Half-metallic graphene nanoribbons

Electronic structure evolution of the transition metals substituted tetragonal graphene: a first-principles investigations

Motivated by the possibilities of tuning the Fermi level of the metallic band structure of the planar tetragonal graphene (T-graphene), by using the transition metals (TMs) substitution (3d, 4dand 5dseries), the electronic structure investigation has been carried out at low concentration level (≈2.7%) throughab initiodensity functional theory method. We have investigated the influence of the valence electrons of the TM on the evolution of the electronic structure and magnetization and the induced magnetic moments at the carbon atoms in the T-graphene network. The investigations also explored the possibilities of inducing long-range magnetic ordering. In the case of multi TMs substitutions we found the dominance signature of the antiferromagnetic correlations for most of the TM substituted cases. The critical analysis of the magnetization densities indicated the important role of the hybridization between the carbonπandσorbitals with the TM-dstates. We explored that the observed non-monotonic nature of the magnetization and evolution of electronic structure was due to the competing energy scales of electronic correlation, hybridization and crystal field splitting. This study opens up the route for further investigations towards the possibilities of using T-graphene as a potential polymorph of graphene for device applications.

Strain effect on electronic structure and transport properties of zigzagα-T3nanoribbons: a mean-field theoretical study

Theα-T₃lattice, a minimal model that presents flat bands, has sparked much interest in research but the finite-size effect and interaction has been rarely involved. Here we theoretically study the electronic structure and transport properties of zigzag-edgeα-T₃nanoribbons (ZαT₃NRs) with and without uniaxial strain, where the exemplary widthsN= 40 and 41 for two series are considered. By adopting the mean-field Hubbard model combined with the nonequilibrium Green's function method, we show that the spin-degenerate dispersionless flat band at the Fermi energy for the pristine ribbons is split into spin-up and -down flat bands under electron-electron Coulomb interaction. Specifically, the two bands are shifted toward in an opposite direction and away from the Fermi energy, which leads to an energy gap opening in the case ofα≠ 1. All three series of ZαT₃NRs with widthN= 3n, 3n+ 1, 3n+ 2 (wherenis a positive integer) exhibit an energy gap. This differs from the simple tight-binding calculations without considering electron-electron Coulomb interaction, for which the gap is always zero in the case ofN= 3n+ 1. Here, the origin of the energy gap forN= 3n+ 1 arises from Coulomb repulsion between electrons. Importantly, the energy gap can be effectively manipulated by an uniaxial strain and Coulomb interaction ifα≠ 1. The gap linearly increases (decreases) when a tensile (compressive) strain increases, and it also monotonously increases as enhancing Coulomb interaction. Interestingly, a ground state of antiferromagnetic to ferromagnetic transition occurs whenαincreases from 0.8 to 1, leading to a semiconductor to metallic transition. Besides, theα-, strain- and interaction-dependent conductance is also explored. The findings here may be of importance in the band gap engineering and electromechanical applications ofα-T₃nanoribbon-based devices.

Lateral Interfaces between Monolayer MoS2 Edges and Armchair Graphene Nanoribbons on Au(111)

The realization of electronic devices based on heterostructures of metallic, semiconducting, or insulating two-dimensional materials relies on the ability to form structurally coherent and clean interfaces between them, vertically or laterally. Lateral two-dimensional heterostructures that fuse together two different materials in a well-controlled manner have attracted recent attention, but the methods to form seamless interfaces between structurally dissimilar materials, such as graphene and transition-metal dichalcogenides (TMDCs), are still limited. Here, we investigate the structure of the lateral interfaces that arise between monolayer MoS₂ flakes on Au(111) and two families of armchair graphene nanoribbons (GNRs) created through on-surface assisted Ullmann coupling using regular organobromine precursors for GNR synthesis. We find that parallel alignment between the GNR armchair edge and MoS₂ leads to van der Waals bonded nanoribbons, whereas a perpendicular orientation is characterized by a single phenyl-group of the GNR covalently bonded to S on the edge. The edge-on bonding is facilitated by a hydrogen treatment of the MoS₂, and temperature control during growth is shown to influence the nanoribbon width and the yield of covalently attached nanoribbons. Interestingly, the temperatures needed to drive the intramolecular dehydrogenation during GNR formation are lowered significantly by the presence of MoS₂, which we attribute to enhanced hydrogen recombination at the MoS₂ edges. These results are a demonstration of a viable method to make laterally bonded graphene nanostructures to TMDCs to be used in further investigations of two-dimensional heterostructure junctions.

Zhang-Zhang Polynomials of Multiple Zigzag Chains Revisited: A Connection with the John-Sachs Theorem

Multiple zigzag chains Zm,n of length n and width m constitute an important class of regular graphene flakes of rectangular shape. The physical and chemical properties of these basic pericondensed benzenoids can be related to their various topological invariants, conveniently encoded as the coefficients of a combinatorial polynomial, usually referred to as the ZZ polynomial of multiple zigzag chains Zm,n. The current study reports a novel method for determination of these ZZ polynomials based on a hypothesized extension to John–Sachs theorem, used previously to enumerate Kekulé structures of various benzenoid hydrocarbons. We show that the ZZ polynomial of the Zm,n multiple zigzag chain can be conveniently expressed as a determinant of a Toeplitz (or almost Toeplitz) matrix of size m2×m2 consisting of simple hypergeometric polynomials. The presented analysis can be extended to generalized multiple zigzag chains Zkm,n, i.e., derivatives of Zm,n with a single attached polyacene chain of length k. All presented formulas are accompanied by formal proofs. The developed theoretical machinery is applied for predicting aromaticity distribution patterns in large and infinite multiple zigzag chains Zm,n and for computing the distribution of spin densities in biradical states of finite multiple zigzag chains Zm,n.

