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Scientific Unit
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.
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 2nm 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.
Metal-based nanoparticle inks for printed electronics usually require sintering to improve the poor electron transport at particle-particle interfaces. The ligands required for colloidal stability act as insulating barriers and must be removed in a post-deposition sintering step. This complicates the fabrication process and makes it incompatible with many flexible substrates. Here, we bind a conjugated, electrically conductive polymer on gold nanorods (AuNRs) as a ligand. The polymer, poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate)] (PTEBS), provides colloidal stability and good electron transport properties to stable, sintering-free inks. We confirm that the polymer binds strongly through a multidentate binding motif and provides superior colloidal stability in polar solvents over months by IR and Raman spectrometry and zeta potential measurements. We demonstrate that the developed ligand exchange protocol is directly applicable to other polythiophenes such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Films of AuNRs coated with above polymers reached conductivities directly after deposition comparable to conventional metal inks after ligand removal and retained their conductivity for at least one year when stored under ambient conditions.
Ultrathin gold nanowires are unusual colloidal objects that assemble into bundles with line contacts between parallel wires. Each molecule in the contact line interacts with many ligand and solvent molecules. We used X-ray scattering and electron microscopy to study how these interactions control assembly.
Inorganic nano-objects with organic shells form an interesting class of nanostructured materials when they are assembled into larger units - hybrid materials. The industrial use of materials produced via this bottom-up route is impeded by the lack of simple production processes. A promising process is their production from colloidal inks; however, the targeted control of the nano-objects’ superstructure formation must be improved. Here interfaces and the organic shells are crucial, as they strongly affect the assembly characteristics of the hybrid nano-objects. Here, the colloidal and supramolecular chemistry of the ligand shell is studied. Ligandstabilized wire-like and rod-like nano-objects were synthesized, modified, and their assembly behavior in conjunction with the dispersant medium was elucidated. After that, two different hybrid materials were produced using an ink-based approach. First, gold nanorods were coated with a conjugated polymer ligand shell that enabled both good colloidal stability and electrical conductivity of the hybrid structures directly after drying of the ink without sintering. Second, ultrathin gold nanowires were spun into hierarchical fibers, exploiting the interaction of their ligand shells with the surrounding dispersant medium.
