Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

This thesis reports on the optical properties of fullerene molecules forming nanoscale crystals grown on ultrathin NaCl films on Au(111) investigated by Scanning Tunneling Microscope (STM)-induced light emission spectroscopy. In this regard, an existing optical setup has been improved. The new instrument facilitates the alignment of the detection lens with the tunnel junction. STM images of the investigated samples show that, upon adsorption on a NaCl covered Au(111) surface, C60 and C70 crystallize into multilayer islands with a hexagonally arranged close-packed top layer in which electron-poor regions on one molecule face electron-rich regions on the adjacent molecule. Luminescence spectra from these nanocrystals induced by electrons tunneling through a double barrier STM junction are presented. Their bias dependence as well as their similarity with laser-induced photoluminescence spectra indicate a hot electron injection mechanism followed by a radiative decay associated with the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap of the molecules. Local fluorescence and phosphorescence of C70 molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover, the molecular luminescence is demonstrated to be selectively enhanced by localized surface plasmons in the STM tip-sample gap, the tunnel junction acting as an optical antenna. Finally, the results of a scanning tunneling microscopy and scanning tunneling spectroscopy investigations of self-assembled islands of Azure A molecules on a Au(111) substrate are presented. Although no final conclusion can be drawn on the molecular arrangement owing to the impossibility to recognize specific parts of the molecule, the presence of these structures is found to modify the Au(111) herringbone reconstruction.

Electronic and optical properties of supported C60 molecules studied by scanning tunneling microscopy

Elizabeta Cavar

This thesis is primarily concerned with optical properties of adsorbed C60 molecules on metal and insulating substrates. These properties were studied on a single-molecule scale, using Scanning Tunneling Microscope-induced light emission spectroscopy (STM-LES). In the first part of the thesis (Chapter 2), we present the investigation of the structural and electronic properties of C60 islands on Au(111) and on ultrathin NaCl films. We further present a study of the structure of ultrathin sodium chloride films grown on Au(111), and the influence of the herringbone reconstruction on the interface state of a NaCl-covered Au(111) surface. In the second part of the thesis (Chapter 3 and 4), we show that applying a local technique such as STM induced optical spectroscopy enables the exploration of the optical characteristics of individual objects on the nanoscale. Contrary to conventional non-local techniques, the local character of this technique offers the unique possibility to select and probe individual atoms, molecules or clusters on surfaces. Here, we demonstrate the capability of STM-induced light emission spectroscopy (STM-LES) for the chemical recognition on a single-molecule scale. In Chapter 3, we present STM-induced light emission spectroscopy (STM-LES) of a clean Au(111) surface, Au(111) surface covered with islands of C60 molecules and of Au(111) covered with ultrathin NaCl films. We have performed an extensive investigation of the dependence of the gold plasmon spectra on tunneling current and voltage. We show the influence of nano-objects on the shape and intensity of the spectra. Furthermore, we investigated the influence of this experimental technique on morphological changes of surfaces and adsorbed layers. In Chapter 4, we present the first observation of energy resolved luminescence from selected C60 molecules excited by electrons tunnelling through a double barrier STM junction. A comparison with the luminescence spectra obtained by non-local spectroscopy from dispersed C60 molecules in rare gas matrices enables us to demonstrate the molecular origin of the detected light and to identify the observed spectral features. The present novel observation of both, fluorescence and phosphorescence constitutes a solid basis for the chemical identification of an individual C60 molecule.

