7 research outputs found
The role of ligands in coinage-metal nanoparticles for electronics
Coinage-metal nanoparticles are key components of many printable electronic inks. They can be combined with polymers to form conductive composites and have been used as the basis of molecular electronic devices. This review summarizes the multidimensional role of surface ligands that cover their metal cores. Ligands not only passivate crystal facets and determine growth rates and shapes; they also affect size and colloidal stability. Particle shapes can be tuned via the ligand choice while ligand length, size, Ï-functionalities, and chemical nature influence shelf-life and stability of nanoparticles in dispersions. When particles are deposited, ligands affect the electrical properties of the resulting film, the morphology of particle films, and the nature of the interfaces. The effects of the ligands on sintering, cross-linking, and self-assembly of particles in electronic materials are discussed
Ultrathin gold nanowires for transparent electronics: breaking barriers
Novel types of Transparent Conductive Materials (TCMs) based on metal nanostructures are discussed. Dispersed metal nanoparticles can be deposited from liquids with moderate thermal budgets to form conductive films that are suitable for thin-film solar cells, displays, touch screens, and nanoelectronics. We aim at new TCMs that combine high electrical conductivity with optical transparency and mechanical flexibility. Wet-processed films of randomly arranged metallic nanowires networks are commercially established and provide a relatively cost-effective, scalable production. Ultrathin gold nanowires (AuNWs) with diameters below 2 nm and high aspect ratios have recently become available. They combine mechanical flexibility, high optical transparency, and chemical inertness. AuNWs carry oleylamine capping ligands from synthesis that cause high contact resistances at their junctions. We investigated different annealing processes based on temperature and plasma treatment, to remove the ligands after deposition and to allow electrical conductivity. Their effect on the resulting nanostructure and on the material properties was studied. Scanning Electron Microscopy (SEM) and optical spectroscopy revealed changes in the microstructure for the different post-treatments. We found that the conductivity and the stability of the TCM depended strongly on its final microstructure. We demonstrate that the best results are obtained using H2-plasma treatment
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Templated Self-Assembly of Ultrathin Gold Nanowires by Nanoimprinting for Transparent Flexible Electronics
We fabricated flexible, transparent,
and conductive metal grids as transparent conductive materials (TCM)
with adjustable properties by direct nanoimprinting of self-assembling
colloidal metal nanowires. Ultrathin gold nanowires (diameter below
2 nm) with high mechanical flexibility were confined in a stamp and
readily adapted to its features. During drying, the wires self-assembled
into dense bundles that percolated throughout the stamp. The high
aspect ratio and the bundling yielded continuous, hierarchical superstructures
that connected the entire mesh even at low gold contents. A soft sintering
step removed the ligand barriers but retained the imprinted structure.
The material exhibited high conductivities (sheet resistances down
to 29 Ω/sq) and transparencies that could be tuned by changing
wire concentration and stamp geometry. We obtained TCMs that are suitable
for applications such as touch screens. Mechanical bending tests showed
a much higher bending resistance than commercial ITO: conductivity
dropped by only 5.6% after 450 bending cycles at a bending radius
of 5 mm
Sintering of Ultrathin Gold Nanowires for Transparent Electronics
Ultrathin gold nanowires (AuNWs)
with diameters below 2 nm and high aspect ratios are considered to
be a promising base material for transparent electrodes. To achieve
the conductivity expected for this system, oleylamine must be removed.
Herein we present the first study on the conductivity, optical transmission,
stability, and structure of AuNW networks before and after sintering
with different techniques. Freshly prepared layers consisting of densely
packed AuNW bundles were insulating and unstable, decomposing into
gold spheres after a few days. Plasma treatments increased the conductivity
and stability, coarsened the structure, and left the optical transmission
virtually unchanged. Optimal conditions reduced sheet resistances
to 50 Ω/sq
Spinning Hierarchical Gold Nanowire Microfibers by Shear Alignment and Intermolecular Self-Assembly
Hierarchical
structures lend strength to natural fibers made of
soft nanoscale building blocks. Intermolecular interactions connect
the components at different levels of hierarchy, distribute stresses,
and guarantee structural integrity under load. Here, we show that
synthetic ultrathin gold nanowires with interacting ligand shells
can be spun into biomimetic, free-standing microfibers. A solution
spinning process first aligns the wires, then lets their ligand shells
interact, and finally converts them into a hierarchical superstructure.
The resulting fiber contained 80 vol % organic ligand but was strong
enough to be removed from the solution, dried, and mechanically tested.
Fiber strength depended on the wire monomer alignment. Shear in the
extrusion nozzle was systematically changed to obtain processâstructureâproperty
relations. The degree of nanowire alignment changed breaking stresses
by a factor of 1.25 and the elongation at break by a factor of 2.75.
Plasma annealing of the fiber to form a solid metal shell decreased
the breaking stress by 65%