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Multitude of PTCDA Superstructures on Ag(111) and Vicinal Surfaces Stefan Schmitt, Achim Schoell, and Eberhard Umbach J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00657 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 20, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

(743)

(321)

390 nm

Fig. 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

a)

b)

[𝟏𝟏𝟏𝟏𝟏𝟏] 26 nm

Fig. 2

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[110]

(552) facet 1 8 −1 3 Area: 2 x 3.31 nm2


𝑎𝑎2

𝑎𝑎1

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(321) facet

a)

0.43 2.28 −0.88 1.74 Area: 1.93 nm2

d)

[𝟐𝟐𝟏𝟏𝟏𝟏] 50 nm

b)

e)

𝑏𝑏2

7 nm

c)

𝑎𝑎2

𝑎𝑎1

[211]

65°

[211]

𝑏𝑏1

f) 5.0 nm

46 nm

5.0 nm

𝑎𝑎2

𝑎𝑎1


Fig. 3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

a)

(532) facet 0 4 −2 0.5 Area: 2 x 2.06 nm2

[𝟑𝟑𝟐𝟐𝟐𝟐]

36 nm

b)

c)

[321]

𝑎𝑎2

18 nm

Fig. 4

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𝑎𝑎1

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2.5 D

b)

a)

[321] SB(111)

SB(111) 𝑎𝑎2

35 nm 3D

Fig. 5

2D


𝑎𝑎1

molecular chain on (751)


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a)

SA(111):

6 1 −3 5

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SB(111):

b)

Area: 33 uc

Area: 36 uc [312]

[312]

c)

SC(111) on Ag(775):

6 1 −5 5

d)

SD(111) on Ag(10 8 7):

Area: 35 uc

Fig. 6

6 0 −3 6


5 3 −3 5

Area: 34 uc

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a)

[101]

b)

[101]


(751)

(39 35 7)

[110]

c)

20°

16°

27°

20°

[110]

[101]

16°

27°

[101]

d)

inc

inc

inc

20°

Fig. 7

[110]

27°

16°

20°

[110]


27°

16°


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(221)

[110]

(552)

[110]

2D com.

2D com.

[321]

[211]

(542)

(321)

p.o.l. to steps (single)

incom.

[312]

[413]

(532)

(743)

1D com. to kinks 1D com. to kinks (doubled) (doubled) p.o.l to steps [110]

(551)

Fig. 8

(39 35 7)

[871]

ACS Paragon Plus Environment 2D com. 1D com. to kinks

p.o.l. to kinks

(trippled)

(331)

[110]

2D com.

(873)

[541]

2D com. densly packed bulk structure

(111)

2D com. (single)

(954)

[514]

incom. [321]

(751)

[101]

(211)

1D com. to kinks (doubled) [312]

(25 13 7)

incom. 1D com. to kinks (almost p.o.l. to steps) p.o.l to kinks

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Structure

uc/Å: 𝒃𝒃𝟏𝟏 𝒙𝒙 𝒃𝒃𝟐𝟐 ; (β/°)

A(uc)/Å2

Matrix

S(221)

12.3 x 22.6; (84.8)

275.6 (115.2%)

S(874)

/

/

3 1 −5 2

S(552)

31.1 x 22.7; (110.5)

330.6 (138.8%)

S(873)

24.0 x 12.1; (100)

285 (120%)

S(542)

22.0 x 10.3; (86)

226 (95%)

S(321)

13.4 x 14.7; (85)

196 (82%)

S(532)

13.0 x 15.9; (87)

206 (87%)

S(743)

12.1 x 16.1; (81)

193 (81%)

S(954)

12.7 x 15.4; (81)

193 (81%)

S(211)

16 x 13; (103)

200 (85±5%)

SA(111)

12.63 x 18.95; (89.0)

239.3 (:=100%)

/

1 8 −1 3

2.5 1.8 −1.25 0.3 2 2.7 −1 0.7

0.43 2.28 −0.88 1.74 0 4 −2 0.5

0 3 −1.8 0.71

1.77 1.77 −1.71 0.54


Tab. 1

0 11 / /

6 1 −3 5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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Structure

uc/Å: 𝒃𝒃𝟏𝟏 𝒙𝒙 𝒃𝒃𝟐𝟐 ; (β/°)

A(uc)/Å2

Matrix

S(331)

17.3 x 17.1; (95.6)

294.4 (123.6%)

S(551)

16.2 x 17.8; (95)

287 (121%)

2 5 −2 3

S(39 35 7)

14.7 x 22.0; (76.6)

310.0 (130.2%)

S(751)

19.1 x 15; (79)

281 (118%)

S(25 13 7)

18.4 x 10.8; (78)

194 (82%)

SA(111)

12.63 x 18.95; (89.0)

239.3 (:=100%)

Tab. 2


0 8 −1.2 5 2 0 0 2

0.66 2.3 −0.66 1.2 0 3 −0.53 1 6 1 −3 5

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Structure

uc/Å: 𝒃𝒃𝟏𝟏 𝒙𝒙 𝒃𝒃𝟐𝟐 ; (β/°)

A(uc)/Å2

Matrix

Symmetry

SA(111)

18.95 x 12.63; (89.0)

238.53 (:=100%)

P2

SB(111)

17.34 x 15.02; (90)

260.21 (109.1%)

6 1 −3 5

SC(111)

18.95 x 14.45; (112.4)

252.98 (106.1%)

SD(111)

20.02 x 12.63; (77.2)

245.75 (103.0%)

Tab. 3


6 0 −3 6

P2gg

7 −1 0 5

p2

−3 8 −5 2

P2


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Multitude of PTCDA Superstructures on Ag(111) and Vicinal Surfaces

