Naphthalene's Six Shades on Graphite: A Detailed Study on the

Article pubs.acs.org/JPCC

Naphthalene’s Six Shades on Graphite: A Detailed Study on the Polymorphism of an Apparently Simple System Falko Sojka,† Matthias Meissner,† Takashi Yamada,‡ Toshiaki Munakata,‡ Roman Forker,† and Torsten Fritz*,† †

Institute of Solid State Physics, Friedrich Schiller University Jena, Helmholtzweg 5, 07743 Jena, Germany Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan

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ABSTRACT: Recently the complex polymorphism of naphthalene, C10H8, on Cu(111) was reported, with the conclusion that the polymorphism and thermodynamic behavior on that substrate are considerably more complex than what was known previously. Likewise, only two different commensurate structures of naphthalene on graphite are known so far. This study now identifies a total of six different highly ordered phases for this apparently simple molecule−substrate system. All these phases are compiled in a detailed phase diagram, together with the information about the physical conditions needed to obtain them. As one remarkable turning point in the phase diagram, it is observed that the effective molecule−molecule interaction changes from repulsive to attractive and vice versa upon temperature variation. Another reversible structural change occurs while the interaction is still attractive and the equilibrium distance of the molecules increases. We further show that not only the coverage and the temperature but also the specific route toward a certain structure is important to grow a desired phase. Based on our detailed structural analysis using distortioncorrected low-energy electron diffraction, we provide the recipes for a controlled growth of C10H8 on graphite. We shed light on an unexpectedly complex phase diagram where in most cases a simple commensurate registry is not the preferred epitaxial relation.

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INTRODUCTION Polycyclic aromatic hydrocarbons (PAH) enjoy an increasing interest due to their outstanding properties like the strong wavelength-selective light absorption and chemical stability. As one of the most intriguing properties, superconductivity was found for some representatives of the PAHs, resulting from doping with alkali metal atoms.1−5 Also for naphthalene (C10H8), the smallest member of the PAHs,6 the existence of Cooper pairs was demonstrated via photoemission.7 While, on the one hand, superconductivity seems to be preferred for bulk-like systems2 and, on the other hand, some theoretical findings favor twodimensional configurations for superconductivity,8 these studies agree that the structure of the material is indeed a key property. On the way toward a deeper understanding of the growth of highly ordered structures several studies about C10H8 adsorbed on metal surfaces9−27 and on graphite28,29 have been published. Therefore, it is known that C10H8 layers crystallize in many different phases where especially on graphite the electronic states depend sensitively on the geometric structure as recently shown by Yamada et al.30 So far, two commensurate phases have been reported for C10H8 on graphite. The transition between the phases was mainly attributed to the coverage when reaching a certain threshold.28,29 By using distortion-corrected low-energy electron diffraction (LEED),31 we reanalyze these systems with much higher precision. Remarkably, we find a total of six different © XXXX American Chemical Society

phases of which only one exhibits a simple commensurate registry. Two of the six phases disappear irretrievably upon a temperature change, while the remaining three phases transform into each other reversibly. Caused by the complexity and the temperature dependence of the system, a comprehensive structural analysis in real space by scanning tunneling microscopy (STM) was not feasible to us. Instead, we clarify the thermal stability of the resulting phases based on extensive LEED measurements and only limited STM measurements. In this study we further show that not only the coverage and the temperature but also the specific route toward a certain structure (regarding both, temperature and coverage) are important to grow a desired phase. Based on our detailed structural analysis including the physical conditions of the growth process, we provide the recipes for a controlled growth of C10H8 on graphite and shed light on an unexpectedly complex phase diagram of this seemingly simple molecule−substrate system.

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EXPERIMENTAL SECTION All experiments except for the STM measurements were carried out in an ultrahigh vacuum (UHV) chamber described in ref 27. Received: July 5, 2016 Revised: August 30, 2016

