Polymerization of Well-Aligned Organic Nanowires on a

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Polymerization of Well-Aligned Organic Nanowires on a Ferromagnetic Rare-Earth Surface Alloy Mikel Abadía,† Maxim Ilyn,†,‡ Ignacio Piquero-Zulaica,† Pierluigi Gargiani,¶ Celia Rogero,†,‡ José Enrique Ortega,†,‡,∥ and Jens Brede*,† †

Centro de Física de Materiales CFM - MPC, Centro Mixto CSIC-UPV/EHU, Paseo Manuel de Lardizabal 5, E-20018 San Sebastián, Spain ‡ Donostia International Physics Center, Paseo Manuel Lardizabal 4, E-20018 San Sebastián, Spain ¶ ALBA Synchrotron Light Source, Carretera BP 1413 km 3.3, E-08290 Cerdanyola del Vallés, Spain ∥ Departamento Física Aplicada I, Universidad del País Vasco, 20018 San Sebastián, Spain S Supporting Information *

ABSTRACT: The high reactivity of magnetic substrates toward molecular overlayers has so far inhibited the realization of more sophisticated on-surface reactions, thereby depriving these interfaces of a significant class of chemically tailored organics such as graphene nanoribbons, oligonuclear spin-chains, and metal−organic networks. Here, we present a multitechnique characterization of the polymerization of 4,4″-dibromo-p-terphenyl precursors into ordered poly(pphenylene) arrays on top of the bimetallic GdAu2 surface alloy. The activation temperatures for bromine scission and subsequent homocoupling of molecular precursors were followed by temperature-dependent X-ray photoelectron spectroscopy. The structural characterizations of supramolecular and polymeric phases, performed by low-energy electron diffraction and scanning tunneling microscopy, establish an extraordinary degree of order extending into the mesoscale. Taking advantage of the high homogeneity, the electronic structure of the valence band was determined with angleresolved photoemission spectroscopy. Importantly, the transition of localized molecular orbitals into a highly dispersive πband, the fingerprint of successful polymerization, was observed while leaving all surface-related bands intact. Moreover, ferromagnetic ordering in the GdAu2 alloy was demonstrated for all phases by X-ray absorption spectroscopy. The transfer of well-established in situ methods for growing covalently bonded macromolecules with atomic precision onto magnetic rare-earth alloys is an important step toward toward studying and controlling intrinsic carbon- and rare-earth-based magnetism. KEYWORDS: polymerization, Ullmann reaction, magnetism, alloy, spinterface he strong hybridization between the molecular π states and the metal 3d bands of ferromagnetic substrates1−5 can be exploited to tailor the spin-injection into organic layers6 and to control the magnetism of the interface by tuning the spin-polarization,1−3,6−8 anisotropy,9 exchange coupling constants,10−13 or Curie temperatures.11 However, precisely, this strong interaction between the organic overlayer with the

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ferromagnetic substrate has hindered more complex on-surface reactions such as the dehalogenative molecular homocoupling.14,15 Such on-surface Ullmann reactions enable the Received: September 7, 2017 Accepted: November 21, 2017 Published: November 21, 2017 12392

DOI: 10.1021/acsnano.7b06374 ACS Nano 2017, 11, 12392−12401

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Cite This: ACS Nano 2017, 11, 12392−12401

