Klein Edge Research Articles

Microscopic Mechanism for Further NO Heterogeneous Reduction by Potassium-Doped Biochar: A DFT Study.

Biomass reburning is an efficient and low-cost way to control nitric oxide (NO), and the abundant potassium (K) element in biomass affects the heterogeneous reaction between NO and biochar. Due to the incomplete simulation of the NO heterogeneous reduction reaction pathway at the molecular level and the unclear catalytic effect of K element in biochar, further research is needed on the possible next reaction and the influencing mechanism of the element. After the products of the existing reaction pathways are referenced, two reasonably simplified biochar structural models are selected as the basic reactants to study the microscopic mechanism for further NO heterogeneous reduction on the biochar surface before and after doping with the K atom based on density functional theory. In studying the two further NO heterogeneous reduction reaction pathways, we find that the carbon monoxide (CO) molecule fragment protrudes from the surface of biochar models with the desorption of N2 at the TS4 transition state, and the two edge types of biochar product models obtained by simulation calculation are Klein edge and ac56 edge observed in the experiment. In studying the catalytic effect of potassium in biochar, we find that the presence of K increases the heat release of adsorption of NO molecules, reduces the energy barrier of the rate-determining step in the nitrogen (N2) generation and desorption process (by 50.88 and 69.97%), and hinders the CO molecule from desorbing from the biochar model surface. Thermodynamic and kinetic analyses also confirm its influence. The study proves that the heterogeneous reduction reaction of four NO molecules on the surface of biochar completes the whole reaction process and provides a basic theoretical basis for the emission of nitrogen oxides (NOx) during biomass reburning.

Photonic Realization of a Generic Type of Graphene Edge States Exhibiting Topological Flat Band.

Cutting a honeycomb lattice (HCL) ends up with three types of edges (zigzag, bearded, and armchair), as is well known in the study of graphene edge states. Here, we propose and demonstrate a distinctive twig-shaped edge, thereby observing new edge states using a photonic platform. Our main findings are (i)the twig edge is a generic type of HCL edge complementary to the armchair edge, formed by choosing the right primitive cell rather than simple lattice cutting or Klein edge modification; (ii)the twig edge states form a complete flat band across the Brillouin zone with zero-energy degeneracy, characterized by nontrivial topological winding of the lattice Hamiltonian; (iii)the twig edge states can be elongated or compactly localized at the boundary, manifesting both flat band and topological features. Although realized here in a photonic graphene, such twig edge states should exist in other synthetic HCL structures. Moreover, our results may broaden the understanding of graphene edge states, as well as new avenues for realization of robust edge localization and nontrivial topological phases based on Dirac-like materials.

Self‐Passivated Metal Klein Edge of 1T‐Phase Transition Metal Dichalcogenides and the Induced High‐Efficient Hydrogen Evolution Reaction Activity

AbstractThe pristine edge of a 2D material generally undergoes an edge self‐passivation into its ground‐state configuration. To date, several self‐passivated edges of 1H‐phase transition metal dichalcogenides (TMDCs) have been reported in theories and experiments. However, no self‐passivated edges of its sister material 1T‐phase TMDCs are found. Here, taking 1T‐VS2 as an example, a self‐passivated V2 dimer‐like metal Klein (DM‐Klein) edge of 1T‐TMDCs is reported. It exhibits high thermodynamic stability and can spontaneously form at V‐rich chemical potential conditions. Unexpected intermetallic d–d orbital interaction of edge metal atoms results in metal dimerization. This edge self‐passivation mode is universal in group IV and V 1T‐TMDCs. Furthermore, strong intermetallic interaction and appropriate edge sulfur saturation make DM‐Klein edges of 1T‐TMDCs show excellent catalytic activity in the hydrogen evolution reaction compared to that of well‐known Pt(111) surface. This study provides opportunities for the further practical application of 1T‐TMDCs in catalysts.

