Exploiting excitons in atomic monolayers for dielectric sensing

Nontechnical description:

Many crystals including semiconductors are made of weakly bonded layers, and as such, can be separated into stable units of atomic thickness. Electrons (negatively charged particles) and holes (positively charged particles) in these isolated atomic membranes are bound by strong attractions to form excitons. Excitons are hydrogen atom-like particles; they possess a series of quantum states and display characteristic peaks in the absorption spectrum.

On the other hand, excitons are much larger than hydrogen atoms; while the electrons and holes are confined in the atomic membrane, their interactions extend substantially outside. This property endows the excitons--both the energy and the intensity of the absorption peaks--with extreme sensitivity to surroundings. When a membrane is placed near a metal, the electron-hole interactions are significantly screened; and the exciton absorption spectrum is substantially altered.

Conversely, a nearby insulator affects the exciton spectrum much less. In this project, the research team exploits this unique property of excitons in atomic membranes to develop a new sensing technique that can be applied to a wide range of materials including those that are inaccessible by conventional techniques. The team applies the technique to probe new forms of insulators and superconductors in two dimensions.

The project supports the research and development of one graduate student and several undergraduate students. Other activities involve modernizing the physics advanced laboratory course at Cornell University and developing materials for outreach activities, including several science, technology, engineering, and mathematics (STEM) programs on campus that specifically target young girls.

Technical description:

Atomically thin transition metal dichalcogenide semiconductors have emerged as a new platform for strong light-matter interactions. The optical response of monolayers is dominated by excitons (bound electron-hole pairs), which are extremely sensitive to the surrounding dielectric environment because most of the electric-field lines responsible for exciton binding are outside the monolayer material.

The project exploits this unique property of excitons in monolayer semiconductors to develop a new optical sensing technique for dielectric function or electronic compressibility of nanoscale materials. The goals are to develop imaging and time-resolved measurement capabilities, as well as a comprehensive understanding of the technique, its applicability and limitations.

Two experiments are selected to focus on each of the measurement capabilities. The first experiment studies the sensitivity of the technique to the quantum Hall effect in graphene and explores the possibility of imaging the chiral edge states in the quantum Hall regime. The second experiment investigates the sensitivity of the technique to superconducting transitions and explores ultrafast dynamics of two-dimensional superconductors following photon-excitations.

The methods involve the fabrication of van der Waals heterostructures and devices and optical spectroscopies, including the reflection contrast, hyper-spectral imaging and pump-probe spectroscopy. The new sensing technique can be applied to a wide range of materials including those that do not form good electrical contacts for conventional capacitance or transport measurements.

It opens up unprecedented opportunities for studies of quantum many-body dynamics in correlated materials, topological chiral edge states, and two-dimensional superconductors.

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.