Arrow uhfaud

We
use a Cypher ES (Asylum
Research, Oxford Instruments, Santa Barbara, CA) to acquire super-resolved
images of a mica layer immersed in sodium chloride solutions. The
mica layer is freshly cleaved and glued on a magnetic disk using UV
cured NOA 81 glue.^{11 (link)} Imaging is performed
in amplitude modulation mode driven by blueDrive photothermal excitation
(laser power 9 mW) and using reflex gold coated, ultra high frequency,
silica probes (ARROW-UHFAuD, NanoWorld, Switzerland).
Images
of the topography are recorded over a scan area of 10 by 10 nm² or 20 by 20 nm² with 256 or 512 points and lines.
The scan rate and set point are varied within the range of 6.5–8
Hz and 15–70 mV to optimize image quality for the various salt
concentrations. AFM data analysis is performed with Gwyddion 2.55
and Python 3.8.

Gold coated (detector side) silicon ultra-high frequency cantilevers (ARROW-UHFAuD, nanoworld) were used. The length and width were 35 μm and 42 μm, respectively. The tip has an arrow shape and the force constant (k), resonant frequency (f) and quality factor (Q) of the first eigenmode as determined in liquid were in the range: k = 1.23–3.35 N m⁻¹, f = 600–1000 kHz and Q = 6. The above values are determined in purified water (Millipore, resistivity 18.2 MΩ cm). The force constant is determined by using the thermal method. Prior to use, the cantilevers are cleaned by putting them in a bath of 1 : 1 ethanol/isopropanol for 15 minutes, after which they were dried using air and placed in a plasma cleaner (PDC-32G-2, Harrick Plasma) for 20 minutes. The tip radius was determined after data collection from the HRTEM imaging, and was found to be around 2 nm for all the levers used in this study (which is an upper limit since this was measured after calibrating the tip upon being pressed into contact). Relative stiff cantilevers were used in order to prevent mechanical instabilities in the presence of attractive force gradients.