Magnetic Properties and Engineering of Nanographene in Ultra-High Vacuum

Hui Zhang; Wei Xiong; Jianchen Lu; Jinming Cai

doi:10.7536/PC210616

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (3) : 557-567 DOI: 10.7536/PC210616

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Magnetic Properties and Engineering of Nanographene in Ultra-High Vacuum

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Abstract

Due to the existence of unsaturated electrons, it is difficult to synthesize magnetic nanographene directly by the wet chemical method. With the help of surface catalysis in ultra-high vacuum, designed precursor molecules can be transformed into magnetic nanographene. Magnetism of carbon nanomaterials or nanographene possess high magnitudes of spin-wave stiffness, weak spin-orbit coupling, hyperfine couplings and large spin coherence lifetimes comparing with the transition metal, which hold the promise for spintronics construction and basic research. Due to the existence of unsaturated electrons, it is difficult to synthesize magnetic nanographene directly by the wet chemical method. With the help of surface catalysis in ultra-high vacuum, the synthesis of magnetic nanographene by artificial designed precursor molecules has been exploding. Except for the fabrication the magnetic nanographene, the investigation of magnetic ground state of nanographene itself and the magnetic engineering by tip-manipulation and construction of spin chains have been attracting much attentions. The atomic precise chemical and electronic structure and magnetic ground state could be identified by scanning tunneling spectroscopy and CO-decorated tip scanning tunneling microscope image. In this review, based on the recent research on nanographene, we introduce magnetic generations, structural and magnetism characterizations of nanographene as well as the magnetic order control by scanning tunneling microscopy in ultra-high vacuum.

Contents

1 Introduction

2 Magnetism of nanographene

2.1 Sublattice imbalance

2.2 Non-Kekuléstructure due to topology constraints

2.3 Non-Kekuléstructure due to size

3 Engineering of magnetic nanographene

3.1 Tip manipulation

3.2 Alternation of homo-coupling positions

4 Conclusion and outlook

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Keywords

nanographene / magnetism / engineering / scanning tunneling microscope

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Hui Zhang, Wei Xiong, Jianchen Lu, Jinming Cai. Magnetic Properties and Engineering of Nanographene in Ultra-High Vacuum. Progress in Chemistry, 2022, 34(3): 557-567 DOI:10.7536/PC210616

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1 Introduction

Searching for novel States of matter has always been an important frontier topic in condensed matter physics, such as topological superconducting state, topological insulating state and Weyl semimetal state, which have been observed in experiments recently. The electronic ground state of simple metals such as gold, silver and copper is called Fermi-liquid state, which is characterized by the saturation of resistivity with the decrease of temperature. However, when there is a magnetic impurity atom with local magnetic moment on the metal surface, the free electrons from the metal substrate have a strong correlation with the local magnetic moment to form a Kondo Singlet, which makes the spin of the magnetic impurity reverse, and macroscopically shows an abnormal phenomenon of a minimum resistance at low temperature, that is, the Kondo effect. In addition to the Kondo effect of magnetic impurity atoms on the metal surface, the unsaturated carbon nanomaterials with single electrons will also be screened by the spin of free electrons of the substrate, thus showing the Kondo effect. Scanning tunneling microscopy (STM) is an effective tool to observe and study the mechanism of magnetism. From the previous magnetic impurity atomic systems on metal substrates, such as,The Kondo effect of Co atoms on Au (111), Ce or Ti atoms on Ag (111), etc., to the systems of organic molecules such as iron phthalocyanine and manganese phthalocyanine combined with magnetic atoms, and to the recent magnetic studies of single organic nano-graphene^[1]^[2,3]^[4]^[5]^{[6⇓⇓⇓~10]}. Compared with the magnetic properties of transition metals, carbon-based nanomaterials or Nanographene (NG) have attracted much attention recently because of their finer and weaker spin-orbit interaction, higher spin stiffness, higher Curie temperature and spin coherence length, which can be used in spintronic devices^[11⇓~13]^[14]^[15]^{[11,16⇓⇓ ~19]}. It should be pointed out that the Kondo effect described in this paper is a Single-channel Kondo effect, that is, the resistivity increases abnormally with the decrease of temperature in the form of -lnT.When Two spin-degenerate free electrons, each with its own spin-1/2, compete equally for the screening effect of a spin-1/2 magnetic impurity, a new non-Fermi liquid state, the two-channel Kondo effect, is generated, which is characterized by an increase in resistivity in a channel Kondo effect manner^[20].

