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Designing Novel Phonon Valve Device through Ferroic Domain Wall Engineering

Periodic Reporting for period 2 - PHONON-VALVE (Designing Novel Phonon Valve Device through Ferroic Domain Wall Engineering)

Reporting period: 2018-09-26 to 2019-09-25

In this project we address the possibility of creating a conceptually new device that aims at controlling the heat transport. The mechanism would be similar to a switch for heat propagation: enabling (state ON) or inhibiting (state OFF) the heat from flowing along a given path. We call this device a “phonon valve” as phonons, quasiparticles accounting for the mechanical vibrations in solids, are the main heat carriers in insulating crystalline solids. Thus, if phonons are blocked or scattered somehow, heat would not flow. Then, the idea is to design a “phonon barrier” that can be opened or closed at will by using an applied electric field. For a practical and realizable device, the activated and deactivated states should be reversible (you should be able to open the valve again when you have closed it and vice versa) and robust (the effect of opening and closing the valve should last).

Hence, the problem is reduced to design the most appropriate material to meet the requirements of active phonon barrier. Ferroelectric materials naturally form internal and distinctive regions, called ferroelectric domains, each of them characterized by the polarization. The borders between these domains, domain walls, are intrinsic interfaces within the material. These interfaces may be effective centres of phonon scattering, so they would accomplish the first requirement for the phonon valve: the existence of a barrier of phonons. Second requirement is the possibility of opening and closing the phonon valve, which is also expected to be met in these materials: an electric field is able to erase and create domain walls. Third requirement is to be reversible and robust. This criterion is also expected to be fulfilled as the domain walls are stable once the electric field is removed and the process of creation/destruction of domain walls is reversible for a large number of cycles. Because of all these reasons ferroelectrics are excellent candidates for this job.

Regarding the benefits for society it is worth observing that many of the major technological breakthroughs occurring in the last few decades rely on our great capability to manipulate two elementary particles: electrons and photons. Both of them are the basis of present-day electronics, photonics and semiconductor industry in general, including all electronic media (TV, computers, smartphones, etc), wire and wireless communications, energy harvesting, magnetic data storage, etc. If we manage to achieve a similar degree of control over phonons, we could start developing a completely new technology: the so-called “phononics”. This new technology may open a new way of communication or create new data storage devices and computers based on phonons. Additionally, as manipulating phonons entails manipulating the heat flow it could lead to control the thermal energy at will, propelling new possibilities for energy management, including storage and harvesting.
The work performed in this period centres on exploring, designing and assessing domain wall configurations in ferroelectric materials in the first place and, in the second place, to determine the effects of these domain-wall configurations on the heat flow, through thermal conductivity measurements.

In order control the domain wall configuration we have used the so-called epitaxial strain engineering technique. This technique consists of growing crystalline materials in the form of a thin film on top of a crystalline substrate which serves, not only as the support of these films, but also, as a structural template to tune the atomic arrangement on the film. The epitaxial engineering of the crystal structure has a tremendous impact on the physical properties of the material. In particular, in ferroelectric materials, the type of domain walls and their distribution can be changed completely.

Thus, we selected an archetype ferroelectric material: Lead Titanate, and we “play” with its structure by growing it onto several different crystalline substrates. We chose more than 7 different substrates (which means 7 different strain values on Lead Titanate) and three different thickness of the ferroelectric film per substrate. We discovered that both strain and thickness have a key role in the formation of ferroelectric domain structures. As a result, the kind of domain wall configuration can be engineered at will by appropriately selecting the strain value and the thickness.

The need of controlling the wide variety of these domain configurations by an electric field becomes evident for creating a phonon valve. We demonstrated that the ferroelectric structures in Lead Titanate are extremely malleable by an electric field. This is a remarkable result, not only for our purpose of creating a phonon valve, but also for the active research is currently taking place in the field of ferroelectrics.

Finally, we determined the effect of this variety of ferroelecric structures in Lead Titanate on the thermal conductivity. In other words, how good these structures are as phonon barriers. We find out that the thermal conductivity of these samples is strongly coupled to the type of domain wall and the density of domain walls of the ferroelectric structure of the sample. Hence, the heat flow in Lead Titanate is proved to be highly tuneable by engineering the kind of domain wall configuration. Furthermore, the modulation of the thermal conductivity by this method is huge, as it can change as much as 70%. Therefore, Lead Titanate is an excellent candidate for being an active phonon barrier.
The exhaustive study of the Lead Titanate led us to make a significant progress in the engineering of ferroelectric structures. These results are not only useful for our specific project of manipulating phonons, but also for the scientific ferroelectric community in general. In particular, if we are capable to engineer, at will, these intrinsic interfaces –the domain walls– and if these interfaces possess interesting semiconducting properties, as many of them do, it is straightforward their use in electronics, with the clear advantage of their reduced dimensionality which makes them ideal for high-density electronic devices.

On the other hand, attempts of modulating the thermal conductivity in materials, and especially in ferroelectrics, have taken place before, reaching a tunability of 10% in granular ferroelectric materials. However, here, in single crystal ferroelectric films we have managed to largely surpass this figure of merit, reaching a change in the thermal conductivity around 61%. Therefore, Lead Titanate is an excellent candidate to exploit its thermoelectric properties, in which a gradient of temperature can be used to produce electricity. If successful, a new generation of thermoelectric devices based on ferroelectrics could start being developed, with significant socio-economic impact on energy harvesting.

Finally, the fact that the thermal conductivity can be varied largely and at will in ferroelectric materials (in particular, in Lead Titanate), together with the possibility to modify the ferroelectric structures with an electric field, put the project on the verge of fulfilling its main goal: designing a phonon valve device. If successful, this proof-of-concept will boost the novel field so-called “phononics”, with unforeseen societal and technological implications.
Thermal conductivity of PbTiO3 versus the ferroelectric domain configurations