5492
J. Phys. Chem. 1995,99, 5492-5499
IR Spectroscopy Evidence for a Substrate-Dependent Organization of Sexithiophene Thin Films Vacuum Evaporated onto SiWSi and SiOdSi P. Lang,* R. Hajlaoui, F. Gamier, B. Desbat: T. Buffeteau: G. Horowitz, and A. Yassar Laboratoire des Matiriaux Mol6culaires, CNRS UPR 241, 2 rue Henry-Dunant, 94320 Thiais, France, and Laboratoire de Spectroscopie Moleculaire et Cristalline, CNRS URA 124, Universitt? de Bordeaux I, 33405 Talence, France Received: August 3, 1994; In Final Form: November 28, 1994@
The orientation of sexithiophene (6T) thin films vacuum deposited on SiOz/Si and SiWSi is studied by IR spectroscopy. IR dichroic spectra in the (C-C) and (C-H)stretching region show a strong dependence on the nature of the substrate. With SiOdSi, a large proportion (70-80%) of molecules is nearly perpendicular to the substrate plane. Annealing, or deposition on a heated substrate, increases this proportion up to 90%. By contrast, around 60% of the molecules lie on the SiWSi surface. IR calculations allowed us to determine the proportion of the perpendicular molecules, and also the relation between the tilt angle y of the molecules and the herringbone angle a,which agree with X-ray diffraction data.
SCHEME 1
I. Introduction Thiophene oligomers (nT) thin films deposited by vacuum evaporation on various substrates have been largely studied. These organic self-assembling semiconducting layers have potential applications in thin film devices such as field-effect transistor (FET) and light emitting diodes (LED).' The crystal packing of nT and polythiophene (PT) powders2and evaporated films3 has been determined from X-ray diffraction (XRD) analysis. It is worth noting that evaporated nT films are polycrystalline and that no amorphous phase is detected from XRD.192d*3Similarly, the mesoscopic structure and orientation of molecular thin films4 have been shown to depend on the chemical structure of their constituting molecules, and also on parameters of the deposition, such as the evaporation rate and the temperature of the substrate. The chemical structure of the individual molecule can be used as a driving force for inducing highly ordered films. IR vibrational spectroscopy has been used for studying the orientation of these films, while W - v i s spectroscopy has turned out to be very sensitive to the mesoscopic crystalline arrangement. The molecular exciton theory can rationalize the observed modifications (band splitting) of the spectra of solid films as compared to those of the free molecules or in solution^.^ We have recently shown that changing the temperature of the substrate during the evaporation of 6T onto Si02 substrate enhances its field-effect mobility by a factor of nearly This improvement has been correlated to a preferential orientation of the molecules with their long axis perpendicular to the substrate plane, and to a larger size of the crystallites, as deduced from XRD and scanning electron microscopy (SEM). Following this approach of the driving parameters that control the film properties, we focus here on the relation between the orientation of 6T films and the nature of the substrate. Two surfaces have been analyzed, Le., Si02 and SiWSi. The nature of the interactions between 6T molecules and the substrate is shown to strongly influence the molecular ordering. In the same way, the electronic properties of these films and their mesoscopic organization analyzed by UV-vis spectroscopy and grazing XRD depend largely on the nature of the substrate and is the object of a forthcoming paper? t Laboratoire de Spectroscopie Moleculaire et Cristalline. Abstract published in Advance ACS Abstracts, March 1, 1995. @
Ex
EY
II. Experimental Section FTIR-ATR Spectroscopy. ATR (attenuated total reflection) IR' spectra were recorded by using a Brucker IFS 48 spectrometer and a 45" incidence 3 9 ~ 1 5 ~ 0mm3 . 4 silicon crystal (48 internal reflections on each face.) The crystals were cut from n-doped silicon wafers (Siltronix; resistivity ES: 150 Q.cm). An ATR KRS-5 crystal (Specac) cut at 45" was also used. Spectra were run in a sample compartment flushed for 20 min with dry air. They were referenced to a background spectrum previously recorded on a bare crystal cleaned under the same conditions as the covered crystal. A resolution of 4 cm-' was chosen for collecting 500 scans. IR dichroic spectra were recorded with a KRS-5 aluminum wire grid polarizer (Specac), which could be rotated over 90" to produce either p-polarization (parallel to the incident plane) or s-polarization (perpendicular to the incident plane and hence parallel to both the substrate and film planes), as shown in Scheme 1. The base line was electronically adjusted to zero absorbance. Some spectra notably that of 6T in KBr pellet, were obtained by infrared transmittance absorption spectroscopy. Substrate Preparation. Silicon wafers were prepared with two different surface states: Si02 and SiWSi.