Effect of Ligand Structure on the Optical and Electronic Properties of Nanocrystalline PbSe Films

Abstract

Films of nanocrystalline PbSe were fabricated with a set of structurally varied short-chain dicarboxylic acids. Oxidation rates were studied via NIR spectroscopy to determine the effect of the structure of the diacid ligands on film stability under ambient conditions. Ligands favoring a non-bridging bonding mode were found to provide the best protection against oxidation, while among ligands expected to bridge between adjacent nanocrystals in the films, those with shorter chain lengths conferred better oxidative stability. Electronic coupling was observed as a red shift in the optical data of the ground excitonic peak of the PbSe films and found to be strongly influenced by the structure of the ligand. Transport measurements were made in air using thin-film transistors that were treated with a thin Al 2 O 3 coating via remote plasma ALD. Films prepared using fumaric, maleic, and oxalic acids yielded mobility numbers of 2.5 × 10 −5 , 3.7 × 10 −5 , and 1.6 × 10 −3 cm 2 / V•s, respectively. Results suggest that the internanocrystal distance is the major contributor to electron mobility through the nanocrystalline films, while the electronic coupling is heavily influenced by multiple factors related to the structure of the surface ligands in addition to the internanocrystal distance.

Nonmonotonic Size Dependence in the Hole Mobility of Methoxide-Stabilized PbSe Quantum Dot Solids

“…Notable changes in μ h appear after several hours, but at this point the S e r S h transition energy of MeO À -and oleate-capped QDS alike have blue-shifted significantly, which is indicative of the formation of an oxide shell. 14 Hence, our data support the view that electronic coupling is only weakly reflected in the red shift but almost entirely due to transition dipole coupling or polarization effects.…”

“…Upon MeO À treatment, all QD samples exhibit a pronounced red shift consistent with results for hydrazine-, ethanedithiol-, or dicarboxylic acid-capped PbSe QDS. 1,4,13,14 As demonstrated in Figure 3a the red shift upon ligand exchange increases with decreasing particle size. For the smallest particles (3 nm), the exciton was not measurable in thin films, possibly due to either extremely large exciton shifts or the dominance of FabryÀPérot cavity modes.…”

“…After an initial red shift due to the ligand exchange, upon air exposure the transition energy gradually blue shifts at a much slower rate, consistent with previous reports. 4,14 …”

Advancing colloidal quantum dot photovoltaic technology

“…Silva et al later showed that the partial hydrolysis of GSH caused some of the sulfur to react with the CdTe to create a CdS shell that provided better surface passivation [126]. Another study found that when comparing bifunctional carboxylic acid molecules, the more acidic and shorter chains performed the best optically, leading to an excitonic absorption peak red shift due to greater electronic coupling between neighboring dots [127]. This electronic coupling, also seen in the study of 1,3-benzenedithiol (1,3-BDT), ethanedithiol (EDT), mercaptopropionic acid (MPA) and ammonium sulfide, was attributed to the extension of the electron wave function outside the individual CQDs [128].…”

Transient absorption studies of CdSe nanocluster passivated with phenyldithiocarbamate ligands.

“…Numerous studies showed that as the length of the ligands decreases, the observed inter-particle spacing is reduced, the electron transportation is facilitated, and the carrier mobility is increased. [87][88][89][90] Adjusting surface ligands will significantly change the optical and electronic properties of QDs [91]. The choice of ligands will impact the charge transfer processes [92], and also determine whether a single QD species acts as a p-type or a n-type semiconductor in the solar devices [93][94][95].…”