Carbon nanotube tips for surface characterization: Fabrication and properties
Introduction
Surface inspection at the nanometer scale has conventionally been performed using either scanning electron microscopy (SEM) or transmission electron microscopy (TEM). However, SEM cannot be applied to non-conducting samples, while TEM is both destructive and expensive. Accordingly, atomic force microscopy (AFM) has emerged as a powerful technique for studying the surfaces of conductors, semiconductors, and insulators at the sub-nanometer scale. Theoretical studies have shown that the attainable resolution of AFM images is limited by two important factors, namely the sharpness of the cantilever tip and its aspect ratio [1]. High-resolution imaging is based on short-range interactions between the tip apex and the atomic-sized features of the surface. Therefore, minimizing the long-range forces between the tip and the surface is highly desirable. Much effort has been made to improve the sharpness of AFM probing tips and to enhance their wear-resistance properties. However, problems still remain in obtaining consistent high-resolution images since the conventional silicon or silicon nitride tips quickly become worn during operation and then tip profile changes occur, particularly when crashing the surface. An ideal AFM probing tip should be mechanically elastic and chemically inert such that changes of the tip profile and chemical reaction are minimized during the scanning process. In practice, the resolution of an AFM is governed by the geometry of its tip apex and the aspect ratio of the cantilever tip. Specifically, to obtain a high resolution, the tip apex should be sharp and the radius of curvature should be small, e.g., not more than a few nanometers.
Since carbon nanotubes (CNTs) were first discovered by Iijima [2] in 1991, researchers have proposed many potential applications for these nanometer-scale structures, including field emitters, sensors, scanning probe microscopy tips, molecular wires and high strength composites. One of the most promising and near-term applications of CNTs is as tips for scanning probe microscopy [3], [4]. Since the imaging resolution of an AFM is determined by the diameter of the scanning probe tip, CNT tips offer the potential for resolution at the nanoscale. Furthermore, the long characteristic length of CNT tips permits the tracing of rough surfaces with steep and deep features. A high CNT tip aspect ratio is also advantageous in reducing the long-range forces between the tip and the surface in the tip–sample interactions, thereby further improving the imaging resolution. In addition, the extraordinary strength of CNT probes and their ability to retain structural integrity until plastic deformation render CNT scanning probes highly robust [5], [6], [7], [8]. As a consequence, CNT probes are relatively hard wearing compared to conventional silicon scanning probes and are therefore more suitable for the characterization of harder surfaces. Clearly, CNTs are ideal potential candidates for commercial AFM scanning applications. Therefore, a requirement exists to develop new methods for their purification and handling such that their fundamental properties and applications can be further explored.
The current study uses electrophoresis [9] and electrostatic attachment techniques [10] to improve the control and fabrication of CNT tips. CNTs are attached to conventional silicon AFM cantilevers and used as probing tips to carry out surface roughness and line width measurements.
Section snippets
Materials and methods
In this study, the CNTs were synthesized by a conventional dc arc-discharge method [11], resulting in nanotubes with a diameter of 5–20 nm and a length of 1–5 nm. Subsequently, carbon nanotube tips were attached to conventional AFM silicon cantilevers using dc electrophoresis [9] and electrostatic techniques [10].
To align the nanotubes, the CNTs were ultrasonically dispersed in isopropyl alcohol (IPA) and the resulting suspension was centrifuged at room temperature until the IPA had evaporated
Results and discussion
The tracing of a rough surface with deep features has important applications in surface metrology. Figs. 4(a) and (b) show AFM images of 10 μm spacing lines with a depth of 300 nm obtained using one of the conventional silicon tip and the current CNT tips, respectively. Since the length of the CNT tip was less than 300 nm, vibration effects caused by sidewall attractive forces were avoided. The bottom width of the line was found to be 330 nm using the CNT tip and 175 nm using the conventional
Conclusion
This study has successfully attached carbon nanotubes to conventional AFM silicon cantilevers using an electrophoretic and electrostatic attachment technique. The results have shown that the CNT tips are suitable for use as probing tips for AFM surface characterization. It has been demonstrated that the CNT tips have a remarkable flexibility and are not broken or damaged in the tip crashes which typically occur during the scanning process. Furthermore, the CNT tip is capable of tracing a narrow
References (12)
- et al.
Appl. Surf. Sci.
(2000) - et al.
Electrochem. Commun.
(2006) - et al.
J. Phys. Chem. Solids
(2004) Nature
(1991)- et al.
Nature
(1996) - et al.
Phys. Rev. Lett.
(1998)