Germanene Nanoribbons Research Articles

Edge-Oxidized Germanene Nanoribbons for Nanoscale Metal Interconnect Applications

Role of elemental oxygen atom as a terminating element on the electronic and transport properties of germanene nanoribbons (GeNRs) has been investigated using the density functional theory and nonequilibrium Green’s function approach. In principle, we have considered all possible stable combinations of oxygen and hydrogen terminations for both zigzag as well as armchair edge states. Our binding energy ( ${E}_{b}$ ) calculations depict that oxygen termination leads to thermodynamically stable configurations with enhanced stability for the enhanced O-atom concentration. Furthermore, dispersion relation ( E-k structures) and density-of-states profile calculations suggest that oxygen-terminated GeNR is mostly metallic in nature or possesses a very narrow bandgap. On the account of such enhanced metallicity, we have proposed these structures for metal interconnects application. To gauge their performance as metal interconnect, we have calculated crucial parameters, such as ${R}_{Q}, {L}_{k}$ , and ${C}_{Q}$ , using the standard two-probe setup. With the least calculated values of kinetic inductance ( ${L}_{k}=\text {4.14}\,\,\text {nH}/\mu \text {m}$ ) and quantum capacitance ( ${C}_{Q}=\text {8.89}\,\,\text {pF}/\text {cm}$ ), zigzag GeNR with both edge-oxidized (O-ZGeNR-O) configurations can be considered as the potential candidate for nanoscale interconnect application.

Theoretical Studies on the Structures, Electronic and Magnetic Properties of Fully and Partically Fluorizated Germanene Nanoribbons†

Theoretical Studies on the Structures, Electronic and Magnetic Properties of Fully and Partically Fluorizated Germanene Nanoribbons†

Tuning the analog and digital performance of Germanene nanoribbon field effect transistors with engineering the width and geometry of source, channel and drain region in the ballistic regime

In this paper, with taking advantage of electrical properties of a germanene nanoribbon, we propose a germanene nanoribbon field effect transistor (GeNR-FET). Here by tuning the width and geometry of the germanene nanoribbon in the source, drain and channel regions, we investigate theoretically the transistor characteristics, analog and digital performances of these several different GeNR-FETs at room temperature. Our simulations are obtained using density functional theory (DFT) combined withnon-equilibrium Green's function (NEGF) method. The simulation results show that for digital applications, by tuning the width of the germanene nanoribbon a GeNR-FET with a finite band gap in the channel region and small band gap in the source and drain regions shows a better Ion/Ioff ratio in transfer characteristics. However, for the analog applications, if the band gap of the channel region has small value and the band gap of the source and drain regions have a finite value, the output characteristic shows a higher peak to valley (PVR) ratio which is an important figure of merit in analog applications. Also from the output characteristics, we find that the T-shape channel shows more desirable (PVR) compared with other devices and it reaches to 17.28 in this case.

Thermal Spin Generator Based on a Germanene Nanoribbon Subjected to Local Noncollinear Exchange Fields

Spintronics relies on generating, manipulating, and detecting spin current. In addition, converting waste heat into electricity is an essential topic for current and future technologies. To explore spin caloritronics, the authors propose a thermoelectric spin-current generator, consisting of a zigzag germanene nanoribbon with thermal leads partially exposed to noncollinear exchange fields. The spin polarization of the current in the leads can reach 100%, 0%, or even \ensuremath{\infty} (pure spin current with no charge flow), but only under a temperature difference, and the giant magnetoresistance can reach extremely large values just by adjusting the relative directions of the exchange fields.

Doped armchair germanene nanoribbon exhibiting negative differential resistance and analysing its nano-FET performance

Abstract Density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) formalism is performed to explore electronic properties (geometrical stability, band structure and density of states) and quantum transport properties (transmission spectrum and I–V characteristics) of armchair germanene nanoribbon (AGeNR) doped with various elements, such as Ga, In, Tl, As, Sb and Bi. A negative differential resistance is observed for each doped AGeNR. Our results indicate that the indium (In) atom doped AGeNR is the most geometrically stable structure and provides a maximum peak to valley current ratio (Ip/Iv = 1.95). Further, In atom doped AGeNR is proposed for field effect transistor (AGeNR-FET) formation using the high dielectric constant value of hafnium dioxide (HfO2 = 25) at different applied gate voltages (−0.5 V–0.5 V). Finally, AGeNR-FET parameters are also calculated which shows high transconductance i.e. 56,196.3 nΩ−1, high charge mobility i.e. 2.6 × 104 cm2 V−1 s−1 and low subthreshold swing i.e. 39.39 mV/decade. Our findings have great application in digital devices and memory devices, and high frequency applications for future nanoelectronics and nanodevices.

