Temperature Dependence Of Dielectric Function Research Articles

Temperature dependence of MXenes plasmons induced hot carrier generation and transport

Unlike conventional optoelectronic devices, plasmon-driven optoelectronic devices can efficiently realize energy conversion and regulate the energy distribution of hot carriers through high-energy, non-equilibrium “hot” electron-hole pairs (hot carriers) generated by surface plasmon non-radiative decay, thereby presenting new opportunities for realizing hot carrier optoelectronic devices. As the basis for the practical application of plasmon optoelectronic devices, searching for plasmon metal materials with exceptional performance has always been an important topic in the field of hot carrier optoelectronic devices. Currently, MXenes can be synthesized on a large scale and has excellent photoelectric properties, so it can be used to build a variety of hot carrier photodetectors with unique structures and functions. Unlike the fixed surface ends of two-dimensional materials such as graphene, MoS<sub>2</sub> and borophene, MXenes has an abundance of surface functional groups. However, the increase of ambient temperature will accelerate the oxidation modification of surface functional groups, thus affecting the life and performance stability of optoelectronic devices. In view of the inherent limitations of experimental research on dynamic characteristics of hot carriers at continuous temperatures, we study the temperature effects on the electronic state distributions and scattering effects by using the theory of multi-body perturbation and quantum mechanics. Particularly, we introduce temperature effect into interband electron transition and phonon-assisted electron transition process to obtain temperature dependent dielectric function. From the perspective of non-radiative decay of surface plasmon, we quantify the hot carrier generation efficiency, energy distribution and transport characteristics by first principles calculations, in order to systematically study the ambient temperature dependence of plasmon-induced hot carriers in MXenes. The results show that the interband transition and the phonon-assisted electron transition in MXenes together efficiently produce high-energy hot hole-dominated carriers with a long lifetime and transport distance, which is comparable to borophene. The increase of ambient temperature significantly improves the hot carrier generation efficiency in the infrared range. Meanwhile, the physical mechanism of hot carrier generation in visible light is almost unaffected by the increase of ambient temperature, and the generated hot holes show excellent ambient temperature stability. In addition, the lifetime and transport distance of hot carriers decrease with ambient temperature increasing, which is mainly due to the enhanced scattering of electrons and optical phonons. The research results will provide theoretical and data support for quantitatively evaluating the ambient temperature stability of MXenes plasmon optoelectronic devices in practical environment.

Study of collective excitations in monolayer-bilayer graphene heterostructures: Temperature effects

In this paper, we study collective excitations in an M-BLG heterostructure, consisting of one BLG and one parallel MLG sheet on an inhomogeneous background dielectric. Theoretical calculations are carried out within the random-phase approximation, taking into account temperature effects. The results illustrate that two solutions, corresponding to optical and acoustical modes, admit the zero equation of the temperature-dependent dynamical dielectric function of the structure. The energy of these collective modes is significantly larger (smaller) than that in DBLG (DLG) systems with similar parameters. As temperature increases, the frequency of the acoustical mode steadily increases while that of the optical mode only increases in large wave vector regions, found differently with different interlayer separations. Interestingly, we explore that the effects of carrier density in graphene on plasmon properties of M-BLG heterostructures sharply differ from those in both BLG and DLG systems.

Detailed analysis for temperature-dependent and temperature-independent Goos–Hänchen shift

In this work, we have presented the deep insight of Goos–Hänchen shift depending upon the temperature when a plane incident electromagnetic beam of light enters from a rare to a denser medium, keeping conditions of total internal reflection intact. A simple well known Drude’s model has been used for the purpose of thermal analysis of this shift. Through this model, the temperature-dependent permittivity of absorbing or complex medium has been found through the temperature-dependent plasma and damping wavelengths. Both transverse electric and magnetic incident electromagnetic fields have been considered in this regard. On an air–metal interface, firstly, Goos–Hänchen shift has been found when a temperature-independent dielectric function of a complex medium has been considered, and secondly, by analyzing the temperature-dependent dielectric function in the simplest way by using the stationary-phase method. This study is useful to understand the behavior of Goos–Hänchen shift around the temperature-dependency of the medium and how the angle of incidence impacts in this scenario.

