Non-crystalline Structure Research Articles

Ab initiotheory of moiré superlattice bands in layered two-dimensional materials

When atomically thin two-dimensional (2D) materials are layered they often form incommensurate non-crystalline structures that exhibit long-period moir{\' e} patterns when examined by scanning probes. In this paper we present an approach which uses information obtained from {\it ab initio} calculations performed on short-period crystalline structures to derive effective Hamiltonians that are able to efficiently describe the influence of the moir{\' e} pattern superlattices on electronic properties. We apply our approach to the cases of graphene on graphene (G/G) and graphene on hexagonal boron nitride (G/BN), deriving explicit effective Hamiltonians that have the periodicity of the moir{\' e} pattern and can be used to calculate electronic properties of interest for arbitrary twist angles and lattice constants.

On the theory of the universal dielectric relaxation

Dielectric relaxation has been investigated within the framework of a modified mean field theory, in which the dielectric response of an arbitrary condensed matter system to the applied electric field is assumed to consist of two parts, a collective response and a slowly fluctuating response; the former corresponds to the cooperative response of the crystalline or noncrystalline structures composed of the atoms or molecules held together by normal chemical bonds and the latter represents the slow response of the strongly correlated high-temperature structure precursors or a partially ordered nematic phase. These two dielectric responses are not independent of each other but rather constitute a dynamic hierarchy, in which the slowly fluctuating response is constrained by the collective response. It then becomes clear that the dielectric relaxation of the system is actually a specific characteristic relaxation process modulated by the slow relaxation of the nematic phase and the relationship governing such a process can be defined as the universal dielectric relaxation law. Furthermore, we have shown that seemingly different relaxation relationships, such as the Debye relaxation law, the Cole-Cole equation, the Cole-Davidson equation, the Havriliak-Negami relaxation, the Kohlrausch-Williams-Watts function, Jonscher’s universal dielectric relaxation law, etc. are only the variants of this universal law under different circumstances.

Coarse-Grain Model for Natural Cellulose Fibrils in Explicit Water

Understanding biomass structure and dynamics on multiple time and length scales is important for the development of cellulosic biofuels. To this aim, we have developed a coarse-grain (CG) model for molecular dynamics (MD) simulations of cellulose fibrils in explicit water based on target observables from fully atomistic simulations. This model examines the significance of the presence of explicit solvent and compares results with the previous, implicit solvent CG cellulose models. The present, constraint-free CG model is used to generate a series of noncrystalline fibril structures using a coupling parameter, λ, between fully crystalline and fully amorphous potentials. By exploring various structural parameters, including the root-mean-square deviation, root-mean-square fluctuations, radius of gyration, and persistence length, we find the crystalline-to-amorphous state transition takes place at λ ≈ 0.386. The persistence length of cellulose fibril in the transition region corresponds to that of native cellulose fibrils. The transition between crystalline and amorphous fibrils occurs at larger values of λ in explicit water than in the implicit case. Detailed analysis of individual energetic contribution to the transition reveals that the nonbonded interactions, in particular, that of cellulose-water interaction, plays a significant role in the observed crystalline to amorphous transition of cellulose fibril. The present study thus highlights the importance of solvent presence that cannot be adequately described with the previous implicit solvent model. The present method provides an accurate and constraint-free approach for deriving a variety of structures of cellulose in water, with a wide range of crystallinity, suitable for incorporation into large-scale models of lignocellulosic biomass.

Heat-sink enhancement of decalin and aviation kerosene prepared as nanofluids with palladium nanoparticles