* SiOz/Si Substrate. The surface of the Si wafer was carefully prepared to avoid contamination from solutions and handlings. First, the (100) oriented silicon wafers were cleaned by sonication in successive degreasing solvents (CH2C12, acetone, and methanol Rectapur) and rinsed with Ultra-pure (UP) water
0022-3654/95/2099-5492$09.00/0 0 1995 American Chemical Society
Substrate-Dependent Organization of Sexithiophene Thin Films
SCHEME 2: 6T Molecular Formula and Its Symmetry Axes
fl 's'
,s,
-jJ
(ELGA UHQ2; e = 18 MQncm). The substrates were then heated under argon up to 900 "C and oxidized for 5 min under flowing oxygen. This was followed by an annealing under argon at 1050 "C for 1 h. The samples were oxidized again under flowing oxygen during 25 min at typically 900 "C. The Si02 thickness was around 80 nm (dark-blue color). After cooling down to room temperature (RT) the SiO2/Si substrates were immediately transferred to the evaporation chamber where the 6T film was deposited under vacuum Pa). SiH/SiSubstrate. Si wafers were cleaned as described above. They were dipped into a hot solution (80 "C) of NH3/H202/ H2O (1:1:4 volume ratio, Merck Normapur) during 10 min. After rinsing with UP water, the silicon oxide was removed by a mixture of HF and NH@ (1:5 volume ratio, t = 5 min). Then, the substrate was chemically oxidized in a solution of hot HCU H202/H20 (1:1:4 volume ratio) during 10 min and rinsed with UP water. Finally, this oxide was removed by a solution of N U (40% in water) for 5 min followed by a short rinsing (30 s to 1 min).3 The wafer was then nitrogen dried. KRS-5 Substrate. When required, the KRSJ substrate was cleaned by sonication in toluene after polishing (A1203 48 h paste in EtOH). 6T Synthesis and Film Preparation. 6T was synthesized and purified according to procedures described elsewhere.' It under vacuum onto was evaporated at a high rate ( ~ 1 0 A-s-') 0 the freshly prepared substrate. The thickness of the film was typically 15-40 nm, as estimated from the absorbance of a 6T film deposited simultaneously onto a glass slide, by using a standard curve d = Aabsorbance) obtained from interference measurements. Reproducibility of Experiments. The reproducibility was quite satisfactory. However, some doubtful samples were discarded. The scatter in the data may be due to some contamination of the substrate during its transfer into the evaporating chamber and the evacuation time. Furthermore, the purity of 6T also appeared as a crucial point. The quantitative results reported hereafter are the average over six samples of SiOz/Si and SiWSi substrates; the relative error was estimated to be 10- 15% (see Quantitative Analysis section). The reproducibility was lower with the SiWSi substrate that contaminates more quickly.
111. General Consideration on IR Spectra Mode assignments and general considerations have already been reported on the IR spectra of oligothi~phenes~ and polythiophene (FT).l0 Assuming an all-trans planar conformation, the 6T molecule belongs to the symmetry group C2h (see Scheme 2). Both the modes polarized along the long (L) and short axes (M) have B, symmetry, whereas the modes polarized along the normal axis (N) have A, symmetry. In this paper, we focus on polarization dependent bands, in order to get information about the molecular orientations. These bands can be depicted as follows: 1. In the (C=C) stretching mode (1400-1500 cm-' region), three main bands appear at 1424, 1440, and 1490 cm-'. The band at 1440 cm-' also develops a shoulder at about 1460 cm-'. The first two bands have been attributed to a symmetric mode
