Investigation of phonon modes in 2H-TaX2 (X = S/Se) flakes with electrostatic doping

Abstract

Electrostatic doping-assisted phonon softening has been intensively investigated in semiconducting transition metal dichalcogenides (TMDCs), in which strong electron–phonon interactions can be achieved in thin layers. Though electron–phonon coupling plays an important role in structure phase transitions for metallic 2H-TaX2 (X = S, Se), it has been rarely studied under electrostatic doping. Here, the effect of electrostatic doping on the vibrational phonon modes of 2H-TaSe2 and 2H-TaS2 thin films is studied with ionic liquid gating by the electric-double-layer gating technique. Under a gate bias (VG) of 1.5 V, 2H-TaSe2 has a blue shift of 7 cm−1 and a red shift of 3 cm−1 for the in-plane (E2g) and out-of-plane vibration (A1g) modes, respectively, indicating a strong electron–phonon interaction in both intralayer and interlayer. By varying the thickness of the flakes, it has been found that there exists a threshold VG value for the stiffening of the E2g mode and the softening of the A1g mode because of the strong electrostatic screening effect at lower doping density. As the intensity of the E2g mode decreases with VG, an anomalous enhancement of the A1g phonons can be achieved, which is further convinced in TaS2. The asymmetric phonon evolution behavior is very different with that in the semiconducting TMDCs, which may provide useful information for understanding electron–phonon interplay in metallic layer materials.