Article pubs.acs.org/Langmuir
The Complex Polymorphism and Thermodynamic Behavior of a Seemingly Simple System: Naphthalene on Cu(111) Roman Forker,*,† Julia Peuker,† Matthias Meissner,† Falko Sojka,† Takahiro Ueba,‡ Takashi Yamada,‡ Hiroyuki S. Kato,‡ Toshiaki Munakata,‡ and Torsten Fritz† †
Institute of Solid State Physics, Friedrich Schiller University Jena, Helmholtzweg 5, 07743 Jena, Germany Department of Chemistry, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
‡
S Supporting Information *
ABSTRACT: Naphthalene, C10H8, is a polycyclic aromatic hydrocarbon (PAH) consisting of two fused benzene rings. From previous studies, it is known to form three different commensurate structures in thin epitaxial films on Cu(111), depending on the preparation conditions. One of these structures even exhibits a chiral motif of molecular rotations within the unit cell. In an attempt to elucidate this polymorphism, we performed in situ low-energy electron diffraction (LEED) as a function of temperature and surface coverage, revealing an unexpected and extraordinarily complex structural and thermodynamic behavior. We present experimental evidence for a phase transition from a two-dimensional gas to a highly ordered molecular solid via an intermediate metastable phase with moderate order (extending over a few lattice constants only) which undergoes a reversible orientational shift upon temperature variation. At monolayer coverage and above, we find that two different point-on-line (POL) coincident epitaxial relations constitute the dominant structures. This is remarkable because, so far, POL structures of naphthalene on Cu(111) and other substrates have either not been recognized or not obtained under the respective experimental conditions. Our results are corroborated by the analysis of characteristic moiré patterns observed in scanning tunneling microscopy (STM), indicative of a noncommensurate epitaxial registry.
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INTRODUCTION Naphthalene (for structural formula, see Figure 1) is a member of the [n]acenes and therefore a polycyclic aromatic hydro-
states) of a semiconductor or a metal substrate is in the focus of such investigations and represents a general, vital topic in surface science. In this respect, naphthalene on (111)-oriented single crystals of face-centered cubic (fcc) metals is often regarded as a model system for studies of interface effects in particular and of the structure−property relations in general. In the past, naphthalene structures have been extensively examined by means of surface sensitive techniques, such as lowenergy electron diffraction (LEED), scanning tunneling microscopy (STM), two-photon photoemission (2PPE) spectroscopy, temperature-programmed desorption (TPD), and many others.13−31 For the unreconstructed fcc metal substrates Cu(111), Pt(111), and Rh(111), STM measurements revealed that single C10H8 adsorbates align with their long molecular axis C2 parallel to one of the primitive surface lattice directions.13,20,26 For Rh(111) and Pt(111), the C10H8 unit meshes are describable by a single (3 × 3) commensurate registry. In these cases the distribution of the three observed molecular orientations, 120° apart, lacks a strict long-range order even though sharp LEED spots are discernible.17,20,26 Cu(111) is a noteworthy substrate, since three distinct commensurate naphthalene adlayer structures were found
Figure 1. Skeletal formula of naphthalene, C10H8, mmolar = 128.17 g mol−1. Chemical Abstracts Service (CAS) registry number: 91-20-3.
