Adsorption of Phenacenes on a Metallic Substrate: Revisited - The

Article pubs.acs.org/JPCC

Adsorption of Phenacenes on a Metallic Substrate: Revisited Song-Wen Chen,† I-Chen Sang,† Hideki Okamoto,‡ and Germar Hoffmann*,† †

Department of Physics, National Tsing Hua University, 101 Section 2 Kuang Fu Road, Hsinchu, 30013 Taiwan, Republic of China Department of Chemistry, Okayama University, 3-1-1 Tsushimanaka, Okayama, Japan 700-8530

‡

S Supporting Information *

ABSTRACT: Phenacenes represent a class of simple hydrocarbons with appealing physical properties ranging from high charge mobility to superconductivity in combination with chemical robustness that are easily modified to serve as versatile building blocks for tailored structures. As a promising candidate for applications in organic devices, phenacenes are the focus of recent investigations. Thereby, the initial growth behavior starting from a single molecule is controversial. Here, we address the growth of [7]phenacene and [9]phenacene on a Ag(111) surface, studying the details of the initial stage of growth by scanning tunneling microscopy. According to our results, a previously introduced model involving a coverage-dependent phase change with the out-of-plane rotation of molecules in the initial growth stage can be disregarded. Instead, we find evidence for the formation of a new phase on top of an in-plane wetting layer during the initial stage of growth.

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INTRODUCTION Organic electronics and, more recently, molecular electronics (i.e., based on molecular films and single-molecules, respectively) are promising advances in current research on novel types of devices. Organic materials are already present in consumer electronics as organic light-emitting diodes (LEDs). Established organics cover a broad range of materials from polymers to solid crystals. In general, these are aromatic systems with delocalized electrons that are often further functionalized with electron-donating and/or -accepting substituents to tune their electron and hole mobilities. In addition to transport properties, chemical stability and the ability to form well-ordered films on a large scale are critical aspects to be considered for applications. Pentacene, which has a high electron mobility, is a prominent, intensively investigated, and well-characterized candidate for device applications.1 However, pentacene is vulnerable to degradation under ambient conditions and photolability, so its potential applications are limited.2 The recently introduced class of phenacenes represents a promising alternative, offering equivalent physical properties combined with enhanced chemical stability. Phenacenes and pentacene share similarities in their chemical structures (see Figure 1); molecular arrangements in bulk materials;3,4 and, notably, their growth behaviors, as discussed below. However, investigations into local processes of phenacenes, specifically their growth, are still at the beginning. Pentacene is a model compound for applications in organic field-effect transistors (OFETs).5 It is a hydrocarbon with a simple, highly symmetric chemical structure of five fused benzene rings arranged in a row. Large and highly ordered crystals of pentacene are easily obtained from solutions. Thin films, down to single molecule adsorption, can also be grown by thermal deposition and, for improved surface homogeneity, by supersonic molecular beam deposition.6 The initial growth of © 2017 American Chemical Society

Figure 1. Chemical structures of pentacene and [n]phenacenes with n = 5, 7, and 9.

pentacene has been studied on a large number of surfaces. On inert SiO2,7 growth starts directly with a three-dimensional structure in a herringbone pattern. On reactive metal surfaces, such as Au(111),8,9 Au(100),10 Ag(110),11 Ag(111),12−15 and Cu(110),16,17 the growth of pentacene starts with isolated, highly mobile, flat-lying molecules. When the coverage is increased to a full monolayer, ordered networks are stabilized. A flat initial layer also serves as a wetting layer for additional growth in three-dimensional fashion upon further adsorption. The growth behavior has been subjected to several theoretical investigations addressing the molecule−metal interactions (see, for example, refs 9 and 18) or the electrostatic potential.19 On less reactive surfaces, growth behavior between the extrema is reported, as for highly oriented pyrolytic graphite (HOPG),20 a three-dimensional rearrangement of the first, initially fully Received: February 26, 2017 Revised: May 5, 2017 Published: May 5, 2017 11390

DOI: 10.1021/acs.jpcc.7b01806 J. Phys. Chem. C 2017, 121, 11390−11398

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The Journal of Physical Chemistry C