J. Phys. Chem., Vol. 99, No. 15, 1995 5493 (S) polarized along the M a x i ~ . ~ The ~ .band ~ at 1490 cm-' corresponds to an antisymmetric mode (AS) polarized along the L axis. The band at 1460 cm-' appears to be very small in the 6T single-crystal spectrumgfwhatever the light polarization. Consequently, this shoulder is connected to the polycrystalline nature (defects, molecules at grain boundaries ) and its polarization is probably the same than those at 1424 and 1440 cm-' . We note that in general, and particularly in the bulk spectrum, the intensity of the band at 1424 cm-' is higher than that at 1440 cm-'. On some samples this order is reversed without connection with the substrate nature (Si02 or SiWSi). To date, we have not found any satisfactory explanation for this phenomenon. To analyze the orientation of the molecules, we have considered the absorption Ab (M) of the M-polarized bands (area under the bands in the 1400-1480 cm-' range) and the absorption Ab (L) of the L-polarized band (area under the bands in the 1480-1510 cm-' range). The ratio
R(C=C) = Ab(L)/Ab(M)
(1)
was evaluated for both the s- and p-polarized spectra, and also for the bulk spectra, and will be termed R,(C=C), Rp(C=C), and Rb(C=C), respectively. A greater value for R,(C=C) than for Rb(C=C) indicates that the dipolar transition moment is parallel to the s-polarization, so that the molecular L axis is parallel to the substrate. We show in the quantitative section how to evaluate the proportion of molecules parallel and perpendicular to the substrate by using the ratio Rb, = [Rb(C=C)]/[R,(C=C) A(M)/A(L)]
(2)
(the factor A(M)/A(L) 1.04 is introduced for taking into account the proportionality between the absorption and the wavelength A for ATR spectra; see Appendix). We make the usual assumption that the oscillator strength of the organic molecule is identical both in the the polycrystalline film and in KBr pellet. This hypothesis is moreover supported by the fact that the wavenumbers of the (C=C) modes are identical whatever the sample nature (in KBr, Si02, and SiWSi) within the limits of the spectra resolution of 4 cm-'. This method is simple and does not require any calculation of the electric fields E, and Ep at the fikdsubstrate interface and in the organic film." In fact, E, is perpendicular to the incident plane and hence strictly parallel to the substrate, whereas, owing to the 45" angle of the incident light, E, contains two components Epyand Epz, perpendicular and parallel to the substrate, respectively. The calculation of these three components and of the dichroic ratio R,, = R,(C=C)/R,(C=C)
(3)
strongly depends on the 6T refractive index and on the geometry of the system and will not be considered in this paper. 2. Aromatic v(C-H) stretching mode (3000-3200 cm-'). Qualitative information was obtained from the four bands a, b, c, d in the aromatic (C-H) stretching region (see Figure 2). The polarization of these bands has already been d i s c ~ s s e d ; ' ~ ~ ~ it has been shown that the couples of bands (b, d) and (a, c) have opposite polarization; recently, the ambiguity of their assignment was removed.* The bands a and d correspond to the vibrations of the C-H bond in the a positions and the bands b and c to the C-H in the @ positions. Our results show that the bands (b, d ) are L-polarized whereas (a, c) are M-polarized. 3. C-H in-plane (i.p.) (M) and out-of-plane (0.p.) (N) bending vibration (650-850 cm-'). This region was only observed in the ATR spectra obtained with the KRS-5 substrate and in the transmission spectra with Si wafers. Some ambiguity
5494 J. Phys. Chem., Vol. 99, No. 15, 1995
Lang et al.
TABLE 1: Values of Parameters and ymu Deduced from IR Data and Proportion of Standing Molecules Deduced from IR and XRD Data38
0,19
Y"(f5)
0,)s
R, RT
0.28
R,
Rps Rb,
0.42 1.5
2.82
Si02 Ann. 0.144 0.27 1.88 5.47 SiOzonHS 0.13 0.29 2.2 6.2 SiH 0.70 0.46 0.66 1.12
0,l 1 vi n
KRS-5
a
0.19
0.30 1.55 4.03
(deg) 32 24 22 -
-
*
r* 0.07 y = 10.6" y = 2 1 " 72% 86% 88% 35% 80%
80% 96% 98% 41%
90%
0,07 0,03
0,l 5 -
-0.0 1 1390
I
I
I
1430
1
I
I
I
1470
01'
-3-
1
. 0,1
1510
v)
n
w avenumberlcm - 1 Figure 1. ATR spectra for 6T (- - -) in KBr pellet (au);deposited onto
a OlO5
SiOz: (0)s-polarized; (0)p-polarized.
0104
0 0,03 -0,os 1390 0,oz
1430
1470
1510
wavenumber/cm-l
ui n
Figure 3. Annealing effect on the s-polarized ATR spectra of 6T/ SiOz: (0)no annealing; (0)annealed 200 "C; (- - -) 6T in KBr pellet
a
(au).
0,o 1
at 1420 and 1440 cm-', the band at 1490 cm-' is weaker under s-polarization than p-polarization and in the bulk product. We see that
0
-0,Ol 3000
3050
31 00
31 50
wavenumbers/cm-1 Figure 2. ATR spectra for 6T (- - -) in KBr pellet (au); deposited onto SiOz: (0)s-polarized; (0)p-polarized.
