Showing papers by "Shuit-Tong Lee published in 2001"
TL;DR: The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies.
Abstract: The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure. © 2001 American Institute of Physics.
332 citations
TL;DR: In this article, Si-containing products with different colors and appearances were formed on the surfaces of the Si wafers over a wide temperature range of 890-1320 °C and a long distance of 85 mm.
Abstract: Silicon-based nanostructures with different morphologies, sizes, compositions, and microstructures were grown on Si wafers by thermal evaporation of SiO powders at 1350 °C for 5 h under 300 Torr of a flowing gas mixture of 5% H2-Ar at a flow rate of 50 standard cubic centimeters per minute (sccm). The SiO powders and Si wafers were placed inside an alumina tube, which was heated by a tube furnace. The local temperature inside the tube was carefully calibrated by a thermal couple. After evaporation, Si-containing products with different colors and appearances were formed on the surfaces of the Si wafers over a wide temperature range of 890-1320 °C and a long distance of 85 mm. Basing on the colors and appearances of the products, five distinct zones, which corresponding to different temperature ranges, were clearly identified from the highest temperature of 1320 °C to the lowest temperature of 890 °C. They are zone I (1250-1320 °C), zone II (1230-1250 °C), zone III (1180-1230 °C), zone IV (930-1180 °C), and zone V (890-930 °C). The deposited products were systematically studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that, besides Si nanowires, many other kinds of Si-based nanostructures such as octopuslike, pinlike, tadpolelike, and chainlike structures were also formed. The temperature distribution inside the alumina tube was found to play a dominant role on the formation of these structures. It is demonstrated that a control over the growth temperature can precisely control the morphologies and intrinsic structures of the silicon-based nanomaterials. This is an important step toward design and control of nanostructures. The growth mechanisms of these products were briefly discussed.
182 citations
158 citations
TL;DR: In this article, the luminance efficiency of the double heterostructure (DH) LEDs was investigated and the authors attributed the efficiency enhancement to the formation of a narrow recombination zone, in which both charge carriers and excitons were confined.
Abstract: Red light-emitting diodes (LEDs) with both a conventional bilayer structure and a double heterostructure (DH) have been investigated. In these LEDs, N,N′-bis-(1-naphthl)-diphenyl-1, 1′-biphenyl-4,4′-diamine (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-quinolinolato) aluminum (Alq3) were used as hole-transporting, hole-blocking, and electron-transporting layers, respectively. The bilayer and DH LEDs had a configuration of ITO/NPB/Alq3:red dopant/Alq3/MgAg and ITO/NPB/Alq3:red dopant/BCP/Alq3/MgAg, respectively. Three kinds of red fluorescent dyes—nile red, DCJTB, and DCM—were used as dopants. Compared with the bilayer structures, the luminance efficiencies of the DH LEDs were found to increase as much as 100%. We attribute the efficiency enhancement to the formation of a narrow recombination zone, in which both charge carriers and excitons were confined. High charge concentrations in the emissive layer resulted in efficient collision capture in the electron–hole recombination proc...
142 citations
TL;DR: In this paper, the 4-Dicyanomethylene-chromene moiety has been introduced as a π-electron acceptor in red fluorescent dye molecules for organic light-emitting devices (OLEDs).
Abstract: 4-Dicyanomethylene-chromene moiety has been introduced as a π-electron acceptor in red fluorescent dye molecules for organic light-emitting devices (OLEDs). On the basis of this moiety, a new family of red dopants with saturated emission, higher fluorescent quantum yield, and convenient synthetic procedures have been designed and synthesized. Their photoluminescent and electroluminescent properties have been examined and compared. On the basis of the results, useful guidelines for the molecular design of saturated red-emission fluorescent dopants for OLED applications are presented.
142 citations
139 citations
TL;DR: In this paper, the photoluminescence spectrum of as-grown ZnO whiskers was studied and the influences of reaction temperature, time, and carrier gases on the formation of the whiskers were investigated.
112 citations
107 citations
TL;DR: In this paper, a target of GaN mixed with gallium oxide (Ga 2 O 3 ) was used to synthesize a core-sheath structure of 80 nm in average diameter and tens of micrometers in length.
