Langmuir 1994,10, 4657-4663
4657
Fourier Transform Infrared Spectroscopy Studies of Hafnium-Alkylbis(phosph0nate) Multilayers on Gold: Effects of Alkylbis(phosph0nate) Chain Length, Substrate Roughness, and Surface Functionalization on Film Structure and Order J. T. O'Brien, A. C. Zeppenfeld, G. L. Richmond, and C. J. Page* Department of Chemistry, University of Oregon, Eugene, Oregon 97403 Received June 9, 1994. In Final Form: September 16, 1994@ The influences of substrate roughness,order in the surface functionalizationmonolayer, and the alkylbis(phosphonate) chain length on the structural order of hafnium-alkylbis(phosph0nate) multilayer films were investigated using external reflectance Fourier transform infrared (F'TIR) spectroscopy. To examine the effectsof substrate roughness, multilayers of hafnium-decylbis(phosphonate) (Hf-CloBP) were grown on as-evaporated gold films and on similar films which were flame-annealed. The influence of alkyl chain order in the surface functionalization monolayers was investigated by preparing Hf-CloBP multilayer films on monolayers of Pmercaptobutylphosphonic acid (SC4P) and 12-mercaptododecylphosphonicacid (SC1pP). Effects of the bis(phosphonate)alkyl chain length on alkyl chain order in multilayer films were examined by assembling multilayer films using hafnium and alkylbis(phosphonic acids) with 6-, 8-, lo-, and 12-carbon chain lengths (C-BPA). FTIR spectra indicate that annealing of the gold substrate prior to functionalization and self-assembly does not significantly affect the order of the alkyl chains in the multilayers. While the IR spectra of SClpP anchor monolayers indicate the alkyl chains are better ordered than monolayers of SGP, the order of the alkyl chains in the Hf-CloBP multilayers grown on the two anchor monolayers is very similar and is less than that of the SClpP monolayer. The overall degree of alkyl chain order in the multilayers is dependent on the bis(phosph0nate) chain length. Chain order decreases as the alkyl chain of the bis(phosphonate) becomes shorter; films grown with C6BPA show essentially liquid-likedisorder, indicating that an eight-carbon alkyl chain is the lower limit for producing multilayers with some crystallinity in the alkyl chains of the organic layers.
Introduction Interest in monolayer and multilayer films has developed rapidly over the last decade, largely because of their potential for modifying the chemical reactivity and physical properties of surfaces.l By use of monolayers with appropriate pendant functional groups, surfaces can be modified to control wettability, inhibit corrosion, passivate semiconductor surfaces, or give selective chemical responses for catalytic applications, electron transfer reactions, or sensor device^.^^^ Multilayer films have even greater potential because they allow for the assembly of organized supramolecular structures with control of orientation, chemical composition, and thickness of each individual layer. A clear illustration of this potential is demonstrated by recent reports on the preparation of thinfilm nonlinear optical materials via sequential selfassembly of oriented nonlinear optical chromophore layers onto a s u r f a ~ e . ~ To. ~fully realize the potential of such films, methods for reproducibly preparing stable, wellordered films with minimal defect densities are needed. Traditionally, multilayer films have been prepared using Langmuir-Blodgett (LB) techniques, involving mechanical transfer of a compressed molecular monolayer formed a t an air-liquid interface to a solid substrate. Abstract published in Advance ACS Abstracts, November 15, 1994. (1)Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. (2)Cao, G.; Hong, H.-G.; Mallouk, T. E. ACC.Chem. Res. 1992,25, 420. (3)Rubinstein, I.; Steinberg, S.;Tor, Y.; Shanzer, A.; Sagiv, J.Nature 1988,332,426. (4) Li, D.; Ratner, M. A.; Marks, T.J.;Zhang, C.; Yang, J.;Wong, G. K. J . Am. Chem. SOC.1990,112,7389. (5) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991,254,1485. @
Although it is possible to produce exceptionally wellordered films using this method, its applications are limited because the procedure to produce multilayers is relatively complex. The resultant LB films are sensitive to impurities, are limited to planar surfaces, and often lack thermal, chemical, and mechanical stability.6 Self-assembled (SA) films have been studied as a n alternative to LB films,6 and SA monolayers of functionalized alkanes have been prepared which are comparable to LB films in monolayer density and degree of order. Self-assembly involves spontaneous adsorption of a n organized molecular monolayer driven by interactions between functional head groups on the molecule and the surface groups of the substrate. When the SA film is composed of molecules with pendant alkyl chains, van der Waals interactions between the hydrophobic alkyl chains can produce densely packed films dominated by a crystalline structure, in which a n all-trans alkyl chain configuration comparable to well-ordered LB films is observed.'-1° The degree of order in these systems is largely determined by the alkyl chain length; SA films prepared by using molecules with short alkyl chains (fewer than ten carbons) apparently lack sufficient attractive interactions between alkyl chains to produce well-ordered monolayers." Although the importance of alkyl chain length has been shown for monolayer systems, less attention has been (6)Ulman, A. An Introduction to Ultrathin Organic Films fiom Langmuir-BlodRettto Self-Assembly, 1st ed.; Academic Press: San Diego, CA,-l991; 442 pp. (7)Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982,86,2700. (8)Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1,45. (9) Allara. D. L.: Nuzzo. R. G. Lanamuir 1986.1,52. (10) Nuzzo, R. G.; Dubois, L. H.; k l a r a , D. L: J:Am. Chem. SOC. 1990,112,558. (11)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987,109,3559.
