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Integrating low dielectric permittivity (low-k) polymers to metals is an exacting fundamental challenge because poor bonding between low-polarizability moieties and metals precludes good interfacial adhesion. Conventional adhesion-enhancing methods such as using intermediary layers are unsuitable for engineering polymer/metal interfaces for many applications because of the collateral increase in dielectric permittivity. Here, we demonstrate a completely new approach without surface treatments or intermediary layers to obtain an excellent interfacial fracture toughness of >13 J/m(2) in a model system comprising copper and a cross-linked polycarbosilane with k approximately 2.7 obtained by curing a cyclolinear polycarbosilane in air. Our results suggest that interfacial oxygen catalyzed molecular ring-opening and anchoring of the opened ring moieties of the polymer to copper is the main toughening mechanism. This novel approach of realizing adherent low-k polymer/metal structures without intermediary layers by activating metal-anchoring polymer moieties at the interface could be adapted for applications such as device wiring and packaging, and laminates and composites.
ACS Appl Mater Interfaces 2010 May
PMID:Ring-opening-induced toughening of a low-permittivity polymer-metal interface. 2041 22

We quantitatively characterized oxygen reduction kinetics at the nanoscale Ptmid R:CsHSO(4) interface at approximately 150 degrees C in humidified air using conducting atomic force microscopy (AFM) in conjunction with AC impedance spectroscopy and cyclic voltammetry. From the impedance measurements, oxygen reduction at Ptmid R:CsHSO(4) was found to comprise two processes, one displaying an exponential dependence on overpotential and the other only weakly dependent on overpotential. Both interfacial processes displayed near-ideal capacitive behavior, indicating a minimal distribution in the associated relaxation time. Such a feature is taken to be characteristic of a nanoscale interface in which spatial averaging effects are absent and, furthermore, allows for the rigorous separation of multiple processes that would otherwise be convoluted in measurements using conventional macroscale electrode geometries. The complete current-voltage characteristics of the Ptmid R:CsHSO(4) interface were measured at various points across the electrolyte surface and reveal a variation of the oxygen reduction kinetics with position. The overpotential-activated process, which dominates at voltages below -1 V, was interpreted as a charge-transfer reaction. Analysis of six different sets of Ptmid R:CsHSO(4) experiments, within the Butler-Volmer framework, yielded exchange coefficients (alpha) for charge transfer ranging from 0.1 to 0.6 and exchange currents (i(0)) spanning 5 orders of magnitude. The observed counter-correlation between the exchange current and exchange coefficient indicates that the extent to which the activation barrier decreases under bias (as reflected in the value of alpha) depends on the initial magnitude of that barrier under open circuit conditions (as reflected in the value of i(0)). The clear correlation across six independent sets of measurements further indicates the suitability of conducting AFM approaches for careful and comprehensive study of electrochemical reactions at electrolyte-metal-gas boundaries.
ACS Nano 2010 May 25
PMID:Nanoscale electrodes by conducting atomic force microscopy: oxygen reduction kinetics at the Ptmid R:CsHSO4 interface. 2041 68

Electrochemical deposition of equiatomic Fe-Pt from complexing electrolytes provides precise tuning of alloy stoichiometry, enables close control of the growth process, and results in limited oxygen incorporation. The films grow epitaxially on oriented substrates and the low oxygen content favors transformation from the as-deposited cubic to the high anisotropy L1(0) phase and magnetic hardening upon thermal annealing at temperatures (400-450 degrees C) much lower than previously achieved by other plating processes.
ACS Appl Mater Interfaces 2010 Apr
PMID:Electrodeposition of Fe-Pt films with low oxide content using an alkaline complexing electrolyte. 2042 17

