Random In-plane Orientation Research Articles

Optical Alignment and Optical Orientation of Excitons in CdSe/CdS Colloidal Nanoplatelets.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron-hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Ferroelectric Al1−xBxN thin films integrated on Si

Ferroelectric Al0.93B0.07N thin films are prepared (100) Si substrates. The necessary c-axis out-of-plane orientation to observe macroscopic ferroelectric switching was achieved by implementing an initial Ar/N2 plasma treatment, followed by a thin layer of AlN to initiate the desired texture and a 150 nm W layer. The plasma treatment facilitates crystallinity enhancement of the AlN template layer, allowing for subsequent growth of highly oriented W and Al0.93B0.07N layers. The W layer exhibits random in-plane orientation and exclusive (110) out-of-plane orientation with a rocking curve width of 1.4°. When grown on these W surfaces, 175 nm thick Al0.93B0.07N films exhibit random in-plane orientation and exclusive (001) texture with rocking curve full-width-half-max values of 1.6° and RMS roughness values less than 1 nm. Polarization hysteresis measurements show robust hysteresis with coercive field values of 5.4 MV/cm and remanent polarization values of 136 μC/cm2. XPS depth profile analysis suggests that the plasma treatment converts the existing native oxide to a nitrogen rich oxynitride with approximate composition Si3O0.5N3.67. Cross-sectional TEM reveals that the oxynitride interlayer is amorphous and ∼3.4 nm thick, more than double the native oxide thickness measured by multiwavelength ellipsometry, implying that (oxy)nitride growth continues after conversion of the native oxide. This new family of ferroelectric wurtzites is interesting from an integration perspective given their chemical compatibility with mainstream semiconductors. Developing synthesis routes that promote needed texture while preserving compatible processing windows is an important step toward practical integration.

Template and Temperature-Controlled Polymorph Formation in Squaraine Thin Films.

Controlling the polymorph formation in organic semiconductor thin films by the choice of processing parameters is a key factor for targeted device performance. Small molecular semiconductors such as the prototypical anilino squaraine compound with branched butyl chains as terminal functionalization (SQIB) allow both solution and vapor phase deposition methods. SQIB has been considered for various photovoltaic applications mainly as amorphous isotropic thin films due to its broad absorption within the visible to deep-red spectral range. The two known crystalline polymorphs adopting a monoclinic and orthorhombic crystal phase show characteristic Frenkel excitonic spectral signatures of overall H-type and J-type aggregates, respectively, with additional pronounced Davydov splitting. This gives a recognizable polarized optical response of crystalline thin films suitable for identification of the polymorphs. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spin-casted thin films depending on subsequent thermal annealing. By contrast, upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene, and gold, the polymorph expression depends basically on the choice of growth substrate. The same pronounced out-of-plane orientation is adopted in all crystalline cases, but with a surface templated in-plane alignment in case of crystalline substrates. Strikingly, the amorphous isotropic thin films obtained by vapor deposition cannot be crystallized by thermal postannealing, which is a key feature for the spin-casted thin films, here monitored by polarized in situ microscopy. Combining X-ray diffraction, atomic force microscopy, ellipsometry, and polarized spectro-microscopy, we identify the processing-dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.

Low-pressure CVD of (Tix,W1-x)Ny from WF6, TiCl4 and NH3

In this work chemical vapour deposited (CVD) coatings of (Tix,W1-x)Ny from TiCl4, WF6, NH3 and Ar were investigated. This coating material has previously been deposited using other vacuum techniques but no publication has so far demonstrated CVD of (Tix,W1-x)Ny. The studied (Tix,W1-x)Ny coatings had a metallic molar ratio (Ti:W) close to 2:1 and 1:1, and were slightly over-stoichiometric with regard to N. The coatings appeared homogeneous and crystallised in a rock salt structure on an α-Al2O3 substrate. The cell parameter varied between 4.16 and 4.23 Å as a function of the deposition conditions, ranging from a pure TiNx to a pure WNx coating. The texture in the normal direction was 〈100〉 for the TiNx and (Tix,W1-x)Ny coatings and 〈111〉 for WNx. Electron backscattered diffraction (EBSD) results showed that a strong correlation to the substrate existed but random in-plane orientation was also present. The microstructure showed columnar grains with well defined facets growing. Adding a mixture of TiCl4 and WF6 to produce (Tix,W1-x)Ny did increase the grain size significantly when compared to the case when only one metal precursor was present. The down-stream thickness profile, using only WF6 and NH3, displayed mass transport control behaviour, with the coating thickness converging to zero within the deposition zone. Using only TiCl4 on the other hand showed a uniform deposition profile, the signs of a surface kinetics controlled process.

