Optical Properties of Strongly Coupled Quantum Dot–Ligand Systems

Jan 31, 2013 - Victor A. Amin received an A.B. in Chemistry and a Certificate in Materials Science from Princeton University in 2008, where he wrote a...
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Perspective pubs.acs.org/JPCL

Optical Properties of Strongly Coupled Quantum Dot−Ligand Systems Matthew T. Frederick, Victor A. Amin, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States ABSTRACT: This Perspective describes the mechanisms by which organic surfactants, in particular, phenyldithiocarbamates (PTCs), couple electronically to the delocalized states of semiconductor quantum dots (QDs). This coupling reduces the confinement energies of excitonic carriers and, in the case of PTC, the optical band gap of metal chalcogenide QDs by up to 1 eV by selectively delocalizing the excitonic hole. The reduction of confinement energy for the hole is enabled by the creation of interfacial electronic states near the valence band edge of the QD. The PTC case illuminates the general minimal requirements for surfactants to achieve observable bathochromic or hypsochromic shifts of the optical band gap of QDs; these include frontier orbitals with energies near the relevant semiconductor band edge, the correct symmetry to mix with the orbitals of the relevant band, and an adsorption geometry that permits spatial overlap between the orbitals of the ligand and those of the relevant band (Se 4p orbitals for CdSe, for example). The shift is enhanced by energetic resonance of frontier orbitals of the surfactant with a high density of states region of the band, which, for CdSe, is ∼1 eV below the band edge. The Perspective discusses other examples of strong-coupling surfactants and compares the orbital mixing mechanism with other mechanisms of surfactant-induced shifts in the QD band gap.

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diversity to these materials and helps us realize their promise as “artificial atoms”,1 that is, tunable building blocks within hierarchical materials. Simultaneously, creating strongly coupled QD−ligand complexes will, in principle, mitigate the deleterious effect of organic ligands on transport of electrons through solid-state arrays of QDs by decreasing interparticle tunneling barriers. In order to realize these possibilities, we must first understand what properties allow for a strong QD− molecule interaction.2

his Perspective describes observations of strong electronic coupling between semiconductor quantum dots (QDs) and organic surfactants that alters the electronic structure of quantum-confined excitonic states in the QDs. Orbital mixing at the inorganic/organic interface in small colloidal QDs, where the diameter of the QD is less than or equal to the Bohr radius of one or both excitonic carriers, potentially influences not only the dynamics of exciton decay but also the energies and wave function distributions of these excited states. Understanding the mechanisms by which interfacial orbital mixing occurs and what characteristics make some classes of ligands more effective at perturbing QD excitonic structure than others will help researchers rationally design materials with new and interesting properties, not by changing the size or material of the semiconductor core, but by utilizing the much larger variety of possible organic surface functionalizations. Altering the QDs in this way suggests new strategies for both development of technology and fundamental research. In this Perspective, we will discuss the chemical and electronic properties that allow for strong organic−inorganic coupling at the QD−molecule interface, survey the literature for systems in which we believe this coupling occurs, explore the use of the QD−molecule interaction as a probe of the fundamental properties of the QD, and examine other possible mechanisms for exciton stabilization. Surface functionalization of colloidal nanocrystals can be accomplished during growth of the particles or postsynthetically, in a dispersion or in a film. Inclusion of the organic adlayer in the total electronic structure of the colloidal system, as is appropriate if the ligands have strong electronic coupling to the states of the QD core, introduces a rich chemical © 2013 American Chemical Society

Inclusion of the organic adlayer in the total electronic structure of the colloidal system, as is appropriate if the ligands have strong electronic coupling to the states of the QD core, introduces a rich chemical diversity to these materials. Ligands that couple strongly to a metal center, so as to create new electronic states with metal−ligand character, are wellknown in the coordination chemistry and electrochemistry communities as “non-innocent ligands”.3−5 For many metal complexes, redox states are primarily localized on either the Received: November 20, 2012 Accepted: January 31, 2013 Published: January 31, 2013 634

