Publications

Publications:

In-Situ Bragg Coherent Diffraction Imaging of Domain Wall Motion in Multiferroic DyMnO3 Nanocrystal [Under Review] (2024) Mansoor A. Najeeb, Robbie Morrison, Ahmed H. Mokhtar, Daniel G. Porter, Frank Lichtenberg, Alessandro Bombardi, Marcus C. Newton Multiferroic materials which exhibit interacting and coexisting properties, like ferroelectricity and ferromagnetism, possess significant potential in the development of novel technologies that can be controlled through the application of external fields. They also exhibit varying regions of polarity, known as domains, with the interfaces that separate the domains referred to as domain walls. In this study, using three-dimensional Bragg Coherent Diffractive Imaging (BCDI), we investigate the dynamics of multiferroic domain wall motion in a single hexagonal dysprosium manganite (h-DyMnO3 ) nanocrystal under varying applied electric field. Our analysis reveals the dynamics of the ferroelectric polarisation where switching of the direction was achieved with an applied field of less than 0.3 V. Further details »

Three-Dimensional Imaging of Topologically Protected Strings in a Multiferroic Nanocrystal [Under Review] (2024) Mansoor A. Najeeb, David Serban, Daniel G. Porter, Frank Lichtenberg, Stephen P. Collins, Alessandro Bombardi, Nicola A. Spaldin, Marcus C. Newton Multiferroic materials can host a plethora of intriguing phenomena due to the presence of multiple ferroic properties that break both spatial inversion symmetry and time reversal symmetry at an observable scale. Hexagonal manganite multiferroics are of particular interest as the properties of their symmetry-lowering phase transition can be described by a Mexican-hat-like potential energy surface. The early universe is proposed to have undergone a symmetry-lowering phase transition that is described by a similar Mexican-hat-like potential that gives rise to the formation of one-dimensional topologically protected defects known as cosmic strings. According to the Kibble-Zurek mechanism, hexagonal manganite multiferroics can host the crystallographic equivalent of cosmic strings and can therefore serve as a testing ground for exploration of concepts in cosmology. To date, however, in-situ imaging of 1D topological defects to demonstrate Kibble-Zurek equivalence and scaling in a condensed matter material system has not been achieved. Here we report on robust three-dimensional imaging of topologically protected strings in a single hexagonal manganite nancorystal, enabled by advances in experimental techniques. Our findings reveal multiferroic strings with a preferred phase vortex winding direction and average separation of ~93 nm. Further details »

Three-Dimensional Domain Identification in a Single Hexagonal Manganite Nanocrystal Nature Communications (2024) Ahmed H. Mokhtar, David Serban, Daniel G. Porter, Frank Lichtenberg, Stephen P. Collins, Alessandro Bombardi, Nicola A. Spaldin, Marcus C. Newton The three-dimensional domain structure in ferroelectric materials determines many of their physical and technological properties, including their electrostatic stability, coercive field and surface charge. The domain structure can be particularly complex in improper ferroelectrics such as the hexagonal manganites since the polarization is a slave to a non-ferroelectric primary order parameter that drives the domain formation. In antiferromagnetic YMnO3 , for example, this leads to an unusual hexagonal vortex domain pattern with topologically protected domain walls, which have been shown to exhibit electrical conductivity and a net magnetic dipole moment at the sample surface. Characterizing the three-dimensional structure of these domains and domain walls has been elusive, however, due to a lack of suitable imaging techniques. Here, we present a multi-Bragg coherent x-ray diffraction imaging (BCDI) determination of the domain walls and domain types in a single YMnO3 nanocrystal. By reconstructing high-resolution, three-dimensional images of the structure and the full strain tensor field, we resolve two ferroelectric domains separated by a domain wall and confirm that the primary atomic displacements occur along the crystallographic c-axis throughout the nanocrystal. By correlating the BCDI experiment with atomistic simulation, we are able to verify the "Mexican hat" symmetry model of domain formation in the hexagonal manganites, and establish that the two domains correspond to adjacent minima in the Mexican hat with opposite ferroelectric polarization and adjacent trimerization domains. Finally, using a circular mean comparison we show that for this sample the two domains correspond to a clockwise winding around the brim of the hat. Our results highlight the potential of multi-Bragg CDI combined with atomistic simulations for revealing and identifying ferroelectric domain structures. Further details »

