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The electronic states of ordered thin films of perylene on Ag (1 1 0)
ARTICLE IN PRESS
Physica B 352 (2004) 36–41
The electronic states of ordered thin films of perylene on Ag (1 1 0) Huang Hana, Mao Hongyinga, Chen Qiaob, Yan Xinzhenga, Qian Huiqina, Zhang Jianhuaa, Li Haiyanga, He Pimoa, Bao Shininga,* b
a Physics Department, Zhejiang University, Hangzhou 310027, China School of Chemistry and Ultrafast Photonics Collaboration, University of St. Andrews, KY16 9ST, UK
Received 4 December 2003; received in revised form 28 May 2004; accepted 13 June 2004
Abstract The growth of perylene on Ag (1 1 0) has been studied by ultraviolet photoemission spectroscopy measurements and low-energy electron diffraction. Four emission features of the organic material are located at 3.5, 4.8, 6.4 and 8.5 eV, respectively, below the Fermi level. An ordered structure of c(6 2) can be observed when the organic film is about a ( thickness). The angle-resolved ultraviolet photoemission spectroscopy measurements show that the monolayer (3 A molecular plane of perylene near the interface is parallel to the substrate. The desorption of the organic material occurs with warming the substrate; the perylene molecules are stable on the Ag (1 1 0) surface and no decomposition is observed below 140 C. r 2004 Elsevier B.V. All rights reserved. PACS: 68.35.p; 68.55.a; 73.61.ph Keywords: Organic semiconductor material; Ultraviolet photoemission spectroscopy; Structure and electronic structure
1. Introduction Organic semiconductor material thin films have received considerable attention over the past few years in view of their potential applications [1–4]. Perylene, which contains a special condensed nuclear structure and a p conjugated electron system, can be used as organic photoconducting *Corresponding author. Tel.: +86-571-87951594; fax: +86571-87951328. E-mail address: [email protected] (B. Shining).
material due to its excellent photoelectric properties and relatively high durability, being resistant to light and heat. Recently, studies on perylene have been involved in the research of organic photoconductors (OPCs), organic solar cells, etc. suggesting a favorable outlook for these applications. In most organic photoconducting devices there are interfaces [5] between organic–organic, organic–inorganic, inorganic–inorganic, etc. In some double-layered OPCs with the interface between the electron-donor layer and the
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.06.035
ARTICLE IN PRESS H. Han et al. / Physica B 352 (2004) 36–41
carrier-transporting layer, electron properties of the interfaces may significantly influence the charge transfer and separation. Therefore, the impact of interfaces on the photosensitivity of OPCs is the key to the operation of organic-based devices. The controlled growth of organic semiconductor materials on inorganic substrates provides an opportunity for producing hybrid organic–inorganic structures with novel electronic properties [6–11]. Since the behavior of organic material on metal surface is mainly dependent on the organic molecules, up to now most of the studies on the interface between organic material and metal were performed on disordered organic ad layers on substrates. The construction of ordered organic ad layers on different substrates plays an important role in both fundamental and applied research. Ordered organic ad layers are proved to be effective in stabilizing the organic molecules on substrates, so that more information of the interaction between the organic ad layer and substrate can be obtained precisely. Perylene, as shown in Fig. 1, is an axisymmetric molecule in which there are five benzene circles and a condensed ring in the center. The molecule shows the evolution from an isotropic disordered structure in the submonolayer regime to a highly (substrate-dependent) ordered monolayer on silver and gold surfaces [12–13]. What we would like to deal with in this paper is, whether, during the thin film growth on Ag (1 1 0), a highly ordered structural transition could be observed on the interface, and along which azimuth the molecule of
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perylene did adsorb on the substrate? How did the functional groups contribute to the interfacial electronic states, and how did the charge transfer between two layers? In this paper, we studied different stages of the perylene film growth on Ag (1 1 0) and then investigated its evolution with warming the substrate by using ultraviolet photoelectron spectroscopy (UPS). We also investigated the molecular orientation by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS).
