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Structural, electronic and vibrational properties of few-layer 2H- and 1T-TaSe2.

Yan JA, Cruz MA, Cook B, Varga K - Sci Rep (2015)

Bottom Line: We present first- principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries.Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material.Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Astronomy, and Geosciences, Towson University, 8000 York Road, Towson, Md 21252, USA.

ABSTRACT
Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. Few-layer tantalum diselenides (TaSe2) are typical metallic TMDs exhibiting rich CDW phase transitions. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. We present first- principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries. Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material. Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.

No MeSH data available.


Related in: MedlinePlus

Phonon dispersions of few-layer 2H-TaSe2 and 1T-TaSe2 as a function of layer number.The phonon bands of bulk 2H-TaSe2 and 1T-TaSe2 along the high symmetry path on the qz = 0 plane are also shown for comparison.
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f7: Phonon dispersions of few-layer 2H-TaSe2 and 1T-TaSe2 as a function of layer number.The phonon bands of bulk 2H-TaSe2 and 1T-TaSe2 along the high symmetry path on the qz = 0 plane are also shown for comparison.

Mentions: The dimensionality effects are also evident on the phonon properties of few-layer TaSe2. Figure 7 illustrates the phonon dispersions as a function of layer number for both 2H and 1T phases. The results of bulk are also shown for comparison. All calculations used the same smearing of σ = 0.02 Ry. Due to the interlayer coupling, there are small splittings on the optical phonon branches. Additional low-frequency phonon branches (below 50 cm−1) can be seen for bilayer and trilayer TaSe2. These modes correspond to breathing and shearing modes, which will be discussed further below. Most importantly, there are interesting evolutions on the acoustic branches which exhibit negative phonon modes, as shown in Fig. 7. In bulk 2H-TaSe2, only the ΓM line shows negative phonon branches (Fig. 7(e)). In contrast, there are additional negative phonon branches along the ΓK line for both 2H-2L and 2H-3L. This result has not been reported before, and possibly indicates different structural phase transitions in few-layer TaSe2 as compared with their bulk form. In the case of 1T phase, the negative phonon branches along the ΓK line are less evident than the ΓM line (Fig. 7(f)). Upon reducing from bulk to 2L, these negative branches are greatly enhanced and become comparable in magnitude with those along the ΓM direction. These results highlight the dimensionality effects on the phonon properties in few-layer TaSe2.


Structural, electronic and vibrational properties of few-layer 2H- and 1T-TaSe2.

Yan JA, Cruz MA, Cook B, Varga K - Sci Rep (2015)

Phonon dispersions of few-layer 2H-TaSe2 and 1T-TaSe2 as a function of layer number.The phonon bands of bulk 2H-TaSe2 and 1T-TaSe2 along the high symmetry path on the qz = 0 plane are also shown for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4644971&req=5

f7: Phonon dispersions of few-layer 2H-TaSe2 and 1T-TaSe2 as a function of layer number.The phonon bands of bulk 2H-TaSe2 and 1T-TaSe2 along the high symmetry path on the qz = 0 plane are also shown for comparison.
Mentions: The dimensionality effects are also evident on the phonon properties of few-layer TaSe2. Figure 7 illustrates the phonon dispersions as a function of layer number for both 2H and 1T phases. The results of bulk are also shown for comparison. All calculations used the same smearing of σ = 0.02 Ry. Due to the interlayer coupling, there are small splittings on the optical phonon branches. Additional low-frequency phonon branches (below 50 cm−1) can be seen for bilayer and trilayer TaSe2. These modes correspond to breathing and shearing modes, which will be discussed further below. Most importantly, there are interesting evolutions on the acoustic branches which exhibit negative phonon modes, as shown in Fig. 7. In bulk 2H-TaSe2, only the ΓM line shows negative phonon branches (Fig. 7(e)). In contrast, there are additional negative phonon branches along the ΓK line for both 2H-2L and 2H-3L. This result has not been reported before, and possibly indicates different structural phase transitions in few-layer TaSe2 as compared with their bulk form. In the case of 1T phase, the negative phonon branches along the ΓK line are less evident than the ΓM line (Fig. 7(f)). Upon reducing from bulk to 2L, these negative branches are greatly enhanced and become comparable in magnitude with those along the ΓM direction. These results highlight the dimensionality effects on the phonon properties in few-layer TaSe2.

Bottom Line: We present first- principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries.Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material.Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Astronomy, and Geosciences, Towson University, 8000 York Road, Towson, Md 21252, USA.

ABSTRACT
Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. Few-layer tantalum diselenides (TaSe2) are typical metallic TMDs exhibiting rich CDW phase transitions. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. We present first- principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries. Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material. Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.

No MeSH data available.


Related in: MedlinePlus