REVIEW OF SCIENTIFIC INSTRUMENTS 86, 053703 (2015)
Fast and reliable method of conductive carbon nanotube-probe fabrication for scanning probe microscopy Vyacheslav Dremov,1,2,a) Vitaly Fedoseev,1 Pavel Fedorov,1,2 and Artem Grebenko1,2 1
Institute of Solid State Physics, RAS, 142432 Chernogolovka, Russian Federation Interdisciplinary Center for Basic Research, Moscow Institute of Physics and Technology, 141700 Dolgoprudniy, Russian Federation 2
(Received 27 June 2014; accepted 7 May 2015; published online 21 May 2015) We demonstrate the procedure of scanning probe microscopy (SPM) conductive probe fabrication with a single multi-walled carbon nanotube (MWNT) on a silicon cantilever pyramid. The nanotube bundle reliably attached to the metal-covered pyramid is formed using dielectrophoresis technique from the MWNT suspension. It is shown that the dimpled aluminum sample can be used both for shortening/modification of the nanotube bundle by applying pulse voltage between the probe and the sample and for controlling the probe shape via atomic force microscopy imaging the sample. Carbon nanotube attached to cantilever covered with noble metal is suitable for SPM imaging in such modulation regimes as capacitance contrast microscopy, Kelvin probe microscopy, and scanning gate microscopy. The majority of such probes are conductive with conductivity not degrading within hours of SPM imaging. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4921323]
I. INTRODUCTION
The problem of the appropriate probe fabrication for nanostructures investigation emerged together with the origin of scanning probe microscopy (SPM). One can consider a rigid high aspect ratio probe having a single atom at its tip to be an ideal one.1,2 However, common commercially available probes for atomic force microscope (AFM) are made of silicon with a tip radius of approximately 10 nm, and probes with tips of few nanometers in diameter are considered to be extremely sharp.2 Silicon cantilevers possess a number of shortcomings, the fragility of tip apex and its degradation while scanning (probe “wearing”) being the most noticeable ones. To tackle these drawbacks, carbon nanotube (CNT) probe was developed.3–5 Generally, the diameter of multi-walled carbon nanotubes (MWNTs) is in the range of 5-30 nm making MWNT probes not worse than silicon ones in terms of tip radius and having much higher aspect ratio. In general, CNT probes demonstrate the following advantages over Si probes: • CNT probes are more durable, preserve their shape while imaging6 thus and so under the excess load the CNT will bend while restoring its initial shape after the load is removed;3 • CNT probe mounted/attached on a metal-covered cantilever possesses sufficient conductivity to be used in scanning gate microscopy, scanning Kelvin probe microscopy, or to measure local current-voltage characteristics.7 Metal-covered silicon cantilever often experiences substantial conductivity degradation during scanning due to wearing of the metal film on its apex while the CNT probe remains conductive during hours of continuous scanning. a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2015/86(5)/053703/5/$30.00
A simple and robust method of attaching a CNT bundle to the cantilever pyramid was developed exploiting dielectrophoresis phenomenon, pulling the tip out of CNT suspension in water.8,9 Furthermore, it was shown3,10 that by applying pulse voltage between the CNT probe and a conducting sample, it is possible to shorten the final nanotube in a controllable way tens nanometers per pulse to achieve desired probe length (rigidity). Knowledge of the probe shape may provide useful information while analyzing a SPM image. The most reliable instrument for that is scanning electron microscope or transmission electron microscope (SEM/TEM). However, frequently, it is more convenient to obtain this information directly in SPM taking into account that probe shape may change while imaging. Scanning a known-beforehand test-structure provides such probe characteristics as its apex radius and aspect ratio. It is been revealed1 that a rigid sharp protrusion on a flat surface is an optimal test-structure to restore the tip surface function. However, fabrication of the protrusion with radius at its tip smaller or comparable to the sharp SPM probe is a complex technological task.1 In Refs. 11 and 12, AFM methods are used to study alumina obtained by anodization. It is shown that carefully selecting anodization parameters, a set of alumina peaks with radius at apexes of about 10 nm can be obtained. As the interface between the alumina and aluminum has a developed structure, the surface of aluminum after selective alumina removal (the dimpled aluminum) is shown to be a suitable test-structure for tip shape characterization.13 The radius of peaks on the surface was estimated to be 1-2 nm. Under some anodization parameters, the surface of the dimpled aluminum is a set of dimples ordered in a hexagonal structure, depicted schematically in Figure 1. At the intersection points of three neighboring walls of the dimples, peaks with radius less than 10 nm are formed. Such highly ordered structure can be observed after selective removal of alumina14 as shown in Figure 1(b).