Influence of Yttria Segregation and Platinum Nanoparticles on the Ionic Conductivity of Yttria-Stabilized Zirconia (YSZ) Films Under Hydrogen Exposure

Yttria-stabilized zirconia (YSZ) films have high ionic conductivities over the range 400-800oC which make them useful for solid oxide fuels cells, oxygen sensors, and other sensing applications. Adding catalytic particles to the YSZ surface can induce reactivities and associated conductivity changes for gases other than oxygen. In this work, the surface reactivity towards hydrogen was investigated on thin films with a 8-YSZ composition (8at.% Y2O3 - 92at.% ZrO2) that were deposited onto Langasite (La3Ga5SiO14) substrates using RF-magnetron sputtering in a 5% O2 - 95% Ar gas mixture. Films were grown to thicknesses up to 200 nm using different growth temperatures (30 - 7000C), deposition rates (0.03 - 0.07 nm/s), and substrate bias (-300 - +300 V). To create Pt nanoparticles on the YSZ film surface, Pt was deposited in-situ via e-beam evaporation to equivalent Pt thicknesses of 1 - 10 nm with the YSZ film held at either 25oC or 400oC and then subsequently annealed in an air furnace up to 1000oC to cause Pt agglomeration and nanoparticle formation. To assess steady-state surface reactivity to hydrogen in the range 400 - 600oC after the 1000oC treatments, film ionic conductivities were measured with electrochemical impedance spectroscopy (EIS) using an impedance/gain-phase analyzer after long annealing times in air, in pure N2, or in a 4% H2 - 96% N2 mixture within the furnace tube. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also used to analyze the characteristics of the as-deposited and annealed YSZ and Pt/YSZ films.YSZ films were synthesized with a variety of crystallographic textures, nanomorphology, and composition using the deposition parameters discussed above. Films deposited at 25, 400, or 700oC all have (111) cubic YSZ preferred texture with random in-plane orientation on the Langasite substrate. The smoothest films were produced using a rate of 0.07 nm/s at 400oC. However, films deposited using the other growth conditions contain significant nano-roughness and hillocks generated by film stress as evidenced by Williamson Hall XRD measurements. In films deposited at 25oC there is also a minor (200) grain population in addition to (111). To create Pt nanoparticles on these various film surfaces, Pt was evaporated with the YSZ film surface being held at either 25oC or 400oC. Pt layers (1 - 10 nm) were uniform at 25oC whereas the Pt started to agglomerate during deposition at 4000C.Following deposition, all the YSZ and Pt/YSZ films were annealed in air up to 1000oC and held for 1 hour prior to measuring conductivity and hydrogen reactivity over the range 400-600oC. This 1000oC annealing treatment was performed to (i) promote Pt agglomeration at high temperature to form individual Pt islands, (ii) stabilize the YSZ nanostructure in terms of film stress and grain size, and (iii) obtain high temperature ionic conductivity measurements over a range of temperatures. XPS indicates that post-deposition air annealing causes significant yttria segregation to the YSZ surface region as evidenced by increases in the Y3d/Zr3d photoelectron peak area ratio upon annealing up to 1000oC. This yttria enrichment decreases the film ionic conductivity and decreases the activation energy for ion transport compared to the as-deposited films. EIS experiments performed in air, N2 or 4%H2-N2 between 400-600oC indicate that the YSZ films without Pt that are most sensitive to hydrogen are those deposited at 400-700oC using a rate of 0.03 nm/s which contain rough surfaces, a population of (200) grains, and minimal yttria surface enrichment. The EIS results also show that YSZ films decorated with Pt nanoparticles all exhibit varying baseline conductivities and sensitivities to hydrogen with a magnitude dependent on the Pt nanoparticle distribution as well as film nanostructure. These YSZ and Pt/YSZ films hold promise as hydrogen sensing films that can be incorporated onto a variety of sensing platforms for H2 gas detection and management. Figure 1

