Complex Adsorption Behavior of a Non-Planar Naphthalene Diimide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Complex Adsorption Behavior of a NonPlanar Naphthalene Diimide on Au(111) Bertram Schulze Lammers, René Ebeling, Elena Dirksen, Thomas J. J. Mueller, and Silvia Karthaeuser J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00554 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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The Journal of Physical Chemistry

Complex Adsorption Behavior of a Non-planar Naphthalene Diimide on Au(111)

Bertram Schulze Lammers,1 René Ebeling,1,2 Elena Dirksen,3 Thomas J. J. Müller,3 Silvia Karthäuser*1

1Peter

Grünberg Institut (PGI-7) and JARA-FIT, Forschungszentrum Jülich GmbH, Jülich 52425,

Germany *E-Mail: [email protected] 2RWTH

Aachen University, Aachen 52062, Germany

3Institut

für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität

Düsseldorf, Universitätsstrasse 1, Düsseldorf 40225, Germany

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ABSTRACT: Naphthalene diimide (NDI) derivatives are extensively studied in the field of supramolecular and materials chemistry due to their versatility in coordination, electron deficiency and planar aromatic core. Here, we report on the self-assembly behavior and the electronic properties of 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI) studied by low-temperature scanning tunneling microscopy. BNTCDI adsorbed on Au(111) is highly mobile and reveals a tendency to form chain structures on the step edges at low coverages. The chains are kept together by intermolecular hydrogen bonds between neighboring NDI backbones. Within the chain structures the orbital appearance of single molecules was clearly identified. Additionally, supramolecular structures consisting of ordered double layers were observed on terraces for higher surface coverages. The molecule-substrate interaction is investigated in detail via scanning tunneling spectroscopy for different molecular arrangements. The results reveal distinct insights into the subtle balance between intermolecular and interface interactions determining the self-assembly behavior of BNTCDI resulting in chain structures at step edges and ordered double layers on Au(111) terraces.


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1.

INTRODUCTION

A very versatile molecule for molecular and organic electronics is 1,4,5,8-naphthalenetetracarboxylic diimide (NDI) with its various applications. NDI derivatives have gained recent interest as small molecule acceptors1 used, for example, in donor-acceptor molecules2, as ntype semiconductors3,4,5,6, as electrochemical sensors7,8 or as an acceptor for a photo-induced electron

transfer

in

nonfullerene

organic

photovoltaics

and

molecular

optoelectronics9,10,11,12,13,14. The benefit of NDIs is their wide variety of possible modifications by core-substitutions to customize their electronic properties that is, to shift the Nernstian two step reversible reduction potentials towards the desired requirements15. In addition the symmetric or asymmetric attachment of functionalized groups by imidation generates a great variety of functionalized NDI-based molecules leading to their numerous applications16. Hereby the substitution on the N-site has an essential influence for the molecules possibility of intermolecular H-bonding, thus allowing for a chemical tuning of structural parameters6,17. For the detailed characterization of NDIs we utilize low-temperature scanning tunneling microscopy (LT-STM) under ultra-high vacuum (UHV) conditions. This technique generates real space images up to atomic resolution by scanning the local density of states (LDOS) between the surface and a sharp metal tip in a height of less than 1 nm above the surface. The barrier for the tunneling current between surface and tip is heavily affected by the LDOS and the topography of molecules on the surface. Furthermore, STM and scanning tunneling spectroscopy (STS) allow the investigation of molecular orbital structures and transport properties at the submolecular level18,19. In preceding studies we analyzed the adsorption behavior and the electronic structure of the NDI-derivative, 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI), deposited on Pt(111)19. At about 0.1 monolayer deposition, single molecules were uniformly 3 ACS Paragon Plus Environment


