Chee Ming Lim

DFT/TDDFT and Experimental Studies of Natural Pigments Extracted from Black Tea Waste for DSSC Application

We report results of combined experimental and theoretical studies of black tea waste extract (BTE) as a potential sensitizer for TiO2-dye-sensitized solar cells (DSSCs). UV-vis absorption data revealed that BTE contains theaflavin. DSSC sensitized with pigment complexes of BTE showed a photon-energy conversion efficiency of %, while a significant increase (%) is observed when pH of the pigment solution was lowered. The HOMO and LUMO energy levels were calculated using experimental data of UV-vis absorption and cyclic voltammetry. These calculations revealed a reduction of the band gap by 0.17 eV and more negativity of HOMO level of acidified pigment, compared to that of original pigment. Combined effect of these developments caused the enhanced efficiency of DSSC. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computational calculations were carried out to study the four theaflavin analogues which are responsible for the dark colour of BTE. According to the calculations, two theaflavin analogues, theaflavin and theaflavin digallate, are the most probable sensitizers in this dye-sensitized solar cell system.

Tunable silver-shell dielectric core nano-beads array for thin-film solar cell application

The absorbance spectra of thin-film solar cells (TFSCs) can be enhanced by constructing the tunable periodic Ag-shell nano-bead (PASNB) arrays in the active material. In this paper, we investigated a plasmonic thin-film solar cell (TFSC) which composed of the arrays of PASNB deposited onto a crystalline silicon layer. By performing three-dimensional finite element method, we demonstrate that near field coupling among the PASNB arrays results in SPR modes with enhanced absorbance and field intensity. The proposed structure can significantly enhance the plasmonic activity in a wide range of incident light and enlarge working wavelength of absorbance in the range of near-UV, visible and near-infrared. We show that the sensitivity of the PASNB arrays reveals a linear relationship with the thickness of Ag-shell nano-bead (ASNB) for both the anti-bonding and bonding modes in the absorbance spectra. The broadband of absorbance spectra could be expanded as a wide range by varying the thickness of ASNB while the particle size is kept constant. Simulation results suggest this alternative scheme to the design and improvements on plasmonic enhanced TFSCs can be extended to other nanophotonic applications.

Tailoring surface plasmon resonance and dipole cavity plasmon modes of scattering cross section spectra on the single solid-gold/gold-shell nanorod

Tunable surface plasmon resonance (SPR) and dipole cavity plasmon modes of the scattering cross section (SCS) spectra on the single solid-gold/gold-shell nanorod have been numerically investigated by using the finite element method. Various effects, such as the influence of SCS spectra under x- and y-polarizations on the surface of the single solid-gold/gold-shell nanorod, are discussed in detail. With the single gold-shell nanorod, one can independently tune the relative SCS spectrum width by controlling the rod length and rod diameter, and the surface scattering by varying the shell thickness and polarization direction, as well as the dipole peak energy. These behaviors are consistent with the properties of localized SPRs and offer a way to optically control and produce selected emission wavelengths from the single solid-gold/gold-shell nanorod. The electric field and magnetic distributions provide us a qualitative idea of the geometrical properties of the single solid-gold/gold-shell nanorod on plasmon resonance.

A simple structure of all circular-air-holes photonic crystal fiber for achieving high birefringence and low confinement loss

We propose a simple structure of photonic crystal fibers (PCFs) with high birefringence and low confinement loss based on one rectangular centric ring of smaller circular air holes (CAHs) in the fiber core, and three rings of larger CAHs in the fiber cladding. This simple geometry (using all CAHs with two different air hole sizes) is capable of achieving a flexible control of the birefringence, B = 5.501 × 10−3, and ultra-low confinement loss, 7.30 × 10−5 dB/km, at an excitation wavelength of λ = 1550 nm. The birefringence value is ∼5.0 times greater than that obtained for conventional CAH PCF. This simple structure has the added advantage from the view point of easy fabrication, robustness, and cost. A full-vector finite element method combined with anisotropic perfectly matched layers was used to analyze the various fiber structures. We have analyzed four cases of CAH PCFs, focusing on the core asymmetry design as opposed to the conventional approach of CAHs or elliptical air holes on the cladding and c...

Manipulating near field enhancement and optical spectrum in a pair-array of the cavity resonance based plasmonic nanoantennas

A 2D periodic pair-array of cavity resonance based (CRB) plasmonic nanoantennas (PNAs) on the tailoring near field enhancement and optical spectrum of surface plasmon resonance (SPR) modes is numerically investigated by using the finite element method. The CRBPNAs consist of a single cavity or double cavities in each antenna arm. Detailed physical explanations of the simulation results and consistent dependencies of the SPR features on the structural and material parameters of CRBPNAs are presented. Compared to the solid case of counterpart, the proposed CRBPNAs display outstanding SPR characteristics and tune the peak resonance wavelength by varying the outline thickness and the cavity material. In addition, the field enhancement and optical spectrum can be precisely controlled by cavity material and outline thickness in the broad band of ultraviolet, visible and near-infrared, resulting in increased sensitivity and supporting resonances with gaps and cavity surface plasmom as well as the high value of Q factors. We interpret the optical properties of the proposed CRBPNAs and show tuning and optimizing through choice of geometric and material parameters.

