Crystal Growth Direction Research Articles

Size-Controlled ZnO Nanoparticles Synthesized with Thioacetamide and Formation of ZnS Quantum Dots

In this work, we report the first attempt to investigate the dependence of thioacetamide (TAA) on the size of ZnO nanoparticles (NPs) in forming ZnS nanostructures from ZnO. Size-controlled B(blue)_, G(green)_, and Y(yellow)_ZnO quantum dots (QDs) and NC (nanocrystalline)_ZnO NPs were synthesized using a sol–gel process and a hydrothermal method, respectively, and then reacted with an ethanolic TAA solution as a sulfur source. ZnO QDs/NPs began to decompose into ZnS QDs through a reaction with TAA for 5~10 min, so rather than forming a composite of ZnO/ZnS, ZnO QDs and ZnS QDs were separated and remained in a mixed state. At last, ZnO QDs/NPs were completely decomposed into ZnS QDs after a reaction with TAA for 1 h irrespective of the size of ZnO QDs up to ~50 nm. All results indicate that ZnS formation is due to direct crystal growth and/or the chemical conversion of ZnO to ZnS.

Fused-Ring Electron-Acceptor Single Crystals with Chiral 2D Supramolecular Organization for Anisotropic Chiral Optoelectronic Devices.

Supramolecular chiral organization gives π-conjugated molecules access to fascinating specific interactions with circularly polarized light (CPL). Such a feature enables the fabrication of high-performance chiral organic electronic devices that detect or emit CPL directly. Herein, it is shown that chiral fused-ring electron-acceptor BTP-4F single-crystal-based phototransistors demonstrate distinguished CPL discrimination capability with current dissymmetry factor exceeding 1.4, one of the highest values among state-of-the-art direct CPL detectors. Theoretical calculations prove that the chirality at the supramolecular level in these enantiomeric single crystals originates from chiral exciton coupling of a unique quasi-2Dsupramolecular organization consisting of interlaced molecules with opposite helical conformation. Impressively, such supramolecular organization produces a higher dissymmetry factor along the preferred growth direction of the chiral single crystals in comparison to that of the short axis direction. Furthermore, the amplified, inverted, and also anisotropic current dissymmetry compared to optical dissymmetry is studied by finite element simulations. Therefore, a unique chiral supramolecular organization that is responsible for the excellent chiroptical response and anisotropic electronic properties is developed, which not only enables the construction of high-performance CPL detection devices but also allows a better understanding of the structure-property relationships in chiral organic optoelectronics.

Fe doping induced selenium vacancy on cobalt selenide for enhanced hydrogen peroxides production

Electrocatalytic two-electron oxygen reduction reaction (2e-ORR) pathway for hydrogen peroxides (H2O2) generation has emerges as an appealing alternative to conventional anthraquinone craft. In this work, Fe doped CoSe was synthesized via hydrothermal and subsequent high temperature (HT) annealing process. The doping of trace amount of Fe could tune the crystal growth direction and induce the generation of Se vacancy during annealing. During the Electrocatalytic measurement, the Faraday efficiency of 2e-ORR can reach 99.1%, 83.2% and 93.6% in 0.1 M KOH, 0.1 M Na2SO4, and 0.05 M H2SO4, respectively. Under the neutral condition, the production rate of H2O2 reached 18.37 mmol L−1 in 120 min. Moreover, Fe-CoSe-HT exhibits good stability and Fe2+ regeneration during flow-Fenton reaction. Hence, the dual role of selenium-vacancy-enriched Fe-CoSe-HT greatly enhanced selectivity of 2e-ORR and accelerated Fe reduction, resulting in promoted ∙OH radical generation and organic pollutants treatment.

Application of Magnetic Field to Accelerate the Crystallization of Scopolamine Hydrobromide

In this paper, a rapid and efficient method for the preparation of scopolamine hydrobromide with high purity was established, named as magnetic field-induced crystallization. Based on the difference in solubility between scopolamine and scopolamine hydrobromide, salifying crystallization was selected and then treated with the synergistic effect of magnetic field to achieve the goal of purifying scopolamine. The influence of crystallization solvents and magnetic field intensity on the crystallization process of scopolamine hydrobromide, as well as the impact of magnetic field on the crystal growth direction, were investigated. The results revealed that treatment under magnetic field led to a shortened induction time (25.64–75.46%), an increased purity of crystals (0.95–2.92%), and an enhanced recovery rate (4.51–10.78%). Furthermore, it was also discovered that magnetic field could destroy hydrogen bonds within the solution, and change the physical properties of the mother liquid, so as to promote the nucleation formation and crystal growth. These findings suggested that external magnetic field could be a promising method for scopolamine preparation.

Microstructural evolution and failure mechanism dominated by Bi, Sb cooperative diffusion at p-type thermoelectric pillar Bi-Sb-Te/Sn interface

Bismuth telluride-based thermoelectric devices have attracted much attention in the field of low-quality waste heat recovery below 200 °C. However, the rapid diffusion at the interface between solder and thermoelectric materials was serious after long-term service. The brittle Sn-Te was believed to cause the electrode failure. In this work, it suggested that the regional eutectic melting at the β-Sn/SbSn interface at hot-end electrode was a more important risk to cause the failure, requiring urgent attention. Co-diffusion of Bi and Sb led to partial melting of the Sn-based solder, which did not wet on the SbSn surface and formed solidification cracks. Besides, the distributions and migrations of Bi, Sb, Te, and Sn elements at the interface were investigated, the crystal growth direction of SnTe, SbSn, and nano-Bi phases were analyzed, and the phase composition and phase evolution in complex intermetallic compounds (IMCs) at the Bi-Sb-Te/Sn interface were clarified, combined with SEM, EPMA, and TEM characterization during the aging at 150 °C for 150 h. Based on this failure mechanism, it suggested that the service temperature should be lower than the Sn-Bi eutectic temperature and as low as possible, in the case of without any coating treatment for the consideration of cost.

Niobium monoxide NbO single crystal growing by floating zone method

In this work, a single crystal of niobium monoxide NbO was grown by crucible-free zone melting method. Polycrystalline NbO with ordered cubic structure (sp.gr. Pm-3m) with vacancies in both metal and nonmetal sublattices served as an initial material. According to XRD data, the grown niobium monoxide NbO single crystal has predominant growth in the [111]B1 direction and contains only one ordered cubic phase NbO, its lattice constant is equal to a = 421.2 pm. The growing mechanism of the NbO single crystal was clarified, in accordance with the Kossel and Stransky theory, it was suggested that the single crystal grows layer by layer. Analysis of the Kikuchi lines geometry allowed the orientations and disorientations of single crystal to be determined to high precision. An analysis of the diffraction pattern and intersection of Kikuchi lines showed that the NbO single crystal has (111)B1 facet. The experimental Laue diffraction patterns confirmed this direction of niobium monoxide NbO single crystal growth.

Giant enhancement of magnetostriction in Pt doped FeGa ribbons

FeGa alloys are among the most promising magnetostrictive materials. How to improve the magnetostriction of FeGa has been a long-term concern. In this work, trace amount of Pt doped FeGa-(Fe0.83Ga0.17)100–xPtx (x = 0, 0.2, 0.4, and 0.6) was fabricated using melt-spinning technique. The magnetostriction, crystal structure, and magnetocrystalline anisotropy of the ribbon samples were studied, revealing that the conforming between the crystal growth direction and the easy magnetization axis, and the enlarged lattice parameter both contribute to the enhancement of magnetostriction, which reached the maximum λ⊥ of −1369 ppm at the optimum composition x = 0.4. Finally, transmission electron microscopy was performed to show that nanoinclusions exist not only in Pt-doped but also in Pt-free samples, demonstrating that the nanoinclusions do not play the dominant role in magnetostriction improvement. This work may accelerate the design of highly magnetostrictive FeGa alloys and other magnetic functional materials, which are required of large magnetostriction.

