High-Temperature Healing of Cracklike Flaws in Mg- and Ca-Ion-Implanted Sapphire

Controlled-geometry voids were introduced into Mg-implanted and Ca-implanted sapphire substrates using microfabrication techniques and ion beam etching, and were subsequently transferred to an internal interface by hot-pressing. The morphological evolution of cracklike and channel-like defects in response to anneals at 1700°C was studied. The healing behavior of defects in the Ca- and Mg-ion-implanted samples differs significantly. Mg additions appear to reduce the directional dependence of the healing characteristics and thus homogenize the evolution. Destabilization of the basal plane by Ca may contribute to the rapid healing of cracks oriented parallel to the basal plane. The healing characteristics of pore channels in Ca-implanted samples indicate a strong residual energetic barrier to healing stemming from surface energy anisotropy. Despite this, defects in Ca-implanted sapphire healed more rapidly than crystallographically and geometrically identical defects in Mg-implanted sapphire. Thus, the results suggest that Ca additions increase transport rates sufficiently to more than compensate for the relatively higher energetic barriers to pore channel breakup.     

Multilayer formation via spinodal decomposition in TiO2-VO2 epitaxial films on sapphire substrates

We present multilayer formation via spinodal decomposition in rutile TiO₂-VO₂ (TVO) epitaxial films on sapphire substrates. (001)- and (101)-oriented TVO solid-solution films are grown epitaxially on TiO₂/Al₂O₃ using a pulsed laser deposition technique and annealed inside the spinodal region. X-ray diffraction measurements and scanning transmission electron microscopy (STEM) observations show that the films are phase-separated along the [001] direction and lamellar structures are formed in a parallel or slanted direction to the sapphire substrates depending on the film orientation. The results indicate the multilayer formation via spinodal decomposition in the TVO films. STEM investigations also reveal a relatively high Ti concentration in the decomposed phases, reflecting the influence of lattice deformation on the phase decomposition in the films. Our work shows that spinodal decomposition is a promising approach for the formation of a multilayer structure in TVO films and helps deepen understanding the spinodal decomposition in TVO system.     

Structure of V-shape twinned VO2 nanocrystals epitaxially grown on sapphire

We report on the synthesis and structure of ‘V’ shape twinned vanadium dioxide (VO₂) nanocrystals epitaxially grown on c-plane sapphire substrates using a vapor transport method. The (100)_M twin plane played a key role in determining the morphology of VO₂ nanocrystals. The growth of VO₂ nanocrystals begins at the twin plane and proceeds toward two possible monoclinic [100] (a_M−axis) direction resulting in ‘V’ shape twinned crystals with the angle between the sides of approximately 115.4°. At a relatively low growth temperature of 900 °C, the growth of the sides of ‘V’ was limited producing ‘coffee-bean’ shape crystals in which flat crystal facet regions are connected with rounded edges. The twinned VO₂ nanocrystals were epitaxial to the c-plane sapphire substrate with the monoclinic [010] (b_M−axis) normal to the substrate. In the in-plane direction, the (001)_M planes of the VO₂ twin crystals were aligned to the direction ±2.3° away from the sapphire $(11\bar{2}0)$ plane. The sides of ‘V’ exhibit a rectangular cross-section truncated by the substrate. In-situ synchrotron x-ray diffraction measurement across the metal-insulator transition of the twinned nanocrystals implies that the phase transition of the coffee-bean shape nanocrystals occurs at a lower temperature with a smaller hysteresis gap than the fully grown V-shape nanocrystals.     

Graphene etching controlled by atomic structures on the substrate surface

We etched graphene on a sapphire (1 ?1 0 2) surface using the reaction between graphene and hydrogen catalyzed by metal nanoparticles. To investigate effects of the atomic structure of the sapphire substrate on graphene etching, we used sapphire substrate with as-polished, air-annealed, and step-ordered surfaces. We investigated the relationship between the atomic arrangement of sapphire and graphene etching and found that graphene is selectively etched in the [1 ?1 0 ?1] direction of sapphire. This indicates that atomic structure of the sapphire surface can be used as a template to control graphene etching. By combining the transfer method for graphene sheets grown on metal substrates with the present etching technique, graphene nanoribbons can be fabricated at a wafer level.     

