently reported Pb-I-SCN layered perovskite. The Pb-I-SCN perovskite was spin-coated on a SiO2 surface as a thin film, with a thickness of ∼100 nm, where the S12CN- anions were isotopically diluted with the ratio of S12CN:S13CN = 5:95 to avoid vibrational coupling and excitation transfer between adjacent SCN- anions. The 12CN stretch mode of the minor S12CN- component was the principal vibrational probe that reported on the structural evolution through 2D IR spectroscopy. Spectral diffusion was observed with a time constant of 4.1 ± 0.3 ps. Spectral diffusion arises from small structural changes that result in sampling of frequencies within the distribution of frequencies comprising the inhomogeneously broadened infrared absorption band. These transitions among discrete local structures are distinct from oscillatory phonon motions of the lattice. To accurately evaluate the structural dynamics through measurement of spectral diffusion, the vibrational coupling between adjacent SCN- anions had to be carefully treated. Although the inorganic layers of typical 2D perovskites are structurally isolated from each other, the 2D IR data demonstrated that the layers of the Pb-I-SCN perovskite are vibrationally coupled. When both S12CN- and S13CN- were pumped simultaneously, cross-peaks between S12CN and S13CN vibrations and an oscillating 2D band shape of the S12CN- vibration were observed. Both observables demonstrate vibrational coupling between the closest SCN- anions, which reside in different inorganic layers. The thin films and the isotopic dilution produced exceedingly small vibrational echo signal fields; measurements were made possible using the near-Brewster's angle reflection pump-probe geometry.

Stabilizing Au2+ in a mixed-valence 3D halide perovskite.

Although Cu2+ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au2+ exceedingly rare, typically stabilized only through Au-Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs4AuIIAuIII2Cl12, an extended solid with mononuclear Au2+ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu2+ and Pd2+, respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au2+/3+ mixed-valence state and the characteristics of the elusive Au2+ ion coordinated to simple ligands. Compared with the perovskite Cs2AuIAuIIICl6, which has been studied since the 1920s, Cs4AuIIAuIII2Cl12 exhibits a 0.7 eV reduction in optical absorption onset and a 103-fold increase in electronic conductivity.

Mechanochemical synthesis of an elusive fluorinated polyacetylene.

Polymer mechanochemistry has traditionally been employed to study the effects of mechanical force on chemical bonds within a polymer backbone or to generate force-responsive materials. It is under-exploited for the scalable synthesis of wholly new materials by chemically transforming the polymers, especially products inaccessible by other means. Here we utilize polymer mechanochemistry to synthesize a fluorinated polyacetylene, a long-sought-after air-stable polyacetylene that has eluded synthesis by conventional means. We construct the monomer in four chemical steps on gram scale, which involves a rapid incorporation of fluorine atoms in an exotic photochemical cascade whose mechanism and exquisite stereoselectivity were informed by computation. After polymerization, force activation by ultrasonication produces a gold-coloured, semiconducting fluoropolymer. This work demonstrates that polymer mechanochemistry is a valuable synthetic tool for accessing materials on a preparative scale.

Tuning the bandgap of Cs2AgBiBr6 through dilute tin alloying.

The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs2AgBiBr6 (1) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here we report that alloying crystals of 1 with up to 1 atom% Sn results in a bandgap reduction of up to ca. 0.5 eV while maintaining low toxicity. Crystals can be alloyed with up to 1 atom% Sn and the predominant substitution pathway appears to be a ∼2 : 1 substitution of Sn2+ and Sn4+ for Ag+ and Bi3+, respectively, with Ag+ vacancies providing charge compensation. Spincoated films of 1 accommodate a higher Sn loading, up to 4 atom% Sn, where we see mostly Sn2+ substitution for both Ag+ and Bi3+. Density functional theory (DFT) calculations ascribe the bandgap redshift to the introduction of Sn impurity bands below the conduction band minimum of the host lattice. Using optical absorption spectroscopy, photothermal deflection spectroscopy, X-ray absorption spectroscopy, 119Sn NMR, redox titration, single-crystal and powder X-ray diffraction, multiple elemental analysis and imaging techniques, and DFT calculations, we provide a detailed analysis of the Sn content and oxidation state, dominant substitution sites, and charge-compensating defects in Sn-alloyed Cs2AgBiBr6 (1:Sn) crystals and films. An understanding of heterovalent alloying in halide double perovskites opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner.