Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride

Subscriber access provided by WEBSTER UNIV

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride (PTCDA) on Bi Terminated Silicon Thomas Schmidt, Christian Ahrens, Jan Ingo Flege, Cherno Jaye, Daniel A. Fischer, and Jens Falta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10396 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride (PTCDA) on Bi Terminated Silicon Thomas Schmidt,∗,† Christian Ahrens,† J. Ingo Flege,† Cherno Jaye,‡ Daniel A. Fischer,‡ and Jens Falta†,¶ †Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany ‡National Institute of Standards and Technology, Gaithersburg, MD 20899, USA ¶MAPEX Center for Materials and Processes, University of Bremen, Bremen, Germany E-mail: [email protected]

Abstract

ous presence of multiple domains, the individual domains show excellent lateral ordering, with larger domain sizes as compared to the case of Agterminated Si(111).

The epitaxial quality of thin films crucially depends on their interaction with the substrate. Up to now, Ag-terminated Si(111) has been employed as the model substrate for the growth of PTCDA on semiconductors. In this study, we will show that Bi termination results in PTCDA films of superior epitaxial quality. We have studied the growth of PTCDA on bismuth passivated Si(111) in detail by means of spot profile analysis low-energy electron diffraction (SPA-LEED), x-ray photoelectron spectroscopy (XPS), near-edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The XPS results reveal the presence of intact PTCDA molecules on the surface upon adsorption. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. STM shows a very smooth growth front of the PTCDA film preserving the step structure of the substrate. High resolution SPA-LEED data demonstrate the presence of a multi-domain surface with a rich variety of PTCDA surface structures, which were identified to be most prominently herring-bone poly-types. However, in the monolayer range, also quadratic brick-wall structures and a nearly square-like structure have been found as well as a perylene-like structure. Despite the simultane-

Introduction The integration of organic thin films into Si technology has great potential for application, e.g. as solar cells 1 , light-emitting devices 2,3 , or infrared sensors 4,5 . Among the vast variety of organic materials, the molecule of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and its relatives, e.g. 3,4,9,10-perylene tetracarboxylic diimide (PTCDI), have developed into model systems of organic film epitaxy. Hence, today PTCDA is one of the most intensely studied materials. Highly ordered films of PTCDA have been achieved on the surfaces of noble metals, mostly on Ag(111). 6,7 Even multi-layered structures of organic films have been realized successfully on metal substrates. 7–12 All these substrates provide interfaces with weak interaction between the organic film and the substrate. Consequently, the growth results in locally undisturbed PTCDA domains, as can be seen from several STM investigations. 13–15 Lateral ordering over larger length

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

small domains. 31 In the submonolayer regime, a Volmer-Weber growth mode of dendritic PTCDA islands is reported. 33 The epitaxial relation of the PTCDA islands seems to depend on the exact conditions of PTCDA deposition. 33,36 In addition, surface steps are reported to act as centers for the nucleation process. 35 H passivation has also been investigated with STM for the growth of PTCDA on Ge(001), 37,38 and similar results with respect to H:Si(001) are found for this system: Again, a Volmer-Weber like growth mode was identified and islands of different registry to the substrate are visible in the STM images. The use of hydrogen as surface passivant has the advantage of being compatible with existing Si technology. However, the quality of the films achieved by H passivation is limited and hence other passivants come into the focus of interest. For growth of PTCDA on Si(111), various metals, like Sn 39–41 and Pb 42 have been tested. So far, however, Ag has received most attention, since the Ag-terminated Si(111) surface has allowed for growing the best films on silicon, 43–47 up to now. (Also, related systems such as PTCDI on silicon have been studied. 48,49 ) Such PTCDA films, however, exhibit structural limitations. For Ag passivation, STM reveals the presence of two distinct phases at sub-monolayer PTCDA coverages, i.e. a square and a herring-bone-like phase. 43 Chemical shifts of the C 1s and O 1s peaks point to charge transfer from the film to the substrate. 43 For Pb 42 and Sn 41 passivation, the reported STM data show smaller domains of PTCDA, and XPS and NEXAFS reveal strong interaction at the interface. In the case of Sn even one-dimensional growth of PTCDA has been reported. 50 In this study, we have tested Bi for its superior suitability in passivant enhanced growth of PTCDA on Si(111). In addition to the analytic tools in earlier studies of PTCDA growth, we have used high-resolution LEED, i.e. SPA-LEED for a detailed structural analysis of the PTCDA films. The results clearly show that Bi passivation leads to far better ordered PTCDA films.

scales however is very limited as shown recently by spot profile analysis of low-energy electron diffraction (SPA-LEED). 16 Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, shows a good vertical ordering of the PTCDA. In most cases flat-lying PTCDA molecules are found at the interface. Also, an impact of steps of vicinal Ag surfaces on the growth mode has been reported, as well as faceting induced by PTCDA growth on these. 11 Less than for metal substrates, however, is known about the physics of organic thin films grown on inorganic semiconducting surfaces. This is despite the fact that semiconducting organic thin films grown on inorganic semiconducting surfaces would allow for a large variety hybrid devices with high-potential applications, most importantly, if compatible with the well established technology of silicon devices. 2,3,17,18 For unpassivated semiconductor substrates like Si(001), a disordered growth mode is found, with no well-aligned orientation of the PTCDA molecules an no ordering with respect to the substrate. 19 Other authors 20,21 speak of an initial PTCDA layer with different adsorption sites and a disordered interface layer. A recent study elaborates four distinct adsorption sites and points out the importance of substrate-anhydride interaction. 22 The reason for this disorder is the presence of unsaturated open bonds at the surface of the unpassivated Silicon substrate. In the course of adsorption and thermal equalization, different molecules arrive at different adsorption sites and, simplified, stick as they arrive—in different orientations. Hence, no ordered adsorption is found. A possible pathway around this problem is the passivation of the semiconductor surface prior to PTCDA growth in an approach similar to surfactant mediated growth of semiconductors 23–26 or the use of interfactants in the growth of epitaxial high-k gate dielectrics on Si(111) 27,28 . This approach has been followed by many groups for various passivants. By H passivation of Si(001), 29,30 the growth of PTCDA nanocrystals of very good structural quality could be found at sub-monolayer coverages. H passivation of Si(111) has been most intensely studied within this field. 31–35 The structural quality of these films, however, has remained rather limited so far, as the film forms 2

ACS Paragon Plus Environment

Page 3 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental

acquisition and processing the open-source Linux based software package GXSM2 was used. 55 In order to reliably calibrate the STM tips, scans on Si(111)-7×7 were performed. The SPA-LEED measurements were performed using a SPA-LEED system 56 by Omicron NanoTechnology GmbH. The NEXAFS experiments were conducted at the NIST U7A soft X-ray beamline at the National Synchrotron Light Source at Brookhaven National Laboratory. NEXAFS spectra were acquired at x-ray beam angles of incidence between 0° and 70° with respect to the surface normal. To eliminate the effects of beam intensity fluctuations and upstream beamline optics features, the acquired partial electron yield signals have been normalized by the incident beam intensity obtained from the photoemission yield of a freshly prepared Au grid located along the beam path. A toroidal spherical grating monochromator with 600 lines/mm enabled the acquisition of C Kedge spectra with an energy resolution of 0.1 eV. The spectra presented have then been pre- and post-edge normalized to unity. A carbon mesh, also located along the beam path, was used for energy calibration of each individual C K-edge spectrum using the π ∗ transition of carbon at 285.0 eV.

