Proceedings Volume 4807

Physical Chemistry of Interfaces and Nanomaterials

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Proceedings Volume 4807

Physical Chemistry of Interfaces and Nanomaterials

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Volume Details

Date Published: 15 November 2002
Contents: 2 Sessions, 26 Papers, 0 Presentations
Conference: International Symposium on Optical Science and Technology 2002
Volume Number: 4807

Table of Contents

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Table of Contents

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Oral Session
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X-ray absorption study of colloidal Ge quantum dots
Tony van Buuren, Christoph Bostedt, Boyd R. Taylor, et al.
X-ray absorption measurements at the L-edge in chemically prepared Germanium nanoclusters show a blue shift of the conduction band edge consistent with quantum confinement theory. Additionally the effects of the surface termination on the electronic properties are probed with x-ray absorption processes.
Nanostructures of organic molecules and proteins on surfaces
Nabil A. Amro, Jayne C. Garno, Maozi Liu, et al.
Patterning bioreceptors on surfaces is a key step in the fabrication of biosensors and biochips. State-of-the art technology can produce micrometer-sized biostructures, however, further miniaturization at the nanoscale will require new methods and lithographic tools. In this proceeding, we report three approaches: nanopen reader and writer (NPRW), nanografting and latex particle lithography; for creating nanostructures of small molecules, DNA and proteins. Using nanografting and NPRW, nanostructures of thiol molecules or thiolated ssDNA are fabricated within self-assembled monolayers. Proteins attach selectively to nanopatterns of thiol molecules containing bioadhesive groups such as aldehyde or carboxylates. Using latex particle lithography, arrays of protein nanostructures are produced with high throughput on mica and gold substrates. Near-physiological conditions are used in structural characterization, thus the orientation, reactivity and stability of proteins and DNA molecules within nanostructures may be monitored directly via AFM. While AFM-based approaches provide the highest precision, nanoparticle lithography can produce arrays of protein nanostructures with high throughput. The nanostructures of proteins produced by these approaches provide an excellent opportunity for fundamental investigations of biochemical reactions on surfaces, such as antigen-antibody recognition and DNA-protein interactions. These methods provide a foundation for advancing biotechnology towards the nanoscale.
Static and dynamic electronic spectroscopy at the interface between water and chemically modified self-assembled monolayers
Ilan Benjamin, John Vieceli
Molecular dynamics computer simulations are used to study the static and dynamic electronic spectroscopy of a chromophore located at the interface between water and self-assembled organic monolayers terminated by either a methyl group or chlorine atom. The roughness and polarity of the monolayer surfaces are varied to determine the dependence of the spectroscopy on the surface composition. Equilibrium trajectories are used to calculate the static electronic spectrum relative to the gas phase and non-equilibrium trajectories are used to monitor the dynamic solvent response immediately following an electronic transition. Relative to bulk water, the interfaces with methyl-terminated monolayers are less polar, while interfaces with chlorine-terminated monolayers are more polar. This is understood in terms of the contribution from each component of the system to the solvation energy. The dynamic solvent response at each interface studied is slower than bulk water. The rate of water relaxation is correlated with the polarization of interfacial water molecules due to the monolayer.
Photophysics of Metal Nanoparticles: Heat Dissipation and Coherent Excitation of Phonon Modes
Gregory V. Hartland, Min Hu
Ultrafast laser excitation of metal nanoparticles can create temperature increases of many hundreds of Kelvin. The aim of this paper is to provide an overview of our recent experimental studies of heat dissipation and the coherent generation of acoustic phonon modes in the particles. Our results show that the rate of heat dissipation depends on the surface area of the particles, and that both impulsive lattice heating and hot-electron pressure contribute to phonon excitation. The measured periods also depend on the pump laser intensity: higher intensities yield slower periods. This softening of the coherently excited phonon modes is due to the temperature dependence of the elastic constants of the particles.
