Formation Of Hexagonal Phase Research Articles

Structure properties of CdTe nanocrystals created in SiO2/Si ion track templates

The synthesis of CdTe nanocrystals was studied by electrochemical (ECD) and chemical (CD) deposition in SiO2/Si-n structures irradiated with 200 MeV 132Xe ions. Both types of electrolyte (sulfate and chloride) used in ECD led to the formation of CdTe nanocrystals in single wurtzite phase. On the other hand, in the case of CD in a sulfate solution, CdTe nanocrystals with a zinc blende structure (ZBS) are formed that coexist with CdO nanocrystals in the hexagonal structure. Furthermore, an increase in the temperature of the CD solution led only to the formation of hexagonal phase of CdO nanocrystals. A model is proposed for the formation of a CdTe nanocrystal inside the ion track, taking into account the influence of a weak external electric field on the region of the inner surface of the ion tracks.

MgO nanostructures at different annealing temperatures for d0 ferromagnetism

Present study is based on the facile development of magnesium oxide (MgO) nanostructures at different annealing temperatures (300°C, 500°C, 700°C & 900°C) for d0 ferromagnetism. The developed MgO nanostructures were characterized for structural, morphological and magnetic properties using versatile techniques. X-ray diffraction (XRD) results revealed the formation of hexagonal phase corresponding to Mg(OH)2 at 300 °C which further transforms to cubic phase of MgO nanostructures at higher temperatures. SEM image showed the variation in morphology with temperature from nanoflakes to flowers and then hexagonal sheets. It has been found that saturation magnetisation increases with temperature. The concept of d0 ferromagnetism is understood on the basis of defects which were confirmed from XPS studies. Further, electron paramagnetic resonance (EPR) measurements were also performed to understand the paramagnetic ions and exchange interaction of spins which support the defects present in the material.

Iron deficient BaNixMnxFe12−2xO19 (x = 0.0–0.5) hexagonal plates: single-domain magnetic structure and dielectric properties

The current investigation was carried out in order to present the structural, magnetic and dielectric properties of BaNixMnxFe12−2xO19 (x = 0.0–0.5) (BNMFO) hexagonal microplates prepared via hydrothermal method followed by calcination at 950 °C for 5 h. The X-ray diffraction patterns of x = 0.0–0.5 contents confirmed the formation of hexagonal phases. The morphology and grain size of the BNMFO was examined by the field emission of scanning electron microscope. The results indicated that the BNMFO exhibited the hexagonal platelet like grains of size ranging from 2.4 to 3.5 µm. Two peaks were formed in the Fourier transform infrared spectra at 585 and 427 cm−1 and indicated the formation of metal–oxygen bonds. It was observed that the band gap was decreased with the increase in ‘x’. From the room temperature M–H curves, it was observed that the high saturation magnetization (45.8–53.9 emu/g) was recorded for all samples. Further, the coercive field was decreased from 3133 to 1098 Oe as a function of ‘x’. The real and imaginary parts of dielectric permittivity parameters were found to be increased with the increase in ‘x’.

Chrysin mediated synthesis, crystallographic structure and optical emission characteristics of ZnO nanoparticles

ZnO nanoparticles were synthesized through chrysin mediated phytochemical reduction mechanism. pH of the reaction mixture was kept proximate to 9 and the reaction was carried out at room temperature. The powdered sample obtained was annealed at 600 °C for four hours. X-ray diffraction pattern analysis revealed the formation of nanocrystalline hexagonal primitive lattice phase of zinc (II) oxide with crystallite size equal to16 nm as estimated from Debye-Scherrer equation. The coalescence resulted from annealing the sample which led to the formation of needle like grains of edge to edge crystallites aggregate was evidenced from FESEM images, with grain size equivalent to crystallite size calculated from x-ray diffraction analysis. Tauc’ plot of uv–visible spectrum employing Kubelka-Munk function provided a direct optical band gap of 3.27 eV. The photoluminescence spectrum using 280 nm optimal excitation wavelength and the related color coordinates estimated from chromaticity diagram revealed that the optical emission in the sample occurs at blue region in the visible spectrum.