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DOI: 10.1021/acs.jpcc.6b06702 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

rather complex phase diagram, while all structural details of the phases shall be discussed beforehand in the following subsection. Structural Analysis. The results of the structural analysis with error margins are given in Table 1. Note that the corresponding epitaxy matrix elements in the text are rounded for the sake of clarity. Previously Known Phases. The first phase in Table 1 is well known from previous studies.28,29 This hexagonal structure (COM) has a commensurate registry with the substrate and can be described by the epitaxial matrix ECOM = −42 22 (cf. Figure 2b,c). Also the second structure listed in Table 1 was already found by Bardi et al., who, however, observed only first-order diffraction features. This likewise hexagonal structure, albeit with a higher order commensurate (HOC) registry with the substrate, 0.5 can be described by EHOC = −3.5 0.5 3 according to a suggestion of Bardi et al. Owing to the limited experimental sensitivity at the time, Bardi et al. alternatively proposed a rectangular structure with two molecules per unit cell (cf. Figure 2i, gray rectangle) to account for their LEED pattern.28 However, for such a rectangular structure one would expect additional reflexes (cf. Figure 2h, white circles), which are clearly absent in our data. This statement is corroborated by the fact that our LEED patterns also display higher-order features as opposed to those reported previously.28 Therefore, we attribute this structure to a hexagonal unit cell as described above, in accordance with the first suggestion of Bardi et al. The Four New Phases. In addition to these two known phases, we found no less than four additional phases that are described in the following. Almost as dense as HOC but with oblique lattices instead of hexagonal ones, two phases with a point-on-line coincidence33 were observed as well (cf. Figure 2d,e). The first of 0 them is described by the epitaxial relation EPoL1 = −3.69 0.35 3 and will be called PoL1 in the following. The second, called PoL2, 0.31 − 3 . Despite their has the epitaxial relation EPoL2 = 3.21 4 apparent similarity (cf. Figure 2b) they are not mirror domains to each other as one might imagine at first glance. Therefore, they lead to different diffraction patterns and have to be described as two separate phases. A third point-on-line structure (PoL3) that occurs at low temperatures and higher coverage is characterized 0.14 (cf. Figure 2f). Interestingly, the unit by EPoL3 = 43 −5.13 cell area of PoL3 is about 45% larger than those of PoL1 and PoL2, which has a certain impact on the molecular density: In the case of one molecule per unit cell the molecular density would be unreasonably small compared to the structures (PoL1 and PoL2), which at this stage of the deposition have already formed on the surface (cf. Figure 2j). From the low temperatures and higher coverage one would expect a higher molecular density, not a smaller one. Therefore, we assume that there are not less than two molecules per unit cell, maybe even more (in fact for the values given in Table 1 two molecules per unit cell are assumed). As the unit cell area is only larger by 45% but the occupancy of the unit cell has doubled, one can see that PoL3 is more densely packed than the other PoL structures. Last but not least we found a very densely packed rectangular structure (REC) with an apparently incommensurate relation (see Table 1) to the substrate. Here we emphasize that a meaningfully large supercell (containing a few tens of molecules at most) does not lead to a description that justifies a coincidence (point-on-line, line-on-line, or higher order commensurability)

There, the purification and deposition procedures of naphthalene as well as the sample temperature calibration have also been described in detail. Here, we used graphite single crystals (GSC), a few millimeters in diameter, purchased from Naturally Graphite, Michigan Technological University. Individual GSC specimens were glued to sample holders using PELCO High Temperature Carbon Paste (purchased from Plano) and then cleaved in air. After the transfer to UHV, the samples were degassed for several hours at more than 800 K in order to achieve a clean surface prior to any experiment. LEED patterns were acquired with a microchannel plate (MCP) LEED from OCI Vacuum Engineering. Due to the high amplification of the diffracted electrons the MCP-LEED is operated at much lower primary electron currents (reducing beam damage drastically) while being more sensitive even to rather weak spots than standard rear-view LEED devices without MCP amplification. For an enhanced quantitative analysis of the lattice parameters we numerically fit diffraction pattern simulations to the LEED images (using LEEDLab 2015) after correcting them for geometrical distortions.31 Thus, the determination of the lattice parameters is much more accurate than what is achievable for manually analyzed LEED images or STM images as used in previous studies.28,29 Two data sets of LEED images are evaluated. They differ in terms of the deposition rate, which, however, is constant for each data set. Therefore, series of LEED images from each data set, taken under comparable conditions, can be employed to relate both data sets in terms of the amount of deposited material. For that we use a well observable and abrupt phase transition (between a commensurate and a higher order commensurate phase, called COM−HOC transition) caused by an increasing coverage (detailed explanation below). We also use this transition for the definition of one monolayerequivalent (MLE, details are given in the next section), which is a value for the amount of molecules deposited on the surface. Figure 1 shows this transition observed in both data sets. The

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Figure 1. Matching of the two data sets and the definition of 1 MLE employed in this study. LEED images of five different deposited amounts are shown for the two separate data sets considered in this study, which differ in the deposition rate. Upper images belong to the data set I (T = 163 K, Ebeam = 37.9 eV). Lower images belong to the data set II (T = 136 K, Ebeam = 35.1 eV). (a) Disc pattern, (b) disc plus ring pattern, (c) COM, (d) COM−HOC-transition and (e) HOC.

additional STM measurements were carried out in another ultrahigh vacuum (UHV) chamber on highly oriented pyrolytic graphite (HOPG) as described in ref 29.

