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A Molecular Approach for Engineering Interfacial Interactions in Magnetic-Topological Insulator Heterostructures Journal: ACS Nano Manuscript ID Draft Manuscript Type: Article Date Submitted by the n/a Author: Complete List of Authors: Cuxart, Marc; Catalan Institute of Nanoscience and Nanotechnology Valbuena, Miguel ; Catalan Institute of Nanoscience and Nanotechnology Robles, Roberto; Centro de Fisica de Materiales, Moreno, Cesar; Catalan Institute of Nanoscience and Nanotechnology Bonell, Frédéric; Catalan Institute of Nanoscience and Nanotechnology Sauthier, Guillaume; Catalan Institute of Nanoscience and Nanotechnology Imaz, Inhar; Catalan Institute of Nanoscience and Nanotechnology Xu, Heng; Catalan Institute of Nanoscience and Nanotechnology Nistor, Corneliu; ETH Zürich Barla, Alessandro; Istituto di Struttura della Materia Consiglio Nazionale delle Ricerche Gargiani, Pierluigi; Consorcio para la Construccion Equipamiento y Explotacion del Laboratorio de Luz Sincrotron, Valvidares, Manuel; Consorcio para la Construccion Equipamiento y Explotacion del Laboratorio de Luz Sincrotron, Maspoch, Daniel; Catalan Institute of Nanoscience and Nanotechnology Gambardella, Pietro; Eidgenossische Technische Hochschule Zurich, Materials Valenzuela, Sergio; Catalan Institute of Nanoscience and Nanotechnology Mugarza, Aitor; Catalan Institute of Nanoscience and Nanotechnology ACS Paragon Plus Environment Page 1 of 16 ACS Nano 1 2 3 4 A Molecular Approach for Engineering 5 6 Interfacial Interactions in 7 8 Magnetic-Topological Insulator 9 10 Heterostructures 11 12 ∗,† † ‡ † 13 Marc G. Cuxart, Miguel Angel Valbuena, Roberto Robles, César Moreno, 14 † † † † ¶ Frédéric Bonell, Guillaume Sauthier, Inhar Imaz, Heng Xu, Corneliu Nistor, 15 § k k †,⊥ 16 Alessandro Barla, Pierluigi Gargiani, Manuel Valvidares, Daniel Maspoch, 17 ¶ ∗,†,⊥ ∗,†,⊥ Pietro Gambardella, Sergio O. Valenzuela, and Aitor Mugarza 18 19 20 †Catalan Institute of Nanoscience and Nanotechnology (ICN2). CSIC and BIST, Campus 21 UAB, 08193 Barcelona, Spain. 22 ‡Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), 20018 Donostia-San 23 Sebastián, Spain. 24 ¶Department of Materials, ETH Zurich, Hönggerbergring 64, CH-8093 Zurich, Switzerland. 25 26 §Istituto di Struttura della Materia (ISM), Consiglio Nazionale delle Ricerche (CNR), 27 I-34149 Trieste, Italy. 28 kALBA Synchrotron Light Source, Carretera BP 1413km 3.3, E-08290 Cerdanyola del 29 Vallès, Spain. 30 ⊥Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona 08070, Spain. 31 32 E-mail: marc.gonzalez-cuxart@tum.de; SOV@icrea.cat; aitor.mugarza@icn2.cat 33 34 35 36 37 Abstract magnetic ions with the surface states. 38 By depositing Co-based porphyrin and 39 Controlling interfacial interactions phthalocyanine monolayers on the sur- 40 in magnetic/topological insulator het- face of Bi2 Te3 thin lms, robust inter- 41 erostructures is a major challenge for faces are formed that preserve undoped 42 the emergence of novel spin-dependent topological surface states as well as the 43 44 electronic phenomena. As for any ratio- pristine magnetic moment of the divalent 45 nal design of heterostructures that rely Co ions. The selected ligands allow us to 46 on proximity eects, one should ideally tune the interfacial hybridization within 47 retain the overall properties of each com- this weak interaction regime. These re- 48 ponent while tuning interactions at the sults, which are in stark contrast with 49 50 interface. However, in most inorganic in- the observed suppression of the surface 51 terfaces interactions are too strong, con- state at the rst quintuple layer of Bi 2 Se3 52 sequently perturbing, and even quench- induced by the interaction with Co ph- 53 ing, both the magnetic moment and the thalocyanines, demonstrate the capabil- 54 topological surface states at each side of ity of planar metal-organic molecules to 55 56 the interface. Here we show that these span interactions from the strong to the 57 properties can be preserved by using lig- weak limit. 58 and chemistry to tune the interaction of 59 60 ACS Paragon Plus Environment 1 ACS Nano Page 2 of 16 Keywords to coordinate the single magnetic impurities 1 2 with organic ligands. Using this approach, 3 Metal-organic molecules, topological insulators, the interaction between the magnetic ion and 4 interfacial interactions, scanning tunneling mi- the TSS can be nely tuned with ligands 5 croscopy, angle-resolved photoelectron spec- that place the magnetic ion at a selected dis- 6 troscopy, X-ray magnetic circular dichroism, tance from the surface. 