Imaging and Ferroelectric Orientation Mapping of Photostriction in a Single Bismuth Ferrite Nanocrystal Nature Computational Materials (2024) Ahmed H. Mokhtar, David Serban, Daniel G. Porter, Gareth Nisbet, Steve Collins, Alessandro Bombardi, Marcus C. Newton The exploration of multiferroic materials and their interaction with light at the nanoscale presents a captivating frontier in materials science. Bismuth Ferrite (BiFeO3 , BFO), a standout among these materials, exhibits room-temperature ferroelectric and antiferromagnetic behavior and magnetoelectric coupling. Of particular interest is the phenomenon of photostriction, the light-induced deformation of crystal structures, which enhances the prospect for device functionality based on these materials. Understanding and harnessing multiferroic phenomena holds significant promise in various technological applications, from optoelectronics to energy storage. The orientation of the ferroelectric axis is an important design parameter for devices formed from multiferroic materials. Determining its orientation in the laboratory frame of reference usually requires knowing multiple wavevector transfer (Q-Vector) directions, which can be challenging to establish due to the need for extensive reciprocal-space searches. Our study demonstrates a method to uniquely identify the ferroelectric axis orientation using Bragg Coherent X-ray Diffraction Imaging (BCDI) measurements at a single Q-vector direction. This method involves applying photostriction-inducing laser illumination across various laser polarizations. Our findings reveal that photostriction primarily occurs as a surface phenomenon at the nanoscale. Moreover a photo-induced crystal length change ranging from 30 to 60 nm was observed, consistent with earlier findings performed on bulk material. Further details »

Three-Dimensional Imaging of the Structural Phase Transition in a Single Vanadium Dioxide Nanocrystal Physica Status Solidi A (2024) Mansoor A. Najeeb, Ahmed H. Mokhtar, David A. Serban, Daniel G. Porter, Steve Collins, Alessandro Bombardi, Marcus C. Newton Vanadium dioxide (VO2) is a strongly correlated electronic material that is considered future enabling due to its potential utility in ultra-fast next-generation pto-electronic devices. VO2 exhibits a number of structural phase transitions (SPT), including an ultra-fast femtosecond symmetry-breaking SPT just above room temperature that is accompanied by a metal-to-insulator transition spanning four orders of magnitude. Knowledge of how the SPT is initiated and propagates through a nanoscale crystal of VO2 would inform the design of devices based on this material. Here we show that Bragg Coherent X-ray Diffractive Imaging (BCDI) combined with machine learning is an effective means to recover three-dimensional images of a single VO2 nanocrystal during an SPT from the room temperature monoclinic phase to the high-temperature rutile phase. Our findings reveal the persistence of multiple phases within the nanocrystal throughout the transition. Further details »

Imaging in-operando LiCoO2 Nanocrystallites with Bragg Coherent X-ray Diffraction Nature Communications Chemistry (2024) David Serban, Daniel G. Porter, Ahmed H. Mokhtar, Mansoor Nellikkal, Uthay Sivaperumal, Min Zhang, Stephen P. Collins, Alessandro Bombardi, Peng Li, Christoph Rau, Marcus C. Newton Rechargeable lithium ion batteries (LIBs) with higher specific energy are a means to affordable and clean energy. Although the LiCoO2 (LCO) cathode material has been widely used in commercial LIBs and show high stability, its practical capacity is limited (≈ 155 mAhg−1 ) and LIBs’ improvements have several challenges that still need to be overcome. In this paper, we will study the in-operando structural properties of LCO within battery cells using Bragg Coherent X-ray Diffraction Imaging (Bragg CDI) to identify ways to optimise the practical capacity and cycling of LCO batteries. We have successfully reconstructed the X-ray scattering phase variation (a fingerprint of atomic displacement) within a ≈ (1.6 × 1.4 × 1.3) µm3 LCO nanocrystal across a charge/discharge cycle. The reconstructions were obtained by running a deep learning model based on a Convolutional Neural Network with a high degree of fidelity. Reconstructions indicate strained domains forming and expanding at the surface of the nanocrystal as higher voltages are applied, with some domains collapsing into multiple smaller ones possibly due to the strain generated by delithiation. While discharging, all domains replicate the effects observed from the charging states, but with a delay that suggests a plastic deformation from charging. Principal component analysis (PCA) performed on the phase data confirms that the greatest differences between the reconstructions happen at the surface, namely the collapsing domain and another that migrates the most towards the inner crystal. The differences from the PCA replicate the stability of the lattice, being more stable when charging begins or completes, and continue to destabilise with the suggested plastic deformation and the following discharging voltages. These findings show the domain dynamics within LCO lattices during cycling and how higher voltages can cause voltage decay from plastic deformities, and will inspire future work on how to extend LCO-based batteries’ lifetimes as well as possibly facilitate lattice self-healing. Further details »

