The ability to selectively arrange nanosized domains of inorganic and/or organic materials into hybrid nanomaterials offers an attractive route to engineer new nanostructured materials with unique combination of properties and multiple tunable functionalities that can be used in (spin) electronics, energy, memory and microwave devices, catalysis, sensor and bio-medical. The BSMA division has strong expertise in the development of synthesis methods (nanoimprinting, electrodeposition in nanoporous templates, thin-film deposition, nanoassembly) to selectively control the composition and shape of such hybrid nanostructures and in the characterization of their physical and physico-chemical properties.
Polymers for nano-electronics
New ferroelectric polymers
Group of Alain M. Jonas
Ferroelectric polymers are a relatively lesser known class of materials which combine the easy processing of plastics with advanced functional properties such as the possibility to store electrical information (ferroelectricity), and to deform reversibly upon application of an electric field (reverse piezoelectricity). This is due to the presence of electric dipole moment son chains, which align in the same spatial direction in the crystalline phase, and can be reversed by application of an electric field, as illustrated below for the beta phase of poly(vinylidene fluoride) (PVDF).
Although PVDF and related copolymers are the seminal examples of ferroelectric polymers, other macromolecules such as odd polyamides and even specific proteins are ferroelectric. We are synthesizing multi-sequenced ferroelectric block copolymers by peptide-based synthetic methodologies, with macromolecular architectures designed in order to force crystallization in specific structures for improved ferroelectricity.
Hybrid devices and materials based on ferroelectric polymers
Group of Alain M. Jonas, Bernard Nysten and Luc Piraux
We have developed the nanoimprinting of ferroelectric PVDF copolymers, and shown how this nanoprocessing methodology can result in improved crystallinity, preferential orientation and therefore reduced coercive field (the minimum field needed to switch the orientation of the dipole moments). We can thus produce ordered arrays of nanoimprinted ferroelectric nanostructures of improved properties, as shown below.
The openings created by this process can then be used to introduce a second material, thereby creating a hybrid layer exhibiting a synergetic combination of properties. Based on this concept, we develop organic semiconducting/ferroelectric layers for memory devices (diodes, transistors), in which the semiconducting polymer is used to measure the state of polarization of the ferroelectric material. An example is shown below. Other explored devices are organic solar cells including nanoimprinted ferroelectric polymers.
Likewise, we fabricate hybrid ferromagnetic/ferroelectric layers using relatively simple molding and electrodeposition operations leading to a regularly-nanopatterned layer made of a continuous ferroelectric plastic with embedded ferromagnetic metal nanopillars. In such composite multiferroic layers, magnetoelectric effects should allow us to modify electric polarization by magnetic fields, and vice-versa. We recently demonstrated that the orientation of the electric polarization of the polymer can be flipped by applying a magnetic field. This magnetoelectric effect happens at room temperature, and is mediated by internal stresses in the polymer building upon applying the magnetic field.
1. processing of ferroelectric polymers and device fabrication : Alain M. Jonas
2. characterization of ferroelectric/semiconducting devices : Alain M. Jonas and Bernard Nysten
3. fabrication and characterization of multiferroic materials : Luc Piraux, Alain M. Jonas, Bernard Nysten
Functional Hybrid Nanostructures
Functional Hybrid Multisegmented Nanotubes and Nanowires
Group of Sophie Demoustier, Alain M. Jonas
The membrane-templating method is combined with layer-by-layer (LbL) assembly and/or electrodeposition to sequentially synthesize multisegmented nanostructures composed of metals, polymers, synthetic and biological polyelectrolytes, and colloids. The electrochemical approach offers the control over the architectural parameters of the resulting structures (in particular, the segment length and morphology), whereas the LbL adsorption technique permits to integrate nonconducting materials, including biomacromolecules, within the nanostructures.
