Contribution Of Free Electrons Research Articles

Fermi level limitation in Na1/2Bi1/2TiO3–BaTiO3 piezoceramics by electrochemical reduction of Bi

The (electro)chemical stability of undoped and Zn-doped 0.94Na1/2Bi1/2TiO3–0.06BaTiO3 lead-free piezoceramics (NBT–6BT) was studied. For this purpose, the Fermi level at the interface between NBT–6BT and Sn-doped In2O3 (ITO) electrode is varied by gradually reducing the ITO film either by annealing in vacuum or by applying a voltage across a Pt/NBT–6BT/ITO. The chemical and electronic changes are monitored in situ by x-ray photoelectron spectroscopy. The experiments reveal the formation of metallic Bi when the Fermi level is reaching a value of 2.23 ± 0.10 eV above the valence band maximum, while no reduction of Ti is observed. The electrochemical reduction of Bi constitutes an upper limit of the Fermi level at ≈1 eV below the conduction band minimum. High electron concentrations in the conduction band and a contribution of free electrons to the electrical conductivity of NBT–6BT can, therefore, be excluded. The reduction occurs for an ITO work function of 4.2–4.3 eV. As typical electrode materials such as Ag, Cu, Ni, or Pt have higher work functions, an electrochemical instability of the electrode interfaces in ceramic capacitors is not expected. Under the given experimental conditions (350 °C, electric fields <40 V/mm), no degradation of resistance and no enrichment of Na at the interface are observed.

Computational Study of the Effect of the Size-Dependent Dielectric Functions of Gold Nanomaterials on Optical Properties

The effect of size on the optical properties of gold nanomaterials has been studied using the theoretical Drude–Sommerfield model. The real and imaginary parts of the dielectric function of bulk as a function of wavelength due to free electron contribution and the real and imaginary parts of the dielectric function of nanogold materials as a function of wavelength due to free electron and bond electron contribution are calculated. The real and imaginary parts of the dielectric function of bulk as a function of wavelength due to the free electron contribution graph and The real and imaginary parts of the dielectric function of nanogold materials as a function of wavelength due to free electron and bond electron contributions are plotted. As we observed from the graphs, the real dielectric functions of both bulk and nanogold materials are inversely proportional to wavelength. The imaginary part of the dielectric function of bulk gold materials is independent of wavelength. At high wavelengths, the size of the gold nanomaterial is highly influenced by both real and imagined dielectric functions at high waves. As the wavelength increases, the effect of the size on the dielectric function also increases. The size-dependent dielectric function of nanomaterials is highly influenced by their optical properties and electrical structure.

Thermal conduction in polycrystalline or amorphous transparent conductive oxide films

With the widespread utilization of transparent conductive oxides (TCO) in various electronic devices, it is beneficial to revisit the fundamentals of heat conduction to formulate a comprehensive physical understanding for thermal properties of TCOs with the aim of improving the thermal design of electronic devices. For degenerate polycrystalline TCO films, both free electrons and phonons act as heat carriers, where free electron contribution can be calculated based on the Wiedemann-Franz law.Interestingly, the phonon thermal conductivity remains almost constant for degenerated polycrystalline TCO films with different dopant concentrations, suggesting the existence of dopant-induced minimum phonon thermal conductivity. For degenerate amorphous TCO film, free electrons act as heat carriers along the conduction path formed by the orbital overlap of cations, which also obey the Wiedemann-Franz law. Moreover, the atomic vibration contribution to thermal conductivity almost keeps unchanged for amorphous TCO films deposited under different conditions, indicating the existence of a minimum thermal conductivity caused by the scattering of vibrational mode due to structural randomness appeared in the bond angle. We anticipate that these fundamental understanding can provide a guide for the discovery of TCO materials with optimized thermal properties.

