J. Phys. Chem. 1995,99, 12858-12862
12858
Characterization of Ultrathin Films of Chloroaluminum Phthalocyanine during Layer-by-Layer Preparation on Graphite: PIES and UPS Study Tibor Pasinszki,' Masaru Aoki, and Shigeru Masuda Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153, Japan
Yoshiya Harada" and Nobuo Ueno Department of Materials Science, Faculty of Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba 263, Japan
Hajime Hoshi' and Yusei Maruyama Institute for Molecular Science, Myodaiji, Okuzaki 444, Japan Received: December 8, 1994; In Final Form: March 28, 1995@
Penning ionization electron spectroscopy and ultraviolet photoelectron spectroscopy were used to characterize ultrathin films (one to several monolayers) of chloroaluminum phthalocyanine during layer-by-layer vacuum deposition onto a graphite substrate. At one monolayer, molecules deposited on the substrate held at room temperature are oriented flat to the substrate with the chlorine atom protruding outside the film surface. With increasing film thickness, molecules are gradually tilted in subsequent layers. The monolayer film deposited on the substrate at -170 "C shows a disordered molecular arrangement (tilted molecules), but the change in the arrangement, disordered (tilted) to ordered (flat), is observed upon heating the film up to
100 "C.
Introduction Phthalocyanines are widely used in science and technology as photoconductors, molecular metals, agents for electrocatalysis, photovoltaics and photocatalysis, materials for dyestuffs, medicines and electric batteries, etc. (see recent reviews'). In the last decade, the interest strongly increased for the use of phthalocyanine thin or ultrathin films in electronic devices2 and gas sensor^,^ where well-ordered films have unique physical and chemical properties. To interpret these properties from a microscopic point of view and further to develop new molecular devices, knowledge of the molecular orientation in the film is essential. Although the structures of phthalocyanine thin films have been investigated by various techniques,"-I0 the orientation of molecules on a given substrate is difficult to predict in advance, because it strongly depends on the conditions of the film preparation. Therefore, the molecular orientation should be observed during the film preparation under well-defined conditions. In this work we have taken up the ultrathin film of chloroaluminum phthalocyanine (ClAlPc) as an example, and we intend to study the change in the molecular orientation and electronic state during layer-by-layer vacuum deposition onto a graphite cleavage plane using the characteristics of Penning ionization electron spectroscopy (see below). In our earlier work we have studied the structure of iron phthalocyanine (FePc) films deposited on graphite by Penning spectroscopy.".I2 The structural change, disordered layer closely packed monolayer island, was observed upon heating the film deposited on the substrate held at low temperature. For
-
-
* To whom correspondence should be addressed.
'Present address: Department of Chemistry and Biochemistry, University
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of P l p h , Guelph, Ontario, NlG2W1, Canada. Present address: Faculty of Engineering, Tokyo Institute of Technology, Oh-okayama, Meguro 152, Japan. Abstract published in Advance ACS Absrructs, August 1, 1995. @
the monolayer and island, FePc molecules were found to be oriented flat on the substrate." In contrast to the FePc molecule having a planar shape, the ClAlPc molecule has a chlorine atom which is bonded to the aluminum atom of the phthalocyanine ring and protrudes outside the ring plane. Therefore, it is interesting to study, compared to the case of FePc, the Orientation of ClAlPc molecules during layer-by-layer deposition. In Penning ionization electron spectroscopy applied in the present work, the energy of electrons ejected from targets T by impact of metastable atoms A* is analyzed:I3
T
+ A* - T+ + A + e-, e- for energy analysis
The widely used electron spectroscopic methods such as photoelectron spectroscopy, Auger electron spectroscopy, and electron impact spectroscopy probe the average character of several top layers, because photons or electrons used for the excitation penetrate into inner layers. Penning ionization electron spectroscopy, however, selectively probes the outermost surface layer because metastable atoms do not penetrate into inner layers. Further, metastables interact with the outermost part of the first-layer molecules. The relative intensity of the bands in Penning spectra, therefore, reflects the electron distribution of individual molecular orbitals exposed outside the surface. Since molecules are held together by weak van der Waals forces in the organic molecular crystal, the electronic structure of a molecule in the film is essentially unchanged from that of a separate molecule. Thus, the analysis of the relative intensity of the Penning bands provides information on the geometrical orientation of the molecules in the outermost layer.I4-l6 In the present work ultraviolet photoelectron spectroscopy is also applied to probe the several top layers of the film.