Figure 2. (a) Full monolayer coverage of densely packed [7]phenacene (5 × 5 nm2, −500 mV, 10 pA). (b) Submonolayer coverage of [9]phenacene with the bare Ag(111) surface appearing between the molecules (5 × 5 nm2, −250 mV, 15 pA). (c) Overview scan of [9]phenacene. In areas with a stable adsorption geometry, molecules are resolved with submolecular resolution. Within fuzzy areas (marked areas), molecules are mobile and are not individually resolved. The arrow wheel in the bottom corner indicates the [11̅0] substrate direction (60 × 60 nm2, + 75 mV, 11 pA).

molecule−molecule interactions due to the presence of two unequally rotated molecules (for illustration, see Figure 9 below). The central argument in this interpretation is the observed apparent height, as the f ull height of a tilted molecule can be expected to be significantly larger. Therefore, the possibility of the adsorption of Phase 2 on top of Phase 1 has been disregarded. In LEED experiments, Hasegawa et al.33 also identified an enlarged unit cell, but the data are not suitable for unambiguously identifying the structure of the very first layer. A combined X-ray reflectivity/diffraction and atomic force microscopy investigation of the initial growth of [5]phenacene on SiO2 and HOPG37 suggested the out-of-plane growth of [5]phenacene on SiO2 from the very first molecule. On HOPG, an in-plane [5]phenacene layer was found to grow first and to serve as a wetting layer for the subsequent growth of [5]phenacene layers in Phase 2. This coincides with the known growth behavior of pentacene on metallic substrates. The preference for the formation of a wetting layer over direct three-dimensional growth already indicates the decisive role of the substrate for a weakly interacting surface such as HOPG, and this role can be assumed to be even more important for reactive metallic surfaces.

planar monolayer upon additional adsorption or upward standing molecules in the initial monolayer on Bi(001),21 as well as for adsorption on BN,22 graphene,23 and MoS2.24 Phenacenes are aromatic hydrocarbons with n benzene rings fused in a zigzag geometry, thereby reducing the symmetry in comparison to that of pentacene. Phenacenes were initially identified as thermally very stable and rather inert side products of the oil processing industry.25,26 The synthesis of extended phenacenes with seven or more benzene rings is difficult because of their insolubility, which delayed research into such phenacenes and was first reported by Mallory et al. in 1996,27 who coined the term [n]phenacene. [5]Phenacene is also known as picene. A robust and efficient prototype organic fieldeffect transistor (OFET) using [5]phenacene was realized in 2008.28 In addition, [5]phenacene gained significant attention because of the emergence of superconductivity when it is doped with potassium.29 However, very few reports so far have addressed the structure and growth mode of phenacenes, with controversial interpretations for [5]phenacene and none for extended phenacenes.

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PREVIOUS RESULTS A few publications exist on the growth of [5]phenacene on Ag(111),30 Au(111),31−35 Cu(100),35 and Ag(100)35,36 as studied by scanning tunneling microscopy (STM), on Au(111) and Ag(100) as studied by low-energy electron diffraction (LEED);33,36 and on SiO2 and HOPG as studied by X-ray reflectivity/diffraction and atomic force microscopy.37 Previous STM experiments on the adsorption of [5]phenacene on metallic substrates gave qualitatively equivalent results and were interpreted with the help of density functional theory (DFT) calculations. At submonolayer coverage, no self-assembled structures are observed, indicating repulsive molecule−molecule interactions30,35 with the mobility sufficiently suppressed at 5 K for local measurements.30,34 At coverages close to a full monolayer, molecules are arranged in a planar geometry and form a close, surface-filling network, called Phase 1. With further increasing coverage, a new structure with a larger unit cell containing two nonequivalent molecules (in terms of their respective environments), called Phase 2, emerges and appears to be elevated by approximately 2.5 Å with respect to the initial layer in STM images. These observations have been interpreted in terms of a phase change of Phase 1 upon increased coverage, driven by attractive

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METHODS The experiments presented herein were carried out in a Unisoku STM setup operated at liquid N2 temperature (77 K). A Ag(111) crystal was cleaned by cycles of argon-ion gas sputtering and annealing. STM probes were made of PtIr wires as cut. The bias voltage was applied to the sample; specifically, positive voltages refer to tunneling into unoccupied states, and negative voltages refer to tunneling into occupied states. [7]Phenacene and [9]phenacene were synthesized as described in refs 38 and 39; thermally sublimed at 420 and 500−540 K, respectively; and deposited at rates of 0.06 and 0.01−0.06 ML/ min, respectively, on the room-temperature Ag(111) surface. The deposition rate was monitored with a quartz crystal balance.

