Photoemission Studies of Polythiophene and Polyphenyl Films

Mar 12, 2005 - Synchrotron Radiation Center, UniVersity of Wisconsin-Madison, Stoughton, Wisconsin 53589-3097. ReceiVed: October 25, 2004; In Final ...
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J. Phys. Chem. B 2005, 109, 7134-7140

Photoemission Studies of Polythiophene and Polyphenyl Films Produced via Surface Polymerization by Ion-Assisted Deposition Sanja Tepavcevic,† Amanda T. Wroble,† Mark Bissen,‡ Daniel J. Wallace,‡ Yongsoo Choi,† and Luke Hanley*,† Department of Chemistry (m/c 111), UniVersity of Illinois at Chicago, Chicago, Illinois 60607-7061, and Synchrotron Radiation Center, UniVersity of Wisconsin-Madison, Stoughton, Wisconsin 53589-3097 ReceiVed: October 25, 2004; In Final Form: February 3, 2005

Conducting polymer films are grown by mass-selected, hyperthermal thiophene ions coincident on a surface with a thermal beam of organic monomers of either R-terthiophene (3T) or p-terphenyl (3P) neutrals. Mass spectrometry and X-ray photoelectron spectroscopy previously verified polymerization of both 3T and 3P by 200 eV C4H4S+ during surface polymerization by ion-assisted deposition (SPIAD). The electronic structure of these films are probed here by ultraviolet photoelectron spectroscopy (UPS) and polarized near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and compared with similar spectra of evaporated films. The conducting polymer films formed by SPIAD display new valence band features resulting from a reduction in both their band gap and barrier to hole injection, which are calculated from the occupied and unoccupied valence band states measured by UPS and NEXAFS. These changes in film electronic structure result from an increase in the electron conjugation length and other changes in film structure induced by SPIAD.

I. Introduction Conjugated polymers and oligomers are currently used for various organic-based applications, including light-emitting devices, anti-corrosion coatings, photovoltaics, rechargeable batteries, and field effect transistors.1 Flexibility, chemical tailoring, processibility, and low cost are among the major advantages of these conjugated systems. Studies of oligomers provide information that may be used to improve the development of novel materials and devices. Oligomers of sufficient conjugation length can also serve as models of structurally similar polymers since there is often little difference in their electronic properties.2 Performance of organic electronic devices depends on interface interactions between organic materials and metallic contacts. Interface properties such as bond formation, electronic states, molecular orientation, molecular order, and charge transfer are interrelated and determine device performance.3,4 For example, control of molecular orientation permits tuning of the organic-metal interaction and energy level alignment. The injection properties of an organic solid can be improved by controlling the band gap and the positions of valence and conduction band edges relative to the vacuum level. An effective injection of a hole occurs from an electrode whose work function is close to or higher that the solid-state ionization potential of the conducting polymer, whereas the injection of an electron into an organic solid takes place from an electrode whose work function is close to or lower than the electron affinity of the molecules forming the solid. The energy gap in oligothiophene and oligophenyls can be tuned by variation of the length of the molecule as well as by use of suitable substituents or doping.2 Replacement of the conventional linear structure in oligomers with a branched structure can also narrow the band gap.5 * Author to whom correspondence should be addressed. E-mail: [email protected]. † University of Illinois at Chicago. ‡ University of Wisconsin-Madison.

Surface polymerization by ion-assisted deposition (SPIAD) has been used previously to deposit novel polythiophene and polyphenyl thin films by hyperthermal, mass-selected thiophene cations coincident with a thermal beam of R-terthiophene (3T) or p-terphenyl (3P) neutrals.6,7 Furthermore, the optical band gaps of 3T SPIAD films produced by non-mass-selected ions were found to be narrower than those of evaporated 3T films (formed without ion deposition).8,9 SPIAD displays several characteristics that make it a highly versatile method for synthesizing conducting polymer films. A wide range of different neutral and ionic species can be used in SPIAD, provided those species can be vaporized and ionized without excessive degradation. The addition of the neutral species allows fast film growth and permits the production of a much wider range of film types than are available to direct polyatomic ion deposition.10 SPIAD films do not have entrained solvent molecules, unlike those prepared by electrochemistry, spincoating, casting, printing, or other solvent-based methods. SPIAD can effectively control film chemistry and can produce chemical gradients across the surface by systematic variation of mass-selected ion fluence. Film thickness can be further controlled down to the subnanometer scale by the overall fluence of ions and neutrals. Thus, SPIAD can be applied in a combinatorial fashion in that the ion energy, ion structure, ion kinetic energy, neutral structure, ion/neutral ratio, and substrate temperature can all be varied to create libraries of candidate films which can then be rapidly probed for morphology, film thickness, or electronic property. Mass spectrometry demonstrated that 3T and 3P SPIAD films display a distribution of oligomers with molecular weights exceeding those of the original oligomers.7 Changes in S/Si and C/Si ratios from X-ray photoelectron spectroscopy (XPS), characteristic vibrations in the Raman spectra, and enhanced stability in a vacuum all indicated that 3T SPIAD films are most efficiently polymerized at a 1/150 ion/neutral ratio and 3P SPIAD films at a 1/100 ion/neutral ratio when 200 eV thiophene

