London Dispersion Directs On-Surface Self-Assembly of [121

London Dispersion Directs On-Surface SelfAssembly of [121]Tetramantane Molecules Daniel Ebeling,*,#,⊥ Marina Šekutor,‡,⊥ Marvin Stiefermann,# Jalmar Tschakert,# Jeremy E. P. Dahl,§ Robert M. K. Carlson,§ André Schirmeisen,*,# and Peter R. Schreiner*,‡ #

Institute of Applied Physics, Justus-Liebig University, Heinrich-Buff-Ring 16, 35392 Giessen, Germany Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany § Stanford Institute for Materials and Energy Sciences, Stanford, California 94305, United States ‡

S Supporting Information *

ABSTRACT: London dispersion (LD) acts between all atoms and molecules in nature, but the role of LD interactions in the self-assembly of molecular layers is still poorly understood. In this study, direct visualization of single molecules using atomic force microscopy with COfunctionalized tips revealed the exact adsorption structures of bulky and highly polarizable [121]tetramantane molecules on Au(111) and Cu(111) surfaces. We determined the absolute molecular orientations of the completely sp3hybridized tetramantanes on metal surfaces. Moreover, we demonstrate how LD drives this on-surface self-assembly of [121]tetramantane hydrocarbons, resulting in the formation of a highly ordered 2D lattice. Our experimental findings were underpinned by a systematic computational study, which allowed us to quantify the energies associated with LD interactions and to analyze intermolecular close contacts and attractions in detail. KEYWORDS: bond imaging, CO-functionalized tips, diamondoids, low temperature noncontact atomic force microscopy, van der Waals interactions

T

molecules together, and therefore is a key interaction in the formation of condensed matter.22,23 For increasingly larger and hence more polarizable structures, LD gains more importance as the attractive effect grows rapidly with molecular size.21 For instance, bulky groups can act as dispersion energy donors (DEDs)24 that stabilize extremely long C−C bonds that would otherwise be fleetingly existent.25 LD interactions have also been reported to play an important role for self-assembly processes of molecular structures on surfaces.26−29 However, in most examples, LD coexists side-by-side with electrostatic interactions or hydrogen bonding, making it difficult to assess and quantify the overall LD contributions. We therefore chose [121]tetramantane for our study, which serves as an archetype system to explore LD due to the absence of other interactions, as recent studies showed that hydrocarbons in general are primarily stabilized by the mechanism of oscillating dipoles (in other words by LD), with electrostatic interactions having only a minor effect.30−32 Our aim was to determine the influence LD

o observe a single molecule and recognize its individual atoms is the chemist’s ultimate fascination, and astonishingly, what once was only a musing became in recent years an exciting reality. Numerous nanostructures have been prepared using on-surface synthesis on metals, with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) emerging as the main tools for product identification and the analysis of surface phenomena.1−4 The AFM technique, using a scanning probe tip terminated by a CO molecule, results in a significant increase of resolution, to the point that individual atoms in molecules can be visualized.5−7 Moreover, the ability to pick up small molecules with the tip provides ways to control single-molecule reactions.8−11 The advances in this field are steadily on the rise,12,13 ranging from elucidation of molecular structures14 to stepwise analysis of intermediates and products formed by on-surface reactions.15−19 Because many self-assembly processes on surfaces are still poorly understood, techniques capable of submolecular resolution could help shed some light on the forces behind them. One such force is London dispersion (LD).20,21 As LD conceptually arises from instantaneous induced dipoles, it is independent of interactions between permanent dipoles, keeps © 2017 American Chemical Society

Received: July 23, 2017 Accepted: August 28, 2017 Published: August 28, 2017 9459

