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1. Crystal structure, stability, spectroscopy, electronic structure, and ultrafast excited-state dynamics of the elusive iron(II) phthalocyanine axially coordinated with DABCO ligands

   https://doi.org/10.1142/S1088424623500670  

   Nevonen, Dustin E; Schaffner, Jacob; Hanrahan, Paul; Shepit, Michael; van_Lierop, Johan; Blank, David A; Nemykin, Victor N (October 2023, Journal of Porphyrins and Phthalocyanines)

   The elusive PcFe(DABCO)₂(Pc = phthalocyaninato(2-) ligand; DABCO = 1,4-diazabicyclo[2.2.2]octane) complex was prepared and characterized by UV-Vis, MCD,¹H NMR, and Mössbauer spectroscopies. The X-ray crystal structure of this complex indicates the longest Fe-N(DABCO) bond distance among all known PcFeL₂complexes with nitrogen donors as the axial ligands. The target compound is only stable in the presence of large access of the axial ligand and rapidly converts into the (PcFe)₂O [Formula: see text]-oxo dimer even at a modest temperature. The electronic structure of the PcFe(DABCO)₂complex was elucidated by DFT and TDDFT methods. The DFT calculations predicted a very small singlet-triplet gap in this compound. The femtosecond transient absorption spectroscopy is indicative of extremely fast ([Formula: see text]200 fs) deactivation of the first excited state in PcFe(DABCO)₂with a lack of formation of the long-lived low-energy triplet state. 

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2. Identifying charge-transfer and trip–multiplet states in Co( i ), Co( ii ), and Co( iii ) phthalocyanines using (magneto)optical spectroscopy and (TD)DFT calculations

   https://doi.org/10.1039/d5dt00628g  

   Muldowney, Breanna E; Nevonen, Dustin E; Jeaydi, Towhidi Illius; Ziegler, Christopher J; McNicholas, Brendon J; Nemykin, Victor N (January 2025, Dalton Transactions)

   The electronic structures, charge-transfer, and triplet–multiplet transitions in cobalt(i), cobalt(ii), and cobalt(iii) phthalocyanines were investigated in detail by UV-vis-NIR, MCD, DFT, and TDDFT methods. 

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3. Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α -(Al _x Ga _1−x ) ₂ O ₃ (0≤x≤1)

   https://doi.org/10.1063/5.0087602  

   Hilfiker, Matthew; Kilic, Ufuk; Stokey, Megan; Jinno, Riena; Cho, Yongjin; Xing, Huili Grace; Jena, Debdeep; Korlacki, Rafał; Schubert, Mathias (August 2022, Applied Physics Letters)

   Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al x Ga 1− x ) 2 O 3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga 2 O 3 and α-Al 2 O 3 , respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga 2 O 3 whereas for α-Al 2 O 3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M 1 type in α-Ga 2 O 3 to M 0 type van Hove singularity in α-Al 2 O 3 . 

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4. A combined helium atom scattering and density-functional theory study of the Nb(100) surface oxide reconstruction: Phonon band structures and vibrational dynamics

   https://doi.org/10.1063/5.0085653  

   McMillan, Alison A.; Thompson, Caleb J.; Kelley, Michelle M.; Graham, Jacob D.; Arias, Tomás A.; Sibener, S. J. (March 2022, The Journal of Chemical Physics)

   Helium atom scattering and density-functional theory (DFT) are used to characterize the phonon band structure of the (3 × 1)-O surface reconstruction of Nb(100). Innovative DFT calculations comparing surface phonons of bare Nb(100) to those of the oxide surface show increased resonances for the oxide, especially at higher energies. Calculated dispersion curves align well with experimental results and yield atomic displacements to characterize polarizations. Inelastic helium time-of-flight measurements show phonons with mixed longitudinal and shear-vertical displacements along both the ⟨[Formula: see text]⟩, [Formula: see text] and ⟨[Formula: see text]⟩, [Formula: see text] symmetry axes over the entire first surface Brillouin zone. Force constants calculated for bulk Nb, Nb(100), and the (3 × 1)-O Nb(100) reconstruction indicate much stronger responses from the oxide surface, particularly for the top few layers of niobium and oxygen atoms. Many of the strengthened bonds at the surface create the characteristic ladder structure, which passivates and stabilizes the surface. These results represent, to our knowledge, the first phonon dispersion data for the oxide surface and the first ab initio calculation of the oxide’s surface phonons. This study supplies critical information for the further development of advanced materials for superconducting radiofrequency cavities. 

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5. Elevated temperature spectroscopic ellipsometry analysis of the dielectric function, exciton, band-to-band transition, and high-frequency dielectric constant properties for single-crystal ZnGa ₂ O ₄

   https://doi.org/10.1063/5.0087623  

   Hilfiker, Matthew; Williams, Emma; Kilic, Ufuk; Traouli, Yousra; Koeppe, Nate; Rivera, Jose; Abakar, Assya; Stokey, Megan; Korlacki, Rafał; Galazka, Zbigniew; et al (March 2022, Applied Physics Letters)

   We report the elevated temperature (22 °C [Formula: see text]  T [Formula: see text] 600 °C) dielectric function properties of melt grown single crystal ZnGa₂O₄using a spectroscopic ellipsometry approach. A temperature dependent Cauchy dispersion analysis was applied across the transparent spectrum to determine the high-frequency index of refraction yielding a temperature dependent slope of 3.885(2) × 10⁻⁵ K⁻¹. A model dielectric function critical point analysis was applied to examine the dielectric function and critical point transitions for each temperature. The lowest energy M₀-type critical point associated with the direct bandgap transition in ZnGa₂O₄is shown to red-shift linearly as the temperature is increased with a subsequent slope of −0.72(4) meV K⁻¹. Furthermore, increasing the temperature results in a reduction of the excitonic amplitude and increase in the exciton broadening akin to exciton evaporation and lifetime shortening. This matches current theoretical understanding of excitonic behavior and critically provides justification for an anharmonic broadened Lorentz oscillator to be applied for model analysis of excitonic contributions. 

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