Structures and Dynamics of Ultra-thin Smectic Films

Smectic films consisting of only one or few molecular layers are prepared by spin-coating from solution. By controlling the concentration of the smectic material in the spin-coating solution, films with a defined small number of smectic layers can be prepared. Unless the concentration is precisely tuned to certain values, the topmost smectic layer of the films is formed only partially, i.e., it is either fragmented into isolated islands or it shows a porous structure. There is a well-defined relation between the concentration of the liquid crystal in the spin-coating solution and the resulting surface structure, thereby enabling the targeted generation of island or pore structures. These films might be of interest for the preparation of structured two-dimensional soft matter systems.

Fig 1: AFM height images (area 20 x 20 μm2) of ultra-thin (two to three molecular layers) smectic films spin coated from solution onto silicon wafers. The concentration of the smectic compound (8CB) in the spin-coating solution was 2.8 mg/ml (a), 3.0 mg/ml (b), 3.2 mg/ml (c), 3.4 mg/ml (d), 3.6 mg/ml (e), and 3.8 mg/ml (f). The images demonstrate the partial formation of the smectic top layer: with increasing concentration, a structural sequence is observed which starts with isolated islands which grow to a porous structure and finally form a complete layer. The height difference between the thicker and the thinner parts of the film is about 3.2 nm, corresponding to the thickness of one molecular smectic layer. — Fig 1: AFM height images (area 20 x 20 μm²) of ultra-thin (two to three molecular layers) smectic films spin coated from solution onto silicon wafers. The concentration of the smectic compound (8CB) in the spin-coating solution was 2.8 mg/ml (a), 3.0 mg/ml (b), 3.2 mg/ml (c), 3.4 mg/ml (d), 3.6 mg/ml (e), and 3.8 mg/ml (f). The images demonstrate the partial formation of the smectic top layer: with increasing concentration, a structural sequence is observed which starts with isolated islands which grow to a porous structure and finally form a complete layer. The height difference between the thicker and the thinner parts of the film is about 3.2 nm, corresponding to the thickness of one molecular smectic layer.

Fig 1: AFM height images (area 20 x 20 μm²) of ultra-thin (two to three molecular layers) smectic films spin coated from solution onto silicon wafers. The concentration of the smectic compound (8CB) in the spin-coating solution was 2.8 mg/ml (a), 3.0 mg/ml (b), 3.2 mg/ml (c), 3.4 mg/ml (d), 3.6 mg/ml (e), and 3.8 mg/ml (f). The images demonstrate the partial formation of the smectic top layer: with increasing concentration, a structural sequence is observed which starts with isolated islands which grow to a porous structure and finally form a complete layer. The height difference between the thicker and the thinner parts of the film is about 3.2 nm, corresponding to the thickness of one molecular smectic layer.

Fig. 2: Mean film thickness, as determined by ellipsometry (black dots, left axis: ellipticity coefficient ρ, right axis: resulting mean thickness h) and by analyzing the AFM images (blue diamonds, right axis only), as function of the concentration c of the liquid crystal compound 8CB in the spin coating solution.

Fig. 2: Mean film thickness, as determined by ellipsometry (black dots, left axis: ellipticity coefficient ρ, right axis: resulting mean thickness h) and by analyzing the AFM images (blue diamonds, right axis only), as function of the concentration c of the liquid crystal compound 8CB in the spin coating solution.

We are currently conducting single molecule diffusion measurements [B. Schulz, D. Täuber, F. Friedriszik, H. Graaf, J. Schuster, and C. von Borczyskowski, Phys. Chem. Chem. Phys. 12, 11555 (2010)] in the ultra-thin films described above as well as in freely suspended smectic films. For this method, one determines the Brownian motion of single fluorescent probe molecules which are dissolved in the liquid crystal matrix. The following animation gives an example of fluorescent single molecules moving within a 16 × 16 μm² area of a smectic film:

Image processing enables the determination of the trajectories of the probe molecules and from the mean square displacement the diffusion coefficient is obtained.

Image processing enables the determination of the trajectories of the probe molecules and from the mean square displacement the diffusion coefficient is obtained.

Find more information:

Surface structure of ultrathin smectic films on silicon substrates: Pores and islands
B. Schulz and Ch. Bahr, Phys. Rev. E 83, 041710 (2011).
DOI: 10.1103/PhysRevE.83.041710

Nanoscale viscoplastic behavior of smectic liquid crystals and its application in nanolithography
B. Schulz, P. Steffen, and Ch. Bahr, J. Appl. Phys. 115, 074302 (2014).
DOI: 10.1063/1.4865796

Single-molecule diffusion in freely suspended smectic films
B. Schulz, M. G. Mazza, and Ch. Bahr, Phys. Rev. E 90 040501(R) (2014).
DOI: 10.1103/PhysRevE.90.040501