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Angular distributions of fluorescence emitted from tetraphenylporphine (H2TPP) near metal surfaces
Surface Science 201 (1988) 311-320 North-Holland, Amsterdam
311
ANGULAR DISTRIBUTIONS OF FLUORESCENCE EMI'I'rED FROM TETRAPHENYLPORPHINE (H2TPP) NEAR METAL SURFACES Y. ISHIBASHI, S. OHSHIMA and T. KAJIWARA Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan Received 18 December 1987; accepted for publication 14 March 1988
Angular intensity distributions of fluorescence emitted from tetraphenylporphine (H2TPP) molecules near metal surfaces were measured; the H2TPP molecules were adsorbed on Ag, Au, Cu, and AI substrates directly, and also on a hexatriacontane (HTC) film formed on the metals. By analysing the data on the basis of a classical model for the system, the complex refractive index of the metal substrates and the orientation of the molecules were deduced. On the Ag, Au, and Cu metal substrates, the molecules are adsorbed with their molecular planes parallel to the surface (" parallel" orientation), whereas on the AI substrate they are arranged slightly oblique ("oblique" orientation). On the HTC film, only a "parallel" orientation is recognized.
1. Introduction There has been considerable interest in the optical properties of a molecule adsorbed on a metal surface, because these properties p;o',hde information about the interaction between the molecule and the surface. For such a system, the lifetime or quantum yield of the luminescence of the molecule has been extensively measured as a function of the distance from the metal [1]. The experimental results have been successfully explained using a simple, classical model, in which the luminescent molecule and the metal are pictured as a point dipole and a medium with frequency-dependent die!ectdc constant, respectively [1,2]. However, there have been only a few experiments on the angular intensity distribution of the luminescence from a molecule on a metal [3,4]. Such a distribution arises from interference effects at the metal surface and strongly depends on the orientation of the molecule, or the emitting dipole. According to Greenler's calculations [5], a dipole parallel to the surface gives a different radiation pattern than a dipole perpendicular to the surface. Thus the angular distribution can be utilized to reveal the molecular orientation relative to the metal surface. Recently we have measured the angular distributions of the fluorescence of tetraphenylporphine (H2TPP) near a silver surface, and indeed determined the orientation of the molecules [6]. In the present work, we extended the 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
312
Y. Ishibashi et ai. / Fluorescence emitted from H2TPP near metal surfaces
!o
(a)
d
(b)
METAL
~illl|lllllllllllll I'lnlll Case 1.
Case 2.
i[lllllllllnllllllnlillllllllllllllllllnilnil II Case 3.
Fig. 1. (a) An oscillating dipole near a metal smi'ace. Direct and reflected waves irtterfere, causing an angular distribution of radiation. The t~hin arrows perpendicular to the direction of propagation of the waves show electric vectors. (b) Three orientations of the radiating dipole. Case 1: Dipole axis perpendicular to the surface. Case 2: Dipole axis parallel to the surface and to the plane of incidence. Case 3: Dipole axis parallel to the surface and perpendicular to the plane of incidence.
measurements to other substrate systems; H2TPP was adsorbed on Ag, Au, Cu, and A1 either directly or via a hexatriacontane film as a spacer. On the basis of the experimental results, the adsorption states of the H2TPP molecules on these surfaces were discussed.
2. Angular distributions of the radiation emitted by a dipole on a metal
An overview of Greenler's calculations [5] is provided in this section, because the present experimental data are ah,,dysed by his results later. Consider an oscillating dipole placed near a plane metal surface (fig. la). It is assumed that the distanc~ between the dipole and the surface is very small compared to the wavelength of the emitted radiation. The amplitude of the wave of emitted radiation in a direction 0 is obtained by superposition of the direct and the reflected waves, where the relative phase of the two waves and the reflectivity of the metal are taken into account. In the present case, the relative phase depends only on the phase shift resulting from the reflection, because the dipole i~ located so near the surface that the geometrical path difference between the two waves is negligible. ~Ihe square of the resultant amplitude gives the intensity of the radiation in the 0 direct'on, I(0). The pattern of the intensity, thus obtained, depends on the orientation of the dipole axis related to the surface. 'Fhree cases can be distinguished, as shown in fig. lb.
Y. Ishibashi et al. / Fluorescence emitted from H2TPP near metal surfaces
313
>2.0 °u
u) c
.. 1.0 N
°oJ
'
3o
60
Detection Angte/deg
90
Fig. 2. Calculated angular distributions of radiation for a dipole on a silver surface emitting 660 nm fight. Results are for the three orientations defined in fig. lb.
Case 1. The dipole oscillates perpendicular to the surface: I ( 0 ) = sin20(1 + rl~ + 2r u cos 81i).
(1)
Case 2. The dipole oscillates parallel to the surface and parallel to the plane of incidence: 1 ( 0 ) = cos20(1 + r,~ + 2rll cos 8,1).
(2)
Case 3. The dipole oscillates perpendicular to the plane of incidence: I ( 0 ) = 1 + r2 - 2 r ± c o s 8x .
