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Langmuir–Blodgett films of octadecanethiol – properties and potential applications
Analytica Chimica Acta 568 (2006) 109–118
Langmuir–Blodgett films of octadecanethiol – properties and potential applications Raj Kumar Gupta a , K.A. Suresh a,∗ , Rui Guo b , Satyendra Kumar b a b
Raman Research Institute, Sadashivanagar, Bangalore 560080, India Department of Physics, Kent State University, Kent, OH 44242, USA
Received 29 July 2005; received in revised form 6 October 2005; accepted 8 October 2005 Available online 9 November 2005
Abstract Octadecanethiol (ODT) is known to form self-assembled monolayer on noble metal surfaces which has potential technological applications. Langmuir–Blodgett (LB) technique is another useful method of obtaining highly ordered assembly of molecules. It is of interest to find whether ODT molecules can also form a stable Langmuir monolayer which facilitates the preparation of LB films. In literature, it has been reported that ODT molecules form an unstable Langmuir monolayer. We have studied the stability of the monolayer of the ODT molecules at air–water interface using surface manometry and microscopy techniques. We find the monolayer to be stable on ultrapure water of resistivity greater than 18 M cm. However, the behavior changes in the presence of even small amount of additives like NaOH or CdCl2 in the subphase. Our AFM studies on the LB films of ODT deposited from ion-free ultrapure water showed streak-like bilayer domains. The LB films of ODT deposited from CdCl2 containing aqueous subphase yield dendritic domains of the complexed unit grown over ODT monolayer. These nanostructures on surfaces may have potential applications in molecular electronics. © 2005 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; Octadecanethiol; Monolayer stability; Atomic force microscopy
1. Introduction Molecular electronics studies form an important step in the miniaturization of devices [1]. Here a bunch of molecules or a single molecule with appropriate electronic properties can be employed for rectification, amplification, negative differential resistance and conductance switching. Hence, the assembly and the ordering of the molecules on appropriate electrodes or substrates are important. In the molecular electronics studies, an ordered assembly of molecules have been obtained by self-assembly, Langmuir–Blodgett and other techniques. Selfassembly of organosulfur compounds on metallic surfaces like gold, silver and copper is a well known technique to obtain an ordered monomolecular film [2]. Self-assembled monolayers (SAM) of organosulfur compounds are generally prepared by immersing clean metallic substrate such as gold, silver and copper into the millimolar solution of the compounds in
∗
Corresponding author. E-mail address: [email protected] (K.A. Suresh).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.10.010
an appropriate organic solvent. The head group (–SH) of the molecules chemisorb spontaneously to the metallic surfaces. Such an adsorption leads to a monolayer of highly ordered molecules. SAM finds applications in the fields of wetting, dewetting, chemical sensors, biological sensors, nanolithography, lubrication, optical waveguides and molecular electronics [3–5]. Langmuir–Blodgett (LB) technique is another important method to obtain a highly ordered monolayer or multilayer on different substrates. A single layer formed by insoluble amphiphilic molecules at the air–water (A–W) interface is called a Langmuir monolayer. An amphiphilic molecule generally comprises of two parts – a hydrophilic (polar) head part and a hydrophobic (non-polar) tail part. When amphiphilic molecules are dissolved in volatile organic solvent and the solution is spread at the A–W interface, the solvent evaporates and the hydrophilic part of the molecule gets adsorbed to the water surface, whereas the hydrophobic part stays away from the surface. This forms a monolayer at the A–W interface. The Langmuir monolayer provides an ideal two-dimensional (2D) system to study the surface thermodynamics where the 2D plane is provided by the smooth
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water surface [6–8]. The monolayer shows a variety of 2D phases depending on the nature of interaction among the molecules and experimental conditions like temperature, pH and ion contents of the subphase [9]. The Langmuir monolayer can be transferred layer by layer onto a solid substrate by vertically moving the substrate in and out of the subphase. This is known as LB technique [10,11]. A stable Langmuir monolayer serves as a precursor for building defectless and ordered LB films. Such films provide useful systems for understanding fundamental phenomena involved at the surfaces and interfaces. The properties of the surfaces can be tailored with the choice of appropriate molecules. Octadecanethiol (C18 H37 SH), an alkanethiol, forms a very stable self-assembled monolayer. The octadecanethiol (ODT) molecules can be used efficiently for patterning gold substrate through microcontact printing technique [12]. The –SH group of the ODT is weakly acidic in nature and thereby the molecule becomes amphiphilic. We can expect it to form a stable monolayer at the air–water (A–W) interface. Though there are many reports on the SAM of ODT on the metallic surfaces, there have not been many studies on the LB films of ODT molecules [13], as ODT is reported to form an unstable monolayer at the A–W interface [14–17]. It is important to have a stable monolayer for the formation of defect free LB films which may find applications in devices. LB technique also provides a means of forming ODT monolayer on non-conducting surfaces. There have been many attempts to stabilize the ODT monolayer by adding additives to the subphase. It has been reported that the ODT monolayer can be stabilized with stearic acid provided the subphase contains barium salt [14]. Livingston and Swingley [15] have checked the stability of ODT monolayer on an aqueous subphase with different pH values and salts. They have observed that the ODT monolayer was stable only on a subphase containing potassium permanganate. Itaya et al. [16] have found that the ODT monolayer is stable only on the subphase containing BaCl2 . Bilewicz and Majda [13] have reported that the ODT monolayer can be stabilized by mixing it with octadecanol. A recent report on the surface manometry and the X-ray diffraction studies on the monolayer of ODT at the air–mercury interface have shown that ODT molecules exhibit a disordered phase of surface-parallel molecules, a condensed phase with tilted molecules and an untilted condensed phase [18]. We have studied the stability of the ODT monolayer at the A–W interface using different techniques. We find that ODT monolayer is stable on a subphase of ultrapure water. We have formed LB films on silicon substrate which exhibit streak-like patterns of bilayer thickness. We have also studied the formation of monolayer of ODT–metal complex by the addition of CdCl2 in the subphase. The LB films of ODT–CdCl2 complex yield dendritic patterns. 2. Experimental The water used in the experiments on ODT was ultrapure ion-free having a resistivity greater than 18 M cm obtained by passing double distilled water through the filtering and deionizing columns of a Milli-Q Millipore unit. The alkaline subphase was prepared by the addition of NaOH in the ion-free water. To form ODT–metal complex, CdCl2 salt was dissolved in the
ion-free water. In surface manometry, the surface density of the molecules adsorbed at the interface is varied and the surface pressure (π) is measured at a constant temperature. This yields a surface pressure–surface density isotherm. The area per molecule (Am ) is defined as the inverse of the surface density. Surface pressure–area per molecule isotherms were obtained using a NIMA 611M trough. The solution of concentration 3.5 mM of ODT in chloroform was prepared and it was spread on the subphase using a microsyringe. About 15 min were allowed for the solvent to evaporate from the surface. The compres˚ 2 /molecule)/min in all the sion speed was maintained at 3.8 (A experiments. For epifluorescence experiments, a fluorescent dye 4-(hexadecylamino)-7-nitrobenz-2-oxa-1,3-diazole (Molecular Probes) was used. The amount of dye was around 1% molar concentration in the ODT monolayer. The dye-doped monolayer was observed under a Leitz Metallux 3 microscope and the images were captured using an intensified CCD camera. The intensity of the emitted light from the dye-doped monolayer depends on the miscibility of the dye molecules in a particular phase of the monolayer. The gas phase appears dark due to the quenching of the dye molecules. The low density liquid phase appears bright in the epifluorescence images. On the other hand, the solid phase appears dark which is due to the expulsion of dye molecules from the highly dense solid domains [19]. Though both gas phase and solid phase appear dark under the microscope, it is possible to distinguish them; the gas phase will appear uniformly dark whereas the solid phase will show a few bright specks of dye particles