STM-induced Light Emission Research Articles

Plasmon-Driven Motion of an Individual Molecule.

We demonstrate that nanocavity plasmons generated a few nanometers away from a molecule can induce molecular motion. For this, we study the well-known rapid shuttling motion of zinc phthalocyanine molecules adsorbed on ultrathin NaCl films by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced light emission. Comparing spatially resolved single-molecule luminescence spectra from molecules anchored to a step edge with isolated molecules adsorbed on the free surface, we found that the azimuthal modulation of the Lamb shift is diminished in case of the latter. This is evidence that the rapid shuttling motion is remotely induced by plasmon–molecule coupling. Plasmon-induced molecular motion may open an interesting playground to bridge the nanoscopic and mesoscopic worlds by combining molecular machines with nanoplasmonics to control directed motion of single molecules without the need for local probes.

Adsorption and Light Emission of a Racemic Mixture of [7]thiaheterohelicene-2,13-carboxaldehyde on Au(111), Cu(001), and NiAl(110) Surfaces Investigated Using a Scanning Tunneling Microscope

The coverage-dependent adsorption of a racemic mixture of [7]thiaheterohelicene-2,13-carboxaldehyde on Au(111), Cu(001), and NiAl(110) surfaces was investigated using a scanning tunneling microscope (STM). At a low coverage, the adsorption process for helicene molecules on Au(111) was strongly affected by surface reconstruction. At monolayer saturation coverage, the dominant molecular structure observed on Au(111) characteristically had separated self-assembled twin rows aligned in ⟨112̅⟩ directions. The helicene molecules within these separated twin rows are preferentialy arranged in a zigzag pattern with alternating (M)- and (P)-enantiomers. With increasing molecular coverage, the molecular structural transition from self-assembled twin rows to self-assembled single rows was observed. STM-induced light emission (STM-LE) investigation of helicene molecules on Au(111) showed the suppression of plasmon light emission over the molecules. The adsorption of helicene molecules on Cu(001) and NiAl(110) was quite different from that observed on Au(111). Neither the formation of self-assembled twin rows nor the molecular arrangement into different domains was observed. The formation of molecular clusters on Cu(001) and NiAl(110) was observed. STM-LE investigation of helicene molecules adsorbed on Cu(001) showed the suppression of plasmon light emission over these molecules. In contrast to metallic Au(111) and Cu(001) substrates, STM-LE investigations revealed the enhancement of light emission above the molecular clusters formed on metallic NiAl(110), suggesting plasmon-enhanced molecular light emission.

Many-Body State Description of Single-Molecule Electroluminescence Driven by a Scanning Tunneling Microscope.

Electron transport and optical properties of a single molecule in contact with conductive materials have attracted considerable attention because of their scientific importance and potential applications. With the recent progress in experimental techniques, especially by virtue of scanning tunneling microscope (STM)-induced light emission, where the tunneling current of the STM is used as an atomic-scale source for induction of light emission from a single molecule, it has become possible to investigate single-molecule properties at subnanometer spatial resolution. Despite extensive experimental studies, the microscopic mechanism of electronic excitation of a single molecule in STM-induced light emission has yet to be clarified. Here we present a formulation of single-molecule electroluminescence driven by electron transfer between a molecule and metal electrodes based on a many-body state representation of the molecule. The effects of intramolecular Coulomb interaction on conductance and luminescence spectra are investigated using the nonequilibrium Hubbard Green's function technique combined with first-principles calculations. We compare simulation results with experimental data and find that the intramolecular Coulomb interaction is crucial for reproducing recent experiments for a single phthalocyanine molecule. The developed theory provides a unified description of the electron transport and optical properties of a single molecule in contact with metal electrodes driven out of equilibrium, and thereby, it contributes to a microscopic understanding of optoelectronic conversion in single molecules on solid surfaces and in nanometer-scale junctions.

STM-induced light emission enhanced by weakly coupled organic ad-layers

We analyze the light emission induced by the tunneling current flowing in a scanning tunneling microscopy experiment. In particular, we study the influence of organic ad-layers on the light emission on the initial monolayer of bismuth (Bi) on Cu(111) in comparison to the well-known case of organic ad-layers on Ag(111). On the Bi/Cu(111)-surface, we find that the scanning tunneling microscopy-induced light emission is considerably enhanced if an organic layer, e.g., the fullerene C60 or the perylene derivate perylene-tetracarboxylic-dianhydride, is introduced into the tip-sample junction. The enhancement can be correlated with a peculiarly weak interaction between the adsorbed molecules and the underlying Bi/Cu(111) substrate as compared to the Ag(111) substrate. This allows us to efficiently enhance and tune the coupling of the tunneling current to localized excitations of the tip-sample junction, which in turn couple to radiative decay channels.

