Showing papers by "Shuit-Tong Lee published in 2007"
TL;DR: A new sensing mechanism based on C=N isomerization, which shows a very significant fluorescence enhancement to the metal cations in a simple and efficient way, is demonstrated.
519 citations
TL;DR: In this article, a two-dimensional silica colloidal crystal template is used to create metal nanohole arrays on a silicon surface, which enables the controlled fabrication of aligned silicon nanowire (SiNW) arrays via metal catalytic etching.
Abstract: Two-dimensional silica colloidal crystal template is used to create metal nanohole arrays on a silicon surface, which enables the controlled fabrication of aligned silicon nanowire (SiNW) arrays via metal catalytic etching. By varying the size of silica colloidal crystals, aligned arrays of SiNWs with desirable diameter and density could be obtained. The formation of ordered SiNW arrays is due to selective and anisotropic etching of silicon induced by the silver pattern. The orientation of SiNW arrays is influenced by silver movement in silicon, and the wire axes are primarily along the ⟨100⟩ direction.
326 citations
TL;DR: In this paper, the photoresponse properties of single-crystal CdSe nanoribbons are studied systematically, and the photodetector shows a high sensitivity towards excitation wavelength with a sharp cut-off at 710 nm, corresponding to the bandgap of CdSE and exhibits a high photo-to-dark current ratio of five orders of magnitude at 650 nm.
Abstract: Photodetectors are fabricated from individual single-crystal CdSe nanoribbons, and the photoresponse properties of the devices are studied systematically. The photodetector shows a high sensitivity towards excitation wavelength with a sharp cut-off at 710 nm, corresponding to the bandgap of CdSe. The device exhibits a high photo-to-dark current ratio of five orders of magnitude at 650 nm, and can function with excellent stability, reproducibility, and high response speed (< 1 ms) in a wide range of switching frequency (up to 300 Hz). The photocurrent of the device shows a power-law dependence on light intensity. This finding together with the analysis of the light intensity-dependent response speed reveals the existence of various traps at different energy levels (shallow and deep) in the bandgap. Coating with a thin SiO 2 isolating layer increases the photocurrent but decreases the response speed of the CdSe nanoribbon, which is attributed to reduction of recombination centers on ribbon surface.
258 citations
TL;DR: The 1−2 nm and 3−4 nm SiQDs can photocatalyze CO2 reduction and dye (methyl red) degradation, while the 1−1 nm and 2−3 nm can photoc atalyze selective oxidation of benzene to phenol, owing to tunable band gap and excellent photoconductive properties ofSiQDs.
Abstract: The 1−2 nm SiQDs can photocatalyze CO2 reduction and dye (methyl red) degradation, while 3−4 nm SiQDs can photocatalyze selective oxidation of benzene to phenol, owing to tunable band gap and excellent photoconductive properties of SiQDs.
226 citations
TL;DR: By varying the process temperatures, the DAPMP nanostructures can be controllably prepared as either nanoribbons, nanotubes, or nanowires with high morphological and chemical purities.
Abstract: Single-crystal one-dimensional (1-D) nanostructures of [2-(p-dimethyl-aminophenyl)ethenyl]-phenyl-methylene-propanedinitrile (DAPMP) have been prepared by a simple solution process without the assistance of added surfactant, catalyst, or template under ambient condition. The approach exploits the directional supramolecular interaction induced by strong donor-acceptor dipole-dipole supramolecular interaction in the growth of 1-D nanostructures. By varying the process temperatures, the DAPMP nanostructures can be controllably prepared as either nanoribbons, nanotubes, or nanowires with high morphological and chemical purities. Significant changes in optical properties were observed for nanostructures of different morphology.
186 citations
TL;DR: In this article, the performance of organic photovoltaic (OPV) devices is dramatically enhanced by doping suitable fluorescent dyes into the donor and/or acceptor layers.
