InGaN as a Substrate for AC Photoelectrochemical Imaging

Abstract

AC photoelectrochemical imaging at electrolyte-semiconductor interfaces provides spatially resolved information such as surface potentials, ion concentrations and electrical impedance. In this work, thin films of InGaN/GaN were used successfully for AC photoelectrochemical imaging, and experimentally shown to generate a considerable photocurrent under illumination with a 405 nm modulated diode laser at comparatively high frequencies and low applied DC potentials, making this a promising substrate for bioimaging applications. Linear sweep voltammetry showed negligible dark currents. The imaging capabilities of the sensor substrate were demonstrated with a model system and showed a lateral resolution of 7 microns.

Keywords: InGaN/GaN epilayer; cell imaging; light-activated electrochemistry; light-addressable potentiometric sensor; photoelectrochemistry.

Figure 1

Schematic of the LAPS setup with a 405 nm diode laser to generate photo-induced charge carriers, a lock-in amplifier to measure AC photocurrent, and an X-Y-Z stage to move the electrochemical cell with respect to the laser beam for imaging.

Figure 2

SEM images of InGaN/GaN: (a) top view and (b) cross-sectional view.

Figure 3

UV-Vis spectrum of InGaN and inset Tauc-plot.

Figure 4

LSV curves of InGaN in the dark and with chopped illumination.

Figure 5

(a) Frequency dependence of the AC photocurrent and the background dark current measured at 1.0 V; (b) Characteristic I−V curve of InGaN/GaN measured in pH 7.4 DPBS at 1 kHz with a focused laser beam at 18% maximum intensity.

Figure 6

AC photocurrent images of a PMMA dot on InGaN measured at 0.6 V (a) and 0 V (b); X axis line scan across the polymer edge (indicated by the red arrow in (a)) at 0.6 V (c) and its corresponding first derivative plot (d).

Figure 7

(a) AC photocurrent image of a mesenchymal stem cell on InGaN surface (cell shapes from (b) superimposed in blue for an attached cell and red for non-attached cells); and (b) its corresponding optical image.

Figure 8

(a,b,c) Fluorescence microscope images of MG-63 cells taken after photocurrent imaging, living cells with intact membranes appeared green, dead cells with collapsed membrane appeared red, and the nuclei of the cells appeared blue. (d,e,f) Images of MG-63 cells that were not subjected to imaging.