distributed in the dark background. The monolayer was also studied using a Brewster angle microscope (BAM). In BAM, a polarized light was allowed to be incident on the water surface at the Brewster angle for the A–W interface. The intensity of the reflected light was minimum for the water surface. However, the presence of a monolayer at the interface alters the Brewster angle condition. This in turn reflects some light which was collected by the CCD to form images of the monolayer domains. The intensity of the reflected light depends on the thickness of the film and the surface density of the molecules [20]. The equilibrium spreading pressure (ESP) was determined by depositing small crystallites of ODT on ultrapure water in a Teflon container and the surface pressure was monitored using a tensiometer. The relaxation of the monolayer was studied by maintaining the monolayer at a given surface pressure and monitoring the variation in the molecular area with time. The monolayer from the aqueous subphase was transferred on solid substrates by LB technique. The silicon (Si) wafers (one sided-polished, 0.4 mm thick) were treated chemically to yield a hydrophilic nature to the surface [11]. The Langmuir films were transferred onto the treated Si substrate at a target pressure of 8 mN/m. The dipper speed was maintained at 5 mm/min. The substrate was immersed in the subphase prior to the spreading of the molecules on the subphase. The films of the ODT or its metal-complexes were transferred on the Si substrate by a single upstroke of the dipper. The atomic force microscope (AFM) imaging of the LB films was carried out using Nanoscope IIIA (Digital Instruments). All the images were taken in the tapping mode. Tapping mode has numerous advantages over contact mode for imaging soft matter and biological samples [21]. For
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the imaging, silicon (Si) tips were used. The spring constants were in the range of 0.1–0.6 N/m. All the experiments were carried out at the room temperature (22 ± 0.5 ◦ C). 3. Results and discussion 3.1. Films of ODT molecules at air–water and air–solid interfaces The surface pressure (π)–area per molecule (Am ) isotherm of ODT monolayer on the ultrapure ion-free water is shown in Fig. 1. The isotherm shows the coexistence of gas and a con˚ 2 . Then there is a sharp change in densed phase up to 19.5 A the slope of the isotherm indicating the onset of the homogeneous condensed phase. The extrapolation of the steep region of the isotherm to zero surface pressure is called limiting area per molecule (A0 ). This is the minimum area to which the molecules can be compressed on the water surface without collapsing the monolayer. The orientational state of the molecules (tilted or untilted) in a phase can be estimated qualitatively by comparing the extrapolated area per molecule with that of molecular crosssectional area. The isotherm of the ODT monolayer yields the ˚ 2 which approximately corresponds to value of A0 to be 19.0 A the cross-sectional area of the alkyl chain [9]. Hence, the steep region of the isotherm may correspond to the untilted condensed phase. The monolayer collapses at the surface pressure of about 14 mN/m. The isotherm reveals a plateau in the collapse region of the ODT monolayer. The stability of the ODT monolayer on ion-free water at a given surface pressure was studied by monitoring the change in the normalized area, At /At=0 , where At is the value of Am at a time t and At=0 is the initial value of Am . This is shown in Fig. 2. Assuming a linear dependence of the normalized area with time, we get the rates of reduction of the normalized area to be 0.076%, 0.10% and 0.117% per minute at the surface pressures of 2, 5 and 9 mN/m, respectively. The monolayer of the ODT molecule was quite stable as indicated by the negligible rate of decrease in the normalized area. According to a model proposed by Smith and Berg [22], the decrease in normalized
Fig. 1. Surface pressure (π)–area per molecule (Am ) isotherm of ODT monolayer on the ultrapure ion-free water.
Fig. 2. The area relaxation curves of the ODT monolayer on ion-free water at a given surface pressure. At is the Am at a time t and At=0 is the Am at the initial time.
area can be due to the dissolution of the molecules in the subphase by diffusion, if it follows the relation ln(At /At=0 ) ∝ −tβ with β equals to 0.5. However, in the present case, fitting such a power law to the data (Fig. 2) for the surface pressures of 2, 5 and 9 mN/m yield the β values to be 0.77, 0.92 and 0.81, respectively. Hence, the small reduction in the normalized area may not be due to the dissolution of the molecules. Such a small reduction can be attributed to the relaxation of the ODT molecules in the monolayer. The equilibrium spreading pressure (ESP) [9] of a material is the surface pressure of its monolayer phase coexisting in equilibrium with its bulk phase. It indicates qualitatively, the spreading capability of the molecules at an interface and the loss due to the dissolution of the molecules into the subphase and its evaporation. The ESP value depends on the material properties, subphase, temperature and humidity. The ESP measurement of ODT molecules is shown in Fig. 3. We find a finite ESP value of 2.5 mN/m for ODT indicating the stability of the monolayer on the ion-free water. The ESP values of the compounds like octadecanol, octadecylamine and stearic acid have been reported to be 34.3, 9 and 5.2 mN/m, respectively [22,23]. Although these molecules are similar in structure, they differ from each other in
Fig. 3. Variation of surface pressure with time for ODT molecules on ion-free water. The relative humidity and temperature were maintained at 90% and 24 ◦ C. The saturated value of surface pressure (2.5 mN/m) is the ESP of ODT.
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Fig. 4. BAM image of the ODT monolayer on the ion-free water captured at different Am : (a) shows the coexistence of gas (dark region) and condensed phase (bright region), (b) shows a homogeneous condensed phase. The scale bar represents 500 m.
Fig. 5. Epifluorescence images of the ODT monolayer on the ion-free water captured at different Am : (a) shows the coexistence of gas (dark region) and condensed phases (bright region), (b) shows a homogeneous condensed phase. The scale bar represents 50 m.
their hydrophilic head groups. The lower value of ESP of ODT can be attributed to the less polar nature of the compound. The BAM images of the ODT monolayer on the ion-free water at different Am are shown in Fig. 4. At a large Am , the image (Fig. 4(a)) shows a coexistence of dark region and bright region. The dark region represents the gas phase, whereas the bright region represents the condensed phase. On compression, the bright region grows at the expense of dark region, yielding an uniform bright texture which corresponds to a homogeneous condensed phase. The monolayer collapses on further compression. Epifluorescence microscope images of the ODT monolayer on ion-free water are shown in Fig. 5. The image at large Am shows dark and the bright regions (Fig. 5(a)). The dark region represents the gas phase. The bright region corresponds to the condensed phase of the ODT monolayer. On compression, the bright region grows at the expense of the dark region, leading to an uniform bright region, indicating a homogeneous condensed phase (Fig. 5(b)). On further compression, the ODT monolayer collapses and the domains of different intensity levels were seen. These observations were in accordance with those of the BAM imaging of the monolayer. Both microscopy techniques show an
uniform texture for the condensed phase (Figs. 4(b) and 5(b)) which indicates the formation of a stable ODT monolayer. An earlier study of the ODT monolayer on water subphase of resistivity of the order of 12 M cm showed the coexistence of gas and condensed phases even in the steep region of the isotherm indicating the unstable nature of the monolayer [17]. However, we find a very stable Langmuir monolayer of ODT provided the subphase is ion-free water with a resistivity greater than 18 M cm. The phase sequence observed are; coexistence of gas and condensed phase, condensed phase and the collapsed state. The formation of a stable Langmuir monolayer of ODT has facilitated us to prepare LB films. The ODT monolayer was transferred from ion-free water on hydrophilically treated silicon (Si) substrates. We have employed AFM to study the assembly of ODT molecules in the LB films. The reference image of the bare Si substrate for a scan range of 1 m × 1 m is shown in Fig. 6. The image revealed an average and a root mean square (RMS) roughnesses to be 0.122 and 0.154 nm, respectively. This shows that the Si substrates were reasonably smooth and featureless. The AFM images of the LB film of ODT on Si substrate for different scan ranges are shown in Fig. 7. The AFM images
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Fig. 6. AFM image of the reference Si substrate. The white line in the image represents the line along which the height profile is drawn. The height profile is shown below the image. The size of the image is 1 m × 1 m.