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[7]PHENACENE AND [9]PHENACENE ON AG(111) At coverages much below 1 ML, molecules are highly mobile, and stable imaging at 77 K is not feasible. Figure 2a−c presents [7]phenacene at full coverage and [9]phenacene at a coverage of ∼0.7 ML. The shapes of the molecules correspond to previous observations for [5]phenacene. Because of the 11391


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Figure 3. STM images of (a,b) [7]phenacene and (c,d) [9]phenacene as recorded at V = 1 V and V = 2 V for identical areas. At 2 V, molecular pairs of nearly identical height can be observed, whereas at 1 V, a pronounced height variation between two nonequivalent molecules is characteristic. The arrows indicate the respective unit cells. (a,b) 30, (c) 20, and (d) 13 pA. (Each image is 5 × 5 nm2.)

Figure 4. (a) STS spectra (set point U = 2 V, 117 pA) of Phase 2 of [7]phenacene averaged over a large surface (full line) and after renormalization by I/V as recorded at positions above the brighter (···) and darker (− − −) appearing molecules. The energy range close to EF is excluded because of the divergence of renormalization in the presence of noise fluctuations at very low current values. At a band gap of ∼3.1 eV, the highest densities of states were recorded at +1.6 and −1.9 eV on bright molecules. Dark molecules exhibited similar spectra with a less pronounced peak for occupied states and a slightly broadened peak for unoccupied states and a significantly earlier onset by 150 mV (see hatched area). (b) STM image of [7]phenacene as recorded over the edge between Phase 1 and Phase 2. The apparent island height is ∼2.5 Å (25 × 25 nm2, + 2 V, 10 pA).

contained two molecules with a characteristic bias-dependent appearance; see Figure 3a,b for [7]phenacene and 3c,d for [9]phenacene. The molecular rows were shifted relative to each other, resulting in a unit cell in the shape of a parallelogram with dimensions of (1.05 ± 0.1) × (1.8 ± 0.2) nm2 and an angle of ∠ba = 78° ± 4° for [7]phenacene and with dimensions of (1.05 ± 0.1) × (2.2 ± 0.3) nm2 and an angle of ∠ba = 80° ± 5° for [9]phenacene.42 During tunneling into unoccupied states at low voltages (V < 1.5 V), a pronounced height contrast between the two molecules of the unit cell of ∼0.8 Å could be observed (see Figure 3a) that vanished at elevated voltages (V > 1.5 V). Figure 4a shows cross sections through scanning tunneling spectroscopy (STS) data acquired at each pixel point of Phase 2 before (solid line) and after renormalization.43 STS data provide a clear indication of the onset of the highest occupied molecular orbital (approximately −1.7 eV) and lowest unoccupied molecular orbital (approximately +1.4 eV), resulting in a band gap of ∼3.1 eV, in quantitative agreement with data from bulk samples.4 Within the band gap, no apparent state was present. However, the observed relative change in the topographical contrast of ∼0.8 Å in the voltage range from 1.5 to 2 V can be attributed to a broadened lowest unoccupied molecular orbital (see hatched area), with a ∼50% increase in conductivity within the given energy range. A tentative interpretation is that the electronic properties of the two different phenacene molecules are essentially identical, but

additional benzene rings, molecules were extended in the long direction. Below 1 ML, depending on the local packing density, the molecules remained mobile. In Figure 2c, a representative surface at a coverage of ∼0.7 ML is imaged across a Ag(111) step edge. As a result of the reduced density, some molecules are misaligned or even remained mobile, and long-range ordering is missing. Instead, a thermodynamically labile system can be studied with areas with clearly resolved single molecules, corresponding to a solid phase, and a fuzzy region, corresponding to a liquid phase. Within the fuzzy region, molecules change their positions during scanning and are not resolved. Because of the time evolution of the local arrangement at the solid−liquid interface, in consecutive scans, melting and solidification of individual molecules can be observed (not shown), and the change of the fuzzy regions can be followed.40 At full coverage (see Figure 2a), that is, when no free and accessible surface area is present, no mobility could be observed. Molecules were aligned in correspondence with the crystal symmetry.41 Within individual rows, the relative orientations of molecules, that is, of their long and short sides, were random within statistical error. Individual rows were shifted by approximately one-half of a molecule with respect to neighboring rows. The unit cell size42 was (0.75 ± 0.05) × (3.6 ± 0.4) nm2 for [7]phenacene and (0.75 ± 0.05) × (4.2 ± 0.5) nm2 for [9]phenacene. As for [5]phenacene, we found an additional, well-ordered structure (Phase 2) at enhanced coverage. Molecular unit cells 11392


