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Penning ionization electron and ultraviolet photoelectron spectroscopies of organic extra-thin films comprising alkyl chains: molecular aggregation, film growth and chemical reaction
531
Journal of Electron Spectroscopy and Related Phenomena, 68 ( 1994) 53 l-539
0368-2048/94/$07.00 @ 1994 - Elsevier Science B.V. All rights reserved
Penning
electron and ultraviolet photoelectron of organic extra-thin films comprising molecular aggregation, film growth reaction
ionization
spectroscopies alkyl chains: and chemical
Hiroyuki Ozakis, Shigeto Maria, Takeshi Yasuhiro Mazakib, Masaru Aokib, Shigeru and Kei j i Kobayashib
Miyashitaa, Takao Tsuchiyaa, Masudan, Yoshiya Haradab
aDepartment of Material Systems Engineering, Tokyo University of Agriculture and Technology, FDepartment of Chemistry, The University of Tokyo,
Koganei,
Tokyo 184, Japan
College of Arts and Sciences, Komaba, Meguro, Tokyo 153, Japan
of long chain alkyl derivatives were The extra-thin evaporated films characterized by Penning ionization electron and ultraviolet photoelectron molecular aggregation were found in films spectroscopies. Two types of films made up of lying prepared under different conditions. In particular, chains were studied in detail, the number of layers being controlled from 1 (4 8. in thickness) to several, and the first stage of film growth was observed layer by layer. Based on these results, a monolayer composed of sash-like and flat-lying macromolecules was prepared by the photopolymerization of dialkyldiacetylene taking place in the outermost surface layer.
l.INTRODUCTION Molecules deposited on a solid surface aggregate in a unique manner ordained by the interaction with the in substrate. In addition, molecules the outermost surface layer (OSL) without neighbors on them are more From mobile than those in the bulk. this standpoint, we intend to utilize OSL of organic extra-thin films (monolayer (4 K in thickness) to several layers) as a place for chemical reactions to obtain new organic substances. For example, an organic monoatomic layer could be yielded if planar or linear molecules were arranged flat on a solid surface to
SSDr0368-2048(94)02155-S
form a monolayer and all of them were combined two-dimensionally by chemical reaction. As a preliminary study for such an attempt, we intend here to link polymerizable molecules in OSL one-dimensionally and prepare a monolayer composed of sash-like and flat-lying macromolecules, the being monitored by Penning process ionization electron spectroscopy[l] and ultraviolet photoelectron specthe former probes OSL troscopy; selectively, whereas the latter detects OSL and the inner layers (IL) simultaneously. When metastable rare gas atoms are introduced onto molecules in solid phase, they do not permeate
532
into the solid, but interact, causing Penning ionization, with an MO spreading outside the surface more effectively than with an MO distributed inside the surface. Therefore, the relative band intensity of a Penning ionization electron spectrum (PIES) reflects the local electron distribution of individual MOs at the externally exposed portion of surface molecules[2,3]. This characteristic has made Penning spectroscopy a useful tool for investigating various organic films[2-91. In this paper, we focus our attention on extra-thin films mainly comprising long alkyl chains because (1) the chains all in trans conformation can pack closely with the plane of the carbon skeleton parallel to the substrate surface, (2) their flexibility will be of advantage for solid phase reaction, and (3) the molecular weight suitable for film preparation is easily adjustable by the number of methylene units without sacrificing packing facility. As the coupler of the a diacetylene unit seems molecule, to be appropriate. Though diacetylenes are known to polymerize in crystalline phase on exposure to UV light[ lo] n R-C=C-C~C-R
-
hv
G ‘3 R
‘c-CEC-c,
Rn
most of them studied well have bulky groups R and .cannot form a closely packed monolayer of flat-lying moleHence, we have chosen 17,19cules. hexatriacontadiyne (C16H33CfCCfCClBH33; HTDY) as the compound for deposiall carbon atoms of an HTDY tion; molecule are expected to lie on the
same plane parallel to the substrate surface before and after neighboring diacetylene units polymerize in the plane. Prior to the description of HTDY, we will survey the aggregation of long alkyl chains in the extra(C17H35thin films of zinc stearate COOZnOCOC17H35;ZnSti), a model compound with structural similarity to HTDY. At first, films prepared under different conditions will be characterized and the growth of piled-up monolayers will be observed layer by layer +
Z.EXPERIMENTAL The film preparation, spectrum measurements, and photopolymerization were carried out with an ultrahigh vacuum electron spectrometer ZnStz obreported previously[6]. tained commercially and HTDY synthesized by Eglington coupling reaction of 1-Octadecyne were purified by sublimation in an other vacuum chamber. A graphite or stainless steel was cleaned by heating substrate under ultrahigh vacuum. The deposited amount of the sample was monitored with a quartz oscillator calibrated in advance and was controlled in the unit MLE (monolayer equivalence) : the substrate surface can be covered with just 1 MLE of molecules packed closely when the carbon skeleton planes are oriented parallel to the substrate surface[ 81. He* ( 23S, 19.82 eV) metastable atoms and the He I (21.22 eV) resonance line were used as the excitation sources. To polymerize the HTDY films, UV light from a mercury lamp was introduced through a sapphire viewing port of the apparitus. For the details of the graphite substrate and estimation of 1 MLE, see ref.8.
