J. Phys. Chem. B 2000, 104, 10627-10634
10627
Spectroscopic Characterization of the Bonding, Orientation, and Coverage of Copper Tetraazaphthalocyanine Monolayer Films on SiO2 Surfaces Kjell O 2 berg, Ulf Edlund, and Bertil Eliasson* Department of Chemistry, Organic Chemistry, Umea˚ UniVersity, SE-901 87 Umea˚ , Sweden
Andrei Shchukarev Department of Chemistry, Inorganic Chemistry, Umea˚ UniVersity, SE-901 87 Umea˚ , Sweden.
Kannan Seshadri and David Allara Department of Chemistry, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802-6300 ReceiVed: July 24, 2000
Self-assembled monolayer films of a metallotetraazaphthalocyanine were prepared using two different reaction conditions. The films were characterized by ellipsometry, AFM, electronic absorption spectroscopy, semiempirical MO calculations, core-level XPS, and projection of a calculated structure of the chromophore onto a surface to obtain its silhouette area for comparison with surface concentration from absorption spectroscopy. A denser packing of the chromophores is found in the SAM film prepared with bromonaphthalene at 250 °C as compared with the film from the more polar solvent DMF at 120 °C. Data is consistent with an almost vertical orientation of the chromophores in the former film. A different number of bonds connecting the chromophore with the coupling molecule is suggested as an explanation to differences in the absorption bands for the two films.
Introduction The interest in ultrathin films with functional organic molecules is steadily increasing because of a large number of potential practical applications. Some notable examples of these applications are optoelectronic and photonic devices.1-7 For these applications, it is important to obtain thermally stable and uniform films with a high degree of ordering and dense packing of the constituent chromophores. An adequate starting point for the preparation of such thin films seems to be the design of self-assembled monolayers (SAMs).8 In a SAM film, the chromophores can be attached with strong bonds to the surface, thereby making the film less fragile than those typically made by Langmuir-Blodgett (LB), spin coating, or vapor deposition techniques.1,2,9-12 The active chromophore also may be attached to the surface of the substrate by an appropriate preassembled coupling layer. For multilayer films, the SAM method typically also provides a good control of film thickness, since each layer is added in a separate step. Many studies of thin films to date have utilized phthalocyanine (Pc) chromophores; a class of compounds that displays an unusually rich manifold of applications and potential uses in different fields.2 While a major part of the work is performed using LB or evaporated films, less is reported about SAMs.2,13 Although considerable information about SAM film structures has been accumulated during the past decade, there is still more to be learned on how the structure and properties of a film using one certain chromophore can be controlled by the choice of film assembly conditions, particularly solvent and temperature. In our emerging studies of thin films of organic materials, we search for more knowledge about factors important for the * To whom correspondence should be addressed. E-mail: Bertil.Eliasson@ chem.umu.se.
packing and arrangement of molecules in SAM films. We have investigated SAM films from the Pc-related compound Copper(II) tetrapyridino[3,4-b:3′,4′-g:3′′,4′′-l:3′′′,4′′′-q]porphyrazine (1Cu) on the native oxide surface of Si and quartz, with trichloro(4-chloromethylphenyl)silane employed to form the coupling layer (Figure 1). We provide an example of two 1-Cu SAM films, where quite different electronic absorption spectra result from changing the solvent and temperature during the preparation of the films on quartz. The average chromophore density and orientation in the films have been estimated using an approach based on integrated electronic absorptions of 1-Cu in solution and in the films, and projection of molecular electron density surfaces (EDSs) as obtained from molecular orbital (MO) calculations.14 Ellipsometry, wetting, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) have also been utilized in order to characterize the 1-Cu films on Si(100) and Si(111). Experimental Section Materials. Trichloro(4-chloromethylphenyl)silane (Aldrich) and 1-Cu (Aldrich) were used as received without further purification. Elemental analysis of 1-Cu provided by Aldrich: Found for C28H12N12Cu: C, 55.51; N, 27.64. Calcd: C, 57.98; N, 28.98; Cu, 10.96. Inductively coupled plasma (ICP) analysis data from Aldrich showed 10% Cu. The organic solvents used in the reactions and in the washing procedures were all p.a. quality if not stated otherwise. All glassware used in the experiments was dried in an oven and cooled under nitrogen or argon prior to use. The undoped n-type Si(100), used for wetting and ellipsometry, and Si(111) wafers, for ellipsometry and XPS experiments, had one side polished and were purchased from Virginia Semiconductor Inc.15 The quartz substrates were optical
10.1021/jp002634y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/19/2000
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10628 J. Phys. Chem. B, Vol. 104, No. 45, 2000
Figure 1. Preparation of a SAM film from a copper(II) tetraazaphthalocyanine.