20 Furthermore, molec- 7 density functional theory ular self-assembly interconnects magnetic im- 8 9 purities and arrange them with regular inter- 10 atomic spacing, facilitating control over Rud- 11 Introduction ermanKittelKasuya Yoshida (RKKY) inter- 12 Interfacing topological insulators (TI) with actions, and promoting ligand-mediated interi- 13 14 magnetic materials can give rise to interesting onic exchange. Indeed, ligand-mediated long- 15 phenomena that result from breaking time re- range magnetism in a metal-organic coordina- 16 versal symmetry, such as band-gap opening in tion network in contact with a TI surface has 17 been predicted to break time-reversal symmetry 18 the topological surface states (TSS) and the emergence of the quantum anomalous Hall ef- and open a band-gap at the Dirac point of the 19 20 fect. 13 It can also enable the interconversion TSS. 21 The interaction between the organic lig- 21 of spin and charge currents. 4 The observation and and the TSS can additionally be exploited 22 of these phenomena, however, demands a ne to tune the spin-charge interconversion. 22 Ul- 23 timately, metal-organic frameworks with coex- 24 control over the Fermi level position, 5,6 and the interfacial hybridization. 7 isting magnetic and electric orderings 23 might 25 26 A way of engineering these properties in a pre- be used to simultaneous induce magnetic prox- 27 dictive and controlled manner is by tuning in- imity and electrostatic gating of TI interfaces, 28 terfacial interactions without substantially per- or as tunable buer layers for controlling the 29 spin-charge conversion eciency to an adjacent 30 turbing the pristine properties of each compo- 31 nent, such that the magnetic and topological ferromagnetic metal. 32 materials can be designed independently. Un- The degree of tunability of charge and spin 33 fortunately, this is not the case when the TI interactions oered by metal-organic layers has 34 already been demonstrated on metallic sur- 35 is interfaced directly with metallic ferromag- nets. 812 The strong interaction leads to unde- faces. 20,2432 The few systematic studies on TIs 36 37 sired eects such as magnetically dead or ill- suggest a similar tunability, driven by either the 38 dened layers induced by intermixing, 12 or the choice of the metal ion 33 or the ligand. 34 Sur- 39 suppression of the TSS. 4 At the single impu- prisingly, despite the considerably larger dis- 40 tance from the surface, as compared to bare 41 rity level the tendency for intermixing expresses as strong lattice relaxations around the impu- impurities, interactions between the molecular 42 43 rity atom, leading to multiple congurations metal ions and the TSS can still be strong 44 that hamper the reliable control of the doping enough to lead to doping levels comparable to 45 of the TSS and of the magnetic moment and those observed for bare impurities, 35,36 form hy- 46 anisotropy of the impurities. 1318 The magnetic brid interface states 3739 and, more dramati- 47 cally suppress the TSS from the rst quintu- 48 exchange required for long-range magnetic or- 49 dering is also hard to control. The interatomic ple layer (QL), as recently found for cobalt ph- 50 distance and doping level, which are tightly thalocyanine (CoPc) on Bi 2 Se3 . 36 A follow-up 51 entangled via the impurity concentration, can study concluded that non-perturbative interac- 52 tions can only be achieved with molecules with 53 alter the size and signature of the interaction by the interplay between the dierent exchange non-planar coordination geometry, where the 54 55 mechanisms. 