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Two-dimensional semiconducting analogues of graphene, like monolayer molybdenum diselenide (MoSe2), provide a rich platform for optoelectronics. We study sharp localized photoluminescence (PL) emission from exfoliated MoSe2 flakes. These emission signatures are present in single layers as well as bi- and few layer flakes at low temperatures. The PL from these defects saturate at a lower power than the neutral and charged excitons of the monolayer MoSe2 and are stable against multiple heating and cooling cycles. We study the Zeeman effect in these emitters through magneto-optical photoluminescence studies and derive a g-factor of around 4 which is similar to the delocalized excitons in MoSe2.
© 2016 Optical Society of America
1. Introduction
Atomically thin layers of transition metal dichalcogenides (TMDCs) are the semiconducting analogues of the semimetal graphene and exhibit a variety of rich nano-optoelectronic properties [1–3]. These quantum well-like two-dimensional (2D) materials have a direct bandgap, exhibit bright photoluminescence emission, [4] and possess polarization selection rules that correlate emitted photon polarization with distinct locations in the material’s reciprocal space. The latter feature has given rise to the emergence of “opto-valleytronics” [5–8]. Recent efforts have endeavored to realize localized exciton light emission in these systems. One approach has been to fabricate quantum dots (QDs) in TMDCs from the top down using lithographic masks [9]. The challenge in fabricating QDs in TMDCs is that accessing the strong confinement regime requires a nanofabrication length scale commensurate with the materials’ small exciton Bohr radius (∼1 nm). Another possibility is to identify defects in the material that can localize excitons. Such exciton localization via defect centers has been recently discovered in the atomically thin semiconducting tungsten dislenide (WSe2) [10–14] as well as the 2D material insulator hexagonal boron nitride [15]. These new optically active defects in WSe2 inherit their electronic structure from the host 2D semiconductor, emit single photons, and have an excited state lifetime on the order of 1 ns and remarkably large excitonic g-factors of 10. As a result of all the previous characteristics, WSe2 defects are being actively studied for applications in integrated solid-state quantum photonics, quantum information processing and quantum metrology.
A natural question to emerge after uncovering defects in WSe2 is if other semiconducting TMDCs host optically active defects. In this study, we provide a partial answer to this question by identifying and studying localized defect-like emission from single and multilayer MoSe2 flakes. We explore the optical properties of these defects through photoluminescence (PL) spectroscopy at low temperature (4K). Finally, magneto-optical photoluminescence spectroscopy is used to characterize the Zeeman effect in these emitters.
2. Sample and experimental setup
The MoSe2 flakes used in this study were exfoliated and transferred onto a silicon substrate with a 300 nm oxide layer by a dry transfer technique [16]. All measurements were performed in a attoDRY cryostat, and the sample was cooled using helium exchange gas. The sample was excited nonresonantly with a 675 nm continuous wave laser source and focused with an 0.82 numerical aperture (N.A.) objective. For polarization resolved measurements a combination of linear polarizers and quarter waveplates were used in both the excitation and collection channels of the microscope to study circularly polarized emissions. Signal from the sample was collected using the same objective and filtered by a 700 nm long pass filter prior to being sent to a spectrometer equipped with a nitrogen-cooled charge-coupled device (CCD) camera for photoluminescence studies. Figure 1(a) shows an optical micrograph of a region in the sample that hosted the defect light emission.
Fig. 1 Sample and PL spectra: (a) Optical micrograph of a region hosting the defect light emission in MoSe2 single layer flakes and near the interface of a bulk/bilayer flake. (b) PL spectrum from a defect free region of a single layer MoSe2 flake. (c) PL spectrum of defect from bilayer/bulk interface [region circled in yellow in Fig. 1(a)]. Inset shows a high resolution spectrum from the shaded region in grey. (d) PL spectrum of defect from single layer region [circled in blue in Fig. 1(a)]. (e) Spectral wandering data collected within a span of 60 seconds (each spectrum is acquired for 1 second). Integrated PL is projected above the 2D plot to show the inhomogeneous broadening from spectral wandering.
