Materials Chemistry C

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1 Journal of Materials Chemistry C REVIEW View Article Online View Journal Adhering magnetic molecules to surfaces Cite this: DOI: /c5tc03225c Received 8th October 2015, Accepted 31st October 2015 DOI: /c5tc03225c Introduction Rebecca J. Holmberg and Muralee Murugesu* It has been over twenty years since the first example of a molecule exhibiting magnetic bistability was put forth. 1 This type of molecule; shown to exhibit slow relaxation of magnetisation of purely molecular origin, is defined as a Single Molecule Magnet (SMM). Researchers working in the field of molecular magnetism continue to target a plethora of potential applications for these nanomagnets, such as: high-density information storage, 2 Department of Chemistry and Biomolecular Sciences, and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie, Ottawa, ON, Canada K1N 6N5. m.murugesu@uottawa.ca; Tel: +1 (613) ext The field of molecular magnetism has a strong focus on the design and improvement of magnetic molecules towards their potential application in molecular electronics; spintronics, quantum computing, information storage, etc. However, despite the growth and expansion of the field, there are still few researchers exploring the fundamental concern of their successful adhesion and interaction with surface structures, while maintaining their inherently desirable magnetic properties. Thus, in this review we aim to present an overview of the work that has been performed on attaching and studying Single-Molecule Magnets (SMMs) on various surfaces, with an emphasis on molecular design for surface interaction and on the magnetic properties before and after adhesion occurs. quantum computing, 3 7 magnetic refrigeration, 8,9 molecular spintronics, etc. With recent advances in understanding the physics of these nanosystems, perhaps the most promising of these potential applications are quantum computing and molecular spintronics. 16 Thus, it is crucial for magnetochemists to use their expertise in studying the adhesion of these monodisperse magnetic units to a conducting surface structure, while controlling and/or maintaining the inherent magnetic properties of the SMM itself. However, rather surprisingly, this is not the focus of the field overall. There are many potential causes which can be attributed to the lacking attention on surface attachment; as (i) SMMs still have not reached the required properties for reasonable employment; (ii) surface attachment is not trivial and most often the magnetic properties of each molecule are Rebecca J. Holmberg received her BSc from Queen s University in 2009, with an Honours thesis on employing NMR in the structural study of polyguanylic acid, completed under the supervision of Prof. Gang Wu. In 2011, she completed her MSc on electrochemically formed coloured passive layers on Ti and Zr, under the guidance of Prof. Gregory Jerkiewicz. Thereafter, Rebecca J. Holmberg she began her PhD in 2012 under the supervision of Prof. Muralee Murugesu at the University of Ottawa. Her PhD research has focused on the study of nanomagnetism through a materials approach; exploring the synthetic development and characterization of unique crystalline nanomaterials. Muralee Murugesu received his PhD from the University of Karlsruhe in 2002 under Prof. A. K. Powell. He undertook postdoctoral positions at the University of Florida ( ) with Prof. G. Christou, and jointly at the University of California, Berkeley and the University of California, San Francisco under the supervision of Prof. J. R. Long and the Nobel Laureate Prof. S. Pruissner Muralee Murugesu ( ). In 2006, he joined the University of Ottawa as assistant professor, became associate professor in 2011, and full professor in His research focuses on the design and development of new synthetic methods towards novel nanoscale materials.

2 significantly affected upon attachment; (iii) their chemical stability is often impractical for study under ambient conditions; and furthermore, (iv) techniques required to study such systems are specialized and often very costly. These, amongst other reasons, can help to explain why there are only a handful of magnetochemists who have ventured forth into the world of surface science in order to attach these remarkable molecules to a substrate and study their properties thereafter. Thus, this review intends to articulate the findings of these researchers, regarding the surface attachment of SMMs; and thus convey the position of the field at this time towards future applicability in the aforementioned areas. Furthermore, through the exploration of unique surface structures, as we will discuss herein, we can also envision SMMs in novel, more exploratory applications. Molecular nanomagnets It is important to note that molecular magnets are discrete magnetic entities, possessing a bistable ground state and magnetic hysteresis. In order to improve them for their proposed applications, researchers within the field of molecular magnetism work with the goal of synthesizing SMMs with higher energy barriers (U eff ) and more accessible blocking temperatures (T B ). These two factors are commonly cited and compared in order to qualify SMMs. The energy barrier is a thermally activated relaxation regime, by which a spin reversal may occur. The blocking temperature is the temperature below which magnet-like behaviour is observed; namely magnetic remanence and coercivity. Thus, this value is often found through the observation of loop opening phenomena in hysteretic measurements (Mvs.H). Since many SMMs are plagued by significant quantum tunneling of the magnetisation (QTM), meaning that instead of crossing an energy barrier spins can tunnel through degenerate states, T B values are often below a measurable range using conventional magnetometers, and thus U eff is a more commonly reported value for comparison. Currently, the pinnacles of the field are different in regards to the two parameters. The highest blocking temperature to date was reported in 2011 for a radical bridged Tb complex (T B =14K), 17 while the highest energy barrier to date was claimed in 2013 by a heteroleptic TbPc 2 complex (U eff = 653 cm 1, 939 K). 18 These complexes are now the benchmarks with regards to these parameters. However, it should be taken into consideration that structural design and stability often create an impediment to the practical employment of such materials. Thus, an important third parameter arises naturally from the nature of these systems: surface adhesion. In order for a nanomagnet to be utilised it must first be immobilized, while retaining its magnetisation on a surface. Herein, SMMs that have been adhered to surfaces, the chosen surfaces, and what effect this attachment took on the original magnetic properties will be discussed. Mn12: the beginning The molecule mentioned in the introductory sentence of this review, often referred to as Mn 12, was not only the first example of a single molecule magnet, but has also been the most studied to date. The original structure: Mn 12 Ac, 1a, as well as a clear representation of many of its derivatizations, which were synthesized towards successful surface attachment, can be seen in Fig. 1. The reasoning behind the extensive study of this molecule, other than it holding fame for being the first SMM, was that it retained records for both the highest energy barrier (U eff = 62 K) and blocking temperature (T B = 4 K) until they were finally broken by a Co radical complex in 2003, 19 and a Mn 6 Fig. 1 Top-down (1a j) and side-on (1k) molecular structures of Mn 12 complexes which have been successfully attached to various surfaces through their displayed functionalities. Structures 1b, 1i, and1j were modelled for visual representation, as crystal structures were not reported. Hydrogen atoms are omitted for clarity.

