Interband Electronic Transitions Research Articles

Covellite Nanodisks and Digenite Nanorings: Colloidal Synthesis, Phase Transitions, and Optical Properties

Copper sulfide nanoparticles represent a class of dual-functional materials whose optical properties are dominated by interband electronic transitions and intraband plasmon resonances, both of whic...

Metallic nanoparticle-on-mirror: Multiple-band light harvesting and efficient photocurrent generation under visible light irradiation

We present a photoanode concept for photoelectrochemical (PEC) water splitting based on metal nanoparticles (Au, Ag, Cu) deposited on a Ti mirror (NPoM) to photosensitize an intermediate TiO2 layer to induce the harvesting of light with sub-bandgap photon energies. The generation of hot electron-hole pairs in metallic nanostructures can occur either by intraband excitation, i.e., plasmon electrons, or by the interband transition of the d-band electrons to the unoccupied conduction band states. Our results demonstrate that the underlying Ti mirror significantly amplifies the PEC activity of such NPoM systems in the visible spectral range. We show that the PEC enhancement in the visible spectral range is not limited to the intraband excitations but is affected mainly by the light trapping pathway within the TiO2 layer, i.e. due to a favorable interplay between thin-film interference cavity modes with both intraband and interband excitations. Among tested metals, Cu and Ag NPs demonstrate a ~3-fold higher enhancement factor than that of Au NPs, while in both former cases the nature of the excited electrons is different. The experimentally determined internal quantum efficiency demonstrates alternating behavior with wavelength showing higher efficiencies at short wavelengths, which is attributed to the reduced Schottky barrier of the NPoMs at constructive interference maxima.

Towards an understanding of the localized surface plasmon resonance decay pathways in bimetallic core–shell nanoparticles

Exploring the energy transfer mechanisms and understanding the localized surface plasmon resonance (LSPR) decay pathways of plasmas are of great importance for realizing effective composition adjustment and optimization of these novel solar energy materials. In this work, the absorption, scattering, and extinction decay channels of bimetallic core–shell nanostructures were calculated using the finite difference time domain method. Moreover, we separated the energy absorbed in the core and shell of the bimetallic nanoparticles to get a better understanding of the decay pathways of LSPR excited energy. Results indicated that the availability of fast inter-band electron transitions in shell metals is the way to preferentially dissipate the surface electric field energy of nanoparticles. It was supported in essence that the larger imaginary part of the permittivity value of the metal shell is crucial for accelerating the decay channel via the surface of the nanoparticle. This work provided a basic physical framework and transparent design principles for material selection of multifunctional LSPR nanoparticles in practical applications.

Retrieval of Optical Constants of Undoped Flash-Evaporated Lead Iodide Films from an Analysis of Their Normal Incidence Transmission Spectra Using Swanepoel’s Transmission Envelope Theory of Non-Uniform Films

Abstract: Normal-incidence transmission-wavelength (T_exp (λ)-λ) spectra of 1 and 1.2 m thick flash-evaporated lead iodide (PbI2) films on 1.1 mm thick glass slides held at 200 ℃ display well-spaced several interference-fringe maxima and minima in the λ-range 520-900 nm, without exhibiting a transparent region and with the maxima lying well below the substrate transmission. Below 520 nm, these T_exp (λ)-λ curves drop steeply to zero (at λ ≼ 505 nm), signifying crystalline-like PbI2 film absorption. As corrections of measured (PbI2 film/substrate) transmittance data for substrate absorption and spectrometer slit-width effect were marginal over the studied λ-range, the observed low transmittance of (PbI2 film/substrate) system was related to PbI2 film thickness non-uniformity (∆d), which causes shrinkage of both maxima and minima and leads to significant film optical absorption that reduces both maxima and minima. The McClain ENVELOPE algorithm was utilized, with a minor modification, to construct maxima T_M (λ_max/λ_min) and minima T_m (λ_min/λ_max) envelope curves, which were analyzed by Swanepoel’s envelope method of non-uniform films using an approach that takes account of dispersive substrate refractive index n_s (λ) and circumvents the non-availability of a high-λ transparency region. In such analytical approach, ∆d was varied till a re-generated T_gen (λ)-λ curve matches the T_exp (λ)-λ curve. An average thickness d ̅ of the film, besides its refractive index n(λ) and absorption coefficient α(λ) in the weak, medium and strong absorption regions, were then obtained. The energy-dependence of α(λ) is discussed in view of interband electronic transition models. The obtained results are consistent with other literature studies on similar flash-evaporated PbI2 films. Keywords: PbI2, Optical constants and bandgap, Swanepoel's transmission envelope method, Non-uniform films.

