2-dimensional polar metals: a low-frequency Raman scattering study

Abstract

The intercalation of a molecule or ion in a layered structure is key to enhancing energy storage, material conductivity, intercalant structural ordering, and the formation of two-dimensional (2D) superconducting states. The process of intercalation modifies the vibrational energy of the host, which can be monitored non-invasively by Raman spectroscopy. However, the detected Raman spectral shifts may originate from a variety of phenomena, generally making the technique an indirect means of identifying intercalation success. Here, we discuss newly discovered low-frequency (LF) (<100 cm −1 ) Raman features due to the formation of unique 2D polar metals (Ag, Cu, Pb, Bi, Ga, In) or metal alloys (In x Ga 1−x ) intercalated at an epitaxial graphene (EG)/silicon carbide (SiC) interface and demonstrate that 2D-Ag and 2D-Ga can have spatially distinct phases with their own unique Raman responses. Additionally, we establish that the 2D-Ga exhibits a structural evolution as a function of temperature, independent of the SiC and EG, that can lead to nucleation of secondary phases. The newly identified LF Raman responses discussed here lay the foundation for rapid, direct, and spatially resolved evaluation of 2D polar metals in ambient.

Shear Modes in a 2D Polar Metal

“…The atomic structure of 2D Ga on SiC used in this work (e.g., Figure a) is consistent with high-resolution high-angle annular dark-field cross-sectional scanning transmission electron microscope (HAADF-STEM) images (Figure S1) and represents the energetically most stable structure according to density functional theory (DFT) calculations . Experimentally, it has been found that the low-frequency Raman spectra are independent of the number of graphene overlayers on top of 2D Ga . Our DFT calculations also show that the graphene shear and breathing modes are essentially decoupled from the Ga and SiC phonons (Table S1).…”

“…This observation is distinct from that in van der Waals layered materials, and arises from the coupling of the 2D metal to the substrate, which results in an asymmetric bonding profile in the system. We expect that the low-frequency shear modes predicted here will also be present in other 2D metals (e.g., In, Ag, alloys) that have similarly been fabricated by heteroepitaxy on SiC. A clear understanding of the low-frequency phonon modes in these 2D polar metals is important for developing an atomic-scale understanding of the coherent, coupled electron and phonon dynamics in these emerging systems …”

Highly Asymmetric Graphene Layer Doping and Band Structure Manipulation in Rare Earth–Graphene Heterostructure by Targeted Bonding of the Intercalated Gadolinium

“…The deterministic stacking order and interlayer twist angles provide highly tunable electronic properties, which are especially demonstrated by the realization of superconducting phases in twisted bilayer graphene. − On the other hand, bottom-up growth provides an alternative for high quality and stability over wafer-scale lengths, with a particular interest in epitaxial graphene layers obtained by thermal decomposition of silicon carbide (0001) substrates. ,,− In this case, the intercalation of heteroatoms under graphene layers is the most promising strategy for the synthesis of designer 2D materials . First, intercalation was used to electronically decouple graphene layers from the SiC substrate, , and subsequently, it was found to be a very effective way to control the electronic properties of the resulting graphene-based heterostructures. − …”

Interface-Mediated Resonant Raman Enhancement for Shear Modes in a 2D Polar Metal

“…In the slab models, the SiC substrate is represented by six layers of SiC passivated by hydrogen atoms, while in the Green’s function approach, the SiC substrate is represented as a semi-infinite substrate. Although the polar Ga layers are covered by graphene overlayers in the experiment, these graphene layers do not have a significant impact on the low-frequency phonon modes or the interband optical transitions in the energy range of interest here ,, and are not included in the theoretical models.…”