Combinatorial catalyst approach for high-density growth of horizontally aligned single-walled carbon nanotubes on sapphire

We studied effects of metal catalyst and gas composition on the chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWCNTs) on r-plane sapphire substrates. The SWCNTs are sitting on the substrate and aligned along [10] direction of the sapphire surface. A combinatorial metal deposition method was applied for single and binary metal catalysts to systematically investigate the thickness and the composition dependence. The horizontally-aligned SWCNTs grown from stripe-patterned catalysts enable the direct comparison of the catalytic activity based on nanotube density. We found that the SWCNT density strongly depends on the metal catalyst in the order Fe > Co ? Ni ≈ Cu, while no nanotubes were grown over Mo. In addition, the methane concentration during CVD strongly influenced the nanotube density, and the optimal concentration varied depending on the metal species and its thickness. The study on the binary metal catalysts revealed that Fe–Co combination increases the SWCNT density (7–9 tubes/μm) about twice of the original metal film. The Co–Cu binary catalyst also showed the high density (8–10 tubes/μm) under a limited methane concentration. Different catalytic activity of each metal is discussed.     

Improvement in Wear Resistance of High-Strength and High-Toughness Silicon Nitride Modified by Ion Implantation

Surface modification by ion implantation has been conducted to improve the tribological properties of a high-strength and high-fracture-toughness unidirectionally aligned silicon nitride (UA-SN). B⁺, N⁺, Si⁺, and Ti⁺ ions were implanted into the planes parallel and normal to the grain alignment of the UA-SN with a fluence of 2 × 10¹⁷ ions/cm² at an energy of 200 keV. The ion implanted UA-SN showed a dramatic improvement in wear resistance. For example, the specific wear rate of the Si⁺-implanted specimen in the direction parallel to the grain alignment was reduced to a value of 3 × 10⁻¹⁰ mm²/N, equal to 1/20 of the unimplanted one. Cross-sectional transmission electron microscopy indicates the high wear resistance was attributed to the amorphous surface caused by the ion implantation.     

Effect of Substrate Orientation on Interfacial and Bulk Character of Chemically Vapor Deposited Monocrystalline Silicon Carbide Thin Films

The high breakdown electric field, saturated electron drift velocity, and melting (decomposition) point of SiC have given continual impetus to research concerned with the development of thin films having minimum concentrations of line and planar defects and electronic devices for severe environments. To this end, epitaxial growth via chemical vapor deposition of monocrystalline films of β-SiC on Si (100) and 6 H -SiC {0001} substrates and 6 H -SiC on vicinal 6 H -SiC {0001} substrates have been conducted. High concentrations of stacking faults, microtwins, and inversion domain boundaries were produced in films grown directly on Si (100) as a result of a lattice parameter difference of ∼ 20% and the presence of single (or odd number) atomic steps on the substrate surface. Growth on Si (100) oriented 3° to 4° toward [011] completely eliminated the IDBs (but not the other defects) due to the preferential formation of double steps with dimerization axes on the upper terraces parallel to the step edges. Growth of β-SiC films on 6 H {0001} lowered the density of all defects but resulted in the formation of a new defect, namely, double positioning boundaries. The latter were eliminated by using 6 H {0001} oriented 3° toward [1120]. The defect density in these last films, relative to those grown on on-axis Si (100), was reduced substantially (to ∼10⁵ cm/cm³). However, the resulting film was 6 H -SiC. Significant improvements in electrical properties of simple devices were obtained as the defect density was progressively decreased.     