ZrC-SiC closed-cell ceramics with low thermal conductivity: Exploiting unique spherical closed-cell structure through tape casting and CVI techniques

Porous ultra-high temperature ceramics (UHTCs) are recognized as novel candidates for fulfilling the requirements of thermal protection systems of hypersonic aircrafts, as they possess excellent high-temperature resistance and low thermal conductivity. Currently, the reported porous UHTCs predominantly exhibit an open pore structure. By contrast, closed-cell UHTCs, formed by employing ceramic hollow microspheres (HMs) as pore-forming agents, hold great potential for achieving superior thermal insulation performance. Unfortunately, the implementation of this strategy has been hindered by the scarcity of raw materials and preparation techniques.In this paper, ZrC-SiC closed-cell ceramics were first successfully prepared through a combination of tape casting and chemical vapor infiltration (CVI) techniques, utilizing the self-developed ZrC HMs as the primary raw material. The morphology, microstructure, and thermal insulation properties of the obtained ZrC-SiC closed-cell ceramics were investigated. The results indicate that when the content of ZrC HMs is 30 vol.%, the density of the prepared porous ceramics is 2.09 g cm–3, with a closed porosity of 14.05% and a thermal conductivity of 1.69 W (m K)–1. The results clearly prove that the CVI process can successfully convert ZrC HMs into closed pore structures within porous ceramics. The introduction of ZrC HMs suppresses the contribution of free electrons to thermal conductivity and brings about a large number of solid-gas interfaces, which increases the interfacial thermal resistance and significantly reduces the phonon thermal conductivity. Consequently, the as-prepared ZrC-SiC closed-cell ceramics show excellent thermal insulation properties. This study provides a new idea and method for the development of porous UHTCs and offers a more reliable material choice for thermal protection systems.

Nonideal effect of free electrons on ionization equilibrium and radiative property in dense plasmas.

The thermodynamic as well as optical properties of strongly coupled plasmas depend crucially on the average degree of ionization and the ionic state composition, which, however, cannot be determined by using the normal Saha equationusually used for the ideal plasmas. Hence, an adequate treatment of the ionization balance and the charge state distribution of strongly coupled plasmas is still a challenge for theory due to the interactions between the electrons and ions and among the electrons themselves. Based on a local density temperature-dependent ion-sphere model, the Saha equationapproach is extended to the regime of strongly coupled plasmas by taking into account the free-electron-ion interaction, the free-free-electron interaction, the nonuniform free-electron space distribution, and the free-electron quantum partial degeneracy. All the quantities, including the bound orbitals with ionization potential depression, free-electron distribution, and bound and free-electron partition function contributions, are calculated self-consistently in the theoretical formalism. This study shows that the ionization equilibrium is evidently modified by considering the above nonideal characteristics of the free electrons. Our theoretical formalism is validated by the explanation of a recent experimental measurement of the opacity of dense hydrocarbon.

Hydrogen diffusion and its electrical properties variation as a function of the IGZO stacking structure

The oxygen vacancies and hydrogen in oxide semiconductors are regarded as the primary sources of charge carriers and various studies have investigated the effect of hydrogen on the properties of oxide semiconductors. However, the carrier generation mechanism between hydrogen and oxygen vacancies in an a-IGZO semiconductor has not yet been clearly examined. In this study we investigated the effect of hydrogen and the variation mechanisms of electrical properties of a thin film supplied with hydrogen from the passivation layer. SiOx and SiNx, which are used as passivation or gate insulator layers in the semiconductor process, respectively, were placed on the top or bottom of an a-IGZO semiconductor to determine the amount of hydrogen penetrating the a-IGZO active layer. The hydrogen diffusion depth was sufficiently deep to affect the entire thin semiconductor layer. A large amount of hydrogen in SiNx directly affects the electrical resistivity of a-IGZO semiconductor, whereas in SiOx, it induces a different behavior from that in SiNx, such as inducing an oxygen reaction and O–H bond behavior change at the interface of an a-IGZO semiconductor. Moreover, the change in electrical resistivity owing to the contribution of free electrons could be varied based on the bonding method of hydrogen and oxygen.