distributed and exhibited commensurability with the hexagonal structure of the Pt(111)surface. For slightly higher surface coverages, yet submonolayer deposition, dimers of BNTCDI were observed due to weak C−H…O hydrogen bonds. LT-UHV-STM investigations of single BNTCDI on Pt(111) with intramolecular orbital resolution allowed us to determine characteristic topographic and electronic features that indicate a weak chemical coupling between BNTCDI and the Pt(111) surface. We identified the NDI-backbone lying flat on the Pt(111)-surface and the phenyl rings standing upright, since all molecules exhibit C2v symmetry and the atomic nodal planes at the upright phenyl rings were resolved. This BNTCDI conformation enables the formation of an effective transport path via hybrid BNTCDI/Pt dstates around the Fermi energy in combination with intramolecular N···H−C hydrogen bonds on each side of the molecule. These transport paths are indicated by two bright spots located at the positions of the nitrogen atoms in the STM images, thus confirming the weak chemisorption of the large conjugated NDI acceptor on the Pt(111) substrate. The coupling between the metal surface and the molecule is crucial for its electronic properties and adsorption geometry. Thus, the appearance of BNTCDI in STM and STS depends heavily on the chosen substrate material and the contact chemistry20. Therefore, the question arises whether hybrid BNTCDI/metal d-states form on less reactive substrates as well. By varying the substrate reactivity the balance between molecule-molecule and moleculesubstrate interactions will be affected and an adapted self-assembly behavior can be expected. Hereby, C−H…O hydrogen bonds are crucial for the self-assembly, as also described for other NDI-derivatives6,17,21,22,23. Here, we investigate the self-assembly behavior and the electronic properties of BNTCDI on Au(111) in order to study the interface interactions with a substrate exhibiting a weaker reactivity than Pt(111)24. Using LT-UHV-STM with orbital


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resolution and STS we will show that the reactivity of the substrate is of utmost importance for the delicate balance between interface interactions and weak intermolecular interactions.

2.

EXPERIMENTAL METHODS

The synthesis and the electrochemical characterization of 2,7-dibenzyl 1,4,5,8naphthalenetetracarboxylic diimide (BNTCDI) are described in a previous work19. We employed a commercial Createc LT-UHV-STM (Germany) for low-temperature scanning tunneling microscopy (LT-STM) and spectroscopy (STS) in ultrahigh vacuum (UHV) (base pressure of 1 × 10-10 mbar). The STM chamber is divided into three parts, a load-lock, a preparation chamber and a measurement chamber, to enable clean in situ preparation of substrates and molecular layers. Using custom-made tungsten tips the LT-UHV-STM measurements were performed in the constant-current mode at 4.2 K. The STM measurements were analyzed and plotted by SPIP. The acquired images are solely plane corrected and unfiltered, if not stated else in the corresponding caption, in order to conserve the information for apparent height profiles (averaged over nine or fifteen lines as mentioned in the corresponding captions). The lengths given for molecule dimensions are determined as full width at half maximum (FWHM) of an observed feature. Differential conductance spectra (dI/dV) were measured with the feedback-loop deactivated and lock-in detection while applying an oscillating bias voltage of ±50 or ±100 mV. These spectra were averaged over four to ten spectra and plotted in OriginPro 9. A polished Au(111) single crystal (purity 99.999%; MaTeck, Germany) cleaned by cycles of Ne+-ion sputtering (1 kV, 20 min., 1.5 µA) and thermal annealing (700 °C, 20 min.) was used as a substrate. These sputtering/annealing cycles were repeated until the Au(111) surface was devoid of impurities and showed at least 50 nm wide terraces. BNTCDI was deposited by 5 ACS Paragon Plus Environment


sublimation from a Knudsen cell (10–10 mbar range and varying deposition times) onto the clean Au(111)-surface at room temperature (RT). Hereby, a submonolayer coverage of BNTCDI was achieved by evaporation for 4 minutes at 120 °C and additional 4 minutes at 135 °C, while approximately half-monolayer coverages resulted from evaporation of BNTCDI for 10 minutes at 135 °C. Immediately after the deposition, the sample was transferred into the measurement chamber and cooled by liquid helium (4.2 K).

3. RESULTS AND DISCUSSION Adsorption Behavior of BNTCDI on Au(111) during the Onset of Self-Assembly. In this study BNTCDI molecules were deposited with different surface coverages on an Au(111) single crystal surface at RT. The symmetric BNTCDI consists of a naphthalene diimide (NDI) backbone and two benzyl groups connected to the nitrogen atoms on both sides (Figure 1b). The methylene linkers introduce kinks into the molecule and enable free rotations of the phenyl groups in gas phase. On adsorption to the Au(111) surface these rotational degrees of freedom are strongly reduced due to the interaction between surface and molecule. However, during deposition at RT BNTCDI is highly mobile on the surface, as will be shown, and a conformational flexibility has to be assumed until the sample is cooled down. This degree of freedom allows the molecule to optimize the molecule-molecule and moleculesubstrate interactions to a large extent. The STM image (Figure 1c) of submonolayer BNTCDI coverage reveals uniform chains on step edges consisting of small recurring units corresponding to BNTCDI molecules. The appearance of the molecular structure resembles three aligned spots analogous to the three significant parts of the BNTCDI (Figure 1b). In comparison with STM images of NDI molecules which appear as one bright ellipse on Au(111)21 we assign the large central spot to the NDI6 ACS Paragon Plus Environment

