Letter pubs.acs.org/JPCL
Achieving Exciton Delocalization in Quantum Dot Aggregates Using Organic Linker Molecules Eyal Cohen,† Itay Gdor,‡ Elisabet Romero,§ Shira Yochelis,† Rienk van Grondelle,§ and Yossi Paltiel*,† †
Department of Applied Physics, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel § Department of Physics and Astronomy, Faculty of Sciences, VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands ‡
S Supporting Information *
ABSTRACT: The design of new complex structures containing semiconductor quantum dots offers a means to create a variety of new meso-solids and molecules. The control of the coupling properties between the dots, accompanied by the energetic tunability of the dots themselves, paves the way toward the application and use of novel quantum properties. Here we present our approach to alteration of interdot coupling using organic linking molecules in a system of covalently bonded, aggregated quantum dots. We used ultrafast transient absorption measurements to identify marks of exciton delocalization over nearest neighbors to some extent. In linking molecules incorporating a benzene ring, the delocalized electron cloud displayed a profound influence over the interdot effects, leading the way to easy coupling control in quantum-based devices, under ambient conditions.
N
anometer scale semiconductor colloidal quantum dots (QDs) are known for their tunable electronic and optical properties.1,2 They provide the means to realize quantum entities with controlled properties for a variety of applications at room temperature. Using QDs, often referred to as “artificial atoms”, as building blocks for the construction of “artificial solids” is expected to yield novel properties.3−5 Indeed, scientific efforts have already attempted to manipulate and control QD coupling. For example, electronic coupling was achieved by the use of molecular linkers,6,7 and dipole−dipole interaction control was performed through encapsulation of the QDs in ordered matrices.8,9 Control of the coupling properties often requires difficult manipulation and organization of the interdot distances and structure. Our aim was to control coupling properties and excitonic delocalization in QD aggregates by using a choice of linker molecules that covalently bind to the dots. Differences in delocalization among neighboring dots were examined while keeping interdot distances approximately constant. We found evidence that some portion of the dots effectively created strong coupling, forming nearest neighbor delocalization. Using a wet chemistry, layer-by-layer method,10−12 we assembled an aggregated structure of CdSe dots covalently bound by the linker molecules (Figure 1b). Further detailed information about the sample preparation process can be found elsewhere13 and in the Supporting Information as well. We chose two relatively short (∼7 to 8 Å) molecules to act as covalent linkers between the dots (Figure 1a). The two molecules, 1,4-butanedithiol (BuDT) and 1,4-benzenedimethanethiol (BDMT), share the same linking head groups (thiols) © XXXX American Chemical Society
Figure 1. (a) Chemical structure of the linking molecules used for adsorption. (b) Schematic illustration of the CdSe quantum dot aggregated structure.
while differing in their carbon backbones (alkyl chain and benzene ring, respectively). Figure 2 shows the steady-state optical absorption spectra of the two samples, alongside the absorption spectrum of the ∼3.3 nm diameter QDs, in toluene solution, in which they retain their original trioctylphosphine oxide (TOPO) ligands. This serves the reference for an isolated, noncoupled QD system. The absorption spectra are normalized to 1 for each sample at the bandgap (1S) peak, around 540 nm. The actual OD of the aggregated BuDT/BDMT-linked samples is on the order of 0.1 to 0.2. The differences between the linked QDs and the isolated are small and are of major significance only at the red tail of Received: December 19, 2016 Accepted: February 14, 2017 Published: February 14, 2017 1014
DOI: 10.1021/acs.jpclett.6b02980 J. Phys. Chem. Lett. 2017, 8, 1014−1018
Letter
The Journal of Physical Chemistry Letters
of the bandgap and above, both the steady-state and the TA spectra reveal relatively minute differences between all samples. When monitoring at bandgap wavelengths on very short time scales (
