Phonons, Localization, and Thermal Conductivity of Diamond Nanothreads and Amorphous Graphene

Study on the thermal behaviour of spontaneous combustion of open-pit minerals

To study thermal behaviour during spontaneous combustion of an open-pit coal mine, mixed slag (coal, oil shale, and coal gangue) was taken as the research object. Laser thermal conductivity analyser and differential scanning calorimetry were used to test thermophysical parameters and heat release characteristics of the minerals. The parameters can be employed to calculate the apparent activation energy using the Arrhenius equation and evaluate the thermal behaviour of open-pit mixed slag. The results indicate that thermophysical parameters have stage characteristics. Thermal diffusivity and thermal conductivity of minerals, especially mixed slag, have a strong correlation with temperature. Heat flow of minerals exhibits five characteristic stages, and heat flow of the samples is consistent with the change in heating rate. During the heating process, thermal diffusivity and heat flow of the mixed slag are between those of a single mineral. Except for the mixed slag at 15 and 20 °C/min, the initial exothermic temperature of the other samples is mainly concentrated at 50-80 °C. Thermal energy release of the sample is mainly concentrated in the accelerated exothermic stage and rapid exothermic stage. Thermal energy release of mixed slag in rapid exothermic stage is always greater than that in accelerated exothermic stage, and the proportion of thermal energy release in these two stages exceeds 98 %. The apparent activation energy during the accelerated exothermic stage is lower, making it easier to release heat, and rapid exothermic stage is relatively high, which can readily lead to heat accumulation. Thermal analysis reveals that the thermal behaviour of mixed slag is significantly different from that of a single mineral. Its unique exothermic characteristics can provide a more accurate theoretical basis for the prevention and control of environmental pollution caused by slag spontaneous combustion.

Anderson Localization of Phonons in Thermally Superinsulating Graphene Aerogels with Metal-Like Electrical Conductivity

In the quest to improve energy efficiency and design better thermal insulators, various engineering strategies have been extensively investigated to minimize heat transfer through a material. Yet, the suppression of thermal transport in a material remains elusive because heat can be transferred by multiple energy carriers. Here, the realization of Anderson localization of phonons in a random 3D elastic network of graphene is reported. It is shown that thermal conductivity in a cellular graphene aerogel can be drastically reduced to 0.9 mW m-1 K-1 by the application of compressive strain while keeping a high metal-like electrical conductivity of 120 S m-1 and ampacity of 0.9 A. The experiments reveal that the strain can cause phonon localization over a broad compression range. The remaining heat flow in the material is dominated by charge transport. Conversely, electrical conductivity exhibits a gradual increase with increasing compressive strain, opposite to the thermal conductivity. These results imply that strain engineering provides the ability to independently tune charge and heat transport, establishing a new paradigm for controlling phonon and charge conduction in solids. This approach will enable the development of a new type of high-performance insulation solutions and thermally superinsulating materials with metal-like electrical conductivity.

Assessing the impact of ultra-thin diamond nanothreads on the glass transition temperature of a bituminous binder

Low-temperature cracking and rutting are the most destructive problems of bitumen that hinder the application of high-performance bitumen engineering, which is dependent on its glass transition temperature (Tg). Through in silico studies, this work has systematically investigated the Tg of a bituminous binder with the addition of diamond nanothread (DNT) fillers with varying filler content, alignment, distribution, and functional groups. In general, the glass transition phenomenon of the bitumen is determined by the mobility of its constituent molecules. Tg is found to increase gradually with the increase in the weight percentage of DNT and then decreases when the weight percentage exceeds 5.05 wt%. The enhancement effect on Tg is weakened when DNTs are distributed vertically or functionalized with functional groups. Specifically, DNT fillers induce inhomogeneity, which promotes the motion of small molecules while hindering the motion of large molecules. The aggregation of DNTs and the molecular environment in the vicinity of DNTs directly affect Tg. In summary, aggregation and adhesion are the dominant mechanisms affecting the mobility of the constituent molecules in the DNT/bitumen system and thus its glass transition temperature. This work provides in-depth insights into the underlying mechanisms for the glass transition of a bituminous binder, which could serve as theoretical guidance for tuning the low-temperature performance of the bituminous binder.