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backbone and the smaller outer spots to the two benzyl groups as shown schematically in Figure 1a. The length of BNTCDI along the long molecular axis crossing the three spots corresponds to 2.5 ± 0.1 nm or 2.9 ± 0.1 nm depending on the exact adsorption geometry of the molecule at the step edges, as will be explained later in detail. These values are somewhat larger than the value obtained for the long axis of BNTCDI adsorbed on Pt(111) which amounts to 1.9 nm19. As opposed to the high occupation of the step edges, no significant structures corresponding to single BNTCDI molecules were found on Au(111) terraces. Nearly bare Au(111) surfaces with unaffected Herringbone reconstruction remain between the step edges indicating the almost absent interaction between BNTCDI and the Au(111) terraces. Interestingly, even for the step edges, no single BNTCDI molecules could be identified despite various attempts at very low surface coverages. At the minimum two molecules form a dimer on a step edge even if almost all step edges are devoid of molecules. These results are in accordance with DFT calculations21 stating that the binding energy of single NDI molecules or their dimers is endothermic on Au(111). Thus, NDI can only be adsorbed exothermally in networks of two or more molecules due to the fact that in these structures the gained stabilization through intermolecular hydrogen bonds is high enough to overcome the endothermic binding energy. Assuming in a first approximation that the main interaction between BNTCDI and the Au(111) surface is determined by the aromatic NDI backbone, the high mobility during deposition and the absence of single molecules on the Au(111) terraces can be explained likewise. Only if all surrounding step edges are nearly completely occupied by chain structures, additional molecules form disordered structures on very narrow terraces and corners between the step edges (Figure 1c). These results demonstrate that BNTCDI molecules are 7 ACS Paragon Plus Environment


highly mobile on Au(111) surfaces and that the formation of chain structures on step edges is energetically favored over the adsorption on Au(111) terraces.

Figure 1: a) Scheme of one possible configuration of BNTCDI on the step edge. b) Chemical structure of BNTCDI. c) LT-UHV-STM image of BNTCDI deposited on Au(111) with submonolayer coverage (setpoint: Ubias = 0.5 V, Iset = 150 pA; scale bar: 20 nm). Only the step edges are covered by BNTCDI single chain or double chain structures, while the terraces exhibit the typical Herringbone reconstruction. Having a closer look at the BNTCDI structures adsorbed on Au(111) step edges reveals that there are single chains and double chains (Figure 2). From the apparent height profiles illustrated in Figure 2 it can be deduced that the single chain structures are located on the upper terrace while double chains consist of molecules building an ordered structure of two adjacent BNTCDI chains across the step edge. NDIs in general are prototypical molecules for the forming of supramolecular networks. In disubstituted NDIs, like BNTCDI, the formation of N-H…O hydrogen bonds is not possible, but they are able to form non-classical intermolecular C-H…O hydrogen bonds6,17. This is possible as a result of the electron withdrawing nature of the imide group producing a partial positive charge on the aromatic C-Hs25. The formation of the C(sp2)-H…imide oxygen hydrogen bonds between neighboring NDI-backbones together 8 ACS Paragon Plus Environment

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with the space requirements of the substituents at the nitrogen atoms define the geometry and usually result in a displacement (Dx) of adjacent molecule pairs along the long molecular axes. These hydrogen bonds are marked exemplarily by grey dotted lines in the schematic of Figure 2a. Two weak C(sp2)-H…O hydrogen bonds between neighboring NDI-backbones are identified as the intermolecular driving force, which stabilizes the BNTCDI chains. According to theoretical calculations21 a comparable molecular arrangement of NDI molecules is stabilized by -0.2 eV per molecule. Thus, we find a strong tendency to form chain structures on Au(111) step edges based on weak intermolecular C(sp2)-H…O hydrogen bonds and preferred interface interactions of BNTCDI, particularly of one benzyl substituent, with the Au(111) step edges. The optimization of both effects leads to BNTCDI chain structures following the geometry of the step edges.

