A Review on the Influence of Applied Potential on Different Electrical Properties of Self‐Assembled

International Journal of Material and Mechanical Engineering (IJMME), Volume 4, 2015 www.ijm‐me.org doi: 10.14355/ijmme.2015.04.009

A Review on the Influence of Applied Potential on Different Electrical Properties of Self‐Assembled Monolayers (SAMs) of Alkanethiols on Gold (Au) Surface M. Jalal Uddin, 2M. Abdul Momin, 2M. Abdur Razzaque, 2M. Shahinuzzaman, 2M. Khairul Islam, 3Wayesh Qarony, and 3Ismail Hossain 1

Department of Radio Science and Engineering, Kwangwoon University, Seoul, Republic of Korea

1

Department of Applied Physics, Electronics and Communication Engineering, Islamic University, Kushtia, Bangladesh 2

Faculty of Engineering, American International University‐Bangladesh (AIUB), Dhaka, Bangladesh

3

Email: mju.aece@gmail.com (M. Jalal Uddin)

1

Abstract Self‐assembled monolayers (SAMs) of alkanethiols are commonly used in surface patterning, as well as in modifying the physical and chemical properties of surface. Therefore, the understanding of the formation of self‐assembled monolayers (SAMs), including its structure and different properties requires significant importance. Among a wide variety of organic molecules used for self‐assembly, alkanethiols and their derivatives on gold surface are the mostly studied. Because the alkanethiols based monolayers on gold layer prepared either using the flexible or rigid substrate are stable and possess well‐packed structure.

Self‐assembled monolayers (SAMs) of alkanethiols on gold (Au) have many significant applications. Among them, the applications in the field of interfacial electron transfer, sensing, developing selected electrodes, and fabricating lipid bilayers on electrodes are remarkable. Hence, the intensive study of the different electrical properties of self‐assembled monolayers (SAMs) is noteworthy for the devices fabrication. This review work is to report on the influence of applied potential on different electrical properties of self‐assembled monolayers (SAMs) of alkanethiols on conductive gold (Au) surface. The work will essentially focus on different techniques of self‐assembling, discussion on the arrangement and structure, and the influence of applied potential on different electrical properties like SAM resistance, capacitance, impedance etc.

Keywords Self‐Assembled Monolayers (SAMs), Alkanethiols, Cyclic Voltammetry, Impedance Spectroscopy, Gold (Au) Electrode

Introduction In the applications of interfacial electron transfer, sensing, developing selected electrodes, and fabricating lipid bilayers on electrodes through surface functionalization, the molecular assemblies are of prior interest [1‐4]. The molecular assembling on the surface may be either by the Langmuir‐Blodgett (L‐B) or Self‐assembly techniques [5].

Both of these Langmuir‐Blodgett (L‐B) and Self‐assembly routes allow the means of engineering to integrate the mono or multilayers of the functional molecules on the electrode surface. The layers formed either by Langmuir‐ Blodgett (L‐B) or by Self‐assembly techniques, modify the electrode surface as well as separate the bare electrode from the bulk of the solution functioning as passivation layer [5]. Therefore, the intensive study on the formation chemistry and properties of such mono or multilayers on the electrode surfaces is important.

Langmuir‐Blodgett (L‐B) Technique The Langmuir film is prepared by spreading amphiphilic molecules on a liquid surface like water. Amphiphilic molecules themselves can be organized at different orientations like liquid‐liquid, liquid‐air, liquid‐solid and air‐solid. The hydrophilic head group of the amphiphilic molecules also has an affinity to the water (at liquid‐liquid orientation) with the hydrophobic end group sticking out on the other side (at liquid‐air orientation) [6]. Therefore,

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considerable order of monolayer or multi‐layers of these molecules can be achieved by repeated dipping of the substrate in appropriate solutions and then transferring the molecules from liquid‐air to air‐solid interface. The film is thus transferred to a solid substrate from the liquid surface which is graphically shown in figure 1. Due to the chemical and mechanical instabilities and disorders of the film prepared by Langmuir‐Blodgett (L‐B) technique [7], most of the research trend focuses on the more stable films from molecular self‐assembled monolayers (SAMs).