10.1021/jp0451445 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/12/2005

SPIAD Polythiophene and Polyphenyl Films

Figure 1. Schematic diagram of surface polymerization by ion-assisted deposition (SPIAD) of evaporated R-terthiophene (3T) or p-terphenyl (3P) by mass-selected 200 eV thiophene ions (C4H4S+).

ions were employed (for substrates at room temperature). Furthermore, thiophene ions were found to incorporate into some of the polymerization products observed by mass spectrometry. These polymerization results using mass-selected ions were found to be generally consistent with those obtained using nonmass-selected ions.8,9 This paper extends our previous work on mass-selected SPIAD films by examining electronic structure of these films using photoemission. Ultraviolet photoelectron spectroscopy (UPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) are used to investigate the electronic structure and the molecular orientation in these novel polymer films. The total density of occupied states is given by UPS while the density of unoccupied states can be derived from NEXAFS.11 Band gap values are estimated from the positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) measured by UPS and NEXAFS, respectively. The barrier for hole injection from the substrate into the valence band of the polymer is determined from UPS data. The band gap of a polymer film affects the spectrum of optical emission and absorption, as well as other crucial conducting polymer device properties.4 The barrier for hole injection from the substrate into the polymer valence band affects the efficiency of charge injection and if reduced will lower operational voltages for devices. II. Experimental Methods Mass-selected C4H4S+ ions used for surface polymerization by ion-assisted deposition (SPIAD) are formed by 80 eV electron impact ionization of thiophene vapor, accelerated, mass selected by a Wien filter, decelerated to 200 eV kinetic energy, refocused, and transmitted at normal incidence to the substrate. A thermal beam of evaporated neutrals is deposited onto the substrate coincident with the thiophene ion beam, as shown schematically in Figure 1.7 The vacuum apparatus used to perform ion deposition and XPS has been described previously.12 3T (99%, Aldrich Chemical Co.) and 3P (Pfaltz & Bauer, Inc.) crystals are used as received. An R-sexithiophene (6T) film is used as a reference instead of 3T for NEXAFS because the latter is not vacuum stable and sublimes quickly in a vacuum. 6T is synthesized following the method of Kagan and Aora in which 3T is lithiated and oxidatively coupled with copper chloride as catalyst,13 as discussed previously.7 Silicon wafers (Atomergic Chemetals Corp., Si (100) p-type, boron doped) are used as substrates for all samples, except where otherwise noted. Hydrogen terminated silicon surfaces or H-Si(100) with a minimum of oxide are produced by HF etching, as previously described.14 XPS of the HF etched Si surfaces display a surface elemental content of ∼5% carbon and 0-6% oxygen immediately prior to deposition. Atomic force micros-

