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Electronic Structure of Aromatic Monomolecular Films: The Effect of Molecular Spacers and Interfacial Dipoles Lingmei Kong,† Frederick Chesneau,‡ Zhengzheng Zhang,† Florian Staier,‡ Andreas Terfort,§ P. A. Dowben,† and Michael Zharnikov*,‡ †
Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln, 855 North 16th Street, P.O. Box 880299, Lincoln, Nebraska 68588-0299, United States ‡ Angewandte Physikalische Chemie, Universit€at Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany § Institut f€ur Anorganische und Analytische Chemie, Universit€at Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany ABSTRACT: The electronic structure of several arene-based monomolecular films on Au(111), with and without an alkyl linker between the aromatic unit and thiol headgroup, has been investigated by photoemission and inverse photoemission. While the HOMOLUMO gap in these films was found to depend on the aromatic backbone, the molecular band offset of the electronic states was strongly affected by the interfacial dipole. The smallest HOMO LUMO gap was found for the strongly conjugated anthracene moiety, intermediate for terphenyl, and widest for the perfluorinated terphenyl. The perfluorinated terphenyl-based films appear to be more n-type as a semiconductor than the terphenyl or anthracenebased monolayers, as indicated by the placement of the Fermi level (chemical potential) relative to the conduction or lowest molecular orbital band edge. Accordingly, the occupied electronic states related to the aromatic rings sink to greater binding energies, well below those for the alkyl linker, and thus for the perfluorinated terphenyl, the aromatic orbital contribution is not to the HOMO but the HOMO-1 orbital (one occupied molecular orbital away from the HOMO). This placement of the perfluorinated terphenyl aromatic orbital contribution is in drastic contrast to the nonfluorinated systems in our study, in which both HOMO and LUMO orbitals are extended throughout the aromatic moieties.
1. INTRODUCTION Self-assembled monolayers (SAMs) are considered an efficient means to tune the physical and chemical properties of surfaces and interfaces in a controlled fashion.14 An important issue in this context is the adjustment of the electronic properties at interfaces. For example, SAMs have been used as an intermediate layer between metal electrodes and organic semiconductors and can be particularly valuable if the interfacial dipole can be controlled.58 In this case, not only can SAMs serve as a structural mediator affecting the growth mode and improving the crystal quality of organic semiconductors but also, in addition to aiding in the formation of the correct choice of the molecular structure, SAM interfacial layers may be able to enhance the charge injection and transport in the respective junctions. This could be realized as improved performance in organic light emitting diodes and field effect transistors.915 By forming a dipole layer at the electrode/semiconductor interface, SAMs can lower the barrier for electron or hole injection into the organic semiconductor.57,1420 On the other hand, SAMs can also serve as ultrathin organic insulators and possibly even good dielectric barrier layers in electronic and spintronic devices, e.g., for use as a gate dielectric in field effect transistors or as an insulating layer in the tunnel junction memory cell.2127 A striking example is the case of a quasi-two-dimensional, cross-linked SAM of r 2011 American Chemical Society
4,40 -terphenyl-4,400 -dimethanethiol (TPDMT) which not only provides an effective barrier against the penetration of a metal adsorbate28,29 but also electrically isolates the fabricated metal film at the SAMambient interface from the substrate.30,31 The advantage of such organic dielectric layers is not only that they are potentially well-defined but also the possibility to adjust certain parameters, such as, e.g., the barrier thickness and electronic structure, by physical or chemical means. In particular, the positions of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of a SAM affect the height of the injection barrier for electrons and holes through the SAMmodified electrode/semiconductor interface or SAM-containing molecular junction.27,32 Further, the width of the HOMOLUMO gap and placement of the chemical potential with respect to an electrode defines the dielectric barrier height of a SAM, affecting its electronic device performance.27 Thus, alkanethiols with band gaps of more than 5 eV are predestined for the preparation of dielectric layers, while aromatic systems with typical band gaps below 4.5 eV can act not only as insulators but also as photo or thermal semiconductors. As a rule of thumb, the semiconducting Received: July 15, 2011 Revised: October 5, 2011 Published: October 05, 2011 22422
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Figure 1. Schematic drawing of the SAM precursor molecules along with their acronyms.
