Showing papers by "Shuit-Tong Lee published in 2008"

Silicon nanowires for rechargeable lithium-ion battery anodes

Abstract: Large-area, wafer-scale silicon nanowire arrays prepared by metal-induced chemical etching are shown as promising scalable anode materials for rechargeable lithium battery. In addition to being low cost, large area, and easy to prepare, the electroless-etched silicon nanowires (SiNWs) have good conductivity and nanometer-scale rough surfaces; both features facilitate charge transport and insertion/extraction of Li ions. The electroless-etched SiNWs anode showed larger charge capacity and longer cycling stability than the conventional planar-polished Si wafer.

Motility of metal nanoparticles in silicon and induced anisotropic silicon etching

Abstract: The autonomous motion behavior of metal particles in Si, and the consequential anisotropic etching of silicon and production of Si nanostructures, in particular, Si nanowire arrays in oxidizing hydrofluoric acid solution, has been systematically investigated. It is found that the autonomous motion of metal particles (Ag and Au) in Si is highly uniform, yet directional and preferential along the [100] crystallographic orientation of Si, rather than always being normal to the silicon surface. An electrokinetic model has been formulated, which, for the first time, satisfactorily explains the microscopic dynamic origin of motility of metal particles in Si. According to this model, the power generated in the bipolar electrochemical reaction at a metal particle's surface can be directly converted into mechanical work to propel the tunneling motion of metal particles in Si. The mechanism of pore and wire formation and their dependence on the crystal orientation are discussed. These models not only provide fundamental interpretation of metal-induced formation of pits, porous silicon, and silicon nanowires and nanopores, they also reveal that metal particles in the metal/Si system could work as a self-propelled nanomotor. Significantly, it provides a facile approach to produce various Si nanostructures, especially ordered Si nanowire arrays from Si wafers of desired properties.

Vertically Aligned p-Type Single-Crystalline GaN Nanorod Arrays on n-Type Si for Heterojunction Photovoltaic Cells

Abstract: Vertically aligned Mg-doped GaN nanorods have been epitaxially grown on n-type Si substrate to form a heterostructure for fabricating p-n heterojunction photovoltaic cells. The p-type GaN nanorod/n-Si heterojunction cell shows a well-defined rectifying behavior with a rectification ratio larger than 10(4) in dark. The cell has a high short-circuit photocurrent density of 7.6 mAlcm2 and energy conversion efficiency of 2.73% under AM 1.5G illumination at 100 mW/cm2. Moreover, the nanorod array may be used as an antireflection coating for solar cell applications to effectively reduce light loss due to reflection. This study provides an experimental demonstration for integrating one-dimensional nanostructure arrays with the substrate to directly fabricate heterojunction photovoltaic cells.

Silicon nanowire array photoelectrochemical solar cells

Abstract: Silicon nanowires (SiNWs) arrays prepared by electroless etching show excellent optical antireflectivity over a wide spectral bandwidth from 300to1000nm and surface defect-induced electrical conductivity. Both characteristics make the SiNWs a promising material for photovoltaic cell applications. Photoelectrochemical (PEC) measurements showed the electroless etching SiNWs are remarkably photoactive and effective in enhancing photovoltaic properties including photocurrent and photovoltage. Since electroless etching can enable simple, wafer-scale fabrication of SiNWs without the need of doping. SiNWs array thus prepared show great promise as low-cost and scalable photovoltaic-type PEC materials.

p-Type ZnO Nanowire Arrays

Abstract: Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on α-sapphire substrates by using N2O as a dopant source via vapor−liquid−solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1−2) × 1018 cm−3 and a field-effect mobility of 10−17 cm2 V−2 s−1. Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type Zn...

