Strong Interband Absorption Research Articles

Tapping the Light Emitting Potential of Chalcogenide Perovskite SrHfS3 via Eu2+ Doping

AbstractChalcogenide perovskites are an emerging family of perovskite materials, which possess excellent optoelectronic properties comparable with halide perovskites, avoiding the drawbacks of toxicity and poor stability. Due to the superiorities of strong light absorption, direct bandgaps, excellent carrier transport abilities, high defect tolerance, robust stability, and non‐toxicity, chalcogenide perovskites have been extensively studied as photovoltaic materials. However, the light emitting potential of chalcogenide perovskites is ignored or less explored. In this work, an Eu2+ doping strategy is proposed to activate the light emitting abilities of chalcogenide perovskite. The synthesized SrHfS3:Eu2+ powder exhibits intense red emission of Eu2+ through the relaxation of 4f65d1 to 4f75d0. The strong interband absorption of SrHfS3, as a distinct character of chalcogenide perovskite, harvests the optical energy and funnels it to the luminescent center Eu2+. Besides, SrHfS3:Eu2+ is characterized by superior semiconducting properties, suggesting its strong potential as suitable emitters in light‐emitting diodes. Moreover, the obtained SrHfS3:Eu2+ powder and its luminescence properties remain stable under ultraviolet irradiation, water, and high temperature testing. The above observations open a new frontier for exploring luminescent chalcogenide perovskites, highlighting their great potential in lighting and displays, rather than being limited to photovoltaic devices.

Optical properties and electronic structures of intrinsic gapped metals: Inverse materials design principles for transparent conductors

Traditional solid-state physics has long correlated the optical properties of materials with their electronic structures. However, recent discoveries of intrinsic gapped metals have challenged this classical view. Gapped metals possess electronic properties distinct from both metals and insulators, with a large concentration of free carriers without any intentional doping and an internal bandgap. This unique electronic structure makes gapped metals potentially superior to materials designed by intentional doping of the wide bandgap insulators. Despite their promising applications, such as transparent conductors, designing gapped metals for specific purposes remains challenging due to the lack of understanding of the correlation between their electronic band structures and optical properties. This study focuses on representative examples of gapped metals and demonstrates the cases of (i) gapped metals (e.g., CaN2) with strong intraband absorption in the visible range, (ii) gapped metals (e.g., SrNbO3) with strong interband absorption in the visible range, and (iii) gapped metals (e.g., Sr5Nb5O17) that are potential transparent conductors. We explore the complexity of identifying potential gapped metals for transparent conductors and propose inverse materials design principles for discovering next-generation transparent conductors.

Efficiency of Hot-Electron Generation in Plasmonic Nanocrystals with Complex Shapes: Surface-Induced Scattering, Hot Spots, and Interband Transitions

The generation of hot electrons is an intrinsic property of all plasmonic nanocrystals under illumination. However, the number of such excited electrons will strongly depend on the shape, material, and excitation wavelength. In this paper, we develop a practical self-consistent formalism to describe the generation of energetic electrons in a plasmonic nanocrystal with an arbitrary shape. We apply our formalism to gold nanospheres, nanorods, and nanostars. Among the investigated shapes, the nanostar geometry demonstrates the best performance, with an internal energy efficiency of ∼25%. This superior capability of hot-electron generation in the nanostars comes from the following factors: strong hot spots in the red spectral region, isotropic optical response, and the absence of interband transitions at the plasmonic resonance. Spherical gold nanocrystals show strong interband absorption at the plasmon resonance, and the related efficiency of the generation of hot holes in the d band can reach a level of 70%. By analyzing the energy performance of nanocrystals under CW illumination, we show that the most relevant parameter to consider is the rate of hot-electron generation, whereas the steady-state numbers of thermalized and nonthermalized electrons play secondary roles. The physical principles formulated in this study can be used to design a variety of plasmonic nanomaterials for applications in photocatalysis and photodetection.

