Manhee Lee

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.

Mechanical properties of the nanoscale molecular cluster of water meniscus by high-precision frequency modulation atomic force spectroscopy

The mechanical properties (viscoelasticity) of the nanoscale molecular cluster of water meniscus, spontaneously formed between a quartz tip (∼100 nm curvature) and a mica substrate were quantitatively studied. The theoretical and experimental investigation was performed on the basis of the quartz tuning fork-based frequency modulation-atomic force microscope system with a high vertical resolution (∼0.5 A). The proposed system is suitable apparatus for the dynamic force spectroscopy of nanoscopic materials with several advantages including high sensitivity, short response time, immunity to the electrical noise, and simple and intuitive interpretation of the results using the frequency shift.

Low-volume liquid delivery and nanolithography using a nanopipette combined with a quartz tuning fork-atomic force microscope

Electric-field-induced low-volume liquid ejection under ambient conditions was realized at a low bias potential of 12 V via a nanopipette (aperture diameter of 30 nm) combined with a non-contact, distance-regulated (within 10 nm) quartz tuning fork-atomic force microscope. A capillary-condensed water meniscus, spontaneously formed in the tip-substrate nanogap, reduces the ejection barrier by four orders of magnitude, facilitating nanoliquid ejection and subsequent liquid transport/dispersion onto the substrate without contact damage from the pipette. A study of nanofluidics through a free-standing liquid nanochannel and nanolithography was performed with this technique. This is an important breakthrough for various applications in controlled nanomaterial-delivery and selective deposition, such as multicolor nanopatterning and nano-inkjet devices.

Quartz tuning fork-based frequency modulation atomic force spectroscopy and microscopy with all digital phase-locked loop

We present a platform for the quartz tuning fork (QTF)-based, frequency modulation atomic force microscopy (FM-AFM) system for quantitative study of the mechanical or topographical properties of nanoscale materials, such as the nano-sized water bridge formed between the quartz tip (~100 nm curvature) and the mica substrate. A thermally stable, all digital phase-locked loop is used to detect the small frequency shift of the QTF signal resulting from the nanomaterial-mediated interactions. The proposed and demonstrated novel FM-AFM technique provides high experimental sensitivity in the measurement of the viscoelastic forces associated with the confined nano-water meniscus, short response time, and insensitivity to amplitude noise, which are essential for precision dynamic force spectroscopy and microscopy.

Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication

This paper introduces a nanopipette combined with a quartz tuning fork-atomic force microscope system (nanopipette/QTF-AFM), and describes experimental and theoretical investigations of the nanoscale materials used. The system offers several advantages over conventional cantilever-based AFM and QTF-AFM systems, including simple control of the quality factor based on the contact position of the QTF, easy variation of the effective tip diameter, electrical detection, on-demand delivery and patterning of various solutions, and in situ surface characterization after patterning. This tool enables nanoscale liquid delivery and nanofabrication processes without damaging the apex of the tip in various environments, and also offers force spectroscopy and microscopy capabilities.

