Electron Microscope Based Fabrication and Nano-Mechanical Testing

The early research involving manipulation and nano-mechanical testing of individual nanostructures was demonstrated by use of atomic force microscope (AFM) and scanning tunneling microscope (STM) based systems [1-4]. For the first time, these microscopes allowed sub-nanometer scale observation as well as interaction with the specimen. Most of the work involving fundamental mechanical properties of nanotubes was done by AFM based surface manipulations on a planar substrate. Using a sharp microscopy tip, individual nanotubes could be located and transported by rolling and sliding; ultimately they could be cut to the right size (by pushing). These microscopes offer advantage in terms of the resolution but there is one major downside to any AFM/STM based manipulation strategy; the lack of real-time imaging and the limitation of manipulation to planar surfaces, which makes some operations impossible.

In the last few years, scanning electron microscope (SEM), focused ion beam microscope (FIB) and transmission electron microscope (TEM) based nano-manipulation systems have started to replace the AFM (based systems) [5-8]. SEM/FIB offers 3-D manipulation of the nano-structures in real time. The more spacious chamber of this kind of microscope allows mounting of bigger specimen. Although the resolution provided by a SEM is an order of magnitude less than that of a TEM or an AFM, it usually is considered good enough for selection, separation, manipulation, assembly as well as testing of nano-mechanical devices.

Our research group has been involved in SEM and FIB based manipulation for fundamental mechanical property evaluation as well as fabrication of ‘proof-of-principle’ devices. Since the whole fabrication and testing is carried out under direct view of SEM, there remains little chance for ambiguity in the experimental data. Dr. Singh and his team [7-8] have demonstrated fabrication and testing to two individual nanotube (NT) based devices: a) an individual NT/sphere device for use as a force sensor [7] and b) a prototype microtome CNT nano-knife for sectioning of biological materials [8].

Figure 1. Nano-devices fabricated and tested by use of a SEM based nanomanipulation system. (a) NT/sphere force sensor device, see reference [7] for details. (b) Carbon nanotube prototype nano-knife, see reference [8] for details.

The NT/sphere device incorporates a polystyrene microsphere bead attached to an individual multi-walled carbon nanotube (MWCNT), shown in Fig. 1(a). The device has applications in studying cell deformation behavior by measuring the deflection of the sphere optically, since the sphere is large enough to be detected accurately with optical methods [7,9]. We have continued to work with research groups at National Institute of standards and Technology (Materials Reliability and Optoelectronics Division) to explore new applications of this device and so far we have been able to demonstrate: (a) calibration of the sensor to much lower the range of forces i.e., piconewton and (b) Utilizing the NT/sphere arrangement for emulating a cell nucleus for calibration studies using optical coherence tomography (OCT) [9].

The prototype nano-knife device consists of a CNT stretched between two tungsten needles (held together on a glass substrate). In-situ transverse load tests on the nano-knife indicated that failure was at the weld (the CNT was unaffected by the force applied), shown in Fig. 1(b). Measured device strength was ~0.14 GPa, corresponding to a weld breaking force of ~10^-7 N. While the cutting experiments performed on a gold-coated epon resin specimen (biological cell plasticizer) showed indentation marks due to the NT [7].

Figure 2. MEMS based tensile tester stretching an individual MWCNT (left) and corresponding stress-strain plot (right), see reference [10] for more details.

Our current research at Nanoscience and Engineering Lab at Kansas State University is focused on synthesis and mechanical testing of polymer-derived ceramic SiCN-Carbon nanotube composite nanowires [11-12]. Polymer-derived ceramics are unique, as they have been shown to exhibit mixed properties of polymers, ceramics and graphene in general. We are developing ways to experimentally determine the mechanical strength of individual nanowires using MEMS based tensile platform (collaboration with Dr. Victor Bright of University of Colorado at Boulder). We have previously demonstrated the tensile testing capabilities of such a MEMS tester in which an individual MWCNT was stretched to fracture demonstrating typical telescopic mode of failure in MWCNTs, Fig. 2 [10]. Bending tests are being performed by use an AFM-based system inside SEM, similar to reference [7].

In conclusion, the introduction of SEM-based manipulation systems into engineering research has enhanced our understanding of the nano-mechanical phenomenon in 1-D nanostructures as well as opened new avenues for fabrication of various prototype nanoscale devices. This will have major effect in shaping the future of nanotechnology research.

Acknowledgements

Gurpreet Singh would like to thank Kansas State University for start-up funds for related research currently being performed in our lab.

References

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