Crystallographic C-axis Orientation Research Articles

Effect of the crystallographic c-axis orientation on the tribological properties of the few-layer PtSe2

Two-dimensional (2D) transition metal dichalcogenides are potential candidates for ultrathin solid-state lubricants in low-dimensional systems owing to their flatness, high in-plane mechanical strength, and low shear interlayer strength. Yet, the effects of surface topography and surface chemistry on the tribological properties of 2D layers are still unclear. In this work, we performed a comparative investigation of nanoscale tribological properties of ultra-thin highly-ordered PtSe2 layers deposited on the sapphire substrates with the in-plane and out-of-plane crystallographic orientation of the PtSe2c-axis flakes, and epitaxial PtSe2 layers. PtSe2c-axis orientation was found to has an impact on the nanotribological, morphological and electrical properties of PtSe2, in particular the change in the alignment of the PtSe2 flakes from vertical (VA) to horizontal (HA) led to the lowering of the coefficient of friction from 0.21 to 0.16. This observation was accompanied by an increase in the root-mean-square surface roughness from 1.0 to 1.7 nm for the HA and VA films, respectively. The epitaxial films showed lower friction caused by lowering adhesion when compared to other investigated films, whereas the friction coefficient was similar to films with HA flakes. The observed trends in nanoscale friction is attributed to a different distribution of PtSe2 structure.

Investigation on microstructure transition and electrical behavior in (1-x)Bi4Ti3O12/xBi4.5K0.5Ti4O15 lead-free composites

The lead-free (1-x)Bi4Ti3O12/xBi4.5K0.5Ti4O15 composites (x = 0–0.4) have been synthesized by solid state reaction method. Evolution of microstructure and electric properties has been investigated with varying composition ratio. X-ray diffraction, Raman and IR absorption confirm the crystal transition tendency from orthorhombic to tetragonal phase. Evident texture along c-axis crystallographic orientation is observed in structure and morphology characteristics with x = 0.05 and 0.1. Through modulating the mole ratio of Bi4.5K0.5Ti4O15, coexistence of ferroelectric order and relaxor state is verified by the variation of ferroelectric hysteresis loops and polarization current density. This behavior can be attributed to the uncertain stability of discernable ferroelectric order induced by electric field among polar nanoregions during symmetry distoration. Temperature dependence of dielectric properties suggests an excellent thermal stability during a wide temperature range. Moreover, the detailed dielectric mechanism has been analyzed via various methods. Electrical conduction in grains and grain boundaries respectively plays the different role during the varying temperature process.

Direct-current piezoelectric nanogenerator based on two-layer zinc oxide nanorod arrays with equal c-axis orientation for energy harvesting

Direct current (DC) piezoelectric nanogenerators, that can harvest mechanical energy into electrical power, have been extensively studied for driving low-power electronic devices without pre-rectification. However, the output power of most developed DC generators is generally too low to satisfy the requirements of diverse micro-electronic devices. Herein, we design a high-output Schottky DC nanogenerator consisting of a top gold electrode, the piezoelectric layer of zinc oxide (ZnO) nanorod arrays grown on polarized polyacrylonitrile film, a bottom copper electrode with ZnO nanorod arrays, where the crystallographic c-axis orientation of both ZnO nanorod arrays trends to be the same. The fabricated DC nanogenerator shows the peak voltage and current density as high as 1.6 V and 7.2 μA/cm2, respectively, which are several times higher than that of a pure ZnO-based nanogenerator. It has been demonstrated to harvest ambient tiny mechanical energy, including human activity, impact/collision, and sound vibration, to power commercial microelectronics directly, and to charge a storage capacitor for driving a heath-care device. Such novel DC nanogenerator may have great application prospects in self-powered electronics, structure and human health monitoring, and many more places.

