Superstructures and Electronic Properties of Manganese

Article pubs.acs.org/Langmuir

Superstructures and Electronic Properties of Manganese− Phthalocyanine Molecules on Au(110) from Submonolayer Coverage to Ultrathin Molecular Films M. Topyła, N. Néel,* and J. Kröger Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany ABSTRACT: The adsorption of manganese−phthalocyanine molecules on Au(110) was investigated using a low-temperature scanning tunneling microscope. A rich variety of commensurate superstructures was observed upon increasing the molecule coverage from submonolayers to ultrathin films. All structures were associated with reconstructions of the Au(110) substrate. Molecules adsorbed in the second molecular layer exhibited negative differential conductance occurring symmetrically around zero bias voltage. A double-barrier tunneling model rationalized this observation in terms of a peaked molecular resonance at the Fermi energy together with a voltage drop across the molecular film.

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INTRODUCTION In surface chemistry and surface physics phthalocyanine molecules have attained substantial attention. Along with their availability and stability, these molecules are supposed to be promising building blocks in nanotechnology.1,2 Indeed, phthalocyanine molecules were demonstrated to exhibit a wide range of functions such as chemical reactivity, optoelectronic conductance, and optical absorbance, and they act as electron donors and acceptors.1,2 Moreover, phthalocyanine molecules on surfaces are interesting owing to their appealing model character for the organic−inorganic interface. Recently, research has been devoted to the phthalocyanine assembly on a variety of substrate surfaces3−14 and to their electronic15−18 and vibrational14,19−21 as well as magnetic22−34 properties. Chemical reactions at the single-molecule level have likewise been reported for a variety of phthalocyanines.35−40 Recent comprehensive review articles are available that summarize different aspects of phthalocyanine molecules on surfaces.41−43 Our investigations into structural and electronic properties of manganese−phthalocyanine (Mn−C 32 N 8 H 16 , MnPc) on Au(110) were motivated by the following issues. The Au(110) surface exhibits a characteristic (2 × 1) missing-row surface reconstruction, which may act as a template for MnPc growth. Moreover, MnPc adsorption studies on Au(110) at the single-molecule level have not been reported to date. Here we show by scanning tunneling microscopy that MnPc adsorption leads to several commensurate molecular superstructures with increasing coverage. The substrate surface is subject to adsorption-induced reconstructions that enable individual MnPc molecules to hybridize with a maximum number of Au atoms. For such adsorbed molecules signatures of molecular orbitals are hardly discernible in spectra of the differential © 2016 American Chemical Society

conductance (dI/dV). In contrast, MnPc molecules adsorbed in the second molecular layer give rise to clear signatures of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, secondlayer MnPc molecules exhibit negative differential conductance (NDC) at symmetric negative and positive bias voltage. The basic physics of the observed NDC is captured by a doublebarrier tunneling model where Mn d states and a voltage drop across the molecular film play an important role.

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EXPERIMENTAL METHODS

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RESULTS

Experiments were performed with a scanning tunneling microscope (STM) operated at 77 K and in ultrahigh vacuum (10−8 Pa). The Au(110) surface was cleaned by Ar+ bombardment and annealing. The MnPc molecules were sublimated from a heated Ta crucible and adsorbed on Au(110) at room temperature. Molecular coverages are expressed as surface densities, which were obtained by dividing the number of molecules in a large-scale STM image by the area of the imaged region. STM images were recorded at constant current with the bias voltage applied to the sample. Spectra of dI/dV were acquired by modulating the sample voltage (5 mVrms, 950 Hz) and measuring the current response with a lock-in amplifier.

