Pz Electrons Research Articles

On‐Surface Synthesis of Triaza[5]triangulene through Cyclodehydrogenation and its Magnetism

AbstractTriangulenes as neutral radicals are becoming promising candidates for future applications such as spintronics and quantum technologies. To extend the potential of the advanced materials, it is of importance to control their electronic and magnetic properties by multiple graphitic nitrogen doping. Here, we synthesize triaza[5]triangulene on Au(111) by cyclodehydrogenation, and its derivatives by cleaving C−N bonds. Bond‐resolved scanning tunneling microscopy and scanning tunneling spectroscopy provided detailed structural information and evidence for open‐shell singlet ground state. The antiferromagnetic arrangement of the spins in positively doped triaza[5]triangulene was further confirmed by density function theory calculations. The key aspect of triangulenes with multiple graphitic nitrogen is the extra pz electrons composing the π orbitals, favoring charge transfer to the substrate and changing their low‐energy excitations. Our findings pave the way for the exploration of exotic low‐dimensional quantum phases of matter in heteroatom doped organic systems.

On-Surface Synthesis of Triaza[5]triangulene through Cyclodehydrogenation and its Magnetism.

Triangulenes as neutral radicals are becoming promising candidates for future applications such as spintronics and quantum technologies. To extend the potential of the advanced materials, it is of importance to control their electronic and magnetic properties by multiple graphitic nitrogen doping. Here, we synthesize triaza[5]triangulene on Au(111) by cyclodehydrogenation, and its derivatives by cleaving C-N bonds. Bond-resolved scanning tunneling microscopy and scanning tunneling spectroscopy provided detailed structural information and evidence for open-shell singlet ground state. The antiferromagnetic arrangement of the spins in positively doped triaza[5]triangulene was further confirmed by density function theory calculations. The key aspect of triangulenes with multiple graphitic nitrogen is the extra pz electrons composing the π orbitals, favoring charge transfer to the substrate and changing their low-energy excitations. Our findings pave the way for the exploration of exotic low-dimensional quantum phases of matter in heteroatom doped organic systems.

Theoretical verification on adsorptive removal of caffeine by carbon and nitrogen-based surfaces: Role of charge transfer, π electron occupancy, and temperature

Eliminating an emerging water pollutant, caffeine molecules, from an aqueous solution using carbon and nitrogen-based adsorbents is of significant interest to public health. These adsorbents have been shown to have decent adsorption capacity toward caffeine due to their surface functionality. Therefore, screening various carbon and nitrogen-based surfaces can be a better option for high-performance adsorbents to remove caffeine efficiently from wastewater. Herein, we present combined first principles and molecular dynamics quantification of the adsorption enthalpies of caffeine molecules on the possible active sites of carbon and nitrogen-based adsorbents (graphene, phagraphene, graphdiyne, single-wall carbon nanotube, fullerene, and graphitic carbon nitride) with the incorporation of Van der Waals interactions. From the DFT calculations, N-doped carbon surfaces show the highest adsorption energies of single and dimer CAF compared to pristine carbon-based adsorbents. A charge density difference and Bader charge analysis display that high charge transfer occurs between the caffeine's oxygen and the surface's nitrogen atoms. An abundance of π-electrons from the nitrogen atoms, composed of large electron clouds of aromatic rings on the graphitic carbon surface, tends to favor extensive π-π interactions with the caffeine molecule. The high value of pz electron occupancy (1.445) of N in the hexagonal ring of the graphitic surface transfers additional charge transfer, which leads to strong adsorption energy of CAF than pristine surfaces. Also, the g-C3N4 surface adsorbs the CAF molecule with higher adsorption than other N-doped carbon surfaces due to the high pz_eo (1.5448) of N atoms on the surface. At 310 K, the water molecules' kinetics aids the single and dimer caffeine molecules to adsorb with the highest adsorption energies on the active sites of g-C3N4 surfaces than graphene adsorbent.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions.

