Open Access
Add to BibSonomyAbstract
Monolayer transition-metal dichalcogenides (TMDs) have grown as fantastic building blocks for optoelectronic applications, owing to their direct band gap, transparency, and mechanical flexibility. Since the luminescence of monolayer TMDs suffers from low light absorption and emission, surface plasmons, which confine light at subwavelength and enhance the local electric field, are utilized to boost both excitation and emission fields of TMDs, enabling strong light-matter interaction at the nano-scale. Meanwhile, radially-polarized beams (RPBs) as new and attractive excitation source have found many applications in surface plasmon polaritons, optical tweezer and so on. Here, by using RPBs, we demonstrate the photoluminescence (PL) enhancement of monolayer molybdenum disulfide (MoS2) hybridized with 210 nm-diameter gold nanoparticle (AuNP) is improved by about 1.37-fold compared with linearly-polarized beams (LPBs). Besides, the PL enhancement with RPBs depends on the size of AuNP as well. With 210nm-diameter AuNP, the PL enhancement is more than 1.5-fold higher than that with 60nm-diameter AuNP. This study highlights that RPBs are superior to LPBs for tuning the near-field system response and shows that RPBs drive a valuable avenue to further study the emerging two-dimentional materials.
© 2016 Optical Society of America
1. Introduction
An emerging field of two-dimensional layer materials, known as transition-metal dichalcogenides (TMDs), has sprung up and drawn the increasing attention of researchers owing to their unique properties [1]. Compared with zero-gap graphene [2], TMDs are considered as the optimum candidate of 2D post-graphene materials due to the presence of band gap ranging from 1 to 2 eV [2–4]. Besides, when mechanically exfoliated from bulk into monolayers, TMDs show layer-dependent property, changing from indirect band gap materials to direct band gap materials. Therefore, TMDs have found many applications in optics and optoelectronics, such as field-effect transistors [5], quantum dots [6,7], photodetectors [8,9], quantum emitters [10], and flexible and transparent optoelectronics [11]. Though the direct band gap of monolayer TMDs enables light absorption, the photoluminescence (PL) of TMDs still suffers from low light emission [12]. Fortunately, surface plasmons, which are localized at the interface between dielectric and metal and can greatly enhance the local electric field at nano-scale, show great potential to boost the emission of monolayer TMDs, and to realize various next-generation devices for nanotechnology [12–15]. Using surface plasmon polaritons (SPPs) to control over the behavior of photons has been widely studied in nanophotonics, data storage, solar cells and bio-sensing [16–18].
Recently, the monolayer molybdenum disulfide (MoS2) hybridized with plasmonic nanoparticles has been demonstrated to be an outstanding platform to deeply study the light-matter interaction [19–22]. Monolayer MoS2 exhibits a direct band gap of 1.8eV, resulting in PL in the visible range [23]. But the PL quantum efficiency of freestanding monolayer MoS2 is demonstrated to be as low as 0.4% (The opacity of graphene is 2.3%) [24,25]. Hence, it is important to enhance the light emission of monolayer MoS2 for further researches, especially for microelectronics where the large-area monolayer MoS2 films are appealing [26–28]. Previous works have demonstrated the PL enhancement of monolayer MoS2 using metal nanoparticles and core-shell structures [14,19,20]. However, those hybrid plasmonic monolayer MoS2 structures are mostly excited with linearly-polarized beams (LPBs), which could excite dipoles in only one direction. Unlike LPBs, radially-polarized beams (RPBs) have axial symmetric polarization vectors along all the radial directions, which means that dipoles of all the directions can be excited at the same time. On the other hand, since RPBs have a strong focus for the longitudinal electric field component, they have been widely utilized for focusing [29], spectroscopy, optical trapping [30] and so on [31,32].
