Semi-infinite Bulk Research Articles

Optical properties of yttria-stabilized zirconia from spectroscopic ellipsometry

Complex dielectric function (ɛ = ɛ1 + iɛ2) spectra of a heat treated single crystal yttria-stabilized zirconia (YSZ) have been determined over a spectral range of 0.03–8.5 eV using spectroscopic ellipsometry. Spectra are collected using three instruments covering different parts of the measured spectrum. The YSZ sample is modeled as a semi-infinite bulk crystal covered by a surface layer described by a Bruggeman effective medium approximation of equal parts YSZ and void.

An intermediate modulator enhanced the near-field thermal radiation of semiconductor vanadium dioxide

We theoretically investigate a three-body near-field radiative heat transfer (NFRHT) system, which consists of three separated parallel plates with different temperatures and certain vacuum spacing. The emitter (body1) and the receiver (body3) are both composed of a monolayer graphene and a VO2 film covered by semi-infinite bulk SiO2. The intermediate (body2) has a composite graphene/VO2/graphene heterostructure. The steady temperature of body2 can be determined based on the fact that the near-field radiative heat flow (NFRHF) emitted by body1 is equal to that received by body3 at thermal steady state. NFRHF can be highly tuned by changing the vacuum gap and the chemical potential of graphene in the three-body system. This paper can provide theoretical guidance for experimental and practical applications of near-field thermal radiation, and are significant for the applications of NFRHT structures containing intermediates in the field of temperature control.

Hydrodynamic simulation of laser-induced shock waves using the Turbulence Problem Solver software package

The problem of irradiation of a semi-infinite bulk metal target by a single laser pulse, being important for many lase technology applications, is considered. The model for simulating the propagation and attenuation of shock waves, occurring when metals are irradiated with single laser pulse, is presented. The paper describes the computational algorithm based on the model and features of its implementation in C++ in a software package Turbulence Problem Solver (TPS) for modeling a wide class of problems described by hyperbolic equations. The problem is modeled using different approaches and the results are compared. The object-oriented polymorphic architecture of the software complex accelerates and facilitates the process of model development and calibration, allowing for flexible configuration and combination of elements of numerical models: Riemann solvers, various types of and reconstructions, and equations of state of matter.

Fundamental limits to attractive and repulsive Casimir-Polder forces

We derive upper and lower bounds on the Casimir--Polder force between an anisotropic dipolar body and a macroscopic body separated by vacuum via algebraic properties of Maxwell's equations. These bounds require only a coarse characterization of the system---the material composition of the macroscopic object, the polarizability of the dipole, and any convenient partition between the two objects---to encompass all structuring possibilities. We find that the attractive Casimir--Polder force between a polarizable dipole and a uniform planar semi-infinite bulk medium always comes within 10% of the lower bound, implying that nanostructuring is of limited use for increasing attraction. In contrast, the possibility of repulsion is observed even for isotropic dipoles, and is routinely found to be several orders of magnitude larger than any known design, including recently predicted geometries involving conductors with sharp edges. Our results have ramifications for the design of surfaces to trap, suspend, or adsorb ultracold gases.

Monte Carlo study on the surface potential measurement using the peak-shift method

The accurate surface potential measurements in charging phenomena of insulating materials are highly desired as important information about charging can be inferred from surface potential. The interaction of the charged particle, induced by probe particles and emitted from sample surfaces, with surface potential enables its determination from the shift in the characteristic peak in the energy spectrum—the “peak-shift” method. However, important features, which are believed to be correlated with charging, are presented in the energy spectrum, and on the other hand, the peak-shift method is not suited for all charging phenomena. We use a Monte Carlo method to evaluate the charging of the semi-infinite SiO2 bulk induced by primary-electron irradiation and clarify the correlation of energy spectral features with charging and the applicability range of the peak-shift method. To take into account the charge trapping by defects, vacancies, and impurities etc., trapping sites are assumed to uniformly distribute in the sample, with the depth distribution described by a Gaussian function. Furthermore, the trapping of low-energy secondary electrons is also included to model their emissions more accurately. With simulation reliability evidenced by the agreement on the peak-shift magnitude with experiment, this work advances our understanding of the peak-shift method.

Measurement of anisotropic thermal conductivity of a dense forest of nanowires using the 3ω method.

The 3ω method is a dynamic measurement technique developed for determining the thermal conductivity of thin films or semi-infinite bulk materials. A simplified model is often applied to deduce the thermal conductivity from the slope of the real part of the ac temperature amplitude as a function of the logarithm of frequency, which in-turn brings a limitation on the kind of samples under observation. In this work, we have measured the thermal conductivity of a forest of nanowires embedded in nanoporous alumina membranes using the 3ω method. An analytical solution of 2D heat conduction is then used to model the multilayer system, considering the anisotropic thermal properties of the different layers, substrate thermal conductivity, and their thicknesses. Data treatment is performed by fitting the experimental results with the 2D model on two different sets of nanowires (silicon and BiSbTe) embedded in the matrix of nanoporous alumina templates, having thermal conductivities that differ by at least one order of magnitude. These experimental results show that this method extends the applicability of the 3ω technique to more complex systems having anisotropic thermal properties.

Research on Electron Emission from Dielectric Materials by a Monte Carlo Method

The charge accumulation in an insulating material under an electron beam bombardment exerts a significant influence to scanning electron microscopic imaging. This work investigates the charging formation process by a self-consistent Monte Carlo simulation of charge production and transportation based on a charge dynamics model. The charging effect in a semi-infinite SiO2 bulk and SiO2 trapezoidal lines on a SiO2 or Si substrate has been studied. We used two methods to calculate the spatial distributions of electric potential and electric field for two different systems respectively: the image charge method was used to deal with a semi-infinite bulk, and, random walk method to solve the Poisson equation for a complex geometric structure. The dynamic charging behavior depending on irradiation time has been investigated for SiO2. The simulated CD-SEM images of SiO2 trapezoidal lines with charging effect included were compared well with experimental results, showing the contrast change of SEM image along with scanning frames due to charging.

Spin accumulation in disordered topological insulator ultrathin films

Topological insulator (TI) ultrathin films differ from the more commonly studied semi-infinite bulk TIs in that the former possess both top and bottom surfaces where the surface states localized at different surfaces can couple to one another across the finite thickness of the film. In the presence of an in-plane magnetization, the TI thin films display two distinct phases depending on which of the inter-surface coupling or the magnetization is stronger. In this work, we consider a Bi2Se3 TI thin film system with an in-plane magnetization and numerically calculate the resulting spin accumulation on both surfaces of the film due to an in-plane electric field to linear order. We describe a numerical scheme for performing the Kubo formula calculation in which we include impurity scattering and vertex corrections. We find that the sums of the spin accumulation over the two surfaces in the in-plane direction perpendicular to the magnetization and in the out of plane direction are antisymmetric in Fermi energy around the charge neutrality point and are non-vanishing only when the symmetry between the top and bottom TI surfaces is broken. The impurity scattering, in general, diminishes the magnitude of the spin accumulation.

Monte Carlo Study on Electron Emission from Dielectric Materials

ABSTRACTThe charge accumulation in an insulating material under an electron beam bombardment exerts a significant influence on scanning electron microscopic imaging. This work investigates the charging formation process by a self-consistent Monte Carlo simulation of charge production and transportation based on a charge dynamics model. The charging effect in a semi-infinite SiO2 bulk and SiO2 trapezoidal lines on a SiO2 or Si substrate has been studied. We calculated the spatial distributions of electric potential and electric field for two different systems by two methods respectively: the image charge method was used to deal with a semi-infinite bulk, and random walk method to solve the Poisson equation for a complex geometric structure. The dynamic charging behavior depending on irradiation time has been investigated for SiO2. The simulated CD-SEM images of SiO2 trapezoidal lines with charging effect included showing the contrast change of SEM image along with scanning frames due to charging.

Modulation of electromagnetic local density of states by coupling of surface phonon-polariton

We studied the electromagnetic local density of state (EM-LDOS) near the surface of a one-dimensional multilayer structure (1DMS) alternately stacked by SiC and Si. EM-LDOS of a semi-infinite bulk appears two intrinsic peaks due to the resonance of surface phonon-polariton (SPhP) in SiC. In contrast with that of SiC bulk, SPhP can exist at the interface of SiC and Si for the 1DMS. The SPhPs from different interfaces can couple together, which can lead to a significant modulation of EM-LDOS. When the component widths of 1DMS are large, the spectrum of EM-LDOS exhibits oscillation behavior in the frequency regime larger than the resonance frequency of SPhP. While the component widths are small, due to the strong coupling of SPhPs, another peak appears in the EM-LDOS spectrum besides the two intrinsic ones. And the position of the new peak move toward high frequency when the width ratio of SiC and Si increases. The influences of distance from the surfaces and period of 1DMS on EM-LDOS have also been studied in detail. The results are helpful in studying the near-field radiative heat transfer and spontaneous emission.