Figure 2: a) High-resolution STM image of a single and a partial double chain structure on an Au(111) step edge (setpoint: Ubias = -0.5 V, Iset = 170 pA; scale bar: 5 nm). The molecules forming the partial double chain in the lower left corner of the STM image exhibit an angle α 9 ACS Paragon Plus Environment


= 36° and a length of 2.9 nm (red solid line). The BNTCDI molecules in the single chain (top right corner) reveal α = 20° and a length of 2.5 nm. Inset: Sketch of BNTCDI chain with: Dx displacement along long molecular axes; Dy distance between molecules along short axis; dnn next neighbor distance along chain direction; α angle between the short molecular axes and the chain direction. The hydrogen bonds are indicated by gray dotted lines. For more details see text. b) Apparent height profiles (averaged over 15 lines) along the profiles depicted in a). A red solid line across a single molecule, a blue dashed line across two molecules of the double chain structure and as reference a gray background across the Au step are plotted.

BNTCDI chains are observed on straight as well as on slightly curved step edges (see Figure 1), which implies certain variability in the chain structure. Starting at the end of a single chain structure the first three spots are easily assigned to a single molecule and thus, the orientation of molecules within the chain becomes apparent. To characterize the intermolecular interactions within a chain in more detail, twelve individual straight single chain structures were analyzed with the focus on the next-neighbor distance dnn along the chain and the angle α between the short molecular axis and the chain direction (Figure 2a, schematic). Furthermore, the intermolecular displacement along the long molecular axes Dx and the intermolecular distance Dy in the direction of the short molecular axis have been calculated therefrom. The angle α is constant within a straight chain, but different values for α are obtained for various BNTCDI chains. Two mean values of the angle α, at 21 ± 3° and at 33 ± 3°, were determined by fitting of Gaussian curves (see Supplementary Information; Figure S1). The unexpected broad distribution of α values can be interpreted in terms of the remaining conformational flexibility of BNTCDI on the Au(111) surface used to enable an adaption of the chain structure to the Au step edge. 10 ACS Paragon Plus Environment

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The next-neighbor distance dnn does not vary significantly for different angles. It is directly measured by line profiles along the chain and averaged across at least four molecules. The resulting value of dnn is 1.0 ± 0.1 nm, which is consistent with supramolecular structures observed for N,N’-bis(N-alkyl)-naphthalenediimides, molecules with a NDI-backbone and alkyl chains from C3 to C18 bonded to the N atoms on both sides6,26. We calculated intermolecular displacements Dx for BNTCDI of 350 ± 20 pm and 540 ± 25 pm, corresponding to α = 21° and 33°, respectively. The first displacement is in good agreement with values of Dx ranging from 336 to 420 pm6,17 in comparable molecular arrangements. The second intermolecular displacement found for the BNTCDI chain structures Dx = 540 pm is, to the best of our knowledge, not yet reported in literature and might be fostered by phenyl-…imide oxygen interactions. Even though these interactions are assumed to be very weak, they may play a role in our scenario as only weak interactions are involved. The area per molecule is roughly determined by the length of the long molecular axis (2.5 or 2.9 nm) times the intermolecular distance Dy (0.9 ± 0.1 nm or 0.8 ± 0.1 nm) and yields 2.30 ± 0.14 nm² with α = 21° and 2.38 ± 0.13 nm² with α = 33°, respectively. The three spots attributed to one BNTCDI show an asymmetry in their apparent heights despite of the symmetrical chemical structure of the molecule due to the adsorption on the step edge (Figure 2b). The bright spot in the middle of the molecule, corresponding to the NDIbackbone, shows an apparent height of approximately 185 ± 10 pm while the two smaller spots on both sides, attributed to the benzyl groups, display values of 110 ± 10 pm on the faceside and 80 ± 10 pm on the edge-side. This asymmetry within each single molecule indicates a significant interaction of the benzyl group with the Au(111) step edge, which will be discussed later in combination with differential conductivity measurements.



