Presence Of Structural Defects Research Articles

Crystal orientation effect on dislocation nucleation and multiplication in FCC single crystal under uniaxial loading

Molecular dynamics simulations are applied to study the effects of orientation and the presence of structural defects on the compression/tension (C/T) asymmetry of copper single crystals. In addition to the perfect crystal, crystals with stacking fault tetrahedra, as a representative internal defect, are considered to investigate both homogeneous and heterogeneous deformation mechanisms. Both the normal stresses to the slip plane and the relative values of Schmid factor in compression and tension impact the C/T asymmetry. The presence of an SFT lowers the applied stress required for plastic deformation, but this effect is highly dependent on the crystal orientation and loading direction. The reduction in yield stress is larger in compression than in tension for almost all orientations. Results show that in general a structural defect would decrease the C/T asymmetry in copper, corresponding closely to previous experiments. The reduction in yield stress in tension is less sensitive to defects than that in compression, suggesting that compression test is a more reliable experimental tool for future size dependence studies, since structural defects are the main reason behind the observed size effects in materials. On the other hand, since the reduction in yield stress is almost constant in tension for all orientations, testing in this geometry is more efficient to determine the orientation dependence of the yield stress.

IR Spectroscopy study of the surface of commercial quartz ceramic specimens

We have obtained the Fourier transform IR reflectance spectra of quartz ceramic in different stages of its heat treatment: after drying at 100°C, calcining at 1180°C, and flame treatment of the surface. The spectroscopic data suggest that in the original material, there are a large number of non-bridging Si–O bonds. This is a favorable factor when doping the ceramic with Eu, Ce, Sm oxides, with the goal of converting the UV emission of laser pump lamps to luminescent emission. Non-bridging bonds do not appear in the final product, but we observe the presence of structural defects potentially reducing the UV resistance of the ceramic.

Intrinsic electrical conductivity of nanostructured metal-organic polymer chains.

One-dimensional conductive polymers are attractive materials because of their potential in flexible and transparent electronics. Despite years of research, on the macro- and nano-scale, structural disorder represents the major hurdle in achieving high conductivities. Here we report measurements of highly ordered metal-organic nanoribbons, whose intrinsic (defect-free) conductivity is found to be 104 S m−1, three orders of magnitude higher than that of our macroscopic crystals. This magnitude is preserved for distances as large as 300 nm. Above this length, the presence of structural defects (~ 0.5%) gives rise to an inter-fibre-mediated charge transport similar to that of macroscopic crystals. We provide the first direct experimental evidence of the gapless electronic structure predicted for these compounds. Our results postulate metal-organic molecular wires as good metallic interconnectors in nanodevices.

Shape-controlled hierarchical ZnO architectures: photocatalytic and antibacterial activities

A simple soft chemical approach has been successfully adopted for the synthesis of ZnO in spherical assemblies (SA), nanorods assemblies (NRA), cauliflower-like assemblies (CFA) and mushroom-like assemblies (MA). The morphology of self-assembled ZnO nanostructures composed of numerous nanocrystals has been confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction and optical studies suggest an anisotropic nature of ZnO and presence of structural defects in ZnO nanostructures, respectively. N2 adsorption–desorption isotherm curves of these nanostructures (except MA) indicate predominantly 3D-mesoporous nature. These nanostructures are useful for their potential application in photocatalytic degradation of organic dyes (e.g. methylene blue, Rhodamine B) and inhibition of bacterial growth (S. aureus). Among these ZnO architectures, CFA exhibits excellent photocatalytic and antibacterial activities. In addition, the inhibition of bacterial growth of S. aureus is more effective under UV light than in dark conditions.

Selective removal of metal impurities from single walled carbon nanotube samples

Large scale production of high quality CNT samples is still challenging. The presence of structural defects and metallic particles in pristine single walled carbon nanotubes (SWNTs) is responsible for the alteration of both their chemical stability and their magnetic and electrical properties. The commonly used purification procedures are based on multi-step treatments that are often too aggressive towards the CNTs, leading to disappointing yields. Here, we propose an alternative process that allows preparing high-quality and high-purity SWNT samples. The proposed process merely consists of heating up SWNT powder under high chlorine partial pressure and high temperature. These thermodynamic conditions favor high chlorine diffusion to metal impurities embedded in carbon shells thus inducing an avalanche process of metal chloride formation and sublimation. The purified samples have been characterized by transmission electron microscopy, thermogravimetric analysis, magnetic measurements and Raman spectroscopy. We show that the developed process combines selective elimination of catalytic impurities and high yields. More importantly, we show that this process preserves the quality of the resulting purified nanotubes.

