Step-by-step monitoring of CVD-graphene during wet transfer by Raman spectroscopy

Transfer of 2D Films: From Imperfection to Perfection

Atomically thin 2D films and their van der Waals heterostructures have demonstrated immense potential for breakthroughs and innovations in science and technology. Integrating 2D films into electronics and optoelectronics devices and their applications in electronics and optoelectronics can lead to improve device efficiencies and tunability. Consequently, there has been steady progress in large-area 2D films for both front- and back-end technologies, with a keen interest in optimizing different growth and synthetic techniques. Parallelly, a significant amount of attention has been directed toward efficient transfer techniques of 2D films on different substrates. Current methods for synthesizing 2D films often involve high-temperature synthesis, precursors, and growth stimulants with highly chemical reactivity. This limitation hinders the widespread applications of 2D films. As a result, reports concerning transfer strategies of 2D films from bare substrates to target substrates have proliferated, showcasing varying degrees of cleanliness, surface damage, and material uniformity. This review aims to evaluate, discuss, and provide an overview of the most advanced transfer methods to date, encompassing wet, dry, and quasi-dry transfer methods. The processes, mechanisms, and pros and cons of each transfer method are critically summarized. Furthermore, we discuss the feasibility of these 2D film transfer methods, concerning their applications in devices and various technology platforms.

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining

Supreme mechanical performance and tribological properties render graphene a promising candidate as a surface friction modifier. Recently, it has been demonstrated that applying in-plane strain can effectively tune friction of suspended graphene in a reversible manner. However, since graphene is deposited on solid surfaces in most tribological applications, whether such operation will result in a similar modulation effect becomes a critical question to be answered. Herein, by depositing graphene onto a stretchable substrate, the frictional characteristics of supported graphene under a wide range of strain are examined with an in situ tensile loading platform. The experimental results show that friction of supported graphene decreases with increasing graphene strain, similar to the suspended system. However, depending on the adherence state of the graphene/substrate interface, the system exhibits two distinct friction regimes with significantly different strain dependences. Assisted by detailed atomic force microscopy imaging, we attribute the unique behavior to the transition between two friction modulation modes, i.e., contact-quality-dominated friction and puckering-dominated friction. This work provides a more comprehensive view of the influence of strain on surface friction of graphene, which is beneficial for active modulation of graphene friction through strain engineering.