For nano-graphene materials with single electrons, the activity of organic molecules is improved due to the existence of single electrons, which increases the difficulty of direct synthesis. In solution chemistry, magnetic molecules can be successfully synthesized by means of group protection, and the magnetic ground state of nano-graphene can be obtained by means of Squid and other characterization methods^[21]^[22,23]. Scanning tunneling microscope (STM) can be used to achieve atomic-level accurate synthesis and characterization of products. "on-surface synthesis" method can be used to synthesize organic magnetic molecules on catalytic substrates, and combined with STM technology to carry out structural observation and magnetic characterization^[24,25]. In this review, we focus on the magnetic properties of carbon-based nanomaterials studied by STM in recent years, including the generation of magnetic properties of graphene nanomaterials, the synthesis and preparation of products, the imaging and signal spectrum under STM, and the regulation of magnetic properties.

2 Magnetic Properties of Nano-Graphene

Kondo effect has been an important direction in the research of magnetic materials for a long time, and the existence of local magnetic moment is a necessary condition for Kondo effect. The interaction between the local magnetic moment and the free electrons from the substrate realizes the process of spin flip through the two intermediate States of "two-electron occupied state" and "empty state" of the magnetic orbit, and produces a resonance state with extremely small energy broadening near the Fermi surface, as shown in Fig. 1^[26]. Scanning tunneling microscopy (STS) can be used to measure the Kondo effect of surface nanostructures. The spatial atomic resolution of STM makes the measurement of Kondo effect reach the scale of single atom and single molecule.

The existence of single electrons in carbon-based nanomaterials can also be screened by the spin from the substrate, and the resulting state signals can be tracked by STM and STS. Therefore, the magnetic study of carbon nanomaterials should first study the situation of carbon nanomaterials generating single electrons, and then study the structural and spectral manifestations of these situations with the help of STM and STS. In terms of structural information, the magnetic properties of nano-graphene are divided into sublattice imbalance and special non-Kekule structure.

2.1 Sublattice imbalance

Like graphene, nanographene is also composed of carbon atoms connected by sp² hybridization, and each six-membered ring is composed of two sets of sublattices (sublattice A and sublattice B) with the same number of carbon atoms. If the number of carbon atoms in the two sets of sublattices is not equal, that is, N_AN_B(N_A and N_B represent the number of carbon atoms in the A and B sublattices, respectively), the so-called sublattice imbalance is formed. According to Lieb's theory, it can be determined that the total spin quantum number in the full six-membered ring nano-graphene is

S (S = ∣ N A - N B ∣ / 2) 。

2019. When Li et al. Used the constant height mode of STM to study the chiral graphene nanoribbons on the Au (111) substrate, they found that under the bias voltage (2 mV) near the Fermi surface, there is an additional electronic state distribution at the fusion position of multiple chiral graphene nanoribbons, as shown in Figure 2C^[27]^[28]. Bond-resolved scanning tunneling microscope (BR-STM) images with higher spatial resolution (Figure 2D) were obtained by further modifying the tip of STM with CO.Far away from the band junction, the bond resolution information of the chiral graphene nanoribbon is clear, and the ring characteristics of the benzene ring are obvious, while at the band junction, due to the influence of the additional electronic States near the Fermi surface, it has a higher brightness, and only a five-membered ring structure can be vaguely seen at the junction position, as shown in Figure 2e.

There are two distributions of electronic States at the junction (inset of Fig. 2F and Fig. 2g). STS in the small energy range is used to study the regions of these two distributions, and it is found that there is an additional Lorentz linear resonance state near the Fermi surface in these regions with additional electronic States. The six-membered ring in the normal graphene is alternately distributed by C in the two sublattices, and the five-membered ring is connected at the junction of the two nanoribbons, so that the balance of the local sublattice distribution is directly broken, unsaturated single electrons are generated, and a Kondo resonance peak is generated on a Fermi surface under the action of spin shielding of a substrate. In order to further verify the Kondo resonance of this resonance state, the experiments on the resonance peak were carried out at variable temperature (1.1 ~ 15 K, Fig. 2h) and variable magnetic field (0 ~ 2.8 T, Fig. 2i), and it was found that the half of the full width at half maximum of the resonance peak near the Fermi surface (Γ (Half width at half maximum, HWHM)) increased with the higher temperature condition of the data, and the peak shape was also consistent with the Frota fitting of the magnetic system^[3,29]^[30]. In addition, with the increase of the applied magnetic field, the HWHM also increases, and Zeeman splitting occurs at 2.8 T. By extracting the value of HWHM and the corresponding temperature to make a new spectral line, we can use the empirical formula to approximately calculate the Kondo temperature

T K

of this system is about

6 K (α = 9.5)

^[3].

