Manipulation of Molecular Spin State on Surfaces Studied by Scanning Tunneling Microscopy

The adsorbed magnetic molecules with tunable spin states have drawn wide attention for their immense potential in the emerging fields of molecular spintronics and quantum computing. One of the key issues toward their application is the efficient controlling of their spin state. This review briefly summarizes the recent progress in the field of molecular spin state manipulation on surfaces. We focus on the molecular spins originated from the unpaired electrons of which the Kondo effect and spin excitation can be detected by scanning tunneling microscopy and spectroscopy (STM and STS). Studies of the molecular spin-carriers in three categories are overviewed, i.e., the ones solely composed of main group elements, the ones comprising 3d-metals, and the ones comprising 4f-metals. Several frequently used strategies for tuning molecular spin state are exemplified, including chemical reactions, reversible atomic/molecular chemisorption, and STM-tip manipulations. The summary of the successful case studies of molecular spin state manipulation may not only facilitate the fundamental understanding of molecular magnetism and spintronics but also inspire the design of the molecule-based spintronic devices and materials.


Introduction
Magnetic molecules have attracted extensive interest during the past few decades due to their potential applications as high-density memories, displays, sensors, and so forth [1][2][3][4]. Recently, magnetic molecules are drawing even more attention, since they are considered as a promising material platform for the blossoming fields of molecular spintronics and quantum computing, in which the spin degree of freedom plays the key role [5][6][7][8][9][10][11][12][13][14]. One of the crucial issues toward the development of magnetic-molecule-based devices and functional materials is the spin state manipulation of the molecular spin-carriers. The relevant studies focusing on the surface-confined molecular systems and at molecular level are of special significance not only for the fundamental understanding of the magnetic characteristics of the molecular systems but also for realizing their practical applications.
The investigation of the molecular spins absorbed on surfaces has been facilitated by the rapid development of surface science techniques over the past few decades, especially scanning tunneling microscopy (STM) whose ultra-high spatial resolution in real space makes it one of the most powerful tools for directly charactering the surface-confined molecular systems at angstrom scale [15][16][17][18][19]. The detection of magnetic properties of molecules on surfaces with STM is enabled by measuring the differential conductance (dI/dV) spectra of the molecules, which refers to scanning tunneling
Liu and coworkers reported one of the earliest works concerning the controlled quenching of spins carried by the pure organic radicals absorbed on surfaces [44]. Their combined STM and density functional theory (DFT) investigations were focused on the verdazyl radicals (Figure 1a), that is, a family of pure organic molecules with an unpaired π-electron delocalized over the N(1)-N(2)-C(3)-N(4)-N(5) portion of the heterocycle, that were deposited on Au(111). They found that some 1,3,5-triphenyl-6-oxoverdazyl (TOV) molecules (denoted as type B in Figure 1b) could preserve their spin upon adsorption on Au(111), which was evidenced by the Kondo resonance seen in their dI/dV spectra (top curve in Figure 1c). As a comparison, other TOV molecules that possessed a protrusion in the center (denoted as type A in Figure 1b) showed no Kondo feature by dI/dV measurement (bottom curve in Figure 1c). Moreover, featureless dI/dV spectra were also found for another verdazyl radical, 1,3,5-triphenyl-6-thioxoverdazyl (TTV, Figure 1a). The absence of Kondo effect in the TOV molecules of type A and TTV was explained as a result of the quenched molecular spin. For the TOV molecules with a protruding center (type A), the variation in the molecular morphology compared with that of type B was attributed to the attachment of a H atom to the oxygen atom of TOV. The addition of atomic H caused the transfer of an additional electron to the verdazyl ring and thus quenched the unpaired electron spin of TOV. As for the TTV molecules absorbed on Au(111), the spin quenching was originated from the charge transfer facilitated by the strong interaction between the S atom in the molecule and the Au substrate. Therefore, it can be concluded that the spin of the TOV radical absorbed on Au(111) could be quenched by either the addition of atomic H or chemical modification of the molecule by replaying the O atom with a S atom. (c) Differential conductance (dI/dV) spectra recorded at TOV molecules of types A and B. Adapted from [44] with permission from The American Chemical Society, 2013.