properties of a molecule improve with decreasing band gap, making, e.g., pentacene with a band gap of 2.21 eV33 an archetypical representative of organic semiconductors.34,35 In this context, we wished to determine the electronic structure of a series of SAMs, and here we present a combination of spectroscopic characterization and theoretical analysis of several different aromatic thiol molecules (Figure 1). A key issue we wanted to address is the relation between the aromatic spacer and the electronic structure of the associated SAM. To avoid ambiguity regarding the quality of the aromatic SAMs, we decided to work with long-chain molecules, having either terphenyl (TPT and TP3), fluorinated terphenyl (FTP3), or an anthracene (Ant3) backbone and a short aliphatic linker (TP3, FTP3, Ant3) as shown in Figure 1. All these molecules produce well-defined SAMs on Au(111), with especially high quality in the case of the molecules with the aliphatic linker (TP3, FTP3, and Ant3).3641 This quality is related to the suitable adjustment between the thermodynamically favorable steric arrangement of the aromatic moieties and the bending potential at the headgroupsubstrate joint, which occurs at a certain number of carbon atoms (odd or even) in the aliphatic linker. This number should be odd in the case of gold, thus enabling, as mentioned above, an optimal packing of the aromatic moieties. Such accommodation is hardly possible in the case of an aliphatic linker with an even number of carbon atoms or the direct attachment of the aromatic backbone to the thiolate headgroup.4249
2. EXPERIMENTAL PART 2.1. Chemicals and SAM Preparation. The syntheses of 4,40 terphenyl-4-thiol (TPT), 4,40 -terphenylyl propanethiol (TP3), 4,40 -perfluoroterphenylyl propanethiol (FTP3), and 2-anthracenyl propanethiol (Ant3) are described in refs 5052 and 40, respectively. The gold substrates were prepared by thermal evaporation of 100 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. The resulting metal films were polycrystalline, with a predominant (111) orientation of the individual grains and a grain size of 2050 nm. The SAMs were prepared by immersion of the freshly prepared substrates into a 1 mM solution of the target compounds in a suitable solvent at room temperature for 36 h.3641 After immersion, the samples were carefully rinsed with pure solvent and blown dry with argon. They were either characterized immediately or stored under an inert gas atmosphere in glass containers until the experiments at the
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synchrotrons (see below). The properties of the TPT, TP3, FTP3, and Ant3 SAMs are described in detail in refs 3640 and 41, respectively. 2.2. Photoemission and Inverse Photoemission Measurements. The occupied and unoccupied molecular orbitals of the target SAMs were characterized by combined photoemission and inverse photoemission spectroscopies.25,26,31,5358 The ultraviolet photoemission (UPS) and inverse photoemission (IPES) spectra were taken in a single ultrahigh vacuum (UHV) chamber to study the placement of both occupied and unoccupied molecular orbitals of the SAM molecular systems at room temperature. The IPES spectra were obtained by using variable kinetic energy incident energy electrons while detecting the emitted photons at a fixed energy (9.7 eV) using a GeigerM€uller detector.25,26,31,5358 The inverse photoemission spectroscopy was limited by an instrumental line width of approximately 400 meV, as described elsewhere.25,26,31,5358 The angle integrated photoemission (UPS) studies were carried out using a helium lamp at hv = 21.2 eV (He I) and a Phi hemispherical electron analyzer with an angular acceptance of (10° or more, as described in detail elsewhere. The core level X-ray photoemission spectra (XPS) were taken with a SPECS X-ray source with a Mg anode (hv = 1253.6 eV). The pass energy was set to 5 eV for UPS and 24 eV for XPS, limiting the resolution to roughly 100 meV for the UPS spectra. The photoemission experiments were made with the photoelectrons collected along the surface normal, while the inverse photoemission spectra were taken with the incident electrons normal to the surface. This restriction of the electron emission (photoemission) or electron incidence (inverse photoemission) to the surface normal was done to preserve the highest point group symmetry and eliminate any wave vector component parallel with the surface. In both photoemission and inverse photoemission measurements, the binding energies are referenced with respect to the Fermi edge of gold in intimate contact with the sample surface, and the photoemission (UPS, XPS) data are expressed, in terms of E EF (thus making occupied state energies negative). Particularly for XPS, fluences were kept low because of the tendency for thiol layers to decompose. As checks against decomposition, spectra were taken on multiple samples of each type of thiol and for different irradiation (total fluences of photons or electrons) exposures. 