Bifurcation-enhanced ultrahigh sensitivity of a buckled cantilever

Significance This work brings together the fields of nonlinear dynamics and precision measurement, aiming to develop a highly sensitive nonlinear mechanical force sensor. We use dynamic force spectroscopy of the buckled cantilever tip in an ambient condition, which allows sensitive detection of the noise-induced flipping near the bifurcation point. Key parameters, such as the fluctuation enhancement and the activation barrier of the buckling-to-flipping transition, lead to realization of the bifurcation-enhanced sensor. We contiguously observe the buckling–flipping dynamic transition of the softened tip resulting from the competition between fluctuation and bifurcation, providing the in situ continuous sensing of the mechanical vibrations. This work not only furthers our understanding of nonlinear dynamics at the nanoscale, but also is a stepping stone toward the highly sensitive mechanical sensor. Buckling, first introduced by Euler in 1744 [Euler L (1744) Opera Omnia I 24:231], a sudden mechanical sideways deflection of a structural member under compressive stress, represents a bifurcation in the solution to the equations of static equilibrium. Although it has been investigated in diverse research areas, such a common nonlinear phenomenon may be useful to devise a unique mechanical sensor that addresses the still-challenging features, such as the enhanced sensitivity and polarization-dependent detection capability. We demonstrate the bifurcation-enhanced sensitive measurement of mechanical vibrations using the nonlinear buckled cantilever tip in ambient conditions. The cantilever, initially buckled with its tip pinned, flips its buckling near the bifurcation point (BP), where the buckled tip becomes softened. The enhanced mechanical sensitivity results from the increasing fluctuations, unlike the typical linear sensors, which facilitate the noise-induced buckling-to-flipping transition of the softened cantilever. This allows the in situ continuous or repeated single-shot detection of the surface acoustic waves of different polarizations without any noticeable wear of the tip. We obtained the sensitivity above 106 V(m/s)−1, a 1,000-fold enhancement over the conventional seismometers. Our results lead to development of mechanical sensors of high sensitivity, reproducibility, and durability, which may be applied to detect, e.g., the directional surface waves on the laboratory as well as the geological scale.

Probing nonlinear rheology layer-by-layer in interfacial hydration water.

Significance The hydration water layer (HWL) is a ubiquitous form of nanoscale water bound to the hydrophilic surfaces and plays a critical role in diverse phenomena in nature. Especially, investigation of its nonlinear fluidic properties is important for better understanding of its rheological contributions at a microscopic level. Using precise control and sensitive detection of HWL, we observe shear thickening at above a critical shear-strain rate of 106 s−1, resulting from the interplay between relaxation and elasticity-induced fluctuation correlation. Interestingly, the results indicate the occurrence of elasticity turbulence above a universal shear velocity of ∼ 1 mm/s. This work not only furthers our understanding of nonlinear nanorheology but is a stepping-stone toward controlled experiments on the nonlinear fluid dynamics near the interface. Viscoelastic fluids exhibit rheological nonlinearity at a high shear rate. Although typical nonlinear effects, shear thinning and shear thickening, have been usually understood by variation of intrinsic quantities such as viscosity, one still requires a better understanding of the microscopic origins, currently under debate, especially on the shear-thickening mechanism. We present accurate measurements of shear stress in the bound hydration water layer using noncontact dynamic force microscopy. We find shear thickening occurs above ∼ 106 s−1 shear rate beyond 0.3-nm layer thickness, which is attributed to the nonviscous, elasticity-associated fluidic instability via fluctuation correlation. Such a nonlinear fluidic transition is observed due to the long relaxation time (∼ 10−6 s) of water available in the nanoconfined hydration layer, which indicates the onset of elastic turbulence at nanoscale, elucidating the interplay between relaxation and shear motion, which also indicates the onset of elastic turbulence at nanoscale above a universal shear velocity of ∼ 1 mm/s. This extensive layer-by-layer control paves the way for fundamental studies of nonlinear nanorheology and nanoscale hydrodynamics, as well as provides novel insights on viscoelastic dynamics of interfacial water.

Buckling-Based Non-Linear Mechanical Sensor

Mechanical sensors provide core keys for high-end research in quantitative understanding of fundamental phenomena and practical applications such as the force or pressure sensor, accelerometer and gyroscope. In particular, in situ sensitive and reliable detection is essential for measurements of the mechanical vibration and displacement forces in inertial sensors or seismometers. However, enhancing sensitivity, reducing response time and equipping sensors with a measurement capability of bidirectional mechanical perturbations remains challenging. Here, we demonstrate the buckling cantilever-based non-linear dynamic mechanical sensor which addresses intrinsic limitations associated with high sensitivity, reliability and durability. The cantilever is attached on to a high-Q tuning fork and initially buckled by being pressed against a solid surface while a flexural stress is applied. Then, buckling instability occurs near the bifurcation region due to lateral movement, which allows high-sensitive detection of the lateral and perpendicular surface acoustic waves with bandwidth-limited temporal response of less than 1 ms.

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.