Germanene Nanoribbons Research Articles

Edge magnetization and spin-valley-caloritronics in germanene and stanene nanoribbons

We proposed a concept to generate the spin-valley Seebeck effect using the Kane-Mele-Hubbard model and electric field in zigzag nanoribbons. For this purpose, we apply a thermal gradient across the stanene and germanene nanoribbons. Then, we investigate the valley thermospin properties. Our results show that the edge magnetization can generate a controllable spin- and valley-polarized current by tuning the electric field in the silicene-like materials with large spin-orbit interaction. Moreover, a pure valley current is observed in a wide range of temperatures. Remarkably, we found that it is possible to produce the spin-valley-locked effect and high values of spin-valley Seebeck coefficients by controlling the chemical potential.

Epitaxial growth of massively parallel germanium nanoribbons by segregation through Ag(1 1 0) thin films on Ge(1 1 0)

We investigate the epitaxial growth of low-dimensional nanostructures formed by surface segregation of germanium on single crystalline Ag(110) thin films supported by a Ge(110) template. Detailed studies of the atomic geometries have been carried out using scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and high-resolution synchrotron radiation core-level spectroscopy. At very low Ge coverages the segregated Ge atoms form two kinds of commensurate and incommensurate quantum structures after annealing. With increasing Ge coverages, narrow stripes, 0.5 nm in width and with lengths ranging from 5 nm to 50 nm, are formed. With further increase of the Ge atoms’ segregation on the surface, the narrow stripes transform into massively parallel one-dimensional Ge nanostructures with specific widths. From a combined analysis, these fascinating quantum structures are proposed to be zig-zag germanene nanoribbons, potentially with very exciting electronic, spintronic, and topological properties.

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.

Fully spin-valley-polarized current induced by electric field in zigzag stanene and germanene nanoribbons

We theoretically investigated the spin-dependent valleytronics in stanene and germanene nanoribbons, considering the electron-electron interaction and the external electric field without any magnetic exchange element. Our results showed that applying an electric field in these two-dimensional materials with a large intrinsic spin-orbit coupling can provide a versatile platform to create spin-valley currents by exploiting the edge magnetism at their zigzag edges. Generally, manipulating the electric field can generate a fully spin-valley-polarized current with a large magnitude even at room temperature. The rich and tunable spin and valley degrees of freedom can turn these structures into ideal candidates for spin-valley applications.

Impact of substitutional metallic dopants on the physical and electronic properties of germanene nanoribbons: A first principles study

Density functional theory (DFT) has been used to investigate doped armchair germanene nanoribbons (AGeNRs) doped by low-concentration metallic atoms (Pt, Ag, Au, In and Sn). The structural stability and electronic properties of these doped nano-structures have been analyzed. The formation energy of the examined ribbons shows that they are thermodynamically stable. Examination of E-k band structures and density of state (DOS) has shown that depending on the type of metal atom, different energy bands can be seen around the Fermi level. Doping of the nano-ribbon by Pt and Sn in N = 7 only reduces the band gap compared to the pristine structure and the nano-ribbon stays semiconducting. However replacing the In, Ag, and Au atoms in AGeNR leads to the semiconducting-metal transition. Moreover, metallic doping of the ribbon in N = 8, yields an increase of the band gap and a transfer is observed from metal to semiconductor.

Band gap determination of graphene, h-boron nitride, phosphorene, silicene, stanene, and germanene nanoribbons

The energy band gap of graphene, h-boron nitride, stanene, silicene, germanene, and phosphorene nanoribbons is investigated theoretically with two approaches. In the first approach, a set of equations to calculate the energy band gap of nanoribbons is deduced using the expression of Fermi velocity. The size dependency of the energy band gap so obtained confirms previous expressions. The second approach, however, determines resistivity by directly connecting quantity to the energy band gap using an electron–acoustical phonon scattering mechanism. For the calculation of the energy band gap and resistivity, the armchair configuration of nanoribbons is considered. It is observed that the transverse magnetic field influences the energy band gap and resistivity. The magnitude of the band gap is negative up to a certain value of the field and turns positive at a sufficiently higher field. The negative band gap can be interpreted as metallic behavior of the materials. The results of the work are useful in the determination of the energy band gap of nanoribbons for their nanoelectronic and optoelectronic applications as well as tuning the band gap using the applied magnetic field.

