Special Issues

Quartz Tuning Fork-based Sensors, Till 20 March 2019

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Atomic force microscopy (AFM) was introduced in 1986 and has since found many applications. AFM can be employed in ambient and liquid environments as well as in vacuum and at low and ultralow temperatures. The technique is an offspring of scanning tunneling microscopy (STM). Measuring the tiny chemical forces that act between tip and sample is more difficult than measuring the tunneling current in STM. Therefore, even 30 years after the introduction of AFM, progress in instrumentation is substantial. The core of the AFM, is the force sensor with its tip and detection mechanism. Initially, force sensors were mainly micro machined silicon cantilevers, mainly using optical methods to detect their deflection. The qPlus sensor, originally based on a quartz tuning fork and now custom built from quartz, is self sensing by utilizing the piezoelectricity of quartz. The qPlus sensor allows to perform STM and AFM in parallel, and the spatial resolution of its AFM channel has reached the subatomic level, exceeding the resolution of STM. Frequency modulation AFM (FM-AFM), where the frequency of an oscillating cantilever is altered by the gradient of the force that acts between tip and sample has emerged over the years as the method that provides atomic and subatomic spatial resolution as well as force spectroscopy with sub piconewton sensitivity. FM-AFM is precise, because of all physical observables, time and frequency can be measured by far with the greatest accuracy. By design, FM-AFM clearly separates conservative and dissipative interactions where conservative forces induce a frequency shift and dissipative interactions alter the power needed to maintain constant oscillation amplitude of the cantilever. As it operates in a noncontact mode, it enables simultaneous AFM and STM measurements. The frequency stability of quartz and the small oscillation amplitudes that are possible with stiff quartz sensors optimize the signal to noise ratio. Here, we discuss the operating principles, the assembly of qPlus sensors, amplifiers, limiting factors and applications. Applications encompass unprecedented subatomic spatial resolution, the measurement of forces that act in atomic manipulation, imaging and spectroscopy of spin-dependent forces and atomic resolution of organic molecules, graphite, graphene and oxides.

High resolution force microscopy of single molecules gives the opportunity to achieve submolecular resolution. Conformational changes of molecules from transoid to cisoid conformation can be observed. Co-deposition of Fe-atoms leads to a dominance of cisoid configurations due to metal complex formation. Larger polycyclic aromatic hydrocarbon molecules were prepared by electrospray deposition. Comparison between room temperature and low temperature results will be shown. In the latter case, a CO molecule is attached to the probing tip, which gives the opportunity to enter the repulsive regime. On-surface chemical reactions give the possibility to grow relatively complex structures, such as graphene ribbons or polymeric chains. One of the challenges in force microscopy is to distinguish between different elements. The probing tip can be used to locally pull ribbons or nanowires. Quantitative information about normal and lateral forces can be gained by comparison with suitable modelling, which is of interest for nanotribology. Internal degrees of freedom, such as torsional motions, may become accessible.

Three-dimensional (3D) imaging of nanostructures with high aspect ratio features is a vital issue for nanoscience and technology. A wide variation of experimental method to improve 3D imaging capabilities have been proposed, including novel probes and algorithms such as the flared tip method and probe tilting approach. While present methods are mainly limited to imaging deep trenches, precise imaging for arbitrary shape structures has not been studied intensively. Herein, we report a tuning fork based atomic force microscopy (AFM), overcoming these challenges by employing a novel 3D scanning algorithm. We measured the depth and imaged sidewall topography of a porous materials (anodic aluminum oxide) and silicon nanopillar (protrusion) with high resolution. Using a carbon nanotube and a tungsten tip, tuning fork based AFM probe with a high aspect ratio (ranging between 5 and 25) was fabricated. It was possible to image the depth of holes down to 1 μm of a via hole in silicon. The 3D scanning algorithm is based on the sidewall touching in a hole and following the rim of the sidewall in XY plane. This scanning strategy is completely different from the conventional raster scanning. Artifact-free images of the sidewall and bottom edge of the hole were achieved. Our findings manifested how the intelligent scanning algorithm with a high-aspect-ratio probe can be utilized to characterize 3D topographical nanostructures. This instrumentation provides the potential capability of non-destructive imaging for wide industrial applications. This experimental demonstration provided strong evidence that the measurement capabilities can be expanded to complicated nanostructures with arbitrary morphologies. While previous attempts employed highly specialized probes that were limited to characterizing specific nanostructures like trenches, this new method possesses a unique advantage because it can image a wide range of morphological features that are present above or below the main scanning surface.

