Matching Items (2)

128049-Thumbnail Image.png

Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Description

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2]. In

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2]. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K point, and therefore, direct optical transitions at H point should be expected in modulated reflectance spectra besides the direct optical transitions at the K point of Brillouin zone. This prediction is confirmed by experimental studies of the electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2] crystals by CER, PR, and PzR spectroscopy, i.e., techniques which are very sensitive to critical points of Brillouin zone. For the four crystals besides the A transition at K point, an A[subscript H] transition at H point has been observed in CER, PR, and PzR spectra a few tens of meV above the A transition. The spectral difference between A and A[subscript H] transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (B[subscript H] transition) overlaps spectrally with the B transition at K point because of small energy differences in the valence (conduction) band positions at H and K points. Therefore, an extra resonance which could be related to the B[subscript H] transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

Contributors

Agent

Created

Date Created
  • 2016-06-21

128513-Thumbnail Image.png

Pressure coefficients for direct optical transitions in MoS2, MoSe2, WS2, and WSe2 crystals and semiconductor to metal transitions

Description

The electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within

The electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: α[subscript A] = 2.0 ± 0.1 and α[subscript B] = 3.6 ± 0.1 meV/kbar for MoS[subscript 2], α[subscript A] = 2.3 ± 0.1 and α[subscript B] = 4.0 ± 0.1 meV/kbar for MoSe[subscript 2], α[subscript A] = 2.6 ± 0.1 and α[subscript B] = 4.1 ± 0.1 meV/kbar for WS[subscript 2], α[subscript A] = 3.4 ± 0.1 and α[subscript B] = 5.0 ± 0.5 meV/kbar for WSe[subscript 2]. It has been found that these coefficients are in an excellent agreement with theoretical predictions. In addition, a comparative study of different computational DFT approaches has been performed and analyzed. For indirect gap the pressure coefficient have been determined theoretically to be −7.9, −5.51, −6.11, and −3.79, meV/kbar for MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], respectively. The negative values of this coefficients imply a narrowing of the fundamental band gap with the increase in hydrostatic pressure and a semiconductor to metal transition for MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals at around 140, 180, 190, and 240 kbar, respectively.

Contributors

Agent

Created

Date Created
  • 2016-05-24