The African Light Source: history, context and future Journal of Synchrotron Radiation (2023) Simon H. Connell, Kathleen Dollman, Gihan Kamel, Sameen A. Khan, Edward Mitchell, Sekazi K. Mtingwa, Marcus C. Newton, Prosper Ngabonziza,Emmanuel Nji, Lawrence Norris and Michele Zema The African Light Source (AfLS) project is now almost eight years old. This article assesses the history, current context and future of the project. There is by now considerable momentum in building the user community, including deep training, facilitating access to current facilities, growing the scientific output, scientific networks and growing the local laboratory-scale research infrastructure. The Conceptual Design Report for the AfLS is in its final editing stages. This document specifies the socio-economic and scientific rationales and the technical aspects amongst others. The AfLS is supported by many national and Pan-African scientific professional bodies and voluntary associates across many scientific disciplines, and there are stakeholders throughout the continent and beyond. The current roadmap phases have expanded to include national and Pan-African level conversations with policy makers through new Strategic Task Force groups. The document summarizes this progress and discusses the future of the project. Further details »

Building a brighter future for Africa with the African Light Source Nature Reviews Physics (2022) Marcus C. Newton, Simon H. Connell, Edward P. Mitchell, Sekazi K. Mtingwa, Prosper Ngabonziza, Lawrence Norris, Tshepo Ntsoane and Daouda A. K. Traore Africa is the only habitable continent that is not yet host to a light source — an important tool across disciplines. Scientists from the Executive Committee of the African Light Source Foundation discuss work towards building an advanced light source in Africa, and what remains to be done. Further details »

Coherent X-ray diffraction of the M1 to M2 structural phase transition in a single vanadium dioxide nanocrystal Appl. Phys. Express (2022) Marcus Newton, Ulrich Wagner and Christoph Rau Correlated electronic materials are of interest due to strong coupling between lattice, spin and orbital degrees of freedom that give rise to emergent behaviour that is often of considerable utility for next generation technologies. Vanadium dioxide is a prototypical material that undergoes a number of structural phase transitions near room temperature. Here are presented the results of coherent x-ray diffraction measurements on a single vanadium dioxide nanocrystal approximately 440 nm in size. Experimental findings are compared with ab-initio simulations to elucidate the origin of distortions that are observed in the diffraction pattern. Further details »

Simulation of Bragg coherent diffraction imaging J. Phys. Commun. Volume 6, 055003 (2022) Ahmed Mokhtar, David Serban, Marcus Newton The arrangement of atoms within a crystal and information on deviations from the ideal lattice is encoded in the diffraction pattern obtained from an appropriately conducted Bragg coherent diffraction imaging (BCDI) experiment. A foreknowledge of how specific displacements of atoms within the unit cell alter the BCDI diffraction pattern and the subsequent real-space image is often useful for interpretation and can provide valuable insight for materials design. Here we report on an atomistic approach to efficiently simulate BCDI diffraction patterns by factorising and eliminating certain redundancies in the conventional approach. Our method is able to reduce the computation time by several orders of magnitude without compromising the recovered phase information and therefore enables feasible atomistic simulations on nanoscale crystals with arbitrary lattice distortions. Further details »

UK-XFEL Science Case STFC Andrew Burnett, Marco Borghesi, Andrew Comley, Mark Dean, Sofia Diaz-Moreno, David Dye, Jason Greenwood, Andrew Higginbotham, Adam Kirrander, Jon Marangos, Malcolm McMahon, Russell Minns, Marcus Newton, Allen Orville, Thomas Penfold, Anna Regoutz, Ian Robinson, David Rugg, Sven Schroeder, Jasper van Thor, Sam Vinko, Simon Wall, Justin Wark, Julia Weinstein, Amelle Zair and Xiaodong Zhang Following the Strategic FEL Review in 2016, this draft science case, led by Professor Jon Marangos (Imperial College) sets out the scientific rationale for a potential sovereign machine in the UK. Further details »

