Research

Phys. Status Solidi B 249, 2515–2518 (2012) DOI: 10.1002/pssb.201200157

Graphene is a two-dimensional material whose electronic and mechanical properties have sparked much research interest. Although graphene is generally perceived as completely flat, wrinkles and folds do form in the material, changing its electrical properties and chemical reactivity. The intentional folding of graphene provides a pathway to tailoring its properties for specific applications.

We have developed a facile method to produce three-dimensional folded structures of graphene over a polymer scaffold using patterned elastomeric stamps. We show that the graphene is continuous across these structures and that this folding of graphene results in an increased surface area in comparison to flat samples. This has potential applications for electronic devices and sensors containing graphene.

Langmuir 28, 13218-13223 (2012)

The ability to control the stability of droplets at fluid interfaces is central to many applications, including mineral and particulate separation techniques and emulsion based technologies such as paints. While much is known about macroscopic droplets the behaviour of micron and nanoscale droplets are less well understood. In particular, standard surface tension values for pure liquids predict that water droplets are unstable at the liquid/air interfaces of many common organic solvents.

Using a novel optical technique we demonstrated the existence of micron size water droplets at these interfaces and provided estimates of their stability. We showed that droplets were stabilized by the presence of a nanoscale solvent layer that reduced the surface energy of the exposed droplet at the solvent/air interface.

J. Phys. Chem. C, October (2012)

Understanding the electronic transport properties of nanoconfined systems under wetting conditions is essential for many applications ranging from molecular nano-junctions to nanoelectronics. The fundamental mechanism of the EC-STM operation in aqueous solution however is still not completely understood.

With the aim of understanding electrochemical scanning tunnel microscopy experiment in aqueous environment we investigate electron transport through ice in the coherent limit. By comparing the decay coefficient for different ice structures and different Au electrode orientations we find that the electron transport occurs via tunneling with almost one-dimensional character. We demonstrate that the slow decay of the current with the ice thickness is largely due to the small effective mass of the conduction electrons and that a finite bias measurement may be capable of sorting polar from non polar interfaces due to the asymmetry of the current-voltage curves for polar interfaces.

Phys. Rev. Lett. 107, 206103 (2012)

By means of ab-initio simulations we provide a comprehensive scenario for hydrogen oxidation reactions at the Ni/YSZ anode of SOFCs. We find that the overall rate limiting step is associated with hydrogen spillover from the metal to the YSZ surface. The simulations have also revealed that in the presence of water chemisorbed at the oxide surface, the active region for H oxidation actually extends beyond the metal/YSZ interface unravelling the role of water partial pressure in the decrease of the polarization resistance observed experimentally.

Optical Materials Express, Vol. 2, Issue 6, pp. 891-892 (2012)

Nanocarbon materials are attractive building blocks for future nanoelectronic and nanooptic devices, because they allow for achieving new degrees of both performance and functionality—a combination unachievable by most conventional materials.

Recent linear and nonlinear spectroscopic and optoelectronic results prove that they have much to offer in the photonics arena.

This feature issue of Optical Materials Express gives a topical and highly interesting overview of the state-of-the-art in the emerging wide-ranging field of nanocarbon photonics research and offers many possibilities for future research. The field spans the entire spectrum from basic materials research to novel materials for laser and optoelectronic device applications.

Physica Status Solidi – Rapid Research Lett. 6, pp. 376 (2012)

Being an all-surface material, the transport properties of graphene are altered by a wide range of interactions with the environment. These interactions can originate from molecular and atomic species adsorbed from the atmosphere on the graphene or from the presence of a specific substrate surface. Such interactions can lead to an unintentional doping but more crucially introduce a variety of charge-carrier scattering centres such as charged impurities, lattice strain or resonant scatterers. Here we report on the development of the fractional quantum Hall state with the filling factor 4/3 in graphene on hydrophobically rendered SiO2 surfaces at 4.2 K. The study demonstrates that this physically fundamental manybody state can be realised by strongly suppressing short-range scattering due to the elimination of localised charged scatterers usually induced by a bare SiO2 surface in contact with graphene.

Specifically, the study reveals that upon reduction of charged scatterers the increase of the ratio of mean-free-path to charge-carrier-separation prevails over the generally believed necessity for an extraordinarily high overall mobility to enable the formation of fractional quantum Hall states in graphene and, thus, provides new insight in the development of the fractional quantum Hall state in relativistic fermion systems.

Biomaterials, Volume 33, Issue 26, September 2012, Pages 6132–6139

Heart disease is a leading cause of death globally. Once damaged by heart attack, cardiac muscle has very little capacity for self-repair and at present there are no clinical treatments available to repair damaged cardiac muscle tissue. This collaboration between the Regenerative Medicine Institute (REMEDI) at the National University of Ireland Galway and CRANN has capitalised on the electrical properties of a widely used nanomaterial to develop cells which may allow the regeneration of cardiac cells.

Our data show that by providing a biomimetic electroactive cue, manipulation of the MSC differentiation pathway can be achieved by harnessing the electrical properties of a carbon nanotube based medium or scaffold. Since proof of principle has been established herein, the biomimetic properties of such a platform can be now exploited even further and tailored for other electroactive environments in the heart, the brain or the spinal cord. Ultimately, this strategy provides an opportunity for future studies in the quest to use CNT and MSCs to promote electroactive tissue repair.

Physical Review B 86, 035318 (2012)

The high stability of the Si(100) surface and the possibility of manipulating and functionalising its properties at the atomic level are opening up new perspectives for a wide range of applications. These range from transistor downscaling, dictated by Moore’s law, to quantum computing. The adsorption of single atoms and small inorganic molecules is a key enabling tool for controlling the passivation, oxidation and epitaxial growth of the surface.

