RESEARCH ARTICLE www.advmatinterfaces.de Surface Deposition Induced Reduction of the Ground State Spin in Cr10 Wheel Elena Bartolomé,* Ludovica Ferrari, Francesco Sedona, Ana Arauzo, Javier Rubín,* Javier Luzón, Julia Herrero-Albillos, Mirco Panighel, Aitor Mugarza, Marzio Rancan, Mauro Sambi, Lidia Armelao, Juan Bartolomé, and Fernando Bartolomé* 1. Introduction We report the deposition of monolayers and multilayers of {Cr10 (OMe)20 (O2 CCMe3 )10 } wheels, hereafter {Cr10 }, onto Au(111) and Efforts to assemble molecular magnets Cu(111) single-crystal substrates, and their characterization combining into layers of variable thickness have been fueled by the goal of develop- scanning tunneling microscopy, X-ray photoelectron spectroscopy, X-ray ing low-dimensional magnetic materi- absorption spectroscopy, X-ray natural linear dichroism, and X-ray magnetic als with molecularly controllable proper- circular dichroism (XMCD). {Cr10 } in bulk shows axial magnetic anisotropy ties, with interest in different fields, such and a cluster total spin S = 9 ground state, stemming from an interaction as molecular spintronics or quantum scheme of two semi-crowns containing four Cr3+ ions interacting computing, among others.[1] Molecules exhibiting magnetic hysteresis, known as ferromagnetically, separated by two Cr3+ antiferromagnetically coupled. The single molecule magnets (SMMs),[2,3,4] one-monolayer (1ML) samples of {Cr10 } sublimated on Ag(111) and Cu(111) have been intensively investigated, as show slightly different applied magnetic field dependence of XMCD signal. they represent the ultimate miniaturiza- The field-dependence of the magnetization evolves from a lower curve for the tion limit of magnetic memories and 1ML {Cr10 } samples to a curve resembling the bulk one as the number of exhibit a rich quantum behavior.[5] The layers is increased, as shown in a 14ML sample. Monte Carlo simulations combination of a large spin of the molecule with an easy-axis magnetic allow rationalizing the magnetization curves of the 1ML samples in terms of a anisotropy results in an energy barrier reduction of the cluster ground-state total spin, S = 3 or S = 6, as a for the reversal of the magnetization. consequence of variations in the intra-wheel coupling interactions induced by Accurate control of anisotropy by chem- the on-surface deposition. The sensitivity of the magnetic configuration of ical design has led in recent years to 4f- {Cr10 } to minor distortions of the intramolecular conformation might be based mononuclear SMMs[6,7] with en- ergy barriers of ≈2000 K[8] at blocking exploited to control magnetism by external stimuli for applications. temperatures as high as 80 K.[9,10] How- ever, a necessary step to fully exploit the E. Bartolomé A. Arauzo, J. Rubín, J. Herrero-Albillos, J. Bartolomé, F. Bartolomé Escola Universitària Salesiana de Sarrià (EUSS) Instituto de Nanociencia y Materiales de Aragón (INMA) Passeig Sant Joan Bosco 74, Barcelona 08017, Spain Universidad de Zaragoza – CSIC E-mail: ebartolome@euss.es Pedro Cerbuna 12, Zaragoza 50009, Spain L. Ferrari, F. Sedona, M. Sambi, L. Armelao E-mail: jrubin@unizar.es; bartolom@unizar.es Dipartimento di Scienze Chimiche A. Arauzo, J. Bartolomé, F. Bartolomé Università di Padova Departamento de Física de la Materia Condensada Via Marzolo 1, Padova 35131, Italy Universidad de Zaragoza Pedro Cerbuna 12, Zaragoza 50009, Spain J. Rubín, J. Herrero-Albillos Departamento de Ciencia y Tecnología de Materiales y Fluidos Escuela de Ingeniería y Arquitectura The ORCID identification number(s) for the author(s) of this article Universidad de Zaragoza can be found under https://doi.org/10.1002/admi.202300146 María de Luna 3, Zaragoza 50018, Spain © 2023 The Authors. Advanced Materials Interfaces published by J. Luzón Wiley-VCH GmbH. This is an open access article under the terms of the Centro Universitario de la Defensa Creative Commons Attribution License, which permits use, distribution Academia General Militar and reproduction in any medium, provided the original work is properly Zaragoza Spain cited. DOI: 10.1002/admi.202300146 Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (1 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de functionalities of SMMs to store and process information is to Monolayers of functionalized fullerene Tb2 @C80 (CH2 Ph) on find ways to deposit monolayers of organized molecules, individ- graphene from solution displayed magnetic bistability up to a ually addressable, onto suitable substrates.[11] temperature of 28 K, which is the highest hysteresis temperature This challenging goal has stimulated the research of chemi- observed among air-stable SMMs.[31] cally stable molecules that withstand surface-deposition meth- As regards polynuclear 3d-clusters, early attempts to demon- ods. The self-assembly of monometallic molecules, such as strate slow magnetic relaxation in Mn12 adsorbates on gold phthalocyanines,[12,13] porphyrins[14] and bis(phthalocyaninato) were unsuccessful owing to redox and structural rearrangement lanthanide-based complexes[15,16] is already well established, phenomena,[32] and even surface layers of bulk Mn12 exhibited no whereas there are only a few examples of polymetallic molecules magnetic hysteresis, likely due to structural deformations.[33] The robust enough to sustain a wide range of processes and ex- first demonstration of SMM behavior retention on surface was periments on surfaces, for example, {Fe4 },[17] {Mn6 },[18] and achieved for a sublimated monolayer of sulfur-functionalized {Cr7 Ni}.[19] Deposition by sublimation in ultrahigh vacuum {Fe4 } cluster grafted on Au(111) at sub-kelvin temperatures.[34] (UHV) is the cleanest method to prepare molecular overlayers, The chemical stability and ease of functionalization of the {Fe4 } something highly important for experiments that are sensitive family have been exploited to control their interaction with vari- to the molecule-surface interaction. However due to the fragility ous substrates.[35,36] These propeller-like tetrairon(III) structures of polymetallic complexes, sublimation has been only success- possess a cluster S = 5 ground state and anisotropy values of fully demonstrated in a limited number of cases,[19–21] while liq- D/kB = −0.30 to −0.65 K, resulting in energy barriers amounting uid phase deposition methods have been predominantly utilized. to U/kB = 7.6 – 16.2 K.[37] The introduction of anchoring insulat- Understanding how the magnetic properties of molecular ing spacers[38] may difficult the integration in devices given the magnets are affected when supported on a substrate is crucial for non-planar molecular geometry. their successful integration into solid-state devices. In archetyp- In this respect, the planar and highly symmetric structure ical TbPc2 , direct interaction with metallic substrate was ob- of cyclic molecules makes them potentially suitable for deposi- served to significantly impact the magnetic properties, and con- tion onto substrates.[20,39] Molecular cyclic complexes have re- sequently, various strategies were proposed to avoid the loss of ceived special attention owing to their interesting intrinsic mag- magnetic bistability, including the use of insulating decoupling netic properties, as model systems for the study of macro- layers,[22] conductive graphene/SiC(0001) substrates,[23] thicker scopic quantum coherent phenomena, and their potential ap- films,[24] or spacers to graft the molecules to the substrates.[25,26] plication as elementary units in high-density information stor- Recently, an extension of the lanthanide-driven SMM approach age and processing.