Abstract
The binary assembly DDA-{Mo132}/OA-γ-Fe2O3 (DDA = didodecyldimethylammonium, {Mo132} = [Mo132O372(CH3COO)30(H2O)72]42−, OA = oleic acid) constitutes one of the two examples in the literature of binary superlattices made of a mixing of nanocrystals and oxo-clusters. In a precedent work, we reported in details the preparation of such magnetic binary systems and studied the effect of the nature of the polyoxometalates (POMs) on the magnetic properties. In the present paper, we study the stability of this model binary assembly under heating at various temperatures. Indeed, especially if magnetic and/or transport properties are targeted, an annealing can be essential to change the phase of the nanocrystals in a more magnetic one and/or to desorb the organic capping of the nano-objects that can constitute an obstacle to the electronic communication between the nano-objects. We gave evidence that the maghemite organization in the binary assembly is maintained until 370°C under vacuum thanks to the presence of the POMs. This latter evolve in the phase MoO3, but still permits to avoid the aggregation of the nanocrystals as well as preserve their periodical arrangement. On the contrary, an assembly made of pure γ-Fe2O3 nanocrystals displays a clear aggregation of the nano-objects from 370°C, as attested by transmission and scanning electronic microscopies and confirmed by magnetic measurements. The stability of the magnetic nanocrystals in such POMs/nanocrystals assemblies opens the way to (i) the elaboration of new binary assemblies from POMs and numerous kinds of nanocrystals with a good control on the magnetic properties and to (ii) the investigation of new physical properties as exchange coupling, or magneto-transport in such systems.
Highlights
Binary superlattices are made of periodical arrangement of nano-objects of different nature, size and shape (Kiely et al, 1998; Redl et al, 2003; Rogach, 2004; Talapin, 2008; Vanmaekelbergh, 2011)
We described the formation of DDA-{Mo132}/OA-γFe2O3 (DDA = didodecyldimethylammonium; OA = oleic acid) binary superlattices made of the assembly of polyoxometalates (POMs) and maghemite nanocrystals (MNCs) (Breitwieser et al, 2016)
In this study we considered 7.7 ± 0.50 nm Maghemite nanocrystals synthesized by thermal decomposition of iron chloride at high temperature by following a method (Ngo et al, 2013) similar to that reported by Park et al (2004)
Summary
Binary superlattices are made of periodical arrangement of nano-objects of different nature (metallic or semi-conducting nanocrystals, micelles, clusters. . . ), size and shape (Kiely et al, 1998; Redl et al, 2003; Rogach, 2004; Talapin, 2008; Vanmaekelbergh, 2011). Due to the needed high temperatures, the annealing procedure can induce coalescence phenomena leading to an increase in the size of the NCs, a modification of the shape and a change in the organization (Klemmer et al, 2003; Alloyeau et al, 2010; Demortière and Petit, 2011) In this context, one of the main challenges is to be able to anneal the nanostructured film while preventing from the aggregation of the nano-objects and keeping the periodicity in the binary system, both for addressing individual magnetic objects and for controlling the charge transport (Dong et al, 2011; Altantzis et al, 2016)
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