Search

• Light shows obvious suppressive effect on the dislocation-based plasticity in hexagonal wurtzite ZnO single crystals. • Activation volume and Peierls barrier under illumination are larger than in darkness. • Indentation-induced dislocations are present in a unique thin-plate shape along one basal slip plane for (0001) 45o off ZnO. Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness. [Display omitted] [ABSTRACT FROM AUTHOR]

The introduction of dislocations is a recently proposed strategy to tailor the functional and especially the electrical properties of ceramics. While several works confirm a clear impact of dislocations on electrical conductivity, some studies raise concern in particular when expanding to dislocation arrangements beyond a geometrically tractable bicrystal interface. Moreover, the lack of a complete classification on pertinent dislocation characteristics complicates a systematic discussion and hampers the design of dislocation-modified electrical conductivity. We proceed by mechanically introducing dislocations with three different mesoscopic structures into the model material single-crystal SrTiO3and extensively characterizing them from both a mechanical as well as an electrical perspective. As a final result, a deconvolution of mesoscopic structure, core structure,and space chargeenables us to obtain the complete picture of the effect of dislocations on functional properties, focusing here on electric properties.

It was recently found that extremely large plasticity is exhibited in bulk compression of single-crystal ZnS in complete darkness. Such effects are believed to be caused by the interactions between dislocations and photoexcited electrons and/or holes. However, methods for evaluating dislocation behavior in such semiconductors with small dimensions under a particular light condition had not been well established. Here, we propose the “photoindentation” technique to solve this issue by combining nanoscale indentation tests with a fully controlled lighting system. The quantitative data analyses based on this photoindentation approach successfully demonstrate that the first pop-in stress indicating dislocation nucleation near the surface of ZnS clearly increases by light irradiation. Additionally, the room-temperature indentation creep tests show a drastic reduction of the dislocation mobility under light. Our approach demonstrates great potential in understanding the light effects on dislocation nucleation and mobility at the nanoscale, as most advanced technology-related semiconductors are limited in dimensions.

Functional and structural ceramics have become irreplaceable in countless high-tech applications. However, their inherent brittleness tremendously limits the application range and, despite extensive research efforts, particularly short cracks are hard to combat. While local plasticity carried by mobile dislocations allows desirable toughness in metals, high bond strength is widely believed to hinder dislocation-based toughening of ceramics. Here, we demonstrate the possibility to induce and engineer a dislocation microstructure in ceramics that improves the crack tip toughness even though such toughening does not occur naturally after conventional processing. With modern microscopy and simulation techniques, we reveal key ingredients for successful engineering of dislocation-based toughness at ambient temperature. For many ceramics a dislocation-based plastic zone is not impossible due to some intrinsic property (e.g.bond strength) but limited by an engineerable quantity, i.e.the dislocation density. The impact of dislocation density is demonstrated in a surface near region and suggested to be transferrable to bulk ceramics. Unexpected potential in improving mechanical performance of ceramics could be realized with novel synthesis strategies.

First-principles molecular dynamics simulations of stoichiometric La9.33(SiO4)6O2were performed to investigate a physical origin on the unusual anisotropic distributions of columnar oxygen ions (O4) reported experimentally. Based on the mechanisms proposed previously, effects of O4 vacancy and interstitial oxygen ion were considered. It was found that interstitial oxygen ions close to the O4 column can induce unusual anisotropic displacements of O4 ions over several adjacent oxygen sites. Although the O4 vacancy also promotes more thermal motion of O4 ions, as compared to those in the perfect crystal, thermal motion of O4 in the presence of O4 vacancy was not large enough to understand the experimentally observed one.

