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Site-specific adsorption of aromatic molecules on a metal/ metal oxide phase boundary Alexander Timmer, Harry Moenig, Martin Uphoff, Oscar Díaz Arado, Saeed Amirjalayer, and Harald Fuchs Nano Lett., Just Accepted Manuscript • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Site-specific adsorption of aromatic molecules on a metal/metal oxide phase boundary Alexander Timmer,†,‡ Harry Mönig,∗,†,‡ Martin Uphoff,† Oscar D´ıaz Arado,†,‡ Saeed Amirjalayer,∗,†,‡,¶ and Harald Fuchs∗,†,§,‡ †Physikalisches Institut, Westfälische Wilhelms-Universität M¨ unster, Wilhelm-Klemm-Strasse 10, 48149 M¨ unster, Germany ‡Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149 M¨ unster, Germany ¶Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität M¨ unster, Wilhelm-Klemm-Straße 10, 48149 M¨ unster, Germany §Institut f¨ ur Nanotechnology, KIT, 76344 Karlsruhe, Germany E-mail: [email protected]; [email protected]; [email protected]

Abstract Nano-structured surfaces are ideal templates to control the self-assembly of molecular structures towards well-defined functional materials. To understand the initial adsorption process, we have investigated the arrangement and configuration of aromatic hydrocarbon molecules on nano-structured substrates composed of an alternating arrangement of Cu(110) and oxygen-reconstructed stripes. Scanning tunneling microscopy reveals a preferential adsorption of molecules at oxide phase boundaries. Non-contact atomic force microscopy experiments provide a detailed insight into the preferred adsorption site. By combining sub-molecular resolution imaging with density functional theory calculations, the interaction of the molecule with the phase boundary was elucidated excluding a classical hydrogen bonding. Instead, a complex balance of different interactions is revealed. Our results provide an atomistic picture for the driving forces of the adsorption process. This comprehensive understanding enables developing strategies for the bottom-up growth of functional molecular systems using nanotemplates.

Keywords Site-specific adsorption, phase boundaries, nano-templates, noncontact atomic force microscopy, molecular self-assembly, density functional theory

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The ability to tailor the physical properties of molecules at interfaces on the nano-scale is crucial for the development and improvement of functional materials with designated electronic properties for possible future applications. 1–11 Despite the tremendous progress in the fields of morphology prediction and molecular selfassembly, it remains a great challenge to control the assembly of molecules into desired structures on surfaces. Considering surfaces without lateral phase boundaries, the two principal forces influencing the structure of the molecular overlayer are molecule-substrate and moleculemolecule interactions. Balancing the relative strength of these non-covalent interactions is the main parameter for a controlled molecular bottom-up construction of functional materials on surfaces. 12,13 Typically, the molecule-substrate interaction is considered to act mainly vertically with respect to the surface, in contrast to intermolecular forces. However, considering nano-structuring at surfaces, the molecule-substrate interaction goes beyond a two-dimensional constraint. For example, the selective decoration of Au(111) step-edges due to site specific differences in the electrostatic potential has been observed. 14,15 Furthermore, the concept of additional surface anisotropy is often employed to direct the growth of atoms and molecules. 16 A popular approach utilizes vicinal coinage metal substrates as templates for parallel one-dimensional molecular self-assembly with long-range periodicity. 17–19 In particular, this approach allows to suppress interfering pathways in complex onsurface reactions, which are lacking the appropriate selectivity. 20 Another related approach is to sterically restrict the self-assembly or growth of large molecular structures with nanopatterned surfaces. 21,22 An example of such nano-template for molecular self-assembly is the partially Cu(110)-p(2×1)O added row (AR) reconstructed Cu(110) surface, which has previously been utilized to influence the arrangement of molecules and beyond that also guide onsurface reactions. 21,23–31 Incomplete oxidation of the Cu(110) surface results in a supergrating formed by alternating rows of bare copper and p(2×1)O AR terraces following the [1¯10]