Spectroscopy and dynamics in GaSe nanoparticles
Viktor Chikan, David F. Kelley
The synthesis, characterization and spectroscopy of GaSe nanoparticles are described in this paper. The particles are synthesized from the reaction of trioctylphosphine selenium with trimethylgallium in a solution of trioctylphosphine and trioctylphosphine oxide at 280 °C. TEM images show that this synthesis produces GaSe nanoparticles in the size range of 2 - 6 nm. These particles may be size segregated by column chromatography or size selective precipitation and relatively monodisperse nanoparticles are obtained. Electron diffraction results indicate that these particles have a two-dimensional single tetra-layer type structure. The particles have absorption onsets in the 360 to 450 nm region, with the smallest particles absorbing furthest to the blue. The particles are emissive, with emission quantum yields of about 10 - 15%. Following 400 nm excitation, these particles exhibit a static emission maximum at 480 nm. This emission is polarized and the anisotropy is largest on the blue edge of the emission spectrum. Both the total (unpolarized) emission kinetics and the emission anisotropy kinetics are obtained. Static emission spectra along with wavelength dependent kinetic results permit the reconstruction of time dependent spectra. The kinetic results are interpreted in terms of an energetic model that is based on the relative energetics of the band edge and trap states in bulk GaSe. The emission kinetics show an 80 ps decay component in the total emission, but not in the anisotropy decay kinetics. There is a ca. 270 cm-1 shift in the emission maximum during this decay. This transient is assigned to direct to indirect band edge relaxation. This is followed by 400 ps and 2.4 ns decay components in both the total emission and the anisotropy kinetics. These transients are assigned to trapping of holes in shallow, followed by deeper acceptor levels. As the 480 nm emission decays, it is replaced by a much weaker, long-lived, unpolarized 520 nm emission. Transient absorption results show a broad absorption in the 500 to 650 nm region, which is assigned to a hoole intraband transition. This band exhibits a 20 ps rise time, indicative of relaxation of holes to the valance band edge.
Longitudinal plasma resonance shifts in gold nanoparticle aggregates
Thaddeus Jude Norman Jr., Christian Grant, Donny Magana, et al.
Recent interest in colloidal gold focuses on understanding the tunability of the longitudinal and transverse plasma resonance. It was reported that the reduction of HAuCl4 by Na2S produces gold nanoparticles with an optical absorption in the near infrared. This absorption blue shifts during the course of the reaction. X-ray photoemission spectroscopy (XPS) measurements on this system indicated that there was little sulfur present in the system. A small angle x-ray scattering (SAX) experiment was used to monitor the reaction while simultaneously the UV-VIS spectrum was measured. During the reaction the fractal dimension decreased from 4.154 ± 0.850 to 0.624 ± 0.146. The decrease in fractal dimension coincided with the blue shift in the longitudinal plasma resonance from the near IR to the visible. This suggests a change from reaction limited colloid aggregation (RLCA) to diffusion limited colloid aggregation (DLCA), caused the shift in the plasma resonance.
Direct observations of metal support interaction in nano-scale systems
Robert F. Klie, Kai Sun, Mark M. Disko, et al.
In this paper, we will describe the experimental processes involved in analytical atomic-resolution scanning transmission electron microscopy (STEM) of supported nano-scale systems. We show that the combination of high-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) provides an analytical tool with unprecedented chemical and spatial sensitivity that is vital for studying interfaces in heterogeneous catalyst systems. We apply the described methods to study two example heterogeneous catalyst systems: Pt/SiO2, and Cu/Al2O3. In particular, the presence of a few monolayers of platinum oxide in Pt/SiO2 can be clearly seen, and changes in the chemistry of the SiO2 support within ~1 nm of the metal-oxide interface can be characterized as a function of the catalyst preparation conditions. The Cu/Al2O3, reduced at various temperatures, exhibits an increasing oxidation of the Cu-particles upon higher temperature reduction.
Structural transformations in self-assembled semiconductor quantum dots as inferred by transmission electron microscopy
Peter Mock, Yuanyuan Lei, Teya Topuria, et al.
Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10 %. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide.
