S. Peter Apell

Plasmonic Near-Field Absorbers for Ultrathin Solar Cells

If the active layer of efficient solar cells could be made 100 times thinner than in todays thin film devices, their economic competitiveness would greatly benefit. However, conventional solar cell materials do not have the optical capability to allow for such thickness reductions without a substantial loss of light absorption. To address this challenge, the use of plasmon resonances in metal nanostructures to trap light and create charge carriers in a nearby semiconductor material is an interesting opportunity. In this Perspective, recent progress with regards to ultrathin (∼10 nm) plasmonic nanocomposites is reviewed. Their optimal internal geometry for plasmon near-field induced absorption is discussed, and a zero thickness effective medium representation is used to optimize stacks including an Al back reflector for photovoltaics. This shows that high conversion efficiencies (>20%) are possible even when taking surface scattering effects and thin passivating layers inserted between the metal and semiconductor into account.

Resonant coupling between localized plasmons and anisotropic molecular coatings in ellipsoidal metal nanoparticles

We present an analytic theory for the optical properties of ellipsoidal plasmonic particles covered by anisotropic molecular layers. The theory is applied to the case of a prolate spheroid covered by chromophores oriented parallel and perpendicular to the metal surface. For the case that the molecular layer resonance frequency is close to being degenerate with one of the particle plasmon resonances strong hybridization between the two resonances occurs. Approximate analytic expressions for the hybridized resonance frequencies, their extinction cross-section peak heights, and widths are derived. The strength of the molecular-plasmon interaction is found to be strongly dependent on molecular orientation and suggests that this sensitivity could be the basis for novel nanoparticle based bio- and chemo-sensing applications.

Intrinsic Fano Interference of Localized Plasmons in Pd Nanoparticles

Palladium (Pd) nanoparticles exhibit broad optical resonances that have been assigned to so-called localized surface plasmons (LSPs). The resonances energy varies with particle shape in a similar fashion as is well known for LSPs in gold and silver nanoparticles, but the line-shape is always anomalously asymmetric. We here show that this effect is due to an intrinsic Fano interference caused by the coupling between the plasmon response and a structureless background originating from interband transitions. The conclusions are supported by experimental and numerical simulation data of Pd particles of different shape and phenomenologically analyzed in terms of the point dipole polarizability of spheroids. The latter analysis indicates that the degree of Fano asymmetry is simply linearly proportional to the imaginary part of the interband contribution to the metal dielectric function.

Competition between surface screening and size quantization for surface plasmons in nanoparticles

We present a theoretical model for analyzing the size dependence of the surface plasmon resonance of metallic nanospheres in a range of sizes down to a single nanometer. Within this model, we explicitly show how different microscopic mechanisms, namely quantization due to size (quantum size effect (QSE)) and dynamical surface screening, affect the energy of the surface plasmon. We demonstrate that the latter mechanism, which can move the surface plasma energy both toward the red or the blue, can be comparable to or even stronger than QSE. Thus, depending on material parameters, QSE may only be observed for ultra-small metal nanoparticles much closer to 1 nm in size than to 10 nm. Results presented herein are in quantitative agreement with recent published experimental results for Ag and Au.We develop a theoretical model of the surface plasmon resonance of metallic nanospheres in the size range down to the single nanometer size. Within this model we explicitly show how different microscopic mechanisms, namely quantization due to size (QSE) and electron spill-out, affect the energy of the surface plasmon. We demonstrate, that electron spill-out effects, which can move the surface plasma energy both toward the red or the blue, can be comparable to or even stronger than QSE. Thus, depending on circumstances, QSE may only be observed for ultrasmall metal nanoparticles much closer to 1 nm in size than to 10 nm. Results presented herein are in quantitative agreement with recent published experimental results for Ag and Au.

Molecular shape, capacitance, and chemical hardness

To elucidate the effects of overall molecular shape upon the electronic response properties of molecules and nanoclusters we recently have considered various jellium cluster models for the mean excitation energy. Here we apply similar models to characterize the relationship among gross molecular shape, the capacitance of an identically shaped spheroidal conductor, and the chemical hardness of the system η=(I−A)/2 (I,A are the first ionization energy and electron affinity, respectively). As with the mean excitation energies, the models possess reasonable predictive capability for these cases. Within a strict density functional interpretation, we also show that, quite unlike a classical capacitor, the capacitance of a nanoscale object is not independent of the way charge is added. Classical behavior is recovered by an average over the final charge state of the nanoscale capacitor.