[40–42] Magnetic wheels of different 3d tran- consisting on surface-supported periodic networks containing sition metals have been studied.[43,44] For example, ferric wheels lanthanides has been proposed,[27] although SMM behavior has Fen (for even n = 6 –18) have been reported,[45–50] showing anti- not been demonstrated yet. ferromagnetic (AF) coupling between the Si = 5/2 spins of the An alternative approach to achieving robust on-surface SMMs Fe3+ ions, resulting in a total S = 0 ground state. Chromium has been through the use of endohedral metallofullerenes,[28,29] wheels of different number of Cr3+ ions with spin Si = 3/2 in which a pair of lanthanides with strong single-ion anisotropy have also been intensively investigated. In particular, {Cr8 } with and exchange coupling are encapsulated within a carbon cage. AF coupling and total S = 0,[51–54] which served as precursor The high thermal stability of fullerenes enabled the formation for the synthesis of other homometallic[55,56] and heterometal- of monolayers by sublimation both on metals and insulators.[30] lic closed[57–61] and open rings[62–66] was thoroughly investigated. The heterometallic {Cr7 Ni}[67,19] wheel containing an AF cou- pled Ni2+ ion, with a total S = 1/2 ground state, has been M. Panighel considered as a candidate for the implementation of molecular CNR-IOM qubits.[41,39] Laboratorio TASC A powerful experimental method to study the on-surface mag- Basovizza, Trieste 34149, Italy netic properties of these polymetallic clusters is X-ray absorption M. Panighel, A. Mugarza spectroscopy (XAS) combined with X-ray magnetic circular Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST dichroism (XMCD). Pertinent to the present work are previous Campus UAB XAS – XMCD studies of Cr3+ wheels as monolayer (ML) or Barcelona 08193, Spain polycrystalline thin-films (TFs). In early works on Cr8 -piv (S = 0) A. Mugarza and Cr7 Ni-piv[68] (S = 1/2) TFs deposited by drop casting, XMCD ICREA – Institució Catalana de Recerca i Estudis Avançats was crucial to determine the AF nature of interactions and the Barcelona 08010, Spain resulting ground state. In a comparative study of the magnetic M. Rancan properties of TF and sub-ML distributions of Cr7 Ni wheels Institute of Condensed Matter Chemistry and Technologies for Energy (ICMATE) grafted on Au(111), functionalized through S-ligands (Cr7 Ni- National Research Council (CNR) 3tpc), S-free derivatives (Cr7 Ni-4mtpp) and other approaches c/o Department of Chemical Sciences (Cr7 Ni-phenoxy, CoPc-Cr7 Ni),[39] XMCD allowed to correlate University of Padova the Ni dichroic signal with the strength of ligand-substrate via F. Marzolo 1, Padova 35131, Italy bonding. An XMCD study of the magnetic properties of sub-ML L. Armelao Department of Chemical Sciences and Materials Technologies (DSCTM) distributions of isolated molecular Cr7 Ni rings either deposited National Research Council (CNR) or grafted on Au(111) by liquid phase showed a weakening of the Piazzale A. Moro 7, Roma 00185, Italy Ni–Cr bond, which was associated to slight ring distortions.[69] Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (2 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 1. a) Structure of the {Cr10 (OMe)20 (O2 CCMe3 )10 } wheel, top view. The close to fivefold symmetry of carboxylate ligands (right) has been outlined by lines connecting the tert-butyl groups above and below the metal ring plane; b) Schematics of {Cr10 } deposited on Au(111), lateral view. Color code: Cr green, O red, C black (light blue) atoms above (below) the plane of metal atoms ring. The different magnetic properties and anisotropy of ML and TFs molecule interactions produced by small distortions in the angles of Cr7 Ni-bu self-assembled by sublimation on Au(111), non-flat of the Cr–O–Cr alkoxy bridges coupling the Cr3+ ions. wheels deposited on HOPG[70] and Cr7 Ni-thiobu grafted on Remarkably, some of us succeeded in depositing {Cr10 } on a Au(111) by functionalization were discussed in ref. [19]. Finally, Ag(110) single-crystal[20] by UHV sublimation, paving the way for in the study of polycrystalline TFs of Cr2n Cu2 rings,[71] XMCD studying the magnetic properties of these interesting wheels self- was used to locally probe the moment of Cr and Cu ions and assembled onto different substrates. In this work, we present the discuss the question of entanglement. magneto-structural properties of UHV-sublimated {Cr10 } wheels In view of obtaining SMM behavior, ferromagnetic (FM) cou- on Cu(111) and Au(111) metallic substrates, investigated com- pling between the metallic ions is necessary. However, most ho- bining scanning tunneling microscopy (STM), X-ray photoelec- mometallic wheels are AF coupled,[72,73] whereas FM coupled tron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X- instances are much scarcer, for example, {CuII 6 }, {MnII 6 },[43] ray natural linear dichroism (XNLD) and X-ray magnetic circular {NiII 12 }, and {CoII 12 }.[44] Interestingly, the study of the family of dichroism (XMCD). We discuss the field dependence of the mag- Cr3+ wheels [Cr10 (OR)20 (O2 CR′)10 ], (with R′ = Me, R = Me (1), netization and the magnetic anisotropy of the deposited {Cr10 } Et (2); R′ = Et, R = Me (3), Et (4); R′ = CMe3 , R = Me (5), Et molecules as a function of the metallic substrate and coverage. (6)), revealed a remarkable dependence of the magnetic coupling We compare the magnetic behavior of the films with that of bulk with R and R′ ligands.[74] From 𝜒(T) susceptibility measurements {Cr10 }. [74,76] Monte Carlo simulations have been used to rational- on powdered samples, it was concluded that 1 and 5 showed an ize the modified magnetic properties of surface-deposited mono- overall FM behavior (with an estimated average interaction con- layer samples. stant equal to J/kB = +5.3 K for 1, and weaker, J/kB = +0.59 K, for 5), whereas all other compounds showed an overall AF be- 2. Preparation of On-Surface {Cr10 } Samples havior. Inelastic Neutron Scattering (INS) studies of 1 revealed the overall interaction constant had to be an average of five dis- 2.1. Structure of the Initial {Cr10 } Single-Crystals tinct Jij between Cri 3+ -Crj 3+ pairs, and suggested the existence of a zero-field-splitting (ZFS) with a negative anisotropy constant D in {Cr10 (OMe)20 (O2 CCMe3 )10 } (from now on {Cr10 }) single crystals the Hamiltonian, ZFS = D(S2z − S(S + 1)∕3). Subsequent elec- were prepared by a solvothermal reaction using a method pre- tron paramagnetic resonance (EPR) measurements of 5 (here- viously described.[20] The crystal structure was checked by X-ray after {Cr10 })[75] showed this wheel behaves as a SMM with a uni- diffraction and coincided with that described by D.M. Low et al. axial anisotropy, D/kB = −0.045 ± 0.004 K and a cluster S = 9 ̄ [74] The {Cr10 } cyclic structure is formed by 10 (triclinic S.G. P1). ground state. 3+ Cr ions lying close to an equatorial plane (Figure 1). The ring Very recently, we unveiled the origin of this intermediate S = 9 diameter is about 0.96 nm considering two opposite Cr atoms, ground state in {Cr10 }.