Nanophysics Group Seminars

Quasicrystals are new materials that are well ordered but not periodic. Their mechanical and electrical properties are very different from those of periodic crystals. Recent progress in the characterization and preparation of quasicrystal surfaces raises new possibilities for their use as substrates and templates in the growth of films having novel structural, electronic, dynamic and mechanical properties. The apparently unusual frictional properties of quasicrystals also evoke interesting fundamental questions about how physical properties are altered by quasiperiodicity.

Our focus for the past several years has been to study the growth of films on quasicrystal surfaces, with a view toward producing 1-component quasicrystal structures, or quasiperiodically arranged nanostructures. Metal films on quasicrystal surfaces have been found to grow in a variety of modes, including the formation of quasiperiodic Cu multilayer structures, as starfish-like clusters of Al in quasiperiodic arrays, and in hexagonal dendrite structures of Ag on icosahedral AlPdMn. To obtain a more fundamental understanding of the growth process, we have recently studied and modeled the growth of Xe on decagonal AlNiCo. This growth process includes the formation of a quasicrystalline first layer followed by a transition to an fcc(111) structure.

This talk will provide an introduction to many of the interesting features of quasicrystals and well as reviewing the recent progress in the area of film growth on their surfaces.

Piezoelectric materials are widespread in applications both seen and unseen in everyday life. Ultrasonic transducers using piezoelectric materials are well known in sonar, biomedical imaging and therapy, materials processing, and non-destructive testing. For most of these, bulk piezoelectric materials are used, however, obtaining the materials in thin or thick film form is becoming increasingly important. For example, a key application for piezoelectric films is filters for high frequency electronics such as mobile phones, where film bulk acoustic resonators (FBARS) are taking over from surface acoustic wave devices.

In this talk I will discuss piezoelectric films and their properties and applications. Piezoelectric materials are the basis of electromechanical devices, and therefore piezoelectric films are especially relevant to micro electro mechanical systems or MEMS. However, due to materials issues, piezoelectrics have not formed a major part of the main stream of MEMS activity. Now, piezoelectric films are being brought together with MEMS for two reasons: in order to obtain better electromechanical sensing and actuation than can be achieved by mainstream Si-MEMS technology; and in order to more easily realise miniaturisation of electromechanical devices in traditional applications such as ultrasonics.

In Microscale Sensors at University of Paisley we are interested in growth, characterisation, and modelling, of aluminium nitride thin films for MEMS and ultrasonics applications. In addition to a general review of piezo films, I will present some of our work in these areas.

Self-assembly is one of the major approaches to fabricating nanostructures under investigation today. Although it has several advantages over “top-down” methods, (focussed ion beam milling, STM manipulation, etc.) precise control over the end result is a challenge.

This talk will discuss ongoing work at Nottingham aimed at understanding and controlling self-assembled systems, specifically hydrogen-bonded networks of organic molecules on surfaces. The influence of substrate interactions, adsorbate binding and commensurability will be considered.

A focus of current research is exploring the possibility of forming these networks on a boron nitride “nanomesh”. A review of this substrate is given, followed by an overview of progress towards this goal, and planned experiments.

The introduction of modern surface science methodologies has led to rapid advances in our current understanding of chemical phenomena at the solid-gas/liquid interface, and has impacted particularly on mechanistic and kinetic studies of heterogeneous catalysis [1]. However the development of new catalytic materials still proceeds by largely empirical trial-and-error methodologies. To a large degree this reflects the dearth of quantitative experimental techniques suitable for the identification of surface intermediates, and the determination of surface dynamical processes during reaction [2]. We have recently demonstrated that Fast X-ray Photoelectron Spectroscopy is an ideal tool for studying such surface reactions [3]; facilitated by the high photon flux and high-resolution available at 3rd-generation synchrotrons. Fast XPS represents a chemically specific, quantitative surface probe for following adsorption/reaction over catalytically active surfaces in a time-resolved fashion. Here we illustrate the power of this technique for elucidating reaction pathways in environmental applications including C-H activation and VOC destruction [4], and explore how more versatile alloy and oxide thin-film model catalysts can also be constructed.

