ANFO vapour detection with conducting polymer percolation network sensors and GC/MS.

Ammonium nitrate mixed with fuel oil (ANFO) is commonly used in improvised explosive devices (IEDs). The development of ANFO vapour sensors that are small, inexpensive, and easy to use will enable widespread IED detection in the context of security and humanitarian demining. Because of concealment and the low vapour pressures of most explosive materials, achieving sufficiently high sensitivity and low limits of detection are some of the main challenges of explosives vapour detection. Here ANFO chemiresistive vapour sensors based on polypyrrole (PPy) percolation networks are presented and compared to gas chromatography-mass spectroscopy (GC/MS) results for ANFO. Improved sensitivities are achieved by using a polymer percolation network instead of a thin film for the gas sensors. Vapour concentrations are detected of 13-180 ppb of ammonia emitted by a variety of different ammonium nitrate-containing fertilisers and fertiliser-diesel mixtures.

Shapes of epitaxial gold nanocrystals on SrTiO3 substrates.

Morphological control of gold nanocrystals is important as their catalytic and optical properties are highly shape dependent. In this paper we report the shapes of gold nanocrystals which deviate from the equilibrium Wulff shape due to the influence of the SrTiO3 single crystal substrates. The gold crystals are characterized by scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The nanocrystals have an equilibrium shape of a truncated octahedron with {111} and {001} facets. On all three substrate surfaces, i.e., SrTiO3(001)-(2 × 1), SrTiO3(001)-c(4 × 2), and SrTiO3(111)-(4 × 4) + (6 × 6), the height-to-width ratio of the gold crystals is not a constant as would be expected for equilibrium crystals, but instead it increases with crystal height. We propose that as the crystals increase in size, their aspect ratio heightens to relax the interfacial strain. The ratio between the {111} and {001} surface areas of our gold crystals is found to differ on the three substrates, which we speculate is due to the selective adsorption of surfactants on the {111} and {001} gold facets resulting from the different substrate surfaces. Reentrant facets of gold crystals that should be present according to their Wulff shape are not observed because these concave sites typically grow out due to kinetic considerations. This study demonstrates the significant effect of the crystal facet termination and surface reconstruction of an oxide substrate on the shape of supported gold nanocrystals.

Magnetic Iron Oxide Nanowires Formed by Reactive Dewetting.

The growth and reactive dewetting of ultrathin films of iron oxides supported on Re(0001) surfaces have been imaged in situ in real time. Initial growth forms a nonmagnetic stable FeO (wüstite like) layer in a commensurate network upon which high aspect ratio nanowires of several microns in length but less than 40 nm in width can be fabricated. The nanowires are closely aligned with the substrate crystallography and imaging by X-ray magnetic circular dichroism shows that each contain a single magnetic domain. The driving force for dewetting appears to be the minimization of strain energy of the Fe3O4 crystallites and follows the Tersoff and Tromp model in which strain is minimized at constant height by extending in one epitaxially matched direction. Such wires are promising in spintronic applications and we predict that the growth will also occur on other hexagonal substrates.

Scanning tunnelling microscopy of epitaxial nanostructures.

Epitaxial nanostructures have generated a great deal of interest because of the applications in catalysis, photonics and nanoelectronics. To study the structure and electronic properties at the nanoscale, scanning tunnelling microscopy (STM) has proven a very effective technique due to its extraordinarily high spatial resolution. Growth modes of epitaxial nanostructures depend predominantly on the surface free energy of the deposited material, and that of the substrate onto which it is deposited, leading to layer-by-layer or island growth modes. The strain due to lattice mismatch plays an important role in the formation of semiconductor quantum dot islands via strain-induced transitions in the morphology of epitaxial nanoislands. Examples of the different growth modes observed with STM are presented in this review within a general framework that uses the surface and strain energies to understand the effects that govern nanostructure shapes. Some self-assembled oxide and metal nanostructures, as well as molecular networks, are also discussed.

Formation mechanism for a hybrid supramolecular network involving cooperative interactions.

