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

Dybała, F.; Polak, Maciej P.; Kopaczek, Jan; Scharoch, P.; Wu, Kedi; Tongay, Sefaattin; Kudrawiec, R.

doi:10.1038/srep26663

Cited by 61 publications

(62 citation statements)

References 60 publications

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“…25 The van der Waals interactions, very important in geometry optimization of this type of systems, have been included via the DFT-D3 dispersion correction as proposed by Grimme et al 26 used together with generalized gradient approximation (GGA) (PBE) functional. 27 This approach provides an excellent agreement of the geometry optimization with the experimental values (less than 1% of discrepancy), as we have proven in our previous studies 28 where a detailed comparison of different approaches and functionals can be found. To improve the description of the band structure of regular LDA/GGA approach, we have used the modified Becke-Johnson exchange potential with LDA correlation (MBJLDA) 29,30 for all our band structure calculations.…”

Section: A Band Structure Calculationssupporting

confidence: 52%

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

Kopaczek

Polak

Scharoch

et al. 2016

Journal of Applied Physics

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Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS2, MoSe2, WS2, and WSe2. 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 MoS2, MoSe2, WS2, and WSe2 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 AH 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 AH transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (BH 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 BH transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

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Section: A Band Structure Calculationssupporting

confidence: 52%

“…This approach has been proven by us to be very effective and efficient numerous times over the years in more traditional III-V semiconductors 31,32 as well as in TMDs in Ref. 28 where a detailed reasoning of this choice is presented. A 10 Â 10 Â 6 Monkhorst-Pack mesh was used, as a result of convergence studies.…”

Section: A Band Structure Calculationsmentioning

confidence: 99%

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

Kopaczek

Polak

Scharoch

et al. 2016

Journal of Applied Physics

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“…For direct gap semiconductors the optical transition with the lowest energy is the band gap transition, but for indirect gap semiconductors the direct optical transitions are observed in MR spectra at energies larger than the fundamental band gap. So far MR spectroscopy was applied few times to study van der Waals crystals 12,39–46 but many of them were not studied yet by this technique. In this work we use MR spectroscopy as a complementary method to study the band gap in van der Waals crystals, which allows to determine the character of band gap observed in PA spectra.…”

Section: Introductionmentioning

confidence: 99%

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS 2 , MoSe 2 , MoTe 2 , HfS 2 , HfSe 2 , WS 2 , WSe 2 , ReS 2 , ReSe 2 , SnS 2 and SnSe 2 ) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.Van der Waals (vdW) crystals are known for a long time 1-14 but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers [15][16][17][18][19][20][21][22][23][24][25] . Since pioneering reports on optical properties of MoS 2 layers 15,16 , it is well established that the electronic band structure of MoS 2 strongly varies with the number of layers and exhibits indirect-to-direct band gap transition with the size reduction to a single layer. Similar studies of optical properties and the electronic band structure have been performed on other van der Waals crystals such as MoSe 2 , MoTe 2 , WS 2 , and WSe 2 22-25 . Some of them were investigated quite recently as bulk materials 13,14 but interest in these materials was low due to no potential applications in semiconductor devices. At this moment the situation is different. VdW crystals are exciting because of interesting physics of two-dimensional (2D) layers and their potential applications in novel semiconductor devices [26][27][28] . Because of this even bulk van der Waals crystals are interesting to study since many of them are not explored experimentally. One method which has never been applied to study most of van der Waals crystals is photoacoustic (PA) spectroscopy. We show that this technique is an excellent tool to study the band gap in van der Waals crystals since it is sensitive to indirect gap transitions which cannot be observed in photoluminescence. In combination with modulated reflectance (MR), which is known as a state of the art absorption-like technique to study direct optical transitions 29 , it is possible to conclude about the band gap character in van der Waals crystals and the spectral separation between the indirect and direct gap. In this work we applied PA and MR to study indirect and direct gap in many van der Waals crystals ...

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“…На рис. 3, b показана энергетическая диаграмма экситонных уровней WSe 2 из работы [12]. Согласно этой диаграмме, усиление интенсивности ГВГ при энергии фотона 1.95 eV соответствует 1s(B)-экситону, что согласуется с литера-турными данными [13].…”

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Оптическая диагностика монослоев WSe-=SUB=-2-=/SUB=-

Брехов¹,

Гришунин²,

Ильин³

et al. 2017

Письма В Журнал Технической Физики

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Представлены результаты исследования пленки WSe2 методом генерации второй гармоники при комнатной температуре в диапазоне длин волн излучения накачки 1200-1420 nm. Предварительные исследования методами фотолюминесцентной и атомно-силовой микроскопии показали, что пленка состоит из протяженных микрокристаллитов, а ее толщина составляет порядка 1 nm, т. е. соответствует толщине одного монослоя. Спектральная зависимость интенсивности генерации второй оптической гармоники позволила определить наличие в спектре 1s (B)-экситона с энергией 1.95 eV. DOI: 10.21883/PJTF.2017.24.45340.16956

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

Cited by 61 publications

References 60 publications

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

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

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Оптическая диагностика монослоев WSe-=SUB=-2-=/SUB=-

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