The Nano-Sandwich in Optoelectronics

By Benedette Cuffari, M.Sc.Mar 3 2017

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Image Credit: shutterstock.com/sakkmesterke

Graphene is an allotrope of carbon, in which the carbon atoms form a single-layered sheet with a hexagonal honeycomb lattice, where the carbon atoms are arranged in the vertices of each hexagon¹. Multiple layers of graphene, when stacked on top of each other, are called graphite. Despite its single atom thickness and extremely lightweight properties, graphene sheets display great mechanical strength, thermal and electrical conductance, and unusual levels of light absorption¹. Due to these exceptional physicochemical properties, the uses of graphene as the first isolated two-dimensional (2D) material, has led to the discovery of many isostructural 2D materials such as graphane, borophene, stanene, many others².

To conduct electricity, electrons must flow across any type of material. Electrons in solids are confined to certain ranges or bands of energy, where the band gap represents the energy required to knock the electrons off the atom³. Electrons can only break free from the atom when enough energy in the form of heat or photons is supplied to overcome the band gap³. In metals, the band gap is zero, whereas in case of insulators, this band gap is typically higher. In semiconductors like silicon and germanium, the band gap is much smaller as compared to the insulators.

Graphene has an extremely small band gap, allowing currents to travel 100-200 times faster than in case of silicon³. The tiny band gap of graphene allows it to absorb photons across the visible spectrum and beyond, converting the energy of photons to electricity, which makes graphene an ideal material to be used for both optics and electronics³. As impressive as it sounds, the lack of a band gap in graphene makes it very difficult to turn off the current once the electrons start flowing, making it unsuitable for on-off switching³. Production of large sheets of graphene is also expensive and has many atomic scale flaws and tears as there is no industrial method for producing laboratory-grade graphene³. Incorporation of different materials as impurities in graphene could create a band gap, but only at the expense of the speed of conduction. Researchers are therefore trying to fine-tune the electrical properties of graphene by stacking other 2D materials such as hexagonal boron nitrate (hBN) and transition metal chalcogenides (TMDCs) to form hetero-structures³.

Nanoclusters (NC) are clusters of near mono-dispersed particles that measure less than 10 nm in diameter⁴. Due to their unique structural, electronic and optical properties, unlike their single particle and bulk counterparts, these NCs have created great interest over the past decade. The electrons in the NCs exhibit quantum-size effects, as they are restricted to spaces of a few atom-widths across⁴. Researchers at Rice University, Department of Civil and Environmental Engineering recently developed a nano-sandwich with magnesium oxide NCs sandwiched between two atom-thick layers of graphene⁶. Magnesium oxide (MgO) nanoparticles have been applied in a wide variety of applications in fields such as electronics, catalysis, ceramics and petrochemical products⁵. MgO nanocrystals therefore also have the potential to be used in a wide range of technological applications such as gas sensors or catalysis. Rouzbeh Shahsavari’s team explored the electronic and optical properties of MgO nanocrystal-encapsulated mono and double layer graphene using high level first principle calculations and found that 2D MgO flakes on graphene displayed surface polarization effects and showed a significant charge transfer unlike the isolated MgO flakes⁷.

In this study, BerkelyGW (BGW) packages were used to analyze the optical absorption of single flake MgO and the absorption of MgO when encapsulated in monolayer and bilayer graphene⁷. The results revealed that while a single MgO flake has a one-quantum emission, the MgO encapsulated mono- and bi-layer graphene had absorptions measuring in wide regions of the visible spectrum⁷. By understanding how these MgO flakes are capable of enhancing the properties of graphene, future research in this field could modulate other 2D material nanosandwiches for electronic and optical applications.

References:

1. "Graphene - What Is It?" Graphenea. N.p., n.d. Web. https://www.graphenea.com/pages/graphene#.WKYtFhiZPdQ.
2. "Two-dimensional Materials as Revolutionary as Graphene'." Phys.org. Web. https://phys.org/news/2016-07-two-dimensional-materials-revolutionary-graphene.html.
3. "Graphene: The Quest for Supercarbon." Nature News. Nature Publishing Group, Web. http://www.nature.com/news/graphene-the-quest-for-supercarbon-1.14193.
4. Aiken, John D., and Richard G. Finke. "A Review of Modern Transition-metal Nanoclusters: Their Synthesis, Characterization, and Applications in Catalysis." Journal of Molecular Catalysis A: Chemical 145.1-2 (1999): 1-44. Web.
5. Mastuli, Mohd Sufri et al. “Growth Mechanisms of MgO Nanocrystals via a Sol-Gel Synthesis Using Different Complexing Agents.” Nanoscale Research Letters 9.1 (2014): 134. PMC. Web.
6. "Nano 'sandwich' Offers Unique Properties." ScienceDaily. ScienceDaily, 27 Feb. 2017. Web. https://www.sciencedaily.com/releases/2017/02/170227150328.htm.
7. Farzaneh Shayeganfar et al, Electro- and Opto-Mutable Properties of MgO Nanoclusters Adsorbed on Mono- and Double-Layer Graphene, Nanoscale (2017).

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