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

在这个由化学系大分子、超分子和纳米化学项目资助的EAGER合作项目中，芝加哥伊利诺伊大学的普雷斯顿·斯尼、威廉和玛丽学院的克里斯汀·乌斯霍尔茨以及普林斯顿大学的霍·杨试图解决量子点闪烁的长期谜团。量子点是纳米尺寸的半导体颗粒，具有在各种技术中应用的潜力，包括节能显示器、低成本太阳能电池以及作为生物医学研究工具。 虽然这些点是荧光的，但单个粒子并不均匀地发射，因为它们随机地关闭和打开。这种“闪烁”现象仍然是一个难以解释的挑战，这种缺乏深入理解的情况限制了量子点技术的实际发展和应用。 Snee，Wustholz和Yang的团队结合了化学合成，先进成像和统计分析方面的专业知识，正在评估各种量子点样品的闪烁行为，这些样品的表面结构已经使用合成化学系统地改变。从事该项目的研究生和本科生研究人员将获得实验和计算化学方面的广泛经验。 合作研究小组还参与公共宣传和免费（在线）物理化学教科书的开发。 这个由美国国家科学基金会支持的项目的结果正在被纳入教科书和推广活动中。量子点闪烁现象通常使用幂律概率统计来描述发射"关闭"和"打开"的时间尺度;然而，对于这种行为没有直接的物理解释。这个EAGER建议的中心假设是量子点闪烁本质上是光生激子与表面陷阱态相互作用的结果，并且发射概率分布函数反映了陷阱势垒激活能的分布。该模型的一般泛函形式归因于Albery（1985），他试图解释半导体表面和吸附分子之间的电子转移。与幂律函数相反，Albery模型预测对数正态函数。此外，Snee小组最近证明，量子点可以通过操纵表面化学来表现出对数正态闪烁。EAGER建议的次要假设是：1）现有大量实验工作中报告的明显的幂律依赖性是不受控制的表面化学和关于光子计数应如何在时间上"分箱"的不适当假设的结果，以及2）缺陷位点与分子氧相互作用，并且这种相互作用导致表现为对数正态分布的离散和的行为，所述对数正态分布看起来几乎与幂律分布相同。 这些假设正在通过联合量子点合成，单粒子成像和光子计数实验进行检查。 闪烁统计分析采用最大似然估计，解决了以前任意合并闪烁数据可能导致错误结论的问题。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.