95 citations
TL;DR: In this paper, the synthesis and optical characterization of GaAs nanowires obtained by oxide-assisted laser ablation of a mixture of GAs and Ga2O3 was reported.
Abstract: This letter reports the synthesis and optical characterization of GaAs nanowires obtained by oxide-assisted laser ablation of a mixture of GaAs and Ga2O3. The GaAs nanowires have lengths up to tens of micrometers and diameters in the range of 10–120 nm, with an average of 60 nm. The nanowires have a thin oxide layer covering a crystalline GaAs core with a [111] growth direction. Raman scattering and photoluminescence (PL) characterizations of GaAs nanowires reveal that the spectral peaks significantly shifted and broadened from those of bulk GaAs material. The changes in these spectra are mainly attributed to impurities, defects, and residual stress in the GaAs nanowires.
87 citations
TL;DR: In this paper, a short voltage pulse was used as an excitation source for transient electroluminescence (EL) from organic bilayer devices having different N-arylbenzimidazole derivatives as electron-transporting layers (ETLs).
TL;DR: In this article, the growth of the GaP nanowires can be described by an oxide-assisted mechanism involving several oxidation-reduction reactions, which is beneficial for further exploration of their fundamental properties and applications.
Abstract: Gallium phosphide (GaP) nanowires of 22 nm in diameter and hundreds micrometers in length were synthesized by laser ablation of a powder mixture of GaP and gallium oxide (Ga2O3). The morphology and microstructure of GaP nanowires were investigated by transmission electron microscopy. Twins and stacking faults were observed on {111} planes of the GaP nanowires with special morphologies, and the formation of these defects was discussed. The growth of the GaP nanowires can be described by an oxide-assisted mechanism involving several oxidation-reduction reactions. The successful synthesis of GaP nanowires without any metallic impurities is beneficial for further exploration of their fundamental properties and applications.
TL;DR: In this paper, boron-doped and undoped silicon nanowires (SiNWs) were observed to have clear and regular nanoscale domains on the SiNW surface.
Abstract: Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements have been performed on boron-doped and undoped silicon nanowires (SiNWs). STM images clearly showed the presence of nanoparticle chains and nanowires in the B-doped SiNWs sample. Clear and regular nanoscale domains were observed on the SiNW surface, which were attributed to boron-induced surface reconstruction. STS measurements have provided current–voltage curves for SiNWs, which showed clearly enhancement in electrical conductivity by boron doping.
TL;DR: The amorphous boron-carbon-nitrogen (BCN) films were deposited by RF magnetron sputtering from a mixed target composed of h-BN and graphite as discussed by the authors.
TL;DR: In this article, a bilayer and trilayer device TPD/TPD with a different ratio of TPD to the Pr-complex was fabricated, and the emission of the 1D2→3H6 transition was promoted and that from the exciplex suppressed.
Abstract: Praseodymium(dibenzoylmethanato)3(bathophenanthroline) [Pr(DBM)3bath] was employed as an emitting and electron transport layer, and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1, 1′-biphenyl-4,4′-diamine (TPD) as a hole transport layer in organic electroluminescent (EL) devices. Bilayer device TPD/Pr(DBM)3bath and trilayer devices TPD/TPD:Pr(DBM)3bath/Pr(DBM)3bath with a different ratio of TPD to the Pr-complex were fabricated. Emission bands at 608 nm (1D2→3H6), 890 nm (1D2→3F2), 1015 nm (1D2→3F3), 1065 nm (1D2→3F4) and 1550 nm (1D2→1G4) originating from the internal f–f transitions of a Pr3+ ion were observed from EL devices using both bilayer and trilayer structures. Decreasing the ratio of TPD to the Pr-complex, the emission of the 1D2→3H6 transition was promoted and that from the exciplex suppressed, which was explained in terms of energy transfer from the ligand to the central ion.
Patent•
14 Sep 2001TL;DR: In this article, an organic electroluminescent device such as a light-emitting diode is disclosed, in which the emission layer comprises a single emitting material at different aggregate state to obtain constant chromaticity white emission.
Abstract: An organic electroluminescent device such as a light-emitting diode is disclosed, in which the emission layer comprises a single emitting material at different aggregate state to obtain constant chromaticity white emission. Correspondingly, a novel configuration has been developed to get white emission and color change in the organic EL devices.
