The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here, we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.

The world is recently witnessing an explosive development of novel electronic and optoelectronic devices that demand more-reliable power sources that combine higher energy density and longer-term durability. Supercapacitors have become one of the most promising energy-storage systems, as they present multifold advantages of high power density, fast charging-discharging, and long cyclic stability. However, the intrinsically low energy density inherent to traditional supercapacitors severely limits their widespread applications, triggering researchers to explore new types of supercapacitors with improved performance. Asymmetric supercapacitors (ASCs) assembled using two dissimilar electrode materials offer a distinct advantage of wide operational voltage window, and thereby significantly enhance the energy density. Recent progress made in the field of ASCs is critically reviewed, with the main focus on an extensive survey of the materials developed for ASC electrodes, as well as covering the progress made in the fabrication of ASC devices over the last few decades. Current challenges and a future outlook of the field of ASCs are also discussed.

Two-dimensional (2D), layered transition metal dichalcogenides (TMDCs) can grow in two different growth directions, that is, horizontal and vertical. In the horizontal growth, 2D TMDC layers grow in planar direction with their basal planes parallel to growth substrates. In the vertical growth, 2D TMDC layers grow standing upright on growth substrates exposing their edge sites rather than their basal planes. The two distinct morphologies present unique materials properties suitable for specific applications, such as horizontal TMDCs for optoelectronics and vertical TMDCs for electrochemical reactions. Precise control of the growth orientation is essential for realizing the true potential of these 2D materials for large-scale, practical applications. In this Letter, we investigate the transition of vertical-to-horizontal growth directions in 2D molybdenum (or tungsten) disulfide and study the underlying growth mechanisms and parameters that dictate such transition. We reveal that the thickness of metal seed layers plays a critical role in determining their growth directions. With thick (>∼ 3 nm) seed layers, the vertical growth is dominant, while the horizontal growth occurs with thinner seed layers. This finding enables the synthesis of novel 2D TMDC heterostructures with anisotropic layer orientations and transport properties. The present study paves a way for developing a new class of 2D TMDCs with unconventional materials properties.

The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.

Supercapacitors represent a major technology to store energy for many applications including electronics, automobiles, military, and space. Despite their high power density, the energy density in supercapacitors is presently inferior to that of the state-ofthe-art Li-ion batteries owing to the limited electrochemical performance exhibited by the conventional electrode materials. The advent of two-dimensional (2D) nanomaterials has spurred enormous research interest as supercapacitor electrode materials due to their fascinating electrochemical and mechanical properties. This Review discusses cutting-edge research on some of the key 2D supercapacitor electrode materials including transition metal dichalcogenides, transition metal oxides and hydroxides, MXenes, and phosphorene. Various synthetic approaches, novel electrode designs, and microstructure tuning of these 2D materials for achieving high energy and power densities are discussed.