- Email: [email protected]
Ultraviolet photoelectron spectroscopy of oriented carboxylic acid films prepared by vacuum deposition
119. number
Volume
5
CHEMICAL
ULTRAVIOLET OF ORIENTED Munehisa Advanced
Naoki
PHOTOELECTRON CARBOXYLIC ACID
MITSUYA,
Yoshio
Research Labormary,
SAT0
‘, KazuIzlko
Received 30 January
PHYSICS
13 September
LEl-l-ERS
SPEC-I-ROSCOPY FILMS PREPARED
BY VACUUM
1985
DEPOSITION
TANIGUCHI
Hlrachr L.rtL. KokubunJi. Tokyo 285, Jrrpon
SEKI
and Hiroo
1985; in final form 16 May
INOKUCHI
1985
Ullrawolel pholoeleclron spectra were measured for films or ahphatic carboxybc acids made b> vacuum depasluon. Their threshold lonizalion pownlials varied within 07 eV accordmg 10 the measure of molecular oricntarion. This varinlion is explained by dimer rormanon in a highly crysrallme film.
1. Introduction Ultraviolet provide tronic
valuable structure
2. Experimental photoelectron
spectroscopy
information of solids.
on the valence Most
studies
for
(UPS)
can
Eight aliphatic monocarboxyhc acids with a normal-chain structure (CH3(CHZ)n_2COOH, n = 1 S-24 and 30), which are commercially available, were used without further purification. They were vacuum deposited on copper, indium oxide and gold substrates_ The copper substrate was polished and rinsed with acetone_ The glass plate covered with a layer of indiurn oxide was obtained commercially and rinsed with acetone. Gold layers with 50 nm thickness were vatuum deposited on glass plates to form the gold substrate_ The apparatus and conditions for vacuum deposition were the same as those reported on zinc stearate fiLns [3] _ The fraction sublimed in early or late stages were discarded, and only the middle fraction was used. Films of three compounds (n = 18,22 and 30) among the eight were examined by X-ray diffraction employing Cu KLYradiation_ The film thickness was 100-300 nm for X-ray diffracbon, and 2030 nm for photoemission measurements_ Details of the photoemission equipment have been described [IS] _ The light source was a hydrogen discharge lamp with a 0.5 m Seya-Namioka-type vacuum-ultraviolet monochromator. Photoelectron spectra were obtained by differentiating the photocurrent
elecorganic
compounds have employed polycrystalline thin films prepared by vacuum deposition_ In many cases such evaporated ftirns are more or less uniaxially oriented [l-3] _ This Letter reports the threshold ionization potential of evaporated carboxyhc acid films measured by UPS, together with the molecular orientation in the film determined by X-ray diffraction. These compounds are important in the formation of LangrnuirBlodgett films [4] relating to molecular devices. It was found that a highly crystalline fiLm shows a threshold ionization potential 0.7 eV smaller than that for an amorphous film. Tlus can be explained by the formation of a carboxylic acid dimer in the oriented film
’ Present address: Deparlmem ence, Kumamo~o Universily. Japan
or Chemisq. Kurokami,
Faculty of SciKumarno~o 860.
0 009-26 14/85/f 03.30 0 Elsetier Science Publishers (North-Holland Physics Publishing Division)
B.V.
431
as a function ofthe retarding potential applied between the sample and the surrounding spherical collector. The energy resolution obtamed was better than 0.2 eV_ Photoelectron spectra were measured using several incident photon energies, rangmg from 8 to 10 eV, and the threshold ionization potential was determmed as an average of the values obtained from these spectra_
3.
