Amorphous Lattice Research Articles

Real-space approach for the Euler class and fragile topology in quasicrystals and amorphous lattices

Real-space approach for the Euler class and fragile topology in quasicrystals and amorphous lattices

Microstructure, properties and damage characteristics of designed PyC interphase in Cf/SiC mini-composites

Pyrolytic carbon (PyC) interphase plays a crucial role in improving the mechanical properties of fiber-reinforced ceramic-matrix composites (FRCMCs). In this paper, three groups of PyC interphase were prepared: the first group (G1) with a mid-high texture PyC (MT-HT-PyC) interphase, the second group (G2) with a low/high texture combination PyC (LT/HT-PyC), and the third group (G3) with a low texture PyC (LT-PyC). The microstructure, intrinsic properties and interfacial properties of these three groups PyC interphase were characterized by SEM, TEM, Raman spectroscopy, nanoindentation tests, and fiber push-in tests. The result showed the LT-PyC with more amorphous PyC lattice and lattice defects is able to reduce the indentation modulus and interfacial properties of PyC interphase. The uniaxial tensile tests revealed that the Cf/SiC mini-composites containing a LT-PyC interphase could significantly improve their tensile strength, exhibiting a 34 % enhancement in tensile strength compared to those counterparts without LT-PyC interphase. The experimental result of scanning ion beam etching (SIBN) also showed that the interfacial crack propagates primarily within the LT-PyC interphase, rather than along the interface between the fiber and the interphase.

Rifampicin adsorption and release study using Santa Barbara amorphous-16 modified Al (SBA-16-Al) for a drug delivery system.

In this study, the surface modification of Santa Barbara Amorphous-16 (SBA-16) with aluminum (SBA-16-Al) was carried out as a rifampicin matrix for the treatment of tuberculosis. Surface modification of SBA-16 was achieved using the direct-synthesis grafting method. Then, the adsorption and release properties of rifampicin from the SBA-16-Al matrix have been studied in batches. In addition, the SBA-16-Al has been characterized using Fourier-Transform Infrared Spectroscopy (FTIR), X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), and Surface Area Analysis (SAA) Brunaur, Emmett and Teller (SAA-BET). The results show that the mesoporous material, the SBA-16-Al has a specific surface area of 843.5 m2 g-1 and 624.3 m2 g-1 for SBA-16, nanometer-sized pore diameters, and an amorphous crystal lattice. The FTIR spectra showed the Al-O bond at 802 cm-1 which indicates the Al group has been successfully added into SBA-16. The adsorption isotherm of rifampicin in SBA-16-Al follows the Freundlich model which illustrates the adsorption is heterogeneous and forms a multilayer. The adsorption of rifampicin is chemisorption which occurs non-spontaneously and is quite stable. The release kinetics of rifampicin in the drug delivery system followed the Higuchi model with k1 0.5472 mg 0.5/hour pH 1.5 and k2 mg 0.5/hour pH 6.5.

Studies of the mechanically induced reactivity of graphene with water using a 2D-materials strain reactor.

Using mechanical force to induce chemical reactions with two-dimensional (2D) materials provides an approach for both understanding mechanochemical processes on the molecular level, and a potential method for using mechanical strain as a means of directing the functionalization of 2D materials. To investigate this, we have designed a modular experimental platform which allows for in situ monitoring of reactions on strained graphene via Raman spectroscopy as a function of time. Both the strain present in graphene and the corresponding chemical changes it undergoes in the presence of a reagent can be followed concomitantly. As a case study, we have experimentally monitored and theoretically modeled the reactivity of a suspended single-layer graphene membrane under strain with water, where the graphene is strained via an applied backing pressure. While exposure of the unstrained membrane to water does not drive a chemical reaction, distortion of the membrane causes a rise in the ID/IG peak ratio, indicating an initial lattice conversion from crystalline to nanocrystalline due to reaction with water. With continued reaction, a decrease in the ID/IG peak ratio is then seen, indicative of a nanocrystalline to amorphous lattice transition. Using density functional theory (DFT) calculations, the reaction of water on graphene has been determined to be nucleated by epoxide defects, with the reaction barrier decreasing by nearly 5× for the strained vs. unstrained graphene. While demonstrated here for graphene, this approach also provides the opportunity to examine a host of force-driven chemical reactions with 2D materials.