(3)
Here rii and r± are the amplitude reflectivities and 8it and 8± the phase shifts upon reflection, for parallel and perpendicular polarizations to the plane of incidence, respectively. The radiation distribution for any other case can be obtained by resolving the dipole into the components represented by these three cases. Typical angular distributions for the three cases are shown in fig. 2 for a dipole on a silver surface emitting 660 nm fight. The case 1 intensity shows a maximum at about 60 °, while both the case 2 and 3 intensities decrease monotonically with increasing detection angle. Under the present experimental conditions, a case I dipole is rather hard to be directly excited, since excitation light irradiates a sample at normal incidence with its electric field parallel to the surface. Therefore, such a dipole is yielded only as a component into which a dipole oriented oblique to the surface is resolved. Nevertheless, the case 1 orientation can con:dbute to the angular distribution because case 1 intensities are about one order of magnitude higher than case 2 and 3 intensities (fig. 2).
314
Y. Ishibashi et al. / Fluorescence emitted from H2TPP near metal surfaces
3. Experimental Two types of samples were prepared in almost the same way as reported previously [6]: H 2 T P P / H T C / m e t a l and H2TPP/metal systems. Since the latter is a simplified one of the former, the preparation method is described only for the former. The metal, then hexatriacontane (HTC) (Tokyo Kasei Kogyo Co.) as a spacer, and finally tetraphenylporphine (H2TPP) (Strem Chemicals Inc.) were successively evaporated onto a glass slide. The thickness of each layer was monitored and determined by the quartz-crystal-oscillator method. The deposition rate was about 1 ,~/s for all the layers. The thickness was 1000 ,A for the metal layer, 100 ,~ for the HTC layer, and 30 ,~ for the H2TPP layer. Metals used were Ag, Au, Cu, AI, Sn, In, and Pb. However, no mirrors were formed on the surfaces of Sn, In, and Pb films thus prepared because of the roughness of their surfaces. Therefore, they were omitted. The apparatus for measuring the angular distribution of fluorescence was also described before [6]. A brief review of the apparatus is presented here (fig. 3). Samples were irradiated with 425 nm light at normal incidence; the light was obtained by filtering the output of a Xe-lamp with a monochromator. The resulting fluorescence was allow~ to pass a silt, a polarizer, a light guide, and an interference-band-pass filter, and detected by a photomultiplier, whose output was processed by a photon counter. The filter was chosen in such a way that the wavelength of maximum transmittance of the filter coincides with that (about 660 rim) of the maximum intensity of the fluorescence. The polarizer was turnable so that the polarization direction could be set parallel or perpendicular to the plane of observation, which is determined by the normal to the surface and the direction of observation. Such a set-up of the polarizer is hereafter referred to as "horizontal" or "vertical" situation, respectively. The detection angle, 0, defined as the angle between the surface normal and the detection direction, was varied over the range 25 < 0 < 75 * by moving the light guide.
SAMPLE N~j~~__~ POLARIZER LIGHTGUIDE )_ FILTER CHOPPER i
i "d~
/.
Xe-LAMP
•
,m- ....
,
r-! ,
t I
~~_~/COUNTER . . . . . . .
Fig. 3. A schematic diagram of the apparatus used for the measurements of the angular distribution of fluorescence.
Y. Ishibashi et al. / Fluorescence emitted from H2TPP near metal surfaces
315
The fluorescence of the molecules is strongly quenched when they are located near metal surfaces [1,2]. Therefore, in some cases, the signal intensity of the fluorescence becomes a serious problem. In such a case, the excitation was modulated with a light chopper and measurements were performed with a lock-in method.
4. Results and discussion 4.1. H 2TPP / H T C / metal s y s t e m s
Fig. 4 shows the angular distributions of the fluorescence from H2TPP adsorbed on a HTC film formed on Ag, Au, Cu, and Al substrates: filled and
i0oo0oo "°'°
A
Au
¢-
.~ ~
" o . , . o.o.o~ "
~C
I
"~
I
'
Cu
,
c
*"a
I1~
"e-
~
.
'~'~'a X
' ~ ' ~ AI ~'*'~., X
"~'o.o ~
u
~u
Detection
~u
~U
Angle/deg
Fig. 4. Angular fluorescence distributions for H2TPP/HTC/metal systems: filled and blanc circles denote experimental data for the "horizontal" and the "vertical"situations, respectively. The best fitting results are also shown w~,~h solid and broken curves. The error bars indicate the statistical uncertainty.
