Influence of Polar Pressure Transmission Medium on the Pressure Coefficient of Excitonic Interband Transitions in Monolayer WSe_2

The influence of the pressure transmission medium (PTM) on the excitonic interband transitions in monolayer tungsten diselenide (WSe_2) is investigated using photoluminescence (PL) spectra under hydrostatic pressure up to 5 GPa. Three kinds of PTMs, condensed argon (Ar), 1:1 n-pentane and isopentane mixture (PM), and 4:1 methanol and ethanol mixture (MEM, a PTM with polarity), are used. It is found that when either Ar or PM is used as the PTM, the PL peak of exciton related to the direct K–K interband transition shows a pressure-induced blue-shift at a rate of 32\pm4 or 32\pm1 meV/GPa, while it turns to be 50\pm9 meV/GPa when MEM is used as the PTM. The indirect \it \Lambda–K interband transition presents almost no shift with increasing pressure up to approximately 5 GPa when Ar and PM are used as the PTM, while it shows a red-shift at the rate of -17\pm7 meV/GPa by using MEM as the PTM. These results reveal that the optical interband transitions of monolayer WSe_2 are very sensitive to the polarity of the PTM. The anomalous pressure coefficient obtained using the polar PTM of MEM is ascribed to the existence of hydrogen-like bonds between hydroxyl in MEM and Se atoms under hydrostatic pressure.