Realizing stable half-metallicity in zigzag silicene nanoribbons with edge dihydrogenation and chemical doping

Although many schemes have been proposed to obtain full half-metallicity in zigzag silicene nanoribbons with edge monohydrogenation (H-H ZSiNRs) by chemical modification, the resulted negligible energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations makes the half-metallicity hardly observable practically. In this work, based on density functional calculations, we find that the ZSiNRs with edge dihydrogenation (H2-H2 ZSiNRs) can be tuned to be half-metallic by replacing the central two zigzag Si chains with two zigzag Al-P chains, and more importantly, the FM-AFM energy difference is significantly increased compared with the H-H cases. The obtained half-metallicity originates from the different potential between two edges of the ribbon after doping, which results in the edge states of two spin channels shifting oppositely in energy. This mechanism is so robust that the half-metallicity can always be achieved, irrespective of the ribbon width. Our finding provides a fantastic way for achieving stable half-metallicity in ZSiNRs.

Antiferromagnetic spin ordering in two-dimensional honeycomb lattice of SiP3

Magnetism in low-dimensional materials has been of sustained interest due to its intriguing quantum mechanical origin and promising device applications. Here, we propose a buckled honeycomb lattice of stoichiometry SiP₃, a two-dimensional binary group-IV and V material that exhibits an antiferromagnetic ground state with itinerant electrons. Here we perform elemental Si substitution in pristine blue phosphorene to downshift the Fermi energy and induce the Fermi instability that results in a spin polarized ground state. Within first-principles calculations, we observe antiferromagnetic spin alignment between adjacent ferromagnetic triangular domains where each Si atom is coupled with three neighboring P atoms with a ferromagnetic interaction. Such unique spin structure and resulting magnetic ground state are unprecedented in defect-free two-dimensional materials made of only p-block elements. This metal-free magnetism can be exploited for advanced spintronic and memory storage applications.

Topological Surface State in Epitaxial Zigzag Graphene Nanoribbons

Protected and spin-polarized transport channels are the hallmark of topological insulators, coming along with an intrinsic strong spin-orbit coupling. Here we identified such corresponding chiral states in epitaxially grown zigzag graphene nanoribbons (zz-GNRs), albeit with an extremely weak spin-orbit interaction. While the bulk of the monolayer zz-GNR is fully suspended across a SiC facet, the lower edge merges into the SiC(0001) substrate and reveals a surface state at the Fermi energy, which is extended along the edge and splits in energy toward the bulk. All of the spectroscopic details are precisely described within a tight binding model incorporating a Haldane term and strain effects. The concomitant breaking of time-reversal symmetry without the application of external magnetic fields is supported by ballistic transport revealing a conduction of G = e²/h.

Spiers Memorial Lecture. Carbon nanostructures by macromolecular design - from branched polyphenylenes to nanographenes and graphene nanoribbons

Nanographenes (NGs) and graphene nanoribbons (GNRs) are unique connectors between the domains of 1D-conjugated polymers and 2D-graphenes. They can be synthesized with high precision by oxidative flattening processes from dendritic or branched 3D-polyphenylene precursors. Their size, shape and edge type enable not only accurate control of classical (opto)electronic properties, but also access to unprecedented high-spin structures and exotic quantum states. NGs and GNRs serve as active components of devices such as field-effect transistors and as ideal objects for nanoscience. This field of research includes their synthesis after the deposition of suitable monomers on surfaces. An additional advantage of this novel concept is in situ monitoring of the reactions by scanning tunnelling microscopy and electronic characterization of the products by scanning tunnelling spectroscopy.

Synthesis and derivatization of hetera-buckybowls

Buckybowls have concave and convex surfaces with distinct π-electron cloud distribution, and consequently they show unique structural and electronic features as compared to planar aromatic polycycles. Doping the π-framework of buckybowls with heteroatoms is an efficient scheme to tailor inherent properties, because the nature of heteroatoms plays a pivotal role in the structural and electronic characteristics of the resulting hetera-buckybowls. The design, synthesis, and derivatization of hetera-buckybowls open an avenue for obtaining fascinating organic entities not only of fundamental importance but also of promising applications in optoelectronics. In this review, we summarize the advances in hetera-buckybowl chemistry, particularly the synthetic strategies toward these scaffolds.

