Difference between revisions of "Research"

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<p style="text-align: center"> Hear from our students on their exciting research and experiences. </p>
  
== Research ==
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=== Coherent X-ray Imaging ===
 
  
[[Image:BCXDI.png|950px|alt=Research Summary.|Experimental Geometry for Bragg Coherent X-ray Diffraction Imaging (BCXDI). ]]
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== Research Interests ==
[[Image:AniAu.gif|frameless|300px]]
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=== Imaging Dynamics in Quantum Materials ===
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The dynamics of materials are governed by their quantum properties. Strong coupling between lattice, charge, spin and orbital degrees of freedom often gives rise to emergent behaviour and multifunctional properties. Such materials are of great interest because the different properties may work together in uniquely different ways and lead to exciting new applications, if we could understand this better. For example, the coupling between magnetic spin and ferroelectric charge ordering can be utilised to develop magnetoelectronic devices where spin transport can be controlled electrically. This is advantageous for technological applications, including novel sensors and ultra-low power memory devices. As a result there is a vibrant effort to understand the underlying mechanisms at play.
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When materials are reduced to the nanoscale, their behaviour is often very different from the bulk material due to quantum confinement effects and the increased surface-to-volume ratio that may result in behaviour that is dominated by surface properties. It is for this reason that our ability to directly image materials in three-dimensions at the surface and in the bulk can greatly increase our understanding of how novel phases develop and influence the material properties.
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The aim of this project is to utilise [[#Coherent Diffraction Imaging | Bragg coherent diffraction imaging]] (BCDI) to image in three-dimensions phenomena in a range of multifunctional quantum materials. This allows us to gain insight into the microscopic properties of the material and facilitates in identifying new and potentially novel applications for the materials.
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=== Ultra-fast Imaging of Strongly Correlated Quantum Materials ===
  
Our group's research is focused on the study of nanomaterials using Bragg coherent X-ray diffraction imaging (BCXDI). BCXDI is a lens-less far field imaging technique that allows imaging of nanometre scale crystalline materials with a sensitivity below a single angstrom. It is largely non-destructive and provides strain information at the surface and throughout the bulk of a material. Conventional BCXDI is performed by illuminating a sample with a spatially coherent X-ray source (a synchrotron X-ray light source) so that the coherence volume exceeds the dimensions of the nanocrystal (~ 1 micron). In the Bragg reflection geometry, scattered light from the entire volume of the crystal interferes in the far-field, producing a three-dimensional k-space diffraction pattern. Iterative phase reconstruction methods (see [[#Non-linear Phase Retrieval Optimisation|Non-linear Phase Retrieval Optimisation]], below) are then used to recover the complex three-dimensional electron density and phase information. The displacement of ions throughout the bulk is directly related to the phase and can be used to obtain three-dimensional strain tensor information (M. Newton ''et al.'' Nature Materials (2010))BCXDI experiments are conducted at synchrtron facilities including the Diamond Light Source (DLS), European Synchrotron Radiation Facility (ESRF), Advanced Photon Source APS, Spring-8 and the SACLA X-ray free electron laser (XFEL) facility.
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Correlated quantum materials are those which the interaction between the valence electrons can strongly influence the materials properties. They are interesting as their unique properties are of considerable utility for device physics, functional materials and the study of fundamental condensed matter physics. Recently, there has been renewed interest in materials that exhibit a solid-solid structural phase transition at a critical temperature. Some of these materials exhibit a reconstructive structural phase transition (SPT) at some critical temperature that occurs on the femto-second time scale. There is still a great deal that is not known about the dynamics of this phenomenon as the ability to probe ultra-fast transitions remains largely inaccessible to conventional techniquesAn example is vanadium dioxide (VO<sub>2</sub>), where spontaneous (non-diffusion limited) atomic rearrangement occurs due to external excitation. Switching speeds on the femto-second timescale have been observed.  
  
=== Strongly Correlated Materials ===
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The aim of this project is to make use of femto-second Bragg coherent diffraction imaging (BCDI) to study ultra-fast SPT's in nanoscale quantum materials. This process became possible with the advent of X-ray Free Electron Laser (XFEL) radiation facilities.  By imaging and hence determining atomic motions during the SPT and making comparison with [[#Ab Inito Molecular Dynamics |ab-initio simulations]], we are able to develop models to describe the observed behaviour which serve as a platform to develop next generation devices.
  
