How fast an organic field effect transistor can operate depends on charge transport in the first few molecular layers near the dielectric interface. However, direct probing of such intrinsic properties in organic thin films is challenging because it is difficult to produce high-quality samples. Now, writing in Physical Review Letters, Xinran Wang and colleagues report the epitaxial growth of 2D pentacene crystals and their conducting properties. They found that charge transport is dominated by hopping in the first conductive layer but becomes band-like in the second conductive layer. As Wang explains, “this few-layer molecular thin film allows us to unequivocally correlate the macroscopic electrical properties of the organic layers with the microscopic geometry of the molecular packing”.
We believe that this study is just the beginning of investigations of 2D organic semiconductors
Until now, intrinsic properties of organic systems, such as band-like charge transport, could only be observed in bulk single crystals because, as the thickness of the material approaches the monolayer limit, the sample response is dominated by disorder and defects. The highly crystalline pentacene layers grown on a boron nitride substrate by Wang and colleagues have thicknesses ranging from a monolayer to four layers and can be used to probe the conducting properties of each individual layer.
A combination of scanning probe microscopy, photoluminescence and density functional theory reveals that the molecular packing is different in the first three layers as a consequence of the competition between molecule–molecule interactions in the same layer and those between different layers. The pentacene molecules in contact with the boron nitride substrate, which form the wetting layer, lie flat owing to their strong interaction with the substrate. This wetting layer shows no conductivity. The pentacene molecules above the wetting layer form the first conductive layer. The intermolecular interactions between pentacene molecules in this layer start to dominate, leading to a more upright packing, with a molecular tilt of roughly 60° with the substrate. A transition to bulk-like packing is observed in the subsequent second conductive layer, in which the interaction between neighbouring layers is practically absent and the molecules are assembled upright, with a tilt angle of roughly 80°.
Measurements of electrical transport in devices with thicknesses ranging from one to four layers show that the first conductive layer transports charge at room temperature, but becomes insulating at low temperatures, indicating a hopping charge transport. This behaviour originates from the tilted molecular packing, which limits the superposition of molecular orbitals and from a strong interaction with the wetting layer. By contrast, in the second conductive layer, the horizontal overlap of molecular orbitals results in an extended density of states, enabling band-like transport. Interestingly, devices with an additional pentacene layer — that is, with a total of four layers — behave as three-layer devices, indicating that it only takes three layers (with a thickness of ∼3 nm) to reach mobility saturation, whereas previous studies of polycrystalline pentacene have suggested that saturation is reached after the sixth layer.
“We believe that this study is just the beginning of investigations of 2D organic semiconductors,” says Wang. “In light of the strong vertical confinement of charges, new physics that was previously impossible to observe in bulk crystals is expected to emerge, in a similar way to what happened with graphene.”
Zhang, Y. et al. Probing carrier transport and structure–property relationship of highly ordered organic semiconductors at the two-dimensional limit. Phys. Rev. Lett. 116, 016602 (2016)
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Pacchioni, G. Organic electronics: Packing tips for charge transport. Nat Rev Mater 1, 16005 (2016). https://doi.org/10.1038/natrevmats.2016.5
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