Three different kinds of palladium nanoparticles modified by octadecanethiol, octadecylamine, and mixture of them have been prepared, which are simply marked as Pd@S, Pd@N and Pd@S&N in turn. The nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and thermo-gravimetric analysis (TGA). It follows that the average diameters of the nanoparticles are 1–3nm. Pd@S shows noncrystalline structure, while Pd@N and Pd@S&N show face-center cubic (fcc) structure. These palladium nanoparticles can be well-dispersed in decalin, and then the nanofluids composed of decalin and palladium nanoparticles are prepared, which can be regarded as pseudohomogeneous systems. The cracking of each nanofluid containing 500ppm Pd is performed under supercritical conditions (3.5MPa, 600–750°C) of decalin with a mass flow rate of 1g/s in an electrically heated tube reactor which simulates a single passage in a practical heat exchanger. The cracking results indicate that these nanoparticles exhibit catalytic activity to a certain extent, and the activity of Pd@N nanoparticles is better than that of Pd@S or Pd@S&N. At 750°C, the conversion of decalin is raised from 70.31wt% of thermal cracking to 73.21wt% with the existence of Pd@S, while that raised to 80.54wt% with Pd@S&N and even raised to 91.85wt% with Pd@N. The heat sink of nanofluids is effectively enhanced, and the nanofluid containing Pd@N shows the highest value, which reaches 3.50MJ/kg and is increased by 0.29MJ/kg in comparison with the heat sink of thermal cracking at 750°C. The cracking of aviation kerosene containing Pd nanoparticles is then performed, the results of which confirm the feasibility of practical application of Pd nanoparticles to hydrocarbon fuels.

Advances in nanomaterials

In many cases new technologies are driven by the development of novel materials providing properties or combinations of properties that were not available before. The use of polymers and silicon-based semiconductor technologies are just two examples of how the availability of certain materials revolutionized existing technologies and products, giving rise to completely new technologies and having an enormous impact on our everyday life. During the past three decades, the field of nanomaterials experienced a tremendous rise from the early beginnings to modern-day technologies, and still, new ideas and breakthroughs keep opening new perspectives for future technologies, from cars to aircraft, from semiconductors and electronics to novel displays, from energy harvesting to energy storage technologies and from biomimetic structures to medical technologies, to mention just a few examples. One recent development in the field of nanomaterials is the ability not only to tailor the properties of nanomaterials (to achieve custom-designed, “tailor-made” values of certain properties) but to tune these properties. Tunable materials allow to change the properties of these materials reversibly and in a controlled manner after fabrication, e.g., by applying an electric field. Another remarkable development is the discovery of “nanoglasses”, based on the idea of introducing internal interfaces on the nanometer scale not between crystalline structures (leading to nanocrystalline materials) but between non-crystalline, amorphous or glassy structures. These glasses differ structurally from present-day glasses and thus are expected to open the way to an age of glass-based technologies comparable to the contemporary, primarily crystalline-based, technologies. This Thematic Series does not attempt to give a comprehensive overview over the vast and thriving field of nanomaterials. It rather gives a sequence of contributions on recent developments within this field. The volume is dedicated to Horst Hahn on the occasion of his 60th birthday. Horst Hahn is Executive Director of the Institute for Nanotechnology at the Karlsruhe Institute of Technology (KIT) and Full Professor at the Technische Universitat Darmstadt. In addition, he is Founding Director of the Helmholtz Institute Ulm for Electrochemical Energy Storage (HIU). He is Distinguished Professor at the Indian Institute of Technology Madras, India, and Honorary Professor at the University of Hyderabad, India, and at the Lanzhou University, China. Moreover, he is Elected Member of the German Academy of Sciences Leopoldina and the European Academy of Science. As a Fellow of the Materials Research Society (MRS), his sustained and distinguished contributions to the advancement of international materials research were recognized. In the past two years, Horst Hahn was recipient of the highest award of the Deutsche Gesellschaft fur Materialkunde (DGM), the Heyn Denkmunze, and of the renowned Robert Franklin Mehl Award of the Minerals, Metals & Materials Society (TMS). Horst Hahn has published more than 300 peer-reviewed articles in various fields of materials science. His main research interests are related to the synthesis and processing of nanocrystalline materials and nanostructures with tailored and tunable properties, with a focus on applications in printed electronics and energy materials. The nucleation point for this Thematic Series was an international symposium held in August 2012 at the Karlsruhe Institute of Technology on the occasion of the 60th birthday of Horst Hahn – the “Nanomaterials Days” – which brought together renowned experts in the field of nanomaterials, each of them giving a short contribution on one recent research highlight in this field. A selection of these contributions is found in this Thematic Series in the form of original research articles reflecting recent advances in nanomaterials. The articles in this Thematic Series highlight recent developments, from nanoporous polymers to graphene quantum dots, from concepts for designing magnetic properties to nanoplasticity and to the remarkable mechanical properties of a novel type of nanowires, just to mention some of the developments described. We would like to thank all the authors for contributing their excellent work to this Thematic Series. Moreover, we would like to thank all referees for their promptly provided reports keeping the publication times short and attractive for contributors. Finally, we thank the team at the Beilstein Institut for their excellent support, and we acknowledge to the open access policy of the Beilstein Journal of Nanotechnology, which provides the basis for unrestricted discussions on “Advances in Nanomaterials”. Herbert Gleiter, Horst Hahn and Thomas Schimmel Karlsruhe, November 2013

