Monovacancies In Graphene Research Articles

Strain-Tuneable Magnetism and Spintronics of Distorted Monovacancies in Graphene.

The electronic and spintronic properties of the monovacancies in freestanding and isotopically compressed graphene are investigated using hybrid exchange density functional perturbation theory. When the effects of electronic self-interaction are taken into account, an integer magnetic moment of 2 μB is identified for a Jahn-Teller reconstructed V1(5-9) monovacancy in freestanding graphene. For graphene with stable ripples induced by a compressive strain of 5%, a bond reconstruction produces a V1(55-66) structure for the monovacancy, which is localized at the saddle points of the ripple. The sizeable local distortion induced by reconstruction modifies both the geometric and electronic properties of rippled graphene and quenches the magnetic moment of the vacancy due to the sp3 hybridization of the central atom. The nonmagnetic V1(55-66) structure is found to be stable on rippled structures, with the formation energy ∼2.3 eV lower than that of the metastable distorted V1(5-9) structures localized at sites other than the saddle points. The electronic ground state of distorted V1(5-9) corresponds to a wide range of fractional magnetic moments (0.50-1.25 μB). The computed relative stabilities and the electronic and magnetic properties of the V1(5-9) structures are found to be closely related to their local distortions. This analysis of the fundamental properties of defective graphene under compression suggests a number of strategies for generating regular defect patterns with tuneable magnetic and electronic properties and may, therefore, be used as a novel technique to achieve more precise control of graphene electronic structure for various application scenarios such as transistors, strain sensors, and directed chemisorption.

Characterization and Manipulation of Intervalley Scattering Induced by an Individual Monovacancy in Graphene.

Intervalley scattering involves microscopic processes that electrons are scattered by atomic-scale defects on the nanoscale. Although central to our understanding of electronic properties of materials, direct characterization and manipulation of range and strength of the intervalley scattering induced by an individual atomic defect have so far been elusive. Using scanning tunneling microscope, we visualize and control intervalley scattering from an individual monovacancy in graphene. By directly imaging the affected range of monovacancy-induced intervalley scattering, we demonstrate that it is inversely proportional to the energy; i.e., it is proportional to the wavelength of massless Dirac fermions. A giant electron-hole asymmetry of the intervalley scattering is observed because the monovacancy is charged. By further charging the monovacancy, the bended electronic potential around the monovacancy softens the scattering potential, which, consequently, suppresses the intervalley scattering of the monovacancy.

Chemical environment dependent Stabilities, electronic properties and diffusions behaviors of intrinsic point defects in novel Two-Dimensional MoSi2N4 monolayer

A novel 2D semiconductor, MoSi2N4 monolayer, was very recently successfully fabricated and shown great potential for future broad applications in nanoelectronic devices and catalysis. For the synthesis and application of the material, intrinsic point defects are required to be considered, because they are inevitably formed during the bottom-up synthesis and following substrate transfer process, and also can be intentionally introduced by defect engineering to modulate properties and explore applications of materials. Herein, eight types of intrinsic point defects including two vacancies (N mono- and di-vacancies; VN(m) and VN2(m)), four antisites (SiMo, SiN(t), MoN(m) and MoSi) and two adatoms (Siada and Nada) are found to have highly thermodynamic stabilities in MoSi2N4 monolayer under various chemical environments. Electronic properties calculations suggest the existence of VN(m) and VN2(m) leads to the emergence of the recombination center, reducing the carrier lifetime and thus showing the potential for high-speed switching device applications. SiN(t) and Siada-defected structures show p-type and n-type conducting characters, respectively, which can be explored for possible PN junction applications. Also, the existence of MoSi may induce unusual resistance–temperature dependence behavior, providing some chances to design high-mobility semiconductors. Combined with the migration behavior of point defects (monovacancies and adatoms), the Schottky defect formation mechanism of VN(m) could be reasonably explained. Besides, high migration barriers of monovacancy defects (VN(m), VSi and VMo) demonstrate their higher kinetic stability in monolayer, compared with monovacancies in graphene and silicene. The migration of N adatom is relatively difficult than that of Si adatom, increasing opportunities to detect it in experiments. This work will provide insight into defect engineering of MoSi2N4 monolayer and the MA2Z4 monolayer family for various applications.

