Role of tip shape in light emission from the scanning tunneling microscope

Abstract

The influence of tip shape on the light produced by scanning tunneling microscopy is analyzed theoretically for the case of nobel metals where collective modes enhance the photon emission. We investigate a hyperbolic tip geometry where the aperture of the tip and its apex curvature can be changed independently. The electromagnetic field in the tip-sample region is calculated with the use of the boundary charge method. The tunneling current is treated in a modified Tersoff-Hamann theory. The aperture of the tip is found to control the overall shape of the emission spectrum, while the radius of curvature of the apex is more important for the intensity. Experimentally observed variations of emission spectra may be understood in terms of different tip shapes. The lateral extent of the tip-induced charge density is strongly dependent on the tip shape, and may reach a near-atomic scale for sufficiently sharp tips. This spatial localization has direct implications for the resolution in photonic maps.

Light emission from Ag(111) driven by inelastic tunneling in the field emission regime

“…It is well established that the spectral distribution of the emitted light can change largely with the tip geometry as described in Ref. [30] for a hyperboloid tip model. Here, we will use the sphere tip model of Refs.…”

Engineering the emission of light from a scanning tunneling microscope using the plasmonic modes of a nanoparticle

“…In most experiments, the emitted light is completely dominated by the resonance of the gap plasmon mode induced by the strong tip-sample interaction when both the tip and the sample are made of plasmonic materials such as gold or silver [22,25,27]. The * eric.le-moal@u-psud.fr † andrei.borissov@u-psud.fr extreme spatial confinement of the gap plasmon inside the junction makes it exceedingly sensitive to very slight yet unavoidable and unpredictable changes in the morphology of the tip apex [28,29]. Consequently, the common wisdom in the field is that STM-induced light emission suffers from this "arbitrary" character [29].…”

Nonequilibrium Green's Function Theory of Scanning Tunneling Microscope-Induced Light Emission from Molecule Covered Metal Surfaces: Effects of Coupling between Exciton and Plasmon Modes

“…Especially the results show that the detailed investigation of variation in spectral profiles of interface plasmon due to energy exchange with molecular excitons requires a quantum mechanical treatment of interface plasmons. The theory developed by us is suitable for such studies, compared to previous theories of STM-LE [25,28,29,42,44,45,47]. Moreover, analysis of the luminescence property of the system from the viewpoint of superposition of the coupled modes provides simple description of luminescence spectral profile of the system, which aids in interpretation of the calculation results as well as experimental results.…”

“…The energy of the interface plasmon mode ℏω p is set to 2.05 eV and U β ′ is set to give a plasmon lifetime of 4.7 fs for ℏg = 0 which are consistent with experiments [36,43] and earlier simulation results [44,45]. The relaxation constant of the molecular exciton for ℏg = 0 is given by ℏΓ ex = ∑ β |U β | 2 πδ (ℏω − ℏω β ) and set to 0.005 eV [40].…”