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Energy Level Alignment at the Fullerene/ Titanium Oxide Ultrathin Film Interface Michael Paßens, Marco Moors, Rainer Waser, and Silvia Karthaeuser J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11386 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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The Journal of Physical Chemistry

Energy Level Alignment at the Fullerene/Titanium Oxide Ultrathin Film Interface Michael Paßens,1 Marco Moors,1 Rainer Waser,1,2 Silvia Karthäuser1* 1

Peter Grünberg Institut (PGI-7) and JARA-FIT, Forschungszentrum Jülich GmbH, Wilhelm-

Johnen Str., 52425 Jülich, Germany 2

IWE2 and JARA-FIT, RWTH Aachen University, Sommerfeldstraße 24, 52056 Aachen,

Germany

ABSTRACT The performance of molecule based electronic devices can be improved by use of transition metal oxides (TMO) as charge injection buffer layers between electrodes and organic semiconductors. It is known that the appropriate energy level alignment at the molecule/TMO interface determines the efficiency of the respective TMO. Herein, scanning tunneling microscopy is employed to characterize the interface formed by fullerenes (C60) deposited on a titanium oxide (TiO) ultrathin film, which is created by oxidation of a Pt3Ti(111) surface. Individual C60 are identified with orbital resolution, and the interfacial and intermolecular interactions are characterized in detail. Furthermore, the energy level alignment at the C60/TiO ultrathin film interface is deduced based on scanning tunneling spectroscopy data. The results demonstrate that the C60/TiO interface corresponds to a Type-I heterojunction and thus, is useful to decouple C60 molecules from the metallic alloy surface.

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INTRODUCTION Charge transfer across the interface between organic molecules and electrodes is of particular importance in order to increase the performance of organic thin-film transistors (OFETs), organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs).1-4 Usually, transition metal oxides (TMO) are used as charge injection buffer layers between metallic electrodes and organic semiconductors to reduce the potential energy barrier and thus, to increase the efficiency of charge injection.5 TMOs are especially suited for this purpose since their electronic and chemical properties can be fine-tuned in a wide range.4,6,7,8 They possess work functions spanning from 3.5 eV, for defective ZrO2, to 7.0 eV in case of stoichiometric V2O5, and thus, are suited as charge-selective interlayers.9 Whereas TMO layers with high work functions are used as hole-injecting buffer layers for anodes, TMOs with low work functions, like ZnO, ZrO2 or TiO2, are used as electron-injection buffer layers for cathodes.9 Rutile(110) (TiO2) as a prototypical TMO has been tested as substrate for the assembly of organic molecules and especially fullerenes (C60 molecules), as prominent components of photovoltaic cells, in a number of studies in the last decade.10-20 It turned out that C60 molecules are highly mobile on TiO2(110) surfaces at room temperature and that nucleation takes place at step edges or substrate defects.13 C60 molecules assemble on the rutile surface in a well-ordered p(5x2) as well as in a (5, 2; 2, -3) phase.16 The molecule/TMO interface is characterized by a large distance of 0.32 - 0.33 nm between the carbon atoms of the C60 and the titanium atoms of the TiO2(110) surface. C60 desorbs completely from the rutile(110) surface for temperatures higher than 600 K, while the sticking temperature is significantly lower, i.e., 420 K and below. This indicates that the intermolecular interactions are stronger than the interface interactions between C60 and TiO2(110).17 In addition, theoretical calculations document that the TiO2(110)


