Tuning of Topological Dirac States via Modification of van der Waals Gap in Strained Ultrathin Bi₂Se₃ Films

Robust massless Dirac states with helical spin textures were realized at the boundaries of topological insulators such as van der Waals (vdW) layered Bi₂Se₃ family compounds. Topological properties of massless Dirac states can be controlled by varying the film thickness, external stimuli, or environmental factors. Here, we report single-crystal-quality growth of ultrathin Bi₂Se₃ films on flexible polyimide sheets and manipulation of the Dirac states by varying the vdW gap. X-ray diffraction unambiguously demonstrates that under uniaxial bending stress the vdW gap substantially changes with interatomic-layer distances unaltered. Terahertz and photoelectron spectroscopy indicate tuning of the number of quantum conducting channels and of work function, by the stress, respectively. Surprisingly, under compressive strain, transport measurements reveal dimensional crossover and suppressed weak antilocalization. First-principles calculations support the observation. Our findings suggest that variation of vdW gap is an effective means of tuning the Fermi level and topological Dirac states for spintronics and quantum computation.