EPFL2005

Single molecule detection and fluorescence correlation spectroscopy on surfaces

Kai Hassler

In this thesis a new approach for single molecule detection and analysis is explored. This approach is based on the combination of two well established methods, fluorescence correlation spectroscopy (FCS) and total internal reflection fluorescence microscopy (TIRFM). In contrast to most existing fluorescence spectroscopy techniques, the subject of primary interest in FCS is not the fluorescence intensity itself but the random intensity fluctuation around the mean value. Intensity fluctuations are induced by thermal noise in a minute observation volume, which is in classical FCS the confocal volume of a confocal microscope. E.g. FCS is commonly utilized to investigate diffusion. In this case, diffusing fluorescent molecules entering or leaving the observation volume cause intensity fluctuations, which are analyzed by calculating the temporal autocorrelation of the observed signal. The autocorrelation is a measure for the self-similarity of a signal and contains information about the average fluctuation strength and duration. The confocal observation volume, i.e. the measurement volume that is actually seen by the detector is approximately given by the product of the optical transfer function with the fluorescence exciting intensity distribution of a focused laser beam. To achieve a high signal-to-background ratio a small observation volume is absolutely essential, first of all because the background from e.g. scattered light increases with the size of the observation volume. Second, a small volume assures for a small average number of fluorophores inside the observation volume and therefore for a high fluctuation amplitude i.e. FCS signal. This thesis proposes and discusses an alternative to confocal FCS specially conceived for measurements on surface-bound molecular systems, such as biological receptors or immobilized enzymes. In contrast to confocal FCS, fluorescence is excited within an evanescent field generated by total internal reflection (TIR) of a laser beam at the interface between a microscope coverslip and the sample. This is achieved by focusing the laser beam off-axis at the back focal plane of a high NA oil-immersion objective. The collimated beam that emerges from the objective is incident at an oblique angle at the coverslip-sample interface and totally internal reflected. In contrast to confocal FCS, the generated observation volume is completely confined to the surface and background fluorescence as well as scattered light from the bulk is efficiently suppressed. Our method, called objective-type TIR-FCS in the following, features an increased collection efficiency compared to existing techniques that combine evanescent wave excitation and FCS. Existing techniques use total internal reflection on the surface of a prism to generate an evanescent field. This leads to a configuration where the choice of objectives is limited to air or water-immersion objectives. In our system we use a high NA oil-immersion objective, specially conceived for TIR applications, which collects light efficiently. The collection efficiency is further enhanced by a naturally occurring change of the emission properties of fluorophores close to interfaces between dielectric media. The presence of the interface favors emission into the optically denser medium so that about 60% of the emitted light can be collected. These factors, together with a reduced observation volume lead to a very sensitive method with a high potential for applications in single molecule detection and analysis. The performance of the proposed method was experimentally shown for measurements on molecules subject to Brownian motion and binding to modified coverslips. In particular, it was experimentally shown that objective-type TIR-FCS features high signal-to-background ratio on a single molecule level. In this thesis, concise derivations of analytical expressions for autocorrelation functions for diffusion and most important, the case of ligands reversibly binding to a single and localized binding site are presented. The derived model allows for the quantitative determination of binding rates for a single receptor. We strongly believe that the application of these results in the context of investigations of receptor-ligand binding kinetics will allow for deeper understanding of cellular signaling. Moreover, this thesis discusses the applicability of the proposed method in enzymology. Enzymes, as most proteins are subject to continuous changes of their structure or conformation. These conformational changes are correlated with the function of the enzyme. In the discussed example the enzyme catalyzes an oxidation where the product is fluorescent but the substrate is not. The function of the enzyme i.e. the recurring product formation leads to observable intensity fluctuations. Since function and conformational states are correlated, conformational fluctuations can be investigated by means of FCS as was already shown for confocal FCS. A technique closely related to FCS is fluorescence lifetime spectroscopy (where lifetime refers to the mean lifetime of the electronic excited state). Whereas in FCS the relaxation after random deviations from thermal equilibrium is investigated, relaxation of excited fluorophores towards their electronic ground-state is investigated in lifetime spectroscopy. The technique is used for e.g. discrimination between fluorophores with different lifetimes. Lifetime spectroscopy combined with imaging is used in many domains of life-science, including microarray reading and medical diagnostics. In preliminary work we developed a novel approach to perform lifetime imaging that is based on a multiplexing technique. The proposed method requires no mechanical scanning stage and only a single-point detector. Furthermore, noise is reduced under certain circumstances if the signal is low. Characteristics of this technique as well as advantages and disadvantages are shortly discussed.

EPFL2006

Scanning Tunneling Microscopy and Luminescence of Individual Nanostructures

Theresa Lutz

A detailed investigation and characterization of the local properties of individual nanoscopic structures is of great importance for the understanding of novel physical phenomena at the nanoscale as well as for the assessment of their possible use in future applications. A unique tool for the study of such structures are scanning probe methods. These methods do not only allow to obtain atomic scale information in both topographic and spectroscopic measurements at surfaces, but can also be employed for example to stimulate local photon emission. In this thesis the growth mechanism of inorganic and organic semiconductor nanostructures and the relation between their local structure and their electronic and optical properties is investigated by scanning probe techniques. In the first part of this thesis, the growth as well as the local electronic and morphologic properties of InAs/GaAs(001) quantum dots grown by molecular beam epitaxy are described. The recorded spectra show evidence of the discretization of the density of states that can be attributed to the zero dimensional confinement of charge carriers to the quantum dots. Moreover, the shape evolution of the quantum dots during the controlled removal of material through an in situ etching gas is studied. High resolution topographic images reveal that hereby an island shape transition takes place. This shape evolution is found to be the reverse of the shape evolution that typically takes place during the growth, indicating that thermodynamic factors play an important role during the growth process and the etching process. In addition, the mechanism of the material removal from the quantum dots is investigated in detail by the analysis of the size evolution of the islands as a function of the nominal amount of etched material. The second part of this thesis is dedicated to the study of individual CdSe nanowires grown by the solution-liquid-solid approach. These nanowires consist of small alternating sections of wurtzite and zincblende lattice structures. This structural arrangement also leads to an alternating electronic structure in the form of a type II band alignment along the long axis of the nanowires. The local electronic properties of the nanowires are analyzed by scanning tunneling spectroscopy. Tunneling current induced luminescence spectra from individual nanowires are acquired by the injection of holes and electrons and their subsequent radiative recombination. In contrast to previous studies on similar systems, the coupling of tip induced plasmons to the radiation process is not required. The shape and the position of the luminescence spectra is compared to photoluminescence spectra and Raman measurements. The photon energy is found to decrease with increasing wire diameter indicating quantum confinement of the charge carriers to the wire. The bulk bandgap, that can be extrapolated from the energy vs. diameter dependence of the emission peaks, indicates that the photons are emitted from zincblende type CdSe sections. Moreover, the diameter dependence reveals that the emission stems from carriers that are confined to quantum dot like parts of the wires. The lack of light emission from the wurtzite type segments is also re ected in the appearance of non-luminescent regions in the NW as shown by spatially resolved light intensity maps. Possible mechanisms for the light excitation are discussed. The investigation of single molecules of the luminescent organic emitter tris-(2-phenylpyridine)iridium(III) (Ir(ppy)3) is the topic of the last part of this thesis. To this end, Ir(ppy)3 molecules are deposited onto metal substrates and different types of thin insulating layers. Besides their structural and electronic characterization, the optical properties of single molecules are studied by tunneling current induced luminescence. No intrinsic light emission originating from the molecules can be observed neither on metal substrates nor on thin insulating layers. This indicates that in these systems, the molecules are still not sufficiently decoupled from the underlying metallic substrate. Preliminary results of tunneling current induced luminescence measurements of Ir(ppy)3 molecules on multilayers of C60 show great promise for exciting light emission on single molecules.

EPFL2011