Bilayer Germanene Research Articles

Moiré-modulated band gap and van Hove singularities in twisted bilayer germanene

Twisting bilayers of two-dimensional topological insulators has the potential to create unique quantum states of matter. Here, we successfully synthesized a twisted bilayer of germanene on Ge2Pt(101) with a 21.8° twist angle, corresponding to a commensurate (√7×√7) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moiré-modulated band gap and a well-defined edge state. This band gap opens at AB/BA stacked sites and closes at AA stacked sites, a phenomenon attributed to the electric field induced by the scanning tunneling microscopy tip. Our study further revealed two van Hove singularities at −0.8 eV and +1.04 eV, resulting in a Fermi velocity of (8 ± 1) × 105 m s−1. Our tight-binding results uncover a unique quantum state, where the topological properties could be regulated through an electric field, potentially triggering two topological phase transitions.

The Renaissance and Golden Age of Epitaxial Dry Germanene

Germanene, as an artificial graphene-like near room temperature topological insulator, compatible with ubiquitous silicon technology, is potentially the most promising artificial Xene for ultra-scale nanoelectronics. Here, we follow its emergence and development when prepared in situ under ultra-high vacuum in clean and controlled conditions by dry epitaxy on prominent metal surfaces (e.g., aluminum, silver, gold). We describe its predicted electronic properties and its birth in 2014, even if it was just a renaissance, as it was only understood after 51 years after an undeciphered birth certificate lost in oblivion. We emphasize the lifting of germanene flakes from an aluminum template with the tip of a scanning tunneling microscope, and their repositioning to form bilayer germanene with Bernal stacking. Finally, we discuss the growth of monolayer germanene in a single phase harboring Dirac fermions, following a bottom-up synthesis strategy by segregation on a gold thin film in epitaxy on a germanium support.

Effect of stacking mode on electronic structure and optical properties of bilayer germanene

The electronic structure and optical properties of germanene and three type of bilayer germanene have been studied by first-principles calculation method. The results show that the stable germanene exhibits warped structure. Monolayer germanene exhibits massless Dirac Fermion near K-point, while bilayer germanene exhibits anisotropy Dirac bands. Meanwhile, in order to control the energy band structure, the band gap of bilayer germanene is adjusted by changing the interlayer distance and stacking mode. At the optimal layer spacing, the band gap of bilayer germanene appears at the K-point with 0.417eV, and the phase transition from semiconductor to metal can be realized by further changing the stacking mode. In terms of optical properties, bilayer germanene has better optical response. The results show that the band gap of bilayer germanene can be modulated successfully using stacking mode. These findings are of great significance to the modulation of electronic structure of germanene and the application of germanene in the field of Dirac band materials.

Effect of warpage on the electronic structure and optical properties of bilayer germanene

The electronic structure and optical properties of bilayer germanene under different warpages are studied by the first-principles method of density functional theory. The effects of warpages on the electronic structure and optical properties of bilayer germanene are analyzed. The results of the electronic structure study show that the bottom of the conduction band of bilayer germanene moves to the lower energy direction with the increase of warpages at the K point, and the top of the valence band stays constant at the K point, and so the band gap decreases with the increase of warpage. When the warpage is 0.075 nm, the top of the valence band of bilayer germanene changes from K point to G point, and the bilayer germanene becomes an indirect band gap semiconductor. This is an effective means to modulate the conversion of bilayer germanene between direct band gap semiconductor and indirect band gap semiconductor by adjusting the band structure of bilayer germanene effectively. The study of optical properties shows that the effect of warpage on the optical properties of bilayer germanene is mainly distributed in the ultraviolet and visible regions, and the warpage can effectively regulate the electronic structure and optical properties of bilayer germanene. When the warpage is 0.069 nm, the first peak of dielectric function and extinction coefficient is the largest, and the energy corresponding to the absorption band edge is the smallest. Therefore, the electron utilization rate is the best when the warpage is 0.069 nm.

Feature-Rich Electronic Properties of Sliding Bilayer Germanene.