2. Experimental The experiments were performed in an ultrahigh vacuum apparatus of VG ADES-400 angleresolved electron energy spectrometer with a residual pressure better than 3 108 Pa. The apparatus was described elsewhere [12]. In brief, it contains an argon-ion gun, a low-energy electron diffraction (LEED) optic, a UV source and a semispherical deflector analyzer (SDA), etc. The ARUPS measurements can be performed by rotating the SDA and the sample in the UHV chamber. A clean and ordered Ag (1 1 0) surface was obtained by executing enough cycles of argonion sputtering (1000 eV, 15 min) and annealing (up to 750 K). The organic thin films were deposited from a resistively heated tantalum boat, at an evaporation temperature below 450 K. The nominal thickness of each deposited layer was monitored by a quartz crystal oscillator. The UPS measurements were performed with an UV source of He I (21.2 eV) and a sample bias of 5.0 V was used to enable observing the low-energy secondary cutoff. The overall resolution is about 0.05 eV. The temperature of the sample was determined by an attached thermocouple.
3. Results and discussion
Fig. 1. Chemical structure of the perylene (C20H12) molecule.
UPS spectra recorded from the Ag (1 1 0) surface with different amounts of the organic material are shown in Fig. 2. The nominal thickness of the organic film is 0, 1, 3, 10, 20, 40, ( respectively. The spectra are collected in and 70 A, the normal direction of the substrate with an
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d
Intensity (arb. units)
c b a θ(Å) 70 40 20
θ(Å) 70 40 20 10 3 1 0
10 3 1 0 -2 0
2
4
6
8 10 12
16
17
18
Binding Energy (ev) Fig. 2. UPS spectra collected in the surface normal with an incidence angle of 30 , the thickness of the organic overlayer ( respectively. was 0, 1, 3, 10, 20, 40, and 70 A,
incidence angle of 30 off the surface normal. From the clean Ag (1 1 0) surface, the UPS spectrum (the bottom one) distinctly shows the characteristic valence structure of the substrate with a d band from 4 to 8 eV in binding energy [14]. With increasing thickness of the organic films, a progressive attenuation of emission features of Ag can be observed, together with an increasing intensity of the features of the perylene molecule. ( a peak When the nominal thickness is about 1 A, appears at 2.3 eV and shifts to higher binding energy with increasing coverage of perylene. Since the most typical emission features coming from the organic material are located between 4 and 9 eV, there is an overlap with Ag substrate d band structures. With further increasing the coverage of perylene, the intensity of the peaks coming from perylene, labelled a, b, c and d in Fig. 2, increases greatly, while the intensity of the peaks from the Ag substrate decreases sharply. When the thick( the valence features of Ag ness is more than 20 A, have almost disappeared, indicating that the metal surface is completely covered and that the present typical emission features come from organic material. The top spectrum, corresponding to the ( 70 A-thickness film, represents the typical emission
features of the multilayer of organic material. These features are located at 3.5, 4.8, 6.4 and 8.5 eV in binding energy, respectively. Based on the variation in the secondary cutoff with the thickness of the organic films, shown in the right part of Fig. 2, we can obtain the change of the work function. With the cutoff energy of 16.7, 16.8, 16.9, 17.0, 17.10, 17.2, and 17.2 eV for ( the thickness of 0, 1, 3, 10, 20, 40, and 70 A, respectively, the work function is 4.4, 4.3, 4.2, 4.1, 4.0, 3.9, and 3.9 eV, respectively. The decrease in work function can be attributed to the dipole formation at the beginning of the deposition, as caused by the polarization of the organic molecule. The saturated value of 3.9 eV represents the work function of the organic film, and the change in work function is typical for the interface between organic semiconductors and metals [15]. Gas phase perylene has photoemission peaks at 7.0, 8.5, 8.7, 8.9, 9.3, 10.4, 11.2, and 12.3 eV. The peaks at 7.0, 8.7, 9.3, 10.4 and 12.3 eV have the strongest intensities, due to the superposition of unresolved states [16]. The peak at 7.0 eV, corresponding to the highest occupied molecular orbit (HOMO), is well resolved, since there are no other states close to this energy. Based on semi-empirical calculations, for the free molecule, the photoemission from 7.0 eV has been assigned to a p state [17]. Since the work function of the organic film on Ag (1 1 0) is 3.9 eV, the features of perylene on Ag (1 1 0), at 3.5, 4.8, 6.4, and 8.5 eV, can be considered as coming from the same orbits of perylene in the gas phase, at 7.0, 8.7, and 9.3, 10.4, and 12.3 eV respectively. The peak at 6.4 eV is assigned to the circular orbit with p character, and the peaks at 4.8 and 8.5 eV are both assigned to the C–C orbits with s character photoemission [16,17]. The peak at 3.5 eV cannot be assigned to any functional group individually. It arises from a new molecular orbit with s character resulting from the interaction among all the benzene of perylene. So, the highest occupied molecular orbit (HOMO) of perylene on Ag is located at 3.5 eV in binding energy. These electronic structures determine the semiconductivity of the organic material. Interfacial interactions occur between ad layers and the substrate. The evidence for such an interaction is the increase in binding energy of