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.

Investigation and comparison of the large-signal characteristics and dynamical parameters of silicene and germanene nanoribbon interconnects

One dimensional materials such as nanotubes and nanoribbons are studied widely in the literature for their peculiar properties. In particular, zigzag graphene and silicene nanoribbons constitute an active research area for the possibility to use them as interconnects. Recently, it is exposed that asymmetric zigzag germanene nanoribbons also show similar metallic characteristics. In this work, the electronic transport properties and dynamical parameters of zigzag silicene and germanene nanoribbons (ZZ-SiNR and ZZ-GeNR) are investigated and compared. First-principles quantum mechanical simulations using density functional theory in conjunction with non-equilibrium Green’s function formalism are utilized to extract the current-voltage behaviours and Fermi velocities of equivalent ZZ-SiNR and ZZ-GeNR samples. The results show that ZZ-SiNR and ZZ-GeNR samples show similar current-voltage characteristics with the ZZ-GeNR having slightly higher conductance. Then, dynamical parameters namely kinetic inductance and quantum capacitance values are obtained. The obtained values show that the kinetic inductance and quantum capacitance of the ZZ-GeNR are lower than those of ZZ-SiNR. It is concluded that both ZZ-SiNR and ZZ-GeNR can be used as nanoscale interconnects with the ZZ-GeNR having slightly higher linearity and lower kinetic inductance and quantum capacitance compared to its SiNR counterpart.

Edge states of hydrogen terminated monolayer materials: silicene, germanene and stanene ribbons

We investigate the energy dispersion of the edge states in zigzag silicene, germanene and stanene nanoribbons with and without hydrogen termination based on a multi-orbital tight-binding model. Since the low buckled structures are crucial for these materials, both the π and σ orbitals have a strong influence on the edge states, different from the case for graphene nanoribbons. The obtained dispersion of helical edge states is nonlinear, similar to that obtained by first-principles calculations. On the other hand, the dispersion derived from the single-orbital tight-binding model is always linear. Therefore, we find that the non-linearity comes from the multi-orbital effects, and accurate results cannot be obtained by the single-orbital model but can be obtained by the multi-orbital tight-binding model. We show that the multi-orbital model is essential for correctly understanding the dispersion of the edge states in tetragen nanoribbons with a low buckled geometry.

The effect of buckling on I–V characteristics of symmetric and asymmetric zigzag germanene nanoribbons: a first-principle calculation

In this paper, we consider zigzag-edge germanene nanoribbons with small buckling and even (symmetric) or odd (asymmetric) widths. Although the band structures of these two type structures are the same, they are noticeably different in terms of conductivity and current. In previous works on silicene, the role of buckling to dominate the effect of symmetry has been ignored and just the buckling changes of the symmetry space group from σ to C2 are shown. In this case, buckling is the main factor responsible for differences in conductivity and current. Fluorine, hydroxyl, and hydrogen are used to passivate the nanoribbon edges. Results show that the current and conductivity are strongly dependent on the kinds of substitutional atoms/groups used to passivate the structure. It is found that symmetry breaking is not the only effective factor in the creation of current—buckling and backscattering are also important.

Prospects of asymmetrically H-terminated zigzag germanene nanoribbons for spintronic application

First-principles investigations have been performed to explore the spin based electronic and transport properties of asymmetrically H-terminated zigzag germanene nanoribbons (2HH ZGeNR). Investigations reveal a significant formation energy difference (ΔEF=EF(2H-H)−EF(H-H)∼−0.49eV), highlighting more energetic stability for asymmetric edge termination compared to symmetric edge termination, irrespective of the ribbon width. Further, magnetic moment analysis and total energy calculations were performed to unveil that these structures have a magnetic ground state with preferred ferromagnetic (FM) coupling. The calculated E-k structures project a unique bipolar semiconducting behaviour for 2HH ZGeNR which is contrast to H-terminated ZGeNR. Half-metallic transformation has also been revealed via suitable p-type or n-type doping for these structures. Finally, transport calculations were performed to highlight the selective contributions of spin-down (spin-up) electrons in the I–V characteristics of the doped 2HH ZGeNR, suggesting their vitality for spintronic device applications.