Abstract: Performance of organic photovoltaic (OPV) devices is dramatically enhanced by doping suitable fluorescent dyes into the donor and/or acceptor layers. By doping rubrene into standard CuPc∕C60 OPV cell, a high JSC of 30.1mA∕cm2, VOC of 0.58V, and an exceptionally high power conversion efficiency of 5.58% are achieved. The performance improvement is mainly attributed to efficient light absorption by rubrene in the range of 460–530nm where two hosts have low absorbance, leading to more effective exciton formation. Their findings motivate the use of fluorescent dyes for maximizing absorption spectral coverage as well as increasing photon harvesting.
164 citations
TL;DR: Si quantum dots, nanoparticles, nanowires, and ordered Si complex micro-/nanostructures can be obtained directly from silicon wafer by a polyoxometalate-assisted electrochemical method.
Abstract: Si quantum dots, nanoparticles, nanowires, and ordered Si complex micro-/nanostructures can be obtained directly from silicon wafer by a polyoxometalate-assisted electrochemical method.
158 citations
TL;DR: In this paper, a large-area array of wurtzite CdS nanowires is synthesized on Cd-foil substrates via a simple liquid reaction route using thiosemicarbazide and Cd foil as the starting materials.
Abstract: Highly ordered large-area arrays of wurtzite CdS nanowires are synthesized on Cd-foil substrates via a simple liquid reaction route using thiosemicarbazide and Cd foil as the starting materials. The CdS nanowires are single crystals growing along the [001] direction and are perpendicular to the surface of the substrate. The characteristic Raman peaks of CdS are red-shifted and show asymmetric broadening, which is ascribed to phonon confinement effects arising from the nanoscale dimensions of the nanowires. Significantly, the uniform CdS nanowire arrays can act as laser cavities in the visible-light range, leading to bandgap lasing at ca. 515 nm with obvious modes. The high density of nuclei and the preferential growth direction induce the formation of aligned CdS nanowires on the metal substrate.
127 citations
123 citations
TL;DR: In this paper, the authors reviewed the recent progress in the nucleation, growth and characterization techniques of cubic BN films and described various successful approaches in interface engineering and growth techniques in increasing film thickness, improving crystallinity and adhesion of cBN films to the substrate, which are the major issues hindering CBN films for both mechanical and electronic applications.
Abstract: Cubic BN (cBN) has a set of extreme properties similar or even superior to diamond The advance of science and technology of cBN has however been severely hampered by the poor quality of the material available (random orientation, limited film thickness, poor crystallinity and adhesion with substrates due to a non-cubic BN interlayer) This paper reviews the recent progress in the nucleation, growth and characterization techniques of cBN films It describes various successful approaches in interface engineering and growth techniques in increasing film thickness, improving crystallinity and adhesion of cBN films to the substrate, which are the major issues hindering cBN films for both mechanical and electronic applications Based on observations of the surface and interface structures, we further discuss the growth mechanisms of cBN films via physical and chemical routes
109 citations
TL;DR: In this article, the authors showed that vacuum level alignment is only valid for certain O/O heterojunctions rather than a general rule for organic junctions, and that the mode of energy level alignment depends on the Fermi level position in the organic energy gap.
Abstract: Electronic structures of organic/organic (O/O) heterojunctions have been studied by photoemission spectroscopy We showed that vacuum level alignment is only valid for certain O/O heterojunctions rather than a general rule for organic junctions The mode of energy level alignment is found to depend on the Fermi level position in the organic energy gap In general, when the Fermi level is near the midgap position, vacuum level alignment at the O/O heterojunction is observed, whereas when the Fermi level is close to the edge of the lowest unoccupied or highest occupied molecular orbital level, Fermi level pinning accompanied by molecular orbital level bending is observed at the O/O heterojunction
TL;DR: In this paper, single-crystal submicrometer rods and tubes of C60 with highly uniform size and shape were produced from a solvent-induced and surfactant assisted self-assembly technique.
Abstract: Single-crystal submicrometer rods and tubes of C60 with highly uniform size and shape were produced from a solvent-induced and surfactant assisted self-assembly technique. The length and length-to-width ratio of both rods and tubes were tunable by controlling the concentration of C60 in the stock solution. The transformation from rod to tube was achieved by simply varying the volume ratio of two solvents. Fourier-transform infrared and Raman spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy revealed the detailed structures of the rods and tubes. We proposed a concentration profile based growth model to describe the self-assembly process of C60 subunits. This study may contribute to better understanding of chemistry of fullerenes in solutions and extend the surfactant-assisted self-organization of inorganic system to fullerene system.