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surface interacts weakly with C60 molecules and that the charge transfer is negligible. This is also indicated by core-level peak spectroscopy measurements.19,20 The C60/TiO2(110) interface is assumed to be a type-II heterojunction, where the LUMO of the molecule overlaps with the conduction band of the TMO, while the HOMO is located in the band gap. This energy-level alignment favors the injection of photoexcited electrons from C60 into TiO2, as it could be proved by time-resolved soft X-ray photoelectron spectroscopy, and makes the C60/TiO2 heterojunction useful for efficient solar cells.20 However, theoretical investigations of the energy level alignment at the C60/TiO2 interface using DFT methods suggest that this heterojunction might not be efficient due to a small energy gap between the LUMO of C60 and the conduction band minimum (CBM) of TiO2 .19 In a different approach C60 molecules are directly assembled on ultrathin titanium oxide films as charge injection buffer layers on metallic electrodes, like it is required for future applications in the field of organic electronics. Most importantly, the properties of ultrathin TMO films can change considerably with respect to their corresponding bulk materials, and the organic-TMO interface properties will be influenced accordingly.21 The surface composition as well as the structure of the ultrathin titanium oxide phases formed on metallic surfaces depend on the preparation conditions. Thus, a variety of different phases can be formed. However, the direct oxidation of the Pt3Ti alloy leads to a phase pure oxide film (w’-TiO) with homogeneous thickness and a significantly lowered defect density in contrast to TMO phases grown by evaporation methods.21 This makes the w’-TiO/Pt3Ti(111) system an ideal TMO model surface. Therefore, we focus on the assembly of C60 molecules on ultrathin w’-TiO films on Pt3Ti(111) single crystal surfaces.22 The w’-phase, a commensurate hexagonal (7x7)R21.8° TiO-structure, is stable, covers the entire Pt3Ti(111)-surface, and can be prepared in a reproducible manner.21 It is


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proposed that the w’-TiO phase consists of a bilayer with the stoichiometry 1:1. While titanium atoms cover the Pt3Ti(111) surface a terminating outer layer is composed mainly of oxygen atoms and in consequence, a polar bilayer results. A similar phase was observed after reactive evaporation of titanium onto a Pt(111) single crystal in the presence of oxygen.23 Additionally, a related bulk material with the composition TiO and a hexagonal bilayer structure, ε-TiO, was reported very recently.24 In this work, the interface between C60 molecules and an ultrathin titanium oxide film (w’-TiO phase) grown on a Pt3Ti(111) single crystal is investigated by UHV-STM methods and compared to the interface properties of C60 on other systems. Moreover, the scanning tunneling spectroscopy measurements give valuable insights into the energy level alignment at the C60/TiO interface and thus, will add a fundamental contribution to the interface engineering process for organic electronics.

EXPERIMENTAL METHODS The Pt3Ti(111) single crystal was cleaned by several cycles of neon sputtering (p(Ne) = 3x10-5 mbar) for 10 minutes and subsequent annealing at 1200 K for 25 minutes until a sharp p(2x2) pattern was visible by LEED. This procedure led to a Pt3Ti(111) alloy surface terminated by two layers of platinum as described recently.22 Exposing this 2Pt-Pt3Ti(111) surface to an oxygen dose of 600 L (p(O2) = 1.33x10-6 mbar, 10 min) at a sample temperature of 1000 K led to the formation of the w’-TiO phase. C60 molecules (Sigma Aldrich, purity 99.9%) were outgassed and then deposited by sublimation at 320 °C using a Knudsen cell onto the w’-TiO covered Pt3Ti(111) surface at room temperature. The C60 deposition rate was 0.05 ML/min. During the


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deposition the background pressure was in the 10-10 mbar range. After deposition the samples were transferred into the LT-UHV-STM and measured at 77 K. STM images were obtained using a commercial LT-UHV-SPM (Createc, Germany) under ultrahigh vacuum conditions at 77 K and homemade tungsten tips. The dI/dV spectra were measured above C60 molecules adsorbed on the w’-TiO/Pt3Ti(111) substrate. Lock-in detection of the ac tunnelling current was achieved by modulating the sample bias after switching-off the feedback loop. The measurement parameters used are given directly in the respective caption of the STM/STS measurements.

