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Journal Club Theme of May 2014: in situ Nanomechanics

Ting Zhu's picture

The in situ nanomechanics is an emerging field that investigates the mechanical properties and deformation mechanisms of nanoscale and nanostructured materials, by integrating the real-time mechanical testing inside electron microscope and the mechanics modeling with atomic resolution. It provides a powerful approach to "visualize" the intrinsic nanomechanical behavior of materials - seeing is believing.

An engineering revolution is currently under way in that small structures and devices are being fabricated at the micrometer to nanometer scales. Understanding the mechanical properties of materials at small length scales is central to the successful design and fabrication of reliable micro/nano-devices, with applications to microelectronics, renewable energy, biotechnology and a host of other fields.

While there are a drastically increasing number of in situ nanomechanics studies and reviews in the recent literature (only cite a few [1-13] among many excellent ones), here I will discuss two works from my group at Georgia Tech and collaborators in other universities/national labs. These studies uncovered a variety of fascinating nanomechanical phenomena and properties in 2D materials and nanostructured engineering/energy materials.

 

- in situ scanning electron microscopy (SEM) study of fracture toughness of graphene

Zhang et al., Nature Communications 5, 3782 (2014)

The in situ nanomechanics study seeks to quantitatively measure the nanomechanical properties. For example, the 2D material such as graphene has exceptional mechanical properties. However, the fracture toughness of graphene has not been measured to date. In this collaborative research with Prof. Jun Lou at Rice, the in situ tensile testing of suspended graphene was conducted using a nanomechanical device in a SEM. During tensile loading, the pre-cracked graphene sample fractures in a brittle manner with sharp edges, at a breaking stress substantially lower than the intrinsic strength of graphene. The combined experiment and modeling verify the applicability of the classic Griffith theory of brittle fracture to graphene. Fracture toughness of graphene is directly measured. This in situ nanomechanics study quantifies the essential fracture properties of graphene and provides mechanistic insights into the mechanical failure of graphene.

 Graphene fracture

Fig. 1 Fracture of pre-cracked graphene from (a-b) in situ SEM and (c-d) molecular dynamics studies.

 

- in situ transmission electron microscopy (TEM) study of lithiation mechanism in Si electrodes

Liu et al.,  Nature Nanotechnology 7, 749-756 (2012)

The in situ nanomechanics study can well address the multiphysics problems where mechanics plays an important role. Lithium ion batteries (LIBs) have revolutionized portable electronics and are key to electrifying cars. The electrochemical reaction between lithium and electrodes in LIBs is a critical process that controls their capacity, cyclability, and reliability. Despite the intensive study of LIBs, the atomic-level mechanisms of electrochemical reactions in solid electrodes remain elusive. In this collaborative research with Dr. Jianyu Huang at Sandia and Prof. Scott Mao at University of Pittsburg, an atomically-resolved process of dynamic lithiation in single-crystal silicon is revealed by using in situ high resolution TEM. A sharp interface of about 1 nm thick is observed between the crystalline Si reactant and the amorphous LixSi product. An atomic ledge flow process is discovered to produce the amorphous LixSi alloy through layer-by-layer peeling of the {111}atomic facets on Si surface, resulting in the orientation-dependent mobility of the interfaces. This in situ nanomechanics study provides crucial insights for the understanding of lithiation-induced stress generation and failure in high-capacity electrodes for LIBs.

 Silicon lithiation

Fig. 2 in situ lithiation of an single Si nanowire, showing the TEM image (a) before lithiation and (b) when lithiation is ongoing. (c) A high resolution TEM image of atomic structure at the lithiation reaction front and (d) corresponding atomistic simulation.