Probing Gold-Doped Germanene Nanoribbons for Nanoscale Interconnects Under DFT-NEGF Framework

Gold-doped germanene nanoribbons (Au-GeNRs) are investigated for their potential as interconnects, using density functional theory combined with nonequilibrium Green’s function formalism. Various stable doping sites for both zigzag and armchair GeNRs (ZGeNR and AGeNR) are investigated. Based on formation energy ($$E_{{{\mathrm{FE}}}}$$) analysis, all considered Au-GeNRs are revealed to be thermodynamically stable. The analysis also shows that near-edge-doped ZGeNR (with $$E_{{{\mathrm{FE}}}} = -3.46$$ eV) is the most stable configuration. It is shown through $$E-k$$ structures and density-of-states profiles that Au-doping results in metallic GeNR irrespective of the edge states and ribbon width. To further explore the prospects for the use of Au-doped GeNR for interconnect applications, important small-signal dynamic parameters (including $$R_Q, L_K,$$ and $$C_Q$$) for various doped configurations are explored. The present investigations also take into account the effect of bias voltage on $$R_Q, L_K, C_Q$$. It is revealed that, with the exception of the edge-doped ZGeNR configuration, bias voltage has a prominent effect on these parameters for every configuration. Thus, edge-doped ZGeNR (with $$L_K = 4.41$$ nH/$$\upmu $$m, $$C_Q = 4.21$$ pF/cm) represents a potential candidate for nanoscale interconnect applications among the considered configurations.

Aluminum Doped Germanene Nanoribbons: A First Principles Investigation

The purpose of current investigation is to analyze the impact of Al when introduced as an impurity from Al-substrate to germanene monolayers using first principles approach. Here, zigzag germanene nanoribbons (ZGeNR) with Al as dopant were studied at various sites viz. center (C-doped), edge (E-doped), nearedge (NE-doped). Formation energy calculations have been performed to show that Al-doping is preferred at the edge site, thus E-doped ZGeNR (EFE = −3.46 eV) emerges as most stable configuration. Further, based on band structure and DOS profile analysis we reveal that unlike semi-metallic pristine ZGeNR, Al-doped ZGeNR are metallic in nature, promoting their utilities as nanoscale interconnects.

Impact of an antidote vacancy on the electronic and transport properties of germanene nanoribbons: A first principles study

In this article, we investigate the effect of various antidote defects on the electronic properties and current characteristics of an armchair Germanene nanoribbon (AGeNR) using density functional theory (DFT) and non-equilibrium Green's function (NEGF) method. The defected AGeNRs are introduced by setting antidote topologies in the pristine nanoribbons, resulting in antidote super-lattice of AGeNRs. It is found that new electronic properties appear due to the presence of defects. The obtained results indicate that bandgap of the defected AGeNRs can be increased or decreased in different cases. Moreover, the transport properties are analyzed based on the various defect locations in the AGeNR when the ribbon is utilized as the channel of a tunneling field effect transistor (TFET). Based on our results, it is found that the presence of antidote defects leads to reduction or increase in the current, drifting the Dirac point, and decreasing or increasing the minimum or off-state current.

Electron Mobility in Defective Nanoribbons of Monoelemental 2D Materials

Statistical atomistic quantum transport simulations are employed to study electron mobility in silicene, germanene, phosphorene and graphene nanoribbons with edge defects. We demonstrate that silicene and germanene nanoribbons can provide the mobility of up to ≈100 cm2/Vs in the sub-6 nm width range. However, none of these nanostructures appear promising for future extremely-scaled transistors when bandgap-mobility trade-off in defective nanoribbons is considered.

Quantum conductance of defected phosphorene and germanene nanoribbons

The electronic and transport properties of the monolayer black phosphorus and germanene nanoribbons are studied in the framework of the tight-binding model (TBM) based upon Landauer-Buttiker formalism using Green’s function method (GFM). The local density of states (LDOS) and electronic conductance of the phosphorene and germanene nanoribbons along zigzag and armchair directions are examined when the various types of defects are introduced into the system. It is found that the transport properties of zigzag phosphorene and germanene nanoribbons are strongly dependent on the number and location of the vacancies. Furthermore, it is shown that the one-/three-atom vacancy induces quasi-states in the conductance around the Fermi energy because of breaking the sublattice symmetry in the zigzag germanene nanoribbons (ZGeNRs). So the metal-semiconductor transition occurs when one-/three-atom vacancy is located at the edges of ZGeNRs; however, this transition is not observed in the zigzag phosphorene nanoribbons (ZPNRs). Besides, the results of the calculations indicate more sensitivity of ZPNRs on conductivity to the edge vacancy disorders than armchair phosphorene nanoribbons (APNRs). In addition, the conductance of ZPNRs decreases with the increment of the ribbon width in the presence of edge vacancy. Importantly, the disappearance of conductance around Fermi energy in ZPNR due to Anderson localization disorder highlights an important conclusion for the possibility of quenching of the conductance near the Fermi energy, making this class of materials appealing for applications in digital transistor devices.