[76] Its magnetic properties could be ra- while it is close to 1.7 nm when considering the C atom con- tionalized within a model where {Cr10 } is formed by two semi- necting the three methyl groups of the tert-butyl crown. Taking wheels, each with four Cr3+ spins FM coupled, separated by two into account all the atoms of the molecule, it occupies a room of Cr3+ ions AF coupled asymmetrically (vide infra, Figure 9a). INS diameter ≈ 2 nm. Each pair of Cr–Cr ions is bridged by one μ2 - allowed us to confirm this coupling model leading to the S = 9 carboxylate and two μ2 -alkoxide groups. The carboxylate groups ground state, and first excited state S = 8. SMM behavior with are alternatively oriented above and below the {Cr10 } plane, but an activation energy of U/kB = 4.0(5) K was measured, in very all of them point away from the ring. The wheel presents a close good agreement with the expected value U/kB = (D/kB )Sz 2 = to fivefold symmetry. The mean distance between two tert-butyl 3.6(4) K from the EPR experimental value of D. We showed that groups above or below the metal ring is about 1 nm. The alkoxide the unusual behavior of {Cr10 } arises from the asymmetry in the groups between nearest neighbor (n.n.) Cr3+ ions point toward Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (3 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 2. STM images of {Cr10 } monolayers on Au(111) (top) and Cu(111) (bottom). The existence of mosaicity is outlined in (a,b). The wheels’ close to fivefold symmetry close-packed model is superimposed on high resolution STM images (e,f). The quasi-hexagonal unit cells are outlined on the large-scale images (c,d). a) 150 × 150 nm2 , 1200 mV, 0.6 nA; b) 200 × 200 nm2 , 1000mV, 1.2nA; c) 50 × 50 nm2 , 1300 mV, 0.200 nA; d) 50 × 50 nm2 , 875 mV, 1.500 nA; e) 10 × 10 nm2 , 1300 mV, 0.200nA; f) 10 × 10 nm2 , 1200 mV, 0.092 nA. and away from the wheel, respectively, with one lying above and of the {Cr10 }/Au(111) and {Cr10 }/Cu(111) monolayers in Figure the other below the metal plane (Figure S1, Supporting Informa- 2c,d reveal that the wheels self-assemble in an ordered hexago- tion). nal structure, as shown by the unit cells outlined in the images Since there is only one {Cr10 } molecule per unit cell the crys- in blue. The analysis of many STM images yields a hexagonal tal can be viewed as packed in columns with all the molecules unit cell of dimension 2.3 ± 0.1 nm, similar in both substrates, showing the same orientation. Although the stacking direction within the experimental error. is parallel to the crystallographic a-axis, the axis perpendicular to The higher resolution images (Figure 2e,f) show that the the molecule’s mean plane (z-axis) is ≈35°away from the a-axis. wheels retain their structure and exhibit a close to fivefold sym- Alternatively, the stacking of molecules can be visualized as layers metry: at each {Cr10 } molecule position, the black pentagons ev- of molecules at slightly different heights with a common z-axis idence the bright spots attributed to the tert-butyl groups of the perpendicular to the molecular ring’s plane, but with slight rel- carboxylate ligands above the equatorial metal plane and the sec- ative shifts between layers. The average distance between these ond blue pentagon, rotated by 36° with respect to the former, can layers of molecules is ≈0.8 nm (Figure S1, Supporting Informa- be ascribed to the tert-butyl groups below the metal plane. The av- tion). erage pentagon side length is close to 1 nm, which agrees with the distance of tert-butyl groups below or above the equatorial plane of the molecule (Figure S1, Supporting Information). The close- 2.2. Preparation and Structure of Substrate-Supported {Cr10 } packed, quasi-hexagonal 2D network, shown by the high reso- lution STM contrast is very similar to that earlier described for {Cr10 } molecules were successfully sublimated onto Au(111) and monolayer {Cr10 }/Ag(110).[20] Cu(111) single-crystal substrates, at conditions similar to those The large scale STM images reported in Figure 2a,b evidences earlier used for the sublimation on Ag(110).[20] {Cr10 } were de- the presence of many hexagonal domains slightly rotated rela- posited from a pyrolytic boron nitride crucible held at tempera- tive to each other. This suggests that the molecules are relatively tures between 520 and 540 K on the Cu or Au substrates, held at free to move on the surface and aggregate with each other in the room temperature, previously cleaned by repeated cycles of 1 keV 2D assembly. The long-range order of the sublimated monolay- Ar+ sputtering and annealing. On-surface {Cr10 } samples of dif- ers in correlation with that of the surface was further investi- ferent thickness could be grown by tuning the deposition time. gated by Low-Energy Electron Diffraction (LEED). In line with Surface coverage was monitored in situ by STM at room temper- the STM observations, the LEED experiments do not show a ature. Figure S2, Supporting Information, shows STM images clear diffraction pattern typical of commensurate overlayers, but of sub-monolayers of {Cr10 } on the two studied substrates. Af- rather diffuse halos. Nonetheless, the comparison of the LEED ter a deposition time of 10′, 1ML coverage of {Cr10 } on Au(111) images for 1ML {Cr10 } on Cu and on Au shows interesting dif- and Cu(111) over large areas were obtained, as shown in the ferences: the pattern obtained on Cu(111) at 28 eV (Figure S3a, large-scale STM images reported in Figure 2a,b. The STM images Supporting Information) shows an hexagonal pattern of rather Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (4 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 3. XPS measurements of the 1ML {Cr10 }/Au(111) sample (top) and 1ML {Cr10 }/Cu(111) sample (bottom) at the C1s region (left), O1s region (center) and Cr2p3/2 region (right). large spots in the same direction of the substrate spots (85 eV), in Figure S6c, Supporting Information), precursor of the {Cr10 } whereas the pattern obtained on Au(111) does not shows any wheel. This compound contains Cr3+ ions with a similar car- overlayer spots at 30 eV (Figure S3b, Supporting Information), boxylic environment, but without methoxy ligands. and also the substrate spots appears quite faint (76 eV). There- The binding energy position of the Cr 2p3/2 peak is in perfect fore, the LEED data suggest a higher level of epitaxial order on agreement with the expected Cr3+ oxidation state. It is important Cu(111) than on Au(111), which probably implies a stronger to note that in the case of {Cr10 } on Cu(111) the Cr 2p3/2 peak is vertical interaction between the molecules and the Cu(111) partially superimposed to the CuLMM Auger signal, thus, to evi- substrate. dence the position of the peak in Figure 3 we report the signal X-ray Photoelectron Spectroscopy (XPS) measurements obtained after subtracting the peak of clean Cu to the peak of further confirm the integrity of sublimated {Cr10 } wheels on 1ML{Cr10 }/Cu(111). Au(111) and Cu(111). The complete XPS analysis of the C 1s, The shape and the binding energy position of these peaks O 1s, and Cr 2p3/2 peaks performed on 1ML samples deposited are very similar regardless of the metal substrate, and quanti- on both surfaces is reported in Figure 3 and Supplementary tative analysis of C 1s, O 1s, and Cr 2p3/2 peaks, reported in (Figures S4 and S5, Supporting Information). In the C 1s region, Figures S4 and S5, Supporting Information, yield C/O, C/Cr, and the XPS signal is deconvoluted in three peaks in accordance with O/Cr atomic ratios in agreement with intact {Cr10 }. Moreover, the molecular structure: CI ascribed to aliphatic carbon in the the XPS peaks observed in monolayer samples were coincident tert-butyl groups (–C4 H9 ), CII associated to oxygen-bound car- with those found in bulk {Cr10 }, as shown in Figures S6 and S7, bon of methoxy groups (–OCH3 ), and CIII the electron-depleted Supporting Information, further confirming the integrity of the carbon of carboxylate groups (COO). The area ratio of the three molecules. components is 4CI :2CII :1CIII in agreement with the expected Additionally, to verify the integrity of {Cr10 } in multilayer sam- nominal values for intact {Cr10 } wheels. ples, XPS analysis was conducted on samples with increasing O 1s peaks have been fitted with two components with the thicknesses. In Figure S8, Supporting Information, we report for same area in agreement with the stoichiometry of the two types comparison the XPS C 1s, O 1s, and Cr 2p3/2 peaks of the mono- of oxygen present in the molecules respectively in the carboxylate layer and two different multilayer coverages on Cu(111). The in- (OIII ) and in the methoxy (OII ) groups, 1OIII :1OII . The OIII bend- crease of the peaks area is linear with the deposition time. More- ing energy (BE = 531.2 eV) has been assigned to the carboxy- over, the shape of the peaks and the area ratio is constant at differ- late groups by comparison with the O 1s high resolution spec- ent coverages, indicating that the molecules keep their structure trum of {[Cr6 F7 (O2 CCMe3 )10 ][NH2 Et3 ]3 }2 [77] (O 1s, BE = 531.5 eV also for thicker films. Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (5 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de 3. Magnetic Properties of Deposited {Cr10 } The magnetic properties of monolayer samples on two different substrates, 1ML {Cr10 }/Cu(111), 1ML {Cr10 }/Au(111), and a thin 14ML {Cr10 }/Au(111) sample, were studied under synchrotron radiation at low temperature (T = 3.4 K). In situ STM experi- ments were performed to guarantee the formation of extensive monolayers. First, to assess the electronic anisotropy of Cr3+ ions in the de- posited {Cr10 } molecules we investigated the X-ray Natural Linear Dichroism (XNLD) by measuring the X-ray Absorption Spectra (XAS) using vertically (V) and horizontally polarized light (H), with respect to the synchrotron ring plane, at the Cr L2,3 edge, under grazing incidence (𝛾 = 70°) and zero applied field. The electric field vector E of linearly polarized X-ray acts as a “search- light” for the valence band empty states in different directions of the atomic volume. For the selected experimental geometry, EV lies in the sample substrate plane for V, while EH is mostly out-of- plane (see sketch in the inset of Figure 4). The XNLD signal is de- fined as the difference, 𝜎 V – 𝜎 H , between the cross-sections mea- sured with vertically polarized (𝜎 V ) photons and with horizontally polarized (𝜎 H ) photons. Figure 4a shows the XNLD signal mea- sured for the 1ML {Cr10 }/Cu sample, normalized to the isotropic adsorption spectrum 𝜎 ISO , calculated from the measured 𝜎 V and 𝜎 H values at 𝛾 = 70°. (A similar XNLD signal was measured for the 1ML {Cr10 }/Au sample, see Figure S10, Supporting In- formation). A small XNLD signal of 3.6% is detected, which is of pure electronic origin, as measurements were performed under H = 0. We simulated the observed XNLD signal within the framework of the ligand field multiplet approach implemented in Quanty- Crispy code (see Supporting Information S5 for details). The {Cr10 } molecule can be viewed as a ring of [CrO6 ] octahedra con- Figure 4. a) Experimental (black) and calculated (red) isotropic XAS, de- fined as 𝜎 ISO = (𝜎 ∥ + 2𝜎 ⊥ ) /3, with 𝜎 V = 𝜎 ⊥ , and 𝜎 H = (sin 2 𝛾)𝜎 ∥ + nected by two sharing O2− ions. Each of the octahedra is oriented (cos 2 𝛾)𝜎 ⊥ for 𝛾 = 70° (see Supporting Information S4 and ref. [23] for such that one of its trigonal axes is nearly parallel to the normal n details). Inset: sketch of the experimental configuration for XNLD experi- to the molecule’s mean plane and substrate (Figure S9, Support- ments: H is parallel to the synchrotron ring plane, and V perpendicular to ing Information). Thus we simulated the XNLD of a single Cr3+ it; b) experimental (black) and calculated (red) XNLD spectra (𝜎 V − 𝜎 H ), under trigonal symmetry (C3v ) for incident X–ray k(1,0,0) and po- normalized to the isotropic signal, for sample 1ML {Cr10 }/Cu(111). larizations EH (0,0,1) and EV (0,1,0). As shown in Figure 4, the ex- perimental XNLD and isotropic XAS could be nicely reproduced considering a cubic crystal field splitting of Dq = 0.195 eV and small distortions from the octahedral symmetry, D𝜎 = 0.045 eV, D𝜏 = 0.01 eV. This value of Dq is quite comparable to the value In the bulk the {Cr10 } wheels are packed along the a-axis, with 0.2182 eV given by Mc Clure[78] for Cr3+ . The same simulation pa- the distance between adjacent molecules being 0.987 nm. If the rameters could be used to satisfactorily reproduce the XAS and deposited molecules would follow the same structure, the wheels XMCD spectra at 6 T (vide infra). The slight overestimation of would not be placed just on top of those of the first layer, but the calculated XNLD signal may be attributed to the existence of slightly shifted, with the distance between layers being ≈0.8 nm an angular distribution of the ten different single-ion Cr3+ trig- (see Figure S1b, Supporting Information). Unfortunately, high- onal axes from the normal to the molecular plane, and disorder resolution STM images of 2ML samples could not be obtained, induced by the mosaicity within the deposited 1ML. as the tip scanning produced the movement of the top-most layer; To characterize the magnetic properties of sublimated {Cr10 }, hence, the actual packing of {Cr10 } in multilayer samples could XAS measurements under left (C− ) and right (C+ ) circularly po- not be ascertained. larized light and X-ray magnetic circular dichroism (XMCD) at In the remaining of this work, we will discuss magnetic results the Cr L2,3 edge were performed under a magnetic field of μ0 H = obtained on monolayer (1ML) samples of {Cr10 } on Cu(111) and 6 T at T = 3.4 K, for different beam incidence angles with re- Au(111), to investigate the influence of the substrate, and a sam- spect to the substrate normal (𝛾). Figure 5a–c shows the XAS and ple with an estimated number of 14 monolayers (14ML) to study XMCD spectra measured for the 1ML {Cr10 }/Cu, 1ML {Cr10 }/Au, the influence of the layer addition on the magnetic properties of and 14ML {Cr10 }/Au samples in normal incidence (𝛾 = 0°). Qual- deposited {Cr10 }. itatively, the spectra for all the samples are very similar. The XAS Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (6 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 5. Cr L2,3 XAS (top) and XMCD (bottom) normalized spectra measured at T = 3.4 K, μ0 H = 6 T in normal beam incidence (𝛾 = 0°) for a) 1ML {Cr10 }/Cu(111), b) 1ML {Cr10 }/Au(111), and c) 14ML {Cr10 }/Au(111). In (a), the simulated XAS and XMCD spectra are also shown, see Supporting Information S5 for details. A double-step function was added to the simulated whiteline to enable the comparison with the experimental XAS. sure (CShM) value of 0.885, calculated with SHAPE2.1[79] from the crystallographic data of the bulk {Cr10 } sample.[74] To search for magnetic anisotropy of {Cr10 } supported on sur- face, we measured the XAS and XMCD spectra as a function of 𝛾. A small angle dependence in the XMCD spectrum is observed when passing from 𝛾 = 0° to 𝛾 = 70°, accompanied by a change in the XAS amplitude, so that the magnetic anisotropy of the nor- malized XMCD spectra is practically unobservable. A similar re- sult was previously reported for {Cr7 Ni} wheels.