Coverage-Dependent Commensurate Superstructures. A sketch of the MnPc molecule used in this work is shown in Figure 1a. Hydrogen (H), carbon (C), nitrogen (N), and manganese (Mn) atoms appear in white, gray, blue, and violet color, respectively. Figure 1b is an overview STM image Received: April 21, 2016 Revised: June 14, 2016 Published: June 20, 2016 6843

DOI: 10.1021/acs.langmuir.6b01529 Langmuir 2016, 32, 6843−6850

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of the Au(110) surface after depostion of MnPc molecules with an average surface density of ≈0.01 nm−2. The characteristic (2 × 1) missing-row reconstruction of Au(110) is visible as alternating bright and dark rows along the [11̅0] direction. Bright rows are separated by 0.81 nm, which corresponds to the double of the Au(110) lattice constant (a = 0.408 nm). At low surface densities MnPc molecules predominatly decorated step edges. The characteristic crosslike shape (Figure 1a) of the molecule was particularly well observed in close-up STM images (Figure 1c). From such close-up views their oblique adsorption configuration at step edges was inferred. Two isoindole groups resided on the upper terrace while the remaining isoindole pair adsorbed on the lower terrace. The azimuthal rotation and the inclination angle of MnPc at step edges was irregular and depended on the local environment. At higher coverage (≈0.05 nm−2) step edges of the surface were saturated, and MnPc molecules likewise adsorbed on terraces (Figure 2). MnPc molecules formed chains along the [11̅0] direction (Figure 2a) and locally modified the surface reconstruction (Figure 2b). Two types of molecules adsorbing in two distinct configurations were discernible and will be referred to as α-MnPc and β-MnPc in the following. α-MnPc molecules were characterized by all isoindole groups appearing with nearly uniform contrast in STM images. They adsorbed between two Au rows separated by 2.05 ± 0.10 nm, which is virtually identical with 5a. Therefore, the adsorption of α-MnPc was accompanied by a local variation of the (2 × 1) to a (5 × 1) missing-row reconstruction of Au(110). Similar observations were reported for other metal−phthalocyanine molecules on Au(110).3,12,13 Chains of fullerene molecules on vicinal Au surfaces were previously shown to induce strong substrate modifications, too.44 Two opposite isoindole groups of β-MnPc molecules adsorbed on top of Au atom rows and were imaged with brighter contrast than the remaining isoindole pair. The adsorption of β-MnPc likewise induced a local modification of the Au(110) surface reconstruction to a (3 × 1) missing-row reconstruction, where the Au atom rows carrying the isoindole groups were separated by 1.21 ± 0.10 nm, which is virtually identical with 3a. H2Pc molecules on Au(110) were also

Figure 1. (a) Sketch of MnPc (H: white; C: gray; N: blue; Mn: violet). (b) Overview STM image of Au(110) covered with MnPc at a low surface density of molecules (≈0.01 nm−2, 1 V, 10 pA, 45 × 45 nm2). Crystallographic directions are indicated. (c) STM image of a single MnPc molecule adsorbed at a step edge of Au(110) (1 V, 10 pA, 5.9 × 5.9 nm2).

Figure 2. (a) STM image of MnPc-covered Au(110) at a surface density of ≈0.05 nm−2 where molecules likewise occupy step edges and terrace sites (1 V, 100 pA, 45 × 45 nm2). (b) Close-up view of (a) showing different types of MnPc molecules and revealing adsorbate-induced surface reconstructions (1 V, 100 pA, 18 × 8 nm2). MnPc molecules adsorbing between two Au atom rows are labeled α. MnPc molecules with two opposite isoindole groups residing atop Au atom rows are labeled β. Surface regions which are not occupied by molecules exhibit a distance of 2a (a: Au(110) lattice constant) between two adjacent Au atom rows. α-MnPc are embedded between two Au rows with a mutual distance of 5a. β-MnPc span adjacent Au rows separated by 3a. The intermolecular distance of α-MnPc is 5b with b the Au(110) nearest-neighbor distance in [11̅0] direction. (c, d) Sketches of top and side views of suggested α-MnPc (c) and β-MnPc (d) adsorption configurations. 6844