13 September 1985
CHEMZCAL PHYSICS LETTERS
Volume 119, number 5
Results and discussion
Fig_ 1 shows the X-ray diffraction patterns for stearic acid (n = 1 S), behemc acid (rz = 22) and melissic acid (n = 30). Stearic acid Nms on three different substrates show simxlar diffraction patterns, whiIe their mtensiiies differ. The peaks can be characterized in terms of (OOZ) diffractions. Each peak has a cammon fwhm (full width at half-maximum) value of 0.21 f 0.01 O. These results indicate that the de.
posited fiLm has both oriented and amorphous re-
gions and that the ratio of the two regions depends on the substrate- Randomly oriented polycrystalline regions do not exist in the film, since no diffmction other than (001) was observed. The periodic distance determined from peak positions of these patterns was 3.99 5~0.01 nm, which is close to the thickness of a double layer for a stearic acid crystal [6J. Thus, the oriented region should consist of alternate monolayers hating head groups oriented both towards and away from the substrate. The behenic acid ftim is poorly oriented in comparison with stearic acid, since fig. Id shows a peak WI+&weak intensity and a large fwhm. The melissic acid film is amorphous, judging from the still lower diffraction intensity shown in fig. le. The lowering of orientaaon with the cham length can be interpreted as follows. A carboxylic acid has both hydrophilic and hydrophobic groups in the moie&e. For an aad with a longer alkyl chain, hydrophobic interactions dominate the intermolecular forces. This IS less kkely to lead to a preferred orientation in the deposited film. In fact, compounds around n = 20 give the highest orientation as studied by the Langmuir-Blodgett method [4] _ Photoelectron spectra of steanc acid frEmsare shown in fig. 2. Arrows indicate maximum kmetic
3
10
I
20 ZB/degree
30
1
Fig. 1. X-ray diffraction patterns of monocarboxylic acid ~UJTIS (thicla~- ti parentheses).(a) Stearicacid on iridium otidc (113 nm). @) copper (100 nm), and (c) gold (330 nrn). (d) Behenicacid on copper (104 nm). (e) Mel&m acid on copper (123 nm?.
432
0 Ktnelic
1
2
3
Energy/cV
Fig. 2. Photoelectronspecha of steaxicacid fii on the kinetic energyscale. (a) Deposited on indium oxule, @I cop per, and (c) gold Incidentphoton energyis 9.76 eV. Amows indicatemaximumkinetic ene*es for photoelecfzons.
Volume 119, number 5
CHEMICAL
PHYSICS LETTERS
13 September 1985
energtes for photoelectrons,E~ax. The threshold ionization potential, Is, was calculated using the relatiOll
IS=hu-EFa
>
where hv is the photon energy. The I, values are 7 -8, 8.1 and 8.4 eV for stearic acrd films on indium oxide, copper and gold, respectively. A deposited Wm with higher orientation has a smaller 1, value. On the other hand, the ratios of the total photocurrent are 1.0, 0.91 and 0.81 for spectra (a), (b) and (c), respectively; a highly oriented film possesses a large photocurrent. The tieshold ionization potentials for carboxylic acrds deposited on copper substrates depend on the carbon number, as shown in fig. 3. The I, value increases with increase m the carbon number, reaclung a constant value of about 8.5 eV, which meets the value of polyethylene [7]. X-ray photoelectron spectroscopy and MO calculations applied to a series of normal akanes (nC,,HZ,,+2) have shown that the electronic structure of tridecane (n-CI,H2S) is approximately that of an infinite linear polymer [8, 9] _ On the other hand, Chattopadhyay et al. pointed out that all the normal dicarboxylic acids with II > 7 would have ul(HOOC(CH2),_,COOH) traviolet photoelectron spectra similar to that of adrpic acid (n = 6) [IO] _Therefore, the dependence of I, on the carbon number should be explained by an effect of molecular aggregation rather than the electronic structure of an isolated molecule_ Higher orientation, caused by appropriate substrate material and chain length, leads to a decrease in Is, as described above. Moreover, the value ofl,
Fig. 4. Relation between the normalized X-ray diffraction intensity and the threshold ionization potential. (a) Stearic acid on indium oxide, (b) copper, and (c) gold (d) Behenlc acid on copper. (e) Melissic acid on copper.