FIGURE 1. SCHEMATIC ILLUSTRATION OF LANGMUIR‐BLODGETT (L‐B) TECHNIQUE [8].

Self‐assembly Techniques Self‐assembled monolayers (SAMs) are ordered and densely packed layers which are formed by spontaneous adsorption of organic molecules on the solid substrates in liquid or gas phases. These organic molecules have strong affinity of its head group to metal thin film patterned on the substrate. The monolayer thus formed with the nanometer sized constituents is more complex with considerably fewer degrees of freedom [9].

The quality of self‐assembling is dominated by the nature of chemical interaction between the surface and adsorbate as well as by the strength of intermolecular interactions of the adsorbates themselves. The chemical interactions and their strength depend on the hydrogen or covalent bonding, or ion pairing between the surface and adsorbates and the molecules themselves [7]. The interests behind SAMs are due to the following reasons: a) self‐assembly is quite simple, reproducible, and easy to be prepared b) it has significant diversity to modify the molecular structure and functions of substrate c) since the preparation parameters can be tuned, it can be used as molecular building blocks with more complex structures d) it can be used to structure the surface in the nanometer regime 1) Basic Components of Self‐Assembled Monolayers (SAMs) Atoms or molecules of the self‐assembled monolayers (SAMs) are the basic units of the system. In the case of thiols and also of silanes, the molecules used in the preparation of self‐assembled monolayers (SAMs) can be divided into some basic units described below with corresponding schematic of those components shown in figure 2.

FIGURE 2. BASIC COMPONENTS OF THE SELF‐ASSEMBLED MONOLAYERS (SAMs).

The head (or linking) group: Each of the functional molecules of interest for self‐assembly possesses a particular covalent linker to couple the molecules with the substrate. For metals, Si and oxidized surfaces, S, O, N and n‐alkyl tails atoms are more promising cross linkers extensively on Au, Ag and some cases on Cu, Ni and Pd [7, 10].

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The backbone (main chain): The backbone works alike chain of variable length between the linker and end group of the functional molecules. The stability of the monolayer is dependent on the Van der Waals force functioning among the neighboring molecules of the chains. The study of the different angle between the backbone and substrate characterizes the orientation and structures of the molecules on the substrate. For alkanethiols, dialkyl sulfides, and dialkyl disulfides, hydrocarbon (CH) is the typical molecular chain or backbone between its end and head groups.

The terminal specific (functional) group: The terminal specific group of SAMs is substantial to the surface chemical and physical properties. The different hydrophobic/hydrophilic/metallophobic behavior, adhesive, reactive and other different properties are dominated by the presence of different functional group attached to the end of the molecular backbone. Generally, the ‐CH3, and ‐CF3 groups exhibit hydrophobic and metallophobic manner of the SAMs surface, whereas the hydrophilic characteristic is yielded with the ‐OH, ‐COOH, and ‐NH2 groups [9].

2) Different Functional Groups Used for Self‐Assembly The first reported self‐assembled monolayers (SAMs) were with carboxylic acid and polar head‐group molecules as thiols, whereas the recent molecules used in the preparation of SAMs incorporate a wide variety of organic materials as functional molecules. Some of the most commonly reported functional groups are listed below.

TABLE 1. SOME COMMONLY USED ORGANIC MATERIALS IN THE FORMATION OF SAMs

Types of Groups

Name

References

HS(CH2)n R

Alkanethiols

[5,11,12]

R(CH2)m S(CH2)n R

Dialkyl sulfides

[13,14]

R(CH2)m S‐S(CH2)n R

Dialkyl disulfides

[15,16]

C2H7NS.HCl

2‐Aminoethanethiol hydrochloride

[17]

HS(CH2)10CO2H

2‐Mercaptoethylamine hydrochloride

[17]

where n and m are the number of methylene units and R represents the end group of the alkyl chain (e.g., CH3, ‐OH, ‐COOH).