By an analogous procedure the spots of the double chain structure are identified as two molecules, one is lying on the upper terrace of the step edge and the other one on the lower terrace. Hereby, both chains forming the double chain usually show the same directionality. Only in some cases the Au step edge acts as a mirror plane between both chains leading to a fishbone decoration. The apparent heights of their NDI-backbones and the benzyl groups pointing away from the step are nearly identical on the upper and lower terrace and similar to the apparent heights measured for the single chain structure. However, on the edge side the benzyl groups of both molecules combine to one feature, which indicates a moleculemolecule interaction. This observation is supported by the reduced length along the long molecular axis over both molecules, which amounts to 4.7 ± 0.1 nm, in the geometry shown in Figure 2, instead of 5.8 nm expected for two not interacting molecules. Slightly increased surface coverages result in a rising amount of double chain structures of BNTCDI. This process of adsorption on the step edges depending on the coverage and especially, the forming of single and double chain structures has been observed alike for another molecule (1-nitronaphthalene) with a weak molecule-surface interaction on Au27. All observations discussed so far demonstrate very small interface interactions between the BNTCDI molecules and the bare Au(111) terrace, which is as well in accordance with theoretical calculations21. They state that individual NDIs should not be able to adsorb on Au(111) due to their endothermal adsorption energy, while in contrast, molecular networks stabilized by hydrogen bonds should form exothermally. Consequently, a high diffusibility of BNTCDI can be assumed on Au(111) and all molecules migrate to a step edge and a neighbor molecule before the substrate is cooled down to 4.2 K. The intermolecular interactions due to hydrogen bonds as well as the molecule step edge interaction, are structure determining and responsible for the observed one-dimensional arrangement of molecules. 12 ACS Paragon Plus Environment

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Adsorption Behavior of BNTCDI on Au(111) at Submonolayer Coverage. Evaporation of approximately half-monolayer BNTCDI coverages results in an occupation of all step edges with BNTCDI and an adsorption of further molecules on the terraces. These molecules form disordered layers on the Au(111) terraces completely covering narrow terraces and layer sizes of up to 3000 nm2 are found on large terraces (lower right side of Figure 3). Similar structures were occasionally observed at lower coverage too, in corners built by fully covered Au(111) steps meeting each other (see Figure 1). If the width of these BNTCDI layers is larger than roughly 10 nm, an ordered supramolecular structure is observed forming a plait pattern often surrounded by a disordered arrangement. Usually, these self-assembled layers of BNTCDI are confined by two step edges occupied with double chain structures and reveal a sharp boundary towards the bare Au(111)-surface.



Figure 3: LT-UHV-STM image of an approximately half-monolayer coverage of BNTCDI on Au(111) (setpoint: Ubias = 1.0 V, Iset = 200 pA; scale bar: 20 nm). All step edges are occupied by double chain structures and additional molecules form ordered double layers (ODL, red) or disordered double layers (DDL, orange). A monolayer (ML, light green) can be identified at the boundary of a double layer structures. i) and ii) are marked as color-coded lines in the STM image and their apparent height profiles (averaged over 9 lines) are plotted below. In profile i) a gray, semitransparent step represents the subjacent Au steps.

In Figure 3 these structures are analyzed in detail with the help of two apparent height profiles. The first colored line (i) presents the profile across three terraces: bare Au(111)surface with BNTCDI double chain on the step edge, disordered and ordered molecular layers on the second terrace, and a disordered layer on the third terrace. This line profile nicely illustrates the different apparent heights of the molecular layers relative to the underlying Au steps, shown in gray. The chains on confining step edges appear less than 200 pm high, whereas the disordered and ordered structures of the molecular layer reach significant larger apparent heights of about 230 pm in average and 300 pm as peak maxima. Concerning the apparent height no distinct differences were found for the ordered and disordered structure. The second profile (ii) in Figure 3 depicts the pathway from the bare Au(111)-surface via the BNTCDI layer boundary and a disordered area to the ordered molecular arrangement on the second terrace. Thereby, it reveals a monolayer at the boundary with an apparent height of about 180 pm, which is in good agreement with the apparent height of the single chain structures observed at low coverage (Figure 2). Consequently, both ordered and disordered layers, are attributed to double layer structures. Hence, the intermolecular interactions, in this case presumably face-to-face -aggregation25,28, are significantly stronger than the 14 ACS Paragon Plus Environment