Anisotropic optical properties of highly doped rutile SnO2: Valence band contributions to the Burstein-Moss shift

The interband absorption of the transparent conducting semiconductor rutile stannic oxide (SnO2) is investigated as a function of increasing free electron concentration. The anisotropic dielectric functions of SnO2:Sb are determined by spectroscopic ellipsometry. The onsets of strong interband absorption found at different positions shift to higher photon energies with increasing free carrier concentration. For the electric field vector parallel to the optic axis, a low energy shoulder increases in prominence with increasing free electron concentration. We analyze the influence of different many-body effects and can model the behavior by taking into account bandgap renormalization and the Burstein-Moss effect. The latter consists of contributions from the conduction and the valence bands which can be distinguished because the nonparabolic conduction band dispersion of SnO2 is known already with high accuracy. The possible originsof the shoulder are discussed. The most likely mechanism is identified to be interband transitions at |k| > 0 from a dipole forbidden valence band.

Phage-directed synthesis of copper sulfide: structural and optical characterization

The growth of crystalline copper sulfide using a viral template was investigated using sequential incubation in CuCl2 and Na2S precursors. Non-specific electrostatic attraction between a genetically-modified M13 bacteriophage and copper cations in the CuCl2 precursor caused phage agglomeration and bundle formation. Following the addition of Na2S, polydisperse nanocrystals 2–7 nm in size were found along the length of the viral scaffold. The structure of the copper sulfide material was identified as cubic anti-fluorite type Cu1.8S, space group . Strong interband absorption was observed within the ultraviolet to visible range with an onset near 800 nm. Furthermore, free carrier absorption, associated with the localized surface plasmon resonance of the copper sulfide nanocrystals, was seen in the near infrared with absorbance maxima at 1060 nm and 3000 nm, respectively.

Optical and magneto-optical properties and electronic structures of single-crystallineRAl2(R=Y, La, Ce, Pr, and Lu)

The diagonal optical conductivity spectra of single crystals of $R{\mathrm{Al}}_{2}$ $(R=\mathrm{Y},$ La, Ce, Pr, and Lu) were measured at room temperature by spectroscopic ellipsometry in the $1.5--5.6\ensuremath{-}\mathrm{eV}$ range. All the compounds exhibit two strong interband absorption peaks at about 1.8 and 3.6 eV for ${\mathrm{YAl}}_{2}$ and ${\mathrm{LuAl}}_{2},$ and at about 2.0 and 3.0 eV for ${\mathrm{LaAl}}_{2},$ ${\mathrm{CeAl}}_{2},$ and ${\mathrm{PrAl}}_{2}.$ Such differences in the second peak position appear in the theoretical optical conductivity spectra calculated from their band structures obtained by the tight-binding linear-muffin-tin-orbitals method. Most of the contributions to the two peaks in ${\mathrm{LaAl}}_{2}$ are from the p and d states, i.e., $\stackrel{\ensuremath{\rightarrow}}{p}d$ and $\stackrel{\ensuremath{\rightarrow}}{d}p$ transitions, while those involving f states are negligible. These results suggest that f character near ${E}_{F}$ for ${\mathrm{LaAl}}_{2},$ ${\mathrm{CeAl}}_{2},$ and ${\mathrm{PrAl}}_{2}$ distorts their conduction bands significantly through hybridization, leading to different optical spectra compared to those of ${\mathrm{YAl}}_{2}$ and ${\mathrm{LuAl}}_{2}.$ The magneto-optical properties of ${\mathrm{CeAl}}_{2}$ and ${\mathrm{PrAl}}_{2}$ were measured at low temperatures. The Kerr rotation $({\ensuremath{\Theta}}_{K})$ and ellipticity $({\ensuremath{\epsilon}}_{K})$ for both compounds show similar spectral variations with maximum ${\ensuremath{\Theta}}_{K}$ of $0.35\ifmmode^\circ\else\textdegree\fi{}$ and $0.55\ifmmode^\circ\else\textdegree\fi{}$ at about 2.1 eV for ${\mathrm{CeAl}}_{2}$ and ${\mathrm{PrAl}}_{2},$ respectively. The evaluated off-diagonal conductivity spectra of the two compounds are also similar, with two structures at about 2.1 and 3.8 eV for ${\mathrm{CeAl}}_{2}$ and 2.1 and 3.4 eV for ${\mathrm{PrAl}}_{2}.$ The energy difference in the second structures is interpreted as due to the different conduction-band structures of the two compounds caused by different hybridization strengths of their f states with conduction bands, because of the difference in their degree of localization.

