Sideband Kelvin Probe Force Microscopy
sidebands was recorded. Two different types of conductive cantilevers, NSC14/Cr-Au and NSC36-C/Cr-Au, were used to analyze the correlation between EFM amplitude sensitivity, the cantilever’s length, and the spring constant. NSC14/Cr-Au and NSC36-C/ Cr-Au were 125 and 130 µm long and had a spring constant of 5 and 0.6 N/m, respectively.
Figure 3: Fourier transform of the vertical deflection comparing off-resonance KPFM versus side- band KPFM.
where Lx Lc
is the distance between the tip and the cantilever, the cantilever’s length, and ΔZ the displacement of the
cantilever oscillation. Terefore, the longer the cantilever, the smaller the displacement. Te EFM amplitude sensitivity, thus the KPFM signal sensitivity, increases when the cantilever has a short length and/or a small spring constant [4].
Eicosylperfluorotetradecane F14 H20 KPFM not only measures the CPD on metals or
semiconductors, but it can also measure the surface potential of organic or soſt samples like the self-assembled molecules of some alkanes. Eicosylperfluorotetradecane, F(CF2 or its abbreviated form F14
)14 H20 (CH2 )20H, , is a semifluorinated alkane with
a perfluorinated chain segment. It consists of two incompatible subunits that segregate into distinct domains. Te mismatch in the cross section between the perfluoro and the perhydro alkyl tail result in distinctive superstructure formations (Figure 4). Te mechanism describing how these compounds order in specific polymorphism is not yet fully understood. Te investigation of semifluorinated alkanes, such as F14 assembly knowledge [5]. F14
H20 H20 , contributes to self- nanomolecular structures have
dipole orientation, rendering considerable surface potential of 0.8V, and thus is suited for studying the assembly of molecules and their electronic properties on the nanoscale using KPFM [6].
Methods In this study, a Park NX10 AFM system, equipped with
SmartScan™ soſtware, was used to measure the local contact potential difference and topography of F14
H20 molecular
structures. Te study shows a comparison between sideband KPFM and conventional off-resonance KPFM image resolution. For the sideband KPFM experiments, lock-in amplifiers were used to send a 3 kHz AC signal to the cantilever, while the standard technique used a 17 kHz AC signal. SmartScan automatically
selected the phase and amplitude of resonances. For these experiments, the average signal of both
Results and Discussion Figure 5 shows a surface potential
comparison between sideband and conventional off-KPFM images obtained with a NSC36-C/
Cr-Au cantilever. Te sideband KPFM measurement of the F14 H20
aggregates shows a surface potential contrast of 600–650mV between the fluorinated parts of the molecules and the substrate, along with a clear lateral resolution where even the in-between space of the aggregates can be distinguished (Figures 5a and 5b). Images of the conventional off-resonance KPFM measurements show that the technique successfully measures the potential difference of the aggregates and the substrate; however, the potential difference gives a value of 300mV (Figures 5c and 5d). Te spatial resolution is not as well defined. Te improvement
the
Figure 5: Sideband and off-resonance comparison: Figure 4: Schematic pictures of the ribbon and the spiral morphology for F14 54 H20 .
(a) sideband topogra-
phy image; (b) sideband surface potential image; (c) off-resonance topography image; (d) off-resonance surface potential image; (e) line profiles taken along the red line in (b) and along the green line in (d).
www.microscopy-today.com • 2021 May
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