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the peaks with increasing coverage with organic thin film, since there is less interaction between the organic multilayer and the substrate. The feature of the monolayer at 2.3 eV has the largest binding energy shift in comparison with that of the multilayer (3.5 eV). This may indicate a stronger interaction between this molecular orbit and the substrate. Ag atoms in the substrate donate electrons to the orbit associated to the peak. The LEED images, near the (0, 0) spot, are taken starting from the surface with very low coverage of perylene. The surface with low coverage of perylene shows a diffuse halo-like structure near the (0, 0) spot, seen in Fig. 3a. With increasing coverage, the halo changes into a ringlike diffuse pattern, seen in Fig. 3b. When the ( thickness, thickness is about one monolayer (3 A corresponding to a saturation monolayer of perylene), the ring decays into some single spots
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denoting an ordered structure as seen in Fig. 3c. The LEED pattern in Fig. 3d, taken with the same coverage as that in Fig. 3c, shows both the fractional spots and the integral spots by moving the (0, 0) spot close to the center of the screen. The superstructure, shown in Fig. 3d, can be described as c(6 2). The LEED pattern of the ordered structure is sketched in Fig. 4a. Since only a ring-like diffuse pattern can be observed when the coverage is less than one monolayer, there is no long-range ordered structure, only a short-range ordered structure, on the surface with low coverage of perylene. Based on
(0,0)
(a)
(a)
(b)
(c)
(d)
(0,0)
(0,1)
(0,0)
Fig. 3. (a)–(c) the LEED images near the (0, 0) spot with the ( (close to a coverage of perylene on Ag (1 1 0) from 1 to 4 A monolayer). The beam energy is 29 eV; (d) a LEED image taken same as c, shows the whole screen with the (0, 0) spot at the center of the screen.
(b) Fig. 4. (a) A sketch map of the LEED pattern in Fig. 4d, (b) possible configuration of the perylene molecules on Ag (1 1 0).
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emission angle of 300 incidence angle= 600 Intensity (arb.units)
the fact that the radius of the ring is close to the value of the separation between two spots from the ordered surface, the local adsorption structure of perylene on Ag (1 1 0) at the lower coverage should be the same as that at the coverage of one monolayer. A possible configuration of the perylene molecules adsorbing on Ag (1 1 0) is shown in Fig. 4b. The ARUPS spectra from the organic thin film of one monolayer thickness recorded in ½1 1% 0 azimuth with fixed emission angle of 30 and fixed incidence angle at 30 are shown in Figs. 5a and b, respectively. The characteristic valence structure of the substrate d band cannot be negligible at the coverage of one monolayer. The emission features in Fig. 2, except peak a, are significantly influenced by the Ag substrate d band structure. Therefore the peak a is the only one which we analyze with ARUPS. A change of the intensity of peak a can be observed while varying the incidence angle or the emission angle. The intensity of peak a increases with increasing incidence angle. The maximum peak intensity is obtained at the incidence angle of 50 . Reducing the intensity with a larger incidence angle is due to the decrease of intensity of the light in a unit area. The peak a with p character, the highest occupied molecular orbit (HOMO) of perylene, arises from the interaction among all the benzene of perylene. Its polarization is perpendicular to the perylene molecular plane. The ARUPS results suggest that the organic molecule plane is parallel to the Ag substrate. The intensity of the peak a also increases with increasing emission angle. The results of ARUPS show that the photoelectron emission from the highest occupied molecular orbit (HOMO) of perylene is larger in off-normal direction. The results taken in [0 0 1] azimuth are similar to that in ½1 1% 0 azimuth. The ARUPS spectra suggest that the organic molecule plane is parallel to the Ag substrate. Without enough specific evidence, however, we cannot confirm the molecular orientation as shown in Fig. 4b. The UPS spectra evolution as a function of temperature has been plotted in Fig. 6. The spectra ( from the 70 A-thickness organic film were collected in the surface normal direction with an incidence
500 400 300 200 100 00
(a)
-2
-1
0
1
2
3
4
incidence angle 300 emission angle= 600 Intensity (arb.units)
40
500 400 300 200 100 00 -2
(b)
-1
0
1
2
3
4
Binding Energy (ev)
Fig. 5. ARUPS spectra from an organic overlayer with the thickness of 1.0 monolayer: (a) recorded with a fixed emission angle at 30 ; (b) recorded with a fixed incidence angle at 30 .
