Projects
The following projects on thin films and coatings are currently running::
- Active substrate approach for switchable multiferroic thin films
- Development of organic vapour phase deposition for the formation of nanostructures and complex films
- Magnetic properties of molecular thin films
- New film architectures for improved performance in molecular solar cells
- Novel multiferroic thin films with artificial superstructure and active substrate
- Ferroelectrics for Nanoelectronics (FERN)
- Nano-Scale SQUID Magnetometry of Oxide Heterointerfaces
- Nanostructured Functional Materials for Energy Efficient Refrigeration, Energy Harvesting and Production of Hydrogen from Water
- Active Plasmonics: Electronic and All-optical Control of Photonic Signals on Sub-wavelength Scales
- Multiferroic Nanostructured Thin Films
- Ultra violet radiation controlled non-linear dielectrics
- Foundations of Molecular Nanospintronics
- Determination of Surface and Interface Processes in Materials Science
- Molecular Spintronics
- Molecular Thin Films: Growth, Magnetism and Spintronic Applications
- Heterointerface control of organic semiconductor devices
- Self-organized nanostructures and transparent conducting electrodes for low cost scaleable organic photovoltaic devices
- Solution-processable High-refractive Index Hybrid Systems for Photonic Structures
- Solution Processable Chemically Derived Graphene for Large-area Electronics
- Engineered Nano-layered Structures for Energy Harvesting Devices
- Fabrication of Nanorods on the Industrial Scale
A list of previous projects is also available here
Active substrate approach for switchable multiferroic thin films
Researchers
Dr Anna-Karin Axelsson, Dr Matjaz Valant
Supervisor
Funding
The Leverhulme Trust, January 2008 to December 2010
In the quest for ever-higher data densities, multiferroics can provide a medium for a four-state, rather than two-state data storage by switching the ferroelectric and magnetic domains. Here, it simply means a management of magnetic domains by other means than a magnetic field such as an electric field, and this is of high interest, for example, for read/write devices. However, the coupling between the magnetic field and electric field (ME coupling) has to be high for commercial interest.
The ME coupling is relatively low in a single-phase multiferroic material but for a two phase magnetostrictive and piezoelectric composite, the ME coupling can be increased dramatically. The ME coupling appears when an applied electric field creates an alteration of the magnetic properties in the thin-film via the interface elastic coupling.
Thin-film growth needs a substrate to grow and by using piezoelectric substrates such as BaTiO3 or PZT, which by themselves will respond to an external electric field, we reduce the normal occurring clamping effect. In a bi-layer like CoFe2O4-BaTiO3 for example, the strain developed in the piezoelectric BaTiO3 substrate will be directly transferred to the thin magnetostrictive CoFe2O4 layer without any reduction in electric displacement, d31 (see illustration below).
Schematic representation of the conventional magnetoelectric thin-film system deposited on the passive substrate and a system with thin film deposited on the piezoelectric active substrate. The coupling between magnetic field and electric field (ME coupling) will be dramatically increased using an active substrate, to the left.
The initial step of this research was to look into the chemistry of CoFe2O4, chosen because of its high magnetostrictive (magnetoelastic) properties. In bulk, the magnetic properties are mainly dominated by volume but in thin-films other sources plays an important role. These factors can be; ordering of Co and Fe ions, epitaxy, oxygen vacancies, broken symmetries and surface defects.
By fine adjusting the thin-film processing (here by Pulsed Laser Deposition) such as laser output, ambience in the deposition chamber, post annealing conditions, different magnetic properties will be achieved. In addition, the interface at the growth template i.e. the active piezoelectric substrate, plays a crucial role as different developed strain and stress can further manipulate the CoFe2O4 magnetic properties.
Crystallographic tools such as XRD, AFM, TEM and Raman are used while the magnetic properties are measured by SQUID, VSM and MFM. The results are compared to see the relation between of film thickness-interface – deposition factors – magnetic properties. After a full understanding of the chemistry and physics of the thin films, the magnetostrictive properties will be investigated to determine how to maximize the ME-coupling.
Initial results prove that the coercivity can be reduced to 200 Oe while retaining the magnetisation high in a 13nm film on a SrTiO3 substrate, which indicates a greatly reduced magnetic field is needed to obtain the domain switching in these ultra thin-films compared with bulk.
Development of organic vapour phase deposition for the formation of nanostructures and complex films
Supervisor
The ultimate control over film thickness and properties can be attained using sublimation techniques, were a beam of single molecules is directed towards a substrate. Conventionally sublimation is attained in a high vacuum chamber, through organic molecular beam deposition (OMBD), but this technique is costly, and not compatible with the motivation for cheap plastic electronics. In organic vapour phase deposition (OVPD), growth is performed in a chamber held at moderate vacuum and enclosed in a furnace, and the molecules are s wept to a cooled substrate using a beam of inert gas. OVPD allows controlled growth at low cost, and more flexibility than OMBD. Particular topics of interest are:
- The correlation between growth conditions and film morphology and structure, with particular focus on high aspect ratio molecular crystals
- Understanding of growth mechanisms, and
- Generation of complex charge-transfer salts, with accurate control over stoichiometry.
