Dan A. Allwood

Forgery: ‘Fingerprinting’ documents and packaging

We have found that almost all paper documents, plastic cards and product packaging contain a unique physical identity code formed from microscopic imperfections in the surface. This covert ‘fingerprint’ is intrinsic and virtually impossible to modify controllably. It can be rapidly read using a low-cost portable laser scanner. Most forms of document and branded-product fraud could be rendered obsolete by use of this code.

Fabrication of Luminescent Monolayered Tungsten Dichalcogenides Quantum Dots with Giant Spin-Valley Coupling

A high yield (>36 wt %) method has been developed of preparing monolayered tungsten dichalcogenide (WS2) quantum dots (QDs) with lateral size ∼8-15 nm from multilayered WS2 flakes. The monolayered WS2 QDs are, like monolayered WS2 sheets, direct semiconductors despite the flake precursors being an indirect semiconductor. However, the QDs have a significantly larger direct transition energy (3.16 eV) compared to the sheets (2.1 eV) and enhanced photoluminescence (PL; quantum yield ∼4%) in the blue-green spectral region at room temperature. UV/vis measurements reveal a giant spin-valley coupling of the monolayered WS2 QDs at around 570 meV, which is larger than that of monolayered WS2 sheets (∼400 meV). This spin-valley coupling was further confirmed by PL as direct transitions from the conduction band minimum to split valence band energy levels, leading to multiple luminescence peaks centered at around 369 (3.36 eV) and 461 nm (2.69 eV, also contributed by a new defect level). The discovery of giant spin-valley coupling and the strong luminescence of the monolayered WS2 QDs make them potentially of interests for the applications in semiconductor-based spintronics, conceptual valley-based electronics, quantum information technology and optoelectronic devices. However, we also demonstrate that the fabricated monolayered WS2 QDs can be a nontoxic fluorescent label for high contrast bioimaging application.

Magnetic domain wall propagation in nanowires under transverse magnetic fields

We have investigated the propagation of transverse domain walls in magnetic nanowires under axial and transverse magnetic fields using three-dimensional micromagnetic modeling. Transverse magnetic fields change the domain wall width and, below the Walker field, either increase or decrease the domain wall velocity depending when the field and wall magnetization are parallel or antiparallel, respectively. Furthermore, differences in the Walker field also appear for opposite transverse fields, and a surprising result is that under relatively high axial and transverse fields, Walker breakdown can be completely suppressed and the domain wall velocity returns to several hundreds of ms−1.

Fabrication and Luminescence of Monolayered Boron Nitride Quantum Dots

Monolayered boron nitride (BN) quantum dots (QDs; lateral size ≈10 nm) are fabricated using a novel method. Unlike monolayered BN sheets, these BN QDs exhibit blue-green luminescence due to defects formed during preparation. This optical behavior adds significant functionality to a material that is already receiving much attention. It is further shown that the QDs are nontoxic to biological cells and well suited to bio-imaging.

Sulfur-Depleted Monolayered Molybdenum Disulfide Nanocrystals for Superelectrochemical Hydrogen Evolution Reaction

Catalytically driven electrochemical hydrogen evolution reaction (HER) of monolayered molybdenum disulfide (MoS2) is usually highly suppressed by the scarcity of edges and low electrical conductivity. Here, we show how the catalytic performance of MoS2 monolayers can be improved dramatically by catalyst size reduction and surface sulfur (S) depletion. Monolayered MoS2 nanocrystals (NCs) (2-25 nm) produced via exfoliating and disintegrating their bulk counterparts showed improved catalysis rates over monolayer sheets because of their increased edge ratios and metallicity. Subsequent S depletion of these NCs further improved the metallicity and made Mo atoms on the basal plane become catalytically active. As a result, the S-depleted NCs with low mass (∼1.2 μg) showed super high catalytic performance on HER with a low Tafel slope of ∼29 mV/decade, overpotentials of 60-75 mV, and high current densities jx (where x is in mV) of j150 = 9.64 mA·cm(-2) and j200 = 52.13 mA·cm(-2). We have found that higher production rates of H2 could not be achieved by adding more NC layers since HER only happens on the topmost surface and the charge mobility decreases dramatically. These difficulties can be largely alleviated by creating a hybrid structure of NCs immobilized onto three-dimensional graphene to provide a very high surface exposure of the catalyst for electrochemical HER, resulting in very high current densities of j150 = 49.5 mA·cm(-2) and j200 = 232 mA·cm(-2) with ∼14.3 μg of NCs. Our experimental and theoretical studies show how careful design and modification of nanoscale materials/structures can result in highly efficient catalysis. There may be considerable opportunities in the broader family of transition metal dichalcogenides beyond just MoS2 to develop highly efficient atomically thin catalysts. These could offer cheap and effective replacement of precious metal catalysts in clean energy production.

