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Energy Transfer in Quantum Dot Solids Natalia Kholmicheva,†,‡ Pavel Moroz,†,‡ Holly Eckard,§ Gregory Jensen,§ and Mikhail Zamkov*,†,‡ †
The Center for Photochemical Sciences, ‡Department of Physics, and §Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43402, United States ABSTRACT: Understanding the energy flow in quantum dot solids represents an important step toward designing artificial systems with configurable optoelectronic properties. The growing complexity of nanoparticle assemblies and deposition techniques calls for advanced methods of characterization and control of the underlying exciton diffusion, which is pervasive in these materials. Along these lines, the Perspective will review recent strategies for measuring the energy transfer processes in assemblies of semiconductor nanocrystals with particular emphasis on emerging avant-garde characterization techniques. We will also shed light on novel experimental methods for controlling the energy diffusion in quantum dots solids, highlighting the role of assembly architecture in ensuing processes of exciton diffusion and dissociation. Novel energy transfer mechanisms recently observed in perovskite quantum dots and triplet-sensitizer nanocrystals will also be discussed.
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Assemblies of semiconductor nanocrystals (NCs) represent a unique class of excitonic solids, where interparticle distances can be tuned through the use of different binding motifs. The variable degree of interparticle coupling plays an important role in device engineering as it allows adjustment of both the length and the rate of exciton diffusion. For instance, in light-emitting applications, the motion of excitons can be restricted to a small volume by using long-chain interparticle linkers, which helps prevent exciton migration toward luminescence-quenching film boundaries.23 Conversely, the diffusion of excitons to chargeseparating interfaces in solar cell applications is beneficial and can be enhanced via short-chain surface ligands.3 Such inherent tunability of interparticle coupling in NC solids has enabled a wide diversity of emerging technological applications in areas of solid-state light-emitting4−12 and photovoltaics.11,13−24 Experimental Strategies for Probing Energy Dif f usion in Nanocrystal Solids. Monitoring exciton diffusion processes in molecular or nanocrystalline assemblies is complicated by the fact that no net charge is transferred between photoexcited species. In this regard, optical techniques provide an almost exclusive probe of the energy flow across excitonic materials25 by relying on either temporal or spectral signaling of the emitted, absorbed, or scattered light. Most methodologies build on a concept of funneling the exciton energy to low-energy “acceptors” strategically positioned across the sample and employing steady-state, time-resolved, or temperature-dependent photoluminescence (PL) as a signal of exciton arrival to
xciton diffusion represents a predominant method of energy transfer (ET) in many polymolecular materials. Cascade-like exciton migration is the first step of the energy conversion during photosynthesis1 and is the primary mechanism of the energy flow in nanostructured (excitonic) solids or organic crystals. Similar energetic processes are observed in living tissues and proteins, where photon energy is transmitted tens of angstroms away from single-site excitations.2 Such exciton-mediated flow of energy carries no charge and is a characteristic feature of quantum confined systems, unlike bulk semiconductors where energy is conveyed primarily via electron or hole diffusion. Our need to understand and control exciton transfer processes in nanoparticle, polymer, or macromolecular assemblies has been gradually evolving over the years with the advances in nanomaterial design. It is becoming apparent that many properties of quantum dot assemblies are directly affected by the length and the rate (diffusivity) of exciton diffusion. Along these lines, the prospective will focus on (1) reviewing some of the recent spectroscopic strategies for unveiling the ET dynamics in excitonic solids, with particular focus on inorganic quantum dot (QD) solids, and (2) highlighting several emerging strategies for enabling experimental control over the energy flow in nanoparticle films.
