Density Of Structural Defects Research Articles

Formation of thermal defects in silicon grown by means of float zone melting

The factors of formation of thermal donors (TDs) and thermal acceptors (TAs) silicon with a low oxygen concentration grown by means of float-zone melting are discussed. The results of thermal treatment in a temperature range of 400–1150°C show that interstitial iron atoms produce the largest contribution to formation of TDs. Substitutional atoms of iron are probable drivers of TA formation in the process of hightemperature treatment (HTT). Iron precipitates formed during low-temperature annealing (400–600°C) also contribute to TA formation. The TD and TA densities after HTT depend on the type and the density of structural defects in the material and the conditions of thermal treatment: the cooling rate and the gas medium (oxygen or argon).

Elucidating the Mechanisms Driving the Aging of Porous Hollow PtNi/C Nanoparticles by Means of COads Stripping.

The oxygen reduction reaction (ORR) activity of Pt-alloy electrocatalysts depends on (i) the strain/ligand effects induced by the non-noble metal (3d-transition metal or a rare-earth element) alloyed to Pt, (ii) the orientation of the catalytic surfaces, and (iii) the density of structural defects (SDs) (e.g., vacancies, voids, interconnections). These SDs influence the "generalized" coordination number of Pt atoms, the Pt-alloy lattice parameter, and thus the adsorption strength of the ORR intermediates (O*, OH*, OOH*). Here, we discuss a set of parameters derived from COads stripping measurements and the Rietveld refinement of X-ray diffraction (XRD) patterns, aiming to show how the leaching of the non-noble metal and the density of SDs influence the ORR activity of porous hollow PtNi/C nanoparticles (PH-PtNi/C NPs). PH-PtNi/C NPs were aged at T = 353 K in an Ar-saturated 0.1 M HClO4 electrolyte during 20 000 potential cycles between E = 0.6 and 1.0 V versus the reversible hydrogen electrode, with an intermediate characterization after 5000 cycles. The losses in the ORR specific activity were attributed to the dissolution of Ni atoms (modifying strain/ligand effects) and to the increase of the crystallite size (dXRD), resulting in a diminution of the density of grain boundaries. In agreement with the Gibbs-Thompson equation, the electrocatalysts that presented larger crystallites (dXRD > 3 nm) were far more stable than the ones with the smallest crystallites (dXRD < 2 nm). We also observed that performing intermediate characterizations (in an O2-saturated electrolyte) results in activity losses for the ORR.

Effects of precursor topology on polymer networks simulated with molecular dynamics

Molecular modeling of crosslinked polymers often follows arbitrary pathways for network generation, with different precursor topology from experimental systems. We use coarse-grained molecular simulation to study the effects of precursor choice on the predicted network structure and properties. Three sets of precursors with different molecular architectures are designed such that they would form identical networks at the limit of perfect conversion. Little difference is observed between the resulting networks in typical properties including the radial distribution function, macroscopic statistics of network connectivity, and glass transition behaviors. However, the stress-strain relationship in tensile deformation clearly depends on the formation pathway when compared at the same crosslinking density. The elastic modulus of the network is found to correlate strongly with the number of elastic strands in the network, except at the highly-crosslinked limit where substantial discrepancy is observed between networks from different precursors. Although these final networks contain a similar average density of structural defects, the choice of precursor has significant impact on their spatial distribution, leading to the precursor dependence of their mechanical properties. Uniform defect distribution and fast defect elimination can be achieved by designing precursor units with a proper stoichiometric ratio of different monomers.

Monovalent Cation Doping of CH&lt;sub&gt;3&lt;/sub&gt;NH&lt;sub&gt;3&lt;/sub&gt;PbI&lt;sub&gt;3&lt;/sub&gt; for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Chemically Reduced Graphene Oxide for the Assessment of Food Quality: How the Electrochemical Platform Should Be Tailored to the Application.

Graphene platforms have been drawing considerable attention in electrochemistry for the detection of various electroactive probes. Depending on the chemical composition and properties of the probe, graphene materials with diverse structural features may be required to achieve an optimal electrochemical performance. This work comprises a comparative study on three chemically modified graphenes, obtained from the same starting material and with different oxygen functionalities and structural defects (graphene oxide (GO), chemically reduced graphene oxide (CRGO), and thermally reduced graphene oxide (TRGO)) towards the electrochemical detection of quinine, an important flavoring agent present in tonic-based beverages. In general, the reduced graphenes, namely CRGO and TRGO, showed enhanced performance in terms of calibration sensitivity and selectivity, due to the improved heterogeneous electron-transfer rates on their surfaces. In particular, CRGO provided the best overall electrochemical performance, which can be attributed to its higher density of structural defects and reduced amount of oxygen functionalities. For this reason, CRGO was employed for the electrochemical detection of quinine in commercial tonic drink samples, showing high sensitivity and selectivity, and therefore representing a valid low-cost alternative to more complicated and time consuming traditional analytical methods.