0022-3654/95/2099-12858$09.00/0 0 1995 American Chemical Society
Ultrathin Films of Chloroaluminum Phthalocyanine
J. Phys. Chem., Vol. 99, No. 34, 1995 12859
H e 1 UPS
f
e
5 MLE
/'
3 MLE
d C
\
G'r
/---J\
a Graphite
15
10
Kinetic
Energy
5 / eV
1
15
1
1
~
1
1
10
1
1
1
1
1
1
5
,
~
1
0
K i n e t i c Energy / eV
0
Figure 1. Change in the He I UPS of ClAlPc layers deposited on a graphite cleavage plane held at room temperature with increasing coverage.
1
Figure 2. Change inthe He(2%) PIES of ClAlPc layers deposited on a graphite cleavage plane held at room temperature with increasing coverage.
Experimental Section The apparatus used in this work has been reported in a previous paper.I5 Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) were measured by an ultrahigh vacuum electron spectrometer with a 180' hemispherical deflection-type analyzer. The overall energy resolution was about 0.2 eV. In PIES measurements, metastable atoms of He*(23S,2'S) were produced by impact of 70-120 eV electrons, and the He*(2'S) component was quenched by a water-cooled dc helium lamp. U P S were measured using a He I resonance line produced by a dc discharge lamp. The graphite substrate (Grafoil sample) was cleaned by heating at 400 "C for 24 h under ultrahigh vacuum (UHV). It consists of graphite crystallites with basal (cleavage) planes parallel to the foil surface. The sample of ClAlPc was commercially obtained and purified by vacuum sublimation. Ultrathin films were prepared by sublimation in the preparation chamber of the spectrometer under UHV. The deposited amount of ClAlPc was monitored with a quartz oscillator calibrated in advance. First, 0.5, 1, 2, 3, 4, 5 , and 10 MLE (monolayer equivalence) amounts of ClAlPc were deposited onto the substrate held at room temperature, and PIES and UPS of the films were measured. The amount of 1 MLE is such that the closely packed molecules with their molecular planes oriented parallel to the substrate plane just form a monolayer. In a separate experiment 1 MLE amount of ClAlPc was deposited onto the substrate held at -170 OC. After the PIES and UPS measurements the temperature of the film was raised and the spectra were measured. During the measurements the temperature of the film was kept constant.
Results and Discussion Figures 1 and 2 show the He I UPS and He*(2%) PIES, respectively, of ClAlpc films prepared on the graphite substrate at room temperature. In each figure the numbers of the abscissa, the kinetic energies, refer to the low-energy cutoff of the substrate spectrum. In Figures 1 and 2 the sharp peak (indicated by Gr) at electron energy E k = 2.9 eV is due to CJ conduction bands of graphite.I7 In the U P S of the ClAlPc films the graphite peak appears up to 10 MLE. This shows that photons penetrate through ClAlPc layers and interact with the substrate. In the
8 0
0
5 NO. OF LAYERS
10
Figure 3. Observed (black dots) and calculated (circles) changes in the relative intensity of the substrate peak in UPS with increasing film thickness. PIES, on the other hand, the graphite peak is reduced to about one-half at 0.5 MLE and almost zero at 1 MLE. Since just 1 MLE of ClAlPc shields the graphite substrate from metastable beams, we can conclude that molecules lie flat in the 1 MLE film, with their molecular planes oriented parallel to the graphite cleavage plane. The flat molecular orientation is also shown by the relative intensity of the PIES bands (see below). Figure 3 shows the observed (black dots) and calculated (circles) changes in the relative intensity of the substrate peak at Ek = 2.9 eV in UPS with increasing film thickness. The calculated change was estimated by assuming that the emitted electrons giving the substrate peak escape in the constant proportion Q when they pass through one overlayer. Using a least-squares fit, we obtained 0.65 as the value of Q, which is reasonable compared to the corresponding value 0.53 of pentacene layers for Ek = 15.7 eV,I8 considering the difference in the kinetic energy of electrons. The good agreement between the observed and calculated relative intensities in Figure 3 indicates that ClAlPc films grow layer-by-layer uniformly on graphite without forming islands and that the tilting angle of the molecular plane does not change much with increasing film thickness. In Figure 1 the UPS of the ClAlPc layers have five bands denoted by A-F. The corresponding bands in the PIES are also indicated in Figure 2, although band E, located on the shoulder of the strong band D, is not clear. Table 1 compares the ionization potentials IP of these bands for the UPS and PIES.