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FIG. 1. Processing of dimpled aluminum. (a) Porous alumina obtained by anodization of aluminum. (b) Dimpled aluminum produced by selective removal of alumina.
Typical lattice constant is approximately 100 nm, the height difference between the peak apex and the dimple bottom (peak height) is 20-50 nm (the lattice constant and peaks height depend on the anodization parameters). Scanning such an object in AFM reveals probe shape information about its last 20-50 nm. Herein, we describe a relatively simple procedure of the MWNT probe fabrication with a single nanotube at its end on the pyramid of a silicon cantilever applying dielectrophoresis technique to the MWNT water suspension. To transform the CNT bundle attached to the pyramid into a probe suitable for SPM imaging, we use the dimpled aluminum sample as a test-structure for tip shape determination as well as for the CNT bundle shortening/modification by applying pulse voltage. The probe produced in this way ends with a single nanotube reliably attached to the cantilever capable of AFM imaging with resolution not worse than that of the silicon cantilever. If Si cantilevers are metal-covered, the majority of the CNT probes produced on them turn out to be conductive.
II. EXPERIMENTAL METHODS A. CNT bundle attachment
To attract a CNT bundle on a cantilever pyramid, the method of dielectrophoresis deposition was chosen. The scheme of our setup is shown in Figure 2(a). It was decided to use commercially available MWNTs produced by electric arc
discharge method (Sigma-Aldrich, Germany). Each nanotube consists of 5-20 graphitic layers being 1-10 µm long and 5-20 nm in diameter. The product contains approximately 10%-40% tubes, the remainder being multi-layer polygonal, amorphous, and graphitized carbon nanoparticles. To prepare a MWNT suspension, 50 mg of MWNTcontaining substance was dispersed in 5 ml of distilled water using ultra-sonic bath. A droplet of the suspension was placed on a platinum wire ring (ring diameter 2 mm and wire diameter 0.25 mm). An AC voltage of 5-10 V, 2 MHz was applied between the wire and the cantilever. Under an optical control, the tip of the cantilever pyramid was immersed into the suspension using an XYZ manipulator 2-5 µm deep and slowly pulled out. The dielectrophoresis force drags CNTs towards the tip, forming a CNT bundle 1-10 µm long reliably attached to the tip (Figure 2(b)). Study of the suspension revealed that conglomerates of nanotubes were still present after dispersing. Likely, these conglomerates were attached to the tip rather than the pure bundle was assembled of individual nanotubes, which might be a reason that the bundle grasped the pyramid similarly to “roots of trees.” Forces of surface tension in the meniscus of suspension around the tip align nanotubes into the bundle. The angle between the pyramid axis and the CNT bundle is defined by joint orientation of the pyramid and surface of the suspension and has distribution of less than ±12◦.8 In some cases, the bundle was not formed or appeared to be not visible in the 400× optical microscope during the first try. Repeating
FIG. 2. CNT bundle attachment. (a) Scheme (top view) of a setup for a MWNT bundle formation on the tip of a cantilever. (b) Acquired 400× magnified optical image of four stages of the bundle formation.
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To be assured that the protruding nanotube is not chemically modified, it was shortened by Joule heat. B. Formation and characterization of the tip
FIG. 3. SEM image of the dimpled aluminum sample reveals long-range order. Inset shows magnified part of the same structure.
the procedure, one can succeed in forming the CNT bundle. Investigation of bundles obtained this way in SEM revealed that a bundle of 200 nm in diameter was already visible in the optical microscope. As soon as the probe is formed, one should anneal it at 120-130 ◦C within 30 min to remove the water from the bundle. It is done to avoid complete bundle destruction during further electrochemical etching and pulse-voltage modification described below. AFM imaging using CNT probes showed that in most cases, the probes possessed longitudinal or transverse instability, if the bundle was 10 µm long or more. For this reason, just after the bundle formation, its length was estimated in the optical microscope. If the visual length was more than 3 µm (as was in most cases), the bundle was shortened using electro-chemical etching in the same setup where we had the second wire ring to keep a droplet of KOH. Applying 4 V, 0.01-1 ms pulses, it is possible to shorten the bundle length in steps of 1 µm and more by etching immersed part of the CNT probe. SEM imaging of etched bundles showed that it extended 2-5 µm from the pyramid and in most cases ended with a single nanotube with length of up to few micrometers.
