X-rays reveal monolayer phase in organic semiconductor

phys.org | 4/12/2019 | Staff
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A team of researchers from Russia, Germany, and France, featuring materials scientists from the Moscow Institute of Science and Technology, has investigated how the electrical properties of dihexyl-quarterthiophene thin films depend on their structure. This material is an organic semiconductor with prospects for flexible electronics.

Once the thin films undergo a transition from the crystal to the liquid-crystal state, they lose some of their electrical conductivity. The team also discovered a "third phase" that does not occur in bulk material and corresponds to a monomolecular layer of the semiconductor. This structure could be favorable for charge transport across the films, with potential implications for microelectronics design. The research findings were published in Nanoscale Research Letters.

Oligothiophenes - Semiconductors - Molecules - Surface - Cycles

Oligothiophenes are promising organic semiconductors. Their rod-shaped molecules can orient at the surface on which they have been deposited, piling up cycles of hydrocarbons containing a sulfur atom known as thiophenes, like stacks of coins. The "coin edges" in the neighboring stacks form a herringbone pattern. This molecular arrangement enables the charge transfer from one molecule to the other.

As the number of thiophenes in the molecule increases, so does the electrical conductivity, at the cost of the compound's solubility. The optimal number of these so-called thiophene moieties is four. To increase solubility, hexyl fragments are grafted to the ends of the conjugated molecular fragment (fig. 1).

Researchers - Dihexyl-quarterthiophene - DH4T - Vacuum - Reactor

The researchers dissolved and evaporated dihexyl-quarterthiophene (DH4T) in a vacuum reactor and deposited the material as thin films on a silicon substrate. They went on to study the crystal structure of the samples using grazing-incidence X-ray diffraction. This technique involves exposing a film to X-rays at a very small glancing angle to maximize the distance the X-ray beam travels in the film, undergoing numerous reflections. Otherwise, the signal from the thin film would be too faint to be distinguishable from the substrate...
(Excerpt) Read more at: phys.org
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