An exact chiral amorphous spin liquid

Topological insulator phases of non-interacting particles have been generalized from periodic crystals to amorphous lattices, which raises the question whether topologically ordered quantum many-body phases may similarly exist in amorphous systems? Here we construct a soluble chiral amorphous quantum spin liquid by extending the Kitaev honeycomb model to random lattices with fixed coordination number three. The model retains its exact solubility but the presence of plaquettes with an odd number of sides leads to a spontaneous breaking of time reversal symmetry. We unearth a rich phase diagram displaying Abelian as well as a non-Abelian quantum spin liquid phases with a remarkably simple ground state flux pattern. Furthermore, we show that the system undergoes a finite-temperature phase transition to a conducting thermal metal state and discuss possible experimental realisations.

Deposition of graphenic nanomaterials from elevated temperature premixed stagnation flames

The work examines the unique nanostructure of carbon nanoparticles deposited from sooting premixed flames with flame temperatures exceeding 2200 K. This flame temperature regime has previously been shown to transition from typical soot formation conditions to a regime whereby the flame-form carbon adopts a nanostructure considerably more ordered than soot. Graphenic carbon deposits observed by High-resolution TEM (HRTEM) are reported here corroborating previous Raman spectroscopy evidence. The use of premixed stretch-stabilized flames enables particle production in the high-temperature regime under a flow field amenable to low-dimensional flame modeling. Although the flame flow configuration is relatively simple, three sample preparation methods are used to assess the representation of true carbon properties as they exist in the flame. HRTEM imaging is carried out on carbon particle samples prepared by rapid-insertion deposition, aerosol dilution probe deposition and carbon particle film deposition. Images from rapid-insertion samples show amorphous particles in the lightly sooting flame and turbostratic particles in the heavy sooting flame. There is trace evidence of graphenic structure in rapid-insertion samples but the most striking particles on the TEM grid are graphite nanocrystals presumably formed by a new artificial crystallization process. HRTEM images of particles collected over time by diluted aerosol deposition and film deposition show clear graphenic structures. Overall, the carbon nanostructure observed by HRTEM is a mixture of amorphous, turbostratic and graphenic carbon lattices depending on the flame condition and sampling method. The current work highlights potential impacts of higher flame temperatures and higher equivalence ratio on deposited flame-formed carbon. Namely, graphenic particle structure is observed in rapid-insertion deposition samples but graphene portions are most abundant in aerosol dilution and carbon particle film deposition samples. This may indicate that graphene structures grow on the deposition surface over time.

Fabrication and structural elucidation of graphene oxide-based nanocomposites (GO-Ag, rGO-Ag) for degradation of methylene blue

Abstract Graphene oxide (GO) and reduced graphene oxide (rGO) are two-dimensional material of carbon having unique optical, electrical, mechanical and thermal properties, which could form composite material with different materials. In this work, we fabricated the nanocomposite of GO and rGO with silver (Ag). Modified Hummer method was adopted for the production of GO from graphite powder. This GO was reduced into rGO using ascorbic acid. On the reduction of GO into rGO, inter-planar distance increases which enhanced the properties of rGO as compared to GO. Turkevich method was adopted to produce the nanocomposite of GO and rGO with Ag. FTIR was performed to confirm the different functional group in the desired product. XRD spectra of rGO have broad hump shaped peak which reflects the amorphous structure or lattice and proposed rGO composite as a better membrane material than GO composite for water purification. Similarly, the Raman spectra shows the lower value of D and G band for rGO than GO. Results revealed that methylene blue dye was degraded up to 79.36 % using rGO-Ag composite. Hence, it is concluded that fabricated material could possibly be employed for degradation of other toxic pollutant from industrial effluents.

Oxygen and iron in interstellar dust: An X-ray investigation

Understanding the chemistry of the interstellar medium (ISM) is fundamental for the comprehension of Galactic and stellar evolution. X-rays provide an excellent way to study the dust chemical composition and crystallinity along different sight lines in the Galaxy. In this work, we study the dust grain chemistry in the diffuse regions of the ISM in the soft X-ray band (<1 keV). We use newly calculated X-ray dust extinction cross sections obtained from laboratory data in order to investigate the oxygen K and iron L shell absorption. We explore the XMM-Newton and Chandra spectra of five low-mass X-ray binaries (LMXBs) located in the Galactic plane and model the gas and dust features of oxygen and iron simultaneously. The dust samples used for this study include silicates with different Mg:Fe ratios, sulfides, iron oxides, and metallic iron. Most dust samples are in both amorphous and crystalline lattice configuration. We computed the extinction cross sections using Mie scattering approximation and assuming a power-law dust size distribution. We find that the Mg-rich amorphous pyroxene (Mg0.75Fe0.25SiO3) represents the largest fraction of dust towards most of the X-ray sources, namely about 70% on average. Additionally, we find that ~15% of the dust column density in our lines of sight is in metallic Fe. We do not find strong evidence for ferromagnetic compounds, such as Fe3O4 or iron sulfides (FeS, FeS2). Our study confirms that iron is heavily depleted from the gas phase into solids; more than 90% of iron is in dust. The depletion of neutral oxygen is mild, namely of between 10% and 20% depending on the line of sight.