New aspects in the oxygen related satellite observed in photoemission for high T c superconducting oxides

A peak in the photoemission process is consistently observed at approximately 9 eV for the superconducting oxides with the feature being absent for CuO. A feature near 12 eV is observed for all the Cu +2 superconducting oxides. We find that the experimental spectra can be accounted for quantitatively with two-electron excitations in addition to the usual one-electron emission. In the two-electron process, a single photon causes one electron to be excited by approximately 8 eV due to a strong interband absorption while the second electron is photoemitted. The shake-up spectra at 9 and 12 eV then reflect the local densities of Cu and O states shifted by approximately 8 eV due to the second excitation. Our model is an alternative to the 5 parameters model based on the Cu-O atomic-like excitations, where it is assumed the 9 eV peak is spurious. Virtual shake-up transitions can be used as the coupling mechanism for the metallic oxide superconductors.

Temperature dependence of the refractive-index dispersion in amorphous germanium at elevated temperatures

Refractive-index dispersion in amorphous germanium was measured in the vicinity of the optical gap (0.2-1.2 eV) and for the first time at elevated temperatures (25\ifmmode^\circ\else\textdegree\fi{}C-400\ifmmode^\circ\else\textdegree\fi{}C). The results at each temperature follow the dispersion of a single oscillator, whose resonance energy and strength decrease linearly with temperature. However, the magnitude of these variations differs markedly (e.g., by a factor of 2 or more, respectively) from temperature derivatives of band gaps or of electron density. It is apparent that in small gap materials, such as amorphous germanium, temperature dependences derived from dispersion do not reflect the temperature dependence of specific interband transitions (within the framework of the single-oscillator model). A new model is developed which accounts for the temperature dependence of the dispersion by replacing the single oscillator with two other interband electronic oscillators. The stronger of these two, which represents the majority of interband transitions, is located at the maximum of the optical transition strength. Assuming that its oscillator strength is close to unity, and that the temperature derivative of its resonance energy is identical with that of the Penn gap, the parameters of the second oscillator are obtained by fitting to the data. This is a weak oscillator located closely above the optical gap thus representing the low-energy portion of the ${\ensuremath{\epsilon}}_{2}$ spectrum. Moreover, the temperature derivative of its resonance energy acquires a value similar to that of the optical gap. In this model the dispersion is sensitive to the location of the weak oscillator due to the low value of its resonance energy and its proximity to the spectral region of interest. The dispersion is similarly sensitive to the location of the low-energy portion of ${\ensuremath{\epsilon}}_{2}$, as obtained by considering its moments. Thus in crystalline germanium, where the weak oscillator and the onset of strong interband absorption lie at higher energies than in amorphous germanium, a smaller temperature derivative of the single-oscillator energy is expected. This prediction is confirmed by experiment. It is concluded that using band-gap derivatives, the two-oscillator model may be utilized to calculate the temperature dependence of the dispersion or vice versa.

Optical properties of single crystals of some rare-earth trifluorides, 5-34 eV

The reflectances of single crystals of the trifluorides of La, Ce, Pr, Nd, Dy, and of polycrystalline Gd${\mathrm{F}}_{3}$ were measured in the 10-34-eV region, along with 5-9-eV transmission measurements on films of these materials. Localized $4f\ensuremath{\rightarrow}5d$, $6s$ transitions on the rare-earth ions give rise to the absorption below 10 or 11 eV. Strong interband absorption gives anisotropic reflectance peaks in the 10-15-eV region, presumably from ${\mathrm{F}}^{\ensuremath{-}}$ $2p$ valence bands to lanthanide $5d$, $6s$ (and other states) conduction bands, accounting for about one-third of the expected ${\mathrm{F}}^{\ensuremath{-}}$ $2p$ osillator strength. Transitions from lanthanide $5p$ levels cause two types of spectra beginning around 20 eV: transitions of the $5p\ensuremath{\rightarrow}5d$ type, apparently localized on lanthanide ions, but with some an isotropy; and transitions of $5p$ electrons to the conduction band, giving a peak of very high reflectance at energies above 27 eV.