of the LB films of the ODT molecules showed some interesting results. The image for the scan range of 5 m × 5 m (Fig. 7(a)) shows streak-like bright domains that had grown on the uniform grey background. Careful observation of the image shows bright and dark spots all over the grey background. The average height of the grey background is 2 nm and that of the streak domains is 4.3 nm. The image for the scan range of 1 m × 1 m is shown in Fig. 7(b). The image is consistent with Fig. 7(a). Here also, the part of the streak-like domains are visible. The uniform grey background is embedded with the bright and dark spots. The height profile along the line drawn over the image reveals the height of the bright streaks and bright spots to be around 4.2 nm. The average height of the grey background is around 2 nm. Fig. 7(c) and (d) also show the similar features. The bright and the dark domains are more clear in Fig. 7(c). The height profile along the line shown in the image reveals an average height of 2.3 nm for the grey background. The image also exhibits bright domains, dark spots and the spots with intensity varying from grey to bright. The height profile over the bright domains yields a value of 4.2 nm. The height profile data yield the thickness of the dark spots to be nearly zero. This indicates that the dark spots are due to the bare Si substrate. The height of the spots with varying intensity lie in the range of 0.5–1.8 nm. Fig. 7(d)
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shows the grey background and the domains with varying intensity. The length of the ODT molecule is 2.1 nm [24]. Hence, it can be interpreted that the grey background corresponds to the monolayer of the ODT molecules in which the aliphatic chains are oriented normal to the Si substrate. The streak-like domains appeared over the grey background. The average height of the streak-like domains (∼4.2 nm) suggests the domains to be a bilayer of the ODT molecules oriented normal to the Si substrate. The films have defects in the form of dark domains and domains with varying intensity. The aggregation of molecules in the LB films depends highly on the nature of the substrates and the experimental parameters, such as temperature, humidity and ion contents in the subphase [25]. The molecules find a different thermodynamical environment when they get transferred from the A–W interface to the solid substrates. Hence, on the substrate the molecules relax in the thermodynamical stable state [26,27]. Here, we have transferred the ODT molecules from the A–W interface in the condensed phase. In the condensed phase, the ODT molecules orient normal to the A–W interface. Through AFM imaging we could see the existence of predominantly condensed phase (grey background). However, the relaxation of the monolayer leads to the formation of streak-like bilayer domains and the defects. We present a schematic model, suggesting a possible arrangement of the ODT molecules in the LB film on the Si substrate. This is shown in Fig. 8. The grey background in the AFM images is denoted by a layer of ODT molecules oriented normal to the Si substrate. The streak-like domains are represented by the bilayer of the ODT molecules oriented normal to the substrate. The defects with varying intensity are depicted by the bunch of randomly oriented molecules and the dark spots are indicated by gaps in the film. 3.2. Films of ODT molecules on alkaline subphase The ODT molecule is weakly acidic in nature and hence we have studied the effect of alkaline subphase on the ODT monolayer. The effect of addition of NaOH in the subphase on the isotherms of ODT is shown in Fig. 9. The presence of even very small quantity of NaOH has a marked effect on the monolayer of the ODT molecules. The isotherms shift systematically towards higher Am with increase in concentration (≤10−3 M) of NaOH in the subphase (Fig. 9). Above the concentration of 10−3 M, the ODT monolayer gets destabilized completely and the isotherms show a continuous and a gradual rise in the surface pressure. The limiting area per molecule (A0 ) values obtained from the isotherms of the ODT monolayer on the aqueous subphases containing 10−5 , 10−4 and 10−3 M of NaOH (Fig. 9) are 20.6, 22.1 ˚ 2 , respectively. The acidic head group (–SH) of the and 22.5 A ODT molecule may tend to dissociate on the alkaline subphase. The increase in A0 values may be due to the electrostatic charge repulsion between such dissociated components [9,28]. It was further observed that holding the monolayer at a given surface pressure leads to a relatively faster decrease in Am as compared to that of the ODT monolayer on ion-free water. This indicates an unstable nature of the monolayer on the alkaline subphase. We have carried out the microscopy experiments on the ODT monolayer for different concentrations of NaOH in the subphase.
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Fig. 7. AFM images of the LB films of ODT for different scan ranges. The white line in the image represents the line along which the height profile is drawn. The height profiles are shown below the respective images.
Fig. 10 shows a BAM image of the ODT monolayer on the alkaline subphase. The presence of very minute quantity of NaOH in the subphase shows a marked change in the BAM images. In Fig. 10, at least three different phases were observed. The dark
region is the gas phase, the bright region with the sharp boundary is the usual condensed phase, whereas the grey domains with diffuse boundary may correspond to the monolayer of the dissociated ODT molecules. These features were seen up to the NaOH
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Fig. 8. A schematic model of the arrangement of the ODT molecules in the LB film transferred from the ion-free water onto Si substrate. The –SH group is represented by black circle and the aliphatic chain is denoted by the zigzag line.
Fig. 11. The surface pressure (π)–area per molecule (Am ) isotherms of the ODT monolayer for different molar concentrations of CdCl2 in the aqueous subphase.
3.3. Films of ODT–CdCl2 complexes at air–water and air–solid interfaces
Fig. 9. Surface pressure (π)–area per molecule (Am ) isotherms of the ODT monolayer with different molar concentrations of NaOH in the subphase.
concentration of 10−3 M in the subphase. Above this concentration, the crystalline flakes of the complex of the ODT molecules and NaOH were observed. There are possibilities of the surface oxidation of the ODT molecules to form dioctadecyl disulfide over the alkaline subphase [29]. The dioctadecyl disulfide does not form a stable film over the aqueous subphase. Further, the dissolution of the charged monolayer is known to be higher as compared to the uncharged monolayer [9]. Hence the instability of the ODT monolayer on the alkaline subphase can be attributed to the dissociation of the ODT head group and the formation of the disulfide during the surface chemical oxidation.
Fig. 10. BAM image of the ODT monolayer on an aqueous subphase containing 10−6 M of the NaOH. The image has been taken for zero surface pressure and at a large Am . The scale bar represents 500 m.
We have studied the effect of divalent ion (Cd2+ ) in the subphase on the films of the ODT molecules at the A–W interface and on air–silicon interface. It is known that a Cd2+ ion complexes with two ODT molecules in order to achieve electroneutrality. In the complex unit, it has been proposed that the orientation of the aliphatic chain of the two ODT molecules are in opposite direction with the Cd2+ ion at the center [16]. The isotherms of the ODT molecules on the aqueous subphases having different molar concentrations of CdCl2 are shown in Fig. 11. We find a drastic change in the π − Am isotherm of the ODT due to the presence of small amount of CdCl2 in the aqueous subphase. For a very low concentration (8.7 × 10−8 M) of CdCl2 in the aqueous subphase, the trend of the isotherm was similar to the one obtained for the ion-free water. However, there is a shift in the limiting area per molecule (A0 ). It shows the ˚ 2 . The isotherm for the concentration value of A0 to be 16.5 A −6 of 10 M shows a similar trend but the surface pressure continues to increase after an initial collapse at the surface pressure ˚ 2 . The of about 13 mN/m. It shows the A0 value to be 10.5 A isotherms for higher concentrations show a different behavior. They show a gradual rise in the surface pressure to very large values (≥60 mN/m). The trend of the isotherms do not change on further increase in the concentration of CdCl2 . Here, the values ˚ 2 . These values are about half of A0 lie in the range of 10.5–11.5 A of those corresponding to the normally oriented molecules in the condensed phase of ODT at the A–W interface. This indicates the formation of bilayer of the complex of ODT and CdCl2 . The ˚ 2 ) for the lower concentration (8.7 × 10−8 M) A0 value (16.5 A is more than that for a bilayer. This can be suggested as the formation of the complex of the ODT molecules in a fraction of the monolayer with CdCl2 . The BAM and the epifluorescence images of the ODT monolayer for an aqueous subphase containing 8.7 × 10−8 M of CdCl2 are shown in Fig. 12. In the BAM image (Fig. 12(a)), there are dark domains and grey domains with the different intensity levels. The dark region represents the gas phase. The grey region may represent the condensed phase of the ODT molecules. The bright domains may represent
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Fig. 12. (a) and (b) represent the BAM and the epifluorescence images of the ODT monolayer on an aqueous subphase containing 8.7 × 10−8 M of CdCl2 , respectively. The images were captured at a large Am and zero surface pressure. The scale bars in (a) and (b) represent 500 and 50 m, respectively.