dx.doi.org/10.1021/jz301905n | J. Phys. Chem. Lett. 2013, 4, 634−640

The Journal of Physical Chemistry Letters

Perspective

composed of CdSe, which alters the electrochemical potential of these materials.17 In order to understand the features of PTC that enable strong coupling to the electronic states of the QD core, we utilize the simple particle-in-a-spherical-box (PIB) model of QD electronic structure. The usefulness of the PIB description of excitonic carriers in QDs is not limited to its prediction of the size-dependent excited-state energies of QDs18 because the confinement energy of the particle depends not only on the size of the box but also on the magnitude of its surface potential. The “walls of the box” within the PIB model, as given by the interfacial states of the QD−organic complex, are not perturbative terms added to the energy of the particle after solving the Schrödinger equation; they are boundary conditions for the solution; therefore, the confinement energy that they produce is intrinsic to the wave function of a charge carrier. As predicted by this simple model, confined carriers tunnel into the confining barrier region with a probability given by the height of these walls. For many types of QD−organic complexes, ignoring the contribution of this tunneling process to the total energy of the carrier is rational because the tunneling barriers presented by the surfactant are large, and the minimal amount of wave function density that does delocalize onto the ligands does not affect the properties of the core. When exchanging one insulating ligand for another (for example, alkylphosphonates to alkylcarboxylates), the change in tunneling probability is small even though the change in barrier height may be on the order of electron volts because the sensitivity of the kinetic energy of excited carriers to the confinement potential at the inorganic/organic interface is low when the absolute magnitude of the confining potential is large. When, however, one exchanges an insulating ligand for a ligand, like PTC, with orbitals of the correct energy and symmetry to create interfacial states near the band edge, ligand exchange causes dramatic shifts in band edge absorption and emission frequency. These shifts are due to large changes in the percentage of total wave function that tunnels into the surfactant. In order to observe shifts in the band gap of the QD due to the ligand, the coupling of the ligands to the core must then be strong enough such that the mixed states of the QD−ligand complex have energies near that of the band edge of the core − the energy that defines the “bottom” of the box. The ability of a ligand to delocalize an excitonic carrier is equivalent to its ability to change the tunneling barrier for the carrier by creating new, less confining states at the QD−organic interface. These new states are created upon mixing of the ligand orbitals with those orbitals of the QD that are delocalized over both the core (where the carrier originates) and the surface (to which the ligand couples directly). When two states couple, the degree of mixing depends on two factors, (i) spatial overlap of orbitals that represent the states, V, and (ii) the energy gap between the states, Δε. Each pairwise coupling between a ligand and a QD orbital results in two new states, split from the original states by an energy approximately equal to V2/Δε. The effectiveness of PTC as a delocalizing ligand lies in both spatial overlap and energetic resonance with orbitals of the QD. We suspect that the orbital overlap, V, between PTC and the QD is larger than that of common surfactants because the chelating geometry presents sulfur atoms that spatially approximate the anionic positions in the crystal lattice; Cd−S bond lengths in chelating dithiocarbamate cadmium complexes range from 2.5 to 3.0 Å,3 while the Cd−Se bond length is ∼2.6

metal center or the ligand. In the case of non-innocent ligands, however, the redox states of the ligand and metal are ambiguous, as the redox center is delocalized over the metal center and one or more of the ligands.6 No ligand is intrinsically non-innocent but becomes so in the presence of a metal center with orbitals of the right symmetry and energy to couple electronically with ligand orbitals. The same criteria apply to ligands for QDs. In the case of QD−ligand complexes, the mixed-character state that forms delocalizes not only over the metal to which the ligand binds, but also over the entire crystal lattice of the QD, such that binding of the ligand creates a new electronic structure without distinct QD or organic phases. The electronic structure of QD−organic complexes lies at the complicated intersection of solid-state physics, described with infinite periodic potentials, and chemistry, described with local bonding. We have found that both sets of models are needed to describe the nanoscopic organic/inorganic interfaces within colloidal QDs. The direct interactions of molecules with the QD surface are local interactions with the QD surface ions and are best modeled with molecular descriptions of the interfacial region. The core of the semiconductor is large enough to be well-approximated by a bulk density of states,7 but a density of states that is confined to a nanoscale region by the interface with a low-dielectric organic species. Hoffman8,9 gives a basic and intuitive set of rules for connecting chemical and solid-state descriptions at heterogeneous interfaces. Our research has found that, thus far, the molecule that exhibits the strongest electronic coupling to the semiconductor cores of CdSe, CdS, and PbS QDs  the most “non-innocent ligand” with respect to QDs  is phenyldithiocarbamate, PTC, Chart 1.10 Upon exchange for the native (as-synthesized) Chart 1. Structure of the X−PTC Acid, Where X Is an Arbitrary Substituenta

a We have explored X = −N(Me2), −OMe, −Me, −H, −Br, −F, −OCF3, and −CF3. The molecule is introduced to the QD dispersion as a salt (usually ammonium or triethylammonium) and has multiple possible binding modes, both monodentate and chelating, to metal cations.

surfactants of QDs such as phosphonates and carboxylates, PTC, which adsorbs as an anion, probably in both monodentate and chelating geometries (the distribution of binding modes as a function of QD surface composition is not yet clear), causes dramatic shifts in the QD’s optical band gap, up to 1 eV in the case of CdS and at least a factor of 6 greater than any other known chemical treatment for CdSe.11 Several research groups have employed chelating, sulfur-containing groups to achieve strong binding of ligands to QD surfaces.12−16 These studies were not focused on the optical response of the QDs to adsorption of these ligands and therefore did not explicitly report shifting the optical spectra of the QDs, but inspection of the spectra in these reports reveals bathochromic shifts of the absorption upon exchange of native ligands for dithiocarbamates. Additionally, dithiocarbamates are known to donate electron density to single-crystal electrodes 635