Effect of structural defects and chemical functionalisation on the intrinsic mechanical properties of graphene

Due to its unique mechanical properties, graphene can be applied for reinforcement in nanocomposites. We analyse the Young's modulus of graphene at the semi-empirical PM6 level of theory. The internal forces are calculated and the Young's modulus is predicted for a finite graphene sheet when external strain is applied on the system. These results are in a good agreement with theoretical and experimental results from the literature giving values of about 1 TPa for the Young's modulus. Stress-strain curves are computed for elongation up to 20%. In addition, the influence of the presence of a single vacancy, as well as for oxygenation of a vacancy, on the mechanical properties of graphene has been analysed. Our results indicate that when applying the deformation locally onto the system, higher local stress can be induced, as confirmed by Finite Element Analysis. Also, the presence of structural defects in the system will stiffen the system upon low strain, but reduces the elastic limit from more than 20% strain for pristine graphene to less than 10% strain when defects are present.

Disorder effects on the static scattering function of star branched polymers

We present an analysis of the impact of structural disorder on the static scattering function of f-armed star branched polymers in d dimensions. To this end, we consider the model of a star polymer immersed in a good solvent in the presence of structural defects, correlated at large distances r according to a power law \sim r^{-a}. In particular, we are interested in the ratio g(f) of the radii of gyration of star and linear polymers of the same molecular weight, which is a universal experimentally measurable quantity. We apply a direct polymer renormalization approach and evaluate the results within the double \varepsilon=4-d, \delta=4-a-expansion. We find an increase of g(f) with an increasing \delta. Therefore, an increase of disorder correlations leads to an increase of the size measure of a star relative to linear polymers of the same molecular weight.

Accessing intermediate ferroelectric switching regimes with time-resolved transmission electron microscopy

BiFeO3 (BFO) is one of the most widely studied magneto-electric multiferroics. The magneto-electric coupling in BiFeO3, which allows for the control of the ferroelectric and magnetic domain structures via applied electric fields, can be used to incorporate BiFeO3 into novel spintronics devices and sensors. Before BiFeO3 can be integrated into such devices, however, a better understanding of the dynamics of ferroelectric switching, particularly in the vicinity of extended defects, is needed. We use in situ transmission electron microscopy (TEM) to investigate the response of ferroelectric domains within BiFeO3 thin films to applied electric fields at high temporal and spatial resolution. This technique is well suited to imaging the observed intermediate ferroelectric switching regimes, which occur on a time- and length-scale that are too fine to study via conventional scanning-probe techniques. Additionally, the spatial resolution of transmission electron microscopy allows for the direct study of the dynamics of domain nucleation and propagation in the presence of structural defects. In this article, we show how this high resolution technique captures transient ferroelectric structures forming during biasing, and how defects can both pin domains and act as a nucleation source. The observation of continuing domain coalescence over a range of times qualitatively agrees with the nucleation-limited-switching model proposed by Tagantsev et al. We demonstrate that our in situ transmission electron microscopy technique is well-suited to studying the dynamics of ferroelectric domains in BiFeO3 and other ferroelectric materials. These biasing experiments provide a real-time view of the complex dynamics of domain switching and complement scanning-probe techniques.

Transport properties of graphene containing structural defects

We propose an extensive report on the simulation of electronic transport in 2D graphene in presence of structural defects. Amongst the large variety of such defects in sp$^2$ carbon-based materials, we focus on the Stone-Wales defect and on two divacancy-type reconstructed defects. First, based on ab initio calculations, a tight-binding model is derived to describe the electronic structure of these defects. Then, semiclassical transport properties including the elastic mean free paths, mobilities and conductivities are computed using an order-N real-space Kubo-Greenwood method. A plateau of minimum conductivity ($\sigma^{min}_{sc}= 4e^2/\pi h$) is progressively observed as the density of defects increases. This saturation of the decay of conductivity to $\sigma^{min}_{sc}$ is associated with defect-dependent resonant energies. Finally, localization phenomena are captured beyond the semiclassical regime. An Anderson transition is predicted with localization lengths of the order of tens of nanometers for defect densities around 1%.

Sliding on a Nanotube: Interplay of Friction, Deformations and Structure

The frictional properties of individual carbon nanotubes (CNTs) are studied by sliding an atomic force microscopy tip across and along its principle axis. This direction-dependent frictional behavior is found to correlate strongly with the presence of structural defects, surface chemistry, and CNT chirality. This study shows that it is experimentally possible to tune the frictional/adhesion properties of a CNT by controlling the CNT structure and surface chemistry, as well as use friction force to predict its structural and chemical properties.

