Photoemission and STM Study of the Morphology and Barrier Heights

Aug 23, 2010 - Department of Chemistry, School of Energy Resources, UniVersity of Wyoming, ... hole barrier heights of 1.30 and 1.10 eV, respectively...
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J. Phys. Chem. C 2010, 114, 15394–15402

Photoemission and STM Study of the Morphology and Barrier Heights at the Interface between Perylene and Noble Metal (111) Surfaces K. Manandhar and B. A. Parkinson* Department of Chemistry, School of Energy Resources, UniVersity of Wyoming, Laramie, Wyoming 82071 ReceiVed: January 28, 2010; ReVised Manuscript ReceiVed: July 16, 2010

Various coverages of perylene thin films on Ag(111) and Cu(111) were investigated using ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and near monolayer (ML) films by scanning tunneling microscopy (STM). A Schottky junction is formed between perylene/Ag(111) and perylene/Cu(111) with hole barrier heights of 1.30 and 1.10 eV, respectively. When combined with previous measurements of the perylene/Au(111) interface, the slope S, in the plot of the hole barrier height as a function of work function, was calculated to be 0.53, which is higher than the S value for the other planar polyaromatic hydrocarbons without heteroatoms, revealing that the slope is material specific. During deposition a wetting layer of approximately 4 Å thickness was initially formed followed by island formation, consistent with Stranski-Krastanov (SK) growth. Room temperature STM investigations of nominal one ML perylene films revealed coincidenceII and double row array structures with commensurate lattices on Ag(111) and Cu(111), respectively. The perylene film growth mode, thin film structure, energy level diagram, and hole barrier height as a function of metal work function are discussed. I. Introduction Small organic molecules show significant potential for future device applications due to their potential for complementing, or even surpassing, the electronic and opto-electronic properties of their inorganic counterparts. A series of new organic-based devices, such as thin film transistors (TFTs), photovoltaics (PVs) cells, and light-emitting diodes (LEDs), have been demonstrated over past decades and their performance has rapidly improved.1-5 These devices employ an organic-metal (OM) thin film architecture where the metals are the electron or hole injection contacts that can determine the performance of these devices.6,7 The efficiency with which charges cross the interfaces in organic Schottky devices, such as OLEDs and OTFTs, is related to the height of the energy barrier (i.e., turn on voltage) at the metalsemicondictor junction. Fabrication of low turn-on voltage contacts to increase the efficiency is a technological challenge and an important avenue for research.8,9 It was initially assumed that OM interfaces followed the Mott-Schottky model where the relevant carrier injection barriers could be predicted from the fundamental properties of the separate constituents (i.e., highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap and ionization potential of the semiconductor and work function of the metal). However, many of the organic-metal interfaces investigated exhibit interface dipole barriers as large as 1-1.5 eV,10-13 resulting in a discontinuity in the vacuum level across the interface and hence significantly affecting the magnitude of the charge injection barrier. Moreover, determination of the electronic structure of many OM interfaces revealed behavior ranging from the Schottky limit to the Bardeen limit.10,13,14 This points out that the physics behind the electronic structure of these interfaces is not yet well understood and requires more investigations of the relationship between the injection barrier and the work function of the contacting metal. * To whom correspondence should be addressed. Tel.: +1 307 766 4363. Fax: +1 307 766 2807. E-mail: [email protected].

In addition to the energy gap between the HOMO and the LUMO frontier orbitals and the Fermi level of the metal at the interface, the overlap of the corresponding energy levels of molecule-molecule and molecule-substrate are important for the establishment of the electrical and optical properties of the interface.15 Because the spatial extent of the molecular wave functions are in general not isotropic, the orbital overlap depends on the orientation of the molecules in the thin film.15 Enhanced field effect carrier mobilities, together with increased electrical conductivity and reduced activation energy for electrical conduction, have been reported when the ordering in the organic thin film is improved.16,17 Tisper et al.18 and Alvarado et al.19 demonstrated, with scanning tunneling microscopy and spectroscopy (STM/STS), that the energy of charge carrier injection from Au(111) into thin films of 3,4,9,10-perylene-tetracarboxylic acid-dianhydride (PTCDA) and copper phthalocyanine (CuPc) strongly depended on the intermolecular packing and crystallographic order of the thin films. As the degree of crystalline order of the thin film improved, the energy gap decreased and charge transport properties improved. Thus, a detailed understanding of the adsorption process of organic molecules on metal substrates, which often yields interesting surface structures and packing motifs, are of prime importance. Perylene, a D2h symmetry polyaromatic hydrocarbon (PAH), attracted considerable interest because perylene thin films showed excellent electro-optical properties. A very high electron mobility of 5 cm2/(V s) along one of the orthogonal principal axes in a single crystal perylene film20 and improved photovoltaic performance for perylene-doped CuPc devices21 have been demonstrated. Thin films of perylene have been studied on different semiconductor and metal surfaces, including Ag(111)22 and Cu(111).23 STM studies of monolayers (MLs) of perylene molecules showed them lying flat on Cu(111) surfaces with multiple domains consisting of incommensurate two-dimensional oblique and rectangle lattices with lattice parameters 10.3 × 8.4 Å and 10.3 × 8.9 Å2, respectively.23 However, STM studies of near ML perylene films on Ag(111)

10.1021/jp1008626  2010 American Chemical Society Published on Web 08/23/2010

Interface of Perylene and Noble Metal (111) Surfaces showed no features related to the perylene surface structure.22 The authors described the surface structure as an orientated “liquid”, in which the molecules are positionally ordered in an incommensurate close-packed superlattice but orientationally disordered and mobile.22 The relative position of perylene frontier orbitals, the HOMO and the HOMO and the LUMO, with respect to the Au(111) Fermi level has been measured using UPS24 and STS,25 respectively. However, the energetics of perylene/Ag(111) and perylene/Cu(111) interfaces, to our knowledge, have yet to be studied. In this contribution, we measure the energy level alignment for perylene on Ag(111) and Cu(111) using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and ultraviolet and visible (UV-vis) spectroscopy and combined with our previous measurement of the perylene on Au(111) interface to estimate the interface parameter S from the plot of barrier height as a function of metal work function. Furthermore, STM results on packing structures formed at coverages near ML are also presented. II. Experimental Section Experiments were performed in a commercial Omicron Multiprobe ultrahigh vacuum (UHV) system (base pressure 5 × 10-10 Torr). This system is equipped with a variable temperature STM (VT-STM) for surface structural characterization. XPS and UPS, using a VSW EA125 single channel hemispherical analyzer, were used to study the electronic structure of the interface. A physical vapor deposition chamber (base pressure 9 × 10-9 Torr) is attached to the UHV system allowing samples and films to be prepared in situ. The silver and copper films were prepared in the load lock of the vacuum chamber by evaporating silver and copper, respectively, onto freshly cleaved mica. The micas were outgassed overnight at around 410 °C and held at a temperature between 300 and 250 °C during silver and copper evaporation, respectively. The deposition rates, as monitored by quartz crystal growth monitor, were about 120 and 260 Å/min for silver and copper, respectively. The silver and copper films were approximately 500 and 1500 Å thick, respectively. Cycles of sputtering at 1 keV energy with Ar-partial pressure of 7 × 10-7 Torr and annealing at 400 °C were used to clean and flatten the Cu(111) and Ag(111) surfaces. The chemical purity of the surface was determined with XPS (Mg kR, 50 eV pass energy), and the presence of flat surfaces was confirmed with STM.26 The surfaces revealed terraces as large as 600 × 500 nm and 1 × 1 µm with monatomic step heights of 2.27 and 1.90 Å in silver and copper surfaces, which are within 8% error from the accepted values of 2.36 and 2.08 Å27, respectively. In small area scanning, the hexagonal structure of the lattice was resolved with the measured lattice constants of 2.60 and 2.30 Å for silver and copper, which are within 10% error from the accepted values of 2.89 and 2.56 Å27, respectively. Perylene (Aldrich Chemical Co.) films were deposited under UHV (base pressure 9 × 10-9 Torr) using a silicone rubber flexible heater to resistively heat a quartz ampule prepared from a UHV series quartz-metal adapter (source temperature, ∼125 °C). The source was extensively degassed at about 102 °C prior to deposition. The quartz ampule containing perylene was heated to obtain a desired deposition rate that was monitored by a Leybold quartz crystal microbalance (QCM). The deposition rate was on average 1.5 ML/min, with 1 ML defined as a thickness of 4 Å as measured by the QCM. The Ag(111) and Cu(111) substrate were maintained at room temperature (RT) during the deposition.

