Halogen compounds are capable of playing an important role in the manipulation of nanoparticle shapes and properties. In a new approach, we examined the shape evolution of CdSe nanorods to hexagonal pyramids in a hotinjection synthesis under the influence of halogenated additives in the form of organic chlorine, bromine and iodine compounds. Supported by DFT calculations, this shape evolution is explained as a result of X-type ligand coordination to sloped and flat Cd-rich facets and an equilibrium shape strongly influenced by halides. Synchrotron XPS measurements and TXRF results show that the shape evolution is accompanied by a modification in the chemical composition of the ligand sphere. Our experimental results suggest that the molecular structure of the halogenated compound is related to the degree of the effect on both rod growth and further shape evolution. This presents a new degree of freedom in nanoparticle shape control and highlights the role of additives in nanoparticle synthesis and their possible in situ formation of ligands.
The performance of devices based on semiconductor nanocrystals (NCs) improves both with stronger interface interactions among NCs and between NCs and solid electrode surfaces. The combination of X-ray photoelectron spectroscopy (XPS) and solid 31P CP/MAS NMR (cross-polarization/magic angle spinning nuclear magnetic resonance) shows that the selective substitution of long organic chains by chlorine atomic ligands during the colloidal synthesis by the hot injection method promotes the adsorption of CdSe NCs to carbon sp2 surfaces, leading to the formation of well-ordered NC monolayers on graphitic materials.
358 wileyonlinelibrary.com www.particle-journal.com www.MaterialsViews.com COMMUNICATION light into the development of these materials, focus of a myriad of applications, from catalysis to aerospace technology. Imaging, along with spectroscopical information at a single particle level, has been previously reported by STM in combination with STS (scanning tunneling spectroscopy), revealing a quantized density of states in different size, shape, and composition-controlled NCs, including hybrid metal-semiconductor structures. [4][5][6][7] Imaging of NCs by STM requires reducing the tunnel barriers, from the STM tip to the NCs surface and from the NCs surface to the substrate, [ 8 ] by desorption of solvents and/or ligands that passivate the NCs surface. This is usually performed by gently heating the samples at temperatures around 150 °C for several hours. [ 9 ] Furthermore, to avoid NCs displacement during imaging, they must be tightly anchored to the surface, which can be resolved by passivating the NP surface with a ligand that binds covalently to the substrate. [ 10 ] Alternatively, imaging can also be controlled by taking advantage of ordered arrangements, where tip effects over the movement of NCs are minimized. [ 5 ] In this work, we report STM and synchrotron XPS studies of pyramidal CdSe NCs self-assembled on highly oriented pyrolytic graphite (HOPG) substrates. The samples have been characterized by STM after different annealing treatments from 80 to 400 °C, evidencing a soft intra-particle ripening at moderate temperatures (up to 200 °C) and a severe inter-particle ripening (Ostwald Ripening) at higher ones. While these effects, studied in detail for colloidal semiconductor quantum dots in solution, [ 11 ] were also previously well known in nanoparticle systems grown by MBE on solid surfaces, [ 12 ] our system, consisting on colloidal NCs deposited on the surface is different at least regarding the presence of the capping ligands. Synchrotron-based, high-resolution XPS experiments reveal indeed that the activation of the ripening processes is correlated with the sequential desorption of the different chemical components of the ligand shell. The adsorption/desorption of ligands in colloidal NCs is regularly investigated by NMR spectroscopy [ 13 ] but rarely in ultrahigh vacuum conditions. In this work correlated synchrotron XPS performed after the annealing treatments allows studying the desorption dynamics of the different ligands. While degradation enhanced by temperature has been widely studied in supported NCs to study the loss of active surface area, [ 14 ] our studies give emphasis to the effects that each ligand desorption at different temperatures provokes on the NC's morphology. The methodology followed in this work can be very valuable to correlate the performance of nanoscale supported materials (i.e., hetero geneous catalysis, fuel cells, etc.) working under high temperature real conditions with modifi cations on their surface, taking also organic capping ligands into account.The last decades h...
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