Determination of Molecular Orientation in Self-Assembled Monolayers

monolayer (SAM). In the past, mainly two methods have been used to extract orientational information from IR data. The application of the absolute met...
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Determination of Molecular Orientation in Self-Assembled Monolayers Using IR Absorption Intensities: The Importance of Grinding Effects Ralf Arnold,† Andreas Terfort,‡ and Christof Wo¨ll*,† Physikalische Chemie I, Ruhr-Universita¨ t Bochum, Universita¨ tsstrasse 150, D-44801 Bochum, Germany, and Institut fu¨ r Anorganische und Angewandte Chemie, Universita¨ t Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany Received February 6, 2001. In Final Form: May 3, 2001 The orientation of organic chain molecules grafted to a metal surface is commonly determined using infrared reflection absorption spectroscopy (IRRAS). Whereas the acquisition of the IR spectrum itself is rather straightforward, the determination of orientational parameters (tilt angle, twist angle) is complicated by the requirement to use the bulk reference spectrum of the substance taken to form the self-assembled monolayer (SAM). In the past, mainly two methods have been used to extract orientational information from IR data. The application of the absolute method requires the precise knowledge of the complex refractive index, but the intensities of the IR bands are influenced by Mie scattering of light in the pellet which depends on the particle size and shape and therefore on the grinding time and conditions. On the other hand, the application of the method using relative intensities requires a second IR-active vibration with a differently oriented transition dipole moment (TDM) to eliminate the relative concentration. Both methods rely on the assumption that the individual TDMs have the same strength in the bulk as well as at the surface. We will demonstrate that this assumption is frequently not true. All these difficulties are demonstrated by the determination of molecular orientation of octadecanethiol and p-terphenylthiol on a gold surface.

Introduction The formation of stable and well-defined molecular adlayers using self-assembly is attracting a still increasing interest.1-3 In addition to rather straightforward applications for corrosion inhibition4 or lithography,5-8 the investigation of these systems has also provided new insight into the complex structure of the molecular adsorption process and the huge variety of different structures that can be formed by molecular chemisorption of molecules on solid substrates. In the two most widespread classes of self-assembled molecule/substrate combinations, namely chlorosilanes9,10 assembled onto Si substrates and organothiols11,3 assembled onto gold, the molecules of the self-assembled monolayer (SAM) can be divided into three parts: first, the anchor group, by which the molecule is tightly bound to the substrate; second, a functional group which defines the properties of the organic surface formed in the adsorption process; and third, a backbone chain (most † ‡

Ruhr-Universita¨t Bochum. Universita¨t Hamburg.

(1) Ulman, A. Self-Assembled Monolayers of Thiols; Academic Press: London, San Diego, 1998; Thin Films 24. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (4) Grundmeier, G.; Reinartz, C.; Rohwerder, M.; Stratmann, M. Electrochim. Acta 1998, 43, 165-174. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (6) Xia, Y.; Qin, D.; Whitesides, G. M. Adv. Mater. 1996, 8, 10151017. (7) Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 2059-2067. (8) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (9) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (10) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304-1312. (11) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66.

often an alkyl chain) linking these two groups together. While the adsorption process is initially driven by the binding of the anchor group to the substrate, the molecular packing in the organic adlayer is largely controlled by the steric requirements of the connecting chains and the interaction between them. In addition, the interactions between the tail groups can have significant influence on the ordering of the films.12-14 A detailed insight into the properties of the selfassembled film requires the availability of precise information on the molecular orientation within the films. Although a number of methods are available to determine the thickness of the films (such as surface plasmon resonance spectroscopy,15-18 ellipsometry,19 X-ray photoelectron spectroscopy,20 X-ray absorption spectroscopy21), a direct determination of molecular orientation is necessary for a detailed understanding of the film properties. It has been demonstrated in previous work that information on the film thickness alone is not sufficient for a reliable determination of the orientation within the films.12 (12) Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, C. Thin Solid Films 1997, 307, 9885-9893. (13) Himmel, H.-J.; Terfort, A.; Arnold, R.; Wo¨ll, C. Mater. Sci. Eng. C 2000, 8-9, 431-435. (14) Himmel, H.-J.; Terfort, A.; Wo¨ll, C. J. Am. Chem. Soc. 1998, 120, 12069-12074. (15) Meldrum, F. C.; Flath, M.; Knoll, W. Langmuir 1997, 13, 20332049. (16) Meldrum, F. C.; Flath, M.; Knoll, W. Thin Solid Films 1999, 348, 188-195. (17) Hausch, M.; Beyer, D.; Knoll, W. Langmuir 1998, 14, 72137216. (18) Rothenha¨usler, B.; Duschl, C.; Knoll, W. Thin Solid Films 1988, 159, 323-330. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (20) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (21) Ha¨hner, G.; Wo¨ll, C.; Grunze, M.; Kinzler, M.; Scheller, M. K.; Cederbaum, L. S. Phys. Rev. Lett. 1991, 67, 851-854, See also Erratum: Phys. Rev. Lett. 1992, 69, 694.