3) Mechanism of Self‐Assembly The formation of a SAMs does not require the help of an external driving force. With the exposing of metal surface of interest into a solution containing the surfactant, self‐assembling occurs into two phases. In the first phase, the surfactants are rapidly attached to the surface and in the second phase, the attachment of surfactant is followed by slow reorganization process and it might takes a fraction of minute to several days for well‐structured monolayer. The well‐studied SAMs are due the strong affinities between the head‐groups and surface that leads the formation of covalent or ionic bonding [7].

FIGURE 3. SCHEMATIC ILLUSTRATION OF SELF‐ASSEMBLING [18]. RAPID PINNING OF THE SAM MOLECULES TOWARDS SLOW REORGANIZATION PROCESS FOR STRUCTURED MONOLAYER IS SHOWN FROM LEFT‐TO‐RIGHT DIRECTION.

Figure 3 is the pictorial overview of self‐assembling which gives an idea behind forming the ordered molecular layer of SAM on a substrate. Quick attaching of SAM constituent molecules is shown in the left whereas the structured monolayer after its slow reorganization process is depicted in the right of the above figure.

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Approaches to Study the Different Electrical Properties of Self-Assembled Monolayers (Sams) Cyclic Voltammetry Gold (Au) is a noble metal prominently used in preparation of SAMs using alkanethiol. The manifold reasons behind the preference of gold are its high thermal and electrical conductivity, chemical inertness, infrequent oxide layer formed on it and ease in preparation over its counterpart like Cu, Pt, Pd [19]. The alkanethiol molecules are used in chemical and electronics research to anchor the organic molecules or nanostructures on the substrate. Hence, it is essentially important to assess the stability and integrity of alkanethiol based monolayer on the gold surface as well as its insulation behavior in any bulk of solution. The passivation behavior of the monolayer against redox probes is the more pronounced study to test its stability, integrity and the insulation properties. These studies could be carried out through cyclic voltammetry with and without in the presence of some redox species in the associating electrolyte. Also, this study reveals the structural quality of the film either from the observation of higher current rather than that of the defect less SAM or from any change in the current‐voltage curve shape.

The passivating behavior can be addressed based on three different voltammetric responses. Firstly, unpassivating attitude of the electrode is shown due to the bare electrode being accessible by the redox species. Secondly, the sigmoidal capacitance‐voltage response renders the defective SAM with pinholes; and thirdly, the qualitative passivating behavior is extracted from the exponential capacitance‐voltage curves.

FIGURE 4. (LEFT) AND FIGURE 5. (RIGHT) (A) CYCLIC‐VOLTAMMOGRAM, AND (B) CAPACITANCE‐VOLTAGE CURVE OF THE RESPECTIVE DEFECT‐FREE (LEFT FIGURE) AND DEFECTIVE (RIGHT FIGURE) SAM OF ODT ON Au ELECTRODE IN 0.5M KCl AQUEOUS SOLUTION WITH 1mM K4Fe(CN)6 REDOX PROBE [10] RESPECTIVELY.

Several groups have demonstrated the approach of studying the passivating behavior. Regarding the passivating behavior, P. Krysinski and M.B. Smolska [10] has reported the current‐voltage and capacitance‐voltage properties of SAM on Au electrode using electrochemical process in the presence of potassium ferrocyanide K4Fe(CN)6 species with supporting 0.5M aqueous KCl. As it is shown in figure 4(a), the exponential increase of current indicates the transferring of electron across the monolayer on the electrode surface with the corresponding increased capacitance of SAMs shown in figure 4(b) for the applied electric field in the electrolysis‐SAM environment. The sigmoidal voltammogram of figure 5(a) is for the defective monolayer. The abrupt increase of capacitance in figure 5(b) at zero potential during reverse oxidation is either due to the accumulation of uncompensation K+ charge from the oxidation of K4Fe(CN)6 within the defective region of the SAM or due to the movement of Cl‐ ions from the electrolysis to optimize the K+ charge in the monolayer. Also, the increased current in the figure 5(a) than 4(a) by a factor of more than 10 is the clear identification of defective monolayer.

