The Effects of Hydrogen Annealing on Carbon Nanotube Field-Effect Transistors.

We have systematically investigated the effects of hydrogen annealing on Ni- and Al-contacted carbon nanotube field-effect transistors (CNTFETs), whose work functions have not been affected by hydrogen annealing. Measured results show that the electronic properties of single-walled carbon nanotubes are modified by hydrogen adsorption. The Ni-contacted CNTFETs, which initially showed metallic behavior, changed their p-FET behavior with a high on-current over 10 µA after hydrogen annealing. The on-current of the as-made p-FETs is much improved after hydrogen annealing. The Al-contacted CNTFETs, which initially showed metallic behavior, showed unipolar p-FET behavior after hydrogen annealing. We analyzed the energy band diagrams of the CNTFETs to explain experimental results, finding that the electron affinity and the bandgap of single-walled carbon nanotubes changed after hydrogen annealing. These results are consistent with previously reported ab initio calculations.

Superstructures of PbS nanocrystals in a conjugated polymer and the aligning role of oxidation.

We present a method to directly align PbS nanocrystals in micron-sized superstructures within a conjugated polymer. First, lead sulfide nanocrystals are directly synthesized in a MEH-PPV suspension via a single pot, surfactant-free method. Post-synthesis precipitation of the composite solution involving mild oxidation of the nanocrystals results in the formation of nanocrystal-polymer and nanocrystal-oxide superstructures. Detailed TEM is used to study the crystallographic nature of these structures and the roles of polymer and lead sulfate. An epitaxial relationship between lead sulfide and lead sulfate at the nanoscale is shown, giving insight into the oxidation rates of the PbS nanocrystals' facets.

A one-step aqueous synthetic route to extremely small CdSe nanoparticles.

By employing Na2Se as a selenium source, we demonstrate that extremely small ( approximately 1 nm) mercapto acid-stabilized CdSe nanoparticles can be conveniently prepared in water. The as-prepared nanoparticles start to show dominant near band-gap photoluminescence in the blue spectral range and show high photoluminescence in the green spectral range.

A versatile double aberration-corrected, energy filtered HREM/STEM for materials science.

A HREM/STEM incorporating aberration correctors in both the probe-forming and imaging lenses has been installed at Oxford University. This unique instrument is also equipped with an in-column energy-loss (Omega-type) filter, HAADF detectors above and beneath the filter, and an EDX system. Initial tests have shown it to be capable of approximately 0.1 nm resolution in both TEM and HAADF STEM imaging modes. Some examples of applications are finally presented.

Encapsulation of quaternary 1D pentlandite-type alloy crystals within conical multi-layer carbon nanotubes.

Ordered 1D crystals of a complex pentlandite-type alloy with the general composition (Fe,Ni,Co)9S8 have been synthesised inside conical Multi Walled Carbon Nanotubes (MWNTs); the crystals are observed as a by-product of an arc-evaporation synthesis of Double Walled Carbon Nanotubes (DWNTs).

Band gap expansion, shear inversion phase change behaviour and low-voltage induced crystal oscillation in low-dimensional tin selenide crystals.

In common with rocksalt-type alkali halide phases and also semiconductors such as GeTe and SnTe, SnSe forms all-surface two atom-thick low dimensional crystals when encapsulated within single walled nanotubes (SWNTs) with diameters below ∼1.4 nm. Whereas previous density functional theory (DFT) studies indicate that optimised low-dimensional trigonal HgTe changes from a semi-metal to a semi-conductor, low-dimensional SnSe crystals typically undergo band-gap expansion. In slightly wider diameter SWNTs (∼1.4-1.6 nm), we observe that three atom thick low dimensional SnSe crystals undergo a previously unobserved form of a shear inversion phase change resulting in two discrete strain states in a section of curved nanotube. Under low-voltage (i.e. 80-100 kV) imaging conditions in a transmission electron microscope, encapsulated SnSe crystals undergo longitudinal and rotational oscillations, possibly as a result of the increase in the inelastic scattering cross-section of the sample at those voltages.

Continuous hydrothermal synthesis of extensive 2D sodium titanate (Na2Ti3O7) nano-sheets.

A novel and rapid and continuous hydrothermal route to the synthesis of extensive ultra-thin 2D sodium titanate (Na(2)Ti(3)O(7)) nano-sheets using a superheated water flow at 450 degrees C and 24.1 MPa as a crystallizing medium is described. High resolution electron microscopy of the sheets revealed that they were a few layers thick and largely uncurled, highly crystalline despite their very short time under hydrothermal flow conditions. The sodium titanate sheets possessed excellent photocatalytic activity for decolourisation of methylene blue dye.

