CAREER: Understanding the Formation Mechanism of Binary SAMs to Create an Experimental Phase Diagram

在这个由化学部化学结构动力学和机理(CSDM-A)计划资助的项目中，圣十字学院的L.Gaby Avila-Bront博士使用了一种能够可视化单个分子的显微镜，以了解决定不同分子的混合物如何结合到表面形成二维图案的基本驱动力。尽管用分子图案修饰的表面有许多应用，包括用作化学和物理传感器，但目前还不可能精确地控制由不同分子组成的分子图案的组成。此外，在混合之前，不知道会形成什么二维图案，也不知道会观察到什么新的表面性质。尽管混合物的行为已经在三维上得到了彻底的研究，但情况并不完整，因为目前的治疗方法没有适当地考虑发生在表面上的化学过程。这对理解分子如何在表面混合以及预测分子在表面混合的模式提出了挑战。成功地解决这些问题可以为科学家提供前所未有的自上而下的控制，通过分子图案化进行表面修饰。本实验室的所有研究都是由本科生进行和推动的。这些学生将进行实验，分析和解释数据，并将被鼓励在科学会议上展示他们的发现，同时与阿维拉-勃朗特博士密切合作，制作手稿供出版。在此过程中，学生们正在发展和磨练他们解决问题的技能，以便他们能够将这些技能转移到他们选择的研究生领域。这项工作的总体目标是了解二维混合过程的机理，并构建双组分(二元)自组装单分子膜(SAM)的实验相图。二元SAM的实验相图将使基于热力学条件的二维材料的浓缩或分离成为可能，以及具有有意预制图案的表面的设计。有机硫醇化合物在贵金属表面的SAMS是一种模型二维体系，已被广泛表征和应用。然而，随着SAM功能的扩展，关于SAM阶段行为机制的核心问题仍然没有得到回答。二元SAM的实验相图从未被报道过，因为很难控制二元SAM的组成。因此，这个项目首先在金的(111)表面建立了一个由脂肪族和芳香族有机硫醇组成的双分子单层库。SAM的结构将在环境条件下使用扫描隧道显微镜(STM)进行分子解析，并将通过接触角测量来量化SAM的有序性。一旦建立了这个文库，就将构建已知组成的模型二元单分子层。单分子膜的组成将用X射线光电子能谱和还原脱附来确定。然后，通过将该模型中存在的二元单层中的相与不同温度下的单层的化学成分相关联来构建实验相图。用它来预测新型自组装膜的单层结构，将检验所得相图的准确性。这项工作的更广泛影响将包括对本科生进行化学研究的培训和指导。此外，Avila-Bront博士正在建立一个社区外展计划，并将远程外展扩展到小学--包括一个本科生与小学生在科学博览会项目上合作的计划--通过虚拟研讨会，旨在鼓励大众媒体对科学进行批判性评估。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。

In this project, funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Dr. L. Gaby Avila-Bront of the College of the Holy Cross is using a microscope capable of visualizing individual molecules to understand the fundamental driving forces that determine how a mixture of different molecules bound to a surface forms a two-dimensional pattern. Although surfaces modified with molecular patterns find many applications including serving as chemical and physical sensors, it is currently not possible to precisely control the composition of a molecular pattern composed of dissimilar molecules on a surface. Furthermore, prior to mixing, there is no knowledge of what two-dimensional patterns may form, or what new properties of the surface will be observed. Even though the behavior of mixtures has been thoroughly investigated in three-dimensions, the picture is not complete because current treatments do not properly consider chemical processes that occur on surfaces. This presents a challenge for understanding how molecules mix on surfaces, and predicting the pattern of a mixture of molecules on a surface. Successfully addressing these issues could provide scientists with unprecedented top-down control over surface modification with molecular patterning. All of the research in this lab is conducted and driven by undergraduate students. These students will conduct the experiments, analyze and interpret the data, and will be encouraged to present their findings at scientific conferences, while working closely with Dr. Avila-Bront to produce manuscripts for publication. In doing so, the students are developing and honing their problem-solving skills so that they can transfer these skills in the post-graduate fields of their choice.The overall goal of this work is to understand the mechanism of two-dimensional mixing processes and construct the experimental phase diagram of two-component (binary) self-assembled monolayers (SAMs). An experimental phase diagram of a binary SAM would enable the enrichment or separation of two-dimensional materials based on thermodynamic conditions, as well as the design of surfaces with deliberate preformed patterns. SAMs of organothiol compounds on noble metal surfaces are model two-dimensional systems that have been extensively characterized and utilized in numerous applications. However, as SAM functionalities have been extended, central questions about the mechanism of SAM phase behavior remain unanswered. An experimental phase diagram for a binary SAM has never been reported because it is very difficult to control the composition of a binary SAM. Therefore, this project begins by creating a library of binary monolayers composed of aliphatic and aromatic organothiols on the (111) surface of gold. The structure of the SAM is to be molecularly resolved using STM (Scanning Tunneling Microscopy) in ambient conditions, and contact angle measurements will be made to quantify the ordering properties of the SAM. Once this library is established, a model binary monolayer of known composition will be constructed. The composition of the monolayer will be interrogated using X-ray photoelectron spectroscopy and reductive desorption. An experimental phase diagram will then be constructed by correlating the phases present in this model binary monolayer to the chemical composition of the monolayer at different temperatures. The accuracy of the resulting phase diagram will be tested by using it to predict the monolayer structure of novel SAMs. The broader impacts of this work will include the training and mentoring of undergraduate students in chemical research. In addition, Dr. Avila-Bront is establishing a community outreach program and expanding remote outreach to elementary schools--including a program where undergraduate students work with elementary students on science-fair projects-- with virtual workshops aimed at encouraging critical assessment of science in the popular media.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.