W. Van Roy

imec Belgium, Louvain, Flanders, Belgium

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Publications (145)405.98 Total impact

  • Nima Arjmandi, Willem Van Roy, Liesbet Lagae
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    ABSTRACT: Nanopores have recently been developed for the detection and physical characterization of nanoparticles, viruses, proteins, nucleic acids and other macromolecules in liquids. The method provides the ability to rapidly estimate the size and electrical charge of analytes over a wide range of concentration, potentially with small sample volumes and low cost. Here, we use the technique to measure the mass of nanoparticles and viruses, and their sedimentation. The analyte sedimentation-time measurement provides an estimate for the nanoparticle mass-density. We also show that the method can be used with samples at low concentration and in small volumes.
    Analytical Chemistry 04/2014; · 5.70 Impact Factor
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    ABSTRACT: We present the top-down synthesis of a novel type of MRI T2 contrast agent with great control over size and shape using a colloidal lithography technique. The resulting synthetic antiferromagnetic nanoparticles (SAF-NPs) yield improved relaxivities compared to superparamagnetic iron oxide alternatives (SPIONs). For T2 weighted imaging, the outer sphere relaxation theory has shown that the sensitivity of a T2 contrast agent is dependent on the particle size with an optimal size that exceeds the superparamagnetic limit of SPIONs. Using the interlayer exchange coupling effect, the SAF-NPs presented here do not suffer from this limit. Adjusting the outer sphere relaxation theory for spherical particles to SAF-NPs, we show both theoretically and experimentally that the SAF-NP size can be optimized to reach the r2 maximum. With measured r2 values up to 355 s-1mM-1 our SAF-NPs show better performance than commercial alternatives and are competitive with the state-of-the-art. This performance is confirmed in an in vitro MRI study on SKOV3 cells.
    ACS Nano 02/2014; · 12.03 Impact Factor
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    ABSTRACT: Surface Plasmon Resonance (SPR) is a well-established technique for studying binding kinetics and is extensively used in assay development as well as in drug discovery. Many biosensors contain an oxide surface instead of the conventional Au surface typically used in SPR sensing, which may introduce additional variables when using established protocols. Therefore, SiOx-covered SPR substrates are of great interest as a benchmarking tool for silicon-based biosensors. Moreover, SiOx has multiple advantages over Au, for instance with respect to the thermal stability of commonly used surface coupling strategies. In this paper, the bulk sensitivity of SiO2-covered Au substrates was evaluated for use in SPR. Both theoretical simulations and experimental results showed that the presence of ∼10 nm of SiO2 resulted in minimal loss of bulk sensitivity compared to Au substrates. This was proven for a prostate specific antigen (PSA) recognition immuno-assay. Thus we clearly demonstrated that SiO2-covered Au substrates can be used for biosensing applications and do not generate significant differences compared to the original Au substrates.
    Sensors and Actuators B: Chemical. 01/2014; 200:167–172.
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    ABSTRACT: We propose a write scheme for perpendicular spin-transfer torque magnetoresistive random-access memory that significantly reduces the required tunnel current density and write energy. A sub-nanosecond in-plane polarized spin current pulse is generated using the spin-Hall effect, disturbing the stable magnetic state. Subsequent switching using out-of-plane polarized spin current becomes highly efficient. Through evaluation of the Landau-Lifshitz-Gilbert equation, we quantitatively assess the viability of this write scheme for a wide range of system parameters. A typical example shows an eight-fold reduction in tunnel current density, corresponding to a fifty-fold reduction in write energy, while maintaining a 1 ns write time.
    Applied Physics Letters 12/2013; 104(1). · 3.79 Impact Factor
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    ABSTRACT: Magnetic vortices in thin films are in-plane spiral spin configurations with a core in which the magnetization twists out of the film plane. Vortices result from the competition between atomic-scale exchange forces and long-range dipolar interactions. They are often the ground state of magnetic dots, and have applications in medicine, microwave generation and information storage. The compact nature of the vortex core, which is 10-20 nm wide, makes it a suitable probe of magnetism at the nanoscale. However, thus far the positioning of a vortex has been possible only in confined structures, which prevents its transport over large distances. Here we show that vortices can be propagated in an unconstrained system that comprises electrical nanocontacts (NCs). The NCs are used as tunable vortex attractors in a manner that resembles the propelling of space craft with gravitational slingshots. By passing current from the NCs to a ferromagnetic film, circulating magnetic fields are generated, which nucleate the vortex and create a potential well for it. The current becomes spin polarized in the film, and thereby drives the vortex into gyration through spin-transfer torques. The vortex can be guided from one NC to another by tuning attractive strengths of the NCs. We anticipate that NC networks may be used as multiterminal sources of vortices and spin waves (as well as heat, spin and charge flows) to sense the fundamental interactions between physical objects and fluxes of the next-generation spintronic devices.
