Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication Sangmin An, Kunyoung Lee, Bongsu Kim, Haneol Noh, Jongwoo Kim, Soyoung Kwon, Manhee Lee, Mun-Heon Hong, and Wonho Jhe Citation: Review of Scientific Instruments 85, 033702 (2014); doi: 10.1063/1.4866656 View online: http://dx.doi.org/10.1063/1.4866656 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quartz tuning fork-based frequency modulation atomic force spectroscopy and microscopy with all digital phaselocked loop Rev. Sci. Instrum. 83, 113705 (2012); 10.1063/1.4765702 Active Q control in tuning-fork-based atomic force microscopy Appl. Phys. Lett. 91, 023103 (2007); 10.1063/1.2753112 Electrostatic force microscopy using a quartz tuning fork Appl. Phys. Lett. 80, 4324 (2002); 10.1063/1.1485312 Tuning-fork-based fast highly sensitive surface-contact sensor for atomic force microscopy/near-field scanning optical microscopy Rev. Sci. Instrum. 73, 1795 (2002); 10.1063/1.1462038 Imaging soft samples in liquid with tuning fork based shear force microscopy Appl. Phys. Lett. 77, 1557 (2000); 10.1063/1.1308058
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 033702 (2014)
Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication Sangmin An,a) Kunyoung Lee, Bongsu Kim, Haneol Noh, Jongwoo Kim, Soyoung Kwon, Manhee Lee,b) Mun-Heon Hong,c) and Wonho Jhed) Department of Physics and Astronomy, Center for Nano-Liquid, Seoul National University, Seoul 151-747, South Korea
(Received 26 September 2013; accepted 10 February 2014; published online 4 March 2014) 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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4866656] I. INTRODUCTION
The nanopipette is a versatile tool for biological research applications, including sensors, scanning ion conductance microscopy (SICM), and cell treatments.1–5 In particular, it is useful for the precise injection of nanomaterials into a target cell for vital reactions.6 Recently, researchers have investigated nanoscale patterning using the atomic force microscope (AFM) system in combination with the nanopipette;7, 8 however, the process is complex and difficult to handle along with the necessary optical apparatus. In addition, most nanopipettes are used in liquid environments with soft materials because of the effects of surface impact on the apex of the tip, and the aperture diameter is limited to more than 100 nm for high surface tension near the apex of the pipette. Here, we introduce a nanopipette in combination with a quartz tuning fork-atomic force microscope system (nanopipette/QTFAFM) along with experimental and theoretical investigations of the nanoscale materials under ambient conditions using a simple electrical detection method and with no surface impact on the nanopipette’s apex. We also describe systematic experimental studies of the pulled nanopipette (elongated length, tilting angle based on the y-axis pushing force, and the x-axis contact region), quantitative force spectroscopy (a confined nano-water experiment), and atomic force microscopy (surface imaging of a CD-R, a hard disk, and 50-nm polystyrene nanospheres), along with a demonstration of nanofabrication processing involving nanomaterial solution delivery through the nanopipette aperture. The proposed tool has several advantages, including in situ surface characterization after nanofabrication, simple attachment of the a) Present address: Center for Nanoscale Science and Technology, National
Institute of Standards and Technology, Gaithersburg, MD 20899.
b) Present address: S. LSI, Giheung Campus, Samsung Electronics, Seoul
(Gyeonggi-do) 446-711, South Korea.
c) Present address: LG Electronics Advanced Research Institute, Seoul 137-
724, South Korea.
d) Electronic mail: [email protected]
0034-6748/2014/85(3)/033702/7/$30.00
AFM tip to the edge of the QTF sensor’s single prong, easy control of the quality (Q) factor, and simple variation of the tip aperture size via the mechanical puller’s control parameters. II. NANOPIPETTE/QTF-AFM
Figures 1(a) and 1(b) show a picture and a schematic of the proposed nanopipette/QTF-AFM system, respectively. The QTF (resonance of ∼32 kHz) and the pencil-shaped nanopipette combine to produce a functional liquid solution delivery along with force spectroscopy/microscopy capabilities. The fabricated pipette tip is perpendicularly attached to one prong of the QTF to oscillate laterally so that it senses the capillary force from the confined nano-water meniscus alone, and is then brought close to the surface. A. Nanopipette
Figure 1(c) shows a schematic diagram of the process used to fabricate the pulled nanopipette, which is mechanically pulled when a CO2 laser melts the center section of a pipette via a commercial puller (P-2000, Sutter Instrument, Co., Novato, CA, USA). The pulled nanopipettes are generally produced from three materials: borosilicate, quartz, and aluminosilicate. The borosilicate and quartz pipettes, with outer and inner diameters of 1 mm and 0.7 mm, respectively, are found in the experiments to suffer the least degradation in the Q-factor of their tips, even when attached to the QTF. The puller has five different control parameter settings: the laser temperature (HEAT; example settings: borosilicate: 350; quartz: 700), the beam spot diameter (FIL, which is generally fixed at 4), the pulling velocity (VEL), the delay time after melting by the laser (DEL; the taper angle is varied by control of this parameter), and the pulling power (PUL). We controlled the aperture sizes of the pulled nanopipettes by varying the PUL parameter. When the value of PUL increases, the aperture size decreases, and the smallest aperture size achieved was approximately 15 nm. The aperture size can be
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tion of the tip parallel to the surface indicates the detection of the lateral interaction force induced by the formation of a nanoscale confined water meniscus, while ignoring the various perpendicularly exerted forces, as the tip approaches the surface. We calculated the mechanical responses of the QTF (effective elasticity, kint , and damping, bint ) by solving for a harmonic motion with respect to the oscillation amplitude (A) and phase (θ ) of the QTF sensor when externally driven by a function generator.
