In what could prove to be a significant advance in fabricating new technologies, scientists discovered a new self-assembly mechanism that surprisingly drives negatively charged molecules to clump together to form islands when graphene is supported by an electrical insulator. Under these conditions, different charge interactions are not diminished, as they are when graphene is supported by a metallic substrate. At low concentrations, individual adsorbed molecules repel each other, but with increasing concentration, the molecules form two-dimensional islands. It was determined by theory that the flow of extra electrons into the islands from graphene keeps the molecules together. The electronic driving forces and stabilization energies are sufficient to overcome the repulsion between the negative charges.
This self-assembly mechanism can be used to tune the electronic properties of graphene layers in devices and control how electrons flow through the graphene. This mechanism permits atomic-scale patterning of electronic properties, which cannot be achieved with conventional lithographic techniques currently being used in the semiconductor industry.
Silicon has been successful because it is an electronically tunable semiconductor material that can be used in electronic devices. Graphene has distinct advantages over silicon for many applications due to its higher electron mobility and a very stable crystal structure, but it can be difficult to precisely tune. One way to tune the electronic properties of graphene is to adsorb molecules onto its surface. For example, negatively charged molecules on a graphene surface pull electrons from the graphene layer, changing its electronic properties. However, efforts to controllably assemble such negatively charged molecules have been limited because negatively charged species repel each other. Now scientists led by the University of California-Berkeley and Lawrence Berkeley National Laboratory have discovered that this repulsion can be overcome and two-dimensional islands can be controllably formed by negatively charged molecules on graphene supported by an insulator. Through microscopy and theoretical modeling, they determined that the underlying insulator was key to altering the nature of the interactions between the negatively charged molecules and graphene. These molecules are known to extract electrons from its substrate. At low surface concentrations, the negatively charged molecules separately accept electrons from the underlying graphene and repel each other, as expected because like charges repel each other.
Remarkably and counterintuitively, at higher concentrations, these charged molecules clump together to form ordered islands. This usual behavior is explained by theory as the donation of extra electrons to the islands of molecules by the graphene when compared to the donation to a single molecule. This extra charge makes it energetically more favorable to form islands. Surprisingly, this behavior observed on graphene substrate supported by an insulator does not occur when graphene is supported by a metal. This molecular self-assembly provides a possible alternative to patterning graphene using conventional lithographic techniques. Atomic-scale tuning of the properties of graphene layers could enable the fabrication of new devices based on graphene that cannot be made using silicon.
Explore further: 2D islands in graphene hold promise for future device fabrication
More information: Hsin-Zon Tsai et al. Molecular Self-Assembly in a Poorly Screened Environment: FTCNQ on Graphene/BN, ACS Nano (2015). DOI: 10.1021/acsnano.5b05322
Journal reference: ACS Nano
UK researchers have taken a step forward in making flexible displays, debuting a warping screen that uses graphene as its electronic material.
Like many of the screens that we view today, the new Cambridge invention uses an electrophoretic display that rearranges particles suspended in a solution by means of an electric field. However, in contrast to most displays the screen is made of flexible plastic and its pixel electronics, also known as backplane, replace the traditional metal electrode with one built from graphene.
According to Cambridge researchers, “Graphene is more flexible than conventional ceramic alternatives like indium-tin oxide (ITO) and more transparent than metal films.” What’s more the 2-dimensional carbon material is also processed and printed very easily making the display simple to produce.
Currently, the Cambridge consortium’s display is only capable of a 150 pixel per inch resolution, however, that may change in short order. Researchers believe they can build a flexible OLED or LCD screen that can project a full color HD image. Looking further into the future the UK team also believes that using graphene-based backplanes might allow them to embed sensors in the displays, making them more capable of interacting with their viewers.
“We are happy to see our collaboration with Plastic Logic resulting in the first graphene-based electrophoretic display exploiting graphene in its pixels’ electronics,” said Professor Andrea Ferrari, Director of the Cambridge Graphene Centre. “This is a significant step forward to enable fully wearable and flexible devices.”
While graphene’s potential has been known since it was first discovered, industries have been slow to leverage its unique abilities. This Cambridge display should do wonders for the development of process engineering for graphene and in the end it might just dazzle us with beautiful, flexible moving images.