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ACS Nano synthesis of not only complex one- and two-dimensional covalently bonded functional networks15 or molecular spinchains16 but also custom-tailored graphene nanoribbons.17,18 The ability to study such in situ synthesized structures, which offer an atomic precision of the product with control even over the edge topology,19,20 under precisely controlled experimental conditions and in direct contact with a ferromagnetic substrate is of fundamental interest, as it is envisioned as a promising route for studying the many phenomena of emergent carbonbased spintronics21 by means of spin-resolved measurement techniques offering submolecular spatial resolution.22 Moreover, the enhanced thermal and structural stability of covalently bonded structures offers possibilities toward applications in particular if synthesized products extend into mesoscale dimensions accessible for device fabrication. Here, we establish using a multitechnique approach that a monolayer of ferromagnetic GdAu2 on Au(111)23 is perfectly suitable for the polymerization of 4,4″-dibromo-p-terphenyl precursors24,25 (DBTP) into mesoscopically well-aligned arrays of organic poly(p-phenylene) (PPP) wires, the narrowest graphene nanoribbon, via the surface-assisted Ullmann coupling reaction. Outline. In the following we briefly introduce the surfaceconfined Ullmann reaction and the material properties of GdAu2 before discussing independent as well as complementary measurements by various techniques: First, using in situ temperature-dependent X-ray photoelectron spectroscopy (XPS), we determine the onset of dehalogenation of DBTP precursors, indicative of a surface-assisted Ullmann coupling reaction, and the temperature at which Br atoms desorb from the surface. The different thus determined phases are subsequently studied locally by scanning tunneling microscopy (STM) resolving well-ordered molecule and polymer lattices. Indeed, it is demonstrated by means of low-energy electron diffraction (LEED) that the local order extends into the mesoscale suitable for angle-resolved photoemission spectroscopy (ARPES) characterization. Next, we discuss the ARPES data and show a clear transition from a localized highest occupied molecular orbital in the case of DBTP to a fully developed and strongly dispersing π-band after polymerization into PPP arrays. Lastly, the magnetic properties of the GdAu2 alloy as determined by X-ray magnetic circular dichroism (XMCD) are discussed. In particular we unambiguously demonstrate that the alloy remains ferromagnetically ordered after catalyzing the reaction. Surface-Confined Ullmann Reaction and Rare-Earth Surface Alloys. On the coinage metal substrates Cu, Ag, and Au aryl halides have been extensively used to study the surfaceconfined Ullmann reaction.20,24,26−32 During the reaction (schematically depicted in Figure 1) the preferential cleavage of the terminal halide is thermally activated and catalyzed by the substrate and thereafter proceeds either via an intermediate organometallic phase (as observed on Cu29 and Ag28) or via fusing of two aryl radicals as is commonly observed on Au(111). The details of the surface-confined Ullmann reaction, including the formation of an organometallic phase, the role of chemisorbed halogen atoms, and the influence of adatoms, are still under debate.29,30,32−34 However, experimental evidence largely agrees that the occurrence of an organometallic phase is suppressed for catalysts with high ionization potentials (e.g., Au(111)24) and/or catalysts in elevated oxidation states (e.g., TiO2(110)25), whereas highly reactive lanthanide adatoms

Figure 1. Schematic representation of the surface-confined Ullmann reaction: Aryl halides are thermally (ΔT) activated on a metal surface acting as the catalyst (cat). Depending on the substrate, either aryl radicals or organometallic intermediates are formed. Aryl radicals recombine to complete the reaction, while the organometallic intermediate is stable until a second thermal activation completes the reaction.

catalyze dehalogenation and promote the formation of an organometallic phase at room temperature.33 Here, GdAu2, which belongs to the class of intermetallic rareearth surface alloys,23,35−38 is used to catalyze the Ullmann reaction. These rare-earth alloys have a 1:2 stoichiometry and can readily be synthesized on top of Au(111) using standard surface science techniques. They exhibit long-range-ordered domains similar to the well-known surface alloy BiAg2.39 Importantly, GdAu2 is ferromagentically ordered up to about 20 K38 and displays thermal stability to about 650 K, while being at the same time chemically comparatively inert due to the trivalent state of the Gd atoms.

RESULTS AND DISCUSSION Monitoring the Polymerization Reaction with Temperature-Dependent XPS. In a first step we have monitored the binding energy (BE) of the Gd 3d5/2, C 1s, and Br 3p3/2 core levels (CLs) of a monolayer of DBTP deposited on GdAu2/Au(111) (Figure 2(a)) as a function of annealing temperature. We directly contrast the results with DBTP deposited on the Au(111) surface (Figure 2(b)). Two transition regions (indicated by dashed lines in Figure 2(a) and (b)) are readily identified by following the maxima (indicated by dotted lines in Figure 2(a) and (b)) of the relevant CLs. On GdAu2/Au(111) (Figure 2(a)) the first transition, which we assign to Br−C bond cleavage (as discussed in detail below), occurs close to 400 K and is identified by a shift of about 1 eV (0.1 eV) toward the lower BE of the Br 3p3/2 (C 1s) CL. Moreover, the Gd 3d5/2 CL maximum shifts gradually by up to 0.5 eV toward higher BE. On Au(111) (Figure 2(b)) the Br−C bond cleavage leads to a 2 eV (0.3 eV) shift of the Br 3p3/2 (C 1s) CL and is observed close to 450 K. The second transition, which we assign to Br desorption, is marked by the disappearance of Br 3p3/2 CL intensity. On GdAu2/Au(111) (Figure 2(a)) this transition takes place around 750 K, and concurrently with the loss of Br 3p3/2 CL intensity a shift of the Gd 3d5/2 CL maximum by about 0.3 eV toward lower BE is observed. On Au(111) (Figure 2(b)) the second transition takes place close to 620 K, as is evident by the loss of Br 3p3/2 CL intensity. Here, the C 1s CL shows a concurrent shift of about 0.2 eV toward higher BE. In Figure 2(c) we have plotted the integrated intensity of the relevant core levels for both 12393


