Plasmonic nanocavity-enabled universal detection of layer-breathing vibrations in two-dimensional materials

TL;DR

This work introduces a universal plasmon-enhanced Raman spectroscopy approach to access interlayer layer-breathing vibrations in 2D materials by coupling them to plasmonic Au/Ag nanocavities. The authors develop the electric-field-modulated interlayer bond polarizability model (E-IBPM), which integrates localized plasmonic field enhancement with interfacial polarizability changes to quantitatively describe LB-mode intensities across NLG, hBN, and vdW heterostructures. By combining experimental observations with a modified linear chain model and FDTD simulations, they demonstrate robust, polarization-sensitive LB signals that reveal interfacial coupling strengths and layer-dependent dynamics, and show wavelength-tunable detection through different plasmonic cavities. The framework provides a quantitative, generalizable tool for probing hidden interlayer interactions and could extend to other quasiparticles such as interlayer excitons in layered quantum materials. Overall, the work establishes plasmonic nanocavities as a universal amplifier for weak interlayer phonons and a platform for characterizing complex interfacial physics in 2D systems.

Abstract

Conventional Raman spectroscopy faces inherent limitations in detecting interlayer layer breathing (LB) vibrations with inherently weak electron-phonon coupling or Raman inactivity in two-dimensional materials, hindering insights into interfacial coupling and stacking dynamics. Here we demonstrate a universal plasmon-enhanced Raman spectroscopy strategy using gold or silver nanocavities to strongly enhance and detect LB modes in multilayer graphene, hBN, and their van der Waals heterostructures. Plasmonic nanocavities even modify the linear and circular polarization selection rules of the LB vibrations. By developing an electric-field-modulated interlayer bond polarizability model, we quantitatively explain the observed intensity profiles and reveal the synergistic roles of localized plasmonic field enhancement and interfacial polarizability modulation. This model successfully describes the behavior across different material systems and nanocavity geometries. This work not only overcomes traditional detection barriers but also provides a quantitative framework for probing interlayer interactions, offering a versatile platform for investigating hidden interfacial phonons and advancing the characterization of layered quantum materials.

Figures (4)

• Figure 1: Plasmon-enhanced Raman modes in AuNCs/$N$LG. (a) Schematic illustration of the AuNCs/$N$LG and the PERS of LB modes. (b) Optical (left panel, scale bar is 20 $\mu$m), SEM (middle panel, scale bar is 40 nm), and TEM (right panel, scale bar is 25 nm) images of AuNCs/$N$LG. (c) Dark-field scattering spectra of AuNCs/18LG and 18LG. (d) Low-frequency Raman spectra of AuNCs/18LG under on-resonance ($\lambda_{\text{L}}=633$ nm) and off-resonance ($\lambda_{\text{L}}=532$ nm) conditions. (e) Raman spectra of AuNCs/$N$LG and the corresponding $N$LG with $\lambda_{\text{L}}$ of 633 nm ($N=1,2,6,18$). The "*" and "+" symbols indicate the LB modes and new low-frequency modes, respectively.
• Figure 2: Plasmon-enhanced LB modes in AuNCs/$N$LG. (a) Low-frequency Raman spectra (up to 140 cm$^{-1}$) of AuNCs/$N$LG, where $N$ varies from 1 to 30. The IMs are marked by "+". (b) Extracted LB mode frequencies from (a) as a function of $N$ (circles), along with the fitting results (lines) by the LCM. The inset illustrates the LCM for AuNCs/$N$LG and the corresponding LB force constants. (c) Linearly polarized (including VV and HV configurations) and (d) circularly polarized (including $\sigma+\sigma+$ and $\sigma+\sigma-$ configurations) Raman spectra of AuNCs/6LG and t(3+3)LG ($\theta$=10.5$^{\circ}$). The C and LB modes are labeled. All spectra were obtained with $\lambda_{\text{L}}=633$ nm.
• Figure 3: Electric-field-modulated interlayer bond polarizability model for plasmon-enhanced LB modes in AuNCs/$N$LG. (a) FDTD-simulated distribution of normalized $x$-component of local field ($|E_{\text{Loc},l,x}|/|E_0|$) in different graphene layers near AuNCs. (b) Simulated maximum $|E_{\text{Loc},l,x}|/|E_0|$ at graphene layers near AuNCs and a schematic of AuNCs/$N$LG with $\alpha_{i,xx}^{\prime}$. (c) Measured PERS spectra with $\lambda_{\text{L}}=633$ nm and (d) the calculated IM and LB modesof AuNCs/$N$LG ($N$=5, 10, 15, 20) by E-IBPM. The IM and LB modes are labeled. The lineshapes of all LB modes were modeled as Lorentzian functions, with FWHM of 10 cm$^{-1}$, 7 cm$^{-1}$, 4 cm$^{-1}$ and 3 cm$^{-1}$ for $N$=5, 10, 15 and 20, respectively. (e) SEM images and the (f) corresponding Raman spectra of as-prepared AuNCs/8LG, AuNCs/8LG annealed at 600$^\circ$C, as-prepared AgNCs/8LG and AgNCs/8LG annealed at 400$^\circ$C. Scale bars in (e): 250 nm. The IMs and LB modes are marked with dashed lines in (f).
• Figure 4: Plasmon-enhanced LB modes in hBN, $N$LG and tMLG coupled to AuNCs or AgNCs. Experimental (Exp.) and calculated (E-IBPM or IBPM) Raman spectra of the LB modes for (a) AuNCs/16L-hBN, (b1) AuNCs/t(3+3)LG (twist angle $\theta=10.5^{\circ}$) and the corresponding pristine counterparts under 633 nm excitation. The schematic diagram for interlayer bond parameters $\alpha_{l,xx}^{\prime}$ of AuNCs/t(3+3)LG is also shown in (b2). (c) Experimental (Exp.) and calculated (E-IBPM or IBPM) Raman spectra of the LB modes for AgNCs/30LG and AgNCs/50L-hBN under 532 nm excitation. Asterisks (*) indicate residual C modes.