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Realizing in situ simultaneous measurement of full vibrational modes in tip-enhanced Raman spectroscopy (TERS) is of immense importance for improving the ability of quantitative analysis and measurement accuracy of the molecular spectrum. In this study, the electric field gradient effect in the TERS system was theoretically investigated by using the finite element method. The respective contributions of electric field and electric field gradient to dipole Raman and gradient Raman modes in the TERS spectrum were quantitatively distinguished. By comparing the relative intensity ratio between electric field gradient and electric field, the TERS configuration could be optimized to achieve a maximum of gradient Raman modes. Theoretical results indicate that molecular symmetries strongly influence the molecular hyperpolarizability, which significantly influences the plasmonic electric field gradient effects. Our theoretical results could reveal the contributions of electric field and electric field gradient to molecular vibration modes, useful for the design of the TERS platform with more spectrum information.

Optical studies on black phosphorus (BP) are crucial for its applications in plasmon‐enhanced spectroscopy, biomedicine, and energy. Plasmonic enhancement, spatial resolution, and directional emission, which related to tip‐enhanced spectroscopy of few‐layer BP, have been theoretically investigated. Calculated Raman and fluorescence enhancement factors are 8 and 3 orders of magnitude, respectively. The spatial resolutions for Raman and fluorescence spectroscopy could reach 6 and 7 nm, respectively, under the 633‐nm laser wavelength. Directional tip‐enhanced spectroscopy emission reveals a tunable optimized collection angle by changing the tilted angle of the tip. Our theoretical results provide a deep understanding of the underlying enhanced mechanism of BP spectroscopy, useful for the design of nanoelectronic and optoelectronic devices.

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