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Publications
| Selected Scientific Highlights |
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Hotspot Decorations map plasmonic patterns with the resolution of scanning probe techniques
Phys. Rev. Lett. 106, 226803 (2011).
V. K. Valev, A. V. Silhanek, Y. Jeyaram, D. Denkova, B. De Clercq, V. Petkov, X. Zheng, V. Volskiy, W. Gillijns, G. A. E. Vandenbosch, O. A. Aktsipetrov, M. Ameloot, V. V. Moshchalkov and T. Verbiest
In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be
completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots
(regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is
correspondingly improved.
Plasmons Reveal the Direction of Magnetization in Nickel Nanostructures
ACS Nano 5, 91-96 (2010)
V. K. Valev, A. V. Silhanek, W. Gillijns, Y. Jeyaram, H. Paddubrouskaya, A. Volodin, C. G. Biris, N. C. Panoiu, B. De Clercq, M. Ameloot, O. A. Aktsipetrov, V. V. Moshchalkov and T. Verbiest
The development of nanophotonics, nanoelectronics and nanomagnetics could result in new generations of devices, capable to store and process
information at increasing speed and decreasing energy cost. However, the relationship between these three nanotechnologies is only beginning
to be clarified. The influence of magnetic fields on surface plasmons combines aspects of photonics, electronics and magnetics at the nano scale.
Surface plasmons are collective excitations of electrons under the influence of light's electromagnetic field and, in magnetic materials,
these electrons can experience the effects of externally applied magnetic fields as well.
We have applied the surface-sensitive nonlinear optical technique of magnetization-induced second harmonic generation (MSHG) to plasmonic,
magnetic nanostructures made of Ni. We show that surface plasmon contributions to the MSHG signal can reveal the direction of the magnetization.
Both the plasmonic and the magnetic nonlinear optical responses can be tuned; our results indicate novel ways to combine nanophotonics,
nanoelectronics, and nanomagnetics and suggest the possibility for large magneto-chiral effects in metamaterials.
More information: ACS Nano 5, 91-96 (2010)
Asymmetric Second Harmonic Generation from Chiral G-Shaped Gold Nanostructures
Phys. Rev. Lett. 104, 127401 (2010)
V. K. Valev, A. V. Silhanek, N. Verellen, W. Gillijns, P. Van Dorpe, O. A. Aktsipetrov, G.A.E. Vandenbosch, V. V. Moshchalkov and T. Verbiest
We present a new electromagnetic phenomenon – the asymmetric second harmonic generation from planar chiral structures.
The effect consists in distinguishing the handedness of a chiral material by rotating the sample in an experiment involving
solely linearly polarized light. This phenomenon originates in the surface plasmon resonance of chiral gold nanostructures,
where homodyne interference of anisotropic and chiral electric and/or magnetic multipoles appears to play an important role.
More information: Phys. Rev. Lett. 104, 127401 (2010)
Linearly polarized second harmonic generation microscopy reveals chirality
Optics Express, 18, 8286-8293 (2010)
V. K. Valev, A. V. Silhanek, N. Smisdom, B. De Clercq, W. Gillijns, O. A. Aktsipetrov, M. Ameloot, V. V. Moshchalkov, T. Verbiest
In optics, chirality is typically associated with circularly polarized light. Here we present a novel way to detect the
handedness of chiral materials with linearly polarized light. We performed Second Harmonic Generation (SHG) microscopy on
G-shaped planar chiral nanostructures made of gold. The SHG response originates in distinctive hotspots, whose arrangement
is dependent of the handedness. These results uncover new directions for studying chirality in artificial materials.
More information: Optics Express, 18, 8286-8293 (2010)
Plasmonic Ratchet Wheels: Switching Circular Dichroism by Arranging Chiral Nanostructures
Nano Letters
V. K. Valev, N. Smisdom, A. V. Silhanek, B. De Clercq, W. Gillijns, M. Ameloot, V. V. Moshchalkov and T. Verbiest
We demonstrate circular dichroism (CD) in the second harmonic generation (SHG) signal from chiral
assemblies of G-shaped nanostructures made of gold. The arrangement of the G shapes is crucial
since upon reordering them the SHG-CD effect disappears. Microscopy reveals SHG 'hotspots' assemblies,
which originate in enantiomerically sensitive plasmon modes, having the novel property of exhibiting a
chiral geometry themselves in relation with the handedness of the material.
These results open new frontiers in studying chirality.