Y. Ishibashi et al. / Fluorescence emitted from HzTPP near metal surfaces
316
Table 1 Optical constants of metals determined from the experiments H2TPP/HTC/Metal
H 2TPP/metal
Literature [7,8]
n
k
n
k
n
k
H a) V
0.080 0.050
3.74 3.69
0.060 0.061
3.48 3.58
0.05
4.48
Au
H V
0.15 0.14
2.64 3.69
0.14 0.14
3.62 3.73
0.14
3.70
Cu
H V
0.23 0.24
3.53 4.06
0.34 0.23
2.82 3.76
0.22
3.75
A1
H V
0.83 0.82
5.33 5.99
0.83 0.76
4.85 5.55
1.30
7.11
Ag
a) ,, H" and "V" denote the '" horizontal" and "vertical" situations, respectively.
blanc circles denote the data for the "horizontal" and the "vertical" situations, respectively. For each situation, the angular distributions present a similar pattern, which indicates that the orientation of the H2TPP molecules is independent of the metal. This is consistent with the fact that in this system the molecules are in contact with the HTC film. With the passage of time, no change was observed in the distributions within experimental error. Since both the HTC film thickness (100 .A) and the H2TPP film thickness (30 .g,) are sufficiently small compared to the emission wavelength ( - 6600 A.), the H2TPP molecules are regarded as located "near" the metal. Hence, Greenler's model describext in section 2 holds for this system. In fig. 4, no maxima can be seen in the angular distributions for the "horizontal" situation. This suggests that dipoles are distributed in case 2 and 3 orientations, i.e., on a plane parallel to the surface (see fig. 2). The least-squares fit was applied to the distributions by using eqs. (2) and (3); the optical constants n and k(~ = n -- ik is the complex refractive index of each metal) were chosen as adjustable parameters. The results are shown in fig. 4, with solid ano broken lines for the "horizontal" and "vertical" situations, respectively. The agreement between the calculated and measured distributions is fairly good. The values of n and k for the metals, determined from the best fitting, are presented in table 1. These values are also in good agreement with those in the literature [7,8], considering the facts that the light observed was not monochromatic
a n d t h a t t h e r e n n r t o r l n n t i c ~ l c ' n n e t n n t e n f th~ mc.t~alc
contain some uncertainty. From these results, it is confirmed that the dipoles are parallel to the surface of each metal. This is consistent with the previous report for the H e T P P / H T C / A g system with a H2TPP thickness of 10 A [6]. As described therein, the electric-dipole transition moment in a H2TPP molecule is considered to be parallel to the molecular plane, i.e., the porphyrin
317
Y. Ishibashi et al. / Fluorescence emitted from Hz TPP near metal surfaces
J
J (a)
(b)
Fig. 5. Orientations of a H 2TPPmolecule; (a) "parallel" and (b) "oblique" orientation.
ring. Thus it follows that H2TPP molecules are arranged with their molecular planes parallel to the surface ("parallel" orientation) in the H 2 T P P / H T C / metal system. The adsorption state of the molecules can be illustrated as in fig. 5a.
4.2. ~2TPP / metal systems Fig. 6 shows the angular distributions of the fluorescence from H2TPP molecules directly adsorbed on Ag, Au, Cu, and AI surfaces. The distributions ior ~g, Au, and Cu are almost the same as those for the corresponding H E T P P / H T C / m e t a l systems. However, in the case of A1, the distribution for the "ht;rizontal" situation exhibits a hump at about 60 °, which is not observed in the H 2 T P P / H T C / m e t a l system (see fig. 4). The hump appeared when the thickness of the H2TPP film was in a range of 30-40 A; for smaller thickness, the signal intensity was too weak to be accurately measured, and for larger thickness, the distribution patterns became blurred because of the reduction of interference effects. Furthermore, the hump gradually grew up with the passage of time, as shown in fig. 7. The growth continued for about 12 h. A similar phenomenon was also observed for Ag. Whereas no hump could be seen for a freshly prepared sample, a small one was clearly recognized about 6 h after the preparation. It became distinct by degrees, as in the case of A1. However, no such phenomenon occurred for Au and Cu. Greenler's model can be applied to the H2TPP/metal system as well, for the same reason as for the H e T P P / H T C / m e t a l system. Since no maxima can be seen in the distributions for HeTPP on Ag, Au, and Cu, it is expected that the dipoles are distributed parallel to the surface. The calculated results under this assumption are in good agreement with the experimental data (see fig. 6 o , , a tablv 1 ~ T h a r ~ f r ~ r , ~ i t ; e A~A,,t-,~,A t h a t t h a H T D D ~ l ~ , r , , , l ~ , z c~r~cRr~tv,~rl with their molecular planes parallel to the surface (fig. 5a). On the other hand, the hump which appears in the distribution for A1 seems to be ascribable to case 1 orientation. The experimental data for AI were indeed reproduced by a lhlear combination of the calculated distributions for cases 1 and 2; the fraction of case 1 ( F ) was adjusted as well as the optical
318
Y. Ishibashi et aL / Fluorescence emitted from HzTPP near metal surfaces
"
~
Ag
"~,~.