Unlike bulk semiconductors where energy is carried by charged particles, exciton diffusion has a significant contribution to the energy transfer in quantum dot solids. © 2016 American Chemical Society
Received: November 1, 2016 Accepted: December 12, 2016 Published: December 12, 2016 154
DOI: 10.1021/acsenergylett.6b00569 ACS Energy Lett. 2017, 2, 154−160
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diffusion in NC solids can be described by two characteristic parameters: the exciton diffusion length, ldiff, and its diffusivity, μ (cm2 s−1). The former gives the average increase in the spatial dispersion of the excitation energy away from the single-site excitation, while the latter reflects the diffusion rate. When the energy disorder in the film does not exceed room temperature kT, the character of exciton diffusion through a solid of uniform NCs can be viewed as random. In this case, we can use the three-dimensional random walk approximation to express μ and as a function of the interparticle “hopping” distance lhop
acceptor sites. These strategies have been recently enhanced with new imaging techniques utilizing time-resolved optical microscopy,26 transient absorption (TA),27 and transient PL quenching.28 Upon photoexcitation, excitons diffuse through a nanostructured solid at rates determined by the strength of the interparticle coupling.3 It is generally accepted that dipole− dipole Förster ET is the dominant mechanism of exciton diffusion in weakly coupled NC arrays (see Figure 1b) as it
elhop2 1 μ= 6kT τ
(1)
ldiff = lhop nhop/6
(2)
where τ is the exciton lifetime (Figure 1a), which is related to the interparticle ET rate, ΓET = ⟨nhop⟩/τ.
The variable degree of interparticle coupling in quantum dot solids allows adjustment of both the length and the rate of exciton diffusion in these materials. Among a plethora of spectroscopic strategies for probing intermolecular ET in nanoparticle solids, we would like to highlight three representative techniques. One of the most effective strategies is bulk quenching. It was first introduced for measuring the singlet exciton ET in molecular solids30 and subsequently adapted by Kagan31 in 1996 and the Klimov group32 in 2002 for measuring exciton diffusion rates in CdSe NC films. The concept of the bulk quenching approach is based on blending the investigated solid of nanoparticles D with randomly distributed “acceptor” nanoparticles A (Figure 1c), which trap excitons in the potential energy minima. Excitons funneled into acceptor sites can be forced to recombine radiatively (bulk activation) or quenched through nonradiative channels (bulk quenching). In both cases, the diffusion length of excitons in a blended solid will be restricted to a smaller volume due to the presence of quenching sites. As a result, the lifetime of excitons in a blended solid will be reduced due to a shorter travel, causing the PL emission as well as the PL lifetime to diminish. If the concentration of quenching nanoparticles is small, nA ≪ nD, the ratio of the emission intensity in a pure sample (D) to that of an acceptor-doped film (DA) will be expressed linearly with the quencher concentration, nA: ID/IDA = 1 + keffnA, where ID is the PL intensity of a pure QD solid, IDA is the emission intensity of a doped film, and keff is an effective ET parameter. The application of the bulk quenching strategy to QD solids is typically performed by doping NC solids with nanoparticles of the same semiconductor material featuring a larger diameter31,33,34 or by introducing “energy gradient” bilayer structures.32,35 These methods have allowed extraction of interdot ET rates in films of CdSe/ZnS NCs linked with oleic (109 s−1),32 octadecylphosphonic (2.6 × 108 s−1),33 and benzylphosphonic (0.6 × 108 s−1)33 acids. Mixing of donor and acceptor nanoparticles has also been applied to study charge carrier dynamics in hybrid organic−inorganic PbS films, which allowed one to reveal the character of diffusion processes under charge-neutral (undepleted) conditions.36
Figure 1. (a,b) Illustration of the PL lifetime-based approach for measuring the exciton transport parameters in NC films. By extracting the fast component of the PL intensity decay, one can determine the lifetime of excitons in the solid (τfast), the exciton diffusivity (μ), and the diffusion length (ldiff). (c) The bulk quenching approach relies on doping the QD solid with randomly distributed acceptors (A), which promote exciton quenching. The resulting reduction in the emission of a blended solid, ID/IAD, is proportional to the concentration of the acceptor A (see text). A linear fit to this equation can then be used to extract the rate of interparticle ET, ΓET(D → A). (d) Illustration of the known mechanisms of ET in NC solids, including Förster, Dexter, and tandem ET processes. A portion of the image is reproduced with permission from ref 29, Copyright ACS.
