Quantitative Scanning Microwave Microscopy of Few-layer Platinum Diselenide

PtSe
₂
is unique among all 2D materials by having simultaneously sizable bandgap, high carrier mobility, and air stability. Moreover, PtSe2 undergoes a semiconductor-semimetal transition when its thickness increases beyond a few atomic layers, which facilitates low-resistance contact. However, there has been a controversy in how abruptly PtSe
₂
transitions from a semiconductor to a semimetal. In this work, scanning microwave microscopy was used to quantify the conductivity in two PtSe
₂
layers 1.5-nm-thick and 3-nm-thick, respectively. The resulted conductivity in 3-nm-thick PtSe
₂
of (1.7 ± 0.2) × 10
⁴
S/m is consistent with that reported for bulk PtSe
₂
, confirming that 3-nm-thick PtSe
₂
is a semimetal. However, although the conductivity in 1.5-nm-thick PtSe
₂
appears much lower, it is too close to that of TiN to be reliably quantified. To characterize low conductivity in few-layer semiconductors, the signal-to-noise ratio of scanning microwave microscopy needs to be further improved.