Quantitative analysis of free-electron dynamics in InSb by terahertz shockwave spectroscopy

The contribution of free electrons to the dielectric function is largely determined by their plasmonic resonance, a collective density oscillation. We studied this broadband and carrier-density-dependent response in the narrow-gap semiconductor InSb with THz shockwave spectroscopy. A synthesized waveform, with a steep onset followed by a short electric-field plateau, gives access to a multioctave spectrum of frequencies from 100 GHz up to the mid infrared. By measuring the entire spectral characteristics of the plasma, we analyze the dynamics of photogenerated free electrons for a wide range of excitation fluences. Thanks to this analysis, we were able to quantify the coefficients of both electron trapping and the Auger process from cryogenic to room temperature.

Influence of external magnetic field on the electronic structures and radiative properties in plasma-embedded multi-electron atomic systems

A method within the fully relativistic frame, based on the reconstructed Dirac–Coulomb Hamiltonian is suggested which applies to the plasma-embedded multi-electron atomic systems perturbed by applied magnetic fields. The proposal is a configuration interaction method and based upon a combination of two concepts, referred to as plasma screening and external magnetic field. An effective potential, derived by introducing a parameter dependent scaling of the radial dependence of free electron contribution in the ion-sphere, is adopted to model the interactions among the charged particles in dense plasmas, and also the range of external weak magnetic field B<105 T is considered. In this instance, numerical calculations are performed for electronic structures and spectroscopic properties of the He atom and Fe14＋ ion as examples. The accuracy of the present approach is shown by comparison to previous theoretical calculations. The behavior of energies of low excited states and radiative transition probabilities for allowed transitions as a function of the plasma screening and field strength is classified. These results are developed in high energy density physics and astrophysics context.

Ionization state and dielectric constant in cold rarefied hydrocarbon plasmas of inertial confinement fusion.

A combined approach to study cold rarefied matter is introduced that includes a semianalytical method based on the free-energy minimization and ab initio calculations based on the finite-temperature density-functional theory. The approach is used to calculate the ionization state of hydrocarbon (CH) under the shock-release conditions in inertial confinement fusion. The dielectric constant of CH is calculated using the Kubo-Greenwood formulation and contribution from atomic polarizabilities is found to be as important as the free-electron contribution. Using the ionization state and dielectric constant, the electron density profile in the rarefaction wave of the shock-release plasma is obtained.

Local field correction to ionization potential depression of ions in warm or hot dense matter.

An analytical self-consistent approach was recently established to predict the ionization potential depression (IPD) in multicomponent dense plasmas, which is achieved by considering the self-energy of ions and electrons within the quantum statistical theory. In order to explicitly account for the exchange-correlation effect of electrons, we incorporate the effective static approximation of local field correction (LFC) within our IPD framework through the connection of dynamical structure factor. The effective static approximation poses an accurate description for the asymptotic large wave number behavior with the recently developed machine learning representation of static LFC induced from the path-integral Monte Carlo data. Our calculation shows that the introduction of static LFC through dynamical structure factor brings a nontrivial influence on IPD at warm/hot dense matter conditions. The correlation effect within static LFC could provide up to 20% correction to free-electron contribution of IPD in the strong coupling and degeneracy regime. Furthermore, a new screening factor is obtained from the density distribution of free electrons calculated within the average-atom model, with which excellent agreements are observed with other methods and experiments at warm/hot dense matter conditions.

