INTRODUCTION
In recent years, the applications and demand for solid surfaces with nanometer to micrometer patterns have increased because of extensive developments in the fields of nanoscience and nanotechnology. Solid surfaces with nanometer- and micrometer-scale patterns have many potential applications in many fields of nanoscience and nanotechnology, such as phase separation of polymer mix-tures,[1-3] localized crystal growth,[4] the arraying and immobilization of proteins,[5] cells,[6,7] nanoparticles,[8] etc. Methods for patterning micrometer features on surfaces are well developed and include microlithography of self-assembled units,[4,9] microcontact printing,[10-12] photopatterning of self-assembled monolayers (SAMs).[13-15] On the other hand, methods for developing nanometer-scale features, such as nanowriting,[16,17] phase-separated Langmuir-Blodgett (LB) films,[18-20] and block copolymers1-21-1 are time consuming and cumbersome.
Self-assembled monolayers consist of amphiphiles, which spontaneously adsorb onto a solid surface from solution to form a densely packed two-dimensional ordered monolayers. Self-assembled monolayers have been extensively used for modification of surface chemistry of solid surfaces. Recently, Fan et al.[22] have demonstrated that nanometer-scale domains of different chemical functionality can be obtained by co-adsorption and phase separation of different silane molecules. Sequential adsorption of self-assembling monolayers had been suggested as an alternate and simpler route for making such surfaces.[23,24] The island growth mechanism in silane monolayer can be exploited to prepare surface with nanodomains. In this article, we demonstrate the use of sequential adsorption of silanes to prepare domains of various sizes by controlling their self-assembly on solid surfaces.[25] We describe a procedure to prepare domains of one chemical functionality, with size from nanometer to micrometer surrounded by another chemical functionality, using sequential adsorption of silane self-assembled monolayers. Partial monolayers of octadecyltrichlorosi-lane (OTS) consisting of condensed islands with controlled size are prepared by varying the deposition conditions. The area surrounding the OTS islands is filled by sequential adsorption of 11-bromo undecyltrichlorosi-
lane (BrUTS) or decyltrichlorosilane (DTS) to obtain nanometer- to micrometer-scale domains of OTS in a monolayer of DTS or BrUTS. First, we describe in detail the methodology to form partial OTS monolayers composed of domains of a desired size. Then, we discuss the procedure and optimum conditions for successful backfilling. These monolayers were analyzed by atomic force microscope (AFM) to obtain the topographical and friction images.
OVERVIEW
In our study we use the silane-based SAMs, which were first studied by Sagiv et al.[26-30] Recent studies[31-34] suggest that the silane monolayers are formed by the adsorption of the hydrolyzed silanes[31-33,35] onto a water layer, which typically exists on a silica surface.[36-38] In addition, there is no evidence of direct binding of the adsorbing silanes to the surface silanols of the substrate. Because of this water layer and the lack of direct bonding, the adsorbed molecules are mobile and can diffuse laterally on the surface.[25,39] This mobility allows OTS molecules to aggregate into islands, which can be exploited to form nanometer-size domains by sequential adsorption. These islands, which are formed by aggregation, have been imaged by atomic force microsco-py[25,33,40-44] and have been found to be ”fractal”-like in structure. This is the result of an adsorption-surface diffusion-aggregation mechanism,[45] in which surface diffusion is rate determining.
