Abstract
Atomic force microscopy (AFM) is an important tool for measuring a variety of nanoscale surface properties, such as topography, viscoelasticity, electrical potential and conductivity. Some of these properties are measured using contact methods (static contact or intermittent contact), while others are measured using noncontact methods. Some properties can be measured using different approaches. Conductivity, in particular, is mapped using the contact-mode method. However, this modality can be destructive to delicate samples, since it involves continuously dragging the cantilever tip on the surface during the raster scan, while a constant tip–sample force is applied. In this paper we discuss a possible approach to develop an intermittent-contact conductive AFM mode based on Fourier analysis, whereby the measured current response consists of higher harmonics of the cantilever oscillation frequency. Such an approach may enable the characterization of soft samples with less damage than contact-mode imaging. To explore its feasibility, we derive the analytical form of the tip–sample current that would be obtained for attractive (noncontact) and repulsive (intermittent-contact) dynamic AFM characterization, and compare it with results obtained from numerical simulations. Although significant instrumentation challenges are anticipated, the modelling results are promising and suggest that Fourier-based higher-harmonics current measurement may enable the development of a reliable intermittent-contact conductive AFM method.
Highlights
Conductive atomic force microscopy (C-Atomic force microscopy (AFM)), a contact-mode technique, has been extensively utilized to investigate local electrical properties of nanoscale systems, such as organic solar cells [1,2,3,4,5,6,7], semiconductors [8,9,10], and metals [11,12,13]
We propose the use of Fourier analysis to implement intermittent-contact mode (ICM) current measurements
In order to demonstrate the second case, we performed a numerical simulation for a dynamic AFM experiment that operates in the attractive tip–sample interaction regime
Summary
Conductive atomic force microscopy (C-AFM), a contact-mode technique, has been extensively utilized to investigate local electrical properties of nanoscale systems, such as organic solar cells [1,2,3,4,5,6,7], semiconductors [8,9,10], and metals [11,12,13]. Contact-mode AFM techniques, where the probe continuously interacts with the surface in a repulsive. For the cases where the sample is rather delicate, intermittent-contact mode (ICM) imaging, where the tip and the sample interact briefly at the bottom of each cantilever oscillation, can be a less destructive technique [16,19,20], and this could be advantageous for performing current measurements on such samples. Scanning tunnelling microscopy (STM) applications may benefit from current measurements during which the tip oscillates above the surface, in the noncontact regime. Any unexpected contact with the surface may lead to a current spike and severely perturb the controller for a period of time, during which the tip apex structure could be damaged further due to additional tip–sample impacts. If a noncontact oscillatory current measurement mode is used, where the control variable is not the instantaneous value of the current, these unexpected tip–sample impacts may be more benign and may not perturb the measurement as drastically as in traditional STM approaches
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