Materials Science Challenges to Graphene Nanoribbon Electronics

Graphene nanoribbons (GNRs) have recently emerged as promising candidates for channel materials in future nanoelectronic devices due to their exceptional electronic, thermal, and mechanical properties and chemical inertness. However, the adoption of GNRs in commercial technologies is currently hampered by materials science and integration challenges pertaining to synthesis and devices. In this Review, we present an overview of the current status of challenges, recent breakthroughs toward overcoming these challenges, and possible future directions for the field of GNR electronics. We motivate the need for exploration of scalable synthetic techniques that yield atomically precise, placed, registered, and oriented GNRs on CMOS-compatible substrates and stimulate ideas for contact and dielectric engineering to realize experimental performance close to theoretically predicted metrics. We also briefly discuss unconventional device architectures that could be experimentally investigated to harness the maximum potential of GNRs in future spintronic and quantum information technologies.

On-Surface Synthesis and Molecular Engineering of Carbon-Based Nanoarchitectures

On-surface synthesis via covalent coupling of adsorbed precursor molecules on metal surfaces has emerged as a promising strategy for the design and fabrication of novel organic nanoarchitectures with unique properties and potential applications in nanoelectronics, optoelectronics, spintronics, catalysis, etc. Surface-chemistry-driven molecular engineering (i.e., bond cleavage, linkage, and rearrangement) by means of thermal activation, light irradiation, and tip manipulation plays critical roles in various on-surface synthetic processes, as exemplified by the work from the Ernst group in a prior issue of ACS Nano. In this Perspective, we highlight recent advances in and discuss the outlook for on-surface syntheses and molecular engineering of carbon-based nanoarchitectures.

The spin-polarized edge states of blue phosphorene nanoribbons induced by electric field and electron doping

Edge states of various two-dimensional materials such as graphene are intrinsically spin-polarized. In other materials, electric field and charge doping are required for introducing magnetism to their edges. In this work, by using first-principles calculations, we studied the effects of transverse electric field on the edge states of the armchair blue phosphorene nanoribbon (ABPNR), and found that a transverse electric field drives the edge electronic state occupied and at the same time spin-polarized. We also doped electrons to the ABPNR and found that these additional electrons occupy and spin-polarize the electronic states of both edges of the nanoribbon.

Diversified Phenomena in Metal- and Transition-Metal-Adsorbed Graphene Nanoribbons

Adatom-adsorbed graphene nanoribbons (GNRs) have gained much attention owing to the tunable electronic and magnetic properties. The metal (Bi, Al)/transition metal (Ti, Fe, Co, Ni) atoms could provide various outermost orbitals for the multi-orbital hybridizations with the out-of-plane π bondings on the carbon honeycomb lattice, which dominate the fundamental properties of chemisorption systems. In this study, the significant similarities and differences among Bi-/Al-/Ti-/Fe-/Co-/Ni-adsorbed GNRs are thoroughly investigated by using the first-principles calculations. The main characterizations include the adsorption sites, bond lengths, stability, band structures, charge density distributions, spin- and orbital-projected density of states, and magnetic configurations. Furthermore, there exists a transformation from finite gap semiconducting to metallic behaviors, accompanied by the nonmagnetism, antiferromagnetism, or ferromagnetism. They arise from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement, edge structure, and spin-dependent many-body effects. The proposed theoretical framework could be further improved and generalized to explore other emergent 1D and 2D materials.

Gap opening in graphene nanoribbons by application of simple shear strain and in-plane electric field

The effects of shear strain and applied in plane electric field on the electronic properties of monolayer graphene nanoribbons (GNRs) are theoretically investigated. Band structures and the probability densities are calculated within the tight-binding model and the mechanical stresses submitted to the GNRs are taken into account by using the theory of linear elasticity with joint modifications in the elongation of the nearest-neighbor vectors and the modification of the hopping parameters. The energy gaps for specific widths of (semiconducting) armchair nanoribbons are verified also in the presence of either strain or field, whereas zigzag nanoribbons are metallic for any value of strain and exhibit a small gap for any value of field. However, our results demonstrate that when both strain and electric field are combined, a significant energy gap is always observed in the band structure, for any width or edge type of the ribbon. Moreover, the obtained total wave function is asymmetric along the ribbon width due to the applied electric field that pushes the electrons to one side of the ribbon and, under shear strain, a peak at the center of the ribbon in the spatial distribution is also observed owing to the preferable localization around the almost undeformed carbon bonds at ribbon center.

Imprinting Tunable π-Magnetism in Graphene Nanoribbons via Edge Extensions

Magnetic carbon nanostructures are currently under scrutiny for a wide spectrum of applications. Here, we theoretically investigate armchair graphene nanoribbons patterned with asymmetric edge extensions consisting of laterally fused naphtho groups, as recently fabricated via on-surface synthesis. We show that an individual edge extension acts as a spin-12 center and develops a sizable spin-polarization of the conductance around the band edges. The Heisenberg exchange coupling between a pair of edge extensions is dictated by the position of the second naphtho group in the carbon backbone, thus enabling ferromagnetic, antiferromagnetic, or nonmagnetic states. The periodic arrangement of edge extensions yields full spin-polarization at the band extrema, and the accompanying ferromagnetic ground state can be driven into nonmagnetic or antiferromagnetic phases through external stimuli. Overall, our work reveals a precise tunability of the π-magnetism in graphene nanoribbons induced by naphtho groups, thereby establishing these one-dimensional architectures as suitable platforms for logic spintronics.

