Elastic and plastic deformation of graphene, silicene, and boron nitride honeycomb nanoribbons under uniaxial tension: A first-principles density-functional theory study
This study of elastic and plastic deformation of graphene, silicene, and boron nitride (BN) honeycomb nanoribbons under uniaxial tension determines their elastic constants and reveals interesting features. In the course of stretching in the elastic range, the electronic and magnetic properties can be strongly modified. In particular, it is shown that the band gap of a specific armchair nanoribbon is closed under strain and highest valance and lowest conduction bands are linearized. This way, the massless Dirac fermion behavior can be attained even in a semiconducting nanoribbon. Under plastic deformation, the honeycomb structure changes irreversibly and offers a number of new structures and functionalities. Cagelike structures, even suspended atomic chains can be derived between two honeycomb flakes. Present work elaborates on the recent experiments [C. Jin, H. Lan, L. Peng, K. Suenaga, and S. Iijima, Phys. Rev. Lett. 102, 205501 (2009)] deriving carbon chains from graphene. Furthermore, the similar formations of atomic chains from BN and Si nanoribbons are predicted.
Silicene (a monolayer of silicon atoms) is a quantum spin-Hall insulator, which undergoes a topological phase transition into other insulators by applying external field such as electric field, photo-irradiation and antiferromagnetic order. We investigate the electronic and transport properties of silicene nanoribbons based on the Landauer formalism. We propose to determine topological phase transitions by measuring the density of states and conductance. The conductance is quantized and changes its value when the system transforms into different phases. We show that a silicene nanoribbon near …
Electric and thermoelectric properties of silicene nanoribbons doped with Al and P impurity atoms are investigated theoretically for both antiparallel and parallel orientations of the edge magnetic moments. In the former case, appropriately arranged impurities can lead to a net magnetic moment and thus also to spin thermoelectric effects. In the latter case, in turn, spin thermoelectric effects also occur in the absence of impurities. Numerical results based on ab-initio calculations show that the spin thermopower can be considerably enhanced by the impurities.
By using first-principles calculations, we predict that an in-plane homogenous electrical field can induce half-metallicity in hydrogen-terminated zigzag silicene and germanene nanoribbons (ZSiNRs and ZGeNRs). A dual-gated finite ZSiNR device reveals a nearly perfect spin-filter efficiency of up to 99% while a quadruple-gated finite ZSiNR device serves as an effective spin field effect transistor (FET) with an on/off current ratio of over 100 from ab initio quantum transport simulation. This discovery opens up novel prospect of silicene and germanene in spintronics.
Using density functional theory and non-equilibrium Greens function technique, we performed theoretical investigations on the structural and transport properties of zigzag silicene nanoribbons with Stone-Wales defect. The calculated formation energy is significantly lower than that of graphene and silicene, which implies the high stability of such defect in SiNRs. Negative differential resistance can be observed within certain bias voltage range in both perfect and SW defected SiNRs. In order to elucidate the mechanism the NDR behavior,the transmission spectra and molecular projected self-cons…
We have performed scanning tunneling microscopy/spectroscopy (STM/STS) study and first-principles calculations to investigate the atomic structure and electronic properties of silicon nanoribbons (SiNRs) grown on Ag(110). Despite of the extensive research on SiNRs in the last decades, its atomic structure is still not fully understood so far. In this report we determine that the structure of SiNRs/Ag(110) is armchair silicene nanoribbon with reconstructed edges. Meanwhile, pronounced quantum well states (QWS) in SiNRs were observed and their energy spectrum was systematically measured. The QWS…
Using ab-initio methods we calculate thermoelectric and spin thermoelectric properties of silicene nanoribbons with bare, mono-hydrogenated and di-hydrogenated edges. Asymmetric structures, in which one edge is either bare or di-hydrogenated while the other edge is mono-hydrogenated (0H-1H and 2H-1H nanoribbons) have ferromagnetic ground state and display remarkable conventional and spin thermoelectric properties. Strong enhancement of the thermoelectric efficiency, both conventional and spin ones, results from a very specific band structure of such nanoribbons, where one spin channel is block…