[[Image:VO2TRICXD.png|thumb|center|950px|alt=Research Summary.|Femto-second time-dependent angular deviation of diffraction pattern for a single nanocrystal. (M. Newton ''et al.'' Nano Letters (2014))]]
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=== Machine Learned Phase Retrieval ===
  
Correlated electronic materials are those which the interaction between the valence electrons can strongly influence the materials properties. They are interesting as their unique properties are of considerable utility for device physics, functional materials and the study of fundamental condensed matter physics. Recently, there has been renewed interest in materials that exhibit a solid-solid structural phase transition at a critical temperature. Some of these materials exhibit a reconstructive structural phase transition (SPT) at some critical temperature that occurs on the femto-second time scale.  There is still a great deal that is not known about the dynamics of this phenomenon as the ability to probe ultra-fast transitions remains largely inaccessible to conventional techniques.  An example is vanadium dioxide (VO<sub>2</sub>), where spontaneous (non-diffusion limited) atomic rearrangement occurs due to external excitation (M. Newton ''et al.'' Nano Letters (2014)). Switching speeds on the femto-second timescale have been observed.
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Deep learning has emerged as a powerful alternative to the iterative phase retrieval approach, that can provide robust reconstruction of Fourier-space diffraction pattern data into three-dimensional real-space images where iterative methods often fail to solve the phase retrieval problem. This project is focussed on further developing machine learning methods to understand better how artificial neural networks function and how to implement efficient and robust machine learning methods that are universally applicable to all Bragg coherent diffraction imaging (BCDI) data.
  
To study these materials, our group makes use of femto-second coherent X-ray diffraction imaging to study ultra-fast SPT's in nanoscale materials. This process became possible with the advent of the X-ray Free Electron Laser (XFEL) synchrotron radiation facility.  Here we combine Bragg coherent X-ray diffraction  imaging (BCXDI) with a cross-correlated femto-second pump-probe scheme that allows us to image structural changes with a time resolution of the order of a few femto-seconds.  BCXDI is a powerful lens-less imaging technique for probing crystalline materials with sub-nanometre sensitivity. When BCXDI imaging is performed using the femto-second timing of an XFEL, we can obtain three-dimensional images of nanoscale structures with femto-second temporal resolution and nanometre spatial resolution. As a result, we are able to address the fundamental problem of how to determine atomic motions during a SPT in solid-phase materials. Our findings will likely aid in the development of a dynamical theory that can fully describe this structural change.
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=== Nanomaterials for Energy Harvesting ===
  
=== Ab Inito Molecular Dynamics ===
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[[File:peng_cxs2.png|center|alt=PENG.| Illustration of a piezoelectric nanogenerator made from a forest of nanowires. ]]
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New materials for the capture of light and ambient vibrational energy such as photovoltaic devices and piezoelectric mechanical recouperation devices can potentially enable low-cost and innovative renewable sources of energy while eliminating negative effects on the environment that are inherent when non-renewable sources of energy are utilised. The development of portable energy harvesting power sources as an alternative to batteries is an attractive prospect that will permit a new class of renewable energy devices that can operate indefinitely.  
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Our group is also engaged in large scale supercomputer simulations of structural phase transitions.  We make use of the Iridis high-performance computing facility to perform ab-initio molecular dynamics (MD) simulations of structural phase transitions under various conditions.  Simulations of the ultra-fast SPT allow us to bridge XFEL experiments to current theory and to make predictions as to how the behaviour of a material changes under various conditions. This might relate to a change in the immediate environment of the material or the materials use in a heterostructure device setting. To the right is an animation resulting from an ab-initio simulation of a supercell of vanadium dioxide undergoing a structural phase transition  from monoclinic to tetragonal crystal structure (D. Plasienka ''et al.'' Phys. Rev. B (2017)).
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The aim of this project is to design, fabricate and test a range of novel energy harvesting devices using state-of-the-art facilities within the Southampton Nanofabrication Centre (Zepler Institute) with the aim of optimising the performance characteristics. Extensions will include exploring the role of defects in nanocrystal heterojunction interfaces formed with organic conductive polymers. The outcome will be a complete modelling framework that is able to accurately predict the performance characteristics of devices for a range of configurations.
  
