Durability of resin bonding to zirconia ceramic after contamination and the use of various cleaning methods

OBJECTIVES

The aim of the study was to evaluate the influence of contamination and different cleaning methods on the tensile bond strength with a phosphate monomer containing luting resin to zirconia ceramic.

METHODS

After the contamination with saliva or silicone disclosing agent, 228 polished and airborne-particle abraded zirconia discs were ultrasonically cleaned with 99% isopropanol. In a second step, the specimens were either treated with argon-oxygen plasma, air plasma, enzymatic cleaning agent or did not undergo an additional cleaning process. Uncontaminated zirconia specimens were used as the control group. X-ray photoelectron spectroscopy (XPS) was used for chemical analysis of the bonding surfaces of specimens. Plexiglas tubes filled with composite resin were bonded to zirconia specimens with a phosphate monomer containing luting resin. Tensile bond strength (TBS) was tested after 3 days or 150 days water storage with 37,500 thermal cycles.

RESULTS

XPS revealed a decrease of the carbon/oxygen ratio after plasma treatment and an increase after treatment with an enzymatic cleaning agent in all groups. All contaminated specimens showed high and durable TBS after cleaning with a combination of isopropanol and a non-thermal atmospheric plasma. After the cleaning with enzymatic cleaning agent the TBS was significantly reduced in all groups after 150 days thermal cycling.

SIGNIFICANCE

The combination of isopropanol and plasma cleaning was effective in removing salvia and disclosing agent contamination. Enzymatic clearing agent was not able to remove contamination effectively and had a negative impact on the TBS of non-contaminated specimens.

Correlating Nanostructure, Optical and Electronic Properties of Nanogranular Silver Layers during Polymer-Template-Assisted Sputter Deposition

Tailoring the optical and electronic properties of nanostructured polymer-metal composites demonstrates great potential for efficient fabrication of modern organic optical and electronic devices such as flexible sensors, transistors, diodes, or photovoltaics. Self-assembled polymer-metal nanocomposites offer an excellent perspective for creating hierarchical nanostructures on macroscopic scales by simple bottom-up processes. We investigate the growth processes of nanogranular silver (Ag) layers on diblock copolymer thin film templates during sputter deposition. The Ag growth is strongly driven by self-assembly and selective wetting on the lamella structure of polystyrene-block-poly(methyl methacrylate). We correlate the emerging nanoscale morphologies with collective optical and electronic properties and quantify the difference in Ag growth on the corresponding homopolymer thin films. Thus, we are able to determine the influence of the respective polymer template and observe substrate effects on the Ag cluster percolation threshold, which affects the insulator-to-metal transition (IMT). Optical spectroscopy in the UV-vis regime reveals localized surface plasmon resonance for the metal-polymer composite. Their maximum absorption is observed around the IMT due to the subsequent long-range electron conduction in percolated nanogranular Ag layers. Using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy, we identify the oxidation of Ag at the acrylate side chains as an essential influencing factor driving the selective wetting behavior in the early growth stages. The results of polymer-templated cluster growth are corroborated by atomic force microscopy and field emission scanning electron microscopy.

The impact of O2/Ar ratio on morphology and functional properties in reactive sputtering of metal oxide thin films

Morphology is a critical parameter for various thin film applications, influencing properties like wetting, catalytic performance and sensing efficiency. In this work, we report on the impact of oxygen partial flow on the morphology of ceramic thin films deposited by pulsed DC reactive magnetron sputtering. The influence of O₂/Ar ratio was studied on three different model systems, namely Al₂O₃, CuO and TiO₂. The availability of oxygen during reactive sputtering is a key parameter for a versatile tailoring of thin film morphology over a broad range of nanostructures. TiO₂ thin films with high photocatalytic performance (up to 95% conversion in 7 h) were prepared, exhibiting a network of nanoscopic cracks between columnar anatase structures. In contrast, amorphous thin films without such crack networks and with high resiliency to crystallization even up to 950 °C were obtained for Al₂O₃. Finally, we report on CuO thin films with well aligned crystalline nanocolumns and outstanding gas sensing performance for volatile organic compounds as well as hydrogen gas, showing gas responses up to 35% and fast response in the range of a few seconds.

