Youwei
Wang
,
Yubo
Zhang
and
Wenqing
Zhang
*
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail: wqzhang@mail.sic.ac.cn
First published on 28th February 2014
We studied the effects of chlorine passivation and iodine passivation on the electronic structures of Cd33Se33 quantum dots through partial chlorine replacement for surface pseudo-hydrogen atoms, taking full pseudo-hydrogen-terminated Cd33Se33 quantum dots as a reference. Our calculations demonstrate that the electrostatic interaction between surface Cd absorbates and the halide passivant removes the dangling-bond-derived states of surface Cd atoms. Due to the high electronegativity, the Cl passivants need to coordinate with three Cd atoms to saturate p orbitals and achieve complete saturation. The modulation of the Cl passivants to the electronic structures of quantum dots depends on the coordination number of the Cl passivants. With coordination numbers of up to three, Cl passivants contribute less to the HOMO state and the QDs are more energetically stable. As the electronegativity decreases from Cl to I, I passivants with a coordination number of 2 are energetically stable enough to passivate QDs, leading to a relatively inactive surface after passivation. The HOMO states of I-terminated QDs are composed of I 5p and Se 4p.
Experiments have shown that surface coordinating ligands have a dramatic effect on the band edges.4–9 For example, the rate of intra-band relaxation can be slowed down by changing the capping organic ligands.4 Organic alkylamines9 and inorganic multi-semiconductor-shells9,10 increase the emission quantum yields by passivating the surface of CdSe QDs. Surface passivants also influence multiple-exciton generation efficiencies.8 Surface passivation with appropriate ligands improves the quantum efficiency of band edges.9 The use of short mercaptopropionic acid for organic ligand passivation achieved the best reported efficiency of 5.6% by reducing the organic ligand interparticle spacing.7 Much attention has been devoted to organic ligands which are used to lower the recombination loss due to the surface dangling bonds.4,7,11,12 However, the bulkiness of the organic ligands that employ long chains (8–18 carbons) prevent organic passivants from passivating other cations on the surface of QDs, leading to low coverage of passivants. The incomplete coverage of organic ligands leaves a high density of defects which leads to recombination loss. Monovalent inorganic ligands5 were recently reported to offer a better passivation considering steric effects, achieving efficiencies as high as 6.0%. Halide ions, which attach to CdSe QDs through electrostatic interactions, lead to different degrees of redispersion and stabilization in polar solvents.13 Moreover, a hybrid passivation scheme6 of short organic ligands and halide anions has improved the efficiencies of colloidal QD solar cells, achieving as high as 7%.
First principles calculations have showed that organic ligands form delocalized orbitals spreading over the QD surface and ligands.12,14 Steric interference of neighboring organic ligands affects the binding strength between oxygen atoms in the organic ligands and cations on the surface.11 Unlike the well-studied effects of organic ligands on both structure and physical properties, the effects of inorganic ligands are rarely studied.15–17 In theoretical studies of nanostructured semiconductors, pseudo-hydrogen (PH) has been proposed to passivate each of the dangling bonds with a fractional charge of (8 − Z)/4, where Z is the valence charge of the surface atom.18 With fractional charge, the PH passivants saturate the dangling bonds and remove surface states from the electronic structure of the band edges.19 However, PH passivants are artificial and do not exist in the real world. There is no way of getting the same intrinsic electronic structure as in the PH passivation case. However, we can take PH passivation as a reference, which is an ideal strategy to remove the dangling bonds. Experiments have shown that atomic halide ions, with the goal of using the shortest imaginable ligands, passivated QDs.5,6,13 However, the coordination effects of halide anions on the electronic structure of QDs have still not been investigated. Herein, we report results of density function theory (DFT) calculations that reveal the effects of chlorine passivation on the morphology and electronic structure of CdSe QDs. We used Cd33Se33 clusters, which have been experimentally identified to be one of the smallest CdSe clusters.20
In this paper we investigated the different impacts of PH and halide (Cl and I) passivations on the electronic properties of passivated QDs. For that purpose, we focused on Cd33Se33 dots capped with chlorine or iodine atoms on topical sites and analyzed the changes in the corresponding electronic structure. We show that the electronic interaction between surface Cd absorbates and halide passivants can remove the surface states from dangling bonds of surface Cd atoms. However, the incompletely saturated Cl passivants affect the states of the highest occupied molecular orbitals (HOMO). Therefore, the effect of chlorine passivation depends on the coordination number of the Cl passivants. With a coordination number of 3, the Cl passivants remove surface states and have a negligible effect on the HOMO states. In iodine passivation, the 2-coordinated I passivants are energetically stable and have energy levels close to those of Cd–Se in QDs, which greatly affects the HOMO states.