The passivating properties of the monolayers of different preparation times are another convenient method of studying the monolayer quality. In this case, the adsorption time of the ODT molecules on the electrodes of different dimensions could be systematically tuned to vary the surface coverage of the electrode. Then the cyclic voltammetry provides corresponding current responses based on the quality of the adsorption time dependent monolayer. The highly passivated monolayer shows lower current in cyclic‐voltammogram, which provides necessary information about the formation time of qualitative monolayer.

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M. Sheffer et al. [20] has reported the cyclic voltammetry (CV) of SAM by ODT to study the case of monolayer formation both on the Au macro and microelectrodes. In cyclic‐voltammogram shown in figure 6, for SAM on Au macro‐electrode (figure 6A) and micro‐electrode (figure 6B), the gradually reduction of current peak for increased preparation time of monolayer from 10 minutes to overnight is recorded. This significant reduction of current for 2 hours and overnight clarifies the necessity of longer adsorption time of the ODT molecules on the electrodes for qualitative monolayer formation.

FIGURE 6. CYCLIC‐VOLTAMMOGRAM OF SAM ON (A) 3 mm DIAMETER Au ELECTRODE, AND (B) 25 μM DIAMETER Au MICROELECTRODE FOR (A) BEFORE, (B) AFTER 10 MIN, (C) 2 HR AND (D) OVERNIGHT IMMERSION IN ODT IN SUPPORTING 0.1M Na2SO4 SOLUTION WITH 2mM Ru(NH3 )63+ [20].

The response of current against applied potential shown in figure 6A and 6B indicates comparatively longer time for the complete modification of the surface by monolayer for the case of microelectrode than macro‐electrode.

Concerning the shape of the cyclic‐voltammogram reported elsewhere in [10], similar result is observed both for the macro‐electrode and microelectrode. The sigmoid shape of cyclic‐voltammogram of the micro‐electrode (figure 6B) inferred the defective SAM with pinholes, whereas the linearly changing tunneling current (figure 6A), is the interpretation of the excellent monolayer; nevertheless the structural differences in the SAMs formed on a micro vs. macro‐electrode cannot be attributed from these studies.

In figure 7(a), D. G. Raya et al. [21] has shown corresponding cyclic voltammogram of adsorption time dependent monolayer of ODT on Au electrode of same dimension. They have considered the bare Au and the adsorption time of 15 minutes, 75 minutes, 7 hours and 18 hours of ODT molecules on Au in 1mM Fe(CN)63‐/4‐ with KNO3 support solution. With varying adsorption time of ODT SAMs on Au, the change in currents is due to the different statuses in the SAM modified electrode surface. As the adsorption time increases from 15 minutes towards 18 hours, the decreasing respective current density is the clear identification of larger surface coverage with completely formed and ordered monolayer so that the current at pinholes has almost disappeared which is well agreed with [20]. In cyclic voltammogram shown in figure 7(b) [22], the reduced oxidation reaction at the thiol modified gold electrode interface is observed due to the insulating behavior of thin monolayer.

FIGURE 7. CYCLIC‐VOLTAMMOGRAMS OF (A) ODT BASED SAM ON Au IN 1 mM Fe(CN)63‐/4‐ WITH KNO3 SOLUTION FOR DIFFERENT ADSORPTION TIME [21], AND (B) GOLD ELECTRODE BEFORE AND AFTER (‐(3‐DIMETHYLAMINOPROPYL)‐3‐ETHYLCARBODIIMIDEHYDROCLORIDE (EDC) BASED THIOL DEPOSITION IN PBS AT PH = 7.5 [22].

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Impedance Spectroscopy Impedance spectroscopy is a standard tool for the analysis of compactness of self‐assembled monolayer on solid state surfaces. Different electrical properties like capacitance, resistance and leakage current can be studied with this technique. The idea behind the impedance analysis is to study the fruitfulness of the monolayer as the blocking layer. The resistance and the frequency dependent capacitance of SAM can be modeled as an equivalent circuit for understanding the underlying science behind SAM capacitance and resistance as well. The block diagram of the corresponding model, its equivalent circuit with simplification based on the reports from several groups [22‐26] have been sketched below in figure 8.

