Atomic Layer Deposition of Amino-Functionalized Silica Surfaces

In the present study N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS) was used as a silylating agent. The characterization of aminosilylated sili...
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9650

J. Phys. Chem. B 2004, 108, 9650-9655

Atomic Layer Deposition of Amino-Functionalized Silica Surfaces Using N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane as a Silylating Agent Satu Ek,* Eero I. Iiskola, and Lauri Niinisto1 Laboratory of Inorganic and Analytical Chemistry, Helsinki UniVersity of Technology, P.O. Box 6100, FIN-02015 Espoo, Finland ReceiVed: January 5, 2004; In Final Form: April 28, 2004

Atomic layer deposition (ALD) technique can be used for the preparation of amino-functionalized silica surfaces and for the study of the gas-solid reactions. In the present study N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS) was used as a silylating agent. The characterization of aminosilylated silica samples was performed by elemental analyses, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and solid-state 13C NMR. Under saturation conditions, viz. at 180 °C and at a pressure of 20-50 mbar, vaporized AAPS molecules were observed to react with the silanols of silica at the silane end of the molecule forming siloxane bridges. Thus, one surface-saturated molecular layer was deposited on the surface. Under these conditions indication of the gas-phase reaction of terminal amino groups with methoxy groups of other AAPS molecules or silanol groups of silica was observed. The surface density of amino groups on the silica surface could be controlled within 2.0-3.4 amino groups/nm2 silica through the pretreatment temperature of silica, i.e., 200-800 °C. The amino group density on silica could also be controlled through a procedure based on sequential gas-phase reactions of AAPS and water. Thus, a high-density aminopropylsiloxane network was grown on the silica surface. With this procedure a surface density of 3.0-5.4 amino groups/nm2 of silica (pretreated at 450 °C) could be obtained depending on the number of AAPS/water cycles. The surface was observed to be saturated with the precursor molecules after four AAPS/water cycles. The gas-solid reactions of AAPS on silica were also compared with those of single-amino-group precursors, viz. γ-aminopropyltrialkoxysilanes.

1. Introduction Atomic layer deposition (ALD), also known as atomic layer epitaxy (ALE), is a gas-phase technique traditionally used in micro- and optoelectronic applications for the deposition of thin films onto planar substrates or 3D objects with excellent conformality.1 ALD is a surface-controlled layer-by-layer deposition process involving alternating, self-limiting surface reactions to achieve controlled atomic-level deposition. The film thickness can be simply controlled by the number of reaction cycles, i.e., atomic layers employed. Metal compounds, such as oxides, sulfides, and nitrides, and metals are typically deposited onto planar substrates as well as onto porous supports by ALD. When ALD is applied to porous supports with a high surface area,2 usually only one or a few molecular layers are deposited, whereas for planar substrates thicker layers, processed from tens to thousands of ALD cycles, are employed. The high surface area of porous supports enables the study of gas-solid reactions of precursor molecules on the solid surface. A wide range of chemical compounds, e.g., β-diketonate-type chelates, halides, or alkoxides, which vaporize without decomposition and have sufficient reactivity can be used as precursors for ALD on porous supports for various applications. According to published data, most studies based on ALD on porous supports are focused on the catalyst applications. Aminopropylalkoxysilanes are generally used in various applications as coupling agents to immobilize different kinds of ions and molecules on the surface.3,4 One of the most * To whom correspondence should be addressed. Phone: 358-9-451 2602. Fax. 358-9-462 373. E-mail. [email protected]; [email protected].

important commercial applications is their use as stationary phases in chromatographic columns.4,5 At the moment, the preparation of amino-terminated silica surfaces from the liquid phase is frequently studied for research and industrial purposes, the organic solvent and sol-gel processes being the most reported techniques in the literature.4 Nevertheless, ALD may offer a useful, and besides a solvent-free, preparative route to amino-functionalized silica surfaces for applications where a controllable and reproducible surface density of amino groups is needed. The ALD technique also enables the preparation of a high-density aminopropylsiloxane network on the surface by using sequential reactions of aminosilanes and water.6 In ALD, hydrolysis of alkoxy groups and condensation of the hydroxyl groups formed can be avoided before deposition on the surface. The gas-phase-deposited product has been observed to possess significantly improved performance characteristics in chromatographic applications.7 We previously studied atomic layer deposition and gas-solid reactions of several γ-aminopropylalkoxysilane precursors containing one amino group on porous silica (Table 1).6,8,9 Recent studies reported in the literature on the ALD processes of various γ-aminopropylalkoxysilanes on porous silicon dioxide surfaces are also listed in Table 1.6-13 The aim of the present study was to show that aminosilylated silica surfaces can also be prepared by using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS) as a precursor by ALD. The deposition and surface densities of AAPS on silica were compared with the previous results of single-amino-group precursors, viz. γ-aminopropyltrimethoxysilane (APTMS) and γ-(aminopropyl)triethoxy-

10.1021/jp0499629 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/04/2004

ALD of Amino-Functionalized Silica Surfaces

J. Phys. Chem. B, Vol. 108, No. 28, 2004 9651

TABLE 1: Gas-Phase Studies Reported on the Deposition of Aminopropylalkoxysilanes on Porous Silica aminosilane APTS APTS APTS APTS APTS APTMS APTMS APDMS APDMES APDMES APDMES a

pretreatment temp./°C

amino groups/nm2

ref

100-160 150 200-800 450 450 200-800 450 200-800 200-820 400 600-820

1.4 1.3 1.3-2.0 2.0 1.9-3.0a 1.1-1.8 1.8-3.0a 1.4-2.1 1.3-1.8 not reported 1.0-1.5

7 10 8,9 11 6 8 6 8 8 12 13

Depending on the number of aminosilane/water cycles performed.

CHART 1

silane (APTS).8 In particular, the effects of heat treatment of silica and sequential gas-phase reactions of AAPS and water on the number of amino groups on silica were studied. 2. Experimental Section 2.1. Precursor. N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS) from ABCR, Germany, was used as received. The molecular structure of the precursor is shown in Chart 1. The FTIR spectrum of liquid AAPS in CCl4 was measured in a Nicolet Magna IR 750 spectrometer. A liquid cuvette with NaCl windows was inertly filled with AAPS under nitrogen atmosphere. The spectra were collected in the 4000-400 cm-1 region at a 2 cm-1 resolution over 32 scans. 2.2. Silica Surfaces. The support material was EP 10x silica gel from Crosfield Ltd., U.K., with a specific surface area of 300 m2/g. The pore volume was 1.2 cm3/g and pore diameter 20 nm, while the average particle size was 100 µm. In silica heat treated at 800 °C, the specific surface area decreased to 280 m2/g and pore volume to 1.0 cm3/g. For the BET (Brunauer-Emmett-Teller) measurements, the silica gel was degassed at 350 °C in a vacuum (10-6 Torr) and the determinations performed using nitrogen as the adsorbate. To prepare silica surfaces with various degrees of dehydroxylation, the silica support was heat treated in air at 200, 450, 600, and 800 °C for 16 h in a muffle furnace in air. Sequential reactions of AAPS and water were performed on silica pretreated at 450 °C. 2.3. Gas-Phase Depositions. 2.3.1. Deposition of One Surface-Saturated Molecular Layer on the Surface. AAPS was deposited onto porous silica via the gas phase by the atomic layer deposition technique.1,2 Prior to the depositions, the heattreated silica support (4-5 g) was pretreated in an ALD reactor (F-120, ASM Microchemistry Ltd., Espoo, Finland) at 180 °C for the removal of physically adsorbed water from the surface. The pressure in the ALD reactor was 20-50 mbar. AAPS was vaporized at 130 °C and deposited onto the silica bed at a reaction temperature of 180 °C. Physisorbed precursor molecules were then purged from the surface with inert nitrogen gas. One reaction sequence of AAPS resulted in one surface-saturated molecular layer. No further heat treatment was performed after the deposition. The as-deposited silica samples were stored in a desiccator to avoid hydrolysis of the methoxy groups by air.

2.3.2. Procedure Based on Sequential Reactions of AAPS and Water on the Surface. In this procedure, AAPS was first deposited onto silica (10 g) under the above-described experimental conditions. Then the AAPS-modified silica was treated with water at 150 °C in the ALD reactor whereupon the free, unreacted methoxy groups were hydrolyzed. At the same time, silanol groups of silica that were not reacted with the methoxy groups of AAPS molecules were exposed.6,8 Next, the sample was further treated with AAPS and followed by water treatment. These sequential AAPS/water cycles were repeated six times. The procedure is described in more detail in our previous publication.6 2.4. DRIFTS Measurements. DRIFTS technique was used for the qualitative inspection of the surface species on heattreated silica and aminosilylated silica samples. The DRIFT spectra were measured in a Nicolet Magna IR 750 spectrometer equipped with a Spectra-Tech diffuse reflectance accessory. The spectrum obtained from a steel mirror was used as the background. The samples were not diluted with KBr14 because only qualitative inspection was performed by means of DRIFTS. The measurements were made in a microsize sampling cup. The samples for analysis were prepared in air as quickly as possible to avoid hydrolysis of the methoxy groups. The spectra were collected in the 4000-400 cm-1 region at a 2 cm-1 resolution over 64 scans. 2.5. 13C CP/MAS NMR Measurements. In addition to DRIFTS, the surface species on aminosilylated silica before and after water treatment were qualitatively studied by solid-state13C CP/MAS NMR. The experiments were carried out at 270 MHz, in a Chemagnetics CMX Infinity spectrometer using a 2 ms contact time (i.e., the time during which the 1H and 13C spin systems are brought into contact with each other), 5 s recycle delay, and 4 kHz MAS speed. The powder samples were handled under dry nitrogen and placed in zirconia rotors of 6 mm o.d. The strength of the rf fields employed was 50 kHz, and between 12 000-15 000 transients were acquired. The chemical shifts are reported relative to TMS using an external sample of hexamethylbenzene. 2.6. Elemental Analyses. The numbers of carbon and nitrogen atoms in the aminosilylated silica samples were calculated from the results of elemental analyses carried out in a LECO CHN-600 elemental analyzer. 3. Results and Discussion 3.1. Silica Surface. For a comprehensive study of the gassolid reactions of precursor molecules on the silica surface, qualitative and quantitative information on the surface species of silica is needed. The silica surface is modified during heat treatment whereupon dehydration and dehydroxylation reactions take place. Physisorbed water is removed from the surface, and the relative amounts of hydrogen-bonded and isolated silanols are altered.4 The numbers of the different types of silanols on silica pretreated at various temperatures as determined by 1H MAS NMR measurements15 are shown in Table 2. At low pretreatment temperatures (450 °C, strained siloxane bridges also appear on the surface. These strained siloxane bridges have been observed to act as reactive sites for alkoxysilanes,16 and thus, aminopropylalkoxysilanes are also likely to react with the siloxane groups on silica. In addition, evolving alcohol molecules may react with both siloxane groups and silanols of silica.17

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TABLE 2: Effect of Heat-Treatment Temperature of Silica on the Number of Nitrogen and Carbon Atoms of Gas-Phase Deposited AAPS on Silica

a

heat treatment of silica/°C

isolated OH groups on silica15/nm2

H-bonded OH groups on silica15/nm2

200 450 600 800

1.9 2.0 1.6a 1.1b

4.6 2.1 0.5a

N atoms/ nm2 silica

AAPS molecules/nm2

C atoms/ nm2 silica

C/N ratio

3.4 3.0 2.5 2.0

1.7 1.5 1.3 1.0

9.2 8.4 7.2 5.5

2.7 2.8 2.8 2.7

Measured on silica pretreated at 560 °C. b Measured on silica pretreated at 820 °C.

TABLE 3: Most Important Frequencies and Band Assignments18,19 in the DRIFT Spectra of AAPS on Silica Before and After Water Treatment and in the FTIR Spectrum of Liquid AAPS peak position/cm-1 AAPS on silica AAPS on silica (pretreated at 450 °C) after water treatment liquid AAPS assignmenta 3678 s,b 3445 m 3372 w 3348 w 3309 m 3170 m 2935 m 2874 m 2843 m (2750 vw) (2699 vw)

3666 s,b

1596 w 1453 w,b 1404 w,b

1597 m 1452 w,b

3364 m 3305 m

3388 m 3321 m

2932 m 2881 w 2822 w

2941 s 2884 w 2840 s 2742 w 1614 m 1576 w 1459 s 1411 m

ν(O-H) ν(N-H) ν(N-H) ν(N-H) ν(N-H) ν(N-H) ν(C-H) ν(C-H) ν(C-H) ν(C-H) ν(C-H) δ(NH2) δ(H-C-H) δ(H-C-H)

a ν ) stretching, δ ) bending, b ) broad, s ) strong, m ) medium, w ) weak, vw ) very weak.

Figure 1. DRIFT spectra of (a) silica pretreated at 450 °C and (b) silica treated with AAPS.

3.2. Gas-Solid Reactions of AAPS on the Silica Surface. The principal reaction of γ-aminopropylalkoxysilanes from the gas phase at moderate deposition temperatures, as well as from the liquid phase, occurs through the silane end of the precursor molecule with the silanol groups of silica. Then the alkoxy groups of aminosilanes react with the silanol groups forming siloxane bridges.8 No physisorbed molecules remain on the surface due to the use of elevated reaction temperatures and nitrogen purging in the ALD reactor. On the basis of our previous study it is known that isolated silanols selectively react with vaporized γ-aminopropylalkoxysilane precursors under saturation conditions, viz. 150 °C at 20-50 mbar.8 However, in the present study we used a somewhat higher reaction temperature, viz. 180 °C at a pressure of 20-50 mbar, than with the single-amino-group precursors. The saturation of the silica surface was corroborated by both elemental analysis and DRIFTS for samples taken from the surface and bottom of the modified silica bed. The DRIFT spectrum of AAPS on silica pretreated at 450 °C is shown in Figure 1, and the band assignments are presented in Table 3. These bands can be compared with the bands shown in the FTIR spectrum of liquid AAPS (Figure 2). The band corresponding to O-H stretching vibrations of free silanols on silica at about 3750 cm-1

Figure 2. FTIR spectrum of liquid AAPS.

disappears (Figure 1 and Table 3) after AAPS treatment. There are, however, silanol groups which have not reacted with AAPS molecules probably due to steric reasons. The existence of these silanols has been confirmed by previous experiments with deuterium oxide.6,8 These silanol groups form hydrogen bonds with the free alkoxy groups of chemisorbed AAPS molecules, and this can be seen as a broad and pertubated band for O-H vibrations in the DRIFT spectrum at 3678-62 cm-1, the exact wavenumber depending on the heat-treatment temperature of silica. The most common stretching vibrations of N-H bonds are seen at 3376-72 and 3309-3401 cm-1 and the bending vibrations at 1616-1596 cm-1 (Table 3 and Figures 1 and 3a). The N-H stretching vibrations of -NH- groups cannot be distinguished from the -NH2 groups. The above-mentioned bands are also observed for the corresponding single-amino-

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Figure 4. 13C CP/MAS NMR spectra of silica (a) after the first AAPS treatment and (b) after the first water treatment.

Figure 3. DRIFT spectra of silica treated with vaporized AAPS and water recorded (a) after the first AAPS treatment, (b) after the first water treatment, and (c) after the second AAPS treatment.

group precursor, APTMS.8 The amino groups of AAPS molecules form hydrogen bonds with the surface silanols, as has been shown for single-amino-group precursors.20-22 An indication of the reactions of amino groups with the methoxy groups of adsorbed AAPS molecules or silanols of silica can be seen in the spectrum because the stretching vibration of N-H bonds in -Si-NH-CH2- appears at 3449-38 cm-1 (Figures 1 and 3a). These secondary reactions, which occur in the gas-phase deposition of aminopropylsilanes at temperatures g150 °C, have been reported earlier.9 The bands at 3348 and 3170 cm-1 are also located in the region where N-H stretching vibrations are generally seen.23 In addition, several stretching C-H vibrations and bending H-C-H vibrations are visible below 3000 cm-1 (Table 3). The chemisorption of AAPS molecules on silica can also be observed by 13C CP/MAS NMR (Figure 4). The methylene groups are clearly distinguishable in the NMR spectra, but vibrations of the carbon atoms of free methoxy groups overlap with the γ- and γ′-carbon atoms and are seen as a shoulder near 50 ppm. When the chemical shifts of the liquid precursor,24 (CH3O)3-Si-C(R)H2-C(β)H2-C(γ)H2-NH-C(γ′)H2-C(γ′′)H2-NH2, and those of the chemisorbed AAPS on silica are compared (Table 4), the largest difference can be observed in the R-carbon, which is affected by the bonding of one or more methoxy groups. 3.3. Effect of Pretreatment Temperature of Silica on the Surface Density of Amino Groups on Silica. The surface

density of AAPS molecules on silica decreases from 1.7 to 1.0 AAPS/nm2 (Table 2) when the pretreatment temperature of silica is increased from 200 to 800 °C. When the surface densities of AAPS on silica are compared with our earlier results8 of singleamino-group precursors, trifunctional APTS and APTMS, and also bifunctional APDMS (γ-aminopropyldiethoxymethylsilane), they are observed to be somewhat lower. One reason for this is the difference in the reaction temperature, which was 150 and 180 °C for single-amino-group precursors and AAPS, respectively. We earlier observed that the nitrogen content in aminosilylated silica distinctly decreases due to the reactions of the amino groups with the ethoxy groups and silanols of silica when the deposition temperature of APTS is raised from 150 to 300 °C.9 We also observed that the size of the precursor molecule, i.e., the number and the type of alkoxy ligands (ethoxy and methoxy), affects the surface density of amino groups of aminosilanes on silica.8 Thus, steric effects may also cause the lower surface density of AAPS on the surface. The relative sizes of AAPS and APTS molecules on β-cristobalite (111)25 are shown in Chart 2, where these molecules are monodentately attached on the surface. One AAPS molecule with the longer carbon chain covers a somewhat larger surface area on silica than APTS (or APTMS) with shorter carbon chains. According to Chart 2, aminosilane molecules overlap some silanols on the surface. The achieved amino-group densities are lower than the density of isolated OH groups on silica, but AAPS molecules are likely to react selectively with isolated silanols instead of hydrogenbonded silanols, as observed for single-amino-group precursors.8 On the basis of the obtained surface densities, approximately 66-89% of the isolated silanols on silica are observed to react with AAPS molecules in the gas phase depending on the pretreatment temperature of silica (Table 2). According to the calculated C/N ratios, viz. 2.7-2.8 (Table 2), both bi- and tridentate bonding modes of AAPS are likely to coexist on the silica surface. The pretreatment temperature of silica does not seem to distinctly affect the bonding mode of AAPS on silica. However, these results do not exclude the possibility of monodentately bound surface species on silica. Also, the solidstate 13C NMR results show that free methoxy groups remain on the surface after the surface reaction, and thus, mono- and/ or bidentate bonding modes of AAPS on silica can coexist. For a more comprehensive characterization of the surface species on silica, a 29Si NMR study would be useful. According to our solid-state NMR results, APTMS and APTS molecules are deposited onto silica heat treated at 600 °C both mono- and bidentately.26 When 3-(2-aminoethyl)(aminopropyl)triethoxysilane has been deposited onto silica in the liquid phase, it is observed to be solely bidentately bound.27

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TABLE 4: Chemical Shifts in the 13C CP/MAS NMR Spectra of Liquid Precursor24 and AAPS on Silica (pretreated 450 °C) Before and After Water Treatment sample

-OCH3

-Si-C(R)H2-

-C(β)H2-

-C(γ)H2-NH-C(γ′)H2-

-C(γ′′)H2-NH2

liquid AAPS AAPS on silica AAPS on silica after water treatment

50.50 51a -

6.75 9.2 9.6

23.25 23.2 23.1

52.55 51.1 51.4

41.95 41.1 40.9

a

Shoulder.

CHART 2

TABLE 5: Surface Densities of Nitrogen and Carbon Atoms on Silica (pretreated at 450 °C) in Depositions Performed with Sequential Reactions of Vaporized AAPS and Water steps 1st AAPS 1st water 2nd AAPS 2nd water 3rd AAPS 3rd water 4th AAPS 4th water 5th AAPS 5th water 6th AAPS 6th water

AAPS N atoms/nm2 molecules/nm2 C atoms/nm2 C/N ratio 3.0 3.1 4.1 4.0 4.8 4.8 5.1 5.2 5.3 5.3 5.5 5.4

1.5 1.6 2.1 2.0 2.4 2.4 2.6 2.6 2.7 2.7 2.8 2.7

8.4 7.4 10.4 10.0 12.3 12.1 12.9 13.0 13.4 13.3 14.0 13.7

2.8 2.4 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

3.4. Effect of AAPS/Water Cycles on the Surface Density of Amino Groups on Silica. A gas-phase procedure based on the sequential gas-solid reactions of trifunctional γ-(aminopropyl)triethoxysilane (APTS) or -trimethoxysilane (APTMS) and water has been previously developed by us.6 In the present study, a similar treatment for silica was performed by using AAPS as a silylating agent. First silica, pretreated at 450 °C, was allowed to react with AAPS, whereupon a single surfacesaturated molecular layer was formed and a surface density of 3.0 amino groups/nm2, i.e., 1.5 AAPS/nm2, of silica was obtained (Table 5). The DRIFT spectra of silica after the first AAPS treatment are shown in Figures 1 and 3a and are discussed above. After the first AAPS step the surface was treated with water, leading to hydroxylation of the free methoxy groups. The formation of free OH groups cannot be observed by DRIFTS on silica pretreated at low temperatures, i.e., < 600 °C, because of hydrogen-bonding of OH groups.6 Nevertheless, this has been observed with single-amino-group precursors on silica pretreated at 600 and 800 °C.6,8 Changes in the C-H stretching region after hydroxylation of methoxy groups cannot be clearly observed (Figure 3b). In our spectrum, the band for the stretching vibrations at 3445 cm-1of N-H bonds for -Si-NHCH2- groups is seen to disappear after hydrolysis. Also, the N-H stretching bands at 3348 and 3170 cm-1 disappear, so they are probably due to the secondary reactions of amino groups with the surface. This is because the -Si-NH- bonds

Figure 5. Effect of AAPS/water cycles on the surface density of AAPS molecules and carbon atoms on silica: (B) after AAPS treatment and (O) after water treatment.

are very sensitive to hydrolysis. At the same time the C/N ratio is diminished from 2.8 to 2.4, which indicates a complete hydroxylation of methoxy groups (Table 5). Also, in the 13C NMR spectrum free methoxy groups of chemisorbed AAPS molecules on silica, seen as a shoulder near 50 ppm, are observed to disappear after water treatment (Figure 4), as expected. Cross-linking reactions between OH groups resulting in a formation of siloxane bridges are likely to occur at these temperatures. After the first AAPS/water cycle the surface was further treated with AAPS. The above-mentioned AAPS/water cycles were repeated several times, in this case six times. The most intense increase of amino groups on silica was observed during the first three cycles, after which a surface density of 4.8 amino groups/nm2, i.e., 2.4 AAPS/nm2, of silica was achieved (Table 5 and Figure 5). In general, the amino-group density on silica pretreated at 450 °C could be controlled through sequential gasphase reactions of AAPS and water within a range of 1.5-2.7 AAPS/nm2. After several, at least four, aminosilane/water cycles a high-density aminopropylsiloxane network was grown on the surface. The growth is assumed to occur through horizontal polymerization of AAPS molecules on the surface because the surface is saturated after four or five cycles and thereafter the number of nitrogen atoms does not significantly increase (Table 5 and Figure 5). 4. Conclusions Similar gas-solid reactions were observed to take place with AAPS and single-amino-group precursors on dehydroxylated silica surface. Vaporized AAPS molecules were observed to react with the silanols of silica from the silane end of the

ALD of Amino-Functionalized Silica Surfaces molecule forming siloxane bridges. The terminal amino groups of AAPS molecules were also observed to react to some extent with the methoxy groups of other chemisorbed AAPS molecules and the silanols of silica at the reaction temperature of 180 °C (20-50 mbar). A high surface density of chemisorbed precursor molecules on the surface could be achieved in a reproducible way also with AAPS. The amino-group density could be controlled on silica through heat-treatment temperature of silica and sequential gas-phase reactions of AAPS and water. The number of amino groups on silica was in the range of 2.0-3.4 amino groups/nm2 on silica heat treated at 200-800 °C, which corresponds to 1.0-1.7 AAPS molecules/nm2. The obtained surface densities were lower than the results achieved with single-amino-group precursors, e.g., APTS and APTMS, due to the higher deposition temperature used and also due to steric reasons. When a gas-phase procedure based on four sequential AAPS/water cycles was performed on silica, a surface density of 2.6-2.7 AAPS/nm2 (corresponding to 5.2-5.4 amino groups/ nm2) could be achieved. Because the surface was observed to be saturated after four or five cycles, the polymerization reaction is assumed to take place in the horizontal direction. These results based on the modification of porous silica can be further applied for thin film deposition processes on planar or curved substrates. Acknowledgment. Financial support for S.E. and E.I. from the Fortum Foundation and for S.E. from the Jenny and Antti Wihuri Foundation is gratefully acknowledged. Dr. Jetta Kera¨nen is thanked for performing the BET measurements of silica and Dr. Andrew Root for the solid-state NMR measurements. Dr. Joseph Campbell is thanked for revising the language of the manuscript. References and Notes (1) (a) Suntola, T. Mater. Sci. Rep. 1989, 4, 261. (b) Suntola, T. In Handbook of Crystal Growth; Hurle, D. T. J., Ed.; Elsevier: Amsterdam, 1994; Vol. 3B, pp 601-603, 605-663. (c) Niinisto¨, L. Proc. 23rd Int. Semicond. Conf. 2000, 1, 33. (d) Ritala, M.; Leskela¨, M. In Handbook of Thin Film Materials; Nalwa, H. S., Ed.; Academic Press: San Diego, 2002; Vol. 1, pp 103-159. (e) Leskela¨, M.; Ritala, M. Thin Solid Films 2002, 409, 138. (f) Leskela¨, M.; Ritala, M. Angew. Chem., Int. Ed. 2003, 42, 5548. (g) Ritala, M. In High-k Gate Dielectrics; Houssa, M., Ed.; Institute

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