Magnetic switching control in Co/TiO2 bilayer and TiO2:Co thin films for Magnetic-Resistive Random Access Memories (M-RRAM)

The classical architecture of the resistive Non-Volatile Memories (NVM) is metal/insulator/metal and, more recently, metal/metal oxides/metal. An alternative configuration, based on Co/TiO2 bilayer and TiO2:Co thin films, like oxide-diluted magnetic semiconductors (O-DMS), are promising nanostructures for a magnetic control performance of resistive switching for NVM. Electrical and magnetic characterization shows a contribution of lattice free electrons, produced for oxygen vacancies, and Co ions randomly located into TiO2 in the SET and RESET states. Experimental results evidence the low energy consumption in the process and the expansion of storage capacity, due to both high and low resistive states induced for the magnetic field applied. An alternative model of conductive filament formation corresponds to the alignment and reorientation of charge carriers supported by TiO2 molecular stability, Gibbs energy analysis, and giant magnetoresistance resulted at room temperature.

Two-dimensional electron gas in a metal/amorphous oxide interface with spin-orbit interaction

The formation of novel two-dimensional electron gas (2DEG) with high mobility in metal/amorphous interfaces has motivated an ongoing debate regarding the formation and novel characteristics of these 2DEGs. Here we report an optical study, based on infrared spectroscopic ellipsometry, of nonmagnetic metal and amorphous semiconducting oxide (Cu/Bi$_2$O$_3$) interfaces that confirms the formation of a 2DEG with spin orbit coupling (SOC). The 2DEG optical response was simulated with a uniaxial diagonal dielectric tensor within a sub-nanometer thin layer, where its $x$ and $z$ components lineshapes resolved in both free-electron and peak-like contributions, resulted very similar to theoretical predictions [M. Xie et al., Phys. Rev. B $\bf{89}$, 245417 (2014)] of a two dimensional electron gas confined in the normal direction of a perovskite interface. In particular, the small but finite conducting character of the $z$ component provides a unambiguous signature of the presence of the 2DEG in the Cu/Bi$_2$O$_3$ system. Although the original constituent materials do not possess spin-orbit coupling (SOC), the resulting interfacial hybridization of such states induce electronic asymmetric wave functions. This work demonstrates the detection of 2DEG in amorphous crystals allowing to study its challenging interfacial phenomena such as SOC and interface-bulk coupling, overcoming an experimental impediment which has hold back for decades important advancements for the understanding of 2DEGs in amorphous materials.

Prediction of Potential Barrier at Crystallite Boundaries in Poly- and Nanocrystalline Semiconductors

The distribution of the potential and the parameters of the potential barrier for electrons in semiconductor crystallite is numerically calculated. The calculations are made in a spherical crystallite with uniformly distributed surface states and uniformly distributed donors. It is considered in the calculations that the surface charge is screened both by the ionized donors and on free electrons; the contibution of free electrons should not be neglected in semiconductors with a high concentration of free electrons. It is demonstrated that the height of the potential barrier depends non-monotonically on the concentration of the donors in the crystallite. Moreover, in the curve of the height of the potential barrier as a function of the concentration of the donors, it is possible to highlight two segments corresponding to the cases of the complete and partial depletion of the crystallite. The height of the potential barrier increases with the concentration of the donors in the first segment and decreases in the second segment. It is established that the height of the potential barrier increases with the increase in the concentration of the surface states. The possibility of the existence of surface potential barriers in nano- and polycrystalline metal-oxide semiconductors, which are applied as a sensitive layer of gas sensors, is estimated. It is concluded that if the crystallite radius in metal-oxide semiconductors does not exceed 10 nm the sensor’s sensitivity to gas could hardly be attributed to the usual barrier model. It is demonstrated that shape of crystallite and the contribution of free electrons to screening of surface charge have to be taken into account to calculation of width of potential barrier.