[1] D.W. Goodman, Surf.Rev.Lett. 2 (1995) 9.
[2] J.M. Thomas and G.N. Greaves, Science 265 (1994) 675.
[3] A.F. Lee, K. Wilson, R.L. Middleton, A. Baraldi, A. Goldoni, G. Paolucci and R.M. Lambert, J.Am.Chem.Soc. 121 (1999) 7969.
[4] A.F. Lee, K. Wilson, J.Vac.Sci.A 21 (2003) 563; A.F. Lee, P.A. Carr, K. Wilson, J.Phys.Chem.B 110 (2006) 907.

Left - Temperature-programmed C 1s Fast XP spectra of a reacting 1,1,1-trichloroethane adlayer adsorbed at 95 K over a Pt(111) catalyst surface; Right - corresponding C 1s 3D image map.

The talk will describe the Leicester programme on measuring the magnetic properties of size-selected magnetic nanoparticles from the simplest case of isolated particles adsorbed on a surface in UHV through to dense interacting assemblies embedded in matrices. The effect of the embedding matrix on the atomic structure and magnetic properties of the clusters will be described. The talk will also focus on the magnetisation and switching behaviour of materials made by assembling pre-formed nanoparticles. The prospect for producing high-moment materials and future directions of the research, including the use of pre-formed core-shell nanoparticles will be presented. In addition some new work on life science applications of nanoparticles will be described.

A three-dimensional medium-energy ion scattering (3D-MEIS) spectrometer has been developed for crystallographic structure analysis of nanomaterials. In 3D-MEIS, a pulsed He⁺ ion beam at a medium energy of 100 keV is incident upon a sample and scattered particles are detected by a three-dimensional detector. The three- dimensional detector used is a position-sensitive and time-resolving micro channel plate detector with two delay-line anodes wound perpendicular to each other. 3D-MEIS has been employed to analyze the structure of Er silicides on Si substrates. Clear blocking patterns based on the structures of Er silicides were observed. The results indicate that 3D-MEIS is effective for the crystallographic structural analysis of nanomaterials.

Time and location: 14:15 in P/T111

Discovery of the giant magneto-resistance (GMR) effect and the tunneling magneto-resistance (TMR) effect opened a new research area called as "spintronics", in which two possessions of an electron, i.e. charge and spin, are utilized at the same time. It has been also shown that a direct current applied into a magnetic nano-pillar may excite a magnetization switching [1] and a spontaneous precession of the magnetization [2] exerting a spin-torque on it. Here, we report a new phenomenon based on the interplay of a current induced spin-torque and a spin-dependent transport. We show that magnetic nano-pillars produce dc voltage, when a small radio-frequency (rf) current is passed. This "spin-torque diode"[3] is realized here by employing high performance magnetic tunnel junctions (MTJ) with a crystalline MgO barrier [4].

We performed experiments on a MTJ of structure Si(substrate)/PtMn(15 nm)/CoFe(2.5 nm)/Ru(0.85 nm)/CoFeB (3 nm)/MgO(0.85 nm)/CoFeB(3 nm) [5]. This multi-layered film was patterned into oval shaped pillars of dimension 200 nm x 100 nm, using electron-beam lithography and ion milling techniques. The bottom anti-ferromagnetically coupled CoFe and CoFeB layers (so-called synthetic antiferromagnetic (SAF) layer) act as a pinned layer, while the top CoFeB layer acts as a free layer. Resistance of MTJs is about 100 Ohm for parallel configuration and is 200 Ohm for antiparallel configuration at room temperature. We applied an external dc field to the MTJs to set a certain angle between the free layer magnetization and that of pinned layer. The external field was in film-plane and tilted by 30 degree from long axis of the oval.

By applying a small rf-current (-15dbm for example) we observed spin-torque diode effect spectra of the samples. The spectra showed clear resonance peaks. The resonance frequency was shifted as a function of applied field and was well fitted by a Kittel’s formula. To make sure about excited modes, we also tried to observe thermal noise spectrum. As a result, center of the resonance like spectra appeared in the diode spectra coincides with that in thermal noise spectra. From analysis based on a single macro-spin model, co-existence of both spin transfer torque and a filed like torque [6] was revealed. The spin-torque diode can be a frequency selective detector in telecommunication circuits and also provides a relevant technique to investigate physical origins of the current induced spin-torque in the magnetic nano-pillars.