The above-mentioned work by Liu et al. [44] shows the relatively poor stability of the pure organic spin systems adsorbed on surfaces, which is an obstacle to their characterization. In this context, controlled chemical reactions have been employed as an efficient approach to creating stable organic spins from close-shelled molecules on surfaces [53][54][55]. One of the successful cases was reported by Karan and coworkers [54]. The authors managed to convert a closed-shell all-transretinoic acid (ReA) molecule ( Figure 2a) on Au(111) into a radical that carries localized spin via intramolecular chemical conversions induced by the biased STM tip. As shown in Figure 2b, ReA molecules were found to form a striped pattern on Au(111) upon adsorption. By placing the tip at a bias of −2.5 V over the neck of the cyclohexene head, the ReA molecules in arrays could be switched selectively among different states of A, B, and C which featured distinct shapes and apparent heights in the STM images (Figure 2c-e). DFT calculations enabled the authors to assign the pristine and switched molecular states to different adsorption configurations. Further spectroscopic investigations revealed the different spin states between the pristine and switched states of ReA. The dI/dV spectra of molecules in states A, B, and C displayed a remarkable peak around the Fermi level which was absent for the pristine molecules ( Figure 2f). The splitting of the zero-bias peaks in the presence of a magnetic field (Figure 2g,h) demonstrated that the zero-bias peaks were originated from the Kondo effect. The latter served as a key proof for the existence of a majority spin residing in the switched molecules, although the pristine ReA is a closed-shell molecule. The follow-up theoretical study [56] attributed the emergence of spin upon molecular switching to the migration and dissociation of the allylic hydrogen from the endocyclic bond, i.e., a sigmatropic reaction, induced by the negatively biased tip. The distorted molecular structures of states A, B, and C, as well as the weak coupling between the molecules and the inert Au(111) substrate were believed to be important for stabilizing the metastable radical states of ReA. The generation of spins in organic systems by controlled on-surface chemical reactions were also reported for cholesterol molecules [53] and 13-cisretinoic acid [55] on Au(111). The above-mentioned work by Liu et al. [44] shows the relatively poor stability of the pure organic spin systems adsorbed on surfaces, which is an obstacle to their characterization. In this context, controlled chemical reactions have been employed as an efficient approach to creating stable organic spins from closed-shell molecules on surfaces [53][54][55]. One of the successful cases was reported by Karan and coworkers [54]. The authors managed to convert a closed-shell all-trans-retinoic acid (ReA) molecule ( Figure 2a) on Au(111) into a radical that carries localized spin via intramolecular chemical conversions induced by the biased STM tip. As shown in Figure 2b, ReA molecules were found to form a striped pattern on Au(111) upon adsorption. By placing the tip at a bias of −2.5 V over the neck of the cyclohexene head, the ReA molecules in arrays could be switched selectively among different states of A, B, and C which featured distinct shapes and apparent heights in the STM images (Figure 2c-e). DFT calculations enabled the authors to assign the pristine and switched molecular states to different adsorption configurations. Further spectroscopic investigations revealed the different spin states between the pristine and switched states of ReA. The dI/dV spectra of molecules in states A, B, and C displayed a remarkable peak around the Fermi level which was absent for the pristine molecules ( Figure 2f). The splitting of the zero-bias peaks in the presence of a magnetic field (Figure 2g,h) demonstrated that the zero-bias peaks were originated from the Kondo effect. The latter served as a key proof for the existence of a majority spin residing in the switched molecules, although the pristine ReA is a closed-shell molecule. The follow-up theoretical study [56] attributed the emergence of spin upon molecular switching to the migration and dissociation of the allylic hydrogen from the endocyclic bond, i.e., a sigmatropic reaction, induced by the negatively biased tip. The distorted molecular structures of states A, B, and C, as well as the weak coupling between the molecules and the inert Au(111) substrate were believed to be important for stabilizing the metastable radical states of ReA. The generation of spins in organic systems by controlled on-surface chemical reactions were also reported for cholesterol molecules [53] and 13-cis-retinoic acid [55] on Au(111).

Main-Group-Metal Phthalocyanines
In addition to the metal-free molecules, some main-group-metal-involved metal-organic complexes, such as main-group-metal phthalocyanine (Pc) molecules, are also of interest in terms of molecular magnetism. Pc, a widely used macrocyclic chelating ligand, possesses four indole rings bridged by four -N= groups with their two -NH-and two -N= groups pointing toward the center of the molecule. The coordination of Pc with metal atoms usually takes place via the dissociation of the two H atoms in the -NH-groups of the indole rings, leading to a -2 valence state of the coordinated Pc ligand. Therefore, the stable metal phthalocyanine (MPc) molecules prefer to form between Pc and the metals that can be stabilized in their +2 oxidation state, such as the 3d-metals. As a comparison, for the main-group metals that not tend to stabilize in their +2 state, e.g., K (+1), Al (+3), and Ti (+4), the synthesis of the Pc-coordinated complexes and the following characterization and manipulation of their spin-related properties are challenging.
Hong et al. [57] employed the recently developed vacuum synthesis method for the preparation of AlPc, a main-group-metal phthalocyanine molecule, via the metalation of pristine H2Pc on Au(111). The formation of the new species was confirmed by its distinct appearance in STM images ( Figure 3a) and the different electronic structures as detected by dI/dV measurements compared with H2Pc. The existence of a spin in AlPc was evidenced by the detection of a zero-bias peak in the dI/dV spectrum acquired at the lobe of AlPc (top-most curve in Figure 3b) which was assigned to Kondo resonance according to its splitting in the increasing magnetic field. The theoretical results provided insights into the origin of the spin by showing a doublet ground state of AlPc where the singly occupied molecular orbital (SOMO) and singly unoccupied molecular orbital (SUMO) were both in a π* state. Therefore, it is concluded that the spin density of the molecule was originated from the unpaired π-electron, which was mainly delocalized over the lobes of the Pc ring. This explained the

Main-Group-Metal Phthalocyanines
In addition to the metal-free molecules, some main-group-metal-involved metal-organic complexes, such as main-group-metal phthalocyanine (Pc) molecules, are also of interest in terms of molecular magnetism. Pc, a widely used macrocyclic chelating ligand, possesses four indole rings bridged by four -N= groups with their two -NH-and two -N= groups pointing toward the center of the molecule. The coordination of Pc with metal atoms usually takes place via the dissociation of the two H atoms in the -NH-groups of the indole rings, leading to a -2 valence state of the coordinated Pc ligand. Therefore, the stable metal phthalocyanine (MPc) molecules prefer to form between Pc and the metals that can be stabilized in their +2 oxidation state, such as the 3d-metals. As a comparison, for the main-group metals that not tend to stabilize in their +2 state, e.g., K (+1), Al (+3), and Ti (+4), the synthesis of the Pc-coordinated complexes and the following characterization and manipulation of their spin-related properties are challenging.