Strong Reinforcement Effects in 2D Cellulose Nanofibril-Graphene Oxide (CNF-GO) Nanocomposites due to GO-Induced CNF Ordering.

Nanocomposites from native cellulose with low 2D nanoplatelet content are of interest as sustainable materials combining functional and structural performance. Cellulose nanofibril-graphene oxide (CNF-GO) nanocomposite films are prepared by a physical mixing-drying method, with focus on low GO content, the use of very large GO platelets (2-45μm) and nanostructural characterization using synchrotron x-ray source for WAXS and SAXS. These nanocomposites can be used as transparent coatings, strong films or membranes, as gas barriers or in laminated form. CNF nanofibrils with random in-plane orientation, form a continuous non-porous matrix with GO platelets oriented in-plane. GO reinforcement mechanisms in CNF are investigated, and relationships between nanostructure and suspension rheology, mechanical properties, optical transmittance and oxygen barrier properties are investigated as a function of GO content. A much higher modulus reinforcement efficency is observed than in previous polymer-GO studies. The absolute values for modulus and ultimate strength are as high as 17 GPa and 250 MPa at a GO content as small as 0.07 vol%. The remarkable reinforcement efficiency is due to improved organization of the CNF matrix; and this GO-induced mechanism is of general interest for nanostructural tailoring of CNF-2D nanoplatelet composites.

Growth and Characterization of Metastable Hexagonal Nickel Thin Films via Plasma-Enhanced Atomic Layer Deposition

There is a great interest in various branches of the advanced materials industry for the development of novel methods (and improvements to existing ones) for the deposition of conformal ultrathin metallic films. In most of these applications, like enhanced solar absorbers and microelectronics, achieving the capacity to deposit a conformal thin film on a three-dimensional structure is an important condition. Plasma-enhanced atomic layer deposition (ALD) is known for its potential for growth of conformal thin films with a precise control over the thickness and its capability for deposition at relatively low temperatures (below 500 °C). This study evaluates the potential of plasma-enhanced ALD for growth of conformal nickel thin films, using bis(ethylcyclopentadienyl)nickel and nitrogen/hydrogen plasma as precursors. A comprehensive analysis of the structure, composition, and physical properties of the films was performed. The results indicate that conformal nickel films with low levels of impurity were successfully deposited on sapphire. The films had a roughness of Ra = 1.5 nm and were seen to be under strain. The deposited nickel had a hexagonal crystal structure, with a random in-plane orientation of the grains, while the grains had their c-axes oriented along the normal to the interface. These results pave the way for conformal low-temperature deposition of high-quality nickel thin films on three-dimensional structures.

Structure characterization and magnetic properties of barium hexaferrite films deposited on 6H-SiC with random in-plane orientation

Abstract Oriented barium hexaferrite films grown on wide band-gap semiconductor substrates (such as SiC) are promising candidates for high-power microwave integrated devices. In this work, BaFe 12 O 19 (BaM) films with c -axis randomly distributed in the film plane were prepared by direct current magnetron sputtering on a -plane 6H-SiC substrates. An insight into the orientation relationship and the epitaxial-like growth mechanism demonstrates that ( 11 2 ¯ 0 ) planes of BaM have been grown on ( 11 2 ¯ 0 ) 6H-SiC substrates. The random alignment of [0001] axis within the film plane was revealed by X-ray diffractions, X-ray pole figures, polarized Raman spectra and magnetic hysteresis loops. The BaM films on 6H-SiC substrates exhibited a large remanence ratio, M r / M s , of about 0.64 along in-plane axis and a relatively small one of ~0.2 for the out-of-plane hard axis, showing the potential of application in self-biased microwave devises.

