Tip-induced Plasmon Research Articles

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.

Submolecular Electroluminescence Mapping of Organic Semiconductors.

The electroluminescence of organic films is the central aspect in organic light emitting diodes (OLEDs) and widely used in current display technology. However, its spatial variation on the molecular scale is essentially unexplored. Here, we address this issue by using scanning tunneling microscopy (STM) and present an in-depth study of the electroluminescence from thin C60 films (<10 monolayers) on Ag(111) and Au(111) surfaces. Similar to an OLED, the metal substrate and STM tip inject complementary charge carriers that may recombine within the molecular film; however, the atomically defined charge injection by the tip enables mapping of the local electroluminescence down to the submolecular scale. We show that the radiative recombination in solid C60 is restricted to various structural defects, whose emission characteristics can be addressed individually. The emission fine structure reveals a coupling to Jahn-Teller active vibrational modes of C60, which implies that its parity-forbidden lowest singlet transition becomes locally allowed at the emission centers. At lateral distances of a few nanometers, only a weak emission from tip-induced plasmons is detectable. Their excitation evidences the injection of both charge carrier types and confirms that they are unable to recombine radiatively at positions far from structural defects. Finally, we demonstrate that the molecular orbital pattern visible in electroluminescence maps enables an unambiguous discrimination between the intrinsic radiative recombination of electron-hole pairs in the organic film and the technique-related emission of tip-induced plasmons. This capability is essential to consolidate STM as a tool to explore the light generation from organic films on the nanoscale.

Spatial Confinement of the Electrical Field Affected by Quantum Effects in Plasmonic Dimer Sandwiching a Single Molecule

The spatial confinement of the tip-induced plasmon is a critical factor to determine the resolution of the tip-enhanced Raman spectroscopy (TERS) system. Despite the compressed optical field, only 1 nm is obtained by the self-interaction effect of molecule; however, the deeper physical laws underlying are still under discussion. In addition, due to the gap between the tip and the substrate is only a few nanometers (or less), the quantum effects should be taken into account. For simplicity, we treat the system of the plasmonic dimer with a molecule in the gap as a TERS-like system. In the framework of the newly developed quantum hydrodynamic model, we propose a model to study the light field enhancement and compression affected by the quantum effects in TERS-like system. The results show that the “hot spot” size depends on both the shape and the boundary location of the molecule in the dimer gap. The mechanism of such light field distribution will make us distinguish the boundary of the molecule more clearly. Therefore, our theoretical model offers a new insight into the confinement of the electromagnetic field in the TERS-like system, hence, the physical mechanism of the subnanometer spatial resolution in TERS system.

Manipulating photon emission efficiency with local electronic states in a tunneling gap

We demonstrate manipulation of photon emission efficiency in a tunneling gap by tuning the rates of elastic and inelastic electron tunneling processes with local electronic states. The artificial local electronic states are created by a scanning tunneling microscope tip on a CuN nanoisland grown on a Cu(100) surface at cryogenic temperature. These local electronic states can either enhance or suppress the excitation of tip-induced surface plasmon modes at specific bias voltages, and thus the induced photon emission rates. A theoretical model quantitatively analyzing inelastic and elastic tunneling processes associated with characteristic electronic states shows good agreement with experiments. We also show that tip-induced photon emission measurement can be used for probing the electronic states in the tunneling gap.

Plasmonic excitation of light emission and absorption by porphyrine molecules in a scanning tunneling microscope

Light emission from a scanning tunneling microscope is investigated from Ag(111) surfaces using tips with a clean metallic apex and a molecule-covered shaft. The measurements are conducted such that no molecules are located in the tunneling gap. Nevertheless, spectra of the emission exhibit absorption and emission features of the tetraphenyl-H${}_{2}$-porphyrine molecules used. The experimental data show that molecules in the vicinity of the tunneling gap are excited through tip-induced plasmons and their radiative decay. The excitation of plasmons is due to inelastic tunneling and does not involve an external light source.

Scanning Tunneling Luminescence of Individual CdSe Nanowires

The local luminescence properties of individual CdSe nanowires composed of segments of zinc blende and wurtzite crystal structures are investigated by low-temperature scanning tunneling luminescence spectroscopy. Light emission from the wires is achieved by the direct injection of holes and electrons, without the need for coupling to tip-induced plasmons in the underlying metal substrate. The photon energy is found to increase with decreasing wire diameter due to exciton confinement. The bulk bandgap extrapolated from the energy versus diameter dependence is consistent with photon emission from the zinc blende-type CdSe sections.

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.

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.

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 &amp; Sons, Ltd.

Photon emission spectroscopy of thin MgO films with the STM: from a tip-mediated to an intrinsic emission characteristic

Electron injection from the tip of a scanning tunnelling microscope (STM) has been used to stimulate local photon emission from the surface of a thin MgO film grown on Mo(001). Depending on the excitation energy, several emission regimes have been identified on the basis of the energy and the spatial distribution of the emitted photons. At low excitation bias, tip-induced plasmons are excited in the tip–sample gap, carrying little information on the oxide film. With increasing tip bias, radiative electron transitions between field-emission resonances dominate the photon response from the MgO surface. Intrinsic optical modes of the oxide material, such as radiative decays of electron–hole pairs, are only observed when operating the STM in the field emission regime, where the direct correlation between the photon signal and the local surface morphology gets partially lost due to the increased tip–sample distance.