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Figure 2. Temperature-dependent XPS of DBTP: Waterfall plots (a, b) and normalized spectra (d) of Gd 3d5/2, C 1s, and Br 3p3/2 core levels, respectively, are shown for DBTP on GdAu2/Au(111) (black traces) and DBTP on Au(111) (red traces). (c) Evolution of the area of the respective core levels as a function of temperature for DBTP on GdAu2/Au(111) (black dots) and DBTP on Au(111) (red dots), respectively. Lines are guides to the eye. Dashed and dotted lines in (a) indicate characteristic transition temperatures and CL shifts (for details see text), respectively. (e) Chemical structures of the different phases for given temperatures.

transition region (see SI Note #1 for details) traces the onset of Br−C bond scission just below 400 K for DBTP on Au(111), and only at temperatures above 450 K have all C−Br bonds been cleaved and remaining Br atoms are only found chemisorbed on the Au(111) surface. Again, in good agreement with previous reports of other aryl halide homocoupling reactions on Au(111)40,41 we find a desorption of chemisorbed Br close to 620 K. The changes of the Br 3p3/2 CL are accompanied by a narrowing of the C 1s CL, due to Br−C bond scission.24,25,34 The deconvolution of the relevant CLs can be found in SI Note #1, Figure S1, and Table S2. On the GdAu2/Au(111) substrate, the fingerprint of the dehalogenation reaction, i.e., the shift of the Br 3p3/2 CL toward lower BE (the C 1s CL again shows the complementary behavior, SI Table S1) is seen close to 400 K as discussed above. However, the chemical shift of the Br 3p3/2 CL amounts only to about 1 eV, which is considerably smaller than on coinage metals. Nonetheless, this relatively small shift is readily understood as the Gd atoms in GdAu2/Au(111) are expected to be formally in a trivalent state. Hence, the reduced magnitude of the chemical shift directly reflects the high

samples as a function of temperature. Again, the two transition regions are conveniently identified in the Br 3p3/2 CL evolution. In a second set of experiments we have recorded spectra corresponding to the three identified phases on both samples, i.e., upon deposition at 300 K and after annealing to 500 and 770 K, respectively. The results are summarized in Figure 2(d). Apart from the observations discussed above, a direct comparison of the molecule-related C 1s and Br 3p3/2 spectra on GdAu2/Au(111) (black traces) and on Au(111) (red traces) reveals that the respective CLs are systematically shifted toward the lower BE on Au(111) by about 0.4 eV due to the higher work function compared to the alloy (see SI Note #2). In order to interpret the XPS data discussed above, we focus in particular on the Br 3p 3/2 CL because previous experimental29,40,41 and theoretical34 works have identified a characteristic shift of more than 1.5 eV in the Br 3p3/2 CL as the fingerprint of molecular Br−C bond scission. As discussed above, we observe a chemical shift of close to 2 eV for DBTP on Au(111) at about 450 K (exact values of CL shifts are summarized in SI, Table S1). Indeed, in good agreement with previous STM studies by Basagni et al.24 an analysis of the 12394


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Figure 3. Geometry of DBTP and PPP on GdAu2/Au(111). LEED pattern (top), STM images (middle) with Fourier transform (middle, inset), and line profiles (bottom) of GdAu2/Au(111) (a), DBTP on GdAu2/Au(111) (b), and PPP on GdAu2/Au(111) (c), respectively. A model illustrating the three phases is shown in (d). Several features are indicated and discussed in the text.