Γ = 12 (α K B T) 2 + (2 K B T K) 2

Where

Г

is half of the peak width at half maximum,

α

is the slope factor at

T ≫ T K

K B 为 玻 尔 兹 曼 常 数

T

is the temperature,

T K

Kondo temperature.

The structural defects caused by the five-membered ring lead to the imbalance of the local sublattice of the graphene nanoribbon, thus introducing a magnetic ground state into the graphene nanoribbon.

In view of this, other non-six-membered heterocyclic rings such as five-membered rings or seven-membered rings directly constructed on the nano-graphene can also make the nano-graphene magnetic. In 2020, when Mishra et al studied the magnetic ground state of nanographene synthesized by 10-bromo-10 '- (2,6-dimethylphenyl) -9,9' -bianthracene (BDMPBA), they found that the product had sublattice imbalance, and observed the signal of Kondo resonance on nanographene^[31]. As shown in fig. 3, fig. 3A is a schematic diagram of the process of synthesizing the nano-graphene to be studied. The precursor molecule BDMPBA is deposited on the metal substrate Au (111) with catalytic activity, and the precursor molecule is subjected to dehalogenation, C-C bonding and dehydrocyclization respectively by means of a two-step heating annealing method to form the prepared nano-graphene structure NG1a^[32]. Affected by the annealing rate, one methyl group will be lost from the precursor molecule, resulting in the defective product NG1b. The difference in the electronic state distributions of NG 1A and NG 1b can also be found from the STM images. Fig. 3B shows the products of the two electronic state morphologies, NG 1A and NG 1b. Fig. 3C and fig. 3G are the high-resolution STM images of NG 1A and 1b, which can further distinguish the difference between the two structures. The terminal of non-defective NG 1A shows the characteristics of double-lobed electronic state, while compared with NG 1A, one terminal of NG 1b still has the characteristics of double-lobed electronic state, and the other terminal shows the characteristics of blurred electronic state due to the existence of five-membered ring. BR-STM of NG 1A and NG 1b (Figure 3D and Figure 3H) can further reveal their structures, indicating that their structures are indeed the same as the structural formulas. A closer look at the BR-STM map of NG 1b shows that at the same height (constant height mode), the brightness of the two terminals is obviously different, and the end without defects is brighter due to the contribution of additional electronic States near the Fermi surface. In addition, BR-STM studies on the two terminals also show that the structure of the five-membered ring with defects is clearly distinguished, while the six-membered ring without defects is wrapped by additional electronic States, which makes its structural information blurred. The structure of NG1b shows that the number of carbon atoms in the two sets of sublattices is different, and the difference of the number of carbon atoms is 1. Because of the existence of the five-membered ring, the Lieb theory can not be directly used to judge the S of the system, but the existence of the five-membered ring makes the system have a non-Kekule structure with a single electron, and the total spin quantum number is 1/2. The presence of a single electron makes the system magnetic and there is a Kondo resonance signal.

In order to explore the origin of the electronic state near the Fermi surface is caused by the Kondo resonance, the magnetic properties of NG are analyzed by theoretical calculations and experiments in a small energy range with variable temperature and magnetic field.

Fig. 4A shows the energy level distribution of NG 1b with S = 1/2 calculated by the tight-binding model without the Coulomb repulsion (U) of valence electrons. It can be found that there is a zero-energy state orbital on the Fermi surface. Fig. 4B shows the energy level distribution when U = 3.5 eV. After applying Coulomb repulsion, the spin-up and spin-down energy levels are calculated. It is found that the orbital of the zero-energy state near the Fermi surface is split into two single-electron occupied orbitals, which enter the valence band and conduction band of the energy band respectively, forming a single-occupied molecular orbital (SOMO) and a single-unoccupied molecular orbital (SUMO). The wave functions at the SOMO and the SUMO are distributed, and there is only a phase difference between the wave functions at the two positions. From the distribution of spin density of States (Fig. 4D), it can be found that the spin States are distributed at the defect-free end of NG 1A, while there is no distribution of spin States at the end with five-membered ring defects, indicating that the additional electronic States near the Fermi surface in BR-STM of NG 1b come from spin States. It is found that there is a resonance peak near the Fermi surface by STS at small energy and variable temperature for NG 1b, and the HWHM-value broadens with the increase of the temperature of signal acquisition, which further proves the existence of Kondo resonance. In addition, the estimated Kondo temperature is T_K=141 K. Nano-graphene introduces five-membered rings to make the number of carbon atoms in the sublattice of the system unbalanced, thus bringing a magnetic ground state to the system. Similarly, a seven-membered ring can also be introduced to unbalance the two sets of sublattices of the system, and then a magnetic state is introduced^[33].

