Random Atomic Arrangement Research Articles

Up-conversion and near-infrared luminescence in Er3+ -doped La2O3-PbO-B2O3-TeO2 glasses for the generation of fibre amplifiers and green LED applications

In this work, we demonstrate the influence of Er3+ ions on the spectroscopic properties of 5La2O3-15PbO-(50-x)B2O3-xEr2O3-30TeO2 glasses doped with x = 0.1, 0.5, 1.0, 2.0 & 3.0 mol% of Er3+ were prepared with the melt-quenching method. The structural, fluorescence and decay time properties were examined, and the theoretical model, JO, Mc, and FL theory, was utilised to confirm the experimental findings. It is evident from XRD results that the glasses do not possess a well-defined crystalline structure. Instead, they exhibit a random arrangement of atoms and molecules, indicating an amorphous characteristic. JO analysis yields the stimulated-emission cross-section (2.12 ×10−20 for LPBTE1 glass, which is highest among other prepared glasses) & intensity parameters (Ω2,4,6) show Ω2 > Ω6 > Ω4 trend indicating the covalency nature. Predominant luminescence in the NIR region, emission peaked at 1537 nm was observed, which was ascribed to the 4I13/2⟶4I15/2 transition. The temporal decay rate decreased from 0.83 ms to 0.27 ms with an increase in the concentration of Er3+ from 0.1 to 3 mol%, which is due to the self-quenching action of the cross-relaxation channels. Furthermore, using Mc-Cumber's theory the peak values for the emission and absorption cross-sections are evaluated and found in good agreement with each other. The gain profile demonstrates more than 40% population inversion between 1530 and 1610 nm in the C +L band. In addition, the up-conversion of 4S3/2⟶4I15/2 transitions at 544 nm exhibits green emission shows two-photon absorptions. Overall, the investigated results demonstrate that the prepared Er3+ -doped La2O3-PbO-B2O3-TeO2 glasses are the best choice for optical fibre applications.

Clarifying the Origin of Ordering Effect of PtCo Intermetallic Catalysts in Catalyzing Oxygen Reduction

Pt intermetallic catalysts exhibit higher activity for oxygen reduction reaction (ORR) than the disordered counterparts, but the origin of exact effect of structural ordering has not been fully clarified yet. Herein, by comparing the PtCo catalysts with changed ordering degree but the exactly same alloying degree, we distinguish the ordering effect from alloying effect of intermetallic catalysts in catalyzing ORR; the ordering effect is identified to increase the ORR activity by ∼1.5 times. The highly ordered PtCo intermetallic catalyst is first prepared by separating the alloying and ordering stages, then the disordered counterpart with the same alloying degree is obtained by quenching the ordered catalyst at high temperatures to freeze the random atomic arrangement. Experimental and theoretical studies reveal that both alloying effect and ordering effect of intermetallic alloy catalysts intrinsically originate from the compressive strain effect, which leads to the weakened adsorbate bonding towards oxygen-containing adsorbate and thus the promoted ORR kinetics.

A localized stress field approach for calculating the critical stress intensity factor for an isotropic solid at atomistic scale

Crack growth initiation in brittle materials is generally studied using the stress intensity factor (SIF) since linear elasticity is assumed to be valid in the crack-tip region. The critical SIF is the SIF just before the onset of crack growth. In atomistic simulations, the “local” critical SIF is usually found indirectly by relating it to the “global” energetically calculated critical energy release rate. Since the crack growth happens at the crack tip, a method for calculating critical SIF (at a state just before crack growth initiation) directly by near-tip stress field has been implemented. The implementation of this methodology for amorphous isotropic solids is not trivial due to the random arrangement of atoms. Further, the presence of two types of elements in amorphous solids (silica is considered here) complicates the process since the volume occupied by each atom will be different (making the use of virial stress extremely complicated). In the present work, it has first been shown that the amorphous solid is indeed linear elastic isotropic by conducting various tests for finding elastic constants and showing that only two of them are independent. Then, a near-tip stress field is calculated by averaging the analytically calculated stress. The near-tip stresses in amorphous silica, validates the presence of linear elasticity beyond a small region from the crack tip. Even with the truly amorphous nature of silica, the minimum reported R2 for the inverse square-root fit (K-dominance) is around 80% at the critical state just before the onset of crack growth. The value of critical SIF calculated directly from the near-tip stresses in the present work comes out to be very close to those predicted by other indirect methods (experiments and atomistic simulations). The proposed method in the present work has a general applicability to measure the critical SIF directly using the near-tip stress field for any amorphous isotropic solid at the atomistic scale irrespective of the number of type of constituent elements.