TL;DR: In this paper, silicon nanowires modified by palladium (Pd) nanoparticles were used for hydrogen detection using a thermal evaporation method using tin (Sn) as the catalyst.
Abstract: Silicon nanowires (SiNWs) modified by palladium (Pd) nanoparticles were investigated for hydrogen detection. SiNWs were fabricated via a thermal evaporation method using tin (Sn) as the catalyst. The as-grown SiNWs were chemically treated to remove surface oxide and then coated with a thin layer of Pd nanoparticles. A gas sensor device was fabricated with the Pd-functionalized SiNWs. The sensor showed better sensitivity to hydrogen and faster responding time than the macroscopic Pd wire hydrogen sensor.
TL;DR: In this article, the authors reported the synthesis of a new starburst molecule TTPPPA and its application as a hole-transporting material in organic light-emitting devices (OLEDs).
Abstract: This paper reports the synthesis of a new starburst molecule, 4,4′,4′′-tris[(2,3,4,5-tetraphenyl)phenyl]phenylamine (TTPPPA) and its application as a hole-transporting material in organic light-emitting devices (OLEDs). Although TTPPPA has almost the same ionization potential as 1,4-bis(1-naphthylphenylamino)biphenyl (NPB), the TTPPPA-based device of ITO/TTPPPA/Alq3/LiF/Al yields much better efficiency of 5.3 cd/A and 4.3 lm/W than the standard ITO/NPB/Alq3/LiF/Al device (3.0 cd/A and 2.9 lm/W). The remarkable performance enhancement is attributed to a better balance of hole and electron injection in the TTPPPA-based device. Further, TTPPPA has a much higher glass-transition temperature (Tg, 202 °C) than NPB (Tg, 98 °C), suggesting that TTPPPA can be an alternative material to NPB especially for high-temperature applications of OLEDs and other organic electronic devices.
TL;DR: In this article, the influence of the n-type layer in the connecting unit on the performance of tandem organic light-emitting devices (OLEDs) was investigated by systematically varying the metal dopants and the electron-transporting hosts.
Abstract: The present work investigates the influence of the n-type layer in the connecting unit on the performance of tandem organic light-emitting devices (OLEDs). The n-type layer is typically an organic electron-transporting layer doped with reactive metals. By systematically varying the metal dopants and the electron-transporting hosts, we have identified the important factors affecting the performance of the tandem OLEDs. Contrary to common belief, device characteristics were found to be insensitive to metal work functions, as supported by the ultraviolet photoemission spectroscopy results that the lowest unoccupied molecular orbitals of all metal-doped n-type layers studied here have similar energy levels. It suggests that the electron injection barriers from the connecting units are not sensitive to the metal dopant used. On the other hand, it was found that performance of the n-type layers depends on their electrical conductivities which can be improved by using an electron-transporting host with higher electron mobility. This effect is further modulated by the optical transparency of constituent organic layers. The efficiency of tandem OLEDs would decrease as the optical transmittance decreases.
TL;DR: In this article, a non-oped organic system of copper hexadecafluorophthalocyanine (F16CuPc)∕copper phthalocynine (CuPC) has been investigated as a connecting unit for deep-blue electrofluorescent tandem organic light-emitting devices (OLEDs) based on 2-methyl-9,10-di(2-naphthyl) anthracene emission.
Abstract: A nondoped organic system of copper hexadecafluorophthalocyanine (F16CuPc)∕copper phthalocyanine (CuPc) has been investigated as a connecting unit for deep-blue electrofluorescent tandem organic light-emitting devices (OLEDs) based on 2-methyl-9,10-di(2-naphthyl) anthracene emission. Such devices exhibited a doubling in current efficiency from 0.63to1.47cd∕A at J=100mA∕cm2 as compared to the single-unit device. The pure organic connecting unit showed superior optical transparency (∼100%), resulting in minimal microcavity effect in the devices. Interface dipole and band bending on both sides of the F16CuPc∕CuPc interface suggested the formation of an intrinsic p-n junction, which is a prerequisite of an effective connecting unit leading to a dramatic performance improvement in the tandem OLEDs.
