DUV Plasmon Resonance in Aluminum nanoparticle Arrays

By a straightforward self-organization approach, arrays of Al/Al2O3 core/shell nanoparticles with a metallic-core diameter in the 12-25 nm range were fabricated, displaying sharp plasmonic resonances at very high energies up to 5.8 eV. Theoretical calculations allowed to correlate each experimental feature to the corresponding plasmon modes.

G. Maidecchi et al, ACS Nano, 7,7,5834-5841 (2013)


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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.

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.

In Fig.2A, we report the optical extinction of the Al NP arrays as a function of photon energy, measured at the BEAR beamline in normal-incidence geometry. The measurements of the extinction were performed in longitudinal or transverse configuration, i.e. with the electric-field vector aligned either parallel or perpendicular to the Al-NP “chains” (Fig. 2, top). In both configurations, a clear extinction peak indicates the successful detection of a LSPR within the NPs. In the transverse configuration, the LSPR is found at the strikingly high energy of 5.8 eV, the highest ever observed in optically-excited metallic NPs. In the longitudinal case, the plasmon hybridization along the NP chains redshifts the LSPR to the slightly lower value of 4.2 eV.
Figure 2: Top: experimental geometry for the optical transmission measurements. Panel B: optical extinction of the Al/LiF NP arrays in longitudinal and transverse geometries (open and full markers, respectively). Panel C: calculated extinction spectra.


Theoretical calculations by the Finite-Integration Technique (Fig. 2 B) reproduce the high-energy character of the LSPR excitation, along with the redshift of the longitudinal LSPR induced by the plasmon hybridization along the NP chains.
The successful observation of DUV LSPR in aluminum NPs represents a fundamental step forward for ushering plasmonics into the ultraviolet regime. Furthermore, the ease of fabrication of Al particles in the small-size regime and in ultradense arrays is extremely promising for application of these systems as high-efficiency plasmonic substrates for DUV plasmon-enhanced optical spectroscopies.
This research was conducted by the following research team:
Giulia Maidecchi, Luca Anghinolfi, Maurizio Canepa, Lorenzo Mattera; CNISM and Dipartimento di Fisica, Università di Genova, Genova, Italy
Grazia Gonella, Hai-Lung Dai; Temple University, Philadelphia (PA) U.S.A.
Remo Proietti Zaccaria; Istituto Italiano di Tecnologia, Genova, Italy
Angelo Giglia, Stefano Nannarone; CNR-IOM, Trieste, Italy
Riccardo Moroni, Francesco Bisio; CNR-SPIN, Genova Italy

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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

Last Updated on Monday, 20 July 2015 16:06