Size-Dependent Lattice Structure and Confinement Properties in CsPbI₃ Perovskite Nanocrystals: Negative Surface Energy for Stabilization

CsPbI₃ nanocrystals with narrow size distributions were prepared to study the size-dependent properties. The nanocrystals adopt the perovskite (over the nonperovskite orthorhombic) structure with improved stability over thin-film materials. Among the perovskite phases (cubic α, tetragonal β, and orthorhombic γ), the samples are characterized by the γ phase, rather than α, but may have a size-dependent average tilting between adjacent octahedra. Size-dependent lattice constants systematically vary 3% across the size range, with unit cell volume increasing linearly with the inverse of size to 2.1% for the smallest size. We estimate the surface energy to be from −3.0 to −5.1 eV nm⁻² for ligated CsPbI₃ nanocrystals. Moreover, the size-dependent bandgap is best described using a nonparabolic intermediate confinement model. We experimentally determine the bulk bandgap, effective mass, and exciton binding energy, concluding with variations from the bulk α-phase values. This provides a robust route to understanding γ-phase properties of CsPbI₃

A Map of the Inorganic Ternary Metal Nitrides

Exploratory synthesis in novel chemical spaces is the essence of solid-state chemistry. However, uncharted chemical spaces can be difficult to navigate, especially when materials synthesis is challenging. Nitrides represent one such space, where stringent synthesis constraints have limited the exploration of this important class of functional materials. Here, we employ a suite of computational materials discovery and informatics tools to construct a large stability map of the inorganic ternary metal nitrides. Our map clusters the ternary nitrides into chemical families with distinct stability and metastability, and highlights hundreds of promising new ternary nitride spaces for experimental investigation--from which we experimentally realized 7 new Zn- and Mg-based ternary nitrides. By extracting the mixed metallicity, ionicity, and covalency of solid-state bonding from the DFT-computed electron density, we reveal the complex interplay between chemistry, composition, and electronic structure in governing large-scale stability trends in ternary nitride materials

Thermochromic Metal Halide Perovskite Windows with Ideal Transition Temperatures

Urban centers across the globe are responsible for a significant fraction of energy consumption and CO2 emission. As urban centers continue to grow, the popularity of glass as cladding material in urban buildings is an alarming trend. Dynamic windows reduce heating and cooling loads in buildings by passive heating in cold seasons and mitigating solar heat gain in hot seasons. In this work, we develop a mesoscopic building energy model that demonstrates reduced building energy consumption when thermochromic windows are employed. Savings are realized across eight disparate climate zones of the United States. We use the model to determine the ideal critical transition temperature of 20 to 27.5 {\deg}C for thermochromic windows based on metal halide perovskite materials. Ideal transition temperatures are realized experimentally in composite metal halide perovskite film composed of perovskite crystals and an adjacent reservoir phase. The transition temperature is controlled by co-intercalating methanol, instead of water, with methylammonium iodide and tailoring the hydrogen-bonding chemistry of the reservoir phase. Thermochromic windows based on metal halide perovskites represent a clear opportunity to mitigate the effects of energy-hungry buildings