Cauliflower-like CeO2-TiO2 hybrid nanostructures with extreme photocatalytic and self-cleaning properties

In recent years, heterogeneous photocatalysis has gained enormous interest due to increasing concerns about environmental pollution. Here we propose a facile approach to synthesize cauliflower-like CeO₂-TiO₂ hybrid structures by magnetron reactive sputtering, exhibiting an extremely high photocatalytic activity. While heating and air-quenching of the sputter deposited TiO₂ thin film (first layer) triggered the formation of a nanocrack network, the second heat-treatment led to transformation of the CeO₂ film (second layer) into CeO₂ nanoclusters (NCs). We attribute the resulting high photocatalytic activity to the confined structure of the CeO₂ NCs and the CeO₂-TiO₂ interface, which allows Ce³⁺/Ce⁴⁺ dynamic shifting. In addition to high photocatalytic activity in an aqueous medium, the prepared CeO₂-TiO₂ hybrid structures exhibited significant self-cleaning properties in air (non-aqueous).

Concept and modelling of memsensors as two terminal devices with enhanced capabilities in neuromorphic engineering

We report on memsensors, a class of two terminal devices that combines features of memristive and sensor devices. Apart from a pinched hysteresis (memristive property) and stimulus dependent electrical resistance (sensing property) further properties like dynamic adaptation to an external stimulus emerge. We propose a three component equivalent circuit to model the memsensor electrical behaviour. In this model we find stimulus dependent hysteresis, a delayed response to the sensory signal and adaptation. Stimulus dependent IV hysteresis as a fingerprint of a memsensor device is experimentally shown for memristive ZnO microrods. Adaptation in memsensor devices as found in our simulations resembles striking similarities to the biology. Especially the stimulus dependency of the IV hysteresis and the adaptation to external stimuli are superior features for application of memsensors in neuromorphic engineering. Based on the simulations and experimental findings we propose design rules for memsensors that will facilitate further research on memsensitive systems.

Tunable polytetrafluoroethylene electret films with extraordinary charge stability synthesized by initiated chemical vapor deposition for organic electronics applications

Bulk polytetrafluoroethylene (PTFE) possesses excellent chemical stability and dielectric properties. Indeed, thin films with these same characteristics would be ideal for electret applications. Previously, the electret properties of PTFE-like thin films produced by rf sputtering or plasma enhanced chemical vapor deposition were found to deteriorate due to structural changes and surface oxidation. In this article, the technique of initiated chemical vapor deposition (iCVD) is evaluated for electret applications for the first time. The iCVD method is known for its solvent-free deposition of conformal, pinhole-free polymer thin films in mild process conditions. It is shown that PTFE thin films prepared in this way, show excellent agreement to commercial bulk PTFE with regard to chemical properties and dielectric dissipation factors. After ion irradiation in a corona discharge the iCVD PTFE thin films exhibit stable electret properties, which can be tailored by the process parameters. Due to the mild deposition conditions, the iCVD technique is suitable for deposition on flexible organic substrates for the next-generation electret devices. It is also compatible with state-of-the-art microelectronic processing lines due to the characteristics of conformal growth and easy scaling up to larger size substrates.

Efficacy of Plasma Treatment for Decontaminating Zirconia

PURPOSE

To evaluate the influence of contamination and plasma treatment on the bond strength of resin to zirconia ceramic.

MATERIALS AND METHODS

After immersion in saliva or the use of a silicone disclosing agent, polished and airborne-particle abraded zirconia specimens were cleaned either ultrasonically in 99% isopropanol or with nonthermal plasma. Uncontaminated zirconia specimens were used as control. For chemical analysis, specimens of all groups were examined with x-ray photoelectron spectroscopy (XPS). Plexiglas tubes filled with composite resin were bonded to ceramic specimens with a phosphate-monomer-containing luting resin. The influence of contamination and cleaning methods on ceramic bond durability was examined by tensile testing after 3 and 150 days of water storage, with an additional 37,500 thermocycles during the 150-day storage.

