Surface Potential Imaging of F14
H20
Armando Melgarejo, Ben Schoenek, and Byong Kim* Research Technical Services, Park Systems, Inc. *
byong@parksystems.com
Abstract: Sideband Kelvin probe force microscopy (KPFM) uses the intermodulation of an electrostatic drive force and a mechanical drive force to upconvert the electrostatic frequency to the first flexural resonance, where the high-quality factor of the resonance yields a more sensitive measurement. The sideband KPFM signal is calculated using a local interaction between the tip apex and the sample, rather than a total interaction between the cantilever and the sample, improving the spatial
resolution over other assembled molecular nanostructures.
Keywords: atomic force microscopy, sideband Kelvin probe force microscopy, molecular nanostructure, topography, surface potential, semi-fluorinated alkanes F14
H20
Introduction From materials science to biological research, scientists
have adopted Kelvin probe force microscopy (KPFM) to measure surface potential and work functions. KPFM follows the electrostatic force microscopy (EFM) operation principle. It measures a contact potential difference (CPD) and determines the sample’s work function using DC bias feedback. However, conventional
off-resonance KPFM produces low spatial
resolution and low signal-to-noise ratios due to long-range crosstalk caused by the interaction between the sample and the total body of the cantilever. Sideband KPFM has an unparalleled spatial resolution and centers on the local interaction between the tip apex and sample, making it useful for analyzing grain boundaries, semiconductor junctions, photovoltaics materials, and even molecular structures [1–3]. KPFM operates by measuring the CPD. CPD is an
electrostatic potential that exists between samples of two dissimilar, electrically connected materials. Considering atomic force microscopy (AFM), the AFM tip and sample are two different materials. Each of the materials has a specific conduction band and a separate work function. Te work function of a material is the amount of energy required to move an electron to infinity from the surface of a given solid [2]. If the tip and the sample are electrically connected, the
electrical connection will induce a natural flow of electrons and will create a potential difference between the two materials (Figure 1). Tis potential difference is measurable by applying the theoretical formula of contact potential difference:
CPD = where ϕtip and ϕsample
ϕϕ e
sample − tip (1) are the work function of the sample and
tip, respectively, and e corresponds to the electronic charge. Obtaining the work function of the material is possible by multiplying the surface potential of the materials by a single electron charge.
52 doi:10.1017/S1551929521000705
This paper covers the sideband KPFM details, including trade-offs and imaging of semi-fluorinated alkanes, such as F14
technique variations. H20
, and self- Equation (1) can be explained using the Kelvin probe
method, which relies on detecting the electric field of the materials composing the AFM tip and sample. By modifying the voltage CPD, the electric field varies. Tereby, when applying an external bias (VDC
) of the same magnitude to the AFM tip but in
the opposite direction, the surface charge of the contact area is nullified. Once there are no electrostatic forces, the work function difference has the same value as the applied voltage CPD; consequently, the work function of the sample can be calculated if the work function of the tip is known (Equation (2)) [3].
ϕ =⋅ +sC ϕPD eV t (2)
Off-Resonance KPFM When considering the AFM tip and the sample as a capacitor,
analyzing the electrostatic forces between them is straightforward, as the amount of energy can be calculated with the difference in voltage and the capacitance. KPFM uses two frequencies to acquire topography and surface potential simultaneously. Te standard procedure for KPFM consists of using two lock-in amplifiers (Figure 2A). Te first lock-in amplifier modulates the frequency used to oscillate the cantilever at its mechanical resonant frequency, using a piezoelectric material to obtain a topography image. Te second lock-in amplifier regulates the frequency, generally at 17 kHz, to measure the surface potential. Tis technique is denoted “off- resonance KPFM,” as we can theoretically choose any resonance frequency as long as it does not match the cantilever’s mechanical resonance [2]. Additionally, off-resonance KPFM obtains surface potential information by measuring the total interaction of the complete cantilever with the sample, limiting its spatial resolution.
Sideband KPFM Sideband KPFM is a technique that exploits the
intermodulation of an electrostatic drive force and a mechanical drive force to upconvert the electrostatic frequency to the first flexural resonance, where the high Q “quality factor” yields a more sensitive measurement [3]. Additionally, the sideband KPFM signal is calculated by using a local interaction between the tip apex and the sample rather than a total interaction between the cantilever and the sample, improving the spatial resolution over off-resonance KPFM. Local interaction allows the system to measure the surface potential of localized features with high resolution, due to the reduced interaction with other parts of the cantilever. Te reason for the substantial reduction of the interaction, induced by the cantilever and the tip, is the downward distance dependence of the lever and cone forces on the experimentally important range [1]. Te electrical driving frequencies of sideband KPFM
appear at the sidebands of the mechanical oscillation of the cantilever (Figure 3), reducing the long-range crosstalk [3].
www.microscopy-today.com • 2021 May
Molecules via Sideband Kelvin Probe Force Microscopy
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