SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Crystal structure

179

Globally ordered colloidal crystal lattices have broad utility in a wide range of optical and catalytic devices, for example, as photonic bandgap materials. However, the self-assembly of stereospecific structures is often confounded by polymorphism. Small free energy differences often characterize ensembles of different structures, making it difficult to produce a single morphology at will. Current techniques to handle this problem adopt one of two approaches: that of the “top-down,” or “bottom-up” methodology, whereby structures are engineered starting from the largest or smallest relevant length scales, respectively. However, recently a third approach for directing high fidelity assembly of colloidal crystals has been suggested which relies on the introduction of polymer co-solutes into the crystal phase [N. A. Mahynski, A. Z. Panagiotopoulos, D. Meng, S. K. Kumar, Nat. Commun., 2014, 5, 4472]. By tuning the polymer’s morphology to interact uniquely with the void symmetry of a single desired crystal, the entropy loss associated with polymer confinement has been shown to strongly bias the formation of that phase. However, previously this approach has only been demonstrated in the limiting case of close-packed crystals. Here we show how this approach may be generalized and extended to complex open crystals, illustrating the utility of this “structure-directing agent” paradigm in engineering the nanoscale structure of ordered colloidal materials. The high degree of transferability of this paradigm’s basic principles between relatively simple crystals and more complex ones suggests this represents a valuable addition to presently known self-assembly techniques.

Concepts: Crystal, Crystal structure, Condensed matter physics, Sol-gel, Materials science, Colloidal crystal, Photonic crystal, Crystals

165

Organic-inorganic hybrid metal halide perovskites, an emerging class of solution processable photoactive materials, welcome a new member with a one-dimensional structure. Herein we report the synthesis, crystal structure and photophysical properties of one-dimensional organic lead bromide perovskites, C4N2H14PbBr4, in which the edge sharing octahedral lead bromide chains [PbBr4 (2-)]∞ are surrounded by the organic cations C4N2H14 (2+) to form the bulk assembly of core-shell quantum wires. This unique one-dimensional structure enables strong quantum confinement with the formation of self-trapped excited states that give efficient bluish white-light emissions with photoluminescence quantum efficiencies of approximately 20% for the bulk single crystals and 12% for the microscale crystals. This work verifies once again that one-dimensional systems are favourable for exciton self-trapping to produce highly efficient below-gap broadband luminescence, and opens up a new route towards superior light emitters based on bulk quantum materials.

Concepts: Crystal, Crystal structure, Quantum mechanics, Chemistry, Atom, Quantum dot, Solid, Materials science

147

Y-shaped ZnO nanobelts are fabricated by a simple thermal evaporation method. Transmission Electron Microscopy (TEM) investigation shows that these ZnO nanobelts are crystals with twinned planes {11-21}. Convergent Beam Electron Diffraction studies show that the two sides of twinned nanobelts are O-terminated towards the twinned boundary and Zn-terminated outwards. The two branches of twinned ZnO nanobelts grow along [11-26] from the trunk and then turn to the polarization direction [0001]. The featured Y-shape morphology and TEM characterizations indicate that the growth of these novel nanostructures is driven by an unusual twinned dislocation growth mechanism.

Concepts: Electron, Crystal structure, Fundamental physics concepts, Electromagnetic radiation, Transmission electron microscopy, Scanning electron microscope, Crystallographic defect, Electron diffraction

41

Controlling the size and shape of semiconducting nanocrystals advances nanoelectronics and photonics. Quantum-confined, inexpensive, solution-derived metal halide perovskites offer narrowband, color-pure emitters as integral parts of next-generation displays and optoelectronic devices. We use nanoporous silicon and alumina thin films as templates for the growth of perovskite nanocrystallites directly within device-relevant architectures without the use of colloidal stabilization. We find significantly blue-shifted photoluminescence emission by reducing the pore size; normally infrared-emitting materials become visibly red, and green-emitting materials become cyan and blue. Confining perovskite nanocrystals within porous oxide thin films drastically increases photoluminescence stability because the templates auspiciously serve as encapsulation. We quantify the template-induced size of the perovskite crystals in nanoporous silicon with microfocus high-energy x-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. Low-voltage electroluminescent diodes with narrow, blue-shifted emission fabricated from nanocrystalline perovskites grown in embedded nanoporous alumina thin films substantiate our general concept for next-generation photonic devices.

Concepts: Oxygen, Crystal structure, Optics, Aluminium, Semiconductor, Photonics, Optoelectronics, Nanocrystal

36

Although broadly admired for its aesthetic qualities, the art of origami is now being recognized also as a framework for mechanical metamaterial design. Working with the Miura-ori tessellation, we find that each unit cell of this crease pattern is mechanically bistable, and by switching between states, the compressive modulus of the overall structure can be rationally and reversibly tuned. By virtue of their interactions, these mechanically stable lattice defects also lead to emergent crystallographic structures such as vacancies, dislocations, and grain boundaries. Each of these structures comes from an arrangement of reversible folds, highlighting a connection between mechanical metamaterials and programmable matter. Given origami’s scale-free geometric character, this framework for metamaterial design can be directly transferred to milli-, micro-, and nanometer-size systems.