Stefan Schmitt, Achim Schöll*, and Eberhard Umbach Experimentelle Physik VII, Universität Würzburg, Am Hubland, D-97074, Germany *corresponding author; email: [email protected] Abstract: Organic molecules that covalently bond to metal surfaces, like perylene tetracarboxylic acid dianhydride (PTCDA) on Ag, may cause large-scale reconstruction of the interface, in particular if the adsorption takes place on a “stepped” (i.e. vicinal) surface. Under certain conditions such surfaces develop a large variety of high-index facets that are stabilized by superstructures of the PTCDA adsorbate. These superstructures can be commensurate, point-on-line, or incommensurate with respect to the substrate facets depending on facet orientation and step direction. The present scanning tunneling microscopy and low-energy electron diffraction data were taken from two different vicinal surfaces, Ag(775) and Ag(10 8 7), both inclined by 8.5° with respect to the (111) direction but in different azimuthal directions. Altogether we identify and evaluate 18 new superstructures and correlate their data with the properties of the corresponding facets. The observed richness of superstructures and the occurrence of large-scale reconstruction may be common for many other combinations of similar organic molecules and metals.

1


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1. Introduction The interfaces between metals and organic molecules are of considerable interest because they determine many properties of such hybrid systems. For metal-organic devices like organic light emitting devices, organic field effect transistors, or organic solar cells these interfaces, e.g. as substrate or contacts, are responsible for organic layer growth, mechanical stability, charge transport, and optical properties including quenching. But also for a fundamental understanding of interface properties, supramolecular structures, nano-systems, or self-organization phenomena organic layers on metal surfaces still provide a wealth of new and partly unexpected discoveries, in particular if large aromatic molecules are chemisorbed on medium reactive metallic substrates. Examples for such systems are polyaromatic hydrocarbons, aromatic heterocycles, or phthalocyanines on metals like Ag, Au, or Pt, just to name a few. In these cases each molecule interacts with many substrate atoms simultaneously, and if the interaction is neither too strong nor too weak it can lead to all kinds of coverage- and temperature-dependent ordering phenomena and phase transitions, to large-scale reconstructions of the interface and to nano-pattern formation. These aspects are addressed in a very large amount of original publications and are summarized in several review articles some of which are given here as example1-4. The key to the understanding of such phenomena is the subtle interplay between the various acting forces, i.e. the chemical bond between each molecule and the number of adjacent metal atoms depending on their relative positions, the interaction between the neighboring substrate atoms, the intermolecular interaction, and the internal molecular forces5-9. 2



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The present work addresses a particular aspect in the multi-parameter space of interactions, namely the dependence of the molecular superstructure on the relative positions of substrate atoms. The latter can be varied by using substrate faces that are vicinal to a low index-surface. Such vicinal surfaces can intentionally be prepared by cutting and polishing a single crystal with the desired orientation provided these surfaces are thermodynamically stable – without and with adsorbates. Alternatively, as done in the present work, one can select a certain (vicinal) surface orientation, which is slightly (few degrees) tilted against a low-index surface orientation. After polishing and cleaning, such a surface usually shows flat terraces of low-index orientation, separated by a regular array of single-atomic steps (in the case of a transition metal). After adsorption of molecules on such a “stepped” surface and after annealing, the surface then may considerably reconstruct involving adsorbate-induced step bunching and massive transport of metal atoms. This large-scale reconstruction may lead to the development of (in general several) vicinal (i.e. high-index) surfaces which are thermodynamically stabilized by the adsorbate10-12. The “selection” of these adsorbate-induced vicinal faces depends on the involved materials and the orientation of the initial surface as well as on adsorbate coverage and annealing temperature10-12. The stable, high-index vicinal faces usually have a larger tilt angle with respect to the low-index plane than the initial (global) surface. The combination of the high-index vicinal faces with the low-index terraces in between sums up to the global tilt angle12. It turns out that on each of such vicinal surfaces different molecular superstructures develop upon molecular adsorption depending on coverage and preparation temperature. The diversity of these superstructures, their commensurability, parameter dependences, 3


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and stability are the topic of the present work. We concentrate on one particular system, PTCDA (perylene tetracarboxylic acid dianhydride) on Ag(111) and its vicinal surfaces but emphasize our expectation that other substrates and especially other molecules may lead to similar behavior provided that the preparation occurs in a similar way and that the requirements mentioned below are fulfilled. A few examples of multiple organic superstructures are found in the literature3, 9, 13-14, but a systematic investigation similar to the present is - to our knowledge - missing. The choice of this particular system is based on a long-standing experience with the adsorption of PTCDA on low-index Ag planes and the detailed knowledge about their chemical, geometric, electronic, dynamic, and optical properties acquired by utilizing the full palette of surface analysis techniques15-29. It turns out that this system is ideally suited to study all aspects of superstructure formation, large scale reconstruction, and selforganization, and that all different possible forces play a significant role with varying contributions. We are convinced, however, that the multitude of observed effects is not limited to this particular system but appears to be of general nature depending on the details of the used molecules, metal surfaces, and preparation conditions.