High-entropy alloy metallene for highly efficient overall water splitting in acidic media

The preparation of stable and efficient acidic overall water splitting catalysts is crucial to advance the progress of proton exchange membrane water electrolyzers. Herein, we successfully prepared IrPdRhMoW HEA metallene with rich amorphous and crystalline structures. In 0.5 mol L−1 H2SO4, the extraordinary catalytic performance (the overpotentials for hydrogen evolution (HER) and oxygen evolution (OER) of IrPdRhMoW/C at 10 mA cm−2 are 15 mV and 188 mV, respectively) is far stronger than that of commercial catalysts (HER: Pt/C, 47 mV and OER: RuO2, 305 mV) and even other reported noble metal-based catalysts. Using IrPdRhMoW/C for the overall water splitting, only a cell voltage of 1.48 V is required to achieve 10 mA cm−2 and 1.59 V required to achieve 100 mA cm−2, which is the best voltage under high current density reported so far. More importantly, the IrPdRhMoW/C still maintains excellent electroactivity and structural stability after 100 h of water splitting at 100 mA cm−2. Theory calculations reveal the self-balanced effect of electronic structures in the HEA due to the co-existence of crystalline and amorphous lattice structures. The strong orbital couplings not only maximize the electroactivity towards both HER and OER but also stabilize the valence states of metal sites for durable electrocatalysis.

Vortex Pinning in Neutron Stars, Slipstick Dynamics, and the Origin of Spin Glitches

We study pinning and unpinning of superfluid vortices in the inner crust of a neutron star using three-dimensional dynamical simulations. Strong pinning occurs for certain lattice orientations of an idealized, body-centered-cubic lattice and occurs generally in an amorphous or impure nuclear lattice. The pinning force per unit length is ∼1016 dyn cm−1 for a vortex–nucleus interaction that is repulsive and ∼1017 dyn cm−1 for an attractive interaction. The pinning force is strong enough to account for observed spin jumps (glitches). Vortices forced through the lattice move with a slipstick character; for a range of superfluid velocities, the vortex can be in either a cold, pinned state or a hot, unpinned state, with strong excitation of Kelvin waves on the vortex. This two-state nature of vortex motion sets the stage for large-scale vortex movement that creates an observable spin glitch. We argue that the vortex array is likely to become tangled as a result of repeated unpinnings and repinnings. We conjecture that during a glitch, the Kelvin-wave excitation spreads rapidly along the direction of the mean superfluid vorticity and slower in the direction perpendicular to it, akin to an anisotropic deflagration.

Amorphous Kane-Mele model in disordered hyperuniform two-dimensional networks

As a prototype system of the quantum spin Hall effect, the Kane-Mele model which was proposed initially in graphene promotes the search for two-dimensional topological materials of hexagonal lattices. Here we generalize the Kane-Mele model to exotic amorphous systems which possess a remarkable structural property called hyperuniformity. We show that, in general, the Quantum spin Hall state still survives in disordered hyperuniform lattices that are constructed by structural transformation involving Stone-Wales defects. However, compared to that in the honeycomb lattice, the gapped topological region in the phase diagram shrinks and the size of the corresponding topological gap decreases remarkably in disordered hyperuniform lattices. By introducing random vacancies in either perfectly ordered or disordered hyperuniform lattices, we further show that the degradation or destruction of hyperuniformity is detrimental to the existence of gapped topological states. We therefore propose that the hyperuniform metric of an amorphous lattice, which quantifies the extent of disordered hyperuniformity, reflects its ability to preserve topological states. Our finding not only establishes the possible underlying link between electronic topology and disordered hyperuniformity but also provides useful guidance for the seeking of topological materials in amorphous states of matter.