0743-746319412410-4657$04.5010 0 1994 American Chemical Society
4658 Langmuir, Vol. 10, No. 12, 1994
~
Anchor
j Inorganic
1 Organic I Inorganic
Figure 1. Schematic representation of a metal alkylbis(phosphonate)multilayer on a gold substrate. focused on this issue for multilayer SA films. In part this is probably due to the limited success of most multilayer SA techniques. 12,13 One successful method for producing self-assembled multilayers utilizes metal 11, I11 or IV interactions with phosphonate group^.'^-'^ By use ofthese systems, multilayer films can be prepared by alternately immersing a n appropriately functionalized substrate in metal cation and alkylbis(phosph0nic acid) solutions to build up the desired number of metal bis(phosphonate1 layers. Resulting multilayer films have alternating organic and inorganic layers. Figure 1shows the proposed structure for these materials, which is based on the structure of the corresponding bulk materials. Multilayer films can be reproducibly prepared and are mechanically, chemically, and thermally stable. Although these films have considerable potential, a better understanding of the factors that control the film order and quality is needed. Previous IR studies show that alkyl chains in metal bis(phosphonate) films appear to be disordered (i.e. in a liquid-like ~ t a t e ) . ’ ~ J *Although J~ many FTIR studies on metal bis(phosphonate) multilayers have been reported, the effects of substrate surface roughness, the importance of order in the initial surface functionalization layer (i.e. the “primer” or “anchor” layer), and the influence of the bis(phosphonate) alkyl chain length on the ordering of the alkyl chains in the organic layer (especiallyover several layers) have not been systematically addressed for films grown on gold substrates. Further, some of the previous reports appear to draw contradictory conclusions about the importance of bis(phosphonate) chain length on film order. For example, pinhole-free films of zirconium ethylbis(phosph0nate) have been grown, suggesting that the alkyl chain length is of minor importance for preparing well-ordered high-quality films,2o while other studies suggest that a chain length of 16 carbons is necessary to produce well-ordered films.21 In addition, evaluation and comparison of previously reported data are complicated because a variety of surface functionalization techniques have been employed prior to film growth, and the effects of surface functionalization on subsequent film growth are not ~ e l l - k n ~ w n . ~ ~ J ~ ~ ~ ~ This study involves examination of the influence of the gold surface preparation, the chain length of the anchor (12)Sagiv, J.;Netzer, L. J . Am. Chem. SOC.l98S,105,674. (13)Tillman, N.;Ulman, A.; Penner, T. L. Lungmuir 1989,5,101. (14)Lee, H.; Kepley, L. J.;Hong, H.-G.; Mallouk, T. E. J.Am. Chem. SOC.1988.110.618. (15)Lee, H.f Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J.Phys. Chem. 1988,92,2597. (16)Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J.Am. Chem. SOC.1993,115, 11855.
(17)Katz, H. E.;Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Muter. 1991,3, 699. (18)Hong, H.-G.; Sackett, D. D.; Mallouk, T. E. Chem. Muter. 1991,
.-, ? 521
(19)Frey, B. L.; Hanken, D. G.; Corn,R. M. Langmuir 1993,9,1815. (20)Hong, H.-G.; Mallouk, T. E. Lungmuir 1991,7 , 2362. (21)Bent, S.F.; Schilling, M. L.; Wilson, W. L.; Katz, H. E.; Hams, A. L.Chem. Muter. 1994,6, 122. (22)Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.;Nagler, S. E.; Talham, D.R.J.Am. Chem. SOC.1994,116,295.
OBrien et al. layer, and the bis(phosphonate) alkyl chain length on the growth and order of multilayer metal bis(phosphonate) films. External reflectance FTIR spectroscopy has been employed to obtain information about the average film structure including the degree of crystallinity, density of packing, and the average orientationofthe alkyl c h a i n ~ . ~ J l The films studied here were grown using identical procedures on as-evaporated and on evaporated, flameannealed gold film surfaces to determine whether substrate roughness influences film order to a significant degree. To examine whether the alkyl chain length of the surface functionalization layer (anchor layer, Figure 1)is an important factor in multilayer film structure and order, a short chain anchor monolayer prepared with mercaptobutylphosphonic acid (SC4P) and a long chain anchor monolayer prepared using mercaptododecylphosphonic acid (SC12P) were used in the growth of hafnium-1,lOdecylbis(phosph0nate) (Hf-CloBP) films. To investigate the importance ofbis(phosph0nate) alkyl chain length with respect to the degree of crystallinity and order in these systems, multilayer films were assembled using hafnium and bis(phosphonicacids) with varying alkyl chain lengths, C,BPA ( n = 6, 8, 10,and 12).
