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Kirigami/origami: Unfolding the new regime of advanced 3D micro-/nanofabrication with 'folding'

Posted By Graphene Council, Wednesday, May 27, 2020
3D micro-/nanofabrication holds the key to build a large variety of micro-/nanoscale materials, structures, devices, and systems with unique properties that do not manifest in their 2D planar counterparts. Recently, scientists have explored some very different 3D fabrication strategies such as kirigami and origami that make use of the science of cutting and folding 2D materials/structures to create versatile 3D shapes. Such new methodologies enable continuous and direct 2D-to-3D transformations through folding, bending and twisting, with which the occupied space can vary "nonlinearly" by several orders of magnitude compared to the conventional 3D fabrications. More importantly, these new-concept kirigami/origami techniques provide an extra degree of freedom in creating unprecedented 3D micro-/nanogeometries beyond the imaginable designs of conventional subtractive and additive fabrication.

In a new paper published in Light: Science & Applications, Chinese scientists from Beijing Institute of Technology and South China University of Technology made a comprehensive review on some of the latest progress in kirigami/origami in micro-/nanoscale. Aiming to unfold this new regime of advanced 3D micro-/nanofabrication, they introduced and discussed various stimuli of kirigami/origami, including capillary force, residual stress, mechanical stress, responsive force and focused-ion-beam irradiation induced stress, and their working principles in the micro-/nanoscale region. The focused-ion-beam based nano-kirigami, as a prominent example coined in 2018 by the team, was highlighted particularly as an instant and direct 2D-to-3D transformation technique. In this method, the focused ion beam was employed to cut the 2D nanopatterns like "knives/scissors" and gradually "pull" the nanopatterns into complex 3D shapes like "hands". By utilizing the topography-guided stress within the nanopatterns, versatile 3D shape transformations such as upward buckling, downward bending, complex rotation and twisting of nanostructures were precisely achieved.

As discussed in this review, the unprecedented micro-/nanoscale geometries created by kirigami/origami have brought about extensive potentials for the reshaping of 2D materials, as well as in biological, optical, and reconfigurable applications. Moreover, 3D transformations of emerging 2D materials (such as graphene, MoS2, MoS2, WSe2 and PtSe2), for example, were briefly introduced and the associated new electrical and mechanical properties were uncovered.

"Advanced kirigami/origami provides an easily accessible approach for the modulation of mechanical, electrical, magnetic and optical properties of existing materials, with remarkable flexibility, diversity, functionality, generality and reconfigurability", they said. "These key features clearly differentiate the facile kirigami/origami from other complicated 3D nanofabrication techniques, and make this new paradigm technique unique and promising for solving many difficult problems in practical applications of micro/nano-devices."

Furthermore, they discussed the current challenges in kirigami/origami-based 3D micro-/nanofabrication, such as the limited strategies of stimuli and reconfigurations, and the difficulties in on-chip and large-scale integration. "When these challenges are met and the advantages are fully adopted," they envisioned, "micro-/nanoscale kirigami/origami will greatly innovate the regime of 3D micro-/nanofabrication. Unprecedented physical characteristics and extensive functional applications can be achieved in wide areas of optics, physics, biology, chemistry and engineering. These new-concept technologies, with breakthrough prototypes, could provide useful solutions for novel LIDAR/LADAR systems, high-resolution spatial light modulators, integrated optical reconfigurations, ultra-sensitive biomedical sensors, on-chip biomedical diagnosis and the emerging nano-opto-electro-mechanical systems."

Tags:  2D materials  Beijing Institute of Technology  Graphene  nanofabrication  South China University of Technology 

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Won’t crack under pressure: stress test reveals graphene can withstand more than one billion cycles before breaking

Posted By Graphene Council, The Graphene Council, Thursday, January 30, 2020
Graphene is a paradox: it is the thinnest material known to science, yet also one of the strongest. Now, research from U of T Engineering shows that graphene is also highly resistant to fatigue — able to withstand more than a billion cycles of high stress before it breaks.

Graphene resembles a sheet of interlocking hexagonal rings, similar to the pattern you might see in bathroom flooring tiles. At each corner is a single carbon atom bonded to its three nearest neighbours. While the sheet could extend laterally over any area, it is only one atom thick.

The intrinsic strength of graphene has been measured at more than 100 gigapascals, among the highest values recorded for any material. But materials don’t always fail because the load exceeds their maximum strength. Small repetitive stresses can weaken materials by causing microscopic dislocations and fractures that slowly accumulate over time, a process known as fatigue.

“To understand fatigue, imagine bending a metal spoon,” says Professor Tobin Filleter (MIE), one of the senior authors of the study, which was recently published in Nature Materials. “The first time you bend it, it just deforms. But if you keep working it back and forth, eventually it’s going to break in two.”

The research team — consisting of Filleter, fellow U of T Engineering professors Chandra Veer Singh (MSE) and Yu Sun (MIE), their students, and collaborators at Rice University — wanted to know how graphene would stand up to repeated stresses. Their approach included both physical experiments and computer simulations.

“In our atomistic simulations, we found that cyclic loading can lead to irreversible bond reconfigurations in the graphene lattice, causing catastrophic failure on subsequent loading,” says Singh, who along with postdoctoral fellow Sankha Mukherjee (MSE) led the modelling portion of the study. “This is unusual behaviour in that while the bonds change, there are no obvious cracks or dislocations, which would usually form in metals, until the moment of failure.”

PhD candidate Teng Cui, who is co-supervised by Filleter and Sun, used the Toronto Nanofabrication Centre to build a physical device for the experiments. The design consisted of a silicon chip etched with half a million tiny holes only a few micrometres in diameter. The graphene sheet was stretched over these holes, like the head of a tiny drum.

Using an atomic force microscope, Cui then lowered a diamond-tipped probe into the hole to push on the graphene sheet, applying anywhere from 20 to 85 per cent of the force that he knew would break the material.

“We ran the cycles at a rate of 100,000 times per second,” says Cui. “Even at 70 per cent of the maximum stress, the graphene didn’t break for more than three hours, which works out to over a billion cycles. At lower stress levels, some of our trials ran for more than 17 hours.”

As with the simulations, the graphene didn’t accumulate cracks or other tell-tale signs of stress — it either broke or it didn’t.

“Unlike metals, there is no progressive damage during fatigue loading of graphene,” says Sun. “Its failure is global and catastrophic, confirming simulation results.”

The team also tested a related material, graphene oxide, which has small groups of atoms such as oxygen and hydrogen bonded to both the top and bottom of the sheet. Its fatigue behaviour was more like traditional materials, in that the failure was more progressive and localized. This suggests that the simple, regular structure of graphene is a major contributor to its unique properties.

“There are no other materials that have been studied under fatigue conditions that behave the way graphene does,” says Filleter. “We’re still working on some new theories to try and understand this.”

In terms of commercial applications, Filleter says that graphene-containing composites — mixtures of conventional plastic and graphene — are already being produced and used in sports equipment such as tennis rackets and skis.

In the future, such materials may begin to be used in cars or in aircraft, where the emphasis on light and strong materials is driven by the need to reduce weight, improve fuel efficiency and enhance environmental performance.

“There have been some studies to suggest that graphene-containing composites offer improved resistance to fatigue, but until now, nobody had measured the fatigue behaviour of the underlying material,” he says. “Our goal in doing this was to get at that fundamental understanding so that in the future, we’ll be able to design composites that work even better.”

Tags:  Graphene  Graphene Composites  nanofabrication  Rice University  Teng Cui  Tobin Filleter  University of Toronto Engineering 

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