Nickel ditelluride (NiTe₂), a recently discovered Type-II Dirac semimetal with topological Dirac fermions near the Fermi energy, is expected to exhibit strong thickness-mediated electronic tunability and intrinsic two-gap superconductivity in the single-layer limit. Realizing such intriguing phenomena requires the fabrication of ultrathin NiTe₂ films and an understanding of the underlying physics that is still under debate. By conducting experimental band mappings of ultrathin films prepared with molecular beam epitaxy, we reveal spectroscopic evidence for the dimensionality crossover of single-crystalline ultrathin NiTe₂ films as a function of film thickness. As the film thickness increases from one to five layers, the gap in the conical topological surface states closes. Comparisons of experimental to first-principles results also highlight difficulties in fabricating atomically smooth single-layer NiTe₂ films. Our results not only provide further impetus for studying emergent phenomena in NiTe₂ but also underscore the limitations of fabricating NiTe₂ films for device applications.

二碲化镍（NiTe ₂）是一种最近发现的II型狄拉克半金属，在费米能量附近具有拓扑狄拉克费米子，有望在单层范围内表现出很强的厚度介导的电子可调性和固有的两间隙超导性。要实现这种引人入胜的现象，需要制造超薄的NiTe ₂薄膜，并了解仍在争论中的基本物理原理。通过对用分子束外延制备的超薄膜进行实验谱带映射，我们揭示了单晶超薄膜NiTe ₂的尺寸交叉的光谱证据。薄膜厚度的函数。随着膜厚度从一层增加到五层，圆锥形拓扑表面状态中的间隙将关闭。实验原理与第一原理结果的比较也突出了制造原子上光滑的单层NiTe ₂膜的困难。我们的结果不仅为研究NiTe _2中出现的现象提供了进一步的动力，而且突显了为设备应用制造NiTe ₂膜的局限性。