III. RESULTS AND DISCUSSIONS A. Systematic investigation of quantitative force measurement
FIG. 1. Nanopipette/QTF-AFM system. (a) Piezoelectric device of QTF combined with the pulled nanopipette. (b) Schematic of the system. The nanopipette is perpendicularly attached to the edge of the QTF sensor prong. (c) Schematic of pipette pulling process. The glass pipette (inner diameter of 0.7 mm, outer diameter of 10 mm) is mechanically pulled when the CO2 laser melts the center.
reduced further by optimizing all the process parameters. Various pulled nanopipette aperture sizes were produced in the experiment. For example, aperture sizes of ∼30, 50, and 100 nm were produced by PUL parameter settings of 190, 180, and 170, respectively, when the other four parameters were fixed as HEAT = 350, FIL = 4, VEL = 20, and DEL = 125. The aperture sizes were determined using a typical electrical current measurement system using artificial sea water (ASW) and scanning electron microscopy (SEM) images. The measured resistance (∼35 M) from the I V curve (gain of 107 for I V -converter) corresponds to the 30-nm aperture.9, 10
Figures 2 and 3 show the experimental conditions and the results from our systematic approaches to define the proposed nanopipette/QTF-AFM system as a force sensor. Figures 2(a) and 2(b) show the dependence of the attached length of the nanopipette on the contact position with the QTF on a single prong’s edge for thin and thick diameter pipettes. Figures 2(c) and 2(d) show the dependence of the tilt angle on the pushing force on the y-axis and the x-axis contact regions, respectively. As a result, the tip resonance frequency decreases with decreasing Q-factor as the length of the attached pipette from the contact position on the QTF increases, as shown in Fig. 3(a). This indicates that attractive and damping forces are exerted on the QTF sensor because of the widening of the pipette diameter at the contact area and the increment in the effective stiffness of the contacted pipette under the influence of the √ effective mass loading effect on the QTF edge (f ∝ 1/2π (k + δk)/(m + δm)) and the imbalance of the center of mass.20, 21 A short attachment length causes the smallest degradation of the Q-factors, which are less than the intrinsic QTF in cases 1–3 of Fig. 3(a). However, the Q-factor decreased drastically when the elongated
B. QTF-AFM
The QTF, which is a piezoelectric device with mechanical resonance, can be used as a high-sensitivity force sensor with a simple electrical detection system in various environments, including vacuum, ambient, and liquid environments.11–15 One useful advantage of the QTF in the AFM system is that it avoids a jump-to-contact problem that occurs with high stiffness (103 ∼ 104 N/m), where the problem is caused by formation of a confined nanoscale water meniscus between the tip and the substrate16 in the conventional cantilever-based AFM system under ambient conditions. The mechanical properties (i.e., the viscoelasticity) of the target materials that are exerted on the apex of the tip can be interpreted using the amplitude modulation (AM)17, 18 and frequency modulation (FM)19 modes of QTF-AFM based on an approximate model of a damped harmonic oscillator. In our experiment, we used a nanopipette/QTF-AFM that serves for small oscillation (50 nm) at distances of less than 10 nm from the surface. Thus, the pulled pipette (total length of ∼5 cm, length from taper starting position to the apex of ∼0.5 mm) that was tightly mounted on the pipette mounting stage is perpendicularly and precisely attached to a single prong of the QTF above the substrate, with the angles being checked via the x- and y-axis CCD cameras. Thus, the inertia of the inserted lead-wires used to apply the electric field, which is induced by movement of the tip during scanning and nanofabrication, has no effect on the QTF sensor’s resonance and phase signals. As a result, the initially detected QTF reference signal is sustained throughout the experiment. The length of the scanning end from the apex of the nanopipette to the contact point is about 300–500 μm, and the length above the QTF is approximately 2–3 mm from the pipette-QTF contact point to the end of the pipette mounting. The Q-factor is easily controlled by changing the length of the scanning end. As the length increases, then the Q-factor of the QTF sensor decreases, with an accompanying effect on the scanning images in the AM mode case of AFM.25, 26 In the AM-mode AFM case, a high Q-factor (>1000) induces a ringing effect and increments in the scanning time with high sensitivity. This is because the changed amplitude signal at the tip needs time to settle down to the next position through interaction forces. At a scanning end length of 300–500 μm with the nanopipette attachment, the Q-factor obtained is 4000–7000, which indicates high force detection sensitivity, but long scanning times, and indicates