This study employs first-principles calculations to elucidate the properties of sliding bilayer germanene (BLGe). The buckled structure of germanene can afford a greater number of metastable stacking configurations than planar graphene and enrich the electronic properties. Herein, a detailed analysis of the structural variety, shift-dependent energy bands, and spatial charge densities of BLGe is presented. The projected density of states (PDOS) reveals diverse structures such as plateaus, dips, symmetric/asymmetric peaks, and shoulders. The low-lying ones of the prominent structures could correspond to single or multiorbital hybridization, depending on the stacking configuration.

Computational characterization of thermal and mechanical properties of single and bilayer germanene nanoribbon

Two-dimensional germanene has provided a cornucopia of new functionalities in the field of nanotechnology owing to its remarkable electronic and thermoelectric attributes. The robust spin–orbit coupling and high carrier mobility give rise to many salient features including non-trivial topological properties, quantum spin-Hall state near room temperature, and topological superconductivity, rendering it an excellent contender for valleytronics, spintronics, and quantum computation. As such, an in-depth characterization of thermal and mechanical properties of germanene is crucial for its practical implication and efficient operation, which remains elusive. Here, we employed equilibrium molecular dynamics simulations utilizing Stillinger Weber potential to reveal the mechanical strength, melting temperature, and phonon thermal conductivity (PTC) of single-layer germanene nanoribbon (SLGeNR) and bilayer germanene nanoribbon (BiLGeNR). Effects of temperature, biaxial tensile and compressive strain, monovacancy defects, length and width of the nanoribbon on the PTC have been rigorously investigated. It has been found that PTC of SLGeNR could be substantially reduced by BiLGeNR. Our simulation results suggest that PTC of SLGeNR demonstrates an inverse relation with temperature, biaxial compressive strain, and monovacancy defects while biaxial tensile strain, length and width of the nanoribbon increases the PTC of SLGeNR significantly. To understand the PTC more profoundly, phonon density of state (PDOS) profiles have been studied. The BiLGeNR demonstrates more tensile strength as well as melting temperature compared to SLGeNR. This study offers a comprehensive guideline for engineering the TC as well as discloses important mechanical and melting characteristics of the SLGeNR and BiLGeNR for a wide range of applications in flexible nano-electronics and thermoelectric nanodevices.

Atomic insights into single-layer and bilayer germanene on Al(111) surface

The artificial postgraphene two-dimensional (2D) materials termed 2D-Xenes, which consist of atoms arranged in atomically thin sheets with a honeycomb lattice, have received extensive attention because of their unique electronic properties. Insights into the growth mechanism at the atomic scale constitute a major challenge for realizing high-quality 2D materials, as often the substrate and deposited atoms tend to form alloys. Here, we present evidence for intrinsic growth of germanene on Al (111), i.e. a postgraphene elemental 2D atomically thin sheet. The atomic-scale extra-high resolution scanning tunneling microscopy (STM) characterization supported by density functional theory–based calculations provides deeper insight into the true atomic arrangement for the (2 × 2)/Al (111) (3 × 3) and (√3 × √3)R (30°)/Al (111) (√7 × √7)R (±19.1°) germanene phases and unambiguously confirms their real 2D honeycomb nature, which till now lacks convincing experimental proof. First-principles calculations suggest atomic models with strongly buckled (2 × 2) and (√3 × √3)R (30°) germanene, with one of eight and one of six Ge atoms protruding upward, respectively, providing perfect atom-by-atom agreement with the true atomically resolved STM images for both germanene phases. Moreover, the experimentally observed phenomena of local removal of the buckling is consistent with the formation of a flat germanene structure. Furthermore, the formation of bilayer germanene with AB-stacking is demonstrated by means of re-deposition of germanene flakes from the STM tip on top of single layer germanene, indicating that germanene flakes can be easily torn off from the aluminum substrate and attached to the STM tip, retaining their 2D configuration. On the other hand, the formation of bilayer germanene suggests an increased electronic decoupling of the bottom germanene layer from the substrate, in contrast to single layer germanene.