The spin-dependent transport properties of defected zigzag graphene nanoribbons with graphene nanobubbles

Zigzag-edged graphene nanoribbons (ZGNRs) have important applications in spintronics and spin caloritronics. While in the preparation of a ZGNR, defects like the graphene nanobubbles often appear, which may affect the physical properties of the ZGNR. In this paper, we studied the transport properties of a defected ZGNR with a graphene nanobubble by performing first-principles quantum transport calculations. The results show that when the nanobubble is intact and locates at the centre, the spin polarization and magnetoresistance tend to drop off in the low bias voltage cases, compared to the ideal ZGNR. While when the nanobubble is split and locates at the edge, all the transport properties are significantly affected and altered, such as the spin polarization, the giant magnetoresistance effect and the spin Seebeck effect. Meanwhile, some new results are obtained from the device, including the negative differential resistance effect and the pure thermal-induced spin-current.

Vacancy ordering induced topological electronic transition in bulk Eu2ZnSb2

Metal-semiconductor transitions from changes in edge chirality from zigzag to armchair were observed in many nanoribbon materials, especially those based on honeycomb lattices. Here, this is generalized to bulk complex Zintl semiconductors, exemplified by Eu₂ZnSb₂ where the Zn vacancy ordering plays an essential role. Five Eu₂ZnSb₂ structural models are proposed to guide transmission electron microscopy imaging. Zigzag vacancy ordering models show clear metallicity, while the armchair models are semiconducting with indirect bandgaps that monotonously increase with the relative distances between neighboring ZnSb₂ chains. Topological electronic structure changes based on cation ordering in a Zintl compound point toward tunable and possibly switchable topological behavior, since cations in these are often mobile. Thus, their orderings can often be adjusted by temperature, minor alloying, and other approaches. We explain the electronic structure of an interesting thermoelectric and point the way to previously unidentified types of topological electronic transitions in Zintl compounds.

Towards chirality control of graphene nanoribbons embedded in hexagonal boron nitride

The integrated in-plane growth of graphene nanoribbons (GNRs) and hexagonal boron nitride (h-BN) could provide a promising route to achieve integrated circuitry of atomic thickness. However, fabrication of edge-specific GNRs in the lattice of h-BN still remains a significant challenge. Here we developed a two-step growth method and successfully achieved sub-5-nm-wide zigzag and armchair GNRs embedded in h-BN. Further transport measurements reveal that the sub-7-nm-wide zigzag GNRs exhibit openings of the bandgap inversely proportional to their width, while narrow armchair GNRs exhibit some fluctuation in the bandgap-width relationship. An obvious conductance peak is observed in the transfer curves of 8- to 10-nm-wide zigzag GNRs, while it is absent in most armchair GNRs. Zigzag GNRs exhibit a small magnetic conductance, while armchair GNRs have much higher magnetic conductance values. This integrated lateral growth of edge-specific GNRs in h-BN provides a promising route to achieve intricate nanoscale circuits.

Self-passivation leads to semiconducting edges of black phosphorene

The edges of black phosphorene (BP) have been extensively explored. The previous experimental observations that all the BP edges are semiconducting implies that the as-cut edges of BP tend to be reconstructed. Here we present a global structural search of three typical BP edges, namely armchair, zigzag and zigzag-1 edges. It is found that all the three pristine edges are metastable, and all of them can be quickly self-passivated by (i) forming P[double bond, length as m-dash]P double bonds (one σ and one π bond), (ii) reconstructing new polygonal rings will all P atoms bonded with three sp³ bonds or (iii) forming a special P(2)-P(4) configuration with a two-coordinated P atom accommodating two lone pair electrons and one four-coordinated P atom without lone pair electrons. Highly different from the pristine edges, all these highly stable reconstructed edges are semiconducting. This study showed that the reconstruction of the edges of a 2D material, just like the surfaces of a 3D crystal, must be considered for both fundamental studies and practical applications. Besides BP, this study also sheds light on the structures and properties of the edges of many other 2D materials.

Two-dimensional magnetic materials: structures, properties and external controls

Since the discovery of intrinsic ferromagnetism in atomically thin Cr2Gr2Te6 and CrI3 in 2017, research on two-dimensional (2D) magnetic materials has become a highlighted topic. Based on 2D magnetic materials and their heterostructures, exotic physical phenomena at the atomically thin limit have been discovered, such as the quantum anomalous Hall effect, magneto-electric multiferroics, and magnon valleytronics. Furthermore, magnetism in these ultrathin magnets can be effectively controlled by external perturbations, such as electric field, strain, doping, chemical functionalization, and stacking engineering. These attributes make 2D magnets ideal platforms for fundamental research and promising candidates for various spintronic applications. This review aims at providing an overview of the structures, properties, and external controls of 2D magnets, as well as the challenges and potential opportunities in this field.

