Research Papers

Study of Insulating Properties of Alkanethiol Self-Assembled Monolayers Formed Under Prolonged Incubation Using Electrochemical Impedance Spectroscopy

[+] Author and Article Information
Damena D. Agonafer

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801
e-mail: agonafer@illinois.edu

Edward Chainani

Department of Chemistry,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

Muhammed E. Oruc

Department of Chemical and Biomolecular
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

Ki Sung Lee

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

Mark A. Shannon

Department of Mechanical Science
and Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801;
Department of Chemical and Biomolecular
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

Manuscript received April 16, 2012; final manuscript received September 12, 2012; published online January 18, 2013. Assoc. Editor: Debjyoti Banerjee.

J. Nanotechnol. Eng. Med 3(3), 031006 (Jan 18, 2013) (8 pages) doi:10.1115/1.4007698 History: Received April 16, 2012; Revised September 12, 2012

The electrochemical interfacial properties of a well-ordered self-assembled monolayer (SAM) of 1-undecanethiol (UDT) on evaporated gold surface have been investigated by electrochemical impedance spectroscopy (EIS) in electrolytes without a redox couple. Using a constant-phase element (CPE) series resistance model, prolonged incubation times (up to 120 h) show decreasing monolayer capacitance approaching the theoretical value for 1-undecanethiol. Using the CPE exponent α as a measure of ideality, it was found that the monolayer approaches an ideal dielectric (α = 0.992) under prolonged incubation, which is attributed to the reduction of pinholes and defects in the monolayer during coalescence and annealing of SAM chains. The SAMs behave as insulators until a critical potential, Vc, is exceeded in both cathodic and anodic regimes, where electrolyte ions are believed to penetrate the monolayers. Using a Randles circuit model for these cases, the variation of the capacitance and charge transfer resistance with applied dc potential shows decreased permeability to ionic species with prolonged incubation time. The EIS data show that UDT (methylene chain length n = 10), incubated for 120 h, forms a monolayer whose critical voltage range extends from −0.3 to 0.5 V versus Ag/AgCl, previously attained only for alkanethiol at n = 15. At low frequencies where ion diffusion occurs, almost pure capacitive phase (−89 deg) was attained with lengthy incubation.

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Grahic Jump Location
Fig. 2

Cyclic voltammograms of a bare polycrystalline gold surface scanned (▾) and an Au substrate with a “defect- free” thiol monolayer (▪), which was grown for 5 days. The supporting electrolyte used was 10 mM (I = 0.0316 M) phosphate buffer solution, pH 7, and scan rate 1 V s−1.

Grahic Jump Location
Fig. 1

AFM measurement of bare gold substrate obtained in intermittent contact mode. The scan area shown is 1 μm by 1 μm.

Grahic Jump Location
Fig. 6

Bode plot of undecanethiol applied at 0 V (versus Ag/AgCl) for different incubation times

Grahic Jump Location
Fig. 3

Phase angle plot of undecanethiol SAMs formed at different immersion times, with an applied potential of −0.2 V. The phase angle at low frequencies of the 48-h grown monolayer exhibits a phase angle less than −88 deg, for 1 mM and 10 mM concentrations. The substrate Agonafer 23 immersed in thiol solution for 5 days shows a phase angle of −88 deg, an indication of a monolayer without defects for 1 mM and 10 mM concentrations.

Grahic Jump Location
Fig. 4

Equivalent circuits of the thiol monolayer: (a) CPE in series with a solution resistance, indicative of a “defect-free” SAM and (b) an equivalent Randle circuit of an SAM with potential induced defects

Grahic Jump Location
Fig. 5

Schematic of alkanethiolate SAM evolution on a gold surface over a range of incubation times

Grahic Jump Location
Fig. 7

Plot of model data (CPE and α) obtained from fitting the EIS data for various incubation times. The dc potential was varied from −0.3 V (versus Ag/AgCl) to 0.5 V. For immersion times shorter than 120 h, the CPE model did not hold over the full dc potential range due to ion penetration.

Grahic Jump Location
Fig. 8

Plot of phase shift at 1 Hz from EIS measurements performed at voltages ranging from −0.3 V to 0.5 V (versus Ag/AgCl)



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