carbon (PAH).1 The existence of Cooper pairs in some PAHs including C10H8 was verified in a recent photoemission study.2 Several other representatives of this material class have indeed been found to become superconducting upon doping (intercalation) with metal atoms.3−8 While superconductivity of doped PAHs has been proposed to be a bulk property,3 theoretical suggestions exist that favor two-dimensional (2D) structures over 3D ones for the electronic mechanisms of this phenomenon, cf. ref 6 and references therein. Therefore, PAH thin films bear a fundamental significance, and the essential question arises how 2D and 3D systems compare in terms of structure and properties.9,10 Furthermore, the electronic characteristics of pristine naphthalene thin films are of special interest.11−13 The interaction of the molecular orbitals with the surface electronic states (i.e., Shockley states or image potential © 2014 American Chemical Society
Received: August 6, 2014 Revised: October 2, 2014 Published: October 31, 2014 14163
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sample preparation. It can be cooled with liquid nitrogen (LN2) or heated via a tungsten filament and electron bombardment. A microchannel plate LEED (MCP-LEED, model BDL600IR from OCI Vacuum Microengineering) with a tungsten−rhenium filament was used for substrate characterization and also during film growth. We operated the MCP at high voltages between 0.9 and 1.0 kV, which is close to its highest possible amplification factor, thereby reducing the primary electron current as far as possible. None of the structures observed showed noticeable signs of degradation even during LEED experiments that lasted several hours. The LEED patterns were recorded with a charge-coupled device (CCD) camera (WAT-902H2 ultimate from Watec) firmly positioned in front of the viewing window flange, ensuring stable and reproducible measurement conditions. In order to improve the signal-to-noise ratio, a series of usually 50 consecutive LEED images, recorded under the same conditions typically within 10 s, was averaged using the software ImageJ.35 Due to unavoidable distortions occurring in LEED measurements, corrections were performed based on the well-known Si(111)-(7 × 7) surface reconstruction.36 Moreover, an energy offset of Eoff = (1.1 ± 0.2) eV of this particular LEED device was determined from the scaling of the reference spot pattern with the primary energy.36 As the commensurate structure A exhibits even more reflexes than the Si(111)-(7 × 7), especially at low primary electron energies, it was used as a reference itself for an improved distortion correction of further LEED images. For this purpose, the lattice parameter of copper (at T = 298.15 K: αbulk = 3.6146 Å, corresponding to |s⃗1| = |s⃗2| = 2.5559 Å for the (111)-surface, ACu(111) = 5.6575 Å2) and its temperaturedependency have to be taken into account; cf. ref 37 and references therein. LEED images of tilted samples were also taken with the intention to access higher-order spots at low energy. The additional distortions associated with the tilting angle ϑ were corrected as introduced recently.38 LEED patterns were analyzed using the commercially available software LEEDLab.39 All LEED images shown were contrast inverted and, when specified, also contrast enhanced in order to improve the visibility of weak spots. Low temperature STM measurements (USM-1400S from UNISOKU) were performed in a different UHV chamber at a sample temperature of 80 K in the constant current mode, as described in ref 13. Materials and Film Growth. A Cu(111) single crystal (MaTecK) was clamped on the manipulator and cleaned by alternating cycles of Ar+-sputtering (600 eV, 3 μA; ϑ = ±45° and 0° with respect to the surface normal, 20 min) and annealing (T ≈ 750 K, 15 min). The quality of this preparation was judged by the LEED patterns and repeated until sufficiently sharp spots were observed for primary electron energies up to 500 eV. Naphthalene (Wako Pure Chemical Industries) was purified by freeze−pump−thaw cycles prior to usage. Gaseous naphthalene40 was introduced into the UHV chamber using a pulsed leak valve (opening time 2 ms, vapor pressure 20 Pa). Owing to the small amount of material allowed in the UHV chamber, the nominal film thickness d is assumed to scale linearly with the pulses applied. One nominal C10H8 monolayer equivalent (1 MLE) is defined throughout this study as 500 pulses. This value was determined with an accuracy of ±20 pulses (see below). The substrate was cooled with liquid nitrogen while being gently heated to various temperatures T > 