Temperature-dependent permittivity of silver and implications for thermoplasmonics

Silver is an extremely appealing metal for plasmonics due to its very low optical losses in the visible and near-ultraviolet range and its relatively low reactivity. Within the emerging field of thermoplasmonics, where light-metal interactions are exploited to generate heat on the nanometric scale, knowledge of temperature-dependent complex permittivities of plasmonic materials is indispensable. We extracted the temperature-dependent complex permittivity of silver ${\ensuremath{\varepsilon}}_{Ag}$ by spectroscopic ellipsometry under high-vacuum conditions. For rising $T$, we observed an increase of the free-electron contribution to the imaginary part of the permittivity $\text{Im}[{\ensuremath{\varepsilon}}_{Ag}]$ and a temperature-dependent absorption band splitting off the interband absorption edge in the 320--360-nm range. Around 340 nm the relative increase of $\text{Im}[{\ensuremath{\varepsilon}}_{Ag}]$ at 600 K with respect to its room-temperature value is around 500%. In order to understand the implications of this behavior on silver thermoplasmonics, we computed the temperature-dependent extinction efficiency of oblate Ag ellipsoids with localized plasmon resonance within the 320--360-nm range. We predict that dramatic damping of the plasmon resonance occurs for increasing temperature, possibly leading to intriguing self-limiting effects in Ag thermoplasmonics.

Calculation of the grain potential barrier in the poly− and nanocrystalline semiconductors

The distribution of potential and parameters of potential barrier in semiconductor crystallite was calculated numerically. The calculation was carried out in spherical crystallite with evenly distributed donors and surface states. The calculation assumed that the surface charge is screened by both ionized donors and free electrons, the contribution of which cannot be neglected in semiconductors with high concentration of free electrons. The height of potential barrier is shown to nonmonotonically depend on the concentration of donors. The dependence of height of potential barrier on the concentration of donors may be divided into two part. One part of dependence describes the fully depleted crystallite and the second part describes the party depleted crystallite. On the first part the height of potential barrier increases with the donor concentration but on the second part the height of potential barrier decreases. The height of the potential barrier increases with increasing of concentration of surface states. The possibility of existing of potential barriers is estimated in nano− and polycrystalline metal oxide semiconductors used as sensitive layers of gas sensors. It is concluded that if the radius of crystal grains in metal oxide semiconductors does not exceed 10 nm, the explanation of the sensitivity of the sensor to gas by using a commonly barrier model seems unlikely. It is demonstrated that shape of crystallite and the contribution of free electrons to screening of surface charge have to be taken into account to calculation of width of potential barrier.

Nonlinear optical response of a gold surface in the visible range: A study by two-color sum-frequency generation spectroscopy. II. Model for metal nonlinear susceptibility.

We present a modeling of the nonlinear optical response of a metal surface in order to account for recent experimental results from two-color Sum-Frequency Generation (SFG) experiments on gold. The model allows calculating the surface and bulk contributions and explicitly separates free and bound electron terms. Contrary to the other contributions, the perpendicular surface component is strongly model-dependent through the surface electron density profiles. We consider three electron density schemes at the surface, with free and bound electrons overlapping or spilling out of the bulk, for its calculation. The calculated SFG signals from the metal rely only on bulk quantities and do not need an explicit definition of the density profiles. In the particular case of gold, when the free electrons overlap with the bound ones or spill out of the bulk, the free electron response completely dominates through the perpendicular surface terms. When the bound electrons spill out, the situation is more balanced, still in favor of the free electrons, with lower amplitudes and different dispersion line shapes. As for silver, the free electron contributions dominate and the calculated slow amplitude growth from blue to red follows the experimental trends.

Electronic structure and properties of (TiZrNbCu)1-xNix high entropy amorphous alloys