polished Vitreosil 066 plates (20 mm diameter, 1 mm thickness) acquired from WG-Lab.16 Wafer Cleaning Procedure. The quartz and silicon substrates (cut to about 2.5 × 1 cm pieces) were rinsed with Millipore (MQ) water and immersed into a Piranha type solution (H2O2/ H2SO4 1:3) for 15 min at ca. 85 °C. The substrates were then rinsed thoroughly with MQ water and dried with the use of nitrogen 5.0 or a Headway photoresist spinner. The silicone wafers were further treated in a UV-ozone chamber for 15 min to oxidize any organic remnants, and subsequently washed again carefully with MQ water and dried as described above. The quartz substrates were not subjected to the ozone treatment. Each substrate plate was cleaned prior to use. Self-Assembly of Trichloro(4-chloromethylphenyl)silane Coupling Layer on Silicon and Quartz. 1. Silicon Wafers. Hexadecane (35 mL, 99.9%+) and HPLC grade CCl4 (15 mL) were mixed in a beaker in a nitrogen-purged glovebox and allowed to stabilize for 30 min. Trichloro(4-chloromethylphenyl)silane (0.5 mL, 2.62 mmol) was added and the mixture was allowed to equilibrate for 50 min. Since it has been reported that moisture is critical for ensuring a high degree of selforganization of the siloxane at the surface,17 some air was let in through the transfer chamber of the glovebox until the humidity had reached about 8-9%. The freshly cleaned wafers were immersed in the 52 mM trichloro(4-chloromethylphenyl)silane solution and the surface reaction was continued for 1h. The wafers were then washed rigorously in CHCl3, followed by acetone three times using ca. 2 min. sonications and dried with a stream of nitrogen or argon. The wafers were directly used for the reaction with 1-Cu. 2. Quartz Wafers. A mixture of anhydrous hexadecane (7 mL, 99,9% +) and HPLC-grade CCl4 (3 mL) in a test tube was allowed to stabilize for 10 min.18 Trichloro(4-chloromethylphenyl)silane (0.12 mL, 0.63 mmol) was added with a syringe, and the solution was stirred and allowed to equilibrate for 15 min. The cleaned quartz substrates were then immersed (one in each test tube) in the 63 mM trichloro(4-chloromethylphenyl)silane solution and the surface reaction was continued for 1 h.19 The wafers were washed rigorously in CHCl3 followed by sonication for three 2 min periods in acetone. The wafers were then dried with nitrogen and directly used for the reaction with 1-Cu. Substitution Reaction between 1-Cu and the SAM Coupling Layer on Silicon and Quartz Surfaces in Dimethylformamide (DMF) and 1-Bromonaphthalene (BN). 1. DMF. 1-Cu (5 mg) and HPLC-grade DMF (15 mL) were added to a test tube equipped with a condenser.20 The prefunctionalized Si or quartz wafers were transferred to the solution and placed under nitrogen or argon using a standard Schlenk-line technique. The mixture was subjected to a 1 min sonication to increase
the solubility of the chromophore in DMF and subsequently heated at 120 °C for 3 days. Immediately after the reaction was stopped, the test tubes were sonicated again for 1 min. The wafers were carefully rinsed three times with acetone and two times with CHCl3, each with sonication for 2 min. The plates were also washed with 1 M HCl and 10% Na2CO3 solutions. Finally, the wafers were dried with nitrogen and stored under nitrogen or argon prior to use. 2. BN. To a dry test tube equipped with a reflux condenser, 1-Cu (4 mg) and freshly distilled BN (5 mL) were added and the reaction mixture was purged with argon. The coupling-layer prefunctionalized substrate was immersed in the solution. After a 1 min sonication, the mixture was heated at 250 °C for 24 h. The washing and drying procedure was then performed as described above. Measurements. 1. Contact Angle. The aqueous contact angle and the advancing aqueous contact angle were measured using MQ water. A Teflon micrometer syringe was used to produce the solvent droplet for the measurements. Three to four tests were made on each silicon wafer to follow the change of the surface after each surface reaction. After measurement of contact angles, the advancing type angles were measured by making the same droplet slowly grow until it advanced. The readings for the latter type of measurements were made just when the droplet started to move. 2. Optical Ellipsometry. The ellipsometric measurements were conducted on a Rudolph AutoEL-II Null Ellipsometer at 70° of incidence and at a single wavelength of 632.8 nm. The optical constants of the uncovered substrate were obtained by measurements on freshly cleaned wafers. Several measurements were performed on each silicon wafer before and after each surface reaction to follow the growth of the SAM layers and to get a good estimate of the quantities of ∆ and Ψ. The value of the refractive index (n) of the coupling layer was assumed to be the same as for neat trichloro(4-chloromethylphenyl)silane (1.548), while n ) 1.6 was used for the resulting bilayer.21 3. AFM. The measurements were performed in the tapping mode on the 1-Cu film from DMF solution with a Nanoscope III AFM from Digital Instruments Inc. 4. Electronic Spectroscopy. The optical absorption spectra for the samples on quartz substrates were recorded with a Shimadzu UV-3101PC Spectrometer. The measurements were made in a dual beam mode with an uncovered, freshly cleaned, quartz plate as reference. Each sample was carefully mounted vertically on the sample holder to ensure perpendicular penetration of the beam. Several measurements were made on each plate to obtain accurate values. The minimum-to-maximum variation in integrated absorbance (over the range of 500-900 nm) for the measured spots on the 1-Cu film from BN and DMF solutions was 8% and 16%, respectively. The integrated isotropic
Copper Tetraazaphthalocyanine Monolayer Films on SiO2 extinction coefficient was derived from the integrated absorbance for 1-Cu in concentrated HCl solution of five different concentrations in the range of 2-8 µM. These measurements were done in two separate series with five samples in each series. Regression coefficients (r) of 0.998 and 0.999 were obtained for the two series, and the extinction coefficient was derived from the series having the better r value. 