Developing new functional TCs

Transparent Conductors (TCs) are increasingly critical to the performance and reliability of a number of technologies. Traditionally based primarily on oxides of Ga, In, Zn and Sn the class is rapidly expanding into new materials including both other oxides and more recently composites of metallic or carbon nanowires. Many of these materials offer unique functionality as well as processing and reliability advantages over some of the historic materials. These compounds are all classically non-stoiciometric and often metastable consisting of oxide, non-oxide and composite materials which are being collectively looked at for an increasingly broad set of applications including photovoltaics, solid state lighting, power electronics and a broad class of flexible and wearable electronics. In this talk, we will focus on two main areas; the development of predictive models to be able to identify dopants and the processing regimes where they can be activated as well as the use of nanowire oxide composites to develop a new generation of tunable high performance TC. The complex set of demands for a desired TC include not only classical performance, but also processibility, cost and reliability necessitating a search for new materials. The ability to use materials genomics to identify new dopable TC materials that are experimentally realizable is rapidly increasing. We will discuss recent work on predicting the dopability of Ga2O3 films, which potentially have broad applicability as buffer layers, TCOs, and in power electronics if the doping level can be well controlled. We will discuss the theoretical predictions for the process windows to activate both Sn and Si as dopants and compare this to experimental results and the literature. We will also present resent results on the theoretical prediction and realization of a new p-type TC based on CuZnS, which has demonstrated conductivities of up to 100 S/cm. The latter while not classically an oxide is certainly non-stoichiometric and properties are enhanced in many cases by the use of complex oxide, sulfide and selenide materials. Together these will illustrate the evolving tools both theory and experiment to develop and realize dopants in wide band gap materials. In cases where single materials may not be sufficient, nanowire (metal or carbon based) composites with oxides is increasingly attractive. For example, Ag, and potentially Cu, nanowires embedded in a metal oxide matrix can potentially produce TCs that can be processed at low temperature, have conductivity and transparency comparable to the best TCOs, control interface stability and electronic properties and are suitable to flexible electronics. We will present work on ZnO, InZnO and ZnSnO composites with Ag nanowires where the performance can be as good as high quality InSnO with films Rs\u3c 10 Ohms/sq. We will discuss the dependence on the interrelationship between the nanowire properties and the oxide properties. We will also discus the concept of employing sandwich oxides to separately optimize the top and bottom interfacial properties. This work was supported, in part, by the Center for the Next Generation of Materials by Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research also supported in part by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, with support from the Office of International Affairs) and the Government of India subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd Nov. 2012

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region

Using Solution Phase Self-Assembly to Control the Properties of Magnetic and Magnetoelectric Nanostructures

Nanostrutured magnetic materials have gained much recent interest because of their application in various electronic systems. These materials, however, often require complex lithography and epitaxy to control the magnetic properties. In this work, solution-phase self-assembly is used to create magnetic and magnetoelectric materials with a variety of nanoscale structures. By engineering the architecture of the system, control over a range of magnetic properties can be realized. The first part of this work focuses on nano-magnetic materials. Here, the organization of nanoscale magnets into different geometries is controlled, and the properties of the systems are studied. In the first chapter, Ni-Cu nanowire stacks are examined to explore the effect of shape anisotropy on the coupling between different elements. This work provides insight into how to design new elements for spin-torque devices. In the next chapter, directed self-assembly of block copolymers is used to create coupled 1D chains of ferromagnetic and superparamagnetic FePt nanoparticles. These nano-patterned are globally aligned on the wafer length-scale using micron-sized lithographic grooves. This system is ideal for studying dipolar coupling between magnetic nanocrystals. Additionally, the processing methods developed here provide a platform for organizing other types of nanomaterials. The second sections explore magnetoelectric materials. These are materials that combine ferromagnetism and ferroelectricity in a coupled manner. One material that does this intrinsically is bismuth ferrite. The first chapter of this section explores ordered nanoporous bismuth ferrite produced by block copolymer templating. It is shown that the ordered porosity of the system creates a unique strain state in the bismuth ferrite, which in turn produces a large change in magnetization upon application of an electric field. Finally, in the last chapter, a nanostructured composite magnetoelectric system is studied. Here, magnetostrictive Ni nanocrystals are coupled to a single-crystalline piezoelectric substrate. The nanocrystals are superparamagnetic and show no net magnetization. Upon application of an electric field, however, strain induced in the piezoelectric substrate strains the lattice of the nanocrystals, creating a preferred magnetic axis along the high strained direction. This locks the magnetization along the strain axis and switches the nanocrystals from a superparamagnetic to a ferromagnetic state

Undoped and Ni-Doped CoO_<i>x</i> Surface Modification of Porous BiVO₄ Photoelectrodes for Water Oxidation

