Figure 2
Electronic structure of monolayer 2H- and 1 T-NbSe2. (a) Plot of valence-band ARPES intensity for monolayer NbSe2 (mixed phase obtained at a substrate temperature TS of 530 °C) measured along the Γ-M cut. ARPES data were recorded at T=40 K at the He-Iα line (hν=21.218 eV). (b) As in a, but for a cleaved surface of bulk 2H-NbSe2. Blue dashed curves in a, b highlight the band dispersions that are commonly seen in a, b. Red dashed curves are a guide, indicating the bands that are absent in bulk 2H-NbSe2. (c) Calculated band structure obtained from the first-principles band calculations for monolayer 2H- (blue) and 1 T- (red) NbSe2 compared with the ARPES-intensity plot of monolayer NbSe2 (mixed phase; as in a, but plotted in grayscale). (d, e) ARPES intensity plotted as a function of wavevector and binding energy for monolayer 2H-NbSe2 (TS=515 °C) in the valence band and near-EF regions, respectively. (f) ARPES-intensity mapping at EF plotted as a function of the two-dimensional wavevector for monolayer 2H-NbSe2. The intensity at EF was obtained by integrating the ARPES intensity within ±10 meV of EF. The solid and dashed lines indicate the high-symmetry lines in the Brillouin zone for the (1 × 1) structure. (g) Constant-current STM image for monolayer 2H-NbSe2 (sample bias voltage Vs=500 mV) measured at 4 K. (h) STM image expanded in the area enclosed by the small rectangle in g (Vs=10 mV). The diamond corresponds to the 3 × 3 unit cell. (i–m) As in d–h but for monolayer 1T-NbSe2 (TS=590 °C). The sample bias-voltage in l, m is Vs=−2 V. The diamond in m corresponds to the 
unit cell arising from the star-of-David clusters.
