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A Novel Surface Plasmon Polaritons (SPPs) Induced Near-Field Raman Spectroscopy Imaging Technique Performed by Ultra-Sharp Tip Silver Nanowires (AgNWs)

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With the rapid development of nano-science and nano-technology, Tip-Enhanced Raman Spectroscopy (TERS) becomes one of the most important and highest performances demanded characterization techniques among many optical microscopes. TERS combines both the Raman spectroscopy, which provide atoms vibration modes of materials and the scanning probe microscopes (SPMs), which can revile the topography of materials surface with nano- or sub-nano-spatial resolution. Although the large success has been achieved by TERS since it was proposed in 1985 and was first demonstrated in 2000, traditional TERS configurations, the directly-excited ones, are suffering from the large background signal leading to degradation of the spatial resolution and arising high chance of sample damaging by the large uncoupled far-field laser energy flux. Separating the incident laser exciting point and the Raman signal scattering point, i.e. the Remote-excited TERS (RE-TERS) configuration, seems to be a reasonable way to tackle the problem. Focused ion beam (FIB) milling has been used to etch the pre-designed nano-grating on the sidewall of a tapered metal (gold) probe, the so-called adiabatic nanofocusing TERS probe, have been demonstrated the 0.1% to 9% coupling efficiency and TERS mapping images with sub-10 nanometers spatial resolution. However, the time-consuming, complicated fabrication process, high-cost and low reproducibility of the probes restricted their applications. Thanks for the high reproducibility and high yield chemical synthesized AgNWs have the crystallized smooth surface and ultra-sharp tip, leading to the superb plasmon performance and the abilities for nanofocusing compressed SPPs mode for an extremely confined nano-light source. To test the AgNW based SPMs performant, an ultra-sharp tip AgNWs based high spatial resolution (HR) and high aspect ratio (HAR) AFM has been achieved. Furthermore, the AgNW-AgNC bundle has been demonstrated high coupling efficiency (4%). Combing with the AgNW-AFM, high spatial resolution and high TERS contrast (largely suppressed background signal) RE-TERS has been achieved. As a benchmark, single-walled carbon nanotube (SWCNT) with ~1nm diameter served as a test sample and its D-, G- and 2D- peaks mapping image has been achieved with 9.7nm spatial resolution.

Besides the near-field TERS imaging, the vapor-assisted clean-stamp transfer method has been developed for TERS sample preparation. Using this method, we prepared graphene-based transistors with low charge-neutral concentration (3 × 1010 cm–2) and high carrier mobility (up to 48 820 cm2 V–1 s–1 at room temperature with SiO2 supported, which is keeping the world record) and heterostructure optoelectronics with high operation speed.

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