RESULTS AND DISCUSSION Structural Characterization The controlled oxidation of the Pt3Ti(111) single crystal terminated by two Pt-layers resulted in an ultrathin layer of w’-TiO as verified by UHV-STM imaging and LEED, like described recently in detail.21,25 This oxidation of the alloy surface leads to an ultrathin TiO film since the formation energy of TiO (-5.37 eV) is higher than that of PtO (-0.74 eV) or PtO2 (-1.39 eV) and in excess of the formation energy of Pt3Ti (-3.57 eV). However, one prerequisite for the oxidation process is that enough activation energy is supplied by choosing the proper preparation conditions. In our case in addition the diffusion barrier for Ti built by the two terminating Ptlayers on the Pt3Ti(111) single crystal had to be surmounted. Since this diffusion barrier is effective it is possible to create the w’-TiO phase in a controlled manner. The resulting w’-TiO phase is homogeneous and flat, and covers the whole surface (Figure 1b, left side, bias voltage +2.5 V). This appearance of the ultrathin w’-TiO phase obtained from STM imaging is voltage


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dependent and has its origin in the electronic surface structure of Pt3Ti(111), which has been modulated by the hexagonal w’-TiO phase.22 The apparent height differences amount to 60 pm at maximum in the regular phase and thus, can be attributed to electronic surface or small corrugation effects.

Figure 1. STM images of C60 on the w’-TiO covered Pt3Ti(111) surface. a) Large scale STM image showing C60 islands attached to step edges (392 x 392 nm2, Uset = 1.96 V, Iset = 0.02 nA,


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77 K). b) Enlargement of an edge of an C60 island with the w’-TiO covered Pt3Ti(111) surface on the left side and the C60 domain on the right side. The white line indicates the alignment of the substrate unit cell in relation to the C60 domain orientation (14 x 14 nm2, Uset = 2.50 V, Iset = 0.07 nA, 77 K). C60 molecules deposited on the w’-TiO phase covering the alloy crystal form well-ordered closepacked islands as shown in Figure 1. All islands are attached to step edges suggesting that the island growth starts thereof (see Figure 1a). The apparent height of the C60 islands on the surface is 0.94 ± 0.03 nm, which is significantly higher than the values obtained on metal surfaces or even on rutile TiO2(110) (0.8 nm).17 Especially, the apparent height of C60-islands in the latter case reflects the different electronic properties of the rutile TiO2(110) surface, which is characterized by rows of titanium or oxygen atoms. Here, the C60 molecules prefer the adsorption sites on the Ti rows in between the oxygen rows. In case of the w’-TiO phase the availability of Ti in the outer layer is reduced and thus, a weaker interaction with the C60 molecules results, as suggested by the increased apparent height. The large size of the incommensurable C60 domains (Figure 1) points also to relatively weak interactions with the substrate and a high mobility of the C60 molecules. Likewise, no small islands or single C60 are observed on terraces pointing to sites with higher adsorption energy due to possible defects in the w’-TiO phase. No evidence was found for a distinct interaction of the C60 molecules with the oxidized alloy surface. The w’-TiO film indeed covers the whole surface and determines the molecule-substrate interaction, that is, reduces it to a minimum. However, a certain interaction between the C60 islands, predominantly located at substrate step edges, and the oxidized alloy surface remains, since we find throughout a constant orientational relation, as shown in Figure 1b. The nearest neighbor distance between the C60 molecules is 1.00 ± 0.01 nm,


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the same distance as in the bulk C60 crystal as well as the distance measured between C60 molecules on the rutile TiO2(110) surface, pointing to a favorable intermolecular interaction.13

Figure 2. High resolution STM images of C60 on the w’-TiO covered Pt3Ti(111) surface showing a) the defect distribution in the C60 domain (49 x 49 nm2, Uset = 1.11 V, Iset = 0.03 nA, 77 K). b) the C60 molecules with molecular orbital resolution. Here, at a positive set-point value, the