Structural characteristics of single crystalline GaN films grown on (111) diamond with AlN buffer

Hexagonal GaN films with the [0001] direction parallel to the surface normal were grown on (111) oriented single crystalline diamond substrates by plasma-assisted molecular beam epitaxy. Pre-treatments of the diamond surface with the nitrogen plasma beam, prior the nucleation of a thin AlN layer, eliminated the inversion domains and reduced the density of threading dislocations in the GaN epilayers. The films have an in-plane epitaxial relationship [1010]GaN//[110]diamond. Thus GaN (0001) thin films of single epitaxial relationship and of single polarity were realised on diamond with AlN buffer.     

Stability and Surface Energies of Wetted Grain Boundaries in Aluminum Oxide

The stability of a calcium-aluminum-silicate liquid film between two near-basal plane surfaces of sapphire at 1650°C was studied. Samples were prepared having an average basal misorientation across the interface of 6–7° about < 〈10 1 0〉. The interfaces varied in orientation from 0° to ∼38° to the [0001] direction. Three types of interfaces were observed: faceted, solid-liquid interfaces; low-angle grain boundaries consisting of aligned arrays of dislocations; and boundaries consisting of alternating regions of dislocations and faceted solid-liquid interfaces. The type of interface observed depended on the orientation of the interface and could be predicted by using a construction based on Wulff shapes. Because the type of interface depends on crystal alignment and interface angle, these results suggest an absolute method of determining the surface free energy of wetted boundaries.     

Raman scattering and electrical conductivity of nitrogen implanted MoO3 whiskers

Whiskers of MoO₃ have been grown by a thermal transport process. A set of samples was then implanted with nitrogen ions at a dose of 5 × 10¹⁶ ion/cm². The implanted whiskers changed from transparent to semi-transparent. Raman spectroscopy of the whiskers was observed and compared with those of unimplanted whiskers. The results revealed that the Raman intensity of the implanted whiskers was decreased about 10 times with respect to that of unimplanted whiskers. Only the case of the wave propagation parallel to the a-axis, a lower suppression ratio of the B_3g modes was observed. No extra mode due to the nitrogen implantation was observed. This indicates that implantation could only induce defects and oxygen vacancies but not the structural transformation. From electrical conductivity and Hall measurement, it was found that the whiskers exhibited an n-type semiconductor and its conductivity drastically increased due to the defects and oxygen vacancies.     

Surface morphology and electrochemical properties of highly boron-doped homoepitaxial diamond films

Highly boron-doped diamond films were grown on (100) diamond substrates that were mechanically repolished at an off-axis angle of 4° with respect to the (100) surface, tilted toward the [110] direction. The surface morphology and crystallinity were examined with atomic force microscopy, and it was found that the deposited surfaces have high crystallinity, with steps running parallel to the [110] direction. The terrace width was ∼30 nm. Atomic resolution images obtained on these terraces showed a disordered atomic arrangement, with no evidence for the 2×1 or 1×2 reconstruction usually observed for non-doped samples, suggesting that the high level of boron doping affects the surface structure. The electrochemical behavior of the films showed a wide working potential window and low capacitance.     

Mechanical properties of ceria nanorods and nanochains; the effect of dislocations, grain-boundaries and oriented attachment