Analysis of Zn diffusion in various crystallographic directions of GaN grown by HVPE

This work presents the diffusion mechanism of zinc in gallium nitride. Halide vapor phase epitaxy layers were deposited on native ammonothermal seeds of three crystallographic orientations: c - (0001), m - (10-10), and a - (11–20). The surfaces were prepared to an epi-ready state by mechanical and chemical-mechanical polishing. Zinc ions were then implanted with energy of 230 keV and fluence 1016 cm−2 to create an infinite-like source of zinc atoms near the surface. The implanted samples were annealed by ultra-high-pressure method in the temperature range 1250–1450 °C at nitrogen pressure of 1 GPa for 4.5 h to remove the post-implantation structural damage and trigger the diffusion of zinc into the depth of the gallium nitride layers. X-ray diffraction measurements were performed for samples at every stage of the process: as-grown, as-implanted and annealed in order to investigate the evolution of the structural quality. Secondary ion mass spectrometry was employed to analyze depth profiles of zinc and concentrations of impurities. Positron annihilation spectroscopy and low-temperature photoluminescence were employed to assess the defect evolution in the samples. The results showed a strong dependence of zinc diffusion on the crystallographic direction of gallium nitride crystal growth. The depth profiles measured for the analyzed crystallographic directions differed in shape and range of zinc penetration. Diffusion coefficients were calculated for all the analyzed temperatures. The pre-exponential diffusion factor and activation energy were determined.

Experimental and simulation studies of surface segregation-limited nitrogen incorporation at the growth front of physical vapor transport-grown 4H-SiC crystals

Experimental and simulation studies were conducted for surface segregation-limited kinetics of nitrogen incorporation into a 4H-SiC crystal during physical vapor transport (PVT) crystal growth. It was revealed that the nitrogen incorporation is kinetically limited by the step-flow velocity on the growing crystal surface of a 4H-SiC crystal; in this study, the surface step-flow velocity at the growth front was deduced from the local inclination angle of the growth front measured from the (0001¯) plane, assuming a uniform growth rate along the c-axis (crystal growth direction) across the growth front, and the nitrogen concentration across the growth front was measured using Raman scattering microscopy. The step-flow velocity dependence of nitrogen incorporation was theoretically analyzed using a two-site-exchange model, and the simulated dependence using the model was in good agreement with the experimental data. On the basis of these experimental and simulation results, kinematical and energetical aspects of nitrogen incorporation at the growth front of a 4H-SiC crystal during PVT growth are discussed.

Directional Crystallization of Conjugated Molecules during Coating Processes.

The coating of organic molecules from the solution phase can result in directional crystal growth under certain conditions, even on a smooth isotropic surface and without the need of any kind of graphoexpitaxial preparation of the substrate. Based on reviewing the results from a variety of coating techniques and coating parameters, we identified that it is crucial for the coating speed to match the growth speed of the fastest growing crystal plane to achieve a high degree of directional crystallization.

Role of the thermal regime in the defect formation of zinc oxide nanostructures prepared by the thermal decomposition process