Excitonic and charge transfer interactions in tetracene stacked and T-shaped dimers

Extended quantum chemical calculations were performed for the tetracene dimer to provide benchmark results, analyze the excimer survival process, and explore the possibility of using long-range-corrected (LC) time-dependent second-order density functional tight-biding (DFTB2) for this system. Ground- and first-excited-state optimized geometries, vertical excitations at relevant minima, and intermonomer displacement potential energy curves (PECs) were calculated for these purposes. Ground-state geometries were optimized with the scaled-opposite-spin (SOS) second-order Møller-Plesset perturbation (MP2) theory and LC-DFT (density functional theory) and LC-DFTB2 levels. Excited-state geometries were optimized with SOS-ADC(2) (algebraic diagrammatic construction to second-order) and the time-dependent approaches for the latter two methods. Vertical excitations and PECs were compared to multireference configuration interaction DFT (DFT/MRCI). All methods predict the lowest-energy S₀ conformer to have monomers parallel and rotated relative to each other and the lowest S₁ conformer to be of a displaced-stacked type. LC-DFTB2, however, presents some relevant differences regarding other conformers for S₀. Despite some state-order inversions, overall good agreement between methods was observed in the spectral shape, state character, and PECs. Nevertheless, DFT/MRCI predicts that the S₁ state should acquire a doubly excited-state character relevant to the excimer survival process and, therefore, cannot be completely described by the single reference methods used in this work. PECs also revealed an interesting relation between dissociation energies and the intermonomer charge-transfer interactions for some states.

On-surface synthesis of graphene nanostructures with π-magnetism

Graphene nanostructures (GNs) including graphene nanoribbons and nanoflakes have attracted tremendous interest in the field of chemistry and materials science due to their fascinating electronic, optical and magnetic properties. Among them, zigzag-edged GNs (ZGNs) with precisely-tunable π-magnetism hold great potential for applications in spintronics and quantum devices. To improve the stability and processability of ZGNs, substitutional groups are often introduced to protect the reactive edges in organic synthesis, which renders the study of their intrinsic properties difficult. In contrast to the conventional wet-chemistry method, on-surface bottom-up synthesis presents a promising approach for the fabrication of both unsubstituted ZGNs and functionalized ZGNs with atomic precision via surface-catalyzed transformation of rationally-designed precursors. The structural and spin-polarized electronic properties of these ZGNs can then be characterized with sub-molecular resolution by means of scanning probe microscopy techniques. This review aims to highlight recent advances in the on-surface synthesis and characterization of a diversity of ZGNs with π-magnetism. We also discuss the important role of precursor design and reaction stimuli in the on-surface synthesis of ZGNs and their π-magnetism origin. Finally, we will highlight the existing challenges and future perspective surrounding the synthesis of novel open-shell ZGNs towards next-generation quantum technology.

Nonvolatile electrical control of 2D Cr2Ge2Te6 and intrinsic half metallicity in multiferroic hetero-structures

The electrical control of two-dimensional (2D) van der Waals ferromagnets is a step forward for the realization of spintronic devices. However, using this approach for practical applications remains challenging due to its volatile memory. Herein, we adopt an alternative strategy, where the bistable ferroelectric switches (P↑ and P↓) of Sc₂CO₂ (SCO) assist the ferromagnetic states of Cr₂Ge₂Te₆ (CGT) in order to achieve non-volatile memories. Moreover, MXene SCO, being an aided layer in multiferroic CGT/SCO hetero-structures, also modifies the electronic properties of CGT to half metal by its polarized P↓ state. In contrast, the P↑ state does not change the semiconducting nature of CGT. Hence, non-volatile, electrical-controlled switching of ferromagnetic CGT can be engineered by the two opposite ferroelectric states of single layer SCO. Importantly, the magnetic easy axis of CGT switches from in-plane to out-of-plane when the direction of electric polarization of SCO is altered from P↓ to P↑.

Realization of an Antiferromagnetic Superatomic Graphene: Dirac Mott Insulator and Circular Dichroism Hall Effect

Using first-principles calculations, we investigate the electronic and topological properties of an antiferromagnetic (AFM) superatomic graphene lattice superimposed on a bipartite honeycomb lattice governed by Lieb's theorem of itinerant magnetism. It affords a concrete material realization of the AFM honeycomb model with a Dirac Mott insulating state, characterized by a gap opening at the Dirac point due to inversion symmetry breaking by long-range AFM order. The opposite Berry curvatures of the K and K' valleys induces a circular dichroism (CD) Hall effect. Different from the valley Hall effect that activates only one valley, the CD Hall effect activates carriers from both K and K' valleys, generating the opposite directions of transversal Hall currents for the left- and right-handed circularly polarized light, respectively.

External electric field modulated second-order nonlinear optical response and visible transparency in hexalithiobenzene

External electric fields (EEFs) offer a unique opportunity to tune certain activity of molecules by orienting the alignment of the electric field along the specific axis. The second-order NLO response of hexalithiobenzene (C₆Li₆) is very poor due to its first mean hyperpolarizability of 0.5 a.u. Therefore, we have analyzed the effect of EEFs on the structural, electronic properties, and NLO response of C₆Li₆ using a density functional approach. We notice that the structure of the C₆Li₆ molecule remains planar, with the slight change in C-C and C-Li bond lengths, but their stability is increased under the effect of EEFs. By applying EEFs, the conductivity or reactivity of C₆Li₆ is increased as their HOMO-LUMO energy gap is decreased. Furthermore, C₆Li₆ attains a finite dipole moment in the presence of EEF, which increases linearly as the EEF increases. More interestingly, the first static hyperpolarizability of C₆Li₆ is significantly enhanced, becoming as high as 3.4 × 10⁴ a.u. for EEF = 50 × 10^-4 a.u. This suggests the EEF as an effective way to enhance the second-order NLO responses, leading to the design of potential NLO materials. Nevertheless, the visible transparency of C₆Li₆ with and without EEF may suggest its possible applications in optical devices.