19 metal ion is protected by the ligand core from 56 An eective way of regulating the param- surface-induced perturbations. In this congu- 57 eters that govern all of the above factors is ration, however, magnetic interactions are lim- 58 ited to dipolar and, as a consequence, relatively 59 60 ACS Paragon Plus Environment 2 Page 3 of 16 ACS Nano weak. 40 surface are formed even at low coverage, re- 1 2 In order to validate the approach of planar vealing a high diusion rate of the molecules, a 3 coordination geometries, we provide here a sys- rst indication of weak interaction with the sub- 4 tematic investigation of the interfacial proper- strate. The molecular appearance changes as 5 ties of a cobalt tetrakis (4-bromophenyl) por- we probe dierent Co-derived orbitals by vary- 6 phyrin (CoTBrPP) monolayer deposited on the ing the bias voltage (inset in Figure 1b), result- 7 8 (0001) surface of a twin-free Bi 2 Te3 thin lm. 41 ing in bright circular centers at positive bias, 9 By combining scanning tunneling microscopy and dumbbell-like structures oriented parallel 10 and spectroscopy (STM/STS), angle-resolved to one of the pyrrole-pyrrole axial directions at 11 photoelectron spectroscopy (ARPES), and x- negative bias. 12 ray absorption and magnetic circular dichroism A complete description of the molecular lat- 13 14 (XAS/XMCD), we are able to probe the TSS tice can be obtained from the atomically re- 15 band dispersion and both the molecular orbital solved STM image shown in Figure 1c. By 16 structure and its magnetic moment. We nd extrapolating the Bi 2 Te3 surface lattice to the 17 that doping eects are negligible, and that the molecular island or, conversely, the molecu- 18 magnetic moment of Co is neither quenched nor lar lattice to the Bi2 Te3 surface, we derive 19 20 Kondo screened. Yet, our STS analysis, sup- the stacking relation and relative angle of the 21 ported by Density Functional Theory (DFT) two lattices, as well as the azimuthal angle of 22 calculations, reveal that the spin carrying Co the molecular axis relative to the surface lat- 23 dz2 orbital undergoes an appreciable hybridiza- tice. The molecular lattice has a commensu- 24 tion with substrate electronic states, as required rate stacking with the surface, with Co ions sit- 25 26 for magnetic exchange interactions. Our results ting on Te bridge sites, in agreement with the 27 show that non-perturbative interfaces can also most stable adsorption conguration found in 28 be attained by using metal-organic molecules DFT calculations (see Figure 1d). The respec- 29 that expose the magnetic ion directly to the sur- tive experimental and theoretical azimuthal ro- 30 31 face, if the ligand and substrate are appropri- tation of the phenyl axis, of φ = 9.0 ± 2.8◦ and 32 ately chosen. By replacing the porphyrin ligand φ = 9.8◦ , are also in close agreement. The 33 with the more planar phthalocyanine, we fur- molecular lattice vectors ( b1 ,b2 ), with a mod- 34 ther demonstrate that the position of the Fermi ulus of |b1 | = |b2 | = 16.6 ± 0.7 Å, form an 35 level and the interfacial hybridization can be angle of ϕ = 91.9 ± 0.8◦ . The slight deviation 36 37 tuned within this interaction regime by select- from the otherwise favourable squared lattice is 38 ing dierent ligands. a consequence of keeping registry with the un- 39 derlying hexagonal lattice of the surface, with 40 a relation of b1 = 4a1 + a2 and b2 = a1 + 4a2 . 41 Results and discussion A previous systematic study of the molecular 42 43 44 Structural characterization self-assembly of dierent metal phthalocyanines on Bi2 Se3 nicely illustrates the trend from a 45 The structural characterization of the Bi 2 Te3 frustrated assembly for the strongly interacting 46 MnPc, to a hexagonal arrangement mimicking 47 surface and the molecular overlayer carried out by STM is summarized in Figure 1. After the underlying surface lattice for the interme- 48 49 the thermal desorption of a protective Te layer diate case of CoPc and, nally, to an incom- 50 (see Methods), the surface of the Bi 2 Te3 thin mensurate square arrangement for the weakly 51 lms exhibit atomically at terraces exposing interacting CuPc. 33 The commensurate, quasi- 52 square arrangement that is observed for CoT- 53 the outermost Te atoms arranged in a hexago- nal lattice (see Figure 1a). After submonolayer BrPP on Bi2 Te3 lies somewhere between the in- 54 55 deposition of CoTBrPP, self-assembled molecu- termediate and weak interaction cases of CoPc 56 lar islands grow from the step edges. Extended and CuPc on Bi2 Se3 . 57 regular islands separated by large areas of clean 58 59 60 ACS Paragon Plus Environment 3 ACS Nano Page 4 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Figure 1: STM topographic images of the Bi 2 Te3 substrate (a) after the Te cap removal ( Vbias = 41 1.4 V , It = 20 pA). Inset: atomically resolved at terrace with its unit cell ( Vbias = 0.4 V , It = 42 540 pA); and (b) after the deposition of CoTBrPP molecules ( Vbias = 1.4 V, It = 10 pA). Insets are 43 44 close-up images of the molecular lattice obtained at Vbias = 1.6 V and Vbias = −1.2 V respectively. 