Preface to the Special Topic on 2D Materials and Devices

The great research progresses in materials sciences in the past decades have revealed that reduced dimension can have significant influences on the properties of materials.Since the successful fabrication of graphene in 2004 by Novoselov and Geim,two-dimensional (2D) crystals have attracted great attention because of their novel properties and great application potentials.     

Preface to the Special Topic on Compound Semiconductor Materials and Devices on Si

The research in silicon photonics has been booming due to its potential for lowcost,reliable,energy-efficient and high-density chip-wise integration using widely available CMOS technology,featuring the tremendous success in modulator,detector and other passive waveguide components in industry.However,the absence of efficient and reliable electrical to optical converter on Si platform has been considered as“the last piece of the puzzle”,hindered by the in-direct bandgap property of Si bulk materials.CompoundⅢ–Ⅴsemiconductor devices offer highly efficient optical light emitting sources and optical amplifiers,hence the compound semiconductor materials and devices on Si platform are drawing more and more attention nowadays as it could make possible the long-dreamed light sources on Si substrates by combining their advantages with silicon ICs,enabling the fabrication of full functional optoelectronic circuits,chip-to-chip and even system-to-system optical chips.     

Preface to the Special Topic on Perovskite Solar Cells

Halide perovskite materials were discovered about one hundred years ago,but the application of this material in solar cells only can be traced back to 2009,when the first perovskite solar cells (PSCs) were demonstrated by Miyasaka et al.in a dye sensitized solar cell configuration showing 3.8％ power conversion efficiency.After seven years of intensive efforts,including controlling the perovskite film crystal growth,perovskite/transport layers interfaces and device architecture,the laboratory efficiency of perovskite solar cells has reached 22.1％ by Seok et al.from South Korea in early 2016.     

Preface to Special Topic on Twisted van der Waals Heterostructures

<正>Twisted van der Waals heterostructures are becoming the building blocks for engineering new device structures, in which their electronic, optical and mechanical properties can be tuned by changing the “twist” angle between layers of 2D materials. Such twisted 2D heterostructures offer a unique opportunity to create a new field of “twistronics” by mechanically stacking different 2D van der Waals materials together.     

Preface to the Special Issue on Si-Based Materials and Devices

It has been well known that the development of microelectronic and integrated circuit (IC), mainly based on silicon materials, have changed the way of our life dramatically and accelerated the development and innovation of new technologies. With the increase of integration density in ICs, the gate lengths of transistors are now scaled down to 7 nm, leading to fundamental challenges to keep up with the Moore’s law. One possible solution is to integrate optical circuits into the Si microelectronic platform to achieve high density electronic–photonic integration. This will combine the advantages of photons in energy-efficient and high bandwidth data transmission with those of electrons in high-speed data processing and high performance logics. It will thereby overcome the interconnect bottleneck and achieve high functionality as an extension to Moore’s law. In this opto– electro integration system, monolithic integrated group IV lasers are of great interest. However, the indirect bandgap nature of crystalline silicon limits its application in optoelectronics. In recent years, many researchers have tried to use the nano-sized Si structure or introduce group IV alloys, such as Ge, GeSn or other compounds, to address the light source issue and establish all Si-based electronic–photonic ICs.     

Preface to the Special Topic on Devices and Circuits for Wearable and IoT Systems

The concept of Internet of Things (IoT) was first proposed by MIT Prof.Kevin Ash-ton in 1999.The implementation of IoT was mainly through RFID in its early time.With advanced technology and manufacture,diverse implementation forms ofIoT are becoming possible.Wearable devices,as an essential branch of IoT,will have broad application prospects in health monitoring and intelligent healthcare.     

Preface to the Special Issue on Flexible and Wearable Electronics: from Materials to Applications

Electronic systems that can cover large areas on flexible/stretchable substrates have received increasing attention in the past several years because they enable new classes of applications that lie outside those easily addressed with wafer-based microelectronics.Some attractive examples include flexible displays,flexible solar cells,electronic textiles,sensory skins,detectors,active antennas,etc.The field expends very fast and great developments have been obtained during the past several years.Recent exciting progresses such as highly flexible/stretchable E-skins,large area flexible displays,highly sensitive sensors,wearable electronic devices suggest that there are still a lot of room needs to be exploited including materials,devices and systems.     

Preface to the Special Issue on Ultra-Wide Bandgap Semiconductor Gallium Oxide: from Materials to Devices

As one of the ultra-wide bandgap {UWBG)semiconducting materials,gallium oxide has attractive properties with a wide bandgap of about 4.8 eV and a high breakdown field of about 8 MWcm,which offers an alternative platform for various applications such as high performance power switches,RF amplifiers,solar blind photodetectors,and harsh environment signal processing.     

Preface to the Special Issue on Flexible Materials and Structures for Bioengineering,Sensing,and Energy Applications

Human body with curved and soft interfaces requests advanced flexible materials and structures for the interaction with organs and signal collection from targets in applications such as bioengineering and diagnostic devices.Among them,it is highly demanded to achieve creative design in flexible materials and structures with great stretchable capability for required applications.To this end,both inorganic and organic materials could be adopted and designed with assembly and self-assembly methods for flexible electronics and electrodes.Soft or flexible materials and structures inspired by nature can lead to highly conformal contacts between devices and the human body.These approaches hold great potential for applications in flexible electronics,medical imaging technology and portable disease diagnostics.Novel strategy on related sensors/actuator and energy storage/generation devices could overcome certain limitations on flexible materials engineering and thus advance the field as well.All these methods would deliver a profound impact to our future intelligent society.     

Preface to the Special Issue on Flexible Energy Devices

Flexible energy devices are the building blocks for next-generation wearable electronics.Flexible energy devices are expected to have multiple functions,such as energy conver-sion from light to electricity and vice versa,energy genera-tion from triboelectric,energy storage and so on.These func-tions can be efficiently realized by solar cells,light-emitting di-odes (LEDs),triboelectric nanogenerators (TENG),batteries and supercapacitors,etc.The flexible energy devices can be in-tegrated into flexible,wearable,and/or portable platforms to enable wide application prospects in the fields of informa-tion,energy,medical care,national defense,etc.However,flex-ible energy devices face more challenges when compared to their rigid counterparts,which requires more breakthroughs and research efforts on fabrication techniques,materials innov-ation,novel structure designs,and deep physical understand-ings.     

Preface to Special Issue on Novel Semiconductor-biochemical Sensors

<正>Since 2020, the global outbreak and continued evolution of the COVID-19 pandemic have brought the concepts of nucleic acid, antigen-antibody, gene sequencing, and other biochemical testing into ordinary families. In this regard, novel semiconductor-biochemical sensors that convert biochemical information into monitorable electrical and optical signals according to specific rules have become increasingly important and indispensable. These sensors deeply fuse the technical advantages of s...