Advanced Technology Institute (ATI) Seminars

The realization of efficient semiconductor based spin filters and manipulators is essential for semiconductor spintronics to achieve its promised potential as a route to faster and more energy efficient electronics. One of the challenges is the creation of spin polarized currents within inherently non-magnetic semiconductors. The conventional approach to achieve this has been via the incorporation of magnetic materials. However, it may be possible to produce non-magnetic spin filters with very high efficiency by exploiting the strong spin-orbit interaction present in a number of semiconductors[1-3].

Laser ablation plumes are an example of complex compositional environments that, in addition to atomic components and depending on the ablation conditions, are constituted by molecules, clusters, nanoparticles and larger aggregates. This talk summarizes results on the use of a novel diagnostic procedure of ablation plumes that provides a wealth of information on the spatiotemporal composition of the laser plasma. The method is based on the generation of the harmonics of a driving laser beam propagating through the plasma.

Combining plasmonics with magneto-optical materials introduces nanoscaleinteractions between light fields and magnetisation, hence opening up the possibility of using one of these fields to control the other. In this talk I will give an introduction to magneto-plasmons which, at planar interfaces, are known to exhibit non-reciprocal propagation i.e. the wave vectors for left and right propagation are unequal.

Furthermore I will discuss my work on surface plasmons in metal-insulator-metal (MIM)  slot waveguides. In a MIM waveguide with magnetic dielectric the symmetry between the upper and lower interfaces is broken by the introduction of the magnetic field; the balance between the field distributions on the two interfaces can be controlled by the applied field. As a result an external magnetic field can switch on and off the coupling of an electric dipole to the surface plasmon cavity waveguide modes. In addition I will show that both the total emission of radiation from the cavity and the distribution of the far-field radiation is strongly modified by tuning the magnetisation of the MIM structure.

Indium Tin Oxide (ITO) is the most widely used transparent conductor in the display industry and also finds applications in photovoltaics, EL lighting and a variety of other optical and electronic applications. However, Indium is in short supply and ITO is expensive. It also cracks when used in the current generation of touch screen displays and it is likely to do so in next-generation rollable displays.  It is therefore imperative that viable alternative technologies be developed. In this talk I will highlight some novel approaches we have developed to produce conducting transparent films, which utilize nanostructures such as carbon nanotubes, graphene and metal nanowires. We show using directed or templated assembly of such nanostructures into monolayers or thin polymer composites allows for precise control of electrical percolation. The resulting thin films are flexible, inexpensive and have potential as candidates for the next generation of transparent electrodes.

In cavity quantum electrodynamics (QED), strong coupling of atoms and light is studied via confinement of light inside a cavity.  The same situation can be achieved in the microwave regime in superconducting circuits with an electrical resonator playing the role of the cavity, and a well designed solid state quantum system being used as an artificial atom.  In this talk I will introduce this field of circuit QED, and discuss recent results and future plans on its use in experiments on the strong coupling and quantum control of a variety of solid state systems, such as superconducting qubits and semiconductor quantum dots.

Imaging is one of the most important tools in science and technology, and there have been solid hints that quantum mechanics can help obtain an increase in the imaging resolution over classical methods. In this talk I give an overview of a few imaging techniques and present a unified theory for determining the imaging resolution for classical and quantum imaging. This allows us to properly compare various imaging techniques, and I give a few examples where quantum imaging outperforms classical imaging.

A new edge lithography utilising conventional cleanroom fabrication to create nanometre feature sizes on large area substrates will be presented. Applications including transparent electrodes and nanoscale mapping of thermal transport in graphene will illustrate the talk. A fabrication platform for the integration of carbon nanotubes in scanning thermal microscopy to provide a better insight of nanoscale thermal transport phenomena in materials will also be discussed.