The next step is to check AFM imaging capabilities of the CNT probe. For that purpose, dimpled aluminum was chosen as a test object (see Fig. 1(b)). In our work, the dimpled aluminum sample was provided by Moscow State University’s chemical department. The further study of the test-structure in AFM revealed that its lattice constant is 105 ± 5 nm with dimples’ depth in range of 25-35 nm. Basing on the SEM image (Figure 3), one can predict what AFM scan with ready CNT probe should look like. If the tip is too long or has multiple apex (Figure 4(a)) in order to improve the tip’s quality, the probe should be shortened/modified. To do it in a more controllable way than electrochemical etching, pulse voltage (30-50 V, 10-100 µs) was applied between the cantilever and the dimpled aluminum sample. The current was limited by a resistor of 1 MΩ inserted in series. Using this method, the probe length was shortened by 50-500 nm per pulse depending on the pulse parameters with feedback turned on in the semi-contact mode. Typically, few pulses were sufficient to gain required rigidity and single apex of the MWNT probe. Dimpled aluminum test-structure has two useful features. The first one is that it allows to perform express evaluation of an AFM probe shape. Due to regular arrangement of sharp peaks, we can estimate probe radius so there is no necessity to directly investigate it in SEM/TEM. The second one is that at same time, one can shorten the CNT bundle which significantly decreases the overall time spent on the CNT probe fabrication. Process of shortening/modification being the most time consuming task usually takes not more than 10 min according to our experience. In Figure 5, typical MWNT probe ready for AFM imaging is shown. Ready MWNT probes contain high amount of amorphous carbon and many nanotubes which are not aligned to the probe axis (Figure 5). To obtain CNT probes without
FIG. 4. Evolution of AFM scans during sharpening. (a) AFM scan (256 × 256 points, 1.5 × 1.5 µm, 1 µm/s) of the dimpled aluminum sample revealing multiple apex probe. The height of the peaks is 15 nm. (b) Image of the dimpled aluminum by a MWNT probe obtained after applying pulsed voltage between the CNT bundle and the dimpled aluminum within 2-5 min. The height of the peaks is 30 nm.
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Initial studies of the MWNT probes produced by the proposed procedure revealed that the majority (approximately 80%) of probes were conductive. C. Electrical properties of the tip
FIG. 5. SEM image of a typical probe, a single nanotube protruding from the bundle. The bundles contain lots of amorphous carbon.
amorphous carbon, the MWNTs should be purified prior to the dielectrophoresis step. However, the presence of amorphous carbon in the probe does not affect quality of AFM images. Study of 20 MWNT probes in SEM revealed that the length of the nanotube part protruded from the bundle is more than 40 nm in half of the cases. To make the CNT cantilevers conductive, a thin film of gold-palladium alloy was preliminarily thermo-deposited on the cantilevers. For such a probe to be conductive, it is not required the final nanotube to be in direct contact with the metal film, rather the bundle provides an electric contact between the final nanotube and the metal film.
To demonstrate conductive capabilities of our probes, we studied CNTs deposited on silicon in capacitance contrast mode (Figure 6). Measurements were performed on air with amplitude of bias voltage modulation equal to 1 V and frequency equal to half of cantilever resonance. High resolution has been achieved due to specific for CNT geometry and high susceptibility of conductive tip.15 Additionally, the local conductivity of biopolymer deposited on a highly orientated pyrolytical graphite (HOPG) was investigated by the CNT probes in the modified contact mode. The probe was brought in contact with the sample to the pressure of 5 nN on air and current-voltage curve was measured. To avoid deformation of the final nanotube, the probe was retracted from the sample before transition to the position of the next point of scanning. Measurements were taken at each point of scan (128 × 128 points, 1 × 1 µm) with voltage in range from −0.5 to 0.1 V. The topography and the map of current are shown in Figures 7(a) and 7(b), respectively. The averaged I-V curve for the outlined area is shown in Figure 7(c). Repeated scanning of the sample during 2 h revealed no change in the current map indicating no conductivity degradation of the MWNT probe. Biopolymer solution that was deposited on substrate consists of threads (10 nm in diameter and 1-5 µm in length), building blocks of these threads and salts. I-V curve obtained from the thread (blue areas) shows no current response to applied voltage. However, the averaged I-V curve on HOPG appeared to be non-linear and looks typically for currentvoltage characteristic for a tunneling current of a commercial gold covered probe. It is not clear whether the non-linearity is caused purely by the tunneling gap between the final nanotube and substrate or there is a non-ohmic contact between the final nanotube and the metal sputtered on the cantilever contributing to non-linearity as well. From this data, we can conclude high resolution of our tips not only
FIG. 6. Tip capabilities for capacitance contrast mode. (a) Image (256 × 256 points, 1.5 × 1.5 µm, 1 µm/s) of CNT on silicon substrate topography. (b) Magnitude signal of the capacitance derivative (capacitance contrast) showing that the probe provides resolution of electrical signal not worse than spacial.
