Free Electron Concentration Research Articles

Evolution of Electronic State and Properties of Silver Nanoparticles during Their Formation in Aqueous Solution.

The electron density of a nanoparticle is a very important characteristic of the properties of a material. This paper describes the formation of silver nanoparticles (NPs) and the variation in the electronic state of an NP’s surface upon the reduction in Ag+ ions with oxalate ions, induced by UV irradiation. The calculations were based on optical spectrophotometry data. The NPs were characterized using Transmission electron microscopy and Dynamic light scattering. As ~10 nm nanoparticles are formed, the localized surface plasmon resonance (LSPR) band increases in intensity, decreases in width, and shifts to the UV region from 402 to 383 nm. The interband transitions (IBT) band (≤250 nm) increases in intensity, with the band shape and position remaining unchanged. The change in the shape and position of the LSPR band of silver nanoparticles in the course of their formation is attributable to an increasing concentration of free electrons in the particles as a result of a reduction in Ag+ ions on the surface and electron injection by radicals. The ζ-potential of colloids increases with an increase in electron density in silver nuclei. A quantitative relationship between this shift and electron density on the surface was derived on the basis of the Mie–Drude theory. The observed blue shift (19 nm) corresponds to an approximately 10% increase in the concentration of electrons in silver nanoparticles.

Comparison between results of optical and electrical measurements of free electron concentration in n-InAs specimens

A theoretical model has been developed for determining the free electron concentration in n-InAs specimens from characteristic points in far IR reflection spectra. We show that this determination requires plasmon-phonon coupling be taken into account, otherwise the measured electron concentration proves to be overestimated. A correlation between the electron concentration Nopt and the characteristic wavenumber ν+ has been calculated and proves to be well fit by a third order polynomial. The test specimens have been obtained by tin or sulfur doping of indium arsenide. The electron concentration in the specimens has been measured at room temperature using two methods: the optical method developed by the Authors (Nopt) and the conventional four-probe Hall method (the Van der Pau method, NHall). The reflecting surfaces of the specimens have been chemically polished or fine abrasive ground. The condition Nopt > NHall has been shown to hold for all the test specimens. The difference between the optical and the Hall electron concentrations is greater for specimens having polished reflecting surfaces. The experimental data have been compared with earlier data for n-GaAs. A qualitative model explaining the experimental data has been suggested.

Improving the electrical conductivity of copper/graphene composites by reducing the interfacial impurities using spark plasma sintering diffusion bonding

The copper/graphene (Cu/Gr) composites made of Gr/Cu/Gr foils with a thickness of 25 μm were prepared using spark plasma sintering diffusion bonding (SPS) and hot press diffusion bonding (HP) in an Ar atmosphere at 900 °C for 20 min under a pressure of 50 MPa. The electrical conductivity of the prepared Cu/Gr composites was measured by the van der Pauw method, the microstructure was characterized using Raman spectroscopy, scanning electron microscope and transmission electron microscope, and the corresponding mechanism was analyzed by employing the first-principles calculations. The result indicates that, SPS process reduces the interfacial impurities of Cu/Gr composites, rendering a much higher electrical conductivity (108.6% IACS) than that prepared by using HP (98.8% IACS). This finding is correlated to the reduced oxygen (O) impurities on Gr/Gr interface, which is attributed to the high temperature plasma sputtering effect of SPS. In the prepared Cu/Gr composites, the impurity-free Gr/Gr interface has an ideal spacing of ∼3.3 Å wherein carbon (C) across the interface do not bond. Despite the generated C–C covalent bonding within the same Gr layer bounds some of the electrons, the remaining electrons of the p orbital on each C atom are free electrons, leading to the excellent electrical conductivity. In contrast, absorption of O impurities on the Gr/Gr interface increases the spacing between the Gr layers to ∼3.8 Å, and generates covalent C–O–C bonding across the interface due to p orbital hybridization. This reduces the free electron concentration and the corresponding electrical conductivity.