-°'~'O,o, la..n. ~ "°'°"
n "
A
"
"~
~
AU
"'o
AI
n
t-
x~ !=_
< v
t/! ¢0 ¢:
"
0
30
60
Detection Angle/deg
90
Fig. 6. Angular fluorescence distributions for HETPi'/metal system: filled and blanc circles denote experimental data for the "horizontal" and the "vertical" situations, respectively. The best fitting results are also shown with solid a qd broken curves.
constants of the metal. The resultant calculated distributions (solid curves) almost agree with the measured ones (fig. 6), and the values of n and k (table 1) are also reasonable. The value of F is 0.018. Though the fraction is very small, the presence of "perpendicular" dipoles (case 1 orientation) is confirmed for the H2TPP/A1 system. In the present experiments, a "perpendicular" dipole is yielded as a component of a dipole which is tilted to the surface (see section 2). If all dipoles are tilted alike at a certain angle, the value of F can be regarded as the ratio of the perpendicular to the parallel component of ov,,.,.,~.,,..,..,=...,.j~,v.,..~,~, ~ r h d i ~ | , = " tha~ t-h,= ,,-~K1;~1,= o,-~nl,-- , ~ ÷1.,,=,-1;.,.,,~1,~ ,,., ,i.... . . . . c..,^..., :. ^^.: . . . . ~,-,,='~.-.zx ,,.,x,~.,. V L . . t U I . ~ , 1.~'~r., ~.4dt.t~:rt'~=, 'V.L Lxa.~r~.., %Z.ltJ~K.P.].~.,.~ kli...9 LI.I.~;; ~ U J . I ~ I L ~ . , ~ ll~. ~ L U I J ~ I L L ~ U
-I ._ LqLJ
be 1.2 °. Thus the appearance of the hump indicates that the molecules as a whole are oriented with their molecular planes oblique to the surface ("oblique" orientation). The', adsorption state for the H ETPP/AI system can be illustrated as in fig. 5b. It should be noted that the case 1 orientation can be clearly observed by the angular distribution of fluoresence despite of its small
Y. Ishibashi et aL / Fluorescence emitted from H2TPP near metal surfaces
319
AI
!
e-
h
.d
I
"ll
e•
0
h
30 60 90 Detection Angle/deg
Fig. 7. Change in the angular fluorescence distributions with the lapse of time (t) after the sample preparation, for the H2TPP/AI system in the "horizontal" situatien. The values of F are 0.018, 0.021, and 0.028 in the distributions at t = 0, 6.5, and 12 h, respectively.
fraction of about 0.02. Especially, this orientation largely contributes to the fluorescence intensity in the case of AI. The ratio of the case 1 to case 2 intensities, which can be calculated using eqs. (1) and (2) is about 20 at 0 = 6 0 . for A1, whereas it is smaller than 9 for Ag, Au, and Cu. The occurrence of an "oblique" orientation in the H2TPP/AI system may be explained as the result of crystallization of the H2TPP molecules adsorbed on the surface. It is thought that the interaction between the surface and the molecules is not very strong. Thus the molecules will be less strongly influenced by the surface and may themselves determine their arrangement, in which the molecules lie slightly oblique to the surface• This explanation is consistent with the fact that the hump in the angular distribution for AI grew with time, because its growth can be ascribed to the progress of the crystallization. In the case of Au and Cu, the surface-molecules interaction may be fairly strong so that the crystallization is inhibited; the molecules are adsorbed flat to the surface and keep the "parallel" orientation, The Ag surface seems to have a character intermediate between those of the A1 and the Au and Cu
320
Y. Ishibashi et al. / Fluorescence emitted from H:TPP near metal surfaces
surfaces wi~h regard to the interaction with the molecules, since the hump was hardly reco?,nized at first but gradually grew with time.
5. Conclusions
The adsorption states of H2TPP molecules on hexatriacontane (HTC) and metal substrates were investigated by measurement of the angular intensity distribution of the fluorescence. The molecules are oriented with their molecular planes parallel to the surface on the HTC and Ag, Au and Cu substrates. However, they are oriented slightly oblique to the surface on AI substrate. The optical constants of the metals are also obtained from the analysis of the angular distributions of fluorescence. The present method is useful for the study of the molecular adsorption state on metals and with the advantage that measurements can be performed in atmosphere in comparison with electron spectroscopic methods. It should also be noted that whereas electron spectroscopic methods mainly concern with the uppermost layer molecules, the present method gives information about all molecules as a whole.
References [1] D.H. Waldeck, A.P. Alivisatos and C.B. Harris, Surface Sci. 158 (1985) 103. [2] R.R. Chance, A. Prock and R. Silbey, in: Advances in Chemical Physics, Vol. 37, Eds. S.A. Rice and I. Prigogine (Wiley-Interscience, New York, 1978) p. 1. [3] K.H. Drexhage and H. Kuhn, in: Basic Problems in Thin Film Physics, Conf. Proc., Clausthal-Goettingen 1965, Eds. R. Niedermayer and H. Mayer (Vandenhoeck-Ruprecht, Goettingen, 1966) p. 339. [4] K.H. Drexhage, in: Progress in Optics, Vol. XII, Ed. E. Wolf (North-Holland, Amsterdam, 1974) p. 163. [5] R.G. Greenler, Surface Sci. 69 (1977) 647. [6] S. Ohshima, 1". Kajiwara, M. Hiramoto, K. Hashimoto and T. Sakata, J. Phys. Chem. 90 (1986) 4474. [7] (3. Hass and J. Waylonis, J. Opt. Soc. Am. 51 (1961) 719. [8] P. Johnson and R. Christy, Phys. Rev. B 6 (1972) 4370.