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direction, while the relative proportion of both areas can be tailored by varying the experimental conditions. 32,33 However, a detailed picture of the interaction and initial adsorption process at these phase boundaries is still missing hindering a rational development of designated nanostructures. In the present study, we combine scanning tunneling microscopy (STM), high resolution noncontact atomic force microscopy (NC-AFM) and X-ray photoelectron spectroscopy (XPS) with density functional theory (DFT) to investigate the interaction of organic molecules with such an interface. Sub-molecular NCAFM imaging reveals the preferred interaction site with the AR reconstruction. Together with the DFT calculations, we identify charge redistribution and the resulting electrostatic interaction as the driving force determining molecular adsorption on the nano-template. To gain insight into the interactions of organic molecules influenced by an oxide phase boundary, dicoronylene (DCLN) was initially chosen as a test molecule (Figure 1a). An atomic model of the phase boundary between the p(2×1)O AR oxide phase and the bare Cu(110) surface is depicted in Figure 1b. The side view exemplifies that the AR terraces are elevated from the bare copper surface. The STM overview

Figure 1: a) Ball-and-stick model of dicoronylene and coronene b) Atomic model of the partial p(2×1)O added row reconstructed Cu(110) surface.

image of the partially oxidized Cu(110) surface prepared with a relatively high oxygen exposure of 0.86 Langmuir (L) leading to narrow Cu stripes is shown in Figure 2a (see also Table S1 in the Supporting Information). After subsequent deposition of 0.06 L of DCLN (bright


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protrusions in Figure 2 a-f), it can be observed, that at such a low dosage DCLN adsorbs exclusively on the narrow Cu(110) stripes. The selective adsorption of molecules on bare Cu stripes can be ascribed to oxygen-induced withdrawal of charge density from the underlying metal within the AR reconstruction resulting in reduced attractive van der Waals interaction. 25 At such low coverage, the formation of molecular islands does not occur. Importantly, it can be clearly observed that DCLN predominantly decorates the phase boundary between the metal and the AR oxide phase. Increasing the distance of the oxide stripes up to about dc = 20 nm by reducing the oxygen dosage still leads to the majority of DCLN molecules adsorbed at the phase boundary while only a few individual molecules adsorb on the bare copper (see Figure 2a-c). By further increasing the spacing between the oxide rows up to dd ≈ 28 nm, the ratio of DCLN molecules attached to the oxide rows versus those on the bare copper balances out at approximately 1:1 (Figure 2d). Weighting with the total surface area it becomes obvious that the adsorption sites at the phase boundary is preferred. The two-fold symmetry of DCLN allows to distinctively determine the adsorption angle between the long axis of the molecule and the [001]-direction from the STM images. In Figure 2e-f exemplary results are shown for both, the molecules adsorbed at the oxide and on the free Cu(110) surface. Here the values for the measured angles are in good agreement at (65.7 ± 0.9) ◦ and (65.1 ± 0.9) ◦ , respectively. This strongly suggests, that in both cases the same relative adsorption site is occupied. This observation is further confirmed by a statistical analysis (sample size ∼1000) of the orientations for those DCLN molecules attached to the phase boundary and for those adsorbing on bare copper (see Supporting Information for details). A histogram presenting the corresponding angular distribution is shown in Figure 2g. Two pronounced maxima centered at about ±65◦ are clearly visible in either case, which is in agreement with symmetry considerations of the surface and the point symmetry of DCLN. This structural analysis strongly indicates no change of the adsorp-