Rev. Sci. Instrum. 85, 033702 (2014)
the need for a trade-off between these results. Thus, the Qfactor is reduced to less than 1000 by increments in the scanning end value. To overcome this issue, we used the FM-mode AFM system to obtain the AFM images, which allows clear AFM images to be acquired with high Q (4000–7000 for a scanning end length of 300–500 μm) and high scanning speed because of in situ high speed detection of the changed frequency caused by interactions. In summary, simple contact of the pipette on the QTF with length, tilt angle and pipette contact spot diameter dependencies easily achieves control of the Q-factor, which is generally addressed by use of an additional electrical feedback circuit, as in active-Q control techniques. Most of the FMmode AFM scanning and liquid-delivered nanofabrication processes are performed with high Q-factors for the QTF sensors in the cases of short scanning end lengths (300–500 μm), small tilt angles (0◦ –2◦ ), and 30–300 nm pipette apertures, which provide reliable results and high system performance, including high resolution (∼50 nm) and small-size patterning (∼60 nm) with reasonable mechanical interpretations. Note that the spring constant of the QTF (∼32 kHz) is small enough to have little effect on the modification of the soft target matter on the surface during scanning. Also, non-contact mode AFM is enabled, and thus the tip and the topography can be maintained after several scanning processes. We scanned various samples using the proposed nanopipette-based shearmode QTF-AFM system, and used a quartz pipette with high stiffness to avoid apex percussion, and an aperture diameter of about 30 nm with high resolution capability. Figure 6 shows the scanned shear-mode topographical and force error images for three test samples ((a) a CD-R, (b) a hard disk, and (c) 50-nm polystyrene nanospheres) acquired using the proposed nanopipette/QTF-AFM system, which operates in FM-mode under ambient conditions. The frequency shift (F) was used as the main signal to maintain the distance from the surface. Even when the tip has a high Q-factor (∼6,000), F can follow the fast scanning feedback system using self-oscillation and all-digital phase locked loop (ADPLL) frequency detection methods with avoidance of the ringing effects that occur in AM-mode AFM. In addition, a digital signal processing (DSP) circuit was used for fast feedback with a commercial piezoelectric transducer (PZT). As a result, the surface characterization is clearly defined, and the scan speeds are approximately (a) 1.5 μm/s, (b) 600 nm/s, and (c) 1 μm/s. The resolution can be increased by optimizing the experimental conditions for the various environments. D. Liquid delivery/nanopatterning and in situ scanning probe microscopy
For nanolithography and nanoscale object fabrication, the low volume of liquid delivery onto the substrate is an important issue in solution-based lithography.27 The most useful advantage of the nanopipette/QTF-AFM system is the low volume liquid delivery through the nanoaperture of the nanopipette and nanopattern formation in the residues by evaporation of the liquid, while accurate distance control is enabled by the AFM system. Figures 7(a) and 7(b) show the liquid filling process for the pulled nanopipette. After the
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FIG. 7. Liquid-filled nanopipette. (a) Insertion of the bottom of the pipette into the liquid starts the apex filling process. (b) A capillary force is exerted between the inner wall and the internal filament to fill the apex. (c) Image of the liquid-filled nanopipette apparatus combined with the QTF-AFM on the substrate.
FIG. 6. Scanned shear-mode topographical and force error images for three samples acquired using the proposed nanopipette/QTF-AFM system. (a) a CD-R; (b) an 850-MB hard disk; (c) 50-nm polystyrene nanospheres.