78 K. Thin films of C10H8 adsorbed on the Cu(111) surface regardless of the sample being shadowed by the LEED apparatus. Therefore, LEED images could be recorded while depositing C10H8 onto Cu(111), yielding thicknessdependent in situ measurements. Earlier studies have unequivocally shown that naphthalene remains intact upon adsorption and heating, hence it also desorbs molecularly, and no contaminants remain on the Cu(111) surface.11,12 Neither could we trace any residue in the diffraction images after heating the sample to 600 K. Instead, the Cu(111) reflexes fully recovered after such treatment, and the surface could be reused many times without changing its position which guarantees measurement conditions identical to those used for the LEED calibration. The substrate temperature T was calibrated with a thermocouple clamped onto a polycrystalline copper dummy sample, see the
under different growth conditions, briefly summarized in Table 1.13 Further, a pronounced dependency of the molecular energy Table 1. Thin Film Structures A, B, and C of Naphthalene on Cu(111) Reported in Ref 13 as well as D and E Found in This Studya Ab B
c
Cd D E
M
n
AC10H8
(−105 55) ( 40 23) (05 31) 3.75 (5.00 0.00 2.96 ) 1.00 ( 4.92 0.17 3.00 )
6
12.5 ACu(111)
120 Ke
Tgrowth
1
12.0 ACu(111)
120 Ke
1
15.0 ACu(111)
140 Ke
1
14.8 ACu(111)
various T
1
14.6 ACu(111)
various T
a
n is the number of molecules per unit cell. AC10H8 is the relative surface area per C10H8. Owing to the temperature dependence of the Cu lattice constants absolute values of AC10H8 are not given here. Instead, average areas per molecule are indicated with reference to the surface unit cell area ACu(111) = |s⃗1|·|s⃗2|·sin 120°. Ratios are thus AC10H8/ACu(111) = det M/n, where det M is the determinant of the epitaxy matrix. b Given in ref 13 in Wood’s notation32 as (5√3 × 5√3)R30°. Chiral motifs in unit cell. cGiven in ref 13 as (2√3 × 3)rect-1C10H8. dGiven in ref 13 as −14 −14 . Different lattice vectors were chosen here for easier comparison with structures D and E discussed below. eValues from ref 13. STM images recorded at 80 K; LEED and 2PPE measured at 90 K.
(
)
levels on the film thickness was measured with 2PPE.13 Though not directly imaged, the formation of a metastable C10H8 structure on Cu(111) was deduced from TPD measurements at 86 K.12 At such low substrate temperatures the growth mode of C10H8 was described as “hit and stick” with little lateral mobility, thus causing disordered submonolayers in the first place and further involving the formation of three-dimensional islands. Repulsive intermolecular interactions lead to a reorientation of molecules when monolayer coverage is approached.11,12 Moreover, a temperature-dependent structural rearrangement was concluded from the observation of a change in the work function after annealing the molecular film.12 Judging from these combined findings, the growth and order of naphthalene on Cu(111) appear to be much more complex than on Pt(111) and Rh(111). In this contribution, we demonstrate that naphthalene exhibits a pronounced coverage-dependency and temperaturedependency of the growth behavior as well as a variety of different structures on Cu(111), among them also previously unknown noncommensurate point-on-line (POL) coincidences.33,34 We follow the growth of naphthalene on Cu(111) by means of in situ real-time LEED from submonolayers to densely packed monolayers and beyond with the intention to track the evolution of the film structures and phase transitions. In addition, our results are meant to help identify the variety of features observed in previous 2PPE measurements,13 directly addressing the structure−property relations for this seemingly simple yet rather complicated model system.
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EXPERIMENTS AND METHODS
Setup and Data Analysis. All experiments were carried out in ultrahigh vacuum (UHV, base pressure p ≈ 10−8 Pa). The UHV chamber is equipped with a four-axial manipulator (x, y, z, ϑ) for 14164
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Supporting Information for details. Heat dissipation through the thin chromel and alumel wires was reduced by means of a small radiation shield around the insulated wires. Following this calibration, the growth temperatures favorable for the formation of structures A and C were found here to be slightly below those given in Ref 13. The cooling and heating rates used for all variable temperature experiments discussed were less than 1 K/min, i.e., very gentle temperature ramps were applied. Nonequilibrium effects are thus unlikely to occur.
