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.

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5. Moes, Jesper R., et al. "Characterization of the Edge States in Colloidal Bi₂Se₃ Platelets." Nano Letters 24.17, 5110-5116 (2024).