nanopipette is pulled by the machine, the bottom of the pipette is then inserted into the liquid solution. The liquid then climbs to the apex of the pipette under the capillary force exerted between the inner wall of the pipette and the internal filament that is pre-installed inside the pipette, and starts to fill the pipette from the apex (1 → 8). After the filling process is complete, the liquid-filled nanopipette is placed on the spot for the experiment by attachment to the QTF’s prong, as shown in Fig. 7(c). Note that the pipette tip is set perpendicularly against the surface, but the QTF sensor itself is not located parallel to the surface, and has a slight tilt angle (20◦ –30◦ ) because of the limitations of the QTF’s edge shape for escaping pre-contact with the edge of the QTF rather than the tip [see Fig. 7(c)]. We have tested the angle dependence of the QTF’s location against the surface. As a result, we cannot observe any critical differences in the results (force spectroscopy/microscopy) between the different tilt angles; this is because the sensed QTF output signals of variations in the QTF location angle are similar to each other in only detecting the lateral interaction force, unless the angle of the pipette tip is tilted. After the liquid-filled nanopipette approaches the surface under application of an electric field between the liquid inside the pipette and the surface, the nanopatterning processes are performed by the ejection of small volumes of Au nanoparticle (5 ± 0.4) nm solutions onto the surface, while the liq-
uid evaporates into the air. Figure 8 shows the results of the nanopatterning processes performed using the proposed system. The AFM images are obtained in situ by the proposed system using the same nanopipette tip after the fabrication process is complete. The nanoscale line patterns with line widths of ∼60 nm were fabricated using ∼30-nm-apertured nanopipettes (Fig. 8(a)). The broadening of the line width when compared with the aperture diameter may be caused by continuous flow of the ejected liquid through the 30-nm aperture. The dot patterns (∼200 nm diameter) are fabricated by sudden retraction of the tip from the surface after the liquid solution is ejected by an electric field from inside the ∼200nm-apertured pipette (Fig. 8(b)). Note that the white single line crossing dots are only intended as a visual guide to define the z-axis profile. Figures 8(c)–8(e) show the resulting vertically grown nanoparticle-aggregated nanowires formed using the proposed system with a 5-nm Au nanoparticle solutionfilled nanopipette. As the retraction speed of the nanopipette tip decreases, the dot pattern evolves into nanoscale vertical wires by stacking of the nanoparticles. Figure 8(c) shows the fabricated nanowires, which have relatively short lengths of ∼800 nm and diameters of ∼200 nm. The white line is intended to guide the eyes to define the line of the z-axis profile. All AFM scan images are obtained in situ using the proposed system with the same nanopipette tip, after the fabrication process is completed. Figure 8(d) shows the dependences of the various fabricated nanowire diameters on the nanopipette aperture size. The diameters of the fabricated nanowires followed the aperture diameters of the pulled nanopipette. Figure 8(e) shows an SEM image of vertically grown nanoparticle-aggregated nanowires with a length of ∼3.3 μm and a diameter of ∼300 nm. Progress is also being made in other fabrication approaches with the realization of a threedimensional (3D) printing technique and investigation of the nanoscale 3D molecular architecture.
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FIG. 8. Nanofabrication using the system. (a),(b) AFM images of line and dot patterns using the 5-nm Au nanoparticle solution (the white line indicates the z-axis profile). (c) AFM image of vertically grown nanoparticle-aggregated nanowires (short lengths). (d) Aperture dependence of the pipette. (e) SEM image of the fabricated nanowire (long length).
IV. CONCLUSION
We have introduced the nanopipette/QTF-AFM system along with experimental and theoretical investigations of the processed nanoscale materials. The system has several advantages over conventional cantilever and QTF-based AFM methods, such as a simple Q-factor control based on the QTF contact position, easy variation of the effective tip diameter via the pulling parameter, electrical detection, and ondemand delivery of various solutions for fabrication. Shear mode AFM images can also be obtained after nanofabrication. Using this tool, we can realize low-volume liquid delivery and nanofabrication processes without damaging the apex of the tip in various environments. ACKNOWLEDGMENTS
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (Grant no. 2013-056344), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant no. 2013R1A6A3A03063900), and Brain Korea 21. Wonho Jhe acknowledges the support of the Center for Nano-Liquid. 1 J.
D. Piper, R. W. Clarke, Y. E. Korchev, L. Ying, and D. Klenerman, J. Am. Chem. Soc. 128, 16462–16463 (2006). 2 M. Böcker, B. Anczykowski, J. Wegener, and T. E. Schäffer, Nanotechnology 18, 145505 (2007). 3 A. Bruckbauer, P. James, D. Zhou, J. W. Yoon, D. Excell, Y. Korchev, R. Jones, and D. Klenerman, Biophys. J. 93, 3120–3131 (2007). 4 Q. Li, S. Xie, Z. Liang, X. Meng, S. Liu, H. Girault, and Y. Shao, Angew. Chem. 121, 8154–8157 (2009). 5 L. Ying, Biochem. Soc. Trans. 37, 702–706 (2009).