Magnetic properties of molecular thin films
Supervisor
We have shown that thin films based on metal phthalocyanines (MPcs), archetypal semiconductors used in some of the most successful organic solar cells to date, can display magnetic ordering, which can be switched depending on the structure adopted by the crystallites. By using organic molecular beam deposition in a range of conditions to create distinct polymorphs on flexible substrates, and developing new methods to enable measurements of magnetic moments in very thin films, we have demonstrated that the crystal phase transition corresponds to the switching of interactions from antiferro- to either paramagnetic (in the case of CuPc) or ferromagnetic (MnPc). Our multidisciplinary team at the London Centre for Nanotechnology was able to rationalise the mechanisms of the magnetic coupling using perturbation theory and ab-initio calculations, attributing it to indirect exchange. Currently, we are focusing on increasing transition temperatures, improving control ov er switc hing, and gaining further understanding of single-spin characteristics or molecular coupling using spin resonance techniques.
See our work reviewed in Nature's News and Views!
New film architectures for improved performance in molecular solar cells
Supervisor
Organic solar cells based on small molecules have attained promising conversion efficiencies, and values of 5% have recently been reported in the literature. The high degree of control and versatility of sublimation methods has allowed us to focus on modifying device architecture in a bid to improve efficiencies. For example, we have doubled the efficiencies of a simple donor/acceptor bilayer structure by moving towards a gradient cell, where the stoichiometries of donor and acceptor vary continuously, forming a composition gradient between the electrodes and creating large interfaces that improves photon harvesting. Now we want to use the new morphologies obtained using organic vapour phase deposition in order to generate nanostructured molecular heterointerfaces with high surface area.
Novel multiferroic thin films with artificial superstructure and active substrate
Researchers
Dr Anna-Karin Axelsson and Dr Matjaz Valant
Supervisors
Professor Neil Alford and Professor David McComb
Funding
EPSRC, January 2008 to December 2010
Multi-functional materials can respond to more than one external stimulus. Here we will study a multiferroic system which couples two or more switchable states such as polarization and magnetisation.
The Problem
Theoretical calculations predict possible magneto-electric (ME)couplings up to >2000mV/cm Oe. The problem is that all true single phase multiferroics possess insufficient coupling to be useful for devices and magneto-electric coupled composites, so far, only reach a fraction of the theoretical predicted coupling.
The Solution
In this research we will make thin films with highly periodic nano-layered structure, in which the alternating ferroelectric and ferromagnetic layers will form an artificial single phase superstructure (multiferroic artificial superstructures). This will be deposited on a lattice-matched piezoelectric substrate for optimizing the magnetoelectric coupling. We will have full control of the crystallographic orientation of the electric and magnetic layer, optimization of the interface and thickness of layers. We will therefore improve the ME coupling by reducing the clamping effect both on substrate–film and film-film interface. By decreasing the layer’s thickness until approaching nanometers we will produce a periodic nanostructure where the layers becomes electrically and/or magnetically coupled and where, eventually, a crossover point of the two modes of couplings (elastically mediated and inherent multiferroic) will be reached.
The Approach
In order to achieve these, the project will:
- Investigate the constituent ferroelectric and ferrimagnetic components: optimize piezoelectricity in lead-free piezoelectric thin films; adjust magnetic properties of CoFe2O4 for the use in the multiferroic artificial superstructure
- Make periodically layered multiferroic thin-films described as an artificial crystallographic superstructure
- Investigate the clamping effect and coupling between interfaces of magnetic and electric layers
- Examine the extent of different coupling modes such as field-induced magnetoelectric, inherent multiferroic and linear - quadratic piezoeffects as a function of film thicknesses
- Optimise the electro-magneto coupling by controlling strain and ionic
substitution.
Nano-Scale SQUID Magnetometry of Oxide Heterointerfaces
Researcher: Clementine Walker
Supervisor: Professor Neil McN Alford
Sponsor: EPSRC Project Studentship
The study of the interplay between the electronic and magnetic properties of complex functional oxide materials is of central importance to the international condensed matter physics community, and for the future development of electronic devices. Recently this field has been set alight by pioneering work at Tokyo and Cornell Universities that showed it is possible to obtain a highly mobile two dimensional electron gas at the interface between two perovskite oxides, SrTiO3 and LaAlO3, both of which are insulating. In that work the oxides were grown in a layer-by-layer manner by pulsed laser deposition (PLD) with atomic level monitoring and control. The work has pushed the capabilities of PLD to a new level. Other researchers have since found indirect evidence for magnetic ordering at this type of interface below ~300 mK and have recently detected a superconducting transition in the two dimensional electron gas at ~200mK. The potential of this work for a new generation of electronic devices is enormous, but so far there are many unresolved issues about the nature of this two-dimensional electron gas, the role of oxygen vacancies close to the interface, and especially the nature of the magnetic ordering and how it relates to the superconducting state.