Chemical reaction of Al-Sm sputtered amorphous electrode for stable encapsulation-free organic electronics

An Al-based amorphous metal alloy (i.e., metallic glass) is fabricated in the form of a thin film electrode for an organic solar cell. The chemical stability of the amorphous alloy thin film is analysed via electrochemical analyses, which reveals that the random atomic arrangement within the electrode suppresses the oxidation reaction upon contact with harsh conditions. Incorporation of an amorphous metal thin film as a top electrode for organic solar cell devices results in considerable enhancement in device stability, as the metallic surface with the amorphous phase hinders the ambient moisture and oxygen from permeating into the underlying organic layers. The electrical parameters of the amorphous electrodes are analysed before and after aging and compared with those of a conventional crystalline metal electrode, which presents the advantage of amorphous electrodes in maintaining high charge transport properties for an extended time.

ОСОБЕННОСТИ НАНОДВОЙНИКОВЫХ СТРУКТУР В СПЛАВАХ ГЕЙСЛЕРА NI2MN1,5IN0,5 И NI2MN1,75IN0,25

This article presents the results of calculating the phase stability and structural properties of the twin structures of Ni2Mn1,5In0,5 and Ni2Mn1,75In0,25 Heusler alloys. The structures with a random and periodic arrangement of Mn-excess atoms in the In sublattice were considered. It is shown that the compositions of Ni2Mn1,5In0,5 and Ni2Mn1,75In0,25, with modulation periods of 2–5 and 3–3, respectively, are stable relative to all twin structures. The distribution of Mn-excess atoms does not affect the structural characteristics of the systems. The nanotwin structures of Ni2Mn1,5In0,5 and Ni2Mn1,75In0,25 alloys possess similar crystal lattice parameters. An increase in the stability of the structures with respect to the decomposition into constituent stable components is observed with an increase in the concentration of Mn.

Structural disorder induced enhanced gas-sensing performance of ZnO with surface amorphous layer

The development of highly sensitive gas sensors is of great significance to the environment and human health. In this work, we demonstrate the enhancement of the gas-sensing performance of ZnO via employing disorder surface. The random arrangement of surface atoms could give rise to a coarse surface for improving gas adsorption. Moreover, due to the absence of carrier diffusion in amorphous layer, the Anderson localization could restrict the carrier interaction. Most crucially, the electron localization led by lattice disorder significantly elevate the gas monitoring sensitivity. As a result, the acetone sensitivity of ZnO is greatly enhanced (up to ∼ 20.8 times compared with the pristine sample). This work theoretically and experimentally investigate the role of surface disorder in enhancing gas-sensing performance, and also present a mild, low-cost method to modify the material for further applications.