angle of 30 off the surface normal. The spectra are recorded with warming the substrate to 30 C, 60 C, 80 C, 90 C, 100 C, 110 C, 120 C, 140 C, and 170 C, respectively. Four emission features of the organic material, located between 3 and 9 eV,
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Intensity (arb. units)
a
b
c
41
are stable on the Ag(1 1 0) surface, no decomposition is observed below 140 C.
d
300c 600c
Acknowledgements
800c 900c 1000c
0
30 c 0 600c 800c 90 c 10000c 1100c 1200c 1400c 170 c
1100c 0
120 c 1400c 1700c -2
0
2
6 8 10 4 Binding Energy(ev)
17.0 17.518.0 18.5
Fig. 6. The UPS spectra evolution as a function of the temperature; the spectra were collected in the surface normal with an incident angle of 30 .
decrease their intensity with warming the substrate due to the desorption of perylene. When the temperature is higher than 140 C, all of the emission features from perylene disappear completely. Some tiny features still can be observed, however, resulting from hydrocarbon, which comes from the decomposition of perylene. The change of the secondary cutoff, seen in the right part of Fig. 6, shows the increase of the work function while desorption and decomposition of perylene occur with warming the substrate [18]. The organic material shows its great heat durability on Ag (1 1 0), and the decomposition does not occur below 140 C.
4. Conclusions According to the UPS measurements, four emission features of the organic material are located at 3.5, 4.8, 6.4, and 8.5 eV, respectively, below the Fermi level. A c(6 2)-ordered structure ( thickcan be observed with one monolayer (3 A ness) of perylene adsorbed on the Ag (1 1 0) surface. The ARUPS measurements show that the molecules of perylene near the interface are parallel to the substrate. The perylene molecules
This work was supported by the National Science Foundation of China (No. 10374079) and the National Science Foundation of China (No. 20240430654).
