Figure 3. STM images and the tunnelling spectra of WSe₂ and MoS₂ grown on HOPG.

(a) STM image of single layer WSe₂ grown on HOPG. The inset shows the atomic resolution image taken on the SL-WSe₂. For the inset, U=−1.4 V, I=10 pA. (b) The dI/dV-V spectrum taken on the SL-WSe₂ flake. The tunnelling conductance dI/dV (with arbitrary unit) is plotted in both the linear scale (upper panel) and the logarithmic scale (lower panel). The green dashed arrows indicate the positions of the local VBM at Γ and K points, which are equal to −1.65 eV and −1.05 eV, respectively. The CBM is assigned at +1.03 eV. (c) The clear threshold, corresponding to VBM at K, can be seen in the dI/dV spectrum taken with much more close tip-sample distance (∼3.5 Å closer than b). For comparison, the spectra taken with the stabilization bias with the gap (−0.8 V) is shown in black, while the one taken on bare graphite is in blue. The similar dI/dV (in black) and ‘close-in' (red in inset) spectra of SL-MoS₂ are displayed in d, while the valence band maxima at Γ and K are marked in the inset. The E_C (blue arrow) corresponds to the CBM. (e) The diagram of band alignments among single-layer MoS₂ and WSe₂. The local VBM at Γ and global VBM at K are shown in cyan and red, respectively.