Experimental Section The C,BPAs were synthesizedusing the Michaelis-Arbuzov reaction with the appropriate a,w-dibromoalkane and triethyl ph~sphite.’~ The chemicals were used as received from Aldrich Chemical Co. The resultant bis(phosphonic acids)were dried in vacuo after washing alternately with dilute HC1 in ultrapure HzO and HPLC grade methanol or acetonitrile. Purity was established by lH NMR analysis. The SClzP anchor was synthesized as follows: 4 equiv of 1,12-dibromododecane was reacted with P(OCzH& to give the monophosphonate via the Michaelis-Arbuzov reaction. The reaction was refluxed in a hot water (38“C)jacketed reflux column attached to a condenser to remove the side reactant, ethyl bromide, to a collection flask. Note: the reflux column was heated before starting the reaction to avoid cracking the carbon chain by superheating. Approximately 2.8 mmol of the resulting 12bromododecylphosphonate was refluxed with 1equiv ofthiourea in 10 mL of ethanol for 5 h. Six milliliters of 1.5 M NaOH was added and refluxing was continued for another 3 h. The solution was then acidified to pH 4.5 with 0.5 M HzS04 and the resulting white precipitatewas filtered,washed with acetonitrile,and dried invacuo. The thiol group was identified in the lH NMR spectrum as a quadrupletin the range 2.509 5 6 5 2.558, and no impurities were observed in the NMR spectrum. The 4-mercaptobutylphosphonicacid was kindly provided by Professor T. E. Mallouk and was used as received.14 HfOCl2-8HzO was provided by Teledyne Wah Chang Albany and was used as received. The water used in all cleaning procedures and in the preparation of aqueous solutionswas purified to a resistivity of 17.0-18.3 MOcm with a Barnstead E-Pure water purification system. The gold substrates were functionalized by dipping in 1.25 mM solutions of the respective anchor molecules in absolute ethanol for 48-96 h. Following immersion,the substrates were rinsed in flowingdeionized water for 10-15 min, and transferred to the hafnium oxychloride solution for another 24-48 h, and then rinsed for another 10-15 min in flowing deionized water. Hf-CsBP, - C a p , -CloBP, and -C12BP multilayerfilms were grown following a procedure reported by Mallouk and coworker~.’~ The functionalizedgold substrates were alternately immersed in a 5 mM aqueous solution of HfOC1243HzO and a 1.2 mM solution of the bis(phosphonic acid) prepared with absolute ethanol from Quantum Chemical Corp. In the case of the CaBPA, a solution of 80% ethanol and 20% water was used to improve the solubility. Immersion times were 4-12 h in the metal solution and 6- 18 h in the bis(phosph0nate) solution. Substrates were rinsed with flowing deionized water for 10-15 min between immersions. Two types of Au substrates were used. The first type was made by resistively evaporating approximately 170 A of chro-
Langmuir, Vol. 10, No. 12, 1994 4659
Hf-C,BP Multilayers on Gold mium followed by 1700A of high-purity (99.99%)gold wire from DF Goldsmith onto a clean Si wafer. The wafers were either 4 x 2.5 cm or 1.5 x 1.5 cm in size and were cleaned prior to deposition by soaking in a hot piranha solution for 15-20 min followed by rinsing in ultrapure water for 15 min. An FTIR spectrum of the clean gold surface was collected immediately after the evaporation to ensure there were no signs of contamination. The second type of gold substrates used were "atomically smooth (prepared for STM imaging). These were made by thermal evaporation of -2000 A of gold onto glass substrates precoated with -20 Achromium and were made smooth by flameannealing and quenching in methanol. While we did not directly assess the "roughness" of the substrates used in this study, previous STM studies23suggest that the films prepared by these two different methods should be significantly different in surface topography and roughness. In this previous work, a comparison is made between STM images of as-evaporated gold films and of similar gold films which had been flame-annealed and quenched (both on chromium-coated quartz substrates). STMimages showed that the as-evaporated gold films consisted of small (-250 A) globular clusters of gold which presented only smallareas offlat surface and had a surface height roughness of 75 & 33 A. The annealed films had much larger flat surface areas (with diameters of -1000-5000 A) and a smaller magnitude of roughness (34 f 20 A). On the basis of this previous work, the two different substrates used here are expected to have differences in surface topography that are significant enough to possibly influence the quality of selfassembled films grown on them. IR spectra were collected using a Nicolet Magna 550 FTIR fitted with a Spectra Tech 80" external reflectance attachment. Spectra were collectedwith 2-cm-' resolution and were averaged over 256-1024 scans dependingon the number of layers in the film. Spectra from the