Free Electron Density Research Articles

Real-time in situ monitoring of dust particle growth in a low-pressure nanodusty plasma based on laser-induced photodetachment

Particulate matter air pollution in the form of ultrafine dust is a growing global concern. In this Letter, we will use a nanodusty Ar/HMDSO plasma as a model system for a heavily contaminated gas and we present the development of a technique for real-time in situ measurements of the dust particle size. The method is based on laser-induced photodetachment of bound electrons from the surface of dust particles. These photo-released electrons are measured as an increase in the free electron density of the plasma using microwave cavity resonance spectroscopy. We show that instead of reconstructing the entire resonance profile, the temporal response of a single microwave frequency was enough to perform the measurements. More specifically, the decay timescale of the cavity response can be interpreted as the re-charging timescale of the dust particles. Then, using a stochastic model, this timescale can be modeled, which eventually retrieves the dust particle size. We found good agreement between the predicted dust particle size and the average dust particle size obtained from ex situ scanning electron microscopy measurements. This method allows for the real-time monitoring of the dust particle size and a controlled production of nanometer-scale dust particles, which gives opportunities both for fundamental dusty plasma physics and models, as well as for applications in monitoring ultrafine dust air pollution.

The invisible black widow PSR J1720-0534: implications for the electron density towards the North Polar Spur

Abstract Radio emission from pulsars can be used to map out their distances through dispersion measure (DM), which quantifies the amount of radio pulse dispersion. However, this method relies on accurately modeling the free electron density in the line of sight. Here, we present a detailed study of the multi-wavelength emission from PSR J1720-0534, a black widow compact binary millisecond pulsar discovered in 2021, which the latest electron density model of the Galaxy (YMW16; Yao et al. 2017) places at only 191 pc. We obtained and analyzed deep multi-wavelength observations in the γ-ray (Fermi-LAT, 2008-2022), optical (LCO, 2.7-h), near-infrared (NOT, 3.5-h) and X-ray (Swift-XRT, 10 ksec) bands. We found no significant detection of γ-ray, optical, near-infrared, or X-ray counterparts around the radio-timing position of PSR J1720-0534, which we thus nickname ‘the invisible black widow’. Employing the most constraining near-infrared limit (J > 23.4 mag), we established a lower limit on the source distance, d > 1.1 kpc, assuming conservative properties for the black widow companion star. This distance lower limit differs drastically (by a factor of more than five) from the YMW16 DM distance estimate. We attribute this difference to the inclusion in the YMW16 model of a large and dense component towards the North Polar Spur. Considering our results and recent parallax distances to other pulsars in this direction, we argue that such a local and large component in the electron density model of the Galaxy is unnecessary.

Modeling of the femtosecond pulsed laser-induced damage of multi-layer dielectric gratings

The laser damage resistance of multi-layer dielectric gratings (MDGs) and metal multi-layer dielectric gratings (MMDGs) for femtosecond lasers directly determines the performance and life span of the high-energy laser system. It remains a significant challenge to predict the laser-induced damage threshold (LIDT) of MDGs and MMDGs for femtosecond lasers. Based on the nonlinear ionization and electric field intensity model, an effective method is proposed to calculate the LIDT of MDGs and MMDGs by introducing the dual temperature model. We take into account the evolutions of thermophysical parameters including the absorption coefficient, heat capacity, and thermal conductivity of the dielectric material as a function of the free-electron density and temperature. The free electron density of the conduction band combined with the electric field strength inside the grating can be calculated by the laser-induced ionization theory, more importantly, the transient distribution of the lattice temperature field generated by femtosecond laser irradiated grating excitation is obtained by the two-temperature model (TTM). This work is beneficial for guiding the design and fabrication of MDGs and MMDGs with high-LIDTs for ultra-short ultra-intense laser systems.

Single-Particle Spectroelectrochemistry: Promoting the Electrocatalytic Activity of Gold Nanorods via Oxygen Plasma Treatment without Structural Deformation.

