- Email: [email protected]
Molecular self-assembly at a pre-formed Langmuir-Blodgett template
768
Tl1in Soliri I;;/txv. 244 ( I YY4) 76X 77 I
Molecular self-assembly at a pre-formed Langmuir - Blodgett template Houston
Byrd, Scott Whipps,
John K. Pike and Daniel
R. Talham*
Department of ChemOtry, lJniwrsit_vof Floridu, Gainesville, FL 3261 l-2046 (USA)
Abstract The use of a Langmuir-Blodgett monolayer as a template for subsequent self-assembly of organic molecules from solution has been investigated. The template is formed by transferring a monolayer of octadecylphosphonic acid to a hydrophobic substrate and then binding Zr4+ ions to the surface from solution. The asymmetric methylene (v,(CH,)) band of the template layer appears at 2918 cm-’ and possesses a full width at half-maximum of 20 cm -’ indicating that an all-trans close-packed template is formed. X-ray photoelectron spectroscopy (XPS) analysis of the template layer shows a 1:1 ratio of Zr:P. Octadecylphosphonic acid, l,lO-decanediyldiphosphonic acid (DDPA) and quaterthiophenediphosphonic acid (QDP) have been self-assembled to the template and characterized by attenuated
total reflection-Fourier transform IR, XPS and UV-visible techniques. Octadecylphosphonic acid self-assembles to the zirconium template and produces a well-ordered close-packed film as followed by TR analysis. Multilayers of DDPA have been assembled to the template layer; however, the increase in the integrated area of the ra(CH2) band is not linear with the number of DDPA layers assembled. A rigid diphosphonic acid molecule such as QDP has also been assembled one layer at a time to the template. A plot of UV-visible absorbance us. the number of layers of QDP shows a linear increase in absorbance after each QDP deposition step.
1. Introduction Organic thin films are being explored as materials for use in areas such as microelectronics [ 11, photoconductors [2] and non-linear optics [3-51. These applications depend on the ability to control the organization and packing density of functional molecules within the film. The Langmuir-Blodgett (LB) technique [6] and organic self-assembly (SA) methods [7, 81 are two approaches used to develop organized organic thin films. Thin films [9, lo] of transition metal phosphonates have been deposited onto surfaces through SA techniques by first anchoring a layer of molecules or polymer bearing the phosphonate functionality to a surface and then alternately adsorbing the transition metal ion (usually Zr4+) and an a,w-diphosphonic acid to build up the film. Zirconium alkane-phosphonate films [ 93 prepared in this manner have been shown to act as insulators in metal-insulator-semiconductor and metal-insulatormetal sandwich devices [SC]. Other groups [lo] have used a similar deposition method to prepare oriented assemblies of functionalized molecules one layer at a time for non-linear optical applications. In any layer-by-layer deposition process, the key step is the first layer or template layer, as this will dictate how
*Author
to whom
correspondence
0040-6090/94/$7.00 SSDI 0040-6090( 93)04 11O-E
should
be addressed
molecules pack in subsequent layers. In this paper, we describe use of the LB method combined with inorganic SA to produce a well-ordered close-packed template surface upon which phosphonic acid molecules can be self-assembled.
2. Experimental
details
2.1. Materials Octadecylphosphonic acid ( C,8H3903P) and 1, lodecanediyldiphosphonic acid (C,,H,,O,P,) (DDPA) were synthesized from octadecylbromide and 1,l O-dibromodecane respectively by the MichaelissArbuzov reaction [ 111, followed by acid hydrolysis. Quaterthiophenediphosphonic acid (QDP) was provided by Katz [ 10~1. Zirconyl chloride (98%) was used as purchased from Aldrich (Milwaukee, WI). Single-crystal ( 100) face silicon wafers, purchased from Semiconductor Processing Company (Boston, MA), were used as deposition substrates. Parallelogram (45”) silicon attenuated-total-reflection (ATR) crystals (50 mm x 10 mm x 3 mm) purchased from Wilmad Glass (Buena, NJ), were used as substrates for all IR experiments. The silicon substrates were cleaned using the RCA [ 121 method before octadecyltrichlorosilane (OTS) was self-assembled to make the substrates hydrophobic.