References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [2] V.-E. Choong, M.G. Mason, C.W. Tang, Y. Gao, Appl. Phys. Lett. 72 (1998) 2689. [3] P. He, F.C.K. Au, Y.M. Wang, L.F. Cheng, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 76 (2000) 1422. [4] P. Lu, H. Hong, G. Cai, P. Djurovich, W.P. Weber, M.E. Thompson, J. Am. Chem. Soc. 122 (2000) 7480. [5] P.J. Unwin, D. Onoufriou, J.J. Cox, C.P.A. Mulcahy, T.S. Jones, Surf. Sci. 482–485 (2001) 1210. [6] S. Park, T.U. Kampen, T. Kachel, P. Bressler, W. Braun, D.R.T. Zahn, Appl. Surf. Sci. 190 (2002) 376. [7] K. Glockler, C. Seidel, A. Soukopp, M. Sokolowski, E. Umbach, M. Bohringer, R. Berndt, W.-D. Schneider, Surf. Sci. 405 (1998) 1. [8] C. Kendrick, A. Kahn, J. Crystal Growth 181 (1997) 181. [9] P.L. Unwin, C.P.A. Mulcahy, T.S. Jones, Surf. Sci. 482–485 (2001) 1222. [10] I. Chizhov, A. Kahn, G. Scole, J. Crystal Growth 208 (2000) 449. [11] Y. Hirose, C.I. Wu, V. Aristov, P. Soukiassian, A. Kahn, Appl. Surf. Sci. 113/114 (1997) 291. [12] P. He, H.J. Zhang, B. Li, Y.Y. Zhuang, Y. Wu, S.N. Bao, H.Y. Li, X.S. Zhang, Y.B. Xu, Chin. Phys. Lett. 18 (2001) 97. [13] C. Seidel, R. Ellerbrake, L. Gross, H. Fuchs, Phys. Rev. B 64. 195418. [14] B. Lu, H.J. Zhang, H.Y. Li, S.N. Bao, P. He, Phys. Rev. B 68. 125410. [15] P. He, S.D. Wong, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 79 (2001) 1561. [16] R. Boschi, J.N. Murrell, W. Schmidt, Faraday Discuss. Chem. Soc. 54 (1972) 116; E. Clar, W. Schmidt, Tetrahedron 35 (1979) 1979. [17] R. Boschi, J.N. Murrell, W. Schmidt, Faraday Discuss. Chem. Soc. 54 (1972) 116. [18] Yang Fan, Yang Xinguo, Wu Yue, et al., Phys. B 334 (2003) 244.
Physica B 352 (2004) 36–41
The electronic states of ordered thin films of perylene on Ag (1 1 0) Huang Hana, Mao Hongyinga, Chen Qiaob, Yan Xinzhenga, Qian Huiqina, Zhang Jianhuaa, Li Haiyanga, He Pimoa, Bao Shininga,* b
a Physics Department, Zhejiang University, Hangzhou 310027, China School of Chemistry and Ultrafast Photonics Collaboration, University of St. Andrews, KY16 9ST, UK
Received 4 December 2003; received in revised form 28 May 2004; accepted 13 June 2004
Abstract The growth of perylene on Ag (1 1 0) has been studied by ultraviolet photoemission spectroscopy measurements and low-energy electron diffraction. Four emission features of the organic material are located at 3.5, 4.8, 6.4 and 8.5 eV, respectively, below the Fermi level. An ordered structure of c(6 2) can be observed when the organic film is about a ( thickness). The angle-resolved ultraviolet photoemission spectroscopy measurements show that the monolayer (3 A molecular plane of perylene near the interface is parallel to the substrate. The desorption of the organic material occurs with warming the substrate; the perylene molecules are stable on the Ag (1 1 0) surface and no decomposition is observed below 140 C. r 2004 Elsevier B.V. All rights reserved. PACS: 68.35.p; 68.55.a; 73.61.ph Keywords: Organic semiconductor material; Ultraviolet photoemission spectroscopy; Structure and electronic structure
1. Introduction Organic semiconductor material thin films have received considerable attention over the past few years in view of their potential applications [1–4]. Perylene, which contains a special condensed nuclear structure and a p conjugated electron system, can be used as organic photoconducting *Corresponding author. Tel.: +86-571-87951594; fax: +86571-87951328. E-mail address: [email protected] (B. Shining).
