Single-crystal Pd and its alloy nanowires for plasmon propagation and highly sensitive hydrogen detection

In this work plasmon propagation and highly sensitive hydrogen sensing are demonstrated in single-crystal Pd and its alloy nanowires, based on an evanescent wave coupling technique using optical fiber tapers. The plasmon propagation losses of Pd nanowires on an MgF₂ substrate are measured to be ∼2 dB/μm (at 635 nm). Using a suspension scheme of the nanowire, the propagation losses decrease to ∼0.6 dB/μm in air. Utilizing an alloying technique, single-crystal PdAg nanowires are fabricated and exhibit decreasing and tunable propagation losses. Furthermore, Pd nanowires are used for plasmonic gas sensing with amplitude sensitivity of ∼11 dB to 3.6% hydrogen gas, which is much higher than those in photonic nanowire sensors. The concentration detection limit is about 0.2%, which is lower than those in photonic nanowire sensors and most surface plasmon resonance sensors. The hysteresis effect of the Pd nanowire in hydrogen sensing is suppressed by using single-crystal PdAu alloy nanowires. This work may find widespread application, ranging from sensing and integrated circuits to materials research.