Surface modification of photoanodes with oxygen evolution reaction (OER) catalysts is an effective approach to enhance water oxidation kinetics, to reduce external bias, and to improve the energy harvesting efficiency of photoelectrochemical (PEC) water oxidation. Here, the surface of porous BiVO₄ photoanodes was modified by the deposition of undoped and Ni-doped CoO_<i>x</i> via nitrogen flow assisted electrostatic spray pyrolysis. This newly developed atmospheric pressure deposition technique allows for surface coverage throughout the porous structure with thickness and composition control. PEC testing of modified BiVO₄ photoanodes shows that after deposition of an undoped CoO_<i>x</i> surface layer, the onset potential shifts negatively by ca. 420 mV and the photocurrent density reaches 2.01 mA cm^–2 at 1.23 vs V_RHE under AM 1.5G illumination. Modification with Ni-doped CoO_<i>x</i> produces even more effective OER catalysis and yields a photocurrent density of 2.62 mA cm^–2 at 1.23 V_RHE under AM 1.5G illumination. The valence band X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption spectroscopy results show the Ni doping reduces the Fermi level of the CoO_<i>x</i> layer; the increased surface band bending produced by this effect is partially responsible for the superior PEC performance

Reversible multicolor chromism in layered formamidinium metal halide perovskites

Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties that allow chemical manipulation of the structure with low energy input. Here, the authors leverage the low formation energy of 2D metal halide perovskites to demonstrate films that reversibly switch between multiple colors using tunable quantum well thickness

Analytical Evaluation of Lead Iodide Precursor Impurities Affecting Halide Perovskite Device Performance

Mirroring established semiconductor technologies, halide perovskite materials synthesized from higher quality reagents display improved optoelectronic performance. In this study, we performed a semiquantitative analytical characterization of five different commercial lead iodide sources to determine the identity and concentration of impurities that affect perovskite devices. It was possible to single out acetate (OAc) and potassium (K) as key species in as-received materials, both plausibly remnant from synthesis or purification. We removed these impurities through aqueous recrystallization revealing contrasting impacts on device performance: removal of OAc was beneficial but reducing K could be detrimental. This observation indicates that the highest purity lead iodide does not guarantee the highest performing perovskite material, since certain extrinsic impurities, such as KI, can improve device performance. Fundamental and applied studies will both benefit from improved purification procedures coupled with analytical studies to better understand and control the effects of individual impurities in halide perovskite materials

General Method for the Synthesis of Hierarchical Nanocrystal-Based Mesoporous Materials

Block copolymer templating of inorg. materials is a robust method for the prodn. of nanoporous materials. The method is limited, however, by the fact that the mol. inorg. precursors commonly used generally form amorphous porous materials that often cannot be crystd. with retention of porosity. To overcome this issue, the authors present a general method for the prodn. of templated mesoporous materials from preformed nanocrystal building blocks. The work takes advantage of recent synthetic advances that allow org. ligands to be stripped off of the surface of nanocrystals to produce sol., charge-stabilized colloids. Nanocrystals then undergo evapn.-induced co-assembly with amphiphilic diblock copolymers to form a nanostructured inorg./org. composite. Thermal degrdn. of the polymer template results in nanocrystal-based mesoporous materials. This method can be applied to nanocrystals with a broad range of compns. and sizes, and the assembly of nanocrystals can be carried out using a broad family of polymer templates. The resultant materials show disordered but homogeneous mesoporosity that can be tuned through the choice of template. The materials also show significant microporosity, formed by the agglomerated nanocrystals, and this porosity can be tuned by the nanocrystal size. The authors demonstrate through careful selection of the synthetic components that specifically designed nanostructured materials can be constructed. Because of the combination of open and interconnected porosity, high surface area, and compositional tunability, these materials are likely to find uses in a broad range of applications. For example, enhanced charge storage kinetics in nanoporous Mn3O4 is demonstrated here