In this paper, we further study the PL of hybrid single gold nanoparticle and monolayer MoS2 (AuNP-1L-MoS2) nanostructure excited by RPBs. In order to understand the physical mechanism about one plasmonic nanoparticle under different polarized beams, we intentionally dilute AuNPs to a fine concentration, easy to find a single particle on monolayer MoS2 to do measurement. We demonstrate the PL enhancement of monolayer MoS2 with a single Au nanoparticle (AuNP). It shows that the enhancement with RPBs is about 1.37-fold higher than that of LPBs. Moreover, similar to LPBs excitation, the PL enhancement under RPBs excitation also depends on the size of AuNP. In our experiment, AuNP with 210 nm diameter has more than 1.5-fold better enhancement compared to AuNP with 60 nm diameter. Our study indicates that radially-polarized beams offer more opportunities in the study of emerging TMDs materials hybridized with nano-scale structures.
2. Experimental
The experimental setup is shown in Fig. 1(a). A continuous-wave laser at 532 nm transmits through a polarizer (and liquid crystal phase-plate (LCPP)) to produce LPBs (RPBs), then excites the sample deposited on a three-axis piezo stage through an objective (OLYMPUS, NA = 0.9, 100 × ). The emissions from the sample are collected with the same objective and detected with a single photon avalanche diode (SPAD). A dichroic plate is used to block the excitation laser and let the PL of MoS2 pass through. Also, for spatial filtering, two lenses (f = 40 mm) together with a pin-hole (30 μm diameter) are used. A white light source going through objective and shinning onto the sample is reflected back to CCD to image the sample. By scanning the sample, we are able to get the PL map of the sample. Except for this, we also use a spectrograph (Princeton Instruments, Acton SP2500) to get the PL spectrum of the sample. Figure 1(b) shows the schematic of the AuNP-1L-MoS2 sample we used. Monolayer MoS2 is grown on the 300 nm-thick silica substrate, then the AuNPs with silica coating (10 nm-thick to avoid quenching [33]) are spin-coated on top of it. The Gaussian beam that is used to shine the hybrid nanostructure has a diameter around 600 nm at focus. In order to avoid exciting multiple AuNPs, we intentionally choose the area with only one AuNP to do measurement. And to prevent damage to monolayer MoS2, the excitation power is kept below 1 mW. All the measurements are performed at room temperature.
Fig. 1 (a) Experimental setup to excite MoS2 and collect the emission from it. LCPP: liquid crystal phase-plate. (b) Schematic representation of the hybrid AuNP-1L-MoS2 heterostructure.
3. Results and discussion
Chemical vapor deposition (CVD) method has been successfully applied to grow two-dimensional crystalline atomic layers of TMDs [34–37]. Here our monolayer MoS2 was grown with high pressure CVD technique, which is able to produce large-area monolayer MoS2 films with high crystalline and uniformity. Figures 2(a) and 2(b) display the microscope image and the atomic force microscopy (AFM) image of our monolayer MoS2. And the inset of Fig. 2(a) is enlarged view of the MoS2 (Cyan area). The Raman spectrum (top panel) and PL spectrum (bottom panel) of the monolayer MoS2 are shown in Fig. 2(c). In the Raman spectrum, two modes E12g (in-plane) and A1g (out-of-plane) separated by ~20 cm−1 are clearly observed, which is a typical spectrum pattern for monolayer MoS2 [38]. And all the samples we used have been identified through the Raman spectra. In the PL spectrum, the strongest peak around 680 nm is also consistent with the results in literatures [23,39].
Fig. 2 (a) Optical microscope image of the CVD MoS2 deposited on SiO2 substrate. Aqua region (gray region) corresponds to MoS2 (SiO2 substrate). (b) AFM image of the CVD MoS2. (c) Raman spectrum (top panel) of the MoS2, in which two Raman modes E12g and A1g are separated by ~20 cm−1, proving that the CVD MoS2 is monolayer. PL intensity spectrum (bottom panel) shows a typical strong emission peak at ∼680 nm.
