A circuit model for defective bilayer graphene transistors

This paper investigates the behaviour of a defective single-gate bilayer graphene transistor. Point defects were introduced into pristine graphene crystal structure using a tightly focused helium ion beam. The transfer characteristics of the exposed transistors were measured ex-situ for different defect concentrations. The channel peak resistance increased with increasing defect concentration whilst the on–off ratio showed a decreasing trend for both electrons and holes. To understand the electrical behaviour of the transistors, a circuit model for bilayer graphene is developed which shows a very good agreement when validated against experimental data. The model allowed parameter extraction of bilayer transistor and can be implemented in circuit level simulators.<br/

Irradiation-based defect engineering of graphene devices

The addition of structural defects modifies the intrinsic properties of graphene–the two dimensional allotrope of carbon. The controlled introduction of such defects is therefore desired to realise specific functions. For instance, the grain boundaries formed between epitaxial grown graphene domains has been observed to mimic a metallic wire. By contrast, the presence of point defects in a graphene channel affects the carrier transport significantly in a manner such as the Fermi-level pinning, transport-gap widening and Anderson localization. Incorporating these defects into conventional device structures can open up a new horizon for device engineering.In this work, I propose and explore the defect engineering of graphene devices via ion bombardment using a helium ion microscope (HIM). The lithographic advantage of HIM is demonstrated for various graphene nanostructures such as fully gated 20nm double quantum dots and 10nm nanoribbons, upon which a hybrid EBL-HIM fabrication technique is developed for device integration. Graphene irradiated with HIM up to 5×1014 cm-2 shows a transition from Stage 1 to Stage 2 disorder as probed by confocal Raman spectroscopy. For the first time, the damage of ion-beam-milling on a graphene-onsubstrate sample is visualised. The spatially resolved Raman map shows that the beam damage can extend to a few hundred nm around the 30nm cut, which is attributed to the damage due to backscattered helium ions and recoils from the substrate.Furthermore, the electrical properties of irradiated graphene nanoribbons (iGNR) is characterised. As irradiation dose increases, the iGNR devices shows an abrupt decrease in mobility and interestingly an asymmetric decrease of conductance in the electron and hole conduction branches. This is then related to the pinning of Fermi level in iGNR, a unique property caused by irradiation. This is believed to be associated with additional dangling bonds (scattering centres) created by irradiation, as supported by XPS analysis. Based on these properties, a new graphene device structure is explored, in which irradiated regions are used as energy barriers. The temperature-dependent conductance shows the signature of thermal-activated variable range hopping (VRH) at intermediate temperature. The localisation lengths extracted from hopping temperature showed good agreement with that from length-dependent conductance. Furthermore, the activationless VRH is observed for relatively high electric field

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

Cobalt(II)-Catalyzed Aerobic Oxidation of Terminal-Capped Alkynyl α‑Cyano Alkanone Systems. An Oxygen-Mediated Radical Chain Reaction

A new <i>N</i>-hydroxyphthalimide (NHPI)/Co­(II)-catalyzed protocol, mechanistically involving a sequence of α-hydrogen abstraction, 5-<i>exo</i>-<i>dig</i> cyclization, oxygen capture, hydrogen transfer, and 1,4-dehydration, has been developed to facilitate aerobic oxidation of aryl-, silyl-, and alkyl-capped alkynyl α-cyano alkanone systems to the corresponding highly functionalized products in an effective manner, thus turning this novel chain reaction, originally occurring spontaneously in low yields, into a practical methodology