Crystalline Directions Research Articles

Revealing the nanostructure of calcium phosphate coatings using HRTEM/FIB techniques

Herein, we report on the micro- and nanostructure of the calcium phosphate coating produced by pulsed laser deposition (PLD), using focused ion beam (FIB) lamella sample preparation and transmission electron microscopy (TEM) as the characterization technique. The initial selected area electron diffraction (SAED) data demonstrated the presence of hydroxyapatite (HA) over any other possible calcium phosphate crystalline structure and the polycrystalline nature of the coating. Moreover, the SAED analyses showed clear textured ring patterns coherent with the presence of a preferred orientation in the HA nano-crystal growth. The SAED data also indicated that the coating appears to be textured in the 〈002〉 crystalline direction. Dark-field images obtained using 002 as the working reflection showed a clear oriented crystal growth in columns, from bottom to top. These columns have a peculiar arrangement of nano-crystals since, in some cases, the preferred orientation appears to start at a certain distance from the substrate. Direct d-spacing measurements on high-resolution TEM images provided further proof of the presence of an HA nano-crystal structure. The reported data may be of interest in the future to adjust the microstructure of the HA coatings.

Nonlinearity of Degenerately Doped Bulk-Mode Silicon MEMS Resonators

We present an experimental study of conservative nonlinearities in bulk acoustic mode (bulk-mode) silicon MEMS resonators with degenerate doping. Three types of bulk acoustic mode resonators oriented in $\langle 110\rangle $ and $\langle 100\rangle $ silicon crystalline directions are used to analyze both linear and nonlinear elastic behavior of silicon with p- ( $N \cong 4.19e+18$ to $1.63e+20$ cm $^{-3}$ ) and n-type ( $N \cong 1.58e+18$ to $5.91e+19$ cm $^{-3}$ ) doping. For accurate characterization of the amplitude-dependent nonlinear stiffness constant, we employ two methods: amplitude-frequency response and ringdown measurement. Experimental results show that the nonlinear behavior of these resonators is dominated by material-dependent mechanical nonlinearities and strongly depends on the doping type and crystal orientation. These results are useful for understanding the material-induced nonlinear properties of doped silicon and design of MEMS resonators with desired dynamic behavior, and may provide a new avenue for tailoring resonator response characteristics. [2015-0301]

Crystal growth and structural analysis of hydroxyapatite nanofibers synthesized by the hydrothermal microwave-assisted method

Hydroxyapatite nanofibers were synthesized by the hydrothermal microwave-assisted method at 170°C using a mixture of CaNO3, KOH, K2HPO4 and glutamic acid as precursors. The reaction took place inside of pressurized Teflon vessels at 80 bars. The reaction time was varied from 15 up to 45min meanwhile temperature, microwave power and precursor composition were kept constant. Crystal phase composition, functional groups, vibrational modes, surface morphology and nanostructure were studied by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, Scanning Electron Microscopy and High Resolution Transmission Electron Microscopy, respectively. Hydroxyapatite nanofibers, with diameters in the order of nanometers and lengths of micrometers, were obtained; their surface seems to be smooth with well-defined shapes and edges. The length increased as the reaction time, so fibers as long as 70µm were obtained with a diameter the order of 450nm. According to the XRD patterns, preferential crystalline orientations in the [211] and [300] directions were observed, which are attributed to the glutamic acid concentration used in the reacting mixture as well as to the reaction time. SEM images showed a hexagonal needle-like morphology of nanofibers and studies by TEM and HRTEM indicate that those nanofibers grow in a preferential crystalline direction along the “c” axis of the HAp structure.

Time dependent gold nanoclusters and nanocrystals formation on BSA at solid-water and air-solid interfaces

A straight forward process of growing nanoclusters and in later stage from nanoclusters to nanocrystals growth is explored on BSA protein thin films using UV–Vis and photoluminescence spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and dynamic light scattering (DLS) techniques. It is observed that if different interaction times are provided between BSA thin films and HAuCl4 aqueous solutions of mM concentration, then initially formed tiny gold nanoclusters transform into nanocrystals of different shapes and sizes. From UV–Vis experiments growth of gold nanoclusters from AuCl4− ions at solid-air interface and in later stage from nanoclusters to nanocrystals growth at solid-air interface is confirmed by the presence of peaks at 320nm and 525nm respectively. If the nanoclusters embedded films are kept undisturbed in air and also in dark condition nearly for 30days then nanoclusters convert into nanocrystals. XRD analysis shows the formation of both gold nanoclusters and nanocrystals of faced-centered cubic lattice along (111) crystalline direction and AFM studies confirm the presence of triangular and spherical nanostructures. DLS technique is used to have the average sizes of the nanocrystals. Langmuir-like growth behavior is identified for BSA films induced gold nanoclusters and nanocrystals formation and it is found that both have nearly the same growth dynamics.