Figures 3(a) and 3(b) show the scanning electronic microscope (SEM) images of the sample we used. Figure 3(a) corresponds to a AuNP with 60 nm diameter (AuNP60), while Fig. 3(b) shows a AuNP with 210 nm diameter (AuNP210). As we can see from the SEM images, the single AuNPs (Bright spots in the white squares) are sitting on top of the monolayer MoS2. Additionally, there are another two bright spots (Red circles) in the center of MoS2. It's the nucleation site where CVD monolayer MoS2 starts growing. All the samples we measure are just one AuNP on monolayer MoS2, without any other particles around. So that, this Au nanoparticle-monolayer MoS2 hybrid structure can reflect the physical mechanism excited by different polarized beams. Figure 3(c) compares the PL spectra of AuNP210-1L-MoS2 excited by LPBs and RPBs with that of bare monolayer MoS2. Each curve includes two peaks located around 680 nm (A1 band) and 627 nm (B1 band), which is consistent with previous works. According to the three peaks at A1 band, the PL enhancement due to AuNP210 is 1.47 with LPBs and 1.94 with RPBs, respectively. The spectra definitely prove that AuNPs could enhance the PL of MoS2, and RPBs improves the enhancement of AuNPs to be better. It's worth noting that the peak of the PL spectrum at A1 band is actually 681 nm for RPBs, slightly red-shifted compared to that for LPBs (679 nm), which could be explained by hot electrons as follows [40]. Then we switched spectrograph to SPAD and scanned the sample, in order to get 2D PL map across the sample. At each position, the PL we collected should be the integral of the spectrum shown in Fig. 3(c). The PL maps for monolayer MoS2 with AuNP60 and AuNP210 under LPBs and RPBs excitations are displayed in Figs. 3(d)-3(g). And the PL intensities along the white dashed lines are plotted in the insets. It's obvious that the area where AuNP sits has the highest PL intensity, indicating that AuNP could enhance the PL of bare MoS2 in all cases, but with different performance. For AuNP60, the enhancement under RPBs excitation [Fig. 3(e)] is only slightly better than that with LPBs excitation [Fig. 3(d)]. However, for AuNP210, RPBs excitation [Fig. 3(g)] improves the PL enhancement of MoS2 a lot compared to LPBs excitation [Fig. 3(f)]. Note that, the PL intensities under LPBs are lower than that under RPBs for bare monolayer MoS2. That is because RPBs have more components oriented in z-direction than LPBs at the focus, while the photoluminescence results solely from in-plane excitons in MoS2 [41].
Fig. 3 (a-b) SEM images of monolayer MoS2 with (a) AuNP60 and (b) AuNP210 sitting on top. Insets: SEM images of single AuNPs. (c) PL spectra of monolayer MoS2 under different experimental condition. (d-g) PL intensity maps of monolayer MoS2 with (d) AuNP60 excited by LPBs, (e) AuNP60 excited by RPBs, (f) AuNP210 excited by LPBs, (g) AuNP210 excited by RPBs, respectively. Insets: PL intensity plots along the white dashed lines for each case.
With the same power, the electric field on the monolayer MoS2 of RPBs is stronger and denser than that of LPBs, which would produce larger quantity of hot electrons. Therefore, more hot electrons are transferred to the conduction band of MoS2, resulting in the slight red-shift of PL peak. The plot of PL intensity depending on incident laser power [Fig. 4], in which the PL intensity increases slower at high excitation power, also verifies the hot electrons effect. In Fig. 4, the PL intensities of 1L-MoS2 and the hybrid AuNP-1L-MoS2 structure show a sublinear dependence on incident laser power. The diameter of AuNP we used is 210 nm. When the power of laser we applied is gradually tuned from 5 to 300 μW, both PL intensities of bare 1L-MoS2 and AuNP-1L-MoS2 are increased swiftly at the beginning, followed with a slow trend in the end. According to the plot, the PL intensity of AuNP-1L-MoS2 increases fast until the laser power reaches 60 μW, and then increases slowly. It can be explained as follows. When the excitation laser power increases, the local electric field which is enhanced by surface plasmons becomes stronger, leading to increasing PL intensity. On the other hand, the hot electrons generated due to photothermal effect, are also transferred to the conduction band of MoS2 and contribute to the PL. However, as the excitation power further increases, the hot electrons would get saturated, slowing down the growing rate of MoS2 PL. Note, since there is no SPP enhancement in the local electric field for bare monolayer MoS2, the hot electrons get saturated at a higher excitation threshold (100 μW) in this case.
