Abstract
Quartz tuning forks that have a probe tip attached to the end of one of its prongs while the other prong is arrested to a holder (“qPlus” configuration) have gained considerable popularity in recent years for high-resolution atomic force microscopy imaging. The small size of the tuning forks and the complexity of the sensor architecture, however, often impede predictions on how variations in the execution of the individual assembly steps affect the performance of the completed sensor. Extending an earlier study that provided numerical analysis of qPlus-style setups without tips, this work quantifies the influence of tip attachment on the operational characteristics of the sensor. The results using finite element modeling show in particular that for setups that include a metallic tip that is connected via a separate wire to enable the simultaneous collection of local forces and tunneling currents, the exact realization of this wire connection has a major effect on sensor properties such as spring constant, quality factor, resonance frequency, and its deviation from an ideal vertical oscillation.
Highlights
Scanning tunneling microscopy (STM) [1] and non-contact atomic force microscopy (NC-AFM) [1,2,3] are powerful methods allowing the visualization of the atomic structure of a surface, with STM probing the electronic properties of the sample and NC-AFM its chemical nature with picoampere, piconewton, and picometer resolution [4,5,6,7,8,9,10,11]
To assess the significance of these changes on the sensing capabilities of the device, let us recall from the discussion in [26] that high-resolution measurements involving qPlus sensors are to date mostly conducted in frequency modulation (FM) mode, where the reduction of the eigenfrequency f0 upon approach to the surface is the measured quantity [32]
We have provided a systematic study based on numerical calculations using finite element models of completely assembled qPlus sensors that include attached tips
Summary
Scanning tunneling microscopy (STM) [1] and non-contact atomic force microscopy (NC-AFM) [1,2,3] are powerful methods allowing the visualization of the atomic structure of a surface, with STM probing the electronic properties of the sample and NC-AFM its chemical nature with picoampere, piconewton, and picometer resolution [4,5,6,7,8,9,10,11]. Quartz tuning forks in qPlus configuration have gained the widest popularity as they offer several advantages such as self-sensing properties, low cost, a freedom in the selection of the materials used as local probes, and physical dimensions that allow experimentalists to assemble sensors right in their own labs with relative ease [19,22,23,24,25] Even though this in-lab assembly of qPlus sensors is a manageable task, the small size of the tuning forks (about 3 mm) and complexity of the overall sensor architecture, in particular when outfitted with a separate wire connection to the probe tip for combined STM/AFM experiments, impedes the assembly of such sensors in a reliable and repeatable way for personnel without considerable experience. Personal skills have often a major impact on the sensing characteristics of the completed device
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