I09-Surface and Interface Structural Analysis
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Carlos
Sotelo-vazquez
,
Raul
Quesada-cabrera
,
Min
Ling
,
David O.
Scanlon
,
Andreas
Kafizas
,
Pardeep
Kumar Thakur
,
Tien-lin
Lee
,
Alaric
Taylor
,
Graeme W.
Watson
,
Robert G.
Palgrave
,
James R.
Durrant
,
Christopher S.
Blackman
,
Ivan P.
Parkin
Abstract: Semiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar-to-chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO3/TiO2 heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO3 layer of vertically aligned nanorods that is then coated with a conformal layer of TiO2. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO3 layer into TiO2. State-of-the-art hybrid density functional theory and hard X-ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO3/TiO2 interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.
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Mar 2017
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I07-Surface & interface diffraction
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Astrid-caroline
Knall
,
Raja Shahid
Ashraf
,
Mark
Nikolka
,
Christian B.
Nielsen
,
Balaji
Purushothaman
,
Aditya
Sadhanala
,
Michael
Hurhangee
,
Katharina
Broch
,
David J.
Harkin
,
Jiří
Novák
,
Marios
Neophytou
,
Pascal
Hayoz
,
Henning
Sirringhaus
,
Iain
Mcculloch
Abstract: Wide-bandgap conjugated polymers with a linear naphthacenodithiophene (NDT) donor unit are herein reported along with their performance in both transistor and solar cell devices. The monomer is synthesized starting from 2,6-dihydroxynaphthalene with a double Fries rearrangement as the key step. By copolymerization with 2,1,3-benzothiadiazole (BT) via a palladium-catalyzed Suzuki coupling reaction, NDT-BT co-polymers with high molecular weights and narrow polydispersities are afforded. These novel wide-bandgap polymers are evaluated as the semiconducting polymer in both organic field effect transistor and organic photovoltaic applications. The synthesized polymers reveal an optical bandgap in the range of 1.8 eV with an electron affinity of 3.6 eV which provides sufficient energy offset for electron transfer to PC70BM acceptors. In organic field effect transistors, the synthesized polymers demonstrate high hole mobilities of around 0.4 cm2 V–1 s–1. By using a blend of NDT-BT with PC70BM as absorber layer in organic bulk heterojunction solar cells, power conversion efficiencies of 7.5% are obtained. This value is among the highest obtained for polymers with a wider bandgap (larger than 1.7 eV), making this polymer also interesting for application in tandem or multijunction solar cells.
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Oct 2016
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I22-Small angle scattering & Diffraction
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Abstract: Grazing incidence wide and small angle X-ray scattering (GIWAXS and GISAXS) measurements have been used to study the crystallization kinetics of the organolead halide perovskite CH3NH3PbI3–xClx during thermal annealing. In situ GIWAXS measurements recorded during annealing are used to characterize and quantify the transition from a crystalline precursor to the perovskite structure. In situ GISAXS measurements indicate an evolution of crystallite sizes during annealing, with the number of crystallites having sizes between 30 and 400 nm increasing through the annealing process. Using ex situ scanning electron microscopy, this evolution in length scales is confirmed and a concurrent increase in film surface coverage is observed, a parameter crucial for efficient solar cell performance. A series of photovoltaic devices are then fabricated in which perovskite films have been annealed for different times, and variations in device performance are explained on the basis of X-ray scattering measurements.
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May 2016
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I11-High Resolution Powder Diffraction
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Yi-yeoun
Kim
,
Mona
Semsarilar
,
Joseph D.
Carloni
,
Kang Rae
Cho
,
Alexander N.
Kulak
,
Iryna
Polishchuk
,
Coit T.
Hendley
,
Paul J. M.
Smeets
,
Lee
Fielding
,
Boaz
Pokroy
,
Chiu C.
Tang
,
Lara A.
Estroff
,
Shefford P.
Baker
,
Steven P.