The atomic-scale picture of the proton conduction mechanism in tin pyrophosphate, SnP2O7, has theoretically been investigated using first-principles calculations, to clarify the intrinsic proton conductivity in the bulk region. Protons in the crystal lattice reside around oxide ions and migrate by rotation around single oxide ions and hopping between adjacent oxide ions by a mechanism similar to that in other proton-conducting oxides. The calculated proton conductivity has weak anisotropy reflecting the monoclinic structure (unique axis: b), particularly in the ca-plane. The main origin of the anisotropic conductivity is the relatively fast long-range migration pathways along the unique b-axis and in the [101] direction (potential barrier: 0.57 eV) versus that along the [101̅] direction (potential barrier: 0.64 eV). The apparent activation energy of the estimated proton conductivity is as high as ∼1.1 eV with the proton trapping effect by dopants (association energy: 0.59 eV), leading to the low proton conductivity in the bulk region. This suggests that the reported fast proton conductivity in the literature may be due to other unexpected proton migration routes, such as surfaces, grain boundaries, and secondary phases with residual phosphoric acid.

The proton conduction behaviors in pyrochlore-structured lanthanum zirconate, La2Zr2O7, have been evaluated theoretically in a first-principles manner based on the transition state theory. The three-dimensional proton-conducting network consisting of two hopping paths was found, which lies along the corner-shared ZrO6octahedral network in the crystal lattice. The calculated potential barrier of the proton-conducting network in the undoped system (0.39 eV) is lower than the apparent activation energies of the experimentally reported conductivities in acceptor-doped systems (0.6–0.9 eV). The effects of alkaline earth metal and yttrium dopants on the proton mobility and conductivity have also been evaluated, so that the calculated association energies between protons and dopants are in the range of 0.1–0.5 eV depending on the dopant species and substitutional sites. This makes the slopes of the proton conductivity curves in the Arrhenius plots steeper, leading to higher activation energies (0.4–0.8 eV) in the intermediate temperature range (573–873 K).

The proton conduction behavior in La3NbO7is theoretically analyzed by first-principles calculations with the aid of the nudged elastic band method and the kinetic Monte Carlo method. Protons in the crystal migrate over a long range through fast conduction channels along the single NbO6octahedral chains in the a-axis direction, whose calculated potential barrier is 0.54 eV. The bridging paths connecting the proton channels in the b- and c-axes directions have a higher potential barrier, 0.64 eV, leading to anisotropic proton diffusivity and conductivity, particularly at low temperatures. The preferential bonding of protons to several specific oxide ions is strongly related to the lower potential barrier in the a-axis direction in addition to the presence of the infinite NbO6chains.

A dislocation in a crystalline material has dangling bonds at its core and a strong strain field in its vicinity. Consequently, the dislocation attracts solute atoms and forms a so-called Cottrell atmosphere along the dislocation. A crystalline dislocation can be used as a template to produce nanowires by selectively doping foreign atoms along the dislocation. However, control of the configuration, spacing, and density of the formed periodic nanowire array has heretofore been extremely difficult. Here we show a method for fabricating ordered, electrically conductive nanowire arrays using periodic dislocations at crystal interfaces. As a demonstration, we fabricated arrays of titanium nanowires arranged at intervals of either 13 or 90 nm and then confirmed by scanning probe microscopy that they exhibit electrical conductivity inside an insulating aluminum oxide. Significantly, we were able to precisely control nanowire periodicity by the choice of crystal orientation and/or crystal planes at the crystal interface. This simple method for the fabrication of periodic nanowire arrays of highly controlled density should be widely applicable to electrical, magnetic, and optical devices.