Concurrent phase retrieval for imaging strain in nanocrystals Phys. Rev. B Volume 102, 014104 (2020) Newton, MC Coherent diffraction imaging is a form of microscopy that permits high resolution imaging of atomic displacements from equilibrium where the use of conventional optics is not feasible. Approaches to date for the recovery of atomic displacements from equilibrium and subsequently strain information occur after phase reconstruction of the complex real-space images from at least three independent Bragg diffraction amplitude measurements. While this is a more accessible and effective approach to recover strain information, there is potential for erroneous results if the recovered phase information is not carefully treated. Here we present a strategy for imaging strain with coherent x rays that eliminates the technical challenges that exist in conventional approaches by constructing the strain field concurrently during the phase retrieval process of recovering phase information. Further details »

Coherent diffraction imaging of a progressively deformed nanocrystal Phys. Rev. Materials, Volume 3, 043803 (2019) Newton, MC ; Shi, X ; Wagner, U ; Rau, C Imaging ordered materials with coherent x rays holds great potential to improve our understanding of phenomena in complex materials systems where emergent behavior can arise due to coupling of spin, lattice, and orbital degrees of freedom. Coherent diffractive imaging (CDI) is a lensless imaging technique for probing the structure of materials in three dimensions. Central to the success of the CDI method is the inversion of propagated wave field information to recover a quantitative image of the illuminated crystalline structure. Present challenges faced with existing approaches to image recovery are often due to nonuniqueness of wave propagated forms of the electron density information that can cause prohibitive stagnation of the reconstruction algorithm. Here we report on a major advancement in image recovery that is able to recover the three-dimensional image of a 492 nm gold single crystal undergoing progressive deformation to a highly strained condition without the use of a priori information. Our findings also demonstrate the significance of robust image recovery techniques for revealing high resolution topological structure. Further details »

Ab initio molecular dynamics study of the structural and electronic transition in VO2 Phys. Rev. B, Volume 96, 054111 (2017) Dusan Plasienka, D ; Martonak, R ; Newton, MC The temperature-induced structural and electronic transformation in VO2 between the monoclinic M1 and tetragonal rutile phases was studied by means of ab initio molecular dynamics, based on density functional theory with Hubbard correction (DFT+U). We compare the structure of both phases, transition temperature and atomic fluctuations both above and below the transition, as well as the phonon density of states and scattering intensity of centroid position, with experimental data. The good quantitative agreement indicates that the chosen DFT+U scheme is able to provide a fairly good description of the energetics of the system. Analysis of the dynamical processes associated with the structural transformation was carried out on the atomic scale by following the time evolution of dimerization amplitudes of vanadium atom chains and the twisting angle of vanadium dimers. The electronic transition was studied by tracing the changes in projected densities of states and their correlation with the evolution of the structural transformation. Our results reveal a strong interconnection between the structural and electronic transformations and show that they take place on the same time scale. Further details »

Deformation of a bismuth ferrite nanocrystal imaged by coherent X-ray diffraction Journal of Physics: Conference Series, Volume 849, conference 1 (2017) Newton, MC ; Pietraszewski, Adam ; Kenny, Anthony ; Wagner, U ; Rau, C Perovskite materials that contain transition metal-oxides often exhibit multifunctional properties with considerable utility in a device setting. BiFeO3 is a multiferroic perovskite material that exhibits room temperature anti-ferromagnetic and ferroelectric ordering. Optical excitation of BiFeO3 crystals results in an elastic structural deformation of the lattice with a fast response on the pico-second time scale. Here we report on dynamic optical excitation coupled with Bragg coherent X-ray diffraction measurements to investigate the structural properties of BiFeO3 nanoscale crystals. A continuous distortion of the diffraction speckle pattern was observed with increasing illumination. This was attributed to strain resulting from photo-induced lattice deformation. Further details »

