Showing papers by "Shuit-Tong Lee published in 2009"

Platinum nanoparticle decorated silicon nanowires for efficient solar energy conversion.

Abstract: High-density aligned n-type silicon nanowire (SiNW) arrays decorated with discrete 5-10 nm platinum nanoparticles (PtNPs) have been fabricated by aqueous electroless Si etching followed by an electroless platinum deposition process. Coating of PtNPs on SiNW sidewalls yielded a substantial enhancement in photoconversion efficiency and an apparent energy conversion efficiency of up to 8.14% for the PtNP-decorated SiNW-based photoelectrochemical solar cell using a liquid electrolyte containing Br(-)/Br(2) redox couple. The results demonstrate PtNP-decorated SiNWs to be a promising hybrid system for solar energy conversion.

Water‐Soluble Silicon Quantum Dots with Wavelength‐Tunable Photoluminescence

Abstract: [*] Prof. Z.-H. Kang, Dr. N.-B. Wong, Prof. S.-T. Lee, Dr. Y. Liu C. H. A. Tsang, Dr. D. D. D. Ma, Dr. X. Fan Center of Super Diamond and Advanced Films (COSDAF) Hong Kong SAR (P.R. China) E-mail: zhkang@suda.edu.cn; bhnbwong@cityu.edu.hk apannale@cityu.edu.hk Prof. Z.-H. Kang, Dr. N.-B. Wong, Dr. Y. Liu, C. H. A. Tsang Department of Biology and Chemistry City University of Hong Kong Hong Kong SAR (P.R. China)

Excellent photocatalysis of HF-treated silicon nanowires.

Abstract: HF-treated silicon nanowires exhibited excellent photocatalysis, which were even better than some noble metal-modified ones, such as palladium, gold, silver, and rhodium. This phenomenon was critical in the application of silicon-related materials as they are normally employed as a catalyst carrier. These HF-treated silicon nanowires were also stable in solution over 1 week; consequently, a possible explanation for the stability was proposed.

ZnO Nanotube Arrays as Biosensors for Glucose

Abstract: Highly oriented single-crystal ZnO nanotube (ZNT) arrays were prepared by a two-step electrochemical/chemical process on indium-doped tin oxide (ITO) coated glass in an aqueous solution. The prepared ZNT arrays were further used as a working electrode to fabricate an enzyme-based glucose biosensor through immobilizing glucose oxidase in conjunction with a Nafion coating. The present ZNT arrays-based biosensor exhibits high sensitivity of 30.85 μA cm−2 mM−1 at an applied potential of +0.8 V vs. SCE, wide linear calibration ranges from 10 μM to 4.2 mM, and a low limit of detection (LOD) at 10 μM (measured) for sensing of glucose. The apparent Michaelis−Menten constant KMapp was calculated to be 2.59 mM, indicating a higher bioactivity for the biosensor.

Vertically Aligned ZnO Nanorod Arrays Sentisized with Gold Nanoparticles for Schottky Barrier Photovoltaic Cells

Abstract: Vertically aligned zinc oxide (ZnO) nanorod arrays coated with gold nanoparticles have been used in Schottky barrier solar cells. The nanoparticles enhance the optical absorption in the range of visible light due to the surface plasmon resonance. In charge separations, photoexcited electrons are transferred from gold nanoparticles to the ZnO conduction band while electrons from donor (I−) in the electrolyte compensate the holes left on the gold nanoparticles. The fill factors of the dye-free Schottky barrier cell reach a value of ∼0.50. However, after incorporation of N719 sensitizing dye, the open circuit voltage increases to 0.63 V from 0.5 V being measured for dye-sensitized solar cells based on the bare ZnO nanorods. The Schottky barrier at the ZnO/Au interface blocks the electron transfer back from ZnO to the dye and electrolyte, and thus increases the electron density at the ZnO conduction band. The efficiency of the gold-coated ZnO nanorod dye-sensitized solar cells is thus increased from 0.7% to 1.2%.

Photo and pH stable, highly-luminescent silicon nanospheres and their bioconjugates for immunofluorescent cell imaging.

