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Interaction at the F CuPc/TiO Interface: A Photoemission and X-ray Absorption Study Sumona Sinha, A.K.M.Maidul Islam, Mykhailo Vorokhta, and Manabendra Mukherjee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10803 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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The Journal of Physical Chemistry

Interaction at the F16CuPc/TiO2Interface: A Photoemission and X-ray Absorption Study Sumona Sinha‡||, A. K. M. Maidul Islam§, Mykhailo Vorokhta†,and Manabendra Mukherjee*‡

‡

Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata – 700064, India

§

Department of Physics, Aliah University, Kolkata–700064, India

†

Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles

University, V Holešovičkách 2, 18000 Prague, Czech Republic

ABSTRACT The interfacial interaction and charge transfer dynamics between F16CuPc molecular thin film and rutile TiO2(110) (1x1) surface have been studied by photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and resonant photoemission spectroscopy (RPES).The evolution of PES spectra as a function of F16CuPc film thickness shows strong coupling between the molecules and the TiO2 surface. Adsorbed molecules experience substrate mediated charge transfer. Electrons being pulled away from nitrogen atoms towards to carbon ring results an opposite direction binding energy shift for C1s and N1s. Moreover, the molecule gets deformed due to their strong interaction with the TiO2 surface. Ultrafast charge transfer from F16CuPc molecules to the TiO2 substrate takes place on the time scale of 10fs due to their strong electronic coupling. The results pave the way for the design and realization of F16CuPc based electronic devices.

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INTRODUCTION The rapid progress of novel electronic devices based on organic semiconductors demands a better and clear understanding of the inherent charge transfer mechanism1. The charge transfer, particularly that following photoexcitation, generally depends on the charge separation at the molecular or organic/inorganic interface. This process can be affected by many phenomena such as energy level alignment, interfacial wave function hybridization, chemical reaction, intermixing and film morphology. Moreover, the molecular packing and molecular orientation at interfaces are also found to noticeably affect the charge transfer mechanism both within the molecular assemblies and at the organic/inorganic interfaces2-6. Hence, understanding of the molecular configuration dependent charge transfer process has crucial implications for improving the performance of organic electronic devices. Most of these studies focused on the interaction of the organic semiconductor with metal substrates such as Ag, Au and Cu, as representatives of model hybrid junctions in electronic devices 7-10. Very recently, molecular ultrathin films started to be studied on the TiO2(110) surface.TiO2is extensively used in organic electronics for dual purposes. Its high dielectric constant makes it a valuable material for the fabrication of low threshold voltage and high output current organic thin film transistors. As well, some of its crystalline surfaces can be easily reduced to become strongly reactive and conductive, a transport property exploited in organic photovoltaics, where crystalline powders are coupled to organic dyes11-12. Particularly, the rutile TiO2(110) surface have drawn much attention for the possibility of changing its catalytic and charge transport properties by adjusting the concentration of oxygen vacancies (either by thermal annealing or by ion bombardment) 13. P. Krüger et al 14 and C. M. Yim et al 15 observed that the desorption of oxygen atoms leads to the appearance of a new electronic states in the band gap, which is associated with a redistribution of the local excess of charge among multiple sites around the Ti atoms. However, the origin of the gap states in TiO2 is


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still a debated issue

13-15

. Among organic semiconducting molecules, phthalocyanines have

good photoelectrical properties associated with their chemical and thermal stability and thus they have attracted much attention in the field of organic electronics16-18. Until now, the studies have been carried out on rutile TiO2 (110) surfaces with p-type phthalocyanine molecules for understanding of molecular adsorption properties3,

19-20

. The surface

morphology, energy level alignment and excited-state dynamics were investigated by depositing TiOPc, CuPc, H2Pc and ZnPc on single crystal TiO2 surfaces

