2 research outputs found
Complex Evolution of Built-in Potential in Compositionally-Graded PbZr<sub>1–<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>3</sub> Thin Films
Epitaxial strain has been widely used to tune crystal and domain structures in ferroelectric thin films. New avenues of strain engineering based on varying the composition at the nanometer scale have been shown to generate symmetry breaking and large strain gradients culminating in large built-in potentials. In this work, we develop routes to deterministically control these built-in potentials by exploiting the interplay between strain gradients, strain accommodation, and domain formation in compositionally graded PbZr<sub>1–<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>3</sub> heterostructures. We demonstrate that variations in the nature of the compositional gradient and heterostructure thickness can be used to control both the crystal and domain structures and give rise to nonintuitive evolution of the built-in potential, which does not scale directly with the magnitude of the strain gradient as would be expected. Instead, large built-in potentials are observed in compositionally-graded heterostructures that contain (1) compositional gradients that traverse chemistries associated with structural phase boundaries (such as the morphotropic phase boundary) and (2) ferroelastic domain structures. In turn, the built-in potential is observed to be dependent on a combination of flexoelectric effects (<i>i.e.</i>, polarization–strain gradient coupling), chemical-gradient effects (<i>i.e.</i>, polarization–chemical potential gradient coupling), and local inhomogeneities (in structure or chemistry) that enhance strain (and/or chemical potential) gradients such as areas with nonlinear lattice parameter variation with chemistry or near ferroelastic domain boundaries. Regardless of origin, large built-in potentials act to suppress the dielectric permittivity, while having minimal impact on the magnitude of the polarization, which is important for the optimization of these materials for a range of nanoapplications from vibrational energy harvesting to thermal energy conversion and beyond
Chemical Phenomena of Atomic Force Microscopy Scanning
Atomic
force microscopy is widely used for nanoscale characterization
of materials by scientists worldwide. The long-held belief of ambient
AFM is that the tip is generally chemically inert but can be functionalized
with respect to the studied sample. This implies that basic imaging
and scanning procedures do not affect surface and bulk chemistry of
the studied sample. However, an in-depth study of the confined chemical
processes taking place at the tip–surface junction and the
associated chemical changes to the material surface have been missing
as of now. Here, we used a hybrid system that combines time-of-flight
secondary ion mass spectrometry with an atomic force microscopy to
investigate the chemical interactions that take place at the tip–surface
junction. Investigations showed that even basic contact mode AFM scanning
is able to modify the surface of the studied sample. In particular,
we found that the silicone oils deposited from the AFM tip into the
scanned regions and spread to distances exceeding 15 ÎĽm from
the tip. These oils were determined to come from standard gel boxes
used for the storage of the tips. The explored phenomena are important
for interpreting and understanding results of AFM mechanical and electrical
studies relying on the state of the tip–surface junction