Shuojin Hang

Fabrication and ab initio study of downscaled graphene nanoelectronic devices

In this paper we first present a new fabrication process of downscaled graphene nanodevices based on direct milling of graphene using an atomic-size helium ion beam. We address the issue of contamination caused by the electron-beam lithography process to pattern the contact metals prior to the ultrafine milling process in the helium ion microscope (HIM). We then present our recent experimental study of the effects of the helium ion exposure on the carrier transport properties. By varying the time of helium ion bombardment onto a bilayer graphene nanoribbon transistor, the change in the transfer characteristics is investigated along with underlying carrier scattering mechanisms. Finally we study the effects of various single defects introduced into extremely-scaled armchair graphene nanoribbons on the carrier transport properties using ab initio simulation.

Unipolar conduction induced by defects in graphene nanowire field effect transistors

We present results on electronic transport in disordered graphene nanowire field effect transistors. We show the emergence of unipolar transport where increasing defects density yield almost an insulating behaviour in n-type graphene whilst showing a metallic behaviour in p-type graphene. It is also shown that the Fermi level is pinned at the Dirac point and the conductivity of n-type graphene is pinned to the minimum conductivity at the Dirac point.

Irradiation induced tunnel barrier in side-gated graphene nanoribbon

We investigated a method of forming tunnel barriers in monolayer graphene nanoribbon (GNR) using controlled ion irradiation. By using a helium ion microscope (HIM), we are able to reduce the width of exposure area down to 5nm. Source-drain conductance of side-gated GNR has been measured and the gate capacitances were extracted.

Metal-Insulating transition in disordered graphene nanoribbons controlled by helium ion irradiation

We report a mobility transition in graphene nanoribbons subject to disorder induced by irradiating helium ions. At small irradiation doses defects are created in graphene, enhancing scattering and decreasing the mobility. As the dose is increased further an abrupt transition into an insulating phase is observed characterised by a decrease in electron mobility by more than 5 orders of magnitude. Transmission Electron Microscopy images reveal that heavily He+ irradiated graphene loses its crystallinity as carbon atoms are dislodged from their position by incident ions and reconstruct into an insulating, topologically disordered two-dimensional sp carbon network, where Anderson localisation may possibly play a major role.

Downscaled graphene nanodevices: Fabrication and ab initio study

In this paper we first present a new fabrication process of downscaled graphene nanodevices based on direct milling of graphene using an atomic-size He+ ion beam. We then study the effects of the He+ ion exposure on the carrier transport properties in a bilayer graphene nanoribbon (GNR) by varying the time of He ion bombardment, along with underlying carrier scattering mechanisms. Finally we study the effects of various point defects in extremely-scaled GNRs on the carrier transport properties using ab initio simulation.