3 complex in 2007, 20 respectively. However, work has continued on Mn 12 systems, eliciting the highest barrier within the family of complexes being reported as recently as The first study on surface adhered Mn 12 Ac was published in 2003, where the SMM was deposited on Au(111). 22 The choice of Au(111) was, and continues to be, due to the accessible atomically flat surfaces, the ability to use scanning tunneling microscopy (STM) for study and molecular manipulation, as well as the well-established affinity for soft donor atoms, such as sulfur. The use of STM is incredibly important as it is capable of single molecule manipulation, and thus is capable of utilising the individual nature of SMMs on a surface. 23 In order for adhesion to occur, a self-assembled monolayer (SAM) type approach was performed in order to functionalize the cluster and surface with 16-sulfanylhexadecanoate ligands for adhesion. Through IR, EA, 1 H NMR and matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF MS) the researchers were able to confirm that the ligand replacement had been successful. Post-attachment studies were performed using X-ray photoelectron spectroscopy (XPS) and STM measurements. SQUID measurements were also attempted on thin mica-supported Au films soaked in a solution of the SMM, however, the signal was not strong enough to confirm their presence magnetically. The first successful magnetic study of surface attached SMMs was performed with 1a and Mn 12 Bz, 1e (Fig. 1). 24 Molecules were attached through two different methods; namely a Langmuir Blodgett film method and a SAM method. The initial method involves the creation of a lipid-based monolayer that provides order for the SMMs within. This technique was previously studied and found to necessitate the use of different cluster types inordertoformthelayer,thusexplainingtheuseof1a and 1e. 25 The layer formation was accomplished and studied using X-ray diffraction and IR, and subsequently by magnetic measurements. The films displayed subtle hysteresis upon transfer to a substrate, and a blocking temperature of 2 K was extracted. Furthermore, this study also explored the attachment of 1a to Au(111) through a SAM method, which also showed hysteretic behaviour up to 2 K. This result was clearly reminiscent of present, however significantly reduced, magnetic character of the Mn 12 cluster. This was encouraging for the magnetic community since Mn 12 molecules still held the record for highest anisotropy barrier and blocking temperature in a SMM at this time, and because maintaining any magnetic behaviour when deposited on a surface was unprecedented. The next step in the fundamental study of SMMs on surfaces was to attempt to make more controlled films, thus potentially allowing for design principles to be applied to the organization of SMMs on surfaces. An important technological advancement was depositing 1a on Au(111) using electrospray deposition. 26,27 The reasoning behind using electrospray to introduce SMMs onto a surface is that surface coverage is controllable using this technique, and the molecule of interest does not require volatility in order to be deposited. A comprehensive analysis of the deposition of 1a on Au(111) was performed, using STM and near-edge X-ray absorption fine-structure (NEXAFS) measurements from a synchrotron in order to confirm the stability of the molecule on the surface, and molecular dynamics (MD) simulations in order to model their potential placement and interactions on the surface. 26 Thus, this study was able to model the molecular aggregation, confirming that the molecules do indeed interact with one another to assemble on the Au(111) surface. Interestingly, this organization elicited a supramolecular templating effect on the surface. A later study used the same technique to deposit 1a and 1e on Au(111), and was able to characterize a reduction of the Mn ions within the core structure of both Mn 12 clusters upon surface attachment. 27 It was proposed that this reduction could be due to charge transfer from the surface causing a rearrangement of the atoms in the core to more thermodynamically stable Mn II oxide. 27 This was an important result, as it confirmed a drastic change in the electronic structure of Mn 12 clusters upon surface interaction, thus rationalizing the previously observed changes in their magnetic properties. This also led to unique design strategies to try to combat reduction, vide infra. Further studies on 1a have been performed, also using SAM techniques, on SiO 2 28 and Si(100). 29,30 These studies were tailored toward the specific employment of SMMs in current electronics-based technologies which employ silicon substrates. Both 1a and cationic Mn 12 bet, 1c (Fig. 1), were studied on SiO 2, which was patterned prior to SMM deposition atop a Si(100) surface. 28 These Si-based substrate studies were consistently able to confirm the successful patterning of the SMM on the surface. However, it is noteworthy that one of the studies was also able to discern the magnetic moment of the molecules on the surface through b-nmr, and through these measurements they confirmed that the magnetic properties were drastically different from that of the bulk. 29 This result was corroborated by the finding of a loss of magnetic properties of 1a on Si(100). 31 This loss of magnetic integrity of 1a was rationalized by the change in Jahn Teller elongation properties of the cluster through the use of more extensive magnetic measurements; namely XMCD and SQUID magnetometry. Thus, due to the change in magnetic moment of Mn 12 through surface adhesion, the SMM no longer retained the same magnetic properties as the bulk material. This finding was essential to the development of the field, as it meant that there were a variety of possible alterations upon surface attachment, which were capable of altering the physical properties of the molecule, and moreover were capable of removing its desirable magnetic properties. More recently the adhesion of 1a has been performed on more specialized surface structures, such as: carbon nanotubes (CNTs), 32 Rh(111)/BN, 33 and Bi(111). 34 These unique surface studies probed the magnetic behaviour of 1a and its relation to the underlying surface structure. When placed on the interior surfaces of CNTs, 1a molecules retained much of their magnetic integrity, however, the energy barrier (U eff = 57 K) and blocking temperature (T B = 2.5 K, calculated) were lessened overall from their reported bulk (U eff =72K,T B =3K,calculated), which was attributed to increased QTM observed in the hysteresis measurements. This was encouraging in regards to the ability of the molecules to retain some of their signature properties,

