CAREER: One-Pot Synthesis and Optical Spectroscopy of 2D Heterobilayers

Nontechnical Description

New technologies benefitting society depend on developing novel materials. Monolayers, materials composed of layers that are only one molecule or one atom thick, have shown promise for next-generation electronics and computing. Heterobilayers made by stacking monolayers with different orientations or chemical compositions have led to materials with novel properties such as unconventional superconductivity.

However, current approaches based on producing bulk materials and then isolating thin layers from them cannot be easily scaled. The aim of this project is to develop new pathways for assembling atomically thin bilayers that are free of wrinkles and impurities at the interfaces. The principal investigator (PI) takes advantage of the most stable configurations that arise when bilayers are built by a bottom-up approach.

This approach consists of putting atoms together one by one to form a crystal. In addition to fabricating novel bilayers, investigators will optically study novel phenomena that can arise by changing the substrate upon which heterobilayers are built. This research goes hand in hand with the goal of forming the next-generation physics and material science workforce through a comprehensive educational program that broadens the workforce base and improves recruitment and retention of undergraduate students in related fields.

The PI is working with an outreach team to design and build demonstrations targeted to K-5 students, with the goal of reaching over 500 students per year. This project implements two Course-Based Undergraduate Research Experiences that are closely integrated with the research objectives to improve the recruitment and retention of undergraduate students and broaden the future workforce base.

Educational activities also include communication workshops to increase students’ science communication skills. Technical Description

Two-dimensional (2D) materials hold great promise for positive impacts on technology and society, including innovations in electronics, clean energy, photonics, and quantum information. Individual 2D layers can be stacked to assemble artificial bilayers bonded by van der Waals (vdW) forces. A critical challenge in heterobilayer development is the elimination of bottlenecks associated with top-down mechanical exfoliation and stacking, including the need for interfacial impurity mitigation protocols.

The first research objective of this project is growing heterobilayers using combinations of semiconducting transition metal dichalcogenides (TMD) with different bandgaps. In this TMD heterobilayers, moiré patterns originate from the difference between lattice parameters among the constituent layers. The second research objective is to identify optical spectroscopy signatures, including moiré phonons and electronic transitions, displayed by the heterobilayers for each TMD combination–including those with type-I and type-II band alignments.

The research approach is based on a single-pot chemical vapor deposition route to grow heterobilayers with a thermodynamically preferred stacking order, in the absence of mechanical exfoliation and transferring techniques. Strain in heterobilayers is controlled through the difference in the lattice parameter of the constituent layers and the coefficient of thermal expansion of both TMD layers and the growth substrate.

This enables the study and understanding of the changes in TMDs’ resonant Raman spectra and the energy variations in interlayer excitons in 2D heterobilayers driven by strain.

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.