Surface tension plays crucial role in diverse fields of science such as microfluidics, granular media and colloidal science. Despite its importance, physical understanding of the surface tension at molecular scale is still in open question. In 1948, Tolmanpredicted that pure water droplet would have lower surface tension than the bulk value when its radius of curvature gets down to few nanometers. However, its direct experimental verification was lacked due to difficulty of observing single nanometer-size droplet. Here, we overcome this limitation by capturing the first moment of phase transition. We use quartz tuning fork-based atomic force microscopy and measure the critical distance at which single meniscus is condensed between two hydrophilic surfaces. By having statistical analysis of the critical distance at different humidity, we measured a slight deviation from the classical nucleation theory described as the Kelvin equation and found the deviation can be accounted by the curvature dependent surface tension. We experimentally determined the Tolman length, an important parameter for curvature dependent surface tension. Our results may provide deep understanding of the molecular mechanism of the surface tension and its effect on general nucleation phenomena.

A quartz tuning fork (QTF) has a unique geometry comprising two vibrating prongs separated by a specific gap. The symmetry of the two prongs reduces the number of possible vibration modes, which endows the QTF with a good quality factor. The high stability and low power consumption of QTFs allow them to be used in a wide range of applications, including in precision watches and force sensors. In addition to these conventional applications, there have been attempts to use QTFs as versatile gas sensors because the resonance frequency of a QTF decreases with increasing mass loading. However, the poor mass sensitivity of a typical 32.768 kHz bare QTF (50 ng/Hz) limits its broader applicability in gas sensors. We addressed this problem by attaching a polymer wire or membrane to the two prongs of a QTF. In this configuration, the polymer wire/membrane is stretched and compressed by the two vibrating prongs; the change in the mechanical force of the polymer due to gas adsorption is easily measured from the change in the resonance frequency. We fabricated various polymer wires and membranes and examined the polymer-coated QTF as a gas sensor. Upon exposure to solvent vapor, the resonance frequency of the polymer-coated QTF decreased due to the decrease in the modulus of the polymer wire, which was induced by the adsorption of solvent. Various methods will be discussed in this presentation for the sensitivity enhancement of the polymer-coated QTF

The Center for Quantum Nanoscience (QNS) is one of research center within Insititute of Basic Science (IBS), focusing on quantum control of single atoms and molecules on surfaces. QNS is a new research center started from 2017 and I’ll briefly introduce current status of QNS including research facility constructions. To achieve the goal of the center, we mainly focus on scanning tunneling microscopy (STM) instrumentation combined with electron spin resonance (ESR). As a first part of my talk, I’ll introduce ESR-STM technique to measure and control spin states of single atoms on surface. Recently, we measure and control the hyperfine interaction of individual atoms on magnesium oxide (MgO) by using spin-polarized ESR-STM . As a second part of my talk, I’ll introduce current status of establish tunning fork based low temperature STM / atomic force mirocscope (AFM) system operated at ultra high vacuum in QNS. To increase the quantum coherence time, investigation of new templates with new materials will be necessary but, until now, STM measurement limits to use only thin insulator films on metallic substrates. Expanded choice of substrate to bulk insulator substrates will open a new opportunity to increase the quantum coherence time of atoms and molecules on surfaces. Recently, AFM technology toward improving spatial resolution and spin sensitivity has been improved tremendously. Even subatomic resolution within a molecule using functionalized tip was demonstrated. To reduce the noise level of tunneling current and frequency shift signals, I devised low temperature preamplifier installed near STM / AFM unit.