the (√3×√3)R(30°) superstructure of the GdAu2 layer and additional spots due to the moiré pattern that arises due to the lattice mismatch with the underlying Au(111) surface.23,38 This hexagonal moiré pattern has a periodicity close to 36 Å in real space and can readily be resolved by STM (Figure 3(a), middle panel). Simultaneously with the moiré lattice (large black rhombus), a second lattice (small black rhombus) with a periodicity close to 5.4 Å is resolved in the inset of the middle panel in Figure 3(a). Line profiles were taken along the indicated and colored lines and are plotted in Figure 3(a), bottom panel. In agreement with previous work23,38 we assign the small lattice to Gd atoms. Note that the unit cell of the GdAu2 lattice (small black rhombus) is rotated by roughly 30° with respect to the unit cell of the moiré lattice (large black rhombus). A model of the surface is given in Figure 3(d). After DBTP deposition at RT the LEED image (Figure 3(b), top panel) changes to a (2√(3)×2√3)R(30°) pattern and the diffraction spots due to the moiré lattice are attenuated. Note that the LEED image is taken at a beam energy of 25 eV. STM imaging of the surface proved difficult at 300 K due to

oxidation state of the Gd atoms, similar to results obtained for Br−C scission on transition metal oxides.25 We note that within our experimental XPS resolution a minor contribution of an organometallic phase,29 i.e., terphenyl interconnected by Gd (and Br) atoms, can not be excluded (see SI Note #1); however, as we will discuss below, our STM and ARPES measurements unambiguously show that the vast majority of DBTP has undergone a complete homocoupling reaction into ordered and well-aligned poly(p-phenylene) arrays. Therefore, in contrast to Dy adatoms, which induced an organometallic intermediate phase but successively inhibited successful oligomerization,33 GdAu2 strongly suppresses an organometallic phase and enables successive polymerization. Molecule and Polymer Alignment as Determined by STM and LEED. After the observation of dehalogenation by XPS, we have characterized the geometry of DBPT on GdAu2/ Au(111) deposited at room temperature (RT) and after annealing (and dehalogenation). The results are summarized in Figure 3. The sharp LEED pattern of the GdAu2/Au(111) taken at a beam energy of 55 eV (Figure 3(a), top) consists of 12395


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Figure 4. Valence bands of DBTP and PPP on GdAu2/Au(111): (a) isoenergy surfaces acquired at a BE of 1.9 eV of the clean, DBTP, and PPP (+Br)-covered GdAu2/Au(111) surfaces, respectively. Parts of the surface Brillouin zones of GdAu2 and DBTP/GdAu2 are marked by solid and dashed black lines, respectively. For comparison also data of the PPP (+Br)-covered Au(111) surface are shown. (b) Energy dependence of the photoemission intensity along the kx direction. Characteristic bands stemming from GdAu2/Au(111) are indicated by letters, while molecular features are highlighted by colored arrows. Energy distribution curves along the colored lines in (b) are displayed in (c), and molecular features are highlighted by arrows. Parabolic fits near the band maxima of the two PPP-covered surfaces are shown in (d).

Upon heating the sample close to 500 K, the characteristic features change as summarized in Figure 3(c): the LEED image is a superposition of diffraction spots due to the (√3×√3) R(30°) superstructure of the GdAu2 layer and new diffraction spots that are rotated by about +2° (−2°) with respect to the ΓK direction. The diffraction spots due to the moiré pattern of the GdAu2/Au(111) while still visible are heavily attenuated. The resulting pattern has an overall star shape. STM images (Figure 3(c), middle panel) of the PPP phase still clearly resolve the characteristic moiré pattern of the GdAu2/Au(111); however, an additional stripe pattern is visible. The stripes are roughly aligned with close-packed rows of the moiré lattice, i.e., in three symmetry-equivalent directions. Neighboring chains are separated by about 6.4 Å, and a periodicity close to 4.3 Å is

thermally induced mobility of the DBTP lattice. However, the large unit cell of the moiré pattern is still readily resolved, and new superperiodicities are apparent in Figure 3(b), middle panel. In particular, a large periodicity close to 19 Å (blue trace) is resolved in the [112] direction. A smaller periodicity of about 11 Å (red and yellow trace) is resolved in the [101] and [011] direction, respectively. Note that a periodicity of 11 Å in the direction of the close-packed rows of the GdAu2 layer is equivalent to a (2√(3)×2√3)R(30°) superstructure with respect to the buried Au(111) interface. Thus, the STM data are in complete agreement with the LEED image (Figure 3(b), top panel). Finally, the periodicity of 19 Å matches closely the periodicity of DBTP molecules arranged in one-dimensional chains.25 For reference, a model is given in Figure 3(d). 12396


