Ferroelectronics Lab

Understanding and utilizing non-volatile properties of materials

New Publication! “Germanium dioxide: A new rutile substrate for epitaxial film growth”

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Abstract: Rutile compounds have exotic functional properties that can be applied for various electronic applications; however, the limited availability of epitaxial substrates has restricted the study of rutile thin films to a limited range of lattice parameters. Here, rutile GeO2 is demonstrated as a new rutile substrate with lattice parameters of 𝑎=4.398 Å and 𝑐=2.863 Å. Rutile GeO2 single crystals up to 4 mm in size are grown by the flux method. X-ray diffraction reveals high crystallinity with a rocking curve having a full width half-maximum of 0.0572°. After mechanical polishing, a surface roughness of less than 0.1 nm was obtained, and reflection high-energy electron diffraction shows a crystalline surface. Finally, epitaxial growth of (110)-oriented TiO2 thin films on GeO2 substrates was demonstrated using molecular beam epitaxy. Templated by rutile GeO2 substrates, our findings open the possibility of stabilizing new rutile thin films and strain states for the tuning of physical properties.

Full text available from Journal of Vacuum Science & Technology A

New Publication! “Effects of local compositional and structural disorder on vacancy formation in entropy-stabilized oxides from first-principles”

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Abstract: Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy composition. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodynamics of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equilibrium bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions associated with the local configuration of chemical species. Vacancies in ESOs exhibit deep thermodynamic transition levels that inhibit electrical conduction. By applying the charge-neutrality condition, we determine that the equilibrium concentrations of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chemistry and associated structural distortions by varying alloy composition acts an engineering principle that enables controlled defect formation in multi-component alloys.

Full text available from npj computational materials

New Publication! “Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization”

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Abstract: Monolayer hexagonal boron nitride (hBN) has been widely considered as a fundamental building block for two–dimensional (2D) heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, we propose and further demonstrate a hBN/graphene (hBN/G) interface–mediated growth process for the controlled synthesis of high–quality monolayer hBN. We discover that the in–plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single–domain hBN, aligned to the underlying graphene lattice. Furthermore, we identify that the deep–ultraviolet emission at 6.12 eV stems from the 1s–exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer–scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

Full text available from Advanced Materials

New Publication! “Two-dimensional charge order stabilized in clean polytype heterostructures”

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Abstract

Compelling evidence suggests distinct correlated electron behavior may exist only in clean 2D materials such as 1T-TaS2. Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers disrupts correlation-driven quantum behavior. Here we demonstrate a route to realizing fragile 2D quantum states through endotaxial polytype engineering of van der Waals materials. The true isolation of 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate charge density wave (tC-CDW) reported herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces endotaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication.

Full text available from nature communications

New Publication! “Superconductivity in a quintuple-layer square-planar nickelate”

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Abstract

Since the discovery of high-temperature superconductivity in copper oxide materials1, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials2. One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd0.8Sr0.2NiO2 (ref. 3). Undoped NdNiO2 belongs to a series of layered square-planar nickelates with chemical formula Ndn+1NinO2n+2 and is known as the ‘infinite-layer’ (n = ∞) nickelate. Here we report the synthesis of the quintuple-layer (n = 5) member of this series, Nd6Ni5O12, in which optimal cuprate-like electron filling (d8.8) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd6Ni5O12 interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality.

Full text available from Nature Materials

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