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RESULTS AND DISCUSSION In this study naphthalene on graphite was found to appear in six different phases (cf. Figure 2), depending on the experimental conditions. We will illustrate this in detail below by means of the B

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Figure 2. Representation of the six observed phases of C10H8 on graphite. (a,c−h,j) LEED images partially superimposed with simulated LEED patterns; primary electron energies: (a) 64.0 eV, (c) 50.9 eV, (d) 60.2 eV, (e) 64.2 eV, (f) 37.9 eV, (g) 76.3 eV, (h) 51.6 eV, and (j) 61.0 eV. White circles in (h) mark the positions at which spots would be expected if HOC were describable by the rectangular structure according to the alternative suggestion of Bardi et al.28 (b,i) Real space lattices; black dots belong to the graphite crystal structure. (i) The gray unit cell represents the rectangular structure according to the alternative suggestion of Bardi et al.

with the substrate. Since the first order spots of the corresponding diffraction pattern appear very weak throughout the observed energy range (Figure 2g) we hypothesize that there are two molecules per unit cell in a mutual alignment similar to a glide plane symmetry so that systematic intensity attenuation can occur.34,35 Since the size of the unit cell (0.41 nm2) is too small to contain two flat-lying molecules that would cover an area of at least 2 × 0.33 nm2 = 0.66 nm2,36 we conclude that the molecules must be both significantly tilted, due to the symmetry considered above (otherwise, one could assume one planar and one tilted molecule). It is noteworthy that the two-dimensional surface unit cell in the (001)-plane of a C10H8 crystal is also rectangular with two molecules and comparable lattice constants (cf. Table 1).32 Our proposition for REC is that the molecules are standing upright with the short edges parallel to the substrate, well comparable to the orientation in the (001)-plane of the bulk crystal, so that this phase appears to be a bulk-like structure. Another peculiarity of this structure is that the spots of the respective diffraction pattern are slightly azimuthally elongated. This means that this structure has only a preferred but not

entirely fixed mutual alignment with respect to the substrate. Because the mutual alignment of adsorbate and substrate is mainly defined by the lateral corrugation of the molecule− substrate interaction one can conclude that this interaction plays only a minor role in the formation of REC, which fits well to the assumption of a bulk-like structure. Further investigations show that after REC was formed (at around 160 K) it is very temperature-stable. It does not change upon cooling to T = 80 K and heating up to T = 220 K. At temperatures T > 220 K REC vanishes very fast. Consequently, one should be able to observe the REC unscathed despite the cooling process, which is necessary for the STM measurement. Figure 3a shows an occupied state image of REC where both the two molecules per unit cell (superimposed, green) and the similarity to a glide plane symmetry are well observable. Figure 3b has the same STMcontrast as Figure 1d in ref 30. From the single depression (marked by a dashed black circle) we conclude that one protrusion corresponds to one molecule (in ref 29 and 30 it was assumed that two protrusions correspond to one molecule aligned parallel to the substrate surface). In the light of our new C


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The Journal of Physical Chemistry C Table 1. Comparison of the Six Observed Phases of C10H8 on Graphite and Two Selected C10H8 Bulk Crystal Planes ρc (nm−2)

epitaxial matrix

lattice vectors (Å)

Γa (deg)

ϕb (deg)

COM

⎛ 4.00(1) 2.00(1)⎞ ⎜ ⎟ ⎝− 2.00(1) 2.00(1)⎠

|a⃗1|: 8.52(5) |a⃗2|: 8.52(5)

120.0(1)

30.0(1)

1.59

1

HOC

⎛ 3.50(2) 0.50(3)⎞ ⎜ ⎟ ⎝− 0.50(3) 3.01(2)⎠

|a⃗1|: 8.06(9) |a⃗2|: 8.10(8)

120.0(5)

7.6(5)

1.77

1

PoL1

⎛ 3.69(1) 0.00(1)⎞ ⎜ ⎟ ⎝− 0.35(1) 2.99(1)⎠

|a⃗1|: 9.07(6) |a⃗2|: 7.82(4)

125.4(1)

0.0(1)