Characterizing and visualizing heterogeneous compositions of various nanomaterials together with a nanoscale morphology is an essential part in a variety of disciplines of nanoscience and technology. The recent advanced photo-induced force microscopy (PiFM), which uses the opto-mechanical force between the sharp metal tip and the sample as the read-out mechanism, has successfully visualized the heterogeneous nanomaterials with a chemical selectivity by a few nm spatial resolution. Compared to photodetection techniques, the optical bandwidth in force probe methods is almost limitless ranging from UV to THz, and it is free from background noise like stray light. Here, we present the theoretical and experimental analysis of linear and nonlinear optical responses such as Raman vibrational molecular states, the excited state absorption in single molecular clusters, IR vibrational transition and surface plasmon polaritons and even the substructure of nano-biomaterial in photo-induced force microscopy. The ability to apply AFM's for nanometer scale optical spectroscopic analysis will open new opportunities in materials science and biology by providing the chemical and mechanical nature of individual molecules as well as probing the substructure of semiconductor for its defect analysis.


Phase changing materials are systems that undergo to phase transition a critical parameter cross the threshold value. Interesting examples of mechanical phase changing materials can be found at very difference scales ranging from atomic scale metallic wires to ionic liquids and even snow and ice. Investigating the mechanical properties of such system sis however a very challenging task expecially if one wants to have insight on the phase transition and the parameter affecting it. Tuning fork atomic force microscope (TF-AFM) is the set-up of choice to study mechanical properties of nanoscale material but despite this it has been usually discarded in studying phase changing materials. In this talk we will discuss how TF-AFM measurements allow to get information on the transition of three different materials : gold quantum point contact submitted to an external shear, ionic liquid under confinement and ice melting during macroscopic gliding.

Soft-condensed matter, commonly referred to as soft matter, is a class of materials including fluids, colloids, polymers, liquid crystals, and biological substances. While such materials are seen and experienced in our everyday lives, there remains much unknown about their physics, especially under confinement. Here, we introduce two ubiquitous forms of soft matter under confinement, nano-bridges of water and thin-films of an ionic liquid, and present their mechanical properties measured by a quartz tuning fork-based atomic force microscope (QTF-AFM). First, for the water bridges, we find that the bulk water bridges exhibit negative elasticity and positive damping, whereas the nanobridges of water show both positive-valued elasticity and damping coefficient. We show that the dimensional transition in water bridge not only alters such rheological responses, but also completely different physics governs their properties. Second, we present our recent study on the thin film of an ionic liquid, [Bmim][BF4], confined between a QTF-AFM tip and a substrate. We find that there arises elasticity in the thin-film as the film thickness decreases down to ~ 100 nm, and this elastic layer implies the role of the ionic liquid film as an excellent lubricant. Finally, various applications of QTF-AFM to probe liquid properties will be discussed.

