Optical Properties of Strongly Coupled Quantum Dot-Ligand

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Optical Properties of Strongly Coupled Quantum Dot LigandSystemsMatthew 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 organicsurfactants, in particular, phenyldithiocarbamates (PTCs), couple electronically tothe delocalized states of semiconductor quantum dots (QDs). This couplingreduces the connement energies of excitonic carriers and, in the case of PTC, theoptical band gap of metal chalcogenide QDs by up to 1 eV by selectively delocalizing the excitonic hole. The reduction of connement energy for the holeis enabled by the creation of interfacial electronic states near the valence band edgeof the QD. The PTC case illuminates the general minimal requirements forsurfactants to achieve observable bathochromic or hypsochromic shifts of theoptical band gap of QDs; these include frontier orbitals with energies near therelevant semiconductor band edge, the correct symmetry to mix with the orbitalsof the relevant band, and an adsorption geometry that permits spatial overlap between the orbitals of the ligand and those orelevant band (Se 4p orbitals for CdSe, for example). The shift is enhanced by energetic resonance of frontier orbitals surfactant with a high density of states region of the band, which, for CdSe, is 1 eV below the band edge. The Perspectivediscusses other examples of strong-coupling surfactants and compares the orbital mixing mechanism with other mechanissurfactant-induced shifts in the QD band gap.

This Perspective describes observations of strong electroniccoupling between semiconductor quantum dots (QDs)and organic surfactants that alters the electronic structure of quantum-conned excitonic states in the QDs. Orbital mixingat the inorganic/organic interface in small colloidal QDs, wherethe diameter of the QD is less than or equal to the Bohr radiusof one or both excitonic carriers, potentially inuences not only the dynamics of exciton decay but also the energies and wavefunction distributions of these excited states. Understanding themechanisms by which interfacial orbital mixing occurs and whatcharacteristics make some classes of ligands more eff ective atperturbing QD excitonic structure than others will helpresearchers rationally design materials with new and interestingproperties, not by changing the size or material of thesemiconductor core, but by utilizing the much larger variety of possible organic surface functionalizations. Altering the QDsin 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 forstrong organic inorganic coupling at the QD moleculeinterface, survey the literature for systems in which we believethis coupling occurs, explore the use of the QD moleculeinteraction as a probe of the fundamental properties of the QD,and examine other possible mechanisms for exciton stabiliza-tion.

Surface functionalization of colloidal nanocrystals can beaccomplished during growth of the particles or postsyntheti-cally, in a dispersion or in a lm. Inclusion of the organicadlayer in the total electronic structure of the colloidal system,as is appropriate if the ligands have strong electronic couplingto the states of the QD core, introduces a rich chemical

diversity to these materials and helps us realize their promise as arti cial atoms ,1 that is, tunable building blocks withinhierarchical materials. Simultaneously, creating stronglycoupled QD ligand complexes will, in principle, mitigate thedeleterious eff ect of organic ligands on transport of electronsthrough solid-state arrays of QDs by decreasing interparticletunneling barriers. In order to realize these possibilities, wemust rst understand what properties allow for a strong QDmolecule interaction.2

Ligands that couple strongly to a metal center, so as to createnew electronic states with metal ligand character, are well-known in the coordination chemistry and electrochemistry communities as non-innocent ligands .3 5 For many metalcomplexes, redox states are primarily localized on either th

Received: November 20, 2012 Accepted: January 31, 2013Published: January 31, 2013

Inclusion of the organic adlayer inthe total electronic structure of

the colloidal system, as is appro-priate if the ligands have strongelectronic coupling to the statesof the QD core, introduces a richchemical diversity to these mate-

rials.

Perspective

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metal center or the ligand. In the case of non-innocent ligands,however, the redox states of the ligand and metal areambiguous, as the redox center is delocalized over the metalcenter and one or more of the ligands.6 No ligand isintrinsically non-innocent but becomes so in the presence of a metal center with orbitals of the right symmetry and energy tocouple electronically with ligand orbitals. The same criteriaapply to ligands for QDs. In the case of QD ligand complexes,the mixed-character state that forms delocalizes not only overthe metal to which the ligand binds, but also over the entirecrystal lattice of the QD, such that binding of the ligand createsa new electronic structure without distinct QD or organicphases.