Abstract: We report a novel kind of oxidized silicon nanospheres (O-SiNSs), which simultaneously possess excellent aqueous dispersibility, high photoluminescent quantum yield (PLQY), ultra photostability, wide pH stability, and favorable biocompatibility. Significantly, the PLQY of the O-SiNSs is as high as 25%, and is stable under intense UV irradiation and in acidic-to-basic environments covering the pH range 2-12. To our best knowledge, it is the first example of water-dispersed silicon nanoparticles which possess both high PLQY and robust pH stability suitable for broad utility in bioapplications. Furthermore, the O-SiNSs are readily conjugated with antibody, and the resultant O-SiNSs/antibody bioconjugates are successfully applied in immunofluorescent cell imaging. The results show that the highly luminescent and stable O-SiNSs/antibody bioconjugates are promising fluorescent probes for wide-ranging bioapplications, such as long-term and real-time cellular labeling.

Ultrastable, Highly Fluorescent, and Water‐Dispersed Silicon‐Based Nanospheres as Cellular Probes

Abstract: Fluorescent cellular probes are powerful tools for studying cellular morphology, behavior, and physiological functions. For optimum imaging and tracking of biological cells, these probes should be water-dispersible, antibleaching, luminescent, and biocompatible. In the last century, organic dyes and fluorescent proteins were mostly used as fluorescent probes in biological and biomedical research; however, they suffer from severe photobleaching that restricts their applications for long-term in vitro or in vivo cell imaging. This shortcoming has led to an intense interest in colloidal fluorescent semiconductor quantum dots (QDs) since II/VI QDs were shown to be promising for cell imaging in 1998. Compared to fluorescent dyes, QDs possess unique advantages such as size-tunable emission wavelengths, broad photoexcitation, narrow emission spectra, strong fluorescence, and high resistance to photobleaching. Consequently, QDs have been widely used as a new class of fluorescent probes in cellular research, for example, in vitro imaging, cell labeling, and tracking cell migration. However, recent investigations have shown that these QDs are cytotoxic, especially under harsh conditions (e.g., UV irradiation), because heavy metal ions (such as Cd or Pb ions) are often released in oxidative environments, which leads to severe cytotoxicity by conventional mechanisms of heavy metal toxicity. Surface modification of QDs, such as epitaxial growth of the ZnS shell, silica coating, or polymer coating, has been used to alleviate the cytotoxicity problem. While these techniques are viable to a certain extent, they are relatively complicated and require additional processing steps. Notably, the risk of cytotoxic problems still remains since heavy-metal ions contained in the modified QDs may invariably be released in physiological environments. Consequently, the inherent problems, that is, severe photobleaching and cytotoxicity, associated with the traditional dyes and the fluorescent II/VI QDs remain unsolved, and have fueled a continual and urgent search for new cellular probes that are more photostable and biocompatible. Silicon is the leading semiconductor material for technological applications because of its wide-ranging applications by the electronics industry. Silicon-based nanostructures, such as nanoribbons, nanowires, and nanodots, are being intensely investigated. The quantum confinement phenomenon in silicon QDs (SiQDs) is a particular focus of research since it would increase the probability of irradiative recombination by indirect-to-direct band-gap transitions, which lead to enhanced fluorescent intensity and the prospect of longawaited optical applications. The biocompatibility and noncytotoxic properties of SiQDs are much better than those of traditional II/VI QDs, although their photoluminescence (PL) intensity is weaker. As such, silicon-containing materials may be useful for biological applications such as cellular imaging and labeling because of their favorable biocompatibility, if water-dispersed silicon nanomaterials with adequate stability and strong fluorescence are properly developed. Herein we report a new variety of silicon-based nanospheres for use as cellular probes, which possess excellent water-dispersibility, strong photoluminescence, and robust photostability. Our design of these nanospheres was formulated from calculations performed by using the B3LYP/6-31G method, a classic calculation method based on hybrid density functional theory. Figure 1a shows that the calculated free energy of the products PSi O ( 15.2 kcal mol ) is higher than that of PSi C ( 33.1 kcalmol ), whereas the energy barrier of forming the Si O bond (ESi O = 41.5 kcalmol ) is much lower than that of the Si C bond (ESi C = 59.5 kcal mol ). The energy consideration implies that an acrylic acid monomer (AAc) would react with the surface of the SiQDs via the carbonyl or alkene groups by forming Si O or Si C bonds, as previously reported. Furthermore, the calculations yield the energy relation of ESi O (41.5 kcal mol )