J. Phys. Chem. C, Vol. 114, No. 36, 2010 15395 Sequential depositions were performed onto silver and copper substrates up to a final film thickness of 1024 and 512 Å, respectively. After each growth step, XPS and UPS (HeI, 21.21 eV; with 10 eV pass energy) were utilized to measure the electronic structure of the surface. XPS spectra were taken with a takeoff angle of ≈20° from the normal emission, while UPS spectra were collected under normal emission. A -10.00 V bias was applied to the sample for the UPS measurements to separate the sample and spectrometer high binding energy cutoffs (HBECs). The spectrometer was calibrated as previously described.28 Work function and HOMO-cutoff positions were determined from the HBECs and HOMO onsets, respectively, of UP spectra. XPS core level peak positions were determined by fitting routine using IGOR Pro (Wavemetrics) data evaluation software. A vacuum-deposited perylene film on quartz was used to obtain the thin film solid-state absorption spectrum that was measured using a Varian Cary-500 UV-vis-NIR spectrophotometer. STM was performed in constant current imaging mode using mechanically cut Pt/Ir tips. For perylene/silver and perylene/ copper films, typical tunneling voltages were -1.00 to -1.90 V and -1.50 to -1.90 V using currents 60-110 pA and 100-200 pA, respectively. All STM images were taken at RT. The Ag(111) and Cu(111) surfaces were renewed before each STM experiment with a sputter and anneal cycle. For STM experiments, a nominal 1 ML of perylene was deposited onto the atomically clean Ag(111) and Cu(111) surfaces. The perylene molecular height and intermolecular distances were calibrated using the in situ measurements of hexagonal closedpack and face-centered cubic domains with the characteristic 23 × 3 herringbone reconstruction of Au(111) and also with the atomic resolution of Au(111), Ag(111) and Cu(111) surfaces. The images were digitally filtered to remove low frequency noise and equalized using the WSxM software.29 III. Results and Discussion XPS and UPS of Perylene/Ag(111). XPS spectra for various thicknesses of perylene films are shown in Figure 1a,b. Figure 1c is a graph of the normalized integrated area intensities for the Ag3d and the C1s emissions as a function of perylene thickness. The intensities were normalized to the peak height of the most intense peaks of Ag3d and C1s emissions. Initially, the Ag3d emission is strongly suppressed, while a large increase in C1s intensity is measured. After depositing approximately 4 Å, the C1s intensity was 27% of its eventual maximum, whereas the Ag3d was 78% of its initial value (Figure 1c). At very long deposition times, only a small further decrease in substrate intensity occurs. The binding energy of the Ag3d5/2 emission, obtained after a careful fitting procedure, is constant at 368.32 eV (Figure 1a) for all deposition steps.13 Initially, the C1s binding energy associated with a small amount of contamination on Ag(111) surface is 284.84 eV.13,14,30 For the initial sub-ML perylene deposition, the BE gradually reduces to 284.54 eV at 4 Å thickness and remains unchanged within 0.05 eV up to a thickness of 512 Å after which charging of the film can cause shifts to higher binding energies.13,30 To confirm that the shifts are related to charging, the binding energies were measured as a function of the excitation intensity by varying the power of the X-ray source. Different shifts were measured at higher intensities. Spectra of thick films showing evidence of charging effects are shown, but these binding energy values are not used in our analysis.

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Figure 1. (a and b) Ag3d and C1s XPS spectra of 0-1024 Å thick perylene, respectively. (c) Plot of normalized intensity of C1s (green curve with filled square) and Ag3d (red curve with filled circle) vs perylene thickness. The inset is the plot for perylene thickness up to 32 Å.

A work function change from 4.61 eV for the clean Ag(111) substrate to 4.31 eV for 4 Å thick perylene films was measured (Figure 2a and d), resulting in an interface dipole potential of 0.30 eV. The work function is constant at 4.31 ( 0.03 eV up to the thickness of 128 Å. The onset of charging starting after 128 Å prevents the measurement of a constant final value for the work function (Figure 2a and d). The obtained value for the interface dipole potential of 0.30 eV is among the smallest measured for organic PAHs on metallic substrates (Table 1).13,14,28,30-34 Table 1 shows the experimentally measured interface dipole for a number of organic PAHs on the (111) surfaces of gold, silver, and copper. The electronic properties of the perylene/Ag(111) interface are summarized in a band energy diagram given in Figure 3b. The HOMO cutoff position was determined from the HOMO onset of UP-spectra (Figure 2c). In the full UP-spectra on Ag(111) only two broad peaks at 8.60 ( 0.20 eV and 10.60 ( 0.20 eV are observable (Figure 2b). After background subtraction, the sections of the perylene/ Ag(111) spectra from 12 to 7 eV clearly revealed the peaks at around 8.50, 9.40, 10.00, and 10.70 eV (Figure 2e,f). Figure 2e and f are the background subtracted UP spectra obtained from 4 and 64 Å thick perylene layers, respectively. The binding energies of the features do not change with increasing film thickness (Figure 2e,f).

