Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Ligand-Dependent Nanoparticle Assembly and Its Impact on the Printing of Transparent Electrodes Thomas Kister,‡,† Johannes H. M. Maurer,‡,† Lola González-García,‡ and Tobias Kraus*,‡,§ ‡
INM - Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany Colloid and Interface Chemistry, Saarland University, Saarbrücken, Germany
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S Supporting Information *
ABSTRACT: Metal grids with submicron line diameters are optically transparent, mechanically flexible, and suitable materials for transparent and flexible electronics. Printing such narrow lines with dilute metal nanoparticle inks is challenging because it requires percolation throughout the particle packing. Here, we print fully connected submicron lines of 3.2 nm diameter gold nanoparticles and vary the organic ligand shell to study the relation between colloidal interactions, ligand binding to the metal core, and conductivity of the printed lines. We find that particles with repulsive potentials aid the formation of continuous lines, but the required long ligand molecules impede conductivity and need to be removed after printing. Weakly bound alkylamines provided sufficient interparticle repulsion and were easy to remove with a soft plasma treatment after printing, so that grids with a transparencies above 90% and a conductivity of 150 Ω sq−1 could be printed. KEYWORDS: nanoparticles, ligand design, nanoimprinting, self-assembly, transparent electrodes
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substrate compatibility.7 One alternative are ultrathin gold nanowires with core diameters below 2 nm that have the tendency to form continuous bundles due to their ligands’ interactions8 and readily form percolating meshes during printing.9,10 The wires make it much easier to print percolating structures and have been successfully employed to create conductive transparent electrodes, but nanowire synthesis is still in its infancy. Wire dispersions prepared using the existing protocols are unstable, and only very few materials can be prepared as ultrathin nanowires. Here, we show that it is possible to print very narrow, continuous lines of spherical nanoparticles depending on their colloidal interactions. This opens the possibility to print a variety of materials as fine meshes: many elegant synthetic routes are available to produce spherical nanoparticles with a variety of core materials, and ligands have been successfully used to control the particles’ interactions and prevent premature self-assembly.11−13 We systematically varied the ligands of gold nanoparticles (AuNPs) to tune the imprinting performance and studied the influence on the particles’ self-assembly, electronic behavior, and sintering characteristics. Grids with submicron line width were printed and analyzed regarding their electrical and optical properties and their changes after plasma sintering. The results show which colloidal interactions are necessary to print
ransparent electrodes are critical components of displays, solar cells, and touch screens. Particularly suitable for such materials are grids of very thin metal wires: regular arrays of metal lines with widths below 1 μm on polymer substrates equal or outperform transparent conductive oxides such as indium tin oxide in conductivity and transparency, can be made from abundant metals, and are mechanically flexible and suitable for new device architectures.1 The common trade-off between conductivity and transparency is readily controlled through the density of the mesh. It is highly appealing to print such grids using nanoparticlebased inks. Functional nanoparticles are proven building blocks for micro- and macroscopic electronic circuits.2 Metal inks composed of nanoparticles dispersed in a solvent or a solvent mixture are already commonly applied.3 Such inks can be printed under ambient conditions, fewer processing steps are required than in top-down technology, and less material is wasted because material is only deposited where needed.4 High-throughput and low-cost processing in large sheet or rollto-roll production becomes possible. Despite of the advantages of printing, it has proven challenging to print transparent conductive lines of common nanoparticles. Spherical nanoparticles tend to agglomerate already at low concentrations.5 This leads to disconnected agglomerates that interrupt the printed lines and prevent macroscopic conductivity. As a result, literature only reports the printing of small areas using either highly concentrated (up to 15% by weight,6 where we use inks with 0.64% here), viscous inks or by using comparatively slow nanodrip printing. Existing reports employ annealing at high temperatures, which limits © XXXX American Chemical Society
Received: December 6, 2017 Accepted: February 5, 2018 Published: February 5, 2018 A
DOI: 10.1021/acsami.7b18579 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
2d−f) on polyimide (Kapton tape) supported the SEM results: with decreasing ligand length, the peaks of the structure factor became narrower and more intense, indicating an increase in agglomerate size. The surface-to-surface distance between the particles increased from 1.16 to 1.96 nm as the ligand length increased from C4 to C12 (see Figure S4). The insets show transmission electron microscopy (TEM) images of the structures that the inks formed upon drying on carbon-coated TEM grids (see also Figure S3). Their morphology resembles that of the printed lines. AuNP@C4-thiol formed large agglomerates with diameters up to 500 nm, while AuNP@ C12-thiol formed a monolayer of particles. All inks initially contained the same particle concentrations, and there were no differences in the rheology or interface properties. Contact angle measurements on glass showed no significant influence of the ligand length on the wetting of the dispersion (see Figure S5). The differences in the printed morphologies must therefore be connected with colloidal properties of the particles that are affected by the ligand length. Lohman and Sorensen indicated that the concentration at which particle assembly sets in depends on ligand length.5 We suggest that it is this concentration that dominates the morphologies. Nanoparticles with shorter ligands assemble at a stage where the liquid film is relatively thick, and the particles can still move freely, whereas nanoparticles with longer ligand assemble at a very late stage, when the liquid film is already extremely thin and confinement is severe. Unconfined assembly leads to disconnected compact agglomerates that are deposited as discontinuous traces; confinement causes assembly in compact, continuous lines. The situation reminds of the formation of regular supraparticles in evaporating emulsion droplets, where the assembly kinetics dominate the level of ordering of the final structure.18 The unconnected clusters of the grids printed from AuNP@ C4-thiol and AuNP@C8-thiol were discontinuous and therefore not conductive. AuNP@C12-thiol formed continuous, percolating lines, but they were not conductive after printing because the organic ligands shell impeded electron transport. A common way to remove the barriers and form direct metal− metal contacts is thermal sintering; the required temperature increases with the length of the carbon chain for alkanethiolcoated gold nanoparticles. A temperature of 120 °C was reported to turn AuNP@C4-thiol layers conductive, while AuNP@C12-thiol layers required temperatures between 170 and 200 °C.15,16,19 Sintering at such temperatures damaged our printed grids: the thin lines (line width