Armes
,
Fiona
Meldrum
Open Access
Abstract: This article describes an experimentally versatile strategy for producing inorganic/organic nanocomposites, with control over the microstructure at the nano- and mesoscales. Taking inspiration from biominerals, CaCO3 is coprecipitated with anionic diblock copolymer worms or vesicles to produce single crystals of calcite occluding a high density of the organic component. This approach can also be extended to generate complex structures in which the crystals are internally patterned with nano-objects of differing morphologies. Extensive characterization of the nanocomposite crystals using high resolution synchrotron powder X-ray diffraction and vibrational spectroscopy demonstrates how the occlusions affect the short and long-range order of the crystal lattice. By comparison with nanocomposite crystals containing latex particles and copolymer micelles, it is shown that the effect of these occlusions on the crystal lattice is dominated by the interface between the inorganic crystal and the organic nano-objects, rather than the occlusion size. This is supported by in situ atomic force microscopy studies of worm occlusion in calcite, which reveal flattening of the copolymer worms on the crystal surface, followed by burial and void formation. Finally, the mechanical properties of the nanocomposite crystals are determined using nanoindentation techniques, which reveal that they have hardnesses approaching those of biogenic calcites.
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Mar 2016
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[9282]
Open Access
Abstract: A room temperature magnetoelectric multiferroic is of interest as, e.g., magnetoelectric random access memory. Bulk samples of the perovskite (1−x)BiTi(1−y)/2FeyMg(1−y)/2O3–xCaTiO3 (BTFM–CTO) are simultaneously ferroelectric, weakly ferromagnetic, and magnetoelectric at room temperature. In BTFM–CTO, the volatility of bismuth oxide, and the complex subsolidus reaction kinetics, cause the formation of a microscopic amount of ferrimagnetic spinel impurity, which complicates the quantitative characterization of their intrinsic magnetic and magnetoelectric properties. Here, a controlled synthesis route to single-phase bulk samples of BTFM–CTO is devised and their intrinsic properties are determined. For example, the composition x = 0.15, y = 0.75 shows a saturated magnetization of 0.0097μB per Fe, a linear magnetoelectric susceptibility of 0.19(1) ps m−1, and a polarization of 66 μC cm−2 at room temperature. The onset of weak ferromagnetism and linear magnetoelectric coupling are shown to coincide with the onset of bulk long-range magnetic order by neutron diffraction. The synthesis strategy developed here will be invaluable as the phase diagram of BTFM–CTO is explored further, and as an example for the synthesis of other compositionally complex BiFeO3-related materials.
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Feb 2016
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I09-Surface and Interface Structural Analysis
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Anna
Regoutz
,
Isha
Gupta
,
Alexantrou
Serb
,
Ali
Khiat
,
Francesco
Borgatti
,
Tien-lin
Lee
,
Christoph
Schlueter
,
Piero
Torelli
,
Benoit
Gobaut
,
Mark
Light
,
Daniela
Carta
,
Stuart
Pearce
,
Giancarlo
Panaccione
,
Themistoklis
Prodromakis
Diamond Proposal Number(s):
[0240]
Open Access
Abstract: TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.
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Jan 2016
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[8657]
Open Access
Abstract: The electronic structure of low temperature, solution-processed indium–zinc oxide thin-film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm2 V?1 s?1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ?200 °C are used. Here, the electronic structure of low temperature, solution-processed oxide thin films as a function of annealing temperature and environment using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop-off in performance at temperatures ?200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub-bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution-processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy-induced donor levels.
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Mar 2015
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I07-Surface & interface diffraction
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Demet
Asil
,
Brian J.
Walker
,
Bruno
Ehrler
,
Yana
Vaynzof
,
Alessandro
Sepe
,
Sam
Bayliss
,
Aditya
Sadhanala
,
Philip C. Y.
Chow
,
Paul E.
Hopkinson
,
Ullrich
Steiner
,
Neil C.
Greenham
,
Richard H.
