Dependences of Thermal Behaviors on the Number of Layers in Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. 2. Annealing Effects on the Films Investigated by Atomic Force Microscopy and Ultraviolet−Visible and Infrared Spectroscopies

Abstract

The annealing effects on Langmuir−Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (abbreviated as octadecyl-TCNQ) have been investigated by atomic force microscopy (AFM) and ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. An AFM image of a one-layer LB film of octadecyl-TCNQ scanned at room temperature shows that the film consists of numerous platelike microcrystal domains. A periodic structure of octadecyl-TCNQ molecules with a period of 0.85 nm can be observed inside these domains. After the annealing, cooling down the film from 130 to 30 °C, some of these domains lie on top of one another, keeping their original morphology. The rest seem to melt into a globular bulk sample. In the case of an 11-layer film, it can be seen from its AFM images that a number of domains with smaller size cover the substrate and that they completely melt after the annealing cycle. The UV−vis spectra of the one- and 11-layer films measured before and after the annealing show that the stacked structure of the TCNQ chromophore recovers partially for the one-layer film, but it recovers little for the 11-layer film. The IR measurements of the one-, three-, seven-, and 11-layer LB films before and after the annealing reveal that the subcell packing of the hydrocarbon chain changes due to the cyclic temperature treatment up to 130 °C. The present study also demonstrates that the annealing effects on the LB films of octadecyl-TCNQ are largely different from those on LB films of simple amphiphilic compounds such as fatty acids.