Surface‐Dominated Transport Properties of Silicon Nanowires

Abstract: Surface effects are widely recognized to significantly influence the properties of nanostructures, although the detailed mechanisms are rarely studied and unclear. Herein we report for the first time a quantitative evaluation of the surface-related contributions to transport properties in nanostructures by using Si nanowires (NWs) as a paradigm. Critical to this study is the capability of synthesizing SiNWs with predetermined conduction type and carrier concentration from Si wafer of known properties using the recently developed metal-catalyzed chemical etching method. Strikingly, the conductance of p-type SiNWs is is substantively larger in air than that of the original wafer, is sensitive to humidity and volatile gases, and thinner wires show higher conductivity. Further, SiNW-based field-effect transistors (FETs) show NWs to have a hole concentration two orders of magnitude higher than the original wafer. In vacuum, the conductivity of SiNWs dramatically decreases, whereas hole mobility increases. The device performances are further improved by embedding SiNW FETs in 250 nm SiO 2, which insulates the devices from atmosphere and passivates the surface defects of NWs. Owing to the strong surface effects, n-type SiNWs even change to exhibit p-type characteristics. The totality of the results provides definitive confirmation that the electrical characteristics of SiNWs are dominated by surface states. A model based on surface band bending and carrier scattering caused by surface states is proposed to interpret experimental results. The phenomenon of surface-dependent transport properties should be generic to all nanoscale structures, and is significant for nanodevice design for sensor and electronic applications.

Controlled synthesis of oriented single-crystal ZnO nanotube arrays on transparent conductive substrates

Abstract: Large-scale arrays of highly oriented single-crystal ZnO nanotubes (ZNTs) are successfully fabricated on transparent conductive substrates by a simple method from an aqueous solution at a low temperature (typically 85°C). The tubular morphology of the ZnO nanostructures is formed by a defect-selective chemical etching of the electrodeposited ZnO nanorods. The size of the ZNT arrays is determined by that of ZnO nanorod arrays which can be readily controlled by tuning several electrodeposition parameters. The present method can be employed to prepare ZNT arrays on flexible, conductive substrates, as well as on patterned conductive substrates.

Silicon nanowires-based fluorescence sensor for Cu(II).

Abstract: Si nanowires (SiNWs) were covalently modified by fluorescence ligand, N-(quinoline-8-yl)-2-(3-triethoxysilyl-propylamino)-acetamide (QlOEt) and finally formed an optical sensor to realize a highly sensitive and selective detection for Cu(II). The QlOEt-modified SiNWs sensor has sensitivity for Cu(II) down to 10-8 M, which is more sensitive than QlOEt alone. Metal ions interferences have no observable effect on the sensitivity and selectivity of QlOEt-modified SiNWs sensor. The SiNWs-based fluorescence sensor is reversible by addition of acid to replace Cu(II). The sensing mechanisms of QlOEt-modified SiNWs to Cu(II) and the rationale for the increase in sensitivity and selectivity of QlOEt-modified SiNWs over QlOEt on Cu(II) are discussed. The current sensor structure may be extendable to other chemo- and biosensors, and even to nanosensors for direct detection of specific materials in intracellular environment.

Highly Efficient Nondoped Blue Organic Light-Emitting Diodes Based on Anthracene-Triphenylamine Derivatives

Abstract: Blue light-emitting anthracene derivatives end-capped with triphenylamine for efficient hole transportation have been designed and synthesized using two-step Suzuki coupling reactions. The compounds possess high glass transition temperatures for good thermal stability and strong blue emission in solution. Typical three-layer organic light-emitting devices (OLEDs) made from these compounds show highly efficient blue emission, which are better than or comparable to state-of-the-art fluorescent OLEDs performance. For example, 9-pyrenyl-10-(4-triphenylamine) anthrancene (PAA)-based nondoped device exhibits efficient blue emission with a maximum efficiency up to 7.9 cd/A (or 6.8 lm/W). Based on the good hole transport of the anthracene-triphenylamine derivatives, deep blue emitting devices with high efficiency were achieved by using the derivatives as both emitter and hole transporter.