Fig. 4 PL intensities of 1L-MoS2 and the hybrid AuNP-1L-MoS2 structure under different incident laser power.
To characterize the improvement accurately, we introduce the enhancement factor as [13]
where I is the PL intensity of MoS2 with AuNP, and I0 is the PL intensity of bare MoS2. Then we obtained the enhancement factors for the four cases, which are 1.7 [Fig. 3(d)], 1.8 [Fig. 3(e)], 2.3 [Fig. 3(f)], and 3.5 [Fig. 3(g)], respectively. Furthermore, we repeated measurements for another two groups of samples, in which the diameters of AuNPs are around 60 nm and 210 nm as well. The enhancement factors for one group are 1.5 (AuNP60, LPBs), 1.6 (AuNP60, RPBs), 1.75 (AuNP210, LPBs), and 2.2 (AuNP210, RPBs), respectively. And for the other group, the corresponding enhancement factors are 1.9, 2.0, 2.47, 3.25, respectively. Here, I0 is taken as the average of the PL intensity excluding the peak. As we can see, using RPBs improves the enhancement factor by 1.37 ± 0.14 times for AuNP210, while only a little bit (1.06 ± 0.01 times) for AuNP60. It's probably because the collection area of our system is about 600 nm wide while the diameter of AuNP is just 60 nm, so that only a small part of the collected PL emission is from AuNP enhancement. We believe that the improvement of enhancement factor due to RPBs would be more remarkable if the collection area is smaller. If we use the plasmonic nanoparticle arrays, this enhancement effect theoretically can be achieved great. However there are other sophisticated problems about plasmonic nanoparticle arrays, this is not conducive to distinctly compare RPBs to LPBs. So what we choose can more clearly study the inside of the physical mechanism.We further did numerical simulations with COMSOL Multiphysics. In the simulations, excitation power is fixed, in order to compare the effects of different AuNPs and different beams. In Fig. 5, the results for both AuNP60 and AuNP210 under LPBs and RPBs excitations are shown. Figures 5(a) and 5(b) display the electric field (E-field) distributions around AuNP60 under LPBs excitation with cross-section view and top view respectively. Note, the top view is the E-field distribution in the MoS2. As can be seen, E-field has two lobes and is concentrated on the two sides of AuNP. While under RPBs excitation [Figs. 5(c) and 5(d)], the E-field is mostly confined in the area below the AuNP, enabling stronger interaction with monolayer MoS2. The same results are plotted for AuNP210 [Figs. 5(e)-5(h)]. According to Figs. 5(g) and 5(h), the E-field also gets enhanced in the area under AuNP for RPBs, rather than the two sides for LPBs [Figs. 5(e) and 5(f)]. As expected, the E-field for AuNP210 under RPBs excitation is stronger than that under LPBs condition, and E-field for AuNP210 overall is stronger than that for AuNP60 no matter if it is excited by LPBs or RPBs. These no doubt validate our experimental results.
Fig. 5 (a-b) Electric field distributions of the hybrid AuNP60-1L-MoS2 nanostructure under LPBs excitation, with (a) cross-section view and (b) top view. (c-d) Electric field distributions of the hybrid AuNP60-1L-MoS2 nanostructure under RPBs excitation, with (c) cross-section view and (d) top view. (e-f) Electric field distributions of the hybrid AuNP210-1L-MoS2 nanostructure under LPBs excitation, with (e) cross-section view and (f) top view. (g-h) Electric field distributions of the hybrid AuNP210-1L-MoS2 nanostructure under RPBs excitation, with (g) cross-section view and (h) top view. All the top view images are the electric field distributions in the monolayer MoS2. All the white lines represent for monolayer MoS2.
