Although the optical and plasmonic properties of noble metal (Ag and Au) nanoparticles (NPs) have been thoroughly studied and reported, less information is available concerning NPs made of non-noble metals or semimetals that present a more complex electronic structure. In this work, we combine experiments and modeling to explore the optical response of bismuth NPs in the near-ultraviolet, visible, and near-infrared, which has not been studied so far, despite the unusual and interesting electronic properties of bulk Bi. Two dimensional distributions of Bi NPs with different topologies have been prepared and embedded in a protecting and transparent dielectric matrix, thus providing robust materials suitable for structural and optical characterizations. The Bi NP distributions display optical resonances whose spectral position and width are topologysensitive. The observed macroscopic optical response has been modeled by quasistatic effective medium models, and the analysis shows that the optical resonances present features similar to those of surface plasmon resonances, such as environmental sensitivity. In contrast to noble metals resonances, important nonradiative damping is evidenced in the whole near-ultraviolet-tonear-infrared range, likely due to interband desexcitation paths available in Bi in relation to its electronic structure. Finally, dynamic calculations of the optical extinction performed as a function of the NP's size and shape show a roadmap for tuning the spectral position of the optical resonances in Bi NPs in the whole near-ultraviolet, visible, and near-infrared range.
For years bismuth (Bi) has appealed to a broad community of scientists due to its peculiar electronic, optical, and more recently plasmonic and photocatalytic properties, which enable both the understanding of basic science phenomena and the development of a wide range of applications. In spite of this interest, a comprehensive spectral analysis of the dielectric function (ε = ε 1 + jε 2 ) of bulk Bi from the far infrared (IR) to the ultraviolet (UV) region is not available. So far, the data have been reported in limited spectral ranges and show a wide dispersion that is especially notorious for the IR region. In this work we report ε for Bi in a wide spectral range from 0.05 to 4.7 eV (24.8 to 0.3 μm, far IR to UV). ε is extracted from spectroscopic ellipsometry measurements of excellent quality (dense and smooth) Bi films by using the transfer matrix formalism and Kramers−Kronig consistent analysis. The higher quality and accuracy of the obtained ε compared with the literature data is demonstrated. The analysis and use of this reference bulk dielectric function provides crucial information for the exploration and understanding of the optical, plasmonic, and photocatalytic properties of Bi nanostructures. From its analysis, it is evidenced that the optical properties of Bi in the mid wave IR-to-UV are driven only by interband transitions, which are responsible for the dominant absorption band peaking at about 0.8 eV. Therefore, the plasmonic behavior and the photocatalytic performance of Bi nanostructures in the visible and UV are likely driven by these interband transitions that make ε 1 turn negative in this region without the need of exciting free carriers. Furthermore, classical electrodynamic simulations using the obtained ε show a strong size dependence for the optical extinction of Bi nanospheres in the far IR-to-near IR with Mie-like resonances broadly tunable across this region.
The strong absorption of light [ 1 ] and the local amplifi cation of the electromagnetic fi eld [ 2 ] at the plasmon resonance of noble metal nanostructures have been the focus of hundreds of studies due to their practical applications for the fabrication of optical devices such as fi lters, non-linear optical components, or Raman enhancers. [ 3,4 ] The control of the plasmon features such as spectral width, [ 5 ] position, [ 6 ] and shape [ 7 ] can be accomplished by different physical deposition routes [8][9][10][11][12] providing adequate growing conditions of metal nanoparticles (MNPs). Pioneer works in the 1990s showed the optical selectivity of elongated Ag deposits on SiO 2 with applications as optical fi lters for windows to control solar heat gain and glare, among others. [ 13 ] Recently, assemblies of parallel stripes of MNPs have been fabricated onto preformed surfaces presenting a 1D periodic roughness [ 14 , 15 ] or bundled SiO 2 nanocolumns. [ 16 , 17 ] A signifi cant macroscopic optical dichroism has been reported for these systems that can be useful for the development of polarized light emitters or materials with an enhanced IR luminescence because of the excitation of two distinct plasmon resonances in the directions parallel (longitudinal mode) and perpendicular (transverse mode) to the stripes. [ 18 ] Architecture control of the metal assemblies plays a determinant role in the functional properties of the material. For this purpose, the softlithographic techniques provide means to accurately tailor the nanostructure of the materials. [18][19][20][21] Laser scanning is a softlithographic technique widely used to modify the shape and structure of metal nanoparticles. [21][22][23][24] Surface modifi cation can be easily achieved by in situ [ 21 ] or ex situ [22][23][24] pulsed laser treatment in the case of random systems of MNPs. In contrast, nothing has been reported about the effect of a pulsed laser on the structure and optical dichroism of autoorganized metal nanostructures. In this paper we show that nanosecond (ns) laser irradiation can be effectively used to control the optical dichroism of Ag stripes supported on SiO 2 nanocolumns (NCs). This dichroism can be effectively tailored along the full visible range. Thus, we propose the utilization of the AgNPs/SiO 2 NCs structures for writing dichroic patterns at the microscale with potential applications for encryption and data storage purposes.AgNPs/SiO 2 NCs fi lms were grown by a two-step process.[ 17 ]First, SiO 2 thin fi lms were deposited by glancing angle vapor deposition (GLAD) with a tilted columnar nanostructure and ≈ 350 nm thickness (see Figure S1a in the Supporting Information and the Experimental Section). [ 25 , 26 ] These structures present an anisotropic surface topography known as "bundling", [ 17 ] consisting of the coalescence of the NCs along the x -direction ( Figure S1b). The silver nanoparticles were then grown by DC sputtering at room temperature. The "bundled" SiO 2 NCs act as a template for the fabrication of Ag...
Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies we present in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, we induce a robust nano-crystalline ordering in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC 71 BM cells are shown to exhibit T 80 lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.
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