Strained HgTe plates grown on SrTiO3 investigated by micro-Raman mapping

HgTe plates have been grown by vapor phase epitaxy on (111) SrTiO3 substrates with a preferred orientation in the (111) crystalline direction, as indicated by x-ray diffraction. Examination of the plates using the micro-Raman mapping shows that the HgTe plates exhibit unusual strain patterns: the Raman peaks from the transverse-optical and longitudinal-optical phonons for the thicker (central) parts of the HgTe plates are at the same frequency as that of the bulk HgTe, while the Raman peaks for the thinner parts of the HgTe plates, which surround the thicker parts and can hardly be seen in a scanning electron microscope, are significantly larger in frequency. The full width at half maximum is smaller in the thinner areas than in the thicker parts. Theoretical analysis shows that the HgTe plates on SrTiO3 substrates suffer from compressive stress, and this may be sufficient to induce the three-dimensional topological insulator behavior in HgTe.

Structural, bonding, anisotropic mechanical and thermal properties of Al4SiC4 and Al4Si2C5 by first-principles investigations

The structural, bonding, electronic, mechanical and thermal properties of ternary aluminum silicon carbides Al4SiC4 and Al4Si2C5 are investigated by first-principles calculations combined with the Debye quasi-harmonic approximation. All the calculated mechanical constants like bulk, shear and Young’s modulus are in good agreement with experimental values. Both compounds show distinct anisotropic elastic properties along different crystalline directions, and the intrinsic brittleness of both compounds is also confirmed. The elastic anisotropy of both aluminum silicon carbides originates from their bonding structures. The calculated band gap is obtained as 1.12 and 1.04 eV for Al4SiC4 and Al4Si2C5 respectively. From the total electron density distribution map, the obvious covalent bonds exist between Al and C atoms. A distinct electron density deficiency sits between Al–C bond along c axis among Al4SiC4, which leads to its limited tensile strength. Meanwhile, the anisotropy of acoustic velocities for both compounds is also calculated and discussed.

Optical and Surface Characteristics of Mg-Doped GaAs Nanocrystalline Thin Film Deposited by Thermionic Vacuum Arc Technique

Magnesium (Mg) is the most promising p-type dopant for gallium arsenide (GaAs) semiconductor technology. Mg-doped GaAs nanocrystalline thin film has been deposited at room temperature by the thermionic vacuum arc technique, a rapid deposition method for production of doped GaAs material. The microstructure and surface and optical properties of the deposited sample were investigated by x-ray diffraction analysis, scanning electron microscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, ultraviolet–visible spectrophotometry, and interferometry. The crystalline direction of the deposited sample was determined to be (220) plane and (331) plane at 44.53° and 72.30°, respectively. The Mg-doped GaAs nanocrystalline sample showed high transmittance.

Piezoelectric 3D actuator for micro-manipulation based on [011]-poled PMN-PT single crystal

This paper sets forth to study and demonstrate the usability of an emerging piezoelectric material, called Lead Magnesium Niobate–Lead Titanate (PMN–PT), as a potential actuator for dexterous micro-manipulation. For this purpose, a generic structure called a duobimorph cantilever is studied. The proposed design takes advantage of a specific transverse elongation property of the PMN-PT poled in the [011] crystalline direction. A static 3D analytical model is presented, that links the applied voltage signals to the displacement and blocking force along each axis. Experimental characterization is conducted on 21.0×0.6×0.41mm3 cantilevers to validate the analytical 3D model. The transverse and longitudinal displacement range (not counting end-effectors) is over 460μm and 40μm respectively, while still generating high forces typically in the range of 16.5–48.2mN, values which are significantly higher than in the case of regular Lead Zirconate Titanate (PZT) ceramics. Results also show that PMN-PT has a good linearity, i.e. a very low hysteresis in the range of 2.8–6%, which is around 5 times lower than in the case of PZT. Next, in order to test the potential of this material for micro-manipulation devices, a pair of duobimorph cantilevers is used to build a microgripper. This device is used to conduct a 3D pick-and-place exercise without assistance from external motorized stages. A number of six useful Degrees of Freedom (DoF) are thus achieved, which is a novelty for cantilevered piezoelectric grippers. The results prove that PMN-PT is a great candidate for precise motion generation while still generating large displacement, which is a key factor for any Micro Electro Mechanical Systems (MEMS) implementation. Furthermore, PMN-PT of [011] type particularly can be used in complex, yet more compact, multi-DoF micro-manipulation applications.

