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Spectroscopic techniques that only produce signals from sample interfaces

Recent research from the research group of Professor Aaron Massari

Field-effect transistors (FETs) are at the heart of every digital electronic device, from laptops to checkout scanners. Typically, the active materials in these transistors are inorganic semiconductors such as silicon. There is a strong drive to replace these heavy, expensive materials with cheap, lightweight, carbon-based semiconductors.

During operation of these organic FETs, or oFETs, a molecularly thin layer of the active material is electrochemically transformed into a conductor, presumably accompanied by structural adjustments following the electron transfer process. The mobility of electrical charges through these devices is then defined by the molecular structures in this thin interfacial slab of the organic semiconductor. Measuring the chemical changes in this crucial region is particularly challenging since it is buried beneath 50 to 100 molecular thicknesses of inactive material. This is akin to trying to measure the behavior of the bottom most centimeter of water in the Mississippi river from a bridge in Minneapolis without disturbing the river's flow. Fortunately, there are spectroscopic techniques such as vibrational sum frequency generation (VSFG) that only produce signals from sample interfaces.

Massari's research group has been utilizing this approach to measure the infrared spectra of the molecules that reside at the buried interface in oFETs in order to understand how the interfacial structure differs from the bulk material, and how it changes in response to device switching. Recent work from this project was published in the Journal of Physical Chemistry C 2010, 114, 17629. VSFG spectroscopy was used to probe the polymer-silica interface of poly(3-hexylthiophene) oFETs in-situ during device operation. The VSFG spectra from the buried interface exhibited dramatic changes upon switching. Notably, the spectral changes were observed when the oFET was activated with both positive and negative voltages, despite unipolar current-voltage responses. This supports a model in which electrons accumulate at the buried interface but are quickly trapped and immobile.