‘(‘8 1994 ~
Elsevier Sequoia.
All rights
reserved
769
H. Byrd et al. 1Self-assemblyat LB templates 2.2.
Phosphonic Acid Langmuir Monolayer
Methods
The LB experiments were performed using a KSV 3000 LB trough (Stratford, CT). A Barnstead NAN0 pure purification system produced water with a resistivity of 18 MR cm for all experiments. For depositions a target pressure of 20 mN m-’ was maintained with deposition speeds of 10 mm min-‘. IR spectra were recorded with a Mattson Instruments (Madison, WI) research series 1 Fourier transform (FTIR) spectrometer using a narrow-band mercury cadmium telluride detector. A Harrick (Ossining, NY) TMP stage was used for the ATR experiments. All spectra consist of 1000 scans at 2.0 cm-’ resolution and were ratioed to the OTS-covered substrates. X-ray photoelectron spectra were obtanied using a Perkin--Elmer PHI 5000 series spectrometer. All spectra were obtained using the Mg Ka line source at 1253.6 eV. Multiplex scans, 140 scans at each peak, were run over a 20-40 eV range with a pass energy of 17.90 eV. UV-visible spectra were obtained with a HewlettPackard 8452A diode array spectrometer.
Deposition
/
\ Sample Vial with Phosphonic Acid Template
Self-Assembled Film
3. Results and discussion 3.1. Deposition
and characterization
of the zirconium
Self-Assemble molecules for 1 hour
)
template
The deposition procedure combining the LB method with inorganic SA to form the LB template is outlined in Fig. 1 and has been described in a previous paper [ 131. Briefly, a single layer of octadecylphosphonic acid is transferred to a hydrophobic substrate by dipping the substrate down through the LB film into a vial which is immersed in the trough. The vial containing the octadecylphosphonic acid-substrate is removed from the onto the trough and Zr4+ ions are then self-assembled phosphonic acid molecules by adding enough zirconyl chloride to produce a 5 mM zirconium solution. At this point, X-ray photoelectron spectroscopy analysis shows that the Zr:P ratio is 1: 1 (Table I), indicating that each phosphonic acid site binds a Zr4+ ion from solution during the inorganic SA step. IR analysis of the Zr4’ ion template layer indicates that it is a well-ordered close-packed monolayer. Figure 2 (upper spectrum) shows the C-H bands for the IR region from 3 100 to 2400 cm-‘. The position and full width at half-maximum of the asymmetric methylene (v,(CH;)) band are useful in determining the order [7b, 1611and packing [ 7b, 171 of the aliphatic chains in monolayers. The position of the v,(CH,) band at 2918 cm-’ is characteristic of the alkyl chains possessing an all-trans [7b, 161 conformation, while the full width at half-maximum of 20 cm-’ indicates that the monolayer is close packed [7b, 171. The template layer then has a high density of Zr4+ ion binding sites and
Fig. 1. Deposition template.
TABLE assembled
procedure
for the preparation
1. X-ray photoelectron zirconium
Film
Elements
type Template
phosphonate Area (counts
spectroscopy layers
eV SK’)
of a zirconium
multiplex
data for self-
Concentration” (%I)
Ratio to Zr
1
Zr P
1306 215
53.01 47.0
0.9
QDP (1 h)
Zr S P
3936 3418 1843
15.0 48.0 37.0
1 3.2 2.5
QDP (7h)
Zr
3228 2433 1507
15.7 45.6 38.6
2.9 2.4
s P
“The concentration is derived tivity factors [ 14, 151.
should phonic
from the atomic
and instrument
be a suitable surface for self-assembling acid molecules from solution.
1
sensi-
phos-
3.2. Self-assembly at the zirconium template Octadecylphosphonic acid is self-assembled to the template layer as shown in Fig. 1. The template-coated substrate is rinsed in pure water and placed into a 5 mM ethanol-water (ethanol:water = 9O:lO) solution
770
H. Byrd et al. / Self-assembly ai LB trmplates
I
A=O.Oi
.
v&342)
ok 0
’
2
I
4
.
