Divacancy Research Articles

Effects of divacancies on the electronic properties of zigzag-edge buckling silicene nanoribbons

In this study, we explore the influence of divacancies (DVs) on the electronic structure and thermoelectric properties of zigzag buckling silicene nanoribbons (ZBSiNRs). We employ the tight-binding approach and determine the electronic structure by numerically diagonalizing a tight-binding Hamiltonian with various configurations of hopping parameters. We investigate impacts of an external electric field perpendicular to the material surface on the band gap's width and the Seebeck coefficient's maximal value. Our results reveal that ZBSiNRs retain their metallic nature even in divacancies. However, the dispersion curves of the flat bands transform into upward-sloping bands. Additionally, our calculations indicate that introducing DVs near the ribbon edges can enhance the conductance of the ribbons. Notably, the width of the band gap and the Seebeck coefficient of the ZBSiNRs exhibit an almost linear increase in response to an applied electric field.

First-principles study of sodium adsorption on defective graphene under propylene carbonate electrolyte conditions.

Hard carbon (HC) has been predominantly used as a typical anode material of sodium-ion batteries (SIBs) but its sodiation mechanism has been debated. In this work, we investigate the adsorption of Na atoms on defective graphene under propylene carbonate (PC) and water solvent as well as vacuum conditions to clarify the sodiation mechanism of HC. Within the joint density functional theory framework, we use the nonlinear polarizable continuum model for PC and the charge-asymmetric nonlocally-determined local electric solvation model for water. Our calculations reveal that the centre of each point defect such as mono-vacancy (MV), di-vacancy (DV) and Stone-Wales is a preferable adsorption site and the electrolyte enhances the Na adsorption through implicit interaction. Furthermore, we calculate the formation energies of multiple Na atom arrangements on the defective graphene and estimate the electrode potential versus Na/Na+, verifying that the multiple Na adsorption on the MV and DV defective graphene under the PC electrolyte conditions is related to the slope region of the discharge curve in HC. This reveals new prospects for optimizing anodes and electrolytes for high performance SIBs.

Structural modification and band gap engineering of carbon nano-onions via sulphur doping: Theoretical DFT study

Density functional theory (DFT) calculations were performed to investigate the structures and electronic properties of S-doped carbon nano-onions (S-CNOs). We report a detailed study of the stoichiometric and defective (monovacancy (MV) and divacancy (DV)) CNOs with different doping contents and distribution patterns. The results indicate the natural tendency of sulphur to occur in groups around the edges of the local deformations and at the defects that are in very good qualitative agreement with the experimental data. [22] More importantly, the five-membered rings containing disulphide bridges are the most stable binding motif in the structure of CNOs. However, the S-S motif can also be located in the six-membered ring either with both two-coordinated S atoms or with one- and three-fold coordinated S atoms. For defective systems, we also studied the energetic stability of the MV- and DV-containing S-CNOs. The results reveal that the most stable position of sulphur is the two-fold coordinated carbon site in the first case, whereas in the second case, it is the one linking octagonal and pentagonal rings. Furthermore, the electronic analysis indicates that the position and concentration of dopant and the type of defect unambiguously influence the band gap energy in both stoichiometric and defective CNOs.

Structural modification enhances the optoelectronic properties of defect blue phosphorene thin films

Active enhancement of the optical absorption coefficient to improve the light converting efficiency of thin-film solar cell materials is crucial to develop the next-generation solar cell devices. Here we report first-principles calculations with generalized gradient approximation to study the optoelectronic properties of pristine and divacancy (DV) blue phosphorene (BlueP) thin films under structural deformation. We show that instead of forming sp-like covalent bonds as in the pristine BlueP layer, a DV introduces two particular dangling bonds between the voids. Using a microscopic (non-) affine deformation model, we reveal that the orbital hybridization of these dangling bonds is strongly modified in both the velocity and vorticity directions depending on the type of deformation, creating an effective light trap to enhance the material absorption efficiency. Furthermore, this successful light trap is complemented by a clear signature of σ + π plasmon when a DV BlueP layer is slightly compressive. These results demonstrate a practical approach to tailor the optoelectronic properties of low-dimensional materials and to pave a novel strategy to design functionalized solar cell devices from the bottom-up with selective defects.

A density functional theory study on the adsorption reaction mechanism of double CO2 on the surface of graphene defects.