DOI: 10.1021/acsnano.7b05204 ACS Nano 2017, 11, 9459−9466

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directly. In addition, the Crommie group developed an STMbased vibrational spectroscopy technique (IRSTM) capable of providing spectroscopic evidence for the adsorbed assemblies consisting of adamantane and tetramantane submonolayers on a Au(111) surface.42,43 By probing IR responses of such diamondoid assemblies, they could not only differentiate between chemical structures but also conclude that intermolecular interactions between tetramantane molecules are stronger than the interactions between tetramantanes and the gold surface, as evident by different tetramantane vibrational resonances.42 Despite these recent advances, direct determination of orientation by visual inspection of single, completely sp3-hybridized molecules remained elusive. Here, we should stress the inherent difference between sp2 and sp3 (hydro)carbons, which is especially obvious when considering two-dimensional materials. An illustrative example is a comparison of graphene (sp2 material) with diamond (sp3 material). In the case of graphene, each carbon atom covalently binds with three neighboring carbons, forming three σ bonds and a conducting π band from the fourth valence electron and ultimately producing a flat sheet.44 On the other hand, a diamond carbon atom forms four covalent bonds with neighboring carbons in a tetrahedral fashion. 45 As a consequence, graphene and diamond have very different physicochemical properties, but they also differ in their structural features, that is, sp2 versus sp3 structural arrangements. Coming back to analogous hydrocarbons (aromatic versus diamondoid molecules), it is no surprise that their structures and, consequently, their possible on-surface orientations when forming monolayers on metal surfaces differ as well. With the aim to broaden the understanding of sp 3 hydrocarbon self-assembly on metal surfaces and to quantify LD contributions responsible for the formation of highly ordered monolayers, our study combines experimental and computational approaches. The AFM technique with a COterminated tip enabled us to observe individual [121]tetramantane molecules with atomic resolution and to

has on the self-assembly of bulky tetramantane molecules and to quantify such attractive interactions in two dimensions. [121]Tetramantane belongs to the class of diamondoid compounds, which are hydrogen-terminated cage molecules that structurally resemble the diamond crystal lattice (Figure 1).33,34 Its first synthesis was accomplished in the 1970s by

Figure 1. Structures of lower diamondoids and [121]tetramantane, hydrogen-terminated nanodiamonds.

McKervey et al. albeit in too low a yield for any practical application.35 Recent isolation of diamondoids from oil36 enabled advances in selective functionalization of higher diam ond oid s 3 7 and depositio n of functionalized [121]tetramantane derivatives on metal surfaces resulted in the formation of self-assembled monolayers with fascinating electronic properties.38−40 An important aspect for studying the self-assembly process in detail is the precise identification of molecular orientation. For bulky, completely sp3-hybridized molecules, this is especially difficult because they may easily reorient on the surface due to small differences in adsorption energy. For example, Wang et al. reported the first single-molecule STM study of [121]tetramantane on a Au(111) surface,41 finding that individual molecules oriented very differently on the surface upon tip manipulation. The authors inferred the orientation of hydrocarbon cages from comparison of density functional theory (DFT) computations with STM data. However, this approach is difficult because STM data depend on the local density of states, which is related to the molecular structure in a nontrivial way. Hence, the molecular structure is not measured

Figure 2. Self-assembly of [121]tetramantane on a Au(111) surface. (a) STM overview image of a large-scale 2D island and the corresponding single line profile (solid and dashed red lines). The color scale is split into two parts to reveal the herringbone reconstruction of the Au(111) surface (left part) and the molecular island (right part) side-by-side. White arrows indicate lattice directions of the Au(111) surface. Blue circles represent a regular 2D lattice. Identical structures have been observed on different islands and different surfaces (average lattice parameters: Au(111): vector a = 10.1 ± 0.1 Å, vector b = 7.6 ± 0.1 Å, angle = 58 ± 1°; Cu(111): a = 10.2 ± 0.1 Å, b = 7.7 ± 0.1 Å, α = 60 ± 1°, Figure S1). (b) High-resolution STM topography image. Zoom-in location is marked in (a) by a blue box. (c) High-resolution AFM frequency shift image revealing submolecular resolution. Parameters: STM: 200 mV, 10 pA; AFM: Δz = −236 pm (with respect to the tunneling gap at 200 mV, 10 pA), Q = 8500, oscillation amplitude = 120 pm. All measurements performed at 5 K in UHV with COfunctionalized tip. 9460


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Figure 3. Adsorption structure of [121]tetramantane. (a−c) AFM images of a small cluster of [121]tetramantane on Au(111) obtained at three different imaging heights, Δz = 360, 330, and 270 pm (with respect to the tunneling gap at 200 mV, 150 pA). Parameters: Q = 21300, oscillation amplitude = 70 pm. Each molecule is composed of five bright spots, which are arranged in an Olympic ring-like pattern (orange circles). (d−g) Top and side views of [121]tetramantane adsorbed on Au(111). Hydrogens in the imaging and surface planes (gray planes) are marked as orange and blue spheres, respectively. Red spheres indicate two hydrogens just below the imaging plane (and corresponding ones just above the surface plane), which determine the on-surface handedness of the adsorbed molecule. All other hydrogens are displayed as light blue sticks.