311
ANGULAR DISTRIBUTIONS OF FLUORESCENCE EMI'I'rED FROM TETRAPHENYLPORPHINE (H2TPP) NEAR METAL SURFACES Y. ISHIBASHI, S. OHSHIMA and T. KAJIWARA Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan Received 18 December 1987; accepted for publication 14 March 1988
Angular intensity distributions of fluorescence emitted from tetraphenylporphine (H2TPP) molecules near metal surfaces were measured; the H2TPP molecules were adsorbed on Ag, Au, Cu, and AI substrates directly, and also on a hexatriacontane (HTC) film formed on the metals. By analysing the data on the basis of a classical model for the system, the complex refractive index of the metal substrates and the orientation of the molecules were deduced. On the Ag, Au, and Cu metal substrates, the molecules are adsorbed with their molecular planes parallel to the surface (" parallel" orientation), whereas on the AI substrate they are arranged slightly oblique ("oblique" orientation). On the HTC film, only a "parallel" orientation is recognized.
1. Introduction There has been considerable interest in the optical properties of a molecule adsorbed on a metal surface, because these properties p;o',hde information about the interaction between the molecule and the surface. For such a system, the lifetime or quantum yield of the luminescence of the molecule has been extensively measured as a function of the distance from the metal [1]. The experimental results have been successfully explained using a simple, classical model, in which the luminescent molecule and the metal are pictured as a point dipole and a medium with frequency-dependent die!ectdc constant, respectively [1,2]. However, there have been only a few experiments on the angular intensity distribution of the luminescence from a molecule on a metal [3,4]. Such a distribution arises from interference effects at the metal surface and strongly depends on the orientation of the molecule, or the emitting dipole. According to Greenler's calculations [5], a dipole parallel to the surface gives a different radiation pattern than a dipole perpendicular to the surface. Thus the angular distribution can be utilized to reveal the molecular orientation relative to the metal surface. Recently we have measured the angular distributions of the fluorescence of tetraphenylporphine (H2TPP) near a silver surface, and indeed determined the orientation of the molecules [6]. In the present work, we extended the 0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
312
Y. Ishibashi et ai. / Fluorescence emitted from H2TPP near metal surfaces
!o
(a)
d
(b)
METAL
~illl|lllllllllllll I'lnlll Case 1.
Case 2.
i[lllllllllnllllllnlillllllllllllllllllnilnil II Case 3.
Fig. 1. (a) An oscillating dipole near a metal smi'ace. Direct and reflected waves irtterfere, causing an angular distribution of radiation. The t~hin arrows perpendicular to the direction of propagation of the waves show electric vectors. (b) Three orientations of the radiating dipole. Case 1: Dipole axis perpendicular to the surface. Case 2: Dipole axis parallel to the surface and to the plane of incidence. Case 3: Dipole axis parallel to the surface and perpendicular to the plane of incidence.
measurements to other substrate systems; H2TPP was adsorbed on Ag, Au, Cu, and A1 either directly or via a hexatriacontane film as a spacer. On the basis of the experimental results, the adsorption states of the H2TPP molecules on these surfaces were discussed.
2. Angular distributions of the radiation emitted by a dipole on a metal
An overview of Greenler's calculations [5] is provided in this section, because the present experimental data are ah,,dysed by his results later. Consider an oscillating dipole placed near a plane metal surface (fig. la). It is assumed that the distanc~ between the dipole and the surface is very small compared to the wavelength of the emitted radiation. The amplitude of the wave of emitted radiation in a direction 0 is obtained by superposition of the direct and the reflected waves, where the relative phase of the two waves and the reflectivity of the metal are taken into account. In the present case, the relative phase depends only on the phase shift resulting from the reflection, because the dipole i~ located so near the surface that the geometrical path difference between the two waves is negligible. ~Ihe square of the resultant amplitude gives the intensity of the radiation in the 0 direct'on, I(0). The pattern of the intensity, thus obtained, depends on the orientation of the dipole axis related to the surface. 'Fhree cases can be distinguished, as shown in fig. lb.
Y. Ishibashi et al. / Fluorescence emitted from H2TPP near metal surfaces
313
>2.0 °u
u) c
.. 1.0 N
°oJ
'
3o
60
Detection Angte/deg
90
Fig. 2. Calculated angular distributions of radiation for a dipole on a silver surface emitting 660 nm fight. Results are for the three orientations defined in fig. lb.
Case 1. The dipole oscillates perpendicular to the surface: I ( 0 ) = sin20(1 + rl~ + 2r u cos 81i).
(1)
Case 2. The dipole oscillates parallel to the surface and parallel to the plane of incidence: 1 ( 0 ) = cos20(1 + r,~ + 2rll cos 8,1).
(2)
Case 3. The dipole oscillates perpendicular to the plane of incidence: I ( 0 ) = 1 + r2 - 2 r ± c o s 8x .