Density functional investigations on the adsorption characteristics of nucleobases on germanene nanoribbons

Abstract In order to explore the possibility of using 2D nanostructures as biosensors , we have studied the adsorption characteristics of nucleotide bases on armchair germanene nanoribbon (AGeNR) using density functional theory with several approximations of exchange-correlation functionals with the addition of dispersion correction. It has been found that the dispersion interactions have the key role in characterizing adsorption phenomena through the non-covalent interactions. The structural and electronic properties of the nucleobase-nanoribbon complexes have been investigated along with the study of the dependence of binding energies on ribbon widths and hence the edge (armchair or zigzag) effects. A physisorption process with binding energies in the range of about 0.83–1.37 eV has been found for 10-AGeNR, which alters the electronic and structural properties of the subsystems indicating the potential use of these complexes as biosensors.

A Theoretical Study on the Thermoelectric Properties of Porous Armchair Germanene Nanoribbons

Since the limits of conventional sources of energy are rapidly approaching, the thermoelectric devices have attracted attention for their potential of power generation directly from waste heat. In this paper, thermoelectric properties of porous armchair Germanene nanoribbons (AGeNRs) have been explored for a range of pore dimensions in order to achieve a high performance two-dimensional nanoscale thermoelectric device. The work has been done to investigate the influence of different nanopore shapes and their associated positions on the thermoelectric performance so as to tune it to the optimum pore shape and position that would enhance the overall thermoelectric efficiency. Also, the effect of passivation of the pore edges on thermoelectric parameters for all shapes has been studied. Further, the influence of temperature dependence on figure of merit has been observed. Ballistic transport regime and semi-empirical method using Huckel basis set is used to obtain the electrical properties while the Brenner potential is used for the phononic properties.

Influence of edge magnetization and electric fields on zigzag silicene, germanene and stanene nanoribbons

Using a multi-orbital tight-binding model, we have studied the edge states of zigzag silicene, germanene, and stanene nanoribbons (ZSiNRs, ZGeNRs and ZSnNRs, respectively) in the presence of the Coulomb interaction and a vertical electric field. The resulting edge states have non-linear energy dispersions due to multi-orbital effects, and the nanoribbons show induced magnetization at the edges. Owing to this non-linear dispersion, ZSiNRs, ZGeNRs and ZSnNRs may not provide superior performance in field effect transistors, as has been proposed from single-orbital tight-binding model calculations. We propose an effective low-energy model that describes the edge states of ZSiNRs, ZGeNRs, and ZSnNRs. We demonstrate that the edge states of ZGeNR and ZSnNR show anti-crossing of bands with opposite spins, even if only out-of-plane edge magnetization is present. The ability to tune the spin polarizations of the edge states by applying an electric field points to future opportunities to fabricate silicene, germanene and stanene nanoribbons as spintronics devices.

First Principles Study of the Ambipolarity in a Germanene Nanoribbon Tunneling Field Effect Transistor

In this article, the effects of hetero-dielectric gate material and gate-drain underlap on the ambipolar and ON-state current of a germanene nanoribbon (GeNR) tunneling field-effect transistors (TFETs) is examined. The simulations are performed using the combination of density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism. It was observed that using high-k dielectric gate material increases the ON-state current while the combination of hetero-dielectric gate material and gate-drain underlap suppresses the ambipolar current and improves the ON-state current. In addition, the effect of various hetero-junctions in the source region on the performance of GeNR-TFET was investigated. Due to the dependency between the width and energy bandgap in GeNR, utilizing a small bandgap in the source improves ON-state current and its ambipolar behavior.

Spin filter effect of germanene nanoribbon controlled by local exchange field and electric field

Germanene, which has been synthesized recently, is a single-layered material composed of germanium atoms. Almost all the striking properties of graphene can be transferred to germanene, because both of them have the same honeycomb lattice structure. In contrast with graphene, germanene has a sizable band gap and spin dependent helical edge states, which make it attractive candidate for spintronic applications. By using the nonequilibrium Green’s function method, the effects of <i>Z</i>-axis local exchange field and electric field on spin-polarized transport properties in germanene nanoribbon are studied theoretically. The results reveal that by exerting an exchange field with a strength greater than twice the effective spin-orbit coupling to the edge region of germanene, the spin resolved band gaps can be generated in different energy ranges, and thus 100% filtering of spin-up or spin-down electrons can be achieved. We theoretically propose a method to reduce the threshold exchange field strength for the spin filter effect by using the electric field. The results show that the spin-filter effect can be achieved in a larger energy range under a weaker exchange field when the electric field are applied to the entire central device region. With the increase of the local exchange field intensity, the energy range corresponding to the spin filter effect will increase significantly.