5. XPS. The measurements were performed with a Kratos Axis Ultra electron spectrometer using a monochromated Al KR X-ray source operated at 225 W. High-resolution spectra of separate photoelectron lines were taken with 0.1 eV step and 20 eV pass energy. The base pressure was 4 × 10-9 Torr. Because the samples are semiconductors and insulators, a lowenergy electron flood gun was used to neutralize the surface charging. Photoelectron takeoff angle (TOA) for angle-resolved XPS was varied within 15°-90° (five angles) by tilting the sample manipulator. The electrostatic magnification mode with an analyzer solid acceptance angle of (1.5° was used for angleresolved measurements. Binding energy (BE) scale was referenced to the C 1s line of aliphatic carbons at 285.0 eV. Spectra processing (atomic concentrations, curve fitting, etc.) was done with software supplied by KRATOS. MO Calculations. The geometry optimizations for (OH)3Sip-C6H4CH2(1-Cu)+ and calculations of EDSs of the 1-Cu moiety in this ion were performed with SPARTAN 4.1 software22 using the semiempirical RHF/PM323 Hamiltonian. Nine conformers were chosen for geometry optimization by using different dihedral angles, i.e., by rotation in steps of 120° around the ipso-C(phenylene)-C(methylene) and C(methylene)-N(pyridinium) bonds. The EDSs were obtained with a grid resolution of 0.12 Å and an iso-charge value of 0.08 electrons/bohr3, by which the EDS approximate a surface being smaller than the van der Waals surface. The EDS with an iso-charge value of 0.08 is denoted “bond-density” surface in SPARTAN and was chosen by us on an empirical basis to allow for some separation between molecules in the fitted model of the film. Electronic transitions for 1-Zn were calculated with SPARTAN after geometry optimization using the semiempirical AM124 Hamiltonian and configuration interaction (CI) including singles and doubles excitations. The CI calculations involved an equal number (five or six) of occupied and unoccupied molecular orbitals in the ground-state configuration (keyword CI ) 10 or CI ) 12). Molecular Projected Area Calculation and Refined Geometries. Cartesian coordinates for the EDS of the chromophore, obtained from the MO calculations, were imported to SHADOW by which the EDS in three dimensions was transformed into a two-dimensional contour.25 Subsequently, the area of this electron density projection (EDP) in the xy plane, i.e., the surface of the substrate, was calculated. The EDS was rotated in small steps (1°) around the x and y axes during the search for the best fit of the projection area to the inverse of the chromophore surface concentration (1/asurf), derived from the absorption spectra of the 1-Cu films. The angle between the normals of the surface and the chromophore plane was calculated from the coordinates of the fitted structure. In addition, the thickness of the molecular layer was estimated using SHADOW. For this, an EDS with iso-charge ) 0.002 electrons/bohr3, resembling the van der Waals surface,22 was calculated for the fitted structure and reentered into SHADOW. Results Contact Angle Measurements. The aqueous wetting experiments were performed on the coupling layer and bilayer films prepared in DMF on Si(100) wafers. The coupling layer had a
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10629 TABLE 1: SAM Film Parameters from Absorption Spectroscopy, Ellipsometry, and Electron Density Projection film
1/asurfa (Å2)
tellipsb (Å)
tEDPc (Å)
θd (deg)
coupling layer 1-CuDMF 1-CuBN
117 46.5
12 15 25
11 16 22
e22 77
a Available surface area per chromophore molecule, as derived from visible absorption spectroscopy. b Film thickness ((3 Å) from ellipsometry. c Film thickness from EDP analysis, see text. d Average angle between normals of surface and chromophore planes.
contact angle of 68° and an advancing contact angle of 78°. The corresponding angles for the 1-Cu layer were 42° and 51°, respectively. The numbers are average values from three to four measurements, all within a range of 2°-3°, on each of the modified surfaces. Ellipsometry. The thickness of the coverage on a substrate is obtained by measuring the change in phase between the incoming perpendicular and parallel components of the wave and the outgoing wave components (∆), and the ratio of the amplitude between the outgoing and incoming wave components (Ψ).26 The average thickness of the trichloro(4-chloromethylphenyl)silane coupling layer and of the bilayer films are given in Table 1. AFM Measurements. Atomic force microscopy was undertaken to investigate the quality of the 1-Cu film from DMF on the oxide surface of silicon. The rms value of the film roughness was 0.87 nm. A scratch was found in the film on one wafer. The depth of the scratch was estimated to 15-17 Å by a bearing analysis with histograms and suggests a film thickness similar to that obtained from ellipsometry, with the assumption that the scratch penetrates only the organic layer. Electronic Spectroscopy. Phthalocyanines and related compounds, such as 1-Cu, have strong absorptions in the visible region due to π f π* excitations. Absorption spectra for the region of 500-900 nm of 1-Cu in concentrated H2SO4, concentrated HCl and CHCl3 are shown in Figure 2a, while spectra of the 1-Cu films on quartz from DMF (1-CuDMF) and BN (1-CuBN) solutions are shown in Figure 2b. Compared to 1-Cu in solution, λmax of 1-CuDMF is blue-shifted, while that of 1-CuBN is slightly red-shifted (from CHCl3 and HCl) or approximately unchanged (H2SO4). The absorption bands of the films are also broader than for 1-Cu in solution, and the absorbance is found to be smaller for 1-CuDMF compared with that for 1-CuBN. To estimate the degree of packing in the films, a surface concentration (asurf) was derived from the Beer-Lambert law (A ) lc) as described below. Instead of applying the BeerLambert law for a specific wavelength, we used the integrated absorption (Ai) for the wavelength region of 500-900 nm.14 The integrated isotropic extinction coefficient (ii) was obtained from Ai of 1-Cu in HCl solution at different concentrations (see Experimental Section).27 The surface concentrations for the films were derived from eq 1,
Ai(film)/2 ) iiasurf
(1)
where the factor of 2 compensates for absorption from monolayers formed on both sides of the quartz substrate. The variable 1/asurf represents the available surface area per chromophore molecule. The value of 1/asurf for 1-CuDMF was approximately 2.5 times larger than that of 1-CuBN (Table 1). MO Calculations. Semiempirical AM1 geometry optimizations followed by CI calculations on 1-Zn having zero to four protons at the pyridino nitrogens, i.e., 1-Zn, 1-ZnH+, 1-ZnH22+
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10630 J. Phys. Chem. B, Vol. 104, No. 45, 2000
TABLE 2: AM1 Calculated Heat of Formation (∆Hf), Electronic Absorption Maxima (λmax) and Oscillator Strengths (OS) for Neutral Zn-tetraazaphthalocyanine (1-Zn) and Differently Protonated 1-Zn Ions ∆Hf (kcal mol-1)a
λmax (nm)b
OSc
1-Zn(H+)n
HF
CI ) 10
CI ) 12
CI ) 10
CI ) 12
CI ) 10
CI ) 12
n)0
418.1
407.1
404.5
n)1
549.3
543.0
541.5
n ) 2, syn
744.8
739.2
738.3
n ) 2, anti
750.4
745.2
745.0
n)3
982.2
977.5
975.3
n)4
1264.1
1255.1
1251.9
783 786 547 600 563 688 478 598 813 608 678 755 767
804 811 556 607 570 701 480 600 818 631 702 789 805
8.38 8.99 5.98 6.47 5.16 10.47 3.89 3.86 3.76 8.20 9.04 8.08 9.97
9.29 7.35 5.81 6.39 5.05 10.21 3.88 3.83 3.73 7.61 8.42 7.41 9.35
a
HF and CI denote the Hartree-Fock and configuration-interaction energy, respectively. Data are reported for CI calculations using 10 and 12 orbitals. b λmax is reported for values >450 nm, where OS > 3. c OS is calculated by SPARTAN software from the square of the transition dipole moment.
