Aluminum has been established as an earth-abundant and low-cost alternative to gold and silver for plasmonic applications. Particularly, aluminum largely tends to combines with oxygen compared with silver. Here, a simple glancing angle deposition technique is presented to prepare Ag-Al alloy nanorods (NRs) with a small amount of aluminum. The effect of aluminum is to combine oxygen or corroded substances under certain conditions, such as in the air and in etchants. Beside this, owing to the large diffusion coefficient of aluminum in a Si wafer, the aluminum diffuses easily into a Si wafer, so the bonding force between the Ag-Al alloy NRs and Si wafer can be improved accordingly. In this work, 3.5 at% Al alloy NRs are optimal to exhibit high surface-enhanced Raman scattering (SERS) sensitivity, long-time stability as well as strong bonding force with a Si wafer. Ag-Al alloy NRs make a metal-metal alloy a promising material platform to develop pretty sensitive as well as stable SERS substrates.

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 (Al 2 O 3 ) 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/MoS 2 /Ag−Al device after annealing was achieved. Further observation revealed that the Al 2 O 3 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. KEYWORDS: Ag−Al alloy, low-temperature annealing, in situ grown Al 2 O 3 layer, oblique angle deposition, ultrathin conductive electrode

Due to a wide range of applications, plasmonic properties of metal nanoparticles have been widely explored, while the understanding of plasmonic behaviors of dielectric-metal composites is still limited. Herein, as a proof of concept, the relationship between compositions and plasmonic behaviors of Ag-SiO2 composites was investigated considering their extensive utilization as SERS substrates. Ag-SiO2 nanorods with regulated compositions were fabricated via glancing angle co-deposition. By increasing Ag relative deposition rate, structure of Ag-SiO2 nanorods evolves from Ag nanoparticles embedded into SiO2 matrix to quasi-one-dimensional Ag structure, inducing plasmon resonance changes over a wide range and corresponding SERS activity variations. Plasmonic resonance resulted from individual Ag nanoparticle, coupling effect of Ag nanoparticles, and quasi-one-dimensional Ag nanorods can be observed successively as Ag content increases in Ag-SiO2 nanorods. Meanwhile, SERS performance of Ag-SiO2 corresponds well with their plasmon behaviors and the maximum SERS signal was obtained from Ag-SiO2 composites with quasi-one-dimensional Ag structure. With the optimization of plasmon and SERS performance of Ag-SiO2 nanorods, we demonstrated the potential use of Ag-SiO2 nanorods in real applications as SERS chemical sensors for the detection of melamine.