6 M.
G. Schrlau, E. M. Falls, B. L. Ziober, and H. H. Bau, Nanotechnology 19, 015101 (2008). 7 M.-H. Hong, K. H. Kim, J. Bae, and W. Jhe, Appl. Phys. Lett. 77, 2604– 2606 (2000). 8 A. Meister, M. Gabi, P. Behr, P. Studer, J. Vörös, P. Niedermann, J. Bitterli, J. Polesel-Maris, M. Liley, H. Heinzelmann, and T. Zambelli, Nano Lett. 9, 2501–2507 (2009). 9 A. De Leebeeck and D. Sinton, Electrophoresis 27, 4999–5008 (2006). 10 X. Xuan, Electrophoresis 29, 3737–3743 (2008). 11 F. J. Giessibl, Science 267, 68–71 (1995). 12 J. A. Hedberg, A. Lal, Y. Miyahara, P. Grütter, G. Gervais, M. Hilke, L. Pfeiffer, and K. W. West, Appl. Phys. Lett. 97, 143107 (2010). 13 J. Polesel-Maris, C. Lubin, F. Thoyer, and J. Cousty, J. Appl. Phys. 109, 074320 (2011). 14 L. González, J. Otero, G. Cabezas, and M. Puig-Vidal, Sens. Actuators, A 184, 112–118 (2012). 15 J. Welker, F. de Faria Elsner, and F. J. Giessibl, Appl. Phys. Lett. 99, 084102 (2011). 16 H. Choe, M.-H. Hong, Y. Seo, K. Lee, G. Kim, Y. Cho, J. Ihm, and W. Jhe, Phys. Rev. Lett. 95, 187801 (2005). 17 M. Lee, J. Jahng, K. Kim, and W. Jhe, Appl. Phys. Lett. 91, 023117 (2007). 18 M. Lee, B. Sung, N. Hashemi, and W. Jhe, Faraday Discuss. 141, 415–421 (2009). 19 S. An, J. Kim, K. Lee, B. Kim, M. Lee, and W. Jhe, Appl. Phys. Lett. 101, 053114 (2012). 20 A. Castellanos-Gomez, N. Agraït, and G. Rubio-Bollinger, Ultramicroscopy 111, 186–190 (2011). 21 A. Naber, J. Microsc. 194, 307 (1999). 22 J. Jahng, M. Lee, H. Noh, Y. Seo, and W. Jhe, Appl. Phys. Lett. 91, 023103 (2007). 23 J. Jahng, M. Lee, C. Stambaugh, W. Bak, and W. Jhe, Phys. Rev. A 84, 022318 (2011). 24 S. An, M.-H. Hong, J. Kim, S. Kwon, K. Lee, M. Lee, and W. Jhe, Rev. Sci. Instrum. 83, 113705 (2012). 25 M. Minary-Jolandan, A. Tajik, N. Wang, and M.-F. Yu, Nanotechnology 23, 235704 (2012). 26 L. Chen, X. Yu, and D. Wang, Ultramicroscopy 107, 275–280 (2007). 27 S. An, C. Stambaugh, G. Kim, M. Lee, Y. Kim, K. Lee, and W. Jhe, Nanoscale 4, 6493–6500 (2012).
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 033702 (2014)
Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication Sangmin An,a) Kunyoung Lee, Bongsu Kim, Haneol Noh, Jongwoo Kim, Soyoung Kwon, Manhee Lee,b) Mun-Heon Hong,c) and Wonho Jhed) Department of Physics and Astronomy, Center for Nano-Liquid, Seoul National University, Seoul 151-747, South Korea
(Received 26 September 2013; accepted 10 February 2014; published online 4 March 2014) 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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4866656] I. INTRODUCTION
The nanopipette is a versatile tool for biological research applications, including sensors, scanning ion conductance microscopy (SICM), and cell treatments.1–5 In particular, it is useful for the precise injection of nanomaterials into a target cell for vital reactions.6 Recently, researchers have investigated nanoscale patterning using the atomic force microscope (AFM) system in combination with the nanopipette;7, 8 however, the process is complex and difficult to handle along with the necessary optical apparatus. In addition, most nanopipettes are used in liquid environments with soft materials because of the effects of surface impact on the apex of the tip, and the aperture diameter is limited to more than 100 nm for high surface tension near the apex of the pipette. Here, we introduce a nanopipette in combination with a quartz tuning fork-atomic force microscope system (nanopipette/QTFAFM) along with experimental and theoretical investigations of the nanoscale materials under ambient conditions using a simple electrical detection method and with no surface impact on the nanopipette’s apex. We also describe systematic experimental studies of the pulled nanopipette (elongated length, tilting angle based on the y-axis pushing force, and the x-axis contact region), quantitative force spectroscopy (a confined nano-water experiment), and atomic force microscopy (surface imaging of a CD-R, a hard disk, and 50-nm polystyrene nanospheres), along with a demonstration of nanofabrication processing involving nanomaterial solution delivery through the nanopipette aperture. The proposed tool has several advantages, including in situ surface characterization after nanofabrication, simple attachment of the a) Present address: Center for Nanoscale Science and Technology, National
Institute of Standards and Technology, Gaithersburg, MD 20899.
b) Present address: S. LSI, Giheung Campus, Samsung Electronics, Seoul
(Gyeonggi-do) 446-711, South Korea.
c) Present address: LG Electronics Advanced Research Institute, Seoul 137-
724, South Korea.