Similar to previously reported results on PTCDA deposition on GaN(0001), 16 the experiments were carried out with two different ultra-high vacuum (UHV) systems with base pressures of 2×10−10 mbar, each. One system is equipped with XPS, LEED, and STM; the other one provides access to SPA-LEED, i.e., high-resolution highsensitivity low-energy electron diffraction. As substrates, we used commercial Si-wafers in (111) orientation with a miscut of 0.2°. The sample size reached up to 10×10 mm2 , depending on the ultra-high vacuum (UHV) system in use. Prior to loading, the samples were cleaned with methanol. After introduction into UHV, the samples were degassed at 600 °C for at least 12 hours. The native oxide layer was removed by short annealing at 1200 °C for about 10 s resulting in a sharp Si(111)-7×7 LEED pattern. Sample heating was achieved by direct current, and temperatures were monitored using an infra-red pyrometer. After preparation of the Si(111)-7×7 surface, Bi was deposited from a Knudsen cell at a substrate temperature of 480 °C for 10 min, sufficiently long to achieve the saturation coverage of 2/3 monolayers (ML) of Bi. 51 Commercial PTCDA was purified twice by gradient sublimation and then deposited from a home-built Knudsen cell onto the Bi-passivated Si(111) surfaces at room temperature. X-ray photoemission spectroscopy (XPS) was used for a chemical analysis of the surface after this procedure. For all XPS investigations, Al Kα radiation was used, in conjunction with an Omicron EA125 electron analyzer equipped with a seven-channeltron parallel detection unit. Peak fitting was done including a Tougaard-shaped background 52 with Voigt profiles 53 , the Lorentz width of which was fixed to 0.15 eV and 0.11 eV for O 1s and C 1s, respectively 54 . The binding energy was calibrated with the Mo 3d5/2 emission from the sample holder, which was recorded prior to each XPS experiment. The STM investigations were carried out at room temperature, in a variable-temperature UHV STM made by Omicron NanoTechnology GmbH. The measurements were performed with chemically etched tungsten tips. Tunneling conditions are provided in the respective figure captions. For data

Interaction In order to investigate the interaction of the PTCDA molecules with the substrate and with neighboring PTCDA molecules, we have performed x-ray photoemission spectroscopy (XPS) measurements as well as varying-angle near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. Upon deposition of PTCDA on Biterminated Si(111) at room temperature, XPS reveals the adsorption of intact PTCDA molecules on the surface. Fig. 1(a) and (b) show the C 1s and O 1s photoemission spectra after deposition of 10 ML PTCDA, respectively. The integral peak ratio of the C 1s contributions shows a ratio of carbonyl to aryl C-atoms of 1:(5.1±0.3) in accordance with the intact PTCDA molecule, see Fig. 2. Similarly, the integral peak ratio of the O 1s contributions as shown in Fig. 1(b) reveal a ratio of carbonylic to anhydride (bridging) oxygen atoms of 1:(2.0±0.1), again in accordance with intact 3



H

C1s

(a) Intensity (arb. units)

C 9.2

H C

O

Caryl !-!*C=C(1) Ccarbonyl !-!*C=O !-!*C=C(2)

C

H

O C

C H

C

C

C

C

O C

C C

C

C

C

C

C

H C

C C

O

H

C

C

O

C

C

H

O H

14.2 290 288 286 Binding energy (eV)

284

(b)

540

Figure 2: Structure of the PTCDA molecule. Carbonyl C atoms are marked red, aryl C atoms marked black; bridging oxygen atoms are marked green, carbonylic oxygen is marked blue.

282

O1s Ocarbonyl Oanhydride !-!*C=O !-!*C-O-C

538

C K edge (a)

536 534 532 Binding energy (eV)

530

528

1 2

3

Intensity

292

Intensity (arb. units)

294

Intensity (arb. units)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

4

.8 (b) .6 2 .4 1 .2 0 0 .2 .4 .6 .8 1 sin2(α)

70o z

60o 40

Figure 1: X-ray photoemission spectra of (a) C 1s and (b) O 1s core level after deposition of 10 ML PTCDA. The contributions to the differently coordinated carbon and oxygen atoms are indicated by different colors in the fitted data. The measured peak ratios are well in agreement with the adsorption of intact PTCDA molecules on the surface.

o

hν

y

x

α

20o 0

E

o

280

285

290 295 Photon energy (eV)

300

Figure 3: (a) NEXAFS spectra of the carbon K egde of 10 ML PTCDA deposited on Bi:Si(111)√ √ 3× 3-R30° for five angles of incidence (shifted, for clarity), where 0° indicates normal incidence to the surface. Peaks 1 and 2 correspond to π ∗ resonances of the perylene core, peak 3 originates from a π ∗ resonance from anhydride C, and peak 4 represents σ ∗ transitions. (b) Integral intensity of peak 1 (dots) and peak 2 (squares), respectively, as a function of sin2 (α), together with a fit (solid lines) according to Eq. 14. The data points for α = 70◦ were neglected for the fit.

PTCDA molecules. The orientation of the molecules with respect to the surface can be determined by NEXAFS experiments measuring the angular dependence of the intensity of the corresponding electronic transition. NEXAFS samples were prepared at the laboratory in Bremen, shipped to Brookhaven and reintroduced to vacuum for the NEXAFS experiments. Figure 3(a) shows the NEXAFS signal of the carbon K edge for five different angles of incidence. While the intensity from the σ ∗ resonance [labeled “4” in Fig. 3(a)] is more or less independent of the incident angle, we find a clear angu-

lar dependence of the π ∗ resonances between 285 and 290 eV photon energy (labeled “1” to “3”), i.e. the resonance of the delocalized π electrons of 4


Page 5 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the molecule. 57 For PTCDA, the transition dipole moments of these orbitals are perpendicular to its molecular plane. The largest π ∗ signal is to be expected for the electrical field vector parallel to the dipole moment of the transition. 58 Hence, the first inspection of our data points to rather flat-lying molecules. For a more quantitative analysis, as described in the following, a limited degree of polarization of the x-ray beam has to be taken into account as well as a partial disorder of the molecular film, mainly arising from surface contamination during the sample transfer process. If we chose a coordinate system as indicated in the inset of Fig. 3 we can write the vector of the electrical field’s amplitude as   Ek cos α ~ =  E⊥  , (1) E Ek sin α

same direction, therefore Jk,⊥

0

p(ϕ, ϑ) =

(2)

with 1 2 sin ϑ0 cos2 α + cos2 ϑ0 sin2 α (11) 2 1 = sin2 ϑ0 (12) 2 1 (13) = J⊥rand. = . 3

Jksharp =

~ , the absorption signal I is proportional to (~µ · E) 59 hence, ¡ ¢ I ∝ µ2 E 2 · P Jk + (1 − P )J⊥ , (4)

J⊥sharp Jkrand.

with 59 µ ˆ · Eˆk

(8)

Then, Eq. (4) becomes o n sharp rand. I ∝ P · f Jk + (1−f )Jk n o + (1−P ) · f J⊥sharp + (1−f )J⊥rand. (10)

2

Jk =

1 p(ϑ) . 2π

For the interpretation of our NEXAFS data, we assumed a fraction f of molecules with a welldefined polar angle ϑ0 , i. e. a sharp distribution, in coexistence with a fraction (1−f ) of randomly arranged molecules, reflecting partial disorder, e. g. due to surface contamination during sample transfer: ( 1 δ(ϑ−ϑ0 ) (sharp) sin ϑ0 . (9) p(ϕ, ϑ) = 2π 1 (random) 4π

which in our case is P = 0.85. For a given transition dipole moment (TDM), which can be written in spherical coordinates as   sin ϑ cos ϕ µ ~ = µ  sin ϑ sin ϕ  , (3) cos ϑ

