Tuning the Fermi Level of Graphene by Two-Dimensional Metals for Raman Detection of Molecules.

Graphene-enhanced Raman scattering (GERS) offers great opportunities to achieve optical sensing with a high uniformity and superior molecular selectivity. The GERS mechanism relies on charge transfer between molecules and graphene, which is difficult to manipulate by varying the band alignment between graphene and the molecules. In this work, we synthesized a few atomic layers of metal termed two-dimensional (2D) metal to precisely and deterministically modify the graphene Fermi level. Using copper phthalocyanine (CuPc) as a representative molecule, we demonstrated that tuning the Fermi level can significantly improve the signal enhancement and molecular selectivity of GERS. Specifically, aligning the Fermi level of graphene closer to the highest occupied molecular orbital (HOMO) of CuPc results in a more pronounced Raman enhancement. Density functional theory (DFT) calculations of the charge density distribution reproduce the enhanced charge transfer between CuPc molecules and graphene with a modulated Fermi level. Extending our investigation to other molecules such as rhodamine 6G, rhodamine B, crystal violet, and F16CuPc, we showed that 2D metals enabled Fermi level tuning, thus improving GERS detection for molecules and contributing to an enhanced molecular selectivity. This underscores the potential of utilizing 2D metals for the precise control and optimization of GERS applications, which will benefit the development of highly sensitive, specific, and reliable sensors.

Type-II quantum spin Hall effect in two-dimensional metals.

The quantum spin Hall (QSH) effect has been observed in topological insulators and long quantum wells using spin-orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the different Type-II QSH effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplored but relativistically gauge-invariant form of the generated 2D QSH Hamiltonian. Instead of using the electric field in the surface of the spin-polarized bands of a topological insulator or across the quantum well width as the probe, ones uses an applied azimuthal vector potential and an applied radial electric field as the tools to generate a spontaneously quantized spin current in an otherwise spin unpolarized 2D metal. A long cylindrical solenoid lies normally through the inner radius of a 2D metallic Corbino disk. The currentISsurrounding the solenoid produces an azimuthal magnetic vector potential but no magnetic field in the disk. In addition, a radial electric field is generated across the disk by imposing either a potential differenceΔvor a radial charge currentIacross its inner and outer radii. Combined changes inISand in eitherΔvorIgenerate spontaneously quantized azimuthal charge and spin currents. The experiment is designed to measure these quantized azimuthal charge and spin currents in the disk consistently. The quantum Hamiltonians for both experiments are solved exactly. A method to control the Joule heating is presented, which could potentially allow the Type-II QSH measurements to be made at room temperature.