Coherent x-ray diffraction imaging of photo-induced structural changes in BiFeO3 nanocrystals New Journal of Physics, Volume 18, 093003 (2016) Newton, MC ; Parsons, A ; Wagner, U ; Rau, C Multiferroic materials that exhibit coupling between ferroelectric and magnetic properties are of considerable utility for technological applications and are also interesting from a fundamental standpoint. When reduced to the nanoscale, multiferroic materials often display additional functionality that is dominated by interfacial and confinement effects. Bismuth ferrite (BiFeO3) is one such material with room temperature anti-ferromagnetic and ferroelectric ordering. Optical excitation of BiFeO3 crystals results in an elastic structural deformation of the lattice with a fast response on the pico-second time scale. Here we report on dynamic measurements to investigate the structural properties of BiFeO3 nanoscale crystals using laser excitation and three-dimensional Bragg coherent x-ray diffraction imaging. Tensile strain beyond 8 × 10-2 was observed predominantly at the surface of the nanoscale crystal as evidenced in the reconstructed phase information and was correlated to photo-induced lattice deformation. Further details »

Crystalline Solids Course Notes for PHYS3004 (2016) Newton, MC ; de Groot, P Solids state physics is concerned with the study of matter in a condensed form as found in the world around us. It is often considered synonymous with the study of crystalline solids but extends to include liquids and amorphous materials that have partial ordering. Its primary aim is to provide an explanation of the macroscopic properties of materials through an understanding of underlying atomic structure. These course notes define the content to be delivered for the Level 3 core module on Crystalline Solids. Further details »

Framework for Automatic Information Extraction from Research Papers on Nanocrystal Devices Beilstein J. Nanotechnology 6, 1872–1882 (2015) Dieb, TM ; Yoshioka, M ; Hara, S ; Newton, MC To support nanocrystal device development, we have been working on a computational framework to utilize information in research papers on nanocrystal devices. We developed an annotated corpus called “ NaDev” (Nanocrystal Device Development) for this purpose. We also proposed an automatic information extraction system called “NaDevEx” (Nanocrystal Device Automatic Information Extraction Framework). NaDevEx aims at extracting information from research papers on nanocrystal devices using the NaDev corpus and machine-learning techniques. However, the characteristics of NaDevEx were not examined in detail. In this paper, we conduct system evaluation experiments for NaDevEx using the NaDev corpus. We discuss three main issues: system performance, compared with human annotators; the effect of paper type (synthesis or characterization) on system performance; and the effects of domain knowledge features (e.g., a chemical named entity recognition system and list of names of physical quantities) on system performance. We found that overall system performance was 89% in precision and 69% in recall. If we consider identification of terms that intersect with correct terms for the same information category as the correct identification, i.e., loose agreement (in many cases, we can find that appropriate head nouns such as temperature or pressure loosely match between two terms), the overall performance is 95% in precision and 74% in recall. The system performance is almost comparable with results of human annotators for information categories with rich domain knowledge information (source material). However, for other information categories, given the relatively large number of terms that exist only in one paper, recall of individual information categories is not high (39–73%); however, precision is better (75–97%). The average performance for synthesis papers is better than that for characterization papers because of the lack of training examples for characterization papers. Based on these results, we discuss future research plans for improving the performance of the system. Further details »

Time-Resolved Coherent Diffraction of Ultrafast Structural Dynamics in a Single Nanowire Nano Letters 14 (5) 2413–2418 (2014) Newton, M ; Sao, M ; Fujisawa, Y ; Onitsuka, R ; Kawaguchi, T ; Tokuda, K ; Sato, To ; Togashi, T ; Yabashi, M ; Ishikawa, T ; Ichitsubo, T ; Matsubara, E ; Tanaka, Y ; Nishino, Y The continuing effort to utilize the unique properties present in a number of strongly correlated transition metal oxides for novel device applications has led to intense study of their transitional phase state behavior. Here we report on time-resolved coherent X-ray diffraction measurements on a single vanadium dioxide nanocrystal undergoing a solid–solid phase transition, using the SACLA X-ray Free Electron Laser (XFEL) facility. We observe an ultrafast transition from monoclinic to tetragonal crystal structure in a single vanadium dioxide nanocrystal. Our findings demonstrate that the structural change occurs in a number of distinct stages attributed to differing expansion modes of vanadium atom pairs. Further details »