21-24

. The

investigations have pointed to a general fact that strong chemical interaction exist at phthalocyanine/TiO2 interfaces. It is found that molecular fluorination plays an important role in modifying the gap, transport properties, electron affinity and ionization potential ultimately affecting the interfacial properties of the molecule 25.Copper hexadecafluorophthalocyanine (F16CuPc) is one of the few high-performance and air-stable semiconducting molecules that show n-type behaviour

26

.The incorporation of both n-type F16CuPc and p-type CuPc

molecules within the active region of a transistor has been shown to be an efficient way for producing ambipolar organic field effect transistors (OFET)27. Besides OFETs, the CuPc/F16CuPc heterojunction is found to play an important role in enhancing the performance of organic optoelectronic devices such as organic light-emitting diodes and photovoltaic cells28, 29. In this work, we employed synchrotron-based photoemission spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and resonant photoemission spectroscopy (RPES) to investigate the interfacial interaction, molecular orientation and charge transfer process between F16CuPc molecular thin film and rutile TiO2(110) (1x1) surface by varying the thickness of F16CuPc films from sub-monolayer to multilayers. We revealed from the evolution of PES spectra with F16CuPc film thickness that a strong interaction between the molecules and the TiO2 surface occurs where adsorbed molecules 3 ACS Paragon Plus Environment


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experience substrate mediated charge transfer. Simultaneously, electrons are being pulled away from nitrogen atoms towards to carbon ring which leads to an opposite direction binding energy shift for C1s and N1s with thickness. Moreover, our PES and NEXAFS data indicate that the molecules get deformed as a result of substrate molecule interaction. It is found that ultrafast charge transfer from F16CuPc molecules to the TiO2 substrate takes place on the time scale of 10 fs due to their strong electronic coupling. Our results have substantial implications for the understanding of the interaction and charge transfer dynamics at F16CuPc /TiO2 interfaces and pave the way for the design and realization of F16CuPc based electronic devices.

EXPERIMENTAL DETAILS The experiment was carried out at the end station of Material Science Beamline of the Elettra synchrotron, Trieste, Italy. This beamline provides a horizontally polarized photon beam. The experimental chamber is equipped with a high luminosity electron energy analyser (Specs Phoibos 150, 150 mm mean radius, with nine channels) and operated at a base pressure of ~2 × 10−10 mbar. Sample preparation was done in the preparation chamber which is connected to the analysis chamber. The chamber is equipped with an Ar sputter gun, sample annealing facility and low energy electron diffraction (LEED) optics, as well as molecular evaporator. After each deposition the samples were characterized by in-situ photoelectron spectroscopy and NEXAFS spectroscopy. Data were collected from different positions on the sample surface to minimize the effect of beam damage. All the depositions and measurements were performed at room temperature. The rutile TiO2(110) single crystal (Sigma Aldrich) was cleaned by repeated cycles of Ar+ sputtering and annealing at 1000K in UHV chamber until no contamination was detected by


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PES and low-energy electron diffraction (LEED) showed a clear 1 ×1 diffraction pattern (figure S1, Supporting Information)13. Before each deposition, the surface was cleaned by several cycles of Ar+ sputtering and annealing in UHV until no contamination was detected by PES and the LEED showed a sharp (1x1) pattern. The F16CuPc molecules (TCI, Japan, >98%) were thoroughly degassed for several hours before sublimating onto the substrates using a homemade Knudsen-type cell after sufficient degassing. The nominal thicknesses of the F16CuPc films were calibrated by monitoring the evaporation rate with a quartz crystal Photoelectron spectra were collected at normal emission. The spectra were collected using different photon energies. The excitation energy was as follows: hν= 110 eV for VB; hν= 410 eV for C1s; hν= 450 eV for N1s; hν= 750 eV for O 1s, F1s and Ti2p and hν= 1000 eV for Cu2p. The energy scale was calibrated to the Fermi level, recorded from the tantalum sample holder. Spectra were first normalized to flux by dividing the sample current with the corresponding mesh current. Curve fitting of core level spectra was done with Gaussian−Lorentzian functions and a background correction by using Peakfit data evaluation software.The background of the C1s and N1sspectra was fitted by a straight line whereas a Shirley background fit function was used for metal XPS spectra30. The NEXAFS spectra were taken at the C, N and F K-edge using the Auger yield in three geometries: at normal (θ = 90o), at grazing (θ = 20o), and close to the magic angle geometry (θ = 50o) incidence of the photon beam with respect to the surface. The polarization of light from the beamline was estimated to be P~ 85% linear, the radiation source being a bending magnet. The raw data were first normalized to the relative intensity of the photon beam. Afterwardsbackground correction and normalization of spectra were carried out according to the established procedure31-33.



