  Three-dimensional (3D) topological Dirac semimetal, when thinned down to 2D
few layers, is expected to possess gapped Dirac nodes via quantum confinement
effect and concomitantly display the intriguing quantum spin Hall (QSH)
insulator phase. However, the 3D-to-2D crossover and the associated topological
phase transition, which is valuable for understanding the topological quantum
phases, remain unexplored. Here, we synthesize high-quality Na3Bi thin films
with R3*R3 reconstruction on graphene, and systematically characterize their
thickness-dependent electronic and topological properties by scanning tunneling
microscopy/spectroscopy in combination with first-principles calculations. We
demonstrate that Dirac gaps emerge in Na3Bi films, providing spectroscopic
evidences of dimensional crossover from a 3D semimetal to a 2D topological
insulator. Importantly, the Dirac gaps are revealed to be of sizable magnitudes
on 3 and 4 monolayers (72 and 65 meV, respectively) with topologically
nontrivial edge states. Moreover, the Fermi energy of a Na3Bi film can be tuned
via certain growth process, thus offering a viable way for achieving charge
neutrality in transport. The feasibility of controlling Dirac gap opening and
charge neutrality enables realizing intrinsic high-temperature QSH effect in
Na3Bi films and achieving potential applications in topological devices.