Understanding of the electrocatalytic activity enhancement in gold nanoparticles is still limited. Herein, we present the effect of the oxygen plasma treatment on the electrochemical activity of gold nanorods (AuNRs). Oxygen plasma treatment resulted in the blueshift and line width narrowing of the localized surface plasmon resonance (LSPR) spectra obtained from individual AuNRs immobilized on an indium tin oxide (ITO) surface. These changes can be attributed to increases in the surface charges of the AuNRs. The formation of a Au-ITO heterojunction provided structural stability to the immobilized AuNRs regardless of the duration of oxygen plasma exposure. The electrocatalytic oxidation of hydrogen peroxide (H2O2) was induced by increases in the free-electron densities on the surfaces of these AuNRs owing to oxygen plasma treatment, and Au did not dissolve under the experimental conditions. However, the potential-dependent LSPR spectra of the individual AuNRs showed similar patterns of LSPR behavior, irrespective of the duration of oxygen plasma treatment and the concentration of H2O2. Therefore, this study based on single-particle spectroelectrochemistry and cyclic voltammetry improves the understanding of the role of oxygen plasma treatment in promoting the catalytic activity of structurally stable AuNRs immobilized on an ITO surface.

Temperature-dependent work function, thermionic emission and bulk modulus of TiC: a study on the identification of free valence electrons and localized valence electrons and their roles played in carbides.

By analyzing the projected states of valence electrons (fatband structures), the localized valence electrons and the free valence electrons of TiC were identified, respectively. After defining the volumes and the magnitudes of localized valence electrons and free valence electrons, the influences of the temperature, including the thermal expansion and the atomic thermal vibration, on the localized valence electron density and the free valence electron density were investigated, respectively. Based on the metallic plasma model (MPM), the temperature-dependent work functions and the thermionic emission current densities of TiC were calculated in terms of temperature-dependent free valence electron densities. The results were in good agreement with experimental results. Furthermore, as it was observed, the linear dependence of the bulk modulus on the localized valence electron density demonstrated that the bulk modulus of TiC was determined by the localized valence electron density. The different roles played by the free valence electrons and the localized valence electrons in the work function and the bulk modulus of TiC could be attributed to their different contributions to the kinetic energy density of valence electrons. The influences of the temperature on the work function, thermionic emission and bulk modulus of TiC indicated that the transition metal carbides with lower free valence electron density, higher localized valence electron density and heavier atomic mass were desired to achieve lower work function, higher current density and higher stability.

Diagnosis of bound electron density by measuring energy loss of proton beam in partially ionized plasma target

Partially ionized plasma contains the bound electrons, which have an effect on the instability of the plasma. The evolution process of bound electron density cannot be obtained by using the existing optical method used for diagnosing the free electron density. In this work, we carry out a high-precision experiment: the energy loss of a 100 keV proton beam penetrating through the partially ionized hydrogen plasma target is measured on the platform of ion beam-plasma interaction at the Institute of Modern Physics, Chinese Academy of Sciences. The bound electron density is obtained according to the energy loss model of Bethe theory. The free electron density is measured by laser interferometry and the electron tempercture is obtained from the measured spectrum (<i>T</i><sub>e</sub> = 0.68 eV; <i>n</i><sub>fe</sub> = 2.41×10<sup>17</sup> cm<sup>–2</sup>). It is found that the bound electron density decreases during plasma lifetime. The diagnosis of bound electron density by measuring energy loss of ion beam has the advantages of on-line, in-situ and high resolution, thus providing a new way to solve the problem about measuring the bound electron density in partially ionized plasma. A COMSOL simulation reveals that the high-temperature free electrons will be ejected quickly out of the plasma area through a mechanical diaphragm, thus reducing the total number of free electrons. In order to maintain a relatively high degree of ionization in this plasma, in principle, more and more bound electrons are ionized into free electrons, the density of bound electrons decreases correspondingly. The simulation result accords well with our experimental data. Based on this finding, more detailed plasma target parameter is obtained, which is helpful in deepening the understanding of the interaction process between ion beam and plasma. In future, more researches of low low-energy highly-charged ions-plasma interaction will be conducted.