Synthesis and Post-Processing of Chemically Homogeneous Nanothreads from 2,5-Furandicarboxylic Acid

Compared with conventional, solution-phase approaches, solid-state reaction methods can provide unique access to novel synthetic targets. Nanothreads-one-dimensional diamondoid polymers formed through the compression of small molecules-represent a new class of materials produced via solid-state reactions, however, the formation of chemically homogeneous products with targeted functionalization represents a persistent challenge. Through careful consideration of molecular precursor stacking geometry and functionalization, we report here the scalable synthesis of chemically homogeneous, functionalized nanothreads through the solid-state polymerization of 2,5-furandicarboxylic acid. The resulting product possesses high-density, pendant carboxyl functionalization along both sides of the backbone, enabling new opportunities for the post-synthetic processing and chemical modification of nanothread materials applicable to a broad range of potential applications.

Solid-State Pathway Control via Reaction-Directing Heteroatoms: Ordered Pyridazine Nanothreads through Selective Cycloaddition

Nanothreads are one-dimensional nanomaterials composed of a primarily sp³ hydrocarbon backbone, typically formed through the compression of small molecules to high pressures. Although nanothreads have been synthesized from a range of precursors, controlling reaction pathways to produce atomically precise materials remains a difficult challenge. Here, we show how heteroatoms within precursors can serve as "thread-directing" groups by selecting for specific cycloaddition reaction pathways. By using a less-reactive diazine group within a six-membered aromatic ring, we successfully predict and synthesize the first carbon nanothread material derived from pyridazine (1,2-diazine, C₄H₄N₂). Compared with previous nanothreads, the synthesized polypyridazine, shows a predominantly uniform chemical structure with exceptional long-range order, allowing for structural characterization using vibrational spectroscopy and X-ray diffraction. The results demonstrate how thread-directing groups can be used for reaction pathway control and the formation of chemically precise nanothreads with a high degree of structural order.

Toward Optimal Heat Transfer of 2D-3D Heterostructures via van der Waals Binding Effects

Two-dimensional (2D) materials and their heterogeneous integration have enabled promising electronic and photonic applications. However, significant thermal challenges arise due to numerous van der Waals (vdW) interfaces limiting the dissipation of heat generated in the device. In this work, we investigate the vdW binding effect on heat transport through a MoS₂-amorphous silica heterostructure. We show using atomistic simulations that the cross-plane thermal conductance starts to saturate with the increase of vdW binding energy, which is attributed to substrate-induced localized phonons. With these atomistic insights, we perform device-level heat transfer optimizations. Accordingly, we identify a regime, characterized by the coupling of in-plane and cross-plane heat transport mediated by vdW binding energy, where maximal heat dissipation in the device is achieved. These results elucidate fundamental heat transport through the vdW heterostructure and provide a pathway toward optimizing thermal management in 2D nanoscale devices.

Scalable Synthesis of Crystalline One-Dimensional Carbon Nanothreads through Modest-Pressure Polymerization of Furan

Carbon nanothreads, which are one-dimensional sp³-rich polymers, combine high tensile strength with flexibility owing to subnanometer widths and diamond-like cores. These extended carbon solids are constructed through pressure-induced polymerization of sp² molecules such as benzene. Whereas a few examples of carbon nanothreads have been reported, the need for high onset pressures (≥17 GPa) to synthesize them precludes scalability and limits scope. Herein, we report the scalable synthesis of carbon nanothreads based on molecular furan, which can be achieved through ambient temperature pressure-induced polymerization with an onset reaction pressure of only 10 GPa due to its lessened aromaticity relative to other molecular precursors. When slowly compressed to 15 GPa and gradually decompressed to 1.5 GPa, a sharp 6-fold diffraction pattern is observed in situ, indicating a well-ordered crystalline material formed from liquid furan. Single-crystal X-ray diffraction (XRD) of the reaction product exhibits three distinct d-spacings from 4.75 to 4.9 Å, whose size, angular spacing, and degree of anisotropy are consistent with our atomistic simulations for crystals of furan nanothreads. Further evidence for polymerization was obtained by powder XRD, Raman/IR spectroscopy, and mass spectrometry. Comparison of the IR spectra with computed vibrational modes provides provisional identification of spectral features characteristic of specific nanothread structures, namely syn, anti, and syn/anti configurations. Mass spectrometry suggests that molecular weights of at least 6 kDa are possible. Furan therefore presents a strategic entry toward scalable carbon nanothreads.