J. Phys. Chem. B, Vol. 109, No. 15, 2005 7135 copy of these H-Si(100) substrates yields RMS roughnesses of less than 1 nm. A polycrystalline gold film evaporated on a quartz disk is used for 6T film deposition. Small carbon and oxygen impurities are present on the Au surface prior to deposition of 6T. A constant 3T flux of 6 × 1017 neutrals/cm2 is used for 3T SPIAD film deposition. These experiments employ a thiophene ion current of 30 nA for a total ion flux of 4.0 × 1015 ions/cm2, a thiophene ion kinetic energy of 200 eV, and a 1/150 ion/ neutral ratio. A constant 3P flux of 4 × 1017 neutrals/cm2 is used for 3P SPIAD film deposition, with the same ion current and ion energy noted above, corresponding to a 1/100 ion/neutral ratio. XPS are recorded with a high-resolution monochromatic Al KR X-ray source (15 keV, 25 mA emission current, model VSW MX10 with 700 mm Rowland circle monochromator, VSW Ltd., Macclesfield, U.K.) and a 150 mm concentric hemispherical analyzer (Model Class 150, VSW Ltd.) with multichannel detector operated at constant energy analyzer mode. More details on the 3T and 3P SPIAD film peak assignments, thicknesses, and composition were discussed previously.7 Ultraviolet photoelectron spectroscopy (UPS) is performed with an achromatic He discharge UV source (Model UV 10, Thermo VG Scientific). The helium discharge lamp is operated in the lower pressure discharge mode to emit chiefly He II radiation at hν ) 40.8 eV. Additionally, the surface is biased at -6.0 eV relative to ground during UPS4 while the samples are electrically grounded for XPS. To align the energy levels at the interface, a polycrystalline gold film mounted in electrical contact with the polymer coated substrate is sputtered with 30 mA of 1 keV He+ ions for 30 min, and the Fermi level (EF) of the clean gold surface is defined as zero binding energy. These same voltage settings in the spectrometer electronics are used to set the EF for all samples.15 One drawback of UPS is that it produces secondary electrons of very low energy that can be particularly damaging for some organic films.16 However, the UPS data reported here show none of the changes in frontier photoemission orbitals over multiple scans which are expected for electron beam damage. Furthermore, all XPS data are collected after the UPS data to further minimize damage or eventual oligomerization by X-ray radiation.17 There is no air exposure prior to UPS or XPS analysis, except as described explicitly below. All film deposition by SPIAD and evaporation as well as the XPS and UPS analyses are performed at the University of Illinois at Chicago. NEXAFS is performed at the Synchrotron Radiation Center of the University of WisconsinsMadison on the MARK II beam line (port 103).18 Absorption spectra are taken at the C 1s K-edge line with the light at least 90% linearly polarized. Total electron yield is used as the detection mode by collecting the electrons of all energies (both photoelectrons and Auger) from the sample.11 Spectra are recorded with Keithley 617 electrometers that collect the electrons emitted by a clean reference gold grid Io and the sample I. The electron yield I from the sample is then normalized with respect to Io.19 For the study of the polarization dependence of the NEXAFS, the samples are rotated around a vertical axes allowing the angle between the incident X-rays and the surface to be varied from 90° (normal incidence) to 20° (grazing incidence). The photon energy scale is calibrated by using the boron (187.0 eV), titanium second order (226.9 eV) and polypropylene (284.3 eV) edges. The energy scale calibration is checked with the minor contamination of the light optics at the C K-edge, in the spectrum of the gold grid.20 Spectra are normalized to a few different mesh signals measured

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Figure 2. Normalized C K-edge near edge X-ray absorption fine structure spectra (NEXAFS) of (a) 6T evaporated film on Au and (b) 3T SPIAD film on Si as a function of incident X-ray angle.

Figure 3. Normalized C K-edge NEXAFS spectra of (a) 3P evaporated film on Si and (b) 3P SPIAD film on Si as a function of incident X-ray angle.

concurrently to remove beam fluctuations. Since electron yield is proportional to the electron escape depth over the absorption length, it changes with the angle of incidence. To remove differences in the absorption depth at different angles, the preedge signal at 280 eV and the post-edge signal at 300 eV are adjusted to 0 and 1, respectively.18 Films for NEXAFS are prepared and analyzed within 1-4 days of air exposure, because of the necessity to transport samples to the synchrotron. Some 3T SPIAD films are intentionally exposed to air for this period to check the extent of oxidation by XPS. An increase in oxygen elemental composition of the 3T SPIAD films from 0-1% up to 2% is observed after 3 days of air exposure. No other sample degradation is observed by XPS. For example, the C/S ratio does not change. All changes in XPS valence band spectra of polythiophene upon air exposure for at least several hours occur at binding energies higher than 10 eV due to the oxygen peaks, as previously described.21 There is no significant change in the UPS of the 3T SPIAD film for the lowest 10 eV region after 3 days of air exposure (data not shown). Four days air exposure of 3P SPIAD film induces an increase in oxygen elemental composition from 0% to 4%. However, no change in the UPS derived valence band is observed in the 3P SPIAD films either.