Publisher’s Note: “Strain induced second-order Jahn–Teller reconstruction and magnetic moment modulation at monovacancy in graphene” [J. Appl. Phys. 130, 034303 (2021)

Publisher’s Note: “Strain induced second-order Jahn–Teller reconstruction and magnetic moment modulation at monovacancy in graphene” [J. Appl. Phys. 130, 034303 (2021)

Strain induced second-order Jahn–Teller reconstruction and magnetic moment modulation at monovacancy in graphene

Using density functional theory simulations, we examine the electronic structure of an isolated monovacancy defect in graphene under symmetry-breaking deformation. Results show that the defect experiences a second-order Jahn–Teller reconstruction at a critical strain of 1.7%. It stabilizes the orientation of the JT bond relative to the loading direction and breaks the threefold degeneracy of the defect structure. We call it Jahn–Teller re-reconstruction (JTRR), and it is mechanically reversible. The reversibility and stabilization of the orientation depend on the direction cosine between the JT bond and the loading direction. Also, a change in the loading direction by 90° can change the orientation of the JT bond by 120°. An atomic-scale analysis suggests that the maximum bond force arising from “the derivative of the kinetic energy of electrons” defines the critical strain. JTRR alters the electron occupation in the individual electronic orbitals at the defect site. The electronic charge redistribution and the density of states at the defective sites reveal that the pz orbitals dominate the reconstruction process. Furthermore, JTRR changes the magnitude of the magnetic moment at the defective site from 1.36 μB to 1.22 μB. This unravels a new way of controlling the magnetic behavior of monovacancy by applying symmetry-breaking mechanical strain. Results also show that passivation of the dangling bond can subside or eliminate the reconstruction process depending on the number of valence electrons available in the passivating atom.

Critical inter-defect distance that modulates strength and toughness in defective 2D sp2-lattice

This paper reveals the existence of a critical separation distance (dc) beyond which the elastic interactions between a pair of monovacancies in graphene or hexagonal boron nitride become inconsequential for the strength and toughness of the defective lattice. This distance is independent of the chirality of the lattice. For any inter-defect distance higher than dc, the lattice behaves mechanically as if there is a single defect. For a distance less than dc, the defect–defect elastic interactions produce distinctive mechanical behavior depending on the orientation (θ) of the defect pair relative to the loading direction. Both strength and toughness of the lattice containing a pair of “interacting monovacancies (iMVs)” are either higher or smaller than that of the lattice containing a pair of “non-interacting monovacancies (nMVs),” suggesting the existence of a critical orientation angle θc. For θ<θc, the smaller the distance between the iMVs, the higher the toughness and strength compared to the lattice containing nMVs, whereas, for θ≥θc, the smaller the separation distance between the iMVs, the smaller the toughness and strength compared to the lattice containing nMVs. The transitional behavior has a negligible dependence on the chirality of the lattice, which indicates that the crystallographic anisotropy has a much weaker influence on toughness and strength compared to the anisotropy induced by the orientation angle itself. These observations underline an important point that the elastic fields emanating from vacancy defects are highly localized and fully contained within a small region of around 1.5 nm radius.

Thermodynamic stability of magnetic states of monovacancy in graphene revealed by ab initio molecular dynamics simulations

The stability of magnetic states is essential for potential spintronic applications. Here we report on the thermal stability of magnetic states of monovacancy graphene using ab initio molecular dynamics simulations. At room temperature, thermal fluctuations of the graphene lattice induce a rapid magnetic switching between two states with a high and low magnetic moment, indicating that due to the instability of the atomic structure of the vacancy, the associated magnetic moment is thermodynamically unstable. Lowering the temperature can significantly reduce the rate of the switching process and enhance the resident time on the high magnetic state. It stabilizes in the high magnetic state at as low as 30 K. Analyzing the atomic trajectories and the instant electronic structures confirms that these two magnetic states in MD simulations correspond to the magnetic and nonmagnetic states reported in the literatures. Such fluctuations of local magnetic moments are associated with the vertical displacement of the carbon atoms with the unsaturated dangling bond. This study reveals the dynamical correlation between atomic movement and the magnetic switching, and a comprehensive picture of vacancy magnetism in graphene. It has implications in graphene based spintronic devices.