Lattice thermal transport in two-dimensional alloys and fractal heterostructures

Engineering thermal transport in two dimensional materials, alloys and heterostructures is critical for the design of next-generation flexible optoelectronic and energy harvesting devices. Direct experimental characterization of lattice thermal conductivity in these ultra-thin systems is challenging and the impact of dopant atoms and hetero-phase interfaces, introduced unintentionally during synthesis or as part of deliberate material design, on thermal transport properties is not understood. Here, we use non-equilibrium molecular dynamics simulations to calculate lattice thermal conductivity of [Formula: see text] monolayer crystals including [Formula: see text] alloys with substitutional point defects, periodic [Formula: see text] heterostructures with characteristic length scales and scale-free fractal [Formula: see text] heterostructures. Each of these features has a distinct effect on phonon propagation in the crystal, which can be used to design fractal and periodic alloy structures with highly tunable thermal conductivities. This control over lattice thermal conductivity will enable applications ranging from thermal barriers to thermoelectrics.

A Review on Recent Advances in the Applications of Boron-Doped Diamond Electrochemical Sensors in Food Analysis

The usage of boron-doped diamond (BDD) material has found to be very attractive in modern electroanalytical methods and received massive consideration as perspective electrochemical sensor due to its outstanding (electro)chemical properties. These generally known facilities include large potential window, low background currents, ability to withstand extreme potentials and strong tendency to resist fouling compared to conventional carbon-based electrodes. As evidence of superiority of this material, couple of reviews describing the overview of various applications of BDD electrodes in the field of analytical and material chemistry has been reported in scientific literature during last decade. However, herein proposed review predominantly focuses on the most recent developments (from 2009 to 2020) dealing with the application of BDD as an advanced and environmental-friendly sensor platform in food analysis. The main method characteristics of analysis of various organic food components with different chemical properties, including additives, flavor and aroma components, phenolic compounds, flavonoids and pesticides in food matrices are described in more details. The importance of BDD surface termination, presence of sp² content and boron doping level on electrochemical sensing is discussed. Apart from this, a special attention is paid to the evaluation of main analytical characteristics of the BDD electrochemical sensor in single- and multi-analyte detection mode in food analysis. The recent achievements in the utilizing of BDD electrodes in amperometric detection coupled to flow injection analysis, batch injection analysis, and high-performance liquid chromatography are also commented. Moreover, actual trends in sample preparation techniques prior to electrochemical sensing in food analysis are referred.

Spectral attributes of sub-amorphous thermal conductivity in cross-linked organic-inorganic hybrids

Organic-inorganic hybrids have found increasing applications for thermal management across various disciplines. Such materials can achieve thermal conductivities below the so-called "amorphous limit" of their constituents' thermal conductivity. Despite their technological significance, a complete understanding of the origins of this thermal conductivity reduction remains elusive in these materials. In this paper, we develop a prototypical cross-linked organic-inorganic layered system, to investigate the spectral origins of its sub-amorphous thermal conductivity. Initially, we study the atomic structure of the model and find that besides polymer chain length, the relative drift of the layers governs the reduction in computed basal spacing, in agreement with experimental measurements. We, subsequently, find that organic cross-linking results in up to 40% reduction in thermal conductivity compared to inorganic samples. An in-depth investigation of vibrational modes reveals that this reduction is the result of reduced mode diffusivities, which in turn is a consequence of a vibrational mismatch between the organic and inorganic constituents. We also show that the contribution of propagating modes to the total thermal conductivity is not affected by organic cross-linking. Our approach paves the path toward a physics-informed analysis and design of a wide range of multifunctional hybrid nanomaterials for thermal management applications among others.

The effects of Stone-Wales defects on the thermal properties of bilayer armchair graphene nanoribbons

We investigate the influence of Stone–Wales (S–W) defects on the thermal properties of bilayer graphene nanoribbons (BGNRs) with armchair edges by nonequilibrium molecular dynamics simulations (NEMD). It is shown that an increasing number of S–W defects leads to a significant decrease of the thermal conductivity of BGNRs at room temperature. Moreover, the AA-stacked BGNRs have significantly higher thermal conductivity than that of the AB-stacked BGNRs for all S–W defect numbers. In the temperature range of 300–700 K, the S–W defects always have a weaker effect on heat transfer of AB-stacked BGNRs than AA-stacked BGNRs, which is closely related to their weaker anharmonic effects induced by structure defects. In addition, the simulation results are further explained by performing an analysis of phonon spectrum properties and phonon vibrational modes.

