Zone-Folded Phonons and the Commensurate-Incommensurate Charge-Density-Wave Transition in 1T-TaSe2 Thin Films.
Bulk 1T-TaSe2 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)(1/2) × (13)(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 Γ. 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 ∼154 cm(-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-TaSe2.
†Nano-Device Laboratory (NDL), Department of Electrical and Computer Engineering, University of California-Riverside, Riverside, California 92521, United States.|‡Phonon Optimized Engineered Materials (POEM) Center, Materials Science and Engineering Program, University of California-Riverside, Riverside, California 92521, United States.
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Samnakay
§Laboratory for Terascale and Terahertz Electronics (LATTE), Department of Electrical and Computer Engineering, University of California-Riverside, Riverside, California 92521, United States.
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Wickramaratne
∥Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.
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Pope
§Laboratory for Terascale and Terahertz Electronics (LATTE), Department of Electrical and Computer Engineering, University of California-Riverside, Riverside, California 92521, United States.
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Lake
∥Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.
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Salguero
†Nano-Device Laboratory (NDL), Department of Electrical and Computer Engineering, University of California-Riverside, Riverside, California 92521, United States.|‡Phonon Optimized Engineered Materials (POEM) Center, Materials Science and Engineering Program, University of California-Riverside, Riverside, California 92521, United States.
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Balandin
2D thin films; Charge density wave; Raman metrology; commensurate and incommensurate phases; phonon zone folding; tantalum diselenide; van der Waals materials