Published: July 14, 2011 Published 2011 by the American Chemical Society 15927 dx.doi.org/10.1021/jp205784g | J. Phys. Chem. C 2011, 115, 15927���15932 ARTICLE pubs.acs.org/JPCC Mobile Surface Traps in CdSe Nanocrystals with Carboxylic Acid Ligands Oleksandr Voznyy* Institute for Microstructural Sciences, National Research Council of Canada, Ottawa K1A 0R6, Canada bSupporting S Information ��� INTRODUCTION Since the discovery of colloidal semiconductor nanocrystals (NCs) they have found tremendous amount of applications in bioimaging,1 lasing,2 diodes,3 single-photon sources,4 photovoltaics,5 etc. owing to a wide range of techniques6 available to tune their optical and electronic properties. Many of those applications rely on emission properties of NCs (quantum yield, emission wavelength broadening and diffusion, Stokes shift, blinking) which are strongly affected by different types of defects, supposedly residing at the surface.7 Surface traps may also affect the multiexciton generation yields8 and charge carriers extraction, relevant, e.g., for photovoltaics. Understand- ing better the source of the trap states can help to develop the synthesis procedures to reduce or ultimately eliminate the traps. In contrast to absorption properties, which are determined mainly by the bulk crystalline structure and the macroscopic properties of the NCs (size, shape),9,10 surface and thus emission properties require a more detailed knowledge on atomic scale. The exact atomistic nature of surface defects remains unknown11 13 and the interpretation of experimental data is thus often based on available theoretical models. Several semi- empirical studies of ligated surfaces are available14 18 but this methodology does not reliably capture surface reconstructions and charge redistributions. Few ab initio studies of the NC sur- faces available to date addressed only the bare surfaces19 25 or weakly bound (L-type) ligands.26 30 However, more and more experimental data suggest that the main type of ligands present on the surface are the covalently bound (X-type) ligands, e.g., depro- tonated carboxylic or phosphonic acids.31 33 Theoretical studies of such ligands on CdSe and PbSe only start to emerge.30,33 35 In this work we investigate from first principles the atomistic nature of the surface states in NCs. To do this, we choose CdSe NCs without structural defects, small enough to be treated within the density functional theory (DFT) but large enough to distinguish delocalized (core) and localized (trap) states, with carboxylic acid ligands bound covalently. We find that even such an idealized and small model is rich enough to create structures with or without surface trap states, depending on the amount of ligands. Contrary to expectations, apparently more passivated structures (with more ligands and less dangling bonds) exhibit more surface traps. Our most important finding is the presence of mobile surface ligands whose energy levels fluctuate respectively, a feature required by several phenomenological models of blinking.36 38 We will discuss whether the observed diffusion on its own is capable of explaining the fluorescence intermit- tency, and whether it is capable of producing switchable long- lived trap states. ��� COMPUTATIONAL METHODS Calculations were performed within DFT using the SIESTA code.39 Generalized gradient approximation in a Perdew Burke Ernzerhoff formulation, Troullier-Martins norm-conserving pseudo- potentials with nonlinear core corrections, semicore d-states included in valence shell for Cd, optimized double-�� plus polarization basis sets, and 300 Ry mesh cutoff for charge density were used throughout. Geometries were optimized until forces on atoms below 40 meV/�� were achieved. Full simulation input files are provided in the Supporting Information. The conver- gence of the simulation parameters and the general validity of our approach were tested by reproducing previous DFT results for bare19 and ligated28 CdSe nanoclusters. Received: June 20, 2011 Revised: July 11, 2011 ABSTRACT: We have performed ab initio calculations of electronic properties of the realistic Cd-rich CdSe nanocrystals with covalently bound carboxylic acid (X-type) ligands. Con- figurations both with and without surface traps can be prepared depending on the amount and geometry of the adsorbed ligands. We find that Cd and Se dangling bonds do not nec- essarily create surface traps, whereas traps originating from ligands can form near the top of the valence band. Some of the ligands are found to be mobile on the surface and this mobility is accompanied by a spectral diffusion of the associated trap energy levels. This provides the first atomistic example of the processes required to explain the emission wavelength and lifetime variations, and blinking of the nanocrystals.