4 Fig. 2 Scanning tunnelling microscopy images displaying the topology of Mn 12 complexes on various substrates; area maps on (a) Cu(001), (b) Au(111), (c) BN/Rh(111); as well as molecules at submolecular resolution (d f); and the as-predicted Mn 12 using DFT simulation (g). Reprinted with permission from ref. 32. Copyright r 2012 American Chemical Society. however, once again there are many potential factors at play which have been shown to alleviate SMMs of their magnetisation. Other surfaces, varying from conducting substrates, such as: Cu(001) and Au(111), to insulating Rh(111) coated with boron nitride (BN), were also studied upon deposition of 1a using electrospray ion beam deposition (Fig. 2). 33 The resulting magnetism was studied using inelastic spin-flip spectroscopy and, unlike the conducting surfaces, the insulating surface was indeed capable of allowing magnetisation to be retained. Since conducting surfaces are more desirable for electronic applications, this could be relevant to the fundamental understanding necessary for future application of SMMs. Thus, in order to study the effects of reducing surface conductance, yet retaining charge transfer capabilities, Bi(111) was employed as a semi-metallic substrate for the surface attachment of 1a, and studied thereafter for its local structural and electronic properties. 34 By using a semi-metallic substrate the interactions were confirmed to be weak between SMMs and the surface, while molecules were still adhered and intact on the surface. This is further proof that SMM substrate interactions are incredibly important in order to maintain the desired magnetic properties of the molecule. However, no magnetism was explored in this particular study. Since SMM surface interactions are indeed crucial to the exploration, and potentially successful functionalization, of magnetic molecules on surfaces, Mn 12 structures have also commonly been designed with varying ligands in place of the acetate groups on the original molecule, 1a. This technique has the potential for better control of the structural, electronic and magnetic properties upon adhesion. A simple functionalization was performed in 2005, 35 prior to attachment to Au(111), with pivalate groups in order to form Mn 12 Piv, 1b (Fig. 1). The reasoning behind the selection of these particular functionalizations was that the complex was soluble in organic solvents, and furthermore aggregation, and thus intermolecular interactions, would be prevented by the peripheral t Bu groups. SQUID magnetic measurements clearly showed magnetic hysteresis up to 1.8 K, however, at higher temperatures the loop closes. Thus, as was previously observed, increased QTM is contributing to the magnetic behaviour of the molecule on the surface. An amalgamation of the different approaches of deposition, SAMs and molecular functionalization, was employed using a cationic SMM, Mn 12 bet, 1c, which was also attached to Au(111) through SAM methods in order to promote electrostatic surface interactions. 36,37 Due to the cationic nature of the molecule, the redox chemistry, which had been shown previously to be a detriment to the magnetisation, was deemed to be less of a risk upon surface attachment. This hypothesis was not proven to be concrete. Instead, it was shown that the redox chemistry of the SMM is highly dependent upon the nature of the interfacial layer. Furthermore, despite the efforts to reduce the impact of surface attachment on the magnetic properties within this system, XMCD studies were still able to discern changes in the overall magnetic properties of the Mn core due to either small structural changes, interactions with the substrate or poor thermal conductivity of the ligands. 36 A similar strategy to the use of a cationic SMM in order to promote superior surface attachment with SAMs, and to avoid redox chemistry within the Mn 12 core, is to functionalize the molecule preferentially along one axis. This was explored with Mn 12 dichloroacetate, 1d (Fig. 1), where only axial carboxylate groups are replaced by dichloroacetate groups for SAM attachment to Si(100). 38,39 Magnetically this technique aligns the anisotropy axes of the SMMs with one another. The authors theorized these axes to be parallel to the surface, however, no calculations or experimental measurements were performed in order to confirm the alignment of the anisotropy axes, or indeed to study the resulting magnetisation. The previously discussed Mn 12 Bz cluster functionalized with benzoate groups, 1e, was more recently studied on Au(111) and TiO 2 (110) surfaces, along with another Mn 12 cluster functionalized with terphenyl groups, Mn 12 tpc, 1j (Fig. 1). 40 The goal of studying two different Mn 12 functionalizations was to explore whether the extended terphenyl ligand structure would aid in the insulation of the Mn 12 core on conducting surfaces, such as: Au(111). The reason that Au(111) was chosen was due to the fact that it was previously shown that 1e was able to be reduced on this type of surface, thus changing its magnetic properties. 27 Furthermore, the addition of a non-metallic wide band gap semiconductor surface allowed for further exploration into the substrate effects on the Mn core oxidation states. Interestingly, the TiO 2 (110) substrate showed the same results as had been previously found with Au(111) for 1e, however, for 1j neither substrate was able to elicit reduction of the Mn ions in the cluster core. Thus, this bulky ligand functionalization was indeed capable of adequate core insulation from magnetically detrimental interactions with a conducting surface. Despite the promising results from insulating ligands, it is still important that conductivity between SMM and substrate is maintained. Thus, one group employed an approach which used a fluorinated version of the Mn 12 cluster, Mn 12 pfb, 1f (Fig. 1), with a short acidic linker. This cluster was synthesized towards a new approach to SAM functionalization, and subsequently adhered to Au(111). 41 The electronic structure was indeed shown to

