Major advances in the research field Marie-Paule PILENI February 2010

My research has been highly interdisciplinary over my entire scientific career. My accomplishments have impacted areas in photochemistry, photobiology, solar energy conversion, nanomaterials, colloidal assembly and self-assembly. I started my scientific carrier by investigating the reaction mechanism for the gas phase photoisomerization of pyrazine to pyrimidine and then focused my attention on problems of greater biological and pharmacological relevance. Specifically, I explored the photophysical
processes of tyrosine, trytophan and other derivatives in aqueous solutions. After the second oil price rise in 1978, I undertook research in solar energy materials. I developed micro-heterogeneous systems to mediate charge separation between the primary photolysis products and hence to increase the efficiency of photo- to chemical energy conversion. Full exploitation of the nanocompartments and reactivity control therein requires a molecular-level understanding of the structure of the employed colloidal systems. I developed a geometric model, supported by kinetic approaches, to show a remarkable dependence of the average location of the reactants in the colloidal system on both the chemical and photochemical reactivities. I pioneered the use of a new series of functionalized surfactants in which the counter ion was the reactive species and, again for the first time, I identified, with such surfactants, the existence of the supraaggregates made up of several colloidal phases in equilibrium. I demonstrated that these supraaggregates are predicted from a parameter as simple as the shape of the surfactant making up these entities. Such supraaggregates are largely produced in the food industry and explain the stability of several emulsions. I extended my studies to colloids containing proteins and enzymes. Structural studies of organized molecular systems containing proteins and/or enzymes have enabled me to demonstrate, both experimentally and theoretically, large modifications of the structural properties of colloids with the appearance of percolation phenomena in diluted systems. The percolation and “liquid-gas” transitions depend on the electrostatic interactions between the micelle interface and the macromolecule and not on the hydrophobic interactions, which are too weak to significantly perturb the structure. These phenomena are due to the sum of the interaction potentials between reverses micelles and macrolecules. The control of chemical reactivity, through the study of structure-reactivity relationships, enabled me to propose a pathway for the chemical synthesis of nanomaterial products. I pioneered the use of reverse micelles in the form of nanoreactors to control size and shape of the in situ nanocrystals generated therein. At this time (1985), this domain still appeared to belong to science fiction or scientific utopias looking for discoveries of new structures with extraordinary properties. I demonstrated that some molecules or electrolytes could also play a key role in controlling the shape of these nanocrystals. Thus, I can “manipulate” the atoms in order to produce nanocrystals with different shapes (spheres, cylinders, rods, disks, cubes, etc.). Very recently, by using an organometallic method I succeeded in producing 30% of single crystals inducing a splitting of the vibrational quadrupolar mode. Furthermore, I found that the homogeneous growth of fcc metal nuclei (single domains and multiply-twinned particles or MTPs) can lead to nanocrystals having the same shapes as the initial clusters. This study was supported by experiments and by optical simulations using the discrete dipole approximation (DDA). This also allows me to claim that, in highly pure media, the same crystal shape exists at various scales from clusters to the bulk phase. Using reverse micelles as nanoreactors, I chemically modified enzymes and proteins in order to make them hydrophobic without changing their activity. This has opened a new approach to enzyme catalysis. Starting in 1992, I
developed a new route to synthesize ferrite nanocrystals by using normal micelles. This synthesis is now widely used both in fundamental and applied research. One of its specific features is the production of controlled-diameter nanocrystals characterized by an identical surface state. This makes it possible to ascertain the magnetic properties of the nanomaterial itself and not those resulting from the surface state changes, as is the case in the so-called classic techniques used to produce magnetic fluids. This also allows controlling the stoechiometry of the ferrite nanocrystals.

In 1995, I demonstrated that nanocrystals are able to self-organize into close-packed networks (2D) and also in thin fcc superlattices (3D). Later on, the stacking of several hundreds of nanocrystal layers enabled the formation of supracrystals (3D) with fcc, hcp, or bcc structures in thermodynamically stable states. Other types of self-organization of nanocrystals like rings or honeycomb arrangements, linked to the Marangoni instabilities, were produced.