material due to its excellent photoelectric properties and relatively high durability, being resistant to light and heat. Recently, studies on perylene have been involved in the research of organic photoconductors (OPCs), organic solar cells, etc. suggesting a favorable outlook for these applications. In most organic photoconducting devices there are interfaces [5] between organic–organic, organic–inorganic, inorganic–inorganic, etc. In some double-layered OPCs with the interface between the electron-donor layer and the
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.06.035
ARTICLE IN PRESS H. Han et al. / Physica B 352 (2004) 36–41
carrier-transporting layer, electron properties of the interfaces may significantly influence the charge transfer and separation. Therefore, the impact of interfaces on the photosensitivity of OPCs is the key to the operation of organic-based devices. The controlled growth of organic semiconductor materials on inorganic substrates provides an opportunity for producing hybrid organic–inorganic structures with novel electronic properties [6–11]. Since the behavior of organic material on metal surface is mainly dependent on the organic molecules, up to now most of the studies on the interface between organic material and metal were performed on disordered organic ad layers on substrates. The construction of ordered organic ad layers on different substrates plays an important role in both fundamental and applied research. Ordered organic ad layers are proved to be effective in stabilizing the organic molecules on substrates, so that more information of the interaction between the organic ad layer and substrate can be obtained precisely. Perylene, as shown in Fig. 1, is an axisymmetric molecule in which there are five benzene circles and a condensed ring in the center. The molecule shows the evolution from an isotropic disordered structure in the submonolayer regime to a highly (substrate-dependent) ordered monolayer on silver and gold surfaces [12–13]. What we would like to deal with in this paper is, whether, during the thin film growth on Ag (1 1 0), a highly ordered structural transition could be observed on the interface, and along which azimuth the molecule of
37
perylene did adsorb on the substrate? How did the functional groups contribute to the interfacial electronic states, and how did the charge transfer between two layers? In this paper, we studied different stages of the perylene film growth on Ag (1 1 0) and then investigated its evolution with warming the substrate by using ultraviolet photoelectron spectroscopy (UPS). We also investigated the molecular orientation by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS).
2. Experimental The experiments were performed in an ultrahigh vacuum apparatus of VG ADES-400 angleresolved electron energy spectrometer with a residual pressure better than 3 108 Pa. The apparatus was described elsewhere [12]. In brief, it contains an argon-ion gun, a low-energy electron diffraction (LEED) optic, a UV source and a semispherical deflector analyzer (SDA), etc. The ARUPS measurements can be performed by rotating the SDA and the sample in the UHV chamber. A clean and ordered Ag (1 1 0) surface was obtained by executing enough cycles of argonion sputtering (1000 eV, 15 min) and annealing (up to 750 K). The organic thin films were deposited from a resistively heated tantalum boat, at an evaporation temperature below 450 K. The nominal thickness of each deposited layer was monitored by a quartz crystal oscillator. The UPS measurements were performed with an UV source of He I (21.2 eV) and a sample bias of 5.0 V was used to enable observing the low-energy secondary cutoff. The overall resolution is about 0.05 eV. The temperature of the sample was determined by an attached thermocouple.
3. Results and discussion
Fig. 1. Chemical structure of the perylene (C20H12) molecule.
UPS spectra recorded from the Ag (1 1 0) surface with different amounts of the organic material are shown in Fig. 2. The nominal thickness of the organic film is 0, 1, 3, 10, 20, 40, ( respectively. The spectra are collected in and 70 A, the normal direction of the substrate with an
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d
Intensity (arb. units)
c b a θ(Å) 70 40 20
θ(Å) 70 40 20 10 3 1 0
10 3 1 0 -2 0
2
4
6
8 10 12
16
17
18
Binding Energy (ev) Fig. 2. UPS spectra collected in the surface normal with an incidence angle of 30 , the thickness of the organic overlayer ( respectively. was 0, 1, 3, 10, 20, 40, and 70 A,
