Annemarie Pucci

Optical Nanoantennas for Multiband Surface-Enhanced Infrared and Raman Spectroscopy

In this article we show that linear nanoantennas can be used as shared substrates for surface-enhanced Raman and infrared spectroscopy (SERS and SEIRS, respectively). This is done by engineering the plasmonic properties of the nanoantennas, so to make them resonant in both the visible (transversal resonance) and the infrared (longitudinal resonance), and by rotating the excitation field polarization to selectively take advantage of each resonance and achieve SERS and SEIRS on the same nanoantennas. As a proof of concept, we have fabricated gold nanoantennas by electron beam lithography on calcium difluoride (1-2 μm long, 60 nm wide, 60 nm high) that exhibit a transverse plasmonic resonance in the visible (640 nm) and a particularly strong longitudinal dipolar resonance in the infrared (tunable in the 1280-3100 cm(-1) energy range as a function of the length). SERS and SEIRS detection of methylene blue molecules adsorbed on the nanoantennas surface is accomplished, with signal enhancement factors of 5×10(2) for SERS (electromagnetic enhancement) and up to 10(5) for SEIRS. Notably, we find that the field enhancement provided by the transverse resonance is sufficient to achieve SERS from single nanoantennas. Furthermore, we show that by properly tuning the nanoantenna length the signals of a multitude of vibrational modes can be enhanced with SEIRS. This simple concept of plasmonic nanosensor is highly suitable for integration on lab-on-a-chip schemes for label-free chemical and biomolecular identification with optimized performances.

Resonances of individual metal nanowires in the infrared

With infrared spectroscopic microscopy using synchrotron light, the authors studied resonant light scattering from single metal nanowires with diameters in the 100nm range and with lengths of a few microns. The Au and Cu nanowires were electrochemically grown in polycarbonate etched ion-track membranes and transferred on infrared-transparent substrates. Significant antennalike plasmon resonances were observed in good agreement with exact light-scattering calculations. The resonances depend not only on length and diameter but also on the dielectric surrounding of the nanowire. The observed maximum extinction at resonance corresponds to an electromagnetic far-field enhancement by a factor of about 5.

Experimental verification of the spectral shift between near- and far-field peak intensities of plasmonic infrared nanoantennas.

Theory predicts a distinct spectral shift between the near- and far-field optical response of plasmonic antennas. Here we combine near-field optical microscopy and far-field spectroscopy of individual infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations corroborate our experimental results. We furthermore discuss the implications of this effect in surface-enhanced infrared spectroscopy.

Longitudinal and transverse coupling in infrared gold nanoantenna arrays: long range versus short range interaction regimes

Interaction between micrometer-long nanoantennas within an array considerably modifies the plasmonic resonant behaviour; for fundamental resonances in the infrared already at micrometer distances. In order to get systematic knowledge on the relationship between infrared plasmonic resonances and separation distances dx and dy in longitudinal and transverse direction, respectively, we experimentally studied the optical extinction spectra for rectangularly ordered lithographic gold nanorod arrays on silicon wafers. For small dy, strong broadening of resonances and strongly decreased values of far-field extinction are detected which come along with a decreased near-field intensity, as indicated by near-field amplitude maps of the interacting nanoantennas. In contrast, near-field interaction over small dx does only marginally broaden the resonance. Our findings set a path for optimum design of rectangular nanorod lattices for surface enhanced infrared spectroscopy.

Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites.

The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high-quality infrared spectroscopic measurements of methylammonium lead halide perovskite (CH3NH3Pb(I/Br/Cl)3) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH3(+) stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites.

