Synthesis and Structural Characterization of p-Carboranylamidine Derivatives.

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = iPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C2H10B10[C(NiPr)2Li(DME)]2 (2) and p-C2H10B10[C(NCy)2Li(THF)2]2 (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me3SiCl) provided the silylated neutral bis(amidine) derivatives p-C2H10B10[C{iPrN(SiMe3)}(=NiPr)]2 (4) and p-C2H10B10[C{CyN(SiMe3)}(=NCy)]2 (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

Tackling Disorder in γ-Ga2 O3.

Ga2 O3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga2 O3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga2 O3 presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure-electronic-structure relationship. Here, density functional theory is used in combination with a machine-learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga2 O3 . The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure.

Small compound - big colors: synthesis and structural investigation of brightly colored alkaline earth metal 1,3-dimethylviolurates.

A series of brightly colored alkaline earth metal 1,3-dimethylviolurates M(Me2Vio)2 have been prepared and fully characterized. The title compounds AE(Me2Vio)2·nH2O (AE = Mg, n = 6 (3); AE = Ca, n = 8 (4), AE = Sr, n = 6 (5); AE = Ba, n = 4 (6)) were obtained by neutralizing 1,3-dimethylvioluric acid monohydrate (=H(Me2Vio)·H2O; 2) with 0.5 equiv. of the corresponding metal dihydroxides AE(OH)2. The hair-like appearance of the Sr derivative 5 prevented the growth of single-crystals. This problem could be solved by crystallizing the crown ether derivative Sr(Me2Vio)2(18-crown-6) (5a). The isolated salts exhibit intense colors ranging from red to purple. Various attempts to prepare the beryllium derivative Be(Me2Vio)2 failed. Instead, work-up of the reaction mixtures provided pink crystals of a new modification of 2 formulated as [H3O][Me2Vio] (2b) as shown by an X-ray diffraction study. An unexpected oxidation reaction of the barium salt Ba(Me2Vio)2 led to formation of the novel mixed-anion salt Ba(Me2Vio)(Me2NO2Barb)·2H2O (8, Me2NO2Barb- = 1,3-dimethyl-5-nitrobarbiturate anion). Compound 8 could also be synthesized deliberately by treatment of Ba(OH)2 with a 1 : 1 mixture of 2 and 1,3-dimethyl-5-nitrobarbituric acid (7, =H(Me2NO2Barb)·H2O). All new compounds were fully characterized by their IR, Raman, NMR (1H, 13C{1H}) and UV-vis spectra as well as elemental analyses. Single-crystal X-ray diffraction studies revealed that the solid-state structures of compounds 3, 4, 5a and 6 are governed by the typical coordination behavior of the alkaline-earth metals, i.e. increasing coordination numbers and a decreasing degree of hydration when going from Mg to Ba. The dimensions of the structures range from hydrogen-bonded ions (3) over monomeric, neutral complex molecules (4, 5a), to polymeric networks (6). The successful isolation of the mixed-anion barium salt 8 adds a new facet to the coordination chemistry of violurate and related ligands.

Rubidium and Cesium Enediamide Complexes Derived from Bulky 1,4-Diazadienes.

The first rubidium and cesium enediamide complexes based on bulky 1,4-diaza-1,3-diene ligands (DADs) have been prepared by metalation of either 1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, = H2DADDipp) or 1,4-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene (2, = Me2DADDipp) with an excess of Rb or Cs metals in coordinating solvents such as tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME). All new complexes were fully characterized by spectroscopic and analytical methods as well as single-crystal X-ray diffraction studies.

Catenated and spirocyclic polychalcogenides from potassium carbonate and elemental chalcogens.

The reaction of potassium carbonate with elemental sulfur or selenium in acetone in the presence of [PPN]Cl (PPN = (Ph3P)2N) produces catena-[S12]2-, the longest structurally characterised polysulfide dianion, or spiro-[Se11]2- as ion-separated [PPN]+ salts.

Multivalence-band calculation of the excitonic dielectric function for hexagonal GaN.

The complex dielectric function of hexagonal gallium nitride (α-GaN) is obtained from a numerical solution of the excitonic Schrödinger equation taking into account the full 6 × 6 valence-band structure. The valence-band parametrization includes anisotropy, nonparabolicity, and the coupling of angular-momentum eigenstates. Spectra of excitonic eigenfunctions are obtained from a time-dependent initial-value problem, which is solved via an exponential split-operator method. In particular, we calculate the dielectric function and the reflectivity of a-plane GaN with polarization vectors parallel and perpendicular to the c-axis of the crystal. The simulated reflection spectra are in excellent agreement with recent experimental data and allow the unambiguous identification of the experimentally observed excitonic resonances. The binding energies of the FXA, FXB, and FXC excitons found in our calculation differ by up to 27%, depending on the chosen parameter set. An important consequence of this observation is that the experimentally observed splittings of the excitons cannot be used for the parametrization of the valence band near the Γ-point, but need to be corrected by the differences of the binding energies. This is of general relevance for all spectroscopic measurements in semiconductors with a wide bandgap.

Si rib waveguide photodetector with an ordered array of Ge islands for 1.5 microm.

This work demonstrates a rib waveguide photodetector based on a vertical Si p-i-n junction with Ge islands operating in the spectral region around lambda=1.55 microm at room temperature. A vertical stack of four layers of Ge islands is grown by molecular beam epitaxy on a silicon-on-insulator. Each layer is organized in a two-dimensional square grid with a period of 460 nm. The spectral response of the detector extends well beyond 1.6 mum at 300 K. The absorption length of approximately 135 microm (at 1/e decrease of intensity) at lambda=1.55 microm along the waveguide allows for relatively small-size devices for all-on-one-platform integration.