In the last decade, one of my major breakthroughs was to demonstrate that the physical properties of nanocrystals either aligned along one direction (1D) or self-ordered in close-packed networks (2D) are neither those of isolated nanocrystals nor those of the bulk phase of the same material. Actually, each nanocrystal influences its neighbors and their assemblies are characterized by collective physical properties. This has been demonstrated experimentally, in agreement with theoretical predictions:

(i) Magnetic nanocrystals self-ordered in 1D superlattices show collective properties with a large increase in the magnetization curve. In such an assembly these changes in the magnetic properties are due to dipolar interactions and to the orientation of the nanocrystal easy axis. To differentiate between these two processes magnetic bacteria have been used. In 2009, I was able to demonstrate that the major contribution is due to dipolar interactions.

(ii) Optical and magnetic collective properties emerge when nanocrystals are ordered in a close-packed network (2D): the appearance of coupled plasmon modes has been observed. Note that this splitting markedly differs with the nanomaterial used (Ag, Au). Magnetic properties of such an assembly markedly differ with the nanocrystal ordering.

(iii)Very recently, I discovered the first chemical intrinsic properties related to the stability with respect to either oxidation of Co nanocrystals or oxygen plasma exposure of Ag nanocrystals when they are selfordered in compact hexagonal networks (2D).

Another breakthrough is related to the following question: “Does the ordering of nanocrystals in supracrystals (3D) induce intrinsic properties?” The first intrinsic properties were discovered with nanocrystals ordered in supracrystals. Hence, in 2005 and confirmed in 2008 by a direct proof, I found collective vibrational properties of Ag and Co nanocrystals self-assembled in fcc supracrystals. According to News & Views in Nature Materials (2005, 4, 364-365), “A new study suggests that in materials made from the regular three-dimensional arrangement of discrete nanocrystals, control of order and periodicity could be exploited at a whole new level ”. Other intrinsic properties like magnetic, mechanical and crystal growth have been discovered. Very recently, I discovered that with fcc Co supracrystals there is a lower distribution of interaction energies, an inhibition of the flipping of the super spins and a slower approach to magnetic saturation compared to the disordered aggregates. Furthermore, when magnetic nanocrystals are exposed to an external magnetic field, fcc columns are produced whereas labyrinths are formed with disordered assemblies (2005). Triangular fcc single crystals, observed under ultra-high vacuum at a high temperature (340°C), are produced by mild annealing (50°C) from ordered Ag nanocrystals. The size of the triangles is correlated to the size of the ordered domains.
The electronic properties markedly change with the nanocrystals either isolated or ordered in 2D and thin 3D.

I demonstrated that the nanocrystal film (3D) shape at the mesoscopic scale is tuned by Van der Walls interactions. Actually, with the same nanocrystals, nanocrystal films differing by their shapes are produced. The magnetic properties markedly differ when a homogeneous film is replaced by one made up of superimposed cylinders.. Hence, the nanocrystal ordering is not the only factor that changes the physical properties of an assembly. I discovered that nanocrystals could be used as masks for nanolithography. Dots with sizes of a few nanometers (distance between dots: 2 nm) with the same shape pattern on semimetals and semiconductors are produced. Another breakthrough has been made with crack patterns in magnetic nanocrystal films. A universal scaling law with the film height over three orders of magnitude has been demonstrated.

In summary in my entire career my major breakthroughs are:
1- A fundamental understanding of the kinetics and mechanisms in colloidal solutions guided me in the preparation of either nanocrystals with different sizes and shapes or the chemical modification of enzymes.
2- Formation of thermodynamically stable states of self-assemblies either by using surfactant molecules (supraaggregates) or inorganic nanocrystals (supracrystals).
3- Collective optical and magnetic properties induced by dipolar interactions and due to the nanocrystal arrangements in 1D, 2D and 3D superlattices.
4- Physical intrinsic properties such as vibrational, magnetic and crystal growth related to the nanocrystals ordering in supracrystals (3D).
5- Chemical intrinsic properties due to nanocrystals ordering in close-packed networks (2D