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.

Electronic and optical properties of ZnSe by theoretical simulation TB-mBJ (Tran-Blaha modified Becke Johnson) associated to analysis techniques XPS (X-Ray Photoelectron Spectroscopy); REELS (Reflective Electron Energy Loss Spectroscopy) and PLS (Photoluminescence Spectroscopy)

Our work aims to establish a correspondence between the obtained theoretical and experimental results on the electronic and optical properties of ZnSe.The calculated DOS (Density of States) using the FP-LAPW method with the Tran-Blaha modified Becke-Johnson exchange-correlation potential reveal that the upper valence energy levels involve an important contribution from the Se-(4p and 3d) electron states with a small contribution from the Zn-2s states.The real (ε1(ω)) and imaginary (ε2(ω)) parts of the dielectric function show the anisotropy character of w-ZnSe.The main peaks appearing in the ε2(ω) spectrum, are calculated by summing over all contributions of electronic transitions between the pairs of bands Vn→Cn′, where Vn is an occupied valence band and Cn′ is an unoccupied conduction band. Experimentally, the XPS (X-Ray Photoelectron Spectroscopy) analysis technique is used to show the clean and homogeneous state of ZnSe. To confirm the calculated electron transitions, we use the REELS (Reflected Electron Energy Loss Spectroscopy) technique of great sensitivity for displaying the electron interband transitions. The electron states in the w-ZnSe gap are determined through the Gaussian deconvolution of the photoluminescence spectrum (PLS).

Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO3 thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.Received 14 September 2020Revised 20 January 2021Accepted 12 March 2021DOI:https://doi.org/10.1103/PhysRevX.11.021023Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDielectric propertiesElectric polarizationElectronic excitation & ionizationElectronic transitionsFerroelectricityLight-matter interactionNonlinear opticsPhononsStrong electromagnetic field effectsUltrafast phenomenaTechniquesTerahertz time-domain spectroscopyUltrafast pump-probe spectroscopyCondensed Matter, Materials & Applied PhysicsNonlinear DynamicsAtomic, Molecular & Optical

Effects of 4d transition metals doping on the photocatalytic activities of anatase TiO2 (101) surface

AbstractAiming at improving the visible‐light photocatalytic activities of TiO2(101) surface we make an in‐depth study on the TiO2(101) doped with 4d transition metal (TM) atoms. It is shown that the 4d TM dopings can not only produce new impurity energy bands in the band gap but also result in the semiconductor–metal phase transition. Consequently, the visible‐light absorption is strongly strengthened due to the dopings of Y, Zr, Nb, Mo, and Ag, while it is only weakly improved for Tc, Ru, Rh, Pd, and Cd dopings. The improvement in visible‐light absorption can be attributed to the intraband or interband transition of electrons. Moreover, the photocatalytic activities are explored, and we find Y and Ag dopings can effectively enhance the photocatalytic activity of TiO2(101) surface. Thus the mechanism of improving photocatalytic activity of TiO2(101) has been clearly addressed, which is beneficial to further experimental and theoretical researches on TiO2 photocatalysts.

A Consistent Scheme for the Precise FDTD Modeling of the Graphene Interband Contribution

The conductivity of graphene, due to interband electron transitions, is evaluated, in this article, as rational functions through an efficient iterative optimization of Padé polynomials. Then, the latter are straightforwardly imported in the text finite-difference text time-domain algorithm, as an equivalent surface current, by means of precise recursive convolution schemes. Numerical examples, considering an incident plane wave toward graphene, the extraction of the corresponding surface wave propagation attributes, and a plasmonic switch, successfully validate the consistent modeling of interband conductivity through the proposed technique.