More information: Published on-line
Fano Resonances in Individual Coherent Plasmonic Nanocavities
Nano Letters
Niels Verellen, Yannick Sonnefraud, Heidar Sobhani, Feng Hao, Victor V. Moshchalkov, Pol Van Dorpe, Peter Nordlander and Stefan A. Maier
We observe the appearance of Fano resonances in the optical response of plasmonic nanocavities due to the coherent coupling between their superradiant
and subradiant plasmon modes. Two reduced-symmetry nanostructures probed via confocal spectroscopy, a dolmen-style slab arrangement and a ring/disk dimer,
clearly exhibit the strong polarization and geometry dependence expected for this behavior at the individual nanostructure level, confirmed by full-field
electrodynamic analysis of each structure. In each case, multiple Fano resonances occur as structure size is increased.
More information: Nano Lett., 2009, 9 (4), pp 1663-1667 Publication Date (Web): March 12, 2009 (Letter)
Type-1.5 Superconductors
Physics World
V. V. Moshchalkov, M. Menghini, T. Nishio, Q.H. Chen, A. V. Silhanek, V.H. Dao, L.F. Chibotaru, N. D. Zhigadlo, and J. Karpinski
We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single
crystalline superconductors where a unique combination of both type-1  and type-2
 superconductor conditions is realized for the two components of the order parameter.
This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short
distances, which stabilizes unconventional stripe- and gossamer-like vortex patterns that we have
visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.
FIG. 3: (a) Magnetic decoration image in the MgB2 single crystal at H = 5Oe. (b) Disordered Abrikosov lattice obtained
at H = 5Oe in the NbSe2 sample. The formation of vortex stripes is also reproduced in numerical simulations to a two-component
type-1.5 superconductor (c) in contrast to a homogeneous vortex distribution in a type-2 superconductor at the same vortex
density (d). The scale bars in the images correspond to 10 µm. Vortex density along lines parallel to the vortex stripe direction
(yellow dashed lines in (a)) for MgB2 and NbSe2 vortex structures. The variation of the vortex density as a function of the
distance measured along the direction perpendicular to the stripes (yellow arrows in (a) and (b)) is shown in the insert of
(b). The curves are normalized by their respective average density. The results of a similar calculation performed on the
simulated vortex structures are shown in the inset of (d).
More information: PDF
http://physicsworld.com/cws/article/news/37806
http://sciencenow.sciencemag.org/cgi/content/full/2009/313/1
http://physics.aps.org/articles/v2/22
Detection, Localization, and Conformational Analysis of Single Polysaccharide Molecules on Live Bacteria
ACSNano 2 (2008) 1921-1929
Grégory Francius, Sarah Lebeer, David Alsteens, Linda Wildling, Hermann J. Gruber, Pascal Hols, Sigrid De Keersmaecker,
Jos Vanderleyden and Yves F. Dufrene
The nanoscale exploration of microbes using atomic force microscopy (AFM) is an exciting, rapidly evolving research field.
Here, we show that single-molecule force spectroscopy is a valuable tool for the localization and conformational analysis
of individual polysaccharides on live bacteria. We focus on the clinically important probiotic bacterium Lactobacillus
rhamnosus GG, demonstrating the power of AFM to reveal the coexistence of polysaccharide chains of different nature on
the cell surface. Applicable to a wide variety of cells, this single molecule method offers exciting prospects for
analyzing the heterogeneity and diversity of macromolecules constituting cell membranes and cell walls.
Hydrogen-induced Ostwald ripening
Phys. Rev. Lett. 100, 236105 (2008).
M. Di Vece, D. Grandjean, M.J. Van Bael, C.P. Romero, X. Wang, S. Decoster, A. Vantomme and P. Lievens
More information: PDF
The cluster size of an ensemble of palladium nanoclusters was found to increase on hydrogenation. This phenomenon
is attributed to hydrogen-induced Ostwald ripening and originates from the higher mobility of palladium atoms resulting from
the low sublimation energy of the palladium hydride when compared to the palladium metal. Future applications of
nano-structured hydrogen storage materials will have to take this phenomenon into consideration.
From left to right:
Hydrogen atoms approaching the palladium nanocluster assembled film;
The palladium atoms within the palladium cluster building blocks become free to move due to the interstitial hydrogen;
Some of the clusters become larger at the expense of the smallest.
Classification and control of the origin of photoluminescence from Si nanocrystals
Nature Nanotechn. 3, 174-178 (2008)
S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. I. Lebedev, G. Van Tendeloo and V. V. Moshchalkov
Silicon dominates the electronics industry, but its poor optical properties mean that III–V compound semiconductors are preferred for
photonics applications. Photoluminescence at visible wavelengths was observed from porous Si at room temperature in 1990, but the origin
of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate
ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the
photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light
from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation
with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination
reintroduces the defects, making them the origin of the light again.
