Atomically tweaking spin-crossover cooperativity to augment molecular memory density

Domain: physics
Published: 2026-01-23
Editorial outcome: minor_revision

Abstract
Spin-crossover complexes are promising candidates for molecular bits owing to their magnetic bistability. However, it’s a formidable challenge to turn densely packed spin-crossover molecules into high-density bits due to cooperative switching dynamics inherent in spin-crossover aggregates. Herein, coordination-field engineering is demonstrated by scanning tunneling microscopy/spectroscopy and density functional theory as an efficient strategy to modulate the cooperative dynamics of surface-confined spin-crossover chains. Within these chains, Ni centers exhibit collective spin transitions upon local stimuli. This spin-crossover cooperativity can be disrupted by coordination-field modification via hetero-metal/ligand doping, which converts partial switchable coordination centers into non-switchable nodes. These nodes divide the single-bit-like spin-crossover chain into multiple independently-switchable segments and thereby augment the bit density. One-by-one erasable write-in of these spin-crossover bits is achieved by scanning tunneling microscopy tip manipulation. These results highlight how to precisely tweak the cooperative dynamics of the spin-crossover aggregates to advance their potential applications for high-density memory.

Introduction
Magnetic bistability underpins modern information storage and processing technologies. Among various mechanisms that induce magnetic bistability, molecular spin-crossover (SCO) has attracted sustained interest since its discovery in the 1930s. SCO molecules, primarily 3d metal-organic complexes, can reversibly switch between low-spin (LS) and high-spin (HS) states upon external stimuli. This transition is accompanied by changes in coordination geometry, with the HS (LS) state exhibiting longer (shorter) metal-ligand distances. This magneto-structural correlation introduces intermolecular elastic interactions when SCO molecules are densely packed, driving cooperative spin transitions across aggregates. Such SCO cooperativity enables sharp and global responses of bulk SCO materials to local excitations, making them promising for high-speed and low-energy-consumption molecular memory devices.

However, as device sizes shrink toward the single-molecule limit to meet the demand for ultrahigh-density information storage, the cooperativity becomes detrimental: It forces collective switching and hinders independent control of individual molecules, thereby rendering reduced memory densities. Therefore, precise control of the SCO cooperativity at the molecular level is crucial for advancing SCO-based data storage.

Here, we employ coordination-field engineering as an efficient strategy to suppress collective spin transitions in SCO aggregates and enhance their memory densities. The SCO phenomena arise from a balance between the 3d electron paring energy and the ligand-field splitting, which yields comparable energies for different spin states of the complex and hence its magnetic bistability. Once this delicate balance is perturbed, spin transitions become energetically unfavorable, producing a non-switchable ground state. When snugged into SCO ensembles, these non-switchable centers would introduce rigid (rather than elastic) interactions with other SCO sites, thereby disrupting the SCO cooperativity. To achieve this, hetero-metal/ligand dopants can be utilized to modify the coordination field of partial SCO centers, converting them into non-switchable ones.

Results
On-surface preparation and modification of the SCO chain

The SCO chains employed as the model system were synthesized via a dehydrogenated coordination reaction between THB and Ni on Au(111) [Reaction (i) in Fig. 2], and their representative STM image is depicted in Fig. 3a. Each Ni atom coordinates to four O atoms to form a NiO coordination center (denoted as the Ni center). The Ni centers in their S=1 HS (NiH, illustrated by the light blue dots in Fig. 3a) and S=0 LS (NiL, marked by the dark blue dots) states appear brighter and darker in the STM images, respectively, and typically alternate along the coordination chain (denoted as the NiO chain), leading to a yield of about 85% for the NiH-NiL motif (i.e., one NiH and one NiL centers being bridged by a molecular ligand), with occasionally observed NiL-NiL and NiH-NiH defects (as marked by the dark and light blue dashed rectangles in Fig. 3a, respectively), as described in detail in our previous studies.

To precisely tweak the SCO coordination field, the Ni chains were doped with either a hetero-metal (Fe) or a hetero-ligand [dehydrogenated 2,5-diamino-1,4-benzoquinonediimine (DABQDI), denoted as the N-ligand]. The Fe-doped Ni chains (denoted as NiO-FeO chains) were synthesized via deprotonated coordination of THB with both Ni and Fe on Au(111) [Reaction (ii) in Fig. 2]. The N-ligand-doped Ni chains (denoted as NiO-NiN chains) were obtained by post-deposition of DABQDI onto the Au(111) surface pre-covered by the Ni chains, followed by thermal treatment at ~430 K [Reaction (iii) in Fig. 2]. STM images of the as-achieved NiO-FeO and NiO-NiN chains acquired at low bias voltages are shown in Fig. 3b, d, respectively. These doped Ni chains manifest a constant metal-metal separation of D=7.6±0.2 Å, being identical to that in the pure Ni chains and commensurate with the Au(111) lattice along the chain orientations (the equivalent <123> directions). The same metal-metal separation in both pure and doped Ni chains can be attributed to the efficient chain-substrate interaction that compensates for the energy penalty arising from the compressed or stretched coordination bonds introduced by the hetero-metal/ligand dopants.

Doping with Fe and N-ligands introduce FeO and NiON coordination centers (denoted as the Fe and Ni centers, respectively) into the Ni chains, as illustrated in Fig. 1b. To identify different coordination sites, differential conductance (dI/dV) measurements were performed. Figure 3g displays representative dI/dV curves acquired at the metal centers in the pure NiO, Fe-O-ligand, and Ni-N-ligand chains (denoted as the Fe and Ni chains, respectively; refer to their STM images in Supplementary Fig. 1). The light and dark blue curves in the top panel of Fig. 3g correspond to NiH and NiL in the NiO chain, respectively. The distinct spectral features at the Fermi level, as shaded in gray in Fig. 3g, arise from the presence and absence of the Kondo resonances at the NiH and NiL centers, respectively (see Supplementary Fig. 2 and Supplementary Discussion for details), which clearly indicates their different spin states. Moreover, the dI/dV intensity right below the Fermi level for the NiH center, which is absent for the NiL counterpart, is likely contributed by the dz2 orbital of the former, according to our DFT calculations.