Application of AgPt Nanoshells in Direct Methanol Fuel Cells: Experimental and Theoretical Insights of Design Electrocatalysts over Methanol Crossover Effect

AbstractBased on theoretical simulations, the best design for obtaining AgPt nanostructures (nanoshells with hollow interior) was unraveled that could exhibit methanol tolerance for oxygen reduction reaction (ORR) that occurs during direct methanol fuel cells (DMFCs) operation. A theoretical investigation of Pt@Ag and Ag@Pt core‐shell nanoparticles and AgPt nanoshells′ interaction with oxygen and methanol revealed that the oxygen interaction is significantly more favorable on AgPt nanoshells′ surface, hindering the methanol oxidation reaction (MOR) due to the random arrangement of Ag and Pt atoms. Experimentally, the nanoshells were prepared by a galvanic substitution and immobilized them onto silica, and the material was finely understood by associating electrochemical and physicochemical studies. Cyclic voltammetry showed the reduction and oxidation processes of the catalyst‘s species; however, XPS precisely showed that significant amounts of oxidized species were present (60.5 % of Ag0 and 39.5 % of Ag+, and 55.1 % of Pt0 and 44.9 % of Pt+2), which could affect the performance of the material. Indeed, the catalyst showed an excellent performance to ORR; the system yielded a 4‐electron ORR mechanism with just 1.0 wt.% Pt loading, with significant stability after 1000 runs. In addition, Koutecky‐Levich and Tafel plots assisted in understanding better the mechanism on the catalyst‘s surface, suggesting a first‐electron transfer for the rate‐determining step. Also, the catalyst resistance to the methanol crossover, theoretically simulated and predicted, was tested, showing remarkable tolerance for the alcohol up to a concentration of 2 M. Hence, a cathode catalyst with improved selectivity, low metal loading, high stability, and easy preparation was obtained.

An Investigation of Monolayer As1−xPx Solid Solutions: From a Theoretical Perspective

AbstractPhase stability and disordered solid solutions should be noticed in theoretical works due to the operating temperature up to several hundred Kelvin in the normal growth process. To better understand the properties of monolayer As1−xPx solid solutions, the phase diagram and properties of both ordered and disordered structures are investigated from a theoretical perspective. It is found that monolayer As1−xPx prefer phase‐separating structures at low temperatures and will form disordered solid solutions with temperature elevating. Considering that random atomic arrangement will destroy the inner symmetry of structures, disordered solid solutions are expected to hold piezoelectric properties which are not available in the phase‐separating ordered structures. The strains induced properties are examined which could give a detailed picture of the electronic and catalytic properties. Here, the systematic investigation of As1−xPx will be valuable for understanding monolayer As1−xPx solid solutions and is believed to set a tone for applications in further electronics and optoelectronic applications.

Physical spread and technical upshift in the band gaps of visible-light photocatalytic bismuth oxyhalide solid solutions

Semiconductor band gap determines the wavelength range of the utilized light, and is a key factor for many optical applications, e.g., photocatalysis and optoelectronic devices. As a superior group of bismuth oxyhalide, binary BiOIxBr1-x solid solutions have the visible-light band gap (εg=2.8∼1.8 eV) that is readily tunable by changing the halogen composition. However, there remains a long-standing problem that the εgs of BiOIxBr1-x have never been exactly confirmed by experimental measurements (with an error ≲0.5 eV). Using density-functional-theory calculations, we find that all of the sampled random structures for BiOIxBr1-x solid solutions have readily accessible stabilities, and the random arrangement of halogen atoms can result in a large physical spread (∼0.8 eV) in electronic εg. Such large band-gap variation with halogen-atom arrangement is attributed to the sensitive dependence of valence-band-maximum level on the local halogen composition. In the optical absorption spectra, we find the remarkable upshift of the optical εg by 0.5 eV when only a small absorption-edge threshold (e.g., 2×104cm-1) is used, which is ascribed to the small amount of conduction-band-minimum states. To further reveal the finite optical absorption within band gap that is responsible for the absorption-edge threshold existing in experimental characterizations, we finally calculate the optical absorption spectra of surfaces and find it is the metallic surfaces induced by halogen deficiency that causes the finite in-gap absorption. Such mechanism derived from the two-dimensional defects (i.e., surfaces) guides us to find similar phenomena on a zero-dimensional defect (i.e., halogen vacancy). The band-gap upshift, together with the physical spread, can explain the band-gap uncertainty existing in experimental results.

