Dmitri Talapin – Plenary

Jun 27, 2018

09:00

45 minutes

Challenges and opportunities for colloidal quantum dots: a chemist’s perspective

Colloidal chemistry has revolutionized the synthesis of inorganic nanomaterials. The field has evolved tremendously, both in the fundamental understanding of nucleation, growth and surface chemistry of nanocrystals, and in the ability to provide a toolset for preparation of functional materials for various applications. However, current methodology has several limitations that need to be carefully explored and addressed.

The lack of atomic precision in nanomaterial synthesis restricts our ability to harness all the power of this broad and diverse class of materials. Heterogeneity introduces broadening of the absorption and emission spectra, reduces charge carrier mobility in nanocrystal solids, and generally restricts our ability to engineer nanomaterials. I will discuss new approach for colloidal synthesis of nanomaterials with minimal, ideally no, size distribution. The concept is inspired by gas-phase atomic layer deposition (ALD) widely used in microelectronics. Our studies show that the ALD concept can be successfully implemented in solution and, when applied to nanomaterials, enables layer-by-layer growth of crystalline lattices with close-to-atomic precision.

The other general limitation of traditional colloidal chemistry is related to thermal stability of organic solvents at high temperatures required for some hard-to-crystallize materials. Very few traditional solvents remain liquid above 400°C, and solvent or ligands decomposition becomes a serious problem at even higher temperatures. The use of inorganic salts as solvents eliminates this issue and offers new opportunities. As an example, we found that III-V quantum dots synthesized in organic solvents generally have high concentration of vacancies and antisite defects. Annealing or ion-exchanging in a molten salt can eliminate these structural defects without sintering. We envision multiple exciting opportunities for colloidal chemistry in molten salts.

I will also discuss the advances in the surface chemistry of semiconductor nanostructures. Molecular inorganic species can be designed to electronically couple individual nanostructures into nanocomposite materials with high electron mobility. By making surface ligands photochemically active, we introduce a general approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials (DOLFIN). Examples of patterned materials include quantum dots, metals, oxides, magnetic and rare earth compositions. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The ability to directly pattern all-inorganic layers using a light exposure dose comparable to that of organic photoresists opens up a host of new opportunities for additive nanomanufacturing.

Colloidal chemistry has revolutionized the synthesis of inorganic nanomaterials. The field has evolved tremendously, both in the fundamental understanding of nucleation, growth and surface chemistry of nanocrystals, and in the ability to provide a toolset for preparation of functional materials for various applications. However, current methodology has several limitations that need to be carefully explored and addressed.

The lack of atomic precision in nanomaterial synthesis restricts our ability to harness all the power of this broad and diverse class of materials. Heterogeneity introduces broadening of the absorption and emission spectra, reduces charge carrier mobility in nanocrystal solids, and generally restricts our ability to engineer nanomaterials. I will discuss new approach for colloidal synthesis of nanomaterials with minimal, ideally no, size distribution. The concept is inspired by gas-phase atomic layer deposition (ALD) widely used in microelectronics. Our studies show that the ALD concept can be successfully implemented in solution and, when applied to nanomaterials, enables layer-by-layer growth of crystalline lattices with close-to-atomic precision.

The other general limitation of traditional colloidal chemistry is related to thermal stability of organic solvents at high temperatures required for some hard-to-crystallize materials. Very few traditional solvents remain liquid above 400°C, and solvent or ligands decomposition becomes a serious problem at even higher temperatures. The use of inorganic salts as solvents eliminates this issue and offers new opportunities. As an example, we found that III-V quantum dots synthesized in organic solvents generally have high concentration of vacancies and antisite defects. Annealing or ion-exchanging in a molten salt can eliminate these structural defects without sintering. We envision multiple exciting opportunities for colloidal chemistry in molten salts.

I will also discuss the advances in the surface chemistry of semiconductor nanostructures. Molecular inorganic species can be designed to electronically couple individual nanostructures into nanocomposite materials with high electron mobility. By making surface ligands photochemically active, we introduce a general approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials (DOLFIN). Examples of patterned materials include quantum dots, metals, oxides, magnetic and rare earth compositions. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The ability to directly pattern all-inorganic layers using a light exposure dose comparable to that of organic photoresists opens up a host of new opportunities for additive nanomanufacturing.

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