A comparison of these dI/dV spectra reveals that the key spectral distinction between Ni and Fe is a broad resonance centered at 0.6 V [Feature (i) in Fig. 3g] that appears only at the Ni centers. Thus, dI/dV mapping (or STM imaging) around V=0.6 V can visually distinguish Ni and Fe. An STM image at V=0.6 V of the same area shown in Fig. 3b is given in Fig. 3c, where most metal sites appear bright and are assigned as Ni, whereas the darker ones highlighted by the orange arrows are identified as Fe. These identifications are further confirmed by direct dI/dV measurements of the NiO-FeO chains (Fig. 3h), showing that the NiH (light blue curve), NiL (dark blue curve), and Fe (orange curve) centers in the NiO-FeO chains exhibit similar spectral features with those sampled in the pure Ni and Fe chains.

Similarly, the Ni and Ni centers in the NiO-NiN chains can be distinguished by their dI/dV measurements. The dI/dV spectra in the top and bottom panels of Fig. 3g reveal a resonance at 1.75 V [Feature (ii)] for the NiN center that is absent for Ni, and a sharp peak at −0.3 V [Feature (iii)] for the Ni centers that is missing for NiN. Thus, Ni and Ni in the NiO-NiN chain can be distinguished by the dI/dV mapping or STM imaging at V=1.75 V and −0.3 V. Figure 3e, f present the dI/dV mappings of the same area shown in Fig. 3d at V=1.75 V and −0.3 V, respectively. Some Ni sites become significantly brighter (Fig. 3e) or darker (Fig. 3f) than the others, and always appear in pairs, as highlighted by the green arrows, which are identified as the Ni centers linked by the N-ligands. These identifications are validated by dI/dV measurements of the NiO-NiN chains (Fig. 3i), revealing the spectra for the NiH (light blue curve), NiL (dark blue curve), and Ni (green curve) centers in the NiO-NiN chains, similar to those acquired in the pure Ni and Ni chains.

Superimposing the positions of the Fe and Ni centers on Fig. 3b, d by the orange and green arrows, respectively, shows that both metal centers appear dark at the low bias voltages. They divide the Ni chains into multiple segments where most NiH and NiL centers (marked by light and dark blue arrows in Fig. 3b, d, respectively) are alternately arranged, just like that in the pure Ni chains.

Disrupted SCO cooperativity by coordination-field engineering

As demonstrated in our previous reports, pure Ni chains exhibit cooperative SCO dynamics that reversibly switch between degenerate configurations of …NiH-NiL-NiH-NiL… (top panel of Fig. 4a) and …NiL-NiH-NiL-NiH… (middle panel) in response to local STM-tip stimulations such as voltage pulses or mechanical forces. Such a collective SCO event is explicitly discerned in the bottom panel of Fig. 4a, as achieved by subtracting the top panel from the middle one. White and black dots represent the Ni centers undergoing LS-to-HS and HS-to-LS transitions, respectively. All 28 visible Ni atoms within the chain switch their spin states collectively, demonstrating a fully cooperative behavior. Consequently, the SCO Ni chains manifest large bit sizes and low memory densities (i.e., one chain = one bit).

To examine the effect of the hetero-metal/ligand dopants on the cooperative SCO dynamics in the Ni chains, specific STM-tip voltage pulses (typically |±70|~|±200| mV, termed as manipulation voltage) were applied to trigger spin transitions of the metal centers. Figure 4b, c are STM images of the NiO-FeO chains shown in Fig. 3b, c before and after application of a voltage pulse to the Ni center marked by the black arrow, located within a Ni segment containing two Ni centers (highlighted by the white dashed frames) between two Fe sites (marked by the orange dots). Both Ni centers within the segment switch their spin states upon the voltage pulse, as indicated by the light and dark blue circles in Fig. 4b, c for NiH and NiL, respectively. Their differentiated image (Fig. 4d) confirms that the SCO behavior is confined to the targeted segment, with no changes observed elsewhere. This is consistent with the line profiles along the manipulated chain (Supplementary Fig. 3a), revealing pronounced variations in apparent heights of the Ni centers within the targeted Ni segment, while their counterparts in neighboring segments remain essentially unchanged. These results indicate an efficient decoupling of the SCO dynamics between adjacent Ni segments separated by the Fe centers. The reversibility of the segment-confined spin transition is further verified by successive manipulations of a Ni segment through sequential STM-tip voltage pulses applied to different NiH or NiL centers within the segment (Supplementary Fig. 4). In contrast to the switchable Ni centers, neither STM topographies nor dI/dV spectral features of the Fe centers respond to the tip voltage pulses up to |±500| mV, confirming their non-switchable nature.

Similar tip-manipulations were performed on the NiO-NiN chains. Upon the STM-tip voltage pulse applied to the Ni center marked by the black arrow in Fig. 4e, four Ni centers between two Ni nodes (marked by the green dots in Fig. 4e, f; see Supplementary Fig. 5 for their identification) switch their spin states, as highlighted by the light and dark blue circles for NiH and NiL in Fig. 4e, f, respectively. Meanwhile, the Ni centers remained unaffected. The differentiated image (Fig. 4g), together with the line profiles along the manipulated chain (Supplementary Fig. 3b), evidently shows that the SCO switching is confined within the Ni segment bounded by the Ni nodes.

