Fluorescence and Phosphorescence from IndividualC60Molecules Excited by Local Electron Tunneling

Abstract

Using the highly localized current of electrons tunneling through a double barrier Scanning Tunneling Microscope (STM) junction, we excite luminescence from a selected C60 molecule in the surface layer of fullerene nanocrystals grown on an ultrathin NaCl film on Au(111). In the observed luminescence fluorescence and phosphorescence spectra, pure electronic as well as vibronically induced transitions of an individual C60 molecule are identified, leading to unambiguous chemical recognition on the single-molecular scale.PACS numbers: 68.37. Ef, 73.20.Mf, Light emission induced by electrons tunneling through the junction formed by the sample and the tip of a Scanning Tunneling Microscope (STM) has been proposed to characterize the optical properties of nanoscale objects at surfaces [1]. Contrary to conventional non-local techniques, the local character of this method offers the unique possibility to select and probe individual atoms, molecules or clusters on surfaces.Photon emission due to the decay of localized surface plasmons, excited by inelastic electron tunneling (IET) has been observed on metal surfaces [2,3], as well as on supported metallic nanoparticles [4]. Luminescence spectra have been acquired from semiconductor heterostructures [5], quantum well states of metallic films [6]. Recently, luminescence from supported molecules has been obtained [7,8] by successfully decoupling them from the metallic substrate in order to avoid quenching of the radiative transitions [9,10], using either a thin oxide film [7] or several molecular layers [8].However, unambiguous chemical identification of single complex molecules requires the observation and identification of several vibrational and/or electronicvibrational transitions, which are the spectroscopic fingerprint of the species. Here we present the first observation of energy resolved luminescence from an individually selected C 60 molecule excited by electrons tunneling through a double barrier STM junction. A comparison with the luminescence spectra obtained by non-local laser spectroscopy from dispersed C 60 molecules in rare gas and glass matrices [11,12,13,14,15,16], and from solid C 60 [17,18,19] enables us to demonstrate the molecular origin of the detected light and to identify the observed spectral features with pure electronic transitions and with vibronic transitions induced via Jahn-Teller (JT) and Herzberg-Teller (HT) coupling [20,21]. The present novel observation of both, fluorescence (singletto-singlet transitions) and phosphorescence (triplet-tosinglet transitions) constitutes a solid basis for the chemical identification of an individual C 60 molecule.C 60 nanocrystals were grown on NaCl layers deposited onto a Au(111) substrate. NaCl was evaporated from a Knudsen cell on a clean Au(111) surface at room temperature. Subsequently, the C 60 molecules were sublimated on the NaCl covered substrate. The experiments were performed with a homebuilt ultrahigh vacuum (UHV) STM operating at a temperature of 50 K, using cut PtIr tips. The photons emitted f...

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…23,55 Photon emission spectroscopy with the STM, where local optical modes are excited by electron/hole injection from the tip, offers the required spatial resolution. The potential of this technique has already been demonstrated with experiments performed on single molecules, 124,125 metal particles 119,134 and quantum dots, 121 while comparable studies on the oxide materials are still scarce. 51,82,119 Photon emission spectroscopy with the STM will however develop into a versatile tool to explore the local optical properties and their interrelation with the photo-catalytic performance of oxide materials.…”

“…in semiconductors, 120,121,122 oxides, 51 quantum wells 123 and single molecules. 124,125 Common to both scenarios is the crucial role of the electromagnetic near-field in the tip-sample cavity, which amplifies the emission cross-section. 126,127 Thanks to this field enhancement, light emission from an STM junction becomes an observable effect, in spite of the extremely local character of the measurement (~nm 2 ), the small electron flux from the tip (~nA) and the finite number of optical modes involved.…”

“…In contrast to bare oxide surfaces, photon emission spectroscopy with the STM has been widely employed to investigate the optical response of single metal particles 119,134 and molecules on oxide supports. 124,125 In the case of metal particles, the emission mechanism is governed by the excitation of collective electronic modes in the particle electron gas (so called Mie plasmons) followed by their radiative decay. 58 An example for the resulting optical response is given in Fig.…”

Stabilization of bimolecular islands on ultrathin NaCl films by a vicinal substrate

“…Growth and characterization of two-dimensional ordered supramolecular structures on ultrathin NaCl films has only been achieved in very few studies [11][12][13], which is due to experimental difficulties related to the low molecular adsorption energy on such insulating layers. All three mentioned studies were performed on NaCl islands having similar morphological characteristics as the ones observed here on Au(1 1 1).…”

“…Furthermore, it has been shown that, upon adsorption of molecules on ultrathin NaCl films on metal surfaces, inherent electronic and optical properties of individual molecules can be studied [8][9][10]. However, to our knowledge, there are only three published STM studies on the growth of two-dimensional molecular islands on insulating NaCl layers, two of them on metal-containing porphyrine molecules [11,12], and the third one on C 60 molecules [13]. The small number of investigations about adsorbates on NaCl layers is at least partly due to weak molecule-substrate interactions, which make the stabilization and STM imaging of such systems a challenging task.…”