Flexible Copper Films Modification via Spontaneous Reduction of Aryldiazonium Gold Salts: Unraveling Surface Properties and Energy Profile

Abstract

In this study, we report the modification of flexible copper films via the spontaneous reduction of aryldiazonium gold salts [X-4-C 6 H 4 N�N]AuCl 4 (X�COOH, NO 2 ). The electroless modification involves dipping of flexible copper films in the aryldiazonium gold solutions for a few seconds, under ambient conditions, followed by a washing step with deionized water to obtain a mechanically robust gold-aryl coating. The chemical composition, morphology, electronic structure, and optical properties of the gold-aryl layer and the flexibility of the modified copper films are supported by the results from X-ray photoelectron spectroscopy (XPS), electrochemistry, contact angle, scanning electron microscopy (SEM), and ultraviolet photoelectron spectroscopy (UPS). XPS surface analysis showed metallic gold in addition to C−C, C−O/C−N, and C�O functional groups from the grafted aryls. Cu 2p showed metallic copper as a major component and a small amount of Cu(II) ions. Wettability studies showed that Au-COOH@Cu increased the contact angle of the bare copper films from 68.0 ± 0.7°to 82.0°± 0.7°, while Au-NO 2 @Cu increased the contact angle to 134.0°± 0.3°. UPS energy profile analysis of [HOOC-4-C 6 H 4 N�N]AuCl 4 (valence band maximum = 1.91 eV) exhibited greater reducibility than [O 2 N-4-C 6 H 4 N�N]AuCl 4 (valence band maximum = 2.91 eV). The lower ionization potential of [HOOC-4-C 6 H 4 N�N]AuCl 4 (IP = 4.33 eV) enhanced the reactivity upon copper film contact, potentially inducing efficient energy level alignment, compared with [O 2 N-4-C 6 H 4 N�N]AuCl 4 (IP = 5.62 eV). UPS results were further supported by electrochemistry investigation which revealed that [HOOC-4-C 6 H 4 N�N]AuCl 4 is easily reducible compared with [O 2 N-4-C 6 H 4 N�N]AuCl 4 . The findings presented here hold significant implications for developing flexible copper films and pave the way for future advancements in electronic material modification for industrial applications.