533
10
C19HN CALCD IP/sV 14 18 22 I , I I I
He I UPS
9
J 16
a 12 ELECTRON
’
.
8 0 4 ENERGY, EK / eV
18
12 ELECTRON
GRAPHITE
!,
4 a ENERGY, El; /
0 eV
(C1~Ha&OO)aZn (2nS.l) film PIES and He I UPS of a zinc stearate Figure 1. tie*(W) . prepared on a graphite substrate held at 193 K. The depositeu amount of ZnSta, 6 is see text). The ionization potentials changed from 0 to 6 MLE (monolayer equfvalence, of all trans n-hexadecane (CjsHar) calculated with an STO-3G basis set are shown on with the The IP scale is shifted so as to give a good agreement the upper left[8]. PIES. Bars labeled wfth open circles and triangles correspond to pseudo-zr (pfi) and and respectively, the other bars to 6~‘ MOB. The van der Waals envelopes 629 lms, three types of MOs are schematically shown on the upper right for molecules IY ing flat.
3.RESULTS
AND DISCUSSION
Figure 1 shows the He* (Z3S) PI’ES and He I UPS of a ZnStz film prepared on a graphite substrate amount held at 193 K, the deposited 6 being changed from 0 to 6 MLE+ In the UPS, where OSL and IL are detected.simultaneously, features due to graphite at Ek 14.0, 7.4 and 3.3 eV become weak with 6 and buried in the growing bands of ZnSt.2, which clearly indicates the thickening of
the film; the intense features below 12.5 eV are attributed to the alkyl chains while the band around 13.3 eV faintly observed at 6 MLE is ascribed to the carboxyl group by the UPS of a normal comparison with alkane extra-thin film[8J. On the the PIES at 1 MLE is other hand, that of entirely different from This observation exhibits graphite. that molecules cover up the graphite surface to form a monolayer and that OSL molecules prevent the substrate
534
from interacting with helium metastable atoms. In addition, molecules are considered to lie flat because 1 MLE of them cannot cover the substrate completely with other orientations. The flat orientation will be supported by the relative intensities of PIES bands as follows. At the top of Figure 1 are shown the ionization potentials of all trans n-hexadecane (C16H34) calculated with an STO-3G basis set together with the schematic drawings of three types of MOs[8]. The IP scale is shifted so as to give a good agreement with the spectra. The PIES features above (bands Rl and R2) and below 4 eV (band R3) at 1 MLE are assigned to the pseudo-x (PR ) and & MOs of the alkyl chain, respectively. Since the pz MOs are composed of C 2p, and H 1s AOs and the 02, MOs are made up of C 2s and H 1s AOs, both types of MOs have large distribution perpendicular to the carbon skeleton plane (xy plane), Therefore, the remarkable enhancement of the p7c and 02, bands in the PIES of the monolayer means that
molecules orient with the skeleton plane parallel to the graphite surface since these MOs interact with metastables effectively in this orientation. The PIES of a ZnSt2 film (12 MLE) formed on a stainless steel substrate at room temperature is shown in Figure 2(a); the PIES of an LB mondlayer of calcium arachidate CaAra) prepared on ((ClQhQC00)2Ca; the same substratef21 is shown in Figure 2(b) for comparison. It was confirmed by PIES that CaArz molecules orient with their methyl ends exposed outside in the LB monolayer [21 and that the c&p MOs having large distribution at the terminal hydrogen atom are responsible for the PIES features[ 8 I; px and crzs MOs scarcely attacked by metastables (see the right of Figure 2) do not contribute to the spectrum. Since the ZnStz film exhibits almost the same PIES as the CaArz LB film, ZnSt2 molecules must stand with the methyl ends exposed outside on the stainless steel substrate. Now let us return to Figure 1 and devote
0
He* PIES
Hd
16
12
ELECTRON
Figure 2.
