Defect States In Crystals Research Articles

Amine‐Free Multi‐Faceted CsPbBr3 Nanocrystals for Complete Suppression of Long‐Lived Dark States

AbstractConventional amine‐capped perovskite nanocrystals (A‐PNCs) often show microscopic fluctuation (blinking) of photoluminescence (PL) between bright and near‐background (dark) states. The transient dark phases spanning milliseconds (ms) to several seconds timescale typically owe to the non‐radiative (NR) fate of charge carriers in crystal defect states, limiting the PNC's efficiency in optoelectronics. A PL study of a novel trioctylphosphine (TOP) capped rhombic cuboctahedron‐shaped amine‐free (AF) PNC is presented, demonstrating their fascinating nonblinking nature. While oleylamine‐capped hexahedron A‐PNCs as a reference system show long dark states, their population is completely suppressed in ≈90% of AF‐PNCs with random large‐amplitude PL fluctuations several times above the background. The PL trajectories of AF‐PNCs primarily comprise constant power spectral densities (PSDs) which are unprecedented and deviate from power‐law distribution as observed for A‐PNCs and most of the other blinking nanocrystals. Such signature of PSDs and frame‐by‐frame intensity jump analysis indicate a fast (<<ms) timescale charge‐carrier dynamics governing the PL emission of AF‐PNCs, likely involving short‐lived metastable states. Besides, long‐term stability and suppressed dark phases in ≈70% AF‐PNCs even 90 days after the synthesis is noteworthy. Altogether the study provides exceptional optical characteristics of photostable amine‐free PNCs which would be attractive to exploit in commercial display applications.

Oriented growth of copper & nickel-impregnated δ -MnO2 nanofilaments anchored onto sulfur-doped biochar template as hybrid adsorbents for removing phenolic compounds by adsorption-oxidation process

In the present study, the oxidative absorption was introduced as a new technique in removing hazardous organic pollutants from water and wastewater. Hence, at first manganese dioxide nanodisks were synthesized on sulfur enriched biochar through the simple oxidation–reduction process to play the role of an efficient adsorbent. In the following, the incorporation of copper and nickel in the MnO2 structure generates strain within that structure. This leads to creating crystal defect states as well as the change of phase from γ -MnO2 to δ-MnO2 and also morphology to nanofilaments which increasing specific surface area. Scanning electron microscopy technologies and X-ray diffraction patterns were used to study the surface morphology, and Brunauer–Emmett–Teller analysis was used to determine the specific surface area of the adsorbents. Then, the efficiency of the prepared nanofilaments for removing phenolic compounds from aqueous solutions in a batch and equilibrium system was investigated. Adsorption isotherms (Langmuir, Freundlich, and Temkin) and kinetic models (pseudo-first-order, pseudo-second-order, and Elovich) were used to explain the adsorption process. Results showed the maximum sorption capacitiy at the equilibrium time 18 h with adsorbent dosage of 1.0 mg/mL and pH=4 was obtained. Langmuir isotherm and the pseudo-second-order kinetic model have the best fitness on experimental data. The thermodynamic study of the adsorption of phenolic compounds by nanofilaments shows that this endothermic process is associated with spontaneous and irregular increases. This research shows that the designed adsorbents may be the effective, low-cost, and environmentally-friendly adsorbents to remediate phenol contaminations in the wastewater.

Low Trap Density Para-F Substituted 2D PEA2PbX4 (X = Cl, Br, I) Single Crystals with Tunable Optoelectrical Properties and High Sensitive X-Ray Detector Performance.

Exploring halogen engineering is of great significance for reducing the density of defect states in crystals of organic-inorganic hybrid perovskites and hence improving the crystal quality. Herein, high-quality single crystals of PEA2PbX4 (X = Cl, Br, I) and their para-F (p-F) substitution analogs are prepared using the facile solution method to study the effects of both p-F substitution and halogen anion engineering. After p-F substitution, the triclinic PEA2PbX4 (X = Cl, Br) and cubic PEA2PbX4 (X = I) crystals unifies to monoclinic crystal structure for p-F-PEA2PbX4 (X = Cl, Br, I) crystals. The p-F substitution and halogen engineering, together with crystal structure variation, enable the tunability of optoelectrical properties. Experimentally, after the p-F substitution, the energy levels are lowered with increased Fermi levels, and the bandgaps of p-F-PEA2PbX4 (X = Cl, Br, I) are slightly reduced. Benefitting from the enhancement of the charge transfer and the reduced trap density by p-F substitution and halogen anion engineering, the average carrier lifetime of the p-F-PEA2PbX4 is obviously reduced. Compared with PEA2PbI4, the X-ray detector based on p-F-PEA2PbI4 perovskite single-crystal has a higher sensitivity of 119.79 μC Gyair -1·cm-2. Moreover, the X-ray detector based on p-F-PEA2PbI4 single crystals exhibits higher radiation stability under high-dose X-ray irradiation, implying long-term operando stability.

