Michael L. Crawford

Bio: Michael L. Crawford is an academic researcher from University of Oregon. The author has contributed to research in topics: Perovskite (structure) & Halide. The author has an hindex of 3, co-authored 8 publications receiving 75 citations.

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs.

Abstract: Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

Surface ligation stage revealed through polarity-dependent fluorescence during perovskite nanocrystal growth

Abstract: Methylammonium lead iodide perovskite (MAPbI3) nanocrystals (NCs) exhibit favorable photophysics for a range of light emitting applications. A comprehensive mechanistic understanding of the nucleation and growth processes for these NCs is still elusive. Absorbance and fluorescence spectra were measured during a NC synthesis with kinetics limited by precursor solvation using a rapid sampling technique wherein syringe filters quench NC growth. The signal from well-capped NCs in the reaction mixture was isolated by the use of polar syringe filters, enabling spectroscopic observation of the surface ligation process. Our results indicate that the formation of these NCs involves a single stage of nucleation and growth, followed by a terminal surface ligation stage.

Evolving Stark Effect During Growth of Perovskite Nanocrystals Measured Using Transient Absorption

Abstract: Methylammonium lead triiodide (MAPbI3) nanocrystals (NCs) are emerging materials for a range of optoelectronic applications. Photophysical characterization is typically limited to structurally stable NCs owing to the long timescales required for many spectroscopies, preventing the accurate measurement of NCs during growth. This is a particular challenge for non-linear spectroscopies such as transient absorption. Here we report on the use of a novel single-shot transient absorption (SSTA) spectrometer to study MAPbI3 NCs as they grow. Comparing the transient spectra to derivatives of the linear absorbance reveals that photogenerated charge carriers become localized at surface trap states during NC growth, inducing a TA lineshape characteristic of the Stark effect. Observation of this Stark signal shows that the contribution of trapped carriers to the TA signal declines as growth continues, supporting a growth mechanism with increased surface ligation toward the end of NC growth. This work opens the door to the application of time-resolved spectroscopies to NCs in situ, during their synthesis, to provide greater insight into their growth mechanisms and the evolution of their photophysical properties.

Rapid sampling during synthesis of lead halide perovskite nanocrystals for spectroscopic measurement

Abstract: The photophysical properties of lead halide perovskite nanocrystals (NCs) are critical to their potential application in light emitting devices and other optoelectronics, and are typically characterized using optical spectroscopies. Measurements of nuclei and nascent NC photophysics during synthesis provide insight into how the reaction can be changed to control the properties of the resulting NCs. However, these measurements are typically only performed ex situ after growth is halted by centrifuging the reaction mixture for several minutes. Here, a method is reported to rapidly sample the reaction mixture during a solvation-limited synthesis to enable multiple spectroscopic measurements during nucleation and NC growth. Absorbance and fluorescence measurements of a reaction mixture during the formation of methylammonium lead triiodide perovskite NCs are reported. The changing positions of spectral features as a function of reaction time show the expected weakening of exciton confinement during NC growth. The evolving fluorescence spectra demonstrate that the capping and surface passivation of nascent NCs changes during the reaction. The species in the reaction mixture, particularly during the early stages of the synthesis, are shown to be unstable. This indicates that, even for a relatively slow solvation-limited reaction, the photophysics of the reaction mixture can only be accurately captured if spectroscopic measurements are completed within seconds of sampling. The common use of centrifugation to quench NC syntheses prior to spectroscopic measurement biases the NC population towards more stable, well-capped NCs and does not accurately report on the full NC population in a reaction mixture.

Diol it up: The influence of NaCl on methylglyoxal surface adsorption and hydration state at the air-water interface.

Abstract: Methylglyoxal (MG)-an atmospherically important α-dicarbonyl implicated in aqueous-phase secondary organic aerosol formation-is known to be surface-active. Due to the presence of carbonyl moieties, MG can hydrate to form geminal diols in solution. Recently, it has been shown that MG exists predominantly as a monohydrate at the neat air-water interface. However, inorganic aerosol constituents have the potential to "salt-out" MG to the interface, shift its hydration equilibria, and catalyze self- and cross-oligomerization reactions. Here, we study the influence of the non-reactive salt, sodium chloride (NaCl), on the MG's surface adsorption and hydration state using vibrational sum frequency spectroscopy. The presence of NaCl is found to enhance MG's surface activity but not to the extent that water is fully excluded from the interface. Perturbations in the interfacial water structure are attributed to shifts in MG's hydration equilibrium at higher ionic strengths. Evidence of surface-active MG oligomer species is presented, but such oligomers are not thought to contribute significantly to the interfacial population. This work builds on the published studies on MG in pure water and gives insight into the interface's perturbation by NaCl, which has important implications for understanding MG's atmospheric fate.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Abstract: Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Colloidal nanocrystal synthesis and the organic-inorganic interface