    Nature Nanotechnology 12/2013; · 31.17 Impact Factor
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    ABSTRACT: Magnetic nanoparticle (MNP) enabled cell visualization with magnetic resonance imaging (MRI) is currently an intensively studied area of research. In the present study, we have synthesized polyethylene glycolated (PEG) MNPs and validated their suitability as MR cell labeling agents in in vitro and in vivo experiments. The labeling of therapeutic potent mesenchymal stem cells (MSCs) with small core and large core MNPs was evaluated. Both MNPs were, in combination with a transfection agent, stably internalized into the MSCs and didn't show an effect on cell metabolism. The labeled cells showed high contrast in MRI phantom studies. For quantification purposes, the MRI contrast generating properties of cells labeled with small core MNPs were compared with large core MNPs and with the commercial contrast agent Endorem. MSCs labeled with the large core MNPs showed the highest contrast generating properties in in vitro phantom studies and in in vivo intracranial stereotactic injection experiments, confirming the size-relaxivity relationship in biological systems. Finally, the distribution of MSCs pre-labeled with large core PEGylated MNPs was visualized non-invasively with MRI in a glioma model.
    Biomaterials 11/2013; · 8.31 Impact Factor
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    ABSTRACT: Measurements of low-temperature magnetotransport in lithographic wires of submicron widths fabricated from high-mobility AlGaSb/InAs/AlGaSb two-dimensional electron system heterostructures are presented. The dependence of the spin and phase coherence lengths on wire width and diffusion constant is investigated by analyzing the conductance in low applied magnetic fields with antilocalization models. Predominantly diffusive boundary scattering is deduced from the magnitude and wire width dependence of the conductance. Diffusive boundary scattering leads to a diffusion constant decreasing with wire width and hence allows the dependence of spin coherence on wire width and diffusion constant to be investigated concurrently. The spin coherence lengths are experimentally found to be proportional to the ratio of the diffusion constant to wire width. The phase coherence lengths follow Nyquist decoherence for low-dimensional wires.
    Physical Review B 11/2013; 88(20). · 3.66 Impact Factor
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    ABSTRACT: Tailoring the properties of superparamagnetic nanoparticles (MNPs) is essential for various nano-based biological applications. Having control over the properties of the MNPs permits a maximum flexibility. Starting from monodisperse iron oxide MNPs produced by thermal decomposition, we report on the optimization and characterization of a first and second seed mediated growth step by varying the surfactant amount and by optimizing the heating steps. We demonstrate the ability to gradually increase the size of crystalline MNPs from 6 over 9 to 12 nm with an improving monodispersity as demonstrated by Transmission Electron Microscopy, Dynamic Light Scattering and X-ray diffraction. The magnetic properties of the MNPs, studied by Vibrating Sample Magnetometry, were in concert with their size increase. We also show the functionalization of these particles with polyethylene glycolated silanes, to render the MNPs stable in water. Different characterization techniques, namely Transmission Electron Microscopy, Dynamic Light Scattering, Fourier-transform InfraRed, Thermo gravimetric analysis and X-ray Photoelectron Spectroscopy, confirmed the successful engraftment of the silanes on the MNP's surface. In conclusion, the proposed route of step-wise synthesis in combination with silane functionalization allows fine tuning the physical properties of iron oxide MNPs for applications in an aqueous environment.
    IEEE Transactions on Magnetics 01/2013; 49(1):219-226. · 1.42 Impact Factor
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    ABSTRACT: We have determined the temperature profile in magnetic nanocontacts under applied current densities typical of spin-torque oscillators (∼10^{8} A/cm^{2}). The study combines experimental measurements of the electrical and magnetic properties of the nanocontacts and full three-dimensional simulations of the heat and current flow in these systems. It is found that the quadratic current-induced increase of the resistance due to Joule heating is independent of the applied temperature from 6 to 300 K. In terms of magnetization dynamics, the measured current-induced vortex nucleation, a thermally activated process, is found to be consistent with local temperatures increases of between 147 and 225 K. Simulations reproduce the experimental findings and show that significant thermal gradients exist out to 450 nm from the nanocontact.
