Tailored Nanoscale Plasmon-Enhanced Vibrational Electron Spectroscopy
Résumé
Vibrational optical spectroscopies can be enhanced by surface plasmons to reach molecular-sized limits of detection and characterization. The level of enhancement strongly depends on microscopic details of the sample that are generally missed by macroscopic techniques. Here we investigate phonons in h-BN by coupling them to silver-nanowire plasmons, whose energy is tuned by modifying the nanowire length. Specifically, we use electron beam milling to accurately and iteratively change the nanowire length, followed by electron energy-loss spectroscopy to reveal the plasmon-enhanced vibrational features of h-BN. This allows us to investigate otherwise hidden bulk phonons and observe strong plasmon-phonon coupling. The new milling-and-spectroscopy technique holds great potential for resolving vibrational features in material nanostructures. Vibrational spectroscopy is key in a wide range of research areas and technological applications, from molecular fingerprinting to fundamental solid-state physics 1,2. The discovery that plasmonic structures can increase the measured vibrational signal has driven the development of ultra-sensitive analytical techniques capable of reaching single-molecule detection, such as in surface-enhanced Raman spectroscopy 3,4 (SERS) and surface-enhanced infrared absorption 5 (SEIRA). Typically, plas-monic structures are designed in advance and molecules are randomly dispersed over them, leading to strongly enhanced signals associated with those sitting on the so-called hotspots 4. Alternatively, a metallic tip can be scanned over a sample to induce SERS locally, leading to chemical mapping at sub-molecular scales, a technique known as tip-enhanced Raman scattering 6 (TERS). Vibrational electron energy-loss spectroscopy (EELS) has been performed for decades mainly as a surface technique 7 using wide beams with poor spatial resolution. The recent development of a new family of electron monochromators 8 has allowed vibration mode measurements to be performed based on EELS 9-11 down to atomic spatial resolution in bulk materials 12. Unfortunately , the signal-to-noise ratio of vibrational EELS is low for materials that are sensitive to the electron beam, thus imposing a lower bound on the volume necessary for analysis. This jeopardizes the high resolution mapping of fragile materials, such as organic molecules 13. Recently, theoretical studies 14-16 have proposed the use of infrared plasmonic fields to develop a new form of enhanced vi-brational EELS which would overcome these limitations by making the molecules interact with the beam at a distance mediated by plasmons extended in a nanoparticle. Here, we demonstrate plasmon-enhanced vibrational electron spectroscopy (PEVES) through the tailored coupling of plasmon resonances in metallic nanowires to phonon modes in h-BN thin flakes. Coupling is achieved by continuously shifting the energies of the plasmon modes of micrometer-long metallic nanowires (Fig. S1) using electron-beam controlled milling to bring them into resonance with specific h-BN vibrational modes (Fig. S2). We reveal three new effects when a plasmon-phonon resonance is encountered: 1) strong coupling between surface phonons and plasmons; 2) enhancement of the bulk vibrational EELS signal; and 3) emergence of previously geometry-forbidden dark phonon modes. EELS measures the distribution of energy losses experienced by free electrons when interacting with a target 17. The energy resolution is determined by the energy spread of the electron source, typically ∼250 meV for a cold field emitter source. This resolution can be improved down to a few meV using an electron monochromator (Fig. S3). To achieve subnanometer spatial resolution, the electron monochromator can be coupled to an electron microscope (Fig. S3). Finally, an electron spectrometer is used to acquire the spectrum of the electron beam after interaction with the sample. Here, we use such a setup implemented on a NION Hermes scanning transmission microscope (STEM), see Methods. The plasmonic silver metallic nanowires were synthesized by chemical seeded growth as detailed elsewhere 18 (Fig. 1A) and subsequently deposited, either entirely (configuration (1)) or partially (configuration (2)), on an h-BN substrate 19. In theory, extended h-BN possesses a arXiv:1905.12503v1 [cond-mat.mes-hall]
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