5 be conserved, however, magnetic properties were not investigated post-attachment. Another approach to maintain the conductivity as well as the magnetic properties of the SMM is direct functionalization. Using functionalized ligands, the molecule can be adhered through peripheral ligands instead of merely deposited on a surface or potentially attached through SAM methods. This has been performed on Au(111) using two different types of S-terminated clusters: Mn 12 Th, 1g, and Mn 12 PhSMe, 1h (Fig. 1). 45,46 Thiophene-terminated Mn 12 (1g) was initially studied on Au(111) using AFM, 43 and was explored for its magnetic properties a few years later. 44 It was shown that 1g displayed similar magnetic properties to that of Mn 12 Ac, 1a, however, magnetic measurements were not performed post-attachment in order to compare the properties. Complex 1h was also placed and studied on Au(111), however, no post-adhesion magnetic studies were performed in this study either. Since direct functionalization is a very important approach to study for future application, it is unfortunate that the magnetic properties were not closely investigated post-attachment in these cases. Direct functionalization was also explored on non-au surfaces, using a cluster which has been studied quite often in comparison with similarly functionalized 1e and 1j: Mn 12 biph, 1i (Fig. 1). This biphenyl functionalized cluster has been attached to Si, 47 highlyordered pyrolytic graphite (HOPG), 48 and polymeric substrates Initially, the hydrophobicity of 1i was attractive for its application atop a surface of Si(100) from solution; 47 where it was subsequently studied for its surface organization using AFM. Due to the chemical and thermal stability of the molecule, the previously desired hydrophobicity, and the propensity for pp interactions between phenyl groups, 1i was also chosen for assembly on HOPG. 48 This study used breath-figures to template the molecules into ring-shaped assemblies across the surface. This was an excellent display of control over surface assembly array, however, no magnetism was investigated in this study. Furthermore, 1i was placed on polymeric substrates to probe the ability for control of SMM assembly on practical surfaces for future information storage applications The initial study performed magnetic measurements on the resulting thin films, eliciting two energy barriers (U eff = 56 and 36 K) which were consistent with 1a. 51 Thus, direct functionalization to a conducting surface was indeed shown to be possible, however, still extremely dependent upon the structure of the molecule and conditions involved. Finally, a rather complicated ligand structure was axially functionalized in order to form Mn 12 adc, 1k (Fig. 1), which was subsequently attached to Au(111). 52 This ligand structure was designed with the intention of orienting the magnetic anisotropy axes of SMMs normal to the surface, and though this was not experimentally or theoretically confirmed, the blocking of the magnetisation was shown to be retained up to 2 K. An interesting study of molecular stability was performed on 1g, as well as 1h and 1i upon attachment to a Au/Ti/Si substrate. The results of which exhibited reduction of the Mn core, unless specific functionalizations and/or deposition methods were employed under an inert, moisture-free atmosphere. 42 This study also found that X-ray radiation damage was a significant hazard, thus cautioning the use of X-ray-based characterisation techniques for surface attached SMMs. Thus, through learning from the work of others within the field, researchers were able to take leaps forward in the fundamental understanding of the interactions between Mn 12 complexes, and their chosen substrates. The extensive surface-based work on the Mn 12 system can be directly linked to the success achieved with more recent systems, discussed herein. The broad array of work and understanding gained through the study of the most ubiquitous SMM, Mn 12, was indeed able to lead the field in a new and productive direction. Other TM SMMs Aside from Mn 12, there have been a number of other transition metal-based SMMs studied on surfaces. The field eventually broke away from Mn III/IV into Fe III, however, some smaller Mn clusters were also explored on surfaces. Two new complexes with Mn III were synthesized with the goal of direct functionalization on Au(111); [Mn 6 O 2 (R-sao) 6 (O 2 C-th) 2 L 4 6 ], where R = H (2a) or Et (2b), and HO 2 C-th = 3-thiophene carboxylic acid, L = EtOH, H 2 O and saoh 2 = salicylaldoxime (Fig. 3). 53 Both complexes have 6 Mn III ions, however, 2a has a spin ground state of S = 4, while for 2b the spin ground state is S = 12. Their magnetic properties are, thus, quite different; with effective energy barriers of: U eff = 28 K for 2a, and U eff = 55 K and 67 K for the fast and slow relaxation processes of 2b, respectively. As in previous Mn 12 studies, STM and XPS measurements were employed in order to study surface attachment and grafting in detail, however, unfortunately no magnetic studies were performed post-attachment. A cationic Mn SMM, [Mn 4 (O 2 CCH 3 ) 2 (pdmh) 6 ] 4+, 3, was later studied on the surface of CNTs [Mn 4 (O 2 CCH 3 ) 2 (pdmh) 6 ] 4+ (Fig. 3). 54 This approach was slightly different than what was previously seen with Mn 12 as the CNTs employed were multi walled (MWNTs) in order to allow for a more robust substrate, and they were pre-functionalized with negatively charged carboxylic groups. The goal was to electrostatically attach the SMMs and thus retain their previously desirable properties, as well as those of the substrate. Surface adhesion was confirmed and visualized through HRTEM, EDS and XPS, and the subsequent magnetic properties were studied using SQUID magnetometry. Overall the magnetic properties were indeed altered through surface adhesion, Fig. 3 Molecular structures of Mn 6 and Mn 4 complexes (2a, 2b and 3) which have been successfully attached to various surfaces through their displayed functionalities. Hydrogen atoms are omitted for clarity.

