CAREER: Raman Spectroscopy of Interlayer Phonons in van der Waals Heterostructures

非技术文摘：范德华异质结构是一类新的材料，可以导致新的新颖的电子和光电子器件。这些材料是由石墨烯、六方氮化硼和过渡金属二硫化物（如二硫化钼）层等原子层垂直堆叠而成的。这些层之间的作用力相对较弱。然而，它们可以极大地改变系统的性质，并诱发在单个层中不存在的现象。通过堆叠或外延生长不同的原子层，人们可以设计和制造具有前所未有的光学、电子和振动特性的人造材料，这些特性在天然晶体中无法实现。拉曼光谱是一种直接、无创、灵敏的光学探针，用于研究这些异质结构中的层间耦合、界面效应以及振动与载流子之间的相互作用。这项研究为设计使用堆叠原子层的新型电子和光电子器件提供了重要的指导。这些研究活动对北爱荷华大学的本科生产生了巨大的影响。首席研究员将研究课题整合到两门现有课程中。鼓励各级本科生，包括新生参加。首席研究员还利用部门与地区高中的联系，并与物理教育学院的同事和“经典向上跳跃”项目的主任合作，让地区高中的教师和学生参与进来。让高中生和本科生参与研究对于推进年轻人的STEM（科学、技术、工程和数学）教育非常重要。技术摘要：本研究利用层间声子（通常在低频）的拉曼光谱和光致发光来探索各种范德华异质结构的基本物理性质。研究项目包括：(1)开发基于层间声子的拉曼技术来表征异质结构超晶格的旋转角和云纹波长。本研究提供了一种简单的方法来表征云纹图案特征，而无需使用扫描隧道显微镜；（ii）探索复杂多层范德华异质结构的电子、光学和振动性质，以及嵌入不同中间层材料的二硫化钼双层的间接-直接间隙跃迁。该研究为设计使用堆叠范德华多层材料的新型电子和光电子器件提供了重要的指导；（iii）研究涉及狄拉克材料的范德华异质结构。特别是，研究小组研究了扭曲双层石墨烯在放置或夹在六方氮化硼之间后电子带结构的变化。本研究探讨了石墨烯在异质界面上的电子和振动特性的变化；（iv）探索双栅双扭曲双层石墨烯器件中载流子与层间声子的相互作用。本研究探讨了两个带电石墨烯层之间的库仑相互作用和门控形成的层间电位。在这些研究活动中建立的技术和方法可以应用于任何二维晶体以任何所需顺序堆叠形成的异质结构。

Non-technical abstract:Van der Waals heterostructures are a new class of materials that could lead to new novel electronic and optoelectronic devices. These materials are formed by vertically stacking atomic layers such as graphene, hexagonal boron nitride, and transition metal dichalcogenide (e.g. molybdenum disulfide) layers. The forces between these layers are relatively weak. However, they can dramatically change the properties of the system and induce phenomena that are absent in individual layers. By stacking or epitaxially growing different atomic layers on one another, one may design and fabricate artificial materials with unprecedented optical, electronic, and vibrational properties that cannot be achieved in natural crystals. Raman spectroscopy, which is a direct, noninvasive, and sensitive optical probe, is used to explore the interlayer coupling, the interface effect, and the interaction between vibrations and charge carriers in these heterostructures. This research provides crucial guidance in designing novel electronic and optoelectronic devices using stacked atomic layers. The research activities have a powerful impact on the undergraduate students at the University of Northern Iowa. The principal investigator integrates the research topics into two existing courses. Undergraduates at all levels, including freshman students, are encouraged to participate. The principal investigator also uses departmental contacts with area high schools and collaborates with Physics Education faculty colleagues and the director of the Classic Upward Bound program to involve teachers and students at regional high schools. Engaging high school students and undergraduates in research is very important in advancing STEM (Science, Technology, Engineering, and Mathematics) education in young people.Technical abstract:This research explores fundamental physical properties of various van der Waals heterostructures using Raman spectroscopy of interlayer phonons (typically at low frequencies) and photoluminescence. The research projects include: (i) Develop an interlayer-phonon-based Raman technique to characterize the rotational angle and Moire wavelength of heterostructure superlattices. This study offers a simple method of characterizing Moire pattern features without using scanning tunneling microscopy; (ii) Explore the electronic, optical, and vibrational properties of complex multilayer van der Waals heterostructures and the indirect-direct gap transition in MoS2 bilayers intercalated with different middle layer materials. This research provides crucial guidance in designing novel electronic and optoelectronic devices using stacked van der Waals multilayers; (iii) Study van der Waals heterostructures that involve Dirac materials. In particular, the research team investigates the change in the electronic band structure of twisted bilayer graphene after it is placed or sandwiched between hexagonal boron nitride. This study probes the change in the electronic and vibrational properties of graphene at the hetero-interface; (iv) Probe the interaction between carriers and interlayer phonons in twisted bilayer graphene devices with a dual-gate. This study explores the Coulomb interaction between two charged graphene layers and the interlayer potential formed upon gating. The techniques and methods established in these research activities can be applied to heterostructures formed from any two-dimensional crystals stacked in any desired sequence.