.
.
’
6
’
a
’
10
Layers of DDPA
W342)
Fig. 3. A plot of the area (full width at half-maximum (fwhm) multiplied by the absorbance (Abs)) of the v,,(CH,) band at 2918 cm-’ 6s. the number of DDPA layers assembled at the octadecylphosphonic acid-zirconium ion template.
3000
2800
2600
2400
Wavenumbers Fig. 2. ATR-FTIR spectra monitoring the formation of the zirconium ion template (upper-spectrum) and a self-assembled octadecylphosphonic acid bilayer (lower spectrum). In each spectrum. three C H bands are resolved: v,(CH,) at 2960 cm-‘, r,(CH2) at 2918cmand v,(CH,) at 2852cm-‘. The intensity of the v,(CH2) band doubles from the template layer to the self-assembled bilayer. The full width at half-maximum for the I’~(CH,) band is 20 cm-’ for both spectra. All spectra are referenced to the OTS-covered silicon ATR crystal.
of octadecylphosphonic acid for 1 h. The film-coated substrate is then removed from the solution of octadecylphosphonic acid and is rinsed with water. Figure 2 (lower spectrum) shows the IR spectrum of the template layer after octadecylphosphonic acid is selfassembled. The position and full width at half-maximum of the v,(CH,) band are 2918 cm-’ and 20 cm-’ respectively, which indicates that the film is still close packed [ 7b, 16, 171. The intensity of the v,( CH,) band doubles from the template layer to the self-assembled octadecylphosphonic acid bilayer, demonstrating that every available binding site in the template layer is capped with an octadecylphosphonic acid molecule from solution. The production of a close-packed bilayer indicates that the packing of the template layer is transferred to the self-assembled layer. SA of multilayered films onto the template layer is accomplished by alternately dipping the template layer in a Zr4+ solution and then a solution of a,w-diphosphonic acid molecules. We followed the deposition method published by Mallouk and co-workers [9] and deposited DPPA from a 1.25 mM solution at the Zr4+ ion template to assemble multilayer films. Figure 3 is a plot of the integrated area of the v,(CH,) band us. the number of DDPA layers assembled. The integrated area of the v,(CHJ band is much greater for the first assembled DDPA layer than for subsequent layers.
Since the template layer has a high density of Zr4T ion binding sites and DDPA is not a rigid molecule, it appears that the DDPA, once bound to the Zr4+ ion template, is able to bend over and bind another Zr4+ ion site within the same plane. This bridging of Zr”+ ion sites by the DDPA molecules limits the number of sites available for binding in subsequent layers and accounts for the smaller increase in the intensity of the v,(CH2) band for each successive layer. After the third layer of DDPA is assembled, it appears that the increase in area with each additional layer of DDPA is linear. At this point, it is reasonable that the available Zr4+ ion sites are spaced in such a manner that bridging of these sites by DDPA is now less likely. However, the overall non-linear increase in the area of the v,(CHJ band demonstrates that, even though mulilayers can be formed by self-assembling DDPA at the Zr4+ ion template, these multilayered films are not close packed. QDP, a rigid [ lOc] diphosphonic acid molecule, was assembled onto the template layer from a 1 mM dimethylsulfoxide (DMSO) -H,O ( DMSO:H20 = 50: 50) solution of QDP that is adjusted to pH 3 with a 10% solution of HCl. Depositions are carried out at 25 “C for 1 h, followed by a 1 min rinse in both 50:50 DMSO-H,O and pure water to remove any excess material. XPS (Table 1) and UV-visible analysis [ 181 of the QDP monolayer film confirm that deposition of QDP is complete after 1 h. The QDP molecule was chosen because it is a rigid molecule and cannot bind both phosphonic acid groups to Zr4+ sites in the same plane. The assembly of QDP was monitored by UVvisible absorbance because of its strong absorption at 390 nm. Figure 4 shows the UV-visible spectrum of one layer of QDP together with a plot of the absorbance at 390 nm us. the number of layers of QDP. The increase in absorbance is linear, indicating that the same amount of material is being deposited during each QDP deposition step in the multilayer process. XPS analysis of a single layer of QDP self-assembled to the LB template is listed in Table 1. A Zr:S ratio of
H. Byrd et al. 1 Self-assembly at LB templates
111
Acknowledgments
I
I
I
I
300
I
500
400
Wavenumbers
I
We thank Dr. Howard Katz for supplying the QDP, the Major Analytical Instrumentation Center at the University of Florida for instrument time, and Mr. Eric Lambers for technical assistance. We also thank Professor Kirk S. Schanze for use of the UV-visible spectrometer. Acknowledgment is made to the National Science Foundation (Grant DMR-9205333) and the University of Florida Division of Sponsored Research for partial support of this research.