First Principles Study on Electronic and Optical Properties of Quinary Copper-based Sulfides and Selenides Cu2HgGe(S1-xSex)4

Electronic and optical properties of Cu2HgGe(S1-xSex)4 compounds (x = 0, 0.25, 0.5, 0.75, and 1) were revealed by density functional theory (DFT), in which the Heyd-Scuseria-Ernzerhof hybrid functional was used. Dependence of band gap on the Se constituent in Cu2HgGe(S1-xSex)4 was reported. The substitution of Se element basically cause a slightly lattice expansion and minor change of the band gap. Meanwhile, the overlap of Cu and S/Se states becomes more dense leading to better electron/hole pair separation and inter-band transition of photo-excited electrons. The Cu2HgGe(S0.75Se0.25)4 compound was predicted to be very promising absorber due to the low band gap, high absorption rate, and low reflectivity in the incoming light energy range from 0 eV to 2 eV.
 

Optical and magneto-optical properties of epitaxial Mn2GaC MAX phase thin film

We report measurements of the dielectric permittivity, optical conductivity and magnetic circular dichroism (MCD) of the epitaxial Mn2GaC MAX-phase thin film in an external magnetic field of up to 200 mT, at temperatures of 296 and 140 K and 1.4 to 3.5 eV. The optical conductivity and MCD spectra show absorption peaks which are consistent with the interband electronic transitions for different positions of Mn, Ga, and C ions as confirmed by theoretical calculations of the spin-dependent density of electronic states. The well-known structural phase transition at 214 K is also seen in the changes of optical, magneto-optical and surface magnetic properties of Mn2GaC in our experiment.

Спектроскопия потерь энергии отраженных электронов γ-Fe-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

The reflection electron energy losses spectra, obtained in a wide primary electron energy range of 200 - 3000 eV, are investigated. From these experimental spectra, for each primary electron energy, the spectra of the inelastic scattering cross section of electrons are calculated as the dependence of the product of the inelastic electron mean free path and the differential inelastic scattering cross section of electrons on the electron energy loss. The analysis of the fine structure of the reflection electron energy losses was carried out by decomposing the electron energy losses spectra in the region of energy losses of valence electrons into elementary peaks. A relationship is established between each of their elementary peaks with single and multiple energy losses due to the excitation of bulk and surface plasmons and interband transitions of electrons from the valence band to free states above the Fermi level. The analysis of the obtained results was carried out on the basis of experimental and theoretical literature data on the band structure of  Fe2O3.

Shape control in seed-mediated synthesis of non-elongated Cu nanoparticles and their optical properties.

Shape and surface chemistry control in copper nanoparticle synthesis is an important research area due to a wide range of developing applications of this material in catalysis, energy conversion, sensing and many others. In addition to being an inexpensive and abundant metal, copper is an attractive photocatalyst due to its optical properties in the visible range. Here, we report a facile, tunable and sustainable methodology for synthesizing Pd-seeded Cu nanoparticles with various shapes, including cubes, spheres, raspberry-like particles and cages stabilized with a bilayer of a cationic surfactant in aqueous media. The experimental and theoretical examination of the optical response in the series of synthesized nanoparticles revealed that the low-energy extinction peak is associated with electronic interband transitions in the metal, in contrast to a widely spread attribution of this peak to a plasmonic response in Cu nanoparticles.

Фотолюминесценция композитов полиметилметакрилат / [(Zn,Cd)S : Mn,Cu,Eu

Nanoscale structures based on zinc and cadmium sulfides doped with Mn, Cu, and Eu ions were synthesized by the method of emerging reagents in a polymerizing medium of methyl methacrylate. The broadband photoluminescence of the compositions is associated with recombination processes at the defect levels of semiconductor structures. Narrow photoluminescence bands occur during electron transitions between the energy levels of Eu3+ ions. The excitation of photoluminescence occurs as a result of the interband transition and electron transitions to the levels of structural defects, as well as during self-absorption and energy transfer to the levels of Eu3+ ions.

