Nanoislands on silicon with switchable topological textures
Artistic representation of the center down-convergent polarization field. It results from the compression of the polarization flux by the sidewalls of the nanoislands. The texture in each nanoisland resembles a swirling vortex of liquid flowing into a narrowing funnel. © Laura Canil /HZB
In each row the sample had a different orientation. The columns show the topography (left) and piezoresponse force microscopy (PFM) images. In the lateral PFM amplitude, the nanoislands display a pattern of dark and light regions, evoking coffee beans, which is typical for textures of center-type polar distribution. © HZB
Nanostructures with specific electromagnetic patterns promise applications in nanoelectronics and future information technologies. However, it is very challenging to control those patterns. Now, a team at HZB examined a specific class of nanoislands on silicon with interesting chiral, swirling polar textures, which can be stabilised and even reversibly switched by an external electric field.
Ferroelectrics at the nanoscale exhibit a wealth of polar and sometimes swirling (chiral) electromagnetic textures that not only represent fascinating physics, but also promise applications in future nanoelectronics. For example, ultra-high-density data storage or extremely energy-efficient field-effect transistors. However, a sticking point has been the stability of these topological textures and how they can be controlled and steered by an external electrical or optical stimulus.
New perspectives:
A team led by Prof. Catherine Dubourdieu (HZB and FU Berlin) has now published a paper in Nature Communications that opens up new perspectives. Together with partners from the CEMES-CNRS in Toulouse, the University of Picardie in Amiens and the Jozef Stefan Institute in Ljubljana, they have thoroughly investigated a particularly interesting class of nanoislands on silicon and explored their suitability for electrical manipulation.
Nanoislands on silicon
“We have produced BaTiO3 nanostructures that form tiny islands on a silicon substrate,” explains Dubourdieu. The nano-islands are trapezoidal in shape, with dimensions of 30–60 nm (on top), and have stable polarisation domains. “By fine tuning the first step of the silicon wafer passivation, we could induce the nucleation of these nanoislands,” says Dong-Jik Kim, a scientist in Dubourdieu’s team.
Domain patterns studied by PFM
These domains can be reversibly switched by an electric field. The domain patterns were studied using vertical and lateral piezoresponse force microscopy (PFM). “Both the PFM measurement data and the phase field modelling indicate a centred, downward convergent polarisation, which fits perfectly well with the information from scanning transmission electron microscopy (STEM),” says Ibukun Olaniyan, PhD student.
Reversible switching
In particular, the scientists were able to detect a swirling component around the nanoisland axis that causes the chirality. “The texture resembles a swirling vortex of liquid flowing into a narrowing funnel," explains Dubourdieu. “The center down-converging nanodomains can be reversibly switched to center up-diverging nanodomains by an external electric field,” she points out.
“In this work, we have shown that chiral topological textures can be stabilised by shaping nanostructures in an appropriate way,” says Dubourdieu. The ability to create and electrically manipulate chiral, swirling, polar textures in BaTiO3 nanostructures is very promising for future applications.
Note: This work was partially supported by the ERC Advanced Grant LUCIOLE (101098216).
- Fascinating archaeological find becomes a source of knowledgeThe Bavarian State Office for the Preservation of Historical Monuments (BLfD) has sent a rare artefact from the Middle Bronze Age to Berlin for examination using cutting-edge, non-destructive methods. It is a 3,400-year-old bronze sword, unearthed during archaeological excavations in Nördlingen, Swabia, in 2023. Experts have been able to determine how the hilt and blade are connected, as well as how the rare and well-preserved decorations on the pommel were made. This has provided valuable insight into the craft techniques employed in southern Germany during the Bronze Age. The BLfD used 3D computed tomography and X-ray diffraction to analyse internal stresses at the Helmholtz-Zentrum Berlin (HZB), as well as X-ray fluorescence spectroscopy at a BESSY II beamline supervised by the Bundesanstalt für Materialforschung und -prüfung (BAM).
- Element cobalt exhibits surprising propertiesThe element cobalt is considered a typical ferromagnet with no further secrets. However, an international team led by HZB researcher Dr. Jaime Sánchez-Barriga has now uncovered complex topological features in its electronic structure. Spin-resolved measurements of the band structure (spin-ARPES) at BESSY II revealed entangled energy bands that cross each other along extended paths in specific crystallographic directions, even at room temperature. As a result, cobalt can be considered as a highly tunable and unexpectedly rich topological platform, opening new perspectives for exploiting magnetic topological states in future information technologies.
- MXene for energy storage: More versatile than expectedMXene materials are promising candidates for a new energy storage technology. However, the processes by which the charge storage takes place were not yet fully understood. A team at HZB has examined, for the first time, individual MXene flakes to explore these processes in detail. Using the in situ Scanning transmission X-ray microscope 'MYSTIIC' at BESSY II, the scientists mapped the chemical states of Titanium atoms on the MXene flake surfaces. The results revealed two distinct redox reactions, depending on the electrolyte. This lays the groundwork for understanding charge transfer processes at the nanoscale and provides a basis for future research aimed at optimising pseudocapacitive energy storage devices.