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Boaz Pokroy

Publications and source records attributed to Boaz Pokroy.

32 records · Page 2Linked to original sources
arXiv2019

Strong Quantum Confinement Effects and Chiral Excitons in Bio-Inspired ZnO-Amino Acid Co-Crystals

Elucidating the underlying principles behind band gap engineering is paramount for the successful implementation of semiconductors in photonic and optoelectronic devices. Recently it has been shown that the band gap of a wide and direct band gap semiconductor, such as ZnO, can be modified upon co-crystallization with amino acids, with the role of the biomolecules remaining unclear. Here, by probing and modeling the light emitting properties of ZnO-amino acid co-crystals, we identify the amino acids role on this band gap modulation and demonstrate their effective chirality transfer to the inter-band excitations in ZnO. Our 3D quantum model suggests that the strong band edge emission blue shift in the co-crystals can be explained by a quasi-periodic distribution of amino acid potential barriers within the ZnO crystal lattice. Overall, our findings indicate that biomolecule co-crystallization can be used as a truly bio-inspired means to induce chiral quantum confinement effects in quasi-bulk semiconductors.

cond-mat.mtrl-sci↗
arXiv2019

Morphology preserving solid-to-solid transformation of minerals mediated by a self-assembled temperature responsive polymer membrane: calcite to hydroxyapatite

Assemblies of nanocrystals with complex morphologies have solicited great scientific interest. Controlling the formation of these self-assembled structures has proved difficult. Here we present a novel transformation reaction for transforming a single crystal of minerals into assemblies of nanocrystals of substances with lower solubility without changing the overall morphology. This study focusses on the transformation of rhombohedral calcite crystals to assemblies of hydroxyapatite nanocrystals to control the transformation process. This yielded particle assemblies of HAP nanoparticles grossly preserving the original rhombohedral calcite morphology.

physics.chem-ph↗
arXiv2019

Synthesis of calcium carbonate in trace water environments

Calcium carbonate (CaCO3) was synthesized from diverse water-free alcohol solutions, resulting in the formation of vaterite and calcite precipitates, or stable particle supensions, with dimension and morphology depending upon the condition used. The obtained results shed light on the importance of water molecules during crystallization of CaCO3 and open a novel synthetic route for its precipitation in organic solvents.

physics.chem-ph↗
arXiv2019

Photocatalytic activity of exfoliated graphite-TiO$_2$ nanocomposites

We investigate the photocatalytic performance of nanocomposites prepared in a one-step process by liquid-phase exfoliation of graphite in the presence of TiO$_2$ nanoparticles (NPs) at atmospheric pressure and in water, without heating or adding any surfactant, and starting from low-cost commercial reagents. The nanocomposites show enhanced photocatalytic activity, degrading up to 40$\%$ more pollutants with respect to the starting TiO$_2$-NPs. In order to understand the photo-physical mechanisms underlying this enhancement, we investigate the photo-generation of reactive species (trapped holes and electrons) by ultrafast transient absorption spectroscopy. We observe an electron transfer process from TiO$_2$ to the graphite flakes within the first picoseconds of the relaxation dynamics, which causes the decrease of the charge recombination rate, and increases the efficiency of the reactive species photo-production.

cond-mat.mes-hall↗
arXiv2018

Thickness Dependence of the Physical Properties of Atomic-Layer Deposited Al2O3

Inspired by nature, we investigate the short-range order effect on the physical properties of amorphous materials. Amorphous Al2O3 thin films exhibit a higher proportion of their 4-coordinated Al sites close to the surface, causing variations in the average short-range order of the film. Below some thickness, the density of these films changes with size. In this work, we address the short-range order effect, through the thickness, on the electronic and optical properties of atomic layer deposited (ALD) Al2O3 thin films. Both the refractive index and the permittivity were found to vary with size. The refractive index increased with thickness, and for thick films (~50 nm) it was comparable to that of bulk amorphous Al2O3. The permittivity increased with thickness as well, but did not attain those of the bulk material. We discuss how these effects correlate with the density and short-range order. These results shed light on the size effects in thin amorphous oxides, and may guide the design of electronic and optical components and devices.

