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1. Enhancing and Extinguishing the Different Emission Features of 2D (EA _{1− x} FA _x ) ₄ Pb ₃ Br ₁₀ Perovskite Films

   https://doi.org/10.1002/adom.202200547  

   Kennard, Rhiannon M. ; Dahlman, Clayton J. ; Morgan, Emily E. ; Chung, Juil ; Cotts, Benjamin L. ; Kincaid, Joseph R. A. ; DeCrescent, Ryan A. ; Stone, Kevin H. ; Panuganti, Shobhana ; Mohtashami, Yahya ; et al ( June 2022 , Advanced Optical Materials)

   2D hybrid perovskites are attractive for optoelectronic devices. In thin films, the color of optical emission and the texture of crystalline domains are often difficult to control. Here, a method for extinguishing or enhancing different emission features is demonstrated for the family of 2D Ruddlesden–Popper perovskites (EA_1−xFA_x)₄Pb₃Br₁₀(EA = ethylammonium, FA = formamidinium). When grown from aqueous hydrobromic acid, crystals of (EA_1−xFA_x)₄Pb₃Br₁₀retain all the emission features of their parent compound, (EA)₄Pb₃Br₁₀. Surprisingly, when grown from dimethylformamide (DMF), an emission feature, likely self‐trapped exciton (STE), near 2.7 eV is missing. Extinction of this feature is correlated with DMF being incorporated between the 2D Pb‐Br sheets, forming (EA_1−xFA_x)₄Pb₃Br₁₀∙(DMF)_y. Without FA, films grown from DMF form (EA)₄Pb₃Br₁₀, retain little solvent, and have strong emission near 2.7 eV. Slowing the kinetics of film growth strengthens a different emission feature, likely a different type of STE, which is much broader and present in all compositions. Films of (EA_1−xFA_x)₄Pb₃Br₁₀∙(DMF)_yhave large, micron‐sized domains and homogeneous orientation of the semiconducting sheets, resulting in low electronic disorder near the absorption edge. The ability to selectively strengthen or extinguish different emission features in films of (EA_1−xFA_x)₄Pb₃Br₁₀∙(DMF)_yreveals a pathway to tune the emission color in these compounds.
2. Physical characterization of BiFeO ₃ ‐based thin films with enhanced properties for photovoltaic applications

   https://doi.org/10.1111/jace.18646  

   Mariano, Marcos A. S. ; Mendez‐González, Yanela ; Silva, Atair C. ; Monte, Adamo F. G. ; Lima, Elton C. ; Guo, Ruyan ; Bhalla, Amar S. ; de los Santos Guerra, José ( July 2022 , Journal of the American Ceramic Society)

   The present paper reports the multifunctional properties of lead‐free BiFeO₃–La (BFO–La) thin films. The structural, microstructural, and optical properties have been investigated as a function of the lanthanum doping concentration. The structural properties at room temperature showed the formation of the perovskite structure, thus suggesting the high quality of the obtained thin film compositions. Raman spectroscopy analysis revealed a slight variation in both the peak position and absolute intensity for the Raman active modes, as lanthanum content increases in BiFeO₃–La. Crystallized thin films with well‐defined grains as well as crack‐free surfaces have been obtained, for all the studied compositions, as inferred from atomic force microscopy images. The optical properties have been measured, and the values for the direct bandgap was significantly lower than those reported for other BFO‐based systems, being the lowest ∼1.87 eV for the Bi_0.90La_0.10FeO₃composition. Results revealed a noteworthy effect of the defect concentrations induced by the lanthanum doping on the long‐range crystallinity and directly affecting the polarizability of the A–O bond as well as the Fe–O and Fe–O–Fe bond lengths in the perovskite structure. The enhanced optical absorption properties registered for the Bi_1–xLa_xFeO₃(x= 0–20) compositions make these perovskite multiferroic thin films as a potential candidate material formore »the high‐performance photovoltaic device applications.

