Electronic and Vibrational Structure of Complexes of Tetracyanoquinodimethane with Cadmium Chalcogenide Quantum Dots

Abstract

This paper describes the use of visible, near-infrared, and mid-infrared steadystate optical spectroscopy to study the geometries in which tetracyanoquinodimethane (TCNQ) adsorbs to the surfaces of highly cadmium enriched and near-stoichiometric CdSe quantum dots (QDs) in the formation of QD-TCNQ charge transfer (CT) complexes. Several TCNQ molecules are spontaneously reduced by chalcogenides on the surface of each CdSe QD. The degree of CT depends on the geometry with which the TCNQ adsorbs and the degree of distortion of TCNQ's geometry upon adsorption. Comparison of the electronic and vibrational spectra of CdSe QD-TCNQ complexes with those of CT complexes of TCNQ with molecular reductants (including molecular chalcogenides) and computer simulations of the geometries and vibrational spectra of the TCNQ-chalcogenide CT complexes show that (i) the Cd-enriched CdSe QDs reduce a factor of 7.4 more TCNQ molecules per QD than nearly stoichiometric CdSe QDs because surface selenides are more accessible in the Cdenriched QDs than in the near-stoichiometric QDs and (ii) TCNQ interacts with surface selenides through several adsorption modes that result in different amounts of charge transfer and different degrees of geometric distortion of TCNQ. This study provides a framework for determining the range of adsorption geometries of small molecules on QD surfaces, and for optimizing QD surfaces to adsorb molecules in configurations with maximal electronic coupling between the QD and the adsorbate. ■ INTRODUCTIONThis paper describes the use of visible, near-infrared, and midinfrared steady-state optical spectroscopy to determine the set of geometries in which tetracyanoquinodimethane (TCNQ) adsorbs to the surfaces of CdSe quantum dots (QDs) in the formation of QD-TCNQ charge transfer (CT) complexes, and the dependence of these geometries on the surface chemistry of the QDs. Metal-chalcogenide QDs have high absorption cross sections, for instance, factors of ten to 100 greater than that of the metal-to-ligand charge transfer absorption band of ruthenium(II)tris(bipyridine) (Ru(bpy) 3 2+ ), 1 for bandgapresonant excitations across the long-UV, visible, and nearinfrared regions of the solar spectrum. 2 This optical bandgap can be tuned with both the material and size of the QD. 2 They also have the ability to accumulate multiple simultaneous excitons or charge carriers within a single nanoparticle. These properties make QDs potentially important as chromophores and redox centers within photovoltaic and photocatalytic active materials. For both types of devices, efficient extraction of charge carriers from the QDs is critical to the performance of the device. We have shown previously that, in order for ultrafast (tens of picoseconds or faster) charge transfer between a QD and a molecule to occur, the molecule must permeate the ligand shell of the QD and adsorb, at least transiently, to its inorganic surface. 3,4 It is therefore important to know what types of surface chemistries promote adsorption of potential molecular redox p...

Ultrafast exciton decay in PbS quantum dots through simultaneous electron and hole recombination with a surface-localized ion pair

“…The spectrum of TCNQ x-in this region, where 1≤x<2, is known to contain a pair of peaks separated by ~25 cm -1 with energies that decrease as x increases if the charge is spread symmetrically over the four nitrile groups. [15] In our samples, when the molar ratio TCNQ:QD is < 3.7:1, the dominant pair of peaks in the spectra in Figure 1B are those at 2179 cm -1 ("2") and 2154 cm -1 ("4"). Based on the FTIR spectrum of mixtures of decamethylferrocene (DFe) and TCNQ, where DFe spontaneously reduces TCNQ to TCNQ 1-, we determined that the peaks ("2") and ("4") correspond to TCNQ 1-(see Supporting Information, Figure S3).…”

“…Spectroscopic signatures of this covalent-type interaction are not clearly discernable in the NIR or visible region of the ground state or TA spectra, probably because they are obscured by the QD spectra, but we have observed them previously for complexes of TCNQ and CdSe QDs [15]. …”

“…We discuss these geometries in detail elsewhere. [15] At molar ratios TCNQ:QD > 30.9:1, we observe contributions from neutral TCNQ (peak 1) in the FTIR spectra of the mixtures, due to saturation of available adsorption sites on the QD surfaces and the presence of a steady-state population of freely diffusing TCNQ.…”

“…The growth of peaks "3" and "5" is correlated with the growth of a peak at 2100 cm -1 ("6"). [15] These peaks correspond to TCNQ x-, where 1<x<2. The different charge states of TCNQ correspond to different adsorption geometries.…”

TCNQ Interlayers for Colloidal Quantum Dot Light‐Emitting Diodes

“…TCNQ molecules are known to be electron withdrawing, which could affect the emission properties of QD layers featuring such molecules . To exploit the effect of TCNQ on QD‐LEDs, we conducted steady‐state and time‐resolved photoluminescence (PL) studies.…”