larger wafers were collected with p-polarizedlight, while those from smallersampleswere collected without polarization to improve signal to noise. Integrated intensities of the methylene stretch bands increased linearly as the number of layers increased for all of the samples studied, indicating that regular layer deposition occurred in all cases. Ellipsometry measurements were performed on a Rudolph thin film ellipsometer, 43702-2003, using 632.8-nm radiation from a tungsten-halogen lamp. Ellipsometrydata show uniform incremental changesin A and Y with sequential layer deposition for all of the samples studied. Thickness determination from ellipsometry data was problematic for films grown on the asevaporated gold substrates because we had difficulty obtaining reliable refractive indices for these gold substrates. In the case of films grown on flame-annealed substrates, incremental thicknesses per layer were determinedto be very similar to those of the same type of multilayers grown on silicon substrates. For example,the average layer thickness of a Hf-CloBP film grown on an annealed gold substrate was determined to be -18-21 A (assuming a film index of 1.60-1.50, and using a gold substrate index determined by ellipsometryto be 0.198-3.031i), which is in the same range as average layer thicknesses observed for Hf-CloBP films grown on silicon.24
Results and Discussion Smooth versus Rough Gold Substrates. The HfCloBP films grown on annealed gold surfaces showed no significant differences in the methylene or PO3 stretch spectral regions of the FTIR spectra in comparison with those grown on an as-evaporated gold surface. All of the peak positions and full width half maximum (fwhm)widths were the same within the resolution of the spectra. Effects of the Alkyl Chain Order of the Anchor Monolayer. Figure 2 shows the methylene stretching region of the FTIR spectra of the SC4P and the SClpP anchor monolayers before (lower) and after (upper) deposition of three Hf-CloBP layers. The spectra show significant differences in the alkyl chain ordering of the short and long anchor monolayers. The peak positions of (23)Clemmer, C. R.;Beebe, J.,T. P. Scanning Microsc. 1992,6,319. (24)Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. J. Am. Chem. SOC.,in press.
SC,P
.: :.. ,:. :.. i
i
: :
3150
/
wi3 layers
-SC, 2P wi3
layers (Hf.C,,BP)
:
., /
2900
2650
Wavenumbers (cm ')
Figure 2. IR spectra of the CH2 stretching region (a) of the SC4PandSClzPanchor monolayersand (b)of the same samples after deposition of three layers of Hf-CloBP.
the methylene asymmetric (Y,(CHz)) and symmetric (v,(CHz)) stretches are 2919 and 2850 cm-l, respectively, for the SClzP monolayer; these peak locations are consistent with a film with a high degree of crystallinity in the alkyl chain layer. The corresponding v,(CH2) and vS(CH2)peaks of the SC4P anchor are shifted to 2924 and 2853 cm-l, indicating a n abundance of gauche confonnations associated with a liquid-like structure.ll Although the SClzP monolayer is better ordered than the SC4P monolayer, difference spectra for the incremental growth of the first few Hf-CloBP layers on these monolayers are nearly identical in peak width and position of the methylene stretches. Thus, the enhanced order of the SClzPmonolayer does not significantly influence the alkyl chain order of subsequently deposited Hf-CloBP multilayers. Accordingly, the Ya(CH2) and vs(CH2) peak positions of the cumulative spectra of multilayers grown on both types of anchor layers shift toward higher energies as the number of Hf-CloBP layers is increased and become nearly the same within the spectral resolution after a few layers (Table 1). The observed shifts indicate that the Hf-CloBP layers are much less ordered than either anchor and that enhanced order from the SClpP is dominated by the tendency toward disorder in the Hf-CloBP layers. Peak widths of the v,(CHz) and vs(CH2)bands are also an indication of the degree of liquidity of the alkyl chains. The methylene stretch bands are broadened by coupling between the bending (scissor) and stretching modes of the methylene groups. As the chains change from the solid to liquid phase, coupling between these modes is enhanced causing a greater dispersion of the stretching modes.25 The fwhm of the SClpP anchor peaks range from 14 to 21 cm-l and 6 to 11 cm-l for va(CH2) and Y,(CHZ), respectively; the narrower peaks in these ranges correspond to longer immersion times of the gold substrate in the anchor molecule solution. The fwhm of the SC4P anchor layer methylene peaks were in general larger, ranging from 27 to 32 cm-l and 15 to 17 cm-l for v,(CH2) and vs(CH2),respectively. This is an indication that the SC4P monolayer is more liquid-like and disordered than the SClzP monolayer, consistent with the conclusions (25)Snyder, R.G.;Strauss, H. L.; Elliger, C. A. J.Phys. Chem. 1982, 86, 5145.