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Angle relative to [001] direction (°) Figure 2: STM images of DCLN deposited onto different partial oxidized Cu(110)-p(2×1)O surface: a) DCLN deposited with a low coverage (O2 dose 0.86 L ,It = 50pA). b) DCLN deposited with a the same coverage but O2 dose decreased to 0.39 L, It = 300pA. c-d) Comparison of the adsorption behaviour of DCLN for different Cu stripe widths (O2 dose 0.19 L resp. 0.14 L) e-f ) Free DCLN molecules adsorb in the same angle with respect to the [001]-direction as those adsorbed at the oxide reconstruction. g) Histogram of adsorption angles in relation to the [001]-direction for DCLN moieties next to the phase boundary. All images recorded with Ugap = 0.1V throughout. It = a) 50 pA, b) 300 pA c-f ) 30 pA

tion site of DCLN on the bare Cu surface and in the presence of the phase boundary. However, as mentioned before, our STM results reveal a significant preference for molecular adsorption at the phase boundary, raising the que-


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stion of the driving force for this adsorption process. Initially, a hydrogen-bonded interaction between DCLN and the partially negative charged oxygen atoms in the AR reconstruction may seem to be a likely explanation. To validate this assumption and to characterize the involved interactions, it is desirable to precisely determine the adsorption site of DCLN with respect to the AR oxide stripes. In Figure 2e, distinct maxima within the (2×1)-reconstructed oxide phase are clearly visible. However, an unambiguous discrimination of the added row O (AR-O) atoms and added row Cu (AR-Cu) atoms within the (2×1)O AR reconstruction via STM is difficult, as they exhibit identical periodicity and either can be imaged with a larger apparent height depending on imaging parameters and tip termination. 33–37 It has been recently shown that NC-AFM experiments utilizing tips with a rigidly bonded terminal oxygen atom at the apex (CuOx tip) enable imaging molecules with sub-molecular resolution, while excluding significant lateral deflection of the tip apex and related artefacts. 38–41 Furthermore, NC-AFM experiments with such a CuOx tip exhibit site specific interactions within the AR-reconstructed stripes, providing the possibility to clearly identify oxygen- and copper sites. Figure 3a shows a constant-height NCAFM image of a DCLN molecule adsorbed at an oxide phase boundary. It reveals not only its sub-molecular bonding structure, but also shows atomic resolution within the oxide rows (Figure 3a). Based on this, it is possible to directly determine the precise adsorption site with respect to the oxide and to the underlying Cu(110) substrate (Figure 1b). 38,42 A representation of Figure 3a with a superimposed grid of the first layer Cu atoms is depicted in Figure S4b. Considering the information obtained from the chemical discrimination within the AR, it becomes apparent that the outer hydrogen atoms are clearly facing towards the ARCu atoms (Figure 3a), contrary to the initial expectation of a hydrogen bonded interaction. In order to elucidate this experimental result, DFT calculations were performed. Before incorporating the phase boundary, the behavior of DCLN on Cu(110) is considered, to identify

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Figure 3: NC-AFM experiments and DFT calculations for DCLN adsorbed on a Cu(110) surface: a) Constant-height AFM image with a CuOxfunctionalized tip recorded on DCLN adsorbed next to an oxide phase boundary. A corresponding structural model of the oxide rows is shown on the right. b) Top view and binding energy of the favored adsorption site obtained after geometry optimization on the free Cu(110) surface and c) next to oxide phase boundary. d) Energy profiles for DCLN translation perpendicular (top) and along (bottom) the CuO strips (see SI for details). The colors correspond to the direction of translation as indicated by arrows in a and c.

the preferred adsorption position of DCLN on the bare metal. The only presumptions used for these DFT calculations were the experimental observation of planar adsorption geometry and the adsorption angle relative to the