Gas sensing properties of single crystalline porous silicon nanowires

Abstract: We demonstrate that the porous silicon nanowires (SiNWs) prepared by metal-assisted chemical etching method could impart sensitivity of nanowire electrical properties to gaseous nitrogen oxide (NO) at room temperature, thus are suitable for sensing NO and air monitoring. Particularly, the sensors made from the porous SiNWs assembly showed fast response and excellent reversibility to subparts per million NO concentrations. The excellent sensing performance coupled with scalable synthesis of porous SiNWs could open up opportunities in scalable production of sensor chips working at room temperature.

Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations

Abstract: Arrays of well-aligned one-dimensional ZnO nanostructures (nanowires, nanorods, nanoribbons, nanobuds, and flocky nanorods) with high aspect ratios have been grown on zinc substrates by a solution-phase method using a mixture of ethylenediamine, ethanol, and water. The morphology of the ZnO nanostructures has been modulated by controlling the concentration of ethylenediamine and ethanol and regulating the reaction temperature. Chemical and structural analyses and emission spectra show that the arrays of ZnO nanorods favor nearly stoichiometric composition and good crystallization quality, whereas the arrays of ZnO nanowires, nanoribbons, nanobuds, and flocky nanorods confine a considerable amount of oxygen vacancies. The photocatalytic effect investigated at decomposition of methyl red correlates with the defect-related emission properties of these nanoarrays. Particularly ZnO nanobuds and flocky nanorods arrays have been found to be effective photocatalysts.

Bipolar Molecule as an Excellent Hole-Transporter for Organic-Light Emitting Devices

Abstract: A new bipolar molecule containing hole-transporting and electron-transporting moieties has been synthesized and characterized. The compound, 4,4′,4′′-tris(8-quinoline)-triphenylamine (TQTPA) exhibited good thermal stability and luminescence properties. A single-layer TQTPA light-emitting device shows sky blue emission with a low turn-on voltage of 2.8 V, a maximum brightness greater than 7500 cd/m2 at 10 V, and a maximum current efficiency of 1.6 cd/A. Bipolar transport properties of TQTPA were investigated via the hole-only and electron-only devices. Using the bipolar molecule as a hole-transporter, a typical bilayer device with a configuration of ITO/TQTPA (60 nm)/Alq3 (50 nm)/LiF (0.5 nm)/MgAg yields a maximum current efficiency of 5.6 cd/A (with Alq3 emission), which is much better (50% higher) than that of the prototypical NPB-based device (3.8 cd/A) with a similar device structure.

Silicon nanowire sensors for Hg2+ and Cd2+ ions

Abstract: High-sensitivity detection of toxic heavy metal cations such as Hg2+ and Cd2+ ions was demonstrated using single silicon nanowire field-effect transistors (SiNW-FETs) The conduct-ance of FET fabricated from thermally oxidized SiNWs functionalized with 3-mercaptopropyltriethoxysilane showed high sensitivity to Hg2+ and Cd2+ ions at a concentration down to 10−7 and 10−4M, respectively Linear relationship between the logarithmic concentration of metal ions and the current change was observed The SiNW sensor could be recycled to regain nearly the same sensitivity Comparative experiments showed that SiNW-FET sensors have great selectivity for detecting Hg2+ and Cd2+ over other metal cations

Strong Luminescent Iridium Complexes with CˆN=N Structure in Ligands and Their Potential in Efficient and Thermally Stable Phosphorescent OLEDs

