CRYSTALLOGRAPHY AND FUNCTIONAL EVOLUTION OF ATOMICALLY THIN CONFINED NANOWIRES

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

This Established Career Fellowship proposal concerns the spatial and time resolved crystallography, structural refinement and functional evolution of one to four atom thick 1D 'Extreme Nanowires' formed inside single walled carbon nanotubes - atomically smooth templates that are thermally robust up to 1130 degrees Centigrade. This is at the practical limit of scalable fabrication, a 'Final Frontier' of materials science and the next and ultimate lowest dimension relative to two-dimensional structures such as graphene or inorganic analogues derived from metal sulphides and similar. It will address the four major aspects of atomically regulated crystal growth deemed to be the most critical in terms of their development: three-dimensional crystallography with atom-by-atom sensitivity; four-dimensional crystallography, addressing the special case of nano-Confined Phase Change Materials, which have potential utility in Non-Volatile Memory; structural refinement based on enhanced information obtained from the forgoing structural studies; and the development of thin film devices, which may be either aligned or misaligned, both for fundamental properties evaluation - including several aspects of Novel Physics - but also for 'Proof of Principle' device creation for potential exploitation in thin film devices including solar cells, chemical sensors, fuel cells, batteries and catalysts, all of which may bring economic benefits.

This project is expected to play a significant role in techniques development both in Warwick and through the Project Partner network based in Oxford, Vienna, Warsaw, Pau and Beijing. The 'Ultimate Scale' physical nature of the materials under examination will both test and help improve the most sensitive characterisation methodologies currently available, especially high performance electron microscopy, associated spectroscopic methods, in situ low-temperature imaging, in situ resistance/conductivity measurements, high performance scanning probe microscopies and thin film device fabrication. This project is expected to have in particular a very significant impact on the current rapidly developing field in high-performance electron microscopy in time-resolved 4D studies in which I will exploit ultrafast imaging and diffraction capabilities available in both Oxford/Diamond and Beijing, taking advantage also of the latest developments in Direct Electron Detection in rapid and more quantitative imaging studies. In this regard, the special category of nC-PCMs will provide an ultimate test being literally the smallest scale (i.e. 1 nm) Phase Change Materials ever observed and these experiments will therefore examine reversible and irreversible phase formation at the smallest scale ever likely to be attempted. The new electron diffraction protocol that I have developed will also allow us to 'scale up' phase transformation in bundles and thin films of the template SWNTs enabling resistance and conductivity changes to be measured in situ at the same time as crystalline/glass transformations and to assess their reversibility. Any exploitable physical properties will then be assessed in simple bolometric-type devices that will be used both in further physical testing but also as exploitable 'Proof of Principle' devices.

These studies will make use of the many state-of-the-art facilities available at the University of Warwick as the project requires but also more dedicated expertise and information available through a Project Partner network based in the Oxford, Vienna, Warsaw, Pau and Beijing all of whom contribute to and benefit from techniques development, reciprocal characterisation and from developing/expanding their own activities in what will be a unique World-leading enterprise lead from Warwick. The PI will lead these activities from the UK which can then potentially add 1D nanostructures to its dominance in 2D Nanomaterials by pursuing this complementary but 'Beyond Graphene' research.

Planned Impact

Nanotechnology and Information: A major driving force of nanotechnology is Moore's Law scalability as the minimum feature size of electronic devices gradually approaches the atomic scale. A societal impact of device miniaturisation has been the immense proliferation of hand-held electronic devices, projected to reach 2.9 billion users out of a world population of ~7.5 billion by 2020. This has produced a rapidly developing 'Information Age' that impinges on all aspects of society from health care, economics and politics. The performance of such devices is contingent on processing power, image pixel density, battery capacity, and 'volatile' memory storage, all driven by nanoscale materials and devices. My Fellowship project potentially impacts on several of these aspects - see also below - but on memory storage in particular. Currently, the minimum feature size memory devices is around 20 nanometres but I wish to explore rewritability in Phase Change nanomaterials with feature sizes twenty times smaller than this which could have an immense impact not only on information storage technology itself but also the societal impact that such devices bring.

New Characterisation Methods: The last twenty years have seen incredible advances in Electron Microscopy imaging techniques in terms of spatial resolution, element sensitivity and three dimensional imaging, all of which are also important to my project as my composites are 'Ultimate Specimens' that test these techniques to their limit and assist in furthering their improvement. The 'next big thing' currently being explored by the microscopy community is however time-resolved (i.e. 4D) imaging and here we can expect many further improvements in imaging methodologies regarding the speed and sensitivity with which time-resolved images can be acquired. One of the most exciting aspects of my project will be following atomic scale microstructural changes side-by-side with properties changes made possible by the latest generation of Micro-Electro-Mechanical System (MEMS) holders. In this respect my specimens are also 'Ultimate Specimens' as I will investigate crystalline to glass transformations at the smallest scale ever attempted and also explore how these translate to larger scale samples, especially thin films and nascent devices using novel diffraction approaches which can then be shared with the microscopy community at large.