A novel mechanism of hybrid assembly of molecules on surfaces is proposed stemming from interactions between molecules and on-surface metal atoms which eventually got trapped inside the network pores. Based on state-of-the-art theoretical calculations, we find that the new mechanism relies on formation of molecule-metal atom pairs which, together with molecules themselves, participate in the assembly growth. Most remarkably, the dissociation of pairs is facilitated by a cooperative interaction involving many molecules. This new mechanism is illustrated on a low coverage Melamine hexagonal network on the Au(111) surface where multiple events of gold atoms trapping via a set of so-called "gate" transitions are found by kinetic Monte Carlo simulations based on transition rates obtained using ab initio density functional theory calculations and the nudged elastic band method. Simulated STM images of gold atoms trapped in the pores of the Melamine network predict that the atoms should appear as bright spots inside Melamine hexagons. No trapping was found at large Melamine coverages, however. These predictions have been supported by preliminary STM experiments which show bright spots inside Melamine hexagons at low Melamine coverages, while empty pores are mostly observed at large coverages. Therefore, we suggest that bright spots sometimes observed in the pores of molecular assemblies on metal surfaces may be attributed to trapped substrate metal atoms. We believe that this type of mechanism could be used for delivering adatom species of desired functionality (e.g., magnetic) into the pores of hydrogen-bonded networks serving as templates for their capture.

Polypyrrole Percolation Network Gas Sensors: Improved Reproducibility through Conductance Monitoring during Polymer Growth.

Conducting-polymer-based electrical percolation networks are promising materials for use in high-sensitivity chemiresistive devices. An ongoing challenge is to create percolation networks that have consistent properties, so that devices based on these materials do not have to be individually calibrated. Here, an in situ conductance technique is used during the electrochemical growth of polypyrrole (PPy) percolation networks. The drain current (i d) across the interdigitated electrodes (IDEs) is a measure of the conductance of the PPy network during electrochemical polymerization. The i d curve is used to determine the percolation region. To improve the reproducibility of PPy percolation networks, an in situ conductance monitoring method based on the value of i d is used. A set of optimal ammonia gas percolation sensors was created using this method with an average sensitivity of ΔR/R 0 × 100% ppm-1 = 11.3 ± 1.2% ppm-1 and an average limit of detection of 15.0 ± 3.6 ppb.

A homologous series of structures on the surface of SrTiO3(110).

Strontium titanate is seeing increasing interest in fields ranging from thin-film growth to water-splitting catalysis and electronic devices. Although the surface structure and chemistry are of vital importance to many of these applications, theories about the driving forces vary widely. We report here a solution to the 3 x 1 SrTiO(3)(110) surface structure obtained through transmission electron diffraction and direct methods, and confirmed through density functional theory calculations and scanning tunnelling microscopy images and simulations, consisting of rings of six or eight corner-sharing TiO(4) tetrahedra. Further, by changing the number of tetrahedra per ring, a homologous series of n x 1 (n > or = 2) surface reconstructions is formed. Calculations show that the lower members of the series (n < or = 6) are thermodynamically stable and the structures agree with scanning tunnelling microscopy images. Although the surface energy of a crystal is usually thought to determine the structure and stoichiometry, we demonstrate that the opposite can occur. The n x 1 reconstructions are sufficiently close in energy for the stoichiometry in the near-surface region to determine which reconstruction is formed. Our results indicate that the rules of inorganic coordination chemistry apply to oxide surfaces, with concepts such as homologous series and intergrowths as valid at the surface as they are in the bulk.

Surface and defect structure of oxide nanowires on SrTiO3.

Processing the SrTiO(3)(001) surface results in the self-assembly of reduced titanate nanowires whose widths are approximately 1 nm. We have imaged these nanowires and their defects at elevated temperatures by atomic resolution scanning tunneling microscopy. The nanowire structure is modeled with density functional theory, and defects observed in the center of the nanowire are determined to be Ti(4)O(3) vacancy clusters. The activation energy for Ti(4)O(3) vacancy cluster diffusion is explicitly measured as 4.98±0.17 eV with an exponential prefactor of µ=6.57×10(29) (s(-1).

PEDOT percolation networks for reversible chemiresistive sensing of NO2.