RESULTS

XPS showed an increase in the amount of oxygen and a decrease in the amount of carbon on the zirconia surface after plasma treatment. After contamination with silicone, XPS revealed a high amount of Si residue on the surface that none of the investigated cleaning processes could completely remove. The tensile bond strength to uncontaminated zirconia ceramic was durable, but was significantly reduced by contamination.

CONCLUSION

Plasma treatment was effective in removing salivary contamination but not silicone disclosing agent residue from the bonding surface of zirconia.

Magnetron-sputtered copper nanoparticles: lost in gas aggregation and found by in situ X-ray scattering

Magnetron discharge in a cold buffer gas represents a liquid-free approach to the synthesis of metal nanoparticles (NPs) with tailored structure, chemical composition and size. Despite a large number of metal NPs that were successfully produced by this method, the knowledge of the mechanisms of their nucleation and growth in the discharge is still limited, mainly because of the lack of in situ experimental data. In this work, we present the results of in situ Small Angle X-ray Scattering measurements performed in the vicinity of a Cu magnetron target with Ar used as a buffer gas. Condensation of atomic metal vapours is found to occur mainly at several mm distance from the target plane. The NPs are found to be captured preferentially within a region circumscribed by the magnetron plasma ring. In this capture zone, the NPs grow to the size of 90 nm whereas smaller ones sized 10-20 nm may escape and constitute a NP beam. Time-resolved measurements of the discharge indicate that the electrostatic force acting on the charged NPs may be largely responsible for their capturing nearby the magnetron.

Remote functionalization in surface-assisted dehalogenation by conformational mechanics: organometallic self-assembly of 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl on Ag(111)

Even though the surface-assisted dehalogenative coupling constitutes the most abundant protocol in on-surface synthesis, its full potential will only become visible if selectivity issues with polybrominated precursors are comprehensively understood, opening new venues for both organometallic self-assembly and on-surface polymerization. Using the 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl (Br4F6BP) at Ag(111), we demonstrate a remote site-selective functionalization at room temperature and a marked temperature difference in double- vs. quadruple activation, both phenomena caused by conformational mechanical effects of the precursor-surface ensemble. The submolecularly resolved structural characterization was achieved by Scanning Tunneling Microscopy, the chemical state was quantitatively assessed by X-ray Photoelectron Spectroscopy, and the analysis of the experimental signatures was supported through first-principles Density-Functional Theory calculations. The non-planarity of the various structures at the surface was specifically probed by additional Near Edge X-ray Absorption Fine Structure experiments. Upon progressive heating, Br4F6BP on Ag(111) shows the following unprecedented phenomena: (1) formation of regular organometallic 1D chains via remote site-selective 3,5'-didebromination; (2) a marked temperature difference in double- vs. quadruple activation; (3) an organometallic self-assembly based on reversibility of C-Ag-C linkages with a thus far unknown polymorphism affording both hexagonal and rectangular 2D networks; (4) extraordinary thermal stability of the organometallic networks. Controlled covalent coupling at the previously Br-functionalized sites was not achieved for the Br4F6BP precursor, in contrast to the comparatively studied non-fluorinated analogue.

Self-organized nanocrack networks: a pathway to enlarge catalytic surface area in sputtered ceramic thin films, showcased for photocatalytic TiO2