Selective-area growth and characterization of cubic GaN grown by metalorganic vapor phase epitaxy

Selective–area growth of cubic GaN (c–GaN) layers grown on stripe–patterned GaAs (100) substrates by metalorganic vapor phase epitaxy was investigated. The formation of hexagonal phase was examined along with the growth morphology and the lateral-to-vertical growth ratio of the selective–area grown c–GaN stripes. It is found that the hexagonal phase formation significantly depends on growth parameters, particularly, the growth temperature, direction of the stripe pattern, and fill factor. For the [011]– and [01¯1]–stripe patterns, the volume fraction of the hexagonal phase in the c–GaN layers reduces significantly with increasing fill factor. In contrast, it gradually increases with the fill factor for the [001]–stripe pattern. GaN layers with the highest phase purity containing > 85% of the cubic phase can be obtained at low growth temperatures (~ 900°C) for the [011]–stripe pattern with the smallest fill factor of 0.35. A lower lateral growth rate is observed as compared to that at higher growth temperatures (930–960°C). Further evidence is provided by correlating the cathodoluminescence and photoluminescence to the growth facets. Transmission electron microscopy results confirm that h–GaN is easily constructed on the (111)B facet due to the formation of stacking faults. In practice, the [01¯1]–stripe pattern with a large fill factor of ~ 0.7 can be applied to obtain a selective–area grown c–GaN layer with the highest cubic-phase purity. Furthermore, the [011]–stripe pattern has also been considered for SAG of c–GaN at low growth temperature.

Novel Low Temperature Route to Produce CdS/ZnO Composite Nanofibers as Effective Photocatalysts

In this work, CdS/ZnO composite nanofibers (NFs) were prepared by the electrospinning of a sol–gel comprised of poly(caprolactone), zinc acetate dihydrate, cadmium acetate dihydrate, and ammonium sulfide. The electrospun NF mats were calcined under vacuum in an argon (Ar) atmosphere at 200 °C for 1 h. Standard physiochemical analysis techniques demonstrated the formation of the crystalline hexagonal phase of CdS and ZnO. Composite NFs showed good photocatalytic degradation of methylene blue (MB) dye under visible light irradiation compared to their counterparts. CdS nanoparticles, ZnO nanofibers, and composite NFs photodegraded 35.5%, 47.3%, and 90% of the MB dye, respectively, within 100 min. The reaction kinetics of MB photodegradation using the composite NFs followed the pseudo-first-order relation. Owing to their facile preparation and good photodegradation ability, the proposed method can be used to prepare various photocatalysts for wastewater treatment.

Formation of Thin NiGe Films by Magnetron Sputtering and Flash Lamp Annealing.

The nickel monogermanide (NiGe) phase is known for its electrical properties such as low ohmic and low contact resistance in group-IV-based electronics. In this work, thin films of nickel germanides (Ni–Ge) were formed by magnetron sputtering followed by flash lamp annealing (FLA). The formation of NiGe was investigated on three types of substrates: on amorphous (a-Ge) as well as polycrystalline Ge (poly-Ge) and on monocrystalline (100)-Ge (c-Ge) wafers. Substrate and NiGe structure characterization was performed by Raman, TEM, and XRD analyses. Hall Effect and four-point-probe measurements were used to characterize the films electrically. NiGe layers were successfully formed on different Ge substrates using 3-ms FLA. Electrical as well as XRD and TEM measurements are revealing the formation of Ni-rich hexagonal and cubic phases at lower temperatures accompanied by the formation of the low-resistivity orthorhombic NiGe phase. At higher annealing temperatures, Ni-rich phases are transforming into NiGe, as long as the supply of Ge is ensured. NiGe layer formation on a-Ge is accompanied by metal-induced crystallization and its elevated electrical resistivity compared with that of poly-Ge and c-Ge substrates. Specific resistivities for 30 nm Ni on Ge were determined to be 13.5 μΩ·cm for poly-Ge, 14.6 μΩ·cm for c-Ge, and 20.1 μΩ·cm for a-Ge.