The electronic structure of QD organic complexes lies atthe complicated intersection of solid-state physics, described with innite periodic potentials, and chemistry, described withlocal bonding. We have found that both sets of models areneeded to describe the nanoscopic organic/inorganic interfaces within colloidal QDs. The direct interactions of molecules withthe QD surface are local interactions with the QD surface ionsand are best modeled with molecular descriptions of theinterfacial region. The core of the semiconductor is largeenough to be well-approximated by a bulk density of states,7 but a density of states that is conned to a nanoscale region by the interface with a low-dielectric organic species. Hoff man8,9gives a basic and intuitive set of rules for connecting chemicaland solid-state descriptions at heterogeneous interfaces.

Our research has found that, thus far, the molecule thatexhibits the strongest electronic coupling to the semiconductorcores of CdSe, CdS, and PbS QDsthe most non-innocentligand w ith respect to QDs is phenyldithiocarbamate, PTC,Chart 1.10 Upon exchange for the native (as-synthesized)

surfactants of QDs such as phosphonates and carboxylates,PTC, which adsorbs as an anion, probably in bothmonodentate 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 greaterthan any other known chemical treatment for CdSe.11 Severalresearch groups have employed chelating, sulfur-containinggroups to achieve strong binding of ligands to QDsurfaces.12 16 These studies were not focused on the opticalresponse of the QDs to adsorption of these ligands andtherefore 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 nativeligands for dithiocarbamates. Additionally, dithiocarbamates areknown to donate electron density to single-crystal electrodes

composed of CdSe, which alters the electrochemical potentialof these materials.17

In order to understand the features of PTC that enablestrong coupling to the electronic states of the QD core, weutilize the simple particle-in-a-spherical-box (PIB) model oQD electronic structure. The usefulness of the PIB descriptionof excitonic carriers in QDs is not limited to its prediction of the size-dependent excited-state energies of QDs18 because thecon nement energy of the particle depends not only on the sizeof the box but also on the magnitude of its surface potentialThe walls of the box within the PIB model, as given by theinterfacial states of the QD organic complex, are notperturbative terms added to the energy of the particle aftersolving the Schro dinger equation; they are boundary conditionsfor the solution; therefore, the connement energy that they produce is intrinsic to the wave function of a charge carrier.

As predicted by this simple model, conned carriers tunnelinto the conning barrier region with a probability given by theheight of these walls. For many types of QD organiccomplexes, ignoring the contribution of this tunneling processto the total energy of the carrier is rational because thetunneling barriers presented by the surfactant are large, and theminimal amount of wave function density that does delocalizeonto the ligands does not aff ect the properties of the core. When exchanging one insulating ligand for another (forexample, alkylphosphonates to alkylcarboxylates), the changin tunneling probability is small even though the change in barrier height may be on the order of electron volts because thesensitivity of the kinetic energy of excited carriers to thecon nement potential at the inorganic/organic interface is low when the absolute magnitude of the conning potential is large.

When, however, one exchanges an insulating ligand for aligand, like PTC, with orbitals of the correct energy andsymmetry to create interfacial states near the band edge, ligandexchange causes dramatic shifts in band edge absorption andemission frequency. These shifts are due to large changes in thepercentage of total wave function that tunnels into thesurfactant. In order to observe shifts in the band gap of theQD due to the ligand, the coupling of the ligands to the coremust then be strong enough such that the mixed states of theQD ligand complex have energies near that of the band edgeof the core the energy that denes the bottom of the box.

The ability of a ligand to delocalize an excitonic carrier iequivalent to its ability to change the tunneling barrier for thecarrier by creating new, less conning states at the QD organicinterface. These new states are created upon mixing of theligand orbitals with those orbitals of the QD that aredelocalized over both the core (where the carrier originates)and the surface (to which the ligand couples directly). Whentwo 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, . Each pairwisecoupling between a ligand and a QD orbital results in two newstates, split from the original states by an energy approximatelyequal to V 2/ .

The eff ectiveness of PTC as a delocalizing ligand lies in bothspatial overlap and energetic resonance with orbitals of the QD We suspect that the orbital overlap, V , between PTC and theQD is larger than that of common surfactants because thechelating geometry presents sulfur atoms that spatiallyapproximate the anionic positions in the crystal lattice; Cd S bond lengths in chelating dithiocarbamate cadmium complexesrange from 2.5 to 3.0 ,3 while the Cd Se bond length is 2.6

Chart 1. Structure of the X PTC Acid, Where X Is an Arbitrary Substituent

a

a We have explored X = N(Me2), OMe, Me, H, Br, F, OCF3 , and CF3. The molecule is introduced to the QD dispersionas a salt (usually ammonium or triethylammonium) and has multiplepossible binding modes, both monodentate and chelating, to metalcations.