Effect of precursor solutions on the structural and optical properties of sprayed NiO thin films

Nickel oxide thin films were deposited by a simple and low-cost spray pyrolysis technique using three different precursors: nickel nitrate, nickel chloride, and nickel acetate on corning glass substrates. X-ray diffraction show that the NiO films are polycrystalline and have a cubic crystal structure, although predominantly with a preferred 111-orientation in the growth direction and a random in-plane orientation. The deconvolution of the Ni 2p and O 1s core level X-ray photoelectron-spectra of nickel oxides produced by using different precursors indicates a shift of the binding energies. The sprayed NiO deposited from nickel nitrate has an optical transmittance in the range of 60–65% in the visible region. The optical band gap energies of the sprayed NiO films deposited from nickel nitrate, nickel chloride and nickel acetate are 3.5, 3.2 and 3.43eV respectively. Also, the extinction coefficient and refractive index of NiO films have been calculated from transmittance and reflectance measurements. The average value of refractive index for sprayed films by nickel nitrate, nickel chloride and nickel acetate are 2.1, 1.6 and 1.85 respectively. It is revealed that the band gap and refractive index of NiO films by using nickel nitrate corresponds to the commonly reported values. We attribute the observed behavior in the optical band gap and optical constants as due to the change of the Ni/O ratio.

Molecular beam epitaxy of 2D-layered gallium selenide on GaN substrates

Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report high-quality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-Oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350–450 °C with complete surface coverage. Higher growth temperature (575 °C) resulted in the formation of single-crystalline ε-GaSe triangular domains with six-fold symmetry confirmed by in-situ reflection high electron energy diffraction and off-axis x-ray diffraction. A two-step growth method involving high temperature nucleation of single crystalline domains and low temperature growth to enhance coalescence was adopted to obtain continuous (002)-oriented GaSe with an epitaxial relationship with the substrate. While six-fold symmetry was maintained in the two step growth, β-GaSe phase was observed in addition to the dominant ε-GaSe in cross-sectional scanning transmission electron microscopy images. This work demonstrates the potential of growing high quality 2D-layered materials using molecular beam epitaxy and can be extended to the growth of other transition metal chalcogenides.

Morphology and crystal texture in tilted columnar micro-structured titanium thin film coatings

Titanium thin films deposited at various polar incidence angles by electron-beam evaporation are investigated by electron microscopy and X-ray diffraction pole-figures. The resulting film morphology and texture are shown to be strongly affected by the vapor flux direction. At normal incidence deposition angle, a fiber texture forms, with the c-axis of Ti crystallites aligned parallel to the surface normal with random in-plane (azimuthal) orientation. At off-normal deposition angles, porous morphologies with inclined micro-columns form. The texture of these films is an inclined fiber texture, where the texture tilt angle increases with increasing deposition angle. Biaxial texture is observed for films grown at polar incidence angles larger than 60°. The micro-column tilt axis, and the c-axis of the crystallites, both lie in the deposition plane but at different polar angles; the mechanism behind micro-column tilt is due to the shadowing effect, whereas the fiber texture tilt angle and the biaxial texture orientations of the films result from evolutionary crystal growth, determined by the kinetic growth habits of crystallites.Changes in the morphology of Ti films deposited at a vapor incidence angle of 84° and nominal thickness of 3μm at substrate temperatures corresponding to homologous temperatures of 0.15, 0.4, and 0.5 are studied, and compared to the Mukherjee and Gall structure zone model, proposed for high temperature glancing angle deposition. The observed morphology changes are consistent with their model of competition between shadowing length and adatom diffusion length. All the films were grown with homologous temperature lower than 0.5, and formed porous and inclined micro-column morphology, resulting from the dominance of shadowing over adatom surface diffusion.