(3)
Here rii and r± are the amplitude reflectivities and 8it and 8± the phase shifts upon reflection, for parallel and perpendicular polarizations to the plane of incidence, respectively. The radiation distribution for any other case can be obtained by resolving the dipole into the components represented by these three cases. Typical angular distributions for the three cases are shown in fig. 2 for a dipole on a silver surface emitting 660 nm fight. The case 1 intensity shows a maximum at about 60 °, while both the case 2 and 3 intensities decrease monotonically with increasing detection angle. Under the present experimental conditions, a case I dipole is rather hard to be directly excited, since excitation light irradiates a sample at normal incidence with its electric field parallel to the surface. Therefore, such a dipole is yielded only as a component into which a dipole oriented oblique to the surface is resolved. Nevertheless, the case 1 orientation can con:dbute to the angular distribution because case 1 intensities are about one order of magnitude higher than case 2 and 3 intensities (fig. 2).
314
Y. Ishibashi et al. / Fluorescence emitted from H2TPP near metal surfaces
3. Experimental Two types of samples were prepared in almost the same way as reported previously [6]: H 2 T P P / H T C / m e t a l and H2TPP/metal systems. Since the latter is a simplified one of the former, the preparation method is described only for the former. The metal, then hexatriacontane (HTC) (Tokyo Kasei Kogyo Co.) as a spacer, and finally tetraphenylporphine (H2TPP) (Strem Chemicals Inc.) were successively evaporated onto a glass slide. The thickness of each layer was monitored and determined by the quartz-crystal-oscillator method. The deposition rate was about 1 ,~/s for all the layers. The thickness was 1000 ,A for the metal layer, 100 ,~ for the HTC layer, and 30 ,~ for the H2TPP layer. Metals used were Ag, Au, Cu, AI, Sn, In, and Pb. However, no mirrors were formed on the surfaces of Sn, In, and Pb films thus prepared because of the roughness of their surfaces. Therefore, they were omitted. The apparatus for measuring the angular distribution of fluorescence was also described before [6]. A brief review of the apparatus is presented here (fig. 3). Samples were irradiated with 425 nm light at normal incidence; the light was obtained by filtering the output of a Xe-lamp with a monochromator. The resulting fluorescence was allow~ to pass a silt, a polarizer, a light guide, and an interference-band-pass filter, and detected by a photomultiplier, whose output was processed by a photon counter. The filter was chosen in such a way that the wavelength of maximum transmittance of the filter coincides with that (about 660 rim) of the maximum intensity of the fluorescence. The polarizer was turnable so that the polarization direction could be set parallel or perpendicular to the plane of observation, which is determined by the normal to the surface and the direction of observation. Such a set-up of the polarizer is hereafter referred to as "horizontal" or "vertical" situation, respectively. The detection angle, 0, defined as the angle between the surface normal and the detection direction, was varied over the range 25 < 0 < 75 * by moving the light guide.
SAMPLE N~j~~__~ POLARIZER LIGHTGUIDE )_ FILTER CHOPPER i
i "d~
/.
Xe-LAMP
•
,m- ....
,
r-! ,
t I
~~_~/COUNTER . . . . . . .
Fig. 3. A schematic diagram of the apparatus used for the measurements of the angular distribution of fluorescence.
Y. Ishibashi et al. / Fluorescence emitted from H2TPP near metal surfaces
315
The fluorescence of the molecules is strongly quenched when they are located near metal surfaces [1,2]. Therefore, in some cases, the signal intensity of the fluorescence becomes a serious problem. In such a case, the excitation was modulated with a light chopper and measurements were performed with a lock-in method.
4. Results and discussion 4.1. H 2TPP / H T C / metal s y s t e m s
Fig. 4 shows the angular distributions of the fluorescence from H2TPP adsorbed on a HTC film formed on Ag, Au, Cu, and Al substrates: filled and
i0oo0oo "°'°
A
Au
¢-
.~ ~
" o . , . o.o.o~ "
~C
I
"~
I
'
Cu
,
c
*"a
I1~
"e-
~
.
'~'~'a X
' ~ ' ~ AI ~'*'~., X
"~'o.o ~
u
~u
Detection
~u
~U
Angle/deg
Fig. 4. Angular fluorescence distributions for H2TPP/HTC/metal systems: filled and blanc circles denote experimental data for the "horizontal" and the "vertical"situations, respectively. The best fitting results are also shown w~,~h solid and broken curves. The error bars indicate the statistical uncertainty.