³

0

with p(ϕ, ϑ) being the directional distribution of these TDMs. Due to the threefold rotational symmetry of our substrates, the azimuthal part of p(ϕ, ϑ) can be considered isotropic:

where the transverse component E⊥ is small compared to the in-plane component Ek , as expressed by the degree of polarization P = Ek2 /(Ek2 + E⊥2 ) = Ek2 /E 2 ,

Z2π Zπ ³ ´2 = dϕ sin ϑdϑ p(ϕ, ϑ) · µ ˆ · Eˆk,⊥ , (7)

This finally yields

´2

½

2

= (sin ϑ cos ϕ cos α + cos ϑ sin α)

f 1−f + · sin2 ϑ0 I = Aµ · 3 2 ¶ ¾ µ 3 2 2 + f P 1 − sin ϑ0 sin α 2 2

(5)

and 59 ³ ´2 J⊥ = µ ˆ · Eˆ⊥ = (sin ϑ sin ϕ)2 .

(6)

(14)

with an unknown constant A. The integral intensities of the most prominent peaks labelled “1” and “2” have been extracted from the NEXAFS data shown in Fig. 3(a). As

In the ensemble of molecules on the surface, not all TDMs of a given resonance may point in the 5



expected from Eq. (14), a linear increase with sin2 α is observed, as shown in Fig. 3(b). This has been fitted to Eq. (14 using A, f, and ϑ0 as free parameters. The best match is found for f = 0.75 ± 0.05 and ϑ0 = 10◦ ± 5◦ , corroborating that most molecules are lying (almost) flat on the surface. The limited ordered fraction f is attributed to disorder mainly arising from contamination during the sample shipment under ambient conditions. As we will show in the following √ sections, √ the deposition of PTCDA on Bi:Si(111)- 3× 3R30° leads to the formation of well-ordered epitaxial PTCDA films with a manifold of different reconstructions and rotational domains. This finding already points to the presence of a weak interaction between the PTCDA molecules and the surface, balanced by a similarly strong interaction between the PTCDA molecules themselves.

289.0 eV, as observed here, is expected if a COOR group is attached to an aromatic system. The binding energy that we determined also fits well to experiments of Gustafsson et al. 63 A distinction of the binding energies of the different carbon atoms in the perylene core is beyond the resolution of the apparatus used in our experiments. However, beside the peaks assigned already, we observe the occurrence of three more carbon related peaks. The signal at 287.3 eV is a shakeup peak of a HOMO-LUMO transition related to the C=C double bond 60 . The peak at 291.8 eV is related to a π-π ∗ shake-up process 61 of the aromatic ring and the peak at 290.0 eV stems from a shake-up of a π-π ∗ transition of the carbonyl group 60 . For the determination of the abundance of carbon species we have also accounted for the ∗ ∗ shake-up peaks: (Caryl +Cπ−πC=C(1) +Cπ−πC=C(2) ): ∗ (Ccarbonyl + Cπ−πC=O ) = 5.1 ± 0.3. This result confirms the presence of intact PTCDA molecules, which contain 20 aryl C atoms and 4 carbonyl C atoms (see Fig. 2).

Table 1: XPS binding energies (EB ) and relative integral intensities (Irel ) of the C 1s peak contributions for 10 ML PTCDA [cf. Fig. 1(a)]. EB (eV)

Irel

Caryl

285.1

0.75

π-π ∗ C=C(1)

287.3

0.04

291.8

0.05

Ccarbonyl

288.8

0.09

π-π ∗ C=O

290.0

0.08

π-π

∗

C=C(2)

Page 6 of 20

Table 2: XPS binding energies (EB ) and relative integral intensities (Irel ) of the O 1s peak contributions for 10 ML PTCDA [cf. Fig. 1(b)].

The presence of such interactions should lead to modified transitions of those parts of the system that are responsible for the interaction. Changes of the binding energy, however, should be visible in a chemical shift of the corresponding molecules. In order to distinguish between interface and inner film effects, the position of the peaks in the XPS spectra was carefully inspected as a function of the PTCDA film thickness. From the data displayed in Fig. 1(a) and (b) we determined the peak positions for the different C 1s and O 1s contributions as given in Tab. 1 and Tab. 2, respectively. These values are well in agreement with those reported in the literature 19,43,60,61 . The C 1s emission lines on anhydride groups in aliphatic bonding is expected 62 between 289.4 eV and 289.5 eV. A smaller binding energy between 288.7 eV and

EB (eV)

Irel

Ocarbonyl

531.9

0.52

π-π ∗ C=O

534.3

0.15

Oanhydride

533.7

0.30

π − π ∗ C−O−C

535.7

0.03

The oxygen spectra show two significant contributions. At a binding energy of 531.9 eV, the contribution from the oxygen atoms in the carbonyl group is found. Interestingly, this value lies between those found in the literature 62 for aromatically bonded COOR groups (531.6 to 531.7 eV) and those for anhydride groups in an aliphatic neighborhood (532.5 to 532.9 eV). Emission of bridging oxygen is found at 533.7 eV in agreement with the literature, giving a first hint on the presence of interaction of the carbonyl oxygen with the substrate. At 534.3 eV an intense shake-up of the HOMO-LUMO transition of the carbonyl oxygen is visible 60 , and, at 535.7 eV, the HOMO-LUMO transition of the bridging oxygen can be identi6


Page 7 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fied 61 . In Tab. 2 we summarize the O 1s binding energies and relative integral intensities. For a further check on the stoichiometry of the adsorbed PTCDA molecules, we calculated the intensity relation of anhydride to carbonyl oxygen, ∗ analogously to carbon: (Ocarbonyl + Oπ−πC=O ) : ∗ (Oanhydride + Oπ−πC−O−C ) = 2.0 ± 0.1. Again, the result is in agreement with a non-dissociative adsorption of PTCDA molecules. In order to further investigate possible interaction of the PTCDA molecules with the substrate, we measured the peak position for decreasing PTCDA film thicknesses, therefore increasing the share of interacting molecules as this interaction will be most pronounced right at the interface of the film and substrate. The results (not shown here) indicate that within the sensitivity of our experiments (about ±0.1 eV), we cannot identify a clear coverage dependence of the individual O 1s and C 1s components; instead, peak positions scatter unsystematically within ±0.1 eV. Hence, we conclude that the interaction of the PTCDA molecules with the Bi-terminated Si surface is as weak or even weaker than with Ag-passivated Si(111) substrates, where a coverage dependence has been observed and chemical shifts of more than 0.4 eV have been reported for PTCDA coverages up to one monolayer. 43

(a)

0.5 Å–1

(b)

30 eV

(c)

0.5 Å–1

0.5 Å–1

30 eV

(d)