Mapping the changes in the optical properties of azobenzene films due to photoisomerization

We used spectroscopic ellipsometry to study the dielectric function of an azobenzene polymer called Poly(disperse Red 1 methacrylate) (PDRM) as it underwent a photoisomerization process. The polymer films were fabricated by dissolving PDRM in tetrahydrofuran, and by spin coating films on silicon substrates. After fabricating the films, we used UV-VIS spectroscopic ellipsometry to determine the index of refraction, extinction coefficient and the thickness of each film. Using a heating cell coupled to the ellipsometer, we obtained temperature dependent ellipsometry data from 34 ℃ to 180 ℃, spanning a spectral region between 300 nm and 1500 nm. Using the raw data, we determined that the glass transition temperature of PDRM was 85 ℃ ± 3 ℃. Employing a three-oscillator model, we deduced the temperature dependent dielectric functions of PDRM films. Subsequently, the films were photoisomerized at two polarizations using a visible laser. For the horizontal polarization, there is nearly a 15% a change in the extinction coefficient as the PDRM films undergo a photoisomerization process.

The geometrical and temperature-dependent local field correction effects on the effective mass renormalization in a 2DEG

The paper aims at providing new theoretical insights into geometrical and local field correction effects on the effective mass renormalization of interacting electrons in a two-dimensional electron gas, at finite temperature. For this purpose, the temperature-dependent dynamic dielectric function is studied within random phase approximation. Then, the short-range effects of exchange-correlation holes around electrons are employed in calculations through local field correction using Hubbard, finite-temperature Hubbard, and Singwi-Tosi-Land-Sjolander approximations. Finally, the effective mass renormalization is calculated utilizing the real part of the electron self-energy. The outcomes indicate that the effective mass renormalization is decreased with increasing of the thickness at all dimensionless electron densities within all approximations. Likewise, utilizing a local field correction will lead to reductions in effective mass renormalization at any temperature and dimensionless electron density.

Performance comparison of near- and far-field temperature-dependent thermophotovoltaic systems with tungsten emitter and GaxIn1-xAs cell

The near-field effect greatly increases the radiative heat flux. The enhanced radiative heat flux results in increasing heat production in photovoltaic cells. A temperature-dependent model of the near-field thermophotovoltaic cell (TPVC) with a tungsten emitter and a GaxIn1-xAs cell is established and optimized to compare the performances of far- and near-field TPVCs, in which the temperature-dependent band-gap energy and dielectric function as well as finite-time heat transfer between the photovoltaic cell and the environment are all taken into account. The effects of voltage, emitter temperature, heat transfer parameter, and mole fraction of Ga on the performance of TPVCs are analyzed. It is found that the maximum power density of the near-field TPVC is above three times of that of the far-field TPVC, while the cell temperature at the maximum power density in the near-field TPVC has risen by 14.7 K in comparison with that in the far-field TPVC even if the heat transfer parameter is as high as 9.5 W/(cm2 K). The performances of the near-field TPVC are more affected by the heat transfer parameter than those of the far-field TPVC. When the heat transfer parameter is reduced to 1.5 W/(cm2 K), cell temperature in the near-field TPVC is increased by 126 K and the maximum power density is decreased by 5.43 W/cm2, while in the far-field TPVC, the cell temperature is increased by 40 K and the maximum power density is decreased by 0.68 W/cm2. Moreover, it is pointed out that increasing the emitter temperature, decreasing the cell temperature, and selecting the suitable mole fraction of component Ga in GaxIn1-xAs are some effective approaches to boost the power density and efficiency of far- and near-field TPVCs.

Ellipsometric and first-principles study on temperature-dependent UV-Vis dielectric functions of GaN.