Friend
Diamond Proposal Number(s):
[8657]
Abstract: Semiconductor nanocrystals are promising materials for printed optoelectronic devices, but their high surface areas are susceptible to forming defects that hinder charge carrier transport. Furthermore, correlation of chalcogenide nanocrystal (NC) material properties with solar cell operation is not straightforward due to the disorder often induced into NC films during processing. Here, an improvement in long-range ordering of PbSe NCs symmetry that results from halide surface passivation is described, and the effects on chemical, optical, and photovoltaic device properties are investigated. Notably, this passivation method leads to a nanometer-scale rearrangement of PbSe NCs during ligand exchange, improving the long-range ordering of nanocrystal symmetry entirely with inorganic surface chemistry. Solar cells constructed with a variety of architectures show varying improvement and suggest that triplet formation and ionization, rather than carrier transport, is the limiting factor in singlet fission solar cells. Compared to existing protocols, our synthesis leads to PbSe nanocrystals with surface-bound chloride ions, reduced sub-bandgap absorption and robust materials and devices that retain performance characteristics many hours longer than their unpassivated counterparts.
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Dec 2014
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B18-Core EXAFS
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Davinder
Bhachu
,
Sanjay
Sathasivam
,
Gopinathan
Sankar
,
David O.
Scanlon
,
Giannantonio
Cibin
,
Claire J.
Carmalt
,
Ivan
Parkin
,
Graeme W.
Watson
,
Salem M.
Bawaked
,
Abdullah Y.
Obaid
,
Shaeel
Al-thabaiti
,
Sulaiman N.
Basahel
Diamond Proposal Number(s):
[8071]
Abstract: This paper reports the synthesis of highly conductive niobium doped titanium dioxide (Nb:TiO2) films from the decomposition of Ti(OEt)4 with dopant quantities of Nb(OEt)5 by aerosol-assisted chemical vapor deposition (AACVD). Doping Nb into the Ti sites results in n-type conductivity, as determined by Hall effect measurements. The doped films display significantly improved electrical properties compared to pristine TiO2 films. For 5 at.% Nb in the films, the charge carrier concentration was 2 × 1021 cm?3 with a mobility of 2 cm2 V–1 s–1 . The corresponding sheet resistance is as low as 6.5 ? sq–1 making the films suitable candidates for transparent conducting oxide (TCO) materials. This is, to the best of our knowledge, the lowest reported sheet resistance for Nb:TiO2 films synthesized by vapour deposition. The doped films are also blue in colour, with the intensity dependent on the Nb concentration in the films. A combination of synchrotron, laboratory and theoretical techniques confirmed niobium doping into the anatase TiO2 lattice. Computational methods also confirmed experimental results of both delocalized (Ti4+) and localized polaronic states (Ti3+) states. Additionally, the doped films also functioned as photocatalysts. Thus, Nb:TiO2 combines four functional properties (photocatalysis, electrical conductivity, optical transparency and blue colouration) within the same layer, making it a promising alternative to conventional TCO materials.
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Aug 2014
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James
Byrne
,
Victoria
Coker
,
Eva
Cespedes
,
Paul L.
Wincott
,
David J.
Vaughan
,
Richard
Pattrick
,
Gerrit
Van Der Laan
,
Elke
Arenholz
,
Floriana
Tuna
,
Martin
Bencsik
,
Jonathan R.
Lloyd
,
Neil
Telling
Abstract: The magnetic moments of magnetite nanoparticles are dramatically enhanced through the addition of zinc in a microbiologically driven synthesis procedure. The particles are produced through the reduction of Fe(III)-compounds containing Zn(II) by the iron reducing bacterium Geobacter sulfurreducens .
Results indicate a signifi cant increase in the saturation magnetization by over 50% compared to magnetite at both room and low temperatures for relatively minor quantities of zinc substitution. A maximum saturation magnetization of nearly 100 emu g −1 of sample is measured at room temperature. Analysis of the cation site ordering reveals a complex dependence on the Zn content, with the combined effect of Zn substitution of Fe 3+ ions on tetrahedral sites, together with Fe 2+ cation oxidation, leading to the observed magnetization enhancement for low Zn doping levels. The improved magnetic properties give superior performance in MRI applications with an MRI contrast enhancement among the largest values reported, being more than 5 times larger than a commercial contrast agent (Feridex) measured under identical conditions.The synthesis technique applied here involves an environmentally benign route and offers the potential to tune the magnetic properties of magnetic
nanoparticles, with increased overall magnetization desirable for many different commercial applications.
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May 2014
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