TL;DR: In this paper, a red organic light-emitting diodes have been fabricated, which used a red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(8-methoxy-1,1,7,7-tetramethyljulolidyl- 9-enyl)4H-pyran (DCJMTB), as the emitter or guest dopant material.
Abstract: By using a series of tris-(8-hydroxyquinoline) metal chelates with central metal ions of Al3+, Ga3+, In3+ as the host materials, red organic light-emitting diodes have been fabricated, which used a red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(8-methoxy-1,1,7,7-tetramethyljulolidyl- 9-enyl)4H-pyran (DCJMTB), as the emitter or guest dopant material. The doped devices with Gaq3 as the host materials produce high efficiencies and saturated red colour chromaticity. The device (1% DCJMTB doped in Gaq3) showed a current efficiency of 2.64 cd A-1, which is about 40% higher than that of the doped Alq3 device, and nearly two times higher than that of the doped Inq3 device at 1% DCJMTB dopant concentration. The colour coordinates of the Gaq3:1% DCJMTB device in the Commission internationale del'Eclairage chromaticity chart are x = 0.63 and y = 0.36.
TL;DR: In this paper, a novel yellow dopant (DCTP) was synthesized for organic light-emitting devices (OLEDs), by introducing a triphenylamine segment as a π electron donor and 4-(dicyanomethylene)-2-(tert-butyl)-4H-pyran moiety as aπ electron acceptor to the molecular structure.
Abstract: We have synthesized a novel yellow dopant (DCTP) for applications in organic light-emitting devices (OLEDs), by introducing a triphenylamine segment as a π electron donor and 4-(dicyanomethylene)-2-(tert-butyl)-4H-pyran moiety as a π electron acceptor to the molecular structure. The resultant dopant has a high fluorescent yield and a sterically well-hindered structure. Its photoluminescence in solution and electroluminescence have been studied. The DCTP-doped OLED with a structure of indium tin oxide/N,N‘-bis(3-methylphenyl)-N,N‘-diphenyl-1,1‘-biphenyl-4,4‘-diamine (TPD)/tris(8-quinolinolato)aluminum (Alq3):2 wt % DCTP/Alq3/Mg:Ag shows a bright yellow emission (chromaticity coordinates: x = 0.47; y = 0.51) and a brightness of 19 383 cd/m2 at a driving voltage of 20 V. The current efficiency of the device reaches 5.3 cd/A at a current density of 20 mA/cm2 and a voltage of 11 V.
TL;DR: Based on a systematic study of gas phase silicon oxide clusters and the subsequent interesting findings such as the gas phase favorable composition and distinctive features in reactivity of the different silicon oxide cluster, this paper elucidated the mechanism of oxide-assisted nucleation of silicon nanostructures.
TL;DR: In this article, a very low field emission was achieved from aligned and opened carbon nanotube arrays, achieving field emission current densities of 10 microamperes per square centimeter at applied fields of 0.6-1 V/mum.
Abstract: A very low-field emission was achieved from aligned and opened carbon nanotube arrays. Field emission current densities of 10 microamperes per square centimeter were observed at applied fields of 0.6-1 V/mum, and current densities of 10 mA/cm(2) have been realized at applied fields as low as 2-2.7 V/mum. These fields are more than 2 times lower than those previously obtained for carbon nanotubes, and also represent the lowest field ever reported for any field emitting arrays at the same current densities, indicating that carbon nanotubes are superior field emitters.
TL;DR: The growth history suggests that these SiNWs were formed by nucleation which originated from the surface of amorphous SiO particle matrixes via phase separation and precipitation followed by growth through oxide-assisted vapor-soild reaction as discussed by the authors.
TL;DR: In this paper, the relationship between the geometry (size and spacing) of Si nanospheres, the initial state (diameter and oxide state) of the Si nanowires, and the experimental conditions are discussed.
Abstract: Bulk-quantity Si nanosphere chains have been fabricated. This is accomplished via the spheroidization of Si nanowires of semi-infinite lengths. The process has been extensively investigated by transmission electron microscopy. The nanosphere chains consisted of equally spaced Si crystalline nanospheres connected by Si-oxide bars. The transition from Si nanowires to Si nanosphere chains was determined by the annealing temperature, ambient pressure, initial Si nanowire diameters, and the oxide state of the outer layers of Si nanowires. The relationships between the geometry (size and spacing) of Si nanospheres, the initial state (diameter and oxide state) of Si nanowires, and the experimental conditions are discussed.
