Controlling the Electronic and Structural Coupling of C60 Nano Films

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Controlling the Electronic and Structural Coupling of C60 Nano Films on Fe(001) through Oxygen Adsorption at the Interface Andrea Picone, Dario Giannotti, Michele Riva, Alberto Calloni, Gianlorenzo Bussetti, Giulia Berti, Lamberto Duò, Franco Ciccacci, Marco Finazzi, and Alberto Brambilla ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09641 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016

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ACS Applied Materials & Interfaces

Controlling the Electronic and Structural Coupling of C60 Nano Films on Fe(001) through Oxygen Adsorption at the Interface Andrea Picone,∗,† Dario Giannotti,† Michele Riva,‡ Alberto Calloni,† Gianlorenzo Bussetti,† Giulia Berti,† Lamberto Duò,† Franco Ciccacci,† Marco Finazzi,† and Alberto Brambilla† †Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ‡Institute of Applied Physics, TU-Wien, Wiedner Hauptstraße 8-10, 1040 Vienna (Austria) E-mail: [email protected]*

Abstract C60 molecules coupled to metals form hybrid systems exploited in a broad range of emerging fields, such as nano-electronics, spintronics and photovoltaic solar cells. The electronic coupling at the C60 /metal interface plays a crucial role in determining the charge and spin transport in C60 -based devices, therefore a detailed understanding of the interface electronic structure is a prerequisite to engineering the device functionalities. Here, we compare the electronic and structural properties of C60 monolayers interfaced with Fe(001) and oxygen- passivated Fe(001)-p(1 × 1)O substrates. By combining scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy, photoemission and inverse photoemission spectroscopies, we are able to elucidate the striking effect of oxygen on the interaction between Fe(001) and C60 . Upon C60 deposition on the oxygen-passivated surface, the oxygen layer remains buried at the

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C60 /Fe(001)-p(1 × 1)O interface, efficiently decoupling the fullerene film from the metallic substrate. Tunneling and photoemission spectroscopies reveal the presence of well-defined molecular resonances for the C60 /Fe(001)-p(1 × 1)O system, with a large HOMO-LUMO gap of about 3.4 eV. On the other hand, for the C60 /Fe(001) interface, a strong hybridization between the substrate states and the C60 orbitals occurs, resulting in broader molecular resonances.

Keywords C60 , Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy, Iron, Auger Electron Spectroscopy, Diffusion, Fullerenes, Oxide

1

INTRODUCTION

Adsorption of fullerene (C60 ) on solid surfaces is a widely investigated topic in modern nanoscience. 1–4 Hybrid nano-systems composed by organic molecular structures interfaced with metals or semiconductor constitute the basic building blocks of organic transistors, 5–8 photovoltaic cells 9 and magnetoelectronic devices. 10,11 In order to tailor the device properties, a crucial issue to be addressed is the interaction of the C60 films with the electrodes on which the molecules are deposited. Generally, upon the formation of the C60 /metal interface, a remarkable charge transfer and/or hybridization between the electronic states of the substrate and the frontier orbitals of the molecules are observed . 12–14 These interactions greatly influence the electronic structure of the molecules, inducing a substantial modification of their molecular identity. The superposition between the electronic states at organic/inorganic interfaces is interesting per se, because it allows obtaining hybrid systems with potentially new chemical and physical properties. For instance, the hybridization between the molecular orbitals and the 2