These results demonstrate that hetero-metal/ligand doping introduces non-switchable Fe or Ni centers into the Ni chains, which are consequently cut into multiple independently-switchable segments. Each Ni segment can act as an individual binary bit, which remarkably enhances the information density (i.e., one chain = multiple bits).

Notably, the N-ligand also shows an impact on the SCO dynamics of the Ni sites in neighboring chains (Supplementary Fig. 6 and Supplementary Discussion): The Ni sites adjacent to the N-ligand in neighboring chains are consistently fixed in their HS state, likely due to the altered inter-chain interactions in the presence of the N-ligand. These fixed NiH centers can also serve as non-switchable nodes that disrupt the SCO cooperativity along the Ni chain.

Read-out and write-in of the SCO bits

To ensure reliable read-out and write-in of the Ni-segment bits, their binary states 0 and 1 are respectively defined as the LS and HS states of their central Ni sites (termed the r/w sites). The central Ni site refers to the (L/2)th (for even L) or [(L+1)/2]th (for odd L) Ni atom along the STM-scanning direction inside a segment that contains L Ni centers. Erasable encoding of these Ni-segment bits can be achieved by reversible spin manipulation using the STM-tip pulses at the manipulation voltages. Their read-out can be realized by probing the r/w sites at low bias voltages, typically < |±60 mV|, to avoid unintended SCO transitions during decoding.

One-by-one encoding of the Ni-segment bits is experimentally demonstrated, as shown in Fig. 5. Figure 5a displays that two neighboring bits separated by the Fe centers (marked by the orange dots and dashed lines) within a NiO-FeO chain are independently manipulated between their 0 and 1 states. As a result, the two-bit system is encoded into one of its four possible configurations, 00, 01, 10, and 11, in a controlled manner. Another example of three-bit encoding in a NiO-FeO chain is presented in Fig. 5b. Similar operations are also applicable for the NiO-NiN chains. As depicted in Fig. 5c, three Ni segments in a NiO-NiN chain are separated by the Ni centers (green dots in the top panel) and a non-switchable NiH site (light blue dot) adjacent to a N-ligand in the neighboring chain (refer to details in Supplementary Fig. 6 and Supplementary Discussion). All 23=8 possible configurations of the three-bit system are successfully realized by STM manipulation. One may notice that some Ni sites adjacent to the Fe or Ni nodes within the same chain remain fixed in their LS state during SCO transitions of other Ni atoms, e.g., the one marked by the dark blue arrow in Fig. 5a, which, nevertheless, does not impair the read-out/write-in operation at the r/w sites. Calculations described below suggest that this phenomenon arises from the lower energy of the LS state compared to the HS counterpart of the Ni center in the neighborhood of an Fe or Ni site (Supplementary Fig. 7).

Discussion

Theoretical calculations were performed to elucidate how the hetero-metal/ligand dopants affect the SCO cooperativity in the Ni chains (refer to the calculation details in the Supplementary Discussion). First, energy profiles of isolated Ni, Fe, and Ni complexes in different spin states (S=1 and 0 for Ni and Ni; S=1 and 2 for Fe) were computed as a function of the metal-ligand distance d, as marked in the insets of Fig. 6a–c. The Ni complex exhibits magnetic bistability with comparable energies for its HS and LS states and an energy crossover in between (Fig. 6a). In contrast, both Fe (Fig. 6b) and Ni (Fig. 6c) complexes possess well-defined ground states, indicating their non-switchability. These findings demonstrate that the SCO properties in metal-organic systems can be efficiently tuned via coordination-field engineering, including metal or ligand substitution.

Subsequently, dimeric motifs containing two metal centers bridged by an O-ligand, i.e., Ni-Ni, Ni-Fe, and Ni-Ni, as schematically shown in Fig. 6d (refer to optimized models in Supplementary Fig. 7), were constructed to explore inter-center interactions. The metal-metal distance D was fixed at 7.6 Å according to the experimental measurements. This value lies in between the relaxed metal-metal separations in the NiL-NiL (7.56 Å) and NiH-NiH (7.71 Å) motifs, thereby introducing elastic frustration between adjacent Ni sites that favors the alternating arrangement of NiH and NiL to lower the total energy, in agreement with our experimental results. Optimization of the NiL-NiH motif (Supplementary Fig. 7) unravels asymmetric coordination of the bridging O-ligand, which locates closer (further) to the NiL (NiH) center (d < d, as illustrated in the top panel of Fig. 6d), agreeing with the atomic force microscopy measurements (Supplementary Fig. 8). All these indicate that the collective spin transitions of adjacent Ni centers involved displacement of their bridging O-ligand, which invokes an elastic interaction between them. Moreover, the distances from the outer ligands to the Ni sites (d’ and d’ in Fig. 6d) in the Ni-Ni motif also vary upon the Ni centers’ spin transitions, which establishes the elastic coupling between the dimeric motif and neighboring Ni centers (illustrated by the dashed circles in Fig. 6d) in the extended chain. Accordingly, once an SCO transition is triggered at one Ni site within a chain, the associated displacement of adjacent O-ligands can induce cooperative spin-state switching of the nearest neighboring Ni sites, which further propagates the SCO dynamics to the next-nearest neighbors along the chain, and ultimately leads to the within-chain SCO cooperativity.