The Time – Dependent Adsorptive Behavior of Biomolecules on/in Mesoporous Silica Materials (MPS) Have Been Studied by Using Transmission Electron Microscopy (TEM). Highly Ordered Hexagonal Mesoporous Silica Materials (SBA-15) Synthesized Were Immersed in Phosphate Buffer Saline (PBS) Solution Containing Cytochrome C for 2, 48, and 120 Hours at 4 ºC. after that, MPS Materials Were Rinsed with PBS Solution, and then Immersed in PBS Solution without Cytochrome C. the Amount of Protein Adsorbed on/in MPS Materials Soaked for 2, 48, and 120 Hours Was 75 Mg/g, 135 Mg/g and 178 Mg/g, Respectively. the Time Profiles for Adsorption of Proteins Can Be Well Described by Intraparticle Diffusion Model. the Holes of MPS Materials Were Observed to Be Overlapped with Stained Proteins for First 2 Hours Immersion. the Stained Proteins Were Observed between Primary Particles and Partly inside the Mesoporous Channels in the MPS Material Immersed for 48 Hours. for MPS Immersed for 120 Hours, Stained Proteins Were Observed in Almost All Meso-Scale Channels of MPS. the Ratio of Desorbed Proteins from those Embedded on/in MPS Materials Was 13 % in Case of the MPS Materials Immersed in PBS Containing Cytochrome C for 2 Hours, while 8 % in Case of the MPS Materials Immersed for 48 and 120 Hours. Cytochrome C May Be Strongly Embedded on/in MPS Materials due to the Electrostatic Effect.

Volatile sulfur compounds (VSCs) such as hydrogen sulfide (H2S), methyl mercaptan and dimethyl sulfide produced in mouth. Some oral (Gram-negative) bacteria produce VSCs, which induces permeability of mucous membrane, cause the oral malordor, dental caries, color change of the dental fillings. Thus, material which adsorbs VSCs should be useful to keep health in mouth. Previously, we reported the H2S adsorptive properties of zeolite and hydrotalcite materials having micro pores. The amount of H2S adsorbed on the zeolite or hydrotalcite at room temperature was found to be around 300-400 ppm, and and 3 % of H2S was desorbed when heated at 400 ºC. The hydrotalcite, whose compositon is Mg1-xAlx(OH)2Ax/n·mH2O, where A is CO32-, and x is 0.25, was heat-treated at 500 ºC, and put into aqueous solution containing H2S. In this study, the relation between heat-treatment tempearture and the sulfide adsorption on hydrotalcite in aqueous solution was investigated. The hydrotalcite materials were hydrothermally synthesized and identified by powder X-ray diffraction method. The change in concentrations of H2S in aqueous solution was measured using FPD gas chromatography (GC). The concentrations of H2S was decreased with time for the hydrotalcite heated at 500 ºC or 600 ºC, and fell to 20 % for 12 hours. But, the concentration was decreased by 40% for 12 hours for the hydrotalcite materials heated at higher temperatue. XRD and FT-IR analysis revealed that the sulfides were into or out of the hydrotalcite structure through topochemical reactions.

The encapsulation and immobilization of biomolecules on/in mesoporous silica materials (MPS) have been studied by using transmission electron microscopy (TEM). The mixture of tetraethyl orthosilicate (TEOS) and the triblock copolymer as a template was stirred, hydrothermally treated to form the mesoporous SBA-15 structure, and heat-treated at 550 ºC. SEM observation indicated that long and narrow particles were linked together in the long axis direction to form secondary particles of SBA-15. UV-spectrometry was performed to determine the amount of bovine serum albumin (BSA), cytochrome C, myoglobin or -lactoglobulin encapsulated on/in SBA-15. The time profiles for adsorption of proteins can be well described by intraparticle diffusion model. The TEM observations of proteins on/in mesoporous SBA-15 revealed that proteins were embedded on/in mesoporous SBA-15 and the protein behaviors given by TEM observations may correspond to the intraparticle diffusion model.

Structure and configuration of boundary dislocations on the low angle tilt grain boundaries in alumina were considered based on the ideas that the boundary is composed of regularly arrayed edge dislocations and that the dislocations could dissociate into partial dislocations as well as glide dislocations in bulk. Moreover, the structure of the dissociated boundary dislocations were evaluated by the calculations based on an elastic theory. The calculations indicated that the largeness of the stacking fault region between partial dislocations formed by the dissociation will decrease with increasing tilt angles. It can be said that the idea and calculations used here will be powerful in considering the dislocation structure of low angle tilt grain boundaries that are not or are difficult to be identified.

