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Showing papers by "Shuit-Tong Lee published in 2006"


Journal ArticleDOI
TL;DR: In this article, a novel strategy for preparing large-area oriented silicon nanowire arrays on silicon substrates at near room temperature by localized chemical etching is presented, which is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution.
Abstract: A novel strategy for preparing large-area, oriented silicon nanowire (SiNW) arrays on silicon substrates at near room temperature by localized chemical etching is presented. The strategy is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. The density and size of the as-prepared SiNWs depend on the distribution of the patterned metal particles on the silicon surface. High-density metal particles facilitate the formation of silicon nanowires. Well-separated, straight nanoholes are dug along the Si block when metal particles are well dispersed with a large space between them. The etching technique is weakly dependent on the orientation and doping type of the silicon wafer. Therefore, SiNWs with desired axial crystallographic orientations and doping characteristics are readily obtained. Detailed scanning electron microscopy observations reveal the formation process of the silicon nanowires, and a reasonable mechanism is proposed on the basis of the electrochemistry of silicon and the experimental results.

650 citations


Journal ArticleDOI
TL;DR: CdS nanoribbon has the response speed substantively faster than those ever reported for conventional film and bulk CdS materials and the size ofnanoribbons has a significant influence on the responsespeed with smaller Cd S nanorIBbons showing higher response speed.
Abstract: The photoconductive characteristics of CdS single nanoribbons were investigated. The device characteristics, including spectral response, light intensity response, and time response, were studied systematically. It is found that CdS nanoribbon has the response speed substantively faster than those ever reported for conventional film and bulk CdS materials and the size of nanoribbons has a significant influence on the response speed with smaller CdS nanoribbons showing higher response speed. The high photosensitivity and high photoresponse speed are attributable to the large surface-to-volume ratio and high single-crystal quality of CdS nanoribbons and the reduction of recombination barrier in nanostructures. Measurements in a different atmosphere demonstrate that the absorption of ambient gas (mainly oxygen) can significantly change the photosensitivity of CdS nanoribbons through trapping electrons from the nanoribbons.

547 citations


Journal ArticleDOI
TL;DR: The etching process features weak dependence on the doping of the silicon wafers and, thus, provides an efficient method to prepare silicon nanowires with desirable doping characteristics.
Abstract: A straightforward metal-particle-induced, highly localized site-specific corrosion-like mechanism was proposed for the formation of aligned silicon-nanowire arrays on silicon in aqueous HF/AgNO3 solution on the basis of convincing experimental results. The etching process features weak dependence on the doping of the silicon wafers and, thus, provides an efficient method to prepare silicon nanowires with desirable doping characteristics. The novel electrochemical properties between silicon and active noble metals should be useful for preparing novel silicon nanostructures and also new optoelectronic devices.

278 citations


Journal ArticleDOI
TL;DR: The results show mesoporous SiO2-modified glassy carbon electrodes, particularly MCM-41-modified electrodes, open new opportunities for fast, simple, and sensitive field analysis of NACs.
Abstract: An electrochemical sensor for ultratrace nitroaromatic compounds (NACs) using mesoporous SiO2 of MCM-41 as sensitive materials is reported. MCM-41 was synthesized and characterized by scanning electron microscope, transmission electron microscopy, and small-angle X-ray diffraction. Glassy carbon electrodes modified with MCM-41 show high sensitivity for cathodic voltammetric detection of NACs (including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrotoluene, and 1,3-dinitrobenzene) down to the nanomolar level. The high sensitivity is attributed to the strong adsorption of NACs by MCM-41 and large surface area of the working electrode resulting from MCM-41 modification. The voltammetric response is fast, and the detection of NACs can be finished within 14 s. SiO2 nanospheres were similarly used to modify glassy carbon electrodes for electrochemical detection of TNT and TNB. The detection limit of SiO2 nanosphere-modified electrodes is lower than that of MCM-41-modified electrodes, poss...

