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Two dimensional (2D) materials are substances with a thickness measured in the nanometer (nm) scale.
This nanoscale thickness allows these materials to have free electron mobility in a 2D plane1. Due to its exceptional physicochemical properties, unlike that which are seen in its bulk counterparts, the first isolated 2D material, graphene has paved the pathway for the discovery of many similar enhanced 2D materials such as graphane, hexagonal boron nitride (HBN), transition metal dichalcogenides (TMDCs), borophene, stanene, phosphorene, and many more2.
Borophene is a material comprised of a sheet of boron atoms arranged in hexagonal lattice, similar to that which is seen in graphene, however, borophene contains an extra boron atom mounted on top of each hexagon3. Although boron in its bulk form is a poor conductor of electricity, borophene is fully metallic and can adopt to several structurally distinct phases depending on how it is processed7.
Borophene has a higher electron density than graphene, suggesting that it could behave as a superconductor with no resistance3. Previous studies on borophene as a 2D material have only been studied in isolation, however, nearly all its technological applications require it to be combined with other materials.
Heterostructures can be formed by reassembling isolated atomic layers of different materials in a precise chosen sequence to form a complex structure which has unusual properties and potential applications4. Even though weak Van der waal forces are strong enough to keep this stack together, these stacks can also be held together by either strong covalent bonds or by weak Van der waal forces, depending on the layers in contact5.
Researchers have faced certain challenges in attempting to form pure heterostructures for several 2D materials due to the contamination of the initial growth front, which can lead to ill-defined interfacial regions as a result of the disturbances in the growth of the second and subsequent 2D layers7. These malformed interfacial regions of the 2D heterostructures can be prevented by alloying or intermixing during its development7.
Due to the extreme reactivity of boron, synthesizing atomic layer thick borophene has been challenging, However, borophene could be combined with other 2D materials to make heterostructures. This ability of borophene to form heterostructures can be used to fine tune its electronic properties by modifying the neighboring materials.
Recently, Researchers at Northwestern University’s Department of Material Science and Engineering have discovered spontaneous borophene/organic lateral heterostructures6.
Marc Hersam’s team, for the first time, has successfully demonstrated and characterized a 2D lateral heterostructure of borophene with a molecular semiconductor, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). PTCDA is capable of self-assembling with different substrates such as metals, oxides, salt crystals and semiconductors. Like other 2D and mixed dimensional heterostructures used in electronics such as gate-tunable p-n diodes, PTCDA form noncovalent interactions with silver (Ag) substrates, forming a delocalized 2D band state. These 2D band states also play a key role in the determination of the electronic structure and optical properties of double walled carbon nanotubes7.
The initial borophene submonolayer was developed on the isotope of silver [Ag(111)] on mica substrate using electron beam evaporation of pure boron rod. Subsequent deposition of PTCDA by thermally evaporating PTCDA molecules from an alumina-coated crucible resulted in preferential assembly on silver (Ag).
This finally led to the formation of dense and well-organized PTCDA monolayers to form lateral heterostructures with the borophene 2D flakes. X-ray photoelectron spectroscopy (XPS) validated the formation of pure 2D boron sheets and the formation of noncovalent bonds between the 2D layers of the heterostructures7.
Molecular dynamics (MD) simulations also revealed that the adsorbed PTCDA molecules formed hydrogen bonds with each other. Furthermore, ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM) and STS measurements further illustrated that the borophene/PTCDA heterostructures are electronically abrupt at the molecular scale.
In summary, the ability of borophene/PTCDA lateral heterostructures to form a metal/semiconductor lateral heterojunction allows it to be used in various electronic devices previously employing graphene/organic or metal/semiconductor junctions, for applications like resistive switching based on metal/PTCDA/metal junctions7. Other borophene-based heterostructures could be developed by making the PTCDA layer serve as a template for additional chemistry7.
This group of Researchers is hopeful that their work represents an important step in elucidating the unique chemistry and development of other borophene heterostructures and borophene-based nanoelectronics.
References and Further Reading
- "Two-dimensional Materials." Nature News. Nature Publishing Group, n.d. Web. http://www.nature.com/subjects/two-dimensional-materials.
- "Two-dimensional Materials 'as Revolutionary as Graphene'." Phys.org. Web. https://phys.org/news/2016-07-two-dimensional-materials-revolutionary-graphene.html.
- Peplow, Mark. "Atom-thin 'borophene' Joins 2D Materials Club." Nature News. Nature Publishing Group, 17 Dec. 2015.
- Geim, A. K., and I. V. Grigorieva. "Van Der Waals Heterostructures." Nature News. Nature Publishing Group, 25 July 2013. Web. http://www.nature.com/nature/journal/v499/n7459/pdf/nature12385.
- Wilson, Neil R., Paul V. Nguyen, Kyle Seyler, Pasqual Rivera, Alexander J. Marsden, Zachary P. L. Laker, Gabriel C. Constantinescu, Viktor Kandyba, Alexei Barinov, Nicholas D. M. Hine, Xiaodong Xu, and David H. Cobden. "Determination of Band Offsets, Hybridization, and Exciton Binding in 2D Semiconductor Heterostructures." Science Advances 3.2 (2017). Web.
- Morris, Amanda. "Serendipity Uncovers Borophene's Potential." Phys.org. N.p., 22 Feb. 2017. Web. https://phys.org/news/2017-02-serendipity-uncovers-borophene-potential.html.
- Liu, Xiaolong, Zonghui Wei, Itamar Balla, Andrew J. Mannix, Nathan P. Guisinger, Erik Luijten, and Mark C. Hersam. "Self-assembly of Electronically Abrupt Borophene/organic Lateral Heterostructures." Science Advances 3.2 (2017). Web.