More Like this

1. Oxygen Incorporation in the Molecular Beam Epitaxy Growth of Sc x Ga 1−x N and Sc x Al 1−x N

   https://doi.org/10.1002/pssb.201900612  

   Casamento, Joseph ; Xing, Huili Grace ; Jena, Debdeep ( January 2020 , physica status solidi (b))

   Secondary‐ion mass spectrometry (SIMS) is used to determine impurity concentrations of carbon and oxygen in two scandium‐containing nitride semiconductor multilayer heterostructures: Sc_xGa_1−xN/GaN and Sc_xAl_1−xN/AlN grown by molecular beam epitaxy (MBE). In the Sc_xGa_1−xN/GaN heterostructure grown in metal‐rich conditions on GaN–SiC template substrates with Sc contents up to 28 at%, the oxygen concentration is found to be below 1 × 10¹⁹ cm⁻³, with an increase directly correlated with the scandium content. In the Sc_xAl_1−xN–AlN heterostructure grown in nitrogen‐rich conditions on AlN–Al₂O₃template substrates with Sc contents up to 26 at%, the oxygen concentration is found to be between 10¹⁹and 10²¹ cm⁻³, again directly correlated with the Sc content. The increase in oxygen and carbon takes place during the deposition of scandium‐alloyed layers.

    

   more » « less
2. Structural Stabilization and Piezoelectric Enhancement in Epitaxial (Ti 1− x Mg x ) 0.25 Al 0.75 N(0001) Layers

   https://doi.org/10.1002/adfm.202001915  

   Wang, Baiwei ; Aryana, Kiumars ; Gaskins, John T. ; Hopkins, Patrick E. ; Khare, Sanjay V. ; Gall, Daniel ( June 2020 , Advanced Functional Materials)

   Epitaxial (Ti_1−xMg_x)_0.25Al_0.75N(0001)/Al₂O₃(0001) layers are used as a model system to explore how Fermi‐level engineering facilitates structural stabilization of a host matrix despite the intentional introduction of local bonding instabilities that enhance the piezoelectric response. The destabilizing octahedral bonding preference of Ti dopants and the preferred 0.67 nitrogen‐to‐Mg ratio for Mg dopants deteriorate the wurtzite AlN matrix for both Ti‐rich (x< 0.2) and Mg‐rich (x≥ 0.9) alloys. Conversely,x= 0.5 leads to a stability peak with a minimum in the lattice constant ratioc/a, which is caused by a Fermi‐level shift into the bandgap and a trend toward nondirectional ionic bonding, leading to a maximum in the expected piezoelectric stress constante₃₃. The refractive index and the subgap absorption decrease withx, the optical bandgap increases, and the elastic constant along the hexagonal axisC₃₃= 270 ± 14 GPa remains composition independent, leading to an expected piezoelectric constantd₃₃= 6.4 pC N⁻¹atx= 0.5, which is 50% larger than for the pure AlN matrix. Thus, contrary to the typical anticorrelation between stability and electromechanical coupling, the (Ti_1−xMg_x)_0.25Al_0.75N system exhibits simultaneous maxima in the structural stability and the piezoelectric response atx= 0.5.

    

   more » « less
3. Optical Properties of Electrochemically Gated La 1− x Sr x CoO 3−δ as a Topotactic Phase‐Change Material

   https://doi.org/10.1002/adom.202300098  

   Chakraborty, Rohan D. ; Postiglione, William M. ; Ghosh, Supriya ; Mkhoyan, K. Andre ; Leighton, Chris ; Ferry, Vivian E. ( May 2023 , Advanced Optical Materials)

   Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La_1−xSr_xCoO_3−δ(LSCO) with 0.00 ≤x≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators forx< 0.18 but lossy metals forx> 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors withx> 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n> 2.5), low‐loss insulating phase, accompanied by a perovskite to oxygen‐vacancy‐ordered brownmillerite transformation at highx. Atx< 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower‐index, lower‐loss insulating perovskite state with Δn >0.6. In simulations of plasmonic metasurfaces, these metal‐insulator and insulator‐insulator transitions support significant, varied mid‐infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room‐temperature phase‐change materials for applications including dynamic thermal imaging, camouflage, and optical memories.

