Samuel W. Eaton

Atomically thin two-dimensional organic-inorganic hybrid perovskites

Flat perovskite crystals Bulk crystals and thick films of inorganic-organic perovskite materials such as CH3NH3PbI3 have shown promise as active material for solar cells. Dou et al. show that thin films—a single unit cell or a few unit cells thick—of a related composition, (C4H9NH3)2PbBr4, form squares with edges several micrometers long. These materials exhibit strong and tunable blue photoluminescence. Science, this issue p. 1518 Several inorganic-organic perovskite materials grown as atomically thin crystals exhibit strong photoluminescence. Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials.

Solution-Phase Synthesis of Cesium Lead Halide Perovskite Nanowires

Halide perovskites have attracted much attention over the past 5 years as a promising class of materials for optoelectronic applications. However, compared to hybrid organic-inorganic perovskites, the study of their pure inorganic counterparts, like cesium lead halides (CsPbX3), lags far behind. Here, a catalyst-free, solution-phase synthesis of CsPbX3 nanowires (NWs) is reported. These NWs are single-crystalline, with uniform growth direction, and crystallize in the orthorhombic phase. Both CsPbBr3 and CsPbI3 are photoluminescence active, with composition-dependent temperature and self-trapping behavior. These NWs with a well-defined morphology could serve as an ideal platform for the investigation of fundamental properties and the development of future applications in nanoscale optoelectronic devices based on all-inorganic perovskites.

Highly Luminescent Colloidal Nanoplates of Perovskite Cesium Lead Halide and Their Oriented Assemblies.

Anisotropic colloidal quasi-two-dimensional nanoplates (NPLs) hold great promise as functional materials due to their combination of low dimensional optoelectronic properties and versatility through colloidal synthesis. Recently, lead-halide perovskites have emerged as important optoelectronic materials with excellent efficiencies in photovoltaic and light-emitting applications. Here we report the synthesis of quantum confined all inorganic cesium lead halide nanoplates in the perovskite crystal structure that are also highly luminescent (PLQY 84%). The controllable self-assembly of nanoplates either into stacked columnar phases or crystallographic-oriented thin-sheet structures is demonstrated. The broad accessible emission range, high native quantum yields, and ease of self-assembly make perovskite NPLs an ideal platform for fundamental optoelectronic studies and the investigation of future devices.

Growth and Anion Exchange Conversion of CH3NH3PbX3 Nanorod Arrays for Light-Emitting Diodes

The nanowire and nanorod morphology offers great advantages for application in a range of optoelectronic devices, but these high-quality nanorod arrays are typically based on high temperature growth techniques. Here, we demonstrate the successful room temperature growth of a hybrid perovskite (CH3NH3PbBr3) nanorod array, and we also introduce a new low temperature anion exchange technique to convert the CH3NH3PbBr3 nanorod array into a CH3NH3PbI3 nanorod array while preserving morphology. We demonstrate the application of both these hybrid perovskite nanorod arrays for LEDs. This work highlights the potential utility of postsynthetic interconversion of hybrid perovskites for nanostructured optoelectronic devices such as LEDs, which enables new strategies for the application of hybrid perovskites.

Lasing in robust cesium lead halide perovskite nanowires

Significance Nanowire lasers are miniaturized light sources with great potential for integration into optoelectronic circuits. Many of the current nanowire lasers either require extreme conditions for synthesis or suffer from poor operational stability. We synthesize nanowires of a promising set of compositions, the cesium lead halides, and accomplish this under near-ambient conditions. These nanowires act as efficient laser cavities and are capable of lasing with relatively low excitation thresholds. They also demonstrate unprecedented stability for a perovskite-based nanowire laser and offer a new nanoscale platform for future study. The rapidly growing field of nanoscale lasers can be advanced through the discovery of new, tunable light sources. The emission wavelength tunability demonstrated in perovskite materials is an attractive property for nanoscale lasers. Whereas organic–inorganic lead halide perovskite materials are known for their instability, cesium lead halides offer a robust alternative without sacrificing emission tunability or ease of synthesis. Here, we report the low-temperature, solution-phase growth of cesium lead halide nanowires exhibiting low-threshold lasing and high stability. The as-grown nanowires are single crystalline with well-formed facets, and act as high-quality laser cavities. The nanowires display excellent stability while stored and handled under ambient conditions over the course of weeks. Upon optical excitation, Fabry–Pérot lasing occurs in CsPbBr3 nanowires with an onset of 5 μJ cm−2 with the nanowire cavity displaying a maximum quality factor of 1,009 ± 5. Lasing under constant, pulsed excitation can be maintained for over 1 h, the equivalent of 109 excitation cycles, and lasing persists upon exposure to ambient atmosphere. Wavelength tunability in the green and blue regions of the spectrum in conjunction with excellent stability makes these nanowire lasers attractive for device fabrication.

