Fabricating Freestanding Ultrathin Materials by Atomic Layer Deposition

Ultrathin materials have risen to an increasingly important role in recent years, both due to the nano-scaling of existing technologies and the advancement of new applications that utilize the distinct properties of ultrathin structures compared to bulk materials. Because these properties strongly depend on thickness and uniformity, vapor-based synthesis is popular. In particular, atomic layer deposition (ALD) is a popular strategy to fabricate precisely controlled ultrathin materials because of self-limiting growth and high conformality. However, to fully realize the potential of ultrathin materials from ALD, there exists a need for freestanding structures, which would allow the exploration of applications such as membranes or photolithographic pellicles. Despite the prominence of ALD, no general method of fabricating freestanding ultrathin materials with cm-scale dimension currently exists. This gap exists due to the nature of ALD, in which precursor molecules chemisorb strongly to and are difficult to remove from the substrate surface. To address the challenge of prepared freestanding ALD materials, we report a strategy of growing materials on a tailored single-crystal NaCl surface. Although NaCl is typically a poor substrate for ALD, the insertion of hydroxyl functional groups across the surface results in a heterogeneous surface that forms both bonding and nonbonding interactions with ALD materials. Once deposited, the material is easily removed under mild conditions and transferred to an arbitrary substrate. This approach yields ALD materials that have cm-scale sizes and are low in defects in cracks and pinholes. The strategy provides access to a diverse range of freestanding ultrathin compositions, creating opportunities to investigate structure-property relationships that are distinct from those in bulk material counterparts.