8
ENERGY,
4
PK
Q2P
a2s
0
Ek/eV
(a) He*(2%) PIES of an ZnSt2 film (12 MLE) formed on a stainless steel substrate at room temperature. (b) He*(23S) PIES of an LB monolayer film of calcium arachidate ( (C~QHXXOO)~ Ca; CaAra) prepared on the same substrate [2]. The van der Waals envelopes and three types of MOs are schematically shown on the right for molecules standing upright.
535
16
12 ELECTRON
8 ENERGY,
4
0 EL /
eV
18
12 ELECTRON
Figure 3. He* ( 2aS) PIES and He I UPS of 17,19-hexatriacontadiyne HTDY) deposited on a graphite substrate held at 123 K.
a
4
ENERGY,
Ek /
Q eV
(C1~Ha&GCzCCℑ
,.-CC
‘\.
‘.
Figure 4. Four IC MOs of dialkyldiacetylene formed by bonding actions between the two A MOs of neighboring acetylene units, (ti) or parallel (R”) to the trans zigzag plane.
and antibonding interwhich are perpendicular
536
attention to the PIES at 6 2 2 MLE. They bear no resemblance to the spectra in Figure 2 at all, but have markedly enhanced bands Rl - R3 like the monolayer film. We feel that the orientation of an alkyl chain in OSL of multilayers is similar to that in the monolayer although there must be a subtle difference in the molecular aggregation or electronic. structure because the relative intensities of these bands are different, which is now under investigation. Figure 3 shows the PIES and UPS of HTDY deposited onto the graphite substrate held at 123 K. Apart from the presence of bands D around 12 and 13.5 eV in the PIES and UPS, respectively, the shapes of the alkyl bands and 6 dependence of both spectra are similar to those of ZnStn spectra in Figure 1. ThereHTDYmolecules are considered fore, to lay their chains and take aggregation analogous to the case of ZnSt2 molecules. Bonding and antibonding interactions between the two acetylene ?r MOs of neighboring units, which are perpendicular (x1) (or parallel (x”)) to the trans afford ni (or 7tl ) 2 igzag plane, and of (or R: ) MOs (see Figure 4). Of,,these four MOs, only the d and MOs are well separated from ra the alkyl pi MO region and are responsible for band D in the UPS of Figure 3, whereas the nf MO does not contribute to band D in the PIES in the because it is distributed zigzag plane and cannot be effectively attacked by metastables. Figure 5 shows the changes in the spectra of an HTDYfilm (5 MLE) prepare4 on the graphite substrate held at 123 K (a) upon exposure to the UV light and raising the subAfter 3 h of UV strate temperature. irradiation at 138 K (b), band D b&
comes rather weak and the shoulders on both sides become intense in the PIES and UPS, The alkyl bands, however, remain unchanged in both spectra, indicating the same aggregation of chains as in (a). In addition, molecules are not desorbed because the trace of the substrate band at 3.3 eV in the UPS does not become intense. Therefore, the weakening of band D in the PIES means that OSL molecules begin to polymerize with the chains laid flat and that the r conjugated system starts to be elongated. Raising the substrate temperature slowly to 263 K with UV irradiation for 11 h (c) changes both spectra markedly. In the PIES, the 7c band D of monomer disappears completely and smooth features tailing to higher Ek due to the x electron system of macromolecule appear. It is also noteworthy that the relabands Rl and tive intensities of Rz are changed a little, which suggests that the aggregation of chains from in (c) is slightly different In the UPS, the intenthat in (b). sity of the alkyl bands is reduced and the substrate features emerge at 14 and 3.3 eV, showing that the film becomes thinner; on referring to the alkyl bands in the UPS of monomer films (see Figure 3), about 1 MLEof alkyl chains must remain on the substrate. Consequently, it seems that OSL molecules alone with sufficient mobility to take the most favorable inter-molecular arrangement polymerize and IL molecules sublime before undergoing reaction. Beating the polymerized film to 408 K causes little change in the PIES and UPS (Figure 5(d)). This stability offers a extraordinary striking contrast to the sublimation of a monomer monolayer property shown in Figure 6 and supports the
537
He I UPS
II
-
12 ELECTRON
8
D
(a)
VI
, 16
4
ENERGY,
Eb/
0 eV
" 16
12 ELECTRON
1 8 4 0 ENERGY, Ek/eV
in the He*(ZJS)PIES and He I UPS of an HTDY film (5 MLE) prepared Figure 5. Changes on a graphite substrate held at 123 K upon exposure to W light and raising the at 138 K; substrate temperature: (a) fresh at 123 K; (b) after 3 h of iJV irradiation slowly to 263 K with 11 h of UV irradia(c) after raising the substrate temperature tion; (d) at 408 K.
