Direct laminating of silver foils on copper substrates

A novel laminating process of Ag foils on Cu substrates is successfully developed. The Ag foil is 280 μm thick. After slight polishing to obtain shining surface, the Ag foil is laminated to Cu substrate with a static pressure of 260 psi at 400 °C in 50 millitorr vacuum to suppress metal oxidation. No bonding medium is used. The Ag foil is directly bonded to the Cu substrate. Laminated samples are cut into halves for cross-section examination using scanning electron microscopy (SEM). Nearly void-free bonding is achieved. The Ag–Cu bonding interface is very sharp. This metal-to-metal lamination technique can be applied to various electronic packages. Ag is much ductile than Cu. The yield strength of Ag is only one-tenth of Cu. Ag layer on Cu substrate functions as a buffer to absorb thermal expansion mismatch between semiconductor chips and Cu substrates through plastic strain. Progress is underway to refine the laminating process.     

Direct nucleation of silver nanoparticles on graphene sheet

Silver (Ag) nanoparticles were synthesized on the surface of graphene sheet by the simultaneous reduction of Ag+ and graphene oxide (GO) in the presence of simple reducing agent, hydrazine hydrate (N2H4 x H2O). Both the Ag+ and GO were reduced and Ag+ was nucleated onto graphene. GO flakes were prepared by conventional chemical exfoliation method and in the presence of strong acidic medium of potassium chlorate. Silver nanoparticles were prepared using 0.01 M AgNO3 solution. The reduced GO sheet decorated with Ag is referred as G-Ag sample. G-Ag was characterized by FTIR (Fourier transform infrared) spectroscopy using GO as standard. An explicit alkene peak appeared around 1625 cm(-1) was observed in G-Ag sample. Besides, the characteristic carbonyl and hydroxyl peaks shows well reduction of GO. The FTIR therefore confirms the direct interaction of Ag into Graphene. SEM (scanning electron microscopy) and TEM (transmission electron microscopy) analysis were performed for morphological probing. The average size of Ag nanoparticles was confirmed by around 5-10 nm by the high-resolution TEM (HRTEM). The Ag quantum dots incorporated nanocomposite material could become prominent candidate for diverse applications including photovoltaic, catalysis, and biosensors etc.     

Direct ink-jet printing and low temperature conversion of conductive silver patterns

A drop-on-demand ink-jet printer has been used in the production of conductive silver tracks onto glass, polyimide, polytetrafluoroethylene, carbon and glass fibre reinforced epoxy substrates. Silver patterns were obtained from an organometallic solution by heat treatment at 150°C in air and were found to have resistivity values of 1.3 to 2 times the theoretical resisitivity of bulk silver. Printed track lateral resolution is a function of the ink/substrate wetting behaviour and a simple model is presented that relates track width to equilibrium contact angle. The influence of printing parameters and substrate surface properties on line quality is discussed.     

Role of a silver buffer layer on the superconducting Y-Ba-Cu-O films deposited by screen printing on alumina substrates

The characteristics of Y-Ba-Cu-O superconducting films, prepared on alumina substrates with an intermediate layer of thick film high-temperature silver paste, are studied. It is found that the critical current density of the film has improved, perhaps due to the prevention of interactions between alumina and the superconducting film by the intermediate layer.     

Switching memory cells constructed on plastic substrates with silver selenide nanoparticles

Programmable metallization cell (PMC) memory is a kind of next generation non-volatile memory that has attracted increasing attention in recent years as a possible replacement for flash memory. In spite of the considerable amount of research focused on the fabrication of non-volatile memories on plastic substrates with lightweight, thin, and bendable characteristics, there have been few studies on the fabrication of PCM memory on flexible substrates. In this study, we synthesized Ag₂Se nanoparticles (NPs) by a positive-microemulsion method and constructed PMC memories on plastic substrates with programmable layers formed by the spin-coating of the Ag₂Se NPs. To the best of the knowledge, this is the first attempt to construct PMC memory on plastic substrates by the spin-coating of Ag₂Se NPs. The Ag₂Se NPs synthesized in this study had a uniform size of 2 nm and interestingly showed α-phase (high temperature phase) stability at room temperature. Switching behaviors were observed through the voltage scanning on the fabricated memories with applicable switching voltages. However, the resistance ratios of the off-state to the on-state were quite small. The possible reasons for the α-phase stability of the Ag₂Se NPs at room temperature and the detailed memory characteristics will be described in this article.     