Sputter deposited photocatalytic thin films offer high adherence and mechanical stability, but typically are outperformed in their photocatalytic properties by colloidal TiO₂ nanostructures, which in turn typically suffer from problematic removal. Here we report on thermally controlled nanocrack formation as a feasible and batch applicable approach to enhance the photocatalytic performance of well adhering, reactively sputtered TiO₂ thin films. Networks of nanoscopic cracks were induced into tailored columnar TiO₂ thin films by thermal annealing. These deep trenches are separating small bundles of TiO₂ columns, adding their flanks to the overall catalytically active surface area. The variation of thin film thickness reveals a critical layer thickness for initial nanocrack network formation, which was found to be about 400 nm in case of TiO₂. The columnar morphology of the as deposited TiO₂ layer with weak bonds between respective columns and with strong bonds to the substrate is of crucial importance for the formation of nanocrack networks. A beneficial effect of nanocracking on the photocatalytic performance was experimentally observed. It was correlated by a simple geometric model for explaining the positive impact of the crack induced enlargement of active surface area on photocatalytic efficiency. The presented method of nanocrack network formation is principally not limited to TiO₂ and is therefore seen as a promising candidate for utilizing increased surface area by controlled crack formation in ceramic thin films in general.

Single target sputter deposition of alloy nanoparticles with adjustable composition via a gas aggregation cluster source

Alloy nanoparticles with variable compositions add a new dimension to nanoscience and have many applications. Here we suggest a novel approach for the fabrication of variable composition alloy nanoparticles that is based on a Haberland type gas aggregation cluster source with a custom-made multicomponent target for magnetron sputtering. The approach, which was demonstrated here for gold-rich AgAu nanoparticles, combines a narrow nanoparticle size distribution with in operando variation of composition via the gas pressure as well as highly efficient usage of target material. The latter is particularly attractive for precious metals. Varying argon pressure during deposition, we achieved in operando changes of AgAu alloy nanoparticle composition of more than 13 at%. The alloy nanoparticles were characterized by x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy. The characteristic plasmon resonances of multilayer nanoparticle composites were analyzed by UV-vis spectroscopy. Tuning of the number of particles per unit area (particle densities) within individual layers showed an additional degree of freedom to tailor the optical properties of multilayer nanocomposites. By extension of this technique to more complex systems, the presented results are expected to encourage and simplify further research based on plasmonic multi-element nanoparticles. The present method is by no means restricted to plasmonics or nanoparticle based applications, but is also highly relevant for conventional magnetron sputtering of alloys and can be extended to in operando control of alloy concentration by magnetic field.

Role of Sputter Deposition Rate in Tailoring Nanogranular Gold Structures on Polymer Surfaces

The reproducible low-cost fabrication of functional polymer-metal interfaces via self-assembly is of crucial importance in organic electronics and organic photovoltaics. In particular, submonolayer and nanogranular systems expose highly interesting electrical, plasmonic, and catalytic properties. The exploitation of their great potential requires tailoring of the structure on the nanometer scale and below. To obtain full control over the complex nanostructural evolution at the polymer-metal interface, we monitor the evolution of the metallic layer morphology with in situ time-resolved grazing-incidence small-angle X-ray scattering during sputter deposition. We identify the impact of different deposition rates on the growth regimes: the deposition rate affects primarily the nucleation process and the adsorption-mediated growth, whereas rather small effects on diffusion-mediated growth processes are observed. Only at higher rates are initial particle densities higher due to an increasing influence of random nucleation, and an earlier onset of thin film percolation occurs. The obtained results are discussed to identify optimized morphological parameters of the gold cluster ensemble relevant for various applications as a function of the effective layer thickness and deposition rate. Our study opens up new opportunities to improve the fabrication of tailored metal-polymer nanostructures for plasmonic-enhanced applications such as organic photovoltaics and sensors.

From Au-Thiolate Chains to Thioether Sierpiński Triangles: The Versatile Surface Chemistry of 1,3,5-Tris(4-mercaptophenyl)benzene on Au(111)