Spectroscopic review of hydrogenated, carbonated and oxygenated group IV alloys

Semiconductors based on group IV elements are widely used in the fields of micro-electronics, optics and photonics. The group IV alloys are processed using plasma enhanced chemical vapor deposition and its opto-electrical properties are a result of the material composition and structure. Infrared and Raman spectroscopy are complementary and powerful tools for providing these essential material characteristics. In this work, the vibrational modes present in hydrogenated, oxygenated and carbonated group IV alloys are investigated in a unique range of amorphous and nano-crystalline SiX≥0Ge1−X:H films and their alloys with C, O and Sn. Measurements are performed both post-deposition and following extended exposure to the ambient and de-ionized water. This comprehensive review is of value in the fields of material science and engineering as a single work of reference for group IV peak identification. Additionally, the effect of electrical screening, the influence of the dielectric medium on the peak frequency of a vibrational mode, is illustrated using the experimentally observed frequency shifts of X-O and X-H (X = C, Si, Ge) vibrational modes. All experimentally observed center frequencies of silicon hydride stretching modes in silicon solids and corresponding silicon-hydride configurations are identified using a straightforward Lorentz-Lorenz model approximation and considering all potential hydrogenated volume deficiencies within a tetrahedrally coordinated amorphous and nanocrystalline lattice. It shows that the stretching mode signature can reveal detailed information on the volume deficiencies in IV-valence alloys.

Interpretation of hydrogen incorporation into radio frequency sputtered amorphous silicon based on Berg modelling

Incorporated hydrogen and its bonding configuration have an effect on the electrical and structural properties of hydrogenated amorphous silicon (a-Si) thin films. On one hand hydrogenization is known to be very efficient in reducing the density of dangling bonds responsible for deep levels in the bandgap; the technological process that carries out the incorporation of hydrogen improves the device parameters. On the other hand, the H–Si–H bonding configuration may negatively affect the microstructure of the amorphous lattice and promote the creation of voids.In this work the incorporation of hydrogen into radio frequency sputtered amorphous silicon thin films is interpreted by means of Berg-model handling the hydrogen incorporation as a reactive sputtering process involving ionization of molecular hydrogen followed by reactions between elemental target atoms to compound molecules. This approach is suitable for the interpretation of hydrogen incorporation into a-Si films based on the fact that the amount of incorporated hydrogen is proportional to the current. The imaginary and the real part of the dielectric function deduced from spectroscopic ellipsometry are investigated as a function of hydrogen flow. Using the results of spectroscopic ellipsometry and elastic recoil detection analysis presented earlier, as well as the equations of Berg model, the metal and compound sputtering rate and the sticking coefficient between silicon and hydrogen atoms are calculated.

Multiband transport enables thermoelectric enhancements in the SrMg2Bi2 compound

A small energy offset (ΔE < 0.2 eV) between Γ(pxy) and Γ(pz) band orbitals in AB2X2 Zintl compounds enables the realization of band alignment for an increased band degeneracy as well as an enhanced electronic performance. The SrMg2Bi2 Zintl compound has been theoretically illustrated to have a small ΔE, suggesting an approximately inherent convergence of two band orbitals. Therefore, this work is devoted to the revelation of the thermoelectric potential of the SrMg2Bi2 Zintl compound in a broad Hall carrier concentration of 0.3–5 × 1019 cm−3 by Ag-doping at the Sr site. The electrical transport properties can be well predicted by the single parabolic band model with acoustic phonon scattering, which reveals a multi-band transporting behavior in the heavily doped samples due to Fermi level deepening, leading to a significant zT-enhancement. In terms of the model, a peak zT of up to 1.0 is achievable by assuming the amorphous minimum lattice thermal conductivity, which demonstrates this compound as a promising thermoelectric material.

Ultrafast and stable ion/electron transport of MnNb2O6 in LIC/SC via interface protection and lattice defects

Interface protection and kinetics optimization could effectively relieve the shortcomings of bimetallic oxides, such as low conductivity, strong hydrophobicity, insufficient ion diffusion rate and metal interatomic instability. In this work, ultrathin amorphous carbon shells and lattice defects (heteroatoms and vacancies) are introduced into the MnNb2O6 nanofiber surface to improve the electron/ion kinetic stability, conductivity and electrochemical activity. The ultrathin carbon interface protects unstable lattice with defects, thus restraining the adverse reaction between bimetallic oxides and electrolyte. Especially, ultrathin amorphous carbon layer enhances the stability and uniformity of ion transport as the substitute of solid–liquid ion exchange membrane. Lattice defects (N doping and oxygen vacancy) also enhance the ionic kinetics of the material. MnNb2O6 nanofiber, being optimized by interface protection and lattice defects, shows excellent electrochemical performances in Lithium-ion battery and supercapacitor.