peak at 286.9 eV is assigned as the transition to the π2* orbital which is the asymmetric counter-pair of the LUMO. The third resonance at 287.6 eV is assigned to the transition to the CR-S σ* orbital. The fourth resonance at 289.5 eV is characterized as mixed valence-Rydberg excitations and is present in all hydrocarbon NEXAFS. All these peaks originally seen in polythiophene films are observed in 3T SPIAD and 6T films, but they differ in shape and intensity: all 3T SPIAD and 6T peaks (Figure 2) are lower in intensity and broader than the corresponding π* or σ* peaks in polythiophene films. The polarization dependence in NEXAFS is due to the change in resonance intensities as a function of the direction of the electric vector E of the incident polarized X-ray, relative to the axis of the π* and σ* orbitals. The dipole selection rules indicate that the π* and σ* transitions occur for E perpendicular and parallel to the molecular long axis, respectively.26,27 The angle dependent NEXAFS in Figure 2b shows effectively no change in π* or σ* peak intensities as the X-ray incident angle is varied from 20° (grazing) to 90° (normal to surface).22-25 The direct ion deposited film also does not show any change in NEXAFS peak intensities with incident X-ray angle (data not shown). B. NEXAFS of 3P SPIAD and Evaporated 3P Films. NEXAFS of (a) evaporated 3P and (b) 3P SPIAD films shown in Figure 3 display both of the peaks seen previously in various polyphenyl films.19,28,29 The first peak at 285.5 eV is assigned as the 1s to π* transition, and the second peak at 288.0 eV is assigned as the 1s to σ*C-H transition. The 3P SPIAD film displays a higher π* peak intensity than that of the 3P evaporated film while the σ* peak intensity is similar in both films. The most significant difference between the NEXAFS of these two films is the strong angular dependence observed in the 3P evaporated film, with the π* peak displaying a maximum intensity at normal irradiation (90°), while the σ*C-H peak is most intense at grazing incidence (20°) (Figure 3a). By contrast, the 3P SPIAD NEXAFS data in Figure 3b do not show any angular dependence. C. UPS of 3T SPIAD and Evaporated 6T Films. UPS of the 6T evaporated, 3T SPIAD, and 3T evaporated films in Figure 4 display their lowest binding energy peaks below 3 eV which are assigned as π* antibonding orbitals.30-32 These π* antibonding orbitals are of greatest interest to conducting polymer

III. Results A. NEXAFS of 3T SPIAD and Evaporated 6T Films. NEXAFS experiments provide both chemical and structural information about the energy and average orientation of unoccupied molecular orbitals for organic films. NEXAFS of complex molecules are often interpreted as a superposition of the spectra of diatomic or larger functional subgroups.11 Figure 2 displays the normalized C K-edge NEXAFS of (a) a 6T evaporated film on polycrystalline Au and (b) a 3T SPIAD film on Si, where the angles correspond to that of the incident radiation (90° is normal to the surface). Peak assignments are based on literature values for CK shell transitions (from the C 1s orbital) observed for various polythiophene, substituted polythiophene, and oligothiophene films.22-25 The lowest binding energy peak at 285.9 eV is assigned as a transition to the π1* lowest unoccupied molecular orbital (LUMO). The second

SPIAD Polythiophene and Polyphenyl Films

Figure 4. He II ultraviolet photoelectron spectra (UPS) of 6T evaporated on Au, 3T SPIAD on Si, and 3T evaporated on Si. The binding energy scale is relative to the Fermi level of a clean gold substrate. The 3T SPIAD film thickness is ∼6.4 nm.7

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Figure 6. Valence band diagram for the 3T evaporated and 3T SPIAD film interfaces. Experimentally derived parameters are ionization potential (IP), Fermi level (EF), vacuum level (VL), highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO).

Si substrates, as described further below. The width of the entire UP spectrum or the position of the secondary electron cutoff (Ec) with respect to the Fermi level is measured experimentally to provide the work function (Φ) of a clean Au sample or the polymer-coated Si substrate:15

Φ ) hν - Ec

Figure 5. He II UPS of 3P SPIAD on Si and 3P evaporated on Si. The 3P SPIAD film thickness is ∼4.4 nm.7

electronic structure and those of the 3T SPIAD film display a continuous band from 1.3 to 1.7 eV binding energy, intermediate between those bands observed for 3T and 6T. All three films display UPS peaks near 4 eV, which are assigned to n nonbonding orbitals, while the groups of peaks from 6 to 10 eV are assigned to π bonding orbitals. D. UPS of 3P SPIAD and Evaporated 3P Films. UPS of 3P SPIAD and 3P evaporated films are shown in Figure 5, with the π* antibonding orbital appearing as a broad band below 4 eV binding energy which is strongly enhanced in the 3P SPIAD film.33 By contrast, the π* band below 4 eV is hardly visible in the 3P evaporated film, as expected since this is obscured by the more intense emission from deeper bands. The n nonbonding orbital of both 3P films appears in the UPS at 5-6 eV binding energy, while the π bonding orbital appears near 8 eV. However, both the π and n orbitals are shifted to slightly lower binding energies in the SPIAD films compared with the 3P only films. E. Estimation of Valence Band Parameters from Photoemission Data. The band gaps for 3T and 3P evaporated and SPIAD films are estimated from the UPS and NEXAFS data using a method similar to that previously described in the literature.34-36 The method estimates the position of the highest occupied molecular orbital (HOMO), substrate work function, Fermi level, and valence band edge of the polymer layer by analysis of UPS data using the widely accepted procedure described by Salaneck and co-workers.15,37 The method further estimates the position of the lowest unoccupied molecular orbital (LUMO) by comparing the NEXAFS and UPS data, as previously described.35,38,39 Figure 6 depicts schematically the valence band electronic structure of evaporated 3T and 3T SPIAD films deposited on