2.3. Work Function Measurements. The photoemission experiments were complemented by work function (WF) measurements to obtain information about the surface dipoles and the charge transfer upon the adsorption of the molecules on the substrate. The measurements were performed with a Kelvin probe with manual translator (System M; KP Technology Ltd., UK) under UHV conditions. 2.4. UVvis Measurements. The UVvis spectra were recorded with a two-beam spectrophotometer “LAMBDA 650” from Perkin-Elmer Inc. using 1 cm quartz cuvettes “Suprasil (QS)” from Hellma, Germany. The substances were dissolved in ethanol “for spectroscopy” at concentrations between 105 and 104 mol L1. Before the measurements, a baseline correction using only the solvent was performed. For all the measurements, a slit width of 2 nm and a lamp change at 320 nm (standard program) was maintained. 2.5. Theoretical Simulations. The orbital energies of the single molecules were calculated using both the semiempirical PM3 (neglect of differential diatomic overlap, parametric model number 3) and the hybrid density functional theory (DFT B3LYP) 22423
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Figure 3. Positions of the HOMO and LUMO orbitals and the HOMO LUMO gap for the TPT, TP3, FTP3, and Ant3 films determined on the basis of the experimental UPS and IPES spectra (Figure 2). Binding energies are denoted in terms of E EF. For FTP3/Au, the positions of the HOMO-1 orbital and the (HOMO-1)LUMO gap are also shown. Note that the position of the HOMO orbital for Ant3/Au correlates well with the literature values for anthracenethiol SAMs (1.6 eV).75
Figure 2. Combined UPS and IPES spectra of the TPT, TP3, FTP3, and Ant3 SAMs (from the bottom to the top), along with the calculated (DFT) ground state molecular orbital energies (vertical tick marks below the spectra) and the theoretical density-of-states (the curves below the experimental spectra) for the respective single molecules. Binding energies are denoted in terms of E EF. The presumable positions of HOMO and LUMO are shown by vertical black lines. For FTP3/Au, the position of the HOMO-1 orbital is also shown by the vertical gray line. Note that the combined resolution (analyzer and light source) for the photoemission spectra, taken at He I (21.2 eV) radiation was 100 and 400 meV for the inverse photoemission spectra. Electrons were collected (UPS) or incident (IPES) normal to the surface.
methods,59,60 as has been undertaken successfully elsewhere.55,57 The DFT calculations were performed with the Spartan 06 and Orca 2.861 software packages, with the standard 6-31 G* basis set. The results obtained by both software packages were similar. Geometric optimization of the system was performed by obtaining the lowest unrestricted HartreeFock (UHF) energy states. The orbital energies of the TPT and TP3 molecules were calculated with their terphenyl units in the coplanar geometry to reflect their molecular structure in the SAM.37,6265 A model density of states was obtained by applying equal Gaussian envelopes
of 1 eV width to each molecular orbital at the ground state binding energies to account for the solid state broadening in photoemission and then summing, together with a rigid energy shift of a typical value of 5.4 and 3.5 eV applied to the calculated electronic orbital energies by PM3 and DFT, respectively.25,26,31,5358 This shift is largely attributable to the work function, but DFT is regretfully flawed in estimating the correct electron attachment energies. Thus, further empirical corrections66,67 or a more complete Greens function formalism (GW)68 may be required, but the latter alone would not be sufficient as corrections for the substrate interactions, final state screening, and matrix element effects should also be considered, which are all beyond the scope of this work.
3. RESULTS AND DISCUSSION The combined UPS and IPES spectra of the TPT, TP3, FTP3, and Ant3 SAMs are presented in Figure 2, along with the calculated (DFT) ground state molecular orbital energies and the density-of-states (DOS) for the respective single molecules. Note that the experimentally determined molecular orbital placements are in far better agreement with the DFT calculations compared to the semiempirical ones; therefore, we present here the results of the DFT calculations only. The approximate positions of the HOMO and LUMO are shown by vertical solid lines in Figure 2 and are schematically visualized once more in Figure 3. As is the common practice, these positions were found using vertical peak energies determined by peak fitting. The use of the vertical energies, as opposed to the onset energies, provides a better indication of the initial state binding energies. The respective molecular orbitals are shown in Figure 4, along with the calculated (DFT) positions of the HOMO and LUMO and HOMOLUMO gaps for the isolated TPT, TP3, FTP3, and Ant3 molecules. 22424
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Figure 4. Calculated (DFT) HOMO and LUMO orbitals of the TPT, TP3, FTP3, and Ant3 molecules, along with their energy positions and the HOMOLUMO gap. For FTP3, the data for the HOMO-1 orbital are also presented. Note that, with the exception of the HOMO orbital of FTP3, all the orbitals have π character. The experimental (UPS/IPES) values of the HOMOLUMO gap are given for comparison.