Carrier Transport and Thermoelectric Properties of Differently Shaped Germanene (Ge) and Silicene (Si) Nanoribbon Interconnects

Here, we report a study of carrier transport and thermoelectric properties of silicene/germanene nanoribbon (SiNR/GeNR) interconnects of varying shapes (namely, “V,” “L,” “U,” and “S” shaped) with empirical tight binding—nonequilibrium Green’s function approach. Our simulation shows that a significant reduction in both electron and phonon transmission in such differently shaped NRs compared to perfect ones. In terms of thermoelectric properties, Seebeck coefficient ( $\mathbb {S}$ ) variation in the range of −72.3 to $3.52~\mu \text{V}$ /K at 300 K for SiNR and −176 to $-\textsf {95.5}\,\,\mu \text{V}$ /K at 300 K for GeNR is observed in different pattern conditions. For Peltier coefficient ( $\pi $ ), these ranges are −0.23 to 0.001 V for SiNR and −0.053 to −0.011 V for GeNR at 300 K. Simulation also shows significant change in thermoelectric figure of merit ( $\textit {ZT}$ ) for a different pattern and temperature with peak $\textit {ZT}~0.09$ at 800 K for “U”-shaped SiNR and 0.80 at 400 K for “L”-shaped GeNR.

The electronic and optical properties of armchair germanene nanoribbons

The electronic and optical properties of armchair germanene nanoribbons (AGeNRs) are studied using the first principles calculations. The band structure, band-gap size, projected density of states (PDOS), and dielectric function of AGeNRs are calculated. Moreover, the variation of these parameters as a function of various ribbon widths is investigated. By increasing the width of ribbons the band-gap size of pristine AGeNRs is decreased according to three different trends. Based on these trends, it is extracted that the AGeNRs can be divided into three categories named as n = 3P, n = 3P+1, n = 3P+2, here n is the number of germanium atoms in the width and P is an integer. Moreover, all these categories are direct band-gap materials and the order of band-gap size is changed as: EG (3P+2) < EG (3P) < EG (3P+1). Due to the direct band-gap size, it can be extracted that all of AGeNR categories are proper for optical applications. Based on the simulation results of this work, it is demonstrated that the AGeNRs are appropriate for optical devices in the range of infrared applications. In addition, the effect of uniaxial tensile and compressive strain on the band-gap size and the dielectric function of AGeNRs is investigated and it is shown that the electronic and optical properties of AGeNRs can be tuned by strain in a wide range.

Magneto-electronic specific heat of germanene nanoribbons

Electronic properties of zigzag and armchair germanene nanoribbons (ZGeNR and AGeNR) in magnetic field are calculated by the tight-binding model including the spin-orbit coupling (SOC). The SOC induces spin-split states changing energy dispersion and band-gap. As field strength increases, band-gap increases for ZGeNRs, but decreases for AGeNRs. At zero field, temperature-dependent electronic specific heat strongly depends on nanoribbon's boundary and width. For ZGeNRs, specific heat declines simply proportional to the inverse of width; however, it depends on the dimer-line number for AGeNRs. At low-temperature, width-dependent specific heat of ZGeNRs rapidly drops with increasing field strength, whereas it exhibits an oscillatory behavior for AGeNRs. As temperature increases, such an oscillatory behavior is smeared out or not that is profoundly related to the dimer-line number of AGeNRs. Low-temperature specific heat with varying magnetic field exhibits peak, peak-dip-peak, or smoothly increasing behaviors that strongly reveals geometry dependence of field-modulated electronic properties.

Spin-current diodes based on germanene and stanene subjected to local exchange fields

We theoretically propose a spin-current diode based on germanene and stanene nanoribbons. The results reveal that the germanene or stanene nanoribbon simultaneously under x- and z-direction local exchange fields exhibits excellent rectification characteristics, and that the spin current is blocked in a certain range of spin voltage. The proposed spin-current diodes have three main features: (1) the breakdown voltage of the diodes can be controlled by the strength of exchange fields; (2) the polarity direction of the diodes can be reversed just by adjusting the direction of local exchange fields; and (3) the spin diode effects can be well maintained at 300 K room temperature. All of the results indicate that the proposed germanene and stanene nanosystems are promising candidates for spintronic devices.