Silicene is a two-dimensional quantum spin-Hall insulator. We study the edge channels of silicene nanoribbons from the viewpoint of the topological protection and the interference between the two edges. It is found that the behaviors of the helical edge channels (HECs) are completely different between the armchair and zigzag edges. The penetration depths $ξ$ of the HEC is antiproportional to the spin-orbit gap for the armchair edge $ξ_{\rm arm} \sim \hbar v_\text{F}/ Δ$ ($v_\text{F}$: the Fermi velocity, $Δ$: the gap due to the spin-orbit interaction), while it remains as short as the lattice …
Transport and thermoelectric coefficients (including also spin thermopower) of silicene nanoribbons with zigzag edges are investigated by {\it ab-initio} numerical methods. Local spin density of such nanoribbons reveals edge magnetism. Like in graphene, one finds antiferromagnetic and ferromagnetic ordering, with spin polarization at one edge antiparallel or parallel to that at the other edge, respectively. Thermoelectric properties, especially the Seebeck coefficient, significantly depend on the electronic band structure and are enhanced when the Fermi level is in the energy gap. However, the…
Armchair silicene nanoribbons with width of 9-39 silicon atoms are investigated by using self-consistent field crystal orbital method based on density functional theory. The carrier mobilities obtained from deformation potential theory oscillate with respect to the width and the values are a fraction of what the graphene nanoribbons have. The buckled structure, hydrogen saturation, edge reconstruction as well as edge roughness decrease the carrier mobilities which are explained with the aid of crystal orbitals.
We report results on the Young's modulus (YM) of defect-free and defective silicene nanoribbons (SNRs) as a function of length and temperature. In this study, we perform molecular dynamics simulations using the Environment-Dependent Interatomic Potential (EDIP) to describe the interaction of the Si atoms. We find that the Young's modulus of pristine and defective SNRs increases with the ribbon length in both chirality directions. It is shown that the Young's modulus of defective SNRs exhibit a complex dependence on the combinations of vacancies. With respect to temperature, we find that YM for…
The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magnetic configuration of the two electrodes, are discussed. Switching on and off the external magnetic field, we may convert the metallic ferromagnetic ZSiNRs into insulating antiferromagnetic ZSiNRs. In …
We study the crystal reconstruction in the presence of monovacancies (MVs), divacancies (DVs) and linear vacancies (LVs) in a zigzag silicene nanoribbon (ZSiNR) with transversal symmetry. Their influence on the electric and thermoelectric properties is assessed by the density functional theory combined with the nonequilibrium Green's functions. In particular, we focus on the spin resolved conductance, magnetoresistance and current-voltage curves. A 5-atom-ring is formed in MVs, a 5-8-5 ring structure in DVs, and a 8-4-8-4 ring structure in LVs. The linear conductance becomes strongly spin depe…
Silicene, a new two-dimensional (2D) material has attracted intense research because of the ubiquitous use of silicon in modern technology. However, producing free-standing silicene has proved to be a huge challenge. Until now, silicene could be synthesized only on metal surfaces where it naturally forms strong interactions with the metal substrate that modify its electronic properties. Here, we report the first experimental evidence of silicene sheet on an insulating NaCl thin film. This work represents a major breakthrough; for the study of the intrinsic properties of silicene, and by extens…
The effect of chemical doping on the ZSiNRs with Mn as passivating element replacing H atoms at one edge are investigated by first principles calculations.The structures optimized in the typical ferromagnetic and antiferromagnetic coupling show that the system leads to an AFM state and achieve half metallic properties.Also,our first principle approach based on the Keldysh nonequilibrium Greens function method gives the spin dependent transport properties of the device. When the system changes from parallel to antiparallel configuration. the spin up current increases rapidly while the spin down…
We report on the oxidation of self-assembled silicene nanoribbons grown on the Ag(110) surface using Scanning Tunneling Microscopy and High-Resolution Photoemission Spectroscopy. The results show that silicene nanoribbons present a strong resistance towards oxidation using molecular oxygen. This can be overcome by increasing the electric field in the STM tunnel junction above a threshold of +2.6 V to induce oxygen dissociation and reaction. The higher reactivity of the silicene nanoribbons towards atomic oxygen is observed as expected. The HR-PES confirm these observations: Even at high exposu…
