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Optical characterization of thin films of some phthalocyanines by spectroscopic ellipsometry
Thin Solid Films, 188 (1990) 181-192 LANGMUIR-RLODGETT
AND RELATED
181
FILMS
OPTICAL CHARACTERIZATION OF THIN FILMS OF SOME PHTHALOCYANINES BY SPECTROSCOPIC ELLIPSOMETRY J. MARTENSSON
AND H. ARWIN
Laboratory of Applied Physics, Department University. S-5$1 83 Linkiiping (Sweden) (Received June 12, 1989; accepted
October
of Physics and Measurement
Technology.
LinkSping
31. 1989)
Thin films of water-soluble tetrasulphonated phthalocyanines containing copper, nickel and zinc have been made by spin casting on gold. The dielectric function of films with a thickness less than 1008, have been measured with a rotating-analyser spectroscopic ellipsometer in the photon energy range 1.5-4.5 eV. The 2 eV 7telectron band has been analysed with respect to resonance energies with line shape analysis. The band could be resolved into three resonances; one of them is shifted towards lower energy as the film thickness decreases.
1.
INTRODUCTION
The phthalocyanines (PCS)are well known for their semiconducting properties, their stability and the possibility of modifying their conductivity optically’ or chemically with gases2. The latter property make them very interesting as potential gas sensors3. In a recent paper by Temofonte and Schoch4, futher references to gas sensor applications of PCs can be found. However, most of the work on gas sensitivity has been focused on conductivity changes. Recently some results on gasinduced changes in optical response of PC have been published. The techniques used were IR absorption’ and surface plasmon resonance6. The work presented here is part of our basic research on thin organic films with emphasis on the effects of oxidizing gases on their optical properties. The development of suitable film preparation techniques and methods for determination of fundamental film properties is of large importance. Specifically we have chosen to investigate the spinning technique for fast and simple film preparation. The thickness and optical response of the films were determined with spectroscopic ellipsometry and further analysed in optical models. It is of special interest to know how the film thickness depends on preparation parameters, and whether the optical response depends on film thickness. Our results on gas-induced changes in properties of thin films of PC will be presented elsewhere. 004~6090/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
.I. MARTENSSON,
182 2.
EXPERIMENTAL
H. ARWIN
DETAILS
The substrates were made by sequential vacuum deposition ofchromium (60 A) and gold (2500 A) onto glass slides. The chromium layer serves as an adhesion promoter between gold and’glass. Immediately before the substrates were used, they were washed in a mixture of H,O, H,O, and NH,’ (ratio, 5:l:l) at 80°C for 5 min. Tetrasulphonated phthalocyanine (TSPc) containing the metal ions copper, nickel and zinc (CuTSPc, NiTSPc and ZnTSPc respectively) were obtained from Clovis A. Linkous at Brookhaven National Laboratory. They were synthesized with the method described by Weber and Busch’. Water (Millipore filtered) solutions of TSPc with concentrations in the range 2.0-10.0 mgml-’ were prepared. Films of TSPc were made by spinning these solutions at 4000 rev min ’ onto the substrates. In this work we have prepared and studied films with thicknesses from 15 to 90 A. The measurements were carried out in air at room temperature (2Ok 1 “C) with a rotating-analyser spectroscopic ellipsometerE, operated in the photon energy range 1.5-4.5 eV (825-275 nm). Our ellipsometric spectra consist of measurements taken at 256 equidistant photon energies. The angle of incidence was 68’. The spot size on the sample was approximately 4 mm x 4 mm. An ellipsometer measures the changes in polarization state for the reflection of polarized light on a sample at oblique incidence. The measured quantity is the complex reflectance ratio p. The physical parameter of interest is then obtained by numerical interpretation in a suitable optical model (see next section). A more complete description of the theory of ellipsometry can be found elsewhere’. 3.
INTERPRETATION
the complex reflectance ratio In a rotating-analyzer ellipsometer, p = RJR, = tan I(/exp(id), (where R, and R, are the complex reflectances (at oblique incidence) for light polarized parallel and perpendicular respectively to the plane of incidence) is determined in the form of tan Ic/and cos d from the phase and amplitude of the photomultiplier signals. In the interpretation of the film properties we must know the complex dielectric function E, for the substrate. In a typical experiment we therefore measure the spectrum of p both before and after deposition of a TSPc film on a gold substrate. From p measured before film deposition, E, can be calculated in the two-phase model (ambient-substrate). If the surface is assumed to be perfectly smooth and film free, E, is obtained as E, = E, sin2 & {l +(ztan&y}
(1)
where E, (here equal to 1) is the dielectric constant of the ambient and 4,, the angle of incidence. After deposition of a film of TSPc, a new spectrum was taken on the same sample. We use numerical inversion in the three-phase model (ambient-filmsubstrate) to calculate the film properties. As we have three unknowns (the film thickness and the real and imaginary parts of the dielectric function of the film given by, E = E~+ iE2) but only two measured data (tan $ and cos A), we use the following
OPTICAL
CHARACTERIZATION
OF THIN FILMS OF SOME PCS
183
strategy. E is calculated by standard numerical inversion techniques for a series of film thicknesses. The correct thickness is then chosen as that for which s2 is zero in the PC spectral window near 3 eV. This means that the layer is assumed to be transparent around this photon energy. With this method, which is illustrated in Fig. 1, it is possible to determine the thickness with a precision of approximately 0.50/dof thickness.