DOI: 10.1021/acs.langmuir.6b01529 Langmuir 2016, 32, 6843−6850

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Langmuir reported to induce this reconstruction.11 In order to identify the suggested adsorption geometry of α-MnPc and β-MnPc sketches of top and side views are presented in Figure 2c,d. These sketches show that α-MnPc molecules adsorb on a flat, that is not reconstructed, (110) surface while β-MnPc molecules reside atop a deep missing row. Previous calculations for H2Pc11 and FePc13 on Au(110) unraveled that the large adsorption energy compensated for the energy cost required for modifying the surface reconstruction. α-MnPc and β-MnPc molecules exhibited two azimuthal orientations. The molecular axis defined by two opposite isoindole groups enclosed an angle with the [001] direction of ±24 ± 1° for α-MnPc and ±14 ± 1° for β-MnPc. Within a molecular chain α-MnPc molecules showed a similar orientation. The distance between molecular centers was 1.44 ± 0.10 nm, which is virtually identical with 5b (b = a/√2 = 0.288 nm is the nearest-neighbor distance between Au atoms in [110̅ ] direction). Consequently, the α-MnPc arrangement was commensurate with the Au(110) lattice. The orientation of βMnPc within a molecular chain was not as uniform as observed for α-MnPc. Moreover, the intermolecular distances within a given chain varied. The shortest distance of 1.78 ± 0.10 nm was measured for adjacent β-MnPc molecules with similar orientation. This distance corresponds to 6b and likewise indicates a commensurate arrangement of β-MnPc molecules with the Au(110) lattice. In previous experiments FePc on Au(110) was reported to exhibit exclusively the α-type adsorption with a smaller and incommensurate intrachain periodicity of 1.38 nm.12,13 The binding was shown to occur via the interaction of the isoindole groups and the central metallic atom with the Au surface.9,45 For H2Pc on Au(110), in contrast, only the β-type adsorption was observed.11 Calculations revealed that in this adsorption geometry the two opposite isoindole groups residing on top of the Au atom rows were slightly bent upward. In addition, these isoindole groups were mainly involved in the binding to the surface by accepting charge transferred from the substrate. As a consequence, the electron density at the isoindole groups was increased. Both effectsthe upward bending and the increased electron density of the isoindole groupsgave rise to the their saddle shape appearance in STM images. Here, we observed that MnPc molecules adopted both adsorption configurations. Annealing at ≈500 K after room temperature deposition did not modify the ratio of α and β molecules, indicating that both adsorption geometries were stable. At a surface density of MnPc molecules of ≈0.3 nm−2 Au(110) was nearly uniformly saturated with α-MnPc chains (Figure 3a). In this regular assembly adjacent rows of α-MnPc molecules oriented along [11̅0] were separated by single rows of Au atoms. The periodicity along [001] and [11̅0] was 5a and 5b, respectively. Therefore, we refer to this superstructure as Au(110)-(5 × 5) MnPc. Within a molecular chain the orientation of α-MnPc molecules was uniform, while in adjacent chains the orientation occasionally changed. The latter observation may hint at a rather low interaction between adjacent molecular chains, which is similar to FePc on Au(110).12,13 At this coverage, α-MnPc molecules were observed more frequently than β-MnPc molecules. This observation may be traced to different interactions between adjacent molecular chains. Adjacent chains of α-MnPc molecules are separated by a single Au atom row, which hints at a low interchain coupling. By contrast, isoindole groups of neighboring β-MnPc chains adsorb on one at the same Au

Figure 3. (a) STM image of MnPc-covered Au(110) at a molecular surface density of ≈0.3 nm−2 (1 V, 10 pA, 43 × 37 nm2). (b) Close-up view of (a) showing a (5 × 5) MnPc superstructure with Au atom rows separated by 5a and intermolecular distances of 5b along [11̅0] (1 V, 10 pA, 8.8 × 9.1 nm2). (c) Sketch of the suggested MnPc adsorption geometry in the (5 × 5) superstructure.

