Dominik Enders

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.

Plasmons in nanoscale and atomic-scale systems

Abstract Plasmons in metallic nanomaterials exhibit very strong size and shape effects, and thus have recently gained considerable attention in nanotechnology, information technology, and life science. In this review, we overview the fundamental properties of plasmons in materials with various dimensionalities and discuss the optical functional properties of localized plasmon polaritons in nanometer-scale to atomic-scale objects. First, the pioneering works on plasmons by electron energy loss spectroscopy are briefly surveyed. Then, we discuss the effects of atomistic charge dynamics on the dispersion relation of propagating plasmon modes, such as those for planar crystal surface, atomic sheets and straight atomic wires. Finally, standing-wave plasmons, or antenna resonances of plasmon polariton, of some widely used nanometer-scale structures and atomic-scale wires (the smallest possible plasmonic building blocks) are exemplified along with their applications.

Optical detection of plasmonic and interband excitations in 1-nm-wide indium atomic wires

Infrared spectroscopy is demonstrated to sensitively detect electronic excitations in 1-nm-wide wires made of indium. The polarization-dependent spectra measured at room temperature show a strong broadband plasmonic absorption feature in the direction parallel to the wires, while in the perpendicular direction the wires stay nearly transparent in the same spectral range. At 88 K the wires do not show this broadband absorption anymore, but instead, several interband-transition features arise for both polarizations, in agreement to the gap opening of the metal-to-insulator transition as known for this one-dimensional structure.

Precisely Controlled Fabrication of a Highly Sensitive Au Sensor Film for Surface Enhanced Spectroscopy

We propose a new method of infrared (IR) monitored two-step wet-chemical preparation of plasmonic Au nanostructures with a high degree of surface enhanced infrared absorption (SEIRA) activity. Au nanoparticles (AuNP) are deposited on SiO2/Si substrate and grown to form a network of densely packed islands very close to the two-dimensional percolation threshold. Monitoring the growth process with IR spectroscopy enables the termination of growth at a well defined state of the film morphology and is a great advantage compared to the previous methods. An octadecanethiol (ODT) monolayer adsorbed on the SEIRA optimized film gives a huge absorption intensity of 16.5% for the antisymmetric CH2 stretching vibration.

Two-Step Desorption Process of Au Nanoparticles in D2O Suspension on Amino-Terminated SiO2/Si Substrate Induced by Small Thiol Molecules

We investigated the desorption process of Au nanoparticles immobilized on an amino-terminated SiO2/Si surface in a D2O environment by in situ attenuated total reflection surface enhanced infrared absorption spectroscopy. The advantages of IR spectroscopy over other methods were demonstrated; the IR spectra are not only sensitive to the changing particle density at the surface, but also to chemical environmental changes. Therefore, the gained information obtained by IR spectroscopy and scanning electron microscopy allows us to propose a microscopic model for the desorption process of the Au nanoparticles from the substrate.

Characteristic IR C=C stretch enhancement in monolayers by nonconjugated, noncumulated unsaturated bonds.

Control over and understanding of single-molecule covalent coatings becomes increasingly important in tailoring surfaces during the fabrication of nanoscale electrical or optical elements, such as organic field-effect transistors and light-emitting devices as well as microelectromechanical systems as the relevant feature sizes decrease. In this work, we develop a model based on IR spectra from public databases and DFT calculations that can be used to semiquantitatively assess the level of double bonds in monolayer coatings. We use the model to show the enhancement of the C=C vibrational mode due to silicon substitution and also from additional unsaturated bonds. Simple models for other functional groups in organic monolayers could be produced similarly.