does not require tunneling of charges between particles. NC films featuring short interparticle distances enable electron exchange processes, which opens up other possibilities of ET, either though Dexter or recently predicted tandem ET29 mechanisms (Figure 1d). Typically, a bound electron−hole pair will “hop” through the thermally accessible energy landscape toward the potential energy minimum. The subsequent decay of excitons can proceed through radiative or nonradiative recombination or the dissociation of excitons into a free electron−hole pair. Along these lines, the character of exciton 155
DOI: 10.1021/acsenergylett.6b00569 ACS Energy Lett. 2017, 2, 154−160
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Figure 2. (a) Experimental time evolution of the PL spatial cross section for CdSe/CdS solids. The linear color scale indicates normalized PL intensity. (b) Schematic representation of the three types of QD samples studied, depicting the ligand and shell thicknesses used to space out the dots and indicating the center-to-center separation as determined by electron microscopy. The right panel shows the change in variance, σ2, of the exciton distribution as a function of time for the three samples. The dashed line represents the hypothetical case of normal diffusion in which the variance grows linearly with time. Reproduced with permission from ref 37, Copyright ACS. (c) Cascaded energy transfer (CET) sample consisting of subsequent layers comprising green-, yellow-, orange-, red-, orange-, yellow-, and green-emitting NCs. Below the CET sample, the HOMO and LUMO are sketched, visualizing the cascaded band gaps used to facilitate CET. Reproduced with permission from ref 38, Copyright ACS. (d) Transmission electron microscope (TEM) image of a mixed (Au + PbS) nanoparticle sample representing a donor (PbS)−acceptor (Au) blend. The apparent ratio of Au to PbS nanoparticles on a grid, nTEM, can be used to estimate the gold-to-gold interparticle distance RAu−Au in a solid. RAu−Au = 3 n TEM DPbS + DAu , where DPbS and DAu are the diameters of PbS and Au NPs, respectively. Reproduced with permission from ref 39, Copyright ACS. (e) Schematic illustration of PL lifetime changes in blended films of Au and PbS NCs. The PL intensity decay curves were fitted using a two-exponential function where the fast component was assumed to represent the exciton dissociation (quenching) time. The lifetime measurements of films containing a known fraction of quenching nanoparticles (Au) were then used to calculate transport parameters. Reproduced with permission from ref 39, Copyright ACS.
Figure 3. (a) Illustration of the proposed spectroscopic approach for measuring the quantum efficiency of ET, ED→A, in systems featuring a significant spectral overlap. By employing a donor nanoparticle solution as an excitation filter, one can efficiently suppress the excitations of donor species in the sample, leading to accurate determination of ED→A. (b) Acceptor emission expressed as a linear combination of D and A excitations (ND and NA). (c) Simulated D and A absorption profiles along with the associated acceptor emission. (d) Application of the D molecule filter in front of the excitation light leads to an asymptotic form of (NFL A /NA). The value of ED→A is then determined from the asymptotic and original (no-filter) value of this ratio, as shown in the figure. (e) Illustration of the excitation light shaping using the “donor” filter.
The spatial profile of the exciton diffusion in nanoparticle solids can be characterized by introducing emission-quenching nanoparticles in lieu of fluorescent acceptors (A). This strategy was demonstrated in a recent study39 that employed blended films of small-diameter Au NCs and PbS QDs (Figure 2d,e). By correlating the Au−Au interparticle distance in the film with corresponding changes in the emission lifetime (Figure 2e), it was possible to obtain important transport characteristics,
including the exciton diffusion length, the number of predissociation hops, the rate of interparticle ET, and the exciton diffusivity. In particular, it was found that for 3mercaptopropionic acid (MPA)-linked solids (interparticle distance = 0.9 nm), excitons diffused to an average length of 5.7 nm in approximately 12 hops, which corresponded to a diffusivity of 0.012 cm2 s−1. Meanwhile, 8-mercaptooctanoic acid (MOA)-linked solids (interparticle distance = 1.7 nm) 156
DOI: 10.1021/acsenergylett.6b00569 ACS Energy Lett. 2017, 2, 154−160
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at short donor−acceptor distances (
