Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods

Abstract

The effect of temperature controlled annealing on the confined valence electron states in CdSe nanocrystal arrays, deposited as thin films, was studied using two-dimensional angular correlation of annihilation radiation. A reduction in the intensity by ϳ35% was observed in a feature of the positron annihilation spectrum upon removal of the pyridine capping molecules above 200°C in a vacuum. This reduction is explained by an increased electronic interaction of the valence orbitals of neighboring nanocrystals, induced by the formation of inorganic interfaces. Partial evaporation of the nanoporous CdSe layer and additional sintering into a polycrystalline thin film were observed at a relatively low temperature of ϳ486°C. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3094751͔The size and shape of colloidal II-VI semiconductor nanocrystals ͑NCs͒ can be well controlled, leading to a pronounced tunability and variation in their optical and optoelectronic properties.1-3 Their promise for applications in light-emitting diodes, solar cells and other opto-electronic devices was demonstrated in several studies. [4][5][6] For example, recently ultrathin solar cells consisting of sintered nanorods ͑a dual set of CdSe and CdTe͒ were developed. 7 The ͑opto͒electronic properties can be further modified by structural tailoring of nanocrystal superlattices. [8][9][10] The electronic interaction between neighboring NCs is a fundamentally important factor for composite nanocrystal devices, which determines both the transport of electron and hole charge carriers and the electronic structure of the active layers. The coupling can, in principle, be tailored by ligand manipulation or by inorganic tunneling barriers between neighboring nanocrystals. 1,11,12 Thus, innovative heterostructures are created.12 For example, enhanced conductivity has been achieved by removal of pyridine ligands by gentle heating ͑150-175°C͒ in a vacuum at moderate temperatures. 1,8,10 This reduces the average distance between neighboring NCs to less than 2 Å and leads to changes in the optical properties due to strong coupling. 1,8,10,11 A recent in situ electron microscopy study on monolayers of PbSe NCs indicates that this is accompanied by rotations of the NCs and the formation of an inorganic interface between neighboring NCs at slightly higher temperatures. 13Recent studies on semiconductor NCs 14-17 show the potential of positron methods to study the electronic structure of nanocrystal solids since the positron can be used as a sensitive probe for detecting the surface composition of the nanocrystals and the confinement of the upper valence electron states. In the present study, we apply high-resolution depth-sensitive positron methods 15,[17][18][19] to show that the electronic interaction between CdSe NCs, deposited as thin layers, can be monitored through observation of the electron momentum distribution of the valence states. Further, the depth-resolved positron studies provide insights into further sintering of the nanocrystal layers at highe...

Meeting challenges in nanomaterials for sustainable energy applications using high‐resolution positron methods

“…In-situ electron microscopy on monolayers of PbSe NCs [18] showed that the removal of ligand molecules by a gentle heating in a vacuum at moderate temperatures is accompanied by the formation of interfaces between the NCs. We demonstrated in temperature-dependent 2D-ACAR experiments [19] that depth-profiling positron-electron momentum density methods may reveal the coupling of valence electrons of neighboring CdSe NCs due to the interface formation ( Fig. 1), which will affect the transport of holes.…”

Exactly soluble model of resonant energy transfer between molecules

“…Recent proposals for improved dye-sensitized solar cells [25,26] involve replacing the liquid dye by nanoparticles attached to a substrate and exploiting FRET to achieve efficient energy transfer [27]. One candidate material is a mixture of QDs and the protein bacteriorhodopsin (bR) [28], [29], [30].…”

Ligand-surface interactions and surface oxidation of colloidal PbSe quantum dots revealed by thin-film positron annihilation methods

“…15 Furthermore, in ligand-exchange studies, it was demonstrated that short-chain diamines effectively strip lead oleate from (100) surfaces of the PbSe QDs. 23,31 The ligand replacement remains incomplete, particularly for the PbSe {100}-facets, which remain partially bare.…”

“…12,13 Clearly, the development of methods to investigate and monitor surfaces of PbSe QDs embedded in a sub-surface layer in such devices is of crucial importance. In this regard, positron annihilation spectroscopy (PAS) offers unique advantages over XAS, OAS and XPS, since it combines a high sensitivity to selectively probe surfaces of nanoparticles [14][15][16] with an established capacity for depth-profiling of films in the range of ~10 nm to a few m. [17][18][19] These unique merits are highly important in studies of photovoltaic devices, where charge carrier separation requires the formation of a p-n junction, 8 and PAS can probe the involved light absorbing and charge separation layers independently.…”

“…The PbSe QD films were examined by 2D-ACAR using the thin-film POSH-ACAR setup at the Reactor Institute Delft. [14][15][16] For comparison, a PbSe single crystal was studied by 2D-ACAR using a 22 Na positron source. The 2D-ACAR distributions were obtained with positron implantation energies of 1 keV for PbSe QDs with EDA and 3.4 keV for PbSe QDs with OLA and OA ligands.…”