Matthias Pauly

Spacing-dependent dipolar interactions in dendronized magnetic iron oxide nanoparticle 2D arrays and powders

Self-assembly of nanoparticles (NPs) into tailored structures is a promising strategy for the production and design of materials with new functions. In this work, 2D arrays of iron oxide NPs with interparticle distances tuned by grafting fatty acids and dendritic molecules at the NPs surface have been obtained over large areas with high density using the Langmuir-Blodgett technique. The anchoring agent of molecules and the Janus structure of NPs are shown to be key parameters driving the deposition. Finally the influence of interparticle distance on the collective magnetic properties in powders and in monolayers is clearly demonstrated by DC and AC SQUID measurements. The blocking temperature T(B) increases as the interparticle distance decreases, which is consistent with the fact that dipolar interactions are responsible for this increase. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples and may be described by using the Vogel Fulcher model.

Monolayer and multilayer assemblies of spherically and cubic-shaped iron oxide nanoparticles

Nowadays, nanoparticles are considered as the building blocks of the future nanotechnological devices and the development of strategies for processing nanoparticles into thin films has become a strategic challenge. In this context, the assembling of spherically shaped iron oxide nanoparticles displaying various sizes and of cubic-shaped nanoparticles has been investigated using the Langmuir–Blodgett technique. Homogeneous and dense monolayer and multilayer films have been obtained on large areas. The organisation in films has been studied by combining GISAXS and image analysis of SEM micrographs. The quality of the film has been determined to be mainly dependent on the chemical nature of the substrate and the amount of surfactant molecules at the surface of the nanoparticles (i.e. the organic coating).

Size dependent dipolar interactions in iron oxide nanoparticle monolayer and multilayer Langmuir–Blodgett films

The dipolar interactions in monolayer and multilayer assemblies of iron oxide nanoparticles have been investigated as a function of the nanoparticle size. The magnetic properties of iron oxide nanocrystals of various sizes have been measured for particles as powders and assembled in mono- and multilayers by the Langmuir–Blodgett technique, and compared to the behavior of non-interacting nanoparticles. It is shown that increasing dipolar interactions lead to higher blocking temperatures and to deviation from the Neel–Brown law. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples. The effect of interactions increases strongly with the nanoparticle size in agreement with simulations, leading to an unusual behaviour for the larger particles assembled in monolayer, which could be a signature of a superferromagnetic state.

Nanotrench for nano and microparticle electrical interconnects

We present a simple and versatile patterning procedure for the reliable and reproducible fabrication of high aspect ratio (10(4)) electrical interconnects that have separation distances down to 20 nm and lengths of several hundreds of microns. The process uses standard optical lithography techniques and allows parallel processing of many junctions, making it easily scalable and industrially relevant. We demonstrate the suitability of these nanotrenches as electrical interconnects for addressing micro and nanoparticles by realizing several circuits with integrated species. Furthermore, low impedance metal-metal low contacts are shown to be obtained when trapping a single metal-coated microsphere in the gap, emphasizing the intrinsic good electrical conductivity of the interconnects, even though a wet process is used. Highly resistive magnetite-based nanoparticles networks also demonstrate the advantage of the high aspect ratio of the nanotrenches for providing access to electrical properties of highly resistive materials, with leakage current levels below 1 pA.

Tunable Magnetic Properties of Nanoparticle Two-Dimensional Assemblies Addressed by Mixed Self-Assembled Monolayers

Assemblies of magnetic nanoparticles (NPs) are intensively studied due to their high potential applications in spintronic, magnetic and magneto-electronic. The fine control over NP density, interdistance, and spatial arrangement onto substrates is of key importance to govern the magnetic properties through dipolar interactions. In this study, magnetic iron oxide NPs have been assembled on surfaces patterned with self-assembled monolayers (SAMs) of mixed organic molecules. The modification of the molar ratio between coadsorbed 11-mercaptoundecanoic acid (MUA) and mercaptododecane (MDD) on gold substrates is shown to control the size of NPs domains and thus to modulate the characteristic magnetic properties of the assemblies. Moreover, NPs can be used to indirectly probe the structure of SAMs in domains at the nanometer scale.

Iron oxide magnetic nanoparticles used as probing agents to study the nanostructure of mixed self-assembled monolayers.

Self-assembled monolayers (SAMs) of organic molecules are of exceptional technological importance since they represent a convenient, flexible, and simple system for tuning the chemical and physical properties of surfaces. The fine control of surface properties is directly dependent on the structure of mixed SAMs which is difficult to characterize at the nanoscale with usual techniques such as scanning probe microscopies. In this study, we report on a general method to investigate at the nanoscale the structure of molecular patterns which consist in SAMs of two components. Iron oxide nanoparticles (NPs) have been used as probing agents to study indirectly the structure of mixed SAMs. Mixed SAMs were prepared by the replacement of mercaptododecane (MDD) adsorbed by mercaptoundecanoic acid (MUA) molecules on gold substrates. Therefore, the SAM surface displays both chelating carboxylic terminal groups and non-chelating methylene terminal groups. As NPs have been previously demonstrated to specifically interact with carboxylic acid groups, the increasing density in NPs was correlated with the evolution of the COOH/CH(3) terminal groups ratio. Therefore the structure of mixed SAMs was studied indirectly as well as the kinetic of the replacement reaction and its mechanism. With this aim, we took advantage of the SPR properties of the gold substrate and of the high refractive index of iron oxide nanoparticles to follow their assembling on mixed SAMs as a time resolved study. The high sensitivity and tuning of the SPR signal over a wide range of wavelengths are correlated with the NP density. Furthermore, SEM combined with image analysis has allowed studying the replacement rate of MDD by MUA in SAMs. We took also advantages of the magnetic properties of NPs to evaluate qualitatively the replacement of thiol molecules.

