Author: Long, Hui
Title: Study of two dimensional materials and its nonlinear optical applications
Advisors: Tsang, Y. H. (AP)
Mak, C. L. (AP)
Degree: Ph.D.
Year: 2016
Subject: Hong Kong Polytechnic University -- Dissertations
Nanostructured materials.
Transition metal compounds.
Department: Department of Applied Physics
Pages: xxviii, 151 pages : color illustrations
Language: English
Abstract: Nonlinear optical (NLO) materials have drawn much attention in the past decades because of the potential applications for protecting delicate optical instruments and human eyes from intense laser beam. A good optical limiter should provide high linear transmission at low power of incident but high non-linear absorption at high power. Two-dimensional transition metal dichalcogenide (TMD) nanosheet e.g. MoS₂, WS ₂, MoSe₂, WSe₂, show great promise for laser photonic applications for their unique properties, such as high layer optical absorption, broad optical band response, large surface area, high mechanical strength, high inplane charge mobility. These 2D materials have strong optical limiting properties for high power near infrared laser. In this study, the nonlinear optical absorption properties of the MoS₂ in liquid N-methyl-2-pyrrolidone (NMP) and solid state matrix Polymethyl Methacrylate (PMMA) have been studied and compared. Also, a simple but effective ultrasonic exfoliation has been used to separate the WS₂, nanosheets into different layer number and size through controlling centrifugation rotation speed. Additionally, the optical limiting property of WS₂ sheet incorporated into PMMA show a close dependence on both size and thickness. Due to the edge and quantum confinement effect, WS₂ quantum dots exhibit the lowest onset threshold (FON) and optical limiting threshold (FOL) among all the samples, which are comparable to the lowest threshold achieved in the graphene based materials. Firstly, MoS₂ powder in suitable size is prepared by hydrothermal process and few-layer MoS₂ nanosheets are prepared using simple ultrasonic exfoliation method. Then MoS₂ nanosheets are homogeneously incorporated into solid state matrix (PMMA) to test its NLO property. Z-scan results of MoS₂,/PMMA composite bulk with 0.016 mg/cm³ MoS₂ nanosheets demonstrate the low NLO FON, 0.01 and 0.04 J/cm², FOL, 0.4 and 1.3 J/cm², low optical limiting differential transmittance (TC), 2% and 3% and high two-phonon absorption coefficient (ß), 70 and 55 cm GW⁻¹ for the nanosecond laser operating at 532 nm and 1064 nm, respectively.
Secondly, the morphology of few layers MoS₂ nanosheet in NMP solution under laser illumination with wavelength of 532 and 1064 nm with various laser energy densities have been analysed. Also, the nonlinear optical limiting properties of the MoS₂ nanosheet in liquid NMP and solid PMMA have been studied and their results are compared. It shows that the nonlinear optical absorption properties of MoS₂ in NMP disappeared but without changes in MoS₂/PMMA composites after laser illumination. The significant changes in NMP caused by the oxidization of molybdenum disulfide (MoS₂) into molybdenum oxide (MoO₃). Due to its stronger absorption in visible range, 532 nm have stronger oxidization effect compared with 1064 nm as illumination wavelength. It confirms that the MoS₂ nanosheet is relatively much more stable within PMMA for preserving nonlinear absorption properties under high laser density illumination. Thirdly, a series of different size and thickness WS₂ nanosheets are obtaind through controlling centrifugation rotation speed effectively. Their surface morphology was characterized by Atomic Force Morphology. The successful size and thickness separation allows the research of thickness-dependent optical limiting properties of WS₂/PMMA composites. Results demonstrate that both NOL FON and FOL of WS2 under the excitation of nanosecond pulsed laser can be tuned over a wide range by controlling its size and thickness. The FON and FOL show a rapid decline with the decrease of size and thickness. The tunable NOL properties have been achieved through the control of size and thickness. Finally, Ultra-small WS2 quantum dots with diameter of 2.4 nanometer are fabricated by ultrasound method followed by high speed centrifugation up to 10000 rpm. Excellent NLO property of the WS₂, QD/ PMMA composite for the nanosecond pulsed laser at both 532 and 1064 nm has been measured. Results illustrate the lower FON, FOL, and higher ß with respect to higher concentration of embedded WS₂ quantum dots. The NOL performance exhibited in WS₂ QDs is comparable to the best result of graphene based materials reported so far, having very low FON (0.01 J/cm², 0.03 J/cm²) and FOL (0.062 J/cm², 0.1 J/cm²) at 532 nm and 1064 nm respectively, which is attributed to the edge and quantum confinement effects. In summary, two-dimensional materials MoS₂ WS₂ nanosheet embedded into solid state matrix PMMA have been studied systematically, and a series of related journal publications has been obtained. These results would advance and further promote the development of two dimensional materials and their optical applications.
Rights: All rights reserved
Access: open access

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