In contrast, dramatic changes occur if one Ni center is replaced by Fe or Ni in the dimeric motif. As shown in Fig. 6a–c, the same degree of ligand displacement that is sufficient to induce the SCO transition of Ni actually fails to alter the spin state of Fe or Ni. Therefore, the bridging O-ligand displacement cannot trigger collective SCO transition across the Ni-Fe or Ni-Ni structure (middle and bottom panels of Fig. 6d). This is validated by the calculation results that there are higher energy penalties for the spin transitions of the Fe and Ni sites in the dimeric motifs compared to those involving solely Ni (Supplementary Fig. 7). These findings explain the experimentally observed non-switchable nature of Fe and Ni. Their fixed spin states cause negligible displacements of their adjacent outer ligands in the dimeric motifs, even if the spin state of the Ni centers switches (middle and bottom panels of Fig. 6d and Supplementary Fig. 7). Consequently, the SCO dynamics of the Ni center at one side of an Fe or Ni site cannot be sensed by Ni at the other side, which makes the Fe and Ni centers act as the non-switchable barriers to terminate the SCO propagation along the coordination chain.

In summary, this study demonstrates that coordination-field engineering is an efficient strategy to control cooperative dynamics of the SCO aggregates. The Au(111)-supported coordination chains containing the SCO Ni centers serve as a prototypical scaffold to experimentally explore cooperative spin transitions triggered by local STM-tip stimulations. Doped with hetero-metal (Fe) or hetero-ligand (N-ligand), the coordination field of the SCO centers is modified, resulting in non-switchable Fe or Ni nodes that divide the Ni chains into independently-switchable Ni segments. Each segment functions as a binary bit, which allows multiple bits to be incorporated in an individual chain and thereby augments the memory density. One-by-one erasable write-in of these bits can be achieved by STM-tip manipulation. These findings exemplify atomically precise control of cooperative dynamics in surface-confined SCO systems, providing a promising avenue to improve the performance of molecular memory devices towards ultrahigh-density information storage.

