Nanocrystal Surface Chemistry

We aim to characterize and manipulate the structure of interfaces and so-called “defects” in colloidal semiconductor nanocrystals. At the nanoscale, the interaction of charges with surface atoms dominates electronic properties like photoluminescence quantum yield, fluorescence blinking, vibrational cooling of excited carriers, and trapping of charges during electrical transport. A deeper understanding of surface structure and its interaction with excited electrons and holes is necessary to understand these phenomena and to apply these tunable solution processable materials in optoelectronic devices.

Our research distinguishes between dative ligands that are adsorbed to nanocrystal surfaces and ligands that balance their charge with the crystal lattice (See below). These findings contrast strongly with the prevailing description of nanocrystal surface structure, particularly of CdSe quantum dots, where the dative ligand binding model is widely accepted. We are building upon this distinction to probe electronic states within the nanocrystals band gap and to design surface modification strategies that preserve the desirable properties of nanocrysatalling semiconductors. 

Phosphine and Amine Complexes of Halide-Terminated Nanocrystals

We have synthesized and characterized nanocrystals with a surface monolayer of cadmium halide that are made soluble by supporting trialkylphosphine or alkylamine ligands. These materials have proven valuable as “inks” for the printing of thin films with strong internanocrystal coupling and efficient electrical conductivity because their supporting organic ligands can be readily desorbed under vacuum. Detailed NMR studies are helping us identify the relative binding constants of various neutral ligands, their exchange kinetics and their relationship to surface structure.

Our research group is developing an atomically precise description of colloidal nanocrystal structure and reactivity. To address this challenge we apply the synthetic methods and experimental techniques of organometallic chemistry to nanocrystals. By studying the kinetics and mechanisms of crystal growth, we seek to prepare nanocrystals with unambiguous composition. With this approach we are pursuing three research goals: 1) To control the interplay between surface structure and charge trapping in semiconductor nanocrystals in solution and in thin solid films. 2) To resolve the microscopic steps that lead to the nucleation and growth of colloidal crystals. 3) To develop practical synthetic methods that afford novel nanomaterials of unambiguous composition including colloidal quantum wells and soluble diamond nanocrystals with nitrogen vacancy and silicon vacancy color centers.

Research Summary

Ligand Exchange and Bu3P Complexation

Stable Clusters of Cadmium Selenide

We have synthesized several classes of cadmium selenide clusters and determined their structures using a combination of single crystal x-ray and pair distribution function analysis of x-ray scattering from powders. These quantum dots provide a unique viewpoint from which to understand the nucleation of much larger quantum dots. Their homogeneous formulas and structure also provides a powerful platform from which to understand the structural origins of quantum dot optical properties.  We are currently investigating these materials using a combination of density functional theory and optical spectroscopy to probe the behavior of their excited states.

NMR Spectroscopy

of Bu3P Ligands

Electron Microscopy

Before                       After

Methodology Development and Mechanistic Studies of Colloidal Crystallization

By studying the mechanism of precursor reactions we aim to relate the kinetics of solute supply to the outcome of the crystallization.  We have found that the rate at which precursors are converted to monomers determines the number of nuclei and the steady state supersaturation during growth. These parameters are the keys to controlling nanocrystal size, size distribution and shape. As part of these studies we developed a library of nanocrystal precursors based on the chalcogenocarbonyl motif.  This library of compounds provides a powerful method to tune reactivity in a synthetically convenient manner.  With this library, we are probing crystallization mechanisms and synthesizing graded alloy core-shell quantum dots for energy efficient, warm white, solid state lighting.

Low Temperature Synthesis of Atomically Precise Quantum Dots

Structurally characterized quantum dots: Single crystal and Pair Distribution Function Analysis of X-Ray Scattering

NMR and UV-Vis Kinetics of Nanocrsytal Growth

Tunable Chalcogenocarbonyl Precursor Library