STM-induced light emission from vacuum-evaporated gold film

A vacuum evaporation system has been used to evaporate gold film on glass substrate in order to probe the scanning tunneling microscope-light emission (STM-LE) from the evaporated film. The surface morphology of the evaporated Au film has been checked by atomic force microscope (AFM). In order to estimate the appropriate thickness of the Au film, which is essential for the enhancement of STM-LE in the prism-coupled geometry, a theoretical calculation has been performed. Our theoretical simulation revealed that the light emission from the prism-coupled STM junction is strongly enhanced when the Au film has a thickness of 40 nm. AFM observation also showed that the morphology of the gold films strongly depends on the cleanliness of glass substrates and the deposition temperature. Relatively smooth surface was observed when a 40-nm-thick Au film was evaporated at room temperature on the preannealed glass substrate. Finally, the evaporated films were deposited on the flat bottom of a hemispherical glass prism, and STM-LE from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. It was found from the experimental results that the prism-side emission is much stronger than the tip-side emission by virtue of the enhancement of the prism-coupled geometry.

High-resolution mapping of plasmonic modes: photoemission and scanning tunnelling luminescence microscopies

Photonic properties of dense metal nanostructures are currently under intense investigation because of the possible local enhancements of electromagnetic fields induced by plasmonic excitations. In this review paper, we present examples of plasmonic-field mappings based on multiphoton photoemission or STM-induced light emission, two techniques among those which offer today's best spatial resolutions for plasmon microscopy. By imaging the photoemitted electrons, using well-established electron optics, two-dimensional intensity maps reflecting the actual distribution of the optical near-field are obtained. The imaging technique involves no physical probe altering the measure. This approach provides full field spectroscopic images with a routine spatial resolution of the order of 20 nm (down to 2 nm with recent aberration corrected instruments). Alternatively, an unfamiliar property of the junction of scanning tunnelling microscope is its ability to behave as a highly localized source of light. It can be exploited to probe opto-electronic properties, in particular plasmonic fields, with ultimate subnanometre spatial resolution, an advantage balanced by a sometimes delicate deconvolution of local-probe influence.

STM-induced light emission from thin films of perylene derivatives on the HOPG and Au substrates

We have investigated the emission properties of N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide thin films by the tunneling-electron-induced light emission technique. A fluorescence peak with vibronic progressions with large Stokes shifts was observed on both highly ordered pyrolytic graphite (HOPG) and Au substrates, indicating that the emission was derived from the isolated-molecule-like film condition with sufficient π-π interaction of the perylene rings of perylenetetracarboxylic diimide molecules. The upconversion emission mechanism of the tunneling-electron-induced emission was discussed in terms of inelastic tunneling including multiexcitation processes. The wavelength-selective enhanced emission due to a localized tip-induced surface plasmon on the Au substrate was also obtained.

Spatially resolved detection of plasmon‐enhanced fluorescence using scanning tunneling microscopy

AbstractWe have measured a nanoscale fluorescence image of copper phthalocyanine (CuPc)/Au nanostructures on an indium tin oxide (ITO) substrate using a scanning tunneling microscope (STM). The quantum efficiency (QE) of the intrinsic fluorescence of CuPc is very low; however, the plasmon enhancement effect on an Au surface increases fluorescence efficiency. To obtain the nanoscale fluorescence image, in this study, we used up‐conversion fluorescence through a triplet‐triplet annihilation (TTA) process. Tunnel electrons induced fluorescence and tip‐induced plasmon (TIP) emission simultaneously; however, the up‐conversion fluorescence from CuPc could be clearly separated from TIP emission by spectroscopic filtering. The TIP showed different properties between the ITO substrate and the Au surface, and the CuPc fluorescence was enhanced only on the TIP of Au nanostructures. This study would lead to the application of a new fluorescence imaging method based on plasmon‐assisted STM‐induced light emission (STM‐LE). Copyright © 2008 John Wiley & Sons, Ltd.