The formation of OTS monolayers is affected by a number of factors during the deposition conditions. Below a critical deposition temperature (28±4°C), the complete monolayers exist as condensed into ordered phases resembling the liquid condensed (LC) states analogous to Langmuir monolayers.[33,39-47] A complete monolayer prepared above this critical temperature showed regions of chain disorder resembling the liquid expanded (LE) state. Increasing the deposition temperature resulted in more disorder, until the monolayers were in a completely LE state. The formation of LE or LC phases is analogous to the phase coexistence region for Langmuir films. The silane monolayers with shorter hydrocarbon chains (e.g., DTS) have lower critical deposition temperature. As a result they do not form LC states at room temperatures.[39]
The intermediate stages of the SAM formation have been investigated by studying the structure of partial monolayers. Atomic force microscope measurements of partial monolayers of OTS at room temperature 22°C) have shown a two-step growth mechanism.[25,33,41,44] In the first step, fractal islands of constant height, ~ 21 A,[25,33,41,44] relative to the background, appear and grow on the surface. This step is referred to as the primary growth step. The height of OTS islands, which are in the LC state, is approximately 25 ±1 A.[25,39,48-50] The AFM images of OTS monolayers during this step show that the height of islands is ~ 21 A, implying that the primary fractal islands are in fact LC phases surrounded by a low-density liquid phase. In the second step, the growth of the primary fractal islands stops, and the difference in height between the islands and the background begins to decrease.[25,41,43,44] These facts indicate that the surrounding is an LE phase whose density, and therefore height, is increasing. As the concentration of the LE phase increases, the surface diffusion to the primary islands is reduced, thereby arresting the primary growth. Finally, nucleation and growth of smaller islands of the same height (as the primary islands) occur in the regions surrounding the primary fractal islands.[25] This second step is referred to as the secondary growth step and it continues until a complete monolayer of uniform height is formed.
The OTS monolayers show a different growth mechanism at deposition temperatures well below room temperature (< 16°C).[25,33,41,43] In this case it is observed that the height of the primary island domains does not change over time and remains constant at ~ 21 A. This suggests that during growth the surrounding phase is either gaseous (G) (if the deposition temperature is below the triple point as supposed by Carraro et al.)[41] or a low-density liquid phase, if the deposition temperature is above the triple point (as suggested by Davidovits et al.).[43] Irrespective of whether the surrounding phase is G or LE, the region surrounding the primary islands is less dense at low temperatures. The primary growth continues longer with less inhibition from the surrounding phase. The growth of secondary phase is observed only when the primary islands are very close to each other.[25] Finally, at temperatures exceeding 40°C, all AFM studies show a uniform monolayer without any islands.[33,41,43]
The presence of water in the solvent and on the substrate also affects the monolayer formation. Octade-cyltrichlorosilane molecules are hydrolyzed by water present in the solvent.[51] Excess water in solvent results in the formation of particle oligomers, which can deposit on the solid surface.[40,52] However, these particulates are not strongly adsorbed on the surface and can be removed by sonication or by mechanical methods, such as scanning AFM tip.[25] The presence of water on the solid surface is required for the formation of a dense OTS monolay-er.[31,49,50,53] The water layer on the solid surface allows the adsorbing OTS molecules to diffuse on the surface. This surface diffusion is the cause of the fractal growth of the primary islands. Thus the structure of OTS mono-layers can be controlled by varying the amount of water on the surface.
The ability of OTS molecules to form island-like aggregates in partial monolayers offers a route to fabricate nanometer-scale domains of one chemical functionality in a continuous phase of a different functionality by sequential adsorption of two different silanes. Various macroscopic controls, such as temperature, amount of water, etc., can be used to control the size and structure of islands. In the second step, a second silane is deposited via sequential adsorption on the area surrounding the islands. In this article, we describe this procedure to control the size and structure of OTS islands by controlling the deposition conditions. The process of sequential adsorption is demonstrated using two different silanes with different terminal groups. The monolayers are imaged using AFM to study the structure of the nanometer domains.
MATERIALS AND EXPERIMENTAL TECHNIQUES USED FOR PREPARATIONS AND CHARACTERIZATIONS OF SILANE SAMS
Decyltrichlorosilane (DTS, 97% purity) and 11-bromo undecyltrichlorosilane (BrUTS, 95% purity) were purchased from Gelest, Inc. Octadecyltrichlorosilane (OTS, 95% purity) was purchased from Sigma Aldrich Company. HPLC grade hexadecane, chloroform, and carbon tetrachloride were obtained from Sigma-Aldrich Co. Nochromix® and sulfuric acid (98%) were purchased from Fischer Scientific. All chemicals were used as received without any further purification. Deionized water, with a resistivity of 18 MO cm from a Millipore® system, was used. Double-side, polished (n type, (111)), single-crystal silicon wafers were purchased from Montco Silicon Technologies Ltd.