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The Journal of Physical Chemistry C that a site-dependent hybridization causes an unequal broadened lowest unoccupied molecular orbital and reflects variations in the adsorption geometry. Figure 4b shows a cross-sectional profile starting on Phase 1 and crossing the border onto Phase 2. The measured apparent height accounts for ∼2.5 Å. This value is in agreement with previous observations on [5]phenacene. Within the precision of previous experiments, identical experimental results were reported for [5]phenacene adsorbed even on different metallic surfaces in terms of the appearance of Phase 1, the appearance of Phase 2 upon increased coverage, the voltage-dependent change in corrugation for Phase 2, and the size of the unit cells. In comparison to [5]phenacene, for [7]phenacene and [9]phenacene, we found comparable structures for the different coverages and voltages along the row, that is, with an identical cell size in the a⃗ direction. In the b⃗ direction, the unit cell size was enlarged by ∼1.95 Å for each additional benzene ring, in good agreement with the expected value of 2.1 Å.44 The STS data were qualitatively similar at a reduced band gap and an energetic shift of the molecular orbitals. Such a change in electronic structure could be expected and was reported on the basis of nonlocal investigations.4 We thus conclude that the investigation of phenacenes of different lengths gives fully equivalent observations in terms of geometric structures and that the interpretation of the experimentally observed structures and numerical simulations must follow in a straightforward manner. The interpretation of Phase 2 in terms of a phase change in previous publications is motivated experimentally solely by the observed apparent height. However, apparent heights can be misleading, as has been addressed, for example, for insulating NaCl layers of different thicknesses.45 On NaCl, electronic decoupling reduces conductivity and decreases the apparent height much below the real height. Already available STS data on [5]phenacene30 and [7]phenacene (see Figure 4), as discussed above, provide an indication and demonstrate the crucial influence of barely visible electronic states resulting in a change in the apparent height of 0.8 Å. The possibility that Phase 2 might indeed be a second layer on top can therefore not be neglected. Experimental data unambiguously support the formation of a new phase from two unequally tilted molecules and served as input parameters for DFT calculations of a semi-infinite molecular slab. Such a choice of input parameters equally imposes a qualitative agreement in the results of such calculations independent of the actual choice of substrate, whether a metallic substrate or a molecular wetting layer, and is not sufficient to qualitatively differentiate between the two. The presence of a tilted molecule requires an interaction with its stabilizing, immediate neighbor, and correspondingly, Phase 2 consists of pairs. A Phase 2 island can be extended by attached, single, flat-lying molecules, resulting in three possible island terminations (see Figure 9 below). In the case of the previously suggested phase change model, the attached molecule is already a molecule of Phase 1 and implies that a variation of the termination site at the boundary between Phase 1 and Phase 2 is not feasible.

Figure 5. STM images of Phase 2 at boundaries and missing-molecule sites with respective cross sections overlaid. (a) The ends of rows preferentially terminate with a pair, but deviations can be found. (b−d) The termination sites can be either the lower or upper molecule of a pair. (a) [7]phenacene (+1 V, 12 pA), (b) [7]phenacene (+1 V, 13 pA), (c) [7]phenacene (+2 V, 12 pA), (d) [9]phenacene (−1.5 V, 19 pA). (Each image is 6 × 6 nm2).

