IR-sintering of ink-jet printed metal-nanoparticles on paper
Introduction
Paper has recently gained attention as a potential substrate [1] for low-cost flexible electronics, partly because of its wide usage, low cost and recyclability. Highly conducting printable conductors are desired in most flexible electronic applications, e.g., for contacts, wires and antennas. While conducting polymers can be used when a relatively high resistance of the conductor is acceptable, metals should be used when high and stable conductivities (> 104 S/cm) are required. Metals can be printed by using inks based on metal particles of different size or low-viscosity inks based on organometallic precursors. When conducting metal structures are to be printed onto paper, high-viscosity particle-based inks are advantageous due to the limited penetration of metal particles into the porous substrate. Among the advantages of nanoparticle (NP) based inks are the relatively small feature size that can be printed and the high conductivity that can be achieved after a sintering process. Ink-jet printing is rather commonly used for applying NP-inks, because of its low-cost, flexibility and low ink consumption. Among the metals that have been developed for NP-based inks are silver (Ag) [2], [3], [4], gold (Au) [4], [5], [6], [7], [8] and copper (Cu) [9], [10], and stable inks are made by capping the metal-NPs with sterically stabilizing organic molecules.
Sintering of printed metal-NPs, to some extent, is required in order to achieve highly conducting structures. The coalescence of NPs at temperatures far below the melting temperature of the bulk material is often ascribed to the strong size dependence of the melting point of NPs, which can be derived by relatively simple thermodynamic considerations [11]. Assigning bulk-like thermodynamic properties to NPs is, however, problematic, and there are other processes than melting that can result in the coalescence of NPs. If given enough time it is possible for the NPs to reduce their surface energy by forming necks between each other through grain boundary and surface diffusion at much lower temperatures than predicted by thermodynamic considerations. However, in order for any sintering to take place, contact between the NPs must be enabled, which requires the removal of most of the organic ligands. This is why the lowest annealing temperature required for achieving conducing structures of a metal-NP ink typically is given by the temperature at which the organic ligands are oxidized, pyrolyzed or sublimated [4].
The most common technique of sintering NPs is by conventional heating in an oven or on a hot plate. However, this method usually requires long annealing time (> 30 min) or high temperatures (> 200 °C), and is consequently incompatible with a fast roll-to-roll manufacturing process or with low-cost flexible substrates. There are, nevertheless, several other sintering techniques [12] that could be used. The electrically induced insulating-to-conducting transition of metal-NPs (by Joule heating) has not only been considered for memory applications [13], but also as an interesting technique for sintering metal-NPs [14]. Among the other techniques for sintering printed metal-NPs are microwave [15], [16] and argon plasma [17] sintering. It has even been demonstrated that sintering of NPs, to some extent, spontaneously can take place at room temperature through surface diffusion [18] when the organic dispersion agents are first removed by dipping in a solvent [19], [20], by tuning the Zeta-potential of the NPs [21], by oxidizing the protective organic ligand [22], by including a destabilizing electrolyte in the dispersion agent [23] or by desorption into a photo paper substrate [24].
Another possibility is to induce the sintering by using various light sources. A popular method is to use a laser, since this enables selective heating of the printed areas on the substrate, and even patterning of conducting structures by sintering or ablation [25]. An additional advantage is that the wavelength of the laser can be chosen to match the local light absorption maximum of the NPs, which improves the energy transfer efficiency. However, using a laser is not only expensive, but also less suitable for larger areas, and best results are typically obtained when using a slow writing speed of around 0.2 mm/s. A different type of light source that has been shown to be promising for sintering metal-NPs is the use of a xenon flash lamp [26], [27] emitting millisecond high-intensity pulses (e.g. 25 kW/cm2). The short sintering time reduced the problems of oxidation of the Cu-NPs in air and of heating a plastic substrate (after filtering away the UV-light).
In this work, we used incandescent lamps for sintering ink-jet printed metal-NPs on a paper substrate. Two different NP-inks were used; one based on alkanethiol protected small Au-NPs, and one commercial Ag-ink consisting of relatively large NPs with a polymeric dispersion agent. Both the resistivity and the optical appearance of the sintered ink-jet printed NP-inks were studied.
Section snippets
Material and methods
The recyclable multilayer-coated paper substrates [28] that were used in this work were developed for having good barrier properties and a low surface roughness and desired porosity. The barrier layer consisted of kaolin (Barrisurf HX, Imerys Minerals Ltd.) blended with 30 pph ethylene acrylic acid copolymer latex (Tecseal, Trüb Emulsions Chemie AG), while the calendered top mineral pigment layer was based on kaolin (Barrisurf FX, Imerys Minerals Ltd.) with 12 pph of latex binder (DL 920, Styron
Calculation
The absorptance (A), reflectance (R) and transmittance (T) can be derived by using the measured intensities (L) of the outgoing light from the integrating sphere e.g. when placing the sample on the reflecting plug area. The light absorptance could be calculated by using a similar approach as was used by Pålsson et al. [30] and de Mello et al. [31] for deriving the photoluminescence. In their approach, the light absorptance is given by A = (Ldiffuse − Ldirect) / Ldiffuse. This way of calculating the
Light absorption of metal-NPs and paper
The measured light transmittance of spin coated Ag-NPs and dodecanethiol-Au-NPs on a glass substrate, before and after thermal annealing on a hot plate, are shown in Fig. 3 as a function of wavelength. The local absorption maximum due to plasmons is around 420 nm of the Ag-NPs and around 515 nm of the Au-NPs. Another important feature is the increase in the transmittance of the unsintered NPs with an increasing wavelength (λ > 600 nm). The large decrease in the light transmission at longer
Conclusions
IR sintering is a fast, low-cost and roll-to-roll compatible method of achieving highly conducting structures of printed metal-NP inks on a mineral pigment coated paper. The technique is especially suitable on paper substrates because of the high diffuse reflectance, low thermal conductivity and high thermal stability of paper. In addition to this, a larger light absorptance of the printed NPs on paper than on plastics or glass was found, and was explained by multiscattering on the paper. The
Acknowledgments
Financial support is acknowledged from the European Regional Development Fund in South Finland and the Academy of Finland through the National Center of Excellence program. The authors also acknowledge EUV Technology for providing the SuMMIT software that was used for calculating the line-edge roughness.
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