The control of the domain size was achieved by controlling the deposition conditions. We used a procedure1-31-1 in which the amount of water in the solvent is controlled. A solvent mixture containing hexadecane, carbon tetrachloride, and chloroform was used for making the silane solutions. Chloroform was saturated with water by keeping it overnight in contact with water. The amount of water in the solvent mixture was estimated to be 4-6 mM and was verified using Karl-Fisher titration. The silane concentration was in the range of 0.2 to 2 mM.
The polished silicon wafers (with roughness of — 1 AA as measured by AFM) were cleaned by sonicating in a mixture of Nochromix® and 98% sulfuric acid for about 30 min, followed by successive water rinsing. The cleaned substrates were stored under water at room temperature and dried in a stream of dry nitrogen just before use. The substrates should not be cleaned by any means that involved physical contact, such as cleaning with tissue paper or cleaning paper. Also, the region that is imaged by AFM should not be touched when holding the substrates. These substrates are hydrated with a water layer of thickness of a few nanometers.[36,54-56] The amount of water on the surface is controlled by heating the surfaces. Partially dehydrated substrates were prepared by heating clean silicon wafers at 100°C and 150°C for an hour and then cooling under vacuum in a desiccator. The vacuum was broken using dry nitrogen immediately before immersing the substrates in silane solution. It is important to take maximum care to prevent the dehydrated substrates from exposure to sources of moisture.
Silane solutions were prepared by adding OTS, DTS, or BrUTS in the required quantities to carbon tetrachlo-ride. The low amount of water in CCl4 prevents the polymerization of silanes in the solution. The required amount of silane and CCl4 solution was added to hexa-decane and chloroform mixture, so that the final volume ratio of hexadecane, carbon tetrachloride, and chloroform was 30:5:3 by volume, respectively. The substrates were immersed in the silane solutions after 30-60 sec. This provided sufficient time for the silanes to hydrolyze by the water present in the solvents. If silane solutions are kept for a longer time, then bulk polymerization occurs. Partial OTS monolayers were prepared by immersing clean the substrates in OTS solution for varying deposition times. The deposition was terminated by rinsing the samples in chloroform for about 15 min. Excess chloroform was dried with a stream of dry nitrogen. All sample preparation procedures are carried out under ambient atmospheric conditions.
The mixed monolayers of OTS and the other silane are prepared in the following manner. First, OTS islands of desired size are formed by immersing a hydrated or dehydrated substrate, prepared as described above, in a 1 mM OTS solution maintained at 10°C for varying deposition times. The substrates are then removed from the OTS solution and rinsed in chloroform, also maintained at 10°C, for approximately 1 sec. The second silane is deposited on these partial OTS monolayers, immersing the substrate immediately in a 1 mM DTS or BrUTS solution at 10°C for 1 hr followed by rinsing in chloroform at 10°C. The structures of the monolayers were analyzed by AFM.
Atomic force microscopy images of the monolayers are obtained in contact and frictional force mode.[25] All AFM images were acquired at room temperatures (22-25°C). For some substrates, tapping mode images were obtained using silicon tips. Water contact angles on the monolayers were measured using a contact angle goniometer (Rame-Hart, Inc.) by placing a 2-p.l water drop on the sample. The contact angle values of at least six different locations were averaged.