The Journal of Physical Chemistry Letters Perspective

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.19 Furthermore, dithiocarbamates are -donors (the PTCHOMOs have -type symmetry); therefore, mixing of theseorbitals with those of the QD systems that we have explored(CdSe, CdS, and PbS, which have valence bands ( VBs)composed of primaril y Se 4p or S 3p orbitals20 22 ), issymmetry-allowed.10 ,11 ,23 The combination of spatial andsymmetric alignment allows for strong orbital overlap betweenthese VB states and the PTC HOMO and, consequently, alarge value for V .

In order to explore the mechanism by which energeticresonance dictates the carrier delocalizing ability of PTC, wemodi ed PTC at the para position with electron-donating or-withdrawing groups, X (Chart 1).23 This modication changesthe energies of the highest occupied MOs of PTC. Weobserved that as X varies from electron-donating (X = N(Me)2)to electron-withdrawing (X = CF3), exciton delocalization intothe surfactant (quantied b y a parameter that we call thedelocalization radius ( R ),10 de ned precisely in Figure 1 A)

increases by more than a factor of 2. The shapes of theHOMOs of X PTC are invariant with X;23 therefore, weattribute the dependence of R on X to changes in the set of energy gaps, , between the orbitals of PTC and those of theCdSe 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, tow ardthe middle of the band, which has a high density of states.7 ,24The larger the number of VB states that mix with the X PTCHOMO, the larger the total interaction energy (orbital

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

As we discussed above, this reduction of tunneling barrieronly aff ects the degree of hole delocalization when the barrier issmall in the rst place, that is, when the absolute energy of theinterfacial state is close to the VB edge. Ligand exchange froman alkylcarboxylate to an alkylphosphonate, for instance, mayreduce the tunneling barrier, but the HOMOs of both ligandsare several electron volts below the VB edge; therefore, thinterfacial states that they create are irrelevant in determiningthe optical band gap of the QD. We can conclude that bothproximity of the ligand HOMO to the VB edge of the QD andsubstantial orbital overlap from proper symmetry matching arethe minimal requirements for delocalization of the hole intointerfacial states by the ligand. If those requirements aresatis ed, then tuning the resonance of the ligand

s orbitals withthe highest density of states region of the VB further optimizethe interfacial interactions and maximizes the bathochromicshift of the absorption spectrum (as we see when we vary thesubstituent X).

The chemical mechanism by which the excitonic hole of theQD delocalizes upon exchange of an insulating ligand with X PTC is donation of electron density into the Se 4p orbitals ofCdSe (or S 3p orbitals of CdS). This added interfacial electrondensity lowers the energy barrier for tunneling of the hole , acarrier of positive charge, into the interfacial region.25 A counterintuitive result of our study of the eff ect of thesubstituent, X, on the delocalization radius is that the -

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

Figure 2. (Left) Schematic depiction of the interaction between X PTC and the QD VB. The density of states in the VB is variable, andas the text discusses, the degree of mixing between the X PTCHOMOs depends on the density of states with which X PTC aligns.The greatest splitting occurs at energies where the number of non-negligible (energetically resonant) states is greatest. (Right) Sketch ofthe potential prole for the excitonic hole within the PIB model. The box is inverted from a normal particle in a box because a hole islower in energy toward the top on a traditional energy scale. The blackdashed line shows the potential prole given from the states accessibleto the hole. The VB edge denes the bottom of the box, and the mixedstates represent the interfacial region. In this diagram, the surfacestates present wells to the hole because the mixed state is above the VBedge. Additionally, the hole wave function (red, sketched) is centeredon the QD because of a parabolic potential provided by the excitedelectron (blue). The total potential prole is described by the heavy black line. (Adapted from ref 23.)


