30 research outputs found
Size-Dependent Lattice Structure and Confinement Properties in CsPbIā Perovskite Nanocrystals: Negative Surface Energy for Stabilization
CsPbIā 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 Ī±, tetragonal Ī², and orthorhombic Ī³), the samples are characterized by the Ī³ phase, rather than Ī±, 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 ā3.0 to ā5.1 eV nmā»Ā² for ligated CsPbIā 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 Ī±-phase values. This provides a robust route to understanding Ī³-phase properties of CsPbIā
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
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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
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
Undoped and Ni-Doped CoO<sub><i>x</i></sub> Surface Modification of Porous BiVO<sub>4</sub> 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 BiVO<sub>4</sub> photoanodes was modified by the
deposition of undoped and Ni-doped CoO<sub><i>x</i></sub> 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 BiVO<sub>4</sub> photoanodes shows
that after deposition of an undoped CoO<sub><i>x</i></sub> surface layer, the onset potential shifts negatively by ca. 420
mV and the photocurrent density reaches 2.01 mA cm<sup>ā2</sup> at 1.23 vs V<sub>RHE</sub> under AM 1.5G illumination. Modification
with Ni-doped CoO<sub><i>x</i></sub> produces even more
effective OER catalysis and yields a photocurrent density of 2.62
mA cm<sup>ā2</sup> at 1.23 V<sub>RHE</sub> 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<sub><i>x</i></sub> layer; the increased
surface band bending produced by this effect is partially responsible
for the superior PEC performance
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
Role of Cation Ordering on Device Performance in (Ag,Cu)InSe Solar Cells with KF Post-Deposition Treatment
CuInSe (CIS) has been proposed as an attractive bottom cell candidate in tandem solar cells. However, to justify the coupling with high-performance top cells (e.g., perovskites, GaAs), significant work on improving the efficiency is required. To this extent, several authors have demonstrated the benefits of alkali post-deposition treatments (PDT) to increase device open-circuit voltage (V) in CIS and how Ag alloyingā(Ag,Cu)InSe (ACIS)āreduces defect density and enhances current collection in devices. Herein, we present a detailed study of the role that KF-PDT plays on CIS and ACIS absorber composition and structure, and propose an explanation for the decreased V observed when silver and potassium coexist in the system (ACIS + KF). Through a suite of synchrotron-based techniques, we investigate the nanoscale chemical distribution of the films and the formation of secondary phases. Through photoluminescence imaging, we observed a high degree of passivation with the addition of KF, and synchrotron-based X-ray diffraction confirmed the absence of a KInSe surface layer usually considered to be a passivating agent. Raman spectroscopy and synchrotron X-ray fluorescence show the increased presence of Cu- and Se-poor clusters in ACIS + KF, which are correlated to significantly reduced X-ray beam-induced current (XBIC). An increase in the intensity of the E/B2 stretching mode of CIS is attributed to cation ordering near the junction and is found to track inversely to bulk V measurements. The cation ordering is hypothesized to arise from the formation and redistribution of defects that normally occur near the surfaces of CIS as a consequence of its polar character. These defects compensate each other, and the overall inhomogeneity of the charge distribution generates electrostatic potential fluctuations that greatly increase the saturation current and hence reduce the open-circuit voltage of the device
Magnetoelectric Control of Superparamagnetism
Here we demonstrate electric-field induced magnetic anisotropy
in a multiferroic composite containing nickel nanocrystals strain
coupled to a piezoelectric substrate. This system can be switched
between a superparamagnetic state and a single-domain ferromagnetic
state at room temperature. The nanocrystals show a shift in the blocking
temperature of 40 K upon electric poling. We believe this is the first
example of a system where an electric field can be used to switch
on and off a permanent magnetic moment
Formation of Nanoscale Composites of Compound Semiconductors Driven by Charge Transfer
Composites
are a class of materials that are formed by mixing two or more components.
These materials often have new functional properties compared to their
constituent materials. Traditionally composites are formed by self-assembly
due to structural dissimilarities or by engineering different layers
or structures in the material. Here we report the synthesis of a uniform
and stoichiometric composite of CdO and SnTe with a novel nanocomposite
structure stabilized by the dissimilarity of the electronic band structure
of the constituent materials. The composite has interesting electronic
properties which range from highly n-type in CdO to semi-insulating
in the intermediate composition range to highly p-type in SnTe. This
can be explained by the overlap of the conduction and valence band
of the constituent compounds. Ultimately, our work identifies a new
class of composite semiconductors in which nanoscale self-organization
is driven and stabilized by charge transfer between constituent materials