Infrared Fingerprints of Natural 2D Talc and Plasmon–Phonon Coupling in Graphene–Talc Heterostructures

Abstract

Two-dimensional (2D) materials occupy a noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphenebased heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics features from plasmons, phonon−polaritons, and hybrid plasmon phonon− polaritons. Here we present a prime study on mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the talc vibrational resonances, assigned to in-and out-of-plane molecular vibrations, are achieved by changing the thickness of the crystals, which configures a tunable infrared signature for the 2D talc. In G-talc nanostructures, we unveil a coupling of the graphene plasmons polaritons with surface phonons polaritons of talc, originating hybrid surface plasmon−phonon polaritons modes. In analogy to G-hBN and G-SiO 2 heterostructures, the coupling in G-talc produces a large increase in the optovibrational activity for the out-of-plane mode as well as it induces a blue-shift for the in-plane mode. Moreover, the coupling can be modulated by an external gate voltage to the heterostructure when mounted in a transistor configuration. Therefore, our results introduce talc and G-talc heterostructures as appealing materials for nanophotonics, especially for applications involving long wavelengths and active electric tuning of opto-vibrational activity.

Probing Polaritons in 2D Materials with Synchrotron Infrared Nanospectroscopy

“…The paper also highlights the main SINS contributions to the study of hybrid polaritonic activity in graphene‐hBN systems, polariton‐assisted mapping of local strain in hBN crystals and plasmonic phenomena in metal oxide conductors and vdW semiconductors . The SINS analysis of talc crystals and graphene‐talc devices is reviewed and highlights the photonic tunability of this new vdW material . Finally, a demonstration of the extreme spectral reach of SINS to probe the far‐IR plasmonics of graphene is revisited, exploiting the long wavelengths sensitivity of a Ge:Cu detector.…”

“…The graphene‐talc heterostructure, constructed in a field effect transistor architecture (Figure a,b), features an atomically flat surface, stable, and free of detectable defects . In Figure c one notes from SINS point‐spectra that the 1025 cm −1 out‐of‐plane mode is significantly increased for graphene‐talc compared to pure talc.…”

“…Graphene plasmonics and the phonon‐polariton properties of talc ( Figure ), a natural atomically flat vdW crystal have been studied in a graphene‐talc nanophotonic device using SINS . As in graphene‐hBN heterostructure, hybrid plasmon phonon‐polaritons were identified in graphene‐talc ( Figure a) revealing appealing aspects for photonic applications .…”

“…Copyright 2016, IOP Publishing Ltd. e) SINS point spectra of a talc nanocrystal as a function of the crystal thickness on a gold substrate. b,c,e) Reproduced with permission . Copyright 2018, The American Chemical Society.…”

“…The vertical dotted line indicates the momentum excited in by the SINS system due to the AFM tip radius. Reproduced with permission . Copyright 2018, The American Chemical Society.…”