Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Summary

A protocol detailing how shape-anisotropic colloidal cadmium chalcogenide nanocrystals can be covalently linked via their end facets is presented here.

Abstract

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

Introduction

The directed assembly of colloidal semiconductor NCs offers a synthetic pathway to the fabrication of nanostructures whose physicochemical properties are either the collective sum of or radically different from their individual NC building blocks^1,2,3,4. Among the various approaches to nanoparticle assembly, the method of oriented attachment - in which NCs are essentially fused with each other - stands out as one that allows for interparticle electronic coupling. However, conventional oriented attachment typically requires the delicate balan....

Protocol

1. Preparation of Precursor Stock Solutions

1. TOP-Se stock solution.
   1. In a nitrogen atmosphere glovebox, weigh out 11.84 g of selenium pellets into a 150-mL conical flask and place a magnetic stir bar in the flask.
   2. Add 100 mL of tri-n-octylphosphine (TOP) to the conical flask and seal the flask with a rubber septum.
   3. Stir the mixture overnight at 800 RPM.
      NOTE: Once the selenium pellets have dissolved fully into the TOP, the TOP-Se stock solution (1.5 M) is ready to be used in further reactions.
2. TOP-S stock solution.
   1. Weigh out 0.611 g of sulfur powde....

Results

Using CdSe-seeded CdS NRs as a model system, as illustrated in Figure 1(a), we demonstrated that we can use a partial Ag⁺ exchange process to specifically transform the facets at the NR tips to Ag₂S. The Ag₂S facets are capped by DDA, which reacts with ODPA via an acid-base reaction to form an insoluble salt⁵. This causes the DDA ligands to be removed from the Ag₂S facets, ca.......

Discussion

The linking technique described in this work allows for shape-anisotropic cadmium chalcogenide nanoparticles that can undergo cation exchange with Ag⁺ to be joined, facet-to-facet, into assemblies such as linear chains or branched networks. Failure to form well-dispersed, extensive assemblies of facet-to-facet linked nanoparticles is often because of two reasons: (i) the ODPA is not dispersed well in the NR-containing solution, which can be addressed by sonicating the mixture for the prescribed amount of time .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Cadmium oxide (CdO), 99.5%	Sigma Aldrich		Highly toxic
Tri-n-octylphosphine oxide (TOPO), 90 % and 99%	Sigma Aldrich		Technical and analytical grade
Cadmium acetylacetonate (Cd(acac)2), 99.9%	Sigma Aldrich		Highly toxic
Hexadecanediol (HDDO), 90%	Sigma Aldrich		Technical grade
1-octadecene (ODE), 90%	Sigma Aldrich		Technical grade
Dodecylamine (DDA), 98%	Sigma Aldrich		Toxic
Cadmium nitrate tetrahydrate ((CdNO3)2.4H2O), 98%	Sigma Aldrich		Highly toxic
Myristic acid (MA), 99%	Sigma Aldrich		Analytical grade
Octyl phosphonic acid (OPA), 97%	Sigma Aldrich		Analytical grade
Oleylamine (Oly), 70%	Sigma Aldrich		Technical grade
Hexadecyltrimethylammonium bromide (CTAB), 95%	Sigma Aldrich		Toxic
Selenium pellets (Se, 5 mm), 99.99%	Sigma Aldrich		Analytical grade
Hexadecylamine (HDA), 90%	Alfa Aesar		Technical grade, toxic
n-tetradecylphosphonic acid (TDPA), 98%	Alfa Aesar		Analytical grade
Silver nitrate (AgNO3), 99.9%	Alfa Aesar		Analytical grade
Oleic acid (OA), 90%	Alfa Aesar		Technical grade
Tri-n-octylphosphine (TOP), 97%	Strem		Analytical grade, toxic, air sensitive
n-hexylphosphonic acid (HPA), 97%	Strem		Analytical grade
n-octadecylphosphonic acid (ODPA), 97%	Strem		Analytical grade
Tellurium powder (Te), 99.9%	Strem		Air sensitive
Tri-n-butylphosphine (TBP), 99%	Strem		Analytical grade, highly toxic, air sensitive
Diisooctylphosphonic acid (DIPA), 90%	Fluka		Technical grade, toxic