Influence of a dielectric layer on photon emission induced by a scanning tunneling microscope

We investigate theoretically the influence of a dielectric layer on light emission induced by a scanning tunneling microscope through a combined approach of classical electrodynamics and first-principles calculations. The modification of the junction geometry upon the insertion of a dielectric layer is treated first by using the density functional theory to calculate the effective potential along the surface normal and then by solving a one-dimensional Schrödinger equation to obtain the exact distance between the tip and the substrate for a given current and bias voltage. The modified external field with the inclusion of a dielectric layer is evaluated by using the Fresnel formula. The local-field enhancement factor and radiated power are calculated by the boundary element method for two typical systems, W-tip/C60/Au(111) and W-tip/Al2O3/NiAl(110). The calculated results indicate that the insertion of a dielectric layer tends to reduce the light emission intensity considerably but hardly changes the spectral profile with no substantial peak shifts with respect to the layer-free situation, in agreement with experimental observations. The suppression of the radiated power is mainly due to the increase in the tip-metal separation and the resultant reduction in the electromagnetic coupling between the tip and metal substrate.This work was partly supported by the National Basic Research Program of China (Grant No. 2006CB922003), CAS (Grant No. KJCX2.YW.H06), and NSFC (Grant No. 10574117).Peer reviewe

Chemical mapping of a single molecule by plasmon-enhanced Raman scattering

Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, molecular nanotechnology and biotechnology. Molecular vibrations provide a valuable 'fingerprint' for such identification. Vibrational spectroscopy based on tip-enhanced Raman scattering allows us to access the spectral signals of molecular species very efficiently via the strong localized plasmonic fields produced at the tip apex. However, the best spatial resolution of the tip-enhanced Raman scattering imaging is still limited to 3-15 nanometres, which is not adequate for resolving a single molecule chemically. Here we demonstrate Raman spectral imaging with spatial resolution below one nanometre, resolving the inner structure and surface configuration of a single molecule. This is achieved by spectrally matching the resonance of the nanocavity plasmon to the molecular vibronic transitions, particularly the downward transition responsible for the emission of Raman photons. This matching is made possible by the extremely precise tuning capability provided by scanning tunnelling microscopy. Experimental evidence suggests that the highly confined and broadband nature of the nanocavity plasmon field in the tunnelling gap is essential for ultrahigh-resolution imaging through the generation of an efficient double-resonance enhancement for both Raman excitation and Raman emission. Our technique not only allows for chemical imaging at the single-molecule level, but also offers a new way to study the optical processes and photochemistry of a single molecule. © 2013 Macmillan Publishers Limited. All rights reserved.This work is supported by the National Basic Research Program of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, the Natural Science Foundation of China and the Basque Government Project of Excellence (ETORTEK).Peer Reviewe

Geometrical Simulation and Analysis of Ball-End Milling Surface Topography

Ball-end milling cutter has a strong adaptability and widely used in machining complex surface of parts. However, the geometry of ball-end milling cutter tooth is complex, and contact points between cutter tooth and part are varying constantly during milling process, which lead that it is difficult to study the surface topography by the traditional experimental method. Based on the time-step method, this paper proposes an improved Z-MAP algorithm to simulate the part surface topography after ball-end milling. On the basis of the cutter tooth movement equation established by homogeneous matrix transformation, the improved Z-MAP algorithm combines servo rectangular encirclement and the angle summation method to quickly obtain the instantaneous swept points that belong to the part, and introduce Newton iterative method to calculate the height of swept points. Comparing to traditional Z-MAP algorithm which discrete segments of cutter tooth can only sweep one discrete point of part during a unit time step, the proposed algorithm need not to disperse cutter tooth, accomplishes higher precision and efficiency. The influence of processing parameters, such as step over, feed per tooth, cutter posture, and cutter tooth initial phase angle difference, upon the surface topography and roughness are analyzed. The experiments are conducted to validate the availability of the proposed algorithm, and the results show that surface topographies simulated by the improved Z-MAP algorithm have a higher consistency with the experiments and costs less time than by the traditional Z-MAP algorithm under the same simulation conditions. Therefore, the proposed algorithm is effective for simulating the machined surface quality in practical production and rational selection of machining parameters