45 (c) STM image of a CoTBrPP island on Bi 2 Te3 with combined atomic and intramolecular resolution 46 (Vbias = −1.8 V, It = 10 pA). The surface and molecular lattices, and respective unit cells are 47 overlaid. (d) Top and side view of the minimum energy adsorption conguration of CoTBrPP on 48 Bi2 Te3 (0001), as obtained by DFT. 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 16 ACS Nano 1 Molecular orbitals orbital signatures in the spectral features, 2 we compare the latter with the spin-resolved Next, the molecular electronic structure is in- 3 PDOS of the atomic orbitals. Figure 2h depicts vestigated by performing local STS measure- 4 the PDOS of the Co d orbitals, which dene 5 ments on single CoTBrPP molecules within the the molecular gap and magnetic moment (more 6 islands. Spectra acquired at representative sites extended data with projection to ligand atoms 7 of the molecule are represented in Figure 2a, 8 is provided in the Supporting Information). A together with that acquired on the bare sub- 9 gap of ∼ 2 eV, similar to the experimental one, strates (see Supporting Information for further 10 is obtained by using a U-J=3 eV term, a value 11 details on the spectra on the bare substrate). comparable to that used in other weakly inter- 12 Resonant features can be assigned to molecu- acting Co porphyrin compounds. 44,45 The or- 13 lar orbitals by comparing their corresponding 14 bital sequence suggested by the conductance maps (Figure 2b-g) with the projected density 15 maps is also conrmed, since the HOMO and of states (PDOS) on dierent atomic orbitals 16 LUMO are respectively of dxz,yz and dz2 charac- 17 (Figure 2h). ter in the PDOS. Interestingly, the unoccupied 18 The rst noticeable characteristic of the dz2 orbital also appears as a broad double-peak 19 dI/dV spectra is a large gap of ∼ 2.4 V dened 20 resonance, emulating the observed experimen- by the onsets of the highest occupied (HOMO) 21 tal feature at positive bias, and indicating that and lowest unoccupied (LUMO) molecular or- 22 hybridization with the underlying Te states is 23 bitals at −1.0 V and 1.4 V. In contrast to the not negligible. 24 smaller gaps of ∼ 1 eV observed for Co por- The structural and spectroscopic analysis of 25 phyrins on noble metals, 27,42 the proximity of 26 the molecular lms reveal a nite albeit mod- the measured value to the 2.5 V calculated 27 erate interaction of the Co ions with the sub- for the free-standing molecule 43 indicates that 28 strate. The eect of this interaction on the 29 molecular states are eectively protected from magnetic moment of the Co ion and the TSS 30 strong surface hybridization and screening ef- 31 is discussed in the next Section. fects. Molecular magnetic moment 32 The HOMO onset is most pronounced in the 33 34 spectrum acquired on the pyrrole sites (the 35 dumbbell protrusion pair seen in the images ac- The magnetic moment of the Co ion can be di- 36 quired at negative bias, see inset in Figure 2a). rectly probed by XMCD. Figure 3 shows XAS 37 Constant height conductance maps acquired at 38 and XMCD spectra acquired for a nearly com- energies below this onset replicate the dumbbell pleted CoTBrPP monolayer at the energy range 39 40 feature, which originates from hybrid Co dxz,yz of the Co L2,3 edge. The spectra are acquired 41 and pyrrole p states. 27 The two-fold symmetry at normal and grazing incidence, which probe 42 of the feature is related to the saddle distor- the out-of-plane and in-plane projection of the 43 tion of adsorbed CoTBrPP, which reduces the 44 magnetic moment of the Co 3d orbitals respec- molecular symmetry from four to two-fold and tively. Since photoelectrons can still be emitted 45 46 consequently splits the dxz and dyz orbitals. across the Bi2 Te3 lm, a Ba M4,5 doublet aris- 47 The LUMO is the broad structured resonance ing from the underlying BaF 2 substrate is also 48 in the spectrum acquired on the Co ion. Indeed, visible. The overlap between the Ba M4,5 and 49 conductance maps around the LUMO onset at 50 Co L2,3 edges components is however minimum +1.6 V reveal a spherical protrusion localized and does not impede a qualitative analysis of 51 52 on the Co ion, suggesting the presence of a dz2 the Co L2,3 lineshape. 