Modeling of structural and energetic parameters of р-Er1-xScxNiSb semiconductor

The energy expediency of the existence of Er1-xScxNiSb substitutional solid solution up to the concentration x≈0.10 was established by modeling the variation of free energy ΔG(x) values (Helmholtz potential). At higher Sc concentrations, x> 0.10, there is stratification (spinoidal decomposition of phase). It is shown that in the structure of p-ErNiSb semiconductor there are vacancies in positions 4a and 4c of Er and Ni atoms, respectively, generating structural defects of acceptor nature. The number of vacancies in position 4a is twice less than in position 4c. This ratio also remains for p-Er1-xScxNiSb. Doping of p-ErNiSb semiconductor by Sc atoms by substitution of Er atoms is also accompanied by the occupation of vacancies in position 4a. In this case, Ni atoms occupy vacancies in position 4c, which can be accompanied by the process of ordering the p-Er1-xScxNiSb structure. Occupation of vacancies by Sc and Ni atoms leads to an increase of the concentration of free electrons, an enlarge of the compensation degree of semiconductor, which changes the position of the Fermi level εF and the mechanisms of electrical conductivity.

Experimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides

We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators.Graphic abstract

Approaching Theoretical Band Gap of ZnSnN2 Films via Bias Magnetron Cosputtering at Room Temperature

Zinc tin nitride (ZnSnN2) is a promising semiconductor candidate for solar cell applications and optoelectronic devices. However, the practical use of this material has been limited by several issues as, among others, oxygen contamination, a very high concentration of free carriers, and the difficulty to reach the theoretically predicted band gap. Here, we deposit thin films of ZnSnN2 by reactive DC magnetron cosputtering at room temperature with an RF bias power (Pb) in the range of 0–50 W. Using first principles calculations, we explore the structural and optoelectronic properties that are favorably compared to experimental results. We demonstrate that the optical band gap energy can be decreased from 1.7 to 1.34 eV, close to the predicted value of 1.37 eV. The free electron concentration is decreased down to 1017 cm–3 which results in the reduction of the absorption by free electrons in the IR range. In addition, in a given range of applied bias powers, we observe a densification of the films and a significant decrease of their oxygen contamination from 6.7 down to 2.0 at. %. The study underlines that a value of 20 W power bias leads to the optimal structural, optical, and electrical properties. Our results provide an interesting method to obtain a potential candidate for photovoltaic applications, in an environmentally friendly way, for low-cost industrialization.

Quenching effect of upper laser level lifetime of Fe2+ ions in ZnSe by free electrons

We theoretically and experimentally examined the assumption that the lifetime excited state of Fe2+ in ZnSe:Fe crystal is quenched by free electrons. The effect of the electron beam current value on the temporal characteristics of the luminescence was studied. Three variants, including a sample in the electronic circuit in the case of Fe2+ ions luminescence excitation in a ZnSe:Fe crystal by a beam of accelerated electrons, were first considered theoretically. These variants differ in external electric field magnitude in the sample, which affects the concentration of free electrons in the excited region of the sample. Equations describing the time dependence of the sample's electron density were derived. The luminescence decay kinetics of Fe2+ ions in ZnSe:Fe crystal excited by electron pulse with different electron beam current values and short-pulse optical radiation were obtained. The values of the quantities that determine the temporal characteristics of the luminescence decay depending on the electron beam current were found.

Preparation of High-Performance Metal-Free UV/Near Infrared-Shielding Films for Human Skin Protection.

A series of metal-free UV/near infrared (NIR)-shielding coatings are successfully fabricated by shielded cathodic arc plasma evaporation (CAPE) and substrate-biased RF magnetron sputtering processes. The UV/NIR-shielding coatings comprising quarter-wave stacks of TiO2/SiO2 multilayers and high-conductivity sputter-deposited ITO films with a thickness in the range of 200–600 nm could block IRA and IRB radiations, respectively. The total thicknesses of UV/near infrared-shielding films are in the range from 375 nm to 1513.8 nm. The anatase-phase TiO2 films with absorption edge located at ~375 nm were deposited by shielded CAPE at ~100 °C. Further, the well-crystallized ITO films were found to have high free-electron concentrations (1.12 × 1021 cm−3), resulting in strong absorption of IRB due to the plasmon resonance absorption. The optimal optical design and ITO film thickness were investigated, and the TiO2(SiO2/TiO2)3 multilayer combined with an ITO film thickness of 400 nm was found to provide a high NIR-shielding rate of 94.8%, UVB to UVA-shielding rate of 92.7%, and average visible light transmittance of 68.1%. Further, human skin cells protected by a UV/NIR-shielding coating showed significantly decreased reactive oxygen species generation and inflammatory cytokine expression as compared to those of unprotected cells. The results demonstrate that the development of multifunction coatings have potential for transparent heat insulation windows and human skin protection against UV/IR radiations.