Resource efficient plasmon-based 2D-photovoltaics with reflective support

For ultrathin (~10 nm) nanocomposite films of plasmonic materials and semiconductors, the absorptance of normal incident light is typically limited to about 50%. However, through addition of a non-absorbing spacer with a highly reflective backside to such films, close to 100% absorptance can be achieved at a targeted wavelength. Here, a simple analytic model useful in the long wavelength limit is presented. It shows that the spectral response can largely be characterized in terms of two wavelengths, associated with the absorber layer itself and the reflective support, respectively. These parameters influence both absorptance peak position and shape. The model is employed to optimize the system towards broadband solar energy conversion, with the spectrally integrated plasmon induced semiconductor absorptance as a figure of merit. Geometries optimized in this regard are then evaluated in full finite element calculations which demonstrate conversion efficiencies of up to 64% of the Shockley-Queisser limit. This is achieved using only the equivalence of about 10 nanometer composite material, comprising Ag and a thin film solar cell layer of a-Si, CuInSe₂ or the organic semiconductor MDMO-PPV. A potential for very resource efficient solar energy conversion based on plasmonics is thus demonstrated.

Diffuse Surface Scattering in the Plasmonic Resonances of Ultralow Electron Density Nanospheres.

Localized surface plasmon resonances (LSPRs) have recently been identified in extremely diluted electron systems obtained by doping semiconductor quantum dots. Here, we investigate the role that different surface effects, namely, electronic spill-out and diffuse surface scattering, play in the optical properties of these ultralow electron density nanosystems. Diffuse scattering originates from imperfections or roughness at a microscopic scale on the surface. Using an electromagnetic theory that describes this mechanism in conjunction with a dielectric function including the quantum size effect, we find that the LSPRs show an oscillatory behavior in both position and width for large particles and a strong blue shift in energy and an increased width for smaller radii, consistent with recent experimental results for photodoped ZnO nanocrystals. We thus show that the commonly ignored process of diffuse surface scattering is a more important mechanism affecting the plasmonic properties of ultralow electron density nanoparticles than the spill-out effect.

Optical enhancement of plasmonic activity of catalytic metal nanoparticles

Noble metals have recently been shown to drive direct photocatalytic reactions in which they both provide hot electrons via the localized surface plasmon resonance (LSPR) and the catalytically active site. Catalytic reactions are also possible on other metals such as platinum or rhodium which, however, exhibit rather poor plasmonic properties (low field enhancements, low resonance quality factors) and their LSPR for nanometer sized particles occurs in the UV, an unfavourable effect when considering sunlight as a photon source. By coupling the LSPR response of catalytic metal nanoparticles to that of a silver nanoparticle we can excite a hybridized resonance that matches the spectral characteristic of the light source and light absorption in the catalytic metal is enhanced by up to one order of magnitude. This is shown for a number of catalytic metals and is further discussed for model Drude and Drude–Lorentz materials. These results provide guidelines for designing catalytic metal nanostructures which absorb the solar spectrum very efficiently.

Surface scattering contribution to the plasmon width in embedded Ag nanospheres

Nanometer-sized metal particles exhibit broadening of the localized surface plasmon resonance (LSPR) in comparison to its value predicted by the classical Mie theory. Using our model for the LSPR dependence on non-local surface screening and size quantization, we quantitatively relate the observed plasmon width to the nanoparticle radius R and the permittivity of the surrounding medium ε(m). For Ag nanospheres larger than 8 nm only the non-local dynamical effects occurring at the surface are important and, up to a diameter of 25 nm, dominate over the bulk scattering mechanism. Qualitatively, the LSPR width is inversely proportional to the particle size and has a nonmonotonic dependence on the permittivity of the host medium, exhibiting for Ag a maximum at ε(m) ≈ 2.5. Our calculated LSPR width is compared with recent experimental data.

Theory of a magnetic microscope with nanometer resolution

We propose a theory for a type of apertureless scanning near field microscopy that is intended to allow the measurement of magnetism on a nanometer length scale. A scanning probe, for example a scanning tunneling microscope (STM) tip, is used to scan a magnetic substrate while a laser is focused on it. The electric field between the tip and substrate is enhanced in such a way that the circular polarization due to the Kerr effect, which is normally of order 0.1% is increased by up to two orders of magnitude for the case of a Ag or W tip and an Fe sample. Apart from this there is a large background of circular polarization which is non-magnetic in origin. This circular polarization is produced by light scattered from the STM tip and substrate. A detailed retarded calculation for this light-in-light-out experiment is presented.