Simulation of the absorption spectra of nanometallic Al particles with core-shell structure: Size-dependent interband transitions

Nanoaluminum combined with an oxidizing polymer binder is representative of a new class of nanotechnology energetic materials termed "structural energetic materials" that can be laser initiated by near-infrared heating of the Al particles. The visible and near-IR absorption spectra of Al nanoparticles passivated by the native oxide Al₂O₃, embedded in nitrocellulose (NC) binder, are simulated numerically using a model for the metallic dielectric function that incorporates the effects of interband transitions. The effects of oxide thickness, nanoparticle size and size distribution, and particle shape on the absorption characteristics are investigated. The nanoparticle spectra evidence an absorption peak and valley in the 550-1,100 nm range that redshift with decreasing nanoparticle size. Calculations indicate that this peakvalley structure results from interband transitions, and the unusual redshift cannot be explained without using an interband transition onset frequency that varies with nanoparticle size.