Effect of Stress and Interface defects on Photo Luminescence of Si nano-crystals embedded in SiO2

Effect of stress and interface defects on photo luminescence property of a silicon nano-crystal (Si-nc) embedded in amorphous silicon dioxide (a-SiO2) are studied in this paper using a self-consistent quantum-continuum based modeling framework. Si-ncs or quantum dots show photoluminescence at room temperature. Whether its origin is due to Si-nc/a-SiO2 interface defects or quantum confinement of carriers in Si-nc is still an outstanding question. Earlier reports have shown that stresses greater than 12 GPa change the indirect energy band gap structure of bulk Si to a direct energy band gap structure. Such stresses are observed very often in nanostructures and these stresses influence the carrier confinement energy significantly. Hence, it is important to determine the effect of stress in addition to the structure of interface defects on photoluminescence property of Si-nc. In the present work, first a Si-nc embedded in a-SiO2 is constructed using molecular dynamics simulation framework considering the actual conditions they are grown so that the interface and residual stress in the structure evolves naturally during formation. We observe that the structure thus created has an interface of about 1 nm thick consisting of 41.95% of defective states mostly Sin+ (n = 0 to 3) coordination states. Further, both the Si-nc core and the embedding matrix are observed to be under a compressive strain. This residual strain field is applied in an effective mass k.p Hamiltonian formulation to determine the energy states of the carriers. The photo luminescence property computed based on the carrier confinement energy and interface energy states associated with defects will be analysed in details in the paper.

DLTS analysis of amphoteric interface defects in high-TiO2 MOS structures prepared by sol-gel spin-coating

High-kappa TiO2 thin films have been fabricated from a facile, combined sol-gel spin - coating technique on p and n type silicon substrate. XRD and Raman studies headed the existence of anatase phase of TiO2 with a small grain size of 18 nm. The refractive index `n' quantified from ellipsometry is 2.41. AFM studies suggest a high quality, pore free films with a fairly small surface roughness of 6 angstrom. The presence of Ti in its tetravalent state is confirmed by XPS analysis. The defect parameters observed at the interface of Si/TiO2 were studied by capacitance - voltage (C - V) and deep level transient spectroscopy (DLTS). The flat - band voltage (V-FB) and the density of slow interface states estimated are -0.9, -0.44 V and 5.24x10(10), 1.03x10(11) cm(-2); for the NMOS and PMOS capacitors, respectively. The activation energies, interface state densities and capture cross -sections measured by DLTS are E-V + 0.30, E-C - 0.21 eV; 8.73x10(11), 6.41x10(11) eV(-1) cm(-2) and 5.8x10(-23), 8.11x10(-23) cm(2) for the NMOS and PMOS structures, respectively. A low value of interface state density in both P-and N-MOS structures makes it a suitable alternate dielectric layer for CMOS applications. And also very low value of capture cross section for both the carriers due to the amphoteric nature of defect indicates that the traps are not aggressive recombination centers and possibly can not contribute to the device operation to a large extent. (C) 2015 Author(s).

Role of interface states associated with transitional nanophase precipitates in the photoluminescence enhancement of SrTiO3 : Pr3+,Al3+

