Copyright 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 5754 5758, 2012 Enhanced Formation of a Confined Nano-Water Meniscus Using a 780 nm Laser with a Quartz Tuning Fork-Atomic Force Microscope Sangmin An 1, Kunyoung Lee 1, Geol Moon 1, Wan Bak 1, Gunn Kim 2, and Wonho Jhe 1 1 Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea 2 Department of Physics and Institute of Fundamental Physics, Sejong University, Seoul 143-747, Republic of Korea Demonstrated herein is the optical-field-induced enhancement of the formation of a confined nanowater meniscus using a distance-regulated quartz tuning fork-atomic force microscope (QTF-AFM) with a 780 nm laser. While a pulled optical fiber tip approaches the surface, the laser is suddenly turned on and focuses on the front spot of the tip by the shape of the pulled optical fiber, which plays the role of an objective lens and induces the gathering effect of the water molecules directed to the electromagnetic-field gradient in air. This phenomenon facilitates a new boundary condition to form a long-range confined nano-scale liquid bridge between the tip and the surface. After the pulling of the optical fiber, 20-nm-thick gold was sputtered on the apex (diameter: 100 nm) of the tip to guide and focus the beam on the spot. The critical power of the laser to overcome the barrier for the formation of a new boundary is 100 W at the distance of 22 nm from the substrate. Keywords: Pulled Optical Fiber, QTF-AFM, Enhanced Formation of a Confined Nano-Water Meniscus. 1. INTRODUCTION In nature, the confined nano-water plays an important role at interfaces. 1 3 There have been many reports of late on the mechanical properties of a confined nano-water meniscus. 4 5 The optical measurement of the system, however, is still a challenging issue due to the diffraction limit of optics, 6 although a few studies were reported to have used a variation of an optical-power-dependent refractive index. 7 In this study, the optical dependencies of the nano-scale confined water were focused on, using a 780 nm-laser-combined standard quartz tuning fork-atomic force microscope 8 (QTF-AFM), and increased water volume due to the optical trapping of the molecules, a well-known technique in the field of molecular and optical physics, was demonstrated. 9 11 The optical-fieldinduced viscoelastic properties of the confined nano-liquid were measured using a pulled optical fiber combined with the distance-regulated QTF-AFM, by approaching the surface 12 within 22 nm. The optical-field gradient attracts the water molecules into a focused lasing spot and produces a new boundary condition between the tip and the surface, although the distance is not short enough to Author to whom correspondence should be addressed. cause capillary condensation. 13 Using this technique, the optomechanical properties of the nano-scale water bridge and its manipulation can be studied. 2. EXPERIMENTAL DETAILS A non-contact, amplitude-modulation, shear-mode QTF- AFM was employed for the detection of the shear motion forces of a naturally confined nano-water meniscus formed 14 15 between the apex of the fiber tip and the surface. The pulled optical fiber was simply attached to a side of the prong of the QTF sensor, which monitored the variations of the viscoelastic force of water by approaching and retracting from the surface along the vertical direction (z-axis). The single-mode optical fiber (core diameter: 8 m) was pulled using a commercial CO 2 laser puller (P-2000, Sutter Instrument, Co.). The puller has five control parameters: laser temperature (HEAT), beam spot diameter (FIL), velocity of pulling (VEL), delay time after melting (DEL), and pulling power (PUL). While the first three parameters were fixed, the apex size (50 nm 2 m) and the tapered angle of the optical fiber were varied according to the parameters of the VEL and PUL. Using the recipe 5754 J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 7 1533-4880/2012/12/5754/005 doi:10.1166/jnn.2012.6256
Enhanced Formation of a Confined Nano-Water Meniscus Using a 780 nm Laser with a QTF-AFM Fig. 1. The pulled optical fiber. Tip shape and tapered angle of the pulled optical fiber. The decrement of the tapered angle is important for the Q-factor of the tip and the concentration of the laser power. After pulling, the tip was coated with 20-nm-thick gold using a commercial sputter, and the apex size was determined by capturing the image using a scanning electron microscope. J. Nanosci. Nanotechnol. 12, 5754 5758, 2012 oscillator model. Although enharmonic effects may come from the stray capacitance of each electrode on the quartz body, such effects were removed with an electronic circuit, using another leg-tied QTF, which has the same stray capacitance value but at a phase opposite that of the freely oscillating QTF in Figure 2.20 The resonance frequency of the QTF was 32,768 Hz. As the tip approached the sample, the frequency of the QTF was increased due to the perturbation of the nucleation of the water bridge. This phenomenon can be expressed by the following equation of motion: mx + b x + kx = F cos t + Fperturb (1) where m is the effective mass of the probe, b is the damping coefficient, k is the spring constant, F is the amplitude of the drive, and Fperturb is the force by perturbation. As the forces interact with the tip of the QTF as the tip approaches the surface within 10 nm, the signal is changed. After solving Eq. (1), the elasticity and viscosity constants can be obtained as follows: F 2 k sin + m k = int A (2) F b= cos bint A where is the resonance frequency due to the perturbation and A is the free oscillation amplitude. Using this equation of motion, the mechanical properties of nano-scale liquids can be studied. The QTF has two different modes: the tapping and shear modes. The shear mode is used for detecting only shearforce interactions, while the tapping mode is for other different orthogonal forces, such as van der Waals and short-range forces. The shear mode was chosen to measure only the shear perturbed forces from the naturally formed nano-water meniscus. Figure 2 shows a diagram of the proposed enhanced formation of the confined nano-water meniscus. For the distance control between the pulled optical fiber and the surface with several pico-meter resolutions, a shear-mode QTF-AFM that acts as a shear motion parallel to the substrate was used. The dithering amplitude of the QTF was about 0.4 nm, which is so small that the nano-water meniscus is not affected by it. The opposite side of the pulled optical fiber was cleaved with a slicer and was directly aligned to an objective lens with a beam spot size of 1 mm. Then this tip was perpendicularly attached onto a side of the prong of the QTF to detect the shear motion of the confined water meniscus. The resonance frequency (Q-factor) of the tip was varied from 32,768 to 32,505 Hz (from 6600 to 3400). On the other hand, the driving amplitude was 0.1 mv. To measure the capillary force, a high Q-factor of at least 2000 is required. By combining the stray capacitance compensation circuit and the active-q control system,21 the high Q-factor of 6000 can 5755 HEAT = 350, FIL = 0, VEL = 40, DEL = 126, and PUL = 145, the 100 nm apex of the optical fiber can be fabricated. The aperture size may be further reduced by optimizing all the parameters. Figure 1 shows the tapered angle, which is related to the distance of the focused spot from the apex of the tip, and the shape of the pulled optical fiber, using an optical microscope. The decrement of the tapered angle is important for sustaining the high quality factor (Q-factor) of the tip and the concentration of the laser power. As the skin depth of gold is 20 nm in the visible spectral range,16 the surface of the pulled optical fiber was coated with 20-nm-thick gold using a sputtering machine, after pulling, to concentrate the laser power on an area near the apex of the tip. The scanning electron microscope (SEM) image confirms the size (147 nm) of the gold-coated apex of the fiber tip, as shown in Figure 1. The QTF is generally used to control time accurately. Due to its various advantages, such as its high sensitivity, many researchers have utilized this novel piezoelectric device as a sensor17 18 to observe phenomena with atomic resolution. Generally, in AFM, Si cantilevers or sharp pulled metal tips are used to detect long- and short-range atomic forces. As the cantilever of the conventional AFM has a small force constant (k), these tips have a jump-tocontact motion while approaching the surface. The motion gives rise to viscoelasticity of confined water. In contrast, the QTF has a high force constant (k), generally 103 104 N/m. Therefore, the QTF tip is rigid enough to resist a jump-to-contact motion. Using a QTF tip, a variety of properties of naturally formed nano-scale water meniscus can be investigated. Recent results show that it is enough to detect a very small perturbation of the forces exerted on a nano-water meniscus (< 10 nn).13 19 The measured force can be interpreted using a simple harmonic
Enhanced Formation of a Confined Nano-Water Meniscus Using a 780 nm Laser with a QTF-AFM (c) Fig. 2. The experiment setup. The non-linear effect coming from the stray capacitance of the QTF s electrode can be removed using an electricalcompensation circuit ( 1.25 pf). Using an identical QTF, the oscillating motion of the legs is removed. The laser beam is directly guided onto the apex of the pulled optical fiber and focused on the spot near the substrate. Over the critical laser power, the water molecules in the air are congregated to the focused region and begin to be confined between the tip and the surface in the distance of about 20 nm. (c) Real image of the system. The pulled optical-fiber tip was attached to one prong of the QTF and triggered shear motion to detect the capillary force of the confined nano-water meniscus. To directly detect the power of the laser, the tip approaches the glass surface of the commercial photo-diode, which is glued on the piezoelectric transducer (PZT). be obtained. A lock-in amplifier (EG and G 7265) was employed for filtering the noise originating from the QTF signal. The experiment procedure consists of three steps. First, the tip approaches the surface within 10 nm to form a confined nano-water meniscus with the QTF-AFM, and measures the distance between the tip and the surface. Second, the 780 nm laser is suddenly turned on and focused on the spot near the apex of the pulled fiber (20 nm away from the surface). Finally, a nano-water meniscus can be formed even in extraordinary conditions of capillary condensation. To check the laser power, a typical photodiode was glued on a piezoelectric displacement tube (full moving range of the z-axis: 400 nm) and a mica substrate was used in a typical cleaning process. The real image is shown in Figure 2(c). The tip easily approaches the surface by monitoring the reflection image with a chargecoupled device (CCD) and the output signal of the QTF. Temperature and humidity are important factors in the experiment. Humidity variation leads to a change in the resonance frequency of the QTF, and the thermal expansions due to the temperature variation can affect the whole system. To maintain the temperature and humidity within the required control range, a double-chamber setup was employed by inserting an acryl chamber inside a Faraday cage chamber for shielding against electromagnetic noise. The thermal drift was 0.3 pm/s. During the experiment, the temperature was 21.5 ± 0.1 C and the humidity was 45.7 ± 0.3%. 