Microscopic View of Tin Phthalocyanine Adsorption on the Rutile TiO2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Microscopic View of Tin Phthalocyanine Adsorption on the Rutile TiO (011) Surface 2

Lukasz Bodek, Aleksandra Cebrat, Piotr Piatkowski, and Bartosz Such J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01043 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

Microscopic View of Tin phthalocyanine Adsorption on the Rutile TiO2 (011) Surface

Lukasz Bodek,* Aleksandra Cebrat, Piotr Piatkowski, Bartosz Such*

* Corresponding authors Lukasz Bodek. Email address: [email protected] Bartosz Such. Email address: [email protected]

Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, ul. S. Lojasiewicza 11, 30-348 Krakow, Poland

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ABSTRACT: The adsorption behavior of tin phthalocyanine (SnPc) on the (011) face of rutile TiO2 was studied via scanning probe microscopy at room temperature. Upon deposition, the molecules form a commensurate structure (A-type phase) that exhibits two mirror domains with respect to the [01-1] direction of the substrate. Annealing of the sample leads to the transition to another incommensurate structure (B-type phase) that mimics the structure of the corresponding molecular crystal. Kelvin probe force microscopy results showed that SnPcs tend to adsorb with the metal atom directed towards the surface unless they are in the B-type phase, where the SnPcs adopt a position in which their molecular board is perpendicular to the surface.

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INTRODUCTION

The development of organic electronic devices, such as field effect transistors1, light-emitting diodes2 and especially photovoltaic devices (e.g., dye sensitized solar cells - DSSCs)3, requires deep understanding of organic-inorganic interfaces4,5. Surface functionalization via thin layer deposition of organic molecules is a very popular and effective method of forming such interfaces. Therefore, the adsorption process of organic molecules on semiconductor and metallic surfaces has received considerable attention in the context of light-harvesting devices over recent decades6. Adsorption as a fundamental phenomenon is closely related to carrier transport and injection processes7,8,9 and strongly depends on molecule-molecule and molecule-substrate interactions10 as well as the type of molecular building blocks used11. In this context, molecules, such as phthalocyanines (Pcs), play a crucial role due to their unusual electro-optical properties12. The physical aspects of phthalocyanine adsorption on semiconductor and metal surfaces have been summarized in comprehensive reviews13,14, where it was shown that in most cases Pcs assemble on metals into well-ordered flat-lying monolayers. For the many applications mentioned above, rutile TiO2 is the most often used transition metal oxide and thus has been substantially studied with spectroscopic and microscopic techniques15. In the field of microscopic studies of Pcs adsorption on titanium dioxide, the vast majority of results concern metal phthalocyanines (MPcs) adsorbed on the most thermodynamically stable (110) face of rutile TiO2, such as CoPc/TiO2(110)-(1x1)16, CuPc/TiO2(110)-(1x2)17, FePc/TiO2(110)-(1x1)18 and metalfree H2Pc/TiO2(110)19. However, there have been only a few studies on the adsorption of Pcs on the second most stable (011) rutile surface20: ZnPc/TiO2(011)-(2x1)21 and CuPc/(011)-(2x1)22,23,24. On the (011) rutile surface, single phthalocyanine molecules tend to adsorb in a flat-lying position with their molecular board parallel to the surface; however, they exhibit a higher mobility compared with the (110) surface, especially along the [01-1] direction21,23. With increasing coverage, both CuPcs and 3 ACS Paragon Plus Environment


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ZnPcs begin to arrange in a quasi-ordered structure. However, the main difference in adsorption behavior is clearly visible during the formation of subsequent layers. In both cases, 2D structures created by upright standing Pc molecules differ in the overall domain shape and appearing phases. Further

experiments21

with

the

same

molecules

adsorbed

on

the

ZnTPP

(Zn(II)meso-

tetraphenylporphyrin) wetting layer on the (011)-(2x1) rutile surface indicated that ZnPcs forms more complex 2D structures than CuPc molecules, despite the similarities in the building blocks between both molecular structures. This behavior shows that the metal atom may play a significant role in the adsorption process. Similar findings were described for ZnPc, CuPc and CoPc molecules deposited on the Si(111) surface25, where the strong dependency of ordering, the molecule orientation within the phase and modulation of p-d orbital coupling on the central metal atom were discussed. All these findings motivated us to initiate ultrahigh vacuum (UHV) microscopic studies (STM/AFM) on tin phthalocyanines (SnPc) on the rutile TiO2(011)-(2x1) surface. We chose a molecule that differs considerably from the mentioned ones. The size of the Sn atom does not allow it to fit into the center of a macrocycle without bending it; thus, it protrudes from the molecular plane, resulting in a molecular shape that is referred to as saucer-shaped26 or shuttlecock-type27, as shown in Figure 1. Due to this nonplanarity, the molecule exhibits a nonzero permanent dipole moment along the four-fold rotational axis (the molecule exhibits C4v symmetry) with the reported value of a dipole moment of approximately 0.6D28,29 and 0.9D30. SnPc was studied in detail on metal and semimetal surfaces, including Ag(111)28,31, Au(111)32 and HOPG(0001)32 to name a few. The nonplanar shape of SnPc leads to repulsive forces acting between the adsorbed molecules on the Ag(111)33 surface when forming a monolayer (ML). For 2ML coverage, layer by layer assembling of SnPcs is observed34; however, when more molecules are evaporated, the growth behavior changes, and molecular islands are preferentially created. For 6ML coverage, the molecules within the molecular islands horizontally arrange on the surface34. 4 ACS Paragon Plus Environment

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Figure 1. a) Molecular structure of Tin (II) phthalocyanine (SnPc); b) Side view of the SnPc molecule.

