Chris Boothroyd

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Size and composition tunable Ag?Au alloy nanoparticles by replacement reactions

Ag?Au alloy nanoparticles with tunable size and composition were prepared by a replacement reaction between Ag nanoparticles and HAuCl4 at elevated temperatures. The formation of homogeneous alloy nanoparticles was confirmed by selected-area energy-dispersive x-ray spectroscopy (SAEDX), UV?visible absorption spectroscopy, high resolution transmission electron microscopy (HRTEM) and electron diffraction. This method leverages upon the rapid interdiffusion of Ag and Au atoms in the reduced dimension of a nanoparticle, elevated temperatures and the large number of vacancy defects created in the replacement reaction. This method of preparation has several notable advantages: (1)?independent tuning of the size and composition of alloy nanoparticles; (2)?production of alloy nanoparticles in high concentrations; (3)?general utility in the synthesis of alloy nanoparticles that cannot be obtained by the co-reduction method.

Grain refinement by Al–Ti–B alloys in aluminium melts: a study of the mechanisms of poisoning by zirconium

AbstractConventional grain refining tests confirm that the presence of zirconium in commercial purity aluminium melts dramatically reduces the effectiveness of Al–Ti–B grain refiners. Quantitative comparisons suggest that this poisoning action cannot be attributed solely to reduction in growth restriction arising from changed solute contents in the melt. Microscopic analysis of TiB2 particles in an aluminium based, zirconium containing metallic glass shows the potential for substitution of zirconium for titanium. Analysis of a conventional Al–5 wt-%Ti–1 wt-%B refiner doped with zirconium shows the potential for similar modification of Al3 Ti. Thus zirconium can affect the nucleation stage of grain refinement by substituting for titanium in the aluminide and/or the boride phase, consistent with earlier work suggesting that the key to the nucleation potency is the existence of a layer of Al3 Ti on TiB2 particles.

Why don't high-resolution simulations and images match?

Computer simulations have been used for many years to understand experimental high‐resolution electron microscope images in a qualitative fashion, but the trend nowadays has been to attempt more quantitative image matching. This has led to the discovery that the contrast in experimental images is much less than in simulated images, typically by a factor of about three. There are many possible causes for this discrepancy, ranging from the mechanisms of scattering of electrons by the specimen through the calculations of the diffracted beam intensities and their focusing by the objective lens to the point spread function of the recording device. No single cause can explain all of the experimental contrast loss, although a combination of many factors could.

Specimen preparation methods for the examination of surfaces and interfaces in the transmission electron microscope

Various techniques for the preparation of cross‐sectional and plan view TEM specimens of surfaces and interfaces are described. Particular emphasis is given to preparative methods which are both generally applicable and which minimize differential thinning of the materials present on either side of the interface of interest, thereby improving the reliability of the approach.

Synthesis of Ag@AgAu Metal Core/Alloy Shell Bimetallic Nanoparticles with Tunable Shell Compositions by a Galvanic Replacement Reaction

The fabrication of metallic nanostructures with controllable shapes and sizes is important in delivering the promise of sizeand shape-tunable properties of nanomaterials. The properties of nanometals can be further modified by incorporating more than one metallic component into a common particle to form, for example, a bimetallic nanoparticle. It has been confirmed that co-operative and synergistic interactions between the metallic components could lead to an overall more useful functionality. The bimetallic nanoparticles may be fabricated as alloy nanoparticles where the two constituent metals are mixed at the atomic level or as core/shell nanoparticles where the two components are separated by distinct phase boundaries. It is known that bimetallic nanoparticles with the same overall composition but different composition distributions can exhibit different properties. The geometric distribution of the metals within a particle between the two extremes of alloy (maximallymixed) and core/ shell (minimally mixed) nanoparticles may therefore be used to increase the versatility in property tuning. In an effort to extend the envelope of possibility further, we will demonstrate the synthesis of a new type of core/shell nanostructure where the core is a pure metal and the shell is the alloy of two metals with adjustable compositions, using the Ag–Au system as an example. The novel Ag@AgAu bimetallic nanoparticles were produced by the replacement reaction between Ag nanoparticles and HAuCl4. [19–24] The replacement reaction between Ag nanoparticles and common Au precursor salts has been used by several groups to produce hollow nanostructures in the

Morphological and structural characteristics of homoepitaxial GaN grown by metalorganic chemical vapour deposition (MOCVD)

MOCVD-grown GaN on the N-polar surface of GaN substrates has been found to exhibit gross hexagonal pyramidal features (typically 10}50 lm in size depending on layer thickness). The evolution of the pyramidal defects is dominated by the growth rate of an emergent core of inversion domain (typically 100 nm in size). The inversion domains nucleate at a thin band of oxygen containing amorphous material (2}5 nm in thickness), being remnant contamination from the mechano-chemical polishing technique used to prepare the substrates prior to growth. Apart from pyramidal hillocks, the #at-topped hillocks are also formed. The arguments are presented on the association between these features and the core dislocations, which constitute the source of the growth steps. Improvement in the substrate polishing procedures allowed the e!ective elimination of these surface hillocks. ( 1999 Elsevier Science B.V. All rights reserved.

Hexagonal close-packed Ni nanostructures grown on the (001) surface of MgO

We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed (hcp) form which we believe is stabilized by heteroepitaxial growth on the (001) face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value (about 2.5 nm thick with a lateral size of ∼5nm). The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c∕a ratio (∼6% larger than ideal hcp) is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.

Novel fabrication method for nanometer‐scale silicon dots and wires

We have discovered that a thin film of SiO2 can be directly reduced to Si under electron beam irradiation. The application of this effect to the fabrication of nanometer‐sized Si dots and wires is demonstrated. In particular, if SiO2 is irradiated with a high intensity 100 keV electron beam of nanometer scale, then a column of Si is formed which can be as small as 2 nm in diameter. If the beam is moved in a straight line, then a very thin wire of Si is formed. These columns and wires are formed directly under electron irradiation with a dose of ≥3×109 C m−2 and no resists or chemical development are required.

Synthesis of Cu2SnSe3 Nanocrystals for Solution Processable Photovoltaic Cells

This paper describes the synthesis of ternary chalcogenide Cu(2)SnSe(3) nanocrystals as an alternative solar absorber material to conventional quaternary CuIn(x)Ga(1-x)Se(2). We used the hot coordination solvent method with hexadecylamine as the capping ligand for the first time for this material system. Using a variety of characterization techniques, such as X-ray diffraction, selected area electron diffraction, convergent beam electron diffraction, and Raman spectroscopy, the nanocrystals were found to be monoclinic Cu(2)SnSe(3) with an optical energy band gap of 1.3 eV and have a narrow size distribution. These nanocrystals are shown to be photosensitive in the range of wavelengths corresponding to the solar spectrum, which makes them highly promising as alternative photon absorber materials for photovoltaic applications.