XPS and UPS of Perylene/Cu(111). Cu2p and C1s XPS spectra are shown in Figure 4a and b, respectively. The normalized intensities of the Cu2p3/2 and the C1s emissions as a function of perylene thickness determined from the XPS measurements are given in Figure 4c. The general trends in intensity are similar to that on Ag(111). Initially, the Cu2p emission is strongly suppressed, while a large increase in C1s intensity is measured. After depositing approximately 4 Å, the C1s intensity was 35% of its eventual maximum, whereas the Cu2p was 70% of its initial value (Figure 4c). As expected, the binding energy of the Cu2p3/2 substrate is constant at 932.60 eV, within the experimental error of literature value 932.70 eV.13,14 As in Ag(111), the C1s BE for the small amount of contamination is 284.64 eV. For the initial perylene deposition, the BE gradually reduces to 284.45 eV at 4 Å and remains unchanged within 0.05 eV up to a thickness of 512 Å.14,30 The clean Cu(111) surface shows the expected Cu3d-valence band states and a work function of 5.10 eV (Figure 5a,b). The initial perylene deposition induces a large change in the work function (see Figure 5a,d). After depositing 4 Å, where a change in growth mode appears, the work function remains constant at 4.60 eV for all subsequent depositions (Figure 5a and inset of Figure 5d). The work function is constant at 4.60 ( 0.04 eV up to the thickness of 64 Å. Unfortunately, the onset of charging

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Figure 3. (a) Visible absorption spectrum of perylene. (b) Band diagram of perylene-Ag(111).

Figure 2. UPS HeI spectra of perylene film onto Ag(111). (a) High binding energy cutoff (HBEC) region; (b) full spectra; (c) highly occupied molecular orbital (HOMO) range; and (d) a plot of the work function of Ag(111) vs perylene thickness. The inset is the plot for the perylene thickness up to 32 Å. (e,f) Backgroundsubtracted UP-spectra from binding energy 12 to 7 eV of 4 and 64 Å thick perylene film, respectively, onto Ag(111).

TABLE 1: Experimentally Measured Interface Dipoles for the Systems of Organic PAHs-(111) Surfaces of Gold, Silver, and Copper Systems interface dipole (δ; eV) molecules chrysene N,N′-diphenyl-N,N′bis(1-naphthyl)-1,1′biphenyl-4,4′-diamine (R-NPD) p-sexiphenyl (6P) naphtho[2,3-a]pyrene perylene pentacene tris(8-hydroxyquinoline)aluminum (Alq3)

Au(111) 13

1.40 1.331

0.8031 1.0032 0.6530 1.0528 0.6531

Ag(111)

Cu(111)

13

0.8013

0.30 0.5814 1.2033

0.40 0.8314 0.7034

0.50

Figure 4. (a and b) Cu2p and C1s XPS spectra of 0-512 Å thick perylene, respectively. (c) Plot of normalized intensity of C1s (green curve with filled triangle) and Cu2p (red curve with filled square) vs perylene thickness. The inset is the plot for perylene thickness up to 8 Å.

prevents the measurement of a constant final value for the work function. This is demonstrated by the broadening of the secondary electron onset (SEO) and the evolution of a second SEO (Figure 5b). The HeI spectra in Figure 5b show that subsequent depositions of perylene molecules gradually suppress the valence states of copper. No states are observed at the Fermi energy (0 eV); however, the HOMO of the perylene film is observed at the lowest binding energy edge of the spectrum, with an onset at 1.10 eV (Figure 5c). The onset of the HOMO remains unchanged within 0.02 eV up to a thickness of 512 Å. The energy difference between the copper Fermi level and the HOMO cutoff of perylene is of interest because it represents the height of the hole injection barrier (Ebh) at the interface (Ebh ) 1.10 eV). Assuming a work function of 4.60 eV for the