3. Photoluminescence spectra from the localized emissions
In Fig. 1(b) the photoluminescence spectrum from a single layer region without defects (marked in Fig. 1(a) by a red circle). From previous studies [7, 17], the peaks in Fig. 1(b) are identified as the neutral exciton and trion from the delocalized excitons in the monolayer flake. However, from certain regions on the sample we see comb-like lines in the PL spectra [Figs. 1(c)–(d)]. These correspond to defect emission within the bandgap of the MoSe2, as they show a significant redshift from the neutral exciton peak in Fig. 1(b). From Figs. 1(c)–(d) the spectrally localized emissions are between the energy range 1.58 eV – 1.64 eV which is lower in energy from the PL of the neutral exciton peak of MoSe2 (∼ 1.66 eV). Hence it is most likely that the peaks correspond to emission from a number of states within the bandgap of the TMDC. However, to predict the exact energy level diagram more theoretical studies are needed which is beyond the scope of this article. Figure 1(c)–(d) show photoluminescence spectra from defect sites present near the bilayer/bulk interface and single layer respectively. Note that the delocalized exciton emission peak from the defect site in single layer [Fig. 1(d)] is significantly quenched and is dominated by the red shifted emission from defects. Comparing with the delocalized exciton emission in monolayer MoSe2, these localized emitters have linewidths almost an order of magnitude sharper than the delocalized excitons.Their full width at half maximum (FWHM) is on the order 200–500 μeV limited by inhomgeneous broadening from spectral diffusion. For any localized emitter exposed at the surface, inhomogeneous broadening is observed from spectral wandering caused by nearby surface states. This has been also observed in the WSe2 quantum dots and is the result of a local fluctuating potential in the vicinity of the emission site [10–14]. A time trace plot showing slow spectral wandering within a window of 60 seconds is shown in Fig. 1(e). The spectrum in each frame is collected for a second.
4. Power, polarization, and temperature dependence
We perform a comparative study of the power dependence from the defect emission with respect to the delocalized excitons of MoSe2. As exhibited in Fig. 2(a) the neutral exciton (open diamonds) and trion (open squares) show a linear dependence as a function of power. However, increasing the optical power results in non-linear dependence of the PL intensity from the defects (solid circle). In Fig. 2(b) we plot the power dependence of emission from two different defects. The power saturation curve [13, 15, 18, 19]
is used to fit the data, where, Isat and Psat are the value of counts and power at saturation respectively. The defects have different saturation power (Psat=8.2 μW and 9.1 μW for D1 and D2 respectively), but they both show saturation after a certain optical power level. This kind of behavior is expected for excitons that are bound to defect sites at the flake. At low power only a small fraction of the free electron and hole pair form exciton. On increasing the excitation power, the defects are populated with more and more excitons and saturate at higher powers. This is also responsible for the nonlinear dependence of PL intensity with excitation power for the defect bound excitons, whereas for the free excitons (neutral exciton and trion), the intensity increases linearly with power. Power dependence studies have been used to identify direct recombination process of free excitons from defect mediated transitions in quantum wells [20] as well as defects in TMDCs [10–14, 21]. Thus we see a clear saturation behavior at higher powers which is a signature of quantized emission. However, we do not observe any antibunching from these emission lines. This could be due to the presence of more than one emitter within the collected signal bandwidth or potentially short anti-bunching timescales compared to the ∼ 400 ps instrument response time of our detection system.Fig. 2 Power, polarization, and temperature dependence: (a) Defect (solid red circles) vs delocalized exciton emission from trion (open green square) and neutral exciton (open orange diamonds) as a function of incident power presenting the nonlinear and linear response to the incident power respectively. (b) Power dependence for two different defects showing saturation at different powers. Data from (a)–(b) are fit to Eq. (1). (c) Polar plot showing linearly polarized emission from two different emitters. The curve is a fit to the data. (d) Emission from defects as a function of temperature. Solid line is guide to the eye. Inset shows the emission spectra from the studied defect at different temperatures.