Previous studies have shown that PTCDA molecules chemisorb on different Ag surfaces3, 15-18, 24, 26-27

and form highly ordered monolayers, i.e. different commensurate

superstructures depending on surface orientation17, 19-21, 29 as studied in detail by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) investigations. By high resolution SPA (spot profile analysis) LEED and ultraviolet photoelectron spectroscopy (UPS) it was discovered that at low temperatures a metastable chemisorbed 4



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precursor state first forms which irreversibly transfers into the stable state upon annealing9. It was found that the electronic structure of the adsorbate layer is different for different surface orientations like (111), (110), and (100)18, 20, 27, 30, in particular concerning the frontier orbitals (the occupied and unoccupied orbitals near the Fermi level) which show significantly different relative shifts. The vibrational modes have been investigated resulting in the finding that the vibrational structures of the first layer are very different comparing different surface orientation3 and that for Ag(111) very strong electron-phonon coupling occurs31. X-ray standing wave experiments yielded different bonding distances and a bending of the PTCDA due to the chemisorptive bond5. Dynamic low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) studies using a spectro-microscope revealed that usually very large (~10 µm) domains are being formed at room temperature despite the fact that there are six (by LEEM distinguishable) types of domains in the monolayer32-33; these experiments showed that the molecules diffuse over very large distances across steps before they find their finite adsorption site (at the rim of an island)32-33. Moreover, the growth of the first few layers has been monitored in detail using LEEM3334

, PEEM33-34, micro near edge x-ray absorption finestructure (µ-NEXAFS)32, SPA-

LEED35 and x-ray techniques28, 36. It was found that very different growth mechanisms occur depending on preparation conditions and substrate surface morphology, and that even true epitaxial growth (i.e. layer-by-layer growth, commensurate to the substrate!) is possible under certain conditions. The different growth mechanisms lead to different transport properties and – surprisingly – to very different optical properties depending on the preparation parameters22-23. 5


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The adsorption on “stepped surfaces” has also been studied. If the orientation of a metal surface is slightly tilted with respect to a low-index direction one finds a regular array of (low-index) terraces separated by monatomic steps provided that the mutual interaction of the steps is repulsive37. The width of the terraces can be adjusted by choosing the angle of inclination with respect to the (111) orientation. In this study we selected an angle of 8.5° tilted in [1-21] direction and chose two different azimuthal (tilt) directions leading to two different nominal surface orientations (vicinal to Ag(111)), namely Ag(775) and Ag(10 8 7). This tilt angle resulted in terrace widths of ~1.6 nm which is slightly larger than the length of the PTCDA molecule (1.42 nm). However, the molecules did not adsorb on these terraces but the entire surface was modified by PTCDA adsorption leading to large-scale reconstructions and faceting depending on surface orientation, PTCDA coverage, and annealing temperature. Generally speaking, the PTCDA adsorption on vicinal Ag faces in the submonolayer range leads to a decomposition of the surface morphology into stepped covered facets and large uncovered (111) terraces38-39. The morphology found is far beyond the effect of site selective adsorption on a substrate with fixed morphology because of the high mobility of the Ag atoms. Thus, the molecule PTCDA can literally create its “desired” (i.e. lowest free energy) surface, generating substantial Ag mass transport. Some of the results including a measurement of the macroscopic bending forces as a function of coverage and superstructure were published previously34, and a tentative explanation of the origin of these large-scale reconstructions and the underlying driving forces was given. In the course of these investigations we discovered a wealth of 6



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superstructures by STM and LEED experiments which are interrelated with the formation of various facets and strongly depend on the preparation conditions. These were studied and understood in detail and are the topic of the present paper.

2. Experiment The experiments were performed in a UHV system (base pressure ~10-9 mbar) using conventional LEED optics and a room-temperature STM (RT-STM). The STM is a commercial beetle-type instrument40-41 that could only be operated at RT, but sample preparation at elevated temperatures could be performed separately before the sample was transferred to the measurement position. The images shown are derivatives of the raw data to enhance contrast. The Ag samples were cut from single crystal rods in two different directions both 8.5° inclined with respect to the (111) direction. The two types of vicinal surfaces differ by the direction of the steps: the clean Ag(775) surface is dominated by (111)-type steps, while on the clean Ag(10 8 7) kinked (100)-type steps are present (see also Fig. 5 in ref. 39). The samples were carefully polished and then cleaned in ultra-high vacuum (UHV) by some tens of sputter-annealing cycles until clear LEED patterns were observed and no impurities were detectable by spectroscopic techniques. An additional Ag(775) crystal was also used for comparison showing massive surface corrugations after very many cleaning and measurement cycles; on this surface we found local disorientations of several degrees both in azimuthal and polar direction. 3. Results and interpretation 3.1

Large-scale surface reconstruction

7


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After the cleaning procedure the Ag(10 8 7) surfaces show a regular pattern of nearly equidistant monoatomic steps (see Fig. 1a of ref. 39) oriented along the [-321] direction with 1.6 nm wide (111) terraces between the steps. After PTCDA deposition and annealing (e.g. at 550 K) the surface morphology changes drastically as seen in Fig.1. In principle the annealing temperature can be between 550 K (desorption of multilayers completed) and 630 K (fragmentation of PTCDA in monolayer). The bright stripes indicate PTCDA-covered facets whereas the dark fields in between are PTCDA-covered, step-free (111) areas. The facets have different azimuthal orientations and are – in this case all – about 20° (polar angle) tilted against the (111) direction. Nevertheless, together with the (111) areas the global overall orientation within the field of view of Fig.1 is still 8.5° as the orientation of the initial cut. Fig.1 represents an example from a multitude of such measurements which differ in their patterns, tilt angles, and facet composition depending on PTCDA coverage and annealing temperature. The facets and their superstructures are very asymmetric, i.e. very long (several tens of micrometer) roughly along the step direction and relatively narrow (several tens of nanometer) perpendicular. We emphasize that the vertical corrugation of the faceted, partly PTCDA covered surface is in the order of 10 nm and hence much larger than the initial corrugation of single atomic steps (~0.1 nm) of the clean surface. This finding clearly demonstrates an enormous mass transport of Ag atoms. It is much larger than that of (conventional) adsorbate-induced reconstructions which only lead to a slight repositioning of the atoms of the topmost layers. We further note that this mass transport can be reversed by the cleaning process (sputter-annealing) thus reinstalling the initial Ag(10 8 7) or Ag(775) surface with monatomic steps. 8