6 as was often observed for Mn 12 ; where no energy barrier could be extracted from the collected ac data. The SMM most responsible for shifting the field away from Mn complexes on surfaces was Fe 4 (Fig. 4). Initially Fe 4 was presented as a star-shaped molecule, {Fe III [Fe III (L 1 ) 2 ] 3 }; H 2 L 1 being N-methyldiethanolamine, 4a, which was attached to HOPG and studied thereafter. 55 The magnetic properties of the original molecule show moderate SMM behaviour, with T B = 1.2 K and U eff = K. Surface attachment was studied and confirmed on HOPG using STM and current imaging tunneling spectroscopy (CITS), and modelled using theoretical calculations. However, no post-adhesion magnetic properties were explored. The next studies performed on this type of Fe 4 system instead chose Si(100) as the surface. The molecules synthesized for this purpose were [Fe 4 (OMe) 6 (tmhd) 6 ], 4b, and [Fe 4 (L) 2 (tmhd) 6 ], 4d (Fig. 4), where; Htmhd is 2,2,6,6-tetramethylheptane-3,5-dione and H 3 L is 2-(4-chloro-phenyl)-2- hydroxymethyl-propane-1,3-diol. 56 The reasoning behind using these particular molecules was that the Fe 4 structure was one of the simplest SMM systems, and in particular with this type of structure the bridging and non-bridging ligands could be varied independently, thus they wished to explore this phenomenon. The surface was pre-functionalized with tripodal receptors prior to attachment, and the success of the attachment was confirmed by XPS and AFM measurements thereafter. Unfortunately, post-attachment magnetic properties were, once again, not explored. Furthermore, a similar study was performed shortly after on [Fe 4 (L) 2 (tmhd) 6 ], 4d, molecules attached to Si(100). 57 The purpose of this study was to reduce the number of steps taken in the previous paper towards surface adhesion. Despite this goal, however, the result was overall similar in nature and once again no magnetic measurements were reported on the SMMs once attachment had occurred. A similar structure to that of 4d was utilised for a surface deposition study on Al foil under UHV conditions; [Fe 4 (Ph-C(CH 2 O) 3 ) 2 (dpm) 6 ], 4c (Fig. 4). 58 Upon deposition the thin films of SMMs were studied with time-offlight secondary ion mass spectrometry (TOF-SIMS), EPR and SQUID measurements, confirming not only the structural integrity, but also that the film retained the magnetic properties of the bulk powder. Despite some QTM contribution, which slightly lowered the barrier from B16 K to U eff = 12.2 K, this was an important success in the attachment of Fe 4 to a surface. A different approach, which was previously discussed with Mn 12, is the method of direct functionalization, which was first explored with [Fe 4 (L) 2 (dpm) 6 ]; where H 3 L = 2-hydroxymethyl-2- (4-(pyren-1-yl)butoxy)methylpropane-1,3-diol, 4e (Fig. 4). 59 The molecule itself was studied magnetically to reveal a modest T B = 1.0 K. Upon attachment to CNTs the material was studied with AFM, Micro-Raman, and high-field EPR to confirm attachment and the resulting electronic properties. This study unfortunately did not study the hybrid material with magnetic measurements. Shortly following the previous work, potentially the most famous example of SMM surface attachment was published with [Fe 4 (L) 2 (dpm) 6 ]; where H 3 L = 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm = dipivaloylmethane, 4f (Fig. 4). 60 The functionalization of this Fe 4 derivative was designed towards the goal of attachment to Au(111), and thus involved S-terminated ligands. A desirable trait of this molecule was its stability in solution, thus allowing for ease of self-assembly from the liquid state onto Au(111). The adhesion was studied using STM to show the size and distribution of molecules deposited on the surface. Furthermore, studies were performed using XAS on the electronic structure, followed by XMCD in order to elucidate the magnetic properties. XMCD was able to show that this molecule retained hysteretic behaviour upon adhesion through its peripheral S-terminated ligands onto the Au(111) surface. Thus, this became known as the first display of a surfaceadhered molecule retaining hysteretic behaviour (T B = 0.50 K). This was followed by a study by the same group on 4f, as well as a nearly identical version with slightly shorter alkyl chains, proceeding to compare and contrast their magnetic behaviours. It was confirmed that the shorter linkers did indeed promote more QTM behaviour. 61 Due to the success of this system, further studies were performed. 62 In one example, an analogue Fig. 4 Top-down (4a and 4b) and side-on (4c f) molecular structures of Fe 4 complexes which have been successfully attached to various surfaces through their displayed functionalities. Hydrogen atoms are omitted for clarity.