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electronic states of the substrate is a key ingredient to induce a magnetic moment on the molecules 15–17 and to observe spin filtering at organic/inorganic interfaces. 18,19 However, a weak substrate-molecule coupling is required in targeted cases. For instance, the presence of sharp molecular resonances is essential for the observation of vibrational modes 20 or fluorescence 21,22 from molecules. From a structural point of view, the development of new crystallographic phases can be favored by a weak molecule-substrate interaction. 23 Moreover, a weak coupling between molecules and metallic substrates is required in those applications involving the separation of photo excited charge, because the molecule/electrode interaction can reduce the exciton lifetime. 24,25 Various strategies have been reported to obtain C60 layers weakly bound to the support. C60 molecules may be deposited on an oxide surface, as in the case of C60 films interfaced with WO2 , 26,27 TiO2 28 or Fe3 O4 . 29 More sophisticated architectures, involving the coupling of C60 molecules with different organic layers, have been successfully implemented to decrease the overlap between the fullerene resonances and the electronic states of the metallic substrates. 24,30 Recently, an efficient decoupling of C60 from the support has been obtained by inserting a graphene layer in between the molecular film and a metallic 31 or semiconducting 32 substrate. Here, we explore the influence of a decoupling oxygen layer by comparing the electronic properties of C60 supported on either the Fe(001) or the Fe(001)-p(1 × 1)O substrates. Previous investigations performed on C60 /Fe(001) heterostructures have been showing that, upon interface formation, a remarkable hybridization between the fullerene frontier orbitals and the Fe(001) 3d states occurs. 15,33,34 Since the Fe(001)-p(1 × 1)O surface differs from Fe(001) for the presence of a single layer of oxygen atoms adsorbed in the fourfold symmetric hollow sites, 35 the question arises as to whether and how such an oxygen layer can change the C60 /Fe(001) interaction. We show that the oxygen monolayer present on top of the Fe(001)-p(1 × 1)O surface induces a dramatic modification on the electronic structure of the C60 /Fe(001) hybrid interface. Upon C60 deposition on Fe(001)-p(1 × 1)O,

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oxygen atoms remain buried between the organic overlayer and the metallic substrate, preserving the intrinsic electronic structure of the molecules.

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EXPERIMENTAL DETAILS

Samples were prepared in an ultra-high vacuum (UHV) system (low 10−10 mbar pressure range) by starting from a UHV-cleaned MgO(001) single crystal substrate, over which a 400 nm-thick Fe(001) film was grown by means of molecular beam epitaxy (MBE). The oxygen passivated Fe(001)-p(1 × 1)O surfaces were obtained by exposing a clean Fe(001) substrate held at 770 K to 30 L (1 L = 1.3 × 10−6 mbar × s) of pure O2 (partial pressure 2.0 × 10−7 mbar). The samples were then heated at 970 K for 10 min to remove the excess oxygen from the surface. This procedure generates the oxygen-saturated and well-ordered Fe(001)-p(1 × 1)O surface, characterized by one oxygen atom per surface unit cell, lying in the fourfold hollow site of the Fe surface lattice. 36 Fullerene films were grown onto Fe(001) and Fe(001)-p(1 × 1)O substrates by MBE under UHV conditions, with a typical growth rate of about 0.03 equivalent monolayers (ML) per minute. 37 The substrates were held at room temperature during C60 deposition. The sample temperature was measured by a thermocouple mounted in close proximity to the sample position. Scanning tunneling microscopy (STM) was performed by using an Omicron Variable Temperature STM in an UHV chamber connected with the preparation system. Images were acquired in constant-current mode with home-made electrochemically etched W tips. Scanning tunneling spectroscopy (STS) was performed at liquid nitrogen temperature (estimated sample temperature 100 K). Tunneling spectra were collected by superimposing to the applied sample bias (V) a modulation voltage with a root mean square amplitude of 20 mV and detecting the dI/dV signal by a lock-in amplifier. Auger electron spectroscopy (AES) data were acquired by means of a Omicron SPECTALEED with a retarding field analyzer (total acceptance angle 102◦ ). A 3 kV, 20 µA

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electron beam was used, with a 3 V peak-to-peak modulation amplitude. UV photoemission spectroscopy (UPS) was performed with He I radiation from a He discharge lamp. Photoelectrons were collected by a 150 mm hemispherical electron analyzer from SPECS GmbH operated at a pass energy of 0.5 eV. Inverse photoemission spectroscopy (IPES) was performed in the isochromat mode by impinging electrons on the sample and collecting photons at a fixed photon energy of hν=9.6 eV. The estimated instrumental resolution is about 0.05 (0.7) eV full width at half maximum for UPS (IPES). 38 The work function was estimated from the threshold energy for the emission of secondary electrons during He I excitation, by applying a negative potential of −10 V to the sample.