Inherent mechanical properties of bilayer germanene coupled by covalent bonding

Germanene, a two-dimensional buckled hexagonal structure of germanium atoms, has attractive mechanical, optical, thermal and electronic features. Recently it has been reported that covalent bonding between two monolayer germanene sheets leads to the integration of intrinsic magnetism and band gap opening that makes it attractive to future nanoelectronics. In order to use the captivating features of this structure, its mechanical characterization needs to be studied. In this study, molecular dynamics simulations have been performed using optimized Tersoff potential to analyze the effect of chirality, temperature and strain rate on the uniaxial tensile properties of this structure. This study suggests that bonded bilayer germanene shows higher mechanical strength compared to monolayer germanene. Uniaxial loading in the armchair direction shows higher fracture strength and strain compared to the zigzag direction which is contrary to the monolayer germanene. It also reports that with increasing temperature, both the fracture strength and strain of the structure decrease. It has been found that at a higher strain rate, the material exhibits higher fracture strength and strain. Mechanical properties and fracture mechanisms of defected structures have also been reported below the curie temperature. Moreover, the interlayer shear characteristics of the bilayer structure have been looked into. These results will provide significant insight to the investigation of this structure as a potential nano-electronics substitute.

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.

Mono and bi-layer germanene as prospective anode material for Li-ion batteries: A first-principles study

To find suitability of two dimensional (2D) mono-layer (ML) and bi-layer (BL) germanene as anode material for Li-ion batteries (LIBs), a series of first-principles calculations based on density functional theory (DFT) have been carried out. Our calculations suggest that adsorption of Li atom at hollow site of germanene is energetically favorable. The role of strain on formation energy has been extensively studied for fully lithiated ML and BL germanene. The Bader charge analysis indicates that the transfer of charge occurs from adsorbed Li atom to 2D germanene sheet, suggesting the formation of ionic bond between Li and Ge atom. We have calculated capacity during discharge cycle of fully lithiated ML and BL germanene sheet equals to 369 mAhg−1 and 276 mAhg−1, respectively. The average intercalation voltage (AIV) of Li-ion has been calculated for ML and BL germanene. We utilized the climbing image nudged elastic band (CI-NEB) method to analyze the energy barrier for Li-ion diffusion along the surface and through the germanene sheet. In order to find stability of Li adsorbed germanene at high temperature, we have performed ab−initio molecular-dynamics (AIMD) calculations at 300 K.

Buckling-dependent switching behaviours in shifted bilayer germanene nanoribbons: A computational study

Bilayer germanene nanoribbons are investigated in different stacks like buckled and flat armchair and buckled zigzag germanene nanoribbons by performing theoretical calculations using the nonequilibrium Greens function method combined with density functional theory. In these bilayer types, the current oscillates with change of interlayer distances or intra-layer overlaps and is dependent on the type of the bilayer. Band gap of AA-stacked of shifted flat bilayer armchair germanene nanoribbon oscillates by change of interlayer distance which is in contrast to buckled bilayer armchair germanene nanoribbon. So, results show the buckling makes system tend to be a semiconductor with wide band gap. Therefore, AA-stacked of shifted flat bilayer armchair germanene nanoribbon has properties between zigzag and armchair edges, the higher current under bias voltages similar to zigzag edge and also oscillations in current like buckled armchair edges. Also, it is found that HOMO-LUMO band gap strongly affects oscillation in currents and their I–V characteristic. This kind of junction improves the switching properties at low voltages around the band gap.

Direct Evidence of Dirac Signature in Bilayer Germanene Islands on Cu(111).

Bernal-stacked bilayer germanene with a stable buckled honeycomb structure has been successfully synthesized on Cu(111). Structural and electronic characterizations as well as theoretical calculations unequivocally demonstrate for the first time the presence of a nearly linear energy dispersion in the vicinity of the Fermi energy, as expected of the Dirac signature for theoretical freestanding germanene.