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FIG. 7. Tip capabilities for I-V measurements. (a) Topography of biopolymer on HOPG obtained by a MWNT probe in the modified contact mode (described in text). (b) The current map obtained during the scan at constant force of 5 nN and the probe potential of −0.3 V. (c) The averaged I-V curves from the outlined areas from (a).
in topography but also in the current map. In any case, CNT probes produced by the proposed procedure are suitable for SPM methods where linear I-V characteristic is not required such as scanning capacitance microscopy, electrostatic force microscope, Kelvin probe force microscopy, and scanning gate microscopy.
Napolsky for providing samples of the dimpled aluminum. Financial support from the Russian Academy of Sciences, RFBR Grant Nos. 13-02-12127, 15-02-04285, and 15-5278023, the Ministry of Education and Science of the Russian Federation Grant Nos. 14Y.26.31.0007 and 14.587.21.0006 (RFMEFI58714X0006) is acknowledged. 1D. Fujita, H. Itoh, S. Ichimura, and T. Kurosawa, “Global standardization of
III. SUMMARY
Herein, we demonstrated how to produce conductive high aspect-ratio durable SPM probes. As multi-wall carbon nanotubes are conductors with high probability, they were chosen to serve as probes attached to silicon cantilevers. To reliably attach CNTs to the pyramid of a cantilever, dielectrophoresis technique was applied. CNTs tend to assemble in conglomerates in distilled water. Due to this phenomenon, such a conglomerate of CNTs grasps the pyramid apex securely during the dielectrophoresis procedure rather than nanotubes are attached one by one. To control the probe shape, dimpled aluminum was chosen which serves simultaneously as a test-structure for tip shape determination and is used as a conductive sample to shorten/modify the attached CNT bundle by Joule heat through applying pulsed voltage. Thus, we demonstrated an express method of converting a CNT bundle into a probe ready for scanning. As silicon cantilevers are poorly conductive, we thermodeposited a thin film of gold-palladium on the cantilevers. To demonstrate conductive properties of the CNT probes produced by the procedure proposed, we studied local conductivity of biopolymer covered HOGP. The shape and conductivity of the produced probes degraded much slower compared to gold-covered Si cantilevers. In addition, we studied CNTs deposited on a silicon substrate in capacitance contrast mode where resolution not worse than spacial was achieved. ACKNOWLEDGMENTS
We are grateful to P. A. Malyshkin for supplying us with cantilevers with the diamond pyramid and K. S.
scanning probe microscopy,” Nanotechnology 18, 084002 (2007). J. Giessibl, “Advances in atomic force microscopy,” Rev. Mod. Phys. 75, 949 (2003). 3H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, “Nanotubes as nanoprobes in scanning probe microscopy,” Nature 384(6605), 147–150 (1996). 4J. H. Hafner, C.-L. Cheung, A. T. Woolley, and C. M. Lieber, “Structural and functional imaging with carbon nanotube AFM probes,” Prog. Biophys. Mol. Biol. 77(1), 73–110 (2001). 5J. H. Hafner, C.-L. Cheung, and C. M. Lieber, “Growth of nanotubes for probe microscopy tips,” Nature 398(6730), 761–762 (1999). 6C. V. Nguyen et al., “Carbon nanotube tip probes: Stability and lateral resolution in scanning probe microscopy and application to surface science in semiconductors,” Nanotechnology 12, 363 (2001). 7C. Maeda et al., “Atomic force microscopy and kelvin probe force microscopy measurements of semiconductor surface using carbon nanotube tip fabricated by electrophoresis,” Jpn. J. Appl. Phys., Part 1 41, 2615 (2002). 8J. Tang, G. Yang, Q. Zhang, A. Parhat, B. Maynor, J. Liu, L.-C. Qin, and O. Zhou, “Rapid and reproducible fabrication of carbon nanotube AFM probes by dielectrophoresis,” Nano Lett. 5(1), 11–14 (2005). 9T. Chikamoto, Y. Shimada, M. Umetsu, and M. Sugiyama, “Integration of single-walled carbon nanotube bundle on cantilever by dielectrophoresis,” in IEEE 26th International Conference on Micro Electro Mechanical Systems (MEMS) (IEEE, 2013), pp. 319–322. 10J. H. Hafner, C. L. Cheung, T. H. Oosterkamp, and C. M. Lieber, “High-yield assembly of individual single-walled carbon nanotube tips for scanning probe microscopies,” J. Phys. Chem. B 105(4), 743–746 (2001). 11Q. W. Sun, G. Q. Ding, Y. B. Li, M. J. Zheng, and W. Z. Shen, “Tip-like anodic alumina,” Nanotechnology 18, 215304 (2007). 12Y. C. Sui and J. M. Saniger, “Characterization of anodic porous alumina by AFM,” Mater. Lett. 48, 127–136 (2001). 13A. N. Belov, S. A. Gavrilov, I. V. Sagunova, A. A. Tikhomirov, Yu. A. Chaplygin, and V. I. Shevyakov, “Test structure to determine tip sharpness of micromechanical probes of scanning force microscopy,” Nanotechnol. Russ. 5(5-6), 377–381 (2010). 14F. Li, L. Zhang, and R. M. Metzger, “On the growth of highly ordered pores in anodized aluminum oxide,” Chem. Mater. 10(9), 2470–2480 (1998). 15M. Zhao, V. Sharma, H. Wei, R. R. Birge, J. A. Stuart, F. Papadimitrakopoulos, and B. D. Huey, “Ultrasharp and high aspect ratio carbon nanotube atomic force microscopy probes for enhanced surface potential imaging,” Nanotechnology 19, 235704 (2008). 2F.
