Collaborative Research: EAGER: Designing Nanomaterials to Reveal the Mechanism of Single Nanoparticle Photoemission Intermittency

University Of Illinois At Chicago, Chicago IL

Investigators

Abstract

In this collaborative EAGER project funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Chemistry Division, Preston Snee of the University of Illinois at Chicago, Kristin Wustholz of the College of William and Mary, and Haw Yang of Princeton University seek to resolve the longstanding mystery of quantum dot blinking. Quantum dots are nanometer size semiconductor particles that have potential for applications in a variety of technologies, including energy-efficient displays, low-cost solar cells, and as biomedical research tools. Although these dots are fluorescent, single particles do not emit uniformly as they turn off and on randomly. This “blinking” phenomenon remains a challenge to explain, and this lack of a deep understanding limits the practical development and application of quantum-dot technology. The team of Snee, Wustholz and Yang combines expertise in chemical synthesis, advanced imaging, and statistical analysis, and is evaluating blinking behavior of various quantum-dot samples whose surface structures have been systematically varied using synthetic chemistry. The graduate and undergraduate researchers working on this project will gain a wide range of experience in experimental and computational chemistry. The collaborating research groups are also engaging in public outreach and the development of a free (online) physical chemistry textbook. The results of this NSF supported project are being incorporated into the textbook and outreach activities where appropriate. The quantum dot blinking phenomenon is generally described using power-law probability statistics to describe the emission "off" and "on" timescales; however, there is no straightforward physical explanation for this behavior. The central hypothesis of this EAGER proposal is that quantum-dot blinking is essentially the result of interactions of photo-generated excitons with the surface trap states, and that the emission probability distribution function reflects a distribution of trap barrier activation energies. The general functional form of this model is attributed to Albery (1985), who sought to explain electron transfer between semiconductor surfaces and adsorbed molecules. In contrast to the power-law function, the Albery model predicts a lognormal function. Furthermore, the Snee group recently demonstrated that quantum dots can exhibit lognormal blinking by manipulating the surface chemistry. Secondary hypotheses of this EAGER proposal are that 1) the apparent power-law dependence reported in the large body of existing experimental works is the result of uncontrolled surface chemistries and improper assumptions regarding how the photon counts should be "binned" in time, and 2) the defect sites interact with molecular oxygen, and that this interaction results in behavior that is a discrete sum of lognormal distributions that appear nearly identical to a power-law distribution. These hypotheses are being examined through combined quantum-dot synthesis, single particle imaging, and photon counting experiments. The analysis of blinking statistics utilizes maximum likelihood estimation, which addresses previous problems with arbitrary binning of blinking data that can lead to erroneous conclusions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.