The combined UPS and IPES data for the various SAMs are in good agreement with the theoretical DOS of the respective precursors. Also, the experimental values of the HOMO LUMO gap for the SAMs, as given in Figure 3, correlate well with the respective theoretical values for the molecules (Figure 4) except for TP3 for which the experimental value is somewhat higher. The good agreement between the experimental values for the molecular films and the theoretical values for the isolated molecules is presumably related to the fact that the proximity to the substrate leads to a well-screened photoemission and inverse photoemission final state.58 Note that a partial coupling between the electronic system of the aromatic moiety and the substrate might be expected for TPT/Au.69 This coupling, however, affects mostly the phenyl ring adjacent to the headgroup, which diminishes its effect regarding the entire molecule.69 In contrast to TPT/Au, the electronic systems of the TP3, FTP3, and Ant3 moieties in the respective films are presumably better decoupled from the substrate due to the presence of the alkyl spacer which is a better insulator than the aromatic core of the molecules. This effect has previously been demonstrated on the basis of theoretical simulations for a series of biphenyl-substituted alkanethiol SAMs on Au(111), in which even the insertion of just a single methylene group between the biphenyl unit and the headgroup resulted in decoupling of the electronic subsystem of the aromatic core and the substrate.69 Also, as shown recently for a series of oligophenyl and oligo(phenyleneethynylene) SAMs on Au(111), the characteristic charge transfer time through the molecular framework increases significantly upon the introduction of a single methylene group between the aromatic unit and the headgroup.70 Therefore, the electronic structure of the target SAMs, given by the respective UPS/IPES spectra, is mostly determined by the identity of the aromatic core, at least regarding the width of the HOMOLUMO gap. Of course there are still substrate polarization contributions68 to adsorbate electronic structure, as well as final state effects.58 However, since the
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substrate was gold for all of the studies reported here, changes in the final state effects between one thiol layer and another should be quite small. The narrowest HOMOLUMO gap of 3.5 eV was observed for Ant3/Au, which can be associated with the extended πsystem of the anthracene unit. Note that this value is somewhat smaller than our theoretical estimate for Ant3 (3.66 eV; see Figure 4) and a gap of 3.59 eV for anthracene suggested by other theoretical studies.33 At the same time, it is wider than the experimental value for anthracenedithiol molecules in metal moleculemetal junctions (∼3.2 eV),71 which can probably be explained by the presence of the alkyl linker in our case. A widest (among the films of this study) HOMOLUMO gap of 4.85 eV is observed for FTP3/Au, which is presumably related to the helical conformation of the FTP3 molecule40 (our model singlemolecule calculations for TPT and TP3 show that the gap increases upon change from the planar to helical conformation, as is expected). The respective theoretical estimate (4.76 eV; see Figure 4) is quite close to this value. Finally, the HOMOLUMO gap for TPT/Au and TP3/Au was found to be 4.1 and 4.75 eV, respectively. Interestingly, the HOMOLUMO gap value for TP3/Au is noticeably larger than that for TPT/Au. This tendency in the HOMOLUMO gap is reproduced in the single-molecule DFT calculations (see Figure 4), but the respective difference between the theoretical values for the TP3 and TPT molecules is much smaller. Probably, the larger difference observed in the experiment for these SAMs is somehow related to the matrix effects (not taken into account by the simulations) since the TP3 films have much higher crystalline perfection and packing density than the TPT SAMs.37 Along with the differences in the width of the HOMO LUMO gap, there are significant differences in the placement of this gap with respect to the substrate Fermi level as shown in Figure 3. Presumably, these changes in molecular band offset are characteristic of the influence of differences in the extended interface dipole.5,55,71 In the case of thiol-derived SAMs, this dipole consists of two parts, viz., the potential energy step at the substratethiol interface reflecting charge redistribution upon bond formation and a dipole associated with the molecular backbone.14,15,69,72 Note that the effect of the molecular dipole will differ for TPT/Au and TP3/Au with respect to FTP3/Au since the molecular dipoles for nonfluorinated and fluorinated oligophenylthiols are directed in the opposite directions,72 as is expected just upon inspection to the moieties. The overall effect of the interfacial dipole may be monitored by WF measurements. The WF change as compared to clean Au(111) was found to be 1.0, 1.0, 0.8, and +0.6 eV for TPT/Au, Ant3/Au, TP3/Au, and FTP3/Au, respectively. The difference between these values correlates surprisingly well with the relative position of the HOMO orbital in these SAMs (see Figure 3), with the highest position for TPT/Au and Ant3/Au, slightly lower (by ∼0.1 eV) for TP3/Au, and the lowest (by ∼1 eV) position for FTP3/Au. Note that it is not a specific property of the aromatic units per se but an effect of fluorination. We attribute the fluorination and resulting influence of the dipole as responsible for the WF offset of FTP3/Au compared to TPT/Au and TP3/ Au. Indeed, the difference in WF on going from FTP3/Au to TP3/Au (1.4 eV) is similar to that going from a monolayer of decanethiol to its fluorinated analogue on gold (1.3 eV),73 indicating that the electron-withdrawing fluorine atoms are responsible for the effect, independent of the identity of the backbone. 22425
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Figure 5. Calculated (DFT) DOS for TP3 (bottom panel) and FTP3 (top panel) and their building blocks, viz., propanethiol (C3-SH) and terphenyl (TP-H), for TP3 and propanethiol (C3-SH) and perfluorinated terphenyl with one terminal fluorine atom substituted by a hydrogen atom (FTP-H) for FTP3. The vertical lines are guides for the eye to monitor the contributions of the building blocks to the total DOS. Binding energies are denoted in terms of E EF.