A comparative study on the mechanical and structural design of nacre in gastropod and bivalve molluscs

The lesson learnt from nature is presented through the examination of shells from four mollusc species (two gastropods and two bivalves). Individual aragonite tablets together with the organic network have been studied in terms of tablet dimensions, tablet stacking sequence, texture and the weight fraction of the organic present in these bio-composite shells. Mechanical properties viz. elastic modulus and hardness at the macro (bulk), micro (tablet) and nano (aragonite nanograins) levels have been determined using the experimental technique of nanoindentation together with the theoretical formulations offered by the Mori-Tanaka method and other such mechanical models for composites. XRD studies confirm the strong c-axis crystallographic orientation - aligned parallel to the direction of growth and crystallization of nacreous tablets, and fractography performed on the shell specimens suggests a difference in the stacking of individual aragonite layers for the gastropod and bivalve molluscan class, respectively. DSC and TG-DTA data helped in characterizing the organic material present in the shell specimens and in calculating the organic weight and volume fractions across species. Further to this, the DSC data revealed that the nature of the organic in the brick and mortar assembly of nacre, was similar in nature across the two molluscan classes reported in this study. EBSD studies performed on our system revealed a difference in crystallographic texture for the two different tablet stacking (columnar and sheet nacre). It has been shown experimentally that the shells of gastropods (columnar nacre) are orthotropic in nature, while the bivalve mollusc shells (sheet nacre) are anisotropic in nature.

Friction control by engineering the crystallographic orientation of the lubricating few-layer MoS2 films

Tribological properties of a surface are controlled by its topography and chemical structure as suggested by theoretical studies. However, the effect of crystallographic orientation on the tribological properties is still controversial. By tuning the heating rate during sulfurization of CVD Mo films, we prepared the few-layer MoS2 films with similar surface topography but different crystallographic c-axis orientation. This resulted in distinctly different tribological behavior both on the nanoscale and macroscale that can be attributed to different surface chemistry and surface electronic and phonon structures. In particular, horizontally aligned MoS2 sheets with c-axis oriented along the surface normal favor the weak van der Waals forces which resulted in a twice lower coefficient of friction comparing to the vertically aligned MoS2 sheets. These results demonstrate the possibility to optimize tribological properties by crystallographic orientation as required for the thin solid-state lubricating layers for extreme conditions.

Carbide-free one-zone sulfurization method grows thin MoS2 layers on polycrystalline CVD diamond

The last few decades faced on the fabrication of advanced engineering materials involving also different composites. Here, we report on the fabrication of few-layer molybdenum disulfide on top of thin polycrystalline diamond substrates with a high specific surface area. In the method, pre-deposited molybdenum coatings were sulfurized in a one-zone furnace at ambient pressure. As-prepared MoS2 layers were characterized by several techniques including grazing-incidence wide-angle X-ray scattering, atomic force microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. We found out that the initial thickness of Mo films determined the final c-axis crystallographic orientation of MoS2 layer as previously observed on other substrates. Even though it is well-known that Mo diffuses into diamond at elevated temperatures, the competing sulfurization applied effectively suppressed the diffusion and a chemical reaction between molybdenum and diamond. In particular, a Mo2C layer does not form at the interface between the Mo film and diamond substrate. The combination of diamond high specific surface area along with a controllable layer orientation might be attractive for applications, such as water splitting or water disinfection.

Deformation of high density polyethylene by dynamic equal-channel-angular pressing.

The effect of high strain rate and large shear deformation on the orientation of crystallites in high density polyethylene (HDPE) was investigated with dynamic equal-channel-angular pressing (D-ECAP). The HDPE samples were processed by two loading routes, route A and route C. Grid lines were used to obtain macroscopic strain distributions, which were substantiated by finite element modeling. Owing to the strain rate effect, the number of D-ECAP processing passes has a minor effect on the shear strain accumulations compared to ECAP. D-ECAP leads to a decrease in the thickness of the crystalline stem, and the crystallinity. After route-A or route-C D-ECAP processing, a new monoclinic phase emerges, and two types of crystallographic c-axis orientations appear: the crystallographic c-axis is approximately parallel to the flow direction (FD), or is tilted at approximately 55° clockwise away from FD. However, only one type of crystallographic c-axis orientation is detected after 2 passes of route-C D-ECAP. It is viable to utilize D-ECAP to control the structure and orientation of crystalline polymers, as a complement to ECAP and other processing techniques.