The unique carrier mobility of monolayer Janus MoSSe nanoribbons: a first-principles study

Charge-carrier mobility is a determining factor of the transport properties of semiconductor materials and is strongly related to the optoelectronic performance of nanoscale devices. Here, we investigate the electronic properties and charge carrier mobility of monolayer Janus MoSSe nanoribbons by means of first-principles simulations coupled with deformation potential theory. These simulations indicate that zigzag nanoribbons are metallic. Conversely, armchair nanoribbons are semiconducting and show oscillations in the calculated band gap as a function of edge-width according to the 3p < 3p + 1 < 3p + 2 rule, with p being the integer number of repeat units along the non-periodic direction of the nanoribbon. Although the charge-carrier mobility of armchair nanoribbons oscillates with the edge-width, its magnitude is comparable to its two-dimensional sheet counterpart. A robust room-temperature carrier mobility is calculated for 3.5 nm armchair nanoribbons with values ranging from 50 cm2 V-1 s-1 to 250 cm2 V-1 s-1 for electrons (e) and holes (h), respectively. A comparison of these values with the results for periodic flat sheet (e: 73.8 cm2 V-1 s-1; h: 157.2 cm2 V-1 s-1) reveals enhanced (suppressed) hole (electron) mobility in the Janus MoSSe nanoribbons. This is in contrast to what was previously found for MoS2 nanoribbons, namely larger mobility for electrons in comparison with holes. These differences are rationalized on the basis of the different structures, edge electronic states and deformation potentials present in the MoSSe nanoribbons. The present results provide the guidelines for the structural and electronic engineering of MoSSe nanoribbon edges towards tailored electron transport properties.

Electronic properties of N-rich graphene nano-chevrons

Theoretical analysis based on density functional theory describes the microscopic origins of emerging electronic and magnetic properties in quasi-1D nitrogen-rich graphene nanoribbon structures with chevron-like (or wiggle-edged) configurations. The study focuses on systems with structural units composed of hexagonal graphitic units featuring one and two nitrogen atoms substituted in the graphitic structure, in positions contrasting with the more commonly considered pyridinic configurations. This type of substitution introduces nitrogen levels close to the Fermi level which in turn induce spin polarization depending on a number of structural features. We demonstrate that these systems present a broader set of electronic and magnetic behaviors relative to their pure hydrocarbon counterparts, with the possibility of engineering the electronic band gap strategically using different spin configurations and positions of the substituting nitrogen atoms.

On-Surface Synthesis of Chlorinated Narrow Graphene Nanoribbon Organometallic Hybrids

Graphene nanoribbons (GNRs) and their derivatives attract growing attention due to their excellent electronic and magnetic properties as well as the fine-tuning of such properties that can be obtained by heteroatom substitution and/or edge morphology modification. Here, we introduce graphene nanoribbon derivatives-organometallic hybrids with gold atoms incorporated between the carbon skeleton and side Cl atoms. We show that narrow chlorinated 5-AGNROHs (armchair graphene nanoribbon organometallic hybrids) can be fabricated by on-surface polymerization with omission of the cyclodehydrogenation reaction by a proper choice of tailored molecular precursors. Finally, we describe a route to exchange chlorine atoms connected through gold atoms to the carbon skeleton by hydrogen atom treatment. This is achieved directly on the surface, resulting in perfect unsubstituted hydrogen-terminated GNRs. This will be beneficial in the molecule on-surface processing when the preparation of final unsubstituted hydrocarbon structure is desired.

Tuning the electronic properties of highly anisotropic 2D dangling-bond-free sheets from 1D V2Se9 chain structures

True one-dimensional (1D) van der Waals materials can form two-dimensional (2D) dangling-bond-free anisotropic surfaces. Dangling bonds on surfaces act as defects for transporting charge carriers. In this study, we consider true 1D materials to be V2Se9 chains, and then the electronic structures of 2D sheets composed of true 1D V2Se9 chains are calculated. The (010) plane has indirect bandgap with 0.757 eV (1.768 eV), while the (111̅) plane shows a nearly direct bandgap of 1.047 eV (2.118 eV) for DFT-D3 (HSE06) correction, respectively. The (111̅) plane of V2Se9 is expected to be used in optoelectronic devices because it contains a nearly direct bandgap. Partial charge analysis indicates that the (010) plane exhibits interchain interaction is stronger than the (111̅) plane. To investigate the strain effect, we increased the interchain distance of planes until an indirect-to-direct bandgap transition occurred. The (010) plane then demonstrated a direct bandgap when interchain distance increased by 30%, while the (111̅) plane demonstrated a direct bandgap when the interchain distance increased by 10%. In mechanical sensors, this change in the bandgap was induced by the interchain distance.