We predict that the presence of extended defects can reduce the mechanical strength of a ceria nanorod by 70%. Conversely, the pristine material can deform near its theoretical strength limit. Specifically, atomistic models of ceria nanorods have been generated with full microstructure, including: growth direction, morphology, surface roughening (steps, edges, corners), point defects, dislocations and grain-boundaries. The models were then used to calculate the mechanical strength as a function of microstructure. Our simulations reveal that the compressive yield strengths of ceria nanorods, ca. 10 nm in diameter and without extended defects, are 46 and 36 GPa for rods oriented along [211] and [110] respectively, which represents almost 10% of the bulk elastic modulus and are associated with yield strains of about 0.09. Tensile yield strengths were calculated to be about 50% lower with associated yield strains of about 0.06. For both nanorods, plastic deformation was found to proceed via slip in the {001} plane with direction <110>--a primary slip system for crystals with the fluorite structure. Dislocation evolution for the nanorod oriented along [110] was nucleated via a cerium vacancy present at the surface. A nanorod oriented along [321] and comprising twin-grain boundaries with {111} interfacial planes was calculated to have a yield strength of about 10 GPa (compression and tension) with the grain boundary providing the vehicle for plastic deformation, which slipped in the plane of the grain boundary, with an associated <110> slip direction. We also predict, using a combination of atomistic simulation and DFT, that rutile-structured ceria is feasible when the crystal is placed under tension. The mechanical properties of nanochains, comprising individual ceria nanoparticles with oriented attachment and generated using simulated self-assembly, were found to be similar to those of the nanorod with grain-boundary. Images of the atom positions during tension and compression are shown, together with animations, revealing the mechanisms underpinning plastic deformation. For the nanochain, our simulations help further our understanding of how a crystallising ice front can be used to 'sculpt' ceria nanoparticles into nanorods via oriented attachment.     

High-Temperature Healing of Lithographically Introduced Cracks in Sapphire

A new method for producing controlled-geometry, controlled-crystallography, cracklike defects using photolithography has been developed. The method has been applied to sapphire, and used to study crack healing behavior at 1800°C. Effects of crack face and crack perimeter crystallography, crack face microstructure, and impurities on healing behavior have been identified.     

Graphene nano-cutting using biologically derived metal nanoparticles

We have developed a novel technique to cut graphene films using catalytic metal nanoparticles derived from ferritin, which is one of the proteins that contain a constant amount of Fe oxide in its inner core. For site-selective adsorption of ferritin molecules to sapphire surfaces that are partially covered with graphene films, two methods, dipping and spin-coating, were used. Graphene films were etched by the Fe-catalytic reaction with hydrogen gas at elevated temperatures. It was found that ferritin adsorption sites are controlled by graphene film edges, atomic steps of the sapphire substrate, and solution condition such as molecular concentration and ionic strength. We demonstrate that high density nanoribbons can be fabricated by using the uniformly-sized catalyst nanoparticles derived from ferritin and the aligned etching technique guided by the atomic structures of the substrate surface.     

Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: I, Effect of Substrate Orientation

Epitaxial thin films of Al-doped zinc oxide have been grown on sapphire substrates by pulsed laser ablation. The effect of substrate temperature, background pressure of oxygen, and substrate orientation (A, M, R, C) on the orientation relationships between ZnO and sapphire have been evaluated using on- and off-axis X-ray diffractometry. Under all growth conditions zinc oxide, on A- and C-plane sapphire, grew with the c -axis perpendicular to the substrate. In contrast, on M and R orientations of sapphire, ZnO grew with its c -axis parallel or perpendicular to the substrate depending on the substrate temperature and background pressure employed during growth. In all cases only one unique in-plane relationship between the sapphire substrate and the zinc oxide film was found with the exception of the M-plane at high substrate temperatures.     

Growth strategy for controlling dislocation densities and crystal morphologies of single crystal diamond by using pyramidal-shape substrates

The growth of millimetre-thick diamond single crystals by plasma assisted CVD is complicated by the formation of unepitaxial defects, particularly at the edges of the crystal. These defects tend to encroach on the top surface hence limiting the maximum thickness to typically a few hundreds of micrometres. Dislocations are another type of defects that are also particularly formed at the edges of the crystal. They thread through the diamond film, strongly affecting its characteristics. The growth on pyramidal-shape substrates having different angles and orientations was carried out in an attempt to solve those issues. It was found that the pyramidal-shape tends to disappear after a certain thickness is grown. The inclined faces of the pyramid not only helped in preserving the crystal morphology over a large thickness but also deviated dislocations towards the edges of the crystal, hence limiting their occurrence at the surface. Using this strategy, millimetre-thick diamond single crystals presenting a reduced dislocation density were successfully grown.     