Room‐Temperature Partial Conversion of α‐FAPbI3Perovskite Phase via PbI2Solvation Enables High‐Performance Solar Cells

AbstractThe two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI3hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI3, is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI3, requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI2into the α‐FAPbI3perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI2precursor from its different states are compared. α‐FAPbI3forms spontaneously and efficiently at room temperature from P2(ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI2) to 17.23(±0.28)% (from solvated PbI2) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA+cation without the need for Cs+or mixed halide formulation.

Calcination effects on europium doped zinc oxide as a luminescent material synthesized via sol-gel and precipitation methods

This work is aimed at the determination of the preferential calcination conditions for maximum thermoluminescence (TL) and optically stimulated luminescence (OSL) emissions of europium doped zinc oxide (ZnO:Eu) pellets after exposure to β-particles when they are synthesized using the precipitation and sol-gel methods, in micro/nanoscales. The verification of the photoluminescence (PL) signals were investigated. The calcination temperature range of 800–1200 °C and the calcination times between 2 h and 24 h were applied in the production of ZnO:Eu pellets. The phase composition and morphology of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The transmittance measurements were carried out by Fourier transformed infrared (FTIR) spectroscopy to understand the vibrational properties of ZnO:Eu nanocrystals. In the XRD and SEM patterns, the formation of hexagonal wurtzite phase of ZnO was observed. It was observed that the samples exhibited a weak green emission when the excitation wavelength is 350 nm. In addition, the ZnO:Eu pellets exhibited two sharp peaks at 579 and 615 nm which related to the Eu transitions. The PL intensities dropped with the increasing calcination temperatures. The TL peaks settled at 130 and 300 °C with 5 oCs−1 heating rate, were found similar to each other when the pellets calcined using different temperatures and times. β-dose linearity extended from 1 Gy to 0.2 kGy for all of the studied samples.

Strain stiffening, high load-invariant hardness, and electronic anomalies of boron phosphide under pressure

New refractory hard materials with a favorable band gap are in high demand for the next-generation semiconductors capable of withstanding high temperature and other hostile environments. Boron phosphide (BP) is such an attractive candidate with exceptional properties; however, it has mainly been studied theoretically because of the difficulty in sample preparation. In this work, we report successful synthesis of large millimeter-sized single-crystal BP. The final product has a zinc-blende structure with a unique electronic structure and is optically transparent with a moderate band gap of \ensuremath{\sim}2.1 eV. Our experiments, in conjunction with ab initio simulations, reveal that the compound exhibits extraordinary strain stiffening and unusually high load-invariant hardness of \ensuremath{\sim}38 (3) GPa, which is close to the 40-GPa threshold for superhard materials, making BP the hardest among all known semiconductors. Based on the first-principles calculations, the fracture mechanisms in BP under tensile and shear deformations can be attributed to the formation of a metastable hexagonal phase. Further spectroscopic measurements indicate that an unusual electronic transition occurs at high pressures of \ensuremath{\sim}13 GPa, resulting in an asymptotically enhanced covalent bonding state. The pressure dependence of multiphonon processes is also determined by Raman measurement. In addition, our studies suggest a phonon-assisted photoluminescence process and evidence for the photon-pumped \'etalon effect at 707 nm.