600
(nm)
References
o.o0
2
4
6
8
IO
Layers of QDP Fig. 4. (a) A UVvisible spectrum of one layer of QDP self-assembled onto the zirconium ion template. (b) A plot of the absorbance of QDP at 390 nm us. the number of layers of QDP assembled at the zirconium ion template: -, linear regression fit of the data.
1:4 is expected if one layer of QDP completely covers the Zr4+ ion template layer by binding one QDP molecule at each Zr4+ ion site. However, the observed Zr:S ratio is 1:3.2 which indicates that 80% of the Zr4+ ion sites are bound by QDP molecules. This is reasonable since the cross-sectional area [ 131 of the Zr4+ ion sites in the template layer is 24 A’molecule-‘, whereas the calculated cross-sectional area [ 191 for QDP is 28 A’.
4. Conclusions Monolayer and multilayered films can be self-assembled at a pre-formed, well-ordered LB template. IR analysis indicates that the order of the template layer can be transferred to the self-assembled monolayer of octadecylphosphonic acid. Multilayers of DDPA can assemble at the template layer, although the films are not close packed owing to the ability of the flexible molecule to bridge Zr4+ ion . sites within the same plane. In contrast, multilayered films of a rigid diphosphonic acid can be deposited layer by layer at the Zr4+ ion template with the same amount of material deposited in each layer.
1 J. D. Swalen, D. L. Allara, J. D. Andrade, E. A. Chandross, S. Garoff, J. Israelachvili, T. J. McCarthy, R. F. Murray, R. F. Pease, J. F. Rabolt, K. J. Wynne and H. Yu, Langmuir, 3 (1987) 932. 2 (a) F. R. Ahmed, P. E. Burrows, K. J. Donovan and E. G. Wilson, Synth. Met., 27 (1988) B593. (b) Y. Yamaguchi, T. Fujiyama and M. Yokoyama, J. Appl. Phys., 70 (1991) 855. 3 R. H. Page, M. C. Jurich, B. Reck, A. Sen, R. J. Twieg, J. D. Swalen, G. C. Bjorklund and C. G. Wilson, J. Opt. Sot. Am. B, 7 (1990) 1239. 4 D. Li, M. A. Ratner, T. J. Marks, C. Zhang, J. Yang and G. K. Wong, J. Am. Chem. Sot., If2 (1990) 7389. 5 A. Ulman, J. Phys. Chem., 92 (1988) 2385. 6 Gaines, G., Jr. (ed.). Insoluble Monolayers at Liquid&Gas Interfaces, Wiley-Interscience, New York, 1966. 7 (a) J. Sagiv, Isr. J. Chem., 18 (1979) 339. (b) R. Maoz and J. Sagiv, J. CoNoid Sci., 100 (1984) 465. 8 (a) S. R. Holmes-Farley, R. H. Reamey, T. J. McCarthy, J. Deutch and G. M. Whitesides, Langmuir, I ( 1985) 725. (b) L. Strong and G. M. Whitesides, Lnagmuir, II0 (1988) 3665. 9 (a) H. Lee, L. J. Kepley, H.-G. Hong, S. Akhter and T. E. Mallouk, J. Phys. Chem., 92 (1988) 2597. (b) H. Lee, L. J. Kepley, H.