34 eV

0.5 Å–1

34 eV

Figure 4: Diffraction patterns measured by SPALEED after surface termination and subsequent √ √ deposition of PTCDA. (a) Ag:Si(111)-√3×√3R30°, (b) 1 ML PTCDA √ on√Ag:Si(111)- 3× 3R30°, (c) Bi:Si(111)-√ 3×√ 3-R30°, (d) 1 ML PTCDA on Bi:Si(111)- 3× 3-R30°. Si(111) on the one hand, and PTCDA on Biterminated Si(111) on the other hand. As a general and prominent feature, however, the peaks for PTCDA on Bi-terminated Si are much sharper than for PTCDA on Ag-terminated Si, giving clear evidence for drastically larger domain sizes in the former case. In the following, we will concentrate on the Bi case. Figure 5 shows three close-up views of the reciprocal space region displayed in Fig. 4(d). The LEED pattern has been analyzed by geometric LEED simulations, accounting for the spot positions and forbidden reflections due to symmetry constraints of the super cell. In order to account for the majority of spots visible in LEED, ten distinct superstructures are required as given in Tab. 3. Herring-bone (HB) and nearly square-like (NS) structures could also be found using STM; for details see below. We now turn to the diffraction pattern and the origin of its different diffraction spots. The LEED pattern in Fig. 4(d) shows hexagonal symmetry. The most intense superstructure spots are arranged in clusters of five spots, each organized

Structure Before going into details on the structure of the PTCDA films grown on Bi-terminated Si(111), we like to compare with growth on Ag-terminated Si(111). Fig. 4(a) and (c) shows the diffraction pattern recorded by SPA-LEED for the Si(111)7×7 surface after Ag and Bi termination, respectively. √Both √ of these starting surfaces exhibit a sharp 3× 3-R30° diffraction pattern. √ √ The firstorder superstructure spots of the 3× 3-R30° reconstruction appear near the border of the scan range in Fig. 4(a) and (c). Upon deposition of 1 ML PTCDA, these reflections remain visible, while a large number of additional superstructure spots appear in both cases, as can be seen from Fig. 4(b) and (d), respectively. Hence, we find a rich manifold of different reconstructions for both growth systems, PTCDA on Ag-terminated 7



Page 8 of 20

identified by SPA-LEED measurements for deposition of 1 ML PTCDA Table 3: Superstructures √ √ on Bi:Si(111)- 3× 3-R30°. b~1 and b~2 define the unit cell in real space, β is the angle between b~1 and b~2 , and α is the angle between b~1 and a~1 , the unit vector of the substrate. HB denotes a herringbone structure, BW refers to brick-wall (BWM for modified brick-wall), PS abbreviates perylene structure, and NS refers to nearly square-like structure. Structure HB1

HB2

HB3

HB4

HB5

HB6

BW

BWM

PS

NS

matrix Ã ! 1.58 −1.84 1.89 1.80 Ã ! 2.97 0.16 0.99 2.18 Ã ! 1.32 −0.81 2.65 3.14 Ã ! 1.54 −0.53 2.34 3.31 Ã ! 1.58 −0.44 2.25 3.31 Ã ! 1.68 −0.35 2.12 3.34 Ã ! 0.50 −1.43 1.92 1.43 Ã ! 1.59 −0.64 1.58 2.23 ! Ã 0.29 −2.22 2.04 2.19 ! Ã 1.76 −0.87 1.83

94.0

˚ b1 (A)

˚ b2 (A)

β(deg)

α(deg)

20.2 ± 0.2 12.7 ± 0.1

90.0 ± 0.5

57.4 ± 0.5

19.8 ± 0.2 12.9 ± 0.1

90.0 ± 0.5

92.6 ± 0.5

20.0 ± 0.2 12.7 ± 0.1

90.0 ± 0.5

22.3 ± 0.5

20.2 ± 0.2 12.7 ± 0.1

90.0 ± 0.5

14.2 ± 0.5

20.2 ± 0.2 12.6 ± 0.1

90.0 ± 0.5

12.0 ± 0.5

20.2 ± 0.2 12.8 ± 0.1

90.0 ± 0.5

9.3 ± 0.5

11.9 ± 0.1 11.9 ± 0.1

91.5 ± 0.5

45.7 ± 0.5

13.6 ± 0.1 13.6 ± 0.1

92.4 ± 0.5

76.2 ± 0.5

14.5 ± 0.1 16.2 ± 0.1 117.5 ± 0.5 63.3 ± 0.5

14.2 ± 0.2 13.7 ± 0.2


94.0 ± 0.5

11.0 ± 0.5

Page 9 of 20

(a)

(a)

0.25 Å–1

34 eV

(b)

(c)

(b) 0.05 Å–1

34 eV

0.10 Å–1

34 eV

0.05

0.10

(d) Intensity (arb. u.)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


–0.10

–0.05

0 ∆kll (Å–1)

0.5 A˚ -1

Figure 6: (a) Comparison of calculated peak positions of the herring-bone structures HB1 to HB6 with the measured SPA-LEED pattern; (b) highlighted peaks (red) of the measured LEED pattern that cannot be explained by diffraction from HB1 to HB6.

Figure 5: (a) SPA-LEED zoom to the LEED pattern of Fig. 4(d), i.e. after deposition of 1 ML PTCDA. The peak positions of diffraction contributions from the different herring-bone structures HB1 to HB6 are marked by colored spots; (b) and (c) zoom to the region marked by the violet rectangle and by the turquois square in (a), respectively. (d) Line scans as indicated by the dashed lines in (b) and (c).

do not vanish as expected for a structure with p2gg symmetry. Hence, one can conclude that the angles in the superstructure are not perfectly rectangular but differ slightly from 90°, lifting the p2gg symmetry, which may be attributed to the interaction with the substrate. In addition to the main herring-bone structure (HB1) five more herringbone structures, each with slightly different structural parameters, were observed in the LEED pattern. Their corresponding peaks are marked in different colors in Fig. 5(a). The spots of HB2 (red) and HB3 (brown) are easily visible. The spots of HB4 (light blue) and HB5 (yellow), however, over-

on a triangular lattice. These spots can be explained by the presence of different herring-bone (HB) structures. The length of the lattice vectors determined from the LEED pattern points to a slightly expanded unit cell in comparison 64 to ˚ 2 and β: bulk α and β PTCDA (α: 19.98×11.96 A ˚ 2 ). When looking at the zoomed 19.30×12.45 A LEED pattern in Fig. 5(a), it becomes quite obvious that the (10) spots of the HB1 structure (see green spot at the bottom of the displayed region) 9



lap and appear as elongated spots in larger-area LEED scans. Only a high-resolution zoom as provided in Fig. 5(b) clearly reveals the occurrence of the spots of these structures, best to be seen from the profile shown in Fig. 5(d) (upper curve). The spots of structure HB6 (black) are very weak and become visible only as shoulders of the HB1 spots, as marked with arrows in Fig. 5(d). Within the accuracy of our measurements, the different HB structures show identical size but different orientation to the substrate. Using the as-determined structural parameters for a simulation of the LEED pattern to be expected from all HB structures, a pattern is obtained as given in Fig. 6(a). As can be seen, most spots are explained by the HB structures. An increasing shift between calculated and measured spots towards larger K-values is caused by a distortion of the SPA-LEED pattern due to non-linearities of the instrument. Despite the good agreement between LEED and the simulated pattern, still a significant number of spots cannot be explained by HB1 to HB6, most of these spots being rather weak. In order to highlight these spots, we have marked them red in Fig. 6(b). Many of these spots, however, can be explained by diffraction from a brick-wall structure (BW) also present on the surface. A BW structure so far has only been reported for adsorption of PTCDA on Ag(110). 15 For this substrate, Glöckner et al. report a BW structure with ˚ 2 , identical with the case a unit cell of 11.9×11.9 A presented in this study (see Tab. 3). Combining our SPA-LEED data with the STM analysis by Glöckner et al. 15 we could derive the structural model for the BW structure as presented in Fig. 7(b). The corresponding LEED spot positions are marked by the red arrows in Fig. 7(a). Looking at the intra-molecular charge distribution of the PTCDA, the brick-wall structure shows negatively charged molecular areas, i.e. the carbonyl end group, that are facing each other. As this is energetically unfavorable, this leads to the conclusion that for the BW structures, moleculesubstrate interaction must be of higher importance than for the HB structures where the negatively charged carbonyl end groups face positively charged molecular areas, leading to binding contributions. Vice versa, molecule-molecule interac-

