Lara A. Estroff

Ultrasmooth organic–inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells

To date, there have been a plethora of reports on different means to fabricate organic-inorganic metal halide perovskite thin films; however, the inorganic starting materials have been limited to halide-based anions. Here we study the role of the anions in the perovskite solution and their influence upon perovskite crystal growth, film formation and device performance. We find that by using a non-halide lead source (lead acetate) instead of lead chloride or iodide, the perovskite crystal growth is much faster, which allows us to obtain ultrasmooth and almost pinhole-free perovskite films by a simple one-step solution coating with only a few minutes annealing. This synthesis leads to improved device performance in planar heterojunction architectures and answers a critical question as to the role of the anion and excess organic component during crystallization. Our work paves the way to tune the crystal growth kinetics by simple chemistry.

Visualizing the 3D internal structure of calcite single crystals grown in agarose hydrogels.

Crystal Growing Kit For single crystals to remain intact, there is a limit to the size and number of defects that can be included before the underlying lattice is destroyed. Biological crystals, however, are known to include large macromolecules. H. Li et al. (p. 1244; see the Perspective by Hollingsworth) used electron tomography to study the crystallization of calcium carbonate inside an agarose gel, observing that the crystals physically entrapped the agarose macromolecules. To accommodate the curvature induced by the polymer chains, both high- and low-energy facets formed at the fiber-crystal interfaces. Thus, physical interactions alone may be sufficient for the incorporation of macromolecules in biological crystals and it may be possible to grow unusually shaped single crystals. Electron tomography shows that physical interactions may be sufficient to incorporate macromolecules into a calcite crystal. Single crystals are usually faceted solids with homogeneous chemical compositions. Biogenic and synthetic calcite single crystals, however, have been found to incorporate macromolecules, spurring investigations of how large molecules are distributed within the crystals without substantially disrupting the crystalline lattice. Here, electron tomography reveals how random, three-dimensional networks of agarose nanofibers are incorporated into single crystals of synthetic calcite by allowing both high- and low-energy fiber/crystal interface facets to satisfy network curvatures. These results suggest that physical entrapment of polymer aggregates is a viable mechanism by which macromolecules can become incorporated inside inorganic single crystals. As such, this work has implications for understanding the structure and formation of biominerals as well as toward the development of new high–surface area, single-crystal composite materials.

Thermally Induced Structural Evolution and Performance of Mesoporous Block Copolymer-Directed Alumina Perovskite Solar Cells

Structure control in solution-processed hybrid perovskites is crucial to design and fabricate highly efficient solar cells. Here, we utilize in situ grazing incidence wide-angle X-ray scattering and scanning electron microscopy to investigate the structural evolution and film morphologies of methylammonium lead tri-iodide/chloride (CH3NH3PbI3–xClx) in mesoporous block copolymer derived alumina superstructures during thermal annealing. We show the CH3NH3PbI3–xClx material evolution to be characterized by three distinct structures: a crystalline precursor structure not described previously, a 3D perovskite structure, and a mixture of compounds resulting from degradation. Finally, we demonstrate how understanding the processing parameters provides the foundation needed for optimal perovskite film morphology and coverage, leading to enhanced block copolymer-directed perovskite solar cell performance.

Crystallization Kinetics of Organic–Inorganic Trihalide Perovskites and the Role of the Lead Anion in Crystal Growth

Methylammonium lead halide perovskite solar cells continue to excite the research community due to their rapidly increasing performance which, in large part, is due to improvements in film morphology. The next step in this progression is control of the crystal morphology which requires a better fundamental understanding of the crystal growth. In this study we use in situ X-ray scattering data to study isothermal transformations of perovskite films derived from chloride, iodide, nitrate, and acetate lead salts. Using established models we determine the activation energy for crystallization and find that it changes as a function of the lead salt. Further analysis enabled determination of the precursor composition and showed that the primary step in perovskite formation is removal of excess organic salt from the precursor. This understanding suggests that careful choice of the lead salt will aid in controlling crystal growth, leading to superior films and better performing solar cells.

Hierarchical Porous Polymer Scaffolds from Block Copolymers

A Complicated Scaffold, Simply Materials with tailored pore structures can be useful as catalysis supports and for lightweight materials. When preparing medical scaffolds, restrictive preparation conditions have to be met, which can prohibit multistep preparation procedures. Sai et al. (p. 530) describe a method for making porous polymers containing both relatively large (several microns) interconnecting pores and a second population of ∼ tens of nanometer pores. The process exploits spinodal decomposition of a block copolymer blended with small-molecule additives and requires a simple washing step with water, methanol, or ethanol. Spinodal decomposition of block copolymers and oligomeric additives produces three-dimensional hierarchical porous polymers. Hierarchical porous polymer materials are of increasing importance because of their potential application in catalysis, separation technology, or bioengineering. Examples for their synthesis exist, but there is a need for a facile yet versatile conceptual approach to such hierarchical scaffolds and quantitative characterization of their nonperiodic pore systems. Here, we introduce a synthesis method combining well-established concepts of macroscale spinodal decomposition and nanoscale block copolymer self-assembly with porosity formation on both length scales via rinsing with protic solvents. We used scanning electron microscopy, small-angle x-ray scattering, transmission electron tomography, and nanoscale x-ray computed tomography for quantitative pore-structure characterization. The method was demonstrated for AB- and ABC-type block copolymers, and resulting materials were used as scaffolds for calcite crystal growth.