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A closer inspection of the image of Fig.1 reveals that all areas are covered by highly ordered PTCDA superstructures which are the main objects of the present work. In order to correlate these superstructures with the substrate atoms we need to know the exact orientation of the facets underneath. These were obtained by a comparison with and careful analysis of corresponding LEED data which were derived from a set of measurements after correcting for misalignment and field distortions42. While the polar angle could easily be determined by measuring the (half) angle between primary beam and specular reflection, the azimuthal angle needed more effort and the consistent evaluation of several images of the reciprocal space from the clean and differently covered surfaces. The accuracy of the given angles depends strongly on the brilliance and the diameter of the recorded spots and hence on the size of the facets; the error bars are between 0.3° and 1° in most cases. In the supporting information we give an example of a LEED analysis (see Fig. S1) in order to make transparent that the information given in the text, figures and tables is a result of a careful and consistent analysis of both, the STM images and the LEED data as function of temperature and coverage.

3.2

PTCDA superstructures on different facets

In the following we pick several examples from different surface preparations (coverage and annealing). We do not show the survey images but concentrate on selected details as examples from a large collection of STM data. The full information extracted from these images is then summarized in some tables and graphs at the end of this section and will be discussed in chapter 4. The geometrical data including the superstructure matrices given in the following figures and in the tables are derived by a careful analysis of 9


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(calibrated) STM data; the indicated azimuthal orientations in the sketched unit cells of the figures are best guesses derived from high resolution STM images. We start with an example of a commensurate superstructure other than that of PTCDA on the low-index (111) plane discussed in several publications3,19. In Fig.2 we display an STM image from a PTCDA covered (552) facet that formed after annealing a PTCDA monolayer of a Ag(775) sample at 550 K. The relatively rare (552) facet is embedded between two (873) facets with different superstructures. The PTCDA superstructure on the (552) facet which is in the following abbreviated as S(552), is commensurate and described by the superstructure matrix

; it has a very low

density and contains 4 molecules per unit cell, azimuthally oriented in a herringbone structure due to the quadrupole moment of the molecule. The large unit cell (with four molecules) is required because of the position of the molecules relative to the substrate steps. The opposite is true for the PTCDA covered (321) facet that preferentially forms on the Ag(10 8 7) surface by deposition of more than 0.2 ML PTCDA at elevated substrate temperatures (e.g. 550 K, see Fig.3). In this case the global surface mainly consists of (743) and (321) facets and has a high concentration of kink sites. If the surface is only partly covered by PTCDA (less than 0.4 ML), as is the case in Fig.3, remaining (uncovered) single steps can be observed (see Fig.3a). The S(321) superstructure has a very high density with slightly overlapping molecules (see Fig.3d) which appears possible due to the rather rough surface topography resulting from many steps and kinks. The apparent optimization of the local adsorption site leads to an incommensurate long 10



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range order expressed by the matrix

Page 22 of 49

and to a Moiré pattern. The latter is

best visible in Fig.3c which shows stripes parallel to the substrate unit cell vector

(indicated in red). The stripes are separated by 5 nm, which is ten times the distance (

of the kink rows (which run along

)

) and which is also the length of the component of

the (3 2) superstructure vector parallel to

(see blue superstructure unit cells in Fig.3f).

Thus the Moiré pattern arises because the (-1 1) direction of the superstructure (indicated by two molecular rows and the dashed red lines in Fig.3f) does not coincide with the direction of the kink rows (described by

).

Another interesting detail can be derived from the close-up view shown in Fig.3b demonstrating submolecular contrast. The symmetry of the latter is very similar to that of the superstructure of PTCDA on Ag(111) and closely resembles the symmetry of the former lowest unoccupied molecular orbital (LUMO) which is pulled down below the Fermi level and (partly) filled by the chemical bonding26-27. From this figure and its evaluation in Fig.3e one can safely derive the molecular overlap and also the angle between the long axes of the two molecules per unit cell which is rather small here (65°) as compared to most other herringbone superstructures of PTCDA (around 90°). A third example is given in Fig.4 showing the PTCDA-covered (532) facet. This facet forms on both substrates, Ag(775) and Ag(10 8 7), upon deposition of roughly half a monolayer PTCDA. On Ag(10 8 7) it has the same step direction (and azimuthal angle) 11


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as the clean surface. It preferably occurs upon deposition at substrate temperatures slightly above room temperature (e.g. 350 K) while it becomes a minority facet and is replaced by (321) and (954) facets if deposition is performed on a hot substrate (e.g. at 550 K), i.e. if the mobility of Ag atoms is very high. Also this superstructure S(532) is densely packed with partly overlapping PTCDA molecules. It was chosen as third example because it is 1D commensurate with respect to the substrate and hence lies between the two examples given above. Actually it looks only like a point-on-line structure at first glance as indicated by the dashed lines in Fig.4c (point-on-line with respect to the distance of the substrate terraces). However, a closer look reveals that S(532) is unidirectionally commensurate along the steps of the substrate and its unit cell exactly coincides with the (fourfold of the) kink distances in this direction (thick blue lines of Fig.4c). Also in this case a close-up (high-resolution) view (see Fig.4b) reveals submolecular contrast (representing again the occupied LUMO) and allows a determination of the relative angles between the molecules. These are 76° for one pair and 66° for the second pair of the four molecules per unit cell (see Fig.4c); interestingly one of the molecules lies exactly parallel to the step direction. The results of this section as well as those from 12 other superstructures are summarized in Tables 1 and 2 which will be discussed in section 4.