A supplementary degree of complexity can be reached by capping or loading the LbL nanotubes with colloidal particles. This new versatile methodology of synthesizing and assembling hybrid multisegmented nanostructures is able to provide multifunctional nanowires and nanotubes of unprecedented complexity and of exquisitely controlled structures that could find applications in fields as diverse as catalysis, electronics, sensing or drug delivery.
Hybrid materials for potential thermoelectric applications
Group of Bernard Nysten (In collaboration with CRISMAT/ENSICAEN, Université de Caen Basse-Normandie)
Currently energy and environment are of major concern for the future. Thermoelectric materials are interesting due to their ability to convert energy for cooling (Peltier effect) and electric power generation (Seebeck effect). It is known that the performance of thermoelectric materials depends on the dimensionless figure of merit, ZT = T (a²/r k), where T is the temperature, a is the Seebeck coefficient, r is the electrical resistivity and k is the thermal conductivity.
Based on this relationship, a good thermoelectric material would be a compromise between a good electronic conductor (metal) and a good thermal insulator (insulating material). In other words, a degenerated semiconductor or a highly doped semiconductor could be a good candidate for this work. Presently, for the best known materials, ZT values at room temperature are close to one.
These main characteristics demonstrate an interesting aspect of hybrid materials and present an innovative alternative to oxide or intermetallic materials. Indeed, hybrid thermoelectric materials can combine an organic network (bad thermal conductivity) and an inorganic network (good electronic conductivity and charge capacity), particularly when it has low dimensionality (1D, 2D). In this context, we synthesize this kind of materials made of an organic network connected to an inorganic network and whose composition and structure can be tuned in order to obtain desired properties. These layered materials can be used as host structures for the in-situ polymerization of conducting polymers.
Metamaterials and microwave devices
Architectured and hybrid systems for EM absorption and metamaterial properties
Group of Christian Bailly, Arnaud Delcorte, Sophie Hermans, Isabelle Huynen (ICTeam), Thomas Pardoen (IMMC), Luc Piraux
Wireless communication via electromagnetic (EM) transmission in the microwave range has become ubiquitous. In parallel, electronic devices are becoming ever more compact. These trends generate a growing issue with electromagnetic interference (EMI) with consequences ranging from annoying to dangerous. Classical EMI shielding is based on Faraday cages which reflect the signal. By contrast, EM absorbers are more attractive because they truly eliminate the bothersome EM wave.
We are designing, testing and modeling several original types of EM absorbers, either broadband or frequency-selective, using clever arrangements (akin to metamaterials) of polymers, metals and carbonaceous nanofillers. One approach is based on hierarchical structures combining nanocomposite foams and a metal honeycomb. They have the unique interest of adding up EM, mechanical and thermal performance. We also have demonstated thin multilayer structures, which are able to effectively absorb microwave radiation over a broad frequency range or selectively reflect desired wavelengths. They are built from alternating films of dielectric polymer and conducting layers. The latter are stacked in a precise gradient of conductivity. Such structures can also be used to develop cloaks of invisibility in the microwave region, i.e. the perturbation of a wavefront by a reflecting obstacle is strongly reduced if the obstacle is embedded in a specially designed multilayer coating.
Simulation of microwave invisible cloaking based on Rohacell®-composite bilayers
Ferromagnetic nanowire substrates for RF electronics
Group of Luc Piraux, Isabelle Huynen (ICTEAM)
On the road towards the size reduction of microwave devices, ferromagnetic nanowires embedded into porous templates are an interesting alternative route to ferrite-based materials. Over the last decade, the groups of I. Huynen and L. Piraux at UCL have successfully prepared a variety of ferromagnetic nanowire substrates to design various prototypes of microwave devices, such as or circulators, isolators and phase shifters useful for wireless communication and automotive systems. In contrast to conventional ferrite devices, ferromagnetic nanowire substrates do not need to be biased by an external magnetic field to operate, since magnetic nanowires act as permanent magnets.
