High-frequency devices are key enablers
of state-of-the-art electronics used in a wide and diverse range of
exciting applications such as inertial navigation, communications,
power conversion, medicine, and parallel computing. However, high-frequency
additively manufactured piezoelectric devices are yet to be demonstrated
due to shortcomings in the properties of the printed transducing material
and the attainable film thickness. In this study, we report the first
room-temperature-printed, piezoelectric, ultrathin (<100 nm) ceramic
films compatible with high-frequency (>1 GHz) operation. The films
are made of zinc oxide (ZnO) nanoparticles via near-field electrohydrodynamic
jetting, achieving film piezoelectricity, without high-temperature
processing, through a novel mechanism that is controlled during the
deposition. Optimization of the printing process and feedstock
formulation results in homogeneous traces as narrow as 213 μm
and as thin as 53 nm as well as uniform field films as thin as 91
nm; the printing technique can be used with flexible and rigid, conductive
and insulating substrates. The crystallographic orientation of the
imprints toward the (100) plane increases if the rastering speed during
printing is augmented, resulting in a larger piezoelectric response.
The resonant frequency of film bulk acoustic resonators increases
monotonically with the rastering speed, achieving transmission values
as high as 4.99 GHz, which corresponds to an acoustic velocity of
2094 m/s, similar to the expected transverse value in high-temperature-grown
ZnO films. Piezoresponse force microscopy maps of printed field films
show local variation in the piezoelectric behavior across the film,
with an average piezoelectric response as high as 21.5 pm/V, significantly
higher than the
d
33
piezoelectric coefficient
of single-crystal, high-temperature-grown ZnO, and comparable with
reported values from ZnO nanostructures.
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