The structures, stabilities, electronic and magnetic properties of fully and partially hydrogenated germanene nanoribbons: A first-principles investigation

Based on the first-principles DFT computations, we systematically investigated the geometries, stabilities, electronic and magnetic properties of fully and partially hydrogenated Ge nanoribbons (fH-GeNRs and pH-GeNRs) with the zigzag and armchair edges. It is revealed that the chair-like configuration is the lowest-lying one for zigzag/armchair-edged fH-GeNRs. Regardless of the edge chirality, the full hydrogenation can effectively widen the band gap of GeNR, and endow fH-GeNRs with the nonmagnetic (NM) semiconducting behaviors, where the band gap decreases with the increase of ribbon width. Comparatively, independent of hydrogenation ratio, all the pH-GeNRs with zigzag edge are the antiferromagnetic semiconductors while all the pH-GeNRs with armchair edge are NM semiconductors. When increasing the hydrogenation ratio, the band gap of pH-GeNRs can increase, but the variation of band gap can exhibit the intriguing three family behavior for the armchair-edged pH-GeNRs. Especially, all these pH-GeNRs can exhibit the almost same electronic and magnetic properties as the remaining pristine GeNRs without hydrogenation. This may offer a potential strategy to realize the “narrow” GeNRs in large scale. Finally, all these hydrogenated GeNRs can possess high structure stability, indicating a great possibility of their experimental realization. These valuable insights can be advantageous for promoting the Ge-based nanomaterials in the application of multifunctional nanodevice.

Thermally induced spin-dependent current based on Zigzag Germanene Nanoribbons

In this paper, using first principle calculation and non-equilibrium Green's function, the thermally induced spin current in Hydrogen terminated Zigzag-edge Germanene Nanoribbon (ZGeNR-H) is investigated. In this model, because of the difference between the source and the drain temperature of ZGeNR device, the spin up and spin down currents flow in the opposite direction with two different threshold temperatures (Tth). Hence, a pure spin polarized current which belongs to spin down is obtained. It is shown that, for temperatures above the threshold temperature spin down current increases with the increasing temperature up to 75K and then decreases. But spin up current rises steadily and in the high temperature we can obtain polarized spin up current. In addition, we show an acceptable spin current around the room temperature for ZGeNR. The transmission peaks in ZGeNR which are closer to the Fermi level rather than Zigzag Graphene Nanoribbon (ZGNRS) which causes ZGeNR to have spin current at higher temperatures. Finally, it is indicated that by tuning the back gate voltage, the spin current can be completely modulated and polarized. Simulation results verify the Zigzag Germanene Nanoribbon as a promising candidate for spin caloritronics devices, which can be applied in future low power consumption technology.

Impact of uniaxial compressive strain on physical and electronic parameters of a 10 nm germanene nanoribbon field effect transistor

In this paper, a top-gated Germanene nanoribbon field effect transistor (GeNR-FET) with and without applying a compressive strain is investigated. Three strain strengths (ε = 3%, 5% and 10%) are applied to the different regions of the proposed GeNR-FET and the obtained transistor characteristics are compared with each other. Moreover, physical and electronic parameters of unstrained and strained GeNR are calculated. The results indicate that the bandgap and electron effective mass of GeNR increase firstly by increasing the strain strength but then decrease and this behavior affects on the transistor figures of merit. We find that the Ion/Ioff ratio and subthreshold swing (SS) of the unstrained FET improves when a 3% compressive strain is applied to the whole of the device. Also, applying a 10% strain to the device increases the ON current of the FET due to strain-induced self-doping behavior, but the obtained transistor parameters for this structure are not very satisfactory. Next, we apply the compressive strain either only to the channel or only to the source and drain of the devices. The obtained results show, due to high lattice mismatch between the channel and the leads, the transistor performance of the strained FETs are almost the same or even worse than that of the unstrained one. A novel engineered device is proposed and the transistor figures of merit and in particular, the analog performance are emphasized. It is found that the new proposed device has a better Ion/Ioff ratio and SS. In addition, negative differential resistance occurs in this transistor. The analog performance of the novel device is obtained and compared with the unstrained and 3% strained GeNR-FET in the subthreshold region. It is observed that the proposed device has a better transconductance and voltage gain so that it is a good candidate for analog applications.