53 orbital in this energy region. The spherical fea- At grazing incidence, the Co absorption spec- 54 ture persists at higher energies up to the end trum is characterized by an L3 multiplet with 55 of the resonance at ∼ 2.1 V, above which the 56 a sharp intense peak on the low energy side fol- spectral weight shifts to the peripheral phenyls. lowed by a doublet, and a broad L2 peak. At 57 58 For an unequivocal identication of Co d normal incidence, the low energy peak of the 59 60 ACS Paragon Plus Environment 5 ACS Nano Page 6 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Figure 2: (a) dI/dV spectra acquired on a CoTBrPP molecule within an island at the points 42 depicted in the topographic image at the inset, and on the bare Bi 2 Te3 surface. Spectra acquired 43 on the molecule are vertically shifted for visualizing the nite dI/dV signal of the bare surface 44 around EF (constant-height mode, Vbias = −1.4 V, It = 10 pA, image size 2.5 × 2.5 nm2 ). (b) - 45 46 (g) Constant height dI/dV maps of the same molecule. (h) PDOS of the Co orbitals calculated by 47 DFT. 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 6 Page 7 of 16 ACS Nano L3 component is practically absent, whereas the spectra measured at grazing incidence reveal 1 2 intensity of the L3 doublet and L2 component the presence of a nite magnetic moment re- 3 increase. The overall lineshape and its angular siding on the Co ions. The dichroic signal is 4 dependence closely resemble those of a Co 2+ ion dominated by a sharp peak at the low-energy L3 5 in a square planar ligand eld with weak inter- component, which corresponds to transitions to 6 actions with the substrate, such as in CoPc mul- the half-lled dz2 orbital. The signal vanishes 7 8 tilayers 46 and CoOEP on graphene. 45 In anal- within the noise level at normal incidence due 9 ogy with these systems, the x-ray absorption to the strong anisotropy of the dz2 spin density, 10 spectra thus indicate that the electronic cong- which results in a pronounced angle-dependent 11 uration of the Co ions is characterized by an dipolar term. 47 The absence of XMCD spectral 12 unpaired electron in the a1g (dz2 ) orbital that intensity at the L2 edge further indicates that 13 14 gives rise to a S=1/2 ground state. Spin-orbit the Co ions have a relatively strong orbital mag- 15 coupling additionally leads to mixing of elec- netic moment, 48,49 as anticipated from the anal- 16 tronic congurations with 2A1g and 2Eg sym- ysis of the XAS lineshape. 46 Indeed the XMCD 17 metry, which results in charge transfer from the sum rules 50,51 applied to the spectra reported in 18 eg to the a1g orbital and a nite in-plane orbital Figure 3 reveal an orbital moment of 0.35±0.04 19 20 moment. 46 µB and an eective spin moment of 1.03 ± 0.09 21 µB. We emphasize, however, that these values 22 do not correspond to the full moment given that 23 the paramagnetic CoTBrPP are not fully satu- 24 rated in a eld of 6 T. Moreover, the presence 25 26 of a nonzero magnetic spin dipole moment pre- 27 vents us from obtaining a precise estimate of 28 the spin moment. 47 29 The XMCD results presented above are in 30 31 contrast to those obtained with CoPc 46 and 32 CoTPP 52 monolayers on noble metals, where 33 the dichroic signal is absent. They reveal 34 that the interaction between CoTBrPP and the 35 Bi2 Te3 surface is weak enough to preserve the 36 37 molecular magnetic moment. 38 The experimental results are supported by 39 our DFT calculations, which nd a half empty 40 dz2 orbital and a total spin moment of 1.11 µB. 41 We note that the good match with the eective 42 43 spin moment obtained experimentally indicates 44 that, at this incidence angle, a nite dipole spin 45 moment adds to the unsaturated spin moment 46 in the XMCD spin sum rule. 47,50 Additionally, 47 Figure 3: (a) Average XAS and XMCD mea- DFT shows that the molecules have easy plane 48 sured on a nearly saturated monolayer of CoT- 49 magnetic anisotropy with an energy barrier of BrPP on Bi2 Te3 at the Co L2,3 edge at normal 50 1.66 meV, consistent with the presence of an (θ = 0◦ ) and grazing (θ = 70◦ ) incidence, with 51 in-plane orbital moment (see Supporting Infor- 52 a magnetic eld of 6 T and at a temperature of mation). Calculations of CoPc on Bi 2 Te3 also 53 7.5 K. Features at 784 and 799 eV are attributed result in a similar spin moment of 1.09 µB, 54 to the Ba M4,5 edge from the BaF2 substrate. 