12860 J. Phys. Chem., Vol. 99, No. 34, 1995
TABLE 1: Values of the Ionization Potential IP of the Bands in Ups and PIES UPS PIES band IP(UPS), eVa IP(PIES), eVb AIP, eVc A 6.1 6.3 0.2 B 8.3 8.5 0.2 9.8 10.0 0.2 C D 10.9 11.2 0.3 1 1.45 E F 13.5 13.7 0.2 a IP(uPs) = hv - E k = 21.2 - Ek (ev). The Ek value for each band is corrected by taking into account the small shift of the lowenergy cutoff of the spectrum. IP(P1ES) = E, - & = 19.8 - Ek (ev); E,, excitation energy of the metastable atom. The Ek value for each band is corrected by taking into account the small shift of the low-energy cutoff of the spectrum. AIP = IP(P1ES) - IP(UPS).
Pasinszki et a1.
71 (ring)
Figure 4. Schematic diagram of a ClAlPc molecule deposited flat on a graphite substrate interacting with a metastable atom. The shaded parts of the orbitals mainly interact with the helium metastable atom.
The IP values were obtained from the spectra of 3-10 layers, where the effect of the substrate emission is rather weak in UPS. In Table 1 we can see that the IP for the UPS are by 0.2-0.3 eV lower than those for the PIES. These IP differences between the PIES and UPS, AIP, are ascribed to the difference in the ...... . . ... . .. ... ... . . ..... ..... . .. . ... .. ....... ... .. . .............. target molecule in the two excitations. In PIES the stabilization 7 ~ , ~ ~ ~ ~ ~---------energy of the final state of ionization, which is due to the -//7/777777polarization of the molecules surrounding the molecular ion left Figure 5. Schematic diagram of a ClAlPc molecule tilted at the in the solid upon ionization, is less than that in UPS, because outermost surface layer interacting with a metastable atom. The shaded electrons are emitted from top-layer molecules in PIES in parts of the orbitals mainly interact with the helium metastable atom. contrast to the case of UPS,where electrons are emitted from molecules in several surface layers. The value of 0.2-0.3 eV gradually tilted in subsequent layers with increasing film for the difference in the stabilization energy (difference in the thickness. When a He* atom approaches a ClAlPc molecule ionization potential) between PIES and U P S is reasonable, deposited flat to the substrate, as shown in Figure 4, it effectively because the stabilization energy of the bulk molecule obtained interacts with the Cl(qI), Cl(nl), and n(benzene) orbitals exposed from the IP difference between the UPS of the gas and solid is outside, giving stronger bands in the spectrum, while the n1-1.5 eV for most molecular crystals.19 Further, the IP (ring) and u orbitals shielded by the C1 and n(benzene) orbitals difference between the top layer and the bulk has been found show weaker bands. This is the reason that bands B, C, and D to be 0.3 eV for anthracene20and 0.3-0.35 eV for pentacene.18 are intense and bands A and F are scarcely seen in the PIES of and t h e ~ r e t i c a l ~ ~ - ~the ~ 1 MLE film. The weakness of band E partly related to the By reference to previous n(benzene, la2,) orbitals may be due to their electron distribustudies of various metal and metal-free phthalocyanines, bands tions shifted toward the inner ring owing to the mixing with A, B, E, and F in UPS have been assigned as follows. Band A the ring n orbitals. The flat molecular orientation at one is related to a n(ring) orbital, which is mainly distributed along monolayer is due to the interaction between n orbitals of ClAlPc the skeleton of the inner porphine-like ring. The corresponding and those of the graphite substrate. Similar flat orientations band also appears in the U P S of tetra~henyl-~~ and octapheon graphite have been observed by PIES in planar n-conjugated nylporphyrins2* having porphine rings. Bands B and E are molecules such as benzene?' pentacene,I* and iron phthalocorrelated to the MO with a large contribution from the outer cyanine.' four benzene rings because these bands correspond well to the first (le@)) and second (3e2g(O) and la&)) bands in the In Figure 2 bands