Nitrogen Monoxide Gas Sensing Characteristics of Transparent p-type Semiconductor CuAlO2 Thin Films

We investigated the detection properties of nitrogen monoxide (NO) gas using transparent p-type <TEX>$CuAlO_2$</TEX> thin film gas sensors. The <TEX>$CuAlO_2$</TEX> film was fabricated on an indium tin oxide (ITO)/glass substrate by pulsed laser deposition (PLD), and then the transparent p-type <TEX>$CuAlO_2$</TEX> active layer was formed by annealing. Structural and optical characterizations revealed that the transparent p-type <TEX>$CuAlO_2$</TEX> layer with a thickness of around 200 nm had a non-crystalline structure, showing a quite flat surface and a high transparency above 65 % in the range of visible light. From the NO gas sensing measurements, it was found that the transparent p-type <TEX>$CuAlO_2$</TEX> thin film gas sensors exhibited the maximum sensitivity to NO gas in dry air at an operating temperature of <TEX>$180^{\circ}C$</TEX>. We also found that these <TEX>$CuAlO_2$</TEX> thin film gas sensors showed reversible and reliable electrical resistance-response to NO gas in the operating temperature range. These results indicate that the transparent p-type semiconductor <TEX>$CuAlO_2$</TEX> thin films are very promising for application as sensing materials for gas sensors, in particular, various types of transparent p-n junction gas sensors. Also, these transparent p-type semiconductor <TEX>$CuAlO_2$</TEX> thin films could be combined with an n-type oxide semiconductor to fabricate p-n heterojunction oxide semiconductor gas sensors.

Size-Dependent Velocity Map Photoelectron Imaging of Nanosized Ammonia Aerosol Particles

The size dependence of the band position, the bandwidth, and the asymmetry parameter in the valence photoelectron spectrum of ammonia clusters and ultrafine ammonia aerosol particles is investigated for aggregates with diameters up to ~7 nm. The neutral aggregates are ionized by single photons at 13.318 eV from a tabletop vacuum ultraviolet laser. The photoelectrons are detected in a velocity map imaging photoelectron spectrometer. The size distributions of the neutral aggregates are determined independently using the Na-doping method. Up to clusters with a few tens of molecules, the first ionization energy (value at band maximum) decreases, the width of the lowest band in the photoelectron spectrum broadens, and the corresponding asymmetry parameter decreases with increasing cluster size. Constant ("plateau") values of these parameters are found for larger aggregates, which might be explained by a predominantly noncrystalline structure of these aggregates with an average coordination similar to that of the liquid. This explanation is in agreement with previous infrared studies and theoretical predictions.

Study of Motion of Impurity Ions Making Influence on Electrical Properties of Polymer Solids in Low Frequency Range

The motion of impurity ions affects the electrical properties of polymer solid. The ions move through free spaces which exist in the non-crystalline part between polymer crystallites. The free spaces have inhomogeneous size distribution depending on a property of polymers or crystallization conditions. The homogeneity or inhomogeneity of the size of the free space affects the ionic motion, and consequently it also affects the electrical properties of polymer solids. In order to study the electrical properties of polymer solids in relation with such motion of impurity ions, the conductivity relaxation time was calculated for isotactic poly (propylene) (iPP), low density poly (ethylene) (LDPE) and plasticized poly (vinyl chloride) with dioctyl phthalate (p-PVC) from the electric modulus as a function of frequency which was observed by means of the Broadband Dielectric Spectroscopy (BDS). The value of the conductivity relaxation time reflects the non-crystalline structure of these polymers.

