Optical properties of chiral plasmonic nanoparticles and mesoporous silicon nanowires

  • Junjun Liu

Student thesis: Doctoral Thesis


Structural engineering plays an essential role in controlling the optical properties of nanostructures, which are of fundamental and practical interest in nanoscience and technology. In this study, two kinds of nanostructural engineering were investigated systematically to enrich nano-optics research: structural helicity was imposed on plasmonic nanoparticles (NPs) with chiroptical activity engineerable in the ultraviolet (UV)-visible region, and porosification was imposed on silicon nanowires (SiNWs) to tune optical interaction and photoluminescence (PL).. The generation of helical metamaterials, which have strong, engineerable chiroptical activity in the UV-visible region, has attracted increasing attention due to the manipulation of the circular polarization state of light to develop diverse homochirality-associated bio-applications. Glancing-angle deposition with fast substrate rotation is performed to generate plasmonic helical NPs (PhNPs) with a helical pitch (P) of less than 10 nm, which is so much smaller than the wire diameter (d) that the PhNPs appear to be achiral NPs. The PhNPs exhibit chiroptical activity that originates intrinsically from hidden helicity, characterized by circular dichroism (CD). With an increase of P from 3 to 66 nm, the plasmonic CD signals barely shift but show a logarithmic amplification. PhNPs made of aluminum, silver, and copper exhibit a stable chiroptical response from the deep UV (~220 nm) region to the visible region. When an achiral plasmonic nanostructure guest is coated on a PhNP host (i.e., a chiral host@achiral guest nanostructure is created), the achiral guest becomes chiroptically active due to helicity transfer from the chiral host to the achiral guest. Such a helicity transfer can be generally adapted to diverse plasmonic metals to tailor the plasmonic chiroptical response flexibly in the UV-visible region. Furthermore, an amplification of the near-field optical chirality induced by the PhNPs would pave a novel way to performing asymmetric syntheses, for which investigations are currently lacking. Silver PhNPs are used to effectively mediate the enantioselective photocyclodimerization of 2-anthracenecarboxylate: left-handed silver PhNPs lead to a positive ee (enantiomeric excess) value, and right-handed silver PhNPs give rise to a negative ee value. The enantioselectivity is enhanced with a decreasing P. The PhNP-mediated enantioselective photocyclodimerization is ascribed to the synergistic contribution from chirally helical surface-induced enantioselective adsorption of 2-anthracenecarboxylate and chiroptically active nanoplasmon-enhanced optical chirality of near-field circularly polarized light.. Metal-assisted chemical etching (MACE) is carried out to generate mesoporous SiNWs (mp-SiNWs) with mesopores from 2 to 50 nm. The porosification imposes two prominent properties onto SiNWs: a high surface-to-volume ratio and quantum confinement ascribed to the shrinkage of silicon skeletons. Hence, engineering the porosity of SiNWs is of fundamental importance. Here, a new method is devised to reduce the porosity of mp-SiNWs without changes in the MACE conditions. After generating the mp-SiNWs with high porosity, the mp-SiNWs are removed from the mother Si wafers with sticky tape, followed by MACE under the same conditions to produce low-porosity mp-SiNWs. Less porous mp-SiNWs reduce optical scattering from the porous Si skeletons and vertically protrude on the wafer without aggregation to facilitate optical trapping. Consequently, low-porosity mp-SiNWs effectively reduce UV-visible reflection loss. Furthermore, optical applications require surface modification of mp-SiNWs with functional chemicals, which has a prerequisite to passivate mp-SiNWs with H-termination using 5% hydrogen fluoride. 40% NH4F, which has been widely used to passivate Si(111) wafers with H-termination, tends to unexpectedly etch mp-SiNWs attributed to surface F-termination caused by the nucleophilic attack of F− anions to Si atoms. It has been used to study systematically the NH4F-etching rate as a function of the doping levels of SiNWs, surface crystalline orientations, and porosity. At a modest temperature of 110°C, 1,4-diethynylbenzene (DEBZ) is grafted via monosilylation grafted on H-terminated mp-SiNWs. The modified mp-SiNWs with chemically active monolayers is facilely subjected to further chemical modification and surface functionalization. In addition, the monosilylation encodes mp-SiNWs with PL of DEBZ, opening a door to flexible engineering of PL of mp-SiNWs for optoelectronic and bio-detection applications.

Date of Award31 Aug 2017
Original languageEnglish
SupervisorJeffery HUANG (Supervisor)

User-Defined Keywords

  • Nanostructures
  • Nanowires
  • Optical properties

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