C and D gradually become weaker and U P S of solid benzene.29 Band F can be ascribed to u orbitals, band F stronger with increasing film thickness. This is ascribed being located in the high-IP region. Finally, bands C and D to the gradual tilt of phthalocyanine rings in the outermost layer. are not found in the U P S of ordinary phthalocyanines and can As shown in Figure 5, in the tilted molecular orientation the be assigned to the nonbonding orbitals of the chlorine atom, Cl(q1) and Cl(nl) orbitals are shielded by other orbitals from Cl(nl1) and Cl(nl), which are distributed parallel and perpenhelium metastables, whereas the u orbitals are exposed outside dicular to the molecular plane. This assignment of band C is the film surface, giving appreciable band intensity. The spectral reasonable, because its IP value (9.8 eV) corresponds well to change in Figure 2 also shows that the interaction between the the Cl(q1) band of gaseous monochlorobenzene (1 1.32 eV)?O n(benzene, 1el g) orbitals and metastables becomes stronger if we take into account the stabilization energy (1-1.5 eV) in owing to the tilt of the molecules and that the Cl(q1) orbital solid ClAlPc. The IP difference between bands D and C is distributed parallel to the ring plane is more effectively shielded due to the fact that the Cl(n1) orbital has bonding interaction than the Cl(nl) with increasing coverage. In the bulk crystal with an A1 3p orbital in contrast to the Cl(q), with pure ClAlPc molecules take a staggered arrangement, with their nonbonding nature. The above assignments for bands A-F are chlorine atoms directed obliquely up and down.32 One might, further supported by the discussion on PIES described below. therefore, consider that the second-layer molecules would be oriented with the chlorine atoms protruding down toward the In Figure 2 the relative band intensity of PIES is seen to plane of the first monolayer. This is not the case, as will be change remarkably as a function of film thickness, while that of U P S scarcely changes (cf. Figure 1). This spectral change described below. Table 2 lists the intensity ratio of bands B is accounted for if the phthalocyanine rings of ClAlPc molecules @(benzene)) and A @(ring)), IBIIA in PIES, which does not are arranged flat on the substrate, with the chlorine atoms show a large change with increasing film thickness. If the protruding outside the film at one monolayer, and they are second-layer molecules took a staggered orientation, the ratio
Ultrathin Films of Chloroaluminum Phthalocyanine
J. Phys. Chem., Vol. 99,No. 34, 1995 12861
He1 UPS
ll
I -170'C
Figure 8. Schematic diagram showing the change in the molecular orientation of the monolayer during temperature change.
1 8 ,
#
I
15
t
,
I
I
I I I . , , I
0
5
10
Kinetic Energy / eV
Figure 6. Temperature dependence of the He I UPS of a ClAlPc monolayer deposited on a graphite cleavage plane held at -170 OC. Spectrum e was measured after cooling the film held at 100 "C.
PIES
He*(23S)
molecular planes. In this case the sharp graphite band appears, and the Cl(s1) and Cl(nl) bands become weaker and the u band stronger compared to the case of the flat molecular arrangement shown in Figure 2 (curve c). Figure 6 shows that the spectrum at - 170 "C gradually changes upon heating the film, and finally at 100 OC it becomes almost the same as that of the film prepared at room temperature (compare curve d in Figure 6 and curve c in Figure 2). This means that under substrate-molecule interaction thermal energy allows molecules to cover the substrate with a flat molecular orientation. This molecular arrangement is stable because the spectrum at 100 "C is almost unchanged upon recooling the film to -90 "C (curves d and e). Conclusion
a
i I
I
I
I
I
15
I
I
1 1 1 1 1 , 1 , 1
5
10
0
Kinetic Energy / eV
Figure 7. Temperature dependence of the He(2)S) PIES of a ClAlPc monolayer deposited on a graphite cleavage plane held at -170 "C. Spectrum e was measured after cooling the film held at 100 "C.