Figure 3. Double connection of 1-Cu to coupling layer molecules; a syn configuration with respect to the two quaternized nitrogens.
Figure 2. (a) Absorption spectra for the visible region of 1-Cu in solution; concentrated H2SO4 (s), concentrated HCl (- - -) and CHCl3 (- ‚ -). (b) Absorption spectra of 1-Cu films on quartz; from preparation in DMF (s) and in BN (- - -).
(syn and anti configurations, see below), 1-ZnH33+, and 1-ZnH44+, were undertaken in order to clarify changes in the absorption spectra as a function of the number of quaternized pyridinium nitrogens (Table 2). Except for modeling the possibility of a different degree of protonation of 1-Cu in the films, the calculations were intended to display possible spectral differences between 1-Cu bonded to a single coupling molecule and 1-Cu having double coupling, i.e., bonded at two quaternized nitrogens; here denoted syn configuration (Figure 3). The
calculations were performed with 1-Zn instead of 1-Cu, since the CI method was not available for open-shell systems, such as 1-Cu. However, reported λmax values for 1-Cu and 1-Zn are identical in H2SO4 (both compounds have maxima at 705 and 680 nm) and in H2SO4/H2O solution (maxima at 680 and 665 nm),28 which suggests that the use of Zn instead of Cu in the calculations is justified. Although the CI calculations may not provide very accurate values of λmax on an absolute scale, we believe that semiquantitative information can be obtained. The use of 10 or 12 orbitals (occupied and unoccupied) in the CI calculations gave reasonable values of λmax. For 1-Zn and 1-ZnH44+, two wavelength maxima close to 800 nm were found by the calculations using CI ) 12, which is approximately 100 nm different from experimental values for 1-Cu in CHCl3 (this work) and for 1-Cu and 1-Zn in H2SO4 and for the tetraoctadecyl derivatives 1-CuR44+ and 1-ZnR44+ in CHCl3.29 With the assumption that neutral and tetraprotonated 1-Cu exist in CHCl3 and H2SO4, respectively (see Discussion), the similarity between the λmax values for 1-Zn and 1-ZnH44+ agrees with experimental results, although both maxima are red-shifted relative to the experimental λmax. A shift of the absorption bands to shorter wavelength is indicated for the structures with protonation numbers of one to three, except for 1-ZnH22+ having protonation at opposite nitrogens, denoted as an anti configuration, which has one maximum at 818 nm (CI ) 12). Generally,
Copper Tetraazaphthalocyanine Monolayer Films on SiO2
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10631 TABLE 3: Atomic Ratios from XPS for Bulk 1-Cu, Coupling Molecule Layer, 1-CuDMF and 1-CuBN Films on Si(111)a sample bulk 1-Cu coupling layer 1-CuDMF 1-CuBN
C/Si
C/Cl
N/Cu
N/Si
Si/O
0 0.06 0.7-1.6
1.4 1.4 1.0-0.7
11 1.4 0.9 3.8-9.2
11 b 25-40
a
Figure 4. Schematic projection (right) of a molecular electron density surface for (OH)3Si-p-C6H4CH2-(1-Cu)+.