Modification of crystallinity and pore size distribution in coagulated cellulose films

In this study the effects of altering the coagulation medium during regeneration of cellulose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate, were investigated using solid-state NMR spectroscopy and NMR cryoporometry. In addition, the influence of drying procedure on the structure of regenerated cellulose was studied. Complete conversion of the starting material into regenerated cellulose was seen regardless of the choice of coagulation medium. Coagulation in water predominantly formed cellulose II, whereas coagulation in alcohols mainly generated non-crystalline structures. Subsequent drying of the regenerated cellulose films, induced hornification effects in the form of irreversible aggregation. This was indicated by solid-state NMR as an increase in signal intensity originating from crystalline structures accompanied by a decrease of signal intensity originating from cellulose surfaces. This phenomenon was observed for all used coagulants in this study, but to various degrees with regard to the polarity of the coagulant. From NMR cryoporometry, it was concluded that drying induced hornification generates an increase of nano-sized pores. A bimodal pore size distribution with pore radius maxima of a few nanometers was observed, and this pattern increased as a function of drying. Additionally, cyclic drying and rewetting generated a narrow monomodal pore size pattern. This study implies that the porosity and crystallinity of regenerated cellulose can be manipulated by the choice of drying condition.

A Simple Method for Identification of the Alloy’s Non-Crystalline Structure

In the thermomagnetism analysis, the ferromagnetic amorphous alloy near or after the Curie temperature within a certain scope was in the paramagnetic condition with low magnetization. When the heating temperature achieved the crystallization temperature of the amorphous alloy, the magnetization increased significantly due to the crystallization of the alloy. As the obvious crystallization of the amorphous alloy, the crystallization phase with a high Curie temperature (Tc) formed and the ferromagnetism of the alloy would not be fully transferred to paramagnetism. The magnetization of the alloy would maintain at a stable high level before it had been fully crystallized. Therefore, the structure of the ferromagnetic amorphous alloy can be identified accurately by the obvious difference between the thermomagnetism curves before and after crystallization.

Substrate Effects on the Microstructure and Mechanical Properties of SiO2 Thin Films

The SiO2 films deposited respectively on K9 and two different orientations of Y3Al5O12 (YAG) crystals were manufactured by electron beam technology. X-ray diffraction (XRD) and nano-scratch tests were used to demonstrate the microstructures and mechanical properties of films. The crystal orientation of SiO2 thin films on K9 is noncrystalline structure and the other two, YAG (100) and YAG (111), are polycrystalline structures. The Young modulus of all films are similar. The nano-scratch tests show that the damage mechanisms of SiO2 thin films on K9 and YAG are different. The adhesive forces of the films on YAG (100) and YAG (111) are much less than homolo- gous film on K9. The microstructures of the films and the divergences of the elastic modulus between films and substrates are taken to explain the different mechanical behaviors of the films on three substrates.

Structural Evolution and Optical Properties of TiO&lt;sub&gt;2&lt;/sub&gt; Thin Films Prepared by DC-Reactive Sputtering Technique

Titanium dioxide (TiO2 ) thin films have been deposited on glass substrates under various conditions by using a homemade reactive DC sputtering technique. The TiO2 has unique characteristics and economical alternative material for transparent conductivity oxide thin films compared with other materials. In this study, titanium (Ti) has been used as a target while argon (Ar) and oxygen (O22&lt;/subthin films has been measured by using a calibrated I-V meter. On the other hand, the transparency, microstructure and component of TiO2 thin films have been investigated respectively by using UV-VIS spectrophotometer, XRD and SEM (EDX). The thickness of TiO2 films, the grain size and the band gap have been also successfully estimated. As a result, the conductivity of films increased for Dt at 1 hour to 3.5 hours and decreased for Dt at 4 hours. It means that the optimum Dt was at about 3.5 hours. It may be related to the thickness (structures) of the films. In addition, the thickness and grain size increased by increasing Dt, while the band gap decreased when the film structure changed from non-crystalline structure to crystalizing structure.

