Showing papers by "Shuit-Tong Lee published in 2003"
TL;DR: These hydrogen-terminated SiNW surfaces seem to be more oxidation-resistant than regular silicon wafer surfaces, because atomically resolved STM images of SiNWs were obtained in air after several days' exposure to the ambient environment.
Abstract: Small-diameter (1 to 7 nanometers) silicon nanowires (SiNWs) were prepared, and their surfaces were removed of oxide and terminated with hydrogen by a hydrofluoric acid dip. Scanning tunneling microscopy (STM) of these SiNWs, performed both in air and in ultrahigh vacuum, revealed atomically resolved images that can be interpreted as hydrogen-terminated Si (111)-(1 × 1) and Si (001)-(1 × 1) surfaces corresponding to SiH 3 on Si (111) and SiH 2 on Si (001), respectively. These hydrogen-terminated SiNW surfaces seem to be more oxidation-resistant than regular silicon wafer surfaces, because atomically resolved STM images of SiNWs were obtained in air after several days9 exposure to the ambient environment. Scanning tunneling spectroscopy measurements were performed on the oxide-removed SiNWs and were used to evaluate the electronic energy gaps. The energy gaps were found to increase with decreasing SiNW diameter from 1.1 electron volts for 7 nanometers to 3.5 electron volts for 1.3 nanometers, in agreement with previous theoretical predictions.
1,095 citations
610 citations
TL;DR: In this paper, the oxide-assisted growth (OAG) technique was proposed for the growth of nanostructured materials. But the OAG technique is not suitable for high-purity silicon nanowires.
Abstract: In this contribution, we outline oxide-assisted growth (OAG) (distinct from the conventional metal-catalytic vapor-liquid-solid (VLS) process) for the growth of nanostructured materials. This synthesis technique, in whichoxides instead of metals play an important role in inducing the nucleation and growth of nanowires, is capable of producing large quantities of high-purity silicon nanowires with a preferential growth direction, uniform size, and long length, without the need for a metal catalyst. The OAG 1D nanomaterials synthesis is complementary to, and coexistent with, the conventional metal-catalyst VLS approach, and can be utilized to produce nanowires from a host of materials other than Si including Ge nanowires, carbon nanowires, silicon and SnO 2 nanoribbons, and Group III-V and II-VI compound semiconductor nanowires.
580 citations
TL;DR: In this article, high transparent conductive, aluminum-doped zinc oxide (ZnO:Al) films with surface work functions between 3.7 and 4.4 eV were obtained by varying the sputtering conditions.
Abstract: Highly transparent conductive, aluminum-doped zinc oxide (ZnO:Al) films were deposited on glass substrates by midfrequency magnetron sputtering of metallic aluminum-doped zinc target. ZnO:Al films with surface work functions between 3.7 and 4.4 eV were obtained by varying the sputtering conditions. Organic light-emitting diodes (OLEDs) were fabricated on these ZnO:Al films. A current efficiency of higher than 3.7 cd/A, was achieved. For comparison, 3.9 cd/A was achieved by the reference OLEDs fabricated on commercial indium–tin–oxide substrates.
529 citations
TL;DR: In this paper, the effects of phonon confinement on the Raman spectra of silicon nanowires were measured and the contribution of pure confinement typical of quantum wires was identified.
Abstract: We measure the effects of phonon confinement on the Raman spectra of silicon nanowires. We show how previous spectra were inconsistent with phonon confinement, but were due to intense local heating caused by the laser. This is peculiar to nanostructures, and would require orders of magnitude more power in bulk Si. By working at very low laser powers, we identify the contribution of pure confinement typical of quantum wires.
344 citations
TL;DR: In this paper, a good epitaxial relationship between the Zn core and ZnO shell was observed, and misfit dislocations were observed at the interface, which accommodated the relatively large lattice mismatch.
Abstract: Coaxial Zn/ZnO nanocables and ZnO nanotubes have been fabricated via a thermal reduction route using ZnS powder as the source material. The samples were characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The as-synthesized Zn/ZnO nanocables consisted of a metallic core (Zn) ≈50 nm in diameter and a semiconductor outer shell (ZnO) ≈5 nm in thickness and several micrometers in length. A good epitaxial relationship between the Zn core and ZnO shell was observed, and misfit dislocations were observed at the Zn/ZnO interface, which accommodated the relatively large lattice mismatch. The outer diameter and wall thickness of the ZnO nanotubes are ≈60 and ≈10 nm, respectively. The possible formation mechanisms for the Zn/ZnO nanocables and ZnO nanotubes are discussed.