Three-dimensional atomic-scale structure of size-selected gold nanoclusters
Nature 451 (2007) 46
Z. Y. Li, N. P. Young, M. Di Vece, S. Palomba, R. E. Palmer, A. L. Bleloch, B. C. Curley, R. L. Johnston, J. Jiang and J. Yuan
An unambiguous determination of the three-dimensional structure of nanoparticles is challenging1. Electron tomography requires a series of images
taken for many different specimen orientations2. This approach is ideal for stable and stationary structures3. But ultrasmall nanoparticles are
intrinsically structurally unstable and may interact with the incident electron beam4–6, constraining the electron beam density that can be used
and the duration of the observation. Here we use aberration-corrected scanning transmission electron microscopy7, coupled with simple imaging
simulation, to determine with atomic resolution the size, three-dimensional shape, orientation and atomic arrangement of size-selected gold
nanoclusters that are preformed in the gas phase and soft-landed on an amorphous carbon substrate. The structures of gold nanoclusters containing
30966 atoms can be identified with either Ino-decahedral, cuboctahedral or icosahedral geometries. Comparison with theoretical modelling of the
system suggests that the structures are consistent with energetic considerations. The discovery that nanoscale gold particles function as active
and selective catalysts for a variety of important chemical reactions has provoked much research interest in recent years8–12. We believe that
the detailed structure information we provide will help to unravel the role of these nanoclusters in size- and structure-specific catalytic
reactions11,12. We note that the technique will be of use in investigations of other supported ultrasmall metal cluster systems.
Nonexponential decay of spontaneous emission from an ensemble of molecules in photonic crystals
Phys. Rev. B 76, 045113 (2007) [Coverpage]
R.A.L. Vallee, K. Baert, B. Kolaric, M. Van der Auweraer and K. Clays
Photonic crystals with relatively low dielectric contrast i.e., with pseudogaps have significant influence on the fluorescence decay of internal
emitters. Fluorescence decays of ensembles of dye molecules measured at different positions in the PCs exhibit a nonexponential behavior, which
is best fitted by a continuous distribution of decay rates. The most frequent decay rates of these distributions are smaller and their widths are
narrower in a PC with a pseudogap acting in the emission range of the emitters than in a PC having the pseudogap out of this range. These
experimental results have been well accounted for by calculations of the local density of states and rate constant for spontaneous emission.
Thermodynamically stable noncomposite vortices in mesoscopic two-gap superconductors
European Physics Letters, 78, 47001-47005 (2007)
L. F. Chibotaru, V. H. Dao and A. Ceulemans
In mesoscopic two-gap superconductors with sizes of the order of the coherence length noncomposite vortices are found to be thermodynamically stable
in a large domain of the T-H phase diagram. In these phases the vortex cores of one condensate are spatially separated from the other condensate ones,
and their respective distributions can adopt distinct symmetries. The appearance of these vortex phases is caused by a non-negligible effect of the
boundary of the sample on the superconducting order parameter and represents therefore a genuine mesoscopic effect. For low values of interband
Josephson coupling vortex patterns with L1 =/= L2 can arise in addition to the phases with L1 = L2, where L1
and L2 are total vorticities in the two condensates. The calculations show that noncomposite vortices could be observed in thin mesoscopic samples
of MgB2
The influence of a Cu buffer layer on the self-assembly of iron silicide nanostructures on Si(111)
Appl. Phys. Lett. 92 (2008) 043111
K. Paredis,K. Vanormelingen, and A. Vantomme
The role of a Cu buffer layer on the formation of iron silicide nanostructures is investigated using scanning tunneling microscopy and Mössbauer spectroscopy.
The deposition of 1 Ã… Fe on the Si(111) -7x7 and the Si(111) -5x5-Cu surfaces results in the self-organization of nanoscale islands. Increasing the deposition
temperature (300-600 °C) leads to an exponential decrease in island density and to an increase of the average island size. At 475 °C, the preferential nucleation
site changes from the terrace to the step edges, i.e., step flow growth is observed. The self-assembled nanostructures exhibit the metastable CsCl–FeSi1+x structure.
Due to the enhanced diffusion, nanodots formed on the 5x5 surface are significantly larger and more separated compared to growth on the bare 7x7 surface.
These results show that a buffer layer provides an additional, experimentally controllable parameter, besides temperature, to tailor the size and distribution of nanodots.
Supramolecular Hydrophobic - Hydrophylic Nanopatterns at Electrified Interfaces
Nano Letters, 2007, 7, 3, 791-795
Andrey Klymchenko, Shuhei Furukawa, Klaus Müllen, Mark Van der Auweraer, Steven De Feyter
We have developed hydrophobic-hydrophilic nanopatterns at electrified surfaces via the self-assembly of amphiphilic molecules.
For this purpose, we selected 5-hexadecyloxy isophthalic acid: this neutral amphiphile forms hydrogen-bonded rows that are commensurate with the Au(111)
surface. The alkyl chains are interdigitated. The molecular organization of these nanopatterns depends strongly on the substrate potential, which reveals
the hierarchical nature of the assembly. The new hydrophobic-hydrophilic nanopatterns are of special interest as templates for the formation of
nanostructures of higher complexity.
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