Surface conductivity of Si(100) and Ge(100) surfaces determined from four-point transport measurements using an analyticalN-layer conductance model

An analytical N-layer model for charge transport close to a surface is derived from the solution of Poisson's equation and used to describe distance-dependent electrical four-point measurements on the microscale. As the N-layer model comprises a surface channel, multiple intermediate layers and a semi-infinite bulk, it can be applied to semiconductors in combination with a calculation of the near-surface band-bending to model very precisely the measured four-point resistance on the surface of a specific sample and to extract a value for the surface conductivity. For describing four-point measurements on sample geometries with mixed 2D-3D conduction channels often a very simple parallel-circuit model has so far been used in the literature, but the application of this model is limited, as there are already significant deviations, when it is compared to the lowest possible case of the N-layer model, i.e. the 3-layer model. Furthermore, the N-layer model is applied to published distance-dependent four-point resistance measurements obtained with a multi-tip scanning tunneling microscope (STM) on Germanium(100) and Silicon(100) with different bulk doping concentrations resulting in the determination of values for the surface conductivities of these materials.

Electron–Hole Asymmetry Driven Surface Charge Expulsion

We study an Ising-like dynamic Hubbard (IDHB) model using dynamical mean field theory with an embedding potential. The prominent characteristic of the IDHB model is the broken electron–hole symmetry. Our calculations indicate that the electron–hole asymmetry enhances electron expulsion toward the surface of a semi-infinite bulk. Since correlated hopping amplitude produces the electron–hole asymmetry, it strongly affects the electron expulsion toward the surface.

Capillarity Effect Controlled Precipitate Growth at the Grain Boundary of Long-Term Aging Al 5083 Alloy

A model was developed to predict thickness and continuity of β phase (Al3Mg2) formed at grain boundaries of long-term aged Al 5083 alloy. In this model, a variable collector plate mechanism was adopted at the early stage of aging, then, at about 1 month (\( 2\sqrt {D_{\text{b}} t} \approx 100\;{\text{nm}} \)), the model transitions to a constant collector plate mechanism. Two concentration profiles of Mg, one for a semi-infinite bulk at short diffusion distances and one for a finite slab at long diffusion distances (\( 2\sqrt {D_{\text{m}} t} \approx 20\,{\text{pct}} \) of the grain size), were applied to this model for different aging times. Capillarity effects were used to determine the morphology of β phase at the grain boundary. Combining different collector plate mechanisms and Mg concentration profiles, the whole β phase growth process was divided into three stages (short-term Mg concentration profile-variable collector plate, short-term Mg concentration profile-constant collector plate, and long-term Mg concentration profile-constant collector plate). Finally, the model was solved numerically. Experimental results of β phase length and thickness were obtained using transmission electron microscopy (TEM) images of Al 5083 aged at 343 K (70 °C) for different thermal exposure times. Modeling results of β phase thickness and continuity agree well with experimental observations.