302 citations
282 citations
TL;DR: In this article, a sensor made by a bundle of etched silicon nanowires is presented, which exhibits a fast response, high sensitivity and reversibility, as well as the effect of silicon oxide sheath on the sensitivity.
243 citations
TL;DR: In this article, a hole-transporting material, 1,3,4,5,6,7-hexaphenyl-2-{3‘-(9-ethylcarbazolyl)}-isoindole (HPCzI), has been synthesized.
Abstract: A new hole-transporting material, 1,3,4,5,6,7-hexaphenyl-2-{3‘-(9-ethylcarbazolyl)}-isoindole (HPCzI), has been synthesized. The new compound is a highly phenylated isoindole with good thermal and chemical stabilities. Thermal analysis using scanning calorimetry shows the compound to have a high melting point at 311.5 °C. Its bulky structure leads to good film-forming properties of the compound via thermal evaporation. Aside from the high hole mobility, the compound possesses other important attributes required for a good hole-transporting host material for applications in organic electroluminescence. The photophysical property of HPCzI and its performance as a hole-transporting material in a double-layered electroluminescent device were investigated. At a drive voltage of 8.8 V and a current density of 20 mA/cm2, the device with a configuration of ITO/HPCzI/AlQ3/MgAg (30 Ω/□:700 A:700 A:2000 A) showed a green AlQ3 emission with a current efficiency of 3.5 cd/A.
180 citations
174 citations
TL;DR: Schiff base 2,3-bis [(4-diethylamino-2-hydroxybenzylidene)amino]but-2enedinitrile (BDPMB) and its zinc complex with donor-acceptor-donor (D-A-D) type ICT properties in the neutral form were used as novel red-emitting dopants in OLEDs as discussed by the authors.
TL;DR: In this article, the electrical properties of ZnO nanowires and intramolecular p-n junctions were characterized by I-V measurements, and it was shown that the average resistivity of the nanowire in AAO templates was about one order of magnitude higher than that of the naked single ZnOWire.
Abstract: Electrical properties of ZnO nanowires and intramolecular p–n junctions were characterized by I–V measurements. These nanowires were grown embedded in anodic aluminum oxide (AAO) templates by vapor-phase-transport growth method. The nanowires were dense, continuous, and uniform in diameter along the length of the wires. I–V measurements showed the average resistivity of the ZnO nanowires in AAO templates was about one order of magnitude higher than that of the naked single ZnO nanowire. The p–n junctions in ZnO nanowires were fabricated by a two-step growth of ZnO with and without dopant of boron (∼1 wt %) in the source. I–V results suggested that p–n junctions in ZnO nanowires were formed by the two-step method.
TL;DR: In this paper, the authors reported efficient white emission from mixing of yellow emission from Re-complexes, (4,4′-dimethyl-2,2′-bipyridine)Re(CO)3Cl (Dmbpy-Re), and blue emission from the N,N′-di-1-naphthyl-N, N′-diphenylbenzidine (NPB) is reported.
Abstract: Efficient white emission from the mixing of yellow emission from the Re-complexes, (4,4′-dimethyl-2,2′-bipyridine)Re(CO)3Cl (Dmbpy-Re), and blue emission from the N,N′-di-1-naphthyl-N, N′-diphenylbenzidine (NPB) is reported. NPB is used as the hole-transporting layer. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is used as the exciton-blocking as well as electron-transporting layer. Dmbpy-Re is doped into the host material (4,4′-N-N′-dicarbazole)biphenyl (CBP). The chromaticity of the white emission can be tuned by adjusting the thickness of Dmbpy-Re:CBP layer or the concentration of Dmbpy-Re in CBP. The maximum luminance, efficiency, and Commission Internationale De L’Eclairage coordinates at 9 V of the devices with 20-nm-thick and 30-nm-thick Dmbpy-Re:CBP layer at a fixed Dmbpy-Re doping concentration of 2 wt % in CBP are 1332 cd/m2, 2.9 cd/A, and (0.30,0.37), and 2410 cd/m2, 5.1 cd/A, and (0.36,0.43), respectively. The turn-on voltage of these devices is ∼4 V.
TL;DR: The efficiency of white organic light-emitting devices has been improved dramatically by the introduction of a phosphorescent material, fac tris (2-phenylpyridine) iridium [Ir(ppy)3] as a sensitizer as discussed by the authors.