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BNTCDI/Au(111) interfacial interactions. Consequently, molecular double layers are formed almost exclusively even in the case of submonolayer coverage. The non-planarity of the molecule is presumably the reason for BNTCDI to prefer the formation of double layers in contrast to the well-known multilayer -aggregates of planar NDI derivatives. Besides the height information the periodicity of the self-ordered structures is observable in the red section of both profiles (Figure 3). Profile (i) shows the periodicity between the straight plait rows, while profile (ii) reveals the periodicity along a plait row. The unit cell of the ordered supramolecular arrangement is obtained from high resolution STM images in combination with a Fast Fourier Transformation (FFT) spectrum (Figure 4). The recurring elongated structures with a length of about 1.6 nm (green ellipse in Figure 4) are attributed to BNTCDI molecules in the upper layer of the self-assembled double layer. Focusing on the space between these molecules allows the definition of the lattice by the dark sites within the plaits. The unit cell parameter, a = 2.13 ± 0.01 nm, b = 1.60 ± 0.01 nm, γ = 107.4 ± 0.1°, are calculated from the FFT spectrum in Figure 3. The cell size is 3.24 ± 0.02 nm² and the cell comprises two molecules in the upper layer. Significant differences are recognized in comparison of the areas occupied by one BNTCDI in the two-dimensional double layer (1.62 nm²) and within the onedimensional chain structures on the step edges (2.30 or 2.38 nm²). The difference in molecular area might be attributed to a changed molecular arrangement of BNTCDI within the double layer.



Figure 4: LT-UHV-STM image of a self-assembled double layer (ODL) (slightly Fourier filtered, Vbias = -0.3 V, Iset = 200 pA; scale bar: 10 nm). Inset: Fast Fourier Transform (FFT) spectrum. Elongated structures with a length of 1.6 nm are marked by light green ellipses and denote single molecules.

Electronic Characterization of BNTCDI in Single Chain Structures. The electronic properties of BNTCDI on Au(111) are thoroughly investigated by STS for the different molecular arrangements and at distinct positions above the molecule. Before and after each sequence of spectra a STM image was recorded to ensure the exact position and to exclude structural changes during the measurements. Figure 5 shows two sequences of normalized STS measurements recorded on distinct positions above BNTCDI as depicted in the sketch Figure 5c. In the normalized wide range spectra from -3.3 to +3.3 V only a sharp peak around 2.9 V can be clearly identified in comparison to the background spectrum (Figure 5a). This corresponds to an unoccupied molecular orbital level of BNTCDI. In contrast, no occupied molecular orbital levels can be distinguished from the background spectrum of the substrate. This problem is elaborated in detail using STS-measurements at selected positions over 16 ACS Paragon Plus Environment

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BNTCDI and Au(111), respectively, and at different tip-sample separations (given in the Supplementary Information; Figure S2). An additional significant and complex feature between 1.0 and 1.4 V arises by taking small range normalized differential conductance spectra from -1.7 to +1.7 V at a smaller tip-sample separation (Figure 5b). This feature has been fitted by Gaussian curves (see Supplementary Information Figure S3) and is attributed to the LUMO of BNTCDI located at 1.23 eV. Complementary to the STS measurements the existence of the LUMO is confirmed by a series of voltage dependent STM measurements. The set point voltage within this series is varied from -2.05 to +2.05 V, while the current is fixed at 200 pA (Supplementary Information Figure S4). In STM images taken at voltages of +1.25 V and higher the observed molecular chain appears significantly brighter compared to images taken at lower voltages indicating enhanced tunneling through a molecular orbital. There is another very small, but nevertheless highly interesting feature in the normalized differential conductance series in Figure 5b located around -0.5 V. Having a look at the background spectra on the Au(111) terrace (cyan, lowest curve) and step edge (green, highest curve) the intensity of the small peak at -0.55 V changes. This phenomenon is explained in literature by bulk-surface states mixing due to the reduced symmetry near a defect, like a step edge29. The typical Au(111)-surface state (cyan curve in Figure 5b) only occurs as the dominant feature for distances larger than 0.4 nm from the step edge. Having a look at the STS spectra taken over BNTCDI the background peak around -0.55 V corresponding to the Au(111) step edge state can be identified in the yellow and magenta curve representing the side of the molecule close to the step edge. On the other parts of the BNTCDI, the black, blue, and red curves taken over the NDI backbone and the benzyl group on the site of the terrace, this peak only remains as a weak shoulder, characteristic for Au(111) surface states. These STS 17 ACS Paragon Plus Environment