TL;DR: One-dimensional boron-doped Si nanoparticle chains synthesized in bulk quantity using laser ablating SiO powder mixed with B2O3 powder have been investigated by transmission electron microscopy and measured by electron field emission as mentioned in this paper.
Abstract: One-dimensional boron-doped Si nanoparticle chains synthesized in bulk quantity using laser ablating SiO powder mixed with B2O3 powder have been investigated by transmission electron microscopy and measured by electron field emission. Transmission electron microscopy showed that the outer diameters of the nanoparticles in the chains were around 15 nm. High-resolution transmission electron microscopy showed that the nanoparticles had perfect lattices with an 11 nm crystalline core and a 2 nm amorphous oxide outerlayer while the distance of the interparticles was 4 nm. Field-emission measurement showed that the turn-on field of Si nanoparticle chains was 6 V/μm, which was much lower than that of undoped Si nanowires (9 V/μm). X-ray photoelectron spectroscopy confirmed that the Si nanoparticles had been heavily doped by boron.
TL;DR: In this paper, diamond-like carbon (DLC) films were prepared on AISI 440C steel substrates at room temperature by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C 2 H 2 /Ar plasma under different conditions.
TL;DR: A benzothiazole derivative (BT) with a high fluorescence quantum yield ( φ f = 0.92) was synthesized and first used as a dopant in a blue organic electroluminescent device as discussed by the authors.
Abstract: A benzothiazole derivative (BT) with a high fluorescence quantum yield ( φ f =0.92) was synthesized and first used as a dopant in a blue organic electroluminescent device. TPD is N , N ′-bis(3-methylphenyl)- N , N ′-diphenyl-1,1′-biphenyl-4,4′-diamine and was used as a hole-transporting material; DPBI is a dimer of N -arylbenzimidazole and was used as an electron-transporting material as well as the host for the blue-emitting dopant, BT. A device with a structure of ITO/TPD/DPBI–2% BT/DPBI/Mg–Ag was fabricated. The devices showed electroluminescence with an emission peak around 475 nm. A luminance of 1260 cd m −2 was achieved at a current density of 400 mA cm −2 . The current efficiency of this device is about 0.50 cd A −1 at 5 V.
TL;DR: The surface properties of silicon nanowires after the removal of the surface oxide were studied in this article, where it was found that the surface silicon of the SiNWs can readily reduce silver (I) and copper (II) ions to metal aggregates of various morphologies.
Abstract: The chemical reactivity of hydrogen-passivated surface of silicon nanowires (SiNWs) towards the reductive deposition of silver and copper ions from solution is reported. SiNWs synthesized by laser ablation were used in the investigation. The surface properties of SiNWs after the removal of the surface oxide were studied. It is found that the surface silicon of the SiNWs can readily reduce silver (I) and copper (II) ions to metal aggregates of various morphologies on the SiNW surface at room temperature. The reaction products have been characterized with scanning electron microscopy, energy dispersive x-ray spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy. By varying the concentration of Ag(I) ions in the solution, nanostructures of silver with different shapes and sizes can be obtained. This approach for synthesis of metal nanostructures offers a potential method for the preparation of desired metal catalysts.
TL;DR: In this article, two functionalized bipyrroles, i.e., 3,3,3′, 5,5,5′-tetraethoxycarbonyl-4,4′-diphenyl-2,2′-b======ipyrrole 1 and 2, are highly luminescent materials at room conditions.
TL;DR: This work shows how ion beams can be used to nanostructure various diamond polytypes, epitaxially aligning them to a silicon substrate, to enable new carbon and non-carbon materials to be nanostructured for a host of applications.
Abstract: Carbon is unique in the variety of configurations it can adopt with itself and other elements. Here we show how ion beams can be used to nanostructure various diamond polytypes, epitaxially aligning them to a silicon substrate. The ready controllability of ion beams, which are already used to manufacture submicrometre-scale devices, means that our findings should enable new carbon and non-carbon materials to be nanostructured for a host of applications.
TL;DR: In this article, diamond-like carbon (DLC) films were deposited on silicon using methane and acetylene plasma induced by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD).