Session Y57: Fabrication and Characterization of Freestanding Complex Oxide Membranes

Emerging phase transitions in lead-free antiferroelectric thin films

As one of the prominent lead-free antiferroelectric alternatives, NaNbO₃ has received considerable research attention owing to its significant potential in energy storage applications. NaNbO₃ is also known for being one of the most structurally complicated perovskite materials, exhibiting a rich set of structural phases characterized by distinct symmetries, ferroic orders, and oxygen octahedral tilting patterns. This intricate energy landscape in NaNbO₃ opens up possibilities for inducing and controlling novel phase transitions via strain, thickness, and electric field. In this presentation, I will introduce our recent studies on the intrinsic size dependence of antiferroelectricity in freestanding NaNbO₃ membranes. Through a wide range of experimental and theoretical approaches, we probe an intriguing antiferroelectric-to-ferroelectric transition that occurs as the membrane thickness decreases. Additionally, I will present our recent findings regarding an intriguing strain-induced phase transition in NaNbO₃ films, where we observed the presence labyrinthine domain patterns that self-organize into intriguing topological polar structures. Our work demonstrates enormous potential of using NaNbO₃ as a fertile ground for exploring emerging phase transitions and phenomena in the family of complex oxides.   

Designing novel strain states in CaTiO3 membranes: a first-principles investigation

CaTiO₃ is the prototypical ABO₃ perovskite oxide, which crystallizes in the orthorhombic Pnma structure. Although bulk CaTiO₃ is non-polar, past theoretical and experimental work has unveiled that it transitions into a polar phase when biaxial strain is applied to a thin film via lattice mismatch with a substrate. Recently, the development of freestanding perovskite oxide membranes, whereby a perovskite oxide is lifted off from a water-soluble substrate and then transferred to a polymer sheet, opens up new possibilities for designing novel strain states. For example, the membrane can be stretched in one (uniaxial strain), two (biaxial strain), or even multiple directions by different amounts, producing unique strain states not previously accessible in other oxide platforms. However, our understanding of how these novel strain states impact targeted properties or phases remains relatively limited. In this work, we combine density functional theory calculations with group theoretic analysis to examine the effect of uniaxial strain applied along different crystallographic directions of CaTiO₃ and investigate the impact of these strains on the octahedral rotation amplitudes in the Pnma structure. We also explore the properties of CaTiO₃ when strains of distinct amplitudes are applied along different axes, and examine instabilities to different polar subgroups of Pnma. Our study offers new insight into tuning structural distortions and stabilizing polar structures under strain in CaTiO₃. We also discuss the applicability of our results to other ABO₃ perovskite oxides.    

Interfacial optical sensing of ferroelectricity in freestanding perovskite oxides with monolayer Transition Metal Dichalcogenides

In this work, we integrate monolayer MoSe₂ with a free-standing BaTiO₃ (BTO) membrane and study the ferroelectricity induced PL modulation in WSe₂. Two-dimensional (2D) transition-metal dichalcogenides (TMDs) can be easily integrated with other functional materials due to their lack of dangling bonds. Also, owing to their atomic thickness, electronic and photoluminescence (PL) properties of 2D TMDs can be modulated by external perturbations, which opens new avenues for quantum sensing and tunable optoelectronic devices [1,2,3]. Recent studies show that BTO can be separated from its oxide substrate using a sacrificial oxide layer, which opens new opportunities in material integration [4,5]. Using this method, we fabricated a dual gate field effect device where the BTO polarization can be switched and sensed simultaneously with monolayer MoSe₂ in-situ. We observe that the relative density of charge carriers in MoSe₂ changes as the polarization switches, and this gives rise to PL intensity modulation. The relative emission intensity of neutral and charged excitons show gate dependent hysteresis, which confirms that MoSe₂ senses and optically reads out the ferroelectricity in BTO.

    

Oral: Growth mechanism(s) of SrTiO3 films on graphene-covered crystalline substrates

The use of substrates coated with an ultrathin two-dimensional (2D) material may bring several advantages when it comes to growing freestanding membranes. These advantages include substrate reusability, simplified film exfoliation, and the potential for reducing dislocation density in the epitaxial layer. However, the exact growth mechanism of perovskite oxides on crystalline substrates covered with a 2D material, such as monolayer or bilayer graphene, remains uncertain. This uncertainty stems from the fact that three possible growth mechanisms—specifically, remote epitaxy, pinhole-assisted epitaxy, and van der Waals epitaxy—can result in similar outcomes of epitaxial growth and film exfoliation [1]. In this talk, we will shed light on these mechanisms by examining the heteroepitaxial growth of SrTiO₃ films on various lattice-mismatched substrates covered with varying thicknesses of graphene.