is correlated with molecular orientation using a common parameter as shown in fig. 4. Here, the abscissa indicates a summation of the diffraction intensity normalized to a unit volume of the specimen film. The difference between the maximum and minimum I, was observed to be 0.7 eV. The lowering of the threshold ioruzation potential with increase in the orientation can be ascribed to the formation of a carboxyhc acid dirner in the oriented film. From gas-phase measurements, the adiabatic ionization potential of formic acid and acetic acid 1s known to decrease by about 0.3 eV due to the orbital splitting on dirnerization [11,12] _Similar behavior is expected in the present case. Another contribution to the lowering ofl, should come from a larger polarization energy through the crystallization accompanying dimer formation. The polarization energy P+, which IS the decrease in the threshoId ionrzation potential from a molecule to a solid due to the electric polarization of surroundmg molecules, can be approxrmated as p+
Ccwbon
Number
of Add
Fig. 3. C&on number dependence of the threshold ionization 1potentinl of J fti deposited on a copper substrate.
= Ke 2 ,p413 _
Here, Cuis the average molecular polanzability, p is the number densrty of molecules rn the solrd, e is the charge of the proton and K is a constant [ 13]_ In general, an oriented film is thought to have a higher density than an amorphous one. This leads to an increase of p and hence a large polarization energy in the oriented film, which is expected to decrease the 1, value. The higher density also explains the 433
Volume 119, number 5
CHEMICAL PHYSICS LETTERS
large photocurrent of the highly onented film mentioned above. The molecular aggregation at the film surface may differ from that in the bulk. Since X-rays have a long penetration depth, X-ray diffraction patterns reflect the bulk structure of the film. On the other hand, photoelectron spectra give information about the near-surface repon of the film, since the photoelectron escape depth is generally estimated to be of the order of several nm. The correspondence between the present measurements of photoemission and X-ray diffraction suggests that molecular orientation persists up to the film surface for carboxyhc acids. This agrees with the result on zinc stearate, w6ere reflection electron diffraction from thick and thin films showed the same pattern [3] _Such preservation of molecular onentation may be common to Iong-chain compounds_
Acknowledgement We are grateful to Drs. Shinjl Tomoda and Yohjl Achiba for valuable discussions. Thanks are also due to Mr. Tsutomu Isbiba for useful comments as well as the X-ray diffraction experiment. This work was carried out durmg the stay of MM at the Institute for Molecular Science (1984-85).
434
13 September 1985
References [1] H. Inokucti, H. Kuroda lnd H. Akamatu, BulL Chem. Sot. Japan 34 (1961) 749. [2] K. Seki, S. Hashimoto, N Sato, Y. Harada. K. Ishii. H Inokuchi and J. Kanbe, J. Chem. Phys. 66 (1977) 3644_ 131 M. Mitsuya, Y. Taniguchi and M. Akagi. J. CoUoid Interface Sci. 92 (1983) 291_ 141 K.B Blodgett, J. Am. Chem. SOL 57 (1935) 1007. [51 T. Hirooka, K. Tanaka, K. Kuchitsu, M. Fujihara, H. Inokuchi and Y. Harada. Chem. Phys. Letters 18 (1973) 390. [61 ASTM card No. 9618. [71 S. Hashimoto, K. Seki. N. Sato and H. Inokuchi, J. Chem. Phys. 76 (1982) 163_ [81 JJ. Pireaux. S. Svensson, E. Basilier. P.-A. Malmqvist, U_ Cclius. R. Caudano and K. Siegbahn, Phys. Rev. Al4 (1976) 2133. [91 K. Scki, N. Sato and H Inokuchi. to be published. r101 S Chattopadhyay, J-L. Meeks, G-L. Ftidley and S-P. McGlynn, J. Phys. Chem. 85 (1981) 968 [II; S. Tomoda. Y. Achiba, K. Nomoto, K. Sato and EC. Kimura, Chem. Phys. 74 (1983) 113. 1121 K. Nomoto. Thesis, Hokkaido University (1979). r131 N. Sate, H. Inokuchi, K. Seki, J. Aoki and S. Iwashima, J Chem. Sot. Faraday Trans. II 78 (1982) 1929.