the calculated values of λmax for the one-, two-, and threeprotonated compounds differ more than those for 1-Zn and 1-ZnH44+. PM3 geometry optimizations on the model molecule (OH)3Sip-C6H4CH2(1-Cu)+ resulted in two stable conformers with a difference in heat of formation of only 0.4 kcal mol-1. The second conformer differs from the first mainly by a twist of the chromophore plane relative to the coupling molecule, i.e., by a rotation around the C(methylene)-N(pyridinium) bond. To obtain a three-dimensional model of the chromophore framework, an EDS was calculated for the 1-Cu part of the structureoptimized R-[1-Cu]+, where R is the coupling molecule. The EDS was rotated in the Cartesian coordinate system while its projection onto the substrate surface was fitted to the experimentally found values of 1/asurf (see below and Figure 4). Electron Density Projection (EDP). It was assumed that the long axis of the coupling molecule is perpendicular to the plane of the substrate. The angle (φ) between this axis and the C(methylene)-N(pyridinium) bond axis was taken to be 111.5° (see Figure 1). This is the weighted average value for the PM3calculated angle φ in the most stable conformer (110.7°) and the corresponding value (112.7°) for the second conformer of 0.4 kcal mol-1 higher energy. The main structural difference between the two conformers was the degree of twist around the C(methylene)-N(pyridinium) bond, which therefore became the important variable in the fit of the projected EDS to the surface concentration from absorption in the visible region of 1-Cu films. After fitting for 1-CuDMF, the average angle between the surface and chromophore normals (θ) was found to be e22°.14 The corresponding angle for 1-CuBN was determined to be ca. 77°, indicating that the chromophore plane is almost perpendicular to the surface. The thickness of the coupling molecule and of the two conformer models for each film were also calculated from the EDSs. The values were ca. 11 Å for the coupling layer only, and ca. 16 and 22 Å for 1-CuDMF and 1-CuBN, respectively. XPS. Selected atomic ratios for bulk 1-Cu, coupling molecule layer, 1-CuDMF and 1-CuBN films on Si(111) are given in Table 3. XPS of a clean Si wafer showed some C contamination but no N or Cl contamination; the atomic ratio between total C 1s and total Si 2p, i.e., C/Si, was e0.1. The C/Si ratio for the coupling molecule SAM was found to be 1.4, which was significantly smaller than that for a 1-CuBN film (see below). Further, the C/Cl ratio was approximately 11. These ratios indicate that Cl3Si-p-C6H4CH2Cl molecules have reacted with hydroxyl groups on the surface and that no more than one Cl per coupling molecule remains, i.e., the Cl in the CH2Cl group. The Cu 2p and N 1s binding energies (BE) and peak shapes of bulk 1-Cu were found to be in good agreement with those reported earlier.30 The N 1s peak at BE ) 399.0 eV was rather
Measurements were performed using 90° takeoff angle on at least two different samples of the coupling layer and the 1-CuDMF and 1-CuBN films. Average values are reported except for measurements where the extreme values differed more than 20%, for which an interval is given. b Could not be estimated because of very low Cu 2p atomic concentration (e0.01%).
Figure 5. (a) XPS N 1s spectrum of a 1-CuBN film. (b) XPS N 1s spectrum of bulk 1-Cu.
symmetric, with a full width at half-maximum (fwhm) of 1.5 eV. All three different types of nitrogens in bulk 1-Cu therefore have very similar BE values. The N/Cu ratio was 11, which agreed with the Cu content reported by Aldrich (see Experimental Section). The XPS spectra of the 1-CuDMF and 1-CuBN films showed an asymmetric N 1s peak (Figure 5a). It could be curve fitted with two Gaussian peaks at approximately BE ) 399.3 and 400.8 eV. The increase of peak intensity at the higher BE side is indicative of quaternary nitrogens, which are not present in bulk 1-Cu (Figure 5b). The absence of Cl 2p peaks for the 1-CuDMF and 1-CuBN films shows that the Cl- counterion is replaced by OH- or another anion in the washing procedure. The C/Si ratio of 0.9 for the 1-CuDMF film was actually smaller than that of the coupling layer. Further, the N 1s intensity
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10632 J. Phys. Chem. B, Vol. 104, No. 45, 2000 Discussion
Figure 6. (a) XPS Cu 2p3/2 and Cu 2p1/2 peaks of a 1-CuBN film. (b) Additional XPS Cu 2p peaks of a 1-CuBN film.
TABLE 4: Angle-Resolved XPS Atomic Ratios for a 1-CuBN Film on Si(111) takeoff angle
C/Si
N/Si
C/N
C/O
N/O
Si/O
90 60 45 30 15
9.2 11.6 16.7 27.2 48.7
1.6 2.3 3.0 5.2 8.9
5.7 5.0 5.6 5.2 5.5
6.4 5.6 6.5 7.3 7.3
1.2 1.1 1.2 1.4 1.4
0.70 0.48 0.39 0.27 0.15
was low and Cu 2p peaks almost undetectable. Still, a film with low chromophore concentration is indicated by the N/Si ratio of 0.06; see Discussion. The 1-CuBN films had a C/Si ratio of 4-9. A C/N ratio of 5-6 indicates some C contamination also for these films. The N/Cu 2p ratio is larger than the expected value of 12 for all samples and reaches 40 for some samples, suggesting loss of Cu during SAM reactions or washings. For several samples, both the main Cu 2p3/2 peak (BE ) 935.1 eV) and the main Cu 2p1/2 peak (BE ) 955.0 eV) had an additional peak at slightly lower BE (Figure 6, a and b). The extra peaks at BE ) 932.7 and 952.5 eV, respectively, may be explained by reduction of Cu(II) to Cu(I) or Cu(0); see Discussion. Angle-resolved XPS was performed on a 1-CuBN film. The study showed increasing values of C/Si and N/Si atomic ratios with decreasing TOA and the opposite trend for Si/O (Table 4). The ratios for C/O and N/O increased slightly at smaller angle while C/N was virtually unchanged at different TOA. Finally, the N 1s peak was curve fitted with two components, showing the peak at lower BE (N1) to be twice as big as N2. The ratio of the peak intensities (N1/N2) was examined but no significant change was found at various TOA.
