RII Track-4: Probing the Electronic States of Quantum-Confined Topological Insulator Nanostructures

The properties of materials can change dramatically when the materials are extremely small. Nanoparticles are already in use in a range of applications ranging from medicine to consumer electronics. One of the most exciting applications for nanoparticles is for use as the bit in a quantum computer. For a quantum computer to work, the bits (called qubits) must remain in the state that they are prepared in. Unfortunately, most qubits that exist today have strong interactions with the environment that cause the state of the qubit to change in an undesirable way. In this project, we will study new materials called topological insulators. When topological insulators are made into nanoparticles, theory predicts that they may act as good qubits that are only weakly perturbed by their environment. The goal of this project is to synthesize topological insulator nanoparticles and study their properties. The nanoparticles will be synthesized and characterized using state-of-the-art tools like molecular beam epitaxy at the University of Delaware and scanning tunneling microscopy and angle-resolved photoemission spectroscopy at Brookhaven National Laboratory. The outcome of this project will be a deep understanding of how topological insulators behave at extremely small size scales which is of use to the broader scientific community and an evaluation of their suitability for a variety of applications.

When materials are confined to nanoscale dimensions, their electrons occupy quantized discrete energy levels. To date, most of the research into quantum confinement has used semiconductor-based materials. Recently, there has been substantial interest in a new class of materials called topological insulators (TIs). These materials contain two-dimensional surface states that house massless electrons that are topologically-protected from backscattering. Recent theoretical proposals indicate that, when confined to nanoscale dimensions, the surface states in topological insulators should exhibit discrete, quantized energy levels that remain topologically-protected. The overarching goal of this fellowship is to understand the properties of discrete states in topological insulator nanoparticles. To accomplish this goal, TI nanoparticles with a range of sizes will be synthesized using molecular beam epitaxy at the University of Delaware. The electronic structure of the nanoparticles will be characterized at Brookhaven National Laboratory using scanning tunneling microscopy and angle-resolved photoemission spectroscopy. The objectives for this fellowship are to 1) determine how the quantized energy level spacing depends on nanoparticle size and temperature; 2) understand how the energy-momentum dispersion relationship changes as a function of particle size and temperature; 3) measure the degree of spin-polarization of the quantized TI states; 4) map the wavefunctions in TI NPs to determine their spatial symmetry. The expected outcome of this project is an experimental spin-resolved band structure diagram for TI nanoparticles of varying dimensions. This will be the first experimental measurement of quantized states in TI NPs and will open the door to new avenues of scientific research and new device possibilities.

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.