OSA's Digital Library

Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 4 — Feb. 25, 2013
  • pp: 4908–4916
« Show journal navigation

Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2

Philipp Tonndorf, Robert Schmidt, Philipp Böttger, Xiao Zhang, Janna Börner, Andreas Liebig, Manfred Albrecht, Christian Kloc, Ovidiu Gordan, Dietrich R. T. Zahn, Steffen Michaelis de Vasconcellos, and Rudolf Bratschitsch  »View Author Affiliations


Optics Express, Vol. 21, Issue 4, pp. 4908-4916 (2013)
http://dx.doi.org/10.1364/OE.21.004908


View Full Text Article

Acrobat PDF (1301 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We mechanically exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact number of layers is unambiguously determined by atomic force microscopy and high-resolution Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.

© 2013 OSA

1. Introduction

2. Sample preparation and crystal structure

3. Raman spectroscopy

Raman spectroscopy has been shown to be a powerful tool to determine the exact number of layers of exfoliated MoS2. The energy, width, and amplitude of the vibrational modes are strongly influenced by the thickness of the flakes [17

17. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, “Anomalous lattice vibrations of single- and few-layer MoS2.,” ACS Nano 4(5), 2695–2700 (2010). [CrossRef] [PubMed]

]. We investigate the vibrational modes of mono- and few-layer MoSe2 and WSe2 with high-resolution Raman spectroscopy. We use a Horiba LabRAM HR spectrometer with an excitation wavelength of 514.5 nm. The measurements are performed in a confocal micro configuration using a 100x microscope objective lens and a motorized xyz stage with 100 nm step size. Using a 2400 l/mm grating and a confocal pinhole of 100 µm the spectral resolution is below 1 cm−1. The power of laser excitation measured below the microscope objective lens is 15 µW for MoS2 and 68 µW for both MoSe2 and WSe2. For bulk WSe2 500 µW is used. All measurements are performed at room temperature. For simplicity we denote all vibrational modes with the irreducible representations of the D6h point group of bulk material. A group-theoretical analysis predicts four Raman active modes for the D6h group [18

18. J. Verble and T. Wieting, “Lattice mode degeneracy in MoS2 and other layer compounds,” Phys. Rev. Lett. 25(6), 362–365 (1970). [CrossRef]

], i.e. three in-plane modes E1g, E12g, and E22g, and one out-of-plane mode A1g (Fig. 2
Fig. 2 Schematic drawing of the four Raman active and two inactive modes of the transition metal dichalcogenides MX2 (M = Mo, W and X = Se, S).
). In our experiment only E12g and A1g are accessible. The E22g mode is at very low frequencies (~30 cm−1), and the E1g mode is forbidden in back-scattering geometry on a basal plane [19

19. G. Frey, R. Tenne, M. Matthews, M. Dresselhaus, and G. Dresselhaus, “Raman and resonance Raman investigation of MoS2 nanoparticles,” Phys. Rev. B 60(4), 2883–2892 (1999). [CrossRef]

].

MoS2

MoSe2

For bulk MoSe2 the A1g mode shifts about 150 cm−1 towards lower wavenumbers compared to MoS2 [25

25. T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura, and E. Matsuura, “Raman scattering and infrared reflectance in 2H-MoSe2,” J. Phys. Soc. Jpn. 49(3), 1069–1077 (1980). [CrossRef]

27

27. S. Sugai and T. Ueda, “High-pressure Raman spectroscopy in the layered materials 2H-MoS2, 2H-MoSe2, and 2H-MoTe2,” Phys. Rev. B 26(12), 6554–6558 (1982). [CrossRef]

], which is also confirmed by calculations [28

28. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B 406(11), 2254–2260 (2011). [CrossRef]

]. Indeed, we observe this out-of-plane mode in the spectral range of 240.5 - 242.5 cm−1, depending on the number of layers (Fig. 3
Fig. 3 Raman spectra of bulk and few-layer MoSe2. Labels ‘1L’ – ‘5L’ indicate the number of layers. Raman spectra are vertically displaced for clarity.
).

As with MoS2, we find the characteristic softening of the mode (redshift) with decreasing thickness of the material. It amounts to 2 cm−1 from bulk to monolayer MoSe2. Our measurements are in excellent agreement with two recent studies, where a Raman signal at 239.4 cm−1 on chemically exfoliated MoSe2 with an unknown number of layers [29

29. H. S. S. R. Matte, B. Plowman, R. Datta, and C. N. R. Rao, “Graphene analogues of layered metal selenides,” Dalton Trans. 40(40), 10322–10325 (2011). [CrossRef] [PubMed]

] and 243 cm−1 on a mechanically exfoliated MoSe2 monolayer [10

10. S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, and J. Wu, “Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2.,” Nano Lett. 12(11), 5576–5580 (2012). [CrossRef] [PubMed]

] were reported.