Computational Study of Metal-Free Magnetism and Spin-Dependent Seebeck Effect in Silicene Nanoribbons with Zigzag and Klein Edges

Nanoribbons based on low-dimensional materials are potential candidates for nanoscale spintronics devices. Here, some ferromagnetic silicene nanoribbons with zigzag and Klein edges (N-ZKSiNRs) are constructed. It is demonstrated that the N-ZKSiNRs with various widths (N) are placed in various spin-resolved electronic situations. With the increase of the width parameter N from 4 to 19, the N-ZKSiNRs pass from the indirect-gap bipolar magnetic semiconducting state (BMS) to the bipolar spin-gapless semiconductor (BSGS) and eventually to half-metallicity (HM). Moreover, applying a temperature gradient through the nanoribbons leads to spin-dependent current with the opposite flowing and spin orientations, demonstrating the spin-dependent Seebeck effect (SDSE). Besides, it was found that the BSGS phase is superior to the BMS and HM for generating SDSE. These findings confirm that the ZKSiNRs are promising choices for spin caloritronics devices.

The Magnetic and Thermally-Induced Spin-Related Transport Features Using Germanene Nanoribbons With Zigzag and Klein Edges

The current work employs the first-principles computations and non-equilibrium Greens function to investigate the magnetic and thermally-induced spin-related transport features using germanene nanoribbons with zigzag and Klein edges (ZKGeNRs). It was demonstrated that the ZKGeNRs with various widths (N) are placed in various spin-resolved electronic states. By increasing the width parameter N from 4 to 9, the ZKGeNRs moves from an indirect-gap bipolar magnetic semiconducting state (BMS) to bipolar spin gapless semiconductor (BSGS), and finally to ferromagnetic metal (FM). Moreover, since the right and the left temperatures of the ZKGeNRs device are different, the spin-up and spin-down currents flow in reverse orientations, demonstrating the spin-dependent Seebeck effect (SDSE). Besides, the threshold temperature decreases as N increases and then disappears, while the spin currents increase as N increases. Simulation results indicated that the ZKGeNRs could be an appropriate choice for spin caloritronic devices and could be utilized in future low-power consumption applications.

Eliminating Edge Electronic and Phonon States of Phosphorene Nanoribbon by Unique Edge Reconstruction.

Edge termination plays a vital role in determining the properties of 2D materials. By performing compelling ab initio simulations, a lowest-energy U-edge [ZZ(U)] reconstruction is revealed in the bilayer phosphorene. Such reconstruction reduces 60% edge energy compared with the pristine one and occurs almost without an energy barrier, implying it should be the dominating edge in reality. The electronic band structure of phosphorene nanoribbon with such reconstruction resembles that of an intrinsic 2D layer, exhibiting nearly edgeless band characteristics. Although ZZ(U) changes the topology of phosphorene nanoribbons, simulated transmission electron microscope, scanning transmission electron microscope and scanning tunneling microscope images indicate it is very hard to be identified. One possible identified method is infrared/Raman analyses because the ZZ(U) edge alters vibrational modes dramatically. In addition, it also increases the thermal conductivity of PNR 1.4 and 2.3 times than the pristine and Klein edges.

Bipolar Magnetic and Thermospin Transport Properties of Graphene Nanoribbons with Zigzag and Klein Edges

Magnetic nanoribbons based on one‐dimensional materials are potential candidates for spin caloritronics devices. Here, we constructed ferromagnetic graphene nanoribbons with zigzag and Klein edges (N‐ZKGNRs, N = 4–21) and found that the N‐ZKGNRs are in the indirect‐gap bipolar magnetic semiconducting state (BMS). Moreover, when a temperature difference is applied through the nanoribbons, spin‐dependent currents with opposite flow directions and opposite spin directions are generated, indicating the occurrence of the spin‐dependent Seebeck effect (SDSE). In addition, the spin‐dependent Seebeck diode effect (SDSD) also appeared in these devices. More importantly, we found that the BMS with a larger bandgap is promising for generating the SDSD, while the BMS with a smaller bandgap is promising for generating the SDSE. These findings show that ZKGNRs are promising candidates for spin caloritronics devices.

Local Carbon Concentration Determines the Graphene Edge Structure.

