We report on the frictional force between an SiN tip and graphene/graphite surfaces using lateral force microscopy. The cantilever we have used was made of an SiN membrane and has a low stiffness of 0.006 N m(-1). We prepared graphene flakes on a Si wafer covered with silicon oxides. The frictional force on graphene was smaller than that on the Si oxide and larger than that on graphite (multilayer of graphene). Force spectroscopy was also employed to study the van der Waals force between the graphene and the tip. Judging that the van der Waals force was also in graphite-graphene-silicon oxide order, the friction is suspected to be related to the van der Waals interactions. As the normal force acting on the surface was much weaker than the attractive force, such as the van der Waals force, the friction was independent of the normal force strength. The velocity dependency of the friction showed a logarithmic behavior which was attributed to the thermally activated stick-slip effect.

Since it was invented by Binnig et al in 1986, atomic force microscopy (AFM) has played a crucial role in nano-scale science and technology. AFM is a microscopic technique imaging a surface topography by using attractive and repulsive interaction forces between a few atoms attached at a tip on a cantilever and a sample. In the case of attractive forces, there are three main contributions causing AFM. These are short-range chemical force, van der Waals force and electrostatic force. As the effective ranges of these forces are different, one of them is dominant depending on distance. Atomic force spectroscopy is the force-versus-distance measurement when using AFM. The atomic force can be detected by cantilever bending caused by a tip-sample interacting force, which is called static AFM. Also, the atomic force can be detected by using the resonant properties of a cantilever, which is called dynamic AFM. Under the on-resonance condition, the frequency, amplitude or phase of the cantilever will be shifted by the interaction force. While the force can be estimated in static AFM, for dynamic AFM it requires complicated formalism to evaluate the force from measured amplitude, phase or frequency data. Recently developed techniques for ultra-high resolution imaging unveil sub-atomic features of the sample, which are facilitated by low temperature, ultra-high vacuum environments together with a stiff cantilever. In this study, progress related to theoretical and experimental imaging and force spectroscopy will be discussed.

Nanometer-sized columns of condensed water molecules are created by an atomic-resolution force microscope operated in ambient conditions. Unusual stepwise decrease of the force gradient associated with the thin water bridge in the tip-substrate gap is observed during its stretch, exhibiting regularity in step heights (≈ 0.5 N/m) and plateau lengths (≈ 1 nm). Such "quantized" elasticity is indicative of the atomic-scale stick-slip at the tip-water interface. A thermodynamic-instabilityinduced rupture of the water meniscus (5-nm long and 2.6-nm wide) is also found. This work opens a high-resolution study of the structure and the interface dynamics of a nanometric aqueous column.PACS numbers: 07.79. 07.79.Lh, 47.17.+e, 62.10.+s Water is one of the most important substances of life and has been studied extensively for hundreds of years. Nonetheless, it is still quite a unique matter that keeps surprising us and exhibits peculiarities, in particular, when confined in a nanometric configuration. For example, water and simple organic liquids exhibit solid-like orderedness in molecularly thin films [1,2,3]. Water molecules inside hydrophobic nanotubes manifest phases of ice that are not found under bulk conditions [4]. However, since bulk water possesses only short-range order [5] and water molecules move incessantly, it is usually difficult to experimentally investigate novel features of confined water structures other than thin films.In this Letter, we have employed an atomic-resolution noncontact atomic force microscope (AFM) in air [6] and achieved the spontaneous formation of a nanometric liquid column consisting of thousands of water molecules. We also have performed the sensitive measurement of the elastic property (or the force gradient) of the thin water column during its mechanical stretch. We have thereby demonstrated several novel phenomena: (i) the unusual stepwise decrease of the force gradient, associated with the atomic-scale stick-slip on the AFM-tip surface, (ii) the abrupt rupture of the thin water meniscus due to the thermodynamic instability of the liquid-vapor interface, and (iii) the manipulation of the thin aqueous column by repeated stretch-relaxation cycles, revealing the atomicscale contact angle hysteresis.Water molecules in ambient conditions produce a nanoscale water meniscus between a hydrophilic Si tip and a hydrophilic mica substrate, when capillary condensation occurs as the stiff AFM tip approaches the substrate within a nanometric distance (Fig. 1). Once the thin aqueous column is formed, it is stretched vertically upward by subsequent retraction of the tip. As the molecular water bridge of sub-zeptoliter (zepto = 10 −21 ) volume is elongated thereby, the force gradient associated with the elasticity of the system is measured by an ex- * Corresponding author: whjhe@snu.ac.kr tremely small amplitude-modulation operation of AFM [7,8]. Figure 1 presents the schematics of a home-built AFM setup used for formation of a nanometric water column by capillary condensation as well as for si...