    Physical Review Letters 12/2012; 109(26):267205. · 7.73 Impact Factor
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    ABSTRACT: Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here we study the fano interference that occurs between localized (LSPR) and propagating (SPP) modes by means of phase sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each others vicinity, accompagnied of largely increased phase differences between the respective plasmon resonances. Moreover we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the Figure-Of-Merit which reaches values between 24 and 58 for the respective plasmon modes.
    Plasmonics 11/2012; · 2.43 Impact Factor
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    ABSTRACT: Nanometer-scale pores are capable of detecting the size and concentration of nanometer-sized analytes at low concentrations upon analyzing their translocation through the pore, in small volumes and over a short time without labeling. Here, we present a simple, widely applicable, robust, and precise method to measure the zeta-potential of different nano-objects using nanopores. Zeta-potential i.e., a quantity that represents electrical charge in nanocolloids, is an important property in manufacturing of pharmaceuticals, inks, foams, cosmetics, and food. Its use is also imperative in understanding basic properties of complex dispersions including blood, living organisms, and their interaction with the environment. The characterization methods for zeta-potential are limited. Using the nanopore technique, the zeta-potential and the charge of nanoparticles can be measured independently of other parameters, such as particle size. This simple method is based on measuring the duration of the translocation of analytes through a nanopore as a function of applied voltage. A simple analytical model has been developed to extract the zeta-potential. This method is able to detect and differentiate nanometer-sized objects of similar size; it also enables the direct and precise quantitative measurement of their zeta-potential. We have applied this method to a wide range of different nanometer-sized particles and compared the results with values measured by commercially available tools. Furthermore, potential capability of this method in detection and characterization of virions is shown by measuring the low zeta-potential of HIV and EBV viruses.
    Analytical Chemistry 08/2012; 84(20):8490-6. · 5.70 Impact Factor
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    ABSTRACT: We have developed and characterized two novel micro flow sensors based on measuring the electrical impedance of the interface between the flowing liquid and metallic electrodes embedded on the channel walls. These flow sensors are very simple to fabricate and use, are extremely compact and can easily be integrated into most microfluidic systems. One of these devices is a micropore with two tantalum/platinum electrodes on its edges; the other is a micro channel with two tantalum /platinum electrodes placed perpendicular to the channel on its walls. In both sensors the flow rate is measured via the electrical impedance between the two metallic electrodes, which is the impedance of two metal-liquid junctions in series. The dependency of the metal-liquid junction impedance on the flow rate of the liquid has been studied. The effects of different parameters on the sensor's outputs and its noise behavior are investigated. Design guidelines are extracted and applied to achieve highly sensitive micro flow sensors with low noise.
    Microfluidics and Nanofluidics 07/2012; 12(1-4). · 3.22 Impact Factor
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    ABSTRACT: Nanopore resistive pulse techniques are based on analysis of current or voltage spikes in the recorded signal. These spikes result from translocation of nanometer sized analytes through a nanopore. The most important information that needs to be extracted is the duration, amplitude and number of the translocation spikes. The recorded signal is usually considerably noisy, with a huge baseline drift and hundreds of translocation spikes. Thus, incorporation of suitable signal processing algorithms is necessary for correct and fast detection of all the translocation spikes and to accurately measure their amplitude and duration. Generally, low-pass filtering is used for denoising, averaging is used for baseline detection, and thresholding is used for spike detection and measurement. Here we present novel algorithms and specifically developed software for nanopore signal processing that are significantly improving the accuracy of the nanopore measurements. It includes an improved method for baseline removing, an optimized algorithm for denoising the nanopore signals, a novel spike detection method that detects all the translocation spikes more correctly, and a novel algorithm for measuring the duration and amplitude of the translocation spikes that is less affected by the measurement bandwidth and is more accurate. The newly developed algorithms are evaluated and optimized by a range of experimentally recorded signals, in addition to different simulated signals.
    07/2012;
