Paired Radical States in Molecular Wires: 1D Topological Insulators and Beyond

Nontechnical description: Significant efforts have been invested to develop molecules that can transport charge efficiently at nanometer scale, driven by their important for electronic application, photovoltaics and artificial photosynthesis. However, most molecules that act as conducting wires show an exponential decrease in conductance as their lengths increase, thus limiting their potential applications. The objective of this research project is to develop molecule-based electronic circuits that can conduct charge over long distances using molecular wires with un-paired electrons (radicals). This research will therefore address the challenge of developing long and highly conductive all-organic molecular wires. Further, this project will provide new design strategies to stabilize radicals within molecules; these strategies are likely to have impact in the emerging area of molecular qubits and quantum computing. Beyond advancing applications in nanoscale electronics, this project seeks to excite K-12 children and undergraduate students and attract them in STEM fields, particularly those who are underrepresented minority students and women, which will be pursued through mentoring, providing undergraduate research experiences in a multidisciplinary environment, and influencing the graduate admissions through service programs. Technical description: Molecular wires that conduct following a coherent and off-resonant mechanism exhibit an exponential decrease in conductance with increasing length, making them impractical for applications where highly conductive long wires are required. One way to overcome this limitation is to design and construct one-dimensional topological insulator wires with a pair of radical states. These wires conduct through their radical-based unique boundary (or edge) states, while their interior remains insulating. Such wires could result in a conductance that increases with wire length. This project aims to design, create, measure, and understand electronic transport in these molecular wires by combining theory and experiment. The three specific objectives of the project are: (1) To probe transport in wires with a single radical pair and understand how the electronic coupling between these states and the source and drain electrodes control transport; (2) To create wires with multiple radical pairs and investigate how conductance changes with length; and (3) To design cyclic wires with multiple radical pairs to explore the effects of quantum interference and spin on transport. The educational and outreach efforts of this project also have three broad objectives: (1) To provide undergraduate research opportunities in a multidisciplinary environment; (2) To integrate research into the Applied Physics undergraduate education and (3) To engage K-12 students in laboratory activities and introduce them to nanoscience. 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.