Article pubs.acs.org/JPCC
Electron-Donor Dye Molecule on ZnO(101̅0), (0001), and (0001̅) Studied by Photoelectron Spectroscopy and X‑ray Absorption Spectroscopy Kenichi Ozawa,*,† Masahiro Suzuki,‡ Ryo Tochikubo,‡ Hiroo Kato,‡ Yuichi Sugizaki,§ Kazuyuki Edamoto,§ and Kazuhiko Mase∥,⊥ †
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan Department of Advanced Physics, Hirosaki University, Hirosaki, Aomori 036-8561, Japan § Department of Chemistry, Rikkyo University, Toshima-ku, Tokyo 171-8501, Japan ∥ Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan ⊥ SOKENDAI (The Graduate University for Advanced Studies), Tsukuba 305-0801, Japan ‡
S Supporting Information *
ABSTRACT: Interaction between an air-stable orangecolored dye molecule, acridine orange base (AOB), and single-crystal surfaces of ZnO(101̅0), (0001), and (0001̅) is examined by synchrotron-radiation-excited photoelectron spectroscopy and X-ray absorption spectroscopy. AOB adsorbs molecularly on the ZnO surfaces to form chemical bonds with the surface O atoms. AOB tends to lie flat on the surfaces, whereas a more stand-up orientation is preferred on the AOBcrowded surfaces. AOB adsorption induces downward bending of the ZnO band with the largest amount on ZnO(0001̅), followed by the ZnO(101̅0) and ZnO(0001) surfaces, indicating an electron donor property of AOB. The energy level alignment at the AOB/ZnO interface is determined and is found to be characterized by a type-II heterojunction, which favors the charge transfer of the excited electron from AOB to ZnO. proceeds via the chemical bond formation.8,9 These molecules show a strong charge withdrawal ability from the substrates. Thus, they are often used as p-type dopants to control the hole carrier density.10 Other examples of the p-type dopants are tetracyanoethylene11 and fluorofullerenes.12 Regarding n-type dopant molecules, tetrathiafulvalene (TTF), a prototypical electron donor molecule in organic charge-transfer complexes has proved to behave as an electron donor on graphene10,11 and noble metal surfaces.13,14 A theoretical study predict that TTF also acts as a donor on ZnO(1010̅ ),15 although a photoelectron spectroscopy (PES) study has suggested that it may withdraw the electrons from the surface.16 It must be of great advantage for developing organic-based devices if a large amount of electrons and holes can be donated to desired substrates by simply changing the organic molecules. However, there are not many π-conjugated organic molecules that exhibit a sufficient electron-donating ability. Thus, strong electron-donor molecules are desired. Acridine orange base (AOB, Figure 1a) is a candidate for such a molecule. Li et al.
1. INTRODUCTION Electronic structures of organic/metal-oxide heterojunctions define the performance of organic-based optoelectronic devices such as organic solar cells, organic display devices, and organic light-emitting diodes, among others.1 Controlling charge transfer across the heterojunction is a key technologies to adjust the energy level alignment at the junction via the formation of an electric double layer and/or a space charge layer in the vicinity of the junction. Extensive efforts have, thus, been devoted to clarify charge donation/withdrawal properties of various organic molecules on metal, semiconductor and oxide surfaces. Among organic molecules, π-conjugated molecules have especially attracted attention because these molecules are stable but are easily functionalized to tune their chemical and physical properties that serve specific applications.2−4 The interaction scheme of π-conjugated molecules with substrate surfaces ranges from weak physisorption to strong chemisorption.5 Phthalocyanine on inert materials such as highly ordered pyrolytic graphite (HOPG)6 and noble metal surfaces7 is an example of the physisorption systems where negligible adsorbate−substrate charge transfer occurs. In contrast, adsorption of tetracyanoquinodimethane (TCNQ) and its derivatives on metal and semiconductor surfaces © XXXX American Chemical Society
Received: January 15, 2016 Revised: April 4, 2016
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DOI: 10.1021/acs.jpcc.6b00454 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. (a) Schematic structural model of AOB. C atoms are labeled by numbers with + , −, and 0 superscripts, indicating charged states. (b) Frontier molecular orbitals (MOs) and (c) densities of states (DOS) of an isolated AOB molecule, obtained by the DFT calculations. Bars in c are the energy levels of the MOs, each of which is broadened by a Gaussian function with a 0.5 eV width to give DOS curves. Contributions of the π and σ MOs are shown by the yellow area and a solid line, respectively, and the total DOS is drawn by a dashed line.
