Temperature behavior of LB films of azobenzene derivative

Temperature behavior of LB films of azobenzene derivative.

 Purpose.

Investigations of the LB films of recently synthesized p-(p-octadecyl-aminophenylazo)benzenesulfamide (mel-7) were made with different SPM modes. The temperature changes of morphology of mono- and multi-layers of mel-7 were studied. Transformations of surface potential of LB film with heating were measured (the molecule of mel-7 has high dipole moment about 12 D). This surface-active substance has interesting features that in the temperature range
from 65°C to 115°C it is in smectic liquid-crystal phase, while at higher temperatures it transfers to liquid isotropic phase. 

Sample preparation.

The formation of the mono- and multilayers was made on the NT-MDT LB5 trough. The trough area was 45x11 cm². The rectangular silicon (111) plates with the thickness of 0.35mm were used as a substrate. Before the deposition of the monolayer the silicon plate was loaded into oxygen plasma. After that, substrate became hydrophilic. The transfer of the monomolecular layer from the water surface to the substrate was performed at the surface pressure of 33mN/m. The temperature of the water and the air was 18-20°C with relative air humidity 60-80%. The speed of the withdrawal of the substrate from the water subphase was equal to 20 mm/min. The transfer coefficient approached unity. Obtained sample was attached to standard heating stage.

Equipment and methods.

The formation of the mono- and multilayers was made on the NT-MDT LB5 trough. Investigations of the surface properties were carried out by SPM SOLVER P47 equipped with heating stage with the temperature range from room temperature up to 150°C. Topography was obtained in the semicontact mode. Surface potential measurements were carried out by the SKPM mode. Silicon cantilevers NSG11 were used for topography and potential measurements. The amplitude of free cantilever oscillations was about 20nm and the set-point was maximal close to the free amplitude.

Results.

1. Monomolecular layer of MEL-7
   
   

   
   
   Fig. 1

   Before heating surface of monomolecular layer of mel-7 was flat (left). Surface potential image obtained simultaneously with topography reveals small electrostatic inhomogeneity (right). Potential changes over all scan are ~0.15V.

   
   
   Fig. 2

   This animation shows the growth of highly anisotropic in shape needle-like crystals at T=75°C. The height of the crystals normally to the substrate constituted an odd number of monomolecular layers. It is remarkable that after 12 hours of keeping the samples at a room temperature the crystals spread over the surface producing a fairly homogeneous monomolecular layer [1].
   The characteristic size of the studied area was 35x35 µm². Dark aureoles around needle-shape crystallites correspond to regions depleted of mel-7 molecules. The aureoles around crystallites are absent for the sample with large exposure time at 75°C. This corresponds to the condition when all the material of the original monolayer was accumulated in the crystallites. It should be noted that the pattern becomes stable with time.

   
   
   
   Fig. 3

   Topography (left) and surface potential images (right) obtained for crystals after 30min of heating at T=75°C. Quantitative analysis of topography revealed that the volume of the crystallites was approximately (within 10 %) equal to that of the original monolayer on the substrate. This suggests that at heating to 75°C the evaporation of molecules of mel-7 takes place rather slowly, the energy of molecule adhesion to the substrate being sufficiently large. The aureoles around the needle-shape crystallites were also present when measuring the surface potential.

   
   
   Fig. 4

   Left figure shows a fragment of a microcrystallite in expanded scale (the area of 2.5x2.5µm²), while the profile of characteristic section AA' is given on right image. Section AA' shows that the crystals have typical steps with the thickness of about 40. The height of separate steps over the substrate surface satisfies the condition d=(20+40·n), where the first item stands for the thickness of one monolayer on the substrate, 40 is the thickness of bimolecular layer in , and n is the number of bilayers in the structure. The highest with respect to thickness crystals had n=7. Other important characteristics of the crystals of mel-7 on the substrate surface include the typical length of
   10-30µm and width of 0.3-1.0µm. This crystals are practically flat, their thickness being
   ~30 nm, which is 10-30 times less than their width.

   
   
   Fig. 5

   Topography (left) and phase image (right) obtained simultaneously for the surface of the crystal of mel-7. The images were obtained at room temperature after heating-cooling cycle. Total difference between topography and phase image can be explained by differences in mechanical properties of the areas of a top layer that have various density.

   0min

   16min

   38min

   
   Fig. 6

   Graphoepitaxy of the crystals of mel-7 at T=65°C. These figures show the consequent stages of the crystal growth in the initial time as well as in 16 min and 38 min period, respectively, at 65°C for the substrate processed with the abrasive. As can be seen, the majority of the crystals are aligned along the scratches [2]. The presence of a small number of crystals having the orientation different from the general direction could be attributed to the possibility of the crystal nucleus with arbitrary orientation to appear on the areas of substrate free from scratches.

   
   
   Fig. 7

   Sample topography after heating up to 150°C and following cooling down to room temperature.

    

2. 5 Bilayers of MAL-7
   
   

   
   
   Fig. 8

   This animation shows the growth of lamellar aggregates from LB film of 5 bilayers of mel-7 at T=75°C on area 38mkm x 41mkm. This situation totally differs from monolayer crystallization.

   
   
   Fig. 9

   Lamellar aggregate obtained from 5 bilayers after heating-cooling cycle.
    

References

1. A.M. Alexeev, E.A. Kosobrodova, I.V. Myagkov, Book of Abstracts 8th Europ. Conf. On
   Organizsed Films, Otranto, Italy (2001), P2.02.
2. A.M. Alexeev, E.A. Kosobrodova, I.V. Myagkov, Procedeengs of SPM-2002, Nizhnii Novgorod, Russia (2002), 135-137.