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A multistep phase sequence following the crystallization of amorphous Al2O3 via solid-phase epitaxy (SPE) points to methods to create low-defect-density thin films of the metastable cubic γ-Al2O3 polymorph. An amorphous Al2O3 thin film on a (0001) α-Al2O3 sapphire substrate initially transforms upon heating to form epitaxial γ-Al2O3, followed by a transformation to monoclinic θ-Al2O3, and eventually to α-Al2O3. Epitaxial γ-Al2O3 layers with low mosaic widths in X-ray rocking curves can be formed via SPE by crystallizing the γ-Al2O3 phase from amorphous Al2O3 and avoiding the microstructural inhomogeneity arising from the spatially inhomogeneous transformation to θ-Al2O3. A complementary molecular dynamics (MD) simulation indicates that the amorphous layer and γ-Al2O3 have similar Al coordination geometry, suggesting that γ-Al2O3 forms in part because it involves the minimum rearrangement of the initially amorphous configuration. The lattice parameters of γ-Al2O3 are consistent with a structure in which the majority of the Al vacancies in the spinel structure occupy sites with tetrahedral coordination, consistent with the MD results. The formation of Al vacancies at tetrahedral spinel sites in epitaxial γ-Al2O3 can minimize the epitaxial elastic deformation of γ-Al2O3 during crystallization. 

Metamaterials and metasurfaces operating in the visible and near‐infrared (NIR) offer a promising route towards next‐generation photodetectors and devices for solar energy harvesting. While numerous metamaterials and metasurfaces using metals and semiconductors have been demonstrated, semimetals‐based metasurfaces in the vis‐NIR range are notably missing. This work experimentally demonstrates a broadband metasurface superabsorber based on large area, semimetallic, van der Waals platinum diselenide (PtSe₂) thin films in agreement with electromagnetic simulations. The results show that PtSe₂is an ultrathin and scalable semimetal that concurrently possesses high index and high extinction across the vis‐NIR range. Consequently, the thin‐film PtSe₂on a reflector separated by a dielectric spacer can absorb >85% for the unpatterned case and ≈97% for the optimized 2D metasurface in the 400–900 nm range making it one of the strongest and thinnest broadband perfect absorbers to date. The results present a scalable approach to photodetection and solar energy harvesting, demonstrating the practical utility of high index, high extinction semimetals for nanoscale optics.

 

Reconfiguration of amorphous complex oxides provides a readily controllable source of stress that can be leveraged in nanoscale assembly to access a broad range of 3D geometries and hybrid materials. An amorphous SrTiO₃layer on a Si:B/Si_1−xGe_x:B heterostructure is reconfigured at the atomic scale upon heating, exhibiting a change in volume of ≈2% and accompanying biaxial stress. The Si:B/Si_1−xGe_x:B bilayer is fabricated by molecular beam epitaxy, followed by sputter deposition of SrTiO₃at room temperature. The processes yield a hybrid oxide/semiconductor nanomembrane. Upon release from the substrate, the nanomembrane rolls up and has a curvature determined by the stress in the epitaxially grown Si:B/Si_1−xGe_x:B heterostructure. Heating to 600 °C leads to a decrease of the radius of curvature consistent with the development of a large compressive biaxial stress during the reconfiguration of SrTiO₃. The control of stresses via post‐deposition processing provides a new route to the assembly of complex‐oxide‐based heterostructures in 3D geometry. The reconfiguration of metastable mechanical stressors enables i) synthesis of various types of strained superlattice structures that cannot be fabricated by direct growth and ii) technologies based on strain engineering of complex oxides via highly scalable lithographic processes and on large‐area semiconductor substrates.