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1. Synthesis of metastable Ruddlesden–Popper titanates, ( A TiO ₃ ) _n A O, with n ≥ 20 by molecular-beam epitaxy

   https://doi.org/10.1063/5.0101202  

   Barone, Matthew R.; Jeong, Myoungho; Parker, Nicholas; Sun, Jiaxin; Tenne, Dmitri A.; Lee, Kiyoung; Schlom, Darrell G. (September 2022, APL Materials)

   We outline a method to synthesize ( ATiO 3 ) n AO Ruddlesden–Popper phases with high- n, where the A-site is a mixture of barium and strontium, by molecular-beam epitaxy. The precision and consistency of the method described is demonstrated by the growth of an unprecedented (SrTiO 3 ) 50 SrO epitaxial film. We proceed to investigate barium incorporation into the Ruddlesden–Popper structure, which is limited to a few percent in bulk, and we find that the amount of barium that can be incorporated depends on both the substrate temperature and the strain state of the film. At the optimal growth temperature, we demonstrate that as much as 33% barium can homogeneously populate the A-site when films are grown on SrTiO 3 (001) substrates, whereas up to 60% barium can be accommodated in films grown on TbScO 3 (110) substrates, which we attribute to the difference in strain. This detailed synthetic study of high n, metastable Ruddlesden–Popper phases is pertinent to a variety of fields from quantum materials to tunable dielectrics. 

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2. Induced ferroelectric phases in SrTiO ₃ by a nanocomposite approach

   https://doi.org/10.1039/D0NR03460F  

   Enriquez, Erik; Li, Qian; Bowlan, Pamela; Lu, Ping; Zhang, Bruce; Li, Leigang; Wang, Haiyan; Taylor, Antoinette J.; Yarotski, Dmitry; Prasankumar, Rohit P.; et al (September 2020, Nanoscale)

   null (Ed.)

   Inducing new phases in thick films via vertical lattice strain is one of the critical advantages of vertically aligned nanocomposites (VANs). In SrTiO 3 (STO), the ground state is ferroelastic, and the ferroelectricity in STO is suppressed by the orthorhombic transition. Here, we explore whether vertical lattice strain in three-dimensional VANs can be used to induce new ferroelectric phases in SrTiO 3 :MgO (STO:MgO) VAN thin films. The STO:MgO system incorporates ordered, vertically aligned MgO nanopillars into a STO film matrix. Strong lattice coupling between STO and MgO imposes a large lattice strain in the STO film. We have investigated ferroelectricity in the STO phase, existing up to room temperature, using piezoresponse force microscopy, phase field simulation and second harmonic generation. We also serendipitously discovered the formation of metastable TiO nanocores in MgO nanopillars embedded in the STO film matrix. Our results emphasize the design of new phases via vertical epitaxial strain in VAN thin films. 

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3. Confocal Raman Microscopy for Measuring In Situ Temperature-Dependent Structural Changes in Poly(Ethylene Oxide) Thin Films

   https://doi.org/10.1177/00037028241310904  

   Koh, Miharu; Kitt, Jay P; Pendergast, Andrew D; Harris, Joel M; Minteer, Shelley D; Korzeniewski, Carol (January 2025, Applied Spectroscopy)

   Crystallization from the melt is a critical process governing the properties of semi-crystalline polymeric materials. While structural analyses of melting and crystallization transitions in bulk polymers have been widely reported, in contrast, those in thin polymer films on solid supports have been underexplored. Herein, in situ Raman microscopy and self-modeling curve resolution (SMCR) analysis are applied to investigate the temperature-dependent structural changes in poly(ethylene oxide) (PEO) films during melting and crystallization phase transitions. By resolving complex overlapping sets of spectra, SMCR analysis reveals that the thermal transitions of 50 µm thick PEO films comprise two structural phases: an ordered crystalline phase and a disordered amorphous phase. The ordered structure of the crystalline PEO film entirely disappears as the polymer is heated; conversely, the disordered structure of the amorphous PEO film reverts to the ordered structure as the polymer is cooled. Broadening of the Raman bands was observed in PEO films above the melting temperature (67 °C), while sharpening of bands was observed below the crystallization temperature (45 °C). The temperatures at which these spectral changes occurred were in good agreement with differential scanning calorimetry (DSC) measurements, especially during the melting transition. The results illustrate that in situ Raman microscopy coupled with SMCR analysis is a powerful approach for unraveling complex structural changes in thin polymer films during melting and crystallization processes. Furthermore, we show that confocal Raman microscopy opens opportunities to apply the methodology to interrogate the structural features of PEO or other surface-supported polymer films as thin as 2 µm, a thickness regime beyond the reach of conventional thermal analysis techniques. 

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4. Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

   https://doi.org/10.1038/s41467-021-22777-x  

   Zhang, Xiaoqian; Lu, Qiangsheng; Liu, Wenqing; Niu, Wei; Sun, Jiabao; Cook, Jacob; Vaninger, Mitchel; Miceli, Paul F.; Singh, David J.; Lian, Shang-Wei; et al (December 2021, Nature Communications)

   Abstract While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe 2 , a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature ( T C ) up to 300 K, an atomic magnetic moment of ~0.21  $${\mu }_{{\rm{B}}}$$ μ B /Cr and perpendicular magnetic anisotropy (PMA) constant ( K u ) of 4.89 × 10 5  erg/cm 3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer ( T C  ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe 2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices. 

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5. Structural degeneracy and formation of crystallographic domains in epitaxial LaFeO3 films revealed by machine-learning assisted 4D-STEM

   https://doi.org/10.1038/s41598-024-54661-1  

   Zhu, Menglin; Lanier, Joseph; Flores, Jose; da_Cruz_Pinha_Barbosa, Victor; Russell, Daniel; Haight, Becky; Woodward, Patrick M; Yang, Fengyuan; Hwang, Jinwoo (December 2024, Scientific Reports)

   Abstract Structural domains and domain walls, inherent in single crystalline perovskite oxides, can significantly influence the properties of the material and therefore must be considered as a vital part of the design of the epitaxial oxide thin films. We employ 4D-STEM combined with machine learning (ML) to comprehensively characterize domain structures at both high spatial resolution and over a significant spatial extent. Using orthorhombic LaFeO₃as a model system, we explore the application of unsupervised and supervised ML in domain mapping, which demonstrates robustness against experiment uncertainties. The results reveal the consequential formation of multiple domains due to the structural degeneracy when LaFeO₃film is grown on cubic SrTiO₃. In situ annealing of the film shows the mechanism of domain coarsening that potentially links to phase transition of LaFeO₃at high temperatures. Moreover, synthesis of LaFeO₃on DyScO₃illustrates that a less symmetric orthorhombic substrate inhibits the formation of domain walls, thereby contributing to the mitigation of structural degeneracy. High fidelity of our approach also highlights the potential for the domain mapping of other complicated materials and thin films. 

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