Abstract The intrinsic defect of ZnO depicts a crucial role in the charge transfer owing to the suppression of the exciton recombination, exhibiting superior semiconducting performance. In this study, the intrinsic defect of ZnO nanostructures prepared by direct thermal activation of 300–900 °C was investigated. X-ray diffraction (XRD) was employed to analyze phase, crystallite size, Zn–O bond length, and dislocation density. The relation of Williamson–Hall (W–H) was used to calculate crystallite size and micro-strain. The atomic coordination was approximated through the Rietveld method. Morphology and crystal growth investigation was carried on by scanning electron microscope (SEM) and tunneling electron microscope (TEM), exhibiting rod-like nanostructures transform to oval shape particle with high residual strain when increasing calcination temperature, exhibiting the crystal growth direction of (101). Specific surface and pore analysis reveals a significant value corresponding to SEM analysis. Fourier transform infrared spectroscopy (FT-IR) detected Zn–O stretching vibration bands, presenting a notable increase in the intensity when heat at 600 °C. Relating to the thermal regime, energy bandgap (Eg) was found to be 3.41–3.50 eV as increasing heat treatment temperatures. Photoluminescence (PL) was applied to determine intrinsic defects through emissive spectra. The surface charge was determined through the zeta potential measurement. The photo-induced dye degradation was measured to understand the effect of the defect in semiconductors. The X-ray photoelectron spectroscopy (XPS) confirms the wurtzite structure appearance, including the intrinsic defects. The observed intrinsic defects are discussed, associating with the structural constants, emissive spectra, cationic dye degradation, and binding energy.

A study of the electrical and optical properties of AZO thin film by controlling pulse frequency of HiPIMS

The transparent conductive oxide (TCO) which is AZO thin film was prepared by controlling pulse frequency at 100 Hz to 900 Hz using high-power impulse magnetron sputtering (HiPIMS). All samples were deposited on silicon (100) and glass slide substrates which the thickness was kept constant at 400 nm. The surface morphology was investigated by field-emission scanning electron microscope (FE-SEM), crystallinity by Grazing Incidence X-ray Diffraction (GI-XRD), optical transparency by UV-Vis-NIR spectrophotometry, and electrical properties using Hall effect instrument. It was found that the AZO films exhibited dense columnar structure. The GI-XRD patterns of AZO films demonstrated the crystal growth direction which was preferred the hexagonal wurtzite structure at (002) and (103) planes. The AZO film prepared by using 700 Hz of frequency (duty cycle 7%) showed the average visible transmittance (Tavg) at 82% in the visible region (380 nm to 780 nm). Additionally, the resistivity, high mobility and carrier concentration of AZO film were found to be 3.0 × 10-3 Ω.cm-1, 10.53 cm2∙Vs-1 and 1.82 × 1020∙cm-3, respectively. The fabrication of AZO film presented excellent electrical and optical properties which could be applied in several optoelectronic applications.

Preparation of High-quality Vapor-deposited Crystalline Organic Thin Films through Crystallization from Their Liquid Crystal Phase

High-quality crystalline organic thin films with large crystalline domain size were fabricated by vacuum deposition, followed by a post-annealing and cooling crystallization from their respective smectic A (SmA) liquid crystal precursor states. Crystallinity, surface smoothness and air exposure stability of ionic liquid crystalline thin films were much improved by this process, and the electric double layer action was for the first time observed at the SmA phase. Moreover, a seeded temperature gradient cooling process from the SmA phase was proposed and its capability was demonstrated in the fabrication of benzothienobenzothiophene derivative thin films, achieving their directional lateral crystal growth with high precision controllability.

Complex microstructural analysis of YBCO single-grain bulks with artificial holes: Effect on superconducting properties

This work presents an investigation of the microstructural and superconducting properties of standard single-grain bulk superconductor Y–Ba–Cu–O (YBCO) and samples fabricated with artificial holes. It was shown that the presence of artificial holes reduces the absolute porosity of the sample. The presence of artificial holes did not improve the homogeneity of oxygen distribution in the samples after oxygenation. The distribution of Y211 particles in the samples has a negligible effect on the macroscopic superconducting properties. In the process of crystal growth, the ellipticity of the introduced holes and their displacement in the direction of crystal growth in the growth sectors arise. Macroscopic superconducting properties, such as levitation force and trapped magnetic field at 77 K, were supplemented by magnetization measurements of superconducting transition temperature and critical current density. The levitation force and the trapped magnetic field were higher for the sample with holes. For the measured trapped magnetic field, the decrease of the porosity in the a-growth sectors, which have a higher volume fraction of Y2BaCuO5 (Y211) particles, and consequently stronger pinning, is significant.