Influence of High-frequency localized surface plasmon polariton effect of Al nanoparticles on luminescence efficiency of deep-blue BCzVBi OLED

It is difficult to enhance the blue or purple luminescence efficiency of organic light-emitting device ( OLED) for practice display applications. In this work, aluminum nano particles (Al-NPs) are inserted into the light-tight TmPyPb electron transporting layer (ETL) of ITO/PEDOT:PSS/TAPC/BCzVBi:BCPO/TmPyPb/Liq/Al OLEDs, in which BCzVBi can emit deep-blue fluorescent light, with the attempts to overcome the above deficiency through the local surface plasmon polariton(LSPP) effect excited in Al-NP at higher resonance frequencies by the luminescence radiations from BCzVBi. The distances of Al-NPs from BCzVBi:BCPO fluorescent layer are chosen as <i>x</i> = 4, 8, 12 nm. The morphologies observed by atom force microscope and scan electron microscope show that the Al film with a thickness of 1 nm, deposited at room temperature by vacuum heat evaporate, is composed of separated Al grains (therefore, called Al-NPs) with sizes on a 10 nm scale. By inserting these Al-NPs into the TmPyPb ETL, both the current density and luminance at the same voltage decrease in comparison with the counterparts of reference devices (i.e. ones without Al-NPs) due to the worsened carrier mobility. However, the current density and luminance both rebound significantly at <i>x</i> = 8 nm. This may be due to the fact that the fluorescence quenching strongly occurs at <i>x</i> < 8 nm, and on the other hand, the local surface plasmon polariton is weakened too much at <i>x</i> > 8 nm due to attenuated radiation from BCzVBi. At <i>x</i> = 8 nm, the voltage (9 V) at which the luminance reaches a maximum value is the same as that for the reference device, but the maximum luminance itself decreases from 4200 Cd/m<sup>2</sup> to 3500 Cd/m<sup>2</sup>. However, the current density also decreases from 335.19 mA/cm<sup>2</sup> to 145.71 mA/cm<sup>2</sup>. This conversely results in a promising great increase of current efficiency from 0.88 Cd·A<sup>–1</sup> to 2.36 Cd·A<sup>–1</sup>. Subsequently, the external quantum efficiency (EQE) is enhanced by 170%, while the efficiency roll-off ratio decreases from 78% to 30.5%, with a decrement of 61% . At a high current density of 270 mA/cm<sup>2</sup>, EQE enhances 66.5%. The coupling between fluorescence excitation state and local surface plasmon polariton is determined by the overlapping between fluorescence emitting peak and plasmon resonance peak. As aluminum has a number density of free electrons, 18.1×10<sup>22</sup> cm<sup>–3</sup>, much larger than those for the other normally used metals (such as gold and silver), its spectrum of local surface plasmon polariton is enough to cover the fluorescence wavelength range of BCzVBi. These research results show that the luminescence efficiency of deep-blue OLEDs can be turned better by LSPP excited in Al-NPs at higher resonance frequencies.

Correlation of work function and stacking fault energy through Kelvin probe force microscopy and nanohardness in dilute α-magnesium

Electronic interactions of the Group 2A elements with magnesium have been studied through the dilute solid solutions in binary Mg-Ca, Mg-Sr and Mg-Ba systems. This investigation incorporated the difference in the ‘Work Function’ (ΔWF) measured via Kelvin Probe Force Microscopy (KPFM), as a property directly affected by interatomic bond types, i.e. the electronic structure, nanoindentation measurements, and Stacking Fault Energy values reported in the literature. It was shown that the nano-hardness of the solid-solution α-Mg phase changed in the order of Mg-Ca>Mg-Sr>Mg-Ba. Thus, it was shown, by also considering the nano-hardness levels, that SFE of a solid-solution is closely correlated with its ‘Work Function’ level. Nano-hardness measurements on the eutectics and ΔWF difference between eutectic phases enabled an assessment of the relative bond strength and the pertinent electronic structures of the eutectics in the three alloys. Correlation with ΔWF and at least qualitative verification of those computed SFE values with some experimental measurement techniques were considered important as those computational methods are based on zero Kelvin degree, relatively simple atomic models and a number of assumptions. As asserted by this investigation, if the results of measurement techniques can be qualitatively correlated with those of the computational methods, it can be possible to evaluate the electronic structures in alloys, starting from binary systems, going to ternary and then multi-elemental systems. Our investigation has shown that such a qualitative correlation is possible. After all, the SFE values are not treated as absolute values but rather become essential in comparative investigations when assessing the influences of alloying elements at a fundamental level, that is, free electron density distributions. Our study indicated that the principles of ‘electronic metallurgy’ in developing multi-elemental alloy systems can be followed via practical experimental methods, i.e. ΔWF measurements using KPFM and nanoindentation.