Synthetic α-Helix Mimetics as Agonists and Antagonists of Islet Amyloid Polypeptide Aggregation

The development of small molecules that can modulate the damaging effects of protein aggregation processes remains a high priority goal in contemporary medicinal chemistry.[1] An important class of these aggregates, called amyloids, has been implicated in numerous degenerative diseases including Alzheimer’s, type II diabetes, senile systemic amyloidosis (SSA), prion diseases and rheumatoid arthritis. The attribute shared by these symptomatically unrelated diseases is that a normally soluble protein undergoes a conformational change resulting in self-assembly into cytotoxic forms culminating in a β-sheet rich fibrillar structure. Islet amyloid polypeptide (IAPP), or amylin, is one such protein that has been implicated in amyloidogenesis in type II diabetes.[2] IAPP is cosecreted with insulin by the β-cells of the islets of Langerhans and an aggregated form of IAPP is believed to play a role in β-cell toxicity in the pathology of type II diabetes.[3]

Hydroxyapatite nanoparticle-containing scaffolds for the study of breast cancer bone metastasis

Breast cancer frequently metastasizes to bone, where it leads to secondary tumor growth, osteolytic bone degradation, and poor clinical prognosis. Hydroxyapatite Ca(10)(PO(4))(6)(OH)(2) (HA), a mineral closely related to the inorganic component of bone, may be implicated in these processes. However, it is currently unclear how the nanoscale materials properties of bone mineral, such as particle size and crystallinity, which change as a result of osteolytic bone remodeling, affect metastatic breast cancer. We have developed a two-step hydrothermal synthesis method to obtain HA nanoparticles with narrow size distributions and varying crystallinity. These nanoparticles were incorporated into gas-foamed/particulate leached poly(lactide-co-glycolide) scaffolds, which were seeded with metastatic breast cancer cells to create mineral-containing scaffolds for the study of breast cancer bone metastasis. Our results suggest that smaller, poorly-crystalline HA nanoparticles promote greater adsorption of adhesive serum proteins and enhance breast tumor cell adhesion and growth relative to larger, more crystalline nanoparticles. Conversely, the larger, more crystalline HA nanoparticles stimulate enhanced expression of the osteolytic factor interleukin-8 (IL-8). Our data suggest an important role for nanoscale HA properties in the vicious cycle of bone metastasis and indicate that mineral-containing tumor models may be excellent tools to study cancer biology and to define design parameters for non-tumorigenic mineral-containing or mineralized matrices for bone regeneration.

An organic hydrogel as a matrix for the growth of calcite crystals

The growth of calcite in an aqueous gel of was studied and the appearance of the crystals was found to change over time. Crystals removed from the gel at progressively longer times showed severely affected surfaces resulting from dissolution. If crystals were removed from the gel after 3.5 hours, at which point there were no etch pits, and then placed in either buffer or pure water, etch pits, similar to those observed on crystals that are left in the gel, were observed. Control calcite crystals exposed to similar conditions (water or buffer) show no significant dissolution after equivalent times. A probable cause of the altered dissolution is the non-specific occlusion of gelator aggregates at sites of imperfection. The gel appears to provide a microenvironment in which the molecules that form the matrix also participate in the crystallization. This system allows the study of the unique properties of a gel for influencing the nucleation and growth of inorganic crystals, some of which may be important for better understanding biomineralization.

Tuning hardness in calcite by incorporation of amino acids

Structural biominerals are inorganic/organic composites that exhibit remarkable mechanical properties. However, the structure-property relationships of even the simplest building unit-mineral single crystals containing embedded macromolecules-remain poorly understood. Here, by means of a model biomineral made from calcite single crystals containing glycine (0-7 mol%) or aspartic acid (0-4 mol%), we elucidate the origin of the superior hardness of biogenic calcite. We analysed lattice distortions in these model crystals by using X-ray diffraction and molecular dynamics simulations, and by means of solid-state nuclear magnetic resonance show that the amino acids are incorporated as individual molecules. We also demonstrate that nanoindentation hardness increased with amino acid content, reaching values equivalent to their biogenic counterparts. A dislocation pinning model reveals that the enhanced hardness is determined by the force required to cut covalent bonds in the molecules.

Porous calcite single crystals grown from a hydrogel medium

This paper describes the internal structure of calcite (CaCO3) crystals grown in an agarose hydrogel and demonstrates that the gel-grown calcite crystals, like biogenic calcite crystals, incorporate the organic matrix, resulting in internal structures with pores on the order of 100s of nanometers.