7 View Article Online of 4f with one Cr located in the place of the central Fe ion, [Fe3Cr(L)2(dpm)6], was synthesized and confirmed to retain its inherent magnetic properties using XMCD on the surface of Au(111).63 These results stimulated the field, promoting researchers to believe that SMMs could indeed be utilised in practical applications. Recently a detailed study was published on 4e, which explores the classical and quantum dynamics of SMM spins on the surface of graphene.64 Through the use of AFM, Raman and XPS methods the SMMs were confirmed to be on the surface, after which the hybrid material was carefully magnetically studied. The magnetic behaviour was shown to be retained within the hybrid material under an applied dc field of H = 1000 Oe, whereas under zero applied dc field QTM dominates, and thus no barrier could be extracted. Following the magnetic study, the spin interaction with the graphene substrate was modeled. Hybridization between the graphene substrate and the SMM introduced a significant transverse term that induced a change in the dynamic properties of the molecule, thus explaining the difference observed in ac properties between the crystalline SMM and the hybrid material. The roles of vibrational and electronic interactions were reported, illustrating the capability of this strategy to be utilized in the future design of materials towards molecular spin control. A unique approach employing a non-fe4 system to the attachment of an Fe SMM on the surface of single-walled CNTs was performed with a polyoxometallate (POM) structure, Na6((CH3)4[Fe4-(H2O)2(FeW9O34)2] 45H2O; Fe6-POM, 5 (Fig. 5).65 Once attached through simple sonication, studies were performed using: HRTEM, EDX, SERS, electrochemistry and finally MicroSQUID. Magnetic measurements displayed that, as was predicted, the tungsten oxide matrix which surrounded the Fe6 core was able to shield the coordination sphere of the magnetic ions from significant alterations. Thus, the inherent magnetic behaviour was retained upon attachment, with a slight decrease in the coercive properties attributed to the lack of intermolecular interactions once the molecules are surface adhered. Overall, Fe4 and Fe6 clusters were able to bring the field of molecular magnetism forward from Mn12 and into a new era of surface functionalization towards application in many exciting areas. However, not all interest was reserved for the cluster approach. A single-ion Fe system (Fig. 5), FePc, 6, was frequently functionalized and studied on surfaces.66 This iron complex employed a phthalocyanine ligand to form a planar Fig. 5 Molecular structures of Fe6POM (5) and FePc (6). Hydrogen atoms are omitted for clarity. structure (Fig. 5). The interest in mentioning this system lies in the fact that, although it possesses merely modest magnetic properties, FePc has been studied on a variety of surfaces, particularly due to its inherent semiconductor and electrochemical properties Due to the planarity, the metal and substrate can directly interact with one another, and as such it is beneficial to study the effects of the substrate composition on the electronic and magnetic properties of the material. Furthermore, as will be observed in the following section, this ligand became ubiquitous to magnetochemists with SMM and surface adhesion interests alike. Lanthanide SMMs The previously discussed ligand system, Pc, quickly became one of the most important ligands in the field of molecular magnetism and magnetic semiconductors. In 2003 the first terbium bisphthalocyanine, TbPc2 complex (7a, Fig. 6 and 7) was published as a single molecule magnet with an energy barrier of 230 cm 1, 331 K.72 Extensive work has since been performed with the goal of optimizing TbPc2 in order to elicit improved magnetic properties. To date, it is still the heteroleptic TbPc2 SMM that holds the aforementioned record energy barrier.18 Thus, due to its remarkable properties, as well as the potential for homoleptic or heteroleptic functionalization to promote surface attachment without necessitating the use of lithography techniques, TbPc2 has been extensively investigated on various surfaces. As early as 1993 there were Langmuir Blodgett films being prepared with 7a, due to interest in their electrochromic and semiconducting properties.73,74 The unsubstituted molecule has since been studied on a number of surfaces for a variety of reasons. Initially, 7a was deposited on the surface of Cu(111).75 The surface attachment and arrangement was studied using STM (Fig. 6), followed by DFT studies that were employed to model the 4f orbitals, thereby confirming that they were unperturbed by surface adhesion and, thus, that the magnetic properties should be theoretically retained. However, the first study that was able to experimentally confirm the crystal field and magnetic properties of 7a to be robust upon surface attachment was in 2010 on the surface of Cu(100).76 This investigation employed XMCD in order to compare the experimental magnetic moment values with those found through calculations. With this method Fig. 6 (a) Top-down molecular structure of TbPc2 (7a), with the same colour code as in Fig. 7 below, (b) schematization of surface imprinting method employed, (c) STM topographical image of TbPc2 film formation on the surface of Cu(111), and (d) simulated image of 7a. Adapted from ref. 73. Copyright r 2008 American Chemical Society.