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RESULTS AND DISCUSSION In the view of the wealth of different naphthalene structures formed on Cu(111) with strong dependency on the nominal film thickness d, we will focus on a few representative experiments. In the beginning, we will discuss LEED images acquired during C10H8 adsorption at the constant substrate temperatures of T ≈ 110 and 133 K. These values were selected with the intention to favor the formation of structures A and C, respectively. As outlined below, several new structures are also observed, and the d-dependent development is different in these cases. Next, we will elucidate the structures obtained at the lowest sample temperature achieved, T ≈ 90 K. Subsequently, the temperature variation of an as-grown submonolayer film will be considered which yields information on a reversible phase transition. LEED Results. Naphthalene Deposition at 110 K. In the first exemplary case, the Cu(111) crystal is kept at a fixed temperature of around 110 K. The initial deposition of C10H8 leads to the formation of a blurred disclike LEED pattern which lacks discernible spots; cf. Figure 2a and b. Hence, no ordered phase is formed for submonolayers, instead the molecules are distributed randomly on the surface. The observation of a “disc” leads to the conclusion that a minimum distance between adjacent molecules exists, as opposed to a preferential distance with random angular distribution that would cause a ringlike pattern in reciprocal space.41 It is further observed that the diameter of this disc grows upon naphthalene coverage increase, corresponding to a shrinking of the minimum intermolecular distance. Therefore, the molecules tend to maximize their separation on the Cu(111) surface as a function of the areal density, which is a characteristic behavior for effectively repulsive intermolecular interactions. The disc continues growing until a slightly brighter ring emerges at the perimeter. This means that the surface coverage reaches a threshold where intermolecular distances cannot be merely reduced anymore. Consequently, molecular ensembles are formed and sterically forced into specific rotations giving rise to an intensity modulation that eventually develops on this ring, whereas its diameter reaches a maximum (Figure 2c). When a certain critical coverage is reached, the observed intensity modulation on said ring converges toward six bright spots; cf. Figure 2d. These correspond to the emergence of a moderately ordered phase (translational symmetry extends over a few lattice constants only), which we will call precursor. Structural imperfections (defects, domain boundaries, local variations of the unit mesh) of the precursor lead to the unobservability of higher-order LEED reflexes. This effect can be described by the Debye−Waller factor in which the displacements of the scatterers parallel and perpendicular to the surface can be separated; see the Supporting Information for details. Within just a few additional pulses of C10H8, the six reflexes attributed to the precursor begin to split, thereby forming six pairs; cf. Figure 2e and f. Moderately ordered naphthalene
Figure 2. LEED data acquired during C10H8 growth on Cu(111) at 110 K. (a−g) Normal incidence, Ekin = 16.0 eV. (h) Sample tilted 12° off normal, Ekin = 34.1 eV. Dark areas correspond to high intensities. ξ is the splitting angle of the precursor spots. The contrast in (g) and (h) has been adjusted in order to improve the visibility of weak spots. LEED simulations of structures A (green), D (blue), and E (purple) containing rotational and mirror domains are partly superimposed. Note that especially in (g) the {3 0}-spots of A are discernible (marked with green arrows).
