Role of Ligand Exchange at CdSe Quantum Dot Layers for Charge Separation

Abstract

Layers of CdSe quantum dots (QDs) with fixed diameter (4.5 nm) and various ligands were deposited on ITO substrates and investigated by photoluminescence (PL) and surface photovoltage (SPV) techniques. The distance between QDs decreased after washing and ligand exchange. The PL signals in the first excitonic peak as well as in the defect range strongly decreased with successive ligand exchange. In contrast to multilayer deposition, no PL signal related to CdSe QDs has been observed for a monolayer of QDs with pyridine surfactants on ITO. Neither modulated nor transient SPV signals were found for pristine CdSe QD layers. The modulated SPV signals strongly increased with successive ligand exchange. SPV transients became longer with successive ligand exchange. The experiments gave evidence for strong generation of surface defect states with ongoing ligand exchange and for fast initial charge separation of electrons from the first monolayer of CdSe QDs to the ITO surface. SPV transients could be described within a single QD approximation model, in which the number of surface defect states per QD, the energy distributions of defect states, hopping and distant dependent recombination were considered.

Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

“…However the details of the trapping dynamics in many materials are still subject of debate and intense research. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, in some cases it is even unclear whether it is the trapping of the electron and/or that of the hole that affects the fluorescence efficiency, as several non-radiative decay components have been observed with different magnitudes (sometimes differing by several orders of magnitude for the same material), prompting the suggestion that different types of traps must be present. This is the case of CdTe CQDs, where recent experimental studies have evidenced fluorescence decay curves that required at least a tri-exponential function to yield good agreement with the observed kinetics.…”

Fabrication and optical studies of epoxy-quantum-dot nanocomposites

“…One representative method is ligand substitution, in which the capping ligands bound to the surface of the nanocrystals are replaced with new ligands having an affinity with dispersing solvents or polymer matrices [12][13][14][15][16][17][18]. However, substitution methods have many drawbacks and limitations such as the surface defects of the nanocrystals, leading to poor optical efficiency, and the dependency upon the kind of capping ligands strongly bound to the surfaces of the nanocrystals because the replacing functional groups must have a higher binding force than the capping ligands.…”

Transient absorption studies of CdSe nanocluster passivated with phenyldithiocarbamate ligands.

“…The rate of the transfer increases as the size of the QDs decrease. [47] Over the past decade, organic ligands that either withdraw electrons from or donate electrons (i.e., withdraw holes) to the QD cores, have drawn great attentions to researchers [31,[48][49][50][51][52]. Ligands with aromatic rings are especially interesting due to their piconjugation structure which offers the strong ability to extract photon-generated charge carriers from the QDs surface [49,[53][54][55].…”

“…This electron withdrawing or donating effect is attributed to the conjugation structures, the aromaticity, and the functional groups of the ligand molecules [269,270]. Ligands with aromatic rings (such as thiophenol [53], aniline [54], phenylenediamine [49,55]) are especially interesting due to their pi-conjugation structures which offer the strong ability to extract photongenerated charge carriers from the QDs core to the passivated ligands.…”

“…It has been reported that substitution of covalently bounded ligands, such as thiophenol [53], aniline [54,272], phenylenediamine [49,55], and phenyldithiocarbamate (PDTC) [226,243,273,274], for native ligands, such as trioctylphosphine oxide (TOPO), trioctyl-phosphine (TOP), tetradecylphosphonic acid (TDPA), or hexadecylamine (HDA), via ligand exchange reactions brought about favorable changes to the optical properties and stabilities of QDs. Especially, studies by Weiss group revealed a decrease of the optical band gap of CdSe QDs when passivated with PDTC ligands [57], which indicates relaxation of the quantum confinement.…”