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unoccupied molecular orbitals (LUMOs) are measured (8.2 x 8.2 nm2, Uset = 2.50 V, Iset = 0.10 nA, 77 K, slightly low pass filtered). A well-ordered close packed C60 island is presented in more detail in Figure 2a. Here, some point defects can be identified, like the randomly distributed dark features (black encircled), corresponding to missing C60 molecules. Such missing C60 molecules are also characteristic for C60 domains on rutile(110) surfaces and are generated most likely during the island formation process due to the fast growth of the islands.13,16,17 Furthermore, some rare features exhibit a very bright contrast (highlighted by a yellow circle in Figure 2a) corresponding to an apparent height of 0.30 nm and can be attributed to rare defects in the underlying w’-TiO phase. However, the majority of C60 molecules displayed in Figure 2a exhibits a medium contrast. Only slight differences between the apparent heights of somewhat brighter (magenta encircled) and somewhat darker C60 molecules are recorded and amount up to 90 pm at maximum. These C60 molecules with varying medium contrast are distributed randomly. An obvious reason for the varying medium contrast of the C60 monolayer adsorbed on the w’TiO phase covering the Pt3Ti(111) single crystal is given by the STM image of the molecular monolayer with orbital resolution (Figure 2b). Here, a wide variety of molecular orientations is depicted, in contrast to C60 monolayers deposited on reconstructed metal surfaces, where the great majority of C60 molecules faces the substrate with a hexagon.26-29 Orientation dependent apparent height differences amount in the case of C60 molecules up to 40 pm.27,28,30 This value and the apparent height differences observed for the w’-TiO phase (60 pm) add up to the observed 90 pm for the C60 monolayer. C60 molecules with different orientations (Figure 2b), that is, different adsorption geometries on the w’-TiO film, can be identified by the appearance of their unoccupied molecular orbitals. C60


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molecules exhibiting a two lobe structure face the surface with a 6:6 bond (blue encircled). Three different orientations of C60 with two lobe structure are depicted marked by black lines that enclose an angle of 60° in each case. In addition, examples of C60 molecules facing the substrate with a 5:6 bond are red encircled. They exhibit three lobes and one of them is significantly brighter than the other two. These C60 molecules show two different rotational orientations as indicated by the white and black labels in Figure 2b. The green-encircled C60 molecules are characterized by a bright spot, which corresponds to a pentagon facing the substrate. These C60 molecules are apparently 30 pm to 40 pm higher than the C60 molecules surrounding them. The white encircled C60 molecules face the substrate with a hexagon and are slightly tilted. These various rotational orientations observed at 77 K may have two reasons. First, C60 can adopt many different adsorption sites on the w’-TiO phase. The same has been observed for C60 monolayers deposited onto ultrathin Al2O3 films grown on Ni3Al(110) or on FeO bilayer structures grown on Pt(111)-surfaces.31,32 In these cases the diversity in adsorption sites was attributed to the complex structure of the oxide films, which is also present here. Second, various rotational orientations are observed for fullerenes deposited on unreconstructed metallic surfaces while self-assembled monolayers of fullerenes with rotational order, in general, are detected on reconstructed metallic surfaces.26-29 Like shown in [33] the molecule substrate interaction energy is an important driving force for the ordering process. Furthermore, if only small differences between adsorption energies of fullerenes on different surface sites are present, no rotational order will be observed.34 Since we observe only a weak interfacial interaction between fullerenes and the w’-TiO/Pt3Ti substrate no rotational order is expected. However, the question remains, is it possible that the intermolecular interactions between neighboring fullerenes in a monolayer may cause a rotational ordering. These interactions can be


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described by a Lennard-Jones potential and in addition by short range Coulomb interactions.33,35,36 For fullerenes the optimization of the electronic interactions between pentagons (i. e. electron poor sites of C60) and 6:6 bonds (i. e. electron rich sites of C60) leads to different configurations of neighboring C60 corresponding to local potential energy minima. Thus, these intermolecular interactions are not suitable to induce an overall orientational order.”