Electrodeposition of portable metal nanowire arrays
E. C. Walter, M. P. Zach, F. Favier, et al.
Parallel arrays of long (> 500 m), dimensionally uniform nanowires composed of molybdenum, copper, nickel, gold, and palladium were electrodeposited. Nanowires with diameters in the range from 15 nm to 750 nm were obtained by electrodepositing these metals, or metal oxides, selectively at the step edges present on the surface of a highly oriented pyrolytic graphite electrode. Depending on the metal, either of two methods were used to carry out electrochemical step edge decoration (ESED): Nanowires of Ni, Cu or Mo were prepared by electrodepositing nanowires of a conductive metal oxide such as NiO, Cu2O, or MoO2. Nanowires of the parent metal were then obtained by reducing the metal oxide nanowires in hydrogen at elevated temperature. Nanowires composed of noble metals and some coinage metal can be obtained by direct electrodeposition of the metal at step edges. Direct electrodeposition involved the application of three voltage pulses in succession: An oxidizing "activation" pulse, a large amplitude, reducing "nucleation" pulse, and a small amplitude reducing "growth" pulse. These nanowires arrays were "portable": After embedding the nanowires in a polymer film, arrays of nanowires could be lifted off the graphite surface thereby facilitating the incorporation of these arrays in devices such as sensors.
Rectified quantized charging of gold nanoparticle self-assembled monolayers by arenedithiol linkages
Shaowei Chen, Fengjun Deng
Gold nanoparticle self-assembled monolayers were fabricated by a sequential anchoring mechanism with rigid arenedithiol chemical linkers. These particle surface assemblies exhibited well-defined quantized capacitance characters in aqueous solutions, and more importantly, in aqueous solutions, the voltammetric responses were rectified in the presense of hydrophobic anions, which was interpreted on the basis of a Randle's equivalent circuit where the particle-ion pairing formation led to the manipulation of electrode interfacial double-layer capacitance. The effective nanoparticle capacitances evaluated from the voltammetric measurements were consistent with those with the particles immobilized onto electrode sufraces by alkanedithiols. In addition, the nanoparticle electron-transfer kinetics was investigated by AC voltammetry and the rate constants were generally found of the order 103 s-1, at least an order of magnitude greater than those with saturated alkyl spacers.
Charge carrier transport in nanoporous zinc oxide electrodes
Alexander Eychmuller, Volker Noack
By applying a simple phenomenological model in the analysis of transient photocurrents arising in particulate ZnO films, several information are gained concerning the transport of electrons through the nanoporous network. From the results of experiments using electrodes with different thicknesses of the ZnO film, the driving force of the transport is identified as a gradient in the electrochemical potential across the electrode. Furthermore, since no effect of the ambient temperature on the transport dynamics at positive potentials is observed, an tunneling mechanism is proposed for the basic electron exchange between adjacent particles. By changing the average size of the ZnO particles, information about the height and the width of the potential barrier are drawn for the different ensembles of nanoparticles.
Electrochemically self-assembled ZnO/dye electrodes: preparation and time-resolved photoelectrochemical measurements
Derck Schlettwein, Torsten Oekermann, Tsukasa Yoshida, et al.
Dye-modified ZnO thin films were prepared by electrochemically induced crystallization from aqueous mixtures of zinc nitrate and water-soluble dyes. A direct crystallization of semiconductor/ dye composites without heat treatment is seen as a significant advantage of this method. Moreover, characterization of these materials has revealed ordered growth of ZnO crystallites as well as formation of ordered dye assemblies, thus characterizing this method as electrochemical self-assembly. The photoelectrochemical properties of these unique ZnO-dye thin film electrodes were investigated in photocurrent transient measurements in the ms-regime and by steady- state voltammetric measurements. Two sets of electrodes are discussed, employing either metal complexes of tetrasulfophthalocyanines (TSPcMt; Mt = Zn, Al, Si) or the xanthene dye Eosin Y. For aggregates of TSPcMt on ZnO, efficient charge-transfer to the electrolyte is found, leading to low surface charging and low surface recombination of photogenerated holes with electrons from the ZnO, at however, rather low injection efficiencies of electrons into the conduction band of ZnO. This efficiency was higher for adsorbed monomers of TSPcMt leading to a considerably higher quantum efficiency of the photocurrent in spite of increased surface charging and recombination of holes. Higher photocurrents were observed for ZnO sensitized with monomers of Eosin Y caused by both, efficient electron transfer from the dye to ZnO as well as hole transfer from the dye to the electrolyte. Not only dye molecules which were directly accessible from the electrolyte, but also those which were enclosed within matrix cavities proved to be photoelectrochemically active.
