Showing papers by "Shuit-Tong Lee published in 2005"

Highly Efficient Non-Doped Blue Organic Light-Emitting Diodes Based on Fluorene Derivatives with High Thermal Stability

Abstract: A new series of blue-light-emitting fluorene derivatives have been synthesized and characterized. The fluorene derivatives have high fluorescence yields, good thermal stability, and high glass-transition temperatures in the range 145-193 °C. Organic light-emitting diodes (OLEDs) fabricated using the fluorene derivatives as the host emitter show high efficiency (up to 5.3 cdA - 1 and 3.0 lm W - 1 ) and bright blue-light emission (Commission Internationale de L'Eclairage (CIE) coordinates of x=0.16, y=0.22). The performance of the non-doped fluorene-based devices is among the best fluorescent blue-light-emitting OLEDs. The good performance of the present blue OLEDs is considered to derive from: 1) appropriate energy levels of the fluorene derivatives for good carrier injection; 2) good carrier-transporting properties; and 3) high fluorescence efficiency of the fluorene derivatives. These merits are discussed in terms of the molecular structures.

Strain energy and electronic structures of silicon carbide nanotubes: Density functional calculations

Abstract: We perform density functional calculations for the geometrics, strain energy, and electronic structures of silicon carbide nanotubes ($\mathrm{SiCNT}$'s). We find that the strain energy in $\mathrm{SiCNT}$'s is as higher as $0.686\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{atom}$ relative to $3\mathrm{C}\text{\ensuremath{-}}\mathrm{SiC}$ for (5,5) $\mathrm{SiCNT}$ and decreases with increasing tube diameter. All the $\mathrm{SiCNT}$'s are semiconductors, the band gap of which increases with increasing tube diameter. In contrast to $3\mathrm{C}\text{\ensuremath{-}}\mathrm{SiC}$, zigzag $\mathrm{SiCNT}$ has a direct band gap at the $\ensuremath{\Gamma}$ point, whereas armchair and chiral tubes have an indirect band gap. The highest occupied valance band and the lowest unoccupied conduction band highly localize to $\mathrm{C}$ and $\mathrm{Si}$ atoms, respectively. Hydrogen-decorated $\mathrm{SiCNT}$'s display the characters of $p$- or $n$-type semiconductors depending on the adsorbing site.

Wavelength-Controlled Lasing in ZnxCd1–xS Single-Crystal Nanoribbons†

Abstract: Znx Cd1-x S single-crystal nanoribbons of controlled composition (where 0 ≤ x ≤ 1) can be synthesized by combining laser ablation with thermal evaporation. The nanoribbons exhibit lasing emission that can be continuously tuned within the ranges of 340-390 nm and 485-515 nm. These results suggest that Znx Cd1-x S nanoribbon lasers of pre-selected "tunable" wavelengths between 340 and 515 nm may be achievable by tailoring the value of x.

A High Tg Carbazole-Based Hole-Transporting Material for Organic Light-Emitting Devices

Abstract: A new branched carbazole derivative, 1,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB), was prepared as a hole-transporting material for organic light-emitting devices (OLEDs) TECEB is comparable to 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) in terms of highest-occupied molecular orbital/lowest-unoccupied molecular orbial energy levels, hole-drift mobility, as well as device performance (maximum luminence of about 10 000 cd m-2 and current efficiency of 327 cd A-1) in a standard hole-transporting layer/tris-(8-hydroxyquinoline)aluminum double-layer device, but is superior to NPB in terms of its higher glass-transition temperature (Tg, 130 °C) and ease of synthesis The latter features suggest that TECEB can be a potential alternative material to NPB especially for high-temperature applications in OLEDs and other organic electronic devices

Silicon Nanowire Sensors for Bioanalytical Applications: Glucose and Hydrogen Peroxide Detection†

Abstract: Silicon nanowire films have been modified with boron and used as sensors to measure glucose in aqueous solution. These sensors have a wide linear range (0–10 mM glucose), high sensitivity (172 nA mmol–1), good reproducibility, and long-term stability. Silicon nanowire films have also been modified with magnesium and shown to perform as sensors for detecting hydrogen peroxide in aqueous solution.