He* PIES
303 K
16
12 ELECTRON
Figure 6. a graphite
8 ENERGY,
4
0 Ek/
sv
16
12 ELECTRON
8 4 ENERGY, gk /
0 sv
Changes in the He*(ZaS) PIES and He I UPS of an HTDYmonolayer prepared on substrate held at 123 K upon raising the substrate temperature to 303 K.
538
Figure
7.
(a)
Sash-like
with a polydiacetylene lying
flat.
made up of rows of alkyd chains stitched up (b) Honolayer comprising columns of the macromolecules
macromolecule
chain.
formation of macromolecules. In the PIES and UPS of Figure 6, the features due to HTDY found at 123 K disappear almost completely upon raising the substrate temperature to 303 K, which indicates that most of molecules sublime at room temperaSuch extreme unstability to ture. sublimation in ultrahigh vacuum has been commonly observed by PIES and UPS for the (piled-up) monolayers of flat-lying planar or linear molecules on graphite{lll. As demonstrated by the observat ion mentioned above, a monolayer comprising columns of sash-like and flat-lying macromolecules has been obtained; each column is made up of alkyl chains stitched up rows of with a polydi.acetylene chain (see Figure 7). It may be also possible
to bring about intra-mono1 ayer polymerization provided that a temperature at which the monomer molecules in contact with the substrate take the best arrangement for polymerization but do not sublime is found out.
4.CONCLUSION It was shown that the characterization of extra-thin organic the detection of chemical films, reaction taking place in OSL and the matepreparation of a new organic can be fruitfully carried out rial by the combined use of Penning ionization electron and ultraviolet photoelectron spectroscopies because (1) the former provides information
539
on the molecular aggregation in OSL selectively; (2) the latter suggests the number of layers; and of course, (3) both detect changes in the electronic structures of molecules, which is the most direct evidence of chemical reaction. This work also indicates the possibility that a sheet of a two-dimensional single organic substance with a complicated structure is prepared under mild and regulated conditions.
thank Professor for his helpful1
4. 5.
6. 7.
8.
ACKNOWLEDGEMENT We Shibasaki sion.
3.
Yoshio discus-
9.
10. REFERENCES 1. V. eerm&, J. Chem. Phys., 44 (1966) 3781. 2. H. Ozaki, Y. Harada, K. Nishi-
11.
yama and M. Fujihira, J. Am. 109 (1987) 950. Chem. Sot,, H. Ozaki and Y. Harada, J. Chem. Phys., 92 (1990) 3184. Y. Harada, H. Ozaki and K, Ohno, Phys. Rev. Lett., 52 (1984) 2269. Y. Harada, H. Ozaki, K. Ohno and T. Kajiwara, Surf. Sci., 147 (1984) 356. Y. Harada and H. Ozaki, Jpn. J. Appl. Phys., 26 (1987) 1201. M. Mitsuya, H, Ozaki, Y. Harada, K. Seki and H. Inokuchi, Langmuir, 4 (1988) 569. H. Ozaki and Y, Harada, J. Am. Chem. Sot . , 112 (1990) 5735. Y. Harada, H, Hayashi, S. Masuda, T. Fukuda, N. Sate, S. Kato, K. Kobayashi, H. Kuroda and H. Ozaki, Sruf. Sci., 242 (1991) 95. See, for example, H. -J. Cantow (ed.), Advances in Polymer Science 63, Polydiacetylenes, Springer, Berlin, 1984. H, Ozaki and Y. Harada, unpublished results.