Applications for carbon fibre recovered from composites

Commercial operations to recover carbon fibre from waste composites are now developing and as more recovered fibre becomes available new applications for recovered fibre are required. Opportunities to use recovered carbon fibre as a structural reinforcement are considered involving the use of wet lay processes to produce nonwoven mats. Mats with random in-plane fibre orientation can readily be produced using existing commercial processes. However, the fibre volume fraction, and hence the mechanical properties that can be achieved, result in composites with limited mechanical properties. Fibre volume fractions of 40% can be achieved with high moulding pressures of over 100 bar, however, moulding at these pressures results in substantial fibre breakage which reduces the mean fibre length and the properties of the composite manufactured. Nonwoven mats made from aligned, short carbon fibres can achieve higher fibre volume fractions with lower fibre breakage even at high moulding pressure. A process for aligning short fibres is described and a composite of over 60% fibre volume fraction has been manufactured at a pressures up to 100 bar with low fibre breakage. Further developments of the alignment process have been undertaken and a composite of 46% fibre volume fraction has been produced moulded at a pressure of 7 bar in an autoclave, exhibiting good mechanical properties that compete with higher grade materials. This demonstrates the potential for high value applications for recovered carbon fibre by fibre alignment.

Selective-area growth and magnetic characterization of MnAs/AlGaAs nanoclusters on insulating Al2O3 layers crystallized on Si(111) substrates

We report on selective-area metal-organic vapor phase epitaxy and magnetic characterization of coupled MnAs/AlGaAs nanoclusters formed on thin Al2O3 insulating layers crystallized on Si(111) substrates. Cross-sectional transmission electron microscopy reveals that poly-crystalline γ-Al2O3 grains are formed after an annealing treatment of the amorphous Al2O3 layers deposited by atomic layer deposition on Si(111) substrates. The ⟨111⟩ direction of the γ-Al2O3 grains tends to be oriented approximately parallel to the ⟨111⟩ direction of the Si substrate. We observe that hexagonal MnAs nanoclusters on AlGaAs buffer layers grown by selective-area metal-organic vapor phase epitaxy on partially SiO2-masked Al2O3 insulator crystallized on Si(111) substrates are oriented with the c-axis along the ⟨111⟩ direction of the substrates, but exhibit a random in-plane orientation. A likely reason is the random orientation of the poly-crystalline γ-Al2O3 grains in the Al2O3 layer plane. Magnetic force microscopy studies at room temperature reveal that arrangements of coupled MnAs nanoclusters exhibit a complex magnetic domain structure. Such arrangements of coupled MnAs nanoclusters may also show magnetic random telegraph noise, i.e., jumps between two discrete resistance levels, in a certain temperature range, which can be explained by thermally activated changes of the complex magnetic structure of the nanocluster arrangements.

Liquid-target reactive magnetron sputter epitaxy of High quality GaN(0001̄) nanorods on Si(111)

Direct current magnetron sputter epitaxy with a liquid Ga sputtering target has been used to grow single-crystal GaN(0001̄) nanorods directly on Si(111) substrates at different working pressures ranging from 5 to 20mTorr of pure N2. The as-grown GaN nanorods exhibit very good crystal quality from bottom to top without stacking faults, as determined by transmission electron microscopy. The crystal quality is found to increase with increasing working pressure. X-ray diffraction results show that the rods are highly (0001)-oriented. The nanorods exhibit an N-polarity, as determined by convergent beam electron diffraction methods. Sharp and well-resolved 4K photoluminescence peaks at ~3.474eV with a FWHM ranging from 1.7meV to 35meV are attributed to the intrinsic GaN band edge emission and corroborate the superior structural properties of the material. Texture measurements reveal that the rods have random in-plane orientation when grown on Si(111) with native oxide, while they have an in-plane epitaxial relationship of GaN[112̅0]//Si[11̄0] when grown on substrates without surface oxide.