Nanowelded Carbon‐Nanotube‐Based Solar Microcells

Abstract: Photovoltaic (PV) cells are of immense interest due to their vast application potential in the fields of energy and communication. The adoption of ideal photoactive material and the design of optimum device structure are critical to achieving low-cost, high-efficiency PV cells. The semiconducting single-walled carbon nanotubes (SWNTs) are potentially an attractive material for PV applications due to their many unique structural and electrical properties. They are almost defect free to greatly decrease carrier recombination, bear a wide range of direct bandgaps matching the solar spectrum, and show strong photoabsorption and photoresponse from ultraviolet to infrared, and exhibit high carrier mobility [16] and reduced carrier transport scattering. Indeed, previous studies had attempted to fabricate SWNT films into photoelectrochemical solar cells. However, due to the inefficient separation and collection of photoexcited carriers and large intertube interaction, the maximum monochromatic incident photo-to-current conversion efficiency (IPCE) obtained for the cell is only 0.15%. Here, we report a novel approach that enables fabricating SWNT PV solar microcells with high power-conversion efficiency. In this cell, a directed array of monolayer SWNTs was nanowelded onto two asymmetrical metal electrodes with high and low work functions, respectively, resulting in a strong built-in electric field in SWNTs for efficient separation of photogenerated electron–hole pairs. Under solar illumination,

Fabrication of diamond nanopillars and their arrays

Abstract: High-density, uniform diamond nanopillar arrays were fabricated by employing bias-assisted reactive ion etching in a hydrogen/argon plasma. Gold nanodots were employed as etching masks. The formation of nanopillar structure is associated with the directional physical etching/sputtering by ion bombardment and selective chemical etching of sp2 carbons by reactive hydrogen atoms and ions. The density and geometry of the nanopillars depend on the initial structure of diamond films and reactive ion etching conditions. The nanopillars with high aspect ratio and large surface area may have potential applications in high-efficiency and high-sensitivity diamond-based biomedical and chemical sensors and in mechanical and thermal management.

High Efficiency and Small Roll-Off Electrophosphorescence from a New Iridium Complex with Well-Matched Energy Levels†

Abstract: It is well-known that, under electrical excitation, singlet and triplet excitons will be formed in organic light-emitting diodes (OLEDs) in the ratio of approximately 1 to 3 in the organic layers, where recombination of opposite charges occurs. The harvesting of triplet excitons for emission (termed as phosphorescence) cannot be obtained in common organic compounds because of the spin-forbidden rule. Thus, metal complexes, possessing the spin-orbit coupling effect caused by the heavy metal, are commonly used as phosphorescent emitters. Hence, intensive efforts have been made towards the development of metal complexes with different colors. Phosphorescent OLEDs (PhOLEDs) with peak efficiencies as high as 8.5 % (or 8.5 lm W) for blue, 19.2 % (or 70 lm W) for green, and 11 % (or 11.2 lm W) for red have been reported in the literature. On the other hand, PhOLEDs, in spite of having the ability of harvesting triplet energy that can result in high efficiency, are usually characterized by high rolloff in efficiency with increasing current density. Typically, the external quantum efficiency (EQE) declines sharply at current densities above 20 mA cm, which is the practical driving condition for passive-matrix OLEDs (PMOLEDs). Furthermore, high driving voltages are usually observed in these devices, which can result in low power efficiencies. As with the issues of generally high roll-off and driving voltage in PhOLEDs, it has been found that the roll-off was attributed to the triplet–triplet (T–T) annihilation and/or the field-induced exciton dissociation when there was a high density of triplet excitons with long lifetimes. The high driving voltage is supposed to be caused by two factors: one is the deep and strong charge traps caused by the doped iridium complex in the emitting layer; the second is related to the highest occupied molecular orbital (HOMO), which transfers holes, and the lowest unoccupied molecular orbital (LUMO), which transfers electrons, of materials used. In PhOLEDs, to match the triplet energy level of the dopant for exothermic energy transfer, high bandgap host materials are usually required. So the difference in HOMO levels and/or LUMO levels between the carrier transporting and the emitting layers is usually large, making charge injection into the emitting layer energetically unfavorable and thus leading to high driving voltage. To achieve high efficiency PhOLEDs with the characteristics of small roll-off and low driving voltage, it is believed that both high-efficiency phosphorescent materials and the practical construction of devices are needed. Of primary importance in device architectures is that the hole/electron injection barrier from the hole/electron transport layer to the emitting layer should be small, which will favor charge injection and avoid high driving voltage. Secondly, the density of triplet excitons should be low enough for the purpose of minimizing T–T exciton annealing. This criterion can be achieved by evenly distributing excitons in the bulk emission layer rather than having excitons located only at the narrow interface sites. Finally, the confinement of the exciton within the phosphorescent material layer is also important. If the exciton is not produced in and/or drifts out of the phosphorescent emitter layer with bias variation, the resulting singlet emission from other materials would not only definitely lead to impurity of color but also result in further roll-off in efficiency, because singlet excitons only take up 25 % of the total number of excitons formed by electrical excitation. In this work, we report a new homoleptic cyclometalated iridium triplet green emitter based on a phenylpyridazine ligand. The complex possesses good thermal stability, strong C O M M U N IC A TI O N