Abstract: Colloidal nanocrystals are nanometer-sized, solution-grown inorganic particles stabilized by a layer of surfactants attached to their surface. The inorganic cores exhibit useful properties controlled by composition as well as size and shape, while the surfactant coating ensures that these structures are easy to fabricate and process. It is this combination of features that makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. But their full potential can only be exploited if we achieve exquisite control over their composition, size, shape, crystal structure and surface properties. Here we review what is known about nanocrystal growth and outline strategies for controlling it.

All-Inorganic Quantum-Dot LEDs Based on a Phase-Stabilized α-CsPbI 3 Perovskite

Abstract: The all-inorganic nature of CsPbI3 perovskites allows to enhance stability in perovskite devices. Research efforts have led to improved stability of the black phase in CsPbI3 films; however, these strategies-including strain and doping-are based on organic-ligand-capped perovskites, which prevent perovskites from forming the close-packed quantum dot (QD) solids necessary to achieve high charge and thermal transport. We developed an inorganic ligand exchange that leads to CsPbI3 QD films with superior phase stability and increased thermal transport. The atomic-ligand-exchanged QD films, once mechanically coupled, exhibit improved phase stability, and we link this to distributing strain across the film. Operando measurements of the temperature of the LEDs indicate that KI-exchanged QD films exhibit increased thermal transport compared to controls that rely on organic ligands. The LEDs exhibit a maximum EQE of 23 % with an electroluminescence emission centered at 640 nm (FWHM: ≈31 nm). These red LEDs provide an operating half-lifetime of 10 h (luminance of 200 cd m-2 ) and an operating stability that is 6× higher than that of control devices.

Perovskite light-emitting diodes

Abstract: Light-emitting diodes based on halide perovskites have undergone rapid development in recent years and can now offer external quantum efficiencies of over 23%. However, the practical application of such devices is still limited by a number of factors, including the poor efficiency of blue-emitting devices, difficulty in accessing emission wavelengths above 800 nm, a decrease in external quantum efficiency at high current density, a lack of understanding of the effect of the electric field on mobile ions present in the perovskite materials, and short device lifetimes. Here we review the development of perovskite light-emitting diodes. We examine the key challenges involved in creating efficient and stable devices, and consider methods to alleviate the poor efficiency of blue-emitting devices, leverage emission in the long infrared region and create spin-polarized light-emitting diodes. This Review examines the development of perovskite light-emitting diodes, exploring the key challenges involved in creating efficient and stable devices.

Three-dimensional direct lithography of stable perovskite nanocrystals in glass

Abstract: Material composition engineering and device fabrication of perovskite nanocrystals (PNCs) in solution can introduce organic contamination and entail several synthetic, processing, and stabilization steps. We report three-dimensional (3D) direct lithography of PNCs with tunable composition and bandgap in glass. The halide ion distribution was controlled at the nanoscale with ultrafast laser–induced liquid nanophase separation. The PNCs exhibit notable stability against ultraviolet irradiation, organic solution, and high temperatures (up to 250°C). Printed 3D structures in glass were used for optical storage, micro–light emitting diodes, and holographic displays. The proposed mechanisms of both PNC formation and composition tunability were verified. Description Perovskite nanocrystals under glass Perovskite nanocrystals (PNCs) such as cesium lead triiodide (CsPbI3) can display bright photoemission with narrow linewidths for display applications, but their long-term stability requires passivation and encapsulation steps after synthesis in solution. Sun et al. created three-dimensional arrays of PNCs in doped metal oxide glasses using ultrafast laser pulses that caused local melting and subsequent crystallization. They tuned the bandgap of PNCs and their photoluminescence between 480- and 700-nanometer wavelengths by transforming the composition from CsPb(Cl1-xBrx)3 to CsPbI3. These encapsulated PNCs exhibited long-term stability after prolonged heating or organic solvent and ultraviolet light exposure. —PDS Melting of a doped metal oxide glass with ultrafast laser pulses created perovskite nanocrystal arrays for optoelectronics.