measurements point to an advantageous interaction of one phenyl group or the imide nitrogen of BNTCDI with the step edge sites of Au(111) and are in accordance with the different apparent heights of the benzyl groups obtained for adsorption on the step edge and terrace sites of Au(111) (Figure 2). In addition, this higher interaction may explain the preferential adsorption of BNTCDI chains on the step edges compared to the terraces. These observations confirm a weak interaction of BNTCDI with the Au(111) step edge and the LUMO is identified at +1.23 eV while the peak at around +2.9 V in the wide range spectra corresponds to the LUMO+1. In addition, we have proven that the STS spectra are not affected by the position of the molecule within the chain structure (Supplementary Information Figure S5). BNTCDI molecules at the ends of the chain do not behave differently from the molecules in the middle of the chain. This verifies the assumption of only weak intermolecular hydrogen bonding.

Figure 5: a) Normalized wide range STS on three different positions over BNTCDI (red, black, yellow) and the Au(111) terrace (background, cyan, color code given in c) (setpoint: Vbias = -3.3 V, Iset = 2.0 nA, offset = 2.0). b) Normalized small range STS on five different positions over BNTCDI (red, blue, black, pink, yellow), the Au(111) step edge (green) and the Au(111) terrace (cyan, color code given in c). The dashed lines through the small maxima around -0.5 18 ACS Paragon Plus Environment

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V in all spectra are given as a guide to the eyes for differences in step edge and terrace states of Au(111) (setpoint: Vbias = -2.5 V, Iset = 5.0 nA, offset = 0.7). c) Schematic depiction of the positions used in STS measurements: the upper Au(111) terrace (cyan), Au(111) step edge (green), lower terrace (cyan striped), and points along the long molecular axis of a single molecule (red, blue, black, pink and yellow). Electronic Characterization of BNTCDI in Double Layer Structures. Furthermore, differential conductance measurements were performed on BNTCDI molecules selfassembled in both supramolecular double layers. The spectra were taken over chain structures, ordered double layers and disordered double layers, like indicated in Figure 6. All molecular spectra reveal a clear feature of an unoccupied state which was fitted with a Gaussian curve in each case to determine the precise values for the respective LUMO-level. For the ordered (red) and disordered (orange) BNTCDI double layer structures the LUMO is determined at 1.62 V, while it is shifted to 1.42 V (dotted black line) for the molecular chain structure between two disordered areas, and to 1.33 V (dashed black line), for a chain structure next to the bare Au(111) terrace. This remarkable shift of the LUMO-level depends on the molecular arrangement whereby the electronic states of confined chain structures next to one or two disordered double layers appear very similar to the previously observed structure between two bare Au(111) terraces. Compared to the LUMO of BNTCDI arranged in a single chain between two bare Au(111) terraces (1.23 V) the LUMO of chain structures next to one or two disordered double layers are shifted consecutively to slightly higher energies. An even larger shift of the LUMO is observed for BNTCDI self-assembled in ordered or disordered double layer structures. For both a value of 1.62 V is measured, while a difference is just observed in the shape and not in the position of the peak (Figure 6, red and orange). Since the molecules in the second layer do 19 ACS Paragon Plus Environment


not directly interact with the substrate, the coupling to the substrate is weakened and hence, the value for the LUMO is increased towards the value of a gas phase state of the molecule. The same phenomenon, especially a shift of the LUMO towards higher energies for molecules in the second and following layers has been observed also for PTCDA on Au(111)30 and for fullerenes31,32.

