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The charge density wave (CDW) state in van der Waals systems showsinteresting scaling phenomena as the number of layers can significantly affectthe CDW transition temperature, $T_{CDW}$. However, it is often difficult touse conventional methods to study the phase transition in these systems due totheir small size and sensitivity to degradation. Degradation is an importantparameter which has been shown to greatly influence the superconductivity inlayered systems. Since the CDW state competes with the onset ofsuperconductivity, it is expected that $T_{CDW}$ will also be affected by thedegradation. Here, we probe the CDW phase transition by the mechanicalresonances of suspended 2H-TaS2 and 2H-TaSe2 membranes and study the effect ofdisorder on the CDW state. Pristine flakes show the transition near thereported values of 75 K and 122 K respectively. We then study the effect ofdegradation on 2H-TaS2 which displays an enhancement of $T_{CDW}$ up to 129 Kafter degradation in ambient air. Finally, we study a sample with localdegradation and observe that multiple phase transitions occur at 87 K, 103 Kand 118 K with a hysteresis in temperature in the same membrane. The observedspatial variations in the Raman spectra suggest that variations in crystalstructure cause domains with different transition temperatures which couldresult in the hysteresis. This work shows the potential of using nanomechanicalresonance to characterize the CDW in suspended 2D materials and demonstratethat degradation can have a large effect on transition temperatures.

Study of charge density waves in suspended 2H-TaS2 and 2H-TaSe2 by nanomechanical resonance