SrTiO3:Pr3+,Al3+ phosphor samples with varying ratios of Sr/Ti/Al were prepared by the gel-carbonate method and the mechanism of enhancement of the red photoluminescence intensity therein was investigated. The photoluminescence (PL) spectra of SrTiO3:Pr3+ show both D-1(2) --> H-3(4) and P-3(0) --> H-3(4) emission in the red and blue spectral regions, respectively, with comparable intensity. The emission intensity of D-1(2) --> H-3(4) is drastically enhanced by the incorporation of Al3+ and excess Ti4+ in the compositional range Sr(Ti,Al-y)(O3+3y/2):Pr3+ (0.2 less than or equal to y less than or equal to 0.4) and SrTi1+xAlyO3+z:Pr3+ (0.2 less than or equal to x less than or equal to 0.5; 0.05 less than or equal to y less than or equal to 0.1; z = 2x + 3y/2) with the complete disappearance of the blue band. This cannot be explained by the simple point defect model as the EPR studies do not show any evidence for the presence of electron or hole centers. TEM investigations show the presence of exsolved nanophases of SrAl12O19 and/or TiO2 in the grain boundary region as well as grain interiors as lamellae which, in turn, form the solid-state defects, namely, dislocation networks, stacking faults and crystallographic shear planes whereby the framework of corner shared TiO6 octehedra changes over to edge-sharing TiO5-AlO5 strands as indicated from the Al-27 MAS NMR studies. The presence of transitional nanophases and the associated defects modify the excitation-emission processes by way of formation of electronic sub-levels at 3.40 and 4.43 eV, leading to magnetic-dipole related red emission with enhanced intensity. This is evidenced by the fact that SrAl12O19:Pr3+,Ti4+ shows bright red emission whereas SrAl12O19:Pr3+ does not show red photoluminescence.

Non-Destructive Evaluation of Defects and Damage in Composite Materials and Structures

The mechanical behaviour of composite materials differs from that of conventional structural materials owing to their heterogeneous and anisotropic nature. Different types of defects and anomalies get induced in these materials during the fabrication process. Further, during their service life, the components made of composite materials develop different types of damage. The performance and life of such components is governed by the combined effect of all these defects and damage. While porosity, voids, inclusions etc., are some defects those can get induced during the fabrication of composites, matrix cracks, interface debonds, delaminations and fiber breakage are major types of service induced damage which are of concern. During the service life of components made of composites, one type of damage can grow and initiate another type of damage. For example, matrix cracks can gradually grow to the interface and initiate debonds. Interface debonds in a particular plane can lead to delaminations. Consequently, the combined effect of different types of distributed damage causes the failure of the component. A set of non-destructive evaluation (NDE) methods is well established for testing conventional metallic materials. Some of them can also be utilized for composite materials as they are, and in some cases with a little different approach or modification. Ultrasonics, Radiography, Thermography, Fiber Optics, Acoustic Emision Techniques etc., to name a few. Detection, evaluation and characterization of different types of defects and damage encountered in composite materials and structures using different NDE tools is discussed briefly in this paper.

Supramolecular control of the magnetic anisotropy in two-dimensional high-spin Fe arrays at a metal interface

Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical(1,2) or quantum(3,4) behaviour. Individual atoms, however, are difficult to arrange in regular patterns(1-5). Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment(6,7). Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films.

Stability of Dimeric Interface in Banana Lectin: Insight from Molecular Dynamics Simulations

Banana lectin (Banlec) is a homodimeric non-glycosylated protein. It exhibits the b-prism I structure. High-temperature molecular dynamics simulations have been utilized to monitor and understand early stages of thermally induced unfolding of Banlec. The present study elucidates the behavior of the dimeric protein at four different temperatures and compares the structural and conformational changes to that of the minimized crystal structure. The process of unfolding was monitored by following the radius of gyration, the rms deviation of each residue, change in relative solvent accessibility and the pattern of inter- and intra-subunit interactions. The overall study demonstrates that the Banlec dimer is a highly stable structure, and the stability is mostly contributed by interfacial interactions. It maintains its overall conformation during high-temperature (400–500 K) simulations, with only the unstructured loop regions acquiring greater momentum under such condition. Nevertheless, at still higher temperatures (600 K) the tertiary structure is gradually lost which later extends to loss of secondary structural elements. The pattern of hydrogen bonding within the subunit and at the interface across different stages has been analyzed and has provided rationale for its intrinsic high stability.

