Source: http://aoot.osa.org/ome/abstract.cfm?uri=ome-9-4-1864
Timestamp: 2019-04-24 04:03:52+00:00

Document:
Lateral heterojunction (HJ) of two-dimensional transition metal dichalcogenides has various optoelectronic applications that utilize in-plane charge separation. However, it has been difficult to identify charge transfer characteristics at HJ due to the limited spatial resolution of optical spectroscopy. In this study, near-field scanning optical microscopy is used to directly image the exciton separation occurring at the lateral MoSe2/WSe2 HJ, which was found to be ∼370 nm in spatial width. Efficient charge separation at HJ was confirmed by inspecting local variations of trion and exciton emissions of MoSe2 and WSe2.
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).
B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nat. Nanotechnol. 6(3), 147–150 (2011).
K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, “Tightly bound trions in monolayer MoS2,” Nat. Mater. 12(3), 207–211 (2013).
A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, O. B. Aslan, D. R. Reichman, M. S. Hybertsen, and T. F. Heinz, “Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2,” Phys. Rev. Lett. 113(7), 076802 (2014).
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).
W. S. Yun, S. W. Han, S. C. Hong, I. G. Kim, and J. D. 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).
M. S. Kim, S. J. Yun, Y. Lee, C. Seo, G. H. Han, K. K. Kim, Y. H. Lee, and J. Kim, “Biexciton Emission from Edges and Grain Boundaries of Triangular WS2 Monolayers,” ACS Nano 10(2), 2399–2405 (2016).
S. Mouri, Y. Miyauchi, and K. Matsuda, “Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping,” Nano Lett. 13(12), 5944–5948 (2013).
S. Tongay, J. Zhou, C. Ataca, J. Liu, J. S. Kang, T. S. Matthews, L. You, J. Li, J. C. Grossman, and J. Wu, “Broad-Range Modulation of Light Emission in Two-Dimensional Semiconductors by Molecular Physisorption Gating,” Nano Lett. 13(6), 2831–2836 (2013).
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 MoS2 through Defect Engineering and Oxygen Bonding,” ACS Nano 8(6), 5738–5745 (2014).
N. Peimyoo, W. Yang, J. Shang, X. Shen, Y. Wang, and T. Yu, “Chemically Driven Tunable Light Emission of Charged and Neutral Excitons in Monolayer WS2,” ACS Nano 8(11), 11320–11329 (2014).
K. P. Dhakal, D. L. Duong, J. Lee, H. Nam, M. Kim, M. Kan, Y. H. Lee, and J. Kim, “Confocal absorption spectral imaging of MoS2: optical transitions depending on the atomic thickness of intrinsic and chemically doped MoS2,” Nanoscale 6(21), 13028–13035 (2014).
M. Amani, D.-H. Lien, D. Kiriya, J. Xiao, A. Azcatl, J. Noh, S. R. Madhvapathy, R. Addou, S. KC, M. Dubey, K. Cho, R. M. Wallace, S.-C. Lee, J.-H. He, J. W. Ager, X. Zhang, E. Yablonovitch, and A. Javey, “Near-unity photoluminescence quantum yield in MoS2,” Science 350(6264), 1065–1068 (2015).
H.-V. Han, A.-Y. Lu, L.-S. Lu, J.-K. Huang, H. Li, C.-L. Hsu, Y.-C. Lin, M.-H. Chiu, K. Suenaga, C.-W. Chu, H.-C. Kuo, W.-H. Chang, L.-J. Li, and Y. Shi, “Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment,” ACS Nano 10(1), 1454–1461 (2016).
M. Amani, P. Taheri, R. Addou, G. H. Ahn, D. Kiriya, D.-H. Lien, J. W. Ager, R. M. Wallace, and A. Javey, “Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides,” Nano Lett. 16(4), 2786–2791 (2016).
J. Kang, S. Tongay, J. Zhou, J. Li, and J. Wu, “Band offsets and heterostructures of two-dimensional semiconductors,” Appl. Phys. Lett. 102(1), 012111 (2013).
M. S. Kim, C. Seo, H. Kim, J. Lee, D. H. Luong, J.-H. Park, G. H. Han, and J. Kim, “Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe2/MoS2 Heterostacked Multilayers,” ACS Nano 10(6), 6211–6219 (2016).
H. M. Hill, A. F. Rigosi, K. T. Rim, G. W. Flynn, and T. F. Heinz, “Band Alignment in MoS2/WS2 Transition Metal Dichalcogenide Heterostructures Probed by Scanning Tunneling Microscopy and Spectroscopy,” Nano Lett. 16(8), 4831–4837 (2016).
K. W. Lau, Calvin, Z. Gong, H. Yu, and W. Yao, “Interface excitons at lateral heterojunctions in monolayer semiconductors,” Phys. Rev. B 98(11), 115427 (2018).