Experimental observation of two types of linear magnetoresistance in topological crystalline insulator heterostructures

We report magnetotransport studies of topological crystalline insulator (TCI) heterostructures SnTe/PbTe and SnTe/PbTe/SnTe grown by molecular beam epitaxy. A giant linear magnetoresistance up to 1715% was observed in SnTe/PbTe heterostructure, which is correlated with the topological surface states in SnTe. A large linear magnetoresistance (order of 102%) is exhibited in SnTe/PbTe/SnTe heterostructures. The large linear magnetoresistance may be attributed to the structural disorder of SnTe induced by the random arrangement of Pb atoms in SnTe crystalline lattice. The findings of these two types of linear magnetoresistance are of great importance to magnetoelectronics and spintronics based on the TCI heterostructure.

Synergistically Enhanced Oxygen Evolution Reaction Catalysis for Multielement Transition-Metal Oxides

Transition metal oxides have been extensively investigated as novel catalysts for oxygen evolution reaction (OER). Partial elemental substitutions are effective ways to increase catalytic performance and such electronic interactions between multiple elements are known as synergistic effects. However, serious issues such as random atomic arrangement and ambiguous roles of constituent elements humper theoretical investigations for rational materials design. Herein, we describe systematic study on OER activity of AA′3B4O12-type quadruple perovskite oxides, in which multiple transition metal ions are located at distinct crystallographic sites. Electrochemical measurements demonstrate that OER catalytic activities of quadruple perovskite oxide series, CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), are all superior to those of simple perovskite counterparts CaBO3. The order of activity of B-site transition metal ions for CaBO3 (Fe4+ > Co4+ ≫ Ti4+, V4+, Cr4+, Mn4+) is retained in CaCu3B4O12, indicating that B-site ...

Performance of shotcrete containing amorphous fibers for tunnel applications

The role played by a number of fiber properties (like fiber length, content, type, hybridization, tensile strength and energy-absorption capability or toughness) is investigated in this paper with reference to shotcrete mixes containing amorphous fibers with random atomic arrangement (well known for their excellent corrosion resistance) and conventional hooked steel fibers. Several prismatic beams and plates were cast by varying the above-mentioned properties. Compared to steel fiber-reinforced concrete (SFRC), amorphous metallic fiber-reinforced concrete (AFRC) exhibits a higher flexural strength, but a lower residual strength in tension after first cracking, a lower energy-absorption capability, and a higher “rebound” ratio during the spraying process. The rebound is favored by the fiber ball in the case of amorphous fibers, while a relatively small number of steel fibers gives shotcrete a more cohesive behavior during the spraying process. Amorphous fibers are, therefore, more suitable for quick repairs (for instance, in sewerage pipes), whenever quick hardening and good short-term mechanical properties in tension are sought for, together with corrosion resistance.

Amorphous iron nanoparticles: special structural and physicochemical features enable chemical dynamic therapy for tumors.