the bilayer of the complex of ODT and CdCl2 . In the epifluorescence image (Fig. 12(b)), the dark domains represent the gas phase and the bright background represents the condensed phase of the ODT molecules. The grey and irregular domains may represent the bilayer of the complex of ODT and CdCl2 . The domains of the complex are darker than that of the condensed phase of the ODT molecules due to poor miscibility of dye molecules in it. The effect of CdCl2 in the subphase on the LB films of the ODT molecules was studied using AFM. The films of the complex of ODT molecules with CdCl2 were transferred on hydrophilic Si substrates by LB technique. The AFM images
of the films for two different scan ranges are shown in Fig. 13. Fig. 13(a) shows a dendrite-like structure over an uniform background. The height variation data reveal that the uniform background has an average height of 2 nm. The average height of the dendrite-like structure above the uniform grey background is 4.5 nm. An image with a lower scan range is shown in Fig. 13(b). This image also shows an uniform background of height of about 2 nm and the average height of the dendritelike domain above the grey background is 4.4 nm. The height variation in the dendrite-like domains is very small suggesting the uniform thickness of the dendritic domains. The features in both the images scale according to the scan range indicating its
Fig. 13. AFM images of the LB films of ODT molecules deposited from an aqueous subphase containing 8.7 × 10−8 M of CdCl2 . The white line in the image represents the line along which the height profile is drawn. The height profiles are shown below the respective images.
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Fig. 14. A schematic model of the arrangement of the molecules in the LB films deposited from the subphase containing 8.7 × 10−8 M of CdCl2 . The polar group of the ODT molecule is represented by the black circle and the chain is represented by the zigzag lines. The Cd2+ ion is represented by grey circle.
reproducibility. The length of the ODT molecule is 2.1 nm, and the length of the complex unit for all trans conformation of the aliphatic chain is around 4.5 nm [24]. Therefore, we propose that the uniform background represents an ODT monolayer on Si substrate with the molecules being oriented normal to the substrate. The height variations obtained from the AFM images (Fig. 13) indicate that the dendrite-like domains may correspond to that of a layer of the complex unit of ODT and CdCl2 . The complex unit behaves like a hydrophobic component that cannot anchor to the water surface. We suggest that the monolayer in the background are composed of ODT molecules with the domains of the complex unit deposited on it. Based on our experimental observations, we present a schematic diagram showing a model for the aggregation and arrangement of the ODT molecules in the LB film at the air–silicon interface. This is shown in Fig. 14. In the model, it is shown that the first monolayer is composed of ODT molecules where all the molecules are normally oriented on the Si substrate. In the layer of the complex unit, which forms a dendrite-like structure in the AFM images, the two ODT molecules face each other with the Cd2+ at the center. The lengths d1 and d2 represent the thicknesses of the monolayer of the normally oriented ODT molecules (grey background in the AFM images) and a layer of the complex unit (dendrite-like domains in AFM images), respectively. The AFM images suggest the values for the thickness d1 and d2 (Fig. 14) to be 2 and 4.5 nm, respectively. 4. Conclusions We have shown that the ODT monolayer is stable at the A–W interface provided the water is ion-free having a resistivity greater than 18 M cm. The monolayer is sensitive to the presence of very small quantities of NaOH or CdCl2 in the aqueous subphase. The ODT molecules in the LB films predominantly prefer an orientation normal to the substrate, as seen in the AFM images. Interestingly, the AFM images reveal streak-like domains which are the bilayer of the ODT molecules. The presence of divalent ion (Cd2+ ) in the subphase yields an ODT–metal complex at the A–W interface. The AFM images of the LB films of the ODT molecules deposited from an aqueous subphase containing low concentration of CdCl2 show dendritic domains representing a layer of the complex unit.
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The LB films of the ODT molecules have several potential applications. The streak-like domains in the LB films of ODT may be employed for the growth of nanowires. The nanometer sized defects may be utilized for the formation of nanoelectrodes. Such defects may find application in selective permeation devices. They may also be used in molecular species recognition and sensors. The ODT molecules may be used as an adhesion promoter for the transfer of LB films of poorly adsorbing materials. Since ODT molecules bind strongly to noble metals, it may be mixed with other components during LB transfer onto substrates like gold, silver and copper. The ODT molecules find many interesting applications. Microcontact printing is an useful technique for directly creating patterns in thin metal films. The ODT molecules are found to be a very suitable inking material for microcontact printing on gold surfaces [12]. The ODT and decanethiol molecules have been used to create high-density alternating thiol nanostructures using the AFM nanografting technique [30]. Acknowledgments We are thankful to V. Lakshminarayanan and G.S. Ranganath for helpful discussions. This work is supported, in part, by a grant from the US National Science Foundation under grant DMR-0423619. References [1] C. Joachim, J.K. Gimzewski, A. Aviram, Nature (London) 408 (2000) 541. [2] A. Ulman, Chem. Rev. 96 (1996) 1533. [3] I. Rubinstein, S. Steinberg, Y. Tor, A. Shanzer, J. Sagiv, Nature (London) 332 (1988) 426. [4] C.J. Zhong, M.D. Porter, Anal. Chem. 67 (1995) 709. [5] G.M. Whitesides, P.E. Laibinis, Langmuir 6 (1990) 87. [6] A. Bhattacharyya, K.A. Suresh, Europhys. Lett. 41 (1998) 641. [7] P. Viswanath, K.A. Suresh, Phys. Rev. E 67 (2003) 061604. [8] P. Viswanath, K.A. Suresh, J. Phys. Chem. B 108 (2004) 9198. [9] G.L. Gaines Jr., Insoluble Monolayers at Liquid–Gas Interfaces, Wiley–Interscience, New York, 1966. [10] G. Roberts, Langmuir–Blodgett Films, Plenum Press, New York, 1990. [11] R.K. Gupta, K.A. Suresh, Euro. Phys. J. E 14 (2004) 35. [12] R.B. Bass, A.W. Lichtenberger, Appl. Surf. Sci. 226 (2004) 335. [13] R. Bilewicz, M. Majda, Langmuir 7 (1991) 2794. [14] H. Sobotka, S. Rosenberg, in: H. Sobotka (Ed.), Monomolecular Layers, Am. Assoc. Advan. Sci., Washington, DC, 1954, p. 175. [15] H.K. Livingstone, C.S. Swingley, J. Colloid Interf. Sci. 38 (1972) 643. [16] A. Itaya, M. Van der Auweraer, F.C. De Schryver, Langmuir 5 (1989) 1123. [17] W. Zhao, M.W. Kim, D.B. Wurm, S.T. Brittain, Y.T. Kim, Langmuir 12 (1996) 386. [18] B.M. Ocko, H. Kraack, P.S. Pershan, E. Sloutskin, L. Tamam, M. Deutsch, Phys. Rev. Lett. 94 (2005) 017802. [19] V. von Tscharner, H.M. McConnell, Biophys. J. 36 (1981) 409. [20] S. Rivi`ere, S. H´enon, J. Meunier, D.K. Schwartz, M.-W. Tsao, C.M. Knobler, J. Chem. Phys. 101 (1994) 10045. [21] P.K. Hansma, J.P. Cleveland, M. Radmacher, D.A. Walters, P.E. Hillner, M. Bezanilla, M. Fritz, D. Vie, H.G. Hansma, C.B. Prater, J. Massie, L. Fukunaga, J. Gurley, V. Elings, Appl. Phys. Lett. 64 (1994) 1738. [22] R.D. Smith, J.C. Berg, J. Colloid Interf. Sci. 74 (1980) 273. [23] Y.L. Lee, K.L. Liu, Langmuir 20 (2004) 3180.
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[24] ACD/ChemSketch, Freeware Version 5.12, Advanced Chemistry Development, Inc., Toronto, Ont., Canada, 2002, http://www.acdlabs.com. [25] J.A. Zasadzinski, R. Viswanathan, L. Madsen, J. Garnaes, D.K. Schwartz, Science 263 (1994) 1726. [26] H.D. Sikes, J.T. Woodward, D.K. Schwartz, J. Phys. Chem. 100 (1996) 9093.