A nonequilibrium Green's function technique combined with density functional theory is used to study the spin-dependent electronic band structure and transport properties of zigzag silicene nanoribbons (ZSiNRs) doped with aluminum (Al) or phosphorus (P) atoms. The presence of a single Al or P atom induces quasibound states in ZSiNRs that can be observed as new dips in the electron conductance. The Al atom acts as an acceptor whereas the P atom acts as a donor when it is placed at the center of the ribbon. This behavior is reversed when the dopant is placed on the edges. Accordingly, an accepto…
Based on atomistic simulations, the nonlinear elastic properties of monolayer graphene nanoribbons under quasistatic uniaxial tension are predicted, emphasizing the effect of edge structures (armchair and zigzag, without and with hydrogen passivation). The results of atomistic simulations are interpreted within a theoretical framework of thermodynamics, which enables determination of the nonlinear functions for the strain-dependent edge energy and the hydrogen adsorption energy, for both zigzag and armchair edges. Due to the edge effects, the initial Young's modulus of graphene nanoribbons und…
The spin-dependent electronic structures of aluminum-(Al) doped zigzag silicene nanoribbons (ZSiNRs) are investigated by first-principles calculations. When ZSiNRs are substitutionally doped by a single Al atom on different sites in every three primitive cells, they become half-metallic in some cases, a property that can be used in spintronic devices. More interestingly, spin-down electrons can be transported at the Fermi energy when the Al atom is placed on the sub-edge site. In contrast, spin-up electrons can be transported at the Fermi energy when the ZSiNRs are doped on sites near their ce…
Using density functional theory and non-equilibrium Greens function technique, we performed theoretical investigations on the transport properties of several ZAZ SiNRs junctions,a similar kind of silicene molecules junction combined by zigzag and armchair silicene nanoribbons. It is found that the differential conductances of the three systems decrease with an order of 5-ZAZ>4-ZAZ>3-ZAZ.Particularly,the Negative differential resistance can be observed within certain bias voltage range only in 3-ZAZ SiNRs. In order to elucidate the mechanism the NDR behavior, the transmission spectra and molecu…
In this Letter, we investigate the strain-induced band-gap modulation of both armchair and zigzag graphane nanoribbons based on the first-principles calculations. Within the elastic range, the band gap changes linearly with the uniaxial strain, where the band-gap of graphane nanoribbonsis more sensitive to compressive than tensile deformation. The band-gap deformation mainly originates from the shift of valence band edge of graphane nanoribbons under stain. Our results imply the great potential of graphane nanoribbons in the pressure sensor and optical electronics applications.
Uniaxial tensile properties of hexagonal boron nitride nanoribbons and dependence of these properties on temperature, strain rate, and the inclusion of vacancy defects have been explored with molecular dynamics simulations using Tersoff potential. The ultimate tensile strength of pristine hexagonal boron nitride nanoribbon of 26 nm x 5 nm with armchair chirality is found to be 100.5 GPa. The ultimate tensile strength and strain have been found decreasing with increasing the temperature while an opposite trend has been observed for increasing the strain rate. Furthermore, the vacancy defects re…
Bilayer silicene has richer physical properties than bilayer graphene due to its buckled structure together with its trigonal symmetric structure. The buckled structure arises from a large ionic radius of silicon, and the trigonal symmetry from a particular way of hopping between two silicenes. It is a topologically trivial insulator since it carries a trivial $\mathbb{Z}_{2}$ topological charge. Nevertheless, its physical properties are more akin to those of a topological insulator than those of a band insulator. Indeed, a bilayer silicene nanoribbon has edge modes which are almost gapless an…
Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which shares almost every remarkable property with graphene. The low energy structure of silicene is described by Dirac electrons with relatively large spin-orbit interactions due to its buckled structure. The key observation is that the band structure is controllable by applying the electric field to a silicene sheet. In particular, the gap closes at a certain critical electric field. Examining the band structure of a silicene nanoribbon, we demonstrate that a topological phase transition occurs from a topol…
Two dimensional allotropes of group IV substrates including silicene, germanene and stanene have recently attracted considerable attention in nanodevice fabrication industry. These materials involving the buckled structure have been experimentally fabricated lately. In this study, first principles density functional theory calculations were utilized to investigate the mechanical properties of single layer and free standing silicene, germanene and stanene. Uniaxial tensile and compressive simulations were carried out to probe and compare stress strain properties; such as the Young modulus, Pois…
