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1. Determination of the Complex Refractive Index of GaSb _1−x Bi _x by Variable‐Angle Spectroscopic Ellipsometry

   https://doi.org/10.1002/pssa.202400017  

   McElearney, John; Grossklaus, Kevin; Menasuta, T_Pan; Vandervelde, Thomas (July 2024, physica status solidi (a))

   Variable‐angle spectroscopic ellipsometry is used to determine the room temperature complex refractive index of molecular beam epitaxy grown GaSb_1−xBi_xfilms withx ≤ 4.25% over a spectral range of 0.47–6.2 eV. By correlating to critical points in the extinction coefficientk, the energies of several interband transitions are extracted as functions of Bi content. The observed change in the fundamental bandgap energy (E₀, −36.5 meV per %Bi) agrees well with previously published values; however, the samples examined here show a much more rapid increase in the spin‐orbit splitting energy (Δ₀, +30.1 meV per Bi) than previous calculations have predicted. As in the related GaAsBi, the energy of transitions involving the top of the valence band are observed to have a much stronger dependence on Bi content than those that do not, suggesting the valence band maximum is most sensitive to Bi alloying. Finally, the effects of surface droplets on both the complex refractive index and the critical point energies are examined. 

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2. Doping and Anisotropy–Dependent Electronic Transport in Chalcogenide Perovskite CaZrSe ₃ for High Thermoelectric Efficiency

   https://doi.org/10.1002/adts.201900060  

   Osei‐Agyemang, Eric; Enninful_Adu, Challen; Balasubramanian, Ganesh (July 2019, Advanced Theory and Simulations)

   Abstract The potential of an environmentally friendly and emerging chalcogenide perovskite CaZrSe₃for thermoelectric applications is examined. The orthorhombic phase of CaZrSe₃has an optimum band gap (1.35–1.40 eV) for single‐junction photovoltaic applications. The predictions reveal that CaZrSe₃possesses an absorption coefficient of ≈4 × 10⁵cm⁻¹at photon energies of 2.5 eV with an early onset of optical absorption (≈0.2 eV) well below the optimum band gap. Seebeck coefficient,S, is inversely proportional to the carrier mobility as the calculated average effective mass for electrons is higher than for holes;p‐type doping enhances the electrical conductivity, σ. The electronic thermal conductivityκ_eremains low at all temperatures. The upper limit of the thermoelectric figure of merit (ZT_e) attains ≈1.0 when doped at specific chemical potentials, while a high Seebeck coefficient contributes to the ZT_e = 1.95 at 50 K forp‐type doping with 10¹⁸cm⁻³carrier concentration, demonstrating high thermoelectric efficiency. 

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3. Photoluminescence from Defects in Be‐Doped GaN

   https://doi.org/10.1002/pssb.202400656  

   Reshchikov, Michael_Alexander; McEwen, Benjamin; Shahedipour‐Sandvik, Shadi (February 2025, physica status solidi (b))

   While GaN is a crucial semiconductor material for bright light‐emitting devices, fabrication of p‐type GaN remains challenging since the Mg acceptor commonly used for p‐type doping is not shallow enough. Doping of GaN with Be is a promising path, yet no reliable p‐type GaN has been achieved by Be doping so far. One of the reasons is a poor understanding of point defects in Be‐doped GaN that can be studied by photoluminescence (PL). The yellow (YL_Be) band at 2.15 eV is the dominant PL band in Be‐doped GaN. In this work, a blue PL band named the BL_Beband is discovered. It has a maximum at 2.6 eV and a lifetime of 0.8 μs at temperatures below 100 K. The BL_Beband is observed in GaN samples with relatively high concentrations of Be (>10¹⁸ cm⁻³). Both the YL_Beand BL_Bebands likely originate from the isolated Be_Gadefect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the Be_Ga. The 0/+ transition level is located at 0.1–0.2 eV above the valence band. Other broad PL bands in Be‐doped GaN were also observed and preliminarily attributed to Be‐containing complexes. 

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4. Band alignments, conduction band edges and intralayer bandgap renormalisation in MoSe ₂ /WSe ₂ heterobilayers

   https://doi.org/10.1088/2053-1583/ad7b51  

   Graham, A_J; Nguyen, P_V; Park, H.; Nunn, J.; Kandyba, V.; Cattelan, M.; Giampietri, A.; Barinov, A.; Xu, X.; Cobden, D_H; et al (September 2024, 2D Materials)

   Abstract Stacking two semiconducting transition metal dichalcogenide (MX₂) monolayers to form a heterobilayer creates a new variety of semiconductor junction with unique optoelectronic features, such as hosting long-lived dipolar interlayer excitons. Despite many optical, transport, and theoretical studies, there have been few direct electronic structure measurements of these junctions. Here, we apply angle-resolved photoemission spectroscopy with micron-scale spatial resolution (µARPES) to determine the band alignments in MoSe₂/WSe₂heterobilayers, usingin-situelectrostatic gating to electron-dope and thus probe the conduction band edges. By comparing spectra from heterobilayers with opposite stacking orders, that is, with either MoSe₂or WSe₂on top, we confirm that the band alignment is type II, with the valence band maximum in the WSe₂and the conduction band minimum in the MoSe₂. The overall band gap isE_G= 1.43 ± 0.03 eV, and to within experimental uncertainty it is unaffected by electron doping. However, the offset between the WSe₂and MoSe₂valence bands clearly decreases with increasing electron doping, implying band renormalisation only in the MoSe₂, the layer in which the electrons accumulate. In contrast,µARPES spectra from a WS₂/MoSe₂heterobilayer indicate type I band alignment, with both band edges in the MoSe₂. These insights into the doping-dependent band alignments and gaps of MX₂heterobilayers will be useful for properly understanding and ultimately utilizing their optoelectronic properties. 

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5. Ti‐Modified Imogolite Nanotubes as Promising Photocatalyst 1D Nanostructures for H ₂ Production

   https://doi.org/10.1002/smtd.202301369  

   Jimenéz‐Calvo, Pablo; Naciri, Yassine; Sobolewska, Anna; Isaacs, Mark; Zhang, Yu; Leforestier, Amélie; Degrouard, Jéril; Rouzière, Stéphan; Goldmann, Claire; Vantelon, Delphine; et al (August 2024, Small Methods)

   Abstract Imogolite nanotubes (INTs) are predicted as a unique 1D material with spatial separation of conduction and valence band edges but their large band gaps have inhibited their use as photocatalysts. The first step toward using these NTs in photocatalysis and exploiting the polarization‐promoted charge separation across their walls is to reduce their band gap. Here, the modification of double‐walled aluminogermanate INTs by incorporation of titanium into the NT walls is explored. The precursor ratiox= [Ti]/([Ge]+[Ti]) is modulated between 0 and 1. Structural and optical properties are determined at different scales and the photocatalytic performance is evaluated for H₂production. Although the incorporation of Ti atoms into the structure remains limited, the optimal condition is found aroundx= 0.4 for which the resulting NTs reveal a remarkable hydrogen production of ≈1500 µmol g⁻¹after 5 h for a noble metal‐free photocatalyst, a 65‐fold increase relative to a commercial TiO₂‐P25. This is correlated to a lowering of the recombination rate of photogenerated charge carriers for the most active structures. These results confirm the theoretical predictions regarding the potential of modified INTs as photoactive nanoreactors and pave the way for investigating and exploiting their polarization properties for energy applications. 

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