193 nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors

Abstract: Deep-ultraviolet (DUV) solar-blind photodetectors based on high-quality cubic boron nitride (cBN) films with a metal/semiconductor/metal configuration were fabricated. The design of interdigitated circular electrodes enables high homogeneity of electric field between pads. The DUV photodetectors present a peak responsivity at 180nm with a very sharp cutoff wavelength at 193nm and a visible rejection ratio (180 versus 250nm) of more than four orders of magnitude. The characteristics of the photodetectors present extremely low dark current, high breakdown voltage, and high responsivity, suggesting that cBN films are very promising for DUV sensing.

Ag-modified silicon nanowires substrate for ultrasensitive surface-enhanced raman spectroscopy

Abstract: We report a unique substrate for surface-enhanced raman spectroscopy (SERS) based on silver nanoparticles-embedded silicon nanowires (SiNWs). The SiNWs were prepared by thermal evaporation of SiO powder via oxide-assisted growth, oxide removed with HF, and then used to reduce silver ions to form a highly decorated Ag-embedded surface. Such modified SiNWs substrates yielded ultrahigh SERS sensitivity, which could detect 25μl of 1×10−16M Rhodamine 6G, 1×10−16M crystal violet, and 1×10−14M nicotine in methanol solutions. An Ag-modified SiNW strand could also enable SERS detection of 25μl of 1×10−8mg∕ml calf thymus DNA. The possible mechanisms for the ultrahigh SERS sensitivity were discussed.

A surface-enhanced Raman spectroscopy substrate for highly sensitive label-free immunoassay

Abstract: Large-scale uniform silicon nanowires (SiNWs) array was fabricated by chemical etching on n-Si(111) wafer. Silver nanoparticles (AgNPs) were loaded on their surfaces. The AgNPs on SiNWs (AgNPs@SiNWs) array exhibit strong surface-enhanced Raman effect. On the substrate surfaces, characteristic Raman signals are generated with trace amount of mouse immunoglobulin G (mIgG), goat-anti-mouse immunoglobulin G (gamIgG), and immune complexes formed from 4ng each of mIgG and gamIgG. The shifted positions and changed intensities in Raman bands indicate the occurrence of immunoreactions. This AgNPs@SiNWs array is a unique substrate for surface-enhanced Raman spectroscopy to show the immune reagents and immunoreactions at higher sensitivity.

Single-Crystal 9,10-Diphenylanthracene Nanoribbons and Nanorods

Abstract: Single-crystal 9,10-diphenylanthracene (DPA) nanoribbons and nanorods with uniform sizes and shapes were synthesized via a simple surfactant-assisted self-assembly process. The shape of the as-prepared nanostructures can be readily controlled by varying the solubility of DPA in the preparation solution. A growth mechanism was proposed for the formation of different morphological structures. Crystal structure analysis demonstrated that the overlap between the two phenyl groups at the opposite positions of the anthracene backbone forms effective intermolecular linking for crystal growth, in good agreement with prediction from quantum mechanical calculations. Electronic and optical properties of the as-prepared nanostructures are investigated.