Effect of arc oscillation on porosity and mechanical properties of 2319 aluminum alloy fabricated by CMT-wire arc additive manufacturing

In this paper, arc oscillation is applied in the fabrication of 2319 aluminum alloy samples by cold metal transition wire arc additive manufacturing process (CMT-WAAM). The porosity, microstructure and mechanical properties of 2319 aluminum alloy under different arc oscillation patterns were discussed. The results showed that the stirring of arc oscillation can influence the molten pool shape and improve the fluidity, change the thermal distribution and the growth direction of columnar crystal. Arc oscillation has a obvious effect on reducing the grain size and proportion of pores. After using the spiral arc oscillation pattern and asymmetrical trapezoid arc oscillation pattern, the volume fraction of pores reduced from 2.64% to 1.09% and 1.22%, and the size of pores was mainly smaller than 40 μm, respectively. The tensile strength in vertical direction and plasticity were improved with arc oscillation, but its effect on the horizontal tensile strength was not noticeable. After solid solution-aging treatment, the tensile strength of the sample with spiral arc oscillation pattern increased to 277 MPa, with a cross-section reduction of 3.6%.

Modeling of epitaxial growth of single crystal superalloys fabricated by directed energy deposition

Single crystal (SX) nickel-based superalloys have been used for turbine blades fabrication in recent years because of their superior mechanical performance at high temperature. The restoration of SX blades is significant considering the high cost to re-manufacture the blade. Directed energy deposition (DED), as a laser additive manufacturing technique, is a promising method for repairing the SX blades. Continuous epitaxial growth from the substrate to deposited layers is vital for successful restoration of SX blades. In this work, a novel single crystal texture cellular automata (SITCA) method is developed to simultaneously predict the columnar to equiaxed transition (CET) and crystal growth direction at the molten pool scale. Based on the grain envelope simplification, the SITCA method introduces a random variable to trace the growth trajectory, and therefore indicates the partition of the growth regions in the molten pool. A continuous nucleation model is introduced for nucleation simulation, and the crystal growth is simulated using the Kurz–Giovanola–Trivedi (KGT) model. The validation of SITCA model is performed by casting solidification cases under uniform temperature and uniform temperature gradient. The SITCA method is then integrated with a thermo-fluid molten pool simulation using finite element method to comprehensively compute the epitaxial growth. SX remelting experiments are carried out with different laser scanning speeds using a flat top laser source, and compared with the numerical results. The experimental and numerical results both indicate that increasing laser scanning speed will increase the ratio of crystals growing in the [001] direction while facilitate the CET on the other hand. This work shows the capacity of the SITCA method to predict epitaxial growth in DED process, and we anticipate to deploy the SITCA method to complex SX additive manufacturing.

Direct Crystal Growth Control: Controlling Crystallization Processes by Tracking an Analogue Twin

Due to the dynamic interactions of various crystallization phenomena, crystallization processes are notoriously difficult to control. In some applications, supersaturation control is aimed for as it affects various phenomena. Direct nucleation control aims at controlling the number of crystals in suspension. Here, we propose direct crystal growth control (DGC). The new method avoids parametrization of the thermodynamic relation between concentration, temperature, and solubility. Instead, a single crystal is fixed in the crystallization vessel, and it acts as an analogue twin. The analogue twin enables inline growth rate measurement and serves as a proxy for the growth of the suspended crystals. Via temperature manipulation, the analogue twin’s growth rate can be controlled to be kept constant. Growth rate of suspended crystals is then observed to be constant, too. However, hydrodynamic conditions for the analogue twin and suspended crystals were different and need to be further explored for a more accurate application of DGC. Temperature-dependent growth rates, the existence of molecular clusters in solution, and the potential use of the new growth rate probe for parameter estimation are discussed.

Effects of TiS2 on Inhibiting Candida albicans Biofilm Formation and Its Compatibility with Human Gingival Fibroblasts in Titanium Implants.