Formation and magnetic properties of hexagonal DO19 phase in Fe2MnGa: An experimental and theoretical investigation

The formation and magnetic properties of Fe2MnGa hexagonal DO19 phase were investigated in detail. Hexagonal Fe2MnGa can be stabilized by furnace cooling, while the FCC phase can be obtained by quenching from high temperature. Strong ferromagnetic character is identified in Fe2MnGa DO19 phase. Its saturation magnetic moment Ms at 5 K is as large as 4.40 μB/f.u. and Curie temperature TC is 563 K. First-principles calculations indicate that Fe and Mn atoms tend to occupy the 6h sites randomly in the DO19 lattice. The DO19 type Fe2MnGa shows a normal ferromagnetic metal character in its electronic structure. Similar to the experimental results, the calculated total magnetic moment is also quite large and mainly comes from the contributions of Fe and Mn spin moments. Both of them are larger than 1.8 μB and are parallel coupled. These properties are quite different from the antiferromagnetic character in FCC Fe2MnGa.

Mechanism of formation of light-emitting silicon hexagonal phase 9R-Si

A method of photoluminescence (PL) spectroscopy has been used to study the mechanism of formation of light-emitting hexagonal 9R-Si phase by krypton ion implantation into thermally grown oxide layer on silicon substrate with subsequent annealing. The PL band at ∼ 1246 nm previously assigned to this phase appears at isochronous step-by-step annealing temperatures of 600 °C and higher as well as for one-step annealing. In addition, the PL bands at ∼1324 and ∼1408 nm previously observed in ion-implanted silicon and assigned to self-interstitial complexes are present in our case. The decrease in their intensities and simultaneous enhancement of the 9R-Si band are observed with increase in annealing temperature. It is concluded that the mechanical stresses arising in SiO2/Si system during implantation are responsible for the formation of the 9R-Si phase.

Effect of Cr3+ Doped on Structural, Magnetic and Electrical Properties of Sol-Gel Synthesized SrFe12O19 Hexaferrite Nanoparticles

Nanocrystalline SrFe12-xCrxO19 hexaferrite particles were successfully synthesized via Sol-gel route from Sr3+ and Fe3+ nitrates at molar ratio of 1:9 (Fe/Sr). The XRD results revealed single hexagonal magnetoplumbite SrFe10Cr2O19 phase formation in the presence of non-magnetic (α-Fe2O3) phases in the heated samples. The dielectric properties as a function of frequency under normal conditions were also measured in the frequency range of 500 Hz to 1 MHz. The variations of dielectric constant are discus on the basis of Maxwell–Wagner and Koops models. In VSM graph, coercivity and saturation magnetization confirms that the synthesized materials are suitable for applications in magnetic recording media.

Heterostructured Nanocomposites of Ag Doped Fe3O4 Embedded in ZnO for Antibacterial Applications and Catalytic Conversion of Hazardous Wastes

Heterostructured ternary nanocomposites of silver doped magnetites with ZnO (Z), namely Ag/ZnO (AZ), Ag/Fe3O4 (AF), ZnO/Fe3O4 (ZF) and Ag/ZnO/Fe3O4 (AZF) were synthesized. X-ray diffraction studies indicated the formation of wurtzite hexagonal phase for ZnO, cubic structure for Fe3O4 and FCC structure for silver nanocomposites. Field emission scanning electron microscopy revealed the uniform distribution of granules of AZF. Fourier transform infrared spectroscopic studies confirmed the presence of M–O bonds. The ultra violet-visible diffuse reflectance spectroscopy and photoluminescence studies indicated the higher photo degradation activity of silver doped nanocomposites. Thermogravimetric analysis point towards the enhanced thermal stability of doped and coupled nanocomposites. Hydrogenation of 4-nitrophenol to 4-aminophenol showed that AZ, AF & AZF completed the reaction within 3, 4 & 2 min respectively while Z and ZF are ineffective. The photo catalytic activity of the nano composites was investigated by studying the degradation of malachite green (MG) & methylene blue (MB) under UV irradiation. Hetero structured and Ag doped nano composites exhibited better degradation efficiency of MG & MB. Using well diffusion method antibacterial activity of the as-prepared nanoparticles was tested against gram-positive bacteria Staphylococus aureus and gram negative bacteria Escherichia coli. AF & AZF nanocomposites exhibited strong antibacterial activity with greater inhibition zones due to the synergistic effect of silver and magnetite nanoparticles.

