Defect pattern self-assembly and guided nanoparticle (NP) assembly in thin liquid crystal films
A limiting factor for the development and application of nanotechnology is the difficulty of aligning and spatially organize functional nanoparticles over large length scales. The classical top-down approach allows to create ordered structures and templates using micro- and nano-fabrication techniques such as ion beam lithography. We propose a bottom-up approach to create patterns and templates by dispersing or conjugating functional NP with an anisotropic LC materials that can self-assemble into ordered and aligned structures over multiple length scales. Periodic patterns of topological defects can be created in thin films of layered materials such as smectic LC and cholesteric LC. The interaction of NPs with the core of the defects determines both a regular spatial distribution and alignment of NPs.
The dispersion of gold nanoparticles (Au NP) in a chiral nematic LC evidence the insurgence of an order change in the LC host. Moreover, a comparative analysis based on dielectric and voltammetric spectroscopies performed on pure LC and on Au NP-doped LC shows that Au NP’s presence besides affecting LC order influences its electric properties: ion conductivity results importantly reduced, and beyond a threshold value of the applied field electrophoresis phenomena are induced.
Another activity deals with the studies on the transport and the organization of NP on scales ranging from microns to centimeters by means of topological defects in LC. These defects are generated and modified by means of the holographic control of the command surfaces (polarization sensitive dye films are used on glass substrates to record polarization holograms ) which confine the LC. The quantum dots dispersed in the medium accumulate in the topological defects and their position can be tuned by laser light.
The results obtained from these experiments expand the prospects for the development of miniaturized devices for the optics, photonics and microfluidics.
Molecular architectures at interfaces
The interfaces between bulk media are often site of reactions and phenomena which are distinct from the bulk substances and frequently dominate the macroscopic properties of the entire system. Understanding how molecules adsorb and react with these surfaces attract interests because of several potential applications in industrial processes and everyday life. Catalyses, combustions, lubrification, adhesion, wetting, electrochemical reactions are only few examples of phenomena of industrial interest which are governed by interfacial properties. Polymer surfaces and interfaces play an increasingly important role in modern electronic and optoelectronic technologies (i.e. organic transistors, OLED), as well as in biomedical applications (i.e., biocompatibility).
The challenging aspects of this surface work are always sensitivity and specificity, i.e. detecting molecularly thin surface layer (below 1015 molecules/cm2), distinguish its properties from the bulk substance ones.
To address this problem, surface-specific analytical technique with monolayer sensitivity has been recently developed successfully applied to various kinds of surfaces and interfaces. IR and visible sum-frequency generation (SFG) vibrational spectroscopy is a powerful and versatile in situ surface probe which permits identification of surface molecular species and provides information about orientation of functional groups at the surface. SFG is non destructive, highly sensitive, and has good spatial, temporal, and spectral resolution. Because the technique works in real time under water and protein solutions (and, theoretically, under blood) it is very well suited for studying biomaterials.
Recently SFG has been demonstrated as novel spectroscopic tool for molecular chirality, with significantly better sensitivity of Circular Dichroism, which allowed chiral vibrational spectra from a molecular monolayer. As a second-order nonlinear optical technique, SFG is symmetry forbidden in centrosymmetric bulk under the electric-dipole approximation , but allowed at interfaces where inversion symmetry is naturally broken. By choosing different input/output polarization combination, information on the orientation of selected atomic groups at sub-molecular level, as well as molecular configuration can be achieved.