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Fast and reliable method of conductive carbon nanotube-probe fabrication for scanning probe microscopy Vyacheslav Dremov,1,2,a) Vitaly Fedoseev,1 Pavel Fedorov,1,2 and Artem Grebenko1,2 1
Institute of Solid State Physics, RAS, 142432 Chernogolovka, Russian Federation Interdisciplinary Center for Basic Research, Moscow Institute of Physics and Technology, 141700 Dolgoprudniy, Russian Federation 2
(Received 27 June 2014; accepted 7 May 2015; published online 21 May 2015) We demonstrate the procedure of scanning probe microscopy (SPM) conductive probe fabrication with a single multi-walled carbon nanotube (MWNT) on a silicon cantilever pyramid. The nanotube bundle reliably attached to the metal-covered pyramid is formed using dielectrophoresis technique from the MWNT suspension. It is shown that the dimpled aluminum sample can be used both for shortening/modification of the nanotube bundle by applying pulse voltage between the probe and the sample and for controlling the probe shape via atomic force microscopy imaging the sample. Carbon nanotube attached to cantilever covered with noble metal is suitable for SPM imaging in such modulation regimes as capacitance contrast microscopy, Kelvin probe microscopy, and scanning gate microscopy. The majority of such probes are conductive with conductivity not degrading within hours of SPM imaging. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4921323]
I. INTRODUCTION
The problem of the appropriate probe fabrication for nanostructures investigation emerged together with the origin of scanning probe microscopy (SPM). One can consider a rigid high aspect ratio probe having a single atom at its tip to be an ideal one.1,2 However, common commercially available probes for atomic force microscope (AFM) are made of silicon with a tip radius of approximately 10 nm, and probes with tips of few nanometers in diameter are considered to be extremely sharp.2 Silicon cantilevers possess a number of shortcomings, the fragility of tip apex and its degradation while scanning (probe “wearing”) being the most noticeable ones. To tackle these drawbacks, carbon nanotube (CNT) probe was developed.3–5 Generally, the diameter of multi-walled carbon nanotubes (MWNTs) is in the range of 5-30 nm making MWNT probes not worse than silicon ones in terms of tip radius and having much higher aspect ratio. In general, CNT probes demonstrate the following advantages over Si probes: • CNT probes are more durable, preserve their shape while imaging6 thus and so under the excess load the CNT will bend while restoring its initial shape after the load is removed;3 • CNT probe mounted/attached on a metal-covered cantilever possesses sufficient conductivity to be used in scanning gate microscopy, scanning Kelvin probe microscopy, or to measure local current-voltage characteristics.7 Metal-covered silicon cantilever often experiences substantial conductivity degradation during scanning due to wearing of the metal film on its apex while the CNT probe remains conductive during hours of continuous scanning. a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2015/86(5)/053703/5/$30.00
A simple and robust method of attaching a CNT bundle to the cantilever pyramid was developed exploiting dielectrophoresis phenomenon, pulling the tip out of CNT suspension in water.8,9 Furthermore, it was shown3,10 that by applying pulse voltage between the CNT probe and a conducting sample, it is possible to shorten the final nanotube in a controllable way tens nanometers per pulse to achieve desired probe length (rigidity). Knowledge of the probe shape may provide useful information while analyzing a SPM image. The most reliable instrument for that is scanning electron microscope or transmission electron microscope (SEM/TEM). However, frequently, it is more convenient to obtain this information directly in SPM taking into account that probe shape may change while imaging. Scanning a known-beforehand test-structure