domains thus rotate away from substrate high-symmetry directions. The splitting is a function of coverage and converges toward a final position that corresponds to the maximum angular separation between both spots of each pair; cf. Figure 2g. Suddenly, new reflexes appear which can be attributed to A. The six pairs of spots originating from the precursor now coincide with {2 1}-spots and symmetry equivalents of A. A possible coexistence of both structures leads to the effect that the features in common are more intense than others, provided that there is an incoherent superposition of the diffracted beams. Several naphthalene structures may coexist (as outlined above) with unknown relative occupancies on the Cu(111) surface, each of them claiming different areas per molecule. Moreover, the dissimilar structures are possibly transformed into one another as a function of the amount of deposited material. For this reason, it is per se impossible to give a generally valid relation between the number of adsorbed molecules and a monolayer (ML). We therefore choose to define a nominal C10H8 monolayer equivalent (MLE) such that 14165
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it corresponds to the number of pulses necessary to produce a LEED image as shown in Figure 2g at 110 K substrate temperature and apply this unit to all experiments discussed here. Below nominally 1 MLE, other structures may also exist in the form of closed layers, however with smaller molecular areal density. Upon further deposition, A disappears which is most evident for its {h 0}- and {0 l}-spots; cf. Figure 2h. The former {1 1}reflexes of A seem to become somewhat elongated, while each one of the former {2 1}-spots develops into a set of multiple features. This development occurs until around 2 MLE. After the spots belonging to A have vanished, the LEED images remain constant, and only the background increases at higher nominal film thickness. The observed patterns can be explained by the coexistence of two previously unknown point-on-line coincident phases, which will be called structures D and E from now on. It is beneficial to analyze LEED images of tilted samples which yield higher-order spots at a given primary electron energy Ekin; cf. Figure 2h. The LEED simulation was optimized for various values of Ekin and several tilting angles of the sample in order to improve the accuracy. The resulting epitaxy matrix for structure D is given in Table 1. For each epitaxy matrix element, the confidence interval of ca. ±0.02 stems from the numerical fit of the simulated reciprocal lattice to the determined LEED spot positions. This matrix has further been refined on the basis of moiré pattern simulations reproducing the observed long-range contrast modulations in STM images (see below). The corresponding unit cell dimensions are |a1⃗ | = 11.49 Å, |a2⃗ | = 7.56 Å, ΓD = 73.9°, and ΦD = 46.1°. Similarly, structure E is given in Table 1. Its unit cell dimensions are |a1⃗ | = 11.48 Å, |a2⃗ | = 7.44 Å, ΓE = 106.0°, and ΦE = 11.1°. Note that both matrices have one distinct integer column in common with MC (first column of MD and second column of ME), cf. Table 1. There are indications in STM data that A does indeed not persist when raising the nominal film thickness above 1 MLE.13 First the unoccupied sites within the chiral motif are filled with additional molecules. With then 7 instead of 6 molecules in the same unit cell dimensions the average area per molecule is only AC10H8 = 10.7ACu(111), which seems to be too densely packed and causes steric repulsion. This triggers a reordering and possibly leads to the metastable structure B13 which was, however, not observed in all LEED experiments presented here. A possible explanation is that the sample is continuously struck by electrons that are in part inelastically scattered and may therefore prevent B from forming in the first place. An alternative scenario might be the preferential transformation of A into the less densely packed structures C, D, or E and perhaps occasionally some clusters for the substrate temperatures and deposition rates used here. Naphthalene Deposition at 133 K. We now turn to the exemplary case where the Cu(111) substrate is kept at 133 K. In the beginning, the growth behavior is characterized by the formation of a disordered gaslike phase of C10H8 which exhibits decreasing intermolecular distances upon deposition; cf. Figure 3a and b. Again, a precursor is observed; however, already at 0.51 MLE, six bright spots are discernible (Figure 3c) which is considerably earlier than that for the adsorption at 110 K. When the coverage is slightly increased, the initial precursor spots also split in two and then veer away from one another, though in this case the separation is smaller (compare Figure
Figure 3. LEED data acquired during C10H8 growth on Cu(111) at 133 K. (a−f) Normal incidence, Ekin = 16.0 eV. (g) Sample tilted 10° off normal, Ekin = 28.0 eV. Dark areas correspond to high intensities in the image. The contrast in (g) has been adjusted in order to improve the visibility of weak spots. LEED simulations of structures C (orange), D (blue), and E (purple) containing rotational and mirror domains are partly superimposed. For d ≥ 1 MLE, D dominates the LEED images, whereas the remaining faint spots can either be attributed to C (orange arrows) or to E (purple arrows). Both C and E may be barely coexistent with D.
