[论文解读] Dry release transfer of graphene and few-layer h-BN by utilizing thermoplasticity of polypropylene carbonate for fabricating edge-contact-free van der Waals heterostructures

Kei Kinoshita, Rai Moriya+6 more|arXiv (Cornell University)|Apr 27, 2019

Graphene research and applications参考文献 44被引用 23

一句话总结

本文提出了一种利用聚碳酸丙烯酯（PPC）薄膜进行石墨烯和少层六方氮化硼（h-BN）的干法转移方法，利用PPC的热塑性实现低污染、无边缘接触的范德华异质结构。该方法可实现高质量异质结构的制备，聚合物残留极少，已通过封装的h-BN/石墨烯/h-BN和垂直隧穿器件得到验证。

ABSTRACT

The dry release transfer of two-dimensional (2D) materials such as graphene, h-BN, and TMDs is a versatile method for fabricating high-quality van der Waals heterostructures. Up until now, polydimethylpolysiloxane (PDMS) sheets have been widely used for the dry release transfer of TMD materials. However, this method has been known to have limitations that make it difficult to transfer few-layer-thick graphene and h-BN because of the difficulty to fabricate these materials on PDMS. As an alternative method, we demonstrate the dry release transfer of single- and bi-layer graphene and few-layer h-BN in this study by utilizing poly(propylene) carbonate (PPC) films. Because of the strong adhesion between PPC and 2D materials around room temperature, we demonstrate that single- to few-layer graphene, as well as few-layer h-BN, can be fabricated on a spin-coated PPC film/290-nm-thick SiO2/Si substrate via the mechanical exfoliation method. In addition, we show that these few-layer crystals are clearly distinguishable using an optical microscope with the help of optical interference. Because of the thermoplastic properties of PPC film, the adhesion force between the 2D materials and PPC significantly decreases at about 70 °C. Therefore, we demonstrate that single- to few-layer graphene, as well as few-layer h-BN flakes, on PPC can be easily dry-transferred onto another h-BN substrate. This method enables a multilayer van der Waals heterostructure to be constructed with a minimum amount of polymer contamination. We demonstrate the fabrication of encapsulated h-BN/graphene/h-BN devices and graphene/few-layer h-BN/graphene vertical-tunnel-junction devices using this method. Since devices fabricated by this method do not require an edge-contact scheme, our finding provide a simples method for constructing high-quality graphene and h-BN-based van der Waals heterostructures.

研究动机与目标

• 克服传统基于PDMS的干法转移方法在转移少层石墨烯和h-BN时存在的局限性。
• 开发一种可最大限度减少二维材料异质结构中聚合物污染的转移技术。
• 实现无边缘接触的器件制备，以提升范德华异质结构中的电学性能。
• 利用聚碳酸丙烯酯（PPC）的热塑性特性，实现二维材料的可逆粘附与洁净释放。
• 实现高质量、封装型异质结构，适用于先进电子器件。

提出的方法

• 在290 nm的SiO2/Si基底上旋涂聚碳酸丙烯酯（PPC）薄膜作为转移介质。
• 通过机械转移法将单层及少层石墨烯和h-BN直接机械剥离至PPC/SiO2/Si基底上。
• PPC在室温下对二维材料表现出强粘附性，可形成稳定的转移薄膜。
• 在约70 °C时，PPC的热塑性使其粘附性显著降低，从而实现二维薄片向目标基底的轻松干法释放。
• 该方法可实现将二维材料直接转移至另一片h-BN基底上，无需边缘接触结构。
• 在环境条件下通过光学干涉对比可清晰识别PPC上的少层h-BN和石墨烯。

实验结果

研究问题

• RQ1 聚碳酸丙烯酯（PPC）能否作为少层石墨烯和h-BN的有效干法转移介质？
• RQ2 PPC在约70 °C时的热塑性转变是否能实现二维材料的洁净、低污染释放？
• RQ3 能否利用这种基于PPC的转移方法实现无边缘接触的范德华异质结构？
• RQ4 与传统的PDMS基转移方法相比，该方法在减少聚合物污染方面能达到何种程度？
• RQ5 能否实现高质量、封装型异质结构，如h-BN/石墨烯/h-BN和垂直隧穿器件？

主要发现

• 使用PPC薄膜成功转移了单层至少层石墨烯和少层h-BN，且聚合物残留极少。
• 由于PPC在约70 °C时表现出热塑性行为，其与二维材料之间的粘附性显著降低，从而实现了洁净释放。
• 通过光学显微镜结合干涉对比，可清晰识别PPC上的少层h-BN和石墨烯。
• 无需边缘接触结构即可制备封装的h-BN/石墨烯/h-BN异质结构，证实了高质量的集成性能。
• 利用该方法成功演示了石墨烯/少层h-BN/石墨烯垂直隧穿结器件。
• 该方法可实现多层范德华异质结构的制备，污染极少，显著提升了器件性能潜力。