Small energy gap of CrBr3 revealed by Scanning Tunneling Microscopy and Spectroscopy (STM/S)

CrBr$_{\mathrm{3}}$ is a 2D magnetic van der Waals (vdW) material. Despite the great attention on the magnetic properties, the electronic properties of the CrBr$_{\mathrm{3}}$ are relatively unexplored. The energy gap of CrBr$_{\mathrm{3}}$ is believed to be in the range of 1.68-2.1 eV based on the optical measurements, while the DFT calculations in literature exhibited an even larger deviation of the energy gap values. Here, we present the measurements of the CrBr$_{\mathrm{3}}$ flakes (both thin and thick) by using scanning tunneling microscopy and spectroscopy (STM/S). Along with the DFT calculations, a small energy (0.57 \textpm 0.04eV) is revealed. The multiple peak d$I$/d$V$ spectra were measured on a defect free topography region. Excellent agreements between the reported optical transitions in literature and peak pair energy differences in our d$I$/d$V$ data further confirm that all the measured peaks are intrinsic to CrBr$_{\mathrm{3}}$, and leads to the small energy gap determined by the two peaks closest to the Fermi energy. DFT calculation with U $=$ 5 eV and J $=$ 3 eV reproduces this energy gap. Last, edge degradation was observed on mono- and bi-layer flakes due to \textasciitilde 15 minutes air exposure during sample transfer. These observations provide important information towards the fundamental understanding of CrBr$_{\mathrm{3}}$.