RPES spectra of the valence band (VB) region were measured with the photon energy scanned across the C, N and F K-edge absorption threshold and were plotted on a binding energy (B.E.) scale with respect to the substrate Fermi level (EF).The binding energies of VB experiments were calibrated to the Fermi edge of the tantalum sample holder. The normal emission photoelectrons were collected with the same X-ray incident angle as that of NEXAFS measurement. The intensities of the spectra were normalized to the incident photon intensities and the binding energy of RPES is aligned with their counterparts measured at photon energy of 110 eV3, 5.

RESULTS AND DISCUSSION:

30000 10000

Raw Data Fitted Data

C-F

50 Å

20000

C-N C-C

π-π* shake-up satellites

Interfacial peak

0 20000

Counts

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12 Å

10000 0 10000 5Å 5000 0 6000 4000

Interfacial peak

2Å

2000 0 292

290

288

286

284

282

Binding Energy (eV)

Figure 1. C1s core level XPS spectra with their numerical fit as a function of the deposited F16CuPc film thickness on TiO2 (111). 6 ACS Paragon Plus Environment

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C1s photoelectron spectra recorded during in-situ stepwise deposition of F16CuPc (C32CuF18N8) thin films on the TiO2 (110) substrate are shown in figure 1 with their numerical fits. All calculated peak parameters are summarized in table I. Three core-level states along with the three accompanying shake-up satellites associated with kinetic-energy loss of photoelectrons due to simultaneous HOMO to LUMO excitation are clearly observed for all the films. The peak CC, CN and CF represents C–C, C–N, and C–F bonds respectively with SCC, SCN and SCF representing the corresponding satellites25, 34-36. An additional peak of varying intensity at about 285.06 eV (interfacial component) was needed to fit the C1s spectra of F16CuPc films on TiO2 substrate correctly. This additional lower binding energy component is assigned to the interfacial molecules, indicating a strong influence from the substrate

21-24

. With increasing thickness the interface component reduces as shown in figure

1 and table I. Table 1. Summary of peak parameters obtained from fitting of C1s peak for F16CuPc/TiO2 interface for various F16CuPc film thicknesses. Thickness (Ǻ) 50

12

5

Peak Assign

Post (eV)

Peak Separ

Interfacial CC CN SCC CF SCN SCF Interfacial CC CN SCC CF SCN SCF Interfacial CC CN

284.23 285.15 286.25 286.80 287.27 287.88 288.87 284.43 285.43 286.54 287.08 287.58 288.40 289.26 284.43 285.48 286.59

Value FWHM (eV) (eV)

CN – CC CF– CC CF– CN SCC– CC SCN– CN SCF– CF

1.1 2.02 1.02 1.65 1.63 1.6


1.11 2.15 1.04 1.65 1.86 1.67

CN – CC CF– CC

1.11 2.14

0.90 0.88 0.88 0.88 0.89 0.99 1.04 1.00 0.92 0.92 0.89 0.92 0.99 1.04 1.10 0.94 0.94


IC-C: IC-N: IC-F

I(C-C+ S_C-C): IInterfac I (C-N+S_C-N): ial-comp I(C-F+S_C-F) (%)