Investigation of dislocation migration in substrate-grade CdZnTe crystals during post-annealing

The migration of dislocations in substrate-grade CdZnTe (CZT) single crystals during temperature gradient annealing under Cd/Zn vapor has been investigated. The etch pit density (EPD) and configuration of dislocations have been evaluated before and after annealing in CZT crystals with and without Cd-rich second phase (Cd-SP) particles, respectively. After Cd/Zn overpressure annealing, dislocation reduction in CZT crystals was observed. However, dislocation walls with 120° intervals along <211> crystalline direction were observed in the both types of CZT crystals. The formation of these dislocation walls can be attributed to the reaction of <110> dislocations. Moreover, it is considered that the release of the restored stress during annealing act as the domain driving force for dislocation migration, by comparing the variation of dislocation configuration in CZT crystals with and without Cd-SP particles.

Growth of gold nanoclusters and nanocrystals induced by lysozyme protein in thin film conformation

Structures and growth behavior of gold nanoclusters and nanocrystals have been explored on thin films of globular protein lysozyme by using UV–vis and photoluminescence spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). A simple and one-step environment friendly method has been used to grow nanocrystals on protein surface from HAuCl4 solution. It has been found that if different interaction times are provided between lysozyme films and HAuCl4 solution, then initially formed tiny gold nanoclusters on protein surface transform into nanocrystals with the passage of time. XRD analysis shows the formation of faced-centered cubic lattice along (111) crystalline direction and AFM images confirm the presence of circular, rod-like, triangular and hexagonal crystal structures. Langmuir-like growth behavior has been identified for both the gold nanoclusters and nanocrystals formation induced by the lysozyme films, however, nanocrystal growth is relatively slower than nanocluster.

Electrochemical Properties of Single Crystalline LiMn2O4 cathode Thin Films

Most commercial rechargeable lithium-ion batteries deliver energy densities far below their theoretical values. The key problem determining these limitations are the slow electrode process kinetics, low ionic diffusion and low electronic conductivity, particularly at the electrode-electrolyte interfaces. Mastering control of these interfaces is identified as a grand challenge in battery research, even more important than designing new electrode and electrolyte materials. All-solid-state microbatteries facilitate miniaturization, which will create more flexibility for the design of stand-alone microelectronic devices with enhanced applicability due to the avoided leakage risks. However, the successful application of all-solid-state microbatteries depends strongly on the enhancement of energy density and lifetime. The cycle-life and lifetime are dependent on the nature of the interfaces between the electrodes and electrolyte, whereas safety is a function of the stability of the electrode materials and interfaces. Therefore, perfect control on the interfacial properties between the electrodes and electrolyte is needed, but remains a great challenge. The crystal structure of the electrode, electrolyte matrix and their interfaces will determine the lithium diffusion mechanism within the complete structure, which will eventually be crucial for the battery performance. To study the variation in electrochemical properties for the different crystalline directions in the spinel crystal structure of the LiMn2O4 cathode material, thin films have been grown by pulsed laser deposition. Control over the deposition parameters (temperature, pressure, etc.) in combination with variation in crystal orientation of the Nb-doped SrTiO3 substrates (e.g. (001), (111) and (110)) enabled the formation of single crystalline LiMn2O4 thin films. The local variations in surface morphology and phase composition have been analyzed by Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS), while the variations in structural properties have been characterized by X-ray Diffraction (XRD). Electrochemical performance was studied by impedance spectroscopy. In our study we have investigated the relation between the electrochemical performance and the crystal orientation in LiMn2O4 cathode thin films. This provides important insight for the subsequent creation of solid-state cathode/electrolyte interfaces with controlled crystal structures for optimal interfacial properties.