We investigate the influence of Stone–Wales (S–W) defects on the thermal properties of bilayer graphene nanoribbons (BGNRs) with armchair edges by nonequilibrium molecular dynamics simulations (NEMD).

Thermal transport of carbon nanomaterials

The diversity of thermal transport properties in carbon nanomaterials enables them to be used in different thermal fields such as heat dissipation, thermal management, and thermoelectric conversion. In the past two decades, much effort has been devoted to study the thermal conductivities of different carbon nanomaterials. In this review, different theoretical methods and experimental techniques for investigating thermal transport in nanosystems are first summarized. Then, the thermal transport properties of various pure carbon nanomaterials including 1D carbon nanotubes, 2D graphene, 3D carbon foam, are reviewed in details and the associated underlying physical mechanisms are presented. Meanwhile, we discuss several important influences on the thermal conductivities of carbon nanomaterials, including size, structural defects, chemisorption and strain. Moreover, we introduce different nanostructuring pathways to manipulate the thermal conductivities of carbon-based nanocomposites and focus on the wave nature of phonons for controlling thermal transport. At last, we briefly review the potential applications of carbon nanomaterials in the fields of thermal devices and thermoelectric conversion.

Nanoarchitecture through Strained Molecules: Cubane-Derived Scaffolds and the Smallest Carbon Nanothreads

Relative to the rich library of small-molecule organics, few examples of ordered extended (i.e., nonmolecular) hydrocarbon networks are known. In particular, sp³ bonded, diamond-like materials represent appealing targets because of their desirable mechanical, thermal, and optical properties. While many covalent organic frameworks (COFs)-extended, covalently bonded, and porous structures-have been realized through molecular architecture with exceptional control, the design and synthesis of dense, covalent extended solids has been a longstanding challenge. Here we report the preparation of a sp³-bonded, low-dimensional hydrocarbon synthesized via high-pressure, solid-state diradical polymerization of cubane (C₈H₈), which is a saturated, but immensely strained, cage-like molecule. Experimental measurements show that the obtained product is crystalline with three-dimensional order that appears to largely preserve the basic structural topology of the cubane molecular precursor and exhibits high hardness (comparable to fused quartz) and thermal stability up to 300 °C. Among the plausible theoretical candidate structures, one-dimensional carbon scaffolds comprising six- and four-membered rings that pack within a pseudosquare lattice provide the best agreement with experimental data. These diamond-like molecular rods with extraordinarily small thickness are among the smallest members in the carbon nanothread family, and calculations indicate one of the stiffest one-dimensional systems known. These results present opportunities for the synthesis of purely sp³-bonded extended solids formed through the strain release of saturated molecules, as opposed to only unsaturated precursors.

Diffused Lattice Vibration and Ultralow Thermal Conductivity in the Binary Ln-Nb-O Oxide System

In the pursuit of low thermal conductivity materials for thermal management, one always tries to increase the material entropy by increasing the number of components in the materials to scatter heat-carrying phonons. However, it also drastically increases the technological complexity to synthesize materials with the target complex composition. Here, a material family is presented with simple composition Ln₃ NbO₇ , which only contains binary oxides of Ln₂ O₃ (Ln = Dy, Er, Y, Yb) and Nb₂ O₅ . The thermal conductivities approach the theoretical minimum limit, where the large chemical inhomogeneity due to the charge disorder and fluctuation of bonding length in Ln₃ NbO₇ plays a major role. Despite the simple composition, Ln₃ NbO₇ demonstrates an unprecedentedly high scattering rate of vibration states, as confirmed by the highest elastic constant/thermal conductivity ratio, as well as the diffused wavevector-frequency dispersion. In contrast to the conventional wisdom that low thermal conductivity materials should be explored in the pool of "complex" multiple-component materials, this work points out an avenue to look into materials with simple composition but large internal chemical inhomogeneity, which would be of both scientific and technological significance in the fields of thermal barrier coating, thermoelectric materials, etc.