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[001]-direction of the Cu(110) substrate. Based on these constraints, four adsorption sites are derived, which originate from moving the DCLN molecule in either the [001] or the [1¯10]direction by half a lattice site of the Cu substrate (Figure S2). The DFT calculations reveal that the adsorption position Cu-3 is the energetically favorable (Figure 3b). The binding energy is EBCu−3 = −4.03 eV , implying a strong dispersive interaction between molecule and surface. Here, the center of the DCLN molecule is almost placed in the long bridge site with regard to the first layer Cu atoms. In all considered cases, the DCLN molecules do not bend or twist significantly. A complete summary of all DFT results is attached in the SI. To gain further insight into the topology of the potential energy surface, which stabilizes the DCLN molecules predominately next to the oxide phase boundary, single point (SP) calculations were performed. Starting from position of the full optimized system on bare Cu(110) the DCLN molecule was translated stepwise along the [1¯10] direction perpendicular to the oxide row (details see SI). The energy profile along the translation coordinate is shown in Figure 3d (top). Two distinct energy minima are revealed, one on the bare Cu surface and one in the proximity of the AR interface. The energy difference between these two adsorption sites is ∆EB⊥ = 0.16 eV . Thus, the site closer to the (2×1)O reconstruction is the more stable configuration. In contrast, SP calculations on bare Cu(110) reveal equal minima separated by one lattice constant after translation along the [1¯10] direction (Figure S3). From the minimum position in Figure 3d (top) the molecule is then translated parallel to the oxide along the [001] direction (Figure 3d (bottom)). Here, the same distinct minimum position is found, which exhibits an energy difference of ∆EB = 1.70 eV to the maximum value. This is consistent with the fact that within individual STM images DCLN molecules are always positioned identical with respect to the topographic features in the oxide phase. The corrugation of the calculated potential energy profile is about 1.7 eV in the [001] direction and about 1.0 eV in the [1¯10] direction, with diffusion barriers of about

0.7 eV , which indicates some mobility of DCLN molecules at room temperature. To determine the absolute difference in binding energy between DCLN molecules on Cu(110) and those adsorbed at the oxide rows, the minimum position in the potential energy landscape close to the oxide row is chosen for an additional geometry optimization. The result is depicted in Figure 3c and the binding energy for DCLN adsorbed at the (2×1)O AR reconstruction is EBoxide = −4.90 eV . Thus, adsorption at the phase boundary results in an increase of the binding energy by ∆EB = 0.87 eV . Comparing the experimental findings in Figure 3a with the DFT results it becomes evident that both results are in very good agreement. Clearly, the adsorption sites for DCLN next to the (2×1)O reconstruction and on bare Cu are also matching well and the molecule does not appear rotated with respect to the [001] direction (see also Figure S4). As discussed above, due to their large electronegativity, the oxygen atoms within the AR exhibit enhanced electron density, 43 facilitating a possible hydrogen bond between AR-O atoms and the hydrogen atoms at the rim of the molecule. However, the SP calculation along the [001] direction depicted in Figure 3d (bottom) do not feature a minimum in a position, which enables such hydrogen bonding. Thus, the potential energy gain due to a hypothetical hydrogen bond in this position is not large enough to keep the molecule from moving to its preferred adsorption position with respect to the underlying Cu(110) surface. This conclusion is consistent with DFT calculations on the bare Cu(110) surface indicating an energy difference of ∆EB = −1.02 eV between the preferred adsorption site (Cu-3) and the position where the hydrogen atoms face toward the AR-O atoms. Furthermore, the DFT calculations for DCLN next to the oxide phase result in a vertical distance of the foremost hydrogen atoms to the ˚ and closest AR oxygen atom of dH−O = 3.49 A 1 H−O ˚ respectively. The values obtaid2 = 3.26 A, ned from the NC-AFM data agree well (dH−O AF M = ˚ (3.34 ± 0.20) A, see Table S2). At such distances hydrogen bonding interactions are reported to be almost absent. 44


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Figure 4: STM images of coronene deposited onto the Cu(110)-p(2×1)O surface: a) STM image of coronene deposited onto the Cu(110)-p(2×1)O surface. b) A enlargement of the area indicated in a. The color scale is optimized to highlight AR-O atoms as bright green maxima (verified via NC-AFM). Insert: Structural model including a sketch of the proposed adsorption position of coronene next to the AR oxide reconstruction. c) STM image with a higher coronene coverage. d) Constant height NC-AFM image of coronene next to oxide stripes. All STM images recorded with I = 100 pA and V = 850 mV .