[1] H. Yoon, T. K. Truttmann, F. Liu, B. E. Matthews, S. Choo, Q. Su, V. Saraswat, S. Manzo, M. S. Arnold, M. E. Bowden, J. K. Kawasaki, S. J. Koester, S. R. Spurgeon, S. A. Chambers and B. Jalan, "Free-Standing Epitaxial SrTiO₃ Nanomembranes via Remote Epitaxy using Hybrid Molecular Beam Epitaxy" Sci. Adv. 8, eadd5328 (2022)   

Moiré-patterned twisted SrTiO3 bilayer membranes

Moiré superlattices formed from two-dimensional materials exhibit diverse emergent physical phenomena such as unconventional superconductivity, a flat band structure, ferroelectricity, and ferromagnetism. Recent advances in fabricating freestanding membranes in three-dimensional materials have now made it possible to fabricate twisted bilayers of three-dimensional materials, in particular membranes of complex oxide materials.

We fabricated high-quality ultrathin SrTiO₃ single crystal membranes using a Sr₂CaAl₂O₆ sacrificial layer. The epitaxial SrTiO₃ ultrathin film and Sr₂CaAl₂O₆ sacrificial layer were grown using pulsed laser deposition with atomic layer control facilitated by high-pressure RHEED. To transfer the membranes to create a bilayer, and to control the twisted angle of the SrTiO₃ membrane, we coated a supporting layer (Polypropylene carbonate and Poly (methyl methacrylate)) then floated the sample on water to etch the sacrificial layer. The transferred twisted bilayer exhibited an atomically flat surface with high crystallinity. Through scanning transmission electron microscopy and electron energy loss spectroscopy, we observed ordered Moiré patterns with locally reconstructed electronic structures at coincidence site lattice conditions. We will discuss the interfacial structural analysis via synchrotron X-ray, local electrical properties through scanning probe microscopy, and macroscopic electrical properties.   

Utilizing Membrane Thickness for Tunable Surface Phonon-Polaritons in Strontium Titanate Membranes

Nanophotonics with polaritons in 2D photonic membranes is emerging as a rapidly evolving field with numerous applications in sensing, nano-optics, and integrated circuits. Here we demonstrate that strontium titanate (SrTiO₃) holds promise as a versatile platform for nanophotonics in oxide membranes. In bulk form, SrTiO₃ exhibits tunable insulating, ferroelectric, metallic, and superconducting states. Recently, advances in strain engineering have also allowed ferroelectric transitions to be engineered in thin-film SrTiO₃. In this work, we investigate the photonic properties of mid-infrared surface phonon-polaritons (SPhPs) in SrTiO₃ membranes of varied thickness from 10 to 100 nm using scanning near-field optical microscopy (SNOM). High resolution near-field imaging clearly resolves propagating polariton modes near membrane edges.  Meanwhile, nano-infrared spectroscopy measurements acquired across these membranes allow us to determine the real-space dispersion of SPhPs in this novel photonic platform. Owing to inter-interface coupling of polarization across the membrane thickness, we observe unconventional polariton dispersion and attendant negative group velocity for polariton wave packets, in accord with predictions based on coupled mode theory. Comparisons of our measured and simulated SPhP dispersion demonstrate the tunability of polaritons with the thickness of the SrTiO₃ membrane. Our work lays the foundation for tunable photonics in a wide range of complex-oxides, including negative-index photonics in SrTiO₃ membranes.   

Polar vortices in twisted BaTiO3 layers

Complex correlated oxides are a class of quantum materials characterized by unscreened d-electrons that interact across competing energy scales, giving rise to a diverse range of functionalities that can be modulated by a variety of external stimuli. However, their cube-on-cube epitaxial structure constrains their growth to single-crystalline substrates, thereby limiting the crystallographic orientations and interfacial phenomena available in sequential layer growth. Recent advances in the manipulation of complex oxides have enabled their isolation from parent substrates, characterization of their properties at the few unit cell limit (2D), and probing as a function of external strains.

Here we demonstrate that twisted BaTiO₃ layers produce unconventional strain configurations that results in the formation of ferroelectric topological structures. We analyze the formation of such non-trivial structures as a function of the twisted angle and the thickness of the different layers by means of aberration-corrected scanning transmission electron microscopy in combination with density-functional theory calculations. These results show the possibility to create non-trivial strain patterns through unique moiré assemblies with deep consequences in the ground-state and responses of complex correlated oxides.   

Raman spectroscopy of free-standing perovskite oxide membranes

Raman spectroscopy is commonly used to distinguish features in ultrathin layered materials such as graphene and transition metal dichalcogenides. Free-standing perovskite oxide membranes are now commonly synthesized, adding a whole new class of 2D materials. In this talk, we present our group theory guided first principles results on simulated Raman spectra of free-standing perovskite membranes of SrTiO₃ (STO) and BaSnO₃ (BSO). We show that the Raman spectra of these films have distinct features that originate from their different symmetries and electronic structure, and put forward a method to identify and study free-standing perovskite oxide membranes using Raman spectroscopy.   

