Two-dimensional (2D) metals have received considerable attention in recent years because of their fascinating physical and chemical properties, as well as potential applications in electrocatalysis, sensors, plasmonics, etc. However, the fabrication of 2D metals, especially atomically thin ones with large lateral size, remains a significant challenge because of the strong and isotropic metallic bonds. Here, we use Pt as a model system and demonstrate a general way to fabricate freestanding, high-quality 2D metals with giant aspect ratios (as large as ∼10 7 ) and controllable thickness down to 1 nm via a combination of room-temperature physical deposition and chemical etching. The cool deposition could suppress the Volmer−Webber growth mode, resulting in the formation of continuous ultrathin Pt with smooth surface and high conductivity. Moreover, the ultrathin 2D Pt exhibit outstanding hydrogen evolution reaction activity with a mass activity of 8.06 mA μg −1 at 0.06 V, ∼18 times higher than that of the commercial Pt/C catalyst. Additionally, the freestanding Pt-based strain sensor exhibits a high gauge factor of up to ∼4643, which is 3 orders of magnitude higher than that of conventional constantan wire-based strain gauges. Our studies pave the way for further developing wafer-scale 2D metal-based devices for various applications.

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Lithium niobate (LiNbO3, LN) thin films have been extensively studied for applications in acoustic and photonic devices, due to their outstanding piezoelectric, ferroelectric and electro-optical properties. With the increasing demand for high speed and low latency wireless communication, LN thin films with high electromechanical coupling coefficients are very attractive to improve the performance of acoustic resonators for radio frequency filters. The current bottleneck for LN-based devices is the synthesis of high-quality LN thin films, which is typically fabricated by expensive and inefficient process of ion slicing and layer transfer from bulk single crystals. This review paper focuses on the direct growth of high-quality LN thin films, which has the potential to scale up and lower the cost of LN thin films. We first introduce the crystal structure and piezoelectric properties of LN, followed by an overview of the state-of-the-art LN acoustic resonators. After a summary of the challenges in the fabrication of LN thin films, we review the direct growth of LN thin films by sputtering, pulsed laser deposition, metalorganic chemical vapor deposition and molecular beam epitaxy. With the progress in optimizing the crystallinity and surface roughness, the quality of the LN thin films synthesized by direct growth has been greatly improved. As a result of the fast-growing industrial interests, we believe that the research works in direct growth of LN thin films will increase exponentially to achieve the same quality of the LN thin films as the bulk single crystals.