Photonic Crystal Fiber for Efficient Raman Scattering of CdTe Quantum Dots in Aqueous Solution

  • Mak J
  • Farah A
  • Chen F
 et al. 
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C olloidal quantum dots (QDs) are sus-pensions of semiconductor nanopar-ticles that offer both the intriguing optical properties of quantum-confined par-ticles, and the practical advantages of solu-tion-based processing. 1À3 In the past two decades, aqueous synthesis of colloidal QDs has gained much popularity and evolved tremedously. 4À8 This simple and cost-effective technique allows very small (2À6 nm), monodisperse, and highly water-soluble QDs to be synthesized in gram quan-tities. QD capping plays a pivotal role in the properties and utility of the material; the use of short-chain thiols, such as thioglycolic acid (TGA), as capping agents has been shown to greatly improve the photoluminescent (PL) quantum efficiency of as-synthesized QDs to values of 40À60%. 4 3-Mercaptopropionic acid (MPA) has also shown to offer a larger range of size and PL tunability and a longer emission decay time. 4 Other thiols, such as 1Àthioglycerol (TG), are found to be more suitable for synthesizing stable coreÀshell QDs. 7 The unique properties of the different colloidal QDs have potentials for a wide field of novel applications ranging from photovoltaics 9 and optoelectronics, 10 to biosensing, 11 bioimaging, 12 and even cancer treatments. 13 One of the most important technological challenges in QD advancement is the devel-opment of a cost-effective, reliable, and sensitive optical monitoring system to con-trol the physical, chemical, and size-depen-dent properties of QDs before, during, and after their fabrication on a nanometer scale. To measure and compare these properties between the different QDs, many analytical techniques have been employed including but not limited to PL, electrolumines-cence (EL), ultravioletÀvisible spectroscopy (UVÀvis), transmission electron microscope (TEM) or high resolution transmission elec-tron microscope (HRTEM), capillary zone electrophoresis (CZE), X-ray diffraction (XRD), and X-ray photoelectron spectrosco-py (XPS). These techniques provide valuable information on the composition and prop-erties of the QDs; yet, none of them de-scribes how the capping agents interact with the core of the QDs. Their impact on the overall molecular structure, molecular complex, and different QD properties also remains unclear. Consequently, this limits our capability to improve the quantum efficiency, stability, and bioconjugating abil-ity further from what has been achieved today. Complex QD designs for increasing performance and functionalities in different applications remain very challenging. Fourier-transform infrared spectroscopy (FT-IR) can be an alternative for determining the molecular interactions between the QDs and their capping agents. However, the strong and broad absorption bands of water often overlap with those from the QDs and stabilizing agents. This limits the number of vibrational modes that can be resolved. A complementary technique to FT-IR is Raman spectroscopy, which is a rapid and nondestructive means of probing mo-lecular vibrations optically through inelastic

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  • Jacky S W Mak

  • Abdiaziz A Farah

  • Feifan Chen

  • Amr S Helmy

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