In addition to constructing defects to introduce a magnetic ground state, a full six-membered ring structure with defect-free sublattice imbalance can also be artificially designed. Triangulene is a sublattice non-equilibrium structure, and by changing its size, N-triangulene with different magnetic order ground States can be constructed (where N represents the number of benzene rings on the edge of triangulene), such as 3-triangluene with S = 1 (Fig. 5A), 3-triangluene with S = 3/2 π electron epitaxy (Fig. 5d), and 7-triangleene with s = 3 (Fig. 5e) and their hole structures^[35]^[37]^[38]^[39]. Because the ground state of these sub-lattice mismatched nano-graphene is the existence of active single electrons, it is difficult to synthesize directly by chemical synthesis, so most of them adopt the design of specific precursor molecules (generally containing methyl,And carry out methyl dehydrocyclization to close that precursor molecules into a nano graphene structure contain a magnetic ground state or directly removing the protecting group to form the nano graphene.

In 2020, Li et al. used the precursor molecule (Figure 6a) to synthesize NG 2a with a triplet ground state (see Figure 5B for the structure)^[36]. The first step of thermal annealing is carried out at the temperature of dehalogenation. Due to the steric hindrance, the free radicals after the C-Br cleavage at the left and right sides of the precursor molecule can not combine with the free radicals from the left and right sides of another molecule, and still maintain the monomolecular state. A second step of thermal anneal is performed at a temperature at which dehydrocyclization occur between that methyl group and the benzene ring and between the benzene ring and the benzene ring, and finally NG2a is for. The BR-STM of NG 2a (Figure 6C) obtained using a CO-modified tip shows that the bond-resolved information of the molecule is clearer, but due to the influence of additional electronic States, the whole outer part shows brighter features. This extra electronic state is attributed to the Kondo resonance and T_K=6 K is estimated from the STS of NG 2a in a small energy interval with varying temperature (Fig. 6d) and magnetic field (Fig. 6e). Both defects (including doping) and artificial direct design of sublattice imbalance will introduce a magnetic ground state into the system, and further magnetic control can be achieved by designing different magnetic ground state products combined with structural defects^[40].

2.2 Topologically constrained non-Kekule open shell structures

There are three theories about the structure of benzene ring, namely valence bond theory, molecular orbital theory and resonance theory. According to molecular orbital theory, the six carbon atoms in benzene molecule are bonded by sp² hybridization, and the six carbon atoms form six carbon-carbon σ bonds by sp² hybridization orbitals. The carbon atoms retain a p orbital perpendicular to this plane, which is parallel to each other, so that the p orbitals of the carbon atoms can overlap the p orbitals of the adjacent carbon atoms in parallel to form a pi bond. For some nano-graphene systems, although the number of carbon atoms in the two sets of sublattices is the same, the topology of the π electronic network is limited, so that not all p_z orbitals can form π bonds, thus hybridizing into an open-shell structure containing free radicals, and then there is a magnetic ground state^[9]. Clar goblet is one of the typical examples, and Figure 7 is the schematic diagram of its structure NG3^[9,10]. Interestingly, Kekule's single and double bond alternation method can not make it fully saturated Kekule structure, because of its topological constraints, it directly forms a non-Kekule structure containing single electrons. NG 3 can be drawn as two one-electron 4-Clar structures (NG 3B) and four one-electron 3-Clar structures (NG 3A) according to Kekule's drawing. According to the Clar principle, the more the number of Clar rings, the higher the aromaticity, the lower the energy of the system, and the more stable the structure^[41]. The more the number of single electrons and the less the number of Clar rings, the higher the activity and the more unstable the system. Therefore, the 4-Clar structure with two single electrons is more stable than the 3-Clar structure with four single electrons.