1.73

1

PoL2

⎛ 0.31(1) − 3.00(1)⎞ ⎜ ⎟ ⎝ 3.21(1) 4.00(1) ⎠

|a⃗1|: 7.79(5) |a⃗2|: 9.04(5)

125.9(2)

−55.1(1)

3.51

bilayer

PoL3

⎛ 3.00(2) − 0.14(2)⎞ ⎜ ⎟ ⎝ 4.01(3) 5.13(3) ⎠

|a⃗1|: 7.56(9) |a⃗2|: 11.5(2)

74.3(5)

−2.3(3)

2.39

2

REC

⎛ 2.64(1) 0.83(1)⎞ ⎜ ⎟ ⎝ 0.71(1) 3.22(1)⎠

|a⃗1|: 5.75(4) |a⃗2|: 7.21(5)

90.0(2)

18.0(1)

4.83

2

|a⃗|: 8.1451 |c⃗|: 8.6649 |a⃗|: 8.1451 |b⃗|: 5.9499

124.189

3.43

bilayer

90.00

4.17

2

phase

(010)-planee (001)-planee

Nd

a Angle between the two lattice vectors. bAngle between the first lattice vectors of adsorbate and substrate, respectively. cMolecular density considering the assumed number N of molecules in the unit cell. dNumber of molecules per unit cell. eBulk crystal structure data from ref 32 at T = 150 K. Uncertainties of the last significant digit are given in parentheses.

represented in Figure 4. At first, we focus on the isothermal phase transitions, which take place due to varying coverage at constant temperature. Because the coverage depends generally not only on the amount of evaporated material but also on the sticking coefficient, which may or may not change with temperature, we cannot simply determine the coverage of the substrate surface. Therefore, caution has to be taken that the given quantitative values in the following refer to the deposited amount rather than to the real coverage. Repulsive Behavior. C10H8 adsorbs on cold graphite substrates at temperatures below 220 K. However, highly ordered structures are only observed for temperatures up to 175 K. When depositing the molecules at 125 K ≤ T ≤ 175 K (Figure 4a, light red shaded area) they act like they repel each other. The diffraction patterns show a transition (arrow ①) from an initially disordered molecular distribution characterized by a minimum distance (disc-shaped LEED pattern) to a denser condensate (ring-shaped LEED pattern) and then further toward ordered structures (Figure 1a−c). Finally it reaches the previously described COM.28,29 Since this seemingly repulsive behavior ceases to be dominant in such a way that the molecules condense into small domains when initially cooling the sample to T ≤ 125 K (arrow ③, described in detail later) it is obviously a thermally induced effect and therefore most likely induced by thermal motion. That means in this case (125 K ≤ T ≤ 175 K), a high thermal motion of the molecules overcomes the attractive van der Waals forces (between the molecules), which leads to separation and prevents the formation of small domains. Such a decreasing separation of the molecules (for T ≥ 125 K in this study) during deposition was also observed for other molecules by means of STM measurements in real-space by Bischoff et al.37 For the system C10H8 on Cu(111), Forker et al. showed that a similar separation takes place, but already at even lower temperatures.27 While the observed result is qualitatively the same, apparently there is an additional effect that causes a different behavior on the copper substrate compared to graphite. The separation of the molecules on Cu(111) even occurs at temperatures lower than 90 K in contrast to the behavior on graphite. We assume that a screening of the attractive London

Figure 3. STM measurements of naphthalene adsorbed on HOPG. C10H8 was deposited at T = 150 K. After REC had formed the sample was cooled to T = 80 K in the STM. (a) 7 nm × 7 nm, VS = −3.6 V, It = 120 pA. Thin black lines indicate proposed azimuthal orientations of the two molecules in the unit cell, supposedly standing upright. (b) 5.4 nm × 5.4 nm, VS = −3.4 V, It = 200 pA. The contrast in this image does not allow for an estimation of the molecules’ azimuthal orientation. This might be either a result of the different tunneling parameters or of a different tip condition as compared to (a). Nevertheless, a single depression is visible (highlighted by the dashed circle).

findings we suppose that the phase described by the matrix E = 23 −31 in ref 29 is more likely the REC found here. We note that the contrast obtained in STM changes considerably even for minor variations of the scanning parameters, as can be inferred from a comparison of both panels in Figure 3. Thus, it is difficult to identify the exact positions and azimuthal orientations of the presumably upright standing molecules, which hampers a more precise clarification of a possible glide-plane symmetry in this structure. Isothermal Phase Transitions. Now that the different phases have been identified in terms of their structural characteristics, we want to describe in more detail under which conditions they appear or disappear. The occurrence of all observed phases and transitions for naphthalene on graphite are