[70] Quantitative magnetic moments were obtained by applying the sum rules to the angle-dependent XAS and XMCD spectra. For the number of holes in the 3d band we considered the nom- inal value Neff = 7 for Cr3+ . The dipolar term 〈Tz 〉 is expected Figure 6. Angular dependence of the total magnetic moment, mtot (𝛾) at to be negligible for Cr3+ ions in Oh symmetry;[80] however, to T = 3.4 K, μ0 H = 6 T, for the three studied {Cr10 } deposited samples. avoid this pre-assumption, we calculated the effective spin mo- The spin correction factor SC = 1.75 in Equation (2) was applied. ment mseff (𝛾) = mS − 7mT (𝛾), and deduced only a posteriori the maximum contribution of the 7mT dipolar term. Thus, we deter- mined the orbital magnetic moment (mL ) and effective magnetic line at the L3 edge displays a shoulder (574.11 eV), 3 peaks at moment (mseff ) per Cr3+ ion as: 575.26, 576.45, 577.24, and a satellite at 579.21 eV, while L2 shows a main peak at 584.24 eV and a smaller one at 586.04 eV. These mL 4qNeff =− (1) features are typical of Cr3+ in an octahedral environment, as pre- 𝜇B 3r viously observed.[68] The XMCD signal exhibits two distinct peaks eff ms (6p − 4q) Neff below the large negative L3 peak. The XAS – XMCD spectra could =− SC (2) 𝜇B r be nicely reproduced by multiplet simulations (see Figure 5a), us- ing the same parameters used to simulate the XNLD. This result where r is the integral of the background-subtracted absorption agrees with the minor distortions of the octahedral coordination spectrum, A and B are the intensities of the dichroic signal at geometry of Cr3+ ions, which shows a continuous shape mea- L3 and L2 edges, respectively, and p = A and q = A + B. The Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (7 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 7. Field dependence of the total magnetic moment per Cr3+ ion at 𝛾 = 0° and 𝛾 = 70° beam incident angles, T = 3.4 K, for sample 1ML {Cr10 }/Cu. spin correction SC is required to account for the partial L2 and L3 mixing in Cr.[81] We used a value SC = 1.75, which had been pre- viously validated for Cr8 and Cr7 Ni-piv[68] and applied to other Cr rings,[69,70,82] given that the Cr L2,3 absorption spectrum of {Cr10 } is identical to that of the aforementioned compounds. The value of the intrinsically isotropic spin moment was determined as the value of the effective moment at the so-called “magic angle,” at which the dipolar term cancels in paramagnetic systems, that is, ms = mseff (𝛾 = 54.7◦ ).[83,84] Then, the angular dependence of the total magnetic moment was calculated as: mtot (𝛾) = ms + mL (𝛾). From this quantitative analysis the following conclusions can be drawn. First, the orbital magnetic moment is negligible, within the experimental error, which agrees with mL ≈0 for Cr3+ in {Cr8 } and {Cr7 Ni} wheels reported by Corradini et al.[68] Sec- Figure 8. a) Comparison of the magnetization of the {Cr10 } wheel as a ond, the dipolar contribution 7mT is estimated to be at most function of the applied field, M (H) = 10mtot (H), for samples 1ML/Cu, ≈10% of mseff , so mT is within the experimental uncertainty. And 1ML/Au, and 14ML/Au obtained by XMCD at T = 3.4 K, μ0 H = 6 T. For finally, there is no detectable local anisotropy at this temperature, comparison, the magnetization measured for the bulk at the same tem- as can be observed in Figure 6, from the hardly noticeable angle perature (3.4 K) is shown; b) Comparison of the susceptibility-temperature product, 𝜒T(T) at 0.1 T, for the bulk and {Cr10 } layered samples at 3.4 K. dependence of mtot (𝛾). The total magnetic moment values deter- mined for the three studied samples at 3.4 K and 6 T are summa- rized in Figure 6. enable the observation of SMM behavior at the temperature of ex- The magnetic field dependence of the dichroic signal periment (3.4 K). We note that the SMM behavior in (Fe4 ), with XMCD(H), was measured at T = 3.4 K changing the helic- an energy barrier of U/kB ≈ 16 K larger than that measured for ity of the beam at fixed incident photon energy corresponding {Cr10 } in bulk (U/kB ≈ 4 K)[76] or that predicted for monolayers to the maximum XMCD signal (576 eV) and ramping the mag- (U/kB ≈1.3-3.1 K, vide infra), could only be observed at sub-kelvin netic field from 6 to −6 T, and from −6 to 6 T. Measurements temperatures.[34] were carried out in normal (𝛾 = 0°) and grazing (𝛾 = 70°) inci- Figure 8a compares the field-dependence of the total magnetic dence. The absolute y-axis value was determined by scaling the moment per Cr3+ ion measured in normal incidence (𝛾 = 0°) XMCD(H) cycles to the total magnetic moment obtained from for the three on-surface studied samples, together with the curve the sum rules at 6 T. Figure 7 shows the XMCD(H) curve for measured for powder {Cr10 } at the same temperature (3.4 K) for the 1ML {Cr10 }/Cu sample, while the data for the 1ML {Cr10 }/Au the sake of comparison. The monolayers show magnetizations and 14ML {Cr10 }/Au samples are collected in Figure S12, Sup- clearly lower than that of the {Cr10 } molecules in the bulk mate- porting Information. The magnetization exhibited no hysteresis rial, except at the highest applied magnetic field values, while the in any of the three studied samples. Moreover, no difference was 14ML {Cr10 }/Au sample displays an intermediate magnetic be- observable between the mtot (H) curves at 𝛾 = 0° and 𝛾 = 70° at havior. (It is noted that the experimental curve reflects the mag- any magnetic field value, within experimental resolution. These netic behavior of the outermost 5–6 layers of the 14ML sam- results indicate that the {Cr10 } cluster anisotropy is too small to ple, as XMCD in TEY probes only a few nanometers into the Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (8 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de sample). Figure 8b compares the susceptibility-temperature where the first term describes the exchange interaction coupling, product, 𝜒T, for the layered samples at 0.1 T and 3.4 K (obtained with 〈i, j〉 denoting the interactions between the nearest neighbor from the initial slope of the M(H) curves) with the 𝜒T(T) curve Si = 3/2 spins in the {Cr10 } ring; spins are numbered from 1 to reported earlier for bulk {Cr10 }. It is clear that deposition onto the 10 following the numbering code in Figure 9a, previously used surface brings along a reduction of the overall FM behavior. in Ref. [76] The second term accounts for the zero-field splitting The significant observed changes in the M(H) and 𝜒T(T) produced by the single-ion anisotropy of each Cr3+ site (Di ), as- curves must be related to the accommodation of the {Cr10 } suming negligible rhombic term, and the last term is the Zeeman molecules onto the substrate and the subsequent modification splitting with g ≈ 2. of Cr–O–Cr angles on which exchange coupling constants are The single-ion anisotropy parameter was kept unmodified very dependent.[76,85] Moreover, this change seems to be progres- with respect to the Di /kB = −0.31 K value previously deduced sive as additional layers are deposited, with an apparent tendency from experimental results for bulk {Cr10 },[76] under the reason- toward the magnetic behavior of the molecules in the bulk crys- able assumption that the local environment of each Cr3+ ion is talline form. As it will be shown below, the low values of the mag- not expected to suffer large variations upon on-surface deposi- netization at low and intermediate magnetic fields with respect to tion. The total cluster anisotropy Da and rhombic term Ea in the single crystal results can be explained consistently as a slight each spin configuration were calculated following the method de- change in the exchange interactions giving rise to a lower spin scribed by D. Bencini and A. Gatteschi,[88] (see details in section ground state. S8, Supporting Information). On the other hand, only a small difference between the In order to deal with such a complex system of 10 Si = 3/2 mtot (H) curves measured for the 1ML {Cr10 }/Au(111) and 1ML spins in a ring, we used the classical Monte Carlo (MC) method {Cr10 }/Cu(111) is observed. Their XAS lineshapes of the MLs on implemented in ALPS[89,90] to calculate M(H) curves, exploring Au(111) and Cu(111) are similar, indicating that the nature of the changes in exchange interaction constants Jij starting from the substrate does not affect the electronic structure of the Cr3+ ions. previously proposed distribution of these constants for the bulk Indeed, due to the local nature of the molecule-substrate bond- system.