CONTROL OF OTS MONOLAYER FORMATION
In this section we show how the structure of partial silane monolayers is affected by varying the deposition conditions. The OTS monolayers are prepared under varying deposition time, OTS concentration in solution, deposition temperature, and amount of water on the surface. The OTS molecules are hydrolyzed by water present in the bulk and on the surface and adsorbed on the surface. If a water layer is present on the surface, the OTS molecules diffuse on the surface and aggregate to form fractal-shape islands by diffusion-limited growth mechanism. The size of the island can be controlled by changing the rate at which the OTS molecules are adsorbed from the solution, the ability of molecules to diffuse on the water layer, and the ability of OTS molecules to form condensed LC phases.
The effect of deposition time on the monolayer structure is shown in the AFM images shown in Fig. 1. Water contact angles, height differences, friction differences, and area coverages are summarized in Table 1. The AFM image of a sample with a 1-sec deposition time is shown in Fig. 1A. It exhibits the onset of the formation of the primary islands. The islands appear diffused with a height of —11 A compared to — 21 A[41] for a condensed (LC) island, suggesting that the islands have not yet condensed. In the friction image, the regions covered by OTS exhibit lower friction than the surroundings.[57,58] The friction force data show that the region surrounding the islands can be either bare silica or G or a very lean LE phase. As the deposition time is increased, more OTS are adsorbed on the region surrounding the islands and diffuse toward the primary islands. On reaching the perimeter of an existing island, they become immobilized and aggregate with the island. This leads to the growth of the fractal features. The surface coverage of the primary islands grows rapidly during the primary growth. An image of a sample, with a deposition time of 5 sec, is shown in Fig. 1B. The islands are much larger in size, between 4 and 8 mm, and cover approximately 30% of the surface. The height of these islands is — 21 A. After 5 sec of OTS deposition, the water contact angle has increased to about 67° compared to 44° for the 1-sec deposition time sample. On increasing the deposition time further, we observe that the rate of growth of the primary islands is reduced. An image of a sample with a 15-sec deposition time is shown in Fig. 1C. The height and friction difference between the primary island and surrounding area indicate that the phase surrounding the primary islands is becoming denser and starting to increase in height. Surface diffusivity of the OTS molecules in the area surrounding the islands is reduced because of the increase in density of the surrounding phase. The reduction in surface diffusivity reduces the growth of the primary islands.
Fig. 1 Height and friction images of partial OTS monolayers deposited at 22±1°C on fully hydrated substrates; image size: 10 mm x 10 mm; OTS concentration: 2 mM; deposition times: (A) ~ 1, (B) 5, (C) 15, (D) 30, (E) 45, (F) 120 sec; in the friction images, brighter regions indicate regions with higher friction.
Table 1 Partial monolayers of OTS on hydrated substrates at 22± 1°C
|
Deposition |
~ 1 |
5 |
15 |
30 |
45 |
120 |
300 |
|
time (sec) |
|||||||
|
Height |
11.6 |
20.9 |
16.9 |
15.2 |
11.4 |
- |
- |
|
difference (A)a |
|||||||
|
Friction |
0.76 |
0.26 |
0.1 |
0.03 |
— 0.01 |
- |
- |
|
difference (V)a |
|||||||
|
Area |
11.1 |
30.7 |
52.1 |
66.5 |
65.3 |
100 |
100 |
|
coverage (%) |
|||||||
|
Water contact |
44 |
67 |
78 |
94 |
97 |
105 |
108 |
|
angle (deg) |
The images of samples with longer deposition times (Fig. 1D) show that the primary islands cease to grow, but the surrounding liquid phase becomes progressively denser. The height and friction difference between the islands and surrounding phases decrease. The primary islands cover ~66% of the surface. After 45 sec of deposition, we observe a new growth regime (Fig. 1E). The height difference between the primary islands and the surrounding phase is now reduced to 11.4 A. Small islands of dimension less than 0.5 mm are observed in the liquid phase surrounding the primary islands. The height of these islands is the same as that of the primary fractal islands. This growth regime is referred to as the secondary nucleation and growth step, in which small, condensed islands nucleate and grow in the surrounding liquid phase. In the friction image, shown in Fig. 1E, the condensed primary and secondary islands exhibit slightly higher friction (— 0.01 V) than the surrounding liquid phase. In all the earlier samples (Fig. 1A-D), the LC primary islands exhibit lower friction than the surrounding phase. This observation suggests that the OTS monolayer in the liquid phase surrounding the islands, as it approaches a ”LE-LC phase transition,” has a lower friction than the corresponding denser LC phase. Upon continued adsorption, the secondary condensed islands continue to nucleate and grow from the liquid phase, until all the substrate is covered with a condensed monolayer. An image of a 120-sec deposition time sample (Fig. 1F) shows an almost uniform monolayer. The water contact angle in this sample is approximately 105°, which increases to 108° when the deposition time is increased to 5 min. The primary islands grow rapidly to their fractallike morphology via adsorption, surface diffusion, and aggregation. On the other hand, the secondary islands grow from the phase surrounding the primary fractal islands as the density of OTS in this phase increases.