3d and discussion below). At 0.54 MLE, the splitting of the precursor spots comes to a rest, and subsequently, additional features begin to emerge (Figure 3e and f). Further deposition to 0.96 MLE leads to the disappearance of the precursor spots, while those of D and, to a lesser extent, those of E become more intense (Figure 3g). A is not formed at 133 K. The final LEED images for ≈2 MLE (not shown here) are similar to those at 110 K; minor differences can be noted in the relative intensity ratios of structures D and E and in the occurrence of an additional very weak set of spots at ≈2 MLE. Consequently, even more naphthalene polymorphs may exist. Upon cooling of the 1 MLE thick (or thicker) naphthalene films, the LEED spot positions remain identical, while the diffuse background becomes weaker. The latter observation can be explained by a reduced Debye−Waller factor, since the molecular vibrational modes of C10H8 and the phonons are gradually frozen out.24 Naphthalene Deposition at 90 K. The submonolayer behavior of naphthalene on Cu(111) deposited at 90 K is similar to the deposition at 110 K, differing merely in the coverage threshold of the precursor’s spot splitting. At d = 1 MLE, the LEED images are nearly identical in both cases. Notable differences arise for d > 1 MLE; cf. the Supporting Information. Here, A is transformed exclusively into E for nominal film thicknesses reaching 2 MLE, whereas D cannot be observed at this substrate temperature. Coverage Dependency. The development of the LEED patterns with increasing naphthalene thickness described above shall now be elucidated quantitatively in more detail. For this purpose, we plot the splitting angle ξ of the precursor spots and the intensities of selected spot families representative for 14166
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specific structures as a function of nominal film thickness d in Figure 4.
and the {2 1}-reflexes of A which is not the case at elevated temperatures. Next, we consider the development of specific spot intensities as shown in Figure 4b, normalized individually for each case. At 110 K (black symbols) the {3 0}-reflexes of A are very weak in the submonolayer regime but suddenly emerge at ca. 1 MLE. As this emergence is connected to the abrupt spot splitting of the precursor, we can unambiguously conclude that the latter is at least partly transformed into A. For d > 1 MLE, the intensity of the {3 0}-reflexes of A decreases again and eventually reaches ca. 10% of the value for 1 MLE. Covering the first layer with additional molecules and assuming an escape depth of the scattered electrons of only one molecular layer, the intensity would be expected to vanish within 1 MLE. However, the decrease of the intensity initially occurs at roughly twice that rate (gray solid line in Figure 4b), slowing down about half way through the growth of the second layer. Therefore, it can be assumed that A is at least partly transformed into other structures, most likely D or E, for increasing nominal film thickness. At 133 K (red symbols), the precursor spots emerge abruptly at ≈0.5 MLE. This intensity diminishes linearly until 0.92 MLE, while that of the {1 1}-reflexes of D increases at approximately the same rate. Hence, in this case, the precursor is obviously transformed into D. The slight deviation from the linear increase of D might be explained by a simultaneous but significantly less likely formation of E and/or C. For d > 1 MLE, the {1 1}-reflexes of D decrease in intensity but remain visible (≥ 60%) at 2 MLE which can be explained either by an increased defect density or by a second layer growing with inferior long-range order on top of D. At 90 K (blue symbols), the precursor and A develop almost simultaneously slightly before 1 MLE is reached. Then A disappears and is fully extinct at about 1.5 MLE. The precursor spots decrease much slower in intensity. From the corresponding LEED images (not shown here), it is apparent that E is formed at d > 1 MLE which exhibits reflexes in close proximity to the precursor spots that could not be resolved individually for this analysis. Note that also at 110 K the precursor spots almost coincide with reflexes stemming from D and/or E so that the decrease of the precursor intensity at d > 1 MLE is actually concealed by the simultaneous formation of these two structures. Moiré Analysis of STM Results. It is important to note that structures D and E are not commensurate but point-online coincident, because the epitaxy matrices contain one column with integers and one with noninteger values each. Therefore, one would expect long-range contrast modulations in STM images because the molecules adopt inequivalent adsorption sites at least in one direction,42 which is indeed observed (Figure 5). In fact, these so-called moiré patterns are very susceptible to variations in the lattice parameters and can therefore be used to verify and even adjust the epitaxy matrices extracted from the LEED data. The molecular unit cell dimensions of structures C, D, and E are indeed similar. Hence, the contrast modulation was previously missed, because the STM images were initially attributed to the commensurate structure C.13 In combination with the LEED data presented in this study, we are able to reassess the previously published STM data and attribute the contrast to the point-on-line structures D and E, respectively. While the molecular positions can be readily inferred from the topographic contrasts in Figure 5, the orientations of the molecules with respect to the unit cell vectors are much less certain. Therefore, we refrain from speculating about the real-
Figure 4. Nominal film thickness dependencies (a) of the splitting angle ξ of the precursor spots (cf. Figure 2) and (b) of the normalized intensities of selected spot families representative for specific structures. Before the splitting, the relative intensities of the precursor spots consisted of the sum of two coincident reflexes, which has been accounted for in this diagram. Substrate temperatures during deposition are indicated for each graph. Solid and dashed lines are guides to the eye.