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    ABSTRACT: The plasmonic properties of individual subwavelength-sized silver nanocubes are mapped with nanometric spatial resolution by means of electron energy-loss spectroscopy in a scanning transmission electron microscope. Three main features with different energies and spatial behavior (two peaked at the corners, one on the edges) are identified and related to previous measurements on ensemble or individual nanoparticles. The highly subwavelength mapping of the energy position and intensity of the excitations shows that the surface plasmon modes, localized at specific areas of the particles, for example, the corners or the edges, are modified by their size, the presence of a substrate, and the very local environment. Helped by discrete dipole approximation numerical simulations, we discuss how local modifications of the environment affect the global modes of the particles. In particular, we show both experimentally and theoretically that absorption resonances at different corners of the same nanocube are largely independent of each other in energy and intensity. Our findings provide a better understanding of the spatial coherence of the surface plasmons in nanoparticles but also give useful insights about their roles in the nanoparticle sensing properties.
    Nano Letters 02/2012; 12(3):1288-94. · 13.03 Impact Factor
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    ABSTRACT: A waveguide optical isolator realized by adhesive bonding of a garnet die, containing a Ce:YIG magneto-optic layer, on a silicon-on-insulator waveguide circuit is demonstrated. The die was bonded on top of an asymmetric Mach-Zehnder interferometer using a 100nm thick DVS-BCB adhesive bonding layer. A static magnetic field applied perpendicular to the light propagation direction results in a non-reciprocal phase shift for the fundamental quasi-TM mode in the hybrid waveguide geometry. A maximum optical isolation of 25 dB is obtained.
    Optics Express 01/2012; 20(2):1839-48. · 3.55 Impact Factor
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    ABSTRACT: In diffusive two-dimensional electron systems (2DESs) with linear spin-orbit interaction (SOI), quasi-one-dimensional (Q1D) confinement in narrow wires of width W is theoretically predicted to result in an enhancement of the spin relaxation length LS, such that LS~1/W. We present our experimental data of the dependence of LS on W for three different systems: 1) ballistic InSb wires with specular boundary scattering and strong Dresselhaus SOI, 2) ballistic InAs wires with diffusive boundary scattering and Rashba SOI, and 3) diffusive bismuth wires with a large density of states at the surface. For all three systems, information on the spin relaxation is gathered from the weak-antilocalization effect (WAL), a magnetotransport measurement sensitive to both the spin-orbit coherence length and the phase coherence length Lφ. We find a dependence of LS on W, where LS increases with decreasing W in all three systems. However, the theory is valid for Q1D diffusive wires with linear SOI, which does not fit the profile of all our systems. Thus, we conclude that the increase in LS in narrow wires is a more universal phenomenon that can be extended to systems outside the theory, which impacts various applications.
    12/2011;
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    ABSTRACT: In this paper we demonstrate an optical isolator on a Silicon-on-Insulator waveguide platform realized by the adhesive bonding of Ce:YIG on top of a Mach-Zehnder interferometer. An optical isolation of 25dB is experimentally obtained.
    Group IV Photonics (GFP), 2011 8th IEEE International Conference on; 10/2011
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    ABSTRACT: We study the starting up phase of a current-controlled oscillator based on a magnetic vortex orbiting around a nanocontact in a spin-valve. From the idle state, current pulses down to a few nanoseconds can create the vortex, which is detected through the electrical signature of its steady-state gyration. Two ns are needed to reach the in-current equilibrium. The process can then be described by an Arrhenius law, with an activation energy that is consistent with the Oersted-field-induced separation of a vortex-antivortex pair. Requirements for deterministic nucleation are deduced, with prospects for instant-on oscillator capability.
    IEEE Transactions on Magnetics 07/2011; · 1.42 Impact Factor

Publication Stats

1k Citations
405.98 Total Impact Points

Institutions

  • 1995–2014
    • imec Belgium
      Louvain, Flanders, Belgium
    • Radboud University Nijmegen
      Nymegen, Gelderland, Netherlands
  • 2011
    • Université Paris-Sud 11
      • Institut d'Electronique Fondamentale
      Paris, Ile-de-France, France
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2004–2010
    • Technische Universiteit Eindhoven
      • Department of Applied Physics
      Eindhoven, North Brabant, Netherlands
  • 2009
    • Inha University
      • Department of Physics
      Seoul, Seoul, South Korea
    • Osaka University
      • Department of Materials Engineering Science
      Ōsaka-shi, Osaka-fu, Japan
  • 2006–2008
    • KU Leuven
      • • Department of Chemistry
      • • Institute of Nanoscale Physics and Chemistry (INPAC)
      Leuven, VLG, Belgium
  • 2007
    • University of Oxford
      • Department of Materials
      Oxford, England, United Kingdom