[27] H.D. Sikes, D.K. Schwartz, Langmuir 13 (1997) 4704. [28] M. Yazdanian, H. Yu, G. Zografi, Langmuir 6 (1990) 1093. [29] J. Ahmad, Langmuir 12 (1996) 963. [30] J. Liu, J.R. Von Ehr, C. Baur, R. Stallcup, J. Randall, K. Bray, Appl. Phys. Lett. 84 (2004) 1359.
Langmuir–Blodgett films of octadecanethiol – properties and potential applications Raj Kumar Gupta a , K.A. Suresh a,∗ , Rui Guo b , Satyendra Kumar b a b
Raman Research Institute, Sadashivanagar, Bangalore 560080, India Department of Physics, Kent State University, Kent, OH 44242, USA
Received 29 July 2005; received in revised form 6 October 2005; accepted 8 October 2005 Available online 9 November 2005
Abstract Octadecanethiol (ODT) is known to form self-assembled monolayer on noble metal surfaces which has potential technological applications. Langmuir–Blodgett (LB) technique is another useful method of obtaining highly ordered assembly of molecules. It is of interest to find whether ODT molecules can also form a stable Langmuir monolayer which facilitates the preparation of LB films. In literature, it has been reported that ODT molecules form an unstable Langmuir monolayer. We have studied the stability of the monolayer of the ODT molecules at air–water interface using surface manometry and microscopy techniques. We find the monolayer to be stable on ultrapure water of resistivity greater than 18 M cm. However, the behavior changes in the presence of even small amount of additives like NaOH or CdCl2 in the subphase. Our AFM studies on the LB films of ODT deposited from ion-free ultrapure water showed streak-like bilayer domains. The LB films of ODT deposited from CdCl2 containing aqueous subphase yield dendritic domains of the complexed unit grown over ODT monolayer. These nanostructures on surfaces may have potential applications in molecular electronics. © 2005 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; Octadecanethiol; Monolayer stability; Atomic force microscopy
1. Introduction Molecular electronics studies form an important step in the miniaturization of devices [1]. Here a bunch of molecules or a single molecule with appropriate electronic properties can be employed for rectification, amplification, negative differential resistance and conductance switching. Hence, the assembly and the ordering of the molecules on appropriate electrodes or substrates are important. In the molecular electronics studies, an ordered assembly of molecules have been obtained by self-assembly, Langmuir–Blodgett and other techniques. Selfassembly of organosulfur compounds on metallic surfaces like gold, silver and copper is a well known technique to obtain an ordered monomolecular film [2]. Self-assembled monolayers (SAM) of organosulfur compounds are generally prepared by immersing clean metallic substrate such as gold, silver and copper into the millimolar solution of the compounds in
∗
Corresponding author. E-mail address: [email protected] (K.A. Suresh).
0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.10.010
an appropriate organic solvent. The head group (–SH) of the molecules chemisorb spontaneously to the metallic surfaces. Such an adsorption leads to a monolayer of highly ordered molecules. SAM finds applications in the fields of wetting, dewetting, chemical sensors, biological sensors, nanolithography, lubrication, optical waveguides and molecular electronics [3–5]. Langmuir–Blodgett (LB) technique is another important method to obtain a highly ordered monolayer or multilayer on different substrates. A single layer formed by insoluble amphiphilic molecules at the air–water (A–W) interface is called a Langmuir monolayer. An amphiphilic molecule generally comprises of two parts – a hydrophilic (polar) head part and a hydrophobic (non-polar) tail part. When amphiphilic molecules are dissolved in volatile organic solvent and the solution is spread at the A–W interface, the solvent evaporates and the hydrophilic part of the molecule gets adsorbed to the water surface, whereas the hydrophobic part stays away from the surface. This forms a monolayer at the A–W interface. The Langmuir monolayer provides an ideal two-dimensional (2D) system to study the surface thermodynamics where the 2D plane is provided by the smooth
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water surface [6–8]. The monolayer shows a variety of 2D phases depending on the nature of interaction among the molecules and experimental conditions like temperature, pH and ion contents of the subphase [9]. The Langmuir monolayer can be transferred layer by layer onto a solid substrate by vertically moving the substrate in and out of the subphase. This is known as LB technique [10,11]. A stable Langmuir monolayer serves as a precursor for building defectless and ordered LB films. Such films provide useful systems for understanding fundamental phenomena involved at the surfaces and interfaces. The properties of the surfaces can be tailored with the choice of appropriate molecules. Octadecanethiol (C18 H37 SH), an alkanethiol, forms a very stable self-assembled monolayer. The octadecanethiol (ODT) molecules can be used efficiently for patterning gold substrate through microcontact printing technique [12]. The –SH group of the ODT is weakly acidic in nature and thereby the molecule becomes amphiphilic. We can expect it to form a stable monolayer at the air–water (A–W) interface. Though there are many reports on the SAM of ODT on the metallic surfaces, there have not been many studies on the LB films of ODT molecules [13], as ODT is reported to form an unstable monolayer at the A–W interface [14–17]. It is important to have a stable monolayer for the formation of defect free LB films which may find applications in devices. LB technique also provides a means of forming ODT monolayer on non-conducting surfaces. There have been many attempts to stabilize the ODT monolayer by adding additives to the subphase. It has been reported that the ODT monolayer can be stabilized with stearic acid provided the subphase contains barium salt [14]. Livingston and Swingley [15] have checked the stability of ODT monolayer on an aqueous subphase with different pH values and salts. They have observed that the ODT monolayer was stable only on a subphase containing potassium permanganate. Itaya et al. [16] have found that the ODT monolayer is stable only on the subphase containing BaCl2 . Bilewicz and Majda [13] have reported that the ODT monolayer can be stabilized by mixing it with octadecanol. A recent report on the surface manometry and the X-ray diffraction studies on the monolayer of ODT at the air–mercury interface have shown that ODT molecules exhibit a disordered phase of surface-parallel molecules, a condensed phase with tilted molecules and an untilted condensed phase [18]. We have studied the stability of the ODT monolayer at the A–W interface using different techniques. We find that ODT monolayer is stable on a subphase of ultrapure water. We have formed LB films on silicon substrate which exhibit streak-like patterns of bilayer thickness. We have also studied the formation of monolayer of ODT–metal complex by the addition of CdCl2 in the subphase. The LB films of ODT–CdCl2 complex yield dendritic patterns. 2. Experimental The water used in the experiments on ODT was ultrapure ion-free having a resistivity greater than 18 M cm obtained by passing double distilled water through the filtering and deionizing columns of a Milli-Q Millipore unit. The alkaline subphase was prepared by the addition of NaOH in the ion-free water. To form ODT–metal complex, CdCl2 salt was dissolved in the
ion-free water. In surface manometry, the surface density of the molecules adsorbed at the interface is varied and the surface pressure (π) is measured at a constant temperature. This yields a surface pressure–surface density isotherm. The area per molecule (Am ) is defined as the inverse of the surface density. Surface pressure–area per molecule isotherms were obtained using a NIMA 611M trough. The solution of concentration 3.5 mM of ODT in chloroform was prepared and it was spread on the subphase using a microsyringe. About 15 min were allowed for the solvent to evaporate from the surface. The compres˚ 2 /molecule)/min in all the sion speed was maintained at 3.8 (A experiments. For epifluorescence experiments, a fluorescent dye 4-(hexadecylamino)-7-nitrobenz-2-oxa-1,3-diazole (Molecular Probes) was used. The amount of dye was around 1% molar concentration in the ODT monolayer. The dye-doped monolayer was observed under a Leitz Metallux 3 microscope and the images were captured using an intensified CCD camera. The intensity of the emitted light from the dye-doped monolayer depends on the miscibility of the dye molecules in a particular phase of the monolayer. The gas phase appears dark due to the quenching of the dye molecules. The low density liquid phase appears bright in the epifluorescence images. On the other hand, the solid phase appears dark which is due to the expulsion of dye molecules from the highly dense solid domains [19]. Though both gas phase and solid phase appear dark under the microscope, it is possible to distinguish them; the gas phase will appear uniformly dark whereas the solid phase will show