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

非技术摘要：货车瓦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器件。 这些材料通过垂直堆叠原子层（诸如石墨烯、六方氮化硼和过渡金属二硫属化物（例如二硫化钼）层）来形成。这些层之间的力相对较弱。 然而，它们可以显着改变系统的属性，并诱导个别层中不存在的现象。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、无损伤、灵敏的光学探针，它被用来研究这些异质结构中的层间耦合、界面效应以及振动和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器件提供了重要指导。研究活动对北方爱荷华州大学的本科生产生了强大的影响。首席研究员将研究课题整合到两个现有的课程中。鼓励包括大一学生在内的各级本科生参加。主要研究者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计划的主任合作，让地区高中的教师和学生参与进来。让高中生和大学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货车德瓦耳斯异质结构的基本物理性质。研究项目包括：（i）开发基于层间声子的拉曼技术来表征异质超晶格的旋转角和莫尔波长。这项研究提供了一个简单的方法，没有使用扫描隧道显微镜的莫尔图案的特征;（ii）探索复杂的多层货车德瓦尔斯异质结构的电子，光学和振动特性和插入不同的中间层材料的二硫化钼双层膜的间接直接的间隙转换。这项研究提供了至关重要的指导，在设计新颖的电子和光电器件使用堆叠货车的德瓦尔斯多层膜;（iii）研究货车的德瓦尔斯异质结构，涉及狄拉克材料。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能带结构的变化。本研究探讨了异质界面处石墨烯的电子和振动性质的变化;（iv）探讨了具有双栅极的扭曲双层石墨烯器件中载流子和层间声子之间的相互作用。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时形成的层间电位。在这些研究活动中建立的技术和方法可以应用于由以任何期望的顺序堆叠的任何二维晶体形成的异质结构。

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.