Polyethylene terephthalate (PET) plates have been exposed to different nitrogen containing plasmas with the purpose of incorporating nitrogen functional groups on its surface. Results with a dielectric barrier discharge (DBD) at atmospheric pressure and a microwave discharge (MW) at reduced pressure and those using an atom source working under ultrahigh vacuum conditions have been compared for N(2) and mixtures Ar + NH(3) as plasma gases. The functional groups have been monitored by X-ray Photoemission Spectroscopy (XPS). Nondestructive oxygen and carbon depth profiles for the plasma treated and one month aged samples have been determined by means of the nondestructive Tougaard's method of XPS background analysis. The surface topography of the treated samples has been examined by Atomic Force Microscopy (AFM), while the surface tension has been determined by measuring the static contact angles of water and iodomethane. It has been found that the DBD with a mixture of Ar+NH(3) is the most efficient treatment for nitrogen and amine group functionalization as determined by derivatization by reaction with chlorobenzaldehyde. It is also realized that the nitrogen functional groups do not contribute significantly to the observed increase in surface tension of plasma treated PET.
ACS Appl Mater Interfaces 2010 Apr
PMID:Surface functionalization, oxygen depth profiles, and wetting behavior of PET treated with different nitrogen plasmas. 2042 18

A central composite rotatable design (CCRD) method was used to investigate the performance of the accelerated thermomolecular adhesion process (ATmaP), at different operating conditions. ATmaP is a modified flame-treatment process that features the injection of a coupling agent into the flame to impart a tailored molecular surface chemistry on the work piece. In this study, the surface properties of treated polypropylene were evaluated using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). All samples showed a significant increase in the relative concentration of oxygen (up to 12.2%) and nitrogen (up to 2.4%) at the surface in comparison with the untreated sample (0.7% oxygen and no detectable nitrogen) as measured by XPS. ToF-SIMS and principal components analysis (PCA) showed that ATmaP induced multiple reactions at the polypropylene surface such as chain scission, oxidation, nitration, condensation, and molecular loss, as indicated by changes in the relative intensities of the hydrocarbon (C(3)H(7)(+), C(3)H(5)(+), C(4)H(7)(+), and C(5)H(9)(+)), nitrogen and oxygen-containing secondary ions (C(2)H(3)O(+), C(3)H(8)N(+), C(2)H(5)NO(+), C(3)H(6)NO(+), and C(3)H(7)NO(+)). The increase in relative intensity of the nitrogen oxide ions (C(2)H(5)NO(+) and C(3)H(7)NO(+)) correlates with the process of incorporating oxides of nitrogen into the surface as a result of the injection of the ATmaP coupling agent.
ACS Appl Mater Interfaces 2010 May
PMID:Exploring molecular changes at the surface of polypropylene after accelerated thermomolecular adhesion treatments. 2043 36

We have demonstrated that by coating with a thin dielectric layer of tetrahedral amorphous carbon (ta-C), a biocompatible and optical transparent material in the visible range, the Ag nanoparticle-based substrate becomes extremely suitable for surface-enhanced Raman spectroscopy (SERS). Our measurements show that a 10 A or thicker ta-C layer becomes efficient to protect the oxygen-free Ag in air and prevent Ag ionizing in aqueous solutions. Furthermore, the Ag nanoparticles substrate coated with a 10 A ta-C film shows a higher enhancement of Raman signals than the uncoated substrate. These observations are further supported by our numerical simulations. We suggest that biomolecule detections in analytic assays could be easily realized using ta-C-coated Ag-based substrate for SERS especially in the visible range. The coated substrate also has higher mechanical stability, chemical inertness, and technological compliance, and may be useful, for example, to enhance TiO(2) photocatalysis and solar-cell efficiency by the surface plasmons.
ACS Nano 2010 May 25
PMID:Ultrathin diamond-like carbon film coated silver nanoparticles-based substrates for surface-enhanced Raman spectroscopy. 2043 94