15928 dx.doi.org/10.1021/jp205784g |J. Phys. Chem. C 2011, 115, 15927���15932 The Journal of Physical Chemistry C ARTICLE Different synthesis procedures of CdSe nanocrystals have been reported, with trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), amines, and carboxylic and phosphonic acid ligands available in solution. The resulting stoichiometry of li- gands on surface, however, is usually not well quantified. For phosphine-based synthesis, species not even considered to exist in solution were found recently to be the main ligands on NC surfaces and were shown to come from impurities in solvents or in source materials.32,40 Modeling of such ligands is also compli- cated by their higher structural complexity and increased amount of possible binding geometries. We thus choose carboxylic acids (known to be the sole ligands in phosphine-free synthesis31,41 45 ) as prototypical ligands for current study. Zincblende structure is used throughout, since it is known to be preferred with carboxylic acid based synthesis,31,41,44,45 in contrast to wurtzite structure usually reported for synthesis with TOP/TOPO. We prepare our models by carving a sphere out of zincblende CdSe bulk and removing all singly bonded atoms. On the facets where the formation of two dangling bonds per atom is unavoid- able we give preference to Cd-termination, leading to Cd- enriched clusters, as suggested by experiments.11,31,32,43 Acetate (CH3COO ) is used as a representative model of the longer fatty acid ligands. The diameter of the NC is adjusted to obtain a structure that matches as close as possible the charge neutrality condition intended to reduce the amount of surface states:14,15 NCd �� �� 2�� �� NSe ��-2�� �� NAc ��-1�� �� 0 ��1�� This condition has its roots in the second Pauling rule and is also similar to the electron counting rule used to determine stable semiconductor surface reconstructions.46,47 Both approaches aim to ensure that the total amount of electrons in the system will match the amount of bonds (in the idealized bulk-like structure this means 4 bonds per Cd or Se and 1 bond per acetate) that is, in general, they are not related to the balance of electronic and ionic charges. In the absence of surface Se dimers, eq 1, counting all NC atoms, remains valid and provides identical results to the more general electron counting rule, which con- siders only the surface atoms. One can see from eq 1 that the amount of ligands should be equal twice the excess of Cd atoms. The structure of the cluster thus can be widely varied by adjusting either the amount of ligands, adding/removing Cd or Se atoms, or artificially charging the cluster as a whole. Prepared in such a way [Cd56Se50(OAc)13]1- cluster is shown in Figure 1. Our model is Cd-rich and has a tetrahedral shape, similar to that of the thiolated ultrastable clusters48,49 and also typically observed for larger CdSe NCs with carboxylic ligands.31 It maintains the bulk-like local geometries for all atoms after optimization, representing well the bigger NCs with surface faceting. The three facets available are beneficial for modeling of ligand absorption all within one model. We believe that our structure is a better representative of the solution-prepared NCs than the highly reconstructed nonligated stoichiometric Cd33Se33 cluster often observed in laser ablation experiments50 and used in previous theoretical works.19,24,28,33 For clarity of the presentation we choose to describe the re- sults for the cluster with ���1.6 nm diameter and minimal amount of ligands. To comply with the electroneutrality condition (eq 1), the model in Figure 1 has several Cd atoms removed from the (001) facets (green arrow in Figure 1). One ���extra��� ligand is added (red arrow), compensated by a net charge 1 of the cluster as a whole. These artifacts do not affect the overall conclusions of the paper: a similar Cd68Se50(OAc)36 charge- neutral model with more Cd and ligands can be built (see Figure S1 in the Supporting Information) and was in fact the starting point in our simulations. Test calculations were also performed on smaller zincblende clusters, as well as a wurtzite NC of ���3 nm diameter. ��� RESULTS Ligand Geometries and Energetics. Figure 2 presents the optimized geometries of the ligands. First we populate all avai- lable adsorption sites on a (001) facet, the remaining ���extra��� ligand is then placed elsewhere on the surface. Binding energies were computed for charge-neutral desorbed species, without corrections for solvent effects. Obtained values should not be used for direct comparison with experiment, nevertheless, they are reliable for relative comparison of different adsorption sites. Adsorption of the ligand on a (001) facet (Figure 2a,b) is the most stable since its departure would leave one (or both) Cd with two dangling bonds. Calculated binding energy of the deproto- nated acetate (Eb 4 eV) is much larger than the values reported previously for protonoated case (Eb 1 eV),27,33 consistent with similar findings for CdSe33 and PbSe34 surfaces. Simulations starting from the ���bridge��� (Figure 2a) or ���chelate��� geometries (similar to Figure 2d) both relaxed to a ���tilted bridge��� geometry (which is ���0.25 eV lower in energy), with one of the oxygens bonding to two Cd atoms (Figure 2b). This geometry is consistent with Cd NMR data for CdSeTe magic-size NCs43 and single-crystal XRD data for some Cd salts.51 Previous theoretical studies on smaller Cd33Se33 clusters could not resolve Figure 1. Optimized structure of the [Cd56Se50(OAc)13]1- nanocrystal used in calculations. Green arrow marks the missing Cd atoms on the (001) facet red arrow, an ���extra��� ligand. Figure 2. Optimized geometries of acetate on CdSe NC surface: (a and b) on (001) Cd-rich surface facet of the NC and (c and d) on (111) Cd-rich facet. The atoms legend is the same as in Figure 1.