on the assignment of the bands and their polarization remains in the l i t e r a t ~ r e .9a,b ~ ~Three ~ ~ ? main bands are observed at 690 cm-' (C-H 0. p., a position), 790 cm-' (C-H in-plane or out-of-plane, or C-S-C stretch), and 830 cm-' (C-H i.p. or o.P., or C-S-C stretch). 4. Si-H stretching region (2000-2200 cm-'). These hydride vibration bands are observed on SiH substrate before and after the 6T film deposition, and after the annealing. They were used to probe the interfacial interaction between the 6T molecules and the silicon surface. Some Si-0 vibrations may also be observed in the 700- 1100 cm-' range after the removing of the adsorbed hydrides.'* IV. Results IV.1. Deposition of 6T onto Si02 Substrates. Deposition ut Room Temperature. Figure 1 shows the spectra of 6T deposited at RT onto Si02 substrate in the (C-C) stretching region for both the s- and p-polarized light, along with that of 6T in KBr pellet. No significant shift of the band wavenumbers is observed within the chosen resolution. Compared to those
The value of these ratios is given in the Quantitative Analysis section (Table 1). The above inequalities mean that the L-polarized band is lowered for an electric field parallel to the substrate. That is, the L anis is more perpendicular to the substrate. An attenuation of band b (3060 cm-', Figure 2 ) compared to band c (3075 cm-') is also observed in the aromatic (C-H) stretching region; we deduce that bands b and d are L polarized, whereas a and c are M-polarized. The effect observed on the a C-H (a, M-polarized at 3046 cm-' and d, L-polarized at 3100 cm-') is somewhat less obvious, since variations of the strong b and c distort the smaller a and d bands. This confirms however the preferential orientation of the molecules perpendicular to the substrate. Effect of Annealing. The substrate was annealed under argon for 20 min at 185-200 "C. The heating up and cooling down were performed slowly and under inert ambient. The resulting spectra are shown in -Figure 3. We observe that the band ai 1490 cm-' has almost completely disappeared in the s-polarized spectrum (Figure 3a); consequently, the Rbs ratio has strongly increased (see Table 1). A similar increase of Rps is also observed in the p-polarized spectrum (Figure 3b), but its variation is less important. Accordingly, the annealing increases the proportion of molecules that are perpendicular to the substrate.
J. Phys. Chem., Vol. 99, No. 15, 1995 5495
Substrate-Dependent Organization of Sexithiophene Thin Films
0,02 5
0,08
0,02
0,06
0,015 .0,04 ln n
io,,, a
a
0,005
0,02
0 0 .0,005
3000
3050
31 00
wavenumber/cm-1 Figure 4. Effect of the annealing on the s-polarized ATR spectra of SiOJ6T (C-H stretching region): (0)not annealed; (0)annealed; (- - -)
3000
31 50
6T in KBr pellet (au).
In the (C-H) stretch region (Figure 4), the L-polarized b and c bands have considerably decreased in the s-polarized spectrum; similar conclusions can thus be drawn, confirming the rising up of the molecules. We note that a small part (10-20%) of the 6T film has probably evaporated during the annealing stage as shown by the lowering of the M-polarized bands in both the s- and p-polarized spectra. Annealing at High Temperature. After a further annealing at 320 "C for 20 min, the shape of the (C-C) symmetric bands drastically changes: whatever the polarization of the IR light may be, the band at 1426 cm-' is lowered or shifted, leading to a broad feature centered at about 1440 cm-'. Simultaneously, the band at 1500 cm-' still decreases, particularly in the s-polarized spectrum. We cannot exclude that the 6T molecules could have chemically reacted. We note that polyhophene does exhibit a single broad band in the region of (C=C) symmetric vibrations.lOb,cHowever, the interpretation in the (C-H) stretch region is difficult; the band at 3075 cm-' (M polarized) has disappeared in both the s- and p-polarized spectra, and two bands predominate at 3064 and 3100 cm-' (broad). The film becomes partially insoluble in CH2C12, in contrast to the moderately annealed film, which confirms that either an alteration of the molecule or an important change of its structure has taken place. Deposition on Heated Substrate (HS).A high temperature of the substrate (180-200 "C) during evaporation leads to nearly the same kind of effect as annealing. The L-polarized bands have largely decreased in the s-polarized spectrum (C=C region), and the Rbs ratio (Table 1) is multiplied by a factor of 2 or 3. Figure 5 shows that the L-polarized (C-H) b and d bands have considerably decreased. Therefore, the direct deposition onto a heated substrate also induces a strong perpendicular orientation of the molecules. Moreover, the reproducibility is increased, owing to the lowering of contaminations, coming from the substrate or impurities in the 6T powder, during the deposition step. IV.2. Deposition of 6T on SiWSi Substrates. The formation and structure of SiWSi monolayers have been extensively studied.8 These layers are currently used for the reduction of SiOn/Si in silicon wafer preparation. 6T Films Deposited at Room Temperature. (C=C)and (CH) Stretch Regions. Figure 6 shows the ATR spectra in the
3050
3100
3150
wavenumber/ cm-1 Figure 5. ATR spectra of 6T deposited on a heated SiOJSi substrate (T = 200 "C): (0)s-polarized; (0)p-polarized (- - -) 6T in KBr pellet
(au).