[76] Different distributions of values of nearest neighbor ing, which does not involve any charge transfer, a direct influence Jij along the wheel lead to different ground state spin configura- of the ligand-substrate bond on the properties of the metal core tions. However, only spin configurations with total spin S = 0, is not expected. However, since the magnetic properties of the 3, 6, 9, 12, and 15 are possible if the local magnetic moments {Cr10 } wheels seem to depend on their capacity to accommodate are assumed to be all (almost) parallel or antiparallel. The con- to the substrate, differences in the atomic termination of the sur- figurations with S = 15 and S = 12 can be excluded as their face can produce structural changes along the chromium wheel, expected magnetization values are too high with respect to those which can result in slightly different magnetization curves. The of the present monolayer even at low fields. The spin configura- distance between n.n. atoms in the two investigated metallic sub- tion with S = 0 must be also discarded, despite many 3d-wheels strates is 0.288 nm for Au(111)[86] and 0.255 nm for Cu(111),[87] reported so far display this AF ground state, because it yields to hence a difference of ≈15% in the substrate lattice may be enough a close to linear field dependence of the magnetization, incom- to produce distortions in the {Cr10 } wheel affecting the total mag- patible with the experimental results (see Figure S13, Supporting netic moment. Information). In the case of bulk {Cr10 },[76] we could simulate satisfacto- rily the magnetization M(H) and susceptibility 𝜒T(T) curves of 4. Discussion single-crystalline and powder samples within a S = 9 ground state spin configuration model (8 spins up ↑, 2 spins down 4.1. Monte Carlo Simulation of Molecular Spin Configurations ↓), with an axial cluster anisotropy of Da / kB = − 0.033 K (Ea / Da = 0.15), assuming no inter-molecular interactions. The n.n. In the following we will try to rationalize the observed changes interactions were taken as identical and FM (JFM / kB = 2.0 K), ex- on the magnetic properties of the {Cr10 } wheels upon deposi- cept around two Cr3+ ions on opposite sides of the wheel, denoted tion onto metallic surfaces. The accommodation of the molecules Cr3 and Cr8 (see Figure 9a for numbering), which were consid- onto the surface is expected to affect more profoundly the more ered AF and asymmetric (J23 / kB = J78 / kB = − 2.0 K ≠ J34 / kB flexible molecular parts, that is, the outer tert-butyl ligands, than = J89 / kB = − 0.25 K, Di / kB = − 0.31 K).[76] The same model the wheel’s skeleton of [CrO6 − ] octahedra connected by edges. and exchange interaction constants used for the bulk were tested However, slight changes in the Cr–O–Cr angles and distances against the experimental data for the 1ML/Au at 3.4 K (see Figure may provoke important modifications in the exchange coupling 9b). The failure of such a model is obvious. Variation of the set constants.[76,85] In contrast, the single-ion anisotropies at the Cr3+ of the exchange interaction constants in the S = 9 model was ions are expected to be much less affected, as they are mainly de- unable to bring the simulations near to the experimental curve, termined by the full octahedral cage. that is, in every case the simulated M(H) was too high. Having in mind this central idea, we will explore the conse- In contrast, a spin configuration with S = 3 yields mag- quences of changes in the exchange coupling constants in order netic curves that reproduce reasonably well the experimental re- to simulate the M(H) curves measured for the ML samples within sults. Indeed, for the case of S = 3, two different spin con- the framework of the cluster multispin Hamiltonian in terms of figurations can be considered, denoted S3A and S3B , both re- the single-ion spins Si : specting the inversion symmetry of the molecule. The exper- ∑ ∑10 ∑10 imental data could be well explained within a S3A configura- CL = −2 ⃗ ⋅ S⃗ j + J S Di S2i,z + g𝜇B S⃗ i ⋅ H ⃗ (3) tion (Figure 9a,b), with the set of exchange coupling constants ⟨ij⟩ ij i i=1 i=1 Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (9 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Figure 9. a) Sketch of the ground state of the single spins configurations for total spin S = 9, 6, and 3 (S3A ). The numbering of the Cr3+ spins of the S = 9 configuration is that of Ref. [76] and it is also used for the S = 6 and S = 3 cases; b) Magnetization M(H) curves for the 1ML/Au, 1ML/Cu, and 14ML samples and MC simulations for different spin configurations yielding to a total spin ground state S = 9, 6, and 3; c) Plot of the sets of exchange coupling constants Jij for the three cases displayed in (b) following numbering shown in (a). J12 / kB = J45 / kB = J67 / kB = J9,10 / kB = − 1 K, J23 / kB = J34 / kB For {Cr10 } in the bulk samples, we had previously shown[76] a = J78 / kB = J89 / kB = J9,10 / kB = − 2 K, J56 / kB = J10,1 / kB = + correlation between the exchange coupling constants Jij and the 3 yielding a mean field value 〈J〉/ kB = − 0.6 K (see Supporting inner (𝜃 inn ) and outer (𝜃 out ) Cr–O–Cr angles connecting i and j Information S7). On the contrary, using the S3B configuration the neighboring Cr3+ ions. In particular, the AF exchange coupling experimental results cannot be explained by simulations under constants around the ion labeled Cr3 corresponded to positive any set of Jij constants (a sample of results is shown in Figure 𝜃 inn − 𝜃 out values, while the remaining FM interactions were as- S14, Supporting Information). sociated to negative 𝜃 inn − 𝜃 out . Therefore, in the case of the {Cr10 } The proposed distribution of Jij values along the wheel for S3A monolayers, the sign flip with respect to the bulk case in the Jij includes only small departures from the values determined for interactions for the Cri-Crj couples neighbors to those of Cr3-Cr2 the bulk system. We may note that the mean field value 〈J〉 is and Cr3-Cr4 could be understood as modifications of the corre- negative, in contrast to the case of the bulk sample, where it sponding Cr–O–Cr angles with a sign flip in the difference 𝜃 inn − was positive, 〈J〉/kB = +0.59 K, according to susceptibility mea- 𝜃 out . Finally, we may note from Figure 9c that the proposed set of surements. Even though for the monolayer samples the full sus- Jij values displays a larger symmetry than that of the molecule in ceptibility curve 𝜒T(T) could not be measured directly, the 𝜒T the bulk compound (P1), ̄ with an additional mirror plane, which determined at 3.4 K is deduced from the slope of the M(H) is introduced by the (111) surface of the metallic substrates. curve at that temperature, and it is smaller than the expected The molecular anisotropy calculated for the S3A configuration, high T bulk value at 300 K (Figure 8b), supporting the existence using single ion values Di / kB = − 0.31 K and Ei = 0 as in the of an overall AF mean field value 〈J〉, as predicted by the MC MC simulations, is Da / kB = − 0.14 K, Ea / Da = 0. Alternatively, simulations. recalling that we are assuming that the