FIG. 5: The sub-bands dispersion and the Hall conductance. The sub-bands dispersion of 2D Bi$_{2}$Se$_{3}$ thin film are plotted for (a) 3QL slab at exchange field=-0.05eV, (c) 3QL slab at exchange field=-0.24eV, (e) 5QL slab at exchange field=-0.01eV, and (g) 5QL slab at exchange field=-0.04eV. The insets of (e) and (g) zoom out the dispersion near Γ point. The corresponding Hall conductance are plotted in the right panels ((b), (d), (f) and (g)) as the function of the chemical potential. The systems in (a) and (e) are normal insulators, which have zero platforms in Hall conductance when the chemical potential locates in the energy gap. The system in (c) and (g) are QAH insulators, in which the Hall conductance have quantized value: e $^{2}$/h . 19 -0.4 -0.2  0  0.2  0.4 K M Energy (eV)  (a) -0.1  0  0.1  0.2  0.3  0.4 -0.2 -0.1  0  0.1 0.2 xy  (e 2 /h)  Chemical potential (eV)  (b) -0.4 -0.2  0  0.2  0.4 K M Energy (eV)  (c) -1 -0.5  0  0.5  1 -0.2 -0.1  0  0.1 0.2 xy  (e 2 /h)  Chemical potential (eV)  (d) -0.4 -0.2  0  0.2  0.4 K M Energy (eV)  (e) -0.02  0  0.02 -0.1  0  0.1  0.2  0.3  0.4 -0.2 -0.1  0  0.1 0.2 xy  (e 2 /h)  Chemical potential (eV)  (f) -0.4 -0.2  0  0.2  0.4 K M Energy (eV)  (g) -0.02  0  0.02  0  0.2  0.4  0.6  0.8  1 -0.2 -0.1  0  0.1 0.2 xy  (e 2 /h)  Chemical potential (eV)  (h)