Lattice instability of B-site substituted SrTiO3 crystals studied by terahertz time-domain spectroscopic ellipsometry

The B-site substituted quantum paraelectric strontium titanate, SrTiO3 (STO), crystals with the perovskite structure were studied regarding their dielectric properties in the terahertz frequency range. When Ti4+ ions at the B-site are substituted by heterovalent Nb5+ ions, free electrons are introduced and the conductivity increases. The complex dielectric constants were accurately determined by the terahertz time-domain spectroscopic ellipsometry. The dielectric spectra were analyzed by the damped harmonic oscillator model for a ferroelectric soft mode and the Drude-Smith model for free electrons. The soft mode frequency and the plasma frequency increased with the increasing Nb content. The square of the plasma frequency is proportional to the concentration of free electrons. It was found that the change in the ferroelectric soft mode frequency is approximately proportional to the concentration of free electrons. This fact suggests that the increase in the free electrons by donor-doping suppresses the ferroelectric instability.

Mobility of dislocations in the iron-based C-, N-, H-solid solutions measured using internal friction: Effect of electron structure

Effect of interstitial elements on the Snoek-Köster relaxation and the amplitude-dependent internal friction is analyzed in terms of a change in the electron structure. The density of electron states in the bcc and fcc iron doped with carbon, nitrogen or hydrogen has been ab initio calculated paying due attention to that at the Fermi level. It is obtained that nitrogen and hydrogen increase density of states in the both iron phases, whereas the effect of carbon is opposite. In consistency, using electron spin resonance in the fcc iron alloys, the concentration of free electrons is found to be increased due to nitrogen and hydrogen and decreased by carbon. Based on the obtained results, a conclusion about corresponding change in the dislocation line tension controlling mobility of dislocations is derived. This correlation between electron structure and mobility of dislocations is confirmed for the Snoek-Köster relaxation in the bcc iron phase and for the internal friction background, ADIF, in the fcc phase.

Catalytic effects of magnetic and conductive nanoparticles on immobilized glucose oxidase in skin sensors

Wearable skin sensors is a promising technology for real-time health care monitoring. They are of particular interest for monitoring glucose in diabetic patients. The concentration of glucose in sweat can be more than two orders of magnitude lower than in blood. In consequence, the scientific and technological efforts are focused in developing new concepts to enhance the sensitivity, decrease the limit of detection (LOD) and reduce the response time (RT) of glucose skin sensors. This work explores the effect of adsorbed superparamagnetic magnetite nanoparticles (MNPs) and conductive nanoparticles (CNPs) on carbon nanotube substrates (CNTs) used to immobilize glucose oxidase enzyme in the working electrode of skin sensors. MNPs and CNPs are made of magnetite and gold, respectively. The performance of the sensors was tested in standard buffer solution, artificial sweat, fresh sweat and on the skin of a healthy volunteer during an exercise session. In the case of artificial sweat, the presence of MNPs accelerated the RT from 7 to 5 s at the expense of increasing the LOD from 0.017 to 0.022 mM with slight increase of the sensitivity from 4.90 to 5.09 μAm M−1 cm−2. The presence of CNPs greatly accelerated the RT from 7 to 2 s and lowered the LOD from 0.017 to 0.014 mM at the expense of a great diminution of the sensitivity from 4.90 to 4.09 μAm M−1 cm−2. These effects were explained mechanistically by analyzing the changes in the concentration of free oxygen and electrons promoted by MNPs and CNPs in the CNTs and its consequences on the the glucose oxidation process.

Free switch between bound states in the continuum (BIC) and quasi-BIC supported by graphene-metal terahertz metasurfaces

An important topic of modern nanophotonics focuses on the bound states in the continuum (BIC) that the non-radiative localized waves lie within the radiative continuous spectrum, which has been introduced into metasurfaces to construct a strong resonance system. The development of theory and application promotes the advanced physics research on metasurfaces supporting dynamic BIC. However, the dynamic conversion between quasi-BIC and BIC of the metasurfaces is still not realized in strict sense. Herein, we report a significant progress that the free switch between BIC and quasi-BIC is successfully achieved by introducing the structured graphene into the metal terahertz metasurfaces, which is based on free-electron concentration of graphene changing from semiconductor-like to metal-like with adjustment of Fermi energy. We propose the efficient-structure model to explain these phenomena, and prove the wide applicability of this model by investigating seven class metal structures which includes a parameter-tuned metasurface and six common symmetry-protected metasurfaces. The further investigation shows such graphene-metal metasurface array has potential application in dynamic display, and it only needs to change the bias voltage applied on each structure to obtain different display contents, without the requirement of preparing additional metasurface array, which is impossible to achieve via traditional static metasurfaces.