The third-generation wide bandgap semiconductor GaN currently occupies a hot spot in the fields of high-power electronics and optoelectronics. Fully exploring its optical and optoelectronic characteristics is of great significance. Here, we provide a systematic study on the temperature-dependent dielectric functions of GaN grown by metal-organic chemical vapor deposition in the spectral range of 0.73-5.90 eV via spectroscopic ellipsometry experiments and first-principles calculations. Ellipsometric measurements identify two typical absorption peaks that originate from the excitonic and phonon-assisted indirect absorption process, respectively. To explore the underlying physics, we perform first-principles calculations using the independent-particle approximation, model Bethe-Salpeter equation (mBSE), and phonon-assisted indirect absorption process (Inabs). In comparison with ellipsometric measurements, the mBSE calculation determines the absorption peak contributed by the many-body excitonic effect, while the Inabs calculation successfully predicts the second absorption peak. When heating the crystal, it observes the redshift and weakening of absorption peaks, intrinsically due to the nontrivial electron-phonon interaction as lattice vibration strengthens. While doping GaN with Fe or Si elements, the introduced free carriers modify the electronic interband transition. As the temperature increases, more free carriers are excited, and the temperature influence on the absorption peak is more significant than that of the undoped one. This work fully explores the physical origins of the temperature and doping effect on UV-Vis dielectric functions of GaN, aiming to promote its application in the fields of high-power electronic devices.

Tunable Dielectric Switching of (Quinuclidinium)[MnCl4] Hybrid Compounds

Inorganic–organic hybrid QMnCl (Q = quinuclidinium) crystals were synthesized and characterized. The X-ray and variable-temperature IR/Raman analysis demonstrate that the crystals undergo a reversible structural phase transition, which originates from an order–disorder process and is related to the dynamics of the organic Q cation. Dielectric function measurements disclose a switchability between low (“OFF”) and high (“ON”) dielectric states centered at around 285 K. Owing to a remarkable temperature-dependent dielectric function, this type of molecular compound can represent an interesting tunable and switchable dielectric material for a diverse range of applications.

Modeling Contact Angle vs. Temperature for the Quartz-Water-Decane System

SummaryInnovative approaches are needed to improve the efficiency of oil recovery technologies to meet the growing demands of fossil-fuel based energy consumption. Enhanced oil recovery (EOR) methods such as low-salinity waterflooding and chemically tuned waterflooding aim to optimize the reservoir’s wetting properties, detaching oil globules from rock surfaces and allowing easier oil flow through pore throats. This wetting behavior is commonly quantified by contact angle measurements of the rock-oil-brine interface, which have been thoroughly investigated and theorized for many systems at ambient temperatures and pressures. However, few studies exist for extending contact angle theories away from ambient conditions. In this paper, we model the contact angles of a quartz-water-decane system at elevated temperatures using the surface tension component (STC) approach. Temperature-dependent van der Waals [Lifshitz-van der Waals (LW)] interactions and hydrogen-bonding (acid-base) interactions were calculated and are incorporated into the model for the quartz-water-decane interface. The Hough and White procedure was used to create temperature-dependent dielectric functions of quartz, water, and normal decane for calculations of Hamaker coefficients. Hamaker coefficients calculated this way are highly linear with temperature and agree well with Israelachvili’s approximation. The acid-base interactions likely contribute the most to system wettability changes. Resulting contact angles of the quartz-water-decane system shift from water-wet (16°) to slightly water-wet (57.4°) as temperature increases. The model was also successfully verified for the quartz-air-water system. Our results can be used in future studies to determine optimal injected water compositions for specific rock-oil-brine and other systems with consideration of reservoir temperature.

Temperature effects on plasmon modes in double-bilayer graphene structures

This paper aims to investigate collective excitations in a double-layer structure consisting of two parallel bilayer graphene sheets within random-phase-approximation. We determine the collective excitations in the system from the zeroes of the temperature-dependent dynamical dielectric function. Numerical results demonstrate that both optical in-phase and acoustic out-of-phase plasmon modes are found in the system, similar to other double-layer structures. Calculations prove that plasmon frequencies increase with the increase in temperature while the imbalance in doping density between two bilayer graphene sheets as well as the increase in carrier density decrease these frequencies. Similar to the case of zero temperature, increasing separation increases (decreases) acoustic (optical) plasmon frequency. Our computations illustrate that temperature affects remarkably and can reduce the effects of other factors on plasmon properties in double bilayer graphene systems.