Depolying Tunable Metal-Shell/Dielectric Core Nanorod Arrays as the Virtually Perfect Absorber in the Near-Infrared Regime

In this paper, the coupled Ag-shell/dielectric-core nanorod for sensor application is investigated and the different dielectric core plasmonic metamaterial is adopted in our design. The operational principle is based on the concept of combining the lattice resonance, localized surface plasmon resonance (SPR), and cavity plasmon resonance modes within the nanostructure. The underlying mechanisms are investigated numerically by using the three-dimensional finite element method and the numerical results of coupled solid Ag nanorods are included for comparison. The characteristic absorptance/reflectance peaks/dips have been demonstrated to be induced by different plasmonic modes that could lead to different responses required for plasmonic sensors. A nearly perfect absorptance and an approximate zero reflectance with a sharp band linewidth are obtained from the proposed system, when operated as an SPR sensor with the sensitivity and figure of merit of 757.58 nm/RIU (RIU is the refractive index unit) and 50.51 (RIU–1), respectively. Our work provides a promising method for the future developments of more advanced metamaterial absorber for chemical sensing, thermal radiation tailoring, field enhanced spectroscopy, and general filtering applications.

Dynamic Light Scattering and Zeta Potential Studies of Ceria Nanoparticles

A Cerium (IV) oxide nanoparticle (nanoceria) is widely used in different applications such as biomedicine and catalysis due to its unique structural, morphological and catalytic properties. In this report, the dispersion of nanoceria in both aqueous and non-aqueous (methanol and ethanol) media were studied. Adsorption-desorption processes were observed upon addition of different classes of surfactants such as citric acid (CA), cetrimonium bromide (CTAB) and diethanolamine (DEA). Stable dispersions were obtained in both aqueous, non-aqueous and electrolyte assisted media with the overall mechanism being hydrolysis, dissolution and adsorption. XRD, FE-SEM, FTIR and DLS have been used in the present study to characterize the nanoceria and to quantitatively analyze their average particle size distributions in a unique electrolyte mixture of (0.1 M NaOH/ 65% HNO3:H2O, 1:1 v/v) which has not been reported previously. The surface charge study was carried out across a wide pH range between 1.4 – 9.6 and the isoelectric points (IEP) with respect to 15 ml H2O and 50 ml H2O dispersed phases occurred at a pH of about 6.5 and 6.7 respectively. The present study could be useful in a wide range of applications including nanoparticle synthesis, stabilization, and adsorption of toxic materials, biomedical and pharmaceutical.

Ruthenium Dye (N3) Removal from Simulated Wastewater Using Bamboo Charcoal and Activated Bamboo Charcoal

The presence of heavy metals such as mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (T1) and ruthenium (Ru) in wastewater, even in trace quantities, could cause a negative impact on our health. The adsorption method has been proven to be the most effective and low-cost method for removing of heavy metals from wastewater. In this study, biomass waste was used as a low-cost precursor for the production of cost-effective charcoal and activated carbon. Solid waste from a common local bamboo species (Gigantochloa sp.) was used to produce charcoal and activated carbon. The simulated wastewater was made with Ruthenium complex (N3) dye solution as the adsorbate. The bamboo charcoal was prepared by carbonization, and activated carbon was prepared by NaOH activation after carbonization. The morphological characteristics, chemical compositions, and the lattice structures of the prepared adsorbents were analyzed using SEM, EDX, and XRD. The adsorption performance of the prepared adsorbents toward N3 dye was evaluated, and the highest adsorption capacity of 1.50 mg/g was obtained from activated carbon. The results showed that the activated bamboo-based charcoal has a better adsorption efficiency when compared to the bamboo charcoal for the treatment of N3 dye in wastewater.

Mg-La Co-Doped TiO2 as Compact Layer for High Efficient Dye-Sensitized Solar Cells

A compact layer of TiO2, between FTO and mesoporous TiO2 layer of DSSC anode, has the ability to reduce the electron recombination while mesoporous TiO2 layer functions to collect and transport electrons injected by the photoexcited dye. In enhancing the desired functions of compact and mesoporous TiO2, we study the effects of co-doping it with Mg, Eu, and La. MgLa co-doped TiO2 as compact layer has demonstrated higher efficiency due to its increased oxygen vacancies and smaller particle size which results reduced recombination. The combination of pure and co-doped TiO2 nanoparticles to be selected as either compact or mesoporous layer of DSSC anode depends on their respective properties, as demonstrated in this paper.

An analysis of DSSC performance based on nanosphere, nanorod, and nanoparticle anode morphologies

The precise nature of dye-sensitized solar cell (DSSC) anodes plays a vital role in inter-particle contact, dye absorption, electrolyte diffusion, and distribution of sub conduction band states. This study compares the charge transfer dynamics at the oxide-dye-electrolyte interface in DSSC anodes composed of either nanoparticles (NPs), nanospheres (NSs), or nanorods (NRs). The crystallinity, morphology, visible light and dye absorption on the films were studied by XRD, SEM, and UV-Vis diffuse reflectance spectroscopy while the optoelectronic properties in the DSSC were evaluated by current–voltage, Incident photon to current conversion efficiency, and Electrochemical Impedance Spectroscopy (EIS) measurements. Visible light absorption was highest in NR films while NS films showed the highest light scattering. However, the DSSC with NP films showed the highest power conversion efficiency (PCE) that was attributed to increased dye adsorption amount. The DSSC with NS films had the least PCE but showed the hig...