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Figure 5. UPS HeI spectra of perylene film onto Cu(111): (a) high binding energy cutoff (HBEC) region; (b) full spectra; (c) highly occupied molecular orbital (HOMO) range; and (d) a plot of work function of Cu (111) vs perylene thickness. The inset is the plot for perylene thickness up to 16 Å. (e) Band diagram of perylene-Cu(111) with interface dipole. (f and g) Background subtracted UP-spectra from binding energy 12-7 eV of 4 and 64 Å thick perylene film, respectively, onto Cu(111).

thick perylene film covering almost entirely the Cu(111) surface, a band energy diagram can be drawn as shown in Figure 5e. The perylene band gap of 2.84 eV obtained from UV-vis spectroscopy was used to determine the LUMO cutoff position (Figure 3a). (It is important to note that the first excitonic

Manandhar and Parkinson transition is only used as an estimate for the HOMO-LUMO gap because the large excitonic BE of the molecule and polarization effects35 are not accounted.) Due to the absence of band bending, the obtained work functions directly translate into an interface dipole potential of δ ) 0.50 eV. Another interesting observation is the different shapes of the perylene emission features in comparison with those of Ag(111). In full UP-spectra, the splitting of perylene features is easily visible in the perylene/Cu(111) measurements (see Figure 5b), whereas the splitting is not easily discernible with perylene/ Ag(111) (Figure 2b). After background subtraction, the perylene peaks in the 12-7 eV region of the perylene/Cu(111) spectra are better revealed (Figure 5f,g) as in perylene/Ag(111) and within experimental error, the binding energy of perylene features do not change with additional film thickness (Figure 5f,g). Film Growth Mode and Band Bending. Upon depositing approximately 4 Å of perylene noticeable changes in the intensities of the core level emissions for Ag3d5/2 and Cu2p3/2 (Figure 1c and inset of Figure 4c) are observed. The constant BE of perylene C1s core level emission (Figures 1b and 4b) and the significant change of the Ag(111) and Cu(111) work functions (Figures 2d and 5d, inset) indicate that silver and copper are uniformly covered with 4 Å of perylene. The decrease in the rate of change of substrate intensities at intermediate coverages and the still discernible Ag3d and Cu2p3/2 signals, after deposition of 1024 and 512 Å of perylene onto silver and copper, respectively, are consistent with a growth mode where initial formation of a wetting layer of perylene is followed by island growth (classic Stranski-Krastanov (SK) growth mode).13 Therefore, regions covered with less than 2 ML, the approximate escape depth for photoemitted electrons,35 must exist even for the thickest perylene films and the wetting layer most likely contains only a single ML of perylene. Band bending occurring in the organic material or polarization energy-related shifts at the interface were determined by measuring the shift of the core level photoemission peaks. The silver and copper XPS peaks are not subject to any band bending-related shifts and Ag3d5/3 and Cu2p3/2 were constant at 368.32 and 932.60 eV, respectively (Figures 1a and 4a).36 As discussed above, there were no shifts in the perylene C1s core level BE. Also, the distinct perylene emission features observed in the 8-11 eV region of UP-spectra after the first deposition step on metallic substrates maintained a consistent value with the energy difference between the two end features of 2.00 ( 0.20 eV (Figures 2e,f and 5f,g) after all subsequent depositions. This observation corroborates the perylene C1s core level XPS results. Because there are no detectable binding energy shifts in the perylene features and HOMO cutoff with respect to the Fermi level in the UP-spectra and no binding energy shifts in perylene C1s in XPS, band bending can be excluded. Therefore, on Ag(111) and Cu(111), the energy differences between the lowest and the heighest BE perylene features and HOMO cutoff after all subsequent depositions are 7.3 ( 0.1 and 9.3 ( 0.1 eV. These constant values are regarded as a material property of perylene.13,35 Morphology Dependent Perylene Features. Despite the fact that the binding energies and the energy splittings are similar on Ag(111) and Cu(111) substrates, there are some differences in their perylene photoemission features, as seen in the full UPS spectra. In the perylene/Ag(111) spectra, after background subtraction, the peaks concealed in full spectra are visible as shoulders at 9.4 ( 0.1 and 10.1 ( 0.1 eV (Figure 2e,f). STM images of the bare substrates showed the copper surface as