Methods
STM experiments were carried out at about 5 K on a Unisoku ultrahigh vacuum STM at a base pressure below 3×10−10 Torr. The clean and atomically flat Au(111) surface was prepared by repeated cycles of Ar+ sputtering and annealing at about 770 K. Ni and Fe were thermally deposited at about 1420 K onto the substrate held at room temperature. Commercially purchased THB and DABQDI molecules were evaporated at about 335 K. The samples covered by the metals and ligands were afterwards annealed at 420~430 K to initiate the dehydrogenated coordination reaction. All STM images were acquired in constant current mode, unless otherwise specified, and processed by the WSxM software. Lock-in amplifier parameters for the dI/dV measurements were as follows: Vrms = 10 mV and f = 1.999 kHz for the Ni and NiO-FeO chains, and Vrms = 30 mV and f = 3.001 kHz for the Ni and NiO-NiN chains.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Jing Liu et al proposes an attempt to tailor the cooperative dynamics of surface-confined spin crossover (SCO) chains using the coordination-field modification via hetero-metal/ligand doping. Probed by scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT), Author provide a demonstration of Ni centers that collectively modulated their spin transitions upon local response. The work is timely and hold its promise for future development of high-density information storage. However, several concerns need to be addressed prior to further consideration:\n1. In order to meet the expectation, a more quantified of descriptive knowledge about the dimension of nanochains is beneficial for the breakthrough aiming for high-density information storage at the nanoscale. therefore, a stable and reliable memory logical bits is required, what is the longest maximum chains length can be made using the proposed STM manipulation?\n2. When compared the Nio chain (Fig 3a) and NiO-FeO chains (Fig 3b), why the former case has irregular gap size between the neighboring chain, whereas the latter did not exhibit such prominently distinct feature? In addition how often that Author found such peculiar resembles a brick wall-pattern for the Nio chain? A statistical finding would be very useful to understand the physical origins. In addition, an individual bright spot was observed, can Author comment what is this feature correspond to?\n3. Can author provide the apparent height variation outlining the emphasis Before manipulation (Fig 4b) compared to the after manipulation (Fig 4c)? Do we see any dramatic changes upon triggeed by tip voltage pulse?\n3. What is the repetition rate of tweaking SCO cooperativity demonstrated in Fig 4. Can we observe these sequential STM- tip voltage pulses in more than 10 cycles?\n5. Page 7:\"... These doped NiO chains manifest a constant metal-metal separation of D = 7.6 ± 0.2 Å, being identical to that in the pure NiO chains and commensurate with the Au(111) lattice along the chain orientations\"\nIt is hard to digest that why the doped NiO chains does not marginally differs in terms of bond length upon coordinated with Fe or N-ligand doped. How to deduce the lateral distance of these cases?", "reviewer_2": "The manuscript entitled \"Atomically Tweaking Spin-Crossover Cooperativity to Augment Molecular Memory Density\" by Liu and coworkers presents a combined STS and DFT study of molecular chains composed of Ni atoms linked with dehydrogenated tetrahydroxybenzene units. The authors demonstrate collective spin-state switching in these chains, extending their previous work (Refs. 39, 40, 42). In this study, they convincingly show that introducing Fe atoms or dehydrogenated diamino-benzoquinone-diimine (DABQDI) linkers effectively disrupt the cooperativity along the chains, dividing them into smaller, individually switchable subunits. The data are of high quality, and the theoretical calculations strongly support the interpretation of the observed phenomena. Overall, the manuscript presents significant findings that will be of interest to the broad readership of Nature Communications. I therefore recommend publication after minor revisions.\nBelow are my specific comments and suggestions that should be addressed prior to acceptance:\ni) Figure 1b): The lower right dashed rectangle and the chosen color scheme for the DABQDI linker are misleading. I suggest using green ellipses to indicate the chain disruption caused by modified linkers (as in Figures 5, 6) and extending the dashed rectangle to include two metal atoms and three linkers for consistency and clarity.\nii) page 8, top: The sentence \"Doping with Fe and N-ligands introduces FeO4 and NiO2N4 coordination centers\" appears to contain a typographical error. Did the authors intend to write NiO2N2 or NiN4?\niii) Figure 3: For the high-spin (HS) Ni_O centers, a peak below zero bias is observed, which is absent in the low-spin (LS) Ni_O species. The authors describe the falling edge of this peak as a superposition of two Fano resonances. In the Supplementary Information, they attribute these spectral features to a molecular spin screened by two independent Kondo channels. I am aware that this feature has been reported in previous work (e.g. Figure 2 in Ref. 39) over a reduced voltage range. However, given that the spectra are now shown over a broader energy window, it would be very informative to know whether comparable features appear in the DFT density of states for the LS and HS Ni complexes that correspond to the observed peak."}, "author_response": "We appreciate the Reviewer’s insightful comment.\nTo date, for the pure Ni chains, the maximum number of Ni centers within a single chain that\nO O\nundergo a collective spin transition trigger by one local STM-tip voltage pulse is 28 (corresponding\nto a chain length of ~21 nm). The corresponding STM images before and after the manipulation are\nshown in Figure 4a in the main text (also shown below as Fig. R1). It’s found that the manipulated\nchain length depends on the pulse conditions (voltage and tunneling current). In experiments, the\nhigher the pulse voltage or tunneling current, the longer the chain that can be manipulated. Owing to\nthe space limitation of the manuscript and the scope of this work on regulation of SCO cooperativity,\nthese experimental details are not included here. However, our next manuscript in preparation will\nquantitatively explore the dependence of the manipulated chain length on the manipulation conditions.\nFig. R1 STM images of a Ni chain before (top panel) and after (middle panel) application of an\nO\nSTM-tip voltage pulse to the Ni atom highlighted by the black arrow (V = 10 mV in constant-height\nmode). Twenty-eight Ni atoms with their spin states switched are labeled.