-G. Hong and T. E. Mallouk, J. Am. Chem. Sot., II0 (1988) 618. (c) L. J. Kepley, D. D. Sackett, C. M. Bell and T. E. Mallouk, Thin Solid Films, 208 (1992) 132. IO (a) T. M. Purvinski, M. L. Schilling, H. E. Katz, C. E. D. Chidsey. A. M. Mujsce and A. B. Emerson, Langmuir, 6 (1990) 1567. (b) H. E. Katz, G. Scheller, T. M. Putivinski, M. L. Schilling, IW. L. Wilson and C. E. D. Chidsey, Science, 254 (1991) 1485. (c) H. E. Katz, M. L. Schilling, C. E. D. Chidsey, T. M. Putvinski and R. S. Hutton, Chem. Mater., 3 (1991) 699. I I A. K. Bhattacharya and G. Thyagarajan, Chem. Reo., 81 (1981) 415. I2 W. Kern and D. Puotinen, RCA Rev., 31 (1970) 187. I3 H. Byrd, J. K. Pike and D. R. Talham, Chem. Mafer., 5( 1993) 709. I4 Perkin -Elmer PHI ESCA Operator’s Manual. Perkin-Elmer, 1989. I5 J. H. Scofield, J. Electron Spectrosc., 8 ( 1976) 129. I6 M. D. Porter, T. B. Bright, D. L. Allara and C. E. D. Chidsey, J. Am. Chem. Sot., 109 (1987) 3559. 17 K. A. Wood, R. G. Snyder and H. L. Strauss, J. Chem. Phys., 91 (1989) 5255. I8 H. Byrd, S. Whipps, J. K. Pike and D. R. Talham, unpublished results, 1993. I9 S. Tasaka, H. E. Katz, R. S. Hutton, J. Orenstein, G. H. Fredrickson and T. T. Wang, Synrh. Met.. 16 ( 1986) 17.
Tl1in Soliri I;;/txv. 244 ( I YY4) 76X 77 I
Molecular self-assembly at a pre-formed Langmuir - Blodgett template Houston
Byrd, Scott Whipps,
John K. Pike and Daniel
R. Talham*
Department of ChemOtry, lJniwrsit_vof Floridu, Gainesville, FL 3261 l-2046 (USA)
Abstract The use of a Langmuir-Blodgett monolayer as a template for subsequent self-assembly of organic molecules from solution has been investigated. The template is formed by transferring a monolayer of octadecylphosphonic acid to a hydrophobic substrate and then binding Zr4+ ions to the surface from solution. The asymmetric methylene (v,(CH,)) band of the template layer appears at 2918 cm-’ and possesses a full width at half-maximum of 20 cm -’ indicating that an all-trans close-packed template is formed. X-ray photoelectron spectroscopy (XPS) analysis of the template layer shows a 1:1 ratio of Zr:P. Octadecylphosphonic acid, l,lO-decanediyldiphosphonic acid (DDPA) and quaterthiophenediphosphonic acid (QDP) have been self-assembled to the template and characterized by attenuated
total reflection-Fourier transform IR, XPS and UV-visible techniques. Octadecylphosphonic acid self-assembles to the zirconium template and produces a well-ordered close-packed film as followed by TR analysis. Multilayers of DDPA have been assembled to the template layer; however, the increase in the integrated area of the ra(CH2) band is not linear with the number of DDPA layers assembled. A rigid diphosphonic acid molecule such as QDP has also been assembled one layer at a time to the template. A plot of UV-visible absorbance us. the number of layers of QDP shows a linear increase in absorbance after each QDP deposition step.