We begin our analysis with the splitting of the precursor spots, expressed by the angle ξ between reciprocal lattice vectors of adsorbate and substrate as also indicated in Figure 2. From Figure 4a, it is evident that at 110 K the splitting sets in rather abruptly and converges quickly to its maximum value of ξ = 10.9°. In this case, the precursor spots coincide with the {2 1}-reflexes of A which form simultaneously. This can be used for a definition of a monolayer equivalent (MLE). Under these experimental conditions ca. 500 ± 20 C10H8 pulses correspond to 1MLE. At 90 K the spot splitting sets in at lower coverages and reaches a maximum of ξ = 12.5° at 0.8 MLE while the distance between the precursor spots and the (0 0)spot is still increasing. Then it drops again and converges toward ξ = 10.9° while A emerges. At 133 K, the development is noticeably different: The precursor spots begin to split already at ca. 0.5 MLE reaching a maximum value of only ξ = 4.8° while A is not formed at all. We conclude that the latter only occurs when the rotation of the small precursor domains is sufficiently large to allow a coincidence of the precursor spots 14167
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bright LEED spots are discernible, a moderately ordered structure has already formed. Upon cooling down below 130 K, each of the six bright spots begins to split in two, which is depicted in Figure 6. As a
Figure 6. Spot splitting angle ξ during temperature variation of a 0.5 MLE naphthalene film on Cu(111) displaying the precursor. The substrate temperature during deposition was 132 K. Arrows indicate the development upon cooling and heating. Solid and dashed lines are guides to the eye. The insets depict typical LEED images at the temperatures indicated. Figure 5. STM images (recorded at 80 K) of naphthalene on Cu(111) deposited at 140 K. (a) 20.0 × 17.3 nm2, Vs = −2.50 V, It = 0.02 nA; (c) 15.8 × 15.8 nm2, Vs = −2.30 V, It = 0.03 nA; (e) 21.5 × 21.5 nm2, Vs = −2.00 V, It = 0.01 nA. Individual molecules are resolved, and long-range contrast modulations are visible in (a) and (c). Several representative directions of the long-range contrast modulations are given as thin solid lines. (b,d) Moiré pattern simulations of D and E reproduce the long-range contrast modulations. Minor differences are caused by drift. (f) Comparison of the unit cells of C, D, and E with respect to Cu(111). Panels (a) and (c) adapted with permission from ref 13. Copyright 2010 American Chemical Society.