(1)

where hν is the energy of the exciting photons (40.8 eV for the He II line used here). The Si substrate is connected electrically to the clean Au sample, so it is assumed their Fermi levels are equal and the Au Fermi level is measured.40,41 The work function of the sputter cleaned, polycrystalline Au film is measured here as ranging from 5.0 to 5.1 eV, in good agreement with one literature value of 5.1 eV,42 but slightly lower than another value of 5.4 eV recorded for ozone cleaned Au.15 The position of the vacuum level (VL) is determined for each polymer from the secondary electron cutoff by subtracting the photon energy. The ionization potential (IP) represents the position of the HOMO position referenced to the VL. The IP is an inherent property of the polymer film and is therefore independent of the substrate and the energy level alignment at the interface.37 The IP is given by

IP ) EFVB + EFVAC ) EFVB + (hν - Ec)

(2)

where EFVB corresponds to the offset between the valence band edge in the polymer layer and the Fermi energy of the conducting gold substrate15 and EFVAC is equal to the work function of the polymer (Φ) when the width of the band-bending region at the substrate/polymer interface is substantially less than the polymer film thickness (Figure 6). Table 1 presents the calculated results for IP, EFVB, and EFVAC for each prepared film and compares them with literature values for related films. The IP value of 5.2 eV obtained here for the 6T film is in good agreement with the 5.0 eV value obtained at room temperature50 and 4.7 eV measured by high-resolution electron energy loss spectroscopy.46 The 5.6 eV IP calculated for the 3T SPIAD film is lower than that calculated for the 3T film by 0.6 eV. The 6.4 eV IP calculated for the 3P SPIAD film is 0.3 eV lower than that calculated for the 3P only film. Equation 2 follows the standard approximation of neglecting to include ∆, the offset between the vacuum levels of the polymer-covered-substrate and the clean gold substrate, when it is within the error of the overall measurement.15 A nonzero ∆ arises from the existence of a dipole layer at the interface, but the value here is negligible compared with the errors in the measurement. ∆ is calculated by subtracting the VL position

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TABLE 1: Ionization Potentials and Energy Level Alignment Parameters Determined from Photoemission Experiments film

IP (eV)

2T (bithiophene) 3T 3T SPIAD 6T 6T 6T poly(3-hexylthiophene) polythiophene 3P 3P SPIAD 6P polyphenyl (coplanar) polyphenyl (twisted)

6.2a 5.6a 5.2a 4.7 ( 0.5 5.0 6.7a 6.4a 6.0 5.65 5.85

a

EFVB (eV) EFVAC (eV) 2.2b 1.5b 1.0b 2.0b 1.8b 1.5 -

6.8 4.0b 4.1b 4.2b 4.6 4.2 4.7b 4.6b 4.5 -

TABLE 2: Band Gap Values for Various Oligothiophene, Oligophenyl, and Related Films by Various Methods ref

43 this paper this paper this paper 44 30 38 45 this paper this paper 41 33 33

Experimental error of (0.3 eV. b Experimental error of (0.2 eV.

of a clean gold film (secondary electron cutoff - hν) from the VL position in SPIAD films. The weak interfacial dipole layer in the case of SPIAD filmss∆ is estimated here to range from -0.2 to -0.3 eVsindicates good vacuum level alignment. The position of the LUMO is referenced to VL to give the electron affinity (EA) by

EA ) VL - LUMO

(3)

where VL is the position of the vacuum level in the NEXAFS and LUMO is the position of the first peak onset for 90° incident radiation. The vacuum level IP is very difficult to distinguish experimentally in NEXAFS since it is obscured by the π* and σ* transitions. Therefore, VL in eq 3 is determined by adding the work function value from UPS (i.e., ΦSPIAD) to the C 1s binding energy value from the X-ray photoelectron spectrum of the deposited film.7 For example, by eq 3 the VL for the 6T evaporated film is 4.1 eV + 284.3 eV ) 288.4 eV. Attractive interaction of the core hole with the electron in the excited state during photoemission leads to an ∼2 eV reduction in the reported transition energy.38,39 This polarization energy has been shown to be independent of the system size and should be therefore similar for all organic polymers.45 The smallest photon energy that enables Auger electrons to escape occurs at 286.2 eV for poly(3-hexyl-thiophene), a compound with similar electronic structure to the 3T SPIAD films prepared here.38 This particular photon energy marks the position of the vacuum level, as the Auger electrons have to overcome the work function of the sample. Thus, this 286.2 eV value is used here to reference the vacuum level position in NEXAFS. Finally, the band gap is given by

Band Gap ) IP - EA

(4)