Abstract: [*] Prof. Z. Q. Gao, Prof. W. Huang, Dr. B. X. Mi Jiangsu Key Lab of Organic Electronics & Information Displays and Institute of Advanced Materials (IAM) Nanjing University of Posts and Telecommunications (NJUPT) Mailbox 386, No. 9 Wenyuan Road Yadong Xincheng Distribution Nanjing 210046 (PR China) E-mail: iamzqgao@njupt.edu.cn; iamwhuang@njupt.edu.cn Prof. S. T. Lee, Prof. C. S. Lee Center of Super-Diamond & Advanced Films (COSDAF) and Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue, Kowloon, Hong Kong (PR China) E-mail: apannale@cityu.edu.hk Prof. P. F. Wang Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beiyitiao No.2, Zhongguancun, Haidian District Beijing 100080 (PR China)

High-Quality Graphenes via a Facile Quenching Method for Field-Effect Transistors

Abstract: Single- and few-layer graphene sheets with sizes up to 0.1 mm were fabricated by simply quenching hot graphite in an ammonium hydrogen carbonate aqueous solution. The identity and thickness of graphene sheets were characterized with transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. In addition to its simplicity and scalability, the present synthesis can produce graphene sheets with excellent qualities in terms of sizes, purity, and crystal quality. The as-produced graphene sheets can be easily transferred to solid substrates for further processing. Field-effect transistors based on individual graphenes were fabricated and shown to have high ambipolar carrier mobilities.

Polyhedral Organic Microcrystals: From Cubes to Rhombic Dodecahedra

Abstract: Research Grants Council of Hong Kong SAR, China [CityU101608]; City University of Hong Kong [7002275]; National Basic Research Program of China [2006CB933000, 2007CB936000]; National Natural Science Foundation of China [50825304, 50903059]

Tuning Electrical and Photoelectrical Properties of CdSe Nanowires via Indium Doping

Abstract: As an important II–VI semiconductor material, CdSe has attracted considerable attention due to its unique properties such as a direct band-gap ( 1.7 eV at room temperature) and excellent photoelectrical characteristics that make it a promising material for applications in photodetectors and photovoltaics. Thus far, 1D CdSe nanostructures, such as nanowires (NWs), nanotubes, nanosaws, nanosheets, and nanoribbons, have been successfully synthesized by using various methods including electrochemistry, solution chemical reactions, self-catalysis thermal evaporation, and laser ablation-assisted chemical vapor deposition (CVD). Besides their usage in single-electron transistors and electrochromic and charge-coupling devices, CdSe nanostructures also show application potential in biomolecular labeling, phosphors, and light-emitting diodes. Notably, photodetectors and field-effect transistors (FETs) based on a single CdSe nanoribbon have been recently realized. Nevertheless, the practical applications of CdSe nanostructures are hindered by poor materials properties, due mainly to lack of control over conductivity. Doping is an efficient approach to tune the electrical properties of semiconductors, and has been widely utilized in the semiconductor industry. Indeed, CdSe films and bulk crystals with tunable conductivity have been realized by using indium as n-type dopant. Doping of CdSe nanostruc-

Surface passivation and transfer doping of silicon nanowires.

