Nanometer-Thick Al₂O₃ Layers on Ag–Al Nanostructures as Conductive Electrodes

Silver (Ag) nanostructures are active functional platforms for catalysts, optical sensors, and transparent conductive networks with outstanding performance but present poor stability depending on their chemical, mechanical, or thermal surroundings. Doping with other elements and passivation with a chemically inert coating of a Ag nanostructure are proven to be efficient in enhancing the stability, while both methods have drawbacks such as cost and sacrifice of the on-demand properties. Here, we present a self-passivated Ag–Al nanostructure with a precisely controlled coating thickness, taking advantage of the intrinsic high conductivity and chemical reducibility of aluminum at the same time, fabricated by oblique angle codeposition of Ag–Al nanostructures followed by postannealing under mild conditions. With the nanometer-thick coating confirmed by transmission electron microscope (TEM) observations, surface-enhanced Raman scattering (SERS) measurements were successfully utilized to further demonstrate the quality of the in situ grown aluminum oxide (Al2O3) layer, including the thickness, coating integrity, and chemical stability. In addition, a mechanically robust nanostructure as well as an ultrathin conductive electrode ∼10 nm in thickness was enabled by dilute doping of Al into the Ag matrix. Interestingly, a bipolar change in the Ag–Al/MoS2/Ag–Al device after annealing was achieved. Further observation revealed that the Al2O3 layer grew on top of the Ag–Al alloys. The methods we developed require dilute doping of Al into a Ag matrix and mild processing conditions, yet precise interface thickness control and pronounced property enhancement are achieved, all of which pave the way for the practical application of Ag nanostructures in optical sensors and integrated circuits.