Figure 6: STS measurement on BNTCDI in double layer structures (Vbias = -1.8 V, Iset = 1.0 nA, offset = 0.1 nA/V). Spectra taken on the bare Au(111) surface (cyan), disordered double layers (orange), ordered double layers (red) and chain structures formed by molecules on the step edge (marked by dashed and dotted lines). Inset: STM image with color coded areas to identify the positions where the spectra were taken. Adsorption Behavior of BNTCDI on Au(111) and Pt(111). So far, interesting dissimilarities have been determined between metal surfaces with varying substrate reactivity by comparison of the adsorption behavior of BNTCDI on Au(111) and Pt(111). Especially the characterization of the molecular appearance of the adsorbed molecule with intramolecular resolution using LT-UHV-STM depicts differences mainly defined by the electronic structure. While, in the case of a Pt(111) substrate, characteristic features, like two bright spots located above the imide nitrogen atoms, indicate a weak chemical coupling via hybrid BNTCDI/Pt d-states. The coupling to Au(111) is considerably weaker. This becomes 20 ACS Paragon Plus Environment

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immediately evident by the considerable change in the orbital appearance of BNTCDI on Au(111) which resembles three bright spots corresponding to three isolated -systems like predetermined by the chemical structure. No predominant molecule-substrate interaction is indicated in the latter case. In contrary, BNTCDI exhibits a high mobility at RT on the noble Au(111) surface which allows the self-assembly in energetically favorable structures with respect to the given coverage. At low coverage, the intermolecular weak C(sp2)-H…O hydrogen bonds and the interaction of BNTCDI with the Au(111) step edge states are identified as the main driving force for the evolution of molecular chains. Thus, already at half-monolayer coverage molecular double layers are formed indicating that the intermolecular face-to-face -aggregation28 is energetically favorable compared to BNTCDI/Au(111) interactions. This sequence of interaction strengths is also confirmed by the LUMO energies keeping in mind that a shift of the gas phase LUMO energy of the molecule towards EF due to adsorption is a measure for the interaction strength between molecule and substrate. The determined LUMO energies for BNTCDI adsorbed on the Au(111) step edges and terraces are 1.23 and 1.62 eV, respectively. The latter one corresponds to the LUMO energy given for a series of Nalkyl and N-alkylaryl naphthalene diimides obtained from UV-Vis spectra16 which amounts to 1.53 ± 0.05 eV with respect to EF(Au(111)). This allows the interpretation that BNTCDI molecules in the self-assembled double layer exhibit almost no interaction with the Au(111) surface, while a shift of about 0.4 eV characterizes the physisorption on the Au(111) step edges. An application of this consideration for the adsorption of BNTCDI on a Pt(111) surface leads to a shifted LUMO energy of about 0.9 eV compared to the gas phase value. Hereby the shift is induced by the weak chemisorption of BNTCDI on Pt(111), which confirms the considerably stronger molecule-substrate interaction. 21 ACS Paragon Plus Environment


4.

CONCLUSION

The adsorption sequence of BNTCDI molecules on an Au(111) surface was investigated using LT-UHV-STM and STS with orbital resolution. While no single molecules are observed even at very low coverages, single chain and with increasing coverage double chain structures grow on Au(111) step edges. Weak C(sp2)-H…O hydrogen bonds between neighboring NDIbackbones and the interaction of BNTCDI with Au(111) step edge states were identified as driving force for the chain structure formation. At higher coverages BNTCDI disordered double layers self-assemble on the Au(111) terraces and form ordered plait patterns, if the double layer width surpasses approximately 10 nm on a single terrace. The first two LUMO levels of BNTCDI were determined using differential conductance spectroscopy performed locally on distinct molecular groups. A shift of the LUMO energy was observed to be characteristic for the observed supramolecular structures and thus, could be attributed to different interaction strengths between the molecule and different Au(111) surface positions. A comparison of these results with the adsorption behavior of BNTCDI on Pt(111) provides valuable insights into the role of the substrate reactivity onto the balance between molecule-molecule and molecule-substrate interactions. While, on one hand, BNTCDI forms a weak chemical contact to Pt(111), useful for applications with a need of electron transport through the interface, on the other hand only a weak physisorption of BNTCDI is determined on Au(111), suitable for all applications based on unaffected molecular properties.


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ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website at DOI: LT-UHV-STM images and differential conductance spectroscopy (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +49 2461 614015 Orcid S. Karthäuser: 0000-0003-3953-6980 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Volkswagen Stiftung through the ‘Optically Controlled Spin Logic’ project and from the DFG through the CRC1238 (project C01).



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Complex Adsorption Behavior of a Non-Planar Naphthalene Diimide

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