10.1021/la010202o CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001

Molecular Orientation in SAMs

The technique most frequently used for this orientation analysis is the determination of the dichroism of vibrational modes seen in infrared (IR) spectroscopy.22,23 In the case of metal substrates, however, this method suffers from a significant drawback, since the metal electrons completely screen the components of the electric field vector E parallel to the surface,24 so that only the component normal to the surface can couple to molecular vibrations. In the case of semiconductor substrates, e.g., Si, also orientations of the E vector parallel to the substrate can be obtained, but the situation is still rather complicated because of the different values of the Fresnel coefficients for E vectors oriented parallel and perpendicular to the surface, respectively.23 The fact that experimental IR spectra on metal substrates can be recorded for one orientation of the electric field vector only makes a direct orientation analysis impossible. Previously, however, two indirect methods have been proposed to infer on the molecular orientation. These methods are based on an analysis of the corresponding IR spectra of the bulk samples and work by either determining the absolute excitation probabilities of a vibrational mode (absolute method23) or by comparing the relative excitation probabilities of two or more modes (relative method22). In the present study we will compare the two different methods by applying them to SAMs made from aliphatic and aromatic organothiols, octadecanethiol (ODT), and p-terphenylthiol (TPT). It will be demonstrated that because of experimental problems with the determination of the absolute IR excitation probabilities significant effort is needed for a reliable orientation analysis using IR data. Experimental Section All absorption spectra reported here were taken using a BioRad “Excalibur FTS-3000” FTIR spectrometer. Monolayer spectra were recorded in the grazing incidence reflection mode using a Bio-Rad “Uniflex” unit. The spectrometer was purged by dry air delivered from a commercial purge gas generator. The incident angle of the p-polarized light was set to 80° relative to the surface normal.25,26 The reflected light was detected by a liquid nitrogen cooled MCT narrow band detector. A aperture stop25,26 of 1 cm diameter located at the light entrance of the sample compartment was used to reduce the aperture to f/10, to protect the detector against overload and to avoid nonlinear effects. All measurements were started 5 min after mounting a new sample. For all measured IRRAS data the resolution was set to 2 cm-1 and 2000 scans were accumulated, averaged, and transformed by using a triangular apodization. All IR spectra from bulk samples were recorded using a DTGS detector, averaged over 100 scans and also transformed by using a triangular apodization. Control experiments have shown that pellet spectra from DTGS and MCT detectors are indistinguishable for concentrations resulting in band heights smaller than 0.5 AU.27 All SAM spectra were normalized by division through data recorded for a gold substrate (background) which was covered by a perdeuterated docosanethiolate monolayer to protect the mirror against contamination from the environment. It was found crucial to use high purity perdeuterated docosanethiolate, since (22) Debe, M. K. J. Appl. Phys. 1984, 55, 3354-3366. (23) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-944. (24) Born, M.; Wolf, E. Principles of Optics, 7th ed.; Cambridge University Press: Cambridge, 1999. (25) The light used for IR measurements actually does not come in at a single angle. In practice a light bundle with an aperture angle of 2.5° is focused onto the sample. Hence, the angle of incidence and the azimuth vary (2.5°. The angle of 80° is referenced to the optical axis of the incident light bundle. The error bar resulting from the aperture in the present apparatus is less than 0.5°. See also ref 26. (26) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (27) Absorbance Units: 0 AU ) 100% transmission, 1 AU ) 10% transmission, 2 AU ) 1% transmission (logarithm scale).

Langmuir, Vol. 17, No. 16, 2001 4981 even small amounts of protonated molecules gave rise to significant problems with data normalization. Sample Preparation. Chemicals. Octadecanethiol (ODT, HS-(CH2)17-CH3, 98%, Aldrich), di-n-octadecyl disulfide (DDS, CH3-(CH2)17-S-S-(CH2)17-CH3, 95%, Lancaster), KBr, KCl (both optical grade, Aldrich), and ethanol (A.R. grade, 99.8%, Baker) were used as received. p-Terphenylthiol (TPT) was synthesized as described elsewhere.14 Monolayers. The Au substrates were prepared by first evaporating 5 nm of titanium and subsequently 100 nm of gold onto Si(100) wafers in a recipient with a residual gas pressure of 10-7 mbar. The adsorption of the organothiolate monolayers was carried out by immersing the substrates into ethanolic solutions of the thiols for 24 h. In case of ODT 0.5 mM solutions were used. In case of TPT the solubility in ethanol and other solvents is very small. To obtain an unsaturated solution with a reproducible concentration, small grains of TPT were placed in ethanol and sonicated for 10 min. Then the solution was refrigerated to 4 °C, filtered, and after heating to room temperature diluted by an equal amount of fresh ethanol. Pellets. The series of ODT28 pellets with shorter grinding times (Figure 2, open circles) were made by grinding 0.3 g of KBr and 0.2 mg of ODT into the capsule of the vibrating mill (PerkinElmer) for the appropriate time, after which all material was removed to produce a single pellet with a diameter of 13 mm. The series with longer grinding times were made by pregrinding 1.5 mg of ODT into 4.5 g of KBr (Figure 2, closed circles), 1.3 mg/4.4 g (Figure 3, closed circles), and 1.0 mg/4.7 g (Figure 3, open circles), and by pregrinding 1.63 mg of ODT into 3.72 g of KCl (Figure 3, squares) for 60 s using a hand mortar in order to obtain a homogeneous dispersion. Subsequently, smaller portions of 0.3 g were further ground in the vibrating mill for times between 30 and 660 s. The ground mixtures were pressed to pellets in the evacuated die for 3 min and installed in the IR spectrometer without further delay. The concentration of ODT was chosen so as to obtain a maximum peak height of 0.5 AU in order to get a minimum of measuring inaccuracy for the pellets with the longest grinding time. All DDS pellets were prepared by grinding 1.04 mg of the sample substance with 4.13 g of KBr using the same procedure. The TPT pellets were made by grinding approximately 0.1 mg of TPT into 0.3 g of KBr.

