  The charge density wave (CDW) state in van der Waals systems shows
interesting scaling phenomena as the number of layers can significantly affect
the CDW transition temperature, $T_{CDW}$. However, it is often difficult to
use conventional methods to study the phase transition in these systems due to
their small size and sensitivity to degradation. Degradation is an important
parameter which has been shown to greatly influence the superconductivity in
layered systems. Since the CDW state competes with the onset of
superconductivity, it is expected that $T_{CDW}$ will also be affected by the
degradation. Here, we probe the CDW phase transition by the mechanical
resonances of suspended 2H-TaS2 and 2H-TaSe2 membranes and study the effect of
disorder on the CDW state. Pristine flakes show the transition near the
reported values of 75 K and 122 K respectively. We then study the effect of
degradation on 2H-TaS2 which displays an enhancement of $T_{CDW}$ up to 129 K
after degradation in ambient air. Finally, we study a sample with local
degradation and observe that multiple phase transitions occur at 87 K, 103 K
and 118 K with a hysteresis in temperature in the same membrane. The observed
spatial variations in the Raman spectra suggest that variations in crystal
structure cause domains with different transition temperatures which could
result in the hysteresis. This work shows the potential of using nanomechanical
resonance to characterize the CDW in suspended 2D materials and demonstrate
that degradation can have a large effect on transition temperatures.