55 suggesting that the magnetic moment of this 56 molecule is also preserved on this surface. 57 Consistently with the S=1/2 ground state 58 inferred from the XAS lineshape, the XMCD 59 60 ACS Paragon Plus Environment 7 ACS Nano Page 8 of 16 1 Surface band structure bulk bands (see Figure 4a and Figure 5a). Nev- 2 ertheless, we nd several evidences of a stronger The remaining question is then if the TSS also 3 interaction as compared to CoTBrPP. First, withstands the interfacial interactions with the 4 the coverage dependent t of the Dirac cone 5 magnetic molecules. In order to assess the im- reveals a small but sizeable doping of 35 ± 6 pact of the CoTBrPP molecules on the surface 28 meV at maximum coverage. A more strin- 7 electronic structure, an ARPES study was car- 8 gent indication is provided by the molecular or- ried out as a function of molecular coverage. 9 bitals that are probed in the wide energy range The latter was calculated from the attenuation 10 ARPES spectra presented in Figure 4b. Sev- 11 of the substrate Bi 4f 7/2 peak obtained in simul- eral molecular orbitals can be identied here 12 taneous XPS measurements, and cross-checked as non-dispersing levels that emerge gradually 13 by the intensity evolution of the C 1s peak (see 14 with increasing coverage at a binding energy Supporting Information). 15 larger than 2 eV, in agreement with the onset The characteristic Dirac cone dispersion of 16 found by STS at this energy range. Dierences 17 the Bi2 Te3 surface state can be tracked in the between the two molecular systems can be best 18 narrow energy range spectra presented in Fig- appreciated in the angle-integrated spectra of 19 ure 4a. The TSS spectral density quenches 20 the highest coverage samples displayed in Fig- gradually with coverage. However, when nor- 21 ure 5c. The broader molecular features of CoPc malized by the valence band intensity to ac- 22 contrast with the sharper multiple peaks found 23 count for the overall photoelectron attenuation for CoTBrPP in the same energy region, and 24 by the molecular overlayer, the TSS intensity provide a direct proof of a stronger hybridiza- 25 remains constant, as reported in Figure 5a. The 26 tion experienced by CoPc orbitals. These con- position of the Dirac point is also unaected by 27 clusions are further supported by the structural the interaction with the molecules, as can be in- 28 dierences observed in the self-assembly of the 29 ferred from the Dirac cones of the pristine and two molecular systems. Indeed, the hexagonal 30 highest coverage samples represented by over- 31 symmetry of the patterns found for CoPc 33 de- laid dashed lines (Figure 4a). These have been 32 notes a stronger stacking interaction than in the determined by tting the momentum distribu- 33 square CoTBrPP lattices. 34 tion curves to Lorentzian pairs, and the result- The dierent charge transfer and hybridiza- 35 ing maxima to a linear dispersion (see Support- tion found for the two molecules on Bi 2 Te3 is 36 ing Information). As indicated by the solid ar- 37 a direct indication of the eectiveness of lig- row that connects the two Dirac points in Fig- 38 ands for ne tuning interactions. On the other ure 4a and the complete evolution presented in 39 hand, the dierent results obtained for CoPc on 40 Figure 5b, there is no detectable charge trans- Bi2 Te3 in this work, and on Bi 2 Se3 in Ref. [36] 41 fer between the CoTBrPP molecular layer and show that substrate eects can still be consid- 42 the Bi2 Te3 surface. 43 erable. We attribute the stronger interactions The lack of attenuation and charge transfer 44 found on Bi2 Se3 to the larger electron an- found for CoTBrPP monolayers on Bi 2 Te3 is in 45 ity of Se, which favours charge transfer from 46 stark contrast to the previously reported evo- the molecules to the substrate and reduces the 47 lution of the Bi2 Se3 TSS upon CoPc adsorp- Co-surface distance. This dierence is accen- 48 tion. 36 Here the TSS is already doped at low 49 tuated by the dierent lattice constants of the coverage, and the spectral weight of the outer- 50 two TIs, which results in a 0.15 Å shorter Co- most QL that is probed by the photon energies 51 chalcogenide distance in Bi 2 Se3 . 