Surface enhanced infrared absorption of octadecanethiol on wet-chemically prepared Au nanoparticle films

Surface enhanced infrared absorption of adsorbates on wet-chemically prepared Au islands was investigated using octadecanethiol as adsorbate. The preparation was done by letting Au nanoparticles adsorb on an oxidized silicon wafer surface followed by wet-chemically increasing the size of the Au nanoparticles. The surface enhanced infrared spectrum was measured in transmission geometry at normal incidence. Comparing the size of the antisymmetric CH2 stretch absorption of adsorbates on Au islands to the corresponding infrared reflection absorption signal on a 40nm thick smooth Au film gives an enhancement factor of about 2000.

Optical phonons in methylammonium lead halide perovskites and implications for charge transport

Lead-halide perovskites are promising materials for opto-electronic applications. Recent reports indicated that their mechanical and electronic properties are strongly affected by the lattice vibrations. Herein we report far-infrared spectroscopy measurements of CH3NH3Pb(I/Br/Cl)3 thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine the frequencies of the transversal and longitudinal optical phonons, and use them to calculate, via appropriate models, the static dielectric constants, polaron masses, electron–phonon coupling constants, and upper limits for the phonon-scattering limited charge carrier mobilities. Within the limitations of the model used, we can place an upper limit of 200 cm2 V−1 s−1 for the room temperature charge carrier mobility in MAPbI3 single crystals. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

The correlation between film thickness and adsorbate line shape in surface enhanced infrared absorption

Various experimental results on surface enhanced infrared absorption reveal asymmetric line shapes. Whereas the order of magnitude of the enhancement can be understood from electromagnetic field enhancement the unusual line shape remains without satisfactory explanation. An interaction with electron-hole pairs would lead to an asymmetric line but this should be restricted to the first monolayer. However, asymmetry is also observed for vibrations at larger distances from the metal-film surface. Here we show strongly asymmetric lines and their enhancement as a consequence of the interaction of adsorbate vibrations with surface plasmons of metal islands. Both the effects and also the baseline change can be estimated by a proper application of well established effective-media models.

Infrared optical properties of nanoantenna dimers with photochemically narrowed gaps in the 5 nm regime.

In this paper, we report on the manipulation of the near-field coupling in individual gold nanoantenna dimers resonant in the infrared (IR) spectral range. Photochemical metal deposition onto lithographically fabricated nanoantennas is used to decrease the gap between the antenna arms down to below 4 nm, as confirmed by finite-difference time-domain simulations. The increased plasmonic coupling in the gap region leads to a shift of the surface plasmon resonances to lower energies as well as to the appearance of hybridized plasmonic modes. All of the occurring electron oscillation modes can be explained by the plasmon hybridization model. Besides the bonding combination of the fundamental resonances of individual antennas, also the antibonding combination is observed in the IR transmittance at normal incidence. Its appearance is due to both structural defects and the small gaps between the antennas. The detailed analysis of individual IR antennas presented here allows a profound understanding of the spectral features occurring during the photochemical manipulation process and therefore paves the way to a full optical process monitoring of sub-nanometer scale gaps, which may serve as model systems for experimental studies of quantum mechanical effects in plasmonics.

ASYMMETRIC LINE SHAPES AND SURFACE ENHANCED INFRARED ABSORPTION OF CO ADSORBED ON THIN IRON FILMS ON MGO(001)

We have measured infrared transmission spectra of CO adsorbed on in situ grown iron films on MgO(001) under ultrahigh vacuum conditions. Even at normal incidence we observed strong absorption lines in the C–O stretch region with number, intensities, positions, and shapes dependent on CO coverage and Fe-film thickness and morphology. This absorption must be due to vibrational dipoles oblique to the substrate surface, e.g., due to molecules at island walls. The distinct absorption lines can be assigned to distinct adsorption sites on crystalline facets of epitaxial Fe islands on MgO(001). For each of the films the strongest CO line shows an asymmetric shape. Additionally, the observed absorption is enhanced by at least two orders of magnitude with respect to adiabatic intensities of, e.g., CO on NaCl. Line shapes and intensity let us suggest nonadiabatic coupling of the adsorbate vibration to electronic transitions.