Our high experimental resolution allows us to observe a splitting of the out-of-plane Raman A1g line for the first time. Starting from one Raman line for mono- and bilayer MoSe2 it splits into two for three and four layer material. For five layers of MoSe2, three Raman lines appear. The spectral positions of the two maxima for three layers of MoSe2 are arranged almost symmetrically with respect to the monolayer line. The range of the observed splittings is 2.4 - 3.2 cm−1. This effect is due to the so-called Davydov splitting, which appears due to the presence of more than one MoSe2 molecule in the unit cell [18

18. J. Verble and T. Wieting, “Lattice mode degeneracy in MoS2 and other layer compounds,” Phys. Rev. Lett. 25(6), 362–365 (1970). [CrossRef]

,24

24. A. Molina-Sánchez and L. Wirtz, “Phonons in single-layer and few-layer MoS2 and WS2,” Phys. Rev. B 84(15), 155413 (2011). [CrossRef]

,25

25. T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura, and E. Matsuura, “Raman scattering and infrared reflectance in 2H-MoSe2,” J. Phys. Soc. Jpn. 49(3), 1069–1077 (1980). [CrossRef]

]. For the Raman active A1g mode the Se atoms in all layers oscillate in phase with respect to the corresponding center molybdenum atom, which does not move (Fig. 2, A1g). The splitting occurs due to a varying number of phase shifts of 180° between the layers (Fig. 4
Fig. 4 Schematic drawing of all Raman active out-of-plane vibrational modes in 1 to 5 layers of MoSe2. Dashed red lines between the layers denote an increase of the mode frequency compared to non-interacting layers. Green dotted lines indicate a decrease of the oscillation frequency. The horizontal dashed line indicates the mirror plane σh of the unit cells with odd number of layers. The black dot marks the center of inversion for the unit cells with even number of layers. Due to a smaller unit cell, the out-of-plane mode for the monolayer is denoted A′1.
). A phase shift of 180° between two layers is defined by the movement of the Se atoms: both top and bottom Se atoms within one layer move away from the molybdenum atom, while at the same time top and bottom Se atoms in the adjacent layer move towards the Mo atom and vice versa. Due to the weak interaction between the layers, the vibration frequencies of the in-phase and out-of-phase modes are almost degenerate [18

18. J. Verble and T. Wieting, “Lattice mode degeneracy in MoS2 and other layer compounds,” Phys. Rev. Lett. 25(6), 362–365 (1970). [CrossRef]

]. However, each two adjacent layers vibrating in phase cause a small shift towards higher frequencies (indicated in Fig. 4 as a red dashed line), while two adjacent layers oscillating out of phase cause a shift towards lower frequencies by approximately the same amount (indicated in Fig. 4 as a green dotted line).

Obviously, for a monolayer only one Raman active out-of-plane mode (A′1, see Fig. 4) appears, since the unit cell contains only one MoSe2 molecule and no translation symmetry along the c axis exists. For a bilayer with two MoSe2 molecules per unit cell the situation is similar to bulk material with only one Raman active out-of-plane mode. However, the mode shifts towards higher wavenumbers compared to the monolayer due to the interlayer interaction. For few layer material with an odd number of layers only out-of-plane modes which preserve the symmetry elements of the monolayer A′1 out-of-plane mode are Raman active. For our consideration, the most important symmetry element for these modes is the mirror plane σh indicated in Fig. 4. Therefore, for three layer material two Raman active out-of-plane modes are expected. For the first mode all layers are in phase, while for the second mode the center layer is 180° out of phase with respect to the two outer layers. The energies of these modes are arranged almost symmetrically to the monolayer mode, because the interlayer interactions induce an increase (decrease) of the energy for the first (second) mode. For an even number of layers the out-of-plane modes with the same symmetry elements as the A1g mode are Raman active. These modes exhibit an inversion center. Thus, for four layer material again two out-of-plane modes are Raman active. For the first mode all layers are in phase. For the second mode, the two outer layers are vibrating in opposite phase to the two inner layers. In five layer material one expects four different Raman active modes with none, one, two, and three layers being 180° out of phase with respect to the two outer layers. Two of these modes (with one and three inner layers vibrating out of phase) exhibit the same interlayer interaction and thus are degenerate. Indeed, we observe three maxima for the five layer material at 242.04 cm−1, 240.12 cm−1, and 238.03 cm−1.