Results In Figure 1A we present a bulk spectrum (same as in Figures 1B and 2, grinding time 360 s) obtained for an ODT pellet. The positions of the different bands are provided in Table 1. The mode assignments are also provided; they are taken from previous works.19,23 The spectrum was baseline corrected by subtracting a spline function29 fitted to the nonabsorbing regions below 2800 cm-1 and above 3000 cm-1 using a commercial software package.30 Figure 1B shows a series of spectra recorded for the ODT pellets made using long grinding times. The spectra were corrected as described above and normalized to the substance content in order to obtain normalized peak heights. Unexpectedly, the intensities of the bands exhibit a pronounced dependence on grinding times. A quantitative analysis of the CH2 asymmetric vibration (d-) reveals an monotonic increase of absorption with grinding time (Figure 2, closed circles). The equivalent series for shorter ground pellets is also shown (Figure 2, open circles). The peak heights of the d- band (CH2 asymmetric) of the spectra obtained for some additional series of pellets of ODT/KBr and ODT/KCl as a function of a longer grinding time are depicted in Figure 3. The solid horizon(28) A ultra-high-precision balance (Sartorius 2104) with a resolution of 1 µg and a reproducibility of better than 5 µg was used. (29) Third-order polynoms were used. (30) Bio-Rad Laboratories, Package of Commercial Software: WinIR-Pro, Cambridge, 1998.

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Figure 3. Peak heights of the d-band (CH2 asym) of the IR spectra of three series of pellets normalized to the substance content of ODT/KBr (circles) and ODT/KCl (squares) as a function of the grinding time. In each case the solid horizontal line represents the average between 360 and 720 s. The vertical line denotes the standard deviation from the mean. Values of 4.36 ( 0.62 AU/mg (all circles) and 4.04 ( 0.28 AU/mg (squares) are obtained.

Figure 1. (A) IR spectrum of a ODT/KBr pellet (same as in Figures 2 and 3, 360 s ground). The bands are assigned in Table 1. (B) IR spectra of the ODT/KBr pellets (see also Figure 3) with the same ODT concentrations but for different grinding times. The spectra are normalized to the ODT content and show the influence of particle size on the absorbance values.

Figure 4. IR spectra of an ODT monolayer on a gold substrate. The bands are assigned in Table 1. Table 1. Positions of IR Bands in ODT Pellets,57 for the ODT SAM and Taken from Ref 23 peak position [cm-1] pellet

Figure 2. Peak heights of the d-band (CH2 asym) of the IR spectra of the series of pellets with short (open circles, directly ground by vibrating mill) and long (closed circles, 60 s preground by a hand mortar and further ground by using the same vibrating mill, as shown in Figure 1B) grinding times normalized to the substance content as a function of the grinding time. The figure shows the difference of grinding efficiency which may caused by influences of the environment (e.g., humidity) or differences of the starting distribution of the substance (e.g., substance at the capsule’s wall or covered by KBr).

tal line represents the average between 360 and 720 s. The vertical line denotes the standard deviation from the mean.

IRRAS

stretching mode

max

fit

max

fit

lit.

CH2 sym (d+) CH3 sym (r+) CH2 sym FR (d+FR) CH2 asym (d-) CH3 sym FR (r+FR) CH3 asym oop (r-) CH3 asym ip (r-)

2850.8 2872.3

2850.9 2872.5 2895.2 2919.1 2928.5 2954.6 2960.9

2850.9 2878.5

2850.8 2878.4 2905.7 2919.9 2936.8 2959.4 2966.3

2850 2876 2889 2917 2932 2954 2961

2919.3 2955.9 2955.9

2919.5 2938.1 2965.8 2965.8

In Figure 4 we present the reflection absorption spectrum of an ODT monolayer adsorbed on a gold substrate. The data correction is identical with that used for the data shown in Figure 1A, and the assignments of the bands are provided in Table 1. IR transmission spectra of DDS/KBr were measured for several grinding times. The peak heights of the dband (CH2 asymmetric) are shown in Figure 5. The intensity ratios of the CH2 symmetric/asymmetric bands of each spectrum (not shown) are the same as in the ODT/ KBr pellets.