It has been noted that in practical experiments QDs probably undergo surface reconstruction, and particularly in cases with no passivation, only a limited number of dangling bonds are left on the surface. This reconstruction changes the bulk-like electronic structures of QDs into surface electronic structures which have a great effect on the performance of QDs. For example, when the stoichiometric CdSe QDs are unpassivated, the electrons transfer from cations to anions with surface reconstruction. This reconstruction removes dangling bonds and opens the band gap, which is considered ‘self-healing’ of QDs.21 However, the self-healing drives sp3-like bonds between Cd and Se atoms to become surface bonds, e.g. the surface cations (Cd) form sp2-like bonds with three neighboring anionic atoms (Se) on the Cd-rich surface. These surface atomic bonds change the bulk-like electronic properties to surface electronic properties which have a high density of states at the top of the valence band. That effect becomes enhanced with decreasing size as the surface-to-volume ratio increases. Moreover, non-stoichiometric CdSe QDs may leave some chemically active bonds at the surface, which have surface states to reduce the energy gap. There are no intrinsic effects from surface reconstruction and non-stoichiometry is not discussed in the current work.
When the surface is unpassivated, inner Cd–Se bonds cause the dangling bonds of the surface Cd to be in the sp3 hybridized orbitals configuration. PH matches the orientation of the dangling bonds to conserve the sp3 configuration. The PH-terminated QDs undergo no surface reconstruction. The bonding states, due to the interaction between PH and surface atoms, locate well below the HOMO state, while the anti-bonding states are above the state of the lowest unoccupied molecular orbitals (LUMOs). With no change to the intrinsic electronic structure of band-edges, PH passivation removes the effects of surface states. The HOMO and LUMO states are mostly located within the interior of the quantum dot with negligible overlap with the states from the PH passivants. Consequently, the electronic structure of the band edges is dominated by the intrinsic chemical interaction within the interior of the QDs and is independent of the bonds between the surface atoms and the PH passivants. Theoretically, PH passivation also provides a platform to judge a different passivation strategy by comparing it with the PH passivation case.
Previous computational11 and experimental22 studies confirmed that organic ligands bind preferentially to the surface Cd atoms. We focus mainly on the effect of halide ligands binding to Cd atoms. To gain insight into the effect of halide passivation, we removed some PH atoms from the surface Cd atoms at different sites and attached halide atoms (Cl or I) to the dangling bonds. The approach is similar to the replacement of H with O in the bond study of SiC.23 Correspondingly, we employed a fully PH-terminated Cd33Se33 quantum dot as a reference to compare with. As the distances between the PH atoms and halide atoms are as large as 4 Å (3.5 Å for the (000)II facet case in chlorine passivation and 3.7 for the (000
)II facet case in iodine passivation), the interaction between them could be reasonably neglected in the following discussion.
The Cd positions on the surface are marked as shown in Fig. 1: A1, A2 and A3 indicate 3-coordinated Cd atoms, each with only one dangling bond to be passivated; B1 and B2 label 2-coordinated Cd atoms, each with two dangling bonds to be passivated. In general, taking chlorine passivations for example, the local structures to be passivated are classified into four categories: (i) one Cl atom passivates two dangling bonds from one 2-coordinated Cd atom (B1 or B2 sites); (ii) one Cl atom simultaneously passivates two dangling bonds from two 3-coordinated Cd atoms (A1 or A2 sites); (iii) one Cl atom passivates two dangling bonds from one 2-coordinated Cd atom and another one from one 3-coordinated Cd atom (A2 and B2 sites); (iv) one Cl atom passivates three dangling bonds from three 3-coordinated Cd atoms (A1 site) on the (000) surface. In cases (i), (ii), and (iii), Cd and Cl atoms conform to the 8-valence electrons rule, which means that the QDs maintain charge balance between cationic and anionic atoms, leaving the coordination number of Cl passivants as less than 3. Case (iv) breaks the charge balance, with one excess electron from cations (Cd), but saturates the Cl passivants with coordination numbers of 3. In case (iii), there are other combinations (for example, B2 and A3 sites) which involve PH atoms bonding with Cl passivants when Cl passivants absorb at the sites. We ruled these combinations out as unstable structures, for the reason that just with surface Cd atoms, the Cl passivants can not be stable and need neighboring PH atoms to coordinate with.
Based on the relaxed structures, the configuration of the above four cases can be divided into two categories: (i) for mono-dentate Cl passivants, i.e. one Cl passivant coordinating to one 2-coordinated Cd atom; the addition of Cl leads to no obvious surface reconstruction of the wurtzite structure of QDs (see discussion in Section 3.1); (ii) for multi-dentate Cl passivants, i.e. one Cl passivant coordinating to more than one surface Cd atom; the addition of Cl affects the bond length of a few Cd–Cd pairs and results in surface reconstruction to some extent (see discussion in Section 3.1).