CAFM Experimental Considerations and Measurement Methodology for In-Line Monitoring and Quantitative Analysis of III–V Materials Defects

To continue technology scaling, a new generation of high-performance devices are considered to be implemented using III–V semiconductors, which need to be grown over the conventional Si substrate. However, due to the lattice mismatch between the III–V and silicon materials, the former tend to develop significant density of structural defects [specifically, threading dislocations (TDs)], which can adversely affect device electrical characteristics. Conductive atomic force microscope (CAFM) technique is among the most promising tools for the identification and analysis of TDs in a nanoscale range although obtaining reliable quantitative data requires precise controls over the measurements conditions. In this study, CAFM technique has been applied for TDs detection and analysis in III–V films, and tool requirements and measurement methodology are discussed.

ChemInform Abstract: Design Directed Self‐Assembly of Donor—Acceptor Polymers

AbstractReview: 76 refs.

Variation of critical current and n-value of 2G HTS tapes in external magnetic fields of different orientation

The in-field orientation dependence of critical current and n-value in second generation high temperature superconductive tapes was investigated. The samples were manufactured by Metalorganic Chemical Vapour Deposition method with BaZrO3 inclusions (SuperPower Inc.) and Pulsed Laser Deposition method (Bruker HTS). For samples of each kind of fabrication techniques we observed higher critical current value in the case of external magnetic field aligned along (or nearby) c-axis direction in comparison with one aligned along ab-plane. We analysed possible reasons for this effect. Angular dependences of the critical current and n-value were investigated. The microstructure images of superconductive layer of studied samples show tilt of BaZrO3 nanorods in MOCVD sample and high density of structural defects for PLD sample.

Exceeding 3 ms Minority Carrier Lifetime in n–type Non-contact Crucible Silicon

The presence of metal impurities and their interactions with structural defects (e.g., dislocations) are deleterious to the performance of Si-based solar cell devices. To achieve higher minority carrier lifetimes that translate into higher solar cell efficiencies, novel growth methods with low dislocation densities and reduced metal impurity concentrations have recently been developed. These methods simultaneously aim to achieve low capital expense (capex), necessary to ensure rapid industry scaling. Monocrystalline Si grown by the non-contact crucible method (NOC-Si) has the potential to achieve high bulk minority carrier lifetimes and high efficiencies at low cost given its low structural defect density. Growth in large-diameter crucibles ensures high throughput consistent with low capex. However, high temperatures, coupled with conditions during Si growth (e.g., crucible and ambient gas) can lead to the in-diffusion of impurities, compromising the potential to achieve high efficiency solar cell devices. Herein, we report high minority-carrier lifetimes exceeding 3 milliseconds (ms) in n-type NOC-Si material, achieved through a strict impurity-control procedure at the growth stage that prevents in-diffusion of impurities to the melt, coupled with a tailored defect-engineering process via optimized phosphorus gettering.

Structural Defect Evolution of TATB‐Based Compounds Induced by Processing Operations and Thermal Treatments

Abstract2,4,6‐Triamino‐1,3,5‐trinitrobenzene (TATB) compounds are commonly used in high performance explosives because of their thermal stability and high detonation velocities compared to other materials. The insensitivity and mechanical properties are related to the stability of their crystalline structure. Crystallographic structure and structural defects evolution of TATB and TATB‐based compounds were studied by X‐ray diffraction for powders, molding powders, and pressed compounds, using Rietveld refinement. The effects of synthesis conditions, thermal treatments, coating and pressing operations on the structure of TATB compounds were evaluated. The results show that the pressing operation results in anisotropic crystallite size, leading to an increase of the structural defects density. It could be due to the anisotropic mechanical response of the TATB crystal under pressure, possibly plasticity. Finally, it is shown that increasing thermal treatment temperature on TATB powders decreases the structural defects density.

Long Charge Carrier Lifetime in As-Grown 4H-SiC Epilayer

Over 150 μm thick epilayers of 4H-SiC with long carrier lifetime have been grown with a chlorinated growth process. The carrier lifetime have been determined by time resolved photoluminescence (TRPL), the lifetime varies a lot between different areas of the sample. This study investigates the origins of lifetime variations in different regions using deep level transient spectroscopy (DLTS), low temperature photoluminescence (LTPL) and a combination of KOH etching and optical microscopy. From optical microscope images it is shown that the area with the shortest carrier lifetime corresponds to an area with high density of structural defects.