Self-assembly of 1,3,5-tris(4-mercaptophenyl)benzene (TMB), a 3-fold symmetric, thiol-functionalized aromatic molecule, was studied on Au(111) with the aim of realizing extended Au-thiolate-linked molecular architectures. The focus lay on resolving thermally activated structural and chemical changes by a combination of microscopy and spectroscopy. Thus, scanning tunneling microscopy (STM) provided submolecularly resolved structural information, while the chemical state of sulfur was assessed by X-ray photoelectron spectroscopy (XPS). Directly after room-temperature deposition, only less well ordered structures were observed. Mild annealing promoted the first structural transition into ordered molecular chains, partly organized in homochiral molecular braids. Further annealing led to self-similar Sierpiński triangles, while annealing at even higher temperatures again resulted in mostly disordered structures. Both the irregular aggregates observed at room temperature and the chains were identified as metal-organic assemblies, whereby two out of the three intermolecular binding motifs are energetically equivalent according to density functional theory (DFT) simulations. The emergence of Sierpiński triangles is driven by a chemical transformation, i.e., the conversion of coordinative Au-thiolate to covalent thioether linkages, and can be further understood by Monte Carlo simulations. The great structural variance of TMB on Au(111) can on one hand be explained by the energetic equivalence of two binding motifs. On the other hand, the unexpected chemical transition even enhances the structural variance and results in thiol-derived covalent molecular architectures.

Real-Time Monitoring of Morphology and Optical Properties during Sputter Deposition for Tailoring Metal-Polymer Interfaces

The reproducible low-cost fabrication of functional metal-polymer nanocomposites with tailored optoelectronic properties for advanced applications remains a major challenge in applied nanotechnology. To obtain full control over the nanostructural evolution at the metal-polymer interface and its impact on optoelectronic properties, we employed combined in situ time-resolved microfocus grazing incidence small angle X-ray scattering (μGISAXS) with in situ UV/vis specular reflectance spectroscopy (SRS) during sputter deposition of gold on thin polystyrene films. On the basis of the temporal evolution of the key scattering features in the real-time μGISAXS experiment, we directly observed four different growth regimes: nucleation, isolated island growth, growth of larger aggregates via partial coalescence, and continuous layer growth. Moreover, their individual thresholds were identified with subnanometer resolution and correlated to the changes in optical properties. During sputter deposition, a change in optical reflectivity of the pristine gray-blue PS film was observed ranging from dark blue color due to the presence of isolated nanoclusters at the interface to bright red color from larger Au aggregates. We used simplified geometrical assumptions to model the evolution of average real space parameters (distance, size, density, contact angle) in excellent agreement with the qualitative observation of key scattering features. A decrease of contact angles was observed during the island-to-percolation transition and confirmed by simulations. Furthermore, a surface diffusion coefficient according to the kinetic freezing model and interfacial energy of Au on PS at room temperature were calculated based on a real-time experiment. The morphological characterization is complemented by X-ray reflectivity, optical, and electron microscopy. Our study permits a better understanding of the growth kinetics of gold clusters and their self-organization into complex nanostructures on polymer substrates. It opens up the opportunity to improve nanofabrication and tailoring of metal-polymer nanostructures for optoelectronic applications, organic photovoltaics, and plasmonic-enhanced technologies.

Light-Controlled Conductance Switching in Azobenzene-Containing MWCNT-Polymer Nanocomposites

We report on reversible light-controlled conductance switching in devices consisting of multiwalled carbon nanotube (MWCNT)-polymer nanocomposites blended with azobenzene molecules and photoisomerization of the latter. Both the azobenzene molecules and MWCNT, which are functionalized with carboxyl groups (MWCNT-COOH), are embedded independently in a poly(methyl methacrylate) matrix, and thin films are prepared by using a simple spin-coating technique. We demonstrate the feasibility of the present concept with a photocurrent switching amplitude of almost 10%.

On-surface Ullmann polymerization via intermediate organometallic networks on Ag(111)

The role of organometallic intermediates during on-surface polymerization via Ullmann coupling was studied on Ag(111). The polymerization progress was monitored by X-ray photoelectron spectroscopy (XPS). Scanning Tunneling Microscopy (STM) was used to characterize organometallic and covalent networks and to identify the temperature regimes for organometallic and covalent bond formation.

X-ray spectroscopy characterization of azobenzene-functionalized triazatriangulenium adlayers on Au(111) surfaces

NEXAFS spectroscopy analysis confirms perpendicular standing azobenzene functionalities connected to flat-lying platform molecules adsorbed on Au(111) single crystals.