Y. Ishibashi et al. / Fluorescence emitted from HzTPP near metal surfaces
316
Table 1 Optical constants of metals determined from the experiments H2TPP/HTC/Metal
H 2TPP/metal
Literature [7,8]
n
k
n
k
n
k
H a) V
0.080 0.050
3.74 3.69
0.060 0.061
3.48 3.58
0.05
4.48
Au
H V
0.15 0.14
2.64 3.69
0.14 0.14
3.62 3.73
0.14
3.70
Cu
H V
0.23 0.24
3.53 4.06
0.34 0.23
2.82 3.76
0.22
3.75
A1
H V
0.83 0.82
5.33 5.99
0.83 0.76
4.85 5.55
1.30
7.11
Ag
a) ,, H" and "V" denote the '" horizontal" and "vertical" situations, respectively.
blanc circles denote the data for the "horizontal" and the "vertical" situations, respectively. For each situation, the angular distributions present a similar pattern, which indicates that the orientation of the H2TPP molecules is independent of the metal. This is consistent with the fact that in this system the molecules are in contact with the HTC film. With the passage of time, no change was observed in the distributions within experimental error. Since both the HTC film thickness (100 .A) and the H2TPP film thickness (30 .g,) are sufficiently small compared to the emission wavelength ( - 6600 A.), the H2TPP molecules are regarded as located "near" the metal. Hence, Greenler's model describext in section 2 holds for this system. In fig. 4, no maxima can be seen in the angular distributions for the "horizontal" situation. This suggests that dipoles are distributed in case 2 and 3 orientations, i.e., on a plane parallel to the surface (see fig. 2). The least-squares fit was applied to the distributions by using eqs. (2) and (3); the optical constants n and k(~ = n -- ik is the complex refractive index of each metal) were chosen as adjustable parameters. The results are shown in fig. 4, with solid ano broken lines for the "horizontal" and "vertical" situations, respectively. The agreement between the calculated and measured distributions is fairly good. The values of n and k for the metals, determined from the best fitting, are presented in table 1. These values are also in good agreement with those in the literature [7,8], considering the facts that the light observed was not monochromatic
a n d t h a t t h e r e n n r t o r l n n t i c ~ l c ' n n e t n n t e n f th~ mc.t~alc
contain some uncertainty. From these results, it is confirmed that the dipoles are parallel to the surface of each metal. This is consistent with the previous report for the H e T P P / H T C / A g system with a H2TPP thickness of 10 A [6]. As described therein, the electric-dipole transition moment in a H2TPP molecule is considered to be parallel to the molecular plane, i.e., the porphyrin
317
Y. Ishibashi et al. / Fluorescence emitted from Hz TPP near metal surfaces
J
J (a)
(b)
Fig. 5. Orientations of a H 2TPPmolecule; (a) "parallel" and (b) "oblique" orientation.
ring. Thus it follows that H2TPP molecules are arranged with their molecular planes parallel to the surface ("parallel" orientation) in the H 2 T P P / H T C / metal system. The adsorption state of the molecules can be illustrated as in fig. 5a.
4.2. ~2TPP / metal systems Fig. 6 shows the angular distributions of the fluorescence from H2TPP molecules directly adsorbed on Ag, Au, Cu, and AI surfaces. The distributions ior ~g, Au, and Cu are almost the same as those for the corresponding H E T P P / H T C / m e t a l systems. However, in the case of A1, the distribution for the "ht;rizontal" situation exhibits a hump at about 60 °, which is not observed in the H 2 T P P / H T C / m e t a l system (see fig. 4). The hump appeared when the thickness of the H2TPP film was in a range of 30-40 A; for smaller thickness, the signal intensity was too weak to be accurately measured, and for larger thickness, the distribution patterns became blurred because of the reduction of interference effects. Furthermore, the hump gradually grew up with the passage of time, as shown in fig. 7. The growth continued for about 12 h. A similar phenomenon was also observed for Ag. Whereas no hump could be seen for a freshly prepared sample, a small one was clearly recognized about 6 h after the preparation. It became distinct by degrees, as in the case of A1. However, no such phenomenon occurred for Au and Cu. Greenler's model can be applied to the H2TPP/metal system as well, for the same reason as for the H e T P P / H T C / m e t a l system. Since no maxima can be seen in the distributions for HeTPP on Ag, Au, and Cu, it is expected that the dipoles are distributed parallel to the surface. The calculated results under this assumption are in good agreement with the experimental data (see fig. 6 o , , a tablv 1 ~ T h a r ~ f r ~ r , ~ i t ; e A~A,,t-,~,A t h a t t h a H T D D ~ l ~ , r , , , l ~ , z c~r~cRr~tv,~rl with their molecular planes parallel to the surface (fig. 5a). On the other hand, the hump which appears in the distribution for A1 seems to be ascribable to case 1 orientation. The experimental data for AI were indeed reproduced by a lhlear combination of the calculated distributions for cases 1 and 2; the fraction of case 1 ( F ) was adjusted as well as the optical
318
Y. Ishibashi et aL / Fluorescence emitted from HzTPP near metal surfaces
"
~
Ag
"~,~.
