Electronic and optical properties of hydrogenated group-IV multilayer materials.

Hydrogenated group-IV layered materials are semiconducting forms of silicene, germanene and stanene. We systematically studied the evolution of the structural, electronic and optical properties of these 2D materials as a function of the number of layers. We verify that the exfoliation energy increases upon the increase of the atomic number (Si → Sn) of the group-IV material. We show that silicane, independent of the number of layers, is an indirect band gap (Γ-M) material. This behavior is different from both germanane and stanane, which are direct band gap (Γ point) semiconductors. The calculated optical spectra show, for all systems, a red shift in the absorption edges and an enhanced absorption of the visible light for the in-plane (α‖) component upon the increase in the number of layers and, also as a function of the increasing atomic number. Our findings also indicate that: (i) (XH2)m(YH2)n vdW heterostructures will always present a type-I band alignment for X = Si and Y = Ge or Sn, whereas (ii) for X = Ge and Y = Sn, the band alignment can be tuned (type-I ↔ type-II) by the number of layers (m,n).

Nanodots of transition metal dichalcogenides embedded in MoS2 and MoSe2: first-principles calculations.

The energetic stability and the electronic properties of nanodots (NDs) composed of transition metal dichalcogenides, XS2 and XSe2 (with X = Mo, W and Nb) embedded in single layer MoS2 and MoSe2 hosts, were investigated based on first-principles calculations. We find that through a suitable combination of the ND and host materials it is possible to control the electron-hole localization. For instance, in NDs of WS2 in the MoS2 host we find the highest occupied (hole) states localized in the ND region, while the lowest unoccupied (electron) states spread out in the MoS2 host. On the other hand, by changing the ND and host materials, the electron states become localized in the MoS2 ND in the WS2 host. Further electronic structure calculations show that the NDs of NbS2 and NbSe2 give rise to a set of spin degenerate empty states within the energy gap of the MoS2 and MoSe2 hosts. The spin degeneracy can be removed by negatively charging the ND system. Such n-type doping was examined by considering a van der Waals (vdW) heterostructure composed of a graphene layer lying on the NbS2 and NbSe2 NDs. Indeed we found a net magnetic moment localized in the ND region.

Directional dependence of the electronic and transport properties of 2D borophene and borophane.

Very recently two dimensional layers of boron atoms, so called borophene, have been successfully synthesized. It presents a metallic band structure, with a strong anisotropic character. Upon further hydrogen adsorption a new material is obtained, borophane; giving rise to a Dirac cone structure like the one in graphene. We have performed a first-principles study of the electronic and transport properties of borophene and borophane through the Landauer-Büttiker formalism. We find that borophene presents an electronic current two orders of magnitude larger than borophane. In addition we verified the direction dependence of the electronic current in two perpendicular directions, namely, Ix and Iy; where for both systems, we found a current ratio, η = Ix/Iy, of around 2. Aiming to control such a current anisotropy, η, we performed a study of its dependence with respect to an external strain. Where, by stretching the borophane sheet, η increases by 11% for a bias voltage of 50 mV.