www.ijm‐me.org International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 doi: 10.14355/ijmme.2016.05.004
Density Analysis on Self‐Assembled Monolayers (SAMs) of Functionalized Alkanethiol on Gold over Polyethylene Terephthalate (PET) Substrate *1M. Jalal Uddin, 1M. Khairul Islam, 1M. Nasir Uddin Khan, 1M. Shahinuzzaman, 2M. Khalid Hossain and 1M. Abdul Momin Dept. of Applied Physics, Electronics and Communication Engineering, Islamic University, Kushtia, Bangladesh 1
Atomic Energy Research Establishment, Savar, Dhaka, Bangladesh 2
E‐mail (*Corresponding Author): mju.aece@gmail.com Abstract In this work, an equivalent circuit from electrochemical impedance spectroscopy (EIS) has been designed and executed for qualitative analyzing of leaky behavior of self‐assembled monolayers (SAMs) of octadecanethiol [CH3(CH2)17‐SH] on gold (Au) suface. Polyethylene terephthalate (PET), the cost‐effective substrate has been utilized for metalization of thin Au layer with subsequent formation of octadecanethiol SAM on it with systematic variation of adsorption time. The SAM layer shows its highly dependency on the adsorption time of the surfactants of octadecanethiol with the metal surface. Keywords Self‐Assembled Monolayers (Sams), Electrochemical Impedance Spectroscopy (EIS), Octadecanethiol, Polyethylene Terephthalate (PET), Relectrolyte, RSAM, and CSAM Introduction
Self‐assembled monolayers (SAMs) are well defined and densely packed organic molecules formed by spontaneous adsorption of the functional molecules on a substrate [1]. The surface functionalization by such molecular assembling in the applications of organic electronics has attracted enormous interest [2‐6] during the past decades. Because this route provides the means of engineering to integrate the mono or multilayers of the functional molecules onto the electrode surface [7,8] in the applications of interfacial electron transfer [9‐12], patterning microarray or selective electrodes for bio‐sensors [7,13‐15], and fabricating lipid bilayers on electrodes [16‐19] among others. These applications are closely related to the blocking properties of monolayer that exhibit both the ionic and electronic transfer between the electrolyte and the metal substrate. The blocking properties are eventually deteriorated by the presence of leakage or pinholes in the monolayer, which can be characterized by several electrochemical methods reported in [20‐22]. Self‐assembled monolayers (SAMs) of alkanethiols on gold are one of the mostly studied molecular assembling [23,24]. The –SH head‐group of functional alkanethiol has strong affinity to gold surface and its methyl‐terminated end‐group with long‐chain form well‐ordered monolayers on gold. [24,25]. In this work, we have prepared Octadecanethiol [CH3(CH2)17‐SH] (ODT) SAMs on Au using polyethylene terephthalate (PET) substrate. We have executed electrochemical impedance spectroscopy (EIS) to find out the blocking properties of the adsorption time dependent monolayers. Furthermore, we have carried out the contact angle measurement to study the hydrophobic/hydrophilic properties of the SAM modified surface. Materials and Methods
Sample Preparation 0.071gm of 1‐octadecanethiol (CH3(CH2)17‐SH) (ODT) obtained from Sigma Aldrich was dissolved within 50ml of 28 International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 www.ijm‐me.org
(99.99% MEK) ethanol to prepare 5mM of ODT solution. Polyethylene terephthalate (PET) substrate of 1cm2 properly cleaned with acetonitrile, N2 gas and UV‐ozone cleaner were used to sputter thin layer of gold (Au) layer on it. To prepare SAM of ODT, the Au sample was immersed inside the thiol solution for different adsorption time such as 30 seconds, 1 minute, 1 hour, 1 day, and 2 days. Then the unbound molecules of the SAM layer were removed by consecutive washing with ethanol and sonication for 2 minutes. To prepare PDMS stamp for electrochemical analysis, sylgard polymer and curing agent in 10:1 ratio within a plastic container was mixed homogenously and placed inside a desiccator until getting the air‐bubble of the mixture totally out. The degased mixer was then transferred to a petri dish carefully avoiding any air‐bubble formation and warmed inside an oven for 1 hour at 70 ̊C. Then the soft mold was cooled taking out of the oven, shaped and punched holes of 3‐5mm. Ultrapure Tris‐buffer (C4H11NO3) obtained from Sigma Aldrich in powder form was used to prepare 10mM of the solution. pH 7.4 of the solution was ensured adding additional 37% HCl drop wise. The reason behind using this value of pH as it is favorable for most of the organic molecules. Analysis of Sams with Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) is a