Photoelectrochemical performance of MoBiGaSe5 thin films deposited by vacuum deposition technique

In the present investigation nanocrystalline mixed metal chalcogenide (MMC) MoBiGaSe5 thin films were successfully deposited by vacuum evaporation technique. The asdeposited and vacuum annealed MoBiGaSe5 thin films were studied for their optical, structural, morphological, compositional, electrical and photoelectrochemical (PEC) performance. Optical absorption studies revealed that asdeposited and vacuum annealed MoBiGaSe5 thin films showed absorption in the visible region. X-ray analysis of vacuum annealed film shows the phase transition from orthorhombic Bi2Se3 to stoichiometric hexagonal Bi2Se3 phase. The phase transition confirms the formation of hexagonal nanocrystalline MoBiGaSe5 pure phase. SEM images of the asdeposited film showed uniform well defined morphology, besides annealed sample, had diffused granular growth of uniform spheres. HRTEM images showed an average particle size 58 nm, and SAED pattern confirms the single crystalline hexagonal structure of MoBiGaSe5 thin films. EDS confirmed the presence of Mo, Bi, Ga and Se elements and XPS confirms Mo4+, Bi3+, Ga3+ and Se2− oxidation states. The AFM images were in good agreement with SEM images. Asdeposited and annealed MoBiGaSe5 thin film show semiconducting behavior with p-type conductivity. The PEC performance was studied, and it showed efficiency (ɳ) 0.004 and 0.097% of asdeposited and vacuum annealed MoBiGaSe5 thin films respectively.

Yb Doping Effects on Structure and Performance of BaCo0.7Fe0.3-XYbxO3-Δ Perovskite

Fe partial substitution for Co on B-site may tradeoff stability-activity for BaCoO3-δ perovskite. However, large radius mismatch between Ba and Co/Fe favors the formation of a low symmetric hexagonal phase BaCo1-xFexO3-δ, which may experience phase structural transitions with varying temperatures and could induce structural reliability issue of associated devices. The B-site doping with high valence elements may facilitate to form cubic BaCo1-xFexO3-δ perovskite, it could also significantly deteriorate their catalytic property. The Yb3+ with ionic radius being larger than those of Co4+ and Fe4+ could be used as B-site dopant to tune Goldschmidt tolerance factor of BaCo1-xFexO3-δ to 1 and therefore stabilize its cubic structure. Additionally, the electronegativity of Yb is lower than that of Co and Fe. This would induce a slightly lower valance of Co and/or Fe when Yb is used as B-site dopant, facilitating oxygen vacancy generation and electrochemical kinetic process. In this research, Yb-doping effects on crystal structure and performance of BaCo0.7Fe0.3-xYbxO3-δ (x = 0.05, 0.10 and 0.15) are systematically evaluated. The results indicate that Yb partial substitution for Fe on B-site is able to effectively stabilize crystal structure with highly cubic symmetry and BaCo0.7Fe0.2Yb0.1O3-δ is among the best as cathode material for intermediate temperature SOFCs.