0,18
0,13
u;
n R'b, we can conclude that the bund ut 830 cm-' is M-polarized whereas that at 790 cm-' is N-polarized. In the Quantitative Analysis section, we shall give a relation between a and 7, the angle between the substrate normal and the L axis of the molecules standing up on the substrate. Si-H Stretching Region. Si-H stretching vibrations of the monolayer can be observed in the 2000-2200 cm-' range. The assignment of the main bands can be depicted by considering three regions: (1) 2000-2090 cm-' for monohydride SiH; (2) 2100-2130 cm-' for the dihydride SiH2; (3) 2130-2170 cm-' for the trihyride SiH3. Figure 9 (p-polarized light) clearly shows that only a part of the adsorbed hydrogen has been removed from the surface by the deposition of 6T, in fact less than 15% of the surface that is covered by the organic film, taking into account that both wafer faces contribute to the ATR spectrum. The band maxima are a bit weakened, particularly those at 2082 cm-l (SiH) and 2103 cm-' (SiH2). By contrast, we observe that the absorption is higher with adsorbed 6T around 2118 cm-' (probably SiH2) and 2160 cm-' (SiH3) indicative of a shift (+6 to +8 cm-') of the band located at 2110 cm-' (SiH2) on the naked substrate. However, it is difficult to further compare the spectra, since band broadening, shifting, or weakening can occur simultaneously. A nearly identical observation can be made on the s-polarized spectrum. This again shows that the 6T molecules do not remove adsorbed hydrogen and interact quite strongly with the adsorbed hydrides (SiH,). Effect of Annealing. Depending on the sample, an annealing of 6T on SiWSi leads to two opposite effects: the thermal treatment may induce either an important tilting up of the molecules, or no significant effect at all. In the first case, the molecule tilting up (Figure 10) is evidenced by the decrease of the L-polarized bands (1490,3060,3100 cm-') and of R,. This tilting up comes along with the total removal of the SiH layer on the wafer face, where the 6T film has been deposited (Figure
Substrate-Dependent Organization of Sexithiophene Thin Films
J. Phys. Chem., Vol. 99, No. 15, 1995 5491
0,13
0,08 v)
Q:
0,03
093
1
L.0 1,6
T.0
This substrate allowed us to get ATR-IR data in the region below 1200 cm-' (Figure 12). We note that the ratio R'dR', 0.9 is lower than 1; this confirms that the band at 830 cm-I is M-polarized whereas that at 790 cm-' is N-polarized.
V. Quantitative Analysis V.l. Evaluation of the Orientation of The Molecules. Table 1 summarizes the data obtained from the comparison of polarized and bulk spectra. We have taken into account several samples and present the average data. We define r as the ratio of the number of the molecules standing up on the substrate plane n,, to that of the molecules that have their L axis parallel to the substrate nxyand r* as the ratio of n, to the total number of molecules: r = n/nq
0 1000 1100 1200 1300 1400 wavenumberdcm-1 Figure 11. ATR spectrum of annealed 6T/SiWSi substrate divided by the spectrum of unannealed 6T/SiHSi showing the presence of Si02 coming from the annealing: (0);(- - -) spectrum of surface Si02 oxide (reference SiWSi).
900
9), and with the formation of Si02 (Figure l l ) , characterized by two broad features at ca. 1050 and 1210 cm-l.I2 In the second case, the effect of annealing effect is very weak and the molecule orientation does not change at all. However, we note a partial removing of the SiH layer (around 45% of the side covered by the 6T film) together with the growth of a thin Si02 layer. Finally, we have checked that the SiH layer by itself is stable up to 200 "C. These results suggest that some defects of the 6T/SiH interface would allow the formation of Si02 during the annealing, which in turn could induce the tilting up of the molecules. The second case would correspond to a very stable 6T/SiH interface, probably better oriented. Deposition onto KRS-5 Substrate. In the (C=C) stretch region, the spectra exhibit the same polarization dependence as for the SiOz/Si substrate: the ratio R,(C=C) is smaller than Rb(C=C) (bulk 6T) and R,(C=C) (Table 1). As a consequence, the molecules are more perpendicular to the substrate.
r* = n/(n,
+ nxy)= r / ( l +
(4) I)
(5)
Two different crystal structures have been reported to date on evaporated 6T films. In the molecule L axis is strictly parallel to the crystal long axis, and the angle between the contact plane and this long cell axis is p = 100.6" or 111.3". In the other structure,2d the molecule forms an angle of around 30" with the long cell axis, and /I= 99.0". The corresponding tilt angle y of the molecule with the substrate normal is 10.6", 21.3", and 21", respectively. We note, however, that the conditions of evaporation were different in both reports and that ours (fast evaporation of 6T) are those of ref 3a. We assume that the 6T films are uniaxial and that there are effectively two molecules populations (see section IV.2) whatever the substrate nature. As a matter of fact, this hypothesis is simplifying since we could expect distributions of molecules around the averages angles. Neglecting the noncrystallized molecules (see Appendix), r can be deduced from ATR data through eq 6.