Strain and deformations engineered germanene bilayer double gate-field effect transistor by first principles

Germanene, silicene, stanene, phosphorene and graphene are some of single atomic materials with novel properties. In this paper, we explored bilayer germanene-based Double Gate-Field Effect Transistor (DG-FET) with various strains and deformations using Density Functional Theory (DFT) and Green’s approach by first-principle calculations. The DG-FET of 1.6nm width, 6nm channel length (Lch) and HfO2 as gate dielectric has been modeled. For intrinsic deformation of germanene bilayer, we have enforced minute mechanical deformation of wrap and twist (5°) and ripple (0.5Å) on germanene bilayer channel material. By using NEGF formalism, I–V Characteristics of various strains and deformation tailored DG-FET was calculated. Our results show that rough edge and single vacancy (5–9) in bilayer germanene diminishes the current around 47% and 58% respectively as compared with pristine bilayer germanene. In case of strain tailored bilayer DG-FET, multiple NDR regions were observed which can be utilized in building stable multiple logic states in digital circuits and high frequency oscillators using negative resistive techniques.

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

The structural and electronic properties of germanene are always strongly modulated by the hybridization effects with metal substrates. In order to see what will happen when a buffer layer is introduced in-between germanene and metal substrates, we study the structural and electronic properties of the recently synthesized monolayer and bilayer germanene on Cu(111) though first-principles calculations. Our results show that the monolayer germanene on Cu(111) displays a nearly flat configuration and interface states form between the Ge pz and Cu sp-like states, with the Ge π states maintaining the Dirac character. For bilayer germanene on Cu(111), interactions with Cu(111) are reduced due to germanene inter-layer interactions, which is beneficial for the transfer of germanene. In comparison with the bottom germanene layer, the Ge pz character of the upper germanene layer can be maintained near the Fermi level. Since the linear band dispersion is at the heart of the novel quantum phenomenon, our results will facilitate research into the synthesis, extraordinary quantum properties, and applications based on the two-dimensional (2D) germanium system.

DFT comparison of structural and electronic properties of graphene and germanene: monolayer and bilayer systems

Two-dimensional honeycomb systems such as graphene and graphene-related materials have attracted enormous technological interest due to the wide range of applications from composites to conductive coatings. Based on density functional calculations we present a comparison of structural and electronic properties of monolayer (and bilayer) graphene and germanene, particularly, germanene have been synthesized on MoS2 with the Dirac cone properties. Our results show that germanene prefers a buckled structural conformation, which is slight more stable respect to its planar conformation. The well-known structure of graphene is compared with the buckled and planar structures of germanene at level of local density approximation. Graphene and germanene, in monolayer pristine conformations, exhibit massless Dirac fermions close to the K point. However, in germanene the top valence and bottom conduction bands are found at lower energies respect to graphene corresponding bands. The Fermi velocity in germanene is around 30% lower compared to graphene. Bilayer graphene show an anisotropic Dirac cone, while in bilayer germanene a small band gap at K point of ∼0.05 eV is detected. Our results are found to be in agreement with previous theoretical reports, and these pretend to give a compressible description from fundamental point of view.

Peripheral-branch specific control of axonogenesis in zebrafish sensory neurons by LIM/homeodomain proteins, Islet-2 and Islet-3

Bilayer germanene nanoribbons are investigated in different stacks like buckled and flat armchair and buckled zigzag germanene nanoribbons by performing theoretical calculations using the nonequilibrium Greens function method combined with density functional theory. In these bilayer types, the current oscillates with change of interlayer distances or intra-layer overlaps and is dependent on the type of the bilayer. Band gap of AA-stacked of shifted flat bilayer armchair germanene nanoribbon oscillates by change of interlayer distance which is in contrast to buckled bilayer armchair germanene nanoribbon. So, results show the buckling makes system tend to be a semiconductor with wide band gap. Therefore, AA-stacked of shifted flat bilayer armchair germanene nanoribbon has properties between zigzag and armchair edges, the higher current under bias voltages similar to zigzag edge and also oscillations in current like buckled armchair edges. Also, it is found that HOMO-LUMO band gap strongly affects oscillation in currents and their I–V characteristic. This kind of junction improves the switching properties at low voltages around the band gap.