The effect of deposition temperature on the primary and secondary growth is shown in Fig. 2, which shows the AFM images of partial monolayer deposited at 10°C. At this reduced temperature, the primary fractal islands grow at nearly constant height of — 21 A, until their edges are very close to each other.[41] However, at later stages (Fig. 2C), we note the appearance of very small islands in the region between the primary islands (Fig. 2C, inset). These small islands are similar to the secondary islands nucleating in an LE phase as observed at room temperature (Fig. 1E). The height difference between the primary fractal islands and the surrounding phase is 15-16 A compared to —11.4 A at 22°C (Fig. 1E). At low temperature, the region surrounding the primary islands is leaner compared to the deposition at higher temperature. As a result, the primary growth continues until the islands are very close to each other. These results show that controlling the deposition time and temperature is an effective way to control the growth of primary islands and the density of OTS molecules in the region surrounding the primary islands.
Fig. 2 Contact mode height images of partial OTS monolayers prepared at 10°C on hydrated substrates. OTS concentration: 2 mM; image size: 10 mm x 10 mm image; height scale: 0-10 nm; deposition times: (A) 5, (B) 10, (C) 30 sec (inset: image size is 2.75 mm x 2.75 mm).
Fig. 3 Contact mode height image of a hydrated substrate immersed for 1 sec in a 0.2 mM OTS solution at 22°C. Image size: 5 mm x 5 mm; height scale 0-5 nm.
The growth of OTS islands is preceded by the adsorption of OTS from the bulk to the surface. The rate of adsorption is also expected to play a role in the formation of OTS islands. Atomic force microscopy image of a hydrated substrate immersed in a 0.2 mM OTS solution for a deposition time of approximately 1 sec is shown in Fig. 3. This concentration is 1/10th of the concentration used for samples described in Figs. 1 and 2. Very small OTS clusters of about 50-100 nm in diameter and 15-20 A in height are distributed almost randomly on the surface. For longer deposition times, the OTS monolayer exhibits various phases (as observed at higher concentrations; Figs. 1 and 2) until a uniform coverage is attained. However, adsorption times to achieve a certain surface coverage are longer at lower concentration. Thus nanometer-scale islands of OTS are obtained when the deposition is carried out at low bulk concentrations.