Tunable phase transitions and high photovoltaic performance of two-dimensional In2Ge2Te6 semiconductors

Ultrathin semiconductors with great electrical and photovoltaic performance hold tremendous promise for fundamental research and applications in next-generation electronic devices. Here, we report new 2D direct-bandgap semiconductors, namely mono- and few-layer In₂Ge₂Te₆, with a range of desired properties from ab initio simulations. We suggest that 2D In₂Ge₂Te₆ samples should be highly stable and can be experimentally fabricated by mechanical exfoliation. They are predicted to exhibit extraordinary optical absorption and high photovoltaic conversion efficiency (≥31.8%), comparable to the most efficient single-junction GaAs solar cell. We reveal that, thanks to the presence of van Hove singularities in the band structure, unusual quantum-phase transitions could be induced in monolayers via electrostatic doping. Furthermore, taking bilayer In₂Ge₂Te₆ as a prototypical system, we demonstrate the application of van der Waals pressure as a promising strategy to tune the electronic and stacking property of 2D crystals. Our work creates exciting opportunities to explore various quantum phases and atomic stacking, as well as potential applications of 2D In₂Ge₂Te₆ in future nanoelectronics.

Designer spin order in diradical nanographenes

The magnetic properties of carbon materials are at present the focus of intense research effort in physics, chemistry and materials science due to their potential applications in spintronics and quantum computing. Although the presence of spins in open-shell nanographenes has recently been confirmed, the ability to control magnetic coupling sign has remained elusive but highly desirable. Here, we demonstrate an effective approach of engineering magnetic ground states in atomically precise open-shell bipartite/nonbipartite nanographenes using combined scanning probe techniques and mean-field Hubbard model calculations. The magnetic coupling sign between two spins was controlled via breaking bipartite lattice symmetry of nanographenes. In addition, the exchange-interaction strength between two spins has been widely tuned by finely tailoring their spin density overlap, realizing a large exchange-interaction strength of 42 meV. Our demonstrated method provides ample opportunities for designer above-room-temperature magnetic phases and functionalities in graphene nanomaterials.

Penta-Hexa-Graphene Nanoribbons: Intrinsic Magnetism and Edge Effect Induce Spin-Gapless Semiconducting and Half-Metallic Properties

Two-dimensional materials with intrinsic long-range ordered magnetic moments have drawn a lot of attention. However, for practical applications, whether or not the magnetism is stable in their nanostructures has not been revealed. Here, based on the recently proposed magnetic penta-hexa-graphene, we study the electronic and magnetic properties of its nanoribbons (named PHGNRs). The results show that the PHGNRs have intrinsic robust magnetic moments that are different from zigzag graphene nanoribbons, where the magnetic moments caused by the edge effect are vulnerable. Moreover, the magnetic ground states, namely, ferromagnetic (FM) or antiferromagnetic (AFM), can be transformed by changing the width of PHGNRs. Most interestingly, under the FM ground state, the spin-polarized electronic properties reveal that the zigzag PHGNRs transform from spin-gapless semiconductors (SGSs) to half-metals, as the width of nanoribbons increases, while all the armchair PHGNRs are magnetic semiconductors. Furthermore, by considering different edge effects caused by the residual carbon atoms on the edges, the PHGNRs can further derive different types of SGSs, as well as half-metals. Our work suggests that the PHGNRs possessing intrinsic robust magnetic moments have potential applications in the field of spintronic devices.

Strain-Controllable High Curie Temperature, Large Valley Polarization, and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Janus VSeTe Monolayer

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are important for spintronics. A highly stable ML (monolayer) Janus 2H-VSeTe with intrinsic ferromagnetism is investigated by density functional theory. The biaxial strain could effectively tune the magnetic and electronic properties of Janus VSeTe. Specifically, the magnetic moment, band gap, Curie temperature (T_c), and valley splitting (Δ) could be modulated, as the states near the Fermi level are mainly contributed by the in-plane atomic orbitals. The VSeTe could be switched from ferromagnetic (FM) order to antiferromagnetic (AFM) ground state, under biaxial strains. And the corresponding T_c is tuned from 360 K (4%) to 0 K (-10.7%). However, VSeTe can be modulated from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), half-metal (HM), and even normal metal as the biaxial strain varies from -13 to 10%. Moreover, the easy and hard axes could be switched from each other, and the magnetocrystalline anisotropy (MCA) energy is also controlled by the strains. The Δ is also increased from 158 to 169 meV as the strain varies from 3.3 to -3.0%. The magnetic and electronic phase transitions in the strained VSeTe are observed, which could help researchers to investigate the controllable electronic and magnetic properties in electronics, spintronics, and valleytronics.