1:0.7:1:7

1:0.82:1.63

25.36

1:0.73:1:84

1:0.88:1.89

13.54


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2

SCC CF SCN SCF Interfacial CC CN SCC CF SCN SCF

287.11 287.62 288.32 289.26 284.56 285.33 286.42 286.91 287.73 288.12 289.08

CF– CN SCC– CC SCN– CN SCF– CF

1.03 1.63 1.73 1.64


1.09 2.40 1.31 1.58 1.7 1.35

0.89 0.94 0.99 1.36 0.96 1.16 1.16 0.8 1.16 1.70 1.40

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1:0.77:1.74 1:0.89:2.05

1:0.46:1:06

4.43

1:0.54:1.37

The standard intensity ratio of (CC+SC): (CN+SN): (CF+SF) is 1:1:2 for F16CuPc molecule. We observe that this ratio increases from 1:0.54:1.37 to 1:0.82:1.63 with increasing thickness from 2 Ǻ to 50 Ǻ of F16CuPc films, i.e. it approaches the standard ratio. Similarly the observed intensity ratio of CC:CN:CF was enhanced from 1:0.46:1:06 to 1:0.7:1:7. The deviation of the ratios at sub-monolayer coverage from the expected value may be explained in terms of molecular deformation or change in molecular orientation

3, 9, 37

by the influence

of TiO2 substrate. We examined F1s, N1s and Cu2p core level XPS spectra as a function of thickness for successive deposition of F16CuPc on the TiO2(110) substrate to acquire more explicit information about which sites of F16CuPc get perturbed on close contact of TiO2. No significant changes in peak position and shape were noticed for F1s and Cu2p core level spectra (data not shown). On the other hand, a striking evolution of N1s XPS spectra with thickness of F16CuPc film was observed. We show the fitted N1s core level XPS spectra as a function of thickness for successive deposition of F16CuPc on a TiO2(110) substrate in figure 2. A main peak (N1) at about 399.2 eV and corresponding shakeup satellite (SN) at around 400.8 eV are observed for 50 Ǻ thick F16CuPc film (upper spectrum). In this context, it can be mentioned that the separation between main and satellite peak of N1s spectra of bulk copper phthalocyanine layer is about 1.8 eV9, 25, 35. 8 ACS Paragon Plus Environment

4.37

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50 Å SN

N1

SN N2

N1

20000 0 40000 20000

Counts

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12 Å

0 30000 20000 10000 0

5Å SN

N2

N1

20000

2Å 10000 0 404

SN

N2

402

N1 400

398

396

Binding Energy (eV)

Figure 2. Fitted N1s core level XPS spectra as a function of the deposited F16CuPc film thickness. But an additional small peak (N2) at intermediate position of main and satellite peaks is needed to fit the data of F16CuPc thin film at lower thickness correctly. This peak is located at about 398.8 eV for 2 Ǻ F16CuPc thin film. This additional small peak is found to diminish with increasing F16CuPc film thickness. The binding energy of the peak, N2 is in the range for nitrogen atoms strongly interacted to oxygen of TiO2 surface

40-42

. This suggests that the

nitrogen atoms of the F16CuPc molecules are strongly coupled 43 to the TiO2 surface through oxygen. Some conformational change of the F16CuPc molecules due to strong coupling with the TiO2 substrate can be expected. Earlier, Y. Wang et al 18 reported the occurrence of such phenomena at phthalocyanine/TiO2 interfaces from their study of adsorption of CuPc on the rutile TiO2(110) surface by scanning tunnelling microscopy and spectroscopy (STM and 9 ACS Paragon Plus Environment


