To identify whether a resonance originates from a longitudinal mode or a transverse mode, well-aligned metal nanowires represent an ideal configuration. For examples, Zong et al. [39–41] reported that a dual peak appeared when the incident light was perpendicular to the surface of the composite film of Ag nanowire arrays

in anodic aluminum oxide (AAO) template. The two peaks were ascribed to the transverse dipole resonance (longer wavelength) and the transverse quadrupole resonance (shorter wavelength), respectively. The quadrupole resonance peak displayed a distinct red Poziotinib price shifting from 350 to 365 nm and became the strong peak when the diameter reached 40 nm. Duan et al. [42] also reported that a dual peak appeared when the incident light was perpendicular to the surface of the composite film of Cu nanowire arrays in ion-track templates. The dual peak with a shorter wavelength was attributed to interband

transition of Cu bulk metal, and the dual peak with a longer wavelength was ascribed to transverse dipolar peak, which displayed red a shift with increasing nanowire length. This result is obviously different from the blue shift reported by Zong et al. In order to clarify the difference, a new procedure to electrochemically fill ordered porous anodic alumina (OPAA) was developed where porous alumina remained on the aluminum substrate and the barrier layer was very thin by using a step-by-step AZD3965 voltage decrement process [43]. The thinning leads to a considerable decrease in the potential barrier for the electrons to tunnel through the barrier MRIP layer, when the metal is deposited at the pore tips. Ag and Cu nanocrystals (NCs) were successfully assembled into the ordered OPAA by a single-potential-step chronoamperometry technique, and the influences of preparation processes on the morphology, structure, and optical property of metallic NCs were deeply investigated.

Methods A highly ordered OPAA template with uniform pore diameters of about 60 nm and smooth pore channels perpendicular to the membrane surface was fabricated by a two-step anodization process plus a step-by-step voltage decrement method as described previously [43, 44]. The high purity alumina foil (99.999%) with size of 2 cm × 2 cm × 0.5 mm was firstly annealed at 500°C for 5 h and ultrasonic cleaned for 3 min in acetone, ethanol, and deionized water, respectively. The native oxide layer was removed in 2 mol/L NaOH solution at 60°C for 2 min. Then, the aluminum foil was anodized in 0.3 mol/L oxalic acid aqueous solution under constant voltage (40 V) and constant temperature (5°C). After anodization for 4 h, the formed alumina was removed by a mixture solution of phosphoric and chromic acids. Afterward, the foil was anodized for 5 h again at the same condition as the first anodization.