First Principles and Experimental Study of Surface Redox Reactions in Li2MnO3

The development of rechargeable lithium-ion batteries (LIB) has progressed rapidly to meet the demand for consumer electronic devices such as cellular phones and laptop computers, and the recent demands for larger energy storage applications (xEV and grid storage) require further development of LIB or alternative energy storage technology. Among possible alternative cathode materials compared to the currently used material LiCoO2, Li-rich compounds xLi2MnO3·(1-x)LiMO2 (M = Ni, Co, Mn) are the most promising candidate with higher capacity, lower cost and absence of toxic element. Within the two-component composite compounds, Li2MnO3 plays a central role in increasing the capacity of the compounds and independently possess a high theoretical capacity of 460 mAh/g (if the entire lithium is utilized). Although the Li2MnO3 is not electrochemically active between 2.0V and 4.4V, it is active to extract lithium ions over 4.5V. During the initial delithiation process, Li2MnO3 is known to transform into layered LiMnO2structure, and the subsequent charge/discharge cycling induces a gradual phase transformation to the spinel structure, resulting in lowering voltage plateau and a large irreversible capacity accompanied by oxygen loss. To overcome these capacity loss problems, diverse approaches have been made to improve electrochemical properties of the cathode materials: adjusting the relative ratio of different transition metal ions, coating layers on the particles, and synthesizing at various temperature to control specific surface area.Even though the bulk properties of cathode materials have been studied by many research groups, the surface properties of cathode have seldom been researched so far. Specifically, theoretical study on cathode surface has been rarely performed even though major reactions and chemical transformations occur near the surface and interfaces between different phases. Recently, several research groups highlighted and reported experimental observations of a phase transformation from the layered structure to spinel phase at the particle surface upon the first charge cycle. However, it is still unclear and controversial for theoretical and fundamental understanding on the mechanism of phase transformations. Without the basic understanding based on detailed atomic scale theoretical analysis, many problems of LIB materials will take extreme efforts and time to resolve the problems.In the present paper, we established a fundamental understanding on the origin of phase transformation at the surface through first principles study of interface models. The theoretical findings suggest unusual redox activities of the surface layers of Li2MnO3which is confirmed by detailed experimental study. These theoretical and experimental findings provide basic understanding to improve performance degradation by phase transformation.For the experimental study, coin cells are made with Li2MnO3 cathode and Li reference electrode, and variation of voltage between 2.0 V and various limiting voltage is applied to the electrochemical test with a constant current at room temperature and under 0.05 C-rate during 10 cycles. There is a small amount of redox capacity (4 mAh/g) at lower voltage as shown in Fig. 1a inset, and Figs. 1d-f show 10 mAh/g capacity during subsequent 10 cycles with lower limiting voltage of 4.1-4.3 V. Fig. 1a shows voltage plateau at 4.6 V extracting Li in the bulk Li2MnO3 with accompanying phase change to layered LiMnO2with lower voltage during subsequent cycles.For the modeling study, in order to elucidate the low voltage redox mechanism observed in Fig. 1, we examined the Li intercalation potential at the surface using the model shown in Fig. 2. The model interface contains stoichiometric ratio of constituents (Li : Mn :O = 2 : 1 : 3) split into two regions of a semi-infinite bulk (SIB), which is inactive at low voltage (4-4.5 V), and a interface layer (IFL) exposed to vaccum, which is calculated to be active at low voltage (~4 V).Finally, detailed electronic structures of the interface model show different oxidation state of surface Mn atoms compared to the fully oxidized bulk Mn4+ within the bulk. Since the high activation potential of 4.6 V of bulk Mn is known to be related to electron extraction from oxygen atoms coordinating Mn4+, any lower Mn oxidation state on the surface would reduce the delithiation potential comparable to those of layered LiMnO2 and spinel (3-4 V). Furthermore, delithiation potential at the surface show strong dependence on the Li location in Mn-layer (4.6 V) and Li-layer (4V) at the surface of Li2MnO3. The preferential delithiation of Li from the surface Li-layer would facilitate the Mn atom migration to the Li-layer causing spinel phase transformation at the surface. These underlying understanding mechanisms of the surface will provide a conceptual basis to develop diverse approached to suppress phase transitions in the cathode materials in LIB, which is our current modeling and experimental research topics.This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)

Monte Carlo calculations of energy deposition distributions of electrons below 20 keV in protein

The distributions of energy depositions of electrons in semi-infinite bulk protein and the radial dose distributions of point-isotropic mono-energetic electron sources [i.e., the so-called dose point kernel (DPK)] in protein have been systematically calculated in the energy range below 20 keV, based on Monte Carlo methods. The ranges of electrons have been evaluated by extrapolating two calculated distributions, respectively, and the evaluated ranges of electrons are compared with the electron mean path length in protein which has been calculated by using electron inelastic cross sections described in this work in the continuous-slowing-down approximation. It has been found that for a given energy, the electron mean path length is smaller than the electron range evaluated from DPK, but it is large compared to the electron range obtained from the energy deposition distributions of electrons in semi-infinite bulk protein. The energy dependences of the extrapolated electron ranges based on the two investigated distributions are given, respectively, in a power-law form. In addition, the DPK in protein has also been compared with that in liquid water. An evident difference between the two DPKs is observed. The calculations presented in this work may be useful in studies of radiation effects on proteins.

Effect of lattice mismatch on phonon transmission and interface thermal conductance across dissimilar material interfaces

When phonons transport across a material interface, they experience reflection, transmission, and mode conversion, which results in a local temperature jump at the interface and thus dramatically changes the thermal conductivity of nanostructured materials. Phonon transmission across lattice-matched interfaces has been studied extensively in recent years with the atomistic Green's function (AGF) approach, which usually uses one unit cell to represent the cross section along the interface. However, modeling phonon transmission across realistic material interfaces is much more challenging because realistic interfaces are usually lattice-mismatched ones with atomic reconstruction, defects, and species mixing, which demands a larger cross-sectional area for the AGF simulation. In this paper, an integrated molecular dynamics (MD) and AGF approach is developed to study the phonon transmission across lattice-mismatched interfaces. MD simulation is used to simulate atomic reconstruction close to the interface. The recursive AGF approach is then employed to calculate frequency-dependent phonon transmission across lattice-mismatched interfaces with defects and species mixing, which addresses the numerical challenge in calculating phonon transmission for a relatively large cross-sectional area with reduced computational cost. The study of the relaxed interface formed from two semi-infinite bulk materials shows that lattice mismatch increases the lattice disorder and decreases the adhesion energy, which in turn lowers phonon transmission and reduces the interface thermal conductance across lattice-mismatched interfaces. Low-frequency phonons can be significantly scattered by increasing the defect size across the interface, while high-frequency phonons can be scattered almost completely (phonon transmission 0.1) across an alloyed layer as thin as 2.27 nm. The effect of lattice mismatch on phonon transmission becomes smaller for interfaces with defects and species mixing. The effect of annealing temperature on the Si/Ge interface thermal conductance was studied. A significant reduction of the Si/Ge interface thermal conductance was observed for a lattice-mismatched interface when annealed at high temperature, which agrees well with the available experimental data in literature.