Solvent effects in the synthesis of CoB catalysts on hydrogen generation from hydrolysis of sodium borohydride

Solvent effects are important in synthetic chemistry, especially in preparing nanosized materials in solution. The sizes, morphologies, and properties of nanoparticles obtained from solution are greatly influenced by the solvents used. CoB catalysts were synthesized in solvents with different viscosity and polarity, including water, methanol, ethanol, and n-propanol. Solvent effects on the size and morphology of CoB were investigated using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and nitrogen sorption measurements. CoB catalysts possessed non-crystalline structures composed of discrete nanoparticles, but became more agglomerated with increasing solvent viscosity. The hydrolysis of NaBH4 showed that catalyst morphology and texture greatly affected catalytic activity. XPS showed that Co and B were each present in elemental and oxidized states in the CoB catalysts, and that the Co/B surface ratio varied greatly with a higher Co/B ratio resulting in better catalytic properties. Solvent effects were analyzed in terms of steric hindrance and viscosity, and a mechanism for CoB synthesis in the different solvents was proposed.

Photoluminescence of obsidian: A particle‐dispersed natural glass

AbstractStructural and optical properties of an obsidian, called Tokachi stone, have been studied from a standpoint of optical glasses. The absorption edge rises from ∼3.2 eV, which is governed by Fe3+ oxides in almino‐silicate glassy matrices. It is discovered that, under excitations of ultraviolet light, the obsidian exhibits visible luminescence, the intensities varying with excitation energy and temperature. The characteristics are related to the localized‐extended excitations in inhomogeneous noncrystalline structures (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Communication: An obligatory glass surface

Theory predicts, and experiments have shown, that dynamics is faster at glass surfaces than in the bulk, allowing the glass to settle into deeper energy landscape minima, or "age more." Is it possible that a glass surface could survive at temperatures where the bulk crystallizes, or that it could remain glassy after the bulk is heated all the way to its melting temperature and re-cooled? We image in real-time and with sub-nanometer resolution the two-state surface dynamics on a cerium-based glass surface, from deep within the glassy regime to above the crystallization temperature. Unlike other surfaces that we have studied, this glass surface remains amorphous even after the bulk re-crystallizes. The surface retains non-crystalline structure and two state dynamics of cooperatively rearranging regions even after heat annealing to just below the bulk melting temperature. The heat-annealed cooperatively rearranging regions are larger than originally, a sign that the surface is well aged. The surface dynamics depends weakly on temperature, showing no sign of the superexponential increase in bulk dynamics expected near T(g).

Effect of Heat Treatment Temperature on Properties of SiO&lt;sub&gt;2&lt;/sub&gt;-ZrO&lt;sub&gt;2&lt;/sub&gt;-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Coatings on Stainless Steel Substrate

The SiO2-ZrO2-Al2O3-Cr2O3 film is coated on the surface of stainless steel using analytical reagent TEOS, ZrOCl2·8H2O, Al(NO3)3·9H2O and Cr(NO3)3·9H2O as precursor and basing on mole ratio to calculate through Sol-gel method. The phase transformation behavior，erosion resistance，thermal shock resistance and abrasion resistance of ceramic coating by different heat treatment are studied. The results show that: 1)The SiO2-ZrO2-Al2O3-Cr2O3 gel coatings has non-crystalline structure after the treatment at 700°C and 800°C. New substance is not created below 700°C 2) The stainless steel substrate with ceramic coating has a higher erosion resistance at high temperature (700°Cand 800°C)than that without coating.3) Thermal shock resistance of the samples treated in 700°C is the best which has reached within 17-21cycles (900°C, air cooling). 4) The stainless steel substrate with ceramic coating has a higher abrasion resistance than that without coating. The samples treated at 700°Cand 800°C have the best abrasion resistance.