Density functional theory study on the effect of NO annealing for SiC(0001) surface with atomic-scale steps

The effect of NO annealing on the electronic structures of the 4H-SiC(0001)/SiO2 interface with atomic-scale steps is investigated. The characteristic behavior of conduction band edge (CBE) states is strongly affected by the atomic configurations in the SiO2 and the step structure, resulting in the discontinuity of the CBE states at the step edges, which prevents electrons from penetrating from the source to drain and decreases the mobile free-electron density. We found that the behavior of the CBE states becomes independent from the atomic configuration of the SiO2 and the density of the discontinuities is reduced after NO annealing.

Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure

Nanometer-thick quantum-well structures are quantum model systems offering a few discrete unoccupied energy states that can be impulsively filled and that relax back to equilibrium predominantly via spontaneous emission of light. Here we report on the response of an indirectly excited quantum-well heterostructure, probed by means of time and frequency resolved photoluminescence spectroscopy. This experiment provides access to the sub-nanosecond evolution of the free electron density, indirectly injected into the quantum wells. In particular, the modeling of the time-dependent photoluminescence spectra unveils the time evolution of the temperature and of the chemical potentials for electrons and holes, from which the sub-nanosecond time-dependent electron density is determined. This information allows to prove that the recombination of excited carriers is mainly radiative and bimolecular at early delays after excitation, while, as the carrier density decreases, a monomolecular and non-radiative recombination channel becomes relevant. Access to the sub-nanosecond chronology of the mechanisms responsible for the relaxation of charge carriers provides a wealth of information for designing novel luminescent devices with engineered spectral and temporal behavior.

Terahertz imaging through emissivity control

Electromagnetic radiation in the terahertz (THz) frequency band has unique potential for future communication and imaging applications. However, the adoption of THz technologies is hindered by the lack of cost-effective THz sources. Here we demonstrate a way to generate and control THz radiation, via spatio-temporal emissivity modulation. By patterning the optical photoexcitation of a surface-passivated silicon wafer, we locally control the free-electron density, and thereby pattern the wafer’s emissivity in the THz part of the electromagnetic spectrum. We show how this unconventional source of controllable THz radiation enables a form of incoherent computational THz imaging. We use it to image various concealed objects, demonstrating that this scheme has the penetrating capability of other THz imaging approaches, without the requirement of femtosecond pulsed laser sources. Furthermore, the incoherent nature of thermal radiation also ensures the obtained images are free of interference artifacts. Our spatio-temporal emissivity control could enable a family of long-wavelength structured illumination, imaging, and spectroscopy systems.

Laser-induced localized and maskless electrodeposition of micro-copper structure on silicon surface: Simulation and experimental study

The simulation and experimental study of laser-induced localized and maskless electrodeposition of micro-copper (Cu) structures on p-type silicon (Si) surface is investigated. Firstly, calculations of the internal temperature field and free electron density distribution within Si under nanosecond laser irradiation were carried out, to demonstrate the selective enhancing effect of laser irradiation on electrical conductivity via the thermal and photoconductive manner. Secondly, experimental tests of the laser-induced electrodeposition of micro-wires, regional coatings and complicated patterns on Si surface without mask were performed successively, to illustrate the effectiveness of the proposed method. The experimental results indicate that laser pulse width and scanning space significantly affect the quality of the deposited layer. Specifically, dense and flat micro wires can be achieved when the laser pulse width is 100 ns, which is associated with a width of about 71.7 μm and thickness of around 10.9 μm. In addition, the laser scanning space should be well optimized to balance the uniformity of the regional coating and the stray deposition near the edge. Finally, the reduction of Cu2+ and deposition of Cu atom at Si surface were discussed, as well as the development of transitional layer, to supply an in-depth analysis of the electrodeposition mechanism.