The purpose of the EDP method is to obtain information about the surface concentration of the chromophore and its average orientation in the thin film. A rationale for the EDP scheme has been described elsewhere and will therefore only be briefly treated here.14 Because of the planar structure of 1-Cu it should have electronic transition dipoles only within a plane.31 The absorption is thus expected to decrease when the orientation of the chromophore plane is changed from being perpendicular to the propagation axis of the light to a parallel orientation. However, for an assembly of 1-Cu molecules, we anticipate the angular dependence of the absorption to be counteracted by a tighter packing of chromophores when going to the perpendicular orientation. A larger absorbance may therefore be predicted for a denser film.14 For 1-Cu in the SAM films, where one N is quaternized and the chromophore does not have C4 symmetry, orthogonally polarized in-plane transitions will probably be of unequal magnitude. The EDP method should be more reliable when the sum of all contributing transition dipoles (both magnitude and orientation) in a film is similar to that of its reference, e.g., a molecule in solution. Hence, the use of the EDP method should be justified by integration of absorbance over a large range of wavelengths, which increases the probability of including differently oriented transition dipoles. In this simplified treatment, we therefore propose that the integrated absorption for a certain chromophore in a film is proportional to asurf. Molecular Arrangement in the Films. The minimum and maximum EDP area of 1-Cu on the surface of the substrate has been determined to be 22.5 and 113 Å2, respectively.14,32 (Note that this is not the projected area of the van der Waals surface, see Experimental). A value of 1/asurf being smaller than the minimum EDP area should indicate a tight packing with stacking of chromophores, while a value larger than the maximum EDP area should imply a chromophore layer which does not fully cover the surface. Hence, the values of 1/asurf (Table 1) suggest a significantly denser monolayer for 1-CuBN as compared to 1-CuDMF. The contact angle and advancing contact angle of 68° and 78°, respectively, for the coupling layer on a freshly cleaned surface are as expected for a film having benzyl chloride groups.5 The smaller values of 42° and 51° for the corresponding angles of the 1-Cu layer indicate a more hydrophilic surface as compared to that of the coupling layer. The variation in contact angle at different locations on the plate was small, suggesting that the coverage on the surface is uniform over a macroscopic area.33,34 The estimated thickness from ellipsometry of the coupling layer of ca. 12 Å supports a vertical (or close to vertical) orientation of the spacer, which is in accord with results reported by other groups.35 Hence, it is reasonable to assume that the difference between the 1-CuDMF and 1-CuBN films is due to differences in the 1-Cu layer, and not to irregularities in the spacer layer. Since the AFM analysis of the 1-CuDMF film indicates a smooth surface, we believe that the chromophore film is uniformly spread on the surface. The regularity of the scratch of 15-17 Å depth in the AFM image also provides some support for a surface with an even distribution of chromophores. The thickness of the 1-CuDMF bilayer of 15 Å from ellipsometer data is indeed in good agreement with both the depth of the scratch and the calculated thickness of 16 Å from the EDP analysis. A somewhat larger difference is found between the ellipsometer and EDP values for the 1-CuBN bilayer (25 and 22 Å, respectively).
Copper Tetraazaphthalocyanine Monolayer Films on SiO2 The XPS C/Si and N/Si ratios of the 1-CuDMF film support the idea of a loose packing of chromophores but indicate that the 1-Cu concentration on Si(111) may be even more dilute than for the quartz surface. By a rough estimate, the displacement of the Cl- in the spacer CH2Cl group by a 1-Cu molecule would give a N/Si ratio for 1-CuDMF of 12 times Cl/Si (0.13) in the coupling layer, i.e., N/Si ) 1.56. The experimental N/Si ratio of 0.06 suggests that the film has 26 spacer molecules per 1-Cu molecule. In contrast, XPS C/Si and N/Si atomic ratios for the 1-CuBN film are in better agreement with a tightly packed film on both Si(111) and quartz. In analogy with the reasoning above, the N/Si ratio of 0.7-1.6 for 1-CuBN can be interpreted as the film having 1-2 spacer molecules per 1-Cu molecule. The purpose with the angle-resolved XPS study was primarily to obtain information about orientation of 1-Cu from N/Cu and N1/N2 atomic ratios, where N1 and N2 are peak components from curve fitting of the N 1s peak. Angle-resolved XPS is a valuable method for probing orientation since atomic composition in overlayer relative to underlayer may show significant variation with TOA. The angle-resolved XPS data showed an expected increase in C, N, and O atoms relative to Si when the TOA was decreased, since a smaller TOA results in a smaller photoelectron escape depth. Although less apparent, the C and N contents increase slightly relative to O at smaller TOA. This and the Si/O ratios are in accord with the oxygen atoms being located between the Si and the chromophore regions. However, since the angle dependence of the C/O and N/O ratios was small, it is understandable that no angle dependence for the N1/N2 or N/Cu ratios was found. The reason for this is that the N being bonded to the CH2 group of the spacer is not resolved from other nitrogens in the N2 peak, and that the distance between the different nitrogens giving the N1 and N2 peaks is not sufficiently large. A similar reasoning can explain the lack of angle dependence of the Cu/N ratio since the nitrogens described by total N or by N2 are not situated far enough from the Cu atom. Further, the Cu composition is somewhat uncertain because of the possible Cu(II) reduction. Reduction of Cu(II) to Cu(I) or Cu(0) has been reported earlier for amorphous 1-Cu, copper 5,10,15,20-tetra(4-pyridyl)porphyrin and a tetraalkylated salt of 1-Cu (both amorphous and as an LB film).30 Similar Cu(II) reductions have also been reported for sublimed CuPc systems on Si(111)36-38 while reduction was not seen for Si(001) substrate.37,38 It has been suggested that reduction can take place because of irradiation in the XPS apparatus.30 Further, we cannot exclude the possibility that the use of a low-energy electron flood gun (see Experimental Section) can cause Cu(II) reduction. Shifts of Absorption Bands. We continue the discussion with emphasis on the absorption spectroscopy results, which are the basis for the EDP analysis in this work. The reasons for the band shifts and possible changes in oscillator strengths for various electronic transitions are clearly of interest in this type of study. In addition to the different values of Ai for the two films, one also finds significantly different values of λmax: ca. 620 nm and ca. 710 nm for 1-CuDMF and 1-CuBN, respectively. Since this shift is likely to originate from structural differences between the films, we will discuss some possible reasons for such spectral changes. For disklike chromophores in the solid state and dimers in solution, a blue shift of an absorption band can be expected in comparison with that of the corresponding monomer.39 A common explanation of such shifts involves exciton theory and a cofacial orientation of the molecules.39,40 In the case of two