Concepts: Crystal structure, Crystallography, Symmetry, Materials science, Crystal system, Crystallographic defect, Origami, Crease pattern

30

Molecular solids and polymers can form low-symmetry crystal structures that exhibit anisotropic electron and ion mobility in engineered devices or biological systems. The distribution of molecular orientation and disorder then controls the macroscopic material response, yet it is difficult to image with conventional techniques on the nanoscale. We demonstrated a new form of optical nanocrystallography that combines scattering-type scanning near-field optical microscopy with both optical antenna and tip-selective infrared vibrational spectroscopy. From the symmetry-selective probing of molecular bond orientation with nanometer spatial resolution, we determined crystalline phases and orientation in aggregates and films of the organic electronic material perylenetetracarboxylic dianhydride. Mapping disorder within and between individual nanoscale domains, the correlative hybrid imaging of nanoscale heterogeneity provides insight into defect formation and propagation during growth in functional molecular solids.

Concepts: Electron, Crystal, Crystal structure, Chemistry, Atom, Chemical bond, Ion, Solid

29

Because of crystal symmetry, body centred cubic (BCC) metals have large differences in lattice friction between screw and edge dislocations, and manifest generally different mechanical behaviours from face centred cubic (FCC) metals. Although mechanical annealing (significant drop in stored dislocation density in response to applied stress) has been observed in FCC metals, it has not been observed in BCC metals so far. Here we show that significant mechanical annealing does occur in BCC Mo pillars, when their diameters decrease to hundreds of nanometers. In addition, there exists a critical diameter for focused ion beam milled pillars, below which the strengthening exponent increases dramatically, from ~0.3 to ~1. Thus, a new regime of size effects in BCC metals is discovered that converges to that of FCC metals, revealing deep connection in the dislocation dynamics of the two systems.

Concepts: Crystal structure, Crystallography, Metal, Crystal system, Dislocation, Atomic packing factor, Diamond cubic, Burgers vector

28

The surface energy is a fundamental property of the different facets of a crystal that is crucial to the understanding of various phenomena like surface segregation, roughening, catalytic activity, and the crystal’s equilibrium shape. Such surface phenomena are especially important at the nanoscale, where the large surface area to volume ratios lead to properties that are significantly different from the bulk. In this work, we present the largest database of calculated surface energies for elemental crystals to date. This database contains the surface energies of more than 100 polymorphs of about 70 elements, up to a maximum Miller index of two and three for non-cubic and cubic crystals, respectively. Well-known reconstruction schemes are also accounted for. The database is systematically improvable and has been rigorously validated against previous experimental and computational data where available. We will describe the methodology used in constructing the database, and how it can be accessed for further studies and design of materials.

Concepts: Metabolism, Crystal structure, Crystallography, Energy, Chemical reaction, Chemistry, Thermodynamics, Adsorption

28

The photocontrolled phase transitions and reflection behaviors of a smectic liquid crystal, 4-octyl-4'-cyanobiphenyl (8CB), tuned by a chiral azobenzene, are systematically investigated. For the smectic 8CB doped with the chiral azobenzene (1R)-(-)-4-n-hexyl-4'-menthylazobenzene (ABE), the initial smectic phase can be switched to cholesteric and then to isotropic upon UV irradiation due to the trans-to-cis photoisomerization of ABE; however, no reflection band is observed. For the smectic 8CB doped with ABE and the chiral agent (S)-(-)-1,1'-binaphthyl-2,2'-diol (BN), a reflection band located in the short-wavelength infrared region is observed, which disappears after further UV irradiation. For the smectic 8CB doped with ABE and a chiral agent with higher helical twisting power, (S)-2,2'-methylendioxy-1,1'-binaphthalene (DBN), a phototunable system with cholesteric pitch short enough to reflect visible light is demonstrated. With a given concentration of the chiral dopant DBN, a reversible reflection color transition is realized tuned by the isomerization of azobenzene. The reverse phase transition from isotropic to cholesteric and then to smectic can be recovered upon visible irradiation. The photocontrolled phase transitions in smectic liquid crystals and the corresponding changes in reflection band switched by photoisomerization of azobenzene may provide impetus for their practical application in optical memories, displays, and switches.

Concepts: Crystal, Crystal structure, Phase transition, Liquid crystal display, Liquid crystal, Cholesteric liquid crystal

28

Dislocation mobility is a fundamental material property that controls strength and ductility of crystals. An important measure of dislocation mobility is its Peierls stress, i.e., the minimal stress required to move a dislocation at zero temperature. Here we report that, in the body-centered cubic metal tantalum, the Peierls stress as a function of dislocation orientation exhibits fine structure with several singular orientations of high Peierls stress-stress spikes-surrounded by vicinal plateau regions. While the classical Peierls-Nabarro model captures the high Peierls stress of singular orientations, an extension that allows dislocations to bend is necessary to account for the plateau regions. Our results clarify the notion of dislocation kinks as meaningful only for orientations within the plateau regions vicinal to the Peierls stress spikes. These observations lead us to propose a Read-Shockley type classification of dislocation orientations into three distinct classes-special, vicinal, and general-with respect to their Peierls stress and motion mechanisms. We predict that dislocation loops expanding under stress at sufficiently low temperatures, should develop well defined facets corresponding to two special orientations of highest Peierls stress, the screw and the M111 orientations, both moving by kink mechanism. We propose that both the screw and the M111 dislocations are jointly responsible for the yield behavior of BCC metals at low temperatures.

Concepts: Crystal structure, Temperature, Aluminium, Metal, Cubic crystal system, Work hardening, Plasticity, Space group