Journal of Electron Spectroscopy and Related Phenomena, 68 ( 1994) 53 l-539
0368-2048/94/$07.00 @ 1994 - Elsevier Science B.V. All rights reserved
Penning
electron and ultraviolet photoelectron of organic extra-thin films comprising molecular aggregation, film growth reaction
ionization
spectroscopies alkyl chains: and chemical
Hiroyuki Ozakis, Shigeto Maria, Takeshi Yasuhiro Mazakib, Masaru Aokib, Shigeru and Kei j i Kobayashib
Miyashitaa, Takao Tsuchiyaa, Masudan, Yoshiya Haradab
aDepartment of Material Systems Engineering, Tokyo University of Agriculture and Technology, FDepartment of Chemistry, The University of Tokyo,
Koganei,
Tokyo 184, Japan
College of Arts and Sciences, Komaba, Meguro, Tokyo 153, Japan
of long chain alkyl derivatives were The extra-thin evaporated films characterized by Penning ionization electron and ultraviolet photoelectron molecular aggregation were found in films spectroscopies. Two types of films made up of lying prepared under different conditions. In particular, chains were studied in detail, the number of layers being controlled from 1 (4 8. in thickness) to several, and the first stage of film growth was observed layer by layer. Based on these results, a monolayer composed of sash-like and flat-lying macromolecules was prepared by the photopolymerization of dialkyldiacetylene taking place in the outermost surface layer.
l.INTRODUCTION Molecules deposited on a solid surface aggregate in a unique manner ordained by the interaction with the in substrate. In addition, molecules the outermost surface layer (OSL) without neighbors on them are more From mobile than those in the bulk. this standpoint, we intend to utilize OSL of organic extra-thin films (monolayer (4 K in thickness) to several layers) as a place for chemical reactions to obtain new organic substances. For example, an organic monoatomic layer could be yielded if planar or linear molecules were arranged flat on a solid surface to
SSDr0368-2048(94)02155-S
form a monolayer and all of them were combined two-dimensionally by chemical reaction. As a preliminary study for such an attempt, we intend here to link polymerizable molecules in OSL one-dimensionally and prepare a monolayer composed of sash-like and flat-lying macromolecules, the being monitored by Penning process ionization electron spectroscopy[l] and ultraviolet photoelectron specthe former probes OSL troscopy; selectively, whereas the latter detects OSL and the inner layers (IL) simultaneously. When metastable rare gas atoms are introduced onto molecules in solid phase, they do not permeate
532
into the solid, but interact, causing Penning ionization, with an MO spreading outside the surface more effectively than with an MO distributed inside the surface. Therefore, the relative band intensity of a Penning ionization electron spectrum (PIES) reflects the local electron distribution of individual MOs at the externally exposed portion of surface molecules[2,3]. This characteristic has made Penning spectroscopy a useful tool for investigating various organic films[2-91. In this paper, we focus our attention on extra-thin films mainly comprising long alkyl chains because (1) the chains all in trans conformation can pack closely with the plane of the carbon skeleton parallel to the substrate surface, (2) their flexibility will be of advantage for solid phase reaction, and (3) the molecular weight suitable for film preparation is easily adjustable by the number of methylene units without sacrificing packing facility. As the coupler of the a diacetylene unit seems molecule, to be appropriate. Though diacetylenes are known to polymerize in crystalline phase on exposure to UV light[ lo] n R-C=C-C~C-R
-
hv
G ‘3 R
‘c-CEC-c,
Rn
most of them studied well have bulky groups R and .cannot form a closely packed monolayer of flat-lying moleHence, we have chosen 17,19cules. hexatriacontadiyne (C16H33CfCCfCClBH33; HTDY) as the compound for deposiall carbon atoms of an HTDY tion; molecule are expected to lie on the
same plane parallel to the substrate surface before and after neighboring diacetylene units polymerize in the plane. Prior to the description of HTDY, we will survey the aggregation of long alkyl chains in the extra(C17H35thin films of zinc stearate COOZnOCOC17H35;ZnSti), a model compound with structural similarity to HTDY. At first, films prepared under different conditions will be characterized and the growth of piled-up monolayers will be observed layer by layer +
Z.EXPERIMENTAL The film preparation, spectrum measurements, and photopolymerization were carried out with an ultrahigh vacuum electron spectrometer ZnStz obreported previously[6]. tained commercially and HTDY synthesized by Eglington coupling reaction of 1-Octadecyne were purified by sublimation in an other vacuum chamber. A graphite or stainless steel was cleaned by heating substrate under ultrahigh vacuum. The deposited amount of the sample was monitored with a quartz oscillator calibrated in advance and was controlled in the unit MLE (monolayer equivalence) : the substrate surface can be covered with just 1 MLE of molecules packed closely when the carbon skeleton planes are oriented parallel to the substrate surface[ 81. He* ( 23S, 19.82 eV) metastable atoms and the He I (21.22 eV) resonance line were used as the excitation sources. To polymerize the HTDY films, UV light from a mercury lamp was introduced through a sapphire viewing port of the apparitus. For the details of the graphite substrate and estimation of 1 MLE, see ref.8.
