Tunable Multipolar Surface Plasmons in 2D Ti₃C₂T_x MXene Flakes

Abstract

2D TiC T MXenes were recently shown to exhibit intense surface plasmon (SP) excitations; however, their spatial variation over individual TiC T flakes remains undiscovered. Here, we use scanning transmission electron microscopy (STEM) combined with ultra-high-resolution electron energy loss spectroscopy (EELS) to investigate the spatial and energy distribution of SPs (both optically active and forbidden modes) in mono- and multilayered TiC T flakes. With STEM-EELS mapping, the inherent interband transition in addition to a variety of transversal and longitudinal SP modes (ranging from visible down to 0.1 eV in MIR) are directly visualized and correlated with the shape, size, and thickness of TiC T flakes. The independent polarizability of TiC T monolayers is unambiguously demonstrated and attributed to their unusual weak interlayer coupling. This characteristic allows for engineering a class of nanoscale systems, where each monolayer in the multilayered structure of TiC T has its own set of SPs with distinctive multipolar characters. Moreover, the tunability of the SP energies is highlighted by conducting in situ heating STEM to monitor the change of the surface functionalization of TiC T through annealing at temperatures up to 900 °C. At temperatures above 500 °C, the observed fluorine (F) desorption multiplies the metal-like free electron density of TiC T flakes, resulting in a monotonic blue-shift in the SP energy of all modes. These results underline the great potential for the development of TiC T -based applications, spanning the visible-MIR spectrum, relying on the excitation and detection of single SPs.

Electrochromic Effect in Titanium Carbide MXene Thin Films Produced by Dip‐Coating

“…To probe only the change in optical properties of the WE, a 0.5 cm diameter hole was made in the Ti 3 C 2 T x CE to avoid contribution of the CE in the UV-vis-NIR spectrum (Figure 1a). [28] According to previous studies, it was suggested that the absorption peak at ≈770 nm corresponds to a plasmonic effect, [42,43] more specifically to a transversal surface plasmon, which would explain the independence of the peak position on the flake size. [28] According to previous studies, it was suggested that the absorption peak at ≈770 nm corresponds to a plasmonic effect, [42,43] more specifically to a transversal surface plasmon, which would explain the independence of the peak position on the flake size.…”

“…The UV-vis-NIR spectrum www.afm-journal.de www.advancedsciencenews.com of a Ti 3 C 2 T x MXene thin film has several characteristic features, such as a broad absorption peak around 760-780 nm and an absorption peak in the UV region ( Figure 1c). [22,43] Electrochromic properties of the Ti 3 C 2 T x device were studied by in situ UV-vis-NIR spectroscopy during electrochemical cycling in 1 m phosphoric acid polyvinyl alcohol gel electrolyte (H 3 PO 4 /PVA gel). [22,43] Electrochromic properties of the Ti 3 C 2 T x device were studied by in situ UV-vis-NIR spectroscopy during electrochemical cycling in 1 m phosphoric acid polyvinyl alcohol gel electrolyte (H 3 PO 4 /PVA gel).…”

“…Recently, El-Demellawi et al, [43] demonstrated a shift of ≈0.3 eV for the surface plasmon at 1.7 eV of Ti 3 C 2 T x flakes upon annealing up to 900 °C. This shift was attributed to the modification of the surface terminations of Ti 3 C 2 T x (in that case desorption of fluorine (F) groups) which involved the increase of the metal-like free electron density.…”

Probing Polaritons in 2D Materials

“…Inset: TEM‐HAADF micrograph of the Ti 3 C 2 T x flake. Reproduced with permission . Copyright 2018, ACS.…”

“…The improvement of the energy solution can extend the detectable modes from the infrared to the visible‐ultraviolet. EELS‐TEM has been widely used to excite ultraviolet plasmonic modes in various traditional 2D materials, such as high energy plasmon in graphene, black phosphorous, topological insulators, a recently discovered new class of 2D crystals‐MXenes, and the bandgap excitation in h‐BN . For example, Eberlein and co‐workers observed π and π + σ‐surface plasmon modes in a few layers of free‐standing graphene sheets, where the EEL spectra is shown in Figure c.…”