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).
- CIGS-perovskite tandem cell achieves record efficiency of 25.5 %A Berlin-based team from HZB and Center for the Science of Materials Berlin (CSMB) at the Humboldt-Universität zu Berlin has set a new record for a tandem solar cell. Using a combination of a CIGS semiconductor layer and perovskite, along with several optimised intermediate layers, they were able to convert 25.5% of sunlight into electrical energy. The previous record for this combination of materials and this size of cell stood at 24.6%. The new record has been certified and is visible in the prestigious Solar Cell Efficiency Tables (the "Green Tables"), which serve as the definitive ledger for the global photovoltaic community.
- Disorder creates new properties in compound semiconductorsAn international research team has demonstrated that the intrinsic disorder of the compound semiconductor CuInSnS₄ can be exploited to influence its optical properties. While the atomic vibrations also sense the local disorder, their response is averaged over many different local environments and therefore appear isotropic, as expected for a cubic crystal. In contrast, the optical excitations, known as excitons, are much more sensitive to the local arrangement of atoms. Surprisingly, they show a direction-dependent optical response even though the average crystal structure is cubic. These findings shed new light on the relationship between disorder and material properties, opening up new options for targeted 'disorder engineering' in optoelectronic and photocatalytic devices.
- Perovskite solar cells: Predictions of long-term stabilityReliable statements about the long-term stability of perovskite solar cells are still difficult to make. However, a new study by Dr Carolin Ulbrich’s team, published in the renowned journal Joule, highlights which methods are useful for this purpose and identifies areas where further research is needed.