provides such probe characteristics as its apex radius and aspect ratio. It is been revealed1 that a rigid sharp protrusion on a flat surface is an optimal test-structure to restore the tip surface function. However, fabrication of the protrusion with radius at its tip smaller or comparable to the sharp SPM probe is a complex technological task.1 In Refs. 11 and 12, AFM methods are used to study alumina obtained by anodization. It is shown that carefully selecting anodization parameters, a set of alumina peaks with radius at apexes of about 10 nm can be obtained. As the interface between the alumina and aluminum has a developed structure, the surface of aluminum after selective alumina removal (the dimpled aluminum) is shown to be a suitable test-structure for tip shape characterization.13 The radius of peaks on the surface was estimated to be 1-2 nm. Under some anodization parameters, the surface of the dimpled aluminum is a set of dimples ordered in a hexagonal structure, depicted schematically in Figure 1. At the intersection points of three neighboring walls of the dimples, peaks with radius less than 10 nm are formed. Such highly ordered structure can be observed after selective removal of alumina14 as shown in Figure 1(b).
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FIG. 1. Processing of dimpled aluminum. (a) Porous alumina obtained by anodization of aluminum. (b) Dimpled aluminum produced by selective removal of alumina.
Typical lattice constant is approximately 100 nm, the height difference between the peak apex and the dimple bottom (peak height) is 20-50 nm (the lattice constant and peaks height depend on the anodization parameters). Scanning such an object in AFM reveals probe shape information about its last 20-50 nm. Herein, we describe a relatively simple procedure of the MWNT probe fabrication with a single nanotube at its end on the pyramid of a silicon cantilever applying dielectrophoresis technique to the MWNT water suspension. To transform the CNT bundle attached to the pyramid into a probe suitable for SPM imaging, we use the dimpled aluminum sample as a test-structure for tip shape determination as well as for the CNT bundle shortening/modification by applying pulse voltage. The probe produced in this way ends with a single nanotube reliably attached to the cantilever capable of AFM imaging with resolution not worse than that of the silicon cantilever. If Si cantilevers are metal-covered, the majority of the CNT probes produced on them turn out to be conductive.
II. EXPERIMENTAL METHODS A. CNT bundle attachment
To attract a CNT bundle on a cantilever pyramid, the method of dielectrophoresis deposition was chosen. The scheme of our setup is shown in Figure 2(a). It was decided to use commercially available MWNTs produced by electric arc
discharge method (Sigma-Aldrich, Germany). Each nanotube consists of 5-20 graphitic layers being 1-10 µm long and 5-20 nm in diameter. The product contains approximately 10%-40% tubes, the remainder being multi-layer polygonal, amorphous, and graphitized carbon nanoparticles. To prepare a MWNT suspension, 50 mg of MWNTcontaining substance was dispersed in 5 ml of distilled water using ultra-sonic bath. A droplet of the suspension was placed on a platinum wire ring (ring diameter 2 mm and wire diameter 0.25 mm). An AC voltage of 5-10 V, 2 MHz was applied between the wire and the cantilever. Under an optical control, the tip of the cantilever pyramid was immersed into the suspension using an XYZ manipulator 2-5 µm deep and slowly pulled out. The dielectrophoresis force drags CNTs towards the tip, forming a CNT bundle 1-10 µm long reliably attached to the tip (Figure 2(b)). Study of the suspension revealed that conglomerates of nanotubes were still present after dispersing. Likely, these conglomerates were attached to the tip rather than the pure bundle was assembled of individual nanotubes, which might be a reason that the bundle grasped the pyramid similarly to “roots of trees.” Forces of surface tension in the meniscus of suspension around the tip align nanotubes into the bundle. The angle between the pyramid axis and the CNT bundle is defined by joint orientation of the pyramid and surface of the suspension and has distribution of less than ±12◦.8 In some cases, the bundle was not formed or appeared to be not visible in the 400× optical microscope during the first try. Repeating
FIG. 2. CNT bundle attachment. (a) Scheme (top view) of a setup for a MWNT bundle formation on the tip of a cantilever. (b) Acquired 400× magnified optical image of four stages of the bundle formation.
