insulators – Lightsources.org

Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate framework

2021/01/292021/01/29

Metal-organic frameworks (MOFs) offer disruptive potential in micro- and optoelectronics because of their chemical versatility and high porosity. For instance, the low dielectric constant (low-k) resulting from their porosity makes MOFs competitive candidates for high-performance insulators in future microchips. Both the MOF and microelectronics communities have been striving towards integrating MOFs in microchips, which requires two key engineering steps: thin film deposition and lithographic patterning. However, conventional lithography techniques use a sacrificial layer, so-called photoresist, to transfer a pattern into the desired material. The use of photoresist complicates the process, and might induce contamination of the highly porous MOF films.

A group of researchers from KU Leuven (Belgium) coordinated by Rob Ameloot has used the deep X-ray lithography (DXRL) beamline at Elettra to demonstrate that MOFs can be patterned by X-ray lithography without the use of resist layer. The method is based on selective X-ray exposure of the MOF film, which induces chemical changes that enable its removal by a common solvent. This process completely avoids the resist layer, thus significantly simplifying patterning while maintaining the physicochemical properties of patterned MOFs intact.

Unlocking the secrets of metal-insulator transitions

2018/11/082018/11/09

X-ray photon correlation spectroscopy at NSLS-II’s CSX beamline used to understand electrical conductivity transitions in magnetite.

By using an x-ray technique available at the National Synchrotron Light Source II (NSLS-II), scientists found that the metal-insulator transition in the correlated material magnetite is a two-step process. The researchers from the University of California Davis published their paper in the journal Physical Review Letters. NSLS-II, a U.S. Department of Energy (DOE) Office of Science user facility located at Brookhaven National Laboratory, has unique features that allow the technique to be applied with stability and control over long periods of time.
“Correlated materials have interesting electronic, magnetic, and structural properties, and we try to understand how those properties change when their temperature is changed or under the application of light pulses, or an electric field” said Roopali Kukreja, a UC Davis professor and the lead author of the paper. One such property is electrical conductivity, which determines whether a material is metallic or an insulator.

If a material is a good conductor of electricity, it is usually metallic, and if it is not, it is then known as an insulator. In the case of magnetite, temperature can change whether the material is a conductor or insulator. For the published study, the researchers’ goal was to see how the magnetite changed from insulator to metallic at the atomic level as it got hotter.

Image: Professor Roopali Kukreja from the University of California in Davis and the CSX team Wen Hu, Claudio Mazzoli, and Andi Barbour prepare the beamline for the next set of experiments.

Missing link between new topological phases of matter discovered

2017/10/172017/11/13

HZB-Physicists at BESSY II have investigated a class of materials that exhibit characteristics of topological insulators.

During these studies they discovered a transition between two different topological phases, one of which is ferroelectric, meaning a phase in the material that exhibits spontaneous electric polarisation and can be reversed by an external electric field. This could also lead to new applications such as switching between differing conductivities.

The HZB researchers studied crystalline semiconductor films made of a lead, tin, and selenium alloy (PbSnSe) that were doped additionally with tiny amounts of the element bismuth. These semiconductors belong to the new class of materials called topological insulators, materials that conduct very well at their surfaces while behaving as insulators internally. Doping with 1-2 per cent bismuth has enabled them to observe a new topological phase transition now. The sample changes to a particular topological phase that also possesses the property of ferroelectricity. This means that an external electric field distorts the crystal lattice, whereas conversely, mechanical forces on the lattice can create electric fields.

Image: The Bismut doping is enhanced from 0% (left) to 2.2% (right). Measurements at BESSY II show that this leads to increased bandgaps. Credit: HZB