Inexpensive activated coke electrocatalyst for high-efficiency hydrogen peroxide production: Coupling effects of amorphous carbon cluster and oxygen dopant

Electrochemical oxygen reduction has been regarded as a promising choice to enable H2O2 on-site production and utilization wherein the exploration of high-efficiency yet cost-effective catalysts is the key. Here, we demonstrate a low-cost activated coke (AC) electrocatalyst with size-tailored amorphous carbon clusters doped by oxygen groups, prepared through a facile CO2 assisted mechanochemistry approach, to deliver among the highest performances reported in a typical alkaline system, including high activity (onset potential of 0.83 V), high H2O2 selectivity (∼90 %) and long-term stability. A series of control experiments, structural characterizations before and after electrochemical tests and density functional theory calculations provide a new insight into the coupling role of carbon cluster size and oxygen doping in H2O2 electrochemical production process, that is, size-reduced amorphous carbon lattices with abundant edges contribute to the high activity, while the basal carbon atoms in ether-doped small-size carbon plane are the most active sites towards H2O2 selectivity.

Topological Weaire–Thorpe models of amorphous matter

Amorphous solids remain outside of the classification and systematic discovery of new topological materials, partially due to the lack of realistic models that are analytically tractable. Here we introduce the topological Weaire-Thorpe class of models, which are defined on amorphous lattices with fixed coordination number, a realistic feature of covalently bonded amorphous solids. Their short-range properties allow us to analytically predict spectral gaps. Their symmetry under permutation of orbitals allows us to analytically compute topological phase diagrams, which determine quantized observables like circular dichroism, by introducing symmetry indicators in amorphous systems. These models and our procedures to define invariants are generalizable to higher coordination number and dimensions, opening a route toward a complete classification of amorphous topological states in real space using quasilocal properties.

Robust Metallic Nanolaminates Having Phonon-Glass Thermal Conductivity.

Heat transfer phenomena in multilayer structures have gained interest due to their promising use in thermal insulation and thermoelectricity applications. In such systems, nanostructuring has been used to introduce moderate interfacial density, and it has been demonstrated that interfacial thermal resistance plays a crucial role in reducing thermal conductivity κ. Nevertheless, the main constraint for actual applications is related to their tiny size because they are extremely thin to establish appreciable temperature gradients. In this work, by severe plastic deformation process of accumulative roll bonding (ARB), a 250 µm thick Cu-Nb multilayer containing more than 8000 interfaces with periods below 40 nm was obtained, enabling the production of bulk metallic nanolaminates with ultralow κ. Multilayers present an ultralow κ of ∼0.81 W/mK at 300 K, which is 100 times smaller than its Cu-Nb bulk counterpart, and even lower than the amorphous lattice limit for the Cu-Nb thin film system. By using electron diffusive mismatch model (EDMM), we argue that both electrons diffusively scattering at interface and those ballistically crossing the constituents are responsible for heat conduction in the Cu-Nb multilayers at nanoscale. Hence, ARB Cu-Nb multilayers are intriguing candidate materials which can prove avenues to achieve stable ultralow κ thermal barriers for robust applications.

Optical reflectivity of heat-treated nanofibrous silicon thin-films induced by high energy picosecond laser pulses

In this study, selective optical properties of Si/SiO2 thin films and their dependence on heat treatment were investigated. Samples were synthesized by pulsed-laser ablation deposition of c-Si on glass substrates and annealed at different temperatures (400 °C, 1000 °C). Nanofabrication parameters were chosen to produce significantly different nanostructures ranging from agglomerated particles to nanofibers. Morphology, crystalline, and elemental characterization techniques were employed to explore the variations in visible range reflection measurements. The adverse effect of annealing temperatures on Vis-range reflection spectra to increase and decrease the intensity was confirmed. These improvements are attributed to crystalline and amorphous lattice structures and elemental composition supported by the X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) method results.

Photonic amorphous topological insulator

The current understanding of topological insulators and their classical wave analogs, such as photonic topological insulators, is mainly based on topological band theory. However, standard band theory does not apply to amorphous phases of matter, which are formed by non-crystalline lattices with no long-range positional order but only short-range order, exhibiting unique phenomena such as the glass-to-liquid transition. Here, we experimentally investigate amorphous variants of a Chern number-based photonic topological insulator. By tuning the disorder strength in the lattice, we demonstrate that photonic topological edge states can persist into the amorphous regime prior to the glass-to-liquid transition. After the transition to a liquid-like lattice configuration, the signatures of topological edge states disappear. This interplay between topology and short-range order in amorphous lattices paves the way for new classes of non-crystalline topological photonic bandgap materials.