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RESULTS

3.0.1

C60 nucleation and interface structure

Figs. 1(a) and 1(b) compare the very early stages of the growth of C60 on the Fe(001) and Fe(001)-p(1 × 1)O substrates, respectively. In both cases, single molecules are randomly distributed over the surface, suggesting a low C60 diffusivity. The apparent height of a single molecule is about 700 pm, in line with previous results on C60 adsorbed on the surface of 3d metals. 39 At higher coverages, the film morphology develops on the two surfaces in a markedly different way. Figs. 2(a) and 2(b) compare the different structures obtained after deposition of 0.15 ML on the Fe(001) and Fe(001)-p(1 × 1)O substrates, respectively. In the former case, just an increase of C60 density is observed, while in the latter the surface morphology is highly inhomogeneous. In particular, for the C60 /Fe(001)-p(1 × 1)O system, regions in which large C60 islands have been nucleated coexist with areas in which well-separated C60 molecules are present. This observation is quite surprising, considering that the nucleation of islands is generally associated with a diffusion-mediated growth, 40–42 while the presence of single molecules is reminiscent of statistic growth. 43 We recall that the Fe(001)-p(1 × 1)O 5



C60/Fe(001)

C60/Fe(001)-p(1x1)O

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

4.0

Position [nm]

Figure 1: Early stages of C60 nucleation on Fe(001) (left column) and Fe(001)-p(1 × 1)O (right column). (a,b) Randomly distributed C60 molecules are present on both the Fe(001) (coverage 0.09 ML) and the Fe(001)-p(1 × 1)O (coverage 0.02 ML) surfaces. Images size is 150 × 150 nm2 . Tunneling parameters are V = 1 V, I = 500 pA. The insets show a blow up of the surfaces, in which single molecules are visible. The size of both insets corresponds to 15 × 15 nm2 . Tunneling parameters for insets of panels (a) and (b) are V = 1.5 V, I = 100 pA and V = 1 V, I = 500 pA, respectively. (c,d) STM profiles corresponding to the line scans displayed in the insets. In both cases the C60 molecule is imaged with an apparent height of about 700 pm with respect to the substrate.

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substrate is highly homogeneous, 35,44–46 thus the presence of different regions cannot be explained by chemical and/or morphological changes across the Fe(001)-p(1 × 1)O surface. The discussion of the unusual nucleation path observed for the C60 /Fe(001)-p(1 × 1)O system is beyond the scope of the present paper, however we suggest that this phenomenon deserves further experimental and theoretical investigations. Fig. 2(c) shows that molecules assembled in islands form a hexagonal two-dimensional lattice, corresponding to the close packed (111) face of bulk face-centered-cubic C60 . The line scan reported in Fig. 2(d) reveals that the lattice constant is 1.1 ± 0.1 nm, similar to that obtained in well-ordered C60 overlayers grown on either weakly 31 or strongly 47 interacting substrates. Fig. 3 focuses on a coverage close to 1 ML. On the Fe(001) surface [Fig. 3(a)] the molecules do not display long-range order, as evidenced by the lack of well defined features in the Fast Fourier Transform (FFT) of the STM image, displayed in Fig. 3(c). It should be underlined that, in the present work, the deposition of C60 was performed at room temperature, without any post-growth thermal treatments. Recently, Wong et al. obtained a higher local degree of order by depositing C60 on Fe(001) at higher temperature or by performing post-growth annealing. 47 On the Fe(001)-p(1 × 1)O substrate, the C60 film displays locally ordered hexagonal domains. However, the FFT reported in Fig. 3(d) reveals that four orientations are present, incommensurate with the Fe(001)-p(1 × 1)O lattice. As a matter of fact, the hexagonal domains do not possess a well defined epitaxial relation with the underlying square substrate, indicating a weak coupling between the fullerene islands and the Fe(001)-p(1 × 1)O surface. In order to investigate the interface chemistry of the C60 /Fe(001)-p(1 × 1)O samples, AES spectra were acquired for the different coverages. Fig. 4(a) displays the spectra related to the high kinetic energy region. On the Fe(001)-p(1 × 1)O substrate, the Auger peak corresponding to the O KLL transitions is visible, along with the LMM transitions characteristic of Fe. After the deposition of increasing amounts of C60 , both the O KLL

