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A new type of hybrid dielectric based on nanoparticles with gold cores with diameters of 2.9-8.2 nm and covalently bound thiol-terminated polystyrene shells (Mn = 5000 Da and Mn = 11000 Da) is introduced. Particle dispersions were spin coated as dielectric films of thin film capacitors. The metal contents were 5-31 vol%, and the particles packed randomly or in face-centred-cubic superstructures, mainly depending on the polymer shell. Films with 9 vol% metal and 2.9 nm cores had dielectric constants of 98@1 Hz. Small angle X-ray scattering, transmission electron microscopy, and impedance spectroscopy indicate that classical random capacitor-resistor network models partially describe the hybrid materials. The covalently attached polymer shells enabled higher metal contents than in conventional nanocomposites without the risk of conductive breakdown. Dielectric properties depended on the metal content and the core size, but not on the network structure. The frequency-dependent dielectric polarization mainly takes place at the interfacial areas, but is not considered in the classical models. Smaller core sizes increased internal interfacial areas at comparable metal fractions by 46 %, resulting in 40 % larger dielectric constants in agreement with the Maxwell-Wagner-Sillars model. Inkjet-printed capacitors were prepared with a capacitance of 2.0±0.1 nF@10 kHz over an area of 0.79 mm² on rigid substrates; they retained their functionality over 3500 bending cycles on flexible substrates.
Hybrid dielectrics were prepared from dispersions of nanoparticles with gold cores (diameters from 2.9 nm to 8.2 nm) and covalently bound thiol-terminated polystyrene shells (5000 Da and 11 000 Da) in toluene. Their microstructure was investigated with small angle X-ray scattering and transmission electron microscopy. The particles arranged in nanodielectric layers with either face-centered cubic or random packing, depending on the ligand length and core diameter. Thin film capacitors were prepared by spin-coating inks on silicon substrates, contacted with sputtered aluminum electrodes, and characterized with impedance spectroscopy between 1 Hz and 1 MHz. The dielectric constants were dominated by polarization at the gold–polystyrene interfaces that we could precisely tune via the core diameter. There was no difference in the dielectric constant between random and supercrystalline particle packings, but the dielectric losses depended on the layer structure. A model that combines Maxwell–Wagner–Sillars theory and percolation theory described the relationship of the specific interfacial area and the dielectric constant quantitatively. The electric breakdown of the nanodielectric layers sensitively depended on particle packing. A highest breakdown field strength of 158.7 MV m−1 was found for the sample with 8.2 nm cores and short ligands that had a face-centered cubic structure. Breakdown apparently is initiated at the microscopic maxima of the electric field that depends on particle packing. The relevance of the results for industrially produced devices was demonstrated on inkjet printed thin film capacitors with an area of 0.79 mm2 on aluminum coated PET foils that retained their capacity of 1.24 ± 0.01 nF@10 kHz during 3000 bending cycles.
A new type of hybrid core–shell nanoparticle dielectric that is suitable for inkjet printing is introduced. Gold cores (dcore ≈ 4.5 nm diameter) are covalently grafted with thiol-terminated polystyrene (Mn = 11000 Da and Mn = 5000 Da) and used as inks to spin-coat and inkjet-print dielectric films. The dielectric layers have metal volume fractions of 5 to 21 vol% with either random or face-centered-cubic structures depending on the polymer length and grafting density. Films with 21 vol% metal have dielectric constants of 50@1 Hz. Structural and electrical characterization using transmission electron microscopy, small-angle X-ray scattering, and impedance spectroscopy indicates that classical random capacitor–resistor network models partially describe this hybrid material but fail at high metal fractions, where the covalently attached shell prevents percolation and ensures high dielectric constants without the risk of dielectric breakdown. A comparison of disordered to ordered films indicates that the network structure affects dielectric properties less than the metal content. The applicability of the new dielectric material is demonstrated by formulating inkjet inks and printing devices. An inkjet-printed capacitor with an area of 0.79 mm2 and a 17 nm thick dielectric had a capacitance of 2.2±0.1 nF@1 kHz.