4660 Langmuir, Vol. 10, No. 12, 1994
O'Brien et al.
Table 1. Peak Position and Peak Widths of the Methylene Stretching Modes, om-l 1layer 5 layers 10 layers 15 layers 20 layers alkyl chain length pos fwhm pos fwhm pos fwhm pos fwhm pos fwhm CS
v,
2913
10
2850 2927 2852 2924 2853 2921 2851 2927 2854
8 29 11 29 16 31 11 34 25
FR vs v,
C8
vB ClO (SC12Panchor) Cl2
va
vs v, vs
ClO (SC~Panchor)
v,
v,
2918 2942 2850 2927 2852 2927 2854 2928 2853 2926 2853
41
2921 2943 2862 2928 2852 2928 2855 2927 2854 2927 2854
32 40 21 35 19 31 17 33 23
w
:
0
PO,- SC,P x (0.5)
0
PO,- SC,,P x (0.5)
fwhm
2941 2863 2932 2857 2928 2854 2927 2854 2927 2854
59 35 45 27 39 24 34 20 35 22
55 29 45 25 36 21 34 20 34 22
2937 2865 2928 2856 2928 2856 2927 2854 2927 2854
0
1.5
25 layers pos
56 33 45 25 36 21 34 20 35 20
2941 2865 2931 2858 2929 2854 2927 2854 2927 2854
59 35 46 26 38 24 33 20 35 21
Metal Pliosphonate Layer
X
m 1.0
a D L
L
E
-? -
H
I
0.5
C
C
n n
I
I
I
1
I
5
10
15
20
25
drawn from the peak positions. The methylene peaks broaden significantly with the addition of Hf-CloBP layers, and the fwhm values become very similar for the Hf-CloBP films after several layers regardless of the anchor used. Thus, both the peak positions and the fwhm values suggest that the influence of the order in the anchor monolayer on the alkyl chain order of the multilayer film is negligible. In accord with these observations, a similar study of Zr-CuBP multilayers grown on well-ordered and poorly-ordered SA anchor monolayers on Si ATR crystals showed that the alkyl chain ordering of the multilayers was essentially independent of the alkyl chain order of the anchor layer.21 Although the methylene stretch peak positions and peak widths indicate that Hf-CloBP films on the two types of anchor monolayers have similar characteristics, the peak intensities indicate that significant differences do exist. Based solely on the number of methylene units in each film, the Hf-CloBP film grown on the SClpP monolayer should always have slightly higher v,( CHz) and v,(CHz) peak intensities than films grown on the SC4P monolayer. However, as can be seen in Figure 2 and in the comparisons of peak areas for the first 25 layers in Figure 3, it is obvious that the film grown on the SC4P monolayer has significantly more intense methylene stretches and somewhat more intense PO3 modes than the corresponding film grown on the SClzP monolayer. Differences in intensities may be accounted for by considering alkyl chain tilt angle, order, and density of packing.ll The alkyl chain tilt angle of the anchor and bis(phosphonate) layers plays a critical role in determining the signal intensity, since it defines the average orientation of the transition dipoles associated with the methylene stretch modes relative to the plane of the surface. Since
:e
I
I
C H C
I
Number of Layers
Figure 3. Integrated areas ofthe PO3 (opensymbols)and CH2 (filled symbols) stretching bands as a function of the number of layers of Hf-CIoBP. Squares correspond to films grown on S C Z monolayers and circles to films grown on SClzP monolayers. The lines are linear regression fits to the data.
,C
C I
I
; I
1
I
I
Metal Phosphonate Layer
Figure 4. An alkyl chain in a metal-ClzBP layer drawn with a tilt angle (6)of 30". 0 (=go" - 9) represents the angle of the dipoles of the CHZstretching modes relative t o the surface normal.
the only significant contribution to the signal intensity comes from interactions of the transition dipoles with the field normal to the surface, the intensity of the modes will increase as the transition dipoles align with the surface n ~ r m a l . The ~ ~ intensity ,~~ of a particular mode varies as
where M = (dp/dq) is the transition dipole (which is aligned along the C-H bond axis) and 8 is the angle between M and the surface normal, as shown in Figure 4.9 As a transition dipole tilts away from the surface normal, 8 increases and lower intensity for that particular mode is expected. Because the transition dipoles for the methylene stretches are oriented perpendicular to the alkyl chain axis for a fully extended conformation, if the alkyl chain is oriented normal to the surface (4 = 0) the intensity of va(CHd and Y,(CHZ)would be expected to be zero. As the alkyl chain tilts away from the normal, a n increase in the intensity of the methylene stretch modes should occur. A fairly small shiR in the chain tilt angle could easily account for the difference in intensity observed. For example, alkanethiol monolayers typically have an alkyl chain tilt angle 4 of 20-30" from the surface normal corresponding to a range of 8 from 70" to 60",respectively.ll A change in 8 from 70" to 60" would result in an -2-fold increase in the intensity of these modes. (The PO3 modes are less sensitive to the changes of the tilt angle, because the effects of shifting one P-0 dipole tend to be canceled by the (26)Greenler, R. G. J . Chem. Phys. 1966,44, 310. (27) Francis, S. A.; Ellison, A. H. J Opt. SOC.Am. 1959,49, 131.