π Magnetism of Carbon Monovacancy in Graphene by Hybrid Density Functional Calculations

Understanding magnetism in defective graphene is paramount to improve and broaden its technological applications. A single vacancy in graphene is expected to lead to a magnetic moment with both a σ (1 μB) and a π (1 μB) component. Theoretical calculations based on standard LDA or GGA functional on periodic systems report a partial quenching of the π magnetization (0.5 μB) due to the crossing of two spin split bands at the Fermi level. In contrast, STS experiments (Phys. Rev. Lett. 2016, 117, 166801) have recently proved the existence of two defect spin states that are separated in energy by 20–60 meV. In this work, we show that self-interaction corrected hybrid functional methods (B3LYP-D*) are capable of correctly reproducing this finite energy gap and, consequently, provide a π magnetization of 1 μB. The crucial role played by the exact exchange is highlighted by comparison with PBE-D2 results and by the magnetic moment dependence with the exact exchange portion in the functional used. The ground state ...

Interaction of Monovacancies in Graphene

We analyze the structural and electronic properties of graphene with high density monovacancies by first-principles and cluster expansion calculations in order to establish fundamental insights into the interaction between monovacancies in such conditions. The highest possible defect density is observed to exceed 7.1 atom% and, on the other hand, is bounded by 12.5 atom%. We demonstrate that the structural stability is controlled by the density of dangling bonds within a structure, for which we apply density functional tight bonding molecular dynamics calculations. Cluster expansion calculations are employed to determine the ground state structures as a function of the defect density. We observe a tendency of the monovacancies to form lines. Band structures and densities of states are calculated to evaluate the electronic properties. We find that the band dispersions around the Fermi energy are enhanced for increasing defect density, which indicates that the carrier mobility can be well controlled by mean...

Effect of Nitrogen Doping on the Migration of the Carbon Adatom and Monovacancy in Graphene

Nitrogen-doped graphene (N-graphene) has important implications in graphene-based devices and catalysts. Nitrogen incorporation into graphene via postsynthetic treatment is likely to produce a non-negligible amount of defects and bond disorders, and the resulting nitrogen content is usually dominated by graphitic N and pyridinic N. To understand the kinetic stability of doped N and the effect of doped N on the self-healing of monovacancy in graphene, we have performed density functional theory calculations to study the adsorption and migration of an adsorbed C atom on undoped and N-doped graphene with and without a monovacancy (MV). The effects of N doping and hydrogenation on the migration of a MV in graphene are also studied. Our results suggest that the graphitic N doped in the vicinity of MV is kinetically unstable, and it could be transformed into a pyridinic N due to the migration of MV when N-graphene is through high-temperature annealing. The presence of a C adatom would easily repair the vacancy ...

Self-healing monovacancy in low-buckled silicene studied by first-principles calculations

A new type of monovacancy (MV), MV-1, with a planelike $s{p}^{3}$ hybridization at its defect core has been found in two-dimensional (2D) low-buckled silicene by using the first-principles study, which has never been found to exist stably in the 2D single-layer nanostructures, including other previously studied graphene, $h$-BN sheets, and single-layer ${\mathrm{MoS}}_{2}$. In addition, other two possible monovacancies (MV-2 and MV-3) with higher energies have also been found to exist in 2D low-buckled silicene. More importantly, it is found that the new type MV-1 is the most stable ground structure among the three possible MVs. And the high-energy MV-3 is unstable, easily decaying into MV-1, but metastable MV-2 could coexist with MV-1 at low temperatures less than 10 K. The diffusion coefficient of MV-1 is calculated to be $2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{s}$, much higher than that of the MV in graphene. Finally, electronic structures of the defective silicene with MV-1 and MV-2 are calculated, showing both of them are metallic.

