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 ultravioletvisible (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 oneand 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.

Dependences of Thermal Behaviors on the Number of Layers in Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. 1. Order−Disorder Transitions in the Films Investigated by Ultraviolet−Visible and Infrared Spectroscopies

“…24 The SEM and AFM measurements revealed that the LB films consist of numerous platelike microcrystal domains which are superposed upon each other, like a mosaic structure (see Figure 7 of the following paper 21 ). It was also proved by the AFM measurements that these domains have periodic layered structure and well-ordered molecular arrangement.…”

Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine Prepared by the Langmuir−Blodgett Technique and Donor Doping. 1. Dependence of Thermal Behavior on the Number of Layers Studied by Ultraviolet−Visible−Near Infrared and Infrared Spectroscopies

“…[21][22][23][24][25][26] In our previous papers concerning LB films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ), we reported studies on their order-disorder phase transition and annealing effects by atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies. 24,25 These studies elucidated that phase transition temperature increases with the number of layers and that the thermal behavior of the LB films depends upon the number of layers. Moreover, it was found that the annealing effects on domain structure in the LB films of octadecyl-TCNQ are clearly different between the one-layer and multilayer films.…”

Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine Prepared by the Langmuir−Blodgett Technique and Donor Doping. 2. Morphology and Annealing Effects of the Films Investigated by Atomic Force Microscopy and Ultraviolet−Visible−Near Infrared and Infrared Spectroscopies

“…2,5,8 Their corresponding AFM images validate that the hydrocarbon chains are highly ordered, well-organized, and in an interdigitated pattern inside the bilayer microcrystal plate-like domains. 5 In contrast, the CH 2 antisymmetric and symmetric stretching bands of octadecyl-TCNQ appear at 2922 and 2852 cm -1 , respectively, in the spectra of CT films. This means that the interactions among the hydrocarbon chains are weak and that, at least, the interdigitated pattern of the chains does not exist.…”

“…Its morphological feature is similar to that of the 11-layer LB film of octadecyl-TCNQ. 5 Some amorphous spots can also be found among the domains. The molecular arrangement inside the domains is different from that before the annealing.…”

“…The molecular arrangement is one-dimensional resolved with the interstack period of 0.84 nm and is almost identical to that observed for the LB film of octadecyl-TCNQ. 5 UV-vis-NIR Spectra. Figure 6 shows annealing effects on a UV-vis-NIR spectrum of the 11-layer CT complex film of octadecyl-TCNQ doped by TMB.…”