Nanowires by solution-phase synthesis
Brian T. Mayers, Byron D. Gates, Yugang Sun, et al.
We have demonstrated a variety of solution-phase approaches for the synthesis of dimensionally confined nanostructures of a wide range of materials. These materials include metals (Ag and Au) and semiconductors (Te, Se, and Ag2Se) with interesting properties such as high electric, thermal, and ionic conductivities, piezoelectricity, and photoconductivity. Direct and indirect routes for the solution-phase synthesis of 1-dimensional nanostructures are presented. Control over morphology, chemical purity, and crystallinity are well maintained. We show that by using solution-phase methods, it is possible to generate not only high yields of nanowires but also more complex structures such as tubes and co-axial nanocables. These nanostructures are ideal for the study of size-confinement effects on electrical and optical properties, and also as the future interconnects and active components in nanoscale electronic and electromechanical devices.
Correlating structure and photophysics in erbium-doped silicon nanocrystals
Jeffrey L. Coffer, Robert A. Senter, Junmin Ji
Silicon's position as the semiconductor of choice in micro/nanoelectronics hinges on its ability to shift to new design paradigms such as opto-electronics. Strategies to produce silicon-based light emitters remain limited, however, relying either on quantum confinement effects or optically-active dopants. We are actively studying the effects of incorporating optically-active Erbium centers into discrete crystalline Si nanocrystals. Such nanocrystals have been prepared via the pyrolysis of disilanein the presence of a suitable Er source. Two rather different types of doped materials have been synthesized to date: one involving a random distribution of erbium centers throughout the nanocrystal; the other forces erbium into a location preferentially-enriched near the surface. This work entails the structural characterization of such materials and their photophysical properties, including spectroscopic measurements under the conditions of high pressure.
Nanoparticles formed within porous hosts: a new type of phosphor and storage device
Alan G. Joly, Wei Chen, Jan-Olle Malm, et al.
Recent results on the structure and luminescence enhancement of Eu2O3, EuS, and ZnS:Mn2+as well as photo-stimulated luminescence of Ag and AgI nanoparticles encapsulated in porous hosts are presented. Eu2O3 nanoparticles encapsulated in MCM-41 display different structures depending on the temperature used to form the guest-host material. Particles formed following heat treatment at 140° C show monoclinic structure with enhanced luminescence efficiency. This increased efficiency may result from a decrease in the radiative lifetime of the emitter within the host cavity. Similarly, EuS and ZnS:Mn2+ show increased luminescence when encapsulated in zeolite-Y. Ag and AgI nanoparticles encapsulated in zeolite-Y show significant photostimulated luminescence with very short lifetimes. The appearance of strong photostimulated luminescence with short decay times demonstrates that nanoparticles encapsulated in porous host materials have potential for digital storage and medical radiology applications.
Molecular alignment in nanofilm crystal polarizers and retarders
Leonid Ignatov, Pavel I. Lazarev, Victor Nazarov, et al.
