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Graphite and silicon nanoneedles are fabricated by different etching processes on highly oriented pyrolitic graphite (HOPG) and SiO2/Si. The masks used to engrave such substrates are needle-shaped ...

The use of masks coming from research field as different as colloids, polymers or nanomaterials is a recently emerging field. Recent advances in this area have developed a variety of practical routes which have a great potential to overcome or at least complete the high-cost lithographic techniques. This review focuses on three techniques that try to reduce to the nanometer range, the size of the mask. The major difference between these procedures is related to the type of mask used. The first technique is called colloidal lithography, the mask is a monodispersed-beads template. The second is the block copolymer lithography and the third technique is the nanocrystal lithography, the mask used is a nano-object. For these three parts, the synthetic routes, the improvements and the applications as well as the limitations will be pre- sented. I. INTRODUCTION Nanotechnology research is led by the demand for ever smaller device features that are required to improve perform- ance and decrease costs in microelectronics, communication and data storage. Lithographic methods are in the centre of this nanotechnology. Rapid replication of features below 50nm is currently a significantly unsolved problem. Generat- ing patterned surfaces at the nanoscale is beyond the limits of standard photolithography techniques. The wavelengths of photons limit the minimum feature sizes in the sub-100nm scale. Thus it appears that whatever technologies are used for fabrication in this region it does not represent simple engi- neering extensions of conventional photolithography. There exists an opportunity to introduce news ideas in replication of meso and nanostructures. Indeed, very few methods pro- vide the ability to work in the sub-50-nm scale (1-3). The well-established methods for the fabrication of nanometer- scale structures (2), such as scanning probe, electron and ion beam lithography provide resolution and integration density required for the industrials demands but the level of throughput rate is far below the requirement. There is a need for alternative simple techniques in order to save processing time and costs (4,5). This need has given rise to different kinds of approaches like, for example, nanoimprint lithogra- phy and soft lithography with the use of self-assembly monolayers (SAMs) (3,6,7). However, for both the mold fabrication problems remain unsolved. In this review we will focus on three techniques that try to reduce to the nanometer range, the size of the mask keep- ing a large surface coverage. The major difference between these procedures is related to the type of mask used while the standard techniques to pattern the substrate like metal depo- sition or ion-plasma etching are kept similar. The main re- quirements present in an efficient method for replication are flexibility in the engraved substrate, fidelity in the replica- tion, low density of defects and large patterned surface. Be- cause all of these characteristics are never present in a single method, it is necessary to combine techniques. Thus we present these three techniques and their extensions combined with others. The first technique is called colloidal lithogra- phy, the mask is a monodispersed-spheres template (typi- cally polystyrene beads with an average diameter of 200 nm). The second is the block copolymer lithography, block copolymers are made of two chemically different chains bonded covalently. When the blocks are incompatible, they spontaneously self-assemble into micro domains, this leads, at equilibrium, to an ordered structure which is utilized as lithographic mask. The third technique is the nanocrystal lithography, the mask used is a nano-object.

Formation of a solid solution of Cd1-yMnyS nanocrystals is demonstrated using EXAFS (extended X-ray absorption fine structure).

In this paper the synthesis and characterization of CdTe nanocrystals are presented. Opposite to what has been observed with Cd1–yMnyS, it has been impossible to include manganese ions in CdTe matrices. Only large rods of telluride were seen. ZnTe could also not be formed and again only Te rods were perceived. This clearly indicates that chemistry in colloidal self-assemblies is not always similar to that in homogeneous solutions.

URL: http://www-spht.cea.fr/articles/s00/001 Effets de taille dans les propriétés magnétiques statiques et dynamiques de nanoparticules de Cd$_{1-y}$Mn$_y$S; Magnetic properties of Cd$_{1-y}$Mn$_y$S nanocrystals have been studied. They vary with the particle size and markedly differ from those observed in the bulk phase, in particular their interactions drastically increase compared to those observed in the latter case. A Mn$^{2+}$-Mn$^{2+}$ interaction enhancement with decreasing size involves changes in the magnetic phase diagram. For 4nm nanocrystals, a spin glass phase can be assumed.

