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.

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 μm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

A simple and inexpensive method to functionalize hexagonal boron nitride (hBN) was achieved by using an acid mixture of phosphoric and sulphuric acid. This functionalization induced the exfoliation of the layered structure of hBN into monolayer to few-layer sheets where the sizes of the sheets were dependent on the parent hBN powder used. Exfoliated hBN was shown to be stable in solvents such as ethanol, acetone, deionized water and isopropyl alcohol, and this stability was linked to sulfur functionalization that was induced during the exfoliation process. Further evidence of the functionalization was observed using transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS). By deconvoluting the high resolution peaks for B 1s, the bonding of boron to oxygen and sulfur was confirmed. The exfoliated hBN nanosheets were crystalline as confirmed from X-ray diffraction and they also exhibited an optically active defect related to sulfur functionalization at 320 nm (3.9 ± 0.1 eV).

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Layered materials, especially the transition metal dichalcogenides (TMDs), are of interest for a broad range of applications. Among the class of TMDs, molybdenum disulfide (MoS2) is perhaps the most studied because of its natural abundance and use in optoelectronics, energy storage and energy conversion applications. Understanding the fundamental structure-property relations is key for tailoring the enhancement in the above-mentioned applications. Here, we report a controlled powder vaporization synthesis of MoS2 flower-like structures consisting of vertically grown layers of MoS2 exhibiting exposed edges. This growth is readily achievable on multiple substrates, such as graphite, silicon, and silicon dioxide. The resulting MoS2 flowers are highly crystalline and stoichiometric. Further observations using contact angle indicate that MoS2 flowers exhibit the highest reported contact angle of ∼160 ± 10°, making the material super hydrophobic. This surface wettability was further tuned by changing the edge chemistry of the MoS2 flowers using an ozone etching treatment. Hydrogen evolution reaction (HER) measurements indicate that the surface treated with UV-ozone showed a reduction in the Tafel slope from 185 to 54 mV/dec, suggesting an increase in the amount of reactive surface to generate hydrogen.