\nFor the doped chains, the voltage-pulse-induced collective spin transitions are confined within\nthe Ni segments separated by the Fe or Ni centers. In other words, the manipulated chain length\nO O N\nis dictated by the length of the Ni segment, which makes the Ni segments quite promising for\nO O\nreliable memory bits. Taking the Ni -Fe chains as an example, we have successfully achieved spin\nO O\nmanipulations in the Ni segments containing up to N = 12 Ni centers; The Ni segments with N >\nO O O\n12 are rarely observed in our experiments. The length of these Ni segments can be calculated by\nO\n0.76*N (nm), where 0.76 nm corresponds to the Ni -Ni distance in the chain.\nO O\nR2\nComment:\n2. When compared the Ni chain (Fig 3a) and Ni -Fe chains (Fig 3b), why the former case has\nO O O\nirregular gap size between the neighboring chain, whereas the latter did not exhibit such prominently\ndistinct feature? …\nAuthor reply:\nWe thank the Reviewer for the comment. The “gap size” mentioned by the Reviewer might mean\ntwo aspects: (1) the inter-chain distances between neighboring Ni chains (as indicated by the solid\nO\nblack arrow in Fig. R2a), and (2) the separation between the bright ellipses along the Ni chains (as\nO\nmarked by the solid white arrow in Fig. R2a). In the following, we will address these two issues\nseparately.\nFig. R2 (a) STM image of the Ni\nO\nchains (V = 10 mV in constant-height mode). Solid white and black\narrow mark the separation between the bright ellipses along the Ni chains and the inter-chain\nO\ndistance, respectively. (b) Line profiles along the dashed arrows in (a). It should be noted that the\nSTM image in (a) was acquired in constant-height mode. Therefore, the y-axis of the line profiles in\n(b) corresponds to the tunneling current.\n(1) “Why do the inter-chain distances between the neighboring Ni chains appear irregular,\nO\nwhereas those between the Ni -Fe chains do not?”\nO O\nIn fact, both pure Ni chains and doped chains (the Ni -Fe and Ni -Ni chains) align parallelly\nO O O O N\non Au(111) to form islands in which all chains are equally spaced with a uniform inter-chain distance\nof 6.6 Å.\n(2) “Why do separations between the bright ellipses along the Ni chains appear irregular when\nO\ncompared between neighboring chains, whereas those in the Ni -Fe chains do not?”\nO O\nThe variation in separations between the bright ellipses along the Ni chains is merely a visual\nO\nartifact introduced during image processing. As shown in Fig. R2b provided above, line profiles 1\nand 2 along the neighboring Ni chains (as marked by the dashed arrows in Fig. R2a) confirm that\nO\nthese chains actually exhibit identical gap sizes. To avoid misleading, we have reprocessed the STM\nimage of the Ni chains in Figure 3a, as shown below.\nO\nFig. 3 … (a) STM image of the Ni chains (imaging conditions: bias voltage V = 10 mV in constant-\nO\nheight mode). Light and dark blue dashed rectangles indicate the NiH-NiH and NiL-NiL defects,\nO O O O\nrespectively. …\nComment:\n… In addition how often that Author found such peculiar resembles a brick wall-pattern for the\nNio chain? A statistical finding would be very useful to understand the physical origins. …\nAuthor reply:\nAs shown in Fig. R3 below, the “brick-like” feature (as illustrated by the orange ellipses) in the\nNi chain arises from two molecular ligands connected by a Ni center in the HS state (NiH), which\nO O O\nappears bright under the imaging conditions (e.g., V = 10 mV for Fig. 3a). The neighboring “bricks”\nalong the coordination chain are linked by the dark LS Ni centers (NiL). When the Ni chains align\nO O O\nparallelly to form an island, the “brick wall-pattern” emerges, as illustrated in Fig. R3. Therefore, this\ntopographic feature stems from the alternate arrangement of the bright NiH and dark NiL centers\nO O\nalong the Ni chain. In this context, the yield of the “brick-like” feature is determined by the\nO\nprobability that the NiH and NiL centers are bridged by a molecular ligand to form a NiH -NiL\nO O O O\nmotif. Therefore, we have counted the proportion of the NiH-NiL motif among all possible cases\nO O\n(NiH-NiL, NiH-NiH and NiL-NiL). The statistical results show a yield of about 85% for the NiH-\nO O O O O O O\nNiL motif, which is consistent with the experimental observation that the assemblies of the Ni\nO O\nchains primarily exhibit the “brick wall-pattern”. We have added phrases in the main text to clarify\nthe yield of the NiH-NiL motif as below:\nO O\nParagraph 1, Page 6: “The Ni centers in their S = 1 HS (NiH, illustrated by the light blue dots\nO O\nin Fig. 3a) and S = 0 LS (NiL, marked by the dark blue dots) states appear brighter and darker in the\nO\nSTM images, respectively, and typically alternate along the coordination chain (denoted as the Ni\nO\nchain), leading to a yield of about 85% for the NiH-NiL motif (i.e., one NiH and one NiL centers\nO O O O\nbeing bridged by a molecular ligand), …”\nFig. R3 STM image of the Ni\nO\nchains featuring the “brick wall-pattern” and the corresponding\nchemical structures.\nR4\nComment:\n… In addition, an individual bright spot was observed, can Author comment what is this feature\ncorrespond to?\nAuthor reply:\nThe bright spot in the pure Ni chain, as highlighted by the dark blue dashed rectangle in the\nO\nrevised version of Fig. 3a shown above, is formed by two dark NiL centers connected by a molecular\nO\nligand, as illustrated in Fig. R4a. These NiL-NiL motifs, together with the NiH-NiH ones marked\nO O O O\nby the light blue dashed rectangle in Fig. 3a, occasionally appear as defects in the Ni chains.\nO\nAccordingly, we have added the following phrases to the main text:\nParagraph 1, Page 6: “…, with occasionally observed NiL -NiL and NiH -NiH defects (as\nO O O O\nmarked by the dark and light blue dashed rectangles in Fig. 3a, respectively), …”\nFig. 3 caption: “Fig. 3 … Light and dark blue dashed rectangles indicate the NiH-NiH and NiL-\nO O O\nNiL defects, respectively. …”\nO\nFig. R4 STM images of (a) the Ni chains, (b) the Ni -Fe chains, and (c) the Ni -Ni chains showing\nO O O O N\nbright spots and their corresponding chemical structures.\nBright spots are also observed in the doped Ni -Fe and Ni -Ni chains, as marked by the\nO O O N\ndashed rectangles in Fig. R4b and c. These structures correspond to the NiL-Fe (or Fe -NiL), NiL-\nO O O O O\nNi (or Ni -NiL), and Ni -Ni motifs, as illustrated in Fig. R4b and c, because the Fe and Ni\nN N O N N O N\ncenters also appear dark at low bias voltages (e.g., V = 30 mV for Fig. R4b and V = 50 mV for Fig.\nR4c). The Fe and Ni centers can be distinguished from the NiL ones by STM imaging at higher\nO N O\nbias voltages, as described in detail in the main text on Pages 9 and 10.\nR5\nComment:\n3. Can author provide the apparent height variation outlining the emphasis Before manipulation\n(Fig 4b) compared to the after manipulation (Fig 4c)? Do we see any dramatic changes upon triggeed\nby tip voltage pulse?