Time-resolved Bragg coherent X-ray diffraction revealing ultrafast lattice dynamics in nano-thickness crystal layer using X-ray free electron laser Journal of the Ceramic Society of Japan 121 (1411) 283-286 (2013) Tanaka, Y ; Ito, K ; Nakatani, T ; Onitsuka, R ; Newton, M ; Sato, T ; Togashi, T ; Yabashi, M ; Kawaguchi, T ; Shimada, K ; Tokuda, K ; Takahashi, I ; Ichitsubo, T ; Matsubara, E ; Nishino, Y Ultrafast time-resolved Bragg coherent X-ray diffraction (CXD) has been performed to investigate lattice dynamics in a thin crystal layer with a nanoscale thickness by using a SASE (Self-Amplified Spontaneous Emission)-XFEL (X-ray Free Electron Laser) facility, SACLA. Single-shot Bragg coherent diffraction patterns of a 100 nm-thick silicon crystal were measured in the asymmetric configuration with a grazing exit using an area detector. The measured coherent diffraction patterns showed fringes extending in the surface normal direction. By using an optical femtosecond laser-pump and the XFEL-probe, a transient broadening of coherent diffraction pattern profile was observed at a delay time of around a few tens of picosecond, indicating transient crystal lattice fluctuation induced by the optical laser. A perspective application of the time-resolved Bragg CXD method to investigate small sized grains composing ceramic materials is discussed. Further details »

Bonsu: the interactive phase retrieval suite Journal of Applied Crystallography 45 840-843 (2012) Newton, MC ; Nishino, Y; Robinson, IK Coherent X-ray diffraction imaging has received considerable attention as a nondestructive method for probing material structure at the nanoscale. However, tools for reconstructing and analysing data in both two and three dimensions have lagged somewhat behind. Bonsu, the interactive phase retrieval suite, is the first software package that allows real-time visualization of the reconstruction of phase information in both two and three dimensions. It comes complete with an inventory of algorithms and routines for data manipulation and reconstruction. Bonsu is open source, is designed around the Python language (with C++ bindings) and is largely platform independent. Bonsu is made available under version three of the GNU General Public License. Further details »

Compressed sensing for phase retrieval Physical Review E 85 (5) (2012) Newton, MC To date there are several iterative techniques that enjoy moderate success when reconstructing phase information, where only intensity measurements are made. There remains, however, a number of cases in which conventional approaches are unsuccessful. In the last decade, the theory of compressed sensing has emerged and provides a route to solving convex optimisation problems exactly via l(1)-norm minimization. Here the application of compressed sensing to phase retrieval in a nonconvex setting is reported. An algorithm is presented that applies reweighted l(1)-norm minimization to yield accurate reconstruction where conventional methods fail. Further details »

Coherent X-ray Diffraction Imaging for Strain Analysis on Single ZnO Nanorod AIP Conference Proceedings 1399 (2011) Xiong, G ; Leake, S ; Newton, MC ; Huang, XJ ; Harder, R ; Robinson, IK Strain induced in nanostructure semiconductor materials can result in different electronic properties. Coherent x-ray diffraction (CXD) has emerged as a non-destructive tool for imaging of strain and defects. In this work CXD is applied on a single ZnO nanorod, diffraction patterns from Bragg reflection are used to reconstruct the strain distribution in the samples at a resolution of 40 nm. Further details »

Coherent x-ray diffraction imaging of ZnO nanostructures under confined illumination New Journal of Physics 13 033006 (2011) Xiong, G ; Huang, XJ ; Leake, S ; Newton, MC ; Harder, R ; Robinson, IK Coherent x-ray diffraction imaging has been used to study a single ZnO nanorod in a confined illuminating condition. The focused beam size is smaller than the length of the nanorod, and the diffraction intensity is strongly dependent on the illumination position. The density maps show that the nanorod width in the radial direction is around 210 nm and has a length of 1.5 mu m, in agreement with the scanning electron microscope measurement. Reconstructed phase maps show a maximum phase change of 0.8 radians. The reconstructed direct space structures reveal the exit wavefront profile, which includes that of the focused x-ray beam. The beam profile presents in reconstructions some 'hill and valley' surface features with a typical size of a few tens of nanometres and are attributed to the noise due to the slow variation of the focused beam intensity along the boundary. A single ZnO tetrapod has been investigated with the same method to recover the beam profile in the horizontal direction. Further details »

