Tailoring of the band gap in semiconductors is essential for the development of novel devices. In standard semiconductors, this modulation is generally achieved through highly energetic ion implantation. In two-dimensional (2D) materials, the photophysical properties are strongly sensitive to the surrounding dielectric environment presenting novel opportunities through van der Waals heterostructures encompassing atomically thin high-κ dielectrics. Here, we demonstrate a giant tuning of the exciton binding energy of the monolayer WSe 2 as a function of the dielectric environment. Upon increasing the average dielectric constant from 2.4 to 15, the exciton binding energy is reduced by as much as 300 meV in ambient conditions. The experimentally determined exciton binding energies are in excellent agreement with the theoretical values predicted from a Mott−Wannier exciton model with parameters derived from first-principles calculations. Finally, we show how texturing of the dielectric environment can be used to realize potential-well arrays for excitons in 2D materials, which is a first step toward exciton metamaterials.
We demonstrate anisotropic tunnel magnetoconductance by controllably engineering charging islands in the layered semiconducting ferromagnet Cr 2 Ge 2 Te 6 . This is achieved by assembling vertical van der Waals heterostructures comprised of graphene electrodes separated by crystals of Cr 2 Ge 2 Te 6 . Carefully applying vertical electric fields in the region of (E ∼ 25-50 mV/nm) across the Cr 2 Ge 2 Te 6 causes its dielectric breakdown at cryogenic temperatures. This breakdown process has the effect of introducing subgap defect states within the otherwise semiconducting ferromagnetic material. Low-temperature electron transport through charging islands reveals Coulomb blockade behavior with a strongly gate-tuneable anisotropic magnetoconductance, which persists up to T ∼ 60 K. We report average tunnel magnetoresistance values of 100%. This work opens new avenues and material systems for the development of nanometer-scale electrically controlled spintronic devices.
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