Volume
5
CHEMICAL
ULTRAVIOLET OF ORIENTED Munehisa Advanced
Naoki
PHOTOELECTRON CARBOXYLIC ACID
MITSUYA,
Yoshio
Research Labormary,
SAT0
‘, KazuIzlko
Received 30 January
PHYSICS
13 September
LEl-l-ERS
SPEC-I-ROSCOPY FILMS PREPARED
BY VACUUM
1985
DEPOSITION
TANIGUCHI
Hlrachr L.rtL. KokubunJi. Tokyo 285, Jrrpon
SEKI
and Hiroo
1985; in final form 16 May
INOKUCHI
1985
Ullrawolel pholoeleclron spectra were measured for films or ahphatic carboxybc acids made b> vacuum depasluon. Their threshold lonizalion pownlials varied within 07 eV accordmg 10 the measure of molecular oricntarion. This varinlion is explained by dimer rormanon in a highly crysrallme film.
1. Introduction Ultraviolet provide tronic
valuable structure
2. Experimental photoelectron
spectroscopy
information of solids.
on the valence Most
studies
for
(UPS)
can
Eight aliphatic monocarboxyhc acids with a normal-chain structure (CH3(CHZ)n_2COOH, n = 1 S-24 and 30), which are commercially available, were used without further purification. They were vacuum deposited on copper, indium oxide and gold substrates_ The copper substrate was polished and rinsed with acetone_ The glass plate covered with a layer of indiurn oxide was obtained commercially and rinsed with acetone. Gold layers with 50 nm thickness were vatuum deposited on glass plates to form the gold substrate_ The apparatus and conditions for vacuum deposition were the same as those reported on zinc stearate fiLns [3] _ The fraction sublimed in early or late stages were discarded, and only the middle fraction was used. Films of three compounds (n = 18,22 and 30) among the eight were examined by X-ray diffraction employing Cu KLYradiation_ The film thickness was 100-300 nm for X-ray diffracbon, and 2030 nm for photoemission measurements_ Details of the photoemission equipment have been described [IS] _ The light source was a hydrogen discharge lamp with a 0.5 m Seya-Namioka-type vacuum-ultraviolet monochromator. Photoelectron spectra were obtained by differentiating the photocurrent
elecorganic
compounds have employed polycrystalline thin films prepared by vacuum deposition_ In many cases such evaporated ftirns are more or less uniaxially oriented [l-3] _ This Letter reports the threshold ionization potential of evaporated carboxyhc acid films measured by UPS, together with the molecular orientation in the film determined by X-ray diffraction. These compounds are important in the formation of LangrnuirBlodgett films [4] relating to molecular devices. It was found that a highly crystalline fiLm shows a threshold ionization potential 0.7 eV smaller than that for an amorphous film. Tlus can be explained by the formation of a carboxylic acid dimer in the oriented film
’ Present address: Deparlmem ence, Kumamo~o Universily. Japan
or Chemisq. Kurokami,
Faculty of SciKumarno~o 860.
0 009-26 14/85/f 03.30 0 Elsetier Science Publishers (North-Holland Physics Publishing Division)
B.V.
431
as a function ofthe retarding potential applied between the sample and the surrounding spherical collector. The energy resolution obtamed was better than 0.2 eV_ Photoelectron spectra were measured using several incident photon energies, rangmg from 8 to 10 eV, and the threshold ionization potential was determmed as an average of the values obtained from these spectra_
3.