Phase retrieval of diffraction from highly strained crystals Physical Review B 82 (16) (2010) Newton, MC ; Harder, R ; Huang, XJ ; Xiong, G ; Robinson, IK An important application of phase retrieval methods is to invert coherent x-ray diffraction measurements to obtain real-space images of nanoscale crystals. The phase information is currently recovered from reciprocal-space amplitude measurements by the application of iterative projective algorithms that solve the nonlinear and nonconvex optimization problem. Various algorithms have been developed each of which apply constraints in real and reciprocal space on the reconstructed object. In general, these methods rely on experimental data that is oversampled above the Nyquist frequency. To date, support-based methods have worked well, but are less successful for highly strained structures, defined as those which contain (real-space) phase information outside the range of +/-pi/2. As a direct result the acquired experimental data is, in general, inadvertently subsampled below the Nyquist frequency. In recent years, a new theory of "compressive sensing" has emerged, which dictates that an appropriately subsampled (or compressed ) signal can be recovered exactly through iterative reconstruction and various routes to minimizing the l(1) norm or total variation in that signal. This has proven effective in solving several classes of convex optimization problems. Here we report on a "density-modification" phase reconstruction algorithm that applies the principles of compressive sensing to solve the nonconvex phase retrieval problem for highly strained crystalline materials. The application of a nonlinear operator in real-space minimizes the l(1) norm of the amplitude by a promotion-penalization (or "propenal") operation that confines the density bandwidth. This was found to significantly aid in the reconstruction of highly strained nanocrystals. We show how this method is able to successfully reconstruct phase information that otherwise could not be recovered. Further details »

Three-dimensional imaging of strain in a single ZnO nanorod Nature Materials 9 (2) 120-124 (2010) Newton, MC ; Leake, SJ ; Harder, R ; Robinson, IK Nanoscale structures can be highly strained because of confinement effects and the strong influence of their external boundaries. This results in dramatically different electronic, magnetic and optical material properties of considerable utility. Third-generation synchrotron-based coherent X-ray diffraction has emerged as a non-destructive tool for three-dimensional (3D) imaging of strain and defects in crystals that are smaller than the coherence volume, typically a few cubic micrometres, of the available beams that have sufficient flux to reveal the material's structure(1). Until now, measurements have been possible only at a single Bragg point of a given crystal because of the limited ability to maintain alignment(2); it has therefore been possible to determine only one component of displacement and not the full strain tensor. Here we report key advances in our fabrication and experimental techniques, which have enabled diffraction patterns to be obtained from six Bragg reflections of the same ZnO nanocrystal for the first time. All three Cartesian components of the ion displacement field, and in turn the full nine-component strain tensor, have thereby been imaged in three dimensions. Further details »

Longitudinal coherence function in X-ray imaging of crystals Optics Express 17 (18) 15853-15859 (2009) Leake, SJ; Newton, MC ; Harder, R ; Robinson, IK The longitudinal coherence function at the Advanced Photon Source beamline 34-ID-C has been measured by a novel method and the coherence length (xi(L)) determined to be, xi(L) = 0.66 +/- 0.02 mu m. Three dimensional Coherent X-ray Diffraction (CXD) patterns were measured for multiple Bragg reflections from two Zinc Oxide (ZnO) nanorods with differing aspect ratios. The visibility of fringes corresponding to the 002 crystal direction for each reflection were found to be different and used to map the coherence function of the incident radiation. Partial coherence was found to be associated with amplitude 'hot' spots in three dimensional reconstructions of the crystal structure. Further details »

ZnO tetrapod p-n junction diodes Applied Physics Letters 94 15 (2009) Newton, MC; Shaikhaidarov, R ZnO nanocrystals hold the potential for use in a wide range of applications particularly in optoelectronics. We report on the fabrication of a highly sensitive p-n junction diode structure based on a single ZnO tetrapod shaped nanocrystal. This device shows a noted response to ultraviolet light with high internal gain. The high reponsivities we have observed exceed 10(4) A/W and are likely due to impact-ionization effects at the p-n junction interface. Further details »

