Jul 25, 2013
Helium-ion microscopy of graphene: from edge definition to controllable modification
Microscopy specialists based at Trinity College Dublin have investigated both controllable modification and sub-nanometre metrology of nanomaterials using a helium-ion beam, with graphene used as an example. The researchers found that the sub-nanometre He+ probe facilitates controllable defect production with extremely high spatial resolution. They have also established the ion doses required to safely image graphene and clarified the effect of a sample support on the results.
The benefits of using a helium-ion microscope (HIM) as a high-resolution imaging tool have been demonstrated by several labs. While HIM is expected to address challenges encountered in nanoscale metrology, the invasiveness of the observation, especially for fairly sensitive samples like graphene, has been raised as an issue. A destructive method for a confined scale characterization, on the other hand, may uncover potential for precise modification. For example, it has been demonstrated that HIM has the capacity to accurately and directly tailor graphene.
Any measurement will inevitably introduce a certain amount of disturbance to the material. In the work, the scientists present the relationship between the HIM characterization and the subsequent damage. With this information, a balance between the tolerable level of damage for applications and the information required from the characterization can then be obtained.
Their investigation establishes the parameters that can be used to image with minimal sample modification, as well as the extent of the modification of the sample during higher dose imaging.
Greater edge contrast
The unique, enhanced edge sensitivity of helium-ion microscopy is evidenced by comparison with the more widely established SEM and TEM techniques. The strong dependence of secondary electron yield on the incidence angle provides greater edge contrast in HIM than the other techniques investigated. This effect, coupled with the efficient generation of secondary electrons by the sub-nanometre helium-ion beam, makes helium-ion microscopy ideally suited to graphene imaging.
The extent of the beam-induced sample modification can be tuned and exploited by following the guidelines set out in this work. Further steps are being taken by the team to fabricate nano-structures in graphene, as well as a range of other 2D materials using the HIM.
More information can be found in the journal Nanotechnology 24 335702
About the author
The research was carried out by the Ultramicroscopy Group based in the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and the School of Physics at Trinity College Dublin, under the supervision of Professor Hongzhou Zhang. Daniel Fox is a PhD student and carried out the HIM, SEM and TEM experiments and ion-beam simulations. Dr Yangbo Zhou is a postdoctoral researcher and carried out the Raman spectroscopy and produced the CVD graphene samples. Dr Arlene O'Neill is a postdoctoral researcher and produced the graphene flakes. Dr Shishir Kumar is a postdoctoral researcher and provided additional CVD graphene samples.