Synthesis of Composition Tunable and Highly Luminescent Cesium Lead Halide Nanowires through Anion-Exchange Reactions

Here, we demonstrate the successful synthesis of brightly emitting colloidal cesium lead halide (CsPbX3, X = Cl, Br, I) nanowires (NWs) with uniform diameters and tunable compositions. By using highly monodisperse CsPbBr3 NWs as templates, the NW composition can be independently controlled through anion-exchange reactions. CsPbX3 alloy NWs with a wide range of alloy compositions can be achieved with well-preserved morphology and crystal structure. The NWs are highly luminescent with photoluminescence quantum yields (PLQY) ranging from 20% to 80%. The bright photoluminescence can be tuned over nearly the entire visible spectrum. The high PLQYs together with charge transport measurements exemplify the efficient alloying of the anionic sublattice in a one-dimensional CsPbX3 system. The wires increased functionality in the form of fast photoresponse rates and the low defect density suggest CsPbX3 NWs as prospective materials for optoelectronic applications.

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of N,N-bis(n-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide), 1, obtained by X-ray diffraction reveals that 1 has a nearly planar perylene core and π-π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π-π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of 1 results in a 140 ± 20% yield of triplet excitons ((3*)1) in τ(SF) = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis.

Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives

Photodriving the activity of water-oxidation catalysts is a critical step toward generating fuel from sunlight. The design of a system with optimal energetics and kinetics requires a mechanistic understanding of the single-electron transfer events in catalyst activation. To this end, we report here the synthesis and photophysical characterization of two covalently bound chromophore-catalyst electron transfer dyads, in which the dyes are derivatives of the strong photooxidant perylene-3,4:9,10-bis(dicarboximide) (PDI) and the molecular catalyst is the Cp∗Ir(ppy)Cl metal complex, where ppy = 2-phenylpyridine. Photoexcitation of the PDI in each dyad results in reduction of the chromophore to PDI•- in less than 10 ps, a process that outcompetes any generation of 3∗PDI by spin-orbit-induced intersystem crossing. Biexponential charge recombination largely to the PDI-Ir(III) ground state is suggestive of multiple populations of the PDI•--Ir(IV) ion-pair, whose relative abundance varies with solvent polarity. Electrochemical studies of the dyads show strong irreversible oxidation current similar to that seen for model catalysts, indicating that the catalytic integrity of the metal complex is maintained upon attachment to the high molecular weight photosensitizer.

Solution-Processed Copper/Reduced-Graphene-Oxide Core/Shell Nanowire Transparent Conductors

Copper nanowire (Cu NW) based transparent conductors are promising candidates to replace ITO (indium-tin-oxide) owing to the high electrical conductivity and low-cost of copper. However, the relatively low performance and poor stability of Cu NWs under ambient conditions limit the practical application of these devices. Here, we report a solution-based approach to wrap graphene oxide (GO) nanosheets on the surface of ultrathin copper nanowires. By mild thermal annealing, GO can be reduced and high quality Cu r-GO core-shell NWs can be obtained. High performance transparent conducting films were fabricated with these ultrathin core-shell nanowires and excellent optical and electric performance was achieved. The core-shell NW structure enables the production of highly stable conducting films (over 200 days stored in air), which have comparable performance to ITO and silver NW thin films (sheet resistance ∼28 Ω/sq, haze ∼2% at transmittance of ∼90%).

Singlet Exciton Fission in Thin Films of tert-Butyl-Substituted Terrylenes

Two terrylene chromophores, 2,5,10,13-tetra(tert-butyl)terrylene (1) and 2,5-di(tert-butyl)terrylene (2), were synthesized and studied to determine their singlet exciton fission (SF) efficiencies. Compound 1 crystallizes in one-dimensional stacks, whereas 2 packs in a slip-stacked, herringbone pattern of dimers motif. Strongly quenched fluorescence and rapid singlet exciton decay dynamics are observed in vapor-deposited thin films of 1 and 2. Phosphorescence measurements on thin films of 1 and 2 show that SF is only 70 meV endoergic for these chromophores. Femtosecond transient absorption experiments using low laser fluences on these films reveal rapid triplet exciton formation for both 1 (τ = 120 ± 10 ps) and 2 (τ = 320 ± 20 ps) that depends strongly on film crystallinity. The transient absorption data are consistent with formation of an excimer state prior to SF. Triplet exciton yield measurements indicate nearly quantitative SF in thin films of both chromophores in highly crystalline solvent-vapor-annealed films: 170 ± 20% for 1 and 200 ± 30% for 2. These results show that significantly different crystal morphologies of the same chromophore can both result in high-efficiency SF provided that the energetics are favorable.