Heat transfer from nanoparticles for targeted destruction of infectious organisms

© 2017 Informa UK Limited, trading as Taylor & Francis Group. Whereas the application of optically or magnetically heated nanoparticles to destroy tumours is now well established, the extension of this concept to target pathogens has barely begun. Here we examine the challenge of targeting pathogens by this means and, in particular, explore the issues of power density and heat transfer. Depending on the rate of heating, either hyperthermia or thermoablation may occur. This division of the field is fundamental and implies very different sources of excitation and heat transfer for the two modes, and different strategies for their clinical application. Heating by isolated nanoparticles and by agglomerates of nanoparticles is compared: hyperthermia is much more readily achieved with agglomerates and for large target volumes, a factor which favours magnetic excitation and moderate power densities. In contrast, destruction of planktonic pathogens is best achieved by localised thermoablation and very high power density, a scenario that is best delivered by pulsed optical excitation

The importance of scattering, surface potential, and vanguard counter-potential in terahertz emission from gallium arsenide

It is well established that under excitation by short (<1 ps), above-band-gap optical pulses, semiconductor surfaces may emit terahertz-frequency electromagnetic radiation via photocarrier diffusion (the dominant mechanism in InAs) or photocarrier drift (dominant in GaAs). Our three-dimensional ensemble Monte Carlo simulations allow multiple physical parameters to vary over wide ranges and provide unique direct insight into the factors controlling terahertz emission. We find for GaAs (in contrast to InAs), scattering and the surface potential are key factors. We further delineate in GaAs (as in InAs) the role of a vanguard counter-potential. The effects of varying dielectric constant, band-gap, and effective mass are similar in both emitter types. © 2012, American Institute of Physics

Strategies to control the spectral properties of Au-Ni thin films

Gold and nickel have quite different dielectric functions. Here we use a combination of calculation and sample manufacture to assess two strategies by which thin films of these elements can be produced with a controlled range of far-field optical properties. In the first approach, control can be achieved by manipulating the density of states of metastable solid solutions, which in turn controls the dielectric function. In the second approach the optical properties of the films are controlled by varying the geometry of stacks fabricated from the constituent elements. We show that the two approaches can produce equivalent results so both are viable options in practice. Modeling is used to reveal how the structure controls the optical properties and to map out the possible color gamut. Predictions are tested with thin film samples fabricated by magnetron sputtering. © 2013 Elsevier B.V

Nature of magnetism in thiol-capped gold nanoparticles investigated with Muon spin rotation

© 2018 Author(s). Muon spin rotation/relaxation measurements show clear evidence for magnetism in 2.2 nm gold nanoparticles capped with butanethiol. At low temperatures (1.8 K), there is significant spin relaxation which decreases as a function of both the applied longitudinal magnetic field and increasing temperature. The results indicate that there are spatially inhomogeneous electronic moments that fluctuate with a wide distribution of correlation times. Possible explanations are discussed

Terahertz surfoluminescence

The cleaving of a solid to form two new surfaces may result in the emission of light. Conventional mechanoluminescence involves the transfer of charge between the two surfaces. We now demonstrate that the ultra-fast separation of charge within a newly-formed surface will lead to the emission of electromagnetic radiation. In contrast to the visible light previously observed and modeled, the intra-surface radiation contains terahertz frequencies. This new mechanism - named here surfoluminescence - introduces a new class of terahertz-frequency emitters. It also may in part explain the recent observation of terahertz emission from peeling adhesive tape. © 2012, Elsevier Ltd

Role of vanguard counter-potential in terahertz emission due to surface currents explicated by three-dimensional ensemble Monte Carlo simulation

