Terahertz Studies of Transient Photoconductivity in Quantum Dots and Electron Transfer in Bacterial Reaction Centers

耶鲁大学的Charles Schmuttenmaer博士获得了化学系物理化学项目的资助，用于量子点瞬态光电导和细菌反应中心电子转移的太赫兹研究。PI将测量孤立量子点（QD）以及QD阵列中的时间依赖性光电导。 该信息在基于QD技术的光电子器件的设计中是重要的，其中理解载流子在吸收光子之后保持移动的的时间尺度以及移动性的实际值是至关重要的。 PI还将开发一种新的直接方法来监测电荷转移事件。 电子本身的运动，而不是吸收或荧光光谱的变化，提供了测量的信号。 这是因为加速电荷产生电磁脉冲，并且如果电荷转移和/或溶剂重取向时间尺度在100 fs至10 ps的量级上，则产生THz脉冲。 该方法已在强电场中定向的染料分子上进行了基准测试，并将用于研究细菌反应中心中电荷转移的主要步骤，这些反应中心通过使用多聚组氨酸标签进行空间定向，该标签将蛋白质的特定残基锚定到功能化的石英载玻片上。在过去的几年里，耶鲁大学的PI小组已经学会了如何使用远红外光的短脉冲进行并正确解释一类新的实验。 由于涉及光源强度和检测器灵敏度的技术原因，与光谱的其他区域相比，光谱的远红外区域在光谱实验中一直被忽略。 关于时间分辨光谱实验，其中人们用一个光脉冲“激发”或改变样品，然后用第二个光脉冲监测样品中的变化（这基本上是1999年诺贝尔化学奖授予艾哈迈德·泽维尔的工作类型），在光谱的远红外区域基本上没有表现。 PI和他的团队将使用这些新的脉冲技术来阐明两个重要的科学问题。 首先，他们将研究量子点中的瞬态光电导性。 量子点是一种正在彻底改变材料科学的新型材料，因为它们的性质不仅取决于它们的成分，还取决于它们的大小（一旦它们足够小，量子效应就变得重要）。 PI将评估这些粒子在吸收光子后导电的程度和时间。 在非常基础和基本的层面上，有许多关于新型光电器件和高速通信的重要应用。 其次，他们将专注于光合作用中的主要电荷分离步骤。 光合作用可以说是这个星球上最重要的过程。 它提供我们呼吸的氧气，同样重要的是，它是所有化石燃料的来源-对我们社会如此重要的能源。 PI将以比迄今为止可能的方式更直接和明确的方式监测该主要电荷转移事件，并提供电荷转移机制的进一步表征。 迫切需要在纳米技术方面受过培训的科学人员，在量子点上工作的学生和博士后将在这个新兴和快速变化的领域接受各种技术的培训。 从事光合作用项目的学生和博士后将接受超快光谱学的培训，并将学习光合系统中的电子转移。

Dr. Charles Schmuttenmaer of Yale University is funded for his research in terahertz studies of transient photoconductivity in quantum dots and electron transfer in bacterial reaction centers by a grant in the Physical Chemistry Program of the Chemistry Division. The PI will measure time-dependent photoconductivity in isolated quantum dots (QDs) as well as in arrays of QDs. This information is of importance in the design of optoelectronics devices based on QD technology, where it is essential to understand the timescales that carriers remain mobile after absorption of a photon, and the actual value of the mobility. The PI also will develop a new and direct method for monitoring charge transfer events. The motion of the electron itself, rather than a change in absorption or fluorescence spectrum, provides the measured signal. This occurs because accelerating charges generate electromagnetic pulses, and if the charge transfer and/or solvent reorientation timescale is on the order of 100 fs to 10 ps, then a THz pulse is generated. This methodology has been benchmarked on dye molecules oriented in strong electric fields, and will be used to study the primary step of charge transfer in bacterial reaction centers that are spatially oriented by using a poly-histidine tag that anchors a specific residue of the protein to a functionalized quartz slide.Over the last several years, the PI's group at Yale University has learned how to carry out and properly interpret a new class of experiments using short pulses of far-infrared light. For technical reasons involving the intensity of light sources and sensitivity of detectors, the far-infrared region of the spectrum has been historically neglected for spectroscopic experiments compared to other regions of the spectrum. With regard to time-resolved spectroscopic experiments, wherein one "excites" or changes a sample with one pulse of light, and then monitors the change in the sample with a second pulse of light (which is essentially the type of work for which the Nobel Prize in Chemistry was awarded to Ahmed Zewail in 1999), there has been essentially no representation in the far-infrared region of the spectrum. The PI and his group will use these new pulse techniques to elucidate two important scientific issues. First, they will investigate transient photoconductivity in quantum dots. Quantum dots are a new type of material that is radically changing materials science because their properties depend not only on their composition, but also on their size (once they are small enough that quantum effects become important). The PI will assess how well and for how long these particles conduct electricity after absorbing photons of light. At a very basic and fundamental level, there are many important applications with respect to new types of optoelectronic devices and high speed communications. Second, they will focus on the primary charge separation step in photosynthesis. Photosynthesis is arguably the most important process that transpires on this planet. It provides the oxygen we breath, and equally important, it is the source of all fossil fuels - the energy which is so important to our society. The PI will monitor this primary charge transfer event in a much more direct and unambiguous manner than heretofore possible, and provide further characterization of the mechanisms of charge transfer. There is an urgent need for scientific personnel trained in nanotechnology, and the students and post-docs working on QDs will be trained in a wide variety of techniques in this emerging and rapidly changing field. Students and post-docs working on the photosynthesis project will receive training in ultrafast spectroscopy and will learn about electron transfer in photosynthetic systems.