We now turn to the in-plane E12g mode, which appears at higher wavenumbers than A1g in MoSe2. In bulk MoSe2 a weak vibrational mode was previously experimentally verified at 286 cm−1 [10

10. S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, and J. Wu, “Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2.,” Nano Lett. 12(11), 5576–5580 (2012). [CrossRef] [PubMed]

,25

25. T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura, and E. Matsuura, “Raman scattering and infrared reflectance in 2H-MoSe2,” J. Phys. Soc. Jpn. 49(3), 1069–1077 (1980). [CrossRef]

28

28. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B 406(11), 2254–2260 (2011). [CrossRef]

]. Here, this mode is found at 287.2 cm−1 for monolayer and at 285.9 cm−1 for bilayer MoSe2, indicating the same stiffening with decreasing numbers of layers as for MoS2. Interestingly, at 353 cm−1 we observe a previously unknown Raman line for few-layer MoSe2. The intensity of this mode is strongest for bilayer material and is reduced as the number of layers increases. Using similar considerations as above, we can assign this line to the B2g mode. This mode is inactive in bulk, but becomes Raman active due to the breakdown of translation symmetry in few layers. Besides these sharp lines (~1 cm−1), there are multiple broader maxima resulting from second order Raman processes (data not shown).

WSe2

For bulk WSe2, we find two distinct Raman signals at 248.0 and 250.8 cm−1 (Fig. 5
Fig. 5 Raman spectra of bulk and few-layer WSe2. Labels ‘1L’ – ‘5L’ indicate the number of layers. Raman spectra are vertically displaced for clarity.
). This doublet was also previously reported in a measurement of a WSe2 crystal at a temperature of T = 80 K [30

30. D. G. Mead and J. C. Irwin, “Long wavelength optic phonons in WSe2,” Can. J. Phys. 55(5), 379–382 (1977). [CrossRef]

]. The two lines are also in agreement with a recent calculation [28

28. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B 406(11), 2254–2260 (2011). [CrossRef]

], predicting both the E12g and the A1g mode to be close to 250 cm−1.

For few-layer WSe2 we only find a single maximum, the position of which changes with the number of layers. Presently, it is not clear whether this single maximum is due to only one of the two lines, or the two modes are almost degenerate and cannot be resolved. Moreover, we also find a broad side maximum at 260 cm−1 and a small signature at 309 cm−1 for the bilayer. The latter can be assigned again to the normally inactive B2g mode [21

21. C. Ataca, M. Topsakal, E. Aktürk, and S. Ciraci, “A comparative study of lattice dynamics of three- and two-dimensional MoS2,” J. Phys. Chem. C 115(33), 16354–16361 (2011). [CrossRef]

,28

28. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B 406(11), 2254–2260 (2011). [CrossRef]

]. As for MoSe2, there are multiple broader maxima resulting from second order Raman processes (data not shown).

In summary, we find that Raman spectroscopy is an excellent tool to unambiguously pinpoint the number of sheets of the layered dichalcogenide materials MoSe2 and WSe2.

4. Photoluminescence emission

Recently, the transformation of MoS2 from an indirect semiconductor in its bulk form to a direct semiconductor in case of a monolayer has been experimentally demonstrated [6

6. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010). [CrossRef] [PubMed]

,7

7. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayer MoS2.,” Nano Lett. 10(4), 1271–1275 (2010). [CrossRef] [PubMed]

]. As a consequence, substantial photoluminescence emission was detected. For bulk MoS2, the fundamental indirect band gap originates from a transition from the top of the valence band at the Γ point to the bottom of the conduction band, which is about halfway between the Γ and Κ point. The direct band gap for bulk MoS2 is higher in energy and occurs at the K point. The situation drastically changes, if the number of layers of MoS2 is decreased. The energy of the fundamental indirect band gap increases, until it crosses the direct gap transition, which is virtually unchanged in energy [31

31. A. Kuc, N. Zibouche, and T. Heine, “Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2,” Phys. Rev. B 83(24), 245213 (2011). [CrossRef]

]. In this way, monolayer MoS2 becomes a direct band gap semiconductor and the photoluminescence intensity drastically increases with decreasing number of layers. We now compare the photoluminescence emission of MoSe2, WSe2, and MoS2. A confocal microphotoluminescence setup is used to excite the sample with a continuous wave laser (λ = 532 nm), which is focused by a 100x microscope objective lens (NA = 0.9). Photoluminescence (PL) emission is collected by the same objective, dispersed with a f = 150 mm spectrometer and detected by a nitrogen cooled charge coupled device. All measurements are performed at room temperature.

