Control of growth kinetics during remote epitaxy of complex oxides on graphene by pulsed laser deposition

Remote epitaxy through 2D materials opens new opportunities for research and application, overcoming some limitations of classical epitaxy and allowing the creation of freestanding layers. However, using graphene as a 2D interlayer for remote epitaxy of metal oxides is challenging, particularly when carried out by pulsed laser deposition (PLD). The graphene layer can be easily oxidized under the typically applied high oxygen pressures, and the impact of highly kinetic particles of the plasma plume can lead to severe damages. In this study, both aspects are addressed: Argon is introduced as an inert background gas in order to avoid oxidation and to reduce the kinetic impact of the plasma species on graphene. The laser spot size is minimized to control the plasma plume and particle flux. As a model system, strontium titanate (STO) is quasi-homoepitaxially grown on graphene buffered STO single crystals. Raman spectroscopy is performed to evaluate the 2D, G, and D band fingerprints of the graphene layer and to assess the defect structure of the interlayer after the deposition. Our results prove that control of the growth kinetics by reducing the laser spot size and by using high argon pressures provides a key strategy to conserve graphene with a low defect density during PLD while allowing a layer-by-layer growth of structurally coherent oxide layers. This strategy may be generalized for the PLD remote epitaxy of many complex oxides, opening the way for integrating 2D materials with complex oxides using widely accessible PLD processes

Large imprint in epitaxial 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3thin films for piezoelectric energy harvesting applications

Tuning and stabilizing a large imprint in epitaxial relaxor ferroelectric thin films is one of the key factors for designing micro-electromechanical devices with an enhanced figure of merit (FOM). In this work, epitaxial 500 nm-thick 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-33PT) films, free from secondary phases and with extremely low rocking curves (FWHM < 0.05°), are grown on ScSmO3 (SSO) and DyScO3 (DSO) substrates buffered with SrRuO3 (SRO). The PMN-33PT is observed to grow coherently on SSO substrates (lattice mismatch of -0.7%), which is c-axis oriented and exhibits large tetragonality compared to bulk PMN-33PT, while on DSO substrates (lattice mismatch of -1.9%), the PMN-33PT film is almost completely relaxed and shows reduced tetragonality. Due to the compressive epitaxial strain, the fully strained PMN-33PT film displays typical ferroelectric P-E hysteresis loops, while the relaxed sample shows relaxor-like P-E loops. Samples present large negative imprints of about -88.50 and -49.25 kV/cm for PMN-33PT/SRO/SSO and PMN-33PT/SRO/DSO, respectively, which is more than threefold higher than the coercive field. The imprint is induced by the alignment of defect dipoles with the polarization and is tuned by the epitaxial strain. It permits the stabilization of a robust positive polarization state (Pr ∼20 μC/cm2) and low dielectric permittivity (<700). In addition, the relaxed PMN-33PT film shows improved piezoelectric properties, with a 33% enhancement in d33,eff relative to the fully strained sample. The obtained low dielectric permittivity and the high piezoelectric coefficients at zero electric field in the studied PMN-33PT films hold great promise to maximize the FOM toward applications in piezoelectric devices