Sputter‐Deposited α‐MoO₃ Interlayers for van der Waals Epitaxy and Film Transfer

Abstract

Integration of functional thin films onto flexible substrates is driven by the need to improve the performance and durability of flexible electronic devices. A van der Waals epitaxy technology that accomplishes the transfer of oxide or metal thin films via exfoliation or dissolution of sacrificial α‐MoO3 layers produced by sputtering is presented. The α‐MoO3 thin films, consisting of weakly bonded 2D layers, grow epitaxially on SrTiO3 (001) substrates, exhibiting mosaic domains rotated by 90°. Metallic Au films grown on the α‐MoO3 are transferred by mechanical exfoliation or by dissolving the α‐MoO3 in water at 45 °C. Spinel‐structured CoFe2O4 thin films grown on α‐MoO3 layers are easily transferred to flexible substrates via mechanical exfoliation, and the magnetic anisotropy of the transferred CoFe2O4 films is modulated by bending.

Mixed-Dimensional Integration of 3D-on-2D Heterostructures for Advanced Electronics

“…In addition to the intrinsically flexible mica, the vdW epitaxy platform based on α-MoO 3 showcases a bendable magnetic membrane, as freestanding epitaxial CFO/α-MoO 3 stack can be detached from the STO substrate through mechanical exfoliation (Figure 5d,e). 42 On the other hand, remote epitaxy�an emerging method for growing single-crystalline thin films on 2D surfaces by reading the atomic registry of the epitaxial substrate across atomically thin 2D material�has greatly expanded their material spectrum, which was previously limited due to stringent lattice and processing restrictions. As a 2D interlayer, graphene has been mostly studied due to its lattice transparency, facilitating remote interaction between the epitaxial oxide layer and the underlying substrate.…”

“…One approach is growing 3D thin films on the relevant templates designed for epitaxial lift-off, such as chemical − and mechanical lift-off, − and then transferring them onto the 2D material-coated substrates. The other approach is directly forming 3D constituent layers on 2D surfaces using 2D material-based epitaxy techniques, such as vdW − and remote epitaxy. ,− Note that we particularly delve into the integration strategies of epitaxial 3D oxide thin films onto 2D materials, as such material systems hold enormous potential for pushing the performance limit of 2D electronics, photonics, and spintronics. Pulsed-laser deposition (PLD) and molecular beam epitaxy (MBE) techniques have been primarily used for single-crystalline 3D oxide growth, while there have been some recent efforts to achieve ALD-based epitaxy that allows for low-temperature operation and excellent film conformality and scalability. , …”

“…Muscovite mica, a typical template for vdW epitaxy, is a 2D layered material with a surface free of dangling bonds and a flexible nature. − , Weakly bonded bendable 3D/2D interfaces herein substantially improve or alter the physical properties of oxide thin films, such as ferroelectricity, magnetostriction, magnetoelectric coupling, opto-spintronic effect, and Rashba effect, due to the absence of substrate clamping effect and the presence of flexoelectric effect and inversion symmetry breaking (Figure a–c). In addition to the intrinsically flexible mica, the vdW epitaxy platform based on α-MoO 3 showcases a bendable magnetic membrane, as freestanding epitaxial CFO/α-MoO 3 stack can be detached from the STO substrate through mechanical exfoliation (Figure d,e) …”

“…(e) Cross-sectional low-magnification (left) and high-resolution (right) TEM images for the exfoliated α-MoO 3 /CFO/Au stack. Reprinted with permission from ref . Copyright 2024 John Wiley & Sons.…”