MoSe2

The energy of the indirect gap of bulk MoSe2 is in the near-infrared [32

32. R. Coehoorn, C. Haas, and R. de Groot, “Electronic structure of MoSe2, MoS2, and WSe2. II. The nature of the optical band gaps,” Phys. Rev. B Condens. Matter 35(12), 6203–6206 (1987). [CrossRef] [PubMed]

] at 1.1 eV (1.13 µm). The direct A and B excitons are higher in energy at 1.57 eV (790 nm) and 1.82 eV (682 nm) [33

33. A. R. Beal and H. P. Hughes, “Kramers-Kronig analysis of the reflectivity spectra of 2H-MoS2, 2H-MoSe2 and 2H-MoTe2,” J. Phys. Chem. 12, 881–890 (1979).

,34

34. A. Anedda and E. Fortin, “Exciton spectra in MoSe2,” J. Phys. Chem. Solids 41(8), 865–869 (1980). [CrossRef]

]. Figure 6a
Fig. 6 Photoluminescence of monolayer and few-layer MoSe2 (a) and WSe2 (b). Spectra are recorded at room temperature, 532 nm excitation wavelength, and a fluence of 2100 W/cm2 (a) and 250 W/cm2 (b). PL spectra are Fourier-filtered with a 0.05 pixel−1 short pass to remove etaloning due to the back-illuminated CCD camera.
shows the photoluminescence spectrum of one, two, and three layers of exfoliated MoSe2. The observed emissions from monolayer and bilayer MoSe2 exhibit a single prominent maximum at 1.57 eV (792 nm) and 1.54 eV (807 nm), respectively.

WSe2

Photoluminescence emission vs. absorption of atomically thin MoS2, MoSe2, and WSe2

To gain insight into the efficiency of the photoluminescence emission process we first investigate the role of absorption for each monolayer material. Due to multiple reflections at the interfaces of the MX2/SiO2/Si structure, the absorption of laser light used for excitation is different as compared to a free-standing MX2 monolayer. Since the Si substrate is non-transparent, one may use simple reflectivity measurements and the intensity of the Raman emission from the Si substrate at 528 nm, i.e. 520 cm−1 in the Raman signal, to extract the absorption of the MX2 monolayers. A sketch of the optical system is given in Fig. 7
Fig. 7 Schematic drawing of reflected and transmitted light in the MX2/SiO2/Si multilayer structure for the determination of the absorption in the MX2 monolayer.
. The absorption of the monolayer (neglecting scattering) is given by
AMX2=R0RMX2+T0TMX2,
where R0 is the reflectivity of the bare SiO2/Si substrate and RMX2 is the reflectivity of the MX2/SiO2/Si system, including all interferences. T0 (TMX2) is the transmissivity into the thick Si wafer without (with) the monolayer on top. To eliminate calibration errors, we determine the ratio of reflected and transmitted light with and without the monolayer on top of the substrate:

r=Irefl,MX2/Irefl,substrate=RMX2/R0.

For the transmissivity ratio, we analyze the light entering the Si substrate. It is simply connected with the ratio of the Raman emission intensity from the Si substrate with and without the monolayer on top by

iR=IRaman,MX2/IRaman,substrate=(TMX2/T0)2.

We hereby assume that the absorption of the Raman laser with a wavelength of 514 nm, the Raman light from the substrate at 528 nm, and the exciting laser for the photoluminescence at 532 nm is about the same. The power of two originates from the fact that the Raman emission from the Si substrate is transmitted back through the same layered structure including multiple reflections before it reaches the detector (Fig. 7). After some algebra and the assumption that no light is absorbed in the SiO2 layer, the absorption of the monolayers on a SiO2/Si substrate is given by

AMX2=1rR0(1R0)iR.

The retrieved values for AMX2 are similar for the three monolayer materials deposited on SiO2/Si substrate: 16 ± 8% (MoS2), 23 ± 8% (MoSe2), and 13 ± 4% (WSe2). In contrast, the photoluminescence emission is always brightest for WSe2 and faintest for naturally grown MoS2, differing by at least an order of magnitude. This behavior might be indicative of a higher quantum yield of monolayer WSe2. However, strong variations from flake to flake render a quantitative analysis difficult. Measurements on different substrates and free-standing layers are necessary to answer this question.

5. Conclusions

In conclusion, we have prepared monolayer and few-layer flakes of the transition metal dichalcogenides MoS2, MoSe2, and WSe2. High-resolution Raman measurements show distinct layer dependent changes of the Raman signal including frequency shifts and spitting of the modes, which may be used to unambiguously identify the number of layers. We detect strong photoluminescence emission from monolayer MoSe2 and WSe2, indicative of direct gap semiconductors. These observations pave the way for optoelectronic devices based on these exceptional materials.

Acknowledgments

We thank Robert Magerle for granting access to the atomic force microscope and Mario Zerson for introducing us to its usage. We thank Andreas Zumbusch for providing an avalanche photodiode and Christian von Borczyskowski for a 100x microscope objective lens.

References and links

1.