Local Structure and Bonding of Carbon Nanothreads Probed by High-Resolution Transmission Electron Microscopy

Carbon nanothreads are a new one-dimensional sp³-bonded nanomaterial of CH stoichiometry synthesized from benzene at high pressure and room temperature by slow solid-state polymerization. The resulting threads assume crystalline packing hundreds of micrometers across. We show high-resolution electron microscopy (HREM) images of hexagonal arrays of well-aligned thread columns that traverse the 80-100 nm thickness of the prepared sample. Diffuse scattering in electron diffraction reveals that nanothreads are packed with axial and/or azimuthal disregistry between them. Layer lines in diffraction from annealed nanothreads provide the first evidence of translational order along their length, indicating that this solid-state reaction proceeds with some regularity. HREM also reveals bends and defects in nanothread crystals that can contribute to the broadening of their diffraction spots, and electron energy-loss spectroscopy confirms them to be primarily sp³-hybridized, with less than 27% sp² carbon, most likely associated with partially saturated "degree-4" threads.

Evidence of a two-dimensional glass transition in graphene: Insights from molecular simulations

Liquids exhibit a sudden increase in viscosity when cooled fast enough, avoiding thermodynamically predicted route of crystallization. This phenomenon, known as glass transition, leads to the formation of non-periodic structures known as glasses. Extensive studies have been conducted on model materials to understand glass transition in two dimensions. However, despite the synthesis of disordered/amorphous single-atom thick structures of carbon, little attention has been given to glass transition in realistic two-dimensional materials such as graphene. Herein, using molecular dynamics simulation, we demonstrate the existence of glass transition in graphene leading to a realistic two-dimensional glassy structure, namely glassy graphene. We show that the resulting glassy structure exhibits excellent agreement with experimentally realized disordered graphene. Interestingly, this glassy graphene exhibits a wrinkled but stable structure, with reduced thermal vibration in comparison to its crystalline counterpart. We suggest that the topological disorder induced by glass transition governs the unique properties of this structure.

Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra

A one-dimensional (1D) sp³ carbon nanomaterial with high lateral packing order, known as carbon nanothreads, has recently been synthesized by slowly compressing and decompressing crystalline solid benzene at high pressure. The atomic structure of an individual nanothread has not yet been determined experimentally. We have calculated the ¹³C nuclear magnetic resonance (NMR) chemical shifts, chemical shielding tensors, and anisotropies of several axially ordered and disordered partially saturated and fully saturated nanothreads within density functional theory and systematically compared the results with experimental solid-state NMR data to assist in identifying the structures of the synthesized nanothreads. In the fully saturated threads, every carbon atom in each progenitor benzene molecule has bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called "degree-6" nanothread), while the partially saturated threads examined retain a single double bond per benzene ring ("degree-4"). The most-parsimonious theoretical fit to the experimental 1D solid-state NMR spectrum, constrained by the measured chemical shift anisotropies and key features of two-dimensional NMR spectra, suggests a certain combination of degree-4 and degree-6 nanothreads as plausible components of this 1D sp³ carbon nanomaterial, with intriguing hints of a [4 + 2] cycloaddition pathway toward nanothread formation from benzene columns in the progenitor molecular crystal, based on the presence of nanothreads IV-7, IV-8, and square polymer in the minimal fit.

Amorphous graphene: a constituent part of low density amorphous carbon

In this paper, we provide evidence that low density nano-porous amorphous carbon (a-C) consists of interconnected regions of amorphous graphene (a-G). We include experimental information in producing models, while retaining the power and accuracy of ab initio methods with no biasing assumptions. Our models are highly disordered with predominant sp2 bonding and ring connectivity mainly of sizes 5-8. The structural, dynamical and electronic signatures of our 3-D amorphous graphene are similar to those of monolayer amorphous graphene. We predict an extended x-ray absorption fine structure (EXAFS) signature of amorphous graphene. Electronic density of states calculations for 3-D amorphous graphene reveal similarity to monolayer amorphous graphene and the system is non conducting.