Since the alkane spacer in the TP3, FTP3, and Ant3 systems is generally a better insulator than the aromatic core, the partial density of states (DOS) related to the alkane spacer unit is located much deeper with respect to the Fermi level compared to the partial DOS associated with the latter moiety.69 In this regard, as shown in Figure 4, the electron density associated with the HOMO and LUMO orbitals of the TPT, TP3, and Ant3 molecules is located on the aromatic core. In contrast, the orbital energies associated with the aromatic weighted component of the occupied molecular orbitals of FTP3 are lowered so much that these states slip down below the HOMO and add weight to the HOMO-1 (i.e., one occupied molecular orbital away from the HOMO) and deeper orbitals as shown in Figure 4. The HOMO is then comprised of the electronic states located at the alkyl linker and thiol headgroup. Note that the different locations of the electronic density associated with the HOMO in the target SAMs are indirectly reflected in the UPS spectra in Figure 2. Indeed, the features dominated by the highest occupied molecular orbitals are very well-defined in the photoemission spectra, with narrow line shapes, indicative of a long-lived photoemission associated excited state, except in the case of FTP3/Au. This is consistent with delocalization of the highest occupied molecular orbitals of the target molecules, with the exception being FTP3/ Au. In the latter case, the HOMO is not highly delocalized over the aromatic moiety, but the molecular orbital weight is close to
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Figure 6. UVvis spectra of the TPT, TP3, FTP3, Ant3, and perfluoroterphenyl (C18F14, PFT) molecules in solution. The presumable position of the optical gap is marked by the vertical solid lines. The respective values are given. Note that to determine these values the (hν A)2 was plotted against hν (hν is photon energy in eV, A is the absorbance). The axis interception of the interpolation of the first flank resulted in the band gap energy in the case of direct semiconductors (as we assume the aromatic SAMs are).
the thiol termination and well screened by the gold substrate,58 so that a decay channel that involves the gold substrate is not only possible, but likely. To distinguish the contributions of the electronic states associated with the HOMO in TP3 and FTP3, the partial DOSs related to their building blocks, viz., a short alkanethiol and the aromatic unit, were calculated separately. The respective partial DOSs are presented in Figure 5 for propanethiol (C3-SH) and terphenyl (TP-H) for TP3 as well as propanethiol (C3-SH) and perfluorinated terphenyl with one terminal fluorine atom substituted by H (FTP-H) for FTP3. It is clearly seen for TP3 that the states associated with the alkyl linker and the headgroup are located at lower energies with respect to EF than the states related to the terphenyl unit. The opposite situation occurs for FTP3, where the HOMO is now comprised by the states associated with the alkyl linker and the headgroup both. The latter finding put some doubt in the value given above for the width of the HOMOLUMO gap in FTP3/Au. Indeed, the electronic densities associated with the HOMO and LUMO orbitals are located at the different parts of the molecule, diminishing the matrix elements (transition probabilities) for the HOMOLUMO transition since the wave functions of the initial and final states in the respective transition integral will have no overlap. In this case, not HOMO but HOMO-1, located similar to LUMO, at the aromatic unit of FTP3, is more representative of the effective gap for electronic transitions in this particular system. In this context, the value of 4.85 eV obtained for the HOMOLUMO gap from the combined photoemission and inverse photoemission spectra of FTP3/Au can be substituted by a value of 5.15 eV for the (HOMO-1)LUMO gap (see Figure 3). Note that the latter 22426
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The Journal of Physical Chemistry C value correlates surprisingly well with a respective theoretical estimate for FTP3 of 5.12 eV according to Figure 4. A complementary test for the transition probabilities and electronic structure is the optical spectra of the respective compounds. The UVvis spectra of the TPT, TP3, FTP3, and Ant3 molecules in solution are presented in Figure 6. The onset of the optical absorption in these spectra corresponds to the optical gap which is generally assumed to be smaller than the HOMOLUMO gap due to the electrostatic interactions between the valence hole with the promoted electron in the exciton. The positions of the absorption onsets, which are a measure for the optical band gap, are marked by the vertical solid lines in Figure 6. The values of the optical gap for the systems in this study correlate well with the UPSIPES derived results (Figure 3), assuming an offset of 0.3 eV. The only exception is TP3, where the optical value is closer to the theoretical one (Figure 4), even taking into account the above offset. The smallest gap is observed for Ant3 and the largest for FTP3. For comparison, the UVvis spectrum of perfluoroterphenyl (C6F5C6F4C6F5, PFT) is also depicted in Figure 6, which is indistinguishable from the one of FTP3. This supports the notion that the relevant transition determining the effective band gap in FTP3/Au is not the HOMOLUMO gap difference but—as mentioned before— the (HOMO-1)LUMO one. There are complications in the vicinity of the Fermi level that may have to be considered. The quantum mechanical requirement that overlapping electronic states must be orthogonal to each other can result in charge being removed at the surface of the Au in an area directly under the center of the molecule. This in turn may lead to a shift in the Au substrate surface state energy.74,75 Sometimes known as the “pillow effect”,7680 this can lead to a small density of states just above the Fermi level, as may occur in the inverse photoemission spectra of the target SAMs.