and 2). Most often, rows are terminated at the boundary by a molecular pair. At lower frequency, termination with a single molecule can be observed (indicated by 2′). Figure 5b−d depicts examples of regularly observed missing-molecule defects that are also present for [5]phenacene. Again, termination of Phase 2 varies in accordance with the observation at the step edges. The respective cross sections show a step height of ∼2.5 Å.46 These observations cannot be intuitively understood by the previously proposed phase change model but suggest a deeper investigation into isolated molecules adsorbed on top. The adsorption of [5]phenacene on different surfaces results in nearly perfect well-organized networks of straight rows in both phases, in general without the presence of molecules adsorbed on top. (Note that, although not further discussed, a small number of such molecules is present on Cu(100); see Figure 2 of ref 35.) The adsorption of [7]phenacene and [9]phenacene is less perfect, as structural defects (i.e., misalignments) are present, and rows are wiggling (see Figure 3a,b) in both phases. In Figure 6a, a large surface area of Phase 1 is presented, with enlarged views depicted in panels b−e of Figure 6. The overview image in Figure 6a shows three large terraces with a domain boundary of differently oriented rows crossing from left to right. The rows are not straight and equally spaced but, rather, are interrupted by misaligned molecules. Although a few non-phenacene adsorbates [see, for example, the solid circle (●) in Figure 6d] can be identified as nucleation sites for irregular growth, the vast majority of misalignments cannot be traced back to such defects. In asgrown surfaces, we found molecules rotated in the plane relative to neighboring molecules, as well as molecules shifted partially out of the rows. Although the origin of this misalignment was not further investigated, we suspect that, because of the increased size of molecules, the thermally induced mobility of the molecules during sample preparation is not sufficient to establish a fully ordered layer. In contrast to

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STEP EDGES AND MOLECULAR MOBILITY Figure 5a shows an image of the boundary between Phase 1 and Phase 2. In Phase 2, the molecules appear to be alternating in apparent height (indicated in the figure by the numbers 1 11393


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Figure 6. (a) Surface areas of four terraces separated by three steps and a domain boundary crossing from left to right (90 × 90 nm2) studied over a period of 72 h in 489 images at V = 1 V and I = 20 pA. The surface shows an organized network of flat-lying [7]phenacene molecules in Phase 1 with many structural defects as a result of misalignments (⊥), domain walls (∼), or adsorbates (●) as demonstrated in (b−e) the enlarged views. On top of Phase 1, isolated and paired (=) molecules are present, as well as out-of-plane rotated molecules, which terminate with either the short (···) or long (····) side.

Figure 7. (a) Within well-organized areas of Phase 1, molecules did not change their positions during the entire measurement time. At structural defects, molecules changed their positions within the increased available space. (b) Multiple translations of the same molecule on top of an unaltered Phase 1. (c−e) Out-of-plane rotating molecules with reduced in-plane space, inducing (c) flipping of neighboring molecules, (d) flipping back of neighboring molecules, or (e) fluctuations between two metastable states. (f) Two paired molecules with unequal apparent heights moving laterally with respect to two neighboring marker molecules (●) and rotating within the plane, ending in paired molecules of equal height. (Time sequence from bottom to top; each image recorded at V = 1 V, I = 20 pA).

[5]phenacene, we also found a significant amount of molecules that were not sitting in full contact with the metallic substrate and were also not yet arranged into Phase 2. Apart from in-plane rotated molecules (⊥) and corresponding holes appearing in the molecular networks, molecules (some examples are depicted in Figure 6) appeared in the STM images to be vertically elevated away from the surface, and therefore, they appeared as protrusions on top of the surface. A large variety of different appearances were observed that had in common that their lengths coincided with the lengths of the phenacenes. Therefore, we suspect that these objects are intact molecules sitting partially or fully on top of Phase 1. We found (a) molecules that appeared to be very narrow, often, depending on the tip, with three (···) or four (····) well-defined

protrusion. We suspect that these molecules were rotated out of the plane and that we had essentially probed the three/four benzene rings, depending on the upward-pointing side. We also found (b) isolated molecules with widths similar to those of molecules in Phase 1 and (c) molecular pairs of approximately twice the size of the single molecules. All of these molecules had heights between 1.2 and 1.7 Å, which is lower than the apparent height of the Phase 2 islands. To verify and more precisely identify the adsorption geometries of the molecules, we studied these surfaces continuously over large periods of time. The discussed data covered a range of 72 h and focused on examples representative of the vast majority of observable processes. Although the experiments were performed at 77 K, these on-top-adsorbed 11394


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Figure 8. (a) Cross-sectional profile starting from a Phase 2 island of ∼2.5-Å height onto Phase 1 with a pair of molecules and two isolated molecules of ∼1.5-Å height. (b,c) Bias-dependent STM images of the same area (arrows indicate structural defects for orientation) with a pronounced topographical contrast at elevated energies. (Each image 16.4 × 16.4 nm2, 10 pA.)