Page 10 of 20

(a)

˚ -1 0.5 A

(b) (d)

(c)

Figure 7: (a) LEED pattern with reciprocal unit vectors of the brick-wall (BW, red) and modified brick-wall (BWM, green) structure, respectively; (b) model of the BW structure; (c) LEED pattern with reciprocal unit vectors (yellow arrows) and spot location (yellow rectangles) of the perylene structure (PS); (d) model of the PS structure. tion must be of less importance for energetic balance of the BW structure. 10


Page 11 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecular head groups. The tendency of the system to form either the BW or the BWM structure could be related to the interaction strength of the molecules with the substrate, which might be modified by the local Bi coverage. (Though for the preparation conditions used in our experiments the formation of the β-phase of Bi:Si(111) is expected, coexisting domains of the cannot √ √ α-phase be excluded, since both show a 3× 3-R30° superstructure. 51,65–67 ) In addition to the diffraction spots discussed so far, another structure can be identified from the LEED pattern. The position of corresponding spots is marked in yellow in Fig. 7(c), their relatively low intensity can be seen in the open rectangles of the same figure. Our notation (PS) stems from a similar structure known from the adsorption of perylene on Au(111). 68 On the basis of this structure, the structural model given in Fig. 7(d) has been developed. While for perylene a headto-head geometry is possible due to its homogeneous charge distribution, this geometry is rather unlikely for PTCDA as it would result in a strong repulsion, as laid out above. We therefore assume rotated molecules, thus avoiding a repulsive geometry. Using all structures discussed so far, we derive the simulated diffraction pattern as displayed in Fig. 8(a) and compared to the experimental LEED data. The vast majority of the experimental peaks is also found in the simulation. However, a few weak spots remain unexplained at this point. As before, those spots are marked by red dots in Fig. 8(b). This issue is revisited at the end of the discussion of the STM results. We now turn to the coverage dependence of the LEED pattern. Figure 9 shows the development of the experimental LEED pattern with progressive PTCDA deposit. Upon initial PTCDA deposition (0.3 ML), only the LEED spots of the HB1 structure are visible. With increasing coverage (0.5 ML), the spots of the other HB structures weakly appear. Already at this coverage with little molecule-molecule interaction, the (10) and (01) spots of the HB1 structure can be found at their later position, pointing at substrate-molecule interaction being responsible for the distortion of the unit cell. After deposition of 1.0 ML, all HB structures are well visible. The analysis of the size

(a)

(b)

0.5 A˚ -1

Figure 8: (a) Comparison of calculated peak positions of the HB, BW, BWM, and PS structures with the measured SPA-LEED pattern; (b) highlighted peaks (red) of the measured LEED pattern that cannot be explained by the structures considered so far. A further close look at the diffraction pattern shows the presence of a second BW like structure as indicated by the green arrows in Fig. 7(a). A detailed analysis, however, shows that the unit vectors of this structure are of slightly different length, hence we named it modified BW, or BWM structure. To the best of our knowledge, this structure has not been reported for PTCDA adsorption, neither on semiconductor nor on metal surfaces. Comparing with the BW structure, one might speculate that in the BWM structure the molecules are slightly rotated in order to lower the repulsive interaction of the negatively charged



(a)

0.5 Å–1

(d)

0.3 ML

Page 12 of 20

(b)

0.5 ML

(c)

1.0 ML

(e)

5 ML

(f)

10 ML

34 eV 3 ML

√ √ Figure 9: LEED patterns for progressive deposition of PTCDA on Bi:Si(111)- 3× 3-R30° at room temperature; PTCDA coverages are indicated in each frame. of the unit cells shows no changes above 1 ML. Their intensity distribution, however, changes at the favor of the HB1 structure, which is strongly pronounced at 10 ML. In addition, the peaks of the BW, BWM, and PS structures are most pronounced at a coverage of 1 ML which we consider a further indication that these structures show a stronger substrate-molecule interaction.

local probe, which is the main aspect in the present section. For this purpose scanning tunneling microscopy (STM) was used. Moreover, as STM also provides structural information, we will show that many of the remaining diffraction spots that could not be explained by the previously discussed superstructures can be attributed to a nearly squarelike structure that has not been reported on so far. The development of the surface morphology with progressive PTCDA deposition can be seen in the scanning tunneling microscopy (STM) scans displayed in Fig. 10. Upon Bi adsorption (Fig. 10(a)) at 480 °C we find a smooth surface with only a few defects in the passivation layer. At this tunneling condition they are visible as small bright spots at the step edges and, even fewer, on the terraces. We attribute these to missing or surplus Bi atoms. After deposition of 0.5 ML, we observe PTCDA in large domains on the terraces (Fig. 10(b)). Moreover, we find only very few PTCDA islands, pointing to a large diffusion

Morphology In the previous section, the structural properties, i. e. the size and orientation of the unit cells, have been addressed by SPA-LEED, since SPA-LEED provides precise atomic-scale resolution (in reciprocal space), and it integrates over macroscopic surface areas with a high dynamic intensity range, thus allowing us to investigate phase coexistence and to analyze the structural properties of minority phases. The growth mode and local details such as defects, however, are more easily accessible with a 12


Page 13 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

√ √ length of PTCDA on the Bi:Si(111)- 3× 3-R30° surface at room temperature. Also, the images show that growth proceeds in a step-flow mode as 48 has also been for PTCDA growth on √ √ reported Ag:Si(111)- 3× 3-R30°. In addition, we find an increased density of bright spots at the step edges, which most likely is PTCDA bonding to imperfectly √ √passivated spots of the initial Bi:Si(111)3× 3-R30° surface. Similar features are visible in Ag:Si(111)√ the √ STM images of the study on 48 3× 3-R30° by Swarbrick et al., though they have not been discussed there. The comparison to our results, however, suggests that these features have also been caused by imperfections in the passivation layer. At higher PTCDA coverages, shown in Fig. 10(c) and (d), pure step-flow growth is found. Moreover, these STM images show that the PTCDA domains do not grow up to the very step edges in all cases; instead depressions and trenches, marked by arrows in Fig. 10(d), are found. At higher resolution, not only the decoration of step edges as mentioned above becomes apparent (see white arrows in Fig. 11(a)), but also point defects on the terraces, as indicated by the black arrows in Fig. 11(a). Both features can be attributed to defects in the passivation layer. However, the step-edge defects seem to lead to an agglomeration of PTCDA molecules, while no such strong impact on the PTCDA film morphology is observed for the defects on the terraces. Hence, the latter can more easily be overgrown by the PTCDA film. On the terraces, another type of point defect can be observed, as marked by white arrows in the high-resolution image in Fig. 11(b). Most likely, these are single PTCDA ad-molecules. In Fig. 11(b), the herring-bone structure of the surface is clearly resolved. Despite a small residual drift of the STM, a clear contrast is visible. The line profile in Fig. 11(c) shows that every second row of molecules is brighter than the other. A similar contrast between alternating rows of the HB structure has already been observed for initial adsorption of PTCDA on Ag(111), 14,15,69 PTCDA on Ag(110), 15 and PTCDA on Cu(111) 13 . The authors conclude on a different adsorption site for every second row and, hence, different bonding to the substrate, leading to√this√contrast. For the case of PTCDA/Bi:Si(111)- 3× 3-R30° however, our