Amorphous metals or alloys, also known as the metallic glasses with a disordered structure on atomic level, have attracted great attentions owing to their special physical properties and extraordinary prosperities [1– 5]. In addition to the excellent plasticity upon heating for easy processing, the absence of grain boundaries leads the bulk metallic glasses (BMGs) to higher strength and better corrosion resistance than conventional metals or alloys, which stimulated widespread researches to hurry their potential as advanced structural materials. However, the severe synthesis conditions, typically the superhigh cooling rate (usually higher than 10 K s) to freeze the molten metal and to suppress the nucleation of crystalline phases, have been being one of the main bottlenecks for the widespread applications of BMGs [6]. Compared with BMGs, nanosized amorphous materials such as amorphous nanoparticles, which combine the advantages of both random arrangement of atoms in amorphous state and nanodimension and are therefore expected to exhibit even more unique and diverse properties, have been rarely explored, most probably due to the difficulties in the successful syntheses of such materials which leaves rooms for researches and developments. Taking amorphous iron nanoparticles (AFe-NPs) as an example, not to mention the possible specificities in electromagnetism such as soft magnetic characteristics and others, it is expected that the randomly arranged iron atoms are active enough to be ionized in mild acidic conditions for on-demand Fe (II) release and to initiate subsequent localized Fenton reaction under the presence of hydrogen peroxide. On the other hand, most solid tumors feature acidic nature (pH = 6.5–6.9) and overproduced hydrogen peroxide, and such a special microenvironment has attracted great interest for the exploration of so-called microenvironment-responsive cancer therapy [7–11]. In fact, acidic microenvironment in tumor has triggered the development of numerous strategies of pHstimuli-responsible controlled drug releases, in which drug is expected to release in acidic tumor but not in neutral normal tissues or blood stream; while cumulative H 2 O 2 in cancer has been exploited to implement specific therapeutic strategies, such as to act as a spontaneous trigger for responsive-drugreleasing chemotherapy or an endogenous O 2 producer for enhanced photodynamic therapy [12–14]. Thus, seemly totally different fields of metallic glasses and cancer therapies find their common points for combination. Amorphous iron nanoparticles: special structural and physicochemical features enable chemical dynamic therapy for tumors

Local phonon mode in thermoelectric Bi2Te2Se from charge neutral antisites

Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here, we report a temperature dependent Raman study that identifies such a mode in a standard thermoelectric material, Bi2Te2Se. One of the modes observed, whose origin has been debated for decades, was shown most likely to be an antisite defect induced local mode. The anomalous temperature independent broadening of the local mode is ascribed to the random arrangement of Se atoms. The temperature renormalization of all modes is well explained by an anharmonic model–Klemens's model.

Theoretical investigation on the relationship between the torque correlation and spin correlation models for the Gilbert damping constant

To gain insight into the relationship between the spin correlation (SC) and torque correlation (TC) models for the Gilbert damping constant, α, we first make a simple consideration on the physical aspect of these models and calculate α for the Fe50Co50 disordered alloy by the two models using the first principles technique. Electron scattering is introduced through the random arrangement of atoms and is treated with coherent potential approximation. The results indicate that in the TC model, vertex correction (VC) does not have significant contribution and the results are insensitive to the infinitesimal positive value, δ, which is used in Green's function. In the SC model, on the other hand, the VC is indispensable and the obtained values of α are found to be considerably sensitive to δ. We confirm that the value of α in the SC model approaches the value obtained in the TC model in the limit δ→+0.

Molecular dynamics simulation of Cr-precipitate demixing in FeCr alloys

The mechanical properties of FeCr alloys rely heavily on atomic distribution and can be affected by phenomena such as Cr precipitation. While precipitation of FeCr alloys of various Cr concentrations has been studied before, dissolution of already existing Cr precipitates in FeCr alloys has not; this was the focus of this study. Our means of investigation was molecular dynamics computer simulation: we set up a number of configurations of FeCr alloys containing Cr precipitates of various sizes embedded in matrices of either pure Fe or with a 15% random Cr distribution, and examined their behaviour after thermal ageing at temperatures T ranging between 600 and 2000 K. The T range was selected so that it would include the (α+α′)−α transition in the standard FeCr phase diagram. High-T results provide insight into the mechanisms that govern the dissolution: Cr precipitates dissolve by vacancy exchange, leading to a random distribution of Cr atoms in an Fe matrix, as the short-range order parameter shifts from a positive value (clustering) to zero (random atomic arrangement). Precipitates at low T were found to be stable, as were those at intermediate T (∼1000 K), in agreement with previous experimental and simulation studies that challenge the standard phase diagram's reliability.