3.3

Additional superstructures on Ag(111)

In the course of the present experiments and by closer inspection of the numerous STM images obtained we also found three new superstructures on the (111) terraces. These are minority species – they cover about 10 % of the (111) terraces - but are nevertheless 12



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discussed here because they exist although being considerably different from the commonly observed and frequently documented commensurate superstructure3, 19. The latter is labeled

in matrix notation and called S A (111) in the present work.

This known superstructure closely resembles the (102) plane of ß-PTCDA single crystals (2-3% lattice difference28) and hence represents a layer with little stress from lateral intermolecular interaction. The minority species were not observed on the common, (111)-oriented substrate surfaces indicating that they either need a substrate under tension (resulting from steps and facets) or neighboring PTCDA-covered islands on facets that may provide lateral stress due to space restrictions. It is noted that these superstructures (also) occur at ~550 K, i.e. they are thermally stable. Moreover, one of them (S C (111)) could be converted from a standard S A (111) by the influence of an STM scan with enhanced tunnel current (3 instead of 1 nA; 2 V) indicating a low activation barrier as compared to S A (111). It is further noted that these three superstructures could unambiguously be identified as commensurate to the substrate (111) face. In Fig. 5a one of the three superstructures is shown as example. The STM image consists of alternating, parallel stripes of PTCDA-covered S B (111) and S(751) superstructures. The former (darker) are 1.5 - 3 unit cells broad, while the latter (brighter) stripes consist of a double row of adsorbed PTCDA molecules. The unit cell of the S B (111) superstructure can be described by the matrix

and contains two molecules. The

molecular density is ~10 % lower than that of the common S A (111) structure. A closer 13


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look reveals that the submolecular contrast of the two molecules in the unit cell is rather similar, at least with the used tunneling parameters (2 V, 1 nA), and that the two molecules are hardly discernible. This is in contrast to the image from the S A (111) structure. Thus it is difficult to derive the azimuthal orientation of the two molecules; in fact they could have the same azimuthal orientation representative of a “brick wall” instead of a herringbone structure. Therefore, on the basis of the present data it cannot unambiguously be decided which unit cell structure is present here, but a closer inspection of the borderline between the (111) and (751) surfaces reveals that for the brick wall structure the molecular overlap would be much (i.e. unrealistically) larger than that for the herringbone structure which is a strong argument to assume also a herringbone structure here (for more details see Fig. S2 in the supporting information). Moreover, other STM images (not shown here) reveal that the S B (111) structure exactly fits to an adjacent S A (111) structure which is only possible if both are of herringbone type. The corresponding model is shown in Fig. 5b suggesting relative positions of the molecules in the S B (111) and S(751) superstructures, respectively. This model is fully consistent with the STM images and LEED data and describes a densely packed adsorbate layer with commensurate border lines between the different domains, but it does not give molecular positions relative to the substrate which could not be derived from the present data. Analogous results were obtained for the other two superstructures S C (111) and S D (111). The quantitative data and the real space models of all four superstructures are summarized in Fig. 6 (the corresponding STM images are displayed in the supporting information in Fig. S3). Figure 6 shows that all four are commensurate to the substrate 14



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and have similar molecular density. The relative azimuthal angles of the two molecules of the unit cell are quite different and vary between herringbone and brick wall. However, the latter statement is more or less a good guess derived from the STM images, the commensurability with neighboring adsorbate islands on adjacent facets and the (rather different) submolecular contrasts. All quantitative data are summarized in Table 3 which will be discussed in the next section.

4. Concluding discussion First we compare the various superstructures on vicinal faces and which facets are stabilized by these adsorbate structures. The evaluated structural parameters are summarized in Tables 1 and 2 which distinguish results from facets with inclination angles ϑ less than 21° with respect to the (111) face (Table 1) and those from facets with larger angles ϑ (Table 2). For a better overview and comparison the data of both tables are also displayed in Fig. 7. Fig. 7a gives the Miller indices of the facets stabilized by PTCDA in a stereographic projection which displays their polar angle with respect to the (111) face and their azimuthal angle. The latter is indicated by the direction of the steps ranging from [-110] to [-101]. It is noticeable that the preferred polar angle is 20° and that nearly all major step directions are present for this polar angle including those with many kinks. Fig. 7b compares the molecular densities of these superstructures to that of the common S A (111) superstructure (100%). It is noted that – with one exception – the superstructure densities on the facets with kink-free steps (“straight steps”) is lower than that of S A (111) while – with three exceptions – the density of superstructures on facets with kinks is 15


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higher. A closer inspection reveals the interesting fact that the differences in density only stem from the direction perpendicular to the borderline of the (elongated) facets while the periodicities along the borderlines are essentially equal (within 2 %). This enables commensurate borderlines between adjacent superstructures and hence optimizes the number of adsorbed molecules and hence lowers the surface free energy. In Fig. 7c the commensurability of the superstructures is compared. Again, the facets with straight steps behave differently: they adsorb superstructures that are 2D or at least 1D commensurate to the substrate surface, while on those with kinks several point-on-line or incommensurate superstructures are observed. Moreover, Fig. 7d displays sketches of the unit cells with their position relative to the step directions. Of course, the unit cells are distorted with respect to the well-known S A (111) unit cell in order to accommodate relative to the steps and substrate atoms. This is better seen in Fig. 8 which compares all new unit cells of the present work (blue) with that of the S A (111) structure (green). One can clearly see that nearly all unit cells are considerably distorted and that some are larger while others are smaller than that of S A (111). Next, we compare the minority superstructures on the (111) face with those on the facets. All three new structures are 2D commensurate and all have a lower density than S A (111). Because this leads to a higher surface free energy their existence is less favorable, and hence there must be a reason why they are formed at all. We speculate that their appearance is either provoked by a large-scale mechanical distortion of the substrate34 or by the existence of boundary conditions in the sense that defects, neighboring facets or neighboring islands force the molecules of these superstructures to rearrange such that they best fit between the boundaries and that as many molecules as possible can be 16