Direct precipitation of silver nanoparticles induced by a high repetition femtosecond laser

Silver nanoparticles were precipitated directly in Ag⁺ doped silicate glass by a 150 fs, 800 nm, 250 kHz femtosecond laser irradiation. The irradiated pattern turned to yellow without heat treatment. The absorption peak about 400 nm, ascribed to the surface plasmon resonance of the formed silver nanoparticles, was observed. We believe that the reduced Ag atoms, via capturing free electrons created by multi-photon process, may aggregate into the Ag nanoparticles due to femtosecond laser inducing heat accumulation effect. This result has promising applications for the fabrication of 3D multi-colored images inside a transparent material.     

Opaline films on patterned substrates by a simple self-assembly method

A simple method for the preparation of opaline films on flat and patterned glass substrates was investigated. Opaline films and opal micro-channels were obtained by a solvent-evaporation-induced self-assembly process. Scanning electron microscopy (SEM) and optical transmission spectroscopy have been used to investigate the structural quality of the films and optical band gap.     

Direct stamping of silver nanoparticles toward residue-free thick electrode

Direct stamping of functional materials has been developed for cost-effective and process-effective manufacturing of nano/micro patterns. However, there remain several challenging issues like the perfect removal of the residual layer and realization of high aspect ratio. We have demonstrated facile fabrication of flexible strain sensors that have microscale thick interdigitated capacitors with no residual layer by a simple direct stamping with silver nanoparticles (AgNPs). Polyurethane (PU) prepolymer was utilized as an adhesive layer to transfer AgNPs more efficiently during the separation step of the flexible stamp from directly stamped AgNPs. Scanning electron microscopy images and energy dispersive x-ray spectroscopy analysis revealed residue-free transfer of microscale thick interdigitated electrodes onto two different flexible substrates (elastomeric and brittle) for the application to highly sensitive strain sensors.     

Direct stamping of silver nanoparticles toward residue-free thick electrode

Abstract

Direct stamping of functional materials has been developed for cost-effective and process-effective manufacturing of nano/micro patterns. However, there remain several challenging issues like the perfect removal of the residual layer and realization of high aspect ratio. We have demonstrated facile fabrication of flexible strain sensors that have microscale thick interdigitated capacitors with no residual layer by a simple direct stamping with silver nanoparticles (AgNPs). Polyurethane (PU) prepolymer was utilized as an adhesive layer to transfer AgNPs more efficiently during the separation step of the flexible stamp from directly stamped AgNPs. Scanning electron microscopy images and energy dispersive x-ray spectroscopy analysis revealed residue-free transfer of microscale thick interdigitated electrodes onto two different flexible substrates (elastomeric and brittle) for the application to highly sensitive strain sensors.     

Fabrication of sticky and slippery superhydrophobic surfaces via spin-coating silica nanoparticles onto flat/patterned substrates

Silica nanoparticles were spin-coated onto a flat/patterned (regular pillar-like) substrate to enhance the surface roughness. The surface was further modified by a self-assembled fluorosilanated monolayer. The advancing/receding contact angle and sliding angle measurements were performed to determine the wetting behavior of a water droplet on the surface. It is interesting to find that a transition from a Wenzel surface to a sticky superhydrophobic surface is observed due to the spin-coating silica nanoparticles. A slippery superhydrophobic surface can be further obtained after secondary spin-coating with silica nanoparticles to generate a multi-scale roughness structure. The prepared superhydrophobic substrates should be robust for practical applications. The adhesion between the substrate and nanoparticles is also examined and discussed.     

An organometallic route to highly monodispersed silver nanoparticles and their application to ink-jet printing

Highly monodispersed silver nanoparticles were successfully synthesized by thermolysis of silver alkanoate precursors and were characterized by X-ray diffraction, TGA/DTA and transmission electron microscopy. The results showed that these nanoparticles exhibit spherical shape with FCC crystal structure. The relationship between the carbon chain length and the monodispersity of the nanoparticles was investigated. Furthermore, the size of the particles was controlled by varying the concentration of the stabilizing surfactants. The silver nanoparticles were easily re-dispersed into n-tetradecane and printed onto various substrates using a Microfab head with a single nozzle. The ink-jet printed patterns were sintered at 250 °C and their electrical resistivity was about 6 μΩ cm.     

