Topological insulator nanostructures

© 2014 Materials Research Society MRS BULLETIN • VOLUME 39 • OCTOBER 2014 • www.mrs.org/bulletin Introduction Topological insulators (TIs) are a class of materials where the bulk is a band insulator, but the surface possesses electronic states carrying electric current. 1 , 2 One unique characteristic of the surface bands is the spin-momentum locking property—electrons have a single spin state perpendicular to their moving direction (i.e., a helical spin state). This unique spin nature of the gapless surface states holds promise for new electronics applications (i.e., spintronics devices and quantum information processes), as well as for applications in energy conversion such as thermoelectrics. TI materials, mostly metal dichalcogenides with small bulk bandgaps, can be synthesized in various forms such as bulk crystals, thin fi lms, and nanostructures. Bulk crystals of topological insulators were important material platforms in the early stage of TI studies—topological surface states can be easily probed using surface-sensitive techniques despite the coexisting background bulk electrons. 3 – 9 However, when it comes to electron transport essential for TI studies and applications, the dominance of bulk carriers over surface conduction becomes a signifi cant challenge. 10 – 13 A tiny amount (<1%) of vacancies and anti-site defects in the crystals easily populate bulk carriers, thus masking the surface state effect in electronic transport. Therefore, the material dimensions matter in order to make use of the surface electrons’ unique properties in the surface-dominant transport regime. TI nanostructures have several unique advantages compared to their bulk counterpart. First, their large surface-to-volume ratio naturally reduces the bulk carrier contribution in the overall electron transport. 14 , 15 Second, fi eld-effect gating in nanostructures allows modulation of the Fermi level in a single device. 16 Together with enhanced surface effects, the gating control in TI nanostructures can achieve TI surface state-dominated electron transport. 17 Third, the unique morphology of nanomaterials may enable the manipulation of 2D surface states at reduced dimensions. 18 An interesting example of this is a nanowire, where its cross-section perimeter gets smaller than the electron mean free path—unusual 1D states emerge, suggesting novel topological electronic states. 18 Last, TI heterostructures can be fabricated within single nanostructures, opening the opportunity for versatile band structure engineering of the surface states. In this review, we describe synthesis methods of topological insulator nanostructures with an emphasis on the material design to overcome material challenges. A few examples of TI nanostructure transport are presented. Finally, we briefl y discuss potential applications and future research directions of TI nanostructures.