By examining the emitted photon’s polarization properties, as shown in Fig. 2(c), it is possible to obtain information regarding the orientation of the dipole mediating the light emission. Figure 2(c) indicates a preferred orientation for the emitted light polarization of two different emission lines excited with the same orientation of linearly polarized light. This evidence is consistent with the observation that localized excitons are responsible for this emission since localized excitons are expected to have preferred orientation for their dipole moment. All emitters we have measured have different preferred orientations of the dipole, but exhibit a linear polarization, independent of the direction of the excitation source’s polarization. This angular dependence of PL suggests that the emitters have an in-plane dipole moment just as the neutral exciton of MoSe2, but unlike the MoSe2 exciton the dipole orientation is not determined by the excitation source’s direction of linear polarization. [14].
The temperature dependence of some of these emitters is also recorded in Fig. 2(d). We observe a quenching of PL intensity with temperature which is restored after the sample is cooled back down. This is likely due to activation of some dissipative processes with temperature which supresses the PL from these emitters. Such behavior is also consistent with the previously observed in the localized emitters in WSe2 [10–13] and quantum dots [22–24]
5. Zeeman effect in the localized emitters
Finally, we perform magneto-optical PL studies on the emission lines to study the Zeeman splitting as a function of magnetic field perpendicular to the sample (Faraday configuration). Figure 3(a) shows the PL spectra of the localized emitters at different magnetic fields. D1 and D2 represent two emitters showing a clear Zeeman splitting as the magnetic field is gradually ramped up. At zero field, no measureable fine structure splitting could be observed. This should be compared with emitters in WSe2 which often show a zero field splitting on the order of 700 μeV [13]. Any fine structure splitting in the MoSe2 defects, if present, could be smeared out either due to spectral diffusion and/or resolution of the spectrometer in our current setup. We extract the peak energies for the splitting as a function of magnetic field and plot these in Fig. 3(b). The split peaks also wander together which is apparent in Fig 3(b). The Zeeman energy splitting (D) as a function of the magnetic field can be fit to the linear equation:
where g is the g-factor, μB is the Bohr magneton, and B is the applied magnetic field. From the fit we find the g-factor to be 4.2 and 4.6 respectively for the two defects. To compare this value with the valley splitting of the delocalized excitons of MoSe2, we perform circular polarization resolved PL studies as a function of magnetic field. Figure 3(e) clearly shows the valley splitting of the trion (shaded in black) and neutral exciton peak (shaded in red) for single layer MoSe2. For zero field, there is no significant splitting but it increases with magnetic field. This Zeeman splitting is due to valley dependent magnetic moment [7]. The Zeeman splitting for MoSe2 neutral exciton and trion is plotted in Fig. 3(f) and from the linear fit we derive magnitude of g-factor for the charged and neutral excitons to be about 3.9 and 4.2 respectively, which are very similar to the localized exciton peaks. This tells us that the studied localized emitters likely derive some of their properties from the delocalized MoSe2 exciton. In contrast, the single photon emitting centers in the other TMDC, WSe2, have large g-factors (∼ 7–10) as compared to their 2D excitons [10–13].Fig. 3 Magneto-optical studies of the defects in MoSe2: (a) Zeeman splitting of two emission lines as a function of magnetic field. The red and blue shaded region are guides to eye showing the trail of the two defect peaks D1 and D2. (b) Extracted peak energies of D1 and D2 from Fig. 3(a) showing the similar spectral wandering from the split doublets. (c),(d) The splitting energy from the two doublets are calculated and fit from where the g factors are calculated to be around 4.6 and 4.2 respectively. The resolution of the grating is within the size of the marker used. (e) Valley Zeeman splitting of the trion (black) and neutral exciton (red) in single layer MoSe2. Circularly polarized excitation source is used and signal is collected at the two senses of circular polarization, σ+ (solid curves) and σ−(dotted curves). (f) Extracted splitting energy from (e) fit to equation 2. The error bars represent the CCD pixel size corresponding to the grating used for the spectra in (e).
