Light–Matter Interaction in Quantum Confined 2D Polar Metals

Abstract

This work is a systematic experimental and theoretical study of the in‐plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type. k‐space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near‐zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model‐based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light–matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum‐)plasmonics and nano‐photonics.

Tunable 2D Group‐III Metal Alloys

“…regions 13 in the near-IR (NIR) and visible range prominently occurring for the DFT-calculated functions (Figure 5b) that are in reasonable agreement with the experimentally determined 𝜀 The superconducting transition temperature Tc can also be tuned by the alloy composition. Figure 6a plots the temperature-dependent resistance of InxGa1-x alloys, where x ranges from 0 to approximately 0.5.…”

“…Previously, the Lorentz resonance was shown to decrease with the effective thickness of the quantum well for bilayer and trilayer Ga and In samples 13 . DFT indicates that the electrostatic potential in 2D In is deeper than that in 2D Ga (Figure S14), indicating that the smaller energy separation between the quantum-confined bands in 2D In results primarily from its larger thickness compared to 2D Ga rather than a weaker crystal potential.…”

“…This is supported by DFT calculations of twenty-five structures (Figure S6, Table S1 A simple nearest-neighbor cluster expansion with distinct pair-wise interactions between In-In, Ga-Ga, and In-Ga within or across lower and upper layers and an on-site energy can reproduce the first-principles total energies of this set of alloy configurations to within a few hundredths of an eV/metal atom (these errors and the associated fitting parameters are given in Tables S1 and S2 respectively The 2D metal alloy exhibits a composition-dependent dielectric function. Figures 5a and 5c show the dielectric functions determined from variable angle spectroscopic ellipsometry (VASE) in combination with a regression analysis using a multilayer model 13 . We have previously applied this approach to obtain the dielectric functions for 2D Ga and 2D In 13 and obtained good agreement with first-principles calculations.…”

“…Figures 5a and 5c show the dielectric functions determined from variable angle spectroscopic ellipsometry (VASE) in combination with a regression analysis using a multilayer model 13 . We have previously applied this approach to obtain the dielectric functions for 2D Ga and 2D In 13 and obtained good agreement with first-principles calculations. The model consists of the SiC substrate, a modified SiC substrate describing the uppermost SiC layers to which the metal atoms are covalently bonded 13 , the 2D metal film, and graphene.…”

“…In contrast, a simulation involving phase separated In0.5Ga0.5 alloy structures with In (Ga) in the bottom layer and Ga (In) in the top layer gives peak positions of 2.10 and 1.59 eV, which are significantly different from experiment and further support the well-mixed nature of the alloys. The interband transitions at ~2 eV result from quantum-confined bands centered around the K-M high symmetry path in the Brillouin zone 13 . These transitions are not affected by the discrepancies between the computed and experimental Fermi levels in ARPES (see also Figure S13).…”