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Figure 5. Peak heights of the d-mode in the IR spectra of DDS/KBr pellets prepared for the same DDS concentrations. The spectra are normalized to the ODT content and show the influence of particle size on the absorbance values. The solid horizontal line represents the average between 360 and 720 s. The vertical line denotes the standard deviation from the mean. A value of 4.02 ( 0.31 AU/mg is obtained.

Figure 7. Orientation of the transition dipole moments (TDM) of molecular vibrations (A, C) and the electric field (B) at the surface.

Figure 6. IR spectrum of (A) TPT/KBr pellet and (B) a TPT monolayer on an evaporated Au(111) surface. The bands are assigned using Wilson’s notation (see Table 2).

The IR spectrum of a TPT pellet is displayed in Figure 6A. The spectrum was baseline corrected over the whole spectral region again by using spline functions. The band assignments are provided in Table 2 using Wilson’s notation31 for the normal modes of benzene. In contrast to the case of ODT, the intensities of the IR bands for TPT (31) Wilson, G.; Bloor, J. E. Phys. Rev. 1934, 45, 706-714.

pellets do not present any dependence on grinding time. Figure 6B shows the spectrum for a TPT SAM on a gold substrate. The spectrum was corrected as described above, and the assignments of the bands are again provided in Table 2. The negative bands in the monolayer spectrum are artificially caused by the absorption of the perdeuterated reference sample (see above). Methods To Determine the Molecular Orientation. Each molecular vibration is characterized by the transition dipole moment (TDM) which for a given molecular orientation can be decomposed into components normal (TDMz) and parallel (TDMx) to the surface (see Figure 7A). If the two components TDMx and TDMz were both known, the tilt angle Θ could be obtained directly from the relation tan Θ ) TDMx/TDMz. Unfortunately, at a metal surface the field component Ex (parallel to the surface) of the incident electromagnetic wave (IR light) is almost completely screened by the electrons of the metal substrate24 (Figure 7B). As a consequence, the component of the TDM parallel to the surface, TDMx, does not contribute. Therefore, for a single vibrational mode, the tilt angle can only be determined if the absolute magnitude of the transition dipole moment, |TDM|, is known. As illustrated in Figure 7A, Θ is given by

ISAM ∝ [E B z TDM B]2 ∝ TDM Bz2 ∝ Ibulk cos2 Θ where ISAM denotes the respective IR band intensity (absorbance) in the SAM spectrum and Ibulk the corresponding band intensity in the bulk spectrum. The situation is improved significantly if two vibrational modes with different orientations of their transition dipole

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Table 2. Positions of IR Bands in the TPT Pellet and the TPT SAM and Taken from Refs 58-60 Assigned Using Wilson’s Notation31 of the Normal Modes of Benzene peak position [cm-1] band

lit.

pellet

SAM

ring, op 4 ring, op 0 ring, op 11 ring, ip par 1,4-axis, 12 ring, op 11 ring, op 17b ring, op 10a ring, op 11 ring, op 10a ring, op 10b,0,10b ring, op 10b,0,10b ring, op 17b ring, ip par 1,4-axis 18a ring, ip par 1,4-axis 18a ring, op, 5 ring, ip perp 1,4-axis, 15 ring, ip perp 1,4-axis, 3 ring, ip perp 1,4-axis, 19b ring, ip par 1,4-axis, 19a

695 723 751 769 770 838 839 840 850 908 911 909 1027 1041 1073 1117 1256 1401 1454

693 722 753 763 780 816 816 840 849 911 911 911 1002 1014 1076 1107 1258 1397 1482

705

moments can be measured (Figure 7C). Since the intensity of a band is proportional to the absolute value of the transition dipole moment |TDM| of the band, the ratio (|TDM1|/|TDM2|)2 of two bands is equal to the ratio of the band intensities, I1bulk/I2bulk, of the bulk spectrum. Furthermore, the ratio (TDM1,z/TDM2,z)2 is equal to the ratio I1SAM/I2SAM of the band intensities in the SAM spectrum. When these ratios have been determined, one can determine the tilt angle using

remarks monosubstituted ring parasubstituted rings monosubstituted ring

762

1002 1013

monosubstituted ring 17b for parasubstituted rings, not resolved 17b for parasubstituted rings, not resolved parasubstituted rings parasubstituted rings 17b from monosubstituted ring, not resolved 17b from monosubstituted ring, not resolved 17b from monosubstituted ring, not resolved parasubstituted rings monosubstituted ring