A. K. Geim, “Graphene: status and prospects,” Science 324(5934), 1530–1534 (2009). [CrossRef] [PubMed]

2.

K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, “Two-dimensional atomic crystals,” Proc. Natl. Acad. Sci. U.S.A. 102(30), 10451–10453 (2005). [CrossRef] [PubMed]

3.

V. Podzorov, M. E. Gershenson, C. Kloc, R. Zeis, and E. Bucher, “High-mobility field-effect transistors based on transition metal dichalcogenides,” Appl. Phys. Lett. 84(17), 3301–3304 (2004). [CrossRef]

4.

B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nat. Nanotechnol. 6(3), 147–150 (2011). [CrossRef] [PubMed]

5.

H. Fang, S. Chuang, T. C. Chang, K. Takei, T. Takahashi, and A. Javey, “High-performance single layered WSe₂ p-FETs with chemically doped contacts,” Nano Lett. 12(7), 3788–3792 (2012). [CrossRef] [PubMed]

6.

K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010). [CrossRef] [PubMed]

7.

A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayer MoS2.,” Nano Lett. 10(4), 1271–1275 (2010). [CrossRef] [PubMed]

8.

Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, and H. Zhang, “Single-layer MoS2 phototransistors,” ACS Nano 6(1), 74–80 (2012). [CrossRef] [PubMed]

9.

H. S. Lee, S. W. Min, Y. G. Chang, M. K. Park, T. Nam, H. Kim, J. H. Kim, S. Ryu, and S. Im, “MoS₂ nanosheet phototransistors with thickness-modulated optical energy gap,” Nano Lett. 12(7), 3695–3700 (2012). [CrossRef] [PubMed]

10.

S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, and J. Wu, “Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2.,” Nano Lett. 12(11), 5576–5580 (2012). [CrossRef] [PubMed]

11.

H. Zeng, G.-B. Liu, J. Dai, Y. Yan, B. Zhu, R. He, L. Xie, S. Xu, X. Chen, W. Yao, and X. Cui, “Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides,” arXiv:1208.5864 (2012).

12.

R. Späh, U. Elrod, M. Lux-Steiner, E. Bucher, and S. Wagner, “pn junctions in tungsten diselenide,” Appl. Phys. Lett. 43(1), 79–81 (1983). [CrossRef]

13.

R. Gordon, D. Yang, E. Crozier, D. Jiang, and R. Frindt, “Structures of exfoliated single layers of WS2, MoS2, and MoSe2 in aqueous suspension,” Phys. Rev. B 65(12), 125407 (2002). [CrossRef]

14.

A. R. Beal, W. Y. Liang, and H. P. Hughes, “Kramers-Kronig analysis of the reflectivity spectra of 3R-WS2 and 2H-WSe2,” J. Phys. C, 9, 2449–2457 (1976).

15.

A. Castellanos-Gomez, N. Agraït, and G. Rubio-Bollinger, “Optical identification of atomically thin dichalcogenide crystals,” Appl. Phys. Lett. 96(21), 213116 (2010). [CrossRef]

16.

M. M. Benameur, B. Radisavljevic, J. S. Héron, S. Sahoo, H. Berger, and A. Kis, “Visibility of dichalcogenide nanolayers,” Nanotechnology 22(12), 125706 (2011). [CrossRef] [PubMed]

17.

C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, “Anomalous lattice vibrations of single- and few-layer MoS2.,” ACS Nano 4(5), 2695–2700 (2010). [CrossRef] [PubMed]

18.

J. Verble and T. Wieting, “Lattice mode degeneracy in MoS2 and other layer compounds,” Phys. Rev. Lett. 25(6), 362–365 (1970). [CrossRef]

19.

G. Frey, R. Tenne, M. Matthews, M. Dresselhaus, and G. Dresselhaus, “Raman and resonance Raman investigation of MoS2 nanoparticles,” Phys. Rev. B 60(4), 2883–2892 (1999). [CrossRef]

20.

T. Wieting and J. Verble, “Infrared and Raman studies of long-wavelength optical phonons in hexagonal MoS2,” Phys. Rev. B 3(12), 4286–4292 (1971). [CrossRef]

21.

C. Ataca, M. Topsakal, E. Aktürk, and S. Ciraci, “A comparative study of lattice dynamics of three- and two-dimensional MoS2,” J. Phys. Chem. C 115(33), 16354–16361 (2011). [CrossRef]

22.

T. Korn, S. Heydrich, M. Hirmer, J. Schmutzler, and C. Schüller, “Low-temperature photocarrier dynamics in monolayer MoS2,” Appl. Phys. Lett. 99(10), 102109 (2011). [CrossRef]

23.