AFM cleaning technique can be a potential tool to clean the surface defects of 2D materials like TMDs, as well as graphene.

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We have studied the effect of perpendicular magnetic fields and temperatures on the nonlinear electronic transport in amorphous Ta superconducting thin films. The films exhibit a magnetic field induced metallic behavior intervening the superconductor-insulator transition in the zero temperature limit. We show that the nonlinear transport in the superconducting and metallic phase is of non-thermal origin and accompanies an extraordinarily long voltage response time.In recent years, the suppression of superconductivity in two-dimensions (2D) by means of increasing disorder (usually controlled by film thickness) or applying magnetic fields has been a focus of attention. Conventional treatments [1,2,3,4,5] of electronic transport predict that in 2D the suppression of the superconductivity leads to a direct superconductor-insulator transition (SIT) in the limit of zero temperature (T = 0). This traditional view has been challenged by the observation of magnetic field (B) induced metallic behavior in amorphous MoGe [6,7,8] and Ta thin films [9]. The unexpected metallic behavior, intervening the B-driven SIT, is characterized by a drop in resistance (ρ) followed by a saturation to a finite value as T → 0. The metallic resistance can be orders of magnitude smaller than the normal state resistance (ρ n ) implying that the metallic state exists as a separate phase rather than a point in the phase diagram. Despite many theoretical treatments [8,10,11,12,13,14,15,16,17], a consensus on the mechanism behind the metallic behavior is yet to be reached. Proposed origins of the metallic behavior include bosonic interactions in the nonsuperconducting phase [10,11], contribution of fermionic quasiparticles to the conduction [12,13], and quantum phase fluctuations [14,15].In a recent paper [9] on the magnetically induced metallic behavior in Ta films, we have reported the nonlinear voltage-current (I-V ) characteristics that can be used to identify each phase. The superconducting phase is unique in having both a hysteretic I-V and an "immeasurably" small voltage response to currents below an apparent critical current I c . The metallic phase can be identified by a differential resistance (dV /dI) that increases with increasing I, whereas the insulating phase is identified by a dV /dI that decreases with increasing I. The contrasting nonlinear I-V in the metallic and insulating phase are shown in Fig. 1(a).The main purpose of this Letter is to report that the origin of the nonlinear transport, particularly in the superconducting and metallic phase, is not a simple reflection of T -dependence of ρ via the unavoidable Joule heating. We describe the effect of B and T on the nonlinear TABLE I: List of sample parameters: nominal film thickness, mean field Tc at B = 0, normal state resistivity at 4.2 K, critical magnetic field at which the resistance reaches 90% of the high field saturation value, and correlation length calculated from ξ = Φ0/2πBc where Φ0 is the flux quantum.

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In this study, we investigated an energy harvesting effect of tensile stress using piezoelectric polymers and flexible electrodes. A chemical-vapor-deposition grown graphene film was transferred onto both sides of the PVDF and P(VDF-TrFE) films simultaneously by means of a conventional wet chemical method. Output voltage induced by sound waves was measured and analyzed when a mechanical tension was applied to the device. Another energy harvester was made with a metallic electrode, where Al and Ag were deposited by using an electron-beam evaporator. When acoustic vibrations (105 dB) were applied to the graphene/PVDF/graphene device, an induced voltage of 7.6 Vpp was measured with a tensile stress of 1.75 MPa, and this was increased up to 9.1 Vpp with a stress of 2.18 MPa for the metal/P(VDF-TrFE)/metal device. The 9 metal/PVDF/metal layers were stacked as an energy harvester, and tension was applied by using springs. Also, we fabricated a full-wave rectifying circuit to store the electrical energy in a 100 μF capacitor, and external vibration generated the electrical charges. As a result, the stored voltage at the capacitor, obtained from the harvester via a bridge diode rectifier, was saturated to ~7.04 V after 180 s charging time.