Direct Evidence of Molecular Aggregation and Degradation Mechanism of Organic Light-Emitting Diodes under Joule Heating: an STM and Photoluminescence Study

Abstract: The Joule heating effect on electroluminescent efficiency is important in the degradation origin of organic light-emitting diodes (OLED). Scanning tunneling microscopy (STM) and photoluminescence (PL) measurements were performed on the guest molecule BT (1,4-bis(benzothiazole-vinyl) benzene), host molecule TPBI (2, 2',2' '-(1,3,5-phenylene)tris-[1-phenyl-1H-benzimidazole]), and their mixture deposited on an HOPG surface to study the OLED degradation mechanism due to thermal heating. At room temperature, BT and TPBI in the mixed layer show good compatibility and high PL intensity, but at higher temperatures, they show phase separation and aggregation into their own domains and a concomitant decrease in PL intensity. The PL intensity loss suggests ineffective energy transfer from TPBI to BT due to phase separation, which may cause OLED degradation. Scanning tunneling spectroscopy (STS) results show that the band gaps of TPBI and BT remain unchanged with the annealing temperature, suggesting that the heat-induced decay of OLED is related to the interfacial structural change rather than the respective molecular band gap. The results provide direct evidence showing how the molecular structures of the mixed layer vary and affect the PL intensity due to temperature.

Novel Starburst Molecule as a Hole Injecting and Transporting Material for Organic Light-Emitting Devices

Abstract: We report the synthesis of a novel starburst molecule, 4,4‘,4‘ ‘-tris(N-3-methylphenyl-N-(9-ethylcarbazyl-3)amino) triphenylamine (PCATA), and its application in organic light-emitting devices (OLEDs). The introduction of PCATA into the standard NPB/Alq3 OLED as the hole injecting and transporting layer dramatically enhanced the device efficiency to 5.7 cd/A and 2.2 lm/W, which are a factor of 2 higher than those of the standard OLED without the PCATA layer. The performance enhancement is attributed to a better balance of hole and electron injection in the PCATA-added OLED.

Structures and energetics of hydrogen-terminated silicon nanowire surfaces.

Abstract: The analysis and density-functional tight-binding simulations of possible configurations of silicon nanowires (SiNWs) enclosed by low-index surfaces reveal a number of remarkable features. For wires along , , and directions, many low-index facet configurations and cross sections are possible, making their controlled growth difficult. The 112 wires are the most attractive for research and applications because they have only one configuration of enclosing low-index facets with a rectangular cross section, enclosed with the most stable (111) facet and the (110) facet next to it. In general, the stability of the SiNWs is determined by a balance between (1) minimization of the surface energy gamma(111) svr(rectangular)>svr(triangular)]. The energy band gaps follow the order of wires > wires > wires > wires. The results are compared with our recent scanning tunneling microscopy and transmission electron microscopy data.

Morphology-controllable synthesis of pyrene nanostructures and its morphology dependence of optical properties.

Abstract: Morphology-controllable synthesis of various pyrene nanostructures from nanoparticles to short nanorods and nanowires (long nanorods) was achieved by a simple self-assembly method. In this approach, aqueous sodium dodecyl sulfate (SDS) micelles were used as templates to direct the self-assembly of the pyrene molecules into nanorods. It was found that changing the concentration ratio of the pyrene to SDS molecules could be employed to control the aspect ratio (length to diameter) of the pyrene nanostructures from 1 to 50 or higher. Moreover, the dimensional variation was accompanied by changes of their optical properties. With the increase of the aspect ratio, the characteristic fluorescence of the isolated pyrene molecules was suppressed and concurrently replaced by the excimer emission of the pyrene nanostructures. A blue-shift was observed in the excimer emission peaks as the length of the nanorods increased. The growth mechanism and the change in optical properties of these pyrene nanostructures were d...

Simulation of water cluster assembly on a graphite surface

Abstract: The assembly of small water clusters (H2O)n, n = 1−6, on a graphite surface is studied using a density functional tight-binding method complemented with an empirical van der Waals force correction,...

Manipulating the electronic structures of silicon carbide nanotubes by selected hydrogenation.