The instability of the 3T evaporated films in a vacuum prevented measurement of their NEXAFS, so to obtain their band gap it is assumed that 3T and 6T display the same LUMO binding energies. This assumption is supported by the negligible shifts in the LUMO position for 3T SPIAD, 6T evaporated, and direct thiophene ion deposited films observed here. Use of the same LUMO position for a group of similar compounds was utilized previously for band gap calculations from photoemission data.36 Table 2 presents the band gaps for 3T SPIAD, 3P SPIAD, 3T evaporated, and 3P evaporated films as calculated by the photoemission method with the HOMO and LUMO peaks both

film (method)

band gap (eV)

ref

3T (UPS-NEXAFS)a 3T (HREELS) 3T (Optical maximum) 3T SPIAD (UPS-NEXAFS) 3T SPIAD (Optical maximum) 6T (UPS-NEXAFS) 6T (HREELS) 6T (Optical onset) polythiophene 3P 3P SPIAD 6P polyphenyl

4.2b 4.2 3.72 3.5b 3.2 3.0b 3.0 3.0 2.30 4.2b 3.7b 3.1 4.0

this paper 46 47 this paper 8 this paper 44 48 45 this paper this paper 41 49

a Value for 6T LUMO is used for 3T LUMO in 3T band gap calculation. b Experimental error of (0.6 eV.

referenced to the vacuum level. Also shown in Table 2 are band gaps from the literature for similar compounds obtained by other methods. IV. Discussion The chemical structures of 3T and 3P SPIAD films have been described previously as mixtures of the original oligomer with additional higher molecular weight components such as hexamer and thiophene ion containing adducts,7 as summarized in the Introduction. UPS maps out the occupied states at the valence band of these novel films and also permits determination of the film ionization potential (IP) and energy alignment with respect to the substrate. NEXAFS maps out the unoccupied states, thereby permitting a calculation of the band gap when considered in conjunction with the UPS data. Band gaps, overall valence band structure, and molecular orientation in the SPIAD films are discussed below and compared with their evaporated film counterparts. A. Reduced Band Gap and Barrier to Hole Injection. The transport band gap in a conducting polymer is defined as the minimum energy for the formation of a separated free electron/ hole pair.48 Table 2 presents the band gaps of 3T and 3P SPIAD films as well as several other poly- and oligothiophene films. Comparison of the band gaps for evaporated versus SPIAD films from the same oligomer show a distinct narrowing of the band gap for SPIAD films that is required for many conducting polymer applications, as displayed in Figure 6. The band gap is narrowed by 0.5-0.7 eV for the 3P and 3T SPIAD films, respectively, compared with their evaporated film counterparts. This band gap narrowing is consistent with the formation of higher molecular weight species that have longer conjugation lengths than the original 3T or 3P oligomers used in SPIAD.7 It is also consistent with the band gap narrowing observed optically for 3T SPIAD films prepared by non-mass-selected ions.8 Another important property in conducting polymer films is EFVB, the barrier for hole injection from the substrate into the valence band of the polymer. This offset is 0.7 eV closer to EF for the 3T SPIAD film than in the 3T evaporated film, while for the 3P SPIAD film the offset is 0.2 eV closer to EF than in the 3P evaporated film (Table 1). To improve the efficiency of charge injection and allow lower operational voltages of conducting polymer devices, the bottom of the conduction band (LUMO) should lie near to EF at the electron injection contact, while EF of the hole injection electrode should be aligned near to the top of the valence band (HOMO).43 The hole injection EFVB values are reduced for SPIAD versus evaporated films for