atom row (Figure 4). The concomitant larger interaction between adjacent chains of β-MnPc may result in a less favorable configuration. Increasing the molecular surface density to ≈0.4 nm−2 led to the appearance of β-MnPc molecular chains in addition to αMnPc chains (Figure 4a). At this surface density 15% of the observed molecules adopted the β configuration. β-MnPc molecules often formed two-dimensional domains of adjacent chains (blue square in Figure 4a,b). Within a chain β-MnPc exhibited uniform orientation and an intermolecular distance of 1.78 ± 0.10 nm. This distance was found as the shortest separation in β-MnPc chains at lower coverage (vide supra) and corresponds to 6b. Isoindole groups of the molecules of two adjacent chains adsorbed on the same Au row giving rise to alternating β-MnPc orientations in adjacent chains (sketch in Figure 4b). The distance between molecular chains in the [001] direction was 3a. Therefore, this superstructure is referred to as Au(110)-(3 × 6) MnPc. α-MnPc molecules formed an additional superstructure at this molecular surface density (red square in Figure 4a,c). Alternating bright and dark chains of α-MnPc were observed in STM images. The dark chains were identical to the embedded chains of the Au(110)-(5 × 5) MnPc superstructure (vide supra). Chains containing molecules that appeared with bright contrast in STM images exhibited a similar adsorption configuration as chains with dark contrast. In particular, molecular orientations and intermolecular distances were identical. Deviations are due to the Au(110) facets on which the bright α-MnPc molecules adsorbed. These facets are 0.144 6845

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Figure 4. (a) STM image MnPc-covered Au(110) at a molecular surface density of ≈0.4 nm−2 (1 V, 10 pA, 45 × 45 nm2). Inset: close-up view of MnPc molecules with oblique adsorption geometry indicated by a bright narrow stripe (arrow) oriented along [11̅0]. (b, c) Close-up STM images acquired in the regions of (a) indicated by the blue (b) and red (c) squares (1 V, 10 pA, 7.8 × 7.8 nm2) together with sketches of top and side views of the suggested (3 × 6) (b) and (7 × 5) (c) molecular superstructures.

nm higher than the facets for α-MnPc appearing with dark contrast. The lateral distance between two nearest elevated and embedded chains along the [001] direction was determined as 2.83 ± 0.10 nm, which corresponds to 7a. This distance indicates a modification of the surface reconstruction. Different orientations of molecules were observed in adjacent chains. The superstructure will be referred to as Au(110)-(7 × 5) MnPc. The same pattern was reported for FePc, CuPc, and CoPc on Au(110).12,46 Accompanying calculations suggested that the elevated molecules were adsorbed on top of three Au atom rows. Similar to the superstructures observed at lower coverage, the larger adsorption energy of the elevated molecules was shown to drive the modification of the reconstruction. Consequently, bright an dark α-MnPc adsorbed on locally nonreconstruced Au(110). In addition to the discussed horizontal adsorption geometries of MnPc molecules oblique configurations were observed at this molecular surface density. These structures were indicated by bright narrow stripes (inset to Figure 4a). As discussed in more detail for the following surface density MnPc molecules adsorbed on (111) facets formed by the edges of compact Au(110) planes. In STM images only the upper isoindole groups of obliquely adsorbed MnPc were visible as bright protrusions (inset to Figure 4a). Chains of oblique MnPc molecules were separated by 1.42 ± 0.10 nm, which corresponds to 5b and thus was commensurate with the Au(110) lattice. At a molecular surface density of ≈0.5 nm−2 (Figure 5a) ≈60% of the MnPc molecules adopted an oblique adsorption configuration. Oblique molecules often occurred in adjacent chains (Figure 5b) with a mutual distance along [001] of 1.2 ± 0.10 nm, which corresponds to 3a. Considering the observed intermolecular distance of 5b within a chain this superstructure is referred to as Au(110)-(3 × 5) MnPc. Oblique adsorption in a (3 × 5) phase was reported for several 3d transition-metal phthalocyanine molecules on Au(110) for a coverage close to the monolayer.3,9,45,47 MnPc molecules started to reside in the second molecular layer at this coverage (Figure 5c). Second-layer MnPc molecules adsorbed atop molecules belonging to the dark chains of the first-layer (7 × 5) superstructure. Adsorption on bright chains appeared before the completion of the embedded (dark) chains. The distance between molecules within the chain