standard tool for analyzing interfacial impedance [26]. Frequency dependent modeling of EIS based measurement setup by the equivalent electrical parameters can provide sensitive and accurate capacitance and resistance of the monolayers from their impedance studied by EIS, which in turn could indicate the blocking and leakage properties of the monolayers. The schematic block diagrams of the electrochemical setup for EIS [27‐30] and proposed equivalent circuit have been sketched in Fig. 1 and Fig. 2. FIG. 1 SCHEMATIC DIAGRAM OF EXPERIMENTAL SETUP WITH CORRESPONDING ELECTROCHEMICAL SETUP MARKED IN RED BOX. The schematic view of the experimental setup (lock‐in amplifier SR830 DSP) with corresponding electrochemical setup marked in red box for impedance measurement of the SAM modified surface has been shown in Fig. 1. The lock‐in amplifier used facilitates the operating frequency from 10 Hz to 100 KHz. In the measurement, ac signal of 10mV Peak‐Peak was applied across the sample through a feedback resistor of 10KΩ as connected in Fig. 1 and measured impedance is given by, Vout  I .R fb 
Z
Vin
.R fb (1) Z
Vin
.R fb (2) Vout
Since the lock‐in amplifier measures the voltage drop across SAM layer, it has limitations to provide the impedance of the SAM layer directly. So the reciprocal relation of impedance, which is the voltage across the fabricated SAM layer, has been interpreted in this paper. In the measurement, the PDMS mold was placed over the sample and then pouring the hole of the PDMS with electrolyte solution, the Ag/AgCl electrode was dipped into the solution to apply signal across the sample and measurements were carried out. Based on electrochemical setup shown in the red marked box, an equivalent RRC and RC circuit shown in Fig. 2 can be considered. 29 www.ijm‐me.org International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016
FIG. 2 EQUIVALENT (a) RRC AND (b) RC CIRCUIT FROM THE ELECTROCHEMICAL SETUP SHOWN IN FIG. 1. In the RRC model shown in figure 2(a), Relectrolyte, RSAM, and CSAM represent the resistance of the electrolyte, resistance, and capacitance of the SAM layer respectively. The parallel combination of capacitor CSAM and resistor RSAM of the circuit models the capacitive and resistive effect of the thin monolayer respectively. The ignorance of the resistance RSAM of the monolayer due to the leakage current at very lower frequencies leads to the more simplicity of the model towards figure 2(b). The impedance of the sample based on the simpler model as shown in Fig. 2(b) as a function of frequency is given by Ztotal() = abs [Relectrolyte (1+1/j..Relectrolyte.CSAM)] (3) With frequency  →, |Ztotal()| R electrolyte, and with frequency  →0, |Ztotal()| 1/.CSAM. Therefore, the measured voltage will correspond to the capacitance of the monolayer at the lower frequencies, whereas the measured voltage at the higher frequencies will represent the electrolyte resistance. Also the respective 90 and zero degree phase shift ensures the contribution capacitance and resistance to the measured voltage at the lower and higher frequency regimes. Analysis of Sams with Contact Angle Measurement Contact angle measurement is the most convenient and well‐suited technique to quantify the surface energies, which has direct consequences on the other surface properties like adhesion, wettability and friction. In principle, the angle between a drop of liquid and a solid surface is termed as the contact angle, which quantifies the energetic interaction between the liquid and the solid surface. This contact angle can be determined with droplet contours using Young’s equation. The contact angle of a drop of liquid settled on a surface is due to the surface tension (γ) at the surface‐liquid (SL), surface‐vapor (SV) and vapor‐liquid (VL) interfaces as shown in Fig. 3(a). FIG. 3 SCHEMATIC DIAGRAM OF A CONTACT ANGLE If the drop of liquid on the surface was considered at equilibrium with the surface and vapor, then the change in the area (dA) of the surface would be zero. But with the expansion of the drop of liquid as shown in Fig. 3(b), the area of the surface‐liquid interface increases by dA with the same decreasing area of the surface‐vapor interface. 30 International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 www.ijm‐me.org
Also the area of the vapor‐liquid interface increases by cosθ.dA. Since the sum of the free energy changes due to the change in the area would be zero, then  SL.dA   SV .dA   VL.dAcos  0 (5) cos 
 SV  SL (6)  VL
This equilibrium equation is known as Young’s equation, which predicts the contact angles where the angle could be measured using video camera based commercial apparatus. Measurements, Results and Discussions