+ cos2a)/2 1 - sin2 y[Rb, + cos2(a/2)1 R,, - (1
r=
Here, Rb, is defined in eq 2.
(6)
5498 J. Phys. Chem., Vol. 99, No. 15, 1995
Lang et al.
Stating n, = 0 (Le., l / r = 0), we obtain the maximum value for 7, Ymax: sin2(ymax)= I/[&
+ cos2(a/2)1
(7)
Table 1 gives the average value of R,, R,, Rps, along Rbs, along with r* calculated with the two possible values of y . At this stage, ymaxagrees with the three values deduced from XRD analysis. Moreover, it appears that with Si02 and KRS-5 a large number of molecules (70-80%) and perpendicular to the substrate; an annealing, or the deposition onto a heated substrate, increases this ratio to ca. 90%. By contrast, with the SiH substrate, 60% of the molecules lie on the surface; this unique behavior would originate from that the molecules can interact strongly with the surface hydrides SiH (see section IV.2). With all the other substrates (Si02, KRS-5, glass [6]) used at room temperature, the crystallization seems to take place independently of the nature of the substrate. The high mobility of the molecules (as compared to the deposition rate), associated with a molecule-substrate interaction lower than the intermolecular energy, leads to a slow formation of crystallites that are weakly interacting with the substrate (i.e., on this kind of molecules, with a quite perpendicular long crystal axis). Indeed, it has been reported as a general rule that in an epitaxial layer, long macromolecules lie parallel to the substrate since their interaction with the substrate is substantially higher.13 Similarly, we notice that oligothiophene molecules deposited on Si02 are mostly lying on the surface when film deposition is carried out at low temperature (the surface mobility is lowered),3bor also when the film thickness is less than one monolayer (there is no interaction between the molecule^).^^ On Si02 substrate, the present results agree with the data reported by Egelhaaf and c o - ~ o r k e r s . ~ ~Finally, f UV-vis spectroscopy measurements on Si02 and SiWSi substrates show that not only the orientation but also the organization and film structure at a mesoscopic scale are strongly modified.6 V.2. Relation between a and y, and I R absorption; Estimation of y. Under the same assumptions as for eq 6, the following relation between a and y can be derived for transmission spectra:
Ab(830) As(790) -
R'bs
=AbOAs(830)-
+ 2r[1 - sin2(a/2) sin2 y] (8) 2 - sin' a + 2 r [ I - c0s2(a/2) sin' y] sin2 a
From (6) and (8), we can extract y : Sin2 y = [(Rbs - k)(R'b, - 1) 1
+ kR'b, + k - l]/[(kR'b, -
+ k)[& + COS2(a/2)] + (Rbs- k)[(l -k R'bs)
X
c0s2(a/2) - l])] (9)
+
where k = (1 cos2a)/2. Taking Rbs = 0.826 f 0.01 and R'bs = 0.73 f 0.01, as obtained from the transmission spectra, and since the numerator of (9) must be positive (the denominator is largely positive with approximate values of the angles), we obtain
a Isax = 62" f 2"
(10)
This result agrees with XRD analysis. If we assume that a = 62"[3a], we have sin2 y 0 f 0.06 and y < 15"
SCHEME 3: 6 E [O,WI
(1 1)
8
(ak) plane
We conclude that eq 9 is only consistent with the data of ref 3a where = 100.6". All these results in agreement with XRD data would also c o n f i i a low amount of noncrystallized molecules.
VI. Conclusion We have shown the important effect of the nature of the substrate on the orientation of 6T molecules. IR spectroscopy in the (C=C) stretching region, which is sensitive to the molecule vibrations (whatever may be their crystalline state) has allowed us to evaluate the proportion of molecules parallel and perpendicular to the surface. With SiOz/Si and KRS-5 substrates, the molecules are more normal to the surface (r' E 70-80%), whereas a reverse trend is observed on SiWSi substrates (r' E 40%). In this last case, the adsorbed hydrogen is not displaced by 6T, which interacts quite strongly with the adsorbed hydrides. The feature of 6T/SiWSi, in regard to annealing, depends on the density of defects. On silicon surfaces, the annealing of the film or the deposition on a heated substrate increases the proportion of perpendicular molecules (90- loo%), in agreement with previous XRD data. The orientation of the molecules parallel to the substrate is associated with a large interaction of 6T with the substrate, whereas a perpendicular orientation reveals a sufficient molecules mobility on the surface, together with a weak interaction with the substrate. In this regard, the deposition temperature induces similar effects. These results are qualitatively confiied by UV-vis studies, which in addition indicate important structural differences6as a function of the nature of the substrate. Relationships between IR data and structural parameters a and y have been established, the results agreeing with XRD data obtained with film prepared under identical conditions. Further work is actually devoted to the determination of the structure of 6T deposited on SiWSi and to the control of the substrate surface for orienting the molecules either on the substrate plane or normal to it. Appendix We derive here the absorbance of L-, M-, and N-polarized transitions of ATR spectra with only the E, component of the electric field. L, M, and N are the transition moments corresponding to the three molecular axes. As inferred from XRD,3awe assume that there are two molecular populations in the 6T film, one standing up (1) and the other lying (2) on the substrate plane whatever the substrate nature; we also assume that the film is uniaxial. Population 1 (concentration n,) has its (a,b) crystal plane parallel to the substrate plane (S). Its molecular axis L forms a small angle y with the normal to the substrate (Scheme 3).