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intense, are also found in the second SBZ of DBTP/GdAu2/ Au(111) (close to kx = 0.67 Å−1), in agreement with the reduction of the SBZ due to the molecular overlayer. Importantly, additional photoemission intensity close to kx = 1.4 Å−1 is found at a BE of about 2.2 eV (marked by a yellow arrow) and is, by comparison with previous work,25 assigned to the highest occupied molecular orbital of DBTP. PPP-covered GdAu2/Au(111) shows, besides the attenuated surface-related features (see also SI Note #5 and Figures S10−S18), also a strongly downward dispersing band. The band maximum is determined at a BE = 1.8(0 ± 4) eV and a wave vector kx = 1.4(6 ± 2) Å−1 from a parabolic fit shown in Figure 4(d). The PPP-covered Au(111) surface shows, besides several surfacerelated features, also a strongly downward dispersing band. The band maximum is determined at a BE = 1.0(1 ± 3) eV and at a wave vector kx = 1.4(4 ± 1) Å−1 by a parabolic fit shown in Figure 4(d). In order to systematically compare the three cases, energy distribution curves (EDCs) have been taken along the solid line indicated in Figure 4(b). The EDCs are displayed in Figure 4(c), and the highest occupied molecular orbital (HOMO) of DBTP and the band maxima (BM) of PPP are indicated by arrows. From a parabolic fit in the vicinity of the BM (displayed in Figure 4(d)) the effective mass for PPP on GdAu2 is determined to be m* = −0.2(7 ± 9)me, while it amounts to m* = −0.2(0 ± 4)me on the Au(111) surface. Here, me denotes the free electron mass. Overall and in agreement with our XPS measurements, we find that the energy level alignment of the HOMO (BM) of DBTP (PPP) on GdAu2 and Au(111), respectively, can be well-described assuming physisorption5 and vacuum level alignment of molecular states (see also SI Note #2 and Figure S5). Thus, the adsorption regime of PPP observed on GdAu2 is distinct from chemisorption5 of PPP on Cu(110).43 On Cu(110) hybridization with Cu d-states leads to a reduction of the band gap of PPP to 1.15 eV (compared to a band gap of more than 3 eV on Au(111)44) along with partial filling of the conduction band and a reduced effective mass of m* = −0.15me. The central conclusion drawn from the ARPES data is the transition of the localized HOMO of DBTP on GdAu2 into the highly dispersive π-band characteristic for PPP chains.25,43,44 Thus, our previous assignment of a complete polymerization of DBTP precursors into PPP wires based on XPS, STM, and LEED data is further corroborated. Moreover, the ARPES data indicate that all surface-related bands are preserved below both the DBTP and the PPP overlayers. Note that for the PPPcovered GdAu2 substrate, surface-related bands are especially attenuated in the kx direction due the presence of the incommensurate molecular overlayer, as discussed in SI Note #5 in more detail. Therefore, measurements in the ky direction as shown in SI, Figure S11−S18, allow for a better discrimination of the substrate bands. Magnetic Properties As Determined by XAS and XMCD. The data of the previous sections clearly demonstrate that DBTP precursors have polymerized into ordered arrays of PPP chains on top of GdAu2/Au(111). Thereby, the first of two necessary conditions of a successful on-surface polymerization on a magnetic surface, i.e., the feasibility of the reaction itself, is met. The second condition, i.e., whether the surface remains magnetic after catalyzing the Ullmann coupling reaction, however, has so far not been addressed. Consequently, we performed X-ray absorption spectroscopy (XAS) to monitor the magnetism of GdAu2/Au(111) throughout the polymerization of DBTP precursors into ordered PPP chains.