cond-mat.mtrl-sci↗
arXiv2016

Hybrid gold single crystals incorporating amino acids

Composite hybrid gold crystals are of profound interest in various research areas ranging from materials science to biology. Their importance is due to their unique properties and potential implementation, for example in sensing or in bio-nanomedicine. Here we report on the formation of hybrid organic-metal composites via the incorporation of selected amino acids histidine, aspartic acid, serine, glutamine, alanine, cysteine, and selenocystine into the crystal lattice of single crystals of gold. We used electron microscopy, chemical analysis and high-resolution synchrotron powder X ray diffraction to examine these composites. Crystal shape, as well as atomic concentrations of occluded amino acids and their impact on the crystal structure of gold, were determined. Concentration of the incorporated amino acid was highest for cysteine, followed by serine and aspartic acid. Our results indicate that the incorporation process probably occurs through a complex interaction of their individual functional groups with gold atoms. Although various organic gold composites have been prepared, to the best of our knowledge this is the first reported finding of incorporation of organic molecules within the gold lattice. We present a versatile strategy for fabricating crystalline nanohybrid composite gold crystals of potential importance for a wide range of applications.

physics.chem-ph↗
arXiv2016

Narrowly distributed crystal orientation in biomineral vaterite

Biominerals formed by animals provide skeletal support, and many other functions. They were previously shown to grow by aggregation of amorphous nanoparticles, but never to grow ion-by-ion from solution, which is a common growth mechanism for abiotic crystals. We analyze vaterite CaCO3 multi crystalline spicules from the solitary tunicate Herdmania momus, with Polarization dependent Imaging Contrast PIC mapping, scanning and aberration corrected transmission electron microscopies. The first fully quantitative PIC mapping data, presented here, measured 0° 30° angle spreads between immediately adjacent crystals. Such narrowly distributed crystal orientations demonstrate that crystallinity does not propagate from one crystal to another 0° angle spreads, nor that new crystals with random orientation 90° nucleate. There are no organic layers at the interface between crystals, hence a new, unknown growth mechanism must be invoked, with crystal nucleation constrained within 30°. Two observations are consistent with crystal growth from solution: vaterite microcrystals express crystal faces, and are smooth at the nanoscale after cryo fracture. The observation of 30° angle spreads, lack of interfacial organic layers, and smooth fracture figures broadens the range of known biomineralization mechanisms and may inspire novel synthetic crystal growth strategies. Spherulitic growth from solution is one possible mechanism consistent with all these observations.

cond-mat.mtrl-sci↗
arXiv2016

Sponge like nanoporous single crystals of gold

Single crystals in nature often demonstrate fascinating intricate porous morphologies rather than classical faceted surfaces. We attempt to grow such crystals, drawing inspiration from biogenic porous single crystals. Here we show that nanoporous single crystals of gold can be grown with no need for any elaborate fabrication steps. These crystals are found to grow following solidification of a eutectic composition melt that forms as a result of the dewetting of nanometric thin films. We also present a kinetic model that shows how this nano-porous single-crystalline structure can be obtained, and which allows the potential size of the porous single crystal to be predicted. Retaining their single crystalline nature is due to the fact that the full crystallization process is faster than the average period between two subsequent nucleation events. Our findings clearly demonstrate that it is possible to form singe crystalline nano porous metal crystals in a controlled manner. INTRODUCTION

cond-mat.mtrl-sci↗
arXiv2016

Formation of Curved Micron-Sized Single Crystals

Crystals in nature often demonstrate curved morphologies rather than classical faceted surfaces. Inspired by biogenic curved single crystals, we demonstrate that gold single crystals exhibiting curved surfaces can be grown with no need of any fabrication steps. These single crystals grow from the confined volume of a droplet of a eutectic composition melt which forms via the dewetting of nanometric thin films. We can control their curvature by controlling the environment in which the process is carried out, including several parameters, such as the contact angle and the curvature of the drops by changing the surface tension of the liquid drop during crystal growth. Here we present an energetic model that explains this phenomenon and predicts why and under what conditions crystals will be forced to grow with the curvature of the micro droplet even though the energetic state of a curved single crystal is very high.