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C60/Fe(001)

C60/Fe(001)-p(1x1)O

(a)

(b)

10 nm (c)

(d) 80 pm

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0

2

4

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Position [nm]

10

12

Figure 2: (a,b) STM images acquired after deposition of 0.15 ML of C60 on the Fe(001) [panel (a)] and Fe(001)-p(1 × 1)O [panel(b)] substrates, respectively. Tunneling parameters are V = 1 V, I = 500 pA and V = 0.1 V and I = 1 pA for panels (a) and (b), respectively. (c,d) Intermolecular resolved STM image showing the hexagonal close packing of C60 inside an island nucleated on Fe(001)-p(1 × 1)O. The measured lattice constant is 1.1± 0.1 nm. Tunneling parameters are V = 0.1 V and I = 1 pA.

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C60/Fe(001)

C60/Fe(001)-p(1x1)O (b)

(a)

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[010] [100]

Figure 3: (a,b) Surface morphology after deposition of 0.8 ML on Fe(001) [panel (a)] and on Fe(001)-p(1 × 1)O [panel (b)]. Images size is 100 × 100 nm2 . Tunneling parameters are I = 500 pA, V = 1 V for panel (a) and I = 300 pA, V = 1.5 V for panel (b). Panels (c) and (d) display the FFT of the topographic images of panels (a) and (b), respectively. The FFT of C60 /Fe(001) is featureless, while in the FFT of C60 /Fe(001)-p(1 × 1)O four distinct hexagonal domains are visible.

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and the Fe LMM peaks are attenuated. However, Figure 4(c) shows that the ratio between their intensities as function of C60 coverage remains nearly constant. Considering that the electrons emitted from O KLL and Fe LMM transitions are characterized by a similar inelastic mean free path (λKLL = 10 Å vs. λ LMM = 11.9 Å), 48 AES data demonstrate that the oxygen layer remains buried between the Fe topmost layer and the C60 film. This phenomenology is quite different with respect to that typically observed upon deposition of transition metals on the Fe(001)-p(1 × 1)O, where oxygen floats on top of the growing film. 49–54 Recently, the occurrence of oxygen buried at the interface has been observed in the case of NaCl films deposited on the Fe(001)-p(1 × 1)O substrate. 55 In the low kinetic energy region [see Fig. 4(b)], the Fe MVV transition is characterized by a metallic peak at about 46 eV and by a shoulder at lower kinetic energy, related to the presence of Fe-O bonds. 56–60 The shoulder is present also after C60 deposition, confirming that the Fe-O bonds are preserved. The low energy electron diffraction patterns acquired on 0.8 ML C60 /Fe(001)-p(1 × 1)O (not shown) are characterized by the same square symmetry of the Fe(001)-p(1 × 1)O surface, suggesting that the oxygen atoms retain a well-ordered p(1 × 1) structure with respect to the substrate also after C60 deposition. 3.0.2

Electronic coupling

The electronic properties of C60 molecules on the Fe(001) and Fe(001)-p(1 × 1)O substrates have been probed by STS and PES/IPES. STS allows measuring both the filled and empty states in a single measurement with high spatial selectivity. On the other hand, combining PES and IPES allows to explore the filled and empty states in a larger energy window, avoiding at the same time the possibility of perturbing the molecules with the STM tip. Figure 5 displays STS spectra acquired on single C60 molecules adsorbed on Fe(001)-p(1 × 1)O (red dashed-dotted line), C60 islands nucleated on Fe(001)-p(1 × 1)O (continuous black line) and single C60 molecules on Fe(001) (dashed blue line). The HOMO resonance for a single C60 molecule/Fe(001)-p(1 × 1)O is detected at 2.4± 0.05 eV below the Fermi level 10


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High kinetic energy (a) 0.83 ML C

Low kinetic energy (b)

60

0.83 ML C60

(iv)

0.44 ML C60

(iii)

0.16 ML C60

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Differential AES Intensity (a.u.)

