Assembly of PCBM and tn-ZnPc Molecular Domains and Phase Separation of Molecular Mixtures from Liquid Solution on Si(111).

We present the results of investigations aimed at understanding the mechanisms by which small, mutually immiscible organic molecules self-assemble into domains during phase separation from liquid solutions in the presence of a solid substrate. As an example system, we investigated molecular mixtures of tetranitro zinc-phthalocyanine (tn-ZnPc), an electron donor, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), an electron acceptor, in chloroform solution, deposited onto native oxide-covered Si(111) substrates. We found qualitatively different behavior in the formation of domains of the two types of molecules, seemingly because of a large difference in their solubilities in the solvent. The morphology of PCBM molecule domains varies widely depending on the solvent evaporation rate, the presence/absence of tn-ZnPc molecules in the solution, solvent annealing conditions, and seemingly the presence/absence of trace impurities. By contrast, we find evidence that the shape of the tn-ZnPc domains is insensitive to evaporation; these form while in liquid suspension, and the assembly of domains into clusters varies according to the rate and mode of solvent evaporation.

Impact of interface roughness on the performance of broadband blackbody absorber based on dielectric-metal film multilayers.

We report on factors affecting the performance of a broadband, mid-IR absorber based on multiple, alternating dielectric / metal layers. In particular, we investigate the effect of interface roughness. Atomic layer deposition produces both a dramatic suppression of the interface roughness and a significant increase in the optical absorption as compared to devices fabricated using a conventional thermal evaporation source. Absorption characteristics greater than 80% across a 300 K black body spectrum are achieved. We demonstrate a further increase in this absorption via the inclusion of a patterned, porous anti-reflection layer.

Broadband and mid-infrared absorber based on dielectric-thin metal film multilayers.

We propose a periodic multilayer structure of dielectric and metal interlayers to achieve a near-perfect broadband absorber of mid-infrared radiation. We examine the influence of four factors on its performance: (1) the interlayer metal conductance, (2) the number of dielectric layers, (3) a nanopatterned antireflective layer, and (4) a reflective metallic bottom layer for backreflection. Absorption characteristics greater than 99% of the 300 K and 500 K blackbody spectra are found for the optimized structures. Incident angle and polarization dependence of the absorption spectra are examined. We also investigate the possibility of fabricating a nanopatterned antireflective layer to maximize absorption.

Suppression of long-range collective effects in meta-surfaces formed by plasmonic antenna pairs.

The collective effects in a periodic array of plasmonic double-antenna meta-molecules are studied. We experimentally observe that the collective behavior in this structure substantially differs from the one observed in their single-antenna counterparts. This behavior is explained using an analytical dipole model. We find that in the double-antenna case the effective dipole-dipole interaction is significantly modified and the transverse long-range interaction is suppressed, giving rise to the disappearance of Wood's anomalies. Numerical calculations also show that such suppression of long-range interaction results in an anomalous spatial dispersion of the electric-dipolar mode, making it insensitive to the angle of incidence. In contrast, the quadrupolar mode of the antenna pair experiences strong spatial dispersion. These results show that collective effects in plasmonic metamaterials are very sensitive to the design and topology of meta-molecules. Our findings envision the possibility of suppressing the spatial dispersion effects to weaken the dependence of the metamaterials' response on the incidence angle.

Effect of gold nanopillar arrays on the absorption spectrum of a bulk heterojunction organic solar cell.

We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.

Enhanced fluorescence by metal nanospheres on metal substrates.

We investigate the metal enhanced fluorescence by silver nanospheres on a thin silver substrate. Experimental measurements for core/shell colloidal nanocrystals embedded in a polymer matrix show a fluorescence enhancement factor of about 9. We apply the discrete dipole approximation method to describe the local-field enhancement factor (LFEF). We find that the observed fluorescence enhancement is related to the broad LFEF profile induced by the substrate.

Spacer layer effect in fluorescence enhancement from silver nanowires over a silver film; switching of optimum polarization.

We explore the role of coupling between silver nanowires and an underlying silver film in fluorescence enhancement from proximal molecules. Variation of the thickness of an oxide layer separating nanowire arrays from the Ag film causes an alternation in the incident light polarization that produces the highest enhancement. Finite difference time domain calculations show that it results from an alternation of regions of high field above and between nanowires as the spacer thickness is increased.

Enhanced fluorescence and near-field intensity for Ag nanowire/nanocolumn arrays: evidence for the role of surface plasmon standing waves.

We use scanning fluorescence microscopy and electron beam lithography to probe the mechanism of fluorescence enhancement by periodic arrays of silver nanostructures, determining the optimum size and spacing of both Ag nanowires and Ag nanocolumns for incident light of different wavelengths and polarizations. Finite difference time domain (FDTD) calculations show a systematic variation with spatial period and incident polarization of the local electric field above the surface of the arrays which correlate well with that of the measured fluorescence enhancement, but a lack of a simple proportionality indicates that the dependence of the radiative and nonradiative decay rates on array geometry must be included in models for this effect. The dependence of the enhancement on spatial period and polarization indicates the importance of surface plasmon standing waves in this effect.