Junghoon Jahng

Quantitative atomic force measurement with a quartz tuning fork

The authors demonstrate a simple yet robust method for quantitative measurement of dynamic atomic force using the quartz tuning fork for both electrically driven mode and mechanically driven mode. It is shown that both modes can be made fully equivalent and also allow accurate force measurement. The quartz tuning fork is now expected to be widely employed as a quantitative force measurement probe in addition to its capability to surface image in the atomic scale.

Active Q control in tuning-fork-based atomic force microscopy

The authors present comprehensive theoretical analysis and experimental realization of active Q control for the self-oscillating quartz tuning fork (TF). It is shown that the quality factor Q can be increased (decreased) by adding the signal of any phase lag, with respect to the drive signal, in the range of θ1 to θ1+π (θ1+π to θ1+2π), where θ1 is the characteristic constant of TF. Experimentally, the nominal Q value of 4.7×103 is decreased to 1.8×103 or increased to 5.0×104 in ambient condition, where the minimum detectable force is estimated to be 4.9×10−14N at 1Hz. The novel Q control scheme demonstrated in the widely used quartz TF is expected to contribute much to scanning probe microscopy of, in particular, soft and biological materials.

Eigenmodes of a quartz tuning fork and their application to photoinduced force microscopy

Author(s): Kim, Bongsu; Jahng, Junghoon; Khan, Ryan Muhammad; Park, Sung; Potma, Eric O | Abstract: We examine the mechanical eigenmodes of a quartz tuning fork (QTF) for the purpose of facilitat- ing its use as a probe for multi-frequency atomic force microscopy (AFM). We perform simulations based on the three-dimensional finite element method (FEM) and compare the observed motions of the beams with experimentally measured resonance frequencies of two QTF systems. The com- parison enabled us to assign the first seven asymmetric eigenmodes of the QTF. We also find that a modified version of single beam theory can be used to guide the assignment of mechanical eigen- modes of QTFs. The usefulness of the QTF for multi-frequency AFM measurements is demonstrated through photo-induced force microscopy (PiFM) measurements. By using the QTF in different con- figurations, we show that the vectorial components of the photo-induced force can be independently assessed, and that lateral forces can be probed in true non-contact mode.

Electrical tuning of mechanical characteristics in qPlus sensor: Active Q and resonance frequency control

We present an electrical feedback method for independent and simultaneous tuning of both the resonance frequency and the quality factor of a harmonic oscillator, the so called “qPlus” configuration of quartz tuning forks. We incorporate a feedback circuit with two electronic gain parameters into the original actuation-detection system, and systematically demonstrate the control of the original resonance frequency of 32 592 Hz from 32 572 Hz to 32 610 Hz and the original quality factor 952 from 408 up to 20 000. This tunable module can be used for enhancing and optimizing the oscillator performance in compliance with specifics of applications.

Nanofluidics through a 30-nm aperture nanopipette by applying electrostatic field based on the QTF-AFM system

Nanofluidics of the liquid solution through a 30-nm aperture nanopipette was investgated using the QTF-AFM system. Used pulled nanopipette was attached on the edge of QTF to control the distance between the tip and substrate. Monitored QTF signal was changed by naturally formed water meniscuss force as tip approached to the sample within 10nm. After forming of water, electric field was applied for the purpose of a filled solutions extrusion onto the surface. Measured the fluid speed was 43.5 um/s and spread area was 136.6 um2 after 5 seconds. During the experiment, temperature was 22 °C and humidity was 20.1 %. Checked thermal drift was 0.3 pm/s.