533
10
C19HN CALCD IP/sV 14 18 22 I , I I I
He I UPS
9
J 16
a 12 ELECTRON
’
.
8 0 4 ENERGY, EK / eV
18
12 ELECTRON
GRAPHITE
!,
4 a ENERGY, El; /
0 eV
(C1~Ha&OO)aZn (2nS.l) film PIES and He I UPS of a zinc stearate Figure 1. tie*(W) . prepared on a graphite substrate held at 193 K. The depositeu amount of ZnSta, 6 is see text). The ionization potentials changed from 0 to 6 MLE (monolayer equfvalence, of all trans n-hexadecane (CjsHar) calculated with an STO-3G basis set are shown on with the The IP scale is shifted so as to give a good agreement the upper left[8]. PIES. Bars labeled wfth open circles and triangles correspond to pseudo-zr (pfi) and and respectively, the other bars to 6~‘ MOB. The van der Waals envelopes 629 lms, three types of MOs are schematically shown on the upper right for molecules IY ing flat.
3.RESULTS
AND DISCUSSION
Figure 1 shows the He* (Z3S) PI’ES and He I UPS of a ZnStz film prepared on a graphite substrate amount held at 193 K, the deposited 6 being changed from 0 to 6 MLE+ In the UPS, where OSL and IL are detected.simultaneously, features due to graphite at Ek 14.0, 7.4 and 3.3 eV become weak with 6 and buried in the growing bands of ZnSt.2, which clearly indicates the thickening of
the film; the intense features below 12.5 eV are attributed to the alkyl chains while the band around 13.3 eV faintly observed at 6 MLE is ascribed to the carboxyl group by the UPS of a normal comparison with alkane extra-thin film[8J. On the the PIES at 1 MLE is other hand, that of entirely different from This observation exhibits graphite. that molecules cover up the graphite surface to form a monolayer and that OSL molecules prevent the substrate
534
from interacting with helium metastable atoms. In addition, molecules are considered to lie flat because 1 MLE of them cannot cover the substrate completely with other orientations. The flat orientation will be supported by the relative intensities of PIES bands as follows. At the top of Figure 1 are shown the ionization potentials of all trans n-hexadecane (C16H34) calculated with an STO-3G basis set together with the schematic drawings of three types of MOs[8]. The IP scale is shifted so as to give a good agreement with the spectra. The PIES features above (bands Rl and R2) and below 4 eV (band R3) at 1 MLE are assigned to the pseudo-x (PR ) and & MOs of the alkyl chain, respectively. Since the pz MOs are composed of C 2p, and H 1s AOs and the 02, MOs are made up of C 2s and H 1s AOs, both types of MOs have large distribution perpendicular to the carbon skeleton plane (xy plane), Therefore, the remarkable enhancement of the p7c and 02, bands in the PIES of the monolayer means that
molecules orient with the skeleton plane parallel to the graphite surface since these MOs interact with metastables effectively in this orientation. The PIES of a ZnSt2 film (12 MLE) formed on a stainless steel substrate at room temperature is shown in Figure 2(a); the PIES of an LB mondlayer of calcium arachidate CaAra) prepared on ((ClQhQC00)2Ca; the same substratef21 is shown in Figure 2(b) for comparison. It was confirmed by PIES that CaArz molecules orient with their methyl ends exposed outside in the LB monolayer [21 and that the c&p MOs having large distribution at the terminal hydrogen atom are responsible for the PIES features[ 8 I; px and crzs MOs scarcely attacked by metastables (see the right of Figure 2) do not contribute to the spectrum. Since the ZnStz film exhibits almost the same PIES as the CaArz LB film, ZnSt2 molecules must stand with the methyl ends exposed outside on the stainless steel substrate. Now let us return to Figure 1 and devote
0
He* PIES
Hd
16
12
ELECTRON
Figure 2.
