scispace - formally typeset
Search or ask a question
Author

Louis Hellemans

Bio: Louis Hellemans is an academic researcher from Katholieke Universiteit Leuven. The author has contributed to research in topics: Dielectric & Conductive atomic force microscopy. The author has an hindex of 27, co-authored 82 publications receiving 2451 citations.


Papers
More filters
Journal ArticleDOI
30 Oct 1998-Science
TL;DR: A new approach to second-order nonlinear optical (NLO) materials is reported, in which chirality and supramolecular organization play key roles.
Abstract: A new approach to second-order nonlinear optical (NLO) materials is reported, in which chirality and supramolecular organization play key roles. Langmuir-Blodgett films of a chiral helicene are composed of supramolecular arrays of the molecules. The chiral supramolecular organization makes the second-order NLO susceptibility about 30 times larger for the nonracemic material than for the racemic material with the same chemical structure. The susceptibility of the nonracemic films is a respectable 50 picometers per volt, even though the helicene structure lacks features commonly associated with high nonlinearity. Susceptibility components that are allowed only by chirality dominate the second-order NLO response.

636 citations

Journal ArticleDOI
TL;DR: In this article, a conductive atomic force microscope (AFM) tip based on B-implanted diamond has been developed for the determination of the spatial distribution of charge carriers in semiconducting structures.
Abstract: A conductive atomic force microscope (AFM) tip based on B‐implanted diamond has been developed for the determination of the spatial distribution of charge carriers in semiconducting structures. The characteristics of this tip have been determined by studying the current–voltage behavior as a function of substrate resistivity and tip load. From this work a model of the electrical properties of the microcontact is emerging. It includes an Ohmic contribution to the overall resistance, which is related to the plastically deformed area, and contributions from a barrier. The tip imprints have been imaged with AFM and their physical dimensions are seen to match the requirements of the model. From resistance measurements on uniformly doped silicon a calibration curve has been established which can be used as a standard to convert measured resistance into resistivity.

105 citations

Journal ArticleDOI
11 Apr 2003-Langmuir
TL;DR: A planar microcavity has been inserted into a self-assembled colloidal crystal by a combination of convective self-assembly in a vertical geometry and the Langmuir−Blodgett technique.
Abstract: A planar microcavity has been inserted into a self-assembled colloidal crystal by a combination of convective self-assembly in a vertical geometry and the Langmuir−Blodgett technique. This planar c...

104 citations

Journal ArticleDOI
TL;DR: In this article, the reported nylon-6/EPDM TPVs show significant strain recovery behavior, even though the matrix consists of semicrystalline nylon, which deforms plastically via shear yielding.
Abstract: Thermoplastic vulcanizates (TPVs), as prepared by dynamic vulcanization, are blends in which cross-linked rubber particles are finely dispersed in a thermoplastic matrix. The reported nylon-6/EPDM TPVs show significant strain recovery behavior, even though the matrix consists of semicrystalline nylon-6, which deforms plastically via shear yielding. Atomic force microscopy and transmission electron microscopy experiments revealed an inhomogeneous plastic deformation of the matrix phase. During straining, the plastic deformation is initiated in those zones where the nylon matrix between the rubber particles is the thinnest. Even at high strains, the thick ligaments of the nylon matrix remain almost undeformed and act as adhesion points holding the rubber particles together. When the external force is removed, the elastic force of the stretched, dispersed rubber phase pulls back the plastically deformed nylon parts by either buckling or bending. This is considered to be the key mechanism for the elastic beha...

91 citations

Journal ArticleDOI
TL;DR: The nano-spreading resistance profiling (nano-SRP) method has been developed and improved such that it can now be used as an accurate tool for quantitative two-dimensional carrier profiling as mentioned in this paper.
Abstract: The nano-spreading resistance profiling (nano-SRP) method has been developed and improved such that it can now be used as an accurate tool for quantitative two-dimensional carrier profiling. Instrumental improvements include the use of batch-fabricated, conducting diamond-coated silicon probes, and a low-noise logarithmic current amplifier. The spatial resolution (10 nm), the dynamic range (1014–1020 atoms/cm3), and the sensitivity (1014 atoms/cm3) of the nano-SRP technique are illustrated by profiling a wide range of state-of-the-art device structures. Two-dimensional measurements of the carrier distribution inside fully processed metal–oxide–semiconductor transistors with gate lengths varying from 2 μm down to 0.25 μm illustrate the strength of the technique to map present and future devices. The nano-SRP method currently has sufficient resolution to demonstrate the small asymmetry in the source/drain profiles from transistors in which the sample was not rotated during the 7° implant. The electrical tra...

83 citations


Cited by
More filters
Journal Article
TL;DR: In this article, a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators were developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of 3D micro-optical and micromechanical structures, including photonic-bandgap-type structures.
Abstract: Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

1,833 citations

Journal Article
TL;DR: The basic bonding mechanism to enamel and dentin of these three approaches is demonstrated by means of ultramorphological and chemical characterization of tooth-biomaterial interfacial interactions and confirms that conventional three-step etch&rinse adhesives still perform most favorably and are most reliable in the long-term.
Abstract: Bonding to tooth tissue can be achieved through an "etch&rinse," "self-etch" or "glass-ionomer" approach. In this paper, the basic bonding mechanism to enamel and dentin of these three approaches is demonstrated by means of ultramorphological and chemical characterization of tooth-biomaterial interfacial interactions. Furthermore, bond-strength testing and measurement of marginal-sealing effectiveness (the two most commonly employed methodologies to determine "bonding effectiveness" in the laboratory) are evaluated upon their value and relevance in predicting clinical performance. A new dynamic methodology to test biomaterial-tooth bonds in a fatigue mode is introduced with a recently developed micro-rotary fatigue-testing device. Eventually, today's adhesives will be critically weighted upon their performance in diverse laboratory studies and clinical trials. Special attention has been given to the benefits/drawbacks of an etch&rinse versus a self-etch approach and the long-term performance of these adhesives. Correlating data gathered in the laboratory with clinical results clearly showed that laboratory research CAN predict clinical effectiveness. Although there is a tendency to simplify bonding procedures, the data presented confirm that conventional three-step etch&rinse adhesives still perform most favorably and are most reliable in the long-term. Nevertheless, a self-etch approach may have the best future perspective. Clinically, when adhesives no longer require an "etch&rinse" step, the application time, and probably more importantly, the technique-sensitivity are substantially reduced. Especially "mild," two-step self-etch adhesives that bond through a combined micromechanical and chemical interaction with tooth tissue closely approach conventional three-step systems in bonding performance.

1,721 citations

Journal ArticleDOI
TL;DR: This study critically discusses the latest peer-reviewed reports related to formation, aging and stability of resin bonding, focusing on the micro and nano-phenomena related to adhesive interface degradation.

1,086 citations