6. Conclusion
In summary we have studied the optical properties of localized defect-like emission from MoSe2 layers. We observe non-linear behavior of these emissions as a function of power and temperature. Magneto-optical spectroscopy reveals similar g-factor values for the defects as for the delocalized excitons in MoSe2. Previous studies with WSe2 has shown the presence of strain to be a key factor in the formation of these defects [19]. Such localized strain can arise either from exfoliation or kink formation while transferring the flakes onto the substrate. However, theoretical modelling is required to gain further insight into the energy levels of these transitions. While the physical origin of the emitters in both the TMDCs still needs to be identified, there are still many opportunities in the nano-optoelectronics and quantum photonics for these defect light emitters. For example van der Waals heterostructures of 2D materials containing optical active defects can enable the electrical control and injection of the localized excitons. Further, these materials can be easily coupled to other photonic devices, like optical cavities, owing to their monolayer geometry and ease of transfer between substrates. Finally, the additional valley degree of freedom offered by the TMDCs [25] may provide a platform to study spin-valley coupled system in these materials. Note added to proof: During the review of this paper we became aware of the following reference [26].
Acknowledgments
A.N.V acknowledges support from the Institute of Optics, the National Science Foundation (NSF-EFMA-1542707 and NSF-DMR-1553788) and the Air Force Office of Scientific Research (AFOSR-FA9550-16-1-002).
References and links
1. 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, 136805 (2010). [CrossRef]
2. H. Zeng, J. Dai, W. Yao, D. Xiao, and X. Cui, “Valley polarization in MoS2 monolayers by optical pumping,” Nature Nanotech. 7, 490–493 (2012). [CrossRef]
3. K. M. Goodfellow, C. Chakraborty, R. Beams, L. Novotny, and A. N. Vamivakas, “Direct On-Chip Optical Plasmon Detection with an Atomically Thin Semiconductor,” Nano Lett. 15, 5477–5481 (2015). [CrossRef] [PubMed]
4. 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, 1271–1275 (2010). [CrossRef] [PubMed]
5. A. M. Jones, H. Yu, N. J. Ghimire, S. Wu, G. Aivazian, J. S. Ross, B. Zhao, J. Yan, D. G. Mandrus, D. Xiao, W. Yao, and X. Xu, “Optical generation of excitonic valley coherence in monolayer wse2,” Nature Nanotech. 8, 634–638 (2013). [CrossRef]
6. Y. Li, J. Ludwig, T. Low, A. Chernikov, X. Cui, G. Arefe, Y. D. Kim, A. M. van der Zande, A. Rigosi, H. M. Hill, S. H. Kim, J. Hone, Z. Li, D. Smirnov, and T. F. Heinz, “Valley splitting and polarization by the zeeman effect in monolayer mose2,” Phys. Rev. Lett. 113, 266804 (2014). [CrossRef]
7. D. MacNeill, C. Heikes, K. F. Mak, Z. Anderson, A. Kormányos, V. Zólyomi, J. Park, and D. C. Ralph, “Breaking of Valley Degeneracy by Magnetic Field in Monolayer MoSe2,” Physical Review Letters 114, 037401 (2015). [CrossRef]
8. K. F. Mak, K. L. McGill, J. Park, and P. L. McEuen, “The valley Hall effect in MoS2 transistors,” Science 344, 1489–1492 (2014). [CrossRef] [PubMed]
9. G. Wei, D. A. Czaplewski, E. J. Lenferink, T. K. Stanev, I. W. Jung, and N. P. Stern, “Valley Polarization in Size-Tunable Monolayer Semiconductor Quantum Dots,” arXiv:1510.09135 [cond-mat] (2015). ArXiv: 1510.09135.