SPIAD Polythiophene and Polyphenyl Films both 3P and 3T, but they are still large in absolute terms. Thus, SPIAD permits an improvement in this crucial charge transfer property, but another combination of ions and neutrals is likely required for useful devices. The band gaps calculated from photoemission data agree well with those obtained by other methods, as shown in Table 2. Estimations of band gaps are most commonly calculated by extrapolation from the onset of UV/Vis optical absorption.4,45 However, one limitation of the optical method is that changes in the UV/Vis absorption spectra can also result from optical scattering effects not directly related to electronic structure. Nevertheless, the photoemission method generates a band gap value of 3.5 ( 0.6 eV for these 3T SPIAD films prepared from mass-selected ions, which is within error of the ∼3.2 eV optically derived band gaps for 3T SPIAD films prepared by non-mass-selected ions.8 High-resolution electron energy lost spectroscopy (HREELS) is another technique for determining film band gap and it has the twin advantages over optical methods of less rigid selection rules and enhanced surface selectivity.44 The 4.2 ( 0.6 eV band gap of 3T evaporated films obtained here by photoemission agrees well with the HREELS result.46 The 6T film band gap obtained here by photoemission is also in good agreement with values obtained with other methods.44 The relatively large error bars quoted here for the photoemission derived band gaps limit the method. These error bars result from uncertainty in assigning peak binding energies ((0.2 eV), shifts in the spectrometer work function over time ((0.3 eV), and inaccuracies in assignment of the vacuum level in NEXAFS ((0.5 eV). The procedure used here assigns the same LUMO position for similar compounds, in agreement with prior work.36 It therefore requires that the changes in band gap observed here result from obvious shifts in the UPS data. Overall, the band gaps observed are reliable (even though they have relatively large error bars) because the photoemission method is well-established.15,34-39 The band gaps are consistent with both those obtained by other methods (see above), including optical measurements made on similar polythiophene films produced by non-mass-selected ions.8 Finally, the reduction in band gap observed here is consistent with the increase in electronic conjugation length that is expected to occur upon the polymerization unequivocally shown to occur by mass spectrometry.7,9 B. Relationship of Electronic Structure to Film Polymerization and Molecular Orientation. Photoemission permits correlation of the valence band electronic structure of these SPIAD films with both their molecular orientation and extent of polymerization. Evidence for polymerization during SPIAD is evident in shifts in the binding energy and changes in the shape of the π* antibonding level measured by UPS. The 3T SPIAD film displays a continuous π* band near ∼1.5 eV binding energy. The emergence of this π* band for poly- and oligothiophenes has been previously interpreted as indicating long-range order exists along the polymeric chains with large interaction between the aromatic rings via R-R linkages.21,31,52 This π* structure appears as three levels in 3T evaporated films and five to six levels in 6T evaporated films and occurs due to π orbital overlap between the aromatic rings.53 The broadening of these discrete π* peaks observed in the UPS of 3T evaporated films into the band observed for 3T SPIAD films is likely due to the presence of multiple components formed during SPIAD.7 The nonbonding band near 4 eV, at the high binding energy side of the bonding π band, is present and similar in both 3T and 3T SPIAD films. Previous work has found this nonbonding

J. Phys. Chem. B, Vol. 109, No. 15, 2005 7139 band to be invariant as the conjugation length is changed across a series of oligothiophenes.31 Polarization-dependent NEXAFS allows determination of the orientational order of a molecular film. A planar conformation of the thiophene or phenyl rings for a given oligomer enhances the electronic conjugation length and subsequent charge-transfer events since inter-ring charge-transfer disappears as the orbital overlap between adjacent rings is reduced.51 However, the polythiophene films produced by SPIAD do not display any preferred molecular orientation by NEXAFS. This may be due to ion-induced disordering of the neutral oligomer or disordering from branched or cross-linked structures in the SPIAD films.22-25,50,54 UPS of the 3P SPIAD film displays a 0.2 eV lowering of the HOMO position from that of the evaporated 3P film. Similar shifts were observed when coplanar and twisted p-polyphenyl films were compared by UPS.33 It appears that when the incident thiophene ion binds to the p-terphenyl oligomer during SPIAD, the result is enhanced delocalization of the π orbitals, presumably by maintaining a more planar conformation that enhances inter-ring π-interactions. The deeper σ valence levels do not shift much upon polymerization except for a slight broadening.53 Figure 3a shows that NEXAFS of the 3P evaporated film displays a π* peak intensity maximum at grazing incidence and a σ*C-H peak intensity maximum at grazing incidence, consistent with 3P oriented with its molecular long axis perpendicular to the surface. By contrast, the angle-dependent NEXAFS data for 3P SPIAD films in Figure 3b do not show any preferential molecular orientation. The molecular orientation normal to the surface observed here for the 3P evaporated film is similar to the 4P and 6P film orientations previously observed on silicon oxide surfaces.41 The conformation of polyphenyls balances between electronic delocalization that favors a planar geometry and steric repulsion between the ortho-hydrogen atoms on neighboring rings that causes molecular twisting.49 The absence of orientation in the 3P SPIAD film likely results from it being a mixture of all polyphenyls and polyphenyl-thiophene oligomers,7 in addition to the ion-induced disordered and crosslinking effects noted above. The binding of thiophene monomers to 3P is expected to hinder the molecular twisting observed for “pure” polyphenyls and mixtures of the two different oriented species, contributing to the overall isotropic orientation in the film.2,26,44,50,55,56 V. Conclusions Surface polymerization by ion-assisted deposition (SPIAD) is used to produce novel polythiophene or polyphenyl films from one mass-selected polyatomic ion (C4H4S+) and two evaporated oligomers, R-terthiophene (3T) or p-terphenyl (3P). Mass spectrometry and XPS previously verified the polymerization of 3T or 3P in their respective SPIAD films.7 UPS of the valence band of the 3T SPIAD film shows a new state 1.3-1.7 eV below the Fermi level which is not observed in 3T evaporated films. This new state is attributed to an extended π bonding band along the conjugated aromatic chain of the polymerized 3T. UPS of the 3P SPIAD film displays more electron delocalization over the π bonding band compared with the 3P evaporated film. Carbon K-edge polarized NEXAFS indicates the 3T and 3P SPIAD films are both disordered. The 3T SPIAD film shows 0.7 eV narrower band gap than the 3T evaporated film while the 3P SPIAD film displayed a 0.5 eV narrower band gap as a result of SPIAD polymerization and preserved planar conformation. Both SPIAD films also show reduced barriers to hole injection compared with their evaporated film counterparts.