13 September 1985
CHEMZCAL PHYSICS LETTERS
Volume 119, number 5
Results and discussion
Fig_ 1 shows the X-ray diffraction patterns for stearic acid (n = 1 S), behemc acid (rz = 22) and melissic acid (n = 30). Stearic acid Nms on three different substrates show simxlar diffraction patterns, whiIe their mtensiiies differ. The peaks can be characterized in terms of (OOZ) diffractions. Each peak has a cammon fwhm (full width at half-maximum) value of 0.21 f 0.01 O. These results indicate that the de.
posited fiLm has both oriented and amorphous re-
gions and that the ratio of the two regions depends on the substrate- Randomly oriented polycrystalline regions do not exist in the film, since no diffmction other than (001) was observed. The periodic distance determined from peak positions of these patterns was 3.99 5~0.01 nm, which is close to the thickness of a double layer for a stearic acid crystal [6J. Thus, the oriented region should consist of alternate monolayers hating head groups oriented both towards and away from the substrate. The behenic acid ftim is poorly oriented in comparison with stearic acid, since fig. Id shows a peak WI+&weak intensity and a large fwhm. The melissic acid film is amorphous, judging from the still lower diffraction intensity shown in fig. le. The lowering of orientaaon with the cham length can be interpreted as follows. A carboxylic acid has both hydrophilic and hydrophobic groups in the moie&e. For an aad with a longer alkyl chain, hydrophobic interactions dominate the intermolecular forces. This IS less kkely to lead to a preferred orientation in the deposited film. In fact, compounds around n = 20 give the highest orientation as studied by the Langmuir-Blodgett method [4] _ Photoelectron spectra of steanc acid frEmsare shown in fig. 2. Arrows indicate maximum kmetic
3
10
I
20 ZB/degree
30
1
Fig. 1. X-ray diffraction patterns of monocarboxylic acid ~UJTIS (thicla~- ti parentheses).(a) Stearicacid on iridium otidc (113 nm). @) copper (100 nm), and (c) gold (330 nrn). (d) Behenicacid on copper (104 nm). (e) Mel&m acid on copper (123 nm?.
432
0 Ktnelic
1
2
3
Energy/cV
Fig. 2. Photoelectronspecha of steaxicacid fii on the kinetic energyscale. (a) Deposited on indium oxule, @I cop per, and (c) gold Incidentphoton energyis 9.76 eV. Amows indicatemaximumkinetic ene*es for photoelecfzons.
Volume 119, number 5
CHEMICAL
PHYSICS LETTERS
13 September 1985
energtes for photoelectrons,E~ax. The threshold ionization potential, Is, was calculated using the relatiOll
IS=hu-EFa
>
where hv is the photon energy. The I, values are 7 -8, 8.1 and 8.4 eV for stearic acrd films on indium oxide, copper and gold, respectively. A deposited Wm with higher orientation has a smaller 1, value. On the other hand, the ratios of the total photocurrent are 1.0, 0.91 and 0.81 for spectra (a), (b) and (c), respectively; a highly oriented film possesses a large photocurrent. The tieshold ionization potentials for carboxylic acrds deposited on copper substrates depend on the carbon number, as shown in fig. 3. The I, value increases with increase m the carbon number, reaclung a constant value of about 8.5 eV, which meets the value of polyethylene [7]. X-ray photoelectron spectroscopy and MO calculations applied to a series of normal akanes (nC,,HZ,,+2) have shown that the electronic structure of tridecane (n-CI,H2S) is approximately that of an infinite linear polymer [8, 9] _ On the other hand, Chattopadhyay et al. pointed out that all the normal dicarboxylic acids with II > 7 would have ul(HOOC(CH2),_,COOH) traviolet photoelectron spectra similar to that of adrpic acid (n = 6) [IO] _Therefore, the dependence of I, on the carbon number should be explained by an effect of molecular aggregation rather than the electronic structure of an isolated molecule_ Higher orientation, caused by appropriate substrate material and chain length, leads to a decrease in Is, as described above. Moreover, the value ofl,
Fig. 4. Relation between the normalized X-ray diffraction intensity and the threshold ionization potential. (a) Stearic acid on indium oxide, (b) copper, and (c) gold (d) Behenlc acid on copper. (e) Melissic acid on copper.