incommensurate with the underlying Au lattice. Furthermore, we observe a virtually unaffected herringbone reconstruction of the Au(111) surface beneath the molecular layer (bright lines that shine through the layer in Figure 2a and Figure S2). As any significant molecule−substrate interaction would inevitably lead to a stress relaxation between the Au atoms in the surface layer,46,47 the interaction between [121]tetramantane molecules and the Au surface atoms must be very small. On Cu(111), the observed 2D overlayer is in accordance with a (3 × 4) superstructure and hence aligns perfectly with the Cu lattice. At first glance, this would indicate relatively strong molecule−substrate interactions.47−49 However, the observed 2D lattice parameters and molecular packing densities on Au(111) and Cu(111) are very similar, and on both surfaces, we observe identical types of intermolecular arrangement (as discussed below, Figure 4). Hence, we rationalize that the presence of the (3 × 4) superstructure on Cu(111) is caused by a random coincidence of the molecular dimensions with the dimensions of the Cu(111) lattice rather than by strong molecule−substrate interactions. Furthermore, there are remarkable similarities between the lattice parameters of adsorbed 2D [121]tetramantane islands and bulk [121]tetramantane crystals.50 The vector c of the 3D monoclinic lattice structure has, for example, a length of 10.33 Å, which is close to the determined 2D vectors of a = 10.1 ± 0.1 and 10.2 ± 0.1 Å for Au(111) and Cu(111), respectively.

study the LD-driven assembly by directly determining their precise position and orientation. Using computational tools, we could not only confirm the experimental findings but also quantify the attractive intermolecular interactions. Such a systematic approach was necessary to gain insight into the role that LD plays in on-surface assembly of bulky hydrocarbons and molecules with dispersion energy donors (DEDs).

RESULTS AND DISCUSSION We deposited [121]tetramantane on two different metal surfaces, namely, Au(111) and Cu(111). On both materials, [121]tetramantane forms well-ordered large-scale islands at room temperature (Figure 2a and Figure S1 in the Supporting Information, SI). The molecular islands can be fitted with regular 2D lattices over large distances on the order of several tens of nanometers (blue circles in Figure 2a). The observed lattice parameters for the two metal surfaces are similar but not identical (Figure 2). This leads to packing densities of 1.53 and 1.47 molecules nm−2 on Au(111) and Cu(111), respectively. It should be noted that the 2D [121]tetramantane island (blue circles) does not align to any of the Au(111) surface vectors (white arrows in Figure 2a). A closer look reveals that the obtained 2D lattice parameters for [121]tetramantane on Au(111) do not coincide with any regular overlayer structures. In other words, the observed molecular overlayer is 9461


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Figure 4. Different types and on-surface chirality of tetramantane islands. (a,c) Two different types (A and B) of structures are considered, which are composed of Msurf- and Psurf-type molecules. (b,d) For each type (A and B), one mirror-symmetric structure with identical binding energy exists. The observed lattice vectors and the corresponding angles for each surface material are given in the caption of Figure 2. Whereas lattice parameters are identical, the orientational arrangement of the molecules is different for types A and B as indicated by dashed lines between specific neighboring atoms of the molecules. (e) Type A (Psurf) island on Cu(111) surface. Parameters: Q = 20100, oscillation amplitude = 160 pm. (f) Two type A (Psurf) islands on Au(111) surface separated by grain boundary (dashed white line). (g) Type A (Msurf) island on Au(111) (same island as in Figure 2). Parameters for (f,g): Q = 8500, oscillation amplitude = 120 pm.

the standard bond imaging technique relies on 2D constant height scanning, the method is very sensitive to any height difference in the sample topography. Hence, to reveal structures of bulky molecules or angled adsorption structures, more sophisticated imaging schemes have to be utilized.19,53−56 Here, we performed constant height imaging at several different tip−sample distances to evaluate the formation of the image contrast with decreasing tip−sample distance and to obtain a 3D frequency shift map,57 which can be used to determine the tilting angle of the molecule. Figure 3a−c shows three AFM images, which have been captured at tip−sample distances of Δz = 360, 330, and 270 pm (with respect to the tunneling gap at 200 mV, 150 pA). The closer the CO tip is approached to the surface, the more bright features gradually appear in the images. Ultimately, each molecule is represented by five bright spots, which are arranged in an Olympic ring-like pattern (orange circles in Figure 3a−c). These bright spots are caused by repulsive interactions between hydrogen atoms in the imaging plane and the CO tip, which lead to positive frequency shifts of the tuning fork sensor. Note that bright lines between the five bright spots, which appear when the tip is brought very close to the molecule (Figure 3c), arise from a tilting of the CO