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donating ability of X PTC with respect to the Se 4p orbitalsincreases as X becomes more electron-withdrawing, Figure 1B.This result is reasonable when one considers that the degree of -donation scales with the mixing coefficient between PTC andSe 4p orbitals, which, in turn, scales with the energeticresonance between the PTC HOMO and the high density of states region of the VB. Apparently, this resonance eff ectoverwhelms any possible inuence of the substituent on theamount of electron density on the dithiocarbamate functionalgroup available to donate.

The delocalization of an exciton into the ligand shell hasseveral practical applications, discussed below, but due to theselectivity of the PTC for the VB of the QD, we are also able toprobe fundamental properties of the excitonic hole using theQD s response to adsorption of PTC. We treated CdSe QDs of a series of physical radii, R , with PTC and monitored thedelocalization radius R , aff orded by PTC as a function of R .11For the smallest QDs, where the kinetic connement energy ismuch larger than the electron hole Coulomb energy, theexcitonic hole is strongly conned within the inorganic core of the QD,26,27 and adsorption of PTC to the surface of the QDdelocalizes the hole to the full extent allowed by the QD/PTCinteraction. Within this strong connement regime, the alloweddelocalization radius, R , is not dependent upon the physicalsize of the QD, R . At the value of R where the hole is no longerstrongly conned (where the connement energy is not able toovercome the Coulombic binding energy and the hole isinstead localized by the electron), R begins to decrease withR . This turning point marks the physical radius at which thehole transitions from strong to intermediate connement, andthe value of R at this turning point is therefore indicative of the size (i.e., coherence length) of the hole. In CdSe, this turningpoint is observable within the set of synthetically accessibleradii and occurs at R 1.9 nm. We note that our measurementof the Bohr radius of the hole is distinct from a measurement of the radius at which the exciton (electron hole pair) is nolonger quantum-conned (that is, when the optical absorptionspectrum looks like that of the bulk semiconductor) becausethat radius corresponds to the point at which the mostdelocalized, lighter carrier (the electron in CdS and CdSe) isno longer conned and provides no information about thecarrier with higher eff ective mass, the hole.

The radius at which we observe the hole to transition fromstrong to intermediate connement in CdSe (1.9 nm) iscoincident with the radius at which the photoluminescencequantum yield of CdSe QDs is maximum, the so-called brightpoint of CdSe uorescence.28 The authors who report this bright point suggest that surface reconstruction occurs as QDsincrease in size and that QDs with a radius of 1.9 nm haveoptimal surface structure with the fewest traps. In light of ourmeasurement that the hole becomes strongly conned at 1.9nm, we suspect that the increase in photoluminescencequantum yield occurs because at this radius, electron holeoverlap (and therefore radiative rate) is maximized. A radius of 1.9 nm is also coincident with an observed maximum inexciton exciton binding energies in biexciton systems inCdSe.29 We again suspect that a maximum in electron holeoverlap is responsible for this result because the increase inelectron hole overlap is correlated with an increase in bindingenergy.

A much more common set of sulfur-containing ligands forQDs than dithiocarbamates is thiols. The treatment of QDsurfaces with thiols, for the purposes of water-solubilization,

cross-linking, and bioconjugation of QDs, is associated withsmall (


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interdigitated electrodes) have measured shifts as large as 60meV.39

Yaacobi-Gross et al.40 implicate QCSE in shifts of the optical band gap of CdSe QDs of up to 14 meV when the QDs nativecapping ligands are exchanged for 4-methylthiophenol. For amixed monolayer of ligands to induce an electric eld acrossthe core of the QD, their dipoles must be inhomogenously distributed on the surface. The authors propose that thisinhomogeneity is reasonable due to diff erent binding energiesof the ligands on diff erent crystal facets, a phenomenon that isresponsible for anisotropic growth of nanocrystals into rods.41 We speculate, however, that the QCSE is not responsible forthe observed shift in this case; rather adsorption of 4-methylthiophenol produces a bathochromic shift in the bandgap of the QDs due to exciton delocalization by thiophenolthrough interfacial orbital mixing.