Much research has been done on reactions of a single CO2 molecule with a graphene surface. In this paper, density functional theory calculations are used to investigate the adsorption and reaction of double CO2 on the surface of single vacancy (SV) and divacancy (DV) defect graphene. The study found that due to the mutual repulsion between CO2 and the size of the SV defect, it is difficult for two CO2 molecular to be adsorbed directly above the SV defect at the same time. Regardless of SV or DV, the adsorption of the first CO2 in the defect center will have a beneficial effect on the adsorption of the second CO2. In addition, the transition state calculation of the CO2 reaction on the DV plane was carried out, and the adsorption behavior was analyzed and studied. This in-depth study is helpful to the understanding of the reaction behavior of CO2 on graphene, and further exploration in the direction of the effective application of graphene to the reaction and adsorption of CO2. Our work explores the adsorption behavior of CO2 on graphene surfaces, the physical and chemical adsorption of double CO2 at the defect was studied and analyzed.

Defects investigation of bipolar exfoliated phosphorene nanosheets

Two-dimensional (2D) phosphorene has gained attention due to its exceptional chemical, physical, and optoelectronic properties. However, defects analysis in exfoliated phosphorene through experimental techniques is still largely missing. In this paper, a combination of high-resolution transmission electron microscopy (HRTEM) imaging and density functional theory calculations were provided to study the point defects, grain boundaries (GBs), and amorphization phenomenon in exfoliated phosphorene nanosheets via bipolar electrochemistry method. The HRTEM results demonstrate that the single vacancies (SV) and di-vacancies (DV), ad-atoms, and GBs defects are formed in phosphorene nanosheets. However, the exfoliated black phosphorus nanosheets maintained its orthorhombic crystal structure. In addition, amorphization on the edges and surface of nanosheets is unavoidable in the presence of oxygen. Our first-principles simulation confirms the breakage of P-P bonds of phosphorene upon surface oxidation, which results in amorphization. The defect analysis of phosphorene nanosheets obtained from this study could benefit both fundamental research and technological applications.

Vacancy tuned thermoelectric properties and high spin filtering performance in graphene/silicene heterostructures

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 Green’s 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 almost perfect thermal spin filtering effect (SFE), and spin switching. A negative differential thermoelectric resistance (NDTR) effect and high spin polarization efficiency (SPE) larger than 99.99% are obtained. The system with the DV defects can show a large spin-dependent Seebeck coefficient, equal to Ss ⁓ 1.2 mV/K, which is relatively large and acceptable. Appropriate thermal and electronic properties of the GSNRs can also be obtained by tuning up the DV orientation in the device region. Accordingly, the step-like GSNRs can be employed to produce high efficiency spin caloritronic devices with various features in practical applications.

Impact of oxygen adsorption on the electronic properties and contact type of a defective epitaxial graphene-SiC interface

Adsorption and dissociation of molecular oxygen on a defective graphene buffer layer (BL) grown on Si-terminated SiC(0001) are herein studied by means of periodic DFT calculations coupled with the nudged elastic band method. We considered a single vacancy (SV) and a divacancy (DV) defective BL on SiC, with or without the uppermost epitaxial monolayer graphene (EG). The adsorption energy of O2 on SV and DV surfaces, producing SV-O and DV-O oxidized systems, is between 2 and 3 times as strong as that on a defect-free BL surface and lacks an energy barrier. In addition, we demonstrate that the n-doping measured in the EG layer of SiC-BL-EG systems, which stems from the charge transfer in the SiC-BL interface, can be fully compensated by oxidizing the DV defective BL. The DV-O-EG interface forms a p-type Schottky contact with a small Schottky barrier height. Instead, a p-type Ohmic contact was measured at the SV-O-EG interface.

On the electronic properties of defective graphene buffer layer on 6H–SiC(0001)

Using first-principles calculations we predict that the electronic properties of graphene buffer layer (BL) on silicon carbide (SiC) can be modified by defect-induced itinerant states. Band structures and effective masses of single vacancy (SV), divacancy (DV) and Stone-Wales (SW) defective BL were calculated. Structural reconstruction at the vicinity of defects as well as spin polarization of silicon dangling bonds in the top bilayer SiC and nearby carbons from the BL, give rise to energetically degenerate magnetic states displaying electronic properties completely apart. This is particularly true for the SW and SV defective models where as many as three different magnetic states, turned out to be degenerate, either displaying a semiconductor nature or becoming half-metallic or even metallic. This is in contrast with previously reported results for the perfect BL (PBL) which suggested that most stable degenerate magnetic configurations share a semiconductor nature. On the other hand, for the DV system, the introduction of two vacancies in the BL causes loss of magnetism whereas a band gap of 0.54 eV is opened. Hole effective masses decay upon removal of one and two carbon atoms in the SV and DV systems where an increase in mobility is conceivable, provided current direction is carefully selected.