 

To summarize, the nanomechanics study via in situ SEM and TEM represents the most direct ways of characterizing the mechanical behavior of materials at the nanometer and atomic scales. There are other powerful in situ testing techniques such as in situ synchrotron and in situ tomography that are not covered in this brief writeup. The above studies also demonstrate that the nanomechanics modeling can provide critical insights and even guidance for in situ nanomechanical testing. Undoubtedly, close collaboration between experimentalists and modelers is essential to advance the in situ nanomechanics field in the future. Ultimately, the in situ nanomechanics research will enable the design of nanostructured materials to realize their latent mechanical strength to the full.

 

References

[1] Uchic, M. D., Dimiduk, D. M., Florando, J. N. & Nix, W. D. Sample dimensions influence strength and crystal plasticity. Science 305, 986-989 (2004).

[2] Haque, M. A. & Saif, M. T. A. Deformation mechanisms in free-standing nanoscale thin films: A quantitative in situ transmission electron microscope study. Proceedings of the National Academy of Sciences of the USA 101, 6335-6340 (2004).

[3] Zhu, Y. & Espinosa, H. D. An electromechanical material testing system for in situ electron microscopy and applications. Proceedings of The National Academy of Sciences of the USA 102, 14503-14508 (2005).

[4] Lee, C., Wei, X. D., Kysar, J. W. & Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321, 385-388 (2008).

[5] Shan, Z. W., Mishra, R. K., Asif, S. A. S., Warren, O. L. & Minor, A. M. Mechanical annealing and source-limited deformation in submicrometre-diameter Ni crystals. Nature Materials 7, 115-119 (2008).

[6] Huang, J. Y. et al. In Situ Observation of the Electrochemical Lithiation of a Single SnO2 Nanowire Electrode. Science 330, 1515-1520 (2010).

[7] Lu, Y., Song, J., Huang, J. Y. & Lou, J. Fracture of Sub-20nm Ultrathin Gold Nanowires. Advanced Functional Materials 21, 3982-3989 (2011).

[8] Jang, D. C., Li, X. Y., Gao, H. J. & Greer, J. R. Deformation mechanisms in nanotwinned metal nanopillars. Nature Nanotechnology 7, 594-601 (2012).

[9] Jacobs, T. D. B. & Carpick, R. W. Nanoscale wear as a stress-assisted chemical reaction. Nature Nanotechnology 8, 108-112 (2013).

[10] Ramesh, K. T. Nanomaterials: Mechanics and Mechanisms.  (Springer, 2009).

[11] Legros, M., Gianola, D. S. & Motz, C. Quantitative In Situ Mechanical Testing in Electron Microscopes. MRS Bulletin 35, 354-360 (2010).

[12] Zhu, T. & Li, J. Ultra-strength Materials. Progress in Materials Science 55, 710-757 (2010).

[13] Greer, J. R. & De Hosson, J. T. M. Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect. Progress in Materials Science 56, 654-724 (2011).
  

Comments

L. Roy Xu's picture

Ting:

Thanks for your pioneering work! I have some
issues which need your valuable discussions:

1)    Measured fracture toughness is much smaller than I
expected (so brittle). I'd know your crack tip sharpness. If a crack tip is not
mathematically sharp, we cannot get a material constant-fracture toughness. Can
you tell me the radius of your crack (or notch) tip? FIB may not make very
sharp crack tips.

2)    Plane strain condition for fracture toughness
measurements. Here your stress state is plane stress or your measured value
would be higher than the intrinsic fracture toughness.

3)    In Fig.3, your stress-strain curves are not very linear
and two curves have different slopes, which imply your system compliances are
different every time. How about other three curves? Of course, you use the maximum
stress value for your fracture toughness measurement. However, repeatable
experiments are important too.

Roy

Ting Zhu's picture

Roy,

Thanks for your interest in our work.

Fracture of pre-cracked graphene can be well characterized by the linear elastic fracture mechanics. Hence the crack-tip blunting effect on fracture toughness should be small. The blunting effect has been studied in Fig. 4f of the paper by atomistic simulations.

