Probing Magnetism in 2D Molecular Networks after in Situ Metalation

Letter pubs.acs.org/JPCL

Probing Magnetism in 2D Molecular Networks after in Situ Metalation by Transition Metal Atoms K. Schouteden,*,† Ts. Ivanova,† Z. Li,† V. Iancu,†,‡ E. Janssens,† and C. Van Haesendonck† †

Solid-State Physics and Magnetism Section, KU Leuven, BE-3001 Leuven, Belgium Extreme Light Infrastructure-Nuclear Physics (ELI-NP)/Horia Hulubei National Institute for R & D in Physics and Nuclear Engineering (IFIN-HH), Bucharest-Magurele, RO-077125, Romania

‡

S Supporting Information *

ABSTRACT: Metalated molecules are the ideal building blocks for the bottom-up fabrication of, e.g., two-dimensional arrays of magnetic particles for spintronics applications. Compared to chemical synthesis, metalation after network formation by an atom beam can yield a higher degree of control and flexibility and allows for mixing of different types of magnetic atoms. We report on successful metalation of tetrapyridyl-porphyrins (TPyP) by Co and Cr atoms, as demonstrated by scanning tunneling microscopy experiments. For the metalation, large periodic networks formed by the TPyP molecules on a Ag(111) substrate are exposed in situ to an atom beam. Voltage-induced dehydrogenation experiments support the conclusion that the porphyrin macrocycle of the TPyP molecule incorporates one transition metal atom. The newly synthesized Co-TPyP and Cr-TPyP complexes exhibit striking differences in their electronic behavior, leading to a magnetic character for Cr-TPyP only as evidenced by Kondo resonance measurements.

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of the adsorbate by the conduction electrons of the nonmagnetic metal support.2 The Kondo temperature of an adsorbed molecule is known to be highly sensitive to both charge transfer and its distance to the surface.12 Recently, it has been predicted that, among metalloporphyrins, Cr-porphyrins have the most potential for use as spin filters.13,14 It has been demonstrated as well that synthesized Cr-porphyrins on a Co substrate exhibit an antiferromagnetic coupling to their support.15 Here, we report on successful in situ metalation of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111) by two different transition metal atoms, i.e., Co and Cr atoms, which are investigated by STM and STS. TPyP consists of four pyridyl legs and a central porphyrin macrocycle that is observed to capture a deposited Cr or Co atom. The magnetic character of Cr-TPyP molecules is evidenced by the presence of a Kondo resonance, which is found to be absent for the Co-TPyP molecules within the measured temperature range (4.5 to 40 K). Our findings illustrate how the functionality of molecular networks can be tuned by in situ metalation via deposition of transition metal atoms. Figure 1 presents typical STM topography images of the TPyP network after deposition of minute amounts of Co and Cr atoms on the room temperature substrate. The TPyP molecules in the network have two different orientations and form alternating rows of molecules.16 Clearly the organization of the network is still intact after the atom deposition. TPyP

unctional molecules are generally reckoned as the ideal building block for the bottom-up fabrication of twodimensional periodic networks. In particular, porous networks can ideally serve as templates for, e.g., preparing well-defined arrays of magnetic particles for spintronics applications.1,2 To grow the particles, atom deposition can be used. However, this approach is often detrimental to the molecular arrangement due to the sensitivity of the molecules in the network, typically interacting by relatively weak van der Waals forces. For magnetic adatom decorations of molecular networks, covalently bound networks may be more stable than porous networks.3 Alternatively, molecules containing a metal atom can be obtained via chemical synthesis. Nevertheless, compared to ex situ synthesized molecules, metalation after network formation by an atom beam under ultrahigh vacuum (UHV) conditions yields a higher degree of control and flexibility for tuning the density of magnetic atoms as well as for mixing different types of magnetic atoms on one and the same network. More important, this route is advantageous whenever direct sublimation of clean metalated molecules is difficult in the case of highly reactive molecules, such as Fe-porphyrins.4−6 Apart from their evident potential in molecular spintronics, metalloporphyrins also attracted great scientific interest in view of their selective interactions with diatomic molecules (NO, CO, etc.) that are relevant in many biological processes.2,6−8 To date, there exist only few reports on successful postgrowth metalation of porphyrin networks.9 Magnetism at the single-molecule level can be traced via Kondo resonance measurements using scanning tunneling microscopy (STM) and spectroscopy (STS).10,11 The Kondo resonance originates from screening of the magnetic moment © 2015 American Chemical Society