Abstract: We show that the electronic and atomic structures of silicon carbide nanotubes (SiCNTs) undergo dramatic changes with hydrogenation from first-principles calculations based on density-functional theory. The exo-hydrogenation of a single C atom results in acceptor states close to the highest occupied valence band of pristine SiCNT, whereas donor states close to the lowest unoccupied conduction band appear as a Si atom being hydrogenated. Upon fully hydrogenating Si atoms, (8,0) and (6,6) SiCNTs become metallic with very high density of states at the Fermi level. The full hydrogenation of C atoms, on the other hand, increases the band gap to 2.6eV for (8,0) SiCNT and decreases the band gap to 1.47eV for (6,6) SiCNT, respectively. The band gap of SiCNTs can also be greatly increased through the hydrogenation of all the atoms.

Fabrication and application of long strands of silicon nanowires as sensors for bovine serum albumin detection

Abstract: Strands of aligned silicon nanowires (SiNWs) with lengths over 2mm and diameters of 35nm were synthesized via a two-step controlled growth. The large dimensionality made it possible to fabricate an individual strand of SiNWs into an electrode for cyclic voltammetric detection of bovine serum albumin (BSA). The strand sensor with the immense surface area and electrical conductivity of SiNWs shows the highest sensitivity and widest linear range ever reported for BSA detection.

Temperature-dependent growth of germanium oxide and silicon oxide based nanostructures, aligned silicon oxide nanowire assemblies, and silicon oxide microtubes.

Abstract: We demonstrate the temperature-dependent growth of germanium oxide and silicon oxide based composite nanostructures (multiple nanojunctions of Ge nanowires and SiO(x) nanowires, Ge-filled SiO(2) nanotubes, Ge/SiO(2) coaxial nanocables, and a variety of interesting micrometer-sized structures), aligned SiO(x) nanowire assemblies, and SiO(x) microtubes. The structures were characterized by SEM, TEM, energy-dispersive X-ray spectroscopy, and electron diffraction. The combination of laser ablation of a germanium target and thermal evaoporation of silicon monoxide powders resulted in the formation of Ge and SiO(x) species in a carrier gas; the nano/micro-sized structures grow by either a Ge-catalyzed vapor-liquid-solid or a Ge-nanowire-templated vapor-solid process.

Fabrication, morphology, structure, and photoluminescence of ZnS and CdS nanoribbons

Abstract: One-dimensional semiconductor nanoribbons of hexagonal wurtzite sulfides (ZnS and CdS) have been prepared in bulk quantity by a thermal evaporation technique using thiol-capped gold nanoparticles as catalysts. Compared to their starting materials, ZnS and CdS powders, the band-gap photoluminescence excited by ultraviolet light from ZnS and CdS nanoribbons at room temperature was significantly enhanced. X-ray-excited optical luminescence at the S K edge confirms the near-band-gap and the defect origin of the luminescence.

Electronic structure of boron nitride single crystals and films

Abstract: We present a comparison between experimental soft x-ray spectra and density of states calculations of hexagonal boron nitride ($h$-BN) and cubic boron nitride ($c$-BN) single crystals. Cubic boron nitride films, grown on both mirror and scratched single crystal silicon wafers, have also been investigated using soft x-ray absorption spectroscopy (XAS) and soft x-ray emission spectroscopy (XES). Spectra measured at the $1s$ thresholds of boron and nitrogen give a complete picture of the occupied and unoccupied partial density of states for these materials. The films are shown to be a mixture of $s{p}^{3}$ bonded nanocrystalline $c$-BN phase and $s{p}^{2}$ bonded $h$-BN phase. As the roughness of the deposition surface increases, a decrease in the amount of $s{p}^{3}$ phase in the resulting film is observed. There are clear differences between the electronic structures of the nanocrystalline films and the single crystal samples. No differences between the spectra of the single crystals and previously reported measurements on powder samples of BN were observed.

One-dimensional zigzag gallium nitride nanostructures

Abstract: Two one-dimensional (1D) single-crystalline gallium nitride (GaN) nanostructures with periodic zigzag (type I) and diameter-modulated (type II) shapes have been synthesized by passing through ammonia over a mixture of gallium and gallium oxide (Ga2O3) powders held at elevated temperature. The process was catalyzed by the dispersion of thio-capped Au nanoparticles on the substrate onto which GaN nanostructures were condensed. The transformation between these two nanostructure morphologies was also observed. A possible growth model for the zigzag-shaped nanostructures is proposed, in which the formation of the zigzag nanostructures results from the construction of two different nanoscale unit cells. This work provides an avenue to a group of 1D nanostructures with a zigzag shape. The possibility to form 1D nanostructures yet to be discovered by changing the stacking direction of the (0001) plane will facilitate the fabrication of nanoscale functional devices as well as our understanding of the growth behavi...

