Charge gradient microscopy

Abstract

Here we present a simple and fast method to reliably image polarization charges using charge gradient microscopy (CGM). We collected the current from the grounded CGM probe while scanning a periodically poled lithium niobate single crystal and single-crystal LiTaO3 thin film on the Cr electrode. We observed current signals at the domains and domain walls originating from the displacement current and the relocation or removal of surface charges, which enabled us to visualize the ferroelectric domains at a scan frequency above 78 Hz over 10 μm. We envision that CGM can be used in high-speed ferroelectric domain imaging and piezoelectric energy-harvesting devices.

Keywords: atomic force microscopy; charge scraping; piezoresponse; screen charge.

Fig. 1.

Schematic diagram of CGM. CGM can characterize polarization charges with a high speed by scraping the screen charges on the surface of the ferroelectric thin film using a conducting AFM tip.

Fig. 2.

CGM images as a function of scan frequency: (A) 5 Hz, (B) 10 Hz, and (C) 20 Hz over an area of 25 × 25 μm on the PPLN single crystal. (D) Repeatability of CGM imaging. Line profiles obtained on the same region across the black dotted line in B with number of repetitions: 1, red; 2, orange; 3, yellow; 4, green; 5, blue; 6, indigo; 7, violet; and 8, black. The width of each domain is 5 μm.

Fig. 3.

Delta current as a function of scan frequency. The difference between maximum and minimum current values, i.e., delta current in the line profiles, is plotted as a function of scan frequency from 5 to 30 Hz. Ten delta current values were averaged in each data point.

Fig. 4.

Trace and retrace CGM images at scan frequency of 78 Hz. CGM images taken on domain boundaries of PPLN sample from (A) left-to-right scan (trace) and (B) right-to-left scan (retrace) with line profiles showing the current peaks at domain boundaries. PFM (C) amplitude and (D) phase images are taken at the same region where dark phase contrast corresponds to positive (upward) domain and bright phase contrast corresponds to negative (downward) domain.

Fig. 5.

Trace and retrace CGM images of artificially decorated domains with different sizes. CGM images taken on ribbon-shaped domains poled by 6 V to the bottom electrode of 85-nm-thick LiTaO₃ films from (A) left-to-right scan (trace) and (B) right-to-left scan (retrace) using Pt tips at scan frequency of 40 Hz. PFM (C) amplitude and (D) phase images are taken at the same region at scan frequency of 1 Hz where bright phase contrast corresponds to positive (upward) domain and dark phase contrast corresponds to negative (downward) domain. A real-time acquisition movie of CGM images is available (Movie S1).