Thickness-dependent gap energies in thin layers of Hf Te5

Abstract Hafnium pentatelluride (Hf Te 5 ) is a layered two-dimensional material with various exotic properties. It is thought to be a topological insulator. Whereas bulk Hf Te 5 has a small band gap, single layers are predicted to be a quantum spin hall insulator with a large band gap. Here we measured band gap energies for samples with varying thicknesses and found a clear increase of gap energies for the thinner samples. With decreasing thickness an increase of the measured band gap energies from 40 to 304 meV is observed.