Substrate-Dependent Organization of Sexithiophene Thin Films
SCHEME 4: o E [0,24
J. Phys. Chem., Vol. 99, No. 15, I995 5499
Ab(N) = N2*E:{ (nJ4) sin2 a
+ (nJ2) [1 -
sin2(a/2) sin2 y ] } 4 N ) (A7) The ratio R', now writes, with R'b = (M/N)? R', = Ab(M)/Ab(N)=
+ 2 r [I - c0s2(a/2) sin2 yl A(M) (A8) sin2 a + 2r [ I - sin2(a/2) sin2 y ] 4N)
2 - sin2 a R'b
In population 2 (concentration nq), ,the crystal long axis c, and, according to XRD analysis, the (120) plane, are parallel to the substrate. Owing to the herringbone packing, this means that the plane of half the molecules is practically parallel to the substrate plane, the angle between them being lower than 0.5"; they also have their M axis parallel to (S). The other half has its M axis tilted by a (herringbone angle) with (S) (Scheme 4). The absorbance of a film is proportional to
Ab = n(Mt.E)2
(-41)
where Mt is the transition dipolar moment, E the electric field, and n the number of molecule^.'^ 1. ATR spectra. For an L-polarized band, the average contribution is obtained by integrating (Al) over [0,2n].
Ab(L) = [(nJ2) sin2 y
+ (nJ2)]L2E:A(M)
(A2)
For a M-polarized band, we obtain similarly
Ab(M) = [ng
+ (nJ4) ( 1 + cos2 a)]M2E:A(M)
(A3)
Here B is the average of (M.EJ2 for population 1, over the variation range of 6 (6 E [0,2n]). As cos(d2) cos y cos(d2) sin y €Asin 6 cos 6 0 ) = M E ~ c o s ( d 2 )cos y sin 6
+
sin(d2) cos 6) (A4)
After integration,
Ab(M) = [(n#
(1 - c0s2(a/2)) sin2 y
+ (nJ4) (1 +
cos2 a)]M2E:A(M) (A5) The R,(C=C) ratio is written, with Rb(C=C) = (L/W2: R,(C=C) = Ab(L)/Ab(M)= Rb(C=C)
+
X
(n/2)sin2 y ( n ~ 2 ) 4L) ( q 2 ) [ 1 - c0s2(a/2) sin2 r l + ( n ~ 2 ) ( 1 cos2 a)n(M) (A61
+
Replacing nz/nq by r leads to eq 6 of the text. Similarly, we can calculate the absorption of an N-polarized band through an integration over the range w E [0,2n] (the contribution of the nz lying molecules can be found by replacing a by ( d 2 ) - a in B )
which gives eq 9 of the text. 2. Transmission Spectra. The absorption spectrum does not depend on the wavelength. The derivation takes into account both the equivalent Ex and Ey components; the expression of r is changed into r', obtained by multiplying the right hand-side of (6) by 2. In the same way, the equivalent expression of eq 8 is obtained by replacing r by J/2. Equation 9 is still valid, because Rbs and R'bs refer to the respective contributions of E, and Ey.