Detection of NO2 plays an important role in various safety applications. However, sensitive and reversible sensing of NO2 remains a challenge. Here we demonstrate the use of poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer percolation networks for chemiresistive sensing of NO2. By adjusting the electrochemical polymerisation and doping conditions of the polymer, we show control over the relative contributions of oxidised and over-oxidised PEDOT to the sensing behaviour. Reversible NO2 sensors using only PEDOT as the sensor material are demonstrated. By operating the sensor near the electrical percolation threshold, a higher sensitivity is achieved compared to more traditional thin film based chemiresistive sensors. A limit of detection of 907 ± 102 ppb was achieved.

Endohedral metallofullerenes in self-assembled monolayers.

A method has been developed for the attachment of a dithiolane group to endohedral metallofullerenes via a 1,3-dipolar cycloaddition reaction. This sulfur-containing functional group serves as an anchor, enabling efficient immobilisation of endohedral fullerenes on Au(111) surfaces at room temperature, directly from the solution phase. The functionalised fullerenes form disordered monolayers that exhibit no long-range ordering, which is attributed to both the strong bonding of the dithiolane anchor to the surface and to the conformational flexibility of the functional group. Endohedral fullerenes Er(3)N@C(80) and Sc(3)N@C(80) have been used as models for functionalisation and subsequent surface deposition. Their chemical reactivity towards dithiolane functionalisation and their surface behaviour have been compared to that of C(60). The endohedral fullerenes appear to be significantly less reactive towards the functionalisation than C(60), however they bind in a similar manner to a gold surface as their dithiolane terminated C(60) counterparts. The optical activity of Er(3)N@C(80) molecules is preserved after attachment of the functional group. We report a splitting of the endohedral Er(3+) emission lines due to the reduction in symmetry of the functionalised fullerene cage, as compared to the highly symmetrical icosahedral C(80) cage of pristine Er(3)N@C(80).

Influence of the support on stabilizing local defects in strained monolayer oxide films.

Two-dimensional materials with a honeycomb lattice, such as graphene and hexagonal boron nitride, often contain local defects in which the hexagonal elements are replaced by four-, five-, seven-, and eight-membered rings. An example is the Stone-Wales (S-W) defect, where a bond rotation causes four hexagons to be transformed into a cluster of two pentagons and two heptagons. A further series of similar defects incorporating divacancies results in larger structures of non-hexagonal elements. In this paper, we use scanning tunneling microscopy (STM) and density functional theory (DFT) modeling to investigate the structure and energetics of S-W and divacancy defects in a honeycomb (2 × 2) Ti2O3 monolayer grown on an Au(111) substrate. The epitaxial rumpled Ti2O3 monolayer is pseudomorphic and in a state of elastic compression. As a consequence, divacancy defects, which induce tension in freestanding films, relieve the compression in the epitaxial Ti2O3 monolayer and therefore have significantly lower energies when compared with their freestanding counterparts. We find that at the divacancy defect sites there is a local reduction of the charge transfer between the film and the substrate, the rumpling is reduced, and the film has an increased separation from the substrate. Our results demonstrate the capacity of the substrate to significantly influence the energetics, and hence favor vacancy-type defects, in compressively strained 2D materials. This approach could be applied more broadly, for example to tensile monolayers, where vacancy-type defects would be rare and interstitial-type defects might be favored.

A chiral pinwheel supramolecular network driven by the assembly of PTCDI and melamine.

The mixing of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (melamine) at room temperature in a ratio of 3 : 4 on Au(111) leads to the formation of a new chiral "pinwheel" structure.

Grating of single Lu@C(82) molecules using supramolecular network.

A supramolecular grating of single Lu@C(82) molecules was obtained by depositing Lu@C(82) molecules onto a room temperature PTCDI-melamine network.

Maximising the resolving power of the scanning tunneling microscope.