Functional correlation of bacterial LuxS with their quaternary associations: interface analysis of the structure networks

Background The genome of a wide variety of prokaryotes contains the luxS gene homologue, which encodes for the protein S-ribosylhomocysteinelyase (LuxS). This protein is responsible for the production of the quorum sensing molecule, AI-2 and has been implicated in a variety of functions such as flagellar motility, metabolic regulation, toxin production and even in pathogenicity. A high structural similarity is present in the LuxS structures determined from a few species. In this study, we have modelled the structures from several other species and have investigated their dimer interfaces. We have attempted to correlate the interface features of LuxS with the phenotypic nature of the organisms. Results The protein structure networks (PSN) are constructed and graph theoretical analysis is performed on the structures obtained from X-ray crystallography and on the modelled ones. The interfaces, which are known to contain the active site, are characterized from the PSNs of these homodimeric proteins. The key features presented by the protein interfaces are investigated for the classification of the proteins in relation to their function. From our analysis, structural interface motifs are identified for each class in our dataset, which showed distinctly different pattern at the interface of LuxS for the probiotics and some extremophiles. Our analysis also reveals potential sites of mutation and geometric patterns at the interface that was not evident from conventional sequence alignment studies. Conclusion The structure network approach employed in this study for the analysis of dimeric interfaces in LuxS has brought out certain structural details at the side-chain interaction level, which were elusive from the conventional structure comparison methods. The results from this study provide a better understanding of the relation between the luxS gene and its functional role in the prokaryotes. This study also makes it possible to explore the potential direction towards the design of inhibitors of LuxS and thus towards a wide range of antimicrobials.

Nanocrystalline Janus films of inorganic materials prepared at the liquid-liquid interface

The interface between toluene and water has been employed to prepare ultrathin Janus nanocrystalline films of metal oxides, metal chalcogenides and gold, wherein the surface on the organic-side is hydrophobic and the aqueous-side is hydrophilic. We have changed the nature of the metal precursor or capping agent in the organic layer to increase the hydrophobicity. The strategy employed for this purpose is to increase the length of the alkane chain in the precursor or use a perfluroalkane derivative as precursor or as a capping agent. The hydrophobicity and hydrophilicity of the Janus films have been determined by contact angle measurements. The morphology of hydrophobic and hydrophilic sides of the film have been examined by field emission scanning electron microscopy.

Influence of non-stoichiometric defects on nonlinear absorption and refraction in Nd:Zn co-doped lithium niobate

Nonlinear absorption and refraction phenomena in stoichiometric lithium niobate (SLN) pure and co-doped with Zn and Nd, and congruent lithium niobate (CLN) were investigated using Z-scan technique. Femtosecond laser pulses from Ti:Sapphire laser (800 nm, 110 fs pulse width and 1 kHz repetition rate) were utilized for the experiment. The process responsible for nonlinear behavior of the samples was identified to be three photon absorption (3PA). This is in agreement with the band gap energies of the samples obtained from the linear absorption cut off and the slope of the plot of Ln(1 − TOA) vs. Ln(I0) using Sutherland’s theory (s = 2.1, for 3PA). The nonlinear refractive index (n2) of Zn doped samples was found to be lower than that of pure samples. Our experiments show that there exists a correlation between the nonlinear properties and the stoichiometry of the samples. The values of n2 fall into the same range as those obtained for the materials of similar band gap.

Partial discharges at low pressures in a dielectric interface between uniform field electrodes

Partial discharges in a gaseous interface due to the presence of a dielectric between two uniform field electrodes in air at different pressures from 0.5 to 685 mm Hg have been studied and measurements of inception and extinction voltages, number of pulses and their charge magnitudes at inception are reported. It has been observed that the extinction voltage can be as low as 70% of the inception voltage suggesting that the working voltage in such cases should be about 30% lower than the observed inception voltage. Small magnitude pulses are found to be more in number than large magnitude pulses. The charge is found to be pressure dependent. The results have been explained on the basis of an equivalent circuit consisting of resistance and capacitance in which the discharge gap functions as a switch.

Interaction Between Two Residues in the Inter-Domain Interface of Escherichia coli Peptidase N Modulates Catalytic Activity

The role of interaction between Asn259 (catalytic domain) with Gln821 (C-terminal domain) in PeptidaseN was investigated. The k(cat) of PeptidaseN containing Asn259Asp or Gln821Glu is enhanced whereas it is suppressed in Asn259AspGln821Glu. Structural analysis shows this interaction to change the relative disposition of active site residues, which modulates catalytic activity.