Hong et al. [57] employed the recently developed vacuum synthesis method for the preparation of AlPc, a main-group-metal phthalocyanine molecule, via the metalation of pristine H 2 Pc on Au(111). The formation of the new species was confirmed by its distinct appearance in STM images ( Figure 3a) and the different electronic structures as detected by dI/dV measurements compared with H 2 Pc. The existence of a spin in AlPc was evidenced by the detection of a zero-bias peak in the dI/dV spectrum acquired at the lobe of AlPc (top-most curve in Figure 3b) which was assigned to Kondo resonance according to its splitting in the increasing magnetic field. The theoretical results provided insights into the origin of the spin by showing a doublet ground state of AlPc where the singly occupied molecular orbital (SOMO) and singly unoccupied molecular orbital (SUMO) were both in a π* state. Therefore, it is concluded that the spin density of the molecule was originated from the unpaired π-electron, which was mainly delocalized over the lobes of the Pc ring. This explained the absence of Kondo resonance at the center of AlPc (middle curve in Figure 3b). Further comparative investigation of chlorinated AlPc (ClAlPc), an AlPc derivative formed by the axial bonding of the Al atom with a Cl atom, on Au(111) (Figure 3c) showed no Kondo signature for the absorbed ClAlPc molecules (Figure 3d), since all molecular orbitals of ClAlPc became doubly occupied upon chlorination. As a consequence, the spin state of AlPc on Au(111) could be tuned by the attachment/detachment of a Cl atom to the Al center.
Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 17 absence of Kondo resonance at the center of AlPc (middle curve in Figure 3b). Further comparative investigation of chlorinated AlPc (ClAlPc), an AlPc derivative formed by the axial bonding of the Al atom with a Cl atom, on Au(111) (Figure 3c) showed no Kondo signature for the absorbed ClAlPc molecules (Figure 3d), since all molecular orbitals of ClAlPc became doubly occupied upon chlorination. As a consequence, the spin state of AlPc on Au(111) could be tuned by the attachment/detachment of a Cl atom to the Al center.

3d-Metal Phthalocyanines
MPcs are one of the most studied molecular systems on surfaces [58,59], among which 3d-MPcs draw special attention due to their diversified magnetic properties. With the different electronic configurations of their 3d orbitals and thus the existence of unpaired d-electrons, the central metal ions in 3d-MPcs can serve as spin-carriers presenting various spin states. Extensive investigations addressing the magnetism of 3d-MPcs confined on surfaces, such as the ones with M = V [60,61], Mn [62][63][64][65], Fe [21,23,26,28,66,67], Co [20,68], Ni [69], and Cu [69], have been reported. Various spin-state manipulation strategies have been developed for the on-surface 3d-MPc systems. In the following, we will describe by examples the two frequently used strategies for tuning the spin state of 3d-MPc molecules on surfaces: tip-induced chemical reactions and reversible atomic/molecular chemisorption.
A well-known case of spin-state manipulation via tip-induced chemical reactions was reported by Zhao et al. [68], which involves the CoPc/Au(111) system. For an intact CoPc molecule absorbed on Au(111), the dI/dV measurement showed no spin-associated signature (black curve in Figure 4f), indicating that the unpaired electron of the Co center in free CoPc was completely quenched upon adsorption due to the molecule-substrate interaction. However, the localized spin was recovered in the dehydrogenated CoPc (d-CoPc) which was generated by the sequential tip pulses at 3.6 V on top

3d-Metal Phthalocyanines
MPcs are one of the most studied molecular systems on surfaces [58,59], among which 3d-MPcs draw special attention due to their diversified magnetic properties. With the different electronic configurations of their 3d orbitals and thus the existence of unpaired d-electrons, the central metal ions in 3d-MPcs can serve as spin-carriers presenting various spin states. Extensive investigations addressing the magnetism of 3d-MPcs confined on surfaces, such as the ones with M = V [60,61], Mn [62][63][64][65], Fe [21,23,26,28,66,67], Co [20,68], Ni [69], and Cu [69], have been reported. Various spin-state manipulation strategies have been developed for the on-surface 3d-MPc systems. In the following, we will describe by examples the two frequently used strategies for tuning the spin state of 3d-MPc molecules on surfaces: tip-induced chemical reactions and reversible atomic/molecular chemisorption.