198 citations


Journal ArticleDOI
TL;DR: In this article, bathophenanthroline (BPhen) was used as an exciton blocking layer (EBL) at the organic/cathode contact of a standard copper phthalocyanine/C60 organic photovoltaic (OPV) device.
Abstract: By using bathophenanthroline (BPhen) as an exciton blocking layer (EBL) at the organic/cathode contact of a standard copper phthalocyanine/C60 organic photovoltaic (OPV) device, power conversion efficiency was substantially increased from 0.86% to 2.64%. The BPhen-based devices showed a 45% increase in power conversion efficiency over that of an equivalent device with an EBL of bathocuproine. The performance improvement was analyzed in terms of the electron energy levels, optical transparencies and electron mobilities of the two EBLs. Based on these results, the roles of and requirements for an effective EBL were discussed. Combining the use of BPhen and a WO3 anodic buffer layer further increased the power conversion efficiency of the OPV device to 3.33%.

177 citations



Journal ArticleDOI
TL;DR: In this article, a bilayer connection unit of a stacked organic light-emitting device was investigated for application in stacked organic devices, which led to a stacked OLED with a luminous efficiency twice that of a single unit OLED.
Abstract: A bilayer connection unit of Mg-doped Alq3 and F4-TCNQ-doped m-MTDATA was investigated for application in stacked organic light-emitting device. This connection unit led to a stacked OLED with a luminous efficiency twice that of a single-unit OLED. Electronic structures, including relevant electron energy levels, of the various interfaces in the stacked OLED were studied by using ultraviolet photoemission spectroscopy and used to discuss the working mechanisms of the stacked OLED. The p-type dopant F4-TCNQ was shown to induce a large band bending of 1.36eV and facilitates efficient carrier injection from the connection units into the carrier-transporting layers.

93 citations


Journal ArticleDOI
TL;DR: In this paper, the intrinsic currentvoltage properties of a ZnO nanowire measured by a four-tip scanning tunneling microscopy (F-STM) were investigated.
Abstract: We report intrinsic current-voltage properties of ZnO nanowire measured by a four-tip scanning tunneling microscopy (F-STM). It is found that after bending the nanowire with the F-STM the conductance is reduced by about five orders of magnitude. The cathodoluminescent spectra indicate that the ZnO nanowires contain a sizable amount of defects in the surface region, responsible for their conduction. It is suggested that the observed huge conductance changes are caused by the shifting of the surface defect states in the ZnO nanowires in response to the applied surface strain.

85 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used silicon nanowires (SiNWs) as supporting matrices for enzyme immobilization to construct glucose biosensors for real-time detection of glucose.
Abstract: Silicon nanowires (SiNWs) were investigated as supporting matrices for enzyme immobilization to construct glucose biosensors. Glucose oxidase was adsorbed onto SiNWs after different treatments, either as grown, HF etched, or carboxylic acid (COOH) functionalized. The amperometric biosensor with COOH-functionalized SiNWs performed the best with a detection limit of 0.01mM glucose (signal-to-noise ratio=3). For real-time detection of glucose, SiNW biosensor showed a linear response in the range of 0.1–15mM. This work demonstrates the utility of SiNWs as a biosensor component and provides a general method to modify the surface of semiconducting nanomaterials for potential biomedical applications.

77 citations


Journal ArticleDOI
TL;DR: An effective optical spacer based on doping of ytterbium (Yb) metal into bathophenanthroline (BPhen) has been developed for applications in organic photovoltaic (OPV) devices as discussed by the authors.
Abstract: An effective optical spacer based on doping of ytterbium (Yb) metal into bathophenanthroline (BPhen) has been developed for applications in organic photovoltaic (OPV) devices. Utilizing Yb:BPhen as an optical spacer in standard copper phthalocyanine/C60 photovoltaic devices, power efficiency can be increased by four times to 3.42%. Ultraviolet photoemission spectroscopy measurements reveal that the good electron transport between C60 and Yb:BPhen is mainly related to the suitable energy level alignment at the interface. Combining with its high optical transparency and electrical conductivity, the Yb:BPhen film provides a useful means for maximizing the power conversion efficiency of OPV devices.

65 citations


Journal ArticleDOI
TL;DR: In this paper, field effect transistors made of individual CdSe nanoribbons were fabricated and characterized and showed high sensitivity to above-bandgap irradiation with four orders of magnitude increase in conductance and millisecond response speed.
Abstract: Field-effect transistors made of individual CdSe nanoribbons were fabricated and characterized. The CdSe nanoribbon showed n-type semiconducting characteristics, while the transistors revealed a threshold voltage of 20.9V, an on-off ratio >104, and an electron mobility of 9.6cm2∕Vs in the dark. CdSe nanoribbons showed high sensitivity to above-band-gap irradiation with four-orders-of-magnitude increase in conductance and millisecond response speed. The increase of electron mobility due to light irradiation was demonstrated to contribute to increased photoconductance.