as well as the distance between the chains were identical to the (7 × 5) superstructure. In contrast to molecules adsorbed in the first molecular layer, STM images of MnPc molecules of the second layer exhibited a voltage dependence (vide infra). Summarizing the structural aspects of MnPc adsorption on Au(110), we found that different molecular superstructures occurred upon increasing the coverage. The observed assemblies were commensurate with the Au(110) lattice and induced rearrangements of the Au(110) atomic structure. Starting from Au(110)-(5 × 5) MnPc with a local molecular surface density of 0.34 nm−2, increasing the coverage led to the appearance of Au(110)-(3 × 6) MnPc (0.47 nm−2) and Au(110)-(7 × 5) MnPc (0.49 nm−2). Further increase of the coverage induced then the Au(110)-(3 × 5) MnPc superstructure (0.57 nm−2). Rather than forming a second layer the superstructures became more compact in order to enable a larger amount of MnPc to be directly adsorbed on Au(110). Such compact assemblies were demonstrated to optimize the interaction between the individual phthalocyanine molecule and the surface, which in turn helped decrease the total energy.12 Orbital Electronic Structure and Negative Differential Conductance. The observed adsorption-induced modifications of the Au(110) lattice hinted at a strong interaction between MnPc and the substrate. It was therefore interesting to examine the molecular electronic structure by spectroscopy of dI/dV. A typical spectrum for MnPc molecules directly adsorbed to Au(110) is presented in Figure 6a. It was acquired atop an isoindole group of an α-MnPc molecule. No clear signatures of molecular orbitals are visible. We suggest that the absence of such signatures is due to the strong hybridization of the MnPc molecules with Au(110), which leads to a considerable broadening of the molecular resonances. These findings are consistent with STM images of MnPc molecules at different voltages (right panels of Figure 6a), where differences were hardly discernible. First-layer α-MnPc and β-MnPc appeared similar in STM images at different bias voltage. For positive bias voltage the molecular central regions appeared with slightly brighter contrast due to Mn d states localized around 1 eV.28 Spectra acquired atop the Mn atom (inset to Figure 6a) gave rise to a weak feature at zero bias voltage. In accordance with previous reports for CoPc on Au surfaces22,48 and MnPc on Ag(100)34 and Au(111),28 we attribute the zero6846

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Figure 6. (a) Spectrum of dI/dV acquired atop an isoindole group of α-MnPc on Au(110). Inset: spectrum of dI/dV acquired atop the Mn center of an α-MnPc on Au(110). Right panels: constant-current (0.1 nA) STM images of the same surface region recorded at the indicated bias voltages (5 × 5 nm2). α-MnPc and β-MnPc molecules are visible. (b) Spectrum of dI/dV acquired atop an isoindole group of a secondlayer MnPc on Au(110). Spectroscopic signatures of the HOMO and the LUMO are visible. Right panels: constant-current (0.1 nA) STM images of the same second-layer MnPc molecule recorded at the indicated bias voltages (3 × 3 nm2). The dI/dV spectra were acquired after disabling the feedback loop at 2 V and 0.1 nA.

Moreover, second-layer MnPc molecules exhibited NDC, which was most pronounced in the central region of the molecules (Figure 7). A typical dI/dV spectrum acquired atop the Mn site of a second-layer molecule is shown as a black line in Figure 7a. The corresponding I−V curve is depicted in the inset to Figure 7a. Pronounced NDC dips are visible at −0.45 and 0.47 V. The NDC was always stronger at negative voltages and occurred at the same voltages across the entire molecule. Constant-current maps of dI/dV at −0.44 V (Figure 7b) and at 0.46 V (Figure 7c) showed a similar spatial distribution of NDC across the molecule. These observations hinted at a common origin for the NDC observed at the different bias voltages. We will show next that Mn d states and a voltage drop between the first and second layer molecules are important to understand the NDC. HOMO and LUMO resonances associated with the MnPc isoindole groups (Figure 6b) were not considered in the simulations. Their spectroscopic signatures appeared at −1 V (HOMO) and at 1.5 V (LUMO) and were thus well separated from the voltage range where the NDC occurred. The dI/dV and I−V characteristics of individual second-layer molecules are very similar to observations reported from semiconductor double barriers,51 which work as Esaki diodes.52 Therefore, the idea of modeling the NDC of individual MnPc molecules on the basis of a double-barrier tunneling junction (DBTJ) was manifest. Previously the DBTJ mechanism was applied to molecules adsorbed on insulating layers53 and to molecular multilayers on surfaces.54 In a DBTJ, tunneling electrons are subject to two tunneling barriers, which extend across the vacuum barrier and across the molecular film. In order to interpret the experimentally observed NDC, the tunneling current at low temperature was described within a one-dimensional single-barrier model55 extended to a DBTJ as

