Editorial Feature

Heterostructures: A “Wonder” Material

Semiconductors are solid materials that conduct electricity in some conditions and not in others, which makes them a good means for the control of an electrical current. The electrical conductance of semiconductors changes depending on the voltage applied to the control electrode, or on the intensity of irradiation by the infrared (IR), visible, ultraviolet (UV) or X rays1.

Two dimensional semiconductors (2D semiconductors) are a group of naturally occurring semiconductors whose thickness is in the atomic scale. The growing attention towards these 2D semiconductors started from the discovery of graphene by Nobel Prize winners, Geim and Novoselov2. The structure of graphene is that of a single-layered 2D honeycomb carbon lattice with extraordinary properties and potential applications in many fields such as solid-state physics. While impressive, such mono-layered structures are not limited to carbon alone.

Hexagonal boron nitrate (hBN) and transition metal chalcogenides (TMDCs) share similar structures to graphene. The structure of hBN is almost identical to that of graphene, however, alternating boron and nitrogen atoms replace the carbon atoms that are found at the vertices of graphene.

Similarly, TMDCs contain similar layered structures to that of graphene with a structural formula of MX2, in which M represents a transitional metal from V or VI groups, and X represents a chalcogen such as sulfur (S), selenium (Se), molybdenum (Mo) or tellurium (Te).3 The layers of TMDCs are bonded strongly in plane but are bound by weak Van der Waals forces between its layers, making it easy to exfoliate using various methods into atomically thin layers, which have unusual and extremely captivating properties that are divergent from their bulk parent compounds4.

Heterostructures are structures formed by reassembling isolated atomic layers such as graphene, hBN or TMDCs, one on top of each other in a precise chosen sequence to form a complex structure which has unusual properties and potential applications5. Strong covalent bonds can be formed between the layers depending on the layers in contact, even though Van der Waal forces are strong enough to hold the stack together.

Recently, researchers have prepared several Van der Waal heterostructures, some of which have been shown to display impressive properties6. For example, graphene/hBN has an unusual electronic structure, while graphene/TMD and some TMD/TMD were shown to generate efficient photocurrents, and graphene/hBN/TMD were successfully used for light emitting diodes. Similarly, ultrafast charge transfer and the formation of interlayer excitons were observed in semiconducting TMD hetero bilayers.

Dr. Neil Wilson and his team of researchers at the University of Warwick’s Department of Physics have recently developed a technique looking at a more efficient and precise way to measure the electronic structures of heterostructures and other two-dimensional materials. In this study, researchers analyzed MoSe2/WSe2 heterostructures to measure the electronic properties within each layer of the stack6. By understanding and quantifying how these two-dimensional materials work, Dr. Wilson’s team studied the momentum of electrons within each layer, and how this electrical energy changes when layers are combined or removed.

Through submicrometer angle-resolved photoemission spectroscopy (m-ARPES) in combination with photoluminescence, researchers determined the previously unknown parameters of these heterostructures into a full determination of the band offsets, hybridization and exciton binding properties6. K-point valleys within these structures were found to be weakly hybridized, in which their valence band offset was measured at 300 meV. This factor regarding the hybridization of these structures shows modification at the bands found at G, whereas the valence band edges remain at the K points.

The weak hybridization states found near the K points also imply that the energy found within the intra-layer excitons is insensitive to that which is present within another layer6. This energy difference between the interlayer and the interlayer excitons has two contributing factors including its valence band offset and its binding energies.The study performed by Dr. Wilson and his team illustrated the possibility of engineering multiple layers of bands that are similar to those which are found in the structure of graphene and hBN.

While further research utilizing higher resolution ARPES measurements is required in order to fully understand the effect hybridization has on these structures, researchers are hopeful that this information can provide some guidance to others interested in producing electronic and optoelectronic devices that require the manipulation of K points and band alignment6.

References:

1.     "What Is Semiconductor?" WhatIs.com. Web. http://whatis.techtarget.com/definition/semiconductor.

2.     Novoselov, K. S. (2004). "Electric Field Effect in Atomically Thin Carbon Films". Science. 306 (5696): 666–669.

3.     Wang, Qing Hua; Kalantar-Zadeh, Kourosh; Kis, Andras; Coleman, Jonathan N.; Strano, Michael S. (2012). “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.” Nature Nanotechnology. 7 (11): 699-712.

4.     Song, Xiufeng; Hu, Jinlian; Zeng, Haibo (2013). “Two-dimensional semiconductors: recent progress and future prospectives.” Journal of Materials Chemistry C. 1 (17): 2952

5.     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.

6.     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.

7.      Image Credit: Shutterstock.com/takito

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