H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, and D. Baillargeat, “From bulk to monolayer MoS2: evolution of Raman scattering,” Adv. Funct. Mater. 22(7), 1385–1390 (2012). [CrossRef]

24.

A. Molina-Sánchez and L. Wirtz, “Phonons in single-layer and few-layer MoS2 and WS2,” Phys. Rev. B 84(15), 155413 (2011). [CrossRef]

25.

T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura, and E. Matsuura, “Raman scattering and infrared reflectance in 2H-MoSe2,” J. Phys. Soc. Jpn. 49(3), 1069–1077 (1980). [CrossRef]

26.

T. J. Wieting, A. Grisel, and F. Levy, “Interlayer bonding and localized charge in MoSe2 and α-MoTe2,” Physica B+C 99(1-4), 337–342 (1980). [CrossRef]

27.

S. Sugai and T. Ueda, “High-pressure Raman spectroscopy in the layered materials 2H-MoS2, 2H-MoSe2, and 2H-MoTe2,” Phys. Rev. B 26(12), 6554–6558 (1982). [CrossRef]

28.

Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B 406(11), 2254–2260 (2011). [CrossRef]

29.

H. S. S. R. Matte, B. Plowman, R. Datta, and C. N. R. Rao, “Graphene analogues of layered metal selenides,” Dalton Trans. 40(40), 10322–10325 (2011). [CrossRef] [PubMed]

30.

D. G. Mead and J. C. Irwin, “Long wavelength optic phonons in WSe2,” Can. J. Phys. 55(5), 379–382 (1977). [CrossRef]

31.

A. Kuc, N. Zibouche, and T. Heine, “Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2,” Phys. Rev. B 83(24), 245213 (2011). [CrossRef]

32.

R. Coehoorn, C. Haas, and R. de Groot, “Electronic structure of MoSe2, MoS2, and WSe2. II. The nature of the optical band gaps,” Phys. Rev. B Condens. Matter 35(12), 6203–6206 (1987). [CrossRef] [PubMed]

33.

A. R. Beal and H. P. Hughes, “Kramers-Kronig analysis of the reflectivity spectra of 2H-MoS2, 2H-MoSe2 and 2H-MoTe2,” J. Phys. Chem. 12, 881–890 (1979).

34.

A. Anedda and E. Fortin, “Exciton spectra in MoSe2,” J. Phys. Chem. Solids 41(8), 865–869 (1980). [CrossRef]

35.

J. Martin, N. Akerman, G. Ulbricht, T. Lohmann, J. H. Smet, K. von Klitzing, and A. Yacoby, “Observation of electron–hole puddles in graphene using a scanning single-electron transistor,” Nat. Phys. 4(2), 144–148 (2008). [CrossRef]

36.

Y. Ma, Y. Dai, M. Guo, C. Niu, J. Lu, and B. Huang, “Electronic and magnetic properties of perfect, vacancy-doped, and nonmetal adsorbed MoSe2, MoTe2 and WS2 monolayers,” Phys. Chem. Chem. Phys. 13(34), 15546–15553 (2011). [CrossRef] [PubMed]

37.

W. S. Yun, S. Han, S. C. Hong, I. G. Kim, and J. Lee, “Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-MX2 semiconductors (M = Mo, W; X = S, Se, Te),” Phys. Rev. B 85(3), 033305 (2012). [CrossRef]

38.

G. Plechinger, F. X. Schrettenbrunner, J. Eroms, D. Weiss, C. Schüller, and T. Korn, “Low-temperature photoluminescence of oxide-covered single-layer MoS2,” Phys. Status Solidi (RRL) 6(3), 126–128 (2012). [CrossRef]

39.

M. P. Deshpande, G. K. Solanki, and M. K. Agarwal, “Optical band gap in tungsten diselenide single crystals intercalated by indium,” Mater. Lett. 43(1-2), 66–72 (2000). [CrossRef]

40.

J. A. Wilson and A. D. Yoffe, “The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties,” Adv. Phys. 18(73), 193–335 (1969). [CrossRef]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(180.1790) Microscopy : Confocal microscopy
(300.6250) Spectroscopy : Spectroscopy, condensed matter
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence
(300.6330) Spectroscopy : Spectroscopy, inelastic scattering including Raman
(300.6450) Spectroscopy : Spectroscopy, Raman
(300.6470) Spectroscopy : Spectroscopy, semiconductors
(160.4236) Materials : Nanomaterials
(180.5655) Microscopy : Raman microscopy

ToC Category:
Spectroscopy

History
Original Manuscript: December 17, 2012
Revised Manuscript: January 21, 2013
Manuscript Accepted: January 21, 2013
Published: February 20, 2013