Moreover, within (110) planes of the monoclinic 3D lattice, the tetramantane molecules are packed with a 2D density of 1.46 molecules nm−2. This almost exactly matches the observed 2D packing density on Cu(111) and deviates by less than 5% for Au(111), which also indicates relatively weak molecule− substrate interactions. To reveal the precise adsorption structure and orientation of [121]tetramantane on Au(111), we performed high-resolution scans with CO-functionalized tips at 5 K. In Figure 2b,c, STM and AFM scans of the region marked with the blue box in Figure 2a are shown. Submolecular resolution is obtained in the AFM image, with each depicted molecule being composed of several bright spots that appear in a specific pattern. It is remarkable that all molecules in the imaged region (around 80) are oriented in the same way except for a single molecule located at the rim of the island (red dashed circles). Next, we determined the precise adsorption structure of the tetramantane molecules. In the beginning, the bond imaging technique has been mainly applied to aromatic molecules, which adsorb planar to the surface.5,13,51,52 For these systems, the method directly allows for a detailed interpretation of the chemical structure of the studied molecules. For 3D objects, however, image interpretation is not as straightforward. Because 9462


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ACS Nano tip.58 Hence, these lines should not be misinterpreted as “bonds” between hydrogen atoms. The adsorption structure of [121]tetramantane on Au(111) is shown in Figure 3d−g. Hydrogen atoms, which correspond to the imaged five-atom pattern, are displayed by orange spheres. Due to the symmetry of the molecule, a counterpart of the five-atom pattern is adsorbed to the Au(111) surface (blue spheres). The AFM images reveal differences in brightness of the five features, which are caused by a slight tilting of the molecule. We determined the tilting angle from the 3D frequency shift map (see Figures S3 and S4 for details). The resulting tilting angles are approximately 5 and 12° with respect to the x- and y-axes, respectively (Figure 3f,g). On Cu(111), we find a similar behavior of the image contrast with decreasing imaging heights. Hence, we conclude that the molecular tilting is rather independent of the surface material and is caused by the nonsymmetric arrangement of atoms within the adsorption plane (and neighboring atoms just above this plane). Note that [121]tetramantane is, in our case, a prochiral molecule when attached to a surface; that is, while being achiral in the gas phase, the molecules adopt an on-surface chirality by adsorbing on the surface plane.59 Therefore, an identification of the five-atom pattern is not sufficient for an absolute determination of molecular orientation. A closer look at the structure reveals that each [121]tetramantane molecule contains four of the mentioned five-atom patterns, which can be obtained by rotating the molecule around its longitudinal axis (cf. x-axis in Figure 3d−g). These patterns can be divided into two different types, which determine the on-surface chirality of the adsorbed molecules (in the following denoted as “Msurf” and “Psurf”-type). To distinguish between the types, two specific hydrogen atoms located just below the imaging plane are depicted as red spheres. These red atoms can be located either on the left or on the right side of the five-atom pattern (cf. orange and blue pattern in Figure 3e, which are Msurf- and Psurf-type, respectively). The two red atoms are not directly revealed in our constant height scans. However, when the CO tip is brought very close to the molecule during imaging, it can even sense the two red hydrogen atoms just below the imaging plane. In Figure 3c, these red atoms appear as a bright halo next to the highest atom within the five-atom pattern, as indicated by red arrows. This feature directly allows for determination of the surface-induced handedness of the molecules as the brightest atom within the five-atom pattern always indicates its on-surface handedness. Corresponding overlays of Msurf- and Psurf-type molecular models are depicted in Figure 3b. After equilibration of the system for sufficiently long times at adequate temperatures (e.g., several minutes at room temperature), we observed that [121]tetramantane forms large-scale well-ordered islands where nearly 100% of the molecules were perfectly arranged with each other (Figure 2c). However, even for smaller islands, which form at temperatures below 100 K within several seconds, there is a strong tendency for the molecules to cluster in the energetically most favorable way. Even within islands of 20−30 molecules, clusters on the order of 8−10 molecules with nearly perfect arrangement can be found (Figure S5). Figure 4a,b shows the observed arrangement of molecules within the ordered clusters. The lattice parameters (i.e., lattice vectors and angles) can be precisely determined by large-scale STM overview scans (Figure 2). The exact orientation of single molecules within the 2D lattice is, however, only accessible

