Preferred Crystallographic Directions Research Articles

The effect investigates of temperature on the optical and electrical properties of sprayed SnS: experimental and theoretical study

In this study, Tin monosulfide SnS semiconductor absorbers was deposited by chemical spray pyrolysis route on the glass substrate. We examined the impact of substrate temperature on the structural, morphological, linear optical and electrical characteristics of SnS absorber at many substrate temperatures such as 50 °C, 375 °C and 400 °C. The SnS films have been analysed by diverse techniques like X-ray diffraction, Raman spectroscopy, Scanning electron microscopy and UV-Vis spectrophotometer. The X-ray diffraction (XRD) spectra revealed that the SnS crystallize in the orthorhombic crystal system with the apparition of the preferential crystallographic direction oriented along (111) planes. The SEM micrographs indicate a great uniformity and granular morphological surface of SnS films. Linear optical constants such as energy gap (Eg), coefficient of extinction (k), index of refraction (n), optical conductivity (σopt), as well as the electrical properties confirm the suitable application of SnS thin films as absorber layer in the optoelectronic device applications. Additionally, we have applied the density functional theory DFT and GGA generalized gradient approximation to study the electronic characteristics; as a result of the electronic band structure the SnS absorber has a suitable energy gap.

(Invited, Digital Presentation) Wafer-Scale Epitaxial Growth of Transition Metal Dichalcogenide Monolayers on Sapphire for Large Area Device Applications

Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on c-plane sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in twin boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is demonstrated on 2” diameter c-plane sapphire substrates enabling large area transfer of monolayers for characterization and device fabrication and testing. Applications for wafer-scale TMD monolayers in nanoelectronics, sensing and photonics will be discussed.

(Invited) Epitaxial Growth of Transition Metal Dichalcogenide Monolayers for Large Area Device Applications

Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on c-plane sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in twin boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is demonstrated on 2” diameter c-plane sapphire substrates enabling large area transfer of monolayers for characterization and device fabrication and testing. Applications for wafer-scale TMD monolayers in nanoelectronics, sensing and photonics will be discussed.

Surface boundary-dendrite interactions in thin metallic Al-alloy samples

Crystallographic orientations and growth directions of dendrites in thin Al-15 wt.%Cu (200 µm) and Al-29 wt.%Ge (500 µm) alloys were investigated employing in situ X-radiography and post-mortem X-ray tomography and electron backscatter diffraction. The Al-Cu alloy was solidified under microgravity conditions, while the Al-Ge sample was processed horizontally on ground. It was found that (i) dendrites nucleate on both sides of the surface in microgravity, but accumulate under normal gravity near the top surface due to buoyancy, (ii) dendrites grow along preferred in-plane crystallographic directions, which are 〈100〉 for Al-15 wt.%Cu and 〈100〉, 〈110〉 and 〈210〉 for Al-29 wt.%Ge, indicating that the solid-liquid interfacial anisotropy of this Al-Ge composition is similar for these 〈xy0〉 directions, (iii) slightly inclined dendrite arms in Al-Cu grow along the sample boundary after contact, whereas tips of inclined dendrite arms in Al-Ge never touch the boundary, but develop new primary tips originating from secondary branches.

Unusual anisotropic magnetic orbital moment obtained from x-ray magnetic circular dichroism in a multiferroic oxide system

The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarging the limit of the spin and orbital sum rules, which usually holds for materials where the magnetic ions exhibit a unique crystal field symmetry This finding translates in a sign-reversal between the average Fe magnetic orbital and spin moments. Large Fe-displacements, among some of the octahedral sites, lower the symmetry of the system producing anisotropic paths for the Fe-O bondings giving rise to a large orbital-lattice interaction akin to a preferential crystallographic direction for the magnetic orbital moment. The latter may lead to a partial re-orientation of the magnetic orbital moment under an external magnetic field that, combined to the ferrimagnetic nature of the GFO, can qualitatively explain the observed sign-reversal of the XMCD integral. The results suggest that a control over the local symmetry of the oxygen octahedra in transition metal oxides can offer a suitable leverage over the manipulation of the effective orbital and spin moments in magnetoelectric systems.