TABLE 2: Intensity Ratio of Bands B and A in PIES no. of layers IdIA
1
2
3
5
10
27.5
23.7
21.5
21.4
21.6
would decrease abruptly, because the n(ring) orbitals were exposed outside the surface in this case. In fact, a thick ClAlPc film prepared on a copper substrate at 100 "C, which is considered to be crystalline, has a ratio of 14.7.33 Furthermore, the ratio of a ClAlPc film prepared on MoS2 is 9.7 at 2 monolayers, showing that the second-layer molecules are arranged more flatly, protruding their chlorine atoms downward.33 Figures 6 and 7 show the temperature dependence of the U P S and PIES, respectively, of a 1 MLE film deposited onto the substrate held at - 170 "C. As before, the U P S shows the effect of the substrate emission and does not change much during the temperature change. On the other hand, the relative band intensity of the PIES markedly changes, reflecting the variation in the molecular orientation of the outermost layer. This spectral change is explained as follows (cf. Figure 8). At -170 "C ClAlPc molecules are "frozen" on the substrate with their random orientation just as deposited. As a result, they partly overlap one another, causing lower coverage and the tilt of the
Using Penning ionization electron spectroscopy, we have sensitively observed the change in the orientation of ClAlPc molecules on graphite. Molecules deposited onto the substrate at room temperature are oriented flat on the substrate, with the chlorine atom protruding outside at one monolayer.34 During layer-by-layer deposition they are gradually tilted in subsequent layers. The monolayer film deposited on the substrate at low temperature shows a random molecular arrangement. Upon heating the film, however, the molecules gradually diffuse to cover the substrate and form almost the same molecular orientation as that of the film prepared at room temperature. This is in contrast with the case of the monolayer of iron phthalocyanine (FePc), where the island structure is the mo. t stable on a graphite substrate at room temperature." The difference in the monolayer structure between ClAlPc and FePc is considered to be due to the difference in the molecular structure; ClAlPc molecules cannot form an island easily, because they have the chlorine atom protruding outside the molecular plane. Acknowledgment. The authors are grateful to Dr. H. Ozaki, Tokyo University of Agriculture and Technology, for helpful discussions. One of the authors (T.P.) thanks the Matsumae International Foundation for a scholarship in support of this research. References and Notes (1) Lever, A. B. P. CHEMTECH 1987,17,506. Woehrle, D. Konfakte Sherrington, D. C. Pure Appl. Chem. 1988, 60,
(Darmsradt) 1986, I , 24.
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(2) Andre, J. J.; Simon, J.; Even, R.; Bondjema, B.; Guill, G.; Maitrot, M. Synth. Mer. 1987, 18, 683. Guilland, G.; Madru, R.; A1 Sadoun, M.; Maitrot, M. J . Appl. Phys. 1989, 66, 4554. (3) Wright, J. D. Prog. Surf. Sci. 1989, 31, 1. Honeyboume, C. L.; Ewen, R. J.; Hill, C. A. S.; Collings, M. S. Sens. Actuators 1988, 1.5, 359. (4) Low-energy electron diffriction: Buchholz, J. C.; Somorjai, G. A. J . Chem. Phys. 1977, 66, 573. (5) Angle-resolved ultraviolet photoelectron spectroscopy: Permien, T.; Engelhardt, R.; Feldmann, C. A.; Koch, E. E. Chem. Phys. Lett. 1983, 98,527. Fahy, M. R.; Fujimoto, H.; Dann, A. J.; Hoshi, H.; Inokuchi, H.;