Spectroscopy In order to characterize in more detail the interface interactions in our system, that is, a C60 monolayer deposited on a polar w’-TiO ultrathin film covering a Pt3Ti(111) alloy single crystal, we performed scanning tunneling spectroscopy (STS) investigations. A typical differential conductance spectrum, dI/dV, representive for individual C60 molecules in the monolayer is shown in Figure 3. Here it should be noticed that the STS spectra do not change with respect to different adsorption configurations of C60 on w’-TiO since the molecules are only weakly physisorbed, like discussed above. Significant changes in STS spectra of fullerenes are observed only, if a considerable coupling between the adsorbed molecule and the substrate is present.17,26,27,37


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Figure 3. Differential conductance spectra (dI/dV) over a C60 on the w’-TiO covered Pt3Ti(111) surface (Uset = 0.08 V, Iset = 0.06 nA, f = 746 Hz, A = 0.02 V, average of 30 spectra, 77 K). The peaks in the spectrum correspond to the occupied and unoccupied molecular orbital energies and can be clearly identified: HOMO at -2.52 ± 0.04 eV, LUMO at +1.32 ± 0.04 eV, and LUMO+1 at +2.01 ± 0.04 eV. The onset of the HOMO (∆EH) is located at -1.8 eV and the onset of the LUMO (∆EL) at 0.95 eV (Figure 3). Values resulting for the HOMO-LUMO gap and the gap between the HOMO and LUMO onsets (∆EL - ∆EH) are given together with data for some selected corresponding systems from literature in Table 1.

Table 1: Values for the HOMO-LUMO gap and the (∆EL - ∆EH) gap of C60 molecules deposited on different substrates

C60/TiO/Pt3Ti(111) C60/ZnO

(HOMO-LUMO) [eV]

(∆EL - ∆EH) [eV]

Reference

3.84

2.75

This work

2.2 - 2.4

[7]


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C60/graphene/SiC

3.5

C60 multilayer film

3.5

C60 bulk crystal

3.7

C60/Au(111)

2.5

[37] 2.3

[38] [39]

1.0 – 1.2

[36] [27]

It is known that coupling of molecular states to surface states of metallic substrates or a screening of charges by the surrounding leads to a reduction of the molecular HOMO-LUMO gap, i.e., a molecule coupled to an electrode or in a molecular solid exhibits a smaller HOMOLUMO gap than a molecule in the gas phase. A comparison of the data given in Table 1 leads to the conclusion that the C60 monolayer investigated here consists of physisorbed molecules without hybridization of molecular orbitals with surface states. Likewise fits our obtained value for the HOMO-LUMO gap best to the value for the C60 bulk crystal. Moreover, it is slightly larger since C60 – C60 interactions in the third dimension are missing. This large HOMO-LUMO gap of 3.84 eV together with the apparent height of the C60 monolayer of 0.94 nm confirm the weak C60/TiO interface interactions, which are gradually even smaller than reported for C60/rutile or C60/graphene interactions.

Energy Level Alignment. The interesting values with respect to a potential use of TMOs in organic electronics are the charge injection barriers (ECBM – EL) and (EVBM – EH) after the alignment of the transition metal oxide and organic semiconductor energy levels (with ECBM = conduction band minimum and EVBM = valence band maximum). The relevant parameters for this energy level alignment are the


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TMO work function (Φ) as well as the molecule's ionization energy (IEorg) and electron affinity (EAorg). Based on these values the positions of the organic molecule’s HOMO and LUMO with respect to the substrate Fermi level (EF) are determined. According to the rule of an universal energy level alignment three different regimes can be distinguished depending on the values of IEorg, EAorg, and Φ relative to each other.5,6,9 If the work function is smaller than the electron affinity (Φ < EAorg) the Fermi level becomes pinned to the LUMO and the HOMO onset, ∆EH, stays constant. In the case that the TMO work function is larger than the ionization energy of the adsorbed molecule (Φ > IEorg), EF is pinned at a constant value to the HOMO. In this case the HOMO onset is observed to be 0.3 eV for different molecules including C60.5,6,41 In between those regions ∆EH decreases linearly as the substrate work function increases and the interaction between the molecule and the substrate is weak. Indeed, for C60 deposited on different types of TMOs three different regions have been determined, so that we can assume that the rule of the universal energy level alignment is also applicable for our system.5 Recently, refined models to describe the ranges of the energy level alignment regimes have been presented. They go without adjustable parameters or include the effect of the density of states of the organic semiconductor.41,42 Here, we deduce the energy level alignment for fullerenes adsorbed on the w’-TiO/Pt3Ti(111) surface (Figure 4) adopting values from our STS measurements and from literature.5,21 The values ∆EL = 0.95 eV and ∆EH = -1.8 eV are directly deduced from Figure 3, while the electron affinity of C60 on TMOs is taken over from [5] EAC60 = 4.2 eV. Insertion of these values into (∆EL - ∆EH) = (IEorg - EAorg) leads to a HOMO-LUMO gap of 2.75 eV and an ionization energy of IEC60 = 6.95 eV. The latter value is still smaller than IEC60,gas = 7.6 eV for C60 molecules in the gas phase, but higher than reported values for C60 molecules adsorbed on solid surfaces, that is,


