At DESY’s X-ray source PETRA III, researchers have investigated a startling 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 naturally occur in the material. This behavior enables nanomaterials with new optical and electrical properties, as reported by the team headed by Patrick Huber from the Technical University of Hamburg (TUHH) in the journal Physical Review Letters.
The scientists had studied a special form of liquid crystals that are 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 then cooled slowly. The initially disorganized disk molecules formed concentric rings arranged like round curved columns. Beginning from the edge of the pore, one ring after the other gradually formed as the temperature dropped, until at about 70 degrees almost the entire cross section of the pore was filled with concentric rings. Upon reheating, the rings gradually disappeared again.
“This change in the molecular structure in the enclosed liquid crystal can be monitored very precisely as a function of temperature using methods of X-ray diffraction,” explains DESY researcher Milena Lippmann from the writing team who prepared and carried out the experiments at DESY’s measuring station P08 at PETRA III , “The combination of symmetry and inclusion leads to new, unexpected phase transitions,” adds co-author Marco Mazza from the Max Planck Institute for Dynamics and Self-Organization in Göttingen, where the observed process was simulated with simulation calculations. MPI researcher Arne Zantop developed a theoretical and numerical model for liquid crystal in limited geometry, which confirms the experimental results and helps to interpret them.
The individual rings formed gradually at certain temperatures. “This makes it possible to turn individual nano-rings on and off by small temperature changes,” emphasizes principal author Kathrin Sentker from the TUHH. It has encountered this process through surprisingly step-like signal changes in laser-optical experiments. Otherwise, such quantized state changes typically only occur at very low temperatures. However, the liquid crystal system shows this quantum behavior even well above room temperature.
As the opto-electrical properties of discotic liquid crystals change with the formation of molecular columns, the nanopore-included variant is a promising candidate for the design of new optical metamaterials whose properties 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 of the German Federal Institute for Materials Research and Testing (BAM), who is interested in the thermal and electrical properties of these systems.
“The observed phenomenon is a good example of how versatile soft matter can adapt to extreme spatial constraints, and how this leads to new insights in physics and to new design and control principles for the self-assembly of functional nanomaterials,” explains Huber.
The study also involved the Helmholtz Center Berlin and the Czestochowa University of Technology in Poland. Sentker and Huber are members of the Collaborative Research Center (SFB) 986 “Tailored Multiscale Material Systems – M3”, which has been funded by the Deutsche Forschungsgemeinschaft (DFG) since 2012 and pools material science competencies in the Greater Hamburg area.
Source : Deutsches Elektronen-Synchrotron (DESY)