We introduce a new group of organic materials and method for the three-step nano-film crystallization of these materials on the surface of glass or plastic. Materials are based on polycyclic aromatic compounds. Chemical modification of compounds changes hydrophobic-hydrophilic balance of disk-shaped molecules and makes them water-soluble with aggregation into rod-like supramolecules in aqueous solution and subsequent formation of supramolecular lyomesophases. Coating techniques provide control of crystallographic axes direction of the final crystal film. Shear force that is applied during deposition controls alignment of supramolecules. Structure of liquid material, wet coating and resulting 100-700 nm thin crystal films has been studied optically and by X-ray diffraction. The properties of the molecular material dictate the properties of Thin Crystal Film (TCF), which exhibit the layered molecular structure with interlayer distance equal to thickness of flat molecules (0.336 nm). Molecular alignment during the shear deposition of Lyotropic Liquid Crystal (LLC) results in formation of film with a strong preferred orientation. TCF order parameter determined from both X-ray analysis and optical data is about 0.9. TCFs exhibit a high optical anisotropy and birefringence which makes them unique nano-scale polarizing films and high efficiency retarders. New submicron film retarders have high refraction coefficient anisotropy. Birefringence varies from 0.3 to 1.0 for 380-900nm wavelength range. TCF polarizers and retarders provide new options for LCD designs.
Poster Session
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Ultrafast studies of photoexcited electronic dynamics in thiol-capped CdTe semiconductor nanoparticles
The charge carrier dynamics of thiol-capped CdTe nanoparticles have been studied using femtosecond laser spectroscopy. High-resolution transmission electronic microscopy shows that thiol-capped CdTe particles are spherical, with 2-3 nm average diameter. Static electronic absorption spectrum suggests that they are in the strongly quantum-confined regime. Steady state emission measurements show only a band edge emission at ~ 505 nm, indicating a low density of deep-trapped states. The electronic relaxation dynamics show a fast decay (a few ps), which is dependent on excitation intensity, a slow decay (~230 ps), and an offset beyond 600 ps. The slow decay is attributed to the electron-hole recombination of surfactant or solvent trapped electrons. The intensity dependent, fast decay component is assigned to exciton-exciton annihilation or photoionization/Auger recombination, with the assistance of nanosecond fluorescence study. The decay profiles appear to be independent of probe wavelength. The overall relatively slow decay at low excitation intensity is consistent with the strong fluorescence, indicating an efficient radiative relaxation process.
Oral Session
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Synthesis and optical properties of silver bicrystalline nanowires
This paper describes a solution-phase route to the large-scale synthesis of silver nanowires with diameters in the range of 30-40 nm, and lengths up to ~50 μm. The initial step of this synthesis involved the formation of Pt nanoparticles by reducing PtCl2 with ethylene glycol (EG) refluxed at ~160 °C. These Pt nanoparticles could serve as seeds for the growth of silver (formed by reducing AgNO3 with EG) through heterogeneous nucleation process because their crystal structures and lattice constants matched closely. In the presence of poly(vinyl pyrrolidone) (PVP), the growth of silver could be led to a highly anisotropic mode with formation of uniform nanowires. UV-visible spectroscopy was used to track the growth process of silver nanowires because different silver nanostructures exhibited distinctive surface plasmon resonance peaks at different frequencies. SEM, TEM, XRD, and electron diffraction were used to characterize these silver nanowires, indicating the formation of a highly pure face-centered cubic phase, as well as uniform diameter and bicrystalline structure. The morphology of these silver nanostructures could be varied from particles and rods to long wires by tuning the reaction conditions, including reaction temperature, and the ratio of PVP to silver nitrate. These silver nanowires could be used as sacrificial templates to synthesize gold nanotubes via a template-engaged replacement reaction. The dispersion of gold nanotubes exhibited a strong extinction peak in the red regime, which was around 760 nm.
Poster Session
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Effect of surface microenvironment on bound spiropyran photochemistry
Rohit Rosario, Antonio A. Garcia, Devens Gust, et al.