Probing divacancy defects in a zigzag graphene nanoribbon through an RKKY exchange interaction

We investigate the effects of vacancy defects on the electronic and magnetic properties of zigzag graphene nanoribbons (zGNRs) by making use of the Green’s function formalism in combination with a tight-binding Hamiltonian. We explain the evolution of indirect exchange coupling, known as the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, including single, double, and multiple 5-8-5 divacancy (DV) defects. Our numerical calculations show that changes in the electronic structure and exchange coupling of zGNRs depend to a significant degree on the location of DV defects with respect to the ribbon edges and the number of DV defects. In the case where both impurities are located on the edge, the magnitude of the exchange coupling is several orders of magnitude higher than when they are placed on the interior of the nanoribbon. Furthermore, a periodic DV causes a dramatic change in the magnetic ground state of the ribbon. In the limit of a high vacancy potential, the strength of the RKKY interaction is approximately independent of the Fermi energy. We demonstrate that, on the one hand, the defect engineering of atomic vacancies is a promising way to modify the magnetic properties of graphene nanoribbons, and, on the other hand, unusual RKKY oscillations around the DVs are a new technique for directly probing the local vacancies in a zGNR through the RKKY exchange interaction.

In situ transmission electron microscopy study of the formation and migration of vacancy defects in atomically thin black phosphorus

Structural defects play an important role in the optimization of material structures and properties, especially in low-dimensional systems such as two-dimensional (2D) materials. In this work, we investigated the formation, aggregation, and diffusion of vacancy defects in atomically thin black phosphorus (BP) via in situ high-resolution transmission electron microscopy. Vacancy defects including di-vacancies (DVs), vacancy clusters (e.g. tetra-vacancy and TV), and vacancy lines were confirmed as the primary forms of structural defects in BP. DV and TV defects were found to be highly mobile. The defects preferentially diffused and migrated along the diagonal and in a zigzag pattern (rather than an armchair pattern). After prolonged thermal excitation and electron-beam irradiation, all these as-formed vacancies tended to aggregate and line up parallel to the zigzag pattern direction to form extended vacancy lines with a total length reaching hundreds of nanometers or even the micrometer scale. Ab initio calculations were conducted to reveal the vacancy migration pathway, energy landscape, and modifications to the electronic structure of the host BP monolayers (MLs). It was found that the migration of a 5-8-5 DV was accomplished via sequential structural transformations including several transitions and intermediate configurations, such as 5-7-7-5 DVs. The associated migration barriers were determined as 2.1 eV for diagonal migration and 2.6 eV along the zigzag path, respectively. Calculations further confirmed that the presence of vacancy defects induced considerable electronic structure modification of the host ML-BP; for example, the bandgap was reduced from 0.9 eV (for defect-free ML-BP) to 0.7 eV in the presence of vacancy lines with a concentration of 1.2 at.%. The present study expands the current understanding of the formation and dynamic behaviors of primary vacancy defects and illustrates methods available to alter the electronic structures of 2D BP materials. It can further serve as a guideline for the function-oriented design and fabrication of BP-based devices via precisely controlled defect engineering.

The influence of point defects on Na diffusion in black phosphorene: First principles study

Two-dimensional layered materials like graphene, phosphorene and silicene are promising materials for use as anodes in Li- and Na-ion batteries. However, during synthesis of these materials, point defects – single vacancy (SV), divacancy (DV) or Stone–Wales (SW) type – are quite likely to form and to change the performance as an anode material. In this study, the influences of these defects on Na adsorption performance in black phosphorene are investigated. We conclude that these defects could affect performance of phosphorene as an anode material: negatively in the case of DV-2, SV and SW-2 defects but positively for DV-1 and SW-1 defects. This impact on the performance is greatest in both paths (zigzag and armchair) for SW-2 defects with the diffusion coefficient almost zero. However, the SW-1 and DV-1 defects could improve Na diffusion in phosphorene making these desirable for phosphorene as anode material for Na-ion batteries.