Abstract: One-dimensional nanomaterials are expected to play a key role in future nanotechnology, in addition to providing model systems to demonstrate the unique characteristics of nanoscale effects. Silicon nanowires (SiNWs) in particular are potentially very attractive, given the central role of Si in the semiconductor industry, and are being extensively studied. A SiNW for use in nanodevices is composed of three sections: SiNW core, surface passivant, and adsorbates or interface compounds. A unique way to modulate the transport properties of SiNWs could depend on the individual sections. Volume doping is a conventional method to control conductivity. In volume doping, impurity atoms are introduced into the crystal lattice in the SiNW core by an in situ process during growth, 5] ion implantation, and related methods. However, volume doping for SiNWs has inherent disadvantages, such as poor controllability and destructive processing. Interestingly, the conductivity of amorphous Si films was found to be sensitive to adsorbates, which indicates the importance of the surface of low-dimensional systems in determining the electrical properties of materials. The large surface-to-volume ratio of SiNWs could potentially be important in influencing their transport properties. Its effect could be exploited through SiNW functionalization. Indeed, recent studies of SiNW-based chemical sensors 10] find strong conductivity responses of SiNWs to environmental conditions. Other relevant observations include conductivity modification by adsorbents in the hydrogen-terminated (H-terminated) surfaces of diamond crystals, conductivity determination by surface states in nanoscale thin silicon-on-insulator (SOI) systems, and conductivity enhancement of hydrogenated SiNWs in air and recovery through vacuum or gas purging. Thus, the possibility to modulate the conductivity of SiNWs using surface effects is promising. The ease of such an approach, economically and nondestructively, would offer a unique advantage for use of SiNWs in device fabrication. However, the success of this approach will depend on its controllability and repeatability, and most importantly on the understanding of the mechanisms of the surface effect on SiNWs. Herein, we present a new doping approach, namely surface passivation doping, built on the known surface transfer doping and based on extensive first-principles theoretical investigations and systematic experiments on the surface effects of SiNWs. We also elucidate the involved mechanism and provide better understanding to predetermine the electrical properties of nanomaterials. Surface hydrogen termination is a natural consequence of the hydrogen fluoride treatment of SiNWs. To reveal the role of hydrogen termination in conductivity, we first performed first-principles calculations based on density functional theory (DFT) with an efficient SIESTA code. 15] We adopted popularly used basis sets with double zeta and polarization functions and the Lee–Yang–Parr functional of generalized gradient approximation. We collected atomic charges from a Mulliken population analysis based on DFT calculation, which gave a reasonable charge distribution, as verified using a water molecule ( 0.46 j e j charges on the oxygen atom and 0.23 j e j charges on each hydrogen atom). Interestingly, we obtained extra charges of 0.06 j e j on average on each surface hydrogen atom of the H-terminated SiNWs (H-SiNWs). Clearly, the partial negative charge on the hydrogen atom is due to the higher electronegativity of the hydrogen atom compared to that of the silicon atom (2.2 vs. 1.9). This partial electron transfer from the silicon core to the surface hydrogen is negligible for bulk silicon but is significant for surface-dominated SiNWs whose carrier concentration could be considerably modified, as is estimated below. Assuming each surface silicon atom is terminated by two hydrogen atoms on average, we can calculate the total number of electrons trapped on the terminating hydrogen atoms using Equation (1):

Fluorescent logic gates chemically attached to silicon nanowires.

Abstract: I'm in the mood for dansyl: A chemically controlled fluorescent logic gate was formed by grafting a dansyl unit onto silicon nanowires (SiNWs; see picture). The logic gate was operated by utilizing pH changes, Hg(II), and Cl(-) or Br(-) ions as inputs and the fluorescence of the modified SiNWs as output. The modified SiNWs could perform as a three-input logic gate that combines the YES and INH operations.

Fabrication and photovoltaic property of ordered macroporous silicon

Abstract: Wafer-scale highly ordered macroporous silicon (macro-PSi) has been fabricated by the combination of lithography and metal-catalyzed Si etching in hydrofluoric acid. The periodicity and dimensions of the pores can be controlled by the lithography and the etching conditions. Significantly, this catalytic etching method is capable of creating uniform pores on Si substrates of any type of conductivity. Photovoltaic application results showed that the as-prepared macro-PSi samples are photoactive, and became more effective in solar energy conversion via surface modification with metal particles. The present fabrication method of macroporous Si structures could open potential applications in solar cells, photonic crystals, sensors, and batteries.

High-performance, fully transparent, and flexible zinc-doped indium oxide nanowire transistors

Abstract: We report the fabrication of fully transparent and flexible nanowire transistors by combining a high-quality In2O3:Zn nanowire channel, a SiNx high-κ dielectric, and conducting Sn-doped In2O3 electrodes on a polyethylene terephthalate substrate. The devices show excellent operating characteristics with high carrier mobilities up to 631 cm2 V−1 s−1, a drain-source current on/off modulation ratio ∼1×106, a high on-state current ∼1×10−5 A, a small subthreshold gate voltage swing of 120 mV decade−1, and a near zero threshold voltage. The devices further show high reproducibility and stable performance under bending condition. The high-performance nanowire transistors would enable application opportunities in flexible and transparent electronics.