1. Introduction Organic thin films are being explored as materials for use in areas such as microelectronics [ 11, photoconductors [2] and non-linear optics [3-51. These applications depend on the ability to control the organization and packing density of functional molecules within the film. The Langmuir-Blodgett (LB) technique [6] and organic self-assembly (SA) methods [7, 81 are two approaches used to develop organized organic thin films. Thin films [9, lo] of transition metal phosphonates have been deposited onto surfaces through SA techniques by first anchoring a layer of molecules or polymer bearing the phosphonate functionality to a surface and then alternately adsorbing the transition metal ion (usually Zr4+) and an a,w-diphosphonic acid to build up the film. Zirconium alkane-phosphonate films [ 93 prepared in this manner have been shown to act as insulators in metal-insulator-semiconductor and metal-insulatormetal sandwich devices [SC]. Other groups [lo] have used a similar deposition method to prepare oriented assemblies of functionalized molecules one layer at a time for non-linear optical applications. In any layer-by-layer deposition process, the key step is the first layer or template layer, as this will dictate how
*Author
to whom
correspondence
0040-6090/94/$7.00 SSDI 0040-6090( 93)04 11O-E
should
be addressed
molecules pack in subsequent layers. In this paper, we describe use of the LB method combined with inorganic SA to produce a well-ordered close-packed template surface upon which phosphonic acid molecules can be self-assembled.
2. Experimental
details
2.1. Materials Octadecylphosphonic acid ( C,8H3903P) and 1, lodecanediyldiphosphonic acid (C,,H,,O,P,) (DDPA) were synthesized from octadecylbromide and 1,l O-dibromodecane respectively by the MichaelissArbuzov reaction [ 111, followed by acid hydrolysis. Quaterthiophenediphosphonic acid (QDP) was provided by Katz [ 10~1. Zirconyl chloride (98%) was used as purchased from Aldrich (Milwaukee, WI). Single-crystal ( 100) face silicon wafers, purchased from Semiconductor Processing Company (Boston, MA), were used as deposition substrates. Parallelogram (45”) silicon attenuated-total-reflection (ATR) crystals (50 mm x 10 mm x 3 mm) purchased from Wilmad Glass (Buena, NJ), were used as substrates for all IR experiments. The silicon substrates were cleaned using the RCA [ 121 method before octadecyltrichlorosilane (OTS) was self-assembled to make the substrates hydrophobic.
‘(‘8 1994 ~
Elsevier Sequoia.
All rights
reserved
769
H. Byrd et al. 1Self-assemblyat LB templates 2.2.
Phosphonic Acid Langmuir Monolayer
Methods
The LB experiments were performed using a KSV 3000 LB trough (Stratford, CT). A Barnstead NAN0 pure purification system produced water with a resistivity of 18 MR cm for all experiments. For depositions a target pressure of 20 mN m-’ was maintained with deposition speeds of 10 mm min-‘. IR spectra were recorded with a Mattson Instruments (Madison, WI) research series 1 Fourier transform (FTIR) spectrometer using a narrow-band mercury cadmium telluride detector. A Harrick (Ossining, NY) TMP stage was used for the ATR experiments. All spectra consist of 1000 scans at 2.0 cm-’ resolution and were ratioed to the OTS-covered substrates. X-ray photoelectron spectra were obtanied using a Perkin--Elmer PHI 5000 series spectrometer. All spectra were obtained using the Mg Ka line source at 1253.6 eV. Multiplex scans, 140 scans at each peak, were run over a 20-40 eV range with a pass energy of 17.90 eV. UV-visible spectra were obtained with a HewlettPackard 8452A diode array spectrometer.
