Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - Strained2DMaterials (Unlocking new physics in controllably strained two-dimensional materials)

Teaser

The overarching goal of Strained2DMaterials is to use strain engineering as an enabling tool to study previously inaccessible or hard-to-study phenomena in two-dimensional atomic crystals (e.g.: graphene, bilayer graphene, and monolayer transition metal dichalcogenides). We...

Summary

The overarching goal of Strained2DMaterials is to use strain engineering as an enabling tool to study previously inaccessible or hard-to-study phenomena in two-dimensional atomic crystals (e.g.: graphene, bilayer graphene, and monolayer transition metal dichalcogenides). We intend to: (i) Demonstrate the dominant effect of commonly neglected flexural phonons on electrical transport, thermal conductivity, and mechanical properties of suspended graphene. (ii) Observe the effects of “pseudomagnetic fields” in controllably strained graphene and TMDCs. These observations may lead to realization of devices employing Quantum Hall effect but operating at zero magnetic field. (iii) Develop the techniques to control and tune excitons in TMDC material using uniform and non-uniform strain fields. These techniques may be used to create efficient photoconverting devices and/or building blocks for excitonic electronics.

Work performed

We designed and implemented a new approach to induce uniform and non-uniform strain distributions in 2D materials. At the heart of our approach is a custom (piezoelectric) atomic force microscope (AFM) integrated into a measurement cryostat with electro-optical access. The setup is all-electrical, integrates into an optical cryostat and allows for optical measurements simultaneous with mechanical “pushing”. Using that new setup, we explored several previously inaccessible physical regimes. First, we created non-uniform strain distributions in TMDCs and realized long-discussed exciton “funneling” regime. In that regime, optically generated excitons are driven towards the region of maximal strain by the entropic forces. We observed optical signatures consistent with funneling that only appear in strained devices. Second, we induced non-uniform strain in multilayer hBN and observed signatures consistent strain-activated single quantum emitters (SQEs). Such SQEs may be interesting for telecommunication technologies.

We experimentally demonstrated mechanical softening of two-dimensional graphene and proved that it is related to static crumpling of graphene. It is universally assumed that two-dimensional materials such as graphene are ultra-stiff, have low bending rigidity, small positive Poisson’s ratio, and obey conventional Hooke’s law with mechanical constants similar to that of bulk crystals. The conclusions of hundreds of experimental and theoretical works that use 2D materials e.g. as mass or force sensors rest on this assumption. We showed that this assumption is incorrect, in general, and demonstrated that the inevitable out-of-plane crumpling of 2D materials completely changes their mechanical response in the regime of small extensions. Specifically: 1) We developed a new all-optical technique to quantify the amount of crumpling and relate it to the geometrical hidden area of a 2D material. 2) We showed that the mechanical response of graphene is either linear or nonlinear depending on the degree of crumpling. 3) We confirmed that in the non-linear regime, graphene obeys the “anomalous Hooke’s law” that has been predicted but hitherto unobserved. 4) We obtained preliminary signatures suggesting that the bending rigidity of monolayer graphene suspended in vacuum is renormalized from about 1eV to higher than thousands of eV due to crumpling. 5) We developed a technique to experimentally probe the Poisson ratio of 2D membranes and are now conducting experiments searching for predicted strain-dependent negative Poisson ratio.

Together with the group of Saikat Ghosh (IIT Kanpur), we investigated 2D material membranes towards applications in NEMS (nanoelectromechanical systems). Specifically, we studied graphene resonators deposited on a much larger and heavier SiNx membrane. We demonstrated widely tunable, broad bandwidth, and high gain all-mechanical motion amplifiers based on graphene/silicon nitride (SiNx) hybrids. In these devices, a tiny motion of a large-area SiNx membrane is transduced to a much larger motion in a graphene drum resonator coupled to SiNx. We obtain a displacement power gain of 38 dB and demonstrate 4.7 dB of squeezing, resulting in a detection sensitivity of 3.8 fm/Hz^0.5, close to the thermal noise limit of SiNx. Furthermore, we discovered that strong coupling to mechanically non-linear graphene induces large non-linearity in normally mechanically linear SiNx. The induced non-linearity in SiNx allowed us to observe a range of behavior previously unseen in SiN resonators including frequency comb generation and Arnold tongues.

Final results

The main achievements of the project so far are as follows:

1) Discovery of a non-linear Hooke\'s law in crumpled graphene
2) Experimental discovery of exciton funneling in non-uniformly strained TMDCs and single quantum emitters in non-uniformly strained hBN
3) Development of NEMS devices based on coupled SiNx/graphene hybrids: ultrasensitive mechanical amplifiers and devices with tunable nonlinearities

Until the end of the project, we plan to achieve the following main goals:

1) Exploration of crumpling-induced renormalization of the Poisson ratio and bending rigidity of 2D materials.
2) Demonstration of tunable pseudomagnetic fields in TMDC and graphene. Exploration of the emergent physical phenomena related to these fields.
3) Creation of highly non-linear NEMS devices based on graphene/SiNx hybrids. Optomechanical manipulation of these devices.
4) Development of techniques to probe surface contamination of 2D materials, towards creation of devices with well-characterized mechanical properties.