I 2.0
I
I
2.5
3.0
3
PHOTON ENERGY (ev) Fig. 1. The imaginary part of film dielectric function of ZnTSPc calculated by numerical inversion of p measured before and after deposition of a film. The film thicknesses used in the calculations are indicated on each curve. It we use the criterion s2 = 0 near 3 eV, we obtain 76 A as the correct thickness for this film.
Once the film thickness is known, the dielectric function of the film can be calculated for all photon energies in a spectrum because there are now only two unknowns; s1 and E*.If our assumption that s2 is zero around 3 eV is incorrect, the correct thickness would be larger; so our calculated thicknesses represent the smallest possible. However, we have used this procedure for all films in order to compare the results for different films. 4.
RESULTS AND ANALYSIS
Figure 2 shows how the thickness depends on the concentration of TSPc in the solution which was used to prepare the films. The dependence is linear and the reproducibility is such that a film can be prepared with a precision in film thickness of *4x. In Fig. 3 we show the dielectric functions of films of NiTSPc, CuTSPc and ZnTSPc calculated from ellipsometric spectra as described above. Recall that with ellipsometry the full optical response is determined and both the real and the imaginary parts of E are determined without the use of Kramer-Kroniger transformation. E can be converted to the complex refractive index N by N = n+ik = el”. At
184
J. MARTENSSON,
H. ARWIN
100
80
= 20 -
0 0
10
2 Concentration
2. Thickness c.5. concentration NiTSPc; ZnTSPc: tit obtained is discussed in
=
CuTSPc
+ NiTSPc A ZnTSPc
TSPc in by least-squares
(mglml] to spin
x. C‘uTSPc;
+.
regression.
nm eV) obtain = fi0.19 CuTSPc is be with fi0.23 by et From we also the coefficient using = where is wavelength. most data the are in of we plotted in 3. results qualitatively similar those from measurements Schechtman Spicer of the studied layers Ed has a at around (Q band)r’ or two broader in the UV region (B N bands). Q band is to rt -+ K* transitions in 18 electron ring. UV region contains a of different transitions also of the metal 4 shows E for of NiTSPc with thicknesses A and 81.8 A respectively. These films in this work. There noticeable difference in both of the 2 peak as well in of for of Table I In the further we focus on Q band in films of different By means of derivation and fitting of mathematical line shape Q band be resolved into three we denote L, and is illustrated in Fig. 5 where we also of fit is much worse if only if smaller photon energy is used in order improve We have used
OPTICAL
CHARACTERIZATION
185
OF THIN FILMS OF SOME PCS
/’
.o
bx a
// I’
/ ,--.
/’
CuTSPc
,I ‘_
./I
0.5 .t
ss.oA -1.5
(b)
ENERGY IsVl
(4
ENERGY feVI
Fig. 3. Ellipsometrically determined dielectric function E = E, +ie, and absorption of (a) NiTSPc (81.8 A),(b) CuTSPc (86.0 A) and(c) ZnTSPc (83.5 A).
coefficient
d of films
186
J. MARTENSSON.
,
,
I
H. ARWIN
,
NiTsPc
I
I
I
I
2.5
3.0
3.5
4.0
ENERGY IeV)
NiTsPc
2.5
3.0
3.5
4.0
ENERGY (eV)
(b)
Fig. 4. The ellipsometrically determined (a) real part and (b) imaginary NiTSPc for two films with thicknesses of 15.0 and 81.8 A.
part of the dielectric
function
of
of the function N
Ai exp(iOi) L(E) = izl (E-_E,i+iri)m1’2 and fitted it to the second derivative
+ B(E) of the measured
(2) spectra. In eqn. (2), E stands for
OPTICAL
CHARACTERIZATION
187
OF THIN FILMS OF SOME PCS
I
I
Best fit
I
I
1.5
I
I
n
I
I
2.0
2.5
t ”
1.5
2.0
1
ENERGY (eV) Fig. 5. (a) Second derivative of the imaginary (right) using eqn. (2) with the three resonances 1.86 and 1.97 eV.
part of 8 with respect to photon energy (left). (b) The best fit 1.77,1.88 and 2.OOeV: - --, best fit using the two resonances
photon energy, E,i is the photon energy for the ith resonance, Ai its relative oscillator strength, Ti its linewidth and Oi a phase factor. B(E) is a background function which we assume to vary slowly with E and therefore neglect in the derivative analysis. We can fit up to four resonance lines simultaneously. The goodness of fit is quantified in terms of the “sum of squares” (often called x2) between data and modei13. This technique of line shape analysis has been further described elsewhere14*“. In Fig. 3 the energy position of the three lines are marked and Table I shows a typical parameter set-up obtained for three thick films and one thin film. At present TABLE
I
TYPICAL
PARAMETER
SET-UP
FOR LINE SHAPE
ANALYSIS
Resonance
CuTSPc CuTSPc CuTSPc NiTSPc NiTSPc NiTSPc NiTSPc NiTSPc NiTSPc ZnTSPc ZnTSPc ZnTSPc
86.0 86.0 86.0 81.8 81.8 81.8 15.0 15.0 15.0 83.5 83.5 83.5
0
E, (W 2.10 7.33 9.09 5.67 15.60 14.91 10.96 28.77 29.48 12.70 15.23 18.67
1.813 1.972 2.191 1.834 1.979 2.186 1.824 1.977 2.128 1.772 1.884 2.001
48 59 55 75 88 85 52 95 68 56 67 74
5.98 0.82 3.86 6.25 1.07 3.57 0.58 1.62 4.03 0.62 2.17 4.30
188
J. MARTENSSON,
H. ARWIN
2.18
F
2.16
L 6 z 5
2.14 NiTSPc
60
60
40 Thickness
1 100
[A]
CuTSPc
6) Fig. 6. Resonance
Thickness energy KS.thickness
for resonance
[A]
L, for (a) NiTSPc and (b) CuTSPc.