Deposition
/
\ Sample Vial with Phosphonic Acid Template
Self-Assembled Film
3. Results and discussion 3.1. Deposition
and characterization
of the zirconium
Self-Assemble molecules for 1 hour
)
template
The deposition procedure combining the LB method with inorganic SA to form the LB template is outlined in Fig. 1 and has been described in a previous paper [ 131. Briefly, a single layer of octadecylphosphonic acid is transferred to a hydrophobic substrate by dipping the substrate down through the LB film into a vial which is immersed in the trough. The vial containing the octadecylphosphonic acid-substrate is removed from the onto the trough and Zr4+ ions are then self-assembled phosphonic acid molecules by adding enough zirconyl chloride to produce a 5 mM zirconium solution. At this point, X-ray photoelectron spectroscopy analysis shows that the Zr:P ratio is 1: 1 (Table I), indicating that each phosphonic acid site binds a Zr4+ ion from solution during the inorganic SA step. IR analysis of the Zr4’ ion template layer indicates that it is a well-ordered close-packed monolayer. Figure 2 (upper spectrum) shows the C-H bands for the IR region from 3 100 to 2400 cm-‘. The position and full width at half-maximum of the asymmetric methylene (v,(CH;)) band are useful in determining the order [7b, 1611and packing [ 7b, 171 of the aliphatic chains in monolayers. The position of the v,(CH,) band at 2918 cm-’ is characteristic of the alkyl chains possessing an all-trans [7b, 161 conformation, while the full width at half-maximum of 20 cm-’ indicates that the monolayer is close packed [7b, 171. The template layer then has a high density of Zr4+ ion binding sites and
Fig. 1. Deposition template.
TABLE assembled
procedure
for the preparation
1. X-ray photoelectron zirconium
Film
Elements
type Template
phosphonate Area (counts
spectroscopy layers
eV SK’)
of a zirconium
multiplex
data for self-
Concentration” (%I)
Ratio to Zr
1
Zr P
1306 215
53.01 47.0
0.9
QDP (1 h)
Zr S P
3936 3418 1843
15.0 48.0 37.0
1 3.2 2.5
QDP (7h)
Zr
3228 2433 1507
15.7 45.6 38.6
2.9 2.4
s P
“The concentration is derived tivity factors [ 14, 151.
should phonic
from the atomic
and instrument
be a suitable surface for self-assembling acid molecules from solution.
1
sensi-
phos-
3.2. Self-assembly at the zirconium template Octadecylphosphonic acid is self-assembled to the template layer as shown in Fig. 1. The template-coated substrate is rinsed in pure water and placed into a 5 mM ethanol-water (ethanol:water = 9O:lO) solution
770
H. Byrd et al. / Self-assembly ai LB trmplates
I
A=O.Oi
.
v&342)
ok 0
’
2
I
4
.
.
.
’
6
’
a
’
10
Layers of DDPA
W342)
Fig. 3. A plot of the area (full width at half-maximum (fwhm) multiplied by the absorbance (Abs)) of the v,,(CH,) band at 2918 cm-’ 6s. the number of DDPA layers assembled at the octadecylphosphonic acid-zirconium ion template.
3000
2800
2600
2400
Wavenumbers Fig. 2. ATR-FTIR spectra monitoring the formation of the zirconium ion template (upper-spectrum) and a self-assembled octadecylphosphonic acid bilayer (lower spectrum). In each spectrum. three C H bands are resolved: v,(CH,) at 2960 cm-‘, r,(CH2) at 2918cmand v,(CH,) at 2852cm-‘. The intensity of the v,(CH2) band doubles from the template layer to the self-assembled bilayer. The full width at half-maximum for the I’~(CH,) band is 20 cm-’ for both spectra. All spectra are referenced to the OTS-covered silicon ATR crystal.
of octadecylphosphonic acid for 1 h. The film-coated substrate is then removed from the solution of octadecylphosphonic acid and is rinsed with water. Figure 2 (lower spectrum) shows the IR spectrum of the template layer after octadecylphosphonic acid is selfassembled. The position and full width at half-maximum of the v,(CH,) band are 2918 cm-’ and 20 cm-’ respectively, which indicates that the film is still close packed [ 7b, 16, 171. The intensity of the v,( CH,) band doubles from the template layer to the self-assembled octadecylphosphonic acid bilayer, demonstrating that every available binding site in the template layer is capped with an octadecylphosphonic acid molecule from solution. The production of a close-packed bilayer indicates that the packing of the template layer is transferred to the self-assembled layer. SA of multilayered films onto the template layer is accomplished by alternately dipping the template layer in a Zr4+ solution and then a solution of a,w-diphosphonic acid molecules. We followed the deposition method published by Mallouk and co-workers [9] and deposited DPPA from a 1.25 mM solution at the Zr4+ ion template to assemble multilayer films. Figure 3 is a plot of the integrated area of the v,(CH,) band us. the number of DDPA layers assembled. The integrated area of the v,(CHJ band is much greater for the first assembled DDPA layer than for subsequent layers.