The presence of water in solvent and on surface plays a very important role in the formation of SAMs. Long deposition times are required when dry solvents are used.[51,59] The possibility of formation of particulate OTS in the bulk increases when the amount of water in solvent is high.[59,60] The optimum amount of water in the solvent is maintained as described in the section ”Materials and Experimental Techniques Used for Preparations and Characterizations of Silane SAMs.” Good OTS monolayers are not formed on dehydrated substrates.[33,35,49,50,53,59] The thickness of the water layer determines the diffusivity of the OTS molecules on the surface, which in turn controls the morphology of the OTS monolayer. This is another effective way of controlling the structure of OTS monolayers. The preparation of substrates with varying degrees of hydra-tion is described in the section ”Materials and Experimental Techniques Used for Preparations and Characterizations of Silane SAMs.” Under ambient conditions, a water layer is present on the native silica surface.[31,49,50,53,61] The thickness of this water layer depends on the ambient humidity and can have values of a few nanometers.[36,54-56] Upon heating the substrate at 100°C, some of the physisorbed water is removed.[31,39,49,50,53] We refer to these surfaces as partially dehydrated. Upon further heating to 150°C, most of the physisorbed water is removed and we refer to these substrates as dehydrated. Partial OTS monolayers are deposited on the fully hydrated, partially dehydrated, and dehydrated substrates by immersing in a 2 mM OTS solution for 30 sec at 22±1°C. An image of the OTS monolayer on a hydrated substrate is shown in Fig. 4A. Primary fractal islands and secondary islands are observed on this surface as expected (Fig. 1). The water contact angle is ~93°. On a partially dehydrated substrate (Fig. 4B), we still observe primary fractal and secondary growth. However, the area occupied by the primary fractal islands is reduced and so is their size, when compared to the hydrated substrate (Fig. 4A). The reduced surface mobility of the OTS molecules on the partially dehydrated substrate reduces the size of the primary islands. The water contact angle is ~90°, which is very close to that of the hydrated substrate. On the dehydrated substrate, the AFM images in contact mode were not stable in successive scans. Therefore the samples were imaged in tapping mode, which overcame this difficulty. Such an image is shown in Fig. 4C. A large number of randomly placed ”dots” are observed instead of the primary and secondary islands. These ”dots” are about 100 nm in diameter and 17-18 A in height. They are clusters of adsorbed OTS molecules, which are unable to form large condensed primary islands because of the negligible surface mobility on the dehydrated substrate. These OTS clusters or ”dots” are not strongly adhered to the substrate, and therefore they are swept away by the AFM tip when imaged in contact mode as seen in our experiments. The height of these aggregates is nearly the same as the monolayer height. The water contact angle is only 78° compared to 93° and 90° for a hydrated and partially dehydrated substrate, respectively. This result shows that OTS monolayers do not exhibit the primary and secondary growth on dehydrated surfaces. Much smaller domains of OTS are observed, which are not strongly adhered to the substrate. This result also confirms that the silane molecules do not bind to the surface silanol groups during the monolayer formation process.[31,49,50,53] At longer deposition times, the number of random ”dots” increases, but no condensed fractal islands are observed. The number of these ”dots” continues to grow, until they begin to touch each other to form a disordered monolayer. On a dehydrated substrate, deposition times of about 15 min are required in order to get a monolayer with water at a contact angle of ~ 105°, compared to a deposition time of 2 min for a hydrated substrate. Similar results are obtained when OTS depositions are carried on dehydrated substrates at 10°C.
These results demonstrate that OTS domains from nanometer to micrometer size can be obtained by proper control of the deposition conditions. The primary and secondary islands are formed during the monolayer formation and are not a result of drying the substrates.[25] Primary and secondary growth is controlled by temperature and deposition time. The nanometer-scale OTS domains are obtained by reducing either the adsorption rate of OTS or surface mobility of OTS molecules. The adsorption rate is reduced by reducing the bulk concentration. The surface mobility of OTS molecules is reduced by dehydrating the surface. The OTS domains are not strongly adhered to dehydrated surfaces. The density of OTS molecules in the region surrounding the condensed OTS domains is lower at low deposition temperature. For any other silane molecule, the optimum deposition conditions needed to form these domains would be different.
Fig. 4 Height images of partial OTS monolayers on substrates with different degrees of hydration. Image size: 10 mm x 10 mm; temperature 22±1°C; OTS concentration: 2 mM; deposition time: 30 sec; height scale: 0-10 nm; (A) fully hydrated substrate (image was acquired in contact mode); (B) partially dehydrated substrate, heated at 100°C (image was acquired in contact mode); (C) dehydrated substrate, heated at 150°C (image was acquired in tapping mode).