Langmuir, Vol. 10, No. 12, 1994 4661
Hf-C,,BP Multilayers on Gold simultaneous shifts of the other two dipoles.) Thus, assuming that the densities are similar for films grown on both anchor layers, the difference in the observed intensities indicates that the SClzP anchor is not as tilted as the SC4P anchor and that this difference is maintained with subsequent film growth. This implies that the alkyl chain tilt angle of the anchor layer determines the chain tilt angle of all subsequent layers. We have observed a similar effect for Hf-CloBP films grown on silicon.24 In this case the tilt angle and layer thickness are apparently determined by the first CloBP layer; the individual layer thickness varies widely from sample to sample but is uniform for multiple layers of the same sample. Talham and co-workers have shown that the tilt angle may also be modified by the method of multilayer deposition.22 However, this observation cannot account for the observed differences in intensities reported here since the procedures used for growing multilayer films on both SC4P and SCl2P monolayers were identical. By use of the external reflectance technique, the tilt angle can be determined by calculatingthe expected signal intensity for a single layer based on the spectrum and the optical constants of the corresponding bulk materiaLg Unfortunately this approach requires that the bulk materials and their film analogues have similiar degrees of order, force constants, normal modes, and charge distributions. Changes in these factors result in shifted vibrational frequencies and altered band shape^.^ Because the degree of order (and possibly also the other factors listed above) is different for the alkylbis(phosph0nate) films and their bulk counterparts,16the tilt angle cannot be calculated reliably using this approach. However, the inability to directly determine the chain tilt angle in these films does not alter the qualitative discussion above suggesting that a difference in tilt angle could explain the observed differences in intensities between films grown on SC4P and SClzP anchor monolayers. The relative degree of order and the density of packing are also factors that will influence the signal intensity of the methylene stretching bands. As disorder increases, the methylene stretches should have a wider range of orientations with a net increase in the alignment with the surface normal. Thus the disorder should result in an increase in the signal intensity; however, this intensity increase should be accompanied by peak broadening and shifting toward the higher energies associated with the liquid state. The density of packing influences the intensity of these bands simply because as the density increases, the number of methylene groups also increases. However, if the packing density decreases significantly, the interactions between neighboring alkyl chains should diminish leading to greater disorder which will give rise to broadened, blue-shifted peaks. On the basis of the peak positions and fwhm values of Hf-CloBPA multilayer films grown on either SC4P or SClpP, there are no significant differences in the relative degree of order in the two types of films. This suggests that differences in the packing density between the two types of films are negligible, lending support to the conclusion drawn above that differences in average chain tilt angle of films are responsible for the observed lower va(CH2) and vs(CH2) band intensities of the films grown on the SClzP anchor layer relative to those grown on the SC4P layer. Variation of Bis(phosphonate) Alkyl Chain Length. Although the films grown on the SClzP anchor monolayer had lower signal intensities, the SClpP anchor was used for the investigation of the influence of bis(phosphonate) chain length on film order since it formed better-ordered anchor monolayers. The FTIR spectra of Hf-CsBP, -CgBP, -CloBP, and -&BP multilayer films
Hf-C,BP 0.006
1
n
Hf-C,BP
0.008
3150
2900
2650
Wavenumbers (cm.')
