Publications by authors named "D Jannis"

8 Publications

Signatures of enhanced out-of-plane polarization in asymmetric BaTiO superlattices integrated on silicon.

Nat Commun 2022 Jan 11;13(1):265. Epub 2022 Jan 11.

MESA+ Institute for Nanotechnology, University of Twente, 7500 AE, Enschede, The Netherlands.

In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon platform. Here, we present asymmetric LaMnO/BaTiO/SrTiO superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize c-axis oriented BaTiO layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO single film and conventional BaTiO/SrTiO superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO as the thickness of BaTiO increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities.
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http://dx.doi.org/10.1038/s41467-021-27898-xDOI Listing
January 2022

Event driven 4D STEM acquisition with a Timepix3 detector: Microsecond dwell time and faster scans for high precision and low dose applications.

Ultramicroscopy 2021 Nov 13;233:113423. Epub 2021 Nov 13.

EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium; NANOlab Center of Excellence, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium. Electronic address:

Four dimensional scanning transmission electron microscopy (4D STEM) records the scattering of electrons in a material in great detail. The benefits offered by 4D STEM are substantial, with the wealth of data it provides facilitating for instance high precision, high electron dose efficiency phase imaging via centre of mass or ptychography based analysis. However the requirement for a 2D image of the scattering to be recorded at each probe position has long placed a severe bottleneck on the speed at which 4D STEM can be performed. Recent advances in camera technology have greatly reduced this bottleneck, with the detection efficiency of direct electron detectors being especially well suited to the technique. However even the fastest frame driven pixelated detectors still significantly limit the scan speed which can be used in 4D STEM, making the resulting data susceptible to drift and hampering its use for low dose beam sensitive applications. Here we report the development of the use of an event driven Timepix3 direct electron camera that allows us to overcome this bottleneck and achieve 4D STEM dwell times down to 100 ns; orders of magnitude faster than what has been possible with frame based readout. We characterize the detector for different acceleration voltages and show that the method is especially well suited for low dose imaging and promises rich datasets without compromising dwell time when compared to conventional STEM imaging.
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http://dx.doi.org/10.1016/j.ultramic.2021.113423DOI Listing
November 2021

Reducing electron beam damage through alternative STEM scanning strategies, Part I: Experimental findings.

Ultramicroscopy 2022 Jan 2;232:113398. Epub 2021 Oct 2.

EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium; NANOlab Center of Excellence, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium. Electronic address:

The highly energetic electrons in a transmission electron microscope (TEM) can alter or even completely destroy the structure of samples before sufficient information can be obtained. This is especially problematic in the case of zeolites, organic and biological materials. As this effect depends on both the electron beam and the sample and can involve multiple damage pathways, its study remained difficult and is plagued with irreproducibility issues, circumstantial evidence, rumors, and a general lack of solid data. Here we take on the experimental challenge to investigate the role of the STEM scan pattern on the damage behavior of a commercially available zeolite sample with the clear aim to make our observations as reproducible as possible. We make use of a freely programmable scan engine that gives full control over the tempospatial distribution of the electron probe on the sample and we use its flexibility to obtain multiple repeated experiments under identical conditions comparing the difference in beam damage between a conventional raster scan pattern and a newly proposed interleaved scan pattern that provides exactly the same dose and dose rate and visits exactly the same scan points. We observe a significant difference in beam damage for both patterns with up to 11 % reduction in damage (measured from mass loss). These observations demonstrate without doubt that electron dose, dose rate and acceleration voltage are not the only parameters affecting beam damage in (S)TEM experiments and invite the community to rethink beam damage as an unavoidable consequence of applied electron dose.
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http://dx.doi.org/10.1016/j.ultramic.2021.113398DOI Listing
January 2022

Coupling Charge and Topological Reconstructions at Polar Oxide Interfaces.

Phys Rev Lett 2021 Sep;127(12):127202

Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628CJ Delft, Netherlands.

In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion symmetry across the heterointerfaces. A notable example is the interface between polar and nonpolar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved the way for the discovery of numerous unconventional properties absent in the bulk constituents. However, control of the geometric structure of the electronic wave functions in correlated oxides remains an open challenge. Here, we create heterostructures consisting of ultrathin SrRuO_{3}, an itinerant ferromagnet hosting momentum-space sources of Berry curvature, and LaAlO_{3}, a polar wide-band-gap insulator. Transmission electron microscopy reveals an atomically sharp LaO/RuO_{2}/SrO interface configuration, leading to excess charge being pinned near the LaAlO_{3}/SrRuO_{3} interface. We demonstrate through magneto-optical characterization, theoretical calculations and transport measurements that the real-space charge reconstruction drives a reorganization of the topological charges in the band structure, thereby modifying the momentum-space Berry curvature in SrRuO_{3}. Our results illustrate how the topological and magnetic features of oxides can be manipulated by engineering charge discontinuities at oxide interfaces.
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http://dx.doi.org/10.1103/PhysRevLett.127.127202DOI Listing
September 2021

Spatially Controlled Octahedral Rotations and Metal-Insulator Transitions in Nickelate Superlattices.

Nano Lett 2021 Feb 20;21(3):1295-1302. Epub 2021 Jan 20.

MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands.

The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal-insulator transitions (MITs) in tetragonal NdNiO/SrTiO superlattices by controlling the NdNiO layer thickness, in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at = 8 to a uniform nontilt pattern at = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.
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http://dx.doi.org/10.1021/acs.nanolett.0c03850DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7883389PMC
February 2021
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