Strong green emission in ZnO films after H2 surface treatment

Using a two-step fabrication technique (pulsed laser deposition (PLD) and H2 surface treatment), we fabricated ZnO thin films that could emit ultra-strong green emission with coexistence of random lasing phenomenon. After PLD deposition, the as-prepared undoped ZnO thin films (200–500 nm) were annealed in Ar 95%–H25% ambient at 500 °C. The H2 treatment led to the formation of a porous structure that creates substantial optical cavities (diameter ∼1.3 µm). Surprisingly, these optical cavities tremendously amplified the green emission rather than ultraviolet (UV) emission. There was insignificant change in emission intensity after high-temperature annealing (700 °C) in O2 and acetone dipping, indicating the samples are thermally and chemically stable. The samples exhibited a high quantum yield of 32%. We studied the origin of this ultra-strong green emission using low-temperature photoluminescence, extensive structural study and cyclic annealing. The results suggested that neither hydrogen nor VO plays a role in green emission. The green emission was attributed mainly to the complex defects and the presence of structural defects in the porous structure. In addition, we demonstrated the feasibility of large-scale green emission ZnO fabrication via micro-size patterning, paving a way to practical optoelectronic applications.

Luminescent isolated diamond particles with controllably embedded silicon-vacancy colour centres

A technique to fabricate isolated diamond particles with controllably embedded silicon-vacancy (Si-V) colour centres is described. Particle growth and Si doping are performed by microwave plasma-enhanced chemical vapour deposition (CVD) using silane as a source of impurity atoms. The Si-V centres have a strong narrow-band photoluminescence (PL) at room temperature. The dependence of PL intensity of the 738 nm zero-phonon line (ZPL) on silane concentration in the feed-gas mixture exhibits a pronounced maximum. A comparison of the PL and Raman spectra shows that there is an evident correlation between the ZPL intensity and the presence of structural defects and nondiamond carbon phases that act as nonradiative recombination centres suppressing radiative recombination. The results open the door for using the powerful CVD technique for large-scale production of photostable near-infrared single-photon emitters and noncytotoxic biomarkers.

MODIFICATION OF OPTICAL TRANSITION PROBABILITY IN SEMICONDUCTOR SUPERLATTICES: EFFECTS OF STRUCTURAL DEFECT AND LAYER THICKNESS

We present the modification of optical transition probability in the superlattices (SLs) due to the presence of structural defect and the variation of layer thickness. The results show that optical transition probability between two bound states is mainly determined by the thicknesses of layer in defect region and its nearest constituent layers for a symmetric structure. When the well layer thickness varies, the variation of transition probabilities between two bound states depend on the parity of the index of minigap n, while those from the bound states to the delocalized scattering is irrelevant to n. It is believed that applying appropriate structure and layer thickness may enhance the optical transition probability in designing optical device.

Spark plasma sintering synthesis and mechanical spectroscopy of the ω-Al0.7Cu0.2Fe0.1 phase

Starting from a mixture of Al–Cu–Fe quasicrystalline (QC) particles and Al powder, a fully dense and almost Al–Cu–Fe ω single-phase alloy was produced by spark plasma sintering. This technique allows synthesising large samples with sizes suitable for mechanical spectroscopy experiments. Mechanical spectroscopy was selected because it is a relevant tool for detecting the presence of structural defects at both nano and microscopic scales. Young’s moduli were measured in the 15 kHz range as a function of temperature by the resonant frequency method. Young’s moduli behave similarly for typical metals and exhibit values that are comparable to those of the Al–Cu–Fe QC phase. The damping coefficient Q−1 was determined at various temperatures between room temperature and 840 K over a large frequency range, i.e. between 10−3 and 10 Hz. The results suggest that solid friction effects do occur. In addition, a relaxation peak is observed in the intermediate temperature range.

Structure analysis of CVD graphene films based on HRTEM contrast simulations

AbstractPhysical properties of graphene depend on the number of atomic layers in the film, presence of structural defects and impurities. Low‐voltage, aberration‐corrected, high‐resolution transmission electron microscopy (HRTEM) has proven to be an excellent tool for structure analysis of graphene films. However, experimental observations of graphene films by HRTEM exhibit several challenges due to low contrast and sensitivity of graphene to intense electron beams. While a majority of electron scattering effects in graphene are purely kinematic, the contrast interpretation still requires computer simulations for reliable structure identification. In this paper, we present HRTEM contrast simulations of graphene films based on multislice algorithms and compare them to experimental results. The effects of stacking sequence, number of layers, and presence of impurities have been analyzed in conjunction with imaging conditions.

Microwave assisted synthesis of flower-like ZnO and effect of annealing atmosphere on its photoluminescence property

Large scale flower-like ZnO microstructures were synthesized by microwave assisted hydrothermal method in short reaction time. These flower-like structures were composed of dozen of rods like building block units radiated from a centre. Similar morphology was not obtained in normal hydrothermal method. TEM microscopy study showed that individual rods of flower-like structures are single crystalline in nature and preferentially grown along [0001] direction. Growth mechanism showed that microwave induction and ethylenediamine (EDA) played an important role in the formation of flower-like ZnO microstructures. Photoluminescence property of flower-like ZnO microstructures showed deep level emission along with UV emission which indicates the presence of structural defects. The nature of defect responsible for deep level emission was examined by annealing the sample in different atmospheres. There was no significant effect of annealing the sample in air and argon atmospheres however in hydrogen atmosphere deep level emissions were passivated.