8
ENERGY,
4
PK
Q2P
a2s
0
Ek/eV
(a) He*(2%) PIES of an ZnSt2 film (12 MLE) formed on a stainless steel substrate at room temperature. (b) He*(23S) PIES of an LB monolayer film of calcium arachidate ( (C~QHXXOO)~ Ca; CaAra) prepared on the same substrate [2]. The van der Waals envelopes and three types of MOs are schematically shown on the right for molecules standing upright.
535
16
12 ELECTRON
8 ENERGY,
4
0 EL /
eV
18
12 ELECTRON
Figure 3. He* ( 2aS) PIES and He I UPS of 17,19-hexatriacontadiyne HTDY) deposited on a graphite substrate held at 123 K.
a
4
ENERGY,
Ek /
Q eV
(C1~Ha&GCzCCℑ
,.-CC
‘\.
‘.
Figure 4. Four IC MOs of dialkyldiacetylene formed by bonding actions between the two A MOs of neighboring acetylene units, (ti) or parallel (R”) to the trans zigzag plane.
and antibonding interwhich are perpendicular
536
attention to the PIES at 6 2 2 MLE. They bear no resemblance to the spectra in Figure 2 at all, but have markedly enhanced bands Rl - R3 like the monolayer film. We feel that the orientation of an alkyl chain in OSL of multilayers is similar to that in the monolayer although there must be a subtle difference in the molecular aggregation or electronic. structure because the relative intensities of these bands are different, which is now under investigation. Figure 3 shows the PIES and UPS of HTDY deposited onto the graphite substrate held at 123 K. Apart from the presence of bands D around 12 and 13.5 eV in the PIES and UPS, respectively, the shapes of the alkyl bands and 6 dependence of both spectra are similar to those of ZnStn spectra in Figure 1. ThereHTDYmolecules are considered fore, to lay their chains and take aggregation analogous to the case of ZnSt2 molecules. Bonding and antibonding interactions between the two acetylene ?r MOs of neighboring units, which are perpendicular (x1) (or parallel (x”)) to the trans afford ni (or 7tl ) 2 igzag plane, and of (or R: ) MOs (see Figure 4). Of,,these four MOs, only the d and MOs are well separated from ra the alkyl pi MO region and are responsible for band D in the UPS of Figure 3, whereas the nf MO does not contribute to band D in the PIES in the because it is distributed zigzag plane and cannot be effectively attacked by metastables. Figure 5 shows the changes in the spectra of an HTDYfilm (5 MLE) prepare4 on the graphite substrate held at 123 K (a) upon exposure to the UV light and raising the subAfter 3 h of UV strate temperature. irradiation at 138 K (b), band D b&
comes rather weak and the shoulders on both sides become intense in the PIES and UPS, The alkyl bands, however, remain unchanged in both spectra, indicating the same aggregation of chains as in (a). In addition, molecules are not desorbed because the trace of the substrate band at 3.3 eV in the UPS does not become intense. Therefore, the weakening of band D in the PIES means that OSL molecules begin to polymerize with the chains laid flat and that the r conjugated system starts to be elongated. Raising the substrate temperature slowly to 263 K with UV irradiation for 11 h (c) changes both spectra markedly. In the PIES, the 7c band D of monomer disappears completely and smooth features tailing to higher Ek due to the x electron system of macromolecule appear. It is also noteworthy that the relabands Rl and tive intensities of Rz are changed a little, which suggests that the aggregation of chains from in (c) is slightly different In the UPS, the intenthat in (b). sity of the alkyl bands is reduced and the substrate features emerge at 14 and 3.3 eV, showing that the film becomes thinner; on referring to the alkyl bands in the UPS of monomer films (see Figure 3), about 1 MLEof alkyl chains must remain on the substrate. Consequently, it seems that OSL molecules alone with sufficient mobility to take the most favorable inter-molecular arrangement polymerize and IL molecules sublime before undergoing reaction. Beating the polymerized film to 408 K causes little change in the PIES and UPS (Figure 5(d)). This stability offers a extraordinary striking contrast to the sublimation of a monomer monolayer property shown in Figure 6 and supports the
537
He I UPS
II
-
12 ELECTRON
8
D
(a)
VI
, 16
4
ENERGY,
Eb/
0 eV
" 16
12 ELECTRON
1 8 4 0 ENERGY, Ek/eV
in the He*(ZJS)PIES and He I UPS of an HTDY film (5 MLE) prepared Figure 5. Changes on a graphite substrate held at 123 K upon exposure to W light and raising the at 138 K; substrate temperature: (a) fresh at 123 K; (b) after 3 h of iJV irradiation slowly to 263 K with 11 h of UV irradia(c) after raising the substrate temperature tion; (d) at 408 K.