Exploring the nature of interaction and stability between DNA/RNA base pairs and defective & defect-dopant graphene sheets. A possible insights on DNA/RNA sequencing

A quantum chemical investigation is performed to understand the adsorption behaviour of DNA/RNA base pairs onto the defective (Di-Vacancy (DV) and Stone-Wales (SW)), boron (B) and silicon (Si) defect-dopant graphene (B-DV, Si-DV, B-SW, and Si-SW) sheets using density functional theory (DFT). The stability of DNA/RNA base pairs on the Si-SW sheet is found to be −80.59 kcal/mol (G-C), −70.21 kcal/mol (A-T), and −69.78 kcal/mol (A-U). The quantum theory of atoms in molecule (QTAIM) analysis concluded that the interaction of DNA/RNA base pair on Si-SW sheet has partially electrostatic and partially covalent (Si⋯N) characters. The natural bond orbital analysis (NBO), electron density difference map (EDDM), and natural population analysis (NPA) are revealed that the charge has been transferred from DNA/RNA base pair to defective and defective-dopant graphene sheet. From the time-dependent density functional theory (TD-DFT), a strong redshift is observed at 482 nm for GC-Si-SW, 494 nm for AT-Si-SW, and 497 nm for AU-Si-SW. Hence, the substantial variations in the HOMO-LUMO gap (∆EHL) and UV spectra of Si-SW sheet after the adsorption of DNA/RNA base pair can be beneficially exploited to design a new bio-sensor or DNA/RNA sequencing devices.

DFT study of adsorption of ions on doped and defective graphene

Heteroatom doped carbon materials are one of the most prominent families of material that are used in various energy-related applications. Herein, we have investigated the effects of metal ions adsorption on Boron-Nitrogen (BN) codoped and Sulphur (S) doped pristine (Pr), and divacancy (DV) defected graphene (Gr) sheets using density functional theory calculation (DFT). In this study, we show the role of defects and doping in enhancing the adsorption of alkali metal ions. It is found that the metal ions adsorption on BN and S doped DV-Gr defect sheet have higher adsorption energy. The interaction between different metal ions and the doped graphene sheet is illustrated by the negative region of ESP, and the more negative region is observed in DV-Gr defect sheet which implies the strength of the metal ion adsorption. The energy gap opened in DV-Gr defect sheet upon BN codoping is found to be 0.42 eV and 0.51 eV for S doping. Moreover, the energy gap values are increased upon the adsorption of metal ions on BN, and S doped DV-Gr defect sheet. The bonding between metal ions and doped graphene sheets have analyzed by using QTAIM and electron localization function. Our results indicate that the BN and S doped DV-Gr defect sheet will be the considered as promising candidate materials in battery applications.

DFT Study of Adsorption of Lithium on Si, Ge-doped Divacancy Defected Graphene as Anode Material of Li-ion Battery

Graphene is an anode material that is expected to be a good alternative for graphite to increase the capacity and rate-capability of lithium-ion batteries. Graphene synthesis is always accompanied by defects in the structure. The most common defect in graphene is the divacancy (DV) defect. In this study, the effect of this defect on the adsorption of lithium was studied by density functional theory method, and also the doping effect of silicon and germanium atoms on the defective graphene structure was investigated. The bandgap energy of DV-defected graphene, which has an inverse relationship with electrical conductivity, is steady with the addition of germanium, but decreases with the addition of silicon. In all cases, along with lithium adsorption, the bandgap energy is increased, so that the germanium doped compound has the highest bandgap and the structure with no doped atom has the least bandgap. However, the difference in the minimum and maximum bandgap in structures is very low. The results show that the addition of silicon and germanium leads to stronger adsorption of lithium which means it is possible to raise the charge-discharge capacity of graphene through doping with elements while the material still has a high charge/discharge capability.

Strong interaction between 5f-electron atoms (Th-Cm) and point-defect graphene

The adsorption of actinides (An) atoms (AnThCm) on graphene surface with mono-vacancy (MV) and di-vacancy (DV) defects were studied with ZORA-DFT calculations. The interaction strengths of An and C in defective graphene are highly enhanced relative to pristine graphene. The magnetic moments of An-graphene with MV and DV defects are reduced by 2μb relative to the isolated An atoms due to the strong coupling between An and the neighboring carbon atoms. The AnC bondings show predominantly covalent nature, as observed by the electron density topological parameters. Bond overlap population (BOP) analysis indicates that the 6d orbitals of An atoms participate in the bonding to a higher degree than the 5f orbitals.

