Nanoscale electronic properties of conjugated polymers and nanocrystals

The objective of this thesis is the exploration and characterisation of the nanoscale electronic properties of conjugated polymers and nanocrystals. In Chapter 2, the first application of conducting-probe atomic force microscopy (CP-AFM)-based displacement-voltage (z-V) spectroscopy to local measurement of electronic properties of conjugated polymer thin films is reported. Charge injection thresholds along with corresponding single particle gap and exciton binding energies are determined for a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] thin film. By performing measurements across a grid of locations on the film, a series of exciton binding energy distributions are identified. The variation in measured exciton binding energies is in contrast to the smoothness of the film suggesting that the variation may be attributable to differences in the nano-environment of the polymer molecules within the film at each measurement location. In Chapter 3, the CP-AFM-based z-V spectroscopy method is extended for the first time to local, room temperature measurements of the Coulomb blockade voltage thresholds arising from sequential single electron charging of 28 kDa Au nanocrystal arrays. The fluid-like properties of the nanocrystal arrays enable reproducible formation of nanoscale probe-array-substrate junctions, allowing the influence of background charge on the electronic properties of the array to be identified. CP-AFM also allows complementary topography and phase data to be acquired before and after spectroscopy measurements, enabling comparison of local array morphology with local measurements of the Coulomb blockade thresholds. In Chapter 4, melt-assisted template wetting is applied for the first time to massively parallel fabrication of poly-(3-hexylthiophene) nanowires. The structural characteristics of the wires are first presented. Two-terminal electrical measurements of individual nanowires, utilising a CP-AFM tip as the source electrode, are then used to obtain the intrinsic nanowire resistivity and the total nanowire-electrode contact resistance subsequently allowing single nanowire hole mobility and mean nanowire-electrode barrier height values to be estimated. In Chapter 5, solution-assisted template wetting is used for fabrication of fluorene-dithiophene co-polymer nanowires. The structural characteristics of these wires are also presented. Two-terminal electrical measurements of individual nanowires indicate barrier formation at the nanowire-electrode interfaces and measured resistivity values suggest doping of the nanowires, possibly due to air exposure. The first report of single conjugated polymer nanowires as ultra-miniature photodetectors is presented, with single wire devices yielding external quantum efficiencies ~ 0.1 % and responsivities ~ 0.4 mA/W under monochromatic illumination.

Synthesis, characterisation and applications of group IV nanocrystals

Group IV materials such as silicon nanocrystals (Si NCs) and carbon quantum dots (CQDs) have received great attention as new functional materials with unique physical/chemical properties that are not found in the bulk material. This thesis reports the synthesis and characterisation of both types of nanocrystal and their application as fluorescence probes for the detection of metal ions. In chapter 2, a simple method is described for the size controlled synthesis of Si NCs within inverse micelles having well defined core diameters ranging from 2 to 6 nm using inert atmospheric synthetic methods. In addition, ligands with different molecular structures were utilised to reduce inter-nanocrystal attraction forces and improve the stability of the NC dispersions in water and a variety of organic solvents. Regulation of the Si NCs size is achieved by variation of the surfactants and addition rates, resulting high quality NCs with standard deviations (σ = Δd/d) of less than 10 %. Large scale production of highly mondisperse Si NC was also successfully demonstrated. In chapter 3, a simple solution phase synthesis of size monodisperse carbon quantum dots (CQDs) using a room temperature microemulsion strategy is demonstrated. The CQDs are synthesized in reverse micelles via the reduction of carbon tetrachloride using a hydride reducing agent. CQDs may be functionalised with covalently attached alkyl or amine monolayers, rendering the CQDs dispersible in wide range of polar or non-polar solvents. Regulation of the CQDs size was achieved by utilizing hydride reducing agents of different strengths. The CQDs possess a high photoluminescence quantum yield in the visible region and exhibit excellent photostability. In chapter 4, a simple and rapid assay for detection of Fe3+ ions was developed, based on quenching of the strong blue-green Si NC photoluminescence. The detection method showed a high selectivity, with only Fe3+ resulting in strong quenching of the fluorescence signal. No quenching of the fluorescence signal was induced by Fe2+ ions, allowing for solution phase discrimination between the same ion in different charge states. The optimised sensor system showed a sensitive detection range from 25- 900 μM and a limit of detection of 20.8 μM

Synthesis, functionalisation and characterisation of germanium nanocrystals & their applications