n.n. environment of the Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (10 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de Cr3+ ions may be considered as unchanged upon deposition on carboxylate and diethanolamine ligands. In the latter case, it the metallic substrates, we can make use of the ab initio results of was shown that the use of carboxylate/alkoxide [CrIII (OR)2 (OCO)] the bulk compound[76] to compute the molecular anisotropy for bridging units with a peculiar orbital counter-complementarity the S3A configuration, which produces the compatible values of effect favored FM coupling. This is precisely the type of het- Da / kB = − 0.131 K and Ea / Da = 0.14. The predicted molecular eroleptic bridging unit used in the {Cr10 } wheels, combining two anisotropy is larger in the 1ML than the bulk {Cr10 }, (Da / kB = alkoxides (OMe) and one carboxylate (O2 CCMe3 ), and thus the − 0.033, Ea / Da = 0.15), as a consequence of the change in the result provided a rationale for the FM interactions observed in spin configuration produced by the modification of the exchange this member of the [Cr10 (OR)20 (O2 CR′)10 ] family. Moreover, the coupling constants along the wheel. DFT calculations by Fraser et al.[91,92] predicted high sensitivity We have also tested the configuration with ground state S = of the sign and magnitude of the coupling constant J on even 6, the closest to that found for the bulk compound (S = 9). We tiny structural variations in the Cr–O–Cr angles. Very recently, also found a set of Jij values, with mean field value 〈J〉/ kB = − we demonstrated that very small differences (<1°) between the 0.6 K as in the S = 3 configuration, for which the MC simulated Cr–O–Cr inner angles of bulk {Cr10 } give rise to positive and neg- M(H) curve approaches the experimental data (Figure 9a,b). This ative exchange coupling constants within the molecular ring in magnetic ground state would not be compatible with the point such a way to produce a wheel’s coupling scheme that is at the symmetry group P1̄ of {Cr10 } in bulk, although the point sym- origin of the intermediate S = 9. In this respect, it is interesting metry of the molecule may decrease upon deposition, losing the to note that these molecular wheels crystallize with different unit inversion symmetry. Since we do not have direct experimental ev- cell according to the employed method, which can lead to slightly idence for the actual symmetry group of {Cr10 } on surface, being different Cr–O distances and Cr–O–Cr angles upon different crys- prudent we can say that although the S = 3 spin configuration tallization conditions. For instance, when {Cr10 } is recrystallized is more likely, the S = 6 one cannot be ruled out. ̄ but slightly from toluene,[20] it yields the same space group (P1), From Figure 9 it could be argued that the S = 3 configu- different unit cell than that found in the single crystals obtained ration should be assigned to the 1ML {Cr10 }/Au(111) sample, from the solvothermal synthesis.[74] Slightly different inner and while that of S = 6 to the Cu(111) substrate. However, this outer Cr–O–Cr angles are found when comparing the two {Cr10 } assignment would be probably an overinterpretation of the re- crystal structures. Consequently, the structural parameters of the sults, as we must be aware of the limitation of the MC simula- {Cr10 } molecules, which are already very sensitive to the crystal tions: first, we must consider the simplification in the Di single- packing, may be further stressed when deposited onto the sub- ion anisotropy constants, and second, that the simulation curves strate or in a thin-film. shown in Figure 9b do not come out from an actual least-squares Previous studies on {Cr7 Ni} wheels as single-crystals and as fit of the Jij , but from a consistent set of these values starting from sub-monolayer samples have shown changes in the values of the bulk case. the exchange coupling constants, but the spin coupling scheme The magnetization curve of the 14ML sample lays between along the wheel is preserved upon the deposition on metallic those of the 1ML and bulk single crystals (Figure 8a), which sug- substrates.[68,69,70] For the Cr7 Ni-bu antiferromagnetically cou- gests that the modification of the spin configuration may vary pled wheel sublimated on Au(111), the total magnetic moment from one layer to another, approaching the bulk magnetic behav- curves, mtot (H), of Cr3+ and Ni2+ measured at different temper- ior as the number of layers increases. Since the XAS spectra with atures on a 1ML sample were above those of a polycrystalline soft X-rays, as in the present measurements, will only probe a few thin-film (a bulk sample used as reference), for both types of nm of {Cr10 } layers which would be separated by distances of not ions,[70] in contrast to our results on {Cr10 }. Those results were more of 1 nm, the derived M(H) curve shows the magnetic be- explained within a single ion spin Hamiltonian model, includ- haviour of only the uppermost ≈6 layers. We have not attempted ing the single-ion anisotropies (Di ), and two different coupling a simulation of these experimental results, since it would imply constants accounting for the JCrCr and JCrNi exchange interactions. over parameterizing the model by introducing a distribution of The values of the AF coupling constants in the thin film (JCrCr / spin configuration changes with the layer number. kB = − 8.47 K, JCrNi / kB = − 9.81 K, expressed as  = ∑ −2 Jij Si Sj ), were reduced by a 22% when the molecules were deposited as 1ML, whereas it was not necessary to change the 4.2. Comparison with Literature Results values of the anisotropies (DCr / kB = − 0.35 K and DNi / kB = − 4.06 K) to fit the experimental results. The structure of the low- The extraordinary dependence of the magnetic properties of energy levels of the grafted rings was unaltered by the reduction {Cr10 } on structural changes imposed by accommodation to the of the Jij values, and only a small compression of the energy spec- surface demonstrated in this work is consistent with previous trum was observed because of the reduced exchange interactions. works reported in the literature. In fact, the early study of the Additionally, functionalization was studied in these Cr7 Ni rings; family of Cr3+ wheels [Cr10 (OR)20 (O2 CR′)10 ] in powder reported the Cr7 Ni-3tpc wheel showed a reduction by 15% of JCr-Cr leav- by Low et al.,[74] already revealed an extraordinary dependence of ing the JCrNi unchanged with respect to the pristine (unfunction- the magnetic behavior on the type of ligand, with some members alized) Cr7 Ni-piv bulk compound,[69] whereas sublimation onto of the family showing overall FM magnetic behavior and others Au(111) produced a further reduction of the AF coupling con- presenting AF behavior. In recent years, H.W.L. Fraser et al.[91,92] stants, amounting JCrCr / kB = − 6.09 K, JCrNi / kB = − 2.9 K.