Received: January 28, 2015 Accepted: March 6, 2015 Published: March 6, 2015 1048

DOI: 10.1021/acs.jpclett.5b00196 J. Phys. Chem. Lett. 2015, 6, 1048−1052

Letter

The Journal of Physical Chemistry Letters

Figure 2. Electronic properties of Ag(111), TPyP, Cr-TPyP, and CoTPyP: (a) (dI/dV)(I/V) spectra, (b) typical I-V behavior, and (c) dI/ dV spectra at more elevated voltages. Set points in panel c are V = +1.0 V and I = 0.1 nA. Spectra in panel c are offset for clarity.

very different appearance of TPyP molecules when recorded at sample voltages that are either above (when so-called orbitalmeditated tunneling occurs20) or below the LUMO-related resonance voltage, as illustrated in Figure 1. The energy maximum of the main resonance varies across the TPyP network by about 100 mV [illustrated in Figure S3c in the Supporting Information (SI)]. This modulation is directly related to the varying interaction of the TPyP molecules with the supporting Ag(111) surface21 that is evidenced by the appearance of Moiré-type patterns in large-scale STM topography images (exemplified in Figure S1 in the SI). To cancel out Moiré-related variations of the resonance maxima, the spectra presented in Figure 2a are an average of multiple spectra recorded on different molecules. Note that the averaging give rise to limited broadening of the resonances. It can be seen in Figure 2a that the spectrum of Cr-TPyP is very similar to that of the precursor TPyP. The resonances have a reduced intensity for Cr-TPyP, yet their energy positions are not affected. In contrast, for Co-TPyP the main resonance is shifted upward to around +770 mV and its shoulder has disappeared. In addition, a new resonance emerges for CoTPyP around −500 mV. Spatial (dI/dV)(I/V) maps of a TPyP network with deposited Co and Cr atoms that visualize the spatial variation of the (dI/dV)(I/V) spectra are presented in Figure S2 in the SI. The observed electronic behavior of CoTPyP is very similar to that of Fe-TPyP4 and other Coporphyrins,21,22 for which the additional resonance (at −500 mV) is interpreted as a result of ionization of the d2z orbital of the respective atom. Related Co- and Cr-porphyrins were previously shown to be in a 2+ oxidation state.4,15,23 The pronounced difference in the electronic properties of Cr-TPyP and Co-TPyP may be ascribed to a different interaction of the metalated complex with the Ag(111) support, which may in turn originate from a different complex-to-substrate distance. In fact, the higher appearance of Co-TPyP in Figure 1a,b compared to Cr-TPyP suggests that the Co−Ag distance is indeed larger than the Cr−Ag distance. At higher voltages (Figure 2c), TPyP exhibits two extra resonances that may be assigned to higher energy molecular orbitals (LUMO+1 and LUMO+2) that are more localized to the legs of the molecule.21 For Co-TPyP, the first resonance is shifted to higher voltages by about 200 mV when compared to

Figure 1. TPyP networks on Ag(111) after Co (a and c) and Cr (b and d) deposition. Dashed rectangles enclosing the same area are added to panels a and c as guides for the eye. Set points are (a) V = −1.5 V and I = 0.1 nA, (b) V = −1.0 V and I = 0.1 nA, (c) V = +1.5 V and I = 0.4 nA, (d) V = +1.0 V and I = 0.1 nA. Image sizes are 15 × 15 nm2.

