Two Two-Dimensional Materials Are Better than One

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Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1298-1299
DOI: 10.1126/science.1239501

Extraordinary electronic or optical properties can result when layered solids are realized as two-dimensional (2D) materials (single or few-layer sheets), as is the case when graphene is formed from graphite. Optical properties can also be enhanced by restructuring materials at subwavelength scales into metamaterials, such as enhancing the plasmonic properties of gold—the coupling of light to electrons—by forming nanoparticles. Combining these approaches can lead to devices with capabilities that are otherwise difficult to realize. For example, for photovoltaic devices or sensors, materials with high electronic conductivity could be optically thick (to efficiently absorb light) but dimensionally thin (to impart flexibility and light weight). On page 1311 of this issue, Britnell et al. (1) combined highly conductive graphene and optically active 2D transition metal dichalcogenides into a heterostructure that photoexcites electron-hole pairs within a band-gap material. These carriers were separated with a p-n junction and extracted as a photocurrent with transparent graphene electrodes (graphene), and the performance was enhanced with plasmonic gold nanoparticles.

How does the light-matter interaction become stronger by making a particular material to become 2D, e.g., by exfoliation of single layers and making it so thin that it effectively has no thickness relative to the wavelength of light? This surprising property is directly related to the presence of critical points that generate in 2D or 1D (but not in 3D) the so-called Van Hove singularities in the electronic structure. Britnell et al. report that for the photoactive transition metal dichalcogenides such as molybdenum disulfide (MoS2), these singularities occur at visible frequencies and enhance the electronic density of states, i.e., the number of electrons per volume that participate in the absorption process at a given energy. Using this fundamental concept, a remarkably high quantum efficiency of up to 30% was achieved experimentally, in part by adding layers of hexagonal boron nitrate to provide homogenization of the doping levels.

Uniting flat materials.

Various ultraflat photonic devices that could be built through the combination of 2D electronic materials and 2D photonic metasurfaces are illustrated. (A) Ultrathin broadband photovoltaic devices could be realized by combining 2D materials and broadband light harvesting (1). (B) Label-free single-molecule detectors could be achieved by exploiting a diffractive coupling between plasmonic nanoantennas functionalized by a graphene layer (10). (C) A polarization-independent terahertz modulator could be made by using gate-tunable graphene embedded in a metamaterial structure (11). (D) Ultrafast and broadband metasurface emitters could be made by tailoring the 2D active material to operate in visible light or in the terahertz regime (8, 12).

The light-matter interaction was further increased by manipulating the local optical density of states. Britnell et al. show that depositing gold nanoparticles on the surface of the 2D heterostructure further enhanced the photogeneration of electron-hole pairs by a factor of 10. The nanoplasmonic particles form a broadband light-harvesting metamaterial (2) that concentrates field energy (or bundles optical modes) in “hotspots” under the particles. Additional generation of carriers takes place where these hotspots overlap with the photoactive layer.

These results not only demonstrate that the simultaneous design of the electronic and the optical density of states is the key to an extreme control of light-matter interaction but also provide a glimpse of how future nanophotonic devices can benefit from the combination of two 2D materials: heterostructures of 2D atomic crystals (“electronic metamaterials”) to control the electronic wave functions and nanostructured metasurfaces (35) to control the light field. This principle, which we call the “United Flatlands” (6, 7) opens up many opportunities for the next generation of active 2D metamaterials (8) with quantum gain (9) and ultraflat photonic devices, such as solar cells, light-emitting diodes, nanolasers, and optical sensors. The figure showcases some of the most striking possibilities. With the potential to break many performance and size limitations of bulk materials, particularly with respect to speed, energy efficiency, and area-footprint, the marriage of electronic and optical 2D (meta-) materials heralds a quantum leap in photonic device technology.

References and Notes

  1. Acknowledgments: Supported by the Leverhulme Trust and the Engineering and Physical Sciences Research Council.

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