TL;DR: In this article, the electronic band structures of hydrogenated silicon nanowires (SiNWs) could be significantly altered by axial stresses and axial compression could cause an indirect-to-direct band gap transition in SiNWs.
Abstract: We show via density functional calculations that the electronic band structures of ⟨112⟩-oriented hydrogenated silicon nanowires (SiNWs) could be significantly altered by axial stresses. In particular, an axial compression could cause an indirect-to-direct band gap transition in ⟨112⟩ SiNWs. As direct energy band may induce strong light-emission properties of Si, the possibility of indirect-to-direct band transition via axial stress has fundamental implications in exploiting SiNWs for optoelectronic applications.
TL;DR: In this paper, the authors show that the optical emission gaps of hydrogenated silicon nanoparticles are remarkably different and dully dependent on the size for those smaller than 1.5nm, based on their excited-state calculations of a series of nanoparticles from Si5H12 to Si199H140 using a time-dependent tight-binding density functional method.
Abstract: It is well known that the electronic and optical absorption gaps of hydrogenated silicon nanoparticles are inversely proportional to the particle size. Here, the authors show that their optical emission gaps are remarkably different and dully dependent on the size for those smaller than 1.5nm, based on their excited-state calculations of a series of nanoparticles from Si5H12 to Si199H140 using a time-dependent tight-binding density-functional method. It is revealed that this unusual size dependence is due to the strong excited-state structure relaxation in the particle core region that becomes significant when the size decreases.
TL;DR: In this article, LiF/Yb:Ag cathode was used as an effective and stable cathode in TOLEDs or top-emitting OLEDs, and the luminance and electroluminescent efficiency of the top/bottom side were 165/167/m 2 and 0.825/0.835/A, respectively.
TL;DR: In this article, the intrinsic and exciplex emissions from a single electroluminescent material, 4,4′,4″-trispyrenylphenylamine (TPyPA), were used to demonstrate high efficiency nonoped white organic light-emitting devices (WOLEDs).
Abstract: High-efficiency nondoped white organic light-emitting devices (WOLEDs) were demonstrated by using both the intrinsic and exciplex emissions from a single electroluminescent material, 4,4′,4″-trispyrenylphenylamine (TPyPA). The simple device structure of indium tin oxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/TPyPA/4,7-diphenyl-1,10-phenanthroline/LiF∕Al exhibited a luminance of 10000cd∕m2 at a low driving voltage of 4.5V, and high current and power efficiencies of 9.4cd∕A and 9.0lm∕W, respectively. Such WOLED showed excellent color stability and purity with the Commission Internationale de L’Eclairage coordinates of (0.31, 0.35), which remained unchanged over a wide range of luminance from 100to20000cd∕m2.
TL;DR: In this article, a patterned aluminum-doped zinc oxide (AZO) layer was used to synthesize vertically aligned ZnO nanowire arrays on silicon substrates without using any metal catalysts.
Abstract: Vertically aligned ZnO nanowire (NW) arrays have been synthesized selectively on patterned aluminum-doped zinc oxide (AZO) layer deposited on silicon substrates without using any metal catalysts. The growth region was defined by conventional photolithography with an insulating template. Careful control of the types of template materials and growth conditions allows good alignment and growth selectivity for ZnO NW arrays. Sharp ultraviolet band-edge peak observed in the photoluminescence spectra of the patterned ZnO NW arrays reveals good optical qualities. The current-voltage characteristics of ZnO NWs∕AZO∕p-Si device suggest that patterned and aligned ZnO NW arrays on AZO may be used in optoelectronic devices.
TL;DR: The ternary CdS1−XSeX nanoribbons of variable composition X were synthesized by the combination of thermal evaporation and laser ablation as discussed by the authors.