Analysis of Voltage Dropped across the Sam Prepared for Different Adsorption Time Fig. 4(a) shows voltage dropped across the SAM layer prepared for different adsorption time, whereas 4(b) represents the corresponding phase. As described in the equation (3), Vout at higher frequencies gives the values of Relectrolyte, which is almost similar for the all the SAM layers. It indicates that there is no interruption from the Relectrolyte to the measurement. For the SAM layer prepared for 1 hour, 1 day and 2 days, the obtained Vout is linear, which shows the capacitive behavior of the SAM layer and this prediction is validated by their corresponding phase of around 90 degree. Also for the SAM layer of 1 minute and 30 seconds, Vout gradually increases with inclined phase shifts towards the zero degree, which confirms the formation of the leaky monolayer for shorter adsorption time. Vout for the adsorption time from 1 hour to 2 days is notably identical, which means the necessity for longer adsorption time to prepare the qualitative monolayers in terms of their capacitive properties. 110
30S
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FIG. 4 (a) IMPEDANCE AND (b) PHASE SHIFT OF SAM OF DIFFERENT ADSORPTION TIME ON Au. Analysis of Voltage Dropped across the Sam Measured with Different Electrode Height To study the measurement artifact using our proposed electrochemical setup shown in Fig. 1, voltage drop across the SAM layer prepared for the adsorption time of 30 seconds was performed. During this measurement varying height of 1 mm, 2 mm and 3 mm for the Ag/AgCl electrode from the sample surface were considered. Fig. 5(a) shows the Vout measured for the different electrode heights. Since the measurement has been performed on the same sample, there is no change in the Vout at the lower frequencies because of the same magnitude of the capacitance. The only change found for Vout is at the higher frequencies with different electrode height. This is because of the changing of the distance between the electrode and sample surface that affects the resistance of the electrolyte. For all of the heights of the reference electrode, the phase shifts shown in Fig. 5(b) at higher frequencies is close to 0 degree and at lower frequencies its 90 degree, which correspond to the electrolyte resistance and capacitance of the SAM. The similar values of Vout for the electrode height dependent measurement ensure the accuracy of equivalent circuit modeled in Fig. 2. 31 www.ijm‐me.org International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016
1
90
70
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Vout (V)
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FIG. 5 (a) IMPEDANCE AND (b) PHASE SHIFT OF SAM OF MEASURED WITH DIFFERENT ELECTRODE HEIGHT. Analysis of Contact Angles The sample was placed over the sample holder, there after a drop of deionized (DI) water was placed on it and the contact angle of the drop was measured. The average values of the measured DI water on each of the SAM samples were summarized in Table 1. From the measured data, a highly hydrophobic behavior of the ODT SAM modified Au surface was confirmed by the increased contact angle from 43.24 to around 100 degree. TABLE 1. CONTACT ANGLE FOR ADSORPTION TIME DEPENDENT SAM ON AU. Sample Contact angle (deg) with ±1deg Bare Au 43.24 SAM with 30S 92.12 SAM with 1Min 95.32 SAM with 1Hr 101.38 SAM with 1Day 101.91 Conclusions
Density analysis of self‐assembled monolayer is a key issue for modifying the surface properties of metal electrode. In this work, electrochemical impedance spectroscopy (EIS) for subsequent density analysis of the adsorption time dependent monolayers has been executed. To quantify the quality of the monolayers interms of their capacitance and resistance, straight forward RC and RRC models for the developed SAM layers have been proposed. RRC model was found feasible to analyze the leaky behavior of the monolayer. This leaky behavior shows its strong dependency on the time dependent adsorption of the SAM surfactants to the Au surface. Furthermore, the contact angle measurement gives hydrophobic tendancy of SAM layers on the Au metal surface. The least contact angle was found for the SAM layer prepared for 30 sec which shows similar hydrophobic behavior for longer time adosrption such as 1 hr to 1day. ACKNOWLEDGEMENTS
This work was carried out within the graduate program “Nanomolecular Science” at the Jacobs University Bremen, Germany. The authors would like to acknowledge the grants of Islamic University, Kushtia, Bangladesh for facilitating the participation in the graduate program at Jacobs University Bremen, Germany. REFERENCES
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