1477

monosubstituted ring parasubstituted rings parasubstituted rings monosubstituted ring

814 814

formation24) when correcting the monolayer data for refraction effects. Unfortunately, the strength of the absorption also depends on the orientation of the corresponding TDM and thus on the orientation of the molecule, so that an iterative procedure has to be used to arrive at a self-consistent description of all refraction effects. The iterative procedure has been described in detail by Parikh and Allara;23 see also refs 11 and 26. Discussion

tan2 Θ )

I1bulk I2SAM I2bulk I1SAM

This relative method was described in detail by Debe.22 The basis for the relative method is the availability of sufficiently intense IR bands for at least two different modes with differently oriented transition dipole moments. Since in practice this condition is frequently not fulfilled, Parikh and Allara23 have proposed an alternative method, which is based on the determination of the absolute excitation probability (corresponding to |TDM|) of a given mode in the bulk. If this information is available, the molecular tilt angle of a TDM relative to the surface normal can be calculated from

cos2 Θ )

( ) TDM Bz

2

|TDM B|

Therefore, the determination of the molecular tilt angle of a molecule relative to the surface becomes possible even if the intensity of only one band in the SAM spectrum having a TDM along the chain axis is known. Unfortunately, the direct (or Parikh and Allara) method suffers from an additional complication: the IR light incident at a grazing angle (80°) is not only reflected by the substrate surface but in part also by the SAM-air interface. The reflectivity of the latter is given by the Fresnel equations24 and depends on the refractive index of the SAM. For wavelengths close to the positions of the absorption bands, the refractive index exhibits anomalous dispersion,32 and accordingly, the absorption strength has to be considered (imaginary part of the complex refraction index, real part obtained by a Kramers-Kronig trans(32) The relative method neglects the effect of anomalous dispersion. The real part of the refractive index is assumed to be constant.

Analysis of ODT Data Using the Relative Method. The orientation of a molecule relative to the surface is given by three angles (tilt angle Θ, twist angle Ψ, and azimuthal rotation angle Φ) (see Figure 8). In the present case, however, the substrate consists of patches of Au single crystallites exhibiting preferentially a (111) orientation, yielding at least three equivalent rotational domains. As a result, the azimuthal angle Φ within a single domain cannot be determined, since information is simultaneously obtained for all three different domains. As can be shown analytically, the averaging over the different domains rotated by 120° with respect to each other (as well as the averaging over the different orientations of the crystallites) is equivalent to using a single, averaged azimuthal angle of Φ ) 45°. In the case of the relative method, an additional unknown quantity is the concentration c of the corresponding organothiol in the bulk pellets, thus yielding three unknown parameters (Θ,Ψ,c). For obtaining the maximum amount of information, it is thus necessary to determine the intensities of three vibrations with linearly independent TDMs. In the case of ODT, a possible choice of sufficiently intense modes consists of the CH2 symmetric vibration which has its TDM oriented along the bisector of the HCH angle in the HCH plane (perpendicular to the molecular axis along the CCC plane and lying in the CCC plane), the CH2 asymmetric vibration with its TDM oriented in the HH direction of the methylene group (perpendicular to the CCC plane), and finally the CH3 symmetric33 vibration with an TDM oriented parallel to the diagonal of the (33) Because the ODT molecule contains 17 methylene groups and only one methyl group, the CH3 sym band is very weak compared to the CH2 bands. Unfortunately, the CH2 bands have no vibrations with TDMs oriented parallel to the chain axis and the intensities of both CH2 modes show the same variation with the tilt angle. Thus it is impossible to average the twist angle of the ODT and determine the tilt angle by using only the CH2 bands. The weak CH3 sym mode has to be considered.

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Figure 8. Orientation of a ODT chain (left) at the surface (x-y plane) and of the TDMs (right) of some CH2 and CH3 vibrations. The orientation of the alkyl chain is defined by the angles Θ, Ψ, and Φ in the following way: first the alkyl chain is oriented normal to the surface with the CCC plane parallel to the z-x plane and the S-C bond pointing in the +x direction. Then the chain is rotated around the z-axis by the angle Ψ, tilted by the angle Θ in the +x direction, and finally rotated around the z-axis by the angle Φ.

pyramid spanned by the methylene group (in the direction of the CC bond of the terminating ethyl group). If the corresponding intensities are known, the orientational parameters can be determined by solving the equation system34 SAM bulk ICH ) ICH c cos2(Θ - 37.65° cos Ψ) 3sym 3sym SAM bulk ICH ) ICH c cos2 Ψ sin2 Θ 2sym 2sym SAM bulk ICH ) ICH c sin2 Ψ sin2 Θ 2asym 2asym

