We study the scattering of electrically excited surface plasmon polaritons (SPP) from individual nanostructures. The tunneling electrons from a scanning tunneling microscope (STM) are used to excite an out-going, circular SPP wave on a thin (50-nm) gold film on which isolated gold nanoparticles (NPs) have been deposited. Interaction of the excited SPPs with the NPs leads to both in-plane (SPP-to-SPP) and out-of-plane (SPP-to-photon) scattering. We use SPP leakage radiation microscopy to monitor the interference between the incident and in-plane scattered SPP waves in the image plane. By changing the location of the STM tip, the distance of the pointlike SPP source to the scatterers can be varied at will, which constitutes a key advantage over other existing techniques. As well, the out-of-plane scattered radiation interferes with the direct light emission from the STM tip in the back focal plane (Fourier plane). This confirms the mutual coherence of the light and SPP emission resulting from the inelastic tunneling of an electron in the STM junction. We use this effect to demonstrate that SPP-to-photon scattering at NPs is highly directional.

The temporal coherence of propagating surface plasmons is investigated using a local, broadband plasmon source consisting of a scanning tunneling microscope. A variant of Young's experiment is performed using a sample consisting of a 200-nm-thick gold film perforated by two 1-mu m-diameter holes (separated by 4 or 6 mu m). The resulting interference fringes are studied as a function of hole separation and source bandwidth. From these experiments, we conclude that apart from plasmon decay in the metal, there is no further loss of plasmon coherence from propagation, scattering at holes, or other dephasing processes. As a result, the plasmon coherence time may be estimated from its spectral bandwidth. (C) 2014 Optical Society of America

Une source de lumière de dimension atomique est réalisée à l’aide d’un microscope à effet tunnel (STM). Cette source de photons est localisée à la jonction entre une pointe, que l’on peut déplacer avec une précision atomique, et un échantillon métallique. Elle se distingue par une excitation de nature électronique (le courant tunnel), et fait également intervenir des plasmons de surface présents dans la jonction tunnel. La jonction tunnel est ainsi une « source électrique » de plasmons de surface. Cependant, on est encore loin de comprendre le fonctionnement de cette source optique et plasmonique et d’en avoir exploité toutes les possibilités.

The highly confined nature of the fields from surface plasmons makes them excellent candidates for future nano-optical devices. Most often, optical excitation is used to excite surface plasmons. However, a local, low energy, electrical method for surface plasmon excitation would be preferable for device applications.The scanning tunneling microscope (STM) is an ideal, low energy, local source of electrons that can excite both localized (LSP) and propagating surface plasmons (SPP). Its local nature, along with the ability to precisely position the excitation source and the absence of any background light from the excitation are essential for our experiments. We have used this technique to locally excite surface plasmons on a variety of metal structures. In our setup, the STM is coupled to an inverted optical microscope and the resulting emitted light is collected through the glass substrate. In such a configuration, both the light emitted from localized plasmons as well as the leakage radiation from propagating surface plasmons may be recorded. Both real plane (spatial information) and Fourier plane (angular information) images may be obtained, as well as emission spectra.In this article we will present the results of STM-SPP excitation on thin Au films on glass and investigate the effect of Au film thickness on the SPP propagation length. These results demonstrate the unique features of STM-excited SPPs: the STM plasmon source may be considered equivalent to a series of oscillating vertical point dipoles, and the resulting plasmons consist of a 2D circular wave with a broadband spectrum. These properties are then exploited to study how SPPs scatter into photons from super and sub-wavelength sized holes. It is found that the larger the hole diameter, the more directional the scattering light. From a type of SPP-Young's experiment we determine that the orientation of the electric field is maintained when SPPs are scattered into photons at holes.

The scattering of electrically excited surface plasmon polaritons (SPPs) into photons at the edges of gold metal stripes is investigated. The SPPs are locally generated by the inelastic tunneling current of a scanning tunneling microscope (STM). The majority of the collected light arising from the scattering of SPPs at the stripe edges is emitted in the forward direction and is collected at large angle (close to the air-glass critical angle, θ(c)). A much weaker isotropic component of the scattered light gives rise to an interference pattern in the Fourier plane images, demonstrating that plasmons may be scattered coherently. An analysis of the interference pattern as a function of excitation position on the stripe is used to determine a value of 1.42 ± 0.18 for the relative plasmon wave vector (kSPP/k0) of the corresponding SPPs. From these results, we interpret the directional, large angle (θ~θ(c)) scattering to be mainly from plasmons on the air-gold interface, and the diffuse scattering forming interference fringes to be dominantly from plasmons on the gold-substrate interface.

We report on the angular distribution, polarization, and spectrum of the light emitted from an electrically controlled nanoscale light source. This nanosource of light arises from the local, low-energy, electrical excitation of localized surface plasmons (LSP) on individual gold nanoparticles using a scanning tunneling microscope (STM). The gold nanoparticles (NP) are chemically synthesized truncated bitetrahedrons. The emitted light is collected through the transparent substrate and the emission characteristics (angular distribution, polarization, and spectrum) are analyzed. These three observables are found to strongly depend on the lateral position of the STM tip with respect to the triangular upper face of the gold NP. In particular, the resulting light emission changes orientation when the electrical excitation via the STM tip is moved from the base to the vertex of the triangular face. On the basis of the comparison of the experimental observations with an analytical dipole model and finite-difference time-domain (FDTD) calculations, we show that this behavior is linked to the selective excitation of the out-of-plane and in-plane dipolar LSP modes of the NP. This selective excitation is achieved through the lateral position of the tip with respect to the symmetry center of the NP.