measured along the chains, matching the phenyl−phenyl distance of about 4.2 Å in PPP. Note that the STM image in Figure 3(c) has specifically been chosen to visualize the three rotationally equivalent PPP domains on GdAu2/Au(111). Typically, domain boundaries are difficult to find, as individual PPP domains are very extended and well-ordered, with PPP chain length in excess of 100 nm (see SI Note #3 for exemplary data). On heating the sample above 700 K, the formation of graphene nanoribbons is observed (see SI Note #4 and Figure S9), but a detailed characterization of this phase will be presented elsewhere. Valence Band Properties As Determined by ARPES. So far we have presented compelling evidence that the transition from DBTP precursors into well-ordered and extended arrays of PPP chains takes place via the surface-assisted Ullmann coupling reaction on GdAu2/Au(111). Here, we corroborate our assignment and performed ARPES measurements of the molecular as well as the polymer phase and contrast the results with data for clean GdAu2/Au(111) in Figure 4. For a direct comparison and to highlight the high quality of the PPP arrays, we also present data obtained for PPP arrays on Au(111), which are known to show long-range order with individual PPP polymers extending over several tens of nanometers.24 At a binding energy of 1.9 eV the isoenergy surface of GdAu2/ Au(111) (Figure 4(a)) shows several bands: the well-known sp-band of the Au substrate and a downward dispersing d-band of Au character and an s,p,d-GdAu-hybrid band, labeled, following the nomenclature of ref 38, “sp”, “B”, and “C”, respectively. The sp-band runs roughly parallel to the ky direction close to kx = 0.85 Å−1. Here, ky corresponds to the ΓK and kx to the ΓM direction, respectively (see also Figure 3(c), top panel). Band “B” gives rise to a circular feature in the second surface Brillouin zone (SBZ). Band “C” is identified by the hexagonal feature in both the first and second SBZ; due to details of the photoemission process, some bands are only visible in the first or second SBZ, respectively.42 The edge of the first SBZ, indicated by the solid black line, is close to kx = 0.67 Å−1. The isoenergy surface of DBTP on GdAu2/Au(111) (Figure 4(a), top right) clearly exhibits replicas of bands “B” and “C” in between the features known from pristine GdAu2/ Au(111). Therefore, the isoenergy surface reflects straightforwardly the reduction of the SBZ due to the molecular overlayer, as previously described for the LEED data presented in Figure 3. For convenience we have superimposed the first three SBZs of DBTP/GdAu2/Au(111) with dashed lines. The isoenergy surface of PPP on GdAu2/Au(111) (Figure 4(a), bottom left) clearly shows two parallel lines running along the ky direction (as indicated by the blue arrows). These lines are roughly centered around kx = 1.45 Å−1. For completeness, we also show the isoenergy surface for PPP on Au(111) in Figure 4(a), bottom right. Here, relevant features originating from the PPP chains are indicated by green arrows. The energy dependence of several bands can be followed up to a BE of 3 eV in Figure 4(b). Below BE = 3 eV the photoemission intensity is dominated by d-bands of the Au substrate (see SI Note #5 for additional data). Besides the already introduced bands an upward dispersing feature “A” with a band minimum close to a BE of 1 eV can readily be identified in agreement with previous work.38 For DBTP/GdAu2/ Au(111) the intensity of surface bands is attenuated due to the DBTP overlayer, but all bands can still be clearly identified (see also SI Note #5 and Figures S10−S18). As discussed above, replicas of all bands, with band “B” appearing most 12397


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Figure 5. Curie temperatures of pristine and molecule-covered GdAu2 as determined by XAS: (a) Magnetic hysteresis curves measured at 2 K for DBTP- and PPP-covered GdAu2 samples in the in-plane (IP) and out-of-plane (OOP) direction, respectively. Arrott plots measured at the indicated temperatures for DBTP on GdAu2 (b) and PPP on GdAu2 (c), respectively. (d) Curie temperature (TC) of pristine GdAu2, DBTP on GdAu2, and PPP on GdAu2 extrapolated from the x-axis intercept of the Arrot plots. Importantly, all samples show ferromagnetic ordering below 15 K.