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two data sets, which were obtained with different deposition rates (cf. Figure 1). Note that each data set itself (several LEED series, cf. Figure 4b) was obtained with the very same deposition rate. Since HOC is the densest structure with almost flat-lying molecules (cf. Table 1), we define one monolayer-equivalent (MLE) as the amount of C10H8 needed to obtain a LEED pattern where the rearrangement of COM to HOC is entirely completed, i.e., shortly after COM is no longer observable and only HOC exists (cf. Figure 1e). The highest temperature where the COM−HOC transition was observed is around 170 K. At this temperature not only HOC grows, but also REC appears almost simultaneously. Likewise, further deposition at temperatures above 150 K starting from a surface completely covered with HOC (arrow ②) drives the formation of REC. Upon a deposited amount of about 3 MLE only REC can be seen. The first possible scenario is a phase transition from HOC to REC. A second possibility would be that nominally 2 MLE REC cover 1 MLE HOC. However, in this case one-third of the HOC monolayer should be left uncovered since REC is much denser than HOC, which contradicts the fact that HOC is not observed in LEED at 3 MLE. Therefore, we conclude that a HOC−REC transition takes place between 1 MLE and 3 MLE upon deposition. Attractive Behavior. During deposition of C10H8 at temperatures T ≤ 125 K (cf. Figure 4a, orange shaded area) an effectively attractive behavior between the molecules can be inferred from our results as mentioned previously. Even at low coverages LEED patterns with sharp spots occur; therefore, we conclude that the molecules condense into small domains as 2D single-layer islands. Besides that, one can make several observations at two different selected temperatures. At 110 K (arrow ③), initially only COM can be observed until the deposition reaches an amount of 0.7 MLE. Only then PoL1 starts growing and after reaching 0.85 MLE also PoL2 arises (cf. Figure 2a). This means that the thermal energy is low enough for the intermolecular forces to be attractive while also being high enough to enlarge the equilibrium distance between the molecules leading to the preferential growth of COM instead of the denser PoL structures at low coverages. Thus, it seems that a full surface coverage (marked by the beginning of the growth of PoL1) is reached before an amount of 1 MLE was deposited (compare definition of MLE in the section before). A possible explanation for that would be an increase in the sticking coefficient due to the lower temperatures. The initial behavior of the system while depositing at 90 K (arrow ④) is analogous to that at 110 K but with a smaller equilibrium distance between the molecules due to the lower thermal energy. This is deduced from the fact that immediately PoL1, with a smaller unit cell area than COM, starts to grow in the absence of COM. Again, while the available space for the molecules decreases, they are forced toward a closer intermolecular distance leading to the growth of the much denser PoL3. Simultaneously PoL2 starts growing (cf. Figure 2j). Since PoL2 has almost the same unit cell area as PoL1 but always grows after PoL1 already covers the surface, we come to the conclusion that PoL2 must be a bilayer. This bilayer presumably grows according to the role model of the (010)-plane of a C10H8 bulk crystal32 (cf. Table 1) where the molecules stand with the long edges parallel to the surface. The formation of PoL3 only takes place when depositing at T ≤ 110 K and cannot be induced by cooling after deposition. Having deposited 4 MLE at 90 K only PoL3 is observable. Warming up PoL3 at 4.6 MLE (arrow ⑤) leads to a disorder (disc-like LEED pattern) and not to a

Figure 4. Phase diagram of C10H8 on graphite in two different representations. (a) Analyzed LEED data sets (each data point represents one image) with different symbols used to discriminate between the various phases observed. The shaded background areas mark differences in the effective molecule−molecule interaction (detailed explanation in the text). The numbered arrows are guides to the eye in order to support the discussion in the text. (b) Interpretation of the coverage- and temperature-dependent occurrence of the different phases. The same data points as those in (a) are plotted with a different color code to distinguish between the two data sets. REC and PoL3 are only observed when grown in the indicated temperature regimes.

forces by the presence of the metal substrate, which does not occur on graphite, could be the responsible effect.38 After reaching COM, further deposition leads to HOC where the transition from COM to HOC is abrupt and well observable (Figure 1c−e). HOC is packed more densely than COM, which is in good agreement with the fact that it appears for higher amounts of deposited material. Considering that the molecules effectively avoid each other, in the temperature range 125 K ≤ T ≤ 175 K, the long-range order is a result of the stronger compression upon further deposition. Consequently, the densely packed HOC can only be formed when the entire surface is covered by flat-lying molecules, independent of the substrate temperature. In turn, one can say that the exclusive observation of HOC requires always the same amount of molecules. Therefore, we used the COM−HOC transition to relate the E