Other advantages of such materials compared to classical ferrites are a higher operation frequency, and a higher saturation magnetization, and a better temperature stability. These advantages make such planar materials highly competitive to classical ferrite-based hybrid solutions used for microwave signal processing, especially for on-board aeronautic and space applications (in collaboration with THALES Alenia Space S.A. and THALES Systèmes Aéroportés S.A.) requiring weight and size reduction together with stability in severe temperature and vibration conditions. Tunability is also of great interest for filters and and novel compact devices using the metamaterial concept, combining negative permittivity and permeability of substrates. Recently, an original processing method based on laser patterning has demonstrated the possibilities of accurate positioning of nanowire structures in porous dielectric templates, thus opening the road to conceive a new generation of monolithic microwave integrated circuits.
Nanomagnetism and spintronics
Nanomagnetism in nanoparticle arrays and multiferroic nanocomposites
Group of Luc Piraux, Bernard Nysten
Arrays of magnetic elongated nanostructures such as nanowires and nanotubes with defined complex architectures are of considerable interest for their potential applications in memory, sensors, microwave and spintronics devices. Proper control and tuning of their magnetic properties requires further understanding of the interplay between intrinsic and shape effects as well as interaction among individual nanostructures. Assemblies of nanowires into complex architectures such as 3D networks, vertically-aligned pillars, core-shell and segmented nanowires, multiferroic heterostructures, … have been successfully fabricated. Besides, the feasibility of bit pattern media was demonstrated by considering ordered arrays of single-domain nanodots magnetized perpendicularly to the barrier layer of anodic alumina templates. Multiferroic composites consisting of arrays of magnetic nanowires surrounded by a ferroelectric material exhibit large magnetoelectric coupling and are also currently investigated.
The magnetic properties are characterized using static magnetometry, ferromagnetic resonance, extraordinary Hall effect and magnetic force microscopy (research teams of L. Piraux and B. Nysten). The work is made in close collaboration with the University San Luis Potosi-Mexico, lnstitut Jean Lamour-Nancy, National Autonomous University of Mexico and Université de Bordeaux.
Spintronics in magnetic nanowires
Group of Luc Piraux
More than 20 years ago, the group of L. Piraux at UCL first reported on the synthesis of arrays of magnetic multilayered nanowires (as Co/Cu and NiFe/Cu nanowires) by electrodeposition into porous templates. The nanowires system has provided an important opportunity in exploring giant magnetoresistance (GMR) effects in the current perpendicular to the planes geometry and in testing theoretical models in various limits. Next, the team has studied spin-transfer torque phenomena in spin-valve multilayered nanowires using a nanolithography based contacting method. Both magnetization switching and the generation of microwave oscillations due to spin-transfer torque were observed via the injection of spin-polarized current at high current density.
Using micromagnetic calculations, it was possible to identify the magnetization dynamics associated with the experimentally determined microwave emission and to study the synchronization of spin-torque vortex oscillators disposed either in series or parallel configuration. The magnetothermopower properties of 3D interconnected GMR nanowire networks are currently investigated.
The work is made in close collaboration with the Unité Mixte de Physique CNRS-THALES, Prokhorov General Physics Institute and CEA Saclay.
Fabrication of nanoporous templates
Group of Luc Piraux, Etienne Ferain, Roger Legras
Track etched polymer membranes (E. Ferain and R. Legras) and nanoporous alumina films (L. Piraux) are currently made in-house at UCL-IMCN-BSMA.
Track etched membranes are produced from polycarbonate, polyester or polyimide films following a two-step process consisting in the irradiation of the polymer by energetic heavy ions to create linear damage tracks, and the chemical etching of these tracks to well-controlled pores.