To obtain the quantitative enhancement, it's clearer to compare the integration over abs(E)^2 at the plane of MoS2, then normalize it with the excitation power. We define the ratio of this normalized integrals as enhancement. The theoretical enhancements with RPBs are improved by 1.47-fold and 1.35-fold compared to LPBs, for AuNP60 and AuNP210, respectively. Although photoluminescence results solely from in-plane excitons [41] as mentioned above, here we take the total electromagnetic field into consideration. Because due to the presence of AuNPs, the hot electrons may also be involved in the interaction between excitation light and MoS2 [40], which are transferred to the conduction band of MoS2 and contribute to the PL emission. In our experiment, the enhancement for AuNP60 is not so remarkable. It’s probably because the size of AuNP is much smaller than that of excitation laser beam, thus a lot of PL emissions from bare MoS2 are collected and cover up the contribution of AuNPs.
4. Conclusions
In conclusion, we demonstrated that plasmonic nanoparticles can enhance the photoluminescence emission of MoS2 due to the couplings with both excitation and emission field. And using radially-polarized beams can further improve the enhancement of plasmonic nanoparticles compared to using linearly-polarized beams. Note that, in our experiment, single nanoparticle is used to clearly characterize the effect of the size of nanoparticle and the excitation polarization on photoluminescence emission. We believe that the enhancement would be more significant as the quantity of plasmonic nanoparticles increases. Furthermore, we plan to use radially polarized beams to excite gap plasmons by coating gold film under MoS2, which will result in better enhancement as well. Our works drive a valuable way for the development of next-generation optoelectronic devices to further exploit the unique optical and electronic properties of two dimensional materials.
Acknowledgments
D. L. and L. Y. contributed equally to this work. This work was funded by the National Key R & D Program of China (No. 2016YFA0301700), the Strategic Priority Research Program of the CAS (No. XDB01030000), the Innovation Funds from the Chinese Academy of Sciences (No. 60921091), the Open Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2015KF12), the National Natural Science Foundation of China (Nos. 11304301, 11374289, 11575172, 61306150, 61590932, and 91421303), and the Fundamental Research Funds for the Central Universities.
References and links
1. G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee, and L. Colombo, “Electronics based on two-dimensional materials,” Nat. Nanotechnol. 9(10), 768–779 (2014). [CrossRef] [PubMed]
2. A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nat. Mater. 6(3), 183–191 (2007). [CrossRef] [PubMed]
3. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides,” Nat. Nanotechnol. 7(11), 699–712 (2012). [CrossRef] [PubMed]
4. D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam, “Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides,” ACS Nano 8(2), 1102–1120 (2014). [CrossRef] [PubMed]
5. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nat. Nanotechnol. 6(3), 147–150 (2011). [CrossRef] [PubMed]
6. X.-X. Song, D. Liu, V. Mosallanejad, J. You, T.-Y. Han, D.-T. Chen, H.-O. Li, G. Cao, M. Xiao, G.-C. Guo, and G.-P. Guo, “A gate defined quantum dot on the two-dimensional transition metal dichalcogenide semiconductor WSe2.,” Nanoscale 7(40), 16867–16873 (2015). [CrossRef] [PubMed]
