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Abstract

The additive manufacturing of functional structures by inkjet printing offers a unique
and powerful alternative to conventional top-down patterning methods. Yet, multi-layer
inkjet deposition is challenging due to problems with the wetting and drying. The wetting
depends on the properties of the ink and the surface of the substrate. The latter can be
regulated by the deposition of a few-nanometres-thick polymeric layer. The chemical nature
of the polymer determines the surface properties of the newly formed surface. A precise
control over the wetting is achieved by modifying the polymer layer, in our case poly(methyl
methacrylate), by (i) a partial thermal decomposition, (ii) an O2-plasma treatment and (iii)
a UV/O3 treatment. A smooth transition in wetting occurs when exposing the polymeric
layer to a modification method, which enables the printing of complex structures with high
resolution. While the thermal decomposition is effective for wetting adjustments only on
unpatterned surfaces, the other two methods can be applied to pre-patterned surfaces.
The solvent composition of the ink determines its wetting behaviour on a selected surface,
the drying process and the morphology of the dried deposit. Mixtures of solvents with
a large difference in the surface tension and the volatility are commonly used to suppress
the coffee-stain effect, which is mainly attributed to the modification of the internal liquid
flows. However, a reconstruction of the drop height profiles during the drying reveals a
distinct difference in the contact-line dynamics for the ink formulations that produce ringlike
or dome-like deposits. A pinned contact line promotes the formation of a ring-like
deposit, whereas a dynamic contact line promotes the formation of a dome-like deposit.
Flat structures are printed only when the contact line exhibits dynamic behaviour; the
latter can be regulated by the solvent composition and the initial contact angle.
These findings were used to establish a reliable inkjet printing process that is highly
efficient and enables the conformal stacking of functional oxide layers according to the
user-defined geometry, sequence arrangement and layer thickness. Ferroelectric thin-film
capacitors (proof-of-concept devices) consisted of lanthanum nickelate electrodes and a
lanthanum-doped lead zirconate titanate (PZT) ferroelectric layer. The capacitors could
be printed on various substrates, including silicon wafers, alumina plates and nickel foils. A
detailed structural and electrical characterization reveals the excellent functional properties
of the printed single-layer devices. These easily achieve the properties of devices prepared
by spin coating. The printed multi-layer devices, however, greatly exceed the performance
of single-layer devices, which is indicated by an increase in the capacitance density and the
electromechanical response in comparison to the single-layer devices.
In the last part, a soft-chemistry technique was devised for the deposition of nanostructured
PZT films with greatly enhanced piezoelectric properties. The method utilizes
the self-assembly of organic precursors in a polymer matrix to fabricate nanostructured
oxide thin films by spin coating. The implemented macro-porosity leads to local elastic
relaxations and greatly enhances the electromechanical response. The large piezoelectric
coefficient—reaching the level of bulk ceramics—is associated with the increased elastic
compliance of the nanostructured films and the highly mobile ferroelastic domain walls.