Since the template layer has a high density of Zr4T ion binding sites and DDPA is not a rigid molecule, it appears that the DDPA, once bound to the Zr4+ ion template, is able to bend over and bind another Zr4+ ion site within the same plane. This bridging of Zr”+ ion sites by the DDPA molecules limits the number of sites available for binding in subsequent layers and accounts for the smaller increase in the intensity of the v,(CH2) band for each successive layer. After the third layer of DDPA is assembled, it appears that the increase in area with each additional layer of DDPA is linear. At this point, it is reasonable that the available Zr4+ ion sites are spaced in such a manner that bridging of these sites by DDPA is now less likely. However, the overall non-linear increase in the area of the v,(CHJ band demonstrates that, even though mulilayers can be formed by self-assembling DDPA at the Zr4+ ion template, these multilayered films are not close packed. QDP, a rigid [ lOc] diphosphonic acid molecule, was assembled onto the template layer from a 1 mM dimethylsulfoxide (DMSO) -H,O ( DMSO:H20 = 50: 50) solution of QDP that is adjusted to pH 3 with a 10% solution of HCl. Depositions are carried out at 25 “C for 1 h, followed by a 1 min rinse in both 50:50 DMSO-H,O and pure water to remove any excess material. XPS (Table 1) and UV-visible analysis [ 181 of the QDP monolayer film confirm that deposition of QDP is complete after 1 h. The QDP molecule was chosen because it is a rigid molecule and cannot bind both phosphonic acid groups to Zr4+ sites in the same plane. The assembly of QDP was monitored by UVvisible absorbance because of its strong absorption at 390 nm. Figure 4 shows the UV-visible spectrum of one layer of QDP together with a plot of the absorbance at 390 nm us. the number of layers of QDP. The increase in absorbance is linear, indicating that the same amount of material is being deposited during each QDP deposition step in the multilayer process. XPS analysis of a single layer of QDP self-assembled to the LB template is listed in Table 1. A Zr:S ratio of
H. Byrd et al. 1 Self-assembly at LB templates
111
Acknowledgments
I
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We thank Dr. Howard Katz for supplying the QDP, the Major Analytical Instrumentation Center at the University of Florida for instrument time, and Mr. Eric Lambers for technical assistance. We also thank Professor Kirk S. Schanze for use of the UV-visible spectrometer. Acknowledgment is made to the National Science Foundation (Grant DMR-9205333) and the University of Florida Division of Sponsored Research for partial support of this research.
600
(nm)
References
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Layers of QDP Fig. 4. (a) A UVvisible spectrum of one layer of QDP self-assembled onto the zirconium ion template. (b) A plot of the absorbance of QDP at 390 nm us. the number of layers of QDP assembled at the zirconium ion template: -, linear regression fit of the data.
1:4 is expected if one layer of QDP completely covers the Zr4+ ion template layer by binding one QDP molecule at each Zr4+ ion site. However, the observed Zr:S ratio is 1:3.2 which indicates that 80% of the Zr4+ ion sites are bound by QDP molecules. This is reasonable since the cross-sectional area [ 131 of the Zr4+ ion sites in the template layer is 24 A’molecule-‘, whereas the calculated cross-sectional area [ 191 for QDP is 28 A’.
4. Conclusions Monolayer and multilayered films can be self-assembled at a pre-formed, well-ordered LB template. IR analysis indicates that the order of the template layer can be transferred to the self-assembled monolayer of octadecylphosphonic acid. Multilayers of DDPA can assemble at the template layer, although the films are not close packed owing to the ability of the flexible molecule to bridge Zr4+ ion . sites within the same plane. In contrast, multilayered films of a rigid diphosphonic acid can be deposited layer by layer at the Zr4+ ion template with the same amount of material deposited in each layer.
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