Fibril orientation redistribution induced by stretching of cellulose nanofibril hydrogels

The mechanical performance of materials reinforced by cellulose nanofibrils is highly affected by the orientation of these fibrils. This paper investigates the nanofibril orientation distribution of films of partly oriented cellulose nanofibrils. Stripes of hydrogel films were subjected to different amount of strain and, after drying, examined with X-ray diffraction to obtain the orientation of the nanofibrils in the films, caused by the stretching. The cellulose nanofibrils had initially a random in-plane orientation in the hydrogel films and the strain was applied to the films before the nanofibrils bond tightly together, which occurs during drying. The stretching resulted in a reorientation of the nanofibrils in the films, with monotonically increasing orientation towards the load direction with increasing strain. Estimation of nanofibril reorientation by X-ray diffraction enables quantitative comparison of the stretch-induced orientation ability of different cellulose nanofibril systems. The reorientation of nanofibrils as a consequence of an applied strain is also predicted by a geometrical model of deformation of nanofibril hydrogels. Conversely, in high-strain cold-drawing of wet cellulose nanofibril materials, the enhanced orientation is promoted by slipping of the effectively stiff fibrils.

Friction behaviour of TiAlN films around cubic/hexagonal transition: A 2D grazing incidence X-ray diffraction and electron energy loss spectroscopy study

The properties at different scales of Ti1−xAlxN films deposited by reactive magnetron sputtering from TiAl sintered (S) targets produced by powder metallurgy are compared with those of a set of films previously deposited in the same conditions from mosaic targets (M) made of pure Ti and Al metals. For compositions close to the hcp/fcc transition (around x=0.6), the friction behaviour, growth directions and organization of crystallized domains are found to be sensitive to the type of target used. The resistance to crack creation is higher for Ti0.54Al0.46N (S) and Ti0.38Al0.62N (S) than for Ti0.50Al0.50N (M) and Ti0.32Al0.68N (M). From the measurement of mechanical properties, toughness, and wear volumes and from the observation of wear tracks, it is found that films prepared from sintered targets exhibit a better wear resistance. Grazing incidence X-ray diffraction and electron energy loss spectroscopy in Transmission Electronic Microscopy are used to investigate the long- and short-range orders within the films. The morphology of Ti0.54Al0.46N (S) film can be considered as an array of crystalline domains having reciprocal-space vectors 111 and 200 directed along the meridian but with random in-plane orientation. Ti0.38Al0.62N (S) Al-rich film presents a random orientation of the crystalline domains whereas Ti0.32Al0.68N (M) deposited from composite targets exhibits a well-oriented fibrillar structure. The N K-edge Electron Energy Loss Near Edge Spectra are discussed with previous results of Extended X-ray Absorption Fine Structure Spectroscopy, which has evidenced different values of Al–N and Ti–N bond lengths, either octahedral (cubic-like) or tetrahedral (hexagonal-like) within Ti0.50Al0.50N (M) and Ti0.32Al0.68N (M) films. For similar compositions, films deposited from sintered alloys contain more nitrogen atoms in octahedral cubic-like environment than coatings made from mosaic targets, which could explain their better resistance to cracking, higher hardness and better wear resistance.

Yttria-stabilized zirconia buffered silicon to optimize in-plane electrical conductivity of [Ca2CoO3]0.62[CoO2] thin films

The monolithic integration of thermoelectric generators and magnetoresistive functionality on the basis of misfit cobaltate [Ca2CoO3]0.62[CoO2] thin films into silicon technology is a prerequisite for their application in miniaturized electric circuits. Here, we report on [Ca2CoO3]0.62[CoO2] thin films grown by pulsed laser deposition on (001)-silicon with a thin epitaxial yttria-stabilized zirconia (YSZ) buffer layer. X-ray diffraction and cross-sectional high resolution transmission electron microscopy analysis reveal that high quality c-axis oriented heteroepitaxial [Ca2CoO3]0.62[CoO2] films with a 12-fold in-plane rotational symmetry can be grown, which exhibit remarkable lower electrical resistivity compared to those with random in-plane orientation. This result is explained by energetically preferred epitaxial growth directions of the pseudo hexagonal [CoO2] sublayer in monoclinic [Ca2CoO3]0.62[CoO2] onto the cubic (001)-YSZ surface leading to a highly symmetric in-plane mutual orientation of the charge transporting CoO2 sublayer domains.

