Controlling the electronic bandgap of materials is an important procedure for solid-state technology applications. Recently, two-dimensional (2D) crystals have offered a new approach for tuning the energy of the electronic states through Coulomb interactions.
A team of international researchers have engineered the surrounding dielectric environment to tune the electronic bandgap and exciton binding energy in WS2 and WSe2 monolayers by hundreds of meV.
The ability to precisely, and efficiently, manipulate the electrons in solid-state devices has improved many technological applications, including those in the information processing, communication, sensing and renewable energy fields. In these applications, the ability to tune the electronic bandgap is paramount to achieving a high operational efficiency.
There are currently many methods in use, from tuning the chemical composition to doping and quantum confinement to help engineer the bandgap of such materials. However, these methods have been found unsuitable for arbitrarily shaped and atomically sharp variations in the bandgap.
The Coulomb interactions between charge carriers in 2D materials has been found to be extremely strong and can lead to a renormalisation of the electronic energy levels and increase the quasiparticle bandgap. The Coulomb strength in these materials arises from the weak dielectric screening in the 2D limit.
The interaction of the charge carriers in 2D materials have been found to be very sensitive to changes in the environment, especially within the local dielectric environment. Such environmental changes have been found to induce changes in the Bohr radius.
This influence has been thought to allow the electronic energy and exciton binding energy to be tuneable and has motivated the researchers to employ Coulomb Engineering through manipulating the local dielectric environment.
The researchers formed a series of heterostructures by placing graphene and hexagonal boron nitride layers above WS2 and WSe2 monolayers to tune the electronic quasiparticle bandgap and the exciton binding energy. The researchers also utilised in-plane and sandwich configurations.
The thickness of each layer was determined using a polymer-stamp transfer technique and the researchers confirmed the structure using optical contrast spectroscopy. Binding energies were calculated using a density functional theory (DFT) approach that employed a Wannier–Mott model.
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Graphene was found to be a well-suited material for the heterostucture as it produced a high dielectric screening and it allowed for multiple layers to be added. The dielectric screening ability was found to be the best when a few monolayers of graphene were used.
The heterostructure was found to form a potential well greater than 100 meV, and the dielectric screening led to a renormalisation of the bandgap which was treated as a semiclassical framework which accounted for both the dielectric environment and the underlying substrate.
Overall, by Coulomb engineering the bandgap using a series of heterostructures with varying layers, the researchers were able to shift the bandgap to between 100 and 300 meV. Through computational studies, it has been estimated that it is possible to extend this to a limit of 500 meV.
The observed energy shifts also produced a larger thermal energy than the energy exhibited in room-temperature environments. As such, the research shows promising signs that devices implementing these materials and the engineered approach will be useful for both room-temperature and high-temperature applications.
The saturation of the screening effect was found to occur on the nanometre scale. The researchers tested multiple configurations, with respect to layers, material combinations, different alignments and found that the screening effect is not restricted by the choice of capping material.
Through realising the opening of the bandgap, it is thought that patterning dielectric layers on top of thin, 2D, semiconductors will allow for a variety of novel devices to be fabricated that utilise the 2D plane.
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This research also opens up new doors into new developments for the optoelectronic industry through implementation into transistors, light emitters and detectors. It may also be possible in the future to create custom-made superstructures, using 2D layers, with integrated photonic cavities, plasmonic nanomaterials or quantum emitters for the fabrication of new hybrid technologies.
“Coulomb engineering of the bandgap and excitons in two-dimensional materials”- Raja A., et al, Nature Communications, 2017, DOI: 10.1038/ncomms15251
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