Abstract: Alloyed ternary CdS1−XSeX nanoribbons of variable composition X were synthesized by the combination of thermal evaporation and laser ablation. High-resolution transmission electron microscopy and x-ray diffraction showed that the ternary CdS1−XSeX nanoribbons were single phase and highly crystalline. Room-temperature optical measurements showed that band-gap engineering could be realized in CdS1−XSeX nanoribbons via modulation in composition X. Lasing emission between the band-gap energy of CdS (512 nm) and that of CdSe (710 nm) was observed for composition 0
TL;DR: In this paper, the authors show that II-VI nanoribbons are capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV).
Abstract: The authors show that II-VI nanoribbons are capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV). This is accomplished by simply using nanoribbons of two ternary compositions, namely, CdSXSe1−X and ZnYCd1−YS. Under optical pumping, CdSXSe1−X nanoribbons lase from NIR (710nm) to green (510nm) as X changes from 0 to 1, while ZnYCd1−YS nanoribbons lase from green (510nm) to UV (340nm) as Y varies from 0 to 1. Furthermore, lasing control shows fine-tuning via composition changes that overlap thermally induced tuning. This demonstrates that II-VI materials can enable lasing at any selected wavelength between 710 and 340nm with continuous tuning capabilities.
TL;DR: In this paper, the authors employ density functional theory within the generalized-gradient approximation to investigate infinitely long [0001] ZnO nanowires with triangular cross-sections, and report on atomic relaxations, formation energies, and electronic structure.
Abstract: The authors employ density functional theory within the generalized-gradient approximation to investigate infinitely long [0001] ZnO nanowires. The authors report on atomic relaxations, formation energies, and electronic structure of bare and hydrogen passivated ZnO wires with triangular cross sections. The authors find that surface reconstruction plays an important role in stabilizing the nanowires. The authors have shown that the band gap can be tuned by changing the wire diameter and by passivating with hydrogen. While bare and completely passivated wires are semiconducting, wires with intermediate hydrogen passivation exhibit metallic behavior.
TL;DR: In this paper, the bipolar carrier transport properties of 2,7-dipyrenyl-9,9′-dimethyl-fluorene (DPF) have been elucidated by using different device structures.
Abstract: Excellent bipolar carrier transport properties of 2,7-dipyrenyl-9,9′-dimethyl-fluorene (DPF) have been elucidated by using different device structures. A nondoped device using DPF as host emitter showed highly-efficient blue emission with a maximum efficiency of 6.0cd∕A and CIE coordinates of x=0.15 and y=0.19. Another device based on rubrene-doped DPF as emission layer gave pure high-efficiency white emission with good color stability, a maximum efficiency of 10.5cd∕A, and CIE coordinates of x=0.28 and y=0.35. The excellent bipolar transport capability and high performance as both emitter and host suggest that DPF is an efficient and versatile material for various applications in organic light-emitting devices.
TL;DR: A facile self-assembly growth route assisted by surfactant has been developed to synthesize tris(8-hydroxyquinoline)aluminium (Alq(3)) nanorods with regular hexagonal shape and good crystallinity, which exhibit field-emission characteristics with a very low turn-on field.
TL;DR: In this article, the effects of carbon nanotubes on polymerase chain reaction (PCR) and restriction digestion reaction were investigated and the results showed that CNT can reduce and even inhibit PCR and restrict digestion reaction, possibly due to the decrease of respective enzyme activity.
Abstract: Effects of multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT) functionalized with and without carboxylic groups on polymerase chain reaction (PCR) and restriction digestion reaction were investigated. The results showed that CNT can reduce and even inhibit PCR and restriction digestion reaction, possibly due to the decrease of respective enzyme activity. The inhibition effect on double restriction digestion reaction and PCR was increased in the order of CNT–COOH > pristine CNT and SWCNT> MWCNT. This study demonstrated that CNT may significantly affect the efficiency of biochemical reactions through different action mechanisms, which is critical for understanding how nanomaterials impact biological systems.