Atomic force microscopy and spectroscopy are based on locally detecting the interactions between a surface and a sharp probe tip. For highest resolution imaging, noncontact modes that avoid tip-sample contact are used; control of the tip’s vertical position is accomplished by oscillating the tip and detecting perturbations induced by its interaction with the surface potential. Due to this potential’s nonlinear nature, however, achieving reliable control of the tip-sample distance is challenging, so much so that despite its power vacuum-based noncontact atomic force microscopy has remained a niche technique. In the first part of this talk, we introduce a new pathway to distance control that prevents instabilities by externally tuning the oscillator’s response characteristics. A major advantage of this operational scheme is that it delivers robust position control in both the attractive and repulsive regimes with only one feedback loop, thereby providing an easy-to-implement route to atomic resolution imaging and quantitative tip-sample interaction force measurement.
In the second part of the talk, we will then analyze the accuracy that measurements of the tip-sample interaction potentials can achieve, both in conventional FM-mode operation as well as in the new mode introduced in the talk’s first part. We will in particular show that the force law obtained from data acquired under vacuum conditions using the most commonly applied methods may deviate more than previously assumed from the actual interaction when the oscillation amplitude of the probe is of the order of the decay length of the force near the surface, which may result in a non-negligible error if accurate absolute values are of importance. Caused by approximations made in the development of the mathematical reconstruction procedures, the related inaccuracies can be effectively suppressed by using oscillation amplitudes sufficiently larger than the decay length. To facilitate efficient data acquisition, we propose a novel data acquisition approach that includes modulating the drive amplitude at a constant height from the surface while monitoring the oscillation amplitude and phase. Such amplitude sweep-based force spectroscopy enables shorter data acquisition times and increased accuracy for quantitative chemical characterization compared to standard approaches that vary the tip-sample distance. An additional advantage is that since no feedback loop is active while executing the amplitude sweep, the force can be consistently recovered deep into the repulsive regime.

Nanopipette/nanorod is useful tool for manipulation of micro/nanoscale objects and their investigation in the field of nanoscale science and technology, especially nano-bio system. Typically, it is used for bio or medical applications in the liquid or soft matter environment, such as biomolecule injection into the cells or handling micro/nanoscale biomolecules, or nano-bio engineering. However, it is difficult to precisely approach and control on and near the hard surface without tip breaking even with a guidance of conventional atomic force microscope (AFM) system due to geometrical issue. Here we show the capability of precise control of the nanopiptte/nanorod combining with a quartz tuning fork (QTF)-based AFM which is a versatile tool in the field of atomic force microscopy.
We demonstrate a nanopipette/QTF-AFM for nanolithography and a nanorod/QTF-AFM for nanoscratching with in situ measurement of shear dynamics during performing. 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) QTF-AFM along with systematic investigation of the system. We fabricate and characterize Au nanoparticle-aggregated nanowires by using the nano meniscus-induced colloidal stacking method by using nanopipette/QTF-AFM. And we demonstrate the bifurcation-enhanced sensitive measurement of mechanical vibrations using the nonlinear buckled cantilever tip (nanorod/QTF-AFM) in ambient conditions. We have demonstrated a nanoscratching technique using in situ direct measurement of the elastic and viscous responses of shear dynamics for a buckled quartz nanorod tip.

Structural transformations originating from diverse rearrangements of the hydrogen bonding network in water create various phases of liquid and ice. Although most water phases have been well investigated down to the molecular level, the molecular structure of the nanomeniscus, a ubiquitous form of nanoscale water produced between hydrophilic surfaces in nature, still remains unexplored. Identification of molecular structure of water, especially in nanoconfinement, has been challenging and even incorrect despite its increasing interest as well as extensive works done to date. We use surface-enhanced Raman spectroscopy to trace the molecular fingerprints of the hydrogen-bonding configuration of water nanomeniscus that is formed in dried manoparticulate of silver, and show that such ubiquitous water exists surprisingly as ice-VII in normal ambient conditions, beyond general expectation of its existence only in extreme conditions such as in Earth’s deep mantle. In particular, the ice-VII structure for such nanoconfined water is evidenced by the spectral independence with respect to the temperature variation as well as the surface types including the material, size and shape of nanoparticles. Our results may be useful for a better understanding of molecular aspects of water at nanoscale such as biological water.

Invited Speakers

Prof. Franz Giessibl

Univ. of Regensburg, Germany

Prof. Ernst Meyer

University of Basel, Switzerland

Prof. Qi-Kun Xue

Tsinghua University, China

Prof. Alessandro Siria

École normale supérieure, France

Prof. Udo Schwarz

Yale University, USA



Organizing Committee

Prof. Wonho Jhe

Seoul National University

Prof. Sangmin An

Seoul National University

Prof. Manhee Lee

Chungbuk National University

Prof. Yongho Seo

Sejong University