Nanomaterials, New Materials and New Devices: Besides improving rewritable memory, I will also investigate sensor properties in simple bolometric devices which will make use of physical and chemical changes as conductivity, resistivity, changes in magnetic properties and polarity. These various aspects may find application in devices and composites including but not limited to solar cells, chemical sensors, smart composites for used for example in fabrics, fuel cells, batteries and catalysis, all of which may bring economic benefits.

People and Knowledge: It is anticipated that all of the people involved in this project based in Warwick, Oxford, Europe and China will benefit hugely from various aspects of this project which I have designed with a strong international component in terms of techniques development, new research initiatives and capacity. While initially the project will operate within the comparatively small Network of researchers based in the immediate circle of researchers named in the proposal, the undoubtedly and already demonstrated high profile nature of this project guarantees its future expansion. I am particularly keen to encourage European expansion, in the current political climate, but will seek expansion beyond the boundaries of the EU also. Members of my research team, especially RK and new PhD (TBA), will benefit from extensive international collaboration, will learn new time-resolved microscopy techniques and in presenting results at high profile International conferences in Europe and the US.
 
Description Preliminary outcomes

WP1. 3D Crystallography, Symmetry and Composition

(Objectives) "The major objectives here are to produce the most accurate quantitative 3D crystal structures of 1-4 atom thick crystals taking into account the 3D disposition of atoms.following preliminary synthesis."

Several new syntheses have been achieved including a new phase change material (i.e. Sb2Se3), superconducting phases based on Ag1-xSnxSe2 and novel ternary phases based on two halide perovskite phases. Some imaging has been done at STFC Daresbury (SuperSTEM) and EXAFS work on one of the encapsulated phases was performed at DIAMOND.

The following papers have either been published or have been submitted during this project so far relevant to WP1:

(published) Unprecedented new crystalline forms of SnSe in narrow to medium diameter carbon nanotubes C. Slade, et al. J. Sloan, Nano Lett. 19, 2979-2984 (2019).

(published) Vibrational dynamics of extreme 2 × 2 and 3 × 3 potassium iodide nanowires encapsulated in SWCNTs. et al. J. Sloan, and E. Faulques, Phys. Rev. B 98, 125429/1-9 (2018).

WP2. 4D (Time-Resolved) Crystallography and Spectroscopy

(Objectives) "Here the principal objective is to extend WP1 to the fourth dimension (4D, i.e. time) and produce time resolved structural transformations of 1-4 atom thick nano-confined Phase Change Materials (nC-PCMs)"

Our first major research visit (PI plus PDRA) was to Prof. Huaixin Yang's group at the Beijing IoP. We carried out time and temperature programmed electron diffraction experiments on phase change materials embedded in carbon nanotubes. In addition the first rapid laser pulse phase transformation experiments were performed showing a second route to reversibly phase transforming crystalline to non-crystalline phases - a 'proof or principle' for phase change memory. Complementary diffraction work was carried out at DIAMOND

WP2 benefitted from the recruitment of two new PhDs students, one of whom was allocated by Warwick as a result of the EPSRC award, a second from a Chinese CSC award Fellowship. One PhD student (Warwick) is being trained to carry out parts of WP2.

(internal report) 'Summary of Laser-Induced Reversible Phase Transition in SWCNT'

WP3. Purification Issues: SWNTs and Nanowire Refinement

(Objectives) "Because of their mode of formation, two thirds of all possible SWNTs are semiconducting while the remainder are metallic and I aim to produce single conformation.SWNTs on a mg scale"

The second CSC PhD student is working on SWCNT and nanowire refinements. Nanotube conformation separation has been achieved using a combination of two-phase separation and ultracentrifuge approaches. Some of this work is being already applied to WP4. No publishable outputs as yet.


WP4. Thin Films and Trial Devices

(Objectives) "ENs and nC-PCMs in SWNTs are most likely to see their novel properties exploited in thin films, arrays and composites.."

We have made some progress on this initial part essential for the rest of WP4. The CSC PhD student is already working on producing pure chirality SWCNT films for various testing experiments and we have demonstrated that such films can easily be filled from the vapour phase with target 'thermoelectric' materials. No publishable outputs as yet.
Exploitation Route The biggest potential Impact of this research so far is the demonstration of reversible phase transformations of phase change memory materials using a combination of electron beam irradiation (previously demonstrated), annealing (newly demonstrated), in situ heating (newly demonstrated), laser pulses (newly demonstrated) and also both the reversibility or non reversibility of this process as a function either of the applied heat or the applied laser energy. These protocols can be developed further to demonstrate phase changes at a size scale of 1 nm^3 which is about 6x smaller than current devices.

In addition to the above, we have demonstrated the formation of at least four new crystalline forms of tin selenide (published in Nano Letters) which presents challenges for theory sciences in terms of establishing their stability and physical properties. We also have (unpublished) several new crystallin forms based on the phase change material antimony selenide, the halide perovskites based on caesium tin iodide and caesium lead bromide and the superconductor Silver-Tin-selenide which will present similar challenges to the above new crystalline forms of tin selenide.
Sectors Aerospace, Defence and Marine,Chemicals,Digital/Communication/Information Technologies (including Software),Education,Electronics,Other