Fig. 4: Bandgap measurement by STS.

a dI/dV spectra acquired near the center of 11 nanobubbles of increasing radii. The inset (scale bar: 10 nm) is a representative STM image of a bubble (R = 9.9 nm), and the location of the tip for the measurements is marked with a cross. Two spectra with different tip apexes are shown for a bubble of R = 2.4 nm. The setpoint for the measurements was V_bias = 1.0–1.2 V, I_t = 0.5 nA. b The energy of the valence and conduction band onsets extracted from the dI/dV spectra, as shown in (a) for the bubble of R = 2 nm. c The calculated bandgap (E_g = E_CBM − E_VBM) as a function of R. Notice that, for R > 10 nm, stabilization at a constant value is expected. The error bars in (b) and (c) are defined as \({{\Delta }}E=\sqrt{{(3.3{k}_{{{{\rm{B}}}}}T)}^{2}+{(2.5{V}_{{{{\rm{mod}}}}})}^{2}}\), where k_B is the Boltzmann constant, T temperature, and \({V}_{{{{\rm{mod}}}}}\) (= 30 mV) the lock-in modulation signal.