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10633 interacting parallel transition dipoles, the in-phase oscillation (of higher energy) is more likely than the out-of-phase oscillation (of lower energy). Therefore, the band envelope becomes blueshifted. In the other extreme case of two collinear (head-totail) transition dipoles, the more favored oscillation is of lower energy, i.e., a red shift of the absorption band is expected. The absorption spectra of R-, β- and x-polymorphs of metalfree phthalocyanine provide examples where the Q-band envelope (π-π* transition) is markedly changed from one form to the other.41 More examples can be found in the crystalline state42-44 and in films from vacuum deposition45 of other phthalocyanines, and from liquid crystals46 and dimers in solution47 for B- and Q-bands of porphyrin compounds. In addition to exciton theory, a change in the environment (often denoted the “crystal shift” or “solvent shift”) has been invoked in the explanation of spectral shifts when going from solution to the solid state.42,46,47 Another factor that can cause band shifts when intermolecular interactions increase, e.g., from solution to solid state, is chromophore-to-chromophore charge transfer (CT).46 These effects require that the interacting species are close to each other. In the crystal structure of β-CuPc, the closest intermolecular contact is ca. 3.4 Å, and several other intermolecular C-C distances are in the range of 3.4-3.8 Å.48 The absorption band of 1-CuDMF is blue-shifted from that of 1-CuBN. This cannot be explained by exciton theory if the chromophores in 1-CuDMF have an orientation being close to the in-line arrangement, as would be the case for θ e 22° (see Results). Further, our XPS and EDP results suggest that the chromophore packing in 1-CuDMF is looser than for CuPc and similar crystalline systems (MnPc, CoPc)49 and therefore intermolecular exciton coupling can be expected to be weaker in the 1-CuDMF film. Elemental analysis of a water-precipitated product from a 60% H2SO4 solution of the 2,3-isomer of 1-Cu, i.e., copper(II) tetrapyridino[2,3-b:2′,3′-g:2′′,3′′-l:2′′′,3′′′-q]porphyrazine, here denoted 2-Cu, has suggested that the product is tetraprotonated 2-Cu.50 It is therefore likely that 1-Cu in concentrated H2SO4 solution also is tetraprotonated (at the pyridinium nitrogens), which explains the better solubility of 1-Cu in this strong acid compared with CHCl3 as solvent. In CHCl3, 1-Cu must reasonably be unprotonated since no acidic hydrogens are available. In HCl, some degree of protonation is expected. Since we believe that one or two pyridino nitrogens are quaternized in the 1-Cu films, we have chosen ii from HCl solution for the evaluation of our absorption spectra. The CI calculations yield similar λmax values for 1-Zn and 1-ZnH44+ in accord with the experimental results for 1-Cu in CHCl3 and H2SO4 solution, respectively (see Results), although both values are shifted to longer wavelength relative to the experimental λmax. In contrast, the experimental value for 1-Cu in HCl does not appear to match the calculated data for protonation numbers of one to three, where blue-shifted absorption bands are indicated. Considering that 1-Cu in HCl may exist as an equilibrium of several structures differing by the number of protons at pyridinium nitrogens, it does not seem fruitful to make a detailed comparison between calculated data and the experimental absorption band of 1-Cu in HCl. However, the blue-shift of 1-CuDMF relative to 1-CuBN may be clarified by the CI calculations. Two approximately equally strong absorptions at 556 and 607 nm are predicted for 1-ZnH+ while syn-1-ZnH22+ has a moderately strong absorption at 570 nm and a stronger absorption at 701 nm (Table 2). The blue shift of 1-CuDMF relative to 1-CuBN may therefore be explained by the presence of only one bond between 1-Cu and the coupling
10634 J. Phys. Chem. B, Vol. 104, No. 45, 2000 molecule in the film from DMF solution as opposed to a double bonding for 1-CuBN in the syn arrangement (Figure 3). An anti type of bonding, i.e., with the quaternized nitrogens at opposite positions in 1-Cu, is highly unlikely because of the strained geometry that would result from such a situation. The double connection for 1-CuBN to the coupling layer also appears reasonable in view of the higher temperature used in the preparation of this film. In addition, a tighter packing seems likely to result at higher temperature due to some back reaction, where the C(methylene)-N(pyridinium) bond is broken before final pinning. It is also worth noting that an SN2 type reaction, i.e., displacement by backside attack, for the reaction between the spacer molecule and 1-Cu should be favored by unhindered backside access at the substrate. Such an attack is most certainly hindered in the present reaction, which explains why the reaction requires high temperature even though DMF is generally considered to be a better solvent than BN for nucleophilic displacement reactions. Summary Markedly different monolayer-film structures result from preparation of the films using DMF at 120 °C compared with bromonaphthalene at 250 °C. This is revealed by data obtained from ellipsometry, XPS, absorption spectroscopy and fitting of the inverse of a surface concentration derived from absorption spectroscopy to the projected molecular area obtained from a calculated electron density surface. The relatively low absorbance for the 1-CuDMF film is explained by the chromophore being orientated rather flat on the substrate. Analogously, the more dense 1-CuBN film prepared at higher temperature appears to have a significantly tighter packing suggesting a high degree of vertically oriented 1-Cu molecules. The different shapes of the absorption bands for the two films, with a larger proportion of blue-shifted absorptions for 1-CuDMF, can be rationalized by a structure with only one bond connecting the chromophore and the spacer layer in this film. In 1-CuBN, two bonding sites in the chromophore (to two spacer molecules) are inferred. Acknowledgment. We are grateful for support from the Swedish Natural Science Research Council (U.E.), Umea˚ University (B.E.), the J. C. Kempe Memorial (stipend to K.O ¨ .), and the US National Science Foundation (D.A.). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Cook, M. J. J. Mater. Chem. 1996, 6, 677. (3) Sasaki, D. Y.; Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 9685. (4) Marks T. J.; Ratner A. Angew. Chem. Int. Ed. Engl. 1995, 34, 155. (5) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (6) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (7) Fox, M. A. Acc. Chem. Res. 1999, 32, 201. (8) Ulman, A. Chem. ReV. 1996, 96, 1533. (9) Penner, T. L.; Matschmann, H. R.; Armstrong, N. J.; Ezenyilimba, M. C.; Williams, D. J. Nature 1994, 367, 49. (10) Hassan, B. M.; Li, H.; McKeown, N. B. J. Mater. Chem. 2000, 10, 39. (11) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197. (12) Bao, Z. N.; Lovinger, A. J.; Dodabalapur, A. AdV. Mater. 1997, 9, 42. (13) Revell, D. J.; Chambrier, I.; Cook, M. J.; Russell, D. A. J. Mater. Chem. 2000, 10, 31. (14) O ¨ berg, K.; Eliasson, B. Mater. Lett., in press. (15) Virginia Semiconductor Inc., 1501 Powhatan St., Fredericksburg, VA 22401. (16) Werner-Glas & Instrument AB, Kungsa¨ngen, Sweden.