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)4Cl(L)]+ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH+, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(μ-pz)Ru(py)4Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

Fe1+yTexSe1-x: a delicate and tunable Majorana material

Abstract We report the observation for the pz electron band and the band inversion in Fe1+yTexSe1-x with angle-resolved photoemission spectroscopy. Furthermore, we found that excess Fe (y>0) inhibits the topological band inversion in Fe1+yTexSe1-x, which explains the absence of Majorana zero modes in previous reports for Fe1+yTexSe1-x with excess Fe. Basing on our analysis of different amounts of Te doping and excess Fe, we propose a delicate topological phase in this material. Thanks to this delicate phase, one may able to tune the topological transition via applying lattice strain or carrier doping.

Fe1+y Te x Se1- x : A Delicate and Tunable Majorana Material

We report the observation for the pz electron band and the band inversion in Fe1 + y Te x Se1–x with angle-resolved photoemission spectroscopy. Furthermore, we found that excess Fe (y > 0) inhibits the topological band inversion in Fe1 + y Te x Se1 – x , which explains the absence of Majorana zero modes in previous reports for Fe1+y Te x Se1 – x with excess Fe. Based on our analysis of different amounts of Te doping and excess Fe, we propose a delicate topological phase in this material. Thanks to this delicate phase, one may be able to tune the topological transition via applying lattice strain or carrier doping.

Spin Alternation Rule in USCF for Through-Bond Magnetic Coupling─A New Look: Why and When Does It Arise and How To See It.

This work takes a new look at the spin alternation rule in unrestricted self-consistent-field (USCF) calculations in terms of structural characteristics such as periodicity, impurity location, and Coulomb exchange. For clarity, the systems considered are biradicals produced from linear conjugated hydrocarbons. Both site-parametrized Hamiltonian models for theoretical analysis and spin unrestricted density functional theory (DFT) calculations are used. Theoretical analysis leads to the following conclusions: (1) The diradical state is an excited state of a linear chain of N conjugated carbon atoms (when N is about ≤ 10). Spin alternation is a consequence of the (truncated) periodic symmetry combined with filling each closed-shell pi orbital with two electrons and each singly occupied molecular orbital (SOMO) with one electron. Spin polarization is evident in triplet (T) and broken symmetry (BS) solutions for an odd N and only in the T solution for an even N. Spin alternation is visible in the BS for an odd N and always remains muted in the calculated T. (2) For a doped chain with two radical centers, spin alternation is generally visible in the BS for an odd N. The sign of spin population on the radical centers in the BS indicates the stable spin. For radical centers separated by an odd (even) number of pz electrons, spin alternation favors T (S) state with FM (AFM) interaction. Spin oscillation remains less transparent for an even N without exchange. (3) In an unrestricted treatment with exchange, spin alternation becomes observable. Without SCF iterations, the more stable state can be identified from a clear spin oscillation in the BS. An irregular oscillation indicates a possible singlet ground state. These observations are supported by density functional calculations using the B3LYP functional and the 6-311+g(d,p) basis set on linear decapentaene diradicals with nitronyl nitroxide moieties substituted on two sets of conjugated atoms, (3,9) and (3,10). Because of the SCF procedure, one finds spin alternation in the T (BS) solution and erratic oscillation in the BS (T) solution of the 3,9 (3,10) diradical in respective equilibrium geometries. The ground state is T (S). DFT adiabatic coupling constants, SOMO energies, spin population plots, and SOMO lobe diagrams compare well with molecular electronic characteristics from theoretical analysis using Hamiltonian parameters.

Magnetic Interactions Between Radical Pairs in Chiral Graphene Nanoribbons.