Graphene Nanoribbons: On-Surface Synthesis and Integration into Electronic Devices

Graphene nanoribbons (GNRs) are quasi-1D graphene strips, which have attracted attention as a novel class of semiconducting materials for various applications in electronics and optoelectronics. GNRs exhibit unique electronic and optical properties, which sensitively depend on their chemical structures, especially the width and edge configuration. Therefore, precision synthesis of GNRs with chemically defined structures is crucial for their fundamental studies as well as device applications. In contrast to top-down methods, bottom-up chemical synthesis using tailor-made molecular precursors can achieve atomically precise GNRs. Here, the synthesis of GNRs on metal surfaces under ultrahigh vacuum (UHV) and chemical vapor deposition (CVD) conditions is the main focus, and the recent progress in the field is summarized. The UHV method leads to successful unambiguous visualization of atomically precise structures of various GNRs with different edge configurations. The CVD protocol, in contrast, achieves simpler and industry-viable fabrication of GNRs, allowing for the scale up and efficient integration of the as-grown GNRs into devices. The recent updates in device studies are also addressed using GNRs synthesized by both the UHV method and CVD, mainly for transistor applications. Furthermore, views on the next steps and challenges in the field of on-surface synthesized GNRs are provided.

On-surface synthesis of singly and doubly porphyrin-capped graphene nanoribbon segments

On-surface synthesis has emerged as a powerful tool for the construction of large, planar, π-conjugated structures that are not accessible through standard solution chemistry. Among such solid-supported architectures, graphene nanoribbons (GNRs) hold a prime position for their implementation in nanoelectronics due to their manifold outstanding properties. Moreover, using appropriately designed molecular precursors, this approach allows the synthesis of functionalized GNRs, leading to nanostructured hybrids with superior physicochemical properties. Among the potential “partners” for GNRs, porphyrins (Pors) outstand due to their rich chemistry, robustness, and electronic richness, among others. However, the use of such π-conjugated macrocycles for the construction of GNR hybrids is challenging and examples are scarce. Herein, singly and doubly Por-capped GNR segments presenting a commensurate and triply-fused GNR–Por heterojunction are reported. The study of the electronic properties of such hybrid structures by high-resolution scanning tunneling microscopy, scanning tunneling spectroscopy, and DFT calculations reveals a weak hybridization of the electronic states of the GNR segment and the Por moieties despite their high degree of conjugation.

Singly and doubly porphyrin-capped graphene nanoribbon segments are reported and their electronic properties are studied by high-resolution scanning tunneling microscopy and spectroscopy, and DFT calculations.

On the Impact of Substrate Uniform Mechanical Tension on the Graphene Electronic Structure

Employing density functional theory calculations, we obtain the possibility of fine-tuning the bandgap in graphene deposited on the hexagonal boron nitride and graphitic carbon nitride substrates. We found that the graphene sheet located on these substrates possesses the semiconducting gap, and uniform biaxial mechanical deformation could provide its smooth fitting. Moreover, mechanical tension offers the ability to control the Dirac velocity in deposited graphene. We analyze the resonant scattering of charge carriers in states with zero total angular momentum using the effective two-dimensional radial Dirac equation. In particular, the dependence of the critical impurity charge on the uniform deformation of graphene on the boron nitride substrate is shown. It turned out that, under uniform stretching/compression, the critical charge decreases/increases monotonically. The elastic scattering phases of a hole by a supercritical impurity are calculated. It is found that the model of a uniform charge distribution over the small radius sphere gives sharper resonance when compared to the case of the ball of the same radius. Overall, resonant scattering by the impurity with the nearly critical charge is similar to the scattering by the potential with a low-permeable barrier in nonrelativistic quantum theory.

cAAC-Stabilized 9,10-diboraanthracenes-Acenes with Open-Shell Singlet Biradical Ground States

Narrow HOMO-LUMO gaps and high charge-carrier mobilities make larger acenes potentially high-efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open-shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO-LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10-diboraanthracenes, which are shown to feature disjointed, open-shell singlet biradical ground states.

B5N5 monolayer: a room-temperature light element antiferromagnetic insulator

We demonstrate theoretically that an intrinsic antiferromagnetic phase exists in monolayer materials consisting of non-magnetic light atoms, and propose that B5N5 with a decorated bounce lattice is a thermodynamically stable two-dimensional antiferromagnetic insulator by performing state-of-the-art density functional theory calculations. The antiferromagnetic phase originates from spontaneous symmetry breaking at the nearly flat bands in the vicinity of the Fermi energy. The flat bands are formed by purely s-p z orbitals and are spin degenerate. A perpendicular electric field can remove the spin degeneracy and a prototype controllable dual spin filter with 100% spin polarization is proposed. Our proposal offers a possible two-dimensional atomically thick antiferromagnetic insulator.

From graphene to graphene ribbons: atomically precise cutting via hydrogenation pseudo-crack

The properties and applications of graphene nanoribbons (GNRs) depend heavily on their shape and size, making precise design and construction at atomic scale significantly important. Herein, we show that pseudo-cracking is a feasible method for creating atomically precise GNRs. By using molecular dynamics (MD) simulation, we find that hydrogenation can act as a pseudo-crack to trigger the fracture of graphene along the hydrogenation line and cut the graphene into a GNR. Precise GNRs with a desired width, edge type and associated properties can be realized in a controllable way by manipulating the position and dimension of the hydrogenation pseudo-crack. We also find that it is better to use hydrogenation pseudo-cracks along the armchair direction to cut graphene at lower forces into GNRs with smooth edges. Our findings suggest a promising approach to cut graphene and other two-dimensional materials into nanoribbons effectively and accurately.