we have no procedures for obtaining correlation coefficients and confidence intervals. We estimate empirically that the error limit in E,, is of the order of 10 meV and the error limit in Ti of the order of 5 meV. If the Q band analysis is repeated for films of different thicknesses, we find that obtained parameter values are independent of film thickness for ZnTSPc. For
OPTICAL CHARACTERIZATION
OF THIN FILMS OF SOME PCS
189
CuTSPc and NiTSPc, one of the three resonances (LJ is lower in energy for thinner films, while the other two resonances (L, and L2) are independent of the film thickness. This result is further illustrated in Fig. 6, which shows the relation between film thickness and the resonance L, for NiTSPc and CuTSPc. The resonance energy of L, depends approximately linearly on thickness in this region. The slope of the curves are 9 meV A -I and 7 meV 8, for NiTSPc and CuTSPc respectively. 5.
DISCUSSION
5.1. Filmpreparation
The preparation of thin films of TSPc with spin casting is fast and simple. Our results also show that the film thickness is reproducible to within +4x. The intersection with the ordinate is slightly negative. This may be either an offset in concentration caused by loss of material due to cell wall adsorption in test tubes or a sigmoidal deviation with its origin in methodological effects, such as anisotropy as discussed below. An important question is whether the film quality is different for films prepared by the spinning technique from that of films prepared by more commonly used methods such as crystallization, sublimation and the LangmuirBlodgett technique. In our work we have used ellipsometry to characterize the films, and the information that we obtain is limited to what can be observed in the optical response interpreted in an assumed optical model. 5.2. Methodological efects
We have found very few reports on ellipsometric measurements on thin films of PC in the literature. Ritz and Lt.ithi6 have determined spectra of monolayers of copper phthalocyanine (CuPc) on GaP, and Arwin and Aspnes” have studied freebase PC. Barret et al. lo have done single- wa velength ellipsometry on CuPc films up to a thickness of 400 nm. They addressed the important question of whether the homogeneous isotropic three-phase model can be used in the interpretation of ellipsometric data or not. They found that films less than 100 nm thick can be satisfactorily represented by a single homogeneous isotropic layer, while thicker films appear to be equivalent to an isotropic inner layer and an anisotropic outer layer. It is rather obvious that each molecule has an intrinsic optical anisotropy due to its planar geometrical structure and the presence of 18 delocalized n: electrons. The approach using an isotropic model would therefore be valid only if the molecular ,orientation in the films were random. However, in our case we have very limited information about the detailed microstructure. Most probably our films are amorphous or of the a-type polymorph. The similarity between our ellipsometric spectra and absorption spectra in the literature” suggests that we are dealing with the c1 form. Even if our films are crystalline locally, they may very well be polycrystalline with a random orientation of the grains in the films. In our work we have used the isotropic model, simply because this is the best we can do at present. We have not taken account of surface roughness. It is also important to realize that our data area averages over a large area of the sample.
190
J. MiRTENSSON.
H. ARWIN
Another question ofmethodological nature is whether the assumption that K*is zero between the Q and B band can be justified or not. We use this as a criterion for determination of film thickness. If >:2has a finite value. our calculated thicknesses and dielectric functions deviate from the correct values. Most data in the literature indicate that most PCS have a small absorption in this photon energy region. One exception is the vapour phase absorption spectra determined by Edwards and Gouterman”. However, the errors possibly introduced by our interpretation procedure are systematic in nature and do not affect our qualitative conclusions. When it comes to analysis in terms of line shape fitting, the choice of line shape function should be discussed. The mathematical function that we use is normally used for analysing critical points in crystalline semiconductors or metals. In its quantum mechanical derivation, a square root behaviour of the density of states close to a band edge in three-dimensional symmetry is found. Broadening is introduced phenomenologically with the parameter r. The line shape function given by eqn. (2) is an approximation in this theory which is valid in the vicinity of a band edge (we call it resonance). In fact it is of a lorenzian type similar to what can be derived in the classical spring-ball model. As curve fitting is carried out on the second derivatives, the weighting of the data is further narrowed down to a small region around the resonance. Of course, other mathematical functions could be used. However, the main differences would be in the definition of the line parameters. The most important parameter. the resonance energy. would probably be rather independent of model. Finally we would like to point out that the resonance energies that we determine are parameters in a physical model attempting to mimic reality. These energies are therefore not identical with the energy positions of”peaks” or “bumps” in the dielectric function spectra. Furthermore, if one compares our spectra with absorption spectra in the literature, one must be aware of the possible shifts in peak positions due to transformation from dielectric function to absorption coefficient. Mathematically, this transformation is described by ‘x = (2n/in)ti,. where one can see that, compared with the physical parameter Lo, the observable parameter #LX is weighted with the wavelength j_ and the real part of the refractive index n. This effect is clearly seen if z and i: are compared in Fig. 3(b).