Open-shell graphene nanoribbons have become promising candidates for future applications, including quantum technologies. Here, we characterize magnetic states hosted by chiral graphene nanoribbons (chGNRs). The substitution of a hydrogen atom at the chGNR edge by a ketone effectively adds one pz electron to the π-electron network, producing an unpaired π-radical. A similar scenario occurs for regular ketone-functionalized chGNRs in which one ketone is missing. Two such radical states can interact via exchange coupling, and we study those interactions as a function of their relative position, which includes a remarkable dependence on the chirality, as well as on the nature of the surrounding ribbon, that is, with or without ketone functionalization. Besides, we determine the parameters whereby this type of system with oxygen heteroatoms can be adequately described within the widely used mean-field Hubbard model. Altogether, we provide insight to both theoretically model and devise GNR-based nanostructures with tunable magnetic properties.

Embedding atomic cobalt into graphene lattices to activate room-temperature ferromagnetism

Graphene is extremely promising for next-generation spintronics applications; however, realizing graphene-based room-temperature magnets remains a great challenge. Here, we demonstrate that robust room-temperature ferromagnetism with TC up to ∼400 K and saturation magnetization of 0.11 emu g−1 (300 K) can be achieved in graphene by embedding isolated Co atoms with the aid of coordinated N atoms. Extensive structural characterizations show that square-planar Co-N4 moieties were formed in the graphene lattices, where atomically dispersed Co atoms provide local magnetic moments. Detailed electronic structure calculations reveal that the hybridization between the d electrons of Co atoms and delocalized pz electrons of N/C atoms enhances the conduction-electron mediated long-range magnetic coupling. This work provides an effective means to induce room-temperature ferromagnetism in graphene and may open possibilities for developing graphene-based spintronics devices.

Twisted Bilayer Graphene Quantum Dots for Chiral Nanophotonics

Twisted bilayer graphene (TBG) is a two-dimensional chiral material whose optical activity is remarkably strong for its atomic thickness. While the chiral optical properties of TBG are currently well understood, the optical activity of quantum dots (QDs) made of TBG has not been analyzed so far. Here, we comprehensively analyze the chiral optical properties of TBG QDs with hexagonal, triangular, and rhomboid symmetries. We show that the dissymmetry factors of TBG QDs can reach 10–2, greatly exceeding the dissymmetry factors of semiconductor QDs and small chiral molecules. The strong optical activity of TBG QDs is due to the out-of-plane polarized transitions of the pz electrons and can be controlled by changing the size and shape of the QDs. These unique features make TBG QDs highly attractive for application in chiral nanophotonics.

Superatom spin-state dynamics of structurally precise metal monolayer-protected clusters (MPCs).

Electronic spin-state dynamics were studied for a series of Au25(SC8H9)18 q and Au24Pd(SC8H9)18 monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = -1, 0, +1. These clusters were chosen for study because Au25(SC8H9)18 -1 is a closed-shell superatomic cluster, but Au25(SC8H9)18 0 is an open-shell (7-electron) system; Au25(SC8H9)18 +1 and PdAu24(SC8H9)18 0 are isoelectronic (6-electron) closed-shell systems. Carrier dynamics for electronic fine structure spin states were isolated using femtosecond time-resolved circularly polarized transient-absorption spectroscopy (fs-CPTA). Excitation energies of 1.82 eV and 1.97 eV were chosen for these measurements on Au25(SC8H9)18 0 in order to achieve resonance matching with electronic fine structure transitions within the superatomic P- and D-orbital manifolds; 1.82-eV excited an unpaired Pz electron to D states, whereas 1.97-eV was resonant with transitions between filled Px and Py subshells and higher-energy D orbitals. fs-CPTA measurements revealed multiple spin-polarized transient signals for neutral (open shell) Au25(SC8H9)18, following 1.82-eV excitation, which persisted for several picoseconds; time constants of 5.03 ± 0.38 ps and 2.36 ± 0.59 ps were measured using 2.43 and 2.14 eV probes, respectively. Polarization-dependent fs-CPTA measurements of PdAu24(SC8H9)18 clusters exhibit no spin-conversion dynamics, similar to the isoelectronic Au25(SC8H9)18 +1 counterpart. These observations of cluster-specific dynamics resulted from spin-polarized superatom P to D excitation, via an unpaired Pz electron of the open-shell seven-electron Au25(SC8H9)18 MPC. These results suggest that MPCs may serve as structurally well-defined prototypes for understanding spin and quantum state dynamics in nanoscale metal systems.

