Langmuir 1993,9, 2156-2160
2156
Structural Studies of Zirconium.Alkylphosphonate Monolayers and Multilayer Assemblies M. L. Schilling, H. E. Katz,’ S. M. Stein, S. F. Shane, W. L. Wilson, S. Buratto, S. B. Ungashe, G. N. Taylor, T. M. Putvinski, and C. E. D. Chidseyt AT&T Bell Laboratories, Murray Hill, New Jersey 07974 Received March 18,1993. I n Final Form:June 4,1993 Zirconium organophosphonates form thermally stable surface multilayers of predictable ellipsometric thickness. Fluorescence and nonlinear optical properties of dyes incorporated in such f i i indicate organized structures;however, other data place limits on the degree of order obtained. Here, we present evidence gleaned from IR, XPS,and cyclic voltammetry that Zr phosphonate multilayers are less perfect than previously thought and contain defecta arising in the initial stages of f i b growth. We still consider the transition metal phosphonate matrix to be attractive for constructing novel optical materials, but in fabricating active structures it will be necessary to compensate for defects. Approaches to doing so are under evaluation.
Introduction
It has been five years since the use of phosphonatezirconium bonds as a basis for surface layer assembly was f i t proposed by Mallouk.’ By starting with a substrate rich in phosphate or phosphonate groups, it is possible to grow films by alternate chemisorption of W +and organodiphosphonates,with stoichiometriescorresponding to analogous layered solids that would be formed from the same components. Since then, his group has established that zirconium alkylydinediphosphonatemultilayers may be reproducibly prepared and fashioned into insulating or rectifying electrode barriem2 We have independently confirmed the intrinsically insulating nature of these materials, prepared related multilayers from electronically donating, accepting: and polar chromophores, and observed second-ordernonlinear optical effects4and evidence of interlayer electron transfer in these systems.6 A single inert spacer layer is sufficient to prevent the interlayer interaction in the latter case. The films may be prepared using simple equipment, and the method is tolerant to many functional groups. Multilayers of this type give ellipsometric thicknesses consistent with those expectedbased on the lengths of the molecular components employed. Furthermore, our optical data indicate that orientational order and interlayer spacing may be controlled in these systems. We might therefore consider zirconium phosphonate multilayers as dense, well-ordered materials, “inorganic analogues of Langmuir-Blodgett films”,S without the thermal and solvent instabilities associatedwith LB f i . On the other hand, films we have prepared with rigid, linear chromophore exhibit neither X-ray diffraction peaks nor linear UV-vis dichroism, and the dielectric constants and
conductivities are strongly hydration dependent? These observations are inconsistent with the notion that these f i i are well-ordered or dense. We expect that the optical effects in our multilayers would be enhanced if the films could be prepared with closely packed, ordered morphologies. Furthermore, for consideration as electro-opticcomponents, the f hwould have to be sufficientlydefect-freeto withstand high electric fields without short-circuits. In order to probe the degree to which zirconium coordination surface chemistry can produce multilayers with the requisite quality for the above experiments and applications, we have undertaken the present study. Although we ultimately wish to construct thick multilayers using rapid, automated processes, we focus here on monolayers prepared slowly and carefully, with the aim of observing the maximum achievable density and order. Compounds that would be expected to self-associate and pack easily were employed. Layers were equilibrated to the maximum extent before characterization. A variety of tests were performed on the layers, including ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), FTIFi spectroscopy, and cyclic voltammetry (CV). In exceptionalcases,fairlydense monolayer coverage was achieved. More generally, however, the results place limits on the degree of order that one should expect from materials prepared using zirconium phosphonate self-assembly.
Experimental Section
Materials. Octadecylphosphonicacid (Cia, Alfa) and nonadecylmercaptan(ClfiH,Pfaltz and Bauer)were uaed aa received. A two-stepreaction sequencewaa used to prepare l.H,lH,,W,Wperfluorotetradecylphosphonicacids(C1329, elementalanalyses within 0.1-0.5 of theory) and the dodecane-,hexadecane-, and + Present address: Stanford University, Stanford, CA 94305. eicosanediylbis(phosphonic acids) (PC1s,PClSp,F’C&, respec(1) (a)Lee,H.;Kepley,L.J.;Hong,H.-G.;Mallouk,T.E.J.Am.Chem. tively) from the bromides by reaction with triethyl phosphite, SOC. 1988,110,618-620. (b) Lee, H.; Kepley, L.J.;Hong, H.-G.;Akhter, followed by conversionto the trimethylailylesters and hydrolysis S.; Mallouk, T. E.J. Phys. Chem. 1988,92,2597-2601. tothephosphonicacids.e Tetraethyl 1,4-benzenediphoephonate (2) Kepley, L. J.; Sackett, D. D.; Bell,C. M.; Mallouk, T. E. Thin Solid Films 1992,208, 132. waa prepared by photochemical reaction of the diiodide with (3) Katz, H.E.; Schilling, M. L.; Ungashe, S. B.; Putvinski, T. M.; sodium diethyl phosphite in liquid a”onia.1° The phosphonate Chidsey, C. E. D. Supramolecular Architecture; ACS Symposium Seriea 499, Bein, T., Ed.; American Chemical Society: Washington,DC, 1992;
24-32. (7) Katz, H. E.; Schilling, M. L. Chem. Mater., in press. -~ (4) Katz, H. E.;Scheller, G.;Putvinski,T. M.;Schilling, M. L.;Wilson, (8)Law, H. H.; Sapjeta, J.; Chidsey, C. E. D.; Putvinski, T. M. W.L.; Chidsey, C. E. D. Science 1991,254, 14861487. Unpublished resulta. (5) Ungashe,S.B.;Wilson,W.L.;Katz,H.E.;Scheller,G.R.;Putvinski, (9) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; T. M . J. Am. Chem. SOC. 1992,114,8717. Mujece, A. M.; Emerson, A. B. Langmuir 1990,6, 1567-71. (6) Cao, G.; Hong,H.-G.; Mallouk, T.E.Acc. Chem. Res. 1992.25, (10) Bunnett, J. F.; W e b , R. H. OrganicSynthesis; Wdey: New York, 1988; Collect. Vol. VI, p 461. 420-427. DD
r r
~~
0743-7463/93/2409-2156$04.00/00 1993 American Chemical Society
Zirconium Alkylphosphonate Monolayers was converted to the corresponding phosphonic acid (PPhP)via the trimethylsilyl ester. (Triethoxysily1)nonadecylacetate (SiClr OAc), the cqmpound used as a monolayer anchor layer on silicon substrates, and 194hiononadecylphosphonic acid (HSCleP), whichis used as the anchor layer or gold substrates, were prepared as described below. Nonadec-18-en-1-01. To a solution of ll-tetrahydropyranyloxyundecyl bromide (45.58 g, 136 mmol) (prepared from 11bromoundecanol and dihydropyran in CHzClz with pyridinium tosylate as catalyst) in anhydrous THF (100 mL) was added anhydrous lithium chloride (50 mg) and anhydrous copper(I1) chloride (80mg)." The solution was stirred under an atmosphere of nitrogen until all the solids had dissolved and then cooled to -15 OC in an ice-methanol bath (10% methanol). A solution of 7-octenylmagnesiumbromide (150 mmol in 100 mL of THF) was transferred to this solution using a Teflon cannula with occasional heating of the canula to dissolve any material that crystallized. The Grignard solution was transferred slowly over the period of about 1h to avoid heating the cooled solution above 0 OC. After all of the Grignard solution had been transferred, the stirred solution was allowed to warm to room temperature, and by that time, solid magnesium bromide started separating out. The mixture was then stirred overnight. The flask was opened and 250 mL of a half-saturated NHlCl solution was added and the two-phase mixture was stirred for 1 h. The mixture was transferred to a separatory funnel; the lower phase was discarded and the upper phase was shaken with saturated NH4Cl solution. The solvent was removed from the upper phase by rotary evaporation and the residue was passed through basic alumina, flushing the column with toluene. The toluene was removed by rotary evaporation to give 47.59 g of a clear oil. Methanol (150 mL) was added to the oil followed by hydrogen chloride gas (about 0.5 g) and the mixture stirred for a few minutes to form a solution. Most of the methanol was removed by rotary evaporation under high vacuum, and another portion of methanol was added. Finally all the methanol was removed to give 38.75 g of an off-white solid. The solid was crystallized from ethyl acetate (100 mL) in the freezer. The yield was 27 g (71%). 1H NMR CDCla 6 1.25 (broad, H, CHz), 1.56 (m, 2H, CH~CHZOH), 2.02 (m, 2H, C H Z C H ~ H Z3.64 ) , (t,2H, CH2OH), 4.94 (m, 2H, C H Z C H ~ H Z5.82 ) , (m, lH, CH&H=CHz). (19-Acetoxynonadecyl)triethoxysilane. A mixture of nonadec-18-enol (2.55 g, 9.0 mmol), acetic anhydride (1.85 g, 18.12 mmol), pyridine (1.80 g, 22.7 mmol), and a catalytic amount of dimethylaminopyridine was heated until a solution was obtained and was then stirred for 20 min without any further heating. To the solution was added hexane (20 mL) and hydrochloric acid solution (20 mL, IO%),and the two-phase mixture was stirred for 15 min. The upper hexane layer was separated, dried with anhydrous NaaSO4 and MgS04, and filtered and the hexane was removed by rotary evaporation to give 2.75 g of a clear oil, pure by proton NMR. Upon standing, the oil partly solidified. It was crystallized from methanol and dried in a vacuum desiccator to give 2.46 g (84% yield) of 19-acetoxynonadec-1-ene.'H NMR CDC13 6 1.25 (broad, 32H, CHz), 1.61 (m, 2H, CHZCHZOCOCH~), 2.02 (m, 2H, CHzCH=Ch2), 2.04 (s,3H, CHZOCOCH~), 4.05 (t, 2H, CHzOCOCHa), 4.94 (m, 2H, CHzCH==CHz), 5.80 (m, lH, CHzCH=CHz). A solution of 19-acetoxynonadecene (2.39 g, 7.36 mmol), chloroplatinic acid solution (2 drops, 10% in 2-propanol), and triethoxysilane (1.81 g, 11.05 mmol) in anhydrous hexane (20 mL) was heated at reflux for 4 h under an atmosphere of nitrogen. The hexane and excess triethoxysilane were removed by rotary evaporation to give 3.44 g (96% yield). The product was purified by distillation at reduced pressure to give 0.93 g (26%),bp 132141 OC (250 mTorr). *H NMR CDCla 6 0.63 (t, J = 8.4, 2H, CHzSi), 1.22 (t,9H, (CH&HzO)aSi), 1.25 (broad, 32H, CHz), 1.61 (m, 2H, CHZCHZOCOCH~); 2.04 (s,3H, CHzOCOCHs), 3.80 (9, 6H, (CH&H20)aSi), 4.05 (t, 2H, CHzOCOCH3). Anal. Calcd for Cz,H&i04: C, 68.6; H, 11.9; Si 5.9. Found C, 67.2; H, 11.6; Si, 5.7. Diethyl Nonadec-18-enylphosphonate. A mixture of 19(11) (a) Schill, G.; Merkel, C. Chem. Ber. 1978,111, 1446-1452. (b) Erdik, E. Tetrahedron 1984,40,641-657. (c) Fouquet, G.; Schlosser,M. Angew. Chem., Znt. Ed. Engl. 1974,13,82-8.
Langmuir, Vol. 9, No. 8, 1993 2157 bromononadecenell(3.76 g, 10.88 mmol) and triethyl phosphite (2.71 g, 16.33 "01) was heated and stirred overnight at 155-160 "C with nitrogen slowly flowing beneath the surface. Upon cooling, the product solidified to a yellowish waxy mass. The excess triethyl phosphite was removed by rotary evaporation using a mechanical vacuum pump and a bath temperature of 60 OC. The product was dissolved in 20 mL of carbon tetrachloride by heating, 20 mL of water was added, and the mixture was stirred overnight in the presence of air. The mixture was transferred to a separatory funnel along with 10 mL of saturated N K C l solution, shaken, and allowed to separate. The organic phase was dried with NazSO, and MgSO, and filtered, and the solvent was removed by rotary evaporation. The white solid was kept under mechanical pump vacuum for 3 h and weighed 4.29 g (98% yield). Diethyl 19-Thioacatylnonadecylphosphonate.A solution of diethyl nonadec-18-enylphosphonate(3.93 g, 9.76 mmol) and thioacetic acid (1.15 g, 15.11 mmol) in 8 mL of n-octane was prepared by heating in a quartz tube. The solution was irradiated at 2 in. from a 200-W medium pressure ultraviolet lamp (Hanovia) for 2 h and the solvent and excess thioacetic acid were removed by rotaryevaporation. An NMR analysis showedthe olefin group was gone and the reaction complete. The product weighed 4.68 g (100% yield). Diethyl 19-thiononadecylphosphonate.To a solution of the above product (4.68 g, 9.77 mmol) in ethanol (35g) was added concentzatedhydrochloric acid (3mL) and the mixture was heated at reflux for 5 h and poured into water. The solid product was filtered off, rinsed with water, and stored in a vacuum desiccator overnight to dry. 19-Thiononadecylphosphonicacid. To a solution of the above compound (3.86 g, 8.8 mmol) in methylene chloride (30 mL) was added trimethylsilyl bromide (4.06 g, 26.5 mmol) and the solution was stirred overnight under nitrogen. After the mixture was heated at reflux for 2 h, 20 mL each of methylene chlorideand water were added and the mixture was ultrasonicated for 30 min to help break up the lumps that had formed. Half of thesolvent wasremovedbyrotaryevaporationand was replaced by toluene. The mixture was heated to the boiling point and the upper toluene layer decanted off. This was repeated twice more with the residue, combining all the toluene extracts. A white solid had remained in the water phase and was discarded. The toluene extracta were heated to boiling to drive off any water present and then transferred to the refrigerator to cool for a few hours. The buff colored solid was fitered off and dried in a vacuum desiccator. The solid was recrystallized from ethyl acetate, after removal of a small quantity of insoluble material. After drying in a vacuum desiccator, the solid weighed 1.96 g (58% yield). lH NMR 6 1.13 (lH, SH), 1.2-1.3 (34H, internal CHz), 1.7 (2H, CHZ), 2.54 (2H, CH sub 2 S). Anal. Calcd for Ci&PSOs: C, 60.0; H, 10.9; S, 8.4. Found: C, 60.1; H, 10.7; S, 8.2. Layer Deposition. The thiols were deposited on freshly evaporated gold surfaces by immersion of the substrates in 51 mM solutions in ethanol/5% HzO for 1-3 days. In order to add a layer, a phosphonic acid terminated substrate was immersed in a 5 mM solution of zirconium oxychloride in HzO (pH 3) for 5-10 min, washed thoroughly with HzO, and spun dry before placing it in a 1 mM solution of the appropriate monolayer component. The deposition conditions for each specific sample are listed in Tables I and 11. After the appropriate time, the substrate was washed with ethanol and spun dry. Ellipsometry and CV measurements were carried out as soon as possible after sample preparation in order to avoid contamination. For the FTIR experiments, a monolayer of SiCleOAc was deposited directly on a freshly cleaned (3:l HzS04-30% Hz02, 20 min of sonication followed by 2 h of standing; 10% aqueous HF, 1min; 1:l NHdOH-30% HzOz, 10 min; deionized HzO soak; CAUTION strong oxidizers and corrosives) silicon internal reflection plate by immersion in a 1 mM solution (20% CCW 80% water-saturated isooctane made slightly acidic with AcOH) overnight at room temperature. The acetate moieties on the surface of the substrate were reduced to hydroxyl groups by in situ reaction with lithium borohydride in anhydrous THF for 2 h at room temperature. Reaction of the hydroxyl-terminated surface with phosphorus oxychloride/2,4,6-collidinein anhydrous
Schilling et al.
