Enhancing light absorption in organic semiconductor thin films by one-dimensional gold nanowire gratings

Dominik A. Gollmer, Christopher Lorch, Frank Schreiber, Dieter P. Kern, and Monika Fleischer

Phys. Rev. Materials 1, 054602 – Published 17 October 2017

Abstract

The interaction of metallic plasmonic nanostructures and organic semiconductor thin films plays a crucial role in engineering light harvesting and energy transfer processes, e.g., for optoelectronic applications. Plasmonic resonances of the metal structures can be used to increase the light emission or absorption of organic molecules. Here small molecules are employed since they can form organic layers with a defined crystalline order and orientation of the transition dipole. Extinction measurements combined with numerical simulations of a hybrid system consisting of a gold nanowire grating and a thin film of diindenoperylene (DIP) are reported. The experimental results are compared to the simulations and indicate an enhanced absorption in the wavelength region corresponding to the transition from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of DIP. This enhancement is found to be related to the localized field enhancement near the individual nanostructures as well as to grating-induced effects. Notably, the hybrid system also exhibits parallel lattice resonances, which have recently been discussed for two-dimensional (2D) gold nanostructure arrays. In this study a hybrid plasmonic-organic small molecule system exhibiting these modes is investigated. The results for this model system show a way to modify the optical properties of plasmonic nanostructures by collective effects to achieve stronger light-matter interaction in a wide range of hybrid plasmonic systems.

Received 3 May 2017
Revised 8 July 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.1.054602

Physics Subject Headings (PhySH)

Energy applications Energy materials Interference & diffraction of light Light propagation, transmission & absorption Light-matter interaction Plasmonics

Atomic, Molecular & OpticalPolymers & Soft MatterCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Dominik A. Gollmer^*, Christopher Lorch, Frank Schreiber, Dieter P. Kern, and Monika Fleischer^†

Institute for Applied Physics and Center for Light-Matter-Interaction, Sensors and Analytics LISA+, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 10 and 15, 72076 Tübingen, Germany

^*dominik.gollmer@uni-tuebingen.de
^†monika.fleischer@uni-tuebingen.de

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Issue

Vol. 1, Iss. 5 — October 2017

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Images

Figure 1
Fabrication scheme of the nanowire grating: (a) substrate with PMMA positive resist, (b) electron beam lithography, (c) evaporation of gold, (d) lift-off, (e) evaporation of diindenoperylene, and (f) scanning electron microscope (SEM) image of the nanowire grating with a period of 394 nm and linewidth of 101 nm. (g) Illumination scheme for TM and TE polarization. (h) Extinction spectrum of 20 nm DIP on a glass/ITO substrate. The inset shows a model of the diindenoperylene molecule, with µ indicating the orientation of the transition dipole moment for the HOMO-LUMO-transition, and a schematic illustration of the energy levels of gold and DIP.
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Figure 2
Experimental [(a) and (c)] and simulated [(b) and (d)] extinction data of gold nanogratings for TM [(a) and (b)] and TE polarization [(c) and (d)]. The solid blue and green curves show the spectra without DIP, and the dotted orange and red curves the spectra of the hybrid system. The vertical dashed lines indicate the positions of the LSPRs.
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Figure 3
Comparison of the extinction of gratings covered with DIP (dotted orange and red curves), as in Figs. 2 and 2 and with aluminum oxide (solid bright and dark green curves) for (a) TM and (b) TE polarization. The vertical dashed lines indicate that the LSPRs for the two hybrid systems in (a) are nearly identical.
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Figure 4
Simulated relative absorption of the DIP layer on a grating and a single wire, respectively, relative to DIP directly on a glass/ITO substrate for (a) TM and (b) TE polarization. Values larger than 1.0 correspond to absorption enhancement in the organic layer.
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Figure 5
Simulated distribution of the relative local electric field strength along a line 15 nm above the substrate (cross sectioning the wires at half their height) for excitation wavelengths of 400 to 800 nm. The distribution is shown for a unit cell, thus the nanowire center is positioned at 0 nm. The relative electric field strength is shown for a grating of nanowires $(period = 394 nm)$ and a single wire with the same dimensions ( $width = 101 nm, height = 30 nm$ ) for both TM and TE polarizations of the exciting field.
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Figure 6
Experimental extinction spectra of gold gratings with about (a) 500 nm and (b) 700 nm period and a 20 nm DIP film, measured with TM polarized illumination under perpendicular incidence. The parallel lattice modes (a) on the substrate side and (b) on the DIP/air side are clearly visible at about 750 and 700 nm.
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