4. CONCLUSIONS The electronic structure of a series of well-defined SAMs with terphenyl and anthracene backbone was studied by a combination of UPS and IPES and complementary work function (WF) and UVvis measurements and analyzed with the help of DFT calculations for the SAM precursors. In most SAMs, the aromatic backbone was separated from the thiol group by a short alkyl linker, which improves the structural quality and packing density of the aromatic matrix. The width of the HOMOLUMO gap was found to depend on the identity of the aromatic backbone, being smallest for the strongly conjugated anthracene moiety, intermediate for terphenyl, and widest for the perfluorinated terphenyl unit. In contrast, the offset of this gap, and especially the position of the HOMO orbital, correlated with the interfacial dipole, which could be monitored by the WF measurements. In the case of perfluorinated terphenyl, which is characterized by the comparatively large dipole moment directed to the surface, this results in a significant downward shift of the occupied electronic states with respect to the Fermi level (chemical potential), so that the film becomes more n-type than the nonfluorinated systems and the orbitals related to the aromatic rings sink below those for the alkyl linker. The electronic states associated with the latter moiety and the headgroup comprise then the HOMO orbital, which is in drastic contrast to the nonfluorinated systems in this study, in which both HOMO and LUMO orbitals consist of the electronic states delocalized over the aromatic moiety.
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The particular character of the LUMO orbital in the case of perfluorinated terphenyl suggests that the electronic and optical properties of this system are better described by the (HOMO-1) LUMO gap than by the HOMOLUMO one.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +49-6221-54 4921. Fax: +49-6221-54 6199. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work has been supported by the Deutsche Forschungsgemeinschaft (ZH 63/10-1 and ZH 63/14-1) and the National Science Foundation through grant CHE-0909580. F.C., F.S., and M.Z. thank M. Grunze for the support. L.K., Z.Z., and P.D. would like to acknowledge the support and assistance of Ning Wu and Jing Liu. A.T. gratefully acknowledges financial support by the Beilstein-Institut, Frankfurt/Main, Germany, within the research collaboration NanoBiC. ’ REFERENCES (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (3) Schreiber, F. J. Phys.: Condens. Matter. 2004, 16, R881–R900. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (5) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605–625. (6) Zhu, X. Y. Surf. Sci. Rep. 2004, 56, 1–83. (7) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Adv. Funct. Mater. 2010, 20, 1371–1388. (8) Cahen, D.; Naaman, R.; Vager, Z. Adv. Funct. Mater. 2005, 15, 1571–1578. (9) Pernstich, K. P.; Haas, S.; Oberfoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431–6438. (10) Hamadani, B. H.; Corley, D. A.; Ciszek, J. W.; Tour, J. M.; Natelson, D. Nano Lett. 2006, 6, 1303–1306. (11) Bock, C.; Pham, D. V.; Kunze, U.; K€afer, D.; Witte, G.; Terfort, A. Appl. Phys. Lett. 2007, 91, 052110/1–3. (12) Hong, J. P.; Park, A. Y.; Lee, S.; Kang, J.; Shin, N.; Yoon, D. Y. Appl. Phys. Lett. 2008, 92, 143311/1–3. (13) Saudari, S. R.; Frail, P. R.; Kagan, C. R. Appl. Phys. Lett. 2009, 95, 023301/1–3. (14) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. Rev. B 1996, 54, R14321–R14324. (15) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528/1–3. (16) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Colorado, R.; Wysocki, V.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690–11699. (17) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. Adv. Mater. 2005, 17, 621–625. (18) Chen, W.; Huang, C.; Gao, X. Y.; Wang, L.; Zhen, C. G.; Chen, D.; Qi, S.; Zhang, H. L.; Loh, K. P.; Chen, Z. K.; Wee, A. T. S. J. Phys. Chem. B 2006, 110, 26075–26080. (19) Demirkan, K.; Mathew, A.; Weiland, C.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Opila, R. L. J. Chem. Phys. 2008, 128, 074705/1–6. (20) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J. Acc. Chem. Res. 2008, 41, 721–729. (21) Dediu, V.; Murgia, M.; Matacotta, F. C.; Taliani, C.; Barbanera, S. Solid State Commun. 2002, 122, 181–184. 22427
