DUV Plasmon Resonance in Aluminum nanoparticle Arrays

Localized surface plasmon resonances (LSPR) are collective oscillations of the free-electron gas of  metallic nanostructures, excited by an incident electromagnetic (EM) field. LSPR are a very general feature of many metals, yet most of the research in plasmonics has so far focused on a handful of materials, typically the noble metals Ag and Au. The appeal of Au or Ag in plasmonics stems from their low dielectric losses in the visible regime and by their unparalleled low reactivity. Exploiting other metals in plasmonics would however have a strong impact on the field. New materials can indeed bring novel functionalities that would significantly broaden the reach of plasmonics, introducing plasmon-enhanced chemical, catalytic or magnetic activity, to name a few.
The free-electron-metal aluminum is considered by many of the most promising materials for the future of plasmonics. In its small-particle regime, aluminum has indeed the potential of exhibiting its LSPR in the deep-ultraviolet (DUV) region of the EM spectrum, inaccessible to most other materials. Furthermore, Al is predicted to outperform most other metals in terms of plasmon-induced EM near-field enhancement, something extremely appealing for plasmon-enhanced nonlinear optical spectroscopies. Finally, Al is significantly more abundant, hence cheaper, than its noble counterparts, a particularly appealing feature for sustainable plasmon-enhanced photovoltaics.
Successfully exploiting aluminum in plasmonics is however more challenging compared to the noble metals. Aluminum is reactive, hard to synthesize in small-particle form, and quickly oxidizes when exposed to atmosphere, thus degrading its plasmonic performances and redshifting its LSPR. In addition, the LSPR energy of Al nanoparticles (NPs) exhibits a very strong particle-size dependence, quickly redshifting for particle size in excess of 10-15 nm diameter.
Altogether, these technical issues have stood in the way of the full development of Al-based plasmonics, preventing in particular the achievement of the theoretically-suggested DUV limit of the LSPR.
In our work, we set out to experimentally test the theoretical limits of DUV plasmonics within ultradense (>10¹¹ particles/cm²) arrays of Al/Al₂/O₃ core-shell NPs. Taking advantage of the capabilities of the BEAR beamline to measure the optical response of materials in the UV-DUV spectral range, we addressed the optical response of the Al NP arrays up to 12 eV photon energy, and found the LSPR in the Al NPs at energies of 5.8 eV, by far the highest value reported so far for Al nanostructures, and well within the DUV range of the EM spectrum.
The Al NPs were realized by means of a self-organization approach (Fig.1, top). The systems were fabricated starting from the spontaneously-nanopatterned LiF(110) surface, that exhibits a regular, coherently-oriented uniaxial ridge-valley morphology with typical periodicity of 25 nm. Grazing-incidence metal evaporation leads to the formation of arrays of nanowires, that are thermally dewetted and then oxidized be exposure to atmosphere to form arrays of disconnected core/shell Al/Al₂O₃ NPs.
In Fig.1, middle, we report a representative AFM image of an Al NP array obtained by dewetting a 1.7 nm equivalent-thickness Al film (image size 800x800 nm²). In the image, small agglomerates arranged in “chains” aligned along the nanopatterned LiF surface ridges can be seen. Their mean in-plane diameter was estimated at (17±5) nm, nearly within the small-particle limit of aluminum for which a DUV LSPR was predicted.
High-resolution X-ray photoelectron spectroscopy (XPS) spectra, in the energy region of the Al 2p core level (Fig.1, bottom) reveal the presence of two well-defined peaks. The low-binding-energy peak corresponds to metallic Al, while the higher binding-energy structure can be assigned to Al oxide. Thus, the small Al NPs retain a metallic core even after exposure to atmosphere, making it possible to observe the LSPR excitation.

Figure 1: Top: Schematic representation of the Al nanoparticle array fabrication procedure. Middle: AFM image of the Al/LiF system obtained by dewetting of a 1.7 nm thick Al film (image size: 800x800 nm²). Bottom: XPS spctra of the Al 2p core level.

Retrieve article

Deep Ultraviolet Plasmon Resonance in Aluminum Nanoparticle Arrays, Giulia Maidecchi, Grazia Gonella, Remo Proietti Zaccaria, Riccardo Moroni, Luca Anghinolfi, Angelo Giglia, Stefano Nannarone, Lorenzo Mattera, Hai-Lung Dai, Maurizio Canepa, and Francesco Bisio, ACS Nano, Articles ASAP, DOI: 10.1021/nn400918n