He* PIES
303 K
16
12 ELECTRON
Figure 6. a graphite
8 ENERGY,
4
0 Ek/
sv
16
12 ELECTRON
8 4 ENERGY, gk /
0 sv
Changes in the He*(ZaS) PIES and He I UPS of an HTDYmonolayer prepared on substrate held at 123 K upon raising the substrate temperature to 303 K.
538
Figure
7.
(a)
Sash-like
with a polydiacetylene lying
flat.
made up of rows of alkyd chains stitched up (b) Honolayer comprising columns of the macromolecules
macromolecule
chain.
formation of macromolecules. In the PIES and UPS of Figure 6, the features due to HTDY found at 123 K disappear almost completely upon raising the substrate temperature to 303 K, which indicates that most of molecules sublime at room temperaSuch extreme unstability to ture. sublimation in ultrahigh vacuum has been commonly observed by PIES and UPS for the (piled-up) monolayers of flat-lying planar or linear molecules on graphite{lll. As demonstrated by the observat ion mentioned above, a monolayer comprising columns of sash-like and flat-lying macromolecules has been obtained; each column is made up of alkyl chains stitched up rows of with a polydi.acetylene chain (see Figure 7). It may be also possible
to bring about intra-mono1 ayer polymerization provided that a temperature at which the monomer molecules in contact with the substrate take the best arrangement for polymerization but do not sublime is found out.
4.CONCLUSION It was shown that the characterization of extra-thin organic the detection of chemical films, reaction taking place in OSL and the matepreparation of a new organic can be fruitfully carried out rial by the combined use of Penning ionization electron and ultraviolet photoelectron spectroscopies because (1) the former provides information
539
on the molecular aggregation in OSL selectively; (2) the latter suggests the number of layers; and of course, (3) both detect changes in the electronic structures of molecules, which is the most direct evidence of chemical reaction. This work also indicates the possibility that a sheet of a two-dimensional single organic substance with a complicated structure is prepared under mild and regulated conditions.
thank Professor for his helpful1
4. 5.
6. 7.
8.
ACKNOWLEDGEMENT We Shibasaki sion.
3.
Yoshio discus-
9.
10. REFERENCES 1. V. eerm&, J. Chem. Phys., 44 (1966) 3781. 2. H. Ozaki, Y. Harada, K. Nishi-
11.
yama and M. Fujihira, J. Am. 109 (1987) 950. Chem. Sot,, H. Ozaki and Y. Harada, J. Chem. Phys., 92 (1990) 3184. Y. Harada, H. Ozaki and K, Ohno, Phys. Rev. Lett., 52 (1984) 2269. Y. Harada, H. Ozaki, K. Ohno and T. Kajiwara, Surf. Sci., 147 (1984) 356. Y. Harada and H. Ozaki, Jpn. J. Appl. Phys., 26 (1987) 1201. M. Mitsuya, H, Ozaki, Y. Harada, K. Seki and H. Inokuchi, Langmuir, 4 (1988) 569. H. Ozaki and Y, Harada, J. Am. Chem. Sot . , 112 (1990) 5735. Y. Harada, H, Hayashi, S. Masuda, T. Fukuda, N. Sate, S. Kato, K. Kobayashi, H. Kuroda and H. Ozaki, Sruf. Sci., 242 (1991) 95. See, for example, H. -J. Cantow (ed.), Advances in Polymer Science 63, Polydiacetylenes, Springer, Berlin, 1984. H, Ozaki and Y. Harada, unpublished results.