incidence angle of 30 off the surface normal. From the clean Ag (1 1 0) surface, the UPS spectrum (the bottom one) distinctly shows the characteristic valence structure of the substrate with a d band from 4 to 8 eV in binding energy [14]. With increasing thickness of the organic films, a progressive attenuation of emission features of Ag can be observed, together with an increasing intensity of the features of the perylene molecule. ( a peak When the nominal thickness is about 1 A, appears at 2.3 eV and shifts to higher binding energy with increasing coverage of perylene. Since the most typical emission features coming from the organic material are located between 4 and 9 eV, there is an overlap with Ag substrate d band structures. With further increasing the coverage of perylene, the intensity of the peaks coming from perylene, labelled a, b, c and d in Fig. 2, increases greatly, while the intensity of the peaks from the Ag substrate decreases sharply. When the thick( the valence features of Ag ness is more than 20 A, have almost disappeared, indicating that the metal surface is completely covered and that the present typical emission features come from organic material. The top spectrum, corresponding to the ( 70 A-thickness film, represents the typical emission
features of the multilayer of organic material. These features are located at 3.5, 4.8, 6.4 and 8.5 eV in binding energy, respectively. Based on the variation in the secondary cutoff with the thickness of the organic films, shown in the right part of Fig. 2, we can obtain the change of the work function. With the cutoff energy of 16.7, 16.8, 16.9, 17.0, 17.10, 17.2, and 17.2 eV for ( the thickness of 0, 1, 3, 10, 20, 40, and 70 A, respectively, the work function is 4.4, 4.3, 4.2, 4.1, 4.0, 3.9, and 3.9 eV, respectively. The decrease in work function can be attributed to the dipole formation at the beginning of the deposition, as caused by the polarization of the organic molecule. The saturated value of 3.9 eV represents the work function of the organic film, and the change in work function is typical for the interface between organic semiconductors and metals [15]. Gas phase perylene has photoemission peaks at 7.0, 8.5, 8.7, 8.9, 9.3, 10.4, 11.2, and 12.3 eV. The peaks at 7.0, 8.7, 9.3, 10.4 and 12.3 eV have the strongest intensities, due to the superposition of unresolved states [16]. The peak at 7.0 eV, corresponding to the highest occupied molecular orbit (HOMO), is well resolved, since there are no other states close to this energy. Based on semi-empirical calculations, for the free molecule, the photoemission from 7.0 eV has been assigned to a p state [17]. Since the work function of the organic film on Ag (1 1 0) is 3.9 eV, the features of perylene on Ag (1 1 0), at 3.5, 4.8, 6.4, and 8.5 eV, can be considered as coming from the same orbits of perylene in the gas phase, at 7.0, 8.7, and 9.3, 10.4, and 12.3 eV respectively. The peak at 6.4 eV is assigned to the circular orbit with p character, and the peaks at 4.8 and 8.5 eV are both assigned to the C–C orbits with s character photoemission [16,17]. The peak at 3.5 eV cannot be assigned to any functional group individually. It arises from a new molecular orbit with s character resulting from the interaction among all the benzene of perylene. So, the highest occupied molecular orbit (HOMO) of perylene on Ag is located at 3.5 eV in binding energy. These electronic structures determine the semiconductivity of the organic material. Interfacial interactions occur between ad layers and the substrate. The evidence for such an interaction is the increase in binding energy of
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the peaks with increasing coverage with organic thin film, since there is less interaction between the organic multilayer and the substrate. The feature of the monolayer at 2.3 eV has the largest binding energy shift in comparison with that of the multilayer (3.5 eV). This may indicate a stronger interaction between this molecular orbit and the substrate. Ag atoms in the substrate donate electrons to the orbit associated to the peak. The LEED images, near the (0, 0) spot, are taken starting from the surface with very low coverage of perylene. The surface with low coverage of perylene shows a diffuse halo-like structure near the (0, 0) spot, seen in Fig. 3a. With increasing coverage, the halo changes into a ringlike diffuse pattern, seen in Fig. 3b. When the ( thickness, thickness is about one monolayer (3 A corresponding to a saturation monolayer of perylene), the ring decays into some single spots
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denoting an ordered structure as seen in Fig. 3c. The LEED pattern in Fig. 3d, taken with the same coverage as that in Fig. 3c, shows both the fractional spots and the integral spots by moving the (0, 0) spot close to the center of the screen. The superstructure, shown in Fig. 3d, can be described as c(6 2). The LEED pattern of the ordered structure is sketched in Fig. 4a. Since only a ring-like diffuse pattern can be observed when the coverage is less than one monolayer, there is no long-range ordered structure, only a short-range ordered structure, on the surface with low coverage of perylene. Based on
(0,0)
(a)
(a)
(b)
(c)
(d)
(0,0)
(0,1)
(0,0)
Fig. 3. (a)–(c) the LEED images near the (0, 0) spot with the ( (close to a coverage of perylene on Ag (1 1 0) from 1 to 4 A monolayer). The beam energy is 29 eV; (d) a LEED image taken same as c, shows the whole screen with the (0, 0) spot at the center of the screen.
(b) Fig. 4. (a) A sketch map of the LEED pattern in Fig. 4d, (b) possible configuration of the perylene molecules on Ag (1 1 0).