In the small energy range of the experimental STS spectrum of NG3, no resonance state is found near the Fermi surface, which indicates that the ground state of the two-electron 4-Clar is an antiferromagnetic ground state with S = 0^[10]. Further study of STS shows that there are symmetric steps on the positive and negative sides of the Fermi surface, and the second-order dI/dV spectrum shows the existence of inelastic signals, which proves that under the action of external energy, the original ground state is S = 0 singlet state, and the electrons in the ground state undergo spin flip to S = 1 triplet excited state, resulting in inelastic signals. It is also interesting to note that the distance between two singulated electrons also changes the ground state of the magnetic order of the system. As the distance between the two singletons becomes greater, their spin exchange interaction decreases and eventually disappears. In this case, the two single electrons can be independently regarded as two with 1/2 spin, and the existence of Kondo resonance is also found in the STS spectrum in the small energy range. This single-molecule self-extension also provides a new idea for spin control, that is, for antiferromagnetic or ferromagnetic systems, the relative position of their single electrons can be adjusted to change the size of their spin exchange interaction, and even to achieve ferromagnetic or antiferromagnetic transition (polymers, structures of different sizes)^[42,43]^[7].

2.3 Non-Kekule open shell structure due to size factor

The magnetism of N_A=N_B system exists or its ground state is greatly affected by the size. When the spin distance between two single electrons is very close, the exchange energy J between the spins is very large, and the closed shell structure is reconstructed^{[6⇓~8,44⇓⇓ ~47]}^[8]. When the distance between two single electrons is further increased, J is gradually reduced, and the ground state of S = 0 antiferromagnetism is transformed, and the structure is also transformed from a closed shell structure to an open shell structure^[48,49]; The ground state changes from antiferromagnetism S = 0 or ferromagnetism S = 1 to paramagnetism by gradually increasing the distance between the two single electrons^[10]. Graphene nanoribbons with zigzag boundaries can also be seen as the extension of specific nano-graphene along a certain axis, increasing the number of carbon in zigzag boundaries. Due to the existence of carbon at the zigzag boundary, there are low-energy boundary localized States at the edge. With the increase of the distance from the edge, the Coulomb repulsion of valence electrons will cause spin polarization, showing the previously predicted magnetic nanoribbon structure^[50⇓~52].

On the other hand, in the chemical structure of nano-graphene, according to the principle of Clar sextets, the more the number of Clar sextets, the more stable the structure is, but the more the number of Cla-sextets is, the more single electrons appear.The activity of the structure is improved, and the stability is reduced, so that the structure competes between an open shell and a closed shell, and changes to an open shell along with the increase of the size, such as peri-acene, rhombic nano graphene and acene^[41]^{[21⇓~23,33,47,53]}^[48]^[44⇓~46]. Figure 8 is the effect of two dimensional changes on the ground state of nanographene. Fig. 8 a-d are schematic diagrams of the structure of the corresponding nanographene and the synthesis path on Au (111). Four kinds of nano-graphenes can be drawn as Kekule type and non-Kekule type. By comparing the BR-STM diagram and the spectroscopy in the small energy range, it is found that the two NGs with smaller size in the diagram are closed shell characteristics, that is, there is no special state in BR-STM and no inelastic signal in spectroscopy. The two larger NGs are in the antiferromagnetic ground state (BR-STM has no special state, but there is an inelastic signal). The effect of size change has not been studied quantitatively, and it is worth providing more examples to characterize and summarize.

For the structure of N_A=N_B, in addition to the transition from the ground state S = 0 to the excited state of higher magnetic order described above, there will also be a transition from the ground state of higher magnetic order to the excited state of lower magnetic order. When Sun et al. prepared NG6 (contact Atomic Force Microscope in the original text) on Au (111) by using the precursor molecules (fig. 9a), the product obtained was two structural PorA₂(2H) passivated by 2H (it is difficult to distinguish the existence and position of 2H passivation from ordinary constant-current STM and BR-STM, but it can be judged from the protrusions in a Non-contact Atomic Force Microscope (NC-AFM), see the sites marked by the red arrows on the right side of fig. 9B)^[54].