Citation
Philipp Tonndorf, Robert Schmidt, Philipp Böttger, Xiao Zhang, Janna Börner, Andreas Liebig, Manfred Albrecht, Christian Kloc, Ovidiu Gordan, Dietrich R. T. Zahn, Steffen Michaelis de Vasconcellos, and Rudolf Bratschitsch, "Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2," Opt. Express 21, 4908-4916 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-4-4908


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. A. K. Geim, “Graphene: status and prospects,” Science324(5934), 1530–1534 (2009). [CrossRef] [PubMed]
  2. K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, “Two-dimensional atomic crystals,” Proc. Natl. Acad. Sci. U.S.A.102(30), 10451–10453 (2005). [CrossRef] [PubMed]
  3. V. Podzorov, M. E. Gershenson, C. Kloc, R. Zeis, and E. Bucher, “High-mobility field-effect transistors based on transition metal dichalcogenides,” Appl. Phys. Lett.84(17), 3301–3304 (2004). [CrossRef]
  4. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nat. Nanotechnol.6(3), 147–150 (2011). [CrossRef] [PubMed]
  5. H. Fang, S. Chuang, T. C. Chang, K. Takei, T. Takahashi, and A. Javey, “High-performance single layered WSe₂ p-FETs with chemically doped contacts,” Nano Lett.12(7), 3788–3792 (2012). [CrossRef] [PubMed]
  6. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS₂: a new direct-gap semiconductor,” Phys. Rev. Lett.105(13), 136805 (2010). [CrossRef] [PubMed]
  7. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence in monolayer MoS2.,” Nano Lett.10(4), 1271–1275 (2010). [CrossRef] [PubMed]
  8. Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, and H. Zhang, “Single-layer MoS2 phototransistors,” ACS Nano6(1), 74–80 (2012). [CrossRef] [PubMed]
  9. H. S. Lee, S. W. Min, Y. G. Chang, M. K. Park, T. Nam, H. Kim, J. H. Kim, S. Ryu, and S. Im, “MoS₂ nanosheet phototransistors with thickness-modulated optical energy gap,” Nano Lett.12(7), 3695–3700 (2012). [CrossRef] [PubMed]
  10. S. Tongay, J. Zhou, C. Ataca, K. Lo, T. S. Matthews, J. Li, J. C. Grossman, and J. Wu, “Thermally driven crossover from indirect toward direct bandgap in 2D semiconductors: MoSe2 versus MoS2.,” Nano Lett.12(11), 5576–5580 (2012). [CrossRef] [PubMed]
  11. H. Zeng, G.-B. Liu, J. Dai, Y. Yan, B. Zhu, R. He, L. Xie, S. Xu, X. Chen, W. Yao, and X. Cui, “Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides,” arXiv:1208.5864 (2012).
  12. R. Späh, U. Elrod, M. Lux-Steiner, E. Bucher, and S. Wagner, “pn junctions in tungsten diselenide,” Appl. Phys. Lett.43(1), 79–81 (1983). [CrossRef]
  13. R. Gordon, D. Yang, E. Crozier, D. Jiang, and R. Frindt, “Structures of exfoliated single layers of WS2, MoS2, and MoSe2 in aqueous suspension,” Phys. Rev. B65(12), 125407 (2002). [CrossRef]
  14. A. R. Beal, W. Y. Liang, and H. P. Hughes, “Kramers-Kronig analysis of the reflectivity spectra of 3R-WS2 and 2H-WSe2,” J. Phys.C, 9, 2449–2457 (1976).
  15. A. Castellanos-Gomez, N. Agraït, and G. Rubio-Bollinger, “Optical identification of atomically thin dichalcogenide crystals,” Appl. Phys. Lett.96(21), 213116 (2010). [CrossRef]
  16. M. M. Benameur, B. Radisavljevic, J. S. Héron, S. Sahoo, H. Berger, and A. Kis, “Visibility of dichalcogenide nanolayers,” Nanotechnology22(12), 125706 (2011). [CrossRef] [PubMed]
  17. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, “Anomalous lattice vibrations of single- and few-layer MoS2.,” ACS Nano4(5), 2695–2700 (2010). [CrossRef] [PubMed]
  18. J. Verble and T. Wieting, “Lattice mode degeneracy in MoS2 and other layer compounds,” Phys. Rev. Lett.25(6), 362–365 (1970). [CrossRef]
  19. G. Frey, R. Tenne, M. Matthews, M. Dresselhaus, and G. Dresselhaus, “Raman and resonance Raman investigation of MoS2 nanoparticles,” Phys. Rev. B60(4), 2883–2892 (1999). [CrossRef]