A well-known case of spin-state manipulation via tip-induced chemical reactions was reported by Zhao et al. [68], which involves the CoPc/Au(111) system. For an intact CoPc molecule absorbed on Au(111), the dI/dV measurement showed no spin-associated signature (black curve in Figure 4f), indicating that the unpaired electron of the Co center in free CoPc was completely quenched upon adsorption due to the molecule-substrate interaction. However, the localized spin was recovered in the dehydrogenated CoPc (d-CoPc) which was generated by the sequential tip pulses at 3.6 V on top of the four indole rings (Figure 4a-e). The change in the spin state of d-CoPc was evidenced by the presence of an intense peak around the Fermi level in the dI/dV spectra which was assigned as a Kondo resonance due to its temperature evolution (colored curves in Figure 4f). Insights into the mechanism of the conversion was obtained by the comparison between the theoretical results of CoPc/Au(111) and d-CoPc/Au(111). Geometric optimizations showed a structural distortion of d-CoPc with respect to CoPc, including a larger Co-substrate separation and a smaller distance between the end carbon atoms and the substrate of d-CoPc compared with CoPc. The chemical-conversion-induced geometric changes of CoPc were accompanied with the variations in the electronic structures. For intact CoPc, the calculated spin-polarized partial density of states (PDOS) displayed the barely polarized orbitals of the Co center, which explained the absence of a magnetic moment in the intact CoPc absorbed on Au(111). As a comparison, polarized orbitals were found for d-CoPc, leading to a magnetic moment of 1.03 µ B , which was the origin of the Kondo resonance.
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 17 of the four indole rings (Figure 4a-e). The change in the spin state of d-CoPc was evidenced by the presence of an intense peak around the Fermi level in the dI/dV spectra which was assigned as a Kondo resonance due to its temperature evolution (colored curves in Figure 4f). Insights into the mechanism of the conversion was obtained by the comparison between the theoretical results of CoPc/Au(111) and d-CoPc/Au(111). Geometric optimizations showed a structural distortion of d-CoPc with respect to CoPc, including a larger Co-substrate separation and a smaller distance between the end carbon atoms and the substrate of d-CoPc compared with CoPc. The chemical-conversioninduced geometric changes of CoPc were accompanied with the variations in the electronic structures. For intact CoPc, the calculated spin-polarized partial density of states (PDOS) displayed the barely polarized orbitals of the Co center, which explained the absence of a magnetic moment in the intact CoPc absorbed on Au(111). As a comparison, polarized orbitals were found for d-CoPc, leading to a magnetic moment of 1.03 μB, which was the origin of the Kondo resonance. It is worth mentioning that in addition to manipulation of spin state as presented in the case shown above, the similar tip-induced chemical reactions were also employed as an efficient approach to tuning the spin-related transport properties in another 3d-MPc system. Li et al. reported the transition from the Kondo state to the magnetic triplet state of FePc on Au(111) upon the tip-induced dehydrogenation [67]. Deprotonated FePc molecules (Figure 5b inset at the bottom right) were generated from the intact FePc molecules (Figure 5a inset) on Au(111) by tip-pulse in the similar way as that reported for the CoPc/Au(111) system [68]. dI/dV investigations uncovered the distinct spectroscopic features of the intact and dehydrogenated species. For intact FePc, the zero-bias anomaly in the dI/dV spectrum (Figure 5a) was interpreted as the Kondo resonances originated from two unpaired d-electrons of the central Fe ion screened by the substrate electron gas. Once deprotonated, the structural distortion of the molecule resulted in the increased Fe-substate separation, which led to the weakened hybridization between the localized d-electrons of Fe and the delocalized electron gas of the Au substrate. As a consequence, the Kondo features were replaced by a double-step feature in the dI/dV spectrum (Figure 5b), which indicated the emergence of inelastic It is worth mentioning that in addition to manipulation of spin state as presented in the case shown above, the similar tip-induced chemical reactions were also employed as an efficient approach to tuning the spin-related transport properties in another 3d-MPc system. Li et al. reported the transition from the Kondo state to the magnetic triplet state of FePc on Au(111) upon the tip-induced dehydrogenation [67]. Deprotonated FePc molecules (Figure 5b inset at the bottom right) were generated from the intact FePc molecules (Figure 5a inset) on Au(111) by tip-pulse in the similar way as that reported for the CoPc/Au(111) system [68]. dI/dV investigations uncovered the distinct spectroscopic features of the intact and dehydrogenated species. For intact FePc, the zero-bias anomaly in the dI/dV spectrum (Figure 5a) was interpreted as the Kondo resonances originated from two unpaired d-electrons of the central Fe ion screened by the substrate electron gas. Once deprotonated, the structural distortion of the molecule resulted in the increased Fe-substate separation, which led to the weakened hybridization between the localized d-electrons of Fe and the delocalized electron gas of the Au substrate. As a consequence, the Kondo features were replaced by a double-step feature in the dI/dV spectrum (Figure 5b), which indicated the emergence of inelastic electron tunneling. These inelastic electron tunneling steps reflected the spin transitions between the α, β and γ states (Figure 5b inset at the bottom left) that were originated from the splitting of the S = 1 state of FePc due to the spin-orbit interaction.
Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 17 electron tunneling. These inelastic electron tunneling steps reflected the spin transitions between the α, β and γ states (Figure 5b inset at the bottom left) that were originated from the splitting of the S = 1 state of FePc due to the spin-orbit interaction. Due to the irreversibility of the tip-induced chemical conversions, the above-described strategy usually shows a one-way effect to the molecular spin systems. As another frequently used strategy, the chemical adsorption of atoms or small molecules provides a reversible approach to tuning the molecular spins. One example involving the MnPc/Au(111) system was reported by Liu and coworkers [64]. MnPc molecules absorbed on Au(111) exhibited a protruding cross feature ( Figure  6a). H2 dosage caused the conversion of the original MnPc molecules into the structures with a depression at the center (Figure 6b). This variation in the molecular morphology induced by H2 dosage was attributed to the chemical adsorption of an atomic H at the Mn ion in MnPc molecules which gave rise to H-MnPc. Furthermore, it was found that the MnPc state could be recovered from the H-MnPc state by applying a positive tip-pulse at H-MnPc molecules which led to the detachment of H. The chemisorption of atomic H to MnPc also gave rise to remarkable changes in the dI/dV measurements. The dI/dV spetra of MnPc on Au(111) showed a step-shaped feature at zero bias (red curves in Figure 6c). This zero-bias anomaly was attributed to Kondo resonance according to its splitting in the presence of a magnetic field (Figure 6d). In contrast, featureless dI/dV curves were found for H-MnPc (blue curve in Figure 6c). Reversible conversions of the dI/dV spectra between the Kondo-featured and featureless curves were achieved by controlled H2 dosages which led to a MnPc to H-MnPc conversion and tip-pulses which resulted in the H detachment of H-MnPc (Figure 6c).  Due to the irreversibility of the tip-induced chemical conversions, the above-described strategy usually shows a one-way effect to the molecular spin systems. As another frequently used strategy, the chemical adsorption of atoms or small molecules provides a reversible approach to tuning the molecular spins. One example involving the MnPc/Au(111) system was reported by Liu and coworkers [64]. MnPc molecules absorbed on Au(111) exhibited a protruding cross feature (Figure 6a). H 2 dosage caused the conversion of the original MnPc molecules into the structures with a depression at the center (Figure 6b). This variation in the molecular morphology induced by H 2 dosage was attributed to the chemical adsorption of an atomic H at the Mn ion in MnPc molecules which gave rise to H-MnPc. Furthermore, it was found that the MnPc state could be recovered from the H-MnPc state by applying a positive tip-pulse at H-MnPc molecules which led to the detachment of H. The chemisorption of atomic H to MnPc also gave rise to remarkable changes in the dI/dV measurements. The dI/dV spetra of MnPc on Au(111) showed a step-shaped feature at zero bias (red curves in Figure 6c). This zero-bias anomaly was attributed to Kondo resonance according to its splitting in the presence of a magnetic field (Figure 6d). In contrast, featureless dI/dV curves were found for H-MnPc (blue curve in Figure 6c). Reversible conversions of the dI/dV spectra between the Kondo-featured and featureless curves were achieved by controlled H 2 dosages which led to a MnPc to H-MnPc conversion and tip-pulses which resulted in the H detachment of H-MnPc (Figure 6c). Further theoretical investigations demonstrated an S = 3/2 state of MnPc on Au(111). The exchange interaction between the localized spin of the Mn ion and the substrate electron gas gave rise to the experimentally observed Kondo resonance. As for H-MnPc, the calculations unraveled an almost unchanged number of d-electrons of the Mn ion but redistributed effective charges in d orbitals compared with MnPc. The latter was responsible for the reduction in the net spin of the molecule from S = 3/2 for MnPc to S = 1 for H-MnPc. Moreover, H attachment also resulted in a larger Mn-substrate separation and thus weakened Mn-substrate coupling for H-MnPc. Both factors contributed to the suppression of the Kondo effect in H-MnPc. In all, the chemisorption of atomic H served as an efficient approach to tuning the spin state of MnPc on Au(111) reversibly. The similar strategy was also employed for the manipulation of the spin state of MnPc on Bi(110) between S = 1 and S = 1/2 states where CO instead of atomic H was used as the adsorbate [65].

Spin Crossover (SCO) Complexes
Spin crossover (SCO) complexes are a family of coordination compounds featuring switchable spin-state bistability in which transition metals with 3d n (n = 4-7) electronic configuration serve as the spin centers. Upon external stimuli such as temperature, light irradiation, pressure, or electric field [70][71][72][73], the metal centers of SCO molecules can switch between the low-spin (LS) state and highspin (HS) state. The origin of the spin-state switching lies in the magneto-structural effect of SCO complexes, that is, the change in coordination geometry can give rise to rearrangement of d orbitals and hence a variation in the spin state of the metal center.
Investigations of SCO phenomena of individual molecules at molecular scale have been widely carried out by STM. One recent example was reported by Kobke et al. [74] showing that the spin state of Ni in a series of Ni-porphyrin complexes could switch between a LS (S = 0) state and a HS (S = 1) state through reversibly changing the molecular structure between the ruffled and flat conformations. The two different molecular conformations were stabilized by the coordination and non-coordination of the central Ni with an axial ligand, respectively. A number of other cases have been overviewed in a recent review [75].
