Photon upconversion is a process by which two or more low-energy photons are absorbed and one higher-energy photon is emitted by a material. Materials that can achieve photon upconversion are desirable for many applications such as optoelectronic devices, drug delivery, and photovoltaics. A key advantage of using semiconductor nanoparticles for photon upconversion is their wide tunability in structure, which consequently affects their absorption and emission properties. We have synthesized colloidal quantum dot (QD) heterostructures for this purpose, in which two QDs with different bandgaps are separated by a wide-bandgap nanorod. While our CdSe(Te)/CdS/CdSe core/rod/emitter structures demonstrate near-infrared (NIR)-to-visible photon upconversion, their upconversion efficiencies show significant room for improvement. The performance of these structures can be enhanced through a better understanding of their underlying properties and the effect of these properties on upconversion efficiency. While we can further our understanding through theoretical modeling, we must also consider the constraints of available synthesis techniques. Synthesis parameters such as ion precursors, organic ligands, reaction time, and temperature, among others, can be tuned within well-studied procedures to create structures with the desired composition, structure, and resulting optical behavior. For example, introducing a bandgap gradient along the nanorod through doping has been found to funnel carriers to the emitter, increasing both quantum yield and upconversion efficiency. We have therefore studied an “inverted” emitter/rod/core structure which allows this gradient to form more naturally during synthesis. We have also found that charge carrier separation can be improved by controlling the position of the core along the rod and by optimizing the length of the rod. This has led us to explore spherical CdTe/CdS/CdSe core/shell/shell structures with varying shell thicknesses and gradients to further tune absorption and emission wavelengths. We present these synthesis methods, the resulting structures, and their impact on upconversion performance as characterized by TEM, absorbance, and photoluminescence measurements. We describe how further upconversion performance enhancement can be achieved through improved structural control, which will allow for future incorporation into various photonic device applications.

Synthesis of colloidal quantum dot-based heterostructures for photon upconversion

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.