Figure 6. IR spectra of the methylene stretch region of HfC,BP multilayers with 1,5,10,15,20,and 25 layers grown on SClzP monolayers. were collected after depositing 1-5,7,10, 15,20, and 25 layers, and the methylene stretch region of the spectra of 1,5, 10,15, 20, and 25 layers for each type of sample is shown in Figure 5. A comparison of the va(CH2) and v,(CH2) peak positions and widths (Table 1)indicates that the length of the bidphosphonate) alkyl chain does indeed play a significantrole in the order ofthe alkyl chain layers. While the spectra indicate that none of these systems has crystalline ordering of the alkyl chains, the peak positions and widths of the v,(CH2) and v,(CH2) bands of the longer chain Hf-bis(phosphonate) multilayers reflect a higher degree of alkyl chain ordering than do those ofthe shorter chain Hf-bis(phosphonate) multilayers. The data presented in Table 1 clearly show that the va(CH2)and vdCH2)bands broaden and shift toward higher energy when shorter bis(phosphonate) alkyl chains are used and as more layers are added. These trends are most pronounced in the Hf-CsBP films for which the 25layer sample va(CH2)and v,(CH2) peak positions are 2941 and 2863 cm-l, respectively. The shifts from the expected
4662 Langmuir, Vol. 10,No. 12, 1994 positions for crystalline methylene stretches are very large in this case, and the positions of these bands are not normally associated with methylene stretches; however, similar v,(CHz) and Y,(CH~) positions have been reported for very similar bulk materials and for liquid-phase alkanes.l6sZ5 The initial splitting of the va(CHd band observed for small numbers of Hf-C&P layers and the peak shapes for larger numbers of layers indicate that this band contains two peaks, but the lower energy peak is ultimately overwhelmed by the peak at 2941 cm-l as the number ofHf-CsBP layers increases. This 2941-cm-l peak has been attributed to a Fermi resonance between the asymmetric methylene stretch and the methylene bending modes28 and is characteristic of liquid-phase alkanes because this coupling is enhanced in the liquid phase. Since this peak is observed only for the Hf-CsBP films, these films must have the most liquid-like alkyl chains. The va(CH2) and v,(CHz) bands of Hf-CBBP, -CloBP, and -C12BP films are not shifted to such high energies as the Hf-C6BP bands, indicating that the alkyl chains in these films are not as liquid-like. However, the fwhm widths of the Hf-CBBP v,(CHz) and v,(CH2) bands are much broader than are those of the Hf-CloBP or Hf-C12BP films, and the peak positions are a t slightly higher energies. These results suggest that an eightcarbon bis(phosphonate) alkyl chain length is probably at the lower limit for producing a film with any crystalline order in the alkyl layer. Hf-CloBP films and Hf-ClZBP films have very similar va(CH2)and vs(CHz ) peak positions and widths, although the Hf-CloBP film on the SCnP anchor does show some broadening as more layers are added. Overall, these films probably have very similar order, and while better ordered than Hf-CsBP and Hf-&BP, the alkyl chains are not as crystalline as monolayers of equivalent alkanethiols on gold or octadecyltrichlorosilane on silicon. However, work by Mallouk's group does suggest longer bis(phosphonates1 do come closer to the crystallinity observed in such monolayer systems.16 Finally, some peculiarities are observed in the ppolarized Ya(CH2) and v,(CHz peak intensities as a function of the number of layers of Hf-CGBP and -CsBP multilayers. Although the intensities grow linearly for the first 11or 12 layers, they grow more rapidly with the addition of subsequent layers. In this case disorder could be a major factor in the incremental intensity increases, since blue-shifted peak positions and broadening widths are also observed. Increased density of packing is a n unlikely factor in the increased intensity per layer, simply because denser films are generally associated with more order. The average tilt could also be increasing as the layers become more disordered, which again would account for increased intensity per layer. However, in spite of the apparent increasing disorder, film growth beyond ten layers is still fairly uniform. The 1500-900 cm-' region ofthe film spectra is difficult to interpret because there are many overlapping broad bands (Figure 6). Two very weak peaks a t approximately 1470 and 1410 cm-' are attributable to methylene bending modes (shown in the inset of Figure 6). Unfortunately these peaks are often masked by water vapor bands, which can only be eliminated by extended nitrogen purging of the IR chamber. For spectra taken after prolonged purging, these peaks are readily observable. The main band in this region is a broad and usually structureless peak between 1100 and 1080 cm-l, corresponding to the asymmetric POs stretch (Table 2). For (28) Snyder, R. G.; Scherer, J. R. J . Chem. Phys. 1979, 71, 3221.
OBrien et al.
0.010
4
0.005
0,000 1500
1300
1100
900
Wavenumbers (cm")
Figure 6. Phosphonate spectral region of the p-polarized IR spectra of four different films with three Hf-C,BP layers. "he Hf-CloBP film was grown on SC,P; the others were grown on SC12P. "he inset in the upper left shows the weak peaks at 1410 and 1470 cm-l from a 25-layer Hf-ClzBP film (magnified 5x1.