Titanium is widely used in medical devices, such as dental and orthopedic implants, due to its excellent mechanical properties, low toxicity, and biocompatibility. However, the titanium surface has the risk of microbial biofilm formation, which results in infections from species such as Candida albicans (C. albicans). This kind of biofilm prevents antifungal therapy and complicates the treatment of infectious diseases associated with implanted devices. It is critical to developing a feasible surface to decrease microbial growth while not interfering with the growth of the host cells. This study reports the influence of titanium surface modification to titanium disulfide (TiS2) on inhibiting C. albicans biofilm formation while allowing the attachment of human gingival fibroblasts (HGFs) on their surface. The surface of titanium parts is directly converted to structured titanium and TiS2 using direct laser processing and crystal growth methods. C. albicans adhesion and colonization are then investigated on these surfaces by the colony counting assay and reactive oxygen species (ROS) assay, using 2',7'-dichlorofluorescin diacetate (DCFH-DA) and microscopy images. Also, the viability and adhesion of HGFs on these surfaces are investigated to show their adhesion and biocompatibility. Titanium samples with the TiS2 surface show both C. albicans biofilm inhibition and HGF attachment. This study provides insight into designing and manufacturing titanium biomedical implants.

Influence of solvent on accelerating deposition time of α-Fe2O3 photoanode via AACVD

The optical and photoelectrochemical (PEC) properties of semiconductors can be manipulated by varying the deposition time and the solvent used. In this work, the study investigates the influence of organic solvent (methanol and ethanol) on accelerating deposition time for the efficient optical and photoelectrochemical performance of the fabricated α-Fe2O3 thin film photoanode below 10 minutes. The α-Fe2O3 film was fabricated via aerosol-assisted chemical vapor deposition (AACVD). The study discovered that the solvent with high polarity enhanced the crystal growth direction of α-Fe2O3 on the (110) orientation. The UV-vis showed that all deposited α-Fe2O3 films exhibit the absorption band in the visible-light region with the estimated energy bandgap of 2.10 eV. Furthermore, the PEC performance exhibited good photoanodic responses with excellent charge transfer behavior and photostability. These findings show that the choice of solvent could influence the rapid deposition time, thus, would be reliable to accelerate the fabrication process of photoelectrochemical water-splitting photoanode.

Tunable Salt-Inclusion Chalcogenides for Ion Exchange, Photoluminescence, and Scintillation

Salt-inclusion chalcogenides (SICs) consisting of covalent and ionic building blocks have emerged as functional materials exhibiting novel topologies that result from the combination of the chalcogenide-based frameworks and the salt inclusions contained within them. Nine compositions of a novel family of SICs [Cs6X]AGa6Q12 (A = Na, K, and Rb; X = F, Cl, and Br; Q = S and Se) were synthesized via halide/polychalcogenide flux crystal growth and structurally characterized. Ex situ powder X-ray diffraction analysis was used to determine the optimal synthesis temperatures at which the titled SIC materials crystallized in the flux. Density functional theory calculations coupled with convex hull construction were performed to predict the stability of [Cs6X]AGa6Q12 compositions as a function of X, A, and Q and to identify their decomposition pathways. A combination of single-crystal X-ray diffraction and energy-dispersive spectroscopy was used to study single-crystal-to-single-crystal (SC-SC) ion-exchange-reaction, a process that also allowed for the synthesis of two additional members of this family, [Cs6F]NaGa6S12 and [Cs6–xRbxBr]RbGa6S12, which did not form during the direct flux crystal growth. Furthermore, single crystals of [Cs6X]AGa6Q12:Mn2+ were obtained through direct flux crystal growth and SC-SC ion-exchange reactions and studied for their photoluminescent behavior using individual single crystals. The emission profile changed as a function of Mn2+-content, the A cation identity, and the synthesis method used. Finally, radioluminescence measurements were carried out on [Cs6Cl]NaGa6S12:Mn2+ bulk samples, resulting in bright orange scintillation.