In the last two decades, semiconductor nanocrystals have been the focus of intense research due to their size dependant optical and electrical properties. Much is now known about how to control their size, shape, composition and surface chemistry, allowing fine control of their photophysical and electronic properties. However, genuine concerns have been raised regarding the heavy metal content of these materials, which is toxic even at relatively low concentrations and may limit their wide scale use. These concerns have driven the development of heavy metal free alternatives. In recent years, germanium nanocrystals (Ge NCs) have emerged as environmentally friendlier alternatives to II-VI and IV-VI semiconductor materials as they are nontoxic, biocompatible and electrochemically stable. This thesis reports the synthesis and characterisation of Ge NCs and their application as fluorescence probes for the detection of metal ions. A room-temperature method for the synthesis of size monodisperse Ge NCs within inverse micelles is reported, with well-defined core diameters that may be tuned from 3.5 to 4.5 nm. The Ge NCs are chemically passivated with amine ligands, minimising surface oxidation while rendering the NCs dispersible in a range of polar solvents. Regulation of the Ge NCs size is achieved by variation of the ammonium salts used to form the micelles. A maximum quantum yield of 20% is shown for the nanocrystals, and a transition from primarily blue to green emission is observed as the NC diameter increases from 3.5 to 4.5 nm. A polydisperse sample with a mixed emission profile is prepared and separated by centrifugation into individual sized NCs which each showed blue and green emission only, with total suppression of other emission colours. A new, efficient one step synthesis of Ge NCs with in situ passivation and straightforward purification steps is also reported. Ge NCs are formed by co-reduction of a mixture of GeCl4 and n-butyltrichlorogermane; the latter is used both as a capping ligand and a germanium source. The surface-bound layer of butyl chains both chemically passivates and stabilises the Ge NCs. Optical spectroscopy confirmed that these NCs are in the strong quantum confinement regime, with significant involvement of surface species in exciton recombination processes. The PL QY is determined to be 37 %, one of the highest values reported for organically terminated Ge NCs. A synthetic method is developed to produce size monodisperse Ge NCs with modified surface chemistries bearing carboxylic acid, acetate, amine and epoxy functional groups. The effect of these different surface terminations on the optical properties of the NCs is also studied. Comparison of the emission properties of these Ge NCs showed that the wavelength position of the PL maxima could be moved from the UV to the blue/green by choice of the appropriate surface group. We also report the application of water-soluble Ge NCs as a fluorescent sensing platform for the fast, highly selective and sensitive detection of Fe3+ ions. The luminescence quenching mechanism is confirmed by lifetime and absorbance spectroscopies, while the applicability of this assay for detection of Fe3+ in real water samples is investigated and found to satisfy the US Environmental Protection Agency requirements for Fe3+ levels in drinkable water supplies.

Universal Quake Statistics: From Compressed Nanocrystals to Earthquakes.

Slowly-compressed single crystals, bulk metallic glasses (BMGs), rocks, granular materials, and the earth all deform via intermittent slips or "quakes". We find that although these systems span 12 decades in length scale, they all show the same scaling behavior for their slip size distributions and other statistical properties. Remarkably, the size distributions follow the same power law multiplied with the same exponential cutoff. The cutoff grows with applied force for materials spanning length scales from nanometers to kilometers. The tuneability of the cutoff with stress reflects "tuned critical" behavior, rather than self-organized criticality (SOC), which would imply stress-independence. A simple mean field model for avalanches of slipping weak spots explains the agreement across scales. It predicts the observed slip-size distributions and the observed stress-dependent cutoff function. The results enable extrapolations from one scale to another, and from one force to another, across different materials and structures, from nanocrystals to earthquakes.

Bimetallic effects in the liquid phase hydrogenation of 2-butanone

A series of bimetallic Ru-containing monometallic and bimetallic catalysts were prepared and tested for their activity for the hydrogenation of 2-butanone to 2-butanol at 30 °C and 3 bar H2. RuPt bimetallic catalysts were the most active for the reaction, with a ratio of 5 wt% Ru:1 wt% Pt on activated carbon (AC) found to be optimum. The activity of this bimetallic catalyst was more than double that of the sum of the activities of the monometallic Ru and Pt catalysts, providing evidence of a “bimetallic” effect. Structural analysis of the bimetallic catalysts revealed that they consisted of clusters of particles of the order of 1–2 nm. Extended X-ray absorption fine structure analysis showed that there were two types of particle on the surface of the bimetallic RuPt catalyst, specifically monometallic Ru and bimetallic RuPt particles. For the bimetallic particles, it was possible to fit the data with a model in which a Ru core of 1.1 nm is enclosed by two Pt-rich layers, the outer layer containing only 13 at% Ru. Pretreatment of the monometallic and bimetallic catalysts in hydrogen had a significant effect on the activity. Both the bimetallic and monometallic Ru-based catalysts showed a trend of decreasing activity with increasing temperature of prereduction in hydrogen. This loss of activity was almost fully reversible by exposure of the catalysts to air after reduction. The changing activity with exposure to different gas phase environments could not be attributed to changes in particle size or surface composition. It is proposed that the introduction of hydrogen results in a gradual smoothing of the surface and loss of defect sites; this process being reversible on introduction of air. These defect sites are particularly important for the dissociative adsorption of hydrogen, potentially the rate-determining step in this reaction.

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.