\nAuthor reply:\nWe thank the Reviewer for the helpful comment. We have now added line profiles along the\nmanipulated chains in Figure 4b-g to the Supplementary Materials as Supplementary Fig. 3 (also\nshown below). A comparison of these line profiles reveals pronounced variations in apparent heights\nof the Ni centers within the manipulated Ni segment (colored in dark red) before and after\nO O\napplication of an STM-tip voltage pulse, whereas the counterparts in the neighboring segments\nexhibit negligible changes. These results demonstrate that the spin transition behaviors are confined\nto the targeted segments. We have now added sentences regarding the line profiles to the main text as\nfollows:\nParagraph 1, Page 12: “Their differentiated image (Fig. 4d) confirms that the SCO behavior is\nconfined to the targeted segment, with no changes observed elsewhere. This is consistent with the\nline profiles along the manipulated chain (Supplementary Fig. 3a), revealing pronounced variations\nin apparent heights of the Ni centers within the targeted Ni segment, while their counterparts in\nO O\nneighboring segments remain essentially unchanged. These results indicate an efficient decoupling\nof the SCO dynamics between adjacent Ni segments separated by the Fe centers.”\nO O\nParagraph 1, Page 13: “The differentiated image (Fig. 4g), together with the line profiles along\nthe manipulated chain (Supplementary Fig. 3b), evidently shows that the SCO switching is confined\nwithin the Ni segment bounded by the Ni nodes.”\nO N\nSupplementary Fig. 3 Line profiles along the manipulated chains. Line profiles along the\nmanipulated chains shown in (a) Fig. 4b and c and (b) Fig. 4e and f before (top curves) and after\n(middle curves) application of STM-tip voltage pulses. Differentiated profiles are presented as the\nbottom curves. Insets: STM images of the coordination chains along which the line profiles are taken\nwith the manipulated Ni segments highlighted by white dashed frames.\nO\nComment:\nR6\n4. What is the repetition rate of tweaking SCO cooperativity demonstrated in Fig 4. …\nAuthor reply:\nIt’s demonstrated in our previous work (ACS Nano 2020, 14, 11283) that the intrinsic switching\nrate of the SCO chain structure depends on the manipulation conditions (bias voltage and tunneling\ncurrent). In general, higher bias voltages and tunneling currents lead to faster SCO switching. For\nexample, the switching rate is on the order of 100 s-1 at V = 90 mV and I ~ 100 pA, and increases to\n102 s-1 at V = 150 mV and I ~ 230 pA. However, in practical experiments, the spin transition of a Ni\nO\nchain or segment is triggered and verified by applying an STM-tip voltage pulse followed by STM\nimaging. The time required for these operations is typically 101~102 s, corresponding to a rate of 10-\n2~10-1 s-1, which is much slower than the intrinsic switching rate of the Ni chain or segment.\nO\nTherefore, the STM operations become the rate-determining steps. In short, the intrinsic switching\nrate of the SCO chain structure depends on the manipulation conditions, whereas the overall\nmanipulation rate is limited by the STM operations.\nComment:\n… Can we observe these sequential STM-tip voltage pulses in more than 10 cycles?\nAuthor reply:\nYes. An example is shown in Fig. R5, where both Ni centers (as marked by the light and dark\nO\nblue circles for the NiH and NiL centers, respectively) within the segment embedded in a Ni -Fe\nO O O O\nchain change their spin states collectively and reversibly for 11 times upon sequential voltage pulses.\nFig. R5 is now added to the Supplementary Materials as Supplementary Fig. 4, and a sentence\ndescribing these data is added to the main text as below:\nParagraph 1, Page 12: “The reversibility of the segment-confined spin transition is further\nverified by successive manipulations of a Ni segment through sequential STM-tip voltage pulses\nO\napplied to different NiH or NiL centers within the segment (Supplementary Fig. 4).”\nO O\nR7\nFig. R5 STM images showing successive manipulations of a Ni segment within a Ni -Fe chain (V\nO O O\n= 0.6 V in constant-heigh mode for the top panel and V = 30 mV and I = 30 pA for others), as triggered\nby sequential STM-tip voltage pulses. STM image in the top panel is used to identify the Fe centers,\nO\nas marked by dark red dots and dashed lines. The locations where the STM-tip voltage pulses were\napplied are marked by black crosses. The NiH and NiL centers are highlighted by light and dark\nO O\nblue circles, respectively.\nComment:\n5. Page 7:\"... These doped Ni chains manifest a constant metal-metal separation of D = 7.6 ±\nO\n0.2 Å, being identical to that in the pure Ni chains and commensurate with the Au(111) lattice along\nO\nthe chain orientations\"\nIt is hard to digest that why the doped Ni chains does not marginally differs in terms of bond\nO\nlength upon coordinated with Fe or N-ligand doped. How to deduce the lateral distance of these cases?\nAuthor reply:\nWe thank the Reviewer for the helpful comment.\nExperimentally, we first determined the nearest neighboring Ni -Ni distance in the Ni chain\nO O O\nby measuring the total length (L) of a pure Ni chain containing N Ni centers (typically N ≥ 10),\nO O\nwhich gave an average Ni -Ni distance of L/N. Repeating such measurement for multiple times, the\nO O\nR8\nweighted average of all data yielded a Ni -Ni separation of 7.6 ± 0.2 Å. This result is further\nO O\nconfirmed by comparing the chain structures with the Au(111) substrate lattice (as highlighted by the\nwhite grids in Fig. R6), revealing that the chains extend along the equivalent <123̅> directions of the\nsubstrate, as indicated by the black arrows in Fig. R6. Moreover, the coordinated metal centers in the\nchains (as marked by the grey dots in Fig. R6) sit on equivalent substrate sites, demonstrating the\ncommensurability between the chain periodicity and the substrate lattice. Therefore, the Ni -Ni\nO O\ndistance can be experimentally derived from the substrate lattice constant along the <123̅> directions,\nnamely, 7.6 Å.\nFig. R6 STM image showing the Ni chains and the Au(111) substrate lattice (V = 50 mV and I = 70\nO\npA) simultaneously. The substrate lattice is highlighted by white grids. Coordinated metal centers are\nillustrated by grey dots. Equivalent <11̅0> and <123̅> directions are marked by white and black\narrows, respectively.\nSubsequently, the Ni -Fe and Ni -Ni distances in the doped Ni chains were measured and\nO O O N O\ncompared with the Ni -Ni separations acquired from the same STM images. Statistically these\nO O\nmeasured values did not show significant differences. Moreover, it’s also found that the dopants did\nnot change the chain orientations, i.e., the doped Ni chains still extend along the equivalent <123̅>\nO\ndirections. Therefore, it’s concluded that “These doped Ni chains manifest a constant metal-metal\nO\nseparation of D = 7.6 ± 0.2 Å, being identical to that in the pure Ni chains and commensurate with\nO\nthe Au(111) lattice along the chain orientations”.