have proved from conductivity, field effect, and Seebeck measurements17 that AOB is a good electron donor to the C60 film, whose conductivity is enhanced by 6 orders of magnitude by an addition of AOB. Regarding the adsorption systems, the effect of AOB on the surface electronic structures of Au18 and Fe19 has been assessed by ultraviolet PES. An integer charge transfer from AOB to the substrate is expected on Au,18 indicating that the AOB/Au system is classified into the physisorption system. Although no such charge transfer is induced at the AOB/Fe interface, the spin polarization at the Fe surface is found to be reduced by AOB adsorption.19 AOB is an orange-colored π-conjugated molecule with an absorption maximum at 435 nm (2.85 eV).20 It is stable in air and is easy to handle, so AOB is a potential molecule which can be used as an electron dopant in organic-based devices. Nevertheless, studies of the AOB-substrate interaction are not sufficient to predict adequately a behavior of AOB on solid surfaces. In the present study, we performed PES and near-edge X-ray absorption fine structure (NEXAFS) measurements for AOB on single-crystal surfaces of ZnO(101̅0), ZnO(0001) and ZnO(0001̅). An adsorption state and molecular orientation of AOB as well as the AOB-ZnO charge transfer were determined, and the influence of the difference in the surface termination of ZnO was examined. It is found that AOB acts as an efficient electron donor on all three surfaces with a largest amount of transferred charge on ZnO(0001̅). The AOB−ZnO interaction is characterized by chemical bonding between adsorbed AOB and the O atoms on the ZnO surface. This interaction affects partially the adsorption geometry of AOB, whose molecular plane is tilted by 40−50° depending on the ZnO surface and the AOB coverage.
2. EXPERIMENTAL METHODS The PES and NEXAFS experiments were performed at beamline (BL) 3B21 and BL-13B22 of the Photon Factory, High Energy Accelerator Research Organization (KEK), utilizing linearly polarized synchrotron radiation light. Details of the measurement systems are described in our previous papers.16,23 Core-level PES and NEXAFS measurements were carried out at BL-13B, whereas the valence-band PES spectra were acquired at BL-3B. All the measurements were done at room temperature. The binding energy (BE) of the PES spectra was referenced to Fermi cutoff positions in the spectra of Ta sample holders. Single-crystal ZnO with (101̅0), (0001), and (0001̅) orientations (10 mm × 10 mm × 0.5 mm; SPC Goodwill, Russia) were used as substrate surfaces. The ZnO surfaces were cleaned in ultrahigh vacuum chambers with base pressures of 75%) was used in the present study. AOB was purified beforehand by vacuum sublimation, which led from brownish powder to bright orange crystal pieces. Deposition of AOB on the ZnO surfaces were carried out in the ultrahigh vacuum chambers, which were connected to the measurement chambers. A homemade effusion cell for organic solids with low sublimation temperatures was newly designed on the basis of the effusion cell reported in the literature.24 The ZnO samples were placed at about 50 mm away from the effusion cell, and AOB was sublimated at about 420 K while the sample was kept at room temperature. The B
DOI: 10.1021/acs.jpcc.6b00454 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C amounts of deposited AOB were determined by intensity variations of the peaks relating ZnO and AOB in the AES spectra (BL-3B) and in the core-level PES spectra (BL-13B).
3. RESULTS 3.1. Core-Level PES. Figure 2 shows N 1s and C 1s corelevel PES spectra of AOB on the ZnO surfaces. Thicknesses of
Figure 3. AOB-overlayer-thickness dependences of the core-level PES spectra for the AOB/ZnO(101̅0) system. The photon energy was 830 eV. A Shirley-type background was subtracted from the measured spectra. Solid lines in the N 1s and C 1s spectra are the results of the peak fitting using Voigt functions. The spectra at 1.0 nm and >9 nm are the same as the corresponding spectra in Figures 2 and 4.
