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Abstract

It has been 24 years since the first synthesis of colloidal CdSe quantum dots (QDs). Today, we can find them in consumer goods: we enjoy sharp colors on television displays, where improved QDs act as chromophores. Two decades of deliberate work has brought understanding of the chemical processes lying in the preparation and surface passivation of these materials. The community has learned to synthesize nanocrystals of pre-defined composition, covering the whole range of industrially important semiconductors, of various shapes, and of desired surface termination. Over these years, we have observed and understood important physical phenomena in these materials, e.g., quantum confinement in quantum dots, exciton generation and splitting, charge hopping from one dot to another in an array, etc. With this understanding, for a number of systems, we can synthesize a material with pre-defined properties, for example, QDs with a desired bandgap width and position for photovoltaic applications, or QDs with a desired PL band position, stability, and environmental compatibility for biological imaging., The two decades of research has raised even more questions. We would like to fully describe a given systems (e.g., understand the complex exciton and multiexcitons dynamics in a QD), elicit maximum performance from the system (e.g., 100% photoluminescence quantum yield and narrow spectral lines in QDs, or bulk carrier mobilities in solution-processed semiconductors), and even push the system beyond temporary conventional limits (e.g., raising the efficiency of solar cell by multiexcitons generation in quantum dots). Ultimately, we would like industry to take advantage of the ease and tunability of solution-processed semiconductors and introduce them in bigger classes of goods. For that, we will need to mature the field to the degree of existing industrial processes (e.g., layer-by-layer deposition, polymer synthesis, etc.). In particular, we will need meters inserted into the system, monitoring its state in situ, providing feedback to the researcher, and manipulating it automatically. We will also need to bring the protocols to the levels where they can be commercialized, i.e., not only bring all figures of merit to the extreme, but account for new aspects of the process like optimization of the atom conversion or minimization of waste., In one direction of the present work, we explore the possibility of potentiometry and amperometry in quantitative characterization of ligand adsorption at the nanocrystal surface and develop a tool that enables us to controllably grow three monolayers of CdS over CdSe QDs. In another direction, we optimize the synthesis of colloidal quantum wells, CdSe and CdSe/CdS nanoplatelets, and observe superior photonic properties from them. Namely, we demonstrate record-low threshold of amplified spontaneous emission, high saturation gain, and suppressed Auger recombination. We demonstrate laser action at four different wavelengths, including the commercially unavailable 575 nm. We observe that perforation of nanoplatelets leads to colloidal particles of new topology: rings and double rings. Finally, we demonstrate the niche for solution-processed electronic materials: ease of synthesis and possibility of patterning., We believe that our electrochemical probe will find its application in the commercial synthesis of bright core-shells. With more work, researchers will demonstrate electrically pumped lasing in CdSe/CdS nanoplatelets. Colloidal nanorings will be a cheap and convenient platform to topology-determined electronic properties of semiconductors. Finally, our contributions to electronic properties of InP QDs will be a step toward understanding InP.

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