We investigate the charging state of an isolated single dangling bond formed on an unpassivated Si(100) surface with c(4×2) reconstruction, by comparing scanning tunneling microscopy and spectroscopy analysis with density functional theory calculations. The dangling bond is created by placing a single hydrogen atom on the bare surface with the tip of a scanning tunneling microscope. We find that the two configurations corresponding to p- and n-doped samples have different scattering properties and phase shift fingerprints. This might open up interesting perspectives for fabricating a switching device by tuning the doping level or by locally charging the single dangling bond state.

Science, Vol. 331, 2012

Many 3-D compounds exist as stacked layers of 2-D materials. If these 2-D layers could be isolated they could become a diverse source of 2-D crystals whose exotic electric properties and high specific surface areas will be important in a wide range of applications, from sensing to electronics to energy storage.

Graphene is the most well-known layered material; however, others do exist such as: transition metal dichalcogenides (TMDs), transition metal oxides (TMOs) and other 2-D compounds such as boron nitride (BN), molybdenum disulphide (MoS2) and bismuth telluride (Bi2Te3).

Work carried out in CRANN, by Prof. Coleman’s and Prof. Nicolosi's research groups, has resulted in a method of producing 2-D materials in a simple, large-scale and relatively high yield process, where 3-D materials are exfoliated down to very thin 2-D platelets using water and common solvents.

Applications for large yields of high quality 2-D flakes are many and varied; including gas barrier systems, composite materials, energy storage, sensing and electronics.

Nature Nanotechnology, Vol. 4, March 2009

Membrane proteins are the most important target for drug-discovery programmes, with half of all marketed drugs affecting membrane proteins. Finding a way to use these proteins in the detection of viruses in a liquid offers potential for the fast detection of illnesses, hazardous microbes and/or contamination.

Work carried out in CRANN, by Prof. Hegner’s research group, describes the development of a quantitative virus biosensor for protein membranes. This biosensor uses an array of resonating microcantilevers to measure virus interactions under physiological conditions. The sensing technique utilises nanomechanical silicon sensors coated with biomembranes that have been shown to detect the presence of viruses in liquid environments within minutes.

These microcantilever-based sensors are small in size, they only consume tiny amounts of immobilized material and analyte, and they are capable of multiplexed detection. Future applications for this technology are based on faster detection of viruses in liquid which can be exploited in the food, medical, health and pharmaceutical sectors. In the future, development of large-scale, parallel cantilever sensors could be used as a tool for label-free and real-time functional

Phys. Status Solidi B 248, No. 10, 2338–2344 (2011)

There is an increasing demand for magnetic materials that exhibit high perpendicular anisotropy to assure thermal stability in nano-scale spintronic devices. Spin-transfer-torque (STT) memory applications require switching current densities of order 109Am_2 to facilitate reliable switching. A magnetic material which exhibits low magnetization and high perpendicular anisotropy combined with high spin polarization is an ideal combination for STT memories. Magnetic recording applications, on the other hand, require materials that exhibit high magnetization with high perpendicular anisotropy to maintain thermal stability in sub-10 nm bits.

In this research tetragonal Mn3-xGa (0≤x≤1) epitaxial films that possess exceptional magnetic and electronic properties are explored. Stoichiometric Mn3Ga crystallizes in the D022 structure and is a collinear ferrimagnet with an easy c-axis. It exhibits a unique combination of low magnetization, high uniaxial anisotropy, high Curie temperature and high spin polarization, which suit the requirements for spin torque memories down to 10 nm in size. Mn2Ga, on the other hand, exhibits much higher magnetization, high perpendicular anisotropy and high Curie temperature but a lower spin polarization, which make it a potential candidate for high density bit-patterned recording with areal densities up to 10 Tb inch_2 (_15 kbmm_2) and 10-year thermal stability. The flexibility of the D022 structure allows a variety of magnetic materials to be synthesized with varying x to suit specific magnetic applications, additionally hexagonal D019-Mn3Ga films are antiferromagnetic, which could be useful for exchange bias.

The high anisotropy combined with low magnetization and high spin polarization of ferromagnetic Mn3Ga can open ways to realize thermally stable solid state magnetic memories switched by spin torque, which can possibly compete with flash memory.

ACS Nano VOL. 5, NO. 6, 4617–4623 (2011)

Self-organized block copolymer (BCP) systems have been shown to have a host of applications, most notably in the fabrication of inorganic structures used in electronic, optoelectronic, and magnetic devices. Absolute and precise control of pattern orientation (i.e., to the surface plane) is central to their possible use and requires a profound understanding of phase behaviour and structure evolution during post-annealing of the BCP films.

The focus of this research is the demonstration of pattern orientation coupled to pattern alignment being achieved for cylinder forming poly-(styrene-b-ethylene oxide) (PS-b-PEO) films. The research highlights unexpected cyclic transitions with anneal time of the polymer structure between perpendicular and parallel arrangements of microphase separated cylinders in these types of films using in situ time-resolved light scattering data combined with ex-situ time evolution AFM experiments. This is the first time such observations have been reported.

This work is another step forward to understanding the structure evolution and also controlling the alignment of block copolymer nanocylinders independently of thickness and external fields. The research demonstrates continuous control of pattern structure and alignment through solvent exposure and in particular, how orientation of the pattern can be determined independently of the initial film thickness, surface segregation effects, and “solvent fields”.