molecules were mobile at low frequency. In Figure 7a, the indicated area of Phase 1 remained stable, whereas within areas with structural imperfections, the molecules were able to laterally translate within the extended available free space (arrows) but not beyond. A significant number of molecules were also adsorbed on top of Phase 1 and translated over the surface without modifying the layer underneath. In the time sequence presented in Figure 7b, a single molecule can be seen to migrate repeatedly.47 The surface configuration was unaltered before and after this motion. Moreover, no change in the neighboring configuration could be observed when the mobile molecule was on top of Phase 1. This suggests that structural changes within Phase 1 are not involved in the migration process. The situation was different for those molecules that appeared protruded and structurally narrower, which we attribute to molecules tilted out of the plane. These molecules occupies an in-plane are smaller than those in Phase 1 with a planar configuration. Three examples of their motion are presented in Figure 7c−e. These molecules did not translate within the plane. When the molecules rotated back into the plane (see Figure 7c), they again occupied their regular area. As a result, they reduced the space available for neighboring molecules, which were forced out of the plane. In Figure 7d, a molecule can be seen to rotate back into the plane, which, considering the available in-plane space, is energetically unfavorable. Consequently, such a flipping process was rarely observed: The environment responded by a local rearrangement, as seen by the molecule indicated by an open circle (○), and the rotating molecule (●) appeared to be structurally disturbed and immediately flipped back. The image sequence presented in Figure 7e also indicates that the out-of-plane rotation was incomplete, that is, not 90°, with the molecule flipping between rightward and leftward tilting. The tilted molecule sits between two molecules that are separated by 0.95 nm instead of 0.75 nm. Therefore, the topographical maximum is laterally displaced by 2.4 Å. Assuming a rotation along the molecular middle axis and considering the extension of the carbon skeleton, a minimum rotation angle of at least 45° can be estimated; a lower-lying rotational axis as well as the extension of the electron cloud of the hydrogen atoms would further increase the tilting angle but not reduce it.

Molecular pairs were found to be significantly less mobile and to rarely decompose into two separate molecules. As shown in Figure 7f, a pair was first in a configuration with one molecule (○) topographically elevated. After a lateral shift, visible in its position relative to two fixed marker molecules (●) in Phase 1, the molecular pair was rotated (indicated by the solid and dotted lines), and both molecules were at identical heights. The observation of molecular mobility demonstrates that tilted molecules, as suggested by the phase change model, can exist in parallel with coadsorbed, planar molecules on top. A cross section (Figure 8a) through a surface with Phase 2 on top of Phase 1 and additional pairs and isolated molecules present, that is, with tip and voltage artifacts excluded, demonstrates that the apparent height remained significantly below that of Phase 2 islands and questions the validity of the apparent height analysis. A final indication of adsorption on top of a weakly ordered wetting layer (i.e., Phase 1) is given by voltagedependent topographs. STM probes electron densities within the energy interval as set by the bias voltage. Therefore, STM images reflect the structural periodicity as experienced by the involved electron states. In Figure 8b, the same area is imaged at low and high voltages. Low and intermediate voltages are preferentially chosen in molecule experiments to avoid tipinduced modifications. Here, Phase 2 was found to be highly stable even when elevated voltages were applied between the tip and the sample, and modifications of the investigated structures were not detected in corresponding control measurements. At low and intermediate voltages up to +3 V, a site-dependent, interpair topographical contrast was barely discernible. This can tentatively be attributed to a well-ordered layer underneath14 or even adsorption directly on the flat surface. However, at elevated voltages (+3.5 V), a very pronounced topographical contrast appeared. This suggests that low-energy states were predominantly localized within the plane whereas high-energy states, which tend to be less localized, carried additional information on the periodicity of the underneath layer. Therefore, we conclude that the layer underneath was not periodically well-ordered, thereby supporting the conclusion of Phase 2 grown on top of a Phase 1 wetting layer. 11395


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Figure 9. On top of a wetting layer, an incommensurate Phase 2 layer grows with three distinctly different possible terminations (1−3). A phase change with tilted molecules, as they appear in misaligned structures (5), within Phase 1 can be excluded. Tilted molecules and isolated, on-topadsorbed molecules (6) have apparent heights less than the height of the Phase 2 islands. In contrast, tilted molecules (4) of Phase 2 have an enhanced apparent height. Free-standing tilted molecules on top of Phase 1 were not observed and cannot be expected. The right side shows the previous interpretation in terms of a phase change with increasing coverage.