Band Gap Tuning of Germanene Nanoribbon Using Chemical Functionalization

Graphene as a 2D carbon nanostructure has drawn tremendous interest worldwide because of its extremely high mobility and unique electronic, magnetic, and surface properties but opening a band gap remains challenging. Other 2D materials have emerged as promising materials that may be more easily altered to have a band gap, high mobility, and on/off ratio. Germanene with its buckled two-dimensional structure exhibits extremely high mobility, massless fermions behavior, and strong spin-orbit coupling which has drawn tremendous interest for high performance devices. However, it has no intrinsic band gap and low structural stability which are needed for logic and switching devices. Despite numerous attractive features in germanene it, like graphene, exhibits a semi-metallic zero band gap. Applying electric field, chemisorption of adatom species, introducing periodic nanoholes, doping and edge functionalization are all techniques aimed at opening a bandgap in germanene. Here we present a density functional theory based study of the influence of edge-functionalization using hydrogen (-H and -2H) and halogen (- F, -Cl, -Br, -I, -2F,-2Cl, -2Br, -2I), -S atoms and –SH, -OH, -CH3 groups termination. An overview is given of the influence of these edge-functional groups attached to different germanene nanoribbon (GNR) structures of varying width ranging from 6 to 19, focusing primarily on band gap. Additionally, the dependence of armchair GNR band gap on functionalization and ribbon width is explored. We found that edge functionalized armchair germanene nanoribbon (AGeNR) opened a band gap as small as 0.012 eV when functionalized by -2H and could be as high as 0.84 eV when functionalized with -2I. Nanoribbons could be classified into three families according to width as follows, W = 3K, 3K+1, and 3K+2. For mono-hydrogenated, fluorinated, chlorinated, hydroxyl termination, the band gap varied as EG (3K+2) < EG (3K) < EG (3K+1). For di-hydrogenated, fluorinated, and chlorinated we found band gap follwed the periodicity EG (3K+ 1) < EG (3K +2) < EG (3K ). Formation energy studies revealed that the AGeNR produced a more stable structure with fluorine functionalization and -SH, -OH, -CH3 group’s termination also provides similar result with sizable band gaps. Simulation results suggest that the electronic structure of germanene is similar to graphene and silicene. Energy band gap tuning of AGeNR using edge functionalization may provide a new means of integrating germanene in optoelectronics, low power and high performance switching devices.

Germanene nanoribbon tunneling field effect transistor (GeNR-TFET) with a 10 nm channel length: analog performance, doping and temperature effects

In this paper, a scheme of the germanene nanoribbon tunneling field effect transistor (GeNR-TFET) is proposed. The characteristics and analog performance of the device were theoretically investigated by exploiting the electrical properties of a germanene nanoribbon and applying the doping concentration in the source and drain regions at 300 K and 4 K temperatures. The device parameters were obtained using a non-equilibrium Green’s function (NEGF) method within the tight binding (TB) Hamiltonian. The TB Hamiltonian was extracted from the density functional theory (DFT) through the Wannier function. We find that by increasing the doping concentration the Ion current increases which leads to an improvement of the Ion/Ioff ratio to 105. Moreover, decreasing the temperature from 300 K to 4 K causes the Ioff to become ten times smaller. We find that the device output characteristic displays a negative differential conductance with a good peak-to-valley ratio which is improved by increasing the doping concentration. The analog performance of the device is also investigated in the subthreshold regime of operation by varying the doping concentration. It is observed that by increasing the device doping concentration, the analog figures of merit can be improved.

Hydrogen sensitive field-effect transistor based on germanene nanoribbon and optical properties of hydrogenated germanene

Physisorption of hydrogen molecules on armchair germanene nanoribbon (GeNR) is studied with density functional methods. The adsorption geometries, adsorption energies and transferred charge are obtained. To take the Van der Waals forces into account, the Grimme correction is added to the calculation method. The physisorption effect on the electrical properties of the ribbon is explored as a function of $$\hbox {H}_{2}$$H2 concentration through the Green's function techniques. Sensing features of the GeNR are investigated as a channel of a back gated field effect transistor. The optical properties of the nanoribbon are obtained for parallel and perpendicular polarizations. The results point out that, the germanene is a suitable substrate for $$\hbox {H}_{2}$$H2 encapsulation. Moreover, $$\hbox {H}_{2}$$H2 physisorption can improve the I---V characteristics and suppress the optical spectrum of the GeNR. The current through the nanoribbon increases by increasing $$\hbox {H}_{2}$$H2 concentration at the same bias voltage. Also, the germanene back gated FET improve the sensing properties. The results show that the GeNR dielectric function is anisotropic and the GeNR becomes more transparent by increasing $$\hbox {H}_{2}$$H2 density. Finally, by applying the spin-orbit coupling (SOC) effect, the obtained results are re-calculated and the changes in the results are studied. The SOC opens up the electronic band gap of the GeNR about 20 meV and increases the current slightly through the GeNR.