The atomically thin sheet of graphene is obvisouly under the plane stress condition. The measured fracture toughness of graphene represents its plane stress value. In linear elastic fracture mechanics, the influence of plane-strain versus plane-stress deformation on fracture toughness can be characterized through the effective elastic modulus.

We have been trying to improve the quantitative accuracy of in situ nanomechanical testing of atomically thin graphene. At the moment, while there are still some variations in the stress-strain curves, the measured fracture stresses are overall consistent and repeatable.

Ting

Rui Huang's picture

Dear Ting:

Thank you for the interesting theme! I am glad to see your recent work on fracture toughness of graphene, showing the robustness of linear elastic fracture mechanics down to the atomic layer. I can image how challenging the experiments were. Despite the limted data, the evidence is strong that graphene is brittle at the room temperature. The obtained toughness value (~16 J/m^2) is much larger than typical values for brittle fracture (~4 J/m^2 for silica glass), but it is consistent with the edge energy of graphene. I would be happier if you had compared to the predicted edge energy for graphene (see Table I in: Excess energy and deformation along free edges of graphene nanoribbons, Physical Review B 81, 155410, 2010). DFT and MD calculations have predicted values around 10-15 eV/nm for the edge energy, while the value you obtained is about 17 eV/nm. Pretty good in my opinion considering the challenging measurements!

Best regards,

RH

Ting Zhu's picture

Dear Rui,

Thanks for your comments. Your paper is very nice. Indeed the MD prediction of Gc is about 30% lower than the experimental measurement. In the paper we discussed the factors that might cause this difference, such as crack blunting, crack orientation, polycrystalline microstructure, defect, lattice trapping, and so on. More systematic studies are needed to identify the most important factor in the future.

Ting 

It is a valuable experimental measurement of the toughness of graphene by profs Zhu and Lou!

The Kc was ever thought to be related to the stress states around the tip, ~ 2.63–3.38 nN Å−3/2, see our predictions http://imechanica.org/node/13272 ,

see  http://apl.aip.org/resource/1/applab/v101/i12/p121915_s1

 

Ting Zhu's picture

Bin, thanks for pointing out your MD work. Indeed the fracture of 2D materials can be affected by the loading mode. In the paper we focus on mode I fracture. Other groups have studied the tearing of graphene. But it is hard to achieve a quantitative control during tearing of atomically thin sheets like graphene in experiments.

Yong Zhu's picture

Hi Ting,

Thank you for initiating this very interesting topic. Your examples nicely demonstrated the power of the synergy between in-situ nanomechanical testing and atomistic simulations. I have a comment
on the importance of interior defects in nanostructures (e.g.,
crystalline nanowires with diameter < 100 nm) in addition to free surfaces. A number of atomistic
simulations have focused on the surface effects for both semiconductor
and metal nanowires, which are certainly critical due to increasing
surface to volume ratio. What's more, nanowires (including bottom-up
synthesized ones that are often assumed defect-free) contain different
types of interior defects. As an example, our recent work on SiC
nanowires showed high density of twins and stacking faults present
inside the nanowires (“Mechanical Properties of Silicon Carbide
Nanowires: Effect of Size-Dependent Defect Density
", Nano Letters 14,
754, 2014). Such defects were found to have important effects on the
crack initiation and propagation. Also of importance are surface
defects. I hope more atomistic simulations will look into these defects
in nanostructures.

As experimentalists we like to advance nanomechanical testing methods. One question is what types of testing capabilities you would like to
have from the simulation perspective. For instance, we recently
developed temperature control capability on top of a MEMS nanomechanical
testing platform (“A microelectromechanical system for thermomechanical
testing of nanostructures
", Applied Physics Letters 103, 263114, 2013).
Another recent work is from Dan Gianola’s group at UPenn ("Temperature
controlled tensile testing of individual nanowires”, Review of
Scientific Instruments, 85, 013901, 2014). What other testing
capabilities would you like to see?