Journal ArticleDOI
TL;DR: In this paper, the intrinsic photoluminescence (PL) peak at 585nm is attributed to Mn dopant and confirms Mn incorporation in ZnS, but does not appear without MnS, which is consistent with increased incorporation of Mn2+ ions.
Abstract: Manganese (Mn) doping of ZnS nanoribbons was achieved by simple thermal annealing. Upon heating ZnS nanoribbons with MnS powder up to 700°C, the intrinsic photoluminescence (PL) of the annealed nanoribbons disappeared and a new PL peak at 585nm gradually emerged. Significantly, the annealing process induced no detectable change in the morphology and uniform hexagonal wurtzite 2H structure of the single-crystal ZnS nanoribbons. The PL peak at 585nm is attributed to Mn dopant and confirms Mn incorporation in ZnS because (1) the peak appears only when ZnS ribbons were annealed with MnS, but does not appear without MnS, (2) its intensity increases with increasing annealing temperature, which is consistent with increased incorporation of Mn2+ ions, and (3) its position is similar to that of Mn-related emission in ZnS, and is independent of the measuring temperature and excitation power. This work demonstrates the capability of doping nanostructured materials by simple postannealing treatment.

Journal ArticleDOI
TL;DR: In this paper, the transport properties of single-crystal CdS nanoribbons were studied by evaluating the characteristics of field effect transistors made of individual Cs.
Abstract: The transport properties of single-crystal CdS nanoribbons were studied by evaluating the characteristics of field-effect transistors made of individual CdS nanoribbons. The nanoribbon transistors exhibited typical normally off n-channel characteristics with an on-off ratio as high as 104 in dark and in air. A barrier of 0.55eV at the metal-semiconductor contact was deduced from the temperature-dependent conductance. The CdS nanoribbons were highly sensitive to visible light and to adsorbed oxygen molecules. The transistor showed substantial improvement in performance under light irradiation and in vacuum.

Journal ArticleDOI
TL;DR: In this article, the electronic properties of copper-phthalocyanine/tris(8-hydroxyquinoline) aluminum (CuPc∕Alq3) heterojunctions on Mg and indium tin oxide (ITO) substrates have been studied by photoemission spectroscopy.
Abstract: The electronic structures of copper-phthalocyanine/tris(8-hydroxyquinoline) aluminum (CuPc∕Alq3) heterojunctions on Mg and indium tin oxide (ITO) substrates have been studied by photoemission spectroscopy. While the typical vacuum energy level lineup occurred at the CuPc∕Alq3 junction on ITO, the same junction formed on Mg displayed vastly different electronic structures, showing a 0.5eV band bending associated with the formation of a space charge layer. The substrate effects were explained by the proximity of the Mg’s Fermi level to the lowest unoccupied molecular orbital of CuPc, resulting in spontaneous charge transfer. The results show the feasibility of tuning the electronic properties of an organic heterojunction via the Fermi level of the substrate.

Journal ArticleDOI
TL;DR: The nucleation efficiency and the films structure were investigated using scanning and transmission electron microscopy and Raman spectroscopy in this paper, where nanodiamond films were deposited using different microwave plasma chemical vapor deposition schemes following several nucleation pretreatment methods.
Abstract: Nanodiamond films were deposited using different microwave plasma chemical vapor deposition schemes following several nucleation pretreatment methods. The nucleation efficiency and the films structure were investigated using scanning and transmission electron microscopy and Raman spectroscopy. C2 dimer growth (CH4 and H2 in 90% Ar) cannot nucleate diamond and works only on existing diamond surfaces. The methyl radical process (up to 20% CH4 in H2) allows some nucleation probability on appropriate substrates. Prolonged bias enhanced nucleation initiates both diamond nucleation and growth. C2 dimer growth results in pure nanodiamond free of amorphous carbon, while prolonged bias enhanced nucleation forms an amorphous carbon/nanodiamond composite.

Journal ArticleDOI
TL;DR: A series of nitrogen incorporated nanocrystalline diamond (ND) films were synthesized by microwave plasma-enhanced chemical vapor deposition (MP CVD) in a gas mixture of CH 4 /H 2 /N 2 with varying nitrogen concentration in a wide range.

