Photoaktive Flüssigkristallnanodispersionen

• Antragsteller: Professor Dr. Heinz-Siegfried Kitzerow, seit 7/2020
• Fachliche Zuordnung: Physikalische Chemie von Molekülen, Flüssigkeiten und Grenzflächen, Biophysikalische Chemie
• Förderung: Förderung von 2016 bis 2022
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 299309634

• Erstellungsjahr: 2023

Zusammenfassung der Projektergebnisse

Liquid crystals – ordered fluids – are ubiquitous and very important because of their essential role for the existence of life and because of their extremely successful technical application in energysaving flat panel displays. For the latter application, the electro-optic properties of a suitable liquid crystal (LC) is used, i. e. the reorientation of the preferred direction of rod-like molecules under the influence of an applied voltage. In contrast, the present research project was focused on exploring the fundamentals and opportunities of optical (rather than electrical) addressing. For this purpose, single crystals and colloidal particles made of iron-doped lithium niobate (Fe:LiNbO3) were used. Fe:LiNbO3 shows an anomalous photovoltaic effect, which is not suitable for application in solar cells. However, illumination of their surface with visible light induces high local electric fields, which can – in turn – reorient a LC, thereby changing its optical properties. The experimental results of this research project confirm that thin films of a LC deposited on the surface of a Fe:LiNbO3 single crystal can be manipulated by a focused laser beam in various ways, depending on the cut orientation of the crystal surface, on the initial alignment of the LC and on its dielectric properties. Under appropriate conditions, defects can be created or erased by a laser beam, which allows, for example, the reversible generation of miniaturized optical phase plates to control the spatial distribution of light intensity, the storage and erasure of optical information, or the trapping and manipulation of very small particles. The project also contributed to basic understanding of interactions between locally illuminated Fe:LiNbO3 and LCs. A further target of the project was developing a light-sensitive soft-matter composite material, which is composed of a LC solvent, Fe:LiNbO3 particles with sub-micrometer size and a stabilizing polymer network. Thus, conditions were explored, under which a Fe:LiNbO3 crystal can be successfully grinded to yield sufficiently small particles with a narrow size distribution. A ball milling protocol was developed for this purpose. In addition, different compositions of photoreactive additives to LCs were tested. By in-situ photopolymerization, a polymer network LC was developed, which shows decently low switching voltages and small switching times. Finally, special photo-initiators were synthesized and characterized, which can be attached to surfaces by a stable (covalent) chemical bond. Basic studies indicate that these novel compounds can bind to the surface of different substrates, including Fe:LiNbO3, as intended. They can be used to fabricate a polymer network that is strongly bound to the surface of a solid. In future experiments, these surface-grafted photo-initiators may be used to stabilize the polar order of Fe:LiNbO3 nanoparticles in an oriented polymer network LC, thereby yielding novel soft matter composites with unusual photo-optical properties.

Projektbezogene Publikationen (Auswahl)

• Laser-induced erasable patterns in a N* liquid crystal on an iron doped lithium niobate surface. Optics Express, 25(21), 26148.
  Habibpourmoghadam, Atefeh; Lucchetti, Liana; Evans, Dean R.; Reshetnyak, Victor Y.; Omairat, Faissal; Schafforz, Samuel L. & Lorenz, Alexander
  
• Optical manipulation and defect creation in a liquid crystal on a photoresponsive surface. Physical Review E, 96(2).
  Habibpourmoghadam, Atefeh; Jiao, Lin; Reshetnyak, Victor; Evans, Dean R. & Lorenz, Alexander
  
• Photovoltaic light valving induced in a vertically aligned nematic liquid crystal on a x-cut Fe:LiNbO3substrate. Optical Data Processing and Storage, 4(1), 8-13.
  Lorenz, Alexander; Jiao, Lin & Evans, Dean R.
  
• Surface grafted crosslinker in polymer network liquid crystals. Journal of Molecular Liquids, 267, 109-114.
  Braun, L.; Schafforz, S.L. & Lorenz, A.
  
• Defect formation in N* LCs via photovoltaic fields: impact of surface treatment. Liquid Crystals, 46(13-14), 2013-2022.
  Schafforz, Samuel L.; Yang, Yu & Lorenz, Alexander
  
• Impact of co-doping concentration in copolymer network liquid crystals. Liquid Crystals, 46(10), 1485-1493.
  Jahanbakhsh, Fatemeh & Lorenz, Alexander
  
• Formation of relocatable umbilical defects in a liquid crystal with positive dielectric anisotropy induced via photovoltaic fields. Journal of Molecular Liquids, 307, 112963.
  Schafforz, S.L.; Nordendorf, G.; Nava, G.; Lucchetti, L. & Lorenz, A.
  
• Surface grafted agents with various molecular lengths and photochemically active benzophenone moieties. Physical Chemistry Chemical Physics, 22(3), 1774-1783.
  Nordendorf, Gaby; Schafforz, Samuel L.; Käkel, Eireen B.; Miao, Shunyi & Lorenz, Alexander
  
• Surface Modification with Benzophenonesilane Derivatives for Liquid Crystal Test Devices, research results presented at the 47th German Liquid Crystal Conference (S26, P26), Magdeburg, 17. - 19.03.2021
  G. Nordendorf, A. Lorenz & H. Kitzerow
  
• Geometric-phase metalens to be used for tunable optical tweezers in microfluidics. Liquid Crystals, 50(7-10), 1193-1203.
  Geromel, René; Rennerich, Roman; Zentgraf, Thomas & Kitzerow, Heinz