0.44 ML C60 (iii)

0.16 ML C60 (ii)

Fe(001)-p(1x1)O (i) Fe(001)-p(1x1)O (i) O KLL Iron Iron Oxide

Fe LMM

450 500 550 600 650 700 750

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Electron Kinetic Energy (eV) AES Intensity Ratio

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Coverage (ML)

Figure 4: (a,b) AES spectra acquired on the C60 /Fe(001)-p(1 × 1)O sample in the high and low kinetic energy regions. Spectra (i) refers to the Fe(001)-p(1 × 1)O substrate. Spectra (ii), (iii) and (iv) have been acquired after deposition of 0.16 ML, 0.44 ML and 0.83 ML of C60 , respectively. (c) Evolution of the intensity of Fe LMM (IFe ) and O KLL (IO ) peaks, as a 0 and I 0 are the intensities measured on the Fe(001)-p (1 × 1)O function of C60 coverage. IFe O 0 ), red dots report the ratio between (I /I 0 ) substrate. Black squares correspond to (IFe /IFe Fe Fe 0 and (IO /IO ). The error bars have been determined by considering the variance of the AES signal in flat regions of the spectra.

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(EF ), while the LUMO and LUMO+1 orbitals are placed at 0.9± 0.05 eV and 2.2± 0.05 eV above EF , respectively. In the case of C60 islands nucleated on the Fe(001)-p(1 × 1)O substrate, the HOMO is 2.6± 0.05 eV below EF , and the LUMO (LUMO+1) is 0.8± 0.05 (2.1± 0.05) eV above EF . The HOMO-LUMO gap of islands and single molecules adsorbed on Fe(001)-p(1 × 1)O is very similar, as recently found also in the case of C60 nucleated on Co/Au(111). 61 In the case of C60 islands on Fe(001)-p(1 × 1)O, however, a clear region of negative differential conductance is visible around 1.5 eV above EF . The negative differential conductance is due to the higher energy barrier experienced by the electrons tunneling into the LUMO states when the voltage increases above the resonance. 62 Generally, negative differential conductance is detected when the molecule possess intense and sharp resonances. 24,30,62 This observation suggests that in C60 islands the electronic coupling with the substrate is further reduced. The spectra acquired on the oxygen-free Fe(001) substrate are markedly different. The HOMO resonance is located at -1.9± 0.05 eV, shifted by about 0.5 eV with respect to the case of single C60 molecule/Fe(001)-p(1 × 1)O. In the empty states side of the spectrum, no clear features related to LUMO and LUMO+1 states are detected. Such a circumstance stem from the broadening of the LUMO peaks, induced by the hybridization of the molecular orbitals with the electronic states of the substrate. Fig. 6 displays PES/IPES spectra acquired on a multilayer C60 film deposited on Fe(001)p(1 × 1)O, 1 ML C60 /Fe(001)-p(1 × 1)O and 1 ML C60 /Fe(001). The electronic structure of multilayer C60 /Fe(001)-p(1 × 1)O is consistent with previous PES/IPES measurements performed on solid C60 , 63 thus it can be taken as a reference for bulk C60 . Interestingly, the 1 ML C60 /Fe(001)-p(1 × 1)O spectrum is very similar to that of the multilayer C60 film, indicating a negligible interaction between C60 and Fe(001)-p(1 × 1)O. On the other hand, the spectrum acquired on 1 ML C60 /Fe(001) reveals a strong influence of the metallic substrate on the molecular orbitals. In good agreement with STS data, the HOMO peak is closer to EF by about 0.6 eV with respect to the corresponding feature measured on the C60 /Fe(001)-p(1 × 1)O sample. Moreover, the LUMO and LUMO+1 peaks are no longer

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Island C60/Fe(001)-p(1x1)O Single C60/Fe(001)-p(1x1)O Single C60/Fe(001)

6 dI/dV (nS)

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4 2 0 -3

-2

0 -1 1 Energy (eV)

2

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Figure 5: STS spectra acquired on single C60 molecules on Fe(001) (dashed blue curve),single C60 molecules on Fe(001)-p(1 × 1)O (red dashed dotted line), C60 islands on Fe(001)-p(1 × 1)O (continuous black line). Notice the region of negative differential conductance dI/dV around 1.5 eV for spectra measured on C60 islands nucleated on Fe(001)-p(1 × 1)O. Before switching off the feedback loop, the tip-sample tunnel junction was stabilized at V = 1.5 V and I = 100 pA.

