Optical Characterization of Advanced Thin Films

A wide range of technologies are impacted by the performance of thin (<100 nm) layers of advanced materials. This is particularly true in the emerging macroelectronics industry and in biomaterials. Rational development of these materials is hampered by the lack of structure-function relationships due to a severe lack of adequate data/measurement tools for molecular level characterization of thin films. The traditional structural tools of x-ray diffraction and NMR are severely limited due to lack of signal and contrast. The focus of this project is developing and demonstrating the ability of bench-top optical tools, combined with advanced physical models to determine the details of molecular structure in thin films and at interfaces. When possible, measurements are complemented and validated by comparison to unique NIST resources such as neutron scattering and synchrotron-based x-ray techniques.

The focus of this project is developing and demonstrating the ability of bench-top optical tools, combined with advanced physical models to determine the details of molecular structure in thin films and at interfaces. Specific emphasis is placed on polarized photon spectroscopies: spectroscopic ellipsometry (SE), IR absorption (IR), sum frequency generation (SFG), and coherent anti-Stokes Raman scattering (CARS). When possible, measurements are complemented and validated by comparison to unique NIST resources such as neutron scattering and synchrotron-based x-ray techniques.

Specific recent technical advances include the extension of analytical models of anisotropic systems to include in-plane order, and the development of the ability to perform both SE and IR measurements at elevated temperatures. An example of these advances comes from our studies of polymer semiconductors. Polymer semiconductors are inexpensive solution processable alternatives to amorphous silicon for applications in flexible large area electronics. Recently, thin films of spun-cast poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene) (pBTTT: see Figure 1) have been demonstrated to exhibit exceptional hole mobilities in thin film transistors (TFTs) after heating into a low temperature mesophase. This mild thermal cycle results in high level of crystalline order in the annealed film with a morphology exhibiting single molecular layer terraces. We have applied polarized optical spectroscopies: spectroscopic ellipsometry and FTIR to the in-situ study of the structural evolution in the two high temperature phases. The thermal processing of the film is captured by the temperature evolution of the frequency of the symmetric stretch of the CH2 group of the side chain, as shown in Figure 2. Low values of the frequency (~2920 cm-1) correspond to highly ordered chains while high values (~2926 cm-1) correspond to liquid–like chains. As the temperature increases, the originally ordered and interdigitated chains disorder and separate, resulting in a well ordered liquid crystal characterized by melted side chains, but retaining excellent vertical lamella order. The behavior of pBTTT can be contrasted with that of poly(dialkylthieno[3,2-b]thiophene-2,5-bithiophene) (pTTBT: also see Figure 1), a newly synthesized isomer of pBTTT with side chains attached to the thienothiophene rather than the bithiophene unit. This subtle structural change results in distinct thermal behavior. The structural transitions of the isomers are generally similar; however, the side chain melting transition Tm occurs about 50°C lower in pTTBT than in pBTTT. The significant drop in Tm appears to correlate with a subtle decrease in main chain packing interactions. Both materials exhibit high hole mobility in their respective liquid crystal mesophases. The decrease in main chain packing interactions in pTTBT results in greater disorder and a moderate decrease in hole mobility in annealed device performance.

The specific level of molecular characterization provided by these studies, will enable rational design of future highly ordered polymers for electronic applications.

Implemented advanced models for characterization of thin films with polarizing spectroscopies.

Characterized critical molecular transformations during thermal processing of organic semiconductor films.

Proposed structure-function design rules for organic semiconductor films.