52 used in our measurements is totally quenched 53 at the monolayer completion. Motivated by 54 such dierent behaviour, we extended our stud- Conclusions 55 ies to CoPc on Bi2 Te3 . Surprisingly, the TSS 56 presents a similar evolution as for CoTBrPP, In summary, we nd that CoTBrPP and CoPc 57 58 without any evidence of attenuation relative to monolayers adsorbed on Bi 2 Te3 form robust in- 59 60 ACS Paragon Plus Environment 8 Page 9 of 16 ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Figure 4: Coverage-dependent evolution of ARPES spectra for CoTBrPP and CoPc. (a) ARPES 55 intensity near EF showing the TSS. (b) Second derivative of the ARPES intensity of a larger spectral 56 range showing the evolution of Bi 2 Te3 bulk bands and molecular orbitals from the pristine surface 57 to the monolayer completion (see Supporting Information for the corresponding ARPES intensity). 58 Blue/red dashed lines in the narrow energy range spectra of (a) are linear ts to the dispersion 59 60 of the TSS Dirac cone. Corresponding ACShorizontal Paragon Pluslines trace the Dirac point of pristine and fully Environment covered surfaces. Insets show spectra of the highest 9 coverage samples with rescaled color range. ACS Nano Page 10 of 16 terfaces where electronic interactions can be 1 2 tuned without strongly perturbing of the in- 3 trinsic properties of each constituent. Our 4 conclusions are supported by consistent struc- 5 tural, electronic and magnetic information de- 6 rived from a combined STM, ARPES, XMCD 7 8 and DFT study. 9 The observed molecular self-assembly pro- 10 vides a rst proof that weak but nite in- 11 teractions exist among the molecules and TI, 12 which result in a close-packed square lattice ar- 13 14 rangement of the Co ions, as preferred by the 15 molecules, with a commensurate stacking on Te 16 sites, as imposed by Bi 2 Te3 . ARPES further 17 reveals that the TSS remains undoped upon 18 molecular deposition and maintains its pristine 19 20 spectral intensity. The only electronic signature 21 of interactions is the broadening of the empty 22 dz2 orbital probed by STM. Interestingly, this 23 is the orbital bearing the spin moment of the 24 Co2+ ion, which suggests that magnetic prox- 25 26 imity eects may be induced by the molecules 27 on the TI. Given that the CoTBrPP molecules 28 are paramagnetic, however, we do not observe 29 signatures of long-range magnetic order in our 30 31 study. 32 The comparative ARPES study reveals 33 stronger interactions at the CoPc/Bi 2 Te3 inter- 34 face relative to CoTBrPP/Bi 2 Te3 . For CoPc, 35 we nd a signicant hybridization between 36 37 molecular and TI states without considerable 38 charge redistribution. The results are consis- 39 tent with the stronger stacking interaction that 40 leads to hexagonal molecular patterns in this 41 system. 33 42 Figure 5: (a) Coverage-dependent evolution of 43 the TSS attenuation, dened as the TSS to bulk The degree of tunability oered by the planar 44 band photoemission intensity ratio relative to molecular compounds spans all the way from 45 that of the pristine surface. The photoemission the weak to the strong perturbation regime, 46 as demonstrated by comparing the results pre- intensity has been integrated over the areas de- 47 sented here for Bi2 Te3 with the case of the 48 ned in the inset ARPES map. (b) Evolution 49 of the Dirac point obtained from the tting of stronger interacting CoPc/Bi 2 Se3 interface. Fi- 50 momentum distribution curves (see Supporting nally, although intermolecular exchange inter- 51 Information). (c) Angle-integrated spectra of actions in this self-assembled layers are ex- 52 pected to be weak, we envisage that they could the highest coverage samples shown in Figure 53 be enhanced by the use of planar 2D covalent 54 4. Molecular orbitals are indicated with verti- 55 cal ticks. or metal-organic coordination networks. 56 57 58 59 60 ACS Paragon Plus Environment 10 Page 11 of 16 ACS Nano Methods age and dierence of the negative and positive 1 2 circularly polarized absorption, respectively. A 3 Methods. Sample preparation. Twin-free molecular coverage of . 1 ML was estimated 4 Bi2 Te3 thin lms of 20 QL were synthesized from the XAS L3 edge jump, previously cali- 5 by coevaporation of elemental Bi and Te (6N brated with in-situ STM measurements of the 6 purity) in a molecular beam epitaxy chamber same molecules on Au(111). 7 8 (MBE) with a base pressure of 2 · 10−10 mbar, Ab initio calculations were performed in the 9 equipped with thermal cracker cells. 