Prediction of ultra-flat bands in twisted moiré oxides

The emergence of flat bands in moiré superlattices of twisted bilayer structures has recently gathered significant attention. Flat bands feature new phases and electronic behavior due to strong electronic correlations. In this work, driven by the recent experimental demonstrations of free-standing oxide membranes [1,2], we explore electronic properties of twisted oxide membranes. These membranes are expected to provide a much stronger coupling between the layers than typical two-dimensional van der Waals structures and thus may lead to a broader spectrum of exotic properties. We consider a prototypical oxide, SrTiO₃, and design SrTiO₃ bilayers with a relative twist between the layers. Using calculations based on density functional theory, we predict the emergence of ultra-flat bands in these bilayers at relatively high twist angles. These bands are formed due to the strong localization of electronic states exhibiting moiré periodicity. Using a tight-binding approach, we create simple toy models which qualitatively explain our results. With proper doping, the flat bands at the Fermi energy may give rise to superconductivity, Mott insulating phases, and other strongly correlated properties.

1. D. X. Ji et al., Nature 570, 87 (2019).

2. H. S. Kum et al., Nature 578, 75 (2020).   

Electronically Reconfigurable Flipped SrTiO3/LaAlO3 Freestanding Membranes

LaAlO₃/SrTiO₃ heterointerfaces have been used to create reconfigurable nanoelectronic devices based on nanoscale control of LAO/STO metal-to-insulator transition. The realization of freestanding membrane based on LAO/STO heterostructures and the capability of creating reversible patterns of nanoscale conducting regions open opportunities to integrate heterostructures with Si-based architectures [1]. In particular, the integration of flipped STO/LAO membranes on Si will enable us to create reconfigurable Si-based nanodevices for storage and gating of electrons and spins. We have fabricated flipped freestanding membranes of LAO/STO heterostructures and transferred them onto Si. First, we deposited epitaxial LAO/STO heterostructures on (001) STO substrates with a Sr₂Ca₁Al₂O₆ sacrificial layer by pulsed laser deposition. Subsequent selective etching yielded freestanding membranes, which were collected by a wire loop. By flipping the wire loop over, flipped STO/LAO membranes were then integrated with Si via van der Waals stacking. The flipped membranes, with dimensions on the millimeter scale, preserved atomically smooth surface with step-terrace structure and high crystallinity. ULV-EBL technique has demonstrated the capability of switching flipped STO/LAO heterointerfaces to conducting state from insulating. In this talk, I will discuss the integration of flipped STO/LAO membranes with Si, which will allow the fabrication of Si-based field-effect transistor devices utilizing STO/LAO 2DEG as metallic gates. We anticipate that the advantage of flipped STO/LAO heterostructures will enable the high-resolution gate patterning of quantum devices, and further provide a new quantum material host system for storing and processing quantum information.

[1] Eom, K., et al. Science advances 7, 33 (2021)   

Fast route to synthesis of free-standing complex oxide membranes using a sacrificial layer method grown by hybrid MBE

Free-standing membranes have gained interest as they overcome the limitations imposed by the use of a rigid substrate, providing new opportunities in the area of twistronics, moiré-tronics, and artificial heterostructure engineering. A sacrificial layer method has been effective in the creation of membranes, where a crystalline layer is selectively etched in solvents, followed by transfer of target film onto a host substrate. However, the commonly used sacrificial layers are difficult to synthesize with regard to the state-of-the-art thin film growth method of molecular beam epitaxy (MBE). In this study, we synthesize SrTiO₃ membranes ranging from tens-to-hundreds of nanometers in thickness using (Ca, Sr, Ba)O system grown by hybrid MBE and compare them with (Ca,Sr)₃Al₂O₆ sacrificial layers. We show fast synthesis of SrTiO₃ membranes with a large lateral size up to 2-inch of developed membranes with minimum cracks and wrinkles. By combining, structural characterization using x-ray diffraction (XRD), atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), we show bulk-like structural properties. And using electrical and optical characterizations, we show bulk-like dielectric response in SrTiO₃ membranes. We discuss the implications of membrane released from the substrate on their physical properties.

      

In-situ Strain Tunning of Perovskite Nickelate Membranes

Transition-metal oxides exhibit a diverse range of emergent phenomena arising from interactions among spin, charge, lattice, and orbital degrees of freedom. Rare-earth perovskite nickelates have been the subject of significant interest because of the metal-insulator and paramagnetic-antiferromagnetic phase transitions depending on ionic radius of the rare-earth element, as well as epitaxial strain ^[1,2]. Recent advances in strain tuning capabilities based on freestanding oxide membranes have enabled continuous strain control in arbitrary symmetries ^[3,4]. To further improve the strain tuning capability, we have built an in-situ strain-tuning stage inside a cryogenic system with precise stress applied to freestanding membranes, either uniaxially or biaxially. With optical readout of the strain value, we investigate the magneto-transport properties of perovskite nickelate membranes under tensile strain down to 2K.

 

[1] S Catalano et al., Rep. Prog. Phys. 81, 046501 (2018)

[2] S. Middey et al., Annu. Rev. Mater. Res. 46, 305–34 (2016)

[3] S. S. Hong et al., Science 368, 71 (2020)

[4] R. Xu et al., Nat. Commun. 11, 3141 (2020)