molecules that contain either a deposited Co or Cr atom, hereafter referred to as Co-TPyP and Cr-TPyP, respectively, can be identified most clearly as the elongated protrusions in STM topography images recorded in the negative voltage range. At voltages around V = −1.0 V, Co-TPyP appears more pronounced than Cr-TPyP (compare Figure 1a to Figure 1b), which allows their discrimination when Co and Cr are both deposited on the same TPyP network (Figure 3c). Note that Co-TPyP and Cr-TPyP molecules show two different orientations that correlate with the alternating TPyP rows of the network. At elevated positive voltages, Co-TPyP and CrTPyP can no longer be discerned from the TPyP precursor molecules (Figure 1c,d), which implies that the orbitals involved in tunneling at positive voltages mainly have contributions from the ligands surrounding the metal center. Next, we determined the electronic properties of the newly formed Co-TPyP and Cr-TPyP molecules on Ag(111) with STS. The pronounced nonlinear I−V behavior of the TPyP molecules (Figure 2b) implies that the relative heights of the electronic features in the derived (dI/dV)(V) spectra [often used as a direct measure of the local density of states (LDOS) of the sample] depend strongly on the set point used for the sample voltage (e.g., positive or negative voltage set point). To account for the strong set point-dependence and to achieve a meaningful comparison of the spectra, we relied on normalized (dI/dV)(I/V) spectra that yield a better measure of the LDOS.17 Figure 2a presents typical (dI/dV)(I/V) spectra of Ag(111), TPyP, Co-TPyP, and Cr-TPyP. The TPyP spectrum is characterized by a pronounced resonance at about +570 mV and a shoulder around +250 mV. These features can be ascribed to the two lowest unoccupied molecular orbitals (LUMOs) that are energetically very close in the isolated porphyrin molecule.4,18,19 These orbitals are at the origin of the 1049

DOI: 10.1021/acs.jpclett.5b00196 J. Phys. Chem. Lett. 2015, 6, 1048−1052

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The Journal of Physical Chemistry Letters

Figure 3. (a) and (b) Modification of a TPyP molecule (indicated by the black dotted circle) by locally applying a voltage pulse of V = +3.0 V. Set points are V = −1.0 V and I = 0.1 nA. Image sizes are 6 × 6 nm2. (c,d) STM topography images of a TPyP network after deposition of Co and Cr atoms, before (c) and after (d) applying a high voltage to all molecules. Set points are V = −1.2 V and I = 0.1 nA. (e,f) Same as panel d, but recorded with a modified STM tip. Set points are (e) V = −1.2 V and (f) V = +1.2 V (I = 0.4 nA). Image sizes for panels c−f are 8 × 8 nm2. Dotted rectangles comprising the same two TPyP molecules are added as guides for the eye.

bare TPyP, whereas the second resonance appears at the same voltage. Within the accuracy of our experiments, these two TPyP-resonances remain unaffected for Cr-TPyP (data not shown). The shifted resonances for Co-TPyP indicate a modified charge transfer between the TPyP molecule and the Ag(111) substrate upon Co incorporation, as well as a different charge transfer for Co-TPyP and Cr-TPyP that may be related to a different complex-to-substrate distance as indicated above. Interestingly, the TPyP molecules have a modified appearance after locally applying a high voltage of V = +3.0 V.24 This can be explained by (voltage-induced) dehydrogenation of the porphyrin macrocycle (one/two hydrogens are removed from the two pyrrolic nitrogens), which results in one/two free bonds that allow TPyP to bond stronger to the Ag(111) substrate. The modified appearance of the TPyP molecules after partial/full dehydrogenation is similar to that of previously reported tetraphenyl-porphyrin (TPP) molecules on Ag(111).25 After full dehydrogenation, the porphyrin macrocycle appears lower than the pyridyl legs of the molecule (illustrated in Figure 3a,b), and the unoccupied orbital at +570 mV is shifted to higher voltages by about +200 mV (see Figure S3 in the SI). Figure 3c,d presents STM topography images of a Co- and Cr-containing TPyP network before and after applying V = +3.0 V at every molecule. Remarkably, the appearance of metalated Co-TPyP and Cr-TPyP in STM topography images (see Figure 3c,d) as well as their LUMO (see Figure S3 in the SI) do not change after applying the high voltage. This indicates that the macrocycle is already dehydrogenated upon Co and Cr incorporation, in agreement with previously reported metalation experiments.4,6,26 It may therefore be concluded that introduction of the Co or Cr atom in the macrocycle triggers H−N bond breaking followed by the release of two hydrogens (one for each of the two pyrrole groups in the porphyrin macrocycle) and the formation of bonds to the metal atom, as is also the case for synthesized metalloporphyrins.2,6−8