Dipole dipole coupling among neighboring QDs can alsoaff ect the energy of an excitonic state (Figure 3C). Somereports suggest that physically tethering QDs in solution resultsin QD QD interactions that stabilize the exciton.31 Thesestudies, however, used dithiolate molecules as tethers and didnot conclusively eliminate hole delocalization by the thiolates as

a mechanism for the observed exciton stabilization. A latepublication by the same authors, which used only singlyfunctionalized alkane thiols, reported the same eff ect. This work led the authors to revise their proposal to include somedelocalization of the excitonic state into the surfactant.32 Wesuspect that thiolate-induced exciton delocalization alsocontributes to the bathochromic shift of the band gap of PbSe QDs reported by Wolcott et al.30 upon replacing thenative carboxylate ligands with short-chain alkanethiols. Theauthors in that case proposed that coupling of transitionmoments of the QDs is responsible for the shift, but wespeculate that while the transition dipole transition dipolecoupling that they observe is likely real, the unaccounted fomagnitude of shift that they observe is due to adsorption ofthiolates.

Solvatochromism is another form of electric-eld-mediatedshift in the optical band gap of a QD (Figure 3D). Leatherdaleand Bawendi published a theoretical description of solvato-chromism42 that predicts shifts in absorption energy of the rstexcitonic peak upon changing solvents. Experimentally, theyonly realized shifts of 2.5 meV upon changing solvents fromhexanes to 3-iodotoluene and hypothesized that the solubilizingsurfactants provide insulation from the dielectric environmenof the solvent. Similarly, deposition of QDs into lms can resultin reversible bathochromic shifts.43 The change in the dielectricenvironment of the QD upon lm deposition stabilizes theexciton by a mechanism analogous to the solvatochromic eff ect.

In this Perspective, we described an orbital mixingmechanism designed to explain our observations of dramatic bathochromic shifts of the absorption spectra of metalchalcogenide QDs upon adsorption of -donating aromaticdithiocarbamate ligands. The shifts in the optical band gap ofthe QD, enabled by large orbital overlap and energeticresonance among the valence orbitals of the ligand and thoseof the QD, are more than a factor of 10 larger than thoseinduced by electric-eld-mediated mechanisms such as thequantum-conned Stark eff ect, solvatochromism, and inter-particle coupling of transition dipoles. The orbital mixingmechanism operative in dithiocarbamates and, to a lesserextent, in thiols is also diff erentiated from other mechanisms by its distinct dependence on the degree of connement of theaff ected charge carrier. In fact, the response of the band gap of aQD to adsorption of phenyldithiocarbamate is, in the case ofCdSe, a quantitative measure of the Bohr radius of the excitonihole, an experimentally elusive quantity.

Our detailed analysis of the electronic interactions betweenQDs and phenyldithiocarbamate has yielded a set of criteria foQD ligand systems to exhibit strong coupling. The next step isto use our mechanistic knowledge to conduct targeted searchesthrough experimental and computational screens for othermolecules that have the potential to engineer the distribution ofelectronic wave functions in colloidal nanocrystals. In additionto possibly realizing an even greater stabilization of the QDexciton by varying the chemical structure of chelating -donors with orbitals tuned to the VBs of other semiconductors, we wisearch for electron delocalizers, ligands that electronicallycouple to the conduction bands of common colloidalsemiconductors. The conduction bands of cadmium chalcoge-nides are composed of s-type orbitals and therefore require asigma-accepting ligand to lower the barrier for electrondelocalization, while the conduction bands of the leadchalcogenides have both s and p character.20 22 As mostQDs are synthesized with anionic surfactant, their surfaces

Figure 3. (A) The largest observed change in the absorption spectrumof QDs upon postsynthetic modication is due to strong-couplingsurfactants like PTC. A strongly coupled ligand increases thedelocalization volume of a carrier by decreasing the potential barrierpresented at the surface of the QD by the surfactant. The maximumobserved change in energy for this process is 970 meV. (B) Thequantum-conned Stark eff ect occurs when surfactants withpermanent dipole moments (arrows) selectively bind to QD crystalfaces. This asymmetric binding induces an electric eld that stabilizesthe exciton. The QCSE has been implicated in stabilization energies of up to 60 meV for an externally applied eld, and up to 14 meV for theligand-induced case. (C) When QDs are deposited in lms oraggregate in solution, their transition dipole moments interact toprovide net stabilization for the excitonic state. This interactionreaches a limit when QD lms are physically sintered, which removesquantum connement. (D) Stabilization of excited states by up to 5meV by solvatochromism has been observed. This observedstabilization energy is smaller than that theoretically predicted,probably because the insulating surfactant shell on the QD screensthe core from solvent dipoles.


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Optical Properties of Strongly Coupled Quantum Dot-Ligand

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