Journal ArticleDOI
TL;DR: A new electrophosphorescent material, iridium(III)tris(5-(2,4-difluoro-phenyl)-10,10- dimethyl-4-aza-tricycloundeca-2, 4,6-triene), Ir(F2-mppy)3, has been synthesized and characterized.
Abstract: A new electrophosphorescent material, iridium(III)tris(5-(2,4-difluoro-phenyl)-10,10- dimethyl-4-aza-tricycloundeca-2,4,6-triene), Ir(F2-mppy)3, has been synthesized and characterized. Ir(F2-mppy)3 shows strong blue emission peaking at 475nm with a high phosphorescent quantum yield (ΦPL) of 0.72 in degassed toluene. Electroluminescent devices with a structure of indium tin oxide (ITO)/NPB (40nm)∕Ir(F2-mppy)3(x):CBP (30nm)/BCP (6nm)∕Alq3 (20nm)∕Mg:Ag (10:1) and 15% (10%) exhibit a maximum luminous power efficiency of 9.58lm∕W (7.68lm∕W), CIE coordinates (x=0.18,y=0.36) (x=0.17,y=0.29), and a maximum luminance of ∼9000cd∕m2 at 12.5V (∼6000cd∕m2 at 11.5V).

Journal ArticleDOI
TL;DR: The ability of increasing structural complexity beyond the one-step "thermal evaporation and condensation" synthesis provides a new dimension to the rational design of building blocks for nanodevices.
Abstract: Nanocantilever arrays were formed on the edges of the ±(0001) planes of pre-synthesized ZnS nanoribbons via catalyst-assisted post-annealing treatment on Si substrate at 600 °C. Similar nanostructu...

Journal ArticleDOI
TL;DR: In this article, a low-temperature photoluminescence (PL) and photoconductivity measurements were performed on Cu-doped CdZnS nanoribbons.
Abstract: Copper-doped Cd1?xZnxS (x~0.16) nanoribbons were prepared by controlled thermal evaporation of CdS, ZnS, and CuS powders onto Au-coated silicon substrates. The nanoribbons had a hexagonal wurtzite structure, and lengths of several tens to hundreds of micrometres, widths of 0.6?15??m, and thicknesses of 30?60?nm. Cu doping and incorporation into the CdZnS lattice were identified and characterized by low-temperature photoluminescence (PL) and photoconductivity measurements. Temperature-dependent PL measurement showed that the PL spectra of both Cu-doped and undoped CdZnS nanoribbons have two emission peaks at 2.571 and 2.09?eV, which are assigned to band edge emission and deep trap levels, respectively. In addition, the Cu-doped nanoribbons present two extra peaks at 2.448 and 2.41?eV, which are attributed to delocalized and localized donor and acceptor states in the band gap of CdZnS resulting from Cu incorporation. Photoconductivity results showed the nanoribbons can be reversibly switched between low and high conductivity under pulsed illumination. The Cu-doped CdZnS nanoribbons showed four orders of magnitude larger photocurrent than the undoped ones. The current jumped from ~2 ? 10?12 to ~5.7 ? 10?7?A upon white light illumination with a power density of ~9?mW?cm?2. The present CdZnS:Cu nanoribbons may find applications in opto-electronic devices, such as solar cells, photoconductors, and chemical sensors.

Journal ArticleDOI
TL;DR: The electronic excitation spectrum of ZnS nanowires (NWs) is characterized by cathodoluminescence (CL) excited using the tip of a scanning tunneling microscope as a highly localized and bright source of low-energy electrons with impinging power densities in the range of up to 60 kW/cm(2) or more.
Abstract: The electronic excitation spectrum of ZnS nanowires (NWs) is characterized by cathodoluminescence (CL) excited using the tip of a scanning tunneling microscope as a highly localized and bright source of low-energy (150−350 eV) electrons with impinging power densities in the range of up to 60 kW/cm2 or more. The CL spectra reveal significant differences when compared with the photoluminescence spectra of ZnS NWs. The differences can be associated with the properties of the surface region of the NWs, which are preferentially emphasized in the CL spectra owing to the small probing depth of low-energy electrons.