Numerical investigation of the surface and microstructure effects on the high cycle fatigue performance of additive manufactured stainless steel 316L

This paper aims to study the individual and competitive effects of surface defects and microstructural components in an AM 316L by numerical methods. Based on the microstructure sensitive modeling framework, numerical simulations of models in different surface states with virtual quasi-realistic explicit microstructures are performed. Problems encountered during the establishment of the numerical polycrystalline aggregate model are discussed, including the grain morphology, the strongly textured microstructure with preferential crystallographic directions, roughness and surface defect. A statistical method based on generalized extreme value distribution along with a non-local method is used to describe the stochasticity of microstructural attributes. Simulations of differently configured explicit microstructure models are analyzed and discussed.

Green synthesis of silver nanoparticles with phytosterols and betalain pigments as reducing agents present in cactus Myrtillocactus geometrizans.

Abstract

Textured multilayered piezoelectric structures for energy conversion

Piezoelectric materials are essential for the conversion between mechanical and electrical energy, for example in ultrasound imaging and vibrational energy harvesting. Here, we are making and exploring the effects of a new design: co-sintered multilayers with texture (grains of a preferential crystallographic direction). The motivation is the combination of increased piezoelectric response in certain crystallographic directions; multilayer structures where thick films rather than bulk materials can allow higher frequency operation and large area; and co-sintering to avoid detrimental effects from gluing layers together. Samples of the lead-free piezoelectric material Li0.06(K0.52Na0.48)0.94Nb0.71Ta0.29O3 with 0.25 mol% Mn (KNNLTM) were made by tape casting and co-sintering. NaNbO3 platelets with (100) orientation which were used as templates to introduce texture, and polymethyl methacrylate (PMMA) was used as a pore forming agent for making porous substrates. The electrical impedances of the co-sintered samples were recorded and analyzed by equivalent electrical circuit modelling. A texture up to 85% in the [100] crystallographic direction was obtained. The samples displayed ferro- and piezoelectricity, with a maximum thickness coupling coefficient (kt = 0.18) between mechanical and electrical energy in the most textured sample. This demonstrates that the introduction of texture in multilayered, co-sintered piezoelectrics shows promise for improving devices for ultrasound imaging or energy harvesting.

The Marangoni convection effects on directional dendritic solidification

The Pore Formation and Mobility Investigation (PFMI) experiments conducted on the International Space Station revealed that Marangoni convection caused by the presence of bubbles could affect the microstructure formation. The experiment provided information on the bubble movement and morphological evolution during remelting and directional solidification under microgravity condition for a succinonitrile (SCN)-water binary alloy. In this work, a three-dimensional lattice Boltzmann (LB)-Cellular automaton (CA) model was utilized to study the effects of Marangoni convection on the solidification microstructure. The results reveal that the induced convection is strong enough to bend dendrites from their preferential crystallographic direction and can affect the dendrite growth rate.

Growth Features of Diamond Films on the Tungsten Carbide Surface with a Copper Sublayer

The surface preparation is a prerequisite for ensuring the required properties of a diamond film formed by gas-phase deposition. The influence of the temperature and concentration of the CuSO4 etchant on the structural and phase compositions of the surface of hard-alloy materials is considered in this work, and the structural and phase compositions of a continuous polycrystalline diamond film at its growth stages is also considered. The adhesion of the diamond films to the surface of hard-alloy materials is determined qualitatively. It is established that the treatment of the hard-alloy surface in the CuSO4 solution at t = 23°C leads to the inhomogeneous removal of a cobalt binder with chipping WC grains and the formation of the porous structure in the surface layer of the WC–6% Co alloy. The treatment by the CuSO4 etchant at t = –2°C provides the homogeneous etching of the Co binder over the boundaries of WC grains and the formation of a chemically homogeneous surface. The orientation growth and adhesion of the diamond film depend on the elemental composition of the WC–Co alloy surface after treatment by the CuSO4 solution. If the treatment was performed at tsln = 23°C, then the removal of copper from the defect surface WC layer is complicated during the synthesis, which provides the multidirectional growth of diamond crystals in the film along two directions, 〈111〉 and 〈110〉, which causes critical biaxial compressive stresses (2.5 GPa) and leads to low film adhesion to the hard alloy surface. If the treatment is performed at tsln = –2°C, then the orientational growth of diamond crystals in the film occurs in one preferential crystallographic direction 〈111〉, which lowers the biaxial compressive stresses (1.7 GPa) and increases the adhesive bond of the film to the hard alloy surface. The structure imperfection calculated from the ratio of the lines of integrated intensities I1333/I1580 using the Raman spectroscopy method decreases with an increase in concentration at negative temperatures and increases at positive temperatures of the CuSO4 solution during surface preparation.