Plasmon modes in light emission from silver nanoparticles induced by a scanning tunneling microscope

This paper deals with light emission induced by a scanning tunneling microscope using a silver nanoparticles film as a sample. We measured both two photon maps of different spectral ranges along with a topography and light emission spectra corresponding to different spots of the samples. The spectra revealed two distinct peaks, corresponding to different-order tip-induced plasmon modes of the tunneling junction.

Setup of a STM with Photon Detection Using an Optical Fiber

We have set up a scanning tunneling microscope (STM) with a light detection capability operated at room temperature. We used an optical fiber situated proximate to the tip-surface tunneling junction for collecting the light emitted during the STM tunneling. The tunneling-induced light emission from a Ag film grown on a Si(111)-7 × 7 surface was successively detected using our optical setup. The spectral intensities in the wavelength range of 430 ∼ 520 nm were observed. The peak position was invariant with respect changes in the bias voltage, but its intensity showed a non-monotonic variation with the sample bias. The observed tunneling-induced light emission was interpreted to be due to the decay of tip-induced plasmons excited in inelastic tunneling and confined between the tip and the surfaces.

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.

Photon mapping of MgO thin films with an STM

The light emission from an STM junction consisting of an MgO thin film on Mo(0 0 1) and an Au tip is analyzed with respect to its spatial distribution for various excitation conditions. The spectral characteristic of the light is compatible with an emission mechanism mediated by tip-induced plasmons that are excited by inelastic electron tunneling involving field-emission resonances in the tip–sample gap. The dependence of field-emission resonances on the MgO work function allows the controlled stimulation of differently thick oxide islands in the photon maps by changing the sample bias.

Characterization of light emission from subphthalocyanine monolayers using scanning tunneling microscopy

Monolayers of subphthalocyanine were prepared by molecular beam epitaxy and self-assembled depositions on the Au (111) surface. Light emission from the monolayers was measured by photoexcitation as well as tunneling current excitation using scanning tunneling microscopy under ultrahigh vacuum. The photoexcited spectra of the monolayers deposited by both methods exhibited the fluorescence bands of π-electronic transition of the subphthalocyanine macrocycle although their lifetime was very shortened due to quenching by the Au surface. The tunneling induced emission was taken from the self-assembled monolayers in which the subphthalocyanine molecules were chemically bound onto the surface by alkylthio substituents. The observed spectra were ascribed to the radiative decay of tip-induced surface plasmon of Au. The molecules in the self-assembled monolayers shifted the emission peak due to modification of the dielectric property and distance between the tip and surface.

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.

Light emission induced by tunneling electrons from surface nanostructures observed by novel conductive and transparent probes

We have developed an ultrahigh-vacuum low-temperature scanning tunneling microscope (STM) equipped with a near-field optical detection system using novel conductive and optically transparent probes. Tunneling-electron induced photons generated in a nanometer-scale area just under the STM probe can be collected directly into the core of the optical fiber probe within the optical near-field region. Firstly, optical fiber probes coated with indium-tin-oxide thin film are applied to quantitative analysis of p-type GaAs(110) surface, where a decrease of light emission in photon mapping clearly extracts the existence of Zn accepter atoms located at the sub-surface layers. Secondly, in order to enhance the efficiency for inelastic tunneling excitation of a tip-induced plasmon mode, a STM probe coated with an Ag/ITO dual-layer film has been developed and applied to an Ag(111) surface, where photon mapping with a step resolution has been achieved by near-field detection.

Light Emission from Organic Molecules on Metal Substrates Induced by Tunneling Currents

Tunneling currents in a scanning tunneling microscope were used to induce light emission by positioning either a gold-coated W tip on the surface of a porphyrin molecular monolayer on Cu(100) or a pure W tip on the surface of a perinone derivative monolayer on Au(100). The emission spectra from the surfaces of both molecules are dominated by the plasmon-mediated emission from substrates. Molecular fluorescence is completely quenched by the metal substrate. The organic molecules act as a spacer to modify the tip-induced plasmon modes, enhance the intensity of the plasmon-mediated emission, and cause blue shift of the spectra.

Role of molecules in tunneling-current-induced photon emission from the surface of a perinone derivative molecular monolayer on Au(100)

The role of molecules in tunneling current induced photon emission from luminescent molecules on metal surfaces is still an open issue despite the recent observation of molecular fluorescence. We found that at low tunneling currents (1 nA), the molecular fluorescence from the surface of a perinone derivative (PD) molecular monolayer on Au(100) is completely quenched through non-radiative energy dissipation to the Au substrate. The PD molecular monolayer acts as a spacer to modify the tip-induced plasmon modes, enhance the intensity of the plasmon-mediated emission, and cause a blueshift of the emission spectra. However, at relatively high tunneling currents $(1\mathrm{nA}{I}_{t}\ensuremath{\sim}5\mathrm{nA}),$ broadened emission spectra were observed, which suggests that molecular fluorescence may be feasible upon further decoupling of the PD molecular electronic states from the metal substrate.

Resonance shift effects in apertureless scanning near-field optical microscopy

We develop a theory to study apertureless scanning near-field optical microscopy which takes into account retardation, higher multipoles of the tip, and the multiple scattering between the tip and the surface. We focus on metallic systems and discuss the implication of the formation of tip-induced surface plasmon modes in the tip-surface system. We discuss the effects associated with the shift in energy of those modes as a function of the tip-surface distance. Both the local field and the scattering cross section are enhanced when the tip approaches the surface, but there is no general correspondence between the two enhancements.