Porous alumina films are formed by a two-step anodization process leads to highly ordered two-dimensional cylindrical pore arrays oriented perpendicular to the surface with a hexagonal pattern over large areas. The two technologies allow the fabrication of 10-100µm thick self-supported membranes and of < 1µm thin porous layer supported on a silicon/glass substrate. Due to their specific characteristics and easily tunable geometrical parameters such as pore diameters and interpore distances, these nanoporous media are also efficient as template for the electrochemical synthesis of arrays of controlled metallic, polymeric, multi-layered, core/shell nanowires, nanorods and nanotubes that are very difficult to obtain using conventional lithographic techniques. These nano-objects exhibit unexpected properties that make them suitable for various nanowire-based devices with specific applications in several fields, such as electronics, energy, photonics, environment, sensor and bio-medical.
In addition to its manufacturing and supply activities, customization of these track etched membranes is a major activity of ‘it4ip’ company (www.it4ip.be).
Three-dimensional interconnected nanowire networks for microbattery architectures
Group of Luc Piraux, Alexandru Vlad
A simple and reliable template approach for reliable synthesis of 3D nanostructured electrodes for microbattery applications was developed by the groups of L. Piraux and A. Vlad at IMCN. The method involves electrochemical processes using hierarchical pore structures as a template followed by controlled thermal conversion to form 3D interconnected core–shell nanowire networks. The interconnected architecture is self-supported and provides rapid ion and charge collection pathways, as well as efficiently accommodates the volume change induced stress resulting in enhanced lithium storage properties. The capacity per footprint area is few tens times higher than that of a 2D system at an equivalent active material thickness. Integration of such protocols with microfabrication techniques could lead to the development of next generation high power and energy microbatteries.
Nanowire array-based electronic devices ans sensors
Group of Luc Piraux
ZnO nanowires have potential applications in highly integrated nanoscale electronic and optoelectronic circuits with improved performance for solid-state display devices, sensors, solar cells, energy harvesting devices, switchable hydrophobic surfaces, ...
The group of L. Piraux has demonstrated the growth at low temperatures (< 100°C) of well-cristallized ZnO nanowires exhibiting a high degree of verticality and orientation, by using a direct electrodeposition technique without using any template or seeding layer, thus being suitable for applications in which the process simplicity and substrate’s thermal toughness are essential requirements. The method is an attractive technique to apply on substrates with lower thermal tolerance like flexible and transparent media and has great potential to be used in the fabrication processes of transparent electronic and optoelectronic devices.
In a recent study, flexible networks of 3D interconnected polypyrrole nanotubes with easily tunable geometrical dimensions and spatial arrangement were synthesized. Such a macroscopic network provides an extremely large active surface with increased electrical connectivity, and offers a cost-effective alternative for large-scale production of easily integrable high performance chemiresistive gaseous sensors for different applications.
Patterning surfaces and functional polymers using nanoimprint and lithography
Patterned surfaces are of interest for a variety of applications; in our groups, we essentially use chemical patterning to provide local signals and cues to biological cells, or to create assembly marks for controlling nano-assembly processes. For this, we develop specific patterning methodologies based on conventional and unconventional lithography (photolithography, electron-beam lithography, nanoimprint lithography, colloidal lithography are typical examples). Patterning is often performed on silica surfaces (Si wafer, glass), although some methodologies allow us patterning more complex surfaces such as polymers. An example of chemically-patterned surface is shown (carboxylic acid-ended dots of 40 nm average diameter, in an alkane background).
In addition, we have developed a specific expertise in nanoimprint lithography applied to functional polymers. This method is most often applied to amorphous polymers; in our case, we use it as a processing tool to nanoshape functional semi- or liquid crystalline polymers. Due to the combined effects of confinement and grapho-epitaxy, preferential crystalline orientation and increased performance result. An example of cavity mold fabricated in our lab is displayed below, together with a scheme of nanoimprinting.
1. Chemical nanopatterns by e-beam or nanoimprint lithography: K. Glinel and A. Jonas
2. Colloidal lithography : C. Dupont
3. Nanoimprinted semicrystalline polymers: A. Jonas