Using tip manipulation, inject electrons from the tip to the sample (small positive bias pulse, bias the sample), gradually remove the excess H atoms, and finally obtain NG6 (fig. 9c shows the structure PorA₂(H) after removing one excess H atom, and fig. 9e shows the structure PorA₂ after removing two excess H atoms). Fig. 9d shows the spin state density distribution of the ground state and the excited state corresponding to the spectroscopic information obtained in the experiment and the theoretical calculation. According to the calculation, it is a ferromagnetic ground state with the spin state density distribution in the same spin direction.The Kondo resonance at Fermi surface and the step-like inelastic tunneling signals at symmetrical positions far away from Fermi surface are observed, which proves that NG 6 is antiferromagnetic ground state S = 1 and transforms to antiferromagnetic excited state S = 0.

3 Tuning the Magnetic Properties of Nano-Graphene

As mentioned above, different structures of nano-graphene bring different ground States of magnetism to the system, so if you want to obtain different magnetic ground States of nano-graphene, you can design different structures of magnetic nano-graphene, and synthesize the corresponding precursor molecules. However, the preparation of precursor molecules is often difficult to obtain, which limits the study of different ground state structures. Therefore, for the magnetic study of nano-graphene, on the one hand, it is necessary to try to obtain more new precursor molecules of magnetic structure; On the other hand, limited precursor molecules are used to directly control the magnetic ground state. At present, the methods to control the magnetic order include tip manipulation, molecular self-binding extension, and co-deposition of different molecules.

3.1 Needle tip manipulation

By applying a pulse voltage through the tip, electrons are injected into the molecule from the tip, which is equivalent to giving the molecule a certain amount of energy. In the process of obtaining the open-shell non-Kekule nano graphene structure, if the precursor molecule has a protecting group, the protecting group can be removed by applying a pulse voltage through a needle tip, so as to obtain the open-shell carbon nano structure^[45]; The precursor molecules containing methyl groups can also be passivated by H atoms during dehydrocyclization during thermal annealing. At this time, the excess H atoms can be removed by applying pulses to the passivated sites, and the magnetic ground state structure can be restored. Tip manipulation can be used to remove the excess H atoms from the saturated site to restore the site to the unsaturated state, and at the same time, tip manipulation can also be used to quench the single electron that restores the unsaturated state. A pulse bias is applied to a site where a single electron is present to cause the single electron to form a bond with the base metal atom, thereby quenching the single electron^[55]. The charge transfer between the organic molecule and the substrate can make the organic molecule change from a closed-shell structure to an open-shell structure^[56]. The adsorption position of the open-shell nano-graphene on the surface also affects the "switch" of the magnetism of the open-shell nano-graphene, and the adsorption position of the nano-graphene on the surface is changed by using needle tip manipulation, so that the magnetism is opened and closed^[10,57]; The above-mentioned single electron bonding with the base metal will quench the magnetism of the site, which can also be manipulated by the needle tip to directly change the adsorption position of the nano-graphene, destroy the carbon-metal bond, and restore the previous magnetism.

3.2 Change the attachment position of homogeneous molecules

As mentioned above, the distance between two spin 1/2 spins will affect the spin exchange energy J of the system, and with the increase of the distance, J will gradually decrease. If two nanographenes with a ground state of 1/2 are connected together by a σ bond, the structure of 1/2 spin on each side will still be maintained because the σ bond connection will not cause resonance reconstruction on both sides. Because the magnetic moment of the S = 1/2 system is localized in a certain part of the nano-graphene, the relative position of the local magnetic moment on both sides can be changed by adjusting the connection position of the two nano-graphenes, thereby changing the exchange energy J. Fig. 10a is a schematic diagram of the molecular combination of NG7 obtained after the rapid annealing of the precursor molecules under further annealing. Due to the existence of five-membered ring defects, NG7 is a sublattice imbalanced structure with S = 1/2. From the result of spin-polarized density functional theory calculation (SP-DFT), the spin state is mainly distributed in half of NG7. NG7 has two sites that can bind to another NG7 (indicated by the red and blue circles in Fig. 10a), and due to the presence of the five-membered ring, there are six cases of binding (C1-C6, inset of Fig. 10h). If where C1 is "points 1-points 1" connected and the five-membered rings are not on the same side, C2 is "points 1-points 1" connected and the five-membered rings are on the same side, C3 is "point 1-points 2" connected, and the five-member rings are not on the same sides, C4 is "points 2-points 1" connecting and the five- member rings are on the same sides, C5 is a "points 2-points 2" connecting and the five- member ring is not on one side, and C6 is a "point 2-points. Figs. 10B to 10e are NC-AFM images of the C1 to C4 structures, and the corresponding structures can be distinguished from the positions of the five-membered rings (indicated by the red arrows in the figure). In the spectral analysis of the structures from C1 to C4 in the small energy range (Fig. 10f), it is found that the structure C1 with the farthest distance between the two spin States has Kondo resonance near the Fermi surface, while the rest of the configurations have step-like inelastic tunneling characteristics at the symmetrical positions on both sides of the Fermi surface. In addition, as the distance between the spin States on both sides approaches (C1 ~ C6), the spin exchange energy J gradually increases from 0 (Fig. 10g), which is also consistent with the experimental calculation results (Fig. 10h).