N atom (the lower BE component) and N in the N(CH3)2 groups (the higher BE component). The weak peak at 406.7 eV may be associated with a shakeup satellite with a shakeup excitation energy of about 5.2 or 6.9 eV. This large energy indicates that the shakeup process should not be related to the electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) because the HOMO−LUMO gap energy of AOB is less than 3 eV.20 The C 1s spectrum of the thick AOB film (>9 nm) also bears a double-peak structure (286.3 and 287.6 eV) with weak satellite peaks at 290.7 and 292.3 eV. The BE difference of the main peaks should reflect the charged state of the C atoms in AOB. According to the theoretical calculations,25 9 out of 17 C atoms are positively charged (the corresponding C atoms are indicated by numbers with superscript “+” in Figure 1a), 6 are negatively charged (numbers with superscript“−”), and 2 are nearly neutral (superscript “0”). From the intensity ratio of the main peaks (1.0:1.15 for the low and high BE components, respectively), the higher BE component can be associated with the positively charged C atoms, whereas negatively charged and neutral C atoms should contribute to the lower BE component. The satellite peaks are separated by ∼4.5 eV from the main peaks. If the satellite is also formed by the shakeup process, then involved molecular orbitals (MOs) should be different from those in the N 1s shakeup process. As the AOB thicknesses are reduced, the influence of the ZnO substrate on adsorbed AOB is obviously detectable. Figure 3 shows the change in the core-level spectra for the AOB/ ZnO(101̅0) system as a function of the overlayer thickness. Aside from the differences in the peak BEs and intensities, the spectral lineshapes of both the N 1s and C 1s spectra are similar at 2.5 and >9 nm. However, a strong modification is induced at 1.0 nm as a result of the influence of the substrate. Curve fitting reveals that, although the main structures of the N 1s and C 1s spectra consist of two components at 1.0 and >9 nm, four components (or alternatively two doublets) are required for the spectra at 2.5 nm. It is obvious that one doublet is attributed to AOB under the influence of ZnO, and the other corresponds to
Figure 2. N 1s and C 1s core-level spectra of the AOB overlayers on the ZnO surfaces. The photon energy of 830 eV was used to acquire the spectra. A Shirley-type background was subtracted from each raw spectrum. Regions at the higher BE sides of the main structures are enlarged to show weak shakeup satellite peaks. Solid lines are results of peak fitting using Voigt functions to reproduce the experimental data indicated by circles. The main peaks of both N 1s and C 1s spectra are given by two components. Additional components are required to reproduce the shakeup satellite peaks.
the AOB overlayers are between 1.0 and 1.4 nm (see Section 1 of the Supporting Information for the determination of the overlayer thickness). For comparison, the spectra of a thick AOB layer (>9 nm), at which the substrate-originating O 1s and Zn 3s peaks are hardly observed in the spectra taken at hν = 830 eV (Figure 3), are also shown in the bottom panels. On the basis of the molecular orientation of adsorbed AOB determined by NEXAFS measurements whose results will be given in section 3.2, a monolayer thickness may correspond to ca. 0.5 nm (or 1 nm) if AOB adsorbs on the ZnO surfaces with its long (or short) edge in contact with the surface (for more details, see Section 1 of the Supporting Information). The N 1s spectrum of the thick AOB layer (>9 nm) bears two intense peaks at 399.8 and 401.5 eV along with a very weak structure at 406.7 eV. The main doublet peaks are safely assigned to, on the basis of the peak intensity ratio, a pyridinic C
DOI: 10.1021/acs.jpcc.6b00454 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C AOB in an environment similar to the thick AOB film. The BE difference of these doublets reflects the charge state of adsorbed AOB as will be discussed in section 3.3. The N 1s and C 1s doublet peaks shift toward the higher BE side as the AOB thickness is increased from 2.5 to >9 nm. Because the AOB molecules outside the influence of the substrate surface give these peaks, the BE difference should not be attributed to the difference in the charge state of AOB. The exact origin is not understood at present, but charging of the AOB overlayer during the PES measurements is not the cause because the O 1s and Zn 3s peaks do not exhibit the same BE shift as that of the N 1s and C 1s peaks. An important conclusion deduced from Figure 3 is that AOB adsorbs on the ZnO surface without dissociation at room temperature. This is because the N 1s and C 1s spectra are composed of one or two doublet structures irrespective of the AOB thickness and that no other extra components are observed (except for the satellites). This adsorption behavior is in sharp contrast to TTF, which undergoes dissociation upon adsorption on the ZnO surfaces.16 A close examination of the N 1s and C 1s spectra at reduced AOB thicknesses (Figure 2) reveals that although the spectral features of AOB/ZnO(0001̅) is very similar to those of the thick AOB film each component of the N 1s and C 1s spectra for the AOB/ZnO(1010̅ ) and AOB/ZnO(0001) systems is broader by ∼1.3 times than the corresponding components of the AOB/ZnO(0001̅) system. More pronounced difference in comparison with AOB/ZnO(0001̅) is the intensity ratio of the two main components in the C 1s spectra; the intensity ratios of the high and low BE components are 1.0:0.9 for AOB/ ZnO(101̅0) and 1.0:0.8 for AOB/ZnO(0001). Such reversed ratios are reasonably explained if one of the positively charged C atoms becomes less charged or even negatively charged upon adsorption. The most probable candidate is the C atom labeled 5+ (Figure 1a) because this atom is the least positively charged one.25 The spectral characteristics of the thin AOB films (
