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Energy Transfer in Quantum Dot Solids. Natalia Kholmicheva, Pavel Moroz, Holly Eckard, Gregory Jensen, and Mikhail Zamkov ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00569 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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Energy Transfer in Quantum Dot Solids. Natalia Kholmicheva,1,2 Pavel Moroz,1,2 Holly Eckard, 3 Gregory Jensen,3 and Mikhail Zamkov1,2,*. 1
The Center for Photochemical Sciences, 2Department of Physics, and 3Department of Chemistry, Bowling Green State University, Bowling Green, Ohio, 43402, USA.
*
[email protected] ABSTRACT. Understanding the energy flow in quantum dot solids represents an important step towards 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 a particular emphasis of 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 tripletsensitizer nanocrystals will also be discussed.
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Exciton 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 energy transfer dynamics in excitonic solids with a 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.
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Assemblies of semiconductor nanocrystals (NC) 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 adjusting 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 preventing exciton migration towards luminescence-quenching film boundaries.23 Conversely, the diffusion of excitons to charge-separating interfaces in solar cell applications is beneficial and can be enhanced via short-chain surface ligands.3 Such inherent tunablity 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 the energy diffusion in nanocrystal solids. Monitoring exciton diffusion processes in molecular or nanocrystalline assemblies is complicated by the fact that no net charge is being 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 upon a concept of funneling the exciton energy to low-energy “acceptors” strategically positioned across the sample and employ steady-state, timeresolved, or temperature-dependent photoluminescence as a signal of exciton arrival to acceptor sites. These strategies have been recently enhanced with new imaging techniques utilizing timeresolved optical microscopy,26 transient absorption,27 and transient photoluminescence quenching.28
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Figure 1. (a,b). Illustration of the photoluminescence lifetime-based approach for measuring the exciton transport parameters in nanocrystal films. By extracting the fast component of the PL
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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, as expressed by Eq. 1. A linear fit to this equation can then be used to extract the rate of interparticle energy transfer, ΓET(D→A). (d). Illustration of the known mechanisms of energy transfer in nanocrystals solids, including Förster, Dexter, and tandem ET processes. The portion of the image is reproduced with permission from Ref. 29 . Copyright ACS.
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 energy transfer is the dominant mechanism of exciton diffusion in weakly coupled NC arrays (see Fig. 1b), as it 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 (Fig. 1d). Typically, a bound electron-hole pair will “hop" through the thermally accessible energy landscape towards the potential energy minimum. The subsequent decay of excitons can undergo through radiative or non-radiative recombination or the dissociation of excitons into a free electro-hole pair. Along these lines, the character of exciton diffusion in nanocrystals solids can be described by the two characteristic parameters: the exciton diffusion length, l diff , and its diffusivity, µ (cm2s-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 the room temperature kT, the character of exciton diffusion through a solid of uniform nanocrystals can be viewed as random. In this case,
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we can use the three-dimensional random walk approximation to express µ and as a function of interparticle “hopping” distance lhop: (1)
2 elhop 1 µ= 6kT τ
:
(2) ldiff = lhop nhop / 6
where τ is the exciton lifetime (Fig. 1a), which is related to the interparticle energy transfer rate, ΓET = n hop τ .
Among a plethora of spectroscopic strategies for probing the intermolecular energy transfer 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 energy transfer in molecular solids,30 and subsequently adapted by Kagan31 in 1996 and Klimov group32 in 2002 for measuring the exciton diffusion rates in CdSe nanocrystal films. The concept of the bulk quenching approach is based on blending the investigated solid of nanoparticles D with randomly distributed “acceptor” nanoparticles A (Fig. 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 non-radiative 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 photoluminescence (PL) emission as well as the PL lifetime to diminish. If the concentration of quenching nanoparticles is small, nA
