book of abstracts - IM2NP

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A B S T R A C T S FRIDAY, JULY 2 N A N O S E A 2 0 1 0 11H20-11H40 Local structure at substitutional Mn2+ ions in small cubic ZnS nanocrystals self-assembled into a mesoporous structure. S. V. Nistor*, M. Stefan, L. C. Nistor, D. Ghica, N. J. Barascu and C. D. Mateescu (National Institute of Materials Physics, Atomistilor 105bis, POB MG 7, Magurele-Ilfov, 077125 Romania).* snistor@infim.ro. Due to the high potential of applications based on outstanding optical and photocatalytic properties, cubic ZnS nanocrystals activated with transition ions, in particular Mn2+, were between the first investigated II-VI semiconductor nanocrystals. Despite intensive research, the Mn2+ impurities localization in the nanocrystals core is still unclear. Electron Paramagnetic Resonance (EPR) investigations reported the unexpected presence of an axial crystal-field (CF) at the substitutional Zn2+ sites of normal cubic symmetry and a large range of values for the associated D-parameter. Determining accurate spectra parameters, essential for explaining the origin of a possible non-cubic local CF, requires well resolved EPR spectra, not available in the previously investigated nanocrystalline powders. We have prepared by a surfactant-assisted liquid-liquid reaction at room temperature a sponge-like mesoporous structure self-assembled from nanocrystals of cubic ZnS doped with Mn2+ ions (cZnS:Mn) [1]. The resulting tight size distribution around 2 nm of the cZnS:Mn nanocrystals and the improved EPR spectra resolution, with the narrowest lines reported so-far, were attributed to the restraining effect of selfassembling [2]. The full analysis, with dedicated software, of the EPR spectra recorded at low and high microwave frequencies confirmed the presence, besides the cubic parameters also found in cubic ZnS single crystals, of an axial CF component described by the parameter |D| = 41x 10-4 cm-1, attributed to the localization of the Mn2+ ions in the nanocrystals core next to an extended planar defect, as a stacking fault or twin. The proposed local structure is supported by earlier EPR results in strongly defective cZnS single crystals [3] and by our High Resolution Transmission Electron Microscopy (HRTEM) study which shows that ~ 30% of the investigated cZnS:Mn nanocrystals contain such extended planar defects. The improved quality of the cZnS:Mn nanocrystals self-assembled into a mesoporous structure did allow us to conclude, based on a correlated EPR and HRTEM investigation, that Mn2+ impurities are preferentially localized in the core of the cZnS:Mn nanocrystals at Zn2+ sites situated next to a stacking fault or twin. It also suggests that doping of Mn2+ ions in small cZnS:Mn and possibly in other cubic II-VI semiconductor nanocrystals is assisted by the extended planar defects. Such an extended lattice defects assisted (ELDA) incorporation mechanism can also offer a valid explanation for the high concentration of impurity ions, such as Mn2+ or Co2+, observed in cubic II-VI nanocrystals grown at low temperatures. [1]. S. V. Nistor, L. C. Nistor, M. Stefan et al. Superlattices and Microstructures 16, 306 (2009). [2]. S. V. Nistor, L. C. Nistor, M. Stefan et al. Rom. Rep. Phys. (2010) (in press). [3]. M. F. Bulanyi, A. V. Kovalenko and B. A. Polezhaev, J. Appl. Spectr. 69, 747 (2002) and ref. cited therein 127
Absorbance (a.u.) Absorbance (a.u.) A B S T R A C T S FRIDAY, JULY 2 N A N O S E A 2 0 1 0 11H40-12H00 Fullerenes C60 self-assembled on functionalized surfaces. Grégory Delafosse1,2,3, Lionel Patrone1,2,3, Didier Goguenheim1,2,3 (1 Aix-Marseille Université, IM2NP; 2 CNRS, IM2NP (UMR 6242); 3 Institut Supérieur de l‟Electronique et du Numérique, IM2NP Maison des Technologies, Place Georges Pompidou, F-83000 Toulon, France). gregory.delafosse@im2np.fr 1 – Introduction Memory devices play an important role in electronics market leading to a growing research interest in the next generation of non-volatile memory cells. Numerous groups [1,2] work on top-down memory cells based on fullerenes C60 that are generally embedded in insulating polymers where they act as storage sites. Beside these studies, a reaction path between amines and fullerenes [3] may be used to covalently bind C60 on amine-functionalized surfaces. Such an approach is interesting since it involves self-assembled monolayers (SAM) [4,5] which constitute a promising strategy to build molecular nano-devices. For applications compatible with microelectronics technology, it is very important to control first the formation of amineterminated SAMs grafted on silicon, second the grafting of a C60 monolayer on top of these SAMs. In this work, we studied these two steps in order to build memory cells using a bottom-up approach based on organic SAMs. 2 – Abstract As substrates, we used silicon covered with its native oxide, and Au(111) for scanning tunnelling microscopy (STM) experiments. Surface modification is observed by contact angle measurements, ellipsometry, UVvisible and FTIR spectroscopy, and AFM/STM microscopy. Surface functionalization is performed using aminopropyltrimethoxysilane (APTMS) molecules for Si/SiO2 surfaces, and aminothiophenol or aminoethanethiol for Au(111). We developed two methods to build APTMS SAMs on Si/SiO2 substrates. Using APTMS in methanol solution, SAM formation was analyzed in order to obtain the right parameters leading to a single amine-functionalized monolayer. A second original way was studied using a dry deposition method. In the latest, freshly prepared clean substrates are exposed to APTMS vapor under a nitrogen flux, leading to the formation of APTMS SAM. This deposition method allowed us to show there is a minimum waiting time of ~4 hours under our conditions for the monolayer to be grafted on Si/SiO2. APTMS SAM formation could be monitored using ATR-FTIR spectroscopy. 3253 3minutes 38minutes 90minutes 130minutes 150minutes 180minutes 214minutes 240minutes 270minutes 300minutes 330minutes 365minutes 2927 2857 3minutes 38minutes 90minutes 130minutes 150minutes 180minutes 214minutes 240minutes 270minutes 300minutes 330minutes 365minutes 3500 3000 Wavenumber (cm -1 ) 3000 2950 2900 2850 2800 2750 2700 Wavenumber (cm -1 ) (a) Figure 1. ATR-FTIR spectra of dry-deposited APTMS SAM showing (a) the increase of OH absorption band, (b) CH2 band narrowing with time. (b) 128
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