STS). In addition, N1s core level peak is observed to shift about 0.2 eV towards higher binding energy with subsequent deposition. Moreover, it is interesting to note here that N1s peak was shifted towards higher binding energy whereas C1s was shifted towards lower binding energy with subsequent deposition of F16CuPc molecules. Upon adsorption on TiO2 F16CuPc experiences substrate mediated charge transfer where electrons are being pulled away from nitrogen atoms towards the carbon rings leading to energy shift of C1s and N1s in opposite direction. Here it may be mentioned that no significant changes in peak position and shape were observed for Ti2p and O1s peaks (data not shown). This may be explained in terms of the fact that the amount of nitrogen at the substrate molecule interface is much less compared to those of Ti or oxygen, hence the change (if any), cannot be detected in their spectra. To get more specific information aboutmolecular deformation and chemical interaction at F16CuPc/TiO2 interfaces, NEXAFS experiments at the nitrogen K- edge was carried out. Figure 3 shows the π* region of N K-edge NEXAFS spectra taken at normal-incidence geometry at the F16CuPc/TiO2 interface as a function of the molecular film thickness. N K-edge 2Å 5Å 12 Å 50 Å

Norm. Intensity (Arb. Unit)

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

396

398 400 402 Photon Energy (eV)

404

Figure 3. π* region of Nitrogen K-edge NEXAFS spectra taken at normal-incidence geometry at the F16CuPc interfaces with TiO2 as a function of the molecular layer thickness.


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It can be noticed from the figure that the NEXAFS spectrum of 2 Ǻ thick F16CuPc film was shifted slightly to the higher photon energy with respect to thick films. This peak shift may be occurred due to the imhomogenous charge transfer for phthalocyanine

24, 43

. One can easily

observe from the figure the two asymmetric broad peaks at 398.7 and 401.0 eV for 50 Ǻ thick F16CuPc film on the TiO2 substrate. Moreover, the shape of the π* region of the NEXAFS spectra (particularly the blue box region) vary prominently at sub-monolayer coverage on TiO2. The asymmetric feature is more prominent for the sub-monolayer film (5Å) than the thicker film (50 Å) on TiO2 surface. Using the available literature31,

38-39

, we have

deconvoluted the π* region of Nitrogen K-edge NEXAFS spectra of 5Å and 50 Å thick F16CuPc thinfilms on TiO2 to get information for the specific bonding configurations of nitrogen atoms, as shown in figure 4. Features near 399 and 401 eV are denoted by transition ‘1’ and transition ‘2’ respectively, as displayed in figure 5. Transition ‘1’ may be ascribed to an excitation from N1s into LUMO and transition ‘2’ represents that from N1s to higher empty π* states. A detail fitting shows both the transitions require two peaks at 398.64, 399.22, and 400.45, 401.31eV respectively for both monolayer and multilayer.



1


50 Å Norm. Intensity (Arb. Unit)

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2

1

2

5Å

398 400 Photon Energy (eV)

402

Figure 4. Fitted π* region of Nitrogen K-edge NEXAFS spectra taken at normal-incidence geometry of 5 Å and 50 Å thick F16CuPc thin films on TiO2 substrate. Ιt can be easily noted from the figure that the comparative ratio between the intensity of component peaks of transition “1” (as well as transition “2”) drastically deviated at submonolayer coverage on TiO2 substrate.The intensity of NEXAFS resonant transition is related to the total number of electrons produced by decay (normal Auger + autoionization), which is proportional to the absorption cross-section. The variation of intensity ratio of component peaks in the π* region of the N K-edge NEXAFS spectra taken at the same incidence geometry indicates that all nitrogen sites are not equally changed with thickness of the molecular layer on the TiO2 substrate. This may suggest that F16CuPc molecules get deformed due to strong coupling between the nitrogen atoms of the molecules and the TiO2 surface.