Fabrication of meta-aramid fibrid by precipitation

“Fibrid”, a type of fibrous particle used for binding, and the meta-aramid fibrids were successfully prepared by injecting the meta-aramid solution into the high shearing coagulation bath in this paper. The meta-aramid solution was prepared by dissolving the meta-aramid fibers in LiCl/DMAc solving system, and the mixture of water and DMAc was used as coagulation bath. The rotating speed, concentration of meta-aramid solution and the concentration of water in the coagulation bath were the main factors on effecting the formation and morphology of fibrids. The results showed that the length of meta-aramid fibrid decreased with the rotating speed and the content of water in coagulation bath increasing, and increased with the concentration of meta-aramid solution increasing. The morphology of meta-aramid fibrid were observed by optical microscope, a lower thermal property of meta-aramid firbid than meta-aramid fiber was observed in TGA which was mainly due to the breaking of hydrogen bonding and crystalline structure of meta-aramid fiber during the dissolving process, the results in TGA were consistent with the XRD, and the XRD curves showed that the non-crystalline structure were observed in meta-aramid fibrid.

Thermodynamic States of Nanoclusters at Low Pressure and Low Temperature: The Case of 13 H2

A confinement model of finite-size systems that embodies an equation of state is presented. The temperature and pressure of the system are obtained from the positions and velocities of the enclosed particles after a number of molecular dynamics simulations. The pressure has static and dynamic (thermal) contributions, extending the Mie-Grüneisen equation of state to include weakly interacting anharmonic oscillators. The model is applied to a system of 13 H(2) molecules under low-pressure and low-temperature conditions in the classical regime. The confining cage in this case is a spherical hydrogen cavity. The Born-Oppenheimer molecular dynamics in conjunction with density functional theory are used for the time evolution of the particle system. The hydrogen molecules form a noncrystalline cluster structure with icosahedral symmetry that remains so in the whole temperature range investigated. The fluctuations of the interatomic distances increase with the temperature, while the orientational order of the enclosed system of molecules fades out, suggesting a gradual order-disorder transition.

Achieving long time scale simulations of glass-forming systems

Glass-forming systems have posed an especial challenge for atomistic simualtions given their complicated non-crystalline structure and the long time scales involved with glass transition and relaxation phenomena. In this article, we review two recent techniques for extending the time scales of these simulations. First, we describe the enthalpy landscape approach, which uses inherent structure and transition point mapping to develop a set of coarse-grained master equations for computing long time dynamics. Accounting for the broken ergodic nature of glass, these master equations can be solved on any arbitrary time scale. Second, we discuss the Kinetic Monte Carlo method and its application to glassy systems. Kinetic Monte Carlo provides an effective means of sampling rare events without losing the detailed atomistic description of the glass structure.

Structure and luminescence properties of Mn-doped Zn 2SiO 4 prepared with extracted mesoporous silica

Zn 2SiO 4:Mn powders were prepared by solid-state reaction using extracted SBA-15 as silica source. The well crystalline willemite Zn 2SiO 4:Mn can be obtained at 800 °C, much lower than the conventional solid-state reaction temperature and lower than using the calcined SBA-15. This can be attributed to the high reactive activity of the extracted SBA-15 due to its high density silanol groups, large surface areas, and non-crystalline structure. Ultraviolet (UV) and vacuum ultraviolet (VUV) excitation spectra reveal the host lattice absorption band around 162 nm and the charge transfer transition band around 245 nm. The Zn 2SiO 4:Mn phosphor exhibits a strong green emission around 527 nm. The Zn 2SiO 4:Mn phosphor with an Mn doping concentration of 0.06, i.e., Zn 1.94Mn 0.06SiO 4, shows the highest relative emission intensity. Upon 147 nm excitation, the luminescence decay time of the green emission of Zn 1.94Mn 0.06SiO 4 around 527 nm is 8.87 ms.