Effects of oxidation on the localized surface plasmon resonance of Cu nanoparticles fabricated via vacuum coating

Cu is a kind of inexpensive plasmonic material but the applications based on the localized surface plasmon resonance (LSPR) of Cu nanoparticles (CuNPs) are limited. This is because CuNPs normally synthesized by chemical methods are prone to oxidization, and the absorption of Cu2O, CuO, the interband transition of electrons from 3d to 4s overlap the LSPR of CuNPs, making the LSPR of CuNPs difficult to be identified. Herein, ultra-thin Cu films composed of CuNPs were fabricated by vacuum coating where oxidization of Cu was expected weak. However, no strong LSPR in the UV–Visible absorption spectra can be seen. In order to understand the mechanism, the as-deposited ultra-thin Cu films were annealed in nitrogen, oxygen and 5% hydrogen diluted nitrogen to mimic oxygen insufficient and sufficient oxidation, and reduction ambient to obtain Cu2O, CuO and CuNPs respectively. By UV–Visible absorption spectroscopy, Field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and surface enhanced Raman spectroscopy, the LSPR of CuNPs, the absorption of Cu2O, CuO/Cu(OH)2 and interband transition were identified. The absorption spectra of the as-deposited ultra-thin Cu films, and their evolutions with the annealing temperature and time were explained. Finally, the oxygen sources are analyzed.

Synthesis and Photoluminescence of Nanostructures Based on Zinc, Cadmium, and Manganese Sulfides in a Polyacrylate Matrix

The photoluminescence of nanostructures based on zinc, cadmium, and manganese sulfides is studied in relation to the conditions of synthesis and doping in a (poly)methyl methacrylate (PMMA) medium. Photoluminescence excitation is defined by interband electronic transitions in the semiconductor structure, by the absorption of optical radiation energy at structural defects in the crystals, and by energy transfer to the excited energy levels of Mn2+ ions. The luminescence signal results from recombination processes at the levels of structural defects mainly at the surface of particles and from 4T1 → 6A1 electronic transitions between the intrinsic energy levels of Mn2+ ions. Based on variations observed in the photoluminescence and photoluminescence excitation spectra of PMMA/(Zn, Cd, Mn)S composites, some inferences about the structure of the particles are made. It is shown that their photoluminescence is influenced by the distribution of Mn2+ ions in the structure of the layers and at the surface of particles.

Discovering Electron-Transfer-Driven Changes in Chemical Bonding in Lead Chalcogenides (PbX, where X = Te, Se, S, O).

Understanding the nature of chemical bonding in solids is crucial to comprehend the physical and chemical properties of a given compound. To explore changes in chemical bonding in lead chalcogenides (PbX, where X= Te, Se, S, O), a combination of property-, bond-breaking-, and quantum-mechanical bonding descriptors are applied. The outcome of the explorations reveals an electron-transfer-driven transition from metavalent bonding in PbX (X = Te, Se, S) to iono-covalent bonding in β-PbO. Metavalent bonding is characterized by adjacent atoms being held together by sharing about a single electron (ES ≈ 1) and small electron transfer (ET). The transition from metavalent to iono-covalent bonding manifests itself in clear changes in these quantum-mechanical descriptors (ES and ET), as well as in property-based descriptors (i.e., Born effective charge (Z*), dielectric function ε(ω), effective coordination number (ECoN), and mode-specific Grüneisen parameter (γTO )), and in bond-breaking descriptors. Metavalent bonding collapses if significant charge localization occurs at the ion cores (ET) and/or in the interatomic region (ES). Predominantly changing the degree of electron transfer opens possibilities to tailor material properties such as the chemical bond (Z*) and electronic (ε∞ ) polarizability, optical bandgap, and optical interband transitions characterized by ε2 (ω). Hence, the insights gained from this study highlight the technological relevance of the concept of metavalent bonding and its potential for materials design.