Transmissivity to reflectivity change delay phenomenon observed in GeTe thin films at laser-induced reamorphization

The article presents the results of an in-situ pump–probe study of the change’s kinetics in the optical properties of GeTe thin films associated with reversible phase transitions initiated by nanosecond laser pulses. It was shown that the change of the transmissivity and reflectivity of GeTe films in case of direct phase transition from the amorphous state to the crystalline starts at the same time, while the reverse transition is observed delay change of the transmissivity relative to the reflectivity. The value of this delay for the 100 nm GeTe thin film was estimated at t100 = 24 ns, and for the 50 nm sample – t50 = 13 ns. This time lag in changes of the transmissivity from the reflectivity in case of reamorphization films depending on sample thickness, can be explained by the high contrast of the concentration of free electrons between amorphous and crystalline states, which determine the reflectivity of the surface layer of the sample, while the transmissivity is determined by the total thickness. The experimental dependences of the kinetics of changes in the optical properties of GeTe samples under laser-induced phase transitions are in good consistent with the numerical simulation data of the Stefan problem.

Constructing on-demand photoreversible mono/multi-color switching fabrics with plasmonic in-doped ZnO catalyzed systems

Photoreversible color switching fabrics have attracted much interest by using plasmonic semiconductor-catalyzed systems with simultaneous high photocatalytic and photothermal effects, while such kinds of semiconductors are rarely reported. In this work, with ZnO as a model, we report the tuning of plasmonic performances of ZnO nanocrystals by doping various concentrations of indium ions through a solvothermal route. Indium doping not only confers size evolution but also incorporates ZnO lattices with a high concentration of free electrons that result in extended plasmonic photoabsorption, simultaneously achieving high photocatalytic and photothermal performances. The plasmonic In-doped ZnO nanocrystals, along with redox dyes, can be conveniently utilized for the design of a series of novel semiconductor-catalyzed color switching systems, including mono-color (red, green, blue) and multi-color switching (red, yellow, blue) systems, which rapidly respond to 365 nm light for photocatalytic discoloration and 980 nm light-induced photothermal heating for recoloration (in air). Furthermore, with methylcellulose as the polymer matrix, the plasmonic In-doped ZnO catalyzed systems can be further processed into switchable fabrics/papers, exhibiting excellent mono/multi-color response features. Based on the manipulation of the interactions between light and plasmonic semiconductor-catalyzed systems, the rewritable fabrics/papers exhibit a broad range of highly reversible photochromic features, showing great potential for application as advanced sensors and rewritable media.

Light-Activated Chemoresistive and Plasmonic-Resonant Optical Sensors for NO2 and H2 Sensing Based on ZnO Doped Nanoparticles

Light-Activated Chemoresistive and Plasmonic-Resonant Optical Sensors for NO2 and H2 Sensing Based on ZnO Doped Nanoparticles

Bi doping stimulation on the visible-light absorption of In2O3 ceramics

Bandgap engineering in semiconductors is a long-term subject in many interests. Doping Bi modifies the wide-bandgap semiconductor In2O3 by generating an in-gap state. The reflectance spectra show that an optical transition occurs at 1.2 eV inside the bandgap of In2O3 as a substitution of Bi atoms, which is in good agreement with the previously reported density functional theory (DFT) calculation by Sabino et al., Phys. Rev. Mater. 3 (2019) 034605. The detailed analyses illustrate that the trivalent Bi dopant sites on the In-site in a limited concentration maintaining the chemical state of In and O. For the higher doping concentration, the secondary phase of Bi2O3 forms even though no chemical state variation of the host In2O3. However, the free electron concentration decreases rapidly with increasing Bi doping concentration while unchanging oxygen vacancies. These results confirm that the trivalent Bi induces an occupied band in the In2O3 electronic structure and In-vacancy as an acceptor, promising host materials for the p-type doping. Furthermore, the extended DFT calculation to the other pnictogen atoms confirms the similar in-gap states and lower In vacancy formation energy, suggesting the possibility toward p-type oxide semiconductors using pnictogen doped In2O3.