7140 J. Phys. Chem. B, Vol. 109, No. 15, 2005 These results demonstrate that SPIAD can be used to produce films of conducting polymers in which the electronic properties have been improved with respect to their evaporated film counterparts. Combinatorial variation of the ion energy, ion structure, ion kinetic energy, neutral structure, ion/neutral ratio, and/or substrate temperature during SPIAD allows the production of arrays of candidate conducting polymer films that can be rapidly probed for favorable optical or electronic properties. SPIAD should therefore prove an important method for the development of novel conducting polymer films, especially in cases where the exclusion of solvent is critical to achieving optimal device performance. Acknowledgment. The component of this work based at the University of Illinois at Chicago is funded by the National Science Foundation (NSF) under Award No. CHE-0241425. This work is based upon research conducted in part at the Synchrotron Radiation Center, University of Wisconsins Madison, which is supported by the NSF under Award No. DMR-0084402. The authors thank the staff at the Synchrotron Radiation Center for assistance with the collection of NEXAFS data. References and Notes (1) Fahlman, M.; Salaneck, W. R. Surf. Sci. 2002, 500, 904. (2) Fichou, D.; Ziegler, C. Structure and properties of oligothiophenes in the solid state: Single crystals and thin films. In Handbook of Oligoand Polythiophenes; Fichou, D., Ed.; Wiley-VCH: Weinheim, 1999; p 185. (3) Sworakovski, J.; Ulanski, J. Annu. Rep. Prog. Chem. 2003, Sec. C, 99, 87. (4) Campbell Scott, J. J. Vac. Sci. Technol. A 2003, 21, 521. (5) Mazzeo, M.; Vitale, V.; Della Sala, F.; Pisignano, D.; Anni, M.; Barbarella, G.; Favaretto, L.; Zanelli, A.; Cingolani, R.; Gigli, G. AdV. Mater. 2003, 15, 2060. (6) Tepavcevic, S.; Choi, Y.; Hanley, L. J. Am. Chem. Soc. 2003, 125, 2396. (7) Tepavcevic, S.; Choi, Y.; Hanley, L. Langmuir 2004, 20, 8754. (8) Choi, Y.; Tepavcevic, S.; Xu, Z.; Hanley, L. Chem. Mater. 2004, 16, 1924. (9) Choi, Y.; Zachary, A.; Tepavcevic, S.; Wu, C.; Hanley, L. Int. J. Mass Spectrom. Ion Process. 2005, 241, 139. (10) Hanley, L.; Sinnott, S. B. Surf. Sci. 2002, 500, 500. (11) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992; Vol. 25. (12) Wijesundara, M. B. J.; Ji, Y.; Ni, B.; Sinnott, S. B.; Hanley, L. J. Appl. Phys. 2000, 88, 5004. (13) Kagan, J.; Arora, S. K. Heterocyc. 1983, 20, 1937. (14) Fuoco, E. R.; Hanley, L. J. Appl. Phys. 2002, 92, 37. (15) Salaneck, W.; Lo¨gdlund, M.; Fahlman, M.; Greczynski, G.; Kugler, T. Mater. Sci. Eng. Rep. 2001, 34, 121. (16) Duwez, A. S. J. Elec. Spectrosc. Relat. Phenom. 2004, 134, 97. (17) Hernandez, J. E.; Ahn, H.; Whitten, J. E. J. Phys. Chem. B 2001, 105, 8339. (18) Peters, R.; Nealey, P.; Crain, J.; Himpsel, F. Langmuir 2002, 18, 1250. (19) Polzonetti, G.; Carravetta, V.; Russo, M.; Contini, G.; Parent, P.; Laffon, C. J. Elec. Spectrosc. Relat. Phenom. 1999, 98/99, 175. (20) Shaporenko, A.; Adlkofer, K.; Johansson, L. S. O.; Tanaka, M.; Zharnikov, M. Langmuir 2003, 19, 4992. (21) Wu, C. R.; Nilsson, O.; Inganas, O.; Salaneck, W. R.; Osterholm, J. E.; Bredas, J. L. Synth. Metals 1987, 21, 197. (22) Tourilllon, G. Surf. Sci. 1988, 201, 171.

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