Synthesis of KCa₂Nb₃O₁₀ Crystals with Varying Grain Sizes and Their Nanosheet Monolayer Films As Seed Layers for PiezoMEMS Applications

The layered perovskite-type niobate KCa₂Nb₃O₁₀ and its derivatives show advantages in several fields, such as templated film growth and (photo)­catalysis. Conventional synthesis routes generally yield crystal size smaller than 2 μm. We report a flux synthesis method to obtain KCa₂Nb₃O₁₀ crystals with significantly larger sizes. By using different flux materials (K₂SO₄ and K₂MoO₄), crystals with average sizes of 8 and 20 μm, respectively, were obtained. The KCa₂Nb₃O₁₀ crystals from K₂SO₄ and K₂MoO₄ assisted synthesis were protonated and exfoliated into monolayer nanosheets, and the optimal exfoliation conditions were determined. Using pulsed laser deposition, highly (001)-oriented piezoelectric stacks (SrRuO₃/PbZr_0.52Ti_0.48O₃/SrRuO₃, SRO/PZT/SRO) were deposited onto Langmuir–Blodgett films of Ca₂Nb₃O₁₀⁻ (CNO) nanosheets with varying lateral nanosheet sizes on Si substrates. The resulting PZT thin films showed high crystallinity irrespective of nanosheet size. The small sized nanosheets yielded a high longitudinal piezoelectric coefficient <i>d</i>₃₃ of 100 pm/V, while the larger sized sheets had a <i>d</i>₃₃ of 72 pm/V. An enhanced transverse piezoelectric coefficient <i>d</i>₃₁ of −107 pm/V, an important input parameter for the actuation of active structures in microelectromechanical systems (MEMS) devices, was obtained for PZT films grown on CNO nanosheets with large lateral size, while the corresponding value on small sized sheets was −96 pm/V

Local Control over Nucleation of Epitaxial Thin Films by Seed Layers of Inorganic Nanosheets

Nanosheets of Ti_0.87O₂ and Ca₂Nb₃O₁₀ were synthesized and transferred onto Si substrates by Langmuir–Blodgett deposition. Using pulsed laser deposition, SrRuO₃ films were formed on top of these samples. The underlying nanosheets determined both the morphology and crystallographic orientation of the films. SrRuO₃ grew preferentially in the [110]_pc direction on Ti_0.87O₂ nanosheets, while growth proceeded in the [001]_pc direction on Ca₂Nb₃O₁₀ nanosheets (pc refers to the pseudocubic unit cell of SrRuO₃). Besides macroscopic control over the out-of-plane crystal direction, single crystal orientations were measured by electron backscatter diffraction on the level of individual nanosheets, indicating that epitaxial growth was achieved on the nanosheets as imposed by their well-defined crystal lattices. The nanosheets also had a clear effect on the magnetic properties of the films, which showed anisotropic behavior only when a seed layer was used. A monolayer consisting of a mixture of both types of nanosheets was made to locally control the nucleation of SrRuO₃. In this context, SrRuO₃ was used as model material, as it was used to illustrate that nanosheets can be a unique tool to control the orientation of films on a (sub-)micrometer length scale. This concept may pave the way to the deposition of various other functional materials and the fabrication of devices where the properties are controlled locally by the different crystallographic orientations

Highly Oriented Growth of Piezoelectric Thin Films on Silicon Using Two-Dimensional Nanosheets as Growth Template Layer

Ca₂Nb₃O₁₀ (CNO<i>ns</i>) and Ti_0.87O₂ (TiO<i>ns</i>) metal oxide nanosheets (<i>ns</i>) are used as a buffer layer for epitaxial growth of piezoelectric capacitor stacks on Si and Pt/Ti/SiO₂/Si (Pt/Si) substrates. Highly (001)- and (110)-oriented Pb­(Zr_0.52Ti_0.48)­O₃ (PZT) films are achieved by utilizing CNO<i>ns</i> and TiO<i>ns</i>, respectively. The piezoelectric capacitors are characterized by polarization and piezoelectric hysteresis loops and by fatigue measurements. The devices fabricated with SrRuO₃ top and bottom electrodes directly on nanosheets/Si have ferroelectric and piezoelectric properties well comparable with devices that use more conventional oxide buffer layers (stacks) such as YSZ, CeO₂/YSZ, or SrTiO₃ on Si. The devices grown on nanosheets/Pt/Si with Pt top electrodes show significantly improved polarization fatigue properties over those of similar devices grown directly on Pt/Si. The differences in properties are ascribed to differences in the crystalline structures and the density of the films. These results show a route toward the fabrication of single crystal piezoelectric thin films and devices with high quality, long-lifetime piezoelectric capacitor structures on nonperovskite and even noncrystalline substrates such as glass or polished metal surfaces

Tiling the Silicon for Added Functionality: PLD Growth of Highly Crystalline STO and PZT on Graphene Oxide-Buffered Silicon Surface

The application of two-dimensional (2D) materials has alleviated a number of challenges of traditional epitaxy and pushed forward the integration of dissimilar materials. Besides acting as a seed layer for van der Waals epitaxy, the 2D materialsbeing atom(s) thickhave also enabled wetting transparency in which the potential field of the substrate, although partially screened, is still capable of imposing epitaxial overgrowth. One of the crucial steps in this technology is the preservation of the quality of 2D materials during and after their transfer to a substrate of interest. In the present study, we show that by honing the achievements of traditional epitaxy and wet chemistry a hybrid approach can be devised that offers a unique perspective for the integration of functional oxides with a silicon platform. It is based on SrO-assisted deoxidation and controllable coverage of silicon surface with a layer(s) of spin-coated graphene oxide, thus simultaneously allowing both direct and van der Waals epitaxy of SrTiO3 (STO). We were able to grow a high-quality STO pseudo-substrate suitable for further overgrowth of functional oxides, such as PbZr1–xTixO3 (PZT). Given that the quality of the films grown on a reduced graphene oxide-buffer layer was almost identical to that obtained on SiC-derived graphene, we believe that this approach may provide new routes for direct and “remote” epitaxy or layer-transfer techniques of dissimilar material systems

In Situ X‑ray Absorption Spectroscopy of LaFeO₃ and LaFeO₃/LaNiO₃ Thin Films in the Electrocatalytic Oxygen Evolution Reaction

We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO3 and LaFeO3/LaNiO3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La0.67Sr0.33MnO3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca2Nb3O10 nanosheet-buffered 100 nm SiNx membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiNx membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)­valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO3 film