Intentional and unintentional channeling during implantation of 51V ions into 4H-SiC

Ion implantation is a commonly used process step in 4H-SiC device manufacturing to implement precise concentrations of dopant atoms in selected areas and depths. This paper reports on vanadium (V) implantation into 4H-SiC(0001) and how the crystal lattice, with preferential directions, channels, for the ions, will influence the final dopant distribution. Concentration versus depth profiles of V-ions, intentionally and unintentionally channelled, has been recorded by secondary ion mass spectrometry. Ion implantations have been performed between 50 and 300 keV at various impact angles and fluence at room temperature as well as at elevated temperatures. Before ion implantation, the samples were aligned utilizing the blocking pattern of 100 keV backscattered protons. In addition to the aligned implantations, our standard beam line for ion implantation has been used for implantations in a ‘random’ direction using the wafer miscut angle of 4°. The electronic stopping has been determined from these ‘random’ cases and the values have been used in 3D simulations to predict preferential crystallographic directions using SIIMPL, a Monte Carlo simulation code based on the binary collision approximation. The results show that, independent of the used impact angle there is always a probability that the vanadium ions will be steered into the [000-1] and the family of 〈11-2-3〉 crystal directions and therefore penetrate deep into the sample, resulting in unwanted ‘spikes’. If the implantation is performed at elevated temperatures, a larger degree of dechanneling is present due to increased thermal vibrations and the penetration depth of vanadium is slightly reduced.

ELABORATION AND CHARACTERIZATION OF ZNO THIN FILMS STRUCTURAL AND OPTICAL STUDY

The Nano-topographical structure of the solid surface is known as a necessary parameter in the physicochemical characterization and wetting properties. In this study, the physicochemical properties are evaluated by calculating the surface energy and by measuring the contact angle. The structural proprieties were determined using XDR. The optical proprieties were studied using the UV-visible technics. Substrates used in this study are the zinc oxide thin films deposited on the glass by sputtering under different powers (150, 200 and 250 watt). The Nano -topographic properties were examined using the atomic force microscopy (AFM) in order to calculate the roughness of different substrates. As results, the images obtained by atomic force microscopy showed that the growth of the power causes the growth of the roughness. XRD diagram assessment revealed that the deposited films have a preferential crystallographic direction according to the (002) plane while maintaining the initial orientation. The optical characterization showed that the bandwidth of these films is in the order of 3.28 eV. It is interesting to mention that the increase in RF power has slightly increased the energy of gap.

Features of the diamond films growth on the tungsten carbide surface by a copper underlayer

Surface preparation is a prerequisite for ensuring the required properties of a diamond film obtained by gas-phase deposition. The paper considers the effect of temperature and concentration of the etchant CuSO4 on the structural and phase composition of the surface of hard-alloy materials. The structural and phase composition of a continuous polycrystalline diamond film at its growth stages was also studied. Adhesion of the obtained diamond films to the surface of carbide materials was qualitatively determined. It has been established that surface treatment of a hard alloy in a CuSO4 solution at a temperature t = 23 °C leads to unequal removal of the cobalt bond with chipping of WC grains and the formation of a porous structure in the surface layer of the WC–6%Co alloy. The treatment with an etchant CuSO4 at t = –2 °С ensures uniform etching of the Co-bond along the WC grain boundaries and the formation of a chemically uniform surface. The orientational growth and adhesion of the diamond film depend on the elemental composition of the surface of the WC–Co alloy after treatment with a CuSO4 solution. If the treatment was carried out at a tsolution = 23 °C, then during the synthesis of the diamond film, the removal of copper from the defective surface layer of WC is difficult. This provides the multidirectional growth of diamond crystals in the film in two directions: <111> and <110>, which causes critical biaxial compressive stresses (2,5 GPa) and leads to low adhesion of the film to the surface of the hard alloy. If the treatment was carried out at tsolution = –2 °C, then the orientational growth of diamond crystals in the film occurs in one preferential crystallographic direction <111>. It reduces the biaxial compressive stresses (1,7 GPa) and increases the adhesive adhesion of the film to the surface of the hard alloy . The structure defect, calculated from the ratio of the lines of integrated intensities I1333 / I1580 using the Raman spectroscopy, decreases with concentration growth for negative temperatures and increases for positive ones of CuSO4 solution during surface preparation.

