1、A Review on Carbon Nanotube Probes for Microscopy Applications,Wei Chen, Yuan Yao and YunYoung KimDepartment of Mechanical Engineering Northwestern University, Illinois 60201, USA,Contents,Introduction (YunYoung Kim) Gluing Method (Wei Chen) Growng Method (Yuan Yao) Biological Applications (YunYoung
2、 Kim),Atomic Force Microscopy (AFM)- Indispensable device for micro/nanoscale phenomena studies- Surface topology measurement- Micro/nano structure manipulation,Introduction,Carbon Nanotube (CNT)- A new material that has exceptional properties- Promising possibilities of applications (composites, FE
3、P display, chemical sensors, etc.),Hafner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).,Hafner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).,CNT-Probed AFM- A CNT (either SWNT or MWNT) is attached on the apex of the conventional silicon AFM cantilever tip.,Introduction,The Synergy Effe
4、ct of CNT-Probed AFM- Resolution enhancement,- Precise measurements of high aspect ratio structures,Chang et al., Jpn. J. Appl. Sci. 43(7B), 4517-4520 (2004).,Guo et al., Appl. Surf. Sci. 228, 53-56 (2004).,Introduction,Si Tip,Si Tip,CNT Probe,CNT Probe,CNT Probe,Si Tip,- High wear-resistance,Chang
5、et al., Jpn. J. Appl. Phys. 43(7B), 4517-4520 (2004).,Si Tip,CNT Probe,CNT Probe,Si Tip,Introduction,- Biomolecule manipulation,Chang et al., Jpn. J. Appl. Phys. 43(7B), 4517-4520 (2004).,Biotin Ligand,Streptavidin Receptor,Overview: Two Ways of CNT-Tip Fabrication- Gluing Method,- Growing Method,Ha
6、fner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).,Tang et al., Nano Lett. 5(1), 11-14 (2005).,Introduction,Gluing Method,Pick-up methodSWNT 0.92.8nm in diameter,Lieber et al. J. Phys. Chem. B 2001,First trial-”glue”MWNT 520nm in diameter 0.251m in length,Dai, H. et al. Nature 1996,Gluing Met
7、hod,Disadvantage,Manual assembly method is time-consuming, needs proficient experimental technique, the fabrication yield is restricted. Spacial resolution is compromised by the large diameters of attached CNTs, since only thick CNTs can be viewed under optical microscope.,Gluing Method,Magnetic fie
8、ld-aligned,Sputter coat the Au film on AFM probes; Fix the AFM tips; Prepare and stir CNTs suspension (5mL); Apply alternative magnetic field (Amplitude B0=0.1T); CNTs are aligned and attached onto the tips.,Hall, A. et al. Appl. Phys. Lett. 2003,Average protruding angle: 35o Length: 100-500nm,Gluin
9、g Method,Physical Principle,Change in flux (Induced potential),Induced Current,Dipole moments,Potential Energy,Attraction Force,Gluing Method,Dip-Coating & Dielectrophoresis,Introduce the CNTs into the TiO2 SG solution; W tip was dipped into SG solution containing CNTs; Apply DC voltage between the
10、tip and the solution; CNTs was deposited on the tip, and the SG was coated on it as well, Withdraw the tip and anneal it under infra-red lamp.,A. Brioude et al. Appl. Surf. Sci. 221 (2004),Diameter: 9.2nm Length: 950nm,Strong adhesion,A. Brioude et al. Appl. Surf. Sci. 221 (2004),Many CNTs entangle
11、together into fiber, CNTs break near the apex of tips since the sudden strong current; The SG thin film works like a sheath, which integrates the CNTs and tip as a whole.,Potential for large-scale fabrication,Gluing Method,Growing Method,Pore-growth method (first direct growth, Hafner et al., Nature
12、 1999),Flatten the conventional silicon (Si) tip at its apex by contact AFM imaging; Anodize the tip in hydrogen fluoride to create nano-pores of 50100 nm diameter along the tip axis; Electrodeposit iron catalyst into the nano-pores from FeSO4 solution; Use Chemical Vapor Deposition (CVD) to grow ca
13、rbon nanotubes with ethylene and hydrogen at 750 for 10 min.,Growing Method,The pore-growth method demonstrated the great potential of thermal CVD to grow directly controlled diameter nanotube tips.,1m,20nm,Growing Method,Limitation: The preparation of a porous layer can be time consuming and may no
14、t place individual SWCNTs at the optimal location on the flattened apex.,Surface-growth method (Hafner et al., J. Am. Chem. Soc. 1999),Deposit catalyst onto the pyramidal tip of a commercial cantilever; (Fe-Mo and colloidal Fe-oxide catalysts are electrophoretically deposited on the tip) grow the SW
15、CNT probe using thermal CVD at 800 for 3 min. (ethylene is used as carbon source - C2H4 : H2 : Ar = 1 : 200 : 300),Growing Method,Growing Method,Surface-growth method,SEM cannot provide an accurate measure of the tip diameters.TEM analysis was able to demonstrate that the tips consist of individual
16、SWCNT and small SWCNT bundles.,20nm,200nm,Individual SWNT tips can be prepared by lowering the catalyst density on the surface such that only 1 nanotube reaches the apex. Hence, by controlling the catalyst density , it is possible to produce well-dened individual SWNT tips.,Surface-growth method,Gro
17、wing Method,10nm,Growing Method,Mass production ?,Erhan Yenilmez, et al., Appl. Phys. Lett. 2002,How to place catalyst on each of the 375 Si tips on a 4-in wafer ?How to grow SWCNTs on a wafer-scale large substrate using CVD?,Growing Method,Catalyst patterning,Spin coat polymethylmethacrylate (PMMA)