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To be assured that the protruding nanotube is not chemically modified, it was shortened by Joule heat. B. Formation and characterization of the tip
FIG. 3. SEM image of the dimpled aluminum sample reveals long-range order. Inset shows magnified part of the same structure.
the procedure, one can succeed in forming the CNT bundle. Investigation of bundles obtained this way in SEM revealed that a bundle of 200 nm in diameter was already visible in the optical microscope. As soon as the probe is formed, one should anneal it at 120-130 ◦C within 30 min to remove the water from the bundle. It is done to avoid complete bundle destruction during further electrochemical etching and pulse-voltage modification described below. AFM imaging using CNT probes showed that in most cases, the probes possessed longitudinal or transverse instability, if the bundle was 10 µm long or more. For this reason, just after the bundle formation, its length was estimated in the optical microscope. If the visual length was more than 3 µm (as was in most cases), the bundle was shortened using electro-chemical etching in the same setup where we had the second wire ring to keep a droplet of KOH. Applying 4 V, 0.01-1 ms pulses, it is possible to shorten the bundle length in steps of 1 µm and more by etching immersed part of the CNT probe. SEM imaging of etched bundles showed that it extended 2-5 µm from the pyramid and in most cases ended with a single nanotube with length of up to few micrometers.
The next step is to check AFM imaging capabilities of the CNT probe. For that purpose, dimpled aluminum was chosen as a test object (see Fig. 1(b)). In our work, the dimpled aluminum sample was provided by Moscow State University’s chemical department. The further study of the test-structure in AFM revealed that its lattice constant is 105 ± 5 nm with dimples’ depth in range of 25-35 nm. Basing on the SEM image (Figure 3), one can predict what AFM scan with ready CNT probe should look like. If the tip is too long or has multiple apex (Figure 4(a)) in order to improve the tip’s quality, the probe should be shortened/modified. To do it in a more controllable way than electrochemical etching, pulse voltage (30-50 V, 10-100 µs) was applied between the cantilever and the dimpled aluminum sample. The current was limited by a resistor of 1 MΩ inserted in series. Using this method, the probe length was shortened by 50-500 nm per pulse depending on the pulse parameters with feedback turned on in the semi-contact mode. Typically, few pulses were sufficient to gain required rigidity and single apex of the MWNT probe. Dimpled aluminum test-structure has two useful features. The first one is that it allows to perform express evaluation of an AFM probe shape. Due to regular arrangement of sharp peaks, we can estimate probe radius so there is no necessity to directly investigate it in SEM/TEM. The second one is that at same time, one can shorten the CNT bundle which significantly decreases the overall time spent on the CNT probe fabrication. Process of shortening/modification being the most time consuming task usually takes not more than 10 min according to our experience. In Figure 5, typical MWNT probe ready for AFM imaging is shown. Ready MWNT probes contain high amount of amorphous carbon and many nanotubes which are not aligned to the probe axis (Figure 5). To obtain CNT probes without
FIG. 4. Evolution of AFM scans during sharpening. (a) AFM scan (256 × 256 points, 1.5 × 1.5 µm, 1 µm/s) of the dimpled aluminum sample revealing multiple apex probe. The height of the peaks is 15 nm. (b) Image of the dimpled aluminum by a MWNT probe obtained after applying pulsed voltage between the CNT bundle and the dimpled aluminum within 2-5 min. The height of the peaks is 30 nm.
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Initial studies of the MWNT probes produced by the proposed procedure revealed that the majority (approximately 80%) of probes were conductive. C. Electrical properties of the tip
FIG. 5. SEM image of a typical probe, a single nanotube protruding from the bundle. The bundles contain lots of amorphous carbon.
amorphous carbon, the MWNTs should be purified prior to the dielectrophoresis step. However, the presence of amorphous carbon in the probe does not affect quality of AFM images. Study of 20 MWNT probes in SEM revealed that the length of the nanotube part protruded from the bundle is more than 40 nm in half of the cases. To make the CNT cantilevers conductive, a thin film of gold-palladium alloy was preliminarily thermo-deposited on the cantilevers. For such a probe to be conductive, it is not required the final nanotube to be in direct contact with the metal film, rather the bundle provides an electric contact between the final nanotube and the metal film.