Strain Effect on the Superconductivity in Borophenes

The effects of strain on the structure stability, electron–phonon coupling (EPC), and superconductivity of two types of monolayer borophenes realized in the experiments are systematically investigated within the framework of density functional theory. We find that the electron–phonon coupling (EPC) in the buckled triangle borophene can be significantly enhanced by the suitable strain (−2–3%) due to the lower acoustic phonon branch softening. Our calculations suggest that a superconducting transition temperature (Tc) ranging from 24 to 32 K may be observed in the experiment. For the β12 borophene, the EPC constant (λ) and Tc exhibit a U-curve variation with the strain ranging from 0 to 12%. The highest Tc of 14.9 K can be obtained in the pristine structure. The stiffness of the lower acoustic phonon branches and the U-curve variation of N(EF) mainly from the pz electrons of boron are responsible for the change of the superconducting transition temperature with the increase of the tensile strain. Although b...

Activating impurity effect in edge nitrogen-doped chevron graphene nanoribbons

Doping has been regarded as one of the most important band structure engineering methods for graphene and its derivatives. Here, we theoretically investigate the chevron-type graphene nanoribbon (CGNR) which is doped by nitrogen in its edges (N-CGNR). The impurity effect can be activated by hydrogen efficiently. When each N atom is adsorbed by a H atom, the σ bond of N induced by self-hybridization is replaced by the N-H sp2 bond, leaving two pz electrons perpendicular to the CGNR plane. Only one pz electron can be bonded with the nearest C atom. Therefore, the residual pz electron is delocalized from the N atom and induces the n-type impurity effect. We have calculated the binding energies of N-H bonds and found they are stable and can be manipulated independently without impacting the other bonds. A molecular dynamic (MD) simulation under high temperature further verifies that the N-H bonds in some specific positions can even be stable at 2000 K. Finally, the activated impurity effect is exhibited in the transport properties of CGNR based devices, indicating its wide application prospects.

Ring type and π electron occupancy decides the Li-ion storage properties of Phagraphene: An example of sp2 hybridized carbon structure

Graphite, a sp2 hybridized layered material is used as an anode material in commercial Lithium ion batteries (LIB's). It is desirable to improve the Li/C ratio in the sp2 hybridized carbon allotropes to increase the specific energy capacity. Using First-principles calculations bulk Phagraphene (contains penta-, hepta- and hexa ring) an example of sp2 hybridized carbon allotropes is identified as a potential high capacity anode material for rechargeable LIB's. The Li adsorption site preference is attributed to the combined effect of the pz electrons of the carbon atoms constituting a ring. It is found that the Li diffusion is favoured during both adsorbed and intercalated state. Bulk-Phagraphene shows desirable negative formation energy, high specific capacity of 558 mAh/g, and stable positive open circuit voltage profile for high Li intercalation. Overall the asymmetry in the pz electron occupancies of the distinct carbon atoms in Phagraphene is identified to be a major contributing factor for the higher activity of Phagraphene as compared to the graphene. This indicates that we can tune the property of sp2 based carbon structures by changing the π electron environments.