Combined in-depth X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectroscopy study of the effect of deposition pressure and substrate bias on the electrical properties and composition of Ga-doped ZnO thin films grown by magnetron sputtering

This work reports the effect of the applied substrate bias and deposition pressure on the bulk composition, electrical and microstructural properties of Gallium-doped Zinc Oxide thin films deposited by DC magnetron sputtering. In-depth Time-of-Flight Secondary Ion Mass Spectrometry and X-ray Photoelectron Spectroscopy studies were endured to determine the Ga content for the varying process conditions. Experiments confirm that the bulk composition of all films is homogeneous and that an optimized Ga doping of 3.9 at.% is obtained for a substrate bias of −100 V and deposition pressure of 0.51 Pa. It was also verified that films with lower electrical resistivity (2.6 × 10−3 Ω∙cm) have a hexagonal wurtzite structure with [001] preferred crystallographic direction. These transparent conductive oxide thin films have potential applications as electrodes in photovoltaics.

Structural and photoluminescent properties of CuGaxIn1−xSe2 thin films prepared by close-spaced vapor transport technique

CuGaxIn1−xSe2 (CIGS) (x = 0, x = 0.3 and x = 1) absorber layers were deposited onto transparent conducting oxide (SnO2) substrates using close-spaced vapor transport technique (CSVT). The aim of this work was to assess the quality of CIGS absorber films with various gallium contents. Composition found by Energy Dispersive Spectroscopy (EDS) showed that the investigated samples were Cu-poor and CuGa0.3In0.7Se2 thin film had a composition close to 0.3 as planned. The structural properties were investigated by X-ray diffraction analysis (XRD) and Raman spectroscopy, while optical properties were investigated by measuring transmittance and photoluminescence spectroscopy. It was found that all investigated samples crystallize with (112) as a preferential crystallographic direction. The shift of the XRD peaks towards a higher value of 2θ with the increase of Ga composition was observed. The grain size determined from the width of 112 XRD peak was within 40–75 nm range with no clear correlation with Ga content. Raman spectra of samples with x = 0 and 0.3 featured mainly a peak assigned to the A1 mode at about 175 cm−1. In CuGaSe2 the secondary Cu2Se phase was detected both by XRD and Raman. The band gap as evaluated by optical transmission measurement is shifted towards lower than expected values due to substantial defect-related bands broadening. A large concentration of defects in the samples was also confirmed by low or absent PL signal in the vicinity of the bandgap, while the emission due to deep defects was prevailing in the spectrum. The optoelectronic properties as indicated by photoluminescence have to be improved if CSVT method of absorber preparation is to be used more widely in photovoltaics.

Tailoring the formation of textured oxide films via primary and secondary nucleation of oxide islands

Complex hierarchical architectures of nano/microcrystals are of great interests for tuning material properties at the micro- and meso-scale. Unfortunately, the fundamental mechanisms underlying the hierarchical organization of nanocrystals, especially the preferred crystallographic directions and their inter-transitions, remain largely elusive. Herein, we demonstrate the tunability of the crystallographic orientations in textured oxide films through a two-stage nucleation process of oxide nanoislands in metal oxidation. Using in situ environmental transmission electron microscopy, we manipulate the oxidation of copper to result in a dually-structured architecture of Cu2O nanocrystalline films via primary nucleation of cube-on-cube epitaxy of oxide islands, and secondary nucleation of oxide islands on side facets of the primary islands. This dual-nucleation mechanism of oxide islands may find broader applicability in making hierarchical architectures of other nanocrystalline metal oxides.