Evolution of the Rashba spin-orbit-split Shockley state on Ag/Pt(111)

We have studied the evolution of the Pt(111) Shockley-type surface state, which can be described as a surface resonance upon deposition of Ag monolayers. We have combined angle-resolved photoemission and scanning-tunneling-spectroscopy measurements with electronic-structure and photoemission calculations for true semi-infinite systems to guarantee a complete description of all surface-sensitive spectral features. An unusual energy shift and a considerable Rashba-type spin-orbit splitting of the surface resonance were observed and are discussed in terms of modifications in the electronic structure and relationship between the semi-infinite bulk and the surface potential via multiple electron scattering. The maximum magnitude of the splitting in momentum space amounts to $0.051\ifmmode\pm\else\textpm\fi{}0.007$ \AA{}${}^{\ensuremath{-}1}$ and appears as one of the highest values so far observed in the $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}$-type surface-state-resonance bands.

Surface versus bulk contributions to the Rashba splitting in surface systems

We report on a quantitative investigation on the Rashba effect in the Shockley state of the model system Ag/Au(111) by means of angle-resolved photoemission spectroscopy with high-energy resolution. The spectra are discussed in detail in comparison with results of one-step photoemission calculations. The systematic overestimation in the wave-vector splitting present in all actual theoretical studies is explained in terms of an insufficient description of the surface potential. Also, we conclude that the coupling between the semi-infinite bulk and the surface potential mediated by the bulk reflection matrix $B$ is of major importance for a quantitative description of the Rashba effect.

Lattice Green’s function for crystals containing a planar interface

Flexible boundary condition methods couple an isolated defect to a harmonically responding medium through the bulk lattice Green's function; in the case of an interface, interfacial lattice Green's functions. We present a method to compute the lattice Green's function for a planar interface with arbitrary atomic interactions suited for the study of line defect/interface interactions. The interface is coupled to two different semi-infinite bulk regions, and the Green's function for interface-interface, bulk-interface and bulk-bulk interactions are computed individually. The elastic bicrystal Green's function and the bulk lattice Green's function give the interaction between bulk regions. We make use of partial Fourier transforms to treat in-plane periodicity. Direct inversion of the force constant matrix in the partial Fourier space provides the interface terms. The general method makes no assumptions about the atomic interactions or crystal orientations. We simulate a screw dislocation interacting with a $(10\bar{1}2)$ twin boundary in Ti using flexible boundary conditions and compare with traditional fixed boundary conditions results. Flexible boundary conditions give the correct core structure with significantly less atoms required to relax by energy minimization. This highlights the applicability of flexible boundary conditions methods to modeling defect/interface interactions by \textit{ab initio} methods.

Substrate-affected indentation contact parameters of elastoplastic coating/substrate composites

In coating/substrate bilayer systems, the indentation contact behavior transitionally varies from coatinglike to substratelike behaviors. Spatial confinement effects of the substrate induce very complicated plastic flows in the coating beneath the indenter, leading to a crucial difficulty that is not accounted for by any of the present quantitative analytical/theoretical predictions for the substrate-affected contact hardness. In this work, the author presents finite-element-based studies on the elastoplastic indentation contact mechanics of coating/substrate systems. The effect of the substrate is taken into account by introducing the spatially variable elastic modulus and the yield stress; this approach quantitatively describes the substrate-affected stress/strain field in the spatially localized area beneath the indenter. The elastoplastic constitutive relationship of the contact hardness for semi-infinite homogeneous bulks combined with these spatially variable material characteristics are successfully applied to analytically as well as quantitatively predict the substrate-affected contact hardness of bilayer composite systems having wide ranges of elastoplastic coating/substrate characteristics. The experimental procedures for determining the elastic/plastic parameters both of the coating and the substrate are also discussed, in which the importance of the experimental determination of substrate-affected indentation contact radius/area is emphasized.