Dislocation structure of {1120}/<1100> 2º tilt grain boundary in alumina was observed by transmission electron microscopy (TEM). The grain boundary consisted of periodical array of basal dislocations, which were dissociated into pairs of 1/3<1010> and 1/3<0110> partials with {1120} stacking-fault in between. The relationship between the separation distance of partials and the stacking-fault was modeled by considering the force balances of periodical dislocations. The estimated stacking-fault energy for 2o tilt grain boundary was consistent with the previous reports.

Near-edge structure of X-ray absorption spectrum (NEXAFS) of various Ti-oxides were investigated by combined with first principles orthogonalized linear combinations of atomic orbitals (OLCAO) method. From experimental and theoretical studies on the NEXAFS of the tetravalent and trivalent Ti-oxides, including rutile, anatase, brookite, columbite, and Ti2O3, it was found that the valence state of Ti can be identified by regarding the positions of the spectral onset and the shoulder in the main-peak of Ti-K NEXAFS.

First-principles pseudopotential calculations were performed to investigate atomic and electronic structures of titanium (Ti) dopants in alumina (Al2O3). It was found that a substitutional Ti3+ defect induced an extra level occupied by one electron within the band gap of Al2O3. When two or more substitutional Ti3+ defects were located closely to each other, the defect-induced levels exhibited strong bonding interactions, and their formation energies decreased with increasing numbers of Ti3+ defects. This indicates that association and clustering of substitutional Ti3+ defects in Al2O3 can take place due to the interaction of the defect-induced levels.

Low-dimensional structures, such as microclusters, quantum dots and one- or two-dimensional (1D or 2D) quantum wires, are of scientific and technological interest due to their unusual physical properties, which are quite different from those in the bulk. Here we present a successful method for fabricating conducting nanowire bundles inside an insulating ceramic single crystal by using unidirectional dislocations. A high density of dislocations (109cm−2) was introduced by activating a primary slip system in sapphire (α-Al2O3single crystal) using a two-stage deformation technique. Plate specimens cut out from the deformed sapphire were then annealed to straighten the dislocations. Finally, the plates on which metallic Ti was evaporated were heat-treated to diffuse Ti atoms inside sapphire. As a result of this process, Ti atoms segregated along the unidirectional dislocations within about 5 nm diameter, forming unidirectional Ti-enriched nanowires, which exhibit excellent electrical conductivity. This simple technique could potentially to be applied to any crystal, and may give special properties to commonly used materials.

Dislocations have been recently introduced as a novel tool to tailor the conductivity of functional ceramics. However, tuning strategies suffer from poor insight into the structural complexity of dislocations and their networks. Here, we demonstrate that dislocations can be used to both enhance and reduce the overall conductivity in the same ceramic material. Accurate control of the arrangement of dislocations within the dislocation network enables tailoring TiO2bulk samples to behave like being chemically modified either with an acceptor or donor dopant. Our approach combines ultra-high voltage electron microscopy, oxygen partial pressure, and temperature dependent electrical conductivity measurements combined with time-of-flight secondary ion mass spectrometry. This allows us to focus on mechanically tailored interaction of next neighbor dislocations and to differentiate between percolating conductive pathways and separated charge carrier zones. This seemingly simple approach purposefully tailors the conductivity of TiO2, opening new avenues to engineer functional ceramics beyond common chemical doping strategies.

Variation in the formation energy of stacking faults (SFs) with the contamination of Na atoms was examined in Si crystals with different Fermi levels. Na atoms agglomerated at SFs under an electronic interaction, reducing the SF formation energy. The energy decreased with the decrease of the Fermi level: it was reduced by more than 10 mJ/m2in p-type Si, whereas it was barely reduced in n-type Si. Owing to the energy reduction, Na atoms agglomerating at SFs in p-type Si are stable compared with those in n-type Si, and this hypothesis was supported by ab initio calculations.