To demonstrate conductive capabilities of our probes, we studied CNTs deposited on silicon in capacitance contrast mode (Figure 6). Measurements were performed on air with amplitude of bias voltage modulation equal to 1 V and frequency equal to half of cantilever resonance. High resolution has been achieved due to specific for CNT geometry and high susceptibility of conductive tip.15 Additionally, the local conductivity of biopolymer deposited on a highly orientated pyrolytical graphite (HOPG) was investigated by the CNT probes in the modified contact mode. The probe was brought in contact with the sample to the pressure of 5 nN on air and current-voltage curve was measured. To avoid deformation of the final nanotube, the probe was retracted from the sample before transition to the position of the next point of scanning. Measurements were taken at each point of scan (128 × 128 points, 1 × 1 µm) with voltage in range from −0.5 to 0.1 V. The topography and the map of current are shown in Figures 7(a) and 7(b), respectively. The averaged I-V curve for the outlined area is shown in Figure 7(c). Repeated scanning of the sample during 2 h revealed no change in the current map indicating no conductivity degradation of the MWNT probe. Biopolymer solution that was deposited on substrate consists of threads (10 nm in diameter and 1-5 µm in length), building blocks of these threads and salts. I-V curve obtained from the thread (blue areas) shows no current response to applied voltage. However, the averaged I-V curve on HOPG appeared to be non-linear and looks typically for currentvoltage characteristic for a tunneling current of a commercial gold covered probe. It is not clear whether the non-linearity is caused purely by the tunneling gap between the final nanotube and substrate or there is a non-ohmic contact between the final nanotube and the metal sputtered on the cantilever contributing to non-linearity as well. From this data, we can conclude high resolution of our tips not only
FIG. 6. Tip capabilities for capacitance contrast mode. (a) Image (256 × 256 points, 1.5 × 1.5 µm, 1 µm/s) of CNT on silicon substrate topography. (b) Magnitude signal of the capacitance derivative (capacitance contrast) showing that the probe provides resolution of electrical signal not worse than spacial.
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FIG. 7. Tip capabilities for I-V measurements. (a) Topography of biopolymer on HOPG obtained by a MWNT probe in the modified contact mode (described in text). (b) The current map obtained during the scan at constant force of 5 nN and the probe potential of −0.3 V. (c) The averaged I-V curves from the outlined areas from (a).
in topography but also in the current map. In any case, CNT probes produced by the proposed procedure are suitable for SPM methods where linear I-V characteristic is not required such as scanning capacitance microscopy, electrostatic force microscope, Kelvin probe force microscopy, and scanning gate microscopy.
Napolsky for providing samples of the dimpled aluminum. Financial support from the Russian Academy of Sciences, RFBR Grant Nos. 13-02-12127, 15-02-04285, and 15-5278023, the Ministry of Education and Science of the Russian Federation Grant Nos. 14Y.26.31.0007 and 14.587.21.0006 (RFMEFI58714X0006) is acknowledged. 1D. Fujita, H. Itoh, S. Ichimura, and T. Kurosawa, “Global standardization of
III. SUMMARY
Herein, we demonstrated how to produce conductive high aspect-ratio durable SPM probes. As multi-wall carbon nanotubes are conductors with high probability, they were chosen to serve as probes attached to silicon cantilevers. To reliably attach CNTs to the pyramid of a cantilever, dielectrophoresis technique was applied. CNTs tend to assemble in conglomerates in distilled water. Due to this phenomenon, such a conglomerate of CNTs grasps the pyramid apex securely during the dielectrophoresis procedure rather than nanotubes are attached one by one. To control the probe shape, dimpled aluminum was chosen which serves simultaneously as a test-structure for tip shape determination and is used as a conductive sample to shorten/modify the attached CNT bundle by Joule heat through applying pulsed voltage. Thus, we demonstrated an express method of converting a CNT bundle into a probe ready for scanning. As silicon cantilevers are poorly conductive, we thermodeposited a thin film of gold-palladium on the cantilevers. To demonstrate conductive properties of the CNT probes produced by the procedure proposed, we studied local conductivity of biopolymer covered HOGP. The shape and conductivity of the produced probes degraded much slower compared to gold-covered Si cantilevers. In addition, we studied CNTs deposited on a silicon substrate in capacitance contrast mode where resolution not worse than spacial was achieved. ACKNOWLEDGMENTS
We are grateful to P. A. Malyshkin for supplying us with cantilevers with the diamond pyramid and K. S.
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