Large scale, highly conductive and patterned transparent films of silver nanowires on arbitrary substrates and their application in touch screens

The application of silver nanowire films as transparent conductive electrodes has shown promising results recently. In this paper, we demonstrate the application of a simple spray coating technique to obtain large scale, highly uniform and conductive silver nanowire films on arbitrary substrates. We also integrated a polydimethylsiloxane (PDMS)-assisted contact transfer technique with spray coating, which allowed us to obtain large scale high quality patterned films of silver nanowires. The transparency and conductivity of the films was controlled by the volume of the dispersion used in spraying and the substrate area. We note that the optoelectrical property, σ(DC)/σ(Op), for various films fabricated was in the range 75-350, which is extremely high for transparent thin film compared to other candidate alternatives to doped metal oxide film. Using this method, we obtain silver nanowire films on a flexible polyethylene terephthalate (PET) substrate with a transparency of 85% and sheet resistance of 33 Ω/sq, which is comparable to that of tin-doped indium oxide (ITO) on flexible substrates. In-depth analysis of the film shows a high performance using another commonly used figure-of-merit, Φ(TE). Also, Ag nanowire film/PET shows good mechanical flexibility and the application of such a conductive silver nanowire film as an electrode in a touch panel has been demonstrated.     

Electrohydrodynamic (EHD) inkjet printing flexible pressure sensors with a multilayer structure and periodically patterned Ag nanoparticles

Flexible pressure sensors are widely employed for accurate pressure sensing on geometrically complex surfaces. As sensing materials, silver nanoparticles (AgNPs) have high electrical conductivity but relatively poor sensitivity as a trade-off. In this work, electrohydrodynamic (EHD) inkjet printing was utilized to directly write patterns of AgNPs tracks with periodic geometries on the flex-substrate surface. The patterns in which the as-printed AgNPs tracks, with a width of several tens of micrometres, exhibited a piezoresistive effect. This work confirmed that introducing multilayered structures into the flexible pressure sensors with AgNPs patterns was a practical path to improve the sensing sensitivity, with the assistance of soft packaging material of Polydimethylsiloxane (PDMS). The sensitivity was improved more than tenfold after fourfold overlapping of the as-printed single-layer sensor. Experimental tests, formula calculations, and numerical simulations of the sensors were conducted. It was concluded that the as-printed single-layer sensor with the AgNPs pattern of concave regular hexagonal structure (CRHTS) had better sensing performance than that of grid-type structure (GTS) or wave-type structure (WTS). For the two-layered CRHTS sensor, the dynamic and quasi-static sensing response characteristics, response recovery duration, cyclic stability, and ability to discriminate different strain frequencies were further measured and analysed. The working principle of the flex sensors was discussed based on the Percolation Theory and the Tunneling Effect. Some application demonstrations of the sensors were also exhibited. The structural design and EHD inkjet printing fabrication path facilitate the development of more versatile flex sensors.

     

Ordering of Ge islands on Si(001) substrates patterned by nanoindentation

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters.     

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by μRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650?°C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.     

The preparation of highly active antimicrobial silver nanoparticles by an organometallic approach

Silver nanoparticles of small size with a high surface to volume ratio have been prepared using an organometallic approach. For this, the complex NBu(4)[Ag(C(6)F(5))(2)] has been treated with AgClO(4) in a 1:1 molar ratio, giving rise to the nanoparticle precursor [Ag(C(6)F(5))] in solution. Addition of one equivalent of hexadecylamine (HDA) and 5?h of reflux in toluene leads to a deep yellow solution containing monodisperse silver nanoparticles (Ag NPs) of ca.?10?nm. This approach leads to nanoparticles with almost uncontaminated surfaces which make them very reactive. Antimicrobial studies show that these nanoparticles are very active as antimicrobial agents. Very low concentrations between 12 and 25?μg?ml(-1) of Ag NPs are enough to produce bacteriostatic and bactericidal effectiveness.