The discovery that short pulses of near-infrared radiation striking a semiconductor may lead to emission of radiation at terahertz frequencies paved the way for terahertz time-domain spectroscopy. Previous modeling has allowed the physical mechanisms to be understood in general terms but it has not fully explored the role of key physical parameters of the emitter material nor has it fully revealed the competing nature of the surface-field and photo-Dember effects. In this context, our purpose has been to more fully explicate the mechanisms of terahertz emission from transient currents at semiconductor surfaces and to determine the criteria for efficient emission. To achieve this purpose we employ an ensemble Monte Carlo simulation in three dimensions. To ground the calculations, we focus on a specific emitter, InAs. We separately vary distinct physical parameters to determine their specific contribution. We find that scattering as a whole has relatively little impact on the terahertz emission. The emission is found to be remarkably resistant to alterations of the dark surface potential. Decreasing the band gap leads to a strong increase in terahertz emission, as does decreasing the electron mass. Increasing the absorption dramatically influences the peak-peak intensity and peak shape. We conclude that increasing absorption is the most direct path to improve surface-current semiconductor terahertz emitters. We find for longer pump pulses that the emission is limited by a newly identified vanguard counter-potential mechanism: Electrons at the leading edge of longer laser pulses repel subsequent electrons. This discovery is the main result of our work.© 2012, American Physical Societ

Microscopic model for exchange bias from grain-boundary disorder in a ferromagnet/antiferromagnet thin film with a nanocrystalline microstructure

Monte Carlo spin simulations were coupled to a Voronoi microstructure-generator to predict the magnitude and behavior of exchange bias in a magnitude/antiferromagnet (AF) thin film bilayer with a nanocrystalline microstructure. Our model accounts for the effects of irregular grain-shapes, finite-sized particles, and the possible presence of local random-fields originating from the antiferromagnet's grain-boundary regions. As the grain-boundary represents a crystal-structure distortion, we model the local effect on the exchange constants in the Gaussian approximation which can cause regions resembling a spin glass confined to an unusual 2D topology. Although an ensemble of completely disconnected AF grains isolated by non-magnetic barriers provides a small exchange bias, the introduction of a spin-glass network at the boundaries causes a four-fold enhancement in the magnitude of the loop-shift. This implies the importance of local grain-boundary behavior in defect-engineered antiferromagnets. © AIP Publishing LLC

Modulating the magneto-crystalline anisotropy and the exchange bias field in CoFe/(Co,Fe)O bilayers using ion-beam bombardment and single crystalline substrates

We report the effects of ion-beam bombardment on the room temperature and low temperature magnetic properties of ferromagnetic CoFe/antiferromagnetic (Co,Fe)O thin film bilayers. The films were deposited onto amorphous SiO(2) and single crystalline MgO(110)/(100) substrates. Magnetometry showed that ion-beam bombardment was capable of modifying the coercivity and loop shape for the thin film system at room temperature, corresponding to alteration of the effective magneto-crystalline anisotropy field. After field cooling to 50 K, a shifted hysteresis loop was seen for those films containing a proportion of the antiferromagnetic rock-salt (Co,Fe)O phase, with an exchange bias magnitude that depended on the ion-beam bombardment conditions. Our results indicate that matching the substrate with appropriate ion-bombardment conditions provides a promising way to engineer selectively two important types of magnetic anisotropy in ferromagnetic/antiferromagnetic bilayers: magneto-crystalline and exchange bias. © 2012, Institute of Electrical and Electronics Engineers (IEEE)

Enhancement of the magnetic interfacial exchange energy at a specific interface in NiFe/CoO/Co trilayer thin films via ion-beam modification

A series of ferromagnetic Ni 80Fe20(55 nm)/antiferromagnetic CoO (25 to 200 nm)/ferromagnetic Co (55 nm)/SiO2(substrate) trilayer thin films were fabricated by ion-beam assisted deposition in order to understand the role of ion beam modification on the interfacial and interlayer coupling. The microstructural study using transmission electron microscopy, X-ray reflectometry, and polarised neutron reflectometry showed that ion-beam modification during the deposition process led to an oxygen-rich Co/CoO nanocomposite interface region at the bottom layer. This interface caused a high exchange bias field for the ferromagnetic cobalt. However, the exchange bias for top permalloy ferromagnet remained low, in line with expectations from the literature for the typical interfacial energy. This suggest that the ion-beam enhancement of the magnetic exchange bias is localized to the Co/CoO interface where local microstructural effects provide the dominant mechanism. © 2020 AIP Publishing LL