Although the structures and properties of various graphene edges have attracted enormous attention, the underlying mechanism that determines the appearance of various edges is still unknown. Here, a global search of graphene edge structures is performed by using the particle swarm optimization algorithm. In addition to locating the most stable edges of graphene, two databases of graphene armchair and zigzag edge structures are built. Graphene edge self-passivation plays an important role in the stability of the edges of graphene, and self-passivated edge structures that contain both octagons and triangles are found for the first time. The obvious "apical dominance" feature of armchair edges is found. The appearance of the experimentally observed ac(56), ac(677), and Klein edges can be explained by the local carbon concentration. Additionally, the graphene edge database is also significant for the study of the open end of nanotubes or fullerenes.

Concentric dopant segregation in CVD-grown N-doped graphene single crystals

Heteroatom doping in graphene leads to bandgap opening and tunable electronic, magnetic and optical properties, which are important for graphene-based electronics applications. In recent years, scalable growth of nitrogen-doped graphene (NG) by chemical vapor deposition (CVD) has been extensively studied because of its potential for practical applications. A phenomenon that occurs exclusively for CVD-grown NG films is the segregation of doping concentration. However, most studies to date are conducted using highly polycrystalline NG films comprising small grain sizes. It is still unknown whether dopant segregation occurs in single crystalline NG domains. Here, we used hexamethylenetetramine ((CH2)6N4) as a single-source solid precursor to grow hexagonal-shaped monolayer NG single crystals of ∼20 µm on Cu substrates. The NG single crystals exhibit discrete concentric hexagonal rings comprising N depleted regions as determined by Raman spectroscopy. Supported by scanning tunneling microscopy experiments, we propose that the segregation of N dopants is caused by a competing N attachment mechanism to either zigzag or Klein edges during growth; where the former should result in higher N concentration and the latter with lower N concentration. This work provides critical insights into the growth mechanism of CVD-grown NG and enables new opportunities to engineer the properties of graphene by fabrication of lateral heterostructures.

Atom-Dependent Edge-Enhanced Second-Harmonic Generation on MoS2 Monolayers.

Edge morphology and lattice orientation of single-crystal molybdenum disulfide (MoS2) monolayers, a transition metal dichalcogenide (TMD), possessing a triangular shape with different edges grown by chemical vapor deposition are characterized by atomic force microscopy and transmission electron microscopy. Multiphoton laser scanning microscopy is utilized to study one-dimensional atomic edges of MoS2 monolayers with localized midgap electronic states, which result in greatly enhanced optical second-harmonic generation (SHG). Microscopic S-zigzag edge and S-Mo Klein edge (bare Mo atoms protruding from a S-zigzag edge) terminations and the edge-atom dependent resonance energies can therefore be deduced based on SHG images. Theoretical calculations based on density functional theory clearly explain the lower energy of the S-zigzag edge states compared to the corresponding S-Mo Klein edge states. Characterization of the atomic-scale variation of edge-enhanced SHG is a step forward in this full-optical and high-yield technique of atomic-layer TMDs.

Capture the growth kinetics of CVD growth of two-dimensional MoS2

Understanding the microscopic mechanism is fundamental for function-oriented controlled chemical vapor deposition growth of two-dimensional (2D) materials. In this work, we reveal the growth kinetics of 2D MoS2 by capturing the nucleation seeds, evolving morphology, edge structure, and edge terminations at the atomic scale during chemical vapor deposition growth using the transmission electron microscopy and scanning transmission electron microscopy. The direct growth of few-layer and mono-layer MoS2 onto graphene-based transmission electron microscopy grids helped us to perform the subsequent transmission electron microscopy characterization without any solution-based transfer. Two seeding centers are observed: (i) Mo-oxysulfide (MoOxS2−y) nanoparticles either in multi-shelled fullerene-like structures or as compact nanocrystals for the growth of fewer-layer MoS2; (ii) Mo-S atomic clusters. In the early stage growth, irregular polygons with two primary edge terminations, S-Mo Klein edges and Mo zigzag edges, appear approximately in equal numbers. The morphology evolves into a near-triangle shape in which Mo zigzag edges predominate. Results from density-functional theory calculations are consistent with the inferred growth kinetics, and thus support the growth mechanism we proposed. In general, the growth mechanisms found here should also be applicable in other 2D materials, such as MoSe2, WS2 and WSe2.