Characterization of chemical doping of graphene by in-situ Raman spectroscopy

We explored single-layer graphene and graphene field-effect transistors immersed in nitric acid using in-situ Raman spectroscopy. Two distinct stages were observed in the chemical doping process. The first stage involved blue shifts of the G and 2D peaks, whose saturation occurred rapidly with a time constant in the range of 10–25 s depending on the molar concentration of the acid. In the second stage, the intensity of the D peak, which was associated with structural defect formation, increased for a relatively long period of time. Since the major doping effects appeared during the first stage, the optimal doping conditions under which no noticeable structural defect formation occurred can be determined by monitoring the frequency shift. Transient doping concentrations along with structural defect densities were obtained from the Raman peak positions and intensities. We found that the doping-induced shift in the Dirac point in graphene field-effect transistors exhibited a fast response with respect to frequency shifts in the Raman spectra, which was attributed to the saturation of electrostatic gating effects.

Effect of potassium on the structure and catalytic properties of K1.2Cu0.4Fe2O4

We have studied the structural properties of the phase K1.2Cu0.4Fe2O4 and the effect of potassium on its catalytic activity for oxidation of carbon. The results demonstrate that the potassium-doped phase differs from the undoped ferrite CuFe2O4 in linear dimensions of its unit cell, high density of structural defects, and low-temperature activity and activation energy for the catalytic process. Contact interaction between K1.2Cu0.4Fe2O4 and carbon in the temperature range 240–420°C leads to the reduction of Cu2+ to Cu+ and the formation of Cu2O, CuFeO2, and K2Fe4O7. Testing results for the phases identified indicate that the catalytic processes in the presence of K2Fe4O7 and K1.2Cu0.4Fe2O4 are identical.

Synthesis, X-ray structure analysis, and Raman spectroscopy of R2TiO5-based (R = Sc, Y) solid solutions

Order–disorder transitions in xR2O3 · (1 − x)TiO2 (R = Sc, 0.4 ≤ x ≤ 0.5; R = Y, 0.5 ≤ x ≤ 0.6) solid solutions with highly imperfect fluorite-derived structures have been studied using monochromatic synchrotron X-ray diffraction and Raman spectroscopy. The results demonstrate that the synthesis process leads to the formation of a fluorite-like (Fm3m) disordered phase and a nanoscale (~10–100 nm) pyrochlore-like (Fd3m) ordered phase of the same composition, coherent with the disordered phase. We have determined their lattice parameters. The Raman spectra of Sc2TiO5 (Y2TiO5) contain broad lines in low- and high-frequency regions: at 190, 350, and 775 (134, 188, 365, 404, and 727) cm−1. These lines are characteristic of a pyrochlore-like phase with a varying degree of order and a disordered fluorite-like phase, respectively. The pyrochlore-like phase Y2Ti2O7 has two strong Raman peaks in the low-frequency region: at 312 and 527 cm−1. The formation of nanodomains with different degrees of order is caused by the internal stress that arises from the high density of structural defects in the unit cells of the solid solutions.

Nanoscale surface modification of AISI 316L stainless steel by severe shot peening

Surface nanocrystallization is gaining increased attention due to its high potential of enhancing mechanical functionality and modulating material's interaction with the surrounding environment. Among mechanical approaches, severe shot peening has recently shown to be a very promising technique for surface grain refinement, considering its efficiency, eco-friendliness, relative low cost and minimum geometrical restrictions. This study evaluates the effect of severe shot peening on microstructural and mechanical properties of 316L stainless steel, which is widely used in biomedical, food preparation, structural and marine applications. 316L samples, shot peened with different peening parameters, were studied in terms of morphological and structural features, defect density, grain size, phase transformation, surface topography, surface wettability, residual stresses and microhardness. The results indicated that severe shot peening induced near surface grain refinement to nano and sub-micron range and transformed the austenite phase into strain-induced α’- martensite in a layered deformation band structure. Severe shot peening also induced compressive residual stresses and work hardening in the top surface layer. Surface roughness and surface wettability, both of which favorably contribute to modulating the interaction of material with biological environment, were also notably enhanced by severe shot peening.

High-Performance Phototransistors Based on PDIF-CN2 Solution-Processed Single Fiber and Multifiber Assembly.

