Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of the band gap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electromechanical device that can apply biaxial compressive strain to trilayer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct band gap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct band gap and reveal a higher tunability of the indirect band gap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.

Developments in semiconductor technology are propelling the dimensions of devices down to 10 nm, but facing great challenges in manufacture at the sub-10 nm scale. Nanotechnology can fabricate nanoribbons from two-dimensional atomic crystals, such as graphene, with widths below the 10 nm threshold, but their geometries and properties have been hard to control at this scale. Here we find that robust ultrafine molybdenum-sulfide ribbons with a uniform width of 0.35 nm can be widely formed between holes created in a MoS2 sheet under electron irradiation. In situ high-resolution transmission electron microscope characterization, combined with first-principles calculations, identifies the sub-1 nm ribbon as a Mo5S4 crystal derived from MoS2, through a spontaneous phase transition. Further first-principles investigations show that the Mo5S4 ribbon has a band gap of 0.77 eV, a Young’s modulus of 300GPa and can demonstrate 9% tensile strain before fracture. The results show a novel top–down route for controllable fabrication of functional building blocks for sub-nanometre electronics.

No abstract

We report a two-dimensional tetragonal Titanium Carbide (TiC) monolayer sheet with distinguished structure and properties based on comprehensive first-principles calculations. The TiC sheet exhibits a novel zigzag-shaped buckling structure with all atoms being quasiplanar tetracoordinate, as favored by strong in-plane C2p-Ti3d bonding and synergetic out-of-plane electronic delocalization. This unique structure endows the sheet with high kinetic stability and anisotropic mechanical properties. Moreover, the TiC sheet displays orientation-dependent electronic properties derived from its special rectangular symmetry, with indirect band gap of ~0.2 eV and substantial ferromagnetism along its edges, thus promising for wide applications in nanoelectronics.

(1 of 22)allotropes of boron have been discovered up to now. Four of them are thermodynamically stable, including α-rhombohedral, [6] β-rhombohedral, [7] γ-orthorhombic, [1f,2d,8] and β-tetragonal boron crystals. [1d,9] These materials are often called boron-icosahedral cluster solids (B-ICSs), [5b,10] consisting of icosahedral closo-cluster B 12 to link with each other or with other clusters to form boron allotropes and B-rich compounds, such as various types of BN, [11] B 12 As 2 , [12] B 12 P 2 , [13] B 12 O 2 , [14] YB 66 , [15] AlB 12 , [16] and boron carbide. [17] Apart from three-dimensional (3D) boron icosahedral solids, 2D boron also exhibits many unique structures owning to its electron deficiency, different from other well-known 2D materials. Generally, 2D boron crystals can be classified into three categories: 1) graphene-like atomically monolayered boron sheets, [18] 2) 2D boron structures with thickness of a single or a few unit cells, [19] and 3) new kinds of 2D boron structures reported recently. [20] For the first category, "borophene" was coined to refer to a general class of atomically thin boron sheets, [21] unlike other structures with the suffix ene, where each name always corresponds to a certain structure. For example, graphene, as the most attractive 2D crystals, is a single monolayer of carbon atoms, while phosphorene (monolayered black phosphorus) exists with puckered layer structures in nature. Both graphene and phosphorene have corresponding bulk counterparts, allowing for facile access to 2D-layered van der Waals structures through mechanical exfoliations. Other elemental 2D materials, such as silicene, [22] germanene, [23] stanene, [24] arsenene, [25] and antimonene, [26] actually do not have layered bulk counterparts. Similarly, 2D single-layered boron could not be produced by exfoliating from its bulk materials because there is no layered bulk boron. As a result, it is suggested that 2D boron sheets can be synthesized via chemical vapor deposition, thermal evaporation deposition, or molecular beam epitaxy. In the past decade, extensive theoretical efforts have been paid to investigate possible boron sheets, many of which have been predicted to have potential applications in electronic devices, [18c,d] photoelectric devices, [27] superconductivity, [20a,28] field-emission (FE) materials, [29] hydrogen storage media, [30] and lithium-ion batteries. [31] However, the fabrication of 2D boron crystals is a great challenge. Until very recently, three types of monolayered Boron, as a unique element nearest to carbon in the periodic table, has been predicted to form many distinctive two-dimensional (2D) structures that significantly differ from other well-studied 2D materials, owning to its exceptional ability to form strong covalent two-center-two-electron bonds as well as stable electron-deficient multi-center-two-electron bonds. Until recently, the successful syntheses of atomically thin crystalline 2D boron sheets (i.e., borophenes) provoked growing passion in 2D boron crysta...

Hexagonal boron nitride (h‐BN), a wide band gap monolayer crystal with structure similar to graphene, is optically transparent with exceptionally high thermal and chemical stability, and should be ideal to serve as an atomically thin coating. However, limited by the challenges in fabricating h‐BN of high quality in large area, the wetting performance of h‐BN has seldom been studied. Here, it is shown that the water contact angle of freshly grown h‐BN film is nearly independent of the underlying materials as well as the h‐BN layer number, but increases gradually to a saturated stable value in air due to the spontaneous adsorption of airborne hydrocarbon. First‐principles calculations and molecular interaction modeling confirm that a monolayer h‐BN coating does efficiently tune the interaction of a water molecule with different substrates to a converging level. The saturated wettability of h‐BN coating is robust against variation of several factors, facilitating its practical applications.

No abstract

Graphene with periodically patterned antidots has attracted intense research attention as it represents a facile route to open a bandgap for graphene electronics. However, not all graphene antidot lattices (GALs) can open a bandgap and a guiding rule is missing. Here, through systematic first-principles calculations, it is found that bandgaps in triangular GALs are surprisingly well defined by a chirality vector R = n a1 + ma2 connecting two neighboring antidots, where a1 and a2 are the basis vectors of graphene. The bandgap opens in the GALs with (n-m)mod3 = 0 but remains closed in those with (n-m)mod3 = ±1, reminiscent of the gap-chirality rule in carbon nanotubes. Remarkably, the gap value in GALs allows ample modulation by adjusting the length of chirality vectors, shape and size of the antidots. The gap-chirality relation in GALs stems from the chirality-dependent atomic structures of GALs as revealed by a super-atom model as well as Clar sextet analyses. This chirality-dependent bandgap is further shown to be a generic behavior in any parallelogram GAL and thus serves as an essential stepping stone for experimenters to realize graphene devices by antidot engineering.