Thermoelectric Properties of n-Mg2Si0.8Sn0.2 Solid Solutions with Addition of SiO2 and TiO2 Nanoinclusions

The results of theoretical calculations show the possibility of increasing the thermoelectric figure of merit in solid solutions of Mg2Si–Mg2Sn using nanoinclusions (NEAT approach). The thermoelectric properties of the Mg2Si0.2Sn0.8 solid solution, doped with antimony to obtain the optimal concentration of free electrons, with the addition of SiO2 and TiO2 impurities as additional scattering centers were studied. During the preparation of solid solution samples by hot pressing, nanoparticles of silicon dioxide and titanium dioxide of 15–200 nm in size at a concentration of 0.5–1 vol % were added. The coefficients of thermopower, electrical conductivity, and thermal conductivity were measured on these samples in the temperature range from 300 to 800 K. An insignificant decrease in thermal conductivity was shown for samples containing additional impurities. In this case, an increase in the thermoelectric figure of merit was not observed due to a decrease in the mobility of majority carriers.

Projected finite dimensional iteratively regularized Gauss–Newton method with a posteriori stopping for the ionospheric radiotomography problem

We investigate a class of finite dimensional iteratively regularized Gauss–Newton methods for solving nonlinear irregular operator equations in a Hilbert space. The developed technique allows to investigate in a uniform style various discretization methods such as projection, quadrature and collocation schemes and to take into account available restrictions on the solution. We propose an a posteriori stopping rule for the iterative process and establish an accuracy estimate for obtained approximation. The regularized Gauss–Newton method combined with the quadrature discretization and the a posteriori iteration stopping is applied to a model ionospheric radiotomography problem. The problem is reduced to a nonlinear integral equation describing the phase shift of a sounding radio signal in dependence of the free electron concentration in the ionospheric plasma. We establish the unique solvability of the inverse problem in the class of analytic functions.

Wide Band Gap Al and In Co-doped ZnO Films for Near-Infrared Plasmonic Application

In transparent conducting oxide films, tuning of plasmonic resonance is directly controlled by free electron concentration and thus by activated dopants. In this study, large area AlxInyZn1-x-yO thin films at various concentrations were prepared by spray coating using water as a solvent. The effect of Al/In dopant ratio on the structural, electrical, optical, and plasmonic properties was investigated. Tuning of optical response to a well-defined plasmon resonance is correlated to the above properties of AlxInyZn1-x-yO films. Theoretical fitting based on the Drude-Lorentz (D-L) theory was utilized for extracting the dielectric spectra and cross-over wavelength (ωc). The studies revealed plasmonic properties in NIR for the films with Al/In ratios of A5I5, A2.5I7.5, and A0I10, indicating In as the most activated dopant. Surface plasmon mode simulated using the extracted permittivity values showed the influence of mobility of these films on the broadening of the dip. The minimum plasmonic loss suggests the suitability as an alternative plasmonic material in the near infrared.

Effect of γ-radiation on magneto-thermoelectric properties of the extruded samples of Bi85Sb15〈Te〉 solid solution

Extruded samples of [Formula: see text] solid solutions doped with 0.0005 at.% Te were obtained and the electrical conductivity [Formula: see text], thermoelectric power (Seebeck) [Formula: see text], Hall [Formula: see text] and thermal conductivity [Formula: see text] coefficients were investigated in the range [Formula: see text]–300 K samples and magnetic field strength up to [Formula: see text] A/m, as annealed after extrusion, non-irradiated with gamma-quanta and the same samples irradiated with gamma quanta at different doses. It was found that at low doses (1 Mrad) of irradiation, radiation defects (RDs) appear in the samples which play the role of donor centers, as a result of which the concentration of free electrons [Formula: see text], and, consequently the electrical conductivity [Formula: see text] increases, and the Seebeck coefficient [Formula: see text] decreases. These defects, scattering the current carriers, reduce their mobility [Formula: see text]. With an increase in the radiation dose, the concentration of defects also increases and free carriers are captured at the level of the RD. In this regard, the concentration of charged carrier defects [Formula: see text] and, consequently, [Formula: see text] of the sample decrease, while the Seebeck coefficient and mobility increase. The effect of a magnetic field on the electrical and thermal parameters of extruded solid solution samples also depends on the radiation dose in the sample.