Charge puddles in germanene

We report an investigation of the electronic inhomogeneities in a single germanene layer grown on a molybdenum disulfide (MoS ₂ ) substrate. Using scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of the Dirac point of germanene. The Dirac point maps reveal the presence of charge puddles in the germanene sheet. The Dirac point varies from -30 meV to +15 meV, corresponding to a charge density in the puddles in the range of 2.6 × 10 ^-3 electrons to 6.6 × 10 ^-4 holes per nm ² . The radius of these puddles is about 10-20 nm, resulting in a total charge of the order of one charge carrier per puddle. The defect concentration in the top layer of the MoS ₂ substrate is very comparable to the density of charge puddles, suggesting that the charge puddles are caused by the charged defects in the top layer of the MoS ₂ substrate.