Chlorine passivation | Iodine passivation | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
PH | (0001)I | (11![]() |
(01![]() |
(000![]() |
(01![]() |
(000![]() |
(0001)I | (000![]() |
(000![]() |
|
Energy gap (eV) | 2.58 | 2.53 | 2.49 | 2.43 | 2.47 | 2.40 | 2.50 | 2.06 | 2.33 | 2.42 |
Bond length (Å) | 1.88 | 2.35 | 2.37 | 2.65 | 2.65 | 2.57/2.70 | 2.83 | 2.66 | 2.94 | 3.14 |
Generally speaking, all passivation cases have LUMOs similar to PH-terminated QDs. With negligible overlap with Cl passivants, the LUMO states are mainly located at the interior Cd atoms (see Fig. 2). The energy levels of the anti-bonding state of Cd–Cl are higher than those of interior Cd–Se. Passivated surface Cd atoms and incompletely saturated Cl passivants do not lead to surface states close to the LUMO states. As a result, the LUMO states of the QDs are composed of intrinsically anti-bonding states of Cd–Se, and the incompletely saturated Cl passivants have a negligible effect on the LUMO states.
When Cl passivants are mono-dentate, the HOMO states of the QDs with Cl passivation on the (0001)I facet and the (110)I facet are composed of the incompletely saturated Cl 3p orbitals (see Fig. 2), partially mixing with neighboring Se 4p orbitals. The states of Cl 3p orbitals are above those of the intrinsic Cd–Se of the QDs. Although Cl passivants remove the surface states of the dangling bonds of surface Cd atoms, the incompletely saturated Cl passivants lead to new surface states. The relatively high density of Cl 3p states actually leaves the QDs unstable, and the QDs prefer to lower the energy level of the Cl p orbitals through surface structure relaxation. As a result, the (0001)I facet and (11
0)I facet are the last possible sites on to which Cl passivants absorb.
In the case of (000)II (or (01
0)II) facet passivation, the bond lengths between Cl and 3-coordinated Cd atoms are shorter than the Cd–Cl distance in bulk CdCl2 (2.70 Å). The HOMO states are also contributed to by the Se 4p and Cl 3p states. Although, the (000
)II and (01
0)II cases reduce the contribution from Cl 3p states to the HOMO states, the mixture breaks the symmetry of the distribution of HOMO states in the PH-terminated case. Moreover, the spatial wavefunction distribution of the top HOMOs shows that those wavefunctions have a significant contribution from the surface Cl passivants of the QDs. Compared with the PH-terminated case, those 2-coordinated Cl atoms significantly affect the electronic structure of band edges. Consequently, the HOMO states of the (000
)II and (01
0)II cases are a mixture of Se 4p and Cl 3p states, compared with those in the PH-terminated case which are Se 4p states. In consideration of this, Cl passivants in bidentate cases (i.e. (000
)II and (01
0)II cases) remove surface states of surface Cd absorbates but perturb the HOMO states of QDs. Compared with the mono-dentate case of Cl passivation, bidentate cases lower the energy level of Cl passivants, which is still higher than that in the tridentate case (see discussion in Section 3.2).
Fig. 3 shows the schematic bonding picture of charge balance cases. The above discussed Cl passivation cases, conforming to the 8-valence electrons rule, remove the dangling-bond-derived states. The LUMO states of the Cl passivation cases can be considered to be the same as those in the PH-terminated case, which means that Cl passivants remove the dangling-bond-derived states and cause negligible perturbation to the LUMO states of the QDs. However, the passivation effect of Cl passivation on band edges is not as strong as that in PH passivation, and the contribution of Cl passivants to the HOMO states can still be clearly observed, depending on the coordination number of the Cl passivants. Additionally, the energy levels of the bonding states of the coupling surface Cd and incompletely saturated Cl passivants are higher than those of the bonding state between Cd and Se atoms. As shown in Fig. 2, for all Cl passivation cases, the incompletely saturated Cl passivants, i.e. the coordination numbers of the Cl passivants less than three, cause great perturbation to the HOMO states. Though Cl passivants can passivate, to a great extent, the dangling bonds of surface Cd absorbates in all cases, the p orbitals of Cl atoms need three neighboring Cd atoms to be saturated. However, the charge balance consideration (i.e. all atoms conforming to the 8-valence electrons rule) leaves the coordination number of Cl passivants at less than three. Also, a mixture of Cl 3p states and Se 4p states composes the HOMO states of the QDs, and the coordination number of the Cl passivants influences the wavefunction distribution of the HOMO states by changing the saturation extent of the Cl passivants. As a result, the energy levels of the bonding states of Cd–Cl in charge balance Cl passivations are not definitely below that of the intrinsic bonding state of Cd–Se but can be considered as a function of the coordination number of the Cl passivants. With the coordination number of the Cl passivants increasing, the energy level of the bonding states of Cd–Cl gets deeper. The system in which one chlorine atom coordinates with two 3-coordinated Cd atoms is more stable than the system in which one chlorine atom coordinates with only one 2-coordinated Cd atom, but the most stable case should correspond to the Cl passivants interacting with 3-coordinated Cd atoms. The HOMO states with incompletely saturated Cl p orbitals are expected to have a negative effect on the performance of QD-based devices.