Revealing the Origins of 3D Anisotropic Thermal Conductivities of Black Phosphorus

Black phosphorus (BP) has emerged as a direct bandgap semiconducting material with great application potentials in electronics, photonics, and energy conversion. Experimental characterization of the anisotropic thermal properties of BP at the micrometer scale is extremely challenging. This study reports measurement results of the anisotropic thermal conductivity of BP along three primary crystalline orientations, using a novel time‐resolved magneto‐optical Kerr effect. The thermal conductivity along the zigzag crystalline direction is 84–101 W m−1 K−1, nearly three times as large as that along the armchair direction (26–36 W m−1 K−1). The through‐plane thermal conductivity of BP ranges from 4.3 to 5.5 W m−1 K−1. This study performs first‐principles calculation to predict the phonon transport in BP along both in‐plane through‐plane directions, and identifies that the strong anisotropy of thermal transport in BP can be attributed to the structural‐asymmetry‐induced group velocity variations along different crystalline orientations, and the relaxation time variation induced by the direction of the applied temperature gradient. This work successfully unveils the fundamental mechanisms of anisotropic thermal transport along the three crystalline directions in BP, as demonstrated by the excellent agreement between the first‐principles‐based theoretical predictions and experimental characterizations on the anisotropic thermal conductivities of BP.

Carbon-nanotube electron-beam (C-beam) crystallization technique for silicon TFTs

We introduced a carbon-nanotube (CNT) electron beam (C-beam) for thin film crystallization and thin film transistor (TFT) applications. As a source of electron emission, a CNT emitter which had been grown on a silicon wafer with a resist-assisted patterning (RAP) process was used. By using the C-beam exposure, we successfully crystallized a silicon thin film that had nano-sized crystalline grains. The distribution of crystalline grain size was about 10 ∼ 30 nm. This nanocrystalline silicon thin film definitely had three crystalline directions which are (111), (220) and (311), respectively. The silicon TFTs crystallized by using a C-beam exposure showed a field effect mobility of 20 cm2/Vs and an on/off ratio of more than 107. The C-beam exposure can modify the bonding network of amorphous silicon with its proper energy.

Superior texture-controlled ZnO thin film using electrochemical deposition

Superior texture-controlled ZnO thin film is prepared using a low cost electrochemical deposition process at open atmospheric condition. FE-SEM shows that the surface morphology can be altered from nanorods to microplates by adding a chemical additive. XRD analysis confirms that the crystalline growth direction and texture can be controlled by changing the additive concentration during the deposition. Using the optimized ZnO based back reflector (BR) in single junction a-SiGe:H solar cell device, we have achieved external quantum efficiency (EQE) as high as 87% at ∼550nm; the EQE corrected short circuit current density (JSC) was 20.63mAcm−2 and solar cell efficiency was 7.73% for the single junction a-SiGe:H solar cell. It proves to us that the high quality ZnO thin films can be produced at much reduced cost using electrochemical deposition for solar cell and other optoelectronic applications.

RF Sputtering of ZnO (002) Thin Films on Top of 3C-SiC-on-Si (100) Substrates for Low Cost Piezoelectric Devices

In this paper, we deposited c-axis oriented ZnO thin films on top of epitaxial 3C-SiC/Si (100) substrates using RF magnetron sputtering. We investigated the effect of O2/Ar ratio and the post-annealing temperature. The grazing angle incident x-ray diffraction results show that ZnO thin-films are highly oriented along the (002) crystalline direction between the O2/Ar ratio of 30% to 50%, at the post-annealing temperature of 600 ˚C in nitrogen environment. The recipe from this work can be used to develop low cost piezoelectric devices such as an energy harvester.

Molecular Dynamics Study of Uniaxial Deformation in Perfect and Defective Aluminum

In materials, strength is usually controlled by the occurrence of grain boundaries, dislocations, voids, and other microstructural defects. molecular dynamics (MD) simulations with the embedded atom method potential have been employed to investigate the uniaxial tension and compression of single crystal aluminum with and without defects. In this paper, defect refers to a void or grain boundary. For perfect crystal aluminum, the ideal strengths and stress-strain curves obtained from the MD simulations agree well with those obtained by first principles calculations. The strength and ductility of Al decrease with the occurrence of void and grain boundaries. The effect of void size on the uniaxial stress-strain relations is investigated. Also, dislocation nucleation and emission from the void free surface are observed. In order to study the effect of a grain boundary, we generate a symmetric tilt grain boundary Σ5(310) and strain is imposed perpendicular to the boundary interface. We can clearly see that fracture starts in the grain boundary. The observed defect pattern is expected to provide some helpful insights into the damage mechanism of ductile materials at the microscale. The effects of temperature, strain rate, and crystalline direction on the ideal strength are also discussed.