    

   more » « less
4. Enhancing Intrinsic Magnetic Hardness by Modulating Antagonistic Interactions in the Rare‐Earth‐Free Magnetic Solid Solution Hf 2 Fe 1−δ Ru 5−x Ir x+δ B 2

   https://doi.org/10.1002/chem.202303381  

   Luong, Diana ; Schumacher, Lars ; Kilic, Sam ; Haddon, Elena ; Pöttgen, Rainer ; Fokwa, Boniface P. T. ( January 2024 , Chemistry – A European Journal)

   The quinary members in the solid solution Hf₂Fe_1−δRu_5−xIr_x+δB₂(x=1–4, VE=63–66) have been investigated experimentally and computationally. They were synthesized via arc‐melting and analyzed by EDX and X‐ray diffraction. Density functional theory (DFT) calculations predicted a preference for magnetic ordering in all members, but with a strong competition between ferro‐ and antiferromagnetism arising from interchain Fe−Fe interactions. The spin exchange and magnetic anisotropy energies predicted the lowest magnetic hardness forx=1 and 3 and the highest forx=2. Magnetization measurements confirm the DFT predictions and demonstrate that the antiferromagnetic ordering (T_N=55–70 K) found at low magnetic fields changed to ferromagnetic (T_C=150–750 K) at higher fields, suggesting metamagnetic behavior for all samples. As predicted, Hf₂FeRu₃Ir₂B₂has the highest intrinsic coercivity (H_c=74 kA/m) reported to date for Ti₃Co₅B₂‐type phases. Furthermore, all coercivities outperform that of ferromagnetic Hf₂FeIr₅B₂, indicating the importance of AFM interactions in enhancing magnetic anisotropy in these materials. Importantly, two members (x=1 and 4) maintain intrinsic coercivities in the semi‐hard range at room temperature. This study opens an avenue for controlling magnetic hardness by modulating antagonistic AFM and FM interactions in low‐dimensional rare‐earth‐free magnetic materials.

    

   more » « less
5. The Role of Antioxidant Structure in Mitigating Oxidation in Ti 3 C 2 T x and Ti 2 CT x MXenes

   https://doi.org/10.1002/admi.202200480  

   Zhao, Xiaofei ; Cao, Huaixuan ; Coleman, Bryan J. ; Tan, Zeyi ; Echols, Ian J. ; Pentzer, Emily B. ; Lutkenhaus, Jodie L. ; Radovic, Miladin ; Green, Micah J. ( June 2022 , Advanced Materials Interfaces)

   The oxidation of 2D MXenes jeopardizes their shelf life, both in colloidal dispersions and in functional devices. Certain compounds have been shown to effectively mitigate oxidation of MXenes (such as sodium L‐ascorbate, ascorbic acid, and polyanions), but the nature of interaction between these antioxidants and MXene remains unknown, which impedes the future selection and design of improved protection. This work systematically examines the interactions between three classes of organic antioxidant candidates, α‐hydroxy acids, polycarboxylic acids, and phenols with Ti_n+1C_nT_xMXenes, specifically Ti₃C₂T_xand Ti₂CT_x. Interestingly, while some antioxidants provide no protection for the MXenes, and some antioxidants even accelerate their degradation, three antioxidants (e.g., citric acid, tartaric acid, and oxalic acid) protect the MXene nanosheets exceptionally well, showing minimum MXene degradation after the 14‐day storage period. Analysis of the antioxidants’ molecular structure and efficacy suggests that chelation interactions with the transition metal atoms of the nanosheets play a key role in effective protection of MXenes from oxidation.

    

   more » « less