Covalent functionalization of germanene employing computational simulations

Computational simulations through density functional theory in conjunction with M06-L and HSE functional have been carried out to investigate the chemical reactivity of the germanene monolayer. It is exceptionally reactive, with an average reaction energy of -60.4 kcal mol-1 for the nineteen functional groups considered: H, F, Cl, Br, O, S, Se, Ge, OH, SH, CH3, CF3, NH, NH2, C6H5, C6H4, CCl2, CBr2, and the azomethine ylide. The results indicate that oxygen is the most reactive reagent (-96.9 kcal mol-1), followed by fluorine (-83.1 kcal mol-1). Germanene presents a rich organic chemistry, and functionalization with azomethine ylides, benzynes, and carbenes can be easily accomplished as indicated by the reaction energies computed, which lie between -45 and -65 kcal mol-1. Furthermore, germanene is significantly more reactive than graphene and hexagonal boron nitride monolayers since the reaction energy for germanene is more than 40 kcal mol-1 lower. Although, in general, germanene is slightly more reactive than black and blue phosphorene and less prone to oxidation, but its oxidation when exposed to air occurs spontaneously. The addition of functional groups works cooperatively. The reaction energies become lower as the number of functional groups increases, thus favouring the agglomeration of functional groups attached unless the steric effect alters this pattern. Finally, we analyzed the electronic properties of functionalized germanene. It is possible to fine-tune the band gap of germanene from 0.1 to 2 eV using different functional groups and coverages. For O-50% and S-50% functionalized germanene, we found that carrier recombination is the most difficult due to the considerable differences between the effective masses of holes and electrons, which is promising for optical applications.

Novel graphene-like two-dimensional bilayer germanene dioxide: electronic structure and optical properties

Using ab initio calculations, we present a two-dimensional (2D) α-2D-germanene dioxide material with an ideal sp3 bonding network which possesses a large band gap up to 2.50 eV. The phonon dispersion curves and molecular dynamics (MD) simulation under the chosen parameters suggest that the novel 2D structure is stable. The dielectric function and absorption spectrum also show the consistent band gap within the electronic structure diagram, suggesting possible application as an ultraviolet light optical detector. The calculated carrier mobility of 4.09 × 103 cm2 V-1 s-1 can be observed along the x direction, which is much higher than that of MoS2 (∼3.0 cm2 V-1 s-1). Finally, we found that α-2D-germanene dioxide could potentially act as an ideal monolayer insulator in so-called van der Waals (vdW) heterostructure devices. These findings expand the potential applications of the emerging field of 2D α-2D-germanene dioxide materials in nanoelectronics.

Germanene on single-layer ZnSe substrate: novel electronic and optical properties

In this work, the structural, electronic and optical properties of germanene and ZnSe substrate nanocomposites have been investigated using first-principles calculations. We found that the large direct-gap ZnSe semiconductors and zero-gap germanene form a typical orbital hybridization heterostructure with a strong binding energy, which shows a moderate direct band gap of 0.503 eV in the most stable pattern. Furthermore, the heterostructure undergoes semiconductor-to-metal band gap transition when subjected to external out-of-plane electric field. We also found that applying external strain and compressing the interlayer distance are two simple ways of tuning the electronic structure. An unexpected indirect-direct band gap transition is also observed in the AAII pattern via adjusting the interlayer distance. Quite interestingly, the calculated results exhibit that the germanene/ZnSe heterobilayer structure has perfect optical absorption in the solar spectrum as well as the infrared and UV light zones, which is superior to that of the individual ZnSe substrate and germanene. The staggered interfacial gap and tunability of the energy band structure via interlayer distance and external electric field and strain thus make the germanene/ZnSe heterostructure a promising candidate for field effect transistors (FETs) and nanoelectronic applications.

Tunable Electronic and Topological Properties of Germanene by Functional Group Modification

Electronic and topological properties of two-dimensional germanene modified by functional group X (X = H, F, OH, CH₃) at full coverage are studied with first-principles calculation. Without considering the effect of spin-orbit coupling (SOC), all functionalized configurations become semiconductors, removing the Dirac cone at K point in pristine germanene. We also find that their band gaps can be especially well tuned by an external strain. When the SOC is switched on, GeX (X = H, CH₃) is a normal insulator and strain leads to a phase transition to a topological insulator (TI) phase. However, GeX (X = F, OH) becomes a TI with a large gap of 0.19 eV for X = F and 0.24 eV for X = OH, even without external strains. More interestingly, when all these functionalized monolayers form a bilayer structure, semiconductor-metal states are observed. All these results suggest a possible route of modulating the electronic properties of germanene and promote applications in nanoelectronics.

Evolution of the linear band dispersion of monolayer and bilayer germanene on Cu(111)

The bottom germanene layer plays a role as a buffer layer preserving the electronic properties of the upper germanene layer.

The electronic properties of the stanene/MoS2 heterostructure under strain

The effect of a MoS₂ substrate on the structural and electronic properties of stanene were systematically investigated by first-principles calculations.

Controllable electronic and magnetic properties in a two-dimensional germanene heterostructure

Based on first-principles calculation, we predict a new kind of ferromagnetic half-metal (HM) with a Curie temperature of 244 K in a two dimensional (2D) germanene vdW heterostructure.