Crystalline Direction Dependence of Spin Precession Angle and Its Application to Complementary Spin Logic Devices.

In a semiconductor channel, spin-orbit interaction is divided into two terms, Rashba and Dresselhaus effects, which are key phenomena for modulating spin precession angles. The direction of Rashba field is always perpendicular to the wavevector but that of Dresselhaus field depends on the crystal orientation. Based on the individual Rashba and Dresselhaus strengths, we calculate spin precession angles for various crystal orientations in an InAs quantum well structure. When the channel length is 1 μm, the precession angle is 550° for the [110] direction and 460° for the [1-10] direction, respectively. Using the two spin transistors with different crystal directions, which play roles of n- and p-type transistors in conventional charge transistors, we propose a complementary logic device.

Reversible changes induced by temperature in the spherulitic birefringence of nylon 6 9

Spherulitic morphologies of nylon 6 9 as an example of an even-odd nylon were studied by optical microscopy. A well-defined dependence on crystallization temperature was found. In particular, positively birefringent spherulites were characteristic for temperatures higher than 232 °C, low birefringence spherulites developed between 232 and 225 °C, positive ringed spherulites were found between 225 and 220 °C, and finally negative fibrillar spherulites were formed at temperatures lower than 220 °C. These optical properties were clearly different from those observed with even–even nylons (e.g., negative and positive birefringence for high and low temperatures, respectively), and may derive from the peculiar crystalline structures determined for even-odd nylons. Furthermore, low birefringence spherulites were characterized by a flat-on lamellar disposition and reversibility of the birefringence sign in the 80–120 °C temperature interval (positive and negative values at high and low temperatures, respectively).Real time WAXD profiles taken during heating and cooling processes demonstrated that different crystalline structures (named I, II and III) developed depending on the temperature and crystallization procedure of samples (e.g., from solution or from the melt state). Crystalline structures were characterized by the formation of hydrogen bonds along two crystalline directions in all cases, a peculiar arrangement that may account for the development of positive and negative spherulites in a more simple way than formulated for conventional polyamides having a single hydrogen bonding direction. DSC and FTIR data also showed a complex structural behavior with structural transitions in the 80–120 °C range, a region that corresponds to birefringence sign reversibility.

Correlation between microstructure and first-order-reversal-curve of Co nanowire arrays

Self-ordered Co nanowire arrays with different diameters of 30, 70 and 75 nm were grown into the anodized aluminum oxide templates by variation of deposition current between 5 and 55 mA. Increasing diameter of the nanowires enhances interaction between the nanowires, thereby decreasing the coercivity. However, increasing the deposition current decreases coercivity due to increase in deposition of light hydrogen ions. The microstructure investigations by x-ray diffraction patterns revealed that varying the diameter of deposited nanowires at constant current caused variation of preferential crystalline growth of the polycrystalline nanowires. Comparing first-order reversal curve (FORC) diagrams of the samples with different diameters and deposition currents revealed magnetization contribution of each FORC feature is proportional with a certain crystalline direction. Magnetic moment contribution of the domains with low and high coercivity and along the H = 0 axis is correlated with intensity of Co (100), (110), (002) and (101), respectively.

Influence of addition of indium and of post-annealing on structural, electrical and optical properties of gallium-doped zinc oxide thin films deposited by direct-current magnetron sputtering

Abstract In this study, both gallium-doped zinc oxide (GZO) and indium-added gallium-doped zinc oxide (IGZO) thin films were deposited on commercial glasses by magnetron dc-sputtering in argon atmosphere. The crystal structure, electrical conductivity and optical transmission of as-deposited as well as post-annealed thin films of both GZO and IGZO were investigated for comparison. A small amount of indium introduced into GZO thin films had improved their polycrystalline structure and increased their electrical conductivity by over 29%. All obtained GZO and IGZO thin films have strong [002] crystalline direction, a characteristic orientation of ZnO thin films. Although post-annealed in air at high temperatures up to 500 °C, IGZO thin films still had very low sheet resistance of 6.6 Ω/□. Furthermore, they had very high optical transmission of over 80% in both visible and near-infrared regions.