10. A. Srivastava, M. Sidler, A. V. Allain, D. S. Lembke, A. Kis, and A. Imamoğlu, “Optically active quantum dots in monolayer WSe2,” Nature Nanotech. 10, 491–496 (2015). [CrossRef]
11. Y.-M. He, G. Clark, J. R. Schaibley, Y. He, M.-C. Chen, Y.-J. Wei, X. Ding, Q. Zhang, W. Yao, X. Xu, C.-Y. Lu, and J.-W. Pan, “Single quantum emitters in monolayer semiconductors,” Nature Nanotech. 10, 497–502 (2015). [CrossRef]
12. M. Koperski, K. Nogajewski, A. Arora, V. Cherkez, P. Mallet, J.-Y. Veuillen, J. Marcus, P. Kossacki, and M. Potemski, “Single photon emitters in exfoliated WSe2 structures,” Nature Nanotech. 10, 503–506 (2015). [CrossRef]
13. C. Chakraborty, L. Kinnischtzke, K. M. Goodfellow, R. Beams, and A. N. Vamivakas, “Voltage-controlled quantum light from an atomically thin semiconductor,” Nature Nanotech. 10, 507–511 (2015). [CrossRef]
14. P. Tonndorf, R. Schmidt, R. Schneider, J. Kern, M. Buscema, G. A. Steele, A. Castellanos-Gomez, H. S. J. van der Zant, S. Michaelis de Vasconcellos, and R. Bratschitsch, “Single-photon emission from localized excitons in an atomically thin semiconductor,” Optica 2, 347 (2015). [CrossRef]
15. T. T. Tran, K. Bray, M. J. Ford, M. Toth, and I. Aharonovich, “Quantum emission from hexagonal boron nitride monolayers,” Nature Nanotechnology 11, 37–41 (2016). [CrossRef]
16. A. Castellanos-Gomez, M. Buscema, R. Molenaar, V. Singh, L. Janssen, H. S. J. v. d. Zant, and G. A. Steele, “Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping,” 2D Materials 1, 011002 (2014). [CrossRef]
17. J. S. Ross, S. Wu, H. Yu, N. J. Ghimire, A. M. Jones, G. Aivazian, J. Yan, D. G. Mandrus, D. Xiao, W. Yao, and X. Xu, “Electrical control of neutral and charged excitons in a monolayer semiconductor,” Nat Commun 4, 1474 (2013). [CrossRef] [PubMed]
18. I. Aharonovich, S. Castelletto, D. A. Simpson, A. Stacey, J. McCallum, A. D. Greentree, and S. Prawer, “Two-Level Ultrabright Single Photon Emission from Diamond Nanocrystals,” Nano Letters 9, 3191–3195 (2009). [CrossRef] [PubMed]
19. S. Kumar, A. Kaczmarczyk, and B. D. Gerardot, “Strain-induced spatial and spectral isolation of quantum emitters in Mono- and Bilayer WSe2,” Nano Letters 15, 7567–7573 (2015). PMID: [PubMed] . [CrossRef]
20. T. Schmidt, K. Lischka, and W. Zulehner, “Excitation-power dependence of the near-band-edge photoluminescence of semiconductors,” Physical Review B 45, 8989–8994 (1992). [CrossRef]
21. S. Tongay, J. Suh, C. Ataca, W. Fan, A. Luce, J. S. Kang, J. Liu, C. Ko, R. Raghunathanan, J. Zhou, F. Ogletree, J. Li, J. C. Grossman, and J. Wu, “Defects activated photoluminescence in two-dimensional semiconductors: interplay between bound, charged, and free excitons,” Scientific Reports3, 2657 EP – (2013). [CrossRef] [PubMed]
22. Y. Zhao, C. Riemersma, F. Pietra, R. Koole, C. de Mello Donegá, and A. Meijerink, “High-Temperature Luminescence Quenching of Colloidal Quantum Dots,” ACS Nano 6, 9058–9067 (2012). [CrossRef] [PubMed]
23. W. Q. Cheng, X. G. Xie, Z. Y. Zhong, L.H. Cai, Q. Huang, and J. M. Zhou, “Photoluminescence from InAs quantum dots on GaAs(100),” Thin Solid Films 312, 287–290 (1998). [CrossRef]
24. S. Fafard, S. Raymond, G. Wang, R. Leon, D. Leonard, S. Charbonneau, J. L. Merz, P. M. Petroff, and J. E. Bowers, “Temperature effects on the radiative recombination in self-assembled quantum dots,” Surface Science 361–362, 778–782 (1996). [CrossRef]
25. D. Xiao, G. B. Liu, W. Feng, X. Xu, and W. Yao, “Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides,” Phys. Rev. Lett. 108, 196802 (2012). [CrossRef]
26. A. Branny, B. Wang, S. Kumar, C. Robert, and X. Lassagne, “Discrete quantum dot like emitters in monolayer MoSe2: Spatial mapping, magneto-optics, and charge tuning,” Appl. Phys. Lett. 108, 142101 (2016). [CrossRef]