Ab-initio investigations on the physical properties of 3d and 5d transition metal atom substituted divacancy monolayer h-BN

In present work, we show that, transition metal (TM) (i.e. 3d and 5d) atom substitution in di-vacancy (DV) h-BN layer can greatly modify its electronic, optical and magnetic behaviors through first-principles investigations based on density functional theory (DFT) method. In terms of electronic structures, it is predicted that 3d and 5d TM atom substitution converts wide bandgap insulating h-BN to semiconducting/half metallic layer depending upon the nature of substituted TM impurity. 3d and 5d TM atom substitution introduces significant finite magnetic moments in h-BN layer. Comparatively, it can be maintained that 3d TM atom substitution introduces larger magnetic moments than 5d TM atom substitution. In PDOS plots, orbital polarization is observed, confirming that, non-magnetic h-BN layer can be converted to magnetic h-BN through TM atom substitution in its lattice. In absorption coefficient plots, it was revealed that, 3d and 5d TM atom substitution produces finite absorption quantity in the low lying energy region of absorption spectrum of h-BN layer. Moreover, blue shift in the absorption coefficient of h-BN occurs in higher energy region after TM impurities addition into DV monolayer h-BN. Higher static reflectivity is gained, while reduced reflectivity peaks are obtained in higher energy regions after 3d and 5d TM impurities substitution in DV h-BN lattice. These findings provide an efficient method to tune the physical properties of h-BN layer to make it functional for spintronic and optoelectronic device applications.

Structure and properties of intrinsic and extrinsic defects in black phosphorus.

The electronic and geometric structures of a range of intrinsic and extrinsic defects in black phosphorus (BP) are calculated using Density Functional Theory (DFT) and a hybrid density functional. The results demonstrate that energy barriers to form intrinsic defects, such as Frenkel pairs and Stone-Wales type defects, exceed 3.0 eV and their equilibrium concentrations are likely to be low. Therefore, growth conditions and sample preparation play a crucial role in defect chemistry of black phosphorus. Mono-vacancies (MV) are shown to introduce a shallow acceptor state in the bandgap of BP, but exhibit fast hopping rates at room temperature. Coalescence of MVs into di-vacancies (DV) is energetically favourable and eliminates the band gap states. Thus MVs are not likely to be the main contributor to p-doping in BP. Extrinsic defects are a plausible alternative, with SnP found to be the most promising candidate. Other defects considered include I, O, Fe, Cu, Zn and Ni in surface adsorbed, intercalated and substitutional geometries, respectively. Furthermore, BP was found to be magnetic for isolated MVs and Fe doping, motivating further research in the area of magnetic functionalisation.

Optical and magnetic properties of free-standing silicene, germanene and T-graphene system

AbstractThe physics of two-dimensional (2D) materials is always intriguing in their own right. For all of these elemental 2D materials, a generic characteristic feature is that all the atoms of the materials are exposed on the surface, and thus tuning the structure and physical properties by surface treatments becomes very easy and straightforward. The discovery of graphene have fostered intensive research interest in the field of graphene like 2D materials such as silicene and germanene (hexagonal network of silicon and germanium, respectively). In contrast to the planar graphene lattice, the silicene and germanene honeycomb lattice is slightly buckled and composed of two vertically displaced sublattices.The magnetic properties were studied by introducing mono- and di-vacancy (DV), as well as by doping phosphorus and aluminium into the pristine silicene. It is observed that there is no magnetism in the mono-vacancy system, while there is large significant magnetic moment present for the DV system. The optical anisotropy of four differently shaped silicene nanodisks has revealed that diamond-shaped (DS) silicene nanodisk possesses highest static dielectric constant having no zero-energy states. The study of optical properties in silicene nanosheet network doped by aluminium (Al), phosphorus (P) and aluminium-phosphorus (Al-P) atoms has revealed that unlike graphene, no new electron energy loss spectra (EELS) peak occurs irrespective of doping type for parallel polarization. Tetragonal graphene (T-graphene) having non-equivalent (two kinds) bonds and non-honeycomb structure shows Dirac-like fermions and high Fermi velocity. The higher stability, large dipole moment along with high-intensity Raman active modes are observed in N-doped T-graphene. All these theoretical results may shed light on device fabrication in nano-optoelectronic technology and material characterization techniques in T-graphene, doped silicene, and germanene.

Electronic structure of bilayer graphene with defects under an external electric field

The electronic structure of bilayer graphene, where one of its layers possesses divacancies (DVs) or Stone-Wales (SW) defects, under an external electric field is studied within the framework of the density functional theory. Our calculations show that the Fermi level of bilayer graphene with DVs strongly depends on gate voltage, while that of bilayer graphene with SW defects is insensitive to gate voltage, indicating that the Fermi level tuning of graphene thin films with defects depends on the defect species and relative arrangement to the gate electrode. The electronic energy band of bilayer graphene under an external electric field is asymmetrically modulated with respect to the gate arrangements.