FIGURE 8. (A) SETUP MODEL, (B) EQUIVALENT RRC, AND (B) SIMPLIFIED RC CIRCUIT IN THE MEASUREMENT OF ELECTRICAL IMPEDANCE.

The RRC model shown in figure 8(b) is used for the measurement of impedance of the sample using electrolyte as conducting solution where Relectrolyte is the contribution from the resistance of electrolyte. 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 8(c). In the analysis of the electrical impedance of the sample based on the simpler model as in figure 8(c), the complex electrical impedance Z() due to the contribution from the resistor and capacitor in series is considered as a function of frequency which is given by Z ( ) 

V ( )  Z o ( ).ei ( ) (1) I ( )

Where,  = 2πf with frequency f and  is the phase shift between AC voltage V and current I. Therefore, the total impedance of the circuit due to the resistance Relectrolyte and capacitance CSAM in series is shown in figure 8(c) and the corresponding absolute value of the impedance, Ztotal() = Relectrolyte(1+1/j..Relectrolyte.CSAM) (2) |Ztotal()|= Relectrolyte{1+1/(.Relectrolyte.CSAM)2} (3) With frequency →,|Ztotal()|  Relectrolyte, and with frequency →0, |Ztotal()| 1/.CSAM. Thus, 1/.CSAM of the absolute impedance corresponds to the capacitance of the monolayer at lower frequencies, whereas the absolute impedance is dominated by the resistance of electrolyte at higher frequencies. Also the respective 90 and zero degree phase shift ensures the contribution of capacitance and resistance to the total absolute impedance at lower and higher frequencies regime. The capacitance C of the plate capacitor can be determined as, C

 r o A d

(4)

Where, r is the relative permittivity of the organic monolayer, 0 = 8.854×10‐12 F/m is the permittivity of free space, A is the area of cross‐section and d indicates the thickness of SAM. Plotting of the absolute impedance as a function of operating frequency provides the determination of the capacitance and resistance with the help of the equations mentioned earlier. The corresponding impedance spectra for the equivalent RRC and simplified RC circuit shown in figure 8(b) and 8(c) are given in figure 9(a) and 9(b) respectively as follows‐

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FIGURE 9. IDEAL IMPEDANCE SPECTRA FOR (A) EQUIVALENT RRC, AND (B) SIMPLIFIED RC MODEL IN THE ANALYSIS OF ELECTRICAL IMPEDANCE.

FIGURE 10. IMPEDANCE SPECTRA OF (A) Au ELECTRODE WITH AND WITHOUT MONOLAYER COVERAGE [22], AND (B) SAM MODIFIED Au ELECTRODE [24].

In the impedance spectra from [22], figure 10(a) shows a comparative absolute impedance of an Au electrode with and without monolayer coverage, which shows increased magnitude of impedance due to surface coverage by thiol molecules. But the impedance spectra requires fitting based on the equivalent circuit shown in figure 8 in accordance to the equations (1‐4) for the study of different parameters of SAMs. This type of fitting based analysis of SAMs has been carried out in another report [24] by D. Shinn‐Jyh et al.. To study the magnitude of SAM capacitance and SAM resistance over the specified frequency range of 1 Hz to 100 kHz, D. Shinn‐Jyh et al. have used three different Carboxyl‐terminated alkanethiols including 4,4‐dithiodibutyric acid ([–S(CH2)3COOH]2,4‐DTBA), 11‐mercaptoundecanoic acid ([HS(CH2)10COOH],11‐MUA), and 16‐mercaptohexadecanoic acid ([HS(CH2)15COOH],16‐MHA). The impedance spectra of one of that thiol based SAMs are shown in figure 10(b) with corresponding fitted data summarized in table 2.