Wide tuning of epsilon-near-zero plasmon resonance in pulsed laser deposited ITO thin films

Oxygen vacancies in indium tin oxide (ITO) thin films provide a direct route to effectively tune the free electron density and thereby, controlling the epsilon-near-zero (ENZ) cut-off wavelength, the wavelength at which the real part of permittivity crosses zero of the permittivity axis. In this report, oxygen vacancies in pulsed laser deposited ITO thin films are systematically tuned using different background gases (O2, N2, Ar, and He). ENZ cut-offs are observed for the films deposited under He and Ar gases. In contrast, no such cut-offs are observed in the case of other two gases. An ITO thin film deposited under He gas exhibits deeper resonance signal than the one deposited under Ar gas. As expected, no such dip in the resonance spectra is observed for the films deposited under O2 and N2 gases. This observation is directly correlated to the change in the number of oxygen vacancies under different ambient gases. A modified transfer matrix method which incorporates surface roughness as an effective medium layer is developed to describe the experimentally observed resonance spectra numerically. Angular invariancy of ENZ plasmon resonance and the difference in absorption values for ITO films deposited under different gases is understood in terms of local field intensity enhancement factor. The study presented here will certainly be very useful in understanding the ENZ plasmon resonance phenomena as a whole. Additionally, ITO films deposited under an inert gas environment could be excellent material platforms for realizing several exotic ENZ applications.

Picosecond to microsecond dynamics of X-ray irradiated materials at MHz pulse repetition rate

Modern X-ray free-electron lasers (XFELs) produce intense femtosecond X-ray pulses able to cause significant damage to irradiated targets. Energetic photoelectrons created upon X-ray absorption, and Auger electrons emitted after relaxation of core-hole states trigger secondary electron cascades, which contribute to the increasing transient free electron density on femtosecond timescales. Further evolution may involve energy and particle diffusion, creation of point defects, and lattice heating. This long-timescale (up to a microsecond) X-ray-induced dynamics is discussed on the example of silicon in two-dimensional geometry. For modeling, we apply an extended Two-Temperature model with electron density dynamics, nTTM, which describes relaxation of an irradiated sample between two successive X-ray pulses, emitted from XFEL at MHz pulse repetition rate. It takes into account ambipolar carrier diffusion, electronic and atomic heat conduction, as well as electron-ion coupling. To solve the nTTM system of equations in two dimensions, we developed a dedicated finite-difference integration algorithm based on Alternating Direction Implicit method with an additional predictor-corrector scheme. We show first results obtained with the model and discuss its possible applications for XFEL optics, detectors, and for diagnostics tools. In particular, the model can estimate the timescale of material relaxation relevant for beam diagnostic applications during MHz operation of contemporary and future XFELs.

Study of the Free Randomly Moving Electron Transport Peculiarities in Metals

In this study, we review some aspects of the application of free randomly moving (RM) electron density and its probability density function distribution to the main free electron transport characteristics of elemental metals. It is shown that metal atom thermal vibrations not only produce free RM electrons, but also produce the same number of electronic defects (weakly shielded ions). The general expressions for the drift mobility, diffusion coefficient, and mean free path of randomly moving electrons are presented. It is shown that the scattering of free RM electrons is mainly due to electronic defects, which cause the distortion of the periodic potential (or the charge density) distribution in the periodic lattice. The resistivity of the elemental metal is caused by electronic defect scattering, taking into account the exchange in the thermal energies between phonons and free RM electrons. Special attention is paid to the analysis of the Hall effect measurement data: the Hall coefficient is presented for two types of RM electrons and holes, taking into account electron-like and hole-like densities of states. The paramagnetism and diamagnetism of the free RM electrons are simply explained using the definition of free RM electron density.

A METHOD FOR MAINTAINING THE ACCELERATION RATE AND INCREASING THE ENERGY OF SELF-INJECTED BUNCH DUE TO THE USE OF INHOMOGENEOUS PLASMA

The paper considers the process of excitation of a wakefield in a plasma by a laser pulse. The plasma density corresponds to the density of free electrons in the metal. A method is demonstrated for keeping self-injected bunch in the accelerating phase of the wakefield as laser pulse and bunch move in plasma with an increasing density gradient. Thus, the rate of acceleration of self-injected bunch is maintained and enhanced.