Effect of a ZnO buffer layer on the properties of epitaxial ZnO:Ga films deposited on c-sapphire substrate

Bi-layer ZnO films with 2wt.% Al (AZO; ZnO:Al) and 4wt.% Ga-doped (GZO; ZnO:Ga) were deposited on the non-buffered and buffered c(0001)-sapphire(Al2O3) substrates respectively by Pulsed Laser Deposition (PLD). The effect of a ZnO buffer layer on the crystallinity and electrical properties of the GZO thin films was investigated. X-ray Diffraction (XRD) peaks and High Resolution Transmission Electron Microscopy (HRTEM) studies showed that the GZO thin film on a buffered substrate was epitaxially grown with an orientation relationship of (0001) [112¯0]GZO||(0001)[112¯0]Al2O3. However, GZO thin film on a non-buffered substrate was grown as a monocrystalline hexagonal wurtzite phase with c-axis preferred, out-of-plane orientation, and random in-plane orientation. The electrical resistivity of the GZO thin films was improved by introducing a ZnO buffer layer from 2.2×10-4Ωcm to 1.2×10-4Ωcm, respectively. In a word, it was found in the films that more preferred c-axis orientation texture and reduction of the defects such as stacking faults and dislocations, with introducing a ZnO buffer layer.It was seen that the ZnO buffer layer had a great influence on the orientation and defect density of GZO thin films from X-ray Diffraction (XRD) peaks and High Resolution Transmission Electron Microscopy (HRTEM) images.

Influence of composite electrical properties on the VHF–UHF electromagnetic shielding characteristics of polyethylene–carbon nanoparticle composites

The influence of composite electrical properties on the plane-wave electromagnetic shielding effectiveness (EM SE) of carbon nanoparticle/polyethylene nanocomposites is reported in the VHF and UHF bands (30 MHz–1.5 GHz). Two highly graphitic carbon nanoparticles (heat-treated Pyrograf® III PR-19 carbon nanofibers and CheapTubes® multi-walled carbon nanotubes) were dispersed in a linear low density polyethylene (LLDPE) matrix to produce a nominally random in-plane particle orientation. For a concentration of 10 vol% nanoparticles and a sample thickness of 2.5 mm, EM SE values of 24 dB and 22 dB were obtained for MWNT HT and PR-19 HT nanocomposites, respectively. At a high concentration of 40 vol%, EM SE values as high as 68 dB and 55 dB were obtained for the PR-19 HT and MWNT HT nanocomposites, respectively. Because such nanocomposites possess only moderate electrical conductivity, the composite permittivity was also used to predict the plane wave EM SE and its components. Based on the material properties of the nanocomposites, the predicted values of EM SE were found to be consistent with the experimental values.

On the role of Fe in the growth of single crystalline heteroepitaxial Au thin films on sapphire

Thin Au–Fe bilayers were deposited on c-plane sapphire (α-Al2O3) substrates at room temperature employing the electron beam deposition method. The layers were found to be single crystalline (i.e. the grain size was much larger than the film thickness), with a [111] and [110] texture for Au and Fe, respectively, and strong heteroepitaxy to the substrate. Au films deposited on sapphire and Au–Fe bilayers deposited on amorphous SiO2 were polycrystalline and exhibited random in-plane orientation of the grains. The effects of Fe and the Fe–sapphire interface on the microstructure of the Au film were investigated and discussed in terms of the orientation relationships, in-plane strain, interface energy and adhesion. The microstructures of annealed and as-deposited films were very similar, indicating that as-deposited films are close to thermodynamic equilibrium in terms of the orientation relationship with the substrate. This is uncommon for non-equilibrium thin film deposition processes, which usually result in a high density of defects in the as-deposited films.