Graphene nanoribbons are widely regarded as promising building blocks for next-generation carbon-based devices. A critical issue to their prospective applications is whether and to what degree their electronic structure can be externally controlled. Here, we combine simple model Hamiltonians with extensive first-principles calculations to investigate the response of armchair graphene nanoribbons to transverse electric fields. Such fields can be achieved either upon laterally gating the nanoribbon or incorporating ambipolar chemical co-dopants along the edges. We reveal that the field induces a…
Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which shares almost every remarkable property with graphene. The low energy structure of silicene is described by Dirac electrons with relatively large spin-orbit interactions due to its buckled structure. The key observation is that the band structure is controllable by applying the electric field to a silicene sheet. We explore the phase diagram of silicene together with exchange field $M$ and by applying electric field $E_{z}$. There appear quantum anomalous Hall (QAH) insulator, valley polarized metal (VP…
We have performed a first-principles study on the deformation effect of the electronic structures of graphite nanoribbon arrays with zigzag edges on both sides, and the edge atoms are terminated with hydrogen atoms. A uniaxial strain is considered to have deformation effect on the graphite nanoribbons. We found that the antiferromagnetic arrangement of the spin polarizing edges of graphite nanoribbon is still more favorable than that with the ferromagnetic arrangement under deformation. We also learned that a tensile strain increases the magnetization of the graphite ribbon while a compressive…
The main contribution of this paper is to study the spin caloritronic effects in defected graphene/silicene nanoribbon (GSNR) junctions. Each step-like GSNR is subjected to the ferromagnetic exchange and local external electric fields, and their responses are determined using the nonequilibrium Greens function (NEGF) approach. To further study the thermoelectric (TE) properties of the GSNRs, three defect arrangements of divacancies (DVs) are also considered for a larger system, and their responses are re-evaluated. The results demonstrate that the defected GSNRs with the DVs can provide an alm…
Using molecular dynamics(MD) simulations, we study thermal transport in graphene nanoribbons (GNR) subjected to uniform uniaxial and nonuniform strain fields. We predict significant thermal rectification (over 70%) in a rectangular armchair GNR by applying a transverse force asymmetrically. The heat flux is larger from the less stressed region to the more stressed region. Furthermore, we develop a theoretical framework based on the non-equilibrium thermodynamics to discuss when thermal rectification under a stress gradient can occur. We conclude with a discussion of details relevant to experim…
Using first principle calculations, we study the structural, optical and electronic properties of two-dimensional silicene-like structures of CSi7 (carbosilicene) and GeSi7 (germasilicene) monolayers. We show that both CSi7 and GeSi7 monolayers have different buckling that promises a new way to control the buckling in silicene-like structures. Carbon impurity decreases the silicene buckling, whereas germanium impurity increases it. The CSi7 has semiconducting properties with 0.25 eV indirect band gap, but GeSi7 is a semimetal. Also, under uniaxial tensile strain, the semiconducting properties …
The effects of tensile strain on the current-voltage (I-V) characteristics of hydrogenated-edge armchair graphene nanoribbons (HAGNRs) are investigated by using DFT theory. The strain is introduced in two different ways related to the two types of systems studied in this work: in-plane strained systems (A), and out-of-plane strained systems due to bending (B). These two kinds of strain lead to make a distinction among three cases: in-plane strained systems with strained electrodes (A1) and with unstrained electrodes (A2), and out-of-plane homogeneously strained systems with unstrained, fixed e…
An exact mapping of the tight-binding Hamiltonian for a graphene's nanoribbon under any armchair uniaxial strain into an effective one-dimensional system is presented. As an application, for a periodic modulation we have found a gap opening at the Fermi level and a complex fractal spectrum, akin to the Hofstadter butterfly resulting from the Harper model. The latter can be explained by the commensurability or incommensurability nature of the resulting effective potential. When compared with the zig-zag uniaxial periodic strain, the spectrum shows much bigger gaps, although in general the state…