Highly Efficient Blue Organic Light-Emitting Device Based on a Nondoped Electroluminescent Material

Abstract: A highly efficient nondoped blue organic light-emitting device was demonstrated by using a novel electroluminescent material DDPFTBC as an efficient electrofluorescence emitter. The device in a simple structure of ITO/NPB/DDPFTBC/TPBI/LiF/Al exhibited stable blue emission with the Commission Internationale de L’Eclairage coordinates of (0.18, 0.36), a high current efficiency of 8.7 cd/A, and a high power efficiency of 9.1 lm/W.

Direct electrochemistry and electrocatalytic activity of cytochrome c covalently immobilized on a boron-doped nanocrystalline diamond electrode.

Abstract: Cytochrome c (Cyt c) was covalently immobilized on a boron-doped nanocrystalline diamond (BDND) electrode via surface functionalization with undecylenic acid methyl ester and subsequent removal of the protecting ester groups to produce a carboxyl-terminated surface. Cyt c-modified BDND electrode exhibited a pair of quasi-reversible and well-defined redox peaks with a formal potential (E0) of 0.061 V (vs Ag/AgCl) in 0.1 M phosphate buffer solution (pH 7.0) and a surface-controlled process with a high electron transfer constant (ks) of 5.2 ± 0.6 s-1. The electrochemical properties of as-deposited and Cyt c-modified boron-doped microcrystalline diamond (BDMD) electrodes were also studied for comparison. Investigation of the electrocatalytic activity of the Cyt c-modified BDND electrode toward hydrogen peroxide (H2O2) revealed a rapid amperometric response (5 s). The linear range of response to H2O2 concentration was from 1 to 450 μM, and the detection limit was 0.7 μM at a signal-to-noise ratio of 3. The sta...

Optimal surface functionalization of silicon quantum dots

Abstract: Surface functionalization is a critical step for Si nanocrystals being used as biological probes and sensors. Using density-functional tight-binding calculations, we systematically investigate the optical properties of silicon quantum dots (SiQDs) with various termination groups, including H, CH(3), NH(2), SH, and OH. Our calculations reveal that capping SiQDs with alkyl group (-Si-C-) induces minimal changes in the optical spectra, while covering the surface with NH(2), SH, and OH results in evident changes compared to hydrogenated SiQDs. The structural deformations and electronic property changes due to surface passivation were shown to be responsible for the above-described features. Interestingly, we find that the optical properties of SiQDs can be controlled by varying the S coverage on the surface. This tuning effect may have important implications in device fabrications.

Interfacial electronic structures in an organic double-heterostructure photovoltaic cell

Abstract: The electronic structure of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/fullerene/copper-phthalocyanine double heterostructure in a typical organic photovoltaic cell was studied by photoemission spectroscopy. The results show that the traditional vacuum energy level lineup is not valid for these organic heterojunctions, which suggests formation of interface dipole and energy level bending across the interface due to charge transfer. Based on the measured energy levels using x-ray and ultraviolet photoemission spectroscopies, important parameters such as the theoretical maximum of the open circuit voltage are extracted, and their impacts on charge photogeneration process are discussed.

Surfactant-assisted alignment of ZnO nanocrystals to superstructures.

Abstract: Self-organization of ZnO nanoparticles into various superstructures (sheet, platelet, ring) has been achieved with the assistance of micelles formed by surfactant cetyltrimethylammonium bromide (CTAB) under one-pot condition. The CTAB-modified zinc hydroxy double salt (Zn−HDS) mesocrystals act as intermediates to form ZnO hexagonal superstructures at temperatures as low as 50 °C. The decomposition temperature of Zn−HDS mesocrystals is much lower than that of the corresponding bulk crystals because the organic additive CTAB effectively decreases the degree of crystallinity. Taking advantage of temperature-induced phase transformation of micelles, two-stage self-organization can form ZnO platelets and ring mesocrystals, that is, ZnO ellipsoidal superstructures formed through vertical attachment on (0001) facets of basic units can further assemble to form ZnO platelets and rings through vertical attachment on (0001) facets of ZnO ellipsoidal superstructures. The structural transformation of micelles as shape...