On the basis of the above, the magnetic units with the ground state of S = 1 are placed in a periodic lattice and connected by Sigma bonds to form magnetic nano-graphene chains or magnetic nano-graphene rings, and the magnetic exchange behavior between the chains and the construction of Haldane phases are studied^[58,59]^[58]. As shown in fig. 11, trigonal alkene belongs to the type of sublattice imbalance, S = 1, which is simplified as two magnetic units with the same spin direction, and any two magnetic units are antiferromagnetically coupled. If the magnetic units are connected end to end to form a closed ring, there is antiferromagnetically coupling at each part of the ring, and an inelastic tunneling signal appears on the dI/dV spectrum; In the case of chain-like connection, the dI/dV spectra show the existence of inelastic tunneling signals, and the two terminals appear as two separate S = 1/2 final States, which is consistent with the Haldane phase results.

4 Conclusions and outlook

Active magnetic molecules can be prepared by the method of "on-surface synthesis" with the help of the catalytic activity of the substrate on the surface, and their structures and magnetic properties can be characterized by scanning tunneling microscopy, which broadens the magnetic research on carbon-based nanomaterials. The magnetism of carbon-based graphene nanomaterials is mainly due to the unsaturated single electrons in the sp orbitals of the structure. According to the structure, the single electrons can be divided into: (1) the number of carbon atoms in the two sets of sublattices is different; (2) Special non-Kekule structures. With regard to the characterization of nano-graphene by scanning microscopy: (1) When a single electron receives spin screening from the substrate, Kondo resonance occurs near the Fermi surface, and there will be additional electronic state distribution when BR-STM is used to characterize the structure; (2) For the structures of ferromagnetic and antiferromagnetic ground States, there may be an inelastic tunneling channel signal from the ground state to the excited state in the spectrum; (3) The ground States of some structures are non-antiferromagnetic (such as the triangulene series, S = 1/2, 1, 3/2 …), but the resonance States generated by spin flip or inversion are covered by the substrate, the resonance peaks near the Fermi surface do not appear in STS spectroscopy, and there is no contribution of additional electronic States in BR-STM. About the regulation of magnetism: (1) The distance between two spins will affect the magnitude of their spin exchange energy J, the closer they are, the larger J is; (2) The "switch" of magnetism can be realized by tip manipulation; (3) From another point of view, artificial defects and doping can also construct the situation of sub-lattice imbalance, which can introduce magnetism^[40]. In addition, it is worth mentioning that compared with transition metals, magnetic nano-graphene and metal substrates produce weaker Kondo States, which may be covered by substrate States, so Au, which is more inert, is often chosen as the substrate^[35].

For magnetic nano-graphene, there are still some problems to be solved and prospects in this field: (1) to construct more ground-state nano-graphene, their magnetic performance is still a research hotspot and focus; (2) In the special open shell structure, it is also worth studying quantitatively the transition from closed shell to open shell and the local magnetic moment due to the increase of size, and on this basis, a feasible judgment basis of open shell and closed shell is summarized; (3) The magnetic properties of magnetic nano-graphene and graphene nanoribbons have been studied (for example, peri-acene extends to a certain dimension to obtain nanoribbons), so the role of magnetic properties in the transition from nano-graphene to nanoribbons also needs more exploration; (4) the doping in the graphene nanoribbon is artificially designed to realize the single-bond connection on the two sides of the doping, and the generated single electron can not be quenched through resonance, so the same situation should exist in the nano-graphene, and the doped magnetic nano-graphene can be constructed^[40]; (5) There have been some attempts to place magnetic nano-graphene in periodic lattices, such as magnetic nano-graphene chains and nano-graphene rings, which need more exploration^[58,59]^[58]; (6) Magnetic electronic devices based on carbon nanomaterials are also worth studying in the application of nano-graphene magnetism.

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