Photoresponse of ZnO tetrapod nanocrystal Schottky diodes IEEE Transactions on Nanotechnology 7 (1) 20-23 (2008) Newton, Marcus C. ; Firth, Steven ; Warburton, P.A. The fabrication of an ultraviolet photodiode employing a single ZnO tetrapod nanocrystal is reported. We have attached two tungsten leads and one platinum lead to three of the arms of the tetrapod. By measuring the transport properties between each pair of leads we show that the tungsten contacts are ohmic and the platinum contacts are rectifying. Photoresponse measurements were carried out with above and below band gap illumination. We observe a much larger ultraviolet photoresponse for the rectifying Pt-ZnO-W junction than the linear W-ZnO-W junction. We conclude that the enhanced photoresponse of our rectifying junction results from a photoinduced reduction of the Schottky barrier height at the Pt-ZnO interface. Further details »

Zinc Oxide Nanostructures and High Electron Mobility Nanocomposite Thin Film Transistors IEEE Transactions on Electron Devices 55 (11) 3001-3011 (2008) Li, F.M. ;Hsieh, G.-W. ; Dalal, Sharvari ; Newton, M.C. ; Stott, J.E. ; Hiralal, P. ; Nathan, A. ; Warburton, P.A. ; Unalan, H.E. ; Beecher, P. ; Flewitt, A.J. ; Robinson, I. ; Amaratunga, G. ; Milne, W.I. This paper reports on the synthesis of zinc oxide (ZnO) nanostructures and examines the performance of nanocomposite thin-film transistors (TFTs) fabricated using ZnO dispersed in both n- and p-type polymer host matrices. The ZnO nanostructures considered here comprise nanowires and tetrapods and were synthesized using vapor phase deposition techniques involving the carbothermal reduction of solid-phase zinc-containing compounds. Measurement results of nanocomposite TFTs based on dispersion of ZnO nanorods in an n-type organic semiconductor ([6, 6]-phenyl-C-61-butyric acid methyl ester) show electron field-effect mobilities in the range 0.3-0.6 cm(2)V(-1)s(-1), representing an approximate enhancement by as much as a factor of 40 from the pristine state. The on/off current ratio of the nanocomposite TFTs approach 10(6) at saturation with off-currents on the order of 10 pA. The results presented here, although preliminary, show a highly promising enhancement for realization of high-performance solution-processable n-type organic TFTs. Further details »

ZnO tetrapod nanocrystals Materials Today 10 (5) 50-54 (2007) Newton, MC; Warburton, PA ZnO has received considerable attention because of its unique optical, piezoelectric, and magnetic properties. It also readily self-assembles into a family of nanocrystalline structures. We review the current status of research into ZnO tetrapod nanocrystals. These crystals consist of a ZnO core in the zinc blende structure from which four ZnO arms in the wurtzite structure radiate. The arms are cylinders of hexagonal cross section, with each arm of equal length and diameter. Possible applications in optoelectronics, photovoltaics, spintronics, and piezoelectricity are discussed. Further details »

ZnO tetrapod Schottky photodiodes Applied Physics Letters 89 (7) (2006) Newton, MC; Firth, S; Matsuura, T; Warburton, PA The fabrication of an ultraviolet photodiode employing a single ZnO tetrapod nanocrystal is reported. This diode structure is prepared by depositing W and Pt electrodes to form Ohmic and Schottky contacts, respectively. Dark current-voltage measurements show rectifying behavior. The properties of the metal-semiconductor interface are studied with above and below band gap illumination. It is found that with increasing UV excitation the device converts from a rectifying to an Ohmic behavior. This effect is attributed to a flattening of the energy bands due to the migration of photogenerated carriers within the space charge region at the metal-semiconductor interface. Further details »

Synthesis and characterisation of zinc oxide tetrapod nanocrystals Journal of Physics Conference Series 26 251-255 (2006) Newton, MC; Firth, S; Matsuura, T; Warburton, PA Zinc oxide is an important group II-VI semiconductor material with optical properties that permit stable- emission at room temperature. We report on the synthesis of highly uniform nanocrystalline ZnO tetrapod (ZnO-T) nanostructures through a modified chemical vapour transport process. These self assembled nanocrystals are characterised by four cylindrical arms with a hexagonal facet all of which are joined at a tetrahedral core. Studies are carried out on ZnO tetrapods using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), photoluminescence spectroscopy (PLS) and Raman measurements. We find a simple technique to quench visible emission found in ZnO tetrapods as grown. We also observe Raman active modes suggesting that nitrogen is incorporated within our samples. Further details »