New Fluorene Derivatives for Blue Electroluminescent Devices : Influence of Substituents on Thermal Properties, Photoluminescence, and Electroluminescence

Abstract: A new series of fluorene derivatives with different substituents at the C-2,7- and C-9 positions has been synthesized and characterized with respect to their luminescence and thermal properties. It was found that substitutions at the C-2,7 positions significantly affected the thermal properties and optical properties, whereas substitutions at the C-9 position mainly affected the thermal properties of the fluorene derivatives. Organic light-emitting diodes (OLEDs) with a structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (15 nm)/4,4‘-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) (50 nm)/fluorene derivative (30 nm)/tris(8-quinolinolato)aluminum (Alq3) (50 nm)/Mg/Ag (200 nm) were fabricated. OLEDs based on the naphthyl-substituted fluorenes gave saturated blue emission centered at 452 nm, while those based on anthryl-substituted fluorenes showed a blue−green emission with a wider spectrum. OLEDs based on 2,7-dipyrenyl-9‘9-dimethylfluorene (DPF) exhibited blue emission with a maximum efficienc...

Controllable Synthesis of Vertically Aligned p-Type GaN Nanorod Arrays on n-Type Si Substrates for Heterojunction Diodes

Abstract: A new catalyst seeding method is presented, in which aerosolized catalyst nanoparticles are continuously self-assembled onto amine-terminated silicon substrates in gas phase to realize controllable synthesis of vertically aligned Mg-doped GaN nanorod arrays on n-type Si (111) substrates. The diameter, areal density, and length of GaN nanorods can be controlled by adjusting the size of Au nanoparticles, flowing time of Au nanoparticles, and growth time, respectively. Based on the synthesis of p-type GaN nanorods on n-type Si substrates, p-GaN nanorod/n-Si heterojunction diodes are fabricated, which exhibit well-defined rectifying behavior with a low turn-on voltage of similar to 1.0 V and a low leakage current even at a reverse bias up to 10 V. The controllable growth of GaN nanorod arrays and the realization of p-type GaN nanorod/n-type Si heterojunction diodes open up opportunities for low-cost and high-performance optoelectronic devices based on these nanostructured arrays.

High-sensitivity pesticide detection via silicon nanowires-supported acetylcholinesterase-based electrochemical sensors

Abstract: We report the use of a silicon-based nanocomplex, i.e., gold nanoparticles-coated silicon nanowires, for the improvement of acetylcholinesterase (AChE)-based electrochemical sensors for pesticide detection. Owing to the high electrical conductivity of the nanocomplex and its compatibility with the enzyme, the sensor exhibited significantly enhanced performance. The AChE enzyme bound to the surface possessed Michaelis-Menton constant of 81 mu M, resembling that in its free form. The sensor showed rapid response toward substrate acetylcholine in the concentration range of 1.0 mu M-1.0 mM. This AChE nanosensor could detect as low as 8 ng/L dichlorvos, an organophosphate pesticide. (C) 2008 American Institute of Physics.

Electrical properties of Be-implanted polycrystalline cubic boron nitride films

Abstract: P-type conductivity of polycrystalline cubic boron nitride (cBN) films was achieved by implantation of beryllium ions The effects of implantation doses and annealing on the phase composition and electrical properties of cBN films were studied A reduction in resistivity by seven orders of magnitude was observed Hall measurement revealed a corresponding hole concentration of 61×1018cm−3 and mobility of 3cm2∕Vs The activation energy was estimated to be 020±002eV from the temperature dependence of resistance The electrical properties of Be-implanted films are comparable to that of Be-doped cBN single crystals synthesized by high-pressure and high-temperature method