We are addressing and answering these key questions by developing new nano-scale sensors and measurement techniques to probe the dc and ac magnetisation of small mesas containing a two dimensional electron gas at an oxide heterointerface. By confining the two dimensional electron gas to a small area ~ 200nm x 200 nm we will minimise issues relating to defects in oxide films. This interface is buried well inside the oxide structure and cannot be probed by surface techniques such as scanning tunnelling microscopy. Instead we are developing sensors based on nano-scale superconducting quantum interference devices (SQUIDs) that are very sensitive detectors of magnetic flux. These consists of a very small loop of superconducting thin film interrupted by two weak links (Josephson elements) which consist of a very narrow track (~150 nm wide) made by a focussed ion beam (FIB). We are designing and optimising such devices to operate at temperatures from 4.2K down to ~ 100mK, and integrating them with oxide structures. SQUID-based instruments are the key tool in many laboratories for performing dc magnetisation and ac susceptibility measurements on macroscopic samples containing a very large number of magnetic moments. By shrinking the devices to the nano-scale we are able to measure much smaller changes in magnetisation and have sufficient resolution to make useful measurements on the relatively small number of magnetic dipole moments expected in our oxide samples.
Solution-processable High-refractive Index Hybrid Systems for Photonic Structures
Researcher: Christopher Burrows
Supervisor: Dr Natalie Stingelin and Professor Molly M Stevens
Sponsor: EPSRC (DTA)
This project addresses the clear need for 1) new materials with increased optical/photonic functionalities; and 2) novel concepts and approaches that will allow better control to manipulate photons and that advance our capability to do so in a more straight-forward fashion. The project will focus on photonic crystals, i.e. structures, in which the refractive index changes periodically. Because of these regular optical modulations, photonic crystals strongly interact with light, provided that the critical dimensions are in the order of optical wavelengths. Within this project, new materials systems will be designed and developed that display both, good processability and sufficiently high refractive indices that can be tuned according to specific requirements.
Solution Processable Chemically Derived Graphene for Large-area Electronics
Researcher: HoKwon Kim
Supervisor: Professor Eduardo Saiz Gutierrez
Sponsor: NSERC (Postgraduate Scholarship) and The Leverhulme Trust
While graphene could be viewed as the material for next generation of electronics, reliable means of fabricating and manipulating it for large-scale integration into devices are presently lacking. To overcome this, fabrication method using solution processable chemically derived graphene via oxidative exfoliation of graphite has been developed. Aqueous dispersions of chemically derived graphene provide solution process for uniform deposition of graphene thin films, facilitating its implementation to devices. However, chemically derived graphene contains defects introduced by the chemical treatments. The aim of the project is to investigate the atomic and nano scale structures of chemically derived graphene and their effects on the macroscopic optoelectronic properties.
Engineered Nano-layered Structures for Energy Harvesting Devices
Researcher: Dr Bin (Kevin) Zou
Supervisors: Professor Neil McN Alford and Dr Peter K Petrov
Sponsor: KAUST
This project is a feasibility study aimed to a particular type of nanolayered energy harvesting structure, which will be used for development of high frequency (THz) rectenna. The study is focussed on three practical problems. Firstly, to deposit continuous ultra-thin (up to 10nm) functional oxide and metal thin films. Secondly, to develop techniques for ultra-thin structural and electrical characterisation. Finally, novel concept development of an energy harvesting device. To date work has mainly focussed on deposition of thin and ultrathin (Ba0.5,Sr0.5)TiO3 (BSTO) films and their electrical and structural characterisation. Work on rectenna device layout has been carried out in parallel. Films of (Ba0.5,Sr0.5)TiO3 with thicknesses varying from 150nm down to 12nm were deposited on LaAlO3 and MgO substrates and their electrical and structural properties characterised. Work is in progress to reduce further the BSTO film thickness and improve their electrical properties.
Fabrication of Nanorods on the Industrial Scale
Researcher: Dr Fang Xie
Supervisor: Dr Jason Riley and Dr Mary P Ryan
Sponsor: KAUST
In recent years, the fabrication of one-dimensional (1-D) nanostructures has attracted ever-increasing interest for its applications in many fields, including magnetics, self-assembly, electronics, biology, catalysis and optics. Among all the synthesis techniques, the template method for 1-D nanostructures synthesis has become a very simple yet powerful process, with the advantages of low cost, high throughput, high volume and ease of production. It is clear that if practical applications, such as solar energy and catalysis, are to be realised, methods for mass-producing template-synthesized nanostructures will be required. Anodic Aluminium Oxide (AAO) template is a well-established nanotechnique and has become a method of choice for scientists wishing to synthesise and characterise small quantities of multisegmented nanostructures. By varying the anodization voltage of the aluminium foil or film and the electrolyte, phosphoric, sulphuric or oxalic acid, the density and diameter of the nanopores can be readily controlled. The aim of this project is to demonstrate that this template electrosynthesis by AAO membrane can be employed to produce large quantities of nanomaterials of defined dimension, for their applications in electronics, optics, solar energy and sensor technology. To objectives that must be met are: production of large area AAO membranes; uniform filling of large area AAO membrane via electrosynthesis; and release of the electrosynthesised nanomaterials from the membrane.