Abstract: The efficiency of white organic light-emitting devices has been improved dramatically by the introduction of a phosphorescent material, fac tris (2-phenylpyridine) iridium [Ir(ppy)3] as a sensitizer. Ir(ppy)3 and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl) are codoped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. The chromaticity of white emission can be tuned by adjusting the thickness of the codoped CBP layer or the concentration of Ir(ppy)3. The luminance efficiency of these devices is affected at the same time due to the severe difference in efficiency between the two primary emission peaks.
TL;DR: In this article, a simple method based on the thermal oxidation of Si wafers has been discovered to provide a large-scale synthesis of very long, aligned silica nanowires.
TL;DR: The thermal stability of the hydrogen-passivated surfaces of SiNWs was investigated by measuring the FTIR spectra after annealing at different elevated temperatures, and it was found that hydrogen desorption of the trihydrides occurred at approximately 550 K, and that of the dihydride occurred at about 650 K.
Abstract: Attenuated total reflection Fourier transform infrared (FTIR) spectroscopy was used to characterize the surface species on oxide-free silicon nanowires (SiNWs) after etching with aqueous HF solution. The HF-etched SiNW surfaces were found to be hydrogen-terminated; in particular, three types of silicon hydride species, the monohydride (SiH), the dihydride (SiH2), and the trihydride (SiH3), had been observed. The thermal stability of the hydrogen-passivated surfaces of SiNWs was investigated by measuring the FTIR spectra after annealing at different elevated temperatures. It was found that hydrogen desorption of the trihydrides occurred at ∼550 K, and that of the dihydrides occurred at ∼650 K. At or above 750 K, all silicon hydride species began to desorb from the surfaces of the SiNWs. At around 850 K, the SiNW surfaces were free of silicon hydride species. The stabilities and reactivities of HF-etched SiNWs in air and water were also studied. The hydrogen-passivated surfaces of SiNWs showed good stabilit...
TL;DR: The first synthesis of ultrafine zinc sulfide (ZnS) nanowires with a sphalerite structure was reported in this paper, where large-area, high-and uniform-density ZnS nanowsires were grown on Au-coated silicon substrates by hydrogen-assisted thermal evaporation.
TL;DR: A new family of isophorone-based red fluorescent materials (A, B, C, and D) with a typical donor-π-acceptor structure was designed and synthesized for use in organic light-emitting devices (OLEDs) as mentioned in this paper.
Abstract: A new family of isophorone-based red fluorescent materials (A, B, C, and D) with a typical donor-π-acceptor structure was designed and synthesized for use in organic light-emitting devices (OLEDs)....
TL;DR: A blue emitting material, 9,10-bis(2 ′ -naphthyl)anthracene (BNA), was synthesized and used as a host emitting material in organic light-emitting devices (OLEDs) as mentioned in this paper.
TL;DR: In this paper, bias-assisted reactive ion etching was used to produce uniform, large-area cone arrays of diamond films, which can be applied to any structure of diamond film ranging from microcrystalline to nanocrystalline.
Abstract: A structuring method capable of producing uniform, large-area cone arrays of diamond films was developed. The technique employs bias-assisted reactive ion etching and is applicable to any structure of diamond films ranging from microcrystalline to nanocrystalline. Variation of the etching conditions enables control of the cone density, geometry, and height. Surface nanostructuring of cone arrays significantly improves the field emission properties of diamond films of all kinds. The turn on field is reduced to 6 and 10 V/μm for nanodiamond and microdiamond films, respectively, (compared to >25 V/μm for as deposited surfaces). Lower cone density yields better field electron emission (lower turn-on electrical field) due to the screening in high-density cone arrays. The field emission properties are determined by both the enhancement factor of the cone array and the emitting properties of the material. The field electron emission properties of nanodiamond arrays are better than cone arrays of single crystalli...
TL;DR: It was demonstrated that zeolite can be used as a pseudo-template to grow very fine and uniform silicon nanostructures via disproportionation reaction of SiO by thermal evaporation.
Abstract: It was demonstrated that zeolite can be used as a pseudo-template to grow very fine and uniform silicon nanostructures via disproportionation reaction of SiO by thermal evaporation. Three distinct types of composite nanowires and nanotubes of silicon and silica were grown on the surfaces of zeolite Y pellets. The first type is formed by an ultrafine crystalline silicon nanowire sheathed by an amorphous silica tube (a silicon nanowire inside a silica nanotube). The second type is formed by a crystalline silicon nanotube filled with amorphous silica (a silicon nanotube outside a silica nanowire). The third type is a biaxial silicon-silica nanowire structure with side-by-side growth of crystalline silicon and amorphous silica. These silicon nanostructures exhibit unusually intense photoluminescence (in comparison to ordinary silicon nanowires).
