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...

Nested assembly: Chiral molecules of 5,6,11,12‐tetraphenylnaphthacene (rubrene) organize spontaneously into homochiral supramolecular architectures of increasing complexity. Adsorbed individual molecules self‐assemble enantioselectively into chiral pentagonal supermolecules, which act as building blocks for the formation of chiral supramolecular decagons (see STM images).

The supramolecular self-assembly of rubrene ͑C 42 H 28 ͒ on Au͑111͒ results in different adsorption conformations of the physisorbed molecules. Owing to the three-dimensional geometry of the molecule providing an inherent decoupling of the molecular states from the substrate, the conformers are distinguished by their submolecular appearance in the scanning tunneling microscopy images and by the corresponding position of the highest occupied molecular orbital in the differential conductance ͑dI / dV͒ spectra. The application of an electric field induces a switching of the electronic and geometric conformation of the self-assembled molecules.

Surface nanostructuring by molecular self-organization is a relevant process in the growing field of nanotechnology. Depending on the characteristics of the molecules and on the type of interactions among them and with the substrate, a variety of surface patterns have been observed by means of scanning tunneling microscopy (STM).[1] Indeed, two-dimensional (2D) tiling constitutes a fundamental issue in topology, [2] with fascinating examples in nature and art, and applications in many domains such as cellular biology, [3] foam physics, [4] and crystal growth.[5] Herein we present the first observation of surface tiling with both nonperiodic and periodic arrangements of slightly distorted pentagons, hexagons, and heptagons formed by rubrene molecules adsorbed on a Au(111) surface. On adjacent regions of the sample, ordered honeycomb and hexagonal close-packed patterns are found. The existence of manifold arrangements in the supramolecular self-assembly of rubrene on gold originates from the three-dimensional nonplanar flexible structure of the molecule, as well as from the nature of the intermolecular bonds.Rubrene (5,6,11,12-tetraphenylnaphthacene, C 42 H 28 ) is a nonplanar chiral aromatic hydrocarbon constituted of a twisted tetracene backbone flanked by four out-of-plane phenyl groups as shown in the inset of Figure 1. The survey STM image (Figure 1) shows a Au(111) surface covered by a single layer of rubrene molecules. Three terraces separated by monatomic steps are visible, as well as the Au(111) herringbone reconstruction underlying the molecular layer. This overview shows areas with variable packing densities and different degrees of supramolecular order, denoted by A, B, and C. Regions A and C appear as ordered arrangements: the porous domain A reveals a honeycomb pattern, while region C displays a hexagonal close-packed structure. In contrast, the inhomogeneous area B presents no translational symmetry.A detail of the latter nonperiodic phase is shown in Figure 2 a. The dashed blue circle surrounds a single rubrene molecule; the submolecular contrast reveals three lobes and a quite regular threefold symmetry.[6] The self-assembled pattern is composed of supramolecular pentagons, hexagons, and filled heptagons, which appear to be randomly distributed over the surface. There exist 11 distinct tilings by regular polygons; [2] however, a combination of regular pentagons, hexagons, and heptagons generates empty gaps and overlapping regions, as inferred from the consideration of the angles at the corners of a regular pentagon (1088), hexagon (1208), and heptagon ( % 1298). Only when three hexagons joined and share a common corner is the angular sum exactly 3608. Nevertheless, three configurations exist that yield an angular sum close to 3608 (Figure 2 b): pentagon-hexagonheptagon ( % 3578), pentagon-heptagon-heptagon ( % 3668), and hexagon-hexagon-heptagon ( % 3698). The introduction of a slight distortion of the polygons allows a plane-filling tessellation. These three configurations are the most frequently...

Coverage-dependent self-assembly of rubrene molecules on different noble metal surfaces, Au(111) and Au(100), Ag(111) and Ag(100), is presented. On Au(111), the homochiral supramolecular assemblies evolve with increasing rubrene coverage from very small structures composed of a few molecules, to honeycomb islets, and to one-dimensional chains of supramolecular pentamers. At higher coverage, the racemic mixture of molecules forms close-packed islands. On Au(100), chains of pentamers and two different types of densely packed islands are formed. On the Ag surfaces, exclusively close-packed islands are created, independently of the rubrene coverage. Moreover, the role of the chiral nature of the molecules in the self-assembly process is discussed, as well as the existence of different molecular conformers depending on the supramolecular assembled phase. The observed differences and similarities reflect the influence of the electronic properties and the geometric structure of the various substrates on molecular self-assembly.