We demonstrate experimentally that a nano-gap could be constructed by using a scanning probe microscope to allow a gold tip to approach a gold nanorod immobilized on a glass coverslip. The nano-gap can enhance Raman scattering of graphene sandwiched between the tip and the nanorod. The Raman intensity was strongly dependent on the incident light polarization. Here, linear, radial, azimuthal, and intermediate states between radial and azimuthal polarization were investigated and compared in detail. The maximum surface-enhanced Raman scattering effect of the nano-gap occurred for the intermediate states between the radial and azimuthal polarized light.
been demonstrated to create photodetectors, photovoltaic devices, and plasmonenhanced infrared spectroscopy. [10][11][12] To optimize the performance of hybrids system, a central question is to understand the electron or energy transfer between 2D material and metallic nanostructures. [13][14][15] Optical method is a versatile way to characterize the time scales for this process. In general, the transfer process is an additional decay channel for surface plasmon resonance of the gold nanoparticles. That would lead to a broadened linewidth and decreased amplitude of localized surface plasmon (LSP) resonances. [16] However, most early optical investigations of the hybrid structures rely on the ensemble measurements. [17,18] The hybrids have been proposed for SERS substrate to increase signal-to-noise ratio [19][20][21][22][23][24] since it can suppress continuum background (i.e., photoluminescence (PL) from the gold nanostructures). [25,26] The decreased background is attributed to the electron or energy transferring from gold to graphene [27,28] but ensemble measurement method cannot quantify the time scale of the transfer because of size and shape inhomogeneity of nanoparticles. [17] By contrast, single particle method enables measurement of homogeneous plasmon linewidth to determinate the transfer time scale. [16] Recently, Hoggard et al. employed single particle spectroscopy to investigate the interaction between gold nanorods (GNRs) and monolayer graphene. They statistically analyzed and compared the scattering spectra of hundreds of individual GNRs on quartz or on graphene substrates. [29] A time scale of ≈160 fs and a transfer efficiency of ≈10% were obtained. However, they did not observe intensity suppression of the scattering and PL spectra with comparison to previous ensemble measurements. That should be explained as the scattering intensity of individual nanoparticles vary more dramatically rather than the linewidth varying despite minor differences in the size. [30] At single particle level, this property hinders the statistical analysis method to determinate the changes of scattering and PL intensity. So far, previous reports about the scattering and PL of the hybrids were not always consistent each other. [17,29] This controversy is due to the limitation of measurement methods.In this study, we proposed to perform in situ single particle measurements to circumvent previous experimental limitation. We combined atomic force microscope (AFM) nanomanipulation technique with single particle spectroscopy method. [31,32] We directly measured and compared the scattering and PL spectra of Electron or energy transfer process is critical to understand the interaction between graphene and metal nanostructures. Combining atomic force microscope nanomanipulation with single-particle spectroscopy enables in situ investigation of the interaction between single gold nanorod and monolayer graphene. This paper reveals that scattering and photoluminescence intensity of the nanorod decreased after coupling with monolayer...
We demonstrate a high-performance apertureless near-field probe made of a tapered metal tip with a set of periodic shallow grooves near the apex. The spontaneous emission from a single emitter near the tip is investigated systematically for the side-illumination tip enhanced spectroscopy (TES). In contrast with the bare tapered metal tip in conventional side-illumination TES, the corrugated probe not only enhances strongly local excitation field but also concentrates the emission directivity, which leads to high collection efficiency and signal-to-noise ratio. In particular, we propose an asymmetric TES tip based on two coupling nanorods with different length at the apex to realize unidirectional enhanced emission rate from a single emitter.Interestingly, we find that the radiation pattern is sensitive to the emission wavelength and the emitter positions respective to the apex, which can result in an increase of signal-to-noise ratio by suppressing undesired signal. The proposed asymmetrical corrugated probe opens up a broad range of practical applications, e.g. increasing the detection efficiency of tip enhanced spectroscopy at the single-molecule level.
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