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CONCLUSIONS We studied the growth of [7]phenacene and [9]phenacene on Ag(111) and observed different highly ordered and stable structures with different molecular packing densities. The initial adsorption starts with flat-lying molecules (Phase 1). At increased coverage, a new Phase 2 emerges with paired and partially rotated out-of-plane molecules. The experimental observations are in excellent quantitative agreement with previous studies on [5]phenacene even on different metallic substrates. This implies that all conclusions are valid for all systems investigated so far. Unit cells are along the long direction extended by the additional benzene rings. Along the short direction, stacking of molecules is identical, and correspondingly, we found a pairing with a pronounced voltage dependence in images of Phase 2. Variations at domain boundaries cannot be intuitively understood by previous DFT calculations, which suggest a reorganization of Phase 1 into Phase 2 upon further adsorption. In contrast to [5]phenacene, a significant amount of isolated molecules were found to be adsorbed on top of Phase 1. Although not further addressed, it seems reasonable to attribute such imperfections to a reduced mobility during film preparation as a consequence of an increased molecular size and a correspondingly increased bonding into metastable states. We followed as functions of time the motion of molecules adsorbed in a planar fashion on top of Phase 1 and the rotation of tilted molecules embedded in Phase 1. We found evidence in both cases that the apparent height was even lower than for Phase 2 islands, thereby falsifying the relevance of the apparent height in previous publications as a valid argument. This conclusion was additionally supported by spectroscopic measurements. Therefore, we conclude that the previously assigned Phase 2 is indeed adsorbed on top of a weakly disordered Phase 1, serving as a wetting layer. Figure 9 summarizes the results. At coverages beyond a complete Phase 1 filling network, a new Phase 2 of paired and nonequivalent molecules is formed with three possible and different terminations (1, 2, 3) on top of a wetting layer. Considering the significant structural differences, one can expect different binding energies and, correspondingly, a (nonexclusive) preference of termination, as observed experimentally. An unequal out-of-plane rotation, as in the case of pentacene, can be expected to counter repulsive molecule− molecule interaction through the positive electrostatic field of the hydrogen atoms, resulting in an attractive molecule− molecule interaction between the positive electrostatic potential of the hydrogens and negative potential on the ring carbons. The growth of Phase 2 is incommensurate with the structure of the wetting layer, which leaves traces in images at elevated energies. Tilted molecules within Phase 1 can be observed (5);

however, although their out-of-plane rotation can be expected to be larger than that within Phase 2, their apparent height but not necessarily their real height remains significantly below the height of the Phase 2 islands and contradicts previous interpretations. The same holds for isolated, on-top-adsorbed molecules (6). Considering the recorded discrepancies, the previously introduced coverage-dependent phase change seems improbable.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01806. Spatial variations of the STS data of Phase 2 (Figure S1), exact positions of panels b−e of Figure 6 and panels a−f of Figure 7 within the overview image of Figure 6a (Figure S2), original STM image of the cross section presented in Figure 8a (Figure S3), and cross sections through the paired molecules of Figure 7f before and after a lateral shift (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Germar Hoffmann: 0000-0003-1692-9618 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.-W.C., I.-C.S., and G.H. acknowledge funding through the Ministry of Science and Technology ROC through Grants 1022112-M-007-011-MY3 and 105-2112-M-007-022-MY3 and NTHU Project 106N505CE1. H.O. thanks the Okayama Foundation for Science and Technology for financial support.

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REFERENCES

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The Journal of Physical Chemistry C (45) Olsson, F. E.; Persson, M.; Repp, J.; Meyer, G. Scanning tunneling microscopy and spectroscopy of NaCl overlayers on the stepped Cu(311) surface: Experimental and theoretical study. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 075419. (46) Therefore, the apparent height depends on tip status and voltage. For missing single-molecule sites, the depths of the respective holes are often smaller than the step height, and we attribute this to the finite width of the probing tip. (47) We also note that, during the migration process along the projected path between the starting and ending points of migration, additional noise often appears in the respective images. This indicates that mobile molecules tend to follow the path of and are mobilized by the probing tip, without limitations for the discussion here.

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Adsorption of Phenacenes on a Metallic Substrate: Revisited - The

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