When the topic of two-dimensional (2D) materials is presented, more often than not, graphene will be involved in this conversation. Due to its high carrier mobility, impressive strength, thermal and electrical conductivity and overall lightweight nature, this single layer of carbon atoms has become an ideal component of a growing number of applications for a variety of industrial purposes. While graphene is the thinnest and strongest material known to man, its lack of a natural bandgap prevents it from being used for important transistor and optoelectronic devices.
With an atom, electrons are placed in an array of states, which represents their different energy level, momentum and spin. These states then form regions known as bands, and the bandgap therefore describes the regions located between those bands. The importance that the presence of a bandgap plays is evident in understanding the role of semiconductors for optoelectronic devices.
An example of the role of the bandgap in a semiconductor material is in silicon, which is the material of choice for numerous optoelectronic devices, particularly solar cells, whose bandgap is wide enough to allow for the electrons within the material to easily cross the bandgap, following the introduction of photons from visible light into the material. While graphene does not inherently contain this bandgap, there are several ways to engineer a bandgap into this wonder material, however, such measures often reduce the material’s ability to conduct electricity.
In an effort to look towards other 2D materials that closely resemble the properties, structure and fabrication process of graphene, Researchers have found that transition metal dichalcogenides (TMDs), particularly monolayered TMDs, exhibit distinctive electronic and optical properties that have enhanced a number of devices including photodetectors, photovoltaics, thin film transistors and many more. The semiconducting properties of TMDs are largely attributed to their inherent and direct bandgaps of around 1-2eV for monolayer TMDs. Additionally, monolayer TMDs exhibit strong excitonic transitions that allow this material to provide an efficient optical-gain for certain applications, such as nanolasers.
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