Here we limit our discussion to the analysis of the Q band for the different TSPcs and to the dependence of the optical response on film thickness. Differences can, however, also be seen in the B band at around 3.8 eV and it is also possible to discuss part of the N band. When analysing the Q band, we find that a model with two resonances gives a very poor fit to the data while three resonances give an excellent fit (see Fig. 5). In most reports on the optical properties of PCS, the Q band is found to be composed of only two resonances, denoted Q(O,O) and Q( 1.0) I’. with Q(O,O) lower in energy. In iron phthalocyanine, three resonances have been observed’2,20. Absorption spectra of metal-free PC have also been resolved into three resonances’6. This has been explained as a splitting of Q(O,O) due to a lower symmetry (D2,, instead of D,,)“. It should also be mentioned that, in most absorption data in the literature, Q(O,O)
OPTICAL
CHARACTERIZATION
OF THIN FILMS OF SOME PCS
191
normally is dominating and Q(l,O) appears as a shoulder on the blue side of the Q band. In our spectra we have a shoulder on the red side of the Q band. This was also found by Schechtman and Spicer I1 . Furthermore, Q(O,O) has been assigned to originate from monomeric structures while Q( 1,O)has a dimeric origin. However, it is clear that, in all our spectra, Q(O,O)is rather weak, which is in agreement with the observation that, in aqueous solutions, Q(O,O) decreases in magnitude as the concentration increases. A film may be regarded as a highly concentrated “solution”. However, there is no doubt that we observe three and not two resonances in the Q band. Probably L, is identical with Q(l,O), and L, and LZ together constitute Q(O,O).A resonance splitting of Q(O,O)may occur when going from the free molecule to the solid state. Such splitting has been discussed in terms of dimer formation for metal-free PC by Sharp and Lardoni8. The effect of layer thickness on optical constants of CuPc has been studied by Ritz and Liith16. For very thin layers (below monolayer coverage) they observed large shifts in transition energies and a sharpening of the spectra. They attribute this to a decrease in intermolecular interaction and an increase in molecule-substrate interaction. In our case the films are well above monolayer coverage and the differences must be discussed mainly in terms of changes in intermolecular interactions. The fact that the energy of L, is lower for thinner films explains why E for those films look sharper (see Fig. 4). The shoulder (L,) on the red side of the Q band also seems to be more pronounced in Eof the thinner films. Quantitatively this can be seen in Table I, where r for Li decreases from 75 meV for a 8 1.8 A NiTSPc film to 52 meV for a 15.0 i( film. This would indicate that thin films are more dilute (porous) and more monomer like. However, the overall increase in polarizability ei for thin films indicates the opposite. We think therefore that the differences may be due to polymorphic differences (~1form vs. /3 form) between thin and thick films. Another possible explanation would be differences in ordering (orientation of grains), giving rise to anisotropic effects as discussed above. 6.
CONCLUSION
Thin films (less than 100 A) of water-soluble metallo-phthalocyanines can be prepared by spin casting on gold substrates. The film thickness shows a linear dependence on the phthalocyanine concentration of spun solution. The Q band can be resolved into three resonances. For one of the three resonances, the energy depends on film thickness for CuTSPc and NiTSPc. This resonance is assigned to the Q( 1,0) vibronic band. The other two are assumed to be the split Q(O,O)band. ACKNOWLEDGMENTS
We would like to thank Clovis A. Linkous at Brookhaven National Laboratory for providing the phthalocyanine compounds used in this study, and Ingemar Lundstrom for valuable comments on the manuscript. Financial support has been obtained from the National Swedish Board for Technical Development.
192
J. MARTENSSON,
H. ARWIN
REFERENCES
2
5 6
8 9
10 II 12 13 14 15 16 17 IX 19 20
T. W. Barret, Thin Solid Films. 10-7( 1983) 23 I. Bott T. Jones, Acrua/or.v. (1984) Y 27. V. P. M. and D. Mol. Liy. 134 137. A. and F. J. Phy.v.. (3) 1350. F. and A. Thin 165 83. P. C. and C. Thin 160 43 J. Weber D. Busch, C’hrm.. (1965) D. Aspnes A. Studna. Op/., (1975) R. A. and M. E/lip.sonw/ry Polurized North-Holland, 1977. A. Z. M. Humphreys R. Thin Films. (1975) B. Schechtman W. Spacer. Mol. 33 2X. Edwards M. J. Sprc~rrorc~.. (1970) P. Bevington, Reduction Error for Plz~sicrd McGraw-Hill. York, H. Arwin. R. Jansson and J. Martensson, to be published. D. E. Aspnes. .Sur/: SC;.. 37(1973)418. A. Ritz and H. Liith, Appl. Phys. A. 3/(19X3) 75. H. Arwin and D. E. Aspnes, Thin Solid Fihm. IM (1986) 195. J. H. Sharp and M. Lardon, J. Phys. Chm.. 72 (1968) 3230. B. Simic-Glavaski, S. Zecevic and E. Yeager. J. Ekctrounal. Chum.. /SO (1983) 469. H. Laurs and G. Heiland, Thin Solid Fihm. 149 (1987) 129.
