Mechanistic Understanding of Excitation-Correlated Nonlinear Optical Properties in MoS2 Nanosheets and Nanodots: The Role of Exciton Resonance

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• 作者：Jianhui Sun ; Yan-Juan Gu ; Dang Yuan LeiShu Ping Lau ; Wing-Tak Wong ; Kwok-Yin Wong ; Helen Lai-Wai Chan
• 刊名：ACS Photonics
• 出版年：2016
• 出版时间：December 21, 2016
• 年：2016
• 卷：3
• 期：12
• 页码：2434-2444
• 全文大小：897K
• ISSN：2330-4022

文摘

Low-dimensional molybdenum disulfide (MoS₂) materials such as nanosheets and nanodots exhibit exotic optical, electronic, and catalytic properties, but so far the limited understanding of their nonlinear optical properties has restricted their potential use in nonlinear optoelectronics. In this work, we demonstrate that chemically prepared MoS₂ nanosheets and nanodots have distinctive nonlinear emission properties: the former shows efficient second harmonic generation (SHG) with maximum intensity at its C-exciton resonance energy while the latter exhibits strong excitation-correlated two-photon luminescence (TPL). We combine two-photon photoluminescence excitation (TPPLE) and the Z-scan spectroscopies to study the second order response of the MoS₂ nanodots and reveal, for the first time, that the most efficient and tunable TPL occurs through resonant two-photon absorption (TPA) induced transition from the 1S_h to 1P_e exciton state, followed by phonon-mediated exciton relaxation (1P_e → 1S_e) and fast transition to surface states at different energies. With this novel nonlinear spectroscopy approach, we determine the energy splitting between the 1S_e and 1P_e excitonic states to be 0.3 eV, which is slightly larger than that for MoS₂ monolayers, probably due to the stronger quantum confinement effect in the nanodots. We also observe a clear TPA saturation behavior in the MoS₂ NDs, and this is attributed to the state-filling effect in the 6-fold degenerate 1P_e state. Finally, we demonstrate that these new fundamental understandings of the nonlinear absorption and emission properties of the MoS₂ NDs are critical for optimizing the performance of MoS₂ TPL-based multicolor cellular imaging.