0.0 ML

100 nm

(b)

0.5 ML

100 nm

(c)

1.5 ML

200 nm

(d)

3.5 ML

200 nm

√ √ Figure 10: STM images of Bi:Si(111)- 3× 3R30° with progressive PTCDA deposition at room temperature of (a) 0.0 ML, (b) 0.5 ML, (c) 1.5 ML, and (d) 3.5 ML, respectively; tip bias UT = −2 V, tunneling current IT = 0.3 nA.



molecules with respects to the C-C bond of the substrate. The angle between the two √ molecu√ lar orientations of PTCDA on Bi:Si(111)- 3× 3R30°, however, cannot unambiguously be determined from our STM data. In addition to the HB structure, upon room temperature deposition of PTCDA on Bi:Si(111)√ √ 3× 3-R30°, we also observed a nearly squarelike structure in STM as displayed in Fig. 12(a). Different from the previous STM images, the PTCDA molecules no longer appear as a single bright spots but as two separated bright ovals which is related to the different tunneling bias, now imaging the highest occupied molecular orbital (HOMO) of the PTCDA molecules. The line scan in Fig. 12(b) shows that only every second row shows a double structure along the scan direction. Hence we can conclude that the molecules of the alternating rows are arranged perpendicularly to each other, as known from an investigation by Swarbrick al. 48 for the case of PTCDA on √ et √ Ag:Si(111)- 3× 3-R30°. A close inspection of the structure reveals a unit cell that is not perfectly square and hence will be called nearly square-like (NS) in the following. The dimensions of the unit cell are given in Tab. 3. The exact dimension of the unit cell can be determined with better accuracy from diffraction experiments. Figure 12(c) shows the comparison of the diffraction spots calculated from LEED simulations with the experimental data. The NS structure explains the presence of a large number of LEED spots which cannot be explained from the structures discussed so far, as can be seen from a comparison with the missing (red) spots in Fig. 8(b).

(a)

20 nm

(b)

5 nm

0.6 height (Å)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c)

0.4 0.2 0 –6

–4

–2 0 2 distance (nm)

5

Page 14 of 20

6

(HB) Figure 11: STM images of the herring-bone √ √ structure of PTCDA/Bi:Si(111)- 3× 3-R30°, at a coverage of 0.5 ML. (UT = −2 V, IT = 0.3 nA.) (a) Step edges and terraces; (b) HB structure with molecular resolution; (c) height profile along the dashed line in (b).

Conclusion Using XPS we could show that room tempera√ √ ture deposition of PTCDA on Bi:Si(111)- 3× 3R30° leads to the formation of an adsorption layer of intact molecules. Using NEXAFS we find that the majority of the PTCDA molecules lies flat on the surface. Structural investigations with SPA-LEED reveal an ordered growth of a multidomain film, mostly consisting of herring-bone (HB) structures. The size of the unit cell is different from volume PTCDA and is affected by

SPA-LEED results clearly rule out the presence of a simple commensurate HB structure, and therefore we can rule out the explanation based on a periodically differing bonding geometry. It is more likely that the contrast of the rows is related to an alternating orientation of the PTCDA molecules of every second row, similar to adsorption of PTCDA on graphite. 70 There, the authors predict a contrast change depending of the orientation of the 14


Page 15 of 20

age range of up to 3.5 ML. Instead, the film grows in a step-flow fashion. This points to a high mobility of PTCDA on the surface at room temperature. Hence, our approach to use Bi as passivating agent allows to grow PTCDA films on Si(111) with unsurpassed structural and morphological quality.

(a)

00 00

00

00

5 nm 0.2 height (Å)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information Available

(b)

SPA-LEED analysis of PTCDA grown on Agterminated Si(111).

0.1

Acknowledgement Certain commercial names presented in this manuscript do not constitute an endorsement by the National Institute of Standards and Technology. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

0 –6

–4

–2 0 2 distance (nm)

4

6

(c)

References (1) Lee, S.; Yeo, J.; Ji, Y.; Cho, C.; Kim, D.; Na, S.; Lee, B.; Lee, T. Flexible organic solar cells composed of P3HT:PCBM using chemically doped graphene electrodes. Nanotechnol. 2012, 23, 344013.

0.5 A˚ -1

(2) Ermakov, O.; Kaplunov, M.; Efimov, O.; Yakushchenko, I.; Belov, M.; Budyka, M. Hybrid organic-inorganic light-emitting diodes. Microelectron. Eng. 2003, 69, 208.

Figure 12: Nearly square-like √ √ structure (NS) of PTCDA on Bi:Si(111)- 3× 3-R30°. (a) Highresolution STM (UT = +2 V, IT = 0.3 nA); (b) line profile as indicated in (a); (c) SPA-LEED pattern and comparison to calculated diffraction peak positions

(3) Guha, S.; Haight, R.; Bojarczuk, N.; Kisker, D. Hybrid organic-inorganic semiconductor-based light-emitting diodes. J. Appl. Phys. 1997, 82, 4126.

the local molecule/substrate interaction. This interaction is also responsible for the appearance of additional PTCDA structures at monolayer coverage, i.e. a brick-wall structure, a modified brickwall structure, a nearly square-like (NS) structure, and a perylene structure (PS). Two of these (HB and NS) have also been proven by high-resolution STM measurements. Using STM, we could not detect any islanding within the investigated cover-

(4) Bednorz, M.; Matt, G. J.; Głowacki, E. D.; Fromherz, T.; Brabec, C. J.; Scharber, M. C.; Sitter, H.; Sariciftci, N. S. Silicon/organic hybrid heterojunction infrared photodetector operating in the telecom regime. Org. Electron. 2013, 14, 1344. (5) Zhao, F.; Luo, X.; Liu, J.; Du, L.; Lv, W.; Sun, L.; Li, Y.; Wang, Y.; Peng, Y. To15



ward high performance broad spectral hybrid organic–inorganic photodetectors based on multiple component organic bulk heterojunctions. J. Mater. Chem. C 2016, 4, 815.

Page 16 of 20

large aromatic adsorbates by molecular design. Nature 2003, 425, 602. (15) Glöckler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bohringer, M.; Berndt, R.; Schneider, W. D. Highly ordered structures and submolecular scanning tunnelling microscopy contrast of PTCDA and DM-PBDCI monolayers on Ag(111) and Ag(110). Surf. Sci. 1998, 405, 1.

(6) Schreiber, F. Organic molecular beam deposition: Growth studies beyond the first monolayer. phys. stat. sol. (a) 2004, 201, 1037, and references therein. (7) Tautz, F. Structure and bonding of large aromatic molecules on noble metal surfaces: The example of PTCDA. Progr. Surf. Sci. 2007, 82, 479, and references therein.

(16) Ahrens, C.; Flege, J. I.; Jaye, C.; Fischer, D. A.; Schmidt, T.; Falta, J. Isotropic thin PTCDA films on GaN(0 0 0 1). J. Phys.: Condens. Matter 2016, 28, 475003.

(8) Ikonomov, J.; Bauer, O.; Sokolowski, M. Highly ordered thin films on perylene3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on Ag(100). Surf. Sci. 2008, 602, 2061.

(17) Sharifzadeh, S.; Biller, A.; Kronik, L.; Neaton, J. B. Quasiparticle and optical spectroscopy of the organic semiconductors pentacene and PTCDA from first principles. Phys. Rev. B 2012, 85, 125307.