d) Electronic mail: [email protected]
0034-6748/2014/85(3)/033702/7/$30.00
AFM tip to the edge of the QTF sensor’s single prong, easy control of the quality (Q) factor, and simple variation of the tip aperture size via the mechanical puller’s control parameters. II. NANOPIPETTE/QTF-AFM
Figures 1(a) and 1(b) show a picture and a schematic of the proposed nanopipette/QTF-AFM system, respectively. The QTF (resonance of ∼32 kHz) and the pencil-shaped nanopipette combine to produce a functional liquid solution delivery along with force spectroscopy/microscopy capabilities. The fabricated pipette tip is perpendicularly attached to one prong of the QTF to oscillate laterally so that it senses the capillary force from the confined nano-water meniscus alone, and is then brought close to the surface. A. Nanopipette
Figure 1(c) shows a schematic diagram of the process used to fabricate the pulled nanopipette, which is mechanically pulled when a CO2 laser melts the center section of a pipette via a commercial puller (P-2000, Sutter Instrument, Co., Novato, CA, USA). The pulled nanopipettes are generally produced from three materials: borosilicate, quartz, and aluminosilicate. The borosilicate and quartz pipettes, with outer and inner diameters of 1 mm and 0.7 mm, respectively, are found in the experiments to suffer the least degradation in the Q-factor of their tips, even when attached to the QTF. The puller has five different control parameter settings: the laser temperature (HEAT; example settings: borosilicate: 350; quartz: 700), the beam spot diameter (FIL, which is generally fixed at 4), the pulling velocity (VEL), the delay time after melting by the laser (DEL; the taper angle is varied by control of this parameter), and the pulling power (PUL). We controlled the aperture sizes of the pulled nanopipettes by varying the PUL parameter. When the value of PUL increases, the aperture size decreases, and the smallest aperture size achieved was approximately 15 nm. The aperture size can be
85, 033702-1
© 2014 AIP Publishing LLC
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An et al.
Rev. Sci. Instrum. 85, 033702 (2014)
tion of the tip parallel to the surface indicates the detection of the lateral interaction force induced by the formation of a nanoscale confined water meniscus, while ignoring the various perpendicularly exerted forces, as the tip approaches the surface. We calculated the mechanical responses of the QTF (effective elasticity, kint , and damping, bint ) by solving for a harmonic motion with respect to the oscillation amplitude (A) and phase (θ ) of the QTF sensor when externally driven by a function generator.
III. RESULTS AND DISCUSSIONS A. Systematic investigation of quantitative force measurement
FIG. 1. Nanopipette/QTF-AFM system. (a) Piezoelectric device of QTF combined with the pulled nanopipette. (b) Schematic of the system. The nanopipette is perpendicularly attached to the edge of the QTF sensor prong. (c) Schematic of pipette pulling process. The glass pipette (inner diameter of 0.7 mm, outer diameter of 10 mm) is mechanically pulled when the CO2 laser melts the center.
reduced further by optimizing all the process parameters. Various pulled nanopipette aperture sizes were produced in the experiment. For example, aperture sizes of ∼30, 50, and 100 nm were produced by PUL parameter settings of 190, 180, and 170, respectively, when the other four parameters were fixed as HEAT = 350, FIL = 4, VEL = 20, and DEL = 125. The aperture sizes were determined using a typical electrical current measurement system using artificial sea water (ASW) and scanning electron microscopy (SEM) images. The measured resistance (∼35 M) from the I V curve (gain of 107 for I V -converter) corresponds to the 30-nm aperture.9, 10
Figures 2 and 3 show the experimental conditions and the results from our systematic approaches to define the proposed nanopipette/QTF-AFM system as a force sensor. Figures 2(a) and 2(b) show the dependence of the attached length of the nanopipette on the contact position with the QTF on a single prong’s edge for thin and thick diameter pipettes. Figures 2(c) and 2(d) show the dependence of the tilt angle on the pushing force on the y-axis and the x-axis contact regions, respectively. As a result, the tip resonance frequency decreases with decreasing Q-factor as the length of the attached pipette from the contact position on the QTF increases, as shown in Fig. 3(a). This indicates that attractive and damping forces are exerted on the QTF sensor because of the widening of the pipette diameter at the contact area and the increment in the effective stiffness of the contacted pipette under the influence of the √ effective mass loading effect on the QTF edge (f ∝ 1/2π (k + δk)/(m + δm)) and the imbalance of the center of mass.20, 21 A short attachment length causes the smallest degradation of the Q-factors, which are less than the intrinsic QTF in cases 1–3 of Fig. 3(a). However, the Q-factor decreased drastically when the elongated
B. QTF-AFM