TABLE 2. FITTED DATA FOR RSAM, CSAM, AND RELECT OF SELF‐ASSEMBLED MONOLAYERS AFTER DIFFERENT ADSORPTION TIME ON Au ELECTRODES

4‐DTBA Time (min)

11‐MUA

RSAM (K)

16‐MHA

CSAM (nF)

Relect ()

RSAM (K)

CSAM (nF)

Relect ()

RSAM (K)

CSAM (nF)

Relect ()

1

6

49

875

56

388

877

44

294

895

10

8

483

889

156

299

846

183

145

827

30

8

478

875

320

254

887

384

150

868

60

10

489

87

396

224

883

561

162

912

120

11

489

881

449

198

874

585

161

916

From the fitted data for RSAM, CSAM, and Relect of SAMs with three different Carboxyl‐terminated alkanethiols for different adsorption time as listed in Table 2, insignificant variation of Relect was observed for all of the thiol based

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SAMs. But the RSAM, and CSAM showed the strong dependence on the adsorption time of alkanethiol molecules on Au surface. In essence, the more dominating behavior of RSAM was found as long as the molecules are adsorbed on electrode surface. Organic molecules of different chain length show the qualitative variations of their monolayer properties. According to the magnitude of RSAM for three different thiol based SAMs, more leaky and more resistive layer have been reported for smaller chain length of 4‐DTBA and longer chain length of 16‐MHA for the respective 1 minute and 120 minutes adsorption times. In accordance to the equation 4, since the thickness of the monolayer is inversely proportional to its capacitance, the trend of monolayer thickness with the variation of adsorption time was significantly similar with RSAMs for all the thiol based SAMs. In the impedance analysis reported elsewhere in [22, 23], the impedance response of the monolayer has been carried out at fixed applied potential. But H. Sahalov et al. [24] has shown impedance properties of ODT monolayer on Au in phosphate buffered saline (PBS) electrolyte at two different applied potentials of 0V and ‐0.5V depicted in figure 11. According to their study, the PBS electrolyte resistance RS is in series with a parallel RC network that corresponds to the monolayer resistance Rm and capacitance Cm. This is in analogy with the model in figure 8. The impedance and phase measurement from the solid‐nonlinear fitting lines, the maximum phase angles are close to 90° and close to 0° at lower frequency of 0.1 Hz both for the corresponding applied potential of 0 V and ‐0.5 V. The maximum phase angle close to 90° indicates the purely capacitive monolayer of about 106 ohm‐cm2 (figure (a)) whereas the phase angle decreases for another case at ‐0.5 V with a constant magnitude of the impedance (figure (b)).

These are the evident features of SAMs as parallel plate capacitor in parallel with a SAMs resistance.

FIGURE 11. IMPEDANCE SPECTRA FOR C18H37‐SH (ODT) SAM ON Au ANALYZED IN PHOSPHATE BUFFERED SALINE (PBS) AT THE APPLIED POTENTIAL OF (A) 0 V AND (B) ‐0.5 V WITH THE CORRESPONDING SOLID FITTING LINES TO CALCULATE THE RESISTANCE AND CAPACITANCE OF MONOLAYER [24].

R. Subramanian and V. Lakshminarayanan [25] have studied the growth kinetics of SAMs of alkanethiols of varying chain lengths on an evaporated gold surface to calculate the capacitance as well as corresponding surface coverage as a function of SAMs preparation time.

FIGURE 12. IMPEDANCE SPECTRA SHOW (A) THE VARIATION OF THE MEASURED CAPACITANCE WITH FREQUENCY FOR EVAPORATED GOLD ELECTRODE IN 1mM DECANETHIOL IN +0.1M LiClO4 ELECTROLYTE AND IN THE INSET FOR BARE GOLD ELECTRODE IN THE SAME ELECTROLYTE WITHOUT DECANETHIOL, AND (B) CAPACITANCE Vs. TIME CURVES FOR EVAPORATED GOLD IN 1mM OF EACH OF OCTANETHIOL (OT); DECANETHIOL (DT); DODECANETHIOL (DDT); HEXADECANETHIOL (HDT); OCTADECANETHIOL (ODT) IN +0.1M LiClO4 ELECTROLYTE AT A POTENTIAL OF 0.1 V [25].

