A Fun Side Project with Monolayers

Earlier this year, a friend of mine, Alex Krotz, and I used a new software (InQuanto™) to explore the quantum chemistry of a fascinating new class of materials called monolayer transition metal dichalcogenides (TMDs for short). The official press release is here, but I thought it’d be good to explain what we were interested in briefly.

In brief, when things get very small, they can behave in surprising ways. A quintessential example of this phenomenon is when one strips down bulk graphite to a single layer of graphene, one goes from a gray material used in pencils to a see-through sheet with no band gap. Having one dimension at the nanoscale (the other two still being macroscopic makes graphene a so-called "2D" material), abrupt yet often desirable changes can be made. 

Here, we were interested in a type of material that still behaves as direct band gap semiconductors when they are 2D: monolayer TMDs. The "TM" refers to a "transition metal," though in practice, we mean tungsten or selenium, just as the “D” from"dichalcogenide" implies two group 16 elements with a particular focus on sulfur, selenium, and occasionally tellurium. For such a broad term, I’ll note that I've never heard of iron oxide or cadmium sulfide being referred to as transition metal chalcogenides even though such nomenclature is technically accurate. 

These 2D TMDs behave in very unique ways. Since they lack inversion symmetry, individual valleys in their band structure can be selectively probed with circularly polarized light. Since you can embed information in the polarization of light, this selectivity opens up all sorts of opportunities in quantum information science. These valleys can be finely controlled with electric fields, by pushing or pulling on the monolayers, and by removing atoms from their structure. I've previously focused on manipulating TMDs via electromagnetism, so Alex and I thought it'd be a nice change of pace to look deeper into that list of methods, seeing what happens when their structures are deformed or when one introduces defects into them. 

We are, naturally, hardly the first to have broached these questions. However, in testing quantum computational algorithms on TMDs, we aimed to add to the ongoing conversation on some truly remarkable materials.