Liquid crystal molecules form nano rings

February 7, 2018, Deutsches Elektronen-Synchrotron
Cutaway-view into a largely self-organised liquid crystal in a nanopore. Credit: A. Zantop/M. Mazza/K. Sentker/P. Huber, Max-Planck Institut für Dynamik und Selbstorganisation/Technische Universität Hamburg (TUHH)

At DESY's X-ray source PETRA III, scientists have investigated an intriguing form of self-assembly in liquid crystals: When the liquid crystals are filled into cylindrical nanopores and heated, their molecules form ordered rings as they cool—a condition that otherwise does not occur naturally in the material. This behavior enables nanomaterials with new optical and electrical properties, as the team led by Patrick Huber from Hamburg University of Technology (TUHH) reports in the journal Physical Review Letters.

The scientists studied a special form of liquid crystals composed of disc-shaped molecules called discotic liquid crystals. In these materials, the disk molecules can form high, electrically conductive pillars by themselves, stacking up like coins. The researchers filled discotic liquid crystals in nanopores in a silicate glass. The cylindrical pores had a diameter of only 17 nanometers (millionths of a millimeter) and a depth of 0.36 millimeters.

There, the liquid crystals were heated to around 100 degrees Celsius and cooled slowly. The initially disorganised disk molecules formed arranged like round curved columns. Starting from the edge of the pore, one after the other gradually formed with decreasing until at about 70 degrees Celsius the entire cross section of the pore was filled with concentric rings. Upon reheating, the rings gradually disappeared again.

"This change of the molecular structure in confined liquid crystals can be monitored with X-ray diffraction methods as a function of temperature and with high accuracy," says co-author and DESY scientist Milena Lippmann, who prepared and participated in the experiments at the High-Resolution Diffraction Beamline P08 at PETRA III. "The combination of symmetry and confinement gives rise to unexpected, new phase transitions," says Marco Mazza from the Max Planck Institute for Dynamics and Self-Organization in Göttingen where the process was modelled with computer simulations. For this purpose, MPI scientist Arne Zantop devised a theoretical and numerical model for the nanoconfined liquid crystals that confirmed the experimental results and helps to interpret them.

The individual rings formed stepwise at characteristic temperatures. "This makes it possible to turn individual nano rings on and off by small changes in temperature," emphasises main author Kathrin Sentker from TUHH. She had noticed this phenomenon through surprisingly step-like signal changes in laser-optical experiments. While such quantised changes usually only occur at very low temperatures, the system shows this quantum behavior already well above room temperature.

As the opto- of change with the formation of molecular columns, the nanopore-confined variant is a promising candidate for the design of new optical metamaterials with properties that can be controlled stepwise through temperature. The investigated nanostructures could also lead to new applications in organic semiconductors, such as temperature-switchable nanowires, explains co-author Andreas Schönhals from the Bundesanstalt für Materialforschung und -prüfung (BAM), the German Federal Institute for Materials Research and Testing, who is interested in the thermal and electrical properties of these systems.

"The phenomenon constitutes a fine example how versatile soft matter can adapt to extreme spatial constraints and how this can lead to new insights in physics as well as new design and control principles for the self-organisation of functional nanomaterials," explains principal investigator Huber.

Explore further: Scientists found and studied complex types of defects in the droplets of liquid crystals

More information: Kathrin Sentker et al, Quantized Self-Assembly of Discotic Rings in a Liquid Crystal Confined in Nanopores, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.067801

Related Stories

New theory on liquid crystals with high symmetry

November 8, 2016

LCD screens use liquid crystals, which have a high degree of order, even though they form a fluid. A new theory detailed in Physical Review X maps out the interplay between order, temperature and symmetry.

Research reveals inner workings of liquid crystals

March 20, 2017

Liquid crystals are used in everything from tiny digital watches to huge television screens, from optical devices to biomedical detectors. Yet little is known of their precise molecular structure when portions of such crystals ...

Antimatter unveils the secrets of liquid crystals

April 21, 2016

Liquid crystals combine such contradictory features as the chaos of liquid molecules, and the ordering characteristics of crystals. Thanks to an innovative application of antimatter, it has been demonstrated at the Institute ...

Recommended for you

Single-celled architects inspire new nanotechnology

July 16, 2018

Diatoms are tiny, unicellular creatures, inhabiting oceans, lakes, rivers, and soils. Through their respiration, they produce close to a quarter of the oxygen on earth, nearly as much as the world's tropical forests. In addition ...

X-ray triggered nano-bubbles to target cancer

July 16, 2018

Innovative drug filled nano-bubbles, able to be successfully triggered in the body by X-rays, have been developed by researchers, paving the way for a new range of cancer treatments for patients.

Smart window controls light and heat, kills microorganisms

July 13, 2018

A new smart window offers more than just a nice view—it also controls the transmittance of sunlight, heats the interiors of buildings by converting solar radiation into heat, and virtually eliminates E. coli bacteria living ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.