O ¨ berg et al. (17) Parikh, A. N.; Schively, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135. (18) Instead of using a glovebox, test tubes kept under nitrogen or argon were used for the Quartz procedure. The amount of moisture could then not be measured, but since films prepared in test tubes under slightly different conditions (temperature, reaction time) yielded similar characteristics, this was taken as an indication that enough moisture was present. The method of using test tubes instead of beakers is convenient. Furthermore, the wafers are kept separated from each other and contact that might damage the surfaces is avoided. (19) Generally, the reaction was completed within 30 min as judged by ellipsometry measurements. (20) For a similar preparation, see: Palacin, S.; Ruaudel-Teixier, A.; Barraud, A. J. Phys. Chem. 1986, 90, 6237. (21) Li, D.; Lu¨tt, M.; Fitzsimmons, M. R.; Synowicki, R.; Hawley, M. E.; Brown, G. W. J. Am. Chem. Soc. 1998, 120, 8797. (22) Wavefunction, Inc., 18401 Von Karman Ave. Suite 370, Irvine, CA 92715. (23) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221. (24) a) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F. J. Am. Chem. Soc. 1985, 107, 3902. b) Dewar, M. J. S.; Reynolds, C. H. J. Comput. Chem. 1986, 2, 140. (25) The calculation routine SHADOW was programmed by Johan Trygg and Anders Berglund, Department of Chemistry, Chemometrics Group, Umea˚ University. (26) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977. (27) Since ii typically is integrated over more than one absorption band, it does not describe the dipole strength or oscillator strength of a specific transition (which is obtained by integration only over the specific absorption band in energy units). (28) Palacin, S.; Barraud, A. Colloids Surf. 1991, 52, 123. (29) From ref 28: 1-CuR44+ and 1-ZnR44+ have octadecyl groups on the pyridino nitrogens. λmax in CHCl3: 1-CuR44+ 675 nm, 1-ZnR44+ 670 nm. (30) Carniato, S.; Roulet, H.; Dufour, G.; Palacin, S.; Barraud, A.; Millie´, P.; Nenner, I. J. Phys. Chem. 1992, 96, 7072. (31) If existing, out-of-plane charge-transfer transitions involving the Cu ion are assumed to make only small contributions to the total absorption envelope. (32) With the suggested bonding of the spacer molecule to the substrate, the chromophore plane cannot become coplanar (0°) with the substrate surface; the MO calculations show an angle of 22° in the ‘most parallel’ orientation. (33) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (34) Koloski, T. S.; Dulcey, C. S.; Haralson, Q. J.; Calvert, J. M. Langmuir 1994, 10, 3122. (35) A thickness of 10 Å is reported in ref 5. (36) Ottaviano, L.; Lozzi, L.; Rispoli, F.; Santucci, S. Surf. Sci. 1998, 402-404, 518. (37) Rochet, F.; Dufour, G.; Roulet, H.; Motta, N.; Sgarlata, A.; Piancastelli, M. N.; De Crescenzi, M. Surf. Sci. 1994, 319, 10. (38) Dufour, G.; Poncey, C.; Rochet, F.; Roulet, H.; Sacchi, M.; De Santis, M.; De Crescenzi, M. Surf. Sci. 1994, 319, 251. (39) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (40) Chau, L.-K.; England, C. D.; Chen, S.; Armstrong, N. R. J. Phys. Chem. 1993, 97, 2699. (41) Sharp, J. H.; Lardon, M. J. Phys. Chem. 1968, 72, 3230. (42) Sharp, J. H.; Abkowitz, M. J. Phys. Chem. 1973, 77, 477. (43) Loufty, R. O. Can. J. Chem. 1981, 59, 549. (44) Cook, M. J. Spectroscopy of New Materials; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, UK, 1993; Chapter 3, pp 87-150. (45) Schmidt, A.; Chau, L.-K.; Back, A.; Armstrong, N. R. Phthalocyanines. Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1996; Vol. 4, Chapter 8, pp 307-341. (46) Yan, X.; Holten, D. J. Phys. Chem. 1988, 92, 409. (47) Gregg, B. A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1989, 93, 4227. (48) Brown, C. J. J. Chem. Soc. (A) 1968, 2488. (49) Mason, R.; Williams, G. A.; Fielding, P. E. J. Chem. Soc., Dalton Trans. 1979, 676. (50) Smith, T. D.; Livorness, J.; Taylor. H.; Pilbrow, J. R.; Sinclair, G. R. J. Chem. Soc., Dalton Trans. 1983, 1391.
