Investigating Edge States in Bi₂Se₃ Nanocrystals with Controllable Dimensions

Nanoscale topological insulators, with bismuth selenide (Bi₂Se₃) being a prototypical example, exhibit intriguing physics due to the combination of topological properties and confinement effects. Three-dimensional Bi₂Se₃ is characterized by an insulating bulk with topologically protected quantum states at its surface. Interestingly, a reduction of the crystal thickness down to 1-6 quintuple layers (QLs) results in gapping of the surface states due to hybridization.^[1,2] Such two-dimensional crystals are predicted to host one-dimensional helical edge states,^[3,4] which can be characterized with cryogenic scanning tunneling spectroscopy (STS). To study the topological properties of 2D Bi₂Se₃, a synthesis method is required to produce nanoscale Bi₂Se₃ with controllable dimensions. By modifying the precursor reactivity in a hot injection synthesis, we were able to synthesize ultrathin, colloidally stable Bi₂Se₃ nanoplatelets (NPLs) with a well-defined lateral size and a thickness of 1-6 QLs. The improved size control and homogeneity of the Bi₂Se₃ nanocrystals enables us to measure the density of states of individual NPLs with STS. For crystals of 4-6 quintuple layers, we observe an 8 nm wide, one-dimensional state along the edge of the crystal^[5]. We use a low-energy continuum model and ab initio GW-Tight Binding theory to investigate the nature of this state.<br/><br/>1. Neupane, M.<i> et al.</i> Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi₂Se₃ films. <i>Nature Communications</i> <b>5</b>, 3841 (2014).<br/><br/>2. Zhang, Y.<i> et al.</i> Crossover of the three-dimensional topological insulator Bi₂Se₃to the two-dimensional limit. <i>Nature Physics</i> <b>6</b>, 584-588 (2010).<br/><br/>3. Liu, C. X.<i> et al.</i> Oscillatory crossover from two-dimensional to three-dimensional topological insulators. <i>Physical Review B</i> <b>81, </b>041307(R) (2010).<br/><br/>4. Licerán, L. M., Koerhuis, S. J. H., Vanmaekelbergh, D., & Stoof, H. T. C. Topology of Bi₂Se₃ nanosheets. <i>Physical Review B</i> <b>109 </b>(2024).<br/><br/>5. Moes, Jesper R., et al. "Characterization of the Edge States in Colloidal Bi₂Se₃ Platelets." <i>Nano Letters</i> <b>24</b>.17, 5110-5116 (2024).