CAREER: Ultrafast Imaging and Spectroscopy of Cooperative Phenomena in Photomagnetic Nanomaterials

数字技术的进步在很大程度上依赖于缩小电子元件的尺寸。在过去的三十年里，计算机芯片的基本构件已经缩小到纳米尺寸，或者说大约是人类头发的1000倍。在这个尺寸范围内，纳米结构表现出了在其体相对应物中没有观察到的新的性质和行为。新的纳米材料暴露在光线下会改变其形状，这种材料的出现可能会影响许多不同的技术。然而，在实验室中观察单个粒子的形状变化是一项独特的挑战，因为从一种形式转换到另一种形式所需的时间非常短，而且最快的相机也无法捕捉到。在这个由化学系化学结构动力学和机制(CSDM-A)计划资助的项目中，伊利诺伊大学厄巴纳-香槟分校(UIUC)的Renske M.Van der Vain教授正在建造一种显微镜，它使用非常短的电子爆发(不到百万分之一秒)来观察单个纳米结构随光而改变形状。范德维恩教授和她的学生正在使用这种显微镜，以获得对光激发后金属-有机纳米材料中发生的级联开关事件的基本见解。这项工作的结果可能会影响从数据存储到电信中的光开关的各种技术，因此可能会对社会产生重大好处。作为Carrer项目的一部分，Van Der Veen教授正在开发新的教育活动，以培训下一代科学家使用这种新兴的实验方法。室温光开关在功能设备中的实际应用是非常必要的。这个项目正在推进我们对一阶热相变以及在室温附近发生的合作滞后现象的理解。在磁滞区内的光激发可以导致在单纳米颗粒水平上有效地、完全地切换磁性、介电和/或结构属性。该项目的重点是不同维度的铁和铁钴自旋交叉配位聚合物，它们在光激发下经历最低d轨道上的电子重新排列。利用瞬时吸收光谱、同步辐射装置上的超快X射线能谱和新研制的超快电子显微镜研究了光激发后结构自由度和电子自由度之间的相互作用。该项目还在研究与合作光开关相关的机制、时间尺度和效率。这项工作的更广泛的影响包括通过为本科生和研究生开发“3天同步加速器训练营”来加强STEM员工队伍，以及为女性和贫困中学生提供研究整合的讲座和演示模块。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Improvements in digital technology rely heavily on reducing the size of electronic components. Over the past three decades, the basic building blocks of computer chips have shrunk to nanometer dimensions, or about 1000 times smaller than a human hair. In this size range, nanostructures exhibit new properties and behaviors that are not observed in their bulk counterparts. The appearance of new nanomaterials that change their shape when exposed to light have the potential to impact many different technologies. However, watching a single particle change its shape in the laboratory presents unique challenges, since the time needed to switch from one form to the other is exceedingly short and cannot be captured by the fastest camera. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Renske M. Van der Veen at the University of Illinois at Urbana-Champaign (UIUC) is building a microscope that uses very short bursts of electrons (less than a millionth of a millionth of a second) to watch as a single nanostructure change its shape in response to light. Professor Van der Veen and her students are using this microscope to gain fundamental insight into the cascade of switching events that occur in metal-organic nanomaterials after light excitation. The results from this work could impact technologies ranging from data storage to optical switches in telecommunication, and thus could have significant benefit to society. As part of this CARRER project, Professor Van der Veen is developing new educational activities that will train the next generation of scientists in this emerging experimental methodology.Room temperature photoswitching is highly desired for realistic applications in functional devices. This project is advancing our understanding of first-order thermal phase transitions with cooperative hysteresis phenomena that occur around room temperature. Photoexcitation inside the hysteresis region may lead to efficient, "complete" switching of magnetic, dielectric and/or structural properties at the single-nanoparticle level. The project's focus is on iron- and iron-cobalt spin-crossover coordination polymers of different dimensionality, which undergo a rearrangement of electrons in their lowest d-orbitals upon light excitation. Transient absorption spectroscopy, ultrafast X-ray spectroscopy at synchrotron facilities, and a newly developed ultrafast electron microscope are used to investigate the interplay between structural and electronic degrees of freedom after photoexcitation. The project is also studying the mechanisms, time scales and efficiencies pertinent to cooperative photoswitching. The broader impacts of this work include STEM workforce enhancement through the development of a "3-Day Synchrotron Boot Camp" for undergraduate and graduate students, as well as research-integrated lectures and demonstration modules for female and underprivileged middle-school students.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.