Vo-Van Truong

Enhancement of current-voltage characteristics of multilayer organic light emitting diodes by using nanostructured composite films

With the aim of improving the photonic efficiency of an organic light emitting diode (OLED) and its display duration, both the hole transport layer (HTL) and the emitting layer (EL) were prepared as nanostructured thin films. For the HTL, nanocomposite films were prepared by spin-coating a homogeneous solution of low molecular weight poly(4-styrenesulfonate) (PEDOT-PSS) and surfactant-capped TiO2 nanocrystals onto low resistivity indium tin oxide (ITO) substrates; for the EL, nancrystalline titatium oxide (nc-TiO2)-embedded Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV+nc-TiO2) conjugate polymers were spin-coated onto the HTL. Also, for a shallow contact of Al/LiF/MEH-PPV instead of Al/MEH-PPV a super LiF thin film was deposited onto the EL by vacuum evaporation. The resulting multilayer OLED had the following structure of Al/LiF/MEH-PPV+nc-TiO2/PEDOT-PSS+nc-TiO2/ITO. Characterization of the nanocomposite films showed that both the current-voltage (I-V) characteristics and the phot...

Optical behaviour of thick gold and silver films with periodic circular nanohole arrays

To better understand the enhanced optical transmission observed with an array of nanoholes on optically thick metallic films, an extensive numerical calculation is presented in this paper to relate the scattering modes observed to transmittance spectral behaviour. Surface plasmon polariton Bloch waves, Woods anomalies and localized surface plasmon resonances at the rim of the nanoholes are investigated based on scattering modes that form the features of the transmission spectrum. The finite difference time-domain method is applied to calculate the transmission intensity from films of gold and silver subjected to variations in geometrical parameters such as film thickness, spatial period of the structures and hole diameter. Analysis of transmission spectra showed that the cut-off frequency of the array of subwavelength holes is mostly defined by the thickness of the film and the diameter of the holes rather than the periodicity of the structures.

Plasmonic modes and optical properties of gold and silver ellipsoidal nanoparticles by the discrete dipole approximation

The discrete dipole approximation (DDA) is used to model the absorption efficiency of isolated gold (Au) and silver (Ag) ellipsoidal nanoparticles. The characteristics of the plasmonic bands of those nanostructures depend strongly on the size and orientation of the particles in both the lab and target frames. At specific rotation and incident angles, the desired plasmonic mode can be excited. The result of the simulation shows the possibility of excitation of three plasmonic modes--one longitudinal mode (LM) and two transverse modes (TM)--corresponding to the redistribution of the polarization charges along each principal axis. At oblique incidence of the incoming light, both the Au LM and a hybrid Au TM are observed whereas three more distinct plasmonic modes can be found in the case of the Ag particle. The effect of length distribution on the characteristics of the plasmonic bands is also examined for the three principal axes. The band position of the plasmonic bands associated with the electronic oscillation along each principal axis is found to vary linearly with the axis length. The linear variation of the band position of the LM is steeper as compared with the one found for the other modes.

Synthesis and characterization of ce-doped Y 3 Al 5 O 12 (YAG:Ce) nanopowders used for solid-state lighting

Nano-Ce-doped Y3Al5O12 (YAG:Ce) powders were synthesized by using a sol-gel low temperature combustion method, followed by thermal annealing. The annealing temperature for enriching nanoparticles was optimized and found to be 1000°C. The process for enriching uniform nanoparticles of YAG:Ce powder was carried out by using the nanosteam technique (NST). The nanoparticles obtained from this NST treatment had a size in the range of 9-20 nm. Measurements of the photoluminescence spectra of the dispersed YAG:Ce nanoparticles solutions showed a blue shift in the photoemission with a value of ca. 10 nm in the green region. WLEDs made from the blue LED chip coated with the nano-YAG:Ce + MEH-PPV composite epoxy exhibit white light with a broad band luminescent spectrum and a high color rending index (CRI). The photoluminescence spectra of the YAG:Ce nanoparticles showed a potential application of the prepared nanostructured YAG:Ce phosphor not only in energy-efficient solid-state lighting, but also in optoelectronic devices, including organic composite solar cells. In addition, it is suggested that NST can be applied for the enrichment of uniform inorganic nanoparticles.