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Figure 6. (a) Plot of interface parameter S and hole barrier height (Ebh) as a function of metal work function (φM).

predominately flat islands with a very small fraction of a “rolling hill” morphology, whereas silver surfaces showed equally large fractions of flat islands and “rolling hill” morphologies (STM images of bare substrates not shown). The molecules could be more randomly oriented on the “rolling hills”. A photoemission peak represents a statistical average value of molecules in a molecular layer.13,35,37 Therefore, different shapes of the photoemission peaks for molecular films on copper and silver surfaces may represent differences in the average orientation of the molecules. Interface Properties. The interface parameter S gives the change of barrier height as a function of the work function of the contacting metal. Jaegermann et al.38 have shown that barrier heights produced by metal deposition on nearly defect free van der Waals (vdW) surfaces, such as WSe2, approach the Schottky limit. These materials show a slope of 1.0 for the change in barrier height with a change of the metal work function. The same plot for a covalently bonded semiconductor, for example, silicon, shows a slope of only 0.1, approaching the Bardeen limit as a result of Fermi level pinning. The S ) 1, which is the Schottky limit for inorganic semiconductor interfaces, would be expected to hold for weakly interacting interfaces formed by organic solids. Studies of poly(2-methoxy-5-(2′-ethyl-hexoxy)-1,4-phenylenevinylene) (MEH-PPV)39,40 and 5,10,15,20tetraphenylporphynatozinc (ZnTPP)41 on metals have shown nearly Schottky-like behavior, while PTCDA,42 pentacene,14 chrysene,13 and 5,10,15,20- tetraphenylporphyrin (H2TPP)43 deviate from Schottky-like behavior and exhibit an S parameter well below unity. Vazquez et al. have extended the concepts of an induced density of interface states (IDIS) and the theory describing the charge neutrality level (CNL), initially developed for understanding the Schottky barrier formation and Fermi level pinning at inorganic semiconductor interfaces, to nonreacting organic semiconductor/metal interfaces.44,45 Due to the interaction with the metal substrate, the organic molecular levels in resonance with the metal density of states (DOS) are broadened, resulting an IDIS. The initial δ-like energy level distribution is therefore transformed into a continuum. The CNL is the highest occupied level in the molecules interacting with the substrate. The CNL acts as the Fermi level of the semiconductor interface. The authors demonstrated that even for weakly interacting systems there are IDIS in the gap of the organic material, which can explain the observed dipoles, Fermi level pinning, and energy level alignment.44,45 They attributed the higher value of S with a smaller density of states induced in the organic gap leading to a greater tendency of the interface Fermi level to move within the organic energy gap. The experimental S value of planar PAHs without heteroatoms never exceeded the value 0.53 for perylene (Figure 6), for example, for chrysene S ) 0.00, and for pentacene S ) 0.37.14,31 For the planar heteroatomic

Figure 7. (a) STM images of nominal 1 ML perylene film on Ag(111) (52.5 × 52.5 nm, -1.60 V, 106 pA). (b) Zoomed-in images (13.7 × 13.7 nm) of the black-square sections shown in the left image. (c) Line profile along the line in (a).

