9 Search Results

Controlling the connectivity and topology of solids is a versatile way to target desired physical properties. This is especially relevant in the realm of hybrid halide semiconductors, where the long-range connectivity of the inorganic substructural unit can lead to significant changes in optoelectronic properties such as photoluminescence, charge transport, and absorption. We present a new series of hybrid metal-halide semiconductors, (phenH₂)BiI₅·H₂O, (2,2-bpyH₂)BiI₅, (BrbpyH)BiI₄·H₂O, (phenH₂)₂Pb₃I₁₀·2H₂O, and (2,2-bpyH₂)₂Pb₃I₁₀ where (phenH₂)²⁺ = 1,10-phenanthroline-1,10-diium, (2,2-bpyH₂)²⁺ = 2,2'-bipyridine-1,1'-diium and (BrbpyH)⁺ = 6,6'-dibromo-2,2'-bipyridium. These compounds allow us to observe how the planarity of the cation, induced either through structural modification in the case of (phenH₂)²⁺ ormore » through non-covalent interactions in (BrbpyH)⁺, both relative to (2,2-bpyH₂)²⁺, modifies the inorganic substructural unit. While the Pb²⁺ series of compounds show minimal changes in inorganic connectivity, we observe large differences in the Bi³⁺ series, ranging from 0-D dimers to corner- and edge-sharing 1-D chains of octahedra. We find that compounds containing (phenH₂)²⁺ and (BrbpyH)⁺ pack more efficiently than those with (2,2-bpyH₂)²⁺ due to their retention of planarity leading to greater inorganic connectivity. Electronic structure calculations and optical diffuse reflectance reveal that the band gaps of these compounds are influenced by the degree of inorganic connectivity and the inorganic substructural unit distances. These results show that the structure and planarity of organic cations can directly influence both the inorganic connectivity and the optical properties that could be tuned for certain optoelectronic applications.« less

Hybrid metal halides yield highly desirable optoelectronic properties and offer significant opportunity due to their solution processability. This contribution reports a new series of hybrid semiconductors, (C₇H₇)MX₄ (M = Bi³⁺, Sb³⁺; X = Cl^–, Br^–, I^–), that are composed of edge-sharing MX₆ chains separated in space by π-stacked tropylium (C₇H₇⁺) cations; the inorganic chains resemble the connectivity of BiI₃. The Bi³⁺ compounds have blue-shifted optical absorptions relative to the Sb³⁺ compounds that span the visible and near-IR region. Consistent with observations, DFT calculations reveal that the conduction band is composed of the tropylium cation and valence band primarily the inorganicmore » chain: a charge-transfer semiconductor. The band gaps for both Bi³⁺ and Sb³⁺ compounds decrease systematically as a function of increasing halide size. Furthermore these compounds are a rare example of charge-transfer semiconductors that also exhibit efficient crystal packing of the organic cations, thus providing an opportunity to study how structural packing affects optoelectronic properties.« less

In this work, we present our discovery and characterization of a new kagome prototype structure, KV₃Sb₅. We also present the discovery of the isostructural compounds RbV₃Sb₅ and CsV₃Sb₅. All materials exhibit a structurally perfect two-dimensional kagome net of vanadium. Density-functional theory calculations indicate that the materials are metallic, with the Fermi level in close proximity to several Dirac points. Powder and single-crystal syntheses are presented, with postsynthetic treatments shown to deintercalate potassium from single crystals of KV₃Sb₅. Considering the proximity to Dirac points, deintercalation provides a convenient means to tune the Fermi level. Magnetization measurements indicate that KV₃Sb₅ exhibits behaviormore » consistent with a the Curie-Weiss model at high temperatures, although the effective moment is low (0.22μ_B per vanadium ion). An anomaly is observed in both magnetization and heat capacity measurements at 80 K, below which the moment is largely quenched. Elastic neutron scattering measurements find no obvious evidence of long-range or short-range magnetic ordering below 80 K. The possibility of an orbital-ordering event is considered. Single-crystal resistivity measurements show the effect of deintercalation on the electron transport and allow estimation of the Kadowaki-Woods ratio in KV₃Sb₅. We find that A/γ²~61μOhm cm mol²_FU K²J^-2, suggesting that correlated electron transport may be possible. KV₃Sb₅ and its cogeners RbV₃Sb₅ and CsV₃Sb₅ represent a new family of kagome metals, and our results demonstrate that they deserve further study as potential model systems.« less