[69] studied the dependence of exchange coupling constants on struc- For Cr7 Ni wheels grafted to Au(111) through different functional tural changes in a large family of Cr3+ -Cr3+ dimers coordinated ligands, the Ni dichroic signal was larger the stronger the ligand- by di-alkoxo bridges using DFT calculations and 𝜒T(T) data and substrate bonding, suggesting that the distortion of the wheel is Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (11 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmatinterfaces.de modulated by the strains due to ligands-substrate bond. In that [Cr10 (OR)20 (O2 CR′)10 ] onto substrates opens an interesting play- work, DFT calculations were performed to estimate changes in ground to study, and eventually tune, the magnetic properties of the Jij constants under possible ring distortions. For a dilation cyclic clusters with non-zero spin for applications. The extreme of 5% in the Cr–Cr(Ni) distance (within the possible range for an sensitivity of the magnetic configuration to minor distortions of isolated molecule compared to a molecule in a molecular crystal), the intramolecular conformation might be exploited to control the calculated JCrCr and JCrNi exchange coupling constants were magnetism by external stimuli such as electric fields, pressure, 29% (60%) smaller as compared to the coupling constants in the solvents etc. not-dilated structure. In addition, a parabolic function of the JCrCr coupling as a function of the bond angles in the carboxylate and fluorine bridges was argued. Like in the above examples, the magnetic properties of {Cr10 } 6. Experimental Section are modulated by wheel distortions produced in the layered con- {Cr10 (OMe)20 (O2 CCMe3 )10 } single crystals were prepared by a solvother- figuration, modifying the Cr–Cr coupling constants. However, in mal reaction using a method previously described,[20] starting from contrast to the Cr7 Ni wheels, the deposition of {Cr10 } on metallic {[Cr6 F7 (O2 CCMe3 )10 ][NH2 Et3 ]3 }2 . The crystal structure was checked by surfaces induces an enhancement of the AF interactions involv- X-ray diffraction and coincided with that earlier described.[74] ing flipping two of the exchange coupling constants from FM to Initial deposition experiments were performed in an ultra-high vac- AF, which produces a change in the total spin of the molecule uum (UHV) chamber (base pressure of 10−10 mbar), equipped with an Omicron VT Scanning Tunneling Microscope (STM), Low-Energy Electron (S = 3 or S = 6), with respect to the total spin observed in the Diffraction (LEED), and X-ray Photoelectron Spectroscopy (XPS) analysis molecules in the bulk (S = 9). tools at Padua University. XPS measurements were performed with a Thermo Scientific ES- 5. Conclusions CALAB QXi spectrometer employing a monochromatic Al K𝛼 X-ray source (1486.6 eV) with a 650 μm × 200 μm spot size. For measurements of Large cluster {Cr10 } wheels of formula {Cr10 (OMe)20 - the bulk, a few mg of {Cr10 } single crystals were mounted on double-side sticky carbon tape attached to the sample holder. The operating vacuum (O2 CCMe3 )10 } were successfully deposited on Au(111) and conditions of the analysis chamber were better than 10−8 mbar. Survey Cu(111) substrates by direct sublimation in UHV. STM images scans were measured in a binding energy range of 0–1350 eV using a con- show the self-assembly of the {Cr10 } wheels into monolayers stant pass energy of 200 eV at 1.0 eV/step. High-resolution spectra were with hexagonal symmetry. XPS analysis demonstrates the in- recorded using a constant pass energy of 15 eV at 0.1 eV/step. The high- tegrity of the deposited molecules. {Cr10 } represents the largest resolution XPS spectra were used for assessment of the elemental state metal-organic cluster that has ever been sublimated. as well as for quantification using the sensitivity factors provided by the From XNLD experiments it was proven that there is a small Thermo Fischer Avantage software and after background correction the with smart-background function implemented in the same software. Fit- anisotropy of electronic origin at the Cr3+ ion with distorted oc- ting of C1s and O1s was performed with the Thermo Fischer Avantage tahedral oxygen coordination. The magnetic-field dependence software. The BE shifts were corrected by assigning to the C1s peak asso- of the magnetic moment measured by XMCD in 1ML samples ciated with aliphatic carbon a value of 284.8 eV. on Au(111) and Cu(111) substrates is slightly different, showing During the synchrotron experiment, samples of {Cr10 } were sublimated the sensitivity of the magnetic properties of {Cr10 } to structural onto metallic substrates in a vacuum chamber in UHV conditions, similar changes imposed by surface deposition. Moreover, mtot (H) mea- to the setup used in Padua, and characterized in situ by STM to assess the formation of monolayers. The deposited samples, mounted on the STM sured for 1ML samples is lower than that of a thin 14ML sam- plate, were then transported through a system of UHV tubes to the XAS- ple, which is in turn below that of the bulk compound. This XMCD experimental chamber. This work reported results on three different result suggests that structural stress imposed by self-assembly samples: a monolayer sample on a polycrystalline Au(111)/mica substrate onto the substrate relaxes in the successive layers. Thus, the mag- (denoted 1ML {Cr10 }/Au), a 14-layer sample deposited onto an Au(111) netic properties of the multilayer samples progressively approach single crystal (denoted 14ML {Cr10 }/Au), and a monolayer deposited onto those of the bulk as coherence with the substrate is lost. a Cu(111) single crystal (1ML {Cr10 }/Cu). The molecules were sublimated in UHV on the metal substrates from a PBN a crucible held at temperature Monte Carlo calculations allowed us to rationalize the mag- of 250 °C. netic field dependence of the magnetization observed in the on- Soft X-ray absorption and magnetic dichroism experiments at the Cr surface 1ML {Cr10 } samples in terms of a change in the spin L2,3 edge were carried out at the BL29 BOREAS beamline in ALBA syn- coupling configuration along the wheel, plausibly induced by on- chrotron radiation facility, provided with a high-field vector magnet (HEC- surface deposition, changing the original total spin ground-state TOR) end station. The sample was oriented with the substrate perpendic- from S = 9 in the bulk compound to S = 3 or S = 6 in the ular to the synchrotron ring. Experiments were performed with the X-ray beam forming an angle between the beam direction and the direction nor- molecules as 1ML samples. Despite the initial total spin of {Cr10 } mal to the plane varying between 𝛾 = 0° – 70°. The detection mode was is reduced upon deposition, the array of self-assembled subli- total electron yield (TEY). mated still supports a non-zero magnetic moment. X-ray linear polarization absorption measurement were performed for The above results emphasize the importance of characteriz- a given incidence angle (𝛾), with the polarization of the electric field E par- ing the on-surface magnetic properties and spin ground-state of allel to the substrate (V), EV , or perpendicular to that direction (H), EH , molecular nanomagnets, as they may be significantly different see inset in Figure 4. In XMCD experiments the incoming light was right from those in the bulk, and from one cluster system to another. and left circularly polarized. XMCD spectra were measured at the maxi- mum available magnetic field (μ0 H = 6 T). The temperature at the sample The present case shows a reduction of the molecules’ ground- position was T = 3.4 ± 0.5 K, as determined by calibration experiments at state total spin upon deposition, but other molecules might dis- BOREAS. play an increase if the bulk exchange coupling constants are In order to avoid sample degradation due to radiation exposure the not very high. The capability of assembling 2D monolayers of beam front-end was closed between measurement runs; experiments were Adv. Mater. Interfaces 2023, 10, 2300146 2300146 (12 of 15) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2023, 20, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300146 by Readcube (Labtiva Inc.), Wiley Online Library on [24/10/2023]. 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