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In order to further elucidate possible molecular orientation, polarization dependent N K-edge NEXAFS spectra of 50 Å thick F16CuPc film on TiO2 is presented in figure 5. It can Norm. Intensity (Arb. Unit)

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


3 (a)

θ=10o θ=50o θ=90o

2

1

0 395

400 405 Photon Energy (eV)

410

Figure 5. Polarization dependent N K-edge NEXAFS spectra 50 Å thick F16CuPc film on TiO2 surface. be easily noted from the angle dependent linear dichroism that F16CuPc molecules are in nearly lying down configuration in 50 Å thick film on both substrates. Similarly, on both substrates, the π* character was more intense with respect to σ* character at sub-monolayer coverage also (data not shown). This suggests that no significant change in the geometry (lying down) of F16CuPc molecules has occurred for the first molecular layers TiO2 substrates. Electronic structure of occupied (valance electronic structure) and unoccupied (NEXAFS) states of the molecules and the substrate is one of the most important factors that influence the charge transfer dynamics, because such a transfer on the femtosecond time scale can proceed only if the substrate density of state matches that of the adsorbate molecules. In other words, the unoccupied molecular orbital of the adsorbates should overlap with the substrate (TiO2) conduction band.



In order to study the energy levels (occupied and unoccupied) of the molecules and the substrate, in figure 6 we have plotted the valence band (VB) and N-K- edge NEXAFS spectra for sub monolayer to multilayer films of F16CuPc. All the NEXAFS spectra in figure 8 have been aligned to the binding energy scale. The energy level alignment has been done by subtracting the binding energy of the N1s PES peak from the photon energy of the corresponding NEXAFS (BEN1s−Ephoton energy) 44. In the figure the valance band spectra from sub-monolayer to multilayer F16CuPc along with that of clean TiO2 are shown. At higher molecular coverage (12 Å and 50 Å), HOMO and various other molecular frontier orbital (H1, H-2, and H-3) peaks from F16CuPc molecules are clearly visible within the binding energy

NEXAFS

VB

5Å

LUMO LUMO+1 LUMO+2 H-3

H-2

50 Å 10

8

6

H1 HOMO

4

2

Intensity (Arb. Unit)

5Å

Clean TiO2 Intensity (Arb.Unit)

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50 Å 0

-2

-4

-6

-8

-10

Binding Energy (eV)

Figure 6. The valence electronic spectra of 5 Ǻ and 50 Ǻ thick F16CuPc films on TiO2 and that of clean TiO2.The relative binding energy scale of NEXAFS (blue line for 50 Å and green for 5 Å as a typical case) was referenced to the N1s core level BE of F16CuPc molecules (BEN1s−Ephoton energy).


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range 2−8 eV. In contrast, a sub-monolayer spectrum contains strong substrate contributions. N-K-edge NEXAFS spectra provide information about the empty orbitals (LUMOs) in the presence of aN1s core hole. Dominant resonances are observed in the transition from N1s to individual π* states between 397 to 403eV and the broad absorption peaks at higher photon energy above 403 eV are assigned to the σ* states as discussed earlier. From the energetic point of view, energy level alignment of NEXAFS to the binding energy scale shows that the transition from N1s to the LUMO ends up at the band gap of the TiO2 substrate and the excited electron is trapped locally within the band gap (~3eV)

21-24

of the

TiO25, 44-45. This may be due to the fact that the coulomb interaction between the core-hole and photo-excited electrons largely pulled down the energy level of photo excited F16CuPc toward the Fermi level, EF 44 Consequently, any charge transfer from this resonance (LUMO, for sub-monolayer as well as multilayer F16CuPc) is prevented by the electronic mismatch with the TiO2 substrate. However, the higher energy resonances of NEXAFS overlap the substrate conduction band and may participate in the interfacial charge transfer. Resonance photoemission spectra have been used for the determination of whether an excited electron stays localized on the molecules during core-hole lifetime or it is transferred to the substrate conduction band. The RPES scans are essentially a collection of valence band spectra recorded at different photon energies across the N K-edge threshold. As photon energies are being scanned, certain transitions are resonantly enhanced. At the same time, during core-hole decay through the auto-ionization process, enhancement at a specific photon energy will be quenched if the charge transfers away from the molecular LUMO (details of basics of RPES is given in figure S2, Supporting Information). In order to evaluate this issue, an RPES contour plots for all the samples (from sub-monolayer to multilayer) were collected across the N K-edge. RPES results of sub-monolayer and multilayer are shown in figure 7 (two images for 5Ǻ and 50Ǻ are plotted as typical cases). In order to compare the intensity 15 ACS Paragon Plus Environment