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adsorbed. In any case it is noticeable that such superstructures exist because they show – together with the various superstructures on the facets – that the system PTCDA/Ag has a strong stimulus to form ordered interfaces. This stimulus is the covalent bond involving the Ag 5s and 4d electrons and the frontier orbitals of PTCDA, highest occupied molecular orbital (HOMO) and LUMO, whereby the latter is pulled below the Fermi level leading to its partial or complete filling by charge transfer26-27. The interaction between LUMO and Ag electrons of course depends on the details of the adsorption site, and the spectroscopic results are indeed different for Ag(111), Ag(100), and Ag(110) 18, 20, 27, 30

. It would be interesting to see the differences in the valence band of the various

superstructures and their correlation with the parameters of Tables 1 to 3, but that would require a local spectroscopic probe with very high spatial resolution like the SMART instrument43. In general, one interesting finding of the present work is the variety of superstructures that a large organic molecule, here PTCDA, may have on a metallic substrate like Ag. The number of superstructures described here (19) which occurred on the low-index plane Ag(111) and its neighboring high-index planes is most likely not very close to the upper limit of superstructures that may occur for PTCDA on Ag in general since ordered PTCDA layers were also found on Ag(100) 29 and Ag(110) 19, 25 and will probably be observable on other high-index planes neighboring at least the (100) plane and on more possible Ag substrate planes. The surprising fact is not that different superstructures form on different substrate faces (which is known for most adsorbates since the early days of surface science) but that several “new” vicinal substrate faces which are usually thermally unstable (without adsorbate) become observable because they are stabilized by 17


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the adsorbate PTCDA. In other words, the projected surface free energy is significantly lowered by the adsorption of this molecule such that local minima of the surface free energy occur into which a “stepped” surface may develop upon adsorption. Based on our extended experience with this particular system but also on the experience with other molecules like NTCDA, phthalocyanines, oligo-thiophenes, various heterocycles, etc. as well as with other substrates (soft and hard metals, semiconductors, insulators) we assume that this is a quite common behavior for large organic adsorbate systems provided at least five pre-conditions are fulfilled: (1) the molecule is highly symmetric, (2) its bonding to the substrate is covalent but nondissociative, (3) the substrate atoms are mobile (“soft” substrate), (4) a significant lateral (intermolecular) interaction is available, and (5) diffusion of the adsorbate must be easy and must be possible over large distances. Requirement (1) enables the molecules to arrange themselves in many different configurations relative to each other in order to optimize the adsorption site and hence to minimize the surface free energy. Requirement (2) is necessary because the bonding must be strong enough (and exothermic) in order to make the bond site-specific and to provide enough energy gain such that the substrate atoms can be forced into the lowest energy positions with respect to the adsorbate hence causing large-scale reconstruction. Requirement (3) is needed because otherwise the diffusion barriers for substrate atoms would be too high to enable the observed largescale reconstructions. Requirement (4) leads to the formation of ordered islands and to densely packed, ordered borderlines between islands which help to minimize the surface free energy. And requirement (5) is helpful to allow the formation of large islands.

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Finally, we note that a similar variety of superstructures should be observable for other organic molecules provided that the above pre-conditions are fulfilled and provided that the formation of facets is supported by suitable preparation conditions, one of which could be a substrate surface orientation slightly off from a low-index plane. Thus the observations made here should be quite common also for real, i.e. polycrystalline substrates whose micro-facets could behave exactly like the facets of the present work.

Acknowledgement The authors acknowledge the preliminary work of Christian Seidel on which the here presented experiments were partly based. They like to thank Wolfgang Moritz and Ms. Wunderlich, University of Munich, for the preparation of the Ag(10 8 7) crystal.