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Å.19 Furthermore, dithiocarbamates are π-donors (the PTC HOMOs have π-type symmetry); therefore, mixing of these orbitals with those of the QD systems that we have explored (CdSe, CdS, and PbS, which have valence bands (VBs) composed of primarily Se 4p or S 3p orbitals20−22), is symmetry-allowed.10,11,23 The combination of spatial and symmetric alignment allows for strong orbital overlap between these VB states and the PTC HOMO and, consequently, a large value for V. In order to explore the mechanism by which energetic resonance dictates the carrier delocalizing ability of PTC, we modified PTC at the para position with electron-donating or -withdrawing groups, X (Chart 1).23 This modification changes the energies of the highest occupied MOs of PTC. We observed that as X varies from electron-donating (X = N(Me)2) to electron-withdrawing (X = CF3), exciton delocalization into the surfactant (quantified by a parameter that we call the delocalization radius (ΔR),10 defined precisely in Figure 1A)

splitting), and the smaller the tunneling barrier presented to the hole by the interfacial states (see Figure 2 for an example of a diagram of this orbital mixing and how it determines the confinement potential for the excitonic hole within the PIB model).

Figure 2. (Left) Schematic depiction of the interaction between X− PTC and the QD VB. The density of states in the VB is variable, and, as the text discusses, the degree of mixing between the X−PTC HOMOs depends on the density of states with which X−PTC aligns. The greatest splitting occurs at energies where the number of nonnegligible (energetically resonant) states is greatest. (Right) Sketch of the potential profile for the excitonic hole within the PIB model. The box is inverted from a “normal” particle in a box because a hole is lower in energy toward the top on a traditional energy scale. The black dashed line shows the potential profile given from the states accessible to the hole. The VB edge defines the bottom of the box, and the mixed states represent the interfacial region. In this diagram, the surface states present wells to the hole because the mixed state is above the VB edge. Additionally, the hole wave function (red, sketched) is centered on the QD because of a parabolic potential provided by the excited electron (blue). The total potential profile is described by the heavy black line. (Adapted from ref 23.)

As we discussed above, this reduction of tunneling barrier only affects the degree of hole delocalization when the barrier is small in the first place, that is, when the absolute energy of the interfacial state is close to the VB edge. Ligand exchange from an alkylcarboxylate to an alkylphosphonate, for instance, may reduce the tunneling barrier, but the HOMOs of both ligands are several electron volts below the VB edge; therefore, the interfacial states that they create are irrelevant in determining the optical band gap of the QD. We can conclude that both proximity of the ligand HOMO to the VB edge of the QD and substantial orbital overlap from proper symmetry matching are the minimal requirements for delocalization of the hole into interfacial states by the ligand. If those requirements are satisfied, then tuning the resonance of the ligand’s orbitals with the highest density of states region of the VB further optimizes the interfacial interactions and maximizes the bathochromic shift of the absorption spectrum (as we see when we vary the substituent X). The chemical mechanism by which the excitonic hole of the QD delocalizes upon exchange of an insulating ligand with X− PTC is donation of electron density into the Se 4p orbitals of CdSe (or S 3p orbitals of CdS). This added interfacial electron density lowers the energy barrier for tunneling of the hole, a carrier of positive charge, into the interfacial region.25 A counterintuitive result of our study of the effect of the substituent, X, on the delocalization radius is that the π-

Figure 1. (A) Absorption spectra of CdSe QDs (1.44 nm) before (black) and after (red) adsorption of PTC. We calculate ΔR by subtracting the apparent radius (determined from the calibration curve of the absorption maximum versus R from Yu et al., ref 18) before PTC exchange from the radius after PTC exchange. (B) Plot of ΔR as a function of the HOMO of X−PTC. The more electron-withdrawing the substituent, the more energetically stabilized the HOMO. As the HOMO of X−PTC decreases in energy, it becomes more energetically resonant with the highest density of states region of the CdSe VB and affects a larger delocalization radius, ΔR, for the excitonic hole. (Adapted from ref 23.)

increases by more than a factor of 2. The shapes of the HOMOs of X−PTC are invariant with X;23 therefore, we attribute the dependence of ΔR on X to changes in the set of energy gaps, Δε, between the orbitals of PTC and those of the CdSe VB. As X becomes more electron-withdrawing, the X− PTC HOMO is stabilized (Figure 1B); it moves from near the band edge of CdSe, which has a low density of states, toward the middle of the band, which has a high density of states.7,24 The larger the number of VB states that mix with the X−PTC HOMO, the larger the total interaction energy (orbital 636

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The Journal of Physical Chemistry Letters

Perspective

cross-linking, and bioconjugation of QDs, is associated with small (