Cd2P2Se6 nanolenses formed at a water-air interface

Self-assembly offers an attractive approach for the organization of nanocrystals (NCs) into functional superstructures. Well-ordered arrays with various configurations have been produced upon solid substrates [1– 6], but there are obvious advantages to be exploited if self-supporting films containing assemblies of functional NCs can be produced. To this end, the selfassembly behavior of NCs on liquid substrates, such as water surfaces, has been investigated. Lim et al. produced close-packed arrays of 5 nm silver NCs at a water–air interface by introducing a NC dispersion in the presence of polysterene [7]. Well-organized monolayer and bilayer regions were embedded in a continuous layer of the polymeric species. In a similar manner, Park et al. produced free-standing photonic bandgap crystals by developing 3-D assemblies of colloidal polysterene particles (∼240 nm in diameter) [8]. In the present letter we describe attempts to apply this approach to tri-n-octyl phosphine oxide (TOPO)coated CdSe NCs. The authors have had considerable success in assembling such NCs into assemblies on solid substrates, even to the point of producing wellordered monolayers with strong crystallographic texture [9]. Attempts to produce free-standing assemblies at a water–air interface, however, led to the serendipitous observation of nanoscale lens-shaped particles of the cadmium hexachalcogeno-hypodiphosphate phase Cd2P2Se6. A summary of these observations is presented here together with an outline of possible routes by which this phase could form and how the morphology might develop. The method for producing TOPO-coated CdSe NCs has been described in detail elsewhere [10]. Briefly, the precursors used are dimethyl cadmium and a solution of selenium in tri-n-octyl phosphine (TOP). The precursors were pre-mixed and subsequently introduced into a reaction flask containing a degassed mixture of TOPO and TOP under a nitrogen atmosphere at an elevated temperature (∼300 ◦C). The growth of the NCs in the reaction solution was monitored by ultraviolet visible spectrometry. The surfaces of the assynthesised CdSe NCs were passivated by a layer of TOPO molecules in the presence of the TOPO/TOP solvent mixture. These solvents were then removed by precipitation and repeated rinsing with methanol. Fractionation was performed using a solvent/non-solvent pair (chloroform/methanol) to further narrow the initial size distribution of the CdSe NCs [11]. This procedure was used to produce NCs of various core diameters, corresponding to different luminescent wavelengths. The CdSe NCs used in this work had a core diameter of 6.20 ± 0.36 nm, i.e. a variation of ±6.0%. Dilute dispersions of these fractionated CdSe nanocrystals in toluene were slowly deposited onto the surface of water contained in a 100 mm diameter Pyrex petri dish. Since toluene and water are immiscible, the dispersion formed a smooth floating circular layer on the water surface. The petri dish was then covered and the toluene allowed to evaporate for a period of 4 days. When the evaporation was completed, a fragmented thin film remained on the water surface. These fragments were carefully transferred on to a TEM copper grid and examined in a Philips EM420T operating at an accelerating voltage of 100 kV. Many regions of the thin film fragments were too thick for the electron beam to penetrate. In those regions that were electron-transparent, no assemblies of close-packed CdSe NCs were observed. Instead, large numbers of disc-like nanoparticles were present. These varied in diameter from 20 nm to 400 nm and were typically ∼100 nm in diameter. Fig. 1 is a brightfield TEM micrograph obtained from a representative area showing such particles. It was not possible to measure the thickness of the discs accurately but tilting experiments indicated that this was far less than the diameter of the discs in each case and probably <10 nm. The composition of the particles was investigated using energy dispersive spectrometry (EDS) in the TEM. The spectra exhibited characteristic peaks corresponding to Cd Lα , Se Lα , and P Kα , together with Kα system peaks for Cu and Zn, which arose from the support grid and specimen holder. Accurate quantification was not possible in the absence of a suitable standard, but standard-less analysis indicated that the particles certainly contained far more Se and P than one would