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2D Oxide and van der Waals layered devices

Periodic Reporting for period 1 - OXWALD (2D Oxide and van der Waals layered devices)

Reporting period: 2019-05-01 to 2021-04-30

Two-dimensional (2D) materials, due to its singular properties mainly flexibility and transparency, have gathered the attention for their potential use in photonic and optoelectronic devices. Initially these studies were restricted to graphene however, its lack of electronic band gap inspired the search of semiconducting counterparts such as MoS2, Bi2Te3, h-BN etc. However, the absence of electronic correlations -i.e. the fact that electrons do not feel each other- restricts their functionalities and applications. Endowing 2D materials with electronic correlations could produce exciting groundstates such as ferroelectricity, ferromagnetism or superconductivity, which could expand their functional capabilities. In complex correlated oxides (CCOs) the unscreened Coulomb repulsion between electrons in 3d bands gives rise to a non-trivial entanglement between charge, spin, orbital and lattice, which is responsible of a wide variety of electronic groundstates. In this project we have searched for new synthesis of 2D CCOs in the strain free -freestanding- form. The general perovskite structure of these compounds requires the use of single crystalline substrates for their growth, followed by a post growth-release. This last step is nowadays a challenge and different methodologies are being considered. The most common is the use of sacrificial buffer layers which are solved under the immersion in water or an acid. However, etching procedures differ for the different materials, and depend also on the growth technique. In this regard, a careful investigation for the release of 2D CCOs must be conducted. The main objective of this project has been the manipulation of correlated states in 2D CCOs layers, and their combination with van der Waals (vdW) layers in heterostructures, in order to achieve tunable responses and device functionalities. We expected that collective states such as ferromagnetism or superconductivity will be modified when the CCOs layers are in the freestanding form when released from its substrate. On the other hand, the assembly of CCOs in vdW heterostructures is expected to yield exciting electronic states possibly with topological properties such as topological superconductivity or magnetic skyrmions. In addition, the possibility to transfer CCOs on top of Si substrates opens a huge number of opportunities to implement the unique functional properties of these materials in the devices of the CMOS technology, expanding the perspectives and functionalities of current electronic devices. This project can be framed in the field of Quantum Materials and Technologies, a global research direction which is expected to develop the so called “second quantum revolution” with exciting scientific and technological outcomes.
Taking advantage of the large expertise of the host group in the growth of CCO heterostructures, the first part of the project has consisted of the elaboration of a systematic methodology for releasing layers of different CCOs grown by high oxygen pressure sputtering with different thicknesses. Different solutions where used, as well as different buffer layers, depending on the material to be released. During the project period, we have released BaTiO3 (BTO) and La2/3Sr1/3MnO3 (LSMO) flakes ranging from 60nm to 5nm (see Fig. 1 and 2 for some examples). Once BTO and LSMO have been released from its on-grown substrate (SrTiO3), and onto the PDMS support prior to transfer, we have characterized the lattice relaxation effects by means of X-Ray diffraction (XRD) technique. From Fig. 3 it can be observed that differences between strained and freestanding layers (black and colored lines, respectively) are present, i.e. the lattice parameters of the different layers is being modified. In particular, both BTO and LSMO trend towards its bulk structure when released. We have collaborated with ICMM-CSIC, institution where the Marie Curie fellow has performed a secondment. He has learned the procedure for deterministic transfer of detached layers, as well as different optical techniques for structural characterization. We have settled a systematic methodology for determining the thickness and the Young modulus of the flakes through simple and fast optical imaging (Fig. 3 and 4). BTO and LSMO have been transferred also to Au/Si substrates in order to perform functional AFM characterization. In particular we have performed preliminary PFM characterization in collaboration with CNRS/THALES (previous host institution of the experienced researcher) on 25nm-thick BTO layers proving its ferroelectric switching behavior, even in the freestanding form (Fig. 6). Further experiments have been postponed due to the COVID-19 situation. MFM characterization is also planned in the LSMO flakes in order to observe magnetic domains in the freestanding structures. Same BTO and LSMO have been also transferred onto TEM and synchrotron grids (see Fig. 2 for an example) in order to characterize its structural and functional properties. Characterization is currently being done with exciting preliminary results. From Fig. 7 it can be observed that the flake shows atomically sharp edges indicating the good quality of the sample, and the release and transfer processes. Further atomic composition will be performed, among others. X-Ray absorption experiments in freestanding BTO layers in the ALBA synchrotron light source reveal information about modification of ferroelectric domains linked to possible oxygen defects in the freestanding layers. We have transferred LSMO flakes onto SiOx substrates and performed a photolithography process to electrically contact them (Fig. 8a). By measuring the resistance as a function of temperature (Fig. 8b), we can observe a similar trend to that obtained in the strained form. Further analysis including Hall measurements and SQUID magnetometry will be addressed. Because of the global COVID-19 pandemic, the 16 months project has focused to the release of LSMO and BTO layers with different thicknesses, and the functional and structural characterization of BTO as mentioned. In parallel, the experienced researcher has performed teaching activities at the Universidad Complutense de Madrid (UCM) as well of tutoring and supervising different final degree projects. Dissemination of topics related to the project and its technological projection have been presented in open seminars for a broad audience during the Semana de la Ciencia at UCM (4-17 Nov. 2019) (see Fig. 9).
Currently CCOs in their freestanding form are materials intensively studied because of their unique properties, which cannot be found in conventional 2D materials such as graphene or h-BN, among others. Our investigations have expanded knowledge on these new materials in the freestanding form using novel characterization methodologies conducted at synchrotron facilities with preliminary exciting results showing a completely new defect zoology with impact on the functional properties which are modified with respect to the bulk/strained counterpart. The exciting results obtained up to now encourage the continuity of this project beyond the duration of the MSCA-IF project. Our future investigations will be the vdW assembly of CCOs with conventional 2D and study the emergent phenomena at the interfaces that, to the best of our knowledge, is completely unexplored field and it is expected to show new phenomena.
BaTiO3 flakes on PDMS released from La2/3Sr1/3MnO3 sacrificial buffer layer. (a) 20nm and (b) 4nm.
Young’s modulus characterization of BaTiO3
Scanning transmission electron microscopy characterization of a freestanding BTO flake.
Out-of-plane PFM-phase characterization of a 25nm-BaTiO3 flake.
BaTiO3 reflectance as a function of thickness.
La2/3Sr1/3MnO3 flakes transferred onto the grid for synchrotron characterization.
X-Ray diffraction of freestanding CCOs.
Photos of Víctor Rouco during the Semana de la Ciencia.
La2/3Sr1/3MnO3 flakes transferred onto SiOx and R(T) electrical transport measurements.