Ultrafine graphene nanodevice fabrication

Graphene, a material made from a single atomic layer of carbon arranged in a two-dimensional honeycomb structure, is attracting massive worldwide interest for its potential to be the foundation of a new generation of nanoscale electronic/photonic/ spintronic devices.^1,2 This is thanks to its remarkable material properties, which include very high carrier mobilities and large current carrying capabilities. The carrier mobility in graphene is weakly dependent on temperature, which implies the mobility is limited only by impurities and/or defects and hence can be further increased by improving material and device fabrication processes. Graphene is a low-noise material, meaning it is highly sensitive to the environment. This makes it an excellent candidate for extreme sensing applications such as a biochemical single molecular sensor.

The electrons in graphene are not much affected by electron–electron interaction and have a long mean free path. In addition, spin-orbit coupling and hyperfine interactions with carbon nuclei are both small in graphene, and a very long spin relaxation length has been demonstrated. All these superior electronic properties encourage us to downscale graphene devices further to the regime where we can fully exploit the coherent natures of electronic and spin states. However, this requires the development of ultrafine patterning technologies that enable accurate nanoscale fabrication beyond the present technique of electron-beam (EB) lithography.

Figure 1. A schematic diagram of the helium (He)-ion-beam milling process for graphene.

Figure 2. Multiple metal contacts fabricated by electron-beam lithography on an exfoliated graphene flake.

Figure 3. Quadruple quantum dots patterned on bilayer graphene using He-ion-beam milling.

Recently, a breakthrough in ultrafine graphene nanofabrication technology has been achieved using an atomic-size beam of helium ions^{3, 4} (see Figure 1) generated in a helium-ion microscope. Helium-ion microscopy is a new surface imaging technique⁵ that involves scanning a focused beam of helium ions across a surface to generate an image from the resulting secondary electron emission, in a similar way to scanning electron microscopy. An atomically sharp and extremely bright source, combined with the larger momentum (and so shorter de Broglie wavelength) of helium ions compared to electrons, enables a sub-nanometer probe size at the sample surface and so high-resolution imaging. Our team at the University of Southampton, UK, and the Japan Advanced Institute of Science and Technology has jointly demonstrated that the tool can also be used to selectively sputter graphene to create intricate nanoscale designs,^{6, 7} offering the potential of resist-free patterning of graphene on a finer scale compared to other techniques.

We first used EB lithography to pattern metal contacts onto the graphene flakes (see Figure 2) and then used the helium-ion-beam milling process to successfully carve graphene into ultrafine structures such as nanoribbons and quantum dots (QDs). We found that complete cleaning of the surface of processed graphene is vital to remove the residues of EB resist and solvents that otherwise prevent a successful milling. Figure 3 shows a pair of single-electron turnstile devices with double QDs patterned onto bilayer graphene, which are separated with an extremely narrow gap of only 5nm. Such fine patterning with an accuracy of 1nm and an excellent yield is virtually impossible using conventional nanofabrication technologies.

In conclusion, this emerging nanotechnology opens a new pathway to design and control electronic and photonic states atomically and enables the development of novel electronic, spintronic, and photonic devices at truly nanometer scale, more functional than anything we could ever have imagined with just conventional technologies. We are currently applying this new atomic fabrication technology to develop ultrathin suspended graphene nanoribbons for extremely sensitive gas molecular sensors and densely integrated graphene QDs for quantum information processing technologies as well as investigating the nature of edge states of the carved graphene nanostructures using atomic-resolution scanning transmission electron microscopy.