Measuring Charge of Individual Quantum Dots

Abstract

Colloidal semiconductor nanocrystal quantum dots (QDs) have been a subject of interest for many years due to their favorable optoelectronic properties arising directly from their small size through quantum confinement effect and the flexibility of synthesis in solution phase growth. Understanding the electrical properties of nanocrystal QDs in environments relevant to functional optoelectronic devices such as QD-Light Emitting Diodes (LEDs) and solar cells is important in informing the design of more efficient devices. Optical properties of QDs are closely related to their unique electrical charging phenomenon. For example, when QDs are used as fluorescent labels in biological analysis, the reason behind the ``blinking" phenomenon is closely related to the charge motion of QDs due to trap states.

Colloidal QD solutions are first crashed out three times to remove excess ligands, and the substrates are plasma cleaned if needed. Samples consisting of a submonolayer array of closely packed PbS QDs on Si substrate with a native SiO_2 layer are prepared by spin-casting. QD solutions are also spin-casted onto other types of substrates such as template-stripped Au coated substrate and Si substrate with a thicker oxide layer. We determine the electric properties of QDs by first measuring the topographic image of our samples using Atomic Force Microscopy, followed by charge state measurement of QDs using Electric Force Microscopy. Previous research shows that a fraction of the photoionization occurs directly via a "hot electron" process during which electrons of high kinetic energy are trapped in certain areas of QDs, forming a space charge, but further research is needed to understand if the trap states responsible for this process are on the particle surface, within the ligand sphere, or within the silicon oxide layer. There are many factors affecting charge states and photoionization of QDs, and we investigate the effects of substrates, shell thickness of QDs, and ligands in particular. In the future, we are interested in studying the charge state of QDs on the surface of organic semiconducting thin films, as well as embedded in these films. To perform these measurements, it is first critical to reliably obtain individual QDs on the surface and successfully measure their charge. This is challenging and developing a procedure at MHC is the focus of this thesis.