The optimal initial configuration of silicon-functionalized graphene for lithium-ion battery anode

Silicon-functionalized graphene in its initial configuration, as anode materials for lithium-ion battery, will directly affect the battery’s reversible capacity, charge and discharge rate and service life. To present its optimal initial configuration, the relaxation and stretching properties of silicon-functionalized graphene were studied using molecular dynamics simulation with the Tersoff potential, the Lennard–Jones potential and the velocity Verlet time-stepping algorithm. In this study, many models of silicon-functionalized graphene with different arrangement of silicon atoms, different Si/C ratios, different vacancy defect ratios, different tension rates and different temperatures were primarily developed respectively to simulate the influence of different configurations on the volume, potential energy, elastic modulus, tensile strain, strength and other properties of the model, and we found that (1) the model with random arrangement of silicon atoms possessed biggest system potential energy, biggest volume and highest mechanics property among all models. (2) With the increasing amount of silicon atoms, the wavy corrugations and the peak became clearer, the potential energy decreased and volume increased. The model with Si/C ratio of 3.28% possessed highest mechanics property. (3) With the increasing vacancy, the system’s potential energy increased and volume decreased. The model with a vacancy defect ratio of 1% possessed highest mechanics property parameters. (4) Mechanics properties were the highest at the temperature of 300 K.

First‐Principles Study on Site Preference of Aluminum Vacancy and Nitrogen Atoms in γ–Alon

The disorder in γ–alon is caused by random arrangement of nitrogen atoms and aluminum vacancies. To understand the properties and electronic structure of γ–alon by theoretical methods, the most reasonable structure model is needed. We examined the site preference of nitrogen atoms and aluminum vacancies by first‐principles density functional theory (DFT) calculations on Al24O24N8 and Al23O27N5. The calculated results for Al24O24N8 with the lowest total energy indicated that nitrogen atoms prefer to be far away from each other, rather than in a completely random arrangement. The further investigation on Al23O27N5 shows that the aluminum vacancies tend to possess octahedral sites and coordinate only with oxygen atoms. Evaluated by lattice variances (Da and Dθ) and simulated XRD pattern, the most reasonable structure model of Al23O27N5 has little deviation from the experimental results. The calculated bulk modulus of 200.9 GPa in Al23O27N5 is slightly lower than the experimental value. The electronic structure reveals that the bonds of Al–N and Al–O have partially covalent and ionic characterization, while the covalent bond strength of Al–N is stronger than that of Al–O. The calculated band gap is 3.99 eV, which is much closer to the experimental 4.56 eV than previous suggestions.

Evidence of Antiblockade in an Ultracold Rydberg Gas

We present the experimental observation of the antiblockade in an ultracold Rydberg gas recently proposed by Ates et al. [Phys. Rev. Lett. 98, 023002 (2007)]. Our approach allows the control of the pair distribution in the gas and is based on a strong coupling of one transition in an atomic three-level system, while introducing specific detunings of the other transition. When the coupling energy matches the interaction energy of the Rydberg long-range interactions, the otherwise blocked excitation of close pairs becomes possible. A time-resolved spectroscopic measurement of the Penning ionization signal is used to identify slight variations in the Rydberg pair distribution of a random arrangement of atoms. A model based on a pair interaction Hamiltonian is presented which well reproduces our experimental observations and allows one to deduce the distribution of nearest-neighbor distances.

The geometry, topology and structure of amorphous solids

Clusters of atoms can be divided into three categories depending on their topology. One of the categories provides the basis for development of a model of a perfectly random structure (ideal amorphous solid) using the non-equilateral triangle topology in the coordination shell. Metallic glasses solidify as amorphous solids with random arrangement of atoms. A model of Zr-based metallic glass has been constructed and described in terms of cluster topology, and compared with a recently published dynamic molecular model of the same alloy. It is shown that the pair distribution function for the ideal amorphous model relates to the pair correlation function obtained from the dynamic model. Debye X-ray scattering computations reveal the presence of vacancies and other flaws relative to the ideal amorphous solid. A shift in the peak position can be predicted using the Erhenfest formula. Two atomic displacement mechanisms involving a five-atom sub-cluster are identified as the fundamental means of compositional redistribution between clusters in the alloy.