Abstract Orienting light‐emitting molecules relative to the substrate is an effective method to enhance the optical outcoupling of organic light‐emitting devices. Platinum(II) phosphorescent complexes enable facile control of the molecular alignment due to their planar structures. Here, the orientation of Pt(II) complexes during the growth of emissive layers is controlled by two different methods: modifying the molecular structure and using structural templating. Molecules whose structures are modified by adjusting the diketonate ligand of the Pt complex, dibenzo‐( f , h )quinoxaline Pt dipivaloylmethane, (dbx)Pt(dpm), show an ≈20% increased fraction of horizontally aligned transition dipole moments compared to (dbx)Pt(dpm) doped intomore » a 4,4′‐bis( N ‐carbazolyl)‐1,1′‐biphenyl, CBP, host. Alternatively, a template composed of highly ordered 3,4,9,10‐perylenetetracarboxylic dianhydride monolayers is predeposited to drive the alignment of a subsequently deposited emissive layer comprising (2,3,7,8,12,13,17,18‐octaethyl)‐21H,23H‐porphyrinplatinum(II) doped into triindolotriazine. This results in a 60% increase in horizontally aligned transition dipole moments compared to the film deposited in the absence of the template. The findings provide a systematic route for controlling molecular alignment during layer growth, and ultimately to increase the optical outcoupling in organic light‐emitting diodes.« less

Two-dimensional perovskites have emerged as more intrinsically stable materials for solar cells. Chemical tuning of spacer organic cations has attracted great interest due to their additional functionalities. However, how the chemical nature of the organic cations affects the properties of two-dimensional perovskites and devices is rarely reported. Here we demonstrate that the selection of spacer cations (i.e., selective fluorination of phenethylammonium) affects the film properties of two-dimensional perovskites, leading to different device performance of two-dimensional perovskite solar cells (average n = 4). Structural analysis reveals that different packing arrangements and orientational disorder of the spacer cations result in orientational degeneracymore » and different formation energies, largely explaining the difference in film properties. This work provides key missing information on how spacer cations exert influence on desirable electronic properties and device performance of two-dimensional perovskites via the weak and cooperative interactions of these cations in the crystal lattice.« less

FMOF-1 is a flexible, superhydrophobic metal–organic framework with a network of channels and side pockets decorated with –CF₃ groups. CO₂ adsorption isotherms measured between 278 and 313 K and up to 55 bar reveal a maximum uptake of ca. 6.16 mol kg^–1 (11.0 mol L^–1) and unusual isotherm shapes at the higher temperatures, suggesting framework expansion. We used neutron diffraction and molecular simulations to investigate the framework expansion behaviour and the accessibility of the small pockets to N₂, O₂, and CO₂. Neutron diffraction in situ experiments on the crystalline powder show that CO₂ molecules are favourably adsorbed at three distinctmore » adsorption sites in the large channels of FMOF-1 and cannot access the small pockets in FMOF-1 at 290 K and oversaturated pressure at 61 bar. Stepped adsorption isotherms for N₂ and O₂ at 77 K can be explained by combining Monte Carlo simulations in several different crystal structures of FMOF-1 obtained from neutron and X-ray diffraction under different conditions. A similar analysis is successful for CO₂ adsorption at 278 and 283 K up to ca. 30 bar; however, at 298 K and pressures above 30 bar, the results suggest even more substantial expansion of the FMOF-1 framework. The measured contact angle for water on an FMOF-1 pellet is 158°, demonstrating superhydrophobicity. Simulations and adsorption measurements also show that FMOF-1 is hydrophobic and water is not adsorbed in FMOF-1 at room temperature. Simulated mixture isotherms of CO₂ in the presence of 80% relative humidity predict that water does not influence the CO₂ adsorption in FMOF-1, suggesting that hydrophobic MOFs could hold promise for CO₂ capture from humid gas streams.« less