Understanding Heterogeneous Nucleation in Nanocrystal Growth with Molecular Probes

纳米晶体是几何上有序的固体，直径约为0.00000005英寸。双金属纳米晶体由两种不同的金属制成，其性能通常优于单金属晶体。两种不同金属在纳米晶体中的相对排列是决定纳米晶体性能的重要因素，为了将一种金属精确地放置在另一种金属的纳米晶体表面上，人们已经付出了相当大的努力。检测和量化沉积的金属仍然是一个巨大的挑战，特别是当纳米晶体仍然悬浮在反应介质中进行生长时。Dong Qin博士通过开发一类分子，异氰化物（含有-NC化学基团的分子）来解决这一挑战，作为通过表面增强拉曼散射（SERS）光谱技术检测异氰化物的原位表征探针。最终目标是为合理合成具有良好控制成分和形状的双金属纳米晶体建立科学基础，这些纳米晶体通过其在各种应用中的需求而具有更广泛的社会影响。该研究项目涵盖了材料科学、化学、胶体科学、固体物理、光子学和表面科学等多个学科，重点关注学生发展的以下组成部分：1)在跨学科领域积极学习，了解纳米材料的结构和性质关系；ii)培训，提供在秦实验室合成纳米材料的实践经验，以及在佐治亚理工学院和国家实验室最先进的设施中对纳米材料进行表征；让社区和社会，包括高中学生和教师，了解纳米尺度的科学和技术概念。在这项研究中，来自佐治亚理工学院的董勤博士在大分子、超分子和纳米化学（MSN）项目的支持下，研究了双金属纳米晶体的非均相成核和过度生长的基本原理。具体来说，开发了一类新型的基于异氰化物的分子探针（R-NC），用于原位检测和分析悬浮在原始生长溶液中的银纳米晶体上的第二贵金属（M: Pd, Pt, Ir， Rh或Ru）的过度生长。由于异氰化物基团与金属表面的结合与一氧化碳的结合相似，因此可以预期，当异氰化物基团分别与Ag和M原子结合时，NC键的拉伸频率不同。因此，通过实时跟踪数控振动的拉伸频率和强度来监测M原子沉积在Ag纳米晶体上是可行的。另一方面，可以将银纳米晶体上的SERS热点设计成与M原子倾向于非均相成核的位置一致，从而实现前所未有的灵敏度和低于单层的检测限。通过利用其对金属原子的完美灵敏度，新型SERS探针为阐明银纳米晶体表面第二贵金属种子过度生长的机制细节提供了新的机会。机制的见解支持合理的设计和知识为基础的合成双金属和多金属纳米晶体的各种应用。该研究项目涵盖了材料科学、化学、胶体科学、固体物理、光子学和表面科学等多个学科，重点关注学生发展的以下组成部分：1)在跨学科领域积极学习，了解纳米材料的结构和性质关系；ii)培训，提供在秦实验室合成纳米材料的实践经验，以及在佐治亚理工学院和国家实验室最先进的设施中对纳米材料进行表征；让社区和社会，包括高中学生和教师，了解纳米尺度的科学和技术概念。

Nanocrystals are geometrically well-ordered solids with diameters on the order of 0.00000005 inches. Bimetallic nanocrystals, made of two different metal species, have properties that are often superior to their single-metal counterparts. The arrangement of the two different metals relative to each other in the nanocrystal is very important in determining the nanocrystal properties, and considerable effort has been made to deposit one metal atop the nanocrystal surface of another metal with precision in location. It remains a grand challenge to detect and quantify the metal being deposited, particularly when the nanocrystals are still suspended in the reaction medium undergoing growth. Dr. Dong Qin addresses this challenge by developing a class of molecules, the isocyanides (molecules containing the -NC chemical group), as probes for in situ characterization with detection of the isocyanides by a spectroscopic technique, surface-enhanced Raman scattering (SERS). The ultimate goal is to establish a scientific basis for enabling the rational synthesis of bimetallic nanocrystals with well-controlled compositions and shapes, which have broader societal impact through their need in a variety of applications. This research project encompasses multiple disciplines such as materials science, chemistry, colloidal science, solid-state physics, photonics, and surface science, with a focus on the following components for student development: i) active learning in interdisciplinary areas involved in understanding the structure and property relationships of nanomaterials; ii) training that provides hands-on experience in the synthesis of nanomaterials in the Qin laboratory, and on the characterization of nanomaterials in the state-of-the-art facilities at Georgia Tech and national laboratories; and iii) exposing the community and society, including high school students and teachers, to nanoscale science and technology concepts.In this research, Dr. Dong Qin from the Georgia Institute of Technology is supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program to study the fundamentals involved in the heterogeneous nucleation and overgrowth of bimetallic nanocrystals. Specifically, a novel class of isocyanide-based molecular probes (R-NC) are developed for in situ detection and analysis of the overgrowth of a second noble metal (M: Pd, Pt, Ir, Rh, or Ru) on Ag nanocrystals suspended in the original growth solution by surface-enhanced Raman scattering (SERS). Because the binding of the isocyanide group to a metal surface is similar to that of carbon monoxide, it is anticipated that the stretching frequency of the NC bond differs when the isocyanide group binds to the Ag and M atoms, respectively. Therefore, it is feasible to monitor the M atoms being deposited onto Ag nanocrystals by following the stretching frequencies and intensities of NC vibration in real time. On the other hand, the SERS hot spots on the Ag nanocrystals can be designed to coincide with the sites favored by M atoms for heterogeneous nucleation, allowing for unprecedented sensitivity with a detection limit below one monolayer. By leveraging their consummate sensitivity toward metal atoms, the novel SERS probes open up new opportunities to elucidate the mechanistic details involved in the seeded overgrowth of a second noble metal on the surface of Ag nanocrystals. The mechanistic insights support the rational design and knowledge-based synthesis of bi- and multi-metallic nanocrystals for a variety of applications. This research project encompasses multiple disciplines such as materials science, chemistry, colloidal science, solid-state physics, photonics, and surface science, with a focus on the following components for student development: i) active learning in interdisciplinary areas involved in understanding the structure and property relationships of nanomaterials; ii) training that provides hands-on experience in the synthesis of nanomaterials in the Qin laboratory, and on the characterization of nanomaterials in the state-of-the-art facilities at Georgia Tech and national laboratories; and iii) exposing the community and society, including high school students and teachers, to nanoscale science and technology concepts.