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emission angle of 300 incidence angle= 600 Intensity (arb.units)
the fact that the radius of the ring is close to the value of the separation between two spots from the ordered surface, the local adsorption structure of perylene on Ag (1 1 0) at the lower coverage should be the same as that at the coverage of one monolayer. A possible configuration of the perylene molecules adsorbing on Ag (1 1 0) is shown in Fig. 4b. The ARUPS spectra from the organic thin film of one monolayer thickness recorded in ½1 1% 0 azimuth with fixed emission angle of 30 and fixed incidence angle at 30 are shown in Figs. 5a and b, respectively. The characteristic valence structure of the substrate d band cannot be negligible at the coverage of one monolayer. The emission features in Fig. 2, except peak a, are significantly influenced by the Ag substrate d band structure. Therefore the peak a is the only one which we analyze with ARUPS. A change of the intensity of peak a can be observed while varying the incidence angle or the emission angle. The intensity of peak a increases with increasing incidence angle. The maximum peak intensity is obtained at the incidence angle of 50 . Reducing the intensity with a larger incidence angle is due to the decrease of intensity of the light in a unit area. The peak a with p character, the highest occupied molecular orbit (HOMO) of perylene, arises from the interaction among all the benzene of perylene. Its polarization is perpendicular to the perylene molecular plane. The ARUPS results suggest that the organic molecule plane is parallel to the Ag substrate. The intensity of the peak a also increases with increasing emission angle. The results of ARUPS show that the photoelectron emission from the highest occupied molecular orbit (HOMO) of perylene is larger in off-normal direction. The results taken in [0 0 1] azimuth are similar to that in ½1 1% 0 azimuth. The ARUPS spectra suggest that the organic molecule plane is parallel to the Ag substrate. Without enough specific evidence, however, we cannot confirm the molecular orientation as shown in Fig. 4b. The UPS spectra evolution as a function of temperature has been plotted in Fig. 6. The spectra ( from the 70 A-thickness organic film were collected in the surface normal direction with an incidence
500 400 300 200 100 00
(a)
-2
-1
0
1
2
3
4
incidence angle 300 emission angle= 600 Intensity (arb.units)
40
500 400 300 200 100 00 -2
(b)
-1
0
1
2
3
4
Binding Energy (ev)
Fig. 5. ARUPS spectra from an organic overlayer with the thickness of 1.0 monolayer: (a) recorded with a fixed emission angle at 30 ; (b) recorded with a fixed incidence angle at 30 .
angle of 30 off the surface normal. The spectra are recorded with warming the substrate to 30 C, 60 C, 80 C, 90 C, 100 C, 110 C, 120 C, 140 C, and 170 C, respectively. Four emission features of the organic material, located between 3 and 9 eV,
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Intensity (arb. units)
a
b
c
41
are stable on the Ag(1 1 0) surface, no decomposition is observed below 140 C.
d
300c 600c
Acknowledgements
800c 900c 1000c
0
30 c 0 600c 800c 90 c 10000c 1100c 1200c 1400c 170 c
1100c 0
120 c 1400c 1700c -2
0
2
6 8 10 4 Binding Energy(ev)
17.0 17.518.0 18.5
Fig. 6. The UPS spectra evolution as a function of the temperature; the spectra were collected in the surface normal with an incident angle of 30 .
decrease their intensity with warming the substrate due to the desorption of perylene. When the temperature is higher than 140 C, all of the emission features from perylene disappear completely. Some tiny features still can be observed, however, resulting from hydrocarbon, which comes from the decomposition of perylene. The change of the secondary cutoff, seen in the right part of Fig. 6, shows the increase of the work function while desorption and decomposition of perylene occur with warming the substrate [18]. The organic material shows its great heat durability on Ag (1 1 0), and the decomposition does not occur below 140 C.
4. Conclusions According to the UPS measurements, four emission features of the organic material are located at 3.5, 4.8, 6.4, and 8.5 eV, respectively, below the Fermi level. A c(6 2)-ordered structure ( thickcan be observed with one monolayer (3 A ness) of perylene adsorbed on the Ag (1 1 0) surface. The ARUPS measurements show that the molecules of perylene near the interface are parallel to the substrate. The perylene molecules
This work was supported by the National Science Foundation of China (No. 10374079) and the National Science Foundation of China (No. 20240430654).
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