7. X.-X. Song, Z.-Z. Zhang, J. You, D. Liu, H.-O. Li, G. Cao, M. Xiao, and G.-P. Guo, “Temperature dependence of Coulomb oscillations in a few-layer two-dimensional WS2 quantum dot,” Sci. Rep. 5, 16113 (2015). [CrossRef] [PubMed]
8. W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G.-B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo, and S. Kim, “High-detectivity multilayer MoS(2) phototransistors with spectral response from ultraviolet to infrared,” Adv. Mater. 24(43), 5832–5836 (2012). [CrossRef] [PubMed]
9. O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, “Ultrasensitive photodetectors based on monolayer MoS2.,” Nat. Nanotechnol. 8(7), 497–501 (2013). [CrossRef] [PubMed]
10. 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,” Nat. Nanotechnol. 10(6), 497–502 (2015). [CrossRef] [PubMed]
11. M. Osada and T. Sasaki, “Two-dimensional dielectric nanosheets: novel nanoelectronics from nanocrystal building blocks,” Adv. Mater. 24(2), 210–228 (2012). [CrossRef] [PubMed]
12. S. Butun, S. Tongay, and K. Aydin, “Enhanced light emission from large-area monolayer MoS2 using plasmonic nanodisc arrays,” Nano Lett. 15(4), 2700–2704 (2015). [CrossRef] [PubMed]
13. S. Najmaei, A. Mlayah, A. Arbouet, C. Girard, J. Léotin, and J. Lou, “Plasmonic pumping of excitonic photoluminescence in hybrid MoS2-Au nanostructures,” ACS Nano 8(12), 12682–12689 (2014). [CrossRef] [PubMed]
14. A. Sobhani, A. Lauchner, S. Najmaei, C. Ayala-Orozco, F. Wen, J. Lou, and N. J. Halas, “Enhancing the photocurrent and photoluminescence of single crystal monolayer MoS2 with resonant plasmonic nanoshells,” Appl. Phys. Lett. 104(3), 031112 (2014). [CrossRef]
15. K. C. J. Lee, Y.-H. Chen, H.-Y. Lin, C.-C. Cheng, P.-Y. Chen, T.-Y. Wu, M.-H. Shih, K.-H. Wei, L.-J. Li, and C.-W. Chang, “Plasmonic gold nanorods coverage influence on enhancement of the photoluminescence of two-dimensional MoS2 monolayer,” Sci. Rep. 5, 16374 (2015). [CrossRef] [PubMed]
16. N. Engheta, “Circuits with light at nanoscales: optical nanocircuits inspired by metamaterials,” Science 317(5845), 1698–1702 (2007). [CrossRef] [PubMed]
17. B. Radisavljevic and A. Kis, “Mobility engineering and a metal-insulator transition in monolayer MoS2,” Nat. Mater. 12(9), 815–820 (2013). [CrossRef] [PubMed]
18. Y. Cui, I. Y. Phang, R. S. Hegde, Y. H. Lee, and X. Y. Ling, “Plasmonic silver nanowire structures for two-dimensional multiple-digit molecular data storage application,” ACS Photonics 1(7), 631–637 (2014). [CrossRef]
19. W. Gao, Y. H. Lee, R. Jiang, J. Wang, T. Liu, and X. Y. Ling, “Localized and Continuous Tuning of Monolayer MoS2 Photoluminescence Using a Single Shape-Controlled Ag Nanoantenna,” Adv. Mater. 28(4), 701–706 (2016). [CrossRef] [PubMed]
20. J. Miao, W. Hu, Y. Jing, W. Luo, L. Liao, A. Pan, S. Wu, J. Cheng, X. Chen, and W. Lu, “Surface Plasmon-Enhanced Photodetection in Few Layer MoS2 Phototransistors with Au Nanostructure Arrays,” Small 11(20), 2392–2398 (2015). [CrossRef] [PubMed]
21. Y. Kang, S. Najmaei, Z. Liu, Y. Bao, Y. Wang, X. Zhu, N. J. Halas, P. Nordlander, P. M. Ajayan, J. Lou, and Z. Fang, “Plasmonic hot electron induced structural phase transition in a MoS2 monolayer,” Adv. Mater. 26(37), 6467–6471 (2014). [CrossRef] [PubMed]
22. B. Lee, J. Park, G. H. Han, H.-S. Ee, C. H. Naylor, W. Liu, A. T. C. Johnson, and R. Agarwal, “Fano resonance and spectrally modified photoluminescence enhancement in monolayer MoS2 integrated with plasmonic nanoantenna array,” Nano Lett. 15(5), 3646–3653 (2015). [CrossRef] [PubMed]