a few bright specks of dye particles distributed in the dark background. The monolayer was also studied using a Brewster angle microscope (BAM). In BAM, a polarized light was allowed to be incident on the water surface at the Brewster angle for the A–W interface. The intensity of the reflected light was minimum for the water surface. However, the presence of a monolayer at the interface alters the Brewster angle condition. This in turn reflects some light which was collected by the CCD to form images of the monolayer domains. The intensity of the reflected light depends on the thickness of the film and the surface density of the molecules [20]. The equilibrium spreading pressure (ESP) was determined by depositing small crystallites of ODT on ultrapure water in a Teflon container and the surface pressure was monitored using a tensiometer. The relaxation of the monolayer was studied by maintaining the monolayer at a given surface pressure and monitoring the variation in the molecular area with time. The monolayer from the aqueous subphase was transferred on solid substrates by LB technique. The silicon (Si) wafers (one sided-polished, 0.4 mm thick) were treated chemically to yield a hydrophilic nature to the surface [11]. The Langmuir films were transferred onto the treated Si substrate at a target pressure of 8 mN/m. The dipper speed was maintained at 5 mm/min. The substrate was immersed in the subphase prior to the spreading of the molecules on the subphase. The films of the ODT or its metal-complexes were transferred on the Si substrate by a single upstroke of the dipper. The atomic force microscope (AFM) imaging of the LB films was carried out using Nanoscope IIIA (Digital Instruments). All the images were taken in the tapping mode. Tapping mode has numerous advantages over contact mode for imaging soft matter and biological samples [21]. For
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the imaging, silicon (Si) tips were used. The spring constants were in the range of 0.1–0.6 N/m. All the experiments were carried out at the room temperature (22 ± 0.5 ◦ C). 3. Results and discussion 3.1. Films of ODT molecules at air–water and air–solid interfaces The surface pressure (π)–area per molecule (Am ) isotherm of ODT monolayer on the ultrapure ion-free water is shown in Fig. 1. The isotherm shows the coexistence of gas and a con˚ 2 . Then there is a sharp change in densed phase up to 19.5 A the slope of the isotherm indicating the onset of the homogeneous condensed phase. The extrapolation of the steep region of the isotherm to zero surface pressure is called limiting area per molecule (A0 ). This is the minimum area to which the molecules can be compressed on the water surface without collapsing the monolayer. The orientational state of the molecules (tilted or untilted) in a phase can be estimated qualitatively by comparing the extrapolated area per molecule with that of molecular crosssectional area. The isotherm of the ODT monolayer yields the ˚ 2 which approximately corresponds to value of A0 to be 19.0 A the cross-sectional area of the alkyl chain [9]. Hence, the steep region of the isotherm may correspond to the untilted condensed phase. The monolayer collapses at the surface pressure of about 14 mN/m. The isotherm reveals a plateau in the collapse region of the ODT monolayer. The stability of the ODT monolayer on ion-free water at a given surface pressure was studied by monitoring the change in the normalized area, At /At=0 , where At is the value of Am at a time t and At=0 is the initial value of Am . This is shown in Fig. 2. Assuming a linear dependence of the normalized area with time, we get the rates of reduction of the normalized area to be 0.076%, 0.10% and 0.117% per minute at the surface pressures of 2, 5 and 9 mN/m, respectively. The monolayer of the ODT molecule was quite stable as indicated by the negligible rate of decrease in the normalized area. According to a model proposed by Smith and Berg [22], the decrease in normalized
Fig. 1. Surface pressure (π)–area per molecule (Am ) isotherm of ODT monolayer on the ultrapure ion-free water.
Fig. 2. The area relaxation curves of the ODT monolayer on ion-free water at a given surface pressure. At is the Am at a time t and At=0 is the Am at the initial time.
area can be due to the dissolution of the molecules in the subphase by diffusion, if it follows the relation ln(At /At=0 ) ∝ −tβ with β equals to 0.5. However, in the present case, fitting such a power law to the data (Fig. 2) for the surface pressures of 2, 5 and 9 mN/m yield the β values to be 0.77, 0.92 and 0.81, respectively. Hence, the small reduction in the normalized area may not be due to the dissolution of the molecules. Such a small reduction can be attributed to the relaxation of the ODT molecules in the monolayer. The equilibrium spreading pressure (ESP) [9] of a material is the surface pressure of its monolayer phase coexisting in equilibrium with its bulk phase. It indicates qualitatively, the spreading capability of the molecules at an interface and the loss due to the dissolution of the molecules into the subphase and its evaporation. The ESP value depends on the material properties, subphase, temperature and humidity. The ESP measurement of ODT molecules is shown in Fig. 3. We find a finite ESP value of 2.5 mN/m for ODT indicating the stability of the monolayer on the ion-free water. The ESP values of the compounds like octadecanol, octadecylamine and stearic acid have been reported to be 34.3, 9 and 5.2 mN/m, respectively [22,23]. Although these molecules are similar in structure, they differ from each other in
Fig. 3. Variation of surface pressure with time for ODT molecules on ion-free water. The relative humidity and temperature were maintained at 90% and 24 ◦ C. The saturated value of surface pressure (2.5 mN/m) is the ESP of ODT.
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Fig. 4. BAM image of the ODT monolayer on the ion-free water captured at different Am : (a) shows the coexistence of gas (dark region) and condensed phase (bright region), (b) shows a homogeneous condensed phase. The scale bar represents 500 m.
Fig. 5. Epifluorescence images of the ODT monolayer on the ion-free water captured at different Am : (a) shows the coexistence of gas (dark region) and condensed phases (bright region), (b) shows a homogeneous condensed phase. The scale bar represents 50 m.
their hydrophilic head groups. The lower value of ESP of ODT can be attributed to the less polar nature of the compound. The BAM images of the ODT monolayer on the ion-free water at different Am are shown in Fig. 4. At a large Am , the image (Fig. 4(a)) shows a coexistence of dark region and bright region. The dark region represents the gas phase, whereas the bright region represents the condensed phase. On compression, the bright region grows at the expense of dark region, yielding an uniform bright texture which corresponds to a homogeneous condensed phase. The monolayer collapses on further compression. Epifluorescence microscope images of the ODT monolayer on ion-free water are shown in Fig. 5. The image at large Am shows dark and the bright regions (Fig. 5(a)). The dark region represents the gas phase. The bright region corresponds to the condensed phase of the ODT monolayer. On compression, the bright region grows at the expense of the dark region, leading to an uniform bright region, indicating a homogeneous condensed phase (Fig. 5(b)). On further compression, the ODT monolayer collapses and the domains of different intensity levels were seen. These observations were in accordance with those of the BAM imaging of the monolayer. Both microscopy techniques show an
uniform texture for the condensed phase (Figs. 4(b) and 5(b)) which indicates the formation of a stable ODT monolayer. An earlier study of the ODT monolayer on water subphase of resistivity of the order of 12 M cm showed the coexistence of gas and condensed phases even in the steep region of the isotherm indicating the unstable nature of the monolayer [17]. However, we find a very stable Langmuir monolayer of ODT provided the subphase is ion-free water with a resistivity greater than 18 M cm. The phase sequence observed are; coexistence of gas and condensed phase, condensed phase and the collapsed state. The formation of a stable Langmuir monolayer of ODT has facilitated us to prepare LB films. The ODT monolayer was transferred from ion-free water on hydrophilically treated silicon (Si) substrates. We have employed AFM to study the assembly of ODT molecules in the LB films. The reference image of the bare Si substrate for a scan range of 1 m × 1 m is shown in Fig. 6. The image revealed an average and a root mean square (RMS) roughnesses to be 0.122 and 0.154 nm, respectively. This shows that the Si substrates were reasonably smooth and featureless. The AFM images of the LB film of ODT on Si substrate for different scan ranges are shown in Fig. 7. The AFM images
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Fig. 6. AFM image of the reference Si substrate. The white line in the image represents the line along which the height profile is drawn. The height profile is shown below the image. The size of the image is 1 m × 1 m.