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== Research Techniques ==
  
=== Strong Phase Structures ===
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=== Coherent Diffraction Imaging ===
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[[File:BCXDI.png|alt=Research Summary.|Experimental Geometry for Bragg Coherent X-ray Diffraction Imaging (BCXDI). ]]
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[[Image:AniAu.gif|frameless|300px]]
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[[Image:soi.png|thumb|center|950px|alt=Research Summary.|Amplitude and Phase Reconstruction of Silicon on Insulator (SOI). Missing amplitude due to phase wrapping occurs with conventional techniques for reconstruction. This influences the resulting phase information yielding incorrect phase reconstruction (a,c). We are investigating techniques that can reliable reconstruct missing amplitude (b,d) to correctly reconstructs the objects amplitude (circled in red). (M. Newton ''et al.'' Phys. Rev. B (2010)) ]]
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Our group's research is focused on the study of nanomaterials using Bragg coherent diffraction imaging (BCDI). BCDI is a lens-less far field imaging technique that allows imaging of nanometre scale crystalline materials with a sensitivity below a single angstrom. It is largely non-destructive and provides strain information at the surface and throughout the bulk of a material (J. Miao ''et al.'' Science (2015)). Conventional BCDI is performed by illuminating a sample with a spatially coherent X-ray source (a synchrotron X-ray light source) so that the coherence volume exceeds the dimensions of the nanocrystal (~ 1 micron). In the Bragg reflection geometry (see image above), scattered light from the entire volume of the crystal interferes in the far-field, producing a three-dimensional k-space diffraction pattern. Provided the diffraction pattern is sufficiently oversampled (R. H. T. Bates, 1982),  iterative phase reconstruction methods (see [[#Non-linear Phase Retrieval Optimisation|Non-linear Phase Retrieval Optimisation]], below) are then used to recover the complex three-dimensional electron density and phase information (see animation above). The displacement of ions throughout the bulk is directly related to the phase information (shown as colour mapped onto the object) and can be used to obtain three-dimensional strain tensor information (M. Newton ''et al.'' Nature Materials (2010)). BCDI experiments are conducted at synchrtron facilities including the Diamond Light Source (DLS), European Synchrotron Radiation Facility (ESRF), Advanced Photon Source APS, Spring-8 and the SACLA X-ray free electron laser (XFEL) facility.
  
Strain engineering plays an important role in the development of devices in the semiconductor industry today.  It is also utilised directly in a number of device applications including piezoelectric actuators and sensors. When the amount of strain exceeds a certain limit (unique to each material), the accompanying strong phase structure makes strain mapping challenging. This research topic aims to investigate the origin of strong phase structures in nanometre scale materials and how they impact the CXD imaging process. We do so by systematically studying the transition into the strong phase structure regime.  Results obtained here are also of value in developing advanced numerical optimisation techniques for phase recovery.
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=== Ab Inito Molecular Dynamics ===
  
=== Non-linear Phase Retrieval Optimisation ===
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[[Image:md.gif|frameless|300px]]
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When Bragg coherent X-ray diffraction imaging (BCXDI) measurements are performed, phase information is in general lost as only intensity measurements can be made. It therefore becomes necessary to reconstruct the phase information subject to some a priori information. In general phase retrieval methods operate by the application of iterative projective algorithms that cause the objective function to converge toward the optimal solution by the repeated application of constraints at each iterationOne important application of phase retrieval methods is to obtain real-space images of nanoscale structures by inverting measured diffraction patterns. Correlated electronic materials that undergo a spontaneous crystalline deformation can experience a significant strain. This can pose a challenge when reconstructing and interpreting phase information obtained from BCXDI. We have previously shown that materials exhibiting strong phase structure require special optimisation methods to reconstruct the real-space object. Our group is continuing to develop algorithms to reconstruct data known to have strong phase structure. Significant developments here will lead to robust algorithms that are universally effectiveWe have also developed the [[Bonsu|Interactive Phase Retrieval Suite]], the first software package that allows for real-time visualisation of the reconstruction of phase information in both two and three dimensions. This is used for analysing diffraction patterns obtained from coherent X-ray imaging experiments and for algorithm development.
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Our group is also engaged in large scale supercomputer simulations of structural phase transitions. We make use of the Iridis high-performance computing facility to perform ab-initio molecular dynamics (MD) simulations of structural phase transitions under various conditionsSimulations of the ultra-fast SPT allow us to bridge XFEL experiments to current theory and to make predictions as to how the behaviour of a material changes under various conditionsThis might relate to a change in the immediate environment of the material or the materials use in a heterostructure device setting. To the right is an animation resulting from an ab-initio simulation of a supercell of vanadium dioxide undergoing a structural phase transition  from monoclinic to tetragonal crystal structure (D. Plasienka ''et al.'' Phys. Rev. B (2017)).
  
