Epitaxial Electrodeposition of Cu(111) onto an l-Cysteine Self-Assembled Monolayer on Au(111) and Epitaxial Lift-Off of Single-Crystal-like Cu Foils for Flexible Electronics

Functional self-assembled monolayers (SAMs) of thiols on single-crystal metals provide two-dimensional (2D) soft templates for the highly ordered growth of crystalline materials. An epitaxial Cu(111) film is electrodeposited on a SAM of the amino acid l-cysteine on Au(111). Epitaxy is confirmed, with Cu(111) following the Au(111) in-plane and out-of-plane orientations with a small amount of twinned [511] orientation, which is shown by X-ray analysis. The mismatch between Cu(111) and the Au(111) substrate is −11.37%. This mismatch is lowered to −0.29% by forming a coincident site lattice in which nine unit meshes of Cu coincide with eight unit meshes of Au. Defect-mediated and coordination-controlled electrodeposition mechanisms are illustrated as two possible deposition mechanisms. The carboxylic (−COOH) and amine (−NH2) functional groups of the l-cysteine molecule are shown to be crucial for the epitaxy of Cu because a 1-butanethiol SAM on Au(111) which has no functional groups yields a textured film with no in-plane order. The (√3 × √3)R30° surface structure of l-cysteine SAM and the c(4 × 2) surface structure of 1-butanethiol SAM on Au(111) are discussed. A 3D model of the Cu lattice on the l-cysteine SAM on Au(111) is proposed. A possible coordination to Cu is shown, which facilitates the epitaxial nucleation and 2D growth of Cu. The Cu(111) films have potential as a substrate for catalysts for CO2 reduction, photovoltaic devices, spintronic devices, and high-temperature superconductors. Direct epitaxial lift-off of the Cu film without etching gives a single-crystal-like Cu(111) foil. The Cu(111) foil exhibits a low resistivity of 3.75 x 10–8 Ω·m and good bending stability, showing only a 12% increase in resistance after 104 bending cycles. Cu(111) foils can be utilized as inexpensive, highly ordered, and conductive substrates for flexible electronics such as wearable solar cells, sensors, and flexible displays. Here, we show an example by electrodepositing epitaxial cuprous oxide on a Cu(111) foil