Sub-5 nm graphene nanopore fabrication by nitrogen ion etching induced by a low-energy electron beam.

A flexible and efficient method to fabricate nanopores in graphene has been developed. A focused, low-energy (5 keV) electron beam was used to locally activate etching of a graphene surface in a low pressure (0.3 Pa) N2 environment. Nanopores with sub-5 nm diameters were fabricated. The lattice structure of the graphene was observed to recover within 20 nm of the nanopore edge. Nanopore growth rates were investigated systematically. The effects of nitrogen pressure, electron beam dwell time and beam current were characterised in order to understand the etching mechanism and enable optimisation of the etching parameters. A model was developed which describes how the diffusion of ionised nitrogen affects the nanopore growth rate. Etching of other two-dimensional materials was attempted as demonstrated with MoS2. The lack of etching observed supports our model of a chemical reaction-based mechanism. The understanding of the etching mechanism will allow more materials to be etched by selection of an appropriate ion species.

Nanopatterning and Electrical Tuning of MoS2 Layers with a Subnanometer Helium Ion Beam.

We report subnanometer modification enabled by an ultrafine helium ion beam. By adjusting ion dose and the beam profile, structural defects were controllably introduced in a few-layer molybdenum disulfide (MoS2) sample and its stoichiometry was modified by preferential sputtering of sulfur at a few-nanometer scale. Localized tuning of the resistivity of MoS2 was demonstrated and semiconducting, metallic-like, or insulating material was obtained by irradiation with different doses of He(+). Amorphous MoSx with metallic behavior has been demonstrated for the first time. Fabrication of MoS2 nanostructures with 7 nm dimensions and pristine crystal structure was also achieved. The damage at the edges of these nanostructures was typically confined to within 1 nm. Nanoribbons with widths as small as 1 nm were reproducibly fabricated. This nanoscale modification technique is a generalized approach that can be applied to various two-dimensional (2D) materials to produce a new range of 2D metamaterials.

Oxide-mediated recovery of field-effect mobility in plasma-treated MoS2.

Precise tunability of electronic properties of two-dimensional (2D) nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS2) has long been thought to be detrimental to the electrical performance of the material. We show that the mobility and conductivity of MoS2 can be precisely controlled and improved by systematic exposure to oxygen/argon plasma and characterize the material using advanced spectroscopy and microscopy. Through complementary theoretical modeling, which confirms conductivity enhancement, we infer the role of a transient 2D substoichiometric phase of molybdenum trioxide (2D-MoO x ) in modulating the electronic behavior of the material. Deduction of the beneficial role of MoO x will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

Ultra-large supramolecular coordination cages composed of endohedral Archimedean and Platonic bodies.

Pioneered by Lehn, Cram, Peterson and Breslow, supramolecular chemistry concepts have evolved providing fundamental knowledge of the relationships between the structures and reactivities of organized molecules. A particular fascinating class of metallo-supramolecular molecules are hollow coordination cages that provide cavities of molecular dimensions promoting applications in diverse areas including catalysis, enzyme mimetics and material science. Here we report the synthesis of coordination cages with exceptional cross-sectional diameters that are composed of multiple sub-cages providing numerous distinctive binding sites through labile coordination solvent molecules. The building principles, involving Archimedean and Platonic bodies, renders these supramolecular keplerates as a class of cages whose composition and topological aspects compare to characteristics of edge-transitive {Cu2} MOFs with A3X4 stoichiometry. The nature of the cavities in these double-shell metal-organic polyhedra and their inner/outer binding sites provide perspectives for post-synthetic functionalizations, separations and catalysis. Transmission electron microscopy studies demonstrate that single molecules are experimentally accessible.

Two-Dimensional Metal Oxide Nanoflower-Like Architectures: A General Growth Method and Their Applications in Energy Storage and as Model Materials for Nanofabrication.

Nanoflower-like architectures represent a unique type of nanomaterials in which thin 2D nanosheets are self-organised into interconnected structures. Lack of restacking between nanosheets and significant internal porosity are the particular advantages of such nanoscale architectures. A general method for the preparation of nanoflowers of a range of oxides (e.g., FeTiO3 , TiO2 , Mn2 O3 ) through a two-step procedure of ball milling and subsequent hydrothermal treatment is outlined. Importantly, the synthetic method is valid not only for a single oxide, but is extendable to a family of oxide materials. It is established that the formation of the nanoflowers from ball-milled powders follows a dissolution-precipitation mechanism; this is confirmed by inductively coupled plasma time of flight mass spectrometry measurements. Additional information on the X-ray photoelectron spectroscopy characterisation and intermediate stage of growth of the nanostructures is included. Furthermore, two applications of Mn2 O3 nanostructures are briefly investigated. Firstly, their properties for energy storage in the electrodes of electrochemical supercapacitors are presented. A capacitive response in the potential window of -0.1-0.9 V versus an Ag/AgCl reference electrode is observed, with an associated increase of the capacitance values over cycling. Secondly, the use of Mn2 O3 nanoflowers as model systems for the development of novel nanofabrication techniques (such as nanopatterning with a He+ beam) is investigated.

Quantitative secondary electron imaging for work function extraction at atomic level and layer identification of graphene.

Two-dimensional (2D) materials usually have a layer-dependent work function, which require fast and accurate detection for the evaluation of their device performance. A detection technique with high throughput and high spatial resolution has not yet been explored. Using a scanning electron microscope, we have developed and implemented a quantitative analytical technique which allows effective extraction of the work function of graphene. This technique uses the secondary electron contrast and has nanometre-resolved layer information. The measurement of few-layer graphene flakes shows the variation of work function between graphene layers with a precision of less than 10 meV. It is expected that this technique will prove extremely useful for researchers in a broad range of fields due to its revolutionary throughput and accuracy.