In this Letter, a model of a Cd1-yMnyS nanocrystal is proposed to explain the appearance of manganese ions isolated in the tetrahedral site, at relatively high composition, whereas magnetic interactions markedly increase in the nanosized range compared to the bulk phase.

In this paper, we present, for the first time, the magnetic properties of Cd1-yMnyS nanocrystals of average size (4 nm) at various compositions, y. We demonstrate that the Mn−Mn interactions are markedly enhanced compared to the bulk material. As in the bulk phase, the photoluminescence and the EPR hyperfine structure, both attributed to isolated Mn2+ ions in a tetrahedral coordination, behave similarly with increasing composition. At the opposite of the bulk phase, isolated Mn2+ ions are observed for a rather large composition (y = 0.08) in nanocrystals. We attempt to relate the change of the band gap with composition with the various manganese interactions.

In this paper we demonstrate that the use of colloidal assemblies permits to control independently the size and composition of Cd 1− y Mn y S nanosized particles. As expected, for a given composition, the increase in the band-edge energy with decreasing the particle size is observed and attributed to a quantum size effect. For a similar average particle size, the band gap does not vary linearly when manganese concentration increases. The minimum is more pronounced and its depth increases with decreasing the particle size. This is attributed to a quantum size effect and due to strong interactions between manganese ions and the conduction and valence bands electrons. For similar size of nanocrystals, the photoluminescence due to isolated Mn 2+ ions in Cd 1− y Mn y S nanoclusters is observed when the particles are aged and the CdS trap emission is observed immediately after the synthesis. The most important parameter in order to observe the Mn 2+ ions luminescence is the aging time of the particles and not a quantum size effect as claimed previously.

Absorption and photoluminescence spectroscopic measurements are presented for particles with sizes from 2 to 4 nm, composition from y = 0 to 0.3,where y is given by Cd1-yMnyS, and different preparation modes. No monotonous variation of bandgap energy with increasing composition is observed. The depth of the minimum increases with decreasing particle size. This is attributed to a marked increase in the interactions between the Mn2+ d electrons and the valence and conduction band electrons. Mn2+ photoluminescence is observed at 77 K, when the particles are aged in the solution. Otherwise only the trap emissions due to CdS defect states are seen. Certain preparation modes favor Mn2+ photoluminescence at room temperature.

New materials made of two- and three-dimensional superlattices of nanoparticles exhibit unique collective properties arising from the ordering of the nanoparticles. Here, dodecanethiol-coated silver nanocrystals self-assembled in 2D were subjected to oxygen plasma using the reactive ion etching process. The careful investigation by transmission electronic microscopy (TEM) of the same areas before and after exposure allows one to distinguish two behaviors under plasma treatment, which are determined by the level of ordering of the 2D organizations. Higher ordered self-assemblies remain unchanged, while less ordered organizations coalesce into larger nanocrystals with spheroidal shapes that could be single crystals. Electron energy-loss spectroscopy (EELS) measurements show that there is no oxidation of the silver either on the large coalesced nanocrystals or on the stable self-assemblies. A new intrinsic property, immunity of highly self-ordered nanocrystals provided by the ordering, is reported, and a mechanism of the coalescence is proposed.

Self-organization of nanocrystals depends on the type of nanomaterial and the coating. With silver nanocrystals, the self-organization is partially perturbed by the latter. With cobalt nanocrystals, the size distribution and thus the self-organization is controlled by the amount of reducing agent added during the chemical reaction and not by the micellar solution. It is possible to make "supra" crystals of cobalt nanoparticles in a face centered cubic (fcc) structure. By applying, during the evaporation process, an external magnetic field to ferrite nanocrystals dispersed in solution, nanocrystal organizations markedly change with their coating. Hence tubes are obtained with nanocrystals coated with citrate ions whereas thick films are produced when the coating is replaced by dodecanoic acid. Collective magnetic properties due to the organization in tubes are observed with a behavior similar to that observed with nanowires. The use of nanocrystals as a mask to produce various patterns on silicon is described.