2158 Langmuir, Vol. 9, No. 8,1993 Table I. PhosDhonic Acid Monolayer Thicknesses thickness (A) deposition estc 25 27 21 26 31 11 26 26 29
ellip 27 30-35 13 24 12 25 38 13 31 21 33
conditions RT, 2 days RT, 2 days RT, 2 h 70 OC, 1day RT, 1day 70 OC, 2 days 70 "C, 3 days 70 "C, 2 h
70°C;3days 70°C,3days HzOsP(CHz)z(CFz)iiCFs(CirFzaP) RT,lday EtOsSi(CH2)lgOAc(SiC1gOAc)b a Thiol attached to Au substrate. b Silaneattached to Si substrate. Estimated from standard bond lengths. Table 11. Differential Capacitance for Monolayer and Bilayer Assemblies. capacitance deposition OcF/cm2) conditions bare Au CleSH
25 l.lb
HSCisP
1.1-1.5
ClrFzaP
1.1 0.8 1.0-1.2
Clap
PClZP PClsP
0.5-0.9 1.1
1.2 0.4
PCaOp
1.2 0.6
I
I
RT, 2 days RT, 2 days 70 OC, 3 days RT, 2 days
70 "C, 3 days 70 "C, 1day 50 "C, 3 days 70 "C, 2 days RT, 2 days 70 "C, 3 days
a Monolayera are thiols anchored to gold; bilayere are a zirconated HSC18 layer + an additional monolayer. Literature value.12
acetonitrile generated a phosphate surface which could then be sequentially reacted with Zr'+and a phosphonic acid for layer deposition as described above. Techniques. Ellipsometry was performed with a rotatinganalyzer ellipsometer (Gaertner),assuming a refractive index of 1-54.' The cyclic voltammetry was carried out as previously described.I2 XPS analyses were carried out on a Physical Electronics XPS Analyzer Model 5600. Samples were placed in the spectrometer, under vacuum, as soon as possible after preparation to minimize surface contamination. Standard spectral recording and analytical methodswere employed duringXPS analysis. Quantitative determination of elemental content (atomic 7%) was made at a 20" takeoff angle in order to investigate surface compositions. For purpose of this study, only relative amounta of zirconium and phosphom were compared. The infrared spectra were obtained with a Nicolet 800 FTIR spectrometer using unpolarized light from a globar infrared source,a HgCdTedetector,and multiple internal reflection (MIR) geometry. The silicon internal reflection plates (Harrick) were 50 mm X 20 mm X 1mm with both edges beveled at 45O, yielding 25 reflections each from front and back. With this geometry, perpendicular and parallel components of the transition dipole are detected with approximately the same sensitivity. Spectra were recorded under dry nitrogen at 2 cm-1 resolution. The force microscope used was a Park Scientific,Inc., Model SFM-BD2. The force tips were made by anisotropically etching SiNx following a standard procedure. The t i p are pyramidal with 70' angle sidesand a tip radius of curvatureof approximately 40 nm. The force constant of the cantilever was 0.032 N/m.
Results Layer Deposition and Ellipsometry. The typical ellipsometric thickness (h3A) achieved in depositing the aliphatic phosphonic acids on Zr+4 surfaces are listed in Table I. All of the compounds could be deposited as (12) Chidsey,C.E.D.;Loianco, D.N.Langmuir 1990,6,6824391.