Peculiar Magnetic Properties in Doped and Edge Functionalized Silicene Nanoribbons

Motivated by the exceptional properties of graphene, other two-dimensional (2D) nanomaterials beyond graphene such as silicene have emerged with growing interest. Silicene is attractive for its exceptional characteristics and compatibility with silicon based nanoelectronics. It is believed that silicene may challenge graphene due to a longer spin-diffusion time, a longer spin coherence length, and a much larger spin-orbit coupling gap. Half-metallic materials and spin-gapless semiconductors (SGS) are highly desirable for spintronics because they provide 100% spin-polarized currents around their Fermi level. Spin filters and spin field effect transistors (FETs) are one of the potential applications of these types of materials. Silicene nanoribbon (SiNR) as an approach to opening a band gap in a silicene sheet is a promising material for spintronics due to its unique magnetic properties. In this study, a first-principle method based on density functional theory (DFT) is used to investigate the stability, electronic, and magnetic properties of edge hydrogenated and fluorinated SiNRs. Edge functionalization of all possible types of SiNRs such as armchair, zigzag, Klein, reconstructed Klein, pentagon-heptagon, and the combination of the edges are studied. It is found that edge fluorinated structures are much more stable than that of edge hydrogenated. It is also discovered that tri-fluorinated Klein edge SiNR and di-hydrogenated armchair edge SiNR are the most stable structures when the edges are terminated with fluorine and hydrogen atoms, respectively. While, the symmetric edge functionalized zigzag SiNRs show antiferromagnetic semiconductor behaviour. Considering spin-polarization, the asymmetry in edge functionalization bring various and extraordinary magnetic states, such as SGS, ferromagnetic metals or semiconductors, and antiferromagnetic metals. In order to discover the effects of doping on the magnetic and electronic properties of edge functionalized SiNRs, N- or B-doped SiNRs are considered. It is revealed that the dopants energetically prefer to be substituted with Si atoms at the edge of SiNRs. Interestingly, the remarkable SGS and half-metal character, as well as ferromagnetic metallic behavior are observed in N- or B- doped armchair, zigzag, and Klein edge SiNRs which are highly advantageous for spintronics applications. These new findings encourage further experimental investigations in the development of SiNRs based nanoelectronics with spin tuning.

Structural Stability of Functionalized Silicene Nanoribbons with Normal, Reconstructed, and Hybrid Edges

Silicene, a novel graphene-like material, has attracted a significant attention because of its potential applications for nanoelectronics. In this paper, we have theoretically investigated the structural stability of edge-hydrogenated and edge-fluorinated silicene nanoribbons (SiNRs) via first-principles calculations. Various edge forms of SiNRs including armchair edge, zigzag edge, Klein edge, reconstructed Klein edge, reconstructed pentagon-heptagon edge, and hybrid edges have been considered. It has been found that fully fluorinated Klein edge SiNRs, in which each edge Si atom is terminated by three fluorine atoms, are the most stable structure. We also discovered that a hybrid edge structure of trihydrogenated Klein edge and dihydrogenated zigzag edge can increase the nanoribbon’s stability up to that of dihydrogenated armchair edge SiNR, which is known as the most stable edge-hydrogenated structure. With the attractive properties of silicene for practical applications, the obtained results will advance experimental investigations toward the development of silicene based devices.

Formation of Klein Edge Doublets from Graphene Monolayers.

With increasing possibilities for applications of graphene, it is essential to fully characterize the rich topological variations in graphene edge structures. Using aberration-corrected transmission electron microscopy, dangling carbon doublets at the edge of monolayer graphene crystals have been observed. Unlike the single-atom Klein edge often found at zigzag edges, these carbon dimers were observed in various edge structure environments, but most frequently on the more stable armchair edges. Observation of this Klein edge doublet over time reveals that its existence enhances the stability of armchair edges and is a route to atom abstraction on zigzag edges.

Magneto-electronic properties of graphene nanoribbons with various edge structures passivated by phosphorus and hydrogen atoms.