Semiconducting Neutral Microstructures Fabricated by Coordinative Self-Assembly of Intramolecular Charge-Transfer Tetrathiafulvalene Derivatives

Abstract: A new class of tetrathiafulvalene(TTF)-based microstructures fabricated by coordinative self-assembly has been successfully prepared by a solution process. The morphology of the TTF coordination polymers is readily tuned by the variation of metal ions. Upon incorporation of Pb2+ and Zn2+ ions, one-dimensional (1D) wirelike microstructures and spherical polymer particles were achieved, respectively. These results indicate that the coordinative approach pursued in this work, in which the building blocks of 1 are linked in a coordination polymer chain by association with metal ions, is an efficient and versatile approach to produce more mechanically robust micro- and nanometer-sized coordination polymer materials. More interestingly, the neutral coordination polymers are conductive and magnetic at room temperature without external manipulation. Such conductivity is reminiscent of the behavior of the neutral conductive TTF in single crystals.

Formation and Photoelectric Properties of Periodically Twinned ZnSe/SiO2 Nanocables

Abstract: Nanoscaled coaxial materials with a periodically twinned ZnSe single crystal core and an amorphous silicon dioxide shell were synthesized by a simple thermal evaporation process. As-fabricated ZnSe/SiO2 core/shell nanowires and nanoribbons were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and a selected area electron diffraction (SAED) pattern. The periodically twinned ZnSe core with a twinning period of about 10−30 nm has a sphalerite crystal structure and a common growth direction of {11-1]. The amorphous SiO2 shell has a thickness of about 5 nm. The formation mechanism can be explained by the surface pressure stress from SiO2 shell to ZnSe core at the solid/liquid interface. The materials show strong cathodoluminescence (CL) related to the mixed light emission composed of band gap emission and defect emission. The single periodically twinned ZnSe/SiO2 nanowire has pronounced a photoconduction effect with a fast response time. The results suggest that the per...

Influence of the donor/acceptor interface on the open-circuit voltage in organic solar cells

Abstract: The donor/acceptor interface in a standard CuPc/C60 organic solar cell was modified by insertion of a thin layer of molybdenum trioxide (MoO3). An ultrathin layer of MoO3 between the donor and acceptor increased the open-circuit voltage (VOC) from 0.45 to 0.85 V. The enhancement in VOC is explained by the increase in the energy level offset between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor (EDHOMO-EALUMO). The explanation is supported by the energy level analysis of the donor/acceptor interface by ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy.

P -type conduction in arsenic-doped ZnSe nanowires

Abstract: Reliable p-type conduction was achieved in ZnSe nanowires (NWs) synthesized by introducing Zn3As2 as a dopant source. The crystal structure and orientation of NWs remained unchanged after doping. The electrical and transport properties of As-doped ZnSe NWs were investigated via the characteristics of NW-based field-effect transistors. The origin of p-type conduction in ZnSe NWs is attributed to the formation of substitutional AsSe and AsZn−2VZn complexes. Arsenic atoms were considered to incorporate into ZnSe lattices partly as As–H pairs; therefore postgrowth annealing could improve p-type conduction by dissociating As–H bonds and activating As acceptors.

A high performance nondoped blue organic light-emitting device based on a diphenylfluoranthene-substituted fluorene derivative

Abstract: A new blue light-emitting fluorene derivative 2,7-di[8-(7,10-diphenylfluoranthenyl)]-9,9-dimethylfluorene (DFDF) with good thermal stability at 420 °C has been synthesized and characterized. An organic light-emitting device (OLED) with the structure of ITO/NPB (70 nm)/DFDF (30 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (100 nm) has been investigated, where DFDF serves as a nondoped host emitter. Such a device possesses high current and power efficiencies of 3.8 cd/A and 2.6 lm/W, respectively, and stable bright blue-light emission at λ = 474 nm with Commission Internationale de L’Eclairage coordinates of (0.16, 0.23) over a wide range of operating voltages. The present results verify that DFDF is a promising candidate for a fluorescent blue-light-emitting OLED.

Surface-Enhanced Raman Scattering from Uniform Gold and Silver Nanoparticle-Coated Substrates

Abstract: Large-area gold and silver nanoparticles inside periodical silicon microhole arrays were prepared via simple electrodeposition. Such metal nanoparticles within Si microholes showed a strong surface...