(9) Mura, M.; Sun, X.; Silly, F.; Jonkman, H. T.; Briggs, G. A. D.; Castell, M. R.; Kantorovich, L. N. Experimental and theoretical analysis of H-bonded supramolecular assemblies of PTCDA molecules. Phys. Rev. B 2010, 81, 195412.

(18) Wang, W.; Ji, Y.; Zhang, H.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. G. Negative differential resistance in a hybrid siliconmolecular system: Resonance between the intrinsic surface-states and the molecular orbital. ACS Nano 2012, 6, 7066.

(10) Stadtmüller, B.; Gruenewald, M.; Peuker, J.; Forker, R.; Fritz, T.; Kumpf, C. Molecular exchange in a heteromolecular PTCDA/CuPc bilayer film on Ag(111). J. Phys. Chem. C 2014, 118, 28592.

(19) Gustafson, J.; Moons, E.; Widstrand, S.; Gurnett, M.; Johansson, L. Thin PTCDA films on Si(001): 1. Growth mode. Surf. Sci. 2004, 572, 23.

(11) Schmitt, S.; Schöll, A.; Umbach, E. Long-range surface faceting induced by chemisorption of PTCDA on stepped Ag(111) surfaces. Surf. Sci. 2016, 643, 59.

(20) Zimmermann, U.; Schnitzler, G.; Karl, N.; Umbach, E.; Dudde, R. Epitaxial growth and characterization of organic thin films on silicon. Thin Solid Films 1989, 175, 85.

(12) Henneke, C.; Felter, J.; Schwarz, D.; Tautz, F. S.; Kumpf, C. Controlling the growth of multiple ordered heteromolecular phases by utilizing intermolecular repulsion. Nature Mater. 2017, 16, 628.

(21) Soubiron, T.; Vaurette, F.; Nys, J. P.; Grandidier, B.; Wallart, X.; Stiévenard, D. Molecular interactions of PTCDA on Si(100). Surf. Sci. 2005, 581, 178. (22) Suzuki, T.; Yoshimoto, Y.; Yagyu, K.; Tochihara, H. Adsorption of PTCDA on Si(001)2×1 surface. J. Chem. Phys. 2015, 142, 101904.

(13) Wagner, T.; Bannani, A.; Bobisch, C.; Karacuban, H.; Möller, R. The initial growth of PTCDA on Cu(111) studied by STM. J. Phys.: Condens. Matter 2007, 19, 056009.

(23) Copel, M.; Reuter, M. C.; Kaxiras, E.; Tromp, R. M. Surfactants in epitaxial growth. Phys. Rev. Lett. 1989, 63, 632.

(14) Eremtchenko, H.; Schaefer, J. A.; Tautz, F. S. Understanding and tuning the epitaxy of 16


Page 17 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Horn-von Hoegen, M.; LeGoues, F. K.; Copel, M.; Reuter, M. C.; Tromp, R. M. Defect self-annihilation in surfactant-mediated epitaxial growth. Phys. Rev. Lett. 1991, 67, 1130.

(33) Chen, Q.; Rada, T.; Bitzer, T.; Richardson, N. Growth of PTCDA crystals on H:Si(111) surfaces. Surf. Sci. 2003, 547, 385. (34) Sazaki, G.; Fujino, T.; Sadowski, J.; Usami, N.; Ujihara, T.; Fujiwara, K.; Takahashi, Y.; Matsubara, E.; Sakurai, T.; Nakajima, K. Epitaxial relation and island growth of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin film crystals on a hydrogen-terminated Si(111) substrate. J. Cryst. Growth 2004, 262, 196.

(25) Falta, J.; Copel, M.; LeGoues, F. K.; Tromp, R. M. Surfactant coverage and epitaxy of Ge on Ga-terminated Si(111). Appl. Phys. Lett. 1993, 62, 2962. (26) Schmidt, T.; Falta, J.; Materlik, G.; Zeysing, J.; Falkenberg, G.; Johnson, R. L. Bi: Perfect surfactant for Ge growth on Si(111)? Appl. Phys. Lett. 1999, 74, 1391.

(35) Sazaki, G.; Fujino, T.; Usami, N.; Ujihara, T.; Fujiwara, K.; Nakajima, K. Effects of vicinal steps on the island growth and orientation of epitaxially grown perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin film crystals on a hydrogen-terminated Si(111) substrate. J. Cryst. Growth 2005, 273, 594.

(27) Gevers, S.; Flege, J. I.; Kaemena, B.; Bruns, D.; Weisemoeller, T.; Falta, J.; Wollschläger, J. Improved epitaxy of ultrathin praseodymia films on chlorine passivated Si(111) reducing silicate interface formation. Appl. Phys. Lett. 2010, 97, 242901.

(36) Sazaki, G.; Fujino, T.; Sadowski, J.; Usami, N.; Ujihara, T.; Fujiwara, K.; Takahashi, Y.; Matsubara, E.; Sakurai, T.; Nakajima, K. Epitaxial relation and island growth of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin film crystals on a hydrogen-terminated Si(111) substrate. J. Cryst. Growth 2004, 262, 196.

(28) Flege, J. I.; Kaemena, B.; Gevers, S.; Bertram, F.; Wilkens, T.; Bruns, D.; Bätjer, J.; Schmidt, T.; Wollschläger, J.; Falta, J. Silicate-free growth of high-quality ultrathin cerium oxide films on Si(111). Phys. Rev. B 2011, 84, 235418. (29) Soubiron, T.; Vaurette, F.; Grandidier, B.; Wallart, X.; Nys, J.; Stiévenard, D. Adsorption of PTCDA on Si(100) and Si(100)-H. NIST-Nanotech 2004, 3, 383.

(37) Zebari, A. A.; Kolmer, M.; PrauznerBechcicki, J. STM tip-assisted engineering of molecular nanostructures: PTCDA islands on Ge(001):H surfaces. Beilstein J. Nanotechnol. 2013, 4, 927.

(30) Vaurette, F.; Nys, J. P.; Grandidier, B.; Priester, C.; Stievenard, D. Growth mechanism of organic nanocrystals on passivated semiconductor surfaces. Phys. Rev. B 2007, 75, 235435.

(38) Zebari, A. A.; Kolmer, M.; PrauznerBechcicki, J. Characterization of PTCDA nanocrystals on Ge(001):H-(2×1). Appl. Surf. Sci. 2015, 332, 403.

(31) Uder, B.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. STM characterisation of organic molecules on H-terminated Si(111). Z. Phys. B 1995, 97, 389.

(39) Zhang, H. M.; Ericsson, L. K. E.; Johansson, L. S. O. PTCDA √ reconstruc√ induced tion on Sn/Si(111)-2 3 × 2 3. Phys. Rev. B 2012, 85, 245317.

(32) Morozov, A.; Kampen, T.; Zahn, D. PTCDA film formation on Si(111):H-1×1 surface: total current spectroscopy monitoring. Surf. Sci. 2000, 446, 193.

(40) Zhang, H. M.; Ericsson, L. K. E.; Johansson, L. S. O. PTCDA √ induced √ reconstruction on Sn/Si(111)-2 3 × 2 3. Phys. Rev. B 2012, 85, 245317. 17



(41) Zhang, H. M.; Johansson, L. S. O. STM study of PTCDA on Sn/Si(111)-23×23. J. Chem. Phys. 2016, 144, 124701.