A new regime of electrochemical anodic oxidation with an atomic force microscope (AFM) is introduced for producing insulating layers on a hydrogenated diamond surface. In this new regime, when a low surface voltage (VS<+2 V) is applied to the sample, an insulating layer is created without any measurable change in the topography. Insulating layers created in this fashion are shown to preserve the high sub-surface conductance of hydrogenated diamond surfaces, contrary to the oxide layers accompanied by a topographic change, which destroy sub-surface conductance.

Electron scattering at graphene edges is expected to make a crucial contribution to the electron transport in graphene nanodevices by producing quantum interferences. Atomic-scale scanning tunneling microscopy (STM) topographies of different edge structures of monolayer graphene show that the localization of the electronic density of states along the C-C bonds, a property unique to monolayer graphene, results in quantum interference patterns along the graphene carbon bond network, whose shapes depend only on the edge structure and not on the electron energy.

We report a description of the SiC(0001) 3 x 3 silicon carbide reconstruction based on single-molecule scanning tunneling microscopy (STM) observations and density functional theory calculations. We show that the SiC(0001) 3 x 3 reconstruction can be described as contiguous domains of right and left chirality distributed at the nanoscale, which breaks the to date supposed translational invariance of the surface. While this surface heterochirality remains invisible in STM topographies of clean surfaces, individual metal-free phthalocyanine molecules chemisorbed on the surface act as molecular lenses to reveal the surface chirality in the STM topographies. This original method exemplifies the ability of STM to probe atomic-scale structures in detail and provides a more complete vision of a frequently studied SiC reconstruction.

Elongated CdSe nanocrystals (nanorods) coated with long phosphonate ligands are deposited by pulse valve injection under ultrahigh vacuum on atomically clean surfaces. The alignment of individual CdSe nanorods is observed on the Si(100)-2x1:H surface by scanning tunneling microscopy, and on the basal plane of graphite by scanning electron microscopy. From calculations, the minimum van der Waals interaction energies between isolated molecular ligands and the surfaces occurs along specific directions. These tiny interactions guiding the orientated soft-landing of the CdSe nanorods show evidence of collective ligand effects.

The concept of molecular nanomachines has become a reality in the past few years in organic and supramolecular chemistry, in biochemistry and in atom-scale manipulation with the scanning tunnelling microscope (STM).

Ion photostimulated desorption (PSD) is a specific surface sensitive process. This paper reviews ion PSD following core level excitation of molecules adsorbed on the Si(111) 7 x 7 and Si(100) 2 x 1 surfaces. Several aspects of ion PSD will be discussed; (i) the use of ion PSD as a tool for investigating the adsorption configurations of O-2 on Si(111) 7 x 7, (ii) the relevance of ion PSD for probing the physisorption-chemisorption transition of benzene molecules on Si(111), (iii) the comparison between ion PSD of methanol adsorbed on Si(111) and photofragmentation of the methanol molecule in the gas phase, (iv) the comparison between ion PSD of formic acid adsorbed on the Si(111) and Si(100) surfaces and (v) the chemical selectivity of ion PSD following core level excitation of NO dissociatively adsorbed on Si(111). Finally, the interplay between ion dynamics and electronic relaxation following core level excitation and Auger processes will be discussed in the case of O+ photodesorption from O-2 adsorbed on Si(111) 7 x 7, as investigated via isotope and temperature effects on both the intensities and kinetic energies of the desorbed ions.

Powering and controlling the operation of a single molecule adsorbed on a surface can be achieved by using the tip of a scanning tunnelling microscope (STM) as an atomic-size source of electrons. We review the various electronic excitation processes induced by the electrons from the STM tip which are able to activate the functions of a molecular nanomachine. In particular, we review recent results illustrating the electronic control of molecular dynamics at the level of a single molecule.

The scanning tunnelling microscope (STM) has been used to study the adsorption at room (300 K) and low (30 K) temperature of oxygen on the Si(1 1 1)-7×7 surface. Subsequently, STM manipulation has been used to find out which of the observed sites can be modified (displacement, transformation, desorption). It was found that the adsorption is quite different at 30 K compared to 300 K and that the manipulation at room temperature is essential in the identification of the sites. These results are in general agreement with recent conclusions from theoretical work [Phys. Rev. Lett. 84 (2000) 1724] and synchrotron radiation experiments [Phys. Rev. B 65 (2002) 035315].

The benzene adsorption on Si(111)7 x 7 was investigated by synchrotron-radiation valence-band photoemission in the temperature range 30-300 K. We found that when benzene is adsorbed on Si(111)7 x 7 at low temperature there is a one-to-one correspondence between the adsorbed and the gas-phase features. Furhermore, the interaction of benzene with the surface states is very weak, hinting at a physisorption process. Upon temperature increase, the adsorption features change gradually, but significantly. In particular, we observe a splitting of the features related to the degenerate orbitals, indicating that the benzene bond to the silicon surfaces is turning into chemisorption. This is the first evidence by photoemission spectroscopy that such a transition occurs, to our knowledge. Furthermore, the spectrum we obtain upon transition from the physisorbed to the chemisorbed state corresponds to the one obtained by dosing benzene at room temperature, thus showing that the chemisorption state does not depend on the way it is achieved. Upon exposure to a benzene multilayer we observe a direct transition from multilayer to chemisorbed state without intermediate transition to a physisorbed state, thus indicating that the multilayer desorption and the physisorption-to-chemisorption transition occur simultaneously. The adsorption of a few benzene overlayers on Si(111)7 x 7 at low temperature allowed us to monitor simultaneously both the first adlayer (hence the physisorption-to-chemisorption transition) and the overlayer features (hence the multilayer desorption), confirming that the physisorption-to-chemisorption transition and multilayer desorption are competing processes, and indicating a way to study substrate-adsorbate systems.