Table 2. Stretching Modes of the PO3 RegionSo 1350-1175 cm-l (unassociated)
P-0 str
RP0s2- str
PO32- str (inorganic salts)
1150-1250 cm-l (associated) 1125-970 cm-' asym 1000-960 cm-l sym 1030-970 cm-I a s p 990-920 cm-' sym
some samples with only a few layers, the main peak is located around 1035 cm-I and gradually shifts to the higher frequency of the asymmetric peak. Unfortunately, this phenomenon is observed irregularly and does not seem to be dependent on the anchor length or bis(phosph0nate) chain length, nor does it always occur in identically prepared films. Interpretation of this observation is therefore not possible a t this time. Another peculiarity in this spectral region is the appearance of two distinct peaks a t 1253 and 1310 cm-I in the H f - c ~ B psample after the tenth layer and in the Hf-CloBP sample grown on the SC4P anchor layer. These peaks probably correspond to shoulders observed between 1130 and 1310 cm-' in the spectra of all other films and are most likely due to a P=O ~ t r e t c h . ~ The ~ , P=O ~~ vibrations are sensitive to association effects such as the degree of hydrogen bonding (or metal binding), so some variation in the peaklshoulder position from sample to sample might be expected.30 The splitting of this peak in the case of the Hf-CsBP and -CloBP films is noteworthy because it could signify that the phosphonate groups are present in two distinctly different configurations. Other F'TIR studies which have examined thin f l m phosphonates and comparable bulk materials have shown only small peaks or shoulders in this region.16 These studies also show that bulk materials have considerably more structure in the lower energy 900-1200 cm-l PO3 spectral region than do SA films, perhaps indicating that the films are less uniform in terms of phosphonate configurations. Although this spectral region may hold important structural information regarding the configuration and uniformity of phosphonate groups in these films, uncertainties in peak assignments currently prevent a structural analysis of this nature.
Conclusions The FTIR studies reported here establish a lower boundary for alkyl chain length of hafnium a,@-al(29) Maoz, R.; Sagiv, J. J . Colliod Interface Sci. 1984, 100, 465. (30) Socrates, G. Infrared Characteristic Group Frequencies, 1ed.; Wiley-Interscience: Bristol, 1980.
H f - C a p Multilayers on Gold kanediylbis(phosph0nate)multilayer films necessary to form films with alkyl chain ordering greater than that associated with liquid-phase alkanes. This lower limit corresponds to an eight-carbon alkyl chain, or l,&octylbis(phosphonate). Longer bis(phosph0nate) alkyl chain lengths produce better-ordered multilayers, but there is not a significant difference between the alkyl chain ordering in Hf-CloBP and Hf-ClZBP multilayer films. Annealing ofthe gold substrate prior to functionalization and multilayer self-assembly appears to have no observable effect on the alkyl chain order in multilayer films; FTIR spectra for multilayers on as-evaporated and on annealed gold-coatedsubstrates are essentially identical. This result is not unambiguous, however, because we did not directly assess the roughness of the substrates used in this study. It is possible that the annealed and asevaporated substrates were more similar in surface topography than was expected based on previous studies.2g Alternatively, if the methods used preparing the asevaporated and annealed substrates did indeed produce the expected different surface topographies, then these results indicate that the surface roughness ofthe substrate does not significantly affect the alkyl chain order of selfassembled multilayers. STM studies of substrates prior to multilayer self-assembly are planned to clarify this issue. The use of SClzP gives a better-ordered anchor monolayer for the initiation of multilayer growth than does the shorter-chain SC4P, but the enhanced order in the SClzP anchor layer does not increase the order of alkyl chains in Hf-CloBP grown on them significantly. The peak positions and fwhm of the Y,(CHZ)and Y&CHZbands of Hf-CloBP films grown on both anchor layers are nearly identical, indicating that the multilayers have essentially
Langmuir, Vol. 10, No. 12, 1994 4663 the same degree of alkyl chain ordering. Thus, enhanced order in the anchor layer is not perpetuated with multilayer growth. However, a striking difference in the intensities of the methylene stretch bands (and to a lesser degree the phosphonate bands) in the spectra of Hf-CloBP multilayers grown on SClzP and SCIP is observed. The intensities of the SClzP-anchored Hf-CloBP methylene stretch bands is only one-third as large as those of the SC4P-anchored Hf-CloBP multilayers. While this effect could be due to a smaller density of the SC1pPanchored Hf-CloBP multilayer, there is no evidence for more disorder in these films which would be a consequence of a lower alkyl chain density. The most likely explanation involves different average alkyl chain tilt angles in the two types of films. This explanation requires that the average alkyl chain tilt of the anchor layer is different for SC4P and SClpP monolayers and that this difference is maintained in the alkylbis(phosphonate) layers as film growth progresses.
Acknowledgment. This research was supported in part by the Department of Energy (DE-FG06-86ER45273). Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this project (PRF 26158-ACS). Support from the German Research Foundation for A.C.Z. is gratefully acknowledged. We thank Professor Thomas Mallouk for providing the SCdP, Teledyne Wah Chang Albany, Inc., for donating the HfOC12.8Hz0, and Dr. Peter Zeppenfeld a t Forschungszentrum Julich (KFA), Germany, for providing the smooth gold substrates. We also thank Professor Martin Wybourne, Jolinda Smith, and Chris Berven for assistance preparing the as-evaporated gold substrates.