Journal ArticleDOI
TL;DR: In this paper, cubic boron nitride/nanocrystalline diamond (cBN/ND) composite films were deposited on silicon substrates using plasma-assisted (PA) CVD processes, and a systematic study of their mechanical properties was carried out.
Abstract: Large-area cubic boron nitride/nanocrystalline diamond (cBN/ND) composite films were deposited on silicon substrates using plasma-assisted (PA) CVD processes, and a systematic study of their mechanical properties was carried out. The direct growth of cBN on ND without an amorphous and turbostratic BN interfacial layer dramatically improves the adherence of cBN films. Because of the strengthened adhesion and improved mechanical properties due to the layered structure and the smooth surface, the composite films could be used in mechanical and some device applications.

Journal ArticleDOI
TL;DR: In this article, a single-layer light absorbing cathode was used to increase the contrast ratio of organic light-emitting devices (OLEDs), which is comparable to that of the traditional OLEDs with Mg:Ag cathode in terms of brightness, electroluminescence efficiency, and turn-on voltage.
Abstract: Sm:Ag, easily fabricated via simple thermal evaporation, was investigated as a single-layer light-absorbing cathode to increase the contrast ratio of organic light-emitting devices (OLEDs). The performance of OLEDs with Sm:Ag cathode was found to be comparable to that of the traditional OLEDs with Mg:Ag cathode in terms of brightness, electroluminescence (EL) efficiency, and turn-on voltage. The maximum EL efficiency of the device with Sm:Ag black cathode is 2.72cd∕A at 15V, while the contrast ratio reaches 390:1 at 10V under 140 lx of ambient light, which is 8 times better than that of the traditional device. The lower EL efficiency and enhanced contrast are due to the reduced optical reflectance of Sm:Ag black cathode, which was calculated to be about 0.15.

Journal ArticleDOI
TL;DR: In this article, the chemistry, electronic structure and electrical behavior at the interfaces between copper phthalocyanine (CuPc) and Mg with a reverse formation sequence were investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photo-emission (UPS), and current-voltage (I-V ) measurements.

Journal ArticleDOI
TL;DR: In this paper, Samarium bath was employed as an emitting and electron transport layer in organic light emitting diodes (OLEDs), and narrow electroluminescent (EL) emissions of a Sm3+ ion were observed in the visible and near infrared (NIR) region, differing from those of the same devices with Eu3+- or Tb3+complex EL devices with the same structure.
Abstract: Samarium (dibenzoylmethanato)3 bathophenanthroline (Sm(DBM)3 bath) was employed as an emitting and electron transport layer in organic light emitting diodes (OLEDs), and narrow electroluminescent (EL) emissions of a Sm3+ ion were observed in the visible and near infrared (NIR) region, differing from those of the same devices with Eu3+- or Tb3+-complex EL devices with the same structure. The EL emissions of the Sm3+-devices originate from transitions from 4G5/2 to the lower respective levels of Sm3+ ions. A maximum luminance of 490 cd m−2 at 15 V and an EL efficiency of 0.6% at 0.17 mA cm−2 were obtained in the visible region, and the improved efficiency should be attributed to introducing a transitional layer between the N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD) film and the Sm(DBM)3 bath film and the avoidance of interfacial exciplex emission in devices. Sharp emissions of Sm3+ ions in the NIR region were also observed under a lower threshold value less than 4.5 V.

Journal ArticleDOI
TL;DR: Fully coordinated ultrathin silica nanotubes were derived by performing first-principles calculations in this article, and they are well ordered and are of three magic configurations, quite different from the conventional amorphous thick silica Nanotubes.
Abstract: Fully coordinated ultrathin silica nanotubes are derived by performing first-principles calculations. The nanotubes are well ordered and are of three magic configurations, quite different from the conventional amorphous thick silica nanotubes. They possess low intrinsic strain energy, tunable band gaps, and high stiffness comparable to carbon nanotubes.

Journal ArticleDOI
TL;DR: In this article, the role of substrate temperature in controlling phase purity, crystallinity, growth rate, and residual stress of CBN films was discussed in the view of phase purity and crystallinity.

Journal ArticleDOI
TL;DR: In this article, the effect of substrate surface roughness on the subsequent growth of BN films at different ion energies (75 −500 eV) was investigated by Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and transmission electron microscopy.

Journal ArticleDOI
TL;DR: In this paper, the process of hole-injection in organic light-emitting devices (OLEDs) can be enhanced by inserting a UV-illuminated fluorocarbon (CFx) layer between indium-tin oxide (ITO) and organic hole-transporting layer (HTL).

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate bulk synthesis of highly crystal Cd-doped ZnO nanowires by using (Cd+Zn) powders at 600°C.