References

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22. Gebauer, W.; Langner, A.; Schneider, M.; Sokolowski, M.; Umbach, E., Luminescence Quenching of Ordered Pi-Conjugated Molecules near a Metal Surface: Quaterthiophene and Ptcda on Ag(111). Physical Review B 2004, 69. 23. Schneider, M.; Umbach, E.; Sokolowski, M., Growth-Dependent Optical Properties of 3,4,9,10-Perylenetetracarboxylicacid-Dianhydride (Ptcda) Films on Ag(111). Chemical Physics 2006, 325, 185-192. 24. Wießner, M.; Ziroff, J.; Forster, F.; Arita, M.; Shimada, K.; Puschnig, P.; Schöll, A.; Reinert, F., Substrate-Mediated Band-Dispersion of Adsorbate Molecular States. Nat Commun 2013, 4, 1514. 25. Wiessner, M.; Hauschild, D.; Schöll, A.; Reinert, F.; Feyer, V.; Winkler, K.; Kromker, B., Electronic and Geometric Structure of the Ptcda/Ag(110) Interface Probed by Angle-Resolved Photoemission. Physical Review B 2012, 86. 26. Ziroff, J.; Forster, F.; Schöll, A.; Puschnig, P.; Reinert, F., Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: Ptcda on Silver. Physical Review Letters 2010, 104. 27. Zou, Y.; Kilian, L.; Schoell, A.; Schmidt, T.; Fink, R.; Umbach, E., Chemical Bonding of Ptcda on Ag Surfaces and the Formation of Interface States. Surface Science 2006, 600, 1240-1251. 28. Krause, B.; Dürr, A.; Schreiber, F.; Dosch, H.; Seeck, O., Late Growth Stages and Post-Growth Diffusion in Organic Epitaxy: Ptcda on Ag (111). Surface science 2004, 572, 385-395. 29. Ikonomov, J.; Bauer, O.; Sokolowski, M., Highly Ordered Thin Films of Perylene-3, 4, 9, 10-Tetracarboxylic Acid Dianhydride (Ptcda) on Ag (100). Surface Science 2008, 602, 2061-2068. 30. Kraft, A.; Temirov, R.; Henze, S. K. M.; Soubatch, S.; Rohlfing, M.; Tautz, F. S., Lateral Adsorption Geometry and Site-Specific Electronic Structure of a Large Organic Chemisorbate on a Metal Surface. Phys. Rev. B 2006, 74, 041402. 31. Tautz, F. S.; Eremtchenko, M.; Schaefer, J. A.; Sokolowski, M.; Shklover, V.; Umbach, E., Strong Electron-Phonon Coupling at a Metal/Organic Interface: Ptcda/Ag(111). Physical Review B 2002, 65, 125405. 32. Marchetto, H.; Schmidt, T.; Groh, U.; Maier, F. C.; Lévesque, P. L.; Fink, R. H.; Freund, H.-J.; Umbach, E., Direct Observation of Epitaxial Organic Film Growth: Temperature-Dependent Growth Mechanisms and Metastability. Physical Chemistry Chemical Physics 2015, 17, 29150-29160. 33. Levesque, P.; Marchetto, H.; Schmidt, T.; Maier, F. C.; Freund, H. J.; Umbach, E., Correlation between Substrate Morphology and the Initial Stages of Epitaxial Organic Growth: Ptcda/Ag(111). The Journal of Physical Chemistry C 2016, 120, 19271-19279. 34. Florian, P., et al., Nanoscale Patterning, Macroscopic Reconstruction, and Enhanced Surface Stress by Organic Adsorption on Vicinal Surfaces. New Journal of Physics 2017, 19, 013019. 35. Kilian, L.; Umbach, E.; Sokolowski, M., Molecular Beam Epitaxy of Organic Films Investigated by High Resolution Low Energy Electron Diffraction (Spa-Leed): 3, 4, 9, 10-Perylenetetracarboxylicacid-Dianhydride (Ptcda) on Ag (111). Surface Science 2004, 573, 359-378. 21


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36. Marchetto, H.; Groh, U.; Schmidt, T.; Fink, R.; Freund, H. J.; Umbach, E., Influence of Substrate Morphology on Organic Layer Growth: Ptcda on Ag(111). Chemical Physics 2006, 325, 178-184. 37. Jeong, H.-C.; Williams, E. D., Steps on Surfaces: Experiment and Theory. Surface Science Reports 1999, 34, 171-294. 38. Seidel, C. PhD thesis, Universität Stuttgart, 1997. 39. Schmitt, S.; Schöll, A.; Umbach, E., Long-Range Surface Faceting Induced by Chemisorption of Ptcda on Stepped Ag(111) Surfaces. Surface Science 2016, 643, 59-64. 40. Frohn, J.; Wolf, J. F.; Besocke, K.; Teske, M., Coarse Tip Distance Adjustment and Positioner for a Scanning Tunneling Microscope. Review of Scientific Instruments 1989, 60, 1200-1201. 41. Besocke, K., An Easily Operable Scanning Tunneling Microscope. Surf. Sci. 1987, 181, 145-153. 42. Schmitt, S. Adsorbatinduzierte Richtungsabhängige Facettierung Und Selbstorganisierte Domänen-Musterbildung Auf Vizinalen Ag(111)-Oberflächen. PhD thesis, Würzburg, 2007. 43. Wichtendahl, R.; Fink, R.; Kuhlenbeck, H.; Preikszas, D.; Rose, H.; Spehr, R.; Hartel, P.; Engel, W.; Schlögl, R.; Freund, H.-J., Smart: An Aberration-Corrected Xpeem/Leem with Energy Filter. Surface Review and Letters 1998, 5, 1249-1256.

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Figure Captions: Fig. 1 STM image of a completely facetted Ag(10 8 7) surface after adsorption of a monolayer of PTCDA at 550 K (derivative of the raw data). The bright areas are PTCDA-covered facets, which have been formed by (adsorption-induced) bundling of the steps that were equally distributed over the entire surface before adsorption. Some facets are labeled by Miller indices. The dark areas represent PTCDA-covered, step-free (111) terraces. The field of view is 390 nm; length scales are equal in both directions.

Fig. 2 a) STM image (derivative of raw data) of the PTCDA superstructure on a (552) facet after deposition of a monolayer on Ag(775) at 550 K (field of view: 40 nm). The (552) facet is the diagonal superstructure in the center of the figure which is embedded by two (873) facets in the upper left and lower right corner. b) Model of molecular arrangement in real space described by superstructure matrix (unit cell vectors in blue) together with the unit cell of substrate facet described by vectors a 1 and a 2 . The step direction [-110] is indicated by the red arrow. Note that the adsorbate unit cell contains four molecules. Area and matrix are accurate results; the relative positions and angles of the molecules with respect to each other are derived from the images, the relative positions with respect to the substrate atoms are “good guesses” derived from the step positions.