In this study, we demonstrate structures and molecular phases created by SnPc molecules on the (011)-(2x1) rutile surface. SnPc molecules form two ordered phases. In both phases, two domains exhibiting mirror symmetry with respect to the [01-1] direction of the substrate were observed. Annealing of the deposited layer above 200 °C leads to molecular reorganization between the phases as determined for coverage up to the equivalent of 7 monolayers. High-resolution STM images allowed us to designate the basic molecular building blocks, and the Kelvin force probe microscopy results showed the local contact potential difference (LCPD) between the molecular phases. Consequently, we determined the arrangement of basic molecular building blocks for both appearing phases. All STM/AFM measurements were conducted at room temperature, and from the point of view of potential applications, this is more realistic than studies performed under cryogenic conditions. 5 ACS Paragon Plus Environment


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EXPERIMENTAL DETAILS The experiment was performed in an ultrahigh vacuum (UHV) system with a base pressure below 2·1010

mbar. All measurements (STM, ncAFM, and KPFM) were conducted with an Omicron STM/AFM

microscope at room temperature. A rutile TiO2 (011) crystal (Mateck GmbH, Juelich, Germany) was placed on a sample holder with a Si wafer underneath acting as a resistive heater. The TiO2 sample was cleaned during consecutive cycles of Ar+ ion bombardment at an energy of 1 keV and annealed to a temperature of 700 °C using an AC electric current flow. SnPc, which was supplied by TCI (Tokyo Chemical Industry Co., Ltd.), was degassed for several hours slightly below its sublimation temperature using a commercially available effusion cell (Kentax), and the subsequent deposition flux rate (0.25 ML/min) was determined using a quartz crystal microbalance. Molecules were evaporated onto an atomically flat and clean surface (checked by STM) that was maintained at 100 °C (samples heated from room temperature to 150 °C during evaporation yielded similar results). Measurements were performed in a wide range of coverages up to the equivalent of 7 monolayers. Here, we define a monolayer as the amount of flat lying molecules required for complete coverage of the substrate, i.e., approximately 0.65 molecules/nm2. STM images were collected in a constant current mode using electrochemically etched Pt-Ir tips, whereas for the ncAFM/KPFM (FM mode) measurements, silicon AFM cantilevers (k = 10 N/m) were used. A bias voltage of approximately 2.2 V and a tunneling current below 5 pA were found to be the most suitable to collect stable STM images of the empty states.

RESULTS AND DISCUSSION Single SnPc molecules tend to be mobile on the rutile (011)-(2x1) TiO2 surface at room temperature. They appear in STM images as streaky lines, suggesting that their residence time under the tip is too 6 ACS Paragon Plus Environment

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short to allow proper imaging. It is not clear whether their mobility is entirely connected to the lack of specific adsorption sites or if the tip-molecule interaction also plays a role. However, a small fraction of molecules was found to be stable on the surface. The flat-adsorbed molecules seem to be constrained by surface defects rather than by some particular molecule-surface interactions. Therefore, highresolution STM measurements remain difficult unless single molecules are probed under cryogenic conditions. With a coverage of up to 1 ML, the molecules adsorb in a flat-lying position without any signs of quasi or long-range ordering and become less mobile due to steric considerations. Nevertheless, the molecular mobility is still noticeable. When the coverage exceeds 1 ML, stable molecular structures appear on the surface. However, the sticking coefficient is likely much smaller than unity since there are always fewer molecules adsorbed on the surface than a quartz microbalance indicates. Complete coverage of the surface with an organized molecular ad-layer was reproducibly achieved for deposition of 5-7 equivalent monolayers, and all the results described in this paper were obtained for this range of coverages. Of note, the structures created on the surface were identical in this range of deposition. In Figure 2a, an empty-state STM image of ordered phases directly after deposition of an equivalent of 7 monolayers is presented. Ordered molecules, creating one molecular phase (A-type phase), cover almost the entire surface; however, small islands of another phase (B-type phase) are observed on the domain borders and near the edges of surface terraces (see blue arrows in Figure 2a). To remove surplus mobile admolecules (indicated by the noisy horizontal lines in Figure 2a), the surface was annealed at 200 °C for 15 min (Figure 2b). As a result, the B-type molecular phase, which was barely visible before, began to slowly grow. Subsequent annealing of the layer to 225 °C for 15 min resulted in further growth of the B-type structures at the expense of the A-type islands (Figure 2c). Regions of the substrate covered by mobile molecules (unordered phase, the same structure as observed for a coverage of

Microscopic View of Tin Phthalocyanine Adsorption on the Rutile TiO2

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