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

Abstract

Dependences of thermal behaviors on the number of layers in Langmuir-Blodgett (LB) films of 7,8, have been investigated for both morphological and molecular structural aspects by using ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies. The temperature-dependent morphological changes have been monitored by the base-line shift in the UV-vis spectra of the LB films. The changes in the base-line in the 450-600 nm region suggest that the domains in the one-layer film decompose progressively with temperature even just above room temperature. In contrast, the domains in the seven-and 11-layer films are stable until 80°C and undergo rather abrupt collapse between 80 and 90°C. These differences in the thermal behaviors of the domain structure between the one-layer film and the seven-and 11-layer films may be attributed to the differences in the film thickness, the effect of the interaction between the first layer and a CaF 2 substrate, and the longitudinal interaction between the domains. The IR spectral measurements for the LB films at elevated temperatures reveal that the order-disorder transition temperature of the one-layer LB film is lower by about 5°C than that of the 11-layer film, which is close to the chain melting point of the bulk material. Of particular interest is that the domain structure of the onelayer film shows the gradual temperature change but that the molecular orientation and structure in the domains experience the clear order-disorder transition.

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

“…The cleaning procedure for these substrates was the same as that previously described. 1 In the AFM experiments, a NanoScopeIII (Digital Instruments) was used to investigate the morphology of the octadecyl-TCNQ LB films which had been built up onto silicon substrates (n-Si(100) surface) to achieve a high resolution. We used a silicon nitride tip on a cantilever with a spring constant of 0.6 N/m.…”

“…1 This paper discusses the dependences of annealing effects on the number of layers in the LB films. Studies of the annealing effects on the structure of LB films are very important from the viewpoints of both basic research and practical applications.…”

Surface Science and Model Catalysis with Ionic Liquid‐Modified Materials

“…[ 100 ] Similar investigations have been performed, for example, on self-assembled monolayers [101][102][103][104][105][106] or Langmuir-Blodgett fi lms. [107][108][109][110] The position of two selected bands, the ν s (CH 2 ) modes (around 2850 cm − 1 ) and the ν as (CH 2 ) modes (around 2920 cm − 1 ), is displayed in Figure 7 b. As their position directly refl ects the conformational and structural changes related to the alkyl chains, they can be used as an indicator of the ordering process (see [ 100 ] and references therein).…”

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

“…For example, the frequencies of the CH 2 antisymmetric and symmetric stretching bands are significantly different between them. 1,2 This implies a different stacking pattern of the hydrocarbon chains of octadecyl-TCNQ. It is well-known that appearances of the CH 2 antisymmetric and symmetric stretching bands at lower wavenumbers (ca 2918 and 2848 cm -1 , respectively) indicate a highly ordered and well-organized arrangement of the hydrocarbon chains, while their appearances at higher wavenumbers (ca 2926 and 2856 cm -1 ) are indicative of the increase in conformational disorder in the hydrocarbon chains.…”