Unsaturated Ligands Seed an Order to Disorder Transition in Mixed Ligand Shells of CdSe/CdS Quantum Dots

Abstract

A phase transition within the ligand shell of core/shell quantum dots is studied in the prototypical system of colloidal CdSe/CdS quantum dots with a ligand shell composed of bound oleate (OA) and octadecylphosphonate (ODPA). The ligand shell composition is tuned using a ligand exchange procedure and quantified through proton NMR spectroscopy. Temperaturedependent photoluminescence spectroscopy reveals a signature of a phase transition within the organic ligand shell. Surprisingly, the ligand order to disorder phase transition triggers an abrupt increase in the photoluminescence quantum yield (PLQY) and full-width at half maximum (FWHM) with increasing temperature. The temperature and width of the phase transition shows a clear dependence on ligand shell composition, such that QDs with higher ODPA fractions have sharper phase transitions that occur at higher temperatures. In order to gain a molecular understanding of the changes in ligand ordering, fourier-transform infrared and vibrational sum frequency generation spectroscopies are performed. These measurements confirm that an order/disorder transition in the ligand shell tracks with the photoluminescence changes that accompany the order disorder ligand phase transition. The phase transition is simulated through a lattice model that suggests that the ligand shell is well-mixed, and does not 1 have completely segregated domains of OA and ODPA. Furthermore, we show that the unsaturated chains of OA seed disorder within the ligand shell.

Observation of ordered organic capping ligands on semiconducting quantum dots via powder X-ray diffraction

“…More broadly, measurement and manipulation of surface ligand ordering can have significant impact on many properties of colloidal quantum dots. Highly ordered ligands on colloidal quantum dot surfaces can have detrimental effects on optical properties 5,[37][38][39] . Disordered surface ligands also increase the solubility of nanoparticles, and understanding the dynamics between ligand type and ligand length on their ordering could enable more precise tuning of solubility 4 .…”

“…T he degree of ligand ordering on colloidal inorganic nanocrystal surfaces has long been a topic of interest [1][2][3][4][5][6][7][8] . Capping ligands are chemically essential, providing protection against aggregation and contributing significantly to solubility 4,9,10 .…”

“…Kinetic and thermodynamic properties of the nanocrystals are also greatly influenced by ligands [7][8][9][10] . There are reports of the degree of ordering led by theoretical works 2,3 and extending to indirect probes, such as infrared spectroscopy 5,11 , small angle X-ray scattering of interparticle spacing 11,12 , and calorimetric measurements that infer entropy 5,7,8,11,12 . Such ordering is interesting fundamentally (these are well-defined finite size order-disorder transitions) and practically (influencing properties including optical, chemical, electrical, and structural).…”

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

“…Low surface coverage could be battled by careful choice of the parameters (notably temperature, solvent, or concentration) during the ligand exchange procedure. A recent report on CdSe@CdS QDs demonstrated how careful calibration of a ligand-composite system (namely oleic acid and an alkyl phosphine) can be used to induce order in the ligand sphere, increasing photoluminescence [78]. Similar approaches might be applicable to increase order in the ligand sphere of PEG-ligand systems, resulting in less surface electron traps.…”

Ligands as a universal molecular toolkit in synthesis and assembly of semiconductor nanocrystals

“…For example, Balan et al quantied the amount of ligands as well as the ligand packing density of ODPA and OA co-passivated CdSe/CdS QDs using 1 H NMR: the average fraction of OA and ODPA is quantied by contrasting areal ratios between the methine protons of OA and methyl protons of ODPA. 63 Recent advances in the characterization of surface ligands using solid state NMR (SSNMR) in combination with numerical simulation enabled quantitative analysis of the dynamic behavior and spatial distribution of ligands at the molecular level. Pang et al analyzed the spatial positioning of entropic ligands composed of myristate and hexanoate by probing 13 C centerband-only detection of exchange (CODEX) SSNMR.…”