Effects of in situ vacuum annealing on the surface and luminescent properties of ZnS nanowires

Abstract: We have monitored the changes that occur in the x-ray-excited optical luminescence, absorption, and photoemission spectra as a function of vacuum annealing time and temperature for ZnS nanowires. All measurements were done in situ. Initial heating causes desorption of surface oxides and a concurrent reduction in the intensity of all the luminescence peaks, which we attribute to the creation of surface states that quench the luminescence. Extended annealing causes diffusion of Au from the particle used to nucleate the wire growth, which results in an increase in intensity of its associated luminescent band at 520nm. Changes were also observed in the ZnL- and SK-edge x-ray absorption spectra, which are consistent with this interpretation.

Strong polarization-dependent photoluminescence from silicon nanowire fibers

Abstract: Fibers of highly oriented Si nanowires (SiNWs) were formed by drawing from a condensed SiNW suspension. The SiNW fiber, excited at 514.5nm, produces a strong photoluminescence (PL) at room temperature. The PL spectrum shows three bands at 565–580, 605–640, and 680–690nm, respectively, which are consistent with the PL of porous silicon. The relative intensity of these bands and the integrated intensity of the PL vary with the angle θ between the electric field of the polarized laser excitation and the fiber axis. The dependence on θ is attributed to the combined effects of the one-dimensional shape of the SiNW and the large dielectric contrast between the SiNW and the ambient.

Molecular orientation and film morphology of pentacene on native silicon oxide surface.

Abstract: The growth morphology and mechanism of pentacene films on native Si oxide surface have been studied by using high-resolution electron energy loss spectroscopy (HREELS), X-ray diffraction (XRD), and atomic force microscopy (AFM). Despite the good agreement between our own and the reported XRD results, the previous XRD interpretation that the pentacene molecules are tilt-standing on the substrate cannot explain our HREELS data. The HREELS results show that a substantial portion of the first two layers of pentacene molecules are tilted-standing or randomly oriented, whereas the upper-layer molecules are mostly lying flat to the substrate. AFM reveals that the first two layers of molecules form a flat and smooth surface, but the upper layers show a rough terrace structure with a mean-square roughness equal to the average thickness (without counting the first two layers). This relationship is explained by a theoretical model which assumes the pentacene molecules to remain on a particular molecule layer after arrival. The observed film growth morphology may have significant implication on the performance of electronic devices based on pentacene thin films. A plausible explanation was proposed for the discrepancy between the HREELS-indicated and the XRD-derived molecular orientations.

Diamond nucleation by energetic pure carbon bombardment

Abstract: Deposition of 1000 eV pure carbon ions onto Si(001) held at 800 degrees C led to direct nucleation of diamond crystallites, as proven by high-resolution transmission electron microscopy and electron energy loss spectroscopy. Molecular dynamic simulations show that diamond nucleation in the absence of hydrogen can occur by precipitation of diamond clusters in a dense amorphous carbon matrix generated by subplantation. Once the diamond clusters are formed, they can grow by thermal annealing consuming carbon atoms from the amorphous matrix. The results are applicable to other materials as well.

Interfacial study of cubic boron nitride films deposited on diamond

Abstract: We have studied the nucleation and growth of cubic boron nitride (cBN) films deposited on silicon and diamond-coated silicon substrates using fluorine-assisted chemical vapor deposition (CVD). These comparative studies substantiate that the incubation amorphous/turbostratic BN layers, essential for the cBN nucleation on silicon, are not vital precursors for cBN nucleation on diamond, and they are inherently eliminated. At vastly reduced critical bias voltage, down to -10 V, cBN growth is still maintained on diamond surfaces, and cBN and underlying diamond crystallites exhibit an epitaxial relationship. However, the epitaxial growth is associated with stress in the cBN-diamond interfacial region. In addition, some twinning of crystallites and small-angle grain boundaries are observed between the cBN and diamond crystallites because of the slight lattice mismatch of 1.36%. The small-angle grain boundaries could be eliminated by imposing a little higher bias voltage during the initial growth stage. The heteroepitaxial growth of cBN films on different substrate materials are discussed in the view of lattice matching, surface-energy compatibility, and stability of the substrate against ion irradiation.