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3. Exploring the Pb _{1− x} Sr _x HfO ₃ System and Potential for High Capacitive Energy Storage Density and Efficiency

   https://doi.org/10.1002/adma.202105967  

   Acharya, Megha ; Banyas, Ella ; Ramesh, Maya ; Jiang, Yizhe ; Fernandez, Abel ; Dasgupta, Arvind ; Ling, Handong ; Hanrahan, Brendan ; Persson, Kristin ; Neaton, Jeffrey B. ; et al ( October 2021 , Advanced Materials)

   The hafnate perovskites PbHfO₃(antiferroelectric) and SrHfO₃(“potential” ferroelectric) are studied as epitaxial thin films on SrTiO₃(001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb_1−xSr_xHfO₃system. The resulting (240)‐oriented PbHfO₃(Pba2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm⁻²at 1.6 MV cm⁻¹, weak‐field dielectric constant ≈186 at 298 K, and an antiferroelectric‐to‐paraelectric phase transition at ≈518 K. (002)‐oriented SrHfO₃films exhibited neither ferroelectric behavior nor evidence of a polarP4mmphase . Instead, the SrHfO₃films exhibited a weak‐field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77–623 K) and electric field (–3 to 3 MV cm⁻¹). While the lack of ferroelectric order in SrHfO₃removes the potential for MPB, structural and property evolution of the Pb_1−xSr_xHfO₃(0 ≤x < 1) system is explored. Strontium alloying increased the electric‐breakdown strength (E_B) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (U_r) and efficiency (η). The composition, Pb_0.5Sr_0.5HfO₃produced the best combination ofE_B = 5.12 ± 0.5 MV cm⁻¹,U_r = 77 ± 5 J cm⁻³, and η = 97 ± 2%, well out‐performing PbHfO₃and other antiferroelectric oxides.
4. Al Incorporation up to 99% in Metalorganic Chemical Vapor Deposition‐Grown Monoclinic (Al _x Ga _1–x ) ₂ O ₃ Films Using Trimethylgallium

   https://doi.org/10.1002/pssr.202300224  

   Bhuiyan, A. F. M. Anhar Uddin ; Meng, Lingyu ; Huang, Hsien-Lien ; Chae, Christopher ; Hwang, Jinwoo ; Zhao, Hongping ( August 2023 , physica status solidi (RRL) – Rapid Research Letters)

   Growths of monoclinic (Al_xGa_1−x)₂O₃thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h⁻¹) of β‐(Al_xGa_1−x)₂O₃thin films on (010), (100), and (01) β‐Ga₂O₃substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(Al_xGa_1−x)₂O₃films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (Al_xGa_1−x)₂O₃films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (01) β‐Ga₂O₃substrates, respectively. Beyond 29% of Al incorporation, the (010) (Al_xGa_1−x)₂O₃films exhibit β‐ to γ‐phase segregation. β‐(Al_xGa_1−x)₂O₃films grown on (01) β‐Ga₂O₃show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (Al_xGa_1−x)₂O₃grown on (100) Ga₂O₃are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(Al_xGa_1−x)₂O₃films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices.
5. Wide and Tunable Bandgap MAPbBr _3−x Cl _x Hybrid Perovskites with Enhanced Phase Stability: In Situ Investigation and Photovoltaic Devices

   https://doi.org/10.1002/solr.202000718  

   Tang, Ming-Chun ; Dang, Hoang X. ; Lee, Sehyun ; Barrit, Dounya ; Munir, Rahim ; Wang, Kai ; Li, Ruipeng ; Smilgies, Detlef-M. ; De Wolf, Stefaan ; Kim, Dong-Yu ; et al ( March 2021 , Solar RRL)

   The current understanding of the crystallization, morphology evolution, and phase stability of wide‐bandgap hybrid perovskite thin films is very limited, as much of the community's focus is on lower bandgap systems. Herein, the crystallization behavior and film formation of a wide and tunable bandgap MAPbBr_3−xCl_xsystem are investigated, and its formation and phase stability are contrasted to the classical MAPbI_3−xBr_xcase. A multiprobe in situ characterization approach consisting of synchrotron‐based grazing incidence wide‐angle X‐ray scattering and laboratory‐based time‐resolved UV–Vis absorbance measurements is utilized to show that all wide‐bandgap perovskite compositions of MAPbBr_3−xCl_xstudied (0 < x < 3) crystallize the same way: the perovskite phase forms directly from the colloidal sol state and forms a solid film in the cubic structure. This results in significantly improved alloying and phase stability of these compounds compared with MAPbI_3−xBr_xsystems. The phase transformation pathway is direct and excludes solvated phases, in contrast to methylammonium lead iodide (MAPbI₃). The films benefit from antisolvent dripping to overcome the formation of discontinuous layers and enable device integration. Pin‐hole‐free MAPbBr_3−xCl_xhybrid perovskite thin films with a tunable bandgap are, thus, integrated into working single‐junction solar cell devices and achieve a tunable open‐circuit voltage as high as 1.6 V.