Article pubs.acs.org/ac
Dynamics of Molecular Orientation Observed Using Angle Resolved Photoemission Spectroscopy during Deposition of Pentacene on Graphite Sang Han Park and Soonnam Kwon* Beamline Division Group of PAL-XFEL Project Headquarters, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, Korea 790-784 S Supporting Information *
ABSTRACT: A real-time method to observe both the structural and the electronic configuration of an organic molecule during deposition is reported for the model system of pentacene on graphite. Structural phase transition of the thin films as a function of coverage is monitored by using in situ angle resolved photoemission spectroscopy (ARPES) results to observe the change of the electronic configuration at the same time. A photoemission theory that uses independent atomic center approximations is introduced to identify the molecular orientation from the ARPES technique. This study provides a practical insight into interpreting ARPES data regarding dynamic changes of molecular orientation during initial growth of molecules on a well-defined surface.
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molecules (1,3-bis(N-carbazolyl)benzene) as measured using angle integrated UPS.16 A simulation of ARPES that assumed that the final state is a plane wave showed that ARPES intensity is proportional to the Fourier transform of the initial state orbital if the emission direction is parallel to the polarization vector of the p-polarized light.17 This approach (orbital tomography), has allowed reconstruction of orbital density in highly oriented monolayer films and can determine molecular orientations.23,24 Azimuthal alignment can be measured by combining ARPES and low-energy electron diffraction.25 In this study, we elaborate on the experimentally observed dynamic changes of ARPES spectra during pentacene growth on graphite as a function of coverage or deposition time. The ARPES spectra were systematically analyzed using IAC in photoemission theory. The changes in spectra were assigned to dynamic changes of molecular orientation. This study gives insight into how to interpret ARPES data collected during phase transition of an adsorbed molecule during its initial growth stage.
aterials for organic electronics have potential applications in cheap and flexible optoelectronic devices such as organic photovoltaics and organic light-emitting diodes. Initial growth behavior of organic materials determines the characteristics of charge transfer between the substrate and organic semiconductor and therefore strongly affects the electrical characteristics of the resulting device. The orientation behavior of the molecules is governed by the interaction between substrate and adsorbate and between adsorbates depending on the coverage; this interaction is the driving force of the change in molecular orientation as the coverage increases. Pentacene is widely used in organic electronics and is well-known for its phase change depending on the surface coverage on highly oriented pyrolytic graphite (HOPG).1−6 Methods that have been used to observe the orientation of molecules on a surface include scanning tunneling microscopy (STM), near edge X-ray absorption fine structure (NEXAFS), transmission electron microscopy (TEM), and Kelvin probe force microscopy (KPFM).1,7−11 As an alternative, angle resolved photoemission spectroscopy (ARPES) had been suggested because it can probe electronic structure and molecular orientation simultaneously; this ability is a big advantage over the other experimental tools.12 However, to interpret and correlate the spectra to molecular orientation, theoretical assistance is imperative.12−17 For an ordered crystalline surface and periodic adsorbates on a crystalline surface, a dynamic theory of ARPES has been developed.18−22 The independent atomic center approximations (IAC)12−16 exploits exact photoemission calculation on each atom to simulate the whole molecule. Simulation using IAC has been used to interpret the orientation of small © 2016 American Chemical Society
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EXPERIMENTAL SECTION A ZYA grade HOPG (Materials Quartz Inc.) and a graphite single crystal (NGS Naturgraphit GmbH) were cleaved using scotch tape outside a UHV chamber, then placed in the chamber immediately. The substrates were degassed at 400 °C for >5 h to remove any adsorbed contaminants. The cleanness Received: March 12, 2016 Accepted: March 21, 2016 Published: March 21, 2016 4565
DOI: 10.1021/acs.analchem.6b00986 Anal. Chem. 2016, 88, 4565−4570
Article
Analytical Chemistry
Figure 1. Simulated ARPES results of pentacene: (a) PES intensity distribution in k-space for selected molecular orbitals. Concentric circles: emission angles from surface normal. (b) Calculated PES spectra with various acceptance angles. Photoemission intensities (arrows) of the MOs depend on the symmetry of each orbital. (c) Simulated ARPES of lying-down (LD) pentacene on HOPG; PES intensity averaged over azimuthal angles. (d) Same as part c, except that pentacene is slightly disordered from LD orientation (inset). (e) PES intensity of HOMO as a function of surface-parallel momenta.
