EAGER: Low energy Ion Irradiation as a Universal Approach to Dope 2D Materials

About This Grant

Nontechnical Description Two-dimensional (2D) materials are atomically thin sheets with atoms strongly bonded within each layer but weakly bonded between layers. This provides unique versatility in their electronic, quantum and topological properties that can be achieved by twisting of the sheets between layers or combining different 2D sheets to form heterostructures. They have shown promise for emergent technologies such as artificial intelligence, quantum computing and photonics. This project focuses on transition metal dichalcogenides (TMDs), a class of 2D materials with layers of transition metal such as molybdenum or tungsten terminated with a chalcogen (sulfur, selenium, or tellurium) on either side. Defect engineering and doping are critical to modulate their electronic and photonic properties but challenging to achieve due to the TMD structure and bonding. The goal of this project is to establish a new and universal approach to achieving precise control of TMD properties with electronic and magnetically active dopants. Success will realize novel magnetic and electronic 2D materials, and advance 2D device technology. To this end, investigators will develop a doping method termed “backdoor doping” where low energy ions are injected into 2D materials. This ideally embeds the dopant atom in a substitutional site while avoiding damage to the material. Developing a highly competent and technologically knowledgeable workforce is at the core of a university education. The technical work will be integrated with education and training of undergraduate and graduate students. Research projects will enable students to develop critical thinking, collaborative work ethics, and a unique set of skills. A team of undergraduate students will be integrated in the EAGER research and tasked with projects including problems related to data analysis, simulation of ion-matter interactions, and image analysis. Technical Description A universal method to dope TMDs and engineer defects is uniquely important and challenging at the same time. This EAGER project will validate the feasibility of “backdoor doping” where the projectile is a recoil target atom generated by irradiation of a thin metal target, ejected from the backside of the foil with low kinetic energy and then implanted in the TMD, as a means to achieve controlled doping with various transition metals, and to predict feasibility for a wide range of dopant-material combinations. Judicious choice of TMDs (light metal, heavy chalcogen or vice versa), varied projectile mass, and use of magnetic dopants is combined with modulation of the ion energies to achieve selective displacement. This leads to preferential replacement on the chalcogen or metal sub-lattices with well-defined bonding states characterized by their ligand field splitting. The challenges lie in generating the low energy ions for a wide range of elements, and to find the right conditions to achieve displacement without incurring unwanted damage. A combined experimental and computational approach will accordingly be used to understand the defect inventory generated by ion impact events. Atomic resolution scanning transmission electron microscopy (STEM) and scanning tunneling microscopy and spectroscopy (STM/STS) are instrumental to this study. The work will also advance the fundamental, mechanistic understanding of ion-TMDC interactions. The proposed “backdoor doping” removes the need for a large ion source, offers clean implantation environment limiting contaminant implantation, and provides high versatility in dopant-material combinations. 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.