Once assembled into extended structures, SCO materials may emerge various spin-state orderings and exhibit a rich variety of collective dynamic behaviors [76]. However, in contrast to the extensively explored individual SCO systems, only a few molecular-level studies have focused on the SCO phenomena in ensembles [77,78]. In a recent work by Liu et al. [78], chemically bonded onedimensional SCO chains that were constructed via coordination between Ni and deprotonated tetrahydroxybenzene (THB) molecules on Au(111) (Figure 7a) were scrutinized by STM. STM imaging resolved the alternate arrangement of two types of Ni atoms, i.e., the bright ones and the dark ones, along the chains (Figure 7b). Theoretical optimization of the coordination structure ( Figure  7c) demonstrated different coordination geometries of the two types of Ni atoms: longer (shorter) Ni-O bonds, smaller (larger) O-Ni-O angles, and a smaller (larger) Ni-substrate separation for the bright (dark) Ni. Different spin states of the two types of Ni atoms were identified by combined dI/dV measurements and theoretical calculations. The dark Ni atoms exhibited featureless dI/dV spectra near the Fermi level (blue curve in Figure 7d). This experimental observation was in accordance with the calculation result which showed barely polarized d orbitals of the dark Ni atoms, indicating their S = 0 (LS) state. As a comparison, zero-bias anomaly was found for the bright Ni atoms (red curve in Figure 7d) and was attributed to Kondo resonances according to the magnetic-field evolution. Theoretical results revealed the highly polarized d orbitals of the bright Ni atoms whose interaction

Spin Crossover (SCO) Complexes
Spin crossover (SCO) complexes are a family of coordination compounds featuring switchable spin-state bistability in which transition metals with 3d n (n = 4-7) electronic configuration serve as the spin centers. Upon external stimuli such as temperature, light irradiation, pressure, or electric field [70][71][72][73], the metal centers of SCO molecules can switch between the low-spin (LS) state and high-spin (HS) state. The origin of the spin-state switching lies in the magneto-structural effect of SCO complexes, that is, the change in coordination geometry can give rise to rearrangement of d orbitals and hence a variation in the spin state of the metal center.
Investigations of SCO phenomena of individual molecules at molecular scale have been widely carried out by STM. One recent example was reported by Kobke et al. [74] showing that the spin state of Ni in a series of Ni-porphyrin complexes could switch between a LS (S = 0) state and a HS (S = 1) state through reversibly changing the molecular structure between the ruffled and flat conformations. The two different molecular conformations were stabilized by the coordination and non-coordination of the central Ni with an axial ligand, respectively. A number of other cases have been overviewed in a recent review [75].
Once assembled into extended structures, SCO materials may emerge various spin-state orderings and exhibit a rich variety of collective dynamic behaviors [76]. However, in contrast to the extensively explored individual SCO systems, only a few molecular-level studies have focused on the SCO phenomena in ensembles [77,78]. In a recent work by Liu et al. [78], chemically bonded one-dimensional SCO chains that were constructed via coordination between Ni and deprotonated tetrahydroxybenzene (THB) molecules on Au(111) (Figure 7a) were scrutinized by STM. STM imaging resolved the alternate arrangement of two types of Ni atoms, i.e., the bright ones and the dark ones, along the chains (Figure 7b). Theoretical optimization of the coordination structure (Figure 7c) demonstrated different coordination geometries of the two types of Ni atoms: longer (shorter) Ni-O bonds, smaller (larger) O-Ni-O angles, and a smaller (larger) Ni-substrate separation for the bright (dark) Ni. Different spin states of the two types of Ni atoms were identified by combined dI/dV measurements and theoretical calculations. The dark Ni atoms exhibited featureless dI/dV spectra near the Fermi level (blue curve in Figure 7d). This experimental observation was in accordance with the calculation result which showed barely polarized d orbitals of the dark Ni atoms, indicating their S = 0 (LS) state. As a comparison, zero-bias anomaly was found for the bright Ni atoms (red curve in Figure 7d) and was attributed to Kondo resonances according to the magnetic-field evolution. Theoretical results revealed the highly polarized d orbitals of the bright Ni atoms whose interaction with the substrate electron gas was responsible for the Kondo features in the dI/dV spectra. A magnetic moment of 1.59 µ B was concluded for the bright Ni atoms, corresponding to an S = 1 (HS) state. As a consequence, the Ni atoms in the coordination chains were at a HS (S = 1) or a LS (S = 0) state alternately along the chains, giving rise to a unique spin-state ordering known as the antiferroelastic phase. With a magneto-structural dependence, the Ni atoms in the chains could play as the SCO centers when being excited by the tip-pulse located upon. Interestingly, it was found that such SCO behavior, i.e., the spin-state switching between HS and LS states, of multiple Ni atoms in the same coordination chain could take place collectively (Figure 7e,f) upon the tip excitation of a single Ni atom (marked by the stars in Figure 7e,f). Such collective conversions actually resulted in the reversible switching of the chain between two degenerate antiferroelastic states, that is, the transition between the spin-state configurations of "...101010..." and "...010101...". A domino-like dynamic magneto-structural relaxation process was proposed to explain the mechanism of the collective SCO phenomenon. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 17 with the substrate electron gas was responsible for the Kondo features in the dI/dV spectra. A magnetic moment of 1.59 μB was concluded for the bright Ni atoms, corresponding to an S = 1 (HS) state. As a consequence, the Ni atoms in the coordination chains were at an HS (S = 1) or a LS (S = 0) state alternately along the chains, giving rise to a unique spin-state ordering known as the antiferroelastic phase. With a magneto-structural dependence, the Ni atoms in the chains could play as the SCO centers when being excited by the tip-pulse located upon. Interestingly, it was found that such SCO behavior, i.e., the spin-state switching between HS and LS states, of multiple Ni atoms in the same coordination chain could take place collectively (Figure 7e,f) upon the tip excitation of a single Ni atom (marked by the stars in Figure 7e,f). Such collective conversions actually resulted in the reversible switching of the chain between two degenerate antiferroelastic states, that is, the transition between the spin-state configurations of "...101010..." and "...010101...". A domino-like dynamic magneto-structural relaxation process was proposed to explain the mechanism of the collective SCO phenomenon.