In operando studies of CO oxidation on epitaxial SrCoO2.5+δ thin films

The high abundance and fast kinetics of select transition metal oxide catalysts are attractive features for many chemical and electrochemical device applications. However, the activity of such catalysts can be accompanied by phase instabilities that prevent their widespread usage. Furthermore, complexities associated with variations in phase behavior and oxygen stoichiometry have hindered studies on the true origins of catalytic activity. Here, we explore the interactions between activity, phase stability, and microstructure using in operando synchrotron X-ray techniques and gas chromatography/mass spectroscopy (GCMS) to probe the behavior of model SrCoO2.5+δ (SCO) catalysts. Pulsed laser deposition was used to prepare SCO thin films on (001) SrTiO3, (111) SrTiO3, and pseudocubic (001) DyScO3 substrates. The GCMS catalytic measurements were performed with a custom-built microreactor compatible with a synchrotron X-ray diffractometer at the Advanced Photon Source. The activity for carbon monoxide oxidation was determined as a function of temperature from 500 °C to 800 °C. We show that the SrCoO2.5+δ films are active for CO oxidation, most likely by direct reaction with lattice oxygen; consequently, the activity was observed to increase as the films become less stable, with the most active film being the one exhibiting the lowest surface and crystal quality. All films decompose at high temperatures, with in operando diffraction indicating the gradual formation of Sr-rich hexagonal and CoO phases. We find that real-time studies of model oxide systems with synchrotron X-rays is a powerful means of gaining insight into the varied processes taking place at catalytic surfaces.

Morphological, optical, photocatalytic and electrochemical properties of hydrothermally grown ZnO nanoflowers with variation in hydrothermal temperature

ZnO nanoflowers were synthesized by hydrothermal method. The growth of the nanoflowers was studied for five different hydrothermal temperatures, viz. 100 °C, 120 °C, 140 °C, 160 °C and 180 °C. Morphology, nanostructure and crystalline phase of the prepared samples were characterized by FESEM and XRD. FESEM images show that with change of hydrothermal temperature ZnO nanoflowers grow in size and their petal density also changes. XRD patterns confirm the formation of polycrystalline hexagonal wurzite phase of ZnO powder. Flowers show intense peaks at (100), (002) and (101). UV–Vis absorption spectroscopy of the samples was taken within 200–800 nm. The band gap of the samples was calculated using the Tauc plot relation and its value was found to be around 3.2 to 3.16 eV. Photoluminescence of the samples was taken within 300–700 nm. Emission spectra show variation of intensity of the peak with change in temperature. Photocatalytic degradation of MB dye by the ZnO nanoflowers shows that degradation was greatly influenced by petal thickness and petal density which are changed with change of temperature. Electrochemical activity of the materials is found to be also influenced by hydrothermal temperature.

Effect of organic ligand on ZnO nanostructures and to investigate the photocatalytic activity under visible light illumination

Controlled morphology of zinc oxide (ZnO) was synthesized by simple hydrothermal method. Morphology and size of ZnO nanostructures was controlled by using pyridine as a passivating agent. Pyridine played a crucial role in modifying the morphological size of ZnO nanostructures. The structural analysis confirms the formation of hexagonal phase of ZnO with good crystalline nature. The elemental and composition analysis confirm the presence of ZnO and the existence of pyridine in the composition. The morphological analysis was performed by field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HR-TEM). The degradation property of organic pollutant was studied under visible light irradiation. Maximum degradation efficiency was observed for 0.6 mL of pyridine sample which disappeared completely after 18 min of irradiation. Furthermore, the degradation property of ZnO was studied by different value of pH, concentration of dye and photocatalyst dosage. The optimum values of solution pH, dye concentration, and photocatalyst dosage was 11, 10 ppm, and 75 mg/L, respectively.

Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties

Abstract Nanocrystalline perovskite hexagonal yttrium manganese oxide (YMnO3) sample have been synthesized through a sol-gel citrate route and then calcination has been done at 700 °C for one hour. The prepared sample was subjected to structural and morphological studies through X-ray diffraction (XRD) technique and scanning electron microscope (SEM). The XRD pattern of the prepared sample confirmed the formation of a pure hexagonal phase of YMnO3. The energy-dispersive X-ray analysis (EDX) results proved the elemental composition of pure yttrium, manganese, and oxygen. The dielectric properties of the YMnO3 nanocrystals were studied using a precision LCR meter at different temperature range 28 °C–500 °C.