Name : Dr. Tushar Sakorikar

Ph.D. graduated in 2019

Title: To crack or not to crack: Geometry Decides!

Walls of a building, bark of a tree, dried mud on a roadside, our foot during winter or a glass vessel just before it breaks into pieces! All these entities have one thing in common which is crack or fracture. Cracks or fractures are a very common sight for all of us whether or not we register it consciously. We consider them in general to be a flaw or something that totally needs to be avoided and to be honest I was on the same page as most of us are as far as these intriguing features are concerned. However, this changed during my PhD work at IIT Madras and this is my story of how I was able to understand or rather “crack these cracks!”

Genesis
As a part of my research, we were trying to study how graphene oxide (GO) , a layered graphene structure decorated with oxygen functional groups, can be used for applications of stretchable electronics. Flexible and stretchable electronics are poised to become the technologies of the future or rather they have already started to be with the advent of flexible displays from companies like LG and Samsung. To be able to be used for such applications, the first step is to understand the correlation between electrical and mechanical properties of layered graphene, our material of interest. We used an elastomer film which is as stretchable as commonly used rubber-band, as a substrate on which layered graphene was deposited to obtain layered graphene/elastomer structure.

The next step was then to test the ability of the film to withstand mechanical deformation like stretching, without compromising on its electrical conduction, to check its feasibility for stretchable electronics. In order to test this, we connected electrical leads to a layered graphene/elastomer sample and the other end to a measuring unit meant to observe the changes in electrical resistance upon gradual straining. Now here starts the interesting part.

Finding the ‘Crack’
On a very usual day (which later turned out be unusual) I started the experiment, wherein, the layered graphene/elastomer sample was progressively stretched till a predefined strain value and then gradually released back to its original position, all the while its electrical resistance being recorded. Such a study is also called electromechanical characterization. The study revealed that the electrical resistance increased with the increasing strain and it decreased gradually to almost reach its original value once the strain was completely released. As if this was not interesting, there was one more peculiar observation. When the layered graphene/elastomer sample strained or stretched, some “line-like” features appeared on the surface and surprisingly they disappeared when the sample was brought back to its original unstrained state. This was also reflected in its electrical resistance behaviour wherein, along with the increasing strain, the electrical resistance increased (i.e. it conducted less electricity) by more than 10 times and as a surprise it almost reached its initial value once the sample was released back to its original state!

The observation was very unusual and therefore, to probe further, especially onto those intriguing lines as the layered graphene/elastomer sample was strained, we observed the sample under an optical microscope. What we found is a beautiful pattern of straight lines oriented perpendicular to the direction of applied strain. The ordered pattern of straight lines really amazed us and to probe further in finer detail we chose to take help of a different kind of microscope which uses a powerful beam of electrons instead of light and due to this reason, they are known as Scanning Electron Microscopes (SEM). These microscopes can help us get 3-D images at a very high magnification capability that can help us visualize at a scale of hundreds of nanometres (less than a hundredth of the size of human hair!). When we observed the top view of layered graphene/elastomer sample under the SEM, after being strained at a certain value, we found quasi straight and almost equally spaced openings in layered graphene with a width in the range of 0.2 µm to 3 µm (human hair is roughly 100 µm in diameter). These openings, which were visible as lines in the optical microscope, were nothing but “cracks” in layered graphene film spread across the length of the film, oriented perpendicular to the direction of stretching. We then visualized the same samples under SEM once they were released back to their original unstrained state. What we found was very surprising. Upon release of strain, the layers of layered graphene film ‘draped’ or ‘covered’ the crack opening by overlapping each other!

A clear explanation for the “reversible” electrical resistance changes as we strained the sample could then be given as follows. Stretching of layered graphene/elastomer samples led to crack formation which disrupted the electrical conduction pathways leading to an increase in the resistance. On the other hand, as the sample was released from strain, the layers of graphene draped or overlapped each other at the crack opening resulting in the restoration of those disrupted or broken electrical conduction pathways, and thus bringing the sample almost back to its initial conductive state.

Cracking the crack
Having established the correlation between electrical resistance change and the crack formation as we strained the layered graphene/elastomer sample, the next inevitable step was to understand the basics of crack formation. This would help us in tuning the electrical response upon stretching by controlling the crack formation. Being an electronics and communication engineer by virtue of my undergraduate studies, studying cracks and then analysing them was a whole different experience for me. It was an unknown but exciting path to tread, especially when it received equal support from my advisor and mentor (a postdoc in our lab). I then started reading more and more about the basics of crack formation and propagation, what factors affect it and how it can be controlled. I then started observing cracks in my surroundings like, in a dried mud, tree trunk, walls, metal pillars, roads and what not. This made me realize that crack is so ubiquitous in our lives and so are many other things which have a deep science embedded in them. The need is just to be observant enough. Coming back, what was found from a preliminary study of literature on crack formation in thin films by Xia and Hutchinson is that by varying the thickness of layered graphene film, one could control the crack formation. To test this concept, we performed similar experiments on layered graphene/elastomer samples with different layered graphene thicknesses and then observed the samples under SEM. We found that there is indeed a strong thickness dependence on the electromechanical response (electrical resistance changes under mechanical deformation) and that it is directly correlated to (a) the number of cracks formed in a given area (density of cracks in other words) and (b) size of the crack opening. By analysing the results from layered graphene of different thicknesses we found that for certain thickness values where the crack density and crack opening width values hit a sweet spot, the sample resisted crack formation. For all other thicknesses it favoured crack formation, which was also reflected in a larger change in the electrical resistance when compared to crack resistant film, upon stretching.

We then thought if such a tunability can be put to use for some useful application. The first natural application to think of was to sense the applied strain by correlating it with the d change in electrical resistance due to crack formation. We performed several cycles of straining-releasing the sample to test the robustness and repeatability of our sample. The layered graphene/elastomer sample with the crack favouring thickness proved to be robust stretchable strain sensor which could sense reliably up to 10% strain with performance comparable and in cases better than the stretchable sensors reported. The concept of thickness corresponding to crack resistant film was put to use by using a layered graphene composite to develop stretchable UV photodetector.

Another aspect of geometry apart from film thickness is the width of the film and this is what we tried next. We reduced the width of layered graphene to 5µm and then performed a similar set of experiments as aforesaid. The idea was to see if layered graphene can be used as an electrode for miniaturized flexible/stretchable electronic devices, where conventionally metals like copper are used. The problem with such metals is that they get cracked permanently unlike layered graphene. From our experiments we found that layered graphene can still show the reversible nature of electrical resistance change. These results are encouraging enough to show the potential of layered graphene in future flexible/stretchable electronics.

The results were published in Scientific Reports,7, 2598, 2017 Link and Carbon, 158, 864, 2020 Link and were also picked up by a media outlet Link

Cracks or fractures which are generally perceived as a sign of failure of a material can also turn out to be a platform for highly sensitive sensors.