Zinc Gallate Spinel Dielectric Function, Band-to-Band Transitions, and Γ-Point Effective Mass Parameters

We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75 eV) into the vacuum ultraviolet (8.5 eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Γ-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24me) at the bottom of the Γ-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Γ-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8 meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements (√ε∞=1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)]

Brillouin Zone Center Phonon Modes in ZnGa\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e

Infrared-active lattice mode properties of melt-grown high-quality single bulk crystals of ZnGa2O4 are investigated by combined spectroscopic ellipsometry and density functional theory computation analysis. The normal spinel structure crystals are measured by spectroscopic ellipsometry at room temperature in the range of 100 cm–1–1200 cm–1. The complex-valued dielectric function is determined from a wavenumber-by-wavenumber approach, which is then analyzed by the four-parameter semi-quantum model dielectric function approach augmented by impurity mode contributions. We determine four infrared-active transverse and longitudinal optical mode pairs, five localized impurity mode pairs, and the high frequency dielectric constant. All four infrared-active transverse and longitudinal optical mode pairs are in excellent agreement with results from our density functional theory computations. With the Lyddane–Sachs–Teller relationship, we determine the static dielectric constant, which agrees well with electrical capacitance measurements performed on similarly grown samples. We also provide calculated parameters for all Raman-active and for all silent modes and, thereby, provide a complete set of all symmetry predicted Brillouin zone center modes

Tunable plasmonic resonances in highly porous nano-bamboo Si-Au superlattice-type thin films

We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. The superlattice-type columns resemble bamboo structures where smaller column sections of gold form junctions sandwiched between larger silicon column sections (“nano-bamboo”). We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous nano-bamboo structures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of the nano-bamboo structure. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of the nano-bamboo structures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water

Anisotropic Dielectric Functions, Band-to-Band Transitions, and Critical Points in \u3cem\u3eα\u3c/em\u3e-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an α-Ga2O3 thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on m-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73 eV to 8.75 eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with M0-type van Hove singularities for polarization perpendicular to the c axis, E0,⊥=5.46(6) eV and E0,⊥=6.04(1) eV, and one direct band-to-band transition with M1-type van Hove singularity for polarization parallel to E0,∥=5.44(2) eV. We further identify excitonic contributions with a small binding energy of 7 meV associated with the lowest ordinary transition and a hyperbolic exciton at the M1-type critical point with a large binding energy of 178 meV

Optical Phonon Modes, Static and High-Frequency Dielectric Constants, and Effective Electron Mass Parameter in Cubic In\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

A complete set of all optical phonon modes predicted by symmetry for bixbyite structure indium oxide is reported here from a combination of far-infrared and infrared spectroscopic ellipsometry, as well as first principles calculations. Dielectric function spectra measured on high quality, marginally electrically conductive melt grown single bulk crystals are obtained on a wavelength-by-wavelength (also known as point-by-point) basis and by numerical reduction of a subtle free charge carrier Drude model contribution. A four-parameter semi-quantum model is applied to determine all 16 pairs of infrared-active transverse and longitudinal optical phonon modes, including the high-frequency dielectric constant, ε∞=4.05±0.05. The Lyddane–Sachs–Teller relation then gives access to the static dielectric constant, εDC=10.55±0.07. All experimental results are in excellent agreement with our density functional theory calculations and with previously reported values, where existent. We also perform optical Hall effect measurements and determine for the unintentionally doped n-type sample a free electron density of n = (2.81±0.01)×1017 cm-3, a mobility of μ=(112±3) cm2/(Vs), and an effective mass parameter of (0.208±0.006)me. Density and mobility parameters compare very well with the results of electrical Hall effect measurements. Our effective mass parameter, which is measured independently of any other experimental technique, represents the bottom curvature of the Γ point in In2O3 in agreement with previous extrapolations. We use terahertz spectroscopic ellipsometry to measure the quasi-static response of In2O3, and our model validates the static dielectric constant obtained from the Lyddane–Sachs–Teller relation

Infrared Dielectric Functions and Brillouin Zone Center Phonons of \u3cem\u3eα\u3c/em\u3e-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e compared to \u3cem\u3eα\u3c/em\u3e-Al\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e

We determine the anisotropic dielectric functions of rhombohedral α-Ga2O3 by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high-frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for α-Al2O3, the binary parent compound for the emerging alloy of α-(AlxGa1−x)2O3, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations, we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single-mode or more complex mode behaviors

Chamber-based insights into the factors controlling epoxydiol (IEPOX) secondary organic aerosol (SOA) yield, composition, and volatility

We present measurements utilizing the Filter Inlet for Gases and Aerosols (FIGAERO) applied to chamber measurements of isoprene-derived epoxydiol (IEPOX) reactive uptake to aqueous acidic particles and associated secondary organic aerosol (SOA) formation. Similar to recent field observations with the same instrument, we detect two molecular components desorbing from the IEPOX SOA in high abundance: C5H12O4 and C5H10O3. The thermal desorption signal of the former, presumably 2-methyltetrols, exhibits two distinct maxima, suggesting it arises from at least two different SOA components with significantly different effective volatilities. Isothermal evaporation experiments illustrate that the most abundant component giving rise to C5H12O4 is semi-volatile, undergoing nearly complete evaporation within 1 h while the second, less volatile component remains unperturbed and even increases in abundance. We thus confirm, using controlled laboratory studies, recent analyses of ambient SOA measurements showing that IEPOX SOA is of very low volatility and commonly measured IEPOX SOA tracers such as C5H12O4 and C5H10O3, presumably 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diols, result predominantly from thermal decomposition in the FIGAERO-CIMS. We infer that other measurement techniques using thermal desorption or prolonged heating for analysis of SOA components may also lead to reported 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diol structures. We further show that IEPOX SOA volatility continues to evolve via acidity-enhanced accretion chemistry on the timescale of hours, potentially involving both 2-methyltetrols and organosulfates.Peer reviewe

Sex Determination:Why So Many Ways of Doing It?

Sexual reproduction is an ancient feature of life on earth, and the familiar X and Y chromosomes in humans and other model species have led to the impression that sex determination mechanisms are old and conserved. In fact, males and females are determined by diverse mechanisms that evolve rapidly in many taxa. Yet this diversity in primary sex-determining signals is coupled with conserved molecular pathways that trigger male or female development. Conflicting selection on different parts of the genome and on the two sexes may drive many of these transitions, but few systems with rapid turnover of sex determination mechanisms have been rigorously studied. Here we survey our current understanding of how and why sex determination evolves in animals and plants and identify important gaps in our knowledge that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying the evolution of sex determination

Growth of WS\u3csub\u3e2\u3c/sub\u3e Thin Photovoltaic Films

Through research for a material that is low cost, earth abundant, and highly efficient, Tungsten Sulfide (WS2) has been identified as a promising solution. From literature, it has been shown that WS2 possesses a band gap of 1.9 eV, which is ideal for a top layer heterostructure cell. While this is a favorable value, other properties needed to understand its value as a photovoltaic material are unknown. Through research, WS2 films will be grown through a sputtering deposition process and electrical characterization measurements will be performed to study these missing properties