Simulated optical properties of gold nanocubes and nanobars by discrete dipole approximation

The absorption spectra for a gold nanocube and for a gold nanobar are calculated by using the Discrete Dipole Approximation (DDA). The results show the excitation of a single albeit broad surface plasmon (SP) band of the gold nanocube. The extinction cross section of the gold nanocube is dominated by the absorption cross section that gains importance as the width increases. Further increasing the nanocube size beyond 80 nmwill result in an optical response mainly characterized by scattering properties. The absorption spectrum of the nanobar shows the excitation of both the longitudinal mode (LM) and the transverse mode (TM). The nanobar is also compared to a cylinder, a spherically capped cylinder, and a spheroid of the same aspect ratio. The band position of the TM of the nanobar is red-shifted as compared to the ones calculated for other morphologies, while the LM is either blue-shifted or red-shifted depending on the morphologies considered.

Ultra-Low-Voltage Schottky-Barrier Field-Enhanced Electron Emission From Gold Nanowires Electrochemically Grown in Modified Porous Alumina Templates

Superior field emission (FE) action has been observed from a tailored array of gold nanowires (AuNWs) grown in porous anodic alumina (PAA) templates. The turn-on voltage was found to be 0.2 V, which is far less than the values reported earlier for diamond-coated cathodes and carbon nanotubes (CNTs). Furthermore, the FE current was unaffected by the ambient gas pressure. Such a low-voltage FE is attributed here to the formation of a Schottky barrier at the interface of gold and amorphous alumina scales that remain on the AuNWs.

Micro-Raman spectroscopy study of colloidal crystal films of polystyrene-gold composites.

Monolayers and multilayers of polystyrene (PS)–gold composite films prepared by two different deposition methods have been investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and confocal Raman microspectroscopy. The intensity of the 1001 cm−1 ring breathing mode of PS is used to evaluate the degree of ordering of monolayers and multilayers within a colloidal crystal. The depth profiling capability of confocal Raman microscopy is used to probe the regions inside the fractures in multilayered films. The intensity profile of the 1001 cm−1 peak revealed the presence of fractures of different shapes with some PS microspheres at the bottom of the fracture. A strong increase in the Raman intensity (by 103 times) has been observed when probing the regions where Au nanoparticles are concentrated in aggregates of different shapes. This enhancement is attributed to the surface plasmons generated by the periodic structure of the gold nanoparticles.

Biosensing Based on Surface Plasmon Resonance of Gold Nanohole and Nanoring Arrays Fabricated by a Novel Nanosphere Lithography Technique

An optical biosensor based on gold nanohole arrays was developed using a new nanosphere lithography technique. A vertical deposition method was used to deposit multilayers of a polystyrene (PS) - gold composite on a silanized glass substrate. The SEM images have shown that the gold nanohole and nanoring arrays are distributed uniformly on the substrate. The sensitivity of the gold surface plasmon resonance (SPR) band to the environment was found to be dependent on the geometry of the array. The new nanostructures have been used as platforms for rapid screening of biomolecular interactions. Fibrinogen and biotin absorption studies conducted with the functionalized gold nanostructures demonstrated the feasibility of using them to monitor biomolecular interactions. The sensitivity was found to be around 300 nmldrRIU-1, which is adequate for biosensing applications. The present technique has the advantage of simplicity; the structures can be prepared without any special equipment and a standard cell can be used for optical measurements.

Ultra-thin, single-layer polarization rotator

We demonstrate light polarization control over a broad spectral range by a uniform layer of vanadium dioxide as it undergoes a phase transition from insulator to metal. Changes in refractive indices create unequal phase shifts on s- and p-polarization components of incident light, and rotation of linear polarization shows intensity modulation by a factor of 103 when transmitted through polarizers. This makes possible polarization rotation devices as thin as 50 nm that would be activated thermally, optically or electrically.

Enhancement of power efficiency and stability of P3HT-Based organic solar cells under elevated operating-temperatures by using a nanocomposite photoactive layer

With the aim to find out an enhanced operating-temperature range for photovoltaic device parameters, two types of the photoactive layer were prepared: poly(3-hexylthiophene) (P3HT) and P3HT+nc-TiO2 (PTC) thin films. The enhancement obtained for the photoelectrical conversion efficiency of the composite based OSCs is attributed to the presence of nanoheterojunctions of TiO2/P3HT. For the temperature range of 30-70°C, the decrease of the open-circuit potential was compensated by an increase of the fill factor; and the increase in the short-circuit current resulted in an overall increase of the energy conversion efficiency. At elevated temperatures of 60-80°C the efficiency of the P3HT- and PTC-based cells reached a maximum value of 1.6% and 2.1%, respectively. Over this temperature range the efficiency of P3HT-based OSC decreased strongly to zero, whereas for the PTC cells it maintained a value as large as 1.2% at the temperature range of 110-140°C. The improved thermal stability of the composite-based device was attributed to the lowered thermal expansion coefficient of the nanocomposite photoactive layer.