=== Nanomaterials and Devices ===
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[[Image:zno.png|thumb|center|950px|alt=Research Summary.| Electron micrograph image of ZnO hexagon, tetrapod and rod shaped nanocrystals (1 μm scale bars shown) ]]
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=== Non-linear Phase Retrieval Optimisation ===
  
The increasing need to make smaller device structures with enhanced functionality has led the move away from conventional top-down lithographic approaches toward the utilisation of self-assembled nanoscale structures. The unique morphologies exhibited by many nanocrystals make them ideal for various applications including biochemical sensors, photodetectors, light emitters, transistors, high frequency oscillators, and waveguides. Our research into nanomaterials and devices is centred on self-assembled metal-oxide nanocrystals which have the advantage of high chemical resistance and low thermal degradance. This includes group II-VI semiconductor materials such as zinc oxide (M. Newton ''et al.'' Materials Today (2007)).
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When Bragg coherent X-ray diffraction imaging (BCDI) measurements are performed, phase information is in general lost as only intensity measurements can be made. It therefore becomes necessary to reconstruct the phase information subject to some a priori information. In general phase retrieval methods operate by the application of iterative projective algorithms that cause the objective function to converge toward the optimal solution by the repeated application of constraints at each iteration (D. Sayer (1952), R.W. Gerchberg and W.O. Saxton (1972), J. R. Fienup (1982)).  One important application of phase retrieval methods is to obtain real-space images of nanoscale structures by inverting measured diffraction patterns. Correlated electronic materials that undergo a spontaneous crystalline deformation can experience a significant strain. This can pose a challenge when reconstructing and interpreting phase information obtained from BCDI. To address this challenge, we have  developed the [[Bonsu|Interactive Phase Retrieval Suite]], software package that allows for real-time visualisation of the reconstruction of phase information in both two and three dimensions. This is used for analysing diffraction patterns obtained from coherent X-ray imaging experiments and for algorithm development.

Latest revision as of 12:13, 26 July 2024

Hear from our students on their exciting research and experiences.


Research Interests

Imaging Dynamics in Quantum Materials

The dynamics of materials are governed by their quantum properties. Strong coupling between lattice, charge, spin and orbital degrees of freedom often gives rise to emergent behaviour and multifunctional properties. Such materials are of great interest because the different properties may work together in uniquely different ways and lead to exciting new applications, if we could understand this better. For example, the coupling between magnetic spin and ferroelectric charge ordering can be utilised to develop magnetoelectronic devices where spin transport can be controlled electrically. This is advantageous for technological applications, including novel sensors and ultra-low power memory devices. As a result there is a vibrant effort to understand the underlying mechanisms at play.

When materials are reduced to the nanoscale, their behaviour is often very different from the bulk material due to quantum confinement effects and the increased surface-to-volume ratio that may result in behaviour that is dominated by surface properties. It is for this reason that our ability to directly image materials in three-dimensions at the surface and in the bulk can greatly increase our understanding of how novel phases develop and influence the material properties.

The aim of this project is to utilise Bragg coherent diffraction imaging (BCDI) to image in three-dimensions phenomena in a range of multifunctional quantum materials. This allows us to gain insight into the microscopic properties of the material and facilitates in identifying new and potentially novel applications for the materials.

Ultra-fast Imaging of Strongly Correlated Quantum Materials

Correlated quantum materials are those which the interaction between the valence electrons can strongly influence the materials properties. They are interesting as their unique properties are of considerable utility for device physics, functional materials and the study of fundamental condensed matter physics. Recently, there has been renewed interest in materials that exhibit a solid-solid structural phase transition at a critical temperature. Some of these materials exhibit a reconstructive structural phase transition (SPT) at some critical temperature that occurs on the femto-second time scale. There is still a great deal that is not known about the dynamics of this phenomenon as the ability to probe ultra-fast transitions remains largely inaccessible to conventional techniques. An example is vanadium dioxide (VO2), where spontaneous (non-diffusion limited) atomic rearrangement occurs due to external excitation. Switching speeds on the femto-second timescale have been observed.

The aim of this project is to make use of femto-second Bragg coherent diffraction imaging (BCDI) to study ultra-fast SPT's in nanoscale quantum materials. This process became possible with the advent of X-ray Free Electron Laser (XFEL) radiation facilities. By imaging and hence determining atomic motions during the SPT and making comparison with ab-initio simulations, we are able to develop models to describe the observed behaviour which serve as a platform to develop next generation devices.