AND RELATED
181
FILMS
OPTICAL CHARACTERIZATION OF THIN FILMS OF SOME PHTHALOCYANINES BY SPECTROSCOPIC ELLIPSOMETRY J. MARTENSSON
AND H. ARWIN
Laboratory of Applied Physics, Department University. S-5$1 83 Linkiiping (Sweden) (Received June 12, 1989; accepted
October
of Physics and Measurement
Technology.
LinkSping
31. 1989)
Thin films of water-soluble tetrasulphonated phthalocyanines containing copper, nickel and zinc have been made by spin casting on gold. The dielectric function of films with a thickness less than 1008, have been measured with a rotating-analyser spectroscopic ellipsometer in the photon energy range 1.5-4.5 eV. The 2 eV 7telectron band has been analysed with respect to resonance energies with line shape analysis. The band could be resolved into three resonances; one of them is shifted towards lower energy as the film thickness decreases.
1.
INTRODUCTION
The phthalocyanines (PCS)are well known for their semiconducting properties, their stability and the possibility of modifying their conductivity optically’ or chemically with gases2. The latter property make them very interesting as potential gas sensors3. In a recent paper by Temofonte and Schoch4, futher references to gas sensor applications of PCs can be found. However, most of the work on gas sensitivity has been focused on conductivity changes. Recently some results on gasinduced changes in optical response of PC have been published. The techniques used were IR absorption’ and surface plasmon resonance6. The work presented here is part of our basic research on thin organic films with emphasis on the effects of oxidizing gases on their optical properties. The development of suitable film preparation techniques and methods for determination of fundamental film properties is of large importance. Specifically we have chosen to investigate the spinning technique for fast and simple film preparation. The thickness and optical response of the films were determined with spectroscopic ellipsometry and further analysed in optical models. It is of special interest to know how the film thickness depends on preparation parameters, and whether the optical response depends on film thickness. Our results on gas-induced changes in properties of thin films of PC will be presented elsewhere. 004~6090/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
.I. MARTENSSON,
182 2.
EXPERIMENTAL
H. ARWIN
DETAILS
The substrates were made by sequential vacuum deposition ofchromium (60 A) and gold (2500 A) onto glass slides. The chromium layer serves as an adhesion promoter between gold and’glass. Immediately before the substrates were used, they were washed in a mixture of H,O, H,O, and NH,’ (ratio, 5:l:l) at 80°C for 5 min. Tetrasulphonated phthalocyanine (TSPc) containing the metal ions copper, nickel and zinc (CuTSPc, NiTSPc and ZnTSPc respectively) were obtained from Clovis A. Linkous at Brookhaven National Laboratory. They were synthesized with the method described by Weber and Busch’. Water (Millipore filtered) solutions of TSPc with concentrations in the range 2.0-10.0 mgml-’ were prepared. Films of TSPc were made by spinning these solutions at 4000 rev min ’ onto the substrates. In this work we have prepared and studied films with thicknesses from 15 to 90 A. The measurements were carried out in air at room temperature (2Ok 1 “C) with a rotating-analyser spectroscopic ellipsometerE, operated in the photon energy range 1.5-4.5 eV (825-275 nm). Our ellipsometric spectra consist of measurements taken at 256 equidistant photon energies. The angle of incidence was 68’. The spot size on the sample was approximately 4 mm x 4 mm. An ellipsometer measures the changes in polarization state for the reflection of polarized light on a sample at oblique incidence. The measured quantity is the complex reflectance ratio p. The physical parameter of interest is then obtained by numerical interpretation in a suitable optical model (see next section). A more complete description of the theory of ellipsometry can be found elsewhere’. 3.
INTERPRETATION
the complex reflectance ratio In a rotating-analyzer ellipsometer, p = RJR, = tan I(/exp(id), (where R, and R, are the complex reflectances (at oblique incidence) for light polarized parallel and perpendicular respectively to the plane of incidence) is determined in the form of tan Ic/and cos d from the phase and amplitude of the photomultiplier signals. In the interpretation of the film properties we must know the complex dielectric function E, for the substrate. In a typical experiment we therefore measure the spectrum of p both before and after deposition of a TSPc film on a gold substrate. From p measured before film deposition, E, can be calculated in the two-phase model (ambient-substrate). If the surface is assumed to be perfectly smooth and film free, E, is obtained as E, = E, sin2 & {l +(ztan&y}
(1)
where E, (here equal to 1) is the dielectric constant of the ambient and 4,, the angle of incidence. After deposition of a film of TSPc, a new spectrum was taken on the same sample. We use numerical inversion in the three-phase model (ambient-filmsubstrate) to calculate the film properties. As we have three unknowns (the film thickness and the real and imaginary parts of the dielectric function of the film given by, E = E~+ iE2) but only two measured data (tan $ and cos A), we use the following
OPTICAL
CHARACTERIZATION
OF THIN FILMS OF SOME PCS
183
strategy. E is calculated by standard numerical inversion techniques for a series of film thicknesses. The correct thickness is then chosen as that for which s2 is zero in the PC spectral window near 3 eV. This means that the layer is assumed to be transparent around this photon energy. With this method, which is illustrated in Fig. 1, it is possible to determine the thickness with a precision of approximately 0.50/dof thickness.