Magnetic monolayer Li2N: Density Functional Theory calculations

Density functional theory (DFT) calculations are used to investigate the electronic and magnetic structures of a two-dimensional (2D) monolayer Li2N. It is shown that bulk Li3N is a non-magnetic semiconductor. The non–spin-polarized DFT calculations show that p electrons of N in 2D Li2N form a narrow band at the Fermi energy due to a low coordination number, and the density of states at the Fermi energy () is increased as compared with bulk Li3N. The large shows instability towards magnetism in Stoner's mean-field model. The spin-polarized calculations reveal that 2D Li2N is magnetic without intrinsic or impurity defects. The magnetic moment of in 2D Li2N is mainly contributed by the pz electrons of N, and the band structure shows half-metallic behavior. Dynamic instability in planar Li2N monolayer is observed, but a buckled Li2N monolayer is found to be dynamically stable. The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms is also investigated to access the exchange field strength. We found that planar (buckled) 2D Li2N is a ferromagnetic material with Curie temperature Tc of 161 (572) K.

Electronic and Magnetic Properties of Highly Correlated Half Metallic Layered Sr2CoO4 Cobaltate Using mBJ Exchange Potential

We have investigated the electronic and magnetic response of layered Sr2CoO4 (SCO) using Tran-Blaha modified Becke-Johnson potential (mBJ) within the density functional theory. The optimized lattice parameters were determined for the ground state energy of ferromagnetic SCO. Present calculations reveal a peculiar half metallic character with an indirect band gap of 1 eV in the minority state. The magnetic moments on Co and O atoms are observed to be antialligned with each other, resulting to a magnetic moment of 1 µB/f.u. Electronic occupancies for the various orbitals of Co and O atoms have been deduced, suggesting the Co-3 dxy and O2- pz orbitals to be majorly possessing magnetic moments in the SCO system. Hybridisation between different orbitals and their contribution in forming the Fermi surfaces (FSs) are discussed. It is revealed that in half-metallic ferromagnetic SCO, FS structures are formed due to majority spin Co-\({3d}_{{x^{2}-y}^{2}}\) and Co-\({3d}_{z^{2}}\) orbitals hybridised with O-2p orbitals. It is inferred that majority spin Co-\({3d}_{{x^{2}-y}^{2}}\) (dominantly) and pz electrons of O atoms are responsible for the conduction mechanism. Present results provide the usefulness of mBJ approach in deducing the electronic and magnetic properties of layered SCO. Deduction of orbital magnetic moment for cobaltates-like systems would be very crucial to further understand its magnetic properties.

First-principles identification of site dependent activity of graphene based electrocatalyst

Graphene based metal free electrocatalysts have been considered as potential candidate for efficient oxygen reduction reaction (ORR) in fuel cell systems. Using density functional theory we investigated the site dependent ORR activity of nitrogen doped and divacancy (DV_555-777) graphene structures considering free energy calculations and correlate with the occupancy of pz(π) electrons. The dioxygen adsorption strength differs for each carbon site in case of N doped and DV_555-777 graphene system. Range of overpotential and on-set potential has been estimated for same system. Among the sites and systems considered, the C2′ site of 2N doped graphene system is most active towards oxygen reduction with lowest overpotential. We have found that the DV_555-777 is more active than the pure system. In this case also we have observed a range of overpotential considering various C sites. We have been estimated the occupancy of pz(π) electrons of each C site of different systems. The occupancy of pz(π) electrons of C atoms increase near the dopant site due to back-donation mechanism in N doped graphene system. A strong correlation has been identified in-between activity of each C site with the occupancy of pz(π) electrons of corresponding site. Overall we concluded that each C site of N doped and defective graphene has different on-set potential and pz(π) electrons play a major role to define the activity.