Optimized HREM imaging of objects supported by amorphous substrates using spherical aberration adjustment.

High Resolution Electron Microscopy (HREM) is often used to characterize objects supported by amorphous substrates, usually amorphous carbon. HREM is currently undergoing step change in performance due to aberration correctors. This paper examines the aberration corrected imaging of objects supported by amorphous substrates. In particular, we show that a substantial increase in the ratio of the object contrast to the substrate contrast can be achieved by utilizing the strong variation of phase contrast with height, which is present when the spherical aberration has been adjusted to a small value. This variation is examined using the familiar Weak Phase Object Approximation model from which it is determined that the contrast ratio achieves a maximum at a small nonzero value of the spherical aberration. This result is confirmed by multislice modelling which allows for deviations from the Weak Phase Object Approximation and delocalization effects. One important practical result of this study is the need to place the object of interest on the correct side of the amorphous carbon substrate.

Correlation of structural and electronic properties in a new low-dimensional form of mercury telluride.

Using high resolution electron microscopy and first principles quantum mechanical calculations we have explored the fundamental physics and chemistry of the semiconductor, HgTe grown inside single wall carbon nanotubes. This material forms a low-dimensional structure based on a repeating Hg2Te2 motif in which both atom species adopt new coordination geometries not seen in the bulk. Density-functional theory calculations confirm the stability of this structure and demonstrate conclusively that it arises solely as a consequence of constrained low dimensionality. This change is directly correlated with a modified electronic structure in which the low-dimensional form of HgTe is transformed from a bulk semimetal to a semiconductor.

Growing and characterizing one-dimensional crystals within single-walled carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) have been used as growth templates for spatially confined crystal growth. The comparative crystallization and high-resolution transmission electron microscopy imaging properties of simple binary halides formed by the alkali iodides MI (M = Li, K, Na, Rb and Cs) within SWNTs are described. The most common structure type observed within SWNTs was the rocksalt archetype, although CsI was observed to form both body-centred cubic (bcc) and rocksalt structure types. ThCl4 was found to form a chain structure of Th[Cl]8 polyhedra. HgI2 crystallized within nanotubes with ultra-narrow (i.e. 0.8 nm) capillaries was observed to form helical 2 x 1 layer crystals.

Structural characterization of atomically regulated nanocrystals formed within single-walled carbon nanotubes using electron microscopy.

The structural chemistry of nanoscale materials encapsulated within single-walled carbon nanotubes (SWNTs) is reviewed. SWNTs form atomically thin channels within a restricted diameter range, and their internal van der Waals surfaces regulate the growth behavior of encapsulated crystals in a precise fashion, leading to atomically regulated growth. The structural properties of these systems are largely dictated by the structural chemistry of the bulk material, although significant deviations from bulk structures are often observed, with lower surface coordinations and substantial lattice distortions.

Characterization of magnetic nanoparticles using energy-selected transmission electron microscopy.

Fe, Co, and Ni magnetic nanoparticles have been characterized using energy-selected imaging in a high-resolution transmission electron microscope. The samples comprised Fe/FeO x and Co/CoO x nanoparticles synthesized by inert gas evaporation and a Ni/C nano-composite prepared by a sonochemical method. All of the particles examined were found to be between 5 and 30 nm in size, with the Fe and Co crystals coated in 5-10 nm of metal oxide layer and the Ni metallic crystallites embedded in an amorphous carbon spherical matrix.

Metastable one-dimensional AgCl(1)-(x)I(x) solid-solution wurzite "tunnel" crystals formed within single-walled carbon nanotubes.

High-resolution transmission electron microscopy and spatially resolved electron loss spectroscopy have revealed that a eutectic mixture of AgCl and AgI crystallizes within single walled carbon nanotubes (SWNTs) as metastable AgCl(1-)(x)I(x) 1D solid solution crystals. The incorporated halide crystals form wurzite "tunnel" structures with locally varying Cl:I ratios and reduced Ag coordination.

An encapsulated helical one-dimensional cobalt iodide nanostructure.

Single-walled carbon nanotubes (SWNTs) can be used as templates for the growth of low-dimensional inorganic materials whose structures and properties often differ greatly from those of the bulk. Here we describe the detailed crystallography of an entire helical one-dimensional cobalt diiodide nanostructure encapsulated within a SWNT. This material has an unprecedented twisted double tetrahedral chain structure arising from a rotation of Co(2)I(4) units along its length. The complete nanostructure comprises two distinct regions with oppositely handed helices separated by a short disordered region. The encapsulating SWNT shows a commensurate ovoid distortion reflecting an unexpectedly strong interaction between the nanostructure and the SWNT.