where c denotes the relative concentration and I denotes the intensity of the different bands (Figures 1 and 4). For the ODT pellet with a grinding time of 360 s (Figure 2) this procedure yields an orientation of Θ ) 64° and Ψ ) 48°, and for the pellet with a grinding time of 60 s (Figure 2) it yields an orientation of Θ ) 15° and Ψ ) 55°. This large difference in the results for the two angles when taking different reference spectra is rather surprising. The main difference between the bulk spectra obtained for pellets with different grinding times is that the spectrum for a grinding time of 360 s is about 4 times more intense than the spectrum with a grinding time of 60 s. Such a linear scaling (as long as the scaling factor is the same for all bands considered) should not affect the results of an orientation determination when using this relative method. A closer inspection of the two different bulk spectra, however, reveals that not only the height of the bands but also the peak shapes show a significant variation and influence the intensity. As a result, the fitting procedure used to extract the weak CH3 symmetric intensity yields different results for the two different bulk (34) The constant azimuthal factor can be taken into the relative concentration factor and further be ignored.

spectra, resulting in the pronounced difference in the orientation angles Θ and Ψ. The differences in peak shapes observed for the two bulk spectra are attributed to differences in size and shape of the corresponding pellets and will be discussed in more detail below. Analysis of ODT Data Using the Absolute Method. Since the organothiol concentration in the pellet and at the surface are known,35 the absolute method of Parikh and Allara23 can be used to determine the twist angle Ψ and the tilt angle Θ by using two IR bands with differently oriented TDMs, e.g., the two methylene bands. The third angle Φ (azimuthal orientation) is again set to 45° (see above). The solid line in Figure 9A shows the theoretical (calculated) spectrum36 resulting from the iterative procedure described above. When using the pellet data for a grinding time of 360 s (Figure 1A, Figure 2) for approximating only26 the CH2 symmetric and CH2 asymmetric bands of the SAM spectrum, we yield a chain tilt angle Θ ) 26.5° and a chain twist angle Ψ ) 45°. Figure 9B compares the calculated spectrum for the pellet with a grinding time of 60 s (Figure 1B, Figure 2) with the SAM spectrum (same as in Figure 9A). For this shortest grinding time (60 s) the iterative procedure (solid line in Figure 9B) shows the best fit to the experimental data: the results are Θ ) 58.5° and Ψ ) 46.5°. The strong variation in the results of the orientational analysis (tilt (35) If the packing densities of the crystallites in the pellet and in the SAM are equal, it is possible to consider the thickness of the substance instead of the concentration. In the pellet, the thickness of the sample is given by the product of the concentration of the substance and the thickness of the pellet. The thickness of the SAM on the substrate can be determined experimentally by XPS or ellipsometry. Alternatively, the known length of a molecule chain can be used to calculate the thickness of the SAM depending on the set tilt angle. In the present work the experimentally determined thickness of 24 Å was used for ODT SAMs. (36) The fit results of the IR bands obtained from the bulk were shifted to the appropriate band positions of the SAM before executing the calculation.

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Figure 9. Measured (dotted line) and calculated (solid line) IRRAS spectra of ODT. The calculation was performed (A) using the bulk spectrum of the pellet made using a grinding time of 360 s for a tilt angle of Θ ) 26.5° and a twist angle of Ψ ) 45° and (B) using the bulk spectrum of a pellet made using a grinding time of 60 s for a tilt angle of Θ ) 58.5° and a twist angle of Ψ ) 46.5°.

Figure 10. Results of the determination of the orientation of the ODT chain using the absolute method (see text).

angles vary between 20° and 60° depending on the pellet used) are due to the fact that the band intensities of the pellet spectra vary strongly with the grinding time37 (see below), and for each grinding time a different orientation is obtained. The twist and tilt angles calculated for the same SAM when referenced to each of the long ground series of ODT pellets of Figure 2 are shown in Figure 10. While the variation of the twist angles is within the error (37) The apparent absorbance depends on the particular type of mortar (laboratory mortars or micromortars). Different persons using the same hand mortar typically also yield different results.

Arnold et al.