Nanowire spintronics for storage class memories and logic

Patterned magnetic nanowires are extremely well suited for data storage and logic devices. They offer non-volatile storage, fast switching times, efficient operation and a bistable magnetic configuration that are convenient for representing digital information. Key to this is the high level of control that is possible over the position and behaviour of domain walls (DWs) in magnetic nanowires. Magnetic random access memory based on the propagation of DWs in nanowires has been released commercially, while more dynamic shift register memory and logic circuits have been demonstrated. Here, we discuss the present standing of this technology as well as reviewing some of the basic DW effects that have been observed and the underlying physics of DW motion. We also discuss the future direction of magnetic nanowire technology to look at possible developments, hurdles to overcome and what nanowire devices may appear in the future, both in classical information technology and beyond into quantum computation and biology.

Symmetric and asymmetric domain wall diodes in magnetic nanowires

Micromagnetic simulations reveal how transverse domain walls couple with triangular diodes in magnetic nanowires. For symmetric diodes, the coupling explains the observed differences in the magnetic field required to depin domain walls traveling in opposite directions. In asymmetric diodes, the wall-triangle interaction can lead to order-of-magnitude differences in the depinning fields of oppositely magnetized walls traveling in the same direction. The asymmetric structures therefore combine the diode function of the symmetric structures with domain wall chirality filtering. We also show how two back-to-back diodes may be used to trap a domain wall and form a memory element.

Stress-based control of magnetic nanowire domain walls in artificial multiferroic systems

Artificial multiferroic systems, which combine piezoelectric and piezomagnetic materials, offer novel methods of controlling material properties. Here, we use combined structural and magnetic finite element models to show how localized strains in a piezoelectric film coupled to a piezomagnetic nanowire can attract and pin magnetic domain walls. Synchronous switching of addressable contacts enables the controlled movement of pinning sites, and hence domain walls, in the nanowire without applied magnetic field or spin-polarized current, irrespective of domain wall structure. Conversely, domain wall-induced strain in the piezomagnetic material induces a local potential difference in the piezoelectric, providing a mechanism for sensing domain walls. This approach overcomes the problems in magnetic nanowire memories of domain wall structure-dependent behavior and high power consumption. Nonvolatile random access or shift register memories based on these effects can achieve storage densities >1 Gbit/In2, sub-10...

Optical coatings for improved contrast in longitudinal magneto-optic Kerr effect measurements

We have studied the increases in the longitudinal magneto-optic Kerr effect signal contrast that can be achieved by the application of optical overlayers on magnetic films. For simple coatings, a factor of ∼3 improvement in signal contrast is possible. Matching the optical impedance of the magnetic material improves the raw Kerr signal and also reduces the sample reflectivity, yielding a large Kerr angle. The contrast can be optimized by increasing the rotated Kerr reflectivity component while maintaining enough of the base reflectivity Fresnel component to produce a strong signal. Calculations and experimental results are presented for single layer ZrO2 dielectric coatings on Ni along with calculations for a three-layer Au–ZrO2–Ni structure. Incidence angle effects are also presented.

Pinning induced by inter-domain wall interactions in planar magnetic nanowires

Pinning Induced by Inter-Domain Wall Interactions in Planar Magnetic Nanowires T.J. Hayward 1 , M.T. Bryan 1 , P.W. Fry 2 , P.M. Fundi 1 , M.R.J. Gibbs 1 , D.A. Allwood 1 , M.-Y. Im 3 and P. Fischer 3 Department of Engineering Materials, University of Sheffield, Sheffield, UK Nanoscience and Technology Centre, University of Sheffield, Sheffield UK Center for X-ray Optics, Lawrence Berkeley Natl Lab, Berkeley, CA, USA PACS: 07.85.Tt, 75.60.Ch, 75.75.+a, 85.70.Kh We have investigated pinning potentials created by inter-domain wall magnetostatic interactions in planar magnetic nanowires. We show that these potentials can take the form of an energy barrier or an energy well depending on the walls’ relative monopole moments, and that the applied magnetic fields required to overcome these potentials are significant. Both transverse and vortex wall pairs are investigated and it is found that transverse walls interact more strongly due to dipolar coupling between their magnetization structures. Simple analytical models which allow the effects of inter- domain wall interactions to be estimated are also presented. There is great interest in developing memory [1] and logic [2] devices based upon the controlled motion and interaction of domain walls (DWs) in ferromagnetic planar nanowires. Such domain walls have particle-like properties which allow them to be propagated around complex circuits using rotating magnetic fields [3,4] or short electric current pulses [5], and hence they may be used to represent binary data in a similar way to electric charge in conventional microelectronics. DWs in planar magnetic nanowires have head-to-head (H2H) or tail-to-tail (T2T) character (Fig 1(a)), and consequently they carry a net monopole moment (i.e. a localised excess of north (H2H) or south (T2T) magnetic poles). Therefore, to a first approximation DWs in adjacent nanowires will interact via a Coulomb-like potential: if the DWs have like monopole moments there will be a repulsive interaction, whereas if they have opposite monopole moments their interaction will be attractive. Understanding these effects and how they affect DW propagation is likely to be important to the development of DW based devices, where large nanowire densities will be desirable. So far there have been relatively few investigations into these effects, with studies characterizing attractive coupling between walls with opposite monopole moments for a limited range of nanowire geometries and DW structures [6,7]. We have also previously demonstrated that DW interaction energies are dependent to some degree on the DWs magnetization structure and chirality [8].