I 2.0
I
I
2.5
3.0
3
PHOTON ENERGY (ev) Fig. 1. The imaginary part of film dielectric function of ZnTSPc calculated by numerical inversion of p measured before and after deposition of a film. The film thicknesses used in the calculations are indicated on each curve. It we use the criterion s2 = 0 near 3 eV, we obtain 76 A as the correct thickness for this film.
Once the film thickness is known, the dielectric function of the film can be calculated for all photon energies in a spectrum because there are now only two unknowns; s1 and E*.If our assumption that s2 is zero around 3 eV is incorrect, the correct thickness would be larger; so our calculated thicknesses represent the smallest possible. However, we have used this procedure for all films in order to compare the results for different films. 4.
RESULTS AND ANALYSIS
Figure 2 shows how the thickness depends on the concentration of TSPc in the solution which was used to prepare the films. The dependence is linear and the reproducibility is such that a film can be prepared with a precision in film thickness of *4x. In Fig. 3 we show the dielectric functions of films of NiTSPc, CuTSPc and ZnTSPc calculated from ellipsometric spectra as described above. Recall that with ellipsometry the full optical response is determined and both the real and the imaginary parts of E are determined without the use of Kramer-Kroniger transformation. E can be converted to the complex refractive index N by N = n+ik = el”. At
184
J. MARTENSSON,
H. ARWIN
100
80
= 20 -
0 0
10
2 Concentration
2. Thickness c.5. concentration NiTSPc; ZnTSPc: tit obtained is discussed in
=
CuTSPc
+ NiTSPc A ZnTSPc
TSPc in by least-squares
(mglml] to spin
x. C‘uTSPc;
+.
regression.
nm eV) obtain = fi0.19 CuTSPc is be with fi0.23 by et From we also the coefficient using = where is wavelength. most data the are in of we plotted in 3. results qualitatively similar those from measurements Schechtman Spicer of the studied layers Ed has a at around (Q band)r’ or two broader in the UV region (B N bands). Q band is to rt -+ K* transitions in 18 electron ring. UV region contains a of different transitions also of the metal 4 shows E for of NiTSPc with thicknesses A and 81.8 A respectively. These films in this work. There noticeable difference in both of the 2 peak as well in of for of Table I In the further we focus on Q band in films of different By means of derivation and fitting of mathematical line shape Q band be resolved into three we denote L, and is illustrated in Fig. 5 where we also of fit is much worse if only if smaller photon energy is used in order improve We have used
OPTICAL
CHARACTERIZATION
185
OF THIN FILMS OF SOME PCS
/’
.o
bx a
// I’
/ ,--.
/’
CuTSPc
,I ‘_
./I
0.5 .t
ss.oA -1.5
(b)
ENERGY IsVl
(4
ENERGY feVI
Fig. 3. Ellipsometrically determined dielectric function E = E, +ie, and absorption of (a) NiTSPc (81.8 A),(b) CuTSPc (86.0 A) and(c) ZnTSPc (83.5 A).
coefficient
d of films
186
J. MARTENSSON.
,
,
I
H. ARWIN
,
NiTsPc
I
I
I
I
2.5
3.0
3.5
4.0
ENERGY IeV)
NiTsPc
2.5
3.0
3.5
4.0
ENERGY (eV)
(b)
Fig. 4. The ellipsometrically determined (a) real part and (b) imaginary NiTSPc for two films with thicknesses of 15.0 and 81.8 A.
part of the dielectric
function
of
of the function N
Ai exp(iOi) L(E) = izl (E-_E,i+iri)m1’2 and fitted it to the second derivative
+ B(E) of the measured
(2) spectra. In eqn. (2), E stands for
OPTICAL
CHARACTERIZATION
187
OF THIN FILMS OF SOME PCS
I
I
Best fit
I
I
1.5
I
I
n
I
I
2.0
2.5
t ”
1.5
2.0
1
ENERGY (eV) Fig. 5. (a) Second derivative of the imaginary (right) using eqn. (2) with the three resonances 1.86 and 1.97 eV.
part of 8 with respect to photon energy (left). (b) The best fit 1.77,1.88 and 2.OOeV: - --, best fit using the two resonances
photon energy, E,i is the photon energy for the ith resonance, Ai its relative oscillator strength, Ti its linewidth and Oi a phase factor. B(E) is a background function which we assume to vary slowly with E and therefore neglect in the derivative analysis. We can fit up to four resonance lines simultaneously. The goodness of fit is quantified in terms of the “sum of squares” (often called x2) between data and modei13. This technique of line shape analysis has been further described elsewhere14*“. In Fig. 3 the energy position of the three lines are marked and Table I shows a typical parameter set-up obtained for three thick films and one thin film. At present TABLE
I
TYPICAL
PARAMETER
SET-UP
FOR LINE SHAPE
ANALYSIS
Resonance
CuTSPc CuTSPc CuTSPc NiTSPc NiTSPc NiTSPc NiTSPc NiTSPc NiTSPc ZnTSPc ZnTSPc ZnTSPc
86.0 86.0 86.0 81.8 81.8 81.8 15.0 15.0 15.0 83.5 83.5 83.5
0
E, (W 2.10 7.33 9.09 5.67 15.60 14.91 10.96 28.77 29.48 12.70 15.23 18.67
1.813 1.972 2.191 1.834 1.979 2.186 1.824 1.977 2.128 1.772 1.884 2.001
48 59 55 75 88 85 52 95 68 56 67 74
5.98 0.82 3.86 6.25 1.07 3.57 0.58 1.62 4.03 0.62 2.17 4.30
188
J. MARTENSSON,
H. ARWIN
2.18
F
2.16
L 6 z 5
2.14 NiTSPc
60
60
40 Thickness
1 100
[A]
CuTSPc
6) Fig. 6. Resonance
Thickness energy KS.thickness
for resonance
[A]
L, for (a) NiTSPc and (b) CuTSPc.