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IEC60 = 6.3 eV to 6.49 eV.5,43,44 This points likewise to a very weak interaction of the C60 monolayer with the w’-TiO/Pt3Ti(111) surface, like suggested before, since ionization energies increase with decreasing electronic screening from neighbors. The work function of the w’-TiO phase covering Pt3Ti(111) is Φ = 5.33 eV,21 the valence-band maximum is EVBM = -3.5 eV (determined from Figure 6 in [21]), and the conduction-band minimum ECBM = 1.7 ± 0.2 eV (determined by STS measurements, see Supporting Information Figure S1). In the case of TiO2(110) the adsorption of C60 does not affect the valance band position of the TMO and the same is assumed here.20 The resulting energy level diagram of the C60/TMO interface is given in Figure 4.

Figure 4. Schematic of the energy level alignment at the C60/w’-TiO interface. The energy level alignment for the C60/w’-TiO interface clearly indicates that the onsets of the HOMO and the LUMO as well as the HOMO and the LUMO levels, both are located in the w’TiO phase band gap. Thus, the C60/w’-TiO interface is characterized by a Type-I heterojunction in contrast to the C60/TiO2 interface, which corresponds to a preferable Type-II heterojunction. The reason for this is the considerably larger band gap of the polar w’-TiO phase (5.2 eV) compared to TiO2(110) (3.0 eV). Type-I heterojunctions do not represent efficient chargeinjection junctions, in contrary, they decouple organic molecules quite well from a metallic alloy surface, as demonstrated in this work.


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Conclusion Large domains of C60 molecules are formed by self-assembly on the w’-TiO phase covering a Pt3Ti(111) single crystal. Nucleation starts at the substrate step edges and a high C60 mobility allows the building of a close-packed hexagonal monolayer with an intermolecular distance of 1.00 nm. Applying UHV-STM at 77 K individual C60 molecules were characterized with molecular orbital resolution and their rotational symmetry was determined. The large apparent height of the C60 monolayer together with a likewise large HOMO-LUMO gap obtained from STS reveal very weak C60/w’-TiO interfacial interactions. Using the experimental data the energy level alignment of the C60/w’-TiO interface was described. Since both, the molecular HOMO and LUMO levels, are located in the band gap of the polar w’-TiO phase, the C60/w’-TiO interface represents a Type-I heterojunction. Therefore, the C60 molecules are indeed well decoupled from the surface. This work shows that the surface modulating properties of ultrathin oxide films determine the energy level alignment at the adsorbate/TMO interface and that the right selection of the respective TMO and its phase is crucial to improve the charge-injection properties at the interface.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS Publication website. Differential conductance spectra on w’-TiO (PDF).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +49 2461 614015 ORCID Silvia Karthäuser: 0000-0003-3953-6980 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the help of Jochen Friedrich and Stephan Masberg.

REFERENCES (1) Gwinner, M. C.; Vaynzof, Y.; Banger, K. K.; Ho, P. K. H.; Friend, R. H.; Sirringhaus, H. Solution-Processed Zinc Oxide as High-Performance Air-Stable Electron Injector in Organic Ambipolar Light-Emitting Field-Effect Transistors. Adv. Funct. Mater. 2010, 20, 3457 - 3465.


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Titanium ... - ACS Publications

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