X. Hong, J. Kim, S.-F. Shi, Y. Zhang, C. Jin, Y. Sun, S. Tongay, J. Wu, Y. Zhang, and F. Wang, “Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures,” Nat. Nanotechnol. 9(9), 682–686 (2014).
F. Ceballos, M. Z. Bellus, H.-Y. Chiu, and H. Zhao, “Ultrafast Charge Separation and Indirect Exciton Formation in a MoS2–MoSe2 van der Waals Heterostructure,” ACS Nano 8(12), 12717–12724 (2014).
B. Miller, A. Steinhoff, B. Pano, J. Klein, F. Jahnke, A. Holleitner, and U. Wurstbauer, “Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures,” Nano Lett. 17(9), 5229–5237 (2017).
J. Yuan, S. Najmaei, Z. Zhang, J. Zhang, S. Lei, P. M. Ajayan, B. I. Yakobson, and J. Lou, “Photoluminescence Quenching and Charge Transfer in Artificial Heterostacks of Monolayer Transition Metal Dichalcogenides and Few-Layer Black Phosphorus,” ACS Nano 9(1), 555–563 (2015).
M. P. Levendorf, C.-J. Kim, L. Brown, P. Y. Huang, R. W. Havener, D. A. Muller, and J. Park, “Graphene and boron nitride lateral heterostructures for atomically thin circuitry,” Nature 488(7413), 627–632 (2012).
Y. Gong, J. Lin, X. Wang, G. Shi, S. Lei, Z. Lin, X. Zou, G. Ye, R. Vajtai, B. I. Yakobson, H. Terrones, M. Terrones, K. Tay, J. Beng, S. T. Lou, Z. Pantelides, W. Liu, P. M. Zhou, and Ajayan, “Vertical and in-plane heterostructures from WS2/MoS2 monolayers,” Nat. Mater. 13(12), 1135–1142 (2014).
C. Huang, S. Wu, A. M. Sanchez, J. J. P. Peters, R. Beanland, J. S. Ross, P. Rivera, W. Yao, D. H. Cobden, and X. Xu, “Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors,” Nat. Mater. 13(12), 1096–1101 (2014).
M.-Y. Li, Y. Shi, C.-C. Cheng, L.-S. Lu, Y.-C. Lin, H.-L. Tang, M.-L. Tsai, C.-W. Chu, K.-H. Wei, J.-H. He, W.-H. Chang, K. Suenaga, and L.-J. Li, “Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface,” Science 349(6247), 524–528 (2015).
F. Ullah, Y. Sim, C. T. Le, M.-J. Seong, J. I. Jang, S. H. Rhim, B. C. Tran Khac, K.-H. Chung, K. Park, Y. Lee, K. Kim, H. Y. Jeong, and Y. S. Kim, “Growth and Simultaneous Valleys Manipulation of Two-Dimensional MoSe2-WSe2 Lateral Heterostructure,” ACS Nano 11(9), 8822–8829 (2017).
W.-T. Hsu, L.-S. Lu, D. Wang, J.-K. Huang, M.-Y. Li, T.-R. Chang, Y.-C. Chou, Z.-Y. Juang, H.-T. Jeng, L.-J. Li, and W.-H. Chang, “Evidence of indirect gap in monolayer WSe2,” Nat. Commun. 8(1), 929 (2017).
D. A. Vithanage, A. Devižis, V. Abramavičius, Y. Infahsaeng, D. Abramavičius, R. C. I. MacKenzie, P. E. Keivanidis, A. Yartsev, D. Hertel, J. Nelson, V. Sundström, and V. Gulbinas, “Visualizing charge separation in bulk heterojunction organic solar cells,” Nat. Commun. 4(1), 2334 (2013).
Y. Lee, S. Park, H. Kim, G. H. Han, Y. H. Lee, and J. Kim, “Characterization of the structural defects in CVD-grown monolayered MoS2 using near-field photoluminescence imaging,” Nanoscale 7(28), 11909–11914 (2015).
Y. Lee, S. J. Yun, Y. Kim, M. S. Kim, G. H. Han, A. K. Sood, and J. Kim, “Near-field spectral mapping of individual exciton complexes of monolayer WS2 correlated with local defects and charge population,” Nanoscale 9(6), 2272–2278 (2017).
Y. Kim, Y. Lee, H. Kim, S. Roy, and J. Kim, “Near-field exciton imaging of chemically treated MoS2 monolayers,” Nanoscale 10(18), 8851–8858 (2018).
K.-D. Park, O. Khatib, V. Kravtsov, G. Clark, X. Xu, and M. B. Raschke, “Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control,” Nano Lett. 16(4), 2621–2627 (2016).