Therefore, it can be concluded that the Cu(110) surface is dominating the general adsorption configuration for DCLN molecules on the bare metal and next to the oxide rows. However, the latter provides an additional attractive energy contribution that renders adjacent adsorption sites advantageous. To gain further insight into the mechanisms that stabilize the adsorption behavior of large hydrocarbon molecules on the AR surface, it is desirable to systematically study the moleculesubstrate interactions without changing the general adsorption characteristics of the molecule. Therefore, coronene was utilized in a subsequent experiment. This molecule is composed of half a DCLN molecule reducing the moleculesubstrate interaction with the copper substrate compared to DCLN. Figure 4a-c depicts a Cu(110)-(2×1)O surface prepared with a relatively high oxygen exposure of 0.95 L leading to a high density of oxide stripes. Initially, 0.03 L of coronene was deposited, resulting in isolated molecules on the surface (Figure 4a). Coronene molecules are only adsorbed on the copper stripes and predominantly decorate the phase-boundary between the metal and the oxide as it was the case for DCLN. This situation is depicted in Figure 4a and zoom in Figure 4b with the bright spots corresponding to individual coronene molecules. Figure 4c illustrates the condition after a significantly higher coverage of coronene on the same surface. It can be deduced that molecules first selectively

occupy the phase-boundary next to the oxide stripes. Afterwards, the bare copper area is sequentially filled. This behavior does not depend on the Cu-stripe width (Figure S7). The contrast in the high-resolution STM image in Figure 4b was optimized to highlight the atomic corrugation within the (2×1)-reconstructed oxide domain. The central axis of coronene molecules in individual STM images is aligned with the same feature in the AR oxide reconstruction (indicated by the orange line), implying that they all adsorb on the same lattice site with respect to the (2×1)O reconstruction. The STM contrast (shape and apparent height) of molecules at the domain boundary does not deviate significantly from molecules adsorbed on bare Cu, e.g. those filling the trenches in Figure 4c. To elucidate the detailed adsorption geometry, NC-AFM experiments were performed (Figure 4d). The NC-AFM image explicitly shows that the axis extending across three central benzene rings of the coronene molecule is orthogonal to the row of AR-O atoms. As the three central benzene rings are not rotated from this axis, it can be deduced that coronene adsorbs symmetrically on Cu surface. Note, even the orientations of the outer hydrogen atoms are recognizable and it is discernable that they, in analogy to DCLN, are also facing towards the AR-Cu atoms. By taking into account the AR-O and AR-Cu atoms positions in the NC-AFM image, a grid corresponding the first layer Cu atoms of the bare surface can be superimposed with high


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precision. This allows determining the exact adsorption site (see Figure S8). The distance between the center of the coronene molecules ˚ and the first AR is dcoro−AR = (7.82 ± 0.2) A. This information was used to derive the adsorption geometry of coronene as depicted in Figure 4b (inset). This structure is consistent with previous experimental and theoretical findings for the preferred molecular orientation in a noncompressed coronene monolayer adsorbed on bare Cu(110). 45 The distance between the foremost hydrogen atoms is determined from the ˚ NC-AFM data to be dH−O = (3.07 ± 0.20) A 1 H−O ˚ which agrees with and d2 = (3.25 ± 0.20) A, the values found for DCLN within the error of measurement. In line with the DCLN results, coronene also favors adsorption at the phase boundary and despite the significantly reduced molecular-substrate interaction, does not change its adsorption site with respect to the Cu(110) surface. Based on our experimental and theoretical results, we can exclude direct hydrogen-bonding interaction with the oxygen atoms of the phase boundary as the driving force for the preferential adsorption. To elucidate if, in addition to pure unspecific van der Waals interactions, contributions based on electron redistributions are involved, we calculated the electron density difference (EDD) at the phase boundary. A slice through this data at the height of AR-O atoms and parallel to the xy plane is given in Figure 5a (slice 1). The EDD plot reveals a partial redistribution of electron density between the coronene molecule and the AR reconstruction, resulting in a charge depletion at the oxide for atoms in proximity to the adsorbed molecule. The vertical cross sections containing the position of the central AR-O atom (slice 2) and the closest AR-Cu atom (slice 3) reveal a distinct interaction between the molecule and AR atoms (Figure 5a). The charge redistribution occurs directly between the atoms of the phase boundary and the coronene without any significant contribution from the underlying substrate copper. In particular, an increase in electron density between AR Cu atoms and the foremost hydrogen atoms is visible, indicating that the adsorption process is governed by a combina-