Influence of the deposition temperature on the optical and electrical properties of TiN film by spectroscopic ellipsometry

Abstract Low emission (Low-e) films are widely used in modern architectural glass. In this presentation, spectroscopic ellipsometry (SE) between 1.12 and 2.75 eV (wavelength range: 450–1100 nm) was used to investigate the temperature dependence of dielectric function of TiN films. A Drude-3Lorentz dispersion model was selected and the results show both the real part e 1 and the imaginary part e 2 increase with the deposition temperature increasing. The structure and performance of the obtained TiN samples were characterized by X-ray diffraction, Raman, scanning electron microscope, UV/Visible (UV/VIS) spectrophotometer and Hall measurements. Combining the measured results, it is found that with the deposition temperature increasing, the grain size of TiN film changed from 15.4 nm to 17.4 nm and the surface becomes rougher. The concentrations of titanium and nitrogen vacancies decrease with increasing deposition temperature. A combined Drude-3Lorentz model was used to analyze the dielectric and optical properties of the obtained TiN films. It is found that TiN samples exhibit typical Drude-Lorentz like behavior in the visible and near-infrared (NIR) regions. The results were verified by Hall measurements and UV/VIS measurements. With the deposition temperature increasing, the reflectivity increased in the NIR region and a peak reflectivity of about 93% can be obtained at the wavelength of 1000 nm. With the deposition increasing, the infrared emissivity decreased. The infrared emissivities of the TiN film deposited at 673 K are 0.26 and 0.08 at wavelength of 2.5 μm and 25 μm, respectively, which shows TiN may be a good candidate for low-e film.

Ultrafast Plasmon Dynamics in Crystalline LiF Triggered by Intense Extreme UV Pulses.

An extreme ultraviolet pump and visible-light probe transmission experiment in crystalline LiF, carried out at the Free Electron Laser facility FERMI, revealed an oscillating time dependence of the plasmon mode excited in the high-density high-temperature electron plasma. The effect is interpreted as a fingerprint of the electron-ion interaction: the ion motion, shaped by the electron dynamic screening, induces, in turn, electron density fluctuations that cause the oscillation of the plasmon frequency at the timescale of the ion dynamics. Fitting the high resolution transmission data with an RPA model for the temperature-dependent dielectric function, which includes electron self-energy and electron-ion coupling, confirms the interpretation of the time modulation of the plasmon mode.

Optoelectronic response of the type-I Weyl semimetals TaAs and NbAs from first principles

Weyl semimetals are materials with topologically nontrivial band structure both in the bulk and on the surface, hosting chiral nodes which are sinks and sources of Berry curvature. Weyl semimetals have been predicted, and recently measured, to exhibit large nonlinear optical responses. This discovery, along with their high mobilities, makes Weyl semimetals relevant to a broad spectrum of applications in optoelectronic, nanophotonic and quantum optical devices. While there is growing interest in understanding and characterizing the linear and nonlinear behavior of Weyl semimetals, an ab initio calculation of the linear optical and optoelectronic responses at finite temperature remains largely unexplored. Here, we specifically address the temperature dependence of the linear optical response in type-I Weyl semimetals TaAs and NbAs. We evaluate from first principles the scattering lifetimes due to electron-phonon and electron-electron interaction and incorporate these lifetimes in evaluating an experimentally relevant frequency-, polarization- and temperature-dependent complex dielectric function for each semimetal. From these calculations we present linear optical conductivity predictions which agree well where experiment exists (for TaAs) and guide the way for future measurements of type-I Weyl semimetals. Importantly, we also examine the optical conductivity's dependence on the chemical potential, a crucial physical parameter which can be controlled experimentally and can elucidate the role of the Weyl nodes in optoelectronic response. Through this work, we present design principles for Weyl optoelectronic devices that use photogenerated carriers in type-I Weyl semimetals.

