Joshua C. Agar

Improved Pyroelectric Figures of Merit in Compositionally Graded PbZr1–xTixO3 Thin Films

Pyroelectric materials have been widely used for a range of thermal-related applications including thermal imaging/sensing, waste heat energy conversion, and electron emission. In general, the figures of merit for applications of pyroelectric materials are proportional to the pyroelectric coefficient and inversely proportional to the dielectric permittivity. In this context, we explore single-layer and compositionally graded PbZr1-xTixO3 thin-film heterostructures as a way to independently engineer the pyroelectric coefficient and dielectric permittivity of materials and increase overall performance. Compositional gradients in thin films are found to produce large strain gradients which generate large built-in potentials in the films that can reduce the permittivity while maintaining large pyroelectric response. Routes to enhance the figures of merit of pyroelectric materials by 3-12 times are reported, and comparisons to standard materials are made.

Unexpected Crystal and Domain Structures and Properties in Compositionally Graded PbZr1‐xTixO3 Thin Films

Synthesis of compositionally graded versions of PbZr(1-x)Ti(x)O3 thin films results in unprecedented strains (as large as ≈4.5 × 10(5) m(-1)) and correspondingly unexpected crystal structures, ferroelectric domain structures, and properties. This includes the observation of built-in electric fields in films as large as 200 kV/cm. Compositional and strain gradients could represent a new direction of strain-control of materials.

Highly mobile ferroelastic domain walls in compositionally graded ferroelectric thin films

Domains and domain walls are critical in determining the response of ferroelectrics, and the ability to controllably create, annihilate, or move domains is essential to enable a range of next-generation devices. Whereas electric-field control has been demonstrated for ferroelectric 180° domain walls, similar control of ferroelastic domains has not been achieved. Here, using controlled composition and strain gradients, we demonstrate deterministic control of ferroelastic domains that are rendered highly mobile in a controlled and reversible manner. Through a combination of thin-film growth, transmission-electron-microscopy-based nanobeam diffraction and nanoscale band-excitation switching spectroscopy, we show that strain gradients in compositionally graded PbZr1-xTixO3 heterostructures stabilize needle-like ferroelastic domains that terminate inside the film. These needle-like domains are highly labile in the out-of-plane direction under applied electric fields, producing a locally enhanced piezoresponse. This work demonstrates the efficacy of novel modes of epitaxy in providing new modalities of domain engineering and potential for as-yet-unrealized nanoscale functional devices.

New modalities of strain-control of ferroelectric thin films.

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Complex Evolution of Built-in Potential in Compositionally-Graded PbZr1- xTixO3 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 PbZr1-xTixO3 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.e., polarization-strain gradient coupling), chemical-gradient effects (i.e., 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.

Novel PDMS(silicone)-in-PDMS(silicone): Low cost flexible electronics without metallization

Future electronics will undoubtedly require natural integration at the system, device and package level in the form of a functional, flexible package. Functional, flexible electronics expand the functionality of devices allowing morphological-electronic response for ergonomic and natural interfaces between the device and its surroundings. Recent technological successes have been able to fabricate functional, flexible electronics, however have all failed to develop a package capable of meeting the stringent cost, reliability and performance required of consumer electronics. We demonstrate the application of electrically conductive adhesive technology to produce low cost, flexible electronics without metallization. We have shown the capability of fabrication of highly conductive Poly(dimethlysiloxane) (PDMS) (ρ~7×10−4 Ω•cm) by incorporation of 80 wt% bimodal distribution of micron sized silver flakes. PDMS is both the ideal substrate and composite matrix material due to its unique properties; PDMS is optically transparent, viscoelastic, chemically and thermally stable, highly flexible, hydrophobic and can easily be molded with high resolution and aspect ratio. These unique properties of PDMS allow for high resolution molds to be prepared from photolithographically defined substrates. Screen printing of electrically conductive PDMS into these molds with micro-sized features creates a low cost, flexible electronic package. We have coined this package PDMS-in-PDMS. We show that PDMS ECA can be prepared by curing a novel formulation of PDMS at curing temperatures of 150 °C for 15 minutes. Upon curing, the ECA undergoes a transition from insulating to conductive. TMA results have shown that this transition is due to ECA shrinkage >20%. Furthermore, we show simultaneous conductivity and tensile strain measurements to show the electrical properties of PDMS ECA are unaffected by tensile strains of >40%. We show the feasibility of this technology to create low cost, flexible devices without the need for metallization.

Electrically conductive silicone nano-composites for stretchable RF devices

The objective of this paper is to show how stretchable conductive composites can be utilized for the fabrication of ultra-low cost stretchable RF devices. We show a method to produce biocompatible highly conductive stretchable silicone composites via an in-situ nanoparticle formation and sintering process. Furthermore, we develop a simple, low cost, processing technique to fabricate stretchable RF transmission lines. These RF transmission lines are highly flexible, stretchable and robust. The S-parameter measurements show stable performance during mechanical deformation up to 6 GHz. Future development of this technology will enable ultra low cost consumer RF devices serving as a platform for future stretchable electronic devices.

Strain-induced growth instability and nanoscale surface patterning in perovskite thin films

Despite extensive studies on the effects of epitaxial strain on the evolution of the lattice and properties of materials, considerably less work has explored the impact of strain on growth dynamics. In this work, we demonstrate a growth-mode transition from 2D-step flow to self-organized, nanoscale 3D-island formation in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 (001) heterostructures as the kinetics of the growth process respond to the evolution of strain. With increasing heterostructure thickness and misfit dislocation formation at the buried interface, a periodic, modulated strain field is generated that alters the adatom binding energy and, in turn, leads to a kinetic instability that drives a transition from 2D growth to ordered, 3D-island formation. The results suggest that the periodically varying binding energy can lead to inhomogeneous adsorption kinetics causing preferential growth at certain sites. This, in conjunction with the presence of an Ehrlich-Schwoebel barrier, gives rise to long-range, periodically-ordered arrays of so-called “wedding cake” 3D nanostructures which self-assemble along the [100] and [010].

Novel stretchable electrically conductive composites for tunable RF devices

Stretchable, flexible and tunable RF devices that are fabricated with Polydimethylsiloxane (PDMS) Electrically Conductive Composites (ECC) are presented. Using this composite material allows mechanical modulation of the device dimensions resulting in tuning of its frequency response. A planar loop antenna and a 5th order stepped impedance low pass filter operating around 1.5 GHz with tunability greater than 15% are shown. The ECC can reach an electrical resistivity as low as 10−4 Ω. cm, which is close to a metal resistivity. The materials are also ultra-low cost for massive fabrication. This technology opens the door for tunable RF devices on flexible and curvilinear packages.

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 μm 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.