\nThe negligible change in metal-metal separations between the undoped and doped chains can be\nattributed to the efficient chain-substrate interaction, as evidenced by the chain-substrate\ncommensurability. In other words, the Au(111) substrate lattice acts as a template that dictates the\nchain growth orientations and constrains the location of the metal centers to the specific equivalent\nsubstrate sites, which results in a constant metal-metal distance. The energy penalty arising from the\ncompressed or stretched coordination bonds introduced by the hetero-metal/ligand dopants is\ncompensated by the chain-substrate interaction. Accordingly, we have now added a sentence to the\nmain text as below:\nParagraph 1, Page 9: “The same metal-metal separation in both pure and doped Ni chains can\nO\nbe attributed to the efficient chain-substrate interaction that compensates for the energy penalty\narising from the compressed or stretched coordination bonds introduced by the hetero-metal/ligand\ndopants.”\nR9\nReviewer #2\nComment:\nThe manuscript entitled \"Atomically Tweaking Spin-Crossover Cooperativity to Augment\nMolecular Memory Density\" by Liu and coworkers presents a combined STS and DFT study of\nmolecular chains composed of Ni atoms linked with dehydrogenated tetrahydroxybenzene units. The\nauthors demonstrate collective spin-state switching in these chains, extending their previous work\n(Refs. 39, 40, 42). In this study, they convincingly show that introducing Fe atoms or dehydrogenated\ndiamino-benzoquinone-diimine (DABQDI) linkers effectively disrupt the cooperativity along the\nchains, dividing them into smaller, individually switchable subunits. The data are of high quality, and\nthe theoretical calculations strongly support the interpretation of the observed phenomena. Overall,\nthe manuscript presents significant findings that will be of interest to the broad readership of Nature\nCommunications. I therefore recommend publication after minor revisions. Below are my specific\ncomments and suggestions that should be addressed prior to acceptance:\ni) Figure 1b): The lower right dashed rectangle and the chosen color scheme for the DABQDI\nlinker are misleading. I suggest using green ellipses to indicate the chain disruption caused by\nmodified linkers (as in Figures 5, 6) and extending the dashed rectangle to include two metal atoms\nand three linkers for consistency and clarity.\nAuthor reply:\nWe thank the Reviewer for the helpful suggestion. According to the Reviewer’s suggestion, we\nhave revised Fig. 1, as shown below:\nR10\nFig. 1 SCO cooperativity and its controlling strategy. (a) Schematic illustration of the cooperative\nspin transition in the Ni chain. (b) Disruption of the SCO cooperativity upon hetero-metal (top panel)\nO\nand hetero-ligand (bottom panel) doping to introduce non-switchable nodes.\nComment:\nii) page 8, top: The sentence \"Doping with Fe and N-ligands introduces FeO and NiO N\n4 2 4\ncoordination centers\" appears to contain a typographical error. Did the authors intend to write\nNiO N or NiN ?\n2 2 4\nAuthor reply:\nWe appreciate the Reviewer’s careful reading. The typo “NiO N ” has been corrected to\n2 4\n“NiO N ” in Paragraph 2, Page 9.\n2 2\nR11\nComment:\niii) Figure 3: For the high-spin (HS) Ni_O centers, a peak below zero bias is observed, which is\nabsent in the low-spin (LS) Ni_O species. The authors describe the falling edge of this peak as a\nsuperposition of two Fano resonances. In the Supplementary Information, they attribute these\nspectral features to a molecular spin screened by two independent Kondo channels. I am aware that\nthis feature has been reported in previous work (e.g. Figure 2 in Ref. 39) over a reduced voltage range.\nHowever, given that the spectra are now shown over a broader energy window, it would be very\ninformative to know whether comparable features appear in the DFT density of states for the LS and\nHS Ni complexes that correspond to the observed peak.\nAuthor reply:\nWe thank the Reviewer for the insightful comment. According to the Reviewer’s suggestion, we\nhave compared the DFT-calculated density of states (DOS) for the Ni centers in the LS (NiL) and\nO O\nHS (NiH) states (Fig. R7). It’s found that for the NiH atom, there are considerable DOS intensities\nO O\nright below the Fermi level, primarily contributed by its d orbital, whereas for the NiL atom, the\n𝑧2 O\nDOS intensities around the Fermi level are negligible. Therefore, the peak below zero bias in the\ndI/dV spectra of the NiH center can be attributed to their d orbital. A brief discussion is now\nO 𝑧2\nadded to the main text as follows:\nParagraph 2, Page 9: “Moreover, the dI/dV intensity right below the Fermi level for the NiH\nO\ncenter, which is absent for the NiL counterpart, is likely contributed by the d orbital of the former,\nO 𝑧2\naccording to our DFT calculations.”\nFig. R7 DFT-calculated DOS of the NiH and NiL centers. Grey dashed line indicates the Fermi\nO O\nlevel.\nR12\nOther Revisions\nIn addition to the above-mentioned revisions, we have also made several non-scientific changes\nto the main text and Supplementary Materials, as listed below:\nParagraph 1, Page 8\n “T he equivalent [123̅] and [213̅] directions” is replaced by “the equivalent <123̅> directions”.\nParagraph 2, Page 9\n “A round the Fermi level” is replaced by “at the Fermi level”.\nParagraph 1, Page 12\n “M oreover” is replaced by “In contrast to the switchable Ni centers”.\nO\nPage 15\n T he section heading “DISCUSSION” is added.\nPage 18\n T he section heading “CONCLUSIONS” is deleted.\nPage 19\n A “DATA AVAILABILITY” section is added with the statement: “The data that support the\nfindings of this study are available from the corresponding author upon reasonable request.”\nAcknowledgements, Page 22\n T wo missed relating grant numbers, 22225202 and T2588301, are added.\nPage 22\n A “AUTHOR CONTRIBUTIONS” section is added with the statement: “K.W. initiated the\nproject, analyzed the data and wrote the manuscript; J.L. conducted the STM experiments,\nanalyzed the data, carried out the DFT calculations and wrote the manuscript; Y.B. and Z.X.\nconducted the STM experiments; Q.C. analyzed the data and wrote the manuscript; J.P.\nconducted the AFM experiments; Y.L., T.H. and H.D. participated in the STM experiments;\nY.G. and L.H. participated in the DFT calculations; Y.W. analyzed the data.”\n A “COMPETING INTERESTS” section is added with the statement: “The authors declare no\ncompeting interests.”\nR13"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have responded fully the raised concerns and comments. Therefore, the current version is recommend for publication in Nature Communications.", "reviewer_2": "The authors have adequately addressed all of my questions, and I am happy to recommend publication in Nature Communications."}, "author_response": ""}]

Decision letter