function of temperature each pair of spots splits further, thereby rotating away from Cu(111) high symmetry directions. Between about 112 and 110 K, several new reflexes emerge which subsequently become brighter; at the same time, the spot splitting reaches a maximum. The LEED pattern then remains constant down to T ≈ 85 K, with the diffuse background slightly decreasing. At these low temperatures, the diffraction images are quite similar to Figure 2g, and all the spots can be explained with just one structure. Therefore, the precursor appears to have changed its orientation with respect to the substrate until it condensed into A. Given that the latter exhibits also higher-order LEED reflexes, a highly ordered structure can be stated. If A emerges from the precursor upon cooling, it should disappear again upon heating, leaving the precursor behind. In fact, as the temperature is increased again, the {h 0}- and the {0 l}-spots associated with A vanish between about 110 and 112 K. Simultaneously, the splitting of the precursor spots (which formerly coincided with the {2 1}-spots and symmetry equivalents of A) is reduced again, converging toward the initial positions; cf. Figure 6. The orientational change of the precursor along with a condensation into A as a function of temperature is thus reversible. Consequently, a phase transition between a moderately and a highly ordered structure occurs. We note that several new LEED spots arise upon heating up again. This seems to be a competing effect, which means that for a certain fraction of the film the back-conversion of A does not yield the precursor, but yet another structure. From the local energy minimum attained for the molecules condensed as A, it may be similarly likely for increasing temperatures to be converted into a competing structure different from the precursor. The corresponding LEED pattern may be attributed to E, although the signal-to-noise ratio of the reflexes observed does not allow for an unambiguous identification in this case. The slight hysteresis in Figure 6 may be explained by the formation of this competing structure which would reduce the
space models of the monolayer structures. Only for structure E one can pinpoint the molecular orientations somewhat more precisely, and this has already been discussed in ref 13. Reversible Phase Transition. Reordering of closed, highly ordered C10H8 monolayers was not observed upon temperature variation. The behavior of dilute submonolayers (less than 1 molecule/10 nm2) is discussed in detail in ref 13. Due to the effectively repulsive interactions, no densely packed islands are formed during the initial growth stage (even at LN 2 temperature), which was also concluded from TPD measurements over a wide temperature range.11,12 Consequently, what remains to be investigated here is the thermodynamic behavior of the precursor. For the above exemplary experiments and also for other substrate temperatures (not shown here), a precursor formed at submonolayer coverage above a certain threshold. Notable differences occurred in each case which we shall now track down by a variation of the temperature of an as-grown submonolayer film. The spot splitting is of particular interest here, as it indicates a certain degree of freedom with respect to surface coverage which might also be thermally influenced. As a starting point, we have thus chosen the precursor structure at 132 K just below 0.5 MLE, that is, before the coverage threshold for the spot splitting was reached. However, as six 14168
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rotational degree of freedom of the coexisting precursor upon reheating. Apart from this effect, we have observed a similar spot splitting behavior of the precursor upon recooling the sample, thus confirming the reversibility of this phase transition.
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SUMMARY AND CONCLUSIONS Summarizing our findings, we have observed two phase transitions for C10H8 films on the Cu(111) surface. The initially formed 2D gas phase is transformed into a molecular solid with moderate order when a critical surface coverage is reached. From there, a highly ordered solid emerges upon further increase of the coverage, exhibiting different structures depending on the growth conditions and on the film thickness. A similar moderate order toward high order phase transition occurs upon cooling of a naphthalene submonolayer and is reversed when heating up again. The polymorphism and thermodynamic behavior is thus considerably more complex than what was known from previous reports. It is also important to note that not all structures observed are commensurate. This is especially true for the moderately ordered precursor with its almost freely tunable rotation relative to the substrate. Further, structures D and E are pointon-line coincident and rather the rule than the exception at d ≥ 1 MLE and/or high temperature. In these cases, a commensurate registry is obviously not energetically favored.
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ASSOCIATED CONTENT
* Supporting Information S
Details of the temperature calibration performed and on the nonobservability of higher-order LEED spots of the precursor structure. Details of selected LEED images of the naphthalene deposition on Cu(111) held at 90 K. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +49 (0)3641 947430. Fax: +49 (0)3641 947412. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. Dr. M. Horn-von Hoegen for fruitful discussions. This collaborative research was funded through the PAJAKO Project No. 56264880 by the DAAD. R.F., J.P., M.M., F.S., and T.F. acknowledge financial support from the DFG Grant No. FR 875/9-3. T.Y. and T.M. acknowledge financial support by Grant-in-Aid for Scientific Research from JSPS (25600004, 24656036, and 24685004). T.U. acknowledges support from JSPS research fellow program (13J01705).
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