Machine Learned Phase Retrieval

Deep learning has emerged as a powerful alternative to the iterative phase retrieval approach, that can provide robust reconstruction of Fourier-space diffraction pattern data into three-dimensional real-space images where iterative methods often fail to solve the phase retrieval problem. This project is focussed on further developing machine learning methods to understand better how artificial neural networks function and how to implement efficient and robust machine learning methods that are universally applicable to all Bragg coherent diffraction imaging (BCDI) data.

Nanomaterials for Energy Harvesting

PENG.

New materials for the capture of light and ambient vibrational energy such as photovoltaic devices and piezoelectric mechanical recouperation devices can potentially enable low-cost and innovative renewable sources of energy while eliminating negative effects on the environment that are inherent when non-renewable sources of energy are utilised. The development of portable energy harvesting power sources as an alternative to batteries is an attractive prospect that will permit a new class of renewable energy devices that can operate indefinitely.

The aim of this project is to design, fabricate and test a range of novel energy harvesting devices using state-of-the-art facilities within the Southampton Nanofabrication Centre (Zepler Institute) with the aim of optimising the performance characteristics. Extensions will include exploring the role of defects in nanocrystal heterojunction interfaces formed with organic conductive polymers. The outcome will be a complete modelling framework that is able to accurately predict the performance characteristics of devices for a range of configurations.

Research Techniques

Coherent Diffraction Imaging

Research Summary.

AniAu.gif

Our group's research is focused on the study of nanomaterials using Bragg coherent diffraction imaging (BCDI). BCDI is a lens-less far field imaging technique that allows imaging of nanometre scale crystalline materials with a sensitivity below a single angstrom. It is largely non-destructive and provides strain information at the surface and throughout the bulk of a material (J. Miao et al. Science (2015)). Conventional BCDI is performed by illuminating a sample with a spatially coherent X-ray source (a synchrotron X-ray light source) so that the coherence volume exceeds the dimensions of the nanocrystal (~ 1 micron). In the Bragg reflection geometry (see image above), scattered light from the entire volume of the crystal interferes in the far-field, producing a three-dimensional k-space diffraction pattern. Provided the diffraction pattern is sufficiently oversampled (R. H. T. Bates, 1982), iterative phase reconstruction methods (see Non-linear Phase Retrieval Optimisation, below) are then used to recover the complex three-dimensional electron density and phase information (see animation above). The displacement of ions throughout the bulk is directly related to the phase information (shown as colour mapped onto the object) and can be used to obtain three-dimensional strain tensor information (M. Newton et al. Nature Materials (2010)). BCDI experiments are conducted at synchrtron facilities including the Diamond Light Source (DLS), European Synchrotron Radiation Facility (ESRF), Advanced Photon Source APS, Spring-8 and the SACLA X-ray free electron laser (XFEL) facility.

Ab Inito Molecular Dynamics

Md.gif

Our group is also engaged in large scale supercomputer simulations of structural phase transitions. We make use of the Iridis high-performance computing facility to perform ab-initio molecular dynamics (MD) simulations of structural phase transitions under various conditions. Simulations of the ultra-fast SPT allow us to bridge XFEL experiments to current theory and to make predictions as to how the behaviour of a material changes under various conditions. This might relate to a change in the immediate environment of the material or the materials use in a heterostructure device setting. To the right is an animation resulting from an ab-initio simulation of a supercell of vanadium dioxide undergoing a structural phase transition from monoclinic to tetragonal crystal structure (D. Plasienka et al. Phys. Rev. B (2017)).


Non-linear Phase Retrieval Optimisation

When Bragg coherent X-ray diffraction imaging (BCDI) measurements are performed, phase information is in general lost as only intensity measurements can be made. It therefore becomes necessary to reconstruct the phase information subject to some a priori information. In general phase retrieval methods operate by the application of iterative projective algorithms that cause the objective function to converge toward the optimal solution by the repeated application of constraints at each iteration (D. Sayer (1952), R.W. Gerchberg and W.O. Saxton (1972), J. R. Fienup (1982)). One important application of phase retrieval methods is to obtain real-space images of nanoscale structures by inverting measured diffraction patterns. Correlated electronic materials that undergo a spontaneous crystalline deformation can experience a significant strain. This can pose a challenge when reconstructing and interpreting phase information obtained from BCDI. To address this challenge, we have developed the Interactive Phase Retrieval Suite, software package that allows for real-time visualisation of the reconstruction of phase information in both two and three dimensions. This is used for analysing diffraction patterns obtained from coherent X-ray imaging experiments and for algorithm development.