An investigation of the effect of metal nanoparticles on the optical properties of silicon nanocrystals

Abstract

This thesis describes the characterization of two types of nanocrystalline material i.e. alkylated silicon nanocrystals (C11-SiNCs) and commercial silicon nanocrystals (SiNCs). The research presented throughout this work also shows that the optical properties of silicon nanocrystals can be affected by erbium ions and metal nanoparticles. The main goal of this characterization is to observe the energy transfer from the excited state of SiNCs to the erbium for optical fiber technology applications. Also, SiNCs have applications in biology as fluorescent labels. Porous silicon was prepared successfully by galvanostatic etching of p-Si(100) wafers followed by a thermal hydrosilation reaction of 1-undecene in refluxing toluene in order to extract the C11-SiNCs from porous silicon. The chemical characterization of C11-SiNCs was carried out using X-ray photoemission spectroscopy (XPS); they are known to be crystalline and of diameter about 5 nm from previous work. The commercial SiNCs have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Atomic force microscopy (AFM), X-ray diffraction (XRD), XPS and Fourier transform infrared spectroscopy (FTIR). It was found that the average diameter of commercial SiNCs is 65 nm and are crystalline with an FCC lattice. Erbium trichloride was added to both types of SiNCs using a simple mixing chemical route. To the best of our knowledge, this is the first investigation on mixing SiNCs with erbium ions (III) by this chemical method. Both SiNCs either C11-SiNCs or commercial SiNCs and their mixtures with Er3+ were investigated using Raman spectroscopy and photoluminescence (PL). The samples showed an orange PL emission peak at around 595 nm which originates from Si. Er/SiNCs mixtures also exhibit a weak PL emission peak at 1536 nm which originates from the intra-4f transition in erbium ions (Er3+). The PL peak of Si in Er/C11-SiNCs and Er/Commercial SiNCs mixtures are increased in the intensity up to four and three times, respectively as compared to pure C11-SiNCs and commercial SiNCs. The collected data suggest that this chemical mixing route leads instead to a transfer of energy from erbium ions to SiNCs. Metal-enhanced luminescence has been studied for mixtures of SiNCs (either C11-SiNCs or commercial SiNCs) with silver nanoparticles (AgNPs). AgNPs of two different sizes were synthesised using photochemical reduction of AgNO3 with sodium dodecyl sulphate (SDS). The synthesized AgNPs (1:5) and (10:50) have a polycrystalline structure with an average particle diameter of 100 nm and 30 nm, respectively. A significant enhancement up to 10 and 4 times in the PL intensity was observed for AgNPs (1:5)/C11-SiNCs and AgNPs (10:50)/C11-SiNCs, respectively using an excitation source of 488 nm. A similar observation was also reported for AgNPs (1:5)/Commercial SiNCs and AgNPs (10:50)/Commercial SiNCs; where the intensity of the PL signal increased up to 9 and 3 times respectively, using 488 nm; whereas the intensity of the PL signal increased up to 7 and 2 times respectively, using 514.5 nm excitation source. The enhancement in SERS intensities occurs as a result of the coupling between the excitation laser light and the plasmon bands of AgNPs; thus this intense field at AgNPs surface couples strongly to SiNCs. The results show that the closer wavelength of the laser excitation source to the surface plasmon resonance absorption bands of silver nanoparticles the greater the emission intensity. Our study also suggests that the larger AgNPs (1:5) caused an optimum enhancement in PL intensity of both types of SiNCs. Under continuous wave (CW) irradiation at 488 nm in a confocal microscope, both types of SiNCs show reversible photoluminescence fading behaviour. This can be interpreted by the same model originally proposed to describe luminescence intermittency, i.e., ’blinking’. When single particles are studied, this leads to the wellknown blinking phenomenon as particles ionize and later discharge by electron-hole recombination. In an ensemble, the result is a reversible photofading as the initial photoluminescence 𝐼0 decays to a steady-state 𝐼∞ controlled by the relative rates of photoionization 𝑘𝑎 and recombination 𝑘𝑒ℎ. Evidence for this interpretation comes from two observations: (i) upon cessation of the irradiation, electron-hole recombination occurs in the dark and the photoluminescence is regained when irradiation recommences and (ii) the initial and steady-state spectra are identical except for a scale factor. The photofading data can be modelled as a simple first order decay with a lognormal distribution of rate constants and therefore characterized by three parameters; 〈𝑘〉 the modal rate constant, 𝛾 which measurs the spread of activiation free energies in units of RT and 𝐼𝑜 /𝐼∞. C11-SiNCs and commercial SiNCs show enhanced luminescence when drop cast as films on glass slides in mixtures with Ag or Au nanoparticles. Such metal-enhanced luminescence is generally explained in terms of the large electric field near the metal surface upon excitation of the plasmon resonance and an increase in the radiative decay rate owing to the effect of the plasmon on the optical density of states. In this work, we find evidence for a third effect: the metal nanoparticles can act as a source of electrons and increase the time integrated luminescence intensity by increasing the rate of electron-hole recombination. In the presence of Ag and Au nanoparticles with alkyl-capped SiNCs, the modal rate constants 〈𝑘〉 increase by factors of up to 4-fold and the ratios 𝐼𝑜 /𝐼∞ decrease by factors up to 5-fold; this is consistent with an increase in the rate of electron-hole recombination facilitated by the metal nanoparticles acting as a source of electrons. It is also should be noted that the presence of either Ag or Au NPs with commercial SiNCs are less effective at enhancing the PL than alkyl-capped SiNCs due to the larger average particle size of commercial SiNCs.