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variation of specific VB peaks, the binding energy scales of the plots were aligned with standard valence band spectra taken at 110 eV. Similarly, the photon energy scales were also aligned with NEXAFS resonances to track the intensity variation at specific photon energy as shown in figure 7 (b) and 7 (d). Details of individual RPES spectra are also shown in figure S3 in the Supporting Information. In the NEXAFS data, the photon energies required for the N1s→π* resonance transitions are marked by 1 (photon energy ~ 399 eV) and 2 (~ 401 eV) as horizontal lines. The spectra for sub-monolayer coverage (figure 7 (a)) were noticeably different from that of multilayer (figure 7 (c)). For the multilayer film, shown in figure 7 (c),

Figure 7: Resonant photoelectron spectra contour plots (image plots) at N-K-edge for (a) 5 Å and (c) 50 Å F16CuPc films on TiO2. The spectra plotted on the images are the corresponding valence band measured with photon energy 110eV. Right panel: corresponding NEXAFS spectra for (b) 5 Å and (d) 50 Å F16CuPc films on TiO2.


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resonant enhancements of individual molecular orbitals between 2 and 8 eV are clearly visible at photon energies ~399 eV. These features are mainly associated with the resonant enhancement of H-1, H-2, and H-3 molecular orbitals. The broad structures above 8eV are mostly due to normal Auger (in the case of charge transfer) or spectator (in case of no charge transfer) process. At photon energy around 401eV, only one resonant feature around H-2 (~ 6eV) orbital was observed. Uniform intensity was observed for photon energies higher than 401eV which was basically due to the TiO2 substrate. There is also a weak contribution from N1s photoelectrons generated by second order light from the monochromator, which superimposes on the RPES a linear trace developing from higher to lower binding energy with increasing photon energy. On the other hand, RPES spectra for sub-monolayer coverage reveal a different scenario, and the strong signals with binding energy in the region 2−8 eV mostly originate from the TiO2 substrate signals as shown in figure 8 (a). However, one can see similar enhancement of the RPES spectra for the photon energy near 399 eV, whereas, no enhancement of RPES intensity (compared to multi-layer) between 2−8 eV was observed at photon energy near 401 eV. This means that apparent resonant enhancements of RPES spectra between 2−8 eV for the photon energy near 401 eV for the sub-monolayer interface are quenched which suggests that the excited electrons transfer from the higher excited states of the molecule to the substrate conduction band 46. Core level XPS and the NEXAFS studies have also confirmed a strong electronic coupling at the sub-monolayer/TiO2 interface which may lead to the charge transfer at the sub-monolayer/TiO2 interface. The charge transfer time at F16CuPc/TiO2 interfaces has been calculated by quantifying the variation of RPES intensities. The resonance enhancement at different photon energies contributed by the participator decay channel has been quantified by integrating each RPES spectra between 2 and 5 eV in binding energy to exclude any Auger-type signal contributions i.e. spectator decay or normal Auger 17 ACS Paragon Plus Environment


decay. The background of integrated RPES spectra contributed by the VB features of the TiO2(110) substrate was subtracted using integrated RPES signals collected on the clean TiO2 (110) under the same measurement conditions. The integrated signals are then re-plotted as a function of photon energy and compared with the NEXAFS spectra as shown in figure 8. Since the transition to the LUMO at 399 eV is not involved in the interfacial charge transfer process, the integrated RPES spectra have been normalized to the height of the NEXAFS peak at 399 eV.