Dielectric function of polycrystalline gold films: Effects of grain boundary and temperature

The experimental data of the dielectric function of gold films are vital in metal optics since optical and optoelectronic properties depend sensitively on the dielectric function. Even though significant discrepancies of the dielectric function exist among the measurements in the past 60 years, the contribution of the grain boundary is less studied. In this work, we measured the dielectric function of three different gold films using two ellipsometers covering the spectral range from 200 nm to 25 μm. Based on the Drude model and the Mayadas-Shatzkes model of the electron-grain boundary scattering, we find that electronic reflection coefficient at the grain boundary is a vital factor to influence the dielectric function of the polycrystalline gold films. Furthermore, the temperature dependent dielectric function of gold is measured from 300 K to 800 K. Performing extended Drude analysis with a frequency dependent relaxation rate, we get a deviation of relaxation rate from the Fermi liquid theory at energies below 0.1 eV from 300 K to 800 K.

Temperature-dependent dielectric functions of bcc transition metals Cr, Mo, and W from ultraviolet to infrared regions: A theoretical and experimental study

In the bcc transition metals Cr, Mo, and W, the existence of the partially filled d bands makes interband transition occur at low photon frequencies and thus, it is difficult to differentiate it from intraband transition. Here, we present a thorough study on decomposing the intraband and interband contribution to finite temperature dielectric functions of these bcc transition metals by performing electron-phonon and electron-electron interaction calculations, as well as ellipsometry experiments. In this work, the Drude model and interband transition theory are applied to quantitatively describe the intraband and interband transition, respectively. To accurately determine intraband transition, the relevant parameters for the Drude model, such as plasma frequency and electron relaxation time, are calculated from first-principles. The electron-electron interaction within the many-body theory and electron-phonon interaction within the density functional perturbation theory are calculated to obtain the electron relaxation time and intraband dielectric function at finite temperature. As for interband transition, the spin-orbit coupling is included and it shows nontrivial influence on the interband dielectric function of Mo and W, especially at low frequencies. To verify theoretical calculations, ellipsometry experiments are performed to measure dielectric functions of Cr, Mo, and W over the temperature range of 300–700 K and energy range of 0.08–4.8 eV. The experimental results are then fitted by the Drude model, and it shows that the electron-phonon interaction rather than electron-electron interaction dominates the frequency dependence of the relaxation time for transition metals Cr, Mo, and W.

Investigating the Electron–Phonon Coupling of Molecular Beam Epitaxy-Grown Hg1−xCdxSe Semiconductor Alloys

Using spectroscopic ellipsometry, the temperature-dependence of the dielectric functions of a series of Hg1−xCdxSe thin films deposited on both ZnTe/Si(112) and GaSb(112) substrates were investigated. Initially, for each sample, room-temperature ellipsometric spectra were obtained from 35 meV to 6 eV using two different ellipsometers. Subsequently, ellipsometry spectra were obtained from 10 K to 300 K by incorporating a cryostat to the ellipsometer. Using a standard inversion technique, the spectroscopic ellipsometric data were modeled in order to obtain the temperature-dependent dielectric functions of each of the Hg1−xCdxSe thin films. The results indicate that the E1 critical point blue-shifts as a function of Cd-alloy concentration. The temperature-dependence of E1 was fitted to a Bose–Einstein occupation distribution function, which consequently allowed us to determine the electron–phonon coupling of Hg1−xCdxSe alloys. From the fitting results, we obtain a value of 17 ± 2 meV for the strength of the electron–phonon coupling for Hg1−xCdxSe alloy system, which compares nominally with the binary systems, such as CdSe and CdTe, which have values around 38 meV and 16 meV, respectively. This implies that the addition of Hg into the CdSe binary system does not significantly alter its electron–phonon coupling strength. Raman spectroscopy measurements performed on all the samples show the HgSe-like transverse optic (TO) and longitudinal optic (LO) phonons (∼ 130 cm−1 and ∼ 160 cm−1, respectively) for all the samples. While there is a slight red-shift of the HgSe-like TO peak as a function of the Cd-concentration, HgSe-like LO peak does not significantly change with the alloy concentration.