bars, the alkyl tilt angle of the chain orientation used to perform the calculation varies so strongly that no reliable result can be obtained. An inspection of the data recorded for the series of ODT pellets made using short grinding times (10-50 s; open circles in Figure 2) approximately shows the same normalized absorbance values of the d- bands as seen for those made using long grinding times (60-360 s; closed circles in Figure 2). We attribute this rise in the normalized peak heights to light scattering effects which depend on the particle sizes and shapes. Although grinding times of 360 s are already considerably larger than those used in the standard procedure for producing pellets for IR transmission spectroscopy, we have additionally explored pellets made by using even longer grinding times. Two such series are presented in Figure 3 (circles). The results reveal that for longer grinding times the increase of absorbance with grinding time levels out and a saturation value of (4.36 ( 0.62) AU/mg (mean value ( standard deviation from both circled series) is reached. When this value is used to determine the tilt angle with the direct method, a value of 24° ( 1° is obtained. A third series of pellets of ODT ground with KCl converged to (4.04 ( 0.28) AU/mg, as shown in Figure 3 (squares). This value corresponds to a tilt angle of 25° ( 1°. In most previous studies on the orientation of ODTSAM bulk spectra recorded for the corresponding dimer, DDS, were used to determine the orientation of alkyl chains instead of ODT used here. To exclude any problems related to using a different reference material, additional bulk spectra of DDS as described in the literature23 were taken for several grinding times. The results are shown in Figure 5. These spectra exhibit the same dependence on grinding time as reported above for the thiol and yield very similar tilt angles. Dependence of ODT Bulk Spectra on Grinding Time. The intensities of the various bands observed in the IR bulk spectra of ODT (Figure 1B) exhibit a linear increase with grinding time (Figure 2) for grinding times below 300 s. Such a dependence is well-known in IR analytical chemistry and is attributed to the scattering of the IR light by the crystallites in the pellets. A theoretical analysis of this effect was presented by Bohren.38,39 From previous IR studies on the relation between the band intensities and the grinding time, a consistent picture has not yet emerged. Sharpless40 has reported experiments using chloranil dispersed in KBr and as single crystals as well as liquid solutions. He has found that the intensities of the IR bands are smallest in the crystal, increase for pellets with decreasing particle size, and are strongest in the solution. In contrast, Duyckaerts41,42 has explored pellets of calcite/KBr and found that the absorbance increases with increasing particle size and reaches a maximum for the single crystal. Other investigations made by Hlavay et al.43,44 for kaolinite/KBr and grinding times (38) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley & Sons: New York, Chichester, Brisbane, Toronto, Singapore, 1983. (39) As described in this work, the Mie theory is the exact solution of the scattering problem for spherical particles. The use of other particle shapes requires a more refined theoretical treatment and will result in different band shapes and intensities. (40) Sharpless, N. E. Appl. Spectrosc. 1963, 17, 47-50. (41) Duyckaerts, G. Spectrochim. Acta 1955, 7, 25-31. (42) Duyckaerts, G. Analyst 1959, 84, 201-214. (43) Hlavay, J.; Jonas, K.; Elek, S.; Inczedy, J. Clays Clay Miner. 1977, 25, 451-6. (44) Hlavay, J.; Inczedy, J. Spectrochim. Acta, Part A 1985, 41A, 783-7.

Molecular Orientation in SAMs

up to 10 days show an increase for the first 2 days followed by decrease of the IR band intensities. The intensities were found to be smaller in the final state than at the beginning. Hlavay suggested that the particle size first decreases and reaches a minimum after 2 days. He attributed the subsequent decrease to smaller intensities to the particles tending to adhere to each other. X-ray diffraction and endothermic difference temperature studies revealed that the particle size decreases over the whole grinding time following an exponential behavior.43,44 From these previous studies it becomes clear that in general a quantitative determination of IR band intensities from pellet data requires precise knowledge of the particle size and shape. A tedious theoretical analysis is required; for reliable results in addition to the pellet size the pellet shape has to be considered explicitly. Even if the intensities of bands converge for longer grinding times or do not vary with grinding time (e.g., the particle size is constant), the absolute excitation probabilities may be affected by the presence of light scattering by the particles in the matrix. The particular dependence of band intensity on grinding time observed here, namely, the approaching of an asymptotic value after long grinding times, is not a universal behavior. According to previous works,43,44 in some cases the band intensities reach a maximum and then decrease when further increasing the grinding time. To independently confirm that the asymptotic values observed here for ODT and DDS do not depend on different scattering as a function of particle size in the KBr powder, we have additionally studied pellets where the two compounds are embedded in a different material, KCl. In this case the refractive index is reduced from nKBr ) 1.54 to nKCl ) 1.47, significantly closer to the refractive index of the two organothiol compounds (nODT ) nDDS ) 1.43, outside of anomalous dispersion). Since the total amount of scattering scales with the difference in refractive index, a comparison of the band intensities for the two different compounds provides direct information on the importance of scattering effects. An inspection of the corresponding results for ODT (Figure 3) reveal that also for KCl an asymptotic value is reached after grinding times of 360 s, and the resulting, asymptotic band intensities for KCl and KBr agree within the error bars. These results strongly indicate that the band intensities determined for grinding times of ODT and DDS (not shown) in KBr and KCl longer than 360 s are largely independent of scattering effects within the pellets. We would like to note that for compounds different from the ones studied here (ODT and DDS) similar systematic investigations are necessary in order to determine reliable bulk band intensities. Recently, attempts have been undertaken to eliminate the grinding effects by recording the amount of scattered light inside the sample compartment of the spectrometer during the measurement and to include it in the analysis using a PLS statistical software.45 A direct application to the problems described here, however, has not yet been reported. Analysis of the TPT IR Data Using the Relative Method. TPT consists of three phenyl rings, where the outer rings are attached to the inner ring in para positions to form a planar molecule.46 In the bulk spectrum (Figure 6A) several bands are observed which have frequencies close to those seen for benzene. The TDMs of these modes (45) Lunige, H. J.; de Koeijer, J. A.; van der Maas, J. H.; Chalmers, J. M.; Tayler, P. J. Vib. Spectrosc. 1993, 4, 301-308. (46) Charbonneau, G. P.; Delugeard, Y. Acta Crystallogr. 1976, 32, 1420.

