**Preface to "Graphene and other Two-dimensional Materials in Nanoelectronics and Optoelectronics"**

I started out as a Master's student working on LPE of GaAs for LEDs and lasers in 2000. It was already an old-fashioned material technology back then, and it was not easy to obtain good devices based on those GaAs. However, I did learn a truth: no good materials, no good devices. Soon after this, I had the opportunity to access more advanced epitaxial techniques (e.g., MBE and CVD) which fortified my belief that novel and decent materials are the ultimate limiting factor on the advancement of solid state electronics.

Graphene has been known to mankind for a long time, but it was not until 2004, when it was first isolated on SiO<sup>2</sup> and its field effect measured, that people understood its revolutionary role in nanoelectronics. Indeed, there had never been any other material simultaneously possessing such outstanding properties, including high mobility, high transparency, high thermal conductivity, high mechanical strength and flexibility. Thus, graphene was given the name "wonder material". Today, there are several ways to produce it, such as micromechanical exfoliation, chemical vapor deposition, etc. In the first article of this book, the authors introduce a new method—glow discharge—to synthesize graphene. Although glow discharge is an old concept, it is "new" here, in the sense that it is the first time it has been used to grow thin film graphene as opposed to flake graphene, which is suitable for device applications.

When I pursued my PhD, I switched from being a material scientist to being a device physicist and engineer, and I spent four years in front of an electron beam lithography machine fabricating InP-based nanoelectronic devices. There I understood another truth: any good material should be compatible with processing devices, otherwise it will be literally useless in applications. Regarding graphene, one of its most obvious advantages over other nanomaterials is that it is two-dimensional, which is compatible with existing semiconductor fabrication. Indeed, in the second paper of this book, the authors show that graphene can be integrated onto GaN wafers as the transparent electrode of microLEDs and the transistor channels of their drivers. This application has a great deal of potential. Graphene is transparent and flexible, which is suitable for transparent and flexible microLED-based displays (with sapphire substrate removed). Nevertheless, graphene processing today also has a bottleneck: the difficulty of transfer. Graphene is typically grown by CVD on a foreign substrate and needs to be mechanically transferred. Holes, wrinkles, and contaminations are inevitable. The third paper offers a solution: to grow graphene directly on GaN and use it in situ as the transparent electrodes for LEDs. It is challenging to obtain high crystalline quality for this direct growth method, but it seems we have to live with that—unless we can accept graphene transfer, which we cannot for real-world applications.

Papers four and five in this book show it is quite possible to go in the opposite direction. That is, to prepare nitride semiconductor materials/devices on graphene and other 2D materials. This is a promising new direction, where 2D materials could potentially serve as cheap/flexible substrates for nitrides. It is worth mentioning that WS<sup>2</sup> has a bandgap, meaning it is a real semiconductor. Of course, it has an advantage, rendering it superior to graphene for fabrication of many electronic devices. If Si, GaAs, and GaN represent first-, second-, and third- generation semiconductors, respectively, I would argue that 2D semiconductors may constitute the fourth generation.

Finally, the thermal aspects of 2D materials are relatively overlooked; namely, they seem to have received less attention than the optical/electrical properties. However, thermal management is vital in today's electronics, as integrated electron devices are increasingly densely packed. Indeed, graphene has been suggested as a heat spreader in electronic chips. Paper six, however, demonstrates the opposite application—graphene can be used as a transparent heater, where a convective heat-transfer coefficient of 60 W/(◦Cm<sup>2</sup> ) is achieved, better than many other heaters. Of course, this is based on the unique thermal properties of graphene and other 2D materials. The seventh paper reviews a resistance thermometer for evaluating the thermal properties of low-dimensional materials, which might be useful for cooling electronic chips.

The authors of these articles are students and professors working actively on semiconductors and 2D materials and devices. Some of these are experts I have already known for two decades. I am impressed by their work and scientific attitude, and I am grateful to them for making the publication of this book possible. An old saying goes: "cast a brick to attract jade". Likewise, I hope this book will inspire new and innovative ideas in the fascinating field of 2D-materials-based nanoelectronics and optoelectronics.

> **Jie Sun** *Special Issue Editor*