K.-D. Park, T. Jiang, G. Clark, X. Xu, and M. B. Raschke, “Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect,” Nat. Nanotechnol. 13(1), 59–64 (2018).
W. Xue, P. K. Sahoo, J. Liu, H. Zong, X. Lai, S. Ambardar, and D. V. Voronine, “Nano-optical imaging of monolayer MoSe2-WSe2 lateral heterostructure with subwavelength domains,” J. Vac. Sci. Technol., A 36(5), 05G502 (2018).
C. Tang, Z. He, W. Chen, S. Jia, J. Lou, and D. V. Voronine, “Quantum plasmonic hot-electron injection in lateral WSe2/MoSe2 heterostructures,” Phys. Rev. B 98(4), 041402 (2018).
H. Kim, G. H. Han, S. J. Yun, J. Zhao, D. H. Keum, H. Y. Jeong, T. H. Ly, Y. Jin, J.-H. Park, B. H. Moon, S.-W. Kim, and Y. H. Lee, “Role of alkali metal promoter in enhancing lateral growth of monolayer transition metal dichalcogenides,” Nanotechnology 28(36), 36LT01 (2017).
S. Park, H. Kim, M. S. Kim, G. H. Han, and J. Kim, “Dependence of Raman and absorption spectra of stacked bilayer MoS2 on the stacking orientation,” Opt. Express 24(19), 21551–21559 (2016).
W. Zhao, Z. Ghorannevis, K. K. Amara, J. R. Pang, M. Toh, X. Zhang, C. Kloc, P. H. Tan, and G. Eda, “Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2,” Nanoscale 5(20), 9677–9683 (2013).
P. Tonndorf, R. Schmidt, P. Böttger, X. Zhang, J. Börner, A. Liebig, M. Albrecht, C. Kloc, O. Gordan, D. R. T. Zahn, S. Michaelis de Vasconcellos, and R. Bratschitsch, “Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2,” Opt. Express 21(4), 4908–4916 (2013).
G. W. Shim, K. Yoo, S.-B. Seo, J. Shin, D. Y. Jung, I.-S. Kang, C. W. Ahn, B. J. Cho, and S.-Y. Choi, “Large-Area Single-Layer MoSe2 and Its van der Waals Heterostructures,” ACS Nano 8(7), 6655–6662 (2014).
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(1), 1474 (2013).
T. Yan, X. Qiao, X. Liu, P. Tan, and X. Zhang, “Photoluminescence properties and exciton dynamics in monolayer WSe2,” Appl. Phys. Lett. 105(10), 101901 (2014).
J. Huang, T. B. Hoang, and M. H. Mikkelsen, “Probing the origin of excitonic states in monolayer WSe2,” Sci. Rep. 6(1), 22414 (2016).
Fig. 1. (a) Optical image of MoSe2/WSe2 lateral HJ made between outer (WSe2) and inner (MoSe2) crystal domains. Blue dashed line indicates domain boundary. (b) Raman spectra of A1g modes of WSe2 (upper) and MoSe2 (lower), respectively. Insets: Raman mapping image of A1g modes of WSe2 (upper) and MoSe2 (lower), respectively. The contrast of MoSe2 Raman signal is not completely dark in the WSe2 region because of spectral overlap between the Raman A1g modes of MoSe2 and WSe2, and relatively higher Raman intensity of MoSe2. (c) PL spectra obtained from P1 through P4 positions shown in Fig. 1(a). Vertical lines are visual guides.
Fig. 2. Estimation of spatial resolution of the near-field (NF) PL and confocal PL imaging. (a) Confocal and (b) NF PL image of monolayer WSe2. (c) Line profiles of the same region obtained across the crack as indicated in (a, b) by dotted lines. The spatial resolutions of the NF PL and confocal PL images were estimated to be 140 nm and 600 nm, respectively.
Fig. 3. (a) NF PL image of MoSe2/WSe2 lateral heterojunction (HJ) (indicated by yellow arrow) composed of WSe2 (outer) and MoSe2 (inner) monolayer domains. Inset: line profile obtained across the HJ (black dash line). (b) Schematic of the sample structure and band alignment of MoSe2 and WSe2. CBM: conduction band minimum. VBM: valence band maximum.
Fig. 4. (a) NF PL image of MoSe2/WSe2 lateral HJ. (b) Representative seven PL spectra obtained from white dashed line in Fig. 3(a), red: fitting curve, pink: A− of MoSe2, magenta: A0 of MoSe2, green: A− of WSe2, blue: A0 of WSe2. (c) A plot of the A−/A0 of WSe2 (upper) and MoSe2 (lower) calculated from the result shown in Fig. 3(b). The inset displays the band alignment of MoSe2/WSe2 lateral HJ.

References: V. 
 V. 
 V. 
 V. 
 V. 
 V. 
 V.