Jongwoo Kim

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.

Direct measurement of activation time and nucleation rate in capillary-condensed water nanomeniscus

We demonstrate real-time observation of nucleation of the single water nanomeniscus formed via capillary condensation. We directly measure (i) activation time by time-resolved atomic force microscopy and (ii) nucleation rate by statistical analysis of its exponential distribution, which is the experimental evidence that the activation process is stochastic and follows the Poisson statistics. It implies that formation of the water nanomeniscus is triggered by nucleation, which requires activation for producing a nucleus. We also find the dependence of the nucleation rate on the tip-sample distance and temperature.

Superwetting of TiO2 by light-induced water-layer growth via delocalized surface electrons

Significance TiO2, which is chemically stable, harmless, and inexpensive, has been widely used for industrial applications. Recently, TiO2-coated materials, exhibiting superwetting under sunlight, have been developed for environmental solutions. However, the mechanism responsible for superwetting of TiO2 is still in controversy despite many studies. We clarified its origin by performing tip-based in situ measurements of the growth dynamics of the photo-adsorbed water layers, as associated with delocalized surface electrons. Combined with molecular dynamics simulations, we provided conclusive clues that the “water wets water” process promotes water adsorption on the water layers, producing superwetting. Titania, which exhibits superwetting under light illumination, has been widely used as an ideal material for environmental solution such as self-cleaning, water–air purification, and antifogging. There have been various studies to understand such superhydrophilic conversion. The origin of superwetting has not been clarified in a unified mechanism yet, which requires direct experimental investigation of the dynamic processes of water-layer growth. We report in situ measurements of the growth rate and height of the photo-adsorbed water layers by tip-based dynamic force microscopy. For nanocrystalline anatase and rutile TiO2 we observe light-induced enhancement of the rate and height, which decrease after O2 annealing. The results lead us to confirm that the long-range attraction between water molecules and TiO2, which is mediated by delocalized electrons in the shallow traps associated with O2 vacancies, produces photo-adsorption of water on the surface. In addition, molecular dynamics simulations clearly show that such photo-adsorbed water is critical to the zero contact angle of a water droplet spreading on it. Therefore, we conclude that this “water wets water” mechanism acting on the photo-adsorbed water layers is responsible for the light-induced superwetting of TiO2. Similar mechanism may be applied for better understanding of the hydrophilic conversion of doped TiO2 or other photo-catalytic oxides.

Effective stiffness of qPlus sensor and quartz tuning fork.

Quartz tuning forks (QTFs) have been extensively employed in scanning probe microscopy. For quantitative measurement of the interaction in nanoscale using QTF as a force sensor, we first measured the effective stiffness of qPlus sensors as well as QTFs and then compared the results with the cantilever beam theory that has been widely used to estimate the stiffness. Comparing with the stiffness and the resonance frequency in our measurement, we found that those calculated based on the beam theory are considerably overestimated. For consistent analysis of experimental and theoretical results, we present the formula to calculate the stiffness of qPlus sensor or QTF, based on the resonance frequency. We also demonstrated that the effective stiffness of QTF is twice that of qPlus sensor, which agrees with the recently suggested model. Our study demonstrates the use of QTF for quantitative measurement of interaction force at the nanoscale in scanning probe microscopy.

Observation of Universal Solidification in the Elongated Water Nanomeniscus.

The ubiquitous capillary water bridge in nature plays an important role in interfacial phenomena under ambient conditions such as adhesion and friction. We present experimental measurements of the mechanical properties of the nanometric water column by using noncontact atomic force microscopy. We observe the universal behaviors that the relaxation time (RT) associated with the meniscus increases with its elongation and ruptures at the same value of RT, independent of the meniscus volume. In particular, the enhancement of RT between formation and rupture of the meniscus is indicative of the increased solid-like response, similar to that observed in nanoconfined water layers. Our results that the longer water column is more solid-like and less stable suggest (i) water at the vapor/liquid interface is more solid-like than that inside the meniscus and (ii) the associated smaller mobility of the interfacial water molecules is responsible for the structural stability of the water meniscus.

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.

Time-resolved observation of thermally activated rupture of a capillary-condensed water nanobridge

The capillary-condensed liquid bridge is one of the most ubiquitous forms of liquid in nature and contributes significantly to adhesion and friction of biological molecules as well as microscopic objects. Despite its important role in nanoscience and technology, the rupture process of the bridge is not well understood and needs more experimental works. Here, we report real-time observation of rupture of a capillary-condensed water nanobridge in ambient condition. During slow and stepwise stretch of the nanobridge, we measured the activation time for rupture, or the latency time required for the bridge breakup. By statistical analysis of the time-resolved distribution of activation time, we show that rupture is a thermally activated stochastic process and follows the Poisson statistics. In particular, from the Arrhenius law that the rupture rate satisfies, we estimate the position-dependent activation energies for the capillary-bridge rupture.

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.

Optimization of Force Sensitivity in Q-Controlled Amplitude-Modulation Atomic Force Microscopy

We present control of force sensitivity in Q-controlled amplitude-modulation atomic force microscopy (AM-AFM) that is based on the high-Q quartz tuning-fork. It is found that the phase noise is identical to the amplitude noise divided by oscillation amplitude in AM-AFM. In particular, we observe that while Q-control does not compromise the signal-to-noise ratio, it enhances the detection sensitivity because the minimum detectable force gradient is inversely proportional to the effective quality factor for large bandwidths, which is due to reduction of frequency noise. This work demonstrates Q-control in AM-AFM is a useful technique for enhancement of the force sensitivity with increased Q or improvement of the scanning speed with decreased Q.

Improved design for a low temperature scanning tunneling microscope with an in situ tip treatment stage

The Low Temperature Scanning Tunneling Microscope (LT-STM) is an extremely valuable tool not only in surface science but also in condensed matter physics. For years, numerous new ideas have been adopted to perfect LT-STM performances-Ultra-Low Vibration (ULV) laboratory and the rigid STM head design are among them. Here, we present three improvements for the design of the ULV laboratory and the LT-STM: tip treatment stage, sample cleaving stage, and vibration isolation system. The improved tip treatment stage enables us to perform field emission for the purpose of tip treatment in situ without exchanging samples, while our enhanced sample cleaving stage allows us to cleave samples at low temperature in a vacuum without optical access by a simple pressing motion. Our newly designed vibration isolation system provides efficient space usage while maintaining vibration isolation capability. These improvements enhance the quality of spectroscopic imaging experiments that can last for many days and provide increased data yield, which we expect can be indispensable elements in future LT-STM designs.