Effect of deep cryogenic treatment on the microstructure and corrosion behavior of the microarc oxidized Mg-2.0Zn-0.5Ca alloy

Biodegradable microarc oxidation (MAO) coatings were prepared on the Mg-2.0Zn-0.5Ca alloys treated with and without deep cryogenic treatment (DCT). Scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to characterize the coating morphology and phase composition. The electrochemical corrosion behavior was investigated using the electrochemical workstation. The effects of DCT on the formation mechanism and corrosion behavior of the MAO coatings were investigated. The results indicate that the DCT has refining effect on the Mg-2.0Zn-0.5Ca alloy. The mechanisms of the coating formation and growth can be influenced by the introduction of the DCT through influencing the grain size, phase precipitation and crystal defect states of the substrate. The changing of the microstructure of the alloy can alter the spark discharge characteristics of the MAO reaction. The DCT has an effect on the controllability of the corrosion rate of the MAO coated samples.

Effective growth and characterization of piperazinium orthophthalate single crystal yielding high second harmonic generation efficiency

Good quality organic non-linear optical single crystal of piperazinium orthophthalate (PPA) was grown successfully by solution growth at room temperature by achieving the supersaturation state in slow evaporation method. The powder XRD analysis confirms the crystalline nature and structural parameters of the grown PPA crystal. The FTIR and Raman spectra confirm the existence of acid-base functional groups along with their vibrational mode exhibited, present in the grown PPA crystal. UV–Vis spectrum exhibits better transparency in the visible region with cut-off wavelength of 330 nm and the energy band gap (Eg) is analyzed to be 3.8 eV. Photoluminescence spectra shows good optical property with the excitation of 342 nm and two emission peaks in near edge UV band with blue emission at 364 and the energy transition in deep level defect states in crystals by exhibiting the green emission at 533 nm respectively. The thermal stability and the thermal absorption of the PPA crystal were identified by the TG - DTA analysis and obtained thermal stability was up to 88 °C. Vickers micro hardness test illustrates the mechanical stability of the grown PPA crystal and it belong to soft category (n = 4.6). Etching studies illustrates the growth pattern of the grown crystal using water as etchant. FE-SEM analysis shows the perfect smooth morphology of the grown PPA crystal. Second harmonic generation study illustrates the grown PPA crystal (λ = 532 nm) exhibit good nonlinear optical response.

Topological defect lasers

We introduce a topological defect to a regular photonic crystal defect cavity with anisotropic unit cell. Spatially localized resonances are formed and have high quality factor. Unlike the regular photonic crystal defect states, the localized resonances in the topological defect structures support powerflow vortices. Experimentally we realize lasing in the topological defect cavities with optical pumping. This work shows that the spatially inhomogeneous variation of the unit cell orientation adds another degree of freedom to the control of lasing modes, enabling the manipulation of the field pattern and energy flow landscape.

Materials issues for quantum computation

Abstract

Influence of defect states on band gaps of the 4340 steel in epoxy in two-dimensional phononic crystal

The band gaps of a two-dimensional phononic crystal are studied by using the plane-wave expansion method and the supercell calculations. The two-dimensional phononic crystal is formed by square-shape arrangement of 4340 steel inserted into epoxy resin. The band gaps of different structures are calculated such as defect-free, 45 crystal defect states, 90 crystal defect states, 135 crystal defect states and 180 crystal defect states. It is found that the bandwidth of defect state is about 31 times larger than that of the defect-free crystal; with F=0.10.9 the band gaps occur in different defect structures and increase with defect state angle increasing. In addition, the influences of filling fraction on the band gap number and the relative width of the minimum band gap are also discussed.

GWApproximation of the Many-Body Problem and Changes in the Particle Number

A stringent test for an exchange-correlation approximation in electronic structure calculations is the equality between the ionization energy of the neutral system and the affinity of the singly positively charged system. All of the commonly used approximations (local, semilocal, hybrid) for the exchange correlation within density functional theory fail badly with this test. They consequently present a localization or delocalization error, resulting in a highest occupied molecular orbital or lowest unoccupied molecular orbital gap over- or underestimation. The GW approximation appears as the best available framework to describe particle number changes. The small remaining error can be further reduced by devising a DeltaSCF-like method within the GW approximation. The proposed approach is necessary as soon as localized states are involved, e.g., in finite systems or defect states in crystals.

Polymer stabilisation of twisted smectic liquid crystal defect states

Polymer stabilisation of a state similar to a twist grain boundary phase is demonstrated. Layers of micrometre thick polymer networks are formed which exhibit discretely varying orientational order. The polymer network shows a high degree of orientational order with an order parameter of approximately S ≈ 0.8, which is equal for all layers formed, and equivalent to that of a polymer network formed in the smectic A* phase. The mechanism of photo-polymerisation in liquid crystalline defect phases is discussed and may offer potential for the synthesis of photonic structures in the visible and IR wavelength range.