The electronic and magnetic structures of graphene nanoribbons (GNRs) with various edge structures passivated by P atoms are investigated systematically, and compared with H passivation as well. GNRs with the entire reconstructed Klein edge or armchair edge are found to be nonmagnetic regardless of P or H passivation. However, if the edge of GNRs is a mixture of zigzag edge and reconstructed Klein edge, they are nonmagnetic for H passivation but significantly magnetic for P passivation, which could be attributed to the "charge transfer doping" effect. And the corresponding magnetic device shows a noticeable negative differential resistance phenomenon and an excellent spin filtering effect under AP configuration, which originate from the special energy band structure. The GNRs with zigzag edge, reconstructed Klein edge, or mixed edge shapes are all metals in the nonmagnetic state regardless of the H or P atoms involved. The relationship between the energy gap and the width in armchair-edged GNRs by P passivation with a dimer structure also satisfies the 3p periodicity, but different in detail from the case of H passivation. The calculated edge formation energy indicates that P-passivated GNRs are energetically more favorable, suggesting that they can stably exist in the experiment.

The transition metal surface passivated edges of hexagonal boron nitride (h-BN) and the mechanism of h-BN's chemical vapor deposition (CVD) growth.

Edge structure and stability are crucial in determining both the morphology and the growth behaviours of hexagonal boron nitride (h-BN) domains in chemical vapour deposition (CVD) growth under near thermal equilibrium conditions. In this study, various edges of h-BN on three typical transition metal surfaces used for h-BN's CVD growth, Cu(111), Ni(111) and Rh(111), are explored with density functional theory calculations. Different from that in vacuum, our study shows that the formation of non-hexagonal rings, such as pentagon, heptagon or their pairs, is energetically not preferred and both zigzag (ZZ) edges are more stable than the armchair (AC) edge on all the explored catalyst surfaces under typical conditions of h-BN's CVD growth, which explains the broad experimental observation of triangular h-BN domains. More importantly, our results indicate that, instead of the pristine ZZ edge terminated with nitrogen atoms (ZZN), the triangular BN domains observed in experiments are likely to be enclosed with ZZ Klein edges having dangling atoms, ZZB + N or ZZN + B. By applying the theory of Wulff construction, we predicted that the equilibrium shape of a BN domain could be a hexagon enclosed with nitrogen-rich AC edges, triangles enclosed with two different types of ZZ Klein edges or a hexagon enclosed with boron-rich AC edges if the growth is in a N-rich, neutral or B-rich environment, respectively. This study presents how the edges and equilibrium shapes of h-BN domains can be controlled during the CVD synthesis and provides guidelines for further exploring the growth behaviours and improving the quality of CVD-prepared h-BN films.

Temperature-driven changes of the graphene edge structure on Ni(111): substrate vs hydrogen passivation.

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.

Extended Klein edges in graphene.

Graphene has three experimentally confirmed periodic edge terminations, zigzag, reconstructed 5-7, and arm-chair. Theory predicts a fourth periodic edge of graphene called the extended Klein (EK) edge, which consists of a series of single C atoms protruding from a zigzag edge. Here, we confirm the existence of EK edges in both graphene nanoribbons and on the edge of bulk graphene using atomic resolution imaging by aberration-corrected transmission electron microscopy. The formation of the EK edge stems from sputtering and reconstruction of the zigzag edge. Density functional theory reveals minimal energy for EK edge reconstruction and bond distortion both in and out of plane, supporting our TEM observations. The EK edge can now be included as the fourth member of observed periodic edge structures in graphene.

Edge structures for nanoscale graphene islands on Co(0001) surfaces.

Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.

Electronic structures of reconstructed zigzag silicene nanoribbons

Edge states and magnetism are crucial for spintronic applications of nanoribbons. Here, using first-principles calculations, we explore structural stabilities and electronic properties of zigzag silicene nanoribbons (ZSiNRs) with Klein and pentagon-heptagon reconstructions. Comparing to unreconstructed zigzag edges, deformed bare pentagon-heptagon ones are favored under H-poor conditions, while H-rich surroundings stabilize di-hydrogenated Klein edges. These Klein edges have analogous magnetism to zigzag ones, which also possess the electric-field-induced half-metallicity of nanoribbons. Moreover, diverse magnetic states can be achieved by asymmetric Klein and zigzag edges into ZSiNRs, which could be transformed from antiferromagnetic-semiconductors to bipolar spin-gapless-semiconductors and ferromagnetic-metals depending on edge hydrogenations.