LETTER pubs.acs.org/Langmuir
Electrochemical Impedance Study of GaAs Surface Charge Modulation through the Deprotonation of Carboxylic Acid Monolayers Fernanda Camacho-Alanis,† Homero Castaneda,§ Giovanni Zangari,‡ and Nathan S. Swami*,† †
Electrical Engineering and ‡Materials Science, University of Virginia, Charlottesville, Virginia 22904, United States § Chemical & Biomolecular Engineering, University of Akron, Akron, Ohio 44325, United States
bS Supporting Information ABSTRACT: Modifications to the space charge region of p+ and p-GaAs due to surface charge modulation by the pH-induced deprotonation of bound carboxylic acid terminal monolayers were studied by electrochemical impedance spectroscopy and correlated to flat-band potential measurements from Mott Schottky plots. We infer that the negative surface dipole formed on GaAs due to monolayer deprotonation causes an enhancement of the downward interfacial band bending. The space charge layer modifications were correlated to intermolecular electrostatic interactions and semiconductor depletion characteristics.
I. INTRODUCTION The modulation of semiconductor surface charge through induced dipoles and conformational changes of bound selfassembled monolayers (SAMs) is widely applied within sensor paradigms based on surface-modulated field-effect transistor devices1,2 to tune the Schottky barrier3 and channel conductivity4,5 of semiconductor device interfaces. Carboxylic acid terminal SAMs ( COOH) are applied routinely for the immobilization of capture probe species to solid surfaces for the development of chemical and biochemical sensors.6,7 Hence, monitoring their degree of deprotonation at the neutral to mildly alkaline pH applied within biochemical applications is of great interest for sensor paradigms based on the control of semiconductor surface charge, where mechanisms are as yet unclear.8 Prior work on the pH-induced deprotonation of COOH SAMs on semiconductor surfaces, such as Si, has focused on Fourier transform infrared (FTIR) spectroscopic methods.9,10 However, these methods cannot be applied to lower-dimensional nanostructures, which are highly sensitive to surface charge modulation;11,12 hence, we consider electrically addressable methods such as electrochemical impedance spectroscopy (EIS). EIS has been applied to study COOH SAM deprotonation on Au surfaces by following the charge-transfer resistance (RCT) of a redox probe13 or the doublelayer capacitance (CDL) upon deprotonation in the absence of a redox probe.14,15 However, modifications of CDL upon SAM deprotonation are not substantial in the moderate- to high-ionicstrength electrolytes required for functional biochemical species,16 and RCT variations are not significant on semiconductors with low substrate doping. Hence, we aim to monitor COOH SAM deprotonation through modifications to the capacitance and r 2011 American Chemical Society
resistance of the semiconductor space charge region (CSC and RSC) by EIS, which has not been reported previously. Because charges due to the deprotonation of short COOH SAMs (