cond-mat.mtrl-sci↗
arXiv2016

Calcite single crystals as hosts for atomic-scale entrapment and slow release of drugs

This study presents a complete structural and biological characterization of Doxorubicin CaCO3 single crystals as a pH responsive drug carrier. Using a biomimetic approach, it was demonstrated that calcite single crystals are able, during their growth in presence of doxorubicin, to entrap drug molecules inside their lattice, along specific crystallographic directions. High resolution synchrotron powder diffraction measurements allowed the determination of the lattice distortion and microstructural parameters. Confocal microscopy confirmed that doxorubicin is uniformly embedded in the crystal and that the drug is not only adsorbed on the crystal surface. A slow release of DOX is obtained that occurs preferentially in proximity of the crystals, targeting cancer cells.

cond-mat.mtrl-sci↗
arXiv2016

Intracrystalline inclusions within single crystalline hosts: from biomineralization to bio-inspired crystal growth

Many crystals in nature exhibit fascinating mechanical, optical, magnetic and other characteristics. One of the reasons for this phenomenon has to do with the presence of specific organic molecules that are tightly associated with the mineral. Over the years, some organic crystals have been found to be located within the lattices of their singlecrystalline biogenic hosts. A number of questions remain unanswered: for example, how do these molecules become incorporated and what is their function? In this review we survey the gradual refinement of the above mentioned finding in biogenic crystals, with the object of tracing the acquisition of our fundamental knowledge in this field during the last 50 years. We highlight the progress made in understanding the function and significance of this intracrystalline organic matter, from the earliest observations of this phenomenon in a biological system to the highly promising recent achievements in bio-inspired material synthesis, where intracrystalline molecules have been used in many studies to synthesize numerous synthetic nanohybrid composites with fascinating new properties.

cond-mat.mtrl-sci↗
arXiv2016

Structural Analysis of Metal doped Calcium Oxalate

Calcium oxalate crystals are the most common biominerals found in plants. They also make their presence known as painful kidney stones in humans and animals. Their function in plants is extraordinarily versatile and encompasses calcium storage and defense mechanisms against herbivores and detoxification processes. Since plants containing calcium oxalate crystals are often exposed to metallic environments, we studied the interaction of such environmental metals with calcium oxalate in vitro. We showed that selected metals are indeed incorporated into synthetic calcium oxalate, and in a manner that depends on their ionic radius when precipitated in solution. One such mechanism of incorporation is based on the replacement of calcium ions by other metal cations within the host unit cell. The unit cell of calcium oxalate expands when incorporating elements with larger atomic radii and shrinks when doped with elements possessing ionic radii smaller than that of calcium. In this systematic study, metal-doped calcium oxalate crystals were characterized by means of high-resolution synchrotron X-ray powder diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and transmission electron microscopy. Better understanding of metal incorporation into mineral hosts might lead to ways of developing new and more efficient sorbent materials for detoxification processes.

physics.med-ph↗
arXiv2014

Size Effect on the Short Range Order in Amorphous Materials

Drawing inspiration from nature, where some organisms can control the short range order of amorphous minerals, we successfully manipulated the short range order of amorphous alumina by surface and size effects. By utilizing the Atomic Layer Deposition (ALD) method to grow amorphous nanometrically thin films, combined with state-of-the-art electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we showed experimentally that the short range order in such films is strongly influenced by size. This phenomenon is equivalent to the well-known size effect on lattice parameters and on the relative stability of different polymorphs in crystalline materials. We also show that the short range order changes while still in the amorphous phase, before the amorphous to crystalline transformation takes place.

cond-mat.mtrl-sci↗