PAHs, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4diamine (R-NPD)10 and 4,4′-N,N′-dicarbazolyl-biphenyl (CBP),10 the S value was approximately 0.50, which closely resembles the value for perylene, while the S for zinc phthalocyanine (ZnPc)31 and copper phthalocyanine (CuPc)45 was measured to be 0.25. However PTCDA31 and 3,4,9,10-perylenetetracarboxylic bisimidazole (PTCBI),31 heteroatomic perylene derivatives, have S ≈ 0.00. The different S values for PAHs may reveal material specific behaviors,10 but as yet, there is no way to reliably predict these values. STM Imaging of Perylene Surface Structures. Scanning tunneling microscopy was utilized to image the near monolayer structures of perylene films on Ag(111) and Cu(111). Only noisy images were obtained at sub-ML coverages due to the perylene molecular motion at RT being faster than the time scale of the STM imaging process, while well-ordered close-packed films formed at near ML coverages (Figures 7a, 8a, and 9a) can be imaged with the STM.46-48 This is consistent with the highly mobile nature of the majority of PAH molecules on metal surfaces near RT46-49 and with the locking of the molecules into ordered regions at sufficiently high coverage to form larger islands. For a nominal ML perylene film on Ag(111) and Cu(111), large area STM scans (as large as 160 × 160 nm) at multiple sites in the sample show that the entire surface is covered with molecules. Figures 7a and 9a show a 52.5 × 52.5 nm multiterraced surface and 30.0 × 10.9 nm surface covered with ordered perylene molecules resulting from 4 Å perylene deposition. Figure 7c shows the height profile along the line in the Figure 7a. The measured step height is 2.12 Å, which is within experimental error of the accepted monatomic step height 2.36 Å for Ag(111).27 In well-ordered close-packed films of perylene on Ag(111), the individual molecules are well resolved (Figure 7a). On all the terraces, the longer axis c1 of perylene (see Figure 8c) is parallel to the step edge direction, as shown by the arrows in the Figure 7b. It is well-known that the majority of step edges on (111) noble metal surfaces run along the closed packed direction [-1-12].47,50,51 For weakly interacting molecules laying flat on the substrate such as CuPc/Au(111),47 iron phthalocyanine (FePc)/Ag(111),52 cobalt phthalocyanine (CoPc)/ Au(111),53 and FePc/Au(111),54 the alignment of one of the two 2-fold symmetric axes parallel to the close-packed direction 〈1j1j2〉 of 3-fold

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Figure 9. (a) STM image of nominal 1 ML perylene film onto Cu(111) (30.0 × 10.9 nm, -2.00 V, 200 pA): a2 and b2 are the overlayer lattice vectors, the oblique is the unit overlayer lattice, and the dashed lines represent the molecular array along a2; (b) line profile along the line in (a).

TABLE 2: Experimentally Measured Lattice Parameters and Calculated Lattice Areas and Molecules per Unit Lattice |a| (Å) |b| (Å) ∠(a,b) (deg) lattice area (Å2) molecules/lattice

Figure 8. (a) High resolution STM image (19.9 × 19.9 nm, -1.90 V, 100 pA) of nominal 1 ML film onto Ag(111). (b) Line profile along the line in (a). (c) Model showing the structure of perylene in a unit lattice of (a): blue arrows c1 and c2 are molecular axes, the green arrows a1 and b1 are the overlayer lattice vectors, the red arrows a and b are the substrate lattice vectors, and the dashed black line is the 〈1j1j2〉 direction of the substrate.

rotational symmetry surfaces has been reported.47,52,53,55 The orientation of longer axis of flat laying perylene being parallel to the 〈1j1j2〉 direction was also reported for perylene/Au(111).25,56 Molecular resolution STM images of the well-ordered films define the overlayer lattice type and registry with an underlying substrate. A high resolution scan of the ordered region reveals that molecules are arranged in a two-dimensional oblique lattice (Figure 8a). On all the terraces, the lattice vectors were measured to be 10.9 ( 0.3 Å by 8.7 ( 0.3 Å (Figure 8b) with the angle between them of 79 ( 2° (Table 2). The overlayer lattice vectors are related through the transformation equation

[] [

][ ]

a1 a 3.13 0 ) b1 -1.44 4.32 b

where a1 and b1 are the overlayer lattice vectors and a and b are the silver substrate lattice vectors (Figure 8c).57 The overlayer lattice is a coincidence type I, where all overlayer molecules lay on the primitive lattice lines of the substrate lattice and it is

perylene/Ag(111)

perylene/Cu(111)