  20. T. Wieting and J. Verble, “Infrared and Raman studies of long-wavelength optical phonons in hexagonal MoS2,” Phys. Rev. B3(12), 4286–4292 (1971). [CrossRef]
  21. C. Ataca, M. Topsakal, E. Aktürk, and S. Ciraci, “A comparative study of lattice dynamics of three- and two-dimensional MoS2,” J. Phys. Chem. C115(33), 16354–16361 (2011). [CrossRef]
  22. T. Korn, S. Heydrich, M. Hirmer, J. Schmutzler, and C. Schüller, “Low-temperature photocarrier dynamics in monolayer MoS2,” Appl. Phys. Lett.99(10), 102109 (2011). [CrossRef]
  23. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, and D. Baillargeat, “From bulk to monolayer MoS2: evolution of Raman scattering,” Adv. Funct. Mater.22(7), 1385–1390 (2012). [CrossRef]
  24. A. Molina-Sánchez and L. Wirtz, “Phonons in single-layer and few-layer MoS2 and WS2,” Phys. Rev. B84(15), 155413 (2011). [CrossRef]
  25. T. Sekine, M. Izumi, T. Nakashizu, K. Uchinokura, and E. Matsuura, “Raman scattering and infrared reflectance in 2H-MoSe2,” J. Phys. Soc. Jpn.49(3), 1069–1077 (1980). [CrossRef]
  26. T. J. Wieting, A. Grisel, and F. Levy, “Interlayer bonding and localized charge in MoSe2 and α-MoTe2,” Physica B+C99(1-4), 337–342 (1980). [CrossRef]
  27. S. Sugai and T. Ueda, “High-pressure Raman spectroscopy in the layered materials 2H-MoS2, 2H-MoSe2, and 2H-MoTe2,” Phys. Rev. B26(12), 6554–6558 (1982). [CrossRef]
  28. Y. Ding, Y. Wang, J. Ni, L. Shi, S. Shi, and W. Tang, “First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers,” Physica B406(11), 2254–2260 (2011). [CrossRef]
  29. H. S. S. R. Matte, B. Plowman, R. Datta, and C. N. R. Rao, “Graphene analogues of layered metal selenides,” Dalton Trans.40(40), 10322–10325 (2011). [CrossRef] [PubMed]
  30. D. G. Mead and J. C. Irwin, “Long wavelength optic phonons in WSe2,” Can. J. Phys.55(5), 379–382 (1977). [CrossRef]
  31. A. Kuc, N. Zibouche, and T. Heine, “Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2,” Phys. Rev. B83(24), 245213 (2011). [CrossRef]
  32. R. Coehoorn, C. Haas, and R. de Groot, “Electronic structure of MoSe2, MoS2, and WSe2. II. The nature of the optical band gaps,” Phys. Rev. B Condens. Matter35(12), 6203–6206 (1987). [CrossRef] [PubMed]
  33. A. R. Beal and H. P. Hughes, “Kramers-Kronig analysis of the reflectivity spectra of 2H-MoS2, 2H-MoSe2 and 2H-MoTe2,” J. Phys. Chem.12, 881–890 (1979).
  34. A. Anedda and E. Fortin, “Exciton spectra in MoSe2,” J. Phys. Chem. Solids41(8), 865–869 (1980). [CrossRef]
  35. J. Martin, N. Akerman, G. Ulbricht, T. Lohmann, J. H. Smet, K. von Klitzing, and A. Yacoby, “Observation of electron–hole puddles in graphene using a scanning single-electron transistor,” Nat. Phys.4(2), 144–148 (2008). [CrossRef]
  36. Y. Ma, Y. Dai, M. Guo, C. Niu, J. Lu, and B. Huang, “Electronic and magnetic properties of perfect, vacancy-doped, and nonmetal adsorbed MoSe2, MoTe2 and WS2 monolayers,” Phys. Chem. Chem. Phys.13(34), 15546–15553 (2011). [CrossRef] [PubMed]
  37. W. S. Yun, S. Han, S. C. Hong, I. G. Kim, and J. Lee, “Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-MX2 semiconductors (M = Mo, W; X = S, Se, Te),” Phys. Rev. B85(3), 033305 (2012). [CrossRef]
  38. G. Plechinger, F. X. Schrettenbrunner, J. Eroms, D. Weiss, C. Schüller, and T. Korn, “Low-temperature photoluminescence of oxide-covered single-layer MoS2,” Phys. Status Solidi (RRL)6(3), 126–128 (2012). [CrossRef]
  39. M. P. Deshpande, G. K. Solanki, and M. K. Agarwal, “Optical band gap in tungsten diselenide single crystals intercalated by indium,” Mater. Lett.43(1-2), 66–72 (2000). [CrossRef]
  40. J. A. Wilson and A. D. Yoffe, “The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties,” Adv. Phys.18(73), 193–335 (1969). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited