Research projects
Anisotropy in carbon nanotube sheets
Carbon nanotube sheets are formed by parallel wires of connected carbon nanotubes. The sheets are aligned but with a degree of orientational order estimated with optical methods. They have also anisotropic properties and we evaluate the optical and electric anisotropic characteristics and interaction with polarized light. Properties are not trivially changed with the number of layers
CNT sheets as multifunctional electrodes for LCDs
The structure and oriented nature of carbon nanotube sheets are capable of aligning liquid crystals but they can be also used as transparent electrodes due to their electrical conductivity and nanometric thickness. We study the alignment and reorientation of thermotropic liquid crystals by electric and magnetic fields in presence of carbon nanotube-based boundary surfaces.
Liquid crystals assume spontaneously a common orientation and the use of two confining surfaces, typically made by rubbed polymeric surfaces, is sufficient for creating a preferred orientation in the overall liquid crystal layer. Aligned carbon nanotubes can supply the function of alignment layers providing unidirectional, planar alignment in liquid crystals. The alignment direction is the same as the alignment direction of the carbon nanotubes. Electric fields are used in displays for changing the orientation of liquid crystal molecules and thus inducing a change in the effective optical properties experienced by light enabling modulation of light. The CNT sheets are spatially heterogeneous and electrically anisotropic which open questions on the reorientation mechanism, also a focus of our research.
Graphene oxide liquid crystal
Flakes of oxidized graphene in water can form liquid crystal phases at extremely low concentrations.
Another remarkable feature is the modulation of light, using polarization selection, obtained when small electric fields are applied to graphene oxide liquid crystal.
Graphene flakes have monoatomic thickness and widths in the micrometer range. This extremely large aspect ratio gives an extremely low concentration threshold for forming liquid crystal phases, of lyotropic type. Due to their tendency to aggregate they are commonly found in the oxidized form and graphene oxide in water can form liquid crystal phases, as experimentally confirmed. The phase formation has to be evaluated since it varies depending on flake characteristics such as dimension, flatness and interaction strength.
Interestingly, graphene oxide liquid crystal is responsive to fields as standard liquid crystals, able to modulate light but, interestingly, at very small electric fields. We want to understand how the electro-optic modulation occurs and the factor influencing it especially studying the dielectric permittivity of graphene oxide at different concentrations.
Not all graphene oxide flakes in water form liquid crystal phases despite having appropriate flake dimensions, it is not clear why this happens but the appearance of birefringence under very strong confinement can be observed. The time stability of graphene oxide is also a matter of discussion and compared to early time of research on graphene oxide liquid crystals, nowadays typical samples can preserve their characteristics over long time span. Nevertheless, we are also interested in the effect of light, especially in the low wavelength range in the visible, on the colloidal stability presumably connected to changes in the flakes towards chemical reduction into reduced graphene oxide with subsequent aggregation. Colloidal instability can be also due the present of salts that can be introduced for tuning the inter-fllakeinteractions strength.