References and Notes (1) Gamier, F.; Yassar, A,; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot; P. J. Am. Chem. Soc. 1993, 115, 8716, and references therein. (2) (a) van Bolhuis, F.; Wynberg, H.; Havinya, E. E.; Meijer, E. W.; Staring, E. G. J. Synth. Met. 1989, 30, 381. (b) Hotta, S.; Waragai, K. J. Mater. Chem. 1991, I , 835. (c) Gavezzotti, D.; Filipini, G. Synth. Met. 1991, 58, 1500. (d) Porzio, W.; Destri, S.; Mascherpa, M.; Briickner, SActa Polym. 1993, 266, 44, and references therein. (3) (a) Servet, B.; Ries, S.; Trotel, M.; Alnot, P.; Horowitz, G.; Gamier, F. Adv. Mater. 1993,5,461.(b) Servet, B.; Horowitz, G.; Ries, S.; Lagorsse, 0.; Alnot, P.; Yassar, A.; Deloffre, F.; Srivastava, P.; Hajlaoui, R.; Lang, P.; Gamier F. Chem. Mater. 1994, 6, 1809 (4) (a) Lazzaroni, R.; Pal, A. J.; Rossini, S.; Ruani, G.; Zamboni, R.; Taliani, C. Synth. Met. 1991, 42, 2359. (b) Egelhaaf, H. J.; Bauerle, P.; Rauer, K.; Hoffmann, V.; Oelkrug, D. J. Mol. Stmct. 1993,293,249; Synth. Met. 1993, 61, 143. (c) Horowitz, G.; Deloffre, F.; Gamier, F.; Hajlaoui, R.; Lang, P.; Yassar, A,; Alnot, P.; Ries, S.; Servet, B. 9th Eur. Hybrid Microelectonics Conf., Nice (Fr.), June 2-4, 1993 Ext. Abstr. pp 60-67. (5) (a) Eckert, R.; Kuhn,H. 2.Elekrrochem. 1960,64, 356. (b) Kuhn, H. Spectroscopy of Monolayers Assemblies, In Physical Methods of Chemistry; Weissberger, A,, Rossiter, B. W., Eds.;Wiley-Interscience: New York, 1972; Vol. 1 Part IIIB, p 593. (c) Schoeler, U.; Tews, K. H.; Kuhn, H. J. Chem. Phys. 1974, 61, 5009. (6) (a) Lang, P.; Hadjlaoui, R.; Dallas, J. P; Yassar, A,; Horowitz, G.; Gamier, F.; J. Chim. Phys., in press. (b) Lang, P.; Hadjlaoui, R.; Dallas, J. P; Yassar, A.; Horowitz, G.; Gamier, F., manuscript in preparation. (7) (a) See for example: Hanick, N. J. Zntemal Rejection Spectroscopy; Wiley: New York, 1967. Hansen, W. N. In Advances in Electrochemistry and Electrochemical Engineering; Delehay, P., Tobias, C., Eds.;Wiley: New York, 1973. (b) Bauman, R. P. Absorption Spectroscopy, John Wiley and Sons, Inc.: New York. (8) (a) Trucks. G. W.: Raehavachari. K.: Hieashi. G. S.: Chabal. Y. J. Phys. Rev. Lett. 1990, 504, 63. (b) Dumas, P.;-Chabal, Y. J ; Jakob, P. Surf: Sci. 1992, 269/270, 867. (c) Bhatta-Charya, A,; Vorst, C.; Carim, H. H. J. Electrochem. SOC. 1985, 132, 1900. (d) Gould, G.; Irene, E. A. J. Electrochem. SOC.1987,134, 1031. (e) Berrada-Gouzi, K. These Universite Paris VII, 1992. (9) (a) Furakawa, Y.; Akimoto, M.; Harada, I. Synth. Met. 1987, 18, 151. (b) Zerbi, G.; Chierichetti, B.; Ingiinas, 0. J. Chem. Phys. 1991, 94, 4637. (c) Hotta, S. J. Phys. Chem. 1991,93,4994. (d) Louam, G.; Mevellec, J. Y.; Buisson, J. P.; Lefrant, S. J. Chim. Phys. 1992, 89, 987. (e) Ehrendorfer, Ch.; Neugebauer, H.; Neckel, A,; Bauerle, P. Synth. Met. 1993, 55, 493. (0 Horowitz, G. et al., manuscript in preparation. (10) (a) Poussigue, G.; Benoit, C.; Sauvagol, J. L.; Lere-porte, J. P.; Chorro, C. J. Phys.: Condens. Matter 1991,3, 8803. (b) Neugebauer, H.; Nauer, G.; Neckel, A.; Tourillon, G.; Gamier, F.; Lang, P. J. Chem. Phys. 1984, 88, 653. (c) Hotta, S.; Soga, M.; Sonoda, N. J. Phys.Chem. 1989, 93, 4994. ( 1 1 ) Buffeteau, T.; Desbat, B.; Devaure, J.; Salimi, A.; Turlet, J. M. J. Chim. Phys. 1993, 90, 1855 and 1871. (12) Denisov. V. N.: Marvin. B. N.: Podobedov. V. B.: Sterin. K. E. Solid State Phys.' 1978, 20, 2016. (13) Wittmann, J. C.; Lotz, B. Pron. - Polym. . Sci.1990, 15, 909 and references therein. JP942015N