Spiropyrans are a group of organic molecules that undergo a reversible photoinduced transformation (i.e. photochromism) from a colorless, non-planar spiropyran form to a colored, planar merocyanine form. Photochromism is accompanied by a large change in the structure and the dipole moment. These changes suggest that such molecules might be useful in light-controlled, 'smart surface' applications. This study examines the effect of the microenvironment near the surface-bound spiropyran on its photochemistry. The surfaces were designed to exhibit a mixture of hydrophobic and hydrophilic components using a mixed silane chemistry on a silica substrate, and the spiropyran was covalently bound to the surface using a carbodiimide linking technique. The solvatochromic behavior of spiropyran derivatives was studied in solution using UV-Vis absorption spectroscopy and fluorescence spectroscopy for comparison with the surface-bound species. Spiropyrans in solution and on the surface both exhibited negative solvatochromism. Based on linear solvation energy relationships using the Kamlet-Taft polarity scales, hydrogen bonding appears to play a prominent role in solvent stabilization of spiropyrans in solution and on surfaces. Correlations between emission maxima of the spiropyrans and Reichardt's ET(30) polarity scale revealed that both the solvent and the substituent groups on the spiropyran affected the spiropyran photochemistry. The solvatochromic behavior of the surface-bound spiropyran was similar to that of a model spiropyran in solution, being very sensitive to the polarity of the surrounding liquid and not significantly affected by the surface components, except in solvents of low polarity where the surface seemed to have an increased influence on the spiropyran's solvation shell.
Chalcogen nanowires: synthesis and properties
Brian T. Mayers, Byron D. Gates, Younan Xia
We have demonstrated a solution-phase approach based on homogeneous nucleation and controlled growth for the synthesis of 1-dimensional nanostructures from a chalcogens such as Se, Te, and Se/Te alloys. These nanostructures include monodispersed nanowires, nanorods, and nanotubes with good dimensional control (lateral dimensions from 10 to 1000 nm, and lengths ranging from a 0.25 to >20 μm). These nanomaterials are ideal components for fabricating devices or composites for photoconductive and piezoelectric applications. In this presentation, we will discuss the mechanisms (as revealed by our SEM and TEM studies) for the formation of these 1-dimensional nanostructures, as well as some preliminary measurements on their properties.
Ultrafast electronic relaxation in gold (I) sulfide nanoparticles
Ultrafast electronic relaxation dynamics in Au2S colloidal nanoparticles have been studied using fs transient absorption spectroscopy. The electronic absorption spectrum of the nanoparticles exhibits a broad featureless absorption with increasing intensity from the near-IR into the visible and UV, indicating that Au2S is an indirect bandgap semiconductor. The electronic relaxation dynamics have been measured with 390 nm excitation and probing at 790 and 850 nm. The transient absorption decay profiles can be fit to a double exponential with time constants of 600 fs and 23 ps. The fast decay can be assigned to trapping of electrons from the conduction band to shallow trap states or from shallow traps to deep traps, while the long decay is assigned to recombination from shallow or deep trap states. The overall fast relaxation can be attributed to a high density of intrinsic or surface trap states. This fast decay is non-radiative and consistent with no observable luminescence at room temperature. EXAFS data show a 20% decrease in the first coordination shell for nanoparticles relative to bulk, which suggests a large number of surface dangling bounds that can contribute to a high density of surface trap states.
Isocrystalline core/shell synthesis of high-quality II-VI diluted magnetic semiconductor quantum dots: ligand-field spectroscopic studies
Pavle V. Radovanovic, Daniel R. Gamelin
Ligand field electronic absorption spectroscopy has been applied as a direct probe of Co2+ dopant ions in II-VI based diluted magnetic semiconductor quantum dots. Synthesis of Co2+-doped CdS (Co2+:CdS) quantum dots by simple coprecipitation in inverted micelle solutions has been found to yield predominantly surface bound dopant ions, which are unstable with respect to solvation in a coordinating solvent (pyridine). The solvation kinetics are biphasic, involving two transient intermediates. In contrast, Co2+ ions are doped much more isotropically in ZnS QDs, and this difference is attributed to the similar ionic radii of Co2+ and Zn2+ ions (0.74 Å), as opposed to Cd2+ ions (0.97 Å). We have developed an isocrystalline core/shell synthetic methodology that enables us to synthesize high quality internally doped Co2+:CdS quantum dots. The effect of Co2+ binding on the surface energies of CdS and ZnS quantum dots is discussed and related to the growth mechanism of diluted magnetic semiconductor quantum dots.
