AsianScientist (Mar. 29, 2018) – A group of scientists at Nagoya University, Japan, have developed a simple method to construct perfectly unidirectional molecular structures on graphene surfaces. They published their findings in Scientific Reports.
Graphene, which are sheets made out of carbon, is attracting interest as a powerful candidate material for manufacturing next-generation electronics. The development of a reliable method that enables the perfect alignment of molecules or molecular assemblies on a graphene surface may allow the fine-tuning of the electric properties of graphene, thereby improving the performance of graphene-based electronic devices.
Although widely studied in recent years, the growth of well-aligned molecular nanostructures along a specific axis has proven to be difficult. This is because the graphene surface has three-fold symmetry, thus making it difficult to align the molecules in a particular direction.
To resolve this problem, a team led by Dr. Yuhei Miyauchi and Professor Kenichiro Itami of the JST-ERATO Itami Molecular Nanocarbon Project and the Institute of Transformative Bio-Molecules (ITbM) focused on the physical changes induced by atomic force microscopy (AFM) tip scanning.
AFM, a technique mainly used for analyzing surfaces, produces images showing the surface unevenness of samples by sliding a probe tip over the surface area. The team suspected that tip scanning modifies the thermodynamic conditions on the surface of graphene, thereby affecting the direction of molecular alignment.
The team investigated how AFM tip scanning leads to changes in molecular alignment on the graphene surface. They used sodium dodecyl sulfate (SDS), a common surfactant molecule, as a model molecule. Studies have shown that SDS forms ribbon-like assembles on graphene surfaces.
Using a microsyringe pump, the SDS solution was slowly injected into a multilayer of graphene within a water droplet. The team compared how the SDS molecules adhered to the graphene, a process called adsorption, with and without AFM tip scanning.
An AFM height image recorded one hour after SDS injection showed random unevenness on the surface, which indicated random adsorption of SDS molecules on the graphene surface. After 15 minutes of intense AFM scanning, however, the pattern of SDS adsorption drastically changed and many ribbon-like molecules were observed. This phenomenon indicated that the strength and direction of AFM tip scanning affected the orientation of the generated SDS ribbons.
“We discovered this phenomenon accidentally when we were carrying out another research project,” said Dr. Hong Liu, a postdoctoral researcher at ITbM who was involved in the study. “We noticed that the SDS ribbons grew in the direction of the AFM tip scanning.”
The team discovered that the SDS ribbons grew easily when the relative angle between the ribbon-growing axis and the scan direction was larger. In addition, computational calculations suggested that adsorbed SDS molecules were removed when they were forced to rotate under the AFM scanning conditions.
Adsorbed SDS molecules with a relatively large angle to the AFM tip scan direction are rotated and are easily removed. Therefore, the molecules adsorbed with small angles to the AFM tip scan direction act as the nuclei and grow to become the SDS ribbon. Using this knowledge, the team tried to construct perfectly aligned SDS molecular assemblies on graphene.
“The most difficult part of this research was how to control the growth and direction of SDS ribbons with precision,” said Hong. “Once the SDS ribbons were grown, their orientations did not change under the AFM scanning conditions. We had to perform rapid AFM scans immediately after the SDS molecules were injected into the water on the graphene surface.”
Under finely tuned AFM scanning conditions, the researchers succeeded in constructing individual one-dimensional molecular assemblies, which are aligned along a selected symmetry axis of the graphene lattice.
“Our method can be used to grow anisotropic molecular patterns on two-dimensional (2D) materials, such as super lattices, which are now essential in both academic and industrial research,” said Hong.
“We hope that our discovery will lead to a distinctive breakthrough in not only chemistry, but in related fields that involve molecular nanostructures and their alignment,” said Itami.
The article can be found at: Hong et al. (2018) Unidirectional Molecular Assembly Alignment on Graphene Enabled by Nanomechanical Symmetry Breaking.
Source: Nagoya University; Photo: Institute of Transformative Bio-Molecules.
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