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RESULTS Photoemission Theory of Pentacene. Pentacene thin film grown on HOPG generally assumes one of three orientations: LD, S(022), and TU, although other disordered geometries may exist. ARPES simulation of the three geometries were performed using IAC. The first simulation considered the PES intensities of highest occupied molecular orbital (HOMO or H) and lower orbitals, (i.e., H-1, H-2) of isolated LD pentacene (Figure 1a,b). If pentacene is perfectly LD without disorder, the PES signal normal to the surface should be absent for most of the molecular orbitals (MOs). This situation can be conceptually understood if the spatial distribution of MOs is carefully inspected. MOs H to H-3 exhibit completely antisymmetric characteristics (Figure 1b, inset), whereas H-4 shows nonzero net distribution.28,29 At normal emission PES from MOs H, H-1, H-2, and H-3 are forbidden but are allowed from H-4. However, as the acceptance angle of the detector is increased, the previously forbidden spectral peaks slowly increase in intensity (Figure 1b). For pentacene on HOPG, in which the micrograins of graphite have no azimuthal orientation order, the PES intensity can be obtained from the average over the azimuthal angles. Therefore, in the PES intensity map vs surface parallel momentum (Figure 1c), the momentum is averaged over the azimuthal angles for the simulated band structure30 and only the magnitude information on the momentum survives (Figure 1a, circles). For perfectly-LD pentacene, the intensity of normal emission is zero except for H-4 (Figure 1c). However, as the momentum increases, the intensity increases slightly; for this reason, angle-integrated measurement can produce nonzero intensity for forbidden MOs. On the contrary, if slight disorder occurs around the long axis of LD pentacene, the previously absent PES shows nonzero intensity (Figure 1d) even at zero
of the substrates was confirmed by examining the XPS core level spectra of nitrogen, oxygen, and carbon. Pentacene (Luminance Technology Corp.) was used as received and fully degassed before deposition to prevent contamination by any exotic molecules. Pentacene was deposited at 0.1 nm/s during ARPES measurement. ARPES was performed at the 8A2 HR-PES beamline at the Pohang Accelerator Laboratory using photon energy from 97.7 to 406.3 eV. The 8A2 beamline provides p-polarized X-ray in the energy range of 100−1600 eV. The electron analyzer was equipped with SES-2002 (VG Scienta) and the angle between analyzer and photon was set to 50°. The angular resolution of the ARPES experiment was 15° shows obviously different photoemission distribution than the experimental results, i.e., TU geometry has a small intensity for H-3 (Figure 2, upper panel) and S(022) geometry has a small intensity for H-2 (Figure 2, lower panel). These MO positions using second derivative can be slightly different because of overlap between MOs. However, these results are well matched with previous reports about crystal and gas-phase pentacene.33,34 For the binding energy >4.0 eV, it is hard to analyze spectra because large background comes from the substrate and higher order MOs which are inseparably mixed. After deposition, 15 Å of pentacene on HOPG and single crystal graphite, the spectrum show significantly decreased intensity at 3.0 eV compared to the both sides (Figure 7b,d). The second derivative spectra show only three minima at the binding energy of 1.4, 2.6, and 3.6 eV. For these spectra, H-2 cannot be indicated by second derivative because of relatively small intensity. This change in ARPES spectra represents a different molecular orientation from the comparison with IAC calculation. The suppression of H-2 and enhancement of the other MOs are characteristics of S(022) or disordered S(022) geometries, although the occurrence of other orientations cannot be discounted. The comparison of experimental data and simulation was also confirmed using the peak fitting process in Supporting Information (Figure S3).When we perform IAC calculation as a function of tilting angles around the molecular long axis, the normal emission spectra changed little at tilting angles between 15° and 35°, but at tilting angle >40°, the spectrum shape changed significantly (Figure S7).
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CONCLUSIONS A practical approach to estimate dynamic change of the adsorbed molecular geometry on a well-defined surface using ARPES was introduced. The changes of ARPES spectra were observed after the deposition of pentacene on graphite. Comparison of ARPES data with a simulation using IAC indicated that at low coverage, pentacene assumes a slightly disordered LD geometry, but that as the coverage increases, this orientation starts to change its orientation, to approach a specific phase, possibly a disordered Siegrist phase. Using this method, dynamic changes of the orientation of pentacene on graphite could be systematically elaborated. This method confirms the predictions of existing PES theory and extends 4569
DOI: 10.1021/acs.analchem.6b00986 Anal. Chem. 2016, 88, 4565−4570
Article
Analytical Chemistry
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the regime of ARPES technique to real-time monitoring of molecular orientation.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00986. Additional procedures for experimental data, PES spectra fitting with simulation for the S(022) and top up geometries, and PES simulations for different tilting angle (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors have approved to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant 2015R1D1A1A02061974). The authors thank the Pohang Accelerator Laboratory for providing the synchrotron radiation sources at the 8A2 beamline used in the study.
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DOI: 10.1021/acs.analchem.6b00986 Anal. Chem. 2016, 88, 4565−4570