The fact that Co-TPyP and Cr-TPyP appear as elongated protrusions in Figure 1a,b can be explained by a saddle-shaped distortion of the planar porphyrin macrocycle, which results in two opposing pyrrole rings facing upward and two downward.4,21,26 Consequently, the observed two orientations of CoTPyP and Cr-TPyP in Figure 1a,b correlate with the alternating (macrocycles in the) TPyP rows of the network. The saddleshaped distortion appears more pronounced for Co-TPyP than for Cr-TPyP, implying a larger distance above the Ag(111) surface for Co than for Cr. During scanning of the TPyP networks with the STM tip, it occasionally occurs that the STM tip picks up a molecular species from the surface, resulting in an abruptly enhanced resolution. Figure 3e,f presents STM topography images with clear submolecular resolution for the same area as the one in Figure 3d that were obtained with such a modified STM tip. We sometimes observe an additional resonance in the STS spectrum in the V = −500 to −1000 mV range. This is indicative of the presence of a TPyP molecule on the STM tip, causing the LUMO state of the TPyP molecule on the STM tip to also appear at negative voltages. The high-resolution images in Figure 3e,f illustrate that the high-voltage pulse did not cause additional structural modification of the molecules and that, hence, mainly their interaction with the substrate was altered due to dehydrogenation. Finally, we note that previous STM studies report a somewhat higher value for the LUMO-related TPyP resonance.4,5 This difference may be explained by some degree of dehydrogenation that may have occurred during STS measurements at elevated voltages in these studies. To probe the magnetism of Cr-TPyP and Co-TPyP, we performed Kondo resonance measurements near zero voltage. The topography image in Figure 4a includes two Cr-TPyPs and one Co-TPyP. Interestingly, a zero-bias peak (ZBP) is observed for Cr-TPyP (see Figure 4c), while the Co-TPyP dI/dV spectra are featureless around zero bias. The ZBP exists for all Cr-TPyP molecules in the TPyP network, both before and after 1050

DOI: 10.1021/acs.jpclett.5b00196 J. Phys. Chem. Lett. 2015, 6, 1048−1052

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as more detailed understanding of the metalation events. Because of the reactivity of the species involved, such experiments require a full in situ approach, which is, at present, beyond the scope of our research. Ultimately, extensive DFTbased calculations of the Cr- and Co-complexes on Ag(111) are required to further elucidate the here reported differences in their electronic and magnetic behavior. In conclusion, in situ metalation of TPyP molecules on Ag(111) surfaces by Co and Cr is achieved by exposure to an atom beam without affecting the molecular organization. Dehydrogenation experiments elucidate that the Co and Cr atoms incorporate in the porphyrin macrocycle. The magnetic character of the newly synthesized metalloporphyrins is investigated by Kondo resonance measurements, revealing a localized Kondo peak for Cr-TPyP only. In situ metalation of molecules offers yet unexploited opportunities for molecular spintronics studies in controlled conditions, especially for reactive metalloporphyrins that can otherwise not be easily synthesized.

Figure 4. (a) STM topography and (b) corresponding constantcurrent dI/dV map recorded at V = −5 mV and I = 0.1 nA. Image sizes are 4 × 4 nm2. (c) dI/dV spectra of Cr-TPyP and Co-TPyP near zero sample voltage taken at the locations indicated in panel a at 4.5 K. Set points are V = −120 mV and I = 0.4 nA. Solid lines represent Fanoline shape fits. Inset: Temperature dependence of the ZBP width (see Figure S4 in the SI for corresponding dI/dV spectra and their Fanoline shape fitting results). The solid line is a fit to eq 1 in ref 28 with α = 4.3 (±0.1).