TL;DR: In this paper, a high performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices.
Abstract: A high-performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices. Standard tris-(8-hydroxyquinoline) aluminum/α-napthylphenylbiphenyl diamine devices with this bilayer cathode showed dramatically reduced operating voltage and a low turn-on voltage of 2.42 V as compared to 3.75 and 2.95 V in devices using, respectively, the Mg:Ag and single-layer Yb cathodes. At a current density of 200 mA/cm2, devices with the CsF/Yb cathode exhibited high luminance efficiency of 3.45 cd/A and power efficiency of 1.27 lm/W. Analysis by x-ray photoemission spectroscopy suggested that the performance improvement is related to the substantial reduction of electron injection barrier at the cathode/organic interface. It was found that upon Yb deposition, CsF dissociates to liberate low work function Cs metal atoms resulting in a cathode with a lower electron injection barrier and thus a better balance of carriers in the devic...
TL;DR: In this article, as-synthesized boron nanowires were characterized by means of scanning electron microscopy (SEM), TEM, selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS).
TL;DR: In this article, the 1H-pyrazolo[3,4-b]quinoxaline derivatives with N,N-dialkylamino electron-donating groups were prepared; their photoluminescence in solution and electroluminecence in thin-film devices have been investigated.
Abstract: Several new 1H-pyrazolo[3,4-b]quinoxaline derivatives with N,N-dialkylamino electron-donating groups were prepared; their photoluminescence in solution and electroluminescence in thin-film devices have been investigated. They show emission at 520–540 nm with a fluorescence quantum yield close to unity in moderately polar solvents. The absorption and emission maxima shift to red with increasing solvent polarity. The fluorescence quantum yield also increases with increasing solvent polarity from non-polar to moderately polar solvents, then decreases slightly with further increase of solvent polarity. It indicates that both “positive” and “negative” solvatokinetic effects co-exist. The electroluminescence of OLED devices fabricated by using these compounds as dopants was investigated. All devices with the configuration of indium–tin oxide coated glass/NPB (60 nm)/Alq3 : Dopant (30 nm)/Alq3
(20 nm)/Mg : Ag (200 nm) show bright green emission with efficiencies of 7.5–9.7 cd A−1 and narrow full bandwidth at half intensity of 68–75 nm peaking at 536–552 nm.
TL;DR: In this paper, the effects of phonon confinement on the Raman spectra of silicon nanowires (SiNWs) were measured and the contributions of pure confinement, heating and carrier photo-excitation were identified.
TL;DR: A review of recent progress in the deposition of thick, adherent, cubic boron nitride (c-BN) films is given in this paper, detailing the success of depositing thick (<20 μm), stress-free, adherent films with a larger crystallite size and significantly better crystalline quality.
Abstract: This article reviews recent progress in the deposition of thick, adherent, cubic boron nitride (c-BN) films. Most of the previous work applied ion-assisted physical vapor deposition methods to deposit c-BN films. The ion impact was successful in nucleating c-BN crystallites, but it resulted in a very small crystallite size and introduced stress, which caused the delamination of films thicker than 100 nm. Recent efforts to reduce the stress and obtain thicker films are described. The limited success of these attempts motivated us to explore chemical vapor deposition methods based on fluorine chemistry. We review this work, detailing the success of depositing thick (<20 μm), stress-free, adherent films with a larger crystallite size and significantly better crystalline quality.
TL;DR: In this article, the growth of GaN nanowires is discussed in terms of the oxide-assisted metal-catalyst vapor-liquid-solid model, and the GaN was systematically characterized by scanning electron microscopy, transmission electron microscope, x-ray diffraction, Raman spectroscopy, and photoluminescence (PL).
Abstract: High-quality GaN nanowires doped with silicon have been synthesized by hot-filament chemical vapor deposition on Au-coated Si (100) wafers. The GaN was systematically characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). The GaN nanowires had a uniform concentration of 3% Si, a uniform diameter around 10 nm, and a hexagonal wurtzite structure grown along the [001] direction. The intense PL peak of GaN nanowires at 344 nm showed a distinct blueshift from the bulk bandgap emission, revealing a clear quantum confinement effect. The growth of GaN nanowires is discussed in terms of the oxide-assisted metal-catalyst vapor–liquid–solid model.
TL;DR: In this paper, the first synthesis of 2H-polytype silicon carbide (SiC) nanowhiskers by the reaction of silicon monoxide (SiO) with methane was reported.