12862 J. Phys. Chem., Vol. 99, No. 34, 1995 Maruyama, Y.; Willis, M. R. Physica Scripta 1990, 41, 550. Ueno, N.; Suzuki, K.; Hasegawa, S.; Kamiya, K.; Seki, K.; Inokuchi, H. J . Chem. Phys. 1993, 99, 7169. (6) Scanning tunneling microscopy: Mizutani, W.; Sakakibara, Y.; Ono, M.; Tanishima, S.; Ohno, K.; Toshima, N. Jpn. J . Appl. Phys. 1989, 28, L1460. Rochet, F.; Dufour, G.; Roulet, H.; Motta, N.; Sgarlata, A.; Piancastelli, M. N.; De Crescenzi, M. Sug. Sci. 1994, 319, 10. (7) Reflection high-energy electron diffraction: Hara, M.; Sasabe, H.; Yamada, A.; Garito, A. F. Jpn. J. Appl. Phys. 1989, 28, L306. (8) Near-edge X-ray fine structure: Guay, D.; Tourillon, G.; Gastonguay, L.; Dodelet, J. P.; Nebesny, K. W.; Armstrong, N. R.; Garrett, R. J . Phys. Chem. 1991, 95, 251. (9) Low-energy electron transmission: Momose, M.; Kamiya, K.; Sugita, K.; Ueno, N. Jpn. J . Appl. Phys. 1994, 33, 319, 4754. (10) X-ray photoelectron spectroscopy: Rochet, F.; Dufour, G.; Roulet, H.; Motta, N.; Sgarlata, A,; Piancastelli, M. N.; De Crescenzi, M. Sui$ Sci. 1994, 319, 10, 251. (11) Harada, Y.; Ozaki, H.; Ohno, K.; Kajiwara, T. Sutf Sci. 1984,147, 356. (12) Ozaki, H.; Harada, Y. J. Chem. Phys. 1990, 92, 3184. (13) CermBk, V. J. Chem. Phys. 1966, 44, 3781. (14) Harada, Y. Sutf Sci. 1985, 158, 455. (15) Harada, Y.; Ozaki, H. Jpn. J . Appl. Phys. 1987, 26, 1201. (16) Ohno, K.; Harada, Y. Theoretical Models of Chemical Bonding; Maksic,Z. B., Ed.; Springer: Berlin, 1991; Part 3, p 199. (17) Willis, R. F.; Feuerbacher, B.; Fitton, B. Phys. Rev. E 1971, 4, 2441. (18) Harada, Y.; Ozaki, H.; Ohno, K. Phys. Rev. Lett. 1984.52, 2269. (19) Inokuchi, H.; Seki, K.; Sato, N. Physica Scripta T 1987, 1, 793. Seki, K. Mol. Cryst. Liq. Cryst. 1989, 171, 255.
Pasinszki et al. (20) Salaneck, W. R. Phys. Rev. Lett. 1978, 40, 60. (21) Htichst, H.; Goldmann, A.; Hufner, S.; Malter, H. Phys. Status Solidi B 1976, 76, 559. Battye, F. L.; Goldmann, A,; Kasper, L. Phys. Status Solidi E 1977, 80, 425. (22) Iwan, M.; Eberhardt, W.; Kalkoffen, G.; Koch, E. E.; Kunz, C. Chem. Phys. Lett. 1979, 62, 344. (23) Berkowitz, J. J. Chem. Phys. 1979, 70, 2819. (24) Almlof, J. Int. J. Quantum Chem. 1974, 8, 915. (25) Spangler, D.; Maggiora, G . M.; Shipman, L. L.; Christoffersen, R. E. J . Am. Chem. SOC.1977, 99, 7470, 7478. (26) Kashiwagi, H.; Obara, S. Int. J. Quantum Chem. 1981, 20, 843. (27) Khandelwal, S. C.; Roebber, J. L. Chem. Phys. Lett. 1975,34,355. (28) Kitagawa, S.; Morishima, I.; Yonezawa, T.; Sato, N. Inorg. Chem. 1979, 18, 1345. (29) For the assignment of the UPS bands in benzene, see: Jonsson, B. 0.;Lindholm, E. Ark. Fys. 1969, 39, 65. (30) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. Molecular Photoelectron Spectroscopy; Wiley: London, 1970; p 29 1, (31) Kubota, H.; Munakata, T.; Hirooka, T.; Kondow, T.; Kuchitsu, K.; Ohno, K.; Harada, Y. Chem. Phys. 1984, 87, 399. (32) Wynne, K. J. Inorg. Chem. 1984, 23, 4658. (33) Aoki, M.; et al. To be published. (34) Recently, we have observed the same monolayer structure of ClAlPc as described here together with the process of molecular diffusion on a graphite substrate at room temperature by an electron emission microscope using metastable atoms as the excitation source (cf.: Harada, Y.; Yamamoto, S.; Aoki, M.; Masuda, S.; Ichinokawa, T.; Kato, M.; Sakai, Y. Nature 1994, 372, 657). JP94326 12