Figure 5. (a) STM image of MnPc-covered Au(110) at a molecular surface density of ≈0.5 nm−2 (−1 V, 100 pA, 54 × 54 nm2). (b) Closeup view of the (3 × 5) molecular superstructure (6.9 × 5.1 nm2) together with sketches of top and side views of the suggested adsorption geometry. (c) Like (b) for a second-layer island exhibiting a (7 × 5) superstructure (9.6 × 6.8 nm2).

bias peak to the spectroscopic signature of Mn d states. Similar spectra were recorded for β-MnPc molecules. Because of the absence of clear spectroscopic signatures of molecular orbitals, inferring a potential influence of the adsorption geometry on, e.g., orbital energies and resonance line shapes was hampered. Spectra of dI/dV are significantly different for second-layer molecules (Figure 6b). Spectroscopic signatures of the HOMO and the LUMO appeared as peaks at −1 and 1.5 V, respectively. These values gave rise to a HOMO−LUMO gap of 2.5 eV, which is only slightly lower than the gap width of 2.9 eV reported for thick MnPc layers by optical absorption spectroscopy.49 Moreover, STM images of second-layer molecules depended on the bias voltage (right panels of Figure 6b). They revealed submolecular patterns reminiscent of the HOMO and LUMO spatial distribution.40 These two observations evidenced the efficient decoupling of second-layer MnPc from the metal substrate. Similar findings were reported for various metal phtalocyanines on metallic surfaces.5,16,50 6847

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Therefore, ϱs (ϱm) was modeled by a Gaussian centered at −0.1 eV (−0.15 eV) with a width of 0.12 eV (0.04 eV). Furthermore, barrier heights Φms and Φtm were set to 4.5 eV according to reported work functions induced by phthalocyanine adsorption on Au(110).45 A distance between the first-layer and secondlayer molecules of 0.32 nm was chosen in agreement with studies of CuPc and FePc multilayers.58,59 The calculated dI/dV and I−V spectra are plotted as red lines in Figure 7a. The simple DBTJ model captures the basic behavior of the experimental data. In particular, the two NDC features are well reproduced at the experimentally observed bias voltages. The depletion of dI/dV around the Fermi energy is likewise reproduced. The best agreement between experimental and calculated data was achieved for ztm = 0. 48 nm and η = 0.4. Although the tip−molecule distance was not determined experimentally, ztm represents a reasonable distance and is similar to findings for FePc on Ag(111).16 We found that variations of zms and ztm on the order of 100 pm only modified the overall intensity. The general behavior of the calculated dI/ dV data was not affected; that is, the two NDC features always occurred at similar bias voltages. Similarly, the calculated data were rather insensitive to the values of Φms and Φtm. Rather, the calculations revealed that the two main parameters determining the NDC at negative and positive bias voltage were the voltage drop across the molecular film (η) and the width of the peak describing the Mn d states (ϱs). Decreasing η led to the disappearance of the NDC. The best qualitative agreement with experimental data was achieved for η = 0.4. For FePc on Ag(111)16 a voltage drop of η ≈ 0.1 was obtained in the effort to explain an observed orbital energy shift in FePc multilayers. While the voltage drop extracted from the DBTJ model is larger by a factor 4, we emphasize that we were striving for qualitative agreement in order to capture the basic mechanism driving the observed NDC. Improvements may be anticipated by using more realistic molecule and substrate density of states. Extending the onedimensional tunneling barrier to three dimensions would also help describing the experimental situation more accurately. The strongest NDC was observed at the central region of MnPc, while NDC was quickly quenched away from the molecular center. This observation may tentatively be related to the hybridization of Mn d states within the molecule. For CoPc, calculations showed that Co d states hybridized with the π states of the isoindole groups and the electronic states of N atoms.60 As a consequence, Co d states were delocalized across the molecule while maintaining the largest charge density at the Co site. Previously, NDC induced by vibrational excitation was reported,61 which led to dI/dV spectra similar to the data presented in Figure 7a. In that work61 the vibrational excitation was accompanied by changes in the molecular conformation, which affected the tunneling current to such an extent that NDC occurred at the threshold bias voltage required for the excitation. In our experiments, by contrast, conformational changes of MnPc were absent. Moreover, energies of MnPc vibrational quanta are below 0.5 eV. Therefore, NDC observed for MnPc here is most likely not mediated by vibrational excitation.