Flaw Tolerance of Nuclear Intermediate Filament Lamina under Extreme Mechanical Deformation

The nuclear lamina, composed of intermediate filaments, is a structural protein meshwork at the nuclear membrane that protects genetic material and regulates gene expression. Here we uncover the physical basis of the material design of nuclear lamina that enables it to withstand extreme mechanical deformation of >100% strain despite the presence of structural defects. Through a simple in silico model we demonstrate that this is due to nanoscale mechanisms including protein unfolding, alpha-to-beta transition, and sliding, resulting in a characteristic nonlinear force-extension curve. At the larger microscale this leads to an extreme delocalization of mechanical energy dissipation, preventing catastrophic crack propagation. Yet, when catastrophic failure occurs under extreme loading, individual protein filaments are sacrificed rather than the entire meshwork. This mechanism is theoretically explained by a characteristic change of the tangent stress-strain hardening exponent under increasing strain. Our results elucidate the large extensibility of the nuclear lamina within muscle or skin tissue and potentially many other protein materials that are exposed to extreme mechanical conditions, and provide a new paradigm toward the de novo design of protein materials by engineering the nonlinear stress-strain response to facilitate flaw-tolerant behavior.

Revising the mechanism of polymer autooxidation

The basic scheme for autooxidation of polymers, originally developed by Bolland, Gee and co-workers for rubbers and lipids, is now widely applied to all types of polymeric materials. According to their scheme, the reaction that makes this process autocatalytic, referred to as the propagation step, is a hydrogen abstraction from the next substrate by the peroxyl radical (ROO˙ + RH → ROOH + R˙). In this study, using advanced quantum-chemical methods, we have shown that this step is actually characterised by largely positive Gibbs free energy (10-65 kJ mol(-1)) for most regular polymers with saturated chains (polypropylene, polyethylene, polyvinyl chloride, polyvinyl acetate, polyurethane, poly(methyl methacrylate) etc.) and even some polymers with unsaturated fragments (polystyrene, polyethylene terephthalate). Neither elevated temperature, nor solvation makes this process thermodynamically favourable. Only when the formed radical centre is conjugated with adjacent double bonds (as in polybutadiene) or captodatively stabilised by two suitable functional groups (such as a carbonyl and a lone pair donor such as oxygen or nitrogen), is the propagation step exoergic. Instead, we show that it is the presence of structural defects, such as terminal or internal double bonds, formed either during polymerisation or in the degradation process itself, that is responsible for the autooxidation of most polyesters and most polyalkenes. Recognition of the real mechanism of autooxidation in polymers is a key to developing strategies for the prevention of their degradation.

Theoretical Investigations on the Healing of Monovacancies in Single-Walled Carbon Nanotubes by Adsorption of Carbon Monoxide

The outstanding properties of single-walled carbon nanotubes (CNTs) are often impaired by the presence of structural defects, such as vacancies, as consequence of peculiar growth techniques or electron irradiation. However, the changes in the electronic structure due to the presence of defects can also induce the formation of sites at the sidewall exhibiting unusual reactive properties. In particular, monovacancies constitute active reaction sites toward the addition of different kinds of adsorbates, which can potentially cure the defects and restore the properties of pristine nanotubes. In this work, the interaction of carbon monoxide with monovacancies on single-walled CNTs is investigated by means of an integrated approach, based on density functional theory, consisting of both static and molecular dynamics calculations for the evaluation of potential-energy minima and free-energy barriers. Our simulations suggest viable reaction routes for the healing of single vacancies on the sidewall by subsequent interaction with two CO molecules, leading to the restoration of a perfect hexagonal carbon-atom network. The outlined results clarify the role of CO as precursor for the growth of nanotubes both as carbon source and as active chemical species for the formation of a defect-free sidewall.

Stacking Faults-Induced Quenching of the UV Luminescence in ZnO

ZnO nanorods grown by a hydrothermal technique on zinc substrate display an extrinsic free-to-bound electron-acceptor (eA°) luminescence with the A° level located at ∼130 meV above the valence band. The correlation between A° and the presence of structural defects is investigated by recording cathodoluminescence, transmission electron microscopy, and high-resolution electron microscopy images at the same location of the nanorod. The presence of dislocations and stacking faults observed in the nanorods is directly connected to the spatial variations of the luminescence intensity. Our study indicates that the (eA°) signal is quenched by structural defects.