Charge transport in hybrid halide perovskites

Charge transport is crucial to the performance of hybrid halide perovskite solar cells. A theoretical model based on large polarons is proposed to elucidate charge transport properties in the perovskites. Critical new physical insights are incorporated into the model, including the recognitions that acoustic phonons as opposed to optical phonons are responsible for the scattering of the polarons; these acoustic phonons are fully excited due to the ``softness'' of the perovskites, and the temperature-dependent dielectric function underlies the temperature dependence of charge carrier mobility. This work resolves key controversies in literature and forms a starting point for more rigorous first-principles predictions of charge transport.

Polarization behavior of zinc oxide thin films studied by temperature dependent spectroscopic ellipsometry

We report on the influence of temperature on the polarization behavior of highly oriented ZnO thin film. First, the investigation of crystal structure change is studied, providing supporting information on the macroscopic-scale polarization of the ZnO thin film. Here, the lattice distortion is investigated by using X-ray diffraction. Furthermore, the role of temperature on the polarization behavior of the ZnO thin film is comprehensively studied by using temperature dependent spectroscopic ellipsometry. Here, the temperature dependent dielectric function analysis and electronic excitation models are used to understand the mechanism of polarization. We found an interesting temperature dependence of electronic transition, where the red-shift absorption and exciton-phonon interaction are observed on the system. This interaction is responsible for the increase of polarization response, which is confirmed by dielectric susceptibility spectra. These results provide important understanding for the control of the polarization dependence on the working temperature of ZnO thin film, which is the essential key in the fabrication of switchable optical devices.

Temperature-Dependent Optical Properties of Plasmonic Titanium Nitride Thin Films

Due to their exceptional plasmonic properties, noble metals such as, gold and silver, have been the materials of choice for the demonstration of various plasmonic and nanophotonic phenomena. However, noble metals’ softness, lack of tailorability, and low melting point, along with melting point depression in nanostructures as well as challenges in thin film fabrication and device integration in standard semiconductor processing, have prevented the realization of practical plasmonic devices for technologically important high temperature and heat-assisted applications. In the recent years, titanium nitride (TiN) has emerged as a promising plasmonic material with good metallic and refractory (high temperature stable) properties. The refractory nature of TiN could enable practical plasmonic devices operating at elevated temperatures for energy conversion and harsh-environment industries such as gas and oil. Here we report on the temperature-dependent dielectric functions of TiN thin films of varying thicknesse...

Temperature-dependent optical properties of gold thin films

Understanding the temperature dependence of the optical properties of thin metal films is critical for designing practical devices for high temperature applications in a variety of research areas, including plasmonics and near-field radiative heat transfer. Even though the optical properties of bulk metals at elevated temperatures have been studied, the temperature-dependent data for thin metal films, with thicknesses ranging from few tens to few hundreds of nanometers, is largely missing. In this work we report on the optical constants of single- and polycrystalline gold thin films at elevated temperatures in the wavelength range from 370 to 2000 nm. Our results show that while the real part of the dielectric function changes marginally with increasing temperature, the imaginary part changes drastically. For 200-nm-thick single- and polycrystalline gold films the imaginary part of the dielectric function at 500 0C becomes nearly twice larger than that at room temperature. In contrast, in thinner films (50-nm and 30-nm) the imaginary part can show either increasing or decreasing behavior within the same temperature range and eventually at 500 0C it becomes nearly 3-4 times larger than that at room temperature. The increase in the imaginary part at elevated temperatures significantly reduces the surface plasmon polariton propagation length and the quality factor of the localized surface plasmon resonance for a spherical particle. We provide experiment-fitted models to describe the temperature-dependent gold dielectric function as a sum of one Drude and two Critical Point oscillators. These causal analytical models could enable accurate multiphysics modelling of gold-based nanophotonic and plasmonic elements in both frequency and time domains.