INFLUENCE OF LEAD IONS ON PHOTOLUMINESCENT PROPERTIES OF POLYMETHYL METACRYLATE /(Zn,Pb)S/CdS/(Zn,Pb)S COMPOSITIONS

The paper presents the results of a study of the effect of lead ions on the photoluminescent properties of core / shell / shell nanostructures based on zinc and cadmium sulfides synthesized in situ during the formation of polyacrylate composition. ZnS/CdS/ZnS and (Zn,Pb)S/CdS/(Zn,Pb)S nanoparticles were obtained by colloidal synthesis in a medium of (poly) methyl methacrylate. Colloidal solutions are solidified to a glassy state by radical thermal polymerization of methyl methacrylate in a block. To initiate the polymerization, benzoyl peroxide was introduced into the solutions in an amount of 0.1% by weight of methyl methacrylate. The conclusion on the formation of nanosized particles of complex composition in the polymer matrix is made by comparing the luminescence and luminescence spectra of the compositions containing ZnS and CdS particles and their structure. In the photoluminescence spectra of the (poly) methyl methacrylate/(ZnS/CdS/ZnS) composition, two complex bands associated with crystal lattice defects of ZnS particles (380 - 530 nm) and CdS (530 - 840 nm) were detected. The excitation of photoluminescence occurs as a result of interband transitions of electrons in each of the semiconductor layers, as well as during transitions of electrons from the conduction band to defect levels located in the forbidden band of the semiconductor structure. In the luminescence excitation spectrum, they correspond to bands of various intensities in the range of 300–420 nm and 300–480 nm. From the overlap of the ZnS luminescence bands and the excitation of CdS luminescence, it was concluded that the radiation of the core is absorbed by the CdS coating layer and that energy is transferred from the levels of structural defects of ZnS to the levels of structural defects of CdS at the layer boundary. Doping of zinc sulfide layers with lead ions leads to a change in the luminescence spectrum in the region of 380 - 530 nm and the disappearance of the CdS luminescence band in the range of 530 - 840 nm. The observed changes are associated with defects that create Pb2+ ions in the crystal lattice of the ZnS outer shell and the screening of the (Zn,Pb)S shell of the inner layers from the penetration of exciting radiation.

Synthesis, physicochemical and optical characterizations of a new isatin hydrazone derivative and its ZnO-complex for potential energy conversion and storage applications

We synthesized a novel isatin hydrazone derivative 3-(((E)-3-ethoxy-2-hydroxybenzylidene)hydrazineylidene)indolin-2-one (IL) via condensation of isatin hydrazide with 3-ethoxy-salicylaldehyde in methanol. Then ZnO was chelated with the ligand (IL) to produce a novel ZnOIL complex. The characteristics of the IL and ZnOIL compounds were compared using various techniques, including nuclear magnetic resonance, ultraviolet–visible, infrared, and mass spectroscopy, and elemental analyses to confirm their chemical compositions. The optical properties of the IL and ZnOIL thin films were studied based on spectroscopic measurements of their absorbance, transmittance, and reflectance within the range of 300–2500 nm. The optical constants, dielectric parameters, and dispersion energies were determined and interpreted. The electronic inter-band transitions of the IL and ZnOIL films were characterized by indirect allowed transitions. The value of the optical band gap (Eg) was determined as 2.14 eV for the IL film, but chelation of IL with ZnO decreased Eg to 2.02 eV. The surface and volume energy loss functions were lower for ZnOIL compared with IL. The structural and the optical features of ZnOIL may be tuned, and thus it is a promising candidate material for applications in energy conversion and storage devices.

Optical absorption window in Na3Bi based three-dimensional Dirac electronic system

We present a detailed theoretical study of the optoelectronic properties of a Na3Bi based three-dimensional Dirac electronic system (3DDES). The optical conductivity is evaluated using the energy-balance equation derived from a Boltzmann equation, where the electron Hamiltonian is taken from a simplified k⋅p approach. We find that for short-wavelength irradiation, the optical absorption in Na3Bi is mainly due to inter-band electronic transitions. In contrast to the universal optical conductance observed for graphene, the optical conductivity for Na3Bi based 3DDES depends on the radiation frequency but not on temperature, carrier density, and electronic relaxation time. In the radiation wavelength regime of about 5μm<λ<200 μm, an optical absorption window is found. This is similar to what is observed in graphene. The position and width of the absorption window depend on the direction of the light polarization and sensitively on temperature, carrier density, and electronic relaxation time. Particularly, we demonstrate that the inter-band optical absorption channel can be switched on and off by applying the gate voltage. This implies that similar to graphene, Na3Bi based 3DDES can also be applied in infrared electro-optical modulators. Our theoretical findings are helpful in gaining an in-depth understanding of the basic optoelectronic properties of recently discovered 3DDESs.