A simple template-free hydrothermal route was used for the synthesis of novel mesoporous CuO dandelion structures formed by self-organized CuO nanorods. A very high surface area approximately 325 m(2)/g and remarkably enhanced photoconductivity under white light irradiation of the CuO dandelions were observed compared to the nanocrystals. The extremely high photoconductivity is attributed to the presence of oxygen related hole-trap states at the large surface area of the dandelions. The fast response (tau = 24 s) of the photocurrent holds promise for the fast photo-sensing device applications.
ACS Appl Mater Interfaces 2010 May
PMID:Template-free synthesis of mesoporous CuO dandelion structures for optoelectronic applications. 2043 59

Sheets of chemically converted graphene (CCG) on the surface of Si/SiO(2) substrates exhibit nanoscopic corrugation. This corrugation has been assumed to be caused by a combination of factors including (a) thermal treatments in the device preparation, (b) different oxygen-containing addends on the CCG, and (c) the substrate roughness. In this paper, we study the interplay of these factors in the corrugation behavior of monolayer CCG flakes, prepared by reduction of graphene oxide (GO) synthesized by Hummers method, and CCG nanoribbons, produced by chemical unzipping of carbon nanotubes, followed by the reduction by hydrazine at 95 degrees C. We have studied the morphology, composition, and electrical properties of the flakes and nanoribbons before and after annealing in Ar/H(2) at 300 degrees C. Our experiments demonstrate that, despite the temperature treatment and the associated removal of the oxygen-containing addends from the basal plane of the CCG, the corrugation pattern of the CCG exhibits almost no change upon annealing. This suggests that the substrate roughness, not the chemical addends nor the thermal cycling, is the predominant determinant in the graphene corrugation. This conclusion is supported by depositing GO flakes on freshly cleaved mica. Such flakes were shown to have extremely low corrugation (rms approximately 70 pm), as dictated by the atomically flat surface of mica. Our experimental observations are in accord with the results of our molecular dynamics simulations, which show that interaction with the substrate greatly suppresses the intrinsic corrugation of graphene materials.
ACS Nano 2010 Jun 22
PMID:Corrugation of chemically converted graphene monolayers on SiO(2). 2044 64

Graphitic nanomaterials such as graphene layers (G) and single-wall carbon nanotubes (SWCNT) are potential candidates in a large number of biomedical applications. However, little is known about the effects of these nanomaterials on biological systems. Here we show that the shape of these materials is directly related to their induced cellular toxicity. Both G and SWCNT induce cytotoxic effects, and these effects are concentration- and shape-dependent. Interestingly, at low concentrations, G induced stronger metabolic activity than SWCNT, a trend that reversed at higher concentrations. Lactate dehydrogenase levels were found to be significantly higher for SWCNT as compared to the G samples. Moreover, reactive oxygen species were generated in a concentration- and time-dependent manner after exposure to G, indicating an oxidative stress mechanism. Furthermore, time-dependent caspase 3 activation after exposure to G (10 microg/mL) shows evidence of apoptosis. Altogether these studies suggest different biological activities of the graphitic nanomaterials, with the shape playing a primary role.
ACS Nano 2010 Jun 22
PMID:Cytotoxicity effects of graphene and single-wall carbon nanotubes in neural phaeochromocytoma-derived PC12 cells. 2048 56

The interaction of rose Bengal (RB) and fluorescein (FL) with poly[diallyldimethylammonium] chloride (PDDA) was studied in layer-by-layer self-assembled thin films and in solution. The spectroscopic behavior is explained in terms of dye-dye, dye-polyelectrolyte, and in solution, dye-solvent interactions. A correlation among dye hydrophobicity, aggregation tendency, polymer folding in solution, and the stability of self-assembled films is obtained. In spite of the very high dye concentration (approximately 1 M), RB-PDDA multilayer thin films are able to photogenerate singlet molecular oxygen, as demonstrated by chemical monitoring and IR phosphorescence detection.
ACS Appl Mater Interfaces 2010 Jun
PMID:Dye-polyelectrolyte layer-by-layer self-assembled materials: molecular aggregation, structural stability, and singlet oxygen photogeneration. 2049 43


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