Exchange and electric fields enhanced spin thermoelectric performance of germanene nano-ribbon

The spin thermoelectric performance in a germanene nano-ribbon is studied by using the nonequilibrium Green’s function method. We demonstrate theoretically that the temperature bias can generate spin thermopower when an exchange field breaks the edge states of germanene leads. However, the spin thermoelectric efficiency is quite low with its maximum . When applying strong electric field in the central region, a relatively large spin-dependent band gap can be opened, and hence the spin figure of merit is predicted to be more than 100 times larger than the case without external field. The remarkably enhancement of (larger than one) comes from the suppression of the thermal conductance and the improvement of the spin Seebeck effect. These striking properties make ferromagnetic leads germanene nano-ribbon a promising pure spin thermoelectric nanogenerator.

Molecular Dynamics Simulations of Thermal Conductivity of Germanene Nanoribbons (GeNR) with Armchair and Zigzag Chirality

Germanene, an allotrope of germanium which is a two dimensional material withsp2hybridization, has almost the same properties with graphene except for its buckled structure. In this study, germanium nanoribbon (GeNR) is use for it is still a new material for nanoscale level of research. In this paper, we investigate the effect of chirality on the thermal conductivity of zigzag GeNR (ZGeNR) and armchair GeNR (AGeNR) chiralities using equilibrium molecular dynamics with varied lengths at fixed temperature and varied temperatures at fixed length. The simulations were carried out in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) using Tersoff potential for the Ge-Ge interactions. The thermal conductivity is calculated using Green-Kubo method. It is found that the chirality can affect the thermal conductivity of GeNR. Our results show that thermal conductivity of AGeNR is higher than ZGeNR in both increasing temperatures and lengths similar to the thermal conductivity behavior obtained in silicene nanoribbons [Int. J. Mech. Mater. Des. 9 (2013) 105].

Molecular Dynamics Simulation of the Thermal Conductivity of Silicon-Germanene Nanoribbon (SiGeNR): A Comparison with Silicene Nanoribbon (SiNR) and Germanene Nanoribbon (GeNR)

This study examines the nature of thermal transport properties of single layer two-dimensional honeycomb structures of silicon-germanene nanoribbon (SiGeNR), silicene nanoribbon (SiNR) and germanene nanoribbon (GeNR) which have not yet been characterized experimentally. SiGeNR, SiNR and GeNR are the allotropes of silicon-germanium, silicon and germanium, respectively, with sp2 hybridization. The thermal conductivity of the materials has been investigated using Tersoff potential through LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) by performing the molecular-dynamics simulations. The temperature is varied (50 K, 77 K, 150 K, 300 K, 500 K, 700 K, 1000 K, and 1200 K) with fixed nanoribbon dimension of 50 nm × 10 nm. The length is also varied (10 nm, 20 nm, 30 nm, 40 nm, and 50 nm) while the temperature is fixed at room temperature and the width is also fixed at 10 nm. The obtained results showed that the thermal conductivity of SiGeNR at room temperature is approximately 10 times higher than GeNR and approximately 6 times higher compared to SiNR. The thermal conductivity increases as the temperature is increased from 50 K – 300 K, and as the temperature is further increased, the thermal conductivity decreases with temperature. Moreover, the thermal conductivity in SiGeNR, SiNR, and GeNR increases as the length is being increased. Predicting new features of SiGeNR, SiNR and GeNR open new possibilities for nanoelectronic device applications of group IV two-dimensional materials.

Symmetry‐Dependent Spin Transport Properties and Spin‐Filter Effects in Zigzag‐Edged Germanene Nanoribbons

We performed the first‐principles calculations to investigate the spin‐dependent electronic transport properties of zigzag‐edged germanium nanoribbons (ZGeNRs). We choose of ZGeNRs with odd and even widths of 5 and 6, and the symmetry‐dependent transport properties have been found, although the σ mirror plane is absent in ZGeNRs. Furthermore, even‐N and odd‐N ZGeNRs have very different current‐voltage relationships. We find that the even 6‐ZGeNR shows a dual spin‐filter effect in antiparallel (AP) magnetism configuration, but the odd 5‐ZGeNR behaves as conventional conductors with linear current‐voltage dependence. It is found that when the two electrodes are in parallel configuration, the 6‐ZGeNR system is in a low resistance state, while it can switch to a much higher resistance state when the electrodes are in AP configuration, and the magnetoresistance of 270% can be observed.