|Title:||Nanomanufacturing of plasmonic materials for sensing applications|
|Advisors:||Law, Wing Cheung Roy (ISE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xi, 89 pages : color illustrations|
|Abstract:||In this thesis, a new approach for manufacturing novel plasmonic nanomaterials (i.e. Copper sulfide nanocrystals (Cu₂-xS NCs)) for sensing applications has been developed. Unlike the traditional plasmonic nanomaterials (e.g. gold nanoparticles and nanorods), in which the plasmon resonance is supported by the collection of surface free electrons, the plasmonic features of Cu₂-xS NCs are originated from the copious free holes. Those varied free holes act as the charge carriers and allow for the condition of plasmon resonance. This condition is very sensitive to the dielectric surrounding of nanocrystals, and thus Cu₂-xS NC can serve as a sensing substrate. Indeed, the sensitivity of this plasmonic substrate is governed by several factors (i.e. size, morphology and nonstoichiometric compositions (number of free hole) of nanocrystals). However, there is little empirical research can be found for the synthesis of Cu₂-xS NCs with controlled size, morphology and nonstoichiometric compositions. Some modification techniques have been developed. The main drawbacks were the incorporation of toxic surfactants and the sophisticated preparation procedures. This study presents a non-toxic and efficient approach for the synthesis of Cu₂-xS NCs. Furthermore, their plasmonic properties and sensing capability were also studied. Firstly, a continuous-flow millifluidic device for synthesizing different morphologies and nonstoichiometric compositions of Cu₂-xS NCs is presented. The device is polydimethylsiloxane (PDMS)- based, fabricated using a three-dimensional (3D) printer followed by moulding with PDMS. This millifluidic device acted as a nanomanufacturing platform and offered an ideal incubation environment, such as uniform heating, higher production rate, precise control on the distribution between precursors and the reaction time for incubation, for the growth of NCs,.|
The as-synthesized Cu₂-xS NCs were characterized by the scanning transmission electron microscope (STEM) and powder X-ray diffraction (XRD). It has shown that through the control of the input flow rates, it was manageable to fabricate spherical Cu₂-xS NCs with diameter 3.6 - 12.6 nm and rod-shaped Cu₂-xS NCs with aspect ratio 2.3 - 3.4. In addition, by altering the molar ratios between precursors, it was able to fabricate four different nonstoichiometric compositions of Cu₂-xS NCs including Cu₁.₁S, Cu₁.₃₉S, Cu₁.₇₅S and Cu₁.₉₇S. Secondly, we present an investigation of sensing capability of Cu₂-xS NCs by varying the refractive index around the NCs surfaces. The shift of the localized surface plasmon resonance (LSPR) of Cu₂-xS NCs was measured due to the change of refractive index in the media. It has been shown that different sized Cu₂-xS NCs have a range of sensitivity from 573 nm / RIU to 850 nm / RIU. Similarly, different aspect ratios of the rod-shaped Cu₂-xS NCs have shown a change of sensitivity from 714 nm / RIU to 985 nm / RIU. For the nonstoichiometric compositions of Cu₂-xS NCs, the delivered range of response was 386 nm / RIU to 714 nm / RIU. Furthermore, a plasmonic quenching effect was proposed and demonstrated by the use of o-phenylendiamine (OPD). OPD is a compound which emitted yellow fluorescence (568 nm) when it was oxidized (electrons donator) by the Cu₂-xS NCs (electrons acceptor). As the concentration of OPD increased, more Cu₂-xS NCs were quenched and the fluorescence was enhanced, therefore the plasmonic quenching effect could be observed. It was shown that the LSPR of Cu₂-xS NCs was quenched effectively as the ratio of Cu₂-xS NCs to OPD increased from 0.003 to 600. Besides, the fluorescence was enhanced from 0.2 × 10³ to 1.1 × 10⁶ by distinct morphologies and compositions of Cu₂-xS NCs. All Cu₂-xS NCs / OPD samples showed that the enhancement was optimized when the ratio reached 0.3. The results showed a satisfying signal-to-noise ratio and sensing capability, revealing the potential for the future development of the Cu₂-xS NCs based nanosensor.
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