Kinetic theory for the formation of diamond nanothreads with desired configurations: a strain-temperature controlled phase diagram

Diamond nanothreads (DNTs) are a brand-new one-dimensional carbon nanomaterial that was synthesized recently by compressing benzene. Compared with sp2 carbon nanotubes, DNTs may possess a much higher interfacial load-transfer ability. However, previous studies have shown that the mechanical properties of DNTs are highly sensitive to the composition of Stone-Wales (SW) transformed sites. Up to now, it remained unclear what roles SWs play in the structure stability and how to engineer its molecular structure for novel mechanical properties. Using ab initio calculations, here we show that the most stable structure of a DNT is composed of alternative SW and hydrogenated carbon nanotube (3,0) units, suggesting that SW plays an essential role in stabilizing DNT. Interestingly, we found that the SW transition barrier is a nearly linear function of the applied strain, enabling strain engineering of its molecular structure. To do so, we propose a strain-temperature-stretching rate phase diagram to guide the construction of desired molecular structures to achieve superplastic behavior of DNTs. Our findings not only enrich our understanding of this novel carbon material, but also provide a strategy to control its structural and mechanical properties for novel applications, such as energy absorption, energy storage and materials reinforcement.

Functionalized diamond nanothreads from benzene derivatives

Diamond nanothreads (DNTs) are fully sp³-bonded one-dimensional carbon nanostructures, synthesized recently through compression of crystalline benzene. They possess outstanding mechanical strength, suitable for the development of novel nanostructured reinforced materials. In this article, we use density functional theory calculations to investigate the feasibility and physical properties of functionalized DNTs. We show that the stacking and covalent bonding of benzene derivative molecules (toluene, aniline, phenol and fluorobenzene) may lead to stable configurations analogous to benzene-derived DNTs, with functional groups (-CH₃, -NH₂, -OH, -F) covalently attached to the surface. The same principle was also applied to pyridine, an aromatic heterocyclic compound, resulting in DNTs containing N heteroatoms within the sp³ C-C chain. We show that the mechanical properties remain practically unaltered compared to the original material, and that the electronic properties can be tuned upon functionalization. The presence of polar functional groups on DNT surfaces are expected to affect their compatibility with other materials and solvents, enabling the development of novel processes and technological applications using DNTs.

Light propagation in binary kagome ribbons with evolving disorder

By introducing evolving disorder in the binary kagome ribbons, we study the establishment of diffusive spreading of flat band states characterized by diffractionless propagation in regular periodic ribbons. Our numerical analysis relies on controlling strength and rate of change of disorder during light propagation while tailoring binarism of the kagome ribbons in order to isolate the flat band with the gap from the rest of the ribbon's eigenvalue spectrum and study systematically its influence on diffusion. We show that the flat band plays a dominant role in the establishment of the diffusion for a given strength and rate of change of disorder, whereas the rest of the ribbon's eigenvalue spectrum induces only quantitative differences in the light spreading regimes. Due to the universality of studied phenomena, our findings may be of interest in various disordered physical systems with flat spectral bands, ranging from photonics to ultracold matter systems and plasmonics.

Fast and slow thermal processes in harmonic scalar lattices

An approach for analytical description of thermal processes in harmonic lattices is presented. We cover longitudinal and transverse vibrations of chains and out-of-plane vibrations of two-dimensional lattices with interactions of an arbitrary number of neighbors. The motion of each particle is governed by a single scalar equation and therefore the notion 'scalar lattice' is used. The evolution of initial temperature field in an infinite lattice is investigated. An exact equation describing the evolution is derived. Continualization of this equation with respect to spatial coordinates is carried out. The resulting continuum equation is solved analytically. The solution shows that the kinetic temperature is represented as the sum of two terms, one describing short time behavior, the other large time behavior. At short times, the temperature performs high-frequency oscillations caused by redistribution of energy among kinetic and potential forms (fast process). Characteristic time of this process is of the order of ten periods of atomic vibrations. At large times, changes of the temperature are caused by ballistic heat transfer (slow process). The temperature field is represented as a superposition of waves having the shape of initial temperature distribution and propagating with group velocities dependent on the wave vector. Expressions describing fast and slow processes are invariant with respect to substitution t by [Formula: see text]. However, examples considered in the paper demonstrate that these processes are irreversible. Numerical simulations show that presented theory describes the evolution of temperature field at short and large time scales with high accuracy.