Figure 5: a) Cross sections through the electron density difference of coronene adsorbed next to the AR interface. Depicted are a slice in the xy plane at the height of AR-O atoms (slice 1) and two cross sections in the zy-plane through the plane containing the central AR-O atom (slice 2) and the adjacent AR-Cu atom (slice 3). The positions of ARO and AR-Cu atoms as well as coordinates of the carbon and hydrogen atoms located in proximity to these sectional planes are indicated for orientation. b) O 1s XPS spectra for clean Cu(110)-p(2×1)O surface and a high-coverage deposition of coronene on the same sample. The O 1s peak exhibits a slight broadening after coronene deposition.

tion of van der Waals interaction and electrostatic effects. The data for DCLN is given in Figure S6, where similar observations can be made. In addition, XPS experiments were performed. The O 1s spectra for a clean Cu(110)p(2×1)O surface (gray) and the same surface


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after complete decoration of the oxide interfaces with coronene (red) is depicted in Figure 5b. In both cases, the peak is centered at about 529.8 eV . Due to the low signal intensity as a consequence of low surface coverage and pass energy, a precise analysis is challenging. Nevertheless, the peak corresponding to the measurement of the completely decorated AR reconstruction (red) appears slightly broadened at higher binding energies. Similar spectroscopic signatures have previously been assigned to electrostatic attraction between two polar moieties. 46–48 Thus the XPS results indicate the same tendency as the EDD data. Both, our experimental and theoretical results indicate that charge redistribution beyond a simple van der Waals interaction at the phase boundary are the driving force for the preferred adsorption at the phase boundary. In summary, we investigated the preferential adsorption of DCLN and coronene molecules on the metal/metal oxide phase boundary at partially (2×1)O-reconstructed Cu(110) surfaces. High-resolution NC-AFM imaging with functionalized probe tips revealed the atomic configuration directly at the lateral moleculeoxide interface with sub-molecular resolution. Although the two molecules exhibit distinct differences in their vertical substrate interaction, similar adsorption sites with respect to the phase boundary were found. Based on our data, a classical hydrogen bonding can be excluded as driving force for the preferential adsorption at the phase boundary. Rather, our DFT calculations and XPS measurements conclusively show a charge redistribution between the molecule and the phase boundary with a significant contribution of the AR-Cu sites. Thus, the resulting adsorption position stems from a delicate balance of interactions. The specific lateral adsorption site is mainly governed by the underlying Cu(110) substrate, whereas a complex combination of attractive electrostatic interactions and additional vdW contributions traps the molecules at the interface. Both contributions promote the preferential adsorption at the phase boundary. Our detailed investigation provides a fundamental understanding of weak lateral interactions determining pre-

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ferred adsorption sites of organic species on nanostructured surfaces. Our results expand the understanding of such phase boundaries in order to control molecular self-assembly on these nano-templates. Acknowledgement Financial support from the Deutsche Forschungsgemeinschaft (DFG) through the SFB 858, the transregional collaborative research center TRR 61, and the grants FU 299/19, AM 460/2-1 and MO 2345/4-1 is gratefully acknowledged. Supporting Information Available: Additional experimental and theoretical details are given. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Nano Letters

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