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resolved.

L U M O + 2 L U M O + 1

H O M O H O M O -1

L U M O

M u ltila y e r C

In te n s ity ( a r b . u n its )

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6 0

o n F e (0 0 1 )-p (1 x 1 )O

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1 M L C

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o n F e (0 0 1 )

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Figure 6: Photoemission (left) and inverse photoemission (right) spectra acquired on multilayer C60 /Fe(001)-p(1 × 1)O (upper spectrum), 1 ML C60 /Fe(001)-p(1 × 1)O (middle spectrum) and C60 and 1 ML C60 /Fe(001) (lower spectrum).

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Discussion

The data presented in the previous section demonstrate a remarkable influence of the oxygen atoms on the electronic properties of the C60 overlayer. Table 1 reports the position of molecular resonances on islands nucleated on Fe(001)-p(1 × 1)O and single C60 molecules adsorbed on Fe(001) measured by means of PES/IPES and STS. Here, it is important to notice that the energy difference between LUMO and HOMO states is about 3.4 eV for C60 nucleated on Fe(001)-p(1 × 1)O, a value considerably larger than that generally found 14


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in C60 adsorbed on metallic substrates (see for instance table I reported by Cho et al. and references therein 32 ). Notice that such a large gap is very similar to the value of 3.5 eV obtained for bulk C60 64 or C60 molecules deposited on inert graphene. 32 Table 1: Energy position (eV) with respect to EF of the HOMO and LUMO resonances for C60 /Fe(001)-p(1 × 1)O and C60 /Fe(001). The values without brackets are obtained from STS measured on islands nucleated on Fe(001)-p(1 × 1)O or single C60 molecules adsorbed on Fe(001). The values in brackets are extracted from PES/IPES spectra acquired on 1 ML C60 . Resonance HOMO-1 HOMO LUMO LUMO+1 LUMO+2

C60 /Fe(001)-p(1 × 1)O (−3.6) −2.6(−2.4) 0.8(0.9) 2.1(2.0) (3.0)

C60 /Fe(001) (−3.2) −1.9(−1.8)

(3.3)

In order to discuss the influence of oxygen on the C60 electronic structure, we recall that the proximity of a metal surface can modify the electronic structure of the molecular overlayer in three different ways: reduction of the HOMO-LUMO band gap, charge transfer across the interface, hybridization between electronic states of metal and molecular orbitals. Despite these effects are intimately interwoven, for the sake of clarity, they will be addressed separately in the following. 1) Reduction of the HOMO-LUMO band gap. In strongly correlated systems, such as C60 molecules, the HOMO-LUMO energy gap can be written as ∆Egap = γ + U, where γ is the mean field molecular orbital splitting and U is the so-called Hubbard term. The U term accounts for on-site Coulomb electron-electron interaction arising when an electron is removed from (or added to) the neutral molecule. Single C60 molecules in the gas phase are characterized by an electron affinity EA = 2.65 eV and a ionization potential IP = 7.6 eV, corresponding to ∆Egap = 4.95 eV. Being γ = 1.6 eV, we obtain for the isolated molecule U = 3.35 eV. 63 On the other hand, when supported by a substrate, C60 usually experiences a considerable reduction of U, because the surrounding medium screens the net charge left after removing or adding an electron. For C60 in the solid state, the screening is provided 15



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by the electric dipole induced on the surrounding molecules. 64 Each neighbor molecule contributes to the reduction of U by a term

e2 α , 4πeR4

being α the molecule polarizability,

R the molecule-molecule distance, e the electron charge and e the dielectric constant. 63 When supported by a metal, an additional screening is provided by image charges induced underneath the metal surface by the charged molecule. 13,63,65 Correspondingly, the band gap is reduced by a term