The Organic Crystalline Materials of Vision: Structure-Function Considerations from the Nanometer to the Millimeter Scale.

Vision mechanisms in animals, especially those living in water, are diverse. Many eyes have reflective elements that consist of multilayers of nanometer-sized crystalline plates, composed of organic molecules. The crystal multilayer assemblies owe their enhanced reflectivity to the high refractive indices of the crystals in preferred crystallographic directions. The high refractive indices are due to the molecular arrangements in their crystal structures. Herein, data regarding these difficult-to-characterize crystals are reviewed. This is followed by a discussion on the function of these crystalline assemblies, especially in visual systems whose anatomy has been well characterized under close to in vivo conditions. Three test cases are presented, and then the relations between the reflecting crystalline components and their functions, including the relations between molecular structure, crystal structure, and reflecting properties are discussed. Some of the underlying mechanisms are also discussed, and finally open questions in the field are identified.

Mechanical Properties of Anhydrous and Hydrated Uric Acid Crystals

A large percentage of molecular compounds can crystallize in different hydration states. Although hydrated and anhydrous crystal forms can exhibit different physical properties (e.g., solubility, stability, mechanical strength), establishing the contribution water makes to the properties can be elusive. Anhydrous (UA) and dihydrate (UAD) crystal forms of uric acid share a remarkably similar two-dimensional layer structure, though the presence or absence of water between the layers imparts these crystal forms with dramatically different mechanical properties. The quantitative and qualitative differences in how these two materials respond to uniaxial stress were investigated with nanoindentation and atomic force microscopy (AFM) imaging normal to the layer direction. Overall, UA was found to be both substantially harder and more brittle than UAD. Load–displacement curves and AFM images of the UA crystal surface postindent reveal slip planes in preferred crystallographic directions and oriented crack formati...

Free-running Sn precipitates: an efficient phase separation mechanism for metastable Ge1\u2212xSnx epilayers

The revival of interest in Ge1−xSnx alloys with x ≥ 10% is mainly owed to the recent demonstration of optical gain in this group-IV heterosystem. Yet, Ge and Sn are immiscible over about 98% of the composition range, which renders epilayers based on this material system inherently metastable. Here, we address the temperature stability of pseudomorphic Ge1−xSnx films grown by molecular beam epitaxy. Both the growth temperature dependence and the influence of post-growth annealing steps were investigated. In either case we observe that the decomposition of epilayers with Sn concentrations of around 10% sets in above ≈230 °C, the eutectic temperature of the Ge/Sn system. Time-resolved in-situ annealing experiments in a scanning electron microscope reveal the crucial role of liquid Sn precipitates in this phase separation process. Driven by a gradient of the chemical potential, the Sn droplets move on the surface along preferential crystallographic directions, thereby taking up Sn and Ge from the strained Ge1−xSnx layer. While Sn-uptake increases the volume of the melt, single-crystalline Ge becomes re-deposited by a liquid-phase epitaxial process at the trailing edge of the droplet. This process makes phase separation of metastable GeSn layers particularly efficient at rather low temperatures.

Anisotropy Induced Switching Field Distribution in High-Density Patterned Media

We present here micromagnetic study of variation of switching field distribution (SFD) in a high-density patterned media as a function of magnetic anisotropy of the system. We consider the manifold effect of magnetic anisotropy in terms of its magnitude, tilt in anisotropy axis and random arrangements of magnetic islands with random anisotropy values. Our calculation shows that reduction in anisotropy causes linear decrease in coercivity because the anisotropy energy tries to align the spins along a preferred crystallographic direction. Tilt in anisotropy axis results in decrease in squareness of the hysteresis loop and hence facilitates switching. Finally, the experimental challenges like lithographic distribution of magnetic islands, their orientation, creation of defects, etc. demanded the distribution of anisotropy to be random along with random repetitions. We have explained that the range of anisotropy values and the number of bits with different anisotropy play a key role over SFD, whereas the position of the bits and their repetitions do not show a considerable contribution.