Komeda and coworkers reported the switch-on and off of a π-electron spin of TbPc 2 on Au(111) by rotating the upper Pc ligand with controlled current pulses [83]. There are two spin systems in TbPc 2 , that is, the Tb ion and an unpaired electron in the ligand π orbital. Although the magnetic signature of the former was invisible by STM in this case, the Kondo effect of the latter was captured by spectroscopic investigations. The dI/dV measurement revealed the site dependence of the Kondo feature: the Kondo peak was prominent around the lobes of the molecule but was dramatically attenuated at the center (Figure 8a). This served as an evidence showing that it was the spin residing on the Pc ligand that was responsible for the Kondo resonance. Then, it was found that the Kondo feature could be switched on and off when the upper Pc ligand was rotated by applying a tip pulse to the molecule. For a TbPc 2 molecule with an azimuthal rotational angle (θ) between the two Pc ligands of 45 • whose center appeared brighter in the STM images (Figure 8b), the Kondo peak could be detected (top curve in Figure 8d). Once pulsed, the center of the molecule became darker, which indicated a change in θ of the molecule to 30 • (Figure 8c). Meanwhile, the dI/dV measurement showed the disappearance of the Kondo resonance for the pulsed molecules (bottom curve in Figure 8d). This θ-dependent switching of the Kondo effect was interpreted as a result of the variation in the molecular spin state. For the TbPc 2 molecules with θ = 45 • , the Pc ligand of the molecule absorbed on Au(111) had an S = 1/2 spin originated from the unpaired π-electron. However, the rotation of the upper ligand to θ = 30 • led to the rearrangement of the frontier molecular orbitals, which resulted in the charge transfer from the surface, therefore quenching the molecular spin and hence the Kondo state. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 17 of the 4f electrons to the tunneling current, the direct detection of the 4f electron states are elusive by STM measurement in some surface-confined LnPc2 systems [83,85,89,93]. In fact, there remain substantial debates on whether the spin states originated from the 4f electrons in LnPc2 molecules can be directly visualized by STM [81,83,85,86,89,91,94]. Nevertheless, the spin state manipulation has been achieved in several LnPc2 systems by tuning the unpaired π-electrons residing on the Pc ligands [83,93]. Komeda and coworkers reported the switch-on and off of a π-electron spin of TbPc2 on Au(111) by rotating the upper Pc ligand with controlled current pulses [83]. There are two spin systems in TbPc2, that is, the Tb ion and an unpaired electron in the ligand π orbital. Although the magnetic signature of the former was invisible by STM in this case, the Kondo effect of the latter was captured by spectroscopic investigations. The dI/dV measurement revealed the site dependence of the Kondo feature: the Kondo peak was prominent around the lobes of the molecule but was dramatically attenuated at the center (Figure 8a). This served as an evidence showing that it was the spin residing on the Pc ligand that was responsible for the Kondo resonance. Then, it was found that the Kondo feature could be switched on and off when the upper Pc ligand was rotated by applying a tip pulse to the molecule. For a TbPc2 molecule with an azimuthal rotational angle (θ) between the two Pc ligands of 45° whose center appeared brighter in the STM images (Figure 8b), the Kondo peak could be detected (top curve in Figure 8d). Once pulsed, the center of the molecule became darker, which indicated a change in θ of the molecule to 30° (Figure 8c). Meanwhile, the dI/dV measurement showed the disappearance of the Kondo resonance for the pulsed molecules (bottom curve in Figure  8d). This θ-dependent switching of the Kondo effect was interpreted as a result of the variation in the molecular spin state. For the TbPc2 molecules with θ = 45°, the Pc ligand of the molecule absorbed on Au(111) had an S = 1/2 spin originated from the unpaired π electron. However, the rotation of the upper ligand to θ = 30° led to the rearrangement of the frontier molecular orbitals, which resulted in the charge transfer from the surface, therefore quenching the molecular spin and hence the Kondo state.  [93] is also inspiring for the manipulation of the ligand spin of LnPc2 molecules. The adsorption of DyPc2 on the ultrathin CuO film on Cu (110) led to four types of molecules, that is A, B, and C in the assembled structures ( Figure 9b) and the isolated molecules D (Figure 9b inset). Similar to TbPc2, DyPc2 also possesses two spin systems in its neutral state, i.e., the Dy ion whose 4f electrons are undetectable by STM measurement, and the unpaired π-electron delocalized on the Pc ligands. Spectroscopic studies were carried out for characterizing the ligand  [93] is also inspiring for the manipulation of the ligand spin of LnPc 2 molecules. The adsorption of DyPc 2 on the ultrathin CuO film on Cu (110) led to four types of molecules, that is A, B, and C in the assembled structures ( Figure 9b) and the isolated molecules D (Figure 9b inset). Similar to TbPc 2 , DyPc 2 also possesses two spin systems in its neutral state, i.e., the Dy ion whose 4f electrons are undetectable by STM measurement, and the unpaired π-electron delocalized on the Pc ligands. Spectroscopic studies were carried out for characterizing the ligand spin. Molecules A and B showed featureless dI/dV curves under zero magnetic field but a remarkable dip at the Fermi level under a magnetic field of 8 T (black and orange curves in Figure 9c). The emergence of the zero-bias feature under the nonzero external magnetic field (B) was attributed to the spin excitation of the molecule from m s = −1/2 to m s = +1/2 which was originated from the Zeeman splitting of the degenerated m s = ±1/2 spin states at B = 8 T (Figure 9d). Therefore, it was concluded that molecules A and B possessed one unpaired π-electron, indicating that they were in neutral states. In contrast, the featureless spectra were found for molecules C and D at both B = 0 and 8 T (red and blue curves in Figure 9c), meaning that their unpaired π-electrons were quenched and thus they were charged. Given the dependence between the spin state and charge state of DyPc 2 in the DBTJ, the manipulation of the charge state could hence lead to the change in the spin state. It was demonstrated that the charge state of DyPc 2 in this system could be tuned by applying different biases of the DBTJ, which would give rise to the switching-on and off of the ligand spin of DyPc 2 .
Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 17 spin. Molecules A and B showed featureless dI/dV curves under zero magnetic field but a remarkable dip at the Fermi level under a magnetic field of 8 T (black and orange curves in Figure 9c). The emergence of the zero-bias feature under the nonzero external magnetic field (B) was attributed to the spin excitation of the molecule from ms = −1/2 to ms = +1/2 which was originated from the Zeeman splitting of the degenerated ms = ±1/2 spin states at B = 8 T (Figure 9d). Therefore, it was concluded that molecules A and B possessed one unpaired π-electron, indicating that they were in neutral states. In contrast, the featureless spectra were found for molecules C and D at both B = 0 and 8 T (red and blue curves in Figure 9c), meaning that their unpaired π-electrons were quenched and thus they were charged. Given the dependence between the spin state and charge state of DyPc2 in the DBTJ, the manipulation of the charge state could hence lead to the change in the spin state. It was demonstrated that the charge state of DyPc2 in this system could be tuned by applying different biases of the DBTJ, which would give rise to the switching-on and off of the ligand spin of DyPc2. Adapted from [93], with permission from American Chemical Society, 2018.

Conclusions
In this review, we summarize the recent investigations of spin state manipulation of surfaceconfined molecules in three categories, i.e., molecules composed of main group elements such as verdazyl radicals, ReA and AlPc, molecules comprising 3d-metals such as 3d-MPcs and the spin crossover coordination chains, and molecules comprising 4f-metals such as TbPc2 and DyPc2.

Conclusions
In this review, we summarize the recent investigations of spin state manipulation of surface-confined molecules in three categories, i.e., molecules composed of main group elements such as verdazyl radicals, ReA and AlPc, molecules comprising 3d-metals such as 3d-MPcs and the spin crossover coordination chains, and molecules comprising 4f-metals such as TbPc 2 and DyPc 2 . Examples are presented to show the efficient effect of several spin state tuning strategies, including chemical reactions, reversible atomic/molecular chemisorption, and STM-tip manipulations, in different on-surface molecular systems. The employment of STM and STS in these studies enables the exploration of the spin-related phenomena and properties at molecular level by detecting the Kondo effect or spin excitation of the molecular systems. These case studies contribute to the fundamental understanding of molecular magnetism and spintronics.
To make further steps toward the practical application of the magnetic molecules as spintronic and quantum computing devices, the studies focusing on the magnetic molecular systems located on carbon-based substrates or nanostructures are of great significance. On the one hand, the work by Cervetti et al. concerning the molecular-spin-graphene hybrids has demonstrated that the interaction between magnetic molecules and graphene has an essential influence on the quantum dynamics of molecular spins [95]. Further explorations in the field have involved a variety of coupled systems formed between different molecular spins and graphene, aiming at elucidating their structures, electronic properties, magnetism, and so forth [61,[96][97][98][99][100][101]. On the other hand, the hybrid systems composed of single molecular magnets and carbon nanotubes (CNTs) also have drawn extensive attention [102][103][104][105]. A strong spin-phonon coupling between molecular spins and CNTs was demonstrated, which can enhance the sensitivity of CNT-based magnetometers and ultimately enable coherent spin manipulation and quantum entanglement [106,107]. Such hybrid systems were also found to be able to play as spintronic devices such as spin valves [108]. More about the research progress focusing on the hybrids consisting of molecular magnets and carbon-based materials can be found in the reviews by Cervetti et al. [109] and Pineda et al. [110], just to name a few. Based on these excellent previous works, the systematic investigation by STM of spin manipulation of magnetic molecules coupled with carbon-based substrates is expected to provide more fundamental insights into the hybrid systems at molecular level, which should be inspiring for the design of molecule-based spintronic and quantum computing devices.