e2 4πe2D ,

where D is the distance between the center of the molecule

and the metallic surface. 63 In the case of C60 grown on Fe(001)-p(1 × 1)O, it is interesting to notice that the HOMO-LUMO gap of C60 single molecules is similar to the gap of the C60 islands, suggesting that the screening is dominated by the substrate. In order to compare the different screening provided by Fe(001)-p(1 × 1)O and Fe(001), we refer to the energy difference between HOMO and LUMO+2, since the broadening of LUMO and LUMO+1 peaks on the oxygen-free surface prevents the comparison of the HOMO-LUMO gap. The C60 HOMO-LUMO+2 energy difference is 5.4 eV on Fe(001)-p(1 × 1)O and 5.1 eV on Fe(001), indicating a lower screening provided by the oxidized surface. To explain such a difference, we suggest that the one-layer-thick FeO accommodated on top of the Fe(001) surface can be regarded as a buffer layer placed between the C60 molecules and the metallic substrate, which increases D and reduces the effect of the image charge. 2) Charge transfer across the C60 /metal interface. Because of the high electronegativity of C60 , a remarkable charge transfer from a metallic substrate to the molecules is generally observed. The transferred electrons induce an interface electric dipole, thus generally the position of molecular orbitals with respect to EF cannot be obtained by a simple alignment of the molecule and metal vacuum levels. 66 In order to evaluate the differences in charge transfer, the work function for a C60 monolayer deposited on either the Fe(001)-p(1 × 1)O or the Fe(001) surface was measured, finding 4.4± 0.1 eV and 4.9± 0.1 eV, respectively. Comparing these results with the values of 4.4 eV and 3.9 eV, computed for Fe(001)-p(1 × 1)O and Fe(001), 67 respectively, we conclude that in the case of C60 /Fe(001)-p(1 × 1)O the charge transfer is negligible, while an electric dipole is present

16


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at the C60 /Fe(001) interface. 3) Hybridization between the electronic states of the metal and the C60 frontier orbitals. STS ad IPES data acquired on C60 /Fe(001) show a remarkable broadening of the unoccupied C60 resonances, which induces the merging of the LUMO and the LUMO+1 peaks. On the other hand, the shape and position of LUMO and LUMO+1 features for C60 /Fe(001)-p(1 × 1)O are similar to those measured on the surface of solid C60 . Accordingly, the hybridization of C60 orbitals with the metal electronic states seems to be hindered by the oxygen. In order to rationalize the different electronic coupling between the C60 and the substrate, we recall that the electronic structure of the Fe(001) around EF is dominated by minority-spin states arising from d-orbitals. 68 According to Tran et al., these electronic states take part in the chemical bonding between C60 and Fe(001). 33 On the other hand, the oxygen layer chemisorbed on the Fe(001)-p(1 × 1)O surface noticeably modifies the minority-spin component of the surface density of states, 36,46 thus it is reasonable to speculate that such a modification hinders the hybridization between the C60 molecular orbitals and the iron electronic states.

5

CONCLUSIONS

The electronic structure of C60 films deposited on either Fe(001) or Fe(001)-p(1 × 1)O substrates has been investigated by combining scanning tunneling microscopy and spectroscopy, Auger spectroscopy, and photoemission electron spectroscopy. The chemisorbed oxygen layer on the Fe(001)-p(1 × 1)O surface remains buried at the C60 /Fe(001)-p(1 × 1)O interface, efficiently decoupling the fullerene layer from the metal. C60 /Fe(001)-p(1 × 1)O is characterized by a large HOMO-LUMO gap of 3.4 eV, comparable to that found on the surface of bulk C60 . Considering the strong interaction generally reported for the C60 /Fe(001) system, we suggest that the adsorption of a single layer of electronegative atoms on metallic substrates

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could be a general strategy to obtain organic monolayers weakly interacting with the substrate. In addition, the presented results show that oxygen contamination should be avoided in those cases in which the goal is to obtain an hybridization between the molecular orbitals and the electronic states of the substrate.

6

Acknowledgment

The authors gratefully acknowledge Fondazione Cariplo for financial support through grant 2013-0623.

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Graphical TOC Entry 8

Oxygen

C60/Fe(001)-p(1x1)O C60/Fe(001)

Iron

6

dI/dV (nS)

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 55 56 57 58 59 60


4 2 0 -3

27

-2

0 -1 1 Energy (eV)

2


3

Controlling the Electronic and Structural Coupling of C60 Nano Films

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