dx.doi.org/10.1021/jp2067342 |J. Phys. Chem. C 2011, 115, 22422–22428
The Journal of Physical Chemistry C (22) Petta, J. R.; Slater, S. K.; Ralph, D. C. Phys. Rev. Lett. 2004, 93, 136601/1–4. (23) Naber, W. J. M.; Faez, S.; van der Wiel, W. G. J. Phys. D: Appl. Phys. 2007, 40, R205–R228. (24) Xu, W.; Szulczewski, G. J.; LeClair, P.; Navarrete, I.; Schad, R.; Miao, G.; Guo, H.; Gupta, A. Appl. Phys. Lett. 2007, 90, 72506/1–3. (25) Xiao, J.; Sokolov, A.; Dowben, P. A. Appl. Phys. Lett. 2007, 90, 242907/1–3. (26) Dowben, P. A.; Rosa, L. G.; Ilie, C. C.; Xiao, J. J. Electron Spectrosc. Relat. Phenom. 2009, 174, 10–21. (27) Velev, J. P.; Dowben, P. A.; Tsymbal, E. Y.; Jenkins, S. J.; Caruso, A. N. Surf. Sci. Rep. 2008, 63, 400–425. (28) Tai, Y.; Shaporenko, A.; Noda, H.; Grunze, M.; Zharnikov, M. Adv. Mater. 2005, 17, 1745–1749. (29) Tai, Y.; Shaporenko, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2005, 109, 19411–19415. (30) Noda, H.; Tai, Y.; Shaporenko, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2005, 109, 22371–22376. (31) Feng, D.-Q.; Losovyj, Ya. B.; Tai, Y.; Zharnikov, M.; Dowben, P. A. J. Mater. Chem. 2006, 16, 4343–4347. (32) Grave, C.; Risko, C.; Shaporenko, A.; Wang, Y.; Nuckolls, C.; Ratner, M. A.; Rampi, M. A.; Zharnikov, M. Adv. Func. Mater. 2007, 17, 3816–3828. (33) Kato, T.; Yamabe, T. J. Chem. Phys. 2004, 121, 2356–2366. (34) Karl, N. Charge carrier mobility in organic crystals. In Organic Electronic Materials; Farchioni, R., Grosso, G., Eds.; Springer: Berlin, 2001. (35) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99–117. (36) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408–2415. (37) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 14462–14469. (38) Shaporenko, A.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Electron Spectrosc. Relat. Phenom. 2006, 151, 45–51. (39) Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; W€oll, Ch. Langmuir 2006, 22, 3647–3655. (40) Chesneau, F.; Sch€upbach, B.; Szela) gowska-Kunstman, K.; Ballav, N.; Cyganik, P.; Terfort, A.; Zharnikov, M. Phys. Chem. Chem. Phys. 2010, 12, 12123–12127. (41) Dauselt, J.; Zhao, J.; Kind, M.; Binder, R.; Bashir, A.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2011, 115, 2841–2854. (42) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359–3362. (43) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; W€uhn, M.; W€oll, Ch.; Helmchen, G. Langmuir 2001, 17, 1582–1593. (44) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; W€oll, Ch. Langmuir 2003, 19, 8262–8270. (45) Cyganik, P.; Buck, M.; Azzam, W.; W€oll, C. J. Phys. Chem. B 2004, 108, 4989–4969. (46) Nakamura, T.; Ueki, K.; Sakai, H.; Matsumoto, M.; Abe, M.; Nakai, I.; Kondoh, H.; Ohta, T. Trans. Mater. Res. Soc. Jpn. 2006, 31, 589–591. (47) Shaporenko, A.; M€uller, J.; Weidner, T.; Terfort, A.; Zharnikov, M. J. Am. Chem. Soc. 2007, 129, 2232–2233. (48) Tao, F.; Bernasek, S. L. Chem. Rev. 2007, 107, 1408–1453. (49) Weidner, T.; Shaporenko, A.; M€uller, J.; Schmid, M.; Cyganik, P.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2008, 112, 12495–12506. (50) Newman, M. S.; Karnes, H. A. J. Org. Chem. 1966, 31, 3980– 3984. (51) M€uller, J.; Brunnbauer, M.; Schmidt, M.; Zimmermann, A.; Terfort, A. Synthesis 2005, 998–1004. (52) Sch€upbach, B.; Zharnikov, M.; Terfort, A. Eur. J. Org. Chem. 2010, 3041–3048. (53) Zhang, J.; McIlroy, D. N.; Dowben, P. A.; Zeng, H.; Vidali, G.; Heskett, D.; Onellion, M. J. Phys.: Condens. Matter 1995, 7, 7185–7194. (54) McIlroy, D. N.; Zhang, J.; Dowben, P. A.; Heskett, D. Mater. Sci. Eng., A 1996, 217/218, 64–68.