we have no procedures for obtaining correlation coefficients and confidence intervals. We estimate empirically that the error limit in E,, is of the order of 10 meV and the error limit in Ti of the order of 5 meV. If the Q band analysis is repeated for films of different thicknesses, we find that obtained parameter values are independent of film thickness for ZnTSPc. For
OPTICAL CHARACTERIZATION
OF THIN FILMS OF SOME PCS
189
CuTSPc and NiTSPc, one of the three resonances (LJ is lower in energy for thinner films, while the other two resonances (L, and L2) are independent of the film thickness. This result is further illustrated in Fig. 6, which shows the relation between film thickness and the resonance L, for NiTSPc and CuTSPc. The resonance energy of L, depends approximately linearly on thickness in this region. The slope of the curves are 9 meV A -I and 7 meV 8, for NiTSPc and CuTSPc respectively. 5.
DISCUSSION
5.1. Filmpreparation
The preparation of thin films of TSPc with spin casting is fast and simple. Our results also show that the film thickness is reproducible to within +4x. The intersection with the ordinate is slightly negative. This may be either an offset in concentration caused by loss of material due to cell wall adsorption in test tubes or a sigmoidal deviation with its origin in methodological effects, such as anisotropy as discussed below. An important question is whether the film quality is different for films prepared by the spinning technique from that of films prepared by more commonly used methods such as crystallization, sublimation and the LangmuirBlodgett technique. In our work we have used ellipsometry to characterize the films, and the information that we obtain is limited to what can be observed in the optical response interpreted in an assumed optical model. 5.2. Methodological efects
We have found very few reports on ellipsometric measurements on thin films of PC in the literature. Ritz and Lt.ithi6 have determined spectra of monolayers of copper phthalocyanine (CuPc) on GaP, and Arwin and Aspnes” have studied freebase PC. Barret et al. lo have done single- wa velength ellipsometry on CuPc films up to a thickness of 400 nm. They addressed the important question of whether the homogeneous isotropic three-phase model can be used in the interpretation of ellipsometric data or not. They found that films less than 100 nm thick can be satisfactorily represented by a single homogeneous isotropic layer, while thicker films appear to be equivalent to an isotropic inner layer and an anisotropic outer layer. It is rather obvious that each molecule has an intrinsic optical anisotropy due to its planar geometrical structure and the presence of 18 delocalized n: electrons. The approach using an isotropic model would therefore be valid only if the molecular ,orientation in the films were random. However, in our case we have very limited information about the detailed microstructure. Most probably our films are amorphous or of the a-type polymorph. The similarity between our ellipsometric spectra and absorption spectra in the literature” suggests that we are dealing with the c1 form. Even if our films are crystalline locally, they may very well be polycrystalline with a random orientation of the grains in the films. In our work we have used the isotropic model, simply because this is the best we can do at present. We have not taken account of surface roughness. It is also important to realize that our data area averages over a large area of the sample.
190
J. MiRTENSSON.
H. ARWIN
Another question ofmethodological nature is whether the assumption that K*is zero between the Q and B band can be justified or not. We use this as a criterion for determination of film thickness. If >:2has a finite value. our calculated thicknesses and dielectric functions deviate from the correct values. Most data in the literature indicate that most PCS have a small absorption in this photon energy region. One exception is the vapour phase absorption spectra determined by Edwards and Gouterman”. However, the errors possibly introduced by our interpretation procedure are systematic in nature and do not affect our qualitative conclusions. When it comes to analysis in terms of line shape fitting, the choice of line shape function should be discussed. The mathematical function that we use is normally used for analysing critical points in crystalline semiconductors or metals. In its quantum mechanical derivation, a square root behaviour of the density of states close to a band edge in three-dimensional symmetry is found. Broadening is introduced phenomenologically with the parameter r. The line shape function given by eqn. (2) is an approximation in this theory which is valid in the vicinity of a band edge (we call it resonance). In fact it is of a lorenzian type similar to what can be derived in the classical spring-ball model. As curve fitting is carried out on the second derivatives, the weighting of the data is further narrowed down to a small region around the resonance. Of course, other mathematical functions could be used. However, the main differences would be in the definition of the line parameters. The most important parameter. the resonance energy. would probably be rather independent of model. Finally we would like to point out that the resonance energies that we determine are parameters in a physical model attempting to mimic reality. These energies are therefore not identical with the energy positions of”peaks” or “bumps” in the dielectric function spectra. Furthermore, if one compares our spectra with absorption spectra in the literature, one must be aware of the possible shifts in peak positions due to transformation from dielectric function to absorption coefficient. Mathematically, this transformation is described by ‘x = (2n/in)ti,. where one can see that, compared with the physical parameter Lo, the observable parameter #LX is weighted with the wavelength j_ and the real part of the refractive index n. This effect is clearly seen if z and i: are compared in Fig. 3(b).
