Graphene Week 2015 looks at the electronic properties of 2d materials

Electronic transport in the ultraclean limit

James Hone of Columbia University in New York began with a discussion of electronic transport in 2d materials in the ultraclean limit.

With electronics applications, graphene is generally laid on a substrate made of a bulk material. The problem is that, owing to the presence of dangling chemical bonds, 3d materials tend to be poor substrates for 2d materials such as graphene. One solution is to use hexagonal boron nitride (hBN), which is both an insulator and 2d material, and so does not have the problematic bonds.

How to create such hBN-graphene heterostructures? Graphene may be transferred onto boron nitride films with the aid of polymers. That is, graphene flakes are exfoliated onto polymers, aligned with a mask, and the flakes stacked on top of each other. Such a heterostructure displays enhanced charge carrier mobility, a lower degree of disorder, and improved field-effect transistor performance.

Looking beyond graphene, few 2d materials have so far been studied, and we have a long way to go before they are fully understood. One 2d material of interest is molybdenum disulphide, with the heterostructure formed with silicon dioxide.

Nanostructured graphene

Guangyu Zhang from the Institute of Physics at the Chinese Academy of Sciences spoke of the fabrication, control and electronic properties of nanostructured graphene.

The electronic properties of graphene are sensitive to geometry. By tailoring the physical structure of the material on the nanoscale, certain features and behaviours can be accentuated. Graphene nanostructures can be created through top-down approaches such as lithography and etching, but precise control is made very difficult at length scales smaller than 10 nanometres. Bottom-up processes such as growth and self-assembly allow for cost-effective mass production.

In his presentation, Zhang focussed on anisotropic etching, and the direct growth of graphene nanostructures for strain sensing applications. He also considered epitaxial graphene on substrates of hexagonal boron nitride.

Tuning the nanoscale energy landscape of graphene

Engineering the local electronic properties of graphene can be achieved in a number of ways, such as with electrostatic fields between metallic electrodes, and the introduction of atomic dopants. Both approaches have their limitations.

Michael Crommie from the University of California, Berkeley, looked at quantum dots made from individual molecules of tetrafluoro-tetracyanoquinodimethane. This organic compound, which is perhaps more easily memorable as F4TCNQ, can be used to tune the energy landscape of graphene.

Placement of F4TCNQ molecules in linear arrays onto graphene surfaces allows for tuneable intermolecular coupling and alternating Coulomb charge patterns at the molecular scale. The interaction of these quantum dot matrices with graphene charge carriers leads to novel super-critical-like phenomena within graphene, enabling new types of electromechanical setups that would otherwise not be possible.

Electronic properties of novel 2d materials

Switching attention from graphene to other 2d materials, Yuanbo Zhang spoke of his investigations into black phosphorus (aka phosphorene), a semiconductor, and tantalum disulphide, a metal.

Interest in phosphorene goes back to 1914. Phosphorene has a structure similar to that of graphene, and with an electronic band gap it occupies a space between zero-gap graphene and the semiconducting 2d materials molybdenum disulphide, molybdenum diselenide and tungsten diselenide.

A significant problem with phosphorene is that the material is unstable in air, with significant degradation after only a few days. It therefore needs to be shielded from the environment and electronic interactions with substrate materials. Zhang is particularly interested in heterostructures of phosphorene with boron nitride and silicon.

Tantalum disulphide, which has also been studied for decades, displays a number of interesting gate-tuneable phase transitions.

Colouring, stitching and twisting for 2d circuitry

Layered materials are like coloured papers that can be glued, stacked, cut and folded to form integrated devices with atomic thickness, noted Jiwong Park from Cornell University in New York. Park showed how different 2d materials can be grown with distinct electrical and optical properties (colouring). He also discussed the means by which 2d materials can be connected laterally to form patterned circuits (stitching), and how their interaction with light is influenced by controlling interlayer rotation and valley degree-of-freedom (twisting).

Park envisages folding various 2d materials into bulk, 3d structures. For example, molybdenum disulphide grown on a silicon substrate detaches automatically from the substrate when put in water, and will float. It can thus transfer easily onto any kind of substrate, in order to generate other structures.

Ghost atoms on monolayer graphene

The final presentation on the Tuesday morning of Graphene Week was given by another US-based researcher. Physicist Abhay Pasupathy of Columbia University spoke of hidden Kekulé ordering of ghost atoms on graphene monolayers.

When certain adatoms are placed onto graphene, they may be subject to strong interactions mediated by the graphene lattice. This interaction can induce Kekulé ordering, in which the symmetry of the carbon-carbon bonds is broken, leading to a tripling of the graphene unit cell.

Pasupathy presented evidence from scanning tunneling microscopy that shows the existence of this ordering in epitaxial graphene deposited on copper. In this case, the Kekulé order is induced by a dilute number of vacancies (ghost atoms) in an otherwise perfect copper lattice.

Photo: copyright © 2015 Anders Frick/Chalmers University of Technology