Transient electron temperature and density changes in water breakdown induced by femtosecond laser pulses

When a femtosecond laser pulse travels through water, optical breakdown occurs when the laser intensity exceeds a certain threshold. This photoionization (PI) process and the resulting laser-induced plasma can strongly influence the laser ablation of substances, including transparent biological tissue, etc. However, numerical models of the initial evolution of the laser plasma and its early electron temperature remain inadequate. Here, a transient ionization rate equation involving the solvation process is proposed for calculating the free-electron number density (FED) generated during the interaction of the femtosecond laser with the water surface, along with the temporal evolution of the electron temperature in the focusing region during the entire femtosecond laser pulse utilizing an improved temperature model. The predictions of the proposed model are in better agreement with the experimentally determined breakdown thresholds. The results indicate that the existence of solvated electrons has a considerable influence on the determination of the optical breakdown threshold in water, which may also be one of the reasons why traditional numerical models consistently fail to forecast the actual breakdown threshold.

A novel diagnostic for dust particle size in a low-pressure nanodusty plasma based on the decay of the electron density released by laser-induced photodetachment

One of the key parameters in low-pressure nanodusty plasmas is the dust particle size. In this work, we introduce a new method for the determination of the dust particle size in a nanodusty plasma, created in a mixture of argon and hexamethyldisiloxane. To this end, an ultraviolet (λ=266 nm) pulsed laser was used to release plasma-collected electrons from the nanoparticles. Subsequently, the response of the free electron density of the plasma was measured using microwave cavity resonance spectroscopy. Using a stochastic model for particle charging using orbital-motion limited (OML) theory, the predicted charging timescale can be directly compared to the experimentally measured decay timescale of the photo-released electron density. Good agreement was found between the experimentally predicted dust particle size and ex situ scanning electron microscopy (SEM) measurements. Furthermore, the sensitivity of the OML model to its input parameters was assessed. Finally, reversing the method can yield an estimate for the positive ion density based on the dust particle size from SEM.

Unique Structural Features of NbS3 Ribbon Whiskers

The X-ray diffraction analysis of NbS3 ribbon whiskers has revealed three structural features: (i) a fine-crystalline structure throughout the entire volume with the preferred orientation along the [001] direction perpendicular to the long b axis of the whisker, (ii) the combination of crystalline macroblocks with a length up to 0.5 mm with small crystallites of various orientations, and (iii) the combination of crystalline macroblocks with the left-hand twisting of planes around the b axis with a pitch angle of 1.25° per every 0.2 mm and with the return to the initial orientation in the next block. Structural features (ii) and (iii) of NbS3 whiskers have not yet been observed, and inorganic crystals with such properties are absent to the best of our knowledge. All crystallites have a unit cell with almost right angles and approximately the same lattice constant c (18.130 Å), whereas the lattice constants a and b are noticeably different in a single sample. All crystallites can be referred to phase IV rather than to phase I, as expected. The right angle between the a and c axes can be explained by the twinning of phase I along the c axis. Differences in the lattice constants in macroblocks indicate large stresses in structures. Such stresses near twins (and/or stacking faults) can significantly affect the free electron density and play a key role in the formation of charge density waves in various phases of NbS3.

Triple signal-enhancing sandwich-type electrochemical immunosensor based on Au@SNHC and PtPdCu/BNC for detection of Cardiac Troponin I

A novel multiple amplification strategy for electrochemical immunoassay was developed elaborately. The realization of this strategy is based on the high efficiency catalysis of boron nitrogen double-doped carbon loaded trimetallic PtPdCu mulberry-like nanospheres (PtPdCu/BNC) and the satisfactory conductivity of gold nanoparticles modified with sulfur nitrogen co-doped hollow porous carbon (Au@SNHC). Single crystal anisotropic Pt octahedral seeds were synthesized with sodium citrate as shape-directing agent, and then three metals were grown in situ to prepare the trimetallic PtPdCu mulberry nanospheres, which had excellent utilization of atoms and a significant number of catalytic active centers. Boron nitrogen double-doped carbon (BNC) with abundant free electrons and high electron density micro-mesoporous can be used as the supported dispersion material of PtPdCu MBNSs, further catalytically enrich H2O2. Moreover, the etching vulcanization of small ZIF template and the co-doping of sulfur and nitrogen endow sulfur nitrogen co-doped hollow porous carbon (SNHC) with shorter the diffusion distance of charge carrier and lower mass transfer resistance. Under optimal conditions, the developed immunosensor exhibits good sensing performance with a detection limit of 4.27 fg⋅mL−1, and the wide linear dynamic range (10 fg⋅mL−1–100 ng⋅mL−1). This study provides a possible strategy for early diagnosis and postoperative monitoring of AMI.