Zone-Folded Phonons and the Commensurate-Incommensurate Charge-Density-Wave Transition in 1T-TaSe sub(2) Thin Films

• Published in: Nano letters 2015-05, Vol.15 (5), p.2965-2973
• Main Authors: Samnakay, R, Wickramaratne, D, Pope, T R, Lake, R K, Salguero, T T, Balandin, A A
• Format: Article
• Language: English
• Subjects: Charge density waves; Film thickness; Mathematical analysis; Phases; Phonons; Reconstruction; Thin films; Transition temperature
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl504811s

Bulk 1T-TaSe sub(2) exhibits unusually high charge density wave (CDW) transition temperatures of 600 and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The (13) super(1/2) (13) super(1/2) C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to Gamma . The C-CDW transition temperatures as a function of film thickness are determined from the evolution of these new Raman peaks, and they are found to decrease from 473 to 413 K as the film thicknesses decrease from 150 to 35 nm. A comparison of the Raman data with ab initio calculations of both the normal and C-CDW phases gives a consistent picture of the zone-folding of the phonon modes following lattice reconstruction. The Raman peak at similar to 154 cm super(-1) originates from the zone-folded phonons in the C-CDW phase. In the I-CDW phase, the loss of translational symmetry coincides with a strong suppression and broadening of the Raman peaks. The observed change in the C-CDW transition temperature is consistent with total energy calculations of bulk and monolayer 1T-TaSe sub(2). Keywords: Charge density wave; van der Waals materials; commensurate and incommensurate phases; tantalum diselenide; 2D thin films; Raman metrology; phonon zone folding