Due to the large difference in the electronegativities between Cl and Cd, Cl 3p and Cd 5s have weak p–s mixing in the upper valence band in CdCl2 crystals. Consequently, the Cl 3p occupied states form the top of the valence band, which is almost 1 eV deeper than those of PH-terminated Cd33Se33 QDs, and a mixture of Cd 5s–Cl 3p unoccupied states form the bottom of the conduction band.24 As shown in Fig 4(b) and (c), in the (000)III case, the HOMO wavefunction of the QD is located at Se sites and has no overlap with Cl passivants, and the LUMO wavefunction slightly overlaps with Cl passivants. The 3-coordinated Cd atoms, as in the case of bulk CdCl2 crystals, saturate Cl passivants and lower the energy level of Cl 3p states below HOMO state, so that the HOMO states are located at the interior Se atoms of the QDs. Energetically, the passivation of one Cl passivant bonding with three 3-coordinated Cd atoms is more stable than the passivations involving one Cl passivant bonding with fewer Cd atoms because the latter cases incompletely saturate Cl which leads to higher energy states of the Cl passivants.
Due to the difference in electronegativity between Cl and I, and the varying bond lengths, the chemical bonding between I–Cd is expected to be weaker than that of Cl–Cd. Consequently, the energy levels of the bonding states of I–Cd are not as deep as those of Cl–Cd. The HOMO states in the (000)III case, which are composed of I 5p and Se 4p, demonstrate that the coordination numbers of the I passivants have less of an effect on the HOMO states in iodine passivation than in chlorine passivation. In chlorine passivation, the energy levels of the bonding states of Cd and 3-coordinated Cl atoms fall deeper in energy than those of the bonding states of Cd and Se atoms within QDs, but the incompletely saturated Cl passivants have relatively high energy levels, which are comparable with the energy levels of the bonding states of Cd–Se in QDs (see Fig. 2). To lower the activity of the incompletely saturated Cl passivants, the Cl passivants tend to interact with nearby or neighboring cations, leading to the aggregation of the QDs. However, in iodine passivation, due to the decrease in the electronegativity, the energy levels of Cd and 3-coordinated I atoms are intrinsically comparable with the energy levels of the bonding states of Cd–Se in the QDs. Also, the coordination number of I passivants has less of an effect on the saturation of I passivants than of Cl passivants, because the passivation of one I passivant to only two 3-coordinated Cd atoms has already been stable in iodine passivation cases. Therefore, QDs with 2-coordinated I passivants have weak interactions with cations from neighboring QDs, leading to no obvious aggregation and change in size.
In the PH-terminated case, PH atoms, exactly matching the direction and fractional charge of dangling bonds, perfectly passivate surface cations. The bonding states and anti-bonding states from PH and surface atoms can be pushed down deep into the valence band and up into the conduction band, respectively. Organic ligands usually passivate surface atoms through surface reconstruction, which leads to charge transfer from cationic to anionic atoms. The binding between organic ligands and 2-coordinated Cd atoms is always stronger than that between organic ligands and 3-coordinated Cd atoms. In addition, the effect of organic ligands depends on the binding capacity of the ligand to the QD as well as steric inter-ligand interactions.12 However, after removing the surface states derived from the dangling bonds of cations of QDs, the effect of Cl passivation depends mainly on the coordination number of the Cl passivants. The incompletely saturated Cl passivants may affect the electronic structure of band edges, while completely saturated Cl passivants have little effect on edge states. For small QDs, the different coordination environment of Cl passivants may lead to different distributions of the HOMO states, which potentially affects the efficiency of QD-based devices. Compared with organic ligand passivation based on binding capacity and complex surface reconstruction, halide passivants perfectly present better passivation due to electrostatic interaction.5 Moreover, due to the different preferred absorbate sites, organic ligands prefer to absorb to one 2-coordinated cations and halide passivants prefer to bind with more than one 3-coordinated cation, a hybrid system of organic and halide ligands can improve the efficiency of solar cells.6 Steric locations to which halide passivants absorb may compensate for the organic ligand-uncovered surface and improve the coverage and thus the passivation of QDs.
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