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EXPERIMENTAL DETAILS All experiments were conducted in a UHV (base pressure in the 10−11 mbar range) system that includes an Omicron lowtemperature STM operated at 4.5 K. Ag(111) films grown on mica were cleaned by Ar+ sputtering and annealing cycles.30 TPyP powder (purchased from Sigma-Aldrich) was sublimated at 548 K using a Knudsen cell and dosed onto the Ag(111) substrate held at room temperature. Submonolayer amounts of TPyP molecules were obtained for a deposition time of 24 min. Co and Cr atoms were electron-beam evaporated from a Co rod (99.9996% purity, Alfa Aesar) and from Cr granules (99.999% purity, Alfa Aesar), respectively. I(V) spectra were recorded with open feedback loop with a grid size of 200 × 200 points, from which normalized (dI/dV)/ (I/V) spectra that reflect the LDOS can be obtained.17 Alternatively, (dI/dV)(V) spectra were acquired by lock-in detection with an open feedback loop (for Kondo resonance measurements; modulation amplitude is 0.4 mV) or closed feedback loop (for high-voltage measurements; modulation amplitude is 50 mV) at 800 Hz. Although the absolute heights of the electronic features of different species cannot be directly compared for STS with closed feedback loop,31 such measurements allow one to explore with acceptable energy resolution32 the electronic properties of the molecules in a voltage range that is not accessible with open feedback loop due to the rapidly increasing tunneling current that frequently gives rise to damaging of the STM tip or the sample surface. For STM tips, we used mechanically cut PtIr (10% Ir) and polycrystalline W wires. The W tips were electrochemically etched and cleaned in situ by thermal treatment. Prior to STM and STS measurements, STM tips were modified by controlled indentation on the bare Ag(111) surface surrounding the TPyP networks until a clean Ag(111) spectrum is resolved. All bias voltages mentioned are with respect to the sample, and the STM tip is virtually grounded. The STM images were analyzed using the Nanotec WSxM software.33

modification of the network by applying high voltages. This additionally indicates that Cr-TPyP is not affected by the applied high voltage that modifies the macrocycle of bare TPyP. Temperature-dependent measurements of the half width at half-maximum (HWHM) of the ZBP (see inset in Figure 4c) elucidate that the ZBP can indeed be interpreted as a Kondo resonance, and a Kondo temperature TK = 70 ± 5 K can be inferred following the procedure described in refs 27 and 28. The existence of the Kondo resonance demonstrates the magnetic nature of the Cr-TPyP complex. The obtained Kondo temperature is comparable to that of magnetic adatoms on noble metal surfaces,29 e.g., TK = 92 ± 6 K for Co on Ag(111). The dI/dV map in (b) reveals that the ZBP has a very localized character and remains restricted to the porphyrin center of the Cr-TPyP complex. Considering the fact that related Coporphyrins adsorbed on Au(111)2 and Cu(111)22 have been previously reported to show magnetic features, the here observed absence of a ZBP for Co-TPyP on Ag(111) may indicate that its Kondo temperature TK is below the minimum experimental sample temperature of 4.5 K. It has been previously demonstrated that the precise charge transfer and the distance between the metal atom in the complex and the supporting substrate (which determines the wave function overlap between molecular orbitals and surface states) crucially determine the Kondo temperature of the system.12 As indicated above, the higher appearance of Co-TPyP in Figures 1a,b and Figures 3c−e compared to Cr-TPyP suggests that the Co−Ag distance is larger than the Cr−Ag distance, which is consistent with a reduced Kondo temperature for Co-TPyP. In addition, it has been reported recently for magnetic Co-TPP on Au(111) that charge transfer (to an adsorbed NO molecule) turns the Co-TPP molecule nonmagnetic.2 Related to this, Co-TPP also turns out to be nonmagnetic upon adsorption on a Cu(111) surface.6,26Use of complementary techniques, such as, e.g., Xray absorption spectroscopy and X-ray magnetic circular dichroism measurements, can provide additional proof as well

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ASSOCIATED CONTENT

* Supporting Information S

Large-scale STM topographies of the bare TPyP networks. dI/ dV maps of the TPyP network after Co and Cr deposition. Spatially resolved dI/dV spectra before and after dehydrogenation. Temperature-dependent Kondo resonance measure1051

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ments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +32 (0)16 32 28 69. E-mail: Koen.Schouteden@fys. kuleuven.be. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the Research Foundation Flanders (FWO, Belgium) and the Flemish Concerted Action research program (BOF KU Leuven, GOA/14/007). Z.L. thanks the China Scholarship Council for financial support (No. 2011624021). K.S. and V.I. acknowledge support from the FWO.

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