41 Subse- framework of the density functional the- 10 quently, a 20 nm thick Te capping layer was ory (DFT) as implemented in VASP. 54 Core 11 deposited in order to prevent the TI surface electrons were treated using the projector 12 oxidation during the transfer in a UHV suit- augmented-wave method. 55,56 For the exchange 13 case to the dierent ex-situ characterization se- and correlation functional the PBE avor of the 14 15 tups for the corresponding STM, ARPES and generalized gradient approximation 57 was used. 16 XMCD measurements. The capping layer was The Dudarev form of the GGA+U method 58 17 removed before measurements by thermal an- was applied to improve the description of the 18 19 nealing at 130◦ C (see Supporting Information). 3d electrons of cobalt with U-J=3 eV. Miss- CoTBrPP molecules were synthesized in-house ing van der Waals forces were included using 20 21 (see Supporting Information) and CoPc were the Tkatchenko-Scheer method. 59 Wave func- 22 commercial (Sigma-Aldrich). Both were de- tions were expanded using a plane wave basis 23 posited on the freshly decapped TI thin lms set with an energy cuto of 400 eV. The Bril- 24 at a base pressure of 5 · 10−10 mbar, with the louin zone was sampled with a (3×3×1) k-point 25 substrate kept at 77 K and subsequently an- mesh. The geometrical structures were relaxed 26 27 nealed to room temperature to promote the until forces were smaller than 0.01 eV/Å. Mag- 28 self-assembly. netic anisotropy energies were determined by 29 ARPES and XPS measurements were per- total energy dierences after including spin- 30 formed with a Phoibos 150 analyzer (SPECS 31 orbit coupling in the calculations, 60 and using 32 GmbH, Berlin, Germany) at a temperature and a (6 × 6 × 1) k-point mesh. Magnetic moments 33 base pressure of 77 K and 5 · 10−10 mbar, with were determined by performing a Bader analy- 34 monochromatic HeI UV ( hν = 21.2 eV) and Al sis. 61 Drawings of the structures were produced 35 Kα (hν = 1486.6 eV) sources at incidence an- by VESTA. 62 36 gles of 55◦ and 50◦ respectively. Measurements 37 38 were performed along the Γ − K direction for 39 CoTBrPP lms, and the Γ − M direction for Acknowledgement 40 CoPc lms. 41 STM measurements were carried out at a tem- The authors thank G. Ceballos and M. Maymó 42 perature and base pressure of 5.4 K and 5·10−10 for their support in the experimental develop- 43 ment. This research was funded by the CERCA 44 mbar. dI/dV spectra and maps were acquired at constant height mode, using a lock-in at a Program/Generalitat de Catalunya, and sup- 45 46 modulation voltage of Vmod =20 mV. ported by the Spanish Ministry of Economy 47 XAS and XMCD measurements were carried and Competitiveness, MINECO (under Con- 48 out at the BOREAS beamline of the Alba tracts No. MAT2016-78293-C6-2-R, MAT2016- 49 75952-R, RTI2018-095622-B-I00, and Severo 50 Synchrotron facility. 53 Spectra were taken in total-electron-yield mode with right (I + ) and Ochoa No. SEV-2017-0706), the Secretariat 51 52 left (I − ) circularly polarized photons at normal for Universities and Research, Knowledge De- 53 (0◦ ) and grazing (70◦ ) incidence, in the presence partment of the Generalitat de Catalunya 2017 54 of a magnetic eld up to ±6 T aligned parallel SGR 827, the European Regional Development 55 Fund (ERDF) under the program Interreg V- 56 to the incident beam, and at a temperature and base pressure of 7.5 K and 1 · 10−10 mbar. XAS A España-Francia-Andorra (Contract No. EFA 57 58 and XMCD spectra are represented as the aver- 194/16 TNSI), the European Commission un- 59 60 ACS Paragon Plus Environment 11 ACS Nano Page 12 of 16 der the H2020 FET Open project Mechanics 5. Winnerlein, M.; Schreyeck, S.; Grauer, S.; 1 2 with Molecules (MEMO) (grant 766864), and Rosenberger, S.; Fijalkowski, K. M.; 3 the European Research Council under Grant Gould, C.; Brunner, K.; Molenkamp, L. W. 4 Agreement No. 306652 SPINBOUND. Epitaxy and structural properties of 5 (V,Bi,Sb)2 Te3 layers exhibiting the quan- 6 tum anomalous Hall eect. Physical Review 7 8 Supporting Information Avail- Materials 2017, 1, 011201. 9 able 10 6. Kondou, K.; Yoshimi, R.; Tsukazaki, A.; 11 Fukuma, Y.; Matsuno, J.; Takahashi, K. S.; Decapping of the Te protective layer, molecular 12 Kawasaki, M.; Tokura, Y.; Otani, Y. 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