The QTF, which is a piezoelectric device with mechanical resonance, can be used as a high-sensitivity force sensor with a simple electrical detection system in various environments, including vacuum, ambient, and liquid environments.11–15 One useful advantage of the QTF in the AFM system is that it avoids a jump-to-contact problem that occurs with high stiffness (103 ∼ 104 N/m), where the problem is caused by formation of a confined nanoscale water meniscus between the tip and the substrate16 in the conventional cantilever-based AFM system under ambient conditions. The mechanical properties (i.e., the viscoelasticity) of the target materials that are exerted on the apex of the tip can be interpreted using the amplitude modulation (AM)17, 18 and frequency modulation (FM)19 modes of QTF-AFM based on an approximate model of a damped harmonic oscillator. In our experiment, we used a nanopipette/QTF-AFM that serves for small oscillation (50 nm) at distances of less than 10 nm from the surface. Thus, the pulled pipette (total length of ∼5 cm, length from taper starting position to the apex of ∼0.5 mm) that was tightly mounted on the pipette mounting stage is perpendicularly and precisely attached to a single prong of the QTF above the substrate, with the angles being checked via the x- and y-axis CCD cameras. Thus, the inertia of the inserted lead-wires used to apply the electric field, which is induced by movement of the tip during scanning and nanofabrication, has no effect on the QTF sensor’s resonance and phase signals. As a result, the initially detected QTF reference signal is sustained throughout the experiment. The length of the scanning end from the apex of the nanopipette to the contact point is about 300–500 μm, and the length above the QTF is approximately 2–3 mm from the pipette-QTF contact point to the end of the pipette mounting. The Q-factor is easily controlled by changing the length of the scanning end. As the length increases, then the Q-factor of the QTF sensor decreases, with an accompanying effect on the scanning images in the AM mode case of AFM.25, 26 In the AM-mode AFM case, a high Q-factor (>1000) induces a ringing effect and increments in the scanning time with high sensitivity. This is because the changed amplitude signal at the tip needs time to settle down to the next position through interaction forces. At a scanning end length of 300–500 μm with the nanopipette attachment, the Q-factor obtained is 4000–7000, which indicates high force detection sensitivity, but long scanning times, and indicates
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the need for a trade-off between these results. Thus, the Qfactor is reduced to less than 1000 by increments in the scanning end value. To overcome this issue, we used the FM-mode AFM system to obtain the AFM images, which allows clear AFM images to be acquired with high Q (4000–7000 for a scanning end length of 300–500 μm) and high scanning speed because of in situ high speed detection of the changed frequency caused by interactions. In summary, simple contact of the pipette on the QTF with length, tilt angle and pipette contact spot diameter dependencies easily achieves control of the Q-factor, which is generally addressed by use of an additional electrical feedback circuit, as in active-Q control techniques. Most of the FMmode AFM scanning and liquid-delivered nanofabrication processes are performed with high Q-factors for the QTF sensors in the cases of short scanning end lengths (300–500 μm), small tilt angles (0◦ –2◦ ), and 30–300 nm pipette apertures, which provide reliable results and high system performance, including high resolution (∼50 nm) and small-size patterning (∼60 nm) with reasonable mechanical interpretations. Note that the spring constant of the QTF (∼32 kHz) is small enough to have little effect on the modification of the soft target matter on the surface during scanning. Also, non-contact mode AFM is enabled, and thus the tip and the topography can be maintained after several scanning processes. We scanned various samples using the proposed nanopipette-based shearmode QTF-AFM system, and used a quartz pipette with high stiffness to avoid apex percussion, and an aperture diameter of about 30 nm with high resolution capability. Figure 6 shows the scanned shear-mode topographical and force error images for three test samples ((a) a CD-R, (b) a hard disk, and (c) 50-nm polystyrene nanospheres) acquired using the proposed nanopipette/QTF-AFM system, which operates in FM-mode under ambient conditions. The frequency shift (F) was used as the main signal to maintain the distance from the surface. Even when the tip has a high Q-factor (∼6,000), F can follow the fast scanning feedback system using self-oscillation and all-digital phase locked loop (ADPLL) frequency detection methods with avoidance of the ringing effects that occur in AM-mode AFM. In addition, a digital signal processing (DSP) circuit was used for fast feedback with a commercial piezoelectric transducer (PZT). As a result, the surface characterization is clearly defined, and the scan speeds are approximately (a) 1.5 μm/s, (b) 600 nm/s, and (c) 1 μm/s. The resolution can be increased by optimizing the experimental conditions for the various environments. D. Liquid delivery/nanopatterning and in situ scanning probe microscopy
For nanolithography and nanoscale object fabrication, the low volume of liquid delivery onto the substrate is an important issue in solution-based lithography.27 The most useful advantage of the nanopipette/QTF-AFM system is the low volume liquid delivery through the nanoaperture of the nanopipette and nanopattern formation in the residues by evaporation of the liquid, while accurate distance control is enabled by the AFM system. Figures 7(a) and 7(b) show the liquid filling process for the pulled nanopipette. After the
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FIG. 7. Liquid-filled nanopipette. (a) Insertion of the bottom of the pipette into the liquid starts the apex filling process. (b) A capillary force is exerted between the inner wall and the internal filament to fill the apex. (c) Image of the liquid-filled nanopipette apparatus combined with the QTF-AFM on the substrate.