8 Fig. 7 Side-on (7a) and top-down (7b d) molecular structures of TbPc 2 complexes which have been successfully attached to various surfaces through their displayed functionalities. Structures 7b, 7c, and7e were modelled for visual representation, as crystal structures were not reported. Hydrogen atoms are omitted for clarity. they were able to conclude that, though the lower Pc ligand has a significant interaction with the substrate, the ligand field and magnetic moment remain unchanged. There were many interesting magnetic studies performed thereafter on TbPc 2 ; studying the coupling between 7a and a ferromagnetic substrate of Ni or Co deposited on Cu(100), 77,78 or of perovskite manganite on strontium titanium oxide. 79 Theoretical studies on the spin dynamics and spin-splitting of the adsorbed molecule were also performed in great detail in order to gain greater understanding of the system and its potential application in the future of spintronics. 80,81 Furthermore, 7a was studied on a wider variety of surfaces, such as: Au(111) for its electron transfer properties, 82 as well as for the Kondo effect properties; Ir(111) for chiral to achiral structural switching; 88 and Mn and CoO for exchange bias properties. 89 Overall, the SMM itself has received extensive attention in a variety of fields for its unique properties and ease of surface functionalization, however, as was discussed with earlier SMMs, ligand functionalization can often be a beneficial method of SMM design towards a specific surface and arrangement. The first study involving a substituted TbPc 2 SMM on a surface was in 2006, where butoxy groups were substituted homoleptically on the Pc ligands (7c) in order to promote selforganized assemblies on HOPG. 90 Both 7a and 7c (Fig. 7) were drop cast onto the HOPG surface, and studied thereafter with STM. Magnetic force microscopy (MFM) was also attempted on the ordered arrays of SMMs, however, no magnetic contrast was observed. This was attributed to either the thermal instability of the molecules, or to the lacking strength of magnetic response from a single monolayer of SMMs on the surface. They proposed that the use of longer alkyl chains could help to prevent the thermal instability from being a concern in the future. A somewhat similar homoleptic TbPc 2 molecule with acetal groups on the Pc ligand, 7b (Fig. 7), was also studied on HOPG. 91,92 Anionic and neutral forms were initially studied using STM, where the neutral compound was found to form ordered arrays on the surface. 91 A subsequent study on the neutral compound used XMCD to confirm that the magnetism was retained upon surface ordering, including its hysteretic behaviour at 7 K. 92 Interestingly, another homoleptic TbPc 2 molecule was also synthesized with long alkylthio chains substituted on the Pc ligands; [Tb{Pc 0 (SR) 8 } 2 ], 7d (Fig. 7). 93 This molecule, as well as an almost identical SMM with slightly longer alkyl chains, were studied for their magnetic properties and then grafted on Au(111). Unfortunately, despite the attractive properties known prior to placing the SMMs on gold, the magnetic behaviour was not studied upon attachment. An important synthetic advancement in the design and control of TbPc 2 molecules on surfaces was heteroleptic substitution, where only one of the Pc ligands was synthetically altered in order to promote more selective and controlled surface adhesion. This substitution involved a pyrenyl group, as was previously seen with 4e, in order to promote favourable interaction with CNT and HOPG surfaces. The structure, 7e (Fig. 7), was initially studied on the surface of CNTs, where the pyrenyl group promoted SMM assemblies on the surface through pp interactions. 94 The surface attachment was studied with HR-TEM, emission spectroscopy, AFM, EDX and EA. Quite remarkably the magnetic properties were not only retained, but also improved upon surface attachment, with an energy barrier of U eff = 351 cm 1, 505 K, which is similar to the bulk value for neutral TbPc 2 (U eff = 410 cm 1, 590 K), 95 as well as hysteretic behaviour shown by micro-squid measurements at 0.04 K. This study clearly presented the promise of this SMM system in future spintronic and electronic applications. This was followed by the attachment of 7e on graphene, where an enhanced Raman signal was observed, thus allowing for a new method of facile detection of small molecules on a surface. 96 Furthermore, a weak electronic interaction between the graphene substrate and the SMMs on the surface was also observed, thus, more specifically illustrating the potential of these SMMs within graphene and other transistor studies. These two investigations opened the door for the fabrication of a CNT- TbPc 2 molecular spin-valve to be synthesized and studied. 13 A more detailed look at the physics within this type of device was published in 2013, 97 where strong spin phonon coupling was observed and well-characterised. This type of strong coupling was proposed to potentially allow for: manipulations of

9 molecular spins within this system using microwave radiation, detection of the mechanical motion of the nanotube, or groundstate cooling of the resonator through spin manipulation by electron spin resonance. These results were indeed promising for the future of molecular magnetic applications in spintronics, and have thus promoted new researchers to have garnered interest in these molecular magnets for such applications. Aside from Tb there has been little exploration into lanthanide SMMs on surfaces thus far. However, recently a non-tb SMM, Er(trensal); where H 3 trensal = 2,2 0,2 00 -tris(salicylideneimino)- triethylamine), 8 (Fig. 8), was studied on Au(111) and Ni coated Cu(100). 