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.

We have used ab initio density functional theory, incorporating van der Waals corrections, to study twisted bilayer graphene (TBLG) where Stone-Wales defects or monovacancies are introduced in one of the layers. We compare these results to those for defects in single layer graphene or Bernal stacked graphene. The energetics of defect formation is not very sensitive to the stacking of the layers or the specific site at which the defect is created, suggesting a weak interlayer coupling. However signatures of the interlayer coupling are manifested clearly in the electronic band structure. For the "γγ" Stone Wales defect in TBLG, we observe two Dirac cones that are shifted in both momentum space and energy. This up/down shift in energy results from the combined effect of a charge transfer between the two graphene layers, and a chemical interaction between the layers, which mimics the effects of a transverse electric field. Charge density plots show that states near the Dirac points have significant admixture between the two layers. For Stone Wales defects at other sites in TBLG, this basic structure is modified by the creation of mini gaps at energy crossings. For a monovacancy, the Dirac cone of the pristine layer is shifted up in energy by ∼ 0.25 eV due to a combination of the requirements of the equilibration of Fermi energy between the two layers with different numbers of electrons, charge transfer, and chemical interactions. Both kinds of defects increase the density of states at the Fermi level. The monovacancy also results in spin polarization, with magnetic moments on the defect of 1.2 -1.8 µB.PACS numbers: 73.22. Pr, 71.15.Mb

Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for 2D materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4′‐bipyridine (DMAP‐OED) as a strong organic dopant, it is established that the doping efficiency of DMAP‐OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. The highest doping levels to date are also achieved in monolayer MoS2 by surface functionalization and demonstrate that DMAP‐OED is a stronger dopant than benzyl viologen, which is the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first‐principles calculations. This work provides a basis for the rational design of OEDs for high‐level doping of 2D materials.

There is a big debate in the community regarding the role of surface states of hematite in the photoelectrochemical water splitting. Experimental studies on non-catalytic overlayers passivating the hematite surface states claim a favorable reduction in the overpotential for the water splitting reaction. As a first step towards understanding the effect of these overlayers, we have studied the system Ga2O3 overlayers on hematite (0001) surfaces using first principles computations in the PBE+U framework. Our computations suggest that stoichiometric terminations of Ga2O3 overlayers are energetically more favored than the bare surface, at ambient oxygen chemical potentials. Energetics suggest that the overlayers prefer to grow via a layer-plus-island (Stranski-Krastanov) growth mode with a critical layer thickness of 1-2 layers. Thus, a complete wetting of the hematite surface by an overlayer of gallium oxide is thermodynamically favored. We establish that the effect of deposition of the Ga2O3 overlayers on the bare hematite surface is to passivate the surface states for the stoichiometric termination. For the oxygen terminated surface which is the most stable termination under photoelectrochemical conditions, the effect of deposition of the Ga2O3 overlayer is to passivate the hole-trapping surface state.

Phonons with chirality determine the optical helicity of inelastic light scattering processes due to their nonzero angular momentum. Here it is shown that 2D magnetic CrBr3 hosts chiral phonons at the Brillouin‐zone center. These chiral phonons are linear combinations of the doubly‐degenerate Eg phonons, and the phonon eigenmodes exhibit clockwise and counterclockwise rotational vibrations corresponding to angular momenta of l = ± 1. Such Eg chiral phonons completely switch the polarization of incident circularly polarized light. On the other hand, the non‐degenerate non‐chiral Ag phonons display a giant magneto‐optical effect under an external out‐of‐plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light. The corresponding degree of polarization of the scattered light changes from 91% to −68% as the magnetic field strength increases from 0 to 5 T. In contrast, the chiral Eg modes display no field dependence. The results lay a foundation for the study of phonon chirality and magneto‐optical phenomena in 2D magnetic materials, as well as their related applications, such as the phonon Hall effect, topological photonics, and Raman lasing.