FIG. 6. Scanned shear-mode topographical and force error images for three samples acquired using the proposed nanopipette/QTF-AFM system. (a) a CD-R; (b) an 850-MB hard disk; (c) 50-nm polystyrene nanospheres.
nanopipette is pulled by the machine, the bottom of the pipette is then inserted into the liquid solution. The liquid then climbs to the apex of the pipette under the capillary force exerted between the inner wall of the pipette and the internal filament that is pre-installed inside the pipette, and starts to fill the pipette from the apex (1 → 8). After the filling process is complete, the liquid-filled nanopipette is placed on the spot for the experiment by attachment to the QTF’s prong, as shown in Fig. 7(c). Note that the pipette tip is set perpendicularly against the surface, but the QTF sensor itself is not located parallel to the surface, and has a slight tilt angle (20◦ –30◦ ) because of the limitations of the QTF’s edge shape for escaping pre-contact with the edge of the QTF rather than the tip [see Fig. 7(c)]. We have tested the angle dependence of the QTF’s location against the surface. As a result, we cannot observe any critical differences in the results (force spectroscopy/microscopy) between the different tilt angles; this is because the sensed QTF output signals of variations in the QTF location angle are similar to each other in only detecting the lateral interaction force, unless the angle of the pipette tip is tilted. After the liquid-filled nanopipette approaches the surface under application of an electric field between the liquid inside the pipette and the surface, the nanopatterning processes are performed by the ejection of small volumes of Au nanoparticle (5 ± 0.4) nm solutions onto the surface, while the liq-
uid evaporates into the air. Figure 8 shows the results of the nanopatterning processes performed using the proposed system. The AFM images are obtained in situ by the proposed system using the same nanopipette tip after the fabrication process is complete. The nanoscale line patterns with line widths of ∼60 nm were fabricated using ∼30-nm-apertured nanopipettes (Fig. 8(a)). The broadening of the line width when compared with the aperture diameter may be caused by continuous flow of the ejected liquid through the 30-nm aperture. The dot patterns (∼200 nm diameter) are fabricated by sudden retraction of the tip from the surface after the liquid solution is ejected by an electric field from inside the ∼200nm-apertured pipette (Fig. 8(b)). Note that the white single line crossing dots are only intended as a visual guide to define the z-axis profile. Figures 8(c)–8(e) show the resulting vertically grown nanoparticle-aggregated nanowires formed using the proposed system with a 5-nm Au nanoparticle solutionfilled nanopipette. As the retraction speed of the nanopipette tip decreases, the dot pattern evolves into nanoscale vertical wires by stacking of the nanoparticles. Figure 8(c) shows the fabricated nanowires, which have relatively short lengths of ∼800 nm and diameters of ∼200 nm. The white line is intended to guide the eyes to define the line of the z-axis profile. All AFM scan images are obtained in situ using the proposed system with the same nanopipette tip, after the fabrication process is completed. Figure 8(d) shows the dependences of the various fabricated nanowire diameters on the nanopipette aperture size. The diameters of the fabricated nanowires followed the aperture diameters of the pulled nanopipette. Figure 8(e) shows an SEM image of vertically grown nanoparticle-aggregated nanowires with a length of ∼3.3 μm and a diameter of ∼300 nm. Progress is also being made in other fabrication approaches with the realization of a threedimensional (3D) printing technique and investigation of the nanoscale 3D molecular architecture.
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FIG. 8. Nanofabrication using the system. (a),(b) AFM images of line and dot patterns using the 5-nm Au nanoparticle solution (the white line indicates the z-axis profile). (c) AFM image of vertically grown nanoparticle-aggregated nanowires (short lengths). (d) Aperture dependence of the pipette. (e) SEM image of the fabricated nanowire (long length).
IV. CONCLUSION
We have introduced the nanopipette/QTF-AFM system along with experimental and theoretical investigations of the processed nanoscale materials. The system has several advantages over conventional cantilever and QTF-based AFM methods, such as a simple Q-factor control based on the QTF contact position, easy variation of the effective tip diameter via the pulling parameter, electrical detection, and ondemand delivery of various solutions for fabrication. Shear mode AFM images can also be obtained after nanofabrication. Using this tool, we can realize low-volume liquid delivery and nanofabrication processes without damaging the apex of the tip in various environments. ACKNOWLEDGMENTS
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (Grant no. 2013-056344), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant no. 2013R1A6A3A03063900), and Brain Korea 21. Wonho Jhe acknowledges the support of the Center for Nano-Liquid. 1 J.
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