Generating in-Plane Orientational Order in Multilayer Films Prepared by Spray-Assisted Layer-by-Layer Assembly

We present a simple yet efficient method for orienting cellulose nanofibrils in layer-by-layer assembled films through spray-assisted alignment. While spraying at 90° against a receiving surface produces films with homogeneous in-plane orientation, spraying at smaller angles causes a macroscopic directional surface flow of liquid on the receiving surface and leads to films with substantial in-plane anisotropy when nanoscale objects with anisotropic shapes are used as components. First results with cellulose nanofibrils demonstrate that such fibrils are easily aligned by grazing incidence spraying to yield optically birefringent films over large surface areas. We show that the cellulosic nanofibrils are oriented parallel to the spraying direction and that the orientational order depends for example on the distance of the receiving surface from the spray nozzle. The alignment of the nanofibrils and the in-plane anisotropy of the films were independently confirmed by atomic force microscopy, optical microscopy between crossed polarizers, and the ellipsometric determination of the apparent refractive index of the film as a function of the in-plane rotation of the sample with respect to the plane of incidence of the ellipsometer.

Anisotropic optical and conductive properties of oriented 1D-nanoparticle thin films made by spray-assisted self-assembly

We report on the fabrication of oriented anisotropic metal nanoparticle thin films made by Grazing Incidence Spraying (GIS) and on the anisotropic plasmonic properties of the resulting thin films. Gold nanorods of two different aspect ratios and silver nanowires were self-assembled as a uniaxially aligned monolayer with the GIS approach. In particular, we examine the influence of the nanowire/nanorod length and diameter on the degree of ordering determined by electron microscopy pictures. Furthermore, we show that the anisotropy of the optical properties (probed by polarized UV-visible-near infrared spectroscopy) strongly depend on the quality of alignment. The prepared monolayer thin films have an orientation order parameter of up to 0.83 for silver nanowires, which is reflected in an optical anisotropy of 0.57 in the UV-visible and 0.76 in the near infrared through the selective excitation of transverse and longitudinal surface plasmon resonance modes. The electronic transport in oriented silver nanowire monolayers is also shown to be highly directional, with the sheet resistance varying over almost an order of magnitude depending on the transport direction. Such anisotropic conductive plasmonic thin films may find applications in various fields like biochemical sensing, energy transport and harvesting or optoelectronic devices.

Combined fitting of alternative and direct susceptibility curves of assembled nanostructures

Experimental ac-susceptibility curves at different frequencies (0.1 Hz ≤ f ≤ 1 kHz) were performed on samples prepared by physical and chemical pathways. By combining the triple fit method and a careful analysis of ac-experimental curves, we demonstrate an unambiguous and consistent determination method of both the magnetic particle size distribution and anisotropy for diluted granular nanostructures of magnetic clusters. Specifically, we highlight the importance of the size distribution in the determination of the magnetic anisotropy constant as well as the low relevance of the deduced parameters by considering alternative measurements alone.

Highly Oriented Nanowire Thin Films with Anisotropic Optical Properties Driven by the Simultaneous Influence of Surface Templating and Shear Forces

The functional properties of nanoparticle thin films depend strongly on the arrangement of the nanoparticles within the material. In particular, anisotropic optoelectronic properties can be achieved through the aligned assembly of 1D nanomaterials such as silver nanowires (AgNWs). However, the control of the hierarchical organization of these nanoscale building blocks across multiple length scales and over large areas is still a challenge. Here, we show that the oriented deposition of AgNWs using grazing incidence spraying of the nano-object suspensions on a substrate comprising parallel surface wrinkles readily produces highly oriented monolayer thin films on macroscopic areas (>5 × 5 mm2). The use of textured substrates enhances the degree of ordering as compared to flat ones and increases the area over which AgNWs are oriented. The resulting microscopic linear arrangement of AgNWs evaluated by scanning electron microscopy (SEM) reflects in a pronounced macroscopic optical anisotropy measured by conventional polarized UV-vis-NIR spectroscopy. The enhanced ordering obtained when spraying is done in the same direction as the wrinkles makes this approach more robust against small rotational offsets during preparation. On the contrary, the templating effect of the wrinkle topography can even dominate the shear-driven alignment when spraying is performed perpendicular to the wrinkles: the concomitant but opposing influence of topographic confinement (alignment along the wrinkles) and of spray-induced shear forces (orientation along the spraying direction) lead to films in which the predominant orientation of AgNWs gradually changes from one direction to its perpendicular one over the same substrate in a single processing step. This demonstrates that exploiting the subtle balance between shear forces and substrate-nanowire interactions mediated by wrinkles offers a new way to control the self-assembly of nanoparticles into more complex patterns.