Action-derived molecular dynamics simulations for the migration and coalescence of vacancies in graphene and carbon nanotubes

We report the results of action-derived molecular dynamics simulations for the migration and coalescence processes of monovacancies in graphene and carbon nanotubes with different chiralities. In carbon nanotubes, the migration pathways and barriers of a monovacancy depend on the tube chirality, while there is no preferential pathway in graphene due to the lattice symmetry and the absence of the curvature effect. The probable pathway changes from the axial to circumferential direction as the chirality varies from armchair to zigzag. The chirality dependence is attributed to the preferential orientation of the reconstructed bond formed around each vacancy site. It is energetically more favourable for two monovacancies to coalesce into a divacancy via alternative movements rather than simultaneous movements. The energy barriers for coalescence are generally determined by the migration barrier for the monovacancy, although there are some variations due to interactions between two diffusing vacancies. In graphene and armchair nanotubes, two monovacancies prefer to migrate along different zigzag atomic chains rather than a single atomic chain connecting these vacancies. On the other hand, in zigzag tubes, the energy barrier for coalescence increases significantly unless monovacancies lie on the same circumference.

Modelling magnetism of C at O and B monovacancies in graphene

The presence of defects can introduce important changes in the electronic structure of graphene, leading to phenomena such as C magnetism. In addition, vacancies are reactive and permit the incorporation of dopants. This paper discusses the electronic properties of defective graphene for O and B decoration. Phonon calculations allow us to address directly the stability of the systems under study. We show that it is possible to obtain magnetic solutions with and without dangling bonds, demonstrating that C magnetism can be achieved in the presence of B and O.

Structural Reconstruction of the Graphene Monovacancy

Two distinct configurations of the monovacancy in graphene have been observed using aberration-corrected transmission electron microscopy (AC-TEM) at 80 kV. The predicted lower energy asymmetric monovacancy (MV), exhibiting a Jahn-Teller reconstruction (r-MV), has been observed, but in addition, we have imaged instances of a symmetric monovacancy (s-MV). We have used geometric phase analysis (GPA) to quantitatively determine the strain in the lattice surrounding these two defect configurations and show that the Jahn-Teller reconstruction generates significant extra strain compared to the symmetric MV case. Density functional theory calculations demonstrate that our experimental images of the two different monovacancies show good agreement with both the low energy r-MV and the metastable structures.

Electronic structure of N-doped graphene with native point defects

Nitrogen doping in graphene has important implications in graphene-based devices and catalysts. We have performed the density functional theory calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. The interference between native point defect and N dopant strongly modifies the role of N doping regarding the free carrier production in the bulk $\ensuremath{\pi}$ bands. For some of the defects and N dopant-defect complexes, localized defect $\ensuremath{\pi}$ states are partially occupied. Discussion on the possibility of spin polarization in such cases is given. We also present qualitative arguments on the electronic structures based on the local bond picture. We have analyzed the 1$s$-related x-ray photoemission and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales defect in connection with the experimental ones. We also discuss characteristic scanning tunneling microscope (STM) images originating from the electronic and structural modifications by the N dopant-defect complexes. STM imaging for small negative bias voltage will provide important information about possible active sites for oxygen reduction reaction.

Fluorinated monovacancies in graphene: Even-odd effect

The electronic and structural properties of fluorinated monovacancies in graphene are studied using density functional theory. Our calculations show that an odd number of F atoms adsorbed on a monovacancy gives rise to a p-type metallic state with a local magnetic moment of 1 μB. In contrast, an even number of F atoms leads to a non-magnetic semiconducting state. We explain the behaviour in terms of local structure properties.

Oxidation of monovacancies in graphene by oxygen molecules

We study the oxidation of monovacancies in graphene by oxygen molecules using first principles calculations. In particular, we address the local magnetic moments which develop at monovacancies and show that they remain intact when a molecule is adsorbed such that the dangling carbon bonds are not fully saturated. However, the lowest energy configuration does not maintain dangling bonds and is found to be semiconducting. Our data can explain the experimentally observed behavior of graphene under exposure to an oxygen plasma.

First-principles study of substitutional metal impurities in graphene: structural, electronic and magnetic properties

We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and find that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon–metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurities present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1μB and are increasingly delocalized. The substitutional Zn suffers a Jahn–Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while it is non-magnetic at the level of generalized gradient approximation (GGA) calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.