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Fast Energy Transfer in CdSe Quantum Dot Layered Structures: Controlling Coupling with Covalent-Bond Organic Linkers Eyal Cohen, Pavel Komm, Noa Rosenthal-Strauss, Joanna Dehnel, Efrat Lifshitz, Shira Yochelis, Raphael D. Levine, Francoise Remacle, Barbara Fresch, Gilad Marcus, and Yossi Paltiel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11799 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018
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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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The Journal of Physical Chemistry
Fast Energy Transfer in CdSe Quantum Dot Layered Structures: Controlling Coupling with CovalentBond Organic Linkers Eyal Cohen,† Pavel Komm,† Noa Rosenthal-Strauss,† Joanna Dehnel,‡ Efrat Lifshitz,‡ Shira Yochelis,† Raphael D. Levine,ǁ, Francoise Remacle,⊥ Barbara Fresch, ∇, Gilad Marcus,† Yossi Paltiel*†,§ †
Department of Applied Physics, §Center for Nano-Science and Nano-Technology, ǁThe Fritz
Haber Center for Molecular Dynamics and Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel ‡
Schulich Faculty of Chemistry, Solid State Institute and Rusell Berrie Nanotechnology Institute, Technion, Haifa 3200003, Israel ⊥
Theoretical Physical Chemistry, UR MolSys, University of Liege, B4000 Liege, Belgium
∇Department
of Chemical Science, University of Padova, Via Marzolo 1, 35131 Padova, Italy
Abstract
Quantum dots (QDs) solids and arrays hold a great potential for novel applications, aiming at exploiting of quantum properties in room temperature devices. Careful tailoring of the QDs
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energy levels and coupling between dots could lead to efficient energy harvesting devices. Here we used a self-assembly method to create a disordered layered structure of QDs, coupled by covalently binding organic molecules. Energy transfer rates from small (donor) to large (acceptor) QDs are measured. Best tailoring of the QDs energy levels and the linking molecules length, result in an energy transfer rate as fast as (30ps)-1. Such rates approach energy transfer rates of the highly efficient photosynthesis complexes, and are compatible with a coherent mechanism of energy transfer. These results may pave the way for new controllable building blocks for future technologies.
Introduction Semiconductor quantum dots (QDs) are known to exhibit optical and electrical characteristics, dependent on size, composition, and shape. When coupled in assemblies, they can be used as building blocks for "artificial molecules" or "artificial solids", which are expected to yield novel properties.1–5 In turn, such new materials can be implemented in a variety of applicative uses, like solar energy harvesting, sensing, light-emission, and information processing.6–15 For many such applications, control of coupling properties and energy transfer (ET) between the QDs is desired in order to enhance efficiency and performance.16 Energy transfer in QD solids was studied in several previous works. These include layered QD solids realized by the Langmuir-Blodgett/spin-coating techniques,17–19 by electrostatic layer-bylayer self-assembly,20–22 or by using dithiol-linkers.16,23,24 ET rates as fast as (71ps)-1 were reported in close-proximity layers of donor and acceptor QDs,20 with a rate of (50ps)-1 derived for a specific subspecies within the ensemble inhomogeneous distribution. In many of the works, it has been pointed out that the ET for case of close-packed QDs deviates from the basic perturbation regime of the dipole-dipole interactions that lead to the known Förster resonance
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The Journal of Physical Chemistry
energy transfer (FRET) mechanism.18,21,22,25–27 Yet, no clear attempt was made to achieve control over coupling properties and to comprehensively study the effect of QD-linkers upon ET. Here we use dithiol-linking molecules, to realize a bi-size QD solids. The sizes of the QDs are such that the first excitonic state of the smaller dot is quasi-resonant with the second exciton of the larger dot. Using ultrafast transient absorption spectroscopy, the fast ET between donor and acceptor QDs is examined. By alternating between different lengths of linking molecule, we aim to control the QDs' coupling strength and ET rates. The measured rates and their dependence on the length of the linker molecules points to a coherent contribution to the energy transfer dynamics beyond the perturbative regime. Experimental Section Sample preparation. Colloidal CdSe QDs were synthesized according to Lifshitz et al.28 QDs of two different sizes were used: smaller QDs with mean diameter of around 2.75nm (here fore, QDs A) or larger ones with mean diameter of 3.45nm(QDs B). A disordered structure of covalently-bond QDs was realized using a wet-chemistry, as described in detail in our prior publications.5,29 Samples were composed of either one size QDs or incorporated both sizes of QDs, to examine energy transfer between small and large QDs (as schematically appears in Figure 1c and 1d, respectively). We used alkanedithiol molecules, of two different lengths, as linkers between dots (Figure 1a). The molecules, presented in Figure 1a, are the shorter 1,3propanedithiol (PrDT) and the longer 1,9-nonanedithiol (NDT), with nominal lengths of 5.5 Å and 13Å, respectively. They form covalent bonds with QD Cd atoms at both ends.30,31 QDs in original toluene solution were measured as well, as a reference of isolated, non-coupled dots. Consecutive dipping of fused silica substrates in QD solution and in linking molecules solution, and repeating the process for ~30 times, yielded a disordered layered structure, as figures 1c and
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1d illustrate. The adsorption procedure was carried out under dry nitrogen environment, and consecutive sealing of the sample was done for their encapsulation, and prevention of oxidization. It should be noted that the adsorption procedure, which leads to a rather disordered structure, was used to obtain a three-dimensional, isotropic structure, with clear signatures of ET. This was done to prevent a preferred energy transfer direction. Yet, it can easily be modified to provide cascaded bi-layer or multi-layer structure of size-gradient QDs, as proposed before.32 (a) HS
(b)
SH
1,3-Propanedithiol (PrDT) SH
HS
1,9-Nonanedithiol (NDT) (c)
(d)
Fused silica
Fused silica
Figure 1. (a) Chemical structure of the alkanedithiol linker molecules used in present work. (b-d) Schematic illustrations of QD samples under investigation: (b) QDs in solution (isolated). (c) Single size QD disordered structures. (d) Disordered structure of QDs of two sizes. Experimental methods. A home-built ultrafast transient absorption (pump-probe) setup was used to investigate electronic dynamics of the samples. A detailed description of the setup and measurement procedures, are found in the supporting information. 400nm pump pulses were used to excite the samples, followed by a white-light probe pulse. At this excitation wavelength, QD of both sizes are excited. Very low power pump pulses were used to achieve a low average number of excitons per dot, N ex
