Welcome to the May 2007 issue. This issue focuses on experimental nanomechanics of nanobuilding blocks. The extremely small dimensions of nanobuilding blocks (for instance, nanoparticles, nanotubes, and nanowires) have imposed great challenges to many existing instruments, methodologies, and even theories.  In this issue, we will discuss – (1) experimental techniques and (2) size-effects. 

The methods that have been developed and used for measuring the mechanical properties of isolated individual nanobuilding blocks include uniaxial tensile loading using a nanomanipulation stage, in-situ compression of nanoparticles and nanopillars, mechanical/electric-field induced resonance, AFM bending, and nanoindentation. The following is a brief summary of these methods for discussion.

Can one perform tensile tests on a nanowire as we normally do on a big dog-bone sample? Prof. Rod Ruoff’s group realized such challenging tests on individual multiwalled carbon nanotubes (MWCNTs) using a testing stage based on a nanomanipulation tool operating inside a SEM. The nanomanipulation stage makes 3-D manipulation possible – picking, positioning, and clamping of individual 1-D nannomaterials. The individual 1-D nanomaterials were attached and clamped to AFM probes by a localized electron beam induced deposition of carbonaceous materials inside the SEM. A single 1-D nanomaterial so clamped between two AFM probes was then tensile loaded by displacement of the rigid AFM probe and the applied force was measured at the other end by the AFM cantilever deflection of the other, compliant AFM probe. The measured force-elongation data were converted, by SEM measurement of the nanomaterial geometry, to obtain a stress-strain curve.  In addition to the Young’s modulus, breaking strength can be measured by this method.  Prof. Rod Ruoff’s paper is the first one under discussion in the present issue of journal club. 

Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS, Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, Science, 287 (2000) 637-640.

Compression tests on small pillars inside a SEM provide a new way to study the sample size effects. To mimic the conventional compression tests, FIB was used to cut a bulk sample to a smaller size (top-down approach).  A flat nanoindenter was then used to perform uniaxial compression tests on the FIB cut sample in situ. Engineering stress-strain curves can be obtained. Sample’s morphology change such as slip bands can be studied by SEM.  The second paper selected for discussion is:

Uchic MD, Dimiduk DM, Florando JN, Nix WD, Sample dimensions influence strength and crystal plasticity, Science, 305 (2004) 986-989.

Prof. C. M. Lieber’s group used an AFM operating in lateral-force mode to bend cantilevered MWCNTs that were deposited on a low-friction MoS2 surface and pinned down at one end by overlaying SiO2 pads using lithography. The bending modulus of individual MWCNTs was calculated from deflection of a cantilevered MWCNT and the lateral force applied by the AFM probe. Another approach is to use AFM to perform indentation three point bending tests on the 1-D nanomaterials deposited on a membrane having nanopores. The suspended 1-D nanomaterial was considered as a double-clamped simple beam that was clamped to the membrane by its high surface energy at the ambient humidity condition. By positioning the AFM tip directly on the center of the 1-D nanomaterial spanning the pore and applying an indentation force, the Young’s modulus of individual 1-D nanomaterials can be obtained from the AFM tip force-deflection curve. Below please find Prof. C. M. Lieber’s paper:

Wong EW, Sheehan PE, and Lieber CM, Nanobeam mechanics: elasticity, strength, and toughness of nanorods and nanotubes, Science, 277 (1997) 1971-1975.

To study the size-effects of nanoparticles is extremely difficult.  Prof. Bill Gerberich’s group performed compression tests on individual silicon nanospheres using in situ nanoindentation techniques. In situ TEM provides insightful information such as dislocation initiation and motion, onset of plasticity, and fracture mechanisms. Here I would like to use one of Prof. Bill Gerberich’s papers for discussion:

Deneen J, Mook WM, Minor A, Gerberich WW, Carter CB, In situ deformation of silicon nanospheres, Journal of Materials Science, 41 (2006) 4477-4483.

In a cantilever vibration test, individual cantilevered 1-D nanomaterials are thermally oscillated using a variable-temperature sample holder in a TEM or oscillated by directly inducing the mechanical resonance using an electric field. Using continuum beam mechanics, the bending modulus of 1-D nanomaterials can be calculated from the measured resonance frequency and the selected nanomaterial geometry. Below, please find the two representative papers:  

Treacy MMJ, Ebbesen TW, and Gibson JM, Exceptionally high young's modulus observed for individual carbon nanotubes, Nature, 381 (1996) 678-680. 

Wang ZL, Poncharal P, and de Heer W A, Measuring physical and mechanical properties of individual carbon nanotubes by in situ TEM, Journal of Physics and Chemistry of Solids, 61 (2000) 1025-1030. 

Nanoindentation techniques and theories have been well established for the mechanical characterization of solid surfaces and thin films. The major challenge we are facing is: can we extend application of traditional nanoindentation approaches to 0-D (nanoparticles) and 1-D nanomaterials (nanowires/nanobelts) for directly measuring their mechanical properties? The early work includes: 

Mao SX, Zhao MH, and Wang ZL, Nanoscale mechanical behavior of individual semiconducting nanobelts, Applied Physics Letters, 83 (2003) 993-995. 

Li XD, Hao HS, Murphy CJ, and Caswell KK, Nanoindentation of silver nanowires, Nano Letters, 3 (2003) 1495-1498. 

The mechanisms (physics/science) of size-effects are still, to a large extent, unknown. One of the basic debates is size-dependency of Young’s modulus. The reported Young’s modulus of many nanobuilding blocks exhibits a large variation relative to the corresponding values of the bulk materials. For many metallic nanowires or small-scale samples, the strength was reported to increase as size decreases. The above selected papers cover the mechanisms of size-effects.  Insightful discussion on this topic is greatly needed.

Previous discussions on experiemntal mechanics can be found in this forum and may help stimulate further discussions.

Please note that due to space restrictions, I am not able to include all the papers that I know of.  I sincerely apologize for any omission of papers that should be included in this post.