using the bond imaging method by identifying the orientation of the five-atom pattern and the on-surface handedness of the molecules (which is obvious from the tilting of the molecule as discussed above, Figure 3). In order to quantify LD contributions and support our experimental findings, we undertook a detailed computational study of LD-bound complexes initially consisting of two [121]tetramantanes. After testing a number of geometries that arise from different orientations of the tetramantane cage and by using several computational methods to verify our results (see SI for details), we found that the energetically most favorable tetramantane orientations were those that maximize the number of close contacts between the hydrocarbons. The computed energetic stabilization attributed to LD thereby amounts to about −10 kcal mol−1. Another important finding was that functionals uncorrected for LD lead to very weakly bound or even dissociated structures, and only dispersioncorrected computational methods give quantifiable LD-bound complexes, confirming the crucial role that LD plays in intermolecular interaction between the cages. These results further validate our rationale that LD is chiefly responsible for the experimentally observed 2D lattice self-assembly process. Additionally, we undertook a DFT study of four interacting [121]tetramantane molecules (see SI for details). We considered the case where only the orientation of the fiveatom pattern with respect to the 2D lattice is known, but information about the on-surface handedness of the molecules is missing. This is illustrated in Figures 4a and c, which display two types of structures (A and B) that exhibit identical lattice parameters and packing densities but are composed of Msurfand Psurf-type molecules, respectively. Note that for each type of structure a mirror symmetric counterpart with identical binding energy exists (cf. Figure 4b,d). However, the binding energies of A- and B-type structures are not equal. This is revealed by the red dashed lines in Figure 4a−d. Whereas the molecules within the rows of both types of structures are organized in the same way, the arrangement between top and bottom rows significantly differs from each other. More detailed information about type A and B structures, including distances for the closest intermolecular contacts, is given in the Supporting Information (Figures S6 and S7). The difference in binding energy of the two structural types is directly reflected by our computations. Whereas tetramers of type A are certainly stabilized by dispersion interactions (by ∼20 kcal mol−1 = 0.87 eV), tetramers of type B are dominated by short-range repulsive forces and are therefore energetically unfavorable. This finding is in agreement with our experimental observations because no structures of type B have been found on any surface material. Atype structures (of Msurf- and Psurf-type), however, have been observed on Au(111) and Cu(111) surfaces (see exemplary images in Figure 4e−g). In addition, we studied the influence of a small tilting angle of the molecules with respect to the x-axis that has been observed in the experiments. As shown in Figure 3f, the side of the five-atom pattern connected to the two red hydrogen atoms is tilted upward (+5°). In the computations, we also studied the case when this side is tilted downward (−5°; see Figure S8 for details), which has a noticeable influence on the binding energy (Table 1). In particular, for structures of type B, the opposite tilting of −5° would even lead to a stabilization of the tetramer. Nevertheless, type B (−5°) tetramers are far less stable than all type A structures (for both, + 5° and −5°). 9463


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were obtained after equilibrating the sample for several hours at room temperature. STM/AFM Measurements. All measurements were performed at 5 K with a commercial ultrahigh vacuum, low-temperature STM/AFM (ScientaOmicron, Germany) using tuning fork sensors and COfunctionalized tips. The electrochemically etched tungsten tips were conditioned by voltage pulses and indentations into the sample surface, prior to the functionalization with single CO molecules using standard recipes.60 All images have been obtained by constant height scanning using the frequency modulation mode. Small drifts of the scanner in z-direction during scanning were regularly adjusted by pausing the scan and switching on the STM feedback. Computational Details. All geometry optimizations were performed with the Gaussian09 program package using several DFT approaches (see SI for details), and the obtained minima were verified by frequency computations. As the B3LYP functional does not account for LD, it needs to be augmented by a correction such as Grimme’s D3 dispersion formulation in connection with Becke−Johnson (BJ) damping. This approach allows for an analysis of the LD contribution in the studied [121]tetramantane complexes. Single-point energies on the DFT-optimized structures (B3LYP-D3(BJ)/6-31G(d,p) level of theory) were computed with the ORCA 4.0.0 program using RI-MP2 and DLPNO-CCSD(T) methods, and complete basis set extrapolation was also performed. All computation methods we used (except for the uncorrected B3LYP, which does not account for LD and therefore cannot provide quantifiable dimers) showed comparable trends in interaction energies for the LD-bound complexes (Table S1). We found that the energetically most stable complexes engage in greatest number of close contacts between tetramantanes, confirming that LD significantly acts between the cages and is responsible for preferable orientation of molecules. Another validity confirmation of our computational approach comes from local energy decomposition analysis, which is implemented in ORCA 4.0.0 for the DLPNOCCSD(T) method. Dispersion contributions from decomposition of CCSD pair energies were in agreement with energies obtained by other methods we used (Table S1). Complete computational details and references are provided in the SI.