Here we limit our discussion to the analysis of the Q band for the different TSPcs and to the dependence of the optical response on film thickness. Differences can, however, also be seen in the B band at around 3.8 eV and it is also possible to discuss part of the N band. When analysing the Q band, we find that a model with two resonances gives a very poor fit to the data while three resonances give an excellent fit (see Fig. 5). In most reports on the optical properties of PCS, the Q band is found to be composed of only two resonances, denoted Q(O,O) and Q( 1.0) I’. with Q(O,O) lower in energy. In iron phthalocyanine, three resonances have been observed’2,20. Absorption spectra of metal-free PC have also been resolved into three resonances’6. This has been explained as a splitting of Q(O,O) due to a lower symmetry (D2,, instead of D,,)“. It should also be mentioned that, in most absorption data in the literature, Q(O,O)
OPTICAL
CHARACTERIZATION
OF THIN FILMS OF SOME PCS
191
normally is dominating and Q(l,O) appears as a shoulder on the blue side of the Q band. In our spectra we have a shoulder on the red side of the Q band. This was also found by Schechtman and Spicer I1 . Furthermore, Q(O,O) has been assigned to originate from monomeric structures while Q( 1,O)has a dimeric origin. However, it is clear that, in all our spectra, Q(O,O)is rather weak, which is in agreement with the observation that, in aqueous solutions, Q(O,O) decreases in magnitude as the concentration increases. A film may be regarded as a highly concentrated “solution”. However, there is no doubt that we observe three and not two resonances in the Q band. Probably L, is identical with Q(l,O), and L, and LZ together constitute Q(O,O).A resonance splitting of Q(O,O)may occur when going from the free molecule to the solid state. Such splitting has been discussed in terms of dimer formation for metal-free PC by Sharp and Lardoni8. The effect of layer thickness on optical constants of CuPc has been studied by Ritz and Liith16. For very thin layers (below monolayer coverage) they observed large shifts in transition energies and a sharpening of the spectra. They attribute this to a decrease in intermolecular interaction and an increase in molecule-substrate interaction. In our case the films are well above monolayer coverage and the differences must be discussed mainly in terms of changes in intermolecular interactions. The fact that the energy of L, is lower for thinner films explains why E for those films look sharper (see Fig. 4). The shoulder (L,) on the red side of the Q band also seems to be more pronounced in Eof the thinner films. Quantitatively this can be seen in Table I, where r for Li decreases from 75 meV for a 8 1.8 A NiTSPc film to 52 meV for a 15.0 i( film. This would indicate that thin films are more dilute (porous) and more monomer like. However, the overall increase in polarizability ei for thin films indicates the opposite. We think therefore that the differences may be due to polymorphic differences (~1form vs. /3 form) between thin and thick films. Another possible explanation would be differences in ordering (orientation of grains), giving rise to anisotropic effects as discussed above. 6.
CONCLUSION
Thin films (less than 100 A) of water-soluble metallo-phthalocyanines can be prepared by spin casting on gold substrates. The film thickness shows a linear dependence on the phthalocyanine concentration of spun solution. The Q band can be resolved into three resonances. For one of the three resonances, the energy depends on film thickness for CuTSPc and NiTSPc. This resonance is assigned to the Q( 1,0) vibronic band. The other two are assumed to be the split Q(O,O)band. ACKNOWLEDGMENTS
We would like to thank Clovis A. Linkous at Brookhaven National Laboratory for providing the phthalocyanine compounds used in this study, and Ingemar Lundstrom for valuable comments on the manuscript. Financial support has been obtained from the National Swedish Board for Technical Development.
192
J. MARTENSSON,
H. ARWIN
REFERENCES
2
5 6
8 9
10 II 12 13 14 15 16 17 IX 19 20
T. W. Barret, Thin Solid Films. 10-7( 1983) 23 I. Bott T. Jones, Acrua/or.v. (1984) Y 27. V. P. M. and D. Mol. Liy. 134 137. A. and F. J. Phy.v.. (3) 1350. F. and A. Thin 165 83. P. C. and C. Thin 160 43 J. Weber D. Busch, C’hrm.. (1965) D. Aspnes A. Studna. Op/., (1975) R. A. and M. E/lip.sonw/ry Polurized North-Holland, 1977. A. Z. M. Humphreys R. Thin Films. (1975) B. Schechtman W. Spacer. Mol. 33 2X. Edwards M. J. Sprc~rrorc~.. (1970) P. Bevington, Reduction Error for Plz~sicrd McGraw-Hill. York, H. Arwin. R. Jansson and J. Martensson, to be published. D. E. Aspnes. .Sur/: SC;.. 37(1973)418. A. Ritz and H. Liith, Appl. Phys. A. 3/(19X3) 75. H. Arwin and D. E. Aspnes, Thin Solid Fihm. IM (1986) 195. J. H. Sharp and M. Lardon, J. Phys. Chm.. 72 (1968) 3230. B. Simic-Glavaski, S. Zecevic and E. Yeager. J. Ekctrounal. Chum.. /SO (1983) 469. H. Laurs and G. Heiland, Thin Solid Fihm. 149 (1987) 129.