of the LB films of the ODT molecules showed some interesting results. The image for the scan range of 5 m × 5 m (Fig. 7(a)) shows streak-like bright domains that had grown on the uniform grey background. Careful observation of the image shows bright and dark spots all over the grey background. The average height of the grey background is 2 nm and that of the streak domains is 4.3 nm. The image for the scan range of 1 m × 1 m is shown in Fig. 7(b). The image is consistent with Fig. 7(a). Here also, the part of the streak-like domains are visible. The uniform grey background is embedded with the bright and dark spots. The height profile along the line drawn over the image reveals the height of the bright streaks and bright spots to be around 4.2 nm. The average height of the grey background is around 2 nm. Fig. 7(c) and (d) also show the similar features. The bright and the dark domains are more clear in Fig. 7(c). The height profile along the line shown in the image reveals an average height of 2.3 nm for the grey background. The image also exhibits bright domains, dark spots and the spots with intensity varying from grey to bright. The height profile over the bright domains yields a value of 4.2 nm. The height profile data yield the thickness of the dark spots to be nearly zero. This indicates that the dark spots are due to the bare Si substrate. The height of the spots with varying intensity lie in the range of 0.5–1.8 nm. Fig. 7(d)
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shows the grey background and the domains with varying intensity. The length of the ODT molecule is 2.1 nm [24]. Hence, it can be interpreted that the grey background corresponds to the monolayer of the ODT molecules in which the aliphatic chains are oriented normal to the Si substrate. The streak-like domains appeared over the grey background. The average height of the streak-like domains (∼4.2 nm) suggests the domains to be a bilayer of the ODT molecules oriented normal to the Si substrate. The films have defects in the form of dark domains and domains with varying intensity. The aggregation of molecules in the LB films depends highly on the nature of the substrates and the experimental parameters, such as temperature, humidity and ion contents in the subphase [25]. The molecules find a different thermodynamical environment when they get transferred from the A–W interface to the solid substrates. Hence, on the substrate the molecules relax in the thermodynamical stable state [26,27]. Here, we have transferred the ODT molecules from the A–W interface in the condensed phase. In the condensed phase, the ODT molecules orient normal to the A–W interface. Through AFM imaging we could see the existence of predominantly condensed phase (grey background). However, the relaxation of the monolayer leads to the formation of streak-like bilayer domains and the defects. We present a schematic model, suggesting a possible arrangement of the ODT molecules in the LB film on the Si substrate. This is shown in Fig. 8. The grey background in the AFM images is denoted by a layer of ODT molecules oriented normal to the Si substrate. The streak-like domains are represented by the bilayer of the ODT molecules oriented normal to the substrate. The defects with varying intensity are depicted by the bunch of randomly oriented molecules and the dark spots are indicated by gaps in the film. 3.2. Films of ODT molecules on alkaline subphase The ODT molecule is weakly acidic in nature and hence we have studied the effect of alkaline subphase on the ODT monolayer. The effect of addition of NaOH in the subphase on the isotherms of ODT is shown in Fig. 9. The presence of even very small quantity of NaOH has a marked effect on the monolayer of the ODT molecules. The isotherms shift systematically towards higher Am with increase in concentration (≤10−3 M) of NaOH in the subphase (Fig. 9). Above the concentration of 10−3 M, the ODT monolayer gets destabilized completely and the isotherms show a continuous and a gradual rise in the surface pressure. The limiting area per molecule (A0 ) values obtained from the isotherms of the ODT monolayer on the aqueous subphases containing 10−5 , 10−4 and 10−3 M of NaOH (Fig. 9) are 20.6, 22.1 ˚ 2 , respectively. The acidic head group (–SH) of the and 22.5 A ODT molecule may tend to dissociate on the alkaline subphase. The increase in A0 values may be due to the electrostatic charge repulsion between such dissociated components [9,28]. It was further observed that holding the monolayer at a given surface pressure leads to a relatively faster decrease in Am as compared to that of the ODT monolayer on ion-free water. This indicates an unstable nature of the monolayer on the alkaline subphase. We have carried out the microscopy experiments on the ODT monolayer for different concentrations of NaOH in the subphase.
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Fig. 7. AFM images of the LB films of ODT for different scan ranges. The white line in the image represents the line along which the height profile is drawn. The height profiles are shown below the respective images.
Fig. 10 shows a BAM image of the ODT monolayer on the alkaline subphase. The presence of very minute quantity of NaOH in the subphase shows a marked change in the BAM images. In Fig. 10, at least three different phases were observed. The dark
region is the gas phase, the bright region with the sharp boundary is the usual condensed phase, whereas the grey domains with diffuse boundary may correspond to the monolayer of the dissociated ODT molecules. These features were seen up to the NaOH
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Fig. 8. A schematic model of the arrangement of the ODT molecules in the LB film transferred from the ion-free water onto Si substrate. The –SH group is represented by black circle and the aliphatic chain is denoted by the zigzag line.
Fig. 11. The surface pressure (π)–area per molecule (Am ) isotherms of the ODT monolayer for different molar concentrations of CdCl2 in the aqueous subphase.
3.3. Films of ODT–CdCl2 complexes at air–water and air–solid interfaces
Fig. 9. Surface pressure (π)–area per molecule (Am ) isotherms of the ODT monolayer with different molar concentrations of NaOH in the subphase.
concentration of 10−3 M in the subphase. Above this concentration, the crystalline flakes of the complex of the ODT molecules and NaOH were observed. There are possibilities of the surface oxidation of the ODT molecules to form dioctadecyl disulfide over the alkaline subphase [29]. The dioctadecyl disulfide does not form a stable film over the aqueous subphase. Further, the dissolution of the charged monolayer is known to be higher as compared to the uncharged monolayer [9]. Hence the instability of the ODT monolayer on the alkaline subphase can be attributed to the dissociation of the ODT head group and the formation of the disulfide during the surface chemical oxidation.
Fig. 10. BAM image of the ODT monolayer on an aqueous subphase containing 10−6 M of the NaOH. The image has been taken for zero surface pressure and at a large Am . The scale bar represents 500 m.
We have studied the effect of divalent ion (Cd2+ ) in the subphase on the films of the ODT molecules at the A–W interface and on air–silicon interface. It is known that a Cd2+ ion complexes with two ODT molecules in order to achieve electroneutrality. In the complex unit, it has been proposed that the orientation of the aliphatic chain of the two ODT molecules are in opposite direction with the Cd2+ ion at the center [16]. The isotherms of the ODT molecules on the aqueous subphases having different molar concentrations of CdCl2 are shown in Fig. 11. We find a drastic change in the π − Am isotherm of the ODT due to the presence of small amount of CdCl2 in the aqueous subphase. For a very low concentration (8.7 × 10−8 M) of CdCl2 in the aqueous subphase, the trend of the isotherm was similar to the one obtained for the ion-free water. However, there is a shift in the limiting area per molecule (A0 ). It shows the ˚ 2 . The isotherm for the concentration value of A0 to be 16.5 A −6 of 10 M shows a similar trend but the surface pressure continues to increase after an initial collapse at the surface pressure ˚ 2 . The of about 13 mN/m. It shows the A0 value to be 10.5 A isotherms for higher concentrations show a different behavior. They show a gradual rise in the surface pressure to very large values (≥60 mN/m). The trend of the isotherms do not change on further increase in the concentration of CdCl2 . Here, the values ˚ 2 . These values are about half of A0 lie in the range of 10.5–11.5 A of those corresponding to the normally oriented molecules in the condensed phase of ODT at the A–W interface. This indicates the formation of bilayer of the complex of ODT and CdCl2 . The ˚ 2 ) for the lower concentration (8.7 × 10−8 M) A0 value (16.5 A is more than that for a bilayer. This can be suggested as the formation of the complex of the ODT molecules in a fraction of the monolayer with CdCl2 . The BAM and the epifluorescence images of the ODT monolayer for an aqueous subphase containing 8.7 × 10−8 M of CdCl2 are shown in Fig. 12. In the BAM image (Fig. 12(a)), there are dark domains and grey domains with the different intensity levels. The dark region represents the gas phase. The grey region may represent the condensed phase of the ODT molecules. The bright domains may represent
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Fig. 12. (a) and (b) represent the BAM and the epifluorescence images of the ODT monolayer on an aqueous subphase containing 8.7 × 10−8 M of CdCl2 , respectively. The images were captured at a large Am and zero surface pressure. The scale bars in (a) and (b) represent 500 and 50 m, respectively.