STM-Induced Light Emission from Substrate-Tethered Quantum Dots

Scanning tunneling microscope-induced light emission from multilayers of colloidal quantum dots (QDs) is found to exceed by up to 2 orders of magnitude the well-known radiation from a local tip-induced plasmon (TIP) at a QD-free gold surface. Emission spectra and the bias-voltage dependence of current and light emission point to inelastic electron-tunneling (IET) excitation of a coupled QD-TIP mode, enhanced by the dielectric properties of the QD multilayer. The threshold voltage for STM-LE is found to depend on the optical gap of the quantum dots, stipulating a simple energy conservation rule for light emission through IET. The results highlight fundamental issues underlying nanoparticle-based optoelectronic devices.

Fiber optic light collection system for scanning-tunneling-microscope-induced light emission

We report a compact light collection scheme suitable for retrofitting a scanning tunneling microscope (STM) for STM-induced light emission experiments. The approach uses a pair of optical fibers with large core diameters and high numerical apertures to maximize light collection efficiency and to moderate the mechanical precision required for alignment. Bench tests indicate that efficiency reduction is almost entirely due to reflective losses at the fiber ends, while losses due to fiber misalignment have virtually been eliminated. Photon-map imaging with nanometer features is demonstrated on a stepped Au(111) surface with signal rates exceeding 10(4) counts/s.

STM-induced light emission of conjugated polymer thin film in different chain aggregations

The STM-induced luminescence (STML) of a conjugated polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)- p-phenylene vinylene] (MEHPPV) with or without annealing on substrate of Ag(1 1 1) were investigated in air at room temperature. The surface morphologies of pure Ag(1 1 1), MEHPPV thin film on Ag(1 1 1) with or without annealing were modified by STM scans under higher positive or negative sample biases. The experimental results show that for MEHPPV film without annealing, the STML intensity increases after aggregates formation induced by STM scans. However, for annealed MEHPPV film, the negative bias scans decrease the STML intensity. The chain aggregations among MEHPPV molecules are believed to be the key factor for the behavior.

Influence of the Au(111) reconstruction on the light emission induced by a scanning tunneling microscope

STM-induced light emission on reconstructed Au(111) is investigated under UHV at room temperature. Light emitted from the tunnel junction is recorded simultaneously with topography using a Au tip. For positive sample biases, a contrasted periodic pattern on the photon map anticorrelated with the reconstruction features of the topography is observed. For negative sample biases, the contrast on the photon map is strongly reduced. In order to understand the link between reconstruction and photon emission, variations in tunnel current and in photon emission are also recorded as a function of tip-sample distance. These measurements show that the contrast on the photon map does not result from variations in the coupling of inelastically tunneling electrons with the radiative tip-induced plasmon mode. Instead, the photon contrast is correlated with the relative variations upon scanning of the local density of final states for elastically and for inelastically tunneling electrons.

STM-induced light emission from the surface of H 2TBP porphyrin/PFP porphyrin/Cu(1 0 0)

Molecular fluorescence from the surface of H 2TBP porphyrin (H 2TBPP) monolayer on the top of PFP porphyrin (PFPP) multilayers on Cu(1 0 0) substrate is studied by an ultrahigh-vacuum scanning tunneling microscope. H 2TBPP molecular luminescence spectra are well defined and in perfect matching with conventional photoluminescence spectra excited from the corresponding bulk molecule. The electronic states of H 2TBPP monolayer are effectively decoupled from the metal surface by controlling the thickness of PFPP multilayers in a nanoscale regime. The excitation mechanism is probably attributed to the hot electron injection. These results provide a new way for the development of nanoscale molecular light source and spectroscopy.

Single nanoparticle manipulation with simultaneously recorded STM-induced light emission

Room temperature manipulation of a single-metal nanoparticle with simultaneously recorded scanning tunneling microscope (STM)-induced light emission is presented. A single nanoparticle was removed from the edge of hexagonal network of particles and placed in a different site by STM tip. When it is mobile, the particle is nonluminescent whereas a fixed particle emits photons. This experiment is demonstrating a site-dependence of light emission of single particle.

Probing individual nanostructures with STM-induced light emission

We present the results of our latest work using the spectroscopy of light emitted by a sample under the scanning tunneling microscope (STM). For the quantum wells in AlGaAs/GaAs layered structures, we have demonstrated that the emission spectra from individual wells can be measured by this method and that the minority carrier diffusion length can be estimated in real space. Furthermore, we have discovered that the emission linewidth depends on the location even within an individual well. On the reconstructed Au(110)-(2×1) surface, the emission from an atomically localized electronic transition in the valley between the atomic rows was found.