The production of large scale ultrathin aligned CNT films by combining AC electric field with liquid flow

We report a simple and versatile technique combining the use of an AC electric field with a liquid shear force to prepare ultrathin aligned CNT films on solid substrates. Multiwalled carbon nanotubes (MWCNTs), which were synthesized by a template method and acid-treated single walled carbon nanotubes (SWCNTs) were dispersed in water and used for the ultrathin film fabrication. A solid substrate was immersed in the CNT dispersions and withdrawn at constant speed under AC electric field. SEM images of the substrate showed that CNTs were aligned with the AC electric field and the withdrawal direction and formed uniform films with a thickness around 10 nm for SWCNTs and 90 nm for MWCNTs. Repeating the deposition process increases the density and size of the film while also maintaining nanometer-scale thickness. Unidirectional alignment of CNTs was also confirmed by Raman spectra and electric conductivity measurements. It was found that ultrathin films of aligned SWCNTs exhibited very high anisotropic electrical conductivity with conductivity measured parallel to the alignment direction 3.3 × 10⁵ times higher than that measured in the perpendicular direction. We demonstrate that use of the aligned ultrathin SWCNT film for a unidirectional alignment of liquid crystal.     

Recognizing nucleosides with transverse electronic transport via perpendicular direction of base planes for DNA sequencing

Putting the four DNA nucleosides in the middle of gold [111] nanoelectrodes with base planes parallel to the electrode surface layer, we study the transverse electronic transport properties of four nucleosides along the direction of electrodes. First, the optimal distance of the electrodes is released. The results show that the optimal electrode distance to study transverse electronic transport characteristics of DNA nucleosides is about 0.68 nm. Second, we theoretically calculate the conductance and current of the four nucleosides via perpendicular direction of base planes in the bias range of [−2, 2] V by exploiting the first principle theory. According to the calculated results, we propose three methods to recognize the nucleoside type in practice application.     

Investigations of defects in Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals grown along [001] direction

The defects of the [001]-oriented grown 0.73Pb(Mg_1/3Nb_2/3)O₃-0.27PbTiO₃ (PMN-0.27PT) single crystals, as well as their influence on electrical properties, were studied by comparing the as-grown and annealed samples. Apart from the anomalies induced by the successive ferroelectric phase transitions, four relaxation processes of the as-grown PMN-0.27PT single crystals grown along [001] direction were discussed and determined by the dielectric spectroscopy, complex impedance spectroscopy, AC conductivity, suggesting the existence of many oxygen vacancies and lead vacancies in the as-grown samples. High-angle annular dark-field imaging of scanning transmission electron microscope (HAADF-STEM) was applied to observe the defects in atomic-scale. The point defects generated more easily when growing the PMNT crystals along [001] direction than along [111] direction can be attributed to the difference of growth mechanism and necessary growth environment induced by different growth directions, which results in the special coloration phenomenon of the [001]-oriented grown PMNT crystals.     

Ferroelasticity of t'-Zirconia: II, In situ Straining in a High-Voltage Electron Microscope

The ferroelastic deformation of t '-ZrO₂, the microstructure of which was described in detail in Part I, was investigated by in situ deformation experiments in the high-voltage electron microscope at 1150°C. During the experiments those two domain variants with their c-axes perpendicular to the [010] tensile direction were transformed into the third one with its c-axis parallel to the tensile direction. The subsequent 'switching' of the domains inside the colonies proceeds much faster than the penetration of the transformation front into a neighboring colony. Therefore, the transformed region, exhibiting a unique tetragonal structure and containing residual defects, preferentially expands into the longitudinal directions of the colonies. The transformation of single domains proceeds instantaneously within the time resolution of the video tape recording.