Enhanced magnetic and ferroelectric properties of textured multiferroic Bi5Ti3FeO15-CoFe2O4 ceramics prepared by plasma activated sintering

Textured multiferroic Bi5Ti3FeO15-CoFe2O4 (BTFO-CFO) ceramics were prepared via microwave hydrothermal synthesis followed by plasma activated sintering. BTFO-CFO ceramics with CFO nanoparticles distributed between the BTFO layers exhibit a high degree of texture (f =0.8) along the c-axis crystallographic orientation, which was confirmed by a combination of XRD, SEM and TEM. BTFO-CFO composite ceramics exhibit desirable magnetic and ferroelectric properties due to the ferromagnetic phase (CFO) and textured ferroelectric phase (BTFO). The current work provides a feasible method to fabricate textured multiferroic composite ceramics.

Microstructural and magnetic properties of self-biased strontium hexaferrite thick films by two-step sintering

SrFe12O19 hexaferrite thick films were prepared by tape casting method followed by a two-step sintering process. X-Ray diffractometer, field emission scanning electron microscope and vibrating sample magnetometer were used to investigate the microstructure and magnetic properties of samples. Results show that high density films with nanocrystalline grains, high crystallographic c-axis orientation of crystals perpendicular to the film plane with high squareness (Mr/Ms = 0.93) and moderate coercivity (Hc = 3,750 Oe) can be obtained with two-step sintering. Grains growth is controllable by this sintering method. The average grain size of the films strongly depends on final stage of sintering and quality of starting powders and ranging between 0.5 and 10 μm. The thick films with starting powders from coprecipitation method are denser with smaller grain size rather than those with starting powders from solid state reactions. This work reveals the feasibility of fabrication of thick hexaferrite films with a simple and effective method for next generation of self-biased planar microwave devices.

Bone mineral as an electrical energy reservoir

Mechanical stress in bone induces an electrical potential generated by piezoelectricity arising from displacement of collagen fibrils. Where and for how long the potential is stored in bone; however, are still poorly understood. We investigated the electrical properties of collagen fibrils and apatite minerals and found that bone, when polarized electrically by applying an external voltage, depolarizes by two mechanisms. Plots of thermally stimulated depolarization current show two significant peaks: one at 100°C, attributed to collagen fibrils because decalcified bone exhibits depolarization peak at 100°C, and the other at 500°C, attributed to apatite minerals because calcined bone exhibits depolarization peak at 500°C and has activation energy similar to that for synthesized apatite. The crystallographic c-axis orientation of calcined bone depends on the direction in which the bone is cut, either transverse or longitudinal, and strongly affects the polarization efficacy.

Crystal Orientation of GaN Nanostructures Grown on Al2O3 and Si(111) with a Zr Buffer Layer

We studied the crystallographic orientation of GaN nanostructures grown on Si(111) and Al2O3 substrates. We evaluated Zr metal as a novel alternative to conventional buffer layers such as AlN and ZnO. One-dimensional structures (nanorods and nanoneedles) were grown by hydride vapor phase epitaxy at 650 and 600 °C, and investigated using field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) analysis. FE-SEM images showed that the GaN nanorods had a uniform diameter along the growth direction. XRD results indicated that the nanostructures had a hexagonal crystal structure, and pole figure measurements revealed that GaN nanostructures grown on the Si(111) substrate had a stronger c-axis crystallographic orientation than those grown on the Al2O3 substrate.

Effect of Substrate and Surface Conditioning on Magnetic Properties and Texture of CoPt Hard Bias Films

Effect of growth surface on CoPt hard bias film performance has been systematically studied using different substrates and substrate surface conditioning. Films deposited on four different types of substrates show different levels of crystallographic quality and magnetic performance. Among them, the films deposited on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -coated substrates are the best, up to 400 Oe higher in coercivity and 0.05 higher in remnant squareness. Proper substrate surface conditioning by ion-beam depositions has been found to be effective in improving inplace <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis orientation; hence, magnetic properties of CoPt hard bias films. In addition, the deposition of a thin amorphous Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> film on substrates prior to the hard bias film deposition helps to promote nearly 100% inplane <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis crystallographic orientation.

