Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particleswarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable 1T-MoS 2 -like ( α-Te), and metastable tetragonal (β-Te) and 2H-MoS 2 -like (γ-Te) structures. The underlying formation mechanism of such tri-layer arrangements is uniquely rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductor-like (e.g., S). In particular, the α-Te phase can be spontaneously obtained from the magic thicknesses truncated along the [001] direction of the trigonal structure of bulk Te. Furthermore, both the α-and β-Te phases possess electron and hole mobilities much higher than MoS 2 , as well as salient optical absorption properties. These findings effectively extend the realm of 2D materials to group-VI monolayers, and provide a new and generic formation mechanism for designing 2D materials. The two-dimensional (2D) materials have been intensively investigated in recent years for their intriguingly emergent properties that can be exploited for electronic, photonic, spintronic, and catalytic device applications [1][2][3][4][5][6][7][8][9][10]. Various 2D monolayers have been synthetized beyond the first member system of graphene [1][2][3], including the group-IV monolayers of silicene [4] and stanene [8], the group-V monolayer of phosphorene [5], and the group-III monolayer of borophene [6,7]. Besides these group-III, -IV, and -V elemental monolayers, transition metal dichalcogenides (TMDCs) have also been attracted much attention because of their relatively wider, tunable, and direct band gaps and inherently stronger spin-orbit coupling [9,10]. Yet to date, somewhat surprisingly, no prediction or fabrication of group-VI elemental monolayers has been made, whose potential existence would not only further enrich our understanding of the realm of the 2D materials world, but could also offer new application potentials stemming from their uniquely physical and chemical properties.In this Letter, we add an attractive new category to the ever increasing 2D materials family by predicting the existence and fabrication of group-VI elemental monolayers centered on the metalloid element Te. Our theoretical calculations reveal that 2D monolayers of Te, named tellurene, can exist in the stable 1T-MoS 2 -like ( α-Te) structure, and metastable tetragonal (β-Te) and 2H-MoS 2 -like (γ-Te) structures. These tri-layer arrangements are driven by the unique multivalency nature of Te, with the central-layer Te behaving more metal-like, and the two outer layers more semiconductor-like. In particular, the monolayer and multilayers of α-Te can be readily obtained via a thickness-dependent structural phase tr...

Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene, whereas for thicker films, the cell size is more consistent with that of the [101[combining macron]0] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.

Monolayer (ML) transition metal dichalcogenides (TMDs) are of great research interest due to their potential use in ultrathin electronic and optoelectronic applications. They show promise in new concept devices in spintronics and valleytronics. Here we present a growth study by molecular-beam epitaxy of ML and sub-ML MoSe 2 , an important member of TMDs, revealing its unique growth characteristics as well as the formation processes of domain boundary (DB) defects. A dramatic effect of growth temperature and post-growth annealing on DB formation is uncovered.

The extraordinary electronic structures of monolayer transition metal dichalcogenides, such as the spin–valley-coupled band edges, have sparked great interest for potential spintronic and valleytronic applications based on these two-dimensional materials. In this work, we report the experimental observation of quasi-particle interference patterns in monolayer WSe2 using low-temperature scanning tunnelling spectroscopy. We observe intervalley quantum interference involving the Q valleys in the conduction band due to spin-conserving scattering processes, while spin-flipping intervalley scattering is absent. Our results establish unequivocally the presence of spin–valley coupling and affirm the large spin splitting at the Q valleys. Importantly, the inefficient spin-flipping scattering implies long valley and spin lifetime in monolayer WSe2, which is a key figure of merit for valley-spintronic applications.