Edge-functionalization of blue phosphorene nanoribbons: Studied from first-principle methods

Through density functional theory calculations, the impact of edge functionalization with O, OH, and alternate termination of them (OHO) on the structural stabilities, electronic and magnetic properties of blue phosphorene nanoribbons (BPNR) are mainly investigated. The formation energies demonstrate that the O-termination on the BPNRs is the most stable, and OHO-termination is more stable than OH-termination, besides the ab initio dynamic simulation show that they are all thermal dynamically stable at room temperature. Both the ground structures of O- and OH-BPNRs are spin-polarized semiconductors, while OH-functionalized BPNRs are nonmagnetic semiconductors. As the ribbon width increasing, the band gaps of O-aBPNRs tend to 1.04eV, but that of OH-aBPNRs tend to 1.97eV, comparable with the band gap of single-layer blue phosphorene, since it is dominated by pz electrons of the inner P atoms. In contrast, the influence of OHO-termination on GNRs, SiNRs, and black PNRs are also studied. Our results demonstrate that OHO-terminated GNRs and SiNRs are not a simple summation of O- and OH-terminated GNRs and SiNRs, and they are nonmagnetic stable both with zigzag and armchair edges, presenting metallic properties. While the OHO-terminated black PNRs present similar electronic and magnetic properties with OHO-terminated blue PNRs, and both the OHO-terminated zigzag and armchair edges are spin-polarized stable. These results provide potential help in the fields of band gap engineering and the designing of phosphorus-based spin devices with control over spin in spintronics.

Peculiarities of interaction of the pz-, π- electrons and the σp-holes at the top 1–6 layers of HOPG

The present work continues the analysis of results of Dementjev et al. (2015) in order to identify the interlayer interactions of the π-bands. Analysis of the N(E) C KVV Auger spectra of highly-ordered pyro-graphite showed the absence of the electron exchange between the π-bands in 1–6 layers. Since the π-bands are formed by the pz→π transitions, one can suggest that the π-band occupation at each graphene layer is formed by the pz-electrons of this layer. Since the pz electrons belong to the σp-bands, the pz→π transitions in the σp-bands in each of 2–6 graphene layers result in formation of holes H, whose concentration is equal to the concentration of electrons in the π-bands [Hi] [πi]. This shows the origin of the ambipolar conductivity in graphene.The absence of the electronic interaction between the π-bands allows a suggestion that the interaction between top six graphene layers is due to the van der Waals electrostatic attractive forces. These forces promote the pz→π transitions in each of the 2–6 graphene layers and depend on the number of graphene layers above. The N(E) C KVV Auger spectra allow identification of number (1–6) of graphene layers and the π-band occupation at each of the layer. For the first time a specification of the van der Waals forces in HOPG was done.

Theoretical study of valence orbital response to guanine tautomerization in coordinate and momentum spaces

The binding energy spectra and electron momentum spectra of eight stable guanine tautomers are calculated in the complete valence space. The present results show that the canonical keto (C=O) guanine N(9)H tautomer (GU1) possesses the largest dipole moment, molecular electronic spatial extent, molecular hardness value, and the minimum first vertical ionization potential (VIP). Valence orbital profile investigations find that several orbitals remain almost unchanged during tautomerization, such as frontier highest occupied molecular orbital 39a and 18a. Several orbitals with interchanged order and inverse direction in charge spatial orientations are also detected. Outer valence orbitals (with smaller VIPs) show more complex orbital shapes in the momentum space than those of inner ones (larger VIPs) due mainly to the relatively strong inter-orbital interaction and delocalized electronic distributions. Proton rotation along C–O(H) and C–N(H) axes within hexagonal ring causes smaller influence to orbital profiles than those of proton migration within pentagonal and/or hexagonal rings. Orbital variation trends between enol (GU3–GU5) and keto (GU1, GU2, GU6–GU8) tautomers are observed, including the signature orbitals of enol form, the variation tendency of total orbital intensity, and the variation order of the maximum orbital intensity. In the outer valence momentum space (outside 26a), orbital composed by pz electrons show single peak with a gradual increasing peak site from 0.5 a.u. of inner valence orbital to 1.0 a.u. of outer valence orbital, whereas orbitals composed by px,y electrons form double peaks with respective sites at about 0.5 and 1.5 a.u., only three px,y-orbitals present single peaks (33a,34a,36a). The general variation trends in the complete valence space for all the valence orbitals on their intensities, peak sites, and orbital components are concluded.