Sub-molecular structural relaxation at a physisorbed interface with monolayer organic single-crystal semiconductors

Abstract

Arranging molecules into highly symmetric, topological crystal structures has been recognized as the best approach to functionalize electronic properties in molecular crystals, where the constituent molecules have been assumed to be rigid in shape. Here, in striking contrast, we demonstrate that the molecules in a monolayer organic crystal can undergo a significant deformation in proximity to the substrate, which is reflected by an asymmetry in the electron density profile. X-ray reflectivity and X-ray absorption spectroscopies in conjunction with density-functional theory calculations reveal that the highly planarized π-core are deformed into a bent shape, while the bulk lattice parameters are maintained. The molecular shape change is found to be perfectly suppressed in a bilayer single crystal, which leads to a 40% increase in mobility in the bilayer crystal. Our finding of a unique, sub-molecular scale shape change in monolayer single crystals can offer possibilities for functionalizing electrical properties via nano-scale physisorption.

Correlation between the static and dynamic responses of organic single-crystal field-effect transistors

“…The substrate was heated to 81°C and moved with a shearing rate of 16 μm s −1 while a blade that sustains the meniscus was fixed 95 μm above the substrate. Bilayer single crystalline films were grown selectively by tuning the substrate temperature 45 . Following annealing of the substrate at 100°C under vacuum to remove any residual solvent, F4TCNQ and Au were subsequently deposited to form the source/drain electrodes, which were patterned by multiple lithographic processes, whereby a negative photoresist (OSCoR4001, Orthogonal inc.) and Au etchant (AURUM S-50790, Kanto Chemical Co. Inc.) were used.…”

Surface Doping of Organic Single‐Crystal Semiconductors to Produce Strain‐Sensitive Conductive Nanosheets

“…[ 13–18,39 ] In continuous edge casting, a self‐assembled molecular nanosheet grows at the vapor–liquid interface and is then laminated onto a substrate. [ 26 ] Bilayer single‐crystalline thin films with a molecularly flat surface were prepared selectively by adjustment of the substrate temperature and the solubility. [ 22,23,25,26 ] Note that single‐crystalline thin films used for transport and X‐ray diffraction measurements were true mono‐domain crystals with no grain boundaries.…”

“…[ 26 ] Bilayer single‐crystalline thin films with a molecularly flat surface were prepared selectively by adjustment of the substrate temperature and the solubility. [ 22,23,25,26 ] Note that single‐crystalline thin films used for transport and X‐ray diffraction measurements were true mono‐domain crystals with no grain boundaries. [ 22,25,26 ] Chemical doping (impurity doping) was performed simply by exposing solid‐state thin films of C 8 –DNBDT to a solution of the dopant (Figure 1b).…”

“…Various groups have recently developed state‐of‐the‐art OSCs, and the electronic properties of organic devices have been considerably improved as the mobility exceeds 10 cm 2 V −1 s −1 . [ 22–26 ] Together with inventions in the printing field, highly reliable, reproducible transistor arrays can be manufactured to produce high‐intensity integrated circuits with relatively high operation speeds up to a few tens of megahertz. [ 27 ] The excellent electronic properties of single‐crystalline OSCs originate from the chemical structure of designed molecules [ 20,21,24 ] and the highly periodic electrostatic potential, even though established in van der Waals bonded organic crystals, and are thus a foundation for the realization of coherent, band electron systems where the electron wavefunction is delocalized over molecular crystals [ 28–30 ] ; therefore, it is essential to control the electronic system while preserving the precisely designed crystal structure.…”

Two-dimensional hole gas in organic semiconductors

“…A uniform optical intensity in polarized microscopy images confirms the existence of a molecularly flat surface without molecular steps. A single-crystalline bilayer is prepared selectively by controlling the substrate temperature 26,32 . A plastic substrate (polyethylene naphtalate: PEN) coated with a parylene layer is a better alternative with regard to the thermal expansion coefficient of OSCs, i.e., less crystal cracks are induced during low temperature measurements.…”