Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. 1H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We fi...

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Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenom...

Prospects of Nanoscience with Nanocrystals

We demonstrate that metal carboxylate complexes (L–M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N′,N′-tetramethylbutylene-1,4-diamine, pyridine, N,N,N′,N′-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L–M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se–2S3/2h absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) an...

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Ligand exchange and the stoichiometry of metal chalcogenide nanocrystals: spectroscopic observation of facile metal-carboxylate displacement and binding.

The possibilities offered by 1H solution NMR for the study of colloidal nanocrystal ligands are reviewed. Using CdSe and PbSe nanocrystals with tightly bound oleate ligands as examples, the solution NMR toolbox for ligand analysis is introduced, highlighting 1D 1H, diffusion ordered (DOSY) and nuclear Overhauser effect (NOESY) spectroscopy as NMR techniques that enable bound ligands to be distinguished from free ligands. For each of the toolbox techniques, it is outlined how dynamic stabilization in the fast exchange regime affects the spectra obtained. Next, it is shown how the perturbation of a purified dispersion by dilution or the addition of excess ligands can be used to analyze the binding of ligands to a nanocrystal. Finally, saturation transfer difference (STD) spectroscopy is presented as an NMR technique that may complement the established toolbox.

A Solution NMR Toolbox for Characterizing the Surface Chemistry of Colloidal Nanocrystals

This review outlines the set of technical approaches to answering three major questions about the surface chemistry of colloidal semiconductor quantum dots (QDs): (i) What is the chemical structure of the ligands on the surface of the QD? (ii) How many of each type of ligand are on the surface of the QD? (iii) What is the intermolecular structure (geometry) of the ligands on the surface of the QD? Each section addresses the accessability of the relevant techniques—which include 1D and 2D NMR, vibrational and electronic absorption and transient absorption spectroscopies, and various elemental analyses—and their sensitivity and applicability to the specified observable of interest.

Chemical, Structural, and Quantitative Analysis of the Ligand Shells of Colloidal Quantum Dots

The ratio of Cd to Se (Cd/Se) within colloidal CdSe quantum dots (QDs) synthesized with 90% trioctylphosphine oxide (TOPO) as the coordinating solvent increases from 1.2:1 for QDs with radius R ≥ 3.3 nm to 6.5:1 for R = 1.9 nm, as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The highest value of Cd/Se reported previously for CdSe QDs was 1.8:1. The dependence of Cd/Se on R fits a geometric model that describes the QDs as CdSe cores with Cd/Se = 1:1 encapsulated by a shell of Cd−organic complexes. Use of 99% TOPO as the coordinating solvent produces QDs with Cd/Se ≈ 1:1 for all values of R, and use of 99% TOPO “doped” with n-octylphosphonic acid (OPA), an impurity in 90% TOPO, produces QDs with values of Cd/Se up to 1.5:1. These results imply that Cd enrichment of the QDs is driven by tight-binding Cd2+−alkylphosphonate complexes that stabilize the interface between the polar CdSe core and the organic medium.

Organic Surfactant-Controlled Composition of the Surfaces of CdSe Quantum Dots

This paper describes the use of a diarylethylene (DAE) ligand, which adopts structures that are sensitive to the wavelength of light, to cross-link films of CdSe quantum dots (QDs) within electrical junctions with photoswitchable conductivity. These QD-DAE films are deposited on indium-tin-oxide/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (ITO/PEDOT:PSS) electrodes and have eutectic Ga-In top-contacts. The photocurrent density of the cross-linked QD films is enhanced by a factor of 6.5 (averaged over all applied voltages) when the DAE ligand is switched from its open, non-conductive form (by illumination with 500–650 nm light) to its closed, conductive form (by illumination with 300–400 nm light). This enhancement is accomplished by changing the inter-particle electronic coupling, not the inter-particle distance. Identical QD films cross-linked with dibenzenedithiol ligands have a photoconductivity that is insensitive to the wavelength of light.

G. Daniel Lilly

Papers

Organic Surfactant-Controlled Composition of the Surfaces of CdSe Quantum Dots

Switchable photoconductivity of quantum dot films using cross-linking ligands with light-sensitive structures