Yong

Ting Zhu's picture

Dear Yong,

Thanks for your stimulating comments.

I totally agree that the internal defects could play as significant roles as surface defects in the mechanical response of nanostructures. Such internal defects could pre-exist or form during deformation. In addition to your work of SiC, ref. 8 from Greer and Gao's team shows an excellent example regarding how the internal defects (i.e., a high density of twin boundaries) affect the mechanical properties of Cu nanopillars by using in situ experiments and MD simulations.

As a modeler, I am very impressed by the nanomechanical testing capabilities being quickly developed in recent years, despite technical challenges. The temperature controlled testing from your group, Gianola’s group and others, in conjunction with strain rate controlled testing, will enable the exciting study of deformation kinetics and related strength-ductility properties. In the future, I would love to see the new testing capabilities, for example, with environmental control (e.g., moisture), multi-field coupling (e.g., electrical measurement), and complex loading (e.g., torsion).

Ting

Dan Gianola's picture

Dear Ting,

 Thanks for leading a nice and timely discussion highlighting the frontier of in situ nanomechanics.  Your collaborative work on 2D materials and the work led by Prof. Zhu represent some of the exciting areas of the field.  In particular, the importance of quantifying the role of small populations of (and even singular) defects in nanostructures has been receiving increased attention, and it interesting to note that some materials are dramatically affected by pre-existing defects, whereas others appear to be robustly defect-tolerant.  Developing some predictive capability to determine, a priori, intrinsic capacity for damage tolerance is, I think, an exciting avenue to pursue.  This topic feeds nicely into the current iMechanica journal theme on discrete dislocation dyanamics , led by Prof. El-Awady. 

 To continue the thread that you and Yong began on new experimental developments in the field, I wanted to highlight a couple of neat examples:

 

  • Electrochemical control of surfaces in nanostructures.  Given the often dominant role of free surfaces (or interfaces) in controlling the properties of nanostructures, an exciting experimental area is where the surface charge/oxidation state can be electrochemically tuned.  This can be particularly importance in properties that are affected by surface stress (e.g. surface dislocation nucleation, molecular adsorption enthalpies and rates, catatlytic activity, elasticity).  Prof. Jörg Weißmüller and his colleagues have reported on the electrochemical control of surfaces in nanoporous metals, which result in large changes to elastic and plastic behavior (see, e.g. Acta Mater. 61 (2013) 6301; Science 300 (2003) 312)
  • Novel in situ scattering methods.  The use of x-ray (both monochromatic and polychromatic), neutron, and visible light scattering experiments during in situ deformation have resulted in key contributions to our understanding of structure property relationships in materials.  Spatially resolving features at the nanoscale, though, has generally been a limitation relative to high energy electron probes.  Several groups have recently reported on the use of highly coherent X-ray diffraction (available at several large synchrotron facilities around the world), which following phase reconstruction of the scattering signals, allow for unprecdented spatial resolution and quantification of a good portion of the strain tensor.  (See, e.g. Nano Letters 13 (2013) 1883; J. Synch. Rad. 19 (2012) 688).  
  • Coupled in situ testing and strain engineering.  Many have already highlighted the importance of simultaneously measuring quantities from different physical fields (e.g. electromechanical coupling) for both emerging devices and for a richer understanding of atomic-scale behavior.  In parallel, the large range of elastic strain that many nanostructures exhibit has been identified as a means to tune other functional properties, as nicely reviewed in a recent MRS Bulletin issue (guest edited by Ju Li, En Ma, and Zhiwei Shan).  Several groups (including Yong's, Prof. Espinosa's at Northwestern, Prof. Kraft's at KIT, and many others) have shown interesting results on the coupling between mechanical and electrical fields in single nanowires. I'd like to highlight a recent study from our own research group (experiments led by Kate Murphy) showing the coupling between strain and thermal conductivity in individual Si nanowires (Nano Letters, ASAP ).

 

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