Jul 22, 2019 | Updated: 09:15 AM EDT

X-Rays Show Monolayer Phase in Organic Semiconductor

Jul 09, 2019 09:18 AM EDT

X-Rays Show Monolayer Phase in Organic Semiconductor
(Photo : Elena Khavina/MIPT Press Office)

How the electrical properties of dihexyl-quarterthiophene thin films depend on their structure is what scientists from Russia, Germany, and France featuring materials scientists from the Moscow Institute of Science and Technology have investigated. The material is an organic semiconductor with prospects for flexible electronics. It turned out that once the thin films undergo a transition from the crystal to the liquid-crystal state, they lose some of their electrical conductivity. Also, the team 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 researchers have published their findings in Nanoscale Research Letters.

Promising organic semiconductors are oligothiophenes. Their rod-shaped molecules can orient at the surface on which they have been deposited, pilling 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.

The team dissolved and evaporated dihexyl-quarterthiophene (DH4T) in a vacuum reactor deposited the material as thin films on a silicon substrate. Then, they studied the crystal structure of the samples using grazing-incidence X-ray diffraction. This technique involves exposing a film to X-rays at a small glancing angle to maximize the distance the X-ray beam travels in the film, undergoing numerous reflections. The signal from the thin film, however, would be too faint to be distinguishable from the substrate signal. The diffraction measurements allowed the team to identify the molecular arrangement in the material deposited on the substrate.

In the first place, H4T was highly crystalline; its molecules formed a herringbone pattern and were positioned almost perpendicular to the substrate. Once they heated it to 85 degrees Celsius, however, the material underwent a phase transition; the molecular arrangement changed forming a liquid crystal phase, and the electrical conductivity of the films dropped.

The researchers further heated the sample to 130 C and subsequently cooled to room temperature. This process partly restored the crystallinity of the material and therefore, conductivity.

A third structure emerged in the x-ray diffraction profile throughout heating, indicated by weak diffraction maxima not corresponding to the liquid crystal phase. Prior research has correlated such maxima with monolayers of compounds like DH4T. Interestingly, this third phase was also observed at 70 C.

The structure of monolayer that the team discovered is favorable for charge transport along the plane of the film, making it significant for flexible electronics applications. Besides that, the newly observed phase could also occur in the thin films of other compounds whose structure is similar to that of DH4T.

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