Graphene nanoribbons with armchair edges are studied for externally enhanced, but realistic parameter values: enhanced Rashba spin-orbit coupling due to proximity to a transition metal dichalcogenide like WS$_{2}$, and enhanced Zeeman field due to exchange coupling with a magnetic insulator like EuS under applied magnetic field. The presence of s--wave superconductivity, induced either by proximity or by decoration with alkali metal atoms like Ca or Li, leads to a topological superconducting phase with Majorana end modes. The topological phase is highly sensitive to the application of uniaxial…
We report a first-principles study on electronic structures of the deformed armchair graphene nanoribbons (AGNRs). The variation of the energy gap of AGNRs as a function of uniaxial strain displays a zigzag pattern, which indicates that the energy gaps of AGNRs can be effectively tuned. The spatial distributions of two occupied and two empty subbands close to the Fermi level are swapped under different strains. The tunable width of energy gaps becomes narrower as increasing the width of AGNRs. Our simulations with tight binding approximation, including the nearest neighbor hopping integrals be…
A caloritronic device setup is proposed that harnesses the intrinsic spin-valley locking of two-dimensional honeycomb lattices with graphene-like valleys, for instance, silicene and stanene. Combining first-principles and analytic calculations, we quantitatively show that when sheets of such materials are placed on a ferromagnetic substrate and held between two contacts at different temperatures, an interplay between the electron degrees-of-freedom of charge, spin, and valley arises. A manifestation of this interplay are finite charge, spin, and valley currents. Uniaxial strain that adjusts th…
Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons upon the presence of stress applying Density Functional Theory within the GGA-PBE approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of +/- 0.02%. The mechanical properties show a linear-response within that range while the non-linear dependence is found for higher strain. The most relevant results indicate that Young's modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrica…
Density functional study of strain effects on the electronic band structure and transport prop- erties of the graphene nanoribbons (GNR) is presented. We apply a uniaxial strain in the x (nearest-neighbor) and y (second nearest-neighbor) directions, related to the deformation of zigzag and armchair edge GNRs (AGNR and ZGNR), respectively. We calculate the quantum conduc- tance and band structures of the GNR using the Wannier function in a strain range from -8% to +8% (minus and plus signs show compression and tensile strain). As strain increases, depending on the AGNR family type, the electric…
Strain-enhanced edge ferromagnetism and bipolar magnetic semiconducting behavior in Janus graphene nanoribbons
School of Physics and Astronomy, and Key Laboratory of Multiscale Spin Physics (Ministry of Education), Beijing Normal University（北京师范大学物理与天文学院，多尺度自旋物理教育部重点实验室）；School of Physics and Electronics, Shandong Normal University（山东师范大学物理与电子科学学院）
We formulate and implement Cyclic Density Functional Theory (Cyclic DFT) -- a self-consistent first principles simulation method for nanostructures with cyclic symmetries. Using arguments based on Group Representation Theory, we rigorously demonstrate that the Kohn-Sham eigenvalue problem for such systems can be reduced to a fundamental domain (or cyclic unit cell) augmented with cyclic-Bloch boundary conditions. Analogously, the equations of electrostatics appearing in Kohn-Sham theory can be reduced to the fundamental domain augmented with cyclic boundary conditions. By making use of this sy…
Silica or SiO$_2$, the main constituent of earth's rocks has several 3D complex crystalline and amorphous phases, but it does not have a graphite like layered structure in 3D. Our theoretical analysis and numerical calculations from the first-principles predict a single-layer honeycomb like allotrope, h$α$-silica, which can be viewed to be derived from the oxidation of silicene and it has intriguing atomic structure with re-entrant bond angles in hexagons. It is a wide band gap semiconductor, which attains remarkable electromechanical properties showing geometrical changes under external elect…
First principles density-functional theory calculations were performed to study the effects of strain, edge passivation, and surface functional species on the structural and electronic properties of armchair graphene nanoribbons (AGNRs) with a particular focus on the work function. The work function was found to increase with uniaxial tensile strain while decreasing with compression. The variation of the work function under strain is primarily due to the shift of the Fermi energy with strain. In addition, the relationship between the work function variation and the core level shift with strain…
