Thermal Stability of Organic Semiconductor Monolayers Monitored by In-situ X-ray Reflectivity

Variable temperature X-ray reflectivity measurements are used to determine the stability and order in organic semiconductor thin films for use in opto-electronic devices. The data reveal information on both the stability and structure of the films so that the temperature at which order (and functionality) is lost can also be determined.

Organic monolayers have applications in many different fields as they make it possible to change the properties of a given surface (e.g. polarity, work function, etc.) to make them suited for various applications. In organic electronics, devices can be produced from self-assembled monolayers (SAMs) to allow single molecule thick devices which have several attractive properties; an example are SAM field-effect transistors (SAMFETs) which can be used to produce code generators or inverters.

For the widespread application of SAM devices, several criteria must be met, including that they are stable and maintain their structure (and function) at elevated temperatures. Such SAMs are typically chemisorbed to a solid substrate via an anchor group (e.g. a silane or phosphonate group) and contain an alkyl spacer, the functional core group, plus a head group which defines the surface properties. The spacer imparts enough flexibility so that the core groups can organize in a structured manner and charge transport is possible through the conjugated cores.

Chemisorption to the substrate should impart improved stability to the organic monolayer. However, the degree of stability which is added has until now not been investigated. It is therefore of fundamental interest to determine the degree of stability imparted by a SAM anchor group.

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