Figure 7. (a) Experimental (black) and calculated (red) dI/dV data of second-layer MnPc. The experimental spectrum was recorded above Mn (feedback loop parameters: 1 V, 50 pA). Inset: Corresponding I− V characteristics. (b, c) Constant-current (50 pA) maps of dI/dV recorded at −0.44 V (b) and 0.47 V (c). Two second-layer MnPc molecules are visible whose positions and orientations are indicated by the dashed lines. The most pronounced negative differential conductance occurs at the central region of the molecules. Dashed crosses in (b) and (c) represent a MnPc molecule.

I(V , η , z tm , zms) ∝

∫0

eV

ϱs(E)ϱt (E − eV )ϱm(E − ηeV )

× Ttm(E , η , eV , z tm)Tms(E , η , eV , zms) dE

(1)

with ϱs, ϱt, and ϱm the density of states of respectively the surface, the tip, and the second-layer molecule. The voltage drop across the molecular film is taken into account by η. Tms (zms) and Ttm (ztm) denote respectively the transmission factors (widths) of the tunneling barriers across the molecular film and the vacuum. They are expressed within the Wentzel−Kramers− Brillouin approximation56,57 as Tms(E , η , eV , zms) ∝ exp( −αzms) Φms − E +

η eV 2

Ttm(E , η , eV , z tm) ∝ exp( −αz tm) Φtm − E +

1−η eV 2

(2)

(3)

with α = 2 2m /ℏ (m: free electron mass; ℏ: Planck’s constant divided by 2π) and Φms (Φtm) the height of the tunneling barrier across the molecular film (vacuum). The tip density of states was assumed to be constant, ϱt(E) = constant, while ϱs and ϱm were extracted from the dI/dV spectra recorded atop the center of first-layer and second-layer MnPc. First-layer MnPc molecules exhibited a peak at ≈−0.1 V, which slightly straddled the Fermi level (inset to Figure 6a), while second-layer molecules showed a sharper peak at ≈−0.15 V (arrow in Figure 7a), completely below the Fermi level.

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CONCLUSION The adsorption of MnPc on Au(110) up to the closed monolayer is characterized by a strong molecule−substrate interaction, which is evidenced by commensurate super6848

DOI: 10.1021/acs.langmuir.6b01529 Langmuir 2016, 32, 6843−6850

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structures and rearrangements of the substrate lattice. The adsorption-induced modifications of the Au(110) missing-row reconstruction enables effective hybridization of MnPc molecules with Au atoms. The molecule−substrate coupling is likewise reflected by the featureless orbital electronic structure. For MnPc molecules residing in the second molecular layer the hybridization with the metal surface is reduced. Scanning tunneling spectra show clear signatures of molecular resonances and scanning tunneling microscopy images unveil the spatial orbital structure. The negative differential conductance of second-layer molecules is due to peaked Mn d states at the Fermi energy combined with a voltage drop across the molecular film.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.N.). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b01529 Langmuir 2016, 32, 6843−6850