Photovoltaic devices (solar cells) based on thin films of polymeric semiconductors represent an attractive technology to harvest solar energy in a sustainable fashion, as they can in principle be manufactured using materials and techniques having low embodied energy. In this talk, I discuss the basic principles of the operation of organic photovoltaic devices, and discuss some of the materials that we are exploring at Sheffield for photovoltaic applications. I also present recent work where we use a spray-coater to deposit various functional layers in a polymeric photovoltaic device, and argue that such techniques could form a useful part of a low-cost manufacturing process.

Abstract:

Advances in nanoscalefabrication allow for the realization of artificial materials with properties that do not exist in nature, metamaterials. They are composed of subwavelengthelectromagnetic structures, placed at close proximity to each other. Due to mutual coupling between individual structures, they present properties to incident electromagnetic radiation that are different from those associated with the material from which the structures are comprised of. If a liquid crystal (LC) material is added to this geometry it provides a variable refractive index pathway between the CNTs and alters the plasmonicfrequency in a complex way. This is not a simple process to understand as the interaction at the surface of the CNT with the liquid crystal is a dominant effect in this geometry and is very difficult to observe experimentally.  In this presentation, the most recent work we have been doing to demonstrate these unusual properties will be reviewed along with other promising hybrid nanophotonic technology options

Professor Alison Walker from the University of Bath will be giving a seminar to ATI PhD students and staff on Thursday 16 May 2013. Further details will follow.

At Bath, we have developed a model of organic devices that links  morphology (packing arrangements) to device characteristics. The  model, based on the dynamical Monte Carlo approach pioneered in surface physics, allows us to include interaction processes between  different species on many different timescales. In this talk I will  show how we have used this approach to compare organic solar cells of  rod, blend and gyroid morphologies andto model the influence of interlayers, layers added to improve  efficiency and lifetime, in organic light emitting devices, OLEDs. We  have developed the model to allow it to distinguish between triplet  and singlet excitons and allows for the interactions of these species  (triplet-triplet annihilation, triplet-singlet annihilation,  triplet-polaron quenching). I will show our predictions for  
current-voltage-illumination characteristics (solar cells) and  current-voltage-luminance characteristics (OLEDs). I will also show  how through prediction of emission zone profiles in an OLED, we can  gain insight into what determines changes in OLED efficiency with  current and how in the longer term this approach can be used to  address degradation.

Very recently, a number of companies announced organic solar cells with power conversion efficiencies well exceeding 10% on lab scale opening pathway towards a cost-efficient exploitation of this young technology, thereby widely exhausting the efficiency potential for common single junction solar cells. Reasons for the strong efficiency limitations in organic solar cells are among others the spectrally limited absorption of organic semiconductors as well as thermalizationlosses during charge carrier relaxation after the absorption of highly energetic photons. A widely discussed concept to overcome this limitation is the use of tandem solar cell architectures, i.e. the (monolithic) integration of two solar cells in series in a single device stack. Their working principle relies on two different light absorbing semiconductors with different band-gap and hence complementary absorption in order to ensure a broader absorption of the solar spectrum and to reduce the energy losses upon the absorption of highly energetic photons. In fabrication processes, the sophisticated tandem solar cell multilayer-architectures offer many degrees of freedom such as choices for materials and layer thicknesses. Hence, understanding their working principle and optimizing their efficiency is one of the most challenging tasks in organic photovoltaics. Besides carefully chosen complementary absorbers there is a strong need for charge carrier transport layers that allow for the fabrication on an ohmicintermediate contact with low resistivity. Both require advanced solutions in particular when low-cost solution deposition processes are considered with respect to future printing processes.

As the front cell of an organic tandem solar cell needs to be transparent for a certain part of the visible spectrum in order to provide light for the back cell, this technology is closely related to a key application for organic photovoltaics: Semi-transparent devices for building or automotive integration.

In this work we present general concepts for the solution fabrication of both semi-transparent and tandem organic solar cells and how to realize devices with decent power conversion efficiencies. In particular, we present promising concepts for charge carrier transport layers for advanced device architectures and solutions how to overcome solubility limitations.