Enhancing piezoelectric properties under pressure

Date:
October 12, 2021
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
Stress enhances the properties of a promising material for future
technologies, with researchers' discovery of a new exotic state
of a promising, room-temperature multiferroic material having
exciting implications for future technologies using these enhanced
properties.



FULL STORY ========================================================================== Stress enhances the properties of a promising material for future
technologies.


==========================================================================
UNSW researchers find a new exotic state of one of the most promising multiferroic materials, with exciting implications for future technologies using these enhanced properties.

Combining a careful balance of thin-film strain, distortion, and
thickness, the team has stabilised a new intermediate phase in one of
the few known room- temperature multiferroic materials.

The theoretical and experimental US-Australian study shows that this new
phase has an electromechanical figure of merit over double its usual
value, and that we can even transform between this intermediate phase
to other phases easily using an electric field.

As well as providing a valuable new technique to the toolkit of all international material scientists working with multiferroics and epitaxy,
the results finally shed light on how epitaxial techniques can be used
to enhance functional response of materials for future application in
next -generation devices.

STRESS CHANGES EVERYTHING If 2020-21 has taught us anything, it's that
stress changes everything. Even the most 'together' person can struggle
and change given enough stress in their life.



==========================================================================
The same applies for crystals, too. When we apply stress to crystals, they become strained and can change their structure and physical properties dramatically. We utilise this in everyday technology, using external
stimuli to bend material properties at will.

When we impose strain on a material, we are usually pushing together
or pulling apart along (at least) one axis, creating compressive and
tensile strain.

When we strain thin films on substrates, the building blocks of the film
will deform to match the sizes of the building blocks of the neighbouring substrate.

If the structural units of the substrate are larger than those of the thin
film (the blue square), the film (white outline) will stretch horizontally
(ie, 'tensile strain') and compress vertically to fit.

On the other hand, a smaller substrate structure cell (green square)
will cause the film structure to be compressed horizontally ('compressive strain') and stretched vertically.



==========================================================================
"In our research, we applied anisotropicstrain to our film. This means
that the strain applied is different depending on which direction you're looking, and this can create complicated strain states that force films
into new phases," says first author Oliver Paull (UNSW).

IT'S MAGNETOELECTRIC, IT'S PIEZOELECTRIC, IT'S PHOTOVOLTAIC... IT'S
GREASED LIGHTNING! BiFeO3 (or BFO) boasts an impressive resume of multifunctional properties, including piezoelectricity, ferroelectricity, magnetism, and optical properties.

BFO is arguably the most popular magnetoelectric material for researchers
(ie, a material that has both magnetic and electrical ordering that can influence each other).

Magnetoelectric materials are highly interesting for spintronics
and memory applications since the coupling between magnetism and ferroelectricity promises low energy technologies. (Writing data with an electric field is much more efficient than writing with magnetic field.)
Not only is BFO magnetoelectric, but it is one of the very few materials
that is magnetoelectric at room temperature, making it viable for use
in applications such as future low-energy electronics, without the
requirement for energy-intensive cryo-cooling.

Only very few multiferroic materials (ie, materials that have both
magnetic and electrical order) exhibit these useful properties at room temperature.

In addition to this, BFO boasts other functional properties:
piezoelectricity, ferroelectricity, photovoltaic effects,and more! It's
also lead free, giving it a clear advantage against most high-performing piezoelectric materials, which unfortunately contain toxic lead.

Piezoelectric materials, which can convert mechanical pressure into
electrical energy, have wide applications as ultra-high-sensitivity
sensors in devices such as smartphone motion sensors and pacemakers
(where obviously avoiding toxic materials is an advantage...).

By using highly miscut substrates, the research team pushed BFO
into a new phase that is essentially the link between the well-known rhombohedral-like and tetragonal-like phases.

This, coupled with the symmetry-related properties of the phase, allows
it to be easily influenced by electric fields.

"We looked through the literature and found that everyone uses fairly
standard commercial substrate orientations," says head investigator
Daniel Sando. "We asked our providers to custom-make different miscut orientations in between the standard orientations, which led to the
discovery of the new phase. We asked ourselves if the reason people
hadn't done this before is that the crystallography involved with these
miscuts is rather complex and can be intimidating!" The international collaboration between researchers at Oak Ridge National Lab, University
of Arkansas, and Monash University, used theoretical calculations and a
suite of experimental techniques to show that this new phase has a much
higher electromechanical response than traditional BFO.

"We additionally show strong evidence that this low-symmetry phase can
be converted into a higher-symmetry phase using an electric field, and
as a result can enhance the electromechanical response even further by
a factor of 3," says Oliver Paull.

A MULTIPURPOSE TOOL: APPLYING THE APPROACH TO A BROAD RANGE OF OXIDE
MATERIALS One of the most appealing aspects of this discovery is its
general methodology and applicability to a broad class of materials
systems.

"We chose to focus on BiFeO3 due to its ferroelectric, magnetic, and piezoelectric properties, but the approach is easily applied to other perovskite oxides," says Oliver Paull.

"We are currently exploring the effect of these high-index substrates
on purely ferroelectric or magnetic systems, but the scope for using
this technique is huge. We expect to find low symmetry phases of
optically interesting materials, as well as novel domain arrangements in ferroelectrics, to name a few," noted Laurent Bellaiche, the theoretical
lead on the project." "If you're dealing with epitaxy, then this
anisotropic technique could prove very fruitful for your research,"
says Daniel Sando.

"This study is just the beginning. We plan to combine this anisotropic
epitaxy approach to oxide superlattices (repeating layers of different compositions, i.e. A-B-A-B etc.), as well as combining the low
symmetry crystal structures with other established routes for improving piezoresponse, including substitution with rare earth elements, for
example. Finally, since BFO is multiferroic, we have a raft of magnetic
studies planned for this new low- symmetry phase." Says UNSW lab leader
Nagy Valanoor.

There are even broader possible applications: Piezoelectrics used
in sensors and actuators are typically lead-based compounds in bulk
form. While the new approach is niche and very research-oriented,
there could be scope for the new methods to work in such industries as nano-actuators or sensors. The key aspect is the use of the anisotropic
epitaxy approach to 1) generate a low-symmetry phase, and 2) facilitate enhancements in response; in this case, the piezoelectric coefficient.

========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
Technologies. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Oliver Paull, Changsong Xu, Xuan Cheng, Yangyang Zhang, Bin Xu,
Kyle P.

Kelley, Alex de Marco, Rama K. Vasudevan, Laurent Bellaiche,
Valanoor Nagarajan, Daniel Sando. Anisotropic epitaxial
stabilization of a low- symmetry ferroelectric with enhanced
electromechanical response. Nature Materials, 2021; DOI:
10.1038/s41563-021-01098-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012095037.htm

--- up 5 weeks, 5 days, 8 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)