5756 3. RESULTS AND DISCUSSION Figure 3 shows the approach/retract curves of the formation and rupture of the confined nano-water meniscus. The tip approach and retract processes are performed at the same position of the mica substrate with the same experiment condition, such as the humidity, temperature, tip diameter, and substrate. If these parameters are varied by any circumstance, the boundary conditions between the tip and surface are easily changed by the use of the YoungLaplace equation. In this experiment, however, these variables were fixed by employing a double-chamber system, and only the dependence of the laser power was defined. After forming the confined nano-water meniscus between the optical-fiber tip and the substrate within 10 nm, the tip was immediately retracted for the measurement of the force gradient from the surface. The approach/retraction speed of the tip was 0.5 nm/s. From the curves, the viscoelasticity, which is associated with the capillary force, can be obtained as a function of the vertical position of the tip. The point where the rupture occurs was also measured. Without the laser, the distance from the surface was determined by repeating the approach/retract process three times. While the 22 nm distance between the tip and the substrate was sustained, the laser power was increased from 0 to 100 W at 10 W intervals. Up to 90 W, the output signal of the QTF showed no noticeable difference, indicating no appreciable formation of a new boundary J. Nanosci. Nanotechnol. 12, 5754 5758, 2012
Enhanced Formation of a Confined Nano-Water Meniscus Using a 780 nm Laser with a QTF-AFM lasing spot surface QTF dithering amplitude [nm] 0.60 0.55 0.50 0.45 80 approach 22nm retract 60 40 20 z-axis displacement [nm] 0 (c) lasing spot surface QTF dithering amplitude [nm] 0.6 0.5 0.4 0.3 0.2 0.1 approach retract 80 60 40 20 z-axis displacement [nm] 0 Fig. 3. Experimental results Approach/retraction curves of the formation and rupture of the confined nano-water meniscus. After the tip approaches and retracts three times at the same position of the mica substrate with the same experiment conditions (humidity, temperature, tip diameter, and substrate), which shows the same results ( ), the laser is turned on and increases the power from 0 to 100 W at10 W intervals at the spot about 22 nm from the surface, stopping the tip movement. At 100 W, the QTF signal was suddenly decreased due to the formation of the confined nano-water meniscus ( ) (the connecting lines are for eye guide). The approach/retraction speed of the tip was 0.5 nm/s. Real side-view CCD camera images of the laser-beam-focused region (laser power: 50 and 100 W) (c) The approach/retraction curve of the manipulation and elongation of the water bridge by lasing. When the laser was turned on, the nano-water meniscus became more stable even far from the surface. condition. At 100 W, however, the QTF signal suddenly decreased because the confined nano-water meniscus was formed 22 nm away from the surface. This phenomenon is a clear evidence of the enhanced formation of a nanowater meniscus due to the gathering effect of the water molecules inward to the field gradient of the laser beam. Figure 3 shows a real side-view CCD image along the y-axis when the laser was turned on with 50 and 100 W power. The confined nano-water meniscus was manipulated and elongated by lasing at several spots, while the tip retracted as shown in Figure 3(c). The decreased QTF signal informs the more bulky and stable confined nano-water meniscus by lasing. 4. CONCLUSIONS In summary, reported herein is the enhanced formation of a confined nano-water meniscus using the pulled optical fiber combined with the QTF-AFM. In the future related works, the dependence of the laser power and the distance between the tip and the surface will be studied, and the optical field induced the relaxation time of the confined nano-water meniscus by turning the laser on/off. With this method, one can apply to a molecular switch with the fabrication of the confined nano-water meniscus at a long distance from the surface using laser. Besides, the manipulation of a tiny volume of liquid can be applied in opto-mechanical systems using tuneable broadband optical laser. Acknowledgment: This work was supported by a National Research Foundation of Korea (NRF) grant within the project Nano-Liquids at Nanoscale: Novel Science and Technology and Brain Korea 21. Wonho Jhe also acknowledges the support given by Center for Nano-Liquid. References and Notes 1. U. Raviv, P. Laurat, and J. Klein, Nature 413, 51 (2001). 2. S. Han, M. Y. Choi, P. Kumar, and H. E. Stanley, Nat. Phys. 6, 685 (2010). 3. B. Mukherjee, P. K. Maiti, C. Dasgupta, and A. K. Sood, J. Nanosci. Nanotechnol. 7, 1796 (2007). 4. R. C. Major, J. E. Houston, M. J. McGrath, J. I. Siepmann, and X.-Y. Zhu, Phys. Rev. Lett. 96, 177803 (2006). J. Nanosci. Nanotechnol. 12, 5754 5758, 2012 5757
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