is correlated with molecular orientation using a common parameter as shown in fig. 4. Here, the abscissa indicates a summation of the diffraction intensity normalized to a unit volume of the specimen film. The difference between the maximum and minimum I, was observed to be 0.7 eV. The lowering of the threshold ioruzation potential with increase in the orientation can be ascribed to the formation of a carboxyhc acid dirner in the oriented film. From gas-phase measurements, the adiabatic ionization potential of formic acid and acetic acid 1s known to decrease by about 0.3 eV due to the orbital splitting on dirnerization [11,12] _Similar behavior is expected in the present case. Another contribution to the lowering ofl, should come from a larger polarization energy through the crystallization accompanying dimer formation. The polarization energy P+, which IS the decrease in the threshoId ionrzation potential from a molecule to a solid due to the electric polarization of surroundmg molecules, can be approxrmated as p+
Ccwbon
Number
of Add
Fig. 3. C&on number dependence of the threshold ionization 1potentinl of J fti deposited on a copper substrate.
= Ke 2 ,p413 _
Here, Cuis the average molecular polanzability, p is the number densrty of molecules rn the solrd, e is the charge of the proton and K is a constant [ 13]_ In general, an oriented film is thought to have a higher density than an amorphous one. This leads to an increase of p and hence a large polarization energy in the oriented film, which is expected to decrease the 1, value. The higher density also explains the 433
Volume 119, number 5
CHEMICAL PHYSICS LETTERS
large photocurrent of the highly onented film mentioned above. The molecular aggregation at the film surface may differ from that in the bulk. Since X-rays have a long penetration depth, X-ray diffraction patterns reflect the bulk structure of the film. On the other hand, photoelectron spectra give information about the near-surface repon of the film, since the photoelectron escape depth is generally estimated to be of the order of several nm. The correspondence between the present measurements of photoemission and X-ray diffraction suggests that molecular orientation persists up to the film surface for carboxyhc acids. This agrees with the result on zinc stearate, w6ere reflection electron diffraction from thick and thin films showed the same pattern [3] _Such preservation of molecular onentation may be common to Iong-chain compounds_
Acknowledgement We are grateful to Drs. Shinjl Tomoda and Yohjl Achiba for valuable discussions. Thanks are also due to Mr. Tsutomu Isbiba for useful comments as well as the X-ray diffraction experiment. This work was carried out durmg the stay of MM at the Institute for Molecular Science (1984-85).
434
13 September 1985
References [1] H. Inokucti, H. Kuroda lnd H. Akamatu, BulL Chem. Sot. Japan 34 (1961) 749. [2] K. Seki, S. Hashimoto, N Sato, Y. Harada. K. Ishii. H Inokuchi and J. Kanbe, J. Chem. Phys. 66 (1977) 3644_ 131 M. Mitsuya, Y. Taniguchi and M. Akagi. J. CoUoid Interface Sci. 92 (1983) 291_ 141 K.B Blodgett, J. Am. Chem. SOL 57 (1935) 1007. [51 T. Hirooka, K. Tanaka, K. Kuchitsu, M. Fujihara, H. Inokuchi and Y. Harada. Chem. Phys. Letters 18 (1973) 390. [61 ASTM card No. 9618. [71 S. Hashimoto, K. Seki. N. Sato and H. Inokuchi, J. Chem. Phys. 76 (1982) 163_ [81 JJ. Pireaux. S. Svensson, E. Basilier. P.-A. Malmqvist, U_ Cclius. R. Caudano and K. Siegbahn, Phys. Rev. Al4 (1976) 2133. [91 K. Scki, N. Sato and H Inokuchi. to be published. r101 S Chattopadhyay, J-L. Meeks, G-L. Ftidley and S-P. McGlynn, J. Phys. Chem. 85 (1981) 968 [II; S. Tomoda. Y. Achiba, K. Nomoto, K. Sato and EC. Kimura, Chem. Phys. 74 (1983) 113. 1121 K. Nomoto. Thesis, Hokkaido University (1979). r131 N. Sate, H. Inokuchi, K. Seki, J. Aoki and S. Iwashima, J Chem. Sot. Faraday Trans. II 78 (1982) 1929.