Structural and luminescent properties of Er-doped ZnO films grown by metalorganic chemical vapor deposition

Structural and luminescent properties of Er-doped ZnO (ZnO:Er) grown by metalorganic chemical vapor deposition were investigated. In comparison to the undoped ZnO, the c-axis crystallographic orientation was deteriorated and the photoluminescence (PL) intensity of the band-edge became lower in the ZnO:Er. The weak band-edge PL in the ZnO:Er was due to the nonradiative recombination centers induced by the Er-doping processes. The band-edge PL intensity of ZnO:Er was drastically increased by the temperature annealing at 800°C in O2 ambient. The annealed ZnO:Er showed clear 1.54μm PL originating from the I13∕24→I15∕24 transitions of the Er3+ ions. These PL properties showed that the local arrangement of the Er ions was changed by the thermal annealing, resulting in a transformation from Er-related nonradiative centers in the as-grown sample to luminescent centers for 1.54μm PL in the annealed one.

Reduction of Transition Layer Thickness Showing Incoherent Switching Behavior in Single CoCrPt–SiO$_{2}$ Perpendicular Media

Magnetic switching behavior and activation volume (Vact) for CoCrPt-SiO2 single layer media on two intermediate layer (IL) types of conventional Ru1/Ru2 (IL1) and RuCr/Ru2 (IL2) are investigated. Ru1 and Ru2 indicate the Ru layer deposited at low and high pressure of Ar, respectively. Replacement of Ru1 with RuCr improves both crystallographic c-axis orientation (Deltathetas50) of <2.5deg and its crystallinity. This improvement helps achieve higher coercivity at thinner magnetic layer thickness (tmag). Both IL exhibit a similar trend in angular dependence of remanent coercivity (Hcr) but minimum value of Hcr 45deg/Hcr 0deg is observed at tmag= 15 nm for IL1 and 9 nm for IL2. The use of IL2 significantly reduces incoherent switching behavior at thinner tmag. Values of Vact for both IL decrease with increasing tmag but IL2 provides considerably higher Vact than IL1. In the media with IL2, higher Hc and more coherent switching behavior at thinner tmag can be understood by contributions of narrower Deltathetas50, thinner transition layer thickness showing incoherent switching behavior, and higher Vact.

Molecular Orientation and Degree of Crystallinity of Piezoelectric Poly(Vinylidene Fluoride) Films Exclusively in the β Phase

Oriented β -phase films were obtained by drawing exclusively β -phase films obtained from dimethylformamide solution crystallization at 60 °C with subsequent pressing. Wide angle X-ray diffraction and pole figure plots showed that drawing at 90°C and with R = 4 of originally β -phase films results in exclusively β -phase films with crystallinity degree of 49% and orientation factor of 0.885. For comparison, with the conventional drawing technique from α-phase material, films oriented at a ratio (R) of 5 still contain about 20% of phase α, a crystallinity degree of 40% and β -phase crystallographic c-axis orientation factor of 0.655.

Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of ∼2.8×10 −4 Ω cm was achieved for a film thickness of 1100 nm (sheet resistance ∼2.5 Ω/□), with a Hall mobility of 18 cm 2/V s and a carrier concentration of 1.3×10 21 cm −3. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed.

Thermal properties of a reaction hot pressed Mo ≤5Si 3C ≤1 ceramic

Mo ≤5Si 3C ≤1 was produced via reaction hot pressing. X-ray diffraction analysis revealed c-axis crystallographic orientation in the hot pressing direction. The orientation, combined with the inherent thermal expansion anisotropy of the structure, led to preferential microcracking and an overall anisotropic thermal expansion in hot pressed ceramics. Thermal conductivity was calculated for temperatures ranging from −120 to 500 °C from measured heat capacity and thermal diffusivity. Over that range, thermal conductivity increased from ∼9.5 to ∼21.5 W/m K. Electrical resistivity measurements allowed the separation of electrical and phononic contributions and revealed that the thermal conductivity of Mo ≤5Si 3C ≤1 was dominated by electron transport.

