Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of new metamaterials. Here we demonstrate a room temperature ferroelectric semiconductor that is assembled using mono- or few-layer MoS2. These van der Waals heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of two 2D crystals, enables ferroelectric domains with alternating out-of-plane polarization arranged into a twist-controlled network. The last can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The observed interfacial charge transfer, movement of domain walls and their bending rigidity agree well with theoretical calculations. Furthermore, we demonstrate proof-of-principle field-effect transistors, where the channel resistance exhibits a pronounced hysteresis governed by pinning of ferroelectric domain walls. Our results show a potential avenue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.

The World Health Organization reported that 4.2 million deaths every year were a direct result of exposure to ambient air pollution (NO 2 , SO 2 , NH 3 , CO 2 , CO, CH 4 ). There is a well-demonstrated global need for high sensitivity, low cost and low energy consumption miniaturised gas sensors to be deployed in a dense network and to be used in an attempt to pinpoint and avoid high pollution hot spots. The high sensitivity of graphene to the local environment has shown to be highly advantageous in sensing applications, where ultralow concentrations of adsorbed molecules induce a significant response on the electronic properties of graphene. This is commonly attributed to the π electrons of graphene, being directly exposed to the surrounding environment. The unique electronic structure makes graphene the 'ultimate' sensing material for applications in environmental monitoring and air quality. In this review, we present the frontiers of graphene-based sensors considering both electrical and optical methods of detection and discuss the topical progress in an attempt to establish whether graphene can be considered as the ideal sensing material. We pay special attention to the optimization of the sensor performance, using various graphene hybrids and doping mechanisms. Furthermore, we present the recent developments in other 2D material-based sensors that have followed in the wake of graphene. We discuss the benchmarked parameters of graphene sensors, such as sensitivity, selectivity, response/recovery time and detection limit, and compare them with other 2D materials as well as existing state-of-the-art sensors currently being used in the field. We also perform a brief market analyses for the environmental sensing industry as well as provide a Strengths-Weaknesses-Opportunities-Threats analysis of graphene technology for environmental sensing.

The miniaturization of ferroelectric devices offers prospects for non-volatile memories, low-power electrical switches and emerging technologies beyond existing Si-based integrated circuits. An emerging class of ferroelectrics is based on van der Waals (vdW) two-dimensional materials with potential for nano-ferroelectrics. Here, we report on ferroelectric semiconductor junctions (FSJs) in which the ferroelectric vdW semiconductor α-In 2 Se 3 is embedded between two single-layer graphene electrodes. In these two-terminal devices, the ferroelectric polarization of the nanometre-thick In 2 Se 3 layer modulates the transmission of electrons across the graphene/In 2 Se 3 interface, leading to memristive effects that are controlled by applied voltages and/or by light. The underlying mechanisms of conduction are examined over a range of temperatures and under light excitation revealing thermionic injection, tunnelling and trap-assisted transport. These findings are relevant to future developments of FSJs whose geometry is well suited to miniaturization and low-power electronics, offering opportunities to expand functionalities of ferroelectrics by design of the vdW heterostructure.

The interest in two-dimensional and layered materials continues to expand, driven by the compelling properties of individual atomic layers that can be stacked and/or twisted into synthetic heterostructures. The plethora of electronic properties as well as the emergence of many different quasiparticles, including plasmons, polaritons, trions and excitons with large, tunable binding energies that all can be controlled and modulated through electrical means has given rise to many device applications. In addition, these materials exhibit both room-temperature spin and valley polarization, magnetism, superconductivity, piezoelectricity that are intricately dependent on the composition, crystal structure, stacking, twist angle, layer number and phases of these materials. Initial results on graphene exfoliated from single bulk crystals motivated the development of wide-area, high purity synthesis and heterojunctions with atomically clean interfaces. Now by opening this design space to new synthetic two-dimensional materials "beyond graphene", it is possible to explore uncharted opportunities in designing novel heterostructures for electrical tunable devices. To fully reveal the emerging functionalities and opportunities of these atomically thin materials in practical applications, this review highlights several representative and noteworthy research directions in the use of electrical means to tune these aforementioned physical and structural properties, with an emphasis on discussing major applications of beyond graphene 2D materials in tunable devices in the past few years and an outlook of what is to come in the next decade.