2800
2850
2900
2950
3000
wavenumber(cm- 1 )
Figure 1. CH2 stretchingregion of the infrared spectrum of a monolayer of SiClsOAc deposited on an internalreflection plate and reduced with LiBfi. approximately monomolecular layers as judged by ellipsometry. It will be shown, however, that ellipsometry paints an overly optimistic picture of phosphonate layers deposited on Zr. For comparison, two long-chain thiols deposited on gold and an alkylsilane deposited on Si are also listed. Besides the monolayers, a 10-layersample of PClSp was prepared on gold. The monolayer ellipsometricthickness was reproducible throughout the multilayer, as long as fresh reagent solutions were employed. The dielectric constant at 1 MHz and the dc resistivity for a film immediately after preparation, determinedusing a Hg drop top contact, were 4.1 and 2 X 108 Q cm, respectively, and, after dehydration, 3.4 and >loll Q cm, respectively. The values and hydration dependence of these quantities are consistent with results obtained on related multilayer^.^ FTIR Spectroscopy. The deposition of SiClgOAc followed by reduction, phosphorylation, zirconation, and deposition of P C l a on a silicon internal reflection plate was monitored by FTIR in order to obtain information about the density and quantity of material deposited. Figure 1 shows the CH2 stretching region of SiClgOAc after deposition and reduction to the alcohol. The positions and width of the bands indicate that this initial layer is well packed.13 The symmetric stretch is observed at 2850 cm-l and the asymmetric peak is at 2918 cm-l. Figure 2 shows the incremental spectral changes (A(absorbance)) obtained by taking the difference of the spectra recorded just before and just after the noted chemical treatment for several subsequent steps in the deposition of PClSp. The order in the anchor layer is reduced on phosphorylation and still further on zirconation as the hydrocarbon chains move to accommodate the sterically demanding inorganic groups, as evidenced by the negativepeaks at the frequency associatedwith ordered hydrocarbon chains and a broad positive increase at the higher frequencies associated with disordered chains, On this sample, the PC1a material appears to go down as a mixture of ordered and disordered domains, as evidenced by the fact that each additional layer contributes a broadened, and in the case of the asymmetric stretch, bimodal, incrementalpeak. The bimodal mexima are 2918 and 2925 cm-l, corresponding to bands attributed to ordered and disordered material, respectively. More commonly, only a broad, red-shifted difference peak is observed. In addition, the quantity of PClsP deposited ~
~
_
_
(13) Porter, M.D.;Bright, T.B.;Allara, D.L.;Chidsey,C.E.D.J. Am. Chem. Soc. 1987,109,3559-3568.
_
_
~
Zirconium Alkylphosphonate Monolayers
Langmuir, Vol. 9, No. 8,1993 2159
CH2 BAND DIFFERENCE SPECTRA PHOSPHORYLAT
w h
VI
.-c
4-
ZIRCONATE
r; v
2800
2850 2900 2950 wovenumber(cm- 1 )
3000
Figure 2. Incremental changes in CH2 stretching bands for consecutive deposition steps, progressing from top to bottom. Zr Content as a Function of Deposition SteD
50
-
40
-
-
C.lc
BP
30-
20
-
10'
I
I
A + Z r wash
PPhP wash
Zr
PPhP
Figure 3. Plot of Zr content 88 atomic percent of total Zr + P content at various stages of the deposition process: top trace, calculated percent based on ideal monolayer depositions; bottom trace, percent observed by XPS.
on the Zr surfaces is only about half of what would be expected based on the intensity of the SiClsOR anchor signals. XPS and AFM. In an attempt to ascertainthe number density of P and Zr atoms as well as the stability of P and Zr surface bonds, deposition of Zr and PPhP was monitored. This compound was selected to minimize the screening of the peaks of interest by carbon. Figure 3 shows the Zr content as an atomic percent of total Zr and P content at each step of the deposition process as comparedto the Zr content expected for perfect deposition. The initial zirconium gives near ideal coverages;however a water wash (pH 3) removed a substantial amount of Zr, suggesting that some of it is only loosely bound to the surface. The Zr content follows the same trend as the ideal prediction but is lower than expected at each step. The second zirconation restores the Zr content to near ideal, but again the next step removes more Zr than can be explained by depth effects in the XPS measurement. In order to check that the Zr density was not inflated by the occasional adhesion of >lo0 A clumps of Zr salts, the reaction of ZrOCla with a P surface was studied by AFM. When fresh, strongly acidic (pH 1-3) Zr solutions were used, the smoothness of the surface was unaffected by Zr deposition. On the other hand, when a less acidic solution (pH 5) was used, particles were indeed observed
on the surface,as expected.'* We have found that solutions of Zr compounds over the pH range 0-4 and containing a variety of counterions may be used to prepare surfaces for organophosphonic acid deposition. In a second series of AFM experiments, samples of 1 and 10 layers of P C l a were deposited on primed, zirconated gold. Coverage of the surface by particulates was 2% of the sample area for the 10-layer sample and