In agreement with previous measurements36,38 a strong X-ray magnetic dicroism is measured at the Gd M4,5-edges for the pristine surface at about 2 K in applied magnetic fields of 6 T, and the X-ray absorption spectra remain almost unchanged for both DBTP- and PPP-covered surfaces (see SI Note #6 and Figure S19). The XMCD of the molecule- and polymercovered surfaces allows us to subsequently study the magnetic properties of the systems by their response to an external magnetic field at different measurement temperatures. First, magnetization loops acquired at 2 K in the out-of-plane (OOP) and in-plane (IP) configurations for the DBTP- and PPPcovered GdAu2/Au(111) are shown in Figure 5(a). Zero remanent magnetization and higher values of the saturation fields observed in both samples for the OOP direction prove the in-plane orientation of the easy axis of these uniaxial systems. In the present work we are in particular interested in whether GdAu2/Au(111) remains magnetically ordered after inducing the polymerization reaction. Therefore, we have extracted the Curie temperature (TC) for both DBTP and PPP samples by means of the Arrot plot technique.45 Figure 5(b) and (c) show the curves of the squared magnetization plotted as a function of the inverse susceptibility H/M for the DBTPand PPP-covered GdAu2, respectively, measured for different temperatures as indicated. It was shown that in the limit of high fields M2 is a linear function of H/M and that the intercept with

the abscissa is positive (negative) above (below) the Curie temperature.45 Following the method described previously37 we have determined the intercept with the abscissa for each measurement temperature as shown in Figure 5(b) and (c). It is clearly seen in Figure 5(d), where we have plotted the values of the intercept H/M (M = 0) for DBTP- and PPP-covered GdAu2 as a function of temperature along with the data for pristine GdAu2 taken from ref 37, that for DBTP-covered (and prisitine) GdAu2 the intercept changes sign between 15 and 20 K, while for PPP-covered GdAu2 it happens at temperatures close to 15 K. This substantial change in the Curie temperature of close to 25% observed for the PPP-covered GdAu2 surface with respect to pristine GdAu2 suggests the possibility to control and tune the magnetic properties of rare-earth surface alloys35,38 by interfacing them with organic layers. However, in the present case we associate the observed reduction in Curie temperature with the formation of Br−Gd complexes and the accompanying slight reduction in structural coherence of the GdAu2 substrate as shown in SI Figure S9. Importantly, however, our XMCD data unambiguously prove that both DBTP- and PPP-covered GdAu2 are ordered ferromagnetically below 15 K. 12398


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302 SPECS monochromator. All binding energies given here were referenced to the Fermi level (EF), i.e., BE = 0 = EF. XMCD measurements were carried out at the BOREAS beamline52 of ALBA synchrotron in Spain, providing circularly polarized light from an AppleII-type helical undulator. The measurements were undertaken in the 2−90 K range with a variable magnetic field up to 6 T, pointing along the direction of the synchrotron light. Measurements were carried out for normal (θ = 0°, out-of-plane) and grazing incidence (θ = 60°, in plane) geometries. Absorption spectra were acquired at the Gd M4,5-edge in total electron yield mode measuring the drain current, setting to 90% the degree of circular polarization to work with the third undulator harmonic. Element sensitive magnetization loops were measured by recording the maximum of the XMCD asymmetry signal at the Gd M5 absorption edges as a function of the applied magnetic field.

CONCLUSIONS In conclusion, we have demonstrated the successful on-surface polymerization reaction of DBTP into ordered arrays of PPP on a ferromagnetic template. Importantly, the reaction leaves the ferromagnetic ordering of the surface alloy intact, as unambiguously established by temperature-dependent XAS and XMCD measurements. This opens a pathway to spin-polarized STM studies22 of carbon-based magnetism due to an imbalance in sublattice occupation such as in chiral GNRs,20,46,47 GNRs with pure zigazag edge topology,19 or the recently synthesized triangulene.48 An alternative route to the purely carbon-based magnetism is the study of on-surface-synthesized molecular magnets such as salophenato(Co) oligomers.16 Another prominent quality of the presented system is the high structural quality that extends well beyond the local scale and enables studies with surface averaging techniques such as (timeresolved) photoemission.49 Specifically, it may in the future be possible to extend the established technique of reconstructing molecular orbitals (and interface states) from photoemission data50 to spin-resolved data, thereby allowing a direct comparison with spatially and spin-resolved measurements by STM22 performed at the local scale. Moreover, it will be interesting to see how lifetimes of molecular states on GdAu2 compare to more traditional interfaces such as Alq3 on Co.49 Lastly, we emphasize that the many possible combinations of quasi isostructural intermetallic rare-earth surface alloys35,38 and the chemical tunability of molecular overlayers offer the perspective of fundamental insights into the complex magnetism of the 4f shell.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06374. Additional experimental data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Maxim Ilyn: 0000-0002-4052-7275 Pierluigi Gargiani: 0000-0002-6649-0538 Celia Rogero: 0000-0002-2812-8853 José Enrique Ortega: 0000-0002-6643-806X Jens Brede: 0000-0002-4946-8160