the bilayer of the complex of ODT and CdCl2 . In the epifluorescence image (Fig. 12(b)), the dark domains represent the gas phase and the bright background represents the condensed phase of the ODT molecules. The grey and irregular domains may represent the bilayer of the complex of ODT and CdCl2 . The domains of the complex are darker than that of the condensed phase of the ODT molecules due to poor miscibility of dye molecules in it. The effect of CdCl2 in the subphase on the LB films of the ODT molecules was studied using AFM. The films of the complex of ODT molecules with CdCl2 were transferred on hydrophilic Si substrates by LB technique. The AFM images
of the films for two different scan ranges are shown in Fig. 13. Fig. 13(a) shows a dendrite-like structure over an uniform background. The height variation data reveal that the uniform background has an average height of 2 nm. The average height of the dendrite-like structure above the uniform grey background is 4.5 nm. An image with a lower scan range is shown in Fig. 13(b). This image also shows an uniform background of height of about 2 nm and the average height of the dendritelike domain above the grey background is 4.4 nm. The height variation in the dendrite-like domains is very small suggesting the uniform thickness of the dendritic domains. The features in both the images scale according to the scan range indicating its
Fig. 13. AFM images of the LB films of ODT molecules deposited from an aqueous subphase containing 8.7 × 10−8 M of CdCl2 . The white line in the image represents the line along which the height profile is drawn. The height profiles are shown below the respective images.
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Fig. 14. A schematic model of the arrangement of the molecules in the LB films deposited from the subphase containing 8.7 × 10−8 M of CdCl2 . The polar group of the ODT molecule is represented by the black circle and the chain is represented by the zigzag lines. The Cd2+ ion is represented by grey circle.
reproducibility. The length of the ODT molecule is 2.1 nm, and the length of the complex unit for all trans conformation of the aliphatic chain is around 4.5 nm [24]. Therefore, we propose that the uniform background represents an ODT monolayer on Si substrate with the molecules being oriented normal to the substrate. The height variations obtained from the AFM images (Fig. 13) indicate that the dendrite-like domains may correspond to that of a layer of the complex unit of ODT and CdCl2 . The complex unit behaves like a hydrophobic component that cannot anchor to the water surface. We suggest that the monolayer in the background are composed of ODT molecules with the domains of the complex unit deposited on it. Based on our experimental observations, we present a schematic diagram showing a model for the aggregation and arrangement of the ODT molecules in the LB film at the air–silicon interface. This is shown in Fig. 14. In the model, it is shown that the first monolayer is composed of ODT molecules where all the molecules are normally oriented on the Si substrate. In the layer of the complex unit, which forms a dendrite-like structure in the AFM images, the two ODT molecules face each other with the Cd2+ at the center. The lengths d1 and d2 represent the thicknesses of the monolayer of the normally oriented ODT molecules (grey background in the AFM images) and a layer of the complex unit (dendrite-like domains in AFM images), respectively. The AFM images suggest the values for the thickness d1 and d2 (Fig. 14) to be 2 and 4.5 nm, respectively. 4. Conclusions We have shown that the ODT monolayer is stable at the A–W interface provided the water is ion-free having a resistivity greater than 18 M cm. The monolayer is sensitive to the presence of very small quantities of NaOH or CdCl2 in the aqueous subphase. The ODT molecules in the LB films predominantly prefer an orientation normal to the substrate, as seen in the AFM images. Interestingly, the AFM images reveal streak-like domains which are the bilayer of the ODT molecules. The presence of divalent ion (Cd2+ ) in the subphase yields an ODT–metal complex at the A–W interface. The AFM images of the LB films of the ODT molecules deposited from an aqueous subphase containing low concentration of CdCl2 show dendritic domains representing a layer of the complex unit.
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The LB films of the ODT molecules have several potential applications. The streak-like domains in the LB films of ODT may be employed for the growth of nanowires. The nanometer sized defects may be utilized for the formation of nanoelectrodes. Such defects may find application in selective permeation devices. They may also be used in molecular species recognition and sensors. The ODT molecules may be used as an adhesion promoter for the transfer of LB films of poorly adsorbing materials. Since ODT molecules bind strongly to noble metals, it may be mixed with other components during LB transfer onto substrates like gold, silver and copper. The ODT molecules find many interesting applications. Microcontact printing is an useful technique for directly creating patterns in thin metal films. The ODT molecules are found to be a very suitable inking material for microcontact printing on gold surfaces [12]. The ODT and decanethiol molecules have been used to create high-density alternating thiol nanostructures using the AFM nanografting technique [30]. Acknowledgments We are thankful to V. Lakshminarayanan and G.S. Ranganath for helpful discussions. This work is supported, in part, by a grant from the US National Science Foundation under grant DMR-0423619. References [1] C. Joachim, J.K. Gimzewski, A. Aviram, Nature (London) 408 (2000) 541. [2] A. Ulman, Chem. Rev. 96 (1996) 1533. [3] I. Rubinstein, S. Steinberg, Y. Tor, A. Shanzer, J. Sagiv, Nature (London) 332 (1988) 426. [4] C.J. Zhong, M.D. Porter, Anal. Chem. 67 (1995) 709. [5] G.M. Whitesides, P.E. Laibinis, Langmuir 6 (1990) 87. [6] A. Bhattacharyya, K.A. Suresh, Europhys. Lett. 41 (1998) 641. [7] P. Viswanath, K.A. Suresh, Phys. Rev. E 67 (2003) 061604. [8] P. Viswanath, K.A. Suresh, J. Phys. Chem. B 108 (2004) 9198. [9] G.L. Gaines Jr., Insoluble Monolayers at Liquid–Gas Interfaces, Wiley–Interscience, New York, 1966. [10] G. Roberts, Langmuir–Blodgett Films, Plenum Press, New York, 1990. [11] R.K. Gupta, K.A. Suresh, Euro. Phys. J. E 14 (2004) 35. [12] R.B. Bass, A.W. Lichtenberger, Appl. Surf. Sci. 226 (2004) 335. [13] R. Bilewicz, M. Majda, Langmuir 7 (1991) 2794. [14] H. Sobotka, S. Rosenberg, in: H. Sobotka (Ed.), Monomolecular Layers, Am. Assoc. Advan. Sci., Washington, DC, 1954, p. 175. [15] H.K. Livingstone, C.S. Swingley, J. Colloid Interf. Sci. 38 (1972) 643. [16] A. Itaya, M. Van der Auweraer, F.C. De Schryver, Langmuir 5 (1989) 1123. [17] W. Zhao, M.W. Kim, D.B. Wurm, S.T. Brittain, Y.T. Kim, Langmuir 12 (1996) 386. [18] B.M. Ocko, H. Kraack, P.S. Pershan, E. Sloutskin, L. Tamam, M. Deutsch, Phys. Rev. Lett. 94 (2005) 017802. [19] V. von Tscharner, H.M. McConnell, Biophys. J. 36 (1981) 409. [20] S. Rivi`ere, S. H´enon, J. Meunier, D.K. Schwartz, M.-W. Tsao, C.M. Knobler, J. Chem. Phys. 101 (1994) 10045. [21] P.K. Hansma, J.P. Cleveland, M. Radmacher, D.A. Walters, P.E. Hillner, M. Bezanilla, M. Fritz, D. Vie, H.G. Hansma, C.B. Prater, J. Massie, L. Fukunaga, J. Gurley, V. Elings, Appl. Phys. Lett. 64 (1994) 1738. [22] R.D. Smith, J.C. Berg, J. Colloid Interf. Sci. 74 (1980) 273. [23] Y.L. Lee, K.L. Liu, Langmuir 20 (2004) 3180.
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