Here we describe the fabrication of organic phototransistors based on either single or multifibers integrated in three-terminal devices. These self-assembled fibers have been produced by solvent-induced precipitation of an air stable and solution-processable perylene di-imide derivative, i.e., PDIF-CN2. The optoelectronic properties of these devices were compared to devices incorporating more disordered spin-coated PDIF-CN2 thin-films. The single-fiber devices revealed significantly higher field-effect mobilities, compared to multifiber and thin-films, exceeding 2 cm(2) V(-1) s(-1). Such an efficient charge transport is the result of strong intermolecular coupling between closely packed PDIF-CN2 molecules and of a low density of structural defects. The improved crystallinity allows efficient collection of photogenerated Frenkel excitons, which results in the highest reported responsivity (R) for single-fiber PDI-based phototransistors, and photosensitivity (P) exceeding 2 × 10(3) AW(-1), and 5 × 10(3), respectively. These findings provide unambiguous evidence for the key role played by the high degree of order at the supramolecular level to leverage the material's properties toward the fabrication of light-sensitive organic field-effect transistors combining a good operational stability, high responsivity and photosensitivity. Our results show also that the air-stability performances are superior in devices where highly crystalline supramolecularly engineered architectures serve as the active layer.

InP Nanoflag Growth from a Nanowire Template by in Situ Catalyst Manipulation.

Quasi-two-dimensional semiconductor materials are desirable for electronic, photonic, and energy conversion applications as well as fundamental science. We report on the synthesis of indium phosphide flag-like nanostructures by epitaxial growth on a nanowire template at 95% yield. The technique is based on in situ catalyst unpinning from the top of the nanowire and its induced migration along the nanowire sidewall. Investigation of the mechanism responsible for catalyst movement shows that its final position is determined by the structural defect density along the nanowire. The crystal structure of the "flagpole" nanowire is epitaxially transferred to the nanoflag. Pure wurtzite InP nanomembranes with just a single stacking fault originating from the defect in the flagpole that pinned the catalyst were obtained. Optical characterization shows efficient highly polarized photoluminescence at room temperature from a single nanoflag with up to 90% degree of linear polarization. Electric field intensity enhancement of the incident light was calculated to be 57, concentrated at the nanoflag tip. The presented growth method is general and thus can be employed for achieving similar nanostructures in other III-V semiconductor material systems with potential applications in active nanophotonics.

Exceptional gettering response of epitaxially grown kerfless silicon

The bulk minority-carrier lifetime in p- and n-type kerfless epitaxial (epi) crystalline silicon wafers is shown to increase &amp;gt;500× during phosphorus gettering. We employ kinetic defect simulations and microstructural characterization techniques to elucidate the root cause of this exceptional gettering response. Simulations and deep-level transient spectroscopy (DLTS) indicate that a high concentration of point defects (likely Pt) is “locked in” during fast (60 °C/min) cooling during epi wafer growth. The fine dispersion of moderately fast-diffusing recombination-active point defects limits as-grown lifetime but can also be removed during gettering, confirmed by DLTS measurements. Synchrotron-based X-ray fluorescence microscopy indicates metal agglomerates at structural defects, yet the structural defect density is sufficiently low to enable high lifetimes. Consequently, after phosphorus diffusion gettering, epi silicon exhibits a higher lifetime than materials with similar bulk impurity contents but higher densities of structural defects, including multicrystalline ingot and ribbon silicon materials. Device simulations suggest a solar-cell efficiency potential of this material &amp;gt;23%.

CARACTERIZACIÓN OPTO-ELÉCTRICA DE BICAPAS n+-ZnO/i-ZnO CRECIDAS IN SITU POR EVAPORACIÓN REACTIVA SIN USAR DOPAJE EXTRÍNSECO

Este trabajo describe un procedimiento para crecer in situ películas delgadas de n+-ZnO e i-ZnO altamente transparentes, depositadas secuencialmente por evaporación reactiva asistido por plasma sin usar dopaje extrínseco. Se logró una buena reproducibilidad del espesor y de las propiedades opto-eléctricas de las películas a través de un control electrónico novedoso desarrollado usando el concepto de instrumentación virtual. Para lo cual se implementó un instrumento virtual (VI) que controlaba el proceso usando PID (proportional integral differential) y PWM (pulse width modulation) como algoritmos de control. Optimizando el diseño del reactor y los parámetros de deposición se obtuvieron con este método películas n+-ZnO e i-ZnO con resistividades alrededor de 6 x 10-4 cm y 104 cm respectivamente y transmitancias mayores al 85% (en la región visible). A partir de medidas de energía Urbach encontramos que las películas n+-ZnO depositadas controlando apropiadamente la cantidad de zinc que llega al reactor tienen una cantidad baja de defectos estructurales. También se reportan resultados relacionados con propiedades de transporte eléctrico de las películas de ZnO, obtenidas de medidas de conductividad y movilidad dependientes de la temperatura.