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The alkanethiols of different chain lengths considered for the study are octanethiol (OT), decanethiol (DT), dodecanethiol (DDT), hexadecanethiol (HDT), and octadecanethiol (ODT). The capacitance spectra of decanethiol (DT) based SAM on gold electrode shown in figure 12(a) and the capacitance spectra of bare gold electrode without SAM layer shown in inset of figure 1 have been carried out to ensure choice of operating frequency range for the measurement. This operating frequency has been validated from the response of pseudo plateau and from the observation of corresponding capacitance at the frequency range of 1 Hz to 3000 Hz. Then as shown in figure 12 (b), the measured capacitance as a function of time for gold electrode at 1mM concentration of different thiols in ethanol 0.1M LiClO4 reaches to its final value within less than 1000 seconds and the individual capacitance of all of the thiol based SAMs is almost unchanged beyond the time of 3200 seconds. Based on the equation (4), the calculated final capacitance Cf is summarized in table 3 considering the length of the thiol molecules. The inset of figure 12 (b) shows the linear fit of final capacitance in terms of inverse number of carbon atoms in each alkanethiol since the thickness of monolayer is dependent on the number of carbon atoms of the alkanethiol. Figure 12 (b) also reveals the formation of the SAM layer by most of the adsorption within first few seconds followed by a slower reorganization period of 20–30 minutes for all the thiols under study.

Based on the measured values of initial and final capacitance, the surface coverage for each of the thiol in ethanol +0.1M LiClO4 has been summarized in table 3 according to the formula [24, 27] given below, Surface coverage,



co  ci (5) c0  c f

where c0 is the capacitance of bare electrode and ci and cf are the initial and final capacitance respectively.

TABLE 3. THE INITIAL AND FINAL CAPACITANCE VALUES OF GOLD ELECTRODE ADSORBED WITH DIFFERENT THIOLS IN ETHANOL +0.1M LiClO4.

Thiol

Initial Capacitance ci (F/cm2)

Final Capacitance cf ((F/cm2)

Initial Coverage i

Octanethiol

6.40

2.45

0.420

Decanethiol

6.38

2.00

0.416

Dodecanethiol

6.07

1.75

0.442

Hexadecanethiol

6.25

1.40

0.401

Octadecanethiol

6.45

1.18

0.360

We mentioned earlier that this study was carried on the growth kinetics of SAMs of alkanethiols of varying chain lengths as a function of SAMs preparation time. Table 3 shows the almost same initial coverage (40%) for all the alkanethiols, which clarifies the extremely rapid formation of monolayer in initial few seconds followed by a slow reorganization process to be well structured. It should be noted also that the measurement of rate constant is not précised at all during the initial stages of adsorption.

Conclusions Alkanethiol based self‐assembled monolayers (SAMs) on gold (Au) potentially contribute to the surface modification. Since alkanethiols of different chain lengths are widely used to tailor such monolayers on gold (Au) for immobilization purposes, so the understanding of the different electrical properties of these self‐assembled monolayers (SAMs) is relevant indeed for its present as well as future applications. Exploiting the highly passivating behavior of the self‐assembled monolayers (SAMs) through cyclic voltammetry has led to the understanding of excellent monolayer formation with corresponding defect, and sticking properties detection. Also the study of self‐assembled monolayers (SAMs) through electrochemical impedance spectroscopy with detail mathematical modeling makes sense to determine the thickness, capacitance, resistance, and impedance as well for the monolayer being viable to be used in many recent and forthcoming applications. In summary, both the cyclic voltammetry and impedance spectroscopy yield the deeper insight into the different electrical properties of self‐assembled monolayers (SAMs) like capacitance, resistance, and impedance which are significantly affected by the preparation time of the monolayer, chain lengths of the alkanethiols, dimension of the electrode and the applied potential among others.

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ACKNOWLEDGEMENTS

This review paper was carried out during graduate program in ‘Nanomolecular Science’ at Jacobs University Bremen, Germany under the study leave from Islamic University, Kushtia, Bangladesh. The authors gratefully acknowledge the financial supports with study leave during graduate program at Jacobs University Bremen, Germany.

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