[Show abstract][Hide abstract] ABSTRACT: The electro-optic characteristics of the semi-insulating and n(+)-type GaAs(001) surfaces passivated with n-alkanethiol self-assembled monolayers were investigated using Kelvin probe surface photovoltage (SPV) and photoluminescence (PL) techniques. Referencing the equilibrium surface barrier height established in an earlier report, SPV measurements demonstrated a significant (>100 mV) increase in the non-equilibrium band-bending potential observed under low-level photo-injection. Modeling of the SPV accounts for these observations in terms of a large (>10(4)) decrease in the hole/electron ratio of surface carrier capture cross-sections, which is suggested to result from the electrostatic potential of the interfacial dipole layer formed upon thiol chemisorption. The cross-section effects are verified in the high-injection regime based on carrier transport modeling of the PL enhancement manifested as a reduction of the surface recombination velocity.
[Show abstract][Hide abstract] ABSTRACT: There are very few published data comparing performance and cost of thermal and photovoltaic (PV) based solar power generations. With recent intense technology and business developments there is a need to establish a comparison between these two solar energy options. We have developed a simple model to compare electricity cost using these two options without any additional fuel source of hybridization. Capital along with operation and maintenance (O&M) costs and other parameters from existing large scale solar farms are used to reflect actual project costs. To compete with traditional sources of power generation, solar technologies need to provide dispatchable electric power to respond to demand during peak hours. Different solutions for energy storage are available. In spite of their high capital cost, adding energy storage is considered a better long term solution than hybrid solar systems for large scale power plants. For this reason, a comparison between the two solar options is also provided that include energy storage. Although electricity storage is more expensive than thermal storage, PV power remains a competitive option. Expenses related toO&Min solar thermal plant are about ten times higher than PV, an important factor resulting in higher energy cost. Based on data from proven commercial technologies, this study showed that PV holds a slight advantage even when energy storage is included. yes yes
Progress in Photovoltaics Research and Applications 03/2011; · 7.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The surface Fermi level of semi-insulating and n+-type GaAs(0 0 1) was determined before and after passivation with n-alkanethiol self-assembled monolayers (SAMs) by X-ray photoelectron spectroscopy. Fermi level positioning was achieved using Au calibration pads integrated directly onto the GaAs surface, prior to SAM deposition, in order to provide a surface equipotential binding energy reference. Fermi level pinning within 50 meV and surface barrier characteristics according to the Advanced Unified Defect Model were observed. Our results demonstrate the effectiveness of the Au integration technique for the determination of band-edge referenced Fermi level positions and are relevant to an understanding of emerging technologies based on the molecular–semiconductor junction.
[Show abstract][Hide abstract] ABSTRACT: The work function of n-alkanethiol self-assembled monolayers (SAMs) prepared on the GaAs(001) surface was measured using the Kelvin probe technique yielding the SAM 2D dipole layer potential (DLP). Direct n-dependent proportionality between the DLP values and the C−H stretching mode infrared (IR) absorption intensities was observed, which supports a correspondence of reported IR enhancements with the electrostatic properties of the interface. X-ray photoelectron spectroscopy is also used to verify the work function measurements. In addition, the principal components of the refractive index tensor are shown to be n-invariant in the ordered SAM phase. Our results suggest that a local field correction to the transition dipole moment accounts for the observed increase in IR activity through an increase to the electronic polarizability.
[Show abstract][Hide abstract] ABSTRACT: Alkanethiol self-assembled monolayers (SAMs) of various methylene group chain lengths [ HS –( CH <sub>2</sub>)<sub>n</sub>– CH <sub>3</sub>] (n=9,11,13,15,17) were fabricated on the GaAs(001) surface followed by characterization using Fourier transform infrared spectroscopy. Modal analysis of the CH <sub>2</sub> stretching mode region (2800–3000 cm <sup>-1</sup>) showed that linear scaling of the n -dependent factors accurately reproduced the spectral data, supporting a chain-length consistent physical model upon which a measurement of the absorption coefficient was based. Evaluated from the linearity of the absorbance data, a peak coefficient of 3.5×10<sup>4</sup> cm <sup>-1</sup> was obtained and a domain for ordered self-assembly was assigned for values n > 9 . Compared with measurements of the absorption coefficient made in the liquid phase, the SAM phase coefficient was determined to be about six times greater. This enhancement effect is discussed in terms of contributions relating to the locally ordered environment and is largely attributed to the chemical properties of the interface. We believe this to be the first demonstration of IR spectral enhancement of a molecular species chemisorbed on the semiconductor surface.