Page 18 of 20

(50) Nicoara, N.; Wei, Z.; GómezRodr´ıguez, J. M. One-dimensional growth of√PTCDA√ molecular rows on Si(111)(2 3 × 2 3)R30◦ -Sn surfaces. J. Phys. Chem. C 2009, 113, 14935.

(42) Nicoara, N.; Méndez, J.; GómezRodr´ıguez, J. M. Growth of ordered molecular layers of PTCDA on Pb/Si(111) surfaces: a scanning tunneling microscopy study. Nanotechnol. 2016, 27, 365706.

(51) Woicik, J. C.; Franklin, G. E.; Liu, C.; Martinez, R. E.; Hwong, I.-S.; Bedzyk, M. J.; Patel, J. R.; Golovchenko, J. √ A. Structural determination of the Si(111) 3-Bi surface by x-ray standing waves and scanning tunneling microscopy. Phys. Rev. B 1994, 50, 12246.

(43) Gustafsson, J. B.; Zhang, H. M.; Moons, E.; Johansson, L. S. O. Electron spectroscopy √ √ studies of PTCDA on Ag/Si(111) 3 × 3. Phys. Rev. B 2007, 75, 155413.

(52) Tougaard, S. Background removal in x-ray photoelectron spectroscopy: Relative importance of intrinsic and extrinsic processes. Phys. Rev. B 1986, 34, 6779.

(44) Zhang, H.; Gustafsson, J.; Johansson, L. STM study of the electronic structure of PTCDA on Ag/Si(111)-3×3. Chem. Phys. Lett. 2010, 485, 69.

(53) Armstrong, B. H. Spectrum line profiles: the Voigt function. J. Quant. Spectrosc. Radiat. Transfer 1967, 7, 61–88.

(45) Ma, J.; Rogers, B. L.; Humphry, M. J.; Ring, D. J.; Goretzki, G.; Champness, N. R.; Beton, P. H. Dianhydride-amine hydrogen bonded perylene tetracarboxylic dianhydride and tetraaminobenzene rows. J. Phys. Chem. B 2006, 110, 12207.

(54) Prince, K.; Vondrácˇ ek, M.; Karvonen, J.; Coreno, M.; Camilloni, R.; Avaldi, L.; de Simone, M. A critical comparison of selected 1s and 2p core hole widths. J. Electr. Spectrosc. Rel. Phenom. 1999, 101-103, 141–147.

(46) Huber, F.; Matencio, S.; Weymouth, A. J.; Ocal, C.; Barrena, E.; Giessibl, F. J. Intramolecular force contrast and dynamic current-distance measurements at room temperature. Phys. Rev. Lett. 2015, 115, 066101.

(55) Zahl, P.; Bierkandt, M.; Schröder, S.; Klust, A. The flexible and modern open source scanning probe microscopy software package GXSM. Rev. Sci. Instrum. 2003, 74, 1222.

(47) Menzli, S.; Hamada, B. B.; Arbi, I.; Souissi, A.; Laribi, A.; Akremi, A.; Chefi, C. Adsorption √ study√of copper phthalocyanine on Si(111)( 3 × 3)R30◦ Ag surface. Appl. Surf. Sci. 2016, 369, 43.

(56) Zahl, P.; Horn-von Hoegen, M. Thirdgeneration conical spot profile analyzing low-energy electron diffraction. Rev. Sci. Instrum. 2002, 73, 2958. (57) Taborski, J.; Väterlein, P.; Dietz, H.; Zimmermann, U.; Umbach, E. NEXAFS investigations on ordered adsorbate layers of large aromatic molecules. J. Electron Spectrosc. Relat. Phenom. 1995, 75, 129.

(48) Swarbrick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. Square, hexagonal, and row phases and PTCDI on √ of√PTCDA ◦ Ag-Si(111)- 3 × 3R30 . J. Phys. Chem. B 2005, 109, 12167.

(58) Stöhr, J. NEXAFS Spectroscopy; SpringerVerlag: Berlin, 1992.

(49) Emanuelsson, C.; Zhang, H. M.; Moons, E.; Johansson, L. S. O. Scanning tunneling microscopy study √ of√ thin ◦PTCDI films on Ag/Si(111)- 3 × 3R30 . J. Chem. Phys. 2017, 146, 114702.

(59) Stöhr, J.; Outka, D. A. Determination of molecular orientations on surfaces from angular dependence of near-edge x-rayabsorption fine-structure spectra. Phys. Rev. B 1987, 36, 7891. 18


Page 19 of 20 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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(60) Wüsten, J.; Berger, S.; Heimer, K.; Lach, S.; Ziegler, C. Interaction of alkali metals with perylene-3,4,9,10-tetracarboxylic-dianhydride thin films. J. Appl. Phys. 2005, 98, 013705.

(69) Tautz, F.; Eremtchenko, M.; Schaefer, J.; Sokolowski, M.; Shklover, V.; Glöckler, K.; Umbach, E. A comparison of the chemisorption behaviour of PTCDA on different Ag surfaces. Surf. Sci. 2002, 502-503, 176.

(61) Gavrila, G.; Zahn, D. R. T.; Braun, W. High resolution photoemission spectroscopy: Evidence for strong chemical interaction between Mg and 3,4,9,10perylene-tetracarboxylic dianhydride. Appl. Phys. Lett. 2006, 89, 162102.

(70) Ogawa, T.; Irie, S.; Isoda, S.; Kobayashi, T.; Whangbo, M.-H. Simulation of long-range contrast modulation in scanning tunneling microscope image of perylene-3,4,9,10tetracarboxylic-dianhydride monolayer on graphite. Jpn. J. Appl. Phys. 1998, 37, 3864.

(62) Beamson, G.; Briggs, D. High resolution XPS of organic polymers: The Scienta ESCA300 database. J. Chem. Educ. 1993, 70, A25. (63) Gustafsson, J. B.; Moons, E.; Widstrand, S. M.; Gurnett, M.; Johansson, L. S. O. Thin PTCDA films on Si(001): 2. Electronic structure. Surf. Sci. 2004, 572, 32. (64) Möbus, M.; Karl, N.; Kobayashi, T. Structure of perylene-tetracarboxylic-dianhydride thin films on alkali halide crystal substrates. J. Cryst. Growth 1992, 116, 495. (65) Wan, K. J.; Guo, T.; Ford, W. K.; Hermanson, J. C. Initial growth of Bi films √ on a Si(111) substrate: Two phases of 3 lowenergy-electron-diffraction pattern and their geometric structure. Phys. Rev. B 1991, 44, 3471. (66) Nakatani, S.; Takahashi, T.; Kuwahara, Y.; Aono, M. Use of x-ray √ reflectivity for determining the Si(111)- 3-Bi surface structures. Phys. Rev. B 1995, 52, R8711. (67) Schmidt, T.; Falta, J.; Materlik, G. Initial stage of the Bi surfactant-mediated growth of Ge on Si(111): a structural study. Appl. Surf. Sci. 2000, 166, 299. (68) Seidel, C.; Ellerbrake, R.; Gross, L.; Fuchs, H. Structural transitions of perylene and coronene on silver and gold surfaces: A molecular-beam epitaxy LEED study. Phys. Rev. B 2001, 64, 195418.



Graphical TOC Entry C 1s binding energy (eV)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2 nm

288.9 288.8 288.7

Ccarbonyl Caryl

285.2 285.1 285.0 0

2 4 6 8 10 PTCDA coverage (ML)


Page 20 of 20

Growth of Epitaxial 3,4,9,10-Perylene Tetracarboxylic Dianhydride

Recommend Documents