Formation mechanism of micro-sized Cu plates using Br ions as basal plane-specific adsorbent and their application in the preparation of highly conductive and flexible Cu films

Abstract

A facile technique based on the aqueous chemical reduction of Cu–lactic acid complexes with ascorbic acid, in the presence of Br⁻ ions as capping agents, was developed to synthesize micrometer-sized Cu plates (Cu MPLs). Crystal and surface analyses suggested that Br⁻ ions were adsorbed on the basal planes of the 14-hedral particle seeds, and Cu was subsequently deposited on the side planes of the seeds, affording Cu MPLs. Electrochemical analysis and pH–potential diagrams of the reaction solutions revealed that the reduction pathway from Cu complexes to Cu without oxide formation was essential for facilitating the anisotropic growth of Cu crystals to obtain Cu MPLs. In addition, Cu thin films prepared by sintering a mixture of Cu MPLs and Cu nanoparticles (NPs) exhibited a lower resistivity (66 μΩ cm) at 200 °C than Cu NPs (129 μΩ cm) and a mixture of Cu NPs and three-dimensional (3D) and micrometer-sized Cu particles (3D MPs) (95 μΩ cm). This is because the Cu NPs act as a filler to bind the Cu MPLs or 3D MPs in the bimodal system. The number of voids in the NP and MPL mixture was lower than that in the mixture of NPs and 3D MPs because of the broad basal planes of the MPLs. Besides, the Cu film consisting of NPs and MPLs exhibited enhanced flexibility compared to the other films. We demonstrate that Cu MPLs are more suitable than 3D MPs for bimodal systems to achieve low resistivity and high flexibility at low sintering temperatures.