Silicon Nanocrystal-radical Systems for Photo Excited, Spin-polarized Triplet- quartet/ Doublet states

With the support of the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Tang of the University of Utah is studying building blocks for quantum computing comprised of silicon nanocrystals, also called quantum dots (QDs), and organic radical compounds. These novel hybrid nanostructures have great potential for quantum information science (QIS) for two key reasons. Firstly, these quantum bits (qubits) can be initiated with light; and secondly, they can support long-lived, coherent spin-active states. The latter is important for information storage at the quantum level and is enabled by the fact that carbon and silicon are light elements with low spin-orbit coupling. Professor Tang and her team will vary the electronic coupling between the silicon QD and organic radical by molecularly engineering their covalent bridge, as well as the conjugated framework of the radical. This synthetic flexibility may allow experimental access to new physics and advance the field by establishing the physical parameters affecting the exchange coupling between the nanocrystal and radical needed to create higher order excited states useful as qubits. Importantly, students working on this project will be exposed to a broad swath of experiments with state-of-the-art equipment conducting spectroscopic and structural characterization in an interdisciplinary environment. This rigorous training is valuable for a career in science and engineering, critical for boosting domestic manufacturing in the United States. All this excitement about qubits for QIS will be shared with students from Whittier elementary school in Salt Lake City school district via a series of chemical demonstrations. With the support of the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor Tang of the University of Utah is studying the synthesis and characterization of silicon nanocrystals covalently functionalized with molecular radicals which are designed to systematically tune the resultant exchange coupling all the way from the weak, to the intermediate and finally to the strong coupling regimes. Here, graduate students will systematically toggle between saturated, aliphatic and unsaturated, conjugated bridges, swapping between meta- and para- linkages, varying the distance between both the nanocrystal and radicals like 1,3-bisdiphenylene-2-phenylallyl (BDPA) and 2,2,6,6-tetramethyl-piperidin-1-oxyl (TEMPO). This approach will vary both the state energies and frontier molecular orbital levels of the radicals anchored on the silicon QD. Optical and electron paramagnetic resonance (EPR) spectroscopy will be used to chart the trajectory of photogenerated spin-states of triplets initially created in the Si QDs, subsequent coupling to the doublet states on the radicals, and evolution to the strongly coupled triplet-doublet and triplet-quartet states. Steady-state and time-resolved optical measurements will establish the rate and yields of photogenerated species. Time-resolved and pulsed EPR will reveal the strength of the exchange coupling and identity of the spin-active species. The data obtained will allow theoreticians to benchmark their predictions to experimental measurements of dipolar and exchange interactions in photogenerated triplet-quartet and triplet-doublet states. If successful, this system will fulfill the DiVincenzo criterion for qubits, i.e addressable, spin-active states with long coherence times. 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.