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Charge transfer between van der Waals coupled metallic 2D layers

Bharti Matta, Philipp Rosenzweig, Craig Polley, Ulrich Starke, Kathrin Küster

TL;DR

The paper investigates how charge transfer occurs in a van der Waals heterostructure that includes a metallic Pb interlayer beneath graphene, modulated by potassium adsorption. Using ARPES, the authors quantify how K electrons distribute among the K adlayer, graphene, and the Pb layer, revealing a substantial but partial transfer to the second-nearest layer Pb. They find that roughly 1.96×10^14 cm^-2 electrons are donated by K, with about 1.10×10^14 cm^-2 going to graphene and 0.86×10^14 cm^-2 to Pb, accompanied by a measurable Pb band shift and an increase in Pb carrier density to ~6.85×10^14 cm^-2. About 56% of donated electrons remain in graphene while 44% populate Pb, indicating interlayer charge transfer beyond the adjacent layer and highlighting distinct coupling behavior compared to Ag-intercalated systems; these results open avenues for tuning charge densities in vdW heterostructures for electronics and spintronics.

Abstract

Van der Waals heterostructures have become a rapidly growing field in condensed matter research, offering a platform to engineer novel quantum systems by stacking different two-dimensional (2D) materials. A diverse range of material combinations, including hexagonal boron nitride, transition metal dichalcogenides and graphene, with electronic properties spanning from insulating to semiconducting, metallic, and semimetallic, have been explored to tune the properties of these heterostacks. However, understanding the interactions and charge transfer between the stacked layers remains challenging, particularly when more than two layers are involved. In this study, we investigate the charge transfer in a potassium-adlayer/graphene/lead-monolayer heterostructure stacked on a SiC substrate. Using synchrotron-based angle-resolved photoemission spectroscopy, we analyze the band structure of each layer, focusing on the charge transfer from K to the underlying 2D layers. Since K forms a $(2 \times 2)$ overlayer with respect to graphene, the amount of charge carriers donated by K can be determined. Our findings reveal that adsorption of K not only leads to a significant $n$-doping of the adjacent graphene layer but also to an electron transfer into the Pb monolayer. Remarkably, $\approx 44\%$ of the electrons donated by the K adlayer are transferred into its second nearest neighbouring layer, i.e. Pb, while $\approx 56\%$ remain in the graphene.

Charge transfer between van der Waals coupled metallic 2D layers

TL;DR

The paper investigates how charge transfer occurs in a van der Waals heterostructure that includes a metallic Pb interlayer beneath graphene, modulated by potassium adsorption. Using ARPES, the authors quantify how K electrons distribute among the K adlayer, graphene, and the Pb layer, revealing a substantial but partial transfer to the second-nearest layer Pb. They find that roughly 1.96×10^14 cm^-2 electrons are donated by K, with about 1.10×10^14 cm^-2 going to graphene and 0.86×10^14 cm^-2 to Pb, accompanied by a measurable Pb band shift and an increase in Pb carrier density to ~6.85×10^14 cm^-2. About 56% of donated electrons remain in graphene while 44% populate Pb, indicating interlayer charge transfer beyond the adjacent layer and highlighting distinct coupling behavior compared to Ag-intercalated systems; these results open avenues for tuning charge densities in vdW heterostructures for electronics and spintronics.

Abstract

Van der Waals heterostructures have become a rapidly growing field in condensed matter research, offering a platform to engineer novel quantum systems by stacking different two-dimensional (2D) materials. A diverse range of material combinations, including hexagonal boron nitride, transition metal dichalcogenides and graphene, with electronic properties spanning from insulating to semiconducting, metallic, and semimetallic, have been explored to tune the properties of these heterostacks. However, understanding the interactions and charge transfer between the stacked layers remains challenging, particularly when more than two layers are involved. In this study, we investigate the charge transfer in a potassium-adlayer/graphene/lead-monolayer heterostructure stacked on a SiC substrate. Using synchrotron-based angle-resolved photoemission spectroscopy, we analyze the band structure of each layer, focusing on the charge transfer from K to the underlying 2D layers. Since K forms a overlayer with respect to graphene, the amount of charge carriers donated by K can be determined. Our findings reveal that adsorption of K not only leads to a significant -doping of the adjacent graphene layer but also to an electron transfer into the Pb monolayer. Remarkably, of the electrons donated by the K adlayer are transferred into its second nearest neighbouring layer, i.e. Pb, while remain in the graphene.
Paper Structure (3 sections, 4 figures)

This paper contains 3 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Experimental Details
  3. Results and Discussion

Figures (4)

  • Figure 1: Dirac cone dispersion of Pb-QFMLG (a) before and (b) after K deposition: $E$-$k$ cuts at the $\overline{\mathrm{K}}$-point of graphene perpendicular to its $\overline{\Gamma\mathrm{K}}$ direction [see inset in (a)], measured at a photon energy of $40$ eV.
  • Figure 2: Pb bands along their $\overline{\mathrm{K}\Gamma\mathrm{K'}}$ direction, measured at $h\nu$ = $40$ eV, (a) before and (b) after K deposition. After K deposition, a band due to the K 4s electrons appears close to the Fermi energy, which is fitted with a parabola (orange) based on EDC and MDC fits.
  • Figure 3: Fermi surface of Pb-QFMLG, (a) before and (b) after K deposition, measured at $h\nu$ = $114$ eV. The Pb and graphene BZs are indicated in black and red, respectively. The (2${\times}$2) superstructure of K with respect to graphene is indicated in green in (b). (c) Fermi surface after K deposition obtained at $h\nu$ = $40$ eV, covering the momentum space within the dotted gray box shown in (b). Note that the Fermi surfaces in (a) and (b) are four-fold symmetrized with respect to the $\overline{\mathrm{M}\Gamma\mathrm{M}}$ and $\overline{\mathrm{K}\Gamma\mathrm{K'}}$ directions of the Pb-BZ for visual enhancement.
  • Figure 4: $E-k$ dispersion along the $\overline{\mathrm{M}\Gamma\mathrm{M}}$ direction of Pb, (a) before and (b) after K deposition, respectively measured at $h\nu$ = $114$ eV. EDC fits through the Pb band dispersion (blue and pink curves) show a clear downshift of the bands after K adsorption, as demonstrated in (c). (d) Schematic drawing of the charge transfer between K, graphene and Pb (not drawn to scale). The dashed (solid) curves indicate the band position before (after) K adsorption. Charge carrier concentrations are given in cm$^{-2}$.