Influence of TiO2 and Si on the exciton-phonon interaction in PbI2 and CdS semiconductors evidenced by Raman spectroscopy

The exciton-phonon interaction, considered as a stimulated Raman scattering process, is studied in different semiconductor mixtures: PbI₂/TiO₂, PbI₂/Si and CdS/Si. Raman spectra recorded at excitation wavelengths of 514.5 and 488 nm for PbI₂ and CdS, respectively, reveal a strong enhancement of the Raman lines peaked at 97 and 305 cm^-1, evaluated by the ratio I _TK/I _{300 K} between the relative intensities of the spectra recorded in the temperature range of 88-300 K. It is found that PbI₂ and CdS exhibit a decrease in the Raman intensity modes with decreasing temperature, while in TiO₂ and Si an increase in the Raman lines intensities peaked at 138 and 520 cm^-1 is observed. This behavior can be explained by an energy transfer process from PbI₂ or CdS towards TiO₂ and Si. This explanation is supported by the schematic potential energy levels diagram obtained from the density of states, which is calculated using the density functional theory. According to this energy levels diagram, the electrons are expected to migrate directly from the conduction band (CB) energetic levels of the PbI₂ and CdS towards the CB levels of TiO₂ and Si.

Temperature and isotope effects on the thermoelectric properties in SnTe

The temperature and Sn isotope effects on SnTe are investigated using the hybrid scheme that combines ab initio molecular dynamics and lattice dynamics (AIMD  +  LD). The unstable softening of TA phonon mode at 0 K in cubic phase SnTe disappears and the TO mode is stiffened by considering the temperature effect, in agreement with the inelastic neutron scattering (INS) observations. A linear dependence on isotope mass of phonon frequency, lifetime, and mean free path for isolate phonon mode is observed with the possibility of positive, negative, and nearly zero shifts. The lattice thermal conductivity ([Formula: see text]) shows saturation characteristic as Sn isotope mass increases to ¹²⁰SnTe, with an increase rate of  ∼2.1% from ¹¹²SnTe to ¹²⁴SnTe. Considering the effects of partial isotope doping, we obtain a reduced [Formula: see text] with respect to the undoped case. The [Formula: see text] is reduced by  ∼14.6% at 600 K when considering the volumetric expansion.

Scaling of slip avalanches in sheared amorphous materials based on large-scale atomistic simulations

Atomistic simulations of binary amorphous systems with over 4 million atoms are performed. Systems of two interatomic potentials of the Lennard-Jones type, LJ12-6 and LJ9-6, are simulated. The athermal quasistatic shearing protocol is adopted, where the shear strain is applied in a stepwise fashion with each step followed by energy minimization. For each avalanche event, the shear stress drop (Δσ), the hydrostatic pressure drop (Δσ_{h}), and the potential energy drop (ΔE) are computed. It is found that, with the avalanche size increasing, the three become proportional to each other asymptotically. The probability distributions of avalanche sizes are obtained and values of scaling exponents fitted. In particular, the distributions follow a power law, P(ΔU)∼ΔU^{-τ}, where ΔU is a measure of avalanche sizes defined based on shear stress drops. The exponent τ is 1.25±0.1 for the LJ12-6 systems, and 1.15±0.1 for the LJ9-6 systems. The value of τ for the LJ12-6 systems is consistent with that from an earlier atomistic simulation study by Robbins et al. [Phys. Rev. Lett. 109, 105703 (2012)]PRLTAO0031-900710.1103/PhysRevLett.109.105703, but the fitted values of other scaling exponents differ, which may be because the shearing protocol used here differs from that in their study.

Ab initio study of the nature and stability of the defects and multi-vacancies in TaN. Comparison with TiN

First principles calculations have been carried out to study the native single-defects and multi-vacancies in TaN and TiN with a cubic rocksalt structure mainly used as diffusion barriers. Our results indicate that vacancies are the most stable single-defects in both compounds and that nitrogen interstitial defects in tetrahedral interstitial site are significantly more stable in TaN than in TiN. The interactions between vacancies are attractive in TaN in contrast to the case of TiN. The vacancies show a much larger tendency to cluster and to form bi- and tri- vacancies in TaN than in TiN. We suggest that the number of d electrons might explain this difference in the defect stability. These results will have impact on the use of these materials as diffusion barrier.