Table 1. Computed Interaction Energies of Type A and Type B [121]Tetramantane Tetramers with Different Tilting Anglesa type Au(111) Au(111) Cu(111) Cu(111)

A B A B

(+5°) (+5°) (+5°) (+5°)

ΔE (kcal mol−1) −19.3 73.6 −20.6 33.7

type A B A B

(−5°) (−5°) (−5°) (−5°)

ΔE (kcal mol−1) −19.1 −4.4 −19.5 −7.0

a Interaction energies are defined as a difference between the energy of the tetramer and the energy of four [121]tetramantane molecules. Computations were performed at the B3LYP-D3(BJ)/6-31G(d,p) level of theory (see SI for details).

These results are remarkable in the context that all computations were conducted without taking the metal surface into account. Certainly, the substrate has some influence on island formation, which is, e.g., reflected by the slightly different lattice parameters and packing densities that have been observed on Au(111) and Cu(111) (Figure 2). Nevertheless, for both type A structures (on Au and Cu), the computed energy gain caused by intermolecular dispersion interactions is on the same order of magnitude (∼20 kcal mol−1). This energy gain clearly arises from the precise intermolecular arrangement as obvious from the differences between type A and B structures (and tilting angles). Therefore, we can follow that even small structural changes can drastically influence the binding energy and, furthermore, that the self-assembly process is driven by LD interactions.

CONCLUSIONS We used STM and AFM imaging to visualize the submolecular structure of [121]tetramantane molecules and to explore their self-assembly on Cu(111) and Au(111) surfaces. Small diamondoids serve as an archetype system to study the role of LD interactions during structure formation because other binding contributions, such as hydrogen bonding, dipole− dipole, π−π, electrostatic interactions, or metal coordination, which predominate self-assembly processes in many other cases, can be neglected. The application of the bond imaging technique allowed a precise and unambiguous determination of the adsorption structure of bulky, completely sp3-hybridized molecules. It also allowed for a means to investigate the dependence of the organization process on their 2D arrangement. A detailed computational study supports the AFM findings, and we successfully quantified the effect LD has on intermolecular interactions between tetramantanes, thus establishing LD as a driving force for directed self-assembly for such class of compounds. Although many high-resolution AFM studies of surface phenomena have been reported, our investigation focuses on an entirely nonaromatic system and contributes to explorations of sp3-hybridized molecules capable of self-assembly on metal surfaces.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05204. Additional STM/AFM images, computational details, and Cartesian coordinates (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Daniel Ebeling: 0000-0001-5829-170X Marina Šekutor: 0000-0003-1629-3672 Peter R. Schreiner: 0000-0002-3608-5515 Author Contributions

D.E. and M.Š. contributed equally to the work, and both contributors are considered first authors. ⊥

METHODS

Notes

Sample Preparation. [121]Tetramantane was isolated from petroleum and purified by multiple HPLC separations as described previously.36 Cu(111) and Au(111) crystals (Mateck, Germany) were cleaned by multiple cycles of sputtering with argon ions and subsequent annealing. To achieve small molecular clusters of 10−30 molecules, the sample was precooled in the STM at 5 K. For evaporation, the sample was removed from the STM with a precooled wobble stick and held for 30−60 s in front of a home-built sublimation device,47 which was kept at approximately 30 °C. Large-scale islands

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) via the GRK (Research Training Group) 2204 “Substitute Materials for Sustainable Energy Technologies”. This project was also supported by the 9464


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Laboratory of Materials Research (LaMa) of JLU and the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E). The work was also supported in part (for J.E.P.D., R.M.K.C., and P.R.S.) by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515. M.Š. thanks the Alexander-vonHumboldt Foundation for a Humboldt Research Fellowship for Postdoctoral Researchers. The computations were conducted on the LOEWE-CSC high-performance computer of the State of Hesse.

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