ARTICLE
(55) Balaz, S.; Caruso, A. N.; Platt, N. P.; Dimov, D. I.; Boag, N. M.; Brand, J. I.; Losovyj, Ya. B.; Dowben, P. A. J. Phys. Chem. B 2007, 111, 7009–7016. (56) Feng, D.-Q.; Wisbey, D.; Losovyj, Ya. B.; Tai, Y.; Zharnikov, M.; Dowben, P. A. Phys. Rev. B 2006, 74, 165425/1–11. (57) Xiao, J.; Zhang, Z.; Wu, D.; Routaboul, L.; Braunstein, P.; Doudin, B.; Losovyj, Ya. B.; Kizilkaya, O.; Rosa, L. G.; Borca, C. N.; et al. Phys. Chem. Chem. Phys. 2010, 12, 10329–10340. (58) Xiao, J.; Dowben, P. A. J. Mater. Chem. 2009, 19, 2172–2178. (59) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. (60) Bechstedt, F.; Fuchs, F.; Kresse, G. Phys. Status Solidi B 2009, 246, 1877–1892. (61) Neese, F. Orca 2.8, TU Bonn. (62) Almenningen, A.; Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin, S. J.; Samdal, S. J. Mol. Struct. 1985, 128, 59–76. (63) Bordat, P.; Brown, R. Chem. Phys. 1999, 246, 323–334. (64) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Johansson, L. S. O.; Grunze, M.; Zharnikov, M. Langmuir 2005, 21, 4370–4375. (65) Goossens, D. J.; Gutmann, M. J. Phys. Rev. Lett. 2009, 102, 015505/1–4. (66) Scheer, A. M.; Gallup, G. A.; Burrow, P. D. Chem. Phys. Lett. 2008, 466, 131–135. (67) Scheer, A. M.; Burrow, P. D. J. Phys. Chem. B 2006, 110, 17751– 17756. (68) Garcia-Lastra, J. M.; Rostgaard, C.; Rubio, A.; Thygesen, K. S. Phys. Rev B 2009, 80, 245427. (69) Heimel, G.; Romaner, L.; Bredas, J.-L.; Zojer, E. Langmuir 2008, 24, 474–482. (70) Hamoudi, H.; Neppl, S.; Kao, P.; Sch€upbach, B.; Feulner, P.; Terfort, A.; Allara, D.; Zharnikov, M. Phys. Rev. Lett. 2011, 107, 027801/ 1–4. (71) Karth€auser, S. J. Phys.: Condens. Matter 2011, 23, 013001/1–16. (72) Schalnat, M. C.; Pemberton, J. E. Langmuir 2010, 26, 11862– 11869. (73) Venkataraman, N. V.; Z€urcher, S.; Rossi, A.; Lee, S.; Naujoks, S.; Spencer, N. D. J. Phys. Chem. C 2009, 113, 5620–5628. (74) Bedwani, S.; Wegner, D.; Crommie, M. F.; Rochefort, A. Phys. Rev. Lett. 2008, 101, 216105/1–4. (75) Scheybal, A.; M€uller, K.; Bertschinger, R.; Wahl, M.; Bendounan, A.; Aebi, P.; Jung, T. A. Phys. Rev. B 2009, 79, 115406/1–6. (76) Bagus, P. S.; Hermann, K.; W€oll, Ch. J. Chem. Phys. 2005, 123, 184109/1–13. (77) Bagus, P. S.; Staemmler, V.; W€oll, Ch. Phys. Rev. Lett. 2002, 89, 096104/1–4. (78) Vazquez, H.; Dappe, Y. J.; Ortega, J.; Flores, F. Appl. Surf. Sci. 2007, 254, 378–382. (79) Rusu, P. C.; Giovannetti, G.; Weijtens, Ch.; Coehoorn, R.; Brocks, G. Phys. Rev. B 2010, 81, 125403/1–17. (80) Kim, B.-S.; Beebe, J. M.; Jun, Y.; Zhu, X.-Y.; Frisbie, C. D. J. Am. Chem. Soc. 2006, 128, 4970–4971.
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