Journal of Applied Physics 05/2009; 105(9):094310-7. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: n-Alkanethiol self-assembled monolayers (SAMs) were prepared on the GaAs(001) surface according to . FTIR modal analysis of the CH2 stretching mode region (2800-3000 cm-1) verified SAM structural coherence, revealed evidence of an assembly threshold and allowed the absorption coefficient of the SAM phase to be directly measured. A 6x enhancement factor was observed relative to coefficients derived from the liquid and polycrystalline phases. This effect is reviewed in terms of the molecular order and is largely attributed to the chemical properties of the interface . Confirmation is provided by Kelvin Probe measurement of the sheet dipole potential, interpreted in terms of the Cooperative Molecular Field Effect .  McGuiness et al., J. Am. Chem. Soc. 128, 5231 (2006).  Marshall et al., submitted.  Cahen et al., Adv. Funct. Mater. 15, 1571 (2005).
[Show abstract][Hide abstract] ABSTRACT: A new 2D molecular imprinting technique based on nanotemplating and soft-lithography techniques is reported. This technique allows the creation of target-specific synthetic recognition sites on different substrates using a uniquely oriented and immobilized template and the attachment of a molecularly imprinted polymer on a substrate. The molecularly imprinted polymer was characterized by AFM, fluorescence microscopy, and ATR-FTIR. We evaluated the rebinding ability of the sites with theophylline (the target molecule). The selectivity of the molecularly imprinted polymer was determined for the theophylline-caffeine couple. The molecularly imprinted polymer exhibited selectivity for theophylline, as revealed by competitive rebinding experiments. Fluorescence microscopy experiments provided complementary proof of the selectivity of the molecularly imprinted polymer surfaces toward theophylline. These selective molecularly imprinted polymers have the potential for chemical sensor applications. Because of its 2D nature, this novel chemical sensor technology can be integrated with many existing high-sensitivity multichannel detection technologies.
[Show abstract][Hide abstract] ABSTRACT: Surface-confined telechelic poly(ε-caprolactone) macroligand with two distinct functional groups per polymeric chain has been synthesized and characterized. The molecular microstructure of the macroligand with regard to the properties of the end-capped functionalities and with those on surface substrate has been studied by solution and surface analytical methods (i.e., X-ray photoelectron spectroscopy (XPS), grazing angle reflectance-Fourier transform IR spectroscopy (GA-FTIR), water contact angle measurements, and atomic force microscopy (AFM)) to elucidate the structure and properties of such multifunctional polymer on gold (111) substrate.
[Show abstract][Hide abstract] ABSTRACT: A novel alkylthioacetyl-capped hydroxyethyl methacrylate monomer and its corresponding homopolymer have been synthesized and characterized. Direct chemisorption of these moieties have been carried out on gold-coated substrate and found to form a strong surface bonding. The surface coverage and the properties of the resultant self-assembled layers have been investigated by multiple surface characterization techniques (i.e. ellipsometry, GA-FTIR, XPS, AFM, and contact angle measurements). These analyses have all confirmed the occurrence of complete chemisorption reactions with typical n-alkanethiol self-assembled characteristics.
[Show abstract][Hide abstract] ABSTRACT: A preparative route for a nanocrystalline CdSe/polymer nanocomposite that is easily integrated into photovoltaic device fabrication has been developed. Microwave assisted synthesis is used to initiate the reaction of relatively safe, inexpensive and air stable precursors. Particles size may be varied from 20 Å to 60 Å in diameter. The CdSe nanocrystals show near band-edge photoemission, are crystalline and may be incorporated into a poly(9,9-dioctylfluorene-2,7-diyl-alt-9-octylcarbazole-3,6-diyl) copolymer. This polymer/CdSe composite can be directly cast, without complicated work-up, for the preparation of photovoltaic devices.
Journal of Materials Chemistry 10/2005; · 6.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Platinum-ruthenium nanoparticles stabilized within a conductive polymer matrix are prepared using microwave heating. Polypyrrole di(2-ethylhexyl) sulfosuccinate, or PPyDEHS, has been chosen for its known electrical conductivity, thermal stability, and solubility in polar organic solvents. A scalable and quick two-step process is proposed to fabricate alloyed nanoparticles dispersed in PPyDEHS. First a mixture of PPyDEHS and metallic precursors is heated in a microwave under reflux conditions. Then the nanoparticles are extracted by centrifugation. Physical characterization by TEM shows that crystalline and monodisperse alloyed nanoparticles with an average size of 2.8 nm are obtained. Diffraction data show that crystallite size is around 2.0 nm. Methanol electro-oxidation data allow us to propose these novel materials as potential candidates for direct methanol fuel cells (DMFC) application. The observed decrease in sulfur content in the polymer upon incorporation of PtRu nanoparticles may have adversely affected the measured catalytic activity by decreasing the conductivity of PPyDEHS. Higher concentration of polymer leads to lower catalyst activity. Design and synthesis of novel conductive polymers is needed at this point to enhance the catalytic properties of these hybrid materials.
The Journal of Physical Chemistry B 09/2005; 109(32):15339-44. · 3.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A study of the magnetic properties of an assembly of non-interacting iron oxide-based nano-particles is presented. The measured average particle size is found to approach the expected thickness of the spin disordered layer. In this regime, the magnetization is characterized by two parameters: Tb above which the particle acts as a single domain, and TE below which intra-particle coupling between pinned and unpinned spins become important.