A comprehensive study of selected properties of four (TiZrNbCu)1-xNix (x ≤ 0.25) amorphous high entropy alloys (a-HEA) has been performed. The samples were ribbons about 20 μm thick and their fully amorphous state was verified by X-ray diffraction and thermal analysis. The surface morphology, precise composition and the distribution of components were studied with a Scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) attachment. The properties selected were the melting temperature (Tm), the low temperature specific heat (LTSH), the magnetic susceptibility χexp and the Young's modulus (E). Whereas LTSH and χexp were measured for the as-cast samples, E was measured both for as-cast samples and relaxed samples (after a short anneal close to the glass transition temperature). The LTSH showed that the electronic density of states at the Fermi level, N0(EF), decreases with increasing x, whereas the Debye temperature (θD) increases with x. This is similar to what is observed in binary and ternary amorphous alloys of early transition metals (TE) with late transition metals (TL) and indicates that N0(EF) is dominated by the d-electrons of the TE. The LTSH also showed the absence of superconductivity down to 1.8 K and indicated the emergence of the Boson peak above 4 K in all alloys. The free-electron like paramagnetic contribution to χexp also decreases with x, whereas E, like θD, increases with x, indicating enhanced interatomic bonding on addition of Ni. The applicability of the rule of mixtures to these and other similar HEAs is briefly discussed.

The features of thermal properties and CEF-influence in intermediate valence compound CeB4 at the temperatures of 2–300 K

Heat capacity and lattice parameters of cerium tetraboride were experimentally determined at 2 – 300K. An anomalously large contribution of free electrons to the heat capacity was influenced by Fermi liquid state in the cerium boride. The parameters of Einstein and Debye contributions to CeB4 heat capacity, as well as free electron gas contribution and Schottky heat capacity have been determined. A negative thermal expansion (NTE) of CeB4 was found (5–25K). We were not able to explain the NTE by the influence of intermediate valence of cerium ions. The NTE was attributed to the influence of the crystal electric field (CEF) on Ce3+ ions.

Experimental and theoretical studies on localized surface plasmon resonance based fiber optic sensor using graphene oxide coated silver nanoparticles

An optical fiber based refractive index sensor using graphene oxide (GO) encapsulated silver nanoparticles (AgNPs) is reported. The AgNPs are encapsulated with a very thin layer of GO as it controls the inter-particle distance thereby preventing aggregation. The encapsulation also enhances the colloidal stability and prevents the oxidation of the AgNPs by separating them from direct contact with the aqueous medium. High-resolution transmission electron microscopy results support the formation of 1 nm thick GO around AgNPs of an average size of 35 nm. A Raman spectrometer and a UV–VIS spectrometer have been used to characterize and study the synthesized nanoparticles along with GO. Further, Raman spectra support a 64.72% increase in D-peak intensity and a 52.91% increase in G-peak intensity of the GO-encapsulated AgNPs (GOE-AgNPs) with respect to GO. Further, the GOE-AgNPs are immobilized on the core of functionalized plastic-cladded silica fiber. FESEM confirms the immobilization of the GOE-AgNPs on the fiber core. We observed that the peak absorbance changes by 87.55% with a 0.05 change in the refractive index. The sensitivity of the proposed fiber sensor is found to be 0.9406 ΔA/RIU along with a resolution of 12.8 × RIU. MATLAB is used to calculate the absorbance of the AgNPs by considering the bound and free electron contribution along with the size-dependent dispersion of the nanoparticles. We found that the simulation results are in good agreement with the experimental results.

Stopping power of a heterogeneous warm dense matter

AbstractThe stopping power of warm dense matter (WDM) is estimated by means of the individual contributions of free electrons and bound electrons existing in this special kind of matter, located between classical and degenerate plasmas. For free electrons, the dielectric formalism, well described in our studies, is used to estimate the free electron stopping power. For bound electrons, the mean excitation energy of ions is used. Excitation energies are obtained through atomic calculations of the whole atom or, shell by shell in order to estimate their stopping power. Influence of temperature and density is analyzed in case of an impinging projectile. This influence becomes important for low projectile velocities and is negligible for high ones. Using free and bound electron analysis, the stopping power of an extended WDM is inferred from a dynamical calculation of energy transferred from the projectile to the plasma, where the stopping range is calculated. Finally, this theoretical framework is used to study a typical plasma density profile of a WDM heated by lasers.