Effect of Oxygen Incorporation on Microstructure and Media Performance in CoCrPt–SiO$_{2}$ Perpendicular Recording Media

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The effect of oxygen incorporation on microstructure and media performance in CoCrPt–SiO<formula formulatype="inline"><tex>$_{2}$</tex></formula> films with various oxygen contents (OC) from 3 to 10 at% at different CoCrPt–SiO<formula formulatype="inline"><tex>$_{2}$</tex></formula> film thicknesses (<emphasis emphasistype="italic">t</emphasis><formula formulatype="inline"><tex>$_{\rm MAG}$</tex></formula>) from 2 to 27 nm is investigated. Nonuniform microstructure with less grain isolation close to Ru and more grain isolation at the top region is clearly seen. Higher density of stacking faults is found at the top region. A higher OC is needed to reduce the thickness of the initial layer with less grain isolation. The increase in coercivity and saturation magnetization with increasing OC is due to the formation of lower Cr and higher Pt-containing core grains caused by the preferred oxidation of Cr. These excess Pt atoms mostly align along the c-axis direction. The magnetocrystalline anisotropy constant enhanced by the excess Pt improves thermal stability factor but it is sensitive to temperature. Crystallographic c-axis orientation and magnetic anisotropy dispersion deteriorate with increasing OC but are independent of <emphasis emphasistype="italic">t</emphasis><formula formulatype="inline"><tex>$_{\rm MAG}$</tex></formula>. </para>

Sputtered zinc oxide coatings: structural study and application to the photoprotection of the polycarbonate

Zinc oxide (ZnO) was deposited by sputtering on silicon substrates. The structure and the composition of these deposits were studied by X-ray diffraction and Rutherford backscattering spectrometry, respectively. It was found that the thin films are polycrystalline with an hexagonal würtzite structure and have a crystallographic c-axis orientation perpendicular to the substrate surface. When the thickness of the coatings increases from 50 to 600 nm, the grain size and the density increase whereas the O/Zn atomic ratio decreases. Then ZnO coatings were deposited on bisphenol-A polycarbonate (PC) films in order to protect this polymer against the solar ultraviolet (UV) radiations. Samples were irradiated in artificial accelerated conditions with a polychromatic light (λ>300 nm). The rate of photodegradation was evaluated by infrared and UV–visible spectroscopies. ZnO coatings decrease the rate of oxidation and the rate of photo-yellowing of PC. The efficiency of the ceramic coating depends on its thickness and its structural properties. As a conclusion, ZnO coatings can improve the long-term behaviour of PC for outdoor applications.

Sputtered zinc oxide coatings: structural study and application to the photoprotection of the polycarbonate

Zinc oxide (ZnO) was deposited by sputtering on silicon substrates. The structure and the composition of these deposits were studied by X-ray diffraction and Rutherford backscattering spectrometry, respectively. It was found that the thin films are polycrystalline with an hexagonal würtzite structure and have a crystallographic c-axis orientation perpendicular to the substrate surface. When the thickness of the coatings increases from 50 to 600 nm, the grain size and the density increase whereas the O/Zn atomic ratio decreases. Then ZnO coatings were deposited on bisphenol-A polycarbonate (PC) films in order to protect this polymer against the solar ultraviolet (UV) radiations. Samples were irradiated in artificial accelerated conditions with a polychromatic light (λ>300 nm). The rate of photodegradation was evaluated by infrared and UV–visible spectroscopies. ZnO coatings decrease the rate of oxidation and the rate of photo-yellowing of PC. The efficiency of the ceramic coating depends on its thickness and its structural properties. As a conclusion, ZnO coatings can improve the long-term behaviour of PC for outdoor applications.