Journal of Magnetism and Magnetic Materials 01/2005; 285(1):204-209. · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stable CoFe(2)O(4) nanoparticles have been obtained by co-precipitation using a microwave heating system. Transmission electron microscopy images analysis shows an agglomeration of particles with an average size of about 5 nm, and X-ray diffraction reveals the presence of a pure ferrite nanocrystalline phase. X-ray photoelectron spectroscopy and thermal gravimetric analysis show the presence of organic matter in the range of about 16 wt%. The magnetic response in DC fields is typical for an assembly of single-domain particles. The measured saturation magnetization is slightly larger than the bulk value, probably due to the presence of small amounts of Co and Fe. AC magnetization data indicate the presence of magnetic interactions between the nanoparticles.
Journal of Colloid and Interface Science 10/2004; 277(1):104-10. · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stable polymer coated platinum–ruthenium nanoparticles were prepared. The effects of several preparation parameters have been studied in view of optimizing the particles' composition and structure characteristics. Solutions of hexachloroplatinic acid and ruthenium chloride in ethylene glycol were reduced at 173 °C in the presence of poly(N-vinyl-2-pyrrolidone) with rapid refluxing in a commercial microwave system. These nanoparticles form stable colloidal solutions in ethanol for several months without any precipitation. Transmission electron microscopy (TEM) analysis revealed the presence of metallic clusters with a uniform size of about 2–3 nm. Photo-correlation spectroscopy (PCS) measurements revealed an average hydrodynamic particle size of about 10 nm. X-Ray photoelectron spectroscopy (XPS) indicates that most of the Pt and Ru atoms are in reduced metallic state. X-Ray diffraction (XRD) revealed alloyed single-crystal clusters with diameters barely smaller than corresponding TEM measurements. The above TEM, PCS, XPS and XRD observations converge on a simple picture of monodisperse, uniformly alloyed Pt–Ru nanoparticles with uniform diameters in the range of 2–3 nm for the PVP-coated samples. Their compositions are comparable with the metal content of the reactants. The PVP layer is less than 7 nm thick, and probably very porous. In the absence of PVP, agglomerated, larger nanoparticles were obtained with a Pt-rich core and Ru-rich shell.
Journal of Materials Chemistry 01/2004; 14(22). · 6.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A simple procedure has been described for the fabrication of multi-wall carbon nanotube (CNT) based electrochemical sensors. 3-Aminopropyltriethoxysilane (APTES) induced solubilization of CNTs allowed for the modification of electrode surfaces. With glucose oxidase (GOx), a flavin (FAD) containing enzyme as a model system, APTES was used as a solubilizing agent for CNTs as well as an immobilization matrix for GOx to construct a mediatorless biosensor. Our biosensor was able to efficiently monitor direct electroactivity of GOx at the electrode surface. A well-defined glucose response was observed at -0.45?V (vs. Ag/AgCl) whereas relevant physiological levels (0.1?mM) of three common interfering species, uric acid, ascorbic acid, and acetaminophen, resulted in no response. Although CNTs modified by APTES acted as semiconductors to reduce the exposed sensing surface, we reasoned nanoscale ldquodendritesrdquo of CNTs modified by APTES formed a network and projected outwards from the electrode surface and acted like bundled ultra-microelectrodes that allowed access to the active FAD site and facilitated direct electron transfer to the immobilized enzyme. The glucose biosensor prepared using a carbon fiber (11?mum) exhibited picoamperometric current response within 5?s with detection limits of 5-10?muM.
[Show abstract][Hide abstract] ABSTRACT: Abstract: This paper describes a new strategy for fabricating continuous gold films based on the self-assembly of the gold colloid monolayer on a poly(diallyldimethylammonium chloride)-modified glass slide, followed by electroless plating. Hydroxylamine-mediated reduction was proven as an excellent route to enlargement of immobilized nanoparticles on polymer-coated glass substrates in comparison to formaldehyde-mediated reduction. Au colloidal surface-catalyzed reduction of Au3+ by hydroxylamine exhibited very fast kinetics as monitored and confirmed by UV-vis spectroscopy in real time. The nanoscale morphology of the gold film was dependent on the initial coverage of gold nanoparticles and thermal annealing. Atomic force micrographs further revealed that enlarged particles were neither spherical nor cyclindrical, but highly complex in shape. The gold film thickness and its corresponding surface roughness could be easily controlled by setting the electroless deposition time. X-ray diffraction certified uniformity of deposits with the Au(111) crystallographic structure as the predominant one. No organic contamination during the course of electroless plating was observed as confirmed by both X-ray photoelectron spectroscopy and contact angle measurements. The stable and continuous gold films were used as electrodes for electrochemical experiments.