Catalytic hydrogenation of tertiary amides at low temperatures and pressures using bimetallic Pt/Re-based catalysts

Hydrogenation of tertiary amides, in particular, N-methylpyrrolidin-2-one, can be efficiently facilitated by a TiO(2)-supported bimetallic Pt/Re catalyst at low temperatures and pressures. Characterisation of the catalysts and kinetic tests have shown that the close interaction between the Re and Pt is crucial to the high activity observed. DFT calculations were used to examine a range of metal combinations and show that the role of the uncoordinated Re is to activate the C=O and that of the Pt is as a hydrogenation catalyst, removing intermediates from the catalyst surface. The rate enhancement observed on the TiO(2) support is thought to be due to the presence of oxygen vacancies allowing adsorption and weakening of the C=O bond. (C) 2011 Elsevier Inc. All rights reserved.

Enhancing the Activity and Tuning the Mechanism of Formic acid Oxidation at Tetrahexahedral Pt Nanocrystals by Au Decoration

Tetrahexahedral Pt nanocrystals (THH Pt NCs), bound by high index facets, belong to an emerging class of nanomaterials that promise to bridge the gap between model and practical electrocatalysts. The atomically stepped surfaces of THH Pt NCs are extremely active for the electrooxidation of small organic molecules but they also readily accommodate the dissociative chemisorption of such species, resulting in poisoning by strongly adsorbed CO. Formic acid oxidation is an ideal reaction for studying the balance between these competing catalyst characteristics, since it can proceed by either a direct or a CO mediated pathway. Herein, we describe electrochemical and in situ FTIR spectroscopic investigations of formic acid electrooxidation at both clean and Au adatom modified THH Pt NC surfaces. The Au decoration leads to higher catalytic currents and enhanced CO2 production in the low potential range. As the CO oxidation behaviour of the catalyst is not changed by the presence of the Au, it is likely that the role of the Au is to promote the direct pathway. Beyond their fundamental importance, these results are significant in the development of stable, poison resistant anodic electrocatalysts for direct formic acid fuel cells.

Epitaxial Magnetic Oxide Nanocrystals Via Phase Decomposition of Bismuth Perovskite Precursors

The phase instability of bismuth perovskite (BiMO₃), where M is a ferromagnetic cation, is exploited to create self-assembled magnetic oxide nanocrystal arrays on oxide supports. Conditions during pulsed laser deposition are tuned so as to induce complete breakdown of the perovskite precursor into bismuth oxide (Bi₂ O₃ ) and metal oxide (M-O_x ) pockets. Subsequent cooling in vacuum volatizes the Bi₂ O₃ leaving behind an array of monodisperse nanocrystals. In situ reflective high energy electron diffraction beam is exploited to monitor the synthesis in real-time. Analysis of the patterns confi rms the phase separation and volatization process. Successful synthesis of M-O_x, where M = Mn, Fe, Co, and Cr, is shown using this template-free facile approach. Detailed magnetic characterization of nanocrystals is carried out to reveal the functionalities such as magnetic anisotropy as well as larger than bulk moments, as expected in these oxide nanostructures.

Structure-reactivity Correlations in the catalytic coupling of ethyne over novel bimetallic Pd/Sn catalysts

The structure, thermal stability, and catalytic behavior of a novel highly dispersed silica-supported Pd/Sn catalyst prepared by an organometallic route have been examined by X-ray photoelectron, X-ray diffraction, and X-ray absorption, fine structure spectroscopies, the latter two measurements being carried outwith an in situ reaction cell. Additional reactor measurements were performed on a more Sn-rich catalyst and on a pure Pd catalyst. Varying the temperature of reduction induced large variations in catalytic performance toward ethyne-coupling reactions. These changes are understandable in terms of the destruction of SnO2-like structures surrounding the Pd core, yielding a skin of metallic Sn which subsequently undergoes intermixing with Pd. The overall thermal and catalytic behavior of these highly dispersed materials accords well with the analogous single-crystal model system.

Ensemble effects in the coupling of acetylene to benzene on a bimetallic surface: A study with Pd{111}/Au

Acetylene coupling to benzene on the Pd(lll) surface is greatly enhanced by the presence of catalytically inert Au atoms. LEED and Auger spectroscopy show that progressive annealing of Au overlayers on Pd(lll) leads to the formation of a series of random surface alloys with continuously varying composition. Cyclization activity is a strong function of surface composition-the most efficient catalyst corresponds to a surface of composition similar to 85% Pd. CO TPD and HREELS data show that acetylene cyclization activity is not correlated with the availability of singleton Pd atoms, nor just with the presence of 3-fold pure Pd sites-the preferred chemisorption site for C2H2 on Pd{111}. The data can be quantitatively rationalized in terms of a simple model in which catalytic activity is dominated by Pd6Au and Pd-7 surface ensembles, allowance being made for the known degree to which pure Pd{111} decomposes the reactant and product molecules.