Klaus Kuhnke

Chemical imaging of interfaces by sum-frequency generation microscopy: Application to patterned self-assembled monolayers

We demonstrate molecule-specific imaging of a chemically patterned self-assembled monolayer by IR-visible sum-frequency microscopy. The pattern on an Au substrate consists of microcontact printed 10 μm wide alkanethiolate stripes embedded in ω-carboxyalkanethiolate adsorbed from solution. We use both electronic and vibrational contrast mechanisms for a quantitative analysis of thiolate density and the coverage of the two molecular species. The evaluation of images taken at three different IR wavelengths suggests a substantial intermixing of the two thiolates occuring in the preparation procedure.

Vicinal metal surfaces as nanotemplates for the growth of low-dimensional structures

Vicinal surfaces, which exhibit a regular array of steps, introduce defined arrangements of surface defects, which have the potential to create specific functionalities of the surface. In particular they can be used as templates for the growth of one-dimensional structures using selective step decoration. In this article we discuss the properties of vicinal metal surfaces and how they can be used as nanotemplates. The requirements for the growth of low-dimensional adsorbate structures at step edges o ft he vicinal (997) and (779) surfaces of platinum will be discussed in detail. Here energetics determined by the different adsorption sites and the kinetics present through the diffusion processes play an essential role. In order to obtain stable arrangements the propensity of the elements for alloy formation must be taken into account. Examples for the properties of the structures obtained and their role in studying one-dimensional systems are discussed, and we give a short outlook on how the principles of step decoration might be extended to a kind of atomic assembly of more complex surface nanostructures.

Sum-frequency generation microscope for opaque and reflecting samples

We report on the performance of a microscope setup, which has been developed for the imaging of sum-frequency generation (SFG) from reflecting, nontransparent samples. In order to maximize the SFG intensity the sample has to be observed from one side at an angle near 60° with respect to the surface normal. The setup is designed (a) to keep focus over the full image field and (b) to compensate for the distortion of the field-of-view, both by means of a blazed grating. In contrast to “specular” SFG spectroscopy, the incident beams reflected from the sample and the generated SF light cannot be separated by angular filtering. In this setup the separation thus relies on spectral filtering only.

Molecular Orbital Gates for Plasmon Excitation

Future combinations of plasmonics with nanometer-sized electronic circuits require strategies to control the electrical excitation of plasmons at the length scale of individual molecules. A unique tool to study the electrical plasmon excitation with ultimate resolution is scanning tunneling microscopy (STM). Inelastic tunnel processes generate plasmons in the tunnel gap that partially radiate into the far field where they are detectable as photons. Here we employ STM to study individual tris-(phenylpyridine)-iridium complexes on a C60 monolayer, and investigate the influence of their electronic structure on the plasmon excitation between the Ag(111) substrate and an Ag-covered Au tip. We demonstrate that the highest occupied molecular orbital serves as a spatially and energetically confined nanogate for plasmon excitation. This opens the way for using molecular tunnel junctions as electrically controlled plasmon sources.

Electroluminescence from Individual Pentacene Nanocrystals

Keywords: exciton ; luminescence ; nanostructures pentacene ; scanning probe microscopy ; Photon-Emission ; Single-Crystals ; Metal-Surfaces ; Molecules ; Spectroscopy ; States Reference EPFL-ARTICLE-172137doi:10.1002/cphc.201000531View record in Web of Science Record created on 2011-12-16, modified on 2017-05-12

Versatile optical access to the tunnel gap in a low-temperature scanning tunneling microscope.

We developed a setup that provides three independent optical access paths to the tunnel junction of an ultrahigh vacuum low temperature (4.2 K) scanning tunneling microscope (STM). Each path can be individually chosen to couple light in or out, or to image the tunnel junction. The design comprises in situ adjustable aspheric lenses to allow tip exchange. The heat input into the STM is negligible. We present in detail the beam geometry and the realization of lens adjustment. Measurements demonstrate the characterization of a typical light source exemplified by emission from tip-induced plasmons. We suggest employing the Fourier transforming properties of imaging lenses and polarization analysis to obtain additional information on the light emission process. Performance and future potential of the instrument are discussed.

Quantitative mapping of fast voltage pulses in tunnel junctions by plasmonic luminescence

An optical read-out technique is demonstrated that enables mapping the time-dependent electrostatic potential in the tunnel junction of a scanning tunneling microscope with millivolt and nanosecond accuracy. We measure the time-dependent intensity of plasmonic light emitted from the tunnel junction upon excitation with a nanosecond voltage pulse. The light intensity is found to be a quantitative measure of the voltage between tip and sample. This permits non-invasive mapping of fast voltage transients directly at the tunnel junction. Knowledge of the pulse profile reaching the tunnel junction is applied to optimize the experiments time response by actively shaping the incident pulses.

Atomic-Scale Imaging and Spectroscopy of Electroluminescence at Molecular Interfaces

The conversion of electric power to light is an important scientific and technological challenge. Advanced experimental methods have provided access to explore the relevant microscopic processes at the nanometer scale. Here, we review state-of-the-art studies of electroluminescence induced on the molecular scale by scanning tunneling microscopy. We discuss the generation of excited electronic states and electron-hole pairs (excitons) at molecular interfaces and address interactions between electronic states, local electromagnetic fields (tip-induced plasmons), and molecular vibrations. The combination of electronic and optical spectroscopies with atomic-scale spatial resolution is able to provide a comprehensive picture of energy conversion at the molecular level. A recently developed aspect is the characterization of electroluminescence emitters as quantum light sources, which can be studied with high time resolution, thus providing access to picosecond dynamics at the atomic scale.

Absence of local magnetic moments in Ru and Rh impurities and clusters on Ag(100) and Pt(997)

6 pages, 5 figures.-- PACS nrs.: 75.20.Hr; 78.20.Ls; 78.70.Dm.-- ArXiv pre-print available at: http://arxiv.org/abs/0708.3975

Second-harmonic spectroscopy of fullerenes

Abstract Optical second-harmonic generation spectra of C 60 and C 70 films are presented for the fundamental energy range 1.0–2.3 eV. The linewidths of the observed resonances are around 0.1 eV and thus much narrower than in linear absorption spectra of fullerene solids. We discuss the assignment of the resonances. Allowed and electric dipole forbidden transitions are observed for C 60 . The resonances observed for C 70 are very weak. Efficient quenching of the signal by the population of excitonic states is observed only for the lowest observed resonance of C 60 at 1.18 eV.