10.9 ( 0.3 8.7 ( 0.3 79 ( 2 98 1

23.0 ( 0.2 13.5 ( 0.2 77 ( 2 306 2

coincidence type II when only a fraction of overlayer lattice points coincide with substrate lattice points.58,59 Because high resolution images consisting of both atomic resolution of substrate and molecular resolution of film in the same image were not obtained, we do not know the relative position of molecules and orientation of the overlayer lattice vectors with respect to the silver substrate lattice points and lattice directions to specify the overlayer lattice as being type I or type II. Based on the fact that all the numbers in the matrix are rational with no integer column in the matrix, the overlayer superstructure is best described as coincidence type II.58,59 As the coincidence type II lattice is more or less independent of the substrate lattice, the thin film growth may be best described as quasiepitaxy.60 The unit cell area is calculated to be 13.52 × 7.23 Å ≈ 98 Å2 with one molecule per unit cell. The density of perylene molecules on Ag(111) is 1.02 × 1014 per cm2 corresponding to 13.5 silver atoms per perylene molecule (Table 2). Furthermore, in the high resolution scan rectangular-shaped individual perylene molecules are observed (Figure 8a). The height profile of the molecule along the c1 axis in Figure 8a (not presented here) is less than 0.10 Å, corresponding to a molecular tilt of about 0.5°. This confirms within experimental error that perylene molecules lie flat on the Ag surface. For a perylene film on Cu(111), a high resolution scan of the ordered film reveals that molecules are not arranged in closed pack array as on Ag(111). The molecules are arranged in the double rows (Figure 9a). The molecules in the double rows are closely packed (Figure 9a). The nearest molecular distance between two molecules in two closely packed rows along the overlayer lattice vector b2 is measured to be 12.7 ( 0.3 Å. The lattice vectors of the oblique overlayer lattice are measured to be 13.5 ( 0.2 Å by 23.0 ( 0.2 Å (Figure 9b) and the angle

Interface of Perylene and Noble Metal (111) Surfaces between them is 77 ( 2° (Table 2). The overlayer lattice vectors are related with copper lattice vectors through the transformation equation

J. Phys. Chem. C, Vol. 114, No. 36, 2010 15401 Acknowledgment. This work was supported by NSF Grant No. CHE-0518563 and University of Wyoming startup funds. References and Notes

[] [

][

a2 -2 6 a ) b2 9 0 b

]

where a2 and b2 are the overlayer lattice vectors and a′ and b′ are the copper lattice vectors.57 The integer numbers in the transformation matrix for the perylene/Cu(111) system refers to the commensuration, suggesting that the film growth is largely determined by the substrate lattice and the growth may thus be characterized as van der Waals epitaxy.60 The unit cell area is calculated as 54 × 5.67 Å2 ≈ 306 Å2 with two molecules per unit cell (Table 2). The density of perylene molecules on Cu(111) is 3.27 × 1013 per cm2. It is noteworthy that CuPc and lead-phthalocyanine (PbPc) also exhibited two different molecular arrangements on highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2).61,62 CuPc and PbPc were close packed on HOPG and were arranged in rows onto MoS2.61,62 The structured first ML of perylene on Ag(111) and Cu(111) determine the magnitude of the interface dipole at the organicmetal interface. From the energy level diagram it is evident that the interface dipoles are pointing from molecules into the substrate resulting in a repulsive interaction between the molecules.30,63,64 The organic film lattice structures are the result of the delicate balance between the molecule-molecule and molecule-substrate interactions.60 In vdW epitaxy in perylene/ Cu(111), the lattice structure is largely determined by moleculesubstrate interactions, whereas for the quasi-epitaxy in perylene/ Ag(111) it is determined by molecule-molecule interactions.60 Thus, in Ag(111), the overlayer lattice structure is probably largely due to the repulsive interactions of molecules through the strong interface dipole formed on the surfaces. IV. Conclusions We investigated thin films of perylene on Ag(111) and Cu(111) using UPS, XPS, and STM. The constant binding energy of C1s core level emissions in XPS and perylene emissions in UPS provided no evidence of band bending in perylene-Ag(111) and perylene-Cu(111). The band line up of perylene on Ag(111) and Cu(111) illustrated that a Schottky barrier is formed between these materials with hole injection barriers of 1.30 and 1.10 eV, respectively. The 4 Å perylene covered interfaces of perylene/Ag(111) and perylene/Cu(111) show interface dipoles of 0.30 and 0.50 eV, respectively. The trend in the S parameter, indicative of Schottky or Bardeen limiting behavior of the interface barriers, for different molecular systems on noble metal (111) surfaces were discussed. The S was calculated to be 0.53, which is closer to the S value of the heteroatomic PAHs, R-NPD and CBP, revealing material specific behavior. The UPS and XPS spectra indicated that an approximately 4 Å thick wetting layer of perylene was formed followed by island growth. Room temperature STM did not show ordered domains for sub-ML coverages presumably due to the high mobility of the perylene molecules. The nominal 1 ML coverage films exhibited molecules laying flat on the Ag(111) surface. Coincidence-type II and commensurate lattice structures with different packing densities were observed, a manifestation of the repulsive interactions of the molecules due to the strong interface dipole formed on the surfaces.30,63,64

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