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The Journal of Physical Chemistry C transition into REC as one might expect. Therefore, also for REC it seems to be a rule that the deposition must happen at 150 K ≤ T ≤ 175 K to stimulate the formation. It is not stimulated by a temperature change after the deposition. Transitions Due to Temperature Changes. The aforementioned change in the equilibrium molecular distance (Figure 4a, bright and dark orange shaded area) can be directly observed by warming up an island-like submonolayer from 90 to 105 K (arrow ⑥). The previously homogeneous PoL1 transforms into COM until COM occurs exclusively. This transformation caused by an increased equilibrium distance starts at a temperature of T = 98 K, which is also the highest temperature where PoL3 was observed. A further noteworthy turning point in the phase diagram is marked by the HOC−PoL transition (arrow ⑦). When cooling down HOC to temperatures below 125 K the LEED pattern of this phase becomes weaker, while PoL1 and PoL2 appear. Inversely, when warming up these PoL-structures (arrow ⑧) they vanish and HOC arises again. Since we know that between 136 K (LEED series referring to arrow ①) and 110 K (LEED series referring to arrow ③) the behavior of the molecules changes from effectively repulsive, due to the high thermal motion (Figure 4a, light red shaded area), to effectively attractive (Figure 4a, orange shaded area) and vice versa, we conclude from the HOC−PoL transition that this change happens at around 125 K. A full conversion of HOC into PoL1 and PoL2 was observed. In the case of cooling REC such a full conversion remains unobserved as only PoL1 arises at around 100 K but also the diffraction pattern of REC becomes sharper. We think that here just small disordered areas have rearranged, and the result was PoL1 while REC remains essentially unaffected. This means that both the formation of the REC at 150 K ≤ T ≤ 175 K and the decay of the PoL3 by warming up the sample to T ≥ 98 K are irreversible. At this point we want to emphasize that another rectangular 0.25 ′ = 1.75 structure EHOC 1.25 3.25 (note that this is not the alternative suggestion of Bardi et al. [gray rectangle in Figure 2i]) would lead exactly to the same diffraction pattern as EHOC when rotational and mirror domains are included. This hypothetical structure would have twice the molecular density (3.5 nm−2) than the hexagonal phase (1.77 nm−2). However, since HOC can be transformed reversibly into other phases (PoL1 and PoL2) with a density fairly equal to the hexagonal description (corresponding to EHOC) and not the rectangular description (corresponding to EHOC ′ ) by cooling, and due to the fact that a decreasing density upon cooling is rather unlikely, we assume that the epitaxial relation described by EHOC is the correct one. The occurrence of HOC can be rationalized by considering that the adsorption lattice points fall either on a top site or on a bridge site, but never on a hollow site. Therefore, a net energy gain is likely.

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In conclusion, we note that the growth of this smallest PAH on graphite depends on the physical conditions like coverage and temperature in a remarkably complex manner where in many cases a simple commensurate registry is not the preferred epitaxial relation. Instead, to obtain a full picture of all occurring phases, epitaxial coincidences such as point-on-line registries have to be considered as well. Remarkably, the REC, once formed, is thermodynamically very stable to the extent that it only disappears from the graphite surface when the desorption temperature (∼220 K) is reached. This is noteworthy because the REC does not readily fall into any category of lattice epitaxy. The adsorption energy required for its particular stability must therefore originate from other mechanisms than merely lock into registry. In fact, we hypothesize that static distortion waves might provide the stabilizing energy for this phase of naphthalene on graphite,39,40 but the expected small local displacements of this effect could not be observed in our STM data at 80 K.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 3641 947400. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support by the Deutsche Forschungsgemeinschaft grants No. FR 875/9 and FO 770/21. This collaborative research was funded in part through the PAJAKO Project No. 56264880 by the DAAD.

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REFERENCES

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CONCLUSIONS We identified six different highly ordered phases of naphthalene on graphite, which are compiled in a detailed phase diagram. The two most important turning points in the diagram are observed while changing the temperature. The first is located at 125 K above which the thermal motion of the molecules becomes high enough to separate the molecules from each other preventing the molecules from condensing into small domains. The second reversible structural change occurs at 98 K during heating. Here the molecule−molecule interaction is still attractive, but the equilibrium distance increases. F


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