23. 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]
24. 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]
25. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008). [CrossRef] [PubMed]
26. Y. Zhan, Z. Liu, S. Najmaei, P. M. Ajayan, and J. Lou, “Large-area vapor-phase growth and characterization of MoS(2) atomic layers on a SiO(2) substrate,” Small 8(7), 966–971 (2012). [CrossRef] [PubMed]
27. K.-K. Liu, W. Zhang, Y.-H. Lee, Y.-C. Lin, M.-T. Chang, C.-Y. Su, C.-S. Chang, H. Li, Y. Shi, H. Zhang, C.-S. Lai, and L.-J. Li, “Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates,” Nano Lett. 12(3), 1538–1544 (2012). [CrossRef] [PubMed]
28. Y.-H. Lee, X.-Q. Zhang, W. Zhang, M.-T. Chang, C.-T. Lin, K.-D. Chang, Y.-C. Yu, J. T.-W. Wang, C.-S. Chang, L.-J. Li, and T.-W. Lin, “Synthesis of large-area MoS2 atomic layers with chemical vapor deposition,” Adv. Mater. 24(17), 2320–2325 (2012). [CrossRef] [PubMed]
29. R. Dorn, S. Quabis, and G. Leuchs, “Sharper focus for a radially polarized light beam,” Phys. Rev. Lett. 91(23), 233901 (2003). [CrossRef] [PubMed]
30. Q. Zhan, “Trapping metallic Rayleigh particles with radial polarization,” Opt. Express 12(15), 3377–3382 (2004). [CrossRef] [PubMed]
31. D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). [CrossRef] [PubMed]
32. N. M. Mojarad and M. Agio, “Tailoring the excitation of localized surface plasmon-polariton resonances by focusing radially-polarized beams,” Opt. Express 17(1), 117–122 (2009). [CrossRef] [PubMed]
33. U. Bhanu, M. R. Islam, L. Tetard, and S. I. Khondaker, “Photoluminescence quenching in gold - MoS2 hybrid nanoflakes,” Sci. Rep. 4, 5575 (2014). [CrossRef] [PubMed]
34. A. M. van der Zande, P. Y. Huang, D. A. Chenet, T. C. Berkelbach, Y. You, G.-H. Lee, T. F. Heinz, D. R. Reichman, D. A. Muller, and J. C. Hone, “Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide,” Nat. Mater. 12(6), 554–561 (2013). [CrossRef] [PubMed]
35. S. Najmaei, Z. Liu, W. Zhou, X. Zou, G. Shi, S. Lei, B. I. Yakobson, J.-C. Idrobo, P. M. Ajayan, and J. Lou, “Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers,” Nat. Mater. 12(8), 754–759 (2013). [CrossRef] [PubMed]
36. X. Wang, H. Feng, Y. Wu, and L. Jiao, “Controlled synthesis of highly crystalline MoS2 flakes by chemical vapor deposition,” J. Am. Chem. Soc. 135(14), 5304–5307 (2013). [CrossRef] [PubMed]
37. S. Tongay, W. Fan, J. Kang, J. Park, U. Koldemir, J. Suh, D. S. Narang, K. Liu, J. Ji, J. Li, R. Sinclair, and J. Wu, “Tuning interlayer coupling in large-area heterostructures with CVD-grown MoS2 and WS2 monolayers,” Nano Lett. 14(6), 3185–3190 (2014). [CrossRef] [PubMed]
38. 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]
39. H. Nan, Z. Wang, W. Wang, Z. Liang, Y. Lu, Q. Chen, D. He, P. Tan, F. Miao, X. Wang, J. Wang, and Z. Ni, “Strong photoluminescence enhancement of MoS(2) through defect engineering and oxygen bonding,” ACS Nano 8(6), 5738–5745 (2014). [CrossRef] [PubMed]
40. Z. Li, Y. Xiao, Y. Gong, Z. Wang, Y. Kang, S. Zu, P. M. Ajayan, P. Nordlander, and Z. Fang, “Active light control of the MoS2 monolayer exciton binding energy,” ACS Nano 9(10), 10158–10164 (2015). [CrossRef] [PubMed]
41. J. A. Schuller, S. Karaveli, T. Schiros, K. He, S. Yang, I. Kymissis, J. Shan, and R. Zia, “Orientation of luminescent excitons in layered nanomaterials,” Nat. Nanotechnol. 8(4), 271–276 (2013). [CrossRef] [PubMed]