-°'~'O,o, la..n. ~ "°'°"
n "
A
"
"~
~
AU
"'o
AI
n
t-
x~ !=_
< v
t/! ¢0 ¢:
"
0
30
60
Detection Angle/deg
90
Fig. 6. Angular fluorescence distributions for HETPi'/metal system: filled and blanc circles denote experimental data for the "horizontal" and the "vertical" situations, respectively. The best fitting results are also shown with solid a qd broken curves.
constants of the metal. The resultant calculated distributions (solid curves) almost agree with the measured ones (fig. 6), and the values of n and k (table 1) are also reasonable. The value of F is 0.018. Though the fraction is very small, the presence of "perpendicular" dipoles (case 1 orientation) is confirmed for the H2TPP/A1 system. In the present experiments, a "perpendicular" dipole is yielded as a component of a dipole which is tilted to the surface (see section 2). If all dipoles are tilted alike at a certain angle, the value of F can be regarded as the ratio of the perpendicular to the parallel component of ov,,.,.,~.,,..,..,=...,.j~,v.,..~,~, ~ r h d i ~ | , = " tha~ t-h,= ,,-~K1;~1,= o,-~nl,-- , ~ ÷1.,,=,-1;.,.,,~1,~ ,,., ,i.... . . . . c..,^..., :. ^^.: . . . . ~,-,,='~.-.zx ,,.,x,~.,. V L . . t U I . ~ , 1.~'~r., ~.4dt.t~:rt'~=, 'V.L Lxa.~r~.., %Z.ltJ~K.P.].~.,.~ kli...9 LI.I.~;; ~ U J . I ~ I L ~ . , ~ ll~. ~ L U I J ~ I L L ~ U
-I ._ LqLJ
be 1.2 °. Thus the appearance of the hump indicates that the molecules as a whole are oriented with their molecular planes oblique to the surface ("oblique" orientation). The', adsorption state for the H ETPP/AI system can be illustrated as in fig. 5b. It should be noted that the case 1 orientation can be clearly observed by the angular distribution of fluoresence despite of its small
Y. Ishibashi et aL / Fluorescence emitted from H2TPP near metal surfaces
319
AI
!
e-
h
.d
I
"ll
e•
0
h
30 60 90 Detection Angle/deg
Fig. 7. Change in the angular fluorescence distributions with the lapse of time (t) after the sample preparation, for the H2TPP/AI system in the "horizontal" situatien. The values of F are 0.018, 0.021, and 0.028 in the distributions at t = 0, 6.5, and 12 h, respectively.
fraction of about 0.02. Especially, this orientation largely contributes to the fluorescence intensity in the case of AI. The ratio of the case 1 to case 2 intensities, which can be calculated using eqs. (1) and (2) is about 20 at 0 = 6 0 . for A1, whereas it is smaller than 9 for Ag, Au, and Cu. The occurrence of an "oblique" orientation in the H2TPP/AI system may be explained as the result of crystallization of the H2TPP molecules adsorbed on the surface. It is thought that the interaction between the surface and the molecules is not very strong. Thus the molecules will be less strongly influenced by the surface and may themselves determine their arrangement, in which the molecules lie slightly oblique to the surface• This explanation is consistent with the fact that the hump in the angular distribution for AI grew with time, because its growth can be ascribed to the progress of the crystallization. In the case of Au and Cu, the surface-molecules interaction may be fairly strong so that the crystallization is inhibited; the molecules are adsorbed flat to the surface and keep the "parallel" orientation, The Ag surface seems to have a character intermediate between those of the A1 and the Au and Cu
320
Y. Ishibashi et al. / Fluorescence emitted from H:TPP near metal surfaces
surfaces wi~h regard to the interaction with the molecules, since the hump was hardly reco?,nized at first but gradually grew with time.
5. Conclusions
The adsorption states of H2TPP molecules on hexatriacontane (HTC) and metal substrates were investigated by measurement of the angular intensity distribution of the fluorescence. The molecules are oriented with their molecular planes parallel to the surface on the HTC and Ag, Au and Cu substrates. However, they are oriented slightly oblique to the surface on AI substrate. The optical constants of the metals are also obtained from the analysis of the angular distributions of fluorescence. The present method is useful for the study of the molecular adsorption state on metals and with the advantage that measurements can be performed in atmosphere in comparison with electron spectroscopic methods. It should also be noted that whereas electron spectroscopic methods mainly concern with the uppermost layer molecules, the present method gives information about all molecules as a whole.
References [1] D.H. Waldeck, A.P. Alivisatos and C.B. Harris, Surface Sci. 158 (1985) 103. [2] R.R. Chance, A. Prock and R. Silbey, in: Advances in Chemical Physics, Vol. 37, Eds. S.A. Rice and I. Prigogine (Wiley-Interscience, New York, 1978) p. 1. [3] K.H. Drexhage and H. Kuhn, in: Basic Problems in Thin Film Physics, Conf. Proc., Clausthal-Goettingen 1965, Eds. R. Niedermayer and H. Mayer (Vandenhoeck-Ruprecht, Goettingen, 1966) p. 339. [4] K.H. Drexhage, in: Progress in Optics, Vol. XII, Ed. E. Wolf (North-Holland, Amsterdam, 1974) p. 163. [5] R.G. Greenler, Surface Sci. 69 (1977) 647. [6] S. Ohshima, 1". Kajiwara, M. Hiramoto, K. Hashimoto and T. Sakata, J. Phys. Chem. 90 (1986) 4474. [7] (3. Hass and J. Waylonis, J. Opt. Soc. Am. 51 (1961) 719. [8] P. Johnson and R. Christy, Phys. Rev. B 6 (1972) 4370.