Molecular dynamics simulations of sliding in an Fe-Cu tribopair system

Experimental evidence suggests that high strain rates, stresses, strains and temperatures are experienced near sliding interfaces. The associated microstructural changes are due to several dynamic an interacting phenomena. 3D non-equilibrium molecular dynamics (MD) simulations of sliding were conducted with the aim of understanding the dynamic processes taking place in crystalline tribopairs, with a focus on plastic deformation and microstructural evolution. Embedded atom potentials were employed for simulating sliding of an Fe-Cu tribopair. Sliding velocity, crystal orientation and presence of lattice defects were some of the variables in these simulations. Extensive plastic deformation involving dislocation and twin activity, dynamic recrystallization, amorphization and/or nanocrystallization, mechanical mixing and material transfer were observed. Mechanical mixing in the vicinity of the sliding interface was observed even in the Fe-Cu system, which would cluster under equilibrium conditions, hinting at the ballistic nature of the process. Flow localization was observed at high velocities implying the possible role of adiabatic heating. The presence of preexisting defects (such as dislocations and interfaces) played a pivotal role in determining friction and microstructural evolution. The study also shed light on the relationship between adhesion and plastic deformation, and friction. Comparisons with experiments suggest that such simulations can indeed provide valuable insights that are difficult to obtain from experiments.

Nanostructured Peptide Fibrils Formed at the Organic-Aqueous Interface and Their Use as Templates To Prepare Inorganic Nanostructures

Formation of fibril-type nanostructures of the Alzheimer's beta-amyloid diphenylalanine (L-Phe-L-Phe, FF) at the organic-aqueous interface and the factors affecting their structures have been investigated. Such nanostructures are also formed by bovine serum albumin and bovine pancreas insulin. The concentration of the precursor taken in the aqueous layer plays an important role in determining the morphology of the nanostructures, The addition of curcumin to the organic layer changes the structure of the self-assembled one-dimensional aggregates of diphenylalanine. By coating the diphenylalanine dipeptide fibrils with appropriate precursors followed by calcination in air, it has been possible to obtain one-dimensional nanostructures of inorganic materials.

Structural effects of a dimer interface mutation on catalytic activity of triosephosphate isomerase.The role of conserved residues and complementary mutations

The active site of triosephosphate isomerase (TIM, EC: 5.3.1.1), a dimeric enzyme, lies very close to the subunit interface. Attempts to engineer monomeric enzymes have yielded well-folded proteins with dramatically reduced activity. The role of dimer interface residues in the stability and activity of the Plasmodium falciparum enzyme, PfTIM, has been probed by analysis of mutational effects at residue 74. The PfTIM triple mutant W11F/W168F/Y74W (Y74W*) has been shown to dissociate at low protein concentrations, and exhibits considerably reduced stability in the presence of denaturants, urea and guanidinium chloride. The Y74W* mutant exhibits concentration-dependent activity, with an approximately 22-fold enhancement of kcat over a concentration range of 2.5–40 μm, suggesting that dimerization is obligatory for enzyme activity. The Y74W* mutant shows an approximately 20-fold reduction in activity compared to the control enzyme (PfTIM WT*, W11F/W168F). Careful inspection of the available crystal structures of the enzyme, together with 412 unique protein sequences, revealed the importance of conserved residues in the vicinity of the active site that serve to position the functional K12 residue. The network of key interactions spans the interacting subunits. The Y74W* mutation can perturb orientations of the active site residues, due to steric clashes with proximal aromatic residues in PfTIM. The available crystal structures of the enzyme from Giardia lamblia, which contains a Trp residue at the structurally equivalent position, establishes the need for complementary mutations and maintenance of weak interactions in order to accommodate the bulky side chain and preserve active site integrity.