Integrated RPES NEXAFS

1 Intensity (Arb. Unit)

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

50Å

1 2

5Å

396 398 400 402 404 406 408

Photon Energy (eV) Figure 8. Integrated RPES and corresponding to N K-edge NEXAFS spectra for 5 Ǻ and 50 Ǻ F16CuPc films on rutile TiO2 (110).

As discussed earlier, the NEXAFS intensity is proportional to the number of electrons excited into unoccupied molecular orbitals, whereas the integrated RPES profile only contains participator signal. Any reduction in RPES integrated intensity can be correlated as a measure of charge transfer from a particular molecular orbital. Therefore, by comparing the ratios of 18 ACS Paragon Plus Environment

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the NEXAFS and integrated RPES for a system of interest in the absence of charge transfer (isolated multilayer for our case) and

the system with charge transfer (coupled sub-

monolayer system), the charge transfer time can be calculated using the following formula 4749

,

=

/

/ −

/

where τCH is the core-hole lifetime which has been reported to be 6 fs for N1s46,

50-51

.

and

are the integrated intensities of LUMOs for RPES spectra as measured in the coupled sub-monolayer and isolated multilayer of F16CuPc.

and

represent the

intensities of LUMOs in NEXAFS spectra for sub-monolayer and multilayer systems. As we have seen earlier that the resonance transition at ~399 eV is forbidden for charge transfer, the ratios RPES/NEXAFS are calculated for the 2nd peak near 401 eV (indicated by transition 2 in figures 8). A reduced intensity ratio for the sub monolayer case (0.518 for 5Ǻ) was obtained as compared to the multilayer case (0.852 for 50 Ǻ), which indicates that part of the photoexcited electrons were transferred at F16CuPc/TiO2 interfaces for the sub monolayer films. Substituting theses values in the above equation, the charge transfer times have been estimated. The obtained values for 5 Ǻ and 12 Ǻ films are ~10fs and ~13fs respectively. The results clearly show that fastest charge transfer occurred at sub-monolayer coverage of F16CuPc.

CONCLUSIONS In conclusion, we have studied interfacial interaction and charge transfer dynamics of F16CuPc on rutile TiO2(110). The evolution of PES spectra with F16CuPc film thickness is indicates strong coupling between the molecules and the TiO2 surface. During substrate mediated charge transfer electrons are pulled away from nitrogen atoms towards carbon ring



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which leads to an opposite direction binding energy shift of C1s and N1s with thickness. Moreover, the molecules get deformed due to this strong interaction. Ultrafast charge transfer from F16CuPc molecules to the TiO2 substrate is found to take place on the time scale of 10 fs due to their strong electronic coupling. The results thus have significant implications for the understanding of the interaction and charge transfer process at F16CuPc /TiO2 interfaces and pave the way for the design and realization of F16CuPc based electronic devices.

ASSOCIATED CONTENT Supporting Information 1)

Picture of the LEED pattern for clean rutile TiO2(110).

2)

Schematic representation of (a) photoexcitation; (b) participator, (c) spectator (d) Auger decay and (e) transfer of excited electron.

3)

RPES spectra for (a) 5 Ǻ and (b) 50 Ǻ F16CuPc on TiO2(110).

AUTHOR INFORMATION *Corresponding Author: [email protected]. ||

Present Address: S. N. Bose National Central for Basic Sciences, Kolkata-700106, India

Notes The authors declare no competing financial interest

ACKNOWLEDGEMENTS The work is partially supported by the Indo-Italian (DST-Elettra) Programme of cooperation (POC) in Science and Technology. We acknowledge Prof. Kevin C. Prince, ElettraSincrotrone and IOM, Italy for his help to perform the experiments at Material science beamline, Elettra-Sincrotrone, Italy and his valuable suggestions for modifying the manuscript. 20 ACS Paragon Plus Environment

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