Langmuir, Vol. 17, No. 16, 2001 4987 Table 3. Results of the Chain Tilt Angle Obtained from the Relative Method22 (See Text)a center [cm-1]

vibration

area [AU‚cm-1]

errer [%]

12 ip par 10a,11,17b op 18a (1) ip par 19a ip par 7b ip par 20b ip par

Fit Results SAM 762.6 0.0039 814.7 0.0024 1001.7 0.0046 1477.4 0.0164 3034.0 0.0016 3062.0 0.0043

16 40 16 5 54 22

12 ip par 10a,11,17b op 18a (1) ip par 19a ip par 7b ip par 20b ip par

Fit Results Pellet 763.1 0.90 816.1 1.35 1001.9 0.17 1481.2 0.83 3030.9 0.21 3055.1 0.12

4 2 16 3 29 89

Tilt of Chain and Inaccuracy band

tilt [deg]

error [deg]

12 ip par 18a (1) ip par 19a ip par 7b ip par 20b ip par

42 20 23 35 17

7 6 6 17 13

a The errors of the results are derived from the standard deviation of the fits.

can exhibit three different orientations: perpendicular to the plane of the rings (op), in the plane parallel to the terphenyl molecular axis (ip par), and in the plane perpendicular to the molecular axis (ip perp). These three TDM directions are linearly independent and can thus be used as a basis for the orientation determination. Unfortunately, in the experimental data (Figure 6B), no ip perp bands could be observed. This reduces the number of independent parameters which can be extracted from the experimental data to two, and accordingly, in addition to the relative intensity only one angle can be determined. Thus, the twist angle was averaged around the chain axis (〈cos2(Ψ)〉 ) 0.5) and the tilt angle of the chain was determined by using band 17b and one of the other ip par bands from

x

Θ ) arctan

SAM Ibulk par Iperp

SAM 0.5Ibulk perpIpar

where Ipar and Iperp denote the intensities of the bands having a TDM parallel and perpendicular (band 17b) to the chain axis. Although the band assigned here to 17b corresponds to an unresolved combination of the benzene derived vibrations 10a/11/17b, it can be used for an orientation determination since for all three bands the TDM points in the same direction. The intensity of the bands 12, 18a, 19a, 7b, 20b (7b and 20b are not shown in Figure 6), and of course 17b were determined by fitting Gaussian functions to the experimental data. The results of this procedure and the calculated tilt angles are listed in Table 3. It is noteworthy that despite the 40% statistical fit error of the intensity for the 17b band the difference in tilt angles amounts to only 6° for the strong bands 18a and 19a. Within the error bars, all bands except 12 yield the same tilt angle. When the weak 7b and 20b bands as well as the anomalous band 12 are excluded, a tilt angle of Θ ) 22° ( 4° relative to the surface normal is obtained. The fact that the tilt angles yielded from band 12 and band 18a or from 19a show large deviations is unexpected. The band intensity ratios Ibulk/ISAM of the pellet bands

4988

Langmuir, Vol. 17, No. 16, 2001

relative to those of the monolayer amount to 37 for band 18a, to 51 for band 19a, and to 231 for band 12. The ratio for band 12 thus deviates strongly from those obtained for band 18a and band 19a. We first attributed this behavior to the wavelength-dependent variation of intensity modulation by anomalous dispersion in pellet grains, known as the Christiansen effect.47 Such an explanation would be supported by the band shape distortion visible on the short wavenumber side of the bands (see Figure 6A). We have undertaken significant efforts to explicitly account for the Christiansen effect by using the known analytical expressions but failed to obtain a consistent picture. Accordingly, we feel that this explanation can be ruled out. The most likely reason for the anomalous behavior seen for the bulk to SAM intensity ratio of band 12 is a variation of the transition dipole moment, caused by the different relative orientation of the molecules in the bulk and in the SAM. This assumption is supported by a 13C NMR investigation of aliphatic organothiols in the absence of metals as well as adsorbed on gold clusters48,49 which show significant differences in the chemical shift of the R-methylene group. This previous study demonstrated that the influence of the gold surface results in a decrease of electron density at the R-carbon. In the case of aromatic systems such as TPT this effect can influence the electron density distribution along the chain axis, will most likely affect the R-phenylene unit, and therefore can yield to a change of the appropriate TDMs. The only possibility to proceed with at this point is to exclude band 12 from the analysis and to determine the orientation from bands 18a, 19a, 7b, and 20b, in which case we yield a tilt angle of 24° ( 12°. Comparison of the Methods. In the case of ODT the IR orientational analysis using the direct method does not provide a single, consistent value for the tilt angle if bulk reference data obtained for short (