Nanowires from Layered van der Waals Crystals: Opportunities for Tuning Structure and Function in 1D-2D Hybrid Nanostructures

半导体纳米线作为一类用于电子、发光、能量转换或量子计算的功能组件，具有技术上的兴趣，可以通过简单的生长过程以极高的晶体质量批量生产。迄今为止，研究几乎完全集中在传统半导体材料的纳米线上，如硅或锗，其中的原子紧密地结合在一起，形成三维晶体。这个项目是由美国国家科学基金会固态和材料化学项目支持的，它探索了在层状半导体纳米线中出现的调谐结构和特性的机会，在层状半导体中，原子紧密地结合在原子薄片上，而原子薄片又被更弱的力固定在一起。协同研究这种层状纳米线的合成，探索其独特的电子和光电子特性，旨在揭示晶体结构、电子结构与电流流动以及光吸收和发射之间的关系，这对技术应用具有重要意义。除了有针对性的技术进步外，该项目还为参与的研究生和本科生提供了深远的教育和培训机会。它包括专门向农村地区的高中生伸出援手，让学生-教师团队有机会参与与研究相关的实践活动。在研究人员的指导下，学生们准备了新的科学学习材料，并将这些材料带回学校，与同龄人分享他们的经验，并为STEM学科的职业生涯建立激情。平面二维和层状硫族化合物半导体由于其优异的电子、光电、机械和化学性能而得到了广泛的研究。相比之下，很少有人关注将蒸汽-液-固（VLS）纳米线生长和范德华（vdW）外延的概念结合起来实现层状硫系纳米线的可能性。现有的少数研究将这些系统与传统的3d晶体纳米线同等对待，而没有解决vdW晶体中出现的材料设计机会。该项目由美国国家科学基金会固态与材料化学项目支持，旨在探索vdW纳米线独特的晶体自由度，并通过定制的VLS生长过程对其进行控制。不同性质的例子包括沿不同方向分层，由于提高晶格匹配要求而易于材料集成，以及自发层间扭曲的趋势，这些趋势加起来形成整体手性，其大小由线直径控制。以各向异性金属单硫系半导体为重点，对选定材料的协调生长和表征提供了对vdW纳米线和异质结构的原子生长机制的见解。结构和形貌与单纳米线电荷输运有关，并与光电性能有关，通过阴极发光光谱在激子半径以下的分辨率下探测，以解决层堆叠和扭曲，约束，激子效应以及异质结构中界面载流子转移的作用。该奖项反映了美国国家科学基金会的法定使命，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。

Part 1: Non-Technical SummarySemiconductor nanowires are of technological interest as a class of functional components for electronics, light emission, energy conversion, or quantum computation, which can be mass-produced with exceptionally high crystal quality by simple growth processes. To date, research has almost exclusively focused on nanowires of traditional semiconductor materials, such as silicon or germanium, in which the atoms are tightly bonded to their neighbors in a three-dimensional crystal. This project, which is supported by the Solid State and Materials Chemistry program at NSF, explores opportunities in tuning structure and properties that arise in nanowires of layered semiconductors, in which atoms are tightly bonded in atomically thin sheets that are in turn held together by much weaker forces. Coordinated efforts in studying the synthesis of such layered nanowires and in exploring their distinct electronic and optoelectronic properties aim to uncover relationships between crystal structure, electronic structure and the flow of electrical current, as well as light absorption and emission, which are import for technological applications. In addition to the targeted technical advances, the project provides far-reaching educational and training opportunities for the involved graduate and undergraduate students. It includes dedicated outreach to high-school students from rural areas that gives student-teacher teams the opportunity to participate in hands-on activities related to the research. Under the guidance of the researchers the students prepare new science learning materials, which they bring back to their school to share their experience with their peers and to help build excitement for careers in STEM disciplines.Part 2: Technical SummaryExtensive research on planar 2D and layered chalcogenide semiconductors has been driven by outstanding electronic, optoelectronic, mechanical and chemical properties of these materials. In contrast, little attention has been paid to the possibility of combining the concepts of vapor-liquid-solid (VLS) nanowire growth and van der Waals (vdW) epitaxy to realize layered chalcogenide nanowires. The few existing studies have treated those systems on an equal footing with conventional 3D-crystalline nanowires without addressing the opportunities for materials design that arise in vdW crystals. This project, which is supported by the Solid State and Materials Chemistry program at NSF, explores the unique crystallographic degrees of freedom of vdW nanowires and their control via tailored VLS growth processes. Examples of distinct properties include layering along different directions, facile materials integration due to lifted lattice matching requirements, and a tendency toward spontaneous interlayer twist that adds up to an overall chirality and whose magnitude is controlled by the wire diameter. Coordinated growth and characterization of selected materials with focus on anisotropic metal monochalcogenide semiconductors provide insight into the atomistic growth mechanisms of vdW nanowires and heterostructures. Structure and morphology are correlated to single-nanowire charge transport, and to optoelectronic properties probed by cathodoluminescence spectroscopy at resolution below the exciton radius to address the role of layer stacking and twist, confinement, excitonic effects, as well as interfacial carrier transfers in heterostructures.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.