EXPERIMENTAL SECTION

Author Contributions

Experiments were carried out in ultrahigh vacuum at base pressures below 2 × 10−10 mbar. Two Au(111) single crystals were prepared by repeated cycles of sputtering (Ar+, 0.8−1.2 keV) and annealing to about 750 K, and cleanliness was monitored by LEED, XPS, STM, and/or ARPES. In particular XPS of the new Au(111) sample, which was used for reference measurements on pristine Au(111) substrates, showed no contamination, while the second crystal, which was used in previous GdAu2 experiments,23,36−38 showed a residual Gd concentration of up to a few percent after sputtering and annealing. Gd was deposited from home-built Gd evaporators at pressures below 6 × 10−9 mbar and rates of 0.03−0.06 ML/min (where a GdAu2 layer corresponds to 1/3 ML of Gd) onto the Au(111) crystal held at a temperature of around 650 K. Scanning tunneling microscopy was carried out at RT using either an Omicron VT-STM or a SPECS Aarhus STM. STM images shown in Figure 3 were recorded at a constant tunneling current of 50−200 pA and constant bias voltage of −2 V, applied to the sample. The unit cell of the GdAu2 moiré lattice, which is superimposed on the STM images, has been scaled to size. STM images in the insets were low-pass filtered to remove the moiré periodicity and enhance the atomic and molecular contrast, respectively. Image processing was done with the WSxM software.51 XPS experiments were performed at a base pressure of 5 × 10−10 mbar with a Phoibos photoelectron spectrometer equipped with an Al Kα Xray source (16 mA, 12.5 kV) as the incident photon radiation. The overall resolution of the instrument is approximately 0.9 eV. The temperature was increased at a rate of roughly 1.2 K per minute during temperature-dependent XPS. Individual spectra (Figure 2(d)) were acquired at RT after heating independent sample preparations to the given temperatures for at least 30 min. The acquisition time for this data was approximately 13 h. No changes were observed in the monitored core levels during that time. Angle-resolved photoemission measurements were performed using a Phoibos 150 SPECS highresolution hemispherical electron analyzer while the sample was cooled to 150 K. He-I (hν = 21.2 eV) radiation was provided by a high-intensity UVS-300 SPECS discharge lamp coupled to a TMM-

J.B. designed the experiments. M.A. and J.B. performed and evaluated the XPS, STM, and ARPES measurements. J.B., M.I., I.P., and P.G. performed and analyzed the XAS measurements. M.I. performed the Arrot plot analysis. J.B. wrote the manuscript with input from all authors. All authors discussed the data and contributed to the interpretations. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge funding from the Spanish MINECO under contract Nos. MAT2013-46593-C6-4-P and MAT2016-78293C6-5-R as well as the Basque Government Grants IT621-13 and IT-756-13. J.B. and M.I. are grateful for insightful discussions with Frederik Schiller. REFERENCES (1) Barraud, C.; Seneor, P.; Mattana, R.; Fusil, S.; Bouzehouane, K.; Deranlot, C.; Graziosi, P.; Hueso, L.; Bergenti, I.; Dediu, V.; Petroff, F.; Fert, A. Unravelling the Role of the Interface for Spin Injection into Organic Semiconductors. Nat. Phys. 2010, 6, 615−620. (2) Brede, J.; Atodiresei, N.; Kuck, S.; Lazić, P.; Caciuc, V.; Morikawa, Y.; Hoffmann, G.; Blügel, S.; Wiesendanger, R. Spin- and Energy-Dependent Tunneling through a Single Molecule with Intramolecular Spatial Resolution. Phys. Rev. Lett. 2010, 105, 047204. (3) Atodiresei, N.; Brede, J.; Lazić, P.; Caciuc, V.; Hoffmann, G.; Wiesendanger, R.; Blügel, S. Design of the Local Spin Polarization at the Organic-Ferromagnetic Interface. Phys. Rev. Lett. 2010, 105, 066601. (4) Sanvito, S. Molecular Spintronics: The Rise of Spinterface Science. Nat. Phys. 2010, 6, 562−564. 12399


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