Investigation of the capacitance minimum of unannealed single-walled carbon nanotube papers in aqueous sodium chloride
The electrochemical properties of carbon nanotube (NT) assemblies are relevant for many potential nanotube applications including super-capacitors, batteries, fuel cells and actuators. In this work, the double-layer capacitance of a paper of single-walled carbon nanotubes is determined for a series of concentrations of NaCl in water. The dependence of capacitance on potential was also determined in an effort to locate the potential of zero charge (PZC) for each NaCl concentration. The double-layer capacitance of the NT paper is seen to increase with electrolyte concentration, while the PZC (capacitance minimum) is seen to depend more on the sequence of electrolyte concentration tested (sample history) than on the concentration of electrolyte itself.
Interfacial controlled growth of AgI nanoparticles
Pandian Senthil Kumar, Channappayya S. Sunandana
AgI nanoparticles with zincblende structure are promising materials for laser based optical communication systems and therefore deserve to be optimally synthesized. We have explored ambient iodization as a strategy for controlled AgI nanoparticle growth on (a) mesoporous Ag foils, (b) Ag-Cu quasi-amorphous thin films and (c) Ag-Sb crystalline alloy films. The modified Ag surfaces obtained by chemical etching, Cu and Sb substitution seem to display an amazingly varied degree of interfacial control over the growth of AgI nanoparticles as investigated by XRD, SEM, optical transmission and photoluminescence. The nature of interfacial control and the associated modification of thin film growth mode are discussed on the basis of the present experimental results and currently available theoretical models.
Photoluminescence and electroluminescence studies on ITO/SiO2/Si tunneling diodes for efficient light emission from silicon
Miin-Jang Chen, Jui-Fen Chang, Chen S. Tsai, et al.
We report the finding of photoluminescence (PL) and electroluminescence (EL) studies at silicon bandgap energy for the indium-tin-oxide (ITO)/SiO2/Si metal-oxide-semiconductor (MOS) tunneling diodes. The characteristics of temporal EL response, temperature dependence of EL and PL intensities, and voltage-dependent PL intensity, were used to investigate the radiative recombination and nonradiative Shockley-Read-Hall (SRH) recombination near the Si-SiO2 interface. The temporal EL response indicates that the radiative recombination coefficient in the light-emitting MOS tunneling diode is about ten times larger than that of the bulk silicon. However, the nonradiative SRH recombination is still the dominant carrier recombination process. The intensity of EL was found to be lesser sensitive with temperature than that of PL, which indicates that the nonradiadiative recombination is less thermally active and less efficient for EL. The voltage-dependent PL study shows that the PL intensity increases with the bias voltage. This observation is attributed to the variations of nonradiative SRH recombination rates due to the change of Fermi level with the bias voltage. This study shows that the nonradiative recombination near the Si-SiO2 interface strongly influences the luminescent efficiency.
Air-assisted ultrasonic spray pyrolysis for nanoparticles synthesis
Shirley C. Tsai, Yu L. Song, C. Y. Chen, et al.
This paper presents new findings regarding the effects of precursor drop size and concentration on product particle size and morphology in ultrasonic spray pyrolysis of zirconium hydroxyl acetate solutions. Large precursor drops (diameter >30μm) generated by ultrasonic atomization at 120kHz yielded particles with holes. Precursor drops 6-9 μm in diameter, generated by an ultrasonic nebulizer at 1.65MHz and 23.5W electric drive power, yielded uniform spherical particles 150nm in diameter under proper control of heating rate and precursor concentration. Moreover, air-assisted ultrasonic spray pyrolysis at 120kHz and 2.3W yielded spherical particles of which nearly half were smaller than those produced by the ultrasonic spray pyrolysis of the 6-9 μm precursor drops, desprite the much larger precursor drop sizes (28 μm peak diameter versus 7 μm mean diameter). These particles are much smaller than those predicted by the conventional one particle per drop mechanism, suggesting that a vapor condensation mechanism may also be involved in spray pyrolysis. It may be concluded that through this new mechanism air-assisted ultrasonic spray pyrolysis can become a viable process for mass production of nanoparticles.