TL;DR: This is the first study on the cytotoxicity of SiNWs on the human hepatocellular carcinoma cell line, HepG2, and their effect on cell adhesion and spreading and the results showed that ACHTUNGTRENNUNGviability of HepG1 cells relative to the untreated control was affected by the SiNWS suspensions in a dose-dependent manner: the higher the concentration of Si NWs suspension, the lower the viability.
Abstract: The unique capabilities of nanomaterials make them good candidates for catalysts, biosensors, and even drug carriers. However, interactions of nanomaterials with biological systems and the environment could lead to toxicity. 2] While there have been reports on the cytotoxicity of carbon nanotubes, quantum dots, 9] gold nanoparticles, and the biocompatibilities of carbon nanotubes, 12] no study has yet appeared on the biological effects of silicon nanowires, which are becoming increasingly important as nanomaterial. 14] Silicon nanowires (SiNWs) are one-dimensional nanomaterials that are typically composed of a single crystalline silicon core and an amorphous SiOx sheath. Their outstanding properties, such as quantum size effects, diameter-dependent thermal conductivity, and large piezoresistance coefficient have attracted a lot of research interest, including for application in biological materials and devices. 14] However, before SiNWs can be incorporated into new and existing biomedical devices, their cytotoxicity and potential adverse effects on biological systems should be thoroughly investigated. Here, we report the first study on the cytotoxicity of SiNWs on the human hepatocellular carcinoma cell line, HepG2, and their effect on cell adhesion and spreading. When the concentration of SiNWs was more than 100 mg mL , the nanowires did not form suspensions and precipitated in the cell-culture plates. Therefore, 100 mg mL 1 was chosen as the highest concentration for the experiments. The incubation time of 48 h has been used in other cytotoxicity studies with nanomaterials, 16] and was also found to be optimal for our experiments. Cytotoxicity of SiNWs was evaluated by using the Alamar blue assay. Cell viability was determined by exposing HepG2 cells to SiNWs suspensions of various concentrations (0.1, 1, 25, 50, 100 mg mL ) for 48 h at 37 8C. Cell viability was normalized with respect to the untreated control sample. The results of the Alamar blue assay showed that ACHTUNGTRENNUNGviability of HepG2 cells relative to the untreated control was affected by the SiNWs suspensions in a dose-dependent manner: the higher the concentration of SiNWs suspension, the lower the viability. It seems that SiNWs exerted a degree of cytotoxicity towards HepG2 cells under these experimental concentrations (Figure 1).
TL;DR: In this paper, the authors used 4,7-diphenyl-1,10-phenanthroline (BPhen) or N,N-bis(1-naphthyl)-N,N′-dipsyl- 1,1′-biphexyl-4,4-4-diamine (NPB) into the blue light-emissive layer.
Abstract: High-efficiency and low operating voltage fluorescent white organic light-emitting devices (WOLEDs) have been realized by doping either 4,7-diphenyl-1,10-phenanthroline (BPhen) or N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) into the blue light-emissive layer. Devices doped with BPhen (or NPB) exhibited a maximum power efficiency of 8.7lm∕W (7.6lm∕W), about 74% higher than that of the reference device (5.0lm∕W). Such performance improvement is ascribed to the incorporation of a better electron-transporting layer and an improved carrier transport through the emissive layer by mixing with the higher drift mobility materials. It provides a simple and general means to improve the power efficiency of WOLED.
TL;DR: It is found that the adsorbed PA molecules slightly affect the structure of silicon core and the modified surface of SiQDs can serve as a reaction substrate to oxygen and solvent molecules, which is responsible for the increase in both PL stability and water solubility.
Abstract: The origin and stability of photoluminescence (PL) are critical issues for silicon nanoparticles to be used as biological probes. Optical and electronic properties of propionic-acid (PA) -terminated silicon quantum dots (SiQDs) were studied using the density-functional tight-binding method. We find that the adsorbed PA molecules slightly affect the structure of silicon core. The PA adsorption does not change the optical properties of SiQDs, while it substantially decreases the ionization potentials in the excited state and results in some new active orbitals with adjacent energies around the Fermi energy level. Accordingly, the modified surface of SiQDs can serve as a reaction substrate to oxygen and solvent molecules, which is responsible for the increase in both PL stability and water solubility.