Effects of Quantum Confinement in Photosensors from Transition Metal Dichalcogenides

Nontechnical Description:

Ultra-thin materials that are just one-atom-thick and the new physics arising from their drastically reduced dimensions are fueling intense research activity worldwide, with tremendous opportunities for discoveries in fundamental science and for potential practical applications, especially in electronics and photonics. The restriction of the charge carriers (electrons and “holes”, or missing electrons) to two dimensions, an example of so-called “quantum confinement”, deeply affects the properties of two-dimensional materials and their interaction with light.

This project involves the study of how the properties of these types of materials can be further modified by additional structure in the layers, for example, by making thin “nanoribbons” or by placing the layers on top of nanoscale holes that will locally affect the charge carriers. The results of this work will help us understand their properties better, as well as potentially lead to improvements in devices such as phototransistors and other types of optical detectors.

The project is a collaboration with a group at the Army Research Lab, and so will also provide an excellent opportunity for students to become involved with cutting-edge research that will support the national defense as well as the optoelectronics industry. Technical Description:

Quantum confinement deeply affects the properties of two-dimensional transition metal dichalcogenides (TMDs) and their interaction with light, yielding a plethora of optical excitations with binding energies larger than a few tenths of an electron volt. This project will push studies of these materials towards even stronger quantum confinement by patterning them into nanoribbons, creating heterostructures of TMD nanoribbons stacked with other TMD layers, and patterning the substrates to suspend these materials on nanoscale holes.

Nanostructuring 2D materials into nanoribbons will introduce new van Hove singularities at the onset of one-dimensional subbands, possibly yielding stronger light-matter interaction and higher responsivity compared to 2D channels with the same active area. Stacking a layer of a different semiconducting TMD on the TMD nanoribbons is expected to result in fast charge transfer and photogating after light absorption, as well as the formation of interlayer excitons having longer lifetimes than the intralayer excitons.

We will study the effect of the longer lifetimes combined with the spatial confinement of the nanoribbons on the exciton condensation. Lastly, this project will study the effect of quantum confinement due to nanopatterning of the substrate. Exciton liquid condensation has been observed in TMD suspended on holes with size of a few microns.

This project will study the effect of reducing the hole size to dimensions that are a few tens or hundreds of the exciton size, thereby studying the effect of spatial confinement in exciton droplets. Statement of merit review:

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.