98 Surface attachment and arrangement were studied with XPS, XAS and STM, and modelled using DFT. Magnetic measurements were performed using XMCD. It was found that on Au(111) the weak interaction with the surface allowed for random molecular orientation, and thus no significant changes were observed in the magnetic behaviour. However, the Ni coated surface formed a covalent interaction with the SMM, which promoted molecular alignment and subsequent antiferromagnetic coupling. It was theorized that this coupling was responsible for stabilizing the magnetic moment, and thus for suppressing QTM. This result was encouraging for further studies on non-tb lanthanide SMMs on surfaces, and will hopefully lead to future interest in this type of fundamental research. Metal nanoparticles Potentially one of the most unique approaches to the study of SMMs wired to surfaces has been in the use of metal nanoparticle substrates in place of more traditional surfaces. Through this design strategy magnetic behaviour can be studied using conventional magnetometry, in place of more specialized techniques like XMCD, and subsequently other analyses can be more simply performed than for a bulk solid substrate material. This is owing to both the ability to study these systems as bulk powders and/or solution suspensions, as well as to the high surface-tovolume ratio of nanomaterials, thus allowing for a large number of magnetic molecules to adhere to the surface of each particle and provide a significant signal. Another point of interest in using nanoparticles is their tunable size, shape, and other physical properties, where interaction between each type of property and the magnetic behaviour inherent to the molecules on the surface could theoretically be probed. Despite the promise of this method as a facile means for investigation of molecular magnetic relaxation on a variety of surface templates, thus far only two such studies have been performed; one using a 3d- 99 and the other a 4f-based magnet. 100 The former employed an Fe 4 structure; similar to the molecule initially wired to a gold surface. 60 In this study, gold nanoparticles were prepared with an average size of 5 nm, where growth was arrested by a commonly used capping agent (hexadecylamine). Particles were subsequently reacted in order to promote surfactant replacement with the 1,2-dithiolane terminated ligands of the Fe-based SMM, 4g (Fig. 9). Thereafter, a comprehensive study was performed using UV-Vis, XPS, XAS/ XMCD, EPR, SQUID Magnetometry, etc. From ac susceptibility the barrier was calculated to be U eff /k B = 8.0(1) K, with t 0 = 1.20(6) 10 6 s. The paper does not directly compare these values with those of the bulk magnet itself, which were calculated to be U eff /k B = 14.0(1) K, with t 0 =6.6(3) 10 8 s. 101 Thus, the magnetic relaxation process speeds up upon surface adhesion, which is indicative of increased QTM. 102 The latter study, performed with a lanthanide SMM, involved the use of a Dy dimer complex, 9, 103 with 3 separate thiocyanate groups available for binding to the gold surface (Fig. 9). A notable component of this study was the methodology with which the gold nanoparticles were synthesized; using a capping agent-free method of photochemical one-electron reduction in water, followed by laser ablation in order to gain consistently sized spherical particles. The reasoning behind using such a unique method was to avoid competition on the surface, thus only allowing nucleation to be arrested by full surface adhesion of the magnetic molecules. Akin to the previous study, a comprehensive investigation was performed using UV-Vis, XPS, IR, SEM, TEM, SQUID Magnetometry, etc. The dc magnetic properties were shown to be retained, however, much like the Fig. 8 Molecular structure of Er(trensal) (8). Hydrogen atoms are omitted for clarity. Fig. 9 Molecular structures of 4g and 9. Structure 4g was modelled for visual representation, as a crystal structure was not reported. Hydrogen atoms are omitted for clarity.

10 Fe 4 work, ac susceptibility studies clearly displayed an increase in QTM upon surface adhesion. In this case the different dynamic behaviour was proposed to be due to the change in local anisotropy of Dy ions, but could also be attributed to surface plasmonic effects from the substrate. Through attachment of molecular magnets to nanomaterials instead of bulk surfaces, the applicability becomes far more open ended, and the techniques for analysis broaden greatly. Thus, this slim area of research is incredibly promising towards advancing the still relatively juvenile field of molecular magnetism. The future of this type of research has the potential for many possible avenues, perhaps the most fascinating of which is the investigation of the electronic and magnetic interplay between a magnetic or non-magnetic nanoparticle substrate and the SMM itself. Conclusions and outlook In conclusion, we have discussed a broad overview of work within the limited field of SMM surface attachment. We attempted to provide a clear representation of the SMM design towards attachment to specific surfaces, which has been shown to be a crucial concern for stability on the surface, as well as for the surface interaction. A comprehensive discussion of the SMMs, surfaces, and resulting effects on the original magnetic properties once surface attachment had occurred was also presented. The surface attachment of magnetic molecules has provided researchers with many challenges, as the inherent and desirable properties of these systems are often not retained. Though the ubiquitous Mn 12 cluster has been studied greatly on surfaces over the years and has successfully moved the field toward a superior fundamental understanding, it was initially overshadowed by Fe 4 systems, which were capable of retaining hysteretic behaviour for the first time. However, now both systems have been surpassed in their magnetic properties by lanthanide-based systems; in particular TbPc 2. TbPc 2 systems continue to be the most promising surface attachment candidates, especially since these systems continue to hold the record for the highest energy barrier. However, since they are significantly outclassed in blocking temperature it is indeed time to explore a wider variety of lanthanide systems on surfaces. Furthermore, due to the potential for the use of the inherent properties of the surface as an advantage, it is important that a variety of different substrate structures continue to be explored for their electronic and magnetic effects on surface assembled SMMs. Finally, many challenges are open for exploration; including but not limited to: surface design and optimization, coverage and assembly, ordering and control over anisotropic axes, electronic coupling, chemical stability, and, of course, a notable improvement in the overall magnetic properties of SMMs. Unfortunately, to this date many researchers have completed beautiful and intricate work on surface attachment of various SMM systems, however, an overwhelming majority of these studies do not include magnetic characterisation. 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