18、 onto the wafer at a low spin speed of 1000 rpm and bake the wafer on a hot plate at 180 for 5 min. Repeat the PMMA spin coating and baking step for three times. Spin coat the catalyst suspension onto the wafer at a low spin speed of 250rpm Liftoff the PMMA layer in acetone. The catalyst coated arou
19、nd the pyramidal surfaces of the tips will be left.,Growing Method,The system is heated to 900 in an Ar flow after a thorough purge of the system by Ar. Then ultrahigh purity methane (99.999% purity, flow rate of 1500 sccm) is piped through the system together with hydrogen (flow rate of 125 sccm) f
20、or a growth time of 7 min. At last, the system is cooled to room temperature in a H2 flow.,Growing Method,Their method yields SWCNTs protruding from more than 90% of the 375 Si tips on a wafer.,CNT location, density, length, and orientation can not be welled controlled by the thermal CVD methods. Al
21、though we can get many tips through one thermal CVD process, seldom of them has individual free-standing and well-oriented CNTs. The yield of readily usable tips is very low. After CNT tip fabrication, a one-at-a-time manipulation approach is required to shorten the extruding CNTs for AFM use. This
22、shortening process decreases the efficiency of mass production. In addition, nearly all of the previous approaches (including both attaching methods and direct growth methods) rely on commercially available silicon tips or prefabricated commercial silicon probe wafers.,Limitations:,Growing Method,Gr
23、owing Method,Qi Ye et al., Nano Lett. 2004,They integrated nanopatterning and nanomaterials synthesis with traditional silicon cantilever microfabrication technology, and produced 244 CNT probe tips on a 4-in. wafer with controlled CNT location, diameter, length, and orientation.,Growing Method,Qi Y
24、e et al., Nano Lett. 2004,PECVD Plasma Enhanced CVD,Growing Method,Qi Ye et al., Nano Lett. 2004,PECVD Plasma Enhanced CVD,Qi Ye et al., Nano Lett. 2004,Growing Method,Growing Method,No prefabricated silicon probe wafers needed; Better control of the CNT location, density, length, and orientation by
25、 PECVD; No need to conduct post-fabrication treatment to remove and/or shorten the CNT tips.,Advantages of their approach:,Qi Ye et al., Nano Lett. 2004,Biological Applications,Biomolecule Structure Characterization,Wong et al., J. Am. Chem. Soc. 120, 603-604 (1998),- Amyloid- fibrils are measured w
26、ith CNT tips.- The small effective tip radii result in 1230% increase of resolution.- Observed widths are 38 nm smaller for MWNT tips than those for Si tips.,An image of A40 fibril,A Y-branch,A defect,A staggered end,Biological Applications,- Significant resolution improvement for SWNT tips (full wi
27、dth at half maximum is 51 nm for SWNT tip and 151 nm for Si tip)- More refined measurements are possible with SWNT tips.,An image of double-stranded DNA on mica,Height cross-section of the DNA,SWNT Tip FWHM=5.6 nm,Si Tip FWHM=14.4 nm,Wong et al., Appl. Phys. Lett. 73, 3465-3467 (1998),Biological App
28、lications,AFM image of DNA taken using a CNT tip,AFM image of DNA taken using a Si tip,- Improvement in resolution by a factor of 2- The diameter ranges from 12 to 15 nm with a Si tip, but 7 to 8 nm with a CNT tip.- Blurred image is observed when the CNT tip with a 10 nm diameter is longer than 500
29、nm,Nishijima et al., Appl. Phys. Lett 74, 4061-4063 (1999),An Issue about the Length of CNTs,Biological Applications,- To avoid a mechanical instability, the length should be less than 500 nm for MWNTs and 50nm for SWNTs1.- Akita et al. also reports that L 500 nm is required for a stable and high-re
30、solution imaging2.,Vibration amplitude,a: outer radii of the nanotube (5 nm) b: inner radii of the nanutube (2.5 nm) Y: Youngs modulus (1TPa),- For example, L 500 nm results in u 0.5 nm and degradation of resolution by a factor of ten.,1) Chang et al., Jpn, J. Appl. Phys. 43, 4517-4520 (2004),2) S.
31、Akita et al., J. Phys. D.: Appl. Phys. 32, 1044-1048 (1999),Biological Applications,Manipulation of Wettability of CNTS,- Hydrophobic CNTs are chemically incompatible in aqueous environments.- Stevens et al. reported a failure of MWNT probe during the measurement in DI H2O.,A MWNT tip before immersi
32、ng in DI water,A MWNT tip after immersing in DI water,Stevens et al., IEEE Trans. Nanobio. 3, 56-60 (2004),Biological Applications,- The wettability can be adjusted by depositing ethylendiamine (DE) on the sidewalls of CNTs.- Amine groups can be absorbed onto CNT sidewalls by exchanging of electrons
33、, and render the CNT to be hydrophilic.- The deposition do not change the chemical property of the tip end, and thus do not affect the probe resolution.,Si tip, in air,CNT tip, untreated, in air,CNT tip, treated, in air,CNT tip, treated, in H2O,Stevens et al., IEEE Trans. Nanobio. 3, 56-60 (2004),Su
34、mmary,1. CNT-probed AFM is reviewed.2. CNT-probes can be fabricated by- the gluing method, in which a grown CNT is picked up by a Si tip using adhesives, electric or magnetic fields.- the growing method, in which a CNT is directly grown on the pyramidal surfaces of a silicon tip.3. Examples of CNT-probe applications for biological studies are presented.,