Fig. 3 a) STM image (derivative of raw data) of the PTCDA superstructure on a (321) facet after deposition of ~ 0.2 ML PTCDA at 550 K (field of view: 50 nm). Only the (321) facets are PTCDA-covered; the remaining uncovered area consists of (111) terraces 23


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crossed by single atomic steps. b) High resolution image with submolecular contrast (field of view: 7 nm). c) STM image of the (321) facet obtained with slightly different tunneling parameters as compared with a) showing a Moiré pattern (field of view: 46 nm). d) Model of real space described by superstructure matrix (unit cell vectors in blue) together with the unit cell of the substrate facet described by vectors a 1 and a 2 . The step direction [-211] is indicated by the red arrow. e) Close-up of the two molecules in the unit cell showing their relative angle and molecular overlap, as derived from the highresolution image b). f) Model of the origin of the Moiré pattern. As in Fig. 2 area and matrix are accurate results; relative positions and angles of the molecules with respect to each other are derived from the images; relative positions with respect to the substrate atoms are “good guesses” derived from the step positions.

Fig. 4 a) STM image (derivative of raw data) of the PTCDA superstructure on a (532) facet after deposition of ~ 0.2 ML PTCDA at 550 K (field of view: 36 nm). Only the (532) facet is PTCDA covered; the remaining uncovered area consists of (111) terraces crossed by single atomic steps. b) High resolution image with submolecular contrast (field of view: 18 nm). c) Model of molecular arrangement in real space described by superstructure matrix (unit cell vectors in blue) together with the unit cell of substrate facet described by vectors a 1 and a 2 . The step direction [-321] is indicated by the red arrow. Note that the adsorbate unit cell contains four molecules. Area and matrix are accurate results; the relative positions and angles of the molecules with respect to each other are derived from the images, the relative positions with respect to the substrate atoms are “good guesses” derived from the step positions. 24



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Fig 5 a) STM image (derivative of raw data) of the PTCDA S B (111) superstructure on (111) terraces (dark) which are separated by molecular double chains on (751) facets (bright) after deposition of a monolayer on Ag(775) at 550 K (field of view: 35 nm). The widths of the (111) terraces are correlated with the diagonal D of the adsorbate unit cell as indicated. b) Model of molecular arrangement in real space described by superstructure matrix (unit cell vectors in blue). The molecules on the (751) facet are also shown together with the unit cell of this facet described by vectors a 1 and a 2 . The step direction [-321] is indicated by the red arrow. The relative positions and angles of the molecules with respect to each other are derived from the images, the relative positions with respect to the substrate atoms are “good guesses” derived from the step positions (see also discussion in the text).

Fig. 6 Models of the real space molecular arrangement of the four superstructures on (111) terraces S X (111) described by their superstructure matrices (unit cell vectors in different colors). These superstructures were either derived by deposition at 550 K (S A (111), S B (111) and S D (111)) or by annealing of a multilayer sample at 550 K (S A (111) and S C (111)). The corresponding STM images are shown in Fig. 5a and in Fig. S3 of the supporting information, respectively. S A (111) is the well-known majority superstructure while the other S X (111) structures are minority species. Their unit cells are graphically compared to that of S A (111); their areas are compared to that of the substrate unit cell (uc) as integer numbers. Interestingly, structure S B (111) has the diagonal in common with the majority structure S A (111) while S C (111) and S D (111) each have one 25


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superstructure vector in common with S A (111). Note that the areas and matrices are accurate results; the relative positions and angles of the molecules with respect to each other are derived from images, while the relative positions with respect to the substrate atoms are only “good guesses”.

Fig. 7 Collection and correlation of the observed facets and data of the corresponding PTCDA superstructures in stereographic projection. The polar angle refers to the normal of the (111) face, the rays belong to selected azimuthal angles and are related to certain step directions, two of which are indicated at the end of the limiting rays ([-110], [-101]). a) The observed and by PTCDA stabilized facets are given by their Miller indices. Three examples of steps with different kink densities are also indicated by “ball models”. b) The sizes of the corresponding adsorbate unit cell areas are given in percent with respect to the common S A (111) superstructure (:= 100%). c) The commensurabilities of the superstructures are indicated by “inc” (incommensurate), “pol” (point on line), “1D” and “2D” (one – and two dimensional commensurate). “pol” refers to the distances of steps, 1D refers to the distances of kinks. d) Positions of the adsorbate unit cells with respect to the step directions; the number of rhombs (elementary cells) times two gives the number of molecules per adsorbate unit cell.

Fig. 8 Comparison of the unit cells of the various superstructures (blue) with that of S A (111) (green). One can easily see the relative distortion of the unit cells as well as their commensurability which is also indicated under each graph.

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Table 1 Data from those superstructures that are found on facets with an inclination angle ϑ less than 21° compared to those of the well-known PTCDA/Ag(111) superstructure (bottom line). The first column contains the notation of the superstructure S(xxx) on the corresponding facet (xxx). The second column gives the lengths of the elementary cell vectors of the superstructure in A (with the “best-guess” angle between the molecules in the elementary cell). The third column contains the area of the elementary cell (in A2) together with its size relative to that of the S A (111) structure (:= 100%). In the fourth column the superstructure is given in matrix notation referred to the unit cell of the high index facet. All values of lengths have an error bar of 2 %, while those of areas have 4 %. Some values, in particular those from commensurate structures and those from superstructures that neighbor S A (111) structures, are much more accurate (

Multitude of PTCDA Superstructures on Ag(111 ... - ACS Publications

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