The usual way to present images from a scanning tunneling microscope (STM) is to take multiple images of the same area, to then manually select the one that appears to be of the highest quality, and then to discard the other almost identical images. This is in contrast to most other disciplines where the signal to noise ratio (SNR) of a data set is improved by taking repeated measurements and averaging them. Data averaging can be routinely performed for 1D spectra, where their alignment is straightforward. However, for serial-acquired 2D STM images the nature and variety of image distortions can severely complicate accurate registration. Here, we demonstrate how a significant improvement in the resolving power of the STM can be achieved through automated distortion correction and multi-frame averaging (MFA) and we demonstrate the broad utility of this approach with three examples. First, we show a sixfold enhancement of the SNR of the Si(111)-(7 × 7) reconstruction. Next, we demonstrate that images with sub-picometre height precision can be routinely obtained and show this for a monolayer of Ti2O3 on Au(111). Last, we demonstrate the automated classification of the two chiral variants of the surface unit cells of the (4 × 4) reconstructed SrTiO3(111) surface. Our new approach to STM imaging will allow a wealth of structural and electronic information from surfaces to be extracted that was previously buried in noise.

C70 ordering on nanostructured SrTiO3(001).

The nanostructured (7 x 4) surface of SrTiO(3)(001) is used as a template to order C(70) into single-molecule-wide chains and linear islands.

Ordering of TiO2-based nanostructures on SrTiO3(001) surfaces.

A class of nanostructured surface phases on SrTiO3(001) is reported and characterized through atomic-resolution scanning tunneling microscopy and Auger electron spectroscopy. These surface phases are created via argon ion sputtering and UHV annealing and form close-packed domains of highly ordered nanostructures. Depending on the type of nanostructures present, the domain ordering exhibit either (6 x 2), (9 x 2), (12 x 2), (6 x 8), or (7 x 4) surface patterning. The nanostructures are composed of TiO2-derived complexes surrounded by a TiO2 surface termination. Such surface ordering phenomena introduce another level of complexity in the chemistry of perovskite oxide surfaces and provide a basis from which potential photocatalytic and molecular-ordering applications may be developed.

Encapsulated Pd nanocrystals supported by nanoline-structured SrTiO3(001).

Palladium nanocrystals were grown on a nanostructured SrTiO(3)(001) surface and annealed in ultrahigh vacuum at 620 degrees C. This leads to the so-called strong metal-support interaction (SMSI) state, characterized by encapsulation of the metal clusters with an oxide layer. Scanning tunneling microscopy (STM) of the oxide adlayer on the Pd(111) cluster surface reveals two superstructures with different lattice parameters and crystallographic rotations. Interpretation of the STM images is most readily achieved via noncommensurate TiO(x)() surface layers which result in two distinct Moiré patterns.

The effect of the size of surface Pd island ensembles on electron transfer of adsorbed perchlorate ions on Au(111).

A progressive increase in the size of Pd ensembles on a mica-supported Au(111) single crystal surface can facilitate electron transfer of perchlorate ions at lower anodic potential in CV curves than pure Au(111) due to a strong ligand effect and Pd-Au neighbouring pairs at edge sites render a higher degree of electron transfer.

Controlled growth of Ni nanocrystals on SrTiO(3) and their application in the catalytic synthesis of carbon nanotubes.

Truncated pyramid-shaped Ni nanocrystals were epitaxially grown on SrTiO(3)(001) surfaces and characterised by scanning tunneling microscopy (STM). These nanocrystals were shown to be catalytically active for the synthesis of carbon nanotubes (CNTs). The narrow size distribution of the Ni nanocrystals results in a similar narrow distribution of CNT diameters.

Synthesis of epitaxial metal oxide nanocrystals via a phase separation approach.

Perovskite phase instability of BiMnO3 has been exploited to synthesize epitaxial metal